1
THE LATE MIOCENE THROUGH MODERN EVOLUTION OF THE ZHADA
BASIN, SOUTH-WESTERN TIBET
by
Joel Edward Saylor
A Dissertation Submitted to the Faculty of the
DEPARTMENT OF GEOSCIENCES
In Partial Fulfillment of the Requirements for the Degree of
DOCTOR OF PHILOSOPHY
In the Graduate College
THE UNIVERSITY OF ARIZONA
2008
2
THE UNIVERSITY OF ARIZONA
GRADUATE COLLEGE
As members of the Dissertation Committee, we certify that we have read the dissertation
prepared by: Joel E. Saylor
entitled: The Late Miocene Through Modern Evolution of the Zhada Basin, South-western
Tibet
and recommend that it be accepted as fulfilling the dissertation requirement for the
Degree of: Doctor of Philosophy
_______________________________________________________________________
Date:
Peter DeCelles
_______________________________________________________________________
Date:
Jay Quade
_______________________________________________________________________
Date:
Paul Kapp
_______________________________________________________________________
Date:
George Gehrels
_______________________________________________________________________
Date:
Mihai Ducea
Final approval and acceptance of this dissertation is contingent upon the candidate’s
submission of the final copies of the dissertation to the Graduate College.
I hereby certify that I have read this dissertation prepared under my direction and recommend
that it be accepted as fulfilling the dissertation requirement.
________________________________________________ Date:
Dissertation Director: Peter DeCelles
________________________________________________ Date:
Dissertation Director: Jay Quade
3
STATEMENT BY THE AUTHOR
This dissertation has been submitted in partial fulfillment of requirements for an
advanced degree at The University of Arizona and is deposited in the University Library
to be made available to borrowers under rules of the Library.
Brief quotations from this dissertation are allowable without special permission,
provided that accurate acknowledgement of source is made. Requests for permission for
extended quotation from or reproduction of this manuscript in whole or in part may be
granted by the head of the major department of the Dean of the Graduate College when in
his or her judgment the proposed use of the material is in the interests of scholarship. In
all other instances, however, permission must be obtained from the author.
Signed: Joel Saylor
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ACKOWLEDGEMENTS
This dissertation would not have been possible without the help of many people.
Foremost, my family: Jeannette, and Eirena for putting up with an absent (or busy)
husband and father; Gary, Lita, and Nathaniel for their support and raising me. I owe a
lifetime debt to my advisors: Drs. Pete DeCelles and Jay Quade for their patience,
tutelage, and unending support. I also owe thanks to Dr. Paul Kapp for reading chapters
(many times over) and many good comments and discussions; Dr. George Gehrels for his
help with detrital zircon analyses and Dr. Mihai Ducea.
Likewise, this work would not have happened without my field assistants:
Jeannette Saylor, Scott McBride, Cai Fulong and our Tibetan crew. A special thanks to
Facundo Fuentes, John Volkmer, Ross Waldrip, Lynn Peyton, Jerome Guynn, Andrew
Leier, Aaron Martin, Matt Fabijanic, Adam and Amy Baker, Ari and Jane Heinze, Ryan
and Janine Wilkinson, Jon Dyhr, Lise Johnson, Carrie Coykendall, Meghan Field, Sam
Jayakanthan and many others from the Dept. of Geosciences, Graduate Christian
Fellowship, and Northwest Community Friends Church for laughs, commiseration, and
generally making life enjoyable. To others who have helped me work through ideas,
given me pointers or hung out: thank you.
Finally, I would like to thank those who I have worked with in the course of this
project and hope to work with again: Dick Heermance for help with cosmogenic nuclide
work; Mike Murphy and Ran Zhang for ideas and research help; Tank Ojha for assistance
in the paleomagnetism lab; David Dettman for advice and assistance with stable isotope
analyses; Majie Fan for assistance with organic material analyses and useful discussions;
Victor Valencia for help with U-Pb analyses; and E. Lindsay and X. Wang for
identification of mammal fossils and biostratigraphy.
This research was supported by grants from the Geological Society of America,
the American Association of Petroleum Geologists, ExxonMobil, Chevron-Texaco, The
Galileo Circle of the U of A and National Science Foundation (grants EAR-0443387 and
0732436) and the National Science Foundation Tectonics Program.
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DEDICATION
Pro Veritas
6
TABLE OF CONTENTS
LIST OF FIGURES………………………………................................………………...9
LIST OF TABLES……………………………............................……………………...11
ABSTRACT…………………………………....................................…………………..12
CHAPTER 1: INTRODUCTION……………………........................................……...13
CHAPTER 2: THE LATE MIOCENE THROUGH PRESENT
PALEOELEVATION HISTORY OF SOUTHWESTERN TIBET...........................17
ABSTRACT………………………………………........................…………………17
INTRODUCTION……..…..……………………................................………....….19
Previous Paleoelevation Work…………………..……........................…....…..20
OXYGEN ISOTOPE PALEOALTIMETRY……………............................……..23
REGIONAL SETTING OF ZHADA BASIN……………....................…………..27
METHODS AND MATERIALS…………………........................…………....…..30
Age Control………………………....................................……………………...30
Modern Water…………………………....................................………………..32
Gastropods………………………………………............................……………33
Zhada Formation Plant Material…………………....................……………...34
RESULTS……………………………………........................................…………...36
Age……………………………………....................................………………….36
Modern Water………………………................................……………………..38
Gastropods……………………………………................................……………39
Zhada Formation Plant Material…………………........…....................……...40
APPLICATION TO PALEOALTIMETRY………………............................…...41
Calculation of Miocene δ18Osw and Related Constraints and Corrections.....41
Source, pathway and amount effect constraints..............................................41
Shell preservation............................................................................................43
Calculation of Miocene δ18Opsw.......................................................................43
Comparison with Models....................................................................................44
Effect of paleotemperature...............................................................................44
Modelling changes in lapse rate......................................................................45
Evaporation vs. Outflow..................................................................................46
Temperature of Modern Gastropod Shell Precipitation..................................48
Paleoelevation models.....................................................................................49
DISCUSSION AND CONCLUSIONS.....................................................................55
Oxygen Isotopes from Zhada Basin...................................................................55
Oxygen Isotopes from Zhongba..........................................................................58
Carbon Isotopes...................................................................................................59
Application to Tectonic Models..........................................................................60
CONCLUSIONS........................................................................................................62
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TABLE OF CONTENTS – Continued
CHAPTER 3: BASIN FORMATION DUE TO ARC-PARALLEL EXTENSION
AND TECTONIC DAMMING: ZHADA BASIN, SW TIBET...................................95
ABSTRACT………………………………………....................................................95
INTRODUCTION………………………………….................................................96
GEOLOGIC SETTING…………………………………........................................98
SEDIMENTOLOGY OF THE ZHADA FORMATION.....................................100
Fluvial Association…………………………………………............................100
Lithofacies Association F1: Gcmi, Gch, Gt, Gcf...........................................100
Lithofacies Association F2: St, Sp, Sh, Sc.....................................................101
Lithofacies Association F3: Sm, Sc, Mm, Mc, Ml, Mh...................................101
Interpretation.....................................................................................................102
Supra-littoral (Lake Margin) Association.......................................................103
Lithofacies Association S1: Gct, Gcmi, Sp, St, Sh, Sm, Sr.............................103
Lithofacies Association S2: Sh.......................................................................104
Lithofacies Association S3: Sf........................................................................104
Interpretation.....................................................................................................104
Littoral Association............................................................................................105
Lithofacies Association L1: Ml......................................................................105
Lithofacies Association L2: Mr, Sr, Srw, Sp, Sh, Sm.....................................105
Interpretation.....................................................................................................106
Profundal Association........................................................................................106
Lithofacies Association P1: Mh, Mm.............................................................106
Lithofacies Association P2: Sh, Sm, Mh........................................................107
Interpretation.....................................................................................................107
Alluvial Fan Association....................................................................................108
Lithofacies Association A1: Gcm, Gch, Gcmi...............................................108
Lithofacies Association A2: Gmm, Gcm........................................................108
Lithofacies Association A3: Gcmi, Gch, Gcf.................................................108
Lithofacies Association A4: Gx......................................................................109
Interpretation.....................................................................................................109
Zhada Formation members..............................................................................110
PALEOCURRENT MEASUREMENTS...............................................................112
PROVENANCE ANALYSIS..................................................................................113
Methods...............................................................................................................113
Sandstone petrography and conglomerate clast counts................................113
U-Pb geochronologic analyses of detrital zircons.........................................113
Results.................................................................................................................115
Sandstone and conglomerate modal compositions........................................115
U-Pb geochronologic analyses of detrital zircons.........................................117
Summary of detrital zircon U-Pb analyses....................................................120
Interpretation.....................................................................................................120
Sandstone and conglomerate provenance.....................................................120
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TABLE OF CONTENTS – Continued
U-Pb geochronologic analyses of detrital zircons.........................................123
SUBSIDENCE CURVE...........................................................................................125
DISCUSSION...........................................................................................................126
Zhada Basin Evolution......................................................................................126
Tectonic Origins of the Zhada Basin................................................................128
SYNTHESIS.............................................................................................................133
CONCLUSIONS......................................................................................................135
CHAPTER 4: CLIMATE-DRIVEN ENVIRONMENTAL CHANGE IN THE
SOUTHERN TIBETAN PLATEAU............................................................................201
ABSTRACT..............................................................................................................201
INTRODUCTION...................................................................................................202
REGIONAL GEOLOGICAL SETTING..............................................................205
METHODS...............................................................................................................207
Sedimentology....................................................................................................207
Correlations........................................................................................................207
Frequency analysis of Zhada Formation cycles..............................................207
Stable Isotopes....................................................................................................209
RESULTS.................................................................................................................211
Sedimentology....................................................................................................211
Depositional Cycles in the Zhada Formation..................................................211
Correlations........................................................................................................212
Frequency analysis of Zhada Formation cycles..............................................214
Stable Isotopes....................................................................................................215
INTERPRETATION OF ZHADA FORMATION CYCLES.............................216
DISCUSSION...........................................................................................................218
Sequence Stratigraphic and Lithostratigraphic Correlations.......................218
Frequency analysis.............................................................................................221
Isotopes in Zhada Formation cycles.................................................................222
Basin history.......................................................................................................224
Global climate change and its impact on the southern Tibetan Plateau......225
CONCLUSIONS...............................................................................................230
WORKS CITED......................................................................................................277
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LIST OF FIGURES
Figure 2.1: Location map…………………………………………………...................... 63
Figure 2.2: Vector component diagrams for the South Zhada Section............................. 64
Figure 2.3: Site and section mean vectors for South Zhada and Southeast Zhada............65
Figure 2.4: Modern water and gastropod sample locations from the Zhongba area..........66
Figure 2.5: δ13C of vascular organic material versus stratigraphic level for the South
Zhada and East Zhada measured sections..............................................................67
Figure 2.6: Measured sections and magnetostratigraphic results……………..................68
Figure 2.7: Stable isotope composition of modern water from the Zhada Basin and
Zhongba area plotted against the Global Meteoric Water Line………….............70
Figure 2.8: X-Ray diffraction data…………………………………………….................71
Figure 2.9: Oxygen and carbon isotope covariance for gastropods……….......................72
Figure 2.10: δ18O vs. stratigraphic height for Miocene – Pleistocene gastropods.............74
Figure 2.11: δ18O vs. mm growth from seasonally sampled Miocene – Pleistocene
gastropods………………………………………………………..........................76
Figure 2.12: Calculated δ18Opw values from gastropods....................................................77
Figure 2.13: Modelled enrichment values.........................................................................79
Figure 2.14: Temperature of aragonite precipitation.........................................................80
Figure 2.15: Modelled percent of modern aragonite in equilibrium with monsoon wetland
waters…………………………………………………………….........................81
Figure 2.16: Δδ18O of modern water vs. mean and maximum catchment elevation.........82
Figure 2.17: Δδ18O values for the most negative modern Zhada water samples and most
negative reconstructed Miocene – Pleistocene water vs. modelled elevation.......83
Figure 2.18: Comparison of the range of Δδ18O values for reconstructed Miocene fluvial
water and modern water from the Zhada Basin verses elevation..........................84
Figure 3.1: Location map……………………………………………….........................136
Figure 3.2: Topographic profile along the Himalayan arc……………...........................138
Figure 3.3: Geologic map of the Zhada region................................................................140
Figure 3.4: Photos of stratigraphic relationships between the Zhada Formation and
basement…………………………………………………………......................141
Figure 3.5: Correlation of the composite magnetostratigraphic section with the GPTS of
Lourens et al. (2004)............................................................................................143
Figure 3.6: Measured sections…………………………………………….....................145
Figure 3.7: Photos of Fluvial association lithofacies…………………….......................151
Figure 3.8: Photos of Supra-littoral association lithofacies……………….....................153
Figure 3.9: Photos of Littoral association lithofacies……………………......................154
Figure 3.10: Photos of Profundal association lithofacies……………….........................155
Figure 3.11: Photos of Alluvial fan association lithofacies…………….........................156
Figure 3.12: Zhada Formation members……………………………..............................158
Figure 3.13: Paleocurrent data for the lower members and the upper members of the
Zhada formation………………………………………………...........................160
Figure 3.14: Photomicrographs of Zhada Formation sandstones…………....................162
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LIST OF FIGURES - Continued
Figure 3.15: QFL diagrams for samples from this study………………….....................164
Figure 3.16: U/Pb concordia plots for detrital zircons samples from this study............. 166
Figure 3.17: U/Pb relative age-probability density diagrams for detrital zircons and
possible source terranes.......................................................................................168
Figure 3.18: Subsidence curves.......................................................................................170
Figure 3.19: Growth structure in Zhada Formation.........................................................172
Figure 3.20: Photos of windgap.......................................................................................174
Figure 3.21: Perspective diagram of basin evolution...................................................... 176
Figure 3.22: Paleogeographic maps.................................................................................178
Figure 4.1: Location map.................................................................................................232
Figure 4.2: South Zhada lithologic section and magnetostratigraphic section and
correlation to the global polarity timescale (GPTS) of Lourens et al. (2004).....233
Figure 4.3: The synthetic wave form constructed for spectral analysis…......................234
Figure 4.4: Idealized forms of parasequence types A and B and interpreted depositional
environments.......................................................................................................236
Figure 4.5: Photos of parasequence types A and B.........................................................237
Figure 4.6: Detailed parasequence correlation................................................................238
Figure 4.7: Basin-wide lithostratigraphic and sequence stratigraphic correlations.........240
Figure 4.8: Photo of second order transgressive surface.................................................243
Figure 4.9: Frequency analysis........................................................................................244
Figure 4.10: South Zhada lithologic section and associated δ 18O values of aquatic
gastropods.............................................................. .............................................246
Figure 4.11: The trajectory of Zhada basin evolution in accommodation and sedimentand water-supply space....................................................................................... 248
Figure 4.12: δ18O and δ13C values of aquatic gastropods plotted against their normalized
height above the flooding surface........................................................................249
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LIST OF TABLES
Table 2.1: Stable isotope and elevation data for modern water from the Zhada Basin and
Zhongba area………………………………………………..................................85
Table 2.2: Oxygen and carbon isotope data (VPDB) from Miocene – Pleistocene
gastropods from Zhada Basin…………................................……....................... 87
Table 2.3: δ13C (VPDB ‰) for plant material from the South Zhada and East Zhada
sections……………………………………………………………...................... 93
Table 3.1: Lithofacies codes and interpretation………………………...........................180
Table 3.2: Paleocurrent data……………………………………………........................182
Table 3.3: Pointcount parameters………………........................................……............183
Table 3.4: Recalculated petrographic pointcount data…………………….....................184
Table 3.5:. Detrital zircon U-Pb data table…………………………………..................185
Table 4.1: Data used in frequency analysis………............................………….............251
Table 4.2: Lithofacies association codes, descriptions, and interpretations....................273
Table 4.3: Stable isotope data………………………………………………..................275
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ABSTRACT
The uplift history of the Tibetan Plateau is poorly constrained in part due to its
complex and extended tectonic history. This study uses basin analysis, stable isotope
analysis, magnetostratigraphy, detrital zircon U-Pb dating, and paleoaltimetry, and
frequency analysis to reconstruct the tectonic, spatial, and environmental evolution of the
Zhada basin in southwestern Tibet since the late Miocene. The Zhada Formation, which
occupies the Zhada basin and consists of ~ 850 m of fluvial, alluvial fan, eolian, and
lacustrine sediments, is undeformed and lies in angular unconformity above Tethyan
sedimentary sequence strata. The most negative Miocene δ 18Opsw (paleo-surface water)
values reconstructed from aquatic gastropods are significantly more negative than the
most negative modern δ18Osw (surface water) values. In the absence of any known
climate change which would have produced this difference, we interpret it as indicating a
decrease in elevation in the catchment between the late Miocene and the present. Basin
analysis indicates that the decrease in elevation was accomplished by two low-angle
detachment faults which root beneath the Zhada basin and exhume mid-crustal rocks.
This exhumation results from ongoing arc-parallel extension and provides
accommodation for Zhada basin fill. Sequence stratigraphy shows that the basin evolved
from an overfilled to an underfilled basin but that further evolution was truncated by an
abrupt return to overfilled, incising conditions. This evolution is linked to progressive
damming of the paleo-Sutlej River. During the underfilled portion of basin evolution,
depositional environments were strongly influenced by Milancovitch cyclicity:
particularly at the precession and eccentricity frequencies.
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CHAPTER 1: INTRODUCTION
The Tibetan Plateau is the largest high-elevation, low-relief plateau on earth.
Average elevations across the plateau are ~5,000 m (Fielding et al., 1994) and the area of
the plateau is > 5,000,000 km2 (Fielding, 1996). The plateau was formed by the early
Paleozoic – Tertiary collision of multiple continental fragments into the southern margin
of Eurasia and subduction of intervening oceans (Allegre et al., 1984; e.g., Dewey et al.,
1988; Leier et al., 2007; Murphy et al., 1997; Sengor and Natal'in, 1996; Yin and
Harrison, 2000). Models developed in the Himlayan/Tibetan orogen drive thinking about
orogens worldwide (e.g., Beaumont et al., 2004; Gerbault et al., 2005; Royden et al.,
1997) and uplift of the Tibetan Plateau is often linked to regional or global climate
change (e.g., Abe et al., 2005; An et al., 2001; France-Lanord and Derry, 1994; Molnar,
2005; Molnar et al., 1993; Raymo and Ruddiman, 1992; Ruddiman et al., 1997).
The temporal and spatial uplift of the Tibetan Plateau is essential to both tectonic
and climatic models. Yet the uplift history is poorly constrained. Paleoenvironmental
proxies from as late as the Pleistocene indicate that conditions on the Tibetan Plateau
were warmer and wetter than today (e.g., Axelrod, 1981; Cao et al., 1981; Li and Li,
1990; Li and Zhou, 2001a, b; Meng et al., 2004; Molnar, 2005; Wang et al., 2006; Wang
et al., 2008c; Xu, 1981; Zhang et al., 1981), indicative of lower elevations. Recent
quantitative paleoelevation studies suggest high elevations extending back at least to the
Eocene – Oligocene (Currie et al., 2005; Cyr et al., 2005; DeCelles et al., 2007; Garzione
et al., 2000a; Graham et al., 2005; Rowley and Currie, 2006; Rowley et al., 2001; Spicer
et al., 2003). This dichotomy and the underlying need to establish a comprehensive
14
paleoelevation record for the Tibetan Plateau was the first motivating factor in this
dissertation.
The second motivating factor is that the Zhada basin, being a large, late Cenozoic
basin surrounded by normal faults at anomalously low elevations on the southern Tibetan
Plateau, represents an unparalleled opportunity to study east-west extension in the
Himalayan-Tibetan orogen. Ongoing deformation in the southern Tibetan Plateau is
dominated by east-west extension, despite continuing northward movement of India (e.g.,
Armijo et al., 1986; Hurtado et al., 2001; Kapp and Guynn, 2004; Molnar et al., 1993;
Molnar and Tapponnier, 1978; Ni and York, 1978; Ratschbacher et al., 1994; Taylor et
al., 2003). East-west extension has variably been attributed to oroclinal bending
(Klootwijk et al., 1985; Ratschbacher et al., 1994), oblique convergence (McCaffrey and
Nabelek, 1998; Seeber and Pecher, 1998), outward radial expansion of the Himalayan
thrust front (Molnar and Lyon-Caen, 1988; Murphy and Copeland, 2005; Seeber and
Armbruster, 1984). East-west extension is best expressed in basins bounded by
approximately north-south trending normal faults. Basin fill in the internally drained
portion of the Tibetan Plateau is largely covered in Quaternary alluvium. Basins on the
southern, externally drained Tibetan Plateau present the best opportunity to study the
ongoing process of east-west extension. However, these basins remain largely unstudied
(e.g., Garzione et al., 2003).
The final motivating factor is that the Zhada basin has been poorly studied. The
Zhada basin contains an abundance of invertebrate, vertebrate and plant fossils (Li and
Li, 1990; Li and Zhou, 2001a, b; Meng et al., 2004; Yu et al., 2007; Zhang et al., 1981).
15
Despite the importance of the basin there is little agreement about even the most basic
aspects of it. Reports of the age, lithologies, paleoenvironment, paleoelevation,
originating cause, and basin history vary widely (e.g., Li and Zhou, 2001b; Meng et al.,
2008; Wang et al., 2008a; Wang et al., 2008b; Wang et al., 2004; Zhou et al., 2000; Zhu
et al., 2004).
The goal of this investigation is to provide a coherent paleoelevation history,
originating cause, basin evolution and paleoenvironmental reconstruction of the Zhada
basin. This data is used to test the proposed models for east-west extension in
southwestern Tibet. The data used in this study include, but are not limited to, >300
stable isotope analyses of carbonate and water, >730 paleomagnetic analyses from 184
sites, >800 paleocurrent measurements at ~ 80 sites, >4.8 km of lithologic section from
14 measured sections, 35 point counted petrographic thin sections and >750 U-Pb
analyses of detrital zircons.
The following chapters represent three manuscripts that are in various stages of
publication. Chapter 2, The late Miocene through present paleoelevation history of
southwestern Tibet, presents stable isotope evidence for high elevations in southwestern
Tibet extending back at least to the late Miocene. It also provides the first stable isotope
evidence for a loss of elevation in southwestern Tibet and tentatively links that loss of
elevation with crustal thinning associated with east-west extension. This manuscript has
been accepted for publication in the American Journal of Science.
Chapter 3, Basin formation due to arc-parallel extension and tectonic damming:
Zhada basin, SW Tibet tracks the evolution of the Zhada basin from a through-flowing
16
river to a closed-basin lake and to its final stage as a deeply exhumed open basin. Basin
evolution is described in terms of sedimentology, sediment provenance and sediment
dispersal patterns. This data set is used to propose that the originating cause for the basin
was subsidence and behind an uplifting sill due to arc-parallel extension. This
manuscript will be submitted to Tectonics (or GSAB).
Chapter 4, Climate-driven environmental change in the southern Tibetan Plateau,
decouples environmental change on the southwestern Tibetan Plateau from uplift of the
plateau through sequence stratigraphic analysis of the Zhada basin fill. Faunal and floral
changes in the basin are shown to be synchronous with changes in basins surrounding the
Tibetan Plateau and likely due to regional or global climate change. Stable isotopes
reveal distinct drying episodes which are linked to Milankovitch cycles. This manuscript
will be submitted to Sedimentology.
17
CHAPTER 2: THE LATE MIOCENE THROUGH PRESENT
PALEOELEVATION HISTORY OF SOUTHWESTERN TIBET
ABSTRACT.
Recent research using stable isotopes of carbon and oxygen from carbonates and
fossil teeth seems to support both a pre- and post-mid-Miocene uplift of the southern
Tibetan Plateau. We examined this issue by analysis of well-preserved fossil mollusks
and plant remains from the Zhada Basin in southwestern Tibet, which ranges in age from
~ 9.2 - <1 Ma. Based on δ18Occ values from shell aragonite, we estimate that oxygen
isotope ratios of Miocene – Pleistocene paleo-surface water (δ18Opsw) in Zhada Basin
ranged from –24.5 to –2.2‰ (VSMOW). The lowest of these calculated values are lower
than δ18Osw values (–17.9 to –11.9‰ (VSMOW)) of modern water in the basin. The
extremely low δ18Opsw values from fluvial mollusks and evaporatively elevated δ18Opsw
values from lacustrine mollusks, show that the peaks surrounding the Zhada Basin were
at elevations at least as high as, and possibly up to 1.5 km higher than today, and that
conditions have been arid since at least 9 Ma. A decrease in elevation since the Miocene
is not specifically predicted by any existing mechanical models for the development of
the Tibetan Plateau.
Paleoenvironmental modelling and physical evidence shows that the climate in
Zhada Basin was cold and arid, indistinguishable from the modern. The δ13Cpm values of
well-preserved vascular plant material increase from -23.4 to -26.8‰ at the base of the
Zhada Formation to as high as -8.4‰ above 250 – 300 m. This shift denotes the
18
expansion of C4 biomass in this high, arid watershed at ~ 7 Ma, and thus corresponds to
the C3 – C4 transition observed in Neogene deposits of the northern Indian sub-continent.
19
INTRODUCTION
The Tibetan Plateau occupies the centre of the largest continent – continent
collision on Earth and yet is actively undergoing extension (for example, Armijo et al.,
1986; Molnar and Tapponnier, 1978; Taylor et al., 2003). Models invoked to explain the
development of the Tibetan Plateau drive our thinking about convergent orogens
worldwide. These models make predictions about the elevation history of the plateau (for
example, Armijo et al., 1986; Beaumont et al., 2004; DeCelles et al., 2002; Guynn et al.,
2006; Kapp and Guynn, 2004; McCaffrey and Nabelek, 1998; Molnar et al., 1993;
Murphy et al., 1997; Ratschbacher et al., 1994; Rowley and Currie, 2006; Seeber and
Pecher, 1998; Tapponnier et al., 2001). In addition, numerous studies have linked
elevation changes on the Tibetan Plateau to changes in precipitation, aridity, and largescale oceanic and atmospheric circulation patterns (for example, Dettman et al., 2001;
Kroon et al., 1991; Molnar, 2005; Molnar et al., 1993; Quade et al., 1995; Raymo and
Ruddiman, 1992; Ruddiman et al., 1997; Zhisheng et al., 2001). Understanding what
drove the Tibetan Plateau to high elevations, and whether those elevations are long lived
and stable, is fundamental to understanding the interaction between asthenospheric,
lithospheric and climatic processes.
In this paper we present a Miocene – Pleistocene sedimentary record in the Zhada
Basin in southwestern Tibet, representing the first paleoelevation study in western Tibet.
The sediments are rich in well-preserved gastropod shells and plant organic matter. From
these archives, we have produced a detailed paleoelevation and paleoenvironmental
record. The oxygen isotopic composition of the shells allows us to reconstruct mean
20
watershed elevation through time, after accounting for the major variables that affect the
oxygen isotopic system in both the modern and ancient record. The oxygen record shows
that Zhada Basin was arid since the late Miocene, and that this part of the Tibetan Plateau
stood as high as today, and possibly higher. The carbon record from plant remains shows
that C4 plants expanded across at least parts of this high watershed during the late
Miocene. The C4 plant expansion at this time agrees with observations from the Gyirong
Basin and Thakkhola graben in south-central Tibet and Miocene deposits in northern
Pakistan and Nepal (France-Lanord and Derry, 1994; Garzione et al., 2000a; Quade et al.,
1995; Wang et al., 2006).
Previous Paleoelevation Work
There is still not unanimity within the scientific community regarding how to
interpret the many complex proxies for Tibetan Plateau paleoelevation (for example,
Molnar, 2005) despite more than two decades of research on the topic. The earliest work
on Tibetan paleoelevation indicated a late Miocene or more recent uplift based on fossil,
sedimentological, and structural data (for example, Li and Zhou, 2001a; Li et al., 1986;
Liu, 1981; Zhang et al., 1981; Zheng et al., 2000). Quade et al. (1995) used a change
from C3 to C4 dominated vegetation in the Himalayan foreland to argue for initiation or
strengthening of the Asian monsoon at about 7 Ma. They linked the change in vegetation
to Himalayan or Tibetan Plateau uplift, insofar as summertime heating of the air above
the high Tibetan Plateau drives current monsoon circulation. This hypothesis was
strengthened by the work of Kroon et al. (1991), Prell and Kutzbach (1992) and Prell et
21
al. (1992), which linked increased upwelling in the Arabian Sea at 7.4 – 8 Ma to onset or
strengthening of the Asian monsoon.
Evidence developed more recently both supports and contradicts the idea of
relatively recent (late Miocene or later) uplift of the Tibetan Plateau. Early in the debate,
Turner et al. (1993) reported late-Miocene, potassium-rich lavas in northern Tibet. Turner
et al. (1993) and Molnar et al. (1993) used this to argue for gravitational removal of
thickened lithosphere and, by inference, uplift of the Tibetan Plateau. However,
subsequent documentation of widely distributed volcanism of Eocene – Miocene age
across the plateau (Chung et al., 1998; Ding et al., 2003; Wang et al., 2001) rendered the
previous line of evidence more ambiguous. Dettman et al. (2001) argued for an onset of
the Asian monsoon by 10.7 Ma by showing that bivalves from the Himalayan foreland
show evidence for seasonal oscillations between wet and very arid conditions, implying
that strong monsoonal circulation was in place between 10.7 and 3 Ma. Even the
occurrence of normal faulting on the Tibetan Plateau, once thought to mark attainment of
high elevations at around 8 Ma (for example, Harrison et al., 1992), has been shown to
extend to at least the mid-Miocene (for example, Blisniuk et al., 2001).
Recent paleobotanical and stable isotope studies, focused primarily on southcentral Tibet, generally point to high elevations in this region since the Eocene –
Oligocene. The earliest quantitative paleoelevation studies on paleosol carbonate,
lacustrine micrite and fossil shells obtained very low δ18Occ values, indicating high
elevations since 10 – 11 Ma in Thakkhola graben (Garzione et al., 2000a; Rowley et al.,
2001), since ~ 8 Ma in Gyirong Basin (Rowley et al., 2001) and since 15 Ma in the Oiyug
22
Basin (Currie et al., 2005). The latter was confirmation of an earlier leaf physiognomy
study in the Namling Basin which also indicated high elevations (Spicer et al., 2003).
Stable isotope studies in the Lunpola Basin (Rowley and Currie, 2006), Nima Basin
(DeCelles et al., 2007), and the Tarim and Qaidam Basins (Graham et al., 2005) argued
for high elevations on the Tibetan Plateau back to at least the Oligocene.
Wang et al. (2006) recently reinvigorated the argument for a post-mid-Miocene
uplift of the southern Tibetan Plateau by presenting carbon isotope data from 7 Ma
mammal fossils in the Gyirong Basin (present elevation: 4,200 m, ~600 km east of the
Zhada Basin) demonstrating that C4 plants composed a significant fraction of their diet.
As C4 grasses today are apparently rare above 3,000 m (Lu et al., 2004; Wang, 2003),
Wang et al. (2006) concluded that the southern Tibetan Plateau attained its current
elevation within the last 7 million years. However, there is some evidence that C4 plants
are present at high elevations on the Tibetan Plateau (Garzione et al., 2000a; Wang et al.,
2008c). The results of paleoelevation studies to date constitute an important beginning to
a still spatially and temporally limited picture of Tibetan uplift. In this paper, we present
the first paleoaltimetry study from western Tibet and address the conflicting conclusions
from stable isotope and paleoenvironmental studies.
23
OXYGEN ISOTOPE PALEOALTIMETRY
Oxygen isotope analysis has emerged as a powerful tool for reconstructing
paleoelevations (for example, Blisniuk and Stern, 2005; Chamberlain and Poage, 2000;
Currie et al., 2005; Cyr et al., 2005; Dettman and Lohmann, 2000; Garzione et al., 2000a;
Garzione et al., 2000b; Poage and Chamberlain, 2001; Rowley and Currie, 2006; Rowley
and Garzione, 2007; Siegenthaler and Oeschger, 1980). The underlying principle of
these reconstructions is that oxygen or deuterium isotopic compositions of meteoric water
(expressed as δ18Omw or δDmw, respectively, in units ‰) vary as a function of elevation,
decreasing by global average values of about –2.8 ‰/km (Poage and Chamberlain,
2001). In the ideal case, the δ18Osw of surface water reflects the average δ18Omw of
rainfall in the catchment. Carbonates, phosphates, and silicates ultimately derive their O
and H from surface water and so record these δ 18Osw and δDsw values. After correcting
for temperature of formation and other factors, paleoelevation can be reconstructed.
The δ18Omw value of precipitation varies as a function of multiple factors (for
example Dansgaard, 1954; Dansgaard, 1964; Drever, 1997; Poage and Chamberlain,
2001; Rowley et al., 2001; Rozanski et al., 1993) resulting in spatial and temporal
departures from the modern global average lapse rate. These local variations can
potentially be redressed by direct measurement and modelling of local lapse rates. For
southern Tibet, this critical information is available (Garzione and others, 2000a; Rowley
and others, 2001).
Rayleigh fractionation models describe moist, adiabatically-rising, thermallyisolated packets of air from which any condensation is immediately removed as
24
precipitation. The effect is an open-system distillation process whereby the water vapour
content of the air is progressively depleted in H218O (Rowley and others, 2001;
Siegenthaler and Oeschger, 1980). These models apply to moisture that is orographically
lifted up the south side of the Himalaya during the summer monsoon. However, because
Rayleigh fractionation models are one dimensional, they must be calibrated for the many
spatial variables that affect air masses and precipitation. By implication, paleoelevation
reconstruction requires knowledge of the source of the water vapour in order to rule out
recycling from continental water sources or variations in oceanic source regions
(Araguas-Araguas et al., 1998). Continental recycling results in precipitation with
anomalous δ18Omw values (Dansgaard, 1964; Drummond et al., 1993), as is the case in
north-central Tibet. Paleoelevation reconstruction also requires knowledge of moisture
pathways, since vapor experiencing a long overland path prior to precipitation (high
continentality) will have the same δ 18Omw value as precipitation at high elevations
(Rozanski et al., 1993). The fractionation factor between liquid and vapour is
temperature sensitive and Rayleigh fractionation models are most sensitive to the source
region temperature (Rowley and others, 2001; Rowley and Garzione, 2007); therefore it
is necessary to be able to constrain paleotemperature. Of the other variables, the amount
effect exerts the strongest influence on δ18Omw, especially at lower latitudes and in
monsoon climates such as in the Himalaya today. In north India today, seasonal δ 18Omw
values vary by up to 10 ‰ (Rozanski and others, 1993; Dettman and others, 2001) owing
primarily to the amount effect.
25
Additional factors influence the δ18Osw values of surface or ground water. The
δ18Osw value is the result of integrating the precipitation that fell at all elevations in the
catchment above the sampling elevation. Thus, the sample represents the precipitation
amount-weighted hypsometric mean elevation of the catchment. In small, high-elevation
catchments, such as those sampled in Zhada Basin, this value varies little from the areaweighted mean elevation of the catchment (Rowley and Garzione, 2007). Postprecipitation evaporation also increases δ18Osw values (Dansgaard, 1964) particularly in
areas with long water-residence times (e.g., lakes, marshes). In Tibet, the combination of
an arid climate and long water-residence times in lakes can sharply increase δ18Osw
values of lake water (1.7 to -7.1 ‰, VVSMOW, Quade, unpublished data (Fontes et al.,
1996; Gasse et al., 1991).
In order to reconstruct the δ18Osw value of water in which minerals formed, we
must also know or be able to estimate the temperature at the time of mineral formation
and be able to constrain the effects of subsequent diagenesis. Diagenesis can change the
δ18O value of oxygen bearing minerals (Garzione et al., 2004), a potential complication
that must be carefully evaluated.
Therefore, before drawing conclusions about paleoelevation and paleoclimate
from stable isotope data, the following factors must be accounted for: (1) comparison
with modern water to establish the modern lapse rate, (2) proof against diagenesis, and
(3) correction for climate change. The third factor includes identifying and assessing
changes in the source region, the amount effect, temperature, and the pathway that the
26
relevant air masses took. Reliable age control must also be established before drawing
tectonic implications from paleoelevation reconstructions.
27
REGIONAL SETTING OF ZHADA BASIN
The Zhada Basin is a late-Cenozoic sedimentary basin located just north of the
high Himalayan ridge crest in the west-central part of the orogen (~32° N, 82° E, Figure
2.1A). The axis of the basin is parallel to the general arc of the Himalaya which, in this
location, is approximately northwest-southeast. The current outcrop extent of the basin
fill is approximately 9,000 km2. The basin fill is undisturbed and lies in angular or
buttress unconformity with underlying deformed Tethyan Sedimentary Sequence (TSS)
strata that were previously shortened in the fold-thrust belt. After deposition, the Sutlej
River incised through to the basement, exposing the entire basin fill in a spectacular
series of canyons and cliffs. The presence of a basin above the TSS in this location is in
contrast to much of the Himalaya where the TSS caps some of the highest mountains in
the world, including Mount Everest.
The Zhada Basin is bounded by the South Tibetan Detachment System (STDS) to
the southwest, the Indus Suture to the northeast, and the Leo Pargil/Qusum and Gurla
Mandhata gneiss domes to the northwest and southeast, respectively (Figure 2.1B). The
STDS is a series of north-dipping, low-angle, top-to-the-north normal faults which place
low-grade Paleozoic – Mesozoic metasedimentary rocks on high-grade gneisses and
granites of the Greater Himalayan sequence. No ages for movement on the STDS in this
area have been published, but elsewhere in the orogen ages range from 21 – 12 Ma
(Murphy and Yin, 2003). Although rocks of Indian affinity are separated from those of
Asian affinity by the Indus Suture, the region north of the STDS is considered the
southern edge of the Tibetan plateau for this study because it is hydrologically integrated
28
with areas north of the Indus Suture. In the Zhada Basin region the Oligo-Miocene Great
Counter Thrust, a south-dipping, top-to-the-north thrust system, modifies the Indus
Suture (for example Ganser, 1964; Yin et al., 1999). Exhumation of the Leo
Pargil/Qusum and Gurla Mandhata gneiss domes (Figure 2.1B) by normal faulting
initiated at ~ 16 Ma and 9 Ma, respectively (Murphy et al., 2002; Thiede et al., 2006) and
is ongoing today. The unique setting of Zhada Basin makes it an ideal place to test
hypotheses about climate, tectonics and paleoelevation in the Himalaya and southwestern
Tibet.
We measured 14 stratigraphic sections covering the basin extent from the Zhada
county seat in the southeast to the Leo Pargil/Qusum rangefront in the northwest (Figure
2.1B). The basin fill consists of approximately 800 m of fluvial, lacustrine, eolian and
alluvial fan deposits and is divided broadly into 3 intervals.
1) The lower part of the section consists of ~ 200 m of trough cross-bedded
sandstone and well-organized, imbricated pebble to cobble conglomerate. Broadly
lenticular bodies of sandstone and conglomerate are interpreted as channel fills. The
presence of 3-4 m thick cross-stratified bedsets in these channel fills suggests the
presence of deep channels and large mid-channel macroforms. We interpret these as
fluvial deposits laid down by large-scale rivers ancestral to the Sutlej or Indus based on
provenance and paleocurrent orientation data. Interbedded with these lithofacies are finegrained, laminated sandstone and siltstone layers showing extensive soft-sediment
deformation. These units contain abundant mammal, gastropod and plant fossils. We
29
interpret these fine-grained intervals as marshy bog or overbank deposits within a lowgradient fluvial setting.
2) The approximately 250 m thick middle unit consists of an upward coarsening
succession of cycles. Individual cycles are up to 17 m thick, coarsen upward, and contain
profundal lacustrine claystone in their lower part and deltaic and wave-worked sandstone
and conglomerate in their upper part. The claystone is devoid of macrofossils but the
sandstone often has well-preserved, robust gastropod shells. Evidence of desiccation
episodes, including gypsum layers and mudcracked mud-flat facies, is also present in the
middle unit interval. Upward coarsening cycles are interpreted as progradational
lacustrine sequences.
3) The upper 350 m of the Zhada Formation continues the upward coarsening
progression displayed in the middle unit but becomes much coarser. The profundal
claystone facies is replaced by deltaic or lake margin deposits. Individual parasequences
contain lake-margin and alluvial-fan and fan-delta conglomerates.
30
METHODS AND MATERIALS
Age Control
We sampled the entire thickness of two measured sections and a portion of a third
for magnetostratigraphic analysis, for a total sampling thickness of ~ 1400 m. We
collected 4-5 samples from 184 sites (102 from the South Zhada section, 5 from the East
Zhada section, and 77 from the Southeast Zhada section) using a cordless, hollow-bit drill
using standard paleomagnetic sampling techniques (Butler, 1992). Samples were stored
in a magnetically shielded room (~300 nT background field) housing the cryogenic
magnetometer and demagnetization equipment, for at least 72 hours prior to measurement
of natural remnant magnetization (NRM) and throughout the analysis process. We
measured NRM using a 2G Model 755R three-axis cryogenic magnetometer with in-line
degaussing system and automated sample handler. All cores were analyzed prior to any
heating to isolate NRM. Using furnaces with programmable temperature controllers and
ten thermocouple temperature sensors on each sample rack, we thermally demagnetized
one sample from each site with temperature steps as follows: 50 degree steps from 100300ºC, 20 degree steps from 300-400ºC and 20 degree steps from 500-700ºC. We based
temperature steps for subsequent batches on these initial results. Temperature steps
within 100 degrees of the Curie temperature were 20 degrees for all batches (see Figure
2.2 for representative vector diagrams). Characteristic Curie temperatures were between
580 ºC and 700ºC, indicating that magnetite and hematite were the primary carriers of
characteristic remnant magnetization (ChRM). However, magnetic intensity in a
31
minority of the samples decreased markedly by 300ºC, indicating the possible presence of
goethite.
The principle component analysis was done using the origin as a separate data
point (―origin‖ line fit of Butler (1992)) and using at least 4 temperature steps. We
discarded samples with line fits yielding a maximum angular deviation of >15º from
further analyses. We then plotted site averages for sites with samples that passed the
maximum angular deviation test on an equal area stereonet and calculated mean vectors
for normal and reversed sites. The mean vectors for both the South Zhada and Southeast
Zhada measured sections were antiparallel and thus passed the reversals test (Figure 2.3;
Butler, 1992).
Most samples show essentially univectoral decay of NRM toward the origin of the
vector endpoint diagram. In order to eliminate potentially inaccurate results, we divided
the sample set into quality sets A, B, C, and D (Figure 2.2). Sites with > 3 samples which
passed the maximum angular deviation test and with a site-mean clustering of ChRM
which yielded a 95% confidence limit (α95) ≤ 15º and K ≥ 30 were then designated class
A sites. Sites with ≥ 3 samples and with a site-mean clustering of ChRM which yielded a
95% confidence limit (α95) > 15º were designated class B sites. Sites with 2 samples
which yielded consistent inclinations and declinations were designated class C sites.
Sites with only 1 sample which passed the maximum angular deviation test or with 2
samples which yielded inconsistent inclinations or declinations were not included in the
magnetostratigraphic column (D sites). We constructed the resultant
magnetostratigraphic columns from 87 class A sites, 60 class B sites and 7 class C sites.
32
Magnetic reversals were placed mid-way between adjacent data points with opposite
polarities. The placement of some reversals based on a single site is warranted given our
caution in processing multiple samples from each site and discarding sites where data
were suspect.
Modern Water
We collected 28 water samples at 20 locations throughout Zhada Basin (Figure
2.1B, Table 2.1), providing the densest sampling coverage for any area in Tibet outside of
Lhasa. Several sites were resampled during consecutive years. Samples were collected
from a number of different settings ranging from the Sutlej main stem to small seep
springs at the base of the Zhada Formation. Virtually no local rain fell during the
sampling campaigns, meaning we were sampling higher elevation run-off. ArcGIS was
used to delineate watersheds for each of our samples. This allowed calculation of the
average elevation at which precipitation feeding these streams and springs fell by
obtaining hypsometric mean watershed elevations for each sample. This approach is
consistent with the work of Garzione et al. (2000b), Rowley et al. (2001), Rowley and
Garzione (2007), Blisniuk and Stern (2005) and oxygen isotope paleoaltimetry theory
(see section OXYGEN ISOTOPE PALEOALTIMETRY).
We also collected modern water samples from modern wetlands and a Tsangpo
River tributary near Zhongba ((N29 ° 39.754’, E84° 10.056’, 4 569 m, Figure 2.4, Table
2.1). Water was collected from ponds, from which we collected modern gastropod
samples (see section below), on the 18th of May and on the 26th of July, 2006. Water
33
sample 180506-4 was collected with gastropod sample TSP16 and sample 180506-5 was
collected with TSP18. Wetland ponds are < 100 m in diameter and < 0.5 m deep and are
inset into both exhumed paleowetlands and modern dune fields. On the 26th of July we
also collected water from the Tsangpo tributary. The river water collection location was
within 10 km of the wetland water and gastropod collection sites for samples 260706-3,
180506-4 and TSP16 and, in the absence of alternative sources, is inferred to be the
source water for the wetlands.
Waters were analyzed for δDsw values using a dual inlet mass spectrometer
(Delta-S, Thermo Finnegan, Bremen, Germany) equipped with an automated chromium
reduction device (H-Device, Thermo Finnegan) for the generation of hydrogen gas using
metallic chromium at 750°C. Water δ18Osw values were measured on the same mass
spectrometer using an automated CO2-H2O equilibration unit. Standardization is based
on internal standards referenced to VVSMOW and VSLAP. Precision is better than ±
0.08‰ for δ18O and ±1‰ for δD.
Gastropods
We sampled fossil gastropods in two measured sections spanning the lower ~ 650
m of the Zhada Formation. No gastropod samples were found above ~ 650 m. Shell
fragments and intact shells were collected from fluvial, marshy, and lacustrine intervals.
We analyzed both homogenized gastropod shell material and micro-drilled gastropod
shells to obtain seasonal information. To check for preservation of biogenic aragonite, 12
representative gastropod samples from fluvial, lacustrine and marshy intervals were
34
powdered and analyzed using the University of Arizona’s D8 Advance Bruker X-ray
powder diffractometer.
We also collected gastropod shells from modern wetlands near Zhongba on the
Tsangpo River (N29 ° 39.754’, E84° 10.056’, 4 569 m, Figure 2.4). Gastropod shells
were collected from the shore of wetland ponds on the 18 th of May, 2006. Gastropods
were recently living and appeared to have died as the water table dropped. We analyzed
one homogenized shell sample and microdrilled another at 0.3 mm increments.
We measured δ18Occ and δ13Ccc values of shell material using an automated
carbonate preparation device (KIEL-III) coupled to a gas-ratio mass spectrometer
(Finnigan MAT 252). Powdered samples were reacted with dehydrated phosphoric acid
under vacuum at 70°C. The isotope ratio measurement is calibrated based on repeated
measurements of NBS-19 and NBS-18 and precision is ± 0.1 ‰ for δ18O and ±0.06‰ for
δ13C (1 ).
Zhada Formation Plant Material
We analyzed 36 samples of organic matter from 29 stratigraphic intervals in two
measured sections. Fossil plant material obtained from our sections is both fragmentary
and locally carbonized, but often preserves primary epidermal cell tissue. Much of this
fossil plant material appears to be grass bladelets rather than leaves or twigs. Organic
material was reacted with sulfurous acid in silver foil boats at least twice to remove
carbonate material prior to drying at 60°C. We measured the δ13Cpm values of plant
material on a continuous-flow gas-ratio mass spectrometer (Finnigan Delta PlusXL).
35
Samples were combusted using an elemental analyzer (Costech) coupled to the mass
spectrometer. Standardization is based on NBS-22 and USGS-24 for δ13C. Precision is
better than ± 0.06 for δ13C (1 ), based on repeated internal standards.
36
RESULTS
Age
We used several temporal anchors independent of magnetostratigraphy to
constrain the magnetostratigraphic correlations. The first was the occurrence of
Hipparion fossils at 310 m in our East Zhada measured section and at 240 m in our South
Zhada measured section. Pilbeam et al. (1996) placed the first appearance of Hipparion
(the Hipparion datum) at 10.7 – 10.8 Ma in northern Pakistan. Hipparion radiated
quickly (Woodburne et al., 1996; E. Lindsay, pers. comm.), thus the onset of
sedimentation in Zhada Basin must be no earlier than 10.7-10.8 Ma. Additional Zhada
fauna include Hipparion zandaense, Nyctereutes, and Paleotragus microdon (Li and Li,
1990; Zhang et al., 1981; X. Wang, pers. comm.). This biostratigraphic evidence
constrains basin filling at Zhada to the late Miocene – Pliocene.
A shift in δ13Cpm values was observed in two of our sections (Figure 2.5). The
δ13Cpm values of plant organic matter are determined by the metabolic pathway that the
plant used. C3 plants, mostly trees, shrubs and cool-growing-season grasses respire CO2
with δ13C values which average -27 ± 6 ‰ globally. C4 plants, including some shrubs,
but primarily warm-growing-season grasses respire CO2 with δ13C which average -13 ± 3
‰ globally (Ehleringer et al., 1991). Quade et al. (1995; 1989) and France-Lanord and
Derry (1994) showed a marked shift in δ13Cpm values in paleosol carbonate and plant
material in the Indian subcontinent at ~ 7 ± 1 Ma. They attributed this shift to a change
from C3 to C4 dominated plants. In far western Nepal, ~300 km SSE of Zhada Basin,
Ojha et al. (2000) place the C3 – C4 transition at 7 Ma. Garzione et al. (2000a) used this
37
same shift in the Thakkhola graben in the southern Tibetan Plateau (500 km SE of Zhada
Basin) as an anchor for their magnetostratigraphic correlation. Finally, the presence of
C4 plants in the diet of fossil herbivores was used as independent confirmation of the post
7 – 8 Ma age of the fossils in Gyirong Basin (Wang et al., 2006).
We first correlated our South Zhada (SZ) and Southeast Zhada (SEZ)
magnetostratigraphic sections using the capping geomorphic surface as a datum (Figure
2.6A). This surface is correlative across the basin and marks the maximum extent of
sedimentation prior to incision and exhumation by the Sutlej River. This approach
assumes that the geomorphic surface is isochron. The validity of this assumption is based
on the interpretation of the geomorphic surface as a depositional surface that extended
across the basin just prior to incision and abandonment. The assumption is also based on
the gross similarities between the upper portions of the South Zhada and Southeast Zhada
magnetostratigraphic (and lithologic) sections. Working downward from the datum, the
longest normal and reversed polarity intervals were correlated between the two sections.
A composite magnetostratigraphic column was then created that incorporated both long
polarity intervals and also the shorter polarity intervals from both sections (Figure 2.6B).
This approach assumes that whereas the shorter polarity intervals may have been missed
in one or the other section, the longer polarity intervals were not. Combining data from
both magnetostratigraphic sections produces a composite magnetostratigraphic section
that is more detailed than either of the individual sections (Figure 2.6B).
The mammal megafaunal fossil anchor described above and the number of
polarity chrons in the composite section indicate that sedimentation extended from the
38
late Miocene to the Pliocene or Pleistocene. Within these constraints we correlated the
composite section with the Global Polarity Time Scale (GPTS) of Cande and Kent
(1995). Intervals P+ through T+ likely correlate with either chron 2An or 3n (Figure
2.6C, E, G). We favour the correlation in figure 2E for several reasons. This correlation
(1) accounts for all of the normal and reversed intervals, (2) places the C 3 – C4 transition
at ~ 7 Ma, which is consistent with its age elsewhere, and (3) yields relatively constant
and reasonable sedimentation rates. The alternative correlations have several drawbacks.
The correlation in figure 2.6C means that the upper geomorphic surface is ~ 3 Myr old
and yet shows no evidence for significant erosion. The correlation of chrons 3r – 4n is
equally problematic in correlation G as evidenced by the large excursions in the sediment
accumulation rates (Figure 2.6H). With those considerations, the base of the composite
section most reasonably correlates to chron 4Ar.1 and the top of the section to chron 1n
(Figure 2.6E). This corresponds to an absolute age interval of < 1 – 9.2 Ma (Cande and
Kent, 1995). While the correlation in figure 2.6E is favoured, both E and C place the
onset of sedimentation at ~ 9.2 Ma. The onset of sedimentation in both correlations E
and C is consistent with an independent magnetostratigraphy study conducted by Wang et
al. (2008b). They differ significantly only in the upper portion, which is not the focus of
this paper.
Modern Water
Modern δ18Osw (surface water) values range from -17.9 to -11.9 ‰, and δDsw
values from -137 to -86 ‰ (VSMOW, Figure 2.7, Table 2.1) for water from the Zhada
39
Basin. The lowest values coincide with small springs draining catchments in the
Himalaya to the south of Zhada Basin. The average δ 18Osw value of the water coming
from the Ayi Shan, to the north of Zhada Basin (Figure 2.1B), is slightly higher than for
water coming from the Himalaya (-14.1 and -15.3 ‰, respectively). The average δ18Osw
value of water from the Sutlej River main stem (-15.1 ‰) reflects input from both of
these sources. The modern δ18Osw values for water from the Zhongba area range from 18.9 to -3.9 ‰ and δDsw values range from -140 to -86 ‰ (VSMOW, Figure 2.7, Table
2.1).
Gastropods
XRD analysis from 11 of 12 samples yielded only aragonite peaks (Figure 2.8).
One sample was too small to yield results and was removed from further consideration.
The δ18Occ values of samples that we analyzed using X-ray diffraction ranged from -20.3
to 0.2 ‰ (VPDB).
δ13Ccc (carbonate) values of gastropods range from -13.8 to 7.5‰ (Table 2.2).
Consideration of the data as a whole shows clear covariance between δ13Ccc and δ18Occ
values (R2 value of 0.61 for the South Zhada section and 0.62 for the East Zhada section;
Figure 2.9). Dividing the data by lithofacies reveals a more complex pattern (Figure 2.9).
δ13Ccc values of samples from fluvial intervals range from -13.8 to -2.6‰ and show
almost no covariance (R2=0.03 for South Zhada section and R2 = 0.06 for the East Zhada
section). δ13Ccc values of samples from supralittoral/marshy intervals range from -13.8 to
+7.5‰ and display significantly more covariance (South Zhada R2=0.36 and East Zhada
40
R2 = 0.19). Samples from littoral intervals yield δ13Ccc values of -3.8 to +3.0‰ and
R2=0.30 for South Zhada and 0.08 for East Zhada. Finally, samples from profundal
lacustrine intervals, which only occur in the South Zhada section, yield δ13Ccc values of 3.0 to +1.9‰ with R2=0.00.
δ18Occ values of gastropods from fluvial intervals range from -21.4 to -9.9‰
(VPDB); from supralittoral/marshy intervals between -21.6 to -1.8‰; from littoral
intervals, -9.3 to +0.7‰ and from profundal intervals, -3.2 to +0.3‰ (Table 2.2, Figure
2.10). Finally, microdrilled samples show a range of δ18Occ values, typically ~ 3‰ but
up to 10.9‰, from a single sample (Figure 2.11). All the snails are aquatic.
δ18Occ values of modern gastropods from Zhongba range from -12.0 to -14.7 ‰
(VPDB) and δ13Ccc values range from -3.6 to -11.2 ‰ (VPDB). The two samples do not
show an obvious covariant trend, though sample TSP18 may show internal covariance
(Figure 2.9).
Zhada Formation Plant Material
δ13Cpm values range from -23.4 to -26.8‰ (VPDB) in the lower 250-300 m of
both sampled sections, and increase to as high as -8.4‰ (VPDB, Table 2.3, Figure 2.5)
above 300 m.
41
APPLICATION TO PALEOALTIMETRY
Calculation of Miocene δ18Osw and Related Constraints and Corrections
Source, pathway and amount effect constraints
Applying the modern δ18O versus elevation lapse rates to the Miocene requires
accounting for potential climate change in the intervening time. Currently, the Indian
subcontinent’s summer monsoon derives its water vapour from the Arabian Sea and
Indian Ocean. Source region temperatures effect the δ18O values of high-elevation
carbonates in two ways: 1) sea surface temperatures effect the δ18O value of source
region water vapour (Blisniuk and Stern, 2005; Jouzel et al., 1997), and 2) low-elevation
temperatures effect the δ18O versus elevation lapse rate (Rowley and Garzione, 2007;
Rowley et al., 2001). Modern mean annual temperature (MAT) for the low-elevation
Himalayan foreland is 25 ºC or 27 – 28 ºC for coastal IAEA/WMO GNIP stations
(Mumbai and Calicut; IAEA/WMO, 2007). Current MAT is close to the MAT of 26.5 ºC
calculated from Miocene soil carbonate nodules in western Nepal (Quade et. al., 1995),
as well as MAT estimates based on fossil flora assemblages (for example, Awashi and
Prasad, 1989; Sarkar, 1989). Lower low-elevation temperatures would elevate the δ18O
versus elevation lapse rate hence reconstructed paleoelevations would overestimate
paleoelevation. As noted above, neither regional nor global late temperature estimates
indicate an increase in temperature between the late Miocene and the modern (Zachos et
al., 2001). Miocene sea surface temperatures have typically been thought to be ~ 10° C
cooler than the modern (Kennett, 1985; Savin et al., 1985; Williams et al., 2005). Lower
Miocene sea surface temperatures would result in low-elevation water vapour with higher
42
δ18O values hence reconstructed paleoelevations would underestimate the actual
paleoelevation. However, there is debate whether measured δ18O values represent sea
surface temperatures or the bottom temperature during early diagenesis (for a brief
summary see Pearson et al., 2002; Zachos et al., 2002). This raises the possibility that
Miocene Indian Ocean sea surface temperatures were comparable to the modern (Stewart
et al., 2004; Williams et al., 2005). Additionally, average paleosol δ18Occ values from
Neogene deposits at low elevation in the northern Indian sub-continent show no change
post-8 Ma (Quade et al., 1995). Araguas-Araguas et al. (1993, Figure 1) indicate that
moisture up to and just north of the crest of the Himalaya is dominated by moisture from
the Indian summer monsoon. The Zhada basin is located just north of the crest of the
Himalaya and all drainages on the southern side of the basin are sourced by glaciers that
originate at the foot of high Himalayan peaks to the south. The implication is that at least
half the water in the basin is coming from the Himalaya. According to Tian et al. (2001),
rainfall with high deuterium-excess (d-excess) values in the Himalaya and just north of
the Himalaya is derived from the Indian summer monsoon. Water from the Zhada basin
has d-excess values of between 3 and 15 (mean: 10) which is consistent with derivation
from the Indian summer monsoon. Finally, the Zhada basin is located far to the west of
the range of penetration of Pacific moisture onto the Tibetan Plateau (Araguas-Araguas et
al., 1993, Figure 1).
Paleosols from northern India would have experienced the same climate changes
and changes in source water δ18O values as the Zhada samples, since northern India is
dominated by the same monsoon climate system (Araguas-Araguas et al., 1998; Dettman
43
et al., 2001; Tian et al., 2001). As no significant change in MAT or δ18Occ values is
evident from the low-elevation late Miocene records, we apply current climatic
conditions in reconstructing δ18Opsw (paleo-surface water) values. Since the monsoon
seems to have been established by at least 10.7 Ma (Dettman and others, 2001), the same
source and pathway applies for Miocene Zhada meteoric water as for modern Zhada
meteoric water. This suggests that we can use the modern δ18O versus elevation
relationships, as measured by Garzione and others (2000b) and modelled by Rowley and
others (2001), to understand the ancient record.
Shell preservation
We are confident that gastropod samples are unaffected by diagenesis and retain
their original δ18Occ values for the following three reasons: (1) all of the samples which
returned usable X-ray diffraction results (11 of 12) were aragonite (Figure 2.8), and none
showed evidence of recrystallization; (2) the samples are visually pristine, retaining a
pearly luster in the interior and obvious growth bands; (3) samples which we microdrilled
showed seasonal variation (Figure 2.11). Such internal variation would not be expected
if the samples were overprinted by a regional δ 18Osw value during diagenesis. The results
from this representative sampling can be confidently applied across the basin because the
sediments were never buried below ~ 800 m and so are not subject to regional
metamorphism or hydrothermal alteration.
Calculation of Miocene δ18Opsw
δ18Opsw values were reconstructed from δ18Oar values using:
44
δ18Opsw (VSMOW) = [(1000 + δ18Occ (VSMOW)) / [exp [(2.559 * 106 * T-2 +
0.715] / 1000)]]] – 1000
(1)
(Dettman et al., 1999; modified from Grossman and Ku (1986)) where T (in K) is the
temperature of CO3 precipitation. We used the modern temperature to constrain the
temperature of aragonite precipitation (see section Source, pathway and amount effect
constraints). The nearest weather station for which there are long term records is at
Shiquanhe (32.5ºN, 80.083ºE, 4280 m) (NCDC, 2007). MAT at Shiquanhe for the period
between 1969 and 1990 is 0°C. Assuming that gastropods grow dominantly during the
warmer months, we calculated the average temperature for the months of April – October
(months when Taverage > 0°C) and set T = 7°C. Warmer temperatures produce higher
δ18Opsw values, thus minimizing paleoelevation estimates. Intrashell variation of ± 1.5 ‰
corresponds to a seasonal variation of ± 7°C and that uncertainty is applied to all samples
(Figure 2.12).
Comparison with Models
Effect of paleotemperature
One possible explanation for the extremely low δ18Occ value of gastropod
aragonite (Figure 2.12) is that it was precipitated in warm water. If we assume no change
in δ18Osw values of water in the Zhada region (i.e., that the most negative δ 18Osw value of
modern Zhada water (-17.9 ‰ VSMOW) is representative of the water in which Miocene
– Pleistocene gastropods lived), we can use the fractionation factor between that water
45
and the gastropod aragonite δ18Occ value (-21.6 ‰ VPDB) to calculate the temperature of
precipitation. However, this exercise yields an unrealistic temperature of aragonite
precipitation of 41° C. Thus, the temperature of aragonite precipitation alone cannot
explain the extremely negative δ18Occ values.
Modelling changes in lapse rate
By changing input parameters for the thermodynamically based δ18Osw versus
elevation models (Rowley and Garzione, 2007; Rowley et al., 2001) we can evaluate the
effect of 1) changes in monsoon intensity on the δ18O versus elevation lapse rate and 2)
changes in low-elevation temperature. We first looked at the effect of increasing
monsoon intensity on the δ18O versus elevation lapse rate. We calibrated the model by
finding the low-elevation temperature that produces a lapse rate that is consistent with the
most negative modern Zhada water (-17.9 ‰ VSMOW with a mean catchment elevation
of 5,419 m). This yielded a low elevation temperature of 297.5 K. While maintaining
the low elevation temperature at 297.5 K, we artificially decreased the saturation vapor
pressure of the system to find a model lapse rate that places the reconstructed Miocene
water value at 5,419 m. Decreasing the saturation vapor pressure artificially forces
increased precipitation, and thus decreased δ18Osw values, at lower elevations. The best fit
was found when the saturation vapor pressure was decreased by 30%. However, there is
no evidence from lowland δ18Occ records of changes in δ18Osw values consistent with a
saturation vapor pressure shift of this magnitude (Dettman et al., 2001; Quade et al.,
1995).
46
We also considered changes in low elevation temperature. Assuming that the δ18O
value of the reconstructed Miocene – Pleistocene water from Zhada Basin (-24.5 ‰
VSMOW, Figure 2.12) is accurate, we changed the low elevation temperature to find a
model lapse rate which places that δ18O value at 5,419 m. A match was found at a low
elevation temperature of 293 K, a decrease of 4.5 ° C from the modern. This decrease in
temperature should show up in the low-elevation δ18Occ and paleobotanical records and in
the climate record in Zhada Basin. However, as noted above, the low-elevation δ18Occ
and paleobotanical records post-8 Ma are consistent with the modern. Moreover, global
climate is thought to have cooled between the Miocene and the present, not warmed as
would be required by this scenario (for example Zachos et al., 2001).
Evaporation vs. Outflow
Comparing δ18Occ values from fluvial and lacustrine gastropods allows
reconstruction of local climate conditions if we can estimate outflow from the basin. All
δ18Occ values are given referenced to VPDB and, as we are interested in the difference
between two δ18O values, there was no need to change them to values referenced to
VSMOW. We used -21.6 ‰ (VPDB) as our inflow value, based on the δ18Occ values of
the fluvial fossil mollusks. The range of δ18Occ values of -3.3 ‰ and -0.4 ‰ (VPDB) for
lacustrine mollusks corresponds to δ18Osw values of -6.2 and -3.3 (VSMOW),
respectively, (assuming a temperature of precipitation of 7 ºC). These paleo-lake water
δ18O values are within the range of -7.1 to +1.7 ‰ (VVSMOW) observed in modern
lakes on the Tibetan Plateau (Fontes et al., 1996; Gasse et al., 1991)(Quade, unpublished
47
data). The range of total isotopic enrichment values (εw-v = 18.7 – 21.7) was calculated
based on our inflow δ18Occ value and the range of lacustrine δ18Occ values.
We compared the calculated Miocene isotopic enrichment values to modelled
modern isotopic enrichment values for relative humidity values between 0 and 70 % and
temperatures between 0 and 10 °C. The modern average relative humidity is 30%,
reaching a maximum of 50% during the monsoon. The modern MAT is 0 °C, average
temperature between April and October is 7 °C, maximum monthly temperature is 14 °C
and the maximum temperature between 1961 and 1990 is 21 °C.
The toal isotopic enrichment, εw-v, is a function of both the equilibrium water –
boundary layer enrichment (εw-bl) and a kinematic enrichment (εbl-v). We calculated the
kinetic enrichment during evaporation using the equation:
εbl-v=14.2 (1-rh),
(2)
where εbl-v is the enrichment in ‰ between a saturated boundary layer above the water
surface and a well-mixed vapor column and rh is the relative humidity fraction
(Gonfiantini, 1986). The δ18O value of evaporation was calculated using a simple mixing
equation such that
δ18Oevap = (δ18Oinflow – f * δ18Olake)/(1 – f))
(3)
where f is the outflow fraction. We assume that f = 0.
The equilibrium enrichment was calculated using:
1000lnα(δ18Ow-bl) = 1.137(106/T2) – 0.4156(103/T) – 2.0667
48
(4)
(Majoube, 1971) where T is temperature in Celsius.
One source of uncertainty is whether or not the basin was closed. Increasing the
outflow fraction would result in increasing the total (outflow + evaporative) isotopic
enrichment. The δ18Occ record in closed-basin gastropod shells could easily be identical
to δ18Occ values for open-lake gastropod shells if conditions were cooler or drier. Hence,
we are unable to determine whether or not outflow has occurred from the δ18Occ values.
However, the occurrence of mudcracks and bedded gypsum in the sedimentary sections,
in addition to the covariance between δ18Occ and δ13Ccc values, suggests that Zhada Basin
was at least periodically, if not continually, closed during lacustrine sedimentation.
The results of modelling the relative humidity, isotopic enrichment, and
temperature confirm that there is a limited range of reasonable values for all of these
variables and that range is consistent with the modern climate in the area (Figure 2.13).
Temperature of Modern Gastropod Shell Precipitation
The δ18Occ values of modern gastropods from near Zhongba, coupled with the
δ18Osw values of water in which those gastropods lived, allows calculation of a
fractionation factor and hence a temperature of aragonite precipitation. In the following
discussion ―pre-monsoon wetland water‖ refers to water with δ18Osw values of between 3.9 and -6.3 ‰ (VSMOW) and ―monsoon wetland water‖ refers to water with δ18Osw
values of -16.6‰ (VSMOW). We first assumed that the gastropods precipitated their
shells in equilibrium with the pre-monsoon wetland water they were near on the date of
collection (18th May, 2006). However, this results in unreasonably high temperatures of
49
aragonite precipitation (Figure 2.14). Alternatively, assuming that the gastropods
precipitated their shells in equilibrium with monsoon wetland water (sample 260706-3)
results in temperatures between ~ 0 – 12 °C (Figure 2.14). These temperatures are
between MAT and the maximum average monthly temperature for the period 1961 –
1990 as recorded by the Shiquanhe weather station.
We can calculate the relative contribution of oxygen from pre-monsoon and
monsoon wetland water to shell aragonite if we assume a range of temperature of
aragonite precipitation of 0 – 12 °C. We used a simple mixing ratio between aragonite
precipitated in equilibrium with monsoon and pre-monsoon wetland waters at 0 and 12
°C. Applying this calculation to sample 180506-4 shows that shell δ18Occ values of -12
‰ (VPDB) require that the shells are 100 % aragonite precipitated in equilibrium with
monsoon wetland water at 0 °C or ~ 70 % aragonite precipitated in equilibrium with
monsoon wetland water at 12 ° (Figure 2.15). A similar calculation for sample 180506-5
(average δ18Occ value ~ -14 ‰ VPDB) shows that if the average temperature was 0 °C the
shell must have been precipitated in water with δ18Osw values more negative than
observed monsoon wetland water, or, if the average temperature was 12 °C, the aragonite
was precipitated in equilibrium with 100 % observed monsoon wetland water at (Figure
2.15).
Paleoelevation models
We compared the most negative δ18Osw and reconstructed δ18Opsw values to
Δδ18Osw versus elevation relationships based on both a simple Rayleigh fractionation
model (Rowley and Garzione, 2007; Rowley et al., 2001) and an empirical data set
50
(Garzione et al., 2000b). In calculating Δδ18Osw, we used the modern, low-elevation
δ18Omw value for New Delhi of -5.8 ‰ (VSMOW, Rozanski et al., 1993) and a Miocene
low-elevation δ18Occ value of -6.0 ‰ (VSMOW) from paleosols from the Siwalik Group
of Pakistan and Nepal (Quade et al., 1995; Dettman et al., 2001). Uncertainties in our
Δδ18Opsw value derive from a ± 0.5 ‰ uncertainty due to variation in the most negative
values of δ18Occ in Miocene low-elevation paleosol carbonates between western Nepal
and Pakistan (Quade and others, 1995), and ± 1.5 ‰ uncertainty due to seasonal changes
in the temperature of aragonite precipitation (see section Calculation of Miocene δ18Opsw).
Additional uncertainty in paleoelevation estimates potentially arises from
variability in the low-elevation temperature or humidity (Rowley and others, 2001;
Rowley and Garzione, 2007). Modern surface water sampled in this study integrates
multiple glacial sources and groundwater. Hence these samples do not represent a single
precipitation event but rather a temporal average. Variability in modern samples that are
derived from the same sources and are collected at the same elevation (see for example
Figure 2.16), interpreted in terms of low-elevation temperature or humidity would lead to
the conclusion that temporally averaged low-elevation temperature or humidity had
varied dramatically. As this degree of variability in low-elevation parameters is not
observed currently, we conclude that this approach incorrectly attributes variability
causation. Hence, in our calculation of paleoelevation we use only the variability inherent
in the reconstructed water Δδ18O.
The modern water samples fall above the Δδ18Osw versus elevation curves based
on both Rayleigh fractionation (with an initial, low-elevation temperature (Ti) = 295 K,
51
and initial relative humidity (rh) = 0.8) and empirical data from Nepal (Figure 2.16). The
implication is that a calculated paleoelevation based on either of these curves will be a
minimum. Δδ18Osw values of modern water samples fall between a variation of the
Rayleigh fractionation model with T i = 303 K and rh = 0.8, and the models described
above. A low elevation Ti = 300 K and rh = 0.8 is consistent with modern, coastal MAT
in the region. The modified Rayleigh fractionation model yields a best-fit polynomial of
z = -0.0014 (Δδ18O)4-0.488(Δδ18O)3-30.88(Δδ18O)2-814.19(Δδ18O),
(5)
where z is elevation in meters. A low-elevation temperature Ti = 303K and rh = 0.8 most
closely matches the Δδ18Osw versus elevation relationship defined by the maximum
catchment elevation of the Zhada water samples. This relationship yields a best-fit
polynomial of
z = 0.011(Δδ18O)4+0.041(Δδ18O)3-29.22(Δδ18O)2-927(Δδ18O).
(6)
We applied the Rayleigh fractionation models with T i = 295 K and Ti = 300 K and
the empirically based model to our data in order to compare the elevations predicted by
Δδ18Osw values of modern water with those predicted by the Δδ18Opsw values of our
reconstructed Miocene – Pleistocene water. Whereas the absolute elevations of our
modern water samples do not match those predicted by several of the models (Figure
2.16), we are interested – at this point – in the difference in elevation predicted by the
models.
52
In all three of the models there is a difference in predicted elevations between our
most negative δ18Opsw value and our most negative δ18Osw values that is greater than the
uncertainty associated with those data points (Figure 2.17). The Rayleigh fractionation
model with Ti = 295 K yields a minimum predicted elevation from Δδ 18Opsw values
(mean elevation – uncertainty) of 5.6 km and a predicted elevation for the modern water
sample of 4.8 km. This adds up to an elevation decrease of at least 0.8 km since the late
Miocene. Doing the same calculation for either the Rayleigh fractionation model with T i
= 300 K or the empirically based curve results in a minimum elevation decrease of at
least 1 km or 1.2 km, respectively. More realistically, comparing the mean values for
each of the Δδ18Osw versus elevation curves yields elevation decreases of 1.0 ± 0.2 km,
1.2 ± 0.2 km and 1.5 + 0.3 - 0.4 km for the Rayleigh fractionation models with T i = 295
K and Ti = 300 K and the empirically based curve, respectively. Although the models are
inconsistent with regards to absolute elevation, yielding an inter-model range of
elevations (z) of 1.4 km for modern water and 1.6 km for reconstructed water, they are
relatively consistent with regards to differences in elevation (Δz), yielding an inter-model
range of only 0.5 km.
The calculations above suggest that paleoelevations during the late Miocene were
higher than those today, but they do not consider the effects of 20th century climate
change. The Siwalik paleosol record does not display any change in average δ18Omw
values over the past ~ 8 Ma. However, the record does not cover the last ~ 100 years, a
time of apparently major changes in δ18Omw water values in this region. Thompson et al.
(2000) noted a 3‰ increase in δ18O values from the Dasuopu glacier in the Himalaya
53
starting in the 20th century. This is consistent with, though larger than, increases
observed at Dunde, Guliya and the Far East Rongbuk glaciers (Kang et al., 2001;
Thompson et al., 1997; Thompson et al., 2000) in the northeastern and northwestern
Tibetan plateau and north-central Himalaya, respectively. As all applicable models have
been developed with reference to the modern system, we add 3‰ to our reconstructed
Miocene water values in order to compare them to modern water δ18O values (Figure
2.17). Even with this correction the models still yield a minimum elevation decrease
(mean elevation – uncertainty) between the Miocene and modern of at least 0.3, 0.4 and
0.5 km for the Rayleigh fractionation models with T i = 295 K and Ti = 300 K and the
empirically based curve, respectively. Again, comparing mean Miocene model elevations
to modern model elevations predicts decreases of 0.6 + 0.2 – 0.3, 0.8 ± 0.3 and 0.8 + 0.3 0.4 km for the Rayleigh fractionation models with T i = 295 K and Ti = 300 K and the
empirically based curve respectively. We caution that the addition of 3‰ to our Miocene
Δδ18Opsw may not be warranted, as interpretation of the glacial record remains unclear
and the inferred increase in δ18Omw values may be due to a short-term, transitory change.
However, as noted above, without the correction for changes in δ18Omw values of modern
water, our estimates of paleoelevation would increase by ~ 0.5 – 0.7 km.
The approach above results in a minimum estimation of paleoelevation
uncertainty and deviates significantly from the approach to uncertainty estimation in
previous paleoelevation studies. In order to facilitate comparison with previous
paleoelevation studies, we compared the range of modern fluvial Δδ18Osw and Miocene
fluvial Δδ18Opsw values, uncertainties associated with those values, and typically cited
54
model 2ζ uncertainties (Figure 2.18). There is considerable overlap in Δδ 18O values.
Elevated Δδ18Opsw likely reflect evaporative enrichment and emphasize that the Miocene
Δδ18Opsw values are minimum estimates. Comparison of the most negative Miocene and
modern Δδ18O values indicates that there is ~ 2.5 km of overlap in the possible
paleoelevation estimates. However, the comparison also admits the previously inferred
elevation loss of up to 1.2 km.
55
DISCUSSION AND CONCLUSIONS
Oxygen Isotopes from Zhada Basin
A key strength of this data set is substantial spatial and some temporal averaging
of δ18Occ values in the shell samples. There is considerable noise in some oxygen isotope
archives from the Tibetan Plateau, such as decadally resolved glacial ice, which would
lead to a very large range in elevation estimates (e.g., Thompson et. al., 2000). However,
water in which the fossil gastropods lived reflects the temporal and geographic average of
multiple glaciers and precipitation events on many mountains surrounding the basin.
Fine-scale temporal and spatial variations would be averaged in this process, accounting
for the very consistent range of δ18Occ values we observe when sorted by
paleoenvironment (Figures 2.9 and 2.10). The δ18Occ record is consistent both across the
basin and through millions of years, indicating that large scale processes are the primary
drivers.
We need to distinguish a precipitation δ18Omw signal from within the range of
δ18Occ values, some of which have been influenced by post-precipitation evaporation, in
order to interpret the δ18O variability in the ancient record. δ18Osw values of water with
low residence times (rivers or streams) are closest to δ18Omw values of rainfall, since there
is the least opportunity for enrichment of H218O due to evaporation. δ18Occ values of
gastropods from fluvial units therefore provide the most reliable record of the δ18Omw
values of precipitation. The extremely low δ18Occ values imply low δ18Omw values and
are due to the extreme elevation of mountains surrounding (and especially south of)
Zhada Basin (Garzione et al., 2000b; Rowley et al., 2001). δ18Opsw values (-12.8 to –
56
24.3‰, VSMOW) are at least as low as δ18Osw values (-11.9 to -17.9‰), indicating that
mountains surrounding the Zhada Basin were at elevations at least as high as today
(>6,000m) during the late Miocene. Our paleoenvironmental modelling indicates that
climate conditions similar to today prevailed in Zhada Basin during the late Miocene,
suggesting an elevation decrease of ~0.6 - 0.8 km in the last 9 million years.
Extremely negative δ18Occ values of gastropods from fluvial intervals establish a
baseline against which we can compare values from other intervals. The higher values,
of between +0.7 and -8.2‰ (VPDB), of gastropods from lacustrine intervals are probably
due to evaporative enrichment due to a longer residence time of average lake water (figs.
11 and 14). Calculated δ18Opsw values from lacustrine intervals of between –2.2 and –
11.7 ‰ (VSMOW) are at least as positive as results obtained from modern Tibetan lakes
(+1.7 to -7.1‰, VSMOW, Quade, unpublished data)(Fontes et al., 1996; Gasse et al.,
1991), implying conditions at least as arid as today. The presence of gypsum and
mudcracked layers within the lacustrine interval at Zhada, as well as Miocene dune fields
support the conclusion that Zhada was arid in the Miocene. The arid conditions implied
by both the isotopic and physical evidence provides additional support for an elevated
Himalayan massif south of the Zhada Basin, insofar as orographic blockage of moisture
in the region today makes it arid.
The shifts in gastropod δ18Occ and δ13Ccc values correlate well with sediment
accommodation creation, which also has a primary control on lithofacies and residence
time of water. Gastropods from fluvial facies show the most negative δ18Occ values
(Figure 2.10) and little covariance between δ18Occ and δ13Ccc values (Figure 2.9), due to
57
low water-residence times (Li and Ku, 1997; Talbot, 1990) in the basin. Although the
discussion of δ18O and δ13C co-variance in closed basin lakes is usually based on trends
in micrites, aquatic gastropods would also be expected to show similar co-variance
because the controls on shell δ18Occ and δ13Ccc are similar, the temperature: δ18Osw and
δ13C of DIC (and secondarily algae) in the lake system (Aucour et al., 2003; Shanahan et
al., 2005). Residence times were low because accommodation creation was low and
water moved through the system quickly resulting in δ18Opsw values, which, like the
modern, reflect the elevation of precipitation. Values from gastropods from
supralittoral/marshy intervals fall between and overlap values from both fluvial and
lacustrine intervals (Figure 2.10), indicating water residence times between that of fluvial
and lacustrine facies. This increasing residence time is also reflected in the covariance
between δ18Occ and δ13Ccc values from samples from these intervals (Figure 2.9).
Although the system was open at this point, the gradient was low, as suggested by
marshy intervals. The increase in residence time is evidenced by higher δ18Occ values
and δ18Occ and δ13Ccc covariance. Samples from intermediate water-residence-time
intervals continue the trend towards increasingly evolved δ18Occ and δ13Ccc values. The
samples still display covariance, but both the δ18Occ and δ13Ccc values are systematically
higher (Figures 2.9 and 2.10). As the rate of accommodation creation increased, the
basin closed, increasing the residence time of water as it began to pond and resulting in
an increased evaporative effect. Gastropods from profundal lacustrine facies show
uniformly high δ18Occ and δ13Ccc values (Figures 2.9 and 2.10), due to water residence
times which were long enough that the system was able to evolve to nearly a steady state
58
(Li and Ku, 1997). Towards the top of the lacustrine interval the return to more negative
δ18Occ values owes to the fact that the rate of accommodation creation was decreasing as
the basin began to fill in (Figure 2.10).
Oxygen Isotopes from Zhongba
The water sampled from wetlands near Zhongba in May is enriched by between
10.3 and 15 ‰ with respect to that sampled in July. Samples 180506-4 and -5 have high
δ18Osw and δDsw values and fall off of the global meteoric water line, implying extensive
evaporation. Only one sample, 260706-2 from the Tsangpo River, falls on the global
meteoric water line. The rest of the samples fall on a mixing line defined by
δD = 4.035 δ18O – 67.88
(7)
between sample 270706-2 and samples 180506-4 and -5. The implication is that water in
the wetlands is replenished during the summer monsoons and subsequently undergoes
evaporation during the remainder of the year so that water sampled in May, just before
the onset of the monsoon, is highly evaporatively enriched in 18O. Water sampled from
the wetlands after the start of the monsoon (sample 260706-3) reflects replenishing and
mixing between fresh fluvial water with low δ18Osw values and evaporated wetland water
with high δ18Osw values.
Modelling shows that gastropod shell material in these open basin wetlands were
most likely precipitated in equilibrium with monsoon wetland water at temperatures
above MAT but no higher than the maximum average monthly temperature. Several
59
conclusions that can be drawn from these results. The first simply reiterates that summer
monsoon water dominates the gastropod δ18Occ record from southern Tibet. This vastly
simplifies the system because we can rule out significant contributions from alternative
sources with unknown δ18Omw values and we can constrain the temperature of aragonite
precipitation. Secondly, it means that the gastropod δ18Occ record is dominated by water
that has been orographically lifted and thus models based on that assumption (for
example Rowley et al., 2001) are valid. Finally, it means that high δ18Occ values imply
closed basins. A basin that is even intermittently open, such as near Zhongba, will result
in low δ18Occ values.
Carbon Isotopes
The increase in δ 13Cpm values appears in two measured sections and can be
confidently dated to the late Miocene, a time of global C4 plant expansion (figs. 2 and 6).
This large shift in δ13C values (Figure 2.5) denotes a major increase in C4 plants. δ 13C
values of organic matter from these results are also consistent with those seen in Neogene
sections across the northern Indian sub-continent (France-Lanord and Derry, 1994; Ojha
et al., 2000; Quade et al., 1995; Quade et al., 1989) and from the southern Tibetan Plateau
(Garzione et al., 2000a; Wang et al., 2006). It is premature to use the presence or absence
of C4 biomass on the plateau to reconstruct paleoelevation as our knowledge of the
modern distribution of C4 plants on the plateau is incomplete. Though CAM plants have
been reported on the Tibetan Plateau (for example, Lu et. al., 2004), the plant remains
from Zhada confirm that C4 grasses, not CAM plants, are the cause of the late Miocene
60
increase in δ13Cpm values in Zhada Basin. Moreover, several lines of evidence hint that
C4 plants are present at high elevations. Wang (2003), Garzione and others (2000a), and
Wang and others (2008) found C4 plants, particularly Chenopodiaceae and Gramineae, in
the elevation range 3,000 – 4,800 m. These are the same families that dominate the
pollen record of the Zhada Formation (Li and Zhou, 2001). The limited ecosystem data
published by Wang (2003) suggests that C4 plants at high elevation prefer some subecosystems, such as river valleys. More importantly, the fossil plants that we sampled in
the Zhada Formation likely grew in or on the margins of the lake or along marshy
watercourses leading to it, given their abundance and associated lithofacies. This
suggests that the global expansion of C4 grasses in the late Miocene was not limited to
well-drained grasslands but also included semi-aquatic grasses in lakes and wetlands.
Semi-aquatic C4 grasses are well known in other riparian or wetland settings (for example
Jones, 1988; Martinelli et al., 1991), but, as far as we know, they remain unstudied in
modern Tibet.
Application to Tectonic Models
This study suggests that the Zhada region of southwestern Tibet underwent a
measurable decrease in elevation during the past 9 Myr, a result that is not specifically
predicted in any existing tectonic model for the development of the Tibetan Plateau. The
late Cenozoic structural setting of the Zhada region motivates us to invoke crustal
thinning in response to mid- to upper-crustal extension (Murphy and Copeland, 2005;
Murphy et al., 2002; Thiede et al., 2006; Zhang et al., 2000) as a mechanism that has
61
contributed to elevation loss. However, future studies are needed to determine whether
elevation loss was restricted to the Zhada region or affected a larger area of the plateau.
62
CONCLUUSIONS
We have accounted for all the major controls of δ 18O change and still the lowest
calculated δ18Opsw is ~3.5 ‰ less than the most negative δ18Osw value of modern water.
At this point, our ability to interpret the record in deep time based on the modern is better
for oxygen than carbon isotopes, and we therefore favor the case for high elevations in
southwestern Tibet at 9 Ma based on the oxygen isotope record. Future research in the
region should focus on better understanding the controls and distribution of C 4 plants in
all ecosystems in Tibet. Twentieth century climate changes can account for part, but not
all of the difference between reconstructed and modern δ18Osw values, raising the
intriguing possibility that mean catchment elevation in southwestern Tibet has decreased
by 1 – 1.5 km since ~9 Ma. The decrease in elevation is indicated by an approach that
assumes a minimum paleoelevation uncertainty estimate. While a solely climatically
driven change in δ18Osw values cannot be completely ruled out, the proxies cited show no
evidence of climate change which would result in the observed change. On the other
hand, there is evidence of crustal thinning through detachment faulting in the Zhada area
that could explain an elevation loss. This decrease in elevation is consistent with tectonic
models that invoke collapse of an over-thickened Tibetan crust due to a change in internal
or boundary conditions.
63
Figure 2.1. A: Elevation, shaded relief and generalized tectonic map of the Himalayan –
Tibetan orogenic system showing the location of the Zhada Basin relative to other
sources of paleoelevation data on the plateau. Sources for paleoelevation data are as
follows; Thakkhola graben: Garzione et. al. (2000b), Oiyug basin: Currie et. al. (2005)
and Spicer et. al. (2003), Lunpola basin: Rowley and Currie (2006), Nima basin:
DeCelles et. al. (2007), locations in the Tarim and Qaidam basins: Graham et. al. (2005),
Hoh Xil basin: Cyr et. al. (2005) and Gyirong basin: Rowley et. al. (2001) and Wang et.
al. (2006). B: Generalized geologic map of the Zhada region. Gastropod samples for this
study come from measured sections whose location is indicated by solid black lines.
Modified from published mapping by Chen and Xu (1987), Murphy et. al. (2000, 2002)
and unpublished mapping by M. Murphy.
64
Figure 2.2. Vector component diagrams for the South Zhada Sections. Site qualities A, B,
C and D are presented by sites S322, S0313, LC6 and S0305 respectively.
65
Figure 2.3. Site mean and section mean vectors for the South Zhada and Southeast Zhada
Sections. The mean vectors for normal and reversed polarity sites are anti-parallel and
thus pass the ―Reversals Test‖ (Butler, 1992).
66
Figure 2.4. Modern water and gastropod sample locations from the Zhongba area. Image
from Google Earth.
67
Figure 2.5. δ13C of vascular organic material versus stratigraphic level for the South
Zhada and East Zhada measured sections. In both sections we see the introduction of C4
plants (δ13C values > -16 ‰) at 250 - 300 m. The shift in δ13C values, from uniformly
negative to a mixture of more and less negative values at 250 – 300 m, is seen in the
northern Indian sub-continent and southern Tibetan Plateau and is associated with a
change in biomass from C3 dominated to a mix of C3 and C4 or C4 dominated biomass
at ~ 7 Ma.
68
69
Figure 2.6. Measured sections and magnetostratigraphic results for the South Zhada and
Southeast Zhada sections. Sections were correlated using the top surface as a datum (A)
to produce a composite magnetostratigraphic column (B). Which was then correlated
with the GPTS of Cande and Kent (1995) (C-H). Our favoured correlation is E. See text
for details. 91 sites were used to construct the South Zhada section and 60 sites were
used to construct the Southeast Zhada section.
70
Figure 2.7. Stable isotope composition of modern water from the Zhada Basin and
Zhongba area plotted against the Global Meteoric Water Line. The thin black line is a
mixing line between evaporated wetland samples and an unevaporated fluvial sample
from Zhongba.
71
Figure 2.8. X-Ray diffraction data for (A) aragonite standard, (B) calcite standard and (C)
a representative sample from the South Zhada section. The sample matches peaks at 2627º, 33º, 36-38º, 46º, 48.5º, 50º and 53º and lacks the prominent peak in calcite at 29.5º.
Spectra from all samples that returned usable results (11 of 12) match the 0.1SZ4.5
spectrum.
72
73
Figure 2.9. Oxygen and carbon isotope covariance for gastropods from the Zhada
formation showing covariance patterns for lithofacies assemblages. Correlation
coefficients for the entire data sets are 0.65 and 0.62 for South Zhada and East Zhada
respectively. Also shown are δ18Occ and δ13Ccc values for modern gastropods from the
Zhongba wetlands.
74
75
Figure 2.10. δ18O (VPDB ‰) and stratigraphic height for Miocene – Pleistocene
gastropods from (A) South Zhada and (B) East Zhada subdivided by lithofacies
assemblages.
76
Figure 2.11. δ18O (VPDB ‰) and mm growth from seasonally sampled Miocene –
Pleistocene gastropods from Zhada Basin. Typical seasonal variations are between 2 - 3
‰. Sample 2EZ16A is from a marshy interval and shows evidence of a drying episode.
Sample 5EZ27 is from a deltaic sequence and shows evidence of a freshening episode.
77
78
Figure 2.12. A) Calculated δ18Opw values from gastropods from the South Zhada
measured section. Error bars represent an uncertainty of ± 7°C due to possible seasonal
variation in the temperature of carbonate precipitation. The range of modern fluvial or
stream water (-17.9 to –11.9 ‰: light grey box) and mean value for modern water (-14.8
‰: dashed vertical line) as well as the range of values obtained from modern Tibetan
lakes (+1.7 to -7.1 ‰: dark grey box) are shown for comparison. δ 18Opsw values from
gastropods from fluvial intervals are at least as negative as the corrected modern,
reflecting elevations at least as high as the modern. δ 18Opsw values from gastropods from
lacustrine intervals are at least as positive as those from modern lakes, reflecting an
environment that, like the modern, was arid. B) South Zhada measured stratigraphic
section. C) Site-mean virtual geomagnetic pole (VGP) latitude for the South Zhada
section plotted against stratigraphic level and correlated with the global polarity timescale
of Cande and Kent (1995). Positive VGP latitudes indicate normal polarity shown by
black intervals and negative VGP latitudes indicate reversed polarity shown by white
intervals.
79
Figure 2.13. Modelled enrichment values (horizontal grey box) for the modern relative
humidity range (vertical grey box) and temperatures between 0 and 10 ºC compared with
Miocene enrichment (diagonally ruled box). Miocene enrichment (21.7 to 18.7 ‰) is
based on the difference in δ18O between fluvial and lacustrine gastropods. Also shown
are the mean annual temperature (0 ºC), average temperature from April – October (7 ºC),
maximum mean month temperature (14 ºC) and absolute maximum temperature (21 ºC)
for 1961 – 1990 at Shiquanhe weather station. Calculated Miocene enrichment values
overlap modelled modern enrichment values, indicating that the climate in the Miocene
was cold and arid, very similar to the modern.
80
Figure 2.14. Temperature of aragonite precipitation reconstructed based on modern
gastropod shell δ18Occ and associated water δ18Osw values.
81
Figure 2.15. Modeled percent of modern aragonite in equilibrium with monsoon wetland
waters given various temperature and pre-monsoon wetland water δ18Osw values. The
range of δ18Occ values of gastropods from modern wetlands along the Tsangpo River are
shown in the horizontal grey shaded box.
82
Figure 2.16. Δδ18O (VSMOW ‰) of modern water from the Zhada Basin and the mean
and maximum catchment elevation for water samples. Δδ18O is calculated assuming a
low-elevation δ18O value for New Delhi of -5.8 ‰. Also shown are thermodynamically
based lapse rate models (based on work by Rowley et. al. (2001) and Rowley and
Garzione (2007)) with low-elevation temperature of 303 K (dashed line), 300 K (thick
black line), 295 K (thin black line) and an empirical lapse rate (from Garzione et. al.,
2000b, light grey line).
83
Figure 2.17. Δδ18O (VSMOW ‰) values for the most negative modern Zhada water
samples and most negative reconstructed Miocene – Pleistocene water and modelled
elevation for thermodynamically based lapse rate models with low-elevation temperature
of 300 K (black line) and 295 K (dark grey line) and an empirical lapse rate (light grey
line). Δδ18O values were calculated using the modern low-elevation δ18O value of -5.8‰
from New Delhi and a Miocene low-elevation δ18O value of – 6‰ from Siwalik group
paleosol carbonates. Uncorrected Miocene – Pleistocene water samples are indicated by
diamonds and those that have been corrected for an ~ 3 ‰ post-industrial revolution
increase are indicated by squares. In both cases the difference between Miocene –
Pleistocene water and modern water δ18O values is greater than the uncertainty associated
with them indicating that the modelled loss in elevation is reliable.
84
Figure 2.18. Comparison of the range of Δδ 18O values for reconstructed Miocene fluvial
water and modern water from the Zhada Basin verses elevation. Also shown is the 2ζ
uncertainty envelope around a mean Δδ 18O versus elevation lapse rate. The low elevation
Ti for the mean Δδ18O versus elevation lapse rate is 295K and low elevation relative
humidity is 80%. Unlike figure 11, which presents a minimum uncertainty estimation,
this figure presents the typically cited 2ζ uncertainty. This results in considerable
overlap in reconstructed elevation. However, it also admits the previously inferred
elevation loss.
85
Table 2.1. Stable isotope and elevation data for modern water from the Zhada Basin and
Zhongba area.
18
Sample
Elevation
(m)
Maximum
Watershed
Elevation
(m)
Mean
Watershed
Elevation
(m)
δ O (‰
VSMOW)
δD (‰
VSMOW)
Lat. (N)
Long. (E)
JQ-38
4325
5943.47
5188.63
-14.3
-106
31.53415
79.975603
Ai Shan
JQ-39
3613
6842.24
5419.08
-17.9
-130
31.46889
79.784636
JQ-40
3523
6856.46
4855.59
-14.3
-100
31.47474
79.74296
Himalaya,
Same
Catchment
as JQ-42
and w2571
Sutlej River
JQ-37
4524
5843.2
5136.76
-16.1
-117
31.45092
80.12007
Ai Shan
JQ-41
3535
6856.46
4855.59
-16.5
-123
30.75320
79.77028
Sutlej River
JQ-42
3851
6842.24
5419.08
-16.4
-116
31.33770
79.790041
JQ-76
3988
5632
4547
-15.4
-117
31.3343
79.901867
Himalaya,
Same
Catchment
as JQ-39
and w2571
Himalaya
JQ-77
3790
6856.46
4855.59
-14.4
-104
31.4005
79.921533
Sutlej River
JQ-80
4074
6253
4881
-13.4
-94
31.16668
80.0719
Himalaya
31.12047
80.117783
Himalaya
80.3501
Himalaya
Sample
Name
Notes
Zhada water
JQ-81
4101
5764
4844
-7.3
-75
JQ-82
w0281
4287
5184
6179
6603.87
5079
5768.95
-15.7
-16.9
-112
-125
30.97107
31.13423
79.450681
Himalaya
w0381
3718
6856.46
4855.59
-15
-110
29.05971
88.000349
Sutlej River
w1771
4409
6043.34
5055.83
-16
-118
31.99252
79.642386
w1772
4410
6257.3
5143.86
-15
-107
32.99252
80.642384
w2371
4724
6069
5325
-13.2
-98
32.22481
79.57032
w2571
3762
6842.24
5419.08
-17.6
-137
31.45323
79.657603
110706-1
160706-3
4678
5026
5028
6048.77
4760
5454.95
-12.6
-15
-90
-112
31.30662
79.57273
Ai Shan,
Same
location as
230706-3
Ai Shan,
Same
location as
230706-4
Ai Shan,
Same
location as
230706-2
Himlaya,
Same
Catchment
as JQ-39
and -42
Himalaya
31.19732
79.5518
Himalaya
160706-4
5042
5806.18
5304.09
-11.9
-86
31.20925
79.546983
Himalaya
200706-1
4341
6202.7
4998.89
-13.4
-99
31.88190
79.7102
Ai Shan
200706-2
4693
5352.13
4904.31
-13.5
-99
31.96550
79.755167
Ai Shan
200706-6
4431
6023
5063.87
-13.8
-100
32.20142
79.423383
Ai Shan
210706-1
4960
6081.74
5569.38
-12.7
-93
32.32415
79.0589
Ai Shan
32.23063
79.57265
230706-2
4738
6069
5325
-13.8
-101
Ai Shan,
Same
location as
w2371
86
230706-3
4368
6043.34
5055.83
-13.4
-102
31.99842
79.64865
230706-4
4368
6257.3
5143.86
-13.2
-102
31.99842
79.64865
240706-1
4863
6255
5680
-14.3
-112
31.82573
79.03238
Ai Shan,
Same
location as
w1771
Ai Shan,
Same
location as
w1772
Ai Shan
-15.2
-108
31.91108
80.10542
Ai Shan
-6.3
-90
29.66257
84.16760
240706-2
Zhongba water
4534
6255
5510
180506-4
4569
180605-5
4591
-3.9
-86
29.08736
83.73856
260706-2
4574
-18.9
-140
29.71140
84.07028
260706-3
4571
-16.6
-140
29.68233
84.15037
87
Table 2.2. Oxygen and carbon isotope data (VPDB) from Miocene – Pleistocene
gastropods from Zhada Basin. Also included are the calculated Miocene – Pleistocene
water values that the aragonite was precipitated in assuming a temperature of
precipitation of 7 C and the stratigraphic height of the gastropod samples.
Sample name
South Zhada
Section
0.1SZ22A
0.1SZ22B
0.1SZ22C
0.1SZ22D
0.1SZ23.25
0.1SZ24.5
0.1SZ25
0.1SZ25A
0.1SZ26.2A
0.1SZ26.2B
0.1SZ26.2C
0.1SZ29.7
0.1SZ31A
0.1SZ31B
0.1SZ38.5
0.1SZ4.5
0.1SZ4.5AD1
0.1SZ4.5AD10
0.1SZ4.5AD10.1
0.1SZ4.5AD10.2
0.1SZ4.5AD11
0.1SZ4.5AD11.1
0.1SZ4.5AD12
0.1SZ4.5AD13
0.1SZ4.5AD14
0.1SZ4.5AD14.1
0.1SZ4.5AD14.2
0.1SZ4.5AD14.3
0.1SZ4.5AD15
0.1SZ4.5AD2
0.1SZ4.5AD4
0.1SZ4.5AD5
0.1SZ4.5AD6
0.1SZ4.5AD7
0.1SZ4.5AD8
0.1SZ4.5AD9
0.1SZ48.7
δ18O
(VPDB ‰)
-16.1
-11.8
-9.9
-11.3
-20.9
-18.9
-10.5
-10.2
-20.2
-21.0
-21.4
-9.9
-14.6
-17.6
-14.2
-1.7
-2.4
-2.2
-2.4
-3.5
-4.1
-3.9
-3.3
-3.5
-3.6
-3.1
-2.0
-4.7
-5.2
-3.4
-4.5
-3.1
-2.1
-1.8
-1.5
-1.8
-13.6
δ 18O
(VSMOW
‰)*
-19.0
-14.7
-12.8
-14.2
-23.7
-21.8
-13.4
-13.1
-23.1
-23.9
-24.3
-12.8
-17.4
-20.5
-17.1
-4.6
-5.4
-5.1
-5.3
-6.4
-7.0
-6.8
-6.2
-6.4
-6.5
-6.0
-4.9
-7.6
-8.1
-6.3
-7.4
-6.0
-5.0
-4.8
-4.4
-4.8
-16.5
13
δ C
(VPDB
‰)
-7.8
-6.0
-10.3
-6.6
-7.7
-6.4
-6.2
-5.7
-7.6
-5.4
-7.0
-3.3
-9.0
-7.3
-8.5
-0.3
0.7
2.2
1.9
1.1
0.4
1.1
1.9
0.6
1.7
1.4
1.4
2.1
2.6
2.5
2.1
1.9
1.9
1.9
3.0
1.9
-5.1
Stratigraphic
height (m)
102.75
102.75
102.75
102.75
104
105.25
105.75
105.75
106.95
106.95
106.95
110.45
111.75
111.75
119.25
85.25
85.25
85.25
85.25
85.25
85.25
85.25
85.25
85.25
85.25
85.25
85.25
85.25
85.25
85.25
85.25
85.25
85.25
85.25
85.25
85.25
129.45
88
0.1SZ52
0.1SZ53.25
0.1SZ53.25A
0.1SZ53.25B
0.1SZ53.25C
0.1SZ53.25D
0.2SZ10
0.2SZ11.5
0.2SZ11.5A
0.2SZ11.5B
0.2SZ15.5
0.2SZ20
0.2SZ25
0.2SZ36A
0.2SZ36B
0.3SZ15.15
0.3SZ22
0.3SZ32.5
0.3SZ38.25
0.3SZ38.25D1
0.3SZ38.25D2
0.3SZ38.25D3
0.3SZ38.25D4
0.3SZ38.25D5
0.3SZ38.25D6
0.3SZ38.25D7
0.3SZ38.25D8
0.3SZ38.25D9
0.3SZ42
0.3SZ6.8AD2
0.3SZ6.8ADA
0.4SZ14.9
0.4SZ24.65
0.4SZ24.8
0.4SZ25
0.4SZ25.35
0.4SZ4.8
1SZ12
1SZ13
1SZ13.8
1SZ18
1SZ2.5
1SZ24
1SZ24.1
1SZ27.9
1SZ32
2SZ16.4
-13.0
-17.3
-20.3
-21.6
-5.2
-18.3
-10.1
-14.2
-18.3
-7.6
-9.1
-15.3
-11.7
-17.7
-17.0
-20.6
-8.1
-0.7
-5.6
-8.3
-7.9
-7.0
-7.1
-6.8
-6.2
-7.1
-8.1
-8.8
-2.5
-11.3
-9.7
-20.3
-3.9
-4.5
-0.9
-1.8
0.2
-3.9
-12.5
-3.2
-1.8
-2.4
-5.4
-4.6
-2.8
-1.5
-12.1
-15.9
-20.1
-23.2
-24.5
-8.1
-21.2
-12.9
-17.0
-21.2
-10.5
-12.0
-18.2
-14.5
-20.6
-19.9
-23.5
-11.0
-3.6
-8.5
-11.2
-10.8
-9.9
-10.0
-9.7
-9.1
-10.0
-11.0
-11.7
-5.4
-14.2
-12.6
-23.1
-6.8
-7.4
-3.8
-4.8
-2.7
-6.8
-15.4
-6.1
-4.7
-5.3
-8.3
-7.5
-5.7
-4.4
-14.9
-4.4
-3.2
-6.5
-7.3
-5.7
-7.1
0.7
-11.0
-6.0
-8.9
-11.2
-10.0
-4.1
-10.6
-7.8
-7.7
-2.5
-0.9
1.6
-2.8
-3.4
-3.7
-3.8
-2.9
-2.3
-2.6
-3.1
-3.3
-0.5
-8.9
-9.3
-9.4
-0.6
1.9
0.6
-3.0
-1.3
-0.9
-3.0
-1.3
2.1
0.6
-0.2
-0.5
0.0
0.5
-5.1
132.75
134
134
134
134
134
160.7
162.2
162.2
162.2
166.2
170.7
175.7
186.7
186.7
217.15
224
234.5
240.25
240.25
240.25
240.25
240.25
240.25
240.25
240.25
240.25
240.25
244
208.8
208.8
278.25
288
288.15
288.35
288.7
268.15
309.65
310.65
311.45
315.65
300.15
321.65
321.75
325.55
329.65
349.5
89
2SZ3.25
2SZ34.5
2SZ35.5
2SZ41A
2SZ41B
2SZ41C
2SZ43
2SZ43.1
2SZ47
2SZ51.5
2SZ51.5AD0.5
2SZ51.5AD10
2SZ51.5AD11
2SZ51.5AD12
2SZ51.5AD13
2SZ51.5AD14
2SZ51.5AD15
2SZ51.5AD3
2SZ51.5AD4
2SZ51.5AD5
2SZ51.5AD6
2SZ51.5AD7
2SZ51.5AD8
2SZ51.5AD9
2SZ55
2SZ7.5
3SZ0.15
3SZ14.45
3SZ14.5
3SZ18.5
3SZ24
3SZ24.1A
3SZ24.25A
3SZ24.25B
3SZ24.25C
3SZ24.3
3SZ27
3SZ3.9
3SZ32.5
3SZ4
3SZ41.5
3SZ49
3SZ5
3SZ50.5
3SZ55
3SZ61.8AD1
3SZ61.8AD2
-1.6
-14.6
-11.6
-13.9
-17.4
-14.6
-1.8
-6.8
-1.9
-2.4
-0.6
-1.2
-2.1
-2.2
-2.1
-2.5
-0.8
-2.2
-2.1
-1.6
-0.8
-0.3
-0.3
-0.4
-2.3
-2.0
-1.7
-2.9
-2.0
-3.1
-2.2
-2.8
-1.4
-1.8
-1.6
-2.0
0.7
-2.0
-15.5
-3.3
-4.6
-13.7
-1.4
-1.0
-2.5
-0.8
-0.9
-4.5
-17.4
-14.5
-16.7
-20.3
-17.5
-4.8
-9.7
-4.8
-5.3
-3.5
-4.1
-5.0
-5.1
-5.1
-5.4
-3.7
-5.1
-5.0
-4.5
-3.7
-3.2
-3.2
-3.3
-5.2
-4.9
-4.6
-5.8
-4.9
-6.0
-5.1
-5.7
-4.4
-4.8
-4.5
-4.9
-2.2
-4.9
-18.3
-6.2
-7.5
-16.6
-4.4
-3.9
-5.4
-3.7
-3.8
0.8
-7.4
-5.3
-2.6
-5.1
-5.5
0.9
-0.4
-0.6
-0.2
-0.6
0.2
0.2
0.1
0.3
0.2
0.8
-0.4
-0.1
0.3
0.9
0.0
0.0
-0.1
-0.7
1.0
0.9
1.9
1.2
-0.8
-0.3
1.6
0.8
-1.1
0.6
0.9
-2.1
-0.4
-5.0
-0.8
0.4
-3.3
1.4
-1.2
1.2
0.2
0.1
336.35
367.6
368.6
374.1
374.1
374.1
376.1
376.2
380.1
384.6
384.6
384.6
384.6
384.6
384.6
384.6
384.6
384.6
384.6
384.6
384.6
384.6
384.6
384.6
388.1
340.6
389.25
403.55
403.6
407.6
413.1
413.2
413.35
413.35
413.35
413.4
416.1
393
421.6
393.1
430.6
438.1
394.1
439.6
444.1
450.9
450.9
90
3SZ62
3SZ67.5
3SZ74A
3SZ74B
3SZ75.6A
3SZ75.6B
4SZ5
5SZ4.6A
5SZ4.6B
East Zhada
Section
1EZ10
1EZ11
1EZ12
1EZ25.5
1EZ25.5A
1EZ25.5BD1
1EZ25.5BD2
1EZ25.5C
1EZ25.5D
1EZ27
1EZ35.5
1EZ42
1EZ44.75
1EZ46
1EZ5.55
1EZ5.55AD1
1EZ5.55AD2
1EZ5.55AD3
1EZ5.55BD1
1EZ5.55CD1
1EZ5.55CD2
1EZ52.2
1EZ63
1EZ8.9A
1EZ8.9AD1
1EZ8.9AD2
1EZ8.9AD3
1EZ8.9AD4
1EZ8.9AD5
1EZ8.9BD1
1EZ8.9BD2
1EZ8.9BD3
1EZ8.9C
2EZ111.5
2EZ115.5
2EZ16.2A
-1.3
-2.4
-3.0
-1.8
-5.0
-4.6
-9.9
-9.9
-7.7
-11.8
-18.3
-16.8
-12.7
-16.1
-9.7
-9.3
-7.9
-15.6
-17.6
-15.1
-14.1
-16.9
-19.0
-14.4
-17.8
-18.7
-19.2
-13.5
-11.6
-11.2
-16.0
-10.7
-15.8
-16.7
-17.1
-17.2
-17.6
-18.2
-19.0
-18.9
-16.1
-17.5
-18.3
-17.1
-13.5
-4.2
-5.3
-5.9
-4.7
-7.9
-7.5
-12.8
-12.8
-10.6
0.0
0.9
0.3
0.1
-0.5
0.0
7.5
1.3
1.4
451.1
456.6
463.1
463.1
464.7
464.7
534.7
642.9
642.9
-13.6
-20.0
-18.6
-14.5
-17.8
-11.5
-11.1
-9.7
-17.3
-19.3
-16.8
-15.8
-18.7
-20.8
-16.1
-19.5
-20.4
-21.0
-15.2
-13.3
-13.0
-17.8
-12.5
-17.6
-18.5
-18.9
-18.9
-19.4
-20.0
-20.7
-20.7
-17.9
-19.2
-20.1
-18.9
-15.3
-5.0
-8.9
-8.2
-4.4
-4.1
-5.4
-5.6
-5.3
-5.8
-5.8
-8.0
-6.5
-10.8
-7.9
-6.8
-9.1
-9.1
-9.6
-6.9
-5.2
-5.6
-7.7
-4.0
-12.8
-6.8
-12.8
-12.4
-12.7
-11.8
-12.9
-12.0
-13.4
-13.6
-7.7
-5.5
-12.4
10
11
12
25.5
25.5
25.5
25.5
25.5
25.5
27
35.5
42
44.75
46
5.55
5.55
5.55
5.55
5.55
5.55
5.55
52.2
63
8.9
8.9
8.9
8.9
8.9
8.9
8.9
8.9
8.9
8.9
167
171
71.7
91
2EZ16AD1
2EZ16AD2
2EZ16AD3
2EZ16AD4
2EZ16B
2EZ22
2EZ39
2EZ60.7
2EZ60.7A
2EZ60.7B
2EZ60.7C
2EZ63
2EZ63AD1
2EZ63AD2
2EZ63AD3
2EZ63AD4
2EZ63B
2EZ63C
2EZ63D
2EZ63E
2EZ89.45
2EZ9.5
3EZ100
3EZ102
3EZ119.5
3EZ131.3A
3EZ131.3B
3EZ136
3EZ146
3EZ152.3A
3EZ152.3B
3EZ158
3EZ173
3EZ180
3EZ180.5
3EZ183
3EZ187.5
3EZ46
3EZ63.5
3EZ69.2
3EZ69.2A
3EZ69.2BD1
3EZ69.2BD2
3EZ69.2C
3EZ69.5
3EZ7
3EZ98
-15.6
-13.3
-4.9
-15.8
-16.5
-16.0
-19.1
-12.6
-17.8
-15.7
-16.2
-11.7
-10.9
-11.1
-11.9
-11.5
-6.9
-11.6
-8.8
-12.2
-9.8
-14.3
-15.3
-16.5
-4.7
-3.8
-2.5
-13.8
0.1
-4.3
-3.1
-2.8
-3.7
-4.1
-2.5
-2.8
-9.3
-10.0
-15.7
-17.4
-16.7
-17.2
-15.6
-18.0
-16.0
-13.9
-14.1
-17.4
-15.0
-6.7
-17.5
-18.2
-17.8
-20.8
-14.3
-19.6
-17.4
-18.0
-13.5
-12.7
-12.8
-13.7
-13.2
-8.7
-13.4
-10.6
-14.0
-11.6
-16.0
-17.1
-18.3
-6.5
-5.6
-4.3
-15.6
-1.7
-6.0
-4.9
-4.6
-5.5
-5.9
-4.3
-4.6
-11.0
-11.8
-17.5
-19.1
-18.5
-18.9
-17.3
-19.7
-17.7
-15.7
-15.9
-7.5
-3.8
-1.8
-3.0
-11.1
-13.8
-8.1
-3.4
-5.5
-3.6
-0.7
-0.3
-5.0
-5.4
-6.5
-6.7
-2.7
-3.5
-3.2
-8.7
-2.8
-4.6
-3.6
-3.5
-3.6
0.0
-0.2
-0.6
-1.1
-3.5
-1.7
0.3
-0.2
0.1
0.4
-0.3
-2.2
-4.4
-5.0
-7.2
-3.4
-5.2
-3.9
-5.2
-5.9
-4.7
-3.8
71.5
71.5
71.5
71.5
71.5
77.5
94.5
116.2
116.2
116.2
116.2
118.5
118.5
118.5
118.5
118.5
118.5
118.5
118.5
118.5
144.95
65
100
276.35
293.85
305.65
305.65
310.35
320.35
326.65
326.65
332.35
347.35
354.35
354.85
357.35
361.85
220.35
237.85
243.55
243.55
243.55
243.55
243.55
243.85
181.35
272.35
92
3EZ99
4EZ1
4EZ23.9
4EZ23.9
4EZ23.9
4EZ23.9
4EZ23.9
4EZ23.9
4EZ23.9
4EZ23.9A
EDGE
4EZ23.9A APEX
4EZ23.9B
EDGE
4EZ23.9B APEX
4EZ23.9C
4EZ23.9D
4EZ23.9E
4EZ23.9 EDGE
4EZ23.9 APEX
4EZ23.9F
4EZ3
4EZ30
4EZ35.3
4EZ4
5EZ1.5A
5EZ1.5B
5EZ27.75
5EZ3
5EZ32
5EZ6.45
-12.6
-3.0
-3.9
-3.9
-4.4
-3.1
-2.2
-1.9
-3.3
-4.2
-14.3
-4.7
-5.7
-5.7
-6.2
-4.9
-4.0
-3.7
-5.1
-5.9
-7.3
-2.7
0.3
1.3
1.0
0.5
1.3
1.7
1.0
-0.2
273.35
414.45
437.35
437.35
437.35
437.35
437.35
437.35
437.35
437.35
-5.0
-3.6
-6.8
-5.4
-0.5
-0.9
437.35
437.35
-5.3
-5.0
-2.0
-3.5
-3.8
-3.0
-2.5
-3.6
-15.3
-10.8
-3.1
-3.1
-2.2
-3.2
-1.5
-4.8
-4.4
-7.1
-6.8
-3.7
-5.3
-5.6
-4.8
-4.3
-5.3
-17.0
-12.6
-4.9
-4.9
-4.0
-5.0
-3.3
-6.6
-6.2
1.1
0.7
1.3
-0.1
1.1
1.3
1.3
-1.8
-0.6
0.4
0.0
-0.2
-2.4
-0.2
-0.1
0.2
-1.1
437.35
437.35
437.35
437.35
437.35
437.35
437.35
416.45
443.45
448.75
417.45
488.35
488.35
514.6
489.85
518.85
493.3
93
Table 2.3. δ13C (VPDB ‰) for plant material from the South Zhada and East Zhada
sections.
Sample
Name
C
(VPDB
‰)
Stratigraphic
height (m)
1EZ5.55
1EZ13
1EZ34.1
2EZ49.1
2EZ68.5
3EZ12.8
3EZ139
3EZ139
3EZ147
3EZ147
3EZ90.8
4EZ20
4EZ3
4EZ3
4EZ6.4
5EZ21.1
5EZ35.6
5EZ35.6
5EZ39.9
South
Zhada
Section
-24.49
-23.93
-24.52
-24.80
-25.03
-23.41
-21.38
-24.07
-11.54
-18.53
-23.64
-23.39
-8.39
-23.24
-24.36
-14.24
-14.78
-11.97
-22.55
5.55
13
34.1
104.65
124.05
150.25
295.05
295.05
303.05
303.05
246.85
371.55
354.55
354.55
357.95
421.05
435.55
435.55
439.85
0.1SZ25
0.4SZ25
0.4SZ25
2SZ 50.35
2SZ35.3
2SZ7.7
2SZ7.7
2SZ5.85
0.3SZ43.35
0.3SZ32.6
0.3SZ32
0.3SZ32
0.3SZ26
0.1SZ52.35
0.1SZ52.35
-25.49
-14.80
-16.84
-18.59
-24.16
-25.24
-24.8
-25.96
-18.68
-21.10
-22.97
-22.83
-24.24
-26.78
-26.08
105.75
288.36
288.36
383.46
368.41
340.81
340.81
338.96
245.36
234.61
234.01
234.01
228.01
133.1
133.1
East Zhada
Section
94
0.1SZ25
4SZ100.2
-25.40
-24.05
105.75
629.9
95
CHAPTER 3: BASIN FORMATION DUE TO ARC-PARALLEL EXTENSION
AND TECTONIC DAMMING: ZHADA BASIN, SW TIBET
ABSTRACT
The late Miocene – Pleistocene Zhada basin in southwestern Tibet provides a
record of elevation loss and basin formation via arc-parallel extension within an active
collisional thrust belt. The > 800 m-thick basin fill is undeformed and was deposited
along an angular unconformity on top of Tethyan strata that were previously shortened in
the Himalayan fold- thrust belt. Modal sandstone petrographic data, conglomerate clastcount data, and detrital zircon U-Pb age spectra indicate a transition from detritus
dominated by a distal, northern source to a local, southern source. This transition was
accompanied by a change in paleocurrent directions from uniformly northwestward to
basin-centric. At the same time the depositional environment in the Zhada basin changed
from a large, braided river to a closed-basin lake. Sedimentation in the Zhada basin was
synchronous with displacement on faults surrounding the basin, particularly the Qusum
and Gurla Mandhata detachment faults, which root beneath the basin and exhume midcrustal rocks along the northwestern and southeastern flanks of the basin, respectively.
These observations indicate that accommodation for Zhada basin fill was produced by a
combination of tectonic subsidence and damming, as mid-crustal rocks were evacuated
from beneath the Zhada basin and uplifted along its northwestern and southeastern
margins in response to arc-parallel slip on the Qusum and Gurla Mandhata detachment
faults.
96
INTRODUCTION
Since they were first recognized (Molnar and Tapponnier, 1978; Ni and York,
1978), normal fault-bounded basins in the high Himalaya have fueled speculation on their
origins and have been used as fundamental arguments in various tectonic models (Armijo
et al., 1986; Beaumont et al., 2004; Garzione et al., 2003; Hurtado et al., 2001; Kapp and
Guynn, 2004; e.g., McCaffrey and Nabelek, 1998; Molnar et al., 1993; Ratschbacher et
al., 1994; Seeber and Pecher, 1998; Tapponnier et al., 2001). These sedimentary basins
provide a record of tectonic and climatic processes essential for discriminating between
competing hypotheses about the evolution of the Himalayan-Tibetan orogenic system.
The Mio-Pleistocene Zhada basin in the western portion of the Himalayan arc
(Figure 3.1) provides an opportunity to study the causes of basin formation in the
Himalayan hinterland (Meng et al., 2008; Saylor et al., in press; Wang et al., 2008a;
Wang et al., 2008b; Zhang et al., 1981; Zhu et al., 2007; Zhu et al., 2004). The Zhada
basin sits at unusually low elevations. Here, elevation averaged over a 100 km interval is
> 1,000 m lower than anywhere else along strike (Figure 3.2). Ganser (1964) noted that
the Zhada basin dwarfs other late Cenozoic sedimentary basins in southern Tibet and the
northern Himalaya. Finally, stable isotope data indicate that the Zhada region has
probably lost up to 1.5 km of elevation since the late Miocene (Saylor et al., in press).
There is little information, and less consensus, regarding the age and tectonic
significance of the Zhada basin despite its abundance of vertebrate, invertebrate and plant
fossils, and location within the tectonically active hinterland of the Himalaya (Murphy et
al., 2002; Ni and Barazangi, 1985; Thiede et al., 2006; Valli et al., 2007; Wang et al.,
97
2008a; Zhang et al., 2000). Basic aspects of the basin geology, such as its stratigraphy
and sedimentology, are thinly documented and variably interpreted. For example, the
Zhada Formation is reported as both an upward-fining fluvio-lacustrine sequence (Li and
Zhou, 2001; Zhang et al., 1981; Zhou et al., 2000) or, alternatively, as being capped by
boulder conglomerates (Zhu et al., 2007; Zhu et al., 2004). Wang et al. (2004) proposed
that the Zhada basin is a supradetachment basin developed above the South Tibetan
Detachment system. This conflicts with the conclusion that the Zhada basin developed in
response to arc-perpendicular compression (Zhou et al., 2000). In contrast to both of
these scenarios, Wang et al. (2008a) concluded that the Zhada basin developed due to
arc-perpendicular rifting and Zhu et al. (2004) suggested that rifting was followed by
recent uplift.
In this paper we examine the development of the Zhada basin and its relationship
to basin-bounding faults as informed by new sedimentologic, provenance, and sediment
dispersal pattern data. Our results show that the basin evolved from a through-going
fluvial system to an internally drained depocenter concomitant with exhumation of midcrustal rocks to the northwest and southeast in the footwalls of major arc-parallel
extensional systems. We present a tectonic model that links development of the Zhada
basin directly to these extensional systems, and which explains basin development
through a combination of sill uplift and basin subsidence due to arc-parallel extension on
crustal-scale detachment faults.
98
GEOLOGIC SETTING
The NW-SE-trending Zhada basin is located just north of the high Himalayan
ridge crest in the western part of the orogen (~32° N, 82° E, Figure 3.1A). The basin is >
150 km long and > 60 km wide and has an outcrop extent of > 9,000 km2.
Major structural features near the margins of the Zhada basin are the South
Tibetan Detachment system (STDS) to the southwest, the Oligo-Miocene Great Counter
thrust and right-slip Karakoram fault to the northeast, and the Leo Pargil and Gurla
Mandhata gneiss domes to the northwest and southeast, respectively (Figure 3.1B). The
STDS is a series of north-dipping, low-angle, top-to-the-north normal faults that place
Paleozoic – Mesozoic low-grade metasedimentary rocks of the Tethyan sequence on
high-grade gneisses and granitoids of the Greater Himalayan sequence (Hodges et al.,
1992). No ages for movement on the STDS in this area have been published, but along
strike displacement is bracketed between 21 and 12 Ma (Cottle et al., 2007; Hodges,
2000; Hodges et al., 1996; Hodges et al., 1992; Murphy and Yin, 2003; Murphy and
Harrison, 1999; Searle et al., 1997; Searle et al., 2003; Yin, 2006). Although rocks of
Indian affinity are separated from those of Asian affinity by the Indus suture, the region
north of the STDS is considered the southern edge of the Tibetan plateau because of the
uniformly high elevations (Fielding et al., 1994). In the Zhada region the south-dipping,
top-to-the-north Oligo-Miocene Great Counter thrust system cuts the Indus suture (e.g.,
Ganser, 1964; Murphy and Yin, 2003; Yin et al., 1999). Exhumation of the Leo Pargil
gneiss dome by normal faulting initiated at either 15 Ma or 9 Ma, as indicated by
40
Ar/39Ar and AFT cooling ages, respectively (Thiede et al., 2006; Zhang et al., 2000).
99
Exhumation of the Gurla Mandhata gneiss dome began at 9 Ma (Murphy et al., 2002).
Faulted Quaternary basin fill indicates that gneiss dome development continues today
(Murphy et al., 2002).
The Zhada Formation occupies the Zhada basin and consists of > 800 m of
fluvial, lacustrine, eolian and alluvial fan deposits (Figure 3.3). The basin fill is
structurally undisturbed and sits above shortened Tethyan sequence strata along an
angular or buttress unconformity (Figures 3.4A, C). The Zhada Formation is capped by a
geomorphic surface that is correlative across the basin (Figures 3.4B, D). The
geomorphic surface is interpreted as a paleo-depositional plain that marks the maximum
extent of sedimentation prior to integration of the modern Sutlej River drainage network.
After deposition, the basin was incised to basement by the Sutlej River, exposing the
entire basin fill in spectacular canyons and cliffs. The best estimate for the age of the
Zhada Formation is 9.2 – < 1 Ma based on magnetostratigraphy (Figure 3.5)(Saylor et al.,
in press; Wang et al., 2008b). Correlations to the geomagnetic polarity timescale
(Lourens et al., 2004) were constrained by Mio-Pliocene mammal fossils and by an
observed shift from pure C3 to mixed C3 and C4 vegetation in the Zhada basin. The
correlation presented in Figure 5 accounts for all of our normal and reversed intervals;
places the C3 – C4 transition at ~ 7 Ma, which is consistent with its age elsewhere in the
Himalaya; and yields relatively constant and reasonable sedimentation rates. For an indepth discussion of alternative correlations see Saylor et al. (in press).
100
SEDIMENTOLOGY OF THE ZHADA FORMATION
We measured 14 stratigraphic sections spanning the basin from the Zhada county
seat in the southeast to the Leo Pargil/Qusum Range front in the northwest (Figures 3.1B
and 3.3B). Sections were measured at centimeter scale and correlated by tracing the
prominent paleo-depositional surface that caps most sections and major stratigraphic
units (Figures 3.4B, D and 3.6). The 14 lithofacies associations below are defined on the
basis of lithology, texture and sedimentary structures, and are grouped into five
depositional-environment associations. Lithofacies codes following the lithofacies
associations below are modified from Miall (1978) and DeCelles et al. (1991) and
described in Table 3.1. Unless otherwise indicated, all deposits are laterally continuous
for 100’s of meters to several kilometers.
Fluvial Association
Lithofacies Association F1: Gcmi, Gch, Gt, Gcf.
F1 deposits are interbedded tabular and amalgamated lenticular pebble
conglomerate bodies. Individual lenticular bodies are gravel- and pebble-filled and up to
3 m thick. Deposits are 2 – 10 m thick and typically have erosive bases (Figure 3.7E).
Deposits are moderately sorted and clast-supported. Clasts are rounded to subrounded
and imbricated (long-axis-transverse). Some units display an upward-coarsening textural
trend. Sedimentary structures include horizontal, trough and planar cross-stratification.
Tabular deposits are massive or contain horizontal stratification, large-scale (up to 4 m)
101
epsilon cross-stratification, and trough cross-stratified coarse-grained sandstone to pebble
conglomerate lenses (Figure 3.7A).
Lithofacies Association F2: St, Sp, Sh, Sc.
F2 sandstone beds are tabular and range in thickness from less than 0.1 m to 5 m
(Figures 3.7C, D). Basal bounding surfaces are erosional and upper bounding surfaces
are either erosional or gradational. Top bounding surfaces frequently display load casts,
convoluted bedding, and ball-and-pillow structures and are often overlain by lithofacies
association F3 (Figure 3.7G). Channel forms up to 3 m thick are common, but occur as
isolated bodies rather than as amalgamated channel bodies. Grain-size varies from fine to
very coarse and floating granules and granule lenses are common. Sorting is poor and
grain size trends are absent. Sedimentary structures include horizontal, planar, and
trough cross-stratification (Figures 3.7C, G). Horizontal stratification is common near
the base of units and planar or trough cross-stratification is more common in the upper
parts of units. Root traces, fragmentary plant material, and shell fragments are common,
though less abundant than in lithofacies F3.
Lithofacies Association F3: Sm, Sc, Mm, Mc, Ml, Mh.
Deposits of dark, organic-rich, massive or horizontally laminated, calcareous
siltstone or sandstone are < 0.1 – 4 m thick (Figures 3.7B, F). Basal surfaces are either
gradational or feature load casts and ball-and-pillow structures. Upper surfaces are either
gradational or erosional. These deposits are often upward-fining. Original sedimentary
structures are in most places not discernible. The grain size is variable and can be up to
medium-grained sandstone, though siltstone or very fine-grained sandstone is most
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common. Shell fragments, including ostracods and small planorbid gastropods (Gyraulus
sp?), plant material, and root traces are abundant in these deposits and are concentrated in
the fine-grained upper portion of upward-fining beds (Figure 3.7B). No clearly
developed soil nodules or soil horizons were observed. The δ 18O values of shells
systematically increase upward within individual units of F3 (Saylor et al., in press).
Lithofacies F3 primarily occurs interbedded with lithofacies F2.
Interpretation.
We interpret lenticular morphologies and tabular, cross-stratified deposits of
interbedded clast-supported conglomerate and sandstone (F1) as the deposits of migrating
or avulsing channels and migrating bars of a braided river system within a low-gradient,
low aggradation rate, fluvial setting (Bristow and Best, 1993; Cant and Walker, 1978;
Collinson, 1996). Horizontally stratified units are interpreted as the product of migrating
bedload sheets (Lunt et al., 2004). Tabular, cross-stratified units are interpreted as the
deposits of gravel dunes (Heinz et al., 2003).
Tabular deposits in lithofacies F2 could be interpreted either as the result of
sheetfloods (terminal splays or lobes) on medial to distal fluvial fans on a marshy
floodplain or marginal lacustrine setting (Hampton and Horton, 2007; Saez et al., 2007)
or as levee and crevasse splay deposits (Bridge, 2003). We favor the sheetflood origin
based on the absence of wedge-shaped sandstone bodies; lateral continuity of deposits;
absence of prograding, steeply-inclined slipfaces; and abundance of convoluted bedding
in lithofacies F2 (Bristow et al., 1999). The abundance of convoluted bedding indicates
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rapid sedimentation into a water-saturated environment, rather than periodic
sedimentation followed by long periods of stability as implied by crevasse splays.
Further, sequence stratigraphic analysis indicates that lithofacies F2 occurs primarily
updip of open lacustrine intervals (Saylor et al., in preparation). The presence of isolated
channel forms associated with these deposits indicates that the flow was not completely
unconfined, but rather occurred within and overwhelmed a distributary network.
Lithofacies association F3 is interpreted as marshy floodplain or lake-margin
wetland deposits (Allen and Collinson, 1986). The upsection increase in δ18O values of
shell from these lithofacies probably resulted from evaporative enrichment of standing
water in paleo-marshes (Saylor et al., in press). Modern analogs to these are widespread
along rivers on the Tibetan Plateau such as the Yarlung/Tsangpo in the vicinity of
Zhongba (Figure 3.1, N26°, E84°).
Supra-littoral (Lake Margin) Association
Lithofacies Association S1: Gct, Gcmi, Sp, St, Sh, Sm, Sr.
This lithofacies association consists of 0.25 – 6 m thick units of medium-grained
sandstone to pebble conglomerate (Figure 3.8B). Deposits consist of sheets or
amalgamated channel forms of upward-coarsening sandstone. Basal surfaces are
erosional, and top surfaces are commonly a sharp transition to lithofacies association L1
and contain reworked shells and shell fragments. Sedimentary structures include
horizontal, planar, and trough cross-stratification (Figure 3.8C). Thin lenses of finegrained sandstone to granule conglomerate are common. Channelized deposits up to 0.5
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m thick are locally present. This lithofacies association contains abundant, wellpreserved, robust gastropod shells (Figure 3.8A), plant and shell fragments, and root
traces. Lithofacies association S1 is often interbedded with F3.
Lithofacies Association S2: Sh.
Lithofacies association S2 is composed of low-angle climbing translatent strata
without internal structure developed in well-sorted, fine-grained sandstone. Stacked
deposits of sandstone laminae are < 0.5 – 5 m thick. This lithofacies can also be massive
or disrupted by soft sediment deformation. Individual laminae range from less than 0.5 to
3 cm thick. Root traces and armored mud balls are common (Figure 3.8E).
Lithofacies Association S3: Sf.
Stacked deposits of sandstone foresets of lithofacies association S3 range from
less than 0.5 m to 1.5 m thick. This lithofacies is characterized by steeply inclined
laminations developed in well-sorted, fine-grained sandstone (Figure 3.8D). Laminations
coarsen upward perpendicular to stratification. As with lithofacies S2, these deposits also
can be massive or contain post-depositional soft-sediment deformation and abundant root
traces. S3 is usually associated with S2.
Interpretation.
Root traces and preferential oxidation and cementation within lithofacies S1 and
its close association with lithofacies F3 indicate deposition in extremely shallow water
associated with lake-margin marshes or just above the shoreline (Ayers, 1986).
Sedimentary structures in lithofacies S1 indicate unidirectional flow. Therefore,
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lithofacies association S1 is interpreted as delta plain and distributary channel deposits
(Dam and Surlyk, 1993; Saez et al., 2007).
Lithofacies S2 is interpreted as vegetated sand-flat deposits on the basis of root
traces, the low angle of the laminations, and lack of internal structure in the laminations
which is consistent with formation be migrating wind ripples (Hunter, 1977). S3 is
interpreted as vegetated eolian dune deposits on the basis of the horizontal and vertical
dimensions of the high angle cross-stratification. Interbedded units of F3 were deposited
in interdune ponds and marshes. Modern analogs to environments proposed for
lithofacies association S1 occur at Kungyu Co (Fig. 1, N30° 36’, E82° 11’) and modern
analogs for environments proposed for lithofacies S2 and S3 occur both at Kungyu Co
and to the west of Zhongba (Fig. 1, N26°, E84°).
Littoral Association
Lithofacies Association L1: Ml.
Lithofacies association L1 consists of papery laminated siltstone with abundant
fossil grasses, fragmentary shell material, and occasional fish fossils (Figure 3.9).
Deposits are < 0.5 m thick and usually cap a coarser-grained deposit. Locally, bedded
gypsum and mudcracks occur in these deposits.
Lithofacies Association L2: Mr, Sr, Srw, Sp, Sh, Sm.
Rippled sandstone deposits are ~ 0.5 – 3 m thick and coarsen upward. Lithofacies
association L2 typically overlies lithofacies association P1 in gradational contact. The
upper surfaces of L2 units are erosional and commonly overlain by either S1 or L1. This
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lithofacies association is massive or contains climbing-ripples, horizontal, wave-ripple,
planar cross-strata, and horizontal laminations. Grain size ranges from siltstone to
medium-grained sandstone. Mud drapes are common.
Interpretation.
Based on modern analogs seen at Kungyu Co (Figure 3.1, N30° 36’, E82° 11’),
L1 represents a grass-rich, shallowly submerged, low-gradient, lake-margin environment
(Allen and Collinson, 1986; Talbot and Allen, 1996). L2 is interpreted as the deposits of
a terminal lobe on a prograding delta front on the basis of consistency in thickness,
sedimentary structures, and grading with modern analogs at Kungyu Co (Allen and
Collinson, 1986; Dam and Surlyk, 1993; Saez et al., 2007; Talbot and Allen, 1996).
Cross-stratification is consistent with both unidirectional and oscillatory currents. The
presence of massive or climbing-ripple cross-stratified beds indicates that flow velocity
was decreasing rapidly (Jopling, 1965; Miall, 1996; Mulder et al., 2003; Mutti et al.,
2003). The presence of wave-ripple cross-stratification in L2 points to a subaqueous
depositional environment above fair-weather wave base. Thus, L2 is possibly the
basinward lateral equivalent of lithofacies association S1.
Profundal Association
Lithofacies Association P1: Mh, Mm.
The P1 siltstone lithofacies association is composed of upward-coarsening,
massive or horizontally laminated silty claystone (Figures 3.10A, B). Deposits are < 1 up
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to 10 m thick. There is no evidence of flow associated with this lithofacies. Dispersed
plant, ostracod, and fish fossils are present in this lithofacies.
Lithofacies Association P2: Sh, Sm, Mh.
This lithofacies association is composed of stacks of very thin, tabular, upwardfining beds of fine-grained sandstone to siltstone (Figure 3.10C). Beds are up 3
centimeters thick and are stacked up to 2 m thick. Individual beds are massive in their
lower part but grade upward into horizontally laminated siltstone in their upper parts.
Basal contacts of individual beds are abrupt.
Interpretation.
The lack of traction sedimentary structures, fine-grain size, and presence of fish
fossils indicate that P1 was deposited below fair-weather wave base in a profundal
setting. P1 mudstones are often massive owing to extensive bioturbation, suggesting that
the lake was not well stratified and hence probably shallow. The sharp bases, massive
lower portions, horizontally stratified upper portions, and upward-fining beds of
lithofacies association P2 represent stacked deposits of dilute turbidity current deposits
(primarily Bouma A and B) (Bouma, 1962; Giovanoli, 1990). The interbedding of
lithofacies P1 and P2 suggests an environment in which stagnant water was occasionally
disrupted by turbid flow, bringing anomalously coarse-grained sediment into the
profundal lake bottom (Lowe, 1982; Mutti et al., 2003).
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Alluvial Fan Association
Lithofacies Association A1: Gcm, Gch, Gcmi.
Deposits of clast-supported, very angular to subrounded, pebble to boulder
conglomerate range from 0.5 – 20 m thick and typically have erosive bases (Figure
3.11A). Clast a-axes are up to 0.5 m long. Deposits are massive and locally horizontally
stratified. They are poorly sorted and contain long-axis-transverse imbrication. Crossstratification and channel forms are extremely rare. However, interbedded mediumgrained sandstone to granule conglomerate lenses are common. In the central part of the
Zhada basin this lithofacies is distinguishable from F1 by its coarser grain-size, poorer
sorting, and abundance of white quartzite clasts.
Lithofacies Association A2: Gmm, Gcm.
Deposits of matrix-supported boulder conglomerate are between 0.5 and 3 m thick
and do not have erosive bases. Grain-size, angularity and sorting are similar to
lithofacies A1. These deposits are massive and highly disorganized (Figure 3.11B).
Lithofacies Association A3: Gcmi, Gch, Gcf.
This lithofacies association is composed of very angular to subrounded, pebble to
boulder conglomerate in beds that range from 0.5 to 3 m thick and typically have erosive
bases. Sedimentary structures include horizontal- and cross-stratification. Lenticular
beds are common. Lithofacies association A3 is poorly sorted, clast-supported and
contains long-axis-transverse imbrication and interbedded medium-grained sandstone to
pebble conglomerate lenses.
Lithofacies Association A4: Gx.
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A4 is characterized by chaotic beds of clast- or matrix-supported, very poorly
sorted boulder conglomerate. Indvidual deposits are < 0.5 m – 5 m thick and often fine
upwards. Clasts are subrounded to angular. Deposits often have a winnowed, matrixpoor, cap < 0.5 m thick. These deposits have abundant load casts and ball and pillow
structures, with up to 2 m of relief on some structures (Figure 3.11C). Lithofacies A4 is
closely associated with lithofacies F3 as well as P1 and P2.
Interpretation.
The presence of imbricated clasts in A1 indicates transport by traction flows.
However, the lateral continuity of these deposits and paucity of channel forms imply
unconfined flow. We interpret lithofacies A1 as high-concentration, sheetflood deposits
(Blair, 2000; Blair and McPherson, 1994b). Traction, dispersive pressures and buoyancy
probably all contributed in varying degrees to transport the sediment (e.g., DeCelles et
al., 1991; Nemec and Steel, 1984; Pierson, 1980, 1981). Alternatively, A1 may represent
the deposits of shear-modified grain flows ("traction carpet" of Heinz and Aigner, 2003;
Sohn, 1997; Todd, 1989), in which dispersive forces combined with traction to produce
the unstratified, but imbricated deposits. Shear- and buoyancy-modified grain flows
commonly develop beneath high-concentration floods (Todd, 1989).
We interpret lithofacies A2 as debris-flow deposits due to the lack of erosion at
the base and matrix-support, which indicates a high viscosity, laminar flow (Pierson,
1980). Massive, disorganized, clast-supported cobble to boulder conglomerate deposits
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are interpreted as non-cohesive debris flows (Blair and McPherson, 1994a; Nemec and
Steel, 1984).
Given the combination of clast imbrication, abundant channel forms and coarse
grain-size, we interpret lithofacies A3 as stream deposits in a stream-dominated alluvial
fan setting (DeCelles et al., 1991).
Following Nemec and Steel (1984) and on the basis of similar deposits seen on
the margins of Kungyu Co, we interpret lithofacies A4 as high-concentration flood or
debris flow deposits that accumulated in a water saturated environment (Miall, 2000;
Postma, 1983), associated with both supra-littoral (F3) and profundal (P1 and P2)
settings. Winnowed cobble intervals capping some A4 deposits are interpreted as the
result of post-depositional wave reworking during subsequent lacustrine transgression
(Blair, 2000; Pivnik, 1990).
Zhada Formation members
We identified five members of the Zhada Formation based on lithofacies
assemblages (Figures 3.3 and 3.12). The lower conglomerate member (Nlc) consists of
lithofacies associations F1, F2, and rare F3. Where present, it extends from the base of
the section to the top of the last F1 interval >1 m thick. Above this, the lower fluvial
member (Nlf) consists of lithofacies associations F2, F3, S2 and S3 and extends from the
top of the lower conglomerate member to the base of the lacustrine claystone. The
lacustrine member (Nl) includes primarily lithofacies associations P1, P2, L1, L2, S1 and
minor S2, S3 and F2. The lacustrine member extends from the first occurrence of
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lacustrine claystone to the first thick (>1 m) interval displaying soft-sediment
deformation. The upper fluvial member (Nuf) is similar to the lower fluvial member with
the exception that it tends to be coarser and contain more abundant soft-sediment
deformation. This member is composed primarily of sand – boulder-sized material in
contrast to the lower fluvial member which is dominantly silt – sand-sized material. The
upper fluvial member extends from the lacustrine member to the first significant (>10 m
thick) conglomeratic interval. The upper alluvial fan member (Nal) includes lithofaces
associations A1-A4, F3, P1 and P2. It extends from the first significant conglomeratic
interval above the lacustrine member to the top of the section, which is defined by a
regional geomorphic surface that caps the Zhada Formation.
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PALEOCURRENT MEASUREMENTS
Paleocurrent data consist of measurements of 298 limbs of trough cross-strata
(method I of DeCelles et al., 1983) at 29 sites and measurements of 568 long-axistransverse imbricated clasts at 47 sites in measured sections throughout Zhada basin
(Figure 3.6).
Paleocurrent measurements fall into two general groups: those that show a
northwestward paleoflow direction and those directed toward the basin-center. The first
group comes from the lower two members of the Zhada Formation (Nlc and Nlf). The
average paleocurrent azimuth from 8 measured sections in the lower members ranges
between 270° and 349° (Figure 3.13A, Table 3.2). The second group of measurements
come from the two upper members (Nuf and Nal) or anomalously coarse-grained tongues
(cobble – boulder) within lower members. These paleocurrent indicators are less uniform
but generally indicate flow towards the center of the basin in seven of the nine measured
sections (Figure 3.13B, Table 3.2). In the northwest end of the basin there is a trend from
uniformly northwestward directed paleo-flow (Qusum section, Figure 3.13A) to a more
complex, but average northwestward directed paleo-flow (1NWZ and 2NWZ sections)
and finally a complete reversal to an eastward average paleo-flow direction (3NWZ
section, Figure 3.13B).
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PROVENANCE ANALYSIS
Methods
Sandstone petrography and conglomerate clast counts
Modal sandstone composition data come from 35 standard petrographic thin
sections from 9 measured sections across the basin. Thin sections were stained for
potassium and calcium feldspar and point-counted (403 - 744 counts per slide) using a
modified Gazzi-Dickinson method (Ingersoll et al., 1984); the modifications involve the
identification of monocrystalline quartz grains that are part of sedimentary lithic
fragments and counting carbonate grains. The petrographic counting parameters are
shown in Table 3.3, and recalculated modal data are given in Table 3.4.
Conglomerate composition data consist of identification of 3447 clasts from 32
sites from measured sections throughout Zhada basin, and bedloads of several modern
streams and rivers. Conglomerate-clast counts involved identification of clast lithology
of ~ 100 clasts at regular intervals from within individual beds. Bedload counts involved
identification of ~100 gravel-sized or larger clasts that were dredged from the streambed.
U-Pb geochronologic analyses of detrital zircons
U-Pb geochronology was conducted on zircons separated from nine sandstone
samples and one paragneiss sample from the Zhada basin and the footwall of the Qusum
Detachment, respectively, and were processed for detrital zircon analysis using standard
procedures described by Gehrels (2000) and Gehrels et al. (2008). Analysis was
conducted using the laser ablation multicollector inductively coupled plasma mass
spectrometry (LA-MC-ICPMS) at the Arizona LaserChron Center. Approximately 100
114
individual zircon grains were analyzed from each sample (with the exception of samples
2EZ88 and 2EZ60.7A). These were selected randomly from all sizes and shapes,
although grains with obvious cracks or inclusions were avoided. In-run analysis of
fragments of a large zircon crystal (generally every fifth measurement) with known age
of 564 ± 4 Ma (2-sigma error) was used to correct for inter- and intra-element
fractionation. The uncertainty resulting from the calibration correction is generally 1-2%
(2-sigma) for both 206Pb/207Pb and 206Pb/238U ages. Common Pb was corrected by using
the measured 204Pb and assuming an initial Pb composition from Stacey and Kramers
(1975) (with uncertainties of 1.0 for 206Pb/204Pb and 0.3 for 207Pb/204Pb).The analytical
data are reported in Table S5. Details of the operating conditions and analytical
procedures can be found in Gehrels et al., (2008).
For each analysis, the errors in determining 206Pb/238U and 206Pb/204Pb result in a
measurement error of ~1-2% (at 2-sigma level) in the 206Pb/238U age. The errors in
measurement of 206Pb/207Pb and 206Pb/204Pb also result in ~1-2% (at 2-sigma level)
uncertainty in age for grains that are >1.0 Ga, but are substantially larger for younger
grains due to low intensity of the 207Pb signal. For most analyses, the cross-over in
precision of 206Pb/238U and 206Pb/207Pb ages occurs at 0.8-1.0 Ga. Interpreted ages are
based on 206Pb/238U for < ~300 Ma grains and on 206Pb/207Pb for > ~300 Ma grains.
Though 206Pb/238U ages are more precise below 1 Ga, the occurrence of numerous zircon
grains with lead loss due to Cenozoic metamorphism necessitated a younger cutoff.
Analyses with > 20% uncertainty, that are >30% discordant (by comparison of 206Pb/238U
and 206Pb/207Pb ages) or >5% reverse discordant are omitted from interpretation.
115
Results
Sandstone and conglomerate modal compositions
Quartz is present in monocrystalline (Qm), polycrystalline (Qp), and foliated
polycrystalline (Qpt) grains, and also monocrystalline grains within sandstone or
quartzite grains (Qms)(Figure 3.14). Qp commonly occurs within granitoid lithic grains.
Qpt commonly occurs within mylonitic lithic grains. Other lithic grains include phyllite
(Lph), schist (Lsm), carbonate (Lc), chert (C), and mudstone (Lsh). Volcanic grains are
common, particularly at the base of the Zhada Formation. The most abundant are felsic
volcanic grains (Lvf). This group includes porphyritic, highly altered quartzofeldspathic
grains, quartz rich grains with disseminated plagioclase, and volcanic grains that include
exotic lithic fragments. The latter are interpreted as volcaniclastic. Mafic volcanic grains
(Lvm) are likewise highly altered; often serpentenized or chloritized. Lath-work volcanic
grains (Lvl) contain lath-shaped plagioclase crystals and are typically felsic in
composition. Vitric volcanic grains (Lvv) are those with pseudo-isotropic textures.
These commonly resemble chert, but, where stained, have taken a pink plagioclase stain.
In most thin sections plagioclase (P) is slightly more abundant than potassium (K)
feldspar. Both biotite and muscovite are present in most samples with muscovite
typically being more common. Accessory minerals include zircon, tourmaline, garnet,
olivine, serpentine, chlorite, and cordierite. The most abundant cement is carbonate with
secondary occurrences of gypsum cementation.
Sandstones from the lower conglomeratic and fluvial members (Nlc and Nlf) of
116
the Zhada Formation consist of poorly lithified carbonate or gypsum cemented
litharenites. Subrounded to subangular lithic grains, including polycrystalline quartz, are
the dominant constituents with lesser amounts of monocrystalline quartz grains (Figures
3.15B, E, H). The lithic fraction is dominated by felsic and vitric volcanic grains (Lvv
and Lvf) (Figures 3.15C, I). Feldspar comprises 10 – 20 % of the total compositions with
plagioclase being slightly more abundant. Whereas volcanic grains are the most
abundant lithic grains in lower member sandstones, in some samples up to 66% of the
lithic portion is composed of either metamorphic (such as Qpt) or sedimentary (such as
limestone, phyllite or mudstone) lithic grains (Figure 3.15C samples 0SZ1 and 0.2SZ30).
As in the lower members, sandstones from the upper members consist of poorly
lithified carbonate or gypsum cemented litharenites. However, starting in the middle of
the lower fluvial member (Nlf) the composition of sandstones changes abruptly. This is
particularly evident in the lithic fraction, which changes from being dominated by
volcanic grains to sub-equal amounts of sedimentary and metamorphic grains (Figures
3.15C, I) or entirely metamorphic grains (Figure 3.15F). Feldspar constitutes a larger
component of upper-member sandstones (10 – 55% of the total), with the proportion of
feldspar increasing upsection. Plagioclase dominates over alkali feldspars. In contrast to
lower member sandstones, volcanic lithic grains are a minor constituent of upper member
sandstones.
Conglomerate clast count data are consistent with modal sandstone analyses.
Sedimentary and volcanic clasts dominate the lower members of the Zhada Formation
and metamorphic and plutonic clasts are a minor component (see clast count pie charts in
117
Figure 3.6). Upsection, metamorphic clasts become more abundant and volcanic clasts
less so (Fig. 6).
U-Pb geochronologic analyses of detrital zircons
A total of 840 new zircon ages from ten samples are reported here (Figure 3.16).
The preferred ages are shown on relative age-probability diagrams (from Ludwig, 2003).
These diagrams show each age and its uncertainty (for measurement error only) as a
normal distribution, and sum all ages from a sample into a single curve (Figure 3.17).
Sample LC58.15 yielded 102 usable ages (Figures 3.16 and 3.17). Zircons ranged
from euhedral to well rounded. The largest population is between 45 and 50 Ma (peak at
48 Ma). Additional populations occurred at 20 – 25 Ma (peak at 23 Ma), 50 – 65 Ma
(peaks at 56 and 61 Ma), 550 – 650 (peak at 620 Ma), 1100 – 1200 Ma (peak at 960 Ma)
and 2450 – 2500 Ma (peak at 2500 Ma).
Sample 2EZ60.7A yielded only 23 usable ages (Figures 3.16 and 3.17) due to a
paucity of zircons in the sample. Zircons were largely euhedral or angular (broken). The
largest age population occurred between 45 and 55 Ma (peaks at 48 and 54 Ma). The
only other reliable age population is between 1150 and 1200 Ma (peak at 1170 Ma).
However, a broad group of ages cluster at 450 – 700 (peak at 500 Ma) and a few (n<3)
occur at 20 – 25 Ma (peak at 20 Ma).
Sample 2EZ66.5 yielded 92 usable ages (Figures 3.16 and 3.17). Zircons were
sub-angular to well rounded. The largest age population is between 55 and 60 Ma (peak
at 58 Ma). Additional populations occur between 45 and 50 Ma (peak at 48 Ma),
between 85 and 95 Ma (peak at 92 Ma), at 520 Ma, between 900 and 1100 Ma (peak at
118
930 Ma) and between 2500 and 2700 Ma (peak at 2640 Ma). There are also a number of
ages at 15 – 25 Ma (dual peaks at 17 and 22 Ma), 1250 – 1400 Ma and 1750 – 1850 Ma.
Sample 2EZ88 yielded 52 usable ages (Figures 3.16 and 3.17). Zircons from this
sample were euhedral to well rounded. The largest age population is between 45 and 60
Ma (peak at 48 Ma). There is a small cluster (n<3) with peaks at 17 and 21 Ma. Older
populations have peaks at 480 Ma and 940 Ma. Additional scattered ages from 15 – 25
Ma (peaks at 17 and 21 Ma) and 2400 – 2700 Ma have too few zircons to compose
reliable age populations.
Sample 4SZ14 yielded 105 usable ages (Figures 3.16 and 3.17). Zircons ranged
from euhedral to well rounded. The largest and youngest age population has a peak at
480 Ma. Additional populations occur in the range of 850 – 1150 Ma (peak at 910 Ma)
and at 2500 Ma. Scattered ages occur between 1300 and 1700 Ma and again around 2000
Ma. However, too few zircons occur in these last groups to consider them reliable
populations.
Sample 2NWZ2.5 yielded 91 usable ages (Figures 3.16 and 3.17). Zircons ranged
from sub-angular to well-rounded, though most were sub-rounded to rounded. The
largest population was at 30 - 40 Ma (peak at 36 Ma). Secondary populations occurred at
40 - 50 Ma (peaks at 44 and 46 Ma), 500 – 650 Ma (broad peak at 590 Ma), 800 – 900
Ma (peak at 860 Ma) and between 1050 and 1150 Ma (peak at 1090 Ma). Two additional
clusters have peaks centered at 1660 Ma and 1890 Ma but each cluster is composed of <
3 zircon ages.
Sample 2NWZ58.2 yielded 92 usable ages (Figures 3.16 and 3.17). Zircons
119
ranged from euhedral or angular to well rounded, though most were sub-rounded to
rounded. The largest age population was from 30 – 50 Ma (peaks at 35, 37, 43 and 48
Ma). Secondary populations occurred at 650 – 700 Ma (peak at 660 Ma) and 900 – 1050
Ma (peak at 960 Ma). One additional cluster occurs between 2450 and 2600 Ma but is
represented by too few zircons to be a reliable age population.
Sample 3NWZ45.9 yielded 85 usable ages (Figures 3.16 and 3.17). Zircons were
sub-rounded to well rounded. Discrete age populations were difficult to identify due to
lead loss during Cenozoic metamorphism. The largest age population was between 750
and 1150 Ma. Additional populations occurred from 450-500 (peak at 480 Ma), 550-650
Ma (peak at 600 Ma) and 2050-2100 (peak at 2080 Ma). As with 2NWZ58.2 another
cluster occurs between 2400 and 2550 Ma but is has too few zircon ages to be a
considered a population.
Sample 3NWZ77 yielded 92 usable ages (Figures 3.16 and 3.17). Zircons were
sub-rounded to well rounded. The largest age population was between 800 and 1100 Ma
(peaks at 830, 950, 1000 and 1080 Ma). Additional populations occurred at 40 – 50 ma
(peaks at 41 and 45 Ma), 550 – 600 Ma (peak at 570 Ma), 650 – 700 Ma (dual peaks at
660 and 690 Ma), 1150 – 1200 Ma (peak at 1170 Ma) and 2050 – 2100 Ma (peak at 2090
Ma).
Sample SMZ 35 yielded 106 usable ages (Figures 3.16 and 3.17). Zircons were
sub-angular to well rounded. The largest age population was between 900 and 1150 Ma
(peak at 1050 Ma) with secondary populations at 450 – 500 Ma (peak at 480 Ma), 550 –
650 Ma (minor peak at 580 Ma), 1300 – 1350 Ma and 1400 – 1450 Ma (peak at 1430
120
Ma).
Summary of detrital zircon U-Pb analyses
Samples fall broadly into two groups: those with significant detrital-zircon ages >
100 Ma and those with almost exclusively < 100 Ma detrital-zircon ages (Figure 3.17).
Analyzed zircons were euhedral to well rounded. There was no correlation between the
degree of rounding and zircon age. The youngest zircon age population is 23 Ma (n=8,
LwrCgm58.15) though a small cluster (n=2) are as young as 17 Ma (2EZ66.5, Figure
3.17, Table 3.5). The youngest detrital zircon ages are well above the oldest depositional
age. The > 100 Ma age population has prominent peaks at 450-550, 800-1200, and 2500
Ma and minor peaks at 550-650, 1300-1450 and 2100 Ma. The < 100 Ma population has
major peaks at ~35, 45-50, and ~55 Ma and minor peaks at 17-23, 60-65 and 90-100 Ma.
Interpretation
Sandstone and conglomerate provenance
Detrital modes of sandstones show an upsection shift from an arc-orogen source
to a recycled-orogen source. The lowermost samples are from sandstones within the
fluvial Nlc and Nlf members. Their lithic fractions fall largely within the arc-orogensource field in Qp-Lv-Ls ternary diagrams and straddle the border between the arcorogen and recycled-orogen fields in Qm-F-Lt and Qt-F-L diagrams (Figure
3.15)(Dickinson, 1985). Sandstone compositions from the top of the Nlf member are
more scattered but trend towards the quartzose, recycled-orogen fields in all ternary
diagrams (Figure 3.15). Samples from the upper members of the Zhada Formation (Nl,
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Nuf, Nal) continue the trend towards more compositionally mature, recycled-orogen
sources, even extending into the continental block field in Qm-F-Lt and Qt-F-L diagrams
(Figures 3.15G, H). These trends are also present in samples from the northwestern part
of Zhada basin (Figure 3.15E, F). Increasing input of detritus from recycled-orogen
terranes is most evident in the Qp-Lv-Ls diagram for the northwest Zhada section, but
lacks the volcanic rich endmember (Figure 3.15F).
The primary source of volcanic detritus for samples from the fluvial interval of
the lower member of the Zhada Formation is interpreted to be the Mt. Kailas region to the
northeast of Zhada basin (north of the Karakoram fault, Figures 3.1 and 3.13). The
region south of the Zhada basin is dominated by high-grade metamorphic rocks and the
only local sources of volcanic detritus are relatively small zones of ophiolitic rocks in the
Tethyan Himalaya. However, ophiolitic rocks are a minor source given the paucity of
mafic volcanic fragments in the Zhada Formation. In contrast, the Mt. Kailas region is
dominated by both volcanic and plutonic rocks associated with the Cretaceous - Tertiary
Gangdese magmatic arc and younger clastic rocks derived from these igneous rocks
(Aitchison et al., 2002; Murphy and Yin, 2003; Murphy et al., 2002; Yin et al., 1999).
This area is also the headwaters of the modern Sutlej River (Figure 3.13B). Probable
sources of the recycled-orogen detritus which dominates the upper members of the Zhada
Formation include rocks in the Ayi Shan mountains northeast of the basin, the Tethyan
sequence strata which underlie the basin, and the Greater Himalayan and Tethyan
sequence rocks southwest of the basin. Greater Himalayan rocks are also exposed in the
core of the Gurla Mandhata metamorphic core complex to the southeast of Zhada basin
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(Murphy, 2007). The modal petrography points to an upsection shift from a northeasterly
source to source located southwest of the Karakoram fault. The enrichment in recycledorogen detritus is also evident in the northwestern Zhada basin. However, being farther
from the inferred source of volcanic detritus, the initial volcanic fraction is reduced and
so the evolution from an arc-orogen dominated source to a more quartzose source is not
as dramatic as in central Zhada basin.
Data from conglomerate clast counts increase the resolution of the provenance
interpretation. Like the petrographic and detrital zircon data, the clast counts show an
upsection decrease in the abundance of volcanic detritus and an increase in the relative
abundance of high-grade metamorphic and plutonic clasts. Particularly on the southern
margin of the basin, where paleocurrent indicators from the upper members of the Zhada
Formation show northward paleoflow directions, the only source of high-grade
metamorphic and plutonic clasts is the Greater Himalayan sequence to the south of Zhada
basin. However, low-grade metasedimentary clasts dominate the clast counts throughout
the basin, indicating that the local Tethyan basement was a significant source of detritus.
In the northern and northwestern parts of the basin, paleocurrent data indicate that highgrade metamorphic and plutonic rocks were derived from the footwall of the Qusum
detachment. Consistent with greater distance from the source of volcanic rocks (the
Kailas Range), the ratio of volcanic clasts to other clasts is lower in the northwestern
Zhada basin than in the central Zhada basin. The conglomerate clast count data indicate
that the source of sediments in the Zhada basin changed from a distal, northeastern
source, with local input, to a predominantly proximal southwestern source.
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An unexpected result of the conglomerate clast counts is the dominance of lowgrade metamorphic rocks over volcanic rocks in the lower members of the Zhada
Formation. There is an apparent disconnect between the relative abundance of detritus
derived from northeast of Zhada basin indicated by the detrital zircon data and the
conglomerate clast count data. One possible reason for this is downstream grain-size
sorting; this explanation is consistent with a more distal northeastern source and a more
proximal local or southwestern source.
U-Pb geochronologic analyses of detrital zircons
Samples with significant detrital-zircon ages < 100 Ma and those with almost
exclusively > 100 Ma ages (Figure 3.17) come from the bottom and top of the Zhada
Formation, respectively. Possible sources for detrital zircons include the Cretaceous –
Tertiary volcanic and intrusive igneous rocks related to Gangdese (Kailas) arc
magmatism, Paleozoic – Mesozoic metasedimentary rocks of the Tethyan Sedimentary
sequence or late Proterozoic – early Paleozoic rocks of the Greater Himalayan seqeuence.
There is good correlation between most of the < 100 Ma peaks with igneous or
detrital zircon age spectra from regions north of Zhada basin (Mt. Kailas or Ayi Shan
regions, Figure 3.17). Specifically, peaks at 17 – 23 Ma, ~ 55 Ma and 78 – 80 Ma
correlate to peaks in the igneous or detrital zircon age spectra from the Mt. Kailas area
(Figure 3.17 ―Source Regions‖). However, there are no good correlations from this
region for peaks between 35 and 50 Ma. A better match for these peaks can be found in
detrital zircons from the Ayi Shan. Ayi Shan zircons also may contribute to the 78 – 80
Ma peak in the Zhada basin (Figure 3.17 ―Source Regions‖). The only documented
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source for ~ 55 Ma zircons is from the Gangdese arc (Figure 3.17).
Ages of detrital zircons from samples from the top of the Zhada Formation (e.g.,
4SZ14, 3NWZ45.9 and 3NWZ77) are dominantly Paleozoic – Proterozoic and those
zircons cannot have been derived from the Mt. Kailas region. The Neogene Kailas
Formation has a greater proportion of < 100 Ma zircons than do the samples mentioned
above (Figure 3.17, ―Kailas‖). Hence, the Kailas Formation may be a minor source, but
the major source of detritus must lie elsewhere. Likely sources include inherited zircons
from Ayi Shan paragneisses or leucogranites which have Tethyan sequence protoliths, the
Tethyan sequence basement, or Greater Himalayan sequence rocks (DeCelles et al., 2004;
DeCelles et al., 2000; Gehrels et al., 2006a, b; Gehrels et al., 2008; Martin et al., 2005).
Consistent with the petrographic data, the detrital zircon data shows a significant
upsection shift from a source in the Mt. Kailas area to a local basement or
southernwestern source.
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SUBSIDENCE CURVE
A tectonic subsidence curve for the Zhada basin was produced from the South
Zhada section by decompacting the sediments using standard basckstripping techniques
(Figure 3.18A "Decompacted subsidence curve") (Allen and Allen, 1990; Dickinson et
al., 1987; Vanhinte, 1978) and then removing the load of the sediment assuming Airy
isostacy (Figure 3.18A ―Tectonic subsidence curve‖). Age control is based on
magnetostratigraphic tie points (Figure 3.5)(Saylor et al., in press). Bathymetry is not
expected to be a major contributor to the accommodation because the lake was never
more than a few tens of meters deep based on the thickest observed upward-coarsening
parasequence within the Zhada Formation. The essentially linear Zhada basin curve best
matches typical subsidence curves from strike-slip or rift basins (Figure 3.18B).
However, the subsidence rates are very low: 0.09 mm/yr and 0.06 mm/yr for the
decompacted and tectonic subsidence curves, respectively. In addition, compared to
classic examples of rift or strike-slip basins, the Zhada basin has a remarkably thin basin
fill. Closer examination of the tectonic subsidence curve indicates that it can be divided
into three linear segments: 9.23 – 5.23, 5.23 – 2.581, and 2.581 – 0.08 Ma. These
segments have subsidence rates of 0.05, 0.1, and 0.03 mm/yr, respectively. The thin
basin fill and the subsidence rate between 5.23 and 2.581 Ma are characteristic of
supradetachment basins (Friedmann and Burbank, 1995).
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DISCUSSION
Zhada Basin Evolution
Sedimentology and paleocurrent data indicate that a large, northwestwardflowing, low-gradient river deposited the lower members (Nlc, Nlf) of the Zhada
Formation. Whereas this may seem to contradict the argument for a northern source for
the volcanic detritus based on petrography, clast composition, and detrital zircon
analyses, we note that the modern Sutlej River has its headwaters in the Mt. Kailas
region, flows southward into, and then northwestward across the Zhada basin and
currently exits the basin to the southwest (Figure 3.13B). The paleo-Sutlej River also
may have followed a similar path into the Zhada basin and northwestward from there.
Paleocurrent indicators at the base of the Zhada Formation are oriented northwestward
even within 10 km of the modern Qusum range-front, indicating that, at the time that the
sediments were deposited, the Leo Pargil/Qusum Range presented no obstacle to
northwestward flow.
Upsection, paleocurrent indicators, provenance data, and sedimentology indicate
that the Zhada basin became closed and that detritus was derived from local highlands
surrounding the basin. The change from a through-going, northwestward flowing river to
a closed basin demands explanation. Northwestward paleo-flow suggests that the cause
of ponding within Zhada basin is to be found to the northwest. Deformation and growth
structures (Figure 3.19) in the Zhada Formation in the northwest of the basin, near the
Leo Pargil/Qusum Range, indicate that movement on the Qusum detachment and
exhumation of the range were synchronous with deposition of the Zhada Formation. It is
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unlikely that the Zhada basin was entirely hydrologically closed because a large river,
such as the Sutlej, would have quickly filled and overflowed a closed basin. Some
outflow probably continued either via a small outlet in the southwest of the basin or
groundwater flow which would have been missed by paleocurrent data. However,
paleocurrent indicators and provenance data show that major surface flow during the
Pliocene-Pleistocene was basin-centric and stable isotope data indicate that the lake
underwent significant evaporation (Saylor et al., in press).
Some of the complexity in the paleocurrent directions results from diversion
around pre-existing topography and sediments being shed from pre-existing topography.
The presence of Zhada basin sediments in buttress unconformity on top of deformed
Tethyan sequence strata indicates that there was some paleo-topography on the basement
prior to deposition of the Zhada Formation. In measured sections located near Tethyan
basement rocks, alluvial fan facies interfinger with fluvial facies. Paleocurrent indicators
from alluvial fan facies are commonly at high angles to those from axial fluvial facies.
This was observed particularly in the northwestern end of the basin and near the Guga
section and is the cause of the anomalous northward oriented petals on the 2NWZ,
3NWZ and 1NZ rose diagrams and east or southward oriented petals on the Guga rose
diagrams (Figure 3.13). These data add detail to environmental reconstructions, but do
not change the overall basin evolution picture.
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Tectonic Origins of the Zhada Basin
Any model for Zhada basin must explain (1) why there was a transition from nondeposition to deposition at ~ 9.2 Ma, (2) the relationship of the Zhada Formation to preexisting topography in the Tethyan fold-thrust belt, (3) the variety and stacking patterns
of depositional environments in the Zhada Formation, (4) the lack of widespread
deformation in the Zhada Formation, (5) the lack of cyclic incision and infilling or
significant hiatuses in sedimentation in the Zhada Formation, and (6) the transition from
deposition to non-deposition and incision at < 1 Ma.
All evidence indicates that closure of the Zhada basin was the result of uplift of
the Gurla Mandhata and Leo Pargil domes. The only basement-involved deformation
observed in the Zhada Formation is a growth structure at the northwestern end of the
basin, ~ 10 km from the Qusum detachment fault (Figure 3.19). The growth-stratal
geometry is best explained by progressive sedimentation and rotation of basin sediments
in the proximal hanging wall above a blind, upward-propagating normal fault (Gawthorpe
and Hardy, 2002; Gupta et al., 1999; Sharp et al., 2000). The elongate shape of the
Zhada basin and its proximity to the Karakoram dextral fault also points to a pull-apart
origin as a possibility. If the Zhada basin is a transtensional pull-apart basin, the Gurla
Mandhata and Leo Pargil/Qusum detachments act as step-over structures. Thus, the
Zhada basin could be a hybrid supradetachment/strike-slip basin. If this is the case, the
lack of widespread deformation in the Zhada Formation implies strain localization on the
detachment faults. These structures can also accommodate arc-parallel extension and be
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the primary controls on Zhada basin evolution in the absence of a large strike-slip fault
on the southwestern margin of the basin (see Synthesis below).
The onset of sedimentation in Zhada basin at ~ 9.2 Ma is within error of the oldest
AFT ages determined by Thiede et al. (2006) from the west side of the Leo Pargil Dome.
The presence of ~15 Ma 40Ar/39Ar cooling ages, from both the west side of the Leo Pargil
Dome and the east side of the Leo Pargil/Qusum Range, has been used to argue for a
mid-Miocene onset of exhumation (Thiede et al., 2006; Zhang et al., 2000). Those ages,
however, may reflect cooling during slip on the Main Central thrust (MCT) (Robinson et
al., 2006; Robinson et al., 2003). This is borne out by the occurrence of abundant early to
mid-Miocene 40Ar/39Ar cooling ages from the Greater Himalayan and Tethyan sequences
on the south flank of the Himalaya (Metcalfe, 1993; Robinson et al., 2006; Thiede et al.,
2005; Vannay et al., 2004). Thus, the mid-Miocene cooling ages obtained from the Leo
Pargil/Qusum Range may be inherited from tectonic events related to arc-normal
thrusting rather than arc-parallel extension. The synchroneity of movement on the
Karakoram fault in the Zhada region (10 Ma; Murphy et al., 2000; Yin et al., 1999) and
exhumation of Gurla Mandhata (9 Ma; Murphy et al., 2002) with the 9-10 Ma AFT ages
from Leo Pargil and the 9.2 Ma onset of sedimentation in Zhada basin points to the late
Miocene as the time of major structural reorganization in the area. The increase in
tectonic subsidence rate at 5.23 – 2.581 Ma was synchronous with the onset of lacustrine
sedimentation in the Zhada basin and also with an increase in exhumation rate indicated
by AFT ages from the west side of the Leo Pargil dome (Thiede et al., 2006). This
further implicates exhumation of the Qusum/Leo Pargil range as a controlling factor in
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Zhada basin development.
The occurrence of an anomalous valley striking orthogonally across the Leo
Pargil/Qusum Range which may be a windgap (32.33°N, 79°E, Figure 3.20) further
supports this explanation. This is the only significant valley in an otherwise unbroken
mountain range. The valley extends through the entire width of the range and yet is
currently occupied by an underfit stream < 1 m deep and < 5 m wide. This stream
originates at a glacier mid-way through the valley and thus cannot account for a valley
transecting the entire range. The valley is mantled by terrace deposits that have recently
been incised (Figure 3.20B). We tentatively interpret this valley as the course of the
paleo-Sutlej River prior to exhumation of the Leo Pargil/Qusum Range. If this is the
case, the paleo-Sutlej may have been the headwaters of either the Indus or the Chennab
Rivers as suggested by Brookfield (1998).
Several explanations for the existence of the Zhada basin can be ruled out based
on the variety and stacking patterns of lithofacies assemblages in the Zhada Formation.
The basin has none of the lithologic, stratigraphic or structural features of flexural or rift
basins. Whereas the subsidence curve superficially matches that of other rift basins, the
Zhada Formation is much thinner than typical rift basin fill (Friedmann and Burbank,
1995) and mapping of the northeastern and southwestern margins of the basin revealed
no geometric relationships other than onlap of basin sediments onto the basin bounding
highlands. In no place along these margins is the basin fill faulted. Further, the tectonic
subsidence curve does not match the classic upward convex foreland basin subsidence
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curve; this is consistent with the absence of either a flexural load or the asymmetric
cross-sectional shape typical of a flexural basin (Figures 3.12 and 3.18).
The duration of the basin and the lack of incision or hiatuses in sedimentation
indicate that climate change was not the direct cause of sedimentation. It is difficult to
envision a scenario in which climate change, acting alone, would initiate sediment
accumulation and basin development, produce the variety of sedimentary environments
archived in the Zhada Formation, drive continuous sedimentation for ~ 9 Myr, and then
abruptly terminate deposition and return to an erosional, incising state. The basin could
also be the result of damming by landslides, glaciers or tectonic activity. However, the
duration of the basin and the lack of glacial deposits argue against the first two causes,
leaving only structural damming as a viable possibility.
Likewise, the South Tibetan detachment system (STDS) is not the primary
controlling structure for the Zhada basin. The Zhada Formation on the southwestern
margin of the basin is undeformed and onlaps preexisting topography on the
southwestern margin of the basin. There is no evidence of active faulting or basin
deformation where mapped on the southern margin of the basin as far southward as the
trace of the STDS. Where documented elsewhere in the Himalayan orogen, the youngest
slip on the STDS predates the onset of sedimentation in Zhada basin (Cottle et al., 2007;
Hodges, 2000; Hodges et al., 1996; Hodges et al., 1992; Murphy and Yin, 2003; Murphy
and Harrison, 1999; Searle et al., 1997; Searle et al., 2003; Yin, 2006). On the other
hand, kinematic analyses of the Gurla Mandhata and Qusum detachments indicate that
these faults accommodate arc-parallel extension and are actively exhuming mid-crustal
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rocks from beneath the Zhada basin (Murphy et al., 2002; Zhang et al., 2000). Structural
and isotopic analysis also indicates that the Gurla Mandhata detachment cuts the STDS,
indicating that slip on the STDS had ceased prior to 9 Ma (Murphy, 2007; Murphy and
Copeland, 2005; Murphy et al., 2002).
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SYNTHESIS
We divide the evolution of the Zhada basin into 5 stages (Figure 3.21). At ages >
9.2 Ma the Zhada basin was occupied by a large, northwestward flowing, braided river
which had its headwaters in the Mt. Kailas region (Figure 3.21A). Exhumation of the
Leo Pargil/Qusum Range along a system of northeast striking detachment faults
beginning at ~ 9.2 – 10 Ma decreased the fluvial gradient and caused ponding and
upstream sediment accumulation. Before being completely defeated, the paleo-Sutlej
River cut and flowed through the valley identified above as a windgap (Figure 3.21B).
The combination of sill uplift and basin subsidence (owing to simultaneous footwall
uplift and hanging wall subsidence during slip on the Qusum detachment) resulted in
basin closure and lacustrine and alluvial fan sedimentation upstream of the dam between
~ 5.6 Ma and < 1 Ma. The Zhada basin became the sink for detritus derived from local
highlands (Figure 3.21C). This situation continued until headward erosion breached a
new sill at the current outlet in the southwestern corner of the basin at < 1 Ma (Figure
3.21D). Following breach of the sill the modern Sutlej River system began to reequilibrate to the new base level resulting in upstream migration of a nickpoint and
incision of the Zhada Formation (Figure 3.21E).
Arc-parallel extension is clearly accommodated by extension across the Leo
Pargil/Qusum Range and Gurla Mandhata, and dextral slip on the Karakoram fault
(Murphy et al., 2002; Thiede et al., 2006; Zhang et al., 2000). It is unclear which
structures accommodate extension on the southwestern side of Zhada basin. No largescale structure comparable to the Karakoram fault is present on this side of the basin. It
134
is possible that extension is taken up by distributed faulting directly south of the basin
(Figure 3.22). Another possibility is that slip on the Leo Pargil and Qusum detachments
is transferred westward or northwestward, a process similar to that described by Murphy
and Copeland (2005) for the Gurla Mandhata detachment (Figure 3.22).
The arc-parallel extension process provides a mechanism to explain the loss of
elevation suggested by Saylor et al. (in press). They inferred a 0.8 - 1.5 km loss of
elevation based on oxygen isotope paleoaltimetry. Assuming Airy isostacy, that loss of
elevation would require between 4.5 and 8 km of crustal thinning. This could be
achieved by two interacting detachment fault systems below the Zhada basin (Figure
3.22). Crustal thinning and the associated elevation loss also explain the anomalously
low elevations in the Zhada region (Figure 3.2).
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CONCLUSIONS
1) Zhada basin evolved from a northwestward-draining river valley to a broad
depositional plain and finally to a largely closed basin. Closure of the basin resulted in
lacustrine and lake-margin alluvial fan sedimentation until a new sill was breached in the
southwest corner of the basin. The change in sedimentation style was accompanied by a
change in sediment provenance, from a volcanic-rich source to the northeast in the Mt.
Kailas region, to more local metamorphic/clastic sources, mainly in the Tethyan and
Greater Himalayan sequence rocks.
2) Depositional environments included braided fluvial, marshy wetland,
lacustrine, dune-field, sand-flat, lacustrine fan-delta, and alluvial fan systems.
Throughout Zhada basin history the climate was sufficiently arid to produce dune fields,
evaporative marshes, evaporative sand flats, and bedded gypsum. These environmental
conclusions are consistent with high δ 18O values from lacustrine gastropods (Saylor et al.,
in press).
3) Zhada basin is the result of arc-parallel extension which was accommodated on
crustal-scale detachment faults at the northwest and southeast extremities of the basin
(Figure 3.22). Sedimentation in the basin was due to a combination of sill uplift and
tectonic subsidence which dammed a previously northwestward flowing river.
4) Excavation (vertical thinning and exhumation) of the mid-crust by detachment
faulting likely provided the subsidence mechanism and resulted in the loss of elevation
indicated by oxygen isotope studies in the basin and by the current, anomalously low
elevation of the basin (Figure 3.2 and 3.22).
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137
Figure 3.1. A: Elevation, shaded relief and generalized tectonic map of the Himalayan –
Tibetan orogenic system showing the location of the Zhada Basin relative to several other
normal fault bounded basins. Image from the UNAVCO Jules Verne Voyager and the
Generic Mapping Tools (GMT). B: Generalized geologic map of the Zhada region. The
location of measured sections presented in Figure 6 are indicated by solid black lines.
Modified from published mapping by Chen and Xu (1987), Murphy and others (2000;
2002) and unpublished mapping by M. Murphy.
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139
Figure 3.2. Topographic profile along an arc with a pole similar to that of Bendick and
Bilham (2001) but with a shorter radius, such that it passes through the highest peaks of
the Himalaya. The Zhada basin stands out as the largest region of low elevation in the
Himalaya.
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Figure 3.3. Geologic map of the Zhada region showing the traces of measured sections
and the relationship between the Zhada Formation and basin-bounding faults.
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142
Figure 3.4. The Zhada Formation is undeformed and lays in buttress unconformity with
deformed Tethyan sequence strata. (A.) The unconformity relationship in the basin
center. (B.) The unconformity relationship at the basin margins. (C.) Deformation in the
central Zhada basin is limited to small-offset faults. The minor offset, lack of basement
involvement and the fact that deformation is limited to a thin stratigraphic interval
indicates that this deformation is related to syn-depositional slumping rather than
significant tectonic activity. (D.) The geomorphic surface which caps the Zhada
Formation
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144
Figure 3.5. Correlation of the composite magnetostratigraphic section with the GPTS of
Lourens et al. (2004). The composite magnetostratigraphic section was constructed from
two magnetostratigraphic sections, each of which spanned the entire thickness of the
Zhada Formation. The stratigraphic location of the C3 – C4 transition is indicated by the
grey box (between 170 and 250 m in the South Zhada measured section. The location of
the first Hipparion fossils is at ~ 240 m in the South Zhada measured section.
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146
147
148
149
150
Figure 3.6. Measured sections, paleocurrent measurements, and clast compositions from
the Zhada basin. See Figure 3.2 for locations.
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Figure 3.7. (A.) Gravel foresets of lithofacies F1. Foresets are 3-4 m tall. (B.) Marshy
wetland deposits of lithofacies F3. White flecks in the center of the picture are gastropod
shells and shell fragments. (C.) Large-scale trough cross-stratification in F2 sandstones.
(D.) F2 sandstones form laterally continuous, tabular deposits. (E.) Moderately sorted,
imbricated pebble conglomerate of lithofacies F1 featuring scoured and filled channel
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forms. Staff is 1.5 m long. (F.) Massive, organic-rich siltstone of lithofacies F3. (G.) F2
(now contorted) overlying marshy wetland deposits (F3). Staff is ~ 50 cm long.
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Figure 3.8. (A.) Robust gastropod shell distinctive of lithofacies S1. (B.) Tabular deposits
and channel forms in lithofacies S1. (C.) Horizontally stratified and planar, and gently
climbing ripple cross-stratification of lithofacies S1. (D.) Dune foreset deposits of
lithofacies S3. (E.) Mudball from lithofacies S2.
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Figure 3.9. Laminated siltstone of lithofacies L1 featuring fossil grasses and fragmentary
shell material.
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Figure 3.10. (A.) Horizontally laminated claystone of lithofacies P1. (B.) Massive
claystone of lithofacies P1. (C.) Upward-fining lamina of lithofacies P2 that are
interpreted as turbidites.
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157
Figure 3.11. (A.) High-concentration flood deposits or modified grain-flow deposits of
lithofacies A1. (B.) Disorganize, matrix-supported boulder conglomerate of lithofacies
A2 that is interpreted as cohesive debris flow deposits. (C.) Convoluted pebble
conglomerate of lithofacies A4. (D.) Coarse progradational sequences at the top of the
Zhada Formation. Visible in this image is an organic-poor example of lithofacies P1 and
lithofacies A4 and A1. Abbreviations; TS: transgressive surface; P: progradation.
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159
Figure 3.12. Stratigraphic cross-section across the Zhada basin showing the relationship
between Zhada Formation members. Cross-section line is indicated in figure 3. Vertical
exaggeration: 10X
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Figure 3.13. Paleocurrent data for (A) the lower members of the Zhada Formation and
(B) the upper members of the Zhada formation superimposed on topography.
Paleocurrent data show an upsection evolution from uniformly northwestward directed to
basin-centric directed paleocurrent indicators. Numbers in rose diagrams identify which
measured section the data comes from (see Table S2). The outline of Zhada basin
sediments is shown in A. For comparison, the modern drainage network is shown in B.
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The modern Sutlej River rises in the region of Rakkas tal and Manasarovar (near Mt.
Kailas) and exits the Zhada basin near the Leo Pargil Dome.
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163
Figure 3.14. Photomicrographs of Zhada Formation sandstones.
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165
Figure 3.15. QFL diagrams for samples from this study. Samples show an upsection
trend from a arc-orogen dominated source to a recycled-orogen dominated source. The
change in source terrane coincides with the reorientation of paleocurrent indicators (Fig.
13). Fields are from Dickinson (1985).
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167
Figure 3.16. U/Pb concordia plots for detrital zircons samples from this study. Error
ellipses are shown for 1-sigma level of uncertainty. See the text for a full discussion.
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169
Figure 3.17. U/Pb relative age-probability density diagrams for detrital zircons from this
study and possible source terranes for Zhada basin sediment. Samples are plotted
stratigraphically upsection from bottom to top. Samples show an upsection change from
a Kailas or Ayi Shan source to a local (Tethyan sequence) or southerly (Greater
Himalyan sequence) source that is coincident with the reorientation of paleocurrent
indicators (Fig. 13). Bin sizes for histograms associated with relative age-probability
diagrams are 5 Myr for ages between 0 and 100 Ma and 50 Myr for ages between 100 Ma
and 3500 Ma. Source region data from Gehrels et al. (2008).
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Figure 3.18. A. Decompacted and tectonic subsidence curves for the Zhada basin. B.
Tectonic subsidence curve for the Zhada basin compared to tectonic subsidence curves
for a variety of tectonic settings (from Allen and Allen, 1990; Angevine et al., 1990).
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Figure 3.19. Growth structure in Zhada Formation in the proximal hanging wall of the
Qusum detachment fault. This is the only significant deformation observed in the Zhada
basin. Telephone poles are 8 m tall. View is toward the northeast. The normal fault
shown is antithetic to the top-to-the-east master Qusum detachment fault (not visible, to
the west). See text for additional discussion of the growth-strata geometry.
174
175
Figure 3.20. (A.) LandSat image of the northwestern corner of the Zhada basin showing a
windgap, possibly representing the course of the paleo-Sutlej River. Inset ―b‖ is enlarged
in B. (B.) A view to the southeast from within the valley in (A). Note the relict surface
that has been abandoned and incised by the modern river which occupies this valley. The
cliff face indicated by ―a‖ is ~ 20 m tall.
176
177
Figure 3.21. Perspective diagram showing the interpreted basin evolution of the Zhada
basin from a through-going river to a closed-basin lake before final breaching of a new
sill resulting in integration of the modern Sutlej River.
178
179
Figure 3.22. Paleogeographic maps showing the influence of arc-parallel extension on the
Sutlej River and Zhada basin. (A.) Prior to 9.2 Ma the paleo-Sutlej River drained regions
north of the Indus suture and flowed to the northwest. (B.) With the onset of arc-parallel
extension and exhumation of the Gurla Mandhata and Leo Pargil/Qusum core complexes
the Sutlej River begins to pond, eventually forming the large Zhada paleo-lake. Arcparallel extension is accommodated on the southwestern side of the basin either by
distributed shortening, shown schematically at (a) or by transferring extension to the west
or northwest (b). (C.) In the Pleistocene a new sill is breached in the west of the basin,
resulting in the integration of the modern Sutlej River system. (D.) NW-SE cross-section
showing the relationship between extension on the Qusum and Gurla Mandhata
detachment faults and sedimentation in the Zhada and Pulan basins. The cross-section
line is indicated in panel C.
180
Table 3.1. Lithofacies codes and interpretation.
Lithofacies
code
Gcm
Description
Interpretation
Conglomerate, clast-supported,
poorly – moderately sorted, angular –
sub-rounded, unstratified, poorly
organized
Deposition by traction currents
modified by shear forces,
dispersive pressure or bouyancy
Gcmi
Conglomerate, clast-supported,
poorly – moderately sorted, angular –
sub-rounded, unstratified, imbricated
Deposition by traction currents
Gch
Conglomerate, clast-supported,
poorly – moderately sorted, angular –
sub-rounded, horizontally stratified
Deposition by traction currents in
relative unconfined sheets
Gt
Gravel – pebble conglomerate, clastsupported, moderately – well sorted,
rounded - sub-rounded, trough crossstratified
Deposition by traction currents in
sub-aqueous, migrating, threedimensional gravel – pebble
dunes
Gcf
Pebble conglomerate, clastsupported, moderately sorted, subangular – sub-rounded, planar or
epsilon cross-stratified
Deposition by lateral or terminal
accretion on bars
Gmm
Conglomerate, matrix-supported,
poorly sorted, angular – subrounded, disorganized, unstratified
Deposition by cohesive mud- or
sand-matrix debris flows
Gx
Conglomerate, matrix- or clastsupported, poorly sorted, angular sub-rounded, disorganized,
unstratified, with soft-sediment
deformation
Deposition into a water saturated
environment; post-depositional
dewatering due to sediment
compaction or due to bioturbation
St
Medium – very coarse sandstone
with trough cross-stratification
Deposition by migrating threedimensional sand dunes
Sp
Very fine – medium sandstone with
planar cross-stratification
Deposition by migrating twodimensional ripples
Sr
Very fine – medium sandstone with
climbing ripple cross-stratification;
occasionally interbedded with
mudstone
Deposition under waning flow
conditions; interbedding with
mudstone indicates surging and
waning flow conditions
Srw
Very fine – medium sandstone with
symmetrical ripple laminations
Deposition under oscillatory
current conditions
Sh
Very fine – medium sandstone with
Deposition under unidirectional
181
horizontal laminations
upper-flow regime conditions
Sm
Very fine – medium sandstone,
unstratified
Deposition under conditions of
extremely rapid sediment
accumulation; alternatively
bioturbation may have destroyed
sedimentary structures
Sf
Very fine – fine sandstone with large
planar foreset laminations
Deposition on the foresets of
migrating sub-aerial dunes.
Sc
Very fine – very coarse sandstone
with soft-sediment deformation, preexisting sedimentary structures are
usually obliterated
Deposition into a water saturated
environment; post-depositional
dewatering due to sediment
compaction or due to bioturbation
Mr
Siltstone with climbing ripple crossstratification
Deposition under waning flow
conditions
Ml
Claystone – siltstone with horizontal
laminations
Suspension settling in shallow
standing water; laminations are
often the result of layers of plant
material
Mh
Claystone – siltstone with horizontal
stratification
Suspension settling in deep
water
Mm
Claystone – siltstone, unstratified
Post-depositional bioturbation
Mc
Claystone – siltstone with softsediment deformation
Post-depositional dewatering due
to sediment compaction or
bioturbation
182
Table 3.2 Paleocurrent data
Table 2. Paleocurrent data presented in Figure 3.13
Lower Members (Fig. 13 A)
Average flow Angular
Number direction
Deviation
Section Name (Fig. 13) (deg.)
(deg.)
Qusum
1
291
3
1 Northwest
Zhada
2
349
47
2 Northwest
Zhada
3
270
43
3 Northwest
Zhada
4
No data
Namru road
west
5
No data
Namru road
east
6
308
Guga
7
331
28
1 Noth Zhada
8
No data
2 North Zhada
9
No data
3 North Zhada
10
No data
South Zhada
11
321
39
East Zhada
12
317
10
Southeast
Zhada
13
292
13
Upper Members (Fig. 13 B)
Qusum
1
No data
1 Northwest
Zhada
2
No data
2 Northwest
Zhada
3
No data
3 Northwest
Zhada
4
92
40
Namru road
west
5
241
42
Namru road
east
6
No data
Guga
7
268
37
1 Noth Zhada
8
8
51
2 North Zhada
9
234
27
3 North Zhada
10
22
14
South Zhada
11
38
20
East Zhada
12
188
Southeast
Zhada
13
29
2
2
Largest
frequency
(%)
100
47
4
25
36
3
67
-
-
-
-
-
-
11
98
1
7
100
29
87
61
7
3
29
67
53
3
33
-
-
-
-
-
-
-
-
-
30
3
33
42
4
25
55
30
30
20
98
15
4
3
3
2
6
1
25
33
33
50
50
100
37
3
100
N
(measurements)
23
N
(sites)
183
Table 3.3. Pointcount parameters.
Table 3. Modal petrographic point-count parameters
Symbol Description
Qm
Monocrystaline quartz
Qp
Polycrystaline quartz
Qpt
Foliated polycrystalline quartz
Qms
Monocrystaline quartz grain within
sandstone or quartzite lithic grain
C
Chert
S
Siltstone
Qt
Total Quartz (Qm+Qp+Qpt+Qms)
K
Potassium feldspar
P
Plagioclase feldspar
F
Total feldspar (P+K)
Lvm
Mafic volcanic grains
Lvf
Felsic volcanic grains
Lvv
Vitric volcanic grains
Lvl
Lathwork volcanic grains
Lv
Total volcanic
grains(Lvm+Lvf+Lvv+Lvx+Lvl)
Lsh
Mudstone/shale
Lph
Phyllite
Lsm
Schist
Lc
Carbonate
Ls
Total sedimentary/low grade
metasedimentary lithic grains
(Lsh+Lc+C+S+Qms+Lph)
Lt
Total lithic grains (Lv+Ls+Qp+Qpt)
L
Total non-quartzose lithic grains
(Lv+Ls+Lc)
Qx
Total polycrystalline quartz (Qp+Qpt)
Accessory minerals include zircon, muscovite,
biotite, garnet, serpentine, chlorite, tourmaline,
cordierite, and olivine.
184
Table 3.4 Recalculated petrographic pointcount data
SAMPLE
1EZ3.25
1EZ29
2EZ26A
2EZ66.5
2EZ117.
3EZ1.5
4
3EZ152
3EZ180.
4EZ24
8
5EZ47.5
1NZ3
1NZ16
1NZ35.5
2NZ29.5
3NZ20
1LC1
1LC24
1LC58.1
0SZ1
5
0.1SZ13.
0.1SZ28
6
0.2SZ30
0.3SZ52.
2SZ42.5
8
3SZ36.2
4SZ14
4SZ27
5SZ8.4
1NWZ37
1NWZ96
.4
2NWZ2.
2NWZ58
5
3NWZ45
.2
3NWZ77
.9
%Qt
50.9
55.3
8
46.1
6
43.3
7
69.5
9
50.8
6
69.0
8
58.7
3
66.9
3
73.1
2
68.9
4
53.1
7
42.3
0
53.6
5
62.0
5
34.6
9
37.5
1
38.5
0
57.8
0
63.0
7
56.8
4
65.2
1
56.8
4
55.3
8
56.9
5
54.6
8
62.4
3
63.7
5
52.1
3
47.5
9
47.4
0
68.0
5
54.5
9
64.8
7
9
%F
11.4
11.7
2
16.5
3
13.2
1
13.1
3
12.7
1
12.0
7
9.02
6
12.6
10.6
2
22.9
4
37.7
1
55.1
9
37.6
0
24.5
9
17.9
6
13.4
0
19.5
6
11.9
9
12.0
9
15.1
6
9.32
7
15.3
17.5
5
17.4
3
17.5
0
15.8
9
13.4
2
8.17
5
6.25
43.1
13.0
4
2.79
7
17.2
4
Table 4. Recalculated modal petrographic data
%L
%Q
%F
%Lt
%Q %Lv %Ls
37.6 33.4
11.4 55.1 18.3
29.8 51.8
m
p
32.9
0 41.8
6 11.7
2 46.4
2 24.4
5 37.2
6 38.3
0
37.3
1 31.5
4 16.5
3 51.9
3 18.5
4 28.2
2 53.2
3
43.3
2 32.6
8 13.2
1 54.0
1 17.2
2 33.0
4 49.6
4
17.3
9 48.9
8 13.1
3 37.9
9 51.5
7 7.55
9 40.8
4
36.3
3 32.6
5 12.7
1 54.6
4 28.3
7 27.6 44.0
8
18.9
5 55.5
1 12.0
7 32.3
2 37.9
6 4.38
4 57.6
0
32.2
1 40.1
6 9.02
6 50.8
9 31.6
6 8.40 59.9
6
20.4
5 47.4
2 12.6 39.9
6 39.3
8 3.79 56.8
2
16.2
6 56.1
2 10.6
2 33.2
6 52.8
4 1.60 45.6
7
8.13
2 52.2
2 22.9
4 24.8
4 63.0
0 6.00 31.0
0
9.11 45.5
2 37.7
1 16.6
8 43.0
0 5.81 51.1
0
2.55 35.4
4 55.1
9 9.44
7 64.8
2 8.11 27.0
6
8.65 46.7
6 37.6
0 15.5 46.9
6 4.94 48.1
3
13.3 40.4
3 24.5
9 35.0
8 57.8
1 2.81 39.3
5
47.4
5 28.4
3 17.9
6 53.7
1 13.3
7 52.3 34.3
3
49.0
9 30.5
0 13.4
0 56.0
0 13.7
9 47.1
0 39.0
1
41.9
4 26.7
3 19.5
6 53.6
1 23.0
9 31.5
3 45.4
8
30.1
1 30.7
5 11.9
9 57.3
6 45.6
3 38.5
5 15.7
3
24.9
5 39.5
1 12.0
9 48.4
0 61.8
4 3.72
9 34.4
7
28.0
0 30.5
3 15.1
6 54.2
2 66.9
6 7.55 25.4
2
25.4
2 50.6
9 9.32
7 40.0
4 23.0
8 18.0 59.0
7
27.7
4 23.4
8 15.3 61.1
0 60.9
0 1.12
0 37.9
0
27.1
7 35.7
8 17.5
5 46.6
7 57.9
7 3.10 38.9
2
25.6
2 44.1
9 17.4
3 38.4
8 41.5
6 14.3 44.1
4
27.7
2 42.1
7 17.5
0 40.2
3 44.2
4 6.25
6 49.5
0
21.7
8 39.6
3 15.8
9 44.5
8 49.0
3 2.17 48.7
2
22.8
3 45.2
1 13.4
2 41.3
7 51.6
7 0.72 47.6
6
39.6
3 25.7
4 8.17
5 66.1
2 56.9
1 20.1 22.9
7
46.2
4 23.1
0 6.25 70.6
4 38.8
7 8.43
2 52.7
1
9.41
5 34.7
3 43.1 22.1
3 61.9
6 15.2 22.8
1
18.8 37.4
1 13.0
4 49.5
6 69.5
0 12.3
4 18.1
6
42.6
4 14.2
4 2.79
7 82.9
0 86.9
2 0.00
0 13.0
8
17.8
4 26.3
1 17.2 56.4
9 96.6
2 0.00 3.39
8
7
3
4
3
1
%Q
74.5
m
78.1
6
65.6
0
71.1
7
78.8
9
71.8
7
82.1
6
81.6
7
78.9
4
84.0
8
69.5
6
54.6
1
39.1
5
55.3
5
62.2
5
61.3
0
69.4
4
57.7
0
71.9
2
76.6
3
66.8
3
84.4
5
60.4
7
67.1
7
71.7
3
70.5
4
71.4
4
77.0
6
75.8
9
78.7
8
44.5
2
74.1
8
83.5
3
60.4
8
3
%P
17.9
10.4
8
21.3
8
14.8
9
11.7
3
15.5
0
12.5
8
9.77
9
7.64
13.9
26.2
4
24.4
3
36.6
2
36.2
2
22.9
2
12.3
2
19.1
7
23.1
3
22.3
6
10.3
7
18.5
4
6.80
4
32.5
17.3
6
13.3
0
29.0
5
14.6
7
10.7
4
16.4
4
14.1
7
26.7
8
11.9
0
14.9
4
26.6
3
2
%K
7.46
11.4
12.9
3
13.9
4
9.43
8
12.5
5.24
5
8.59
13.3
1.99
8
4.26
20.9
24.2
3
8.43
3
14.8
26.2
8
11.4
9
19.1
8
5.70
2
13.0
14.6
3
8.74
1
6.98
15.5
14.9
7
0.39
1
13.9
12.1
0
7.65
7
7.09
28.7
13.9
2
1.49
3
12.9
5
185
Table 3.5. Detrital zircon U-Pb data table.
Table S5. U-Pb (zircon) geochronologic analyses by Laser-Ablation
M u l t i c o l l e c t o r I C P M a s s Isotopic
S p e c ratios
trometery
Apparent ages (Ma)
Analysis
U
206Pb U/Th 207Pb* ±
206Pb* ±
error206Pb*±
207Pb*±
206Pb* ±
Best
(ppm) 204Pb
age
235U
(%) 238U (%) corr.238U (Ma) 235U (Ma) 207Pb* (Ma) (Ma)
±
(Ma)
2EZ881-1
2EZ881-2
2EZ881-3
2EZ881-4
2EZ881-5
2EZ881-6
2EZ881-7
2EZ881-8
2EZ881-9
2EZ881-10
2EZ881-12
2EZ881-13
2EZ881-14
2ez882-1
2ez882-2
2ez882-3
2ez882-4
2ez882-5
2ez882-6
2ez882-7
2ez882-8
2ez882-9
2ez882-10
2ez882-11
2ez882-12
2ez882-13
2ez882-14
2ez882-15
2ez882-16
2ez882-17
2ez882-18
2ez882-19
2ez882-20
2ez882-21
2ez882-22
2ez882-23
2ez882-25
2ez882-26
2ez882-27
2ez882-28
2ez882-30
2ez882-29
2ez882-31
330
377
198
416
1020
531
255
1927
868
610
759
195
649
513
165
1183
464
582
175
355
653
2062
370
979
401
347
573
289
396
82
394
266
248
301
119
243
381
623
779
148
306
602
223
17.2
25.3
9.2
47.9
31.1
26.3
41.4
31.0
214.
7
0.9
17766 1.6
12680 1.5
3744 1.4
2156 5.0
54545 1.5
58630 1.8
43013 1.5
280928 8.2
7178 2.0
1268 1.5
2124 1.2
1916 0.8
68623 2.7
13539 1.2
1196 0.9
81711 1.2
2320 0.9
2085 0.7
9882 0.6
14960 1.6
4411 0.8
2843 1.8
51539 1.6
5193 0.8
12300 0.9
24726 3.7
39281 1.6
1882 0.8
2067 0.6
33432 1.1
4159 1.2
2435 0.8
2073 0.6
2410 0.7
7562 0.4
94967 1.3
44590 1.4
5642 0.8
3608 0.5
4555 0.8
31046 1.6
82602 20.8
26955 1.0
0.61063 4.7 0.077243.7 0.79 479.6 17.2 484.0 18.2
1.26719 6.3 0.133976.2 0.98 810.5 47.0 831.1 35.7
0.66263 10.9 0.076862.0 0.18 477.4 9.2 516.2 44.2
0.76704 15.8 0.0798415.6 0.99 495.2 74.3 578.1 69.6
0.60280 6.8 0.079306.6 0.97 491.9 31.1 479.0 25.9
2.78532 6.7 0.221296.5 0.98 1288.776.1 1351.6 49.8
4.16810 3.7 0.286973.0 0.80 1626.442.9 1667.8 30.6
8.07293 4.1 0.380713.7 0.90 2079.665.6 2239.2 37.2
0.81214 13.7 0.076028.4 0.62 472.3 38.4 603.7 62.2
0.02241 12.5 0.003214.5 0.36 20.7 0.9 22.5 2.8
0.02298 7.2 0.003293.2 0.45 21.2 0.7 23.1 1.6
0.05223 11.3 0.007403.8 0.34 47.5 1.8 51.7 5.7
1.12754 3.7 0.120633.4 0.93 734.2 23.8 766.6 19.8
1.27879 16.2 0.1290916.0 0.99 782.7 118.1 836.3 92.4
0.07837 32.4 0.009968.6 0.27 63.9 5.5 76.6 23.9
10.145553.4 0.437602.8 0.82 2339.954.5 2448.1 31.4
0.05665 9.7 0.007714.8 0.49 49.5 2.4 56.0 5.3
0.05781 13.9 0.007745.7 0.41 49.7 2.8 57.1 7.7
1.67426 4.2 0.168232.7 0.64 1002.324.9 998.8 26.6
0.51027 3.1 0.067372.6 0.84 420.3 10.6 418.6 10.7
0.05628 7.6 0.007483.2 0.41 48.1 1.5 55.6 4.1
0.01790 8.3 0.002664.7 0.56 17.1 0.8 18.0 1.5
1.51792 2.4 0.156941.3 0.57 939.7 11.7 937.6 14.4
0.05925 5.6 0.008922.3 0.42 57.3 1.3 58.5 3.2
0.60937 4.6 0.077282.0 0.44 479.8 9.3 483.2 17.8
1.54427 3.6 0.154922.5 0.69 928.5 21.6 948.2 22.3
7.14936 5.7 0.375832.9 0.51 2056.851.5 2130.2 50.8
0.05348 13.2 0.007405.7 0.44 47.6 2.7 52.9 6.8
0.05839 8.9 0.007463.3 0.37 47.9 1.6 57.6 5.0
12.695811.7 0.495961.1 0.67 2596.424.1 2657.3 15.8
0.06370 11.1 0.008755.3 0.48 56.2 3.0 62.7 6.7
0.05197 15.2 0.007485.2 0.34 48.0 2.5 51.4 7.6
0.10033 19.1 0.011554.6 0.24 74.1 3.4 97.1 17.7
0.04999 14.2 0.007504.7 0.33 48.2 2.2 49.5 6.9
0.68718 4.4 0.081052.1 0.49 502.4 10.4 531.1 18.2
10.815582.3 0.465311.2 0.51 2463.024.0 2507.4 21.5
1.69005 3.0 0.166641.6 0.53 993.5 14.9 1004.8 19.4
0.05053 7.9 0.007313.3 0.42 47.0 1.6 50.1 3.9
0.05301 8.2 0.007516.4 0.78 48.2 3.1 52.4 4.2
1.67951 6.1 0.155284.8 0.79 930.5 41.9 1000.8 39.1
2.19280 8.2 0.199707.9 0.97 1173.785.1 1178.8 57.3
2.60620 3.7 0.216613.2 0.87 1263.937.2 1302.4 27.5
1.63548 4.3 0.164073.9 0.91 979.3 35.3 984.0 26.9
504.4 64.3
886.7 25.3
692.1 229.7
918.8 47.9
417.5 37.8
1452.6 26.3
1720.2 41.4
2388.5 31.0
1133.7 214.7
220.7 270.0
224.6 148.9
249.5 245.4
862.1 27.4
981.6 44.7
495.1 703.7
2539.3 32.7
340.3 191.5
377.5 287.2
991.1 65.2
409.6 37.9
393.8 155.9
135.4 162.4
932.7 39.6
107.7 120.9
498.9 92.0
994.2 53.2
2201.8 85.0
302.0 270.9
482.0 183.7
2704.1 20.5
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116.6 317.7
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2EZ665-28
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2EZ665-58
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2EZ665-61
2EZ665-62
2EZ665-63
2EZ665-64
2EZ665-65
2EZ665-66
2EZ665-67
2EZ665-68
2EZ665-69
2EZ665-70
2EZ665-71
2EZ665-72
2EZ665-73
2EZ665-74
2EZ665-75
2EZ665-76
2EZ665-77
2EZ665-78
2EZ665-79
2EZ665-80
2EZ665-81
265
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188
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2EZ665-85
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2EZ665-94
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2EZ665-98
2EZ665-99
2EZ665-100
2EZ665-101
2EZ665-103
2EZ665-104
2EZ665-102
4SZ14-1
4SZ14-2
4SZ14-3
4SZ14-4
4SZ14-5
4SZ14-6
4SZ14-7
4SZ14-8
4SZ14-9
4SZ14-10
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4SZ14-29
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552
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LWRCGM58154
LWRCGM58155
LWRCGM58156
LWRCGM58157
LWRCGM58158
LWRCGM58159
LWRCGM581511
LWRCGM581514
LWRCGM581515
LWRCGM581516
LWRCGM581517
LWRCGM581518
LWRCGM581520
LWRCGM581521
LWRCGM581522
LWRCGM581523
LWRCGM581524
LWRCGM581525
LWRCGM5815S6
LWRCGM581527
LWRCGM581528
LWRCGM581529
LWRCGM581530
LWRCGM581531
LWRCGM581532
LWRCGM581533
LWRCGM581534
LWRCGM581535
LWRCGM581536
160
653
229
170
400
272
339
350
1451
60
2301
401
412
114
173
384
546
425
432
315
595
311
434
288
1006
945
813
1098
388
186
503
559
657
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191
257
654
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524
229
101
597
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4050
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11228
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1.4
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13.6532
1.5969
0.5843
0.5837
1.5473
0.3523
10.1380
0.6966
0.6369
1.3405
0.9779
2.6139
0.6196
1.5472
0.6185
0.5921
1.55812
0.14765
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0.02365
1.69715
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13.9898
2
0.78826
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0.04806
0.05725
0.04839
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8.14814
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2.56256
0.08992
0.80448
4.43353
3.63601
1.12678
2.66482
0.01935
0.10844
1.60652
2.16061
0.04066
0.05047
0.02500
0.57435
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9
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0.0752
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0.0750
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0.0735
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0.0465
0.4673
0.0824
0.0802
0.1390
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0.2189
0.0778
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3
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3
0.4660
3
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6
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1
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191
LWRCGM581537
LWRCGM581538
LWRCGM581540
LWRCGM581539
LWRCGM581541
LWRCGM581542
LWRCGM581543
LWRCGM581544
LWRCGM581545
LWRCGM581546
LWRCGM581548
LWRCGM581549
LWRCGM581550
LWRCGM581551
LWRCGM581553
LWRCGM581554
LWRCGM581555
LWRCGM581556
LWRCGM581557
LWRCGM581558
LWRCGM581559
LWRCGM581560
LWRCGM581561
LWRCGM581562
LWRCGM581563
LWRCGM581564
LWRCGM581566
LWRCGM581570
LWRCGM581572
LWRCGM581574
LWRCGM581575
LWRCGM581576
LWRCGM581577
LWRCGM581578
LWRCGM581579
LWRCGM581580
LWRCGM581581
LWRCGM581582
LWRCGM581583
LWRCGM581585
LWRCGM581586
LWRCGM581587
LWRCGM581588
LWRCGM581589
LWRCGM581592
LWRCGM581593
LWRCGM581594
LWRCGM581595
LWRCGM581597
LWRCGM581598
LWRCGM581599
LWRCGM5815100
LWRCGM5815101
584
788
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79
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312
1322
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601
632
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LWRCGM5815102
LWRCGM5815103
LWRCGM5815104
LWRCGM5815105
LWRCGM5815106
LWRCGM5815108
LWRCGM5815109
LWRCGM5815110
LWRCGM5815111
LWRCGM5815112
LWRCGM5815113
LWRCGM5815114
LWRCGM5815115
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2NWZ25-42
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2NWZ25-76
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43. 0.0052 8.4
3
0.9
8
0.9
8
0.7
8
0.8
9
0.7
9
0.7
7
0.9
7
0.8
7
0.3
0
0.2
4
0.5
6
0.8
0
0.9
8
0.1
8
0.4
9
0.6
8
0.6
7
0.9
4
0.8
3
0.6
1
0.4
2
0.7
0
0.8
1
0.9
1
0.8
0
0.6
8
0.7
6
0.9
6
0.7
1
0.7
0
0.2
0
0.7
7
0.4
4
0.7
1
0.7
6
0.7
0
0.7
2
0.4
7
0.6
3
0.2
6
0.4
0
0.7
2
0.3
9
0.9
1
0.5
1
0.2
4
0.1
4
0.6
6
0.2
8
0.4
0
0.3
8
0.9
3
0.1
9
424.7
820.5
35.5
46.4
668.9
459.1
693.1
46.6
43.7
34.1
1102.
7
1680.
8
1090.
4
35.2
34.9
922.7
1591.
1
702.8
37.1
37.0
23.8
916.3
445.0
716.5
38.9
35.9
1089.
3
228.1
38.0
818.0
34.1
1125.
5
35.8
1235.
4
36.7
438.7
36.2
33.7
33.5
35.2
35.1
748.8
39.1
1540.
3
43.8
41.3
34.9
1075.
7
39.7
36.1
41.1
650.1
33.2
43.6
42.5
0.8
1.2
20.3
11.1
39.4
3.7
0.6
0.7
14.2
53.3
51.2
1.2
1.4
13.9
14.1
33.1
0.8
0.7
0.7
8.5
11.8
17.5
1.2
1.0
13.1
12.1
1.6
12.4
1.0
19.1
0.5
11.4
0.9
9.4
0.8
0.3
1.0
1.0
2.5
10.2
0.4
34.5
0.9
0.4
0.4
16.6
0.5
1.2
0.7
18.2
2.8
452.2
895.6
35.0
45.4
662.3
474.3
740.6
48.0
45.1
34.0
1105.
2
1806.
3
1095.
8
32.2
34.7
924.3
1619.
9
733.3
40.0
37.5
27.6
913.9
471.4
747.9
39.0
38.6
1090.
0
245.8
40.5
838.6
32.2
1147.
0
37.3
1254.
5
38.8
457.4
35.0
33.1
36.6
24.7
42.5
772.7
37.9
1592.
0
45.0
42.5
36.8
1089.
9
41.0
38.5
42.7
646.5
49.0
39.5
33.6
0.9
1.3
19.6
12.3
32.6
4.3
1.9
3.0
16.9
38.1
34.9
5.8
2.7
14.5
12.1
27.6
1.0
1.2
1.9
8.7
12.8
15.1
1.5
1.5
11.5
12.3
2.4
13.2
4.6
16.5
1.1
10.3
1.2
11.7
1.1
0.7
1.7
2.7
7.4
10.8
0.9
22.1
1.7
1.7
3.0
16.9
1.7
3.0
1.9
15.2
20.7
594.8 47.7 594.8 47.7
1086. 24.0 1086.0 24.0
0
-2.1
41.5 35.5
0.8
-8.1
30.9 46.4
1.2
639.9 53.0 639.9 53.0
548.5 44.7 548.5 44.7
886.8 32.3 886.8 32.3
116.7 105. 46.6
3.7
120.3 3
96.2 43.7
0.6
25.8 208. 34.1
0.7
6
1110. 41.4
1110.2 41.4
2
1954.
48.6 1954.3 48.6
3
1106. 20.1 1106.6 20.1
6
-186.8 448. 35.2
1.2
16.5 6
165. 34.9
1.4
928.2 7
36.2 928.2 36.2
1657. 20.6 1657.7 20.6
7
827.6
37.1 827.6 37.1
216.2 33.0 37.1
0.8
71.6 62.1 37.0
0.7
371.6 139. 23.8
0.7
908.0 9
21.3 908.0 21.3
602.0 43.6 602.0 43.6
842.8 25.0 842.8 25.0
47.0 56.2 38.9
1.2
209.3 67.3 35.9
1.0
1091. 22.3 1091.2 22.3
2
418.3
37.4 418.3 37.4
192.4 98.9 38.0
1.6
893.6 34.3 893.6 34.3
-112.1 350. 34.1
1.0
1187. 6
30.4 1187.7 30.4
7
138.0 64.2 35.8
0.5
1287. 19.6 1287.5 19.6
5
171.7
48.5 36.7
0.9
552.6 50.0 552.6 50.0
-44.0 51.9 36.2
0.8
-9.1
45.1 33.7
0.3
249.9 85.3 33.5
1.0
-910.8 310. 35.2
1.0
479.5 5
362. 35.1
2.5
3
842.5 28.7
842.5 28.7
-41.9 57.0 39.1
0.4
1661. 20.6 1661.1 20.6
1
112.4
80.4 43.8
0.9
108.6 93.6 41.3
0.4
163.6 190. 34.9
0.4
1118. 4
37.7 1075.7 16.6
3
117.5 94.5 39.7
0.5
192.2 172. 36.1
1.2
1
133.7 98.2
41.1
0.7
634.2 25.9 650.1 18.2
909.9 916. 33.2
2.8
7
194
2NWZ582-11
2NWZ582-13
2NWZ582-14
2NWZ582-15
2NWZ582-16
2NWZ582-17
2NWZ582-18
2NWZ582-19
2NWZ582-20
2NWZ582-21
2NWZ582-22
2NWZ582-23
2NWZ582-25
2NWZ582-26
2NWZ582-24
2NWZ582-27
2NWZ582-28
2NWZ582-29
2NWZ582-30
2NWZ582-31
2NWZ582-32
2NWZ582-34
2NWZ582-35
2NWZ582-37
2NWZ582-38
2NWZ582-39
2NWZ582-40
2NWZ582-41
2NWZ582-42
2NWZ582-43
2NWZ582-44
2NWZ582-45
2NWZ582-46
2NWZ582-47
2NWZ582-48
2NWZ582-50
2NWZ582-51
2NWZ582-52
2NWZ582-53
2NWZ582-54
2NWZ582-55
2NWZ582-56
2NWZ582-57
2NWZ582-58
2NWZ582-59
2NWZ582-60
2NWZ582-61
2NWZ582-62
2NWZ582-63
2NWZ582-64
2NWZ582-65
2NWZ582-67
2NWZ582-68
888
330
386
499
646
529
1265
768
185
294
638
234
1820
613
2441
387
47
1105
970
107
311
855
709
194
2447
81
207
3902
1425
1590
1167
77
707
523
728
2135
1762
3403
133
175
3554
252
858
941
6297
49
676
464
1071
534
241
1999
1496
16101
337
465
568
11402
740
1175
794
125
321
647
3849
1981
744
2764
278
1814
41778
1127
1911
405
1010
239
13695
1871
2060
265
2608
1427
1468
1662
2116
457
14370
17108
1563
2307
3910
2846
2436
5102
5992
1190
888
2785
1400
38116
510
1193
9373
10946
2819
1993
2.5
1.8
1.3
1.7
4.8
1.1
1.2
1.1
0.6
0.7
0.4
2.1
0.8
1.3
1.4
0.6
0.7
1.4
1.6
1.8
1.2
1.3
0.4
2.5
1.1
1.1
0.6
1.3
0.9
1.4
0.5
0.7
0.6
1.8
1.0
0.7
1.7
3.7
3.8
3.6
3.1
0.5
1.1
1.2
0.5
1.6
5.1
0.9
1.6
2.6
1.7
4.5
1.1
4.3172
0.0381
0.0513
0.0511
0.7242
0.0477
0.0366
0.0514
0.0430
0.0410
0.0378
1.6578
0.0386
0.0420
0.0364
0.0360
2.0935
11.6325
0.0448
0.9252
0.0449
0.0453
0.0467
11.0716
0.0406
1.6034
0.0496
0.0331
0.0439
0.0479
0.0476
1.1333
0.0481
1.5714
1.7508
0.0414
0.0435
0.0444
0.9333
1.6088
0.0450
1.3078
0.0397
0.0372
0.0429
1.7530
8.0481
0.0496
0.0388
1.5305
3.0448
0.0449
0.0360
2.5 0.3031 1.0 0.4
0
13. 0.0053 5.3 0.4
1
8.8
0.0075 4.2 0
0.4
8
7.2 0.0075 5.4 0.7
2.8 0.0901 2.6 5
0.9
8.1 0.0069 1.0 2
0.1
11. 0.0051 4.7 3
0.4
3
4.3 0.0076 1.0 2
0.2
23. 0.0054 2.7 3
0.1
6
1
23.
0.0052 9.7 0.4
9
0
4.2
0.0058 1.7 0.4
1
3.0 0.1556 2.6 0.8
4.9 0.0058 2.4 7
0.5
10. 0.0061 5.4 0
0.5
7
5.1 0.0055 2.2 0
0.4
11. 0.0051 3.7 2
0.3
3
2.0 0.1930 1.2 2
0.6
0
2.2 0.4813 1.8 0.8
2
6.3 0.0066 2.3 0.3
6
2.4 0.1076 1.0 0.4
1
12. 0.0063 1.5 0.1
8
6.0 0.0067 1.1 2
0.1
28. 0.0057 1.9 8
0.0
7
1.5 0.4896 1.0 6
0.6
10. 0.0058 3.7 8
0.3
1
6
2.4
0.1608 1.4 0.5
9
15. 0.0068 2.4 0.1
1
6
8.3
0.0048 5.1 0.6
5.1 0.0066 2.0 2
0.3
12. 0.0067 1.9 8
0.1
6
3.5 0.0073 2.5 5
0.7
3.2 0.1256 2.1 1
0.6
27. 0.0058 3.1 5
0.1
4
1
2.3
0.1610 2.1 0.9
0
5.3 0.1727 3.9 0.7
3
3.0 0.0064 1.0 0.3
3.9 0.0066 1.2 3
0.3
2.8 0.0067 1.0 1
0.3
3.5 0.1083 2.2 6
0.6
4.2 0.1592 3.0 3
0.7
3.5 0.0068 1.0 1
0.2
9
1.9 0.1423 1.5 0.7
9
4.7 0.0059 1.4 0.3
0
10. 0.0053 2.9 0.2
4
8
8.8
0.0060 4.2 0.4
3.7 0.1700 1.5 7
0.4
1.7 0.3984 1.1 1
0.6
29. 0.0059 1.7 3
0.0
5
4.3 0.0059 2.5 6
0.5
8
4.1 0.1562 2.1 0.5
0
2.4 0.2485 2.0 0.8
1
3.9 0.0069 3.0 0.7
6
4.6 0.0055 1.0 0.2
2
1706. 15.0 1696.
9
34.3
1.8 7
38.0
48.2 2.0 50.8
48.4 2.6 50.6
555.9 13.6 553.1
44.3 0.5 47.4
33.0 1.6 36.5
48.9 0.5 50.9
34.8 0.9 42.7
33.2 3.2 40.8
37.3 0.6 37.7
932.4 22.5 992.5
37.1 0.9 38.5
39.1 2.1 41.8
35.3 0.8 36.3
32.8 1.2 35.9
1137. 12.7 1146.
4
7
2532.
37.3 2575.
8
3
42.5
1.0 44.5
658.9
40.7
42.8
36.7
2569.
1
37.4
961.1
43.8
31.0
42.2
43.1
46.6
762.5
37.5
962.6
1027.
1
40.8
42.6
43.0
662.8
952.5
43.8
857.7
38.2
34.1
38.7
1012.
2
2161.
7
37.9
37.9
935.4
1430.
6
44.6
35.2
6.3
0.6
0.5
0.7
21.2
1.4
12.5
1.1
1.6
0.8
0.8
1.2
14.9
1.1
18.5
36.9
0.4
0.5
0.4
13.9
26.3
0.4
12.1
0.5
1.0
1.6
14.2
20.0
0.6
0.9
17.9
25.3
1.3
0.4
665.1
44.6
45.0
46.4
2529.
2
40.4
971.5
49.1
33.0
43.6
47.5
47.2
769.3
47.7
959.0
1027.
4
41.2
43.3
44.1
669.4
973.6
44.7
849.2
39.6
37.1
42.6
1028.
3
2236.
4
49.2
38.7
942.7
1418.
9
44.6
35.9
20.8
4.9
4.4
3.6
11.9
3.7
4.0
2.2
9.9
9.6
1.5
18.8
1.8
4.4
1.8
4.0
13.9
20.4
2.8
11.8
5.6
2.6
13.0
13.8
4.0
14.8
7.3
2.7
2.2
5.9
1.6
17.2
12.8
14.3
34.4
1.2
1.7
1.2
17.1
26.2
1.5
11.1
1.8
3.8
3.7
23.8
15.5
14.2
1.6
25.2
18.7
1.7
1.6
1684. 42.8 1706.9
0
276.4
275. 34.3
9
177.0 180.
48.2
2
156.3 112.
48.4
541.6 0
24.6 555.9
203.9 186. 44.3
267.8 0
235. 33.0
145.5 2
99.2 48.9
513.1 521. 34.8
8
514.8 486.
33.2
3
63.0 90.4
37.3
1127. 28.9 932.4
8
124.3 99.0 37.1
203.1 215. 39.1
105.4 0
110. 35.3
246.0 3
246. 32.8
1164. 4
31.9 1137.4
2
2608.
21.0 2532.8
9
151.4
138. 42.5
4
686.3 47.1
658.9
259.3 293. 40.7
162.4 0
137. 42.8
579.4 9
633. 36.7
2497. 5
18.4 2569.1
3
219.8 218. 37.4
7
995.3 39.0
961.1
318.0 340. 43.8
9
179.8 151.
31.0
123.7 3
111. 42.2
277.0 7
286. 43.1
74.8 8
58.7 46.6
789.2 50.9 762.5
594.3 600. 37.5
5
950.7 20.6
962.6
1028. 73.5 1027.1
1
64.3
67.1 40.8
80.3 88.8 42.6
102.5 62.0 43.0
691.5 57.6 662.8
1021. 59.6 952.5
6
96.0 79.6 43.8
827.0 24.8 857.7
123.6 105. 38.2
4
235.1 230.
34.1
2
270.4 178.
38.7
1062. 3
67.4 1012.2
5
2305. 22.8 2161.7
5
639.3 647. 37.9
90.4 0
83.2 37.9
959.7 72.4 935.4
1401. 27.6 1430.6
4
45.8
60.4 44.6
83.8 107. 35.2
5
15.0
1.8
2.0
2.6
13.6
0.5
1.6
0.5
0.9
3.2
0.6
22.5
0.9
2.1
0.8
1.2
12.7
37.3
1.0
6.3
0.6
0.5
0.7
21.2
1.4
12.5
1.1
1.6
0.8
0.8
1.2
14.9
1.1
18.5
36.9
0.4
0.5
0.4
13.9
26.3
0.4
12.1
0.5
1.0
1.6
14.2
20.0
0.6
0.9
17.9
25.3
1.3
0.4
195
2NWZ582-69
2NWZ582-70
2NWZ582-71
2NWZ582-72
2NWZ582-73
2NWZ582-75
2NWZ582-76
2NWZ582-77
2NWZ582-78
2NWZ582-80
2NWZ582-81
2NWZ582-82
2NWZ582-83
2NWZ582-84
2NWZ582-85
2NWZ582-86
2NWZ582-87
2NWZ582-88
2NWZ582-89
2NWZ582-90
2NWZ582-91
2NWZ582-92
2NWZ582-93
2NWZ582-94
2NWZ582-95
2NWZ582-96
2NWZ582-97
2NWZ582-98
2NWZ582-99
2NWZ582-100
3NWZ459-1
3NWZ459-3
3NWZ459-4
3NWZ459-5
3NWZ459-6
3NWZ459-7
3NWZ459-8
3NWZ459-9
3NWZ459-11
3NWZ459-12
3NWZ459-13
3NWZ459-14
3NWZ459-16
3NWZ459-17
3NWZ459-18
3NWZ459-19
3NWZ459-20
3NWZ459-21
3NWZ459-22
3NWZ459-23
3NWZ459-25
3NWZ459-26
3NWZ459-27
264
1926
314
1213
2700
250
1378
1347
449
1157
2197
1200
270
1989
7068
1854
1194
453
2423
938
579
391
663
3340
1049
593
4584
102
1741
1213
640
519
322
704
206
64
414
250
177
552
550
161
866
1237
930
1499
436
134
77
265
493
362
88
372
3150
12125
1765
1965
9374
782
1725
39538
437
2082
1782
12110
2336
8998
483
610
15376
4414
15802
821
6084
908
6943
5111
18897
1278
5923
18306
879
823
11495
8637
16916
4422
3747
14924
12700
2385
7218
10885
14249
31245
9007
29153
2763
36254
10121
3168
16984
20645
21738
3879
0.7
0.5
1.2
0.8
0.4
2.8
0.9
1.2
1.0
0.9
0.8
2.7
0.8
2.4
1.2
0.5
0.9
2.0
3.0
2.5
1.5
3.2
0.7
4.6
5.5
2.7
0.5
1.0
0.9
0.7
1.3
10.0
2.2
1.0
1.4
1.4
1.9
1.3
1.0
2.4
1.7
1.1
0.6
1.9
2.1
1.8
0.9
1.8
2.4
1.8
2.6
3.1
1.1
0.0388
0.0389
2.7926
0.0519
0.0363
1.5645
0.0367
0.0440
10.2904
0.0490
0.0394
0.0403
1.6035
0.0418
0.0442
0.0144
0.0255
1.9195
0.0471
1.5057
0.0377
0.7703
0.0482
0.0473
1.2970
1.6924
0.0384
2.8138
1.0365
0.0466
0.0645
0.8454
1.2705
1.2585
0.6215
4.2591
1.9034
1.9583
0.6355
0.9214
0.8717
2.9038
1.8027
1.0361
1.8294
0.6170
6.7831
6.7143
1.8290
2.5405
8.4298
6.3735
1.9184
12. 0.0054 1.6
2
4.7
0.0058 1.3
4.8 0.2213 4.3
8.9 0.0075 2.2
10. 0.0052 3.1
4
3.0 0.1597 1.6
5.9 0.0056 3.0
4.7 0.0066 2.9
3.8 0.4623 2.7
12. 0.0070 1.5
4
6.3
0.0060 4.2
13. 0.0055 3.4
8
1.8 0.1617 1.4
7.0 0.0061 3.6
16. 0.0060 4.3
1
28. 0.0050 5.4
4
15. 0.0051 3.2
2
3.9
0.1768 2.9
4.9 0.0070 1.9
2.3 0.1497 1.6
9.3 0.0055 2.3
4.5 0.0905 3.5
9.1 0.0069 1.0
4.9 0.0073 4.8
9.4 0.1328 8.6
3.2 0.1670 2.6
11. 0.0054 2.7
4
2.8
0.2200 1.0
1.7 0.1175 1.4
10. 0.0066 3.4
3
5.0 0.0096 1.5
3.3 0.0993 1.7
2.2 0.1381 1.9
2.1 0.1358 1.0
4.9 0.0772 3.5
2.4 0.2855 1.9
2.7 0.1817 2.4
1.7 0.1857 1.0
6.3 0.0768 1.0
2.0 0.1035 1.0
4.1 0.1010 3.7
3.6 0.2388 3.5
3.6 0.1741 3.3
2.6 0.1072 2.1
1.8 0.1780 1.1
3.9 0.0743 2.1
3.3 0.3834 3.1
1.4 0.3802 1.0
2.8 0.1760 1.1
1.5 0.2229 1.0
3.3 0.3758 3.0
5.4 0.2939 5.1
2.2 0.1842 1.5
0.1
3
0.2
8
0.9
0
0.2
4
0.3
0
0.5
2
0.5
0
0.6
2
0.7
0
0.1
2
0.6
6
0.2
5
0.8
1
0.5
2
0.2
7
0.1
9
0.2
1
0.7
4
0.3
9
0.7
1
0.2
4
0.7
8
0.1
1
0.9
6
0.9
1
0.8
0
0.2
4
0.3
6
0.8
1
0.3
3
0.2
9
0.5
1
0.8
6
0.4
8
0.7
3
0.8
1
0.8
7
0.5
8
0.1
6
0.5
0
0.9
1
0.9
6
0.9
3
0.7
9
0.5
9
0.5
5
0.9
5
0.7
0
0.3
8
0.6
6
0.9
1
0.9
4
0.6
8
34.6
37.6
1288.
9
48.0
33.2
955.4
35.9
42.5
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5
44.8
38.3
35.5
966.1
39.4
38.7
32.1
32.9
1049.
2
45.1
899.3
35.0
558.8
44.2
46.7
803.7
995.5
34.8
1281.
9
716.2
42.4
61.4
610.1
833.7
820.6
479.2
1618.
8
1076.
0
1098.
1
476.9
635.0
620.2
1380.
3
1034.
5
656.6
1056.
3
462.2
2092.
0
2077.
2
1044.
9
1297.
0
2056.
7
1660.
9
1090.
0
0.5
0.5
50.4
1.0
1.0
14.0
1.1
1.2
54.4
0.7
1.6
1.2
12.7
1.4
1.7
1.7
1.0
27.8
0.9
13.8
0.8
18.8
0.4
2.2
65.1
23.8
1.0
11.6
9.5
1.4
0.9
9.8
14.9
7.7
16.3
27.6
23.7
10.1
4.6
6.0
22.0
43.1
31.8
13.0
10.4
9.5
55.9
17.8
10.1
11.7
52.3
74.2
14.8
38.7
38.8
1353.
6
51.4
36.2
956.2
36.6
43.7
2461.
2
48.6
39.2
40.1
971.6
41.5
43.9
14.5
25.6
1087.
9
46.8
932.7
37.6
580.0
47.8
46.9
844.4
1005.
7
38.3
1359.
2
722.2
46.2
63.4
622.1
832.6
827.2
490.8
1685.
5
1082.
3
1101.
3
499.5
663.1
636.5
1382.
9
1046.
4
722.0
1056.
1
488.0
2083.
5
2074.
5
1055.
9
1283.
7
2278.
4
2028.
6
1087.
5
4.6
1.8
35.9
4.5
3.7
18.7
2.1
2.0
35.5
5.9
2.4
5.4
11.0
2.8
6.9
4.1
3.8
26.0
2.2
14.0
3.4
20.0
4.2
2.3
54.2
20.5
4.3
20.8
9.0
4.6
3.1
15.4
12.6
11.8
19.0
19.6
18.2
11.6
24.7
9.8
19.4
27.4
23.4
13.6
11.8
15.0
29.1
12.6
18.4
11.1
29.6
47.3
14.5
302.8
111.8
1457.
3
211.7
240.9
958.2
87.5
110.8
2470.
9
238.8
99.9
325.3
984.1
168.9
338.7
2428.
-622.3
9
1166.
0
134.7
1012.
6
201.5
663.9
233.2
59.8
952.9
1028.
0
262.0
1482.
9
740.7
249.0
142.9
666.0
829.7
845.0
545.2
1769.
5
1094.
9
1107.
6
604.2
759.8
694.8
1386.
8
1071.
4
930.8
1055.
6
611.1
2075.
1
2071.
8
1078.
7
1261.
6
2483.
8
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7
1082.
5
275. 34.6
8
106.
37.6
7
40.2 1288.9
200. 48.0
6
228. 33.2
5
52.5 955.4
121. 35.9
3
86.9 42.5
46.4 2449.5
285. 44.8
5
111.
38.3
5
304.
35.5
8
21.0 966.1
139. 39.4
7
352. 38.7
3
#### 32.1
#
407. 32.9
0
52.3
1049.2
106. 45.1
6
32.5
899.3
209. 35.0
2
60.8 558.8
208. 44.2
3
32.3 46.7
79.1 803.7
38.7 995.5
254. 34.8
2
49.0
1281.9
21.7 716.2
222. 42.4
9
111. 61.4
3
60.9 610.1
23.4 833.7
38.2 820.6
73.3 479.2
25.4 1618.8
26.5 1076.0
28.1 1098.1
134. 476.9
0
37.0 635.0
36.6 620.2
20.1 1380.3
26.6 1034.5
33.0 656.6
29.2 1056.3
70.1 462.2
17.6 2092.0
17.9 2077.2
52.1 1044.9
22.5 1297.0
22.4 2056.7
31.0 1660.9
31.9 1090.0
0.5
0.5
50.4
1.0
1.0
14.0
1.1
1.2
54.4
0.7
1.6
1.2
12.7
1.4
1.7
1.7
1.0
27.8
0.9
13.8
0.8
18.8
0.4
2.2
65.1
23.8
1.0
11.6
9.5
1.4
0.9
9.8
14.9
7.7
16.3
27.6
23.7
10.1
4.6
6.0
22.0
43.1
31.8
13.0
10.4
9.5
55.9
17.8
10.1
11.7
52.3
74.2
14.8
196
3NWZ459-28
3NWZ459-29
3NWZ459-30
3NWZ459-32
3NWZ459-33
3NWZ459-34
3NWZ459-35
3NWZ459-36
3NWZ459-37
3NWZ459-38
3NWZ459-39
3NWZ459-41
3NWZ459-43
3NWZ459-44
3NWZ459-45
3NWZ459-46
3NWZ459-47
3NWZ459-48
3NWZ459-49
3NWZ459-50
3NWZ459-51
3NWZ459-53
3NWZ459-54
3NWZ459-55
3NWZ459-56
3NWZ459-57
3NWZ459-59
3NWZ459-60
3NWZ459-61
3NWZ459-62
3NWZ459-63
3NWZ459-65
3NWZ459-66
3NWZ459-67
3NWZ459-70
3NWZ459-71
3NWZ459-72
3NWZ459-73
3NWZ459-74
3NWZ459-75
3NWZ459-76
3NWZ459-77
3NWZ459-78
3NWZ459-79
3NWZ459-80
3NWZ459-81
3NWZ459-82
3NWZ459-83
3NWZ459-84
3NWZ459-85
3NWZ459-86
3NWZ459-87
3NWZ459-88
54
203
934
321
252
143
634
266
132
782
354
224
1428
342
104
1338
28
60
396
45
558
124
64
302
247
137
84
511
182
157
205
1153
267
848
295
192
1467
257
609
156
215
380
348
156
243
721
45
335
128
633
157
126
186
1375
12334
15829
10543
4877
3719
26073
8767
6527
17734
17512
12544
37474
26098
6904
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5
1367
2871
18602
3898
46912
4032
2725
14491
14835
3155
4006
22743
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5808
6732
32418
9696
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5494
7392
10974
21917
19306
6967
5555
8493
21076
6048
6967
28353
1865
14960
16102
17083
5005
6955
9518
1.3
2.3
4.0
2.7
3.7
4.5
8.2
4.4
1.2
71.3
1.4
0.7
4.9
2.2
1.0
5.1
1.6
1.8
1.0
2.1
9.9
2.9
1.1
2.0
5.7
4.5
1.3
22.0
1.4
1.9
2.1
154.
8
1.0
1.9
1.9
2.9
3.7
1.0
6.2
2.3
5.3
4.3
2.2
3.5
3.6
7.0
1.4
1.8
0.9
3.1
2.6
2.9
1.8
0.8838
2.6437
1.1466
1.4949
0.6466
1.1202
1.3836
0.9226
1.9441
0.7048
2.0213
1.9326
9.0271
3.7826
6.6370
9.9733
1.7185
1.6145
1.8089
5.7146
3.0360
0.8438
1.0344
1.4847
2.3007
1.6412
1.0833
1.3303
5.5080
1.5078
1.3163
0.8122
1.2515
1.7972
1.4219
2.0184
6.3138
9.9794
1.2266
2.3493
1.3008
0.8220
11.7658
1.3694
1.3863
1.9191
1.6272
1.8530
10.3225
0.9871
1.7699
2.0060
1.7548
12. 0.0968 1.7
7
2.1
0.2295 1.2
1.4 0.1264 1.0
1.8 0.1571 1.5
3.3 0.0813 2.1
2.5 0.1242 1.0
1.8 0.1452 1.4
2.1 0.1059 1.8
2.2 0.1846 1.0
2.6 0.0870 1.7
2.3 0.1928 1.7
3.6 0.1826 2.4
4.0 0.4153 3.7
2.1 0.2796 1.3
3.4 0.3152 3.1
2.0 0.4612 1.2
4.1 0.1680 1.2
2.6 0.1616 1.8
4.4 0.1660 3.3
3.0 0.3354 1.6
8.3 0.2314 6.7
2.1 0.1001 1.4
3.8 0.1153 1.3
1.8 0.1539 1.5
3.8 0.1727 2.0
3.7 0.1586 1.6
3.4 0.1221 2.8
3.3 0.1408 3.0
3.1 0.3240 2.6
2.7 0.1521 1.0
1.9 0.1350 1.0
2.2 0.0971 1.2
1.6 0.1324 1.2
5.2 0.1681 1.4
4.4 0.1501 3.2
1.9 0.1905 1.0
4.0 0.2675 2.0
1.8 0.4620 1.0
2.2 0.1330 1.0
2.5 0.2098 1.3
8.7 0.1334 8.3
4.9 0.0966 4.3
4.4 0.4488 3.5
2.2 0.1442 1.7
1.7 0.1440 1.4
5.9 0.1781 3.5
5.4 0.1607 2.6
2.0 0.1790 1.1
1.5 0.4723 1.0
8.7 0.1073 8.6
2.9 0.1732 2.7
3.3 0.1846 2.3
1.8 0.1706 1.0
0.1
3
0.5
7
0.6
9
0.8
3
0.6
4
0.4
0
0.8
2
0.8
7
0.4
5
0.6
5
0.7
5
0.6
7
0.9
3
0.6
0
0.9
0
0.6
1
0.3
0
0.6
8
0.7
5
0.5
3
0.8
1
0.6
8
0.3
3
0.8
3
0.5
2
0.4
2
0.8
3
0.9
1
0.8
2
0.3
7
0.5
5
0.5
5
0.7
5
0.2
7
0.7
3
0.5
1
0.5
0
0.5
6
0.4
5
0.5
1
0.9
6
0.8
7
0.7
8
0.7
8
0.7
8
0.5
9
0.4
7
0.5
6
0.6
5
0.9
9
0.9
2
0.6
9
0.5
5
595.6 9.6
1331. 14.6
9
767.4
7.2
940.7 13.3
503.9 10.0
754.7 7.1
874.0 11.8
649.0 11.4
1092. 10.0
0
537.6
8.6
1136. 17.6
7
1081.
24.1
0
2239. 69.6
0
1589. 17.9
3
1766. 47.3
1
2444. 24.6
9
1001. 11.5
3
965.9
16.0
990.2 30.0
1864. 25.6
3
1341.
81.2
9
615.3 8.4
703.5 8.5
922.9 13.2
1027. 19.0
0
948.8
13.8
742.5 19.5
849.1 24.0
1809. 40.5
4
912.7 8.5
816.2 7.9
597.4 7.0
801.5 9.1
1001. 12.8
7
901.7
27.3
1124. 10.3
0
1528. 27.3
0
2448. 20.4
4
804.8 7.6
1227. 14.4
6
807.3 62.8
594.2 24.3
2390. 68.9
1
868.6
13.9
867.2 11.0
1056. 33.6
4
960.6 22.8
1061. 10.8
6
2493. 20.7
8
657.0
53.6
1029. 25.7
5
1091.
22.8
9
1015.
9.4
6
643.0 60.4 813.3 263.
3
1312. 15.6 1282. 33.9
9
0
775.7
7.9 799.5
22.0
928.3 11.1 898.9 21.1
506.4 13.0 517.6 55.2
763.1 13.4 787.5 48.0
882.0 10.4 902.0 20.8
663.7 10.3 714.0 22.5
1096. 14.9 1105. 39.6
4
2
541.6
10.8 558.9
42.3
1122. 15.4 1095. 30.2
7
7
1092.
24.0 1115.
53.0
4
2340. 36.2 3
2430. 24.9
8
1589. 16.9 7
1588. 31.2
1
2064. 30.2 7
2376. 25.9
3
2432. 18.4 7
2421. 26.8
3
1015. 26.6 8
1046. 79.9
5
1
975.8
16.4 998.4
39.1
1048. 28.5 1172. 57.0
7
5
1933.
25.7 2008.
44.8
6
7
1416.
63.4 1531.
92.0
7
621.2 9.8 1
643.0 33.2
721.1 19.8 776.2 76.0
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1212. 27.2 1559. 61.7
5
8
986.2
23.3 1070.
67.1
3
745.2 17.7 753.3
39.7
859.0 19.2 884.7 28.0
1901. 27.1 2004. 32.3
8
933.6 16.4 2
983.2 50.9
852.9 10.8 949.6 32.0
603.7 10.1 627.5 39.9
824.1 9.1 885.4 21.8
1044. 33.9 1135. 99.9
4
0
898.2
26.3 889.6
61.9
1121. 13.2 1117. 33.4
7
2020. 34.9 3
2569. 57.5
4
2432. 16.6 4
2419. 25.3
9
812.8 12.3 9
834.8 40.9
1227. 18.0 1227. 42.7
4
846.1 49.7 0
949.1 52.4
609.1 22.5 665.1 51.9
2585. 41.4 2743. 45.6
9
2
875.9
12.9 894.4
28.3
883.1 10.3 923.2 22.2
1087. 39.2 1151. 94.2
8
980.8 34.2 1
1026. 97.3
1064. 13.0 2
1070. 32.7
5
2464. 14.2 4
2439. 19.7
1
7
697.2
43.9 829.1
29.1
1034. 19.1 1045. 23.7
5
0
1117.
22.2 1167.
46.7
6
8
1028.
11.8 1057.
30.6
9
3
595.6
1331.9
767.4
940.7
503.9
754.7
874.0
649.0
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537.6
1136.7
1081.0
2239.0
1589.3
1766.1
2444.9
1001.3
965.9
990.2
1864.3
1341.9
615.3
703.5
922.9
1027.0
948.8
742.5
849.1
1809.4
912.7
816.2
597.4
801.5
1001.7
901.7
1124.0
1528.0
2448.4
804.8
1227.6
807.3
594.2
2390.1
868.6
867.2
1056.4
960.6
1061.6
2493.8
657.0
1029.5
1091.9
1015.6
9.6
14.6
7.2
13.3
10.0
7.1
11.8
11.4
10.0
8.6
17.6
24.1
69.6
17.9
47.3
24.6
11.5
16.0
30.0
25.6
81.2
8.4
8.5
13.2
19.0
13.8
19.5
24.0
40.5
8.5
7.9
7.0
9.1
12.8
27.3
10.3
27.3
20.4
7.6
14.4
62.8
24.3
68.9
13.9
11.0
33.6
22.8
10.8
20.7
53.6
25.7
22.8
9.4
197
3NWZ459-89
3NWZ459-90
3NWZ459-91
3NWZ459-92
3NWZ459-93
3NWZ459-95
3NWZ459-96
3NWZ459-98
3NWZ459-99
3NWZ459-1
3NWZ459-3
3NWZ459-4
3NWZ459-5
3NWZ459-6
3NWZ459-7
3NWZ459-8
3NWZ459-9
3NWZ459-11
3NWZ459-12
3NWZ459-13
3NWZ459-14
3NWZ459-16
3NWZ459-17
3NWZ459-18
3NWZ459-19
3NWZ459-20
3NWZ459-21
3NWZ459-22
3NWZ459-23
3NWZ459-25
3NWZ459-26
3NWZ459-27
3NWZ459-28
3NWZ459-29
3NWZ459-30
3NWZ459-32
3NWZ459-33
3NWZ459-34
3NWZ459-35
3NWZ459-36
3NWZ459-37
3NWZ459-38
3NWZ459-39
3NWZ459-41
3NWZ459-43
3NWZ459-44
3NWZ459-45
3NWZ459-46
3NWZ459-47
3NWZ459-48
3NWZ459-49
3NWZ459-50
3NWZ459-51
41
34
152
282
343
107
318
285
216
640
519
322
704
206
64
414
250
177
552
550
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1.9
0.5
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1.3
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2.2
1.0
1.4
1.4
1.9
1.3
1.0
2.4
1.7
1.1
0.6
1.9
2.1
1.8
0.9
1.8
2.4
1.8
2.6
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1.1
1.3
2.3
4.0
2.7
3.7
4.5
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4.4
1.2
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1.4
0.7
4.9
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1.6
1.8
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2.2441
0.7835
1.6026
0.6059
8.3966
1.5433
1.8488
2.1161
1.6554
0.0645
0.8454
1.2705
1.2585
0.6215
4.2591
1.9034
1.9583
0.6355
0.9214
0.8717
2.9038
1.8027
1.0361
1.8294
0.6170
6.7831
6.7143
1.8290
2.5405
8.4298
6.3735
1.9184
0.8838
2.6437
1.1466
1.4949
0.6466
1.1202
1.3836
0.9226
1.9441
0.7048
2.0213
1.9326
9.0271
3.7826
6.6370
9.9733
1.7579
1.6145
1.8089
5.7146
3.0360
4.2
5.5
2.7
2.7
1.6
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2.4
3.9
1.7
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3.3
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2.4
2.7
1.7
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3.6
3.6
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7
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2.6
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0.0922
0.1588
0.0777
0.3975
0.1586
0.1793
0.1903
0.1674
0.0096
0.0993
0.1381
0.1358
0.0772
0.2855
0.1817
0.1857
0.0768
0.1035
0.1010
0.2388
0.1741
0.1072
0.1780
0.0743
0.3834
0.3802
0.1760
0.2229
0.3758
0.2939
0.1842
0.0968
0.2295
0.1264
0.1571
0.0813
0.1242
0.1452
0.1059
0.1846
0.0870
0.1928
0.1826
0.4153
0.2796
0.3152
0.4612
0.1680
0.1616
0.1660
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1.6
1.9
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1.2
1.0
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1.5
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1.7
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0.6
8
0.8
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198
3NWZ459-53
3NWZ459-54
3NWZ459-55
3NWZ459-56
3NWZ459-57
3NWZ459-59
3NWZ459-60
3NWZ459-61
3NWZ459-62
3NWZ459-63
3NWZ459-65
3NWZ459-66
3NWZ459-67
3NWZ459-70
3NWZ459-71
3NWZ459-72
3NWZ459-73
3NWZ459-74
3NWZ459-75
3NWZ459-76
3NWZ459-77
3NWZ459-78
3NWZ459-79
3NWZ459-80
3NWZ459-81
3NWZ459-82
3NWZ459-83
3NWZ459-84
3NWZ459-85
3NWZ459-86
3NWZ459-87
3NWZ459-88
3NWZ459-89
3NWZ459-90
3NWZ459-91
3NWZ459-92
3NWZ459-93
3NWZ459-95
3NWZ459-96
3NWZ459-98
3NWZ459-99
SMZ35-1
SMZ35-2
SMZ35-3
SMZ35-4
SMZ35-5
SMZ35-6
SMZ35-7
SMZ35-9
SMZ35-10
SMZ35-11
SMZ35-12
SMZ35-13
124
64
302
247
137
84
511
182
157
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1
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8
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1.0344
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1.9 1.4219
2.9 2.0184
3.7 6.3138
1.0 9.9794
6.2 1.2266
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5.3 1.3008
4.3 0.8220
2.2 11.7658
3.5 1.3694
3.6 1.3863
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1.4 1.6272
1.8 1.8530
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1.6
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2.4
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751.8
1089.
6
500.4
462.7
2438.
1
952.9
1002.
3
866.2
989.8
9.8
19.8
11.2
27.2
23.3
17.7
19.2
27.1
16.4
10.8
10.1
9.1
33.9
26.3
13.2
34.9
16.6
12.3
18.0
49.7
22.5
41.4
12.9
10.3
39.2
34.2
13.0
14.2
43.9
19.1
22.2
11.8
29.8
24.7
17.1
10.4
14.2
12.2
15.7
27.0
10.8
15.3
11.3
14.6
12.9
28.3
9.2
11.1
30.8
11.4
22.1
17.7
16.0
643.0
776.2
927.2
1559.
8
1070.
3
753.3
884.7
2004.
2
983.2
949.6
627.5
885.4
1135.
0
889.6
1117.
3
2569.
4
2419.
9
834.8
1227.
0
949.1
665.1
2743.
2
894.4
923.2
1151.
1
1026.
2
1070.
4
2439.
7
829.1
1045.
0
1167.
8
1057.
3
1239.
5
661.3
33.2
76.0
20.9
61.7
67.1
39.7
28.0
32.3
50.9
32.0
39.9
21.8
99.9
61.9
33.4
57.5
25.3
40.9
42.7
52.4
51.9
45.6
28.3
22.2
94.2
97.3
32.7
19.7
29.1
23.7
46.7
30.6
72.7
114.
0
40.8
1019.
8
519.1
34.0
2381. 17.2
8
987.8 34.3
1062. 31.8
7
1212. 64.3
6
978.2 28.2
1092. 37.9
9
461.2
24.5
1049. 39.2
5
929.2
44.1
1117. 29.1
3
479.5 43.3
1121. 41.3
4
2595. 38.4
0
952.4
28.0
1203. 63.2
5
1012.
27.1
4
1022.
42.2
5
615.3
703.5
922.9
1027.0
948.8
742.5
849.1
1809.4
912.7
816.2
597.4
801.5
1001.7
901.7
1124.0
1528.0
2448.4
804.8
1227.6
807.3
594.2
2390.1
868.6
867.2
1056.4
960.6
1061.6
2493.8
657.0
1029.5
1091.9
1015.6
1170.5
568.5
949.9
482.5
2157.7
948.9
1063.1
1123.1
997.7
1092.9
461.2
1049.5
929.2
1117.3
479.5
1121.4
2595.0
952.4
1203.5
1012.4
1022.5
8.4
8.5
13.2
19.0
13.8
19.5
24.0
40.5
8.5
7.9
7.0
9.1
12.8
27.3
10.3
27.3
20.4
7.6
14.4
62.8
24.3
68.9
13.9
11.0
33.6
22.8
10.8
20.7
53.6
25.7
22.8
9.4
22.0
8.5
16.3
10.1
22.0
8.8
17.4
22.2
9.2
37.9
24.5
39.2
44.1
29.1
43.3
41.3
38.4
28.0
63.2
27.1
42.2
199
SMZ35-14
SMZ35-15
SMZ35-16
SMZ35-17
SMZ35-18
SMZ35-19
SMZ35-20
SMZ35-21
SMZ35-23
SMZ35-24
SMZ35-25
SMZ35-26
SMZ35-27
SMZ35-28
SMZ35-29
SMZ35-30
SMZ35-32
SMZ35-33
SMZ35-34
SMZ35-35
SMZ35-36
SMZ35-37
SMZ35-38
SMZ35-39
SMZ35-40
SMZ35-41
SMZ35-42
SMZ35-43
SMZ35-44
SMZ35-45
SMZ35-46
SMZ35-47
SMZ35-48
SMZ35-49
SMZ35-50
SMZ35-51
SMZ35-52
SMZ35-53
SMZ35-54
SMZ35-55
SMZ35-56
SMZ35-57
SMZ35-58
SMZ35-59
SMZ35-60
SMZ35-61
SMZ35-63
SMZ35-64
SMZ35-65
SMZ35-66
SMZ35-67
SMZ35-68
SMZ35-69
219
109
140
363
1910
139
391
141
984
428
31
332
1916
752
661
1168
272
94
204
643
756
325
787
1085
203
128
1385
231
157
431
396
90
323
58
193
263
238
492
1501
369
141
78
399
383
436
341
256
19
465
1211
116
201
64
13629
7056
24558
33252
60366
10437
19623
8121
53502
11247
2544
17103
14030
1
27300
37611
58761
16137
7701
15933
58947
12588
24849
22437
44835
11301
6696
74766
11328
11664
33882
29127
5496
21516
3012
2364
41181
9699
92751
11831
4
19815
9372
5388
31194
10437
40314
22728
18033
2244
25626
91206
5928
5925
11298
1.1
1.3
2.5
2.9
3.4
1.5
1.5
1.7
5.9
1.9
2.0
2.4
22.6
1.9
4.0
3.0
3.0
1.4
2.4
4.3
8.1
0.7
1.1
1.0
0.6
1.2
18.6
2.6
1.2
2.1
2.3
0.9
1.4
0.8
4.8
2.2
1.4
7.6
12.1
2.0
2.4
1.7
8.8
1.8
0.5
0.9
2.1
2.2
1.2
2.3
1.9
1.6
1.0
1.1915
1.1275
9.6267
2.0623
0.5747
1.7928
1.4202
1.0975
1.1742
0.6318
1.9723
0.8061
1.6436
0.6212
0.8146
0.7738
1.0050
1.7457
1.6129
1.9406
0.5975
1.8242
0.6006
1.3823
1.5493
1.8907
1.4422
1.3503
1.6084
1.6936
2.3295
1.5578
1.9698
4.0173
0.4556
8.9268
1.8342
6.9436
1.9580
1.6490
1.5232
1.4811
1.7655
0.6497
2.8835
1.8239
1.5177
3.4575
1.4915
1.8409
1.8142
2.2126
5.2914
2.2
2.9
1.4
9.5
2.9
2.0
5.0
4.1
1.7
7.3
3.6
4.1
2.1
3.3
3.4
4.0
5.4
2.3
2.1
2.2
2.5
4.3
2.0
6.3
2.0
3.2
3.4
2.1
2.4
4.0
4.6
3.5
1.7
7.1
5.2
2.0
2.4
1.4
1.7
2.1
2.4
2.7
2.3
4.0
2.2
1.8
1.8
3.8
3.6
1.6
3.2
11.
5
3.0
0.1285
0.1182
0.4376
0.1903
0.0754
0.1739
0.1413
0.1176
0.1259
0.0748
0.1693
0.0933
0.1655
0.0644
0.0976
0.0957
0.1052
0.1670
0.1541
0.1859
0.0738
0.1795
0.0771
0.1407
0.1558
0.1776
0.1476
0.1374
0.1599
0.1660
0.1995
0.1515
0.1838
0.2675
0.0385
0.4039
0.1774
0.3843
0.1829
0.1627
0.1537
0.1475
0.1724
0.0799
0.2315
0.1785
0.1536
0.2401
0.1487
0.1790
0.1690
0.1803
0.3307
1.8
1.0
1.0
2.1
1.8
1.0
2.5
1.4
1.0
1.0
1.9
2.8
1.5
1.5
1.3
1.0
1.5
1.4
1.0
1.0
1.0
1.2
1.0
5.7
1.0
2.9
1.1
1.2
1.0
1.0
4.1
1.2
1.0
1.3
1.5
1.7
1.5
1.0
1.0
1.0
1.8
2.0
1.7
1.0
1.0
1.3
1.0
1.9
3.1
1.0
1.0
2.3
1.0
0.8
1
0.3
5
0.7
1
0.2
2
0.6
1
0.5
0
0.5
0
0.3
4
0.5
9
0.1
4
0.5
3
0.6
9
0.7
2
0.4
5
0.3
7
0.2
5
0.2
7
0.6
1
0.4
7
0.4
5
0.3
9
0.2
7
0.5
0
0.9
1
0.5
1
0.8
9
0.3
2
0.5
8
0.4
1
0.2
5
0.8
9
0.3
5
0.5
9
0.1
8
0.2
8
0.8
6
0.6
4
0.7
1
0.5
9
0.4
8
0.7
5
0.7
5
0.7
4
0.2
5
0.4
5
0.7
2
0.5
6
0.5
1
0.8
4
0.6
3
0.3
1
0.2
0
0.3
3
779.3
720.2
2339.
8
1123.
1
468.5
1033.
5
852.0
716.5
764.6
465.0
1008.
2
575.0
987.3
402.4
600.3
589.4
644.8
995.6
924.1
1099.
1
458.8
1064.
3
478.5
848.9
933.5
1054.
0
887.8
829.9
956.2
989.8
1172.
7
909.6
1087.
9
1528.
3
243.7
2187.
0
1052.
8
2096.
2
1083.
0
971.6
921.4
887.1
1025.
2
495.6
1342.
1
1059.
0
921.1
1387.
2
893.6
1061.
4
1006.
4
1068.
8
1841.
7
12.9
6.8
19.6
21.3
8.0
9.5
19.7
9.4
7.2
4.5
17.7
15.6
13.5
5.8
7.2
5.7
9.0
12.8
8.6
10.1
4.4
11.4
4.6
45.5
8.7
27.7
9.1
9.3
8.9
9.2
43.7
10.5
10.0
17.0
3.5
31.7
14.9
17.9
10.0
9.0
15.1
16.7
16.3
4.8
12.1
12.4
8.6
23.8
25.4
9.8
9.3
22.2
16.0
796.7
766.5
2399.
7
1136.
4
461.0
1042.
9
897.5
752.1
788.6
497.2
1106.
1
600.2
987.1
490.6
605.1
581.9
706.4
1025.
6
975.2
1095.
2
475.6
1054.
2
477.6
881.4
950.2
1077.
8
906.7
867.7
973.5
1006.
1
1221.
3
953.6
1105.
2
1637.
7
381.2
2330.
6
1057.
8
2104.
2
1101.
2
989.2
939.8
922.7
1032.
9
508.3
1377.
6
1054.
1
937.6
1517.
6
926.9
1060.
2
1050.
6
1185.
0
1867.
5
12.1
15.5
13.0
65.1
10.7
13.1
29.5
21.9
9.3
28.7
24.2
18.7
13.0
12.8
15.5
17.5
27.3
14.7
13.4
15.0
9.6
28.3
7.6
37.1
12.1
21.3
20.3
12.1
15.1
25.5
32.7
21.6
11.4
57.6
16.6
18.1
15.6
12.6
11.3
13.2
14.5
16.2
15.0
15.9
16.9
11.5
10.9
29.6
22.2
10.4
21.3
80.8
25.7
845.5
904.1
2450.
9
1161.
9
423.7
26.9
55.6
16.9
184.
4
50.9
35.2
87.0
80.5
28.5
155.
7
59.3
1062.
6
1011.
2
859.5
857.1
648.7
1303.
9
696.8
63.9
986.8 29.1
926.7 60.4
622.9 68.0
552.9 83.6
907.2 106.
3
1090. 36.1
2
1092. 37.7
3
1087. 40.1
4
557.5 50.9
1033.
4
473.1
964.1
989.0
1126.
3
952.9
965.4
1012.
8
1041.
8
1308.
3
1056.
6
1139.
5
1781.
1
1332.
9
2458.
7
1068.
0
2112.
1
1137.
3
1028.
2
983.0
1008.
8
1049.
2
566.2
1433.
0
1043.
8
976.4
1704.
5
1007.
1
1057.
7
1143.
8
1403.
8
1896.
3
84.1
38.4
53.9
34.3
29.2
65.7
34.5
44.6
77.9
41.2
65.8
27.1
127.
2
97.0
16.9
36.4
17.5
27.1
37.1
32.0
35.6
31.3
83.5
38.4
24.5
30.0
59.5
40.6
24.6
61.4
217.
3
51.1
845.5
904.1
2450.9
1161.9
423.7
1062.6
1011.2
859.5
857.1
648.7
1303.9
696.8
986.8
926.7
622.9
552.9
907.2
1090.2
1092.3
1087.4
557.5
1033.4
473.1
964.1
989.0
1126.3
952.9
965.4
1012.8
1041.8
1308.3
1056.6
1139.5
1781.1
1332.9
2458.7
1068.0
2112.1
1137.3
1028.2
983.0
1008.8
1049.2
566.2
1433.0
1043.8
976.4
1704.5
1007.1
1057.7
1143.8
1403.8
1896.3
26.9
55.6
16.9
184.
4
50.9
35.2
87.0
80.5
28.5
155.
7
59.3
63.9
29.1
60.4
68.0
83.6
106.
3
36.1
37.7
40.1
50.9
84.1
38.4
53.9
34.3
29.2
65.7
34.5
44.6
77.9
41.2
65.8
27.1
127.
2
97.0
16.9
36.4
17.5
27.1
37.1
32.0
35.6
31.3
83.5
38.4
24.5
30.0
59.5
40.6
24.6
61.4
217.
3
51.1
200
SMZ35-70
SMZ35-71
SMZ35-72
SMZ35-73
SMZ35-74
SMZ35-75
SMZ35-76
SMZ35-77
SMZ35-78
SMZ35-79
SMZ35-80
SMZ35-81
SMZ35-82
SMZ35-83
SMZ35-84
SMZ35-86
SMZ35-87
SMZ35-88
SMZ35-89
SMZ35-90
SMZ35-91
SMZ35-92
SMZ35-94
SMZ35-95
SMZ35-96
SMZ35-97
SMZ35-98
SMZ35-99
SMZ35-101
SMZ35-102
SMZ35-103
SMZ35-105
SMZ35-106
SMZ35-107
SMZ35-108
SMZ35-109
SMZ35-110
SMZ35-111
SMZ35-113
SMZ35-114
SMZ35-115
125
471
404
206
174
1164
1438
623
1503
502
210
153
744
131
200
225
142
383
476
483
381
204
148
168
280
334
107
164
227
501
1476
855
309
1011
246
338
361
599
515
470
481
12411
14409
44697
18972
12078
39027
40986
9591
61512
35904
15747
29223
28626
13119
45159
26802
6612
18972
5982
39861
38235
20307
10605
16815
37740
24546
11115
7452
20322
36885
10913
4
49074
18417
34551
19890
14526
14427
29667
45750
41310
44379
1.0
1.0
2.3
1.3
2.0
4.3
2.3
4.1
7.1
2.3
1.3
0.8
4.7
1.6
3.0
2.3
1.4
1.6
6.6
1.5
2.0
1.8
1.4
3.0
2.9
8.1
2.0
1.8
1.9
1.2
7.2
2.8
0.8
1.9
1.7
1.7
9.7
3.0
2.7
2.1
6.2
3.1526
0.7705
4.4225
2.7883
1.7978
1.1849
0.5826
0.5125
0.7851
1.7732
1.5476
8.2361
0.5989
1.9962
6.2898
4.1448
0.9051
1.5356
0.6114
1.9647
2.3472
2.0576
1.7773
1.9197
3.0456
1.0391
1.7581
0.7496
1.6498
1.9555
1.4440
1.8512
1.1828
0.5832
1.6658
1.5768
0.7705
1.3832
2.3738
2.4451
2.6114
1.6
3.3
2.7
1.5
2.5
6.9
1.6
6.3
2.0
2.2
1.9
1.9
3.0
2.1
2.7
1.7
2.9
3.4
11.
0
2.7
3.2
2.1
2.9
2.6
3.1
10.
1
3.2
2.7
2.7
2.6
1.6
1.9
1.8
1.7
1.5
2.0
7.7
2.7
4.9
2.2
1.8
0.2486
0.0919
0.2912
0.2245
0.1707
0.1204
0.0753
0.0512
0.0962
0.1750
0.1576
0.4216
0.0772
0.1899
0.3545
0.2798
0.1023
0.1525
0.0565
0.1880
0.2128
0.1947
0.1662
0.1839
0.2353
0.1127
0.1689
0.0829
0.1660
0.1857
0.1482
0.1744
0.1281
0.0744
0.1645
0.1556
0.0755
0.1350
0.1931
0.2159
0.2131
1.0
2.5
1.0
1.0
1.4
6.5
1.0
6.2
1.1
1.0
1.5
1.1
1.9
1.0
2.5
1.0
1.1
1.5
9.2
1.0
1.9
1.0
2.7
1.9
1.6
8.8
1.0
1.0
1.0
1.1
1.0
1.0
1.1
1.2
1.0
1.0
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201
CHAPTER 4: CLIMATE-DRIVEN ENVIRONMENTAL CHANGE IN THE
SOUTHERN TIBETAN PLATEAU
ABSTRACT
Zhada basin is a large Neogene extensional sag basin in the Tethyan Himalaya of
southwestern Tibet. In this paper we examine environmental changes in Zhada basin
using sequence-, isotope- and litho-stratigraphy. Sequence stratigraphy reveals a longterm tectonic signal in the formation and filling of Zhada basin, as well as higher
frequency cycles which we attribute to Milankovitch forcing. The record of
Milankovitch cycles in Zhada basin implies that global climate drove lake and wetland
expansion and contraction in the southern Tibetan Plateau from the late Miocene to the
Pleistocene. Sequence stratigraphy shows that the Zhada basin evolved from an
overfilled to underfilled basin but continued evolution was truncated by an abrupt return
to fluvial conditions. Isotope stratigraphy shows distinct drying cycles; particularly
during times when the basin was underfilled.
A long-term environmental change observed in Zhada basin involves a decrease
in abundance of arboreal pollen in favor of non-arboreal pollen. The similarity between
the long-term environmental changes in Zhada basin and those observed elsewhere on
and around the Tibetan Plateau suggests that those changes are due to global or regional
climate change rather than solely the result of uplift of the Tibetan Plateau.
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INTRODUCTION
Uplift of the Tibetan Plateau, has long been viewed as a major forcing factor in
regional and global climate change (e.g., Abe et al., 2005; An et al., 2001; France-Lanord
and Derry, 1994; Molnar, 2005; Molnar et al., 1993; Raymo and Ruddiman, 1992;
Ruddiman et al., 1997). Uplift is also thought to have directly driven environmental
change on the Tibetan Plateau (e.g., Liu, 1980; Wang et al., 2006; Zhang et al., 1981; Zhu
et al., 2004). However, recent work suggests that global climate change drives climate
and environmental change on the Tibetan Plateau (e.g., Dupont-Nivet et al., 2007).
These new studies call into question the direct link between uplift and environmental
change on the Tibetan Plateau. Moreover, uplift histories of the Tibetan Plateau based on
faunal or floral associations differ significantly from those based on stable isotope and
other quantitative paleoelevation studies. Paleo-floral assemblages from Pleistocene
deposits on the Tibetan Plateau are consistent with modern floral assemblages at low
elevations (e.g., Axelrod, 1981; Li and Zhou, 2001a, b; Meng et al., 2004; Molnar, 2005;
Wang et al., 2006; Xu, 1981; Zhang et al., 1981) and are used to argue for recent uplift of
the Tibetan Plateau. A similar argument is based on the abundance of mammal megafauna on the Tibetan Plateau in the late Miocene – Pliocene and their relative paucity
now (e.g., Cao et al., 1981; Li and Li, 1990; Meng et al., 2004; Wang et al., 2008c; Zhang
et al., 1981). In contrast, other lines of evidence indicate that the southern Tibetan
Plateau has been at high elevations since at least the mid-Miocene (Currie et al., 2005;
Garzione et al., 2000a; Rowley et al., 2001; Spicer et al., 2003) and central Tibetan
203
Plateau since at least the Oligocene (Cyr et al., 2005; DeCelles et al., 2007; Dupont-Nivet
et al., in press; Graham et al., 2005; Rowley and Currie, 2006).
The environmental effects of tectonics and climate change can best be addressed
in basins that contain all of the proxies mentioned above: pollen, leaf fossils, mammal
fossils, and carbonates used in stable isotope studies. A case in point is the Zhada basin
in southwestern Tibet. However, a lack of coherent, comprehensive basin analysis
integrating all the paleoenvironmental proxies has hampered efforts to untangle the
climatic and tectonic signals in the Zhada record. The Zhada Formation is described as
both upward-fining (Li and Zhou, 2001b; Zhang et al., 1981; Zhou et al., 2000) and
capped by boulder conglomerates (Zhu et al., 2007; Zhu et al., 2004). There is similarly
little consensus regarding the basin’s tectonic origin. The Zhada basin is presented as
having developed as a supradetachment basin above the South Tibetan Detachment
system (Wang et al., 2004), as a flexural basin responding to arc-perpendicular
compression (Zhou et al., 2000), or as a half-graben produced by arc-perpendicular
rifting (Wang et al., 2008a) which was followed by uplift (Zhu et al., 2004). Until
recently, the Zhada basin was understood to have been at low elevations until as late as
the Pleistocene (e.g., Li and Zhou, 2001a; Zhang et al., 1981; Zhou et al., 2000; Zhu et
al., 2004).
In several recent papers (Saylor et al., in review; Saylor et al., in press) we have
documented the chronostratigraphy and geodynamic mechanism of formation of Zhada
basin. Here we provide basin-wide lithologic and sequence stratigraphic correlations,
frequency analysis of the record of environmental change, and a detailed isotope
204
stratigraphy. Our results suggest that global climate change, possibly in conjunction with
regional climate change, controlled environmental variability in the southwestern Tibetan
Plateau during the late Miocene - Pleistocene. The data also point to the possibility of
establishing a high-resolution climate record for this high-elevation basin extending from
the Pleistocene to the Miocene. Finally, paleo-environmental data are used to address the
sharply differing uplift histories proposed for the southern Tibetan Plateau.
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REGIONAL GEOLOGICAL SETTING
The Zhada basin is the largest late Cenozoic sedimentary basin in the Himalaya.
It is located just north of the high Himalayan ridge crest in the western part of the orogen
(~32° N, 82° E, Figure 4.1A). The basin measures at least 150 km long and 60 km wide
and the current outcrop extent of the basin fill is at least 9,000 km2 (Figure 4.1B).
The Zhada basin is located in a zone of active arc-parallel extension (Murphy et
al., 2002; Ni and Barazangi, 1985; Thiede et al., 2006; Valli et al., 2007; Zhang et al.,
2000). It is bounded by the South Tibetan Detachment system (STDS) to the southwest,
the Indus suture to the northeast, and the Leo Pargil and Gurla Mandhata gneiss domes to
the northwest and southeast, respectively (Figure 4.1B). The STDS is a series of northdipping, low-angle, top-to-the-north normal faults that place low-grade metasedimentary
rocks of the Tethyan sequence on high-grade gneisses and granites of the Greater
Himalayan sequence. Along strike, ages for movement on the STDS range from 21 – 12
Ma (Cottle et al., 2007; Hodges et al., 1996; Hodges et al., 1992; Murphy and Yin, 2003;
Murphy and Harrison, 1999; Searle and Godin, 2003; Searle et al., 1997). To the
northeast of the Zhada basin the Oligo-Miocene Great Counter Thrust, a south-dipping,
top-to-the-north thrust system, cuts the Indus Suture (e.g., Ganser, 1964; Murphy and
Yin, 2003; Yin et al., 1999). Exhumation of the Leo Pargil and Gurla Mandhata gneiss
domes (Figure 4.1B) by normal faulting began 9-10 Ma (Murphy et al., 2002; Saylor et
al., in review; Thiede et al., 2006; Zhang et al., 2000) and continues today.
The Zhada Formation fills the Zhada basin and consists of > 800 m of fluvial,
lacustrine, eolian and alluvial fan deposits. The sedimentary basin fill is undisturbed and
206
forms an angular or buttress unconformity with underlying Tethyan sequence strata that
were previously shortened in the Himalayan fold-thrust belt (Saylor et al., in review).
The Zhada Formation is capped by a geomorphic surface that is extends across the basin
and is interpreted as a paleo-depositional plain that marks the maximum extent of
sedimentation prior to integration of the modern Sutlej River drainage network. After
deposition, the basin was incised to basement by the Sutlej River, exposing the entire
basin fill. The best estimate for the age of the Zhada Formation is between ~ 9.2 and < 1
Ma based on vertebrate fossils and magnetostratigraphy (Figure 4.2)(Lourens et al., 2004;
Saylor et al., in press; Wang et al., 2008b).
207
METHODS
Sedimentology
We measured 14 stratigraphic sections spanning the basin extent from the Zhada
county seat in the southeast to the Leo Pargil Range front in the northwest (Figure 4.1B).
Sections were measured at centimeter scale.
Correlations
The geomorphic surface which caps the Zhada Formation, being correlative
across the basin, provides the datum for sequence stratigraphic and lithologic correlation.
Correlations are based on major stratigraphic members that can be physically traced
(Saylor et al., in review). Magnetostratigraphy linking the South Zhada, Southeast
Zhada and East Zhada sections provides additional constraints. A final independent
constraint is the switch from exclusively C3 to mixed C3 and C4 vegetation which is
observed between 130 and 230 m in the South Zhada section and at ~ 300 m in the East
Zhada section (Saylor et al., in press). The expansion of C 4 vegetation is observed across
the Indian subcontinent and southern Tibet at ~ 7 Ma (France-Lanord and Derry, 1994;
Garzione et al., 2000a; Ojha et al., 2000; Quade et al., 1995; Quade et al., 1989; Wang et
al., 2006).
Frequency analysis of Zhada Formation cycles
The sedimentological record of the Zhada Formation archives the cyclical
expansion and contraction of a large paleolake. Frequency analysis was conducted by
208
spectral analysis and also by calculation of the average duration of cycles. In order to
apply spectral analysis to this record, a waveform was created by assigning numerical
values to each of the depositional environments as follows: fluvial and alluvial fan
associations, 5; supra-littoral associations, 4; littoral associations, 3; profundal
associations, 2 or 1 based on the presence or absence of terrestrial clastic or plant
material, respectively. Depositional environments in the South Zhada measured section
were identified at 0.5 m increments or where the depositional environment changed. The
series was converted from the depth domain to the time domain by linear interpolation
between magnetostratgraphic tie points, justified by the generally linear
subsidence/sediment accumulation rates (Saylor et al., in review). The result is a clipped
waveform with uneven sample spacing and temporal resolution better than 4,000 yrs
(Figure 4.3, Table 4.1). Progradation of basin margin depositional environments leads to
waveform saturation and loss of resolution at ages < 3.3 Ma. In order to evaluate the
effect of this saturation on spectral analysis, both the 5.23 – 2.581 Ma and the 5.23 – 3.3
Ma intervals were analyzed (labeled ―Entire Series‖ and ―Short Series‖ in Figure 4.3,
respectively).
The Lomb-Scargle Fourier transform method was applied using the SPECTRUM
program which allows analysis of unevenly spaced time series without interpolation
(Schulz and Stattegger, 1997). We conducted univariate spectral analysis (Welch
method), and harmonic analysis to determine the dominant frequencies in the record.
Cross-spectral analysis was used to determine the coherence between the Zhada record
and the record of summer insolation for 65°N (Laskar et al., 2004).
209
Stable Isotopes
Stable isotopes of oxygen and carbon (expressed as δ 18O and δ13C in units ‰,
respectively, and referenced to Vienna PeeDee Belemnite (VPDB) or Vienna Standard
Mean Ocean Water (VSMOW)) are a sensitive indicator of hydrologic conditions. The
principal controls on surface water δ18O (δ18Osw) values in southern Tibet are increasing
elevation (which decreases δ18Osw values) and evaporation (which increases δ 18Osw
values) (Dansgaard, 1954; Dansgaard, 1964; Garzione et al., 2000b; Poage and
Chamberlain, 2001; Rowley and Garzione, 2007; Rowley et al., 2001; Rozanski et al.,
1993). Freshwater gastropods precipitate shells with oxygen-isotopic ratios (δ18Occ) in
equilibrium with ambient water, dependent on the temperature-dependent fractionation
factor (Fritz and Poplawski, 1974; Leng et al., 1999) between aragonite and water. The
δ13C values of gastropod shells (δ13Ccc) are controlled by the δ13C value of dissolved
inorganic carbon (δ13CDIC) in the ambient water (Bonadonna et al., 1999; Lemeille et al.,
1983; Leng et al., 1999). The δ13CDIC value is controlled primarily by the residence time
of water and secondarily by factors including the local vegetation and substrate. The
δ13CDIC value of surface water is increased by photosynthesis or equilibration with the
atmosphere (Li and Ku, 1997; Talbot, 1990). Particularly in productive lakes, increased
water residence time increases the δ13CDIC value. Thus, both δ13Ccc and δ18Occ values of
gastropod shells are useful in reconstructing paleo-hydrologic and paleo-environmental
conditions (e.g., Abell and Williams, 1989; Hailemichael et al., 2002; Purton and Brasier,
1997; Smith et al., 2004).
210
Fossil gastropod shell fragments and intact shells were collected from fluvial,
marshy, and lacustrine intervals from the lower ~ 650 m in two measured sections.
Shells were powdered and homogenized prior to analysis. To check for preservation of
biogenic aragonite, 12 representative gastropod samples from fluvial, lacustrine and
marshy intervals were powdered and analyzed using the University of Arizona’s D8
Advance Bruker X-ray powder diffractometer (Saylor et al., in press).
We measured δ18Occ and δ13Ccc values using an automated carbonate preparation
device (KIEL-III) coupled to a gas-ratio mass spectrometer (Finnigan MAT 252).
Powdered samples were reacted with dehydrated phosphoric acid under vacuum at 70°C.
The isotope ratio measurement is calibrated based on repeated measurements of NBS-19
and NBS-18 and precision is ± 0.1 ‰ for δ18O and ±0.06‰ for δ13C (1 ).
211
RESULTS
Sedimentology
We identify 14 lithofacies associations and five depositional-environment
associations based on lithology, texture and sedimentary structures (Table 4.2). Unless
otherwise indicated, all deposits are laterally continuous for 100’s of meters to several
kilometers. Only abbreviated descriptions and interpretations are presented here; for
details see chapter 2.
Depositional Cycles in the Zhada Formation
Deposits in the Zhada Formation occur in two types of cycles that mark periods of
lake or wetland expansion and contraction. The bulk of a typical Type-A cycle (Figures
4.4A and 4.5A) consists of a 1 – 10 m thick unit of fluvial or alluvial fan sandstone or
conglomerate unit (lithofacies association F1 or rarely A1-A4) with an erosional base, no
grain-size trend and a capping, upward-fining sandstone bed (lithofacies association F2 or
occasionally S1). This is overlain by an organic-rich, fine-grained unit that contains
convoluted bedding (lithofacies association F3).
An idealized Type-B cycle (Figures 4.4B and 4.5B, C) is characterized by an
upward coarsening succession of, in ascending order, fossil-rich siltstone (lithofacies
association L1), laminated or massive siltstone or sandy turbidites (lithofacies association
P1-P2), rippled and cross-stratified sandstone (lithofacies association L2), sandstone
containing planar, trough or climbing-ripple cross-stratification (lithofacies association
F2 or S1-S3) and conglomerate beds (lithofacies associations A1-A4 or F1). The
212
uppermost sandstone beds have both erosional and gradational basal surfaces. The basal
surface of the capping conglomeratic unit is either erosional or marked by soft-sediment
deformation. Organic-rich convoluted siltstone (lithofacies association F3) can occur at
any point within the capping sandstone or conglomerate succession. In all cases, the
boundary between the fluvial or alluvial fan association and littoral or profundal
association is abrupt while the transition from the profundal association to the fluvial or
alluvial fan association is gradational, indicating a rapid transgression followed by
gradual progradation. Parts of both Type-A and -B cycles may be missing from the
idealized version depicted in Figure 4.4.
Correlations
Type-A and -B cycles stack in predictable patterns within a larger sequence
stratigraphic hierarchy (Figure 4.6). Because of the difficulty of establishing a hierarchy
of continental sequences based on sequence duration as determined in marine sequences
(e.g., Vail et al., 1991), we follow Catuneanu (2006) and establish a unique hierarchy for
Zhada basin. The Zhada Formation does not have significant intraformational
unconformities which might represent extended periods of non-deposition or extensive
sub-aerial exposure and erosion. However, it does have unconformities that represent
rapid progradation of basin margin facies and occasionally non-deposition of a lithofacies
association. It is on the basis of these minor unconformities and associated shifts in
depositional environments that we define sequences of all orders.
213
At the finest scale, 56 Type-A and –B cycles are present in the Zhada Formation
(Figure 4.7). Four second order sequences are evident above the cycles described above.
Nomenclature used identifies sequence order, systems tract or bounding surface, and
stratigraphic position from lowest to highest (hence 2HST2 is the second order highstand
systems tract second from the base of the Zhada Formation). Second order lowstand
systems tracts (2LST-) are characterized by Type-A cycles arranged in a retrogradational
stacking pattern (Figure 4.6). They are fluvially dominated and become increasingly
marshy upsection. Second order transgressive surfaces (2TS-) are identified by an abrupt
transition to thick, profundal claystone (Figure 4.8). Modern Tibetan lakes are typically
broad, shallow, and occur on low-relief plains. The lateral continuity of depositional
units implies that these conditions also existed during deposition of the Zhada Formation.
When transgression occurred, it would have quickly flooded the depositional plain
resulting in rapid retrogradation. As a result, second order transgressive systems tracts
(2TST-) are thin. They are characterized by Type-B cycles arranged in a retrogradational
stacking pattern and are capped by widespread profundal lacustrine sedimentation.
Second order highstand systems tracts (2HST-) are characterized by Type-B cycles
arranged in prograding or aggrading stacking patterns. At the coarsest scale, the entire
Zhada Formation can be seen as a first-order sequence (~ third order sequence of Vail et
al., 1977). 1LST is below the first major lacustrine transgression and is composed of
2LST1 and 2TST1. 1TST occurs between the first major lacustrine transgression and the
most widespread profundal lacustrine sedimentation (maximum flooding surface) and is
composed of 2HST1, 2LST2, and 2TST2. 1HST occurs between the most widespread
214
maximum flooding surface and the top of the Zhada Formation and is composed of
2HST2, 2LST3 - 2HST3, and 2LST4 – 2HST4.
Type-A and –B cycles can be correlated from stratigraphic sections spanning the
entire thickness of the Zhada Formation (South Zhada and Guga sections) toward the
basin margins. Sediment accumulation was greatest in the region of the South Zhada and
Guga sections. However, the maximum thicknesses of fine-grained material were
deposited to the northwest of there, in the region of the Namru Road West section. The
implication is that, though subsidence was greatest in the region of the South Zhada and
Guga sections, these were also close to the source of coarse-grained material (identified
by Saylor et al. (in review) as both the Kailash region to the north of the basin and also
the mountain ranges immediately surrounding the basin).
Frequency analysis of Zhada Formation cycles
The best time control based on magnetostratigraphy in the Zhada basin is between
chrons 2An (2.581 Ma) and 3n (5.23 Ma)(Lourens et al., 2004). Twenty-eight Type-B
cycles occur within this interval, each with an average duration of 95 kyr. Spectral
analysis of both the entire series and the 5.23 - 3.3 interval indicates statistically
significant peaks at 91.7 kyr at the 95% confidence level and at 22.4 kyr at the 85%
confidence level (Figure 4.9A). Harmonic analysis of the entire series reveals peaks at
91.7 ± 2, 126 ± 4, 140 ± 4, 221 ± 12, 379 ± 40, 662 ± 287, and 1330 ± 2000 kyr at the 99
% confidence level (Figure 4.9B). However, in the analysis of the 5.23 – 3.3 Ma interval
all of these peaks except for the 379 and 91.7 kyr peaks are suppressed (Figure 4.9C).
215
This indicates that the suppressed peaks are likely the result of red noise due to waveform
saturation at ages < 3.3 Ma. Coherence analysis of the shorter interval also reveals peaks
at 80 ± 13, 26 ± 1, and 18.9 ± 0.6 kyr (Figure 4.9D).
Stable Isotopes
X-ray diffraction analysis from 11 of 12 samples yielded only aragonite peaks
(Saylor et al., in press). The twelfth sample was too small to yield results. The δ18Occ
values of samples that we analyzed using X-ray diffraction ranged from -20.3 to +0.2 ‰
(VPDB)
Clearly identifiable trends in multiple cycles were found only in the densely
sampled South Zhada section, particularly in the 250 – 470 m interval of the South Zhada
section where we focus our discussion. For analysis of the entire data set see Saylor et al.
(in press). δ13Ccc values of gastropods in this interval range from –3.3 to +2.1‰ (VPDB)
and δ18Occ values from -13.7 to +0.7‰ (VPDB, Table 4.3).
Seventeen Type-B cycles occur within the 250 – 470 m interval of the South
Zhada Section (Figure 4.10). Of those, eight had sufficient sampling densities that trends
in δ18Occ values should be evident. Five cycles show a clear trend of increasing δ18Occ
values with stratigraphic height above the cycle boundary (Figure 4.10). One additional
cycle shows a similar, but muted, trend (Figure 4.10). The final two cycles do not show
any trend in δ18Occ values (Figure 4.10).
216
INTERPRETATION OF ZHADA FORMATION CYCLES
Zhada Formation Type-A and –B cycles are best interpreted as parasequences.
Parasequences are typically thin (<20 m) and correspondingly short lived (~100 kyr). We
conclude that facies are controlled primarily by lake or wetland expansion and
contraction which are related by the interplay of sedimentation and base-level change at
the shoreline. This is most evident in Type-B parasequences where flooding surfaces are
easily identifiable as the sharp basal contact of the fine-grained, fossil-rich, often papery
interval capping a coarser grained unit (Figure 4.4B). In Type-A parasequences flooding
is probably recorded by the transition from the fluvial association to marshy deposits of
the supra-littoral association rather than by an abrupt surface as in Type-B parasequences
(Figure 4.4A). However, Type-A parasequences have clearly identifiable erosive
surfaces which can be correlated to sub-aerial exposure surfaces in Type-B
parasequences (Figures 4.4, 4.5, 4.6). Thus, the maximum regressive surface in both
Type-A and -B parasequences is defined as the erosional surface at the base of the
coarsest grained interval, if an erosional surface is present, or at the base of the lowest
sandy interval showing signs of unidirectional traction transport if no erosional surface is
present.
Type-A parasequences occur at the base of Zhada Formation sequences (Figure 4.
6). Marshy deposits become more prominent components of Type-A parasequences
higher in the sequences consistent with the general retrogradational stacking pattern. The
upward-fining textural trend, retrogradational stacking pattern and location at the base of
the Zhada Formation sequences suggest that Type-A parasequences represent onset of
217
lacustrine transgression. The associated rise in the water table resulted in increased
marshy conditions, although the system was still dominated by fluvial processes (e.g.,
Bohacs et al., 2000).
Type-B parasequences occur in the middle to upper Zhada Formation (Figure 4.6)
and coarsen upward from a profundal lacustrine lithofacies association to a supra-littoral
or fluvial lithofacies association. Thus, they represent progradational parasequences in a
lacustrine setting. The persistence of these cycles to the top of the Zhada Formation
indicates that lacustrine conditions prevailed until the onset of incision by the modern
Sutlej River despite progradation causing replacement of the fine-grained littoral or
supra-littoral deposits by basin-margin alluvial fans.
Zhada Formation cycles obey Walther’s Law. Within individual cycles, facies
that are superposed occurred side-by-side spatially (e.g. Middleton, 1973; Posamentier
and Allen, 1999). This is consistent with sequence stratigraphy theory (Van Wagoner et
al., 1988) but contrasts with reports of non-Waltherian cycles from the Green River
Formation and underfilled lacustrine basins in the Qaidam basin and Death Valley
(Lowenstein et al., 1998; Pietras and Carroll, 2006; Yang et al., 1995).
218
DISCUSSION
Sequence Stratigraphic and Lithostratigraphic Correlations
The overfilled, balanced-fill and underfilled intervals of the Zhada basin were
delineated using definitions modified from Bohacs et al. (2000). In contrast to the
evaporative facies association presented by Bohacs et al. (2000) as typical of underfilled
lake basins, evaporites are present, though not dominant within the Zhada sections. It
may be argued that no sections were measured in the basin center and so the possibility
exists that that is the locus of evaporite deposition. However, that is unlikely given the
lateral facies continuity in the Zhada Formation and the number of measured sections
close to the basin center. A more plausible explanation is that discharge by the paleoSutlej River was consistently large relative to basin volume and that the lake rarely
desiccated (e.g.: Lake Naivasha, Barton et al., 1987; Duhnforth et al., 2006).
The overfilled interval was determined based on the prevalence of fluvial input,
indistinctly expressed parasequences (Bohacs et al., 2000), and the dominance of
sedimentary structures indicating traction transport. The overfilled interval extends from
the base of the section to 1TS (which is the same surface as 2TS1, Figure 4.7). δ18Occ
values from this interval are extremely negative due to the low water-residence times
associated with river throughflow (Saylor et al., in press).
The balanced-fill interval is identified by a dominantly retrogradational
parasequence stacking pattern. The balanced-fill interval is characterized primarily by
the rising water table inferred from the increased prevalence of marshy intervals. Though
the basin was intermittently open, fluvial influx was greater than efflux via outflow and
219
evaporation and so the basin was being slowly drowned. The balanced fill interval
extends from 1TS to 2MFS1 (Figure 4.7). The trend towards more positive δ 18Occ values
in this interval and the inferred increase in water residence times (Saylor et al., in press)
are consistent with this interpretation.
The underfilled interval is well represented in the Zhada Formation and was
identified based on the occurrence of well-expressed flooding surfaces that separate
distinct lithologies. Parasequences are well developed and record a combination of
progradational and aggradational stacking patterns. Depositional geometries (flooding
surfaces) are generally parallel or sub-parallel and well-expressed parasequences
converge and become indistinct towards the basin center. Within parasequences,
transgressive deposits are thin (< 0.5 m) or absent, whereas progradational deposits are
thick, well-developed, and dominated by traction transport (oscillatory current ripples,
climbing ripples). The underfilled interval extends from 2MFS1 to the paleo-depositional
surface.
Type-B parasequences occur primarily in the underfilled portion of the Zhada
basin. However, they differ from previous descriptions of underfilled basin lithofacies
(e.g., Bohacs et al., 2000; Carroll, 1998). The primary difference is that coarse-grained
facies are presented as the result of transgression by Bohacs et al. (2000) and Carroll
(1998), whereas in Zhada basin they typically constitute the regressive portion of the
parasequnce. There are several reasons for interpreting coarse-grained facies as the
regressive part of the cycle in Zhada basin. Unlike the cycles presented by Bohacs et al.
(2000) and Carroll (1998), fine-grained, sub-aerial exposure surfaces do not directly
220
underlie the coarse-grained facies. Rather, the profundal lacustrine facies coarsen
upwards gradually and shows evidence of traction transport, including oscillatory-current
ripples, throughout regression. The coarse-grained facies exhibit evidence of sub-aerial
exposure including preferential weathering and cementation, and root traces.
Additionally, the coarse-grained facies are often interbedded with organic-rich siltstone
and sandstone facies (lithofacies association F3) indicative of marshy wetlands such as
might occur on lake margins or between fluvial channels. We therefore interpret the
coarse-grained facies as the maximum progradation of lake-margin depositional
environments (Figures 4.4C and D, panel I). One possible explanation for the difference
between Type-B cycles and those in the basins studied by Bohacs et al. (2000) and
Carroll (1998) is that fluctuations in influx were not as great in Zhada basin as and that
the Zhada basin rarely desiccated. If water, and thus sediment, influx were relatively
stable, regression would be marked by progradation and during maximum regression,
which marks the time when the lake has the smallest volume and is the most restricted,
the relative influence of fluvial input would be greatest.
The evolution of Zhada basin followed a typical pattern from a fluvial system to
an underfilled lacustrine basin (Figure 4.11)(Bohacs et al., 2000). However, the top of
the Zhada formation is dominated by coarse-grained, basin-margin equivalents of Type-B
sequences. There is no change in large-scale sedimentary environment indicated prior to
an abrupt truncation of the Zhada Formation by a paleo-depositional plain. By
implication, there was no return to a balanced-fill or overfilled basin type. The return to
221
fluvial conditions often observed was discontinuous in that it bypassed the balanced-fill
and overfilled intervals (Figure 4.11).
Frequency analysis
The two independent time-series analyses above indicate that ~ 100 kyr cycles are
present in the Zhada Formation. In addition to a peak at 91.7 kyr, spectral analysis
reveals a peak at 22 kyr. These are within 1/2 bandwidth (6 dB bandwidth = 2.4) of the
eccentricity and precession frequencies. It is possible that the 100 kyr signal is the result
of clipping of a 20 kyr precession signal (Figure 4.3)(e.g., Weedon, 2003 and references
therein). However, in either case climate change related to orbital cyclicity has been
recorded in environmental change on the Tibetan Plateau.
Harmonic analysis does not reveal the 22 kyr peak indicated by univariate
analysis but does show peaks at 91.7 and 379 kyr, both of which are consistent with the
eccentricity cycle (Figures 4.9B and C). Finally, coherence analysis shows both
eccentricity and precession frequencies (Figure 4.9D).
Second-order sequences in the Zhada Formation are of ambiguous origin, but
parasequences are consistent in duration with insolation-driven climate changes (fourth
order sequences of Vail et al., 1977) due to changes in the orbital characteristics of the
earth (i.e., Milankovitch cycles). If parasequences are representative of Milankovitch
cycles, the driving process behind environmental cyclicity in the Zhada basin was not
tectonics. Rather, lacustrine expansion and contraction was caused by a change in the
precipitation to evaporation ratio linked to strengthening or weakening of the monsoon
222
due to (respectively) increases or decreases in insolation (Gupta et al., 2001; Shi et al.,
2001; Thompson et al., 2006). It is not surprising that climatically driven sequences are
most distinctly expressed in the underfilled interval of the Zhada Formation because
during this interval the lake would be most susceptible to changes in hydrology (Bohacs
et al., 2000; Kelly, 1993).
Isotopes in Zhada Formation cycles
Lakes respond to changes in hydrology on much shorter time-scales than do
oceans because they have much smaller water and sediment volumes (see Bohacs et al.,
2000; Kelts, 1988; Sladen, 1994). Additionally, in lacustrine settings the relative
proportion of water influx and efflux (usually climatically driven) and movement on
faults (tectonically driven) are both primary controllers of systems tracts. Finally,
sediment supply is linked to water influx. This creates the paradoxical situation where
the influx of water and lake volume can be high, and yet lake volume can be decreasing
(net evaporation > net influx, Figures 4.4C and D).
Water and sediment influx are thus decoupled from base-level changes but are a
primary control of shoreline trajectory and therefore on parasequence evolution. As
mentioned above, lithofacies distribution and thus lithologic stacking patterns appear to
be controlled primarily by the location of the shoreline and so are also decoupled from
base-level. This means that parasequence flooding surfaces correspond to lake expansion
due to a drop in Evaporation/Precipitation. Thus, the lowest δ18Occ values of aquatic
gastropods and, implicitly, of the lake water, are found at the flooding surface even
223
though the coarsest material is associated with maximum regression (Figure 4.12).
Particularly when the basin was underfilled, the highest values occur at the time of
maximum regression (Figures 4.4D and 4.10). This apparent discrepancy can be
accounted for by understanding that though water and sediment influx were both
relatively high and stable, climatically driven Evaporation/Precipitation controlled lake
level and thus the δ18Osw value of lake water. When efflux was greater than influx, the
δ18Osw value increased, the lake shrank, and the coarse grained material was carried
further into the basin (Figures 4.4C, panel I). Conversely, when influx was greater than
efflux, the δ18Osw value decreased, the lake grew, and the coarse grained material was
trapped at the basin margins (Figures 4.4C, panel II).
The foregoing discussion indicates that the primary control of δ 13Csw and δ18Osw
values was volume-weighted average water residence time. Just prior to flooding, when
lake volume was small, the average water residence time, and hence δ 13Csw and δ18Osw
values, was significantly altered by addition of a small volume of water. On the other
hand, after significant flooding, the lake was sufficiently large and the water sufficiently
evolved that the continued input of water during flooding had only a minor effect on
average water residence time. Though the discussion above refers primarily to individual
parasequences, the effect may span several parasequences and point to climatic control at
multiple frequencies (Figure 4.10).
The correlation between low δ18Osw values and flooding described above is
confirmed in the modern analog of Kungyu Co. Water samples collected on 25 July,
2006 from the lake and from the sole river flowing into the lake had δ 18O values of -14.8
224
and -15.6 ‰ (VSMOW), respectively. Stranded shorelines with aquatic grasses and
evaporites on the lake margins showed that the lake was recently at higher levels. The
samples were collected at the start of the monsoon season and the interpretation is that
the lake level had fallen to extremely low levels and was now in the process of refilling
(Figure 4.4D: black star denotes the interpreted location of Kungyu Co within the
filling/emptying cycle at the time of sampling).
Basin history
Combining the observations made above with previous studies (Saylor et al., in
review; Saylor et al., in press) points to the following basin history. Through arc-parallel
extension a sill was created which caused ponding of the river, leading to deposition of
the lowest strata of the Zhada Formation. The accumulating sediment onlapped the preexisting Tethyan Sequence topography, forming the observed buttress or angular
unconformities. The ancestral Sutlej River continued to flow from its source, increasing
the sediment pile. The exhumation rate of the Leo Pargil/Qusum range to the northwest
of the Zhada basin between 10 and 5.6 Ma is the same as the sediment accumulation rate
in the Zhada basin (Saylor et al., in review; Thiede et al., 2006), indicating that the
uplifting range may have acted as a sill. After 5.6 Ma both the exhumation rate and the
sediment accumulation rate increased, the basin became closed, and lacustrine
sedimentation commenced. These conditions continued, despite progradation of basinmargin alluvial fans, until a new sill was eventually breached at < 1 Ma. At this point,
the system abruptly returned to fluvial conditions and began incising through the Zhada
225
Formation. The sudden return to fluvial conditions via the integration of the modern
Sutlej River system truncated the typical basin evolution pattern described by Bohacs et
al. (2000).
Global climate change and its impact on the southern Tibetan Plateau
Numerous authors have reported 100 and 400 kyr cycles in the Miocene (Di
Celma and Cantalamessa, 2007; Holbourn et al., 2007; Kashiwaya et al., 2001; Van
Wagoner et al., 1988; Zachos et al., 2001), although none from high elevations such as
the Tibetan Plateau.
The Zhada Basin therefore presents an excellent opportunity to
study high-frequency climatically driven environmental change at high-elevations in the
Miocene - Pleistocene. Expansion and contraction of lakes and wetlands has been linked
to variability in the strength of the Asian monsoon (Shi et al., 2001). The Quaternary
Asian monsoon is thought to be modulated by orbital cyclicity (Clemens et al., 1991; Jian
et al., 2001; Nie et al., 2008; Prell and Kutzbach, 1992; Wang et al., 2005; Wang et al.,
2008d) though there is disagreement about which frequencies are dominant (Clemens and
Prell, 2003; Nakagawa et al., 2008). Data from this study support previous work
indicating that the monsoon has long varied at precessional frequencies (Bloemendal and
Demenocal, 1989). Our data also show enivronmental variation at eccentricity
frequencies (Dupont-Nivet et al., 2007).
We turn next to another challenge presented by the Zhada basin: the explanation
of the floral and faunal changes observed within the Zhada Formation and between the
late Miocene and the present. The Zhada basin contained a host of plants that are
226
typically thought of as native to warm, humid, and, by implication, low-elevation
climates (Li and Zhou, 2001a, b; Zhu et al., 2007; Zhu et al., 2004). Additionally, a
broad cross-section of mammal mega-fauna lived in Zhada including Hipparion
zandaense, Nyctereutes, Palaeotragus microdon and rhinoceri that have variously been
identified as Hyracodon or Dicerorhinus (Li and Li, 1990; Liu, 1981; Meng et al., 2004;
Zhang et al., 1981; X. Wang, pers. comm.; E. Lindsay, pers. comm.). This is in striking
contrast to the basin today, in which the only large mammalian fauna are the kiang
(Tibetan wild asses) and extremely rare chiru (small, long-horned antelope). However,
oxygen isotope analysis points to high elevations and a climate which was cold and arid;
very similar to the modern (Saylor et al., in press).
We compiled published pollen data from several measured sections around the
Zhada basin and correlated them with the South Zhada measured section. In the
following discussion, stratigraphic heights and ages are based on the South Zhada
section. The three lowermost sections come from the region of the Guga and South
Zhada sections (Yu et al., 2007; Zhu et al., 2007). Two additional sections are from the
region of the Namru Road West section and span the uppermost ~250 m of the Zhada
Formation (Zhang et al., 1981; Zhu et al., 2006). Due to differences in data presentation,
a precise correlation of the stratigraphy presented by Zhang et al. (1981) to the Namru
Road West section is not possible. Zhang et al. (1981), Zhu et al. (2006, 2007), and Yu
et al. (2007) note an increase in grass and shrub pollen and a decrease in arboreal pollen
which begins at ~ 170 – 230 m and culminates at ~ 630 m (Zhu et al., 2007). The
culmination is marked by the replacement of spruce and fir pollen with shrub between
227
~500 – 630m. This places the onset of the transition at ~ 7.2 – 6 Ma and its end at 3.6 2.6 Ma based on magnetostratigraphy (Figure 4.2) (Saylor et al., in press).
The transition from arboreal to non-arboreal dominated pollen associations in the
Zhada Formation has long been interpreted as indicating substantial cooling and drying
due to recent, rapid uplift of the southwestern Tibetan Plateau (Li and Zhou, 2001a, b;
Meng et al., 2004; Zhang et al., 1981). However, this same floral turnover was observed
in the low-elevation Gangetic foreland deposits of Nepal and northern India (the Siwalik
group, e.g., Awashi and Prasad, 1989; Hoorn et al., 2000; Mathur, 1984; Prasad, 1993;
Sarkar, 1989). Quade et al. (1995) and Hoorn et al. (2000) concluded that the decline in
arboreal species and increased prominence of grasses was part of the regional shift from
C3 to C4 dominated floral populations recorded by a regional increase in the δ 13Ccc values
of paleosol carbonates in northern India and Pakistan. The increase in δ 13Ccc values has
been observed across the northern Indian subcontinent (Cerling et al., 1997; Freeman and
Collarusso, 2001; Quade et al., 1995; Quade et al., 1989), in the Bengal fan (FranceLanord and Derry, 1994), in the southern Tibetan Plateau (Garzione et al., 2000a; Wang
et al., 2006; Yoshida et al., 1984), and in the northern Tibetan Plateau (Ma et al., 1998).
Significantly, it has also been observed in Zhada basin (Saylor et al., in press) and occurs
synchronously, within the sampling resolution, with the onset of the transition to nonarboreally dominated pollen assemblages. The expansion of C4 plants in the Zhada basin
thus appears to be related to a global phenomenon and points to a global climate change,
rather than local or even regional uplift (Cerling et al., 1997; Fox and Koch, 2003;
Freeman and Collarusso, 2001; Latorre et al., 1997; Pagani et al., 1999). In the low-
228
elevation Siwalik group, the change in habitat from forest to open grasslands and
savannas was accompanied by a major mega-faunal turnover (Barry et al., 1990; Barry et
al., 1995; Barry et al., 1991; Flynn et al., 1995; Flynn et al., 1998; Patnaik, 2003; Pilbeam
et al., 1996).
The recognition of Milankovitch cycles in the Zhada Formation indicates that
insolation driven global or regional climate change drove environmental changes in
basins at high elevations on the southern Tibetan Plateau. Thus, we can reasonably
expect that floral and faunal communities on the Tibetan Plateau would have responded
to global climate change. The shift from forest to grassland observed in the Zhada basin
was not the result of basin uplift, because an identical change is observed in lowelevation deposits in nearby northern India. Further, analysis of oxygen isotopes from
aquatic gastropod shells from the Zhada Formation indicates a probable decrease in
elevation of the basin since the late Miocene (Saylor et al., in press). A more likely
scenario is that a regional or global climatic change affected both low- and high-elevation
environments and favored a shift from C3-dominated forests to mixed C3 and C4 or C4dominated grasslands. However, it appears that the transition from forest to grassland
may have taken as long as 3.5 Myr in Zhada basin.
A possible scenario, then, is that the vegetation shift began at high elevations due
to a global or regional climate change. Suggested factors include the onset of rapidly
decreasing global temperatures in the latest Miocene – Pliocene (Zachos et al., 2001) or
increased monsoon intensity (Kroon et al., 1991) and associated increased aridity and
seasonality of precipitation (Garzione et al., 2003; Guo et al., 2002; Molnar, 2005).
229
Increased warm-season precipitation and increased aridity favor C4 grasses (An et al.,
2005). This situation continued, with variability due to strengthening or weakening of
the monsoon, until the onset of a rapid and sustained increase in monsoon intensity and
increased aridity between 3.6 - 2.6 Ma (An et al., 2005; An et al., 2001; Guo et al., 2002;
Wan et al., 2007; Zhu et al., 2007). Summer monsoon strength after 2.6 Ma is highly
variable (An et al., 2005; An et al., 2001; Kroon et al., 1991). The final transition to open
grassland may have been driven directly by this variability, increased aridity, or the
continuing decline in global temperatures. As is thought to be the case in the foreland,
the floral shift was accompanied by faunal change at high elevations. The possibility
remains that these climate changes were driven by expansion of the region of high
elevations, particularly on the northern and eastern margins of the Tibetan Plateau (e.g.,
An et al., 2001). However, any such models must take into consideration long-lived high
elevations in the southern and central Tibetan Plateau (Currie et al., 2005; Cyr et al.,
2005; DeCelles et al., 2007; Dupont-Nivet et al., in press; Garzione et al., 2000a; Rowley
and Currie, 2006; Rowley et al., 2001) and local or regional loss of elevation in the
southwestern Tibetan Plateau since the late Miocene (Saylor et al., in press). In
proposing this scenario, we suggest that continuing global cooling since the late Miocene
may account for the apparent paucity of C4 vegetation on the Tibetan Plateau currently
(Wang et al., 2008c).
230
CONCLUSIONS
1)
Lithologic cycles (Types-A and –B) in the Zhada basin are Waltherian
parasequences.
2)
Sedimentology and sequence stratigraphic analysis indicates that the Zhada basin
evolved from a fluvial system to an overfilled basin. The overfilled basin was marked by
a broad depositional plain dominated by wetlands bordering a large braided river. From
there, the basin evolved sequentially to a balanced-fill basin and an underfilled basin. The
final stage was marked by open lacustrine conditions which give way to prograding
basin-margin alluvial fans. The typical regression through the basin-type sequence was
bypassed by an abrupt return to fluvial conditions. This agrees with overtopping of a
basin sill and integration of the modern Sutlej drainage network (Brookfield, 1998;
Saylor et al., in review).
3)
Two orders of sequences are recognized in Zhada basin in addition to the
parasequences mentioned above. The first order sequence is the result of tectonically
created accommodation and infilling. The second order sequences are of ambiguous
origin but may be linked to continued fault movement.
4)
Where best expressed and dated parasequences have and average duration of ~92
kyr and are likely the result of climatic changes associated with Milankovitch cycles.
This is the first time that 100 kyr cycles have been reported for late Miocene – Pliocene
deposits on the Tibetan Plateau and presents an unparalleled opportunity to study highfrequency climate change at high elevations.
231
5)
Within parasequences, the lowest δ18Occ values and, by implication, of Zhada
paleo-lake water, are associated with flooding. From the flooding surface through
maximum regression δ18Occ values increase. This trend is the result of low
Evaporation/Precipitation during flooding and the associated increase in lake volume and
decrease in volume weighted average water residence time.
6)
The data presented in this paper are consistent with a tectonic origin of the Zhada
Basin. Possible tectonic originating causes include crustal thinning or tectonic damming
due to arc-parallel extension.
7)
It is likely that regional or global climate change, rather than basin uplift, was the
cause of the observed floral and faunal turnover in Zhada basin. The turnover, marked by
decreased arboreal pollen in favor of shrub and grass pollen and a decline in the megafaunal populations, is similar in age and character to that observed in other basins on and
surrounding the Tibetan Plateau. In the Himalayan foreland and elsewhere the turnover
is attributed to regional or global climate change. Though the introduction of C 4
vegetation had previously been documented on the southern Tibetan Plateau, the largescale change from forest to grassland and the accompanying change in fauna observed in
the low-elevation Siwalik group had not been extrapolated to high elevations.
232
Figure 4.1. A. Elevation, shaded relief and generalized tectonic map of the Himalayan –
Tibetan orogenic system showing the location of the Zhada basin relative to major
structures. B. Generalized geologic map of the Zhada region. Modified from published
mapping by Cheng and Xu (1987), Murphy et al. (2002; 2000) and unpublished mapping
by M. Murphy.
233
Figure 4.2. (A) South Zhada lithologic section and associated magnetostratigraphic
section and correlation to the global polarity timescale (GPTS) of Lourens et al. (2004).
234
235
Figure 4.3. The synthetic wave form constructed for spectral analysis. Depo-codes relate
to lithofacies associations (alluvial fan and fluvial associations were assigned a value of
5; supra-littoral associations, 4; littoral associations, 3; and profundal associations, 2 or 1
based on the presence or absence of terrigenous clastic or biologic material). At ages <
3.3 Ma the waveform saturates at values of 5 due to the infilling of Zhada paleo-lake and
the progradation of lake-margin depositional environments. Similarly, the inability to
distinguish fluctuations in water level during times of profundal or alluvial fan/fluvial
sedimentation results in clipping of the waveform. The record of insolation variation
(Laskar et al., 2004) is provided for comparison.
236
Figure 4.4. (A and B) Idealized forms of sequence types A and B and interpreted
depositional environments (C). (D) Simplified representation of the relationship between
lake level, water and sediment flux and lake δ 18O values. The simplifications involve the
assumption that endmember influx and efflux δ 18O values are invariant and that efflux via
evaporation is proportional to lake area. Vertical grey boxes indicate the time of
retrogradation (sediment influx<water influx) associated with flooding. The black star
denotes the temporal location of Kungyu Co within the systems tracts at the time of
sampling (25 July, 2006). The legend for sedimentary structures is found in Figure 10.
237
Figure 4.5. A: Type-A cycles. Cliff is ~15 m. B. Photomosaic of typical progradational
sequences in the lacustrine portion of the Zhada Formation (Nl). C. Type-B cycles.
Lowermost cycle is ~ 9 m. Lower slope-forming interval represents upward-coarsening
profundal to littoral mudstones and siltstones (P1, L1, L2) which are capped by cliffforming littoral or supra-littoral sandstones (L2 or S1).
238
239
Figure 4.6. Portion of Figure 4.7 showing detailed parasequence scale correlations. See
Figures 4.7 and 4.10 for legend.
240
241
242
Figure 4.7. basin-wide lithostratigraphic and sequence stratigraphic correlations for (a) a
north – south transect and (B) a southeast – northwest transect. See Figure 4.1B for
locations of transects.
243
Figure 4.8. Second order transgressive surface showing the abrupt transition from
lithofacies association S1 and F2 sandstone to lithofacies P1 profundal claystone.
244
Figure 4.9. (A.) Power spectrum of the entire interval (5.23 – 2.581 Ma). The spectrum
has peaks at ~100 kyr at the 95% confidence level and ~ 23 kyr at the 85% confidence
245
level. (B.) Harmonic analysis of the entire interval reveals dominant peaks at 379 and
91.7 kyr but also has significant red noise. (C.) Harmonic analysis of the interval from
5.23 – 3.3 Ma reveals the same dominant peaks but red noise peaks are significantly
suppressed. (D.) Coherence analysis reveals peaks at 80 ± 13, 26 ± 1, and 18.9 ± 0.6 kyr.
Vertical error bars indicate the 95% confidence interval.
246
Figure 4.10. South Zhada lithologic section and associated δ18O values of aquatic
gastropods. Horizontal black lines represent parasequence boundaries. Thick vertical
green boxes indicate the sequences that were used to construct Figure 12. Within all five
sequences where a trend is evident, δ18Occ values increase from the flooding surface to
247
the maximum regression surface. Vertical orange boxes indicate sequences which show a
possible, though not clear, trend. Vertical red boxes indicate sequences with sufficient
sampling density that trends in δ18Occ values may be expected but where no trends are
observed.
248
Figure 4.11. The trajectory of Zhada basin evolution in accommodation and sedimentand water-supply space. Also shown are fields which are occupied by overfilled,
balanced-fill and underfilled basins. The Zhada basin followed a typical evolutionary
pattern from fluvial to underfilled basin due to an increase in accommodation (Solid
black arrow A) until a new sill was breached (B). At this point the basin underwent a
discontinuous return to fluvial conditions, bypassing the usual progression back through
the balanced-fill and overfilled fields (Dashed black arrow C). Modified from Bohacs et
al. (2000).
249
250
Figure 4.12. δ18O and δ13C values (VPDB) of aquatic gastropods from five sequences
(indicated in Figure 10) are plotted against their normalized height above the flooding
surface. The lowest values occur just above the flooding surface and represent lake
expansion associated with a decrease in Evaporation/Precipitation. However, continued
evaporative enrichment and isotopic evolution means that δ18Occ and δ13Ccc values
increase through most of the regressive sequence.
251
Table 4.1. Data used in frequency
analysis
Stratigraphic
level (m)
251.41
251.91
252.21
252.41
252.91
253.06
253.41
253.91
254.41
254.51
254.91
255.41
255.91
256.41
256.91
257.41
257.91
258.41
258.91
259.41
259.91
260.06
260.41
260.91
261.41
261.91
262.41
262.76
262.91
263.41
263.91
264.41
264.91
265.41
Depositional
Abs. Age environment
(Ma)
code
5.23
2
5.226433
2
5.224292
3
5.222865
3
5.219298
3
5.218228
4
5.215731
4
5.212163
4
5.208596
4
5.207882
2
5.205029
2
5.201461
2
5.197894
2
5.194326
2
5.190759
3
5.187192
3
5.183624
3
5.180057
3
5.17649
3
5.172922
3
5.169355
3
5.168285
2
5.165788
2
5.16222
2
5.158653
2
5.155086
2
5.151518
2
5.149021
2
5.147951
4
5.144384
4
5.140816
4
5.137249
4
5.133682
4
5.130114
4
252
265.91
266.21
266.41
266.91
267.41
267.51
267.91
268.41
268.56
268.91
269.01
269.41
269.91
270.41
270.91
271.41
271.91
272.41
272.91
273.41
273.91
274.41
274.56
274.91
275.41
275.91
276.41
276.91
277.16
277.41
277.91
278.41
278.91
279.41
279.91
280.41
280.56
280.91
281.41
281.91
5.126547
5.124406
5.122979
5.119412
5.115845
5.115131
5.112277
5.10871
5.10764
5.105143
5.104429
5.101575
5.098008
5.094441
5.090873
5.087306
5.083739
5.080171
5.076604
5.073037
5.069469
5.065902
5.064832
5.062334
5.058767
5.0552
5.051632
5.048065
5.046281
5.044498
5.04093
5.037363
5.033796
5.030228
5.026661
5.023094
5.022023
5.019526
5.015959
5.012392
4
3
3
3
3
4
4
4
3
3
1
1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
3
3
3
3
3
3
3
3
2
2
2
2
253
282.41
282.76
282.91
283.41
283.91
284.41
284.91
285.41
285.91
286.31
286.41
286.91
287.21
287.41
287.91
288.01
288.41
288.91
289.41
289.91
290.41
290.91
291.41
291.91
292.41
292.91
293.41
293.91
294.41
294.91
295.41
295.91
296.41
296.91
297.06
297.41
297.91
298.41
298.91
299.41
5.008824
5.006327
5.005257
5.001689
4.998122
4.994555
4.990987
4.98742
4.983853
4.980999
4.980285
4.976718
4.974578
4.973151
4.969583
4.96887
4.966016
4.962449
4.958881
4.955314
4.951747
4.948179
4.944612
4.941045
4.937477
4.93391
4.930342
4.926775
4.923208
4.91964
4.916073
4.912506
4.908938
4.905371
4.904301
4.901804
4.898236
4.894669
4.891102
4.887534
2
3
3
3
3
3
3
3
3
4
4
4
3
3
3
1
1
1
1
1
1
1
3
5
5
5
5
5
5
5
5
5
5
3
3
1
1
1
1
1
254
299.91
300.41
300.91
301.41
301.81
301.91
302.41
302.56
302.91
303.41
303.91
304.41
304.91
305.41
305.66
305.91
306.41
306.66
306.91
307.21
307.41
307.91
308.41
308.91
309.41
309.91
310.41
310.91
311.41
311.91
312.16
312.41
312.91
313.41
313.91
314.41
314.91
314.96
315.41
315.91
4.883967
4.8804
4.876832
4.873265
4.870411
4.869697
4.86613
4.86506
4.862563
4.858995
4.855428
4.851861
4.848293
4.844726
4.842942
4.841159
4.837591
4.835808
4.834024
4.831884
4.830457
4.826889
4.823322
4.819755
4.816187
4.81262
4.809053
4.805485
4.801918
4.79835
4.796567
4.794783
4.791216
4.787648
4.784081
4.780514
4.776946
4.77659
4.773379
4.769812
1
1
1
1
3
3
3
4
4
4
4
4
4
4
3
3
3
4
4
1
1
1
1
1
1
1
1
1
1
1
3
3
3
3
3
3
3
4
4
4
255
316.41
316.91
317.41
317.81
317.91
318.41
318.91
319.41
319.91
320.41
320.91
321.16
321.41
321.91
322.06
322.36
322.41
322.91
323.41
323.91
324.41
324.91
325.41
325.91
326.41
326.91
327.41
327.91
328.41
328.76
328.91
329.41
329.76
329.91
330.41
330.91
331.01
331.41
331.91
332.41
4.766244
4.762677
4.75911
4.756256
4.755542
4.751975
4.748408
4.74484
4.741273
4.737705
4.734138
4.732354
4.730571
4.727003
4.725933
4.723793
4.723436
4.719869
4.716301
4.712734
4.709167
4.705599
4.702032
4.698465
4.694897
4.69133
4.687763
4.684195
4.680628
4.678131
4.677061
4.673493
4.670996
4.669926
4.666358
4.662791
4.662078
4.659224
4.655656
4.652089
4
4
4
5
5
5
5
5
5
5
5
4
4
4
3
1
1
1
1
1
1
1
1
1
1
1
1
1
1
4
4
4
5
5
5
5
3
3
3
3
256
332.91
333.21
333.41
333.91
334.41
334.91
335.41
335.91
336.41
336.91
337.41
337.51
337.91
338.41
338.91
339.41
339.91
340.41
340.91
341.06
341.41
341.91
342.41
342.91
343.41
343.91
344.41
344.91
345.41
345.91
346.41
346.91
347.41
347.91
348.41
348.91
349.16
349.41
349.91
350.01
4.648522
4.646381
4.644954
4.641387
4.63782
4.634252
4.630685
4.627118
4.62355
4.619983
4.616416
4.615702
4.612848
4.609281
4.605713
4.602146
4.598579
4.595011
4.591444
4.590374
4.587877
4.584309
4.580742
4.577175
4.573607
4.57004
4.566473
4.562905
4.559338
4.555771
4.552203
4.548636
4.545068
4.541501
4.537934
4.534366
4.532583
4.530799
4.527232
4.526518
3
1
1
1
1
1
1
1
1
1
1
3
3
3
3
3
3
3
3
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
4
4
4
5
257
350.41
350.91
351.41
351.91
352.41
352.91
353.41
353.86
353.91
354.41
354.76
354.91
355.41
355.91
356.41
356.91
357.41
357.91
357.96
358.41
358.91
359.41
359.76
359.91
360.41
360.91
361.41
361.91
362.41
362.91
363.41
363.56
363.91
364.41
364.91
365.41
365.91
366.41
366.91
367.41
4.523664
4.520097
4.51653
4.512962
4.509395
4.505828
4.50226
4.49905
4.498693
4.495126
4.492628
4.491558
4.487991
4.484424
4.480856
4.477289
4.473721
4.470154
4.469797
4.466587
4.463019
4.459452
4.456955
4.455885
4.452317
4.44875
4.445183
4.441615
4.438048
4.434481
4.430913
4.429843
4.427346
4.423779
4.420211
4.416644
4.413076
4.409509
4.405942
4.402374
5
5
5
5
5
5
5
3
3
3
5
5
5
5
5
5
5
5
4
4
4
4
3
3
3
3
3
3
3
3
3
5
5
5
5
5
5
5
5
5
258
367.91
368.41
368.91
369.41
369.91
370.41
370.91
371.41
371.91
372.41
372.76
372.91
373.41
373.91
374.41
374.91
375.41
375.91
376.41
376.91
377.41
377.91
378.11
378.41
378.91
379.41
379.91
380.41
380.91
381.16
381.41
381.91
382.41
382.91
383.41
383.91
384.41
384.91
385.41
385.76
4.398807
4.39524
4.391672
4.388105
4.384538
4.38097
4.377403
4.373836
4.370268
4.366701
4.364204
4.363134
4.359566
4.355999
4.352432
4.348864
4.345297
4.341729
4.338162
4.334595
4.331027
4.32746
4.326033
4.323893
4.320325
4.316758
4.313191
4.309623
4.306056
4.304272
4.302489
4.298921
4.295354
4.291787
4.288219
4.284652
4.281084
4.277517
4.27395
4.271453
5
5
5
5
5
5
5
5
5
5
3
1
1
1
1
1
1
1
1
1
1
1
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
1
259
385.91
386.11
386.41
386.61
386.91
387.41
387.91
388.06
388.41
388.51
388.91
389.41
389.51
389.91
390.01
390.41
390.91
391.41
391.91
392.41
392.91
393.41
393.76
393.91
394.41
394.51
394.91
395.41
395.91
396.41
396.91
397.41
397.91
398.41
398.46
398.71
398.91
399.41
399.71
399.91
4.270382
4.268955
4.266815
4.265388
4.263248
4.25968
4.256113
4.255043
4.252546
4.251832
4.248978
4.245411
4.244697
4.241844
4.24113
4.238276
4.234709
4.231142
4.227574
4.224007
4.220439
4.216872
4.214375
4.213305
4.209737
4.209024
4.20617
4.202603
4.199035
4.195468
4.191901
4.188333
4.184766
4.181199
4.180842
4.179058
4.177631
4.174064
4.171924
4.170497
1
4
4
1
1
2
2
3
3
4
4
4
3
3
1
1
1
1
1
1
1
1
2
2
2
4
4
4
4
4
4
4
4
4
1
2
2
2
3
3
260
400.41
400.91
400.96
401.41
401.91
402.41
402.91
403.41
403.91
404.41
404.91
405.41
405.91
406.41
406.46
406.91
407.36
407.41
407.91
408.21
408.41
408.91
409.41
409.76
409.91
410.41
410.91
411.11
411.41
411.91
412.41
412.91
413.41
413.91
414.41
414.91
415.41
415.91
416.41
416.91
4.166929
4.163362
4.163005
4.159795
4.156227
4.15266
4.149092
4.145525
4.141958
4.13839
4.134823
4.131256
4.127688
4.124121
4.123764
4.120554
4.117343
4.116986
4.113419
4.111279
4.109852
4.106284
4.102717
4.10022
4.09915
4.095582
4.092015
4.090588
4.088447
4.08488
4.081313
4.077745
4.074178
4.070611
4.067043
4.063476
4.059909
4.056341
4.052774
4.049207
3
3
5
5
5
5
5
5
5
5
5
5
5
5
3
3
3
5
5
2
2
2
5
3
2
2
2
3
3
3
3
3
5
5
5
5
5
5
5
5
261
417.41
417.91
418.41
418.91
419.41
419.91
420.41
420.91
421.41
421.91
422.41
422.91
423.41
423.91
424.41
424.91
425.26
425.41
425.91
426.36
426.41
426.91
427.11
427.41
427.91
428.01
428.41
428.91
429.41
429.91
430.41
430.91
431.41
431.91
432.41
432.76
432.91
433.41
433.91
434.21
4.045639
4.042072
4.038505
4.034937
4.03137
4.027803
4.024235
4.020668
4.0171
4.013533
4.009966
4.006398
4.002831
3.999264
3.995696
3.992129
3.989632
3.988562
3.984994
3.981784
3.981427
3.97786
3.976433
3.974292
3.970725
3.970011
3.967158
3.96359
3.960023
3.956455
3.952888
3.949321
3.945753
3.942186
3.938619
3.936122
3.935051
3.931484
3.927917
3.925776
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
3
3
3
4
4
4
3
3
3
1
1
1
1
1
1
1
1
1
1
2
2
2
2
4
262
434.41
434.86
434.91
435.41
435.91
436.41
436.91
437.41
437.91
438.16
438.41
438.91
439.41
439.91
440.16
440.41
440.91
441.41
441.91
442.41
442.91
443.41
443.91
444.41
444.91
445.41
445.91
446.41
446.91
447.26
447.36
447.41
447.91
448.41
448.91
449.41
449.91
450.41
450.91
451.41
3.924349
3.921139
3.920782
3.917215
3.913647
3.91008
3.906513
3.902945
3.899378
3.897594
3.895811
3.892243
3.888676
3.885108
3.883325
3.881541
3.877974
3.874406
3.870839
3.867272
3.863704
3.860137
3.85657
3.853002
3.849435
3.845868
3.8423
3.838733
3.835166
3.832668
3.831955
3.831598
3.828031
3.824463
3.820896
3.817329
3.813761
3.810194
3.806627
3.803059
3
1
1
1
1
1
1
1
1
3
3
3
3
3
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
3
2
2
2
2
2
5
5
5
5
5
263
451.91
452.41
452.91
453.41
453.91
454.41
454.91
455.41
455.91
456.41
456.76
456.91
457.41
457.91
458.41
458.66
458.91
459.41
459.91
460.41
460.91
461.26
461.41
461.91
462.36
462.41
462.91
463.41
463.91
464.16
464.41
470.56
470.91
471.41
471.91
472.06
472.41
472.81
472.91
473.41
3.799492
3.795925
3.792357
3.78879
3.785223
3.781655
3.778088
3.774521
3.770953
3.767386
3.764889
3.763818
3.760251
3.756684
3.753116
3.751333
3.749549
3.745982
3.742414
3.738847
3.73528
3.732783
3.731712
3.728145
3.724934
3.724578
3.72101
3.717443
3.713876
3.712092
3.710308
3.66643
3.663933
3.660365
3.656798
3.655728
3.653231
3.650377
3.649663
3.646096
5
5
5
5
5
5
5
5
5
5
4
4
4
4
4
5
5
5
5
5
5
3
3
3
1
1
1
1
1
4
4
5
5
5
5
3
3
1
1
1
264
473.91
474.36
474.41
474.91
475.41
475.91
476.11
476.41
476.91
477.41
477.91
478.41
478.91
479.16
479.41
479.91
479.96
480.41
480.56
480.91
481.41
481.91
482.41
482.91
483.41
483.56
483.91
484.41
484.91
485.41
485.91
485.96
486.41
486.91
487.41
487.91
488.41
488.91
489.41
489.91
3.642529
3.639318
3.638961
3.635394
3.631826
3.628259
3.626832
3.624692
3.621124
3.617557
3.61399
3.610422
3.606855
3.605071
3.603288
3.59972
3.599364
3.596153
3.595083
3.592586
3.589018
3.585451
3.581884
3.578316
3.574749
3.573679
3.571182
3.567614
3.564047
3.560479
3.556912
3.556555
3.553345
3.549777
3.54621
3.542643
3.539075
3.535508
3.531941
3.528373
1
3
3
3
3
3
2
2
2
2
2
2
2
4
4
4
3
3
2
2
2
2
2
2
2
4
4
4
4
4
4
3
3
3
3
3
3
2
2
2
265
490.41
490.86
490.91
491.41
491.91
492.41
492.91
493.41
493.91
494.41
494.91
495.26
495.41
495.91
496.41
496.91
497.41
497.91
498.41
498.91
498.96
499.41
499.91
500.41
500.91
501.41
501.91
502.41
502.91
503.41
503.91
504.41
504.56
504.91
505.31
505.41
505.91
506.41
506.91
507.41
3.524806
3.521595
3.521239
3.517671
3.514104
3.510537
3.506969
3.503402
3.499834
3.496267
3.4927
3.490203
3.489132
3.485565
3.481998
3.47843
3.474863
3.471296
3.467728
3.464161
3.463804
3.460594
3.457026
3.453459
3.449892
3.446324
3.442757
3.439189
3.435622
3.432055
3.428487
3.42492
3.42385
3.421353
3.418499
3.417785
3.414218
3.410651
3.407083
3.403516
2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
3
3
2
2
5
5
5
5
5
5
266
507.91
508.41
508.91
509.41
509.91
510.41
510.51
510.91
511.01
511.41
511.91
512.41
512.91
513.01
513.41
513.66
513.91
514.41
514.91
515.16
515.41
515.91
516.26
516.41
516.91
517.41
517.91
518.41
518.91
519.41
519.91
520.41
520.76
520.91
521.41
521.91
522.41
522.91
523.41
523.91
3.399949
3.396381
3.392814
3.389247
3.385679
3.382112
3.381398
3.378545
3.377831
3.374977
3.37141
3.367842
3.364275
3.363562
3.360708
3.358924
3.35714
3.353573
3.350006
3.348222
3.346438
3.342871
3.340374
3.339304
3.335736
3.332169
3.328602
3.325034
3.321467
3.3179
3.314332
3.310765
3.308268
3.307197
3.30363
3.300063
3.296495
3.292928
3.289361
3.285793
5
5
5
5
5
5
3
3
5
5
5
5
5
4
4
1
1
1
1
4
4
4
3
3
3
3
3
3
3
3
3
3
5
5
5
5
5
5
5
5
267
524.41
524.91
525.41
525.91
526.36
526.41
526.91
527.41
527.91
528.41
528.91
529.41
529.91
530.41
530.91
531.16
531.41
531.91
532.36
532.41
532.91
533.41
533.91
534.41
534.91
535.41
535.91
536.41
536.91
537.41
537.91
538.41
538.91
539.41
539.91
540.41
540.66
540.91
541.41
541.91
3.282226
3.278659
3.275091
3.271524
3.268313
3.267957
3.264389
3.260822
3.257255
3.253687
3.25012
3.246553
3.242985
3.239418
3.23585
3.234067
3.232283
3.228716
3.225505
3.225148
3.221581
3.218014
3.214446
3.210879
3.207312
3.203744
3.200177
3.19661
3.193042
3.189475
3.185908
3.18234
3.178773
3.175205
3.171638
3.168071
3.166287
3.164503
3.160936
3.157369
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
3
3
3
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
3
3
3
3
268
542.41
542.66
542.91
543.41
543.91
544.41
544.91
545.41
545.91
546.41
546.91
547.41
547.91
548.41
548.91
549.41
549.91
550.41
550.91
551.41
551.91
552.41
552.91
553.41
553.91
554.41
554.91
555.41
555.91
556.41
556.91
557.41
557.91
558.41
558.91
559.41
559.91
560.41
560.91
561.41
3.153801
3.152018
3.150234
3.146667
3.143099
3.139532
3.135965
3.132397
3.12883
3.125263
3.121695
3.118128
3.114561
3.110993
3.107426
3.103858
3.100291
3.096724
3.093156
3.089589
3.086022
3.082454
3.078887
3.07532
3.071752
3.068185
3.064618
3.06105
3.057483
3.053916
3.050348
3.046781
3.043213
3.039646
3.036079
3.032511
3.028944
3.025377
3.021809
3.018242
3
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
269
561.91
562.41
562.91
563.41
563.91
564.41
564.91
565.41
565.91
566.41
566.51
566.91
567.41
567.91
568.41
568.91
569.41
569.91
570.41
570.91
571.41
571.91
572.41
572.91
573.41
573.91
574.41
574.91
575.41
575.91
576.41
576.91
577.26
577.41
577.91
578.41
578.91
579.06
579.41
579.91
3.014675
3.011107
3.00754
3.003973
3.000405
2.996838
2.993271
2.989703
2.986136
2.982568
2.981855
2.979001
2.975434
2.971866
2.968299
2.964732
2.961164
2.957597
2.95403
2.950462
2.946895
2.943328
2.93976
2.936193
2.932626
2.929058
2.925491
2.921924
2.918356
2.914789
2.911221
2.907654
2.905157
2.904087
2.900519
2.896952
2.893385
2.892314
2.889817
2.88625
5
5
5
5
5
5
5
5
5
5
4
4
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
4
4
4
4
4
3
3
3
270
580.41
580.76
580.91
581.41
581.91
582.41
582.91
583.41
583.91
584.41
584.91
585.31
585.41
585.91
586.41
586.76
586.91
587.41
587.91
588.01
588.41
588.91
589.41
589.81
589.91
590.41
590.91
591.21
591.41
591.91
592.31
592.41
592.91
593.41
593.91
594.31
594.41
594.91
595.41
595.91
2.882683
2.880186
2.879115
2.875548
2.871981
2.868413
2.864846
2.861279
2.857711
2.854144
2.850576
2.847723
2.847009
2.843442
2.839874
2.837377
2.836307
2.83274
2.829172
2.828459
2.825605
2.822038
2.81847
2.815616
2.814903
2.811336
2.807768
2.805628
2.804201
2.800634
2.79778
2.797066
2.793499
2.789932
2.786364
2.78351
2.782797
2.779229
2.775662
2.772095
3
5
5
5
5
5
5
5
5
5
5
3
3
3
3
4
4
4
4
3
3
3
3
4
4
4
4
3
3
3
5
5
5
5
5
4
4
4
4
5
271
596.41
596.91
597.41
597.91
598.41
598.91
599.41
599.91
600.41
600.91
601.41
601.91
602.21
602.41
602.91
603.21
603.41
603.91
604.41
604.91
605.41
605.91
606.41
606.71
606.91
607.41
607.91
608.41
608.91
609.41
609.91
610.41
610.91
611.41
611.91
612.41
612.71
612.91
613.41
613.91
2.768527
2.76496
2.761393
2.757825
2.754258
2.750691
2.747123
2.743556
2.739989
2.736421
2.732854
2.729287
2.727146
2.725719
2.722152
2.720011
2.718584
2.715017
2.71145
2.707882
2.704315
2.700748
2.69718
2.69504
2.693613
2.690046
2.686478
2.682911
2.679344
2.675776
2.672209
2.668642
2.665074
2.661507
2.657939
2.654372
2.652232
2.650805
2.647237
2.64367
5
5
5
5
5
5
5
5
5
5
5
5
4
4
4
5
5
5
5
5
5
5
5
1
1
1
1
1
1
1
1
1
1
1
1
1
4
4
4
4
272
614.41
614.46
614.91
615.41
615.91
616.41
616.91
617.41
617.91
618.41
618.91
619.41
619.91
620.41
620.91
621.41
621.91
622.41
622.71
2.640103
2.639746
2.636535
2.632968
2.629401
2.625833
2.622266
2.618699
2.615131
2.611564
2.607997
2.604429
2.600862
2.597295
2.593727
2.59016
2.586592
2.583025
2.580885
4
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
273
Table 4.2: Lithofacies association codes, descriptions, and interpretations. We identify
14 lithofacies associations and five depositional environments. Depositional
environments are as follows: F-: Fluvial, S-: Supra-littoral, L-: Littoral, P-: Profundal,
and A-: Alluvial fan.
Lithofacies
association
Description
Interpretation
F1: Gcmi,
Gch, Gt, Gcf
- Amalgamated channel forms and tabular units of
moderately sorted, clast-supported pebble
conglomerate featuring abundant traction sedimentary
structures
- Deposits are 2 – 10 m thick
- Erosive bases.
- Deposits of migrating channels, bars, bedload
sheets, and gravel dunes of a braided river within
a low-gradient fluvial setting (Bristow and Best,
1993; Cant and Walker, 1978; Collinson, 1996;
Heinz et al., 2003; Lunt et al., 2004)
F2: St, Sp,
Sh, Sc
- Tabular units 0.1 – 5 m thick of horizontally or crossstratified sandstone
- Abundant load casts and ball-and-pillow structures
and rare channel forms
- Sheet flood deposits on medial to distal fluvial
fans in a marshy floodplain or marginal lacustrine
setting (Hampton and Horton, 2007; Saez et al.,
2007)
F3: Sm, Sc,
Mm, Mc, Ml,
Mh
- Massive, organic-rich, strongly calcareous siltstones
and sandstones 0.1 – 4 m thick featuring softsediment deformation but no pedogenesis
- Contains ostracods, root traces, and small planorbid
gastropods (Gyraulus sp?)
- Marshy floodplain or lake-margin wetland
deposits (Allen and Collinson, 1986)
S1: Gct,
Gcmi, Sp, St,
Sh, Sm, Sr
- Tabular, upward coarsening sandstone to pebble
conglomerate 0.25 to 6 m thick
- Erosional bases and abrupt tops
- Horizontal stratification, climbing-, trough-, and
planar cross-stratification
- Channelized deposits up to 0.5 m thick are locally
present
- Abundant, well-preserved, robust gastropod shells
- Product of sedimentation within a delta complex
(Allen and Collinson, 1986; Saez et al., 2007)
S2: Sh
- Low-angle laminae up to 3 cm thick without internal
structure
- Developed in well-sorted, fine-grained sand
-Root traces and soft-sediment deformation are
common
- Vegetated eolian sand-flat (Hunter, 1977)
S3: Sf
- Stacked units (0.5-1.5 m thick) of steeply-inclined,
upward-coarsening laminations
- Developed in well-sorted, fine-grained sandstone
- Root traces and soft-sediment deformation are
common
- Lacustrine coastal plain dune deposits
L1: Ml
- Papery, laminated siltstone
- Abundant fossil grasses, fragmentary shell material
and occasional fish fossils
- Deposition with a grass-rich, shallowly
submerged, low-gradient, lake-margin
environment (Allen and Collinson, 1986; Talbot
and Allen, 1996)
L2: Mr, Sr,
Srw, Sp, Sh,
Sm
- Upward coarsening sandstone deposits 0.5 – 3 m
thick
- Gradational bases and erosional tops
- Climbing-ripple and wave-ripple cross-stratification
and mud drapes
- Deposits of a terminal lobe on a prograding
delta front (Allen and Collinson, 1986; Dam and
Surlyk, 1993; Jopling, 1965; Saez et al., 2007;
Talbot and Allen, 1996)
P1: Mh, Mm
- Upward coarsening massive or horizontally
- Profundal lacustrine deposition (below fairlaminated silty claystone
weather wave base)
- Up to 10 m thick, includes dispersed, plant, ostracod,
and fish fossils
P2: Sh, Sm,
Mh
- 2 m-thick stacks of upward-fining laminae up to 3 cm
thick
- Basal surfaces of individual laminae are abrupt and
the upward transition to siltstone is gradational, lower
portion is massive and grades upward to horizontally
laminated
- Stacked deposits of turbidity currents (primarily
Bouma A and B) (Bouma, 1962; Giovanoli, 1990;
Lowe, 1982; Mutti et al., 2003)
A1: Gcm,
Gch, Gcmi
- Poorly sorted, very angular to subrounded, pebble to
boulder conglomerate
- High-concentration, unconfined sheet-flood
deposits (Blair, 2000; Blair and McPherson,
274
A2: Gmm,
Gcm
- 0.5 – 20 m thick
- Erosive bases, horizontal stratification and long-axis
transverse imbrication
1994b; DeCelles et al., 1991; Nemec and Steel,
1984; e.g., Pierson, 1980; Pierson, 1981)
- Massive, unorganized, matrix-supported boulder
conglomerate 0.5 – 3 m thick
- No erosive bases
- Debris flow deposits (Blair and McPherson,
1994a; Pierson, 1980)
A3:Gcmi,Gch, - Imbricated, lenticular, clast-supported, pebble –
- Deposited by streams in alluvial fan channels
Gcf, Gt
boulder conglomerate bodies
(DeCelles et al., 1991)
- Erosive bases and horizontal- and cross-stratification
A4: Gx
- Chaotic matrix- and clast supported boulder
conglomerate deposits
- Abundant load casts and ball and pillow structures
(up to 2 m of relief on some structures)
- High-concentration flood or debris flow deposits
into a water-saturated environment (Blair, 2000;
Miall, 2000; Nemec and Steel, 1984; Pivnik,
1990; Postma, 1983)
275
Table 4.3. Stable isotope data
13
Sample Name
1SZ12
1SZ13
1SZ13.8
1SZ18
1SZ24
1SZ24.1
1SZ27.9
1SZ32
2SZ43
2SZ43.1
2SZ47
2SZ51.5
2SZ51.5AD0.5
2SZ51.5AD10
2SZ51.5AD11
2SZ51.5AD12
2SZ51.5AD13
2SZ51.5AD14
2SZ51.5AD15
2SZ51.5AD3
2SZ51.5AD4
2SZ51.5AD5
2SZ51.5AD6
2SZ51.5AD7
2SZ51.5AD8
2SZ51.5AD9
2SZ55
3SZ0.15
3SZ24
3SZ24.1A
3SZ24.25A
3SZ24.25B
3SZ24.25C
3SZ24.3
3SZ27
Stratigraphic height
(m)
309.65
310.65
311.45
315.65
321.65
321.75
325.55
329.65
376.1
376.2
380.1
384.6
384.6
384.6
384.6
384.6
384.6
384.6
384.6
384.6
384.6
384.6
384.6
384.6
384.6
384.6
388.1
389.25
413.1
413.2
413.35
413.35
413.35
413.4
416.1
δ C
(‰,
VPDB)
-0.9
-3.0
-1.3
2.1
-0.2
-0.5
0.0
0.5
0.9
-0.4
-0.6
-0.2
-0.6
0.2
0.2
0.1
0.3
0.2
0.8
-0.4
-0.1
0.3
0.9
0.0
0.0
-0.1
-0.7
0.9
-0.3
1.6
0.8
-1.1
0.6
0.9
-2.1
18
δ O
(‰,
VPDB)
-3.9
-12.5
-3.2
-1.8
-5.4
-4.6
-2.8
-1.5
-1.8
-6.8
-1.9
-2.4
-0.6
-1.2
-2.1
-2.2
-2.1
-2.5
-0.8
-2.2
-2.1
-1.6
-0.8
-0.3
-0.3
-0.4
-2.3
-1.7
-2.2
-2.8
-1.4
-1.8
-1.6
-2.0
0.7
276
3SZ49
3SZ50.5
3SZ55
438.1
439.6
444.1
-3.3
-1.2
1.2
-13.7
-1.0
-2.5
277
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