Climate-driven environmental change in the Zhada basin,
southwestern Tibetan Plateau
Joel Saylor*
Peter DeCelles
Jay Quade
Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA
ABSTRACT
The Zhada basin is a large Neogene
extensional sag basin in the Tethyan Himalaya of southwestern Tibet. In this paper
we examine environmental changes in the
Zhada basin using sequence stratigraphy,
isotope stratigraphy, and lithostratigraphy.
Sequence stratigraphy reveals a long-term
tectonic signal in the formation and filling of
the Zhada basin, as well as higher-frequency
cycles, which we attribute to Milankovitch
forcing. The record of Milankovitch cycles
in the 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 the Zhada basin involves a
decrease in abundance of arboreal pollen
in favor of nonarboreal pollen. The similarity between the long-term environmental changes in the 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.
INTRODUCTION
Uplift of the Tibetan Plateau has long been
viewed as a major forcing factor in regional and
global climate change (e.g., Raymo and Ruddi-
man, 1992; Molnar et al., 1993; France-Lanord
and Derry, 1994; Ruddiman et al., 1997; An
et al., 2001; Abe et al., 2005; Molnar, 2005).
Uplift is also thought to have directly driven
environmental change on the Tibetan Plateau
(e.g., Liu, 1981a; Zhang et al., 1981; Zhu et
al., 2004; Wang et al., 2006). However, recent
work suggests that global climate change
drives climate and environmental change on
the Tibetan Plateau (e.g., Dupont-Nivet et al.,
2007). 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. Paleofloral assemblages from Pleistocene deposits on the Tibetan Plateau are similar to modern floral assemblages at low elevations (e.g., Axelrod, 1981; Xu, 1981; Zhang et
al., 1981; Li and Zhou, 2001a, 2001b; Meng et
al., 2004; Molnar, 2005; Wang et al., 2006) and
are used to argue for plateau uplift of at least
1 km since the Late Miocene. 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; Zhang et al., 1981;
Li and Li, 1990; Meng et al., 2004; Y. Wang et
al., 2008a). In contrast, other lines of evidence
indicate that the southern Tibetan Plateau has
been at high elevations since at least the Middle Miocene (Garzione et al., 2000a; Rowley
et al., 2001; Spicer et al., 2003; Currie et al.,
2005; Saylor et al., 2009) and central Tibetan
Plateau since at least the Oligocene (Cyr et al.,
2005; Graham et al., 2005; Rowley and Currie, 2006; DeCelles et al., 2007; Dupont-Nivet
et al., 2008). These paleoelevation studies also
show that uplift predated widespread Late
Miocene climate change (see Molnar, 2005,
for a summary of evidence for Late Miocene
climate change). These studies call into ques-
tion the direct link between uplift and environmental change on the Tibetan Plateau.
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, the lack of a 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 (Zhang et al., 1981; Zhou
et al., 2000; Li and Zhou, 2001b) and capped
by boulder conglomerates (Zhu et al., 2004;
Zhu et al., 2007). There is similarly little consensus regarding the basin’s tectonic origin.
The Zhada basin is presented as having developed in the hanging wall of the low-angle South
Tibetan detachment system or as a half-graben
produced in response to arc-normal extension
(Wang et al., 2004; S.F. Wang et al., 2008a). It
is also proposed to be a flexural basin responding to arc-perpendicular compression (Zhou
et al., 2000). The presence of capping boulder
conglomerates has led to the suggestion that the
basin was recently uplifted (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., Zhang et al., 1981; Zhou et
al., 2000; Li and Zhou, 2001a; Zhu et al., 2004).
In a recent paper (Saylor et al., 2009) we
documented the chronostratigraphy and stable
isotope record of the Zhada basin. Here we provide basin-wide lithologic and sequence stratigraphic correlations, frequency analysis of the
record of environmental change, and a detailed
isotope stratigraphy. Our results suggest that
global climate change, possibly in conjunction with regional climate change, controlled
*Present address: Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas 78712-0254, USA.
Geosphere; April 2010; v. 6; no. 2; p. 74–92; doi: 10.1130/GES00507.1; 12 figures; 1 table; 2 supplemental tables.
74
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© 2010 Geological Society of America
Sequence stratigraphy and climate cycles in southwestern Tibet
A
80°
85°
90°
95°
100°
105°
40°
50
50
500
00
0 km
km
Tar
m
Tarim
Basin
Bas
in
n
Alty
h
Tag
t
faul
Qa
Q
a da
dam
dam
Qaidam
B
Bas
n
Basin
35°
So
Son
S
o pa
onpan
pan
p
an-G
a
Ga
Ga
anzi
nzi
nz
z T
Te
Terra
errane
errane
rra
rra
ane
ne
Sonpan-Ganzi
Terrane
JS
SZ
Z
JSZ
Qi
Qia
Q
ia
angt
gta
an
ang
ng T
Te
erra
e
r ane
rr
rra
ne
Qiangtang
Terrane
KF
KF
Th s Study
Th
This
Stud
S
Stu
tudy
tud
ttu
dy
B
BSZ
30°
Lha
L
ha
hasa
sa T
Ter
e
erran
rane
ane
e
Lhasa
Terrane
Himalayan Thr
ust Belt
Great Counter
thrust
Kailas Conglomerate
Mesozoic Tethyan rocks
Paleozoic Tethyan rocks
Gangdese Batholith
Legend:
Thrust fault
Detachment/normal fault
Strike-slip fault
Suture zone
EZ
Trace of measured section
North-south transect
Northwest-southeast transect
Great Counter
thrust
ent
Leo Pargil Detachm
i
Ay
Q
1NWZ
2NWZ
Z 3NWZ
Sh
ha
da
?
an
32°00
Qusum
Detachment
Zada Basin Fill
JSZ : Jinsha Suture Zone
BSZ : Bangong Suture Zone
ISZ : Indus Suture Zone
MFT : Main Frontal thrust
KF : Karakoram fault
Higher Himalayan rocks
ISZ
S
SZ
MFT
T
Quaternary Alluvium
Abbreviations:
B
as
in
Karakoram Fault
System
2NZ
NRW
1NZ
NRE
Guga
31°20
EZ
Indus-Yalu Suture
3NZ
SZ SEZ
Zone
Great Counter
thrust
Ch
Ind ina
ia
So
ut
10
100
00
0 km
km
Pulan
Basin
et
an
in
Ma
De
tac
hm
Ce
79°00
hT
ib
n
t ra l
Thrust
ent
30°00
80°00
Lake Lake
Gurla Mandhata
7728 m
Ind
ia
Ne
pa
l
30°40
81°00
Figure 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 mapping by Cheng and Xu
(1987), Murphy et al. (2000, 2002), and mapping by M. Murphy (2005, 2006, 2007, personal commun.).
Geosphere, April 2010
75
Saylor et al.
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;
Fig. 1A). The basin is at least 150 km long and
60 km wide, and the current outcrop extent of
the basin fill is at least 9000 km2 (Fig. 1B).
The Zhada basin is located in a zone of
active arc-parallel extension (Ni and Barazangi,
1985; Zhang et al., 2000; Murphy et al., 2002;
Thiede et al., 2006; Valli et al., 2007; Murphy
et al., 2009). It is bounded by the South Tibetan
detachment system to the southwest, the Indus
suture to the northeast, and the Leo Pargil and
Gurla Mandhata gneiss domes to the northwest
and southeast, respectively (Fig. 1B). The role
of each of these structures in the development of
the Zhada basin is an area of ongoing research.
The South Tibetan detachment system is a series
of north-dipping, 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, both to the
east and west, ages for movement on the South
Tibetan detachment system range from 21 to
12 Ma (Hodges et al., 1992, 1996; Noble and
Searle, 1995; Searle et al., 1997; Murphy and
Harrison, 1999; Searle and Godin, 2003; Cottle
et al., 2007). To the northeast of the Zhada basin,
the Oligocene–Miocene Great Counter thrust, a
south-dipping, top-to-the-north thrust system,
cuts the Indus suture (e.g., Gansser, 1964; Yin et
al., 1999; Murphy and Yin, 2003). Exhumation
of the Leo Pargil and Gurla Mandhata gneiss
domes (Fig. 1B) by normal faulting began 9–
10 Ma (Zhang et al., 2000; Murphy et al., 2002;
Thiede et al., 2006) and continues today.
The Zhada Formation occupies the Zhada
basin and consists of >800 m of fluvial, lacustrine, eolian, and alluvial fan deposits. The sedimentary basin fill is undisturbed and forms an
angular or buttress unconformity with underlying Tethyan sequence strata that were previously shortened in the Himalayan fold-thrust
belt (Saylor, 2008). The Zhada Formation is
capped by a geomorphic surface that extends
across the basin and is interpreted as a paleodepositional plain that marks the maximum extent
of sediment aggradation prior to integration
of the modern Sutlej River drainage network.
76
After deposition, the basin was incised to basement by the Sutlej River, exposing the complete
thickness of the Zhada Formation. The best
estimate for the age of the Zhada Formation is
between ca. 9.2 and after 1 Ma, based on vertebrate fossils and magnetostratigraphy (Fig. 2)
(Lourens et al., 2004; S.F. Wang et al., 2008b;
Saylor et al., 2009).
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 (Fig. 1B). Sections were measured at centimeter scale.
Correlations
The geomorphic surface that caps the Zhada
Formation is correlative across the basin, and
provides the datum for sequence stratigraphic
and lithologic correlation. Correlations are
based on major stratigraphic members that can
be physically traced (Saylor, 2008). 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 that is observed
between 130 and 230 m in the South Zhada
section and at ~300 m in the East Zhada section (Saylor et al., 2009). The expansion of C4
vegetation is observed across the Indian subcontinent and southern Tibet ca. 7 Ma (Quade
et al., 1989, 1995; France-Lanord and Derry,
1994; Garzione et al., 2000a; Ojha et al., 2000;
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 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
Global
polarity
Age
Polarity
time
(Ma) scale Chrons
0
1n
1
South Zhada Section
Stratigraphic height (m)
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.
800
800
2
700
700
3
600
600
500
500
400
300
300
200
200
2An
4
5
400
2n
6
C3/C4 Transition7
3n
3An
3Bn
4n
100
8
100
9
CS SS Cgm -90
4An
-45
0
45 90
VGP latitude (°)
10
5n
Figure 2. South Zhada lithologic section and associated magnetostratigraphic section and correlation to the geomagnetic polarity time scale
(GPTS) of Lourens et al. (2004). VGP—virtual geomagnetic pole; C—
claystone; S—siltstone; SS—sandstone; Cgm—conglomerate.
Geosphere, April 2010
Sequence stratigraphy and climate cycles in southwestern Tibet
environments as follows: 5—fluvial and alluvial
fan associations; 4—supralittoral associations;
3—littoral associations; 2 or 1—profundal
associations, 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 magnetostratigraphic
tie points, justified by the generally linear
subsidence/sediment accumulation rates (Saylor, 2008). The assumption of a linear sediment
accumulation rate likely breaks down at short
time scales, implying that interpretation of
cycles <100 k.y. must await a more finely tuned
basin chronology. The result is a clipped waveform with uneven sample spacing and temporal
resolution better than 4 k.y. (Fig. 3; Supplemental Table 11). Progradation of basin margin
depositional environments leads to waveform
saturation and loss of resolution at ages younger
than 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,” respectively, in Fig. 3).
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 autospectral analysis (Welch method) to determine the
dominant frequencies in the record. We also
conducted harmonic analysis using Siegel’s test
to discriminate periodic components from noise
in the Zhada 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).
Stable Isotopes
Stable isotopes of oxygen and carbon
[expressed as δ18O and δ13C in units ‰, respectively, and referenced to Vienna Peedee belemnite (VPDB) or Vienna standard mean ocean
water (VSMOW)] are sensitive indicators 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
δ18Osw values) (Dansgaard, 1954, 1964; Rozan-
ski et al., 1993; Garzione et al., 2000b; Poage
and Chamberlain, 2001; Rowley et al., 2001;
Rowley and Garzione, 2007). Freshwater gastropods precipitate shells with oxygen isotopic
ratios (δ18Occ [Shell carbonate oxygen isotope
ratio]) 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 (Lemeille et al.,
1983; Bonadonna et al., 1999; 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 (Talbot, 1990; Li and Ku, 1997).
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 paleohydrologic
and paleoenvironmental conditions (e.g., Abell
and Williams, 1989; Purton and Brasier, 1997;
Hailemichael et al., 2002; Smith et al., 2004).
Fossil gastropod shell fragments and intact
shells were collected from fluvial, marshy, and
lacustrine intervals from the lower ~650 m in 2
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., 2009).
We measured δ18Occ and δ13Ccc values using an
automated carbonate preparation device (KIELIII) 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 (1σ).
RESULTS
structures (Supplemental Table 22). Unless
otherwise indicated, all deposits are laterally
continuous for hundreds of meters to several
kilometers. Only abbreviated descriptions and
interpretations are presented here (for details,
see Saylor, 2008).
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 (Figs. 4A and 5A) consists
of a 1–10-m-thick unit of fluvial or alluvial fan
sandstone or conglomerate (lithofacies association F1 or rarely A1–A4) with an erosional base,
no grain-size trend, and a capping, upwardfining 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 (Figs. 4B, 5B, and
5C) 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 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 type B cycles may be missing from
the idealized version depicted in Figure 4.
Sedimentology
Correlations
We identify 14 lithofacies associations and
five depositional-environment associations
based on lithology, texture, and sedimentary
Type A and type B cycles stack in predictable
patterns within a larger sequence stratigraphic
1
Supplemental Table 1. Word document containing data used in frequency analysis. If you are viewing the PDF of this paper or reading it offline, please visit http://
dx.doi.org/10.1130/GES00507.S1 or the full-text article on www.gsapubs.org to view Supplemental Table 1.
2
Supplemental Table 2. Word document containing the lithofacies association codes, descriptions, and interpretations. If you are viewing the PDF of this paper or
reading it offline, please visit http://dx.doi.org/10.1130/GES00507.S1 or the full-text article on www.gsapubs.org to view Supplemental Table 2.
Geosphere, April 2010
77
Saylor et al.
Legend
South Zhada (SZ)
Record of summer insolation
lithologic section Depo-codes at 65°N (Laskar et al., 2004)
End leg 10 at
N 31˚ 22.910’
E 79˚ 45.075’
4299 ± 8 m 620
St
Gcmi
Oscillatory current ripples
Unidirectional ripples
Erosional surface
Gastropods
Mc
Sc
St/Gcmi
Plants/Plant fragments
St
Sc
St
Mh
Sc
Hcs
St
Sc
Mc/St
St
Sh
St
Sr
Sc
Gcmi
Sc
560
n=10
Start
leg 10 at
540
N 31˚
n=28
23.112’
E 79˚ 44.981’
4197 ± 6 m
End leg 9
520
GPS
unavailable
3
Ostracods
Root Traces
Sh
Fish Skeletons/Fragments
St
Sh
St
Mh
Gmt
Gcmi
Terrestrial Mammal Fossils
Climbing ripples
Sr
Shell fragments
Mh
Sc
St
Mh
Gmm
Gcmi
St
Sf
Sr
Mh
Sh
Sr
Ml
n = 17
3.5
Sh
Mh
Sf
Sr
St
Mh
Ml
St
Mh
Sr
Mh
Sc
Short Series
St
Gmm
Mh
Sc
St
Mh
Mr/Mc
St
Mh
Sr
Sf
St
n = 17
440
Mh
Sh
Start leg 8 at
N 31˚ 24.584’ 340
E 79˚ 45.371’ n = 13
4001 ± 9 m
End leg 7 at
N 31˚ 25.280’
E 79˚
320
44.916’
4001 ± 10 m Start leg 7 at
N 31˚
25.275’ n = 14
E 79˚
44.989’ 300
3966 ± 6 m
End leg 6 at
N 31˚ 25.281’
E 79˚ 44.986’
3966 ± 7 m 280
Start leg 6 at
N 31˚ 25.507’
E 79˚ 45.118’
3933 ± 8 m
End leg 5 at 260
N 31˚ 26.121’
E 79˚ 45.421’
3929 ± 10 m
240
P
Mh
Sh
St
Hcs
St
Gct
Mr
St
Mr
St
St
Sr
Mh
St
Ml
St
Mh
Sh
Mh
Mh
St
Sr
Mh
Ml
Sr
St
Mh
Sr
St
St/Gct
Mh
St
Sf
St
Sh
4.5
St
Sr
Mr
Mh
Ml
Mh
St
Mh
Ml
St
Ml/St
Sr
Mh
Ml
St
Ml
St
Sr
Ml
Mh
Ml
St
Mh
St
Mh
Ml
Sr
Mh
Sr/Sc
Mh
Sr/Sc
Mh/Sr
5
Mh
Ml
St
Mh/Sh
St
Mh
Ml
Sh/St
Ml
Sh
Mh
St
St/Sh
Mh
Mh
Start leg 5
N 31˚ 26.260’
E 79˚ 45.143’
3875 ± 7 m
4
St
Mr/Sr
Ml
220
Paleocurrent direction from
groundwater tubes
Sh
Mh
Mh
St
Mh
Ml
St
Mh
Ml
Ml
St
Mh
1 2 3 4 5
Depo-code
Lake/wetland
expansion
(Stronger
monsoon)
-60-40-20 0 20 40 600
Variation in
Insolation (W/m2)
0.02
0.04
Eccentricity
Mh/Sh
Sf
Mh
Sf
Mh/Ml
Sh
CS SS Cgm
78
Paleocurrent direction from
trough cross beds
Age (Ma)
Sr
Mh
Mh/Sr
460
n = 19
Number of paleocurrent
measurements
Paleocurrent direction from
imbricated clasts
Entire Series
480
360
n = 16
Bivalves
Gcmi
500
Stratigraphic height (m)
Hummocky crossstratification
Gmm
St
Mc
Mh
St
Mc
Gcm
St
Mh/Gcm
St/Gcmi
Mh/Sh
Gmt
Mc
St
Mc
St
Mr/Mh
St
580
n=19
Start leg 9 at 400
N 31˚ 24.158’
E 79˚ 45.442’
4057 ± 6 m
End leg 8 at
380
N 31˚
24.449’
E 79˚ 45.342’
4057 ± 13 m
Convoluted bedding/
soft sediment deformation
Mh/Sr
Gcmi
600
420
Mud cracks
2.5
Mr
Geosphere, April 2010
0.06
Figure 3. The synthetic wave
form constructed for spectral
analysis. Depositional codes
relate to lithofacies associations
(5—alluvial fan and fluvial associations; 4—supralittoral associations; 3—littoral associations;
2 or 1—profundal associations,
based on the presence or absence
of terrigenous clastic or biologic
material). At ages younger than
3.3 Ma, the waveform saturates
at values of 5 due to the infilling of the Zhada paleolake and
the progradation of lake-margin
depositional environments. Similarly, the inability to distinguish
fluctuations in water level during times of profundal or alluvial
fan and/or fluvial sedimentation
results in clipping of the waveform. The record of insolation
variation (Laskar et al., 2004)
is provided for comparison.
GPS—global positioning system;
C—claystone; S—siltstone; SS—
sandstone; Cgm—conglomerate.
Sequence stratigraphy and climate cycles in southwestern Tibet
margin facies and occasionally nondeposition
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.
At the finest scale, 56 type A and type B
cycles are present in the Zhada Formation
(Fig. 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
hierarchy (Fig. 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 the Zhada basin.
The Zhada Formation does not have significant
intraformational unconformities that might
represent extended periods of nondeposition
or extensive subaerial exposure and erosion.
However, it does have Waltherian unconformities that represent rapid progradation of basin
B Type B cycle
Fluvial or Alluvial fan Association
Supra-littoral Association
Sub-aerial exposure
(Panel III.)
{
Fluvial or alluvial fan
deposition
(Panel I.)
~ 10 m
Gradual progadation
(Panel II. - III.)
Fluvial or Alluvial fan Association
Profundal deposition
(Panel II.)
Low energy
transgression drowns
lake-margin semi-aquatic
grasses (Panel I.)
Exposure or sediment
bypass (Panel III.)
Wetland deposition
(Panel II.)
Sandstone or conglomerate
Papery laminated siltstone or claystone
C Depositional Setting Panels
Supra-littoral Association
Littoral Association
Profundal Association
Littoral Association
Fluvial or Alluvial fan Association
Massive or laminated siltstone or claystone
Paleo-Sutlej River
II.
III.
Zhada Co
(lake)
Type B cycle
Marshy wetlands
D Lake level, water influx, and lake water δ18O
Depo-setting Panels:
Progradation
III I
Massive, laminated or rippled siltstone
Alluvial fan
submerged lake
Possible locations of margin grasses
Type A cycles
I.
Fluvial or Alluvial fan Association
1 - 10 m
A Type A cycle
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 (landward stepping of depositional settings resulting in an
increase in lake and/or wetland area) stacking
pattern (Fig. 6). They are fluvially dominated
and become increasingly marshy upsection.
Second-order transgressive (flooding) surfaces
(2TS) are identified by an abrupt transition to
thick, profundal claystone (Fig. 8). Modern
Flooding
II
III I
Progradation
Flooding
II
Greater
lake area
Net +
water
influx Greater
δ18O
values
Time (t)
Figure 4. (A, B) Idealized forms of sequence types A and B. (C) Interpreted depositional environments. (D) Simplified representation
of the relationship between lake level, water, and sediment flux and lake δ18O values. The simplifications involve the assumption that
end-member influx and efflux δ18O values are invariant and that efflux via evaporation is proportional to lake area. Vertical gray
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.
Geosphere, April 2010
79
Saylor et al.
Tibetan lakes are typically broad and 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 (basinward stepping)
or aggrading (no stepping pattern) stacking patterns. At the coarsest scale, the entire
Zhada Formation can be seen as a first-order
sequence (approximately third-order sequence
of Vail et al., 1977). Tract 1LST is below the
first major lacustrine transgression and is
composed of 2LST1 and 2TST1. Tract 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. Tract 1HST occurs between the
most widespread maximum flooding surface
and the top of the Zhada Formation and is
composed of 2HST2, 2LST3–2HST3, and
2LST4–2HST4.
Type A and type 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
A Type
Typ
yp
pe
e A cycles
cycl
cy
cles
c
cl
les
e
es
F1
F1
F3
F3
F2
2
F1
F1
E
Er
ros
sio
on/
n/Se
Sedi
ed
diiment
me
entt b
ypas
yp
pas
ass
Erosion/Sediment
bypass
Floo
Fl
Flood
ood
Ero
Er
Erosion/Sediment
os
sio
on/
n/S
Se
edi
dime
me n
ntt b
bypass
yp
y
pas
pas
ss
B
Prograde
C Type B cycles
Flood
Flood
Prograde
Flood
S1
S1
L2
P1
P1
L1
1
Prograde
Flood
Figure 5. (A) Type A cycles. Cliff is ~15 m high. (B) Photomosaic of typical progradational sequences in the lacustrine portion of the Zhada Formation (Nl). Each cycle in B is ~ 10 m. However, the focus of photo B is to show the
lateral continuity of the Zhada deposits. (C) Type B cycles. Lowermost cycle is ~9 m high. Lower slope-forming
interval represents upward-coarsening profundal to littoral mudstones and siltstones (P1, L1, L2), which are
capped by cliff-forming littoral or supra-littoral sandstones (L2 or S1).
80
Geosphere, April 2010
Sequence stratigraphy and climate cycles in southwestern Tibet
northwest of there, in the region of the Namru
Road West section. The implication is that,
though relative subsidence was greatest in the
region of the South Zhada and Guga sections,
these were also close to the source of coarsegrained material (identified in Saylor, 2008,
as both the Kailash region to the north of the
basin and also the mountain ranges immediately surrounding the basin).
1MFS
1TS
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). There are 28 type B cycles
within this interval, each with an average duration of 95 k.y. Spectral analysis of both the
S.E. Zhada
C S
SS
entire series and the 5.23–3.3 interval indicates
statistically significant peaks at 91.7 k.y.
at the 95% confidence level and at 22.4 k.y. at the
85% confidence level (Fig. 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 k.y. at the 99% confidence
level (Fig. 9B). However, in the analysis of the
5.23–3.3 Ma interval all of these peaks, except for
S. Zhada
Figure 6. Portion of Figure 7 showing
detailed parasequence scale correlations. See Figures 7 and 10 for legend. 1TA—first-order transgressive
(flooding) surface; 1MFS—first-order
maximum flooding surface.
Cgm
: Type A Cycles
: Type B Cycles
Tethyan
fold-and-thrust
belt (basement)
C S
SS
Geosphere, April 2010
81
82
A
C S
SS
Cgm
3 N. Zhada
Geosphere, April 2010
C S
SS
50 m vertical
Sequence stratigraphic surfaces and
sequence number (first & second order)
Independent constraints
Cgm
Sh
Sf
Sf
Ml
St
Ml
Ml
St
Ml
St
Ml
St
St
C S
SS
Cgm
1 N. Zhada
CS
SS
Cgm
Namru Road
East
6km
C S
SS
East
Zhada
6km
Cgm
Tethyan fold-and-thrust belt (basement)
25km
Figure 7 (continued on following page). Basin-wide lithostratigraphic and sequence stratigraphic correlations. (A) North-south transect.
#TS#
6km
2 N. Zhada
Fluvial lithofacies assoc.
Parasequence boundaries
System tract boundaries (first &
second order)
System tract interval and
#LST#
sequence number (first & second order)
Supra-littoral lithofacies assoc.
Littoral lithofacies assoc.
Profundal lithofacies assoc.
Legend
Alluvial fan lithofacies assoc.
Paleo-depositional
plain (datum)
25km
C S
SS
Cgm
S. Zhada
2TS1
2MFS1
2SB2
2TS2
2MFS2
2SB3
2MFS3
& 2TS3
2SB4
2MFS4
2TS4
Saylor et al.
Geosphere, April 2010
1LST
1TS
1TST
1MFS
C S
SS
Cgm
S.E. Zhada
Southeast
20km
C S
Cgm
C S
SS
Guga
Cgm
2LST1
2TST1
2HST1
2LST2
2TST2
2HST2
2LST3
2TST3
2HST3
2LST4
2TST4
2HST4
35km
SS
Cgm
2TS1
2MFS1
C S
2SB2
2TS2
2MFS2
2SB3
2TS3
2MFS3
2SB4
2MFS4
2TS4
?
C S
SS
Cgm
Namru Road West
10km
Tethyan fold-and-thrust belt (basement)
SS
S. Zhada
10km
?
?
40km
C S
SS
Cgm
ult
t Fa
n
e
m
ach
Det
?
um
Qus
?
Northwest
8km
8km
C S
SS
Cgm
C S
SS
Cgm
C S
SS
1 N.W. Zhada Qusum
2 N.W. Zhada
50 m
vertical
3 N.W. Zhada
Paleo-depositional plain (datum)
10km
Cgm
Figure 7 (continued). (B) Southeast-northwest transect. See Figure 1B for locations of transects. TS—transgressive (flooding) surface; MFS—maximum flooding surface; SB—
sequence boundary; HST—highstand systems tract; LST—lowstand systems tract; assoc.—association; TST—transgressive systems tract; assoc.—association.
Overfilled
Balance filled
Underfilled
1HST
B
Sequence stratigraphy and climate cycles in southwestern Tibet
83
Saylor et al.
the 379 and 91.7 k.y. peaks, are suppressed (Fig.
9C). This indicates that the suppressed peaks
are likely the result of red noise due to waveform saturation at ages younger than 3.3 Ma.
-Both the entire series and the shorter interval
pass Siegel’s test, indicating that the record is not
the result of white noise. A random time series of
similar length showed no statistically significant
peaks and did not pass Siegel’s test. Coherence
analysis of the shorter interval also reveals peaks
at 80 ± 13, 26 ± 1, and 18.9 ± 0.6 k.y. (Fig. 9D).
P
1
P1
2TS2
2TS2
S1/F2
S
1/F2
Stable Isotopes
The X-ray diffraction analyses from 11 of 12
samples yielded only aragonite peaks (Saylor
et al., 2009); the 12th 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).
Figure 8. Second-order transgressive surface showing the abrupt
transition from lithofacies association S1 and F2 sandstone to lithofacies P1 profundal claystone. Homo sapiens (circled in red) for scale.
0
Eccentricity Obliquity
(100,000 yrs) (41,000 yrs)
Precession Precession
(23,000 yrs) (19,500 yrs)
0.08
BW
0.07
Relative Power
Log relative power
-5
-10
-15
-20
95% CL
85% CL
-25
A
0
10
20
30
40
50
60
70
0.03
126 ± 4 kyrs
140
±
4 kyrs
99% CL
0.01
0
B
80
Frequency (1/Ma)
0
2
4
6
8
10
12
14
16
18
20
Frequency (1/Ma)
0.12
0.6
0.1
Relative Power
0.5
Coherence
221
±
12 kyrs
0.04
91.7 ± 2 kyrs
0.02
Spectral
background
y = 0.003x2 -0461x - 8.833
R2 = 0.879
-30
379 ± 40 kyrs
1.33
±
2 Myrs
0.06
662
±
287
kyrs
0.05
0.4
0.3
0.2
False alarm
0.08
0.06
0.04
99% CL
0.1
0.02
0
D
0
10
20
30
40
Frequency (1/Ma)
50
60
0
C
0
2
4
6
8
10
12
14
16
18
20
Frequncy (1/Ma)
Figure 9. (A) Power spectrum of the entire interval (5.23–2.581 Ma). The spectrum has peaks at ~100 k.y. at the 95% confidence level
(CL) and ~23 k.y. at the 85% confidence level. Y axis is log (base 10) of relative power. (B) Harmonic analysis of the entire interval
reveals dominant peaks at 379 and 91.7 k.y., but also has significant red noise. (C) Harmonic analysis of the interval 5.23–3.3 Ma
reveals the same dominant peaks, but red noise peaks are significantly suppressed. (D) Coherence analysis reveals a peak at 91 k.y.
Vertical error bars indicate the 95% confidence interval.
84
Geosphere, April 2010
Sequence stratigraphy and climate cycles in southwestern Tibet
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., 2009.) The δ13Ccc values of gastropods in this interval range from
–3.3‰ to +2.1‰ (VPDB), and δ18Occ values are
from −13.7‰ to +0.7‰ (VPDB; Table 1).
There are 17 type B cycles within the 250–
470 m interval of the South Zhada section (Fig.
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 (Fig. 10). One additional
cycle shows a similar, but muted, trend (Fig.
10). The final two cycles do not show any trend
in δ18Occ values (Fig. 10).
INTERPRETATION OF ZHADA
FORMATION CYCLES
Zhada Formation type A and type B cycles are
best interpreted as parasequences (a conformable
succession of beds separated by flooding sur-
faces; Van Wagoner et al., 1988b). Parasequences
are typically thin (<20 m) and correspondingly
short-lived (~100 k.y.). 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 (Fig. 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 (Fig. 4A). However, type A parasequences have clearly identifiable erosive surfaces that can be correlated to subaerial exposure
surfaces in type B parasequences (Figs. 4, 5, and
6). Thus, the maximum regressive surface in both
type A and type B parasequences is defined as
the erosional surface at the base of the coarsestgrained 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.
TABLE 1. STABLE ISOTOPE DATA
δ 18O
Stratigraphic height
δ13C
(‰, VPDB)
(‰, VPDB)
Sample name
(m)
1SZ12
309.65
–0.9
–3.9
1SZ13
310.65
–3.0
–12.5
1SZ13.8
311.45
–1.3
–3.2
1SZ18
315.65
2.1
–1.8
1SZ24
321.65
–0.2
–5.4
1SZ24.1
321.75
–0.5
–4.6
1SZ27.9
325.55
0.0
–2.8
1SZ32
329.65
0.5
–1.5
2SZ43
376.1
0.9
–1.8
2SZ43.1
376.2
–0.4
–6.8
2SZ47
380.1
–0.6
–1.9
2SZ51.5
384.6
–0.2
–2.4
2SZ51.5AD0.5
384.6
–0.6
–0.6
2SZ51.5AD10
384.6
0.2
–1.2
2SZ51.5AD11
384.6
0.2
–2.1
2SZ51.5AD12
384.6
0.1
–2.2
2SZ51.5AD13
384.6
0.3
–2.1
2SZ51.5AD14
384.6
0.2
–2.5
2SZ51.5AD15
384.6
0.8
–0.8
2SZ51.5AD3
384.6
–0.4
–2.2
2SZ51.5AD4
384.6
–0.1
–2.1
2SZ51.5AD5
384.6
0.3
–1.6
2SZ51.5AD6
384.6
0.9
–0.8
2SZ51.5AD7
384.6
0.0
–0.3
2SZ51.5AD8
384.6
0.0
–0.3
2SZ51.5AD9
384.6
–0.1
–0.4
2SZ55
388.1
–0.7
–2.3
3SZ0.15
389.25
0.9
–1.7
3SZ24
413.1
–0.3
–2.2
3SZ24.1A
413.2
1.6
–2.8
3SZ24.25A
413.35
0.8
–1.4
3SZ24.25B
413.35
–1.1
–1.8
3SZ24.25C
413.35
0.6
–1.6
3SZ24.3
413.4
0.9
–2.0
3SZ27
416.1
–2.1
0.7
3SZ49
438.1
–3.3
–13.7
3SZ50.5
439.6
–1.2
–1.0
3SZ55
444.1
1.2
–2.5
Note: VPDB—Vienna Peedee belemnite; SZ–South Zhada.
Geosphere, April 2010
Type A parasequences occur at the base
of the Zhada Formation sequences (Fig. 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 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 (Fig. 6) and coarsen
upward from a profundal lacustrine lithofacies
association to a supralittoral 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 basinmargin 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., 1988b) but contrasts
with reports of non-Waltherian cycles from the
Green River Formation and underfilled lacustrine basins in the Qaidam basin and Death Valley (Yang et al., 1995; Lowenstein et al., 1998;
Pietras and Carroll, 2006).
DISCUSSION
Sequence Stratigraphic and
Lithostratigraphic Correlations
The overfilled, balanced-filled, 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 into the basin by the paleo–Sutlej
River was consistent and large enough that the
85
Mh
Ml
St
Mh
Sr
Mh
480
Saylor et al.
Sc
460
n = 19
Stratigraphic height (m)
Legend
Sr
Mh
Mh/Sr
Mud cracks
Convoluted bedding/
soft sediment deformation
Hummocky crossstratification
St
Gmm
Mh
Sc
St
Mh
Mr/Mc
St
Mh
Sr
Sf
St
n = 17
440
420
P
Oscillatory current ripples
Unidirectional ripples
Mh
Sh
Erosional surface
Mh
Sh
St
Hcs
St
Gct
Mr
St
Mr
St
St
Sr
Mh
Gastropods
Plants/Plant fragments
Bivalves
Ostracods
Root traces
St
Ml
St
Mh
Sh
Mh
Mh
St
400
Start leg 9 at
N 31˚ 24.158’
E 79˚ 45.442’
4057 ± 6 m
End leg 8 at
N 31˚ 24.449’
E 79˚ 45.342’ 380
4057 ± 13 m
Fish skeletons/fragments
Terrestrial mammal fossils
Climbing ripples
Sr
Mh
Shell fragments
Sh
Mh
Rip-up clasts
n = 17
St
Mr/Sr
Ml
Sr
St
Mh
Lithofacies
Sr
360
n = 16
Gcmi
St
St/Gct
Mh
St
Sf
St
Sh
Gm
Gt
St
Sr
St
340
Start leg 8 at
n = 13
N 31˚ 24.584’
E 79˚ 45.371’
4001 ± 9 m
End leg 7 at
N 31˚ 25.280’
E 79˚ 44.916’
4001 ± 10 m 320
Sh/Sm
Sr
Mr
Sf
Mh
Ml
Mh
St
Sc
Mr
Ml
Mh
Mh
Ml
St
Mm
Ml/St
Sr
Mc
Mh
Ml
St
Ml
St
Sr
Ml
Mh
Ml
n = 14
Start leg 7 at
N 31˚ 25.275’
E 79˚ 44.989’300
3966 ± 6 m
End leg 6 at
N 31˚ 25.281’
E 79˚ 44.986’
3966 ± 7 m
St
Mh
St
Mh
Ml
Sr
Mh
Sr/Sc
Mh
Sr/Sc
280
Mh/Sr
Mh
Start leg 6 at
N 31˚ 25.507’
E 79˚ 45.118’
3933 ± 8 m
End leg 5 at
N 31˚ 26.121’260
E 79˚ 45.421’
3929 ± 10 m
Ml
St
Mh/Sh
St
Mh
Ml
Sh/St
Mh
St
Mh
CS
240
86
SS
Number of paleocurrent
measurements
Paleocurrent direction from
trough cross beds
Paleocurrent direction from
imbricated clasts
Paleocurrent direction from
ground water tubes
Cgm
Ml
Sh
Mh
St
St/Sh
-20-18-16-14 -12 -10 -8 -6 -4 -2 0
δ18O (VPDB%)
Geosphere, April 2010
Figure 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 the maximum
regression surface. Vertical orange boxes
indicate sequences that 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. VPDB—Vienna Peedee
belemnite; C—claystone; S—siltstone;
SS—sandstone; Cgm—conglomerate.
Sequence stratigraphy and climate cycles in southwestern Tibet
not directly underlie the coarse-grained facies.
Rather, the profundal lacustrine facies coarsens upward gradually and shows evidence of
traction transport, including oscillatory current
ripples, throughout regression. The coarsegrained facies exhibit evidence of subaerial
exposure including preferential weathering
and cementation, and root traces. In addition,
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 (Figs. 4C, 4D, 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 the Zhada basin, and that the Zhada
basin rarely became 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 the Zhada basin followed
a typical pattern from a fluvial system to an
underfilled lacustrine basin (Fig. 11) (Bohacs et
al., 2000). However, the top of the Zhada Formation is dominated by coarse-grained, basinmargin 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 paleodepositional
plain. By implication, there was no return to a
balanced fill or overfilled basin type. The return
to fluvial conditions often observed was discontinuous in that it bypassed the balanced fill and
overfilled intervals (Fig. 11).
co
Ac
Accommodation < supply
Fluvial
c
In
ter
val
s
In
te
rva
Underfilled
ls
A
B
C
Accommodation > supply
r
ou
kS
Thic
e
ce
ur
So
Sediment and water supply
ly
Balanced fill
pp
u
~s
n=
io
at
od
m
m
Overfilled
Thin
lake rarely desiccated though surface outflow
was minimal (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;
Fig. 7). The δ18Occ values from this interval
are extremely negative due to the low waterresidence times associated with river throughflow (Saylor et al., 2009).
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 evaporation and so the
basin was being slowly drowned. The balanced
fill interval extends from 1TS to 2MFS1 (Fig.
7). The trend toward more positive δ18Occ values
in this interval and the inferred increase in water
residence times (Saylor et al., 2009) 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
subparallel and well-expressed parasequences
converge and become indistinct toward 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 paleodepositional 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., Carroll,
1998; Bohacs et al., 2000). The primary difference is that coarse-grained facies were presented as the result of transgression by Bohacs
et al. (2000) and Carroll (1998), whereas in
the Zhada basin they typically constitute the
regressive portion of the parasequence. There
are several reasons for interpreting coarsegrained facies as the regressive part of the cycle
in the Zhada basin. Unlike the cycles presented
by Bohacs et al. (2000) and Carroll (1998),
fine-grained, subaerial exposure surfaces do
Eolian
Accommodation (height of sill above base level)
Figure 11. The trajectory of Zhada basin evolution in accommodation and
sediment-supply and water-supply space. Also shown are fields 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).
Geosphere, April 2010
87
Saylor et al.
Frequency Analysis
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 Kelts, 1988; Sladen, 1994; Bohacs et
al., 2000). In addition, in lacustrine settings the
relative proportions of water influx and efflux
(usually climatically driven) and movement on
faults (tectonically driven) are both primary
controllers of systems tracts. 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;
Figs. 4C, 4D).
Water and sediment influx are thus decoupled
from base-level changes but are primary controllers of shoreline trajectory, and therefore
parasequence evolution. Lithofacies distribution
and thus lithologic stacking patterns appear to
be controlled primarily by the location of the
shoreline and so are also decoupled from lake
volume. This means that parasequence flooding surfaces correspond to lake expansion due
to a drop in the evaporation/precipitation ratio.
Thus, the lowest δ18Occ values of aquatic gastropods and, implicitly, of the lake water, are
found at the flooding surface, even though the
coarsest material is associated with maximum
regression (Fig. 12). Particularly when the basin
was underfilled, the highest isotopic values
occur at the time of maximum regression (Figs.
4D and 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 (Fig. 4C, panel
I). Conversely, when influx was greater than
efflux, the δ18Osw value decreased, the lake grew,
1
Parasequence 1
Parasequence 2
Regression
Parasequence 3
0.75
Parasequence 4
Parasequence 5
0.5
0.25
Flooding
Normalized height above flooding Sfc
The two independent time-series analyses
described above indicate that ~100 k.y. cycles
are present in the Zhada Formation. In addition
to a peak at 91.7 k.y., univariate spectral analysis reveals a peak at 22 k.y. These are within
1/2 bandwidth (6 dB bandwidth = 2.4) of the
eccentricity and precession frequencies. Harmonic analysis does not reveal the 22 k.y. peak
indicated by univariate analysis, but does show
peaks at 91.7 and 379 k.y., both of which are
consistent with the eccentricity cycle (Figs. 9B,
9C). Coherence analysis shows coherence with
both eccentricity and insolation records (Laskar
et al., 2004) only at the eccentricity frequency
(Fig. 9D). The fact that both frequency analysis
and an average cycle duration shows 100 k.y.
cyclicity indicates that this signal is robust.
Sequences and parasequences in the Zhada
Formation are either tectonic or climatic in
origin. The correlation between the first-order
transgressive surface (Fig. 7, 1TS) and major
tectonic reorganization in the Zhada region
(Saylor, 2008) points to a tectonic origin for the
first-order sequence. Likewise, the correlation
between the first second-order transgressive
surface (Fig. 7, 2TS1) and maximum flooding
surface (Fig. 7, 2MFS1) with the major tectonic
reorganization and an increase in the exhumation rate on of the Leo Pargil Range, respectively,
(Thiede et al., 2006; Saylor, 2008) also points
to a tectonic origin for second-order sequences.
The number and consistent and short duration
of parasequences rule out a tectonic origin.
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 high-frequency environmental
cyclicity in the Zhada basin was not tectonics. Rather, lacustrine expansion and contraction was caused by a change in the long-term
precipitation to evaporation ratio. Long-term
changes in the precipitation/evaporation ratio
have been linked to strengthening or weakening
of the monsoon due to increases or decreases,
respectively, in insolation (Kutzbach, 1981;
Prell and Kutzbach, 1992; Gupta et al., 2001;
Shi et al., 2001; Ruddiman, 2006; Thompson et
al., 2006). Shi et al. (2001) suggested a causal
link between monsoon strength and Tibetan
lake expansion and, in the absence of a change
in winter rainfall in Tibet, we link Zhada paleolake size to insolation-driven monsoon intensity. It is not surprising that climatically driven
parasequences 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
(Kelly, 1993; Bohacs et al., 2000).
0
-4
-3
-2
-1
0
1
2
3
-16
-14
δ13Ccc(‰ VPDB)
-12
-10
-8
-6
-4
-2
0
2
δ13Ccc(‰ VPDB)
Figure 12. δ18O and δ13C values (Vienna Peedee belemnite, VPDB) of aquatic gastropods from five sequences (indicated in Fig. 10) are plotted against their normalized height above the flooding surface (Sfc). The lowest values occur just above the flooding surface and represent
lake expansion associated with a decrease in the evaporation/precipitation ratio. However, continued evaporative enrichment and isotopic
evolution means that δ18Occ and δ13Ccc values increase through most of the regressive sequence.
88
Geosphere, April 2010
Sequence stratigraphy and climate cycles in southwestern Tibet
and the coarse-grained material was trapped at
the basin margins (Fig. 4C, panel II).
The foregoing discussion indicates that the
primary control of δ13Csw 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 δ13Csw and δ18Osw values, was
significantly altered by addition of a small volume of water. However, 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 here refers primarily to individual parasequences, the effect may span several parasequences and point to climatic control at multiple
frequencies (Fig. 10).
The correlation between low δ18Osw values and flooding described here 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 δ18O values of −14.8‰ 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 (Fig. 4D; black star denotes the interpreted
location of Kungyu Co within the filling and/or
emptying cycle at the time of sampling).
Basin History
Combining the observations made above
with previous studies (Saylor, 2008; Saylor et
al., 2009) points to the following basin history.
Through arc-parallel extension, a sill was created that 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 was
the same as the sediment accumulation rate in
the Zhada basin (Thiede et al., 2006; Saylor,
2008), 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
basin-margin alluvial fans, until a new sill was
eventually breached after 1 Ma. At this point,
the system abruptly returned to fluvial conditions and began incising through the Zhada
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 k.y. cycles in the Miocene (Van Wagoner
et al., 1988a; Kashiwaya et al., 2001; Zachos et
al., 2001; Di Celma and Cantalamessa, 2007;
Holbourn et al., 2007), 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 have
been linked to variability in the strength of the
Indian monsoon (Shi et al., 2001). The Quaternary monsoon is thought to be modulated by
orbital cyclicity (Clemens et al., 1991; Prell and
Kutzbach, 1992; Jian et al., 2001; Wang et al.,
2005; Nie et al., 2008; Y. Wang et al., 2008b),
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 eccentricity frequencies (Dupont-Nivet et al., 2007).
We turn next to another challenge presented
by the Zhada basin, i.e., 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 typically thought
of as native to warm, humid, and, as inferred
by some, low-elevation climates (Li and Zhou,
2001a, 2001b; Zhu et al., 2004, 2007). In addition, a broad cross section of mammal megafauna lived in the Zhada basin area, including
Hipparion zandaense, Nyctereutes, Palaeotragus microdon, and rhinoceri that have variously
been identified as Hyracodon or Dicerorhinus
(Liu, 1981b; Zhang et al., 1981; X. Wang, 2006,
personal commun.; E. Lindsay, 2006, personal
commun.; Li and Li, 1990; Meng et al., 2004).
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).
The recognition of Milankovitch cycles in
the Zhada Formation indicates that insolationdriven global or regional climate change drove
environmental changes in basins at high elevations on the southern Tibetan Plateau. Thus,
Geosphere, April 2010
we can reasonably expect that floral and faunal
communities on the Tibetan Plateau would also
have responded to global climate change. The
shift from C3-dominated forests to mixed C3
and C4 or C4-dominated grasslands observed in
the Zhada basin (Zhang et al., 1981; Zhu et al.,
2006, 2007; Yu et al., 2007; Saylor et al., 2009)
was not the result of basin uplift, because an
identical change is observed in low-elevation
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., 2009). A
more likely scenario is that a regional or global
climatic change affected both low- and highelevation environments and favored a shift from
forest to grassland.
A possible scenario 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 (Guo et al., 2002; Garzione et al., 2003;
Molnar, 2005). Increased warm-season precipitation and increased aridity favor C4 grasses (An
et al., 2005). 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 (Garzione et al., 2000a; Rowley et al.,
2001; Currie et al., 2005; Cyr et al., 2005; Rowley and Currie, 2006; DeCelles et al., 2007;
Dupont-Nivet et al., 2008; Saylor et al., 2009).
CONCLUSIONS
1. Lithologic cycles (types A and B) in the
Zhada basin are Waltherian parasequences.
2. Sedimentology and sequence stratigraphic
analysis indicate 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.
89
Saylor et al.
This agrees with overtopping of a basin sill and
integration of the modern Sutlej drainage network (Brookfield, 1998; Saylor, 2008).
3. Two orders of sequences are recognized in
the Zhada basin in addition to the parasequences
mentioned herein. 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 an average duration of ~92 k.y.
and are likely the result of climatic changes
associated with Milankovitch cycles. This is
the first time that 100 k.y. cycles have been
reported for Late Miocene–Pliocene deposits on
the Tibetan Plateau and presents an unparalleled
opportunity to study high-frequency climate
change at high elevations.
5. Within parasequences, the lowest δ18Occ
values and, by implication, the lowest δ18O values of Zhada paleolake water, are associated
with flooding. From the flooding surface through
maximum regression, δ18Occ values increase.
This trend is the result of low evaporation/
precipitation ratios 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 arcparallel 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 the
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 C4 vegetation had previously been documented on the southern Tibetan Plateau, the
large-scale 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.
ACKNOWLEDGMENTS
We thank David Dettman and Majie Fan for
assistance with stable isotope analyses, and our field
assistants, Cai Fulong, Jeannette Saylor, and Scott
McBride. Reviews by an anonymous reviewer and
J. Pelletier helped to significantly strengthen this
manuscript. Additional support was provided by the
National Science Foundation Tectonics Program,
ExxonMobil, Chevron-Texaco, and the Galileo Circle
of the University of Arizona.
90
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REVISED MANUSCRIPT RECEIVED 26 OCTOBER 2009
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