Dynamics of deformation and sedimentation in the northern Sierras
Pampeanas: An integrated study of the Neogene Fiambalá basin,
NW Argentina
Barbara Carrapa†
Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071, USA
Jörn Hauer*
Institut für Geowissenschaften, Universität Potsdam, 14476 Golm, Germany
Lindsay Schoenbohm
School of Earth Sciences, Ohio State University, Columbus, Ohio 43210, USA
Manfred R. Strecker
Institut für Geowissenschaften, Universität Potsdam, 14476 Golm, Germany
Axel K. Schmitt
Department of Earth and Space Sciences, University of California, Los Angeles, California 90095-1567, USA
Arturo Villanueva
José Sosa Gomez
Instituto de Mineralogía y Geología, Universidad Nacional de Tucumán, Miguel Lillo 209, San Miguel de Tucumán, Argentina
ABSTRACT
The thick-skinned Sierras Pampeanas
morphotectonic domain of western and
northwestern Argentina (27°S–33°S) is characterized by reverse-fault–bounded basement blocks that delimit internally deformed,
Neogene sedimentary basins. Forelandbasin evolution in this part of the Andes is
still not very well understood. For example,
challenging questions exist as to how thickskinned deformation develops, if there are
distinct spatiotemporal trends in deformation and exhumation, how such deformation
styles influence sedimentation patterns, and
whether or not broken foreland basins are
related to regional plate-tectonic processes,
such as flat-slab subduction.
The Fiambalá basin of the northwestern
Sierras Pampeanas is the largest of several
intermontane basins in the transition to the
southern margin of the Puna Plateau. This
basin preserves a thick continental Neo†
E-mail: bcarrapa@uwyo.edu
*Present address: Department of Geosciences,
University of Montana, Missoula, Montana 59812,
USA.
gene sequence that provides information on
the dynamics of thick-skinned deformation
and resulting sedimentation. The Fiambalá
basin contains ~4 km of fluvial-alluvial sedimentary rocks that comprise the Tamberia,
Guanchin, and Punaschotter Formations.
U-Pb geochronology of ashes intercalated
within the Fiambalá stratigraphic sequence
demonstrates that these sedimentary rocks
are late Miocene to Pliocene (8.2 ± 0.3 Ma
to 3.05 ± 0.4 Ma) in age. Sedimentology and
provenance data indicate that the source of
the Tamberia Formation was located to the
west of the modern western basin-bounding
range. The Guanchin and Punaschotter Formations record input from local sources,
including the modern basin-bounding range
to the west and the southern Puna Plateau to
the north, suggesting reorganization of the
catchment area at ca. 5.5 Ma.
The coarsening-upward trends recorded
by the fluvial Tamberia and Guanchin Formations indicate enhanced tectonics and
relief during sedimentation. The Punaschotter conglomerates record alluvial-fan sedimentation and local sources. Fault kinematic
data document a contractional regime, characterized by E-W and NE-SW shortening,
active throughout the middle-late Miocene
and Pliocene. Furthermore, a comparison
between the Fiambalá basin and similar sedimentary basins in the Sierras Pampeanas
(e.g., Bermejo foreland basin) and the Eastern Cordillera leads us to propose that the
study area originally constituted an integral
part of a continuous and more extensive foreland-basin system (thin-skinned) for much
of its early history. Our data suggest coeval
intrabasin deformation along strike from the
Bermejo region northward to the Eastern
Cordillera. The coeval change at ca. 6 Ma
from a regional to more compartmentalized
(thick-skinned) tectono-sedimentary environment in the regions adjacent to the Eastern
Cordillera, the southern Puna margin, and
other sectors within the Sierras Pampeanas
domain may thus reflect a regional tectonic
process related to flat subduction. Our data,
combined with existing sedimentological and
petrological evidence, imply that the passage
from steep to flat subduction occurred synchronously from ~30°S to ~26°S.
Keywords: Sierras Pampeanas, foreland, sedimentation, thick-skinned deformation, thinskinned deformation, Andes.
GSA Bulletin; November/December 2008; v. 120; no. 11/12; p. 1518–1543; doi: 10.1130/B26111.1; 13 figures; 2 tables; Data Repository item 2008143.
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For permission to copy, contact editing@geosociety.org
© 2008 Geological Society of America
Dynamics of deformation and sedimentation in the northern Sierras Pampeanas
INTRODUCTION
The Sierras Pampeanas morphostructural
domain of western and northwestern Argentina
constitutes a broken foreland with basement
ranges and intervening sedimentary basins (e.g.,
Jordan et al., 1983). The Sierras Pampeanas are
an integral part of the Cenozoic Andes orogen
and are located between ~27°S and 33°S (e.g.,
González Bonorino, 1950; Allemendinger, 1986;
Jordan and Allmendinger, 1986), a region that
broadly coincides with a subhorizontal segment
of subduction of the oceanic Nazca plate (Jordan
et al., 1983; Ramos et al., 2002). This domain is
composed of crystalline, ~N-S–oriented basement blocks reaching elevations in excess of 5
km that are bounded by reverse faults (Caminos,
1979). Sedimentary basins in the intervening
low sectors are either fault bounded on their
western and eastern margins or, in the case of
asymmetric range uplift, bounded on one side
by exhumed basement erosion surfaces that dip
underneath the basins (Jordan and Allmendinger,
1986; Strecker et al., 1989; Mortimer et al.,
2007). The structural uplifts and isolated, faultbounded basins in the foreland are akin to the
Late Cretaceous to Eocene Laramide province
of North America (Dickinson and Snyder, 1978;
Jordan et al., 1983). Investigation into clastic
sedimentary rocks deposited in intermontane
basins within the Sierras Pampeanas provides
fundamental information regarding the pattern
and timing of source deformation and exhumation, basin dynamics, and sedimentary environments in evolving foreland-basin systems.
North of 27°S, the Sierras Pampeanas domain
merges into the Puna Plateau, which is part of the
larger Central Andean Plateau and the Eastern
Cordillera (e.g., Mon and Salfity, 1995). Similar to the Sierras Pampeanas, both domains are
characterized mainly by reverse-fault–bounded
blocks and intervening intermontane basins.
The Puna-Altiplano region, also known as the
Central Andean Plateau, is characterized by
average elevations over 3.7 km, and it extends
along strike for 1500 km (e.g., Isacks, 1988).
Former foreland-basin deposits are preserved
within the plateau interior and at its eastern
margin (i.e., Eastern Cordillera) and have been
used to constrain the pattern and timing of contractional deformation, erosion, and sedimentation (Jordan and Alonso, 1987; Viramonte et
al., 1994; Vandervoort et al., 1995; Reynolds et
al., 2000; Marrett and Strecker, 2000; DeCelles
and Horton, 2003; Uba et al., 2005; Jordan and
Mpodozis, 2006). Based on sedimentological, structural, and geophysical data, different
authors have proposed that crustal thickening,
eastward propagation of deformation, and initiation and development of a foreland-basin system
began during the mid-Cretaceous and continued
throughout the Tertiary (e.g., Horton et al., 2001;
McQuarrie and DeCelles, 2001; DeCelles and
Horton, 2003; Carrapa et al., 2005; Arriagada et
al., 2006; Carrapa and DeCelles, 2008).
In the transitional region between the plateau
to the north and the Sierras Pampeanas domain to
the south, the structural and sedimentary evolution remains ambiguous. A crucial issue, which
we address with this study, is the structural and
sedimentary behavior of broken forelands and
their relationships with large-scale plate-tectonic
processes such as flat-slab subduction. Our data
also contribute to a better understanding of the
transition from unbroken foreland (thin-skinned)
to broken foreland (thick-skinned) styles of
deformation and related sedimentation.
Sedimentological evidence suggests that a
foreland-basin system in the Bermejo region
(30°S) commenced ca. 20 Ma but was later disrupted in the late Miocene by uplift of intrabasin
crystalline ranges (Jordan et al., 2001). A oncecontiguous foreland basin likely existed east of
the Eastern Cordillera of northwest Argentina
as well at ~26°S–27°S, where pre-Miocene
foreland-basin sedimentary rocks have been
recognized (Starck and Vergani, 1996; Mortimer
et al., 2007). A comparable setting existed at
~24°30′S in the Quebrada del Toro region of the
Eastern Cordillera between 8 and 6 Ma (Hilley
and Strecker, 2005) and prior to 3.5 Ma in the
Quebrada de Humahuaca at 23°30′S (Walther et
al., 1998; Strecker et al., 2007). All of these locations record the transition to a broken foreland
and ensuing intermontane basin sedimentation
characterized by Miocene-Pliocene strata that
are sedimentologically similar to those observed
in the Fiambalá basin and are interpreted to
have been deposited in an intermontane, thickskinned setting (Hilley and Strecker, 2005; Carrapa et al., 2006a, 2006b; Coutand et al., 2006;
Strecker et al., 2007). It is not clear, however,
if a continuous (thin-skinned) foreland-basin
system ever existed in the Fiambalá region. It
is also unknown how the Fiambalá basin relates
to the structurally and sedimentologically similar basins farther north in the Eastern Cordillera
and northernmost Sierras Pampeanas (Horton,
1998; DeCelles and Horton, 2003; Coutand et
al., 2006; Mortimer et al., 2007).
Here, we present new geochronological,
sedimentological, and structural data from a
Miocene-Pliocene clastic sequence deposited
in the Fiambalá basin in order to: (1) constrain
the timing of sediment deposition in this part
of the Andes; and (2) resolve the sedimentary
and tectonic evolution of the Fiambalá basin
within the Sierras Pampeanas morphotectonic
domain. Ultimately, our new data set relates the
dynamics of deformation and sedimentation in
the Sierras Pampeanas region to the history of
the Eastern Cordillera foreland-basin system.
Structural and sedimentological data were collected mainly along two transects, referred to as
Guanchin River and Northern transects (Fig. 1).
GEOLOGICAL SETTING
The study area is located in the southern
Central Andes of northwest Argentina between
27°45′S and 67°45′S at the transition between
the southern margin of the Puna Plateau and
the reverse-fault–bounded, thick-skinned Sierras Pampeanas morphotectonic domain. Along
strike to the south, at ~30°S, the Miocene to
Holocene stratigraphic sequence preserved in
the Bermejo basin has been interpreted as retroarc foreland-basin deposits characterized by
three different tectono-sedimentary intervals:
between 20 Ma and 7.3 Ma, a broad, wedgelike foreland basin developed to the east of the
Western and Central Precordillera thrust belt;
between 7.3 Ma and 6.5 Ma, a symmetrical
foreland basin formed; and between 6.5 Ma and
the present day, the area was characterized by
thick-skinned deformation and segmentation
of the contiguous foreland basin into small,
reverse-fault–bounded basins typical of the
present-day Sierras Pampeanas domain (Jordan
et al., 2001). The Sierras Pampeanas domain is
thought to be associated with diachronous, thickskinned deformation, which has been attributed
to southeastward migration of flat-slab subduction (Ramos et al., 2002). Westward migration
of deformation and exhumation has also been
proposed (Coughlin et al., 1998).
Diachronous deformation in the Eastern Cordillera, to the northeast of the Fiambalá basin,
has resulted from propagation of the deformation front within a zone of structural inheritance
related to a formerly extensional tectonic regime
(e.g., Allmendinger et al., 1983; Allmendinger,
1986; Mortimer et al., 2007). In the neighboring
El Cajón basin, this is recorded by both thermochronological and seismic-reflection data. Apatite
fission-track (AFT) detrital ages document the
appearance in the sedimentary section of a distinct
grain-age population that is typical of the eastern
basin-bounding range and that indicates uplift
and erosion of the Sierra de Quilmes (Fig. 1) at
6 Ma (Mortimer et al., 2007). Seismic-reflection
data show that loading and basin flexure were
caused by reactivation of out-of-sequence structures bounding ranges to the west (Chango Real;
Fig. 1) and east (Sierra de Aconquija; Fig. 1).
Intrabasin deformation occurred at 6 Ma, resulting in exhumation of the Sierra de Quilmes range
(Fig. 1) (Mortimer et al., 2007).
In the Fiambalá basin, there is no clear evidence of Tertiary rocks older then ca. 8 Ma.
Geological Society of America Bulletin, November/December 2008
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Carrapa et al.
68°0'0"W
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Figure 3
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Figure 2
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Pliocene (Punaschotter Formation) and Quaternary
C
Fiambalá stratigraphy
Miocene (Guanchin and Tamberia Formations)
Sed. rates
(mm/yr)
a b
4 km
Punaschotter Fm.
3.6 ± 0.8 Ma (ZFT)
5.5 ± 0.1 Ma (U/Pb)*
~ 0.4
3.7 ± 0.1 Ma (U/Pb)*
Quaternary volcanic rocks
Quaternary evaporites
Miocene-Pliocene volcanic rocks (andesite-dacite)
Guanchin Fm.
Triassic-Jurassic gabbro
~ 0.6
3 km
late Miocene-Pliocene distal facies:
a) Guanchin Formation, b) Punaschotter Formation
Permian-Triassic granite
2 km
Permian-Triassic red sandstone, conglomerate, marl, and
related volcanic rocks (Paganzo Group)
8.2 ± 0.3 Ma (U/Pb)
Cambrian-Ordovician volcanic (dacites) and metamorphic rocks
1 km
Tamberia Fm.
Ordovician phyllite, schist, and minor metavolcanic
5.7 ± 0.8 Ma (ZFT)
5.9 ± 1.2 Ma (ZFT)
~ 9 Ma (pmg)
mrl fs
ms
~. 2.5
Ordovician granodiorite and minor gabbro
Cambrian phyllite, schist, gneiss, and metavolcanic rocks
Paleozoic granite
gr
cgl
Figure 1. (A) Digital elevation model (DEM) of the study area and morphotectonic domains (shown on inset map) (modified after Jordan et
al., 1983; Isacks, 1988). White line corresponds to the area of the Puna Plateau characterized by internal drainage. (B) Geological map of the
Fiambalá basin (location shown by white box in A and locations of the mapped areas and investigated transects (modified after Martinez,
1995). FN—Filo Negro apatite fission-track (AFT) sample; SP—Sierra de las Planchadas AFT sample; AG—Alto Grande AFT sample; CN—
Cerro Negro AFT vertical profile (Carrapa et al., 2006). NPc—location at the northern end of the Fiambalá basin where extra investigations
have been conducted (refer to text). (C) Simplified stratigraphy of the Fiambalá basin; ZFT (zircon fission-track) ages are after Tabutt (1986);
paleomagnetic (pmg) data are from (Reynolds, 1987). (U/Pb)* ages are from dated ashes from this study (see Fig. 4). The best age for the
Guanchin Formation (029) and one single mean age for the Punaschotter Formation are reported. Mrl—marls; fs—fine sand; gr—granule;
cgl—conglomerate; ms—medium sand. Sedimentation rates were calculated using the ca. 9 Ma constraint from paleomagnetostratigraphy
(Reynolds, 1987) for the base of the Tamberia Formation and our new U-Pb ages for the rest of the stratigraphy.
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Geological Society of America Bulletin, November/December 2008
Dynamics of deformation and sedimentation in the northern Sierras Pampeanas
However, apatite fission-track data from basinbounding ranges at 28°S document a thick pile
of sediments overlying these ranges at ca. 20
Ma (Coughlin et al., 1998). Furthermore, thermal modeling of apatite fission-track detrital
populations suggests that the sediment sources
for the Fiambalá basin strata were once buried,
reaching temperatures up to ~90 °C (i.e., over
4 km of sediments considering a paleogeothermal gradient of 20 °C/km) until ca. 20 Ma,
and they were subsequently removed (Carrapa
et al., 2006). Such reworked sediments must
have been deposited in areas to the east of the
Fiambalá basin since evidence of reworking is
not apparent in the Fiambalá strata. Thus, the
sedimentary record observed today in the Fiambalá basin represents only a part of the complete
sedimentary history in this region of the Andes;
sedimentation may have started much earlier
than the oldest sedimentary rocks preserved in
the basin today.
GEOLOGY OF THE FIAMBALÁ BASIN
AND ITS BOUNDING RANGES
ized by migmatite, schist, granite, and orthogneiss of Cambrian age (Turner, 1967). Apatite
fission-track cooling ages from a vertical profile
from the northern area range (Cerro Negro: CR
in Fig. 1B) are between 23.1 ± 1.2 and 14.8 ±
0.7 Ma and document early to middle Miocene
source cooling and exhumation (Carrapa et al.,
2006b). Detrital apatite fission-track thermochronology and provenance data from the late
Miocene–Pliocene Fiambalá stratigraphic section also suggest that the southern Puna Plateau margin, including the northern and eastern
basin-bounding ranges (Cerro Negro and Alto
Grande: CN and AG in Fig. 1B), acquired a similar (or greater) relief by 6 Ma, and that this was
most likely associated with late-stage plateau
uplift (Carrapa et al., 2006b). A single sample of
granite from the eastern basin-bounding range
(Alto Grande: AG in Fig. 1B) has an apatite
fission-track age of 13.1 ± 1.7 Ma (Carrapa et
al., 2006b), which may represent cooling during
exhumation and removal of the sediments that
once covered this range.
Tertiary Clastic Sequence
The Fiambalá basin contains a late Cenozoic
continental clastic sequence bounded to the
west by the northern termination of the Famatina Range (Fig. 1A). To the north and east, the
basin is bordered by basement ranges belonging
to the southeastern margin of the Puna Plateau.
Western Basin-Bounding Ranges
Immediately to the west of the Fiambalá
basin, the Sierra de las Planchadas (part of the
Famatina Range) is mainly composed of Precambrian granite, aplite, and diorite; Ordovician-Devonian siltstone-mudstone; Permian
dacitic volcanic rocks, clastic sedimentary
strata, and basaltic conglomerates (La Cuesta
Formation); and Tertiary andesitic volcanic
rocks (Fig. 1) (Turner, 1967; Martinez, 1995).
Previous studies have shown that the Filo Negro
(FN; Fig. 1B) and the Sierra de las Planchadas
(SP, Fig. 1B) Ranges, to the west of the Fiambalá basin, are characterized by apatite fissiontrack cooling ages of 43.9 ± 4.4 and 34.6 ± 4.2
Ma, respectively, suggesting that deformation
and associated exhumation of these ranges had
already started by the end of the Eocene (Carrapa et al., 2006b). This is also supported by
apatite fission-track data obtained south of the
study area (Coughlin et al., 1998).
Northern and Eastern Basin-Bounding
Ranges
Basement rocks located to the north and
northeast of the Fiambalá basin are character-
The Tertiary basin fill is divided into the
Tamberia and Guanchin Formations; the unconformably overlying conglomerates are referred
to as the Punaschotter Formation, following
the terminology introduced by Penck (1920)
(Fig. 1C). Whereas the Guanchin and Punaschotter Formations are exposed in the north as
well as to the south of the basin, the Tamberia
Formation crops out only along and south of
the Guanchin River (Fig. 2A). Distal facies of
the Punaschotter Formation crop out along the
Guanchin River transect (Fig. 2). Prior to this
study, little was known about the stratigraphic
age of these sediments. Reynolds (1987) proposed an age of ca. 9 Ma for the base of the
Tamberia Formation based on magnetostratigraphy (Fig. 1C). One ash layer in the Tamberia
Formation was dated at 5.9 ± 1.2 Ma (zircon fission-track age) by Tabutt (1986). Two ash layers
in the Guanchin Formation were dated at 5.3 ±
0.7 Ma and 3.6 ± 0.8 Ma, respectively, by Tabutt
(1986) (Fig. 1C); however, these ashes were
collected from a highly deformed stratigraphic
section (Fig. 2A). It must be noted that most of
the ashes in the study area display some degree
of reworking. Therefore, a careful reassessment
of the stratigraphy of these sedimentary rocks
is necessary.
where detailed mapping and sedimentological
investigations were conducted during this study.
An additional ash from the lower portion of the
Guanchin Formation and one from the distal
Punaschotter Formation were collected along
the Northern transect (Fig. 3) to correlate the
different mapped areas and stratigraphy. Results
are presented in Figure 4 and Table 1. The
stratigraphic position of the collected samples
was carefully selected on the basis of detailed
mapping across the entire basin (Figs. 2 and 3).
Mineral separation was conducted at the University of Potsdam following established procedures. Zircon analysis was performed using the
University of California–Los Angeles (UCLA)
Cameca IMS 1270 ion microprobe with a
mass-filtered, ~15 nÅ 16O– beam focused to a
25–30-μm-diameter spot. For further details on
zircon U-Pb analysis, refer to Appendix 1.1
U-Pb Results
Sample 006 is an ash collected from the upper
portion of the Tamberia Formation (member B,
discussed in the following). The age obtained
from eight grain analyses is 8.20 ± 0.33 Ma,
demonstrating that the base of the Tamberia
Formation is likely older than 8.2 Ma. This
is consistent with magnetostratigraphic data,
which suggest an age of ca. 9 Ma for the base of
the Tamberia Formation (Reynolds, 1987).
Two ashes (029; conglomeratic facies) and
(055; sandstone facies) were collected in the
Guanchin Formation along the Guanchin River
transect (Fig. 2A). Seven out of ten grains in
sample 029 give a mean age of 5.51 ± 0.15 Ma.
The youngest grain out of five zircons from
sample 055 has an age of 5.23 ± 0.30 Ma. The
range of ages of other grains in sample 055,
from late Oligocene to late Miocene, suggests
considerable reworking; a lack of material prevented additional analyses. The youngest zircon
age is, however, consistent with the age from
sample 029 and sample J26/04 collected farther
north (Fig. 3A). Ash J26/04, from the lower part
of the Guanchin Formation, was collected along
the Northern transect (area indicated in Fig. 3A),
and it yielded an age of 5.90 ± 0.20 Ma. Results
from samples 029 and 055 corroborate a late
Miocene age for these sedimentary rocks. In the
following, we use the better constrained age of
sample 029 for our geological interpretations.
Three ashes (001, 074, and 003 from top to
bottom) along a ~20 m vertical section were
collected from the top of the Punaschotter
U-Pb GEOCHRONOLOGY
We present new U-Pb geochronological data
from eight ashes, six of which were collected
along the Guanchin River transect (Fig. 2A),
1
GSA Data Repository Item 2008143, U-Pb geochronology, is available at www.geosociety.org/
pubs/ft2008.htm. Requests may also be sent to editing@geosociety.org.
Geological Society of America Bulletin, November/December 2008
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Geological Society of America Bulletin, November/December 2008
27°45'S
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A
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006
Pc
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Pc
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Basement rocks
(Carboniferous-Permian)
Tamberia Formation
(facies a:Ta; facies b:Tb)
Guanchin Formation
Punaschotter Formation
Undifferentiated
10
20
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Pc19
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A' B' location of cross section
e.g. 029: ash sample for U-Pb analysis
reverse fault
syncline
main river
fractures in pebbles-cobbles
F
anticline
detailed logs
L
Pc pebble counting locations
B'
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Figure 2. (A) Geological map of the Guanchin River transect; for location refer to Figure 1B. (Continued on following page.)
70
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Carrapa et al.
Geological Society of America Bulletin, November/December 2008
-2000
0
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A’
(1)
Tb
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(2)
P: Punaschotter Formation
G: Guanchin Formation
Tb: Tamberia Formation (facies b)
Ta: Tamberia Formation (facies a)
1
10
Ta
(3)
Figure 8
2
15
Distance (km)
G
P
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20
Tb
Figure 10 Figure 9
?
25
P
?
G
30
?
B’
Figure 2 (continued). (B) Structural profile along the Guanchin River transect. 1, 2 and 3 are locations along the profile where kinematic analysis was undertaken. Fault planes
are plotted on the stereoplot; the small arrows on the cyclographic traces indicate the movement of the hanging wall based on slickenside measurements. Kinematic results are
presented as “fault plane solution” (cf. Allmendinger, 2001) indicating pressure-tension (P-T) quadrants. The gray quadrants indicate extensional fields from which shortening direction is derived. (1) Thrust contact between the Carboniferous-Permian red beds and the Punaschotter Formation. (2) Thrust contact between the Tamberia and upper
Guanchin Formations. (3) Thrust within the Guanchin Formation. See text for more explanations.
Elevation (m)
B
Dynamics of deformation and sedimentation in the northern Sierras Pampeanas
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Carrapa et al.
67°54' W
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A
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PS1
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Pc
5j
Pc
4j
67°48' W
67°46' W
Pc
3j
70
11
18
J28/04
Pc
1j
Pc
2j
10
60
48
14
29
18
55 16
L2
27°20' S
10
Pc
12j
L3
Palo
Blanco
L4
17
15
55
PS2
?
?
54
12
Pc
11j J26/04
16
Pc J4/04
10j
42
27°20' S
L1
B'
50
?
Pc
7j
27°22' S
60
24
77
22
70
74
18 9j
24
12
16
24
70
80
do
olora
Rio C
80
82
Pc
20
20
27°22' S
45
Pc
8j
85
27°24' W
27°24' S
85
80
74
0
1
2 km
67°54' W
67°52' W
67°50' W
67°48' W
67°46' W
Holocene (undifferentiated sediments)
Carboniferous-Permian
strike & dip
Early to late Pliocene (Punaschotter Fm.)
Ordovician-Carboniferous
overturned layer
Late Miocene to early Pliocene (Guanchin Fm.)
A' B'
thrust fault
location of cross section
e.g. J26/04: ash sample for U-Pb analysis
NW
SE
P
1000
2000
P
Tb
-1000
-2000
Ta
-3000
P: Punaschotter Formation
G: Guanchin Formation
Tb: Tamberia Formation (facies b)
Ta: Tamberia Formation (facies a)
-4000
-5000
-6000
-7000
0
2
P
1000
G
0
-8000
SE
3000
2000
Elevation (m)
NW
3000
Elevation (m)
B
anticline
4
6
8
10
Distance (km)
P
G
0
Tb
-1000
-2000
Ta
-3000
-4000
-5000
-6000
décollement
12
14
-7000
-8000
0
2
4
8
6
10
Distance (km)
12
14
Figure 3. (A) Geologic map of the Northern transect; for location refer to Figure 1B. (B) Balanced cross section along the Northern transect
showing both the box fold (left) and blind thrust (right) scenarios; for location refer to Figure 3A.
1524
Geological Society of America Bulletin, November/December 2008
Dynamics of deformation and sedimentation in the northern Sierras Pampeanas
A
n
(
)
A
(
)
n
n
n
n
n
n
F
n
Figure 4. Zircon U-Pb results; 207Pb/206Pb versus 238U/206Pb and 206Pb/238U model ages (histograms and probability density curves). Plot shows
fixed-intercept regression (207Pb/206Pb = 0.83) and disequilibrium-corrected concordia curve between 3 and 20 Ma. Weighted average ages for
individual samples are stated with 2σ uncertainties. MSWD—mean square of weighted deviates.
Geological Society of America Bulletin, November/December 2008
1525
Carrapa et al.
TABLE 1. ZIRCON U-Pb RESULTS
Sample-grain-spot
006-g1-s1
006-g2-s1
006-g5-s1
006-g1-s2
006-g4-s1
006-g8-s1
006-g9-s1
006-g14-s1
006-g18-s1
006-g19-s1
Pb*/238U
(x10–3)
3.059
76.76
4.004
1.268
12.74
1.300
1.922
4.748
1.336
8.627
206
Pb*/238U
(1 s.e.)
(x10–3)
0.131
3.81
0.198
0.0666
0.464
0.0466
0.0919
0.186
0.0368
0.279
207
Pb*/235U
(x10–3)
235.5
618.9
107.9
15.25
1387
11.55
74.67
407.4
14.02
860.9
207
Pb*/235U
(1 s.e.)
(x10–3)
11
31.2
10.4
1.44
53.1
0.882
7.03
15.2
1.03
29.4
Correlation
of concordia
ellipses
0.80
0.90
0.78
0.29
0.89
0.22
0.81
0.52
0.48
0.80
Disequilibrium corrected, 207Pb corrected†
Pb/238U
±
Radiogenic
U
age (Ma)
Age (Ma)
%
(ppm)
6.88
1.04
34.5
417
476
23
99.8
256
20.9
1.4
80.9
215
7.80
0.44
94.8
431
4.12
4.22
4.9
188
8.26
0.30
97.7
403
8.72
0.71
69.9
298
8.11
1.47
26.3
236
8.35
0.24
96.2
695
7.53
2.44
13.4
341
206
Th
(ppm)
353
59
164
626
239
323
316
333
613
114
055-g6-s1
055-g5-s1
055-g4-s1
055-g3-s1
055-g2-s1
4.539
4.282
5.081
0.8194
1.562
0.207
0.191
0.234
0.0231
0.0617
293.6
95.51
413.9
7.623
36.93
13.1
6.04
24.4
0.459
2.66
0.69
0.62
0.84
0.31
0.32
13.5
23.6
10.0
5.23
8.54
1.5
1.3
2.0
0.15
0.42
45.9
85.3
30.4
97.3
84.0
174
157
523
818
186
79
91
186
273
110
029-g14-s1
029-g13-s1
029-g11-s1
029-g10-s1
029-g9-s1
029-g8-s1
029-g6-s1
029-g5-s1
029-g4-s1
0.8567
5.027
0.8630
0.8623
0.8588
0.9248
0.8907
85.93
0.9154
0.0251
0.225
0.0389
0.0303
0.0305
0.0283
0.0294
3.17
0.0267
6.058
45.97
9.175
9.332
9.11
15.55
8.373
710.1
13.02
0.43
3.55
1.06
0.78
1.2
1.2
0.601
31.9
0.555
0.39
0.53
0.49
0.65
0.14
0.63
0.28
0.80
0.48
5.59
31.6
5.42
5.42
5.41
5.42
5.68
530
5.57
0.16
1.5
0.26
0.20
0.21
0.19
0.19
19
0.17
99.3
97.5
96.0
95.9
96.1
90.3
97.2
99.7
92.7
1101
168
336
669
284
2088
775
38
1514
215
107
249
296
149
3707
179
23
141
074-g1-s1
074-g2-s1
074-g6-s1
074-g7-s1
074-g9-s1
074-g12-s1
074-g13-s1
074-g14-s1
074-g15-s1
074-g17-s1
074-g19-s1
1.009
0.6217
0.5889
0.6118
0.592
0.6827
0.7122
0.6245
0.5698
3.881
0.5815
0.0355
0.0214
0.017
0.0294
0.0203
0.028
0.0378
0.0268
0.0153
0.205
0.0195
9.599
5.553
4.746
9.592
8.801
21.38
18.67
10.09
4.541
384.3
4.954
0.946
0.681
0.349
1.78
0.977
1.39
2.36
0.875
0.327
22.8
0.595
0.20
0.08
0.29
–0.11
–0.11
0.36
0.37
0.01
0.24
0.89
0.03
6.40
4.00
3.82
3.68
3.58
3.46
3.82
3.76
3.71
3.61
3.76
0.23
0.14
0.11
0.21
0.14
0.20
0.28
0.18
0.10
1.84
0.13
97.1
97.6
98.4
91.4
92.1
76.9
81.6
90.9
98.5
14.1
98.0
376
886
1931
150
523
352
272
418
1553
158
870
157
399
1421
107
556
277
217
133
569
109
427
001-g5-s1
001-g6-s1
001-g7-s1
001-g8-s1
001-g9-s1
001-g13-s1
001-g14-s1
001-g17-s1
001-g18-s1
001-g19-s1
1.510
0.5858
1.937
0.5739
4.896
0.6144
1.245
0.5671
0.6224
0.5984
0.0405
0.02
0.0673
0.0177
0.8950
0.0218
0.0356
0.0293
0.0317
0.0233
105.4
4.814
162.6
6.073
507.4
8.786
81.92
7.734
7.879
9.668
3.91
0.556
7.55
0.468
103
0.723
2.19
1.04
1.07
1.01
0.59
0.06
0.54
0.44
0.88
0.47
0.85
0.39
–0.13
0.29
4.09
3.79
3.57
3.64
3.17
3.74
3.69
3.48
3.85
3.58
0.34
0.13
0.61
0.12
8.16
0.15
0.26
0.20
0.21
0.16
41.2
98.3
28.1
96.1
9.8
92.6
44.9
93.2
94.2
90.9
1400
579
336
690
1246
606
1221
191
207
666
796
424
360
429
908
680
562
176
190
565
003-g1-s1
003-g2-s1
003-g3-s1
003-g4-s1
003-g5-s1
003-g6-s1
003-g7-s1
003-g9-s1
003-g10-s1
003-g14-s1
0.6558
0.6062
3.176
0.6571
0.5898
0.6680
0.6263
0.7904
0.5831
0.6579
0.0284
0.0235
0.101
0.0201
0.0186
0.0307
0.0202
0.0781
0.0152
0.0449
13.89
9.734
294.4
10.35
4.894
13.07
9.836
22.96
5.216
12.59
1.7
0.875
11.7
0.647
0.617
1.13
0.684
3.12
0.358
2.08
0.16
0.34
0.77
0.37
0.28
0.42
0.51
0.61
0.48
–0.10
3.72
3.62
4.16
3.96
3.81
3.86
3.76
4.10
3.73
3.82
0.21
0.16
0.92
0.13
0.12
0.21
0.14
0.53
0.10
0.31
86.3
91.0
19.9
91.3
98.2
87.8
91.3
79.0
97.6
88.2
287
479
247
983
1065
299
679
63
1323
142
241
590
226
411
937
243
606
47
1623
98
1.567
1.398
79.29
43.84
0.8928
0.7881
93.19
181.7
26.34
76.51
1.126
0.5845
1.029
0.099
0.112
1.95
0.879
0.0474
0.0364
1.88
6.45
0.819
2.88
0.128
0.026
0.106
0.84
0.85
0.63
0.83
0.78
0.52
0.86
0.77
0.89
0.78
0.60
0.67
0.84
2.97
3.25
479
274
2.87
3.16
571
1034
5.09
472
3.00
2.83
3.17
0.92
0.93
12
5
0.41
0.29
11
36
7.50
17
0.98
0.21
0.94
29.1
35.3
97.2
99.1
49.5
61.2
99.3
95.8
3.0
99.2
40.7
74.6
46.6
307
341
139
779
416
752
376
61
332
458
142
717
298
617
360
47
368
1018
1151
74
29
450
256
243
1713
180
J26-04-g1-s1
1.384
0.0447
59.88
3.38
0.84
5.83
0.34
65.8
J26-04-g2-s1
1.099
0.0321
25.96
1.87
0.63
5.97
0.23
84.0
J26-04-g3-s1
1.184
0.0337
34.42
2.78
0.71
6.11
0.27
78.9
J26-04-g4-s1
1.201
0.0349
39.03
2.93
0.71
5.92
0.28
75.8
J26-04-g5-s1
1.056
0.0288
24.67
1.66
0.73
5.81
0.21
84.2
J26-04-g6-s1
1.17
0.0253
31.13
1.34
0.55
6.21
0.18
81.2
J26-04-g7-s1
1.778
0.0942
114.2
9.88
0.70
5.40
0.82
46.3
J26-04-g8-s1
1.494
0.0755
71.13
6.58
0.85
6.03
0.61
61.8
J26-04-g9-s1
37.06
0.743
286.3
6.51
0.86
233.1
4.6
99.3
J26-04-g10-s1
1.349
0.0561
58.31
4.83
0.67
5.80
0.45
65.8
Note: 207Pb was corrected using common Pb compositions: 206Pb/204Pb = 18.86; 207Pb/204Pb = 15.62 (Sañudo-Wilhelmy and Flegal, 1994).
*Radiogenic.
†230
Th disequilibrium correction factor was calculated from (Th/U)zircon/(Th/U)melt, where (Th/U)melt = 3 (average from Siebel et al., 2001).
911
2488
1217
1056
2291
1420
342
612
1292
493
3498
5522
672
1535
2038
1056
182
428
794
434
J4-04-g1-s1
J4-04-g2-s1
J4-04-g3-s1
J4-04-g4-s1
J4-04-g5-s1
J4-04-g6-s1
J4-04-g7-s1
J4-04-g8-s1
J4-04-g9-s1
J4-04-g10-s1
J4-04-g11-s1
J4-04-g12-s1
J4-04-g13-s1
1526
206
129.8
106.5
853
356.2
54.3
37.96
830.6
2644
2924
661
79.14
19.71
65.84
11.8
10.4
33.1
8.66
4.92
2.94
20.2
143
95.1
25.6
9.51
2.15
11.4
Geological Society of America Bulletin, November/December 2008
Dynamics of deformation and sedimentation in the northern Sierras Pampeanas
Formation, along the Guanchin River transect,
at the highest elevation reached by those sedimentary rocks in the basin (Fig. 2A). In all
three samples, more than 10 grains yielded
overlapping Pliocene ages; from top to bottom, sample 001 gave an age of 3.69 ± 0.06
Ma, sample 074 gave an age of 3.74 ± 0.05
Ma, and sample 003 gave an age of 3.77 ± 0.05
Ma. Only one late Miocene grain was dated,
and based on the lack of detrital contamination,
reworking of these ashes appears to have been
minor. Ash J04/04 was collected from distal
facies of the Punaschotter Formation (Fig. 3);
eight grains yielded an age of 3.05 ± 0.44 Ma.
These results, therefore, date the deposition of
the Punaschotter Formation in the Fiambalá
basin to the early-middle Pliocene.
Tabutt (1986) suggested an age of 3.6 ± 0.6
Ma for the Guanchin Formation on the basis
of zircon fission-track dating of one ash collected along the highly deformed portion of
the Guanchin River transect (Fig. 2A). This
result implies that the overlying Punaschotter
conglomerates must be younger than ca. 3.6
Ma. However, our new data document that the
onset of the Punaschotter deposition must have
been older than 3.77 ± 0.05 Ma, as indicated
by sample 003 from the top of the Punaschotter conglomerates along the Guanchin River
transect. These new ages thus refine the stratigraphic record of the basin fill and provide the
opportunity to assess the onset of conglomerate
deposition in adjacent intermontane basins.
SEDIMENTOLOGY
The following descriptions are based on
detailed facies analysis and measured sections,
conducted by centimeter-scale tape and compass measurements, along the Guanchin River
and Northern transects (Figs. 5, 6, and 7).
Tamberia Formation
The Tamberia Formation crops out only in the
central (Guanchin River transect) and southern
parts of the basin (Figs. 2 and 3), where it consists of over 2000 m of red sandstone, siltstone,
and clast-supported, well-organized, and wellsorted conglomerate. On the basis of distinct
lithofacies, we have divided the Tamberia Formation into two members (members A and B).
The lower member A (Figs. 5A and 5B) consists of ~1000 m of reddish medium-grained
sandstone bodies of meter-scale thickness, intercalated with centimeter- to meter-scale mudstones with large mud cracks (up to 1 m scale);
the sandstone to mudstone ratio is >>1. These
units generally lack sedimentary structures,
have a massive to tabular character, and show
only local parallel and trough cross-stratification. Up-section, this member becomes coarser
and grades into well-sorted granular sandstone
and fine- to coarse-grained, horizontally stratified, generally normally graded conglomerates that constitute the upper member B. This
member reaches a thickness of ~1000 m and has
an overall massive and tabular character, with
local crude horizontal bedding and imbricated
clasts, and a general lack of deep scour surfaces
(Fig. 5C). The overall external geometry of the
sedimentary bodies of the Tamberia Formation
is tabular; the lateral extent of the sedimentary
bodies constituting this formation is on the order
of several tens of kilometers.
The member A strata show a lack of features, such as lateral accretion foresets, prominent levees, oxbow lake deposits, and crevasse
splay deposits, which would indicate meandering channels and anastomosing fluvial systems.
For this reason, and because of the massive and
laterally extensive character of the sand bodies, we suggest that the lower Tamberia strata
were deposited in low-sinuosity, sandy, braided
channels (Smith, 1970; Allen, 1983; Miall and
Turner-Peterson, 1989; Hein and Walker, 1977;
Smith and Perez-Arlucea, 1994; Miall, 1996;
Makaske, 2001; Lunt and Bridge, 2004; Wooldridge and Hickin, 2005).
Fine-grained deposits are often intercalated
among the sandstone beds, but they lack paleosol
development and plant remains, suggesting river
bank instability and poorly confined flood flows
(Nemec and Steel, 1984; Miall, 1996). Furthermore, the presence of mud cracks in the mud layers suggests an episodically dry environment.
The horizontally stratified, imbricated, and
locally trough cross-stratified conglomerates
of member B, which lack sharp erosional contacts, are interpreted to be the result of bed-load
deposits of broad channels on the floodplain
(Nemec and Steel, 1984; Miall, 1996). The limited trough cross-stratified sandstones are interpreted as channel deposits (Nemec and Steel,
1984; Miall, 1996); the associated siltstone layers were derived from waning flow conditions
following channel deposition and abandonment.
The general tabular and coarse character of
these sedimentary bodies suggests deposition
through poorly confined flows in a high-energy
river representing the proximal part of a large
braided fluvial system.
We interpret the coarsening-upward trend
from member A to member B to be the result
of progradation of the depositional system and
possible enhancement of tectonic activity driving source exhumation, erosion, and sedimentation. The provenance data discussed later document erosion of source rocks directly west of the
Fiambalá basin, supporting this interpretation.
Guanchin Formation
The Guanchin Formation consists predominantly of sandstone and conglomerate. We estimate a minimum thickness of ~1500 m along
the Northern transect (Fig. 3A). The contact
with the stratigraphically lower Tamberia Formation along the Guanchin River transect is a
high-angle reverse fault (Fig. 2A); the contact
is not exposed along the Northern transect. The
Guanchin Formation is better exposed along
the Northern transect (Fig. 3), which is essentially undeformed, whereas the Guanchin River
transect is deformed and cut by several structures (Figs. 2A and 2B). The following facies
descriptions are based primarily on observations
and logging along the Northern transect, complemented by additional observations and limited logging along the Guanchin River transect
(Figs. 6A and 6B).
The Guanchin Formation consists of
medium- to coarse-grained lenticular gray to
pink, centimeter- to meter-scale sandstones,
with intercalations of fine- and medium-grained
conglomerates and mudstones. A coarseningand thickening-upward trend is observed along
both transects. In the sandstone bodies, planar
to trough cross-stratification, up to 2 m high, is
common, but massive sandstone bodies are also
present. The sandstone bodies generally show
erosional bases. Dewatering structures occur
locally. Centimeter- to meter-scale conglomeratic lenses are common and frequently show
fining-upward trends and express sharp erosional
basal contacts. Massive- to planar-laminated red
mudstone layers, which range in thickness from
0.5 cm up to 40 cm, are typical of these deposits.
In some cases, the mudstones are reworked and
deposited as muddy intraclasts (diameter up to
25 cm; Fig. 6C) within sandstones. The mud layers must have consolidated prior to erosion and
reworking, suggesting a periodically dry environment. Poorly sorted and moderately rounded,
gray to pink conglomerate occurs toward the top
of the formation. The conglomerate beds have a
massive character and local horizontal stratification and erosional basal contacts.
On the basis of its lithofacies association
(Table 2) and the absence of typical features
associated with meandering channels, the
Guanchin Formation is interpreted to have
been deposited in an ephemeral braided fluvial
environment. We infer the planar and trough
cross-stratified sandstones to be representative of channel deposits. The structureless,
meter-scale sandstone bodies are interpreted to
be the result of rapid deposition from suspension during floods. The conglomerate bodies
with sharp erosional contacts are considered to
represent channel fills resulting from bed-load
Geological Society of America Bulletin, November/December 2008
1527
Carrapa et al.
A
Log 3-Tb (facies B)
Tamberia Fm.
(Guanchin transect)
Log 2-Tb (facies B)
Tamberia Fm.
(Guanchin transect)
Log 1-Ta (facies A)
Tamberia Fm.
(Guanchin transect)
+~1200 m (estimated cover)
Sr
Gh
+~750 m (estimated cover)
Sh
St
50 m
50 m
Sh
Gc
Pc8
Pc9
+~300 m (estimated cover)
Gc
Sh/St-Gt
Gc
Sh
Sr
St
Sh
25 m
25 m
Gh
Sm
25 m
25 m
St/Gt
Gh
Pc10
Sr
Sh
Sh
Pc7
Mrl
FS
MS
Gr
Cgl(~5cm)
Cgl(~10cm)
Mud cracks
Conglomerate
Ripples
Trough cross-stratification
Sandstone
Intraclasts
Ignimbrite
(1) Sandstone with granules
(2) with conglomeratic lens(1)
and channel(2)
Pc: pebble counting locations
e.g. Sp: facies after Miall (1978), refer to Table 2.
Figure 5. (A) Sedimentological logs of the Tamberia Formation along the Guanchin River transect; refer to Table 2 for facies explanation and
to Figure 2A for location. Mrl—marlstone; fs—fine sandstone; ms—middle sandstone; gr—granule; cgl—conglomerate. (Continued on following page.)
1528
Geological Society of America Bulletin, November/December 2008
Dynamics of deformation and sedimentation in the northern Sierras Pampeanas
PHOTO 1
Tabular, massive sandstones
member A
Tamberia Fm.
PHOTO 2
Massive sandstones
with mud cracks
member A
Tamberia Fm.
1m
person for scale
B
PHOTO 1
Horizontally laminated and stratified,
clast-supported conglomerates (Gh)
member B
Tamberia Fm.
PHOTO 2
Imbricated, clast-supported conglomerates (Gi)
member B
Tamberia Fm.
imbrications
1m
1m
C
Figure 5 (continued). (B) Typical facies A of the Tamberia Formation: massive and tabular sandstones (photo 1) with occasional large mud
cracks (photo 2). Refer to Table 2 for facies explanation. (C) Typical facies of the Tamberia member B Formation: horizontally laminated and
stratified (photo 1), occasionally imbricated (photo 2), clast-supported conglomerates. Refer to Table 2 for facies explanation.
Geological Society of America Bulletin, November/December 2008
1529
Carrapa et al.
A
Mud cracks
Conglomerate
Ripples
Trough cross-stratification
Sandstone
Intraclasts
Ignimbrite
(1)
(2)
Sandstone with granules and
conglomeratic channel(1) and lens(2)
Log 5
Guanchin Fm.
(Guanchin transect)
+~140 m (estimated cover)
15-10-03/09#*
Log 4
Guanchin Fm.
(Guanchin transect)
Pc: pebble countings
e.g. Sp: facies after Miall (1978); refer to Table 2.
50 m
Log 6
Guanchin Fm.
(Guanchin transect)
+~150 m (estimated cover)
Pc13
Pc12
25 m
25 m
25 m
Ash 055
Pc14
Pc11
Mrl.
FS
MS
Gr
Cgl(~5cm)
Cgl(~10cm)
Figure 6. (A) Sedimentological logs of the Guanchin Formation along the Guanchin River transect; refer to Table 2 for facies explanation and
to Figure 2A for location. Mrl—marlstone; fs—fine sandstone; ms—middle sandstone; gr—granule; cgl—conglomerate. (Continued on following two pages.)
1530
Geological Society of America Bulletin, November/December 2008
Dynamics of deformation and sedimentation in the northern Sierras Pampeanas
B
Guanchin Fm.
(Northern transect)
158.63 m
(2)
(1)
Sh
Gc
150 m
Sh, Gh
150 m
150.26 m
Sh
Fl
Sh
Fl
Gh
125 m
125 m
Fl
Sp
Fl
Sh
cobbles & boulders
water escape structures
intraclasts (mudstone)
sandstone with conglomeratic lens(1)
and channel(2)
geometrically
estimated
distance from
L3 top to L4
basis: 383 m
Gc
125 m
122.15 m
Sh
Sh, Gh
fining upward
coarsening upward
Fl
100 m
Sh
Sh
Gc
Sh
Gc
pc:
pebble countings
e.g. Gh: main facies after Miall (1978)
Sh
Sh
Gh
Gc
Fl
Sh
Sh
Figure 6 (continued). (B) Sedimentological
logs of the Guanchin Formation along the
Northern transect; refer to Table 2 for facies
explanation. Refer to Figure 3A for location.
(Continued on following page.)
Gc
Sh
Fl
Sh
100 m
100 m
Gh
Gh
Sh
Gh
Gp
Sh
Fl
Sh
Gc
Sh
Sh
220 m
Gh
Gh
Sh
Sh
Sh
Sh
Fl
Gh
Sh
trough cross-stratification
occasional boulders
(>50cm diameter)
Fl
75 m
pebbly sandstone with granules
ash layer/ignimbrite
Gc
100 m
planar cross-stratification
Quaternary cover
Gh
Sh
parallel lamination or bedding
Sh
Sh
125 m
mudstone
sandstone
Sh
Gh, Sh
Fl
75 m
75 m
Fl
Gp
Gh
Fl
Fl
Gh
Sh, Sm
75 m
Sh, Gh
200 m
Gh
Gh
Fl
Sh
Sh
Sh
Gh
Sh
Fl
50 m
Sh
Fl
50 m
50 m
Gc
Gh
Gc
Sp
Fl
Sh
Gh
25 m
Gh
Sh
Sh, Sm
Sh, Fl
Gp
Fl
Sh
Sp
Sh
Gh
Gt
Sh
Fl
Sh
Gh
St
Fl
25 m
Gh
175 m
Sh
Sh
Gh
Sh
Sh, Sm
Sh, Gh
Sh, Gh
Fl
Fl
Gc
Gh
Fl
Gh
Sh
25 m
Sh
Sh
J26/04
Sh, Gh
50 m
Sh, Sm
Gc
25 m
Fl
Sp
Sh
150 m
Fl
Sh
Sh
Sh
Sp
0m
Mrl
FS
MS
Gr
Cgl (~5 cm)
Cgl (~10 cm)
L1
Bottom
Gh
pc 11J
Sh
Gh
pc 12J
Fl
Sh
Sp
0m
Mrl
FS
MS
Gr
Cgl (~5 cm)
Cgl (~10 cm)
distance: 558 m
L2
Gh
Sh
Sh
0m
Mrl
FS
MS
Gr
Cgl (~5 cm)
Cgl (~10 cm)
distance: 879 m
L3
0m
J6/04#
Fl
Gh
Fl
Gh
Mrl
FS
MS
Gr
Cgl (~5 cm)
Cgl (~10 cm)
Mrl
FS
MS
Gr
Cgl (~5 cm)
Cgl (~10 cm)
distance: 1973 m
Sh, Fl
L4
Top
Geological Society of America Bulletin, November/December 2008
1531
Carrapa et al.
C
30 cm
PHOTO 2
Trough cross-stratified sandstones of the
Guanchin Fm.
1m
PHOTO 1
Trough cross-stratified conglomerates (Gt)
and trough cross-stratified sandstones (St)
of the Guanchin Fm.
Figure 6 (continued). (C) Typical facies of the Guanchin Formation: trough cross-bedded conglomerates (Gt) (photo 1), trough cross sandstones (St), and
reworked mud clasts (photo 3). Refer to Table 2 for
facies explanation.
30 cm
PHOTO 3
reworked mud clasts
in the Guanchin Fm.
1532
Geological Society of America Bulletin, November/December 2008
Dynamics of deformation and sedimentation in the northern Sierras Pampeanas
A
Location in the basin
Northern transect
Location in the basin
Guanchin transect
West center East (distal)
West
East
(distal)
(pc6)
Pc18-P
001: 3.69 ± 0.12 Ma
074: 3.74 ± 0.10 Ma
003: 3.77 ± 0.10 Ma
mrl
n=50
fs
ms
mcg
cgl
Pc20-P
n=49
Punaschotter Formation
Pc15-P
Pc4j
n=48
Pc4-G
Pc16-G
Pc12-G
1km
Pc3-G
Guanchin Formation
Pc5j
Pc19-P
Pc17-P
Pc6-G
Pc10j
n = 100 (PS100)
J4/04:
3.05 ± 0.44 Ma
Pc3j
PS2
mrl
fs
ms
mcg
Pc2j
cgl
Pc1j
n = 10 (PS31_A)
Pc9j
Pc7j
n = 15 (PS31_B)
055:
5.23 ± 0.30 Ma
Pc11-G
PS1
1km
Pc5-P
0.5 km
Punaschotter Formation
Pc6jjj
n = 12 (PS2_4)
Pc8j
mrl
fs
ms
mcg
n = 6 (PS1_4)
cgl
Pc13-G
2 km
n=11
member B member A
Pc10-T
006:
8.20 ± 0.33 Ma
Pc2-T
Pc8-T
Pc1-T
Pc7-T
1 km
Tamberia Formation
Pc9-T
1 km
Guanchin Formation
Pc14-G
Pc11j
Pc12j
J26-04:
5.91 ± 0.22 Ma
cgl
mrl
fs
ms
mcg
Typical source lithologies present in the source and relative abundance in %
cgl
West of Guanchin profile West of Northern profile
Granite
Pelitic intraclast
Aplite
Red sandstone
Dacitic volcanic
and metasediment
Andesitic volcanic
White sandstone
Mud crack
Paleosol
Plant remain
mrl
fs
ms
mcg
Qz. and Quartzite
Schist-Gneiss/Phyllite
Northeastern (southern Puna) and eastern sources
50%
Diorite
e.g. 055, J26/04: U-Pb age of zircons from ashes (Figures 2, 3, 4)
Figure 7 (continued on following page.)
Geological Society of America Bulletin, November/December 2008
1533
Carrapa et al.
B
Typical source
lithologies
100%
Typical clast lithologies
along the Guanchin
River transect
Typical clast lithologies
along the Northern transect
Granite
Aplite
Dacitic volcanic and metasediment
Andesitic volcanic
Pelitic intraclast
Red sandstone
80%
White sandstone
Qz. and Quartzite
Schist-Gneiss/Phyllite
60%
Diorite
40%
Facies
codes
Gm
Gc
Gh/i
Gt/p
Sm
St
Sp
Average Punaschotter Fm.
Average Guanchin Fm.
Average Punaschotter Fm.
Average Guanchin Fm.
TABLE 2. FACIES TABLE FOLLOWING THE SCHEME OF MIALL ET AL. (1978)
Lithofacies
Sedimentary structures
Interpretation
Conglomerate, matrix
supported
Conglomerate, clast
supported
Massive, faint gradation
Conglomerate, clast
supported
Conglomerate, clast
supported
Horizontal lamination,
occasional imbrication
Trough and planar cross-beds;
average foreset thickness of 50
cm to 1 m
Massive or faint lamination
Sandstone, fine to
coarse
Sandstone, fine to very
coarse, may be pebbly
Massive, faint horizontal
lamination, imbrication
Trough cross-beds,
10–50 cm average scale
Sandstone, fine to very
coarse, may be pebbly
Sandstone, fine to very
coarse, may be pebbly
Sandstone, very fine to
coarse
Sandstone, fine to
medium, well sorted
Sandstone, siltstone,
mudstone
Planar cross-beds
Fsm/m
Siltstone, mudstone
Massive, desiccation cracks
P
Paleosol carbonate
Pedogenic features: nodules,
filaments
Sh/l
Sr
Seod
Fl
1534
Average Tamberia Fm.
Source to the northeast
(Puna margin) and east
Source west of
Northern transect
Source west of
Guanchin transect
20%
Figure 7 (continued). (A) Clast count data collected along the Guanchin River and Northern transects; for location refer to Figures 2
and 3. Pc—clast count data; for location see
Figure 2A. Mrl—marlstone; fs—fine sandstone; ms—middle sandstone; mcg—granule;
cgl—conglomerate; e.g. 055 indicates sample
code for ashes indicated in Figures 2B, 3B,
and 4; PS—Punaschotter paleocurrent (pc)
data indicated in Figure 3B. (B) Cumulative
histograms for the clast-count data collected
along different transects and from different sediment source lithologies. The relative
abundance of source lithologies represents a
volumetric estimation of those lithologies in
the source area. The source lithologies are
shown in rough stratigraphic order.
High-strength (cohesive)
debris flow
High-strength or lowstrength (noncohesive)
debris flow
Longitudinal gravel bars,
lag deposits
Minor channel fills,
transverse bed forms
Distal debris flow
Subaqueous 3D dunes,
sustained unidirectional
currents
Subaqueous 2D dunes
Horizontal laminations/low-angle
(<15°) cross-beds
Ripple cross-lamination
Scour fills
Large-scale cross-lamination
(>25°)
Fine laminations, very small ripples
Eolian dunes
Ripples (lower flow regime)
Overbank, abandoned
channel or waning flood
deposits
Overbank, abandoned or
drape deposits
Soil with chemical
precipitation
Geological Society of America Bulletin, November/December 2008
Dynamics of deformation and sedimentation in the northern Sierras Pampeanas
deposits in small channels on the floodplain.
Mudstone bodies are interpreted as pond-fill in
shallow floodplain areas after flooding events
or crevasse splays. The horizontally stratified
conglomerates occurring at the top of this formation are likely the result of varying discharge
in an alluvial-plain environment (Flint, 1985;
DeCelles, 1991).
Overall, we interpret the Guanchin Formation
to be the result of individual flood events on an
alluvial plain (Nemec and Steel, 1984; Miall,
1996). The coarsening- and thickening-upward
trends expressed by the Guanchin rocks could
be the result of a second event of progradation
of the sedimentary system coupled with a further pulse of source tectonic activity that may
have been related to eastward propagation of
deformation, enhanced source exhumation,
and resulting reduction of the distance between
source and basin.
Punaschotter Formation
The Punaschotter Formation consists of
medium- to coarse-grained conglomerates
(Figs. 7A and 7B). The boundary with the underlying Guanchin Formation is both transitional
and unconformable in places. The thickness of
this unit is ~600 m. These conglomerates are
mainly massive and disorganized; parallel layering and slight normal gradation occur locally.
The conglomerates are mainly clast-supported
and generally poorly sorted. The clasts are wellto subrounded and range in size from 5 cm up
to 50 cm. Beds and clast assemblages are disorganized, and in some places, imbrications were
observed and measured (Fig. 7A). Ash layers
with thicknesses up to 2 m are commonly intercalated. We interpret these conglomerates to be
representative of hyperconcentrated flows. This
facies association was likely the result of highdensity stream flows (Todd, 1989), which are
typically associated with proximal alluvial-fan
environments (DeCelles et al., 1991).
PROVENANCE
Clast counts from the Tamberia, Guanchin,
and Punaschotter conglomerates help constrain
sediment source lithologies and spatiotemporal
changes in the sediment source. New data from
the Guanchin and Punaschotter Formations have
been collected and integrated with existing data
(Carrapa et al., 2006) in light of the new stratigraphic constraints. Clast-count analyses were
performed at 20 localities along the Guanchin
River transect and at 12 locations along the
Northern transect (Figs. 7A and 7B). Pebbles
of different lithologies were counted every 5–10
cm (depending on granulometry) within a 50 ×
50 cm grid. The grid was shifted parallel to
bedding until at least 100 clasts were counted
at each locality. Paleocurrent directions were
recorded in imbricated conglomerates by measuring 10–100 imbrications per location. These
data were then restored to horizontal using the
bedding plane and are reported as rose diagrams
in Figure 7.
Clast-Count Data
The conglomerates of the Tamberia Formation, member B, are mainly composed of dacite, red sandstone, and minor andesite, granite,
and quartzite clasts (Fig. 7A). These lithologies
represent the Permian-Triassic red sandstone,
Cambrian-Ordovician volcanics (Fig. 1B), and
Paleozoic granites and granodiorites, all present in western bounding ranges (Fig. 7B). Dacite clasts are more abundant in the northwestern part of the basin than directly to the west;
the upward increase in dacite clasts suggests a
reorganization of the drainage system toward
the northwest (Figs. 1B and 7B).
Conglomerates of the Guanchin Formation
are compositionally similar to Tamberia Formation conglomerates (member B), except that
they also contain schist and phyllite clasts. Both
conglomerates investigated along the Guanchin
transect and those along the Northern transect
are from more basinward (eastern) locations
compared to the Tamberia locations, and they
likely record more distal contributions (or a
more extensive source). However, most of the
conglomerates investigated along the Northern
transect are proximal with respect to the western basin-bounding ranges, and therefore they
are likely to record more local sources. Along
the Guanchin River transect, the conglomerates
record a strong contribution from andesite and
granite lithologies and minor red sandstone,
dacite, quartzite, aplite, and pelite lithologies;
the latter document reworking from underlying mudstone layers within the same formation
(Fig. 6C; photo 3). The up-section increase in
granitic clasts may reflect a contribution from
a reorganization of the sediment-source region
to include granites present both to the north and
to the east of the basin. Also, schists and phyllites, typical of northern and eastern sources,
suggest a reorganization of the sediment source
toward the north and northeast. This reflects
exhumation and erosion of the Puna Plateau
margin, which is also corroborated by apatite
fission-track data (Carrapa et al., 2006). Two
clast counts from the Northern transect record
a major input from granite, phyllite, and limited
red sandstone, biotite schist, and other minor
components. The lower volcanic contribution in
the Northern transect compared to the Guanchin
River transect reflects the different abundance of
such lithologies in the original sediment source
in the late Miocene and highlights its local provenance signal.
The composition of conglomerates of the
Punaschotter Formation along both transects
reflects a wider contribution of sources with
respect to those recorded in the Tamberia Formation conglomerates, but it is similar to those
in the Guanchin Formation. In particular, the
Punaschotter conglomerates of the Guanchin
River transect record a strong contribution from
schist, phyllite, and gneiss, which is typical of
northern and eastern sources (Puna-margin
related). Distal facies, which crop out basinward
(east), record a wider spectrum of lithologies,
suggesting a larger drainage including western,
northern, and eastern sources. This is also supported by our paleocurrent data (Fig. 7A).
In order to evaluate the relative effects of
unroofing versus reorganization of the source
area, we compiled a clast-count diagram that
shows the typical lithologies of different sediment sources (Fig. 7B). However, in a regional
reconstruction such as this, some spatial information is lost. In the case of simple unroofing due to tectonic exhumation along a single
structure, lithologies typical of shallower crustal
levels should decrease and eventually disappear
up-section, while lithologies typical of deeper
crustal levels should appear at a certain stratigraphic level and increase up-section (Graham
et al., 1986). This trend is not observed in Figure 7B, suggesting that a reorganization of the
source to involve different and more abundant
specific lithologies (i.e., schist, phyllite, and
gneiss) was superimposed on simple initial tectonic unroofing.
Taken together, the conglomerates along both
transects document first the erosion of western
bounding ranges and then a reorganization of
the source area toward the north and northeast
at the time of deposition of the upper Tamberia
and Guanchin Formations, between ca. 8.2 Ma
and ca. 5.2–5.5 Ma. This trend continued to the
time of deposition of the Punaschotter Formation conglomerates, which record a contribution typical of western, northern, and eastern
sources.
STRUCTURAL DEFORMATION
Structural investigations were conducted
along the two mapped transects (Figs. 2 and
3); detailed structural analyses were carried
out mainly along the Guanchin River transect
where structures are better exposed (e.g.,
Figs. 8, 9, and 10). Fold axes and fault-plane
lineations were used to reconstruct the kinematics of deformation. The kinematic analysis was
Geological Society of America Bulletin, November/December 2008
1535
Carrapa et al.
E
W
Ta mberia
Ta mberia
Tamberia (anticline)
Punaschotter
Guanchin
1m
Tamberia (syncline)
NE
SW
Ta mberia
1m
Figure 8. Open folds in the Tamberia Formation; for locations, refer to Figures 2A and 2B.
performed using the program FaultKinWin 1.1
(Allmendinger, 2002) following the method outlined in Marrett and Allmendinger (1990). This
method allows the contouring of two pressuretemperature (P-T) axis distributions; these two
populations are projected as P-T quadrants, similar to earthquake focal mechanisms (Fig. 2B).
Furthermore, ubiquitous transgranular pebble
and cobble fractures developed at contact points
between neighboring clasts have been used as a
proxy for the direction of tension and to infer the
direction of compression (e.g., Eisbacher and
Hopkins, 1977). The directions of transgranular
fracture planes in clasts are represented in stereonets in Figure 11. These data were used ultimately to distinguish different phases of deformation. Next, we describe deformation along the
Guanchin River transect, the Northern transect,
and a separate location (NPc in Figs. 1B and 11)
at the northern end of the Fiambalá basin.
W
Guanchin River Transect
person for scale
The westernmost reverse fault along the
Guanchin River transect strikes NNW, dips
76ºSW, and places Permian basement rocks of
the Paganzo Group (Figs. 1 and 2B) over the
Tamberia Formation (conglomeratic facies of
member B). Lineations plunge 71ºS, indicating
dominantly dip-slip displacement and NE-SW–
directed shortening (Fig. 2B). Along strike to
the north, the same structure carries Ordovician
metavolcanics and Carboniferous granitic rocks
over Punaschotter conglomerates (Fig. 12).
1536
Figure 9. Tight fold in the Tamberia Formation; for location, refer to Figures 2A and 2B.
Immediately east of the main bounding structure, the Tamberia Formation is overturned, and
to the east, it is tightly folded into a series of
anticline-syncline pairs (Fig. 2). Folds in the
Tamberia Formation indicate ~E-W to NE-SW
shortening (Figs. 8 and 9). The Tamberia For-
mation is unconformably overlain by the Punaschotter Formation, which is folded into a gentle,
slightly north-plunging syncline that has a N-S
axis (Fig. 2B).
To the east, a second reverse fault, which
strikes N-S (Figs. 2A and 2B), places the
Geological Society of America Bulletin, November/December 2008
Dynamics of deformation and sedimentation in the northern Sierras Pampeanas
W
Guanchin Fm.
E
1m
Figure 10. Example of open folds (syncline-anticline pair) in the Guanchin Formation; for location, refer to Figures 2A and 2B.
Transgranular fractures in pebbles and cobbles
Guanchin River transect
F4: upper Punaschotter
Northernmost Fiambalá Basin
NPc: Punaschotter undifferentiated
F3: Punaschotter
n = 50
n = 78
F2: upper Guanchin Fm.
n = 85
F1: lower Guanchin Fm.
n = 83
n = 85
Figure 11. Transgranular pebble-cobble fractures constraining the orientation of tension
from which the direction of shortening may
be inferred. For location of the measurements,
refer to Figures 1 and 2A.
Geological Society of America Bulletin, November/December 2008
1537
Carrapa et al.
person for scale
Figure 12. Granitic basement carried (310/35°) over the Punaschotter Formation; view is to
the NNE along the Northern transect. For location, refer to Figure 1.
deformed Tamberia Formation (member A) to
the west over the Guanchin Formation to the
east. Minor faults associated with this structure
were measured (Fig. 2B) and indicate NE-SW
shortening. Ash sample 029 has an age of 5.51 ±
0.15 Ma and was collected just under this fault
contact, indicating that deformation took place
sometime after ca. 5.5 Ma. To the east of this
fault, strata of both the Guanchin and Punaschotter Formations dip gently (~10º) to the east, and
there appears to be a transitional contact, rather
than an unconformity, between the two formations. Farther east, a thrust within the Guanchin
Formation strikes NW-SE and dips shallowly
(24º) to the west (Fig. 2B). Kinematic analysis
of fault striae indicates ENE-WSW shortening (Fig. 2B). East of this thrust, the Guanchin
Formation is folded into an open fold, indicating E-W to NE-SW shortening (Fig. 10). Farther east, the dip increases near the GuanchinTamberia contact. The Tamberia strata near this
location appear to be tightly folded (Fig. 9).
Although no structures are exposed in the
eastern part of the section, two are suggested by
the mapped relationships. The first would place
strata of the Tamberia Formation on those of the
Guanchin Formation, but it is covered by Punaschotter conglomerates. The second, to the east
of the section, would account for open folds and
back-tilting in the easternmost Guanchin rocks.
Both are indicated with dashed lines in the cross
1538
section (Fig. 2B). The Punaschotter Formation
in this region is deformed into a gentle syncline.
It is in clear unconformity with the Tamberia
Formation to the west, and while the eastern
contact with the Guanchin Formation is not
exposed, the dips are approximately the same in
both units (~30ºW), indicating a possible conformable contact.
Northern Transect and Northernmost
Fiambalá Basin
A similar style of deformation is observed
along the Northern transect, though the amount
of deformation within the basin is less (Figs. 3A
and 3B). The major basin-bounding fault is
present, but there are more structural complications in the basement hanging-wall rocks to the
west. The Tamberia Formation is not exposed,
and the Guanchin and Punaschotter Formations
are folded but apparently not broken by faults
along this transect (Fig. 3B).
Along the west side of the basin, basement
rocks are carried over late Miocene to Pliocene
sedimentary rocks of the Fiambalá basin along
the basin-bounding fault, which here is a NNWstriking reverse fault (Fig. 12). These basement
rocks are internally deformed by several minor
faults, the strikes of which vary from NNW to
NNE and both east and west vergence. Fault
zones within the granite commonly include
fault breccias and gouge of strongly fractured
and cataclasized granite, which varies in color
from white to green to violet. Whereas most
structures show evidence for thrust-sense displacement, striae on one basement fault indicate
strike-slip (dip 6° toward 349° on a NNW-SSE–
striking fault plane).
Adjacent to the basin-bounding fault, the Punaschotter conglomerates are complexly, tightly
folded, and in places overturned (Fig. 3A). To
the east, the Guanchin and the Punaschotter
Formations are folded into a broad, asymmetric anticline with a NNE-SSW–striking fold
axis. Here, the Punaschotter-Guanchin contact
is transitional. The eastern limb of the fold dips
~20°ESE, whereas the western limb is steeper,
with dip angles around 65°WNW. This fold
can either be interpreted as a west-vergent box
fold or as a fault-propagation fold above a westdirected blind thrust (Fig. 3B and 3C). Due to
the lack of subsurface data (e.g., seismic-reflection profiles), this question cannot be resolved.
However, fault-propagation folds above westvergent blind faults in the Bermejo Valley to the
south (Zapata and Allmendinger, 1997) exist in
the region and suggest that similar structures
could also be present in the Fiambalá basin.
If the structure is a box fold, the décollement
would be located within the basement at a depth
of 10 km (Fig. 3B). A west-vergent propagating
blind thrust below the fold could have a deeper
décollement depending on the dip angle of the
fault (Fig. 3C). Similar results for décollement
depth (10–20 km), based on seismic-reflection
profiles, have been reported in the Bermejo Valley (Zapata and Allmendinger, 1997; Allmendinger and Zapata, 2000; Jordan et al., 2001). A
minimum horizontal shortening of 18% is calculated along the Northern transect. This value
also agrees with estimates for the Bermejo basin
(Allmendinger and Zapata, 2000). The orientation of fold axes generally indicates E-W contraction. Minor folds in the Punaschotter Formation along the Rio Colorado have fold axes that
strike NW-SE, reflecting NE-SW shortening.
The basin-bounding fault, which can be traced
north from the Guanchin River and Northern
transects, is a reverse fault that dips 75ºW in the
northernmost part of the basin. This is steeper
than along the Northern transect but similar to
the dip along the Guanchin River transect. Only
a few hundred meters of the top of the Guanchin
Formation are exposed in this area, deposited
directly on bedrock. In most places, a series
of thick ignimbrite units is deposited directly
on basement. The ignimbrites likely originated from the Cerro Blanco Caldera complex
(Viramonte et al., 1994; Coira and Kay, 2004)
on the Puna Plateau to the north. Although the
ignimbrites are not dated here, elsewhere, Cerro
Geological Society of America Bulletin, November/December 2008
Dynamics of deformation and sedimentation in the northern Sierras Pampeanas
Blanco units have yielded 40Ar/39Ar ages of 0.2
± 0.1 Ma (Siebel et al., 2001) and 0.55 ± 0.1 Ma
(Seggiaro et al., 2008). The ignimbrite units are
interbedded toward their top with Punaschotter
Formation or facies-equivalent conglomerates.
The ignimbrites and conglomerates are gently
folded and are cut by two east-vergent reverse
faults, which document NE-SW–directed
shortening (Schoenbohm and Strecker, 2005).
Younger N-S extension is accommodated along
an E- to NE-striking normal fault to the north
of the Fiambalá basin, which likely corresponds
to a younger phase of deformation (late Pliocene to present) documented in many locations across the Central Andean Plateau (e.g.,
Allmendinger, 1986; Allmendinger et al., 1989;
Marrett et al., 1994).
Punaschotter Contact
The base of the Punaschotter Formation is
unconformable with the Tamberia Formation
in every exposure (e.g., western and eastern
ends of the Guanchin transect), indicating that
structural deformation and erosion predate
deposition of the Punaschotter conglomerates.
The relationship between the Punaschotter and
Guanchin Formations, however, is more complex. In the central part of the Guanchin transect
(Fig. 2), and in the Northern transect (Fig. 3), the
Punaschotter-Guanchin contact is transitional,
indicating little or no deformation. To the east,
we were not able to observe the contact directly,
but dips are similar in the Guanchin Formation and overlying Punaschotter conglomerates
(30ºW). The Punaschotter conglomerates that
are transitional with the Guanchin Formation,
along the Guanchin transect, are younger than
5.2 Ma and older than 3.7 Ma (Figs. 2 and 4).
There are no stratigraphic constraints, however,
for the unconformably overlying Punaschotter
conglomerates that crop out just east of the main
basin-bounding fault in the Guanchin transect
(Fig. 2B). These relationships probably reflect a
combination of propagation of deformation into
the basin, and variation in the age of the base
of the Punaschotter Formation. Deformation
occurred both before and during deposition of
the Punaschotter conglomerates.
STRUCTURAL EVOLUTION OF THE
FIAMBALÁ BASIN
Faults and blind faults within the basin
accommodate varying amounts of displacement and apparently terminate along strike
without carrying on into adjacent regions. The
style of deformation, however, is consistent in
all study areas. Most structural indicators, such
as fault and fold-axis orientations and striae on
major and minor fault planes, document E-W to
NE-SW contraction. Transgranular fractures in
clasts document NE-SW and NW-SE tension
(Fig. 11), which can be reconciled with fault
kinematic indicators. The nature of the Punaschotter contact, overthrusting of Punaschotter
conglomerate by the basin-bounding fault, and
overthrusting of Guanchin ash-bearing strata
indicate deformation from at least ca. 5.5 Ma to
3.7 Ma. Warped and offset fluvial terraces suggest protracted deformation to the present day.
A regional kinematic shift in Pliocene-Quaternary time (Allmendinger, 1986; Allmendinger et
al., 1989; Marrett et al., 1994) is documented
to the far north of the Fiambalá basin, where a
normal fault offsets basement and ignimbrite and
accommodates N-S extension (Schoenbohm and
Strecker, 2005). Changes in the tectonic stress
field have been inferred from the southern Puna
Plateau margin in the past 2–4 m.y. as a consequence of plateau uplift, lateral extrusion of crust
from the plateau to the south (Allmendinger,
1986), late Cenozoic reorganization of South
American plate motion (Marrett and Strecker,
2000), and gravitational extensional spreading
(Schoenbohm and Strecker, 2005).
No evidence of faulting along the east side
of the Fiambalá basin has been detected. Thus,
the Fiambalá basin seems to have been formed
through progressive loading along reverse faults
farther to the west (e.g., Chilean Precordillera;
Fig. 1) at the time of Tamberia deposition, just
west of the Fiambalá basin during Guanchin and
Punaschotter deposition, and back-tilting of the
Sierra de Fiambalá range along a fault(s) along
its eastern edge. This is consistent with the
structural geometry in other Puna-margin basins
where the main active bounding structure loading the basin is located to the west—as deformation propagates eastward, it encounters inherited
zones of crustal weakness, and it locks, enhancing both loading from the west and back-tilting
from eastern and intrabasin structures (e.g.,
Mortimer et al., 2007).
SEDIMENTARY EVOLUTION AT 28°S
The sedimentary record preserved in the
Fiambalá basin suggests that sedimentation
and deformation occurred in a distal to proximal braided fluvial system and in alluvial fans
within a regional to broken foreland-basin setting. The oldest sedimentary rocks preserved
in the basin are ca. 9 Ma (Reynolds, 1987).
However, the basin history may be significantly
longer, as suggested by the presence of poorly
documented Miocene (Fig. 1) and pre-Miocene
sedimentary rocks preserved today to the west
of the basin (Turner, 1967). Our provenance and
structural data suggest that the basin received
sediments and subsided in response to progressive tectonic activity and loading.
At ca. 6 Ma, a reorganization of the paleodrainage system occurred to include northern and
eastern source areas. The Punaschotter conglomerates record coarse alluvial-fan sedimentation
characterized by a local drainage pattern, with
sources both to the west and northeast. We interpret the reorganization of paleodrainage at ca. 6
Ma to have been due to the combination of the
Puna Plateau growing southward and uplift of
ranges east and north of the basin (Alto Grande;
AD in Fig. 1B). The systematic coarsening
upward of the investigated strata is consistent
with progradation of the sedimentary system in
response to eastward migration of deformation.
The characteristic unconfined to poorly confined
facies of the Neogene Fiambalá basin sequence
suggest dry climate that prevented vegetation
and enhanced bank instability and avulsion. We
speculate that the Tamberia Formation strata represent the last stage of a more extensive foreland
basin, as supported by the facies association that
documents deposition in a large river system
with source in ranges located farther to the west
with respect to the Sierra de las Planchadas.
Correlation with coeval strata along strike in the
Bermejo basin supports this interpretation. Furthermore, sedimentation rates (Fig. 1C) appear
to have been much higher during Tamberia
deposition (>1 mm/yr) than during Guanchin
and Punaschotter deposition (<1 mm/yr), which
is consistent with a large drainage and regional
foreland at the time of Tamberia deposition. The
two cycles of coarsening (first in the Tamberia,
then in the Guanchin Formation) are interpreted
to reflect propagation of the sedimentary system
related to growth and emplacement of two distinct thrust sheets. Numerous faults have been
documented within and to the west of the Sierra
de las Planchadas (Fig. 1B). However, it was
beyond the scope of this study to further investigate the timing of emplacement of these thrust
sheets.
Collectively, our data suggest that the Fiambalá basin might have once formed a continuous, unbroken foreland with other Neogene
basins within the present-day Sierras Pampeanas
region. In the study area, this basin was disrupted
at ca. 6 Ma (Fig. 13). Basin disruption was coeval
with paleodrainage reorganization and establishment of the high relief along the southern Puna
margin as indicated by apatite fission-track data
(Carrapa et al., 2006b). Tertiary sedimentary
rocks have been reported west of the study area
(Turner, 1967). These sedimentary rocks document an older history of the Fiambalá basin that
could date to Eocene-Oligocene time and would
be in agreement with recent thermochronological data, which suggest that the present-day
Geological Society of America Bulletin, November/December 2008
1539
Geological Society of America Bulletin, November/December 2008
SF
10-5 Ma (AFT m.a.) (12)
11-9 Ma (AFT m.a.) (12)
15-10 (AFT c.a.) (12)
10-5 Ma (AFT m.a.) (12)
reverse fault activation
and exhumation at ca.6 Ma (16)
50 100 150 200 250
Km
64°W
25-20 Ma exhumation age after
burial inferred from AFT modeling
pre-Miocene range exhumation
(AFT age)
Indirect evidence of foreland basin
deposits covering present-day
topographic highs:
15-20 Ma exhumation age after burial
inferred from AFT modeling (m.a.)
CQ
65°W
38-29 Ma(13)
Sierra Pampeanas
Bj
SM
SU
Fb
EC
A
(16)
66°W
Ea
s
t
e
r
n Co
rdille
ra
SVZ: Sierra de Valle Fertil
SF: Sierra Famatina
SM: Sierra La Maz
SU: Sierra Umango
Bj: Bermejo foreland Basin
Fb: Fiambalá Basin
CQ: Corral Quemado
EC: El Cajón
A: Angastaco
AFT c.a.: apatite fission-track cooling age
AFT m.a.: inferred age of final exhumation from AFT modeling
e.g., (1) see references in the text
0
creation of high relief
by 6 Ma (14)
ca. 25 Ma(15)
Puna
Plateau
67°W
4km
0 km
13.5Ma(2)
1 km
11Ma(2)
9.8Ma(2)
9Ma(2)
8Ma(2)
3 km
7Ma(2)
20 Ma
possible older sedimentary
rocks not preserved today (12, 14)
9 Ma(3)
1km
2km
8.2 Ma(3)
3km
4km
5.5 Ma(3)
3.7 Ma(3)
Fiambalá (Fb) Basin
Conglomerate
Fluvial sandstone
Eolian sandstone
Floodplain mudstone
10.7 Ma(6)
5.7 Ma(7)
Basins
A
EC
CQ
Fb
S Bj
N
time 10
5
0
(8)
(8)
Paleocurrent direction average
provenance (e.g. from NE to SW in this example) inferred from
typical clast count composition
regional deformation (continuous foreland basin)
local, intrabasin deformation (broken foreland)
e.g. (1); reference for chronostratigraphic ages
and paleoflow indicators; refer to figure caption for details
1 km
9 Ma(4)
2 km
7.14 Ma(5)
3.66 Ma(5)
3 km
1 km
El Cajon (EC) Basin
Corral Quemado (CQ) Basin
less constrained
possible
well constrained
Spatio-temporal distribution for intrabasin thick-skinned deformation and sedimentation
17.5 Ma(1)
16 Ma(1)
1 km
14.5 Ma(1)
2 km
13.5 Ma(1)
13 Ma(1)
3 km
12.3 Ma(1)
12 Ma(1)
11 Ma(1)
5 km
4 km
5Ma(2)
5 km
3.7Ma(2)
2.5Ma(2)
6 km
Bermejo (Bj) Basin
thick-skinned deformation (TKD)
thin-skinned deformation (TND)
1 km
13.4 Ma(9)
3 km
9.1 Ma(10)
5 km
6 km
(11)
(12)
Angastaco (A) Basin
Figure 13. Satellite image of the southern Central Andes and location of the Neogene basins discussed in the text. The stratigraphy of the Bermejo foreland basin is after Jordan et
al. (2001), the Fiambalá basin stratigraphy is from the present work, Corral Quemado basin stratigraphy is after Muruaga (2001), El Cajon basin stratigraphy is after Mortimer
et al. (2007), and Angastaco basin stratigraphy is after Coutand et al. (2006) and Carrapa et al. (2006a). Local intrabasin deformation in the Fiambalá, Corral Quemado, El Cajon,
and Angastaco basins is inferred from activation of high-angle faults, basin-bounding ranges (both to the west and east), change in provenance, paleocurrent indicators, and the
presence of unconformities. (1) Jordan et al. (2001) and Reynolds et al. (2001); (2) Johnson et al. (1986), Fernandez (1996), Jordan et al. (2001); (3) present work; (4) Sasso (1997);
(5) Latorre et al. (1997); (6) Strecker (1987) and Strecker et al. (1989); (7) Bossi et al. (1997); (8) Mortimer et al. (2007); (9) Coutand et al. (2006); (10) Marshall et al. (1983); (11)
Carrapa et al. (2006a); (12) Coughlin et al. (1998); (13) Coutand et al. (2001); (14) Carrapa et al. (2006b); (15) Carrapa et al. (2005); and (16) Deeken et al. (2006).
32°S
31°S
30°S
29°S
28°S
27°S
26°S
25°S
Precordil
lera
68°W
TKD
TND
69°W
TKD
24°S
70°W
F
SV
Precordillera
TKD
1540
TKD
23°S
Carrapa et al.
Dynamics of deformation and sedimentation in the northern Sierras Pampeanas
basin-bounding ranges were once covered by
sedimentary units (>4 km thick) that are no
longer preserved (Carrapa et al., 2006). Indeed,
in addition to data presented in this study, thermochronological data from Sierras Pampeanas
blocks, and related sedimentary rocks, suggest
that these ranges were once covered by forelandbasin sediments (Coughlin et al., 1998). Regional
isopach mapping (Fielding and Jordan, 1988;
Re, 1995; Re et al., 2003) indicates that Sierras Pampeanas basement rocks, farther south,
were buried beneath ~11 km of combined Upper
Paleozoic to Neogene strata. In the northernmost
Sierras Pampeanas Cumbres Calchaquíes and
Sierra Aconquija blocks (Fig. 1), up to 1600 m
of Cenozoic sediments were eroded after ca. 6
Ma (Sobel and Strecker, 2003).
A scenario in which a continuous foreland
basin is disrupted by thick-skinned deformation
is also supported by independent stratigraphic
evidence from the Corral Quemado basin (east
of the Fiambalá basin; Fig. 1A). Here, Upper
Miocene (ca. 12[?]–9 Ma) (Sasso, 1997) sedimentary rocks of the Hualfin, Las Arcas, and
Chiquímil Formations are characterized by
channels and high-energy flow facies that document axial flow and are derived from the Puna
margin located to the northwest (e.g., Muruaga,
2001). The Andalhuala Formation (ca. 7 Ma;
LaTorre et al., 1997) is characterized by more
proximal fluvial facies (Muruaga, 2001). The
Pliocene Corral Quemado Formation (3.53 ±
0.04 Ma; Butler et al., 1984) is composed of
coarse alluvial-fan conglomerates that originated from proximal local sources (Fig. 13).
These Pliocene rocks are overthrust by basinbounding basement rocks (Muruaga, 2001),
documenting ongoing deformation after ca. 3.5
Ma. Overall, the Corral Quemado stratigraphy
records a situation where proximal, intrabasinbounding ranges started to provide sediments to
the basins sometime between 7 Ma and 3.5 Ma.
Northeast of the Corral Quemado basin, Tertiary sedimentary basins, such as the El Cajon
and Angastaco basins, record similar sedimentary-tectonic features, suggesting that sedimentation began in a continuous foreland-basin
system that was later disrupted by thick-skinned
deformation at ca. 6 Ma (Grier and Dallmeyer,
1990; Mortimer et al., 2007; Coutand et al.,
2006) (Fig. 13). In particular, in the Angastaco
basin (Eastern Cordillera), the oldest record of
foreland-basin deposition, is represented by the
Quebrada de los Colorados Formation (ca. 37
Ma; Carrapa et al., 2006; Hongn et al., 2007).
This formation was deposited in a foredeep
(Starck and Vergani, 1996) or “wedge-top”-like
environment (Hongn et al., 2007), which was
part of a contiguous and large foreland basin
that extended along the southern Central Andes
from Bolivia to Argentina (DeCelles and Horton, 2003; Carrapa et al., 2006; Carrapa and
DeCelles, 2008). The El Cajón basin at ~27°S
contains mainly middle Miocene to Pliocene
sedimentary rocks (ca. 11 to <5.7 Ma; Strecker et
al., 1989; Bossi et al., 2000, 2001). The tectonosedimentary record of the El Cajón basin documents a disruption of a foreland-basin system at
ca. 6 Ma, when the basin was incorporated into
the deformation front and subsequently disrupted
(Mortimer et al., 2007) (Fig. 13). Similar settings exist in the present-day Quebrada del Toro
and Quebrada de Humahuaca at ~24°30′S and
23°30′S, respectively (e.g., Walther et al., 1998;
Reynolds et al., 2001; Hilley and Strecker, 2005;
Strecker et al., 2007). In all of these locations,
the intermontane basin stage, associated with
tectonic compartmentalization of the formerly
contiguous foreland, is recorded by deposition of
coarse and locally sourced facies, similar to the
ones recognized in the Guanchin and Punaschotter Formations in the Fiambalá basin.
An extensive foreland-basin system has also
been described south of the study area at 30°S
(Zapata, 1998; Jordan et al., 2001). The Bermejo foreland basin was formed at ca. 20 Ma in
response to shortening, loading, and fault tilting
of western bounding ranges that constitutes the
Eastern Precordillera (Zapata, 1998). The Bermejo basin is considered to have passed from
a continuous to a broken foreland-basin setting
with crystalline basement uplifts at ca. 6.5 Ma
(Fig. 13; Jordan et al., 2001). This transition
coincided with deformation and uplift of crystalline basement blocks within the basin in late
Miocene and Pliocene time.
In summary, there is evidence for both eastward propagation of deformation in the Fiambalá basin (two coarsening-upward sequences
with appropriate, though unstudied, faults to the
west), and disruption of a continuous and larger
foreland basin at ca. 6 Ma. This basin was of
regional importance and stretched from the Bermejo basin in the transition between the Sierras
Pampeanas and the Precordillera in the south to
the Eastern Cordillera in the north. Our new data
from the Fiambalá basin bridge the gap between
these two regions. The continuous foreland basin
was disrupted throughout its extent at around 6
Ma by thick-skinned deformation through the
uplift of crystalline basement blocks. The fact
that very little to no direct record of a continuous foreland basin (>8–9 Ma) is preserved in the
Fiambalá basin may suggest that exhumation
was greater in the study area than to the south
and to the north, where such a record still exists.
This could be a result of the location of the basin
at the southernmost end of the Puna Plateau, the
growth of which might have enhanced exhumation of early foreland-basin strata.
CONCLUSIONS
1. The investigated sedimentary rocks of the
Fiambalá basin were deposited between 8.2 and
3.05 Ma. Deposition of the lower Tamberia Formation (8.2 Ma) occurred in the distal part of
an arid ephemeral fluvial system that originated
from an exhuming region located farther to the
west with respect to the modern westernmost
basin-bounding range. Sedimentary rocks of
the upper Tamberia Formation were deposited
in the proximal part of the same ephemeral fluvial system and record more proximal sediment
sources. Overall, the Tamberia strata document
deposition in a regional foreland-basin system.
The sedimentary rocks of the Guanchin Formation (ca. 5.5 Ma) document sedimentation
on an alluvial plain that had sediment sources
located to the west and to the north-northeast in
the southern Puna margin. Both the Tamberia
and Guanchin Formations indicate coarseningand thickening-upward trends resulting from
the progressive deformation and exhumation
of east-verging reverse faults and subsequent
decrease in distance between source and basin
due to eastward fault propagation and sedimentary progradation. A reorganization of the drainage system, to include northern and northeastern
sources (southern Puna margin), is recorded at
ca. 5.5 Ma. The Punaschotter Formation consists of alluvial-fan conglomerates; by the time
of deposition of these rocks (ca. <3.7 Ma), the
basin was overfilled and received detritus from
western, northern, and northeastern sources. The
Guanchin and Punaschotter strata document
deposition in a broken (thick-skinned) forelandbasin system characterized by local sources.
2. Structural analyses of folds, faults, and fault
striae document E-W to NE-SW contraction
during basin formation. Deformation occurred
within the basin from at least 5.5 Ma to 3.7 Ma.
The regionally recognized shift in kinematics
from E-W or NE-SW shortening to N-S extension can be identified only in the far northern
part of the Fiambalá basin, along the southern
margin of the Puna Plateau.
3. Comparisons between the Fiambalá basin,
the Bermejo foreland basin to the south, and
basins in the transition between the Sierras Pampeanas and the Eastern Cordillera to the north
suggest a similar structural-sedimentological
evolution. In all three regions, thick-skinned,
intrabasin deformation started at ca. 6 Ma. If the
transition from thin- to thick-skinned deformation was indeed related to flat-slab subduction
(e.g., Ramos et al., 2002; Siame et al., 2005),
then our study suggests that flat-slab subduction is recorded quasi simultaneously along
strike, from 26°S to 32°S, at ca. 6 Ma, rather
than in a time-transgressive manner. Existing
Geological Society of America Bulletin, November/December 2008
1541
Carrapa et al.
thermochronological data suggest that older
rocks, possibly deposited in a regional foreland basin, once covered the Sierras Pampeanas
basement and began to be stripped off at ca. 20
Ma. We propose that the Fiambalá basin was
once part of a larger, continuous (thin-skinned)
foreland-basin system including areas to the
north in the Eastern Cordillera and to the south
in the Sierras Pampeanas. If true, such a regional
foreland-basin system existed despite the differences in the geological details of these areas.
The present-day Fiambalá basin is thus a snapshot of a longer basin history, related to a larger,
contiguous older foreland basin that may have
extended along the central southern Andes from
north to south over a length of 1000 km.
ACKNOWLEDGMENTS
Marc Hendrix, Ken Ridgway, Jim Schmitt, Cari
Johnson, and Karl Karlstrom are kindly acknowledged for their careful and constructive reviews. We
thank Chiara Barbieri, Gregorio Araoz, and Christian
Reinoso for field assistance, and Ricardo Alonso for
fruitful scientific discussions. Estelle Mortimer provided invaluable scientific input. We are grateful to
the Alexander von Humboldt Foundation, Deutsche
Forschungsgemeinschaft (DFG) (project CA 481/5-1
and a Leibniz grant to M. Strecker) and the U.S.
National Science Foundation (NSF; Tectonics; project
EAR-0635630) for funding this research.
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