Effect of late Cenozoic aridification on sedimentation in the Eastern
Cordillera of northwest Argentina (Angastaco basin)
Sharon Bywater-Reyes1, Barbara Carrapa1, Mark Clementz1, and Lindsay Schoenbohm2
1
Department of Geology and Geophysics, University of Wyoming, 1000 E University Avenue, Laramie, Wyoming 82071, USA
School of Earth Sciences, Ohio State University, 275 Mendenhall Laboratory, 125 South Oval Mall, Columbus, Ohio 43210, USA
2
75° W
70° W
65° W
A
25°30' S
ABSTRACT
This study evaluates the effect of climate on facies, grain size,
and sedimentation rates using sedimentology, geochronology, and
stable isotope geochemistry for Miocene–Pliocene deposits in the
Angastaco basin (Eastern Cordillera, northwest Argentina). U-Pb
zircon data from ash layers constrain the transition between the finer
grained fluvial-lacustrine Palo Pintado Formation and the coarser
grained fluvial-alluvial San Felipe Formation to ca. 5.2 Ma and the
first deposition of sediment derived from the present-day orographic
barrier to ca. 4 Ma. δ13C values from pedogenic carbonate nodules
range from −15.4‰ to −10.2‰ for the Palo Pintado Formation and
from −9.5‰ to −8.2‰ for the San Felipe Formation; this can be best
explained by increased, sustained aridity since ca. 5 Ma. The δ18O values range from −9.6‰ to −5.9‰ for the Palo Pintado Formation and
from −6.1‰ to −5.2‰ for the San Felipe Formation, corroborating
this interpretation. The shift toward more arid conditions correlates
with a significant increase in grain size but no significant change in
sedimentation rate. Because aridity precedes the development of an
orographic effect, we interpret the grain size increase in the Angastaco
basin since ca. 5 Ma to be a response of the sedimentary system to
aridification resulting from regional climate change.
INTRODUCTION
A major challenge in determining the interaction of climate and tectonics on erosion and deposition is measuring all the variables that affect
these processes (e.g., Molnar, 2009). In the Central Andes of northwest
Argentina, late Cenozoic conglomerates are preserved along the margin
of the Puna Plateau in a series of basins. One such basin, the Angastaco
(Fig. 1), has a well-understood structural history (e.g., Carrera and Muñoz,
2008; Trimble et al., 2008), allowing for comparison between tectonics and
the sedimentary record. The aim of this study is to evaluate important variables affecting sedimentation via sedimentology, U-Pb geochronology, and
stable isotope geochemistry in order to determine, for the Angastaco basin
in the Miocene–Pliocene, whether there is a correlation between deposition
of coarse fluvial-alluvial deposits and climate, or whether local tectonics
can explain the grain size increase. This study comes to the conclusion that,
although the basin was affected by deformation during the Miocene–Pliocene (Carrera and Muñoz, 2008), deposition of coarse-grained deposits in
the Angastaco basin is a function of climate, and not tectonics.
GEOLOGICAL SETTING
The Angastaco basin preserves >6 km of Eocene–Pliocene continental clastic strata of the Payogastilla Group (e.g., Díaz and Malizzia, 1983;
B
Angastaco Basin
Range
sin
ra Ba
Cerro Negro
Balbuena Subgroup (Cretaceous-Eocene)
Puca
Santa Barbara Subgroup (Paleocene)
ange
Luracatao R
Measured section
Cumbres de
Anticline
25°45' S
25° S
Thrust
nge
Syncline
Angastaco
Ra
dos
lora
400 km
s Co
e lo
20° S
ra d
Sier
15° S
N
Pirgua Subgroup (Cretaceous)
Intrusive basement (Neoproterozoic-Ordovician)
San
Carlos
4 km
Puncaviscana Fm. (Precambrian-Paleozoic)
66°15'W
66°00'W
Figure 1. A: General map of Andes with tectonomorphic domains. B: Geologic map of Angastaco basin and vicinity (modified from Coutand
et al., 2006) indicating location of measured stratigraphic sections.
© 2010 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.
GEOLOGY,
March
2010
Geology,
March
2010;
v. 38; no. 3; p. 235–238; doi: 10.1130/G30532.1; 2 figures; Data Repository item 2010064.
235
Starck and Vergani, 1996). The basin is bounded to the west by an east-vergent reverse fault placing the Precambrian–early Cambrian metasedimentary Puncoviscana Formation over the Cenozoic basin strata. To the east,
a west-vergent reverse fault places Cretaceous rocks of the Sierra de los
Colorados over the basin strata (Fig. 1B; e.g., Carrera and Muñoz, 2008).
Starck and Anzotegui (2001) interpreted the middle to upper Miocene fluvial-lacustrine strata of the Palo Pintado Formation to reflect a
period of increased humidity coinciding with the onset of the South American monsoon ca. 9 Ma. A poorly documented source-area shift within
the San Felipe Formation, which overlies the Palo Pintado Formation, has
been linked to uplift of the Sierra de los Colorados to the east and development of orographic aridity within the basin (Coutand et al., 2006).
locations within the lower San Felipe Formation, and 5 locations within
the upper San Felipe Formation (n = 100 for each location; Fig. 2C). Average clast compositions within the Palo Pintado Formation and lower San
Felipe Formation consist mainly of metamorphic, intrusive, volcanic, and
clastic sedimentary rocks characteristic of lithologies exposed in ranges
to the west and southwest. A marked change in clast composition occurs
between the lower and upper San Felipe Formation with the first occurrence of limestone clasts, which are only exposed in ranges to the east
(Fig. 1B; Sierra de los Colorados; Hongn et al., 1999). From these data, it
can be deduced that the Sierra de los Colorados was not a source of sediment for the basin until upper San Felipe Formation time (ca. 4 Ma; see
discussion of U-Pb geochronology).
SEDIMENTOLOGY
Nine stratigraphic sections were measured within the Angastaco
basin in order to determine depositional environment (Fig. 1B; our own
work). The Palo Pintado Formation consists of ~2000 m of trough crossstratified sandstones interpreted by us as fluvial channel sand bodies (e.g.,
Davis, 1983) interbedded with green ripple- and planar-laminated mudstones interpreted as associated lacustrine and floodplain deposits (e.g.,
Allen, 1970; Miall, 1996). Overall, facies associations indicate that the
Palo Pintado Formation was deposited within a relatively fine grained,
nonmarine setting with meandering rivers, floodplains, and lakes. The
San Felipe Formation is transitional with the Palo Pintado Formation,
and consists of ~650–1000 m of clast-supported cobble conglomerates
and coarse sandstones with crude horizontal stratification and occasional
imbrications. In contrast to the Palo Pintado Formation, we interpret the
San Felipe Formation strata as higher energy, high-density stream flows
deposited in an ephemeral fluvial to alluvial fan environment (e.g., Stanistreet and McCarthy, 1993; DeCelles et al., 1991).
Paleocurrents
Paleocurrents were measured along measured stratigraphic sections
from trough cross-stratification axes and clast imbrication directions at
12 locations within the Palo Pintado Formation, 23 locations within the
lower San Felipe Formation, and 7 locations within the upper San Felipe
Formation (n = 10 for each location; Fig. 2C). Average flow directions for
the Palo Pintado, lower San Felipe, and upper San Felipe Formations are
northeast, east, and southwest, respectively. These measurements corroborate provenance data and convincingly show that the Sierra de los Colorados was not a source of sediment for the basin until upper San Felipe
Formation time.
Provenance
Conglomerate provenance analyses were conducted along measured
stratigraphic sections at 4 locations within the Palo Pintado Formation, 18
U-Pb ZIRCON GEOCHRONOLOGY OF ASHES
We collected two samples from ash beds from the Palo Pintado Formation and seven from the San Felipe Formation (see the GSA Data Repository1 and Table DR1). Zircons were analyzed for 206Pb/238U dates (see the
Data Repository) in order to determine geochronologic age (Fig. 2D;
Table DR1) and deposition rate (Fig. 2D). These dates constrain the timing
of the transition from the Palo Pintado Formation to the San Felipe Formation to ca. 5.2 Ma (interpolated using a constant sedimentation rate and the
dates of two ashes, AT7–1 and AT2–2; Table DR1) and the timing of the
Palo Pintado Fm.
lower
San Felipe Fm.
upper
San Felipe Fm.
Figure 2. A: Stable isoB Generalized
A Stable isotope
C Paleocurrent
D Geochronologic
tope values of pedogenic
values
stratigraphy
age (Ma)
and provenance
carbonate nodules and
8 7 6 5 4 3 2
rodent incisor enamel
plotted as error weighted
2000 m
0.25 mm/yr
means (Tables DR3,
DR4; for more informa0.20 mm/yr
13
C Nodule
tion, see the Data Repos18
1.64 mm/yr
O Nodule
0.14 mm/yr
13
itory [see footnote 1]).
C Enamel
18
VPDB—Vienna Peedee
O Enamel
1.58 mm/yr
belemnite. B: General0.49
mm/yr
ized stratigraphy show1000 m
ing Palo Pintado Forma0.34 mm/yr
tion, lower San Felipe
Formation, and upper
San Felipe Formation
0.32 mm/yr
boundaries in bold black
Phyllite/
Gneiss
lines; mr—mudrock; fs—
Quartzite
fine sandstone; ms—meIntrusive
Schist
dium sandstone; gr—
Volcanic
Limestone
0m
granule conglomerate;
mr
-16
-14
-12
-10
-8
-6
-4
Clastic
fs
Quartz
ms
cgl—cobble conglomerSedimentary
gr
(‰)
V-PDB
ate. C: Respective avercgl
age paleocurrent and
conglomerate clast count data for three formations. D: Geochronologic age with 2σ error (see Table DR1 and the Data Repository) versus generalized stratigraphy (note that the youngest age, 2.3 ± 0.02 Ma, is from John Trimble [2009, personal commun.]); sedimentation rate (in mm/yr)
is shown. Data presented here indicate increasing aridity within basin starting ca. 6 Ma and sustained by ca. 5 Ma (A) correlating with increase
in grain size (B). These changes occur prior to significant changes in paleocurrent and provenance data (C), indicating that aridity and grain
size increase occur prior to uplift of Sierra de los Colorados, and with no significant change in sedimentation rate (D).
1
GSA Data Repository item 2010064, description of analytical procedures, further discussion, and tables of raw analytical data, is available online at
www.geosociety.org/pubs/ft2010.htm, or on request from editing@geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.
236
GEOLOGY, March 2010
first appearance of sediments derived from the Sierra de los Colorados and
the paleocurrent shift to ca. 4 Ma (interpolated using a constant sedimentation rate and the dates of two ashes, AT6–7 and AT6–1; Table DR1). The
possibility of the grain size shift representing a time-transgressive progradation of coarse material into the basin was ruled out (for discussion, see
the Data Repository). Calculated sedimentation rates are displayed in Figure 2D. Throughout the Palo Pintado Formation and over the transition to
the San Felipe Formation sedimentation rates do not change significantly,
changing from 0.32 ± 0.02 mm/yr to 0.34 ± 0.05 mm/yr, and finally to 0.49
± 0.05 mm/yr over the formation boundary. The sedimentation rates throughout the San Felipe Formation are much more varied, changing from 0.49
± 0.05 mm/yr to 1.58 ± 0.15 mm/yr, 0.14 ± 0.03 mm/yr, 1.64 ± 0.95 mm/yr,
0.20 ± 0.02 mm/yr, and finally to 0.25 ± 0.02 mm/yr. We propose that this
variability is inherent to the depositional system: the higher sedimentation
rates represent short-term high-flux events within the alluvial fan system,
whereas the lower sedimentation rates represent times of depositional hiatus
between these events. Although the sedimentation rates for the San Felipe
Formation are more varied, on average they are not significantly different,
suggesting that the increase in grain size was not accompanied by a significant increase in average sedimentation rate, and presumably source erosion
rate. The lack of a distinct change over the transition from the finer-grained
fluvial-lacustrine Palo Pintado Formation to the coarser-grained fluvialalluvial lower San Felipe Formation suggests that neither subsidence nor
sediment supply was controlling grain size distribution at this time, and that
an external forcing, such as change in source rock or climate, was (Paola et
al., 1992). Because no known change in source rock occurred during the
transition of interest, climate is favored as the primary control.
STABLE ISOTOPE ANALYSIS
Pedogenic Carbonate Nodules
In order to evaluate the late Cenozoic paleoenvironment, stable isotope analyses were performed on pedogenic carbonate collected from 11
stratigraphic locations along measured stratigraphic sections (Table DR2
in the Data Repository). δ13C and δ18O values (Vienna Peedee belemnite,
VPDB) are reported as error-weighted means and range, respectively,
from −15.4‰ to −10.2‰ and −9.6‰ to −5.9‰ for the Palo Pintado
Formation and from −9.5‰ to −8.2‰ and −6.1‰ to −5.2‰ for the San
Felipe Formation (Fig. 2A; Table DR2).
δ13C values of pedogenic carbonate reflect, in part, the proportion
of plants in an ecosystem that used either the C3 photosynthetic pathway (e.g., most plants as well as grasses from regions with cool growing seasons) versus those that use the C4 photosynthetic pathway (e.g.,
warm growing season grasses; O’Leary, 1988; Farquhar et al., 1989).
Carbon isotope values for C3 plants growing today range from −30‰ to
−25‰ and are typically lower than δ13C values for C4 plants (−13‰ to
−11‰) (Kohn and Cerling, 2002). Typically, carbonate precipitated in a
completely C3 environment can be as high as −10‰, and carbonate precipitated in a completely C4 environment can be as low as −8‰ (Cerling,
1984; Cerling et al., 1989). As plants become stressed, respiration rates
slow, allowing the diffusion of atmospheric CO2 to greater soil depth,
which results in higher δ13C values of pedogenic carbonates (Garzione et
al., 2008). δ18O values of pedogenic carbonate reflect the combined effects
of temperature and soil water composition, which is related to precipitation composition (Koch, 1998). As precipitation moves inland and through
increasing elevation, the heavy isotope of oxygen, 18O, is preferentially
removed, leaving the remaining precipitation depleted in this species;
thus there is a predictable relationship between δ18O and elevation. If soil
waters are evaporatively enriched (light species, 16O, evaporates), this relationship may be overprinted (Faure, 1977).
The δ13C values in the Palo Pintado Formation suggest a C3 woodland or montane C3 grassland environment (Cerling et al., 1993). The
GEOLOGY, March 2010
increased range in δ13C values (5.3‰) in the upper part of this formation
suggests a shift from wetter conditions at the base to a mosaic habitat with
significant spatial heterogeneity in vegetation type. At the very top of the
Palo Pintado Formation and through the San Felipe Formation, δ13C values
become more positive, which suggests either contribution from C4 plants
or increasingly stressed conditions, and these higher values are associated
with a significant decrease in total range (1.3‰), which implies that the
environment was becoming less varied spatially. Likewise, the δ18O values in the Palo Pintado Formation are initially low (−9.6‰) but increase
ca. 6 Ma (−5.9‰); aside from a few high values (−4‰), this is maintained
through the remainder of this formation and into the San Felipe Formation. As with the carbon isotope values, variation in δ18O values in the San
Felipe Formation (1.3‰) is much less than that determined for the Palo
Pintado Formation (3.7‰); this suggests that environmental conditions
had stabilized during this depositional period. These higher δ18O values
and the significant correlation (see the Data Repository) between δ13C and
δ18O values for the Palo Pintado Formation (Pearson Product Moment
[PPM], r = 0.549, P = 0.03) and San Felipe Formation (PPM, r = 0.615,
P = 0.01) support an interpretation of progressive aridification starting
ca. 6 Ma, with sustained aridity by ca. 5 Ma.
Fossil Rodent Enamel
Enamel from fossil rodent incisors was sampled from 75 m below
and 84 m above the Palo Pintado Formation–San Felipe Formation transition and was analyzed for δ13C and δ18O values (VPDB) in order to evaluate the presence of C3 versus C4 plants (Table DR4; for discussion, see the
Data Repository). The values from below the transition indicate the predominance of C3 plants, whereas the values above the transition indicate
C4 grasses or 13C-enriched plants that perform Crassulacean acid metabolism (CAM) during photosynthesis (e.g., cacti and other succulents; Kluge
and Ting, 1978; Osmond, 1978) made up at least 35% and possibly as
much at 87% of the diet of these rodents (see the Data Repository). This
indicates that prior to ca. 5 Ma, C4 and CAM plants were not present in
any substantial quantity. Therefore, the trend in stable isotope values of
samples older than ca. 5 Ma (from the Palo Pintado Formation) can be
attributed to a change in environmental stress on resident C3 plants. C4
grasses and CAM plants generally take advantage of arid conditions, since
they can tolerate drier conditions than C3 grasses, so it seems reasonable
that the ecosystem would be replaced by C4 grasses or include a greater
proportion of CAM plants as conditions became drier (Ehleringer et al.,
1997). For a discussion of δ18O values, see the Data Repository.
DISCUSSION AND CONCLUSIONS
Provenance and paleocurrent data demonstrate that the eastern basinbounding range, the Sierra de los Colorados, was not a sediment source
for the Angastaco basin until ca. 4 Ma, suggesting that it did not exhibit the
relief necessary to inhibit moisture as an orographic barrier until that time.
The timing of the shift from the fluvial-lacustrine Palo Pintado Formation
to the fluvial-alluvial San Felipe Formation is constrained to ca. 5.2 Ma.
Sedimentation rates, and presumably source erosion rates, did not change
significantly for the studied time interval. This suggests that tectonic subsidence and change in sediment supply are not controlling grain size distribution (Paola et al., 1992) and that, in the case of the Angastaco basin,
aridity is not associated with a significant increase in sedimentation, and
thus erosion, rates (e.g., Molnar, 2001).
Since formation of the eastern orographic barrier and structural
deformation are not synchronous with the initiation of coarse-grained
deposition in the Angastaco basin, another mechanism is required. Stable isotope data from pedogenic carbonates demonstrate relatively more
humid conditions prior to ca. 6 Ma, consistent with previous interpretations (e.g., Starck and Anzotegui, 2001). Aridity increased in the basin
since ca. 6 Ma, with sustained aridity by ca. 5 Ma, synchronous with the
237
shift from fine-grained fluvial-lacustrine deposition to coarse-grained
fluvial-alluvial deposition ca. 5.2 Ma, suggesting a genetic link between
climate and change in facies. Stable isotope values of fossil rodent incisor
enamel reveal the expansion of C4 plants synchronous with aridity and
increased grain size ca. 5 Ma. Expansion of glaciation, particularly of the
West Antarctic Ice Sheet ca. 5–6 Ma, is linked to an increase in aridity in
Chile (e.g., Lamb and Davis, 2003; Hepp et al., 2006). Regional climatic
changes are thus likely to have affected the Angastaco basin. The data
presented here show a temporal link between the onset of coarse-grained
fluvial-alluvial deposition in the Angastaco basin and regional climatedriven aridification, supporting a cause-and-effect relationship between
climate and deposition of coarse clastics.
ACKNOWLEDGMENTS
The National Science Foundation funded this research (grant EAR-0635630).
The ion microprobe facility at UCLA is partly supported by a grant from the Instrumentation and Facilities Program, Division of Earth Sciences, National Science
Foundation. We thank Shikha Sharmathe and the University of Wyoming Stable
Isotope Facility. We also thank Snehalata Huzurbazar, Peter DeCelles, Paul Heller,
Peter Molnar, Andrew Leier, Gregory Hoke, Nicoletta Mancin, and an anonymous
reviewer for constructive criticism. Laura Vietti is kindly acknowledged for field
support and Tim Reed for analytical support.
REFERENCES CITED
Allen, J.R.L., 1970, Studies in fluviatile sedimentation: A comparison of finingupward cyclothems, with special reference to coarse-member composition
and interpretation: Journal of Sedimentary Petrology, v. 40, p. 298–323.
Carrera, N., and Muñoz, J.A., 2008, Thrusting evolution in the southern Cordillera Oriental (northern Argentine Andes): Constraints from growth strata:
Tectonophysics, v. 459, p. 107–122, doi: 10.1016/j.tecto.2007.11.068.
Cerling, T.E., 1984, The stable isotopic composition of modern soil carbonate
and its relationship to climate: Earth and Planetary Science Letters, v. 71,
p. 229–240, doi: 10.1016/0012-821X(84)90089-X.
Cerling, T.E., Quade, J., Wang, Y., and Bowman, J.R., 1989, Carbon isotopes in
soils and paleosols as ecology and paleoecology indicators: Nature, v. 341,
p. 138–139, doi: 10.1038/341138a0.
Cerling, T.E., Wang, Y., and Quade, J., 1993, Expansion of C4 ecosystems as an
indicator of global ecological change in the late Miocene: Nature, v. 361,
p. 344–345, doi: 10.1038/361344a0.
Coutand, I., Carrapa, B., Deeken, A., Schmitt, A.K., Sobel, E., and Strecker, M.R.,
2006, Orogenic plateau formation and lateral growth of compressional basins and ranges: Insights from sandstone petrography and detrital apatite
fission-track thermochronology in the Angastaco Basin, NW Argentina: Basin Research, v. 18, p. 1–26, doi: 10.1111/j.1365-2117.2006.00283.x.
Davis, R.A., Jr., 1983, Depositional systems: A genetic approach to sedimentary
geology: Englewood Cliffs, New Jersey, Prentice-Hall, 669 p.
DeCelles, P.G., Gray, M.D., Ridgway, K.D., Cole, R.B., Pivnik, D.A., Pequera,
N., and Srivastava, P., 1991, Controls on synorogenic alluvial-fan architecture, Beartooth Conglomerate (Paleocene), Wyoming and Montana: Sedimentology, v. 38, p. 567–590, doi: 10.1111/j.1365-3091.1991.tb01009.x.
Díaz, J.I., and Malizzia, D.C., 1983, Estudio geológico y sedimentológico del
Terciario Superior del valle Calchaquí (departamento de San Carlos, provincia de Salta): Bolletino de Sedimentológia, v. 2, p. 8–28.
Ehleringer, J.R., Cerling, T.E., and Helliker, B.R., 1997, C4 photosynthesis, atmospheric CO2, and climate: Oecologia, v. 112, p. 285–299, doi: 10.1007/
s004420050311.
Farquhar, G.D., Ehleringer, J.R., and Hubick, K.T., 1989, Carbon isotope discrimination and photosynthesis: Annual Review of Plant Physiology and
238
Plant Molecular Biology, v. 40, p. 503–537, doi: 10.1146/annurev.pp.40
.060189.002443.
Faure, G., 1977, Principles of isotope geology: New York, John Wiley and Sons,
464 p.
Garzione, C.N., Hoke, G.D., Libarkin, J.C., Withers, S., MacFadden, B., Eiler,
J., Ghosh, P., and Mulch, A., 2008, Rise of the Andes: Science, v. 320,
p. 1304–1307, doi: 10.1126/science.1148615.
Hepp, D.A., Moerz, T., and Gruetzner, J., 2006, Pliocene glacial cyclicity in a
deep-sea sediment drift (Antarctic Peninsula Pacific Margin): Palaeogeography, Palaeoclimatology, Palaeoecology, v. 231, p. 181–198, doi: 10.1016/
j.palaeo.2005.07.030.
Hongn, F., Seggiaro, R., and Ramalio, E., 1999, Hoja Geológica 2566-III Cachi:
Instituto de Geología y Recursos Minerales, Servicio Geológico Minero
Argentino.
Kluge, N.W., and Ting, I., 1978, Crassulacean acid metabolism: Analysis of an
ecological adaptation, in Billings, W.D., et al., eds., Ecological studies.
Analysis and synthesis: Volume 30: Berlin, Springer-Verlag.
Koch, P.L., 1998, Isotopic reconstruction of past continental environments:
Annual Review of Earth and Planetary Sciences, v. 26, p. 573–613, doi:
10.1146/annurev.earth.26.1.573.
Kohn, M.J., and Cerling, T.E., 2002, Stable isotope compositions of biological
apatite, in Kohn, M.J., et al., eds., Phosphates: Geochemical, geobiological, and materials importance: Reviews in Mineralogy and Geochemistry
Volume 48, p. 455–488, doi: 10.2138/rmg.2002.48.12.
Lamb, S., and Davis, P., 2003, Cenozoic climate change as a possible cause for
the rise of the Andes: Nature, v. 425, p. 792–797, doi: 10.1038/nature02049.
Miall, A.D., 1996, The geology of fluvial deposits: Berlin, Springer Verlag, 582 p.
Molnar, P., 2001, Climate change, flooding in arid environments, and erosion rates:
Geology, v. 29, p. 1071–1074, doi: 10.1130/0091-7613(2001)029<1071:
CCFIAE>2.0.CO;2.
Molnar, P., 2009, The state of interactions among tectonics, erosion, and climate:
A polemic: GSA Today, v. 19, p. 44–45, doi: 10.1130/GSATG00GW.1.
O’Leary, M.H., 1988, Carbon isotopes in photosynthesis: BioScience, v. 38,
p. 328–336, doi: 10.2307/1310735.
Osmond, C.B., 1978, Crassulacean acid metabolism: A curiosity in context: Annual Review of Plant Physiology, v. 29, p. 379–414, doi: 10.1146/annurev.
pp.29.060178.002115.
Paola, C., Heller, P.L., and Angevine, C.L., 1992, The large-scale dynamics of
grain size variations in alluvial basins, 1: Theory: Basin Research, v. 4,
p. 73–90.
Stanistreet, I.G., and McCarthy, T.S., 1993, The Okavango Fan and the classification of subaerial fan systems: Sedimentary Geology, v. 85, p. 115–133, doi:
10.1016/0037-0738(93)90078-J.
Starck, D., and Anzotegui, L.M., 2001, The late Miocene climatic change—Persistence of a climatic signal through the orogenic stratigraphic record in
northwestern Argentina: Journal of South American Earth Sciences, v. 14,
p. 763–774.
Starck, D., and Vergani, G., 1996, Desarrollo tecto-sedimentario del Cenozoico
en el sur de la Provincia de Salta-Argentina: Congreso Geológico Argentino, v. XIII, p. 433–452.
Trimble, J., Carrapa, B., Stockli, D., and Stutz, J., 2008, New constraints on the
timing and magnitude of deformation and basin exhumation in the Eastern
Cordillera of NW Argentina: American Geophysical Union, Fall Meeting
2008, abs. T53B–1925.
Manuscript received 16 July 2009
Revised manuscript received 8 October 2009
Manuscript accepted 14 October 2009
Printed in USA
GEOLOGY, March 2010