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TECTONICS, VOL. 27, TC1015, doi:10.1029/2007TC002127, 2008
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Eocene exhumation and basin development in the Puna of
northwestern Argentina
B. Carrapa1 and P. G. DeCelles2
Received 6 March 2007; revised 16 July 2007; accepted 24 September 2007; published 22 February 2008.
[1] The Puna is part of the larger Puna-Altiplano
Plateau (also known as the Central Andean Plateau),
characterized by high elevation, low relief, and aridity,
located in the central Andes of Bolivia and Argentina.
Tertiary sedimentary rocks preserved within the Puna
contain a unique archive of information regarding the
paleogeography, depositional environments, and
timing of sediment source exhumation during the
early stages of Andean mountain building. The Eocene
Geste Formation in the Salar de Pastos Grandes area
(within the central Puna of northwestern Argentina)
consists of deposits that are the result of confined to
unconfined flows in a sandy to gravelly, braided
fluvial system and alluvial fans proximal to the source
terrane. Paleocurrent data document an overall
eastward flow direction. Up-section coarsening of
the Geste Formation suggests that topographic relief in
the source area increased through time, possibly owing
to enhanced tectonic activity and source terrane
unroofing. Sandstone petrography and conglomerate
clast-count data document quartzose and phyllitic
compositions typical of Ordovician rocks preserved
just west of the Salar de Pastos Grandes area.
Paleocene-Eocene detrital apatite fission track age
populations (P1: 35–52 Ma; P2: 52–65 Ma) of the
Geste Formation and their consistent trends up-section
suggest moderate to rapid (0.4 mm/a to >1 mm/a)
exhumation of western sediment sources during the
early to mid-Tertiary stages of Andean mountain
building. Sedimentation rates increase up-section from
0.1 mm/a to 1 mm/a. Our data, when combined with
other structural, stratigraphic and seismic evidence
from surrounding regions, suggest that the Geste
Formation was deposited in response to crustal
shortening and resulting erosion and sedimentation,
which started as early as Cretaceous in the Chilean
Cordillera de Domeyko and in the Salar de Pastos
Grandes area by Eocene time. The Geste Formation
could be interpreted either as a local wedge-top
1
Department of Geology and Geophysics, University of Wyoming,
Laramie, Wyoming, USA.
2
Department of Geosciences, University of Arizona, Tucson, Arizona,
USA.
Copyright 2008 by the American Geophysical Union.
0278-7407/08/2007TC002127$12.00
accumulation on the eastward propagating
central Andean orogenic wedge, or as a local
intermontane basin. The similarities between wedgetop deposits preserved in Bolivia and Eocene deposits
in northwestern Argentina, south of 25°S, lead us to
favor the wedge-top scenario for the Geste Formation.
If correct, this implies that the deformation front of the
Andean orogenic wedge incorporated both thin- and
thick-skinned structures as it migrated, possibly
unsteadily, from the Cordillera de Domeyko during
the Cretaceous-Paleocene to areas within the Puna
and Eastern Cordillera by mid-late Eocene time.
Contemporaneously, a regional-scale foreland basin
system developed over an along-strike distance of at
least 650 km. Citation: Carrapa, B., and P. G. DeCelles
(2008), Eocene exhumation and basin development in the Puna of
northwestern Argentina, Tectonics, 27, TC1015, doi:10.1029/
2007TC002127.
1. Introduction
[2] The Puna is the southern part of the Central Andean
Plateau, or Puna-Altiplano Plateau (which is the second
largest orogenic plateau on Earth after Tibet) and is characterized by mean elevation of >3700 m, peaks over 6000 m,
internal drainage, and aridity [Isacks, 1988; Strecker et al.,
2007] (Figure 1). The tectonic processes responsible for the
formation of the Central Andean Plateau and its marginal
areas are related to subduction of the oceanic Nazca plate
under the continental South American plate. Such processes
mainly include distributed crustal shortening [Allmendinger
et al., 1997], emplacement of regional basement thrust
sheets [Kley et al., 1997; McQuarrie, 2002], underthrusting
of cratonal material beneath the plateau region and later
lithospheric thinning [Isacks, 1988] following delamination
[Kay et al., 1994; Sobolev and Babeyko, 2005] or convective removal of lithosphere [Schurr et al., 2006].
[3] The interconnections between such processes, the
timing and mode of upper crustal shortening and the
resulting sedimentation patterns within the plateau interior
are subjects of ongoing discussion. Particularly vexing is
the Paleogene history of crustal shortening, orogeny, and
basin development in the region presently situated within
the plateau interior. Although Paleogene clastic sedimentary
rocks are widespread [Jordan and Alonso, 1987] the depositional settings and mechanisms of sediment accommodation for these strata are not well understood. It is also not
clear when the Eastern Cordillera, presently located along
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Figure 1. Shaded regional topography of the central
Andes of northern Argentina and southern Bolivia, showing
tectonomorphic zones (labeled), thrust faults (barbed lines),
and volcanoes (circles) (modified after Sobel et al. [2003]).
Grey thick lines indicate the Cordillera de Domeyko thrust
belt (modified after Maksaev and Zentilli [2000]). Dashed
larger box identifies the Salar de Pastos Grandes area,
shown in more detail in Figure 2. AD, Salar de Atacama;
AR, Arizaro Basin; SA, Salar de Antofalla; P, La Poma
Basin; A, Angastaco; TT, Tin Tin; CR, Chango Real.
the eastern margin of the Puna, began to experience regional
uplift and exhumation.
[4] Jordan and Alonso [1987] and Kraemer et al. [1999]
suggested that Eocene-Oligocene sedimentary rocks in the
Puna were deposited along the eastern side of an orogenic
highland located in the Chilean Precordillera (including the
Cordillera de Domeyko). Eocene shortening and uplift in
this region is generally referred to as the Incaic phase of the
Andean orogeny [Steinmann, 1929]. Other authors [Hartley
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et al., 1992; Flint et al., 1993; Charrier and Reutter, 1994]
have interpreted Upper Cretaceous and Paleogene coarsegrained deposits in the Salar de Atacama of northern Chile
as the result of back-arc extension. This interpretation is
partly supported in recent papers by Pananont et al. [2004]
and Jordan et al. [2007] on the basis of interpreted
reflection seismic profiles correlated to strata in a borehole
in the Salar de Atacama. The amounts of extension inferred
from seismic data in Oligocene and lower Miocene sedimentary rocks are, however, very slight. In any case the
extensional basin interpretation begs a tectonic explanation
for regional extension in the South American plate coeval
with rapid absolute westward motion during opening of the
South Atlantic Ocean [Müller et al., 1997]. In contrast,
Mpodozis et al. [2005] proposed that the Salar de Atacama
basin formed as a consequence of tectonic inversion of a
Jurassic – Early Cretaceous back-arc basin within a continuous contractional regime in the Cordillera de Domeyko
from mid-Cretaceous through Paleogene time. Similarly,
other recent work that combines reflection seismic and
outcrop data documents mid-Cretaceous through Tertiary
shortening and synkinematic proximal foreland basin sedimentation in the Salar de Atacama region (Figure 1)
[Arriagada et al., 2006]. The presence of a Cretaceous –
early Tertiary thrust belt in northern Chile [Arriagada et al.,
2006] implies that a contemporaneous foreland basin system should be preserved in the Puna and Eastern Cordillera
of northwestern Argentina. However, the rocks that occupy
this time interval in northwestern Argentina have been
associated with regional extension in the Salta rift complex
during the Early to mid-Cretaceous, followed by tectonothermal subsidence during Late Cretaceous through Eocene
time [Galliski and Viramonte, 1988; Salfity and Marquillas,
1994; Marquillas et al., 2005]. Moreover, the existing
majority opinion holds that crustal shortening and related
foreland basin development in northern Argentina were
delayed until latest Oligocene time [e.g., Allmendinger et
al., 1997, and references therein] or perhaps even later
[Jordan et al., 2007]. On the other hand, recent studies in
Bolivia [Elger et al., 2005; Ege et al., 2007] and northwestern Argentina suggest local crustal shortening and rapid
exhumation during Eocene and Oligocene time [Kraemer et
al., 1999; Coutand et al., 2001; Carrapa et al., 2005;
Hongn et al., 2007]. Thus the key question is whether the
Paleogene strata in the Puna of northwestern Argentina
reflect distal foreland basin deposition [e.g., Kraemer et
al., 1999], regional extension and thermal subsidence
[Marquillas et al., 2005], or local uplift and tectonic
partitioning of the Puna region [Coutand et al., 2001;
Hongn et al., 2007].
[5] We focus here on Eocene coarse-grained clastic rocks
in the Salar de Pastos Grandes area in the central Puna
(Figure 2). We present detailed sedimentological, modal
petrographic, and detrital apatite fission track (AFT) data in
order to constrain depositional environments, paleogeography, sediment provenance, and exhumation ages of bedrock
source terranes. The results of our study, though confined to
a local region, nevertheless have bearing on the larger-scale
scientific issue of whether the Puna was occupied by a
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Figure 2. (a) Geological map of central Andes (modified after Reutter et al. [1994]). Square corresponds to area
shown in Figure 2b. 1, Eocene deposits in the Salar de Pastos Grandes area; 2, Eocene deposits in the La Poma Basin
[Hongn et al., 2007]. (b) Geological map of Salar de Pastos Grandes area (modified after Alonso [1992]). (c) Cross
section modified after San Antonio de los Cobres geological map [Blasco et al., 1996].
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regional-scale foreland basin system or local extensional
basins during Eocene time.
2. Geological Setting
[6] The Puna resides in the hinterland of the Andean
thrust belt, bounded to the west by the modern Andean
magmatic arc and to the northeast by the Eastern Cordillera.
East of the Eastern Cordillera lies the Santa Barbara thrust
belt and its along-strike equivalent Subandean thrust belt
(Figure 1). The Eastern Cordillera and Santa Barbara belt
both contain basement-involved reverse faults, whereas the
Subandean belt is a typical thin-skinned thrust belt [Kley et
al., 1999; Allmendinger and Zapata, 2000; Echavarria et
al., 2003]. Directly west of the magmatic arc is the
Cordillera de Domeyko in northern Chile (Figure 1), which
comprises Paleozoic and Mesozoic rocks that experienced
significant uplift and erosion during at least PaleoceneEocene time [Hartley et al., 1992; Flint et al., 1993;
Charrier and Reutter, 1994; Maksaev and Zentilli, 2000;
Mpodozis et al., 2005; Arriagada et al., 2006].
[7] The Puna interior is characterized by internally
drained topographic basins separated by generally northtrending mountain ranges composed of Precambrian and
lower Paleozoic low-grade metasedimentary and plutonic
rocks, Neogene ignimbrites and volcanic rocks, and Paleogene-Pliocene clastic and evaporitic sedimentary rocks.
Numerous, generally steeply eastward and westward dipping, reverse faults cutting the older rocks and locally
juxtaposing them against the Tertiary sedimentary rocks,
are depicted in the cross section shown in Figure 2c derived
from the 1:250,000 (San Antonio de los Cobres) geological
map [Blasco et al., 1996] and integrated with the
1:1,000,000 geological map [Reutter et al., 1994].
[8] The Salar de Pastos Grandes area contains more than
3.5 km of Eocene to Quaternary strata [Alonso, 1992]. The
section investigated here is part of the Geste Formation of
late Eocene age [Pascual, 1983; Alonso, 1992; DeCelles et
al., 2007]. In this area the Geste Formation unconformably
overlies Ordovician quartzite and phyllite of the Copalayo
Formation in the Copalayo Range. Other pre-Tertiary rocks
west of the Ordovician outcrops include Precambrian plutonic rocks and sparse Paleozoic plutonic rocks. To the east
of the study area, the Eastern Cordillera is composed of
Precambrian-Cambrian meta-sedimentary (Puncoviscana
Formation) and plutonic rocks, and Cretaceous sedimentary
rocks.
[9] Detrital zircon U-Pb ages show that fluvial sandstones in the Geste Formation were derived in part from
quartzite in the underlying Ordovician Copalayo Formation
[DeCelles et al., 2007]. Volcanogenic air fall derived
zircons constrain the depositional age of the Geste Formation
to the late Eocene (39.0 ± 0.6 Ma to 35.4 ± 0.55 Ma;
[DeCelles et al., 2007]). A late Eocene depositional age also
is consistent with vertebrate paleontological ages from the
Geste Formation in the Salar de Pastos Grandes area [Pascual,
1983] and in the La Poma Basin to the east [Hongn et al., 2007].
[10] Recently published seismic, sedimentological, and
structural data from the Salar de Atacama basin in northern
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Chile, approximately 200 km west of the Salar de Pastos
Grandes, document the Cordillera de Domeyko fold-thrust
belt and associated proximal syntectonic deposits of midCretaceous through Eocene age [Arriagada et al., 2006].
Contractional growth structures in these strata imply that
they were deposited in the wedge-top depozone of a
foreland basin system that may have extended into northern
Argentina. Thermochronological AFT data from plutonic
intrusions in the Cordillera de Domeyko document Eocene
to Oligocene cooling ages (between 50 Ma and 30 Ma)
[Maksaev and Zentilli, 2000].
[11] In the Eastern Cordillera, sparse AFT data from the
Chango Real Range document late Eocene – Oligocene
(38.3 ± 3 Ma to 29.0 ± 3 Ma) cooling ages [Coutand et
al., 2006]; however, it remains unclear if the Chango Real
(CR in Figure 1) and other ranges in the Eastern Cordillera
constituted topographically uplifted areas and sediment
sources at that time. Recent sedimentological and detrital
U-Pb geochronological data from the Angastaco and Tin
Tin areas (A, TT, in Figure 1) suggest that a genetic link
may exist between the Eocene sedimentary deposits of the
plateau interior and those located within the Eastern Cordillera [Carrapa et al., 2006a]. AFT data and thermal
modeling support this concept, showing that ranges within
the Eastern Cordillera must have been buried by a thick pile
of pre-Miocene sediments and that exhumation of those
ranges commenced at 20 Ma [Coutand et al., 2006;
Deeken et al., 2006].
3. Sedimentology
3.1. Facies Analysis
[12] The following sedimentological descriptions and
interpretations are based on detailed, bed-by-bed measured
stratigraphic sections (Figure 3). Individual beds were
measured at centimeter scale. We measured a composite
section totaling 2019 m in thickness, and correlated between
offset sections by tracing marker beds and projecting along
strike. Because the lithofacies encountered are well known
in the sedimentological literature, we employ the lithofacies
codes of Miall [1978] with some minor modifications
(Table 1). Paleoflow directions were determined by measuring limbs of trough cross strata according to method I of
DeCelles et al. [1983], and the dip directions of imbricated
clasts in conglomerates. Approximately 20 trough limbs or
at least 10 imbricated clasts were measured per station.
Mean paleocurrent directions are plotted in Figure 3.
3.2. Depositional Environments of the Geste Formation
3.2.1. Description
[13] Overall the Geste Formation exhibits a marked
upward coarsening trend in grain size over the >2 km thick
section that we measured (Figure 3). On the basis of
lithofacies assemblages, we divide the Geste Formation into
informal lower, middle, and upper members.
[14] The lower member is 370 m thick and extends
from the base of measured section 1SP to the base of section
2SP (Figures 3 and 4). It mainly consists of fine- to
medium-grained sandstone, bright red siltstone, and pebble
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Figure 3. Stratigraphic sections of the Geste Formations based on detailed bed-by-bed field measurements.
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Table 1. Facies Codes After Miall [1978] Integrated With Our Descriptions
Facies Codes
Lithofacies
Gmm
Gcm
conglomerate, matrix-supported
conglomerate, clast-supported
Gch
conglomerate, clast-supported
Gct
Gci
Sm
St
conglomerate, clast-supported
conglomerate, clast-supported
sandstone, fine- to coarse-grained
sandstone, fine- to very coarse-grained,
locally pebbly
sandstone, fine- to very coarse-grained,
locally pebbly
fine-grained sandstone, siltstone, mudstone
Sh
Fsm
Sedimentary Structures
structureless, disorganized
structureless to crude horizontal
stratification, imbrication
horizontal stratification,
local imbrication
trough and planar cross-beds
imbricated clasts
massive or faint lamination
trough cross-beds
horizontal laminations/low angle
(<15°) cross-beds
massive, desiccation cracks
to cobble conglomerate. Sandy lithofacies include massive
(Sm), horizontally laminated (Sh), and trough cross-stratified
(St) sandstone in 1- to 5-m-thick, broadly lenticular beds with
erosional bases. These beds commonly exhibit upward fining
grain size trends. Siltstone (Fsm) intervals are massive,
bioturbated, and locally mottled, with clay cutans and peds.
Conglomerate beds are lenticular and typically intercalated
within units of St. The most abundant gravelly lithofacies
are clast-supported, imbricated (Gci), horizontally stratified
(Gch), and trough cross-stratified (Gct) conglomerates.
[15] The middle member is 1482 m thick (Logs SP2 and
SP3 in Figures 3), and consists of 1- to 10-m-thick lenticular
bodies of upward fining conglomerate (lithofacies Gcm,
Gch, and Gci) with erosional basal surfaces; the tops of
these units are capped by thin beds of medium- to coarsegrained sandstone (lithofacies Sm, Sh, and St). Massive,
burrowed siltstone (Fsm) is abundant in the lower 250 m of
the middle member, but becomes sporadic from there
upward (Figures 3 and 4).
[16] The upper member of the Geste Formation (Log SP
4 in Figure 3) consists of thick, amalgamated beds of cobble
to boulder conglomerate (Figure 4). Although we measured
only 167 m of the upper member before encountering
poor outcrop, it is clear that at least an additional several
hundred meters of this unit are present [cf. Alonso, 1992].
The upper member is mainly characterized by massive to
horizontally stratified, clast- and matrix-supported conglomerates (Gcm, Gch, Gmm), with limited massive to horizontally stratified, medium- to coarse-grained sandstone (Sm,
Sh). The conglomerates occur in beds ranging from 1 m to
16 m thick, and no obvious grain size trends are present.
[17] Bedding in the Geste Formation dips eastward away
from the Copalayo Range. Although the exposure is insufficient to document unequivocally the presence of a growth
structure, dip of bedding decreases systematically up-section and eastward from 60°E in the lower member to 20°E
in the upper member.
[18] Sedimentation rates are calculated using the stratigraphic thickness presented in Figure 3 and the mean U-Pb
ages of Eocene zircon grains (refer to Table 3 in section 5.1)
for samples in which they were found [DeCelles et al.,
2007]. Sedimentation rates are on the order of 0.08 mm/a
Interpretation
high-strength (cohesive) debris flow
clast-rich (noncohesive) debris flow
and sheetflood deposits
longitudinal gravel bars, lag deposits
minor channel fills, 3D gravelly bedforms
channel fills and longitudinal bar deposits
hyperconcentrated flows, slurry flows
subaqueous 3D dunes, sustained
unidirectional currents
shallow supercritical flows
overbank deposits, abandoned channel
fills and drape deposits
between samples 1SP32 and 1SP238, and 1.1 mm/a, between
sample 2SP277 and 3SP431. These rates, and their increase
up-section, are typical of sedimentary basins related to
contractional orogenic systems such as wedge-top, foreland,
and intermontane basins [Allen, 1983; Horton, 1998; Allen
and Allen, 2005].
3.2.2. Interpretation
[19] The Geste Formation contains abundant evidence for
deposition in fluvial and alluvial fan environments, including lenticular bodies of conglomerate and sandstone with
erosional basal surfaces, sedimentary structures formed by
traction currents, paleosols, and sediment-gravity flow
deposits. The sandstone and conglomerate with sedimentary
structures formed by unidirectional traction currents (e.g.,
trough cross stratification, imbrication) were deposited in
fluvial channels and gravelly and sandy macroforms [Smith,
1970; Hein and Walker, 1977; Miall and Turner-Peterson,
1989; Lunt and Bridge, 2004; Wooldridge and Hickin,
2005]. Upward fining in these channel deposits resulted
from gradual filling and abandonment. The absence of
typical features associated with meandering channels and
anastomosing fluvial systems, such as lateral accretion
deposits, prominent levees, oxbow lake deposits, and crevasse splay deposits [Smith and Perez-Arlucea, 1994; Miall,
1996; Makaske, 2001], suggests low-sinuosity bed load –
dominated braided channels. Although fine-grained deposits
are abundant in the lower member, paleosols are neither
well developed nor particularly common, and in general we
found little evidence for abundant vegetation, again suggesting that channel banks were unstable. The middle
member is dominated by coarse conglomerate and sandstone, with relatively little fine-grained sediment. The
abundance of imbricated and horizontally stratified conglomerate in beds with erosional bases suggests deposition
in shallow, gravelly braided channels [Nemec and Steel,
1984]. The upper member contains evidence for deposition
by viscous debris flows, hyperconcentrated flows, and
coarse-grained fluvial channels. The massive matrix-supported conglomerates and extreme coarseness are best
explained by deposition in an alluvial fan system [e.g.,
Heward, 1978; Pierson, 1980; Nemec and Steel, 1984;
Schultz, 1984; Flint, 1985; DeCelles et al., 1991]. Most
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Figure 4. Typical facies of the (a) lower, (b) middle, and (c) upper members; (d) typical burrows. Refer
to Figure 3 and text for more details.
likely, these alluvial fans experienced a combination of sheet
flood and sediment-gravity flow deposition. The abundance
of boulders larger than 50 cm in diameter indicates deposition within a few kilometers of the source terrane. Clast
composition data, discussed below, support this concept of
proximal deposition.
4. Provenance
4.1. Methods
[20 ] Sandstone petrography and conglomerate clast
counts were conducted throughout the Geste Formation in
order to identify the sediment source area and its erosional
history. Sixteen sandstone samples (Figure 5 and Table 2)
were analyzed using a modified Gazzi-Dickinson method
[Gazzi, 1966; Dickinson, 1970, 1974; Ingersoll et al., 1984].
Half of each thin section was stained for K and Ca feldspars.
All constituents and at least 250 framework grains were
counted, and an additional 100 lithic grains were counted in
each thin section in order to provide better control on source
rock types. The point-counting parameters and normalized
modal petrographic data are given in Table 2 and plotted on
standard ternary diagrams in Figure 5. Conglomerate compositions (Figure 6) were determined by counting at least
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present, but chert was not detected (Table 2). Lithic grains
(grain size < 62 mm) consist mainly of quartzite, schist, and
phyllite (Table 2 and Figure 5). No volcanic lithic grains
were found (Table 2).
[22] Sandstone samples from the Geste Formation show a
trend from quartzose to quartzolithic compositions, with
average Qm/F/Lt = 66/8/25. Following the classification of
Dickinson and Suczek [1979] and Dickinson [1985], these
compositions overlap the fields of continental block and
recycled orogenic provenance (Figure 5a) and are typical of
Andean-type orogenic systems [DeCelles and Hertel, 1989;
Garzanti et al., 2007]. The Qm/P/K diagram is mainly
dominated by quartz with wavy extinction, Qz (grain size
> 62 mm) in polycrystalline and monomineralic grains and
Qz (grain size > 62 mm) in plutonic and metamorphic rocks.
The Qm/P/K diagram shows a decrease up-sequence of both
P and K (Figure 5b). The extra diagram for fine-grained
lithics (with grain constituents <62mm; refer to Table 2 for
more details) shows a source initially (lower member)
characterized by quartzite (90% quartz) and fine polycrystalline Qz (Quartzite/Qz pole, in Figure 5c). An increase upsection in schist and phyllite fragments is observed (Figure 5c).
[23] Conglomerate clasts are composed of phyllite,
quartzite, fine-grained quartz-mica schist, and milky vein
quartz (Figure 6). Quartzite and phyllite are typical of
Ordovician metasedimentary rocks in the Copalayo Formation. Mica-schist may be representative of Cambrian and
Ordovician metamorphic rocks [Coira et al., 1982]. These
rock types are typical of western source terranes, in agreement with paleocurrent data documenting eastward flow
(Figure 3).
5. Apatite Fission Track Thermochronology
5.1. Method
Figure 5. Modal compositions of Geste Formation
sandstones calculated using the Gazzi-Dickinson method
(for more details refer to text): (a) QmFLt diagram based on
the technique described by Gazzi [1966] and Dickinson
[1970, 1985]; (b) QmPK; and (c) Quartzite/Qz-SchistPhyllite. See text for explanation, and see Table 2 for data
and parameter definitions.
100 clasts per location within a 10- to 20-cm grid that was
moved along strike until enough data were collected.
4.2. Sandstone and Conglomerate Provenance
[21] Framework grains in the Geste Formation sandstones
include monocrystalline quartz (Qm), quartz (Qz) (>62 mm)
in plutonic and metamorphic rocks, plagioclase (mainly
calcic, P), K-feldspar (K), and P or K in plutonic and
metamorphic rocks. Fine-grained polycrystalline quartz is
[24] AFT thermochronology provides information on
the timing and rates of cooling between temperatures of
60°C and 120°C. This temperature range defines the AFT
Partial Annealing Zone (PAZ) [Wagner, 1968; Gleadow and
Fitzgerald, 1987]. The exact temperature of the base of the
PAZ depends on the kinetic characteristics of the apatites
and the cooling rate. Apatite kinetic characteristics can be
quantified by measuring the diameter of fission track etch
pits, which is defined as Dpar [Donelick et al., 1999;
Ketcham et al., 1999]. In general, apatites with smaller Dpar
are typical of flourine-rich apatite and are characterized by
cooler temperatures of the upper PAZ boundary, whereas
apatites with larger Dpar are typically chlorine-rich and are
characterized by hotter temperatures of the upper PAZ
boundary. Fission track lengths provide information about
the proportion of the cooling history that the sample
experienced within the PAZ, and hence how quickly the
apatite passed through the PAZ. Therefore, in order to
interpret the AFT data in terms of a temperature-time path,
an integrated analysis of fission track age, track length
distribution, and kinetic characteristics of the apatite grains
is required. In the case of detrital AFT thermochronology,
multiple populations of apatites may be present in a single
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Qz straight extinction
Qz extinction
Coarse policrystalline
Qz (>62 mm)
Fine policrystalline Qz
Chert
Qz (>62 mm) in
plutonic rock
Qz (>62 mm) in
metamorphic rock
Qz (>62 mm) in
volcanic rock
monocrystalline K
not altered
K (>62 mm) in plutonic
rock not altered
K (>62 mm) in
metamorphic rock
K (>62 mm) in
volcanic rock
Monocrystalline Ca-P
not altered
Monocrystalline Na-P
not altered
P (>62 mm) in plutonic
rock not altered
P (>62 mm) in
metamorphic rock
P (>62 mm) in
volcanic rock
Metamorphic lithic
Volcanic lithic
Clastic lithic
Single biotited/chlorite
Single muscovite
Mica in crystalline
rock fragment
Single heavy mineral
Heavy mineral in
rock fragment
Calc-schist
Sparry calcite
Dolomite
Impure calcareous grain
Mudstone-wackstone
Packstone-grainstone
Total framework grains
Altered grain of
unknown origin
Sandstone Parameters
9 of 19
Qm
Qm
Qm
Qm
K
K
K
K
P
P
P
P
P
Qm
Lt
Lt
Qm
Qm
Qm
K
K
K
K
P
P
P
P
P
Lt
Lt
Lt
Lt
Lt
Lt
Qm
Qm
Qm
Qm
QmFLt QmPK
Lithics
3.1
2.7
0.2
0.2
56
0.2
1.1
57
5.4
0.4
0.8
0.2
7.6
0.4
1.2
2.5
0.4
22.5
2.3
12.2
3.3
0.7
0.9
0.2
0.9
2.4
0.7
2.4
25.2
1.3
2.2
16.7
2.9
56
3.3
55
0.4
0.6
1.0
0.2
2.0
0.2
3.9
0.2
2.0
0.2
0.8
0.2
1.8
3.3
1.6
17.0
4.3
18.4
2.2
0.2
0.6
1.8
0.2
1.6
2.9
2.6
0.4
16.3
3.7
23.4
1.3
57
5.7
0.7
1.1
1.8
0.0
3.5
2.6
22.9
0.2
2.2
16.5
2.9
53
0.2
1.2
0.2
1.9
3.6
0.4
1.0
3.5
0.0
0.2
0.8
4.2
2.9
11.7
1.9
23.2
2.0
53
0.2
1.4
0.2
1.0
0.8
12.7
0.2
1.2
0.4
1.2
0.4
0.6
1.2
2.5
1.0
4.3
2.5
24.9
2.4
69
0.3
0.3
23.4
0.3
0.5
1.0
0.3
1.6
12.3
4.7
3.9
0.3
0.3
20.2
2.8
61
0.2
0.5
0.7
0.2
0.7
1.2
18.5
0.9
0.7
2.6
0.0
4.2
2.8
4.0
2.8
24.1
2.5
60
0.5
1.8
0.7
0.5
16.0
0.7
1.1
0.2
2.3
0.0
0.2
4.1
5.9
0.2
22.2
6.9
0.5
65
0.2
0.2
0.2
24.9
0.7
0.0
0.5
0.5
6.5
0.9
8.2
2.3
20.5
0.4
51
1.0
0.2
0.2
0.4
27.9
0.4
0.4
0.0
7.1
1.0
3.4
1.4
8.9
4.7
52
0.2
0.6
0.6
0.6
1.2
24.7
0.2
0.8
1.2
0.2
0.2
0.4
5.5
0.8
2.6
0.4
1.0
14.0
2.8
46
0.3
0.5
0.2
1.0
0.3
17.8
0.2
0.2
0.0
6.0
5.2
1.3
15.2
0.4
50
0.9
0.7
0.2
22.8
0.4
0.5
0.0
0.5
0.0
7.1
0.5
3.3
0.2
1.3
12.2
CARRAPA AND DECELLES: EOCENE BASIN DEVELOPMENT IN NW ARGENTINA
2.6
47
0.2
1.6
6.6
0.4
0.2
0.4
0.5
2.4
2.0
3.5
4.6
1.6
9.3
0.9
14.4
SP11 SP23 SP77 SP99 SP123 SP201 SP237 SP239 2SP100 3SP10 3SP95 3SP149 3SP203 3SP627 3SP925 4SP111
Table 2. Sandstone Petrography Raw Data and Parametersa
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10 of 19
QmFLt QmPK
Refer to text for further explanations.
a
Fine QZ
Quarzites
Schist QZ + Biotite
Schist QZ + Biotite + Musc.
+ K + heavy min.
Phyllate
Microcrystalline mica
Schist QZ + Muscovite
Schist QZ + Musc. + heavy min.
Total Fine Lithics
Quartzite-Qz
Schist-phyllite
phyllite
Fine Lithic Components
Authigenic mineral in
unknown grain
Coarse silt
Siliciclastic matrix
Carbonatic matrix
Quartzitic cement
Phyllosilicatic cement
Carbonatic cement
Other cements
Inter grains porosity
Calcite overgrowth
Total Rock
Qm
F
Lt + C
Qm
P
K
Sandstone Parameters
Table 2. (continued)
2.7
2.5
100
71
19
10
62
32
6
7.3
1.3
100
83
9
8
81
7
12
0.0
0.4
100
66
20
14
69
5
25
0.4
4.7
12.4
0.0
0.2
28.1
2.6
2.6
2.4
100
83
13
4
81
7
13
0.6
7.7
0.4
0.2
5.9
19.1
0.0
4.0
100
83
6
10
88
12
0
0.2
35.7
8.8
11.0
9.6
0.6
100
80
13
7
80
12
8
1.3
0.2
0.4
8.4
0.4
0.0
4.0
100
82
11
7
85
13
3
0.8
1.0
0.2
0.4
12.5
21.5
4.5
0.8
100
66
10
24
86
8
6
0.4
3.5
3.5
0.2
6.5
22.0
6.3
0.0
100
60
6
34
91
5
4
9.4
6.0
4.7
1.6
0.3
11.5
0.2
100
62
7
31
89
11
0
4.2
4.2
0.2
1.4
11.2
0.5
2.1
100
66
8
27
90
10
1
6.4
19.7
0.2
0.5
3.7
1.1
28.7
100
43
2
55
96
4
0
100
59
3
39
95
2
3
6.5
0.2
1.2
10.5
0.2
2.3
16.1
0.5
0.2
1.9
12.1
0.7
100
46
6
48
88
9
3
0.8
23.1
0.8
0.2
0.4
14.4
0.4
20.6
0.3
100
60
1
39
99
1
0
8.6
5.5
1.0
11.5
1.6
100
49
3
48
94
6
0
38.6
1.3
0.0
1.5
1.5
6.2
0.0
18.3
18.3
26.8
4.2
29.6
2.8
100
37
32
31
phyllite
schist
schist
schist
41.2
3.9
33.3
5.9
100
16
39
45
7.8
7.8
30.5
3.2
45.3
2.1
100
13
54
34
6.3
1.1
11.6
27.8
6.3
39.2
6.3
100
20
46
34
7.6
6.3
66.2
1.5
16.9
1.5
100
9
23
68
4.6
4.6
4.6
28.0
0.0
19.0
5.0
100
45
27
28
4.0
41.0
1.0
2.0
0.0
39.8
4.8
28.9
100
2
58
40
0.0
1.2
3.6
46.5
2.0
28.7
8.9
100
8
43
49
7.9
2.0
3.0
38.4
0.9
39.3
0.9
100
9
52
39
11.6
0.9
8.0
39.3
12.1
29.9
5.6
100
2
47
51
0.0
1.9
1.9
9.3
46.3
5.6
28.7
8.3
100
3
45
52
0.9
1.9
1.9
5.6
41.7
8.3
41.7
5.6
100
0
50
50
1.9
49.3
0.7
28.9
14.1
100
4
46
50
3.5
0.7
2.1
65.7
15.7
13.0
1.9
100
3
16
81
1.9
0.9
0.0
0.9
60.7
3.7
17.8
6.5
100
10
25
64
10.3
0.9
39.3
9.8
100
2
72
26
25.0
0.9
0.9
13.4
8.0
SP11 SP23 SP77 SP99 SP123 SP201 SP237 SP239 2SP100 3SP10 3SP95 3SP149 3SP203 3SP627 3SP925 4SP111
0.6
8.1
0.4
0.2
6.0
20.0
0.9
11.4
1.3
0.2
6.0
10.5
SP11 SP23 SP77 SP99 SP123 SP201 SP237 SP239 2SP100 3SP10 3SP95 3SP149 3SP203 3SP627 3SP925 4SP111
Qz
quartzite
schist
schist
Lithics
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CARRAPA AND DECELLES: EOCENE BASIN DEVELOPMENT IN NW ARGENTINA
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2006b; Coutand et al., 2006; Mortimer et al., 2007; van der
Beek et al., 2007] because it places constraints on cooling
events at temperatures lower than those recorded by zircon
fission tracks and other higher temperature thermochronometers [e.g., Garver et al., 1999]. Detrital AFT thermochronology is particularly suitable in geological settings
where shallow exhumation and erosion (4 – 6 km; considering a paleogeothermal gradient of 20°– 30°C/km and a
closure T of 120°C) has occurred, such as in the central
Andes.
[26] Samples were prepared and analyzed following the
procedure described by Sobel and Strecker [2003]. An
average of 100 grains for each detrital sample of the Geste
Formation was dated (Data Set S1 in auxiliary material1).
Confined track lengths, when present, were measured in the
same grains counted for age determination. The angle
between the confined track and the C-crystallographic axis
(C-axis projected data) was measured in order to mitigate
track-measurement bias [Barbarand et al., 2003], because
confined tracks anneal anisotropically as a function of
orientation [Donelick et al., 1999; Ketcham et al., 1999].
Grain shape was also determined for each analyzed grain
(Figure S1 in auxiliary material). For each detrital sample,
fission track grain-age distributions were decomposed following the binomial peak-fitting method [Galbraith and
Green, 1990] incorporated in the Binomfit program [Brandon,
2002]. All populations are calculated in automatic mode, in
which the program provides an iterative search of peak ages
and number of peaks to find the optimal (best fit) solution.
The best fit solution is determined by directly comparing the
distribution of the grain age data to a predicted mixed
binomial distribution. The related best fit peaks are reported
by age, uncertainty, and size (Table 3). The uncertainty for
the peak age is given at 95% confidence intervals. The size
of the individual peaks is reported as a fraction (in percent)
of the total (Table 3). Track length and Dpar data were
compiled for each detrital population to allow comparison
(Table 2). The complete AFT data set, including radial plots
and probability density diagrams, is represented in Figure 7
and Table 3.
5.2. AFT Data and Interpretation
Figure 6. Conglomerate clast-composition data from the
Geste Formation based on 100 clast counts per location;
arrows indicate mean flow directions based on imbrications
(see text for explanation). For location, refer to Figure 3.
sample. Therefore careful measurement of Dpar values and
track lengths for each population is necessary.
[25] Assuming that the apatites in the sedimentary basin
never were subjected to temperatures high enough to
overprint the original thermochronological signal (80°C;
discussed below), detrital AFT thermochronology provides
information about characteristic cooling ages of rocks
originally present in the source terrane and the timing, rates,
and spatial patterns of exhumation [e.g., Carrapa et al.,
2006a; Coutand et al., 2006]. Detrital AFT has received
increasing attention in recent years [e.g., Carrapa et al.,
[27] Five sandstone samples from the Geste Formation
exhibit four distinctive AFT populations (P1 – P4; Table 3).
The depositional age of each sample is determined from the
youngest cluster of U-Pb zircon ages in each sample
[DeCelles et al., 2007]. This approach assumes that the
depositional age is equal to the zircon U-Pb age (mean or
single age), which is consistent with paleontological data
[Pascual, 1983] and arguments for the volcanogenic air fall
origin of the Eocene zircons [DeCelles et al., 2007]. Sample
1SP238 only produced one Eocene grain age (39.8 ± 0.6 Ma);
we used this age as a maximum stratigraphic age constraint.
Overall the AFT age populations can be traced up-section
and the youngest ones show consistent upward younging
trends (Figure 8). The AFT populations are generally older
1
Auxiliary material data sets are available at ftp://ftp.agu.org/apend/tc/
2007tc002127. Other auxiliary material files are in the HTML.
11 of 19
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Table 3. Detrital AFT Populations Calculated Using Binomfit in Automated Modea
Sample
Depositional Age, Ma
N
Code
4SP7
(UP80-10)
34.7 ± 3.5b
100
3SP431
(UP80-9)
35.4 ± 0.55 Ma
100
2SP277
(UP80-5)
35.9 ± 6.4
27
2SP38
(UP 80-6)
37.3 ± 1.5
100
1SP32
(UP80-3-4)
39.8 ± 0.6c
100
P1
35.4 ± 3.3
46.3%
L(n:1): 13.9
Dpar/stdev: 1.9/0.2
35.8 ± 1.7
39.8%
L(n:7)/stdev: 13.3/0.9
Dpar/stdev: 2.3/0.4
46.5 ± 3.25
25.3%
L(n:11)/stdev: 13.2/0.8
Dpar/stdev: 2.0/0.3
43.7 ± 3.2
41.0%
L(n:3)/stdev: 12.9/0.6
Dpar/stdev: 1.9/0.2
51.6 ± 4.1
27%
L(n:6)/stdev:11.6/3
Dpar/stdev: 1.9/0.2
P2
51.7 ± 4.3
51.1%
L(n:4): 13.8/0.7
Dpar/stdev: 1.9/0.2
54 ± 5.5
40.2%
L(n:10)/stdev: 12.7/2
Dpar/stdev: 2.4/0.5
ND
ND
ND
ND
56.2 ± 2.7
58.0%
L(n:18)/stdev: 12.8/0.8
Dpar/stdev: 1.9/0.2
65.7 ± 6.9d
63%
L(n:27)/stdev:11.5/1.2
Dpar/stdev: 1.9/0.2
P3
ND
ND
ND
ND
64.7 ± 13.8
20.0%
L(n:2)/stdev: 13.3/1.6
Dpar/stdev: 2.4/0.7
ND
ND
ND
ND
88.1 ± 31.1
1%
ND
Dpar: 1.6
93.3 ± 10.9
10%
L(n:1): 13.0
Dpar/stdev:2.0/0.2
P4
106.3 ± 15
2.6%
L(n:2):11.4/1.8
Dpar/stdev: 2/0.1
ND
ND
ND
ND
112.3 ± 26.4
1.7%
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
a
Detrital AFT populations are calculated using Binomfit from Brandon [2002]. Refer to text for mode details. N, number of grains counted; ND, no data;
stdev, standard deviation; Dpar, etch pit diameter (mm). L denotes length (mm); the values reported are not corrected for c axis.
b
Depositional age calculated assuming a sedimentation rate of 1.1 mm/a (calculated between sample 2SP277 and 3SP431).
c
Depositional age is assumed equal to the youngest zircon U-Pb age of sample 1SP238 [after DeCelles et al., 2007].
d
Population is modeled in Figure 9.
than, or equal to (for the stratigraphically higher samples),
the associated zircon U-Pb ages from the same samples.
Detailed inspection of individual AFT ages (Figure 7)
indicates that some grains have mean ages younger than
the depositional age inferred from zircon U-Pb geochronology. However, less than 10% of the AFT ages are younger
than the associated depositional age (i.e., fewer than
10 grains); moreover, these young ages have errors generally greater than 20%. Therefore, statistically they cannot be
considered meaningful. This supports the hypothesis that
the investigated samples did not suffer significant annealing
after deposition. Had these samples been annealed after
deposition, older, more deeply buried samples would have
experienced greater amounts of annealing and, in turn,
recorded younger ages than the stratigraphically higher
samples. This would most likely result in detrital populations becoming older up section, which is the opposite trend
from the younging up-section trend observed in Figure 8. In
the same way, track lengths, even though limited, do not
show an apparent decreasing trend down-section as would
be expected if the Geste Formation apatites experienced
significant postdepositional annealing. All available confined track lengths were measured on double mounts for the
investigated samples (i.e., no extra length data were available). Negligible postdepositional annealing (<80°C) is also
supported by the modeling results described in the following paragraph. Therefore we consider the AFT age populations as representative of regional cooling events owing to
tectonic exhumation and erosion affecting source rocks
located to the west of the study area.
[28] The youngest population P1 is composed of early to
late Eocene ages (35.4 ± 3.3 Ma to 51.6 ± 4.1 Ma) and shows
relatively short to extremely short lag times (11 –0.5 Ma).
P1 could have different explanations discussed below.
[29] 1. The Eocene grains reflect plutonic or volcanic
input. A plutonic origin is unlikely because no Eocene
plutons have been reported in the region. The presence of
zircons in Geste Formation sandstones with Eocene U-Pb
ages might support this interpretation, but we prefer a
volcanogenic origin for the Eocene zircons because of their
fresh and angular appearance, coupled with the absence of
Eocene plutons in the region [DeCelles et al., 2007]. In
addition, Eocene tuffs have been documented in northern
Chile [Ramı́rez and Gardeweg, 1982; Hammerschmidt et
al., 1992; Mpodozis et al., 2005]. However, the ashfall
derived zircons account for less than 5% of the total detrital
zircon age spectrum, whereas the ages belonging to P1
constitute more than 30% of the detrital apatite age spectrum in each sample. Moreover, the measured track lengths
(even though limited) are not typical of instantaneously
cooled ashfall material, for which values of 15– 16 mm are
expected. A detailed analysis of grain shapes on all counted
grains (Data Set S1 and Figure S1 in auxiliary material)
failed to show any distinguishing shape characteristics of P1
grains. We therefore view volcanic input as an unlikely
explanation for P1.
[30] 2. The Eocene grains could represent partial thermal
overprinting of the apatite-bearing source rocks owing to
Tertiary magmatic intrusions that may not surface today. If
such a process were responsible for the Eocene AFT signal
then we would expect to find a broader spectrum of Tertiary
12 of 19
Figure 7. Detrital AFT data from the Geste Formation. Radial plots are calculated using Trackkey program after I.
Dunkl (Trackkey, windows program for calculation and graphical presentation of EDM fission track data, version 4.2,
2002, http://www.sediment.uni-goettingen.de/staff/dunkl/software/trackkey.html). Probability density plots are
calculated using Binomfit [Brandon, 2002] in automated mode; refer to text for more details.
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Figure 8. Lag time plot, showing AFT age populations on the x axis and depositional ages on the y axis
(from Table 3). Depositional ages are based on the youngest U-Pb ages reported for the Geste Formation
samples by DeCelles et al. [2007]. Refer to text for more details.
ages in the original source, characterized by younger ages
closer to the locus of the intrusive body and older ages
farther away from it. Sediment derived from a partially reset
source terrane would contain a broad spectrum of Tertiary
detrital ages, which, most likely, would not result in a
consistent up-section trend.
[31] 3. The short lag time may indicate rapid source
terrane exhumation and cooling. In this case we would
expect a general up-section decrease in population mean
age, which is evident in P1. Moreover, the clear younging
up-section trend of P1 is consistent with the trend observed
for P2, suggesting real exhumation and erosional signals
affecting either a single source characterized by different
ages belonging to populations P1 and P2 (e.g., from
different elevations) or different sources characterized by
similar exhumation patterns. Given the objections raised to
the other two explanations, either of these two scenarios
would seem to be equally valid. We assume a conservative
paleo-geothermal gradient of 20°C/km, which is consistent
with modeling results presented below and with AFT data
and thermal modeling presented by Deeken et al. [2006].
We use the minimum total annealing temperature of 105°C
(in order to limit possible overestimations), indicated by
modeling of P2 presented in Figure 9; note that P1 and P2
apatites have the same Dpar values and therefore the same
total annealing temperature (Ta). A crustal thickness of
5.2 km is obtained with the constraints specified above.
Considering a lag time of 0.4 Ma (obtained using the
youngest sample 3SP431 for which the lowest error is
reported in Table 3) and 11.8 Ma (obtained using the oldest
sample 1SP32 for which the lowest error is reported in
Table 3) we obtain exhumation rates on the order of 0.4 mm/a
to 1.0 mm/a. A lower paleo-geothermal gradient and a
higher Ta would result in even higher cooling and exhumation rates. We consider paleo-geothermal gradient >30°C/km
for Eocene time as unlikely. The lowest estimated exhumation rates are similar to values reported from the Oligocene
and middle-upper Tertiary in neighboring areas [Carrapa et
al., 2005; Coutand et al., 2006; Deeken et al., 2006]
whereas the highest exhumation rates are much greater than
values previously reported. This could be representative of a
strong tectonic exhumation signal related to the early stages
of Andean mountain building, or an overestimate related to
the fact that we are using detrital populations as proxies for
a specific exhumation age. Even considering an overestimation of 50% of the maximum calculated exhumation rate,
we still obtain values that are much higher than previously
reported for the Andes. We interpret this result as a signal of
strong tectonic exhumation related to early Tertiary Andean
shortening and crustal thickening.
[32] Limited volcanogenic contamination remains a possibility that we cannot completely rule out and its potential
influence may have been greater in the upper (younger)
samples for which lag times <5 Ma are observed. In any
case, the fact that locally derived coarse-grained Eocene
conglomerates were deposited in this region unequivocally
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Figure 9. Modeling results of P2 in sample 1SP32 obtained using HeFty model after Ketcham [2005];
T-t constraints are explained in the text. Ta is total annealing T and T-window (gray transparent area)
calculated using the oldest track age.
documents active exhumation and erosion of the sediment
source during the Eocene.
[33] P2 is composed of early Paleocene to early Eocene
ages (51.7 ± 4.3 to 65.7 ± 6.9 Ma) showing a clear younging
up-section trend. P2 in sample 1SP32 was modeled in order
to gain a better understanding of the thermal history of the
original sediment source and of the sedimentary basin; the
method and results are discussed in the following section.
P3 and P4 consist of limited Paleocene and Late to midCretaceous ages (64.7 ± 13.8 to 112.3 ± 26.4 Ma). Such
ages may correspond to rocks in the Cordillera de Domeyko
fold-thrust belt undergoing deformation and exhumation
during middle Late Cretaceous-Paleocene and Oligocene
time [Maksaev and Zentilli, 2000; Mpodozis et al., 2005;
Arriagada et al., 2006].
6. AFT Length Modeling
6.1. Method
[34] Track-length modeling was attempted on sample
1SP32 for which the most track length data per population
exist (Table 3). Inverse modeling is used to test hypotheses
concerning the thermal history of the sediment source prior
to and after deposition, and to define the likely exhumation
history of the specific population source and of the host
sedimentary basin. An important issue when modeling a
detrital population is whether the ages belonging to a single
population might be derived from a broad spectrum of
possible sources, and/or elevations, which experienced
similar but not necessarily identical thermal histories. In
such a case, the spectrum of track lengths in a specific
population may reflect both multiple cooling events prior to
exhumation and variations owing to different elevations of
the source rock, potentially overprinted by reheating due to
burial and subsequent cooling. Considering that multiple
heating/cooling events may have affected the original
source prior to deposition, correctly constraining this portion of the cooling path in the model is challenging. Inverse
modeling was performed using HeFTy [Ketcham, 2005].
6.2. Track-Length Modeling Constraints and Results
[35] The initial time constraint is set at approximately
double the pooled age of the sample or detrital population to
ensure that the first-formed tracks are all completely
annealed, thereby avoiding potential boundary condition
artifacts [Ketcham, 2000]. Other geologically plausible constraints were applied in order to test for: (1) exhumationburial of the source before deposition of the Geste Formation
(65 – 45 Ma); (2) Eocene source exhumation (50 –
37 Ma); (3) peak burial heating during deposition of the
Geste Formation (38– 30 Ma) and subsequent cooling;
and (4) the possibility that the maximum temperature of the
deepest sample in the basin was not high enough (<80°C) to
reset the AFT signal. A slight overlap of temperature-time
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CARRAPA AND DECELLES: EOCENE BASIN DEVELOPMENT IN NW ARGENTINA
TC1015
logical evidence and therefore the output can be considered
realistic.
7. Discussion and Conclusions
Figure 10. Block diagram showing different, direct
(growth-structure apparent in seismic and field relationships; black boxes) and indirect (based on coarse-grained
sedimentation, proximal source, rapid syn-tectonic sourceterrane exhumation; grey boxes) evidence for wedge-top
deposition in the Chilean Precordillera, Puna-Altiplano
Plateau, and Eastern Cordillera regions. Sources are as
follows: (1) Arriagada et al. [2006]; (2) Voss [2002],
Kraemer et al. [1999], and Carrapa et al. [2005]; (3) Jordan
and Mpodozis [2006]; (4) present work; (5) Hongn et al.
[2007]; and (6) Horton [1998].
constraints was applied in order to allow the model a higher
degree of freedom.
[36] The best solution, with the highest number of good
fits (50 good solutions), is the one presented in Figure 9.
This model shows that the main cooling events occurred
between 90 Ma and 75 Ma; and between 60 Ma and 45 Ma
when the sample reached surface temperature. This is
generally in agreement with provenance data documenting
Ordovician rocks as a source for the Geste sediments.
[37] Between 40 Ma and 10 Ma the sample was heated
owing to burial following deposition and subsequently
exhumed. The model predicts that maximum temperature
in the basin never exceeded 80°C, consistent with our
inference that negligible fission track annealing took place
after deposition of the investigated samples. Even if we do
not attempt to draw any major conclusions based on tracklength modeling (because of the limited number of length
measurements), overall the exercise reproduces other geo-
[38] The fluvial and alluvial deposits of the Geste Formation in the central Puna record a response to source
terrane exhumation and deformation within the plateau
interior and its marginal areas during Eocene time. Sedimentological data indicate that the lower and middle members of the Geste Formation are the results of confined to
unconfined flows in a sandy to gravelly, braided fluvial
system characterized by shallow, unstable channels and
overbank deposits. The upper member was deposited in
alluvial fans proximal to the source terrane. Paleocurrent
data document an overall eastward flow direction. The upsection coarsening of the Geste Formation suggests that
topographic relief in the source area increased through time,
possibly owing to enhanced tectonic activity and source
terrane unroofing.
[39] Sandstone petrographic data show compositions that
overlap the field of continental block and recycled orogenic
provenance but may be considered more typical of a
recycled orogenic provenance [Dickinson, 1985; Garzanti
et al., 2007]. Conglomerate clast-count data document that
the source of the Eocene conglomeratic facies was composed of quartzose and phyllitic rocks typical of the
Ordovician metasedimentary basement (Copalayo Formation) directly beneath the Geste Formation and in ranges to
the west. Detrital zircon U-Pb ages support the interpretation of Ordovician provenance [DeCelles et al., 2007].
Sedimentation rates increased up-section from 0.1 mm/a
to more than 1 mm/a, consistent with deposition in a wedgetop depozone or proximal foreland basin setting [e.g.,
Johnson et al., 1986; Jordan, 1995; Ojha et al., 2000].
[40] AFT data indicate primarily Paleocene-Eocene (P1:
35– 52 Ma; P2: 52– 65 Ma) and limited mid-Cretaceous
cooling ages (P3 and P4: 88 –112 Ma) of the rocks in the
source terranes of the Geste sediments. The Cretaceous
signal may be related to a distal source located in the
Chilean Cordillera de Domeyko thrust belt, which is known
to have been actively shortening at that time [Mpodozis et
al., 2005; Arriagada et al., 2006]. The Paleocene and
Eocene cooling ages (35– 65 Ma) document active tectonic
exhumation and suggest that the orogenic belt directly west
of the Salar de Pastos Grandes area was in a constructional
orogenic phase at that time. Lag times suggest that exhumation rates increased from 0.4 mm/a to 1 mm/a
between 40 Ma and 35 Ma, again consistent with proximity
to a rapidly exhuming, tectonically active source terrane.
[41] Overall our new data are consistent with existing
sedimentological, structural, seismic, and the thermochronological evidence from the Salar de Atacama basin and
Cordillera de Domeyko documenting that the Chilean
Precordillera thrust belt and areas to the east were actively
deforming since at least the Paleocene (35 – 65 Ma),
and possibly as early as the mid-Late Cretaceous (88–
112) [Mpodozis et al., 2005; Arriagada et al., 2006]. Also,
our study corroborates existing data from the southern Puna
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CARRAPA AND DECELLES: EOCENE BASIN DEVELOPMENT IN NW ARGENTINA
and the Eastern Cordillera in Argentina and Bolivia documenting proximal foreland basin sedimentation and local
crustal shortening and uplift during the early Tertiary
[Horton, 1998; Carrapa and DeCelles, 2005; Hongn et
al., 2007].
[42] However, we emphasize that the regional tectonic
context of the Geste Formation remains in question. In
southern Bolivia, Eocene deposits clearly fit into a longterm Paleocene through Miocene stratigraphic sequence that
can be interpreted as a regional foreland basin succession
[Horton et al., 2001; DeCelles and Horton, 2003], whereas
in the case of the Geste Formation, the deposits are isolated
and rest directly upon Paleozoic basement [Alonso, 1992].
The isolated nature of Geste depocenters, and their clear ties
to local source terranes suggest that the Geste Formation
represents either intermontane basin deposition (i.e., it is not
part of a single regional foreland basin system), or local
wedge-top basins. Wedge-top basins are characterized by
growth structures related to progressive syndepositional
deformation, local sources of coarse-grained sediment, and
increasing up-section sedimentation rates on the order of
0.5mm/a. They usually overlie proximal foredeep deposits
[DeCelles and Giles, 1996]. In northern Argentina, however,
vertical stacking of regional foreland basin depozones could
be missing because of progressive deformation and reworking of older strata following thrust propagation, coupled
with inherited topographic highs. The systematic up-section
decrease in dip of bedding in the Geste Formation may
indicate syndepositional growth related to a blind structure
beneath the Copalayo Range as suggested by Figure 2.
Overall, the Geste Formation has many of the characteristics
common to wedge-top deposits, including a 2000 m thick
upward coarsening sequence with sedimentation rates that
increase up-section from 0.1 to 1 mm/a, deeply eroded
local sources, and transverse paleoflow directions. For these
reasons we favor the interpretation that the Geste Formation
represents wedge-top deposition rather than local intermontane basin deposits.
[43] The presence of middle Eocene growth structures
within the Eastern Cordillera (La Poma Basin; Figure 2)
[Hongn et al., 2007] supports the wedge-top interpretation,
and suggests that the Geste Formation is genetically equiv-
TC1015
alent to the Quebrada de los Colorados Formation to the
east (Figure 2). This implies that the orogenic wedge, which
exhibits both thin-skinned and thick-skinned deformation
(Figure 2c), was located between the Puna interior and the
Eastern Cordillera of northwestern Argentina during the
middle to late Eocene. Similar coarse-grained deposits of
Oligocene-Miocene age are preserved in the Tupiza Basin
complex of the Bolivian Eastern Cordillera [Horton, 1998,
2000], directly along strike from the Salar de Pastos Grandes
area (Figures 1 and 10). As in the Salar de Pastos Grandes,
Tertiary sedimentary rocks in the Tupiza Basin were deposited directly on top of Paleozoic rocks in a wedge-top
depozone characterized by growth structures related to both
west- and east-verging thrusting [Horton, 1998].
[44] Overall, what is clear from our data and other recent
work in Chile, Bolivia, and Argentina is that by Eocene
time the Puna-Altiplano Plateau and Eastern Cordillera were
experiencing local tectonic shortening, rapid exhumation,
and coarse-grained basin development (Figure 10). Unequivocal growth structures within Paleocene units (Naranja
Formation) in the Salar de Atacama basin [Arriagada et al.,
2006] to the west and Eocene growth structures in the La
Poma Basin (Quebrada de los Colorados Formation [Hongn
et al., 2007]) to the east of the study area, together with our
new multidisciplinary data set, suggest that the orogenic
front was migrating eastward and incorporating areas located
within the central Puna and Eastern Cordillera by Eocene
time. If correct, our interpretation would suggest that the
Andean orogenic strain front shifted eastward from middle
Cretaceous through Eocene time (Figure 10), though not
necessarily smoothly, incorporating both thin-skinned and
thick-skinned structures.
[45] Acknowledgments. This work was supported by DFG (Deutschen Forschungsgemeinschaft) and in part by National Science Foundation
grant EAR 0710724 (Tectonics program) and a generous grant from
ExxonMobil. B. Carrapa gratefully acknowledges M. Strecker for invaluable scientific support and the University of Potsdam for providing
analytical facilities. We kindly thank Brian K. Horton, Ricardo Alonso,
Daniel Starck, and Suzanne M. Kay for useful scientific discussions.
Salvatore Critelli, Eduardo Garzanti, and Marco Malusà are kindly acknowledged for their constructive reviews.
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B. Carrapa, Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82071,
USA. (bcarrapa@uwyo.edu)
P. G. DeCelles, Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA. (decelles@
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