Detrital zircon U-Pb ages provide provenance and
chronostratigraphic information from Eocene synorogenic
deposits in northwestern Argentina
P.G. DeCelles Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA
B. Carrapa Universität Potsdam, Institut für Geowissenschaften, 14476 Golm, Germany
G.E. Gehrels Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA
ABSTRACT
Paleogene clastic sedimentary rocks in the Puna plateau of northwestern Argentina contain
valuable information about the timing and location of early mountain building in the central
Andes. Because these rocks generally lack tuffaceous facies, only paleontological ages have been
available. We present U-Pb ages from detrital zircons in the conglomeratic Eocene Geste Formation of the central Puna plateau. The zircon ages indicate that the Geste Formation was derived
from nearby high-relief ranges composed of Ordovician metasedimentary rocks. A small population of ca. 37–35 Ma grains also confirms the late Eocene stratigraphic age of the Geste Formation, and suggests that U-Pb detrital zircon ages may provide a new tool for determining depositional ages and provenance of widespread Paleogene deposits in the central Andes.
Keywords: Andes, Argentina, syntectonic sedimentation, U-Pb geochronology.
INTRODUCTION
Acquisition of U-Pb ages of detrital zircons
from clastic sedimentary rocks is revolutionizing the discipline of sedimentary petrology
by providing information about both provenance and, in some cases, chronostratigraphy
(Gray and Zeitler, 1997; DeCelles et al., 1998,
2004; Gehrels et al., 1999, 2000; DeGraaffSurpless et al., 2002; Dickinson and Gehrels,
2003; Amidon et al., 2005; Link et al., 2005;
Surpless et al., 2006). Although there have
been detrital zircon geochronological studies
of the pre-Cenozoic rocks of the southern and
central Andes (Hervé et al., 2003; Augustsson
et al., 2003; Finney et al., 2003, 2005; Fanning
et al., 2004; Thomas et al., 2004; Gelcich et al.,
2005; Do Campo and Ribeiro Guevara, 2005),
no attempt has been made to apply the method
to Cenozoic strata. Widespread Paleogene strata
of the central Andes (Fig. 1; Jordan and Alonso,
1987; Alonso, 1992) are especially attractive
targets for detrital U-Pb geochronology because
these sparsely dated, coarse-grained clastic
strata potentially contain information about the
timing and location of early mountain building
in the central Andes, a topic that generates vigorous debate (Allmendinger et al., 1997; Sempere
et al., 1997; Lamb and Hoke, 1997; Kley et al.,
1997; Jordan et al., 1997, 2001; Coutand et al.,
2001; Horton et al., 2002; DeCelles and Horton,
2003; Elger et al., 2005; Carrapa et al., 2005;
Arriagada et al., 2006). Historically, the ages of
Paleogene strata in the central Andes have been
difficult to document owing to a paucity of datable volcanogenic sediment.
We set out to document U-Pb ages of detrital zircons in Cenozoic basin fills of the central
Andes of northwestern Argentina primarily as a
provenance tool. In the course of our work on
samples of the Geste Formation in the Salar de
Pastos Grandes area in the central Puna plateau,
we discovered a small but systematic group of
Eocene grains in several samples. These data
are consistent with previous vertebrate biostratigraphic ages (Pascual, 1983; Alonso, 1992), and
provide some of the best constraints yet available on the exact ages of these strata. Moreover,
our data raise the possibility that U-Pb ages of
detrital zircons in general may provide useful
chronostratigraphic information in retroarc settings such as the Andean orogenic belt, where
comagmatic zircons may be prevalent even in
apparently ash-free sediments.
GEOLOGICAL SETTING AND
STRATIGRAPHY
The Geste Formation is one of many local
accumulations of coarse-grained Cenozoic sedimentary rocks of generally pre-Neogene age in
northwestern Argentina and southern Bolivia
(Fig. 1; Jordan and Alonso, 1987; Sempere et al.,
1997; Horton, 1998). The unit is ~2 km thick
(Fig. 2) and is exposed in the Salar de Pastos
Grandes area of the Puna plateau, a vast highelevation (>4 km average elevation) region in
the interior of the central Andean orogenic system. The Geste Formation unconformably overlies Middle to Upper Ordovician phyllite and
quartzite of the Copalayo (or Colquena) Formation (Fig. 1; Alonso, 1992; Blasco et al., 1995),
which forms the bulk of the range directly to the
west of our investigated section.
The Geste Formation consists of an overall
upward-coarsening succession of sandstone,
conglomerate, and subordinate mudstone (Fig. 2;
Alonso, 1992). The lower part of the Geste
Formation consists of exclusively fluvial facies,
and coarse-grained alluvial-fan deposits become
progressively more abundant in the upper part
of the formation (Fig. 2; Carrapa and DeCelles,
2006). Conglomerates in the Geste Formation
contain clasts of vein quartz, quartzite, and
phyllite that are identical to rocks in the underlying Ordovician Copalayo Formation (Azeñolaza, 1973). Paleocurrent data indicate chiefly
eastward paleoflow directions (Carrapa and
DeCelles, 2006). Middle to upper Eocene fossils have been recovered from the middle part of
the Geste Formation (marsupials, notoungulates,
and armadillos; Pascual, 1983; Alonso, 1992).
The ranges to the east of the Salar de
Pastos Grandes are composed of Ordovician–
Silurian granitoid rocks and pegmatites, and
upper Precambrian metasedimentary rocks
(Puncoviscana Formation, mainly phyllite
and quartzite), gneisses, and orthogneisses.
Pliocene andesites form the >6 km peaks of
the Nevados de Pastos Grandes, directly north
of the study area (Fig. 1B).
METHODS
Six samples of medium- to coarse-grained
sandstone from the Geste Formation and one
sample of medium-grained quartzite from the
underlying Ordovician Copalayo Formation were
collected and processed by standard methods
for separating zircons. U-Pb geochronology of
zircons was conducted by laser-ablation–multicollector inductively coupled plasma–mass spectrometry at the University of Arizona LaserChron
Center. Details of the method were described in
Gehrels et al. (2000, 2006).
Analyses that yielded isotopic data of acceptable discordance, in-run fractionation, and precision are in GSA Data Repository Table DR1.1
Because 206Pb/238U ages are generally more
precise for younger ages, whereas 206Pb/207Pb
ages are more precise for older ages, we rely on
206
Pb/238U ages up to 1000 Ma and 206Pb/207Pb
ages if the 206Pb/238U ages are older than 1000
Ma (Gehrels et al., 2006). We report here a total
of 606 zircon ages. These analyses are plotted
on relative age-probability diagrams (Fig. 3),
which represent a sum of the probability distributions of all analyses from a sample, normal1
GSA Data Repository item 2007079, Table DR1,
U-Pb geochronologic analyses, is available online at
www.geosociety.org/pubs/ft2007.htm, or on request
from editing@geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.
© 2007 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.
GEOLOGY,
April
2007
Geology,
April
2007;
v. 35; no. 4; p. 323–326; doi: 10.1130/G23322A.1; 3 figures; Data Repository item 2007079.
323
Queva
6130m
Salars/
peaks
Cover
s de
Pozuelos Fm.
Pleistocene
upper
Tertiary
middle
Volcanics
lower
vad
o
Pliocene
Upper
Tertiary
Oire Fm.
Capalayo Fm.
Measured sect.
24°27'54''S
¥Ne
Sa
la
rd
Nevad
os de P
alermo
eA
riz
a
ro
Salar Pocitos
Sijes Fm.
Geste Fm.
C
Ordovician Eocene Miocene
66°
s
67°
B
Gr a
nde
68°
Pas
tos
24°
Santa Rosa
de los
Pastos Grandes
A
ara
25°
Pu
c
Peru
Bolivia
Salar de Pastos Grandes
16?S
SIJES
km
Neogene
plutonic rocks
Neogene
volcanic rocks
Tertiary
sedimentary rocks
Paleogene
sedimentary rocks
Cretaceous
sedimentary rocks
Quaternary, locally PlioPleistocene volcanic rocks
Carboniferous
sedimentary rocks
Upper Paleozoic
plutonic rocks
Cambrian
sedimentary rocks
Paleozoic
plutonic rocks
Ordovician sedimentary
and local volcanic rocks
Precambrian/Cambrian
plutonic rocks
Precambrian/Cambrian
sedimentary rocks
Faults
El Morro
Elevation
6500
C
S.
0
en
te
n
ar
io
66°42'18W''
3 km
24°45'54''S
66?W
Quaternary salt lakes and
salt flats
Quaternary continental
sedimentary rocks
Neogene/Quaternary
volcanic rocks
Neogene/Quaternary
ignimbrites
Quaternary, locally PlioPleistocene basalt
de
Sij
es
74?W
20
a
26°
0
r
Pacific
Ocean
er
Argentina
28?S
Si
Study
area
Copalayo
24?S
Salar de Pozuelos
Chile
20?S
Salar de
Hombre Muerto
> 3 km
elevation
1000
66°27'54W''
Figure 1. A: General location map of central South America showing location of study area in northwestern Argentina. Shaded region represents area in which elevation exceeds 3 km. B: Geological map of central Andes (modified after Reutter et al., 1994). Inset shows location
of area within context of broader Andean orogenic belt; rectangle corresponds to C. C: Geological map of Salar de Pastos Grandes area
(modified after Alonso, 1992) draped on digital elevation model. Fm.—formation.
ized such that the areas beneath the probability
curves are equal for all samples. Age peaks on
these diagrams are considered robust if defined
by several analyses.
RESULTS
Sample 1SP0 is a quartzite from the Ordovician Copalayo Formation that crops out directly
west of and stratigraphically below the Geste
Formation. It is characterized by age probability peaks at ca. 521 Ma, 796 Ma, 1045 Ma, and
2130 Ma (Fig. 3). The majority of the grains
from this sample are clustered in the ca. 490–
700 Ma and ca. 1000–1200 Ma ranges. The
six Geste Formation samples exhibit grain-age
populations very similar to those from the Ordovician sample, suggesting that the Geste zircons
were derived locally from Ordovician rocks. In
four of the Geste samples, we also found Eocene
grains, which tend to be more angular and
euhedral than the older detrital grains. The mean
ages of these grains for each sample in which
more than one grain was found range from 37.3
324
± 1.5 Ma to 35.4 ± 0.55 Ma (Fig. 3). The presence of previously reported Eocene mammal
fossils (Pascual, 1983) in the Geste Formation
suggests that the Eocene zircon grains are close
to the depositional age of the Geste Formation.
The fact that the mean ages of the Eocene grains
decrease systematically upsection also gives
credence to the idea that they are essentially the
same age as the depositional ages of the host
sediments. This suggests that the Eocene zircons are volcanogenic.
IMPLICATIONS AND DISCUSSION
Our detrital zircon data provide new information about the provenance of the Geste Formation
and establish a maximum depositional age of late
Eocene (ca. 37 Ma). Most significant, our data
suggest that U-Pb ages of detrital zircons in Tertiary clastic sedimentary units of the central Andes
may provide a powerful new tool for chronostratigraphic studies. The method may be particularly
useful in the sparsely dated, nontuffaceous, lower
Tertiary rocks of the central Andes. The ages of
these rocks should help to constrain the timing
of initial crustal thickening and uplift in the Puna
plateau. In this case, our data require proximal
sediment sources in high relief, presumably tectonically active terranes directly to the west of
the present Geste Formation outcrops. It is clear
that the Ordovician rocks in the ranges directly
west of the Salar de Pastos Grandes were uplifted
and widely exposed during the Eocene. Our data
provide no support for the hypothesis that nearby
ranges farther east (e.g., the Nevados de Palermo)
provided sediment to the Geste Formation in this
region. Eocene uplift in the central Puna plateau
is consistent with recent sedimentological and
apatite fission-track thermochronological studies that suggest deformation and exhumation of
ranges in the central and southern Puna and along
its eastern margin during Eocene–Oligocene time
(Kraemer et al., 1999; Coutand et al., 2001, 2006;
Ege et al., 2003; Carrapa et al., 2005).
The presence of Eocene grains in the Geste
Formation also indicates that a coeval magmatic
source of zircons must have existed in the region.
GEOLOGY, April 2007
silt
sand
pebble
boulder
4SP700
n = 79
2000
4SP700
3SP431
n = 82
2SP38
0
1SP32
1SP0
Base of Geste Formation
Ordovician
Figure 2. Generalized log of measured stratigraphic section through Geste Formation in
Salar de Pastos Grandes area.
However, no Eocene igneous rocks are present in
this region, and the Geste Formation lacks tuffaceous material. Moreover, the absence of any zircons in the Geste Formation with ages between
Eocene and early Paleozoic suggests that the
Eocene grains did not enter the Geste depocenter
by overland travel, because zircons of post–early
Paleozoic age (e.g., derived from Mesozoic igneous rocks that are present in the central Andes
to the west of our study area) would also be
expected. Therefore, the Eocene zircons probably were provided as air fall from distant eruptive
centers in an active Andean magmatic arc.
CONCLUSIONS
The U-Pb ages from detrital zircons in
Cenozoic deposits of the central Andean orogenic belt show a close match with U-Pb ages
from zircons in probable Ordovician source
rocks. Coupled with the coarse grain size, eastward paleocurrent indicators, and conglomerate compositions, the zircon data demonstrate
that local high-relief uplifts in the central Puna
plateau were responsible for generating the
Geste Formation. The detrital zircon ages (37–
35 Ma) overlap with biostratigraphic ages from
GEOLOGY, April 2007
206
38
34
2SP238
n = 92
2SP38
n = 94
Sandstone
Partially covered
Siltstone
Phyllite
3SP431 Sample site
1SP32
n = 85
Ordovician
Copalaya Formation
500
2SP277 LEGEND
2SP238
Conglomerate
39
37
35
33
36
Pb/238U Age (Ma)
1000
2SP277
n = 92
206
3SP431
Eocene Geste Formation
1500
Pb/238U Age (Ma)
Mean = 35.4 ± 0.55 Ma [1.6%] 95% conf.
MSWD = 0.76 (error bars = 2σ)
Mean = 35.9 ± 6.4 Ma [18.0%] 95% conf.
MSWD = 1.9 (error bars = 2σ)
39
37
35
Mean = 37.3 ± 1.5 Ma [4.0%] 95% conf.
MSWD = 6.2 (error bars = 2σ)
1SP0
n = 82
0
0.5
1.0
1.5
2.0
2.5
3.0
Age (Ga)
the Geste Formation (Pascual, 1983). Together
these data indicate that local uplift in the central Puna was under way by late Eocene time.
The presence of Eocene zircons in the Geste
Formation also suggests that magmatic sources
were active during Eocene time. In general, the
U-Pb detrital zircon method may be a powerful tool for determining ages of apparently ashfree Paleogene strata in the Andes, as well as
clastic strata in other retroarc settings where
syndepositional arc magmatism is likely.
ACKNOWLEDGMENTS
This research was supported by the ExxonMobil
Corporation and the German Science Foundation
(Deutsche Forschungsgemeinschaft project CA481/
5-1 to Carrapa and the Leibniz Prize to Manfred
Strecker). We thank Ricardo Alonso for logistical support and for sharing his knowledge of Paleogene stratigraphy in Argentina. Jim Schmitt, Tim Lawton, and
Amy Weislogel provided constructive reviews that
helped us to improve the paper.
Figure 3. Age probability plots of U-Pb ages
of detrital zircons from Salar de Pastos
Grandes area. MSWD—mean square of
weighted deviates; conf.—confidence.
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Manuscript received 28 August 2006
Revised manuscript received 1 December 2006
Manuscript accepted 6 December 2006
Printed in USA
GEOLOGY, April 2007