Oligocene–Miocene age of aridity in the Atacama Desert revealed
by exposure dating of erosion-sensitive landforms
Tibor J. Dunai Faculty of Earth and Life Sciences, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam,
The Netherlands
Gabriel A. González López Departamento de Ciencias Geológicas, Facultad de Ingenierı́a y Ciencias Geológicas,
Universidad Católica del Norte, Avenida Angamos 0610, Antofagasta, Chile
Joaquim Juez-Larré Faculty of Earth and Life Sciences, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam,
The Netherlands
ABSTRACT
The age of onset of hyperaridity in the Atacama Desert, Chile, which is needed to
validate geological and climatological concepts, has been heretofore uncertain. Measurement of cosmogenic 21Ne in clasts from erosion-sensitive sediment surfaces in northern
Chile shows that these surfaces have been barely affected by erosion since 25 Ma. Surface
exposure ages of sediment clasts give replicate values at 25, 20, and 14 Ma and individual
values at 37 and 9 Ma. Predominantly hyperarid conditions are required to preserve these
oldest continuously exposed surfaces on Earth. Our findings are compatible with the hypothesis that the onset of aridity in the Atacama Desert could be the reason for, rather
than the consequence of, uplift of the high Andes.
and southernmost Peru was near sea level in
late Oligocene–early Miocene time (Noble et
al., 1985; Tosdal et al., 1984). In this period,
regional erosion formed the rounded-planate
summits of the Coastal Cordillera with sediments grading from sand to cobble conglomerate filling the valleys in a coastal plain set-
Keywords: Atacama Desert, Andes, desertification, climate change, erosion, exposure age.
INTRODUCTION
The Atacama Desert is one of the major hyperarid deserts on Earth. It represents an extreme habitat for life on Earth and serves as
an analogue for dry conditions on Mars (McKay et al., 2003). Aridity in the Atacama Desert is primarily caused by the cold water of
the Humboldt Current running parallel to the
Chilean and southern Peruvian coast, preventing precipitation in the coastal areas (Houston
and Hartley, 2003). The aridity is intensified
by the pronounced rain-shadow effect of the
Andes to the east, which effectively block
moisture transfer from the Amazon Basin
(Houston and Hartley, 2003).
The onset of aridity in the Atacama Desert
and changes in its intensity were governed by
the onset and fluctuations in strength of the
proto-Humboldt Currents and the timing and
rate of uplift of the Andes (Lamb and Davis,
2003). In turn, the arid climate in the Atacama
Desert influences the rates and patterns of uplift and denudation of the Andes (Lamb and
Davis, 2003). It has been suggested that the
arid conditions of the Atacama Desert are the
cause rather than the result of the uplift of the
high Andes (Lamb and Davis, 2003). The
driving force would be the climate-controlled
sediment starvation in the Peru-Chile trench,
causing high shear stress, focusing the plate
boundary stresses that support the high Andes
(Lamb and Davis, 2003). In order to test the
possibility of this causal link, reliable information on the timing of aridification of the
Atacama Desert is required.
AGE OF ARIDITY IN THE ATACAMA
DESERT
Our present knowledge of the timing of desertification of the Atacama Desert mostly re-
lies on two lines of evidence. One is the timing of cessation of supergene alteration of
orebodies in the Precordillera (Alpers and
Brimhall, 1988; Sillitoe and McKee, 1996)
(Fig. 1), the other is the nature and timing of
changes of sediment input into the Central Depression (Hartley and Chong, 2002). The termination of supergene enrichment of orebodies points to an early regional desiccation,
starting ca. 35 Ma and completed by ca. 14
Ma, whereas the sedimentological evidence
points to a relatively recent change from semiarid to hyperarid conditions ca. 3 Ma. Both
lines of evidence are derived from investigation of areas mostly more than 100 km inland.
The rare precipitation in the Atacama, however, comes from the east and reaches the upper regions (.3500 m) of the western flank
of the Precordillera (Houston and Hartley,
2003). Even in the present-day hyperarid conditions in the central Atacama, discharge from
the Precordillera into the Central Depression
occurs, both occasionally as sheetflows on
vast alluvial fans and continuously by rivers
that have their headwaters deep in the Precordillera (e.g., Rı́o Loa). No measurable precipitation is observed in the elevated regions of
the Coastal Cordillera. Therefore it is possible
that previously used records might overestimate past precipitation and underestimate the
age of aridity in the Atacama Desert in general
and specifically in the Coastal Cordillera, the
coastal desert proper.
Here we assess the age of cessation of erosion, as a consequence of aridification, on
erosion-sensitive landforms in the Coastal
Cordillera. The large-scale morphostructural
units in the study area (Figs. 1 and 2) formed
when the coastal region of northernmost Chile
Figure 1. Geographic setting of study area
in Atacama Desert (digital elevation model
based on GTOPO30). Black circles with
numbers give locations and K-Ar ages of supergene alteration products of orebodies
(Alpers and Brimhall, 1988; Bouzari and
Clark, 2002; Sillitoe and McKee, 1996). Large
dark gray circles indicate sedimentary deposits used for paleoclimatic studies (Hartley and Chong, 2002). Dashed lines give calculated mean annual precipitation based on
topographic elevation (Houston and Hartley,
2003).
q 2005 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected].
Geology; April 2005; v. 33; no. 4; p. 321–324; doi: 10.1130/G21184.1; 4 figures; Data Repository item 2005053.
321
Figure 2. Enlarged portion of Landsat ETM image (panchromatic band,
P002R073p7P20000329) showing sampling sites and geographic features of study
area. Major fault scarps and gravitational collapse structures are indicated. WSWENE-trending reverse fault is expression of trench-parallel shortening of Coastal
Cordillera (Allmendinger et al., 2005). Ephemeral rivers have deeply incised into
Coastal Cordillera. Valleys and fault scarps protect areas around sampling sites
A–C from runoff from precipitation in Precordillera to east. Quebrada is canyon or
valley; oficina indicates (nitrate) plant.
ting (Tosdal et al., 1984). The sources of these
sediments are in the Precordillera and/or the
Coastal Cordillera. In northern Chile these
sediments belong to the Azapa Formation
(Wörner et al., 2002); their equivalents in Peru
belong to the Moquegua Formation (Tosdal et
al., 1984; Wörner et al., 2002). Regionally,
sedimentation ended at the latest ca. 18 Ma
(Tosdal et al., 1984), the bulk occurring between 22 and 25 Ma (Mortimer et al., 1974;
Tosdal et al., 1984; Wörner et al., 2002). Sedimentation in our study area (Figs. 1 and 2)
ended shortly after 21.8 6 0.3 Ma (Mortimer
et al., 1974). For the present study we chose
depositional surfaces on Azapa sediments that
have been effectively protected from runoff
from the Precordillera since their deposition
(Figs. 2 and 3). Consequently these surfaces
have been exclusively affected by local precipitation since deposition of the Azapa Formation. The traces of fluvial transport and erosion on these surfaces therefore record pluvial
episodes in the coastal desert since 25 Ma.
SAMPLES
Here we report exposure ages of quartz
clasts collected from a sediment surface just
inland of Pisagua (Fig. 2). Photographs and
descriptions of sampling sites are provided in
GSA Data Repository1. The surface is at elevations between 910 m and ;1000 m (Fig. 3)
and is dissected by the Quebrada de Tiliviche
1GSA Data Repository item 2005053, Table DR1
and Figure DR1, cosmogenic isotope data and images of the study area, is available online at
www.geosociety.org/pubs/ft2005.htm, or on request
from [email protected] or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301–
9140, USA.
322
and the Quebrada de Jazpampa (quebrada
means canyon or valley). The deeply incised
valleys provide a local low base level for erosion (Figs. 2 and 3). The gradient on the sediment surface (Fig. 3) would be conducive to
fluvial sediment transport of sand and cobbles
of the Azapa Formation, if running water were
available.
We collected samples at three sites that are
indicative of erosion processes. Site A, located
between the two quebradas, is protected from
hillslope runoff by a 10–20-m-deep depression parallel to and at the foot of the NW-SEtrending fault scarp in Azapa sediments (Fig.
2). In the current setting, site A is only affected by runoff from precipitation below the
1000 m isoline (Fig. 3). Due to the slightly
convex form of the area around site A (Fig.
3), it is well protected from runoff, erosion,
and/or deposition of material from higher
areas.
Sites B and C are located at the axis of a
wide topographic low. Hence most runoff
from the surface to the south of the Quebrada
de Jazpampa will flow across these two sites.
Therefore, in contrast to site A, sites B and C
are very sensitive to erosion and/or deposition
by runoff from higher areas (Fig. 2).
Site B is at the bottom of one of the first in
a series of steep-walled salt-karst depressions,
mostly ;2 m deep, that occur between site B
and the Quebrada de Jazpampa. These depressions act as a perfect sediment trap for debris
carried by runoff from higher areas (Fig. 3).
The salt karst is formed in old evaporites that
were deposited in a salina-mudflat setting,
when Azapa sediment surfaces were still at or
close to sea level (Tosdal et al., 1984). The
quartz clasts collected at this site were lying
on salt (mostly halite) in the karst pit. The
clasts are probably not residual material from
dissolved evaporites, but were carried by runoff from higher up in the catchment onto the
evaporites (Fig. 3). The rock clasts found in
the karst pits are indistinguishable from clasts
found on surfaces adjacent to the karst pits.
Site C is on a flat, undulating (decimeter
scale) surface. All runoff that will eventually
end up in the karst pits around site B must
cross the area around site C (Fig. 3). Site C
can be affected by runoff erosion and is the
location of (temporary) deposition of clasts
from higher up in the catchment.
In addition to the sediment surface described here, we collected quartz clasts in the
Quebrada de Jazpampa (site D) and on an alluvial fan (site E) situated on tectonically
Figure 3. Elevation contour plot based on Shuttle Radar Topography
Mission (SRTM) 90 m
data showing hydrographic situation of sites
A, B, and C. Below 900 m
only 50 m contours are
shown; above 900 m, 10
m contours are provided.
White areas in steep section in NW corner indicate missing data. Sites
B and C are on axis of
wide topographic low
draining to Quebrada de
Jazpampa. Runoff of all
precipitation in this
catchment will cross
these sites.
GEOLOGY, April 2005
rates, since the sediments sampled at ;950 m
were deposited near sea level (Noble et al.,
1985; Tosdal et al., 1984). If the uplift was
decelerating or accelerating, and was not constant as we assume for our age calculation, the
corresponding exposure ages would decrease
or increase, respectively. For example, ages
would change by ;10% if the uplift rate decreased/increased fourfold after half the exposure time.
Figure 4. Cumulative
probability plot of clast
exposure ages. Individual ages of clasts are also
indicated, as is age of
amalgamated
sample
from site A. Minimum
and maximum deposition
ages indicated are independent age constraints
for onset and cessation
of sedimentation of Azapa sediments in study
area (Mortimer et al.,
1974; Tosdal et al., 1984,
1981; Wörner et al.,
2002).
downfaulted sediments of the surface sampled
in sites A–C (Fig. 2). The Quebrada de Jazpampa is the only overflow of the Pampa Tamarugal (Fig. 1), and records spilling events
from this large salt pan that serves as a local
base level for all discharge from the Precordillera for the next ;200 km to the south. The
435 m elevation of site E on the coastal cliff
is in the elevation range of 300–1000 m,
where rain is occasionally observed along the
coastal cliff, e.g., in major El Niño events.
Consequently sites D and E record the last geomorphologically relevant precipitation in the
Precordillera and on the coastal cliff,
respectively.
EXPOSURE AGES
Exposure ages were determined from the
concentration of in situ–produced cosmogenic
21Ne in quartz clasts. Details on the samples,
experimental procedure, age calculation, and
the isotope data are provided in Table DR1
(see footnote one). The exposure ages of the
clasts from the sediment surface are generally
very old. From sites A, B, and C, 4 individual
clasts were analyzed per site along with 1
amalgamated sample (Repka et al., 1997) of
24 clasts from site A. The majority of the ages
are older than 19 Ma (n 5 9) with clusters at
20 Ma (n 5 3) and 25 Ma (n 5 5) (Fig. 4).
One clast yielded an exposure age of ca. 37
Ma. Few clasts are younger: one age is ca. 9
Ma, and two identical ages are ca. 14 Ma. The
site best protected from runoff erosion (A)
yielded no clasts younger than 19 Ma and contained the oldest clast. The amalgamated sample gives a mean age of 23.3 6 0.2 Ma (61s)
for site A. The samples of the other two sites
(B and C) contain the three younger clasts,
together with five clasts of ages indistinguishable from those of site A. The samples from
the riverbed of the Quebrada de Jazpampa (D)
and the alluvial fan (E) both give ages ca.
120 ka.
When dating sedimentary deposits accumulation of cosmogenic nuclides at the source
area or during transport has to be considered.
GEOLOGY, April 2005
At sites A, B, and C it was impractical to obtain shielded samples to correct for this preexposure (Repka et al., 1997). We use the
samples from site D to assess preexposure of
quartz clasts in the Azapa Formation; the sediments in the riverbed are recycled Azapa sediments that were cannibalized from the valley
flanks that are cut into the Azapa Formation.
The two amalgamated samples from this site
indicate that average clasts from the Azapa
Formation have no significant preexposure.
The 21Ne concentration found in these samples is in agreement with a single-stage exposure for ;120 k.y. at the surface and at 90
cm depth, respectively. This finding does not
exclude the possibility that individual clasts
have significant preexposure, as suggested by
the oldest clast age of ca. 37 Ma. However,
based on the rather tight clustering of the remaining clasts—ca. 14, ca. 20, and ca. 25
Ma—we judge that it is unlikely that these
clasts had a significant, necessarily random,
preexposure exceeding 1 Ma.
The source regions of the quartz clasts analyzed are in magmatic rocks and medium to
high-grade metamorphic rocks. Any preexposure would have occurred during erosion of
these rocks and not during previous sedimentary cycles, as could be the case if sedimentary rocks were the source of the clasts. In
order to mimic an exposure age of 1 Ma by
preexposure, erosion rates in the higher source
areas in the Precordillera (1500 m elevation at
25 Ma; Lamb and Davis, 2003) would have
been ,1 m/m.y. Denudation rates during the
pluvial episode in the emerging Precordillera
that led to the deposition of the Azapa Formation were, however, probably much higher
(Lamb and Davis, 2003; Tosdal et al., 1984).
The sediments forming the depositional surface investigated in this study were deposited
at the end of this pluvial episode. By this time
most relicts of older landscape surfaces probably had vanished by erosion.
We use an average uplift rate of 40 m/m.y.
to calculate time-integrated 21Ne production
OLIGOCENE–MIOCENE ONSET OF
ARIDITY IN THE ATACAMA DESERT
The majority of clasts sampled on the geomorphologically old sediment surface have
exposure ages that are indistinguishable from
the sediment deposition age, 22–25 Ma, or are
slightly (;10%) younger. The age of the
amalgamated sample from the best-protected
surface (site A) is 23 Ma. The concordant exposure and deposition ages (Fig. 4) leave little
chance for erosive modification of this sediment surface since their deposition in the early
Miocene. The sediment surface sampled is by
far the oldest continuously exposed geomorphologic surface on Earth, being about twice
as old as ancient surfaces in Antarctica (e.g.,
Schäfer et al., 1999; van der Wateren et al.,
1999, and references therein).
The events that led to the deposition of the
younger clasts at site B and C did not significantly erode this region, otherwise the older
clasts present at site C would have been removed. It is likely that the younger clasts
come from the higher areas surrounding sites
B and C (Figs. 2 and 3). We interpret the ages
of the younger clasts as evidence for pluvial
episodes ca. 20 Ma, ca. 14 Ma, and ca. 9 Ma.
The runoff that drained through the trough
axis at sites B and C (Fig. 3) was able to locally dissolve ;2 m of evaporites to form the
salt karst at site B and farther north. Given the
;25 m.y. period available to dissolve the salt,
we conclude that only marginal precipitation
has occurred since that time.
Generally the climate must have resembled
the present-day hyperarid climate for most of
the past 25 m.y. The proposal of a semiarid
climate in the central Atacama Desert until 3
Ma (Hartley and Chong, 2002) is clearly incompatible with our findings. Our findings
are, however, in agreement with an estimate
of 21 Ma for the end of supergene weathering
in the Coastal Cordillera, as may be inferred
from the only orebody dated in a climatic setting equivalent to that of our study area (Fig.
1; Sillitoe and McKee, 1996), i.e., the coastal
desert proper. Based on the large-scale causes
for the regional climate, we assume that the
roughly coast- and/or orogen-parallel zoning
in precipitation (Fig. 1) also existed in the
past. Thus, noting the absence of conditions
conducive to a special microclimate in our
study area, we infer that the long-term prev-
323
alence of hyperarid climatic conditions we
find in our study area is probably valid for
most of the hyperarid portion of the presentday Atacama Desert.
The oldest exposure age of ca. 37 Ma, obtained from a single clast, gives evidence of
the existence of remnants of old surfaces in
the source region of the Azapa sediments at
the time of deposition. This is in line with the
fact that regionally the oldest supergene
weathering ages of ca. 34–35 Ma were obtained in an orebody to the east of our study
area (Cerro Colorado; Bouzari and Clark,
2002; Sillitoe and McKee, 1996), providing
evidence that very old landforms were present
in the source region of the Azapa sediments.
The actual exposure age of the sample is probably somewhat younger than calculated for the
sampling elevation, as production rates of cosmogenic nuclides increase with altitude, i.e.,
the production rate during exposure of this
sample in the source region was higher. A
slowly eroding surface (;0.1 m/m.y.) that was
at 1500 m ca. 25 Ma could be a model source
for the clast with the exceptionally high exposure age. Such low erosion rates usually
only occur in desert environments (van der
Wateren and Dunai, 2001), and indicate that
the region had an arid climate prior to the deposition of the Azapa sediments. Potential
source areas in the Precordillera were at
;1500 m ca. 25 Ma (Lamb and Davis, 2003).
OROGRAPHIC RAIN SHADOW VS.
GLOBAL CLIMATE CHANGE AS
DRIVING FORCE FOR
ENVIRONMENTAL CHANGE IN THE
ATACAMA DESERT
Most ages for the ancient pluvial phases in
the Precordillera and Coastal Cordillera, as inferred from the present and previous studies
(Alpers and Brimhall, 1988; Sillitoe and
McKee, 1996; Tosdal et al., 1984) as ca. 9, ca.
14, ca. 20, and ca. 25 Ma, broadly coincide
with periods of global cooling in the middle Miocene and cool climates across the
Oligocene-Miocene boundary (Zachos et al.,
2001a, 2001b). Termination of a potential earlier pluvial period ca. 34 Ma recorded in the
Precordillera (Bouzari and Clark, 2002; Sillitoe and McKee, 1996) coincides with global
cooling shortly after the Eocene-Oligocene
boundary (Zachos et al., 2001a).
These global climate changes were the result of opening or closing of oceanic pathways
and orbital forcing (Houston and Hartley,
2003; Zachos et al., 2001a, 2001b), and influenced the availability and transport of humidity from the Amazon Basin to the Atacama
Desert (Houston and Hartley, 2003). This
trans-Andean transfer has been the main
source of humidity in the Atacama Desert
324
since the establishment of a proto-Humboldt
Current in conjunction with the opening of the
Tasmania-Antarctic passage ca. 33 Ma (Zachos et al., 2001a). The climatic connection of
these two regions is illustrated by ages of inferred wetter periods in the Amazon Basin ca.
12–17, ca. 20, ca. 24, and 33–35 Ma (Vasconcelos et al., 1994) that correspond to the pluvial phases in the Atacama Desert identified
in the present and previous studies (Alpers
and Brimhall, 1988; Sillitoe and McKee,
1996; Tosdal et al., 1984).
The dominantly hyperarid conditions we infer for the Coastal Cordillera since ca. 25 Ma,
and prevailing arid conditions since ca. 34
Ma, are equivalent to the postulated early aridity (Lamb and Davis, 2003) that is required
for the hypothesis that the onset of aridity in
the Atacama Desert is the cause (Lamb and
Davis, 2003), rather than the result of the uplift of the high Andes. The ensuing positive
feedback between increasing altitude of the
Andes and increasing rain shadow could create and maintain hyperarid conditions in the
Atacama Desert. Only exceptional global climatic disturbances have occasionally permitted humidity transfer across the Andes into the
driest regions of this coastal desert since ca.
25 Ma.
ACKNOWLEDGMENTS
Reviews by R. Anderson, P. Bierman, and F. Stuart
helped to improve this manuscript; Juez-Larré was supported by a Netherlands Organization for Scientific Research (NWO) grant to Dunai. We thank R. van Elsas
for density separations, and B. van der Wagt for U and
Th determinations.
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Manuscript received 13 September 2004
Revised manuscript received 17 December 2004
Manuscript accepted 17 December 2004
Printed in USA
GEOLOGY, April 2005