Modern and ancient fluvial megafans in the foreland basin system of

Transcription

Basin Research (2001) 13, 43±63
Modern and ancient ¯uvial megafans in the foreland
basin system of the central Andes, southern Bolivia:
implications for drainage network evolution in foldthrust belts
B. K. Horton*.and P. G. DeCelles².
*Department of Geology and Geophysics, Louisiana State
University, Baton Rouge, LA 70803, USA
²Department of Geosciences, University of Arizona, Tucson, AZ
85721, USA
ABSTRACT
Fluvial megafans chronicle the evolution of large mountainous drainage networks, providing a
record of erosional denudation in adjacent mountain belts. An actualistic investigation of the
development of ¯uvial megafans is presented here by comparing active ¯uvial megafans in the
proximal foreland basin of the central Andes to Tertiary foreland-basin deposits exposed in the
interior of the mountain belt. Modern ¯uvial megafans of the Chaco Plain of southern Bolivia are
large (5800±22 600 km2), fan-shaped masses of dominantly sand and mud deposited by major
transverse rivers (Rio Grande, Rio Parapeti, and Rio Pilcomayo) emanating from the central Andes.
The rivers exit the mountain belt and debouch onto the low-relief Chaco Plain at ®xed points along
the mountain front. On each ¯uvial megafan, the presently active channel is straight in plan view
and dominated by deposition of mid-channel and bank-attached sand bars. Overbank areas are
characterized by crevasse-splay and paludal deposition with minor soil development. However,
overbank areas also contain numerous relicts of recently abandoned divergent channels, suggesting a
long-term distributary drainage pattern and frequent channel avulsions. The position of the primary
channel on each megafan is highly unstable over short time scales.
Fluvial megafans of the Chaco Plain provide a modern analogue for a coarsening-upward,
>2-km-thick succession of Tertiary strata exposed along the Camargo syncline in the Eastern
Cordillera of the central Andean fold-thrust belt, about 200 km west of the modern megafans.
Lithofacies of the mid-Tertiary Camargo Formation include: (1) large channel and small channel
deposits interpreted, respectively, as the main river stem on the proximal megafan and distributary
channels on the distal megafan; and (2) crevasse-splay, paludal and palaeosol deposits attributed to
sedimentation in overbank areas. A reversal in palaeocurrents in the lowermost Camargo succession
and an overall upward coarsening and thickening trend are best explained by progradation of a
¯uvial megafan during eastward advance of the fold-thrust belt. In addition, the present-day
drainage network in this area of the Eastern Cordillera is focused into a single outlet point that
coincides with the location of the coarsest and thickest strata of the Camargo succession. Thus, the
modern drainage network may be inherited from an ancestral mid-Tertiary drainage network.
Persistence and expansion of Andean drainage networks provides the basis for a geometric model
of the evolution of drainage networks in advancing fold-thrust belts and the origin and development
of ¯uvial megafans. The model suggests that ¯uvial megafans may only develop once a drainage
network has reached a particular size, roughly 104 km2 ± a value based on a review of active ¯uvial
megafans that would be affected by the tectonic, climatic and geomorphologic processes operating
in a given mountain belt. Furthermore, once a drainage network has achieved this critical size, the
river may have suf®cient stream power to prove relatively insensitive to possible geometric changes
imparted by growing frontal structures in the fold-thrust belt.
Correspondence: B. K. Horton, Department of Geology and
Geophysics, Louisiana State University, Baton Rouge, LA
70803, USA.
# 2001
Blackwell Science Ltd
43
B. K. Horton and P. G. DeCelles
INTRODUCTION
Fluvial megafans constitute volumetrically signi®cant
depositional elements of sedimentary basins adjacent
to mountain belts. A ¯uvial megafan is a large
(103x105 km2), fan-shaped (in plan view) mass of clastic
sediment deposited by a laterally mobile river system that
emanates from the outlet point of a large mountainous
drainage network (Gohain & Parkash, 1990.; DeCelles &
Cavazza, 1999.). Modern ¯uvial megafans have been
recognized in nonmarine foreland basin systems at the
outlets of major rivers that drain fold-thrust belts,
particularly in the Himalayas (Geddes, 1960.; Sinha &
Friend, 1994.; Gupta, 1997.; DeCelles & Cavazza, 1999.)
and northern Andes (RaÈsaÈnen et al., 1992.). Although
¯uvial megafans are similar to sediment gravity ¯owdominated and stream-dominated alluvial fans in terms of
their piedmont setting, planform geometry and sedimentation related to expansion of ¯ow downslope of a
drainage outlet, ¯uvial megafans are distinguished by
their greater size (alluvial fans rarely exceed 250 km2),
lower slope, presence of ¯oodplain areas and absence of
sediment gravity ¯ows (see also Blair & McPherson, 1994.,
for a discussion of alluvial-fan classi®cation). The term
`terminal fan' is commonly used for a large, distributary
¯uvial system in which surface water in®ltrates and
evaporates before it can ¯ow out of the system (Friend,
1978.; Nichols, 1987.; Kelly & Olsen, 1993.; Newell et al.,
1999.). A ¯uvial megafan therefore may be considered
`terminal' in cases where ¯uvial channels run dry before
reaching bodies of water downstream. Although ¯uvial
megafans are clearly related to the emergence of a large
mountain river onto a low-relief alluvial plain, their
stratigraphic evolution in nonmarine foreland basins may
also be critically dependent on variables such as sediment
¯ux, water discharge, drainage catchment size, catchment
lithology and subsidence rate, factors that are ultimately
controlled by tectonic, climatic and geomorphic processes
(e.g. Burbank & Raynolds, 1988.; Heller et al., 1988.;
Flemings & Jordan, 1989.; Fraser & DeCelles, 1992.;
Tucker & Slingerland, 1996.; Gupta, 1997.; Schlunegger
et al., 1997., 1998.; Robinson & Slingerland, 1998.).
In an effort to explore both the modern and the ancient
dynamics of these systems, this investigation utilizes an
actualistic approach to sedimentological analysis of ¯uvial
megafans. We ®rst document the geomorphic features
and depositional processes of active ¯uvial megafans in
the central Andean foreland basin system of southern
Bolivia (. Fig. 1.). This provides a modern analogue for
Tertiary foreland basin deposits exposed within the
central Andean fold-thrust belt to the west. Analysis of
these deposits in the Eastern Cordillera demonstrates that
¯uvial megafans are identi®able in the stratigraphic record
and may be dominant elements in the stratigraphic
Fig. 1. Map of the central Andes and
modern foreland basin system showing
¯uvial megafans (Grande, Parapeti,
Pilcomayo) and their source drainage
basins, main rivers, major geological
structures and national borders.
Geological zones include the Altiplano
(AP), Eastern Cordillera (EC),
Subandean Zone (SZ), Chaco Plain
and Beni Plain. AP±EC boundary is
de®ned by the western (hinterland)
drainage divide. EC±SZ boundary is
shown by long dashed line. Barbed
lines represent east-directed thrust
faults present at the mountain front
and at depth in the westernmost Chaco
Plain. Approximate axis of forebulge is
drawn after Horton & DeCelles (1997.).
The polygon in the western Chaco
Plain shows the location of a composite
Landsat image of ¯uvial megafans
(Fig. 2.). Several rivers that deposit
¯uvial megafans bifurcate and/or
terminate in swampland regions (RV:
Rio Viejo area; IZ: Izozog swamp; PA:
PatinÄo swamp). Rectangle in Eastern
Cordillera locates Camargo syncline
study region (Fig. 4.).
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# 2001
Blackwell Science Ltd, Basin Research, 13, 43±63
Fluvial megafans in the central Andes, Bolivia
evolution of nonmarine foreland basin systems. We
anticipate that recognition of ¯uvial megafans in the
geological record will improve attempts to reconstruct the
large-scale geomorphology of ancient mountain belts,
possibly providing estimates on the size and geometry of
palaeo-drainage networks.
G EO LO G I C A L B A C K G R O U N D
Mountain building in the Andes is associated with
subduction of the oceanic Nazca plate beneath western
South America. During the Cenozoic, the central Andes
have grown progressively eastward and continental crust
has thickened by regional horizontal shortening (Isacks,
1988.; Gubbels et al., 1993.; Jordan et al., 1997.). Crustal
thickening and eastward propagation of the central Andes
has developed a topographic load on South American
lithosphere, producing a retroarc foreland basin system
(Jordan & Alonso, 1987.; Jordan, 1995.; Horton &
DeCelles, 1997.). Synorogenic erosion and transport of
clastic sediment to this evolving basin system has
produced a substantial record of Cenozoic nonmarine
sedimentation, including young deposits ®lling the
modern foreland region and ancient deposits exposed
within the central Andes (e.g. Jordan & Alonso, 1987.;
Beer et al., 1990.; Jordan, 1995.; Horton & DeCelles, 1997.;
Sempere et al., 1997.; Horton, 1998.).
The eastern slope of the Bolivian Andes consists of a
belt of generally east-vergent folds and thrust faults in the
Eastern Cordillera and Subandean Zone (Fig. 1.). This
fold-thrust belt is ¯anked on the east by the Chaco Plain
(Fig. 1.), a low-relief, low-elevation region interpreted as
the aggradational surface of the proximal foreland basin
system (the wedge-top and foredeep depozones; Horton &
DeCelles, 1997.). During most of Tertiary time, however,
the foreland basin system was located to the west within
the area now occupied by the fold-thrust belt (Sempere
et al., 1990., 1997.; Horton, 1998.). Eastward advance of the
fold-thrust belt and foreland basin system throughout
Tertiary time has uplifted and exposed older basin ®ll
within the interior of the mountain belt (Horton, 1998.;
DeCelles & Horton, 1999.; Horton et al., in press).. In this
paper, we discuss modern ¯uvial megafans of the Chaco
Plain in order to provide a depositional analogue for
Tertiary strata exposed in the Camargo syncline of the
Eastern Cordillera.
FLUVIAL MEGAFANS IN THE MODERN
FORELAND BASIN
Geomorphology and depositional processes
Fluvial megafans of the Chaco Plain constitute the most
extensive depositional systems of the proximal sector
(wedge-top and foredeep depozones) of the modern central
Andean foreland basin system. The apex of each megafan is
located along the Subandean topographic front at the outlet
point of an extensive mountainous catchment draining the
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fold-thrust belt. Three ¯uvial megafans exist along a
400-km-long segment of the north-striking topographic
front of the central Andes between 18uS and 22uS (Fig. 1.).
They are, from north to south, the Rio Grande (or Guapay),
Rio Parapeti and Rio Pilcomayo megafans (.Fig. 2.).
Additional ¯uvial megafans exist further south in the
Andean foreland of Argentina, including the Rio Bermejo
(23.3uS), Rio Salado (25.3uS), Rio Huaco (30.2uS) and Rio
Jachal (30.7uS) megafans (Iriondo, 1984., 1993.; Damanti,
1993.). The Andean megafans (. Table 1.) are comparable to
modern Himalayan megafans (Geddes, 1960.; Sinha &
Friend, 1994.; DeCelles & Cavazza, 1999.) in terms of
channel gradient (0.03±0.12u), drainage catchment area
(< 104x105 km2), and depositional area (< 103x105 km2)
(Table 1.). In the intermegafan areas between adjacent
¯uvial megafans, the westernmost Chaco Plain is composed
of stream-dominated alluvial fans fed by small catchments
in the Subandean Zone (Fig. 2.). These alluvial fans
(Table 1.) are distinguishable from the ¯uvial megafans on
the basis of their steeper slopes (1±4u), smaller depositional
areas (< 5 km2) and smaller catchment areas (< 200 km2).
Most alluvial fans are fed by small catchments con®ned to
the easternmost anticlinal ridge (Fig. 2.; Table 1.), indicating
that these fans are consequent features developed only after
initial structural growth of the present-day Subandean
topographic front. In contrast, the rivers depositing the
¯uvial megafans exhibit relatively straight courses across
the Subandean Zone (Fig. 2.), suggesting they are antecedent features that pre-date motion on the frontal
structures of the fold-thrust belt, structures that have
been active since about 5±10 Ma (Gubbels et al., 1993.).
Upon entry onto the Chaco Plain, rivers that deposit
¯uvial megafans are no longer con®ned by lateral
topographic barriers and are free to migrate over a large
area. Although each ¯uvial megafan exhibits a single,
relatively straight, sand-dominated, active channel in
which the majority of surface water and sediment is
presently transported (Fig. 2.), evidence outlined below
indicates that the location of this channel is highly
unstable over relatively short time frames.
Drainage patterns on the Chaco Plain can be attributed
to large-scale abandonment of distributary streams in the
lower segments of the ¯uvial megafans. The Rio Grande
and Rio Parapeti each make broad < 90u turns to the
north over a downstream distance of 90 km, starting from
the apex of their respective megafans (Figs 1 and 2.). The
Rio Pilcomayo makes a sharp 40u turn to the south-southeast 60 km downstream from its megafan apex (Figs 1 and
2.). In all three cases, these river in¯ections are ¯anked to
the east by a number of separate small catchments that
have headwater reaches limited to the present ¯oodplain
(i.e. they are not connected to the active primary channel)
(Fig. 2.). Individual streams within these small catchments
¯ow approximately eastward and are roughly parallel or
radiating in plan view. On the Rio Parapeti megafan
(. Fig. 3.), there are numerous large channels similar in size
to, but south of, the presently active channel. These
straight channels commonly bifurcate downstream into
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Fig. 2. Composite Landsat Multispectral Scanner (MSS) image and interpretive line drawing of ¯uvial megafans of the western
Chaco Plain of Bolivia. Fluvial megafans include active channels (black), abandoned or ephemeral megafan channels (densely
stippled pattern) and overbank areas (lightly stippled pattern). Intermegafan areas contain alluvial fans (generally too small to
depict at this scale) and aeolian dunes. Table 1.contains information on alluvial fans directly north of the Rio Parapeti drainage
outlet. Downslope of the ¯uvial megafans, the alluvial plain consists of swampland regions, aeolian dune ®elds and small channels
with headwaters con®ned to the present-day alluvial plain. See Fig. 1 for location.
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Table 1. Geomorphic characteristics of selected modern subaerial fans.
River±Fan
Location
Andes ¯uvial megafans
Grande (Guapay) Chaco Plain, Bolivia
Parapeti
Chaco Plain, Bolivia
Pilcomayo
Chaco Plain,
Bolivia-Argentina
Bermejo
Chaco Plain, Argentina
Catchment
area (km2) Fan area (km2) Slope
< 70 000
< 8000
81 300
< 12 600
< 5800
< 22 600
16 000
< 10 000 (?)
Salado
Huaco
Chaco Plain, Argentina
< 30 000
Bermejo basin,
7100
Argentina
Jachal
Bermejo basin,
27 700
Argentina
Pastaza-Maranon Amazon basin, Peru
?
Central Andes alluvial fans in intermegafan area between Rio Grande
Saipuru
Chaco Plain
< 118
Sanja Honda*.
Chaco Plain
< 2.8
Taputa
Chaco Plain
< 30.2
Tapulami
Chaco Plain
< 22.2
Acae*.
Chaco Plain
< 8.1
Tacuarembo*.
Chaco Plain
< 9.9
Huirapucuti*.
Chaco Plain
< 5.2
Sereque*.
Chaco Plain
< 3.6
Piriti*.
Chaco Plain
< 3.8
Capihuazuti*.
Chaco Plain
< 4.6
Ovai*.
Chaco Plain
< 8.1
Charagua
Chaco Plain
< 168
Himalayan ¯uvial megafans (Nepal and India)
Kosi
Gangetic Plain
59 000
Gandak
Gangetic Plain
45 000
Tista
Gangetic Plain
< 12 000
Other
Okavango
south-central Africa (Botswana) 180 000
Reference
< 10 000 (?)
700
IGM (1972, 1. 998.); this study
IGM (1980a,.b, 1. 998.); this study
Guyot et al. (1990.);
IGM (1981, 1. 998.); this study
< 0.06u (?) Iriondo (1984).;
Guyot et al. (1990).;
OAS (1971).
< 0.06u (?) Iriondo (1984).; OAS (1971).
?
Damanti (1993.)
1400
?
60,000±70 000
and Rio Parapeti
< 5.0
< 0.5
< 3.0
< 1.2
< 0.9
< 0.8
< 0.5
< 0.5
< 0.8
< 0.6
< 0.8
< 2.8
0.03±0.10u
0.10±0.12u
0.04±0.06u
Damanti (1993.)
< 0.06u (?) RaÈsaÈnen et al. (1992).
megafans (Bolivia)
< 1±2u
IGM (1974.); this study
< 4u
< 2u
< 2u
< 3u
< 3u
< 3u
< 4u
< 4u
< 3u
< 3u
< 1±2u
< 16 500
< 17 500
< 16 000
0.06u
0.03u
0.04u
Sinha & Friend (1994.)
Sinha & Friend (1994.)
Geddes (1960).;
DeCelles & Cavazza (1999).
18 000
0.02u
Stanistreet & McCarthy (1993.)
*Alluvial-fan catchment area is limited to frontal (easternmost) anticlinal ridge of Subandean Zone
smaller channels. Channels to the north, closer to the
active Rio Parapeti, are better de®ned than the southern
channels due to a greater amount of aeolian dune activity
to the south (Figs 2 and 3.). For the Rio Pilcomayo system,
separate channels east of the main river de¯ection exhibit
a distributary ¯ow pattern that continues well beyond the
downstream limits of the ¯uvial megafan (Fig. 2.). The
separate drainage networks feeding these channels expand
dramatically downstream such that the mainstem rivers
(the Rio Melo, Rio Apa, Riacho Yacare Norte and Rio
Verde) are widely dispersed by the time they reach the
south-¯owing Paraguay River about 500 km downstream
(Fig. 1.). Based on the Chaco Plain channel geometries
described above, we suggest that the ¯anking stream
catchments were once the site of distributary ¯ow on the
megafan surface. We therefore attribute the overall
planform morphologies of the megafan rivers and these
smaller catchments to recent abandonment of distributary
channel systems on the lower, downslope ¯anks of ¯uvial
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megafans. For the two northern systems (Rio Grande and
Rio Parapeti) in which the active channels ¯ow abruptly
northward (Figs 1 and 2.), these abandonment episodes
could represent recent, large-scale stream capture by the
Amazon drainage system.
Historical records for the past few hundred years also
attest to high channel mobility on the central Andean
megafans. For example, rapid lateral shifts in river course
from several to tens of kilometres have occurred in the Rio
Pilcomayo and Rio Bermejo systems since the late 1800s,
mostly associated with ¯ooding during rainy seasons
(Baker, 1978.; Iriondo, 1984., 1993.). Changes in river
course may also be related to documented stream-capture
events that affect reaches several hundred kilometres long
and may rapidly change the magnitude of water discharge
and sediment ¯ux (Baker, 1978.; Iriondo, 1984.). Collectively, these geomorphologic and historical observations
suggest that megafan construction is dominated by shortterm deposition in one location followed by avulsion to an
47
Fig. 3. Near-vertical, space-shuttle
photograph of Rio Parapeti ¯uvial
megafan. North is to the right; north±
south distance across ®eld of view is
< 70 km. Abandoned, divergent
channels are better de®ned near the
main active channel than they are to
the south where linear (north-trending)
aeolian dunes obscure the inactive
channels.
adjacent, lower-elevation, overbank area. Therefore, the
locus of active sedimentation is constantly shifting about
the megafan through channel avulsion, preventing simple
lateral migration of channels by point-bar accretion (i.e.
meandering rivers with scroll topography) and minimizing the development of relief between the channel and
adjacent overbank areas.
Downstream transport
Fluvial megafans generally display downslope decreases
in grain size and channel size compatible with a
proximal-to-distal change from ¯ow con®ned to a
proximal feeder channel to more distributed ¯ow, either
as smaller distributary channels or crevasse splays, on
distal parts of the megafan (e.g. Friend, 1978.; Nichols,
1987.; Kelly & Olsen, 1993.; Newell et al., 1999.). For the
Chaco Plain ¯uvial megafans, the sizes of both active and
abandoned channels generally decrease downstream. The
mainstem rivers, approximately several hundred to
2000 m wide on the ¯uvial megafans, become abruptly
narrower (generally less than a few hundred metres) at the
downstream terminations of each megafan (IGM, 1998.).
The downstream distance from the megafan apex to the
megafan margin is 150 km, 130 km and 250 km for the Rio
Grande, Rio Parapeti and Rio Pilcomayo, respectively.
Whereas proximal megafan channels are presumably 10 m
deep or greater, the distal channels are commonly less
than a few metres deep (Iriondo, 1993.). Grain size also
decreases downstream, from sand- and limited gravelsized detritus of the upper reaches of the megafans to the
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sand- and mud-sized detritus in lower parts of the
megafans. Historical data and stream gauge data indicate
that the rivers depositing the ¯uvial megafans carry
substantial suspended silt and clay, whereas coarser
sediment is largely transported as bedload in mid-channel
and bank-attached bars developed during individual
¯oods (Iriondo, 1984., 1997.). For the Pilcomayo, the
annual sediment discharge is approximately 8 r 107 tons;
90% of this material is transported in suspension (Guyot
et al., 1990.).
Whereas most river systems of Himalayan ¯uvial
megafans merge downstream into a major trunk river (the
Ganges River), the processes operating downstream of the
central Andean ¯uvial megafans are drastically different.
Little surface water derived from the central Andes ¯ows
out of the Chaco Plain. The Rio Pilcomayo enters the
broad (10 000 km2) PatinÄo swamp directly downstream of
its megafan (Fig. 1.; Iriondo, 1984.). Most water in®ltrates
here or is lost by evapotranspiration, effectively isolating
the proximal segment of the river from a downslope
continuation of the Pilcomayo that ultimately ¯ows into
the Parana River, the second largest river in South
America (and world's ®fth largest drainage area).
Similarly, the Rio Parapeti enters a swampland region
at the terminus of its megafan, ¯owing into the 6800-km2
Izozog swamp (Figs 1 and 2.) (Iriondo, 1993.). In the Rio
Grande, ¯ow may be moderately stalled in the Rio Viejo
area 260±400 km downstream of the megafan apex
(Fig. 1.). In this area, the Rio Grande bifurcates into a
number of smaller channels and much of the ¯oodplain is
subject to periodic inundation before ¯owing into the Rio
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Mamore and ultimately the Amazon River (the world's
largest drainage area). These observations suggest that a
limited solid load is currently transported beyond the
megafans of the Chaco Plain, a condition analogous to the
Okavango terminal fan (Stanistreet & McCarthy, 1993.).
Therefore, the vast majority of clastic sediment derived
from the central Andean fold-thrust belt between about
18u and 22uS (an area of roughly 160 000 km2) may be
deposited on the Chaco Plain, primarily on ¯uvial
megafans, rather than transported to the major river
deltas at the Atlantic margin. Such proximal storage of
orogenic sediment within the wedge-top and foredeep
depozones has implications for estimates of sediment
budgets for the Amazon and Parana Rivers.
STRATIGRAPHIC FRAMEWORK OF THE
CAMARGO SYNCLINE
About 200 km west of the modern Chaco Plain ¯uvial
megafans, a nearly 3-km-thick section of Upper Cretaceous to Tertiary strata is spectacularly exposed in the
Camargo syncline (.Fig. 4.). A persistent, north-trending
ridge about 65 km long with up to 1500 m of relief exposes
clastic deposits of the Tertiary Camargo Formation.
These strata form the bulk of the stratigraphic section in
the syncline and are the focus of this investigation.
Although the Cretaceous±Tertiary stratigraphic interval
generally represents foreland-basin conditions (Sempere
et al., 1997.), it can be further divided into separate genetic
intervals attributable to sedimentation in different
depozones within the evolving foreland basin system
(. Fig. 5.; DeCelles & Horton, 1999.).
The succession begins with the Coniacian±Campanian
Aroi®lla and Chaunaca Formations, a 100-m-thick
interval of red ¯uvial mudstones and minor sandstones
that unconformably overlies Ordovician phyllite. This
interval is overlain by light grey carbonate strata of the
Maastrichtian-early Palaeocene El Molino Formation, a
150-m-thick interval of lacustrine and marginal marine
deposits. Upsection, the mid Palaeocene Santa Lucia
Formation contains 80 m of cross-strati®ed, red ¯uvial
sandstones and ¯oodplain siltstones that are overlain by
the Impora Formation, a distinctive, 60-m-thick interval
of paludal and ¯oodplain deposits dominated by nodular,
grey, calcareous palaeosols. Cross-strati®ed, quartzose,
white ¯uvial sandstones and interbedded red ¯oodplain
siltstones comprise the overlying, 190-m-thick Cayara
Formation. Above the Cayara, the remaining section is
composed of the Camargo Formation, a well-de®ned
coarsening- and thickening-upward succession composed
of a lower interval (< 1100 m) of red ¯uvial sandstones
and ¯oodplain siltstones transitionally overlain by an
upper interval (< 1200 m) of sandy, buff-red ¯uvial
conglomerates (Fig. 5.).
Fig. 4. Map of the Camargo syncline,
Eastern Cordillera, depicting local
geology and measured section location.
Note the persistent, north-trending
ridge with up to 1500 m of relief along
the syncline axis. The highest peaks,
the Cerro Tonka peaks, occur directly
north and south of the east-¯owing
Rio Tumusla. The measured section
consists of a lower interval (A) of
Santa Lucia through Cayara
Formations and an upper interval (B
and C) of Camargo Formation (Fig. 6.).
See Fig. 1.for location.
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49
Fig. 5. Schematic diagram of the Upper Cretaceous±Tertiary
stratigraphic column in the Camargo syncline. Well-de®ned
age control for Aroi®lla through Santa Lucia section is based
on previous studies (see text). The poorly constrained age of
the Impora±Cayara±Camargo succession is based on age data
for possible Camargo-equivalent strata to the west (Potoco
Formation) and north (Mondragon and Bolivar Formations).
Column at left depicts interpretation of depositional setting
(or depozone) within the foreland basin system (DeCelles &
Horton, 1999.).
DeCelles & Horton (1999.) ascribe the Cretaceous±
Tertiary succession in the Camargo syncline to an
eastward-migrating foreland basin system (Fig. 5.). In
this interpretation, the Santa Lucia and possibly El
Molino Formations represent deposition of ®ne-grained
sediment in the distal part of the foreland basin system
beyond the forebulge, a region called the back-bulge
depozone (DeCelles & Giles, 1996.). The thick interval of
stacked palaeosols in the Impora Formation, representing
a long time interval of limited or no deposition (possibly
up to 10±20 Myr), is consistent with stratigraphic
condensation in the forebulge depozone. The Cayara
strata and overlying thick Camargo interval are considered indicators of deposition in progressively more
proximal locations of the foredeep depozone as the foldthrust belt advanced eastward. The absence of unambiguous growth strata in the upper part of the Camargo
Formation suggests that the wedge-top depozone was not
preserved in the Camargo syncline. Palaeocurrent data
support the interpretation of a migrating foreland basin
system, indicating: (1) an eastern sediment source for the
Santa Lucia Formation, presumably cratonic cover rocks
exposed to the east in Palaeocene time; (2) an eastern
sediment source for the Cayara Formation, possibly a
forebulge developed to the east in roughly mid Tertiary
50
time; and (3) a western sediment source for the Camargo
Formation, clearly the fold-thrust belt active during mid
Tertiary time in the central and western areas of the
Eastern Cordillera (Sempere et al., 1997.; DeCelles &
Horton, 1999.).
Although a well-de®ned chronostratigraphy exists for
the lower interval of this Cretaceous±Tertiary succession,
the age of the upper part is poorly constrained (Fig. 5.).
The El Molino Formation is de®ned as Maastrichtian to
early Palaeocene based on invertebrate and mammal
fossils, magnetostratigraphy, and volcanic tuffs dated in
correlative strata in the Altiplano to the west (Sempere
et al., 1997.). The Santa Lucia Formation contains
mammal fossils and magnetostratigraphic data that limit
it to 60±58 Ma (Sempere et al., 1997.). The ages of the
Impora Formation and younger deposits, however,
remain poorly known. Whereas Sempere et al. (1997.)
argue for a late Palaeocene age for the Impora through
Cayara Formations, new palynological age data for a
probable age equivalent of the Camargo Formation, the
Potoco Formation of the Altiplano (< 200 km to the west;
Horton et al., in press)., suggest that these units may be
signi®cantly younger. Based on these new data, and on
early Miocene tuff ages (Kennan et al., 1995.) for the
Mondragon and Bolivar Formations (probable upper
Camargo Formation equivalents 100±200 km to the north;
Sempere et al., 1997.), we tentatively assign a late Eocene
to Miocene age for the Camargo Formation. We report
here on the sedimentology of the Camargo Formation that
comprises the majority of the Cretaceous±Tertiary
succession in the Camargo syncline.
SEDIMENTOLOGY OF THE CAMARGO
FORMATION
Methods
A measured stratigraphic section in the Camargo syncline
includes the Santa Lucia through Camargo Formations.
The lower part of the section, Santa Lucia through
Cayara strata, was measured along the gently dipping east
limb of the syncline. The overlying Camargo interval
(. Fig. 6.) was measured along the nearly complete exposure
afforded by cliff faces comprising the Cerro Tonka peaks
(. Fig. 7.), two massive topographic features with up to
1500 m of relief that ¯ank an intervening river, the Rio
Tumusla (Fig. 4.). Due to inaccessibility of the steep east
face of the southern Cerro Tonka peak (at 3824 m, the
higher of the two peaks), we measured the upper part of
the section along the west face (Fig. 4.). This upper
interval is correlated with the lower interval by tracing
laterally continuous sandstone bodies from the east face to
the west face, a distance of about 400 m. The complete,
laterally continuous exposure of the Camargo interval
allowed collection of detailed lithofacies data and
observation of bed geometries over lateral distances of
hundreds of metres.
Palaeocurrent data sets were measured throughout the
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stratigraphic section at localities consisting of a single
channel sandstone composed of trough-cross strata. The
average trough-axis orientation for each locality was
calculated from 15 to 25 measured trough limbs (method I
of DeCelles et al., 1983.). Additional measurements
included primary current lineations, erosional furrows
and clast imbrications. More than 360 palaeocurrent
indicators were measured at 24 localities in the Camargo
syncline (Fig. 6.). These data indicate generally westward
palaeo¯ow for the Santa Lucia and Cayara Formations
and eastward palaeo¯ow for the Camargo section (Fig. 6.).
In general, the Camargo Formation coarsens and
thickens upward from a lower interval of thin-bedded
siltstone and lenticular sandstone to an upper interval of
very thick-bedded sandstone and conglomerate (Figs 6
and 7.). The Camargo succession is divisible into ®ve
lithofacies described below and summarized in T
. able 2..
Overbank facies
Description
The lower 250 m of the Camargo succession is dominated
by facies 1, brick-red, mottled, structureless, noncalcareous siltstone (. Fig. 8A.). The thin (0.25±2.5 m), laterally
continuous beds of this facies commonly exhibit gradational basal contacts with underlying sandstone facies,
contain spheroidal siltstone aggregates (peds) coated by
clay skins and slickensided surfaces, and display gypsum
veins and nodules. Root traces, mudcracks, carbonate
nodules and thin calcretes are rare. Facies 2 is more
common in the middle Camargo interval. It is composed
of light-red to grey, laminated, calcareous, clayey siltstone
(Fig. 8A,B.). Beds of this facies are 0.05±0.20 m thick,
®nely laminated, typically overlie siltstones of facies 1,
and nearly always underlie sandstones of facies 4
(Fig. 8B,C.). In several places, beds of facies 2 ®ll
lenticular scours up to 3 m deep by several tens of
metres wide (Fig. 8A.).
Facies 3, common throughout the lower Camargo
Formation, is composed of laterally continuous beds of
red, ®ne- to medium-grained sandstone. Beds are
0.20±0.80 m thick by several tens to hundreds of metres
wide and exhibit nonerosional bases, horizontal strati®cation, ripple cross-lamination, primary current lineations
and bioturbation. These sandstones are interbedded with
siltstones of facies 1 and 2. Although individual sandstone
beds do not display upward grain-size trends, the
beds are commonly organized into upward thickening,
1±5-m-thick packages that underlie sandstone beds of
facies 3 and 4.
Interpretation
We interpret facies 1±3 as nonchannellized, ¯uvial
overbank deposits (Table 2.). Variegated, mottled siltstones of facies 1 are interpreted as palaeosols in
¯oodplain settings; laminated siltstones of facies 2 are
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interpreted as suspension fallout deposits in local marshy
environments; and thin sandstone beds of facies 3 are
attributed to crevasse splays in overbank areas. Palaeosol
units (up to 2.5 m thick) developed in the siltstones of
facies 1 indicate that some areas of the ¯oodplain
sustained intervals of nondeposition suf®cient for soil
development. These palaeosols generally lack carbonate
material, suggesting intense leaching under well-drained
conditions (Mack, 1993.) and/or a lack of carbonate in the
sediment source area. The thin, laterally continuous,
nonerosional sandstones of facies 3 are clear indicators of
uncon®ned ¯ow on the ¯oodplain, probably crevasse
splays formed by overtopping of channel banks or
breaching of levees during ¯ood-stage conditions.
Upward-thickening packages composed of facies 3 may
represent progradation of splay complexes (Morozova &
Smith, 2000.) and/or increased proximity to active
channels. Paludal deposition of facies 2 represents local
ponding of water in the topographically low areas of the
¯oodplain. In nearly every case, the paludal siltstones are
abruptly overlain by a channel sandstone (facies 4) with
limited relief on the basal scour surface (Fig. 8B,C.). We
attribute this stratigraphic pattern to avulsion of channels
into the lowest areas of the ¯oodplain (e.g. DeCelles et al.,
1998.). A lack of well-de®ned epsilon cross-bedding for
channel facies (described below) further indicates that
channel mobility involved rapid, avulsive changes rather
than continuous channel migration and point-bar deposition. Several cases of lenticular channel scours ®lled with
siltstone (Fig. 8A.) attest to the importance of rapid
abandonment of channels and subsequent plugging by
overbank facies. Facies 1±3 all lack evidence for widespread, prolonged desiccation. In addition, most palaeosols are thoroughly leached of carbonate. These features
may suggest an overall wet and perennial depositional
system.
Channel facies
Description
Facies 4 dominates the Camargo succession and is de®ned
by lenticular, 0.5±6.0-m-thick beds of red-brown,
medium- to coarse-grained sandstone that typically ®ne
upward from a trough cross-strati®ed base to a
horizontal- and ripple-strati®ed top (Fig. 8C.). The base
of this facies is erosive and commonly contains siltstone
intraclasts. Pebbles are locally present as clasts ¯oating
within a sandstone matrix or as lenticular, clast-supported
beds 2±10 cm thick. Dewatering structures, bioturbation
and gypsum veins are present locally. In the lower
Camargo Formation, this facies commonly forms the base
of ®ning-upward packages that are overlain by siltstones
of facies 1 and 2. Upsection, in the middle to upper
Camargo Formation, these beds are commonly organized
into larger multistorey sandstone bodies that are up to
50 m thick and persist laterally for 200±500 m (Figs 7 and
8D.).
51
Fig. 6. Measured stratigraphic section of the Camargo Formation in the Camargo syncline (see Fig. 4.for location). An overlying
< 800-m-thick interval of coarse-grained, uppermost Camargo strata was not measured due to inaccessibility of the highest cliffs
of the Cerro Tonka peaks. Note the upsection decrease in palaeosols and siltstones and the upsection increase in grain size and
bed thickness.
52
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Fig. 6. (continued).
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53
Fig. 7. View to the west of Camargo Formation exposures in the Camargo syncline. About 1500 m of relief exists between the
basal Camargo contact (de®ned as the base of the lower ridge) along the vegetated Rio Tumusla (lower right) and the crest of the
upper ridge (Cerro Tonka peak, 3824 m elevation, upper left). Note the upsection increase in frequency and thickness of laterally
continuous sheets of lower Camargo sandstone (lower ridge) and middle±upper Camargo conglomerate (upper ridge).
Facies 5 is composed of very thick-bedded, wellorganized, buff-red sandy conglomerate and is only found
in the upper Camargo Formation (.Fig. 9.). Individual,
broadly lenticular beds are 0.5±10.0 m thick and form
large amalgamated bodies up to 80 m thick and over 400 m
in lateral extent (Figs 7 and 9A.). These bodies have
erosional basal contacts and commonly ®ne upward
(Fig. 9B.). Sedimentary structures include moderately
developed trough cross-strati®cation and subhorizontal
strati®cation with imbricated clasts (Fig. 9C.). This facies
is interbedded exclusively with sandstone beds of facies 4.
Slingerland, 1998.). These values are similar to estimates
of ancient ¯uvial systems in the Himalayan foreland basin
system (Willis, 1993.; DeCelles et al., 1998.). Facies 4 and 5
lack epsilon cross-bedding, suggesting that the channels
were not characterized by substantial lateral migration
of point bars. In fact, the common pattern of paludal
siltstones (facies 2) abruptly overlain by channel sandstones or conglomerates (facies 4 or 5) suggests that the
¯uvial channels avulsed, rather than migrated, into low
areas of the ¯oodplain.
Interpretation
DEPOSITIONAL MODEL AND
COMPARISON WITH MODERN FLUVIAL
MEGAFANS
We attribute facies 4 and 5 of the Camargo succession to
deposition in large, bedload-dominated ¯uvial channels
(Table 2.). Deposition was primarily by migrating sand
dunes, bars and gravel sheets. Most individual sandstone
and conglomerate beds are 0.5±10.0 m thick, ®ne upward
and display sedimentary structures indicative of decreasing ¯ow velocity. We therefore interpret these units as the
deposits of individual ¯ood events characterized by initial
dune or bar migration followed by lower ¯ow velocities
and plane-bed and ripple conditions. Observed channel
dimensions (up to 10 m deep by 200±500 m wide), grain
size (0.125±200 mm) and sedimentary structures indicate
bank-full discharges in the range of 500±8000 m3 sx1
(calculation according to method outlined by Robinson &
54
The Tertiary Camargo Formation of the Camargo
syncline forms a well-de®ned, upward-coarsening,
upward-thickening succession attributable to progradation. An upsection increase in the dimensions and average
grain size of channel bodies is coupled with an upsection
decrease in the amount of ®ne-grained ¯oodplain deposits
(Figs 6 and 7.). Palaeocurrent data indicate that Camargo
strata were derived from a western sediment source area
within the fold-thrust belt (Fig. 6.), whereas the underlying lower Tertiary deposits (about 335 m thick)
composed of the Santa Lucia, Impora and Cayara
Formations were derived from the east (Sempere et al.,
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Table 2.0Summary of lithofacies in Camargo Formation.
Facies
Description
Stratigraphic occurrence
Interpretation
1
Mottled, brick-red siltstone; generally non-calcareous;
beds 0.25±2.5 m thick; no primary sedimentary
structures; basal contacts gradational with underlying
unit (commonly facies 3 and 4); local occurrences of
soil aggregates (peds) 0.5±2 cm in diameter that exhibit
clay-skin coatings and slickensided surfaces; gypsum
nodules 1.0±3.0 cm in diameter and gypsum veins are
common in lower 150 m; minor root traces, mudcracks,
carbonate nodules and thin calcretes.
Laminated, light-red to grey, clayey siltstone;
calcareous; beds 0.05±0.20 m thick; characteristically
occurs on top of facies 1 and below facies 4; uncommon
channel plugs up to 3 m thick
Thin, laterally continuous, red, ®ne- to medium-grained
sandstone; beds 0.20±0.80 m thick; non-erosional basal
contacts; ripple cross-strati®ed and horizontally
strati®ed; primary current lineations; bioturbation;
interbedded with facies 1 and 2; commonly occurs in
upward-thickening, 1±5-m-thick packages that underlie
facies 4.
1±50-m-thick bodies of lenticular, red-brown, mediumto coarse-grained or conglomeratic sandstone; beds
0.6±6.0 m thick; erosional basal contacts (commonly
lined by siltstone intraclasts); upward ®ning; trough
cross-strati®cation; plane-parallel lamination; ripple
lamination; dewatering structures; local bioturbation;
gypsum veins common in lower 250 m; pebble clasts
include Palaeocene sandstone, Cretaceous limestone and
sandstone, and Ordovician phyllite; commonly occurs as
basal unit of ®ning-upwad package capped by units of
facies 1 and 2.
2±80-m-thick bodies of well-organized, sandy, pebble to
small boulder conglomerate; beds 0.5±10.0 m thick;
erosional basal contacts; upward ®ning; cross-strati®ed
or subhorizontally strati®ed and imbricated; clasts
include Cretaceous limestone, sandstone and volcanics,
and Ordovician sandstone, quartzite and phyllite.
Present throughout
Camargo interval,
dominant in lower 250 m
of Camargo Formation;
bed thickness decreases
upsection.
Palaeosols in
overbank areas.
General lack of
pedogenic
carbonate suggests
thorough leaching
of soils and/or lack
of carbonate in
source area.
Paludal deposits in
overbank areas.
2
3
4
5
1997.; DeCelles & Horton, 1999.). We attribute this
reversal in sediment dispersal direction and the upwardcoarsening and thickening trend to long-term, eastward
progradation of the Camargo depositional system.
Progradation may have been related to eastward propagation of the fold-thrust belt during mid Tertiary time
(DeCelles & Horton, 1999.). The interpreted upsection
progression from distal to proximal facies provides the
basis for a Camargo depositional model that replicates the
main features of the modern ¯uvial megafans of the Chaco
Plain (.Fig. 10.).
The Camargo depositional system is similar to the
modern ¯uvial megafans in terms of channel and
overbank facies, avulsive channel behaviour, proximal-to-distal trends in grain size and channel size, and
approximate plan-view dimensions. First, similar facies
attributes in the Chaco Plain ¯uvial megafans and
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Present mainly in middle
Camargo Formation.
Present mainly in lower±
middle Camargo
Formation.
Crevasse splay
deposits.
Dominant facies of
Camargo interval; unit
thickness increases
upsection: lower Camargo
contains 1±5-m-thick
units; middle±upper
Camargo composed of
packages of stacked,
multistorey standstones
forming units up to 50 m
thick.
Present in middle±upper
Camargo Formation;
thicknesses of multistorey
conglomerate bodies
increase upsection (up to
80 m thick).
Fluvial channel
deposits on
megafan. Lower
Camargo contains
small (few m)
channels of distal
megafan; middle±
upper Camargo
contains large
(<10 m) channels
of proximal megafan.
Large ¯uvial
channel deposits on
proximal megafan;
channels up to
10 m deep.
Camargo system include comparable-sized channels
®lled by sand and/or gravel and ¯oodplain settings
exhibiting crevasse splay, paludal and pedogenic processes. These depositional systems are distinguished from
smaller alluvial fans by the presence of ¯oodplain
environments and the lack of sediment gravity ¯ows.
These systems are distinguished from meandering and
anastomosed rivers by a lack of point bars, scroll-bar
topography, epsilon cross-bedding and permanent vegetated islands. Second, recent abandonment of channels
and avulsive activity of the Chaco Plain megafans
provides a modern analogue for the Camargo system,
particularly in the case of laminated ¯oodplain siltstones
abruptly overlain by newly established sandstone channels. Third, the decrease in both channel size and grain
size from proximal to distal facies of the Camargo
progradational succession matches the downslope trends
55
Fig. 8. Representative mudstone and sandstone facies of the lower Camargo Formation. (A) Mottled siltstone palaeosols (visible
in lower right) overlain by trough cross-strati®ed sandstone (0.3 m thick at right, 3 m thick at left), an interval of laminated
siltstone (5 m thick to right, 3 m thick at left) and overlying section of multistorey sandstone bodies. Laminated siltstone interval
has basal erosive scour (arrows) and is interpreted as the ®ne-grained plug of an abandoned channel. (B) Basal, massive to
laminated siltstone overlain by low-angle cross-strati®ed, medium-grained sandstone that has a cap displaying poor horizontal and
ripple cross-lamination. Note the limited scour relief (a few cm) along the sharp base of the sandstone. Knife (15 cm) for scale.
(C) Ledges of 1±6-m-thick, lenticular, cross-strati®ed sandstones overlying laminated siltstone interval. Strata dip 60u to left.
Shrub at lower left (2 m tall) for scale. (D) Cliff face composed of multistorey sandstone bodies roughly 5±50 m thick. Exposed
thickness is < 250 m.
in the modern ¯uvial megafans. Although a downslope
reduction in grain size would be expected in nearly any
¯uvial system, most continental-scale rivers have contributary ¯ow patterns that will cause channel size (and
presumably water discharge) to increase, not decrease,
downstream. The Camargo depositional system is more
compatible with ¯uvial megafans in which discharge
diminishes downslope (e.g. Friend, 1978.). Such a
decrease in relative discharge from proximal to distal
settings in the Camargo system may best be attributed to
distributary ¯ow in which the main feeder channel
(relatively high discharge) at the exit point of the
mountain belt bifurcates into numerous smaller channels
(relatively low discharge) downstream (Fig. 10.). Finally,
56
although the original area of the Camargo depositional
system cannot be con®dently estimated, continuous
exposure of these strata in a ¯ow-normal direction
provides an estimate of megafan width. The 25 km
outcrop length of upper Camargo conglomerate in a
north-trending ridge (including Cerro Tonka peaks) that
is erosionally truncated on its northern and southern
margins (Figs 4 and 7.) represents a minimum width across
the proximal part of the megafan. This value is similar to
the widths of the proximal parts of the Chaco Plain
megafans (Fig. 2.).
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Fig. 9. Representative conglomeratic facies of the Camargo Formation. (A) Coarsening- and thickening-upward interval in the
middle Camargo Formation. Laterally continuous sheets of multistorey sandstone and sandy conglomerate are 10±80 m thick.
Total exposed thickness of subhorizontal strata is < 400 m. (B) A lower, ®ning- and thinning-upward interval of cliff-forming
conglomerate and slope-forming sandstone that is overlain by a cliff-forming conglomerate displaying an erosive basal contact
(arrow). Total exposed thickness is < 40 m (wingspan of condor in foreground is < 3 m). (C) Close-up view of sandy
conglomerate facies showing sandy matrix, rounded clasts, imbrication and subhorizontal strati®cation. Hammer (30 cm) at top
for scale (arrow).
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57
Fig. 10. Fluvial megafan depositional model for the Camargo Formation. Key elements include an extensive drainage network, a
large feeder canyon, gravel (G) deposition at the apex of the ¯uvial megafan, sand (S) and mud (M) deposition in middle to
lower megafan areas, and an overall distributary channel pattern. Proximal to distal trends include a progressive increase in the
number of channels and associated decrease in the size (width and depth) of channels. The model shows the approximate
geomorphic con®guration of the region during deposition of the Camargo Formation, including the mountain front roughly
20 km to the west, and a major drainage network approximately similar in scale to the present-day network de®ned by the Rio
Cotagaita and Rio Tumusla catchment areas.
D I S C U S S I O N : A N C I E N T FL U V I A L
MEGAFANS AND IMPLICATIONS FOR
DRAINAGE NETWORK EVOLUTION
Identi®cation of ¯uvial megafans in the ancient record
may provide valuable information about the history of
erosional denudation in a mountain belt. First, the
existence or nonexistence of ¯uvial megafan deposits in an
ancient foreland basin system provides constraints on the
size of the source drainage network. Although in¯uenced
by tectonic, climatic, lithological and geomorphological
parameters unique to a given mountain belt, modern
¯uvial megafans in the central Andes (this study) and the
Himalayan foreland (DeCelles & Cavazza, 1999.) suggest a
minimum required drainage area of roughly 104 km2 to
generate a ¯uvial megafan. Thus, megafans are genetically
linked to relatively large drainage networks, suggesting
their deposits may house the most direct record of largescale erosion in adjacent mountain belts (e.g. Schlunegger
et al., 1997., 1998.; DeCelles et al., 1998.). In contrast,
sediment gravity ¯ow-dominated and stream-dominated
alluvial fans of the foreland basin will provide only local
erosional histories of the frontal-most structures of the
58
orogenic belt. Furthermore, nonmegafan river systems
(such as major axial rivers) in the foreland will contain
erosional records that are subject to complicating factors
such as mixing of multiple drainage networks and possible
cratonic sources.
For an incipient orogenic wedge, initial small drainage
networks will develop on the foreland-sloping topographic surface of the fold-thrust belt (Beaumont et al.,
1992.). Assuming no large antecedent drainages, ¯uvial
megafans would not be expected in the early stratigraphic
record of a foreland basin. Drainage networks would grow
as the orogenic wedge advanced (widened), ultimately
reaching a critical size for the production of ¯uvial
megafans. As the larger drainage systems continually
incorporate smaller watersheds through stream capture
(Koons, 1995.), there would be some point when nearly all
surface water in the mountain belt would be focused into
these large drainage systems (e.g. Oberlander, 1965.). At
this stage, the majority of sediment eroded from the
mountain belt would be funnelled through a few outlet
points at the orogenic front. Owing to their high stream
power, these major drainage networks would be ®rmly
imprinted on the evolving landscape, preventing sig# 2001
ni®cant changes in drainage networks through stream
diversion or ponding upslope of growing frontal
structures (e.g. Burbank et al., 1996.; Gupta, 1997.;
Schlunegger et al., 1998.). This stage could represent a
sort of dynamic equilibrium or steady-state condition in
which tectonic input to topography may be balanced by
erosional out¯ux.
We present a simple geometric model of drainage
network evolution that highlights these concepts (.Fig. 11.).
In the initial stages of fold-thrust belt evolution,
numerous small streams feed sediment to small-scale
alluvial fans ¯anking the mountain front (Fig. 11A.). The
limited topography and areal extent of the orogen
preclude integrated drainage networks, favouring smaller
isolated catchments. The trunk streams within these small
catchments have limited stream power, and are therefore
more likely to be defeated by growing structures
downslope (e.g. Burbank et al., 1996.). Such structural
damming would lead to a greater proportion of closed
drainage basins (including piggyback basins) in the early
stages of mountain building. With advance of the foldthrust belt, individual drainage networks expand in area
Fig. 11. Schematic model for the evolution of drainage networks and ¯uvial megafans during long-term advance of a fold-thrust
belt. Three successive phases are depicted in which the structural front of the mountain belt propagates toward the foreland
(bottom). Different stippled and shaded patterns for subaerial fans represent different relative fan areas and corresponding
catchment areas. Note that the hinterland drainage divide (top) is arbitrarily ®xed throughout mountain building. (A) Early
mountain building: small, poorly integrated drainage networks sourcing numerous alluvial fans (stippled patterns), large number
of closed drainage basins due to topographic damming, and no ¯uvial megafans. (B) Intermediate stage of mountain building:
headward growth and stream capture leads to larger, better integrated drainage networks and initial ¯uvial megafans (shaded
patterns). (C) Late stage of mountain building: well integrated drainage networks sourcing a few, very large ¯uvial megafans.
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59
as tributary and trunk streams grow headward (Fig. 11B.).
This elongation of drainage networks (Fraser & DeCelles,
1992.) is augmented by events of stream capture in which
drainage basins expand their catchment area instantaneously. By this stage of mountain building, major trunk
rivers in large drainage networks have become established. These rivers have more or less straight planform
morphologies, are clearly antecedent to deformation in
the frontal parts of the fold-thrust belt and deposit ¯uvial
megafans in the proximal foreland basin system. Alluvial
fans remain an important element at the mountain front.
Continued propagation of the fold-thrust belt leads to
further integration of drainage networks and the production of very large ¯uvial megafans (Fig. 11C.). Stream
capture and headward erosion yield a simple drainage
morphology in which the fold-thrust belt is ef®ciently
drained by a few very large drainage networks (i.e. there
are little to no areas of closed drainage within the orogen).
At this point, a few large trunk rivers transport the
majority of water and synorogenic sediment. The power
of these rivers exceeds the uplift rate of any growing
structure in the frontal part of the fold-thrust belt,
preventing any substantial topographic damming and
ponding of sediment. Although these rivers may owe their
origin to the early stages of mountain building, they are
clearly antecedent to the majority of fold-thrust structures
in the orogen. It should be noted that this model considers
the case where the hinterland topographic divide is ®xed
in space and does not account for possible migration of the
divide and establishment of hinterland plateaus. The
model also ignores complexities related to variations in
lithology, climate, local tectonics and geomorphic thresholds. Despite its simplicity, this model proposes a link
between drainage network evolution and foreland-basin
sedimentation that can be tested and modi®ed through
further study of ancient foreland basin systems.
The existence of distinct drainage outlets along the
orogenic front would produce a foreland basin dominated
by a few major point sources of sediment. This would
generate large, along-strike variations in sediment ¯ux,
foreland-basin stratigraphic architecture, and degree of
sediment loading. Furthermore, the spacing of large
drainage outlet points may maintain a uniform geometric
relationship such that the ratio of orogen width to outlet
spacing is < 2 throughout the growth of the orogenic
wedge (see Hovius, 1996.). This ratio applies reasonably
well in southern Bolivia where the three modern outlets
are spaced < 135 km apart and the width of the foldthrust belt from the hinterland drainage divide to the
thrust front is < 300 km. If this ratio is maintained during
orogenesis, early stages of mountain building would
display numerous closely spaced drainage outlets whereas
later stages (with greater orogen width) would exhibit
more widely spaced outlets (see Fig. 11.). Therefore,
thrust-front advance and widening of the mountain belts
must be accompanied by lateral expansion of mountainous
drainage networks through sideward erosion and/or
capture of smaller, laterally adjacent networks.
60
On a more local scale, the history of a particular
drainage catchment may be assessed through identi®cation of ancient ¯uvial megafans in the stratigraphic
record. Questions about river course and antecedence
provide good examples. Simply stated, the existence of an
ancient ¯uvial megafan deposit in a particular area of a
foreland basin system implies that the area was receiving
sediment near the outlet point of a relatively large
drainage network in the ancient mountain belt. For the
Camargo syncline of the Eastern Cordillera, we suggest
that the location of the coarsest and thickest deposits of
the interpreted Tertiary ¯uvial megafan (preserved in the
resistant conglomeratic masses that form the Cerro Tonka
peaks) directly north and south of the modern Rio
Tumusla is not fortuitous. Rather, we infer that this
modern river system represents a large ancestral river that
dates back several tens of millions of years. Roughly 20 km
upstream of the Cerro Tonka peaks, two trunk rivers (the
Rio Tumusla and Rio Cotagaita) merge (Fig. 10.). The
total area of the present-day drainage network upstream
of the point where the Rio Tumusla enters the Camargo
syncline is roughly 14 000 km2 (Fig. 10.), greater than the
area of the modern Rio Parapeti network. We suggest that
a precursor to this modern Tumusla±Cotagaita drainage
system, similar in geometry and area, formed the source
catchment for the late Eocene to Miocene ¯uvial megafan
deposits in the Camargo syncline. In this interpretation,
the mid Tertiary mountain front and Camargo megafan
were located directly downstream of the con¯uence of the
Tumusla and Cotagaita trunk rivers (Fig. 10.). The
combined power of these rivers ensured the long-term
existence of a notch in the mountain front through which
large volumes of sediment were transported to the east.
We conclude that the Tumusla±Cotagaita catchment,
now a part of the drainage network that feeds the modern
Pilcomayo ¯uvial megafan of the Chaco Plain (Fig. 1.), is
largely inherited from an ancestral drainage system that
fed the Camargo ¯uvial megafan during Tertiary time.
We speculate that the persistence of such drainage
networks, and their incorporation into larger integrated
networks, is a natural progression in the evolution of
mountain river systems in fold-thrust belts (Fig. 11.).
Because topographic lows are dif®cult to destroy, it is
possible that such drainage systems may even outlive the
tectonic processes that produced them.
CONCLUSIONS
1. Fluvial megafans form ®rst-order geomorphic
elements of the modern foreland basin system adjacent
to the central Andes. On the Chaco Plain of southern
Bolivia, ¯uvial megafans are large (5800±22 600 km2),
fan-shaped masses of sediment deposited where major
rivers emanate from the fold-thrust belt at ®xed outlet
points along the Subandean topographic front. Fluvial
megafans contribute the greatest volume of sediment to
the modern foreland basin, much more than the
# 2001
stream¯ow-dominated alluvial fans derived from the
frontal-most structures. Most erosion of the fold-thrust
belt presently takes place in large mountain catchments
such as the Rio Pilcomayo drainage network
(81 300 km2). Megafan deposits therefore provide one
of the most direct records of the erosional history of a
mountain belt.
2. An ancient ¯uvial megafan system is recognized in
the well-exposed, 2300-m-thick, Camargo Formation
in the Camargo syncline of the Eastern Cordillera. This
depositional system is similar to active ¯uvial megafans
of the Chaco Plain in terms of approximate dimensions,
lithofacies and depositional processes. The Camargo
system had a ¯ow-normal (north±south) minimum
width of 25 km, comparable to modern megafans.
Ancient facies include thick bodies of lenticular
channel sandstone and conglomerate as well as overbank deposits composed of mottled siltstone palaeosols,
laminated paludal siltstones and crevasse-splay sandstones. Similar channels and overbank areas exist on
the modern megafans. In both the modern and the
ancient examples, channels exhibit little stability over
time, constantly avulsing into new areas and abandoning previously active channels.
3. The
upward-coarsening,
upward-thickening
Camargo succession documents the progradation of a
¯uvial megafan as the fold-thrust belt advanced eastward. The coarsest and thickest strata in the upper
Camargo Formation spatially coincide with the location
of a modern drainage outlet for the present-day Rio
Tumusla ± Rio Cotagaita drainage network (14 000 km2).
This network may represent an antecedent catchment
that initially provided a point source of sediment to the
Camargo megafan and has been inherited by the presentday Pilcomayo drainage system.
4. A simple geometric model for the development of
drainage networks in advancing fold-thrust belts
suggests that drainage networks become larger and
better integrated over time. A given network must
reach a critical size (roughly 104 km2) before ¯uvial
megafans will be produced in the proximal foreland
basin. Expansion of drainage networks leads to a
situation whereby most sediment eroded from the
orogenic belt is delivered by point sources (¯uvial
megafans) to the foreland. Mainstem rivers in the
larger drainage networks will have high stream power
and their courses should not be affected by growing
frontal structures.
ACKNOWLEDGMENTS
This research was supported by NSF grant EAR9804680. We are grateful to Richard Fink for ®eld
assistance and Sohrab Tawackoli, Reinhard Roessling,
Carlos Riera and Juan Huachani of Sergeomin (La Paz)
for logistical assistance. We have bene®ted from discus# 2001
sions with Thierry Sempere, Justin Wilkinson and
Nadine McQuarrie. Constructive reviews by Rebecca
Dorsey, Fritz Schlunegger and Gerilyn Soreghan
signi®cantly improved the manuscript.
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Received 12 April 2000; revision accepted 6 November 2000
63

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