AN INTEGRATED SEDIMENTARY SYSTEMS ANALYSIS OF THE …

Journal of Sedimentary Research, 2016, v. 86, 1359–1377

Research Article

DOI: http://dx.doi.org/10.2110/jsr.2016.82

AN INTEGRATED SEDIMENTARY SYSTEMS ANALYSIS OF THE RIO BERMEJO (ARGENTINA):

MEGAFAN CHARACTER IN THE OVERFILLED SOUTHERN CHACO FORELAND BASIN

MICHAEL M. MCGLUE,1 PRESTON H. SMITH,2 HIRAN ZANI,3 AGUINALDO SILVA,4 BARBARA CARRAPA,5 ANDREW S. COHEN,5 AND

MARTIN B. PEPPER5

1Department of Earth and Environmental Sciences, University of Kentucky, Lexington, Kentucky 40506, U.S.A.2Department of Earth and Planetary Science, University of New Mexico, Albuquerque, New Mexico 87131, U.S.A.

3ESRI-Brazil, SIG Sul, Quadra 4, CEP 70610440 Brasılia, Distrito Federal, Brazil4Departamento de Geografia, Universidade Federal de Mato Grosso do Sul, Campus do Pantanal, Brazil 79304-020

5Department of Geosciences, University of Arizona, Tucson, Arizona 85721, U.S.A.

e-mail: [email protected]

ABSTRACT: The Chaco foreland is well known for its spectacular fluvial megafans, and this basin frequently serves asa modern analog for studies that use the geological record to investigate tectonic processes in convergent orogenicsystems. Yet little sedimentary data has been recovered from the southern Chaco, particularly at the latitude ofnorthern Argentina, which limits both its utility as an analog and our understanding of Andean megafan developmentin the subtropics. The southern Chaco is particularly amenable for a modern time-slice sedimentary systems analysis,as environmental gradients controlled by tectonics and climate are relatively well documented. Deposition in thesouthern Chaco foreland is dominated by the Rıo Bermejo, which was studied by integrating geomorphologicalobservations made using geographic information systems with analyses of composition, texture, and U-Pb ages ofmodern detrital sediments. Remote-sensing data reveal changes in Rıo Bermejo channel width, sinuosity, andbifurcation from west to east across the foreland, which are compatible with aspects of geomorphic models fordistributive fluvial systems. The Rıo Bermejo fully spans the foreland and connects with the south-flowing RıoParaguay; fluvial sediments and reworked loess bury and obscure any surficial expression of a flexural forebulge.Therefore, the Rıo Bermejo megafan (RBM) stands as an important example of a megafan in an overfilled forelandbasin. Variability in RBM morphology and landforms appears to have only subtle linkages to flexural partitioning ofthe Chaco, which is likely a consequence of sediment supply overwhelming available accommodation. Trend-surfaceanalysis highlighted topographic contrasts in RBM architectural elements and indicates the presence of distributarychannels with variable surficial expression. Shallow accommodation, marked by ponds, is most pervasively developedat the eastern end of the RBM; this pattern is consistent with model-predicted locations of a back-bulge depozone.Although three environmentally distinct hinterland areas contribute sediment to the RBM, sands from the forelandcannot distinguish these zones. Rather, provenance is controlled chiefly by parent lithologies and transport distance.Sands in the RBM plot within the recycled orogen provenance field (mean QtFL ¼ 77\6\17), and compositionalmaturity increases with distance from the thrust front, such that quartzose sublitharenites dominate in the distal back-bulge. Distinct intra-foreland gradients are captured in sand texture and clay-mineral abundances. Rıo Bermejo sandsizes become progressively finer towards the forebulge and back-bulge (mean¼~ 98 lm), whereas mean particle sizesin the wedgetop and foredeep are~ 280 and 195 lm, respectively. Detrital clays suspended in the Rıo Bermejo consistof illite, smectite, chlorite, and kaolinite (in rank order of abundance), with smectite concentrated in the foredeep dueto reworking of the semiarid floodplain by water and wind. The results of this study provide novel sedimentaryground truth for distributive fluvial systems in overfilled retroarc forelands that can be used to improve facies modelsand interpretations of the geological record.

INTRODUCTION

The ~ 7000-km-long Andes mountain belt is the archetype of an active

Cordilleran-style orogen (Ramos 2009). The modern high Andes formed

as a result of crustal thickening and lithospheric-scale magmatic processes,

which began as early as Cretaceous time, attributable to eastward

subduction of the Nazca plate beneath the South American plate. Crustal

thickening due to horizontal shortening, most likely in concert with a

viscous coupling between the Nazca and South American plates via a

mantle wedge, produced flexure-inducing topographic and dynamic loads

on the South American lithosphere (Jordan 1995; DeCelles and Giles

1996; Chase et al. 2009; DeCelles 2012). As a consequence, a laterally

mobile, continental-scale foreland basin formed adjacent to the thrust belt

(Horton and DeCelles 1997).

Published Online: December 2016Copyright � 2016, SEPM (Society for Sedimentary Geology) 1527-1404/16/086-1359/$03.00

Throughout its history, the Andean foreland has been a repository for

sediment generated by weathering and erosion of the adjacent fold-and-

thrust belt. Distribution of these sediments is accomplished mainly through

alluvial fans and fluvial megafans. These syn-orogenic sediments are an

important archive of tectonic and surficial processes, which have helped to

broaden our understanding of the evolution of the Andean cordillera

(Jordan and Alonso 1987; Horton et al. 2001; Carrapa et al. 2006; Uba et

al. 2006; DeCelles et al. 2007; Barnes and Heins 2009; Hulka and

Heubeck 2010; Siks and Horton 2011). Although several lines of evidence

indicate that a flexural foreland basin once occupied areas in the heart of

the extant Andes (DeCelles and Horton 2003; McQuarrie et al. 2005),

debate continues to surround the tectonic interpretations of Paleogene and

Neogene continental strata in northwestern Argentina (del Papa and

Quattrochio 2002; Marquillas et al. 2005; DeCelles et al. 2011; Carrapa et

al. 2012; Hain et al. 2011; del Papa et al. 2013). In this context, a robust

understanding of the modern foreland is critical, because the Chaco plains

serve as a frequently referenced analog for ancient strata now preserved in

the fold-thrust belt.

In an important study, Horton and DeCelles (1997) used seismic

reflection, gravity, and topographic datasets to define the modern Andean

foreland from 18 to 248 S as a four-part foreland basin system (sensu

DeCelles and Giles 1996), comprised of discrete wedgetop, foredeep,

forebulge, and back-bulge depozones. This work helped to establish the

modern basin as an analog for older foreland deposits in an eastward-

propagating flexural foreland basin. To date, few other studies have

considered the impact of flexural partitioning on depositional patterns and

landforms in the Chaco. Rather, most modern time slice studies of the

Chaco have focused on the geomorphology and accumulation history of

Bolivian fluvial megafans, which are spectacularly developed north of

~ 238 S latitude (Horton and DeCelles 2001; Leier et al. 2005; Barnes and

Heins 2009; Latrubesse et al. 2012; Cohen et al. 2015). Fluvial megafans

and their deposits archive a history of Cordilleran surficial and geodynamic

processes, with implications for lowland biogeography (Wilkinson et al.

2006). As noted by Hartley et al. (2010), large distributive fluvial systems

are critical building blocks of stratigraphic architecture for aggrading

continental basins globally, regardless of tectonic setting or climate.

However, the significance of large distributary fluvial systems in the

geologic record remains a topic of debate (Hartley et al. 2010; Fielding et

al. 2012). It is clear that megafans are diverse, and a wide range of facies

models have been developed to encapsulate this variability (Nichols and

Fisher 2007; Chakraborty and Ghosh 2010; Weissmann et al. 2010;

Rossetti et al. 2012; Latrubesse et al. 2012; Assine et al. 2014).

Comprehensive sedimentary studies of the Chaco are rare in the

literature, most likely because the vast scale of the lowland wilderness

makes extensive sampling a challenge. This is particularly true of the

southern Chaco foreland in Argentina (23–268 S latitude), where the Rıo

Bermejo and its tributaries drain a structurally complex area of the Andes

marked by Cenozoic basement-involved structures, thrusts, and Cretaceous

rift-related deformation (e.g., Kley and Monaldi 2002; Carrapa et al. 2013;

Pearson et al. 2013). This data gap is troublesome, considering the value a

well-developed modern analog from the Chaco could hold for improving

interpretations of contentious ancient deposits, as well as for improving

megafan facies models more generally. Limited data on modern sand

petrography and clay mineralogy have been generated for regions of the

northern Andean foreland, for example in the Amazon and Madre de Dios

basins, which have proven useful for testing provenance models or

developing insights on climatic and weathering-intensity controls on

sediment composition (Franzinelli and Potter 1983; Johnsson et al. 1988;

DeCelles and Hertel 1989; Rasanen et al. 1992; Potter 1994; Vital et al.

1999; Guyot et al. 2007). Broad extrapolation of results from these studies

to the southern Chaco is difficult, however, considering the dramatic along-

strike variability in basement geology, style and extent of deformation,

patterns of erosion, climate, and vegetation encountered in the Andes

(Montgomery et al. 2001; Pelletier 2007).

The goal of this paper is to provide the first modern sedimentary

systems analysis for the little-studied southern Chaco foreland basin, with

special emphasis on the Rıo Bermejo megafan (RBM) (Fig. 1). Our dataset

encompasses samples from proximal, medial, and distal sites in the

foreland, as well as numerous sites in the adjacent fold-and-thrust belt

(hereafter, ‘‘hinterland,’’ sensu Heins and Kairo 2007). In contrast to many

other studies of modern forelands and distributive fluvial systems, we

adopted a hierarchical approach to studying the RBM that integrates basin-

scale geomorphological observations from remote sensing with detrital

provenance analyses for both coarse and fine-grained siliciclastic

sediments. Our study makes clear that aspects of the southern Chaco

and the RBM sedimentary system represent a departure from models of

underfilled foreland basins, where a prominent forebulge compartmental-

izes accommodation and hydrologic networks (DeCelles and Giles 1996).

The results of the analysis suggest that the influences of flexural

partitioning on the surficial character of the RBM are somewhat limited,

and restricted primarily to the expression of landforms in the distal back-

bulge depozone. As a consequence, our study is best suited to explaining

patterns of megafan sedimentation and morphology in an overfilled

foreland, where both foredeep and back-bulge depozones are connected

hydrologically and forebulge topography is subdued. We also provide new

data on sediment composition and texture in the RBM, which can be used

to improve interpretations of ancient sediments from analogous orogenic

settings.

SETTING

The name ‘‘Chaco’’ comes from the Quechua word chacu, meaning

‘‘hunting ground,’’ and the Gran Chaco lowlands cover ~ 1,200,000 km2 in

Argentina, Bolivia, and Paraguay (Fig. 1). We confine our description of

the southern Chaco to the hinterland sediment routing system and the

lowland plains encompassing the RBM, which is the dominant fluvial

system building the foreland clastic wedge at 23–268 S latitude (Fig. 1).

The South-Central Andean Hinterland

At the broadest scale, the hinterland sediment routing system for the

RBM consists of four tectonomorphic provinces, including (from west to

east): (1) the Puna Plateau, (2) the Eastern Cordillera, (3) the Subandean

Zone, and (4) the Santa Barbara Ranges. The westernmost limit of the

hinterland occurs around 65.58 W longitude, at the transition from the

high-elevation (~ 4 km mean), and relatively low-relief Puna Plateau to the

high-elevation and high-relief Eastern Cordillera (Fig. 2). The Argentine

Puna is effectively the southern equivalent of the Bolivian Altiplano

Plateau, and is characterized by an arid climate, dry grassland or scrubland

vegetation, and a mean annual temperature of ~ 9–118C. (Allmendinger et

al. 1997). The Eastern Cordillera is an externally drained fold-and-thrust

belt. Here, steeply dipping east- and west-vergent faults that putatively

merge into regional detachments characterize the dominant deformation

pattern (Coutand et al. 2001). Annual rainfall in the Eastern Cordillera is

highly variable (up to 1100 mm/yr), and the vegetation consists dominantly

of Yungas montane forest (Olson et al. 2001). The deformation front

spanning the border of Bolivia and Argentina is known as the Subandean

Zone, a westward-dipping, thin-skinned fold-and-thrust belt (Fig. 2;

Echavarria et al. 2003). South of 238 S latitude, the frontal thrust belt

consists of the Santa Barbara Ranges, which are dominated by east-dipping

reverse faults (Kley and Monaldi 2002). Annual rainfall along the thrust

front is ~ 1000 mm/yr, vegetation consists of dense Yungas forest, and

mean annual temperature ranges from 19 to 218C (Olson et al. 2001). Thin

and poorly developed soils (lithosols, eutric fluvisols) are found on steep

slopes in the hinterland, whereas well-drained, clay-rich luvisols are more

M.M. McGLUE ET AL.1360 J S R

common in hinterland valleys (Fig. 2; FAO 2012). Seasonally shifting

winds around convergent orogens are a well-established transport

mechanism of mineral dust (Gaiero et al. 2004; Kapp et al. 2011), and

loess is common in superficial deposits of the Andean hinterland (Sayago

et al. 2001).

The Southern Chaco Foreland Basin

Tracing the zero flexural contour produced by the model of Chase et al.

(2009) indicates that the southern Chaco foreland at the latitude of the RBM

consists of a ~ 250-km-wide wedgetop plus foredeep, a ~ 200-km-wide

forebulge, and a diffuse back-bulge depozone on the order of 300–400 km

wide (Fig. 1). Precisely establishing the location of depozones in foreland

basins is a difficult task, as topographic variability is often subtle, diagnostic

features may be buried or obscured by erosion, and basement deformation

patterns are spatially complex (e.g., Uba et al. 2006). These challenges are

manifest in the Andean foreland, as the early Horton and DeCelles (1997)

interpretation for flexural partitioning of the modern system has been

questioned by other researchers. For example, the geographic position of the

forebulge and back-bulge appeared incompatible with geophysical evidence

from the Pantanal wetlands (Ussami et al. 1999). Ussami et al. (1999) noted

the occurrence of extension in the Pantanal (Brazil) and suggested that

reactivation of basement structures by a nearby Andean forebulge was a

plausible mechanism to explain this deformation. By contrast, the Pantanal

was diagnosed as a back-bulge using Bouguer gravity anomalies and stratal-

thickness isopachs by Horton and DeCelles (1997). Although active normal

faults are known to influence river courses in the Pantanal, the precise

genetic relationship between the Pantanal Basin and Andean orogenesis

remains unclear (Assine et al. 2015). However, diverse hydrological

pathways and craton-derived sources feeding water and sediment into the

eastern Amazon, Pantanal, and humid Chaco lowlands are consistent with a

back-bulge setting, and indeed the results of Chase et al. (2009), which

utilizes an effective elastic thickness of 50 km to model flexure, broadly

supports the interpretation of Horton and DeCelles (1997). Thus, we

contextualize environmental variability in the southern Chaco and the RBM

using the constraints provided by this model.

Several environmental gradients relevant to the geomorphology and

sedimentation of the RBM exist in the foreland. Annual precipitation

declines from east to west, such that some areas of the back-bulge receive

. 1300 mm/yr, whereas rainfall in the foredeep is generally � 700 mm/yr

(Fig. 2). The seasonal distribution of precipitation is summer-dominated and

influenced by northerly flow (e.g., Amazonian moisture; Garreaud et al.

2009) and a low-level jet driven by the so-called Chaco Low (Seluchi et al.

2003). Mean annual temperature across the Chaco plains varies from 20 to

228C, but extreme summer heat associated with the Chaco Low has been

recorded (Iriondo 1993). The Dry Chaco ecoregion (locally known as Chaco

Seco) characterizes the lowlands along the Rıo Bermejo, stretching from the

wedgetop to the eastern margin of the forebulge at ~ 25.18 S latitude, 60.78

W longitude. Vegetation of the Dry Chaco consists of xeric forests of

quebracho and mesquite trees, as well as grasslands (Prado 1993; Torrella

FIG. 1.—A) Google Earth e satellite image

(April 2013) of the southern Chaco foreland basin

in northern Argentina. Three distinct hinterland

zones route sediment into the foreland via the Rıo

Bermejo megafan. B) Inset map marks the

position of the Chaco in central South America

(red box). C) Foreland depozones of the southern

Chaco based on the flexural model presented in

Chase et al. (2009).

MODERN SEDIMENTARY SYSTEMS ANALYSIS OF THE RIO BERMEJO MEGAFAN, ARGENTINAJ S R 1361

FIG. 2.—A) Geology of the RBM and its hinterland sediment generation zones. 1:1,000,000-scale map (modified after Reutter et al. 1994) illustrates the major rock types in

the hinterland. B) Shuttle Radar Topography Mission 90 m digital elevation model. Note the topographic contrast between the Puna (P), Eastern Cordillera (EC), Sub-Andes

(SA), Santa Barbara Ranges (SB), and the Chaco foreland (C). C) Mean annual precipitation (mm/yr; Hijmans et al. 2005). Note contrast in rainfall between proximal and

distal sites in the megafan. D) FAO global soils map. Soils become increasingly well-developed and organic-rich in the distal megafan. E) WWF ecozones marking vegetation

transitions. A strong contrast exists between the vegetation of the Andean foothills (Yungas montane forest) and the Chaco Seco, which covers most of the foredeep and

forebulge depozones of the megafan system.


and Adamoli 2005). Soils are dominated by haplic xerosols and eutric

fluvisols (FAO 2012) in the wedgetop and foredeep, whereas soils of the

forebulge region consist of organic-rich haplic kastanozems. In the back-

bulge, the lowland ecoregion is characterized by the Humid Chaco (locally,

Chaco Humedo). Here, vegetation consists of a mosaic of grasslands and

palm savannas. Soils in the Humid Chaco grade from eutric fluvisols in the

east to haplic and luvic kastanozems near the forebulge-to-back-bulge

transition (Fig. 2; FAO 2012). Secondary (reworked) loess is admixed with

fluvial sands and silts across the Chaco, and is estimated to constitute~ 50%

of the surficial cover (Sayago et al. 2001).

Rıo Bermejo Hydrology

The south-central Andean hinterland is drained by several major rivers

and smaller streams that are tributaries of the Rıo Bermejo (also known as

the Rıo Teuco), which flows southeast across the Chaco foreland before

reaching its confluence with the south-flowing Rıo Paraguay (Fig. 1). The

Rıo Bermejo discharge varies seasonally between 20 and 14000 m3/s (Varis

et al. 2008). Peak flow occurs in the austral summer, and it accounts for

~ 75% of the annual flow (Sambrook Smith et al. 2016). Hinterland rivers

form a trellis drainage pattern and generally fall into one of three categories:

(1) northern tributaries of the Rıo Bermejo, extending to the headwater

region of southern Bolivia; (2) southern tributaries of the Rıo Bermejo,

including the rıos San Francisco and Grande; and (3) coincident drainages

along the eastern flank of the Santa Barbara Ranges, which predominantly

flow to the northeast across the proximal Chaco and form the southern

boundary of the RBM in the foredeep (Fig. 3). Channel morphologies of

coincident tributaries draining the slopes of the Eastern Cordillera,

Subandean Zone, and Santa Barbara Ranges are frequently braided, whereas

the larger antecedent rivers exhibit diverse patterns that vary from braided to

meandering. The Rıo Bermejo has been aggradational with periodic

avulsions in the recent past; Sambrook Smith et al. (2016) estimate channel

migration rates on the order of 20–30 m/yr for 2000–2012. Those authors

noted that the dynamic lower reach of the Rıo Bermejo near its confluence

with the Rıo Paraguay exhibited very high suspended load and a channel bed

consisting of sandy silt. Annual sediment yield from the Rıo Bermejo~ 100

3 106 tons, with ~ 98% of the sediment load transported in suspension

(Sambrook Smith et al. 2016). The Rıo Bermejo lacks major modification

by dams, though some water is diverted to sustain a small reservoir, Laguna

Yema, in the Formosa province.

METHODS

The geology and geomorphology of the southern Chaco foreland was

studied using publicly available remote-sensing datasets (e.g., Shuttle

Radar Topography Mission 90 [SRTM] digital elevation data) integrated

FIG. 3.—Google Earth e satellite image (April

2013) of the Rıo Bermejo hinterland and megafan

apex region hydrology. The rıos Bermejo, San

Francisco, and Grande are the main rivers feeding

into the foreland. Several coincident drainages

from the Santa Barbara Ranges flow into the Rıo

Bermejito, which forms the southern margin of

the megafan. Sample stations for sand petrogra-

phy, clay mineralogy, particle size, and detrital-

zircon geochronology are shown for the hinterland

and proximal megafan. Additional sample stations

from farther east on the megafan are detailed in

Table 1.


into a geographic information system (GIS). The GIS was used to define

the Rıo Bermejo watershed and megafan boundaries, as well as for surface

area calculations for the various bedrock types exposed in the study area.

Modified trend-surface analysis was applied to SRTM data to produce a

residual-elevation map that enhances subtle relief on the Chaco plains,

following the method of Zani et al. (2012). The residual-elevation map

provides a detailed perspective on the location and morphology of fluvial

architectural elements, particularly those with low relief. This technique, in

concert with examination of Landsat and Google Eartht imagery, was used

to identify zones of accommodation and analyze megafan landforms (e.g.,

distributary channels, channel levees, and fan lobes). Flexural contours

from Chase et al. (2009) were also imported into the GIS, in order to

observe the geomorphology of the various foreland depozones. A transect

running along the Rıo Bermejo from the megafan apex to its termination at

the confluence of the Rıo Paraguay was traced using Landsat imagery

within ArcGIS. Channel-width measurements were taken at one kilometer

intervals along the ~ 1150 km transect and SRTM90 provided elevation

data for each interval. For the purposes of this study, channel width was

considered to be the distance between the stable outermost banks, as

measured at a 908 angle to the river axis. Sand bars and mid-channel

islands were included in this measurement. Sinuosity was calculated for

each 20 km stream length by measuring the thalweg in ArcGIS.

Modern sediments from the RBM and its watershed were sampled in

2007–2008 during austral winter months, when access to the lowlands

was most feasible. Sand was collected from 46 point bars and side bars

from 35 rivers in northern Argentina and southern Bolivia (Table 1).

Samples were collected from active middle bar surfaces only; care was

taken to retrieve a representative sample. One clear epoxy grain mount

was made for each sample, and a standard thin section was cut from each

grain mount. A combination staining treatment was applied to half of

each thin section, in order to aid in the identification of potassium and

plagioclase feldspars. Each slide was point counted (~ 400 grains per

slide) using the Gazzi-Dickinson method (Ingersoll et al. 1984). This

method allows for results to be compared with similarly analyzed modern

sands from elsewhere in the foreland, as well as traditional provenance

models (Dickinson and Suczek 1979; Garzanti et al. 2007). Point-

counting parameters and count results can be found in Tables 2 and 3,

respectively. The petrographic data are presented on standard ternary

diagrams, and they capture a partial record of sediment generation,

TABLE 1.—Locations for Rıo Bermejo hinterland and megafan sediment

samples. Zone numbers correspond to areas shown in Figure 1. P, thin-

section petrography; PSA, particle-size analysis; CM, clay mineralogy;

DZ, detrital-zircon U-Pb geochronology.

Station

Name Zone River Latitude Longitude Samples

B1 1 R. Bermejo –22.1907 –64.6525 P, PSA, CM,

DZ

B2 1 R. Bermejo –22.7259 –64.3672 P, PSA, DZ

B3 1 R. Bermejo –23.2476 –64.1394 P, DZ

B17 1 R. Bermejo –22.2477 –64.5878 CM

BAR 1 R. Baritu –22.5002 –64.7655 P

BLA 1 R. Blanco –23.0873 –64.3284 P

CAN 1 R. Candado –22.1899 –64.6778 P

EMB 1 R. Emborozo –22.2727 –64.5371 P

PES 1 R. Pescado –22.9643 –64.3637 P

SEC 1 R. Seco –23.0278 –63.9175 P

TOL 1 R. Los Toldos –22.2873 –64.6754 P

GDT 1 R. Grande de Tareja –22.5783 –64.2452 P

SRO 1 R. Santa Rosa –22.1785 –64.6684 P

GAU 1 R. Gaundacay –22.3428 –64.5000 P

LPI 1 R. Los Pinos –22.1936 –64.6377 P

LPZ 1 R. Los Pozos –22.5885 –64.4212 P

COL 2 R. Colorado –23.2974 –64.2184 P

GRA 2 R. Grande –22.9603 –65.4989 P

GRB 2 R. Grande –23.4462 –65.3493 P

GRC 2 R. Grande –23.7859 –65.4779 P

SZO 2 R. San Lorenzo –23.7911 –64.7906 P

LED 2 R. Ledesma –23.9076 –64.8099 P

LEO 2 R. Leon –24.0307 –65.4383 P

MJT 2 R. Mojotoro –24.7337 –65.0288 P

NEG 2 R. Negro –24.0958 –64.8217 P

PER 2 R. Perico –24.3648 –65.0994 P

PIE 2 R. Piedras –23.5738 –64.4961 P

PUR 2 R. Purmamarca –23.7434 –65.4690 P

REY 2 R. Reyes –24.1596 –65.3833 P

SFA 2 R. San Francisco –23.7105 –64.5373 P

SFB 2 R. San Francisco –23.3562 –64.4854 P

SLD 2 R. Saladillo –26.4404 –58.4003 P

TUM 2 R. Tumbaya –23.8290 –65.4714 P

YAC 2 R. Yacoraite –23.3676 –65.3416 P

LOZ 2 R. Lozano –24.0757 –65.4116 P

YAL 2 R. Yala –24.1186 –65.4042 P

DVL 3 R. del Valle –24.7028 –64.1889 P

DOR 3 R. Dorado –24.4183 –63.9909 P

SRA 3 R. Santa Rita –23.9005 –64.3334 P

B4 4 R. Bermejo –23.7259 –63.1559 P, PSA, CM,

DZ

B5 4 R. Bermejo –25.3882 –60.3053 P, PSA, CM





SAL 4 R. Salado –26.4404 –58.4003 P

ZAP 4 R. Zapiran –26.8657 –58.7634 P

B10 4 R. Bermejo –25.0138 –60.8467 PSA

B11 4 R. Bermejo –23.7649 –63.0700 PSA

B12 4 R. Bermejo –24.2553 –61.9257 PSA, CM

B13 4 R. Bermejo –24.4583 –61.5824 PSA, CM

B14 4 R. Bermejo –25.6551 –60.1295 PSA, CM

B15 4 R. Bermejo –26.3342 –59.3610 PSA, CM, DZ

B16 4 R. Bermejo –26.3326 –59.3599 PSA, CM

B18 4 R. Bermejo –23.1485 –64.2071 CM

B19 4 R. Bermejo –23.3856 –63.6699 CM

B20 4 R. Bermejo –23.3856 –63.6698 CM

B21 4 R. Bermejo –25.1668 –60.5401 CM

B22 4 R. Bermejo –26.5231 –58.8279 CM

TABLE 2.—Point counting parameters

Symbol Definition

Qm Monocrystalline quartz

Qpq Polycrystalline quartz

P Plagioclase feldspar

K Potassium feldspar

Lv Volcanic-hypabyssal lithic grains

Cht Chert

M Phyllosilicates

U Unidentifiable grains

Q Total quartzose grains (¼ Qm þ Qpq þ Cht)

F Total feldspar grains (¼ P þ K)

L Total unstable lithic grains (¼ Lv þ Lmh þ Lml þ Lss þ Lsa)

Lm Metamorphic lithic grains (¼ Lmh þ Lml)

Ls Sedimentary lithic grains (¼ Lss þ Lsa)

Qp Total polycrystalline quartzose grains (¼ Qpq þ Cht)

Lvm Volcanic-hypabyssal metavolcanic lithic grains (¼ Lv)

Lsm Sedimentary and metasedimentary grains (¼ Lmh þ Lml þ Ls)

Lt Total lithic grains (¼ Qp þ Lm þ Lv þ Ls)


transport, and modification processes in the hinterland sediment routing

system, as well as a detailed perspective on the compositional

characteristics of the basin fill along a proximal-to-distal transect.

Select samples (n¼ 18) from the modern sand dataset were analyzed to

determine quantitative particle size distributions using a Malvern Master-

sizer, which has a measurement range of 0.2–2000 lm. Samples were

chemically pre-treated with sodium hypochlorite to remove any traces of

organic matter. Sonication and stirring were applied to maintain proper

sample dispersion during the analysis, which was completed in triplicate

and presented as an average value on size-frequency curves. Particle-size

information along a proximal-to-distal transect on the Rıo Bermejo

quantifies downstream fining and abrasion in the sand fraction, as well as

ground-truth for interpretations of the controls on channel-pattern

variability.

TABLE 3.—Recalculated point-count data. Samples B1 to LPZ are located in Zone 1. Samples COL to YAL are located in Zone 2. Samples DVL to SRA

are located in Zone 3. Samples B4 to ZAP are located in Zone 4.

Station Q Qm Qpq F P K L Lt Lm Lv Ls Cht Qp Lsm M U

B1 206 160 5 53 34 19 145 160 91 5 54 5 10 145 25 1

B2 286 239 0 28 14 14 91 138 44 7 42 5 5 87 3 1

B3 345 295 4 30 26 4 101 151 53 14 36 2 6 91 4 0

BAR 133 122 0 49 16 33 165 176 20 14 129 1 1 151 12 3

BLA 272 219 6 12 6 6 159 212 112 6 47 6 12 205 6 0

CAN 67 51 1 25 20 5 337 353 110 18 215 6 7 402 17 0

EMB 377 301 2 15 8 7 53 129 36 3 30 16 18 53 1 0

PES 267 206 4 34 25 9 156 217 109 17 35 10 14 146 12 2

SEC 412 337 0 9 3 6 37 112 24 4 18 11 11 37 0 0

TOL 229 195 0 5 2 3 212 246 100 7 103 1 1 279 3 1

GDT 442 404 2 17 1 16 58 96 25 2 34 7 9 58 0 2

SRO 223 197 1 29 9 20 161 187 69 16 81 5 6 174 14 0

GAU 192 186 0 3 2 1 13 19 11 0 4 2 2 15 0 0

LPI 111 102 2 3 3 0 61 70 40 7 14 0 2 57 6 0

LPZ 129 123 2 4 2 2 71 77 41 0 32 2 4 99 0 0

Average 246 209 2 21 11 10 121 156 59 8 58 5 7 133 7 1

Std Dev 113 95 2 16 11 9 83 82 36 6 55 4 5 103 8 1

COL 341 289 0 19 8 11 56 108 14 7 35 2 2 47 4 0

GRA 131 101 5 21 12 9 178 208 94 13 79 8 13 192 1 0

GRB 218 175 1 60 40 20 146 189 38 11 94 2 3 141 3 0

GRC 227 196 0 29 20 9 143 174 94 15 39 5 5 166 9 0

SZO 278 250 0 40 20 20 129 157 55 6 57 2 2 129 1 0

LED 232 163 27 38 23 15 139 208 82 18 47 9 36 147 5 1

LEO 104 76 2 17 12 5 262 290 186 2 77 10 12 361 0 0

MJT 275 187 28 27 15 12 125 213 103 4 24 7 35 139 0 1

NEG 222 141 23 43 21 22 87 168 32 8 48 2 25 79 6 1

PER 284 243 0 30 20 10 159 200 135 3 26 14 14 184 2 0

PIE 400 326 4 29 13 16 61 135 19 3 38 4 8 55 1 0

PUR 142 116 0 23 21 2 169 195 106 5 63 5 5 189 4 0

REY 137 107 3 11 7 4 269 299 215 9 44 4 7 314 7 0

SFA 154 134 0 22 14 8 73 93 38 8 21 0 0 67 7 0

SFB 391 340 0 48 21 27 87 138 47 9 31 4 4 88 7 0

SLD 378 302 26 55 41 14 79 155 61 4 17 3 29 93 2 0

TUM 191 171 0 40 33 7 158 178 86 6 71 6 6 186 4 1

YAC 195 162 0 91 49 42 148 181 58 12 67 7 7 146 5 0

LOZ 104 92 2 13 10 3 84 96 70 3 13 2 4 107 1 0

YAL 70 66 0 7 6 1 100 104 87 0 14 1 1 131 0 0

Average 224 182 6 33 20 13 133 174 81 7 45 5 11 148 3 0

Std Dev 99 84 10 20 12 10 59 56 52 5 24 3 11 79 3 0

DVL 357 306 23 43 17 26 62 113 20 9 35 5 28 57 1 3

DOR 409 369 0 10 2 8 37 77 7 1 26 3 3 31 1 1

SRA 273 224 29 19 9 10 139 188 33 7 103 4 33 155 7 0

Average 346 300 17 24 9 15 79 126 20 6 55 4 21 81 3 1

Std Dev 69 73 15 17 8 10 53 57 13 4 42 1 16 65 3 2

B4 328 270 26 27 15 12 61 119 24 8 34 6 32 56 1 1

B5 401 335 1 44 19 25 110 176 62 8 42 2 3 112 4 0

B6 193 159 1 12 4 8 36 70 28 4 5 1 2 33 1 0

B7 220 193 6 10 5 5 39 66 30 4 5 0 6 35 0 0

B8 183 162 0 8 2 6 80 101 35 4 39 0 0 74 2 0

B9 221 195 0 26 7 19 64 90 34 6 26 2 2 59 1 0

SAL 315 280 1 18 6 12 24 59 16 2 10 6 7 21 0 0

ZAP 266 234 0 38 34 4 46 78 25 3 20 3 3 43 0 0

Average 266 229 4 23 12 11 58 95 32 5 23 3 7 54 1 0

Std Dev 76 63 9 13 11 7 28 38 14 2 15 2 10 29 1 0


Detrital zircons were separated from five Rıo Bermejo point-bar sand

samples using standard techniques for dense minerals. Uranium-lead

geochronology was determined for each sample at the University of

Arizona LaserChron Center using laser ablation multicollector inductively

coupled plasma mass spectrometry, following the methods described in

Gehrels et al. (2008). The number of U-Pb zircon ages per sample varied

between 69 and 90, with isotopic data quality assured by in-run

fractionation, discordance, and precision. A total of 405 ages were

collected. Following the method of Gehrels et al. (2008), we report206Pb/238U ages up to 1000 Ma and 206Pb/207Pb ages if the 206Pb/238U ages

were older than 1000 Ma. The results are plotted as relative age-probability

diagrams, and peaks are considered robust if defined by several zircon ages

(e.g., DeCelles et al. 2011). Detrital-zircon ages provide insights on

hinterland bedrock provenance and hold potential for broadening our

understanding of sand transport and modification processes prior to burial

in the foreland.

Suspended-sediment samples were collected from stations (n ¼ 19) on

the Rıo Bermejo in a transect that stretched from the headwaters in

southern Bolivia to the confluence with the Rıo Paraguay in eastern

Argentina. In the laboratory, the , 2 lm fraction of each sample was

isolated using centrifugation (Moore and Reynolds 1989). Oriented clay-

mineral mounts were created using the modified filter-membrane peel

technique outlined in Pollastro (1982). Clay mounts were scanned from 0.5

to 358 2H on a Siemens D-500 X-ray diffractometer with a fixed-slit

assembly, Bragg–Brentano geometry, and graphite monochromator. The

maximum current and voltage were set at 30 mA and 40 kV, respectively.

Each slide was X-rayed four times to produce air-dried, ethylene-

glycolated, 4508C heat, 6008C heat treated patterns (Starkey et al. 1984).

Clay minerals were identified using the techniques of Moore and Reynolds

(1989), and relative abundances were calculated by the weighted-peak-area

method of Biscaye (1965). Data are presented on a ternary diagram, with

the 7 A clays (kaolinite, chlorite) grouped together.

RESULTS

Geomorphology

Our analysis indicates that the RBM covers an area of ~ 73,900 km2.

The hinterland sediment routing system, which is defined by the boundary

of the Rıo Bermejo watershed plus rivers draining the Santa Barbara

Ranges, covers an area of ~ 70,000 km2. Here, the maximum altitude is

5735 m a.s.l., the minimum is 273 m, and the mean is 1700 m. As

observed on a number of megafans globally (DeCelles and Cavazza 1999),

the lateral migration of the RBM is limited by the presence of other large

rivers. The RBM is constrained to the north by the Rıo Pilcomayo

megafan, whereas northeast-flowing rivers draining the Santa Barbara

Ranges constrain lateral migration of the megafan southward (Figs. 1, 3).

The apex of the RBM is located to the east of the confluence of the rıos

Bermejo and San Francisco, near the town of Embarcacion in the Salta

province of Argentina. Within the RBM, the maximum altitude above sea

level (a.s.l.) is 299 m, the minimum is 45 m, and the mean is 131 m. The

minimum altitude occurs near the confluence with the Rıo Paraguay, thus

defining a general gradient of lower elevations to the east. The average

slope of the RBM is ~ 0.028.

Wedgetop.—Because the wedgetop is best defined by growth strata,

structural features observable only with geophysical imaging (e.g., blind

thrusts), and stratal thickness patterns, the extent of this depozone in the

modern southern Chaco is not precisely known. We consider the wedgetop

to be the zone of syn-orogenic RBM sedimentation west of longitude

~ 63.58 W, which includes the lowlands surrounding the Subandean Zone,

the Santa Barbara Ranges, and the Lomas del Olmedo (Fig. 1). Therefore,

the wedgetop encompasses all lowlands with discernible indications of

subsurface deformation in the region. Beginning in the wedgetop, the Rıo

Bermejo emerges from the hinterland (after merging with the Rıo San

Francisco) and flows east towards the craton. From the RBM apex (~ 280

m a.s.l.) to the wedgetop-to-foredeep transition at ~ 240 m a.s.l., the Rıo

Bermejo exhibits a ~ 0.5–4-km-wide braided to anastomosing channel

system that is confined within a valley (Figs. 1, 4). The sinuosity of the

river is relatively low (~ 1.1) and invariant in the wedgetop. The width of

the megafan increases eastward, away from the apex, and reaches a

maximum of ~ 22 km (Fig. 5). The density of oxbows, scroll bars and

paleo-channels also increases towards the east; these features are relatively

rare at the narrow RBM apex (Fig. 3). An eastward expanding paleo–

channel belt defined by moderate residual elevation values is evident

between ~ 260 and 200 m a.s.l., north of the Rıo Bermejo (Fig. 6). This

lobate landform limits accommodation north of the river in the wedgetop,

foredeep, and forebulge depozones (Fig. 5).

Foredeep.—The foredeep at the latitude of the RBM is defined as the

region of accommodation adjacent to the easternmost buried structure of

the wedgetop; this depozone notionally thins towards the western edge of

the forebulge at ~ 62.58 W longitude. In the foredeep, the Rıo Bermejo

transitions from a relatively wide (~ 1.5 km mean) braided channel to a

relatively narrow (~ 560 m mean) meandering channel, with sinuosity

values increasing to a mean of 1.8 (Fig. 4). The megafan widens to a

maximum of ~ 150 km in the foredeep, and several vintages of abandoned

meander belts are present on both sides of the river (Fig. 6). The presence

of remnant accommodation is greatest from 220 to 235 m a.s.l. (Fig. 5), but

the density of oxbows, scroll bars, and paleo-channel limits active

floodplain sedimentation to narrow tracts , 3 km wide.

Situated ~ 20–40 km south of the main river channel, the Rıo Bermejito

occupies one of the most prominent meander belts in the foredeep. A dense

network of sandy paleo-channel features separates these two rivers (Fig. 6).

The Rıo Bermejito flows east, parallel to the Rıo Bermejo, and forms the

southern margin of the RBM in the foredeep. The Rıo Bermejito collects

several of the northeast-flowing rivers draining the Santa Barbara Ranges

(Fig. 3), thus forming a conduit for sediment generated from those frontal

thrusts to the distal RBM back-bulge.

Forebulge.—The forebulge is a region of potential uplift on the craton

side of the foredeep, and the flexural model of Chase et al. (2009) suggests

that the forebulge is situated from 62.58 W to 60.58 W on the RBM (Fig. 1).

However, the flexural wave is not expressed in the surface topography of

the southern Chaco foreland, as most evidence of a forebulge is obscured

by a cover of Quaternary reworked loess and alluvium (Iriondo 1993;

Sayago et al. 2001; Cohen et al. 2015). Rather, only minor variations in

gradient with potential relations to the forebulge can be detected. The Rıo

Bermejo crosses the forebulge between ~ 200–130 m a.s.l. Zones of low

residual-elevation values appear to capture the presence of distributary

channels in the forebulge depozone, mostly contained in the active

floodplain (Fig. 5). These distributary channels are broadly arranged both

north and south of the Rıo Bermejo mainstem. Older paleo-channel

elements are less common than in the foredeep. The Rıo Bermejito channel

also bifurcates and takes on a distributary pattern along the forebulge (Fig.

5).

Rıo Bermejo sinuosity varies across the forebulge, with several areas

marked by some of the lowest values east of the wedgetop (Fig. 4). The Rıo

Bermejo channel sinuosity transitions to a tight meandering habit at the

eastern margin of the forebulge, ~ 610 m from the apex (Fig. 4).

Back-Bulge.—The back-bulge is an area of potential accommodation

between the forebulge and the craton (DeCelles and Giles 1996). The

flexural model for the southern Chaco places the eastern margin of the

forebulge at ~ 60.58 W, and therefore defines the transition zone into the


FIG. 4.—A) Downstream changes in channel width (gray) and sinuosity (black) for the Rıo Bermejo, from the megfan apex to the confluence with the Rio Paraguay. Note

marked downstream narrowing of the channel. Modified after Weissmann et al. (2015). B) Longitudinal profile (gray) and best fit line (black) of the Rıo Bermejo, with

concave-up profile.

FIG. 5.—A) Residual-elevation map of the Rıo

Bermejo megafan produced by modified trend

surface analysis. The rıos Bermejo and Paraguay

are marked in black. Hot colors mark areas with

limited accommodation, whereas cold colors

denote areas with more remnant space for water

and sediment. Note that anthropogenic features

are defined by strong geometric shapes (e.g.,

agricultural fields). Distributary and spring-fed

channels are prominent features in the forebulge

and back-bulge depozones. B) Histogram of

ponds on the Rıo Bermejo megafan with respect

to elevation. The vast majority of natural ponds

exist from 75 to 95 m.a.s.l. on the megafan,

marking areas of back-bulge accommodation.


back-bulge, which extends to the megafan margin at ~ 58.48 W, where the

Rıo Bermejo intersects the Rıo Paraguay (Fig. 6). The residual elevation

map indicates that the back-bulge is a zone of moderate relative

accommodation in the RBM (Fig. 5). At least two distributary fans appear

to prograde into the back-bulge, and the distal channels of these lobes are

marked by high relief relative to the surrounding floodplain. One of these

distributary networks can be traced up dip to the avulsion node present on

the Rıo Bermejo in the forebulge region (Fig. 5). The Bermejo channel

width narrows substantially in the back-bulge (mean of ~ 225 m), and

numerous parallel channels that terminate near the Rıo Paraguay are

prominent on the residual elevation map and on satellite images (Figs. 5,

6). These smaller rivers are marked by vegetated levees, and they are likely

fed by springs (Hartley et al. 2013). A defining characteristic of the RBM

back-bulge is the high density of small (� 1 km2) ponds, which are

clustered in accommodation between 73 and 88 m a.s.l., below the

apparent spring-line elevation (Fig. 5). Natural ponds are rare in all other

regions of the RBM (Cohen et al. 2015).

Satellite images show that the Rıo Bermejito deflects to the northeast

and intersects the Rıo Bermejo mainstem in the back-bulge at 110 m.a.s.l.

(~ 25.78 S latitude, 60.28 W longitude). Thus, sediments derived from the

Santa Barbara Ranges that remain confined within the Bermejito meander

belt may bypass parts of the foredeep and forebulge depozones.

Modern Sand Composition

Hinterland Sediment Generation.—We separated the hinterland into

three zones of potential sand generation for our samples with distinct

sediment routing systems (Figs. 1, 7). Zone 1 (~ 28,500 km2) consists of

rocks in the Puna, Eastern Cordillera, and Subandean zones drained by the

Rıo Bermejo and its northern tributaries (Fig. 1). Zone 1 is composed of

Cenozoic sedimentary rocks (Metan Group) and recent alluvium, Paleozoic

marine sedimentary rocks (Meson and Santa Victoria Groups), and minor

amounts of Precambrian metasedimentary rocks (Puncoviscana Formation)

and Mesozoic igneous rocks. Zone 2 (~ 26,400 km2) consists of rocks in the

Puna, Eastern Cordillera, and the western Santa Barbara Ranges. This zone

is drained by the Rıo San Francisco and its tributaries, which includes the

Rıo Grande, thus providing a pathway for sediment from the eastern Puna to

the RBM (Fig. 3). Parent lithologies in Zone 2 consist of Cenozoic rocks and

recent alluvium (dominantly Oran Group), Paleozoic marine sedimentary

rocks (Unchime Formation), and minor amounts of Mesozoic continental

sedimentary rocks (Salta Group) and Proterozoic–lower Cambrian meta-

sedimentary rocks (Puncoviscana Formation). Zone 3 is ~ 14,800 km2 and

consists dominantly of Mesozoic Salta Group sedimentary rocks that make

up the Santa Barbara Ranges and Cenozoic alluvium. This zone is drained

by rivers of the eastern Santa Barbara foothills, including the rıos Dorado

and del Valle. Zone 4 denotes samples from sand bars in the Rıo Bermejo

where it occurs in the Chaco foreland, as well as samples from small spring-

fed channels in the back-bulge.

Petrography.—Modern sands from the hinterland and foreland are

enriched in quartz and lithic grains and deficient in feldspar (Fig. 8). A

FIG. 6.—Landsat images of the Chaco at the latitude of the Rıo Bermejo megafan.

A) The Rıo Bermejo in the Chaco foredeep. The Rıo Bermejo intersects the north-

flowing Rıo San Francisco near Embarcacion (marked with a yellow star). Here, the

river is wide and exhibits a braided morphology at low flow. The sandy well-drained

floodplain is marked by xeric vegetation. This is the most cultivated area of the

megafan, but population density is relatively low. B) The Rıo Bermejo narrows and

becomes highly sinuous ~ 160 km from the megafan apex. Note the density of

paleo-channel features (white/tan) adjacent to the Rıo Bermejo. C) The narrow,

sinuous Rıo Bermejo in the back-bulge. The floodplain is waterlogged and marked

by spring-fed rivers (SFR) with vegetated (palm savanna) levees.


suite of commonly recurring grain types was encountered, including: (1)

monocrystalline quartz; (2) low-grade metamorphic lithic grains, including

metasiltstone and schist; (3) polycrystalline quartz, including chert; (4)

sedimentary lithic grains; (5) plagioclase feldspar; and (6) potassium

feldspar (Fig. 9). The sands of Zone 1 can be generalized as sublitharenites

(average %QFL¼64\5\31), with samples from proximal stations in the Rıo

Bermejo headwaters trending into the litharenite class due to abundant

mudrock clasts. The majority of sands from Zone 2 are likewise

sublitharenites (average %QFL ¼ 57\8\35), although the data exhibit

broader spread on QtFL diagrams, with more numerous litharenites. The

sands of zones 3 and 4 (average %QFL ¼ 77\5\18 and 77\6\17,

respectively) are mature (quartzose) sublitharenites. Feldspars are minor

throughout, but slightly enriched in plagioclase relative to potassium

feldspar. Zone 3 contains a more heterogeneous lithic sedimentary suite

(average %LmLvLs¼25\7\68) that includes lithic carbonate grains (ooids,

mollusk hash from Cretaceous Salta Group lake beds). By contrast, lithic

sedimentary grains in the other zones are less abundant (36–43%). The

QFL and QmFLt compositions almost exclusively fall within the recycled-

orogen provenance type of Dickinson et al. (1983). Sands from zones 1 and

2 exhibit significant scatter on the QmFLt ternary, encompassing the lithic,

transitional, and quartzose recycled fields, whereas sands from zones 3 and

4 are more uniformly quartzose (Fig. 8).

Particle Size of Rıo Bermejo Sand

Figure 10 illustrates particle-size distributions of Rıo Bermejo point-bar

sands from the wedgetop, foredeep, forebulge, and back-bulge depozones,

as well as a single sample from the river’s headwaters in the Eastern

Cordillera. The sands from the foreland exhibit unimodal size distributions,

whereas the headwater sample is bimodal and coarser. A general west-to-

east pattern of fining is recorded throughout the dataset. The mean particle

of size of samples from the hinterland and wedgetop is ~ 280 lm

(maximum D90 ¼ 1118 lm). The mean particle size of samples from the

foredeep is fine-grained sand (~ 195 lm; maximum D90¼ 375 lm). Sand

particle size is slightly finer where the Rıo Bermejo becomes highly

sinuous (D90¼ 285 lm), whereas upstream samples in the braided section

are coarser (D90 . 341 lm). Values of weight-percent silt plus clay are

relatively low in both the wedgetop and foredeep (� 10%). Samples from

the forebulge and back-bulge are in the very-fine-sand class, with mean

particle of sizes of ~ 88 and 102 lm, respectively (maximum D90 ¼ 179

and 261 lm). In both of the distal depozones, mean values of silt plus clay

content are 25–30%.

Rıo Bermejo Detrital Zircon U-Pb Geochronology

The dominant zircon U-Pb age population in the dataset is ~ 490–690

Ma, which is present in the Rıo Bermejo system from its headwaters in the

Eastern Cordillera to the back-bulge (Fig. 11). Moving downstream, a

second broader U-Pb age population spanning ~ 960–1290 Ma becomes

increasingly well-defined, particularly in the foredeep and back-bulge. The

youngest robustly defined U-Pb age population in the Rıo Bermejo

headwaters is Coniacian (~ 86–89 Ma). Very few Cenozoic zircons are

present in the data, with the exception of the most distal back-bulge

sample, which contains a Miocene peak defined by several zircons ~ 23–8

Ma (Fig. 11).

Clay Mineralogy

Typical glycol-solvated X-ray diffraction patterns from oriented, , 2 lm

Rıo Bermejo suspended sediments indicate the presence of illite, chlorite,

kaolinite, and smectite. Illite is the dominant clay mineral, and it exceeds

50% in all samples from the southern Chaco foreland and the hinterland

(Fig. 12). The highest abundance of illite occurs in the wedgetop, where

values routinely exceed 60%. In the foredeep, illite abundance decreases

and smectite increases significantly, often exceeding 30%. Abundances of

chlorite and kaolinite are low, and do not exceed 15% anywhere in the

foreland or hinterland.

INTERPRETATIONS AND DISCUSSION

The Overfilled Southern Chaco Foreland

Low relative accommodation coupled with elevated erosion rates

indicates that the southern Chaco is best classified as an overfilled retroarc

foreland basin (Jordan 1995; Pelletier 2007; DeCelles 2012). Basin

overfilling is expressed by the hydrologic connectivity between the Rıo

Bermejo and the Rıo Paraguay; satellite imagery and residual-elevation

data make clear that the Rıo Bermejo does not debouch into a single large

distributary fan in the Chaco. This hydrologic connectivity suggests that a

combination of both upstream (climate, tectonism in the Andean

hinterland) and downstream (Rıo Paraguay stage height, level of incision)

controls could influence patterns of fluvial sedimentation (Holbrook et al.

2006). If depositional models for retroarc foreland basins developed from

ancient strata are broadly applicable (DeCelles and Giles 1999), then

lithofacies data from modern systems should follow predictable trends in

individual depozones partitioned by the flexural wave. DeCelles (2012)

provides a useful summary of stratal stacking patterns in ancient flexural

forelands produced by convergence. Briefly, an idealized vertical section of

continental strata controlled by migration of a flexural profile, at the

kilometer-scale, can be expected to coarsen upward from lowland fluvial–

lacustrine back-bulge deposits through a thin, time-rich forebulge

(potentially marked by paleosols or unconformities), into thick, coarser

fluvial megafan and alluvial strata of the foredeep and wedgetop,

respectively. The modern snapshot of continental sedimentation captured

in the RBM illustrates some aspects of this model, especially with respect

to arrangement of depositional environments and down-dip particle size

trends (Fig. 13). For example, narrowing of the Rıo Bermejo mainstem and

a decrease in mean particle-size mark the transition from the foredeep into

FIG. 7.—Surface areas and bedrock compositions of hinterland sediment source

zones 1–3. The Chaco hinterland covers ~ 70,000 km2. See text for details.


the forebulge region, which is broadly consistent with foreland model

predictions. In the Chaco Seco, the wide and straight Rıo Bermejo channel,

amalgamated paleochannels, and a paucity of fine-grained floodplain is

strongly consistent with accumulation of upward-coarsening and

-thickening megafan deposits in ancient foredeep depozones, such as

those observed in Bolivia and Argentina (DeCelles and Horton 2003;

DeCelles et al. 2011). However, the RBM lacks substantial topography in

the forebulge region and extensive paleosols were not encountered in our

surveys, which represent a departure from the conceptual model of an

underfilled foreland partitioned by flexure. In ancient continental

forelands, identification of the forebulge is critical for establishing the

presence and history of a flexural basin. For example, DeCelles et al.

(2011) interpreted Eocene paleosols as markers of forebulge position in the

Andes. These ancient soils were found stacked among thin channel and

overbank deposits in discrete stratal packages (i.e., ‘‘forebulge supersols;’’

DeCelles and Horton 2003; DeCelles et al. 2011). In the modern RBM,

soil types do not align precisely with the modeled position of the

forebulge, though the inception of luvic kastanozems occurs near the

transition to the back-bulge (Figs. 1, 2). We interpret the apparent absence

of paleosol development along the RBM forebulge to be a consequence of

progradation of distributary fluvial channels across remnants of flexural

topography; megafans in other overfilled retroarc forelands may share

similar characteristics. Therefore, accurate interpretation of ancient

overfilled foreland strata will necessarily rely on preservation of back-

bulge deposits. From a large-scale perspective, interaction between the

eastward-flowing Rıo Bermejo with the orogen-parallel, south-flowing Rıo

Paraguay more than 700 km from the Andes validates predictions for

variable back-bulge paleocurrents made from ancient flexural foreland

strata (DeCelles and Horton 2003).

The Modern Forebulge and Back-Bulge

If forebulge topography had acted as a hydrological sill in the recent

past, westward-directed paleochannel networks might be expected on the

RBM ~ 130–200 m a.s.l.; no such features have been identified in the

residual elevation or Landsat data. In fact, the eastward expanding paleo-

channel lobes evident on satellite data all suggest flow from the Andes,

which is consistent with an overfilled foreland basin (e.g., Flemings and

Jordan 1989) and subdued forebulge topography in the late Quaternary.

The Rıo Bermejo channel varies in the distal depozones. Discharge,

sediment load, and active tectonic deformation exert important controls on

the valley-floor slope of alluvial rivers, which could result in a change in

channel morphology (Schumm 1986). Holbrook and Schumm (1999)

noted that variability in meander sinuosity could arise as river channels

adjust to different valley gradients. A significant transition in Rıo Bermejo

FIG. 8.—Ternary diagrams for modern sand composition from zones 1–4, with provenance fields. Although compositional maturity increases towards the back-bulge, the

weathering of Puncoviscana Fm. provides adequate lithic metamorphic grains to yield sublitharenites throughout the megafan.


sinuosity occurs around the transition from the modeled forebulge into the

back-bulge (Chase et al. 2009). Across the crest of the forebulge (~ 360–

600 km from the apex), average sinuosity of the Bermejo mainstem is the

lowest of any location in the system. However, along the transition into the

back-bulge depozone (~ 610 km from the apex), Rıo Bermejo sinuosity

increases markedly (Fig. 4). Our elevation data lack the resolution to

validate whether or not flexural effects on slope control this sinuosity

change, yet the position of this transition appears to support both the model

solutions of Chase et al. (2009) and the gravity data of Horton and

DeCelles (1997) for the approximate position of the eastern forebulge

margin. Intriguingly, the influence of a buried forebulge on river gradient

was interpreted for the Beni River (Bolivian foreland), based on differential

GPS measurements (Aalto et al. 2003), and a similar approach should be

considered for future research on the RBM. Alternatively, this change in

channel pattern may have resulted from reduced discharge, associated with

either greater evaporation or infiltration through floodplain soils. These

processes are likely to have influenced the Rıo Bermejo much farther to the

west, where: (1) the channel belt narrows in the foredeep; and (2) the

channel habit transitions from braided to meandering (Fig. 4).

Our residual-elevation data support the interpretation of greater

accommodation in the back-bulge versus the forebulge, which is well

represented by the elevation distribution of topographic closure (ponds).

Continental back-bulges are common sites for low-gradient fluvial and

lacustrine deposition in both modern and ancient retroarc forelands

(Demko et al. 2004; DeCelles et al. 2011; McGlue et al. 2011). In some

areas of the Andean foreland, lake formation has been linked to wind

erosion during intervals of aridification (Iriondo 1993; Latrubesse et al.

2012; May 2013; Cohen et al. 2015). Unlike many of those lakes, the back-

bulge ponds of the RBM: (1) contain hard cohesive bottoms, (2) frequently

support rooted emergent vegetation, and (3) exhibit indistinct shorelines.

Instead of thick green pond facies, cores from these ponds commonly

recovered crumbled, organic-rich soils. As a consequence, these ponds may

best be described as lowland subaqueous soils (cf. Demas et al. 1996;

Demas and Rabenhorst 1999). Hartley et al. (2013) noted longitudinal

transitions in soil types along the RBM, with an inferred spring line and

increase in soil moisture ~ 200 km west of the Rio Paraguay, a location

which appears to sit east of the modeled eastern forebulge margin. Narrow

spring-fed channels are prominent on the residual-elevation map in the

back-bulge (Fig. 5), and groundwater discharge into available accommo-

dation in this region help to explain the density of ponds in the RBM back-

bulge.

FIG. 9.—Photomicrographs (crossed polars, 103) of common framework grains from the Rıo Bermejo system. Scale bar is 100 lm and applies to all photos. A)

Monocrystalline quartz. B) Plagioclase feldspar. C) Potassium feldspar. D) Lithic metamorphic grain (metasiltstone). E) Lithic metamorphic grain (metasediment). F) Lithic

metamorphic grain (schist). G) Lithic volcanic grain. H) Polycrystalline quartz. I) Lithic sedimentary grain from Zone 3 sands (likely lacustrine ooid).


Sediment Provenance

The results of Gazzi-Dickinson point counts demonstrate that: (1)

samples from both the RBM and its hinterland fall within the recycled

orogen provenance field, (2) compositional maturity increases slightly with

distance from the thrust front, and (3) in spite of environmental differences

among the hinterland zones, the composition of sand in the RBM is largely

homogenized and set near the apex (Fig. 8). Overlap of foreland and

hinterland composition in part reflects multiple phases of recycling, an

interpretation in accord with sand provenance findings from parts of the

Amazon and Madre de Dios basins (Franzinelli and Potter 1983; DeCelles

and Hertel 1989). This comes as no surprise, since the Paleozoic and

Mesozoic lithologies of zones 1 and 3 are themselves deformed and

uplifted conglomerates, sandstones, and mudrocks. Thus, classic indicators

of multiple cycles of erosion such as syntaxial quartz overgrowths and

sedimentary lithic grains were found in the RBM sands (Fig. 9; Johnsson et

al. 1988). Extensive overlap in QtFL space among the sands of zones 1, 3,

and 4 suggests that hinterland lithology exerts the strongest control on

foreland-basin sand character. This interpretation is consistent with the

predictions of sand composition generated by the probabilistic model of

Heins and Kairo (2007). Many Zone 2 sands plot within the transitional

recycled field, due to relative enrichment in lithic metamorphic grains (Fig.

9). The origin of this grain assemblage is linked to weathering of

Puncoviscana Fm. outcrops along the eastern margin of the Puna. The

steep slopes, thin vegetation, poorly developed soil cover (fluvisols and

luvisols), and flashy seasonal discharge could lead to preferential erosion

of Puncoviscana Fm. rocks, but the maturity trend exhibited by Zone 2

sands indicates that these grains are likely less stable under transport

distances of several hundred kilometers. Sands from Zone 2 rivers exhibit

the lowest average mineralogical maturity (Q/[F þ L]) across our dataset,

but close examination of these data shows that sands from the Rıo San

Francisco trunk are similar in maturity to Rıo Bermejo sands from Zone 4.

Nonetheless, contributions of Puna-derived lithic metamorphic grains

distinguishes southern Chaco sands from Plio-Pleistocene foreland basin

deposits (Emborozu and Guandacay Fm.) from the central Chaco, which

exhibit similar quartz and feldspar content but greater volcanic lithic

content (Hulka and Heubeck 2010). This relationship is likewise true for

modern sands of the Amazon foreland, as plutonic rocks are more

widespread in the hinterland of Peru and northern Bolivia (DeCelles and

Hertel 1989).

Rıo Bermejo detrital U-Pb zircon ages reflect erosion of: (1) Ordovician

sediments and Precambrian metasediments in the Eastern Cordillera, (2)

Devonian sediments in the Inter-Andean fold-and-thrust belt, and (3)

Carboniferous, Permian, and Cretaceous sediments of the Sub-Andean

wedgetop (Figs. 2, 11). The dominant zircon U-Pb age populations across

the Bermejo megafan are 490–690 Ma and 960–1290 Ma, which are

known to reflect the Famatinian and Ocloyic orogenies that helped

construct South America (Ramos et al. 1986). Cretaceous zircon U-Pb ages

that capture evidence of Salta rifting are present in sand from the Rıo

Bermejo headwaters, which were most likely derived from erosion of small

igneous outcrops in southern Bolivia (Marquillas et al. 2005). Miocene

zircon U-Pb ages (~ 23–9 Ma) present in the back-bulge must represent

more recent volcanism. Multiple well-dated tuffs have been documented

within Miocene–Pliocene Andean foreland basin deposits (Uba et al. 2009;

Carrapa et al. 2012) and may share a genetic heritage with the young

zircons of the southern Chaco back-bulge.

FIG. 10.—Particle-size distributions for sands collected from the Rıo Bermejo in

the four depozones of the foreland. Note the tails present on distributions from the

forebulge and back-bulge, signifying higher silt and clay content. A) Wedgetop

depozone. B) Foredeep depozone. C) Forebulge depozone. D) Back-bulge depozone.


The abundance of illite across the RBM is typical of Andean-derived

immature clays (Guyot et al. 2007), but the increased smectite

concentration in the foredeep likely reflects a combination of weathering

reactions, the composition of floodplain soils (including reworked loess),

and the dry climate of the Chaco Seco (Fig. 12). As pointed out by

Johnsson and Meade (1990), lengthy residence times are not uncommon

for lowland floodplains adjacent to the Andes, and seasonal chemical

weathering during storage can alter primary mineralogy. A potential source

of smectite is through alteration of Zone 1 and 2 feldspars (combined mean

%QtFL¼59\7\34) with minor contributions from weathered lithic volcanic

grains (combined mean %LmLvLs ¼ 55\6\39) stored in paleochannels.

Minor pitting is observed on some plagioclase grains in thin section, which

FIG. 11.—Relative-probability detrital-zircon

U-Pb age spectra for samples collected on a

downsteam transect on the Rıo Bermejo, from the

hinterland (samples A and B) to the confluence

with the Rıo Paraguay (samples C, D, and E).

Note the dominance of 490–690 Ma and 960–

1290 Ma age populations, which reflect the

Famatinian and Ocloyic orogenies. The sample

from the Rıo Bermejo headwaters (HW) in

hinterland (sample A) contain Cretaceous-aged

zircons, reflecting contributions for Salta Rift-

related volcanic rocks in southern Bolivia.


supports this origin (Fig. 9). This material may have been reintroduced into

the Bermejo suspended load through scavenging of the floodplain by

floods in the austral summer or by winds during the semiarid winter.

Sedimentary System of the Rıo Bermejo Megafan

Importantly, over 105–107 yr, the apex of a foreland basin megafan is not

fixed in its location. Rather, depozones and their associated depositional

environments migrate towards the underthrusted craton with the flexural

wave, thus resulting in the characteristic vertical stacking of foreland

depozone strata (DeCelles and Giles 1999). However, at shorter time

scales, variability in the ratio of accommodation to sediment supply could

result in basinward progradation of proximal zones of these distributive

fluvial systems (DFS; Weissmann et al. 2013). This is particularly true in

overfilled foreland settings, where accommodation space is notionally

limited and avulsions may shift the active channel belt(s). Based on

planform geometry and the hydrologic configuration between the rıos

Bermejo and Bermejito (Fig. 13), the RBM closely resembles the single-

thread sinuous-anabranching distributive-fluvial-system (DFS) endmember

discussed by Davidson et al. (2013). Weissmann et al. (2013) present a

generalized model of progradational DFS based on image analysis of

modern rivers as well as case-study data from several ancient fluvial

FIG. 12.—Clay-mineralogy ternary diagram. Although illite is dominant in all

samples, only the foredeep contains significant smectite, which is attributable to

chemical weathering of the Chaco Seco floodplain.

FIG. 13.—Synthesis of sedimentary and geo-

morphic observations from the Rio Bermejo

megafan (RBM). Trends in textural and compo-

sitional maturity from the RBM share similarities

to Cenozoic foreland strata preserved elsewhere in

the Andean thrust belt (DeCelles and Horton

2003), despite basin overfilling in the extant

southern Chaco. Increasing levels of downstream

bifurcation, sinuosity, and channel narrowing are

reflected in maximum grain size and silt:clay

content of Bermejo sediments. Forebulge paleo-

sols are not present in the southern Chaco, which

is overridden and buried by medial-megafan

fluvial sediments and reworked loess. Ponds

(black) and flooded soils mark the back-bulge

depozone. See text for details.


successions. As very few comprehensive modern sedimentary datasets

exist for this type of sedimentary system, our data provide some context for

the model’s longitudinal predictions, with special relevance for subtropical

megafans in active retroarc foreland settings. In the RBM, depositional

environments in the wedgetop and proximal foredeep are relatively coarse-

grained (mean D90¼ 563 and 334 lm, respectively), highly amalgamated

channels with negligible floodplain deposits; such features are expected in

proximal settings of DFS, as this is typically a narrow zone of deposition

where sediment supply is rarely outpaced by subsidence (Weissmann et al.

2013). Transmission of water through these deposits (‘‘infiltration loss’’)

may indeed lead to channel narrowing as suggested by Weissmann et al.

(2013), and the active Bermejo channel captures dramatic evidence of a

transition from a wide, braided system to a much narrower, more sinuous

river (Fig. 4; Weissmann et al. 2015). It should also be noted that sand bars

become increasingly fine across this transition (Fig. 2).

The Weissmann et al. (2013) model depicts DFSs becoming wider in

their medial zones, and spacing between channels likewise increasing.

These predictions are consistent with the RBM across the foredeep and

portions of the forebulge depozone. We observe an avulsion node on the

right bank of the Rıo Bermejo and increased bifurcations of the Rıo

Bermejito channel in the forebulge region on the residual elevation dataset

(Fig. 5). Levee breaching and channel migration via avulsion in medial

positions are characteristics of many mapped DFSs globally, and help to

establish the width of the megafan (Weissmann et al. 2013). Progradation

of distributary channels into back-bulge areas of accommodation is

common on the RBM. Deposition in this distal setting occurs on fan lobes

by lateral accretion and on floodplains by vertical accretion; these

characteristics are shared by many distal DFSs (Weissmann et al. 2013).

In the Chaco back-bulge, low residual-elevation values and the widely

spaced channels apparent on satellite images suggest more significant

tracts of poorly drained floodplain than in the forebulge or the foredeep.

These overbank areas may be occupied by ponds or waterlogged savanna

soils (Figs. 2, 13). Channels in the Chaco back-bulge frequently produce a

strong contrast in residual-elevation values relative to the surrounding

floodplain, which we interpret to reflect vegetated levees supported by

nutrient-rich, fine-grained sediments (Fig. 5). A number of these channels

are discrete features that are disconnected from distributary channel

networks that originate up dip. These channels intersect the Rıo Paraguay

at right angles and can reach lengths that exceed 60 km (Fig. 6). Invariably,

these channels are shallow and filled with very fine grained sandy or silty

muds. Thus, the prediction of isolated ribbon sands encased in floodplain

mud as a common distal DFS facies type is validated in the RBM system.

Similar perennial channels have been observed on the Kosi megfan and

have been linked to springs (Chakraborty et al. 2010).

CONCLUSIONS

This study provides the first comprehensive sedimentary systems

analysis of the Rıo Bermejo Megafan (~ 71,000 km2) that integrates

geomorphological observations with modern provenance insights from

both coarse-grained and fine-grained sediments. The southern Chaco is

overfilled, and therefore the surficial expression of depozones differs from

underfilled models for foreland basin systems. The Rıo Bermejo fully

traverses the foreland and links with the south-flowing Rıo Paraguay,

which is consistent with a buried forebulge that is not presently acting as

hydrological sill or barrier. As an active depositional tract, this depozone

does not appear to be marked by extensive paleosol development. Megafan

sediments plus reworked loess bury any topographic expression of the

forebulge, such that its position can be inferred only using data from geoid

anomalies and flexure data. Using the modeling insights of Chase et al.

(2009) to mark the relative position of depozone boundaries, our study

defines an end-member snapshot of a distributive fluvial system in an

overfilled retroarc foreland.

Significant geomorphological variability exists on the Bermejo mega-

fan, and remote-sensing imagery plus trend-surface analysis highlights

areas with variability in channel patterns and floodplain landforms. The

Rıo Bermejo is straight, wide, and entrenched near the megafan apex, in

the wedgetop depozone. The river’s sinuosity increases markedly ~ 160

km from the megafan apex, a pattern consistent with decreasing gradient,

stream power, and grain size moving into the foredeep depozone.

Expansion of the megafan, progressive narrowing of the Rıo Bermejo

mainstem, and higher sinuosity characterize the distal back-bulge, as well

as the presence of numerous spring-fed plains rivers that flow

approximately parallel to the Rıo Bermejo. Trend-surface analysis and

residual-elevation data indicate that accommodation marked by ponds

increases substantially at 75–85 m.a.s.l., which is consistent with model

predictions for a back-bulge depozone.

Sediment and water is routed to the Bermejo megafan from a complex

hinterland region (~ 70,000 km2) that varies along strike in terms of its

basement geology, style of faulting, topography, annual precipitation, and

vegetation. Sand generated in the hinterland plots in the recycled orogen

field on QtFL ternary diagrams (average %QtFL ¼ 60\7\33), suggesting

that basement geology exerts the main control on the composition of

foreland-basin sediments, which is also reflected in U-Pb detrital-zircon

ages.

Weathering and abrasion associated with long-distance transport from

the hinterland results in sands with higher compositional maturity in the

megafan itself (average %QtFL ¼ 77\6\17). Textural maturity is likewise

conditioned by transport in the hinterland, as particle sizes near the

megafan apex in the wedgetop are similar to fine sands in the foredeep. A

downstream decline in particle size characterizes the megafan, which

results in deposition of very fine sand (mean of ~ 90 lm) with abundant

silt and clay across the distal forebulge and back-bulge.

Detrital clays in the Rıo Bermejo consist of illite, smectite, chlorite, and

kaolinite (in rank-order abundance). Although illite dominates in this

system, smectite increases markedly in Rıo Bermejo sediments in the

semiarid region of the foredeep, suggesting that locally derived clays from

concentrated soils enter the system during floods or by wind. The clay

mineralogy of the foredeep suggests that both infiltration and evaporation

influence this depozone, both of which have implications for variability in

channel patterns in the proximal megafan environment.

ACKNOWLEDGMENTS

We are grateful to ExxonMobil for funding the COSA Project at the

University of Arizona. Fieldwork logistics in Argentina and Bolivia were

arranged by J. Omarini. We thank J. Lucas for generating the particle-size data.

This work benefited from discussions with P. DeCelles, G. Weissmann, C.

Chase, C. Gans, and W. Benzel. J. Nickou produced the graphic designs

throughout this manuscript. Comments from two reviewers, the associate editor,

and corresponding editor were greatly appreciated.

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Received 10 May 2016; accepted 11 October 2016.


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