BasinResearch Plio

Plio-Quaternary sediment budget between thrust belterosion and foreland deposition in the central Andes,southern BoliviaJ. B. Barnesn,w and W. A. Heinsn

nExxonMobil UpstreamResearch Co., Houston,TX, USAwDepartment of Geological Sciences, University of Michigan, AnnArbor, MI, USA

ABSTRACT

Estimates of the physical boundary conditions on sediment source and sink regions and the £uxbetween them provide insights into the evolution of topography and associated sedimentary basins.We present a regional-scale, Plio-Quaternary to recent sediment budget analysis of the Grande,Parapeti and Pilcomayo drainages of the central Andean fold-thrust belt and related deposits in theChaco foreland of southern Bolivia (18^231S).We constrain source-sink dimensions, £uxes and theirerrors with topographic maps, satellite imagery, a hydrologically conditioned digital elevation model,reconstructions of the San Juan del Oro (SJDO) erosion surface, foreland sediment isopachs andestimated denudation rates.Modern drainages range from 7453 to 86798 km2 for a total source area of153 632 km2. Palaeo-drainage areas range from 9336 to 52 620 km2 and total 100 706 km2, suggestingbasin source area growth of �50% since �10Ma. About 2.4^3.1� 104 km3 were excavated frombelow the SJDO surface since �3Ma.The modern foredeep is132 080 km2with £uvial megafan areasand volumes ranging from 6142 to 22511km2 and from1511 to 3332 km3, respectively. Since Emborozu¤Formation deposition beginning 2.1� 0.2Ma, the foreland has a ¢ll of �6.4� 104 km3.The volumeand rate of deposition require that at least �40^60%of additional sediment be supplied beyond thatincised from below the SJDO.The data also place a lower limit of � 0.2mmyear�1 (perhaps� 0.4mmyear�1) on the time- and space-averaged source area denudation rate since �2^3Ma.These rates are within the median range measured for the Neogene, but are up to 2 orders ofmagnitude higher than some observations, as well as analytic solutions for basin topography andstratigraphy using a two-dimensional mathematical model of foreland basin evolution. Source-to-sink sediment budget analyses and associated interpretations must explicitly and quantitativelyreconcile all available area, volume and rate observations because of their inherent imprecision andthe potential for magni¢cation when they are convolved.

INTRODUCTION

The sediment-routing system links sources to sinks, de-termining how mountains erode, how topography evolves,and how landscapes translate into the sedimentary record(Allen, 2008). Sediment sources and sinks are coupledthrough various surface processes and their £uxes to theextent that mountain belt deformation can be in£uencedby deposition downstream (e.g. Flemings & Jordan, 1989;Beaumont et al., 2000; Simpson, 2006). Unfortunately,questions remain aboutwhat combination of factors in£u-ence the volume and rate of sediment production, the spa-tial variability of sediment production within the source,and the rate of sediment delivery to the sink (Tucker &Slingerland, 1996; Stock et al., 2006; Phillips & Gomez,

2007). Sediment delivery rates are a particularly importantcontrol on the dimensions and physical characteristics ofbasin- ¢lling sediments (Hovius & Leeder, 1998). If esti-mates of the volume and mass £ux (among other things)from the source area are available, then quantitative toolscan be used to predict sedimentary architecture (Robinson&Slingerland,1998a,b;Geslin etal., 2001, 2002;Clevis, 2003;Clevis et al., 2003; Van Wagoner et al., 2003; Overeem et al.,2005; Robin etal., 2005) and reservoir quality (Lander &Wal-derhaug, 1999; Perez et al., 1999; Bray et al., 2000; Bonnell &Lander, 2003) in sedimentary basins.

A mass balance approach has been used to quantify se-diment budgets for the Alps, Appalachians, HimalayasandRockyMountains by integrating river sediment loads,palaeogeographic reconstructions, seismic data and thestratigraphic record (Hay et al., 1992; Le Pichon et al., 1992;Curray,1994; Einsele etal.,1996; Pazzaglia&Brandon,1996;Kuhlemann et al., 2001, 2002; Schlunegger et al., 2001;Clift et al., 2002; Clift, 2006; McMillan et al., 2006).These

Correspondence: Jason Barnes, Department of GeologicalSciences, University of Michigan, 1100 N. University, Ann Arbor,MI48109^1005,USA. E-mail: [email protected].

BasinResearch (2009) 21, 91–109, doi: 10.1111/j.1365-2117.2008.00372.x

r 2009 The AuthorsJournal Compilationr Blackwell Publishing Ltd, European Association ofGeoscientists & Engineers and International Association of Sedimentologists

91

sediment budgets provide some of the best constraints forinferring mountain palaeotopography and estimating de-nudation rates, but uncertainties are often large and/ornot quanti¢ed because of the scales over which they areapplied.

Active fold-thrust belts and their foreland basin systemsare sources and sinks closely linked in space and time thatpossess a variety of evidence that can be used to constraintheir sediment budget (Fig. 1) (DeCelles & Giles, 1996;Critelli, 1999; Critelli et al., 2003). For example, manythrust belts have palaeosurfaces, formed by periods ofprotracted erosion (Widdowson, 1997), that have beenused as markers to (a) estimate uplift magnitudes (de Sitter,1952; Epis & Chapin, 1975; Scott, 1975; Kennan, 2000;Barke & Lamb, 2006), (b) estimate exhumation magni-tudes (Sobel & Strecker, 2003; Babault et al., 2005;McMil-lan et al., 2006), (c) reconstruct palaeo-drainage networks(Kennan et al., 1997; Kennan, 2000), (d) constrain the de-formation history (Gubbels et al., 1993; Clark et al., 2006)and (e) calculate the amount of material removed from be-low the surface by post-formation incision (Kennan et al.,1997;McMillan et al., 2006). In the sink, £exure associatedwith the adjacent topographic load creates a foreland basinconsisting ofwedgetop, foredeep, forebulge and backbulgedepozones (DeCelles & Giles, 1996). Fluvial megafans (ty-pically 103^105 km2, with low gradients of 0.01^0.11) aredistinguishable sediment bodies that can be dominant fea-tures of some forelands (Gohain & Parkash, 1990; Gupta,1997; DeCelles & Cavazza, 1999; Leier et al., 2005). Addi-tionally, isopach maps constructed from measured sec-tions, geochronology, seismic data, and well logs provideconstraints on the spatio-temporal distribution of theforeland- ¢lling sediments (e.g.Uba etal., 2006).This fore-land sedimentary record is shaped by thrust belt topogra-phy, tectonics, climate, erosion, lithology, drainagepatterns and base level (Dickinson, 1974; Flemings & Jor-dan, 1989; Damanti, 1993; Devlin et al., 1993; Patterson

etal., 1995;VanWagoner, 1995; Burgess &Allen,1996;Tuck-er & Slingerland, 1996; Schlunegger et al., 1997; Leeder etal., 1998; Geslin et al., 2002). Although prior studies havecharacterized sediment source and sink dimensions anddetermined erosion rates, few attempts have been made toquantify regional- scale sediment budgets and associateduncertainties in thrust belt-foreland settings.

The goal of this paper is to quantify the sediment bud-get for the central Andean fold-thrust belt and foreland insouthern Bolivia since the Plio-Quaternary ( �3^0Ma).We account for the area, volume and rates of sediment re-moved from the upland sources and deposited within thedownstream sink, speci¢cally £uvial megafans and theforedeep. The following logic governs our analysis. Theamount of sediment produced must fall within limits im-posed by the size of the drainage, the rate and duration ofdenudation, and the volume of deposited sediment. Theamount of sediment generated must be at least as great asthe amount of sediment deposited in the proximal fore-deep.The generated sediment cannot be greater than theamount denuded from the present-day drainage at themaximum estimated rate of denudation over the longestpossible denudation time. This lower sediment-produc-tion limit excludes some combinations of size, rate andduration placing improved constraints on the large rangeof denudation rates estimated.

WHY SOUTHERN BOLIVIA?

The central Andean fold-thrust belt and Chaco forelandof southern Bolivia (18^231S) is well suited for quantifyinga Plio-Quaternary sediment budget (Fig. 2). Fluvial mega-fans have been important foreland depositional featuressince the mid-Tertiary and currently occupy most of theChaco plain (Horton &DeCelles, 2001). Isopachs quantifythe spatial and temporal distribution of the Chaco sedi-

T1

T1 T1

T2

T2

T3

T3

T1

T2

T2

T3

T3

sedimentdelivery sediment

deposition

SOURCE SINK

km3

km3/Makm3

km3

km3

km3

foredeep

wedgetop

forebulgebackbulge

fold-thrust belt foreland basin

Fig.1. Schematic thrust belt-foreland basin system sediment budget in cross- section. Eroded anddeposited sedimentvolumes (grey towhite shaded regions and boxes) for time slicesT1 T̂3 (increasing to the present) from a thrust belt hinterland source to an adjacentforedeep sink, respectively.The hinterland topographic evolution from ancient (dashed) to modern (solid) time and the equivalent sinkforeland sedimentary evolution are also shown. In this ideal case, boxesT1 T̂3 in the source are the same size as the equivalent boxes inthe sink. For simplicity, no thrust belt propagation is shown.

r 2009 TheAuthorsJournal Compilationr Blackwell Publishing Ltd, EuropeanAssociation of Geoscientists & Engineers and International Association of Sedimentologists

92

J. B. Barnes andW. A. Heins

ments since the late Oligocene (Uba et al., 2006). Recon-structions of the widespread late Miocene San Juan delOro (SJDO) erosion surface provide an unusual constrainton timing and volume of thrust belt erosion (Servant et al.,1989; Gubbels, 1993; Gubbels et al., 1993; Kennan et al.,1997). Finally, source region erosion rates have been esti-mated across multiple spatial and temporal scales (e.g.Barnes & Pelletier, 2006 and references therein).

GEOLOGIC SETTING

Crustal shortening associated with Cenozoic Andeanmountain building has resulted in a retroarc plateau,fold-thrust belt, and foreland basin system in westernBolivia (Fig. 2) (Jordan&Alonso,1987; Isacks,1988; Jordan,1995; Kley, 1996, 1999; Allmendinger et al., 1997; Horton &DeCelles, 1997; Jordan et al., 1997; McQuarrie, 2002; De-Celles &Horton, 2003; McQuarrie et al., 2005).The domi-nantly east-vergent fold-thrust belt steps down instructural and topographic elevation from the Altiplanoto the Eastern Cordillera, Interandean zone, Subandesand Beni/Chaco plains (Kley, 1996; McQuarrie, 2002).Rocks involved in the deformation range from Palaeozoicmarine siliciclastics to Mesozoic non-marine clastics andCenozoic synorogenic deposits (McQuarrie, 2002 and re-ferences therein). In southern Bolivia, the fold-thrust beltis £anked on the west by the Altiplano basin and on theeast by the Chaco plain (Fig. 2). The Altiplano is a low-

relief, internally drained, intermontane depression (e.g.Placzek et al., 2006). The Chaco plain is a low-relief, low-elevation slope thought to be the aggradational surface ofthe wedge-top and foredeep depozones of the modernforeland (Horton &DeCelles, 1997).The thrust belt is tra-versed by three large rivers, the R|¤ o Grande (or Guapay),R|¤ o Parapeti and R|¤ o Pilcomayo, which form £uvial mega-fans in theChaco (Fig. 2b) (Horton&DeCelles, 2001).Therelatively straight river courses across the Subandes sug-gest the rivers are antecedent from the late Miocene andhence the source drainages somewhat long lived. Megafanapexes begin at the frontal-most Subandes structure im-plying a more recent origin (Fig. 2b) (Horton & DeCelles,2001).

Timing of initial thrust belt deformation ranges fromlate Eocene to lateOligocene ( �27^40Ma) with deforma-tion concentrated in the Subandes since the early to lateMiocene ( �10^20Ma) (Elger et al., 2005; McQuarrieet al., 2005, 2008; Ege et al., 2007; Barnes et al., 2008). Sedi-ment deposition in the Chaco foreland commenced withthe late Oligocene Petaca Formation and continues todaywith the Emborozu¤ Formation (Uba et al. 2006). Structur-al, stratigraphic and geophysical data from southernBolivia constrain the regionalNeogene evolution, particu-larly in the Subandes (Baby et al., 1992, 1995; Dunnet al., 1995; Roeder & Chamberlain, 1995; Kley, 1996, 1999;Moretti et al., 1996; Mˇller et al., 2002; Uba et al., 2005)and Chaco (Marshall et al., 1993; Hulka et al., 2006; Ubaet al., 2006).

BENIPLAIN

Peru

LakeTiticaca

(a)

(b)

300 km

16°

14°

80°W

150 km

Pilcomayo Parapeti

Grande

1 2

ALWC EC IASA

Bolivia

Argentina

ChilePacificOcean CHACO

PLAIN

RVIZ

PA

Elev6000

0 m

70° 68° 66° 64° 62°

22°

20°

18°

10

30°S60

PACIFICOCEAN

PERU

ARG.

BOL.

BRAZIL

Fig. 2. Central Andean fold-thrust belt and Chaco foreland in Bolivia. (a) Topography (GTOPO30 1km) and major tectonic zones(modi¢ed fromMcQuarrie, 2002; Uba et al., 2006):WC,Western Cordillera; AL, Altiplano; EC, Eastern Cordillera; IA, Interandeanzone; SA, Subandes.Megafans are outlined in black. Inset shows location in west-central South America. (b) Satellite image of studyarea draped over topography (SRTM90m) showing the R|¤ o Grande, R|¤ o Parapeti and R|¤ o Pilcomayo channels (solid lines whereperennial, dashedwhere ephemeral), their drainage areas, and megafans. RV, RioViejo area; IZ, Izogog swamp; PA, Patino swamp; 1,white line representing the eastern basin edge of Pilcomayo1 (Table1); 2, black line representing the eastern basin edge of Pilcomayo 2(Table 1) at the megafan apex (see text and Supporting Information for discussion).


93

CentralAndes sediment budget

SAN JUAN DELORO SURFACE

Here, we summarize age constraints and reconstructionsof the SJDO erosion surface that we adopt to quantify thepalaeo-drainage morphology and sediment volume re-moved from below the surface by Plio-Quaternary inci-sion. The SJDO surface is identi¢ed by spatiallycorrelative, remnant, low-relief surfaces at ca. 2000^3800m elevations, which have been mapped throughoutthe Eastern Cordillera and Interandean zone of southernBolivia (Fig. 3).TheSJDO surface is a composite landformof (1) low-relief erosional uplands, (2) coalesced pedimentsand (3) an unconformity beneath undeformed Tertiary se-diments and ignimbrites that is the stratigraphic equiva-lent to surface types 1 and 2 (Fig. 4) (Servant et al., 1989;Gubbels et al., 1993; Kennan et al., 1995, 1997; Barke &Lamb, 2006). All surface types are subhorizontal, truncatedeformed bedrock, decrease in elevation eastward, and aresometimes mantled by sediments up to 250m thick withinter-bedded tu¡s and fossiliferous layers (Gubbels et al.,1993; Kennan et al., 1995, 1997). Surveying the surfaces,40Ar/39Ar dating of the tu¡s, and ages of mammalian fos-sils bracketing the unconformity, show that the age of theSJDO is time-transgressive from �12 to 3Ma with inci-sion beginning 3 � 1.5Ma (Gubbels, 1993; Gubbels et al.,1993; Kennan et al., 1995, 1997; Barke & Lamb, 2006).Thelack of deformation and a dominantly �10Ma age forthe SJDO surface suggests (a) cessation of deformation inthe Eastern Cordillera and its migration eastward into theSubandes, and (b) 1.1^2.5 km of surface uplift has occurredin the region since surface formation (Figs 2 and 3) (Gub-bels et al., 1993; Kennan et al., 1997; Barke & Lamb, 2006).

Two di¡erent models for SJDO surface formation char-acterize it as a pediment and palaeo-drainage base level,respectively. Gubbels and co-workers proposed a ‘cut and¢ll’ model for the SJDO surface whereby as deformationceased, aggradation and pediment development began(Fig. 4) (Gubbels,1993;Gubbels etal., 1993). Eventually, in-cision isolated the surface remnants. In this model, theSJDO surface slopes down to the east from �4.2 km ele-vation in the Eastern Cordillera to �3 km in the Interan-dean zone over �150km (see Fig. 2.33 of Gubbels, 1993).This model suggests a regional gradient of �0.461 andimplicitly allows that the SJDO pediment was not ubiqui-tous and that intervening highlands existed (Fig. 4). Ken-nan and coworkers proposed that the SJDO surfacerepresents the regional base level associated with two pa-laeo-drainage basins (Fig. 5a) (Kennan etal.,1997;Kennan,2000).This model suggests regional, upstream basin gra-dients of �0.461 that decrease to �0.23^0.271 in thedownstream reaches. In both models, the preserved extentof the SJDO surface represents the minimum size of thedrainage basin source area that supplied sediment to theforeland.

Key aspects of the SJDO surface relevant for quantify-ing a sediment budget include: (a) it formed �10Ma and(b) it experienced rapid incision at �3 � 1.5Ma (Gubbelset al., 1993; Kennan et al., 1995, 1997; Barke & Lamb, 2006).

68°W 66° 64° 62°23°S

21°

19°

17°200 km Elev

6000

0 m

Bolivia

Argentina

Potosi

Tupiza

Cochabamba

Fig. 3. Preserved remnants of the San Juan del Oro surface insouthern Bolivia. Remnant surfaces are mapped in white(simpli¢ed fromKennan et al., 1997) with the modern Grande,Parapeti andPilcomayo basins andmegafans outlined in black forcomparison.

IH

IH

IH

active deformation;relief generation

waning deformation;early aggradation

continued aggradation;initial surface formation

surface expansion;basin onlap

canyon incision

T1

T2

T3

T4

T5

12

3

~12 Ma

~10 Ma

~3 Ma - present

W E

Fig.4. Schematic of the cut and ¢ll model for San Juan del Orosurface evolution in cross- section (modi¢ed from Fig. 4.1ofGubbels,1993). Five time steps are shown from pre-Miocene (T1)to present (T5) with absolute ages indicatedwhere possible.Numbers1^3 inT4 indicate the three surface types as discussed inthe text. IH, intervening highlands;W, west; E, east.


94


Additionally, an estimated 1^2 � 104 km3 of material waseroded from the pre-existing topography above the pa-laeo-drainage base levels that together form the SJDOsurface (Kennan et al., 1997). All of this sediment was ap-parently transported out to the foreland (and possibly be-yond) because neither (1) adequate local sinks exist in theEC or IA to store the estimated sediment nor (2) any majordepositional hiatus exists in the Subandes source regionbetween 12 and 3Ma (Coudert et al., 1993; Kennan et al.,1997).

FORELAND SEDIMENTS

Isopachs constrain the spatio-temporal distribution ofOligocene to recent foredeep sediments in theChaco plain(Uba et al., 2006). The sedimentary unit most correlatedwith sediment exported from the thrust belt since thePlio-Quaternary is the Emborozu¤ Formation (Uba et al.,2006; their Fig. 15e).The Emborozu¤ Formation is the cur-rently depositing, sedimentary unit characterized by £u-vial megafan-dominated conglomerates inter-beddedwith sandstone and mudstone (Uba et al., 2005). A seismicN5 interval is equivalent to the Emborozu¤ Formation,which has a maximum thickness of �1500m at the moun-tain front and tapers rapidly eastward (Fig. 6) (Uba et al.,2006). Beginning of Emborozu¤ Formation deposition hasbeen variously estimated at 3.3Ma (Moretti et al., 1996),

2.1� 0.2Ma (Hulka, 2005) and 1.8Ma (Echavarria et al.,2003). The basal age of 2.1 � 0.2Ma is preferred (Ubaet al., 2006) because it agrees with the 1.8Ma documentedcorrelative strata in Argentina by Echavarria et al. (2003).

METHODS

We account for the sediment budget in the Andean fold-thrust belt and Chaco foreland across a range of scales:spatially, from the drainage basin and megafan to entirehinterland drainage and proximal foredeep; and tempo-rally, from recent to the late Miocene^Pliocene. Here, webrie£y outline the datasets and methods. Further details,particularly related to the de¢nition and quanti¢cation ofuncertainties, are available as Supporting Information.

We usedArcGISt 9.2 and the following datasets to esti-mate the modern morphology and area of the R|¤ o Grande,R|¤ o Parapeti and R|¤ o Pilcomayo catchments and mega-fans: 1 : 250 000 topographic maps from the InstitutoGeo-gra¢co Militar (IGM) in Bolivia, 15^150m LANDSATTM-7 satellite imagery, a hydrologically conditioned digi-tal elevation model (HydroSHEDS: http://hydrosheds.cr.usgs.gov/), and digital topography derived fromNASA’s2000 Shuttle Radar Topographic Mission (SRTM). Allmapping and calculations reported were carried out inthe Geographic and Universal Transverse Mercator (Zone20 South) coordinate systems with the datumWGS84.

68°W 66° 64° 62°23°S

21°

19°

17°200 km

Paleo

-forelan

d b

asinPaleosurfaceelevation (m)

> 3200

2800-3200

2400-2800

< 2400

(a)

0 2 4 6 8 10 12 14Age of Onset (Ma)

0

20

40

60

80

100

120

140

160

Hin

terla

nd A

rea

(km

2 x

1,00

0)

Orig DA & onset uncertainty

Orig DA & onset, most likely

Mod Area uncertainty

Emb base, oldest & youngest

Emb base, likely (2.1 Ma)

Area Trajectory, likely

Area Trajectory, max & min

Smallest D, earliest Emb onset

Drainage at Emb mid-life, likely

Largest drainage today

(b)

Bolivia

Argentina

Potosi

Sucre

Tupiza

Cochabamba

Fig. 5. The lateMiocene ( �10Ma) palaeo-drainage model for San Juan del Oro surface evolution overlaying the modern topography.(a)Map of the palaeo-drainage basins (dashed black lines5 boundaries, shaded regions are distributions of SJDO surface elevationstoday) fromKennan etal. (1997)with minor modi¢cations at the outlet convergence to mimic the modern basins. Other features includelocal highlands (jackstraw pattern), river networks (solid black lineswith arrows),mountain front (thrust fault) and palaeo-foreland basin(nowoccupied by theSubandes).Modern drainage basins andmegafans (backgroundgrey lines) are shown for comparison.Two things tonote; (1) the area between Potosi and Sucre used to be part of the palaeo-Grande basin, but has been captured by the Rio Pilcomayo(compare with Fig.1b), and (2) the SJDO surface elevation range suggests typical regional gradients of 0.4^0.8% (Kennan etal., 1997). (b)Various potential evolutionary trajectories between the palaeo- and modern drainages. Orig, original; DA, drainage area;Mod,modern; Emb, Emborozu¤ Formation.


95


We de¢ned and mapped megafan margins by one orseveral of the following criteria; (1) at the transition fromforeland-convex to mountain-front parallel contours, (2)the boundary between (a) well-de¢ned distributary chan-nels and their £anking overbank areas (both of which canbe clearly linked back to the fan apex) and (b) inter-mega-fan areas (with drainages originating from the frontal anti-clinal ridge, not from the fan apex), (3) systematic changesin local slope aspects and their magnitude and (4) con-sistent contrasts in colour, morphology and texture from

15m satellite images arti¢cially enhanced by topographicshading from multiple sun angles (criteria 2 and 3 afterHorton & DeCelles, 2001; B. K. Horton, pers. comm.,2006).

We overlaid the palaeotopography associated with theSJDO surface reconstructions ofGubbels (1993) andKen-nan et al. (1997) onto the modern topography in order tocompare them.We created gridded surfaces correspondingto the reconstructed SJDO surfaces for both models to es-timate the volume of material incised from below the sur-

150 km

500

1000

15000m

0m

EC

4.2

2.4km

N

IA

SA

Chaco

Elev

0.46

RVIZ

PA

4200m

2400(a) cut and fill 3600 3000

SJDO

150 km

4200m

3500

3500 32003000

3200

2800

2800

2400

3300

32003000

2800

EC

4.2

<2.4km

N

IA

SA

Chaco

Elev

500

1000

15000m

0m

RV

IZ

PA

(b) paleo-drainage

SJDO

Fig. 6. San Juan del Oro (SJDO) surface reconstructions and Chaco basin isopachs in southern Bolivia. Contours (solid lines; dashedwhere inferred) used to recreate the SJDO surface before Plio-Quaternary incision. Foreland isopachs (dashedwhere inferred in thisstudy) are for the �2.1^0Ma Emborozu¤ Formation (fromUba et al. (2006); their Fig.15e). Abbreviations are the same as in Fig. 2.Schematic block diagrams of the idealized SJDO surface reconstructed (left) and the distribution of elevation contours used to createthe gridded surface (right) on top of the modern topography for the cut and ¢ll (a) and palaeo-drainage (b) models. Idealized trellisdrainage pattern shown for the Subandes. Grey-shaded region is the extent of the reconstructed SJDO surface in each model. Blockdiagram in B shows how basin slopes grade inward to the centre and eastward to the foreland. Contours in B are from Fig. 5 plusadditional, inferred lines (grey dashed) where necessary.


96


face by measuring the volume di¡erence relative to themodern topography (Fig. 6). For the cut and ¢ll model(Gubbels, 1993), we created a surface by interpolatingbetween four N^S contours that span 66.5^64.41W to 17^231S. The contours have decreasing values from west toeast of 4200, 3600, 3000 and 2400m to replicate a regionalgradient of 0.461.This surface has a calculated mean slopeof 0.46 � 0.061 (1s). For the palaeo-drainage model (Ken-nan et al., 1997), we created a surface by interpolating be-tween contours tracing the distribution of regionalpalaeosurface elevations (compare Figs 5 and 6b). Addi-tional contours were added for this interpolation to prop-erly recreate the palaeotopographic highlands andintermediate palaeosurface elevations. However, the spa-tial extent of the contours was limited to that estimatedby the palaeo-drainage reconstructions (Kennan et al.,1997). The resultant surface slopes mimic the estimatedvalues of 0.46^0.231, but locally possess slopes of o0.21in the downstream regions and411 in very limited areasof the mid-to-upper reaches.

The nature and geometry of £uvial megafan basal sur-faces have yet to be studied. Therefore, we calculatedmegafan volumes between the modern topographic sur-face and two alternate basal- surface geometries: (1) a hor-izontal, planar, basal surface equal in elevation to theminimum megafan surface elevation, and (2) a basal sur-face that is the mirror image of the fan surface about a hor-izontal plane of symmetry at the lowest elevation. Underassumption (1), the volume of the megafan is equal to(average elevation ^ lowest elevation)� surface area. Un-der assumption (2), the volume is just twice the value of as-sumption (1). Assumption (1) is a minimum estimate and

assumption (2) is a more realistic estimate (see SupportingInformation for further discussion).

We created a gridded surface corresponding to the baseof the N5 seismic interval de¢ned by isopachs (Uba et al.,2006) in order to estimate the sediment volume in theforedeep. To encompass the entire study area, isopachswere extended parallel to the mountain front both northand south.We inferred the zero isopach to be parallel tothe 500m isopach and east of the Pilcomayo megafan mar-gin because there are no data to constrain its location morespeci¢cally (Fig. 6).

RESULTS

Modern drainage areas

Drainage basin area estimates for theGrande, Parapeti andPilcomayo are 59381 � 1188, 7453� 149 and86798� 1736 km2, respectively (Table 1). These estimatesare within 7^15% of those previously reported (Horton &DeCelles, 2001; Leier et al., 2005) for reasons related tochoice of basin outlet position and/or di¡erences in mapprojection and datum (Table 1 and Supporting Informa-tion). For example, variation among area estimates usingidentical catchment boundaries, but di¡erent projectionsis �10%. Minimum, maximum and average elevations,as well as relief, are also summarized inTable 1.

Palaeo-drainage areas

The palaeo-R|¤ o Grande and R|¤ o Pilcomayo drainagebasins, as de¢ned by the SJDO surface, may have covered

150 km

Elev6000

0 m

64°65°66°

18°

19°

20°

21°

62°RV

IZ

PA

(a)150 km

Elev6000

0 m

18°

19°

20°

21°

64°65°66° 62°RV

IZ

PA

(b)

Fig.7. Spatial distribution of Plio -Quaternary ( �3^0Ma) incision for both San Juan del Oro (SJDO) surface models. Black areasindicate volume loss when comparing the reconstructed SJDO surfaces with the modern topography for the cut and ¢ll (a) and palaeo-drainage (b) models. Signi¢cantly less volume has been lost from the Grande basin in the palaeo-drainage model.


97


an area of4 �100 000 km2 at �10Ma (Fig. 5a).Taking thepalaeo-drainage model of the SJDO surface at face value,we estimate the size of the palaeo-Grande, Parapeti andPilcomayo drainage basins (Fig. 5) to be 52 620, 9336 and38750km2, respectively (Table 1). Apparently, the palaeo-Grande basin was larger than the palaeo-Pilcomayo basinbecause the Potosi-Sucre area was subsequently capturedby the R|¤ o Pilcomayo (compare Figs 2b and 5a). In total,the preserved remnants of the SJDO surface delineate aminimum drainage area of 100 706 km2, which is roughlytwo-thirds of the modern surface area of the three drai-nage basins (Table1).The modern drainage (Fig. 2b) repre-sents the maximum area that could have been covered bythe SJDO surface. Figure 5b shows the range of uncer-tainty in palaeo and modern area estimates and potentialevolutionary trajectories between the two of them.

Volume excavated below the SJDO surface

We estimate 23920^30 900 km3 has been removed by inci-sion from below the SJDO surface since 3� 1.5Ma.Figure 7 shows regions that have experienced volume lossbetween the SJDO surface and the modern topography forboth surface reconstruction models. The distribution ofincision below both models is similar in the Pilcomayo ba-sin.This re£ects the fact that (1) the cut and ¢ll model wasbased almost exclusively on remnants located in the Pilco-mayo drainage (see Gubbels et al., 1993; Fig.1) and (2) mostof the aerial extent of surface remnants is preserved theretoday (Fig. 3), providing most of the control for both mod-els. Signi¢cantly less incision below the SJDO surface intheGrande basin is determined from the palaeo-drainagemodel because this model predicts a lower local base level

Table1. Source and sink physical dimensions in southern Bolivia

Grande Parapeti Pilcomayo (1) Pilcomayo (2) Total

Drainage BasinMaximum elevation (m) 4988 3303 5741 5741Minimum elevation (m) 442 631 389 320Relief (m) 4546 2672 5352 5421Average (m) 2631 1706 2851 2788

Area (km2)Horton &DeCelles (2001) �70 000 �8000 �81300 ^Leier et al. (2005) 59 000 �8000 81506 ^This study 59381 7453 80 832 86798 153632This study error ( � 2%) � 1188 � 149 � 1617 � 1736 � 3073�10Ma 52 620 9336 ^ 38750 100 706

Basin outlet, fan apexModern long (1W) 63.4 63.19 63.47 63.01Modern latitude (1S) 18.91 20.02 21.27 21.56�10Ma long (1W) �64 �64 ^ �64�10Ma latitude (1S) �18.5 �20 ^ �21

MegafansMaximum elevation (m) 575 646 ^ 360Minimum elevation (m) 299 384 ^ 198Relief (m) 276 262 ^ 162Average (m) 423 507 ^ 272

Area (km2)Horton &DeCelles (2001) �12 600 �5800 ^ �22 600Leier et al. (2005) 9944 6726 ^ 17294This study 12985 6142 ^ 22511This study error ( � 20%) � 2597 � 1228 ^ � 4502(Average-min) elevation (km) 0.124 0.123 ^ 0.074

Volume (km3)Max (area � 1.2 � average-min � 2) 3864 1813 ^ 3998 9675Likely (area � average-min� 2) 3220 1511 ^ 3332 8063Min (area � 0.8 � Average-min) 1288 604 ^ 1333 3225

Chaco ForedeepArea (km2) 132 080Error ( � 2%) � 2642Volume of Emborozu¤ Fm (km3) 63772Error ( � 20%) � 12754

1, pilcomayo basin boundary1; 2, pilcomayo basin boundary 2 (see Fig. 2b).Max, maximum area estimate and mirror-image basal- surface assumption.Likely, area estimate and mirror-image basal- surface assumption.Min, minimum area estimate and planar-basal-surface assumption.


98


relative to the Pilcomayo basin (Fig. 5).The nature of thedi¡erence between the two models suggests the cut and ¢llsurface represents an upper bound and the palaeo-drai-nage surface represents a lower bound on the volume ofmaterial removed.The results are reported this way.

Megafan areas

The £uvial megafans extend4150 km across the foredeepfrom their mountain-front apexes to their distal lobes.To-tal surface area of the megafans is �42 000 km2, whereasthe total surface area of the proximal Chaco foredeep is�132 000 km2 (Table 1).We estimate the megafan surfaceareas to be 12985, 6142 and 22511km2� 20% for theGrande, Parapeti and Pilcomayo, respectively (Table 1 andSupporting Information). Our mapping criteria are su⁄ -ciently restrictive that the estimate of the Pilcomayomegafan is an order of magnitude less than the210 000 km2 reported by Iriondo (1993).

Megafan and foredeep basin fill volumes

Megafan volumes corresponding to the planar-basal- sur-face andmirror-image assumptions are reported inTable1.The estimates range from 604 km3 for the Parapeti mega-fan assuming a planar surface, to 3332 km3 for the Pilco-mayo megafan assuming a mirror image between the fansurface and the basal surface.The foredeep volume of theEmborozu¤ Formation is 63772 km3� 20% (Table 1 andSupporting Information).

Denudation-rate estimates

Barnes & Pelletier (2006) compiled denudation-rate esti-mates from avariety of methods for southernBolivia. Esti-mates range from 0.04 to 1.6mmyear�1 (5 kmMa�1)(Fig. 8).These estimates integrate sediment removal overtemporal scales of 101^107 years and spatial scales from100 to 105 km2 (Fig. 9) (see Supporting Information forfurther discussion).

The relevant denudation rate for our source-to-sinkcalculation is an idealized average over the whole hinter-land (105 km2) and the whole depositional history of theEmborozu¤ Formation (106 years) (see Supporting Infor-mation for additional discussion). Although observationsspan a range ofvalues, the highest rates come from smallerspatial scales and larger temporal scales than the relevantanalytic scale (Fig.9). Observations that come from the re-levant analytic scales (black oval in Fig. 9) fall into a muchsmaller range of 0.1^0.4mmyear�1.The only observationthat matches the relevant analytic spatial and temporalscale is �0.2mmyear�1 (grey circle in Fig.9).

Sediment production and depositionestimates

Boundary conditions

This source-to-sink sediment budget starts with todayand integrates back to the Plio-Quaternary ( �3^0Ma).The chronologic boundary is either initial incision intothe SJDO surface or initial deposition in the Emborozu¤

68°W 66° 64° 62° 60°24°S

22°

20°

18°

16°200 km

A A'

Y'YX

B

AP

SA

VI

0.1-0.6

0.13-0.27

0.04-0.09

0.33

0.33-0.35

0.08-0.16

0.98-1.04

0.07-0.22

MAP KEY

Erosion rate(mm/yr)Symbol Method Time Span

0.43-0.84

0.15-0.27

0.13-0.2

0.89-0.93

B A'

X Y'

A A'

Y Y'

0.04-0.09

0.04-0.080.9-1.6

AFT

seismic

ES/DEM

basin fill

x-section

x-section

x-section

x-section

mass bal

sed flux

sed flux

sed flux

ES/DEM

ES/DEM

AFT

10-40 Ma

0-2 or 3 Ma

0-2 or 3 Ma

years

years

years

0-10 Ma

0-5 Ma

0-40 Ma

0-15 or 20 Ma

0-15 or 20 Ma

0-40 Ma

0-10 Ma

0-10 Ma

0-12 Ma

AP

SA

VI

Elev6000

0 m

Pilcomayo

Parapeti

Grande

Bolivia

BENIPLAIN

ArgentinaChileCHACOPLAIN

Fig. 8. Map showing locations of erosion-rate estimates for the central Andean fold-thrust belt in southern Bolivia (modi¢ed fromBarnes & Pelletier, 2006).Method, method used for calculating the estimate;Time Span, time span over which the erosion rate isaveraged; sed £ux, sediment- £ux data with range of published data fromAalto et al. (2006) and Barnes & Pelletier (2006); AFT, apatite¢ssion-track thermochronology; x- section, cross-section; mass bal, mass balance; ES/DEM, erosion surface andDEM analysis;seismic, seismic cross- sectional area; basin ¢ll, basin ¢ll rate.


99


Formation. In space, the budget starts with the modernlandscape, bounded by the modern drainage divides ofthe R|¤ os Grande, Parapeti and Pilcomayo on the sourceside and by the zero-isopach of the Emborozu¤ Formationon the sink side.The spatial boundary ends with the pre-incision SJDO surface and bordering highlands and thebasal surface of the Emborozu¤ Formation.

Volume balance

The source area sediment volume produced must be atleast as large as the volume of sediment deposited in theEmborozu¤ Formation. The maximum volume of sedi-ments excavated from below the preserved area of theSJDO surface ( �2.4^3.1 � 104 km3) is smaller than theminimum volume of the Emborozu¤ Formation( �5.1� 104 km3) (Table 1). Initiation of incision into theSJDO surface (4.5^1.5Ma) overlaps with initial Emborozu¤Formation deposition (3.3^1.8Ma) within error, but inci-sion (most likely age 3.0Ma) probably predates initial de-position (most likely age 2.1Ma). The volume disparity

between incision and deposition means that erosion fromthe intervening highlands and the drainage regionsbeyond the SJDO surface extent (Fig. 7) contributed atleast �40^60% of the sediment to the Emborozu¤or �2^2.7 � 104 km3 by volume.

Treatment of ions and pores

Exposed source-area bedrock is mostly Mesozoic and Pa-laeozoic siliciclastic sedimentary rock (e.g. McQuarrie,2002). These rocks have some preserved porosity. Somefraction of the rocks is also lost to dissolution during con-version of bedrock to transportable sediment.We estimatethat the volume lost to dissolved ions (�1^15%) plus theoriginal rock porosity (� 12%) is similar to the volume ofvoid space among sedimentary particles deposited in thebasin (16^32%) (see Supporting Information for quantita-tive justi¢cation).Thus,we treat gross volumes of denudedand deposited material as equivalent because the solid/void ratio is similar between source and sink.

apatite fission-track thermochronology

balanced cross section

basin fill

incision below erosion surface

mass balance

sediment flux

seismic volume estimate

scales relevant to this study

scales that match this study

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8Denudation rate (km/Ma = mm/yr)Temporal scale (years)

Spa

tial s

cale

(km

2 )

101100100

101

102

102

103

103

104

104

105

105

106

106

107 108

0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Den

udat

ion

rate

(km

/Ma

= m

m/y

r)

Temporal scale (years)101100 102 103 104 105 106 107 108

Fig.9. Erosion-rate estimates vs. their integration in space and time for the central Andean fold-thrust belt in southern Bolivia (datafrom Barnes & Pelletier, 2006). Ovals highlight values that are both relevant (black) and speci¢cally match (grey) the scale of this study.


100


Estimates

There are two paths for estimating sediment-productionrates from the source area based on the data in this paper.First, we divided the volume of sediment excavated frombeneath the SJDO surface by the time incision began.Table 2 summarizes the range of sediment-productionrates calculated from the estimated volumes and time. Se-diment-production rates range from 5316 to20 600 km3Ma�1 with a middle value of 9137 km3Ma�1.Second, we integrated linear denudation rates over thehinterland area. Table 2 also summarizes the sediment-production rates calculated from a range of denudationrates and potential hinterland areas. Denudation rateswere chosen to uniformly cover (on a log2 scale) the rangereported inFig.8.We picked areas fromFig.5b to representthe smallest, likely, and largest regions that could havebeen encompassed by the drainage from the earliest initia-tion of Emborozu¤ deposition to today. These sediment-production rates show a much wider range, from 5261 to250 733 km3Ma�1with a middle value of 57215 km3Ma�1.

There is only one path to estimate sediment-depositionrates.We divided the Emborozu¤ Formation sediment vo-lume by the time since deposition began. Sediment-de-position rates calculated from a range of estimatedvolumes and times are inTable 2. Rates range from 22182to 42515 km3Ma�1 with a middle value of30368 km3Ma�1.The deposition duration of 3.3Ma is in-cluded for completeness, but is considered unlikely (seediscussion in Uba et al. (2005)).

Reconciling the estimates

We calculated the minimum extra upland area required toproduce the Emborozu¤ sediment by subtracting the vo-lume of SJDO surface excavation from the volume of theEmborozu¤ then dividing by a linear denudation rate.Thevalue is only a minimum because some SJDO surface-de-rived sediment might have been deposited elsewhere (e.g.in an older formation or bypassed downstream).Neverthe-less, this exercise excludes denudation-rate estimates thatare impossible for the relevant scale because they requiremore upland area than exists today. Table 3 summarizesthe results within the ranges of rates, space and time con-strained by observations. Space limits encompass thesmallest total palaeo-drainage size (corresponding to theearliest onset of Emborozu¤ deposition) to the largest pos-sible modern drainage size (estimate plus error).Time lim-its were derived from the oldest potential onset ofEmborozu¤ deposition and the youngest possible onset ofincision. Incision probably began earlier than the oldestonset of Emborozu¤ deposition, but that case is not relevantto this calculation of minimum area. Table 3 rows are or-dered by increasing mass £ux required to ¢ll the Emboro-zu¤ . The results are shaded to indicate possibility:impossible (dark shading) because the combination of vo-lumes and rates imply a hinterland area greater than themodern drainage, possible results (light shading) becausethey are within the range of potential hinterland areas,

and certainly possible (no shading) results because theyare smaller than the smallest drainage size.

Results indicate that any denudation rateo0.1kmMa�1 is impossible as the average is over thecomplete (time, space) that generated the Emborozu¤ sedi-ments (Table 3). Denudation rateso0.2 kmMa�1 are im-possible unless the volume of sediments in the Emborozu¤is near the low end of the likely range, and the onset of de-position is towards the old end of the likely range. Giventhe most likely volumes of SJDO surface excavation, Em-borozu¤ deposition and deposition duration, the averagedenudation rate should have been � 0.2 kmMa�1.

Modern sediment-production estimates

Boundary conditions

This source-to-sink sediment budget starts with todayand integrates backward to the onset of modern megafandeposition.The budget starts with the modern landscapesurface, bounded by the modern drainage divides of the

Table2. Potential Plio-Quaternary sediment production anddeposition rates

Sediment-production rates (km3Ma�1)

Volume removed belowSJDO (km3)

Excavation duration (Ma)

1.5 3 4.5

Minimum 23920 15947 7973 5316Likely 27410 18273 9137 6091Maximum 30 900 20 600 10300 6867

Hinterland denudationrates (kmMa�1)

Potential hinterland area (km2)

131517 143 037 156708

Highest 1.6 210 428 228 859 250 733Median sediment £ux 1.0 131517 143 037 156708Median AFT, highbalanced cross-section

0.8 105214 114 429 125366

Low sediment £ux 0.4 52 607 57215 62 683Median balancedcross-section

0.2 26303 28 607 31342

Low balanced cross-section, AFT

0.1 13152 14304 15 671

Lowest 0.04 5261 5721 6268

Sediment-deposition rates (km3Ma�1) in theEmborozu¤ Formation

Volume deposited (km3)Deposition duration (Ma)

1.8 2.1 2.3 3.3n

Minimum 51018 28343 24294 22182 15 460Likely 63772 35 429 30368 27727 19325Maximum 76526 42515 36 441 33272 23190

Middle values are shaded gray, see text for discussion.nValues in this columnunlikely, see text andUba etal. (2005) for discussion.


101


Tab

le3.

Totalhinterlandarea

requ

ired

(km

2 )to

supp

lyde¢cient

volume(Emborozu

êxcavation

belowSJDO)n

Excavated

below

SJDO(km

3 )Observedin

Emborozu¤(km

3 )Extravolume

requ

ired

(km

3 )

Excavation

duration

(Ma)

Production

rate

ofextravolume

(km

3Ma�

1 )

Potentialdenud

ationrates(km

Ma�

15mmyr�1 )

Highest

Median

sediment

£ux

MedianAFT,

high

balanced

cross-section

Low

sediment

£ux

Medianbalanced

cross-section

Low

balanced

cross-section,

AFT

1.6

1.0

0.8

0.4

0.2

0.1

30900

51018

20118

3.3

6096

1025

00104786

106310

113930

129171

159652

27410

51018

23608

3.3

7154

103161

105843

107632

116574

134459

170228

23920

51018

27098

3.3

8211

103822

106901

108954

119218

139747

180804

30900

51018

20118

2.1

9580

104677

108269

110664

122639

146589

194488

30900

63772

32872

3.3

9961

104915

108651

111141

123593

148496

198302

27410

63772

36362

3.3

11019

105576

109708

1124

63126237

153784

208878

27410

51018

23608

2.1

11242

105716

109931

112742

126794

154898

211107

23920

63772

39852

3.3

12076

106237

110766

113785

128881

159071

219453

23920

51018

27098

2.1

12904

106754

111593

114819

130949

1632

08227726

30900

51018

20118

1.5

13412

107072

112101

115454

132219

165748

232807

30900

76526

45626

3.3

13826

107331

112516

115972

133255

167821

236951

27410

76526

49116

3.3

14884

107992

113573

1172

94135899

173108

247527

30900

63772

32872

2.1

15653

108473

1143

43118256

137823

176956

255223

27410

51018

23608

1.5

15738

108526

114428

118363

138036

177382

256074

23920

76526

52606

3.3

15941

108653

114631

118616

138543

178396

258103

27410

63772

36362

2.1

17315

109512

116005

120334

1419

781852

66271842

23920

51018

27098

1.5

18065

109980

116755

121271

143852

189015

2793

4023

920

63772

39852

2.1

18977

110550

117667

122411

146133

193575

288461

30900

76526

45626

2.1

21727

112269

120417

125848

153007

207324

315958

30900

63772

32872

1.5

21915

112386

120604

126083

153476

208263

317836

27410

76526

49116

2.1

23389

1133

08122078

127926

157162

215633

332577

27410

63772

36362

1.5

24241

113841

122931

128991

159293

219896

341103

23920

76526

52606

2.1

25051

1143

46123740

130003

161316

223943

349196

23920

63772

39852

1.5

26568

115295

125258

1319

00165110

231530

364370

30900

76526

45626

1.5

30418

117701

129107

136712

174734

250778

402866

27410

76526

49116

1.5

32744

119155

131434

139620

180550

262411

426132

23920

76526

52606

1.5

35071

120609

133761

142528

186367

274044

4493

99Min

23920

51018

20118

1.5

Max

30900

76526

52606

3.3

Upp

erLow

erLikely

27410

63772

36362

2.1

Lim

it5156708

km2

Lim

it5131517km

2

nMinim

umarea

ofpreservedSJDOsurface5

98690km

2 ;source

areasrequ

ired

tosupp

lyde¢cit

5extravolprodu

ctionrate/denud

ationrate

198

690;dark

shading,im

possibleresults

;light

shading,po

ssibleresults

;no

shading,certainlypo

ssibleresults;seeFig.8forabb

reviations.

Outlin

edrowismostlikelyresultbasedon

bestcombination

oftimeandam

ount

ofexcavatedsedimentb

elow

theSJDOandob

served

intheEmborozu¤Formation.


102


Grande, Parapeti and Pilcomayo rivers in the source, andbounded by the megafan extents in the sink.

The megafan sediment volume is probably equal to, orslightly less than, the volume produced in the source areafor the following reasons.Very little surface water escapesthe Chaco foredeep because the R|¤ o Parapeti and Pilco-mayo terminate into swamps just downstream of theirmegafans and the R|¤ o Grande stalls as it bifurcates intosmall channels, drops sediments in the adjacent £ood-plains in the R|¤ o Viejo area beyond the megafan margin,and consequently severs its connection to theR|¤ o Paraguay(Fig. 2b) (Iriondo, 1984; Iriondo, 1993; Horton &DeCelles,2001). In particular, the R|¤ o Pilcomayo presently depositssuch a sediment excess that it blocks its own channel,£oods its levees and spills into nearby swamps (Wilkinsonet al., 2006). Furthermore, tectonic depressions, vegeta-tive-debris accumulations and abandoned channels facili-tate water and sediment ponding in lakes both on andaround the megafans (Iriondo, 1993; Wilkinson et al.,2006). Finally, it is estimated that most of the R|¤ o Pilco-mayo sediment load is trapped in the Chaco plain beforejoining the Parana river (Latrubesse et al., 2005).The Sup-porting Information outlines observations that suggest themegafans themselves might not be entirely closed systems.

Sediment production

Integrating a linear denudation rate over the modern drai-nage area yields the modern sediment-production rate.Table 4 summarizes sediment-production rates calculatedfrom a range of denudation rates and measured drainageareas.Denudation rates were chosen to cover the range re-

ported with particular emphasis on rates estimated fromthe basin outlet on each of the rivers: 0.89^0.93mmyear�1

for the Grande (point AP in Fig. 8), 0.98^1.04mmyear�1

for the Parapeti (point SA in Fig. 8), and 0.33^0.35mmyear�1 for the Pilcomayo (point VI in Fig. 8).Thehighest rate used represents the highest observed ratethroughout the Neogene (apatite ¢ssion-track thermo-chronology) whereas the lowest rate is the lowest possiblecalculated in Table 3. For each river, the best sediment-production rate estimate (shading) is based on the mostlikely drainage size and the measured denudation rate forthat drainage.

Age of megafan initiation

If all sediment produced in the drainages is deposited onthe megafans, as observations documented above suggest,then onset of modern megafan deposition can be esti-mated by dividing the megafan sediment volume by therate of sediment production. Table 5 summarizes the re-sults of this calculation with rate and volume ranges con-strained by observation for each megafan and theiraggregate.

For most likely values for drainage area, sediment deliv-ery and megafan volume, estimated age of megafan initia-tion varies considerably from 52^55 ka for the Grande, to110^116 ka for the Pilcomayo, and 218^228 ka for the Para-peti (Table 5 and Supporting Information). It is possiblethat this result is correct and megafan initiation is dia-chronous. Alternatively, modern denudation rates may beinaccurate estimates of the average rate since the (com-mon?) initiation time of the megafans because they arebased on only a few years, compared with the hundreds of

Table4. Potential sediment-production rates from the modern landscape (km3Ma�1)

Observed denudation rates (kmMa�15mmyr�1)

Drainagearea (km2)

Neogenehighest

Grandehighest

Grandelowest

Parapetihighest

Parapetilowest

Neogenemedian

Pilcomayohighest

Pilcomayolowest

Neogenelowest

1.60 1.04 0.98 0.93 0.89 0.80 0.35 0.33 0.20

GrandeMaximum 60569 96910 62991 59357 56329 53906 48 455 21199 19 988 12114Likely 59381 95 010 61756 58193 55224 52 849 47505 20 783 19596 11876Minimum 58193 93109 60521 57030 54120 51792 46555 20368 19204 11639

ParapetiMaximum 7602 12163 7906 7450 7070 6766 6082 2661 2509 1520Likely 7453 11925 7751 7304 6931 6633 5962 2609 2459 1491Minimum 7304 11686 7596 7158 6793 6501 5843 2556 2410 1461

PilcomayoMaximum 88534 141654 92 075 86763 82 337 78795 70 827 30 987 29216 17707Likely 86798 138 877 90270 85 062 80 722 77250 69 438 30379 28 643 17360Minimum 85 062 136 099 88 465 83361 79108 75705 68 050 29 772 28 070 17012

TotalMaximum 250727 162973 153571 145735 139 467 125364 54 847 51713 31341Likely 245811 159 777 150559 142 878 136732 122906 53771 50 699 30 726Minimum 240 895 156582 147548 140 020 133998 120 447 52 696 49 685 30112

Shading, best sediment-production rate estimate.


103


ka over which the megafans must have been accumulating.If we apply the median denudation rate observed acrossthe entire Neogene to the total volume of sediments in allmegafans, onset of deposition would be �66 ka. Table 5essentially presents a series of hypotheses about the ageof the modernChacomegafans that can be tested bydatingthe actual basal surface. Radiocarbon or pollen ages fromrelatively shallow boreholes could provide the necessaryinformation.

DISCUSSION AND IMPLICATIONS

Sediment production volumes through timeand space

Estimated sedimentvolumes have implications for erosionvariability through time and the distribution of sedimentproduction within the source region.The ca. 1000m reliefbetween the SJDO surface and intervening highlands ledKennan et al. (1997) to estimate that �1^2� 104 km3 wasexcavated from the original palaeotopography to make theSJDO surface presumably before �10Ma. Since �2^3Ma, a minimum of �5.1� 104 km3 has been depositedinto the Emborozu¤ Formation, of which at least �40^60% (2.4^3.1� 104 km3) came from below the SJDO sur-face via incision. Although, we cannot quantify the sourcearea extent at any point before SJDO formation, we spec-ulate relative denudation rateswere low for some time per-

iod before �10Ma because the sediment volumeproducedwas only �20^40% of the volume deposited inthe last 3Myr.This is already implied because most of thelong-term (410Myr) averaged denudation rates areo0.4mmyear�1 (Fig. 9), and we already demonstratedthey were most likely � 0.4mmyear�1 during the Plio-Quaternary.

Comparison of estimated sediment volumes betweensource and sink over the last 2^3Myr shows at least�40^60% came from incision into the SJDO surface.The remainder must have come from some combinationof the intervening highlands and drainage areas outsidethe current SJDO surface extent. In map view (Fig. 7), thelargest source areas not accounted for by SJDO incisionare the modern Subandes and the intervening highlands.The Subandes probably contributed to the Emborozu¤ For-mation, but sediments probably can get trapped locally intheTertiary piggyback basins before reaching the peren-nialGrande, Parapeti andPilcomayo trunk rivers.The bestcandidate source might be the intervening highlands be-cause they extend over signi¢cant areas and exhibit thesteepest slopes.

Plio-Quaternary tomodern denudation rates

Measurements of denudation rates, drainage areas andvo-lumes of sediment produced or deposited are inherentlyimprecise. No singular observation, or even a range of ob-servations on a single feature, can be considered accurate

Table 5. Potential age (Ma) of megafan initiation

Megafan volume (km3)

Potential sediment production rates from the modern landscape (km3Ma�1)

Absolute highest High end of likely Low end of likely Absolute lowest

96 910 61756 58193 11639

GrandeMaximum 3864 0.040 0.063 0.066 0.332Likely 3220 0.033 0.052 0.055 0.277Minimum 1288 0.013 0.021 0.022 0.111

Absolute highest High end of likely Low end of likely Absolute lowest12163 6931 6633 1461

ParapetiMaximum 1813 0.149 0.262 0.273 1.241Likely 1511 0.124 0.218 0.228 1.034Minimum 604 0.050 0.087 0.091 0.414

Absolute highest High end of likely Low end of likely Absolute lowest141654 30379 28 643 17012

PilcomayoMaximum 3998 0.028 0.132 0.140 0.235Likely 3332 0.024 0.110 0.116 0.196Minimum 1333 0.009 0.044 0.047 0.078

Absolute highest Neogene median Absolute lowest250 727 122906 30112

All megafansMaximum 9675 0.039 0.079 0.321Likely 8063 0.032 0.066 0.268Minimum 3225 0.013 0.026 0.107

Shading, best sediment-production rate estimate.


104


in isolation. Observations can only be evaluated based ontheir internal consistency. Our analysis demonstrates thatestimates of denudation o0.1kmMa�1 cannot (and esti-mates o0.2 kmMa�1 probably do not) characterize theentire Bolivian-Andes hinterland of the Chaco forelandover the Plio-Quaternary (last 2^3Myr) even though ob-servations demonstrate that such rates may be locally vi-able (Fig. 8). Modern estimates suggest the Pilcomayobasin erodes at a rate (0.34mmyear�1) near the minimumthat characterized the Plio-Quaternary. In contrast, boththe Grande and Parapeti basin rates (0.91 and1.01mmyear�1) are signi¢cantly higher (Fig. 8).This var-iation in erosion rates could be the result of the generalsouthward aridi¢cation (e.g. Barnes & Pelletier, 2006),anthropogenic e¡ects, and/or sediment discharge varia-tions resulting from the type of dominant erosion processand precipitation storminess (e.g. Fuller et al., 2003).

Evolution of topography

Topographic evolution can be better understood by deter-mining the amount and rate of morphologic change acrossdi¡erent spatial and temporal scales.Here,we use the phy-sical dimensions determined in this study to comment oncentral Andean fold-thrust belt and Chaco foreland topo-graphic variations over the lateMiocene^Quaternary (last�10Myr).TheGrande, Parapeti and Pilcomayo basins collectively

expanded by �50% from �100 000 to �150 000 km2

since �10Ma (Table 1). Migration of the drainage dividewestward was �100 km since �10Ma as was the migra-tion of the drainage outlet eastward (Fig.5).This migrationrate of10mmyear�1 is similar to locally estimated rates ofthrust belt propagation (6^8mmyear�1), shortening( �4^8mmyear�1) and foreland basin migration

interveninghighlands

Altiplano

(a)

~10 Ma foredeep

wedgetop

forebulgebackbulge

SOURCE: central Andean fold-thrust belt SINK: paleo-foreland

~100 km

future Subandes

SJDO surface models:cf = cut & fill (Gubbels)pd = paleo-drainage (Kennan)

pd

>10 Ma

cf

paleo-basins: 9,336-52,620 km2

present foredeep

wedgetop

forebulge

backbulge

T2T2

Altiplano

(b)

minor export?

SOURCE: central Andean fold-thrust belt SINK: Chaco foredeep

SJDO surface models:cf = cut & fill (Gubbels)pd = paleo-drainage (Kennan)

pd

cf

Emborozú foredeep since ~2.1 Ma: 63,772 km3

megafans: 604-3,998 km3

deposited over last 52-228 kyr

modern basins: 7,453-86,798 km2

SJDO incision since ~3 Ma: 2.4(pb) - 3.1 (cf) x 104 km3

erosion rates ≥ 0.2 mm/yrT1

T1

T1

T1

time = 2.1 Ma

>10 Ma

Fig.10. Schematic central Andean fold-thrust belt and Chaco foreland basin system sediment budget in cross-section at �201S. cf,cut and ¢ll model surface representation (dashed line in source); pd, palaeo-drainage surface representation (solid concave up line insource). (a) Source and sink features during peak San Juan del Oro (SJDO) formation at �10Ma before incision. (b) Source and sinkfeatures after incision at present.Time sliceT1 is the Plio-Quaternary to recent ( �2 or 3^0Ma) represented by the volume eroded byincision into theSJDO surface in the source anddepositedwithin theEmborozu¤ Formation in the sink, respectively.Time sliceT2 is veryrecent time ( �230^0 ka) represented by the modern drainage areas and the megafan volumes in the foreland, respectively. Additionalsource region solid lines represent the modern maximum (jagged line) and minimum (lowest line) topography.


105


( �13mmyear�1) (McQuarrie et al., 2005; Barnes et al.,2008). Westward, headward erosion, stream piracy andeastward drainage expansion into the Subandes probablycontributed to drainage basin growth. In particular, streampiracy by the R|¤ o Pilcomayo of the Sucre/Potosi regionprobably contributed the most to the Pilcomayo basin’sgrowth of �115% from �40 000 to �87000 km2 (Table2).Despite the area lost to thePilcomayo, theGrande basinstill grew in overall size by almost10%.Finally, the Parape-ti basin actually decreased in size by �15% probably viathe encroachment of the two larger basins on either sideof it. These data suggest that large (104^5 km2) drainagesin (potentially protracted) semi-arid climates still evolvesubstantially over 10Myr time frames.

The modern £uvial megafans are estimated to be up to�228 kyr old (Table 5). Unfortunately, to the best of ourknowledge, no studies have estimated the age of any othermodern megafans for comparison. Regardless, the�228 kyr age suggests that large sediment bodies can bedispersed over distances of 4200 km across low-sloped(mostly o0.351) regions rather rapidly even in semi-aridclimates. Furthermore, the currently active megafans re-present only a small portion (in either time or sedimentvolume) of the most recent seismically resolvable sedi-mentation history in the basin. In fact, sedimentary evi-dence suggests the Subandes megafans have existed forthe last �8Ma (Uba et al., 2007).

Thrust belt-foreland geodynamics

Thrust belt deformation and erosion are dynamicallycoupled to their associated foreland basin systems viadeformation, foreland £exure and erosion (e.g. DeCelles& DeCelles, 2001). Models of this coupling predict thatregions of reduced erosion are characterized by wedgegrowth, a wide, rapidly propagating thrust belt with dom-inantly wedgetop deposition and an under¢lled foredeep,whereas regions of enhanced erosion possess wedge recy-cling, a narrow thrust belt with more constant width, anddominantly foredeep deposition in awide and largely ¢lledforeland (Simpson, 2004, 2006).These predictions are, to¢rst-order, consistentwith the central Andean fold-thrustbelt where observations mentioned above suggest the dry,southern Chaco foredeep is basically under¢lled and thewet, northern Beni foredeep is over¢lled because �50%of the sediment bypasses it and enters the Amazon(Fig. 2) (Horton,1999; Aalto etal., 2006; Barnes &Pelletier,2006). Unfortunately, the models have only beendeveloped for the general case. They could be tested bycalibrating them to speci¢c regions and constraining thesurface process parameters with such datasets as thosepresented here.

SUMMARY

Figure 10 schematically illustrates the central Andeanthrust belt-Chaco foreland basin system sediment budget

presented here. Selected, important values derivedthroughout this study are indicated. Comparison of Figs 1and 10 illustrates the contrast between the idealized andour applied thrust belt-foreland system sediment budgetanalysis.

ACKNOWLEDGEMENTS

Financial support was provided to J. Barnes by an Exxon-Mobil summer internship andNSFgrant EAR 0409289 toT. A. Ehlers.We thank Bob Brovey for providing the en-hanced satellite images. This paper bene¢ted from earlydiscussions with Brian Horton, Lorcan Kennan and PeterDeCelles, and very thorough reviews byKennan, an anon-ymous reviewer, and editor DeCelles.

Supporting Information

Additional Supporting Information may be found in theonline version of this article:

Appendix S1: Details on some of the methodologiesand uncertainties presented in the main text.

Please note: Blackwell Publishing is not responsible forthe content or functionality of any supporting materialssupplied by the authors. Any queries (other than missingmaterial) should be directed to the corresponding authorfor the article.

REFERENCES

Aalto, R., Dunne,T. & Guyot, J.L. (2006) Geomorphic con-trols on Andean denudation rates. J. Geol., 114, 85^99.

Allen,P.A. (2008) From landscapes into geological history.Nat-ure, 451, 274^276.

Allmendinger, R.W., Jordan,T.E.,Kay, S.M. & Isacks, B.L.(1997) The evolution of the Altiplano-Puna Plateau of theCentral Andes.Annu. Rev. Earth Planet. Sci., 25, 139^174.

Babault, J.,van denDriessche, J.,Bonnet,S.,Castelltort,S. & Crave, A. (2005) Origin of the highly elevated Pyreneanpeneplain.Tectonics, 24, doi: 10.1029/2004TC001697.

Baby,P.,Herail,G.,Salinas,R.& Sempere,T. (1992)Geome-try and kinematic evolution of passive roof duplexes deducedfrom cross section balancing; example from the forelandthrust system of the southern Bolivian subandean zone. Tec-tonics, 11, 523^536.

Baby, P., Limachi, R., Moretti, I., Mendez, E., Oller, J.,Guiller, B. & Specht, M. (1995) Petroleum system of thenorthern and central Bolivian sub-Andean zone. In: PetroleumBasins of South America (Ed. by A.J. Tankard, R. Suarez & H.J.Welsink),Am. Assoc. Petrol. Geol.Mem., 62, 445^458.

Barke,R.&Lamb, S. (2006) Late Cenozoic uplift of the EasternCordillera, BolivianAndes.Earth Planet. Sci. Lett., 249, 350^367.

Barnes, J.B., Ehlers,T.A.,McQuarrie,N.,O’Sullivan, P.B.& Tawackoli, S. (2008) Thermochronometer record of cen-tral Andean Plateau growth, Bolivia (19.51S).Tectonics,TC3003,doi: 10.1029/2007TC002174.


106


Barnes, J.B. & Pelletier, J.D. (2006) Latitudinal variation ofdenudation in the evolution of the Bolivian Andes. Am. J. Sci.,306, 1^31.

Beaumont, C., Kooi, H. & Willett, S. (2000) Coupled tec-tonic-surface process models with applications to rifted mar-gins and collisional orogens. In: Geomorphology and GlobalTectonics (Ed. by M. Summer¢eld), pp. 29^55. John Wiley &Sons, Chichester, UK.

Bonnell, L.M. & Lander, R.H. (2003) Reservoir quality pre-diction in deep water to tight gas sandstones using a process/stochastic modeling approach. Am. Assoc. Petrol. Geol. Bull., 87,AAPGDistinguished Lectures.

Bray, A.A., Lander, R.H., Watkins, C.A., Lowrey, C.J. &Owen, M.Anonymous (2000) Characterisation and predic-tion of clastic reservoir quality; an integrated model for use inexploration, appraisal and production projects. Am. Assoc. Pet.Geol. Bull., 1408.

Burgess, P.M. & Allen, P.A. (1996) A forward-modelling ana-lysis of the controls on sequence stratigraphical geometries.Geol. Soc. Lond. Spec. Publ., 103, 9^24.

Clark, M.K., Royden, L.H., Whipple, K.X., Burchfiel,B.C., Zhang, X. & Tang,W. (2006) Use of a regional, relictlandscape to measure vertical deformation of the easternTibe-tan Plateau. J. Geophys. Res., 111, doi: 10.1029/2005JF000294.

Clevis,Q., de Boer, P.&Wachter,M. (2003) Numerical mod-elling of drainage basin evolution and three-dimensional allu-vial fan stratigraphy. Sediment. Geol., 163, 85^110.

Clevis,Q.J.W.A. (2003)Three-dimensional modelling of thrust-controlled foreland basin stratigraphy. Doctoral Thesis, Uni-versiteit Utrecht, Utrecht.

Clift, P.,Gaedicke,C.,Edwards,R.,Lee, J. II,Hildebrand,P., Amjad, S.,White, R.S. & Schlueter, H.-U. (2002) Thestratigraphic evolution of the Indus Fan and the history of sedi-mentation in the Arabian Sea.Mar. Geophys. Res., 23, 223^245.

Clift, P.D. (2006) Controls on the erosion of Cenozoic Asia andthe £ux of clastic sediment to the ocean.Earth Planet. Sci. Lett.,241, 571^580.

Coudert,L.,Sempere,T.,Frappa,M.,Viquier,C.&Arias,R.(1993) Subsidence and crustal £exure Evolution of the Neo-gene Chaco Foreland Basin. Third International Symposiumon Andean Geodynamics, Oxford, extended abstracts, pp.291^294.

Critelli, S. (1999) The interplay of lithospheric £exure andthrust accommodation in forming stratigraphic sequences inthe Southern Apennines foreland basin system, Italy. Att. Ac-cad. Naz. Lincei. Rendiconti Lincei. Sci. Fisiche Natur., 9, 257^326.

Critelli, S., Arribas, J., Le Pera, E., Tortosa, A., Marsa-

glia, K.M. & Latter, K.K. (2003) The recycled orogenicsand provenance from an uplifted thrust belt, Betic Cordillera,southern Spain. J. Sediment. Res., 73, 72^81.

Curray, J.R. (1994) Sediment volume and mass beneath the Bayof Bengal. Earth Planet. Sci. Lett., 125, 371^383.

Damanti, J.F. (1993) Geomorphic and structural controls on fa-cies patterns and sediment composition in a modern forelandbasin. In:Alluvial Sedimentation (Ed. byM.Marzo&C.Puigde-fabregas), Int. Assoc. Sediment. Spec. Publ. 17, 221^233.

DeCelles, P.G. & Cavazza,W. (1999) A comparison of £uvialmegafans in the Cordilleran (Upper Cretaceous) and modernHimalayan foreland basin systems. Geol. Soc. Am. Bull., 111,1315^1334.

DeCelles, P.G. & DeCelles, P.C. (2001) Rates of shortening,propagation, underthrusting, and £exural wave migration incontinental orogenic systems.Geology, 29, 135^138.

DeCelles, P.G. & Giles, K.A. (1996) Foreland basin systems.Basin Res., 8, 105^123.

DeCelles,P.G.&Horton,B.K. (2003) Early to middleTertiaryforeland basin development and the history of Andean crustalshortening in Bolivia.Geol. Soc. Am. Bull., 115, 58^77.

de Sitter, L.U. (1952) Pliocene uplift of Tertiary mountainchains.Am. J. Sci., 250, 297^307.

Devlin, W.J., Rudolph, K.W., Shaw, C.A. & Ehman, K.D.

(1993) The e¡ect of tectonic and eustatic cycles on accommo-dation and sequence-stratigraphic framework in the UpperCretaceous foreland basin of southwestern Wyoming. In: Se-quence StratigraphyandFaciesAssociations (Ed. byH.W.Posamen-tier, C.P. Summerhayes, B.U. Haq & G.P. Allen), Int. Assoc.Sediment. Spec. Publ. 18, 501^520.

Dickinson,W.R. (1974) Plate tectonics and sedimentation. In:Tectonics andSedimentation (Ed. byW.R.Dickinson),SEPMSpec.Publ. 22, 1^27.

Dunn, J.F., Hartshorn, K.G. & Hartshorn, P.W. (1995)Structural styles and hydrocarbon potential of the sub-An-dean thrust belt of southern Bolivia. In: Petroleum Basins ofSouth America (Ed. by A.J. Tankard, R. Suarez & H.J.Welsink),Am. Assoc. Petrol. Geol.Mem., 62, 523^543.

Echavarria, L., Hernandez, R., Allmendinger, R. & Rey-

nolds, J. (2003) Subandean thrust and fold belt of northwes-ternArgentina; geometry and timing of theAndean evolution.Am. Assoc. Petrol. Geol. Bull., 87, 965^985.

Ege,H., Sobel, E.R., Scheuber, E. & Jacobshagen,V. (2007)Exhumation history of the southern Altiplano plateau (south-ern Bolivia) constrained by apatite ¢ssion-track thermochro-nology.Tectonics, 26, doi: 10.1029/2005TC001869.

Einsele, G., Ratschbacher, L. &Wetzel, A. (1996) The Hi-malaya-Bengal Fan denudationâccumulation system duringthe past 20Ma. J. Geol., 104, 163^184.

Elger,K.,Oncken,O.&Glodny, J. (2005) Plateau-style accu-mulation of deformation: SouthernAltiplano.Tectonics, 24, doi:10/1029/2004TC001675.

Epis,R.C.&Chapin,C.E. (1975)Geomorphic and tectonic im-plications of the post-Laramide, lateEocene erosion surface inthe southern Rocky Mountains. In: Cenozoic History of theSouthern Rocky Mountains (Ed. by B.F. Curtis), Geol. Soc. Am.Mem., 144, 45^74.

Flemings, P.B. & Jordan,T.E. (1989) A synthetic stratigraphicmodel of foreland basin development. J. Geophys. Res., 94,3851^3866.

Fuller, C.W.,Willett, S.D., Hovius, N. & Slingerland, R.(2003) Erosion rates forTaiwan mountain basins: newdetermi-nations from suspended sediment records and a stochasticmodel of their temporal variation. J. Geol., 111, 71^87.

Geslin, J.K., Demko, T.M., Drzewiecki, P.A., Feldman,H.R., Hasiotis, S.T., McCrimmon, G.G., Van Wagoner,J.C. & Wellner, R.W. (2001). Sediment £ux, paleoclimate,and sequence stratigraphy; lessons learned from numericalmodeling. 7th International Conference on Fluvial sedimen-tology Proceedings, Open-File Report, University of Nebras-ka-Lincoln, Conservation and Survey Division, Report:OFR-60, 106.

Geslin, J.K., Demko, T.M., Drzewiecki, P.A., Feldman,H.R., Hasiotis, S.T., McCrimmon, G.G., Van Wagoner,J.C. & Wellner, R.W. (2002). Relative role of stream dis-charge, sediment £ux, and baselevel change in stratal architec-ture of continental and nearshore sequences; results fromforward numerical modeling. AAPG Annual Meeting Ex-panded Abstracts, 63.


107


Gohain, K. & Parkash, B. (1990) Morphology of the KosiMegafan. In: Alluvial Fans: A Field Approach (Ed. by A.H. Ra-chocki &M. Church), pp.151^178. JohnWiley & Sons, Chiche-ster, UK.

Gubbels,T.L. (1993) Tectonics and geomorphology of the east-ern £ank of the central Andes,18^231S latitude.DoctoralThe-sis, Cornell University, Ithaca.

Gubbels,T.L., Isacks,B.L.&Farrar,E. (1993)High-level sur-faces, plateau uplift, and foreland development, Bolivian cen-tral Andes.Geology, 21, 695^698.

Gupta, S. (1997) Himalayan drainage patterns and the originof £uvial megafans in the Ganges foreland basin. Geology, 25,11^14.

Hay, W.W., Wold, C.N. & Herzog, J.M. (1992) Preliminarymass-balanced 3-D reconstructions of the Alps and sur-rounding areas during the Miocene. In: Computer Graphics inGeology (Ed. by R. P£ug & J.W. Harbaugh), Lect. Notes EarthSci., 41, 99^110.

Horton, B.K. (1999) Erosional control on the geometry and ki-nematics of thrust belt development in the central Andes.Tec-tonics, 18, 1292^1304.

Horton, B.K. & DeCelles, P.G. (1997) The modern forelandbasin system adjacent to the Central Andes. Geology, 25, 895^898.

Horton, B.K. & DeCelles, P.G. (2001) Modern and ancient£uvial megafans in the foreland basin system of the CentralAndes, southern Bolivia; implications for drainage networkevolution in fold-thrust belts.Basin Res., 13, 43^63.

Hovius,N. & Leeder,M. (1998) Clastic sediment supply to ba-sins.Basin Res., 10, 1^5.

Hulka, C. (2005) Sedimentary and tectonic evolution of theCenozoic Chaco foreland basin, southern Bolivia. DoctoralThesis, Freie Universitat, Berlin.

Hulka, C., Grafe, K.-U., Sames, B., Uba, C.E. & Heubeck,C. (2006) Depositional setting of theMiddle to LateMioceneYecua Formation of the Chaco Foreland Basin, southern Boli-via. J. South Am. Earth Sci., 21, 135^150.

Iriondo,M. (1993) Geomorphology and late Quaternary of theChaco (South America).Geomorphology, 7, 289^303.

Iriondo,M.H. (1984) The quaternary of northeastern Argenti-na. In: Quaternary of South America and Antarctic Peninsula (Ed.by J. Rabassa). 2, 51^78.

Isacks, B.L. (1988) Uplift of the Central Andean Plateau andbending of the Bolivian Orocline. J. Geophys. Res., 93, 3211^3231.

Jordan,T.E. (1995) Retroarc foreland and related basins. In:Tec-tonics of Sedimentary Basins (Ed. by C.J. Busby & R.V. Ingersoll),pp. 331^362. Blackwell Science, Oxford.

Jordan,T.E. & Alonso, R.N. (1987) Cenozoic stratigraphy andbasin tectonics of the Andes Mountains, 201^281 south lati-tude.Am. Assoc. Petrol. Geol. Bull., 71, 49^64.

Jordan,T.E., Reynolds, J.H. III & Erikson, J.P. (1997) Varia-bility in age of initial shortening and uplift in the Central An-des. In: Tectonic Uplift and Climate Change (Ed. by W.F.Ruddiman), pp. 41^61. Plenum Press, NewYork, NY.

Kennan, L. (2000) Large-scale geomorphology of the Andes;interrelationships of tectonics, magmatism and climate. Ed.by M.A. Summer¢eld), pp. 167^199. JohnWiley & Sons, Chi-chester, UK.

Kennan,L.,Lamb, S.&Rundle, J. (1995) K-Ar dates from theAltiplano and Cordillera Oriental of Bolivia; implications forCenozoic stratigraphy and tectonics. J. South Am. Earth Sci., 8,163^186.

Kennan, L., Lamb, S.H. &Hoke, L. (1997) High-altitude pa-laeosurfaces in the Bolivian Andes; evidence for late Cenozoicsurface uplift. In: Palaeosurfaces; Recognition, Reconstruction andPalaeoenvironmental Interpretation (Ed. by M.Widdowson), Spec.Publ. Geol. Soc. Lond., 120, 307^323.

Kley, J. (1996) Transition from basement-involved to thin-skinned thrusting in the Cordillera Oriental of southern Boli-via.Tectonics, 15, 763^775.

Kley, J. (1999) Geologic and geometric constraints on a kine-matic model of the Bolivian Orocline. J. South Am. Earth Sci.,12, 221^235.

Kuhlemann, J., Frisch,W., Dunkl, I. & Szekely, B. (2001)Quantifying tectonic versus erosive denudation by the sedi-ment budget; theMiocene core complexes of theAlps.Tectono-physics, 330, 1^23.

Kuhlemann, J.,Frisch,W.,Szekely,B.,Dunkl, I.&Kazmer,M. (2002) Post-collisional sediment budget history of theAlps; tectonic versus climatic control. Int. J. Earth Sci., 91,818^837.

Lander, R.H. & Walderhaug, O. (1999) Predicting porositythrough simulating sandstone compaction and quartz cemen-tation.Am. Assoc. Petrol. Geol. Bull., 83, 433^449.

Latrubesse, E.M., Stevaux, J.C. & Singha,R. (2005) Tropicalrivers.Geomorphology, 70, 187^206.

Le Pichon,X.,Fournier,M.& Jolivet,L. (1992) Kinematics,topography, shortening, and extrusion in the IndiaÊurasiacollision.Tectonics, 11, 1085^1098.

Leeder,M., Harris,T. & Kirkby, M. (1998) Sediment supplyand climate change; implications for basin stratigraphy. BasinRes., 10, 7^18.

Leier, A.L., DeCelles, P.G. & Pelletier, J.D. (2005) Moun-tains, monsoons, and megafans.Geology, 33, 289^292.

Marshall, L.G., Sempere,T. & Gayet,M. (1993) The Petaca(late Oligocene-middle Miocene) and Yecua (late Miocene)formations of the Subandean-Chaco Basin, Bolivia, and theirtectonic signi¢cance. In: Documents des Laboratoires de Geologie,Lyon, 125 (Ed. by M. Gayet), 291^301. Departement desSciences de laTerre, Universite Claude Bernard.

McMillan, M.E., Heller, P.L. & Wing, S.L. (20060)History and causes of post-Laramide relief in theRocky Mountain orogenic plateau. Geol. Soc. Am. Bull., 118,393^405.

McQuarrie,N. (2002) The kinematic history of the central An-dean fold-thrust belt, Bolivia; implications for building a highplateau.Geol. Soc. Am. Bull., 114, 950^963.

McQuarrie, N., Barnes, J.B. & Ehlers, T.A. (2008) Geo-metric, kinematic, and erosional history of the centralAndean Plateau, Bolivia (15^171S). Tectonics, TC3007, doi:10.1029/2006TC002054.

McQuarrie,N.,Horton,B.K.,Zandt,G.,Beck,S.&DeCel-

les, P.G. (2005) Lithospheric evolution of the Andean fold-thrust belt, Bolivia, and the origin of the central Andeanplateau.Tectonophysics, 399, 15^37.

Moretti, I.,Baby, P.,Mendez,E.&Zubieta,D. (1996)Hydro-carbon generation in relation to thrusting in the Sub AndeanZone from18 to 221S, Bolivia. Petrol. Geosci., 2, 17^28.

Mˇller, J.P.,Kley, J. & Jacobshagen,V. (2002) Structure andCenozoic kinematics of the Eastern Cordillera, southern Boli-via (211S).Tectonics, 21, doi: 10.1029/2001TC001340.

Overeem, I., Syvitski, J.P.M., Hutton, E.W.H. & Kettner,A.J. (2005) Stratigraphic variability due to uncertainty in mod-el boundary conditions; a case study of the New Jersey shelfover the last 40,000 years.Mar. Geol., 224, 23^41.


108


Patterson, P.E., Sprague, A.R., Hill, R.E. & McDonald,K.M.Anonymous (1995). Sequence stratigraphy and £uvialfacies architecture, Farrer and Tuscher formations (Campa-nian),TusherCanyon,UT.AAPG and SEPMAnnualMeetingAbstracts 4, 74.

Pazzaglia, F.J. & Brandon, M.T. (1996) Macrogeomorphicevolution of the post-Triassic Appalachian Mountains deter-mined by deconvolution of the o¡shore based sedimentary re-cord.Basin Res., 8, 255^278.

Perez, R., Ghosh, S., Chatellier, J.-Y. & Lander, R.Anon-ymous (1999). Application of sandstone diagenetic modelingto reservoir quality assesment of theMisoa Formation, Bacha-quero Field, Maracaibo Basin, Venezuela. AAPG AnnualMeeting Expanded Abstracts, A107.

Phillips, J.D.&Gomez,B. (2007) Controls on sediment exportfrom the Waipaoa River basin, New Zealand. Basin Res., 19,241^252.

Placzek,C.,Quade, J. & Patchett, P.J. (2006) Geochronologyand stratigraphy of late Pleistocene lake cycles on the southernBolivian Altiplano; implications for causes of tropical climatechange.Geol. Soc. Am. Bull., 118, 515^532.

Robin, C., Rouby, D., Granjeon, D., Guillocheau, F.,Allemand, P. &Raillard, S. (2005) Expression and model-ling of stratigraphic sequence distortion. Sediment. Geol., 178,159^186.

Robinson, R.A.J. & Slingerland, R.L. (1998a) Grain-sizetrends, basin subsidence and sediment supply in the Campa-nian Castlegate Sandstone and equivalent conglomerates ofcentral Utah.Basin Res., 10, 109^127.

Robinson, R.A.J. & Slingerland, R.L. (1998b) Origin of £u-vial grain-size trends in a foreland basin; the Pocono Forma-tion on the central Appalachian Basin. J. Sediment. Res., 68,473^486.

Roeder,D. & Chamberlain, R.L. (1995) Structural geology ofsub-Andean fold and thrust belt in northwestern Bolivia. In:PetroleumBasins of SouthAmerica (Ed. byA.J.Tankard, R. Suarez&H.J.Welsink),Am. Assoc. Petrol. Geol.Mem., 62, 459^479.

Schlunegger, F., Jordan,T.E. & Klaper, E.M. (1997) Con-trols of erosional denudation in the orogen on foreland basinevolution; theOligocene central SwissMolasse Basin as an ex-ample.Tectonics, 16, 823^840.

Schlunegger, F.,Melzer, J. & Tucker, G.E. (2001) Climate,exposed source-rock lithologies, crustal uplift and surfaceerosion; a theoretical analysis calibrated with data from theAlps/North Alpine foreland basin system. Int. J. Earth Sci., 90,484^499.

Scott,G.R. (1975) Cenozoic surfaces and deposits in the south-ern RockyMountains. In:Cenozoic History of the Southern RockyMountains (Ed. by B.F. Curtis), Geol. Soc. Am. Mem., 144,227^248.

Servant,M., Sempere,T., Argollo, J., Bernat,M., Feraud,G. & Lo Bello, P. (1989) Cenozoic morphogenesis and uplift

of the Eastern Cordillera in the Bolivian Andes. Comp. Rend.Acad. Sci. Paris, 309, 416^422.

Simpson,G. (2004) Dynamic interactions between erosion, de-position, and three-dimensional deformation in compres-sional fold belt setting. J. Geophys. Res., 109, doi: 10.1029/2003JF000111.

Simpson,G. (2006)Modelling interactions between fold-thrustbelt deformation, foreland £exure and surface mass transport.Basin Res., 18, 125^143.

Sobel, E.R. & Strecker, M.R. (2003) Uplift, exhumation andprecipitation: tectonic and climatic control of Late Cenozoiclandscape evolution in the northern Sierras Pampeanas, Ar-gentina.Basin Res., 15, 431^451.

Stock,G.M., Ehlers,T.A. & Farley,K.A. (2006) Where doessediment come from? Quantifying catchment erosion withdetrital apatite (U-Th)/He thermochronometry. Geology, 34,725^728.

Tucker,G.E.&Slingerland,R.L. (1996)Predicting sediment£ux from fold and thrust belts.Basin Res., 8, 329^349.

Uba,C.E.,Heubeck,C.&Hulka,C. (2005) Facies analysis andbasin architecture of the Neogene Subandean synorogenicwedge, southern Bolivia. Sediment. Geol., 180, 91^123.

Uba, C.E., Heubeck, C. & Hulka, C. (2006) Evolution of thelate Cenozoic Chaco foreland basin, southern Bolivia. BasinRes., 18, 145^170.

Uba, C.E., Strecker,M.R. & Schmitt, A.K. (2007) Increasedsediment accumulation rates and climatic forcing in the cen-tral Andes during the lateMiocene.Geology, 35, 979^982.

Van Wagoner, J.C. (1995) Sequence stratigraphy and marine tononmarine facies architecture of foreland basin strata, BookCli¡s, Utah, U.S.A.. In: Sequence Stratigraphy of Foreland BasinDeposits; Outcrop and Subsurface Examples from the Cretaceous ofNorth America (Ed. by J.C.VanWagoner & G.T. Bertram), Am.Assoc. Petrol. Geol.Mem., 64, 137^223.

VanWagoner, J.C.,Beaubouef,R.T.,Hoyal,D.C.J.D.,Dunn,P.A., Adair, N.L., Abreu,V.L.D.,Wellner, R.W., Awwil-

ler, D.N., Sun, T., Deffenbaugh, M. & HUH, C. (2003).Energy dissipation and the fundamental shape of siliciclasticsedimentary bodies. AAPG and SEPM Annual MeetingExtended Abstracts, 175.

Widdowson, M. (1997) The geomorphological and geologicalimportance of palaeosurfaces. In: Palaeosurfaces: Recognition,Reconstruction and Palaeoenvironmental Interpretation (Ed. by M.Widdowson), Spec. Publ. Geol. Soc. Lond., 120, 1^12.

Wilkinson, M.J., Marshall, L.G. & Lundberg, J.G. (2006)River behavior on megafans and potential in£uences on diver-si¢cation and distribution of aquatic organisms. J. South Am.Earth Sci., 21, 151^172.

Manuscript received 10 September 2007; manuscript accepted 28April 2008


109
