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Geological Society of America Bulletin

doi: 10.1130/0016-7606(2001)113<1387:PSSITA>2.0.CO;2 2001;113;1387-1400Geological Society of America Bulletin

B.K. Horton, B.A. Hampton and G.L. Waanders implications for initial mountain building in the central AndesPaleogene synorogenic sedimentation in the Altiplano plateau and

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For permission to copy, contact [email protected] 2001 Geological Society of America 1387

GSA Bulletin; November 2001; v. 113; no. 11; p. 1387–1400; 9 figures; 1 table.

Paleogene synorogenic sedimentation in the Altiplano plateau andimplications for initial mountain building in the central Andes

B.K. Horton*B.A. HamptonDepartment of Geology and Geophysics, Louisiana State University, Baton Rouge Louisiana 70803, USA

G.L. Waanders1611-C Rancho Santa Fe Road, San Marcos California 92069, USA

ABSTRACT

Sedimentologic data and palynologicalages from Paleogene clastic deposits of thenorthern and central Altiplano plateausuggest foreland basin development in thecentral Andes by mid-Paleocene time. Thenonmarine Potoco Formation (3000–6500m thick) constitutes the majority of Ce-nozoic basin fill. The Potoco overlies theSanta Lucia Formation (50–300 m thick),previously dated as mid-Paleocene bymammal fossils and magnetostratigraphy.New geochronologic data for the PotocoFormation include late Eocene to Oligo-cene palynomorph assemblages recoveredthroughout lower to upper stratigraphiclevels. These ages and published 40Ar/39Arand K-Ar ages from overlying upper Oli-gocene–lower Miocene volcaniclastic rocksindicate (1) nondeposition or greatly re-duced deposition (average sediment-accu-mulation rates ,10 m/m.y.) from mid-Pa-leocene to middle Eocene time (top SantaLucia to lowermost Potoco Formation),followed by (2) rapid deposition (averagesediment-accumulation rates up to 500 m/m.y.) throughout late Eocene and Oligo-cene time (majority of Potoco Formation).

Lithofacies and paleocurrent data con-firm both depositional phases. A 20–100-m-thick interval of superimposed paleosols inthe basal Potoco supports reduced sedimentaccumulation and low rates of subsidenceduring mid-Paleocene to middle Eocenetime. The overlying main body of the Po-toco contains facies assemblages and strati-graphic architecture (dominantly nonero-

*Present address: Department of Earth and SpaceSciences, University of California, Los Angeles,California 90095-1567, USA; e-mail: [email protected].

sive fluvial sheet sandstone interbeddedwith overbank mudstone) consistent withan aggradational fluvial system in a rapidlysubsiding late Eocene–Oligocene environ-ment. An upsection reversal in paleocur-rents across the Santa Lucia–Potoco con-tact, from west-directed to east-directedpatterns, also indicates a major change indepositional systems.

The switch from low to high rates of sub-sidence and reversal in paleocurrents canbe explained by eastward migration of aforeland basin system. Basin migration mayhave induced a change from a mid-Paleo-cene–middle Eocene, broad-wavelength,low-amplitude forebulge to a late Eocene–Oligocene, rapidly subsiding foredeep. Ad-vance of such a forebulge-foredeep pairwould require a topographic load to thewest that propagated eastward throughtime. Although synchronous loading east ofthe Altiplano is plausible, paleocurrent datasuggest a greater influence of sediment dis-persal systems derived from a belt to thewest. In general, the Paleogene sedimentaryhistory of the Altiplano is compatible withshortening and crustal thickening in aneastward-propagating contractional belt tothe west in the modern arc and forearc re-gions of westernmost Bolivia and Chile.

Keywords: Andes, Altiplano, foreland ba-sins, fold-and-thrust belts, palynology,plateaus.

INTRODUCTION

The high topography of the Altiplano pla-teau (3.7 km average elevation) provides afirst-order signal of Cenozoic mountain build-ing in the central Andes (Isacks, 1988). Esti-mates for the timing of shortening and asso-

ciated crustal thickening and surface upliftrely on the synorogenic stratigraphic recordwithin and adjacent to the orogenic belt (e.g.,Jordan and Alonso, 1987; Sempere et al.,1990; 1997; Kennan et al., 1995; Allmendin-ger et al., 1997; Horton and DeCelles, 1997;Jordan et al., 1997; Lamb and Hoke, 1997;Lamb et al., 1997; Horton, 1998). Most in-vestigations attribute crustal thickening in thecentral Andes to Neogene-age shortening inthe fold-thrust belt east of the Altiplano (Shef-fels, 1990; Schmitz, 1994; Baby et al., 1997),primarily on the basis of upper Oligocene andyounger synorogenic strata identified in theforeland and Altiplano basin (Isacks, 1988;Sempere et al., 1990; Gubbels et al., 1993).As a result, kinematic and geodynamic modelsattempting to explain the mechanisms andrates of plateau uplift and fold-thrust defor-mation (e.g., Isacks, 1988; Gubbels et al.,1993; Wdowinski and Bock, 1994; Pope andWillett, 1998; Liu et al., 2000) generally uti-lize this estimate of shortening since 25–30Ma. However, several studies propose thatpre-Neogene deformation may have played animportant role in the tectonic history of thecentral Andes (e.g., Horton and DeCelles,1997; Sempere et al., 1997). If pre-Neogenemountain building occurred in regions west ofthe eastern fold-thrust belt, then most modelsunderestimate not only the duration but alsothe total amount of crustal shortening acrossthe orogenic belt. The purpose of this paper isto provide sedimentologic, provenance, andpalynological data from Paleogene sedimen-tary rocks of the northern and central Altipla-no that suggest initial foreland basin condi-tions and associated mountain building in thecentral Andes by mid-Paleocene time.

TECTONIC FRAMEWORK

The Andes are the type example of a moun-tain belt developed along an ocean-continent



1388 Geological Society of America Bulletin, November 2001

HORTON et al.

Figure 1. Map of the central Andes, including international borders, topographic-geologiczonations, and area over 3 km elevation (shaded). Dotted line defines axis of LongitudinalValley (LV). CC—Coastal Cordillera, SA—Salar de Atacama, SB—Santa Barbara. Naz-ca–South America convergence at the trench (178S, 758W) is represented by NUVEL-1Avector (DeMets et al., 1994) and GPS vector (Norabuena et al., 1998). Boxed area showslocation of Altiplano region (Fig. 2).

convergent margin (Dewey and Bird, 1970).In the central Andes, from about 158S to 278S,the subducted Nazca plate dips ;308 eastwardbeneath the South American plate (Isacks,1988). NUVEL-1A and global positioningsystem (GPS) calculations indicate roughlyeast-west convergence at 77 mm/yr and 68mm/yr, respectively (DeMets et al., 1994;Norabuena et al., 1998). From west to east,the continental margin at about 15–278S con-sists of (Fig. 1): (1) the Peru-Chile trench andcontinental slope; (2) the Coastal Cordillera, aremnant of a Mesozoic magmatic arc; (3) theLongitudinal Valley (also called Central De-pression or Pampa del Tamarugal), a modernforearc basin; (4) the Precordillera (includingCordillera Domeyko) and Preandean Depres-sion (including Salar de Atacama basin), thatlargely define the western Andean slope; (5)the Western Cordillera, an active volcanic arcalong the eastern border of Chile; (6) the Al-tiplano-Puna plateau, a high-elevation hinter-land plateau; (7) the Eastern Cordillera, a rug-ged interior of the eastern fold-thrust belt; (8)the Subandean Zone and Santa Barbara Zone,frontal active parts of the eastern fold-thrustbelt; and (9) the Beni Plain and Chaco Plain,parts of a low-elevation foreland basin under-lain by the Brazilian shield.

Modern shortening in the central Andes isfocused in the fold-thrust belt on the easternslope of the mountain belt (Norabuena et al.,1998; Horton, 1999). Most studies considertotal central Andean shortening to be approx-imated by Neogene shortening in the EasternCordillera and Subandean–Santa BarbaraZone (Fig. 1), a maximum value of about200–250 km (see summaries by Allmendingeret al., 1997; Kley and Monaldi, 1998). Thisestimated shortening is insufficient to explainpresent-day crustal thickness beneath the Al-tiplano, modern arc (Western Cordillera), andmodern forearc region (Precordillera, Longi-tudinal Valley, and Coastal Cordillera), lead-ing some workers to propose additional mech-anisms of crustal thickening such as magmaticaddition or tectonic underplating (Schmitz,1994; Baby et al., 1997). Alternatively, pre-Neogene shortening in these western regionsmay account for the discrepancy (Horton andDeCelles, 1997). Structural, stratigraphic, andpaleomagnetic studies indicate Late Creta-ceous–Paleogene shortening, basin develop-ment, and vertical-axis rotations in the modernarc to forearc region (Chong and Reutter,1985; Bogdanic, 1990; Hammerschmidt et al.,1992; Hartley et al., 1992; Scheuber and Reut-ter, 1992; Charrier and Reutter, 1994; Mpo-dozis et al., 1999; Somoza et al., 1999; Arria-gada et al., 2000; Roperch et al., 2000a,2000b). However, the extent of any pre-Neo-

gene mountain belt and associated forelandbasin system, as well as the total amount ofpre-Neogene shortening, remain poorlyunderstood.

CRETACEOUS–TERTIARYSTRATIGRAPHY

The Altiplano (Fig. 2) contains (1) a lowersuccession (250–900 m thick) of regionallyextensive, Maastrichtian to mid-Paleocene,marginal marine and nonmarine sedimentaryrocks (El Molino and Santa Lucia Forma-tions), (2) an intermediate, poorly dated suc-cession (3000–6500 m thick) of nonmarinesedimentary rocks (Potoco Formation), and(3) an upper interval (1000–4000 m thick) ofupper Oligocene to Quaternary nonmarinesedimentary and volcanic rocks (Fig. 3).

Lower Succession

The lower succession is composed of the ElMolino Formation and conformably overlyingSanta Lucia Formation (Fig. 3). The El Mol-ino is a 200–600-m-thick section of carbonateand subordinate mudstone. Strata were depos-ited over much of the Altiplano and EasternCordillera (Fig. 2) in restricted shallow ma-rine, lacustrine, and distal fluvial settings. Thisformation marks the final marine conditions in

the Altiplano–Eastern Cordillera region(Lundberg et al., 1998). The stratigraphic ageis well defined by Maastrichtian marine fos-sils, 40Ar/39Ar ages of two tuffs from the lowerEl Molino (71.59 6 0.25 Ma, 72.57 6 0.15Ma) in the east-central Altiplano near the townof Chita (Fig. 2), and magnetostratigraphicsections in the Eastern Cordillera (Gayet et al.,1991; Sempere et al., 1997).

In the Altiplano, the Santa Lucia Formationis 50–300 m thick and contains a coarsening-and thickening-upward section of interbeddedmudstone and sandstone deposited in distalfluvial settings. Although trough cross-strati-fied sandstone is rare in the lower two-thirdsof the formation, it is abundant in the upperSanta Lucia. Paleocurrent indicators consis-tently yield westward paleoflow directions(Fig. 4). Some investigators have interpretedthe sandstone-dominated, upper Santa Luciainterval as a separate unit, the Cayara For-mation (Marocco et al., 1987; Sempere et al.,1997). For the Altiplano, we retain the origi-nal stratigraphic designation due to an overallconsistency in composition, provenance, anddepositional setting. Different workers havecorrelated the Santa Lucia (including Cayara)regionally with different stratigraphic units ofvarying age, including units in Peru and Ar-gentina (see Palma, 1986; Marocco et al.,



Geological Society of America Bulletin, November 2001 1389

PALEOGENE SYNOROGENIC SEDIMENTATION IN THE ALTIPLANO PLATEAU

Figure 2. Geologic map of the Altiplano andsurrounding region, depicting structuralrelationships and locations of measuredsections, lakes, towns, and internationalborders (after Pareja et al., 1978, andMarsh et al., 1992). HFTB—Huarina fold-thrust belt.

1987; Mourier et al., 1988; Quattrocchio et al.,1990; Marshall et al., 1997; Sempere et al.,1997). However, the age of the Santa Lucia isbest defined by magnetostratigraphy and Pa-leocene mammal fossils in the Eastern Cor-dillera of Bolivia (Gayet et al., 1991; Sempereet al., 1997; Marshall et al., 1997). Sempereet al. (1997) reported exclusively reversedmagnetic polarities from paleomagnetic sitesin the uppermost El Molino and entire SantaLucia section, attributing these rocks to chronC26r (61.6–57.8 Ma) of the geomagnetic po-larity time scale (Cande and Kent, 1992). Thetop of the Santa Lucia therefore is assigned anage of ;58 Ma. Despite successful dating ofthe Santa Lucia in several Bolivian locales,regional lithostratigraphic correlations overhundreds of kilometers remain tentative.

Middle Succession

The 3000–6500-m-thick Potoco Formation,the focus of this study, comprises the greatestvolume of Tertiary deposits in the Altiplanoand possibly the entire central Andes. Thissuccession is identified by its mixed litholo-gies of sandstone, mudstone, and limitedevaporite, its fluvial-lacustrine facies associa-tions, and its conformable stratigraphic rela-tion with underlying Santa Lucia strata andoverlying Neogene rocks (Fig. 3). Despite thepresence of these deposits in a nearly contin-uous outcrop belt that trends north-northwestfor ;300 km in the northern and central Al-tiplano (Fig. 2), Potoco-equivalent rocks havebeen named, from north to south, the Tia-huanacu, San Andres, Ballivian, Pando, Ber-enguela, Turco, Azurita, Chuquichambi,Huayllamarca, and Huayllapucara Formations(Evernden et al., 1977; Suarez and Diaz, 1996;Lamb and Hoke, 1997). Inconsistencies instratigraphic nomenclature may result frompoor age control, postdepositional deforma-tion (including complex fold-thrust structuresinvolving evaporite units), and insufficient ex-posure of the lower section and basal SantaLucia contact (e.g., sections 5–8, 10 in Fig.4). Reported ages for the upper TiahuanacuFormation include K-Ar ages of two tuffs(29.2 6 0.8 Ma, 29.6 6 0.8 Ma; no samplelocations given; Swanson et al., 1987; Sem-pere et al., 1990). These data and general




HORTON et al.

Figure 3. Chronostratigraphic diagram of the uppermost Cretaceous–Cenozoic rocks inthe northern and central Altiplano, including symbols denoting available age control.Rocks underlying and overlying the Potoco Formation have been dated by previous studies(see text). New palynological age data for the Potoco Formation (see text) are based onthe plotted age ranges of several key species (from Regali et al., 1974a, 1974b). Shadingrepresents the palynological age restrictions for the majority of the Potoco Formation.Time scale from Cande and Kent (1992).

stratigraphic relationships require that the Po-toco Formation be post–mid-Paleocene andmostly pre–early Miocene in age. We presentadditional age constraints below.

Upper Succession

The upper succession is a 1000–4000-m-thick interval of upper Oligocene to Quater-nary sedimentary and volcanic rocks (Fig. 3).Basal conglomerate and sandstone include,from north to south, the Penas, Coniri, Luri-bay-Salla, lower Totora, Tambillo, and San Vi-cente Formations (Evernden et al., 1977; Sem-pere et al., 1990; MacFadden, 1990; Suarezand Diaz, 1996; Lamb and Hoke, 1997).These strata were deposited in alluvial-fan andfluvial systems derived from the east, presum-ably from the incipient Eastern Cordillera(Sempere et al., 1990; Rochat et al., 1998).This assertion is supported by upper Oligo-cene–lower Miocene growth strata reported ina west-vergent backthrust belt (the Huarinafold-thrust belt) at ;178S along the Altiplano–Eastern Cordillera boundary (Sempere et al.,1990) and studies demonstrating shortening inthe Eastern Cordillera by late Oligocene time(Baby et al., 1990; Sempere et al., 1990; Hor-ton, 1998). A late Oligocene to early Mioceneage for the coarse-grained interval is based on(1) K-Ar ages of four tuffs (28.0 6 0.9 Ma to25.1 6 0.7 Ma), fission-track ages, magneto-stratigraphy, and mammal fossils in the Luri-bay-Salla beds (Salla region; MacFadden,1990), (2) K-Ar ages of two tuffs (25.5 6 1.7Ma, 24.5 6 0.6 Ma) in the Coniri Formation(no sample locations given; Swanson et al.,1987; Sempere et al., 1990), (3) K-Ar ages ofa single tuff (23.9 6 1.3 Ma, 23.0 6 0.8 Ma)from the lowermost Totora Formation (nearthe town of Corque; Kennan et al., 1995), and(4) K-Ar ages of three volcanic horizons (25.26 0.9 Ma to 23.1 6 1.2 Ma) in the lowermostTambillo Formation (Tambo Tambillo region;Kennan et al., 1995).

The coarse-grained rocks are overlain bylower Miocene to Quaternary volcaniclasticdeposits, including, from north to south: theKollu Kollu, Mauri, Abaroa, Crucero, upperTotora, and Quehua Formations; several tuffsof regional distribution (Ulloma tuff, Callapatuff, and Tuff 76); and numerous other strati-graphic units (see Evernden et al., 1977; Sua-rez and Diaz, 1996). These rocks have beendated by 40Ar/39Ar and K-Ar methods, mag-netostratigraphy, and mammal fossils (Lavenuet al., 1989; Marshall et al., 1992; Roperch etal., 1999).

PALYNOLOGICAL AGE DATA

Palynomorph assemblages (Figs. 5 and 6)from 11 Potoco siltstone samples collectedabove of a basal paleosol zone yield agesranging from late Eocene through Oligocene(Table 1; Fig. 3). Age assignments utilizeSouth American palynological age ranges thatare based on correlation of palynomorph spe-cies with existing age zonations for marinefauna (foraminifera and nannofossils) andnonmarine fauna (ostracods) of the Braziliancontinental margin (Regali et al., 1974a,1974b), including supportive age informationfrom global palynological studies (e.g., Ger-meraad et al., 1968). Seven samples are fromthe lower few hundred meters of the PotocoFormation; two are defined as late Eocene,

and five are defined as late Eocene to earlyOligocene. From the upper levels of the Po-toco Formation, four samples are defined asearly Oligocene or Oligocene. Seven addition-al samples yielded no palynomorphs. All sam-ples were collected from several-centimeters-thick, nonoxidized siltstone beds (see Fig. 4for sample locations within measured strati-graphic sections). For each sample, approxi-mately 5–10 g of material was digested in hy-drochloric and hydrofluoric acids, screenedwith a 10 mm sieve, floated in a solution ofzinc bromide (specific gravity of 2.0), andmounted on standard microscope slides. Theslides were examined and species identifica-tions made using a standard light microscope.

Ages for individual species (Fig. 3) providethe following age restrictions for the 11 pa-





lynomorph assemblages (Table 1). Late Eo-cene to early Oligocene ages for seven sam-ples are defined by occurrences ofEchiperiporites estelae (Fig. 5, photo 1), Zon-ocostites ramonae (Fig. 5, photo 14), and Po-docarpidites sp. 1 and 2 (Fig. 6, photos 1, 2,5, and 6), which do not range below late Eo-cene, and Scabraperiporites nativensis (Fig. 5,photos 2–5), which does not range above earlyOligocene. Two of these samples are restrictedto late Eocene by the additional presence ofMargocolporites vanwijhei (Fig. 5, photo 8)and Retitricolpites americana (Fig. 5, photo9), which have a range no younger than Eo-cene. Oligocene ages for four samples are de-fined by Podocarpidites sp. 1 and 2, whichrange from late Eocene through Oligocene,Scabraperiporites nativensis, which does notrange above early Oligocene, and Psilaperi-porites robustus (Fig. 5, photos 6 and 7),which is generally restricted to Oligocene.

In summary, palynomorphs consistently in-dicate a late Eocene–Oligocene age for themain body of the Potoco Formation. Lack ofspecimens from a basal paleosol zone pre-cludes direct age assignment for the lower fewtens of meters of the formation. However, thecombination of Potoco palynological data andpublished ages from the underlying Santa Lu-cia Formation require that the paleosol zonecomprising the lowermost Potoco is post–mid-Paleocene and pre–late Eocene in age.

PALEOGENE SEDIMENTATION

New palynological age data for the PotocoFormation, combined with existing stratigraph-ic ages for underlying and overlying forma-tions, indicate greatly reduced sediment accu-mulation during mid-Paleocene to mid-Eocene(lowermost Potoco) time and rapid sedimentaccumulation throughout late Eocene and Ol-igocene (majority of Potoco) time. We con-ducted sedimentologic analyses of the Paleo-gene succession in order to evaluate thelithofacies and provenance associated withthese sedimentation patterns. Facies analysesand paleocurrent determinations are presentedfor ten measured sections (Fig. 4). The SantaLucia–Potoco contact is exposed in five sec-tions and covered in the other five sections.Paleocurrent data are based on measuredlimbs in trough cross-stratified sandstone(method I of DeCelles et al., 1983) and mea-sured primary current lineations, sole marks,and ripple marks.

Mid-Paleocene to Middle Eocene

The lower Santa Lucia Formation is a suc-cession of thin-bedded, laminated to massive

mudstones, horizontally and ripple cross-strat-ified, fine-grained sandstones, and thin calcar-eous paleosols (Fig. 7A). The proportion andthickness of sandstone beds increase upsec-tion. The upper Santa Lucia contains predom-inantly trough and ripple cross-stratified, me-dium-grained sandstone (Fig. 7B). Weattribute these facies to deposition in a distalfluvial environment, both in overbank (lowerSanta Lucia) and channel (upper Santa Lucia)settings. Paleocurrent measurements indicateeast-to-west sediment dispersal (Fig. 4).

The sharp contact between the Santa Luciaand overlying Potoco Formation (Fig. 7B) isdefined by an abrupt change from cross-strat-ified sandstone to an overlying 20–100-m-thick section of numerous superimposed pa-leosols (Fig. 7C). The paleosol intervalcontains very few nonpedogenic facies or pri-mary sedimentary structures. Individual pa-leosols are massive 10–200-cm-thick layers ofmottled siltstone and fine-grained sandstonethat commonly contain clay-skin (argillan)coatings and slickenside surfaces around soilaggregates or peds 0.5–2 cm in diameter; inplaces they exhibit root traces, calcareous nod-ules, and calcite-filled fractures (Fig. 7D).Whereas the bulk of each paleosol is moder-ately calcareous, the upper 10–50 cm of in-dividual paleosol units is commonly noncal-careous (Fig. 7D). We attribute this zonationto an upper zone of leaching and lower zoneof accumulation during soil development.

In contrast to the Potoco Formation, SantaLucia paleosols are very thin (1–5 cm aver-age, 20 cm maximum) and interbedded withstratified sandstone and laminated siltstone ona centimeter scale (Fig. 7A). Given their thick-ness and stratigraphic occurrence, Santa Luciapaleosols are interpreted as products of rela-tively short-lived episodes (probably ,105 yr)of pedogenesis between successive sedimen-tation events in an overbank fluvial system.Similar interpretations have been made formodern and ancient soils in floodplain envi-ronments (e.g., Kraus and Bown, 1986; Willis,1993). Basal Potoco paleosols, however, aresubstantially thicker and invariably occurwithin a stacked paleosol interval tens of me-ters thick (Fig. 4). We therefore interpret thePotoco paleosol zone as the product of a rel-atively long-lived episode (.106 yr) of sus-tained soil development. Clearly, source li-thology, climate, and biologic activity areadditional controls on the facies type andthickness of individual paleosols. However,the remarkable composite thickness and lackof nonpedogenic facies suggest that, regard-less of the local climatic and biogeochemicalconditions, substantial time and minimal sed-iment input were required for such soil devel-

opment. These conditions may be attributableto extremely low rates of subsidence (e.g., At-kinson, 1986; Alonso-Zarza et al., 1999).

Late Eocene and Oligocene

The basal Potoco paleosol zone is overlainconcordantly by the main body of the PotocoFormation (Fig. 4), an interval of laterallycontinuous sheet sandstone and interbeddedmudstone (Figs. 7B, 8A and B). Sandstonebeds contain horizontal, ripple, and troughcross-stratification, are up to 3 m thick, exhibithigh width-to-thickness ratios (50/200), andare characterized by basal contacts that arenonerosive or very slightly erosive (,30 cmof scour relief) (Fig. 8C). These beds are at-tributed to channel and crevasse-splay depo-sition. The channel sandstones, however, gen-erally lack lateral-accretion surfaces,multistory bedding, high-relief scours, andbasal lags. Similar fluvial deposits have beendescribed by Friend (1978) and Miall (1980).Such deposits contrast with common modelsof channelized fluvial systems in which mo-bile channels (meandering or braided) inciseand rework laterally adjacent deposits (Friend,1983). Avulsion, rather than channel migra-tion, may dominate these sheet-sand systemswherein the active stream channel, seeking thetopographically lowest area, rapidly redirectsits course into low flood-plain areas (Smith etal., 1989). In the Potoco Formation, a com-mon lithofacies assemblage is a 5–10-m-thicksection composed of overbank siltstones andcrevasse-splay sandstones that coarsen andthicken upward and are overlain by a 1–3-m-thick sandstone cap (Fig. 8D). We attributethis assemblage to increasing volumes of cre-vasse-splay sedimentation during successivefloods followed by the ultimate avulsion of thechannel into the adjacent floodplain area (e.g.,Willis, 1993). In general, facies associationsand stratigraphic geometries within the mainPotoco interval suggest deposition in an ag-gradational fluvial system, possibly implyinghigh sediment-accumulation rates and rapidsubsidence (e.g., Friend, 1978; Bentham et al.,1993) during late Eocene–Oligocene time.

Our paleocurrent data consistently indicatetransport from west to east (Fig. 4). Althoughprevious interpretations hold that the PotocoFormation was derived from the Eastern Cor-dillera to the east (e.g., Kennan et al., 1995;Lamb and Hoke, 1997; Lamb et al., 1997),those studies present no supporting paleocur-rent data or facies analyses. Additional evi-dence for west-to-east transport includes awell-defined proximal-to-distal facies trendobserved in outcrops on opposite limbs of theCorque syncline, a palinspastically restored




HORTON et al.

Figure 4. Measured stratigraphic sections of Paleogene deposits in the north-ern and central Altiplano (locations shown in Fig. 2), including facies types,formation names, stratigraphic correlations, paleocurrent data, published K-Ar dates, and locations of palynological samples. Rose diagrams depict east-directed paleocurrents in the Potoco Formation and west-directed paleocur-rents in the Santa Lucia Formation. Lithostratigraphic correlations for lowerintervals are made on the basis of distinctive, carbonate-bearing strata of theEl Molino Formation and gypsum beds of the Chuquichambi member of thePotoco Formation. Correlations for the upper interval are based on the strat-igraphically lowest occurrence of volcanic tuffs.





Figure 4. (Continued).




HORTON et al.

Figure 5. Photomicrographs of selected spore and pollen taxa representative of assemblages recovered from Paleogene deposits of thenorthern and central Altiplano (sample locations shown in Fig. 4). All figures 31000. (1) Echiperiporites estelae Germeraad et al., sampleR99P7; (2 and 5) Scabraperiporites nativensis Regali et al., sample R99P7; (3) S. nativensis Regali et al., sample PA79.0; (4) S. nativensisRegali et al., sample PB; (6 and 7) Psilaperiporites robustus Regali et al., sample PA79.0; (8) Margocolporites vanwijhei Germeraad etal., sample R99P7; (9) Retitricolpites americana Wymstra, sample 997111; (10) Monoporites annulatus Van der Hammen, sample 997121;(11 and 12) Psilaperiporites minimus Regali et al., sample R99P7; (13) Pxilatricolporites operculatus Van der Hammen, sample R99P7;(14) Zonocostites ramonae Germeraad et al., sample R99P7.





Figure 6. Photomicrographs of selected spore and pollen taxa representative of assemblages recovered from Paleogene deposits in thenorthern and central Altiplano (sample locations shown in Fig. 4). All figures 31000. (1 and 2) Podocarpidites sp. 1 sensu Regali et al.,sample 997121; (3 and 4) Cedridites sp. sensu Regali et al., sample 997121; (5 and 6) Podocarpidites sp. 2 sensu Regali et al., sample997121; (7) Ulmoideipites krempii Anderson emend. Elsik, sample PB; (8) Tricolpopollenites hians (Stanley) Elsik, sample PA79.0; (9)Jussitriporites sp. sensu Regali et al., sample R99P7; (10) Bombax sp., sample PA79.0; (11) T. hians (Stanley) Elsik, sample R99P7; (12)Pediastrum sp. (freshwater alga), sample PB.




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TABLE 1. PALYNOLOGICAL AGE DATA FOR POTOCO FORMATION

distance of about 70–80 km. Potoco strata onthe western limb of the syncline (Fig. 8B) aresignificantly coarser than equivalent strata(Fig. 8A) on the eastern limb (Fig. 4). We at-tribute this trend to alluvial fan and proximalfluvial deposition in the west, passing down-slope into more distal fluvial deposition to theeast.

DISCUSSION

Sedimentologic and geochronologic datafor the Altiplano indicate (1) a mid-Paleocenesediment source to the east during distal flu-vial deposition of the Santa Lucia Formation,(2) a mid-Paleocene to middle Eocene intervalof paleosol development, indicative of verylow sediment-accumulation rates during de-position of the lowermost Potoco, and (3) alate Eocene through Oligocene phase of rapidfluvial aggradation of the Potoco Formationadjacent to a western sediment source.

These temporal variations in sediment-ac-cumulation rates and sediment-dispersal direc-tions, as well as pre-Neogene structural rela-tionships in areas farther west, are consistentwith eastward migration of a Paleogene fore-land basin system (Fig. 9). The foreland basinsystem (DeCelles and Giles, 1996) in this in-terpretation includes a distal back-bulge de-pozone located in the Altiplano and EasternCordillera during Late Cretaceous to mid-Pa-leocene time (El Molino–Santa Lucia deposi-tion), a forebulge depozone located in the Al-tiplano to western Eastern Cordillera duringmid-Paleocene to middle Eocene time (low-

ermost Potoco deposition), and a foredeep de-pozone located in the Altiplano during lateEocene through Oligocene time (main Potocodeposition). Predicted rates of flexural subsi-dence in a migrating foreland basin systemvary from initial low rates in the back-bulgedepozone to extremely low rates in the fore-bulge depozone to high rates in the foredeepdepozone (Fig. 9C). Such a trend is recordedby the upsection variations in sediment-accu-mulation rates from Santa Lucia to lowermostPotoco to main Potoco strata. A change inoverall sediment-dispersal directions is alsoexpected as the topographic load, an incipientmountain belt to the west, propagated east-ward. The upsection change from an eastern,cratonic (Brazilian shield) source for the SantaLucia to a western, orogenic source for thePotoco suggests a progressively greater influ-ence of western regions on Altiplano sedi-mentation during Paleogene time. This pat-tern, however, does not rule out the possibilityof additional loading by shortening-related to-pography east of the Altiplano (i.e., the west-vergent Huarina fold-thrust belt).

It is important to note that accumulation ofsediment in the proposed forebulge depozone,albeit minimal (;20–100 m over a ;15–20m.y. time span), suggests that the forebulgewould not have formed a true topographic fea-ture. Rather, it would have been buried be-neath limited basin fill. This consideration andthe highly consistent pattern of Potoco sedi-ment dispersal (Fig. 4) in a direction trans-verse rather than parallel to the forebulge sug-gest an overfilled foreland-basin geometry

(Jordan, 1995). A possible exception exists inthe southern Altiplano region, where Maroccoet al. (1987) and Welsink et al. (1995) reportan erosional unconformity characterized byupper Santa Lucia to Potoco strata resting di-rectly on the El Molino Formation. Here, theforebulge may have formed a low-amplitudetopographic feature subject to minor erosion.Angular discordance in an area of severalsquare kilometers (Marocco et al., 1987) couldbe attributed to localized minor faulting on theforebulge, similar to reactivated faults in themodern forebulge region of the central An-dean foreland (Horton and DeCelles, 1997;Ussami et al., 1999).

Although our interpretation explains arange of observations, alternative models thatinvoke nonflexural mechanisms could con-ceivably explain Paleogene deposition in theAltiplano. For instance, Paleocene–Eocenestrata of the Santa Barbara Group in north-western Argentina have been considered torepresent postrifting thermal subsidence fol-lowing Cretaceous development of the Saltarift system (Salfity and Marquillas, 1994;Comınguez and Ramos, 1995). A similar in-terpretation could be offered for the Altiplano(e.g., Welsink et al., 1995), but accumulationof 3000–6500 m of Potoco sediment from lateEocene through Oligocene time would requireaverage thermal subsidence rates much higherthan the typical range of values in continentalrifts (McKenzie, 1978; Friedmann and Bur-bank, 1995). The large magnitude of sedimentaccumulation over ;10–15 m.y. effectivelyeliminates subsidence due to cooling and ther-





Figure 7. Photographs of lowermost Potoco and Santa Lucia Formations. (A) View of ;50-m-thick section (foreground) of interbeddedsiltstone, fine-grained sandstone, and thin calcareous paleosols in the lower-middle Santa Lucia Formation. Most resistant ledges areformed by ;10-cm-thick calcareous paleosols. Section 2; arrow points to seated person. (B) View of ;200-m-thick section composed ofupper Santa Lucia sandstone (narrow ridge at left), lowermost Potoco paleosol interval (slope-forming unit), and lower Potoco resistantsandstone (wide ridge at right). Section 1; arrow points to person standing at approximate top of paleosol interval. (C) Interval of ;2.5m of stacked nodular paleosols developed in fine-grained sandstone and siltstone of basal Potoco Formation. Section 2; 30-cm-longhammer is parallel to bedding. (D) Paleosol in basal Potoco Formation exhibiting upper horizon of mottled, bioturbated, fine-grainedsandstone containing root casts and lower horizon (below hammer) of calcareous nodules and discontinuous calcite-filled fractures withina fine-grained sandstone matrix. Section 3; hammer is parallel to bedding.

mal contraction alone. We note, however, thatpostrift thermal subsidence may have contrib-uted to total subsidence during El Molino–Santa Lucia deposition (latest Cretaceous tomid-Paleocene time). Nevertheless, the abruptincrease in sediment accumulation during lateEocene–Oligocene time must be related tosome other tectonic cause, most likely flexuralsubsidence related to thrust-sheet loading orfault-induced subsidence in an extensional orstrike-slip regime. At this point, we consideran extensional or strike-slip setting unlikelydue to a lack of demonstrated normal orstrike-slip faults along margins of the Potocodepositional system (however, compare All-mendinger et al., 1997; Lamb and Hoke, 1997;and Rochat et al., 1998), the regional extent

of the original Potoco depositional system,continuous for a minimum distance of ;300km along strike, and general tectonic recon-structions in which extension along the west-ern margin of South America (including theSalta rift system) is consistently assigned aMesozoic, not Cenozoic, age (Coney and Ev-enchick, 1994; Salfity and Marquillas, 1994,Welsink et al., 1995; Tankard et al., 1995). Incontrast, a flexural subsidence mechanism isin agreement with structural and paleomag-netic evidence for Late Cretaceous–Paleogeneshortening and vertical-axis rotations to thewest in the Longitudinal Valley and Precor-dillera region of Chile (e.g., Chong and Reut-ter, 1985; Hammerschmidt et al., 1992; Scheu-ber and Reutter, 1992; Somoza et al., 1999;

Arriagada et al., 2000; Roperch et al., 2000a,2000b). We speculate that a Paleogene andpossibly Upper Cretaceous foredeep may berepresented by the 4000–6000-m-thick non-marine Purilactis Group in the Salar de Ata-cama (Preandean Depression) region (Hartleyet al., 1992; Charrier and Reutter, 1994; Mpo-dozis et al., 1999) and equivalent strata in thePrecordillera (Bogdanic, 1990; Hammer-schmidt et al., 1992).

Our model of Paleogene shortening andforeland basin development provides an inter-nally consistent explanation for a variety ofobservations, including spatially and tempo-rally varying patterns of sediment accumula-tion (or subsidence), sediment dispersal, andpaleosol development in the Altiplano. One




HORTON et al.

Figure 8. Photographs of Potoco Formation. (A) View of ;450-m-thick section composed of interbedded sheets of laterally continuous,resistant sandstone (;1–3 m thick) and less resistant siltstone. Section 5. (B) View of ;50-m-thick section of stacked, cross-stratifiedsandstones. Foresets are best developed in lower resistant ledge at right. Section 6; person in center is ;1.8 m tall. (C) Prominent, ;2-m-thick sandstone overlying thin interbedded sandstone and siltstone. Tabular geometry and lack of basal erosional scour for thicksandstone indicate limited or no channel incision. Section 5; person at lower right is ;1.8 m tall. (D) View of ;10-m-thick, coarsening-and thickening-upward section of thinly interbedded siltstone and sandstone (left) overlain by thicker sandstone (right). Interval isinterpreted as progradation of a crevasse splay into a flood-plain environment. Section 5; hammer at lower right (arrow) is 30 cm long.

implication of this model is that shorteningwest of the Eastern Cordillera–Subandeanfold-thrust belt, particularly in the arc and fo-rearc regions, may play an important role intotal Andean crustal thickening.

CONCLUSIONS

Palynomorph assemblages from possiblythe thickest Paleogene succession in the cen-tral Andes, the 3000–6500-m-thick PotocoFormation in the Altiplano, indicate that themajority of the succession is of late Eocene–Oligocene age. When combined with the pub-lished mid-Paleocene age of the underlyingSanta Lucia Formation, these data require;15–20 m.y. of greatly reduced sediment ac-cumulation (,10 m/m.y.) during mid-Paleo-cene to middle Eocene deposition of the low-

ermost Potoco (20–100 m thick). Theminimum age for the Potoco succession isprovided by published late Oligocene–earlyMiocene ages from overlying volcaniclasticrocks. Average sediment-accumulation ratesup to 500 m/m.y. throughout late Eocene–Ol-igocene time (;10–15 m.y.) suggest rapidsubsidence in the Altiplano during depositionof the main body of the Potoco Formation.

Sediment dispersal patterns for the PotocoFormation indicate a persistent late Eocene–Oligocene sediment source to the west, in thepresent-day Western Cordillera or Precordil-lera. This contrasts with the underlying SantaLucia Formation, which was derived from theeast. We attribute the mid-Paleocene to middleEocene reduction in sediment-accumulationrates and associated paleocurrent reversalacross the Santa Lucia–Potoco contact to a

broad-wavelength, low-amplitude forebulge inthe Altiplano to Eastern Cordillera region.This is consistent with Paleogene shortening,and presumably crustal thickening and flex-ural loading, to the west. A possible Paleo-cene–Eocene foredeep is preserved in the Sa-lar de Atacama and Precordillera region ofChile. By late Eocene–Oligocene time, the lo-cus of deformation and flexural loading wouldhave propagated eastward, inducing rapid sub-sidence and foredeep development in the Al-tiplano. Such long-term shortening may re-quire modification of existing models ofcentral Andean uplift that consider only Neo-gene shortening east of the Altiplano. Fur-thermore, the currently cited discrepancy be-tween total shortening and crustal thickness inthe central Andes may be resolvable if Paleo-gene shortening in the modern arc and forearc





Figure 9. Conceptual model of a migrating foreland basin system and predicted history of stratigraphic stacking and sedimentaccumulation. (A) Lateral migration of fold-thrust belt and adjacent foreland basin system through time (after DeCelles and Giles,1996). (B) Predicted stratigraphic stacking pattern attributable to basin migration. A thin interval representing the back-bulgedepozone is overlain by a condensed section (or disconformity) representing the forebulge depozone. This section is in turn cappedby a thick succession representing the foredeep depozone. (C) Predicted long-term sediment accumulation history due to variationsin subsidence rates during migration of a foreland basin system. Initial back-bulge deposition is followed by limited or no subsidenceof the forebulge, then rapid subsidence of the foredeep. Application of this model to the Altiplano suggests that the Santa Lucia,lowermost Potoco, and main Potoco rocks represent back-bulge, forebulge, and foredeep depozones, respectively, of the Paleogeneforeland basin system.

regions is incorporated into regional balancedcross sections.

ACKNOWLEDGMENTS

This research was supported by grants from theNational Science Foundation (EAR-9908003) andthe Louisiana State University Council on Researchawarded to Horton, and grants from the GeologicalSociety of America and American Association ofPetroleum Geologists awarded to Hampton. Fieldwork was facilitated by Sohrab Tawackoli, ReinhardRoessling, Carlos Riera, and Juan Huchani of Ser-geomin (La Paz). We thank Enrique Diaz, EddyBaldellon, Thierry Sempere, Peter DeCelles, andNadine McQuarrie for helpful discussions. RichardFink and Danielle Horton assisted in the field. Themanuscript was improved by the constructive re-views of Teresa Jordan, Timothy Lawton, and Ger-ilyn Soreghan.

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