Climate and Lake-Level History of the Northern Altiplano, Bolivia, As Recorded in Holocene Sediments of the Rio Desaguadero

Home , Altiplano, Cabana (ancient lake), Depositional environment, Lake Minchin, Lake Tauca, Lake Titicaca

CLIMATE AND LAKE-LEVEL HISTORY OF THE NORTHERN ALTIPLANO, BOLIVIA, AS RECORDED IN HOLOCENE SEDIMENTS OF THE RIO DESAGUADERO

PATTIE C. BAUCOM* AND CATHERINE A. RIGSBY Department of Geology, East Carolina University, Greenville, North Carolina 27858, U.S.A. e-mail: [email protected]

ABSTRACT: Strata exposed in terraces and modern cutbanks along the Desaguadero terraces record both seasonal precipitation changes (indicated Rio Desaguadero contain a variety of lithofacies that were deposited by interbedded coarse- and ®ne-grained strata preserved in modern cutbank in four distinct facies associations. These facies associations document exposures) and longer-term water-level ¯uctuations of Lake Titicaca (pre- a history of aggradation and downcutting that is linked to Holocene served in multiple terrace exposures dissected by ¯uvial downcutting). Dur- climate change on the Altiplano. ing lake highstands, when Lake Titicaca was not a closed-basin lake, ¯uvial Braided-stream, meandering-stream, deltaic and shoreline, and la- and lacustrine sediments accumulated on the downstream areas of the Al- custrine sediments preserved in multi-level terraces in the northern Rio tiplano. During the intervening times of low lake level, these sediments Desaguadero valley record two high-water intervals: one between 4500 were dissected, exposing ancient sediments as terrace remnants. The lateral and 3900 yr BP and another between 2000 and 2200 yr BP. These wet and vertical facies relationships preserved in terraced sediments offer a periods were interrupted by three periods of ¯uvial downcutting, cen- unique opportunity to examine the effects of lake-level and climate changes tered at approximately 4000 yr BP, 3600 yr BP, and after 2000 yr BP. on ¯uviolacustrine deposition. Our work also provides insight into the re- Braided-river sediments preserved in a single terrace level in the south- lationship between precipitation and evaporation in the Altiplano lake and ern Rio Desaguadero valley record a history of nearly continuous ¯u- river system, offering an alternative approach with which to investigate the vial sedimentation from at least 7000 yr BP until approximately 3200 Holocene water-level ¯uctuations of Lake Titicaca. yr BP that was followed by a single episode (post±3210 yr BP) of downcutting and lateral migration. GEOMORPHIC AND CLIMATIC SETTING The deposition and subsequent ¯uvial downcutting of the northern strata was controlled by changes in effective moisture that can be cor- The Bolivian Altiplano is a tectonically quiescent plateau situated be- related to Holocene water-level ¯uctuations of Lake Titicaca. The de- tween the fold-and-thrust belt of the eastern Cordillera and the active arc position and dissection of braided-stream sediments to the south are of the western Cordillera. It spans 200,000 km2, extends from 16Њ to 22Њ more likely controlled by a combination of base-level change and sed- S latitude and 65Њ to 69Њ W longitude, and has an average altitude of 4000 iment input from the Rio Mauri. m (Wirrmann and de Oliveira Almeida 1987). The Rio Desaguadero valley lies very near the center of the Altiplano and has been little affected by tectonic activity during the Holocene. At its INTRODUCTION closest, the river valley is 75 km from the marginal ranges, and the Ho- locene ¯uvial and lacustrine strata within the Desaguadero valley are ¯at The Altiplano of Bolivia has been occupied by large lakes since the Early lying. Seismically imaged late Quaternary strata in Lake Titicaca are also Quaternary. Paleoshoreline studies indicate that it was the site of at least ¯at lying, exhibiting no intrabasinal surface faulting and no young folding ®ve expansive paleolakes (Lakes Mataro, Cabana, Ballivian, Minchin, and (G.O. Seltzer, Syracuse University, and P.A. Baker, Duke University, per- Tauca), prior to the existence of Lake Titicaca, the world's highest great sonal communication). Approximately 10 m of regional-scale longitudinal lake (Servant 1977; Clapperton 1993b). Lake Titicaca is smaller and exists tilting has occurred in the central Altiplano (south of the Rio Desaguadero) at an average elevation considerably lower than the Pleistocene paleolakes. over a distance of about 100 km during the last 13,000 years (B.G. Bills, Nevertheless, water-level ¯uctuations of Lake Titicaca continue and have NASA Goddard, personal communication; Bills et al. 1994). If extrapolated contributed to a complicated and interesting Holocene lake-level history. northward, this implies a maximum of about5moftilting distributed along This history, along with ice-core data (Thompson et al. 1984; Thompson the length of the river over the time period recorded in the strata discussed et al. 1988; Thompson 1998) and anthropogenic evidence from the regions here. The regional geophysical data, the ¯at-lying nature of Quaternary surrounding the lake (Binford et al. 1992) is being used by scientists work- strata in Lake Titicaca, and the ¯at-lying nature of Desaguadero strata all ing in the area to reconstruct both regional and global climatic changes. suggest that tectonics probably did not have a signi®cant in¯uence on Ho- The most recent studies have used seismic data and sediment cores from locene sedimentation in this area. the lake (e.g., Wirrmann and Mourguiart 1992; Seltzer et al. 1998; Baker The northern and central parts of the Altiplano are occupied by two large, et al. 1997; Baker et al. 1998; Abbott et al. 1997a) to reconstruct this permanently ¯ooded lakes, Lake Titicaca and Lake Poopo. The more arid climate history and to document changes in the level of Lake Titicaca. southern part contains the great salars (salt pans), Coipasa and Uyuni. The Today, Lake Titicaca is linked to lakes and salars in the southern Alti- surface of Lake Titicaca lies at 3810 m above sea level, and the lake plano by a single outlet, the Rio Desaguadero. It seems logical that sedi- occupies an area of about 9000 km2 (Clapperton 1993a). In¯ow into the mentation, erosion, and terrace development along this river would respond lake is predominantly from rivers fed by glaciers and snow®elds of the to the Holocene lake-level and climatic changes. This study attempts to Cordillera Real to the east and the Cordillera Apolobamba to the north. characterize that ¯uvial response by examining the Holocene history of Under the present hydrologic situation, evaporation accounts for more than sediments exposed in terraces and cutbanks of the modern Rio Desaguadero 90% of the water loss from the lake. The only out¯ow is the Rio Desa- valley. guadero. The Rio Desaguadero ¯ows 390 km southward from Lake Titicaca Except for reconnaissance work by Servant (1977) and Lavenu (1992), to Lake Poopo, a shallow lake (Ͻ 6 m deep) (Fig. 1). During wet periods Rio Desaguadero strata have not previously been analyzed in the context Lake Poopo empties into the Salar de Coipasa and, in turn, to the Salar de of paleoclimatology. Our work shows that the sediments preserved in the Uyuni (elevation 3657 m). Annual precipitation on the Altiplano is highly correlated with the annual * Present address: USGS, 384 Woods Hole Rd., Woods Hole, Massachusetts cycle of atmospheric convection in the central Amazon basin and with the 02543, U.S.A. e-mail: [email protected] deep convection that develops over most of tropical South America in the

JOURNAL OF SEDIMENTARY RESEARCH,VOL. 69, NO.3,MAY, 1999, P. 597±611 Copyright ᭧ 1999, SEPM (Society for Sedimentary Geology) 1073-130X/99/069-597/$03.00 598 P.C. BAUCOM AND C.A. RIGSBY

FIG. 1.ÐLocation of Lake Titicaca, Rio Desaguadero, and Lake Poopo on the Altiplano of northern Bolivia and southern Peru. austral summer (Lenters and Cook 1997; Philander 1996). The annual cycle gradient in the northern part of the river basin is very low (0.00012) and results in seasonal water-level ¯uctuations of Lake Titicaca. During the dry that during times of prolonged high water, this marshy region of the river season (between May and October) the water level falls by about 75 cm. basin merges with the southernmost part of Lake Titicaca, becoming so It rises a comparable amount during the wet season. Larger ¯uctuations wet that it is dif®cult to distinguish the river from a southern extension of (up to 5 m this century) occur on about a decadal time scale in response the lake. During the Holocene, the Rio Desaguadero was a very different to events such as El NinÄo, which is often associated with a prolonged dry system, affected by much larger-scale climate and Lake Titicaca water- season on the Altiplano and a lake-level drop (Martin et al. 1993). level changes than those known from the modern instrumental record (e.g., Because the Rio Desaguadero is the only outlet for Lake Titicaca, lake- Abbott et al 1997a; Cross et al. in press; Seltzer et al. 1998). It is reasonable level history has had a dramatic in¯uence on the river. Similar to the clas- to expect that the sediments of that ancient river system provide a record si®cation of several modern river systems (e.g., the Squamish, Cimarron, of the high lake levels and of the timing of lake-level changes. Williams, and Red Rivers) described in detail by other workers (Brierley 1989, 1991; Brierley and Hickin 1991; Shelton and Noble 1974; Smith and METHODOLOGY Smith 1984; Schwartz 1978), the modern Rio Desaguadero is a transitional ¯uvial system. From its headwaters at Lake Titicaca to its terminus at Lake Terrace and modern cutbank exposures were mapped and measured us- Poopo, Rio Desaguadero displays end-member morphologies characteristic ing standard sedimentologic techniques. Emphasis was placed upon the of a marsh, a meandering river, and a braided river, as well as non-end- identi®cation and description of lateral and vertical facies changes as in- member braided and meandering channel morphologies (Baucom and Rigs- dicated by variations in grain size, sedimentary structures, bed morphology, by 1996, 1997). Of particular interest to this study is the fact that the and the presence of interbedded lacustrine sediments. After terrace expo- CLIMATE AND LAKE-LEVEL HISTORY, NORTHERN ALTIPLANO, BOLIVIA 599

TABLE 1.ÐSample number, location, lithofacies, dated material, and radiocarbon TABLE 2.ÐSummary of lithofacies described in this study. and calibrated radiocarbon ages for all the samples used in this study. Facies Sedimentary Sample Litho- Material Radiocarbon Calibrated Code Description Structures Interpretation Number Location facies Dated Age (yr) Age (yr) Gmsh Gravel, massive, matrix-sup- Horizontally bedded, locally Channel ®ll, bar deposits LD-mc 16Њ 58Ј12ЉS, 68Њ 47Ј19ЉW Fmol *gastropods 2380 Ϯ 35 2125 Ϯ 85 BP ported channelized CII-1 17Њ 14Ј29ЉS, 68Њ 63Ј33ЉW Fmp wood 4020 Ϯ 50 4530 Ϯ 120 BP Smg Sand, ®ne to coarse, locally None Channel ®ll, bar deposits CIII-1 17Њ 15Ј24ЉS, 68Њ 36Ј06ЉW Fmp wood 4210 Ϯ 50 4670 Ϯ 100 BP dispersed pebbles CIII-3 17Њ 15Ј24ЉS, 68Њ 36Ј06ЉW Fmp wood 3750 Ϯ 50 4140 Ϯ 160 BP Spw Sand to silty sand, localized Planar, wavy, to ripple lami- Deltaic sediments QC-1 17Њ 12Ј07ЉS, 68Њ 40Ј53ЉW Fmol *gastropods 4085 Ϯ 50 4280 Ϯ 140 BP clay balls nated QC-2 17Њ 12Ј07ЉS, 68Њ 40Ј53ЉW Fmol *gastropods 3540 Ϯ 40 3900 Ϯ 100 BP Fpwr Silt, mud, clay with mud Planar, wavy, ripple or climb- Overbank and/or ¯ood plain, QC-3 17Њ 12Ј07ЉS, 68Њ 40Ј53ЉW Fml *gastropods 3890 Ϯ 95 3950 Ϯ 300 BP drapes ing-ripple lamination and inactive channel or deltaic PKI-2 17Њ 12Ј19ЉS, 68Њ 40Ј39ЉW Fil *gastropods 4035 Ϯ 60 4240 Ϯ 170 BP convolute bedding sediments PKI-3 17Њ 12Ј19ЉS, 68Њ 40Ј39ЉW Fmol *gastropods 4265 Ϯ 60 4525 Ϯ 125 BP Fmt Silt or mud with rare sand and None, mottled Soil NT-I 17Њ 12Ј19ЉS, 68Њ 40Ј39ЉW Fmp wood 6125 Ϯ 65 7000 Ϯ 100 BP locally dispersed pebbles HC-N 17Њ 12Ј19ЉS, 68Њ 40Ј39ЉW Smg wood 3150 Ϯ 75 3390 Ϯ 180 BP Fmp Mud or silt, massive, sandy, Locally laminated Overbank and/or ¯ood plain local pebble stringers Notes: Dates over 6000 yr BP may be larger than the correction capabilities of the calibration program. Fsm Silt, massive Planar laminated Overbank and/or ¯ood plain, * A reservoir correction (Abbott et al. 1997) of 250 years was substracted from the radiocarbon age of all inactive channel or deltaic gastropod samples prior to calibration. sediments Fml Mud, massive Planar laminated Overbank and/or ¯ood plain, inactive channel or deltaic sediments sures were identi®ed and mapped, the sediments composing the terraces Fmol Clay, blue to blue-gray, muddy Thinly laminated Lacustrine sediments were described in detail, key elevations were surveyed, and datable organic with evaporite layers and material was sampled. mollusks common Fmoo Mud, brown, massive, mol- Locally, wavy laminated Deltaic sediments Although gastropods are present in most of the lacustrine sections, dat- lusks and ostracods rare able organic material is scarce. Where present, it consists of small amounts Fil Mud, interlayered green-gray Poorly laminated Interbedded lake and deltaic to blue-gray, sandy to silty sediments of disseminated organic detritus (woody material, peat, and charcoal) with- upsection, mollusks common in the slightly sandy mud lithofacies of the ¯uvial strata. Eleven samples yielded a suf®cient quantity of material for dating. These samples were subjected to radiocarbon analysis at the National Ocean Sciences Accel- erator Mass Spectrometry Facility (NOAMS), Woods Hole Oceanographic and Fmt) are common in FA-A and two (Fpwr and Fsm) are present only Institution. Radiocarbon ages were normalized to a ␦13C ϭϪ25½ (rela- locally. Lithofacies Fml is a massive to parallel-laminated mud that is typ- tive to PDB) and reported with all errors quoted Ϯ 1 standard deviation. ically interbedded within other muddy, sandy, and gravelly lithofacies. OxCal 2.18 (Stuiver and Kra 1986) was used to convert the conventional Lithofacies Fmp is a sandy mud to sandy silt with locally abundant isolated radiocarbon ages to calibrated years BP. Dates derived from gastropod pebbles (Fig. 5A). Lithofacies Fmt, common to all FAs, is a massive, mot- samples were corrected (prior to calibration) for reservoir effects using the tled silty mud with rare sand and rare isolated pebbles. The less common 250 year correction determined by Abbott et al. (1997a) for gastropods lithofacies Fpwr is a planar- to wavy- to ripple-laminated mud. Lithofacies from Lake HuinÄaimarca, the southernmost restricted part of Lake Titicaca, Fsm is rare in FA-A, but is present locally as isolated 25-cm-thick beds of which discharges directly into the modern Rio Desaguadero. Discussion in massive to planar-laminated silt. All ®ne-grained lithofacies in FA-A are the text focuses on the 2␴ calibrated range; both calibrated and uncalibrated laterally continuous, rarely Ͼ 10 cm thick, and contain rare disseminated, ages for all samples used in this study are presented in Table 1. woody organics and local organic-rich lenses. The sandy component of FA-A is dominated by lithofacies Smg, a mas- FACIES ASSOCIATIONS sive ®ne- to coarse-grained sand with locally dispersed pebbles (Fig. 5B). Smg is present both within interbedded sequences and as part of ®ning- Terrace exposures along the Rio Desaguadero contain a diversity of lith- and coarsening-upward packages. This lithofacies is typically only a few ofacies preserved in four distinct facies associations (FAs): braided-stream deposits, meandering-stream deposits, deltaic and shoreline deposits, and lacustrine deposits. The lithofacies that constitute these FAs (Table 2) were TABLE 3.ÐDescription, dominant lithofacies, and depositional environment of the classi®ed on the basis of grain size, sedimentary structures, biological com- facies associations preserved in cutbank and terrace exposures of Rio Desagua- ponents, and stratigraphic position, following the procedures of Miall dero. (1978) as modi®ed by Rust (1978). The lateral and vertical distribution of lithofacies, along with their characteristic groupings, formed the basis for Facies Dominant or characteristic Depositional association lithofacies Description Environment the FA de®nitions (Table 3). Descriptions of the FAs and their character- A Gmsh, Smg, Fml interbedded, locally laminated braided-stream deposits istic lithofacies, as well as interpreted depositional environments, are pre- muds, silts, sands, and sented below. gravels that compose local ®ning- and rare coarsening- upward sequences Braided-Stream Deposits (FA-A) B Fsm, Fmt, Fml local Fpwr massive to planar-laminated meandering-stream deposits silts and muds, commonly Braided-stream deposits include the coarsest-grained strata identi®ed in compose ®ning-upward se- this study and are well exposed at three locations along the northern, cen- quences C Spw, Fpwr, Fmoo stacked coarsening-upward deltaic and lake-shoreline sed- tral, and southern parts of the Rio Desaguadero, near the towns of Calacoto, deposits of laminated mud, iments Huito Churo, and Nequela, respectively (Fig. 2). FA-A strata are typi®ed silt, and sand, and interbedded laminated mud, silt, by thick (0.5 m to Ͼ 1 m) deposits of interbedded sand, mud, and gravel and sand, and convolute and by stacked ®ning- and coarsening-upward sequences (Fig. 3, 4, 5). The bedding lithofacies that constitute these interbedded sediment packages are de- D Fmol massive, thin-bedded, light lacustrine muds blue to blue-gray, mollusc- scribed below in order of decreasing abundance: ®ne-grained lithofacies, rich clay with evaporitic sandy lithofacies, and gravelly lithofacies. layers, plant debris, and Description of Lithofacies.ÐThree ®ne-grained lithofacies (Fml, Fmp, rare ostracods 600 P.C. BAUCOM AND C.A. RIGSBY

FIG. 2.ÐLocation and facies association (FA) designations of the 13 Rio Desaguadero measured sections used in this study. Note that names in italics are section location names, not speci®c town locations. centimeters thick. It is present as laterally continuous beds and, rarely, as calated bar and channel sequences (Miall 1978; Reineck and Singh 1980). discontinuous sand lenses. Interbeds of ®ne sand, silt, and mud, such as lithofacies Fsm and Fml, The only gravel lithofacies in the study area is found in FA-A. Litho- record overbank deposition or sedimentation from suspension in active facies Gmsh is locally channelized (Fig. 5C) but is typically a horizontally channels. The intervening coarser units, such as lithofacies Gmsh and Smg, bedded, matrix-supported, sandy gravel in beds 2±12 cm thick (Fig. 5D). are channel deposits (Miall 1982). Pebble stringers and channelized gravels Within individual beds, the gravels are moderately sorted with angular to more likely represent winnowed lags on scour surfaces (Reading 1996). moderately rounded clasts ranging from 2 mm to 3 cm in diameter; clasts The ®ning- and coarsening-upward units preserved in FA-A record chan- up to 6 cm are present locally. It is interbedded with the sandy and ®ne- nel and bar sequences that suggest cyclical ¯uctuations of energy regimes, grained lithofacies described above and in coarsening- and ®ning-upward probably related to annual to millennial-scale ¯uctuations. Generally, ®n- sequences. ing-upward sequences are a product of sedimentation in channels (Miall Interpretation.ÐFA-A was deposited by a sandy braided ¯uvial system 1982; Reineck and Singh 1980), but may also develop as a response to with localized coarse-grained sedimentation (recognized by rare, isolated waning water ¯ow over bars and channels as they accrete, accompanied by gravels and abundant pebble stringers) from surrounding highlands. Evi- (or in response to) migration of the active channel (Davis 1983). The coars- dence supporting this interpretation includes distinct grain-size variations, ening-upward units of FA-A are most likely associated with progradation the presence of complexly interbedded coarse and ®ne sediments, stacked or downstream migration of bar deposits (Miall 1982). As a bar forms, the ®ning- and coarsening-upward sequences, abundant gravel layers, and the coarse material is deposited in the central axis and upstream end of the scarcity of primary sedimentary structures in the sandy lithofacies. The bar, thus grain size decreases upward and downstream (Leopold and Wol- sedimentary packages of FA-A suggest preservation of complexly inter- man 1957). Once formed, the bar may grow forward by slipface avalanch- CLIMATE AND LAKE-LEVEL HISTORY, NORTHERN ALTIPLANO, BOLIVIA 601

The scarcity of primary sedimentary structures in the sandy lithofacies of FA-A is related to braided-stream depositional processes. Sediment deposition in a braided stream results from large and rapid ¯uctuations in river discharge, abundance of coarse sediment, a high rate of sediment supply, and easily erodible banks (Shelton and Noble 1974; Cant and Walk- er 1978). Under these conditions, bar structures form during periods of high discharge as the channel is rapidly choked with bedload. A rapid drop in velocity causes instantaneous deposition. Consequently, sediments are poorly sorted, and sedimentary structures, other than channelized gravel beds and laminated muds, are not preserved. This was likely the case for the braided-stream deposition recorded in the sediments of FA-A.

Meandering-Stream Deposits (FA-B) Facies association B (FA-B) is exposed at four locations in the northern reaches of the Rio Desaguadero, near the towns of Callapa, Nazacara, and Janko Wichinca (Fig. 2). This FA is characterized by 1.3-m-thick sedimentary packages dominated by ®ning-upward sequences and interbedded FIG. 3.ÐLegend for measured stratigraphic sections. Facies codes and descriptions silt and mud (Figs. 3, 6). The ®ning-upward sequences are commonly over- are in Table 2. lain by sediments of FA-A. FA-B is distinguishable from FA-A by the absence of complexly interbedded sediments, the lack of gravel, the dom- ing, rif¯e migration, or accretion of new bar structures superimposed upon inance of ®ning-upward sequences, and a more homogeneous, ®ner grain- the previously deposited bars (Cant and Walker 1978; Reineck and Singh size distribution (Figs. 4, 6). 1980). In the Rio Desaguadero strata, however, the preservation potential Description of Lithofacies.ÐThe dominant lithofacies in FA-B are Fsm, of coarsening-upward sequences is low because of penecontemporaneous Fml, and Fmt (Table 2). Lithofacies Fsm consists of massive to laminated bar erosion resulting from lateral migration of channels. This low preser- quartzose silt in beds ranging from a few centimeters to 1 m thick (Fig. vation potential explains the scarcity of coarsening-upward sequences. 7A). It occurs as the basal unit of ®ning-upward sequences and as thin

FIG. 4.ÐMeasured sections characteristic of FA-A (braided-stream sediments). Arrows indicate ®ning- and/or coarsening-upward sequences. See Figure 2 for section locations, Figure 3 for legend, and Table 2 for lithofacies descriptions. 602 P.C. BAUCOM AND C.A. RIGSBY

FIG. 5.ÐLithofacies characteristic of FA-A. A) Sandy silt of lithofacies Fmp. Note the presence of pebble stringers. B) Lithofacies Smg overlying lithofacies Fsm. C) Stacked channel sequences of lithofacies Gmsh. D) Channelized, continuous, and discontinuous units of lithofacies Gmsh. See Figures 4 and 10 for measured sections, Figure 2 for section locations of FA-A, Figure 3 for legend, and Table 2 for lithofacies descriptions. interbeds within lithofacies Fml. Lithofacies Fml, a planar-laminated mud, sediments (Allen 1970; Shelton and Noble 1974; Cant 1978; Reineck and also occurs within ®ning-upward sequences, typically overlying lithofacies Singh 1980). Fsm. Lithofacies Fpwr occurs locally atop ®ning-upward sequences. It is characterized as a planar- to wavy- to ripple-laminated mud with common Deltaic or Shoreline Deposits (FA-C) and Lacustrine Deposits (FA-D) mud drapes (Fig. 7B). The only sandy lithofacies in this FA (Smg) occurs locally at the base of a ®ning-upward sequence. The strata that constitute FA-C and FA-D record deposition in lacustrine, Interpretation.ÐFA-B records deposition in a meandering river. This lake shoreline, and deltaic environments. FA-C consists of coarsening-up- FA is dominated by thick (approximately 0.6 m to Ͼ 2.5 m) overbank ward sequences of brown clay overlain by interbedded silt and sand and and/or ¯oodplain deposits (lithofacies Fsm, Fml; Fig. 6). Bar deposits are planar-, wavy-, and ripple-laminated mud with convolute bedding and sand less common. Evidence for the meandering-river interpretation includes the interbeds. This FA is present at three locations in the northernmost terraces relatively homogeneous grain-size distribution within the FA-B vertical of the Rio Desaguadero, near the town of Parko Kkota (Fig. 2). Although sections, the presence of thick overbank deposits and ®ning-upward se- the dominance of both sand and mud lithofacies makes this FA similar to quences, and the absence of interbedded channel and bar lithofacies. FA-A and FA-B, FA-C contains no gravelly lithofacies and is characterized Thick overbank and ¯oodplain deposits are present in all measured sec- by lithofacies with more abundant and variable sedimentary structures tions of FA-B. Brierley and Hickin (1991) demonstrated that meandering (Figs. 8A, B, C). Furthermore, FA-C is always associated with the blue and braided systems are distinguishable by examination of their respective muds of FA-D (either stratigraphically above or laterally continuous with overbank and ¯oodplain sediments. Their analysis of the Squamish River, it) and consistently coarsens upward. FA-D occurs beneath FA-C in the a transitional river in Canada, reveals that modern ¯oodplain sediments Parko Kkota locations and beneath meandering-stream deposits (FA-B) at consistently increase in thickness from the river's upstream braided reach Nazacara and Janko Wichinca (Fig. 2). to its downstream meandering reach. Meandering channels similarly dem- Description of Lithofacies.ÐFA-D is characterized by massive, thin- onstrate slower rates of lateral shifting than braided channels. The stacked bedded, light blue to blue-gray, gastropod-rich clay with thin, in situ evap- channel and bar sequences typical of braided rivers do not appear in FA- oritic layers, plant debris, and rare ostracods (Lithofacies Fmol; Fig. 8E). B. Instead, thick accumulations of ¯oodplain sediments (lithofacies Fsm, These clayey strata typically thin upward. Where both FA-D and FA-C are Fml) are present. These ®ne-grained accumulations, along with the ®ning- present, the strata are characterized by a coarsening-upward sequence of upward sequences distinctive of this FA, are typical of meandering-river mud, silt, and sand (Figs. 3, 9). In those sequences, Fmol is overlain by CLIMATE AND LAKE-LEVEL HISTORY, NORTHERN ALTIPLANO, BOLIVIA 603

FIG. 6.ÐMeasured sections characteristic of FA-B (meandering-stream sediments). Arrow indicates ®ning-upward sequence. See Figure 2 for section locations of FA-B, Figure 3 for legend, and Table 2 for lithofacies descriptions. either lithofacies Fil or lithofacies Fmoo. Fil is a greenish-gray to blue- Interpretation.ÐFA-D was deposited in a lacustrine environment that gray, poorly laminated mud that grades into a brown, coarsening-upward, was locally in®lled by ¯uvial progradation (FA-C). Evidence for this in- sandy mud. Gastropods and plant debris are abundant and ostracods are terpretation includes the presence of thick accumulations of blue, gastro- rare within Fil. Fmoo is a massive, dark brown, slightly silty clay with pod-rich mud overlain by brown mud that coarsens upward into sand with minor disseminated organic detritus, common gastropods, and rare ostra- interbedded mud, and by the coexistence of muddy sediments dominated cods (Fig. 8D). The next lithofacies in this coarsening-upward sequence is by abundant lithofacies Fpwr with convolute bedding and interbedded sand lithofacies Fpwr. As in FA-B, Fpwr is composed of silty and muddy sed- layers. Deposition and preservation of blue to dark brown sediments gen- iments with planar, wavy, and ripple lamination. In FA-C, however, Fpwr erally indicate a reducing environment associated with saturation, whereas is also characterized by climbing-ripple lamination and convolute bedding, red, yellow, orange, and brown sediments are indicative of an oxidizing is more common than in FA-B, and is typically overlain by lithofacies Spw environment and are often associated with unsaturated, dry environments (Fig. 9). Lithofacies Spw, common in this FA, contains planar, wavy, and (Miall 1996). Hence, the blue clayey muds with gastropods (FA-D) indicate ripple lamination with localized mud clasts (Fig. 8B). It is typically up to two possible depositional environments: a lacustrine environment or a poor- 45±50 cm thick and is often interbedded with lithofacies Fml. Lithofacies ly drained backswamp or ¯oodplain environment. It follows that FA-C may Smg is less common. Where present, Smg is thinly bedded (about 2 cm) represent a lacustrine deltaic environment or crevasse-splay sedimentation and unstructured (Fig. 9). atop a backswamp environment. Both depositional environments are ca-

FIG. 7.ÐLithofacies characteristic of FA-B. A) Massive silt of lithofacies Fsm. Note the presence of thin soil layers. B) Laminated silt of lithofacies Fpwr. This lithofacies is preserved locally in meandering-stream sediments at the base of the Calacoto terraces (C-II and C-III). See Figures 6 and 10 for measured sections, Figure 2 for section locations of FA-B, Figure 3 for legend, and Table 2 for lithofacies descriptions. 604 P.C. BAUCOM AND C.A. RIGSBY

FIG. 8.ÐLithofacies characteristic of FA-C (deltaic and lake-shoreline sediments) and FA-D (lacustrine muds). A) Planar, wavy, ripple, to climbing-ripple lamination of lithofacies Fpwr in FA-C. B) Planar- to wavy-laminated sand of lithofacies Spw in FA-C. C) Convolute bedding preserved locally in lithofacies Fpwr at the base of FA- C. D) Brown mud of lithofacies Fmoo in FA-C (deltaic silt and mud). E) Blue mud of lithofacies Fmol in FA-D (lacustrine mud). Note the gastropod and evaporite layers at the top of Fmol and the sharp contact between lithofacies Fmol and lithofacies Fmoo. See Figure 9 for measured sections, Figure 2 for locations of FA-C and FA-D, Figure 3 for legend, and Table 2 for lithofacies descriptions. pable of producing the sediment types and sedimentary structures charac- by deltaic sedimentation. Convolute bedding and planar-, wavy-, and rip- teristic of these FAs. Similar sedimentary sequences have been described ple-laminated sand and silt within the QC-m sediments also supports the by other workers (Ridgeway and Decelles 1993; Smith and Putman 1980) interpretation of a paleoshoreline (Figs. 8C, 9; Link and Osborne 1978). and interpreted as deposition by anastomosing systems. There are distinct Convolute bedding can be produced by several mechanisms that are as- differences, however, between the lacustrine and overbank sediments of an sociated with current activity and rapid sedimentation, the most common anastomosing system and the ones preserved in FA-C and FA-D. For ex- being liquefaction or sediment overloading (Reineck and Singh 1980). Con- ample, in an anastomosing system, black organic-rich material (roots casts, volute bedding has been observed in ¯uviatile environments, especially plant fragments, leaf imprints, etc.) is abundant and lacustrine muds are ¯oodplain and point-bar sediments where there is a source of sand and mud almost always interbedded with sand and silt layers as a result of frequent associated with episodic deposition (Reineck and Singh 1980). Here, how- overwash and/or channel avulsion (Smith 1983; Ridgeway and Decelles ever, they are associated with the shoreline of the lake that deposited the 1993). The Ͼ 1.1 m thick, uninterrupted deposit of the blue-lacustrine mud blue muds of FA-D. Sediments of QC-m were probably deposited in a of FA-D contains no clastic interbeds and no plant remains; it is indicative paleo±¯oodplain or marsh environment that was subsequently reworked by of a ``permanent'' small lake, not of a wetland frequented by overwash sedimentation. The FA-C and FA-D sequence is similar to, but signi®cantly waves to produce a lacustrine shoreline. The horizontally laminated sand smaller in scale than, coarsening-upward mudstone and sandstone facies and the laterally continuous bench-like geometry of the QC-m outcrop fur- described by Ridgeway and Decelles (1993) and by Van-Dijk et al. (1978), ther support agitation by small waves and local reworking of a shoreline which also record the progradation of ¯uvial systems into lacustrine basins. (Link and Osborne 1978). The deposition of sands and muds was most Additional evidence of a prolonged lake at Parko Kkota is supported by likely episodic and related to cycles of fair-weather and ¯ood conditions, the FA-C strata in measured section QC-m (Fig. 9). These sediments are resulting in sediment dewatering and the preservation of convolute bedding laterally continuous with lithologically homogeneous shoreline sediments (Martel and Gibling 1991), and to the formation of distributary channels and have geometric relationships suggestive of a paleolake that was in®lled feeding into the lake. Such channels incise levee and other backswamp CLIMATE AND LAKE-LEVEL HISTORY, NORTHERN ALTIPLANO, BOLIVIA 605

FIG. 9.ÐMeasured sections characteristic of FA-C and FA-D. Arrows indicate ®ning- and/or coarsening-upward sequences. See Figure 2 for section locations, Figure 3 for legend, Table 2 for lithofacies descriptions, and Figure 10 for complete PK-composite measured section. deposits, and typically form small, delta-like distributary systems that shal- (sections QC, PK-composite), the distinct contact between lacustrine and low away from the main channel (Miall 1996). deltaic sediments at Parko Kkota, and a similar distinct and physically traceable contact between braided-stream and meandering-stream sediments FLUVIAL HISTORY at Calacoto. This correlation, along with our lithofacies analysis and the radiocarbon dates for all of the northern terraces (Fig. 10; Table 1), provide The lateral and vertical distribution of FAs in the Rio Desaguadero ter- the basis for the following ¯uvial-history reconstruction of the northern Rio races record two distinct ¯uvial histories for the Rio Desaguadero valley, Desaguadero valley. one for the northern Rio Desaguadero and one for the southern Rio De- Between approximately 4700 and 4000 yr BP, the northern part of the saguadero. This distinction warrants their separate discussion. In the north, Rio Desaguadero at Parko Kkota was dominated by a lacustrine environ- the exposed Holocene-age terraced sediments include all of the FAs just ment (Fig. 11A). The area of the ancient Parko Kkota lake cannot be de- described, and multiple terrace levels are present. In the south, only one termined exactly, but an approximation was made by examining the lateral terrace level is present, and all of the exposed terraced sediments are braid- extent of lacustrine and deltaic sediments outcropping along PK-composite ed-river sediments (FA-A). This difference in terrace con®guration and and QC, and by examining the local geomorphology. The depth of the sediment depositional environment is coincident with the presence of an Parko Kkota paleolake is also dif®cult to estimate, but sediment thickness- important tributary, the Rio Mauri, which drains a watershed composed of es, traceable blue muds, and surveyed elevations provide a means for in- Tertiary volcanic terrain of the western Andes and adds a large volume of terpretation. The thickest exposed accumulation of lacustrine mud is 1.1 m sediment and water to the Rio Desaguadero system. (FA-D at QC; the base of section is not exposed) at an elevation of 3795 m (between the FA-D and FA-C contact; Fig. 9). The shoreline strata in Terraces of the Northern Rio Desaguadero section QC-m are at 3798 m and are 1.8 m thick (Fig. 9). The blue mud The terraced sediments north of the Rio Mauri preserve lacustrine, del- thins and becomes blue-green to green-brown downstream, toward PK- taic, and ¯uvial strata above a modern-day meandering reach of the Rio composite, and pinches out entirely between PK-composite and the Cala- Desaguadero. Radiocarbon dates from the Calacoto and Parko Kkota ter- coto terraces. In situ evaporite layers in the upper part of the blue mud at races (C-II, C-III, PK-composite, and QC; Table 1) allow lithologic and both QC and PK-composite indicate periods of extensive drying. Root casts chronologic correlation of lacustrine (FA-D) and deltaic sediments (FA-C) or siltstone lenses that would suggest that the lake was shallow enough for with meandering-river and braided-river sediments (FA-B and FA-A; Fig. bottom sediments to support plants are absent. Together, these observations 10). The radiocarbon dates indicate that the lacustrine and deltaic sedi- suggest that the Parko Kkota lake was at least 4 km long, approximately mentation at Parko Kkota was contemporaneous with meandering-stream 0.5 to 2 km wide, and at least 4.1 m deep, and that it underwent periods and braided-stream deposition downstream at Calacoto. The correlation be- of evaporation before being completely in®lled with clastic sediment. At tween these laterally distinctive environments is also supported by physi- Janko Wichinca, the presence of gastropod-bearing mud (undated) at the cally traceable gastropod layers and blue lacustrine muds at Parko Kkota same elevation as the Parko Kkota lake (3798 m) suggests that the lake 606 P.C. BAUCOM AND C.A. RIGSBY

FIG. 10.ÐChronologic and stratigraphic correlation diagram for the northern Rio Desaguadero terraces, including FA-D and FA-C at Parko Kkota with FA-B and FA-A near Calacoto.

FIG. 11.ÐPaleogeographic reconstructions of the northern and southern parts of the paleo Rio Desaguadero as interpreted from sediments and radiocarbon dates. A) Holocene reconstruction of the northern Rio Desaguadero based on correlation of the Parko Kkota (QC, QC-m, PK- composite) and Calacoto (C-II, C-III) terraces, as shown in Figure 10. B) Holocene reconstruction of the southern Rio Desaguadero based on sediments and radiocarbon dates from the Huito Churo (HC-N, HC-S) and Nequela (NT) terraces. CLIMATE AND LAKE-LEVEL HISTORY, NORTHERN ALTIPLANO, BOLIVIA 607

lake indicates a rapid change in the entire depositional system. The sharp contact between the lacustrine and deltaic sediments and the absence of lacustrine sediments above this contact suggest that the lacustrine period was followed by an abrupt increase in sediment load or discharge (and, possibly, increased seasonality of precipitation). Subsequently, at least two episodes of ¯uvial downcutting occurred, exposing upper PK-composite and QC (approximately 4000 yr BP) and the lower PK-composite (approximately 3600 yr BP). These are the maximum ages of downcutting. Esti- mated sediment accumulation rates suggest that the second episode of downcutting may not have occurred until approximately 3300 yr BP. During the lacustrine sedimentation at Parko Kkota, meandering-river sedimentation was occurring at Calacoto (Fig. 11A). The lower 1.25 m of strata at C-II and C-III were deposited by a meandering river (Fig. 10). Radiocarbon dates suggest that the sediments of C-II and C-III were deposited contemporaneously. The small-scale stratal variations between C- II and C-III most likely record the lateral facies changes that are commonly associated with river systems. The sharp contact between lake and deltaic sediments at Parko Kkota (FA-D and FA-C) is correlative with the contact between meandering-river and braided-river sediments at Calacoto (FA-B and FA-C; Fig. 10). Soil formation and ¯uvial downcutting followed the cessation of braided-river and deltaic sediment accumulation. If that cessation was contemporaneous, the maximum age of downcutting (calculated from sedimentation rates) is between 3800 and 3300 yearsÐprobably also contemporaneous with the youngest downcutting at QC and PK-composite. There is no record of ¯uvial deposition (in either the northern or southern Rio Desaguadero) after this second downcutting episode. At Nazacara, however, blue gastropod-rich lacustrine muds are present in the modern cutbank (Fig. 13). These muds are identical to, but younger than, the non- evaporitic muds at QC and PK-composite. Gastropods from the Nazacara lake strata yield a calibrated age of 2125 Ϯ 85 yr BP (Table 1), indicating FIG. 12.ÐMeasured section of lacustrine (FA-D) and meandering-river (FA-B) that the 3300 yr BP downcutting was followed by another high-water la- sediments in modern cutbank at Janko Wichinca. custrine episode. Downstream of these lakes (especially south of the Rio Mauri, which provides even more water and sediment to the system) this high water interval may have resulted in erosion rather than deposition. may have been even larger than is indicated by the PK and QC terraces The exact timing of the desiccation of the lake is dif®cult to establish, and (Fig. 12). no evaporite layers are present in the strata. The age of the lake strata and The paleolake was adjacent to (and ¯ooded by) a meandering river (as their position in the modern cutbank, however, indicate a third, post±2000 recorded in the Calacoto terraces) much like the modern Rio Desaguadero yr BP episode of ¯uvial downcutting in this region. in this area (Fig. 11A). The lack of evaporite layers in the lower blue muds indicates that the lake was probably permanent early in its history. Abun- Terraces of the Southern Rio Desaguadero dant pottery shards around the paleolake (paleoshoreline) perimeter further support the hypothesis of a prolonged freshwater lacustrine environment The terraces preserved south of the Rio Mauri con¯uence, at Huito Churo and represent rare and important evidence of abundant early human pop- (HC) and Nequela (NT), record a different ¯uvial history. The sediments ulation in the region. This environment may have been similar to modern in these terraces record braided-stream and alluvial deposition similar to conditions along the shoreline of Lake Titicaca and in the northernmost that of the modern southern Rio Desaguadero system, and suggest that segment of the modern Rio Desaguadero (characterized by marshy inun- channel morphology and depositional processes in this region have not dated areas and small lakes), where a large human population currently changed signi®cantly since 7000 Ϯ 100 yr BP, the oldest date recorded in prospers. the southern Rio Desaguadero terraces (0.70 m from the base of the Ne- The Parko Kkota paleolake strata exposed in terraces QC and PK-com- quela terrace; Table 1). The terraces also suggest that (1) the river has posite are overlain by the deltaic sediments of FA-C. The deltaic sediments migrated southward to reach its modern position (as indicated by the po- are thickest (Ͼ 4 m thick), and the contact between the deltaic sequence sition of the Nequela terrace relative to the modern river channel; (Fig. and the underlying lacustrine muds is the sharpest, at QC (Fig. 10). This 11B), (2) only one episode of ¯uvial downcutting has occurred south of indicates that the main distributary of the paleo±Rio Desaguadero was prob- the Rio Mauri, and (3) deposition has been nearly continuous in this region. ably near the QC terrace and that sedimentation began abruptly in this area. It is important to note, however, that the large sediment load and abundant Rapid in®lling of the lake is also suggested by the presence of convolute alluvial-fan sedimentation in this area could have resulted in buried terraces bedding (Fig. 8C). Floodplain sediments dissected into a wave-cut shoreline and overlapping or inset alluvial deposits such as those noted by Leopold (as discussed previously) suggest that the paleo-Desaguadero existed ap- et al. (1964) for similar ¯uvial systems. Nevertheless, our data suggest that proximately 50 m north of the modern river system. Eventually, the river braided-stream deposition was occurring prior to 7000 yr BP (Table 1) and migrated southward, ®lled the lake with sediment, and occupied its present that a single episode of ¯uvial downcutting occurred sometime after 3210 position (Fig. 11A). Fluvio-deltaic sedimentation began sometime after yr BP, the age of the youngest strata dated in the southern terraces (1.6 m 3900 Ϯ 100 yr BP and lasted until at least 3300 yr BP (judging by the from base of HC-N; Table 1). Although the relationship between precipi- sediment accumulation rate estimated from terrace C-III; Table 1). tation and sedimentation rate is not fully understood, it has been studied The lateral progradation of the stream and the subsequent in®lling of the by many workers (for example, Wilson 1973; Yair and Enzel 1987; Graf 608 P.C. BAUCOM AND C.A. RIGSBY

velopment. Because as presented above, only minor tectonism has been noted in this region during the late Holocene (e.g., Bills et al. 1994), cli- matically controlled changes in discharge are the most likely cause of Rio Desaguadero deposition and downcutting. The strata preserved in Rio De- saguadero terraces and cutbanks record valley aggradation followed by ¯u- vial incision and channel migration, all of which can be linked (directly or indirectly) to water-level ¯uctuations of Lake Titicaca. The distribution and characteristics of the terraces and terrace sediments also correspond with the modern hydrologic conditions of the Rio Desa- guadero. Seasonal water-level ¯uctuations of Lake Titicaca correlate well with the discharge variability of the Rio Desaguadero at the Calacoto gaug- ing station (above the Mauri con¯uence), indicating that at Calacoto the river is mostly supplied by the out¯ow of Lake Titicaca and is affected signi®cantly by even minor changes in lake level (Guyot et al. 1990). Downstream of Calacoto, however, the in¯uence of the Rio Mauri prevails, resulting in a typical maximum of discharge from January to March (wet season), followed by a slow decrease in discharge, but no marked low- water periods. The discharge variations at Calacoto (associated with the minor ¯uctuations of Lake Titicaca) are recorded in modern cutbank exposures by the presence of coarse- and ®ne-grained strata that record a change between braided and meandering stream types. Because, during the modern instrumental period, the small-scale water-level ¯uctuations of Lake Titicaca cause discharge changes in the Rio Desaguadero north of the Rio Mauri that are recorded in modern sediments (Baucom and Rigsby 1996, 1997), it is reasonable to assume that larger-scale Holocene lake-level ¯uctuations were recorded by larger-scale patterns of sediment deposition (during lake highstands) and by ¯uvial dissection and terrace development (during lake lowstands). The geographic position and sediment composition of the Rio Desa- guadero terraces complement these data. As discussed, multiple terraces exist just north and west (upstream) of the Rio Mauri, but only single terraces are present south of the Rio Mauri. This observation suggests that the older terraced strata north of the Rio Mauri were in¯uenced by water- level ¯uctuations of Lake Titicaca in the same manner (but on a larger scale) as the modern cutbank sediments at Calacoto. Deposition of the sediments preserved within the terraces is most likely the result of an increase in regional effective moisture (precipitation minus evaporation) that FIG. 13.ÐMeasured section of lacustrine (FA-D) and meandering-river (FA-B) resulted in increased sediment and water supply from major tributaries. sediments in modern cutbank at Nazacara. This increase in effective moisture also would correspond with a highstand of Lake Titicaca and increased discharge from the lake to the upper reaches of Rio Desaguadero. Likewise, each dissection of the terraced sediments 1988), and several general relationships are fairly well established. Wilson likely corresponded to decreased effective moisture, a drop in the water (1973) demonstrated that, although increased rainfall usually results in in- level of Lake Titicaca, and a drop in the level of downstream Lake Poopo. creased sediment yield, increased seasonality of precipitation often results In the southern part of Rio Desaguadero, high water levels in Lake Ti- in increased ¯uvial erosion. Smith (1994) recognized similar relationships ticaca contributed to increases in sediment load and discharge. However, a in Quaternary strata of the arid southwestern United States, where he drop in lake level or a decrease in precipitation was not necessarily re- showed that erosion (¯uvial downcutting and terrace dissection) occurred corded as a signi®cant sedimentological change because there was a pe- during wet, tectonically quiescent periods, such as those that may have rennial source of sediment and water (the Rio Mauri) in the southern Rio occurred in the Rio Desaguadero valley around 2200 yr BP. It is likely Desaguadero channel. Consequently, terrace formation was dif®cult be- that the same high water levels that led to the deposition of lake strata in cause abandonment of a surface did not occur rapidly enough to isolate the the low-relief, northern part of the Rio Desaguadero (at Nazacara) resulted surface and protect it from erosion. The single, southern terrace level con- in ¯uvial erosion in the higher-relief, southern part of the river valley. ®rms this hypothesis. It is likely that terrace formation in this region was controlled by changes in the amount of water and sediment being carried DISCUSSION down the Rio Mauri. Paleo±Lake Tauca once occupied the southern Altiplano and reached its The mechanisms responsible for the ¯uvial dissection and eventual ex- highest level at about 13,000 yr BP (Clapperton 1993a; Bills et al. 1994; posure of the Rio Desaguadero depositional facies are most likely related Servant et al. 1995). The exact timing of the desiccation of Lake Tauca is to climate. Climate changes affecting both discharge and vegetation may uncertain, but it probably emptied by 10,000 yr BP and possibly as late as alter the hydrologic regimen of a river, including the delivery of both water 8500 yr BP (Servant and Fontes 1978; Clapperton 1993a; Bills et al. 1994; and sediment to the river system (Bull 1991; Leopold et al. 1964; Smith Servant et al. 1995; Sylvestre et al. 1998). Our estimate for the timing of 1994). In many cases, a combination of tectonic and climate change occurs braided-stream deposition (FA-A) in this area and the subsequent down- simultaneously to produce ¯uvial downcutting and subsequent terrace de- cutting of the southern terraces (ϳ 7000 yr BP and 3200 yr BP, respec- CLIMATE AND LAKE-LEVEL HISTORY, NORTHERN ALTIPLANO, BOLIVIA 609

FIG. 14.ÐHolocene lake-level curves and the elevation and age of lake strata in the Parko Kkota (PK) and Nazacara (LD-mc) measured sections. Lake-level curves were constructed using data from Wirrmann and Mourguiart (1995) and Abbott et al. (1997a; ®gure 4). Water-level data are plotted relative to the 3804 m ``over¯ow'' level described by Abbott et al. (1997a). Error bars (6 m) are used to represent the variability of lake level over the past century and/or radiocarbon dating constraints (after Abbott et al. 1997a, ®g. 4). tively) is consistent with both the age and position of Lake Tauca and the lake-level change than do the earlier studies, but they tend to con®rm that timing of wet conditions recorded by lacustrine strata in the northern part Lake Titicaca was below the modern spill level until at least 3000 yr BP. of the Rio Desaguadero. In fact, the Lake HuinÄaimarca studies suggest that from 3500 yr BP until In the northern part of the Rio Desaguadero, as recorded in the terraces 700 yr BP (when the lake once again rose above its modern spill level of at Parko Kkota, a lake highstand occurred around 4525 yr BP and lasted 3804 m) the Rio Desaguadero was not an effective outlet from Lake Titi- until about 3900 yr BP. This highstand resulted in inundation of the Rio caca (Fig. 14). Desaguadero valley and deposition of lacustrine and meandering-river sed- These lake-level studies based on coring of Lake HuinÄaimarca or Lake iments at Parko Kkota and Calacoto (Fig. 11A). The Parko Kkota lake Titicaca are excellent for identifying low lake stands and the timing of lake reached an elevation of at least 3798 m (the maximum elevation of the ¯ooding, but they provide no constraints on the magnitude of lake high lake shoreline sediments in FA-C at this location; Fig. 9; Rigsby and Bau- stands. In fact, the maximum lake level obtainable using such data is that com 1997, 1998). This lake-level high was followed by either increased of the modern lake bottom. Our work on strata outside the modern lake out¯ow from Lake Titicaca or ¯ashier discharge in the Rio Desaguadero basin is better suited to identifying high lake levels. It also allows us to valley, either of which could explain the deposition of deltaic sediments identify periods of higher effective moisture, even during times when Lake atop lacustrine muds and braided-river sediments atop meandering-river Titicaca was below the over¯ow level. A further complication arises be- sediments at Parko Kkota and Calacoto. Sedimentation rates calculated cause the composition and thickness of sediment above bedrock at the sill from dated samples in terrace C-III suggest that the highstand lasted until separating Lake Titicaca from the Rio Desaguadero is unknown and some at least approximately 3550 yr BP and was followed by dissection of mul- evidence shows that the elevation of the over¯ow level actually varies tiple terraces (preserved along the modern river). The terrace con®guration signi®cantly over time, possibly even annually (Ing. Julio Campos, Auto- indicates at least two rapid water-level falls and southward migration of ridad de Lago Titicaca, personal communication). It is likely that the over- the Rio Desaguadero after 3300 yr BP. A third high-water episode, re- ¯ow level has changed in the past; therefore we use the 3804 m elevation corded by the blue lacustrine muds (FA-D) in the Nazacara cutbanks, re- (of Abbott et al. 1997a) here only as a convenient datum. Because the lake sulted in a 3819-m-high lake (Rigsby and Baucom 1997, 1998). Subsequent core studies do not provide great insight into how lacustrine, meandering- downcutting occurred after approximately 2000 yr BP. The modern river river, and braided-river sediments of the paleo-Desaguadero were accu- occupies the channel carved by this last downcutting episode. mulating on the Altiplano prior to 2000 yr BP, a reexamination of the Rio Workers describing the Holocene water-level ¯uctuations in Lake Hui- Desaguadero ¯uvio-lacustrine system is warranted. nÄaimarca, the southernmost basin of Lake Titicaca, previously postulated There are two possible explanations for the major FA changes in the Rio a minor lake-level rise (6±8 m) around 4000 yr BP (Wirrmann and Mour- Desaguadero sediments: (1) changes in the level of Lake Titicaca, probably guiart 1995) and a rapid 15±20 m lake-level rise around 3500 yr BP, fol- responding to precipitation changes (Cross et al. in press), are effectively lowed by a lowstand spanning several centuries and punctuated by several causing spill-over into the river, and/or (2) during times when Lake Titicaca century-scale lake-level rises (Fig. 14; Abbott et al. 1997a) that are cor- was still below spill level, local precipitation changes resulted in wet pe- relative with increase in precipitation recorded elsewhere in the region (Ab- riods that added signi®cant amounts of water to the Rio Desaguadero val- bott et al. 1997b). Although the timing and magnitude of these rises and ley. As discussed, it is likely that sediment cores from Lake Titicaca record falls correspond with the timing of lacustrine sedimentation and terrace the direction of lake-level rises and falls but not the true magnitude of high dissection along the Rio Desaguadero, the reported ¯uctuations occurred lakestands. If the ¯uvio-lacustrine sediments deposited by the paleo±Rio when the lake was thought to still be signi®cantly below its present level Desaguadero are a product of Lake Titicaca spillover (and not local pre- of 3810 m and, importantly, below its modern spill level of 3804 m. Ad- cipitation in the river valley), they may record the magnitude of lake high- ditional studies, utilizing sediment cores and seismic data from the larger stands more accurately than the sediments of Lake Titicaca. This would and deeper, main basin of Lake Titicaca (Baker et al. 1998; Cross 1998; explain why only below-present lake levels have been recorded in the la- Cross et al. in press; Seltzer et al. 1998) suggest a different magnitude of custrine record of the last 10,000 years even though a whole spectrum of 610 P.C. BAUCOM AND C.A. RIGSBY braided-river, meandering-river, deltaic, and lacustrine sediments accumu- 4. Sediments preserved in terraces south of the Rio Mauri record braid- lated in the ancient Rio Desaguadero valley during this period. Alterna- ed-river deposition prior to approximately 3500 yr BP at Huito Churo and tively, if the highstands of Lake Titicaca are accurately recorded by the 7100 yr BP at Nequela, indicating that the Rio Desaguadero south of the data of Abbott et al. (1997a), for example, then lacustrine deposition in the Rio Mauri has been a braided stream for the entire period recorded in Rio Rio Desaguadero valley is local and unrelated to Lake Titicaca spillover Desaguadero terraces. The river was aggrading from at least 7100 yr BP but provides insight into the relative importance of changes in evaporation until at least 3500 yr BP. Incision and southward migration occurred after versus changes in precipitation. Our current data do not allow us to con®rm 3500 yr BP. a direct connection between the Rio Desaguadero strata and Lake Titicaca, 5. The history of sediment deposition and terrace dissection north of the only a correlation between Lake Titicaca rise and increased water supply Rio Mauri has been controlled by precipitation ¯uctuations and is either and lacustrine sedimentation in the Rio Desaguadero valley. The data sug- directly (Lake Titicaca spillover) or indirectly (effective moisture changes gest, however, that a direct link was possible, and that hypothesis is cur- on the Altiplano) correlative with changes in the water level of Lake Ti- rently being invested with further geochemical and provenance studies. ticaca. South of the Rio Mauri, deposition and dissection are controlled by Even if the Parko Kkota, Janko Wichinca, and Nazacara lakes were not the discharge of Rio Mauri, by channel migration, and by ¯uctuations in directly connected to Lake Titicaca, their presence in the northern Rio the level of the southern lakes. Desaguadero valley is important. The presence of these lakes, especially in the absence of signi®cant tectonism, suggests that rises in the level of Lake ACKNOWLEDGMENTS Titicaca (even rises that occurred when the lake was below the spill level) are driven by increases in precipitation, not simply decreased evaporation. This work was supported by grants from the National Science Foundation (#ATM-9709035) and by the East Carolina University Research/Creative Activities It is likely that the Rio Desaguadero valley lakes correspond to brief in- Grants Committee. We thank the personnel at ALT (Autoridad Binacional Lago tervals (perhaps 250 to 500 years) of increased effective moisture in the Titicaca), especially Julio Sanjines, Mario Revollo, and Julio Campos, for allowing basin. Ice-core data from the Huascaran and nearby Sajama ice caps us to work in Bolivia and the staff at GEOBOL (Servicio GeoloÂgico de Bolivia), (Thompson et al. 1988; Thompson 1998) show temperature changes of only especially Herberto PeÂrez, who provided access to aerial photographs and maps. about 1ЊC in the late Holocene. These small temperature changes are not Paul A. Baker, Robin W. Renault, Mark B. Abbott, and Elizabeth Gierlowski-Kor- enough to cause signi®cant changes in evaporation (Blodgett et al. 1997); desch provided insightful comments on the manuscript. therefore, large lake-level changes must be the result of changes in precipitation. REFERENCES

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