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

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Climate and Lake-Level History of the Northern Altiplano, Bolivia, As Recorded in Holocene Sediments of the Rio Desaguadero 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 down- cutting 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 168 to 228 more likely controlled by a combination of base-level change and sed- S latitude and 658 to 698 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 (, 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 q 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).
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