UPPER PLEISTOCENE FLUVIAL SEQUENCES IN THE TARIJA BASIN (SOUTHERN BOLIVIA)

The Tarija Valley Basin (Southern Bolivia) is filled with a ca. 150 m thick Quaternary fluvial sequence (Tolomosa Fm), containing very rich mammal faunas known since the XIX Century. A new detailed sedimentological and stratigraphical study has been carried out in the Basin, coupled with geomorphological investigations. Tephra layers, buried paleosoils and C14 dates on peat and organic rich soils allowed thorough correlations between the sections. The sequence is subdivided in two parts. The lower Ancòn Grande Unit, 50 m thick, is made of shallow channels filled with planarly or trough cross-bedded gravels and sands and gravity flow sediments. In the central part of the Basin there are mainly slightly weathered or bioturbated pinkish-reddish sands and silts. On top of the Unit, Ancòn Grande Log, there is a pedosequence made of several reddish illuvial Bt or Btk horizons. The depositional environment is typical of a braided plain, with the transition from coarser-grained alluvial fans, next to the slopes, to finer-grained distal plain with periods of dense vegetational cover. This Unit is conformably buried under the 100 m thick yellowish-greyish and finer-grained S.Jacinto Unit. Close to the slopes, alluvial fans, whose flat morphology is in place still preserved, are made of massive or trough cross bedded gravels and sands. In the central part of the Basin there is a cyclic repetition of channels filled with planarly cross bedded sands and thin gravelly lags. On top of the channels massive or laminated slightly weathered silts, peat layers, CaCO3 crusts and gypsum are present. Most of the mammal fauna was found within this Unit deposited in a sandy-dominated meandering alluvial plain with frequent channel avulsions and deposition of organic-rich layers inside swamps reached by occasional crevasse channels and splays. The upper part of the sequence is abruptly truncated and locally capped by trough and planarly cross bedded coarse-grained gravels arranged in a series of unpaired alluvial terraces observed all across the Basin. The deposition of the lower Unit, out of the C14 range, might be related to alternating Stadials and Interstadials at the beginning of the Upper Pleistocene. The upper Unit, mostly younger than 40 ka, locally reveals very high sedimentation rates (up to 2 cm/y).