The Late Pleistocene-Holocene Fluvial Dynamics in the Arroyo La Estacada (~33?34º S), Mendoza Province Andean Piedmont, Argentina.

MEHL, A. E.

Vista Flores, Tunuyàn, Mendoza

Conferencia; 4th Southern Deserts Conference; 2014

Laboratorio de Paleoecologìa Humana - Universidad Nacional de Cuyo

The late Quaternary alluvial sequence of Arroyo La Estacada (33°28?S, 69°02?W) and its tributary Arroyo Anchayuyo is a useful record for reconstructing prevailing paleoenvironmental conditions of the eastern Andean piedmont between 33° and 34° S (Mendoza Province,Argentina), particularly during Late Pleistocene?Holocene transition. The alluvial sequence that crops out along the arroyos, including several paleosols and datable organic levels, makes it possible to perform sedimentological,stratigraphical and geochronological analyses which collectively provide a source of paleoenvironmental information. The data were interpreted based on the geomorphological units identified at this piedmont fluvial systems. Late Quaternary deposits of Arroyo La Estacada comprise three main geomorphological units, an extensive aggradation plain (AP), a fill terrace (FT), and the present floodplain (PF). In the, AP a distal alluvial fan lithofacial association was determined between 50,000 BP and the early Holocene; it was mainly related to overbank sheet fluid overflows and probably temporary inactive channels of sandy-like braided streams. During the Late Glacial, a paleosol developed in the uppermost part of likely aeolian deposits aggraded into the floodplain environment. It shows variable grade of development along the arroyo outcrops; the dominant paleosol formation processes, in accordance with the location of the South American Arid Diagonal at this latitude, were organic matter humification,carbonate accumulation, and redox processes. By the early Holocene, paleosol formation was outpaced by renewed alluvial aggradation with a likely high magnitude flooding events affecting the arroyo?s floodplain environment. Other Holocene paleosol formation intervals are recorded in this aggradation unit.The aggradation of the FT started after an erosive event that incised AP deposits before 5270 BP,according to the oldest age obtained at the base of FT deposits. The mid to late Holocene fining upward alluvial sequence of the FT, including several paleosols, corresponds to a sinuous fluvial system lithofacial association. A second incision event, ~400 BP, placed the arroyo aggradation level at its present position (PF),almost 15 meters below the FT surface, creating a narrow fluvial valley constrained by the Late Pleistocene and Holocene fluvial sedimentary sequence. The occurrence of the paleosol formation intervals and the incision events are indicative of the intermittency of the fluvial aggradation processes in the basin during the Holocene. The Late Glacial and Holocene alluvial record of Mendoza?s Andean piedmont (33° and 34° S) constitutes a suitable record of the Late Pleistocene?Early Holocene climatic transition in the extra-Andean region of Argentina. Its paleoenvironmental reconstruction and the chronology of major fluvial landscape landforms contributes to the general understanding of fluvial system dynamics during the late Quaternary. These observations are consistent with many other late Quaternary records from the southern tip of South America?which also display evidence of alternating episodes of sedimentation and pedogenesis at the end of the Pleistocene and the Holocene?making it possible to suggest climate as one of the forcing factors controlling the dynamics of the sedimentary environments.