Telescopic alluvial fans and pleistocene-holocene temporary lakes in the quebradas del Río Toro and Tastil (Salta, NW Argentina): main sedimentary features

Abstract The Toro and Tastil Quebradas (river valleys) present different terrace-like geoforms made up of deposits of coarse-grained clastic materials. These terraces display convex-upward surfaces, a highly variable distribution, have a small lateral extension and depend on the lithology of the geological basement, which differs significantly along the quebradas . The number of terraces between one area and another indicates that these terraced geoforms should not be ascribed to one single cause, but rather to a variety of factors, i.e. local variations at base-level. The coarse-grained materials of the terraces are usually arranged in roughly horizontal layers that characterize the outcrops. These are interfingered with fine-grained mud-rich materials. The freshwater gastropod remains in the mud-rich materials indicate that they accumulated in lacustrine-like sedimentary environments. These developed from alluvial fans that spread out from the lateral quebradas to occupy the entire main valley (q uebrada) , giving rise to natural dams and temporary lakes upstream. Absolute data obtained by radiocarbon ( 14 C) methods strongly suggest that the alluvial fans were coeval with the temporary lakes in the Upper Pleistocene - Middle Holocene. The El Niño Southern Oscillation (ENSO) effects could have brought about the local variations in base-level that generated the younger temporary lakes, whereas the older ones were probably controlled by precession cycles (at about 21,000 y). The large number of temporary lakes scattered across the region suggests the overlapping of forcing factors at local and regional scales.