New evidence concerning the Plio-Pleistocene landscape evolution of southern Santa Cruz region

Remnants of an old aggradational landscape, Cerro Cuadrado Proglacial, are preserved on top of the high mesetas Pampa Alta and La Meseta on both sides of the upper Santa Cruz river valley, South Patagonia. A first dissection of the mesetas, attributable to extended river erosion,: predates the expansion of glacier lobes down the piedmontane area. The glac ial advance is represented by the moraines of Pampa Alta Glaciation display ed on the top of Meseta Pampa Alta. Glacifluvial outlets contribute to the proglacial plain, Pampa Alta Proglacial, which is widespread to the southea st. Strong and persistent fluvial erosion followed the retreat of the ice masse s leading to the formation of several terrace levels in the main upper vall ey, La Australasia Terraces and San Fernando Terraces, and a step, Cordon A lto, that truncates the Meseta Pampa Alta. These foreland features and the relief covered by the basalts at Cerro Fraile in the cordillera, are probab ly a consequence of a diastrophic phase that affected both areas during thi s stage. Late Pliocene basaltic lavas draining into the main and tributary valleys o verran this landscape. The evidence indicates that during the eruption of t he basalts the glaciation was active in the cordillera and that coeval fluv ial and lacustrine aggradation took place in the extra-andean valleys. During the Middle Pleistocene subsequent lava flows covered the high pampas and partially occupied the fluvial valleys again. After this last volcanic episode the glaciers reached their maximum expansion to the east. (C) 1999 Elsevier Science Ltd. All rights reserved.