Cenozoic sedimentary evolution in the lower Colorado River Basin (Northern Patagonia, Argentina): The origin of La Salada Shallow Lake

SEITZ, C.; VELEZ CAICEDO, M.I.; PERILLO G.M.E.

ANDEAN GEOLOGY

SERVICIO NACIONAL GEOLOGIA MINERVA

Lugar: Santiago de Chile; Año: 2018

0718-7092

Climatic changes and eustatic sea levels have been presumed to be the most important controllers of the alluvial fan of the Colorado River in northern Patagonia. Although the alluvial fan occurs in a region considered tectonically stable, there are pieces of evidences that the Miocene Andean orogeny has reactivated inherited structures, with subsequent geomorphological changes that date back to the Pleistocene. Besides, the clear evidence of neotectonism in the region and their effects on the evolution of this fan, it has not been studied in detail yet. In this study, we map and analyze six sections outcropping in different terraces of the alluvial fan with the primary aim of disentangling the role of tectonism, climate and eustatic changes on the evolution of the alluvial fan. This study is part of a bigger project aimed at understanding the origin of the shallow lakes in northern Patagonia. Our results indicate that the alluvial fan of the Colorado River was established in the area around the Middle Pleistocene. Evidence of deformations in Miocene to Pleistocene units indicates significant neotectonism during the Upper Pleistocene. By the Pleistocene- Holocene transition, tectonism produced incision generating a set of terraces. After this time, an important climate change from semiarid to arid favored the calcretization of some terraces. By the Pleistocene- Middle Holocene, the terraces were covered by ancient eolian sediment accumulated during dry conditions. By the Middle Holocene, a broad alluvial fan developed in the region under a warmer and more humid climate generating the Alluvial Colorado River- III deposit at the T3 terrace. Finally, by the late Holocene conditions turn arid again causing the reworked of ACR-III deposit. At the same time, aggradation process was favored by a high sea level and temperate-arid climate, producing T4 terrace. Simultaneously, local deflation and sedimentation of modern eolian deposits (mE) over the T3 terrace occurred. The depressions generated by the deflation were, later on, occupied by shallow lakes when the climate turn more humid. Subsequently, during regressive sea level condition, c. 2000 years BP, the T4 terrace was partially eroded and the modern alluvial plain formed.