Late cenozoic geology and geomorphology of the Laguna de Agnia Area, Argentina

Abstract

Laguna de Agnia is located within an endorheic basin in arid extra-Andean Patagonia. A variety of erosional and depositional landforms, most of which are relict, are well preserved in the basin. Geological, geomorphological, and sedimentological studies, 14C and 40Ar/39Ar ages, and paleomagnetic data allow us to modify the published interpretation of the late Cenozoic stratigraphy of the area and provide an improved understanding of local landscape evolution, and paleoenvironments and paleoclimates in the region. The basin formed during or before the late Oligocene. Miocene pyroclastic deposits, which are widely distributed in this part of Patagonia, were not found within the basin. However, a bajada sloping down toward the east side of the lake likely dates to the Miocene. Basalt lava flows reached the west margin of the Laguna de Agnia depression 3.39 ± 0.02 Ma. A lacustrine phase is manifest in numerous shorelines and related features east of and above the modern lake. This shoreline system, one of the most extensive in Patagonia, provides evidence for high paleo-lake levels associated with cooler and wetter conditions during the late Pleistocene and even in some periods during the Holocene, when Southern Hemisphere Westerlies were more intense than today.