Andean retroarc-basin dune fields and Pampean Sand Sea (Argentina): Provenance and drainage changes driven by tectonics and climate

Abstract

We here review in terms of tectono-magmatic setting and Quaternary landscape dynamics what is known about the provenance of Argentine dune fields and their fluvial feeder systems draining the Andean Cordillera. The detrital signatures of these eolian sediments were previously investigated based on either framework petrography and heavy minerals or detrital-zircon geochronology, and their peculiar volcaniclastic nature was long recognized. Compositional variability, however, was only broadly evaluated, and quantitative provenance analysis based on a systematic multimethod approach across the entire region was not carried out so far. For this reason, here we integrate original and previously obtained petrographic, heavy-mineral, and detrital-zircon geochronology data to present the first comprehensive provenance study of dune fields stretching for 1000 km across central Argentina from the Andean piedmont to the Atlantic Ocean. In dune fields along the Andean retroarc basin, sediment composition defines a steady northward decrease in volcanic detritus. This reflects active magmatism in the Southern Volcanic Zone and Payenia province (38°-34°S), in contrast with the ~600-km-long Pliocene-Quaternary magmatic gap in the Pampean flat-slab segment (33°-27°S), where sediment is derived from deeper-seated tectono-stratigraphic levels of the continental arc and uplifted blocks of retroarc-basin basement. In distal Pampean lowlands extending across the bulge and backbulge depozones, instead, sand dunes display notably homogeneous compositional signatures, indicating that detritus was mostly generated north of 34°S and transported by a paleo-Desaguadero trunk river that formed during southward-progressing diachronous uplift of the Sierras Pampeanas since the late Miocene. In contrast with huge African and Arabian deserts that contain multiply recycled quartzose to pure quartzose sand, even very fragile volcanic clasts, plagioclase feldspar, and unstable ferromagnesian minerals are widespread, testifying to largely first-cycle volcanic provenance and only minor effects of mechanical breakdown and chemical weathering in the Pampean Sand Sea. During the Late Pleistocene, after a first southward shift of the Desaguadero trunk river possibly induced by increased water and sediment discharge at the end of the penultimate glacial maximum, tectonic uplift eventually outpaced stream power during the last glacial period. Paleo-rivers were thus forced to shift farther southwards, leading to the formation of an integrated paleo-Desaguadero+Colorado drainage system. During the latest Pleistocene-early Holocene, such a large trunk river fostered the rapid progradation of a wide delta and littoral sand transport all along the shores of the Buenos Aires Province. Climate change and repeated waxing and waning of glaciers through the Quaternary have left a prominent mark on sediment distribution, dominated by fluvial processes during periods of high fluvial discharge but alternating with arid phases characterized by limited transport capacity and vegetation cover, extensive wind deflation of floodplains, and sand accumulation in the dune fields.