The Glacial and Loess Record of Southern South America

Abstract

The geomorphological, sedimentary and pedological evidence related to the various glacial periods which took place in the Patagonian and Fuegian Andes between the Late Miocene and the Late Pleistocene are herein presented and discussed. The occurrence of basaltic flows interbedded with till and other glaciogenic deposits provides radiometric dating and paleomagnetic information for several Patagonian regions which may be applied to the related glacial episodes. The modern glacial chronology of the Patagonian terrestrial glacial record is basically supported by 40Ar/39Ar dating on volcanic rocks associated with glacial deposits, and cosmogenic isotope dating techniques on erratic boulders and glacial erosion surfaces. These sequences, chronologically calibrated, compared with 47 Marine 18O Isotope Stages (MIS) and constrained by global magnetostratigraphy, enable the interpretation of climatic changes over about seven million years in the Southern Hemisphere. In addition, the loess and loess-like record of southern South American extends back at least to the Late Miocene. These deposits cover the Chaco-Pampean region of central eastern Argentina, between 28–38° S, which formed a large continental sedimentary basin during the Neogene. The oldest glacial evidence is composed of till and glaciofluvial deposits interbedded with 5–7 Ma old basalts found in at least two areas, 38° and 46° S. It has been interpreted that the Pliocene–Pleistocene glaciations were of local ice cap and piedmont glacier type. These ancient glacial deposits have been also affected by Andean tectonics and deformation. At least parts of the Southern Andes were already covered by a local mountain ice sheet during the Late Pliocene. This statement is supported by the finding of glacial boulders in between basaltic flows at 41° S, lava flows overlying glacial and glaciofluvial units at 39° S, and glacial deposits interbedded with flows and till overlying basalt flows between latitude 46°–52° S. Whether the development of glaciers was promoted by climate deterioration, by tectonic forces, or both kinds of processes is still unknown. Thus, already during the Pliocene, local icefields would have been very extensive, covering part of the westernmost, extra-Andean tablelands all along Patagonia. A minimum of seven glaciations were recognized at Cerro del Fraile locality (50° S) during the Early Pleistocene and interpreted as repeated piedmont glaciations, synchronous to active volcanism. The most extensive glaciation recognized in many parts of Patagonia is known as The Great Patagonian Glaciation (GPG) and bracketed between 1.15–1.05 Ma. However, until precise chronology is available for all localities in which GPG has been found, GPG should be considered as a multiple-event glaciation. In several transversal Patagonian valleys three glacial advances took place after GPG and before Last Glacial Maximum (LGM). The absolute chronologies of these post-GPG events have not been closely defined yet though, based upon paleomagnetic data, all till deposits along the Atlantic coast of Tierra del Fuego would belong to the Brunhes epoch. The Last Glaciation is very well known in all Patagonian glacial valleys and coastal channels and fjords. The estimated ages of these deposits begin close to 60 ka, but full glacial times (LGM) have been dated between 25–20 ka. Correlation of all these glacial events with the Pampean loess units is possible, but more detailed studies and absolute dating is needed.