Drumlins and drumlinoid forms of the Lower Pleistocene in southern Patagonia, Province of Santa Cruz

Abstract

Drumlins and drumlinoid forms have been identified at Estancia Sofía (Southwestern Santa Cruz Province, Argentina). The drumlin field extends over 150 km². It is composed of drumlins, megaflutes and other drumlinoid features, ranking from 200 m to 2800 m in length and 35 m to 200 m in width. Thirty-seven drumlins have been measured: two of them are oviodal in shape (w/l ratios between 0.2 and 0.3), 8 are ridges (w/l ratios between 0.12 and 0.2), and 27 are flutes and megaflutes (w/l ratios <0.12). These are all very low landforms, with less than 20 m in local relief over the surrounding depressions, corresponding to the spindle-shaped drumlin type. The surface and sides of the drumlins are covered by scaterred erratic boulders, at least up to 6x4x3 m in visible size, pertaining to Andean Cordillera lithologies. The westernmost portion of the field is oriented N 85°E and over 6 km long; the eastern part is 17.5 km long, showing a clear turn in the main axis direction towards the NE, with a bimodal distribution (N 71°E and N 75°E), thus suggesting a changing ice direction. These landforms are part of the Bella Vista Drift, deposited by a piedmont-type glacier, developed during the "Great Patagonian Glaciation" (GPG), which took place sometime between 1.168 and 1.016 Ma (Early Pleistocene, Oxygen Isotope Stages 30-34). The area where the drumlins field is located was never glaciated again, since the glaciers reached inner positions in subsequent glaciations and did not cover this sector. These ancient landforms have been preserved thanks to the extremely dry climate of the region, which has been dominant at least since the middle Pleistocene. The Estancia Sofía drumlins are amongst the oldest glacigenic landforms preserved in temperate regions of the world (i.e., out of Antartica).