The 'Rodados Patagonicos' (Patagonian shingle formation) of eastern Patagonia: environmental conditions of gravel sedimentation

The gravel mantels known as 'Rodados Patagonicos' of Eastern Patagonia are one of the most distinctive features of the regional landscape. Their significant roundness, along with additional sedimentary and geomorphological evidence, indicates their origin is linked to fluvial action. Currently, no natural processes exist on Earth potentially generating equivalent deposits, either in Patagonia or elsewhere around the world. The hydrological conditions responsible for the gravel formation were thus likely very different from those currently prevailing in Argentine Patagonia, suggesting the gravel mantels formed during the Late Cenozoic, when surface drainage networks of relatively high energy existed in the region. Such high energy was induced by powerful flow rates and/ or strong water gradients, attributable to tectonism, epirogenesis and/ or lowering of sea level. Periods with water flow regimes significantly higher than at present characterized the full glacial phases lasting several thousands of years, corresponding to each of the many glaciations of the Pliocene and Pleistocene. The capacity for erosion and water transport would have been increased by the sea level drop characteristic of each full glacial phase. There were also much shorter periods, known as ` terminations', representing the abrupt ending of the main glacial events during which large amounts of water were released as a result of the intensive melting of the Cordilleran ice sheet. During interglacial periods, in the absence of large masses of ice in the mountains and with average temperatures similar to or higher than at present, layers of gravel were deposited as a result of tectonic or epirogenic movement or by reactivation of drainage networks produced by sea level lowering (glacio-eustatism). These endogenous landscape reactivations were also independent of and longer than the climatic fluctuations and therefore overlapped with both interglacial and glacial periods. Finally, some units of the ` Rodados Patagonicos' may also have been deposited during major pluvial events, characterized by higher mean annual precipitation over sufficiently long periods. The current evidence suggests that, at various times since the Late Miocene, varied climatic conditions have allowed the production and accumulation of large gravel beds covering much of extra-Andean Patagonia. (C) 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 103, 336-345.