Neogene paleoenvironmental evolution of the Northern Patagonian extra-Andean foreland basin, Argentina

Abstract

This paper includes a detailed sedimentologic and stratigraphic analysis of the fill of the western part of the Neogene Northern Patagonian extra-Andean foreland basin of Argentina. Using facies analysis, stratigraphic relationships, independent age indicators and physical correlation between sections, the depositional history of the basin is reconstructed and linked to regional climatic and tectonic events. The fill of the basin comprises the El Sauzal, Barranca de los Loros, El Palo, Bayo Mesa and Río Negro formations and it reaches up to 60 m thick in outcrop. On the basis of fourteen detailed sedimentologic logs located in the western side of the basin, a total of nine facies associations are recognized: braided permanent fluvial channel belt, pedogenized floodplain, shallow floodplain lakes, eolian dune field, alkaline lake, saline lake, shallow freshwater lake, and permanent braided gravelly channel belt facies associations. Basin sediments were deposited on an erosive unconformity on Late Cretaceous/Paleocene to early Eocene sedimentary rocks and are divided into five depositional episodes. The first depositional episode is represented by a shallow freshwater lacustrine system of local occurrence, tentatively assigned to the late Miocene. The second depositional episode is represented by permanent braided sandy fluvial system that occurred in the late Miocene-early Pliocene in a depression from the center and south of the study area. The large rivers of this episode flowed toward the east and northeast. The third depositional episode is represented from south to north by ephemeral fluvial, eolian dune and saline-alkaline lacustrine deposits that were recorded in all sections and is tentatively assigned to the Pliocene. Eolian dunes migrated toward the northeast and east. The fourth depositional episode is composed of permanent sandy braided fluvial deposits occurring only in the center of the study area. The fifth depositional episode (early Pleistocene?) is represented by permanent braided gravelly fluvial deposits that rests on a regional erosive unconformity interpreted as a large fluvial fan sourced from the south. These deposits resulted calcretized prior to the middle Pleistocene. The paleoclimatic evolution suggest a wetter period (depositional episodes 1 and 2) tentatively linked to the middle member of the Río Negro Formation and the Paranense transgression, followed by a progressive aridization during the Pliocene and Pleistocene.