Surficial geology and Quaternary paleoenvironments of Arroyo La Horqueta, Chascomús, Buenos Aires Province

Abstract

The Arroyo La Horqueta is a 7.5 km long creek located in the northeast region of Buenos Aires province and tributary of Salado River in Chascomús District. It runs along three gentle depressions constituted by filled up eolian paleobasins surrounded by Quaternary terrains. Eight stratigraphic units crop out in the region; the oldest one is Ensenada Formation of Middle Pleistocene age, which constitutes the regional substrate. The following units are of eolian origin and span from the Maxi- Glacial to the Little Ice Age, i.e.: Buenos Aires, La Postrera I, III and IV Formations and the paleosols Sin Nombre, La Horqueta and Berrondo. The La Horqueta paleosol is a new pedostratigraphic unit for the region. In the infilling paleobasins sediments five lithostratigraphic units from the Late Pleistocene to the present were recognized. The oldest unit is made up of fluvio-lacustrine sediments with gypsum of the La Chumbiada Member of the Luján Formation, overlain by clastic deposits and gypsum, containing fresh and brackish water remains of the Lobos Member of the Luján Formation,. The third unit is made up of palustrine deposits with freshwater remains, volcanic ash lenses and gypsum of the Río Salado Member of the Luján Formation. The fourth is an aeolian deposit and the last one a pelitic layer of recent alluvial origin. The basins were excavated during the aeolian episodes that accumulated Buenos Aires Formation. Several humid and dry alternate climatic stages followed, extending from the Upper Pleistocene to the present and comprise the three aquatic paleoenvironments described and the four exposed clay dune and loessic deposits.