Análisis de los procesos de transporte y depositación de fragmentos pumíceos en un sistema fluvial: el caso de la Formación El Palo entre las localidades de General Roca y Villa Regina (Río Negro)

Abstract

There are numerous studies analysing volcaniclastic supply to continental environments in distal areas from source volcanoes. However, there are few examples where large pumice fragments are mentioned in distal fluvial deposits. In the present work, the sediments of the Chichinales and El Palo formations in the area of Alto Valle, Río Negro were studied. The deposits of the latter unit include pumiceous fragments that exceptionally exceed 20 cm and were accumulated in a fluvial context more than 200 km from the current Andean volcanic arc. Although previous works mention the presence of pumice in this unit, an analysis of the origin or the transport and depositional processes of these fragments was not carried out. Based on the study of outcropping sections between General Roca and Villa Regina cities, it was determined that the sediments of the Chichinales Formation were deposited in a low to medium energy fluvial environment with development of wide floodplains and paleosols formation during stability periods. The El Palo Formation corresponds to a moderate to high energy braided fluvial system with occasional high discharge periods. The pumiceous fragments present in this unit were derived from the reworking of primary volcaniclastic deposits related to the Andean arc, associated with an important explosive volcanic activity during the Miocene. These fragments were transported and deposited by both dilute and sediment gravity flows with high concentrations of pumice. Petrographic analysis of El Palo Formation sandstones showed a provenance mostly related to the erosion of a magmatic arc. The main source areas would have been the Andean arc and the North Patagonian Massif. On the other hand, a maximum depositional age of 14.6 ± 1 Ma was obtained in a sample from the El Palo Formation, which constitutes the first U-Pb dating of detrital zircons from this unit in the study area. This allowed comparison with other sedimentary units in different depocenters of northern Patagonia.