Sedimentary paleoenvironments in the Upper Cretaceous Plottier Formation (Neuquen Group), Confluencia, Neuquén

Abstract

The Río Neuquén Subgroup (lateTuronian-late Coniacian) of the Neuquén Group, crops out to the west of the Senillosa locality. Sedimentological studies in order to determine paleoenvironments, their evolution, and relevant controls on deposition were carried out in this area. Twelve lithofacies were defined and included in six facies associations. The nature and arrangement of facies associations suggest the development of braided and anastomosig fluvial systems as well as dunes and aeolian sandsheets. The fluvial systems were subject to changes in the discharge regime, interpreted to be related to short term climatic variability. The variations of dominant grain size of the bed load, thickness of channel bodies, and relative proportions of channel/flood plain elements, respond to long term fluctuations of discharge levels. It is suggested that the modifications in base-level controlled the large-scale fluvial architecture. At the base of Portezuelo Formation thick channels show a braided design, levees, crevasse splay and a strong multilateral relation are developed. These relation of channel units, is related with a rate of sediment supply that, in general, compensated by the subsidence rate. At the flood plain aeolian dunes and sheet were developed. Gradually, a decreasing on the channel/floodplain relation is recorded. The fluvial system is characterized by isolate lenticular channels in flood plain deposits and frequent crevasse splays events.were recorded. In the top of the unit, an increase of subsidence rate resulted in the development of flooded areas within the flood plain and the anastomosed fluvial style. At the base of Plottier Formation a 6th order discontinuity is defined. This is the border for a stacked braided channel belts succession. A sharp and planar surface discontinuity and an important architectural design change suggest a strong decreasing on the accommodation/supply.