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

Abstract

A sedimentary succession was studied on the north margin of Los Barreales Lake, in northeastern of Neuquén, Argentina. These outcrops correspond to the Portezuelo and Plottier Formations, which are formally grouped as the Río Neuquén Subgroup of the Neuquén Group. The Plottier Formation is considered as upper Coniacian in age. In order to describe the distribution pattern and evolution of the paleoenvironments of this unit, eleven lithofacies were recognized and grouped into seven lithofacies associations that characterize different fluvial styles, which are named here as fluvial styles I, II and III. The fluvial style I is at the base of these outcrops and represents an ephemeral fluvial system characterized by isolate lenticular channels in flood plain deposits and frequent sheet floods events. Gradually, an increasing on the channel/floodplain relation is recorded. Channels becoming thicker and showing up a braided design, levees, crevasse splays and a strong multilateral relation are developed. These features are defining the fluvial style II. Large scale geometry of fluvial styles I and II are pointing out a progressive decrease on the accommodation/supply. Weather probably varied from sub-humid or semi-arid to humid climatic conditions with strong seasonal variations. These provoked important changes on the water and sediment discharge onto the fluvial system. At the middle part of Plottier Formation a 6° order discontinuity is defined. This is the border for a stacked channel belts succession belonging to the fluvial style II. A sharp and planar surface discontinuity and an important architectural design change suggest a strong decreasing on the accommodation/supply. The climate, probably changed from subhumid or semiarid to more humid climatic conditions but with seasonal variations marked in the rainfalls regime, which provoked strong changes in the water and sediments discharge to the fluvial system. A new discontinuity of 6° order distinguished by an eminent surface of erosion, limits the base of the fluvial style III that corresponds to sinuous channels where the deposits dominate of lateral accretion. These constitute the top of the Formation Plottier and indicate an abrupt modification in the evolution of the fluvial systems. The erosive surface, followed for tabulate and thick bodies of sandstones indicates the period of fall in the level of base and early stadium of ascent.