Clay mineralogy and calcareous nannofossils from Jagüel and Roca formations in the eastern sector of Pellegrini Lake, Neuquen Basin, República Argentina.

Abstract

This study presents new mineralogical, sedimentological, and micropaleontological data from the Jagüel (Late Maastrichtian-Early Danian) and Roca (Danian) formations, located in the eastern sector of Pellegrini Lake, Río Negro province, República Argentina. According to the physical characteristics and the age of the sediments that conform these formations, two siliciclastic lithofacies and a carbonate one were recognized, they are named: Maastrichtian grayish yellow calcareous mudstones, Danian olive gray calcareous mudstones, and Danian carbonates. The rocks which constitutes siliciclastic lithofacies were classified as allochemical mudstones and the mineralogy is characterized by high proportions of clay minerals and, to a lesser extent, calcite. The clay mineral association is dominated by a random (R0) I/S mixed-layers (70-90% Sm) accompanied by lower proportions of illite and kaolinite. The mineralogy and the microfabric of the mudstones reflect that these sediments have not been affected significantly by diagenesis. Clay minerals would be of detrital origin and the predominance of the highly smectitic I/S along all sections would be associated with an active volcanism coming from the western magmatic arc, located in the Neuquén Basin during the Late Cretaceous and Paleocene. Micropalaeontological analyses confirmed the presence of Cretaceous-Paleogene transition in the Jagüel Formation beds. The calcareous nannofossils allowed to relate changes in the the mineralogical associations along the K/P transition. In this way, it was possible to detect an increase in kaolinite content close to the K/P boundary (>10%), that decreases towards higher positions of the Danian, which could indicate a warmer and wetter climate during this interval, that it would tend to be drier with seasonal conditions in the late Danian. The Maastrichtian calcareous nannofossil assemblages suggest oligotrophic environments and a well stratified photic zone, nevertheless, in some horizons, the nannoflora is abundant with lower richness which would indicate more restricted marine conditions.