Fácies sedimentares das unidades Siluro-devonianas aflorantes na porção sudeste do município de Presidente Figueiredo, borda norte da Bacia do Amazonas - AM

Abstract

In the region of Presidente Figueiredo, on the North edge of the Amazon Basin, an elongated outcroppings belt follows a general E-W trend, related to Ordovician-Silurian and Devono-Tournasian sequences. The belt is bordered to the South by a Cretaceous stratigraphic unit. Based on data from rock samples and outcrops, the Paleozoic sequence has been best studied in function of the oil industry, with emphasis on regional geological studies that characterize shallow and distal platform environments that predominated in the basin at that time. However, little focus has been given to the main events that acted in the basin, as the two Silurian and Devonian glaciations, which left records in the sedimentary package. Studies in outcrops (roads and rivers) in the Presidente Figueiredo municipality propitiated the individualization of 9 sedimentary and glacial facies related to shoreface (lower and upper) and foreshore zones, and particularly, to a superimposed glacial event. This study describes for the first time the occurrence of coarse diamictite (polymictic) near the Balbina Village. Together with earlier studies, it was possible to delimit the area of glacial deposits, of general E-W direction, which extends approximately 80 km in this portion of the north border. This diamictite presents restricted areal distribution, being constituted predominantly by blocks and pebbles of quartz-sandstones, mudstones, and quartz, of various shapes and sizes, immersed in a poorly selected fine and coarse sandstone matrix. The intraformational distribution of this facies, which occurs interbedded in the foreshore facies, combined with the occurrence of faceted and striated pebbles, allows assigning it to a glacial origin. The palynological analysis, carried out in the studied facies, was inconclusive in the definition of age, allowing to positioning them in a wide Siluro-Devonian interval. However, based on palynological data obtained from previous works by many investigators, the origin of the diamictite was associated with the Silurian glaciation which occurred in the basin. The stratigraphic succession of the Siluro-Devonian units in the studied area, allowed to interpret 3 depositional episodes: A) sediments deposition on shoreface and foreshore environments during a higher sea level, B) glacial event with polymictic diamictites deposition terminal moraines and/or supraglacial during a period sea level fall and C) deposition of foreshore deposits related to a new sea level rise covering the remaining units.