Petrografia e diagênese da formação Pimenta Bueno - Bacia dos Parecis, Rondônia

Abstract

The Parecis Basin is one of the largest Brazilian intracratonic basins and although of great importance, studies dedicated to its genesis and fossil content, essential to understanding the evolutionary history of a basin, are scarce. This work focused on the rocks of the Pimenta Bueno Formation whose samples were acquired from the PB 01- RO core and allowed to characterize the depositional environment, the diferents diagenetic events as well as the factors that influenced them and allowing inferring the diagenetic evolution of the basin. The petrographic studies allowed classified the sandstones as subarcósios constituted by quartz, feldspar, rock fragments and heavy minerals. The diagenetic evolution has proved complex, characterized by a number of processes that occurred in diferents stages. The eodiagênese is marked, in siliciclastic rocks, by cementation of hematite, mechanical infiltration clays, framboibal pyrite, feldspar overgrowth, gypsum and halite cementation. In the mesodiagenesis occurred cementation by silica, calcite, dolomite and anhydrite, mechanical and chemical compaction, recrystallization, dissolution, replacement, autigênica clay, cementation zircon and titanium oxide precipitation. Electron microscopy analysis revealed the presence of hydrocarbon filling pore. In the carbonate rocks in the eodiagenesis precipitated framboidal pyrite, calcite "Dog tooth" and gypsum. In the mesodiagenesis occurred mechanical and chemical compaction, cementation by anhydrite, celestite, calcite, dolomitization, dissolution and replacement. No have been identified diagenetic processes related to telodiagenese. Analysis of diffraction X - ray allowed to characterize the clay minerals present in the clay fraction and obtain the parameters of the Kübler Index (Crystallinity Index of Illite). The results indicate that the rocks are at the limit of Anquizone/ Epizone and the average width measured at half height (FWHM) is 0.16 °. Diffraction analysis X - ray indicated the presence of chlorite, illite, pyrophyllite, kaolinite and smectite (montmorillonite). In addition to the well-defined species, it were identified also interstratified of clay minerals of illite/ smectite (regular and irregular) and chlorite/smectite. The microfossils found were related to correlate them to literature, which suggest that the age of deposition of this formation is neoproterozoic.