Datação em amostras carbonáticas fossilíferas e em ankaramito, para a caracterização temporal das fácies, formulação de paleoambientes e evolução da bacia de Itaboraí, paleógeno do estado do Rio de Janeiro (sudeste do Brasil)

Abstract

This dissertation focuses on the dating of mineral and fossil samples from the Itaboraí Basin, the only geological unit that has preserved macrofossils in the State of Rio de Janeiro. The Itaboraí Basin is composed of continental limestone rocks interspersed with dissolution channels (erosive phases), which are filled with highly fossiliferous clays and marls. Within these dissolution channel deposits (referred to as the "intermediate sequence"), numerous and significant vertebrate fossils have been recovered, constituting a valuable paleontological record for the study of paleofaunas that inhabited Brazil during the early Cenozoic period. The carbonate sequences deposited in the basin were intersected by an ankaramite dike, which is the only rock unit to have been dated through geochronological methods. The dike's age was estimated at 52.6±2.4 Ma (K/Ar) and 54.89±1.4 Ma (Ar/Ar). For the purposes of this research, the U-Pb geochronological method was employed to obtain absolute dates for fossil and mineral samples associated with the extinct mammal Tetragonostylops apthomasi (Astrapotheria, Mammalia), the extinct reptile Sebecus indet. (Crocodyliformes, Reptilia), and chemical precipitates derived from the S2 sequence of the Itaboraí Basin, identified as MCT1324-LE and MCT1320-LE. The ages obtained, ranging between 19±5 Ma and 55.89±1.40 Ma, suggest isotopic re-homogenization in two of the four samples analyzed using the U-Pb method, likely resulting from events associated with marine transgressions and regressions during the Miocene-Pliocene. These processes, until now, have been scarcely discussed in the context of the Itaboraí Basin.