Traços fósseis de crustáceos da Formação Pirabas, mioceno da Plataforma Bragantina, NE do Pará.

Abstract

The Miocene deposits of the Pirabas Formation present in their fossil records decapods crustaceans of epifauna and infauna. These organisms marked by the presence of ichnofossils that allow the discrimination of the main activities carried out during the construction of these bioturbation. Descriptions of the microfacies in association with the main occurrences of fossil traits in the Pirabas Formation allowed greater interpretations on the distribution and behavior of decapod crustaceans, as well as the main salinity, bathymetry and oxygenation conditions of the paleoenvironment. The described microfacies are mudstone, massive wackestone with echinoderms, packstone rich in foraminifera and mollusks, grainstone with terrigenous and algae, bafflestone with bryozoan and terrigenous, packstone / wackestone with flat lamination, packstone / grainstone with foraminifera and rudstone with fragments of mollusks. The main ichnogenus described are Thalassinoides, Gyrolithes and Sinusichnus individualized in six icnoespecies Thalassinoides suevicus, Thalassinoides paradoxicus, Thalassinoides isp, Gyrolithes dravexi, Gyrolithes krameri, and Sinusichnus sinuosos. This trace assembly presents characteristics typical of the Cruziana icnofacies composed mainly of predominantly horizontal buildings of housing (domichnia) and food (fodinichnia) produced under energy conditions ranging from low to moderate levels. The quantification of bioturbation allowed the distinction of three icnofábrica denominated icrofábrica Gyrolithes, icnofábrica Sinusichnus and icnofábrica dominated by Thalassinoides whose rates of bioturbation vary between ii = 2, with rates of 10-15% up to ii = 4 whose rates are of 35-65%.These indices indicate the progressive increase of bioturbation intensity from the association of tidal flat, laguna to the association of bioclastic barrier. Petrographic analysis, SEM (Scanning Electron Microscope), EDS (Dispersive Energy Spectroscopy) and DR-X (Xray Diffraction) indicated different compositions in the filling of the trace compatible with reduced microenvironments also proven by the presence of pyrite concretions. and siderite, interpreted as a reflection of the interaction of microorganisms with organic matter and the reduction of available sulfate in and around excavations.