Ictiólitos da Formação Pirabas, mioceno do Pará, Brasil, e suas implicações paleoecológicas

Abstract

The Pirabas Formation (Oligo-miocine), which is represented by carbonates deposits through the Brazilian Coastal Area between the States of Pará and Piauí, is recognized for its fossil richness. Among the many fossils paleoichthyofauna stands out for its abundance in outcrops, but the absence of stratigraphic and facies control of these fossil restricted their potential use in paleoenvironmental interpretations. The joint use of the technique of wet sieving for the recovery of ichthyoliths and microfacies and facies analysis of sediments taken from Pirabas Formation deposits, which were exposed in the B17 mine (Capanema-PA), allowed for the first time the use of vertebrate fossil for the elaboration of an interdisciplinary paleoenvironmental reconstruction model. The 3594 recovered ichthyoliths, along with the 5 facies associations and the 4 carbonate microfacies, led to the conclusion that these deposits of Pirabas Formation were formed in marine-marginal depositional system, with different environments genetically associated as shore face (surf zone), beach, lagoon, tidal channel and tidal delta, which would be part of an estuarine system with the influence of wave, marked by four different depositional cycles related to possible transgressiveregressive episodes. The estuarine environment was confirmed by the assembly of ichthyoliths, observing that this assembly is formed by both freshwater specimens, represented by the Characidae family, an unheard occurrence for the unit until then, as for marine specimens, for example, the sharks. Several dental types, possibly attributed to representatives of brackish waters, such as the genera Dasyatis and Sarpa, both equally registered for the first time, also strengthens the proposed environment. The distribution of the ichthyological community evidences palaeoenvironmental control, having registered their highest abundance in deposits of tidal channel. However, the influence of relative sea level resulted in the development of transgressive-regressive depositional cycles, which also exerted strong control on the stratigraphic distribution of the B17 Mine registered ichthyoliths, reinforcing the importance of interdisciplinary studies in refining palaeoecological reconstruction of this unit.