Paleoambiente e icnofósseis do arenito guamá (Siluriano), regiões de São Miguel do Guamá e Irituia, estado do Pará.

Abstract

The Guamá Sandstone is a restricted sedimentary unit that occurs in the Bragantina Platform, mainly in the São Miguel do Guamá and Irituia regions northeast of the state of Pará still needs a better geological detail. It is characterized by thick layers of well-rounded and selected, mature medium-grained quartz-sandstone with medium granulometry, well rounded, well selected and with high degree of textural and compositional maturity. Based on the petrographic, faciological and ichnological characteristics similar to the quartz sandstones of the Nhamundá Formation (Amazon Basin), a Silurian age for the Guamá Sandstone has been suggested. Commonly, the quartz-sandstone layers have a massive appearance and few preserved primary sedimentary structures (e.g., cross-stratified ribbed stratification, plane-parallel stratification and wavy and planar lamination). The facies association indicates that the studied deposits were formed in a sandy coastal region that covered the foreshore and shoreface zones. In the Guamá Sandstone the icnoespecies Skolithos linearis, Psammichnites isp., cf. Schaubcylindrichnus coronus, single vertical tube undetermined and Lingulichnus verticalis were identified, they commonly compose a mixture of the Cruziana and Skolithos ichnofacies. Periodic energy changes and sedimentation rate in the depositional environment are suggested by the regular distribution between Skolithos and Psammichnites in the sandstone layers. Skolithos occurs, generally, in massive layers and can reach up to 1 meter in length indicating high energy conditions and/or high sedimentation rates. While Psammichnites occur at the top of these layers and suggest lower energy and/or low sedimentation rate. Skolithos exhibit a bioturbation index (ii) ranging from 2 to 3 (5-40%). The Psammichnites exhibit ii between 4 and 5, which indicate a 40 to100% reworking of the substrate by excavating organisms. The low ichnodiversity and high abundance of ichnofossils suggest stressful conditions during the depositional period. The mixture of Skolithos-Cruziana ichnofacies is commonly associated with coastal marine environments of brackish waters. The reason for environmental stress conditions related to the presence of brackish water could be the influx of melting/defrosting waters in periglacial environments. The correlation of the Guamá Sandstone with the Silurian deposits of the Nhamundá Formation supports this idea, as this unit exhibits glacial, post-glacial, and coastal deposits that record the Silurian glaciation of Gondwana. Therefore, the Guamá Sandstone can be considered as the record of sandy coastal deposits influenced by the glaciations that affected the Amazon region during the Lower Silurian.