Moldes de Cristais de Gipsita como Pseudofósseis no Arenito Botucatu, RS
Palavras-chave:pseudofossils, gypsum molds, eolian sandstones
Sedimentary features in eolian sandstones with plane-parallel lamination of the Botucatu Formation, situated in a quarry in Novo Hamburgo (RS), southern Brazil, were preciously interpreted as possible plant imprints. They show very variable shapes, comprising: a) small, isolated, lenticular or lamellar molds with oblique borders; b) aggregated molds, comprising divergent or intercepting, rarely radiated geometry; c) large, lenticular or lamellar molds with small molds transversally to their margins; d) rounded or oval-shaped molds, some with small molds along their margins. The shapes and distribution of these imprints indicate their origin from the dissolution isolated and aggregated gypsum (CaSO42H2O) crystals, an evaporitic mineral common in desertic environments. The gypsum was precipitated as large crystals (“selenitic gypsum”) within eolian sheet sands shortly after their deposition. Gypsum dissolution was promoted by the later percolation of meteoric groundwaters throughout the sandstones, leaving molds delineated by iron and manganese oxides. Therefore, the studied imprints represent pseudofossils, without any relationship with plants.