RECONSTITUIÇÃO PALEOAMBIENTAL DE DEPÓSITOS ALBIANOS NA BORDA LESTE DA BACIA DE GRAJAÚ, MA

Abstract

The Albian deposits of the Itapecuru Group exposed at the margins of the Itapecuru River, near the town of Coroata, eastern Grajau Basin, were investigated concerning their sedimentological, stratigraphic and palaeontological aspects. The study showed that these strata include sandstones, shales and limestones formed in depositional environments usually found in deltaic systems. They include: 1. delta front, consisting of sandstone lobes, either amalgamated or separated by thin shale layers, and which are internally characterized by massive, deformed or cross-stratified sandstones arranged into thickening- and coarsening upward successions; 2. wave-dominated delta front, consisting of sandstones in a variety of wavy laminations, including swaley cross-stratification, undulating parallel-lamination, low- angle cross- stratification and large- scale scour-and-fill structures; 3. distal bar, characterized by sandstones with sedimentary structures similar to delta front deposits, but forming thinner lenses isolated within shales; and 4. prodelta/restricted shelf/ mud flat, which includes the finest-grained lithologies of the study area, consisting of laminated to massive shales and limestones. Features attributed to subaereal exposure, such as root marks, mudcracks, karstic holes formed by dissolution under influence of meteoric water, and dinosaur footprints, are all locally found in association with these deposits. The analysis of facies architecture revealed three stratigraphic units, defined by discontinuity surfaces marked by one or more of the following elements: calcitic cement, concentrations of fossil fragments, and a lag of coarse-grained sandstone and conglomerate. Despite the internal organization into prograding cycles, typical of deltaic systems, the distribution of the stratigraphic units and facies associations shows that muddier deposits, attributed to low-energy environments ranging from prodelta to restricted shelf and mud flat, become more abundant from southeast to northwest in the study area, as well as towards the top of the sections, recording progressive drowning of the deltaic system. Punctuated floodings took place, being represented by discontinuity surfaces, characterizing a retrogradational stratal pattern, suggesting abandonment of the deltaic system due to an overall transgression.