At the eastern edge of the São Luís-Grajaú Basin, in the municipality of Codó, deposits of the Codó Formation have been the target of sedimentological, stratigraphic and paleontological studies. However, at the southern edge, in the municipality of Grajaú, such deposits lack detailed studies that allow a better reconstitution of the sedimentary processes and depositional environments. The present work was aimed at filling part of this gap through the faciologic analysis of geological profiles and the definition of carbonate microfacies under petrographic microscopy. The studies focused on the analysis of two geological perus, from which 13 thin slices were extracted. In the first profile, composed essentially of limestone and argillite, five facies were recognized, these being: laminated argillite (AI); crystalline limestone (Cc); mudstone (M); stromatolite (St), framestone; and fenestral (packstone) limestone (Cf). In the second profile, composed mainly of evaporite and argillite, the facies recognized were: fibrous gypsum (Gf); massive gypsum (Gm); gray argillite (Ac); tufa (T); and banded limestone (Cb) (crystalline limestone). Although the Codó Formation occurring in other areas, as in the Codó region, is characterized by abundant black bituminous shales, in the Grajaú area light-colored argillites and limestones represented mainly by mudstone predominate, suggesting an oxygenated, low-energy environment. However more turbulent moments are evidenced by the presence of reworked gypsum grains and packstone. Structures such as pop com, teppees, fenestras, and dissolution features in the gypsum facies, in addition to the presence of tufa (T), indicate moments of subaerial exposure, characterizing marginal lacustrine environments, even when represented by evaporitic facies. This is an important finding, since the evaporitic facies of this unit occur more commonly associated with central lacustrine environments and at times of anoxia. The causes for this facies development in the Grajaú area remain to be better defined.
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