Cretaceous deposits are well represented in the São Luís-Grajaú Basin, ~. State of Maranhão. The Codó Formation is the basal unit of this basin, being composed essentially of evaporites, shales and carbonates, which in the Grajaú region were deposited in a predominantly continental salt complex (i.e. sabkha). The characterizations of the argillomineral groups along the Chorado and Barreirinho Mines, located at the southern edge of the basin, contributed to a better understanding of the climatic conditions prevailing during the time of deposition, and also allowed a discussion of the upward strike-slip cycles that characterize the Codó Formation deposits. X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive analysis (EDS) data allowed the identification of the clay mineral groups of the Codó Formation in the Grajaú area, characterization of their morphology and sentiment-quantitative determination of their chemical compositions. In addition to these methods, calculations of relative peak intensity measurements in X-ray diffractograms were used to determine the proportions between the argillominerals and the smectite crystallinity. The argillomineral groups of the Codó Formation in the study area include, in order of abundance: smectite, kaolinite, and illite. Smectite, for the most part, is of detrital origin, and locally authigenic smectite may occur in the form of honeycomb patterned honeycomb crystals. Kaolinite is preferentially autigenic, and detrital kaolinite may occur locally, while illite is exclusively autigenic. The distribution of the proportions among these groups of argillominerals proved to be a very applicable parameter to better define the upward dragging cycles. Smectite occurs in higher proportion at the base of the cycles due to the predominance of the central lake environment. The highest values for the crystallinity of smectite are observed in this environment. Kaolinite forms more frequently at the top of the cycles, where these deposits are attributed to marginal lake environment, which favored the formation of kaolinite by pedogenic argillization processes.
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