Upper Cretaceous Archean deposits, located at the northeastern edge of the Grajaú Basin (MA), were analyzed in order to reconstruct their depositional paleoenvironments. Five facies associations were described and assigned to the environments of: (1) transgressive tidal plain (?); (2) mouthbar; (3) distal prodeltalbar; (4); distributary channel; and (5) wave/tidal influenced mouthbar. These facies associations characterize deltaic environments, based on: i) deposits with thickening of sets and upward granocrescence; ii) distributary channels associated with mouth bars influenced by wave/tide processes; iii) gravitational collapses (e.g., growth faults, slides) accompanied by convoluted strata. Given the lithofaciological and stratigraphic similarity, these deposits are correlatable with a bay delta complex outcropping in the central-western portion of the São Luís Basin, which records environments proximal to the estuarine system represented by the Cujupe Formation. Based on these comparisons it is suggested that the studied deposits belong to a contemporary estuarine system. Three stratigraphic intervals (Sl-S3), defined by erosional discontinuities (D l-D3) and marked by paleosols and/or fossil traces attributed to Glossifungites ichnofacies, were recognized in the studied unit. These data lead to the interpretation of multiple episodes of progradation, favored by high frequency/low amplitude relative sea level cycles, within a larger cycle marked by estuarine valley incision and abandonment. The data presented reinforce the paleogeographic reconstruction previously proposed, in which the São Luís and Grajaú basins behaved as a single basin at least from the Upper Cretaceous
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