Depositional facies in Late Cretaceous-?Early Tertiary deposits in northwestern Maranhão, Brazil

Abstract

Late Cretaceous-?Lower Tertiary deposits from northwestern Maranhao State (northern Brazil) were investigated using facies analysis in outcrops in combination with subsurface data provided from gamma-ray profiles. Four facies associations were recognized in this area and attributed to: a) mouth bar (facies association 1); b) braided fluvial/distributary channel (facies association 2); c) distal mouth bar/crevasse lobe (facies association 3); and d) interdistributary bay/prodelta (facies association 4). Mouth bar deposits consist of well-sorted, fine- to very-fine grained sandstone with a variety of sedimentary structures, mostly including: convolute lamination; overturned foresets; sigmoidal, trough, and tabular cross stratifications; and massive bedding. Growth faults and slumpings are common features associated with these deposits. Braided fluvial/distributary channel deposits are characterized by poorly-sorted, coarse-grained cross-stratified sandstone with sharp, locally concave-up, basal surfaces mantled by lags. Distal mouth bar/crevasse lobe deposits consist of a succession of strata formed by coarsening upward cycles represented by massive and cross-laminated sandstones that are interbedded with mudstones containing abundant plant remains. Finally, interdistributary bay/prodelta deposits include mudstones with abundant plant remains and streaks of silt and very-fine grained sandstones. The four facies associations described above characterize depositional environments typical of delta settings, an interpretation consistent with the funnel-shaped gamma-ray patterns observed in well logs. Correlation of these deposits with a wave-dominated estuarine system (i.e., Cujupe Formation) exposed in the eastern of Sao Luis Basin led to attribute them to a bayhead delta complex located in the innermost portion of this estuary. The sedimentological and stratigraphic data presented in this paper support the continuity of the Cujupe estuarine system southward of the Ferrer-Urbano Santos Arch. These results thus suggest a paleogeographic reconstruction in which the Sao Luis and Grajau basins behaved as a unique basin at least from the late Cretaceous on.