Faciológica e estratigráfica de depósitos do cretáceo-(?) terciário inferior na borda centro-oeste da bacia de São Luís (Ma)

Abstract

The present research aimed at the paleoenvironmental and stratigraphic characterization of Upper Cretaceous- (?) Lower Tertiary deposits located along the BR-3l6 highway, between Nova Olinda and Zé Doca (MA), mid-western edge of the São Luís Basin. This is a rift-type marginal basin, formed as a result of the origin of the South Atlantic Ocean, beginning in the Upper Jurassic/Lower Cretaceous. Between the Upper Cretaceous and Lower Tertiary the basin filling is represented by 2 incised valley estuarine sequences, corresponding to the Alcântara and Cujupe formations. Faciologic analysis, together with subsurface data, suggest that the outcropping deposits in the study area consist of a deltaic succession correlating to the Cujupe Formation. In subsurface, the funnel pattern of the gamma-ray profiles reveals the progradational nature of these deposits, typical of deltaic environments. Additionally, facies analysis at outcrop led to the recognition of 5 facies associations, which were assigned to subenvironments common to deltaic deposits. Association 1 is assigned to a deltaic plain environment and is characterized by facies of: a) sandstones with large-scale channeled, tabular and inclined cross-stratification; bioturbation; planar-parallel lamination intercalated with channeled microlamination, b) sandstones and pelites with heterolithic lamination; and c) intraformational conglomerate. Association 2, interpreted as deltaic front deposits, consists of a) sandstone with channeled, tabular, and sigmoidal cross stratification; deformations; massive structure; b) pelite with heterolithic bedding; and c) laminated pelite. Association 3 represents a prodelta environment and is characterized by laminated pelite and laminated pelite/sandstone intercalation. In this paleoenvironmental context, it is proposed that these deltaic deposits correspond to the most proximal portion (i.e., bayhead delta) of the wave-dominated estuarine system formed in the early stages of the incised valley evolution represented by the Cujupe Formation.