CARACTERIZAÇÃO PALEOAMBIENTAL DE DEPÓSITOS ALBIANOS NA BORDA SUL DA BACIA DE SÃO LUÍS-GRAJAÚ: MODELO DE DELTA FLUVIAL INFLUENCIADO POR TEMPESTADE

Abstract

PALEOENVIRONMENTAL CHARACTERIZATION OF ALBIAN DEPOSITS FROM SOUTHERN SÃO LUÍS-GRAJAÚ BASIN: A STORM-INFLUENCED FLUVIAL DELTA MODEL Exposures of Albian deposits in the Grajaú-Arame area, southern São Luís-Grajaú Basin, are documented for the first time in this paper. In that area, a composite section up to 135 m thick traced over 70 km of horizontal distance, allowed the recognition of six depositional environments, including: delta front (mouth) bar, distal bar/ prodelta, upper shoreface/foreshore, interdistributary bay/crevasse, fluvial channel and distributary channel. A deltaic system influenced by storms is proposed with basin on: 1. facies characteristics typical of deltaic deposits; 2. arquitectural arrangement of facies association into an overall prograding succession; 3. well-developed lobate or sigmoidal sandstones forming bodies up to 6 m thick in the deposits representative of mouth bar setting; 4. abundance of small- and large-scale combined-flow generated structures (such as wave-scour, swaley cross-stratification, undulating parallel lamination, quasi-planar cross stratification, cross strata with undulating set boundaries and abundant reactivation surfaces and mud drapes); and 5. abundance of soft-sediment deformation structures, attesting high rates of sediment deposition. Paleocurrent data show a main northeastward-oriented dip direction for the delta system, which prograded into a marine-influenced basin, as suggested by the ichnological assemblage including Thalassinoides, Skolithos, Ophiomorpha, Planolites, Teichichnus and Taenidium. The interpretation provided in this paper lead to suggest a basinwide incursion of a broad, epicontinental-like sea over the São Luís-Grajaú Basin during the Albian. Keywords: