SEQUENTIAL ANALYSIS OF THE APTIAN DEPOSITS FROM THE SÃO LUÍS AND GRAJAÚ BASINS, MARANHÃO STATE (BRAZIL) AND ITS IMPLICATION FOR UNRAVELING THE ORIGIN OF EVAPORITES.

DILCE DE FÁTIMA ROSSETTI, JACKSON DOUGLAS PAZ, ANA MARIA GÓES, MOACIR JOSÉ BUENANO MACAMBIRA

Resumo


Sequential analysis of Aptian deposits of the São Luís and Grajaú Basins using gamma-ray logs resulted in the recognition of a 2nd -order depositional sequence bounded by two regional unconformities and internally characterized by a tripartite subdivision attributed to lowstand (LST), transgressive (TST) and highstand (HST) systems tracts. The LSI consists of two parts, a marine and a non-marineinfluenced one, respectively located northward and southward of the Ferrer-Urbano Santos Arch. The TST consists of transgressive marine mudstones that cover the LST. The HST consists of aggressing, marine-influenced deposits that grade upward and southward into fluvial and deltaic strata. More than one episode of evaporite deposition is recorded in this sequence, with the most extensive beds confined to the nonmarine portion of the LST. This, together with sedimentological, paleontological and strontium isotope data, suggest continental-brines as the most likely source for the evaporites of the São LUÍS and Grajaú Basins. Less extensive evaporite minerals associated with shallow marine deposits formed at the transition from the TST to HST, and are probably derived from marine brines.

Palavras-chave


Lacustrine setting; Evaporites; Sedimentology; Sequence stratigraphy; Strontium isotope.

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