Análise de minerais pesados de depósitos cretáceos e terciários na borda norte da Bacia do Grajaú, Maranhão

Abstract

The stratigraphy of Cretaceous and Tertiary deposits outcropping in the northern portion of the Grajaú Basin has been problematic. However, faciological and sequence stratigraphy studies now in progress lead to the separation of Aptian deposits (Codó Formation) from those formed in the middle to upper Albian and upper Cretaceous (?)-lower Tertiary, the latter tentatively correlable to the deposits referred to as the Âlcântara and Cujupe Formation in the São Luís Basin, respectively. Overlying these occur, still, Upper Cenozoic sequence (Barrier Formation and Post Barriers). Such subdivision is preliminary and should be tested by independent parameters. The objective of this research was to determine the heavy mineral assemblages of these units in order to contribute to their better stratigraphic individualization. The results obtained indicate that the Aptian deposits have zircon and garnet as common minerals, while tourmaline, staurolite, rutile, anatase, epidote, apatite and chlorite were rare minerals. The Albian deposits have zircon and tourmaline as common minerals, and staurolite, garnet, rutile, anatase, epidote, apatite, barite, melilite(?), andalusite, chlorite, zoizite, sillimanite, and biotite as rare to trace minerals. The Turonian-(?)Lower Tertiary and Post Barrier samples show zircon and tourmaline as abundant minerals, staurolite as common mineral, rutile garnet, anatase, chlorite, epidote and cyanite as trace minerals. Therefore, according to the identified heavy mineral assemblages, it can be suggested that although the most stable minerals are common to the different stratigraphic levels, these show distinct stable mineral assemblages, possibly due to source rock variations