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Factors Controlling the Stratigraphic Evolution of Eastern Brazil Basins

Hung K. Chang, Renato O. Kowsmann, Antonio M. Figueiredo, Laercio G. Aranha

The east Brazilian marginal basins offer the best opportunity for understanding the interaction between geological processes and stratigraphic response for Atlantic-type margins.

Tectonic subsidence, eustatic changes of sea level, and paleoclimate are the factors that primarily control the nature of the sedimentary infill. Two tectonic stages--rift and thermal--are attributed to lithospheric extension as deduced from crustal sections.

The Late Jurassic-Neocomian rift sequence is characterized by sediments deposited in a slowly subsiding regime during early stages of basin formation. Subsequent late Neocomian diastrophism resulted in rapid basin subsidence and infill and regional basaltic magmatism. The occurrence of the latter is related to the extent of the area subjected largely to lithospheric thinning.

The Aptian rift-thermal transition is marked by a regional unconformity which is most prominent in the shallow portions of the basin. The first overlying sequence is characterized by the presence of evaporites favored by the action of arid climate over narrow seas during the incipient stages of continental drifting.

As drifting proceeded, a carbonate platform developed during Albian time under rising eustatic sea level regime combined with still significant thermal subsidence. Platform drowning during the Cenomanian was followed by the deposition of an onlapping sequence of deep-water sediments.

Decay of the thermal anomaly combined with a standstill and a subsequent drop of sea level controlled the generalized regressive phase, characteristic of the Late Cretaceous and Cenozoic sedimentation. This last sequence encompasses the frontier targets for hydrocarbon exploration in offshore Brazil.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.