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Subsidence History and Tectonic Evolution of Campos Basin, Offshore Brazil

Webster U. Mohriak, Garry D. Karner, John F. Dewey

The tectonic component of subsidence in the Campos basin reflects different stages of crustal reequilibration subsequent to the stretching that preceded the breakup of Pangea. Concomitant with rifting in the South Atlantic, Neocomian lacustrine rocks, with associated widespread mafic volcanism, were deposited on a very rapidly subsiding crust. The proto-oceanic stage (Aptian) is marked by a sequence of evaporitic rocks whose originally greater sedimentary thickness is indicated by residual evaporitic layers with abundant salt flow features. An open marine environment begins with thick Albian/Cenomanian limestones that grade upward and basinward into shales. This section, with halokinetic features and listric detached faulting soling out on salt, is characterized by an inc eased sedimentation rate. The marine Upper Cretaceous to Recent clastic section, associated with the more quiescent phase of thermal subsidence, is characterized by drastic changes in sedimentation rate.

Stratigraphic modeling of the sedimentary facies suggests a flexurally controlled loading mechanism (regional compensation) with a temporally and spatially variable rigidity. Locally, the subsidence in the rift-phase fault-bounded blocks shows no correspondence with the overall thermal subsidence, implying that the crust was not effectively thinned by simple, vertically balanced stretching.

Deep reflection seismic sections show a general correspondence between sedimentary isopachs and Moho topography, which broadly compensates for the observed subsidence. However, even the Moho is locally affected by crustal-scale master faults that apparently are also controlling the movement mechanisms during the rift-phase faulting.

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