--> ABSTRACT: Sequence Cyclicity in Bredasdorp Basin, Offshore South Africa--Comparison with Global Relative Sea Level Cycles, by Sheila Doherty; #91022 (1989)

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Sequence Cyclicity in Bredasdorp Basin, Offshore South Africa--Comparison with Global Relative Sea Level Cycles

Sheila Doherty

A seismic-stratigraphic study of Cretaceous strata in Bredasdorp basin has evaluated potential stratigraphic hydrocarbon plays in a clastic postrift basin where structural traps are rare. The study showed that the basin fill consists of cyclic sequences deposited in response to relative changes in sea level. Cyclic sequence concepts were applied to delineate hydrocarbon plays, a process that doe snot depend upon a global system. Nevertheless, once the sequences were identified, mapped, and placed within a chronostratigraphic framework using age dating, these cycles were compared with Exxon's global system. The similarity was startling--most of Exxon's type 1 unconformities can be correlated with Bredasdorp type 1 unconformities. Where the correlation differed, for example where type 2 unconformities correlate with type 1, the difference was attributed to local tectonic influences. Despite the limitations of the correlation procedures, the comparison was useful because it improved our understanding of the tectonic and sedimentary history of the basin, corroborated by other factors such as sedimentation style and reservoir distribution. Thus, although processes of global cyclicity as proposed by Exxon can be observed in Bredasdorp basin, local tectonics have overprinted this global effect to produce a unique sedimentation history.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.