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L. F. Brown1, G. J. Brink2, E.H.A. Jungslager3, N.J.S. van Wyk3
(1) The University of Texas at Austin, Austin, TX
(2) Consultant, Franschhoek
(3) Petroleum Agency SA, Parow, Cape Province, South Africa

Abstract: Cretaceous (Post-Valanginian) fourth- to second-order sequence-set stacking architecture, offshore South Africa, indicates accommodation and sediment-supply variations induced by relative cycles of sea level produced by lower frequency tectonic and higher frequency eustatic cycles

Depositional sequences of fourth-, third- and second-order frequencies stack into sets indicating multiple, contemporaneous accommodation cycles of several frequencies. Sequence sets reflect cycles of relative sea level and accommodation: (1) basinal shifts of relative sea level and erosion followed by deposition of (2) progradational to aggradational (lowstand) sets, (3) retro-gradational (transgressive) sets, and (4) aggradational to progradational (highstand) sets. Stacking of sets defines “T-R shoreline cycles” of earlier workers. Sequences are seismically resolvable except for fifth- and higher-order sequences/sets. Sequence sets (“systems tracts”) of ~300 to 400 ky, ~3 to 4 my, and ~30 to 40 my in duration compose, respectively, sequences of third- (~1 to 2 my), second- (~10 my), and first-order (~100 my) frequencies.

Sequence-set stacking patterns within second- and first-order sequences correlate with subsidence and uplift events by geohistory analysis. Lower frequency set boundaries display limited diachroneity with “global cycles” (Haq et. al., 1987) and may represent an interplay of composite global and basinal tectonics and long-term changes in sediment supply. Third- and fourth-order sequences match “global” cycles within microfossil age resolution. Higher frequency sequences/sets correlate among basins reflecting eustatic cycles of uncertain origin. Sequence stacking patterns support interpretation of tectonic histories, hydrocarbon systems, and lithofacies.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana