Deposition of Fourth-Order, Post-Rift Sequences and Sequence Sets, Lower Cretaceous (Lower Valanginian to Lower Aptian), Pletmos Basin, Southern Offshore, South Africa
G.J. Brink, J.H.G. Keenan, L.F. Brown, Jr., 1993. "Deposition of Fourth-Order, Post-Rift Sequences and Sequence Sets, Lower Cretaceous (Lower Valanginian to Lower Aptian), Pletmos Basin, Southern Offshore, South Africa", Siliciclastic Sequence Stratigraphy: Recent Developments and Applications, Paul Weimer, Henry Posamentier
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Post-rift Cretaceous siliciclastic rocks of early Valanginian to early Aptian age in the Pletmos Basin, southern offshore, South Africa, display well-developed, seismically resolvable, type 1, fourth-order (simple) depositional sequences arranged in progradational and aggradational sequence sets. Each set is a depositional systems tract within third-order composite sequences. The fourth-order stacking patterns displayed by the sequence sets primarily reflect the strong influence of third-order eustatic cycles superimposed on tectonic subsidence. Tectonic subsidence episodes and their terminal uplift events are documented by the stacking patterns of component sequence sets and the tectonically enhanced erosional boundaries of respective second-order supersequences. Conjunction of second-, third-, and even fourth-order falling inflection points produces maximum subaerial erosion and most favorable lowstand reservoir sandstone potential.
Following uplift and second-order erosion of the drift onset unconformity (126 Ma), initial rapid subsidence was followed by a gradual decrease in subsidence (accommodation) rates from the early Valanginian (126 Ma) to latest Hauterivian (117.5 Ma). This first post-rift tectonic episode occurred in response to inferred diminishing transtensional (rift) stress and coincided with the deposition of 17 fourth-order sequences arranged in sequence sets within third-order composite sequences. The sets alternately comprise three or five individual simple sequences averaging 0.33 and 0.12 m.y. in duration, respectively. The sets display stacking patterns grading (upward) from fully aggradational to progradational. This first post-rift supersequence (10-m.y. duration) was terminated by uplift and intensive second-order erosion at 117.5 Ma.
From earliest Barremian (117.5 Ma) to early Aptian (112 Ma) time, initial accelerated subsidence was followed by decreasing
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Applying depositional sequence stratigraphic concepts to the interpretation of siliciclastic depositional systems is becoming an increasingly important tool in petroleum geology. After a succession of breakthroughs during the 1970s and 1980s, sequence stratigraphic concepts now have entered a phase of intense application and documentation, especially with regard to successful implementation in the field of petroleum geology. Workers have applied these concepts to a variety of databases, ranging from outcrop to cores to electric logs and to multifold seismic data. Clearly, sequence stratigraphic concepts embody–not a rigid model or template–but rather a way of looking at geology. This volume has two purposes: to compile some recent applications of siliciclastic sequence stratigraphic concepts, and to present new studies focused on refining conceptual models. This memoir grew out of a 1991 symposium, "Variations in Depositional Systems Within a Sequence Stratigraphic Framework: Applications to Exploration," organized by the authors at the AAPG annual meeting at Dallas. Robert Loucks and Rick Sarg have edited a companion volume, also published by AAPG,“Entitled Carbonate Sequence Stratigraphy: New Developments and Applications.”