Recognition of Seismic Facies, Genetic Elements, and Depositional Environments in Lowstand System Tracts, Pletmos Basin, Offshore South Africa

Heinz W. Pferdekamper

Postrift Cretaceous strata in the Pletmos basin comprise 67 cyclic depositional sequences that were deposited in response to third-, fourth-, and perhaps fifth-order sea level oscillations. Seismic facies characteristics, identified by analyzing 200 seismic profiles totaling 7,000 km, depict an extensive spectrum of lowstand lithogenetic elements and inferred clastic depositional systems.

Mid-Valanginian to late Hauterivian lowstand deposition was generally restricted to submarine channel infill and mounded and/or sheetlike marine fill lapping out onto type 1 unconformities. Deposition took place during falling sea level during valley and submarine canyon incision. The rarity of lowstand progradation reflects sea level rises that exceeded sediment supply.

Late Hauterivian to middle Aptian areally restricted prograding wedges were deposited in response to apparent equilibrium between fluvial lowstand sediment input, basin subsidence, and slow sea level rise. The wedges clearly exhibit coastal onlap onto type 1 unconformities and pinch out updip against a relict shelf edge. High-angle, oblique clinothems with associated toplap are common and indicate active lowstand deltaic progradation. Clinothems may baselap onto (1) channelized slope fans typified by marine onlap and chaotic reflections, (2) mounded basin-floor fans, or (3) the basin floor itself. Detached mounded basin-floor fans distal from lowstand wedges are numerous throughout the sequences.

Excellent seismic data quality and moderate to shallow depths of burial permitted resolution of the genetic elements of lowstand system tracts in this unique tectonic setting.

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