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Permian Passive Margin Submarine Fan Complex, Karoo Basin, South Africa: Possible Model to Gulf of Mexico

BOUMA, A. H., Louisiana State University, Baton Rouge, LA, and H. DEV. WICKENS, Soekor, Parow, South Africa

The tectonic development of the southwestern part of the intracratonic Karoo basin was related to subduction zones. However, the entire Lower Permian Ecca Group basin-fill succession reflects passive-margin setting depositional characteristics. Subbasins, developed during subsidence, were filled with submarine fan complexes.

The subbasin fill in the west-southwestern area (Tanqua Karoo) is about 250 m thick and is comprised of five arenaceous submarine fan systems separated by basinal shales. The upper one is covered by marine shales followed by prodelta shales and siltstones, overlain by delta-front and deltaic deposits. The complex displays middle and outer fan deposits over an area of about 650 km2. The individual fans vary in maximum thickness from 24 to over 60 m. The cyclicity of the fan deposition was likely induced by relative sea level lowering with an average periodicity of about 100 kA. Paleocurrent directions vary between the five units, reflecting changes in deltaic point source locations between south-southeast and west-southwest. A sixth fan system is located to the south, downlaps onto the last of the five sequences, and may present a slope fan with major slumps and channels.

The strata are not affected by folding or vegetation, permitting three-dimensional viewing of the channel and its levee overbank relationships, lateral migration of channel fills, transition from channelized to nonchannelized, and sheet deposits. Each depositional subenvironment seems to have its own typical sedimentary structures, making it possible to start defining the placement of thin-bedded turbidites and their potential relationships to channel deposits.

These well-exposed outcrops and general setting contain many characteristics known from the Mississippi River delta and fan, such as the gentle tectonics and basin gradients, wide coastal plains, and overall fine grain size of the basin fill. Therefore, the area can provide important elements that can be used for the development of models of deep-water sands in the northern margin of the Gulf of Mexico.

 

AAPG Search and Discovery Article #91006 © 1991 GCAGS and GC-SEPM Meeting, Houston, Texas, October 16-18, 1991 (2009)