ABSTRACT

The tectonic development of the southwestern part of the intracratonic Karoo Basin was related to a subduction zone and an orogenic fold belt. However, the entire Lower Permian Ecca Group basin-fill succession exhibits the depositional characteristics of a passive-margin setting. The subbasins that developed during subsidence accumulated large submarine fan complexes that were succeeded by a river-dominated deltaic system.

The subbasin fill in the west-southwestern area (Tanqua Karoo) is approximately 250 m thick and is comprised of six arenaceous submarine fan systems separated by basinal shales. The uppermost fan system is overlain by prodelta shales and siltstones, followed by delta front and delta plain sediments. The fan 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 is believed to have been controlled by fluctuations in relative sea-level with an average periodicity of about 100,000 years. Paleocurrent directions vary between the six units, reflecting changes in deltaic point source locations between SSE and WSW. The sixth fan system is located to the south, downlaps onto the last of the five sequences, and may represent a slope fan with major slumps and channels.

The strata are not affected by folding or vegetation, permitting three-dimensional viewing of channel fills and levee overbank relationships, lateral migration of channels, and the transition from channelized to non-channelized lobe and sheet deposits. The sediments of the different subenvironments seem to have their own typical sedimentary structures which makes it possible to start defining the placing of thin-bedded turbidites and their potential relationships to channel-fill deposits.

The gentle tectonic development of the southwestern Karoo Basin, as is reflected in the distribution of the lower shale formations, low sand/shale ratio, and overall fine grain size throughout the entire submarine fan/deltaic mega-sequence reflects a passive margin setting. These well-exposed outcrops contain many sedimentary characteristics that can be related to those of the Mississippi Delta and Fan. Therefore, this area can provide important elements that will be useful for the development of models of deep-water sands in the northern margin of the Gulf of Mexico.