A Passive Margin-type Submarine Fan Complex, Permian Ecca Group, South Africa

WICKENS, H. DEVILLIERS, SOEKOR (Pty) Ltd., Parow, South Africa, and ARNOLD H. BOUMA, Louisiana State University, Baton Rouge, LA

A submarine fan complex, comprising five arenaceous fan systems separated by basinal shale units, occurs in the southwestern part of the intracratonic Karoo basin in South Africa. Although basin development is related to a subduction zone bordering the palaeo-Pacific ocean to the south of Gondwanaland and the evolution of the Cape Fold Belt, the entire Lower Permian Ecca Group basin-fill succession reflects depositional characteristics of a passive-margin setting.

The submarine fan complex, 250 m thick, originated from sediments supplied by Mississippi-type deltas dominating the Ecca coastline. The fine grain-size and low sand/shale ratio of the submarine fan and deltaic deposits reflect the maturity of the ancient river systems.

Outcrops of the fan complex are well exposed and cover an area of 650 sq km. The strata are not affected by folding, and deep erosion allows three-dimensional viewing of mid-fan to outer-fan deposits. Features of interest include stacked lobe deposits displayed along 2.5 km of a 60 m high cliff section, and a transverse cliff section through channel-fill deposits 500 m wide. Paleocurrent directions reveal that each sequence had its own main source area located to the northwest and south of its present geographic location.

The cyclic nature of the fan complex is attributed to relative sea-level changes; deposition took place on the basin floor in water depths that do not exceed 500 m. Shoaling of the basin to wave base depths is reflected in the pro-delta and delta front deposits overlying the uppermost fan sequence. Major factors in controlling direction of fan progradation were delta switching and basin floor topography.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)