King, Rosalind¹, Belinda Van Lente¹, Graham Potts¹, David Hodgson¹, Daniel Andersson¹, Richard Worden1, Stephen Flint¹, De Ville Wickens² 
(1) University of Liverpool, Liverpool, United Kingdom 
(2) University of Stellenbosch, Stellenbosch, South Africa

ABSTRACT: Extra-Foreland Source to Turbidite Sink in the Early Karoo Basin, South Africa

Foreland basins commonly fill with sediment derived from the adjacent fold/thrust belt, providing a source-to-sink configuration. The early Karoo foreland basin of South Africa is bounded to the west and south by the Cape Fold Belt (CFB) and provides an opportunity to study the linkages between early foldbelt development, oroclinal bending and deepwater turbidite sandstone provenance, routing, storage and deposition. The CFB is characterised by folding rather than thrusting at exposed crustal levels and strain estimates indicate 93-km and 134-km of shortening in the N-S and W-E striking arms respectively. The oroclinal bend between the two arms includes a complex NE-striking fold. The CFB comprises lower Paleozoic siliciclastic sediments. Petrographic analysis of the early foreland basin turbidite sandstones from both the Tanqua and Laingsburg depocentres indicate a near uniform provenance dominated by granitic and metamorphic materials, not derived from the CFB, despite southwesterly paleocurrents. Chemostratigraphic analysis of the shales also supports a source terrain comprising bimodal igneous rocks. Our model suggests that the early basin-fill was derived from hinterlands outboard of the CFB and sourced through long-lived structurally controlled synforms within the oroclinal bend. Local fold growth history controlled early routing into the sink of the Laingsburg area, followed by a later switch to the Tanqua depocentre. Thus, depending on the style of structural evolution, foreland basin reservoirs may exhibit quite different petrographic and geochemical characteristics to the bounding fold/thrust belt, leading to potential errors in prediction of reservoir quality and sediment provenance at depth in sedimentary basins.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.