Peter J. Sixsmith¹, Stephen Flint², Graham Potts², Martin Grecula², DeVille Wickens³

(1) Imperial College, London, United Kingdom
(2) University of Liverpool, Liverpool, United Kingdom
(3) University of Stellenbosch, South Africa

ABSTRACT: Turbidite Geometries and Palaeotransport Patterns deposited on an Actively Deforming Basin Floor: Laingsburg Formation, Karoo Basin, South Africa

The southernmost part of the main Permian Karoo Basin comprises basinal mudstones, mud- and sand-rich turbidites, slope and deltaic deposits in a retro-arc foreland basin, The first turbidite fan of the Laingsburg Formation (Fan A) is a 350m thick sand-rich system that records the onset of sedimentation on an actively deforming basin floor. Application of high-resolution sequence stratigraphic techniques facilitated the subdivision of Fan A into seven genetically unrelated units, separated by laterally continuous condensed intervals. Each sand-dominated unit exhibits varied vertical and lateral facies trends constrained in part by syn-depositional basin floor growth fold topography. Across-fold facies relationships show subtle thickening of depositional units into synforms, while along-axis thickness variations can be correlated with along-axis changes in fold plunge. Slumped units up to 30 m thick occur in close proximity to the antiforms. Facies architecture coupled with palaeocurrent analyses reveal that specific areas were prone to flow reflection and flow stripping. Flow reflection off an antiform was concomitant with flow stripping over that antiform, thus maximising sand volumes (and hence reservoir potential) in adjacent subtle synforms. Beyond the antiform are sand-depleted (poor reservoir) successions. Palaeocurrents also reveal specific sites of cross-fold sand transport, which correlate exactly with syn-depositional lows (saddles). By interpreting facies distributions, unit thicknesses and palaeocurrents, estimations can be made of relative rates of fold growth during deposition of Fan A. Accurate mapping of facies trends and palaeocurrent indicators is critical to developing accurate predictive models for the location of sand-rich accumulations on tectonically active basin floors.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado