Anatomy and Stratigraphic Development of a Basin Floor Turbidite System in the Laingsburg Formation, Main Karoo Basin, South Africa

Abstract

ABSTRACT Six individual turbidite systems, informally called Fans A-F, were deposited in the Laingsburg area of the southwestern Karoo Permian foreland basin. This study concentrates on the lowermost, 300 m thick system. Facies associations include channel fills and correlative lateral overbank deposits consisting of thin sheet-like and rippled sandstones. Massive- and thin-bedded frontal sheet sandstones form down-dip extensions to channel systems. Identification and correlation of mudstone-dominated intervals from field mapping and oblique aerial photostratigraphy delineates a high-resolution internal stratigraphy of the fan system which, coupled with 4000 m of logged section, allows detailed geometrical and architectural analysis. Seven individual depositional units have been mapped within Fan A, and these are interpreted as the deep-basin sedimentological expressions of high-frequency lowstand systems tracts, separated by high-frequency transgressive and highstand condensed intervals. Stacking patterns of the seven lowstand fan units that make up Fan A record early progradation (units 1 to 3), a backstep (unit 4), followed by further progradation (units 5 and 6). Retrogradation during unit 7 records abandonment of the whole Fan A deposystem. Coupled with facies analysis, paleocurrents reveal unusual paleotransport patterns that are interpreted as a consequence of structural deformation of the basin floor. Paleotransport indicators reveal that sediment pathways are strongly parallel to the structural grain, while some point to sediment pathways that crosscut the dominant structural grain.