New Insights into Basin-floor Fan Growth Using Inter-fan Ash Bed Correlation, Permian Skoorsteenberg Formation, Tanqua-Karoo Basin, South Africa

Veronica Rubio-Gallegos, Stefan Luthi, Salomon Kroonenberg, and Jon Noad, Delft University of Technology, Department of Geotechnology, Delft, The Netherlands, [email protected]

The Permian Skoorsteenberg Formation in the Tanqua-Karoo sub-basin provides excellent exposure of a series of basin-floor fans separated by interfan shales. These outcrops have allowed detailed studies of the architecture and growth mechanisms of the fans to be developed. The fans are considered lowstand system tracts, while the interfan shales correspond to highstand system tracts. The Karoo is considered a Gondwana foreland basin which formed in response to accretion along the southern margin of the supercontinent. Subduction along this margin led to a magmatic arc whose existence is proved by numerous ash beds in the interfan shales of the Skoorsteenberg formation.

We correlated these ash beds in outcrops and research boreholes on the basis of their stratigraphic position, composition, colour and thickness, thereby providing a more complete picture of their three-dimensional distribution. Using the ash beds as time lines to constrain the interfan stratigraphy, they suggest that the stratigraphic base of the fans is diachronous, with sedimentation prograding towards the basin center. This is in line with previous results from intrafan correlation, which showed that fan initiation was gradual and progradational, while during the main fan activity vertical aggradation prevailed.