Morpho-tectonics and mechanism of emplacement of the dolerite rings and sills of the western Karoo, South Africa

TLDR

The dolerite sill‑ring structures of the Karoo Basin have long been debated, with their emplacement mechanism remaining unclear despite recent increases in geological data and mapping. The study proposes a new emplacement model for these Karoo dolerite sill‑ring systems. The model suggests that dolerite dykes feed into inclined sheets that extend into an outer sill and subsequently into an inner sill. Analysis of three western Karoo sill‑ring systems shows a saucer‑shaped configuration with an inner sill, an arcuate ring sheet, and an outer sill, and reveals that many arcuate dykes branch onto the ring structures.

Abstract

The dolerite sill and ring-like structures of the Karoo Basin have been a matter of debate for a considerable period of time. However, the mechanism for their emplacement still remains an enigma. A review of the available literature shows that very little structural work has been carried out on these intrusions, which outcrop over two thirds of South Africa. A large quantity of geological information, maps, and field data have become available over the last 15 years. A comparative morphotectonic analysis of three sill-ring systems of the western Karoo, that is, Williston, Fraserburg, and Victoria West complexes, indicates that their shape is saucer-like with an inner sill at the bottom, an arcuate inclined sheet (the ring) on the periphery, and an outer sill on the rim. Many arcuate dykes are seen branching onto the ring structures. A mode of emplacement is proposed whereby dolerite dykes feed into the inclined sheets, which then propagate into an outer sill and thereafter into an inner sill.