Breccia formation during intrusion of a dolerite sill: an example from Sheffield Beach, KwaZulu-Natal North Coast, South Africa.

Abstract

The breccia documented here is the first recorded occurrence of a sub-volcanic dolerite breccia developing due to the intrusion of overlapping dolerite sill segments in the Karoo Igneous Province (KIP) along the KwaZulu-Natal North Coast, South Africa. Recognising these breccias can assist in indicating the crustal level at which the dolerite sills intruded, as fragmentation of the country rocks will not occur beyond a certain depth. The breccia developed when two overlapping sill segments intruded into water-saturated Permian country rocks creating a laterally-extending dolerite-hosted breccia. The characteristics of the breccia, such as clast angularity and size, have been used to identify the fragmentation process of the rocks and infer the environment where it formed. Fragmentation of the lithified country rock is considered to be a consequence of heating between the sills causing vapourisation of the pore fluids, resulting in a dramatic increase of pressure beyond the yield strength of the rock and lateral propagation of the fragmentation. This brecciation process indicates that the dolerites intruded at a high crustal level and were thus intruded into a thin or thinning crust, possibly related to the rifting phase of the breakup of the supercontinent Gondwana.