The tectono-sedimentary history of the coal-bearing Tshipise Karoo basin

Abstract

The Tshipise Basin is considered to be a fault bounded remnant of a larger Karoo aged basin that was fragmented and preserved in the outline seen today by a number of ENE-WSW and NW-SE trending basement faults. It consists of several long, narrow blocks in which the Karoo strata dip, on average, at 12° to the NNW (354). These blocks are bounded to the north by the basement faults that juxtapose the Karoo strata against the high-grade metamorphic rocks of the Limpopo Mobile Belt. The orientation and form of these faults were greatly controlled by the geometry of the metamorphic foliation of the underlying basement rocks. The pre-Karoo topography consisted of a number of ENE-WSW trending palaeo-valleys, with drainage occurring towards the WSW. The southern-most Makhado valley contains a much thicker accumulation of sediments compared to any of the blocks to the north and is considered to have been the main depository of the Tshipise Basin. The sediment ratio maps indicate that the N-S and NW-SE trending faults influenced the deposition of most of the lower Karoo (Tshidzi, Madzaringwa and Fripp formations) and were areas of enhanced subsidence. The coarse-grained sandstone of the Fripp Formation marks the beginning of a major tectonic event that resulted in active uplift in the SE of the basin. The thick accumulation of this unit in the Makhado Block, suggest that the faults bounding the valleys underwent movement during the pre-Fripp tectonic event. Palaeocurrent measurements in the eastern and northern part of the basin as well as the Tuli Basin indicate a unimodal transport direction towards the NW. However measurements in the central and western part of the basin deviate towards the W and SW. The change in palaeocurrent direction, from NW in the east to WSW in the west, is ascribed to the localised changes in basin relief from differential movement of the individual blocks during the pre-Fripp tectonic event. It is either during this Fripp event, or by the time of the Klopperfontein Formation that the ENE-WSW trending faults separated the Tshipise and Tuli basins. The units above the Madzaringwe Formation are all rift-related sediments while the Bosbokpoort Formation possibly contains a series of unconformity bounded sequences which relate to the cannibalization of the underlying strata. Sedimentation within the basin ceased with the onset of the Karoo Igneous Province; which was a product of the splitting of Gondwana into the African and Antarctic continents. Magmatism was initially focused around at the Mwenezi Triple Junction, which firstly led to the outpouring of lava and the intrusion of the Okavango Dyke Swarm (ODS), the Save- Limpopo Dyke Swarm (SLDS) and the Lebombo Monocline. It was during this event that the vast array of dolerite sills were intruded into the Karoo sediments. The ENE-WSW trending SLDS was the first to evolve as a result of stretching and thinning of the crust in a NW-SE direction at ±182-174 Ma (NE-SW directed SHmax). This extension was roughly perpendicular to the underlying basement structures and led to the reactivation of the ENEWSW trending faults which down-faulted and rotated the Karoo strata to the north and preserved them within half-grabens. A second tectonic event, with a SHmax towards the NW/NNW-SE/SSE, led to some degree of inversion of the structures and is likely related to a change in the motion of the African and Antarctic continents after separation. This event probably initiated dextral transpression and compression which led to the reactivation of the primary structures (normal faults) created during the initial event to form relatively high-angled (40-60°) reverse faults across the basin. The event also led to the formation of the subtle east-west trending lowamplitude folds in the Karoo strata, preferentially localised along the ENE faults. Drilling induced fractures and borehole breakout from the ATV logs indicates a neotectonic SHmax parallel to the underlying WSW-ENE trending basement fabric. The large and small scale structures existing in the Limpopo Mobile Belt played a significant role in the development of the Tshipise Karoo Basin. These structures shaped the depository into which the Karoo sequences were laid down, affected the sedimentation and ultimately controlled the fragmentation of this basin along these faults into the preserved blocks seen today.