Variations in isochore thickness and depositional surface of the Dwyka, Ecca and Beaufort Groups in the Western Cape Province of South Africa as deduced from 2.5D gravity profile models

Zusakhe Nxantsiya,Oswald Gwavava,Christopher Baiyegunhi

doi:10.1016/j.heliyon.2021.e06478

Abstract

A 2.5D gravity modelling along seven selected profiles that covers the Western Cape Province of South Africa was carried out to deduce the depositional surface and isochore (true vertical) thickness of the Dwyka, Ecca and Beaufort Group sediments. The results revealed that the Karoo Basin deepens to a depth of about 4600 m in the south-western region, near the front of the Cape Fold Belt. Also, the model gives indication that the Karoo dolerite intrusions are interconnected at depth and are mostly concentrated at the centre of the basin. The isochore thickness maps show that the Beaufort Group is the thickest group in the Karoo Supergroup, with a maximum thickness of about 6046±277 m, followed by the Ecca and Dwyka Groups with thicknesses of around 3720±183 m and765±69 m, respectively. The maximum depositional surface (elevation) for the Dwyka, Ecca and Beaufort sediments are approximately 1696 m, 1247 m and 1491 m, respectively, whereas the maximum depth below sea level are around 3668 m, 3209 m and 480 m, respectively. Furthermore, the isochore thickness maps indicate that the Ecca Group, which is the main target for hydrocarbon exploration in the Karoo, thickens toward the south, away from the centre of the basin and reaches thickness of greater than 3680 m. The correlation of the depositional surfaces with the isochore thickness maps revealed that the sediments in structural high areas were subsided, eroded and deposited in structural low areas. Consequently, the structural low areas are characterised by thick sediments cover and vice versa.

Highlights

The Karoo Basin, the area under investigation, is of interest to scientists and resource economists
This was done for all the geological groups and the grids were presented as isochore thickness maps
This study has shown that the applications of geophysical surveys combined with geological investigations provide a powerful tool in unravelling the configuration of the south-western Karoo Basin and the isochore thicknesses of the various geologic groups

Summary

Introduction

The Karoo Basin, the area under investigation, is of interest to scientists and resource economists. From the perspective of the economists, shale gas reserves of the Karoo Basin in South Africa have been estimated to have a potential to positively influence the economy of the country. The south-western Karoo Basin is considered to be one of the prospective areas for shale gas in South Africa due to the presence of deeply buried, thermally mature black shales (Baiyegunhi et al, 2017). The study area covers part of the south-western Karoo and Cape Supergroups in the Western Cape Province of South Africa. The Cape Fold Belt comprises of the Palaeozoic clastic sedimentary rocks of the Cape Supergroup that rests unconformably on the Precambrian basement rocks (Bachtadse et al, 1987) and are in turn overlain by the rocks of the Karoo Supergroup (Tankard et al, 2012). The Cape Supergroup is reported to have been deposited during the rising and falling of the ocean along a shallow continental-shelf that flanks the southern margin of Gondwana (Linol et al, 2016)

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Results

Conclusion