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

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

Heliyon. 2021 Mar 12;7(3):e06478. doi: 10.1016/j.heliyon.2021.e06478. eCollection 2021 Mar.

Authors

Zusakhe Nxantsiya^{1

2}, Oswald Gwavava², Christopher Baiyegunhi³

Affiliations

¹ Council for Geoscience, Private Bag X112, Pretoria, 0001, Gauteng Province, South Africa.
² Department of Geology, Faculty of Science and Agriculture, University of Fort Hare, Private Bag X1314, Alice, 5700, Eastern Cape Province, South Africa.
³ Department of Geology and Mining, School of Physical and Mineral Sciences, University of Limpopo, Private Bag X1106, Sovenga, 0727, Limpopo Province, South Africa.

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 $6046 \pm 277$ m, followed by the Ecca and Dwyka Groups with thicknesses of around $3720 \pm 183$ m and $765 \pm 69$ 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.

Keywords: Depositional surface; Gravity modelling; Isochore thickness; Karoo basin.