An Integration of Magnetic, Gravity and Seismic Data in Evaluating the Algoa Basin in the Eastern Cape Province of South
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Pure and Applied Geophysics
An Integration of Magnetic, Gravity and Seismic Data in Evaluating the Algoa Basin in the Eastern Cape Province of South Africa for Stratigraphic and Structural Geodynamics NOMVELISO CAKU,1 OSWALD GWAVAVA,1 KUIWU LIU,1 and CHRISTOPHER BAIYEGUNHI2 Abstract—The Algoa Basin is a hydrocarbon potential basin which lies off the southern continental margin of South Africa, along the coastal region of Port Elizabeth area with both onshore and offshore components. Despite the continued consensus on the basin’s hydrocarbon potential, both its structural and stratigraphic geodynamics are still not well documented. In this study, gravity data constrained by borehole data was modelled to reveal onshore subsurface geology whereas the seismic sections were used for offshore. In addition, magnetic data was used to delineate magnetic zones and lineaments. The onshore magnetic results show the localised occurrence of large-scale geologic bodies, with high and positive anomalies forming bounding rims coincidental with the edges of the basin. The average depths to the top of the shallowest and deepest magnetic sources were estimated from the power spectrum to be approximately 0.4–0.6 km and 9.5 km, respectively. The onshore Bouguer gravity map is characterised by a gravity low zone over the basin’s sediment where it attains its lowest value over the depocentre. 2 D gravity modelling results along five profiles show that the basin comprises of two to three ‘‘‘V’’ shaped’ sub-basins of shallow crustal depth with sediment thickness of up to 4.6 km. The models also revealed discontinuity in sediment units, confirming the diachronous nature of the Uitenhage Group. Analyses of offshore seven seismic sections show that the stratigraphic sequence consists of diachronous geological units, which are characterised by displacements, forming a series of minor and major faults. Complementary to onshore gravity models, seismic sections also shows relatively shallow vertical extension to the Paleozoic crustal basement between 1.3 and 5.3 km. Keywords: Algoa Basin, hydrocarbon potential, lithostratigraphy, structure, gravity, magnetic and seismic sections.
1
Department of Geology, Faculty of Science and Agriculture, University of Fort Hare, Private Bag X1314, 5700 Alice, Eastern Cape, South Africa. E-mail: [email protected]; [email protected]; [email protected] 2 Department of Geology and Mining, School of Physical and Mineral Sciences, University of Limpopo, Private Bag X1106, 0727 Sovenga, Limpopo, South Africa. E-mail: [email protected]
1. Introduction The Algoa Basin was a ‘‘depository for Cretaceous sediments’’, which eroded from ‘‘continental hinterlands’’ of the Cape Orogeny (Tinker et al. 2008a). The basin developed during the JurassicCretaceous period, with the latter being an important period associated with hydrocarbon development (Macdonald et al. 2003). According to Malan (1993), the Creataceous period was favourable for oil generation and conducive migration. It is in this regard that the bas
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