A provisional basin analysis of the Karoo Supergroup, Springbok Flats Basin, South Africa

Abstract

The Springbok Flats (SBF) Basin is one of the smaller basins associated with the Karoo basins of the Late Carboniferous–Middle Jurassic age interval. The preserved SBF basin is a topographically flat area with very few outcrops. It has a NE-SW orientation and is approximately 205 km long and 30 km wide. This study is based on borehole log data captured by the Council for Geoscience, which has been collected from various exploration companies throughout the history of the investigation of the SBF Basin area. The purpose of this study is to identify an evolutionary history of the basin by utilising methods of basin analysis and literature search, and to establish how the basin relates to other Karoo Supergroup basins in southern Africa. The postulated genetic model of a retroarc fore-bulge rift basin was compared to the inferred depositional environments. The geophysical interpretations and structural contour maps of the various strata indicate the presence of the major Zebedelia Fault, which is part of the Thabazimbi Murchison Lineament (TML) relay system. This fault runs along the northern boundary of the basin and has caused the strata of the SBF Basin to be down-faulted by 800 to 1000 metres. The isopachs of the identified Karoo successions do not indicate thickening towards this lineament, which suggests that the faulting along this lineament post-dates the Karoo sedimentation. The Thabazimbi Murchison Lineament played a significant role during the later stages of the SBF sedimentation. Once the depocentre became more centrally located in the depository, it began to migrate towards the TML. Although the major faulting was yet to occur, the weakness in the craton was apparent. During the breakup of Gondwana, the Zebedelia Fault shifted the strata down and allowed the extrusion of the Letaba Basalt, along with the multi-intrusion of dykes throughout the strata. The onset of the deposition of the Karoo Stratigraphy in the SBF was due to uplift resulting from the mid-carboniferous assembly of Pangea. During the Lower Karoo deposition, lithospheric subsidence was facilitated by crustal-scale faults, resulting in the deposition of the glacial Dwyka and Lower Ecca sediments. Flexural subsidence was occurring in the forebulge due to the relaxing of the initial compression of the Cape Fold Belt (CFB). The later Ecca succession was characterized by large subsidence with little accompanying brittle deformation. The lower Beaufort was a deltaic basin and was terminated towards the end of the Permian period, identified by a significant loss of fauna and flora. There was a ± 3km uplift, known as the Namaqua Uplift and erosion north of the fold belt. This marked the structural inversion during deposition of the Beaufort Group and Early Molteno Formation. These uplift events resulted in uplift in the foredeep which resulted in the compression of the forebulge during the deposition of the Molteno Formation. Once these events subsided, the forebulge relaxed and underwent subsidence and extension. Elliot Formation formed during this unloading of structural relief and relaxation of basinforming stresses. The upper Elliot and Clarens formations and Letaba Basalts exhibit the transition from sinistral strain of the late Karoo Basin to the dextral tectonics of the Gondwana breakup that terminated the basin deposition. The Karoo sediments in the SBF Basin clearly represent the broad spectrum of the same set of palaeoenvironments that are recognised in the Main Karoo Basin rocks. These reflect the progressive infilling of the Karoo Basins, the changing tectonic framework as well as the migration of Gondwana from polar to tropical latitudes. However, due to the development of the SBF basin on the forebulge, the compression of the CFB had the opposite effect, where it resulted in uplift of the fore-bulge and subsidence of the foredeep. This subsequently resulted in the SBF correlated Karoo sedimentary successions being markedly thinner than those of the Main Karoo Basin, and in some cases, certain strata are completely absent. An extensional basin formed by reactivation of older structures, such as the TML, as a result of displacement on the principle shear zones. This resulted in the preservation of the SBF strata in the basin today. This study is a baseline and preliminary investigation into the SBF Basin, and may act as a canvas to which more in-depth investigations may be added. Various questions have been identified that require further understanding and are listed under recommendations. Many of the questions put forth may be answered with a thorough Quality Assurance-Quality Control (QAQC) of the database.