Integrated geophysical investigation of the Karoo Basin, South Africa

[摘要] The possibility of extensive shale gas resources in the main Karoo Basin hasresulted in a renewed focus on the basin, and particularly the Whitehill Formation.The main Karoo Basin has been the subject of geological studies since before the1920s, but geophysical data provides an opportunity to shed new light on thebasin architecture and formation. In this thesis, I use regional gravity, magneticand borehole data over the basin, as well as vintage seismic data in the southernpart of the basin. Modern computational capacity allows for more information tobe extracted from these seismic data, and for these data to be better integratedwith potential field data. The integration of datasets in a three-dimensional model(3D) has allowed for a better understanding of the shape of the basin and itsinternal structure, in turn shedding light on basin formation.A new depth map of the basin constructed using this extensive databaseconfirms that the basin deepens from on- to off-craton. The basin is deepest alongthe northern boundary of the Cape Fold Belt (CFB), with a depth of ~4000 m inthe southwestern Karoo and ~5000 m in the southeastern part of the basin.Sediment thickness ranges from ~5500 to 6000 m. The Whitehill Formation alongthis boundary reaches a depth of ~ 3000 m in the southwest and ~4000 m in thesoutheast. Despite limited boreholes in this region, the basin appears to broadlydeepen to the southeast. These seismic and borehole data also allow for mappingof the Cape Supergroup pinch-out below the Karoo basin (32.6°S for theBokkeveld and 32.4°S for the Table Mountain Group), with the basin reaching athickness of around 4 km just north of the CFB. The gravity effect of thesesediments in the south is not sufficient to account for the low of the Cape IsostaticAnomaly near Willowmore and Steytlerville. This ~45 mGal Bouguer gravity lowdominates the central region of the southern Karoo at the northern border of theCFB. The seismic data for the first time show uplift of lower-density shales of theEcca Group (1800 – 2650 kg/m3) in this region, and structural and seismic datasuggest that these lower density sediments continue to depth of 11 to 12 km alongnormal and thrust faults in this region. Two-dimensional density models show thatthese shallow crustal features, as well as deeper lower crust compared tosurrounding regions, account for the anomaly.These seismic and borehole data also allow for constraints to be placed onthe distribution and geometry of the dolerite intrusions that intruded the basin afterits formation, and in some cases impacted on the shale layer, to be constrained. Thehighest concentrations of dolerites are found in the northwest and east of the basin,pointing towards two magma sources. The region of lowest concentration is in thesouth-central part of the basin. Here the intrusions are confined to the BeaufortGroup, ~1000 m shallower than the shale reservoir, suggesting it should be thefocus of exploration efforts. These dolerite sills are shown to be between 5 and 30km wide and are saucer-shaped with ~ 800 m vertical extent, and dips of between2° and 8° on the edges. The sheets in the south of the basin extend for over 150km, dipping at between 3° and 13°, and are imaged down to ~ 5 km. This changein dip of the sheets is linked to deformation within the Cape Fold Belt, withgreater dips closer to the belt, although these sheets do not appear to intrude stratadipping at more than 15 to 20°.In order to understand the shape of the Karoo basin and construct a 3D modelof the basin, an understanding is needed of the underlying basement rocks. TheBeattie Magnetic Anomaly (BMA) that stretches across the entire southern part ofthe basin forms part of the basement Namaqua-Natal Belt. Filtered magnetic dataconfirm that the Namaqua and Natal Belts are two separate regions with differentmagnetic characteristics, which is taken into account during modelling. The BMAis shown to be part of a group of linear magnetic anomalies making up the NatalBelt. The anomaly itself will therefore not have an individual effect on basinformation, and the effect of the Natal Belt as a whole will have to be investigated.An in-depth study of outcrops associated with one of these linear magneticanomalies on the east coast of South Africa suggest the BMA can be attributed toregions of highly magnetic (10 to 100 x 10-3 SI) supracrustal rocks in Proterozoicshear zones. Along two-dimensional magnetic models in the southwestern Karooconstrained by seismic data, these magnetic zones are modelled as dipping slabswith horizontal extents of ~20-60 km and vertical extents of ~10-15 km. Bodydensities range from 2800- 2940 kg/m3 and magnetic susceptibilities from 10 to100 x 10-3 SI.These, as well as other geophysical and geological constraints, are used toconstruct a 3D model of the basin down to 300 km. Relatively well-constrainedcrustal structure allows for inversion modelling of lithospheric mantle densitiesusing GOCE satellite gravity data, with results in-line with xenolith data. Theseresults confirm the existence of lower density mantle below the craton (~3270kg/m3) that could contribute to the buoyancy of the craton, and an almost 50kg/m3 density increase in the lithospheric mantle below the surroundingProterozoic belts. It is this change in lithospheric density along with changes inMoho depths that isostatically compensate a large portion of South Africa’s hightopography (<1200 m). The topography higher than 1200 m along the edge of theplateau, along the Great Escarpment, are shown to be accommodated by anasthenospheric buoyancy anomaly with a density contrast of around 40 kg/m3,while still mimicking the Bouguer gravity field. These findings are in line withrecent tomographic studies below Africa suggesting an 'African Superplume” or'Large Low Velocity Seismic Province” in the deep mantle.The basin sediment thickness maps were further used to investigate theformation of the main Karoo Basin. This was accomplished by studying the pastflexure of the Whitehill Formation using north-south two-dimensional (2D)profiles. Deepening of the formation from ~3000 m in the southwest to ~4000 min the southeast is explained using the concept of isostasy, i.e., an infinite elasticbeam that is subjected to an increasing load size across the Cape Fold Belt. Loadheight values increase from 4 km in the southwest to 8 km in the southeast. Thislarger load is attributed here to 'locking” along a subduction zone further to thesouth. The effective elastic thickness (Te) of the beam also increases from around50 km over the Namaqua and Natal Belts in the southwest to80 km over theKaapvaal Craton and Natal Belt in the southeast. The changes in Te values do notcorrelate with changes in terrane, i.e., a north to south change, as previouslythough. The large extent and shape of the Karoo basin can therefore, in general,be explained as a flexural basin, with the strength of the basement increasingtowards the southeast. Therefore, while factors such as mantle flow could havecontributed towards basin formation, reducing the load size needed, it is no longernecessary in order to account for the large extent of the basin. This flexure modelbreaks down further to the southeast, most likely due to a very high Te value. Thiscould be the reason for later plate break in this region during Gondwana breakup.It is inferred that this increase in Te is linked to the buoyancy anomaly in theasthenospheric mantle.

[发布机构] University of the Witwatersrand