Thesis (M.S.)--Oregon State University, 2010. / Printout. Includes bibliographical references (leaves 51-56). Also available on the World Wide Web.

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A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy Johannesburg, August 2015 School of Geosciences, University of the Witwatersrand / The possibility of extensive shale gas resources in the main Karoo Basin has resulted 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 the 1920s, but geophysical data provides an opportunity to shed new light on the basin architecture and formation. In this thesis, I use regional gravity, magnetic and borehole data over the basin, as well as vintage seismic data in the southern part of the basin. Modern computational capacity allows for more information to be extracted from these seismic data, and for these data to be better integrated with 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 its internal structure, in turn shedding light on basin formation. A new depth map of the basin constructed using this extensive database confirms that the basin deepens from on- to off-craton. The basin is deepest along the northern boundary of the Cape Fold Belt (CFB), with a depth of ~4000 m in the southwestern Karoo and ~5000 m in the southeastern part of the basin. Sediment thickness ranges from ~5500 to 6000 m. The Whitehill Formation along this boundary reaches a depth of ~ 3000 m in the southwest and ~4000 m in the southeast. Despite limited boreholes in this region, the basin appears to broadly deepen to the southeast. These seismic and borehole data also allow for mapping of the Cape Supergroup pinch-out below the Karoo basin (32.6°S for the Bokkeveld and 32.4°S for the Table Mountain Group), with the basin reaching a thickness of around 4 km just north of the CFB. The gravity effect of these sediments in the south is not sufficient to account for the low of the Cape Isostatic Anomaly near Willowmore and Steytlerville. This ~45 mGal Bouguer gravity low dominates the central region of the southern Karoo at the northern border of the CFB. The seismic data for the first time show uplift of lower-density shales of the Ecca Group (1800 – 2650 kg/m3) in this region, and structural and seismic data suggest that these lower density sediments continue to depth of 11 to 12 km along normal and thrust faults in this region. Two-dimensional density models show that these shallow crustal features, as well as deeper lower crust compared to surrounding regions, account for the anomaly. These seismic and borehole data also allow for constraints to be placed on the distribution and geometry of the dolerite intrusions that intruded the basin after its formation, and in some cases impacted on the shale layer, to be constrained. The highest 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 the south-central part of the basin. Here the intrusions are confined to the Beaufort Group, ~1000 m shallower than the shale reservoir, suggesting it should be the focus of exploration efforts. These dolerite sills are shown to be between 5 and 30 km wide and are saucer-shaped with ~ 800 m vertical extent, and dips of between 2° and 8° on the edges. The sheets in the south of the basin extend for over 150 km, dipping at between 3° and 13°, and are imaged down to ~ 5 km. This change in dip of the sheets is linked to deformation within the Cape Fold Belt, with greater dips closer to the belt, although these sheets do not appear to intrude strata dipping at more than 15 to 20°. In order to understand the shape of the Karoo basin and construct a 3D model of the basin, an understanding is needed of the underlying basement rocks. The Beattie Magnetic Anomaly (BMA) that stretches across the entire southern part of the basin forms part of the basement Namaqua-Natal Belt. Filtered magnetic data confirm that the Namaqua and Natal Belts are two separate regions with different magnetic characteristics, which is taken into account during modelling. The BMA is shown to be part of a group of linear magnetic anomalies making up the Natal Belt. The anomaly itself will therefore not have an individual effect on basin formation, 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 magnetic anomalies on the east coast of South Africa suggest the BMA can be attributed to regions of highly magnetic (10 to 100 x 10-3 SI) supracrustal rocks in Proterozoic shear zones. Along two-dimensional magnetic models in the southwestern Karoo constrained by seismic data, these magnetic zones are modelled as dipping slabs with horizontal extents of ~20-60 km and vertical extents of ~10-15 km. Body densities range from 2800- 2940 kg/m3 and magnetic susceptibilities from 10 to 100 x 10-3 SI. These, as well as other geophysical and geological constraints, are used to construct a 3D model of the basin down to 300 km. Relatively well-constrained crustal structure allows for inversion modelling of lithospheric mantle densities using GOCE satellite gravity data, with results in-line with xenolith data. These results confirm the existence of lower density mantle below the craton (~3270 kg/m3) that could contribute to the buoyancy of the craton, and an almost 50 kg/m3 density increase in the lithospheric mantle below the surrounding Proterozoic belts. It is this change in lithospheric density along with changes in Moho depths that isostatically compensate a large portion of South Africa’s high topography (<1200 m). The topography higher than 1200 m along the edge of the plateau, along the Great Escarpment, are shown to be accommodated by an asthenospheric buoyancy anomaly with a density contrast of around 40 kg/m3, while still mimicking the Bouguer gravity field. These findings are in line with recent 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 the formation of the main Karoo Basin. This was accomplished by studying the past flexure of the Whitehill Formation using north-south two-dimensional (2D) profiles. Deepening of the formation from ~3000 m in the southwest to ~4000 m in the southeast is explained using the concept of isostasy, i.e., an infinite elastic beam that is subjected to an increasing load size across the Cape Fold Belt. Load height values increase from 4 km in the southwest to 8 km in the southeast. This larger load is attributed here to “locking” along a subduction zone further to the south. The effective elastic thickness (Te) of the beam also increases from around 50 km over the Namaqua and Natal Belts in the southwest to 80 km over the Kaapvaal Craton and Natal Belt in the southeast. The changes in Te values do not correlate with changes in terrane, i.e., a north to south change, as previously though. 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 increasing towards the southeast. Therefore, while factors such as mantle flow could have contributed towards basin formation, reducing the load size needed, it is no longer necessary in order to account for the large extent of the basin. This flexure model breaks down further to the southeast, most likely due to a very high Te value. This could 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 the asthenospheric mantle.

The epeiric Transvaal Sea covered the Kaapvaal Craton of southern Africa during the Early Proterozoic and its remnant strata represent one of the oldest known carbonate depositories. A genetic stratigraphic approach has been used in this research on the evolution and syndepositional tectonics of the Transvaal Sea; research also emphasized the development of basement precursors, which influenced the Transvaal Sea. Eight subfacies were initially recognized and their interrelationships through Transvaal Sea time and space were used to identify ten depositional systems. Paleogeographic reconstructions indicate that the depositional systems developed on morphological variations of a distally-steepened carbonate rarp and that the depositional character of each was simply a function of water Backstripping of the depositional systems indicates that the Transvaal Sea was compartmentalized; three compartments are preserved on the Kaapvaal Craton. Backstripping also indicates that the depositional center of the Transvaal Sea lay over the western margin of an underlying rift. Rifting had developed a major, north-south-trending structure, and its geographical interrelationships with the east-west-trending Selati Trough created the compartment architecture of the basement. Interpretation of syndepositional tectonics suggests that six stages of subsidence influenced the Transvaal Sea. Early subsidence consisted of mechanical (rift) subsidence followed by

Tectonic signatures such as growth strata, clastic progradation, detrital composition, thickness trends, paleoflow shifts, lithofacies distribution, and vertical stratigraphic stacking patterns provide the basis for a range of tectonic/structural interpretations. Complete understanding of the application and limitations of tectonic signatures is important to maintain consistency and reduce uncertainty of interpretations that use them. This study provides insight into the external controls on two frequently used tectonic signatures in foreland basins: (1) growth strata, and (2) clastic wedge progradation. First, two syntectonic unconformity types are recognized in non-marine, Cenomanian growth strata adjacent to the Sevier thrust-belt in southeastern Nevada, USA. Unconformities with larger angular discordance (>10°, “Traditional Type”) developed when uplift outpaced sediment accumulation. More subtle unconformities with less discordance (2-10°, “Subtle Type”) developed when sediment accumulation nearly kept pace with uplift. Increasing sediment supply with positive net accommodation, allows syntectonic deposits to aggrade above a growing structure, with no change in uplift rate. Hence, sediment supply and regional accommodation impart an important control over growth strata geometries that are often interpreted on the basis of tectonics alone. Identification of unconformity types in growth strata can therefore document additional phases of uplift, particularly for intervals where sediments aggraded above an active structure due to higher sediment supply during regional subsidence, or sea level rise. Second, an anomalous, Campanian clastic wedge is identified in Cordilleran Foreland basin fill, Utah and Colorado. The complex internal architecture, tide-dominated facies and characteristic flat-to-falling shoreline stacking patterns of the wedge reflect rapid progradation of wide (60-80 km), embayed, tide-influenced shorelines; these characteristics distinguish the anomalous wedge from the underlying and overlying clastic wedges in the basin. A high-resolution regional correlation and isopach maps for the anomalous wedge provide evidence that extensive clastic progradation was coeval with both Sevier- and Laramide-style deformation. Stratigraphic relations suggest that development of the anomalous character of Wedge B was due to uplift of a Laramide structure within the foredeep, and possibly enhanced by reduced dynamic subsidence. / text

The research involves mapping of subsurface at a scale of 1:25,000 the top of three geological formations in the Southern Part of Sumatra – the Airbenakat Formation, the top of the Talangakar Formation, and the top of structural basement in the Jambi Trough. Isopach maps of the formations will be constructed. These maps will form the basis of a basin analysis and hydrocarbon source rock assessment of the Jambi Trough using Basin Mod basin modeling software (Rockworks Software). The studies utilize the L. Bogue Hunt Southeast Asia database housed in the Department of Geological Sciences at Ball State University. Seismic record sections, geophysical logs, cutting descriptions, and paleontological reports will provide basic geological data to enable mapping of the three horizons. Although hydrocarbon accumulations are abundant in Central and Southern Sumatra, the nature of the source rocks is only partially understood. The proposed research will map the Airbenakat and Talangakar Formations while identifying the areas of thermally mature source rock is the main goal of the research. This study will identify characteristics which will enable the identification of thermally mature rocks in other regions of Sumatra. The area of the project is located at the Southeastern part of Asia in Indonesia and mainly the Jambi trough located in Southern Sumatra. Generally, the geology and tectonics of this area (Sumatra) is controlled by the subduction of the Indian plate towards the east and beneath the Eurasia plate. / Department of Geological Sciences

According to the Ade observation (Ade, W., pers. Comm.) “95% of all commercial oil fields in the Sumatra region occur within 17 km of seismically mappable structural grabens in the producing basins”. The Ade observation proposes a link between the subsidence of the source rocks (the Talang Akar Formation) in the grabens and the maturity of the organic material. To test the validity of the Ade observation, subsurface mapping of the region was carried out using geophysical logs. Using the well log information, the basement and the formation tops have been mapped with a special emphasis on Talang Akar and Air Benakat Formations. The isopach maps of these formations show that most of the producing wells on the Sunda shelf are in fact located in and around the major structural basins. Trends in the occurrence of the oil fields have also been observed which are analogous to the orientation of the grabens. Structural mapping of the basins have identified several wrench faults. These are of particular interest as wrench faults provide good structural traps for oil in the Los Angeles and the North Sumatra Basins and may prove to be very important for future exploration in southern Sumatra and northwest Java. In South Sumatra Basin, 77.78% of the potential oil fields are located in the 17 km margin from the grabens. For Sunda/Asri Basins and the Ardjuna Basin, it is 100 and 92 respectively. Identifying the source rocks in this 17 km window will enhance the success rate of oil exploration in the Sundaland Basins. / Department of Geological Sciences

Samples of shallow-water carbonates were collected from Jurassic and Cretaceous Italian carbonate platforms and subjected to petrographic, diagenetic and chemostratigraphic analyses (⁸⁷Sr/⁸⁶Sr, δ¹³C_carb, δ¹³C_org, δ¹⁸O). In general, the new chemostratigraphic data generated reflect trends established by previous work, some of which has been carried out on biostratigraphically calibrated reference sections. Consequently, chemostratigraphic correlations (⁸⁷Sr/⁸⁶Sr, δ¹³C_carb) of isotope profiles taken from platform carbonates with well-dated reference sections have allowed the application of high-resolution dating frameworks to the biostratigraphically poorly constrained carbonate platforms. The increased resolution in dating of the Italian carbonate platforms has, furthermore allowed a detailed investigation into the facies response of these carbonate platforms to major geological events. In particular, platform responses to oceanic anoxic events and other periods of major perturbation in the global carbon cycle are analysed (early Toarcian, Aalenian-Bajocian, Oxfordian-Tithonian, Valanginian-Hauterivian, Aptian-Albian, Cenomanian-Turonian, Coniacian-Santonian). Lower Jurassic levels of the Trento Platform record platform devastation in the early Toarcian synchronous with a major negative δ¹³C_carb excursion, followed by platform recovery synchronous with a pronounced δ¹³C_carb positive excursion and return to background values. The Campania-Lucania Platform shows negligible response to the oceanographic events of the early Toarcian even though the characteristic carbon-isotope profile is readily identifiable. The Trento Platform drowned at approximately the Aalenian-Bajocian Stage boundary, synchronously with a reproducible negative followed by positive δ¹³C_carb excursion, whereas the Campania-Lucania Platform underwent a facies transition from oolite to cyclically bedded micrite. The Friuli Platform showed negligible depositional response to the carbon-cycle perturbations of the Kimmeridgian-Tithonian, Valanginian-Hauterivian, Aptian-Albian and Cenomanian- Santonian (as registered in the δ¹³C_carb record). The Campania-Lucania Platform registered flooding and increased levels of organic-matter preservation coincident with pronounced positive δ¹³C_carb excursions at Cenomanian-Turonian and Coniacian-Santonian levels. Observations on the responses of carbonate platforms to oceanographic conditions during periods of global carbon burial lead to the conclusion that temperature excess is a hitherto neglected control on global carbonate accumulation rates.

Bibliography : leaves 214-246. / xiii, 246 leaves : ill., maps ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Geology and Geophysics, 1992

Thesis (Ph. D.)--Macquarie University, Division of Environmental & Life Sciences, Department of Earth and Planetary Sciences, 2005. / Includes bibliographical references (leaves 202-224).