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  • 1
    Publication Date: 2014-08-28
    Description: The reconstruction of thermal history is essential for evaluating the potential for hydrocarbon generation within sedimentary basins. Magnetic techniques provide an alternative to more traditional methods to study the geothermal history of sedimentary basins (such as illite crystallinity and vitrinite reflectance), which are often associated with significant uncertainty. In this paper the application of various magnetic geothermometers to the western Karoo Basin of South Africa are evaluated. Three magnetic experiments were conducted on samples from stratigraphic borehole G39977 to determine the thermal effect of large scale dolerite intrusions on the sedimentary strata of the Karoo Supergroup in western South Africa. Alteration index (A 40 ) data indicate maximum acquired temperatures for the sedimentary units ranging between 200°C and 650°C, with the highest temperatures restricted to short distances (less than half the sill thicknesses) within the contact aureoles. Both magnetostratigraphy and anisotropy of low field magnetic susceptibility (AMS) data confirm that re-magnetization of magnetic fabric does not exceed distances more than half the sill thicknesses. Our results indicate the general elevation of the palaeotemperatures of the organic-rich sedimentary rocks of the Ecca Group to temperatures where hydrocarbons are normally converted into gas. Importantly, it is clear from this study that the greatest thermal effects of the sill intrusions on the sedimentary strata are limited to the contact aureoles, suggesting that there is an, as yet unquantified, potential for hydrocarbon resources remaining between these intrusions.
    Print ISSN: 1012-0750
    Topics: Geosciences
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  • 2
    Publication Date: 2019
    Description: 〈span〉〈div〉Abstract〈/div〉The ultramafic-mafic layered igneous Molopo Farms Complex straddles the border between South Africa and Botswana. Younger cover obscures this igneous complex and its country rocks, which are generally assigned to the Paleoproterozoic Transvaal Supergroup. The area intruded by the complex is characterized by abutting and contrasting successions of the upper Transvaal Supergroup (i.e., the Pretoria and Postmasburg groups), the correlation of which is critical to understanding the first significant build-up in atmospheric oxygen, also known as the Great Oxidation Event. Recent dating of the Postmasburg Group necessitates a reinterpretation of Transvaal Supergroup stratigraphy involving a 200 million year downward revision of the Postmasburg Group relative to the Pretoria Group. The geology of the area intruded by the Molopo Farms Complex may provide important insights into this correlation model. Here we report a 〈sup〉207〈/sup〉Pb/〈sup〉206〈/sup〉Pb ID-TIMS baddeleyite date of 2054 ± 5 Ma from a gabbroic sample from the Molopo Farms Complex in South Africa, and an U-Pb zircon date of 2056 ± 10 Ma from a highly altered and metamorphosed quartzite in direct contact with ultramafic rocks of the complex in Botswana. We interpret these as crystallization ages of the Complex, which are within error of the 2056 to 2055 Ma age of the Bushveld Complex. Also reported from drill core intersections are U-Pb LA-ICP-MS detrital zircon age data from quartzite samples of both the floor and roof country rock. The roof rock detrital zircon age populations are comparable to those of the Paleoproterozoic Waterberg Group. Age populations in floor rocks are generally similar to those of the Pretoria Group, with the addition of ~2050 Ma populations, which likely reflect the metamorphic aureole of the complex on distinctly recrystallized country rock. A revised pre-Kalahari regional geology of the Molopo Farms Complex in South Africa, that incorporates the Pretoria Group in the area, implies an unconformable relationship with the Potsmasburg Group. Future recognition of such an unconformable relationship in drill core will ultimately resolve the problem of Transvaal strata correlation.〈/span〉
    Print ISSN: 1012-0750
    Electronic ISSN: 1996-8590
    Topics: Geosciences
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  • 3
    Publication Date: 2019
    Description: 〈span〉〈div〉Abstract〈/div〉The ultramafic-mafic layered igneous Molopo Farms Complex straddles the border between South Africa and Botswana. Younger cover obscures this igneous complex and its country rocks, which are generally assigned to the Paleoproterozoic Transvaal Supergroup. The area intruded by the complex is characterized by abutting and contrasting successions of the upper Transvaal Supergroup (i.e., the Pretoria and Postmasburg groups), the correlation of which is critical to understanding the first significant build-up in atmospheric oxygen, also known as the Great Oxidation Event. Recent dating of the Postmasburg Group necessitates a reinterpretation of Transvaal Supergroup stratigraphy involving a 200 million year downward revision of the Postmasburg Group relative to the Pretoria Group. The geology of the area intruded by the Molopo Farms Complex may provide important insights into this correlation model. Here we report a 〈sup〉207〈/sup〉Pb/〈sup〉206〈/sup〉Pb ID-TIMS baddeleyite date of 2054 ± 5 Ma from a gabbroic sample from the Molopo Farms Complex in South Africa, and an U-Pb zircon date of 2056 ± 10 Ma from a highly altered and metamorphosed quartzite in direct contact with ultramafic rocks of the complex in Botswana. We interpret these as crystallization ages of the Complex, which are within error of the 2056 to 2055 Ma age of the Bushveld Complex. Also reported from drill core intersections are U-Pb LA-ICP-MS detrital zircon age data from quartzite samples of both the floor and roof country rock. The roof rock detrital zircon age populations are comparable to those of the Paleoproterozoic Waterberg Group. Age populations in floor rocks are generally similar to those of the Pretoria Group, with the addition of ~2050 Ma populations, which likely reflect the metamorphic aureole of the complex on distinctly recrystallized country rock. A revised pre-Kalahari regional geology of the Molopo Farms Complex in South Africa, that incorporates the Pretoria Group in the area, implies an unconformable relationship with the Potsmasburg Group. Future recognition of such an unconformable relationship in drill core will ultimately resolve the problem of Transvaal strata correlation.〈/span〉
    Print ISSN: 1012-0750
    Electronic ISSN: 1996-8590
    Topics: Geosciences
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  • 4
    Publication Date: 2016-10-19
    Description: The Karoo Basin of South Africa is of economic importance for its large coal reserves but has in recent years also been in the spotlight due to the possibility of extensive shale gas reserves. Reconstruction of the thermal history of the Karoo Basin is essential for evaluating the potential hydrocarbon generation within this Late Carboniferous – Middle Jurassic sedimentary basin. Magnetic techniques provide an alternative approach in comparison to more traditional methods to study the geothermal history of sedimentary basins (such as illite crystallinity and vitrinite reflectance), which are often associated with significant uncertainty. In this paper variations in the thermal history across the Karoo Basin as a result of heating by the Karoo LIP are evaluated using different magnetic "geothermometers". These include palaeomagnetism (baked contact test), thermomagnetic analysis (alteration index method) and anisotropy of magnetic susceptibility (AMS). Although these techniques were successful in identifying a variation in metamorphic effects adjacent to contact aureoles, only the alternating index (A 40 ) provides a means of estimating peak temperatures. Our results indicate a regional elevation of palaeotemperatures of the organic-rich sedimentary rocks of the Ecca Group to temperatures where hydrocarbons are normally converted into gas. This study shows that the greatest thermal effects of the sill intrusions on the sedimentary strata are limited to the contact aureoles, suggesting that there is an, as yet unquantified, potential for hydrocarbon resources remaining in strata between these intrusions. An increase in the paleotemperatures from 200°C in the southwest to 400°C in the northeast of the basin is observed. We hypothesize that this trend is mainly due to differences in thermal conductivity of the different sedimentary rock types across the basin as the Karoo Basin transgresses from tight low porosity marine shales in the south and southwest towards more lacustrine mudstone and porous sandstone towards the northeast.
    Print ISSN: 1012-0750
    Topics: Geosciences
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