ALBERT

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.

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.

Description: New 40Ar/39Ar data from dykes intruded into Sverdrupfjella and Ahlmanryggen, Dronning Maud Land, Antarctica, indicate that dyke emplacement commenced at c. 207 Ma and lasted until c. 178 Ma. Whereas the ages ascribed to the Karoo-age magmatism contributing to Gondwana breakup are typically inferred as being c. 182 Ma, the data indicate that ages older than c.192 Ma in the broader Karoo Province are restricted to western Dronning Maud Land, Antarctica, indicating the locality where breakup was initiated. Limited palaeomagnetic data from c. 178–185 Ma dykes combined with published palaeomagnetic data from similar-aged dykes in Vestfjella and the Ferrar Province, suggest that Antarctica had already drifted/rifted significantly away from southern Africa from c. 207 to c. 180 Ma, earlier than previously thought. The data, if correct, require a re-evaluation of the ages ascribed to ocean-floor anomalies used to constrain reconstructions of Gondwana and may provide insight into the history of microcontinental blocks including the Falkland Islands, Haag nunataks, Ellsworth–Whitmore block and Maurice Ewing Bank.Supplementary material: Petrography, Ar isotope data from all samples and summary of ages are available at https://doi.org/10.6084/m9.figshare.c.5612838