Deconstructing the Transvaal Supergroup, South Africa: implications for Palaeoproterozoic palaeoclimate models

Current correlations between the Pretoria and Postmasburg Groups of the Transvaal Supergroup are shown to be invalid. The Postmasburg Group is also demonstrated to be broadly conformable with the underlying Ghaap Group and therefore considerably older (~2.4 Ga) than previously supposed. The new stratigraphy documents an extensive (100 Ma) and continuous cold-climate episode with a glacial maximum at the Makganyene Formation diamictite. Iron formations of the underlying Asbesheuwels and Koegas Subgroups and overlying Hotazel Formation have similar origins, related, respectively, to the onset and cessation of the glacial event. This interpretation of the Transvaal Supergroup stratigraphy has significant implications for various Palaeoproterozoic environmental models and for the timing of the development of an oxygenated atmosphere

The geology and geochemistry of the Palaeoproterozoic Makganyene diamictite

The Palaeoproterozoic Earth experienced a global glacial event at 2400 Ma that occurred during the transitional period from anoxic to aerobic conditions in the atmosphere and oceans. The Transvaal Supergroup in the Griqualand West Basin, South Africa, hosts glacial deposits and associated major iron and manganese deposits that are apparently related to these global changes. The focus of this study is to assess the stratigraphy and geochemistry of the glaciogenic Makganyene Formation, in order to constrain its palaeoenvironmental settings. The Makganyene Formation forms the base of the Postmasburg Group and has been regarded as resting on an erosive regional unconformity throughout the Northern Cape Province. Systematic regional field observations and regional mapping carried out during this study demonstrate that this stratigraphic relationship is not universal. The Makganyene Formation is, in fact, conformable with underlying formations of the Koegas Subgroup in the deep southern Prieska basin and rests on an unconformity only on the shallow Ghaap platform to the north-east. The Makganyene Formation displays lateral facies changes that reflect the palaeogeography of the study area, and the advance and retreat of ice sheets/shelves. Geochemical investigations of glacial strata of the Makganyene Formation demonstrate that underlying banded iron formations of the Transvaal Supergroup acted as the main clastic source for the diamictite detritus. Geographic variations in bulk composition of the diamictites correlate well with field observations, and show that sorting processes were controlled largely by the morphology of the palaeobasin. Carbon isotope results emphasize the transitional nature of the Makganyene Formation in terms of the environmental conditions that resulted in widespread global glaciation in the Palaeoproterozoic. On the basis of the above geological evidence, it is proposed that the Transvaal Supergroup in the Northern Cape Province represents a continuous depositional event that lasted approximately 250 Ma and hence provides a unique opportunity for assessing the transitional changes experienced by the Palaeoproterozoic Earth

Monitoring Of CO2 Leakage Using High-Resolution 3D Seismic Data – Examples From Snøhvit, Vestnesa Ridge And The Western Barents Sea

Source at https://doi.org/10.3997/2214-4609.201802965.Injection of CO2 in subsurface reservoirs may cause overburden deformation and CO2 leakage. The aim of this study is to apply technologies for detection and monitoring of CO2 leakage and deformation above the injection reservoirs. The examples of this study include data from the Vestnesa Ridge natural seep site, the Snøhvit gas field and CO2 storage site region, and the Gemini North gas reservoir. Reprocessing of existing 3D high-resolution seismic data allows resolving features with a vertical and lateral resolution down to c. 1 m and c. 5 m respectively. The current acquisition systems could be modified to image structures down to one meter in both the vertical and horizontal directions. We suggest a monitoring workflow that includes baseline and time-lapse acquisition of highresolution 3D seismic data, integrated with geochemical, geophysical, and geotechnical seabed core and watercolumn measurements. The outcome of such a workflow can deliver reliable quantitative property volumes of the subsurface and will be able to image meter-sized anomalies of fluid leakage and deformation in the overburden

Breakup volcanism and plate tectonics in the NW Atlantic

The seismic, magnetic, and gravity data presented in this study were provided by TGS. Seismic interpretation was done using HS Kingdom software. Grid interpolations and map compilations were established using Geosoft Oasis Montaj and ArcGis softwares. We would like thank Craig Magee, Alexander Lewis Peace, the Editor and the Associated Editor for helpful comments and guidance that improved the paper. We acknowledge the support from the Research Council of Norway through its Center of Excellence funding scheme, project 223272 (CEED).Peer reviewedPostprin

The age and correlation of the Postmasburg Group, southern Africa: Constraints from detrital zircon grains

Age determinations were conducted on detrital zircon grains from two stratigraphic levels in the Postmasburg Group, Transvaal Supergroup - the top of the Makganyene Formation and the base of the Hotazel Formation - to constrain the age of the group and for comparison with purported correlatives in the Segwagwa Group of the Kanye basin and Pretoria Group of the Transvaal basin. Detrital zircon grains are interpreted as being derived mainly from the underlying Ghaap Group and its basement of Ventersdorp Supergroup and Kraaipan granite/greenstone rocks, indicating a proximal source to the east and north on the Vryburg arch. The maximum age of the Postmasburg Group was constrained by the youngest detrital-zircon age to 2436. ±. 7. Ma and the broad age range of the group, as a consequence, to somewhere between 2.43 and 2.38. Ga. Comparison with various stratigraphic units in the Kanye and Transvaal basins indicated close similarities in zircon populations with the Duitschland Formation in the Transvaal basin. The Pretoria and Segwagwa Groups contain younger zircon populations at 2350-2320 and 2240-2200. Ma that are related to syn-depositional volcanic activity and are not present in the Postmasburg Group samples, indicating the younger age of these groups. Based on compatibility of zircon populations, new stratigraphic correlations between the upper groups of the Transvaal Supergroup in the Griqualand West, Kanye and Transvaal basins are proposed. These include the probable restriction of the lower Timeball Hill - Hekpoort portion of the Pretoria Group to the Transvaal basin. An older age for the Postmasburg Group - ≈2.4. Ga as opposed to the generally accepted 2.22. Ga - will affect various models proposed for the evolution of the Earth's atmosphere/hydrosphere that have been based on studies of rocks of the group, supporting an older ≈2.4. Ga age for global oxidation transformations

Sill emplacement and contact metamorphism in a siliciclastic reservoir on Svalbard, Arctic Norway

Igneous intrusions in sedimentary basins are associated with contact aureoles that influence rock properties such as maturation, porosity and permeability. On Svalbard, an extensive dolerite complex (i.e., the Diabasodden Suite) was emplaced in a heterolithologic sandstone, siltstone, shale and carbonate succession during the Early Cretaceous (c. 124.5 Ma). The sedimentary host rocks include the predominantly siliciclastic, Upper TriassicMiddle Jurassic Kapp Toscana Group, which is currently being investigated as a storage unit for potential CO2 sequestration in the vicinity of Long yearbyen. As part of the baseline reservoir characterisation, a 2.28 metre-thick dolerite sill and its associated contact aureole was drilled and fully cored in the lower part of the target aquifer. Geochemical data indicate that the intrusion belongs to the Diabasodden Suite, which also crops out 18 km from the planned injection site. Samples spanning the contact aureole show significant thermal effects around the thin sill. The total organic carbon content is lowered towards the contact (from 1-2 wt.% to zero) accompanied by a reduction of the pyrolysis output and higher Tmax values (500°C vs. 320°C). A count of the visual fractures along the Dh4 borehole shows that the sill itself is characterised by a fracture frequency of 8-10 fractures per metre, well above the background fracturing in the host rock above the sill (0-1 fractures per metre). Increased background fracturing (4-10 fractures per metre) is, however, evident in the host rock below the sill. Our results show that the total aureole thickness is 160-195% of the sill thickness and that the sill and aureole together represent a six metre-thick geochemical and mechanical perturbation in the sedimentary succession. We conclude that even very thin sills and related aureoles may affect the CO2 storage aquifer by locally reducing porosity in the host rock, but also by enhancing permeability along the fractured intrusion-host rock interfaces