Sub-volcanic Intrusions in the Karoo Basin, South Africa
The Karoo Basin in South Africa contains the world’s best exposed sub-volcanic part of a Large Igneous Province. Dolerite sills and dikes crop out across the 630,000 km2 large basin, from base to top of the stratigraphy. We present data from a compilation of 32 boreholes drilled since the 1960’s, showing that the sill percentage in the stratigraphy for individual boreholes varies from 0 to 54 %. Borehole depth is the key factor determining the sill proportion in specific regions, as shorter boreholes give higher sill proportions. When focusing on eight boreholes that were drilled through almost complete stratigraphic sections, the cumulative sill content is between 250 and 720 meters (average 440 m), yielding a proportion of sills to total thickness of 32 %. Using this average number as a proxy for the average sill content in the basin, the resulting sill volume is on the order of 250,000 to 300,000 km3 when extrapolating to basin scale. The volume of dikes remains unknown, but all are quite thin and so have relatively small volume, estimated to less than 15 % of the sill volume. The sills may have been a thermal source for the generation of oil and gas, as well as leading to their volatilization and escape to the Early Jurassic atmosphere.
We gratefully acknowledge support from the Norwegian Research Council (a grant to H. Svensen and a Centre of Excellence grant to CEED, project number 223272). We also would like to thank the Council of Geoscience and the National Core Library in Pretoria for providing borehole material and logs (handled by David Motloi).
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