Physical volcanology of a large crater-complex formed during the initial stages of Karoo flood basalt volcanism, Sterkspruit, Eastern Cape, South Africa

The Sterkspruit Complex, exposed at the base of the Karoo flood basalts in South Africa, is a complex of mostly massive volcaniclastic rocks, dikes and lava that fills a crater > 45 km2 cut into country rock and is surrounded by a blanket of more-distal bedded volcaniclastic rocks. The moderately to steeply inward-dipping margins of the crater are delineated by country-rock breccia that grades into rocks within the complex. Intra-complex rocks are dominated by massive, polymict lapilli tuff and tuff breccia > 150 m thick, with intercalated lavas, and minor bedded volcaniclastic rocks. Pillow lava > 50 m thick and thinly bedded, inferred lacustrine deposits cap the complex succession and are in turn covered by subaerial Karoo flood basalts. Beyond the crater a succession of bedded, finer-grained volcaniclastic rocks (100–60 m thick) rests on country rock and thins distally over at least 5 km.Magma emplaced over a broad area through a network of feeder dikes and sheets created many small vents that grew into their neighbours through lateral quarrying and vent migration, and whose deposits overlap both within and outside the complex. An abrupt end to eruptions that formed volcaniclastic deposits is recorded by flooding of the lowest part of the crater by first water then lava, then subsequently by inundation of the complex and the surrounding landscape by voluminous effusion of flood basalt. Formation of the Sterkspruit Complex, and many others like it in South Africa, confirms that the opening phases of Karoo flood basalt volcanism were partly explosive. The abundant volcaniclastic products at Sterkspruit and in other flood basalt provinces result from eruptions that disperse ash and volcanic aerosols differently than effusive eruptions of lava, and thus may have important implications for climate change and landscape development associated with the initial stages of flood basalt provinces.