Physical volcanology of the Sterkspruit flood basalt crater complex, South Africa

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McClintock, M. K. (Murray Kevin)

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Abstract:

Volcanism associated with the onset of Karoo flood basalt eruptions at Sterkspruit began with emplacement of thin lava flows before abruptly switching to phreatomagmatic and magmatic activity that formed a nest of craters, spatter and tuff rings and cones that collectively comprise a crater-complex >40 km2 filled by 9-18 km3 of volcaniclastic debris. Phreatomagmatic activity driven by interaction of Karoo magma with groundwater hosted in country rock and crater-filling debris quarried broad, mainly shallow craters (hundreds of metres, but not kilometers deep) into wall-rock. Closely spaced individual vents, the consequence of magma emplaced over a broad area through a network of feeder dikes and stocks, were active at the same time or over short periods of time. Highly ephemeral access of external water to vents drove repeated and reversible switches between explosive to effusive magmatic and explosive phreatomagmatic activity, resulting in vents and craters that grew laterally and vertically into adjacent ones through quarrying and vent migration. Deposits within the Sterkspruit crater-complex are dominated by 7-15 km3 of massive, unsorted polymict lapilli tuff and tuff breccia juxtaposed with localised fountain-fed lava and strombolian spatter deposits. Transport within the complex was dominated by jets and fountains of volcaniclastic debris and by mass movement. Country-rock breccias indicate that craters grew via a combination of mechanical fragmentation, granulation and mass-movement of 7-12 km3 of wall-rock, adding mass and previously locked-up pore-water to the volcanic system. Ash and lapilli, the deposits of plumes 5-15 km high, form a 50-110 m-thick ejecta blanket mantling Clarens Formation country rock that thins gradually away from the crater-complex margins. Explosive volcanism was succeeded by brief period of reworking of volcaniclastic debris by fluvial and eolian processes accompanied by formation of a shallow crater lake 12 km2 in extent, and then by voluminous effusion of flood basalt that inundated the Sterkspruit crater-complex with lava. Flood basalt magmas involved in Sterkspruit eruptions were chemically heterogenous. This study documents the rapid (perhaps simultaneous) eruption of multiple, chemically distinct basaltic magmas, that cannot be simply related to each other, from one vent site, and possibly many others, within the Sterkspruit crater-complex. Five distinct magma types were involved in eruptions at Sterkspruit, indicating that (i) magma batches may be small and not simply related to one another, (ii) heterogeneities in the magma source region may be close to each other in time and space, and (iii) eruptions of chemically distinct magmas may take place over short intervals of space and time without significant hybridisation in flood basalt fields. Formation of the Sterkspruit Complex, and many others like it in South Africa, confirms that the opening phases of Karoo LIP volcanism were explosive, and that the volume of the products of explosive volcanism may have important implications for climate change and landscape development associated with the emplacement of LIPs.

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xi, 266p. Map in pocket (folded).

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2006McClintock

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http://download.otagogeology.org.nz/temp/Abstracts/2006McClintock.pdf

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McClintock, M. K. (Murray Kevin), “Physical volcanology of the Sterkspruit flood basalt crater complex, South Africa ,” Otago Geology Theses, accessed March 29, 2024, http://theses.otagogeology.org.nz/items/show/467.

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