Sub-Volcanic Intrusions and the Link to Global Climatic and Environmental Changes

Abstract

Most of the Large Igneous Provinces (LIPs) formed during the last 260 million years are associated with climatic changes, oceanic anoxia, or extinctions in marine and terrestrial environments. Current hypotheses involve (1) degassing of carbon from either oceans or shallow sea-bed reservoirs, (2) degassing from flood basalts, or from (3) sedimentary basins heavily intruded by LIP-related sills. These hypotheses are based on detailed geological and geochemical studies from LIPSs or relevant proxy data sequences. Here we present new data on gas generation and degassing from a LIP, based on the LA1/68 borehole north of the Ladybrand area in the Karoo Basin, South Africa. The borehole was drilled in the middle of a phreatic breccia pipe and penetrated 11 sills before reaching the basement at 1710 m depth. We present new data on the lowermost 15 m thick sill emplaced in shale, and on the breccia comprising the uppermost 154 m of the core. We show that (1) a reduction in organic matter within a contact aureole can be explained by heating and the formation of CH4, (2) a phreatic eruption and breccia formation was initiated from pore fluid boiling around sills emplaced in Beaufort Group sandstones at 420–570 m depth, (3) the phreatic eruption cut through a cover of solidified and partly molten lava flows that subsequently filled the crater, and (4) the pipe has been used as a fluid flow pathway for millions of years, demonstrated by fossil and active oil seeps. We conclude that the sub-volcanic LIP environment hold the key to understand the relationships between large scale volcanism and rapid environmental perturbations.