Stephane Polteau

U–Pb geochronology of Cretaceous magmatism on Svalbard and Franz Josef Land, Barents Sea Large Igneous Province

The opening of the Arctic oceanic basins in the Mesozoic and Cenozoic proceeded in steps, with episodes of magmatism and sedimentation marking specific stages in this development. In addition to the stratigraphic record provided by sediments and fossils, the intrusive and extrusive rocks yield important information on this evolution. This study has determined the ages of mafic sills and a felsic tuff in Svalbard and Franz Josef Land using the isotope dilution thermal ionization mass spectrometry (ID-TIMS) U–Pb method on zircon, baddeleyite, titanite and rutile. The results indicate crystallization of the Diabasodden sill at 124.5 ± 0.2 Ma and the Linnevatn sill at 124.7 ± 0.3 Ma, the latter also containing slightly younger secondary titanite with an age of 123.9 ± 0.3 Ma. A bentonite in the Helvetiafjellet Formation, also on Svalbard, has an age of 123.3 ± 0.2 Ma. Zircon in mafic sills intersected by drill cores in Franz Josef Land indicate an age of 122.7 Ma for a thick sill on Severnaya Island and a single grain age of ≥122.2 ± 1.1 Ma for a thinner sill on Nagurskaya Island. These data emphasize the importance and relatively short-lived nature of the Cretaceous magmatic event in the region.

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.

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

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.