Paper No. 8
Presentation Time: 9:45 AM
THE SHIVA CRATER: IMPLICATIONS FOR DECCAN VOLCANISM, INDIA-SEYCHELLES RIFTING, DINOSAUR EXTINCTION, AND PETROLEUM ENTRAPMENT AT THE KT BOUNDARY
Evidence is accumulating that there were multiple impacts across the Cretaceous-Tertiary boundary such as the Chicxulub crater in Mexico, Boltysh crater in Ukraine, Silverpit crater in North Sea, and the Shiva crater offshore western India. Many of these impact structures are among the most productive hydrocarbon sites on the planet. Among these, the submerged Shiva crater is the largestabout 600 km long and 400 km wideand has proved to be a rich source of oil and gas. It has the morphology of a complex-type impact crater having a central uplift as a series of peaks, surrounded by an annular trough and a collapsed outer rim. The central uplift (Bombay High) has a core of Neoproterozoic granite that rebounded upward for more than 5 km. Probable pseudotachylite veins are observed within the granite target rock that may be linked to the impact melting event. The age of the crater is inferred from its brecciated Deccan lava floor and the overlying Paleocene sediments within the basin, isotopic dating of the presumed ejecta melt, and the magnetic anomaly of the Carlsberg Ridge. Concentric geophysical anomalies, seismic reflection, and structural and drill core data indicate the possible impact origin of the Shiva structure. The KT boundary sections in India, though mostly destroyed by the Deccan lavas, have yielded an iridium anomaly, iridium-rich alkaline melt rocks, shocked quartz, nickel-rich spinels, nickel-rich vesicular glass, sanidine spherules, fullerenes, glass-altered smectites, and tsunami deposits. The last dinosaurs occur immediately below the iridium anomaly layer. The synchroneity of the Deccan Traps with the KT boundary, their geographic proximity with the crater, and the occurrence of a thick shocked quartz layer below the lowermost lava flow strongly imply that the Deccan volcanism may have been triggered by the Shiva impact. The impact was so intense that it led to several geodynamic anomalies: it sheared and deformed the lithospheric mantle across the western Indian margin and contributed to major plate reorganization in the northwestern Indian Ocean. This resulted in a 500-km displacement of the Carlsberg Ridge and initiated rifting between India and Seychelles. The oblique impact may have generated spreading asymmetry, possibly linked to the sudden northward acceleration of the Indian plate in Early Tertiary.