For more than a decade, a number of impact sites have been linked to the mass extinction at the KT (Cretaceous/Tertiary) boundary. The prime candidate today is the Chicxulub Crater in Yucatán Peninsula, Mexico. Recently another potential KT impact scar - the Shiva Crater - has been identified from subsurface data at the India-Seychelles rift margin. The crucial evidence in support of this impact structure comes from the Bombay High field, a giant offshore oil basin in India, and associated alkaline intrusives within the Deccan Traps. The KT boundary age of the crater is inferred from its Deccan lava floor, Palaeocene age of the overlying sediments, isotope dating (∼65Ma) of presumed melt rocks, and the Carlsberg rifting event (chron 29R) within the basin. Seismic reflection data and India-Seychelles plate reconstruction at 65Ma reveal a buried oblong crater, 600km long, 450km wide and 12km deep, carved through Deccan Traps and into underlying Precambrian granite. It represents the largest impact structure of Phanerozoic age. The crater shows the morphology of a complex impact structure and basin, with a distinct central uplift in the form of a series of peaks, an annular trough and a slumped rim. The oblong shape of the crater and the asymmetric distribution of fluid ejecta indicate oblique impact in a SW-NE trajectory. We speculate that a 40km diameter meteorite crashed on the western continental shelf of India around 65Ma, excavating the Shiva Crater, shattering the lithosphere and inducing the India-Seychelles rifting. The crater appears to narrow in the form of a teardrop to the NE or downrange where the ejecta melt rocks were emplaced radially outward by the impact shock. The shape of the Shiva Crater and the asymmetric ejecta distribution mimic those of artificial craters produced by oblique impacts in laboratory experiments. The synchrony and near-antipodal positions of the Shiva and Chicxulub Craters may indicate two alternative modes of their origin. Either, both craters originated from splitting of a larger diameter meteorite, or, large impact on one side of the Earth produced a similar signature on the far side by axial focusing of seismic waves. Since India was ground zero for both an impact and Deccan volcanism, their causal relationships and biotic effects were assessed. It appears that Deccan volcanism began 1Ma before the KT event and was not triggered by the impact. Its origin is attributed to the Deccan-Reunion hotspot. The extensive areal distribution of Deccan Traps is owing to intercanyon flows along the drainage of the Narmada, Godavari and the Cambay basins. During the early stage of Deccan eruption, sauropods, theropods and ankylosaurs flourished in India, but they died out suddenly at the KT impact boundary. Although both impact and Deccan volcanism are hypothesized as contributing to the deleterious environmental consequences leading to biotic crisis at the KT boundary, the impact is suggested as having played the major role as the killing mechanism.
|Number of pages||44|
|Journal||Memoirs of the Queensland Museum|
|State||Published - Dec 20 1996|
- Cretaceous-Tertiary boundary