ENVIRONMENTAL AND BIOTIC EFFECTS OF DECCAN VOLCANISM IN MEGHALAYA, NE INDIA

Um Sohryngkew in Meghalaya, NE India, boasts the most complete Cretaceous-Tertiary boundary (KTB) transition in India. All critical global K-T markers are present, including a major Ir anomaly, negative carbon-13 shift, mass extinction and evolution of first Danian species. In addition, this section also provides evidence of the detrimental effects of Deccan volcanism upon marine life leading up to the mass extinction. Sediment deposition across the K-T transition occurred in a shallow but fluctuating marine environment marked by a succession of conglomerate, glauconitic sandstone, sandy shale, calcareous shale and shell beds, alternating shale and limestone layers and rare coal pockets. Clay assemblages dominated by kaolinite and illite indicate humid conditions, whereas the high detrital content indicates high influx from nearby continental areas. The K-T boundary (zone P0) is in a silty-sandy shale with a 1 cm thick “rust colored” clay layer that contains anomalies in platinum group elements (PGEs) with high concentrations in Ir (11.8 ppb), Ru (108 ppb), Rh (93 ppb), Pd (75 ppb) and Pt (86 ppb). High Ir concentrations suggest a cosmic source, but high Pd and Pt values also suggest a significant volcanic input in PGEs, opening the possibility for potential twin effects of volcanic and cosmic events in the K-T mass extinction.

In addition to the PGE anomalies, the KTB is identified by the negative shift in carbon-13 values that marks this boundary globally, disappearance of most Cretaceous planktic foraminifera, the nannofossil Micula prinsii and dynoflagellates Dinogymnium and Alisogymnium. In the latest Maastrichtian M. prinsii zone planktic foraminifera are highly impoverished and only the disaster opportunist Guembelitria cretacea thrived under eutrophic conditions. These pre-KT high stress conditions correlate with the main Deccan volcanic phase in C29r prior to the KTB and directly link Deccan volcanism to the K-T mass extinction and Ir anomaly. Whether the PGE anomaly is more cosmic than volcanic in origin remains to be determined.