WAS THE CRETACEOUS-TERTIARY TRANSITION INITIATED BY A LARGE BOLIDE IMPACT IN THE PROTO-PACIFIC OCEAN PRIOR TO THAT NEAR CHICXULUB ON THE YUCATÁN PENINSULA?

Although the Cretaceous-Tertiary (K-T) transition is generally accepted as having been caused by a single large bolide impact at ~65 Ma near Chicxulub on the Yucatán Peninsula, a ‘comet shower’ or multiple impacts hypothesis has also been proposed to explain multiple extinction pulses in the Late Cretaceous. The contributory effects of contemporaneous Deccan volcanism and sea level changes also remain controversial. Spherule layers apparently below the K-T boundary (Chicxulub impact layer) in northeastern Mexico and the Badlands of South Dakota have been interpreted as ejecta related to an earlier impact or impacts. Moreover, cristobalite spherules have been found 2 m below the K-T boundary in piston core GPC-3 from the northeastern Pacific Ocean. These spherules are dispersed throughout a highly disrupted interval of sediments (2 m thick), and are here interpreted as ejecta from a nearby oceanic impact farther to the east on what is now subducted crust. Disruption of sediments and dispersal of the cristobalite spherules was probably caused by megatsunami waves associated with the impact. Such waves (initially up to 4 km high) would cause erosion of the sea floor near the impact site and on topographic highs at greater distances, and might have caused the major hiatus at the K-T transition which is most pronounced in DSDP and ODP cores from the northern Pacific Ocean. The megatsunami waves might also account for an abrupt anomaly in the seawater 87Sr/86Sr ratio preceding the K-T boundary by washing vast amounts of 87Sr-rich soils from the continents into the ocean. In addition, erosion of the shallow continental margins, or deposition of sediments related to higher energy environments, by the waves might have left a stratigraphic record that has been erroneously interpreted as indicating a rapid regression and transgression pulse in sea level at that time.