SEA-LEVEL CHANGE ACROSS THE CRETACEOUS-TERTIARY BOUNDARY, A CHIMERA?

Major regression of sea-level has been inferred at Cretaceous-Tertiary (K-T) boundary sections in the Gulf Coastal Plain where some workers place a sequence boundary. However, the discovery of the K-T asteroid impact at Chicxulub, Mexico and continuous cored material from New Jersey yield new insights that requires reinterpretation of sea-level across the boundary. The devastating effect of a high-energy impact (earthquakes, tsunami, ejecta) caused features of deposition (ejecta deposits, tsunamites) and erosion (surface of erosion, slumps) at the K-T boundary in the Gulf Coast. This has made it difficult to determine whether the impact also coincided with a significant change in sea-level. The record in New Jersey is clearer where the K-T boundary lies in the Bass River borehole within an unconformity-bounded, depositional sequence (age ca. 69.1 to 64.5 Ma). A 2.2 m.y. hiatus separates this sequence from underlying Campanian sequences while a ca. 1.5 m.y. hiatus separates Danian Zone P1a from Zone P1c and younger sequences. A 6-cm-thick spherule layer, marks the K-T at this site. Benthic foraminifera indicate that during deposition of the K-T sequence relative sea-level fell from 100-150 m above present sea-level in the lower part of the sequence (transgressive systems tract) to ca. 50 m (highstand systems tract) at the K-T. Three significant events are inferred from the K-T depositional record at Bass River: 1) a ~5°C warming of sea-surface temperatures perhaps began about 500 k.y. and ended about 22 k.y. before the K-T; this warming may be related to the main outpouring of the Deccan Traps in India, 2) the K-T event was clearly caused by an asteroid impact at Chicxulub, and, 3) a tsunami event influenced the NJ margin, possibly triggered by massive slumping on the Atlantic slope. There was no significant change, if any, in sea-level across the K-T.