THE LATE CRETACEOUS/EARLY PALEOGENE RECORD AT BASS RIVER BOREHOLE, NEW JERSEY COASTAL PLAIN

At the Bass River Borehole (New Jersey Coastal Plain Drilling Project, ODP Leg 174AX) the Cretaceous/Paleogene boundary (K/P) lies within an unconformity-bounded, depositional sequence (age ca. 69.1 to 64.5 Ma). A 6-cm-thick spherule layer that contains shocked minerals and an iridium anomaly within this sequence marks the K/P boundary. Calcareous planktonic biostratigraphy indicates that sedimentation was continuous across the K/P and benthic foraminiferal biofacies indicate that 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) across the K/P.

Three significant events are inferred from the depositional record: 1) The K/P event caused by an asteroid impact at Chicxulub, Mexico, 2) a tsunami event following the ballistic fallout of tektites from the Chicxulub ejecta vapor cloud, possibly triggered by massive slumping on the Atlantic slope, and 3) a ~5°C warming of sea-surface temperatures related to the main outpouring of the Deccan Traps in India that began about 500 k.y. and ended about 22 k.y. before the K/P. There is no relationship between the first two events and sea-level change during K/P deposition. The third event probably led to a slight rise in sea-level prior to the K/P due to thermal expansion of ocean waters.

Carbon 13 values of benthic and planktonic foraminifera across the K/P show the well-known carbon isotopic shift that indicates a reduction in oceanic primary productivity, the "Strangelove" ocean. Oxygen 18 values of early Paleogene (Danian) planktonic species indicate that they occupied a narrow range in the water column, suggesting a thin, restricted deep chlorophyll maximum (DCM) layer. Post Danian oxygen 18 values show that a greater vertical range of the water column was used by a greater diversity of species with shallow, intermediate, and deeper-dwelling habitats, indicating a return to pre-K/P oceanic fertility conditions.