SEDIMENTOLOGY, PALEONTOLOGY, AND GEOCHEMISTRY OF THE K/T BOUNDARY IN ODP HOLE 1259B, DEMERARA RISE

Drilling during Ocean Drilling Program (ODP) Leg 207 recovered six K/T boundary sections that included ejecta. Three of these experienced only minor drilling disturbance and preserve a beautiful record of sedimentologic and paleontological changes across the boundary. We had thin sections made from a continuous, 22 cm long slab from ODP Hole 1259B that spans the upper 8.5 cm of the Maastrichtian through the lowest 13.5 cm of the Danian (including a basal, 1.9 cm thick ejecta bed).

The sequence is remarkable for how closely it matches the stratigraphy expected given an impact-induced mass extinction. Cretaceous foraminifera in a carbonate-rich matrix are abundant in the Maastrichtian and continue to be found within the clays of the ejecta bed. We have yet to identify any Cretaceous taxa from above the ejecta layer. The Tertiary assemblage consists of small, thin-walled forms including Guembelitria, Parvularugoglobigerina, and Eoglobigerina. Changes in chemistry across the boundary are well illustrated by major and trace element X-ray maps obtained by electron microprobe. The ejecta bed is normally graded (spherules decrease from 2 mm to 0.3 mm in diameter across its 1.9 cm thickness) and sits on a 1-2 mm thick layer of Maastrichtian chalk that shows evidence of dewatering and minor soft sediment deformation. Sulfides (pyrite, chalcopyrite, and galena) occur selectively in the bottoms of some spherules near the base of the ejecta bed, between spherules in the top 5 mm of the ejecta layer, and as fill in spherules immediately below this zone. Small radiating clusters of rectangular sphalerite crystals that cut across spherule edges are present 7 – 10 mm below the top of the ejecta bed. Unfortunately, such evidence for diagenetic alteration complicates interpretation of boundary chemistry. We attribute the unusually well preserved physical sedimentology and the apparent absence of reworking to Demerara Rise having been close enough to the Yucatan to accumulate a ~2 cm of ejecta while being far enough away (and perhaps sheltered by the curve of northern South America) to not have been greatly effected by impact induced waves.