Iridium and other element concentrations and magnetostratigraphy across the Cretaceous/Tertiarty boundary in ODP Hole 122-761C

Samples from Ocean Drilling Program Hole 761C, collected on both sides of the Cretaceous/Tertiary boundary have been analyzed for their chemical and mineralogical content. The sediment consists of nannofossil ooze with variable amounts of clay. The boundary is marked by a color change associated with a nearly step-like decrease of the carbonate fraction. Paleomagnetic data and the drop of the carbonate content indicate that a strong reduction of the sedimentation rate occurred at the boundary and persisted for million of years. An iridium anomaly of 80 ng/cm**2, together with overabundances of Cr and Fe, are found in close coincidence with the planktonic crisis. These enrichments can be explained by the infall of =0.16 g/cm2 of Cl-like chondritic material. Co and Ni enrichments and a great quantity of Ni-rich magnetites are also observed in the basal Danian. These elements and minerals excepted, the composition of the insoluble fraction appears to be nearly unchanged across the boundary. Chemical and mineralogical observations support a cosmic origin for the Cretaceous/Tertiary event but do not reveal the presence of any significant impact ejecta.

Benthic foraminifers of the Barrow Group, Northwest Shelf, Australia

Micropaleontological analysis of the Barrow Group of Sites 762 and 763 has been undertaken with a view to determining the stratigraphic age and depositional environment of the unit. The stratigraphic age of the unit is Berriasian-Valanginian at both sites, in line with palynological findings. The unit is interpreted as having been deposited in a marine deltaic environment. Paleobathymetry at Site 763 (proximal) and Site 762 (distal) is interpreted as having been of the order of 100 m and 200-500 m, respectively. Paleontological evidence for the presence of deep-water submarine fans at Site 763 is lacking. The paleobathymetric significance of the observed variations in the benthic foraminiferal populations at Site 763 remains unclear.