Ocean Drilling Program, United States

The Cretaceous-Tertiary (K-T) boundary interval recovered by the ODP Leg 171 at Site 1049 (Blake Nose, NW Atlantic) contains a thick (9-17 cm) spherule bed marking the boundary. The spherules are mainly perfect spheres with a lesser proportion of oval spherules. They usually range from 100 to 1000 mu m. This bed represent the diagenetically altered impact ejecta from Chicxulub and further supports this structure as the site of the K-T impact. Mineralogical and geochemical investigations indicate that impact-generated glass was altered to smectite. Transmission electron microscopy observations revealed in some spherules that smectite is forming from a Si-rich or Ca-rich material, which could suggest a precursor similar to Haitian glasses. The variable thickness and the presence of some Cretaceous planktonic foraminifera and clasts of Cretaceous chalk suggest reworking of the ejecta material. However, the spherule bed confirms that a large volume of the Chicxulub ejecta material reached the Blake Nose Plateau.