Age and stratigraphy of near-K/T boundary siliciclastic deposits in northeastern Mexico

Biostratigraphic analysis of four Cretaceous/Tertiary (K/T) boundary sections in northeastern Mexico (Mimbral, Lajilla, Mulato, Parida) reveals that all, except Mimbral, contain a thin (5-to 10-cm) layer of Maastrichtian marl above a siliciclastic deposit that was considered to be an impact generated tsunami-deposit. This marl layer contains typical late Maastrichtian planktic foraminiferal assemblages with no apparent size sorting, grain-size gradation, or reworked shallow-water benthic foraminifera, suggesting an interval of normal hemipelagic sedimentation between deposition of the siliciclastic sediments and the K/T boundary. Siliciclastic sediments were deposited prior to the K/T boundary and within the last 170 to 200 k.y. of the Maastrichtian, as indicated by the presence of the latest Maastrichtian index species Plummerita hantkeninoides and Micula prinsii either below, within, or above this deposit but always below the K/T boundary. Deposition of the siliciclastic sediments occurred not as a single event but as a series of events, as indicated by the presence of bioturbation in several layers within unit 3 and at the base of unit 2 of this deposit. Sedimentologic, biostratigraphic, and trace fossil data all point to multievent deposition prior to the K/T boundary, with depositional events separated by scouring and erosion as well as alternating episodes of terrigenous influx and hemipelagic sedimentation. The spherule-rich sediments of unit 1, with its rare glass fragments of composition similar to that of the glass spherules from Beloc, Haiti, represent an unusual feature of the siliciclastic deposit. Whatever the origin of these glasses, the event that produced them, whether impact or volcanism, preceded the K/T boundary.