Abstract:
Evidence of physical disruptions caused by the postulated bolide impact at the close of the Maastrichtian is clearly defined in the record of the K/T boundary (KTB) layer from different sites in the Southern Peninsula of Haiti. Lithologic and biostratigraphic record of the KTB layer from the different sites also show varying degrees of mixing, yielding faunal components within a time range consistent with bioevents characteristic of the traditional boundary zone equivalent to the uppermost part of the Abathomphalus mayaroensis Zone, part of the Guembelitria cretacea Zone, and the Parvularugoglobigerina eugubina Zone, respectively. The calcareous nannofloras also show transitional taxa that concur with the foraminiferal data, and are indicative of the Micula murus and M. prinsii Subzones as well as the CP1a, Cruciplacolithus primus Subzone, of the Early Paleocene. The Boundary layer shows variation in thickness with a maximum of 75cm at the Beloc stratotype, and the topmost part of the main tektite layer is coincident with an iridium peak. Geochemical analyses and radiometric dating have also demonstrated that the spherules are tektites (Premo and Izett, 1991) that can be chronologically related to the impact event recorded at Chicxulub, Yucatan, Mexico, 65 million years ago. Most importantly faint primary sedimentary structures within the boundary layer are constant at all outcrops, but discrete spatial differences exist even within short distances. In addition, in the areas adjacent to the stratotype (Platon Piton and Madame Toussaint) a volcanogenic layer occurs below the main tektite layer assigned to the Chicxulub event, and it also shows conspicuous, as well as cryptic, cross-lamination indicative of complex, multiphase, subaqueous flow processes that affected sedimentation of the layers similar to the KTB layer. Such structures are known to characterize oscillatory wave processes that affect cohesionless sediments. Water motion associated with seiche is the only known modern analog of a subaqueous flow that provides a plausible mechanism to explain how various levels of the water column in a large basin can oscillate to develop the structures observed. Because of the magnitude of the bolide impact, "megaseiches" must have developed in the oceans worldwide, and subsequently more localized "megaseiches" developed during major crustal readjustment. These phenomena may thus explain the heterogeneity of patterns and faunal discrepancies observed at KTB sites of different water depths worldwide. The structures represent a record of water movement and resuspension of sediment at different times. As observed in smaller-scale modern seiche, various oscillatory modes controlled the duration and attenuation of the water movement, the magnitude of bottom traction and resuspension that led to complex sedimentary structures and reworking of the microfossils.