A PROBABLE NEW CRETACEOUS-PALEOGENE IMPACT BOUNDARY SECTION IN THE UPPER COASTAL PLAIN OF SOUTH CAROLINA

Evidence of the K-Pg impact has been considered lost to erosion in the Atlantic Coastal Plain south of New Jersey. We have identified a 10 cm section of clay capped by a 7 cm-thick spherule bed at the contact between uppermost Maastrichtian and lower Paleogene sediments exposed near Gaston, South Carolina. The spherules exhibit accretionary textures and contain shocked quartz. Stratigraphic placement, along with similarities to other K-Pg boundary sections, leads us to conclude that the section represents a nearly complete sequence of ejecta emplaced across a delta ~1500 kilometers from Chicxulub.

The clay unit can be divided into three parts. The lowermost 5 cm of gray kaolinite, with minor illite/smectite and lepidocrocite, lie above a sharp contact with white kaolinitic sands of the Sawdust Landing Formation. Rare mm-sized clay spherules are observed. The middle 4 cm is composed of dark brown, organic-rich kaolinite with minor amounts of illite/montmorillonite and greater concentrations of lepidocrocite and a trioctohedral smectite (stevensite?). This zone contains abundant carbon and preserves fossil leaves. The uppermost 1 cm consists chiefly of tan flint kaolin.

The clay unit is capped by a razor-sharp contact along which a thin ferricrete has developed beneath a 7 cm-thick orange, sandy, planar-laminated stratum composed of densely accumulated spherules (including doublets, triplets, and splash-form morphologies), typically 1 to 5 mm in diameter. They exhibit accretionary textures and are composed primarily of very angular quartz sand in a matrix of woohouseite-svanbergite-goyazite. Rare rounded quartz grains contain one to four sets of planar deformation features (PDFs) or basal twin planes. Some spherules contain irregular patches of clay containing diverse assemblages of inclusions, including embayed quartz and feldspar, and one example of an unusual C-bearing Ni-V almandine. We interpret the spherules as diagenetically altered accretionary lapilli that contain shocked, unshocked, and devitrified glassy impact ejecta. Trace element analyses, including PGEs, of the sediments and spherules are in progress.

This section could be important for refining models of Chicxulub ejecta emplacement. The lapilli bed also may be a useful analog for understanding the genesis of some Martian spherules.