PETROLOGY OF THE CRETACEOUS OWL CREEK AND PALEOCENE CLAYTON FORMATIONS AT A NEW EXPOSURE OF THE K-PG BOUNDARY IN NORTHERN MISSISSIPPI, USA

Recent road construction in Union County, Mississippi temporarily exposed the K-Pg boundary between the Cretaceous Owl Creek Formation and the Paleocene Clayton Formation. Due to the site’s potential to contain evidence of the Chicxulub bolide impact and resulting tsunami, a team from the University of Mississippi Department of Geology and Geological Engineering was rapidly mobilized to investigate, document, and sample the site before it was covered by continued construction. Our present goals are to determine whether the boundary is accompanied by changes in properties resulting from the impact, as well as to differentiate between primary depositional features and diagenetic alteration. To address these research questions, we began a petrographic study using thin sections made from samples collected throughout the exposed strata. The Owl Creek is dark gray to greenish gray and ranges from muddy micrite to micritic sandstone, following the mixed carbonate-siliciclastic classification of Mount (1985). Grains are dominantly quartz, but glauconite, forams, and high opacity coated grains are common. Intraparticle porosity is present within a limited number of skeletal grains throughout most of the formation, while the upper 25 cm contains abundant intra- and interparticle porosity. The boundary between the Owl Creek and the Clayton is sharp and slightly undulatory. The Clayton bed immediately above the boundary is 15 cm thick, tan, massive, fairly well lithified, and contains abundant whole and fragmental macrofossils. The bed is micritic sandstone with abundant sparry calcite cement. Grains are dominantly quartz and are more angular and more poorly sorted than within the Owl Creek. High opacity coated grains are abundant and glauconite is rare. Highly rounded and spherical rinds of altered glass spherules are common; some are completely filled with spar, some contain porous internal bubble cavities, and some contain only pore space. Many pore-enclosing rinds are fractured and offset, likely as the result of post-burial compaction. Some spherules have visible angular, abraded external margins, which is indicative of transport before deposition; this suggests that the basal Clayton was not directly deposited by the impact tsunami, but rather represents reworked tsunami deposits.