FIRST RECORDED OCCURRENCE OF IMPACT SPHERULES AT THE K/PG BOUNDARY IN MISSISSIPPI

Although evidence of a bolide impact at the K/Pg boundary has been observed in the northern Gulf of Mexico in Texas, Missouri, and Alabama, such evidence has never been reported from Mississippi. Two sites from Union and Chickasaw counties, Mississippi, have yielded the first recorded impact spherules from that state. The section at Union County comprises the top 1.75 m of the Owl Creek Formation and bottom 1.5 m of the Clayton Formation. The Owl Creek is composed of a muddy to silty very fine sand that includes the ammonite Discoscaphites iris, marking the uppermost Maastrichtian. There is a sharp contact with the overlying Clayton Formation, which consists of two distinct units. Above the sharp contact is a muddy, poorly sorted quartz sand which ranges from 10 to 15 cm in thickness, with whole to fragmentary Cretaceous fossils. Abundant impact spherules that are 0.5 to 1 mm in diameter are interspersed within the unit. All are hollow containing multiple smaller globules, consistent with other K/Pg impact spherule sites. The spherules and the shell material have been altered to clay. SEM-EDS analysis suggests that the composition of altered spherules is smectite. Above this spherule rich unit the Clayton Formation is composed of a massive clastic quartz rich sand devoid of fossils and impact spherules. The second locality in Chickasaw County is represented by a similar section. The base of the section is characterized by a marl that makes up the Prairie Bluff Chalk. Discoscaphites iris is common at the top of the section suggesting it is temporally equivalent with the Owl Creek Formation to the north (uppermost Maastrichtian). There is a sharp contact with an overlying muddy, poorly sorted quartz sand 5 cm thick. Clay spherules, mostly 1 mm in diameter, are apparent, as well as abundant fragments of altered shells material similar to the more northerly section. Deposited above this is a 1.5 m thick cross-laminated fine sand with no mud. This clastic unit is very similar to others reported around the Gulf Coast and Mexico such as La Ceiba and El Mulato. The impact spherules directly below the clastic units at both sites strengthens the interpretation that these layers are tsunami deposits related to the Chicxulub impact, or shelf collapse shortly after the impact, and not transgressive sands.