EVIDENCE OF CHICXULUB IMPACT RECORDED FOR THE FIRST TIME IN ARKANSAS

Evidence of the end-Cretaceous impact has been widely observed in the eastern Mississippi Embayment and northeastern Mexico. Surprisingly, given its proximity to the impact site, indicators of the Chicxulub impact are missing in the western Mississippi Embayment between the sites at Brazos River, TX and Crowley’s Ridge, MO. This study documents impact spherules and a possible tsunami deposit related to the Chicxulub impact for the first time in Arkansas. A nine meter section was exposed along the Ouachita River near Malvern, AR that includes the Arkadelphia Formation and the base of the Midway Group. The Arkadelphia Formation is composed of thick beds of marl interbedded with thin beds of limestone. The uppermost limestone contains large pieces of broken shell material and phosphatic nodules, consistent with the vertebrate bed described by Becker et al. (2006). The occurrence of Discoscaphites iris indicates an uppermost Maastrichtian age, which correlates with the CC26b nannofossil sub-zone. The contact with the overlying Midway Group is sharp. A clastic unit is observed at the base of the Midway consisting of a laminated, poorly sorted, glauconitic, lithic quartz sand with impact spherules (0.5-2mm diameter) scattered throughout. Shell fragments are common along with shark teeth and phosphatic nodules. The impact spherules from the clastic unit were analyzed with SEM-EDS and XRD and found to have a composition of smectite with calcite infill. This, in addition to a globular internal structure, is consistent with impact spherules found throughout the Gulf Coast. The sand is normally graded with rip-ups of Arkadelphia lithology (2-5cm diameter) occurring at the base. This supports the interpretation that the clastic unit was formed as a result of a high-energy event such as a tsunami. Our study helps complete the regional picture of the impact of the end-Cretaceous event on the Gulf Coast.