Provenance of a boulder-bearing impact breccia from surface outcrops and a drill-core from the Wetumpka Impact Structure, Elmore County, Alabama

Abstract

The Wetumpka crater is a shallow marine impact structure located in the Elmore County, Alabama. The impact event occurred at approximately 85 Ma, during Late Cretaceous. The target was a shallow marine environment with a water depth of about 30 m or less and the lithology consisted of soft sediments belonging to the Tuscaloosa Group and Eutaw Formation, and crystalline pre- Cretaceous Appalachian quarzitic schistose and gneissose basement rocks. The rim of the crater is made of the same crystalline Appalachian rocks as the local basement, whereas the crater floor consists of a mixed lithology of target sediments of the Tuscaloosa Group and Eutaw Formation, which form the trans-crater slide, are the provenance of the crater-filling impactite sands, and a boulder-bearing breccia deposit. The present study was aimed at understanding the mode of emplacement of this breccia unit, which occurs in the central crater floor area and resembles polymict proximal ejecta deposits as in other craters. In the present study, outcrop mapping, drill- core logging, and petrographic studies were completed to understand the nature and origin of this breccia unit. Field mapping was done to characterize the breccia deposit and identify and map large oriented boulders in the deposit. Petrographic analyses indicated poorly sorted nature, which is characteristic of mass-movement deposition such as a debris flow. Possible shock features such as planar fractures in quartz, kink-banded micas, as well as potential impact-generated spherules were identified in thin sections and analyzed with electron microprobe. AU scientific drill core #09-02 was logged in order to characterize the subsurface nature of the breccia. All results indicate that the breccia is very poorly sorted with clasts ranging in size from less than 0.5cm to more than 10m. Water did not play any significant role in the formation or emplacement of this breccia unit. Presence of large boulders in the breccia is best explained by a large mass movement of proximal ejecta from the crater rim to the crater floor during early modification. The oriented nature of the boulder sized clasts has been attributed to rotation during mass transport of the boulders in to the crater center. The emplacement of this breccia is interpreted as the last impact-related process to occur before the marine water resurge with accompanying deposition of chalk.