Provenance and Composition of Impactite Sands; AU Drill Core #09-04, Wetumpka Impact Structure, Alabama

Abstract

The Wetumpka impact structure is a Late Cretaceous shallow-marine impact crater about 6 km in diameter located in central Alabama. The target consisted of Upper Cretaceous sediments that were unconformably overlying Piedmont schists and gneisses. An arcuate crystalline crater rim is surrounded on the east and northeast by Upper Cretaceous sedimentary units, on the north by Piedmont basement, and on the west by Quaternary alluvium. There are several shallow drill cores at Wetumpka, including Auburn University drill core #09-04, which penetrated a depth of 217.7 m (715 feet) near the southeastern portion of the rim. The upper ~ 60 m (197 feet) of core is interpreted as a segment of slumped, overturned sedimentary section of megablocks that was formerly on the rim. Below this overturned section are 152 m (500 feet) of impactite sands with sedimentary blocks. The objective of the present project is to determine the provenance of the nearly 152 m (500 feet) of impactite sand in the lower part of drill core #09-04. Thin-sections were made from 43 samples taken from impactite sand intervals in the lower portion of the drill core. Fining-upward trends were detected in eight intervals and this pattern is interpreted as the result of an aqueous settling process. Point-counting and statistical analysis of framework grain characteristics within the loosely consolidated sands indicate that the grains do not originate from a single target unit and provide reasonable evidence that they are derived from a mixture of the sedimentary and metamorphic target units.