Abstract

The Wetumpka shallow marine impact structure (about 3.7 mi [6 km] in diameter) is a good field laboratory for study of both sedimentary crater-filling breccias and fine-grained marine resurge facies. This paper focuses on the resurge facies. Formed in relatively shallow water during the initial stages of deposition of the Upper Cretaceous Mooreville Chalk, the Wetumpka marine impact structure has a mixed-target stratigraphy of Upper Cretaceous unconsolidated sediments, several hundred feet in thickness, that overlies crystalline basement rock. Utilizing a shallow drill-core of about 298 ft (90 m), crater-filling mega-slumped sands and their overlying glauconitic, calcareous mudstone resurge unit have been studied in detail. This paper reports on a sedimentologic and stratigraphic investigation of the resurge facies based on core description, adjacent outcrop description, thin-section petrology, and x-ray computed tomography. Results from x-ray computed tomography of cores have revealed internal, inclined bedding structures, brecciation of penecontemporaneous sea floor mud clasts, and other features that are revealing the hydrodynamic genesis of these deposits. This gives us a better understanding of the mode of emplacement of the impact-derived, fine-grained resurge facies at Wetumpka, and by extension, at other marine impact structures.