IMPACTITE SANDS OF WETUMPKA IMPACT CRATER, ALABAMA

Wetumpka impact structure, Alabama, is a well-preserved, Late Cretaceous marine target feature of about 5 km in diameter that is situated in the inner Gulf Coastal plain. The age of impact is at or near the Santonain-Campanian boundary. Formations penetrated by this impact include the local pre-Mesozoic, metamorphic basement rock (mainly the Emukfaw schist and gneiss), Tuscaloosa Group (Cenomanian-Turonian) and the overlying Eutaw Formation (Coniacian-Santonian). Previous work has shown that the crater moat-filling impactite sands are composed of a mixture of all target rocks and that the sedimentary component of the sands is dominant. Drill cores contain blocks of target sedimentary rock (Tuscaloosa and Eutaw) up to several meters in diameter, but no metamorphic blocks. Smaller sedimentary clasts within the impactite sands have distinctive angular shapes with penetrative features such as marginal divots and holes, which are likely related to explosive fragmentation of clayey facies. The general texture of impactite sand matrix is an homogenous mixture of clay to coarse sand, and the color is usually light tan to light grey. Shock features are not evident in framework silicate grains on the impactite sands. Impactite sands are interpreted to have been formed by a slurry of disintegrated target rocks with larger fall back blocks and fragments that filled the transient crater during the early modification phase of this shallow marine impact structure.