WETUMPKA IMPACT STRUCTURE'S CENTRAL BRECCIA LOCALITY, BRECCIA HILL (ELMORE COUNTY, ALABAMA)

Wetumpka impact structure is a Late Cretaceous marine target impact feature located in central Alabama. The total structural diameter is about 6 km, but the inner crystalline rim has a diameter of about 5 km. Wetumpka’s submarine target formations include (in reverse age order): a few m of lower Mooreville Chalk; the clastic paralic Eutaw Formation; the clastic fluvial Tuscaloosa Formation; and basal weathered crystalline Piedmont metamorphic rocks (mainly schists and gneisses). This impact structure consists of three surficial terrains: a crystalline rim; an interior unit (intracrater sediments and broken formations); and an exterior area consisting of structurally disturbed target formations. Intracrater sediments include chalky resurge deposits containing fine ejecta and coarser, polymict impact-breccia resurge deposits. The chalky resurge deposits are located in various low-lying sites in the interior unit and exterior areas, but the resurge breccias are centrally located at higher elevation. The “type locality” for the central breccia occurs at Breccia Hill, an outcrop consisting of about 10.5 m of mixed crystalline and sedimentary megablocks with polymict impact breccias. Based on a measured and described section on Breccia Hill, we observed the following succession. The lower polymict impact breccia at Breccia Hill rests disconformably upon fractured blocks of internally deformed sediments of the Tuscaloosa Group. This lower gray impact breccia contains of a mixture of clasts of both sedimentary and metamorphic provenance and includes a significant component of shocked quartz cobbles and pebbles. This lower breccia is succeeded upward by a reddish impact breccia of mainly sedimentary target derivation. Above this reddish unit is another gray impact breccia with similar features as the lowermost breccia unit. A sedimentary megablock unit succeeds this gray impact breccia and another gray impact breccia overlies the megablocks. The uppermost unit in the Breccia Hill succession is a breccia with a sandy matrix surrounding both large crystalline and large sedimentary clasts. Considering the differences in lithic character of these units and their sharp contacts, a sequence of gravity-driven flows coming from the adjacent rim may explain this succession.