STRUCTURE-FILLING BRECCIAS OF WETUMPKA IMPACT STRUCTURE (ALABAMA): THE RECORD OF TWO DRILL CORES

Wetumpka impact structure (7.6 km) is a crescent-shaped impact feature in the inner coastal plain of Alabama. Late Cretaceous impact occurred in 30-100 m of sea water and a few 10s of km offshore. Target stratigraphy included < 60 m of unconsolidated sandy Upper Cretaceous strata and underlying pre-Cretaceous crystalline basement (mainly schists and gneisses). In 1998, two core holes were drilled a few 100 m apart at crater center, each to a depth of ~ 200 m. Well 1 (Schroeder) was spudded in the structure-filling unit (the ‘Wetumpka Mélange'), which consists of boulders and blocks of sedimentary target formations within a sandy matrix of disaggregated target strata. This mélange has been interpreted as an early slump-and-resurge crater-filling deposit due to rim collapse. At ~ 100m depth in well 1, there is an abrupt lithologic change to underlying impactites, which consist of target rock blocks and breccias. The underlying impactites may be divided into two units, I and II. The upper impactite unit I (~ 51 m thick) has a fine-medium polymict breccia matrix and includes boulder- and block-sized (< 7.5 m), internally deformed sedimentary and crystalline target rock fragments. Crystalline target rock blocks dominate the upper part of impactite unit I, and sedimentary target rock blocks dominate in the lower part of unit I. The lower impactite unit (II) consists of impactite sands, from disintegrated sedimentary and crystalline target formations, including boulders and blocks (< 8 m). The boulders and blocks in this lower part are almost entirely from sedimentary target units, but some boulders are from crystalline basement. Both impactite units are crudely graded with respect to boulder and block fragments. Well 2 (Reeves) is much less complete than well 1 and shows some stratigraphic differences and similarities. In well 2, there are some intervals of relatively fine polymict breccia (resedimented ejecta?) intercalated with the mélange. The impactite interval in well 2 consists entirely of material much like the upper part of unit II in well 1. As in well 1, the impactites are crudely graded and the larger target rock blocks (~ 6-7 m) are near the top of the impactite interval. We provisionally interpret the impactite stratigraphy as being related to different processes near the central peak during early phases of crater modification.