This Is AuburnElectronic Theses and Dissertations

Wetumpka Impact Structure Modeled as the Exposed Remains of a Large, Shallow-Water, Marine-Target Impact Crater for Analysis and Interpretation of Two Drill Cores Taken from Near the Structure’s Geographic Center




Johnson, Reuben

Type of Degree



Geology and Geography


The early Campanian, Wetumpka impact structure has a raised 7.6-km-diameter crystalline rim of pre-impact metamorphic basement. Filling the region within the crystalline rim is a mélange of impact-related sedimentary megaclasts in a sandy matrix ~100 m thick overlying a fallback breccia of unknown thickness. Just outside the crystalline rim are two impact-related structurally disturbed regions. To date, all published works on Wetumpka depict the crystalline rim as the outermost rim. However, when compared to other marine impact structures, the above characteristics indicate Wetumpka is the deeply eroded remains of a larger impact structure. The principle activity of this study was the description and analysis of two drill cores extracted from near Wetumpka’s geographic center. Drill core was cleaned by abrasive blasting, digitally photographed in high resolution, reassembled according to lithologic characteristics, and described using drill-core-logging software. Ancillary investigations, such as clarifying structures and patterns in the drill cores, determining the positions of the drill cores relative to the impact structure’s central peak, elucidating an apparent intra-crater paleosol, and investigating the age of shock metamorphism added supplementary quantitative data. Results were compared to other marine-target impact structures to model Wetumpka as a shallow-water, marine-target impact structure having a speculative 16.5-km-diameter outer rim in Upper Cretaceous target strata. Structural and morphological features at Wetumpka collectively model very well as a large, deeply eroded marine-target impact structure, and crater-filling materials are consistent with those at other marine-target impact structures. Washback, flowin, and fallout units have been completely eroded away, but a fallback breccia and a mixed surgeback/slumpback breccia are preserved within Wetumpka’s inner basin. No intra-crater paleosol divides these two main crater-filling units, but the fallback breccia shows evidence of reworking by surgeback. Within both drill cores, twelve recognizable facies comprise three correlative units, and several enigmatic structures and patterns were modeled and interpreted. Normal faults within disturbed Upper Cretaceous strata outside the crystalline rim do not cut into the regional metamorphic basement even though pre-impact target sediments were >170 m thick. The impact event was not hot enough to cause much loss of radiogenic 40Ar from shocked muscovite, but it was able to vaporize and/or disintegrate Mooreville Chalk. Wetumpka’s central peak is not exposed, but one drill core may have penetrated this feature’s upper edge at depth.