Early modification stage dynamics of shallow crater-filling units, Wetumpka impact structure, Alabama

Abstract

Wetumpka, Alabama, USA is the site of a Late Cretaceous, marine-target impact crater structure located on the inner Coastal Plain of Alabama. During Late Cretaceous, Wetumpka was located offshore from a barrier-island shoreline, and the impact target area was a shallow continental shelf of the sea that formed the northern reaches of the Gulf of Mexico. The target consisted of Upper Cretaceous sediments that were unconformably overlying Piedmont schists and gneisses.

This thesis focuses on the systematic analysis of shallow crater-filling sediments by conducting field work to produce a geologic cross section through the northwestern interior of the 5-km diameter Wetumpka impact structure. Thus, the timing of impact-related events and mega- and microscopic features are related in order to understand the dynamics of the syn- and post-impact modification events.

From northwest to southeast, the transect encountered deformed crystalline rim terrain, a zone of crystalline blocks, sedimentary target megablocks, a folded and faulted overturned rim flap sequence (i.e., a trans-crater slide), resurge chalk deposits, and interior polymict impact breccia. Core drilling near the transect indicates that sedimentary target megablocks occur below sections of impactite sand, with resurge chalk deposits resting upon these facies in topographically low or down-faulted areas and an interior impact breccia unit lying above other units. The relative timing of the resurge chalk deposits, impactite sand injection dikes, trans-crater slide, and emplacement the interior impact breccia is determined based on the formative events that generated the units seen in the geological transect. In some respects, the crater-filling sequence at Wetumpka is similar to some other marine impact structures on Earth. All units studied are related to the early modification stage of the Wetumpka impact structure’s development and likely represent the last few minutes in Wetumpka’s early sequence of events.