BURIAL AND EXHUMATION HISTORY OF WETUMPKA IMPACT STRUCTURE, ALABAMA

Wetumpka impact structure is a well-exposed and relatively well-preserved Late Cretaceous impact crater, which is located within the inner coastal plain of Alabama. Wetumpka has a 5-km east-west diameter, but an exterior disturbed (partial annular brim zone) terrain extends its north-south diameter to 7.6 km. The U/Th-He age date for Wetumpka is 84.4 +/- 1.4 million years. At that time, sea level was about 250 m above present, and the crater rim was an island. According to the current global sea level chart, by about 75 million years ago, sea level had declined to the point where the area of Wetumpka impact structure would have been dry land. Wetumpka impact structure was then progressively buried in sediment, which was being shed from the adjacent Appalachian Mountains. By early Cenozoic, a land surface formed across the inner coastal plain area of Alabama, which is known as the “high terrace.” At that point in time, Wetumpka impact structure lay below the sedimentary cover of the high terrace. Major coastal plain rivers such as the Coosa, Tallapoosa, and Chattahoochee flowed across this land surface, and left behind a record of fluvial sedimentation and paleosols. Part of this ancient land surface is still visible today, as it is not completely eroded away, but it is not recognized as a stratigraphic unit. Widespread sand and gravel deposits from major river and stream systems characterize this land surface, and there are notable outcrops of this high terrace deposit in a few places in and around Wetumpka impact structure, particularly at high elevations. During late Miocene, an orogenic uplift of the southern Appalachians caused extensive erosion of the area, and thus reorganization of major rivers, and the exhumation of Wetumpka impact crater. This orogenic event, which may be related to detachment of Appalachian mountain roots, is reflected in stream capture (e.g. the Coosa and Tallapoosa rivers) and development of antecedent streams such as Corn Creek which cuts through the rim of Wetumpka impact structure. In this report, we estimate that Wetumpka was buried in sediment from during most of Cenozoic, up to late Miocene (c. 7-8 million years ago). This burial period helps account for the well-preserved nature of the crater and its present exposure.