Cryptoexplosive structure near Wetumpka, Alabama

Abstract

A Cryptoexplosive structure that probably resulted from a meteorite impact is located near Wetumpka, Alabama (long 32°31′42″N, lat 86°14′12″W), along the boundary between the Gulf Coastal Plain and the Piedmont physiographic provinces. Its main features are (1) an approximately concentric structural system about 6.5 km in diameter, its rim formed by metamorphic rocks and surrounded by Cretaceous sedimentary rocks; (2) an arcuate ridge of schist extending two-thirds of the way around the structure, which stands 60 to 150 m above the adjacent Piedmont peneplain; (3) the chaotic orientation of Cretaceous and metamorphic units in the center of the structure, perhaps corresponding to the central rebound area; (4) concentric marginal faults with an estimated displacement of 240 to 300 m; and (5) shock effects in feldspar and quartz crystals in rock units that form the rim. A southward-sloping metamorphic rock surface and a northward-thinning Cretaceous cover afforded a unique geologic setting for the formation and possible preservation of the structure. A shallow marine setting may best explain the shape and distribution of rocks. Nowhere else along the 1,600 km of Coastal Plain—Piedmont boundary is there any faulting of the magnitude described in this report. The age of the Wetumpka Cryptoexplosive structure is post-Mooreville (Late Cretaceous) and pre-terrace (Pleistocene?). The name “Wetumpka Astrobleme” is proposed for this structure.