Results from aeromagnetic survey of the Alabama Piedmont, 1974, Neathery et al., showing deep negative anomaly associated with the Wetumpka impact structure (shaded area). Contour lines are in nanoteslas (nT).
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Results from aeromagnetic survey of the Alabama Piedmont, 1974, Neathery et al., showing deep negative anomaly associated with the Wetumpka impact structure (shaded area). Contour lines are in nanoteslas (nT). |
The southwest quadrant of the Wetumpka impact crater is faulted to form a series of graben-like structures defined by a long, narrow linear outcrops belts of Mooreville Chalk (actually a marl). There are two graben structures identified near the Wetumpka impact crater: an interior graben, which is closer to the central crater area, and an exterior graben, exposed on U.S. 231. During the initial studies of Neathery et al. in 1970-1972, excellent exposures on the Interior Graben were visible along the east side of Jasmine Hill Road from the location of the entrance to Jasmine Hill Center south for about 200 yards. Since these intial studies, much of the area has been urbanized and the exposure has been covered up.
Exposures of the grabens are preserved by a few relatively unweathered outcrops of the Mooreville marl and its characteristic grayish-yellow-brown or olive gummy clay residuum containing white caliche nodules. It is notable that no rock of the Arcola Limestone Member is preserved here or elsewhere. Perhaps the impact is pre-Arcola age.
At this stop, the Mooreville Chalk is bounded on both sides by clastic units of the Eutaw Formation. The Eutaw Formation consists of reddish tan sand and clay. Several hundred yards to the southeast and northwest, the Mooreville is bounded by the Gordo Formation of the Tuscaloosa Group. Scattered blocks of marl contain fossil casts and fossils indicative of the Mooreville.
The graben structures may have been formed by extensional adjustments during the central-core rebound. It is not known if the grabens penetrate the crystalline basement of only affect the Cretaceous cover rock. Aeromagnetic data indicate a northeast-trending magnetic low coincident with the general configuration of the crater. Magnetic isogal relief on the structure is 1,548 gammas. A small magnetic low on the northeast side of the craer area may represent a graben feature in the crystalline basement but there is no apparent surface expression of the feature. The grabens on the southwest side of the crater do not have any magnetic signature and may be only in the Cretaceous cover rock.
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