 | 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 following information is excerpted from three papers included in The Wetumpka Impact Structure and Related Features, a guidebook prepared for the 1997 annual meeting of the Southeastern Section Geological Society of America by Neathery et al., 1997 .) Guidebook purchasing information
Anyone driving north on US 280 from Montgomery has seen the northwest rim of the impact crater jutting from beneath sandy coastal-plain and alluvial cover on the highway's eastern side as one enters Wetumpka's city limits. A small forest of transmitting towers crowns Bald Knob, highest point on the eroded northwest rim (elevation ~ 180 m or ~ 600 ft). The impact-crater rim, breached on the south and southwest, is clearly visible on both the USGS 7.5-minute Wetumpka quadrangle. In false-color satellite imagery from a 920-km orbit, the impact crater is clearly visible as a dark-red spot (see State of Alabama Uncontrolled ERTS-1 Mosaic by Svehlak and Sapp, 1972-3). Wetumpka's impact crater center is located at 32 degrees 31' 42" N, 86 degrees 14' 12" W (Neathery et al., 1976).
The horseshoe-shaped structure provides morphological evidence for identifying the Wetumpka feature as an impact crater (Neathery et al., 1976a). Neathery et al. (1976a) speculate that the rimless southwestern part of the structure is the result of the impact occurring in the submarine environment the near the ancient shoreline (see Figure 3 from King's paper). An impactor having a low-angle trajectory is consistent with the arcuate rather than circular form of the Wetumpka feature (Melosh, 1989). East-west trending faults along the structure's open end are interpreted to have resulted extensional rebound and gravitational sliding of sediments into the central area. Concentric or arcuate bounding faults have been observed on the outside margins of the Wetumpka structure (Neathery et al., 1976a) and may represent breaks (detachment surfaces) commonly found in association with crater impacts (Melosh, 1989).
Indications of shock metamorphism are considered the only indisputable evidence for an impact origin (Koeberl and Anderson, 1996). Some microscopic evidence for impact-related structures within rocks of the Wetumpka structure is presented in Neathery et al.(1976a), but additional efforts to search systematically for such evidence have yet to be pursued. Photomicrographs interpreted to illustrate shock features are shown in Neathery et al. (Fig. 6; 1976).
Gravity and seismic studies of terrestrial impact craters have successfully delineated crater diameter, extent of lateral disturbance, central uplift (if present) and crater depth at a wide variety of scales (e.g., Plescia, 1993; Sharpton et al, 1993; Krischner et al., 1992; Jansa et al., 1989). Published aeromagnetic, aeroradioactivity and gravity maps show moderate to distinct northeast-southwest trends that coincide with the structural trend of folded schists and gneisses (Neathery et al., 1976b; Steigert, 1982).
| 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). |
Although these regional maps show local anomalies over the Wetumpka feature they lack sufficient detail to provide good evidence that the structure was formed by an impact. In 1994, a detailed gravity study was made over the Wetumpka feature to determine whether a local anomaly, consistent with that seen at other impact craters, exists. Results from survey.
The following is from The Wetumpka Impact Structure and Related Features, a guidebook prepared for the 1997 annual meeting of the Southeastern Section Geological Society of America by Neathery et al., 1997 .
The following paragraphs are excerpted from "The Wetumpka Impact Crater And The Late Cretaceous Impact Record" by Dr. David T. King found in The Wetumpka Impact Structure and Related Features, a guidebook prepared for the 1997 Annual Meeting of the Southeastern Section Geological Society of America by Neathery et al., 1997 .
The Wetumpka structure in Alabama is believed to be a 6.5 km-diameter complex crater that likely formed during the early Campanian (81.5 +/- 1.5 Ma). Wetumpka had an estimated original rim height of 210 to 360 m, and the center may have experienced an estimated 59 m of stratigraphic uplift resulting from impact. Wetumpka's impact probably occurred in 0 to 150 m of water and 0 to 50 km offshore from a barrier-island coastline.
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| Late Cretaceous Paleogeography North America | Location of impact crater | View of possible Alabama coastline during Campanian |
The impactor, a stoney and (or) iron asteroid or meteorite, is estimated to have been about 350 m in diameter. When the asteroid vaporized approximately 10 -2 sec after surface impact, energy equivalent to between 10 2 and 10 3 MT of TNT (or between 4.2 x 10 24 and 4.2 x 10 25 erg) was released and thus opened a 3.3 to 4.2 km-diameter transient crater within approximately 11.5 sec. On a vegetated plain, both atmospheric peak-overpressure wave and infrared flash-burn combusion due to impact would have devastated a region estimated to have comprised 2.15 to 9.95 x 10 5 hectares (or 830 to 3,840 mi2). In shallow seawater, this impact would have generated a tsunami (giant sea wave) estimated to have been as much as 239 m high at a distance of five asteroid radii (875 m) from the target center. Terrestrial surface-wave magnitude of seismic shock-wave energy is estimated to have been 8.4 to 9.0. Wetumpka's impact-crater melange, at least 25 m thick, was likely formed by mass movement from the transient crater rim commencing after the first 18 to 21 sec. Wetumpka's horseshoe-shaped crater rim and associated interior northeast-trending aeromagnetic anomaly are evidence of an oblique impact by a cosmic object arriving upon a low-angle trajectory.
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