STOP 1 Northwest Flank of the Impact Structure

Rocks exposed here are coarse- to medium-grained biotite-garnet-feldspathic schist and gneiss (metagraywacke), quartz-graphic-muscovite schist and graphitic-muscovite schist that make up most of the rim of the Wetumpka impact crater and the surrounding basement rock. A light colored granitic rock is found occasionally on the eastern side of the structure. The general strike orientation of the metamorphic rocks of the northern Alabama Piedmont is northeast and the dip is to the southeast. At Stop 1, the rocks have a pronounced northwest dip (60 NW) and a northerly strike (N 16 E) orientations that are contrary to the general trend. Around the rim, the strike directions follow the circular trend of the crater and the foliations dip outwardly from the center.

STOP 2 Bald Knob, North Rim

The road to the top of Bald Knob follows along apart of one of the ridges that define the northwest rim of the structure. Note the steep slopes on the north side of the road as you drive toward Bald Knob. These slopes are parallel to the foliation of the ridge rock as seen at Stop 1. The south side of the ridge is very steep and represents weathered fault scarps in the interior of the structure.

The Bald Knob stop provides a panoramic view of the concentric nature of the impact crater. Trees and vegetation have grown up during the past 20 years and now partially block the view. The above picture was taken from Bald Knob. The water tower is approximately 4 km away and is included to give the viewer a sense of scale. Elevation of the rim ranges from 308 m to nearly 462 m above sea level except where it is missing in the southwest quadrant of the structure. The elevation of the floor of the crater averages ~ 100 m. Near the center of the structure is a hill standing approximately 186 m. This feature coincides with the general position of a central rebound area of an impact structure.

If you look away from the Wetumpka impact crater, you may note the topographic elevation of the surrounding rock: to the north the peneplain of the Piedmont is clearly evident; and to the west and south you see the flat to gently dipping strata of the Cretaceous sedimentary units.

The Rim

The exposed metamorphic rock units of the rim are primarily coarse- to medium-grained biotite-garnet-feldspathic schist and gneiss (metagraywacke), quartzite, quartz-graphic-muscovite schist. Metasedimentary rocks have been found northeast of the structure. Foliations of the rim rocks dip outward from the center to define an eroded circular dome-like structure.

Locally along the southern rim, a complex set of foliation attitudes suggest a possible pre-impact fold system was imprinted on the metamorphic rocks, similar to those seen farther northeast in the main body of the crystalline rock. Several concentric faults are suggested by the presence of steep scarps that face inward toward the center of the structure. Surface expressions of these faults are poor. However, brecciated zones and the close spacing of several mappable stratigraphic units exposed in stream beds and gullies indicate their location. Total relief of the rim is approximately 200 m above the Cretaceous-Piedmont nonconformity.

Central interior of the impact crater

Most of the sedimentary rock strata exposed along the road is of Quarternary or Late Cretaceous age. Many of the smaller hills contain chaotically oriented metamorphic rock fragments up to 30 m across. Good exposures of the rock strata in the interior part of the impact structure are few and difficult to see. The best exposures are found in nearly inaccessible borrow pits where steeply dipping, chaotically oriented unconsolidated sediments can be observed (STOP 3). Several exposures occur along the central access road and illustrate, to a small degree, the complex relationships among the various rock strata contained within the central interior part of the structure. Locally thick zones composed of fragmental blocks of metamorphic rock mixed with sedimentary material may represent massmovement (mega-slump) deposits off the horseshoe rim. In some areas the strata appear flat-lying. In other areas, the strata are vertical or are steeply inclined and may be overturned.

The floor of the structure consists of irregular, chaotically distributed blocks of sedimentary and metamorphic rock, which are locally covered with Late Pleistocene-Holocene alluvial sands and gravels. Sedimentary units recognized in the floor area includes the Coker and Gordo Formations of the Tuscaloosa Group, the Eutaw Formation, and the Mooreville Chalk.

At the time of the initial investigation in 1972, only five deep wells had been drilled in the interior of the crater. These wells penetrated unique stratigraphic relationships and abnormal rock-unit thicknesses, which Neathery et. al. (1976) interpreted to indicate the complex geometry of the observed structure. Neathery et. al., (1976) interpreted all these phenomena to indicate post-deposition disruption of the soft sediment deposits in response to a meteorite impact. The following three stop descriptions provide a brief look at the complex nature of the interior structure.

STOP 3 Abandoned borrow pit and associated road cuts

Mooreville Chalk

Residuum exposed in ditches along private roads is from the underlying Mooreville Chalk. Note the characteristic olive residuum and the white caliche nodules.

Highly inclined sedimentary beds in abandoned borrow pit

This stop is a visit to one of several abandoned borrow pits in the interior of the impact structure that expose steeply inclined, sedimentary beds of the Cretaceous formations. Steeply dipping beds are exposed here. The rock strata would not normally be deposited at this angle of repose, nor would deposition produce such irregular relationships.

This borrow pit apparently contains only red and tan colored Tuscaloosa Group (fluvial and overbank) sediments, with small embedded blocks of schists up to one meter long. The interpreted photomosaic shows four megablocks 10s of meters across, each distinguished by sheared marginal discontinuities and differing strikes and dips of beds within. In megablock B we are looking at the tops of the beds from our observation position on the western rim of the borrow pit. This megabreccia was likely an early transient rim slump deposit constituting the crater melange.

The borrow pit contains a clayey sandy unit of the Eutaw Formation. The Eutaw is distinguished by white, clay lined Ophopmorpha burrows that may be seen at this outcrop.

STOP 4 Chaotic sedimentary beds in road cut (on north side of road)

The disturbed strata exposed in the road cut belong to the Gordo Formation of the Tuscaloosa Group and consist of sands interbedded with clays. By tracing individual units, one can visualize the complex relationship that must exist throughout the crater area. Rock strata strike N 30 W and dip 47 SW. Further up the road, 462 m to the east, another poorly preserved road cut exposure contains sediments that may indicate plastic flow.

STOP 5 Chaotically oriented metamorphic rocks of crater melange in central rebound area

Although the outcrops along this road are now partly covered with vegetation, the irregular distribution of the foliations in the fragments of the individual schist pieces can still be seen. Commonly they are at right angles to the foliation in the adjoining rock. Some of the individual blocks are small, less than 30 cm in diameter, whereas others appear to be very large, several meters in diameter. During their initial studies in this area, Neathery and others noted fragments of schists surrounded by sediments that resembled those of the Eutaw and Gordo Formations and those of the Mooreville. The area of schist fragments appears to cover approximately 200 acres in this area. No rock material suitable for thin section study could be found that might add to the current paucity of shock data.

STOP 6 Graben structures on southwest side of the impact crater

The southwest quadrant of the Wetumpka impact crater is faulted to form a series of graben-like structures defined by 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 initial studies in 1970-1972, Neathery and others found excellent exposures of the interior graben along the east side of Jasmine Hill Road near the entrance to Jasmine Hill Center. Since then, much of the area has been urbanized and the exposures have 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. This absence may suggest that the impact is pre-Arcola in 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 meters 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 or if they of only affect the Cretaceous sedimentary rock.