MINERALOGY OF IMPACTITE SANDS FROM AU DRILL CORE #09-04, WETUMPKA IMPACT STRUCTURE, ALABAMA

The Wetumpka impact structure is a Late Cretaceous shallow-marine impact crater about 6 km in diameter, which is located in central Alabama. The target consisted of Upper Cretaceous sediments that were unconformably overlying Piedmont schists and gneisses. An arcuate crystalline rim is surrounded on the east and northeast by Cretaceous units, on the north by Piedmont basement, and on the west by Quaternary alluvium.

There are several shallow drill cores at Wetumpka. AU drill core #09-04 sampled 217.7 m (715 feet) near the southeastern portion of the rim. The upper ~ 60 m of core is interpreted as a segment of slumped, overturned sedimentary section that was formerly on the rim. Below this overturned section are nearly 160 m of impactite sands with sedimentary blocks. The specific origin and provenance of the fine clastics in this impactite interval is not yet understood.

One of the methods we are using to preliminarily characterize these impactite sands is bulk-sample X-ray diffraction. This XRD allows us to characterize changes in the bulk mineral composition and possible shock petrology of the section. Possible shock levels are interpreted using methods presented by Harris et al. In so doing, we analyzed systematic changes in the coherent scattering domain (CSD) lengths for diffracting quartz, plagioclase, and muscovite planes observed in successively deeper samples. We also examined the changes with increasing core depth with regard to CSD length for each indexed crystallographic plane. Thus far, we have found that the upper portions of the core, including upper impactite sands, show little evidence of shock deformation. They may show changes in the degree of deformation associated with {10-13} quartz plane, but the diffraction peak associated with these planes typically have very low intensities and may not be as reliable as other peaks. Deeper impactite sands show evidence for moderate shock deformation, similar to the levels responsible for producing {10-12} planar deformation features in quartz. We are working to compare the thin-section petrography of the impactite sand section to the X-ray diffraction data.