--> Abstract: Wetumpka Melange, A New Stratigraphic Unit in Alabama, by D. T. King, Jr.; #90932 (1998).

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Abstract: Wetumpka Melange, A New Stratigraphic Unit in Alabama

KING, DAVID T., JR.
Auburn University, Auburn, AL

A horseshoe-shaped, Late Cretaceous asteroid-impact crater at Wetumpka, Alabama, has a diameter of approximately 6.5 km. Impact target layers were: 0-50 m of water, 100 m of watersaturated Upper Cretaceous sediment, and underlying pre-Cretaceous, metamorphic basement rock.

The Wetumpka Melange is a lens-shaped mass occupying the crater floor and attaining maximum thickness of approximately 50 m. The stratotype for this unit is exposed in various pits and road cuts within the crater rim. The melange's upper boundary is erosional and is partially covered by Quaternary alluvium and soil. The melange's lower boundary is generally a sharply defined surface not of sedimentary origin.

Facies within the Wetumpka Melange include: (1) monomict megablock breccia, (2) clastic dike-injected deformed strata, and (3) polymict megaconglomerate (corresponding to inner rimslump, moat, and rebound-peak areas, respectively). Time for initial transientcrater rim collapse (calculated using transientcrater diameters between 3.3 to 4.2 km) scales as 18 to 21 sec. All melange facies developed within that brief time.

Melange ("a mappable body of rock characterized by the inclusion of fragments and blocks of all sizes embedded in a matrix of more tractable material") is the best general, nongenetic lithic rank term for this unit. According to the North American Stratigraphic Code (1983), this melange must be a lithodemic unit (lithodeme) because it (1) is "highly deformed", (2) is not "internally stratified", and (3) "does not generally conform to the Law of Superposition."

AAPG Search and Discovery Article #90932©1998 GCAGS/GCS-SEPM Meeting, Corpus Christi, Texas