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The AAPG/Datapages Combined Publications Database

GCAGS Transactions


Gulf Coast Association of Geological Societies Transactions
Vol. 48 (1998), Pages 151-158

Wetumpka Melange, a New Stratigraphic Unit in Alabama

David T. King, Jr.


A 6.5-km semi-circular, Late Cretaceous impact crater at Wetumpka, Alabama, was developed in target material consisting of several tens of meters of water-saturated Upper Cretaceous sediment overlying pre-Cretaceous crystalline basement rocks. The Wetumpka Melange, a proposed new stratigraphic unit, occupies the impact-crater floor of this structure and attains a maximum thickness of approximately 50 m.

Facies within the Wetumpka Melange include: (1) monomict megablock breccia; (2) clastic dike-injected deformed strata; and (3) polymict megaconglomerate (corresponding to inner-rim, crater-moat, and rebound-peak areas, respectively). Time for initial transient-crater rim collapse and rise of the central peak is estimated to have been a few seconds, and all melange facies developed almost entirely during 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, non-genetic lithic rank term for this unit. This impact-generated melange qualifies as a lithodemic unit because it is highly deformed, has lost its primary structure, and no longer conforms to the Law of Superposition.

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