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Regional Analysis of the Lower Mississippian Maccrady Formation of the Central Appalachians: New Evidence for Mississippian Unconformity and Tectonism

WARNE, ANDREW G., University of North Carolina, Chapel Hill, NC

The Lower Mississippian Maccrady Formation was the first salt and gypsum deposit to be commercially exploited in the United States. These thick commercial deposits are found in a limited area of southwestern Virginia, although thinner gypsum and anhydrites occur in the Maccrady Formation throughout much of southwestern Virginia and southern West Virginia. Thick Maccrady salt and gypsum deposits are the result of local, syndepositional subsidence, a change from humid to arid climate as the North American Plate drifted northward, and a eustatic sea level rise.

Regional mapping of the Maccrady Formation and adjacent strata, which incorporated biostratigraphic information, more clearly demonstrates the presence of a regional unconformity at the top of the Maccrady Formation, and establishes the amount of missing strata at the hiatus. Topography on this unconformity is considered to control hydrocarbon accumulation in the overlying Big Lime.

Important Appalachian hydrocarbon reservoirs occur immediately above (Big Lime) and below (Big Injun and Weir sands) the Maccrady Formation. Regional mapping indicates that intra-Mississippian tectonics were active before, during, and after Maccrady deposition, resulting in several local to intraregional uplifts in southwestern Virginia, eastern Kentucky, and central West Virginia. The uplifts are thought to be near vertical and basement controlled. Hydrocarbon production and potential exploration areas can be related to these uplifts.

A major Mississippian uplifted area along Pine Mountain and as far east as Norton, Virginia, is thought to have been a barrier or sill that restricted normal seawater flow to the east, resulting in Maccrady evaporite deposition.

 

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)