Regional stratigraphic analysis of the Lower Mississippian Maccrady Formation of the central Appalachians

The Lower Mississippian Maccrady Formation was the first salt and gypsum deposit to be commercially exploited in the United States and is currently the only source of gypsum for plasterboard in the Southeast. These thick commercial deposits occur in a limited area of southwestern Virginia, although thinner gypsum and anhydrite 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 sealevel rise. These evaporites may have been locally thickened and concentrated into boudins by motion along the Saltville fault.Regionally, the Maccrady Formation is a fining-upwards sequence composed of a lower red siltstone and very fine-grained sandstone unit, and an upper red and green mottled, variegated shale and mudstone unit which locally contains carbonates and evaporites. The Maccrady Formation records the transition from a clastic-dominated (deltaic) to a carbonate-dominated (shallow marine) sedimentary regime. Strata laterally adjacent to the Maccrady Formation are, in part, composed of limestone, dolostone, chert, glauconite, phosphate nodules, geodes, anhydrite, and gypsum, which is a typical stratigraphic assemblage associated with evaporitic conditions.Regional analysis of the Maccrady Formation and adjacent strata, which incorporated biostratigraphic information, demonstrates the presence of a regional unconformity at the top of the Maccrady Formation, and establishes the amount of strata omitted at this hiatus.Regional analysis also demonstrates that intra-Mississippian tectonic movements occurred before, during, and after Maccrady deposition, resulting in several local to intra-regional uplifts in southwestern Virginia, eastern Kentucky and central West Virginia. Evidence indicates that at least some of these uplifts were near-vertical and basement-controlled, and were active several times during the Late Precambrian and Paleozoic Eras.Mississippian tectonic uplift along Pine Mountain and as far east as Norton, Virginia, is thought to have resulted in a barrier or sill that restricted normal seawater flow to the east, resulting in Maccrady evaporite deposition.