NATURE AND PROBABLE ORIGIN OF THE MISSISSIPPIAN-PENNSYLVANIAN UNCONFORMITY IN
EASTERN UNITED STATES.
Ettensohn, Frank R., Department of Geology, University of Kentucky, Lexington,
Kentucky 40506, U.S.A., and Chesnut, Donald R., Jr., Kentucky Geological
Survey, University of Kentucky, Lexington, Kentucky 40506, U.S.A.

Recent investigations of the so-called Mississippian-Pennsylvanian
unconformity indicate that it is in fact an intra-Pennsylvanian unconformity of
Early Pennsylvanian Age. Throughout most of the Appalachian area and on
adjacent parts of the North American craton, rocks of Mississippian and
Pennsylvanian age are separated by a substantial unconformity. However, in
three Appalachian areas, east-central Pennsylvania; western Virginia and
southern West Virginia; and northern Alabama and adjacent parts of Georgia, the
unconformity is absent, and Mississippian and Pennsylvanian rocks are
interpreted to be vertically and laterally gradational. Each of the three areas
represents a depocenter, and each lies just cratonward of areas that were
promontories on the former continental margin.
	In the Virginia-West Virginia area, where the Mississippian-Pennsylvanian
transition sequence probably is best known, the youngest Mississippian rocks
(Bluestone Formation, Pennington Group) are gradational with rocks of the Early
Pennsylvanian-age Pocahontas Formation. The unconformity actually begins atop
the Pocahontas and locally separates the Pocahontas and overlying New River
formations. A widespread episode of post-Pocahontas uplift and erosion
represented by the unconformity progressively truncated the Pocahontas and Upper
Mississippian rocks in a westward direction, so that to the west, in east-
central Kentucky late Early Pennsylvanian equivalents of the New River Formation
unconformably overlie Early Mississippian clastics of the Borden Formation.
	The post-Pochahontas unconformity represents an episode of widespread
cratonic uplift that accompanied the collision of Gondwanaland with a trench off
the southeastern coast of North America at the beginning of the Alleghenian
Orogeny.
	The effects of uplift and erosion apparently were not experienced in the
areas just cratonward of the promontories. This exclusion was caused by an
increase in deformational intensity at the promontories during convergence.
Because of their protuberant nature, collision and resulting deformation would
have been more intense in these areas. Isostatic adjustment to the increased
deformational loading in these areas would have caused intense downwarping in
peripheral basins cratonward of the promontories. This downwarping would have
resulted in major depocenters in which the larger amplitude of downwarp would
have offset the effects of uplift and erosion, resulting in conformable
sequences. Away from these promontory-controlled depocenters, where subsidence
dominated, a marked decrease in subsidence would have permitted uplift and
erosion to dominate. This decrease in subsidence, combined with the
reactivation of many cratonic structures such as the Cincinnati Arch, would have
resulted in greater uplift to the west, so that Lower Pennsylvanian and Upper
Mississippian rocks were progressively truncated in that direction. Hence, even
though the uplift and erosional event was Early Pennsylvanian in age, the
unconformity truncates Mississippian rocks throughout most of its distribution.