An alternative to the barrier-shoreline model for deposition of Mississippian and Pennsylvanian rocks in northeastern Kentucky: Summary

Along the western edge of the Appalachian Basin in eastern Kentucky (Fig. 1) traditional interpretation of the Carboniferous showed that most Carboniferous units are widespread and tabular, and that Mississippian and Pennsylvanian rocks are separated by a prominent unconformity (Butts, 1922; McFarlan, 1943; McFarlan and Walker, 1956; Pryor and Sable, 1974; Rice and others, 1979). Recent work in the area by the cooperative mapping program of the Kentucky Geological Survey and by a number of independent workers has revealed problems in applying these concepts. The unexpected absence of units, abrupt facies changes, deep erosional truncation, and prominent intraformational disconformities observed in these rocks made these concepts seem tenuous at best. Workers such as Sheppard (1964) and Englund (1976) attributed these stratigraphic variations to a sequence of erosional episodes related to sea-level fluctuation during the Mississippian and near the Mississippian-Pennsylvanian transition. Although these observations were based largely on small, scattered exposures, the proliferation of major highways and recreational areas in eastern Kentucky during the 19709s produced many large, extensive exposures, which indicated to others (Wilson, 1970; Dever, 1973, 1977; Dever and MacQuown, 1974; Ettensohn and Dever, 1975; 1979a, 1979b, 1979c; Ettensohn, 1975a, 1975b, 1976, 1977; Anderson and Hester, 1977) a similar explanation. The major difference was that later workers were able to relate periods of erosion to episodes of synsedimentary tectonism in the area. Other explanations were also proposed, and one of the most intriguing of these appeared in a series of articles by J. C. Ferm, J. C. Horne, and others (Horne and Ferm, 1970, 1976, 1978; Ferm and others, 1971; Ferm, 1972, 1974; Horne and others, 1971a, 1971b, 1974, 1976; Swinchatt, 1970). In a model developed along a segment of Interstate 64 in northeastern Kentucky, these writers suggested that the discontinuous nature of units was not caused by erosion, but rather by lateral gradation from one depositional environment to another. In this model, called the Lee-Newman Barrier-Shoreline Model, these authors argued that large parts of the Mississippian-through-Pennsylvanian sequence in northeastern Kentucky represent a single, coherent offshore (Mississippian) to onshore (Pennsylvanian) transition (Fig. 2). Furthermore, they argued that there is no evidence supporting the tabular nature of Carboniferous units or a prominent systemic unconformity. Regarding these interpretations, Horne, and Ferm (1970, p. 217) stated, “ Studies along newly constructed Interstate Highway 64 have shown that marine limestone and shale designated ‘Mississippian’ represent offshore carbonate bars which merge laterally with marine red and green clays. These marine rocks, in turn, intertongue with orthoquartzitic beaches of the ‘basal Pennsylvanian.’ Landward these shoreline sediments grade into coal-bearing fluvio-deltaic Pennsylvanian strata. Physical evidence for a systemic breaks not present, and major faunal differences can be attributed to depositional environments.” These interpretations leave a certain ambiguity regarding age relationships between Mississippian and Pennsylvanian rocks. Most of their diagrams (for example, see their Figs. 2, 3) can lead only to inferences of intertonguing and contemporaneity between rocks bearing Mississippian marine fossils (Newman Limestone and Pennington Formation) and coal and clastics (Lee and Breathitt Formations) traditionally considered to be Pennsylvanian. In fact, Horne and others (1974, p. 102) stated, “Moreover, on a large scale, from one end of the section to the other, the Breathitt, Lee, Pennington, and Newman all grade laterally into one another,” and this is clearly reflected in their diagrams (Ferm and others, 1971, Fig. 2, 13; Horne and others, 1974, Fig. 1; Horne and others, 1974, Fig. 2, 3).