SYNDEPOSITIONAL FAULTING IN THE POTTSVILLE GROUP OF THE DISTAL APPALACHIAN BASIN

The Pottsville Group in southeastern Ohio was deposited outboard of the distal foreland basin that was forming adjacent to the rising Appalachian Mountains during the Middle Pennsylvanian. The Pottsville is composed of a complex heterolithic assemblage of sandstones, siltstones, mudstones, coals, and limestones that is typical of the Pennsylvanian. Recent road construction has resulted in new exposures that contain both normal and thrust faults that may be syndepositional. Detailed measured sections show that the strata are composed primarily of marine mudstones and ironstones interbedded with thin marine and non-marine sandstones, siltstones, paleosol horizons, and coals that record two major fluctuations of relative sea level. The marine sandstones are bioturbated and contain a green clay interpreted to be glauconite; the nonmarine sandstones and siltstones are heavily rooted. At least four thin coals are present in the section.

Tracing the strata across the outcrop using photomosaics shows that fault movement was complex. The most obvious fault trace, which trends 077/50N, has a minimum displacement of 4 m. The orientation of the beds on the northern, downthrown, side of this fault (006/45E) suggests a rotational component to the fault movement. Some of these beds are convolute laminated. Beds at the same stratigraphic level 100-150 m to the north contain more subtle small thrust faults within paleosol, coal, and rooted siltstone beds that formed in response to lateral shortening associated with movement on the main fault. These deformed beds appear to be onlapped by undeformed marine mudstones indicating paleotopographic relief. The presence of convolute lamination in conjunction with the onlap of the upper marine mudstones shows that fault motion was intermittent and probably coeval with Middle Pennsylvanian sedimentation.