File:Mississippian-Pennsylvanian unconformity ("Sharon" Conglomerate over Logan Formation; Jackson North roadcut, Ohio, USA) 16 (36815965994).jpg

Pennsylvanian-aged conglomerate over Mississippian-aged sandstone in Ohio, USA. (geology hammer for scale)

This stratigraphic section in southern Ohio has a nice exposure of the Mississippian-Pennsylvanian unconformity. Unconformities are surfaces of erosion and/or non-deposition of sediments. They can represent geological short intervals of time (for example, a couple million years) or immense periods of time (e.g., a billion years or more). In America, the Mississippian-Pennsylvanian boundary is usually an unconformity. In Ohio, the Middle Mississippian, the Upper Mississippian, and the lower half of the Lower Pennsylvanian are usually missing.

The left part of the photo has sandstones of the Lower Mississippian Logan Formation, a marine mixed siliciclastics succession.

Stratigraphically above the Logan (= center & right portions of the photo) is the lowermost Pottsville Group, a Pennsylvanian-aged, cyclothemic succession in eastern Ohio that contains nonmarine shales, marine shales, siltstones, sandstones, coals, marine limestones, and chert ("flint"). The lower Pottsville dates to the late Early Pennsylvanian. The upper part dates to the early Middle Pennsylvanian. The Lower-Middle Pennsylvanian boundary is apparently somewhere near the Boggs Member (?).

At this site, the basal Pottsville is a spectacular quartz-pebble conglomerate, with a quartzose sandstone unit above it, plus coal and shale above that. The latter two units are visible, but not easily accessible for examination.

In eastern and northeastern Ohio, the conglomeratic base of the Pottsville Group is called the Sharon Conglomerate (a.k.a. Sharon Sandstone; Sharon Formation; Sharon Member). The literature points out that the Sharon elsewhere in Ohio is not directly correlatable with the conglomerate exposed at this site near Jackson, Ohio. Thus, it has been suggested that the unit at this locality be referred to as the "Sharon" Conglomerate.

Note that the erosive contact between the "Sharon" and the underlying Logan is not planar and horizontal, as might be expected in Ohio. The contact has significant relief here. This suggests that, by the Early Pennsylvanian, the Logan Formation had already undergone diagenesis and was lithified, or at least semi-lithified. The rocks were eroded and had near-vertical relief before low areas were filled with pebbles and sand.

Stratigraphy: "Sharon" Conglomerate, lowermost Pottsville Group, Lower Pennsylvanian

   over

Logan Formation, upper Waverly Group, Osagean Stage, upper Lower Mississippian