File:Silicified limestone clast in quartz-pebble conglomerate ("Sharon Conglomerate", Lower Pennsylvanian; Jackson North roadcut, Ohio, USA) 1 (37715072766).jpg

Silicified limestone clast in conglomerate in the Pennsylvanian of Ohio, USA.

This outcrop in southern Ohio exposes 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.

Here, the rocks are generally massive (= non-bedded) quartz-pebble conglomerates. Other clast lithologies and sizes are also present, including angular shale clasts, angular sandstone clasts (both derived from erosion of the underlying Logan Formation of Early Mississippian age), rounded to irregularly-shaped ironstone clasts, rounded quartz sandstone pebbles, rounded quartzite pebbles, and rounded silicified limestone pebbles. A very unusual clast type present at this outcrop is ferruginous, pyrite-cemented and hematite-cemented, quartz-pebble conglomerate (= conglomerate in conglomerate ! ).

The mottled yellowish clast in the middle of this photo is composed of silicified limestone. It is possibly (probably?) derived from the Maxville Limestone, a Middle to Upper Mississippian unit in parts of Ohio. If derived from outside of Ohio (as are the quartz pebbles), it may have been eroded from a Maxville-equivalent unit. The yellowish coloration is from iron oxide staining. Click on the photo to zoom in. Small, subrounded, silicified oolites are visible in places. Oolites are sand-sized, rounded to subrounded, concentrically layered grains of calcium carbonate (calcite or aragonite). They form by back-and-forth rolling on shallow seafloors by wave action. Microcrystalline calcium carbonate (CaCO3) precipitated in layers on the rolling grains. Bacteria may or may not have played a role in oolite carbonate mineralization.

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