File:Ferruginous quartz-pebble conglomerate (boulder in "Sharon Conglomerate", Lower Pennsylvanian; Jackson North roadcut, Ohio, USA) 18 (37093784823).jpg

Ferruginous quartz-pebble conglomerate boulder in matrix in the Pennsylvanian of Ohio, USA. (geology hammer for scale)

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 above photo shows this material.

Seen here is the exposed interior and the edge of a boulder composed of ferruginous conglomerate, which was observed in only two clasts at this outcrop. The ferruginous conglomerate itself is older than the surrounding "Sharon" conglomerate matrix, seen at the far left and upper left. This is known by applying the Principle of Included Fragments, also known as the Principle of Components (fragments of rock, which occur in the matrix of another rock, are older). How much older the ferruginous conglomerate is, is unknown. The "Sharon" matrix surrounding the boulder is quartz-pebble conglomerate and pebbly sandstone, mixed with abundant, dark gray-colored mudshale clasts, derived from the underlying Logan Formation.

The ferruginous quartz-pebble conglomerate was observed to have pyrite cement between clasts on fresh crack surfaces. Hematite cement is also present on both weathered and fresh surfaces. Some limestone clasts are present - usually silicified.

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