File:Oolitic limestone pebble in ferruginous quartz-pebble conglomerate (boulder in "Sharon Conglomerate", Lower Pennsylvanian; Jackson North roadcut, Ohio, USA) 2 (37715581616).jpg

Ferruginous quartz-pebble conglomerate boulder in the Pennsylvanian of Ohio, USA. (part of 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 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 (not visible in this shot). 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. The whitish clast at center is composed of silicified oolitic limestone. Click on the photo to zoom in and look around. Notice the rounded, projecting grains in the limestone pebble. Each rounded grain is an oolite. Oolites are rounded to subrounded, sand-sized, concentrically layered grains composed of calcium carbonate (CaCO3 - either calcite or aragonite). In this case, the original CaCO3 appears to have been replaced by quartz. Oolites form by rolling back-and-forth 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. The oolitic limestone pebble may be derived from the Maxville Limestone, a Middle to Upper Mississippian unit in parts of Ohio. If derived from outside Ohio (as are the quartz pebbles), it is likely from a Maxville-aged equivalent.

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