Clay dikes in pedogenic breccia (Conemaugh Group; Caldwell, Ohio)
English: Clastic dikes in breccia in the Pennsylvanian of Ohio, USA. The irregular, dark gray structures are apparent clastic dikes composed of soft clay that are hosted by an unusual sedimentary breccia. This is part of the Conemaugh Group, an Upper Pennsylvanian cyclothemic succession in eastern Ohio that contains nonmarine shales, marine shales, siltstones, sandstones, coals, marine limestones, and chert ("flint"). The outcrop consists of a mudshale matrix with irregularly-shaped, light-colored, micritic limestone clasts of varying size. Similarly described rock units in the literature have been referred to as "desiccation conglomerates", because the clasts can be rounded. At this site, "desiccation carbonate breccia" is probably a better term. The breccia at this locality is similar to that depicted in Condit (1912, pl. 1, fig. B) Nonmarine lacustrine limestones are moderately common in the Upper Pennsylvanian to Lower Permian of eastern Ohio. Occasionally, the lakes dried out and the fine-grained sediments cracked. With the return of aqueous conditions, the semilithified sediments were disturbed and became pebbles. Condit (1912, pp. 21-22) provides a description of this type of breccia in a different unit of the Conemaugh Group: "It is not unusual to find the Pittsburgh limestone with a conglomeratic structure. This consists of fragments of limestone cemented in a limestone matrix. Some outcrops show more or less rounded fragments, whereas others are made up of sharply angular pieces, cemented so as to have the appearance of a true breccia. Certain rounded, pebbly conglomerates are termed desiccation conglomerates by J.E. Hyde, who regards them as having been formed in lime-mud flats exposed to the sun. The surface on drying became covered with sun-cracks. Subsequent submergence which produced a breaking and rolling into pebbles is then supposed. Mr. Hyde describes an interesting examples of "a conglomerate formed by the breaking up of a sun-cracked lime-mud surface" near Bellaire, Belmont County [Ohio]. The exact locality is an abandoned brick plant on the south side of McMahon Creek, about one and one-half miles above the point where it empties into the Ohio River at Bellaire. In the excavation is exposed on of these conglomerates in process of formation. A limestone layer, about two feet below the Pittsburgh coal, shows a conglomeratic layer on the upper surface, consisting of flattened, somewhat angular pebbles, ranging from pea size to over four inches in diameter. The pebbly layer is overlain by shale, and is cemented to the underlying limestone with only a slight amount of matrix in the interspaces, so that the fragments stand out in relief. Some portions of the limestone show fragments in the relative positions in which they were formed by the cracking. It is evident that the flattened, polygonal pebbles of the conglomerate are the result of sun cracking. A view of this conglomerate is shown on Plate I. Similar conglomerates found in the "freshwater" limestones of the Conemaugh may be largely "desiccation conglomerates," formed in a manner similar to that described for the Bellaire locality." Stratigraphy: several meters below the Skelley Member, upper Conemaugh Group, lower Virgilian Series, Upper Pennsylvanian Locality: creek cut along unnamed east-flowing tributary of the West Fork of Duck Creek, southern side of Rt. 78, just west of the town of Caldwell, west-central Noble County, eastern Ohio, USA (vicinity of 39° 44' 20.70" North latitude, 81° 32' 19.22" West longitude) Some info. from: Condit (1912) - Conemaugh Formation in Ohio. Geological Survey of Ohio Bulletin, Fourth Series 17. 363 pp. 16 pls. Montanez & Cecil (2013) - Paleoenvironmental clues archived in non-marine Pennsylvanian-lower Permian limestones of the Central Appalachian Basin, USA. International Journal of Coal Geology 119: 45-55.