File:Ohio Shale-Olentangy Shale disconformity (Upper Devonian; Highbanks Park, Lewis Center, Ohio, USA) 3.jpg

English: Shales in the Devonian of Ohio, USA.

This is a formation contact section in central Ohio. The unit in the upper part of the photo has relatively hard, iron oxide-stained, chippy-weathering, dark marine mudshales of the basal Ohio Shale (Upper Devonian). The unit below that has soft, gray-colored, marine clayshales of the uppermost Olentangy Shale (also Upper Devonian). The boundary between them is an unconformity - a surface of erosion and/or non-deposition of sediments. Unconformities having horizontal sedimentary rocks atop horizontal sedimentary rocks are called disconformities.

According to Over & Rhodes (2000), conodont biostratigraphy shows that both units are Late Devonian in age. The Late Devonian is subdivided into two parts (in ascending order): the Frasnian Stage and the Famennian Stage. The Frasnian-Famennian boundary marks one of the five most significant mass extinctions events in Earth history (it's one of the "Big Five"). In Delaware County, Ohio, Over & Rhodes (2000) identified the Frasnian-Famennian boundary several centimeters below the Ohio Shale-Olentangy Shale disconformity.

The Ohio Shale has been subdivided into three parts (in ascending order): Huron Shale Member, Chagrin Shale Member, and Cleveland Shale Member. The outcrop appearances of the three different members are best developed in northeastern Ohio, in the Cleveland area. The Huron and Cleveland are black shales and the Chagrin is gray shale. In central Ohio, the Chagrin Shale lithofacies occurs in a relatively thin, poorly developed interval - it is referred to as the Three Lick Bed. The Three Lick consists of interbedded gray and black shales.

The dark shales in the upper part of the photo are the basal Huron Shale Member. It characteristically contains moderately large concretions - many are present up-section at this locality.

Stratigraphy: disconformity at the contact between the Ohio Shale (above) and the Olentangy Shale (below), Frasnian Stage to Famennian Stage, Upper Devonian

Locality: creek cut in ravine a little east of the Olentangy River, Highbanks Park, Lewis Center, southern Delaware County, central Ohio, USA (vicinity of 40° 08' 45.83" North latitude, 83° 02' 10.79" West longitude)

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Reference cited:

Over & Rhodes (2000) - Conodonts from the Upper Olentangy Shale (Upper Devonian, central Ohio) and stratigraphy across the Frasnian-Famennian boundary. Journal of Paleontology 74: 101-112.