English: Carbonate concretion from the Devonian of Ohio, USA. (crack surface)

Concretions are post-depositional structures found in some sedimentary successions, usually in shales. They vary in mineralogy, size, and shape, but often have slightly flattened spheroidal shapes. Concretions form by post-depositional mineralization around some nucleus (a rock fragment or fossil fragment).

This hard concretion fragment is from the Ohio Shale, a thick Upper Devonian unit representing deposition in a moderately deep, usually-anoxic seafloor environment. The formation 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.

Black shale was a widely-distributed lithofacies during the Late Devonian. The Ohio Shale is equivalent to the Chattanooga Shale, the Antrim Shale, and the New Albany Shale in surrounding states.

Large carbonate concretions are common in the Huron Shale Member - they are composed of calcite, dolomite, pyrite, and quartz (Clifton, 1957). This sample bubbles weakly in acid.

Stratigraphy: basal Huron Shale Member, Ohio Shale, Famennian Stage, upper Upper Devonian

Locality: stream cut along Lazarus Run, just north of Winter Road & just east of Chapman Road & just east of the Olentangy River, west of Route 23, northeastern Liberty Township, western Delaware County, central Ohio, USA

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Reference cited:

Clifton (1957) - The carbonate concretions of the Ohio Shale. Ohio Journal of Science 57: 114-124.