File:Large carbonate concretion in black shale (Huron Shale Member, Ohio Shale, Upper Devonian; ravine just east of the Olentangy River, Highbanks Park, Lewis Center, Ohio, USA) 5.jpg

English: Concretion in shale in the Devonian of Ohio, USA.

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

Portions of the Late Devonian-aged Ohio Shale succession are famous for having large, subspherical concretions. Analyses have shown that they are multimineralic and are dominated by carbonate minerals (calcite, dolomite), silicate minerals (quartz, clay minerals), sulfides (pyrite), sulfates (barite), and organic carbon (Criss et al., 1988). Some are composite concretions - two or more individual masses have coalesced.

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

Locality: creek cut in ravine, east of the Olentangy River, Highbanks Park, Lewis Center, southern Delaware County, central Ohio, USA (vicinity of 40° 08' 46.40" North latitude, 83° 02' 07.39" West longitude)

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Reference cited: Criss et al. (1988) - An organic origin for the carbonate concretions of the Ohio Shale. United States Geological Survey Bulletin 1836. 21 pp. (<a href="https://pubs.usgs.gov/bul/1836/report.pdf" rel="noreferrer nofollow">pubs.usgs.gov/bul/1836/report.pdf</a>)

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See info. at:

<a href="https://geosurvey.ohiodnr.gov/portals/geosurvey/PDFs/GeoFacts/geof04.pdf" rel="noreferrer nofollow">geosurvey.ohiodnr.gov/portals/geosurvey/PDFs/GeoFacts/geo...</a>