Concretion Depression Features (Lazarus Run, Orange Township, Ohio, USA)

Apparent depression features in black shale formed post-deposition from large siderite, or calcite concretions.

 

These features are set in the Ohio Shale. A large, thick lithologic unit in Ohio and other surrounding states (known as the New Albany Shale in IN and KY and the Chattanooga Shale in TN). It is gray to a light orange if weathered, and black on a fresh surface. It is organic rich with little to no fossils and The base of it closely aligned with the Frasnian-Famennian boundary, which is one of the top 5 worst extinction in Earth's history.

 

Concretions are large accrections of a mineral growth around a solid nucleus like a loose clast or even animal remains. In the Ohio Shale, they are siderite or carbonates (mostly carbonates). Their weight affects the surrounding rocks, resulting in pseudo-folds, as seen here. In the Ohio Shale, they occur in the lowermost member; the Huron Member.

 

The features above were once the point of concretion formation, and then later it eroded away both the overlying shale and the concretions themselves, resulting in the depressional and upheaval features, though they are not true folds.

 

Straigraphy: base of the Huron Member, lowermost member of the Ohio Shale.

 

Locality: small creek outcrop on west side along Lazarus Creek ~2000 ft east of Home Road, Orange Township, south-central Delaware County, Central Ohio, USA. (40°13'55.8" North latitude 83°03'12.9" West longitude).

 

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Ohio Shale:

- Clifton, Edward H., "The Carbonate Concretions of the Ohio Shale" Ohio Journal Of Science 57(2): 114, March 1957.

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