File:Cone-in-cone limestone interbed in shales (Ohio Shale, Upper Devonian; Glen Echo Ravine, Columbus, Ohio, USA) 1 (35847223904).jpg

Cone-in-cone limestone & shales in the Devonian of Ohio.

Light-colored rock at center = cone-in-cone limestone Grayish & blackish rocks above that = shales

Cone-in-cone structures are strange features found as interbeds in fine-grained siliciclastic rocks and sometimes found in the outer layers of concretions. They consist of sets of conical structures stacked up within each other. Mineralogically, they are often composed of finely-crystalline fibrous calcite. Numerous hypotheses have been proposed over the years that try to explain cone-in-cone structures. None has emerged as the most popular or most likely (see Lugli et al., 2005 for a long list of proposed formation mechanisms). These structures remain a mystery.

The thin cone-in-cone limestone bed shown here is in a central Ohio outcrop consisting of marine mudshales of the Late Devonian-aged Ohio Shale. 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.

These rocks were deposited in a moderately deep, anoxic seafloor environment. Black shale was widespread 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.

Several cone-in-cone limestone interbeds are present at this outcrop - they are thin and laterally discontinuous.

For ~close-up photos of other examples, see the first four photos in: <a href="https://www.flickr.com/photos/jsjgeology/albums/72157646824751150">www.flickr.com/photos/jsjgeology/albums/72157646824751150</a>

Stratigraphy: Ohio Shale, Famennian Stage, upper Upper Devonian

Locality: creek cut in Glen Echo Ravine, Columbus, north-central Franklin County, Ohio, USA (~40° 01' 09.94" North latitude, ~82° 59' 58.35" West longitude)

For more info. on cone-in-cone structures, see (as examples):

Melichar & Shkovira (2001) - Case study of the cone-in-cone structure based on Czech and Crimean samples. GeoLines 13.