<p>Pyrite concretion in the Devonian of Kentucky, USA.
</p><p>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).  This concretion is composed of iron sulfide (FeS2) - pyrite and/or marcasite.
</p><p>The concretion's host rock is the Chattanooga Shale, which consists of dark-colored, chippy-weathering, marine mudshales of Late Devonian age.  These black shales were deposited in a moderately deep, anoxic seafloor environment.  This was a widespread lithofacies during the Late Devonian's Global Anoxia Event.  The Chattanooga Shale is equivalent to the Ohio Shale, the Antrim Shale, and the New Albany Shale in surrounding states.
</p><p>Stratigraphy: Chattanooga Shale, Upper Devonian
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Locality: roadcut on the northern side of Route 90, just west of Burkesville, central Cumberland County, southern Kentucky, USA (36° 47’ 51.17” North latitude, 85° 23’ 10.62” West longitude)
<a rel="nofollow" class="external text" href="https://www.flickr.com/people/47445767@N05">James St. John</a>
CC BY 2.0
<a rel="nofollow" class="external text" href="https://www.flickr.com/photos/jsjgeology/42300453012/">Pyrite concretion (Chattanooga Shale, Upper Devonian; Burkesville West Rt. 90 roadcut, Kentucky, USA) 1</a>