Bleeding unconformities in weathered black shales in the Devonian of Kentucky, USA.
These are weathered black shales in the lower New Albany Shale, a Devonian-aged formation in Kentucky, Indiana, Illinois, and eastern Missouri. The unit is dominated by dark-colored 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 New Albany Shale is equivalent to the Ohio Shale, the Antrim Shale, and the Chattanooga Shale in surrounding states.
The basal unit in the New Albany is the Blocher Member - it consists of dolomitic black shales (dolosiltites, actually). The fissile nature of Blocher rocks is due to post-depositional compaction. Blocher beds are rich in organic carbon and have been homogenized by bioturbation.
The reddish- / orangish-brown areas in the photo are bleeding unconformities - surfaces of erosion and/or non-deposition of sediments. These are paraconformities, with horizontal sedimentary rocks above and below the contacts, which have no obvious paleotopography. Oxidative weathering of pyrite (FeS2 - iron sulfide; "fool's gold") in the rocks has produced iron oxide minerals such as reddish-brown hematite and yellowish-brown limonite. The iron oxides have stained the underlying rocks via descending meteoric waters (rain and runoff).
Stratigraphy: Blocher Member, lower New Albany Shale, Frasnian Stage, lower Upper Devonian
Locality: roadcut along the western side of the south-bound entrance ramp to Interstate 65 at the Route 245-Interstate 65 interchange, north-northeast of Belmont & south of Sherpherdsville, south-central Bullitt County, north-central Kentucky, USA (37° 55' 24.45" North latitude, 85° 41' 18.33" West longitude)
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Info. at:
Schieber, J. & R. Lazar (eds.). 2004. Devonian black shales of the eastern U.S. New insights into sedimentology and stratigraphy from the subsurface and outcrops in the Illinois and Appalachian Basins. Field Guide for the 2004 Annual Field Conference of the Great Lakes Section of SEPM. Indiana Geological Survey Open-File Study 04-05. 90 pp.
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