The rocks in the upper half of the photo are black mudshales of the Upper Devonian Chattanooga Formation. The rocks below that are fine-grained dolostones of the Upper Ordovician Cumberland Formation. The intensely orangish-brown stains covering much of the Cumberland Formation formed by oxidation of pyrite at the formation contact. Considerable missing time is represented by this boundary - the entire Silurian, the Lower Devonian, and the Middle Devonian are all gone here - that's about 55 million years missing. Stratigraphic contacts with missing time are called unconformities - surfaces of erosion and/or non-deposition of sediments. This is a disconformity, with horizontal sedimentary rocks above & below the contact. The basal Chattanooga Shale is relatively rich in pyrite (FeS2 - iron sulfide). With oxidative weathering, the pyrite alters to various iron oxide minerals such as reddish-brown hematite and yellowish-brown limonite. These iron oxides end up staining underlying rocks as meteoric waters (rain & runoff) descend. The end result is a "bleeding unconformity".
This contact is a megasequence boundary - the Tippecanoe-Kaskaskia Megasequence boundary. Regionally recognizable packages of sedimentary rocks bounded by major unconformities are known as megasquences, or Sloss sequences (named after geologist L.L. Sloss, 1913-1996). The bounding unconformities were caused by erosion during regressions. Continental margins have more complete sedimentary successions, whereas craton interiors have more incomplete successions. The names for North America's megasequences are (from oldest to youngest): Sauk, Tippecanoe, Kaskaskia, Absaroka, Zuni, and Tejas.
Stratigraphy: Chattanooga Shale (Upper Devonian) over Cumberland Formation (Upper Ordovician)
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)
Tags: Chattanooga Shale mudshale shales mudshales Devonian dolostone dolomudstone dolosiltstone Cumberland Formation Ordovician Burkesville Kentucky bleeding unconformity disconformity
The upper part of this roadcut consists of fine-grained siliciclastics of the Portwood Member, an offshore marine succession at the base of the New Albany Shale / Ohio Shale. The lower part of the cut consists of carbonates of the Boyle Formation.
The contact is an unconformity - a surface of erosion and/or non-deposition of sediments. The orangish-brown stains on the Boyle are iron oxides derived from weathering of pyrite at or near the unconformity. These are called "bleeding unconformities".
Stratigraphy: Portwood Member of the basal New Albany Shale / Ohio Shale (middle Polygnathus varcus zone, Givetian Stage, upper Middle Devonian) over the Boyle Formation (lower Polygnathus varcus zone, Givetian Stage, upper Middle Devonian)
Locality: Drowning Creek East outcrop - roadcut along the northern side of Route 52, east of Drowning Creek, far-western Estill County, east-central Kentucky, USA (37° 42' 41.30" North latitude, 84° 05' 01.40" 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.
Tags: Portwood Member New Albany Shale Ohio Boyle Formation Devonian Drowning Creek East outcrop roadcut Route 52 Estill County Kentucky bleeding unconformity unconformities disconformity disconformities
The rocks in the upper half of the photo are black mudshales of the Upper Devonian Chattanooga Formation. The rocks below that are fine-grained dolostones of the Upper Ordovician Cumberland Formation. The intensely orangish-brown stains covering much of the Cumberland Formation formed by oxidation of pyrite at the formation contact. Considerable missing time is represented by this boundary - the entire Silurian, the Lower Devonian, and the Middle Devonian are all gone here - that's about 55 million years missing. Stratigraphic contacts with missing time are called unconformities - surfaces of erosion and/or non-deposition of sediments. This is a disconformity, with horizontal sedimentary rocks above & below the contact. The basal Chattanooga Shale is relatively rich in pyrite (FeS2 - iron sulfide). With oxidative weathering, the pyrite alters to various iron oxide minerals such as reddish-brown hematite and yellowish-brown limonite. These iron oxides end up staining underlying rocks as meteoric waters (rain & runoff) descend. The end result is a "bleeding unconformity".
This contact is a megasequence boundary - the Tippecanoe-Kaskaskia Megasequence boundary. Regionally recognizable packages of sedimentary rocks bounded by major unconformities are known as megasquences, or Sloss sequences (named after geologist L.L. Sloss, 1913-1996). The bounding unconformities were caused by erosion during regressions. Continental margins have more complete sedimentary successions, whereas craton interiors have more incomplete successions. The names for North America's megasequences are (from oldest to youngest): Sauk, Tippecanoe, Kaskaskia, Absaroka, Zuni, and Tejas.
Stratigraphy: Chattanooga Shale (Upper Devonian) over Cumberland Formation (Upper Ordovician)
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)
Tags: Chattanooga Shale mudshale shales mudshales Devonian dolostone dolomudstone dolosiltstone Cumberland Formation Ordovician Burkesville Kentucky bleeding unconformity disconformity
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.
Tags: New Albany Shale Devonian black shales weathered Interstate 65 entrance ramp roadcut roadcuts outcrop outcrops Bullitt County Kentucky Blocher Member unconformity unconformities paraconformity paraconformities bleeding iron oxide staining oxidation oxidative chemical weathering pyrite
Weathered black shales disconformably overlying limestones in the Devonian of Kentucky, USA.
The rocks in the upper part of the photo are weathered black shales of the 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 New Albany here 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 rocks in the lower part of the picture are the Middle Devonian North Vernon Limestone, which is part of a widespread sheet of Devonian carbonates that extends from New York State to the Midwest. The North Vernon Limestone represents deposition in a subtropical, shallow-water, carbonate platform environment. The limestone here is fossiliferous, with decent-sized camerate crinoid columnals. An encrinite bed is present in the top-preserved North Vernon.
Just above the top of the North Vernon Limestone is a thin, lensoidal lag unit with phosphatic nodules, glauconitic pellets, and conodonts.
The New Albany-North Vernon contact represents missing time - such stratigraphic boundaries are called unconformities, which are surfaces of erosion and/or non-deposition of sediments. This is a disconformity, with horizontal sedimentary rocks above and below the contact.
Oxidative weathering of pyrite (FeS2 - iron sulfide; "fool's gold") at the New Albany-North Vernon boundary 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). The end result is a "bleeding unconformity".
Stratigraphy: lowermost-preserved New Albany Shale (uppermost Givetian Stage to lower Frasnian Stage, uppermost Middle Devonian to lower Upper Devonian) disconformably over the Beechwood Member of the North Vernon Limestone (Givetian Stage, upper Middle 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.
Tags: New Albany Shale Devonian black shales weathered Interstate 65 entrance ramp roadcut roadcuts outcrop outcrops Bullitt County Kentucky Blocher Member Beechwood North Vernon Limestone unconformity unconformities disconformity disconformities bleeding iron oxide staining oxidation oxidative chemical weathering pyrite