--> Abstract: Deposition and Sequence Stratigraphic Framework of Late Devonian Black Shales in the Eastern Us, by Juergen Schieber; #90078 (2008)

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Deposition and Sequence Stratigraphic Framework of Late Devonian Black Shales in the Eastern Us

Juergen Schieber
Geological Sciences, Indiana University, Bloomington, IN

Rather than being a continuously deposited stratigraphic interval, the Late Devonian black shale succession of Tennessee, Kentucky, Indiana, and Ohio consists of stacked shale packages that are separated by erosion surfaces and their lateral continuities. Corroborating evidence for substantial sea level drops and erosion exists in the form of increasing depth of erosion towards the Cincinnati Arch and chamosite ooids on erosion surfaces. Erosion surfaces are also marked by sandy lags, bone beds, pyritic lags, and exceptionally sharp contacts between black shale packages. Position of ooid deposits relative to the Cincinnati Arch and material from underlying carbonates in lags, suggests partial exposure of the Arch during low stands of sea level. Lateral tracing of erosion surfaces with the help of careful outcrop description, gamma-ray logs (outcrop and subsurface), and petrographic comparisons shows that these erosion surfaces are laterally extensive and can be traced over distances of as much as 900 km’s.

Black shale packages that drape across erosion surfaces record sea level rise and contain large concentrations of algal cysts that suggests sediment-starved conditions and very slow sediment accumulation during high stand conditions. The absence of storm deposits in the upper portions of shale packages suggests water depth in excess of storm wave base, potentially 50 meters or more.

Conodont data indicate that recognized sequences record regressive-transgressive events on a million year scale and correlate with global sea level variations. Parasequences, identifiable through careful study of sedimentary features, record fluctuations on a 100,000 year scale and could represent climatic fluctuations.

 

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas