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Jürgen Schieber1
(1) University of Texas at Arlington, Arlington, TX
Abstract: Sequence stratigraphic correlations in Devonian black shales of the eastern US: Relationship to global sealevel variations
Late Devonian black shales of Tennessee, Kentucky, Indiana, and Ohio have long been thought of as deposited in the deepest portions of stagnant basins. Several years of reexamination, however, reveal a substantially more complex picture.
While they are undoubtedly deep basin deposits in some areas, there is good evidence that in other areas these black shales were deposited in shallow water, within reach of storm waves and with abundant benthic life. “Shallow” water black shales are most common on and along the flanks of the Cincinnati Arch. Outcrop studies in Tennessee and Kentucky revealed extensive erosion surfaces, formed in response to sea level drop, that are the basis of a sequence stratigraphic subdivision of these black shales. In the subsurface of Indiana, sequence boundaries are identified by a combination of core studies and tracing of gamma ray signatures from the outcrop belt. In addition, truncation of gamma ray motifs provides independent confirmation of sequence boundaries in the subsurface.
At present, the succession of black shales has been subdivided into 14 sequences, ranging in age from Givetian to uppermost Famennian. Sequences may diminish in thickness or disappear completely as we approach the Cincinnati Arch, reflecting onlap as well as erosion during emergence of the arch. Utilizing available biostratigraphic data, lithostratigraphic correlations, and matching of transgressive-regressive cycles, it is possible to link the sequences in the study area to equivalent Devonian strata in Iowa, New York, and to the global Devonian sealevel curve. Because there are more sequences than can be accommodated by global eustatic variations, some sequences may have a tectonic origin. Ongoing biostratigraphic studies are likely to clarify assignment of sequences to global eustatic variations.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana