Tectonic and Flexural Significance of Middle Devonian Graben-Fill Sequence in New Albany Shale, Central Kentucky

Stephen F. Barnett, Frank R. Ettensohn, Charles Mellon

The third tectonic phase of the Acadian orogeny began in the late Middle Devonian, and the sedimentary record of that event is largely restricted to the deeper, more proximal portions of the Appalachian foreland and Illinois intercratonic basins. Much of the intervening area, on and near the Cincinnati arch, was uplifted and subjected to erosion by movement on the peripheral bulge accompanying the initiation of the third tectonic phase.

However, bulge movement also reactivated basement fault systems in Kentucky and created a series of grabens that were filled with eroded sediments and debris flows from adjacent horsts. Although rarely preserved, a buried Devonian graben along Carpenter Fork in Boyle County, central Kentucky, reveals such a sequence. The graben is bounded by upthrown blocks of Middle Devonian Boyle Dolomite, which also floors the graben. Within the graben a black-shale unit, apparently absent elsewhere, conformably overlies the Boyle and grades upward into debris-flow deposits represented by the Duffin breccia facies of the New Albany Shale. The Duffin contains clasts of the shale, as well as of chert, silicified fossils, and fine to boulder-size dolostone clasts eroded from the Boyle high on the flan s of the graben. The underlying shale also exhibits evidence of penecontemporaneous soft-sediment deformation related to the debris-flow emplacement of Boyle residue in the graben and due to later loading by the Duffin.

The character and age of the beds preserved in the graben strongly suggest tectonic control of sedimentation and are consistent with lithospheric-flexure models of bulge-basin development.

AAPG Search and Discovery Article #91023©1989 AAPG Eastern Section, Sept. 10-13, 1989, Bloomington, Indiana.