Thinning marine zones of the Breathitt Group along Kentucky Highway 15, Breathitt County, Kentucky

Abstract

The coal-bearing Breathitt Group is divided into formations by major, laterally correlative marine zones such as the Betsie Shale, Kendrick Shale, and Magoffin members. In much of the basin, these members are easily recognized by their thickness (10s of m), basal, variably fossiliferous black or very dark shales, and vertical profiles comprising one or more coarsening-up successions. The shales represent major marine transgression followed by highstand progradation of delta front mouth bars into relatively deep water. Large (m-scale) carbonate concretions are common. Major marine zones contrast with “minor” and more marginal marine zones, which are typically a few meters thick, lack black shale, rarely show well-developed coarsening-up profiles and rarely contain marine fossils. Minor marine zones represent delta-top lake or bay-fill successions.

On KY 15, which is located on the western cratonic margin of the basin, major marine and minor marine units are difficult to distinguish because gray shales in the major marine zones are thin (0-5 m), and the distinctive coarsening-upward part of the highstand tract is truncated by overlying sharp-based fluvio-estuarine lowstand-transgressive sandstones. In addition, major marine zones on the basin margin may lack carbonate concretions, and only the base of the Magoffin Member contains abundant marine fossils. Stratigraphic correlations aid in narrowing down the interval in which the major marine zones occur. The Kendrick Shale is thick near Jackson, but mostly truncated northward. In the Betsie Shale, thin intervals of graded beds representative of deeper-water mouth bar or bayhead delta facies help to differentiate it from shallower water gray shale facies. Also, the Betsie Shale along KY 15 contains several large slumps in which part of the original coarsening-up strata have rotated below the incision level of the overlying sandstone. The slump fill provides evidence of at least some of the eroded section towards the basin margin.

AAPG Datapages/Search and Discovery Article #90258 © 2016 AAPG Eastern Section Meeting, Lexington, Kentucky, September 25-27, 2016