Refined Lithostratigraphy of Upper and Middle Devonian Shales in West Virginia
The evolution of the formal lithostratigraphy for the Middle and Upper Devonian strata in the central Appalachian basin has been complex. The original terminology (Chemung, Portage, Marcellus, Catskill, and others) was assigned late in the 19th century. However, by the 1930s, the realization that these units changed in nature dramatically when traced laterally resulted in a crisis in stratigraphy that forced the differentiation of chronostratigraphy from lithostratigraphy. Further, the sheer size of the basin creates challenges, as stratigraphic terminology defined by examination along the outcrop belts at the basin margins is difficult to extend into the subsurface due to the distances involved, the different nature of outcrop and subsurface data, the different perspective of workers attempting to extend units either from the north and west or from the east, and the complex facies changes within the strata. As a result, the facies architecture and associated lithostratigraphic nomenclature for many Middle and Upper Devonian units remained unsettled, particularly in the basin center. This study uses log data from nearly 400 wells from West Virginia to produce detailed maps of the economically important, organic-rich facies and identifies various associated vertical and lateral lithostratigraphic unit boundaries. The study focuses on the Middle Devonian Hamilton Group and its constituent Marcellus and Mahantango formations. Within the Marcellus, a lower Union Springs Member, a middle Cherry Valley Member, and an upper Oatka Creek Member are defined within northeastern West Virginia only. Throughout the rest of the subsurface of the State, the Marcellus has no distinguishable members. In the Upper Devonian, the occurrence and limits of the Harrell Shale (and its basal Burket Shale Member), and its westward lateral transition into the largely-correlative Genesee Formation (with basal Geneseo Shale and upper West River Shale members) are mapped. Maps also detail the position at which the Sonyea Formation (with basal Middlesex Shale and upper Cashaqua Shale members), West Falls Formation (with basal Rhinestreet Shale and upper Angola Shale members), Java Formation (undifferentiated), and lower part of the Huron Member of the Ohio Shale transition eastward into age-equivalent strata of the Brallier Formation.
AAPG Datapages/Search and Discovery Article #90335 © 2018 AAPG 47th Annual AAPG-SPE Eastern Section Joint Meeting, Pittsburgh, Pennsylvania, October 7-11, 2018