HOW SCHERR IS IT? STRATIGRAPHIC REVISION OF THE UPPER DEVONIAN GREENLAND GAP GROUP IN WEST VIRGINIA AND VIRGINIA BASED ON MAPPING WITH LIDAR-DERIVED ELEVATION IMAGERY

Upper Devonian marginal marine clastic strata of the Greenland Gap Group were defined by Dennison (1970) to include the Scherr and Foreknobs Formations. The type section was measured along State Route 42 at Scherr, West Virginia. Today, exposures at the type section are so weathered that unit divisions are unrecognizable from the original descriptions; however, lidar-derived elevation imagery clearly illustrates the packages of ridge-forming sandstone and siltstone upon which Dennison (1970) had based stratigraphic divisions. Subsequent mapping on lidar-derived basemap imagery has also illustrated that the individual beds used to define the unit divisions pinch out along strike, and thus the contacts of the Greenland Gap Group as originally defined have not been mapped with consistency. Moreover, Dennison (1970) applied an arbitrary cutoff for the mapping of the Scherr Formation as the state line between West Virginia and Virginia, thus causing problems for seamless map compilation across the state line.

This presentation will provide justification for the following revisions to the stratigraphic nomenclature: 1) abandon the Greenland Gap Group and Scherr Formation, retaining only the Foreknobs Formation; 2) abandon the lowest Mallow member of the Foreknobs Formation due to the inability to trace a mappable contact among adjacent quadrangles; and 3) include strata of the Scherr Formation and Mallow member within the Brallier Formation. The newly proposed contact between the Brallier and overlying Foreknobs is defined by the mappable ridge-forming sandstone package equivalent to the base of the Briery Gap member of the Foreknobs. This contact is readily distinguishable by its topographic expression, and by the clear lithologic change moving up the section marked by the first appearance of a coarse, poorly sorted sandstone that is often conglomeratic. Newer roadcuts (c. 2010) on U.S. Hwy 48 that run parallel to the original type section located just 4 km to the northeast along strike provide a new reference section, and support the revisions proposed above.

Reference:

Dennison, J.M., 1970, Stratigraphic divisions of Upper Devonian Greenland Gap Group ("Chemung Formation") along the Allegheny Front in West Virginia, Maryland, and Highland County, Virginia: Southeastern Geology, v. 12, no. 1, p. 53-82.