Stratigraphy and conodont paleontology of late Silurian-early Devonian strata of western Virginia
Biostratigraphic study of the Late Silurian-Early Devonian strata of western Virginia reveals the presence of at least three potentially useful multielement conodont apparatuses. Two multielement species of the genus Ozarkodina may prove to be useful in determining the boundary between the Silurian and Devonian Systems in the Appalachian Basin. Currently, a single form species, Icriodus woschmidti woschmidti, which was found in the lowermost New Creek Limestone at Low Moor, Virginia, is used by previous agreement to denote the base of the Devonian system. In the absence of this standard paleontologic indicator, multielement species of the genus Ozarkodina aay be useful. Ozarkodina steinhornensis eosteinhornensis is identified from the Silurian Tonoloway Formation in the area of New Castle, Virginia. Ozarkodina steinhornensis remscheidensis, which is transitional to Oz. s. eosteinhornensis, has been identified in the overlying Late Silurian-Early Devonian Keyser Formation and Early Devonian New Creek Limestone near New Castle and Low Moor. Oz. s. remscheidensis, which is reported to occur first in the Gedinnian, has been suggested as a possible alternative indicator of lowest Devonian strata (Walliser, 1971). However, because of the transitional relationship of these two multielement species, abundant yields of conodonts are necessary to determine the first appearance of Oz. s. remscheidensis.
Elements belonging to a third conodont apparatus, Delotaxis elegans, have been recovered from the Keyser and New Creek intervals at the same locations. Delotaxis elegans may ultimately prove to be a significant biostratigraphic marker with further study of these units.
An overview of the Late Silurian-Early Devonian strata in Alleghany, Botetourt, Craig, and Roanoke Counties is developed to provide a regional perspective of this stratigraphic package and to illustrate significant biostratigraphic markers. Unconformities, wedge-outs, facies changes, and thickness variations are examined within the study area based on nine measured sections and a review of pertinent literature. Clastic units and quartzose carbonates within the Helderberg Group are discussed with regard to source area.