Patterns of Species Diversity in Helderberg Group of West Virginia and Virginia: ABSTRACT

Abstract

Twenty-five outcrops of the Lower Devonian Helderberg Group in West Virginia and Virginia were analyzed for stratigraphic, petrographic, and paleontologic variations to interpret depositional environments. Fossil counts, by species, were made along 260 line-transects of standard sampling lengths in 14 of the 25 outcrops. The stratigraphic study suggests that Helderberg sediments were deposited in the Virginias during an overall Early Devonian transgressive phase of deposition, and that formations comprising the Helderberg Group in the study area are rock-stratigraphic rather than time-stratigraphic units. Thirty-three interspecific fossil associations involving 10 of 34 species of New Scotland invertebrate macrofossils were identified using a chi-square test for independence. Species showing significant associations were grouped into a primary recurrent group and a related secondary recurrent group by the Fager method. There is a direct relation between the number of interspecifically associated strophomenids in a sample and the percentage of fine-grained sediment. An index of species diversity was calculated for each outcrop using the Fisher, Corbert, and Williams logarithmic End_Page 1589------------------------------ series equation, d = (s-1)/1nN where S is the number of species showing significant associations and N is the number of individuals. A species diversity map constructed by contouring the computed indices of diversity reveals that at least three fossil communities existed in the Helderbergian sea throughout the study area. The boundaries of these communities are undulatory and subparallel with the axis of the Appalachian basin. Areas with lower species diversity are interpreted as deeper water environments. End_of_Article - Last_Page 1590------------