BIOSTRATIGRAPHY OF THE UPPER DEVONIAN IN THE CENTRAL SOUTHERN APPALACHIAN BASIN

In a continuing biostratigraphic study of the Devonian in Virginia, more than 30,000 macrofossils have been collected, identified, and correlated. One hundred sixty taxa were identified and placed in a normalized biostratigraphic chart each with their complete local geologic range. Taxa include: brachiopods (85), bivalves (42), gastropods (16), cephalopods, tentaculitids, echinoderms, and arthropods. The data revealed 16 biozones and the Frasnian/Famennian extinction event occurring in two pulses. A critical factor in defining biozones and interpreting patterns of extinction in this data set is the validity of the correlations between stratigraphic sections. Correlations were initially based on index fossils and the order in which the fossils occurred. Graphic correlation methods (CONOP9 and GraphCor) were used to test the correlations, both including and excluding lithostratigraphic data. Another key factor in interpreting these data is the reliability of biostratigraphic ranges. Confidence intervals on stratigraphic ranges provide one method for gauging the reliability of range endpoints and for statistically testing extinction patterns. The results of confidence interval analyses support the initial correlations and pattern of the Frasnian Famennian extinction event. As additional data has been added, only minor adjustments have been made to the initial 16 biozones suggesting that the zones are consistent and may be reliable in a broader region. The 16 biozones almost double the resolution of previous brachiopod biozonation. There are 42 species of brachiopods before the extinctions pulses. After the first pulse, there are only 32 species despite 5 first occurrences. After the second pulse, there are 22 species despite the first occurrences of 4 species. Overall, brachiopod diversity was reduced by 47%. Bivalves and gastropods were unaffected.