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Significance of Integrating Conodont, Graptolite, and Chitinozoan Biostratigraphy in Middle Ordovician of Southern and Central Appalachians

Stig M. Bergstrom

Systematically collected samples from more than 70 sections of Middle Ordovician rocks in the Appalachians from Alabama to Maryland have been used to establish a detailed biostratigraphic framework of integrated conodont, graptolite, and chitinozoan data. Conodont control is available from most parts of the study area, but graptolite data are restricted to the eastern thrust belts, and chitinozoans have been studied only in Alabama, Georgia, and Tennessee. Presently, conodont data give better biostratigraphic resolution than graptolite and chitinozoan data. In particular, morphotypes in rapidly evolving lineages of Amorphognathus, Baltoniodus, Cahabagnathus, Eoplacognathus, and Pygodus have proved useful for local and regional correlations. Incorporating graptolite and ch tinozoan data into the conodont-based biostratigraphic scheme not only improves its resolution, but also makes it possible to use it across facies boundaries into units such as dark shales from which conodonts cannot be readily obtained. Two southeast-northwest transects across the thrust belts in Virginia, two in Tennessee, and one in Alabama provide instructive illustrations of the temporal relations of facies changes in the lower Middle Ordovician transgressive successions. For example, in the easternmost southern Appalachians, the influx of east-derived clastics, generally taken to mark the onset of the Blountian orogenic phase, started in the D. murchisoni graptolite zone and the P. serra conodont zone. Toward the northwest, the base of this clastic wedge becomes progressively young r, being of late N. gracilis graptolite zone and B. gerdae conodont subzone age at the Saltville fault in south Virginia.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.