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Biostratigraphic Utility of Organic-Walled Phytoplankton, Upper Ordovician-Lower Silurian of Appalachian Basin

G. Kent Colbath

Upper Ordovician-Lower Silurian marine mudstones in the Appalachian basin, which have not been subjected to extensive heating or oxidation, contain abundant organic-walled phytoplankton (prasinophycean algal phycomata and "acritarchs"). In most areas graptolites and conodonts have not been recovered from these rocks, making the phytoplankton particularly important for biostratigraphic correlation. Recent advances have improved the precision with which these microfossils can be used.

By tabulating relative abundance data carefully, an abrupt change in the composition of phytoplankton associations can be recognized at the Ordovician-Silurian boundary. Where these associations can be identified in the subsurface, the Ordovician-Silurian boundary can be located with greater precision and confidence than is possible using the stratigraphic ranges of individual species.

Many supposedly long-ranging species have relatively short stratigraphic ranges, and thus greater utility, as a result of detailed taxonomic studies. Therefore, type and comparative material are important considerations. Also, vesicle wall architecture and dehiscent structures are valuable taxonomic characters. Scanning electron microscopy examination has improved our understanding of small forms (less than 20 µm in diameter), and has thus increased the number of taxa available for use in biostratigraphy.

Further study of samples from vertically extensive stratigraphic sections of established age should help workers refine the biostratigraphy of these microfossils.

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