QUANTIFYING SEQUENCE STRATIGRAPHIC ARCHITECTURE USING FOSSIL ASSEMBLAGE DATA: AN EXAMPLE FROM THE MIDDLE DEVONIAN OF NEW YORK STATE

Fossil biotas are often more sensitive recorders of environmental parameters than simple lithologic characters. In this sense, fossil data can be a useful tool in understanding and comparing consecutive parasequences. For example, faunas collected from two successive maximum flooding surfaces (MFS) can be compared to determine whether both have a similar taxonomic composition, indicating similar environmental conditions. If faunas are found to be dissimilar, however, one MFS may have been deposited in slightly more sediment starved conditions, or during a period of relatively deeper sea level. While these slight differences may not be reflected in lithology, they are often reflected in biotic assemblages. Knowledge about the life habitats and environmental preferences of taxa can help to elucidate why the MFSs are different. Similarly, assemblages collected within a parasequence can be helpful in delineating the shallowest and deepest deposits, and how abrupt the changes between the two conditions are. If a parasequence records a gradual rise, then faunas within it should show a gradual transition from deep water taxa to shallow water taxa. However, if conditions rapidly change from shallow to deep (i.e. at a flooding surface) then faunal changes will be dramatic and distinct groups of taxa will be found.

Parasequences from the Hamilton Group and Tully Formation of the Middle Devonian Appalachian Basin of New York State were analyzed using several multivariate statistical techniques to quantify similarities and differences among successive cycles. A variety of environments from both units were sampled and taxa were counted from closely spaced stratigraphic horizons. Taxonomic composition was controlled by sedimentologic/environmental variables, as distinct faunas are found in specific lithofacies. In particular, Detrended Correspondence Analysis (DCA) was found to array samples along a gradient based upon their position within systems tracts. DCA further provides a metric by which to evaluate the similarity of successive parts of parasequences. Fossil assemblages provide an independent tool that can be used to help delineate important sequence stratigraphic horizons and to understand how environmental conditions within successive cycles may or may not have changed with time.