Abstract: A Middle Devonian Temperate Water Limestone - Isotopes, Stromatoporoids and Shallow Water Facies

Thomas H. Wolosz

The Edgecliff Member of the Middle Devonian Onondaga Formation has long been of interest to exploration geologists because its pinnacle reefs and bioherms are potential natural gas reservoirs. Current evidence indicates that the Edgecliff was deposited in a shallow, temperate water environment suggesting that application of standard tropical carbonate models will be misleading. Three lines of evidence support the temperate water model for the Edgecliff. Carbon and oxygen isotopic analyses performed on 29 samples from the non-luminescent portions of 15 brachiopods with prismatic ultrastructure yield ^dgr¹⁸O values which are isotopically heavier than accepted values for Devonian sea-water, suggesting cool water conditions. The distribution of stromatoporoids (ass med warm water organisms) ranges from rare and small in the eastern part of New York State, to more common and larger in western New York, to large and locally common in Ontario, Canada. This trend in stromatoporoids appears to represent an increase in size and abundance as the distance from the paleo-equator decreases. Finally, previously unrecognized shallow water facies in the Edgecliff including thin, dolomitized and bioturbated carbonate muds, solitary rugosan biostromes and ridge-like fringing coral bioherms negate any possibility that the isotopic and paleobiologic data reflect deposition in deep, cooler waters as opposed to an overall temperate water environment.

AAPG Search and Discovery Article #90954©1995 AAPG Eastern Section, Schenectady, New York