Abstract

Reefs of the Eifelian Edgecliff member of the Onondaga Formation crop out along the Siluro-Devonian strike belt in New York State from south of Albany westward to Buffalo and into Ontario. Subsurface reefs follow a northeast-southwest trend from south-central New York into Pennsylvania. Reef exposures range from small patches (6 m diameter) to large mound/flank structures up to .8 km diameter and 22 m thick. Subsurface pinnacles attain 60 m in thickness and diameters of 3 km.

These reefs exhibit two basic facies, each reef consisting of one or more bafflestone mounds built by phaceloid rugosans (Acinophyllum, Cylindrophyllum, Cyathocylindrium) surrounded by gently dipping (8° to 15°) crinoidal packstone to grainstone flank beds heavily colonized by massive favositids (Favosites, Emmonsia) which are too widely spaced to form a constructional framework. Large overturned favositid and rugosan colonies suggest growth in shallow water; the inability of the reef-core building rugosans to withstand near surface, high energy conditions is shown by the presence of rugosan mound/crinoidal sand bank depositional cycles within both subsurface reefs and the larger surface exposures.

These reefs are unusual due to the near total absence of two important components of typical Devonian reef building communities – stromatoporoids and algae. Their absence is attributed to possible cool water conditions in the basin caused by its being effectively bounded to the east, west and north; the open (southern) end of the basin having been fed by cooler southern temperate currents.