COBLESKILL MEMBER WITH HALYSITES (CORAL): RECONSTRUCTION OF DEPOSITIONAL ENVIRONMENTS, ROSENDALE, ULSTER COUNTY, NEW YORK

During the Ordovician period, marine life flourished. It was during this period that Earth’s first coral “Lichenaria” was formed. Warm temperatures with calm and shallow seas provided an environment that was not only favorable to the prolific formation of limestones, also to the growth and diversification of corals. In Rosendale, upstate New York, widespread coral development is evident in the abundance of Halysites fossils found in sedimentary rocks such as Silurian limestones and shales. The Cobleskill (Glasco) limestone member – one of the three members that comprise the Rondout formation – exhibits a prominent presence of index fossil Halysites, and is known to be a key stratigraphic marker in the field. Halysites are chains of ancient tabulate corals that only form when specific environmental conditions were met. The ecological conditions in which Halysites proliferated were temperatures between 24 to 26 degrees Celsius and pH from 8.0 to 8.5. Halysites predominated the Silurian period up until the point where conditions changed during the formation of the Cobleskill member. It can be deduced that the temperature and pH levels in the water were no longer optimal for the survival of the Halysites, which lead to its rapid decline and eventual die off. The aim of this research is to show how a slight change in the pH levels during the formation of the Cobleskill member could have led to the demise of the Halysites in the Rosendale, up-state New York.