The lower Middle Devonian Onondaga Formation is a restricted marine limestone that extends throughout southern New York State. Thicknesses locally range from 15 to about 260 ft (5 to 78 m) with the thicker parts generally attributed to organic reefal buildups. The reefs may be algal and mudbank as well as wave-resistant structures, however, they contain porous sections that form natural-gas reservoirs if coincident with structural closure. Early interpretations considered the reefs and tectonic structures independent; however, recent investigations suggest that incipient folding was the main control on reefal formation.

Structure and isopach maps constructed from tops picked on about 1,400 wells drilled in New York State in the area bounded by 75°30^prime and 78°30^prime indicate that anticlinal folds are coincident with isopach thicks that indicate reefal buildups. Organic accumulations thus are interpreted as related directly to Appalachian folding. Breaks in trends accompanied by changes in thickness are related to contemporaneous organic reaction to tectonic relief. This relation not only dates the inception of folding but also simplifies exploration for natural-gas reservoirs. Because the reefs are confined to the anticlines, there is no need for stratigraphic tests off structure.

End_of_Article - Last_Page 1591------------