DEPOSITIONAL ENVIRONMENTS OF THE UPPER DEVONIAN OF WESTERN NEW YORK STATE: FACIES ANALYSIS OF THE CONNEAUT AND CONEWANGO GROUPS

The Upper Devonian sedimentary rocks of western New York State contained the key petroleum producing horizons from the mid-1800’s to the mid 1900’s with well over 10,000 wells on record, drilled in Allegany and Cattaraugus counties. Despite all of the early activity in the region of southwestern New York and western Pennsylvania, the Famennian-aged sediments arranged in upsection order of the Canadaway, Conneaut and Conewango groups have not been studied since the advent of modern sedimentary concepts e.g., storm deposition, sequence stratigraphy. For the past 10 years we have measured 5,760 m. of stratigraphic section (at the cm scale) of the Upper Devonian sedimentary rocks in Allegany and Cattaraugus counties. By documenting observed sedimentary structures and trace fossil assemblages we have been able to trace the changes in depositional environment with the goal of developing a sequence stratigraphic model for the Late Devonian sediments of Allegany and Cattaraugus counties of western New York State. During the period of time in which the Machias Fm. of the Upper Canadaway Group and the entire Conneaut Group were deposited the area was an offshore, shallow water system that was heavily dominated by storms. The units within the Machias Fm. and the Conneaut Group contain thick storm-deposition sandstone packets (1.5 –12.5 m) that typically display a wide lateral variation in thickness, consistent with offshore sandbar deposition. Lithologically, the storm-deposition sandstone packets are indistinguishable from one another, and present difficulties in recognizing the Cuba Fm. at the base of the Conneaut Group from sandstone packets that occur higher and lower in the stratigraphic section. The transition from the Conneaut Group to the Conewango Group displays an abrupt shallowing from the offshore marine sediments to nearshore sandstones and terrestrial shales and siltstones. The Wolf Creek and Salamanca conglomerates are thick (2 - 4m), lenticular, and probably represent tidal estuaries; they may be incised valley deposits that mark the Conneaut-Conewango boundary. Overlying the conglomerates, the Conewango Group is comprised of the nearshore to alluvial plain red-beds of the Cattaraugus Fm. that in turn are overlain by the nearshore, marine-deposited green sandstones and marine shales of the Oswayo Fm.