The Acadian orogeny, the second mountain building episode in the Appalachians, occurred during the Devonian period (418-362 mya). In the latest Silurian, a subduction zone developed off eastern Laurentia. The Iapetus Ocean seafloor was subducted, and volcanoes developed on the Laurentian margin. This subduction seems to have pulled the east Laurentian margin down, with the result that Lower Devonian marine rocks occur only east of Syracuse. Fossiliferous Lower Devonian limestones form the Helderberg Escarpment south of Albany and extend south to Port Jervis (Orange Co.). Thin volcanic ashes in the Lower Devonian (seen as clay seams in road cuts on US 20 just west of Sharon Springs in Schoharie Co.) are known to have come from volcanoes in central Maine. The end of the early Devonian featured a sudden deepening, recorded by the nearly nonfossiliferous, low-oxygen environments of the black Esopus Shale from the Helderbergs to Port Jervis. This first mountain-building pulse was followed by quiescence in the early Middle Devonian, when New York was covered by a shallow tropical sea extending from the present Hudson River to Lake Erie and beyond. The sea-level rise brought reef-bearing limestones across the state and into the Midwest; these marine deposits are the Onondaga Limestone, the last major tropical limestone in New York State. Buffalo is built on the Onondaga Limestone, which is also visible in road cuts on the Thruway just north of Catskill (Greene Co). A major pulse of the Acadian orogeny ended Onondaga Limestone deposition by downwarping eastern New York State and depositing a blanket of mud, called the Marcellus Formation, across the state.

fig 41
Acadian Orogeny - Collision of Avalon and Proto North America

The Iapetus Ocean closed with the collision of eastern Laurentia and the Avalon microcontinent. Avalon, once a New Zealand-sized land mass, is recognized by similarity in the rocks and fossils from Rhode Island to northern Nova Scotia and eastern Newfoundland and includes modern Wales, England, and Belgium. As the west margin of Avalon approximately corresponds to the north-south trend of the lower Connecticut River, projection of this trend south suggests that the deep bedrock of eastern Long Island is Avalonian and not part of ancestral North America.

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Continuing Acadian Orogeny and Erosion of Acadian Mountains

The Peekskill Granite was another result of the Acadian orogeny. It appeared in the Devonian as liquid rock punched through the Hudson Highlands of southeastern New York and formed a circular mass almost 10 miles (16 km) in diameter. However, the primary evidence of the Acadian orogeny is a broad, thick apron of sedimentary rocks termed the “Catskill delta.” Sands and muds deposited by Devonian rivers of the Catskill delta now form a thickness of 2,800 feet (850 m) of rock in eastern New York, where the youngest sandstones and conglomerates comprise the Catskill High Peaks. These rocks include subaerial, lacustrine, and river deposits with earth’s oldest forests preserved in Middle Devonian shale and sandstone at Gilboa (Schoharie Co.). Further west, marine deposition continued, and the gray limy shales of the Hamilton Group yield diverse marine fossils from the northern Finger Lakes to the Lake Erie bluffs. Rising and falling sea levels and intervals of greater and lesser sediment drove shorelines east and west across the state through the later Devonian. By the end of the Devonian, shallow seas were limited to westernmost New York State.

fig 43
Erosion; Deposition of the "Catskill Delta"

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The New York State Geological Survey (NYSGS) is a bureau of
the New York State Museum in the New York State Education Department.