Abstract: Tectonic Implications of Crown Point Section, New York

B. Baldwin

A 115-m thick section of Middle Ordovician carbonate rocks is well exposed around old Fort Crown Point on Lake Champlain. The Crown Point Formation (Chazy) includes laminated dolomite (algal mats?) and algae. The Valcour Formation (Chazy) includes shallow channels (intertidal?) and cross-bedded calcarenite (high energy; beach zone?). The Orwell Limestone (Black River and lower Trenton) includes massive micrite (zero energy; lagoon?) and biomicrite with algal masses, corals, bryozoans, some brachiopods and clams, gastropods, nautiloids, and pelmatozoans (shallow marine?). Thin biomicrite beds of Glens Falls Limestone (Trenton) have bryozoans, brachiopods, and trilobites, but no algae or corals. The section indicates a progression from supratidal to moderately deep shelf co ditions.

This model fits rates of crustal subsidence, measured as thickness in meters per million years. Cambrian through Canadian rocks in the Middlebury synclinorium, 20 km east of Crown Point, accumulated at a rate of 17 m per m.y. In Chazy time, subsidence slowed to 10 m per m.y. (5 m at Crown Point) and reefs formed farther north. At Crown Point, subsidence increased to 30 m per m. y. during Orwell and Glens Falls deposition. Data from Shell's 3,808-m test a few kilometers into Canada, St. Armand-Ouest 1, indicated similar thicknesses per m.y. (Canadian, 21 m; Chazy, 5 m; Orwell and Glens Falls, 38 m). Well records indicate 1,450 m of Trenton shale above the Glens Falls, for a rate of 200 m per m.y. Tectonically, the Crown Point section records the end of a subsiding continental platform; beginning of crustal collapse elsewhere about this time was accompanied by emplacement of the Taconic klippe and Thetford Mines ophiolite.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC