New York in the Middle to Late Ordovician

        Middle to Late Ordovician rocks are exposed at the surface in New York State in a band running along the Lake Ontario shore, in the Tug Hill Plateau, and the St. Lawrence and Hudson valleys.   They are especially well exposed in Mohawk River Valley (see NYS map).  These sedimentary layers were deposited in a foreland basin produced by the crumpling of the crust as the eastern edge of the North American plate was subducted below the European plate as these two plates collided (see illustration in the excellent Union College website by Raymond Gildner). 

        This collision produced the Taconian Orogeny (approx. 450 Myr ago), which gave rise to what are now the highland in eastern NYS and western New England.  The North American plate material was heated as it was driven down in the subduction zone, melting, and giving rise in part to an arc of volcanic islands to the east of the foreland basin.  Ash from these volcanoes was carried into the basin by the prevailing winds.  These ashfalls are preserved as widespread thin light-colored layers of bentonite, the decomposed product of the ash.  As these bentonite layers apparently represent discrete short term eruption events, they provide useful markers to correlate strata in different parts of the sedimentary basin.

        The Middle to Late Ordovician rocks exposed in the Mohawk Valley and its tributaries were deposited in a variety of marine environments ranging from shallow intertidal flats (such as the limestones of the Lowville Formation of the Black River Group) to the deep dysoxic or anoxic basin (such as the Utica Shale of the Trenton Group).  A partial strategraphic correlation chart for the Middle to Upper Ordovician is shown in Figure 1.

Ordovician Fieldsites
   The following sites serve as some examples of fossiliferous Ordivician exposures:

- Wolf Hollow Creek, Old City, Herkimer County
- East Canada Creek at Dolgeville, Herkimer County
- East Canada Creek at Ingham Mills, Herkimer County

References:

Isachsen, Y.W., Landing, E., Lauber, J.M., Rickard, L.V., and Rogers, W.B.,  Geology of New York - A Simplified Account, Educational Leaflet No. 28, New York State Museum/Geological Survey, Albany (1991).

Gildner, R., "Mohawk Valley Fossils", http://zircon.geology.union.edu/Gildner/stack.html

Baird, G.C., Brett, C.E., and Lehmann, D. (1992) The Trenton-Utica Problem Revisited: New Observations and Ideas Regarding Middle-Late Ordovician Stratigraphy and Depositional Environments in Central New York.  In April, R.H., ed., New York State Geological Association 64th Annual Meeting, Field Trip Guidebook, pp 1-40.

Page last revised Nov. 1, 2001