ENVIRONMENTAL SIGNIFICANCE OF A GEOGRAPHICALLY AND STRATIGRAPHICALLY PERSISTENT LATE ORDOVICIAN TRILOBITE BIOFACIES

Late Ordovician outer ramp/basinal environments, characterized by black shale or laminated lime mudstone, typically contain low diversity benthic faunas limited to sponges and a few genera of trilobites rarely found above storm wave base. In Laurentia, this environment is often inhabited by a cryptolithid-dominated trilobite assemblage that also includes raphiophorids and remopleuridids. Ranging from the Mackenzie Mountains in northwestern Canada, to the Viola Group of south-central Oklahoma, to the Edinburgh Limestone of Virginia, this fauna is geographically widespread and also stratigraphically persistent from the Turinian to the Gamachian. Facies yielding this recurrent trilobite association are interpreted as representing relatively deep water that was cool and dysoxic, suggesting that the cryptolithid-raphiophorid-remopleuridid biofacies has potential as an environmental indicator. The Upper Ordovician Sugar River Formation of New York yields an unusual, relatively shallow-water, cryptolithid-rich fauna whose occurrence may reflect changes associated with the development of the Taconic Foreland Basin. Onset of foreland basin development and propagation westward into New York State is expressed in the Trenton Group by the appearance of black shale interbedded with dark, organic-rich limestone typical of the Sugar River Formation. Cryptolithus and the bryozoan Prasopora increase upward through the underlying Kings Falls Formation and into the Sugar River. Abundant bryozoan colonies, both branching and domal, make up a large portion of the clasts in the grain- to rudstone of the Sugar River, suggesting deposition above storm wave base. New collections indicate that Cryptolithus dominates the trilobite fauna at most rudstone horizons, but diversity is slightly higher than in similar biofacies containing cryptolithines; associated genera include Flexicalymene, Isotelus, Ceraurus and "Cyphoproetus". Emergence of a low diversity, cryptolithine-dominated trilobite fauna into relatively shallow water supports suggestions by previous authors that cool, poorly oxygenated water inundated inner ramp/inner shelf environments during formation of the Taconic Foreland Basin.