DYSOXIA AND HIGH SEDIMENTATION RATE: A BAD COMBINATION DURING THE LATE ORDOVICIAN

During Edenian and part of the Maysvillian (Late Ordovician), much of the Appalachian basin was a foredeep in which the seafloor and substrate cycled around dysaerobic conditions. Fossil fauna from New York and Pennsylvania suggest that although Ordovician shelled invertebrates were capable of tolerating decreased dissolved oxygen, they were not well equipped to survive a combination of dysoxia and slightly elevated sedimentation rates. The early Maysvillian, Gulf Stream Shale of New York ("Utica" black shale magnafacies) is laterally correlative to silty turbidites of the Frankfort Formation and is overlain by silty turbidites and tempestites of the Whetstone Gulf Formation. In Pennsylvania, the Edenian (?) Antes Shale of the west-central portion of the Valley and Ridge is laterally correlative to silty turbidites of the lower Reedsville Shale in the Reedsville area. The Reedsville Shale also progrades over the Antes, so that the Antes is overlain by silty turbidites and tempestites. Although superficially barren of benthic fauna, close examination of the black shale (Gulf Stream and Antes) reveals a low diversity benthic fauna that can be relatively abundant at some horizons. This fauna predominantly contains a small orthid brachiopod (Onniella), small ramose bryozoa, crinoids (including Ectenocrinus?), and the trilobites Triarthrus and Flexicalymene. Alternatively, laterally correlative siltstone typically lacks benthic body fossils (although silty, dark shales intermediate between the black shale and siltstone facies do contain Onniella and the trilobite Cryptolithus). Being sessile and easily buried, the small lophophorates were not well suited to survive major sedimentation events and for this reason, are absent from the more proximal dysaerobic facies. It is less clear why crinoids (with stems longer than the thickness of most silt turbidites) and trilobites are largely absent from the siltstone facies. Despite the difference in shelled benthic fauna between the black shale and distal siltstone facies, both facies contain horizons of small Chondrites burrows, indicating that, at least episodically, the substrate was not anoxic.