EUSTACY AND TECTONICS OF THE LATE ORDOVICIAN OF EASTERN LAURENTIA: IMPLICATIONS FROM SEQUENCE STRATIGRAPHIC COMPARISONS OF NEW YORK, PENNSYLVANIA, OHIO, AND KENTUCKY

Sequence stratigraphic comparisons of biostratigraphically correlative late Turinian-Edenian age strata from the Mohawk Valley of New York State, western Pennsylvania, southern Ohio, and central Kentucky (~900 km) show marked similarities. Using abrupt facies dislocations and flooding surfaces, a series of decameter scale sequences are recognized throughout these areas. Despite local variations, these successions show broad similarities. The detailed correlations of Late Ordovician strata between the Appalachian Basin-Sebree Trough-Lexington Platform suggest like conditions over a large area of eastern Laurentia. In these regions Upper Turinian-Rocklandian strata record pulses of the Blountian tectophase of the Taconic Orogeny in the form of widespread k-bentonites. However, these strata also exhibit a layer-cake distribution recording an interlude of relative tectonic quiescence. Shermanian strata show evidence of the onset of the Vermontian tectophase with the appearance of multiple thin bentonites and widespread zones of soft sediment deformation (seismites). These strata also exhibit more abrupt facies change than do the underlying units, but are far from being a facies mosaic. A brief incursion of stromatoporoids and algae may record a short warming period in the middle Shermanian. Deepening of the foreland basin was paralleled by the growth of the Sebree Trough throughout this time. The Sebree Trough apparently became connected to the Appalachian Foreland Basin during the late Shermanian, synchronous with major phases of thrust loading in the Vermontian tectophase, changing circulation patterns, and feeding siliciclastics as well as a new fauna into the Lexington Platform. The repeated appearance of inarticulate brachiopod, graptolite (O. ruedemanni to G. pygmaeus zones), and trilobite (Triarthrus) bearing dark shale tongues occur in the Bromley, lower Kope (Fulton) and upper Kope formations. These dark shale events in the Cincinnati region appear to correlate with episodes of widespread black shale deposition in the Appalachian Foreland Basin associated with maximum flooding during highstands. These correlations further imply eustacy as a driving mechanism for episodes of dysoxia during the Late Ordovician.