The large-scale spatial zonation of the Late Ordovician epeiric sea in North America is reflected in the distributions of bryozoans and other invertebrates. Their major life zones (biomes) reflect changing patterns of sedimentation across the platform, from major siliciclastic to carbonate lithotopes. Deeper water within the epeiric sea most likely existed in an area centered on the Michigan Basin and between the clastic wedges of the Taconic Orogen and the extensive cratonic carbonate platform. Stratigraphic gradients in the Cincinnati Arch area indicate a first-order cycle of transgression-regression throughout the Late Ordovician, on which second-order fluctuations in sea level have been superimposed. The second-order fluctuations indicate three transgressive-regressive subcycles, closely corresponding to the Eden, Maysville, and Richmond stages.
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