The Sebree Trough is a relatively narrow, shale-filled sedimentary feature extending for several hundred kilometers across the upper Middle and Late Ordovician carbonate platform of the Midcontinent U.S.A. The trough shales, containing several epipelagic oceanic graptolite species, stand in contrast to the coeval bryozoan-brachiopod-echinoderm-rich limestones on the flanking platforms. We infer from regional stratal patterns, bounding hardground omission surfaces, thickness and facies trends, and temporal relations established by biostratigraphy as well as K-bentonite and 13C stratigraphy that the Sebree Trough initially began to develop during late Turinian to early Chatfieldian time (Mohawkian Series) as a linear bathymetric depression situated over the failed late Precambrian-Early Cambrian Reelfoot Rift. At the same time, rising sea level and the development of a subtropical convergence zone along the southern margin of Laurentia caused the rift depression to descend into cool, oxygen-poor, phosphate-rich oceanic waters that entered the southern reaches of the rift from the Iapetus Ocean. The trough apparently formed in an epicontinental estuarine circulation system marked by a density-stratified water column. Trough formation was accompanied by cessation of carbonate sedimentation, deposition of graptolitic shales, development of hardground omission surfaces, substrate erosion, and local phosphogenesis. The carbonate platforms on either side of the trough are dominated by bryozoan-brachiopod-echinoderm grainstones and packstones that apparently were deposited in zones of mixing where the cool, nutrient-rich waters encountered warmer shelf waters. Concurrently, lime mudstone and wackestone were deposited shoreward (northern Illinois, Wisconsin, Iowa, Minnesota, Michigan) in warmer, more tropical shallow seas. Coeval upward growth of the flanking carbonate platforms sustained and enhanced development of the trough shale facies.