The Ordovician Sebree Trough: An oceanic passage to the Midcontinent United States

The Sebree Trough is a relatively narrow, shale-filled sedimentary feature extending far several hundred kilometers across the Middle and Late Ordovic ian carbonate platform of the Midcontinent United States. The dark graptoli tic shales within the trough stand in contrast to the coeval bryozoan-brach iopod-echinoderm-rich limestones on the flanking platforms. We infer from r egional stratal patterns, thickness and facies trends, and temporal relatio ns established by biostratigraphy and K-bentonite stratigraphy that the Seb ree Trough initially began to develop during late Turinian to early Chatfie ldian time (Mohawkian Series) as a linear bathymetric depression situated o ver the failed late Precambrian-Early Cambrian Reelfoot Rift. Rising sea le vel and positioning of a subtropical convergence zone along the southern ma rgin of Laurentia caused the rift depression to descend into cool, oxygen-p oor, phosphate-rich oceanic waters that entered the southern reaches of the rift from the Iapetus Ocean. The trough apparently formed in a system of e picontinental estuarine circulation marked by a density-stratified water co lumn. Trough formation was accompanied by cessation of carbonate sedimentat ion, deposition of graptolitic shales, development of hardground omission s urfaces, substrate erosion, and local phasphogenesis. The carbonate platfor ms on either side of the trough are dominated by bryozoan-brachiopod-echino derm grainstones and packstones that were deposited in zones of mixing wher e 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 en hanced development of the trough shale facies. Five widespread diachronous late Mohawkian and Cincinnatian omission surfac es are present in the carbonate fades of the Midcontinent. These surfaces i nclude sub-Deicke K-bentonite, DSI; top of Black River Limestone, DS2; base and top of the Guttenberg Limestone Member of the Decorah Formation, DS3 a nd DS4; and top of the Trenton Limestone, DSS. Some of the surfaces corresp ond to previously described depositional sequence boundaries. All five surf aces, which embody deepening phases on top of high-stand-systems tracts, co nverge in the Sebree Trough, indicating that the trough was a long-lived fe ature and was the source of eutrophic waters that episodically spread acros s the adjacent platforms, terminating carbonate production. Late Turinian a nd early Chatfieldian incipient drowning episodes were followed by a final drowning event that began in the Sebree Trough during the late Chatfieldian (Climacograptus spiniferus Zone) and reached southernmost Minnesota and ot her regions far within the platform interior by Richmondian Time (Amorphogn athus ordovicicus Zone).