NATURE AND REGIONAL SIGNIFICANCE OF UNCONFORMITIES ASSOCIATED WITH THE MIDDLE ORDOVICIAN HAGAN K-BENTONITE COMPLEX IN THE NORTH-AMERICAN MIDCONTINENT

Stratal patterns of the Middle Ordovician Hagan K-bentonite complex an d associated rocks show that the Black River-Trenton un conformity in the North American midcontinent formed through the complex interplay o f eustasy, sediment accumulation rates, siliciclastic influx, bathymet ry, seawater chemistry, and perhaps local tectonic uplift. The unconfo rmity is diachronous and is an amalgamated surface that resulted from local late Turinian lowstand exposure followed by regional early Chatf ieldian transgressive drowning and sediment starvation. The duration o f the unconformity is greatest in southern Wisconsin, northern Illinoi s, and northern Indiana, where the Deicke and Millbrig K-bentonite Bed s converge at the unconformity. On the basis of published isotopic age s for the Deicke and Millbrig beds, it is possible that in these regio ns erosion and nondeposition spanned a period of as much as 3.2 m.y. T wo broad coeval depositional settings are recognized within the North American midcontinent during early Chatfieldian time. (1) An inner she lf, subtidal facies of fossiliferous shale (Spechts Ferry Shale Member and Ion Shale Member of the Decorah Formation) and argillaceous lime mudstone and skeletal wackestone (Guttenberg and Kings Lake Limestone Members) extended from the Canadian shield and Transcontinental arch s outheastward through Minnesota, Wisconsin, Iowa, and Missouri. (2) A s eaward, relatively deep subtidal, sediment-starved, middle shelf exten ded eastward from the Mississippi Valley region to the Taconian forela nd basins in the central and southern Appalachians and southward throu gh the pericratonic Arkoma and Black Warrior basins. In the inner shel f region, the Black River-Trenton unconformity is a composite of at le ast two prominent hardground omission surfaces, one at the top of the Castlewood and Carimona Limestone Members and the other at the top of the Guttenberg and Kings Lake Limestone Members, both merging to a sin gle surface in the middle shelf region. The inner and middle shelves r edeveloped later in approximately the same regions during Devonian and Mississippian time.