Following a period of Whiterockian erosion, Middle Ordovician seas transgressed into the North American continental interior, and a diachronous sheet of clastic sediments (St. Peter, McLish, Winnipeg) lapped onto the margins of the Transcontinental arch. The arch served as a major clastic source region during subsequent deposition (especially Decorah). Middle Ordovician clastic sediments adjacent to the Transcontinental arch and in the Williston basin were probably deposited in a humid climatic regime, and carbonate deposition predominated in an arid belt that included Wisconsin, southeastern Iowa, Missouri, and Illinois, where carbonate oolites and evaporites are noted. Clastic source areas on the Transcontinental arch were inundated, as the marine transgression continue , and a continuous sheet of carbonate sediments (Galena, Viola, Red River, Bighorn) spread across most of the continental interior by the close of the Edenian. Migration of North America across latitudinally-related climatic belts brought the Williston basin area into an arid climatic regime by middle-Late Ordovician as evidenced by the deposition of upper Red River evaporites.

Widespread carbonate deposition was replaced by Dubuque-Maquoketa and Stony Mountain carbonate-clastic deposition as source regions emerged on the Transcontinental arch and Ozark uplift. The lower Maquoketa is conformable with the underlying Dubuque over most of the eastern Mid-Continent, and the joint occurrence of organic-rich shales and phosphorites is attributed to a low-oxygen, phosphate-rich water mass in the Maquoketa sea. The Ordovician closed with red clastic deposition, intermixed with carbonates and/or ironstones, adjacent to the arch.

End_of_Article - Last_Page 805------------