UPPER ORDOVICIAN THROUGH LOWER SILURIAN CONODONT BIOSTRATIGRAPHY OF THE CENTRAL WILLISTON BASIN: AGE CONTROL FOR RECOGNIZING SIX GLOBAL POSITIVE δ13C EXCURSIONS

Five drill cores from the center of the Williston Basin provide the unique opportunity to document the conodont biostratigraphy of the basin at its depocenter. This continuous succession is comprised of, from oldest to youngest, the Red River Fm., Stony Mountain Fm. (Stoughton, Gunton, and Williams Mbrs.), Stonewall Fm., and the (Lower) Interlake (Sub)Group. The studied interval begins in the Aphelognathus grandis Zone (Late Ordovician, Katian) in the upper Red River Fm. and ends in the Oulodus expansus? Zone (lower Silurian, Telychian) in the lower Interlake Gp. This conodont biostratigraphy provides age control for recognizing six global positive δ¹³C excursions; (1) Whitewater (=Moe) in the upper Red River Fm. , (2) Elkhorn (=Paroveja) in the Gunton Mbr. of the Stony Mountain Fm., both within the Aphelognathus divergens Zone; (3) LHICE (KaH?) in the lower Stonewall Fm., Aphelognathus shatzeri Zone; (4) HICE in the upper Stonewall Fm., Ozarkodina hassi Zone; (5) early Aeronian and (6) late Aeronian in the lower Interlake Gp., Oulodus strena? and Oulodus expansus Zone, respectively. Conodonts are relatively abundant and are typical of the shallow-water conodont fauna of the North American Midcontinent in the Aphelognathus grandis Zone through the Aphelognathus shatzeri Zone. There is a sharp drop in conodont abundance in the Hirnatian near the Ordovician-Silurian boundary, where the Aphelognathus biofacies is replaced by an impoverished Ozarkodina biofacies. Although the absence of graptolites in this shallow-water environment hinders the precise location of the O-S boundary, the faunal shift in the upper Stonewall Fm. and the presence of the O. strena? in the Interlake Gp. support the idea that the O-S boundary is at the top of the Stonewall Fm.