UPPER ORDOVICIAN-LOWER SILURIAN CHEMOSTRATIGRAPHY TIED TO CONODONT BIOSTRATIGRAPHY IN THE WILLISTON BASIN, SUBSURFACE NORTH DAKOTA
Conodont biostratigraphy indicates that the studied interval spans from the Upper Ordovician (Katian) into the Lower Silurian (Telychian). The maximum age range for the composite is from the Upper Ordovician Aphelognathus grandis Zone (upper Red River Fm.) into the Lower Silurian Oulodus expansus? Zone (lower Interlake Gp.). At least six global positive δ13C excursions are identified. From oldest to youngest, these include (1) Whitewater (=Moe) in the upper Red River Fm. and (2) Elkhorn (=Paroveja) in the Gunton Mbr. of the Stony Mountain Fm., both within the Amorphognathus 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. This suggests that the δ13C record of the Williston Basin, although perennially modified by the restriction of its seawater and associated deposition of evaporites, retained a high-resolution record of global δ13C fluctuations.