UPPER ORDOVICIAN-LOWER SILURIAN CHEMOSTRATIGRAPHY TIED TO CONODONT BIOSTRATIGRAPHY IN THE WILLISTON BASIN, SUBSURFACE NORTH DAKOTA

Difficulty in correlating Upper Ordovician-Lower Silurian sections from the Williston Basin with coeval successions from other major basins of North America has hindered efforts to determine the precise order, duration, and timing of events influencing the biotic, climatic, oceanic, and sea-level evolution during this time interval. This study integrates δ¹³C and conodont biostratigraphy to yield a high resolution and continuous Upper Ordovician-Lower Silurian Williston Basin composite. Samples of bulk carbonate powders (n=202) were collected at intervals of 30-90 cm from five cores located in western North Dakota and analyzed for δ¹³C. 152 conodont samples were collected from the same cores at intervals of ~2 meters. Gamma Ray, neutron-porosity and bulk density logs were used to identify lithostratigraphic unit boundaries and regional marker beds, which facilitated correlation between the studied cores and the construction of a composite δ¹³C curve tied to biostratigraphy.

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.