SEA-LEVEL FLUCTUATIONS, CARBON-ISOTOPE EXCURSIONS, AND CONODONT FAUNA AROUND THE ORDOVICIAN-SILURIAN TRANSITION, WILLISTON BASIN

During the Late Ordovician-early Silurian, the Williston Basin occupied the southern tropics and was characterized by deposition in dominantly shallow-marine environments that were perennially isolated from the Cordilleran shelf and the surrounding epicratonic basins of North America. To test if the basin succession contains a high-resolution record of sea-level and δ¹³C fluctuations, this study uses five continuous cores from the basin center spanning the upper Red River (Aphelognathus grandis Zone, Late Ordovician, Katian) through the Lower Interlake interval (Psuedolonchodina expansa – D. staurognathoides zones, early Silurian, Aeronian – early Telychian). The cores were logged in detail and sampled for microfacies analysis, δ¹³C_carb, and conodont biostratigraphy. Correlation between the studied cores was based on wireline logs (gamma ray, combined neutron-porosity and bulk density) with several regional marker beds identifiable. Sixteen identified depositional facies form twelve complete long-term sequences, ten of which are bounded by evaporites recording changes from humid (semi-arid) TSTs to arid late HSTs-LSTs. Six global positive δ¹³C excursions are recognized and tied to conodont biostratigraphy: (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 within the Distomodus kentuckyensis – Pranognathaus tenuis zones and (6) late Aeronian – early Telychian within the Psuedolonchodina expansa – D. staurognathoides zones.

The Williston Basin retained a high-resolution record of sea-level fluctuations and carbon-isotope excursions that, when integrated with conodont biostratigraphy, enable correlations between Laurentian margins and its interior during the major biotic and environmental changes around the Ordovician-Silurian boundary.