DEPOSITIONAL SEQUENCE FRAMEWORK TIED TO δ13C EXCURSIONS AND CONODONT BIOSTRATIGRAPHY, UPPER ORDOVICIAN (KATIAN) TO LOWER SILURIAN (TELYCHIAN) WILLISTON BASIN, USA

The ~190 m thick Upper Ordovician (Katian)-lower Silurian (Telychian) Williston Basin succession retains a high-resolution record of δ¹³C and sea-level fluctuations spanning the ~13 myr period of extraordinary biotic, environmental, climatic, and sea-level changes. By using a comprehensive core, wireline log, and thin-section dataset integrated with δ¹³C and conodont biostratigraphy, this study improves the understanding of the climatic and sea-level oscillations around the end-Ordovician glaciation. Based on their texture, structures, fossil content, and mineral composition, 16 distinctive facies are identified within the studied upper Red River through the Lower Interlake interval. Twelve complete long-term sequences are identified, 10 of which are bounded by evaporites. Most sequences record a change from humid (semi-arid) TSTs to arid late HSTs-LSTs. All six positive δ¹³C excursions are recorded during periods of sea-level lows in the basin, with their peak values either occurring within tidal-flat laminites or restricted dolomudstones, both of which are associated with lowstand anhydrites of the basin center.

The negative δ¹³C excursion within the Stoughton Mbr. (A. divergens Zone) is likely part of the Boda global warming event, which may have contributed to increasingly more humid conditions around the Williston Basin at that time. The subsequent Elkhorn (=Paroveja) positive δ¹³C excursion in the Gunton Mbr. (upper A. divergens Zone) represents a mid-Boda cooling episode and return to arid conditions. It is followed by a pronounced sharp negative shift (>2‰) that likely records the latest Katian warming before the initiation of the Hirnantian cooling, the latter indicated by the positive LHICE excursion in the lower Stonewall Fm. (A. shatzeri Zone). The significant negative shift (>3‰) in the middle Stonewall Fm. is interpreted to record the mid-Hirnantian interglacial when continental ice temporarily retreated from Gondwana. This is followed by the HICE and major faunal turnover in the upper Stonewall (lower O. hassi Zone), and Early and Late Aeronian excursions in the Lower Interlake (O. strena? and O. expansus? Zones, respectively). The study presents a possible missing link in in correlations between Laurentian margins and its interior for the late Katian-early Telychian interval.