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