Late Ordovician climate and sea-level record in a mixed carbonate-siliciclastic-evaporite lithofacies, Williston Basin, USA

Abstract

The Late Ordovician late Katian was a period of generally warm, transitional greenhouse to icehouse climate with sea-level oscillations dominated by moderate-amplitude eustasy. This study aims to improve the understanding of the climatic and sea-level fluctuations prior to the end-Ordovician glaciation, using a comprehensive core, wireline log, and thin-section dataset spanning the topmost Red River-Stony Mountain Formation within the subsurface of North Dakota, Williston Basin, North America. Eleven facies are identified, interpreted, and grouped into four facies associations, including salina to peritidal evaporative, restricted-marine subtidal, open-marine subtidal, and open-marine deeper water. Stratigraphic trends, basin-wide facies variations, and correlation of individual surfaces for ~380 km across the study area, suggest that the facies were deposited under different climatic and sea-level regimes as part of the long-term Stony Mountain sequence. The sequence (37–52 m) is bounded by anhydrites (basin center) and their correlative thin exposure-related breccias (updip proximal basin), and contains a set of high-frequency sequences (HFS; 2–26 m) that can be traced across the study area. The lowstand systems tract (LST) anhydrites formed in a shallow and well‑oxygenated setting under an arid climate during relatively short periods of the basin restriction. The sea-level rise during the early transgressive system tract (TST) resulted in a higher evapotranspiration rate and an increasingly humid climate that peaked during the late TST with the input of siliciclastics off the Transcontinental Arch. The increased precipitation and runoff resulted in quasiestuarine circulation. Return to semi-arid conditions characterizes the highstand systems tract (HST), with increasingly more restricted, shallow-marine and oxygenated setting with antiestuarine circulation. The work provides insight into the interaction of siliciclastic input, climate, and sea-level changes on depositional dynamics and sequence architecture of a mixed carbonate-siliciclastic-evaporite succession that formed in a tropical epicontinental basin setting prior to the end-Ordovician glaciation.