Ichnology, depositional environments and stratigraphy of the Upper Ordovician Stony Mountain Formation in the Williston Basin, Canada

Abstract

Ordovician epeiric sea carbonates in intracratonic basins of Laurentia are enigmatic due to their unique depositional settings in the absence of modern analogs. Multiple approaches, including the study of both lithofacies and biofacies, as well as trace-fossil analysis, are essential to better understand the depositional setting of epeiric seas. The Stony Mountain Formation is ideal for applying ichnologic analysis to refine subenvironments of Ordovician epeiric sea carbonate due to the abundance of well-preserved trace fossils. Six subenvironments, neritic marine, nearshore marine, open lagoon, restricted lagoon, peritidal sand shoal and peritidal flat, are interpreted along the depositional profile. Whereas ichnofacies analysis records changes from open to restricted conditions along the epeiric ramp, ichnofabrics yield insights into depositional dynamics within subenvironments. Different subdivisions of the Stony Mountain Formation have been adopted not only between Saskatchewan and Manitoba but also between outcrop and subsurface. This confusion most likely resulted from marked sedimentary facies variability across the basin. As a result, the Williams Member is redefined to include only the deposits of the last stage Stony Mountain Formation with two suggested correlations. Progradational successions with diachronous facies boundaries are visible if the proposed correlations were adopted. This study shows the environmental changes and depositional patterns are evident in the Williston Basin with a higher-resolution depositional model. Such depositional sequence implies an impact of eustatic fluctuations overprinting on the second order, tectonic-driven sequence.