Paleoenvironmental and Sequence-stratigraphic Reinterpretation of the Upper Devonian–Lower Mississippian Bakken Formation of Subsurface Saskatchewan Integrating Sedimentological and Ichnological Data Solange Angulo 1 , Luis Buatois 1 , and Steve Halabura 2 Angulo, S., Buatois, L., and Halabura, S. (2008): Paleoenvironmental and sequence-stratigraphic reinterpretation of the Upper Devonian–Lower Mississippian Bakken Formation of subsurface Saskatchewan integrating sedimentological and ichnological data; in Summary of Investigations 2008, Volume 1, Saskatchewan Geological Survey, Saskatchewan Ministry of Energy and Resources, Misc. Rep. 2008-4.1, CD-ROM, Paper A-3, 24p. Abstract The Upper Devonian–Lower Mississippian Bakken Formation of the Williston Basin (Canada) is one of the most significant hydrocarbon reservoirs in southeastern Saskatchewan and represents an ideal hydrocarbon system, comprising the source rock, reservoir rock, and cap rock all within the same formation. The Bakken Formation is subdivided into three members: the Lower and Upper members both consisting of shelf black shale, and the sandy- silty Middle Member. Previous interpretations have suggested that the Middle Member was deposited in an open- marine system. Based on detailed core analysis, we propose a revised interpretation. The Lower Member and the lower interval of the Middle Member form an open-marine coarsening-upward parasequence. The parasequence consists of black shale deposited on a shelf; a lower to upper offshore siltstone interbedded with thin layers of silty, very fine-grained sandstone; an offshore-transition of regularly interbedded siltstone and very fine-grained sandstone; and a lower shoreface very fine-grained sandstone. With the exception of the anoxic to dysaerobic shale, these open-marine deposits are intensely bioturbated, containing Phycosiphon, Nereites missouriensis, Planolites, Asterosoma, Chondrites, and Teichichnus. Overlying this succession is an erosionally based, high-angle planar cross-stratified sandstone, followed by thinly interlaminated sandstone and siltstone. A decrease in bioturbation index, the small size of trace fossils, low ichnodiversity, syneresis cracks, and mud drapes all suggest tidal influence and brackish water rather than an open-marine system. A transgressive lag then occurs and open-marine conditions are re-established with deposition of upper offshore interbedded highly bioturbated siltstone and microhummocky cross-stratified sandstone, followed by the shelf black shale of the Upper Member. Three system tracts have been defined in this study: a basal transgressive systems tract, a highstand systems tract, and an upper transgressive systems tract. The latter two are separated by a co-planar surface at the base of the marginal-marine cross-stratified sandstone. In contrast to previous studies, the contact between the Lower and Middle members is regarded as a facies change rather than a surface of allostratigraphic significance. Integration of sedimentological and ichnological data within a sequence-stratigraphic framework provides a robust depositional model for the Bakken Formation. Keywords: Bakken Formation, Late Devonian, Early Mississippian, Williston Basin, Western Canada Sedimentary Basin, sedimentology, sequence stratigraphy, ichnology, ichnofacies, sedimentary facies, Saskatchewan. 1. Introduction Recent oil discoveries in northeastern Montana, North Dakota, and Saskatchewan have focused much petroleum- industry attention on the Late Devonian to Early Mississippian Bakken Formation. Although several studies have been conducted on the Bakken Formation, an ichnological analysis has not yet been presented. The purpose of this paper is to provide a detailed, integrated study of the sedimentological, ichnological, and sequence-stratigraphy characteristics of the Bakken Formation in order to better constrain the formation’s depositional history and thus to improve the understanding of basin evolution during depositional times. Ichnology is a powerful tool for paleoenvironmental reconstructions because trace fossils effectively provide an in situ record of environment and environmental change, based on factors that influence benthic organisms (Bromley, 1996). It can constrain important environmental variables such as salinity, oxygen, and food supply, which are not 1 Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, SK S7N 5E2. 2 North Rim Exploration Ltd., Suite 210, 3502 Taylor Street East, Saskatoon, SK S7H 5H9. Saskatchewan Geological Survey 1 Summary of Investigations 2008, Volume 1