'False Bakken' interval- sediment patterns and depositional architecture at the facies boundary between siliciclastic mudstones and carbonates, Lodgepole Formation, Mississippian in the Williston Basin, ND

Abstract

The lateral facies transition on deep shelves between carbonates and siliciclastic mudstones is largely enigmatic. Based on detailed facies descriptions and interpretations, this study explores which processes have shaped the sedimentary rocks on both sides of this lithological divide, and adds to our understanding of processes operating on deep shelves in general. Both siliciclastic and carbonate rocks of the 'False Bakken' and 'Scallion' intervals of the lower Lodgepole Formation in the Williston Basin, ND, can be grouped into twelve facies: these facies are graded argillaceous mudstone (F1), massive siliciclastic-argillaceous mudstone (F2a), massive calcareous-argillaceous mudstone (F2b), bioturbated pyrtitized bioclast-bearing mudstone (F3), lenticular mudstone (F4), bioclast-rich wavy mudstone (F5), siliciclastic siltstone (F6), glauconitic siltstone (F7), calcareous siltstone (F8), massive to bioturbated carbonate mudstone (F9), nodular skeletal wackestone (F10), and laminated skeletal packstone (F11). These facies are here presented in order of increasing grain size, carbonate content, and bioturbation from F1 to F11. They are arranged in three fining- and coarsening-upward units that can be identified throughout the basin within the succession. These twelve facies are interpreted to represent distinct processes on a low-inclined shelf system with carbonate occupying the proximal, and siliciclastic mudstones the distal portions of this transect. An overall decrease in energy is reflected from the proximal carbonate to distal siliciclastic facies in this sedimentary system. Nevertheless, most of the mudstone facies still reflect high energy processes operating within the distal portions of the basin; in fact, only one mudstone facies is interpreted to reflect suspension settling under tranquil conditions. Therefore, this study suggests that storm wave base is best placed within the distal siliciclastic mudstones instead of in the proximal carbonates. Carbonate mudstones, deposited above storm wave base but lacking tempestite deposition are therefore interpreted as having been subject to intense degradation of storm-derived bioclasts. A decrease in oxygen concentration is inferred from proximal carbonates to distal siliciclastics as indicated by the decrease in size and type of burrows; yet, the presence of burrows within the most distal facies belt indicates that at least dysoxic conditions prevailed throughout the Williston Basin during the deposition of the 'False Bakken'. Three transgressions and regressions are identified within this succession based on laterally correlated facies patterns and indicate an overall increase in sea level from the beginning to the end of 'False Bakken' times. Sediment starvation occurred in the northeastern and/or southwestern portions of the basin as indicated by the presence of glauconitic siltstones and/or lenticular mudstones at various locations within the succession. However, a source of sediment input is interpreted to be located in the northwestern part of the basin based on a high abundance of detrital silt. In addition, a shift in the basin depocenter southwards from Bakken to lower Lodgepole times is reflected in this succession most likely mirroring an increase in subsidence south of Mountrail County during 'False Bakken' deposition.