Facies characteristics and stratigraphy of an Upper Cretaceous mud‐dominated subaqueous delta: Medicine Hat Member (Niobrara Formation), Alberta, Canada

Abstract

AbstractThe Medicine Hat Member in Alberta, Canada, is a muddy interval with thin sandstones. However, the sedimentology and stratigraphy are poorly understood – a reflection of the limited knowledge of mud‐dominated successions. Here facies analysis and stratigraphy from core and wireline logs were used to construct a depositional framework for the interval in a stratigraphical context. Sedimentological and ichnological results show that the Medicine Hat Member was deposited in shallow water within a mud‐dominated subaqueous delta system under the combined influence of river flow, waves, storms and longshore currents. Four facies are recognized, which are interpreted as distal delta front, proximal prodelta, distal prodelta and interdistributary bay deposits. The distal delta front is dominated by sandy storm deposits and thin storm‐flood hyperpycnites. The prodelta, on the other hand, is characterized by muddy storm deposits and wave‐enhanced sediment gravity flow deposits, with only subordinate hyperpycnites. Ichnological assemblages in both the delta front and prodelta are impoverished, indicating persistent physico‐chemical environmental stresses. Interdistributary bay deposits are uncommon and are differentiated based on more robust ichnological assemblages and higher bioturbation intensities. Subsurface correlations reveal shallowing‐upward parasequences that have aggradational and progradational stacking patterns and a very low depositional gradient (ca 0.03° to 0.1°). The stratigraphy reflects rapid progradation with fixed accommodation in a low‐gradient setting; coeval flexural subsidence in the foredeep was dampened by underlying basement structures. Furthermore, previous suggestions that the Bow Island Arch impacted sediment distributions are not substantiated by the low angle geometry of clinoforms. Data from this study supports the interpretation that Medicine Hat deltas were fed by rivers flowing north through Montana because parasequences are sandier towards the south. This study fills a crucial gap in the literature by providing a rock record example of a mud‐dominated subaqueous delta system and serves as a basis for comparison in core studies of similar units globally.