Hyperpycnal Flow Deposits in Jurassic Units of the Western Canada Sedimentary Basin: Key Evidence for Deltaic Sedimentation during the Early Development of the Foreland Setting

Evidence for hyperpycnal flows during the early phase of foreland basin development in Western Canada is present in the subsurface Jurassic Nikanassin Formation of Alberta. The Nikanassin Formation has remained an enigma in many respects due to limited published work, sparse core control, and the complex nature of the depositional setting. Through understanding the paleogeographical evolution of the Nikanassin Formation, insights into the development of the foreland basin can be ascertained. Depositional models are critical to exploration for tight gas sands in the Nikanassin Formation, an important target in the Deep Basin of Alberta.

Strata in the lower portion of the Nikanassin Formation consist of upward coarsening cycles 15-25 m thick on average. The basal fine-grained portions of these cycles are dominated by thin (0.5-5 cm) beds which grade from very fine sandstone to mudstone. Massive and planar laminated divisions characterize the basal sandier portions of the beds; syneresis cracks are characteristic of the upper muddier portion of the beds. The bioturbation index is generally low however some beds are intensely bioturbated. The trace fossil assemblage is dominated by Phycosiphon, although diminutive Chondrites, Cosmorhaphe, Planolites and Thalassinoides are present locally.

Hyperpycnal flows are usually generated when high-density river effluent enters a standing body of lower density water. These depletive gravity flows commonly deposit thin normally graded beds. Their presence in the lower portions of upward coarsening cycles coupled with evidence for significant salinity stresses, including a low-diversity trace fossil assemblage and syneresis cracks, leads to the interpretation that hyperpycnal flows were an important depositional mechanism during deposition of the units studied. The recognition of these deposits in the Nikanassin Formation provides evidence that deltas were a major constituent of the paleogeographic setting in the developing foreland basin.