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Keywords:

  • Bed-load transport;
  • combined flow;
  • Jurassic;
  • sand ridges;
  • stratigraphy

In south-eastern Alberta, the Oxfordian Swift Formation comprises two unconformity-bounded sequences. Sequence 1 consists of the shale member. It unconformably overlies Bathonian calcareous shale of the Rierdon Formation and consists dominantly of shale with uncommon hum- mocky cross-stratified siltstone. The shale member comprises up to four parasequences that show a subtle upward-fining stacking pattern, suggesting overall deepening in a storm-dominated fully marine basin. Deposition of the shale member was terminated by a fall in relative sea-level, which in turn initiated sequence 2. During lowstand, a network of north-east- to south-west-trending meandering channels incised the top of sequence 1. Despite the preserved morphology of these channels, observations in core suggest that lowstand deposits are absent. These strata were most likely thoroughly reworked by ravinement processes associated with the ensuing transgression and are recorded only by a thin chert pebble lag. Also associated with transgression were significant changes in the physical and ecological conditions in the local Swift basin. These changes were related to modifications in basin configuration and the development of a low-energy strait, which probably formed as a result of uplift of the Sweetgrass Arch. Within this newly configured basin, brackish-water conditions prevailed, and sedimentation was dominated by suspension deposition of mud. Bed-load sediment, composed mostly of very fine and fine sand, made up only a small part of the total sediment flux into the area and was transported principally by low-energy combined flows with variable-speed microtidal currents and a low-energy oscillatory component related to storm waves. Sand was generally deposited in the form of small, discontinuous sand ridges that developed locally throughout the study area – it is these features that form the principal hydrocarbon reservoir bodies in the Swift Formation in the study area. After initiation, ridges built upwards and migrated laterally in response to transport and preferential local deposition of bed-load sediment. Although common near the unconformity along the top of sequence 1, sand ridges also occur at several higher stratigraphic levels in sequence 2. These latter ridges, however, are not associated with regionally correlatable discontinuities but, instead, were initiated by local bed irregularities, possibly related to breaking internal waves, and were supplied with new sediment transported into the study area. The origin of these sand ridges is therefore not related to changes in relative sea-level but, instead, to intrabasinal processes in a low-energy strait.