Abstract

The Upper Cretaceous Chungo Member of the Wapiabi Formation at Mount Yamnuska, Alberta is composed of three cycles: two coarsening-upward progradational successions overlain by a fining-upward transgressive succession. The basal 3 m of the transgressive sequence is made up of interbedded sandstone and conglomerate, with minor shale that was deposited in a nearshore, shallow-marine setting. Discrete "coarse-grained storm beds" in this interval are characterized by: 1) a sharp-based conglomeratic division with local sole marks and onshore-imbricate intra- and extraformational clasts; 2) a cross-stratified conglomeratic division with cross-strata dipping predominantly onshore and capped by symmetrical coarse-grained ripples; 3) a horizontal to sub-horizontally laminated sandstone division containing pebble horizons near the contact with underlying conglomerate and commonly capped by symmetrical ripples; and 4) a capping shale division. Deposition of the coarse-grained storm beds took place in two stages: 1) transport from a beach by offshore-directed, storm-generated combined flows, followed by 2) reworking and selective sorting of bed material by asymmetrical oscillatory currents generated by shoaling swell waves propagating onshore. Stage 2 involved the onshore transport of selected coarse grain sizes, which deposited as the reworked portions of coarse-grained storms beds (divisions 2 and above), concurrent with offshore transport of fine sediment that may be deposited as the top portions of HCS storm beds.

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