Event Sedimentation, Deposition Rate, and Paleoenvironment Using Crowded Rosselia Assemblages of the Bluesky Formation, Alberta, Canada

Spectacular examples of well-preserved, stacked Rosselia occur in cores of the Lower Cretaceous Bluesky Formation from Alberta's Peace River oil-sands deposit. Stacked Rosselia segments reflect burrow readjustments of single tracemakers following erosion and sedimentation events. Where present, these traces help to refine paleoenvironmental interpretations and can be used as proxies for assessing the magnitude and frequency of depositional events. In this study, six cores were logged in detail. Two representative cores containing densely packed (crowded), stacked Rosselia were described sedimentologically and ichnologically and burrow lengths and numbers were measured. The Bluesky sections are interpreted to represent storm-influenced, wave-dominated delta-front deposits on the basis of modal sedimentation measurements and the presence of storm- and wave-generated sedimentary structures. The stacked Rosselia burrows record up to four post-depositional re-establishments per tracemaker, with each readjustment representing a response to abrupt sediment accumulation. Decimeter-length burrows occurring in the core datasets suggest multiple decimeter-scale depositional events in a relatively short time frame, i.e., months to perhaps a few years, depending on the lifespan and growth rate of the organism. The use of Rosselia in this study illustrates a method of evaluating sedimentation events within a brief temporal window, one lying beyond the resolution of more traditional dating methods. Furthermore, the assemblages of crowded, stacked Rosselia are presented as the first documented occurrence from the Mesozoic and the first recognized in a cored succession.