Assessing Event Sedimentation in the Bluesky Formation of the Peace River Oil Sands using the Ichnogenus Rosselia

Summary Spectacular examples of well-preserved, stacked Rosselia Dahmer, 1937 have been observed in Cretaceous Bluesky Formation core from Alberta’s Peace River oil sands deposit. Stacked Rosselia segments reflect burrow re-adjustments of a single tracemaker following sedimentation. These traces can be used as a ‘measuring stick’ for determining the magnitude and frequency of sedimentation events (MacEachern and Pemberton 1992; Nara, 1995, 1997). In this study, two cores with Rosselia were logged, sedimentological and ichnological characteristics were scrutinized and the length and number of stacked segments were measured. These cores are interpreted to represent shoreface deposition in a wave-influenced bay. Rosselia burrows record one to two post-depositional re-establishments per tracemaker, where each re-adjustment represents an average 3.6 cm to 5.6 cm of sediment deposition. In extreme cases, up to four re-adjustments totaling nearly 30 cm were observed, reflecting multiple depositional events in a relatively short time frame--months to perhaps two years depending on the lifespan/growth rate of the organism (Seitz and Schaffner, 1995). The use of Rosselia in this study provides high-resolution analysis of modal sedimentation in the Bluesky Formation, a depositional parameter rarely measurable in the rock record but fundamental to understanding sedimentary processes.