THE MIDDLE CAMBRIAN STEPHEN FORMATION AT MARBLE CANYON, KOOTENAY NATIONAL PARK, BRITISH COLUMBIA

The newly discovered Burgess Shale biota from Marble Canyon lies 40 km southeast of the type area of the Burgess Shale in Yoho National Park. Extensive investigations in the Marble Canyon region reveal an expression of the basinal Stephen Formation that is different from the type area and from other sections previously described. Nevertheless, physical depositional processes and overall paleoenvironmental conditions represented in the Stephen Formation are shared between the Yoho and Marble Canyon areas. Olistoliths, megabreccias and prevalence of slump features and slide masses indicate that sections in both areas accumulated at the front of the Cathedral Escarpment. However, the thickness and stratigraphic architecture of the unit exhibits prominent differences. At Marble Canyon, the Stephen Formation is significantly thinner (160- 100 m) than at Yoho (up to 350 m), and may be divided into two members, a lower carbonate unit and an upper shale. These are informally termed the Whymper Carbonate member and the Marble Canyon Shale member. Both members are exclusively fine-grained and comprised of variable quantities of clay and micrite with very few allochems and no detrital silt or sand present. The Whymper Carbonate member, which exhibits large lateral variations in thickness (<20 to >80m), is comprised of thin bedded micrite with shale partings and internal mm-laminations. It appears to represent the offshore equivalent of the Narao member of the thin Stephen Formation, which locally is comprised of laminated algal carbonates that accumulated atop the escarpment. In the Whymper member, large-scale bed geometry and variation in thickness reflect the migration and coalescence of distributary fans that delivered micrite across the Escarpment to prograding fine-grained fan systems at its front. The transition to the Marble Canyon Shale member reflects a local drowning of the carbonate-producing environments atop the escarpment and an influx of fine-grained clastics. Its mm-to-cm bedded claystones bear evidence of rapid deposition from density currents that had strength to transport rip-up clasts as well as members of the benthic fauna. Ongoing work seeks to provide geochemical constraints on stratigraphic relationships and water column chemistry across the Cathedral Escarpment and between the two regions.