A New Fossil Lagerstätte from the Stephen Formation, Kootenay National Park, British Columbia: Broadening the Search for Burgess Shale-Type Deposits in the Canadian Rockies  

The Burgess Shale–type (BST) deposits of the Canadian Rocky Mountains, including the classic localities on Fossil Ridge, occur along the basinal edge of the Cathedral Escarpment.  This geological feature has traditionally been considered important in the preservation and spatial distribution of the Burgess Shale biota.  Above the escarpment, the “platformal” or “thin” Stephen Formation is widespread and well-exposed regionally.  BST fossils collected from talus material in 1996 from the Stanley Glacier area, Kootenay National Park (British Columbia) were thought to belong to this formation.  However, the exact stratigraphic origin of the fossils remained unknown.  In 2008, a Royal Ontario Museum party conducted detailed geological studies in this area.  Fossils were excavated from a three-meter thick interval including non-biomineralized animals within mm-thick mudstone beds with diagenetic carbonate cement.  This interval occurs a few meters below the contact with the overlying Eldon Formation in the upper part of the Waputik Member, an upward shallowing succession comprised of six shale–wackestone parasequences.  This succession lies within the Ehmaniella zone and fills an important temporal gap between two of the principal BST biotas, the Burgess Shale and the Wheeler Formations. 

Sedimentological attributes, trace fossils and species composition indicate consistent environmental conditions throughout the section sampled and suggest the recurrence of a single community type.  The epibenthic hyolithid Haplophrentis and the brachiopod Lingulella are numerically dominant, followed by nekto-benthic or pelagic raptorial arthropods, Sidneyia, Tuzoia, Hurdia, and a new Hurdia-like organism.  Chancelloria, the hexactinellid sponge Diagonella, the alga Margaretia and Anomalocaris are also present.  Many of these species are particularly abundant in some basinal localities (e.g., Raymond Quarry) suggesting environmental and/or ecological similarities.  One arthropod and several species of worms represent new species possibly endemic to this environment.  A specimen of Tuzoia with preservation of frontal appendages, which suggest a potential predatory habit, is recorded for the first time in a BST deposit.  Evidence of in situ burial includes an indeterminate worm preserved in its burrow; complex rhabdoglyphid structures; dwelling burrows of endobenthic predators; small traces associated with non-biomineralized carapaces; clusters of articulated Haplophrentis with specimens of various sizes; and putative moult assemblages of Sidneyia and Hurdia.

The presence of diminutive trace fossils directly associated with non-biomineralized animals suggests that carcasses or moults remained near the water-sediment interface during short-lived episodes of dysoxic bottom water conditions.  However, time of residence and decay prior to burial was limited; disarticulation of the carcasses was minimal, suggesting a low-energy environment affected by frequent events of rapid sedimentation.  This conclusion is corroborated by the presence of complete specimens of the delicate sponge Diagonella. The presence of this sponge further indicates a low sedimentation rate and a relatively deep water environment.  The palaeoenvironment was a muddy ramp that lay below storm wave base.  This depositional setting is distinct from the setting of the Burgess Shale localities on Fossil Ridge, but is similar to that of other BST deposits worldwide.  It is likely that BST preservation is regionally widespread in the Waputik Member, compared to the patchy distributions of the classic basinal deposits.