A NEW ‘PHYLLOPOD BED’-LIKE ASSEMBLAGE FROM THE BURGESS SHALE: STRATIGRAPHIC SETTING AND THE CATHEDRAL ESCARPMENT IN KOOTENAY NATIONAL PARK

The Burgess Shale fauna is best known from Walcott’s Quarry on Fossil Ridge. Here, and at several other localities in the vicinity of Field, British Columbia, the Stephen Formation lies in direct contact with the Cathedral Escarpment, a prominent submarine cliff. A new Burgess Shale locality, discovered in 2012 in Kootenay National Park ca. 40 km southeast of the type area, contains a rich ‘phyllopod bed’ like assemblage in the uppermost Stephen Formation. Local stratigraphy indicates that the locality was situated immediately offshore of a major topographic break, and lies <400 m from the classical ‘thin’ Stephen Formation, yet is distinguished by a significantly greater thickness and fundamental differences in cycle architecture. The ‘thin’ Stephen in the study area is ~35 m thick and conformably overlies cryptalgal laminate, whereas the basinal Stephen Formation is ~140 m thick and is separated from the underlying Cathedral Formation by a megatruncation surface interpreted to represent platform margin collapse. The ‘thin’ Stephen locally is typical of the Yoho-Kootenay region and is characterized by shale-packstone depostional cycles 3-5 m thickness with amalgamated tops. In contrast, the basinal Stephen Formation contains shale – carbonate mudstone depositional cycles 20 -30 m in thickness with no evidence of condensation or reworking at cycle tops. The basinal Stephen Formation at the new locality contains pervasive syn-sedimentary slumps and slide dislocations ranging from 0.1 to 3 m in thickness, indicating significant slope of the depositional surface. The markedly reduced thickness of this section compared to that of the Stephen Formation in the vicinity of Fossil Ridge (>300 m) may represent: 1. A previously unrecognized transitional expression of the Stephen Formation that lay offshore of a topographic break, but was deposited on a steep slope rather than at the toe of an escarpment; 2. Differential subsidence/accommodation space, or; 3. Diachronous platform margin collapse during deposition of the Stephen Formation regionally. The new assemblage underscores the importance of the Cathedral Escarpment to the preservation of the richest fossil assemblages, specifically in facilitating rapid downslope transportation of fossils and fine sediments en masse across chemical gradients.