139-3 The setting of the Burgess Shale at Marble Canyon

Session: Evolution of Life in the Cambrian Seas: Biotic, Biogeochemical, and Sedimentological Contexts, Part II

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Abstract:

The Middle Cambrian Burgess Shale, discovered in Yoho National Park in 1909, fundamentally transformed paleontology’s understanding of the Cambrian Explosion and remains the archetypal Cambrian lagerstätte. A century of subsequent research revealed a small cluster of Burgess Shale fossil localities within the immediate vicinity of Walcott’s Quarry, occurring at or near the toe of the Cathedral Escarpment, a submarine cliff that formed the shelf break regionally. The discovery in 2012 of a rich soft-bodied fossil assemblage at Marble Canyon, ~40 km south of Walcott’s Quarry in Kootenay National Park, not only greatly expanded the range of the Burgess Shale biota but has yielded a striking proportion (~25%) of new taxa. Intensive regional exploration over the last twelve years has now revealed the extent and paleoenvironmental settings of the Burgess Shale at Marble Canyon, allowing for comparison with the type localities in Yoho. The Marble Canyon area, here defined, occurs within a previously unrecognized thrust sheet that is exposed over an area of 21 km long (NW-SE) and up to 9 km across (SW-NE). Faulting has duplicated much of the Middle Cambrian succession regionally by thrusting slope-deposited strata over coeval platform deposits. Several key aspects of the Burgess Shale at Marble Canyon are distinct from the type area: 1. Most importantly, richly fossiliferous soft-bodied assemblages occur in a nearly continuous band ~11 km in length and limited only by exposure. While these assemblages have some faunal similarities, pronounced differences in fossil composition, which have only begun to be explored, are apparent along this transect. 2. There is no evidence for an escarpment locally, although a sharp shelf break is indicated. 3. Rapid facies changes occur in the SW-NE (offshore-offshore) direction and indicate the presence of habitable (oxic) benthic environments, several kilometers in width, that lay downslope of the shelf break, but upslope from the zone of exceptional fossil preservation. 4. Evidence for an oversteepened slope (slumps, slide masses, etc.) increases in the offshore direction, indicating that the most fossil rich localities lay on the edge of a basin that deepened seaward. These findings provide critical context for understanding the relationships of early Phanerozoic ecosystems to the environments that they inhabited.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10944

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