ABSTRACT: Middle Cambrian Outer Platform Collapse: Implications for the Paleoenvironmental Setting of the Burgess Shale

STEWART, W. D., University of Ottawa and Ottawa-Carleton Geoscience Centre, Ottawa, Ontario, Canada

The Burgess Shale was deposited at the foot of the Cathedral Escarpment, which marks the seaward termination of about the upper third of the Middle Cambrian Cathedral carbonate platform. The Cathedral Escarpment has traditionally been interpreted as a depositional feature that formed in response to a sustained, rapid relative sea level rise. Significantly, however, it is comparable in scale and geometry to large-scale erosional surfaces that truncate outer platform strata belonging to the overlying Eldon and Pika formations. These features, termed megatruncation surfaces, are the product of large-scale, outer platform collapse.

At the level of the Eldon and Pika formations, the megatruncation surfaces cut back as much as 1 km into the platform, and visibly truncate up to 215 m of platform strata. All are sharply overlain by deep-water carbonate and siliciclastic sediments. The surfaces are typically listric in form, and most merge platformward with horizontal platform strata. One notable example has a near-vertical headwall and passes basinward into a subtle, subhorizontal surface virtually free of allochthonous debris. Regional stratigraphic information and comparisions with modern analogs suggest that individual surfaces extend 56 km or more parallel to depositional strike, and perhaps tens of kilometers across strike.

There is every reason to suspect that similar collapse events occurred at other times in the history of the Middle Cambrian margin. In this context, the Cathedral Escarpment is most plausibly interpreted as the near-vertical headwall of a regional slide scar. This interpretation is supported by observations at a newly discovered locality about 60 km southeast of the Burgess Quarry and reaffirms that the Burgess Shale accumulated in relatively deep water at the foot of a submarine cliff.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)