Abstract The boundary limestone (Middle Cambrian), exposed in the Main Ranges of the Southern Canadian Rocky Mountains, accumulated in a deep-water (≃200 m) embayment adjacent to a near vertical, shelf-margin reef, the Cathedral escarpment. Prior to accumulation of the boundary limestone, reef growth was terminated and the adjacent basin, which had been a site of exclusive carbonate deposition, received argillaceous sediments (Stephen Formation). The boundary limestone accumulated during the only significant period of restored carbonate sedimentation to interrupt the argillaceous basin filling. The boundary limestone accumulated along a broad front from the deposition of lime mud and granular carbonate sediment swept basinward off the adjacent shelf. Reef growth was not re-established on the adjacent escarpment; rather it acted as a sediment by-pass slope, contributing rare redeposited material either as small submarine talus blocks or clasts in thin debris flows. The resulting deposit is a proximal bench of allochthonous debris, approximately 100 m thick, abutting the escarpment. This proximal bench is flanked on its basinward side by a mud wedge tapering to less than 12 m thick. In outcrop, the proximal bench exceeds 180 m in width and its basinward flanking wedge exceeds 3 km in width. There is a distinct difference in the lithofacies of the proximal bench compared to those of the distal wedge. Bench lithofacies consist of medium to dark-grey, thin-bedded to massively bedded intervals of heterogeneously mixed, bioclastic peloidal wackestone to grainstone interbedded with uniformly thin-bedded, dark-grey, basinal lime mudstone intervals. Many of the peloids were derived by micritization of such shallow-water grains as oolites, oncolites and coated bioclastic fragments. In contrast, the interfingering distal wedge consists of dark-grey, thin-bedded, “normal” basinal lime mudstone that is interrupted only by widely spaced thin beds of fine allochthonous calcarenite. Submarine sliding was important in extending the flanking wedge of mudstone further basinward. Periodic slope failure in mud accumulating along the upslope portion of the distal wedge resulted in sliding along mappable slip surfaces. Repetition of these events produced a basinward-thinning stack of lime mudstone slide masses of which the youngest extends furthest into the basin. At carbonate shelf margins dominated by growing reefs, the redeposited margin material interrupting adjacent basin mudstone will include talus blocks and debris-flow deposits. However, if reef growth is terminated, the shelf margin becomes a sediment by-pass slope and the resulting basinal deposit is a debris apron of allochthonous, shelf-derived mud and granular sediment lacking reef debris; such is the case of the boundary limestone. The overall wedge-shaped geometry of the debris apron distinguishes this type of accumulation from the cones formed in submarine channel-fan systems that breach a shelf margin.

Abstract The Walcott Quarry was discovered in 1909 by the Smithsonian Institute's Charles Doolittle Walcott (1850–1927). The Cambrian Burgess Shale (505 Ma, Miaolingian) crops out in the quarry and the lagerstätte is the nexus of ongoing vigorous debate about fossil preservation (including taphonomy and diagenesis), taxonomy, classification, phylogeny, and the origin of phyla and baupläne. Smithsonian Institute's field crews collected from 1909–24, and the quarry was subsequently expanded by Harvard University (1930), the Canadian Geological Survey (1966–67), and the Royal Ontario Museum (1992–2000). Approximately 250 000 fossils, including soft-bodied forms, have been collected, making the Walcott Quarry with exposures of the Middle Cambrian Burgess Shale, a significant geoheritage site and an important representation of the Cambrian Explosion.