Rocky Mountain Section - 69th Annual Meeting - 2017

Paper No. 19-3
Presentation Time: 2:20 PM


COLLOM, Christopher J., MacCallum Geological Consulting, 326 Wildwood Drive SW, Calgary, AB T3C3E3, Canada,

Aside from its proximity to the renowned Burgess Shale lagerstätten, the middle Cambrian Cathedral Escarpment is a poorly understood feature in terms of its lateral extent, outline, and history of localized destabilization. Here I describe one of two known scallop-like features of the Cathedral carbonate platform margin, referred to as the Natalco Embayment (NatEmb hereafter). This feature is best exposed in the peaks of the Pharaoh Range near Talc Lake, BC. Unique to this area are multiple orientations of the Cathedral Escarpment, as defined by the sharp contact between fossiliferous Burgess Shale and adjacent Cathedral limestone and dolostone. Tracing segments of the NatEmb on satellite photos defines a northwest-southeast oriented ‘notch’ — 1.3 km wide and 1.4 km long — within the greater embayment. The NatEmb extends past The Monarch to the SE, and is defined by basinal fine-grained clastic facies to the west, and platformal carbonate facies to the east. However, continuous outcrop of the Cathedral Escarpment along the ridge separating Talc Lake from Mummy Lake, has platformal carbonates on the west side. Although this would seem to be a reversal of the normal facies relationship and escarpment orientation, the SE extent of the quasi-isolated platform remnant can be determined from a well-exposed NE-SW oriented segment. This is referred to here as the Talc Lake Sub-basin (TLS). The geometry is similar to the Tongue of the Ocean reentrance on the Bahamas Platform, although the Cambrian example is smaller in scale. Within the Monarch Fm (Glossopleura Zone) the earliest sediments deposited adjacent to the Cathedral Escarpment consist of a >30 metre thick sequence of talcose-bedded deposits.These deposits were briefly mined commercially, and are geochemically much different than typical detrital mudstones of the Chancellor Group. Taken in combination with the distribution of soft-tissue fossil preservation in the overlying Burgess Shale and presence of other seep-related features, the talc-rich beds within the TLS are interpreted to have accumulated in situ. The ascending brines and hydrothermal fluids producing these rocks may have been dammed or locally contained by a seafloor topographic sill or barrier preventing the denser brines from gravity drainage into deeper waters along the Laurentian western margin.