DEEP SUBTIDAL DEPOSITS, EARLY CAMBRIAN SEKWI FORMATION, NORTHWEST TERRITORIES, CANADA

The well-exposed Early Cambrian Sekwi Formation, Mackenzie Mountains, Northwest Territories, Canada preserves a relatively intact record of both carbonate platform and coeval basinal sediments. The lower Sekwi Formation (~480 m thick) consists of deep-water carbonate and shale intercalated with abundant debris flows and turbidites. Deep subtidal deposits of the Sekwi Formation are separated into seven facies: 1) shale; 2) carbonate mudstone; 3) nodular limestone; 4) slope-derived conglomerate; 5) slope- and shelf-derived conglomerate; 6) sandstone; and 7) bioclastic packstone. Both shale and carbonate mudstone were deposited in quiet deepwater, and are commonly interbedded with calcisiltite generated by turbidity currents. Nodular limestone indicates the presence of bioturbation on the slope. Matrix and clast composition of slope-derived conglomerate consists of both carbonate mudstone and calcisiltite, indicating redeposition of these slope facies by debris flows. Slope and shelf-derived conglomerate consist of carbonate mudstone and calcisiltite clasts as well as clasts of shelf-derived sandstone, pelletal/ooid packstone, bioclastic packstone, and algal-archeocyathan reef. Slope and shelf-derived conglomerate record westerly debris flow transport to the lower slope. Sandstone and bioclastic packstone are interpreted as shelf-derived storm and turbidity current deposits. Both conglomerate facies commonly grade upward into laminated and cross-laminated calcisiltite or sandstone, indicating deposition of two-layered sediment gravity flows. Debris flow deposits are difficult to correlate across the basin because of later thrusting. Slope angles of the Sekwi Platform were limited by the lack of reefal organisms and abundance of fine-grained siliciclastic sediment. Extensive debris flow deposits across the basin, widespread shale deposits (~130–200 m thick) that thin dramatically to the east, and slump folds all -suggest syndepositional faulting. We hypothesize that down-to-the basin faulting along the western (outboard) platform edge promoted slope instability, generated debris flows, and created accommodation space for the shale.