Constraining the age and depositional framework of the late Paleoproterozoic to early Mesoproterozoic Muskwa Assemblage, northern British Columbia

The Muskwa Assemblage is one of several Proterozoic successions that developed as part of an extensive passive margin succession along the present-day western margin of Laurentia between 1.74 and 1.45 Ga in a back-arc setting facing Bonnetia and northeastern Australia. The >7 km of exposed strata have an almost identical detrital zircon age distribution to that of the Athabasca Basin. The maximum depositional age of 1.766 ± 0.014 Ga is based on a zircon from the Tuchodi Formation. The upper age is constrained by cross-cutting 0.777.3 ± 0.003 Ga Gunbarrel dikes. Direct U/Pb dating of mudstone indicates deposition of the Aida Formation at around 1.571 ± 0.02 Ga, roughly overlapping with sedimentation of the Douglas Formation in the Athabasca Basin. Strata of the Chischa Formation at the base of the assemblage include carbonate, sandstone, and minor mudstone deposited largely above storm wave base in an inner- to mid-shelf setting. The low-angle unconformity at the base of the Tetsa Formation indicates minor tectonic flexure. The carbonaceous mudstone and minor sandstone in the Tetsa Formation were deposited in a distal ramp setting, prior to progradation of shallow-water carbonate of the George Formation. Mixed dolostone, sandstone, and mudstone in the overlying Henry Creek Formation indicate renewed deepening on the ramp. Sandstone and minor mudstone in the overlying Tuchodi Formation were deposited on a wave-, storm-, and tide-influenced shelf. Mudstone and minor carbonate in the overlying Aida and Gataga formations accumulated in progressively deeper water on the outer-shelf and slope.