SEDIMENTATION AND DEFORMATION OF THE GEORGE FORMATION, A PALEOPROTEROZOIC CARBONATE PLATFORM IN THE MUSKWA ASSEMBLAGE OF NORTHEASTERN BRITISH COLUMBIA

The ~ 1.6 Ga Muskwa assemblage is a > 6 km thick mixed carbonate–siliciclastic marine succession that crops out in the northern Rocky Mountains and extends into the adjacent subsurface of British Columbia and Northwest Territories. Shallow water shelf deposits of the basal Chischa Fm., are unconformably overlain by deeper water thin-bedded mudstone and sandstone of the Tetsa Fm., exhibiting common soft-sediment deformation. The comformably overlying George Fm., is > 500 m thick. The lower part consists of thin- to medium- and wavy- to lenticular-bedded, sandy and silty dolomitic lime mudstone, with HCS, and cross-lamination indicating progressive shallowing within a storm-influenced, mid- to outer shelf. Disrupted, convolute and microfaulted strata occur locally. At several levels between +50 and +70 m large, sandstone-filled gutters ~0.3–2 m wide, are oriented ~NE/SW in the north and NW/SE in the south. Molar-tooth structure first appear at +60 m, become common after +90 m. They are progressively more abundant in the upper part of the section. Molar-tooth exhibit a wide variety of shapes and densities; in plan view the dominant lineation is NW/SE. It occurs together with ~0.10–0.5 m wide, often nested, gutters, filled with molar-tooth rudstone and/or flat-pebble mudstone intraclasts or quartz sand. In plan view gutters are straight to sinuous and oriented ~NNE/SSW to ENE/WSW. Metre-sized stromatolite mounds (Baicalia) occur sporadically above +140 m. Higher in the George Formation beds tend to be more massive, suggesting an inner shelf setting of a platform or ramp that prograded substantially to the SW. The conformably overlying Henry Creek Fm., 650 m thick, signals increased influx of silt and fine sand and a drastic reduction in carbonate deposition.