Tidal/littoral offshore shelf deposits—Lower Cambrian Gog Group, southern Rocky Mountains, Canada

Abstract

Nearly 600 m of shallow marine fine- to coarse-grained quartzites interbedded with shales of the Lower Cambrian Gog Group are exposed at the Spiral Tunnels section, Kicking Horse Pass, Yoho National Park. Facies are defined and interpreted on the basis of sedimentary structures as follows: (1) interbedded shale and sandstone-pelagic sediments with interbedded coarse-grained bottom current deposits; (2) massive to laminated shale-pelagic sediments; (3) conglomerate-storm deposits; (4) linsen-bedded sandstone in shale-bedload deposits with pelagic or suspended sediments; (5) planar tabular cross-bedded sandstone-small scale: bedload deposits from two-dimensional megaripples; large scale: sand wave or sand ridge deposits; (6) trough cross-bedded sandstone: three-dimensional ripple and megaripple deposits; (7) low-angle to horizontally stratified standstone-shoaling wave deposits; and (8) massive sandstone-well-sorted deposits. Large-scale (1–5 m thick) lateral accretion (epsilon) deposits also occur. Facies trends are interpreted as reflecting the migration and aggradation of offshore sand ridges. Larger sand ridge complexes are comprised of amalgamated individual sand ridge accumulations. Progradation of sand ridge complexes yield coarsening-upward sequences; whereas retreat of sand ridge complexes yield fining-upward trends. Overall the sequences are poorly defined-mainly a consequence of the interaction of sand ridge and inter-ridge swales. Theoretical considerations of the Early Cambrian configuration of continents, Earth-Moon separation, and paleoceanographic conditions suggest that storms were not that important in the emplacement of the sand ridge complexes. Major offshore current systems are interpreted as being tidally driven, with perhaps a secondary littoral or geostrophic current influence. The thick sequence of the section (> 500 m), along with a consistent trace fossil signature, suggests that the Gog Group was deposited under shallow marine offshore shelf settings of a rapidly subsiding passive continental margin. This is consistent with the geophysical model for the proto-Pacific margin-which during the Early Cambrian was characterized by very rapid thermal subsidence.