THE CUNNINGHAM FORMATION: A MIXED CARBONATE/SILICICLASTIC RAMP IN THE TERMINAL PROTEROZOIC OF WESTERN CANADA

The Cunningham Formation is a mixed carbonate/siliciclastic, ramp to platform succession, which tops a thick section (kilometer scale) of passive margin sediments in eastern British Columbia. This formation is one of the youngest, well-preserved carbonates from the terminal Neoproterozoic, a dynamic time in earth history characterized by global changes in tectonics, climate and metazoan evolution. Defining the platform geometry and isotopic chemostratigraphy of the Cunningham Fm. provides one of the best means of recognizing and interpreting variations in seawater chemistry in Western Canada at this time and can lead to a greater understanding of Neoproterozoic Earth.

The Cunningham Fm. has been split into three main facies belts, based on groundwork by Ferguson (1994), that extend for nearly 40 km from Northeast to Southwest; proximal nearshore peritidal deposits in the northeast, an eastern middle siliciclastic storm dominated facies, and a platform/ramp shelf edge facies in the south. The shelf edge facies is the thickest and comprises thin-bedded limestone grading upward into a thick-bedded grainstone-packstone facies, interpreted as a high-energy shoal environment at the outer edge of the platform. The middle siliciclastic facies is composed of varied lithologies including, laminated limestone, ooid-pisoid packstone, siliciclastic mudstone and hummocky cross stratified and parallel laminated quartz arenite. This facies is interpreted to have been deposited mostly by storm event sedimentation. The siliciclastic facies formed a massive wedge tapering to the southwest, filling the lagoon landward of the outer grainstone-packstone deposits. The inner platform also contains cyclic sediments in thin, dolomitized beds, and is the only area in the Cunningham Fm. that contains stromatolites. This zone is interpreted as near shore since the siliciclastic rocks are interbedded with dolostone (possibly primary) and rare stromatolites. The restriction of stromatolites to relatively quiet water lagoonal and nearshore facies may indicate that substrate mobility of the outer platform inhibited stromatolite colonization.