Abstract

The Mississippian seas covering Western Canada shallowed northeastwards towards a shoreline on the Canadian Shield and deepened southwestwards to the present Cordilleran area. Various types of limestone, controlled in the main by depth of water and the degree of contamination by terrigenous sediment, were formed in belts across this shelving epicontinental sea. Changes in sea level caused these belts to move back and forth. The genesis and diagenesis of the resulting cyclic sequence of limestones and dolomites are discussed in terms of the typical succession exposed in the Moose Dome inlier of the Foothills of southern Alberta.

The Banff formation at the base consists predominantly of argillaceous, cherty, pasty limestones formed by the anaerobic rotting of skeletal detritus below wave base. Bioclastic lime-sandstones (Pekisko formation) appeared as the seas shallowed, first with interstitial pasty matrix and then clean-washed as the energy of the depositional environment increased. Thin oolitic beds are the first sign of increased salinities. Continued shallowing segmented the seas into lagoons where lime-muds were precipitated to form the lithographic limestones of the Shunda formation. Associated dolomite-muds accumulated in local, more saline lagoons.

A return to open shelf conditions started the second cycle with the deposition of clean-washed lime-sands (Turner Valley formation) composed predominantly of crinoid ossicles and bryozoan fragments. Reservoir rocks of porous and permeable dolomite are common in these sediments, particularly those formed from the more heterogeneous bioclastic detritus. The dolomitization is believed to be epigenetic, — formed during a late phase of diagenesis.

Cryptocrystalline dolomites, brecciated by the solution of associated anhydrite, occur in the overlying Mount Head formation, which represents the evaporitic phase of the second cycle.