Abstract

Beds composed of laminated calcite, dolomite, and anhydrite form the basal unit of the Muskeg and Prairie evaporitic succession in the Middle Devonian Elk Point Basin of western Canada. These laminated rocks (laminites) are restricted to the basal intermound succession between carbonate mounds and banks of the Keg River and Winnipegosis Formations. Laminae occur as couplets composed of a microcrystalline mineral layer and a non-skeletal organic detrital film; bioclasts of benthonic organisms are absent. Individual dolomitic and anhydritic laminae in various sectors of the Elk Point Basin in northern Alberta have been correlated visually and statistically over distances up to 25 km. Graded clastic carbonate beds with eroded basal contacts, conformable tops, tabular clasts, and occasional benthonic bioclasts are common in some dolomitic laminite sections and apparently are more frequent close to the flanks of carbonate buildups.

By comparison with modern depositional basins of various size and water chemistry, the Devonian Elk Point laminites are interpreted as having accumulated subaqueously in a physically and chemically stratified water mass in which the lower water stratum was anoxic. The probable generative mechanism was precipitation within the upper stratum, seasonal in effect and possibly biochemically controlled. Bioturbation and physical disruption of the deposited laminae were prevented by the anoxic bottom environment and the physical stratification of the water mass. The interbedded graded beds are interpreted as turbidites or debris flows related in source to proximity of carbonate buildups.

As both carbonate and sulfate laminae are involved in this interpretation, and as these laminae grade upward into bedded anhydrite and halite deposits, the same subaqueous environmental interpretation, albeit with evolving hypersaline water chemistry, is suggested for the overlying evaporites.

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