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AAPG Bulletin

Abstract


Volume: 55 (1971)

Issue: 10. (October)

First Page: 1759

Last Page: 1786

Title: Carbonate and Evaporite Deposition and Diagenesis, Middle Devonian Winnipegosis and Prairie Evaporite Formations of South-Central Saskatchewan

Author(s): N. C. Wardlaw (2), G. E. Reinson (3)

Abstract:

Algal oncolites, intraformational conglomerates, and dark carbonaceous-argillaceous layers with planktonic or necktonic fossils are interbedded in the upper part of the lower member of the Winnipegosis ("platform" member) in south-central Saskatchewan. Transitions from agitated open-marine waters to quiet restricted conditions apparently occurred abruptly, as did changes from oxidizing to reducing conditions. The algal oncolites and breccias probably accumulated in shallow water, and their extensive areal distribution indicates a sea floor with little relief, a condition which did not prevail during deposition of the overlying upper Winnipegosis carbonates.

In the study area, upper Winnipegosis carbonate banks separate a northern basin from a southern basin and influenced deposition of interbank sediments. Between banks, organic-rich carbonate laminites, which display continuity of specific millimeter laminations over a distance of miles, suggest accumulation under very uniform, quiet-water conditions. Associated anhydrite, some of which grew by displacement in buried sediments, was protected locally from compaction by early diagenetic calcitization and dolomitization.

Nodular anhydrites, up to 200 ft thick, overlie carbonate laminites and encase carbonate banks, occurring preferentially on the southeast sides of the banks within the area of study. In an offbank direction, halite appears to be a facies equivalent of anhydrite. Apparently, the banks influenced deposition between banks and may have been the cause of local marine restriction and evaporite formation.

A distinctive pisolite cap on Winnipegosis banks is interpreted as an inorganically formed vadose pisolite and provides evidence of subaerial exposure at the termination of bank development. An equivalent pisolite unit is present in anhydrite overlying halite in offbank areas, a situation which suggests that the upper part of the carbonate banks may be younger than the carbonate laminites, anhydrite, and halite between banks. Thus, there were two major phases of bank development, one before deposition of interbank anhydrites and halite and one after.

Winnipegosis beds are relatively rich in organic matter, but the relatively low degree of maturation or carbonization may be an important factor in explaining the absence of producing reservoirs in this formation.

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