--> Abstract: Dolomitization of Middle Devonian Winnipegosis Reefs, Manitoba and Saskatchewan, by M. Teare and B. Martindale; #91012 (1992).

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ABSTRACT: Dolomitization of Middle Devonian Winnipegosis Reefs, Manitoba and Saskatchewan

TEARE, MARK, and BILL MARTINDALE, Home Oil Company Limited, Calgary, Alberta, Canada

Winnipegosis reefs in the subsurface of southeastern Saskatchewan and in the Manitoba outcrop belt consist of isolated buildups and larger complexes. Although extensively dolomitized, the remarkable characteristic of these reefs is the exquisite preservation of allochems, textures, and fabrics. In the Manitoba outcrop belt, fabric-retentive dolomites dominate the upper buildup sediments whereas fabric-destructive dolomites occur lower in the buildup and in the underlying Lower Winnipegosis platform.

Isotope geochemistry (87Sr/86Sr, 0.7080; oxygen 18, -4.1 to -6.4 o/oo; Carbon 13, +0.5 to +2.9 o/oo) and a slightly enriched Mg:Ca ratio indicate that subsurface and outcrop dolomites were derived from normal to slightly hypersaline Devonian marine waters. The slightly depleted Oxygen 18 value relative to Winnipegosis limestones deposited in normal marine waters (oxygen 18, -4.3o/oo) suggests that dolomitization occurred at slightly elevated temperatures, probably at depths of 600 m or less. In a number of cored wells in southeastern Saskatchewan, deeper water mudstones of the Lower Winnipegosis platform have retained their original limestone lithology.

The foregoing suggests that dolomitization was accomplished by downward-flowing Devonian brines and began at or near the sea floor and continued in the shallow burial environment. Seepage reflux of brines, formed during a period of hypersalinity that terminated reef growth, may have been responsible for early dolomitization. More aggressive Mg-rich fluids, left over after anhydrite precipitation, may have been responsible for dolomitization of the deeper parts of the Manitoba reefs and of reef-flank sediments in southeastern Saskatchewan.

 

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)