--> Abstract: Early Diagenesis of Middle Devonian Winnipegosis Reefs, S.E. Saskatchewan, by B. Martindale; #91012 (1992).

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ABSTRACT: Early Diagenesis of Middle Devonian Winnipegosis Reefs, S.E. Saskatchewan

MARTINDALE, BILL, Home Oil Company Limited, Calgary, Alberta, Canada

Winnipegosis reefs in southeastern Saskatchewan consist of codiacean algal grainstone shoals capped by a framework of red algae, corals, and sponges. Although dolomitized, reefs display a variety of well-preserved diagenetic fabrics, some of which are facies specific and syndepositional in origin.

Locally, framework and shelter porosity was filled by peloidal marine sediment. On windward margins, interparticle pores were lined by isopachous crusts of fibrous cement (originally aragonite or magnesian calcite). Precipitation of this contemporaneous cement stabilized shoals but reduced porosity and permeability significantly.

Reef growth was terminated by subaerial exposure, resulting in the formation of calcretes and other pedogenic fabrics. Freshwater dissolution at this time generated both fabric and nonfabric selective porosity. Solution vugs and fissures may be lined by vadose cements

and filled by laminated internal sediment, washed into the pore network by storms.

Isotope geochemistry of reefs in the Manitoba outcrop belt and geochemical analysis of Saskatchewan buildups suggest that Winnipegosis reefs were dolomitized in the near-surface environment by normal to slightly hypersaline Devonian brines. Undolomitized limestones in the underlying Lower Winnipegosis platform indicate that dolomitizing fluids flowed down through buildups. Near-surface matrix dolomitization preserved fabrics and textures and generated widespread intercrystalline porosity. During shallow burial, larger intercrystalline pores were lined by isopachous crusts of euhedral dolomite. Later dissolution corroded dolomite rhombs and detached crusts from pore margins. Resultant porosity may be occluded by anhydrite.

Intermediate and deeper burial events are represented by stylolites and associated small fractures and by migrated hydrocarbons.

 

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