Abstract

Carbonate buildups in the Middle Devonian Winnipegosis Formation constitute important hydrocarbon reservoirs in the Williston Basin and are exposed in outcrop in central Manitoba. These buildups were deposited near the southeastern limit of the Elk Point Basin during the initial stages of the Kaskaskia transgression.

The basal Ashern shale is a thin green dolomitic mudstone which blankets a post-Silurian exposure surface and is overlain by crinoid-brachiopod wackestones of the Lower Winnipegosis. This shallow subtidal sequence formed a widespread platform upon which Upper Winnipegosis buildups developed. These buildups grew up to 60 metres above the surrounding basin floor and consist primarily of bioclastic mudstones and wackestones. Framestone-boundstone lithologies are locally developed but overall appear to comprise a very small proportion of these buildups. Algal laminated mudstones with excellent fenestral porosity and teepee structures are present on the crests of the thicker buildups. Thinly laminated kerogenous mudstones were deposited between the buildups and are interbedded with re-sedimented bioclastic detritus along their flanks. Porosity is enhanced in, but not restricted to the coarse bioclastic portions of the buildups. Early marine cements and pervasive dolomitization have greatly modified the primary textures and occluded most interparticle porosity.

The Winnipegosis buildups and associated kerogenous mudstones were apparently deposited during a rapid rise in relative sea level; their growth was subsequently terminated by a drop in sea level which resulted in basin restriction and deposition of the overlying evaporites.