Manetoe Facies--A Gas-Bearing, Megacrystalline, Devonian Dolomite, Yukon and Northwest Territories, Canada

Abstract

The Manetoe facies is a broadly stratiform and regionally developed (38,000 km2 or 15,000 mi2) body of white sparry dolomite hosted in Lower and Middle Devonian strata, and is the northwestward continuation of the similar, but better known, Presqu'ile facies dolomite. Several large gas fields have been discovered within anomalously thick developments of the Manetoe dolomite close to the northwest limit of the Devonian Elk Point basin of Alberta. The external geometry of the Manetoe facies indicates that it may have been a cavern or karst system excavated in the late Middle Devonian by a large, vertically confined coastal aquifer. The local regions of anomalously thick Manetoe facies may have developed where the aquifer stoped upward into strata above the confining aquiclude of the Headless Formation shale. Dolomite cementation may have occurred during any of three possible episodes of large-scale fluid movement. Fluid-inclusion and strontium isotope data favor the hypothesis that Manetoe dolomites precipitated from heated (150°-210°C or 300°-410°F) hypersaline evaporitic brines that have interacted with the underlying Precambrian crust, probably in the late Devonian. Lead-isotope data support the general concept of pre-Mesozoic dolomite cementation, and largely preclude a late Early Tertiary deep burial setting. Spectacular zones of detrital dolomite cement within the Manetoe are most readily accommodated by a near-surface, very early Middle Devonian aquifer mixing-model origin, but the presence of these zones does not preclude the other possible origins. Carbon and xygen isotope data from the Manetoe facies support the concept that the Presqu'ile and Manetoe facies are physically continuous and had a common origin.