Sedimentology and diagenesis of the Bakken Formation in Daly Field, southwest Manitoba
Edwards, W. Ward
The Bakken Formation (Devonian-Mississippian) is a relatively thin unit that can be found throughout the Wiltiston Basin. The unit consists of upper and lower black, organic-rich shales separated by a calcareous siltstone. These units have been informally defined as the Upper, Lower, and Middle Bakken respectively. In the Daly field, southwestern Manitoba, the Lower Bakken is absent and the Middle Bakken has a sharp erosional contact with the underlying Devonian Lyleton Formation. The Upper Bakken has a conformable contact with the overlying Mississippian Lodgepole Formation. The Middle Bakken is subdivided into three subunits: 1) the Massive Unit, 2) the Horizontal Laminated Unit, and 3) the Wavy Laminated Unit. These subunits are composed mainly of quartz, dolomite, and clay minerals, with variable amounts of anhydrite, gypsum, K-feldspar, pyrite, plagioclase, calcite, and halite. The amount of quartz decreases upwards in the section whereas the clay mineral content increase. The Bakken Formation was deposited on a shallow marine, non-barred, low wave energy shoreline. The Lower Bakken was deposited during a transgression and then later eroded from the Daly field area during a period of regression. The Middle and Upper Bakken were then deposited during a subsequent transgression. After deposition, the Bakken Formation experienced a series of chemical diagenetic changes. Shortly after deposition, there was the formation of authigenic pyrite and dolomite which was later followed by the formation of authigenic illite and the dissolution of detrital K-feldspar. Later in diagenesis, there was continued formation of authigenic illite and formation of quartz overgrowths on detrital quartz grains. Somewhat later in diagenesis illite stopped precipitating and authigenic K-feldspar precipitation occurred. Towards the end of the diagenetic history of the Bakken Formation there was formation of coarsely crystalline, pore-lining dolomite, conversion of pyrite into hematite, and precipitation of anhydrite, gypsum, and halite.