Sedimentology, organic petrology, organic geochemistry, and petroleum potential of the Middle Devonian Winnipegosis formation in southwestern Manitoba, Canada
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The Middle Devonian Winnipegosis, Elm Point, and Ratner formations are parts of the Elk Point Group, which is a cyclical succession of carbonate and evaporite rocks that were deposited within the Hume-Dawson sequence in the Western Canada Sedimentary Basin. In Manitoba, these formations occur in the subsurface in the southwestern part of the province and are partly exposed in the Devonian outcrop belt along the eastern erosional margin of the Elk Point Basin. The lower and upper members of the Winnipegosis Formation are composed of ramp and isolated reef/platform units respectively. Within the study area, the western platform margin is approximately coincident with the Birdtail-Waskada Axis, which reflects the Precambrian Superior Boundary Zone, and the northern margin is approximately coincident with the contact between the Precambrian Uchi and English River domains, which raise the issue of tectonically controlled basin differentiation. At the end of Winnipegosis time, the Elk Point Basin became restricted and the basin was filled by interlaminated carbonate and anhydrite of the Ratner Formation followed by the deposition of anhydrite, halite, and bittern salts (Muskeg and Prairie Evaporite formations) as a result of evaporative drawdown. Based on detailed core examination, fifteen lithofacies have been recognized in the Winnipegosis, Elm Point, and Ratner formations in the study area: A) non-fossiliferous mustone, B) mottled-nodular skeletal wackestone-mudstone, C) nodular-patterned mudstone, D) bituminous laminite, E) interlaminated mudstone-anhydrite, F) massive-mottled mudstone, G) septarian-type nodular skeletal packstone, H) bedded-mottled peloidal packstone, I) intraclast floatstone, J) bituminous skeletal wackestone-floatstone, K) peloidal-udoteacean packstone, L) stromatoporoid floatstone, M) coral-stromatoporoid rudstone-framestone, N) laminated mudstone-peloidal packstone, and O) microbial bindstone. These lithofacies have been grouped into five lithofacies associations: ramp, platform interior, platform slope, isolated reef, and basinal salina. The ramp association (lithofacies, A, B, C, D) comprises the Lower Winnipegosis Member/Elm Point Formation and includes deposits of inner to mid-ramp settings as well as off-shore tidal flat islands. The platform interior association (lithofacies D, H, I, O) and platform slope association (lithofacies D, F, G, H, I, M, N, O) comprise the Upper Winnipegosis Member and include deposits of open to slighly restricted lagoonal and lower to upper slope settings respectively. The isolated reef association (lithofacies I, J, K, L, M, N, O) occurs in the Upper Winnipegosis Member isolated reef deposits and includes proto-reef, lagoonal, fore-reef, reef-crest, and reef flat deposits. The basinal salina association (lithofacies D, E) comprises the Ratner Formation. Bituminous laminites (lithofacies D) occur in the ramp-to-platform and ramp-to-basin transitions, platform interior and platform slope successions, and in basinal successions. Based on examination of representative samples of these bituminous laminites using white and ultra-violet incident light microscopy, organic facies indicative of either normal or elevated phytoplankton productivity (blooms) were interpreted based on the maceral assemblages present. Bituminous laminites in the study area were deposited primarly as a result of elevated phytoplankton productivity; three bloom facies were identified: BLI, cyanophyte blooms; BL2, cyanophyte and Prasinophyte blooms; and...