Stratigraphy and sedimentology of the uppermost Frasnian Jean Marie Formation of Northeastern British Columbia and adjoining Northwest Territories
Hudema, Timothy C.
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The Jean Marie Formation is a limestone unit in the subsurface of northeastern British Columbia, northwestern Alberta, and the southwestern Northwest Territories, where it crops out south of the Mackenzie River. This limestone sequence consists of an extensive carbonate platform about 12 m thick, which thickens westward to over 160 m, then "pinches out" in the Fort Simpson shales at about 122o W. To the south and east, the Jean Marie and overlying Redknife shales grade into continuous carbonates. Various rock types observed in the Jean Marie Formation have been described, and eleven distinct lithofacies have been identified within three depositional settings. The three depositional settings reconstructed include the platform setting, the patch reef setting, and the barrier reef setting. The platform setting consists of three lithofacies. These reflect sedimentation in a subtidal-, a shallow subtidal-, and a deeper subtidal environment. Four lithofacies were discriminated in the patch reef setting. They are interpreted to have been deposited in a shallow shoal-, a moderate-energy reef-, a sub-turbulent reef flank-, and a low-energy lagoonal evironment. Six lithofacies were described in the barrier reef setting. These lithofacies reflect sedimentation in a turbulent reef flank-, a high-energy reef-, a moderate- to low-energy back-reef-, and a shoal environment, as well as in an open-marine environment with an influx of shale. The depositional history of the Jean Marie Formation has been summarized in four distinct growth stages. Growth Stage 1 is reflected by the formation of a relatively subdued carbonate platform sequence, which thickens abruptly along a north-south line, then terminates in "basinal" shales westward. Shallowing of the platform, indicated by biostromal growth, marks Growth Stage 2 of the Jean Marie Formation. During this stage, patch reefs also formed on the platform, and a barrier reef developed on the thickened platform margin sequence. Growth Stage 3 is reflected by the presence of deeper water sediments over the carbonate platform and most of the platform patch reefs. Backstepping and pronounced vertical upbuilding occurred beyond this stage in some areas of the barrier reef. During Growth Stage 4, a final terminating event ended carbonate sedimentation on the Jean Marie platform. The end of this stage is marked by the deposition of the dark shales of the Redknife Formation over the Jean Marie carbonates. The diagenetic history of the Jean Marie Formation reflects periods of both submarine and burial cementation. Calcite, Fe-calcite, and later dolomite, are represented in various lithofacies of the Jean Marie. Ostracod assemblages identified from the Jean Marie place the formation in the Lower DFr 5 ostracod zone. The Jean Marie ostracods can be correlated with those of the Cynthia Member of the Nisku Formation of west-central Alberta, the Escarpment Member of the Hay River Formation in the southwestern Northwest Territories, and the lower Birdbear Formation in southeastern Saskatchewan.