Abstract

The stratigraphic interval discussed in this paper includes the Elk Point group and Dawson Bay formation of Middle Devonian age. The Elk Point group (60-800 ft. thick) comprising in ascending order the Ashern, Winnipegosis, and Prairie Evaporite formations is overlain by the Dawson Bay formation (100-200 ft. thick). The Ashern consists of 20-50 ft. of brick red shale or claystone resting unconformably on the Silurian Interlake group. The Winnipegosis includes reef and off-reef deposits. The off-reef material (approximately 40 ft. thick) consists of cream to brown secondary dolomite (locally limestone) with black shale partings. The reef material has maximum thickness of 335 ft. and is composed largely of cream-colored secondary dolomite. On the basis of relict textures and fossils and comparison with data on the geologic framework of other reefs, four depositional environments are interpreted within Winnipegosis reef masses: 1. Open marine quiet water. Scattered crinoid ossicles occur in earthy to microsucrosic cream white to light buff secondary dolomite. 2. Open marine to slightly restricted highly agitated water. Oolitic and pisolitic textures are associated with corals, stromatoporoids, and algae(?). 3. Slightly restricted relatively quiet water. Very fine-grained pseudo-oolitic texture is present in dense to microcrystalline buff dolomite with only a trace of fossils. 4. Highly restricted relatively quiet water. Anhydrite occurs with interbedded secondary dolomite similar to the rock type described in 3. The distribution of these interpreted environments suggests that atoll-like reefs existed in the Middle Devonian sea. It is believed that anhydrite was deposited in the central parts of the atoll lagoon during late stages as well as after cessation of reef growth. The reef masses were buried by a wide- spread salt unit (Prairie evaporite) 650-700 ft. thick in off-reef areas and 350-400 ft. thick over reefs. Although it is believed that much of this salt has been removed by post-depositional subsurface leaching, some of the reefs remain covered by 300- 400 ft. of salt. During the final phases of Prairie Evaporite sedimentation, red and green dolomitic shales (locally containing blebs of anhydrite) were deposited to form the "Second Red" (lowermost Dawson Bay). Thence open marine limestones containing brachiopods, crinoids, and stromatoporoids accumulated. This sequence culminated in deposition of salt and anhydrite (uppermost member of the Dawson Bay) followed by red dolomitic shale ("First Red"). Porosity and permeability within the Winnipegosis and Dawson Bay formations depend largely on two factors: (1) the original lithofacies and biofacies (the best porosity occurs in the interpreted Winnipegosis reef front and in dolomitized biostromes within the Daws Bay), and (2) the amount of infilling of pores by sec- ondary anhydrite and salt. Petroleum accumulations may be expected at the updip limits of those Winnipegosis reefs that are presently buried and sealed by salt. Petroleum traps in the Dawson Bay formation may be caused by the possible irregular infilling of pores with salt and anhydrite. The most promising method of oil exploration in these units involves detailed lithofacies and biofacies analyses before and in conjunction wit stratigraphic test drilling.

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