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Data from the “C” interval of the Ordovician Red River Formation in Divide and Sheridan counties show depositional environments of the “C” burrowed carbonate, “C” laminated carbonate, and “C” anhydrite are remarkably persistent. Diagenetic events, however, are highly localized, and dolomitic porosity is discontinuous.

Environmental conditions changed during deposition of the Red River “C” interval. At the base of the interval is a burrowed carbonate deposited in a well-circulated, subtidal, normal marine environment. Gradually a restricted subtidal, relatively quiet water environment developed and the “C” laminated carbonate was deposited. Intertidal conditions may have existed intermittently. The sequence is capped by the “C” anhydrite deposited in a hypersaline, subtidal restricted environment. Red River paleotopographic highs coincide with “C” laminated carbonate thins and contain vertical microfractures healed with anhydrite. In contrast to laterally persistent dolomite of the “C” laminated mudstone, dolomite of the “C” burrowed carbonate is discontinuous. Anhydrite crystals within the “C” burrowed carbonate are always associated with dolomite and may have been derived from downward percolating sulphate-rich brines from the overlying “C” laminated dolomite, or “C” anhydrite.

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