FACIES AND DEPOSITIONAL ENVIRONMENTS OF THE UPPER ORDOVICIAN RED RIVER “C” INTERVAL, WESTERN NORTH DAKOTA

The Upper Ordovician Red River Formation is one of the oldest lithostratigraphic units in the Williston Basin of North and South Dakota, Montana, Manitoba and Saskatchewan, consisting largely of burrowed limestone, laminated dolomite, and up to three evaporite horizons in the upper part. The upper part of the formation is traditionally subdivided into 4 intervals labeled “D”, “C”, “B”, and “A”. Over 510 feet (155 meters) of core and wireline logs in four widely spaced wells in western North Dakota (Green Valley, Divide, Bowman, and McKenzie counties) were used to study the uppermost “D” and the entire “C” interval. The uppermost 20 to 50 feet of the “D” interval occurs near the bottom of each core investigated. It is characterized by heavily burrowed lime mudstone to skeletal wacke-packstone (shallow subtidal, lagoon) that rarely grades into skeletal and peloid packstone. These subtidal cycles rarely contain basal dark-colored, kerogenous skeletal mudstone-wackestone likely indicating a stratified water column. The contact between the “D” and “C” intervals is evident on neutron/porosity logs as a shift from high (top “D”) to low (basal “C”) porosity; in cores it is commonly associated with a change in lithology from porous dolomite or dolomitized limestone to non-porous burrowed limestone, rare pyritized hardgrounds, and friable, rubble-like horizons. The “C” zone consists of three members that are recognized both in cores and on well-logs, from bottom to top: burrowed limestone, laminated dolomite, and laminated and nodular anhydrite. Non-cyclic burrowed member is predominated by shallow subtidal skeletal mudstone to wackestone with abundant fragments of crinoids, gastropods, brachiopods, and bryozoans; corals, ostracods, trilobites, and green algae are rare. Thin (5 cm) sheets of skeletal packstone to grainstone occur near the boundary between the burrowed and laminated members. Cyclic peritidal facies of the laminated member (~35 feet thick) is predominated by flat and crinkly laminated dolomite. Ooid and pellet packstone-grainstone or flat-pebble breccia lags are found on desiccated laminites with tepees, indicating flooding of previously exposed tidal flats. The supratidal laminated and/or nodular, chicken-wire anhydrite member caps the shallowing-upward depositional sequence of the “C” zone.