The upper Red River Formation (Upper Ordovician) in North Dakota contains four porosity zones which are divided into two different styles of syndepositional dolomitization.

The "D" zone is the base of the upper Red River and comprises two regularly interbedded, primary facies: mottled, partially dolomitized, porous mudstone and wackestone overlain by impermeable, organic wackestone and packstone. The burrowed wackestone represents very shallow-subtidal to low-intertidal deposition. The organic packstone is a product of an intertidal to supratidal pond or an evaporite flat.

Overlying the "D" zone are three repetitive depositional sequences, each consisting of basal wackestone-packstone, overlain by dolomitized mudstone (porosity zones "A," "B," and "C") and capped by nodular anhydrite. Each sequence represents a sabkha environment, progressing from subtidal wackestone-packstone through supratidal dolomite to anhydrite. Intercrystalline porosity resulting from syndepositional dolomitization is common to both the "D"-zone burrowed wackestone and the laminated dolomitic mudstone of the other three zones. Sedimentary structures and dolomite petrography indicate syndepositional origin of the dolomite. In all four porosity zones, limited post-burial dolomitization has resulted in porosity occlusion.

Dolomitization of the "D" burrowed facies occurred within the sediment body without subaerial exposure; the burrows are preferentially dolomitized, and the pervasiveness of the dolomitization was controlled by proximity of an underlying impermeable bed, resulting in a mottled texture in the burrowed facies. In contrast, dolomite in the sabkha sequences results from supratidal exposure, and the duration of exposure controlled the amount of dolomitization. Little preferential dolomitization is present in the supratidal dolomite.

End_of_Article - Last_Page 430------------