Cores have been examined in detail from two wells located six miles apart in northwest South Dakota. They exhibit different degrees of dolomitization in the Upper Ordovician Yeoman (Upper Red River) Formation.
Depositional characteristics and early diagenetic alteration of these carbonates is similar to that of the Yeoman of the northern Williston Basin. Early diagenetic cementation as previously proposed (Kendall, 1977) is now questioned.
In addition to dolomite-mottled carbonates that arose by preferential dolomitization of burrow networks, the Yeoman sequence contains carbonates that exhibit nodular structures (produced by preferential lithification of burrows) and those that lack conspicuous burrow structures. These different sediment types have markedly affected the pathways taken by later diagenesis. Variations in skeletal carbonate abundance are primary and have not been significantly affected by later dolomitization. Stratigraphic units composed of skeletal-poor wackestones and mudstones parallel thin euxinic organic rich beds and are thus considered isochronous. One unit (the “D” porosity zone) has a basinwide distribution and has been particularly prone to being dolomitized. It may even correlate with the Cat Head Member of the Manitoba outcrop. Variations in Yeoman lithology are ascribed to variations in water oxygenation during deposition and many Yeoman carbonates were deposited under dysaerobic conditions. Yeoman deposition is believed to have occurred in deep shelf environments.
Chemical compaction has had a profound effect and is responsible for the removal of matrix from many dolomites and the creation of diagenetic grainstone textures by concentrating the more resistant components in the matrix. Breccias composed of broken and reoriented dolomite mottles closely resemble depositional features but also result from matrix elimination by pressure solution. Fractures associated with pressure solution seams and stylolites are identified: previously they had been regarded as being early diagenetic. Relations between dolomitization and chemical compaction are ambiguous and require further study.
