Date of Award
Master of Science (MS)
F.D. Holland, Jr
The Tyler Formation, of Early Pennsylvanian age, in southwestern North Dakota may be divided into upper and lower units, reflecting both a change in lithology and depositional environments. The lower unit is dominated by varicolored, noncalcareous shales and mudstones, siltstones, thin coal beds, and medium-grained sandstones. The upper unit in the areas of the Square Butte to Fryburg fields may be divided into a lower subunit, dominated by dark gray to grayish-black, argillaceous limestones and calcareous shales, and an upper subunit dominated by grayish-red, anhydritic limestones, varicolored to reddish-brown, calcareous shales, and locally, thin anhydrite. In the area of the Dickinson field, the upper unit is dominated by a variety of lithologies which indicates rapid changes in depositional environments.
In the depositional model suggested for the Tyler Formation, the lower unit is interpreted to represent sedimentation on a progradational delta plain. Distribution of sandstones in the lower unit indicates the presence of two trends: an east-west trend (in Golden Valley to Stark Counties) of medium-grained, well-sorted, quartz arenites, interpreted as a delta front deposit, and to the south, a northwest-southeast trend of medium-grained poor to fairly well-sorted, quartz arenites, interpreted to be distributary channel-fill deposits.
The controlling influence affecting depositional environments in the upper unit was the formation of barrier islands in what is now Billings and Stark Counties. Barrier island development was followed by north-to-northwestward progradation of estuarine-lagoonal environments in the Dickinson field area (Stark County) and transgression of a shallow anoxic sea in the area of what is now Golden Valley and Billings Counties. Continued north-to-northwestward shoreline migration in the Dickinson field area, and the withdrawal of the Tyler sea in the areas of the Square Butte to Fryburg fields (Golden Valley and Billings Counties) created similar depositional environments throughout the region in the uppermost Tyler. Depositional environments are characterized as tidal-flats in the area of the Square Butte to Fryburg fields and marsh in the remaining areas bordering the Tyler sea.
Each sandstone depositional environment had its own affect on diagenesis and porosity development. Characteristically, channel sandstones have low original porosity and permeability due to the large percentage of detrital clay matrix. Porosity and permeability are reduced by the emplacement of authigenic kaolinite and late stage ankerite cement. Delta-front sandstones are tightly cemented by anhydrite: as much as 30 percent of the original porosity has been eliminated by precipitation of calcium sulfate derived from local hypersaline lakes. Detrital clay coatings on quartz grains inhibited overgrowth and total cementation. Late-stage authigenic kaolinite has further reduced primary inter-granular porosity and permeability.
Sturm, Stephen D., "Depositional environments and sandstone diagenesis in the Tyler Formation (Pennsylvanian), southwestern North Dakota" (1982). Theses and Dissertations. 290.