Date of Award
Master of Science (MS)
Pressure-solution features of the Birdbear Formation (Devonian), Williston Basin, North Dakota were the subject of this study. In the area studied, the Birdbear Formation is primarily a limestone with mud-rich textures. The investigation contained three principal phases: 1) a study of the parameters influencing the size and shape of the pressure-solution response, 2) the process of reactate dolomite formation and its relation to calcite neornorphism, and 3) the relation of neomorphism to pressure solution.
The principal pressure-solution response types are stylolites and solution seams. Stylolites are pressuresolution seams with a tooth-and-socket, serrated shape. Pressure-solution seams are gently undulating to smooth in shape.
In the first phase of the study, the parameters judged to be the most important in determining the shape and size of the pressure-solution response were depth of burial, texture of the host rock, and the cementation history. The amount of change with depth in individual wells had no detectable influence on the pressure-solution response, whereas a weak relation of increasing amplitude with increasing depth was found between wells. Texturally, the packstones had more peaked pressure-solution seams than did the other limestone textures, and mudstones hosted pressure-solution seams with the largest amplitudes. Because of the predominantly mud-rich textures of the samples, the role of cementation history in influencing pressure-solution response was difficult to determine.
The second phase of the study concentrated on the formation of reactate dolomite, dolomite that grows in or near a pressure-solution seam. It was deduced that the physical environment of the pressure-solution seam is ideally suited to the formation of reactate dolomite. This environment is in the form of a "pressure-solution sandwich" composed of two layers of host rock, surrounding two boundary layers of water, and both around a central layer of clay-rich insoluble residue. These components provide nucleation sites, diffusion pathways, and ion sources for the formation of reactate dolomite.
The relation between neomorphism and pressure solution is that they co-occur in a similar physico-chemical environment. In addition, reactate dolomite and another type of mesogenetic dolomite, point-source burial dolomite, can help propagate neomorphic calcite. The two dolomite types act as magnesium sinks, removing magnesium ions from the local fluid system allowing neomorphism to proceed.
Collier, James C., "Pressure-Solution Features of the Birdbear Formation (Devonian), Williston Basin, North Dakota" (1988). Theses and Dissertations. 59.