Date of Award
Master of Science (MS)
The Birdbear Formation is a subsurface unit that is present throughout North Dakota except where truncated by post-depositional erosion. An angular unconformity is present between the Birdbear and younger strata in areas where the Three Forks Formation does not overlie the Birdbear. An isopach map of the Birdbear, constructed from drill-hole log data, indicates that the formation generally thickens gradually from the erosional limit to a maximum of 119 feet north of the center of the Basin. A structure map o~ the top of the Birdbear shows a basin that reaches 9000 feet below sea level.
The Birdbear is predominantly a fossiliferous limestone and a dolomitic muddy limestone. The formation can be divided into seven lithofacies based on the results of core and thin-section petrography. Each lithofacies is characterized by fossil fauna and other lithologic features.
Epeiric-sea sedimentation is the depositional model proposed for the Birdbear Formation. Lithofacies analysis suggests, after an initial shallow-water depositional environment, the development of three energy zones of an epeiric depositional setting. In addition, a supratidal or supralittoral zone was present. A stromatoporoid bank-complex developed approximately during middle Birdbear time and may be considered to have been superimposed on the epeiric setting. Lithologic and other features of the Birdbear xiii also suggest sedimentation similar to that documented from the present-day tidal-flat complex of the Caribbean Andros Island area and the sabkha of the Persian Gulf Abu Dhabi area.
Four diagenetic zones can be assigned to Birdbear rocks. Dolomite occurrence and interpreted mechanisms of dolomitization best define each zone. A mixing-zone mechanism is believed to be the most significant telogenetic process. This process probably accounts for bleached Birdbear rocks and intercrystalline porosity, such as that found near the Billings anticline. Late diagenesis is indicated by 1) saddle-dolomite rhombs, and 2) anhydrites that have replaced some of these rhombs.
Nodules, stylolites, microst ylolites, some dolomites, and other diagenetic features are interpreted to be effects of pressure response. Pressure-response carbonate diagenesis probabl y occurred through ionic mobilization and precipitation, induced by a particle edge, fluid-film mechanism and a proposed intraunit solution mechanism.
Petroleum occurrence in the Birdbear is related to lithofacies that represent bank and back-bank areas, and to dolomitization.
Loeffler, Peter T., "Depositional environment and diagenesis, Birdbear Formation (Upper Devonian) Williston Basin, North Dakota" (1982). Theses and Dissertations. 179.