Geology and Diagenesis of the Dawson Bay Formation in the Saskatoon Potash Mining District, Saskatchewan
Ahlstrom, Janice H.
Historically, the Dawson Bay Formation has always been considered a good cap rock for the underlying Prairie Evaporite Formation, but recent flooding of potash mines suggests that this assumption is invalid. This study describes the sedimentology, mineralogy, and diagenetic history of the Dawson Bay Formation in the Saskatoon Potash Mining District, and was undertaken to provide a detailed geological background for future mine planning. The Dawson Bay Formation in the study area ranges in thickness from 53 metres in the east to 31 metres in the west. It is divided into three (locally four) members. They are, in ascending order, the Second Red Bed, Burr, and Neely Members, capped locally by the Hubbard Evaporite Member. The red to grey dolomitic mudstones of the Second Red Bed Member are disconformably overlain by the carbonates of the Burr Member. These carbonates are rich in hardgrounds and a marine fauna of crinoids, brachiopods, and corals. The contact between the Burr and Neely Members is represented by a major hardground. The lower Neely sediments are argillaceous carbonates, which are overlain by a dolomitized, bituminous limestone that contain abundant stromatoporoids and Stringocephalus brachiopods. Overlying these carbonates are organic-rich, peloidal carbonates which contain dolomitized microbial mats; these, in tum, are overlain by interbedded dolomitic mudstone and anhydrite. Locally the Neely Member is overlain by the Hubbard Evaporite which is composed primarily of halite. When the Hubbard Evaporite Member is absent there is a transitional contact between the Neely Member and the First Red Bed of the Souris River Formation. The Dawson Bay sediments were deposited as shallow water carbonates in an epeiric seaway. The presence of hardgrounds suggests periods of little or no sedimentation. Open marine conditions were present during the deposition of the upper Burr and lower Neely sediments. Sabkha-like conditions prevailed during the deposition of the Second Red Bed and upper Neely Members. The main diagenetic events that led to porosity formation in the Dawson Bay Formation are dolomitization, and post-depositional leaching of evaporitic and unstable minerals. Most porosity is secondary, although minor primary intrapartic1e porosity is present. The most significant diagenetic event that affected porosity and fractures was filling by a late halite event. This event significantly reduced the porosity but did not block it completely. Petrography and SEM reveals that the Dawson Bay Formation is more porous than originally thought. There are significant fractures and porosity within the formation. Although most of the porosity is filled with halite, its removal through dissolution would create a reservoir that could prove detrimental to the potash mining.