Katherine M. Bergman

Early dolomitization and recrystallization of carbonate in an evaporite basin: the Middle Devonian Ratner laminite in southern Saskatchewan, Canada

This study provides an example of early dolomitization of basinal limestone associated with a giant evaporite deposit. The Middle Devonian Ratner carbonate is extensively to completely replaced by microcrystalline to finely crystalline dolomite in the southern and central part of southern Saskatchewan in the Elk Point Basin. In the northern part, however, individual cored intervals of the Ratner laminite are either completely dolomitized or contain no microcrystalline to finely crystalline dolomite. Stratigraphic, petrographical and Sr-isotopic data suggest that the Ratner dolomite has formed penecontemporaneously from dense, evaporitic seawater that percolated downward. The distribution of the Ratner dolomite is highly localized in the northern part of the study area, suggesting that the downward flow of Mg2+-rich fluids was restricted by interbedded anhydrite, but where the fluids penetrated, the underlying carbonate deposits were dolomitized. The depletion in 18O isotope (varying from −4.5 to −6.6‰ PDB), and the coarser and inhomogeneous textures of the Ratner dolomite are considered to be the result of recrystallization, which is interpreted to occur at elevated temperatures related to a thermal anomaly during the Late Devonian and Carboniferous burial.

Sequence stratigraphy of the Middle Devonian Winnipegosis carbonate-prairie evaporite transition, southern Elk Point Basin

The Middle Devonian Winnipegosis reefs in the southern Saskatchewan portion of the Elk Point Basin contain extensive vadose diagenetic features, such as dissolution breccia, and cavities/caves filled by microbialite, pisolite and anhydrite. Basinal facies adjacent to reef buildups are characterized by the Ratner laminite, which consists of three brining-upward successions of laterally continuous, laminated dolomite and anhydrite. The basal cycle starts with a relatively thick unit of anhydrite-free, millimeter-scale dololaminite, changing upward into interlaminated carbonate and anhydrite, and ending with enterolithic, nodular to mosaic anhydrite. Subsequent cycles generally lack the dololaminite of the basal cycle. The Ratner laminite grades upward into the bedded to massive mosaic anhydrite of the Whitkow Member (lower Prairie Evaporite) in areas adjacent to reefs. Deposition of the Ratner laminite and the Whitkow Anhydrite is interpreted as genetically related to the vadose diagenetic processes, when the Elk Point Basin became restricted. The carbonate laminite in the basal Ratner was accumulated when seepage of fresh marine water through the barrier kept pace with the rate of evaporation, preventing a complete drawdown and desiccation of the basin. Precipitation of the laminated carbonate was stimulated by vadose diagenesis of the carbonate buildups and by microbial activity. Each Ratner brining-upward succession represents a progressive drawdown when the rate of basin brine evaporation exceeded seepage of marine water into the basin. Marine water seeping through the reefs was enriched in calcium cations by Mg++−Ca++ exchange with the limestone (dolomitization) and by dissolution of reef rocks, and was responsible for the precipitation of calcium sulphate in areas adjacent to the carbonate reefs through brine mixing processes. During vadose diagenesis of the Winnipegosis reefs and deposition of the Ratner laminite and Whitkow Anhydrite, brine level in the barred Elk Point Basin was controlled by the rate of seepage of marine water through the barrier, which in turn was controlled by eustatic sea level changes in the open ocean. The basal anhydrite-free dololaminite of the Ratner represents a Falling Stage System Tract when evaporative drawdown was largely compensated by seepage of marine water into the basin. The interlaminated carbonate and anhydrite of the middle and upper Ratner are interpreted as a Lowstand System Tract associated with evaporative drawdown and increased cyanobacterial activity under near-desiccation conditions when evaporation exceeded seepage. The Whitkow Anhydrite represents a Transgressive System Tract deposited during sea level rise in the open ocean that led to an increased rate of seepage, higher basin brine level and diminished microbial influence on bedding structures.

Stratigraphic relationships between the Middle Devonian Shell Lake Member and the Upper Winnipegosis reef mounds, Saskatchewan Sub-basin

ABSTRACT The spatial distribution of the Shell Lake Member and its relationship with the Upper Winnipegosis reef mounds in the Saskatchewan Sub-basin record an important aspect of the transitional history of the Elk Point Basin from fully marine to desiccation. The Shell Lake Member in the study area is composed of anhydrite and laminated dolomite, and produces a good reflection on seismic profiles. The top of many Winnipegosis reef mounds gives a seismic reflection similar to the Shell Lake Member, leading to the interpretation in the literature that the Shell Lake Member is a continuous unit overlying the Winnipegosis reef mounds in the Sub-basin. Data presented in this paper demonstrate that the Shell Lake Member is not continuous because it terminates against the fully developed reef mounds. The anhydrite on top of the reef mounds previously interpreted as the Shell Lake Member is diagenetic anhydrite that occurs as replacement of the Winnipegosis carbonate at the top of reef mounds and as cement in the vugs. This interpretation is based on differences recorded in the depositional, lithological and diagenetic features of these two units. The formation of the diagenetic anhydrite of the Winnipegosis reef mounds is distinct from the primary depositional anhydrite in the Shell Lake Member based on the lithologic and diagenetic features, spatial distribution and stratigraphic relationship of the Winnipegosis carbonate and the Shell Lake Member in the study area. The spatial relationships of various lithologic units may serve as a predictor for the location of new Winnipegosis reef mounds and potash deposits. 1 Current address: Department of Geology, Brandon University, Brandon, MN R7A 6A9 End_Page 156------------------------