Elk Point Basin Research Articles

Stratigraphy of the Middle Devonian Keg River and Prairie Evaporite formations, northeast Alberta, Canada

Stratigraphy of the Middle Devonian Keg River and Prairie Evaporite formations, northeast Alberta, Canada

Geological Structure and Subsurface Stratigraphy of the Middle Devonian-age Saskatchewan Basin

The Middle Devonian-age Saskatchewan potash deposit represents approximately half of the proven global potash reserve. Salts of the Saskatchewan Basin were laid down in a basin of deposition referred to as the Elk Point Basin that consists of multiple marine transgression-regression cycles and multi-stage marine basins sitting in northwest-southeast direction. In south Saskatchewan, there are three evaporite cycles thinly deposited within the Saskatchewan Basin during the Middle Devonian age. The lowermost evaporite termed as “the Prairie Evaporite” is characterized by four potash members representing the largest potash deposit all over the world. The Prairie Evaporite accumulated of carbonate, anhydrite, halite and sylvite was deposited due to the combined effect of repeated transgressions, intensive evaporation and sea level lowering. Some iconic markers are used for prospecting, such as “the second red bed” and the Dawson Bay Group. Researching in the geological structure and the subsurface stratigarphy has great support to the exploration and development of local potash.

Dedolomitization and calcite cementation in the Middle Devonian Winnipegosis Formation in Central Saskatchewan, Canada

AbstractLimestone consisting of finely to medium crystalline calcite mosaics is present in the upper part of the Winnipegosis Formation on the east‐central margin of the Elk Point Basin where the overlying Prairie Evaporite deposits have been removed. This type of crystalline limestone is interpreted as dedolomite, based on petrographic observations. The δ18O and δ13C values of the Winnipegosis dedolomite vary from −12·8‰ to −11·9‰ VPDB (Vienna Pee Dee Belemnite) and from −0·5‰ to +1·7‰ VPDB, respectively; both values are significantly lower than those for the corresponding dolomite. The 87Sr/86Sr ratios of the dedolomite are significantly higher, between 0·7082 and 0·7087. The spatial distribution and geochemical data of the Winnipegosis dedolomite suggest that dedolomitization was related to an influx of fresh groundwater and dissolution of the Prairie Evaporite anhydrite during the latest Mississippian to the Early Cretaceous when the basin was subjected to uplift and erosion. The Winnipegosis dedolomite displays a series of replacement fabrics showing progressive calcitization of dolomite, including the occurrence of dedolomite restricted along fractures and adjacent areas, dolomite patches ‘floating’ in the dedolomite masses and massive dedolomite with sparsely scattered dolomite relicts. However, the characteristic fabrics resulting from dedolomitization documented in the literature have not been observed in the Winnipegosis dedolomite. Coarsely to very coarsely crystalline, subhedral to euhedral calcite cement is restricted in the dedolomite. The petrographic features, isotopic compositions and homogenization temperatures, coupled with the burial history of the Winnipegosis Formation, constrain the precipitation of the calcite cement from a mixing of basinal brines and fresh groundwater during Late Cretaceous to Neogene time. The more negative C‐isotopic signatures of the calcite cement (−5·3‰ to −2·3‰ VPDB) probably reflect a hydrocarbon‐derived carbon.

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 Mg 2+ -rich fluids was restricted by interbedded anhydrite, but where the fluids penetrated, the underlying carbonate deposits were dolomitized. The depletion in 18 O 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.

Paleokarst in Middle Devonian Winnipegosis mud mounds, subsurface of south-central Saskatchewan, Canada

Abstract Paleokarst of the Winnipegosis mud mounds is mainly characterized by extensive solution features and cavity deposits. Solution features vary from millimetre-size vugs/channels to metre-scale caverns. Most solution voids are filled with anhydrite and/or carbonate deposits. Swiss-cheese type porosities appear as oval to irregular pore networks and most of them remain open. Erosional surfaces are observed in several cores. Fractures and breccia fragments are small-scale and commonly associated with solution features or calcretes. Cavity sediments are dominantly detrital dolomite, interpreted as a product of weathering of the host rocks. Speleothems occur in vugs and channels but are not abundant. Caverns and large vugs likely formed at or just below the water table in the phreatic zone or in a freshwater-saltwater mixing zone during subaerial exposure of the mounds. Porous Swiss-cheese fabrics resemble sponge-like pores that form in mixing zones of modern carbonate platforms and islands. Porosity in the Winnipegosis mounds was extensively modified by karstification and subsequent anhydrite cementation. Paleokarst occurs only in the middle and upper parts of relatively high Winnipegosis mounds with respect to the basin floor. Multiple levels of caverns and vugs are probably related to various positions of freshwater lenses corresponding to recurrent subaerial exposure and water level changes in the Elk Point Basin. Occurrence of caverns and large vugs at 55 m below the top of the mounds indicates that the mixing zone or freshwater has extended downward to this depth.

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------------------------

Dolomitized calcrete in the Middle Devonian Winnipegosis carbonate mounds, subsurface of south–central Saskatchewan, Canada

Dolomitized calcrete is a widespread diagenetic feature in the uppermost portions of the Middle Devonian Winnipegosis mud mounds in the subsurface of south–central Saskatchewan. Individual calcrete profiles are composed of 2 to 5 horizons, including (in descending order) laminar crust, massive, pisolitic, breccia, chalky and transitional horizons. The calcrete profiles are interpreted to be pedogenic in origin, formed by diagenetic alteration of the host carbonate deposits during subaerial exposure of the mud mounds. This interpretation is based on the occurrence of an orderly set of well-differentiated horizons, disconformities, vadose pisoids, micritic stringers, circumgranular cracks, linkage coatings, and gradational contacts between the calcrete and underlying host carbonate rocks. Lack of root-related structures, such as rhizoconcretion and alveolar–septal fabric in the Winnipegosis calcrete, suggests a limited influence of macrophytes on the formation of calcrete. The occurrence of up to three discrete pedogenic calcrete profiles in a single Winnipegosis calcrete succession indicates the mounds underwent three periods of subaerial exposure that resulted from drops of water level in the Elk Point Basin of Saskatchewan during Middle Devonian time.

New insights into the origin and migration of brines in deep Devonian aquifers, Alberta, Canada

Analysis of hydraulic heads and chemical compositions of Devonian formation waters in the west central part of the Alberta Basin, Canada, characterizes the origin of formation waters and migration of brines. The Devonian succession in the study area lies 2000–5000 m below the ground surface, and has an approximate total thickness of 1000 m and an average slope of 15 m/km. Four Devonian aquifers are present in the study area, which form two aquifer systems [i.e., a Middle–Upper Devonian aquifer system (MUDAS) consisting of the Elk Point and Woodbend–Beaverhill Lake aquifers, and an Upper Devonian aquifer system (UDAS) consisting of the Winterburn and Wabamun aquifers]. The Ireton is an effective aquitard between these two systems in the eastern parts of the study area. The entire Devonian succession is confined below by efficient aquitards of the underlying Cambrian shales and/or the Precambrian basement, and above by overlying Carboniferous shales of the Exshaw and Lower Banff Formations. The formation water chemistry shows that the Devonian succession contains two distinct brine types: a ‘heavy brine,’ located updip, defined approximately by TDS >200 g/l, and a ‘light brine’ with TDS <200 g/l. Hydraulic head distributions suggest that, presently, the ‘light brine’ attempts to flow updip, thereby pushing the ‘heavy brine’ ahead. The interface between the two brines is lobate and forms large-scale tongues that are due to channeled flow along high-permeability pathways. Geological and hydrogeochemical data suggest that the following processes determined the present composition of the ‘light’ and ‘heavy’ brines: original seawater, evaporation beyond gypsum but below halite saturation, dolomitization, clay dehydration, gypsum dewatering, thermochemical sulfate reduction (TSR), and halite dissolution. The influx of meteoric (from the south) and metamorphic (from the west) waters can be recognized only in the ‘light brine.’ Albitization can be unequivocally identified only in the ‘heavy brine.’ The ‘heavy brine’ may be residual Middle Devonian evaporitic brine from the Williston Basin or the Elk Point Basin, or it may have originated from partial dissolution of thick, laterally extensive Middle Devonian evaporite deposits to the east of the study area. The ‘light brine’ most probably originated from dilution of ‘heavy brine’ in post-Laramide times.

The gas-bearing Devonian Presqu'ile Dolomite of the Cordova embayment region of British Columbia, Canada: Dolomitization and the stratigraphic template

White dolospar of the Presqu'ile Dolomite forms the reservoir for many gas fields of northeastern British Columbia within the Keg River, Sulphur Point, and Slave Point formations along the shelf margin of the Cordova embayment. The embayment fill succession of the organic-rich Klua Shale, the Otter Park shale, and marlstones equivalent to the Sulphur Point and Slave Point formations are aquicludes that blocked the upward and lateral flow of dolomitizing brines. Presqu'ile dolomitization along the Slave Point shelf edge is interrupted in places where the Klua Shale extends shelfward underneath the Slave Point Formation. The Watt Mountain shale was also a regional aquiclude that prevented upward circulation of dolomitizing fluids in areas behind the Slave Point shelf edge. Flow of dolomitizing NaCl-CaCl2-MgCl2-H2O brine solutions began in the shallow subsurface and precipitated dolomite cements (type 1) at temperatures less than 50 degreesC, possibly even during Slave Point deposition, as northwestward-directed shallow subsurface brine reflux from the Elk Point basin began. This was followed by upward-directed subsurface convective flow of NaCl-CaCl2-MgCl2-H2O and NaCl-MgCl2-H2O brines and precipitation of dolomite cements at temperatures of about 130 degreesC during the Late Devonian-Carboniferous. Lower salinity brines that precipitated calcite cements and some dolomite (type 2) may have interacted with connate fluids of marine origin or, less likely, with meteoric fluids. Modeling of organic maturation indicates that the Muskwa shales above the Slave Point Formation passed through the oil generation window in the Permian and the onset of dry gas generation in the middle Mesozoic. Postdolomitization calcite cements precipitated during these times contain abundant liquid hydrocarbon-filled and methane-filled inclusions. High temperatures of close to 200 degreesC experienced by Slave Point strata in the Mesozoic caused stretching, but no leakage or refilling, of fluid inclusions in all dolomite cements.

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.

Petrography and geochemistry of early‐stage, fine‐ and medium‐crystalline dolomites in the Middle Devonian Presqu’ile Barrier at Pine Point, Canada

The petrography and geochemistry of fine‐ and medium‐crystalline dolomites of the Middle Devonian Presqu’ile barrier at Pine Point (Western Canada Sedimentary Basin) are different from those of previously published coarse‐crystalline and saddle dolomites that are associated with late‐stage hydrothermal fluids. Fine‐crystalline dolomite consists of subhedral to euhedral crystals, ranging from 5 to 25 μm (mean 8 μm). The dolomite interbedded with evaporitic anhydrites that occur in the back‐barrier facies in the Elk Point Basin. Fine‐crystalline dolomite has δ18Ο values between −1·6 to –3·8‰ PDB and 87Sr/86Sr ratios from 0·7079–0·7081, consistent with derivation from Middle Devonian seawater. Its Sr concentrations (55–225 p.p.m., mean 105 p.p.m.) follow a similar trend to modern Little Bahama seawater dolomites. Its rare earth element (REE) patterns are similar to those of the limestone precursors. These data suggest that this fine‐crystalline dolomite formed from Middle Devonian seawater at or just below the sea floor.Medium‐crystalline dolomite in the Presqu’ile barrier is composed of anhedral to subhedral crystals (150–250 μm, mean 200 μm), some of which have clear rims toward the pore centres. This dolomite occurs mostly in the southern lower part of the barrier. Medium‐crystalline dolomite has δ18O values between −3·7 to −9·4‰ PDB (mean −5·9‰ PDB) and 87Sr/86Sr ratios from 0·7081–0·7087 (mean 0·7084); Sr concentrations from 30 to 79 p.p.m. (mean 50 p.p.m.) and Mn content from 50 to 253 p.p.m. (mean 161 p.p.m.); and negative Ce anomalies compared with those of marine limestones. The medium‐crystalline dolomite may have formed either (1) during shallow burial at slightly elevated temperatures (35–40 °C) from fluids derived from burial compaction, or, more likely (2) soon after deposition of the precursor sediments by Middle Devonian seawater derived from the Elk Point Basin.These results indicate that dolomitization in the Middle Devonian Presqu’ile barrier occurred in at least two stages during evolution of the Western Canada Sedimentary Basin. The geochemistry of earlier formed dolomites may have been modified if the earlier formed dolomites were porous and permeable and water/rock ratios were large during neomorphism.

Abstract: Sequence Stratigraphy of the Middle Devonian Winnipegosis Vadolite and Ratner Laminite of the Elk Point Basin, Southern Saskatchewan: A Falling Stage System Tract

Abstract: Sequence Stratigraphy of the Middle Devonian Winnipegosis Vadolite and Ratner Laminite of the Elk Point Basin, Southern Saskatchewan: A Falling Stage System Tract

Paleomagnetic and isotopic evidence for the diagenesis and alteration of evaporites in the Paleozoic Elk Point Basin, Saskatchewan, Canada

In the Paleozoic Elk Point Basin, Saskatchewan, Canada, hematite in evaporite rocks of the Middle Devonian Prairie Formation records two magnetizations that are very different from those expected when the evaporites were originally deposited. The first (X magnetization) corresponds to Cretaceous or Cenozoic paleofield directions and contains both normal and reversed polarities. The second (Y magnetization) appears to be a composite of Cretaceous–Cenozoic and late Paleozoic (Kiaman) reversed polarity directions. The X magnetization occurs in anomalous ores and is associated with fine-grained hematite occurring predominantly along grain boundaries. The Y magnetization occurs in normal ore and is associated with sylvite that has hematite both along grain boundaries and within sylvite crystals, the latter inferred to be of Kiaman age. K–Ar ages of the host sylvites are also composite, and are consistent with those inferred from paleomagnetic directions. Stable isotopic compositions of fluid inclusions in halite and the associated hematite in the Prairie Formation indicate that the hematite carrying the X magnetization formed at low temperature (about 60 °C) by fluids similar to those currently resident in overlying formations. The hematite carrying the inferred late Paleozoic magnetization was also formed at low temperatures by fluids having δD and δ18O values significantly lower than Paleozoic seawater. Paleomagnetic, petrographic, and isotopic data, and K–Ar ages indicate that evaporites in the Elk Point Basin have been affected by major fluid events that occurred during the late Paleozoic and Cretaceous–Cenozoic. These fluids are most probably related to brines mat have their origins within the basin, which were mobilized by major tectonic events.

Origin of Satin Spar Veins in Evaporite Basins

ABSTRACT Satin spar (fibrous gypsum) veins occur in rocks overlying evaporites in the Amadeus Basin, Australia; Appalachian Basin, USA; Cheshire Basin, England; Elk Point Basin, Canada; Palo Duro Basin, USA; Paradox Basin, USA, and Zechstein Basin, England. These antitaxial veins, which are characterized by central partings and near-vertical fibers, fill horizontal and inclined fractures. Most satin spar veins occur in highly fractured or brecciated clastic strata that overlie or are associated with bedded halite, anhydrite, and gypsum. Saline springs are commonly present, indicating that halite dissolution is active. The similarities in morphology and occurrences of these veins suggest a common genesis. Satin spar veins result from several simultaneously active processes. Recharge of low-salinity surface water results in dissolution of shallow ( 200 m to 750 m deep) halite, development of cavernous porosity, and formation of extensional fractures in the rock column overlying salt-dissolution zones. Greater solubility of anhydrite than gypsum at low temperatures and at salinities below halite saturation causes hydration of anhydrite to gypsum, which takes place without significant volume change. Once groundwater is saturated with respect to gypsum, any further anhydrite hydration and solution must result in supersaturation, and the excess CaSO4 carried by ground water flowin from dissolution zones precipitates as gypsum in open, high-permeability fractures.

Bromine Function in Halite Geochemistry

Of the halogens or salt formers, bromine is the only nonmetal which occurs naturally as a poisonous liquid much denser than water. The power of its atoms, expressed by a valence of 1 and 5, makes it unite directly with a large number of metallic elements to form salts. As a rare and strongly electronegative element of group VII in the periodic table, bromine exists in seawater and evaporitic brines as bromide with a ratio to chlorinity of 0.00348. Most water detains only about 1 ppm bromide for each 300 ppm of chloride. The most abundant source of bromine is ocean water (65 ppm Br), but richer peps occur in salt deposits and primarily in mineral brines. Atomic absorption spectrophotometric resolutions of Permian Castile halites exposed low values of bromine compared with its higher quantities in modern oceans like the Mediterranean. Bromine analyses of the two petrographically distinct forms of halite that characterize many ancient evaporite deposits, as in the Elk Point basin fields of Alberta, imply they crystallized from brines of noticeably different concentrations. Bromine in halite has been used as a paleosalinity indicator and a stratigraphic marker. Bromine liquid, with an atomic weight of 79.904 and atomic numbermore » 35, is used in producing gasoline antiknock mixtures, fumigants, photographic chemicals, drilling fluids, and fire retardants. It is also highly toxic and corrosive as bromine gas. Bromine contents greater than 1 ppm may be unsafe in the atmosphere, and a dose of 500 ppm can lead to death in less than an hour.« less

Reinterpretation of the Eastern Platform-to-Basin Facies Transition, Winnipegosis Formation, North-Central North Dakota

Previous interpretations of the eastern platform-to-basin facies transition of the Winnipegosis Formation in north-central North Dakota were part of more extensive, regional studies of the Middle Devonian Elk Point basin sediments (Jones, 1965; Holter, 1969; Ehrets and Kissling, 1987). These studies apparently did not take into account (1) the known spatial distribution of wells penetrating the Ratner Member of the Winnipegosis Formation, which is diagnostic of basinal environments, versus the distribution of known basinal Winnipegosis carbonate buildups; and (2) the control of structural movement along the north-south trending Precambrian Churchill-Superior Boundary on Winnipegosis sedimentation, particularly the eastern platform-to-basin facies transition and basinal carbonate buildups of the Winnipegosis Formation. As a result, the location and configuration of the eastern platform-to-basin facies transition was misinterpreted to be along a north-south trending series of basinal carbonate buildups along the Glenburn-Wiley trend, 20-25 mi west of the present interpretation.

SEISMIC INTERPRETATION OF UPPER ELK POINT (GIVETIAN) CARBONATE RESERVOIRS OF WESTERN CANADA1

AbstractThe seismic signatures of three reefs of the Upper Elk Point Subgroup (Givetian Stage) of the Western Canada Sedimentary Basin are documented and analysed on the basis of variations in seismic image of particular lithologic units, lateral amplitude and/or phase changes, structural relief and velocity‐generated relief, as rendered by the reflection data. The effects on seismic signatures of spatial geological variations resulting from such phenomena as differential compaction, reef‐focused salt dissolution, palaeotopography, lateral and vertical facies variations, regional dip, and reservoir morphology are discussed. The usefulness of seismic data in clarifying relationships between reefs and their adjacent sedimentary sections, particularly in cases where well control is sparse, is also considered. Such documentation of seismic signatures from known reefs using geophysical and geological analysis can establish criteria to enable recognition of similar buildups elsewhere.Three example reefs are presented, each typical of a particular area and environment of W. Canada. The first is from the Winnipegosis Formation of SE Saskatchewan, the second and third from the Rainbow Member and Upper Keg River Reef Member, respectively, of the Keg River Formation of NW Alberta. All three of these carbonate buildups developed in the evaporitic Elk Point Basin. However, the degree of salt encasement and subsequent dissolution varied greatly, as do the resulting seismic effects. For these three reef types, the typical elements of their seismic signatures have been compiled and are here summarized.

The petrogenesis of brines in Devonian potash deposits of western Canada

Recent mine-level floods in some potash mines in the Elk Point Basin of Saskatchewan, Canada, have demonstrated the need to identify accurately the source of water leaks so as to assist in remedial or preventative action. Most chemical constituents of the waters are inadequate tracers of the origin of these leaks because the concentrations of most elements change substantially during migration of the fluids to the mining level. In marked contrast to most of the chemical constituents dissolved in the water, oxygen and hydrogen isotopes are conservative elements of the water. Within the Elk Point Basin, δD- and δ 18O-values of basinal waters normally increase with depth because of mixing between trapped formation waters, and local surface waters such that the water in each aquifer within the basin generally has a unique isotopic composition. Floodwaters in the potash mines of Saskatchewan have chemical and stable isotopic compositions that indicate three different origins for mine-level fluids: (1) halite- and sylvite-saturated basinal brines from Devonian formations directly above the deposits, which are predominantly connate waters; (2) undersaturated waters of predominantly meteoric origin, from stratigraphically higher aquifers such as the Cretaceous Mannville Group; and (3) Ca-rich brines that may represent Devonian fluids associated with recrystallization of the evaporites. The salt-undersaturated formation waters most likely are directed to lower stratigraphic units through collapse structures or related features which may be reactivated through mining.

Fluid inclusion evidence for the deposition and diagenesis of the Patience Lake Member of the Devonian Prairie Evaporite Formation, Saskatchewan, Canada

Low bromine concentrations, radiometric ages, textural relations and large-scale dissolution and collapse features in Devonian potash deposits of the Elk Point Basin in southern Saskatchewan indicate that diagenetic processes and late influxes of water have variably recrystallized the salts. Lack of correlation between homogenization temperature ( T h), δD or δ 18O values of fluid inclusions in the salt with stratigraphic position further indicates that the sequence is not primary. The stable isotopic compositions of fluid inclusion waters, including those in chevron-texture halites, are not those expected for primary crystallization waters and, in conjunction with T h, indicate recrystallization occurred at temperatures of 35–70°C from water originating in formations above the Prairie Evaporite Formation. Using T h as an estimate of the depth of burial and stable isotopes as indicators of the source of fluids, three major recrystallization events related to the evolution of the Elk Point Basin can be recognized.

Brine mixing in the Middle Devonian of western Canada and its possible significance to regional dolomitization

The desiccation that most large evaporite basins experienced in their history created large elevation heads. These heads must have induced flows of ground- or formation-waters into the evaporite basins. The consequences of such groundwater flows are believed to be (1) provision of the necessary hydrodynamic drive to allow dolomitization of carbonates beneath and adjacent to the evaporite basin, and (2) the formation of anomalously distributed evaporites in the basin adjacent to sites of groundwater inflow: sites where the groundwaters mixed with marine-derived brines. Such brine mixing may also have been responsible for depleting the marine-derived brines of their contained sulfate, so explaining later development of sulfate-impoverished potash deposits. Anomalous developments of anhydrite surrounding older carbonate buildups in the distal parts of the Middle Devonian Elk Point Basin of western Canada may represent such products of brine mixing and provide evidence of dolomitization of underlying Lower Palaeozoic and Devonian carbonates coevally with the filling of the evaporite basin. Regional dolomitization in the Williston Basin is stratigraphically confined beneath the Middle Prairie Evaporite suggesting a genetic link between the dolomitization and evaporite deposition. Dolomitization by refluxing brines generated in the evaporite basin appears unlikely because of the presence of an aquitard at the base of the evaporite succession. Developments of laminar and pisolitic carbonates adjacent to the carbonate buildups are believed to indicate the former presence of subaerial springs that carried calcium-rich groundwaters into the evaporite basin. Anhydrite envelopes around the buildups are interpreted to be reaction products that formed when calcium-bearing spring waters mixed with sulfate-rich but calcium-depleted marine brines. The calcium in the spring waters is believed to have entered the groundwaters as a by-product of the dolomitization of carbonates underlying the evaporite basin. Precipitation of gypsum (now anhydrite) as a consequence of brine mixing must have depleted the sulfate content of the marine-derived brines. It is possible that the sulfate-depleted potash deposits in the Prairie Evaporite originate by this mechanism. On the other hand, the stratigraphic position of these deposits lends little support for this hypothesis.