Detrital versus chemical sediments
"The union of alkaline earths with carbonate and calcium with sulfate are unlikely in Saskatchewan."
Sedimentologists have long recognized three basic genetic types of sediment in most lacustrine basins; (a) allogenic (i.e., detrital): that material derived from weathering and erosion of the soils and bedrock of the watershed and transported to the lake by fluvial, sheetwash, gravity, or aeolian processes; (b) endogenic: sediment originating from biological or inorganic processes occurring entirely within the water column of the lake; and (c) authigenic (i.e., diagenetic): material resulting from mainly chemical and biological processes occurring once the sediment has been deposited. The endogenic and authigenic fractions are sometimes collectively referred to in a nongenetic sense as 'chemical sediment', as opposed to 'clastic sediment', which is usually dominated by the allogenic fraction. However, use of this latter designation should be avoided when discussing the deposits of saline lakes because of the presence of such things as 'clastic' sediment comprised of fragmented endogenic components, or 'chemical' sediment derived largely by post-depositional diagenetic processes.
The allogenic component in most of the lakes of western Canada consists mainly of a mixture of (in approximate decreasing order of abundance): clay minerals, quartz, carbonates, feldspars, and ferromagnesian minerals. The relative proportion of each of these detrital mineral groups is generally similar to that of the surrounding glacial debris, except sorting by the transporting medium and diagenetic processes operating within the watershed soils can also influence the final detrital mineralogy. The less chemically stable mineral components, such as detrital carbonates and feldspars, are particularly susceptible to loss by weathering processes in the drainage basin [234
]. Superimposed on this element of chemical stability of the detrital components is the inherent size fractionation of various minerals caused due to physical abrasion and breakage of grains by the action of glacial ice. For example, because the terminal grade for glacier-comminuted dolomite is medium to coarse silt [238
], the modern sedimentary facies of the larger lakes in the region show a gradation from dolomite-enriched sediment nearshore to dolomite-depleted deposits offshore [46
Although the sediments of only a small number of lakes have been examined for detailed clay mineralogy, the most common layered silicates are kaolinite, illite, smectite, and chlorite [228
]. A variety of mixed-layer clays, most commonly illite-smectite and illite-chlorite species, have also been reported, but at least some of these interstratifed minerals have been attributed to possible authigenic processes within the lake basins [204
]. In lakes west of the Manitoba Lowland, the sediments are almost completely dominated by smectitic clays, undoubtedly a reflection of the bentonitic Cretaceous shales, which underlie most of the area west of the Manitoba Escarpment. The abundances of kaolinite, illite and smectite in the eastern part of the region tend to be approximately subequal.
The only detrital carbonate minerals identified in the lakes of the Great Plains are dolomite and calcite. In bulk (i.e., non-size fractionated) samples, CaMg(CO3
(dolomite) is usually considerably more abundant than CaCO3
(calcite). This is most likely a reflection of the relative abundance of these two minerals in the tills and bedrock of the region, as well as the higher solubility and, hence, lower weathering stability of calcite relative to dolomite [239
]. Similarly, potassic feldspars are usually more abundant than plagioclase in the lakes due to the partial loss of the relatively unstable Na and Ca feldspars by hydrolysis in the tills and watershed soils.
Considering the great range of water compositions in lakes of the Great Plains, it is not surprising there is an equally significant breadth of endogenic and authigenic minerals found in these lakes. In the playas of the Great Plains, there are two main types of endogenic precipitates in the modern sediments: (a) very soluble salts, comprising mainly sodium and magnesium sulfates and carbonates, and (b) sparingly soluble precipitates, including mainly carbonates, sulfates, and silicates (Figure ). There have now been close to fifty non-detrital minerals identified from the lakes (Table ). These endogenic and authigenic minerals can also be subdivided according to their genesis and occurrence within the lake: (a) surficial efflorescent crusts and hardgrounds, usually occupying nearshore and seasonally flooded areas; (b) massive and bedded precipitates, most often found blanketing the floor of the basins from shallow marginal zones down to deep central offshore areas; and (c) accumulations of salts and precipitates associated with either subaqueous or subaerial springs.
Figure 21 Endogenic and authigenic salts. Examples of modern chemical precipitates from the salt lakes of the northern Great Plains of western Canada; modified from [10, 59, 117]. A. Shoreline accumulation of rounded accretionary grains of mirabilite; B. Floating (more ...)
Endogenic and authigenic minerals in saline lakes of the northern Great Plains; modified from [10, 59, 116].
Distinguishing endogenic lacustrine minerals from those formed by diagenetic processes after the sediment has been deposited (i.e., authigenic) is often an exceedingly difficult task. Many times, detailed petrographic and geochemical studies are required to convincingly demonstrate the precise origin for the specific minerals. To date, few lakes and lacustrine sequences in western Canada have been examined in this amount of detail. Although a complete discussion of the genesis (viz. endogenic versus authigenic) of the minerals listed in Table is beyond the scope of this paper, some of the more common or noteworthy occurrences will be summarized briefly.
Among the endogenic and authigenic carbonate minerals identified in these lakes, aragonite, magnesian calcite (i.e., hi-Mg calcite), and dolomite are the most common. Inorganic precipitation of carbonate minerals due to thermodynamic supersaturation is very common in lakes of all types on a global basis [245
]; the Great Plains region is no exception. Nearly all of the 131 lakes that were surveyed in the early 1980's were found to be strongly supersaturated with respect to these carbonate minerals [157
]. Supersaturation and precipitation of carbonates can take place for a variety of reasons, including photosynthetic uptake of CO2
and consequent increase in pH, concentration changes brought about by evaporation or dilution, temperature changes, and mixing of brines of different compositions. In most of the lakes of the Great Plains, carbonate mineral supersaturation is due mainly to, but not exclusively, the seasonal uptake of CO2
by primary organic productivity.
The specific carbonate mineral to be precipitated from the supersaturated solution is controlled mainly by the cations in solution (in particular the ratio of Mg to Ca in the water [247
]). The elevated Mg/Ca ratios that characterize the lake waters of the Great Plains give rise to a carbonate mineral assemblage dominated by aragonite (the orthorhombic form of CaCO3
) and Mg-bearing carbonates, such as dolomite, magnesite, huntite and Mg-calcite.
The discovery of non-detrital dolomite in the lakes of the Great Plains has been an important contribution to our understanding of the genesis of this economically important mineral [249
]. Dolomite formation in the sedimentary realm is a subject of long-standing interest and study. Probably no other mineral or sedimentary rock has attracted as much speculation regarding its genesis as dolomite [248
]. The "dolomite problem" has been summarized in many recent reviews [251
] and in most sedimentary geology textbooks. In essence, the dolomite problem is the enigma that the mineral dolomite (CaMg(CO3
) is a very common component of ancient rocks, but is very rare in modern and Holocene sediment. It does not appear to occur as a primary precipitate from marine water of normal salinity and composition, nor cannot be readily synthesized in the laboratory at low temperatures and pressures.
The occurrence of very early diagenetic (i.e., penecontemporaneous) dolomite in the surficial offshore sediments of Devils Lake in northeastern North Dakota [255
] was one of the earliest documented examples of lacustrine dolomite formation. This occurrence of dolomite also emphasized that dolomitization could take place in solutions of moderate
salinities (~20 ppt TDS) and in water with high sulfate contents. Others, working mainly in highly evaporitic marginal marine settings, have suggested that dolomite formation is favoured by elevated salinities (greater than 35 ppt TDS) and greatly reduced SO4
Modern primary and early diagenetic dolomite occurs in numerous lakes in the Canadian Great Plains [249
]. Waldsea and Deadmoose Lakes are two adjacent, meromictic saline lakes located in central Saskatchewan. Two distinct types of non-detrital dolomite have been identified in these lakes [260
]. Most is very finely crystalline with subhedral to anhedral crystallites forming aggregates of about 2 μm in diameter. This dolomite, like most Holocene dolomite, is poorly crystalline and poorly ordered (ordering refers to the regular alteration of the layers of cations and anions within the crystal structure), but has a composition close to that of the ideal, stoichiometric mineral (i.e., Ca=Mg). The other type is also poorly crystalline, but is considerably enriched with calcium: Ca(Ca0.2
. Like that of Devils Lake, the dolomite in both Waldsea and Deadmoose is forming subaqueously as a primary inorganic precipitate in sulfate-rich waters.
Another important occurrence of dolomite is in Freefight Lake, a 25 m deep, hypersaline meromictic lake in southwestern Saskatchewan. Preliminary sedimentological examination of this lake [261
] indicates a considerable range of formative environments and dolomitization processes. Calcian (i.e., Ca-enriched) dolomite occurs in the deep-water, offshore sediments and also in the subaerially exposed mudflats surrounding the basin. Within the mudflat sediments, it is often associated with aragonite and Mg-calcite, and petrographic evidence suggests it may be an early diagenetic product forming in response to the strong evaporative flux experienced on the mudflats during the ice-free season. In contrast, in the organic-rich, highly reducing offshore sediments, dolomite is associated with a variety of soluble sulfate salts and pyrite. It is forming at the sediment-water interface as a primary precipitate in response to the increased alkalinity brought about by sulfate reduction in the anoxic monimolimnion of the lake.
A final example showing the diversity of Mg-carbonate mineral genesis in these lakes is that of Chappice Lake in southeastern Alberta. Chappice Lake is a small, hypersaline, groundwater-fed playa whose brine is dominated by Na and SO4
]. Although endogenic carbonates are not forming in the lake today, the 7000 year long stratigraphic record preserved in the basin contains thick sequences of very finely laminated carbonates [266
]. The carbonate crystals and crystal aggregates making up these laminae are euhedral and contain no petrographic evidence of reworking, abrasion, corrosion, or diagenetic alteration, thus suggesting the laminae were generated by inorganic precipitation from within the water column. The lack of detrital grains in these laminae and the absence of rhythmicity indicate relatively rapid and non-annual precipitation events. As shown in Figure , it is clear from the detailed mineralogical composition of the sequence that the brine underwent striking compositional changes in both carbonate alkalinity and cation ratios.
Chappice Lake laminated carbonates. Photograph of a section of core from Chappice Lake showing the fine lamination and the complex carbonate mineral assemblage present in this otherwise relatively simple playa basin; modified from [10, 59].
The mineralogy of the endogenic and authigenic sulfates in the Great Plains' lakes is diverse and the genesis of these inorganic components complex. Not only is the stable sulfate mineral suite controlled by the ionic composition and cation ratios in the brine, but also the temperature at which the precipitation occurs is also very important (Figure ). For example, in a simple binary salt system (e.g., Na-SO4-H2O, Mg-SO4-H2O, or Na-Cl-H2O) the solubility of mineral phases such as mirabilite (Na2SO4 10H20), and epsomite (MgSO4 7H2O) show enormous ranges over temperatures normally encountered by the brine on a diurnal and seasonal basis. In contrast, common non-sulfate salts, such as halite or bischofite (MgCl2 6H2O), exhibit comparatively little solubility change over these near-surface temperature ranges. In more complex aqueous systems, such as Na-Mg-Cl-SO4-H2O, prediction of the stable phase(s) becomes somewhat more complicated, but the influence of temperature remains important.
Temperature-solubility relationships of binary salt systems. Relationship between temperature and solubility of commonly occurring evaporite minerals in the lakes of the northern Great Plains. Modified from .
The effect of temperature on the mineral suite in these lakes is most obvious in the playas and shallow lakes/wetlands [219
] where the annual cycle of sediment accumulation (precipitation) and dissolution is readily apparent (Figure ). However, the deep perennial salt lakes are also strongly influenced by the annual (and diurnal) temperature regime. For example, in Freefight Lake, the surficial waters become supersaturated with respect to mirabilite and epsomite during the winter due to low temperatures of the upper part of the mixolimnion (-10 to -15°C depending on salinity) and the formation of an ice cover which increases the brine concentration in the upper metre. The precipitated Na2
O and MgSO4
O crystals undergo corrosion and dissolution as they settle through the understaturated (-5 to 5°C) mixolimnion below the ice cover. Dissolution of these sulfate salts in the mixolimnion is of sufficient magnitude to, in turn, affect the temperature of the water. The dissolution of mirabilite is a strongly endothermic reaction. When 1 mol of mirabilite dissolves the reaction absorbs over 4 MJ of heat. This heat must be acquired from the surrounding water, thereby lowering the temperature. However, at the chemocline sulfate salt precipitation begins anew, despite a still higher temperature, due to the greater salinity. Gypsum shows a similar pattern of precipitation from the surficial water; dissolution within the mixolimnion, and precipitation through the monimolimnion, except the supersaturation at the surface is usually brought about during the ice-free season due to evaporative concentration of the brine. Thus, the petrography of the resulting deep-water sulfate salts in this basin records a complex series of multiple precipitation and dissolution events that are controlled largely by seasonal temperature fluctuations within the water column.
Sedimentary cycles in cold saline playa lakes of the northern Great Plains. Schematic illustration showing the major cycles (diurnal, annual, evolutionary) characteristic of playas in the northern Great Plains; modified from [10, 59].
A spectrum of lacustrine sedimentary processes
"I often say that if you measure that of which you speak, you know something of your subject; but if you cannot measure it, your knowledge is meager and unsatisfactory." (Lord Kelvin)
The lakes of the northern Great Plains offer an excellent opportunity to examine the processes of lacustrine sedimentation on both a local and regional scale. As is the case with most other depositional settings, these lakes exhibit a continuous spectrum of sedimentary regimes. This continuum exists in terms of nearly every parameter that can be evaluated for the basins: morphology, hydrology, degree of permanence, brine salinity and ionic composition, sediment character, composition, etc. This great diversity presents somewhat of a problem in studying and discussing the lakes.
In order to subdivide this continuum, several "end member" types of lakes can be recognized (Figure ). A basic sedimentological distinction must be made between the basins whose brines are shallow enough to permit periodic drying and those whose brines are deep enough to maintain a relatively permanent water body. The suite of processes operating in shallow intermittent basins (i.e., playas) includes: cyclic flooding and desiccation of the playa surface, formation of salt crusts, efflorescent crusts, hardgrounds, spring deposits, and intrasedimentary salts, formation of solution pits and chimneys, and periodic detrital sedimentation by sheet flow and wind (Table ; Figure ). Most previously published summaries of these basic playa processes deal with lacustrine systems occurring in warm and dry climatic regimes [10
]. The five to six months of subfreezing temperatures experienced in western Canada, as well as the occurrence of snow and high seasonal runoffs associated with snowmelt, dictate some modifications and additional processes be considered when studying the playas of the northern Great Plains.
Lacustrine sedimentary processes operating in saline lakes of the northern Great Plains; after [10, 117, 158, 178].
The large number of individual playa basins in the Great Plains, the tremendous diversity of chemical and hydrologic settings, and the variable impact of humans are the main factors that have given rise to the complex suite of interrelated sedimentary processes in these lakes. To better understand the relative importance of the many and diverse processes, Q-mode cluster analyses has been applied to help quantitatively group various classes of playas in the region [77
]. This analysis identified five groups of playa basins. One group of lakes, typified by the Chaplin, Bigstick, and the Quill Lakes complex, are characterized by having large areas, moderate salinities but high Cl contents, and high detrital to endogenic sediment ratios. Mudflat and sand flat sedimentary processes dominate. In contrast, basins such as Snakehole, Whiteshore, and Muskiki are marked by brines with very high salinities that are almost completely dominated by sodium and sulfate ions. Sedimentation in this group of lakes is controlled largely by chemical processes and the basins contain thick sequences of bedded to massive salts. Chemical sedimentation processes also dominate in lakes such as Ceylon, Ingebright, and Vincent, but they exemplify a third group of playas distinguished by their relatively small areas, and high shoreline length and development indices (i.e., very irregular shorelines or very long, linear basin morphology). Lakes belonging to the final two groups include those whose brines are characterized by calcium, magnesium, and bicarbonate ions rather than sodium and sulfate. Sedimentation is controlled by either mixed chemical/physical processes (e.g., Verlo, Corral, Lydden) or mainly chemical (e.g., Metiskow Lake).
Despite the range of sediment types and hydrology of these lakes, the processes in these basins create a discrete suite of modern sedimentary facies. Although significant variations in the development of these facies do exist from basin to basin, a basic facies pattern common to most of the playas in western Canada can be recognized. The outer shoreline/nearshore complex comprises colluvium, mudflat/sand flat, and beach facies, and grades basinward into a salt pan complex. This basic pattern shares many of the same features recognized in saline lakes in British Columbia [269
] and is broadly similar to the facies distributions in playas in Spain [273
The colluvium facies consists of an often chaotic mixture of coarse to fine detrital material derived from the adjacent glacial deposits by mass wasting and creep. The mudflats are characterized by a mixture of sandy to silty detrital sediments often capped by thick efflorescent crusts and cemented hardgrounds. Unlike efflorescent crusts on mudflats in playas elsewhere [199
], these crusts are not monomineralic. They usually consist of a complex mineral assemblage of both hydrated and anhydrous Na, Na+Mg, and Ca sulfate and sulfate-chloride salts [277
]. In some basins, carbonate hardgrounds and crusts are also present. The mudflats are areas of active beach processes during seasonally high water levels, but for most of the ice-free season evaporation is the dominant process. Evaporation during the warm summer months results in upward capillary movement of groundwater, ionic concentration, and intrasedimentary mineral precipitation. Once efflorescent crusts have formed, further evaporation is curtailed, and the marginal areas of the lake can maintain high moisture contents throughout the dry season.
The mudflats can be colonized by extensive areas of algae and cyanobacterial mats (Figure ). The sediments in these areas are distinctively laminated [266
], organic-rich, and are usually sites of biogenic carbonate mineral genesis and diagenesis. Many of the more exotic carbonate mineral species found in the Great Plains (e.g., hydromagnesite, psuedohydromagnesite, kutnahorite, siderite, tychite, huntite) have been identified in these biolaminated algal flat sediments. To date, the biology and biogeochemistry of these mats have undergone only cursory examination [207
Figure 25 Microbial mats and other sedimentary-biological features. Examples of modern bio-sedimentary components of the salt lakes of the northern Great Plains of western Canada. A and B. Modern bio-sedimentary facies in Freefight Lake. Colluvium and mudflats/salt (more ...)
The salt pan complex usually comprises over 50% of a typical playa basin. It can be covered by brine for much of the ice-free period, but is often exposed by late summer. This facies is usually characterized by elevated endogenic to detrital sediment ratios; however, a complete gradation exists from salt-dominated pans to mud-dominated basins. The most distinctive feature of the salt pan facies is its distinctive annual cycle [208
]. During spring, relatively dilute inflow from melting snow and rainfall dissolves much of the very soluble salt that may have been precipitated the pan during the previous dry episode. This dissolution results in a significant increase in brine salinity and usually dramatic changes in ion ratios. These brine compositional changes at this time drive many of the penecontemporaneous diagenetic reactions that take place during the initial seasonal flooding of the playa [237
]. Throughout the rest of the ice-free period brine salinities continue to increase due to evaporation. As discussed above, even minor diurnal temperature changes can cause massive salt precipitation or dissolution. If brine remains in the basin after the onset of freezing conditions, considerable thicknesses of salts can occur due to freeze-out precipitation.
The playa pans can be further subdivided into several subfacies on the basis of mineralogy and crystal morphology [68
]. Near the margins of the pan where the salts interfinger with the mudflat facies, a zone of large dog-tooth mirabilite crystals develop, which grow displacively downward into the soft, water-saturated mudflat sediments (Figure ). Further out into the center of the basin, the floor of the salt pan usually consists of a mosaic of large, interlocking, bladed crystals. The specific mineralogy in the salt pan is controlled by the ionic content of the brine and the diurnal and seasonal temperature fluctuations experienced. Elevated temperatures favour minerals such as thenardite, hexahydrite, anyhydrite, and thermonatrite whereas cooler temperatures encourage mirabilite, epsomite, natron, and gypsum.
Superposed on these salt pan evaporites or clastics can be zones of mirabolites and/or sediments associated with spring openings. Mirabolites are rounded accretionary grains (analogous to carbonate pisolites) composed of Na2SO4 10H2O or MgSO4 7H2O that form in the shallow, supersaturated, and wind agitated brine (Figure ). These mirabolites can be moulded into bedforms, shoals, and ridges on the pan surface or form beach deposits on the marginal mudflats. Spring openings can be sites of massive salt precipitation because these are areas where water of different temperatures and compositions mix (Figure ). Frequently the springs continue to discharge onto the playa floor after the pan has been completely desiccated. This continued discharge can form large, low mounds and ridges.
In summary, present-day sedimentation in the playa basins of the Canadian Plains region is controlled by the interplay of: (a) flooding of the playa which causes dissolution of soluble minerals of the salt pan facies and efflorescent crusts of the mudflat and sand flat facies; (b) evaporative concentration of the brine which results in supersaturated conditions and precipitation of various soluble and sparingly soluble salts; (c) detrital influx by streamflow, wind, and spring discharge; and (d) organic productivity. In addition, evaporative pumping of shallow groundwater in the mudflats and sand flats causes growth of intrasedimentary displacive and poikilitic salt crystals in the near-surface clastic sediment.
This suite of interrelated sedimentary mechanisms operating in playa basins of the Great Plains is in contrast to the deposition in the region's perennial lakes, which appears to be controlled by a much smaller array of processes (Table ). The majority of processes, such as shoreline erosion, beach formation, wave/current distribution of sediments, deltaic sedimentation, and turbidity flow and interflow, are basic sedimentological processes common to any perennial basin. Numerous papers and compilation volumes already provide excellent summaries of these processes in perennial lakes [281
], as do most current introductory geoscience textbooks. However, several processes are unique to the perennial basins of western Canada or are of such fundamental importance that they will be introduced here briefly.
Probably the single most critical process operating in the perennial saline lakes is development of stratification of the water column. The influence of temperature stratification on seasonal carbonate mineral saturation and equilibria in the lakes is well known. However, the superposition of temperature stratification on an already chemically stratified water column complicates many mineral precipitation and dissolution reactions. In Deadmoose Lake, for example, both evaporitic and biologically-induced carbonate precipitation occurs in the well-mixed epilimnion. Most of these endogenic carbonates, however, are dissolved upon passing through the thermocline into the anoxic, lower pH water of the hypolimnion, such that the modern lake only contains carbonate-rich muds at water depths of less than 8 m [261
]. In contrast, endogenic CaSO4
O precipitation occurs below about 12 m depth. This evaporitic mineral is found only in the modern sediments of the monimolimnion. A further complication in the gypsum genesis in this meromictic lake is that the surface waters become supersaturated with respect to CaSO4
O during the winter and gypsum precipitation occurs. However, sediment trap data indicate that this gypsum originating in mixolimnion is re-dissolved before reaching the chemocline. The same complexity arises for mirabilite: precipitation of this sulfate mineral occurs both at the chemocline and in the surface water during winter. However, it is only preserved in the sediments of the monimolimnion at water depths greater than 20 m. Although this inhomogeneity can be relatively easily modelled once sufficient details are known about the chemical budgets of the stratified brine, any interpretation of the stratigraphic record is made much more obscure because even subtle chemical and/or temperature changes can result in substantial changes in mineral precipitation and preservation.
Two other important aspects of deposition in these perennial lakes of the Great Plains are the generation and accumulation of deep water-soluble salts, and the extraordinarily high rates of sedimentation experienced by some basins. Many of the non-playa lakes in the region are characterized by either mixed endogenic-allogenic sediments or entirely allogenic deposits [157
]. However, it has been shown that in some basins soluble and sparingly soluble salts are forming in deep water, offshore environments [283
]. Sedimentologists have recognized for some time that there are very few examples of modern sedimentary environments in which deep water evaporite mineral formation is occurring. This lack of modern deep water settings in which evaporites occur is problematic since many ancient evaporitic sequences have been interpreted as forming in deep water [287
]. Consequently, the perennial lakes in the Great Plains in which deep water salts are forming today provide a critical analogue in helping to understand the sedimentology, geochemistry, and stratigraphy of these ancient deposits.
To date, three basins in the Great Plains have been identified in which soluble evaporite minerals are forming and accumulating in their deep, offshore areas: Deadmoose, Little Manitou, and Freefight Lakes. It must be emphasized that there are probably numerous such basins among the many perennial lakes of the region, but our knowledge of the deep basinal facies of most of the basins is limited. Furthermore, subaqueous salt precipitation by freeze-out mechanisms have been reported from numerous other lakes in the region [80
] but presumably these salts are seasonal and re-dissolve during the ice-free period.
Both Deadmoose and Freefight Lakes are hypersaline and meromictic, whereas Little Manitou experiences 'temporary' meromixis. All three lakes are dominated by sodium, magnesium and sulfate ions but the mineralogy of the modern offshore precipitates is somewhat different in each lake as controlled by the specific ionic ratios in the brines. As outlined above, the diurnal and seasonal temperature fluctuations of the mixolimnions in Freefight and Deadmoose Lakes and the elevated salinities of the monimolimnions in these basins provide a complex multi-site source for the endogenic evaporite precipitates within the water columns. In Little Manitou, it appears that brine mixing associated with subaqueous spring discharge and the irregular breakdown of the weak meromixis initiates precipitation.
In addition to the formation of these deep water salts, the rate
at which they are accumulating is noteworthy. For example, sedimentation rates at the sediment-water interface in the deepest part of Freefight Lake average nearly 30 kg m-2
. While these sediment trap data do not represent net
accumulation, nonetheless the stratigraphic sequence recovered in the offshore areas of the basin suggest linear accumulation rates in excess of 2 cm yr-1
, values that are entirely consistent with the extraordinary mass/year sedimentation rates. As discussed elsewhere [290
], such high rates of chemical sedimentation should be expected in an evaporitic regime. However, until the discovery of these salt lakes in western Canada, such rates were not adequately documented in modern deep water environments.