Salt dissolution features in saline lakes of the northern Great Plains, western Canada

Geomorphology, 8 (1993) 321-334 Elsevier Science Publishers B.V., Amsterdam

321

Salt dissolution features in saline lakes of the northern Great Plains, western Canada William M. Last Department of Geological Sciences, Universityof Manitoba, Winnipeg, R3T 2N2, Canada (Received June 22, 1992; revised July 19, 1993; accepted August 20, 1993 )

ABSTRACT Large-scale salt dissolution is an important process affecting the sediments of many of the saline lakes in the northern Great Plains region of western Canada. The most easily recognized features of this salt karst are water-filled chimneys, vertical shafts, and collapse structures. The largest individual chimneys can be up to 20 m deep and 50 m in diameter, with volumes exceeding 25,000 m 3. Large, mud-filled chimneys and cavities, and salt-filled chimneys have also been identified in both the modem lakes and in the Quaternary sediments of the basins, which can adversely affect the salt mining potential of the basins. Because these salt karst features can affect large vertical sections of the sediment fill in the lakes, their recognition is of fundamental importance in attempting to use the stratigraphic records of the basins for paleoenvironmental research.

Introduction

Karstic phenomena are common in many areas of carbonate bedrock terrain having suitable geologic structure and groundwater plumbing systems. Much less frequently reported and studied are large-scale dissolution features involving highly soluble evaporitic sediments. Despite the rare occurrence of nearsurface evaporite karst in most temperate-humid to subhumid climatic settings, relatively deep subsurface (interstratal) salt karst is important in many areas of the world, including the southern High Plains of the United States (Gustavson et al., 1982; Johnson, 1989), the western and northern interior of Canada (Christiansen, 1967, 1971; Wigley et al., 1973; Tsui and Cruden, 1984), eastern Canada (Sweet, 1977; Ford and Williams, 1989), northern Spain (Benito and Gutierrez, 1987 ), and extensive areas of eastern Europe and Asia (Popov et al., 1972; Gorbunova, 1977, 1981 ). These and several other major areas of evapo-

rite karst have been summarized and discussed by Ford and Williams (1989), White (1988), Quinlan et al. (1986) and Nicod (1976). Syndepositional dissolution of highly soluble evaporitic minerals by dilute, undersaturated water is a process very common in most evaporite-forming environments (Sonnenfeld, 1984; Warren, 1989; Handford, 1991). Indeed, centimetre-scale dissolution features are one of the most easily recognized characteristics of the shallow saline pan/salt playa depositional setting (Warren, 1982; Lowenstein and Hardie, 1985; Smoot and Lowenstein, 1991). However, larger-scale syndepositional salt dissolution features, such as metersize channels, pits, sinkholes, and collapse structures, have been reported from surprisingly few modern salt pans (Death Valley and salars of northern Chile: Hunt et al., 1966; Stoertz and Ericksen, 1974 ) and interpreted in only a small number of ancient evaporite sequences (e.g., Permian Salado and San Andres

0169-555X/93/$06,00 © 1993 Elsevier Science Publishers B.V. All rights reserved.

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Fig. 1. Northern Great Plains of western Canada. Hachured areas are areas of internal drainage. Numbers indicate locations of the saline lakes in which large-scale dissolution features have been identified: 1: Alsask; 2: Berry; 3: Bitter; 4: Boot; 5: Ceylon; 6: Chain; 7: Corral; 8: Deadmoose; 9: Freefight; 10: Horseshoe; 11: Ingebright; 12: North Ingebright; 13: Little Manitou; 14: Lydden; 15: Metiskow; 16: Muskiki; 17: Snakehole; 18: Sybouts; 19: Verlo; 20: Vincent; 21: Whiteshore.

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SALT DISSOLUTION FEATURES IN SALINE LAKES OF THE NORTHERN GREAT PLAINS, CANADA

formations, southwestern United States: Powers and Hassinger, 1985; Hovorka, 1987; Devonian Prairie Evaporite, western Canada: Baar, 1974; Coode, 1987). The purpose of this paper is to describe large-scale salt solution (salt karst) features in modern and Holocene lake sediments in the northern Great Plains of western Canada and to emphasize the importance of recognition of these features when attempting to interpret paleoclimatic and paleohydrological fluctuations on the basis of the preserved stratigraphic records in these basins.

Regional setting The northern Great Plains physiographic province of western Canada is a vast region of over 350,000 km 2 stretching from the Precambrian Shield near Winnipeg, Manitoba, to the foothills of the Rocky Mountains in western Alberta (Fig. 1 ). Pleistocene continental glaciation has resulted in a thick mantle of unconsolidated glacial, glaciofluvial, and glaciolacustrine sediment overlying the generally southwesterly-dipping Cretaceous and Tertiary sedimentary bedrock. The geomorphology of the region is characterized by a fiat to gently rolling topography interspersed with numerous deep, often terraced, river valleys. The climate is cold and semi-arid, with a mean annual temperature of about 3 °C and a precipitation of approximately 35 cm. Warm summer temperatures and high winds combine to create high evaporation/precipitation ratios ( 3-10) from open water bodies. Large areas of closed surface drainage occur between the major river systems: in total, over 160,000 km 2 or nearly 45% of southern Saskatchewan and eastern Alberta is characterized by internal drainage (Fig. 1 ). The greatest concentration of saline lakes occurs in these areas of endorheic drainage.

The salt lakes and lacustrine evaporites These geologic, climatic, and hydrologic conditions have given rise to a large number of

323

saline lakes. Estimates of the number of salt lakes and saline wetlands in the region range as high as 6-10 million, with densities in some areas being as high as 120 lakes/km ~ (Last, 1989a). Nearly all of these lakes can be categorized into one of four end-member types: ( I ) shallow, ephemeral lakes (playas) dominated by clastic sediment; (2) playas dominated by salt; (3) deep-water, perennial lakes dominated by clastic sediment; and (4) deep-water, perennial lakes dominated by chemically precipitated sediments. Based on a statistical sampling of about 500 of these basins (Last, 1992 ), approximately 80% of all the lakes are shallow, ephemeral basins that exhibit playa characteristics (i.e., either type 1 or 2), although the region also contains several of the largest and deepest saline lakes in North America. As might be expected considering the large number of individual basins and the influence of varied topography, climate, and geology over this large geographic area, there is substantial variation in brine composition and concentration, and sediment composition in these salt lakes. Water salinities range from a few ppt (parts per thousand) total dissolved solids (TDS) to greater than 400 ppt TDS. The more saline brines are usually dominated by sodium, magnesium, and sulfate ions, whereas the somewhat less saline lake waters are most often enriched in calcium, magnesium, and bicarbonate ions. Detailed discussions of the brine chemistry of these saline lakes and their spatial and temporal variations can be found in the work by Rutherford (1970), Hammer ( 1978, 1986), Last and Schweyen (1983), and Last (1988). Like the brines, the Holocene sediments filling these salt lake basins show a surprisingly diverse spectrum of compositions, ranging from fine-grained, clastic-dominated sequences to massive, coarsely crystalline, soluble and sparingly soluble endogenic and authigenic precipitates (Last, 1989a, 1992). Although it is difficult to generalize over such

324

W.M. LAST

a broad geographic region, thicknesses of soluble evaporitic sediments in the salt-dominated lakes typically range from several meters to several tens of meters. The thickest sequence of salt penetrated to date in any of the lakes is approximately 50 m. Many of the saltdominated playas contain sufficient quantities of economically valuable industrial minerals to form the basis of a $50 X 106 per year salt mining industry (Broughton, 1984; Last and Slezak, 1987 ). Likewise, several of the deep-water perennial lakes contain high enough volumes of solutes to be commercially exploited

(Tomkins, 1953; Slezak and Last, 1985; Barry, 1986). The most common non-detrital components of the Holocene sediments in these lakes are sodium, magnesium, and sodium + magnesium sulfates (mirabilite, thenardite, bloedite, epsomite) and carbonates (magnesite, natron, nahcolite, trona, burkeite). Gypsum and halite, soluble evaporite minerals that are very common in salt lakes in many other areas of the world, are both rare in the lacustrine sediments of western Canada. The salts are typically equigranular, uniformly

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SALT DISSOLUTION FEATURES IN SALINE LAKES OF THE NORTHERN GREAT PLAINS, CANADA

coarsely crystalline, well consolidated, and massive to very thickly bedded with occasional m u d partings and laminae.

Description of dissolution features "The permanent crystal bed covers an area of approximately 460 acres with an average depth of 6 feet. Scattered through the crystal bed are numerous mud holes or chimneys, varying from a few feet to many feet in diameter. These caused considerable trouble in moving the drill from place to place on the deposit... Wherever the mud chimneys occur there is a corresponding hole in the crystal around the edges of which the mirabilite forms beautiful crystalgroups... In the crystalbed only one spring was observed. The flow from this spring seemed almost negligible, as it never overflowed on the crystal surface, but there was a hole, 15 feet in diameter, from top to bottom of the crystal bed, which at this place was 80 feet thick. This chimney increases in diameter in depth and was encountered at a depth of 50 to 70 feet when drilling hole No. 5, 100 feet distant. It appears to be a true solution cavity in the crystal and to be filled with nothing but water or very thin mud..." (Cole, 1926)

The above quotation, describing large-scale salt dissolution features in the sediments of several saline playas in southern Saskatchewan, is the first published identification o f syndepositional salt karst in North America. Cole (1926) indicated the presence o f vertical mud-filled pipes and water-filled chimneys in about half o f the twenty-two salt playas he studied in the region. More recently, Last (1989b, 1990) m a p p e d and briefly described large dissolution pits and chimneys in Ceylon Lake, another salt-dominated playa south of Regina, Saskatchewan. Syndepositional salt karst features have now been identified in the sediments of over twenty saline lakes in the northern Great Plains (Fig. 1 ). Figure 2 summarizes the m a i n types of salt karst and largescale salt dissolution features found in these basins: (1) water-filled chimneys and shafts; (2) collapse chimneys; (3) mud-filled chimneys and cavities; and (4) salt-filled chimneys and cavities.

325

Water-filled chimneys The large-scale salt dissolution that is occurring in the saline lakes o f the northern Great Plains is most easily recognized by the presence of water-filled shafts or chimneys which occur in the massive and bedded evaporitic sediments of the lakes. Today, the best developed and most numerous chimneys are present in Ceylon, Lydden, North Ingebright, Berry, and Muskiki lakes, although prior to salt mining, c o m p a n y personnel and local prospectors indicate that well developed water-filled chimneys also occurred in Snakehole, Ingebright, Sybouts, and Whiteshore lakes. Most of the lakes in which water-filled chimneys have been found are salt-dominated playa basins having relatively thick (greater than 1 m ) sections of massive to thickly bedded evaporites overlying fine-grained lacustrine muds and coarsegrained fluvial a n d / o r ice-laid clastic sediment (till). Figure 3 shows typical Holocene stratigraphic sections through several of the lakes. The chimneys are approximately circular to elliptical in plan view and characteristically have nearly vertical walls. More rarely, the shafts display an irregular cavernous cross-sectional morphology, with overhanging walls. They range in surface area from just a few m : to 50 m 2, although diameters greater than 5 m are unusual (Fig. 4 ). The larger chimneys can be readily identified on aerial photographs (Fig. 5 ) taken during low water or dry conditions in the playa. The depth o f the chimneys ranges from less than 1 m to over 20 m, and appears to be controlled mainly by the thickness of salt in the basin. The walls o f the shafts are smooth and hard, and composed of rounded, tightly-interlocked salt crystals. The chimney bottoms are usually fiat to gently tapering, and consist of soft non-laminated m u d which grades downward into firm, fine-grained clastic sediment. In m a n y of the deeper chimneys, the bottom material is gelatinous and contains abundant organic matter. Occasion-

326

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ally this clastic debris at the bottom also contains large intrasedimentary salt crystals and clastic evaporites. The surface openings of the chimneys can change considerably depending on the time of year and flow of water in them. During winter, the orifices are usually nearly completely covered by a combination of ice and newly precipitated salt. In some cases, such as at Ceylon, North Ingebright, and Muskiki lakes, contin-

ued flow of water out of the chimneys during winter can create salt and ice cones up to several meters high and several hundred m E in area (Fig. 6; see also Cole, 1926; Last, 1989b). With warmer temperatures and reflooding of the playas during spring and early summer, dissolution of the previously precipitated salt and melting of the ice reopens the orifices. During the final stages of playa surface desiccation in late summer and fall, the openings can again

SALT DISSOLUTION FEATURES IN SALINE LAKES OF THE NORTHERN GREAT PLAINS, CANADA

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Fig. 4. View of Ceylon Lake during the final stages of playa desiccation. The arrows point to locations of two waterfilled chimneys. A person is standing next to the chimney on the right.

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b e c o m e e n c r u s t e d a n d c o v e r e d b y large, n e w l y p r e c i p i t a t e d salt crystals g r o w i n g i n w a r d f r o m the walls o f the c h i m n e y . I n u n u s u a l instances, this salt crust c o v e r i n g c a n be o f sufficient t h i c k n e s s a n d s t r e n g t h to s u p p o r t the weight o f a person. In o t h e r cases, the n e w l y p r e c i p i t a t e d

Fig. 6. Examples of salt and ice cones built up over chimney orifices. The cone shown in (A) is from Ceylon Lake and is approximately 40 cm high and 60 m E in area. The depth of water in the chimney below the cone is 4.9 m. (B) The remnants of the same cone during mid-summer of the following year. The salts comprising the remnants of the cone are composed of sodium and magnesium sulfates. (C) A large 1.5 m high cone in Muskiki Lake (from Cole, 1926 ).

328

W.M. LAST

salt forms raised rims around the surface opening. The composition and salinity of the brine in these water-filled chimneys also varies considerably. Lowest salinities generally occur during spring and early s u m m e r when rain and melting snow provide an influx of relatively dilute water to the shallow groundwater systems, and during winter when low temperatures trigger massive freeze-out precipitation of many of the soluble salts, such as mirabilite, epsomite, and natron, from the saturated brines. During much of the ice-free season, however, the water in the chimneys is of high salinity. In some of the deeper chimneys the water column is both chemically and thermally stratified. In contrast to observations made by Cole (1926), who reported "fresh water" chimneys in several playas, all of the samples analyzed from the water-filled chimneys as part of the present study are either brackish (hyposaline: 5-10 ppt TDS), saline

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(10-50 ppt TDS), or hypersaline ( > 50 ppt TDS). Finally, although these large dissolution chimneys and shafts can be most readily observed in the playa basins, the features are not restricted to ephemeral lakes. Similar solution structures also occur in several deep-water perennial lakes. For example, Little Manitou Lake, Deadmoose Lake, and Freefight Lake are relatively deep (mean depths greater than 3 m ) , perennial, hypersaline lakes whose modern offshore sediments consist of coarsely crystalline magnesium and sodium + magnesium sulfates (Last, 1993). Although the m a x i m u m thickness of salt in these basins is not yet known, preliminary coring and sampling has penetrated in excess of 1 m of soluble evaporites in each lake. Punctuating this layer of salt in at least several locations in each of these basins are deep (greater than 1 m ) , roughly cylindrical holes (Fig. 7 ). Even though the precise morphology of these holes or chimneys is

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329

SALT DISSOLUTION FEATURES IN SALINE LAKES OF THE NORTHERN GREAT PLAINS, CANADA

yet to be determined, their locations are well known and readily identified by local residents because the flow o f water emanating from the chimneys greatly decreases the thickness o f ice formed on the overlying water during winter.

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Collapse chimneys Collapse structures have developed from the chimneys and shafts in several basins. These features consist o f circular depressions up to several meters deep. The sides of the depressions are irregular but slope gradually inward at relatively low angles ( 15-30 ° ), in contrast to the very steep to overhanging walls that characterize the water-filled chimneys. Ranging up to 30 m in diameter, these collapse structures are also m u c h larger than the vertical shafts. The base of the central pit is usually comprised of a mixture of soft, water-saturated m u d and salt.

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Mud-filled chimneys Cole (1926) reported a relatively large n u m b e r of " m u d chimneys" in the salt deposits o f the playas he studied. However, his description of these features was somewhat ambiguous and frequently referred to water flow and discharge of water from the " m u d chimneys". Subsequent investigation of the m u d chimneys he identified reveals that these features are actually water-filled chimneys (as described above) in which the water contains a relatively high concentration of suspended finegrained siliciclastic material and organic detritus. True mud-filled chimneys (i.e., dissolution shafts that have filled with clastic material) have not been found in any o f the m o d e m salt playas investigated to date. Nonetheless, investigation o f the stratigraphic records of several o f the salt playas reveals features that are most likely pre-modern mud-filled chimneys and cavities (Fig. 8 ). For example, Fig. 8A shows a portion o f an isopach map o f the total salt thickness in a playa

Fig. 8. (A) Isopach map of the total salt thickness in a portion of a salt-dominated playa near Verlo, Saskatchewan. The contour lines were drawn on the basis of data collected by augering and drill holes on a 250 m spacing grid. The line labelled A-A' is the cross-sectionshown in (B). The dots labelled A and B are the locations of cores described in Fig. 9. (B) Cross-sectionA-A' showing the presence of mud-filled chimneys within the salt of a playa near Verlo, Saskatchewan. (C) A generalizednorth-south cross-section through Alsask Lake prior to mining showing the presence of several mud-filled chimneys and cavities (drawn from data presentedby Cole, 1926). This lake also contained a 9 m deep water-filled chimney (Cole, 1926). near Verlo, Saskatchewan. This map and crosssection (Fig. 8B) were constructed using drill and auger data from closely spaced locations and show highly irregular salt thicknesses over very short horizontal distances in the playa. One of the most plausible explanations for this uneven salt isopach is that sections o f the stratigraphic sequence have been dissolved and replaced with non-evaporitic m u d material. The cross-section shown in Fig. 8C illustrates an

330

W.M. LAST

example of several large mud-filled cavities that occur within the 10 m thick salt section in A1sask Lake. These mud pockets in this 2 km 2 salt playa are up to 2.5 m thick and cover a composite area of approximately 1.8 ha.

Salt-filled chimneys The water-filled chimneys and collapse structures described above are dynamic features, actively forming and changing morphology in response to fluctuating brine chemistry and groundwater flow conditions. Several of the larger chimneys in Boot Lake and Ceylon Lake have been monitored through extended periods of drought during which the entire surface of the playas has been dry for several years. Despite the complete drying and desiccation of the playas, the water level within the chimneys has not fluctuated significantly. However, the morphology and, in particular, the depth of the

chimneys can show significant variation on a relatively short (several years) basis. During the extended desiccation episodes which occurred in these two lakes through the mid1980's, the chimneys and shafts became sites of intense and extremely rapid sedimentation. For example, one of the water-filled chimneys, originally 7.5 m deep, filled with 5.1 m of salt and mud in a 2.5 year period between 1983 and 1986. Another chimney which was 9.1 m deep in August, 1982, had a water depth of 2.8 m in February, 1983 (Last, 1984). The evaporites filling these chimneys are quite distinct from the surrounding salts. In contrast to the hard, massive, coarsely crystalline salts of variable and complex composition which characterize the normal Holocene stratigraphic sequences in these two basins, the newly precipitated evaporites are soft to poorly consolidated, finely crystalline with a dominant acicular morphology, and are composed

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SALT DISSOLUTION FEATURES IN SALINE LAKES OF THE NORTHERN GREAT PLAINS, CANADA

of a "simple" evaporite mineral assemblage. In the case of Ceylon Lake, the shaft-filling salts are entirely mirabilite; in the chimneys of Boot Lake, both mirabilite and bloedite occur. In addition to these modern salt-filled chimneys and shafts in which salt precipitation and in filling is actively occurring, salt-filled chimneys can also be recognized in the stratigraphic records of the lakes, despite the absence of any evident surface morphology or water discharge. Figure 9 shows the stratigraphic variation in soluble salt composition in two cores taken from the salt playa near Verlo, Saskatchewan. The core locations are about 500 m apart. The core from location A shows a stratigraphic sequence in which the salts grade from carbonates in the lower 50 cm upward into about 1.5 m of mainly sodium-dominated sulfates, and, finally, in the uppermost 75 cm, into a mixture of mainly magnesium and magnesium + sodium sulfates together with smaller proportions of sodium sulfates. This general sequence of salt composition has been recognized in other playas in the region (Last and Slezak, 1988; Last, 1990) and represents a commonly occurring Holocene brine evolutionary trend from early carbonate-rich lake water to brines dominated by sodium and sulfate ions and, finally, to water with increasing magnesium content. Core B, however, reveals a considerably different sequence in which the lowermost carbonate-rich salts pass sharply upward into Mg and Mg + Na sulfates with relatively little sodium sulfate salts present. Cole (1926) also remarked about the significant lateral variation in the composition of the salts in several playa basins that have subsequently been mined. In Snakehole Lake, for example, the percent of sodium sulfate salts varied by nearly 50% in two adjacent cores less than 100 m apart located in the center of the basin. These dramatic changes in salt composition over very short horizontal distances cannot be readily explained by any reasonable penecontemporaneous brine fractionation or salt facies change mechanism. Rather, the lateral

3 31

compositional changes reflect large-scale salt karst and subsequent refilling of the solution cavity or chimney by evaporites deposited under a considerably different hydrochemical regime. Discussion

Large-scale salt dissolution is an important post-depositional process in the saline lakes of the northern Great Plains. The presence of various modern and Holocene salt karst features has been known for some time to adversely affect the mining potential and industrial mineral reserve of the lacustrine deposits in the region. For example, in North Ingebright Lake, a small salt playa with estimated reserves of more than 3 × 106 tonnes of NaESO4, present-day solutional activity has removed about 25,000 m 3 of salt or approximately 10% of the total reserve. The presence of an estimated 20,000 m 3 of mud-filled solution cavities within the Alsask deposit has similarly reduced the marketable salt yield from this basin. As important as salt karst is with respect to the economic potential of the basins, equally significant are the implications that this largescale salt solution has on the paleoenvironmental use of the stratigraphic records in the lakes. As discussed by Last (1992), the salt lakes and saline playas provide nearly the only source of detailed, high resolution paleoenvironmental information for the Holocene of the region. Although the dissolution phenomena discussed in this paper appear to affect only relatively small areas of any single basin, the significance of the karst lies in the fact that extensive vertical sections of a salt lake deposit can be modified. This modification can consist of either partial or complete removal of the evaporitic sequence at any given locality in the lake, and replacement by either non-evaporites or salts that may be precipitated under much different hydrochemical conditions. Thus, considerable caution must be exercised when using the stratigraphic records in these

332

basins to interpret past hydrologic changes and brine chemistry fluctuations. Large-scale salt solution also poses important questions regarding the sediment budgets of the lakes in which these features occur. It is often tempting to view a closed basin saline lake simply as a container of water in which relatively dilute inflow and solutes are concentrated by evaporation ultimately to the point of salt precipitation. In this simple approximation, the chemical mass balance and, hence, the brine composition, salinity, and rate of mineral precipitation in the basin can be predicted by estimating several basic environmental parameters such as stream and groundwater composition and inflow, and evaporation (e.g., Wood and Sanford, 1990; Donovan, 1993; Komor, 1993). However, large-scale remobilization of previously precipitated salts can significantly increase (or decrease as in the case of chimney filling) the dissolved solute flux to the modern lake, resulting in considerable error in short-term chemical mass balance and rate determinations. The distribution of water-filled chimneys and shafts in the salt lakes of the northern Great Plains is still not fully understood. The striking linear arrangement of solution chimneys in Ceylon Lake (see fig. 4 in Last, 1989a) suggests that their distribution within this basin may be related to subsurface fractures or joints in the underlying till. Fractures in the till of the Interior Plains have long been recognized as important conduits for shallow groundwater through this otherwise low permeability substrate (Grisak et al., 1976; Hendry et al., 1986). However, in the case of Ceylon Lake, it is not evident how any such fracture pattern or permeability enhancement within the till is transmitted up through as much as 6 m of intervening fine-grained lacustrine muds to control dissolution of the overlying salt. It is now generally well accepted that dilute, shallow groundwater plays a pivotal role in controlling the hydrology and hydrochemistry of these saline lakes (Donovan, 1993; Last,

W.M.LAST

1988). The recognition of large-scale salt karst features in the sediments of numerous saline lakes of the region further stresses the importance of this dilute subsurface inflow and emphasizes the dynamic nature of continental evaporite sedimentation.

Acknowledgements This research was made possible through funding from the Natural Sciences and Engineering Research Council of Canada. Thanks are also extended to D. Bell, T. Ewing, L. Kovac, G. Sayers, L. Sack, T. Schweyen, and L. Slezak for their valuable assistance in the field. The manuscript was improved by the comments of two anonymous reviewers.

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