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

Sedimentary Geology, 64 (1989) 271-285

271

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

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

C. K E N D A L L

*

Department of Geology, University of Toronto, 170 College Street, Toronto, Ont. M5S 1A 1 (Canada) Received September 12, 1988; revised version accepted June 22, 1989

Abstract Kendall, A.C., 1989. Brine mixing in the Middle Devonian of western Canada and its possible significance to regional dolomitization. In: R.W. Renaut (Editor), Sedimentology and Diagenesis of Evaporites. Sediment. Geol., 64: 271-285. 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 infow: 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.

Introduction

source of the magnesium dolomitizing

In recent years interpretations

of dolomitiza-

water

and

or the nature more

nature of the hydrodynamic

concern

of the

with

the

drive required to move

tion on a regional scale have apparently changed

the dolomitizing

t h e i r focus. T h e r e a p p e a r s t o b e less i n t e r e s t i n t h e

( M o r r o w , 1 9 8 2 ; M a c h e l a n d M o u n t j o y , 1986, 1 9 8 7 ;

waters

through

the limestones

H a r d i e , 1987). I n p a r t t h i s s w i t c h i n e m p h a s i s is due to the recognition that much regional dolomi* Present address: School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, U.K. 0037-0738/89/$03.50

© 1989 Elsevier Science Publishers B.V.

t i z a t i o n is p r o b a b l y

of late diagenetic origin and

that dolomitization

i n t h e s e h o t t e r d i a g e n e t i c set-

272

tings is possible from waters with relatively low magnesium to calcium ratios (Hardie, 1987). On the other hand, Land (1985) maintained the emphasis on the source of magnesium and concluded that only seawater (or its modifications) is capable of massive dolomitization and that late diagenetic dolomites are probably recrystallized or remobilized earlier diagenetic dolomites which obtained their magnesium from seawater. This second conclusion is highly controversial and appears to be inconsistent with the distribution and paragenesis of late diagenetic dolomite in some settings (Mattes and Mountjoy, 1980; Machel, 1986; Newton and Hardie, 1986). Elevation- or density-heads may induce the movement of seawater (or magnesium-enriched and concentrated brines derived from seawater) and may cause wholesale dolomitization along the flowpaths. Elevation head provided by evaporite drawdown in large evaporite basins was briefly mentioned by Land (1985, p. 119) but was dismissed as only causing local flow through a sill, and because "precipitation of evaporite salts in the basin will quickly fill the basin, cancelling the head difference and choking off flow". Here it is suggested that large evaporite basins do in fact induce large-scale flow, not only of seawater through the sill but of formation waters present in rocks and sediments beneath the evaporite basin, and may well have been responsible for regional dolomitization of limestones beneath, or adjacent to, the evaporite basin. The proposed model will first be outlined, followed by an application to a specific b a s i n - - t h e Middle Devonian Elk Point Basin of western Canada. This evaporite basin, I suggest, was responsible for the regional dolomitization of Lower Palaeozoic carbonates of the underlying Williston Basin. In particular, the model provides an explanation for an anomalous distribution of anhydrite within the evaporite basin and may also have application to the formation of sulfate-impoverished potash salt deposits. Artesian inflow in evaporite basins Basin desiccation

Fig. 1 is a summary of the physical aspects of the model. All large evaporite basins that accu-

EVAPORATIVELOSSESFROMBASIN

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1 • BASIN BECOMES ISOLATED. 2. EVAPORATION LOSSES IN BASIN EXCEED SEAWATER INFLUX. 3. BASIN DESICCATES AND HYDRAULIC HEAD ESTABLISHED, 4. GROUNDWATERS FLOW INTO BASIN IN RESPONSE TO THE HYDRAULIC HEAD AT LOCATIONS WHERE BASAL SEAL OF THE BASIN IS BROKEN. 5. GROUNDWATERS ENTERING BASIN SUFFER EVAPORATIVE LOSSES~ CONTINUING THE GROUNDWATER INFLUX. 6. GROUNDWATERS (AND EVAPORATED GROUNDWATERS) MIX WITH MARINE-DERIVED BRINES AT LOCA'I1ONS NEAR GROUNDWATER INFLOWS.

Fig. 1. Evaporativebasin model, h Physicalaspects.

mulated halite must have been isolated from the sea. Lucia (1972) demonstrated this when he outlined the relationship that must exist between (1) the maximum salinity that can be developed in the basin and (2) the balance that existed between the relative rates of water loss (by evaporation) and water influx in the basin--this balance represented by the ratio between the areas of the basin and its inlet. For halite precipitation the inlet must be at least eight orders of magnitude smaller in its cross-sectional area than the surface area of the basin. Such small inlets are not geologically persistent (either in existence or size) and Lucia concluded (1972, p. 167) that halite deposition must indicate "complete surface disconnection from the ocean". Once isolated an evaporite basin can only be fed from the sea by seepage through the barrier as is the Holocene MacLeod Evaporite Basin recently monographed by Logan (1987). Even if highly permeable barriers are present, seepage cannot offset losses in the basin due to evaporation, brine level in the basin falls (evaporite drawdown of Maiklem, 1971), and the basin desiccates. Even when Maiklem (1971) used a value of nearly 73 darcies (!) for the permeability of the Presqu'ile Barrier, calculated rates of seepage inflow of seawater into the Middle Devonian Elk Point basin of western Canada would have been

273 EVAPORATIVELOSSESFROMBASIN

unable to offset evaporite losses with the basin. Using more "reasonable" permeability values for the basin (0.9-2.7 darcies) resulted in evaporative losses in the basin calculated as being 200-800 times the inflow.

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Groundwater flow into a desiccated basin A desiccated basin constitutes a very large hole. Large deep holes will attract ground- and formation-waters from rocks beneath and adjacent to the evaporite basin. An elevation head will exist between the floor of the desiccated basin and the groundwater table in the area surrounding the evaporite basin. Relatively small topographic effects are capable of inducing, or controlling deepseated groundwater flow (Back, 1960, 1966; Toth, 1972, 1980) and under appropriate conditions the depth of penetration of the flowpaths may reach several thousand feet under the effect of local topography of only a few tens of feet (Toth, 1963). Present-day depressions in Iran constitute artesian basins and have deep-seated water movements (Issar and Rosenthal, 1968). These waters also experience significant chemical changes (changing from weakly to highly concentrated brines and from bicarbonate through sulfate to chloride water types) as they are drawn through and interact with the essentially clastic sediment fill of the basins. I suggest that if the underlying sediments or rocks were limestones they also would have interacted with such brines and could have undergone dolomitization.

Chemical changes affecting inflowing groundwaters If dolomitization occurs as formation waters are driven towards desiccated evaporite basins, these waters will become progressively enriched in calcium. Calcium is introduced as a consequence of the dolomitization reaction (Fig. 2) as each magnesium ion displaces a calcium ion. Basinal brines are usually impoverished (relative to chloride) in sulfate and bicarbonate (White, 1965; Collins, 1975) so that the calcium released by dolomitization will be balanced by chloride. In other words, dolomitization should generate a calcium chloride brine and this eventually will

E V A P O R I T E BASIN M O D E L II - C H E M I C A L ASPECTS 1. GROUNDWATERSDOLOMITIZE CARBONATES BENEATH BASIN 2. GROUNDWATERS BECOME Ca- ENRICHED

2CaCO 3 + Mg 2+ = CaMg(CO3) 2 + Ca 2+

2 C a C O 3 + Mg2+ ~ C a M g ( C 0 3 ) 2 + C a 2+

3. MARINE BRINES PRECIPITATE GYPSUM IN PROXIMAL PARTS OF BASIN AND BECOME Ca-DEPLETED BUT RETAIN 2/3 OF THEIR SULFATE. 4. MIXING OF BRINES CAUSES PRECIPITATION OF REACTION

{Ca2+ + 2CI-} + {Mg 2+ + SO42"} + 2 H 2 0 = CaSO4.2H20 + Mg 2+ + 2CI-

GYPSUM 5. SEAWATER-DERIVED BRINES BECOME SULFATE-DEPLETED, AND WITH FURTHER CONCENTRAllON PRECIPITATE K- AND KIMg-CHLORIDES INSTEAD OF Mg.SULFATES.

Fig. 2. Evaporativebasin model, II: Chemicalaspects.

enter the evaporite depositional basin, there to undergo evaporation. Some evidence for the introduction of this brine into the evaporite basin thus would be expected.

Interaction between different brines Turning back now to the evaporite basin itself, seawater entering the basin through a barrier by seepage inflow experiences progressive evaporative concentration and will successively precipitate carbonates, gypsum and halite as it moves into the basin. Precipitation of carbonates and, especially gypsum, causes the brine to become depleted with respect to calcium (Hite, 1985). In distal parts of a basin, where halite is precipitated, the seawaterderived brine will be essentially calcium-depleted but will still retain two-thirds of its original sulfate content. If a calcium-depleted marine brine in the distal parts of an evaporite basin mixes with a calcium chloride brine derived from groundwater inflow, calcium sulfate will be precipitated as a reaction product. Calcium is derived from the basinal brine whereas the sulfate originates from the concentrated seawater brine. The evaporite basinal model proposed here therefore predicts that there should be an anomalous distribution of calcium

274

sulfate (anhydrite where the gypsum has been buried sufficiently) at locations where brine-mixing occurred. This, as will now be demonstrated, is the situation in the Middle Devonian Elk Point Basin.

PRESQU'ILE BARRIER

The Elk Point Basin of Western Canada

Initially, the Elk Point Basin was an reef-barred arm of the sea in which pinnacle-reefs flourished (now presented by Keg River and Winnipegosis carbonate buildups) (Grayson et al., 1964; McCamis and Griffith, 1967; Nelson, 1970). Eventually, however, the reefal barrier in northern Alberta closed and evaporation converted the basin into a desiccated brine-pan. In this basin the Prairie Evaporite and Muskeg Formations formed (Fig. 3). In southern Saskatchewan and neighbouring parts of Manitoba, Montana and the Dakotas the Prairie Evaporite rests upon a thick (up to 450 m), carbonate sequence of the Williston Basin (Fig. 4)--a structural basin upon which the Devonian Elk Point Basin rests and cross-cuts. In the Williston Basin almost all carbonates beneath the Prairie Evaporite (Lower Palaeozoic as well as the immediately underlying Middle Devonian) have been regionally dolomitized--except where some have apparently been protected by an overlying Ordovician argillaceous unit (Fig. 4; Kendall, 1976a,b; 1978). Above the Prairie Evaporite, the Upper Devonian and Mississippian carbonates are predominantly limestones and nowhere have been subjected to wholesale and regional dolomitization. This distribution alone strongly suggests that some form of genetic link exists between the Prairie Evaporite and the formation of underlying dolomites.

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2000 Km Fig. 3. Diagrammatic map and cross-section of the Elk Point Basin of western Canada during the time of Muskeg and Prairie Evaporite deposition (map greatly simplified from Grayson et al., 1964). Marine brines entering the basin across the Presqu'ile Barrier became progressively concentrated as they crossed the basin and precipitated a regular succession of facies from carbonates (most proximal to the barrier) to halite (most distal). At distal locations, however, older Winnipegosis carbonate buildups (shown very diagrammatically and at an incorrect scale relative to the basin size) are mantled by anomalous anhydrite envelopes.

There is abundant evidence that the Elk Point Basin was desiccated when the Prairie Evaporite was being deposited. Halites from near the base of

Fig. 4. Dolomitization in the Williston Basin. A. Diagrammatic stratigraphic section through the Palaeozoic sequence in southern Saskatchewan. Regional developments of dolomite are confined to locations between the Prairie Evaporite (where dolomitization is essentially complete), whereas above this evaporite unit carbonates are regionally undolomitized (numerous thin and facies-controlled dolomites are ignored in this diagram). In the Lower Palaeozoic, only Red River carbonates have survived dolomitization at locations where they are overlain by a Stony Mountain Shale cover. B. Map of dolomitization in the Red River Formation showing the lack of correspondence between the extent of dolomitization and the distribution of evaporites within the Red River Formation. The 80% dolomite contour corresponds approximately with the feather-edge of the overlying Stony Mountain Shale. This unit may have acted as an umbrella shielding underlying limestones from dolomitization by refluxing brines (Kendall, 1976b), or it created an underlying stagnant area of formation waters at a time when groundwaters elsewhere in the basin were moving upwards towards the overlying and desiccated Elk Point Basin--an interpretation favoured in this paper. Dolomite data from Porter and Fuller (1964) and Forster (1972); Red River evaporite cover from Kendall (1976a) and Forster (1972).

275

the Prairie Evaporite exhibit chevron crystal fabrics, dissolution surfaces (Wardlaw and Schwerdtner, 1966) and teepee structures (Fig. 5),

and this combination of features is consistent with deposition in a salt-pan environment (Lowenstein and Hardie, 1985). Significant vertical changes in

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5cm Fig. 5. Bedded Lower Prairie Evaporite halite with horizontal and inclined laminae of anhydrite. Well developed chevron structure in the halite layers is tilted in layers bounded by dipping anhydrite laminae, suggesting that they represent sections through tepee structures. All the features of this deposit are consistent with deposition on salt flats. White Rose St. Denis 1-22 well (1-22-37-1W3), 3716-3721 ft (1132.6-1134.1 m), south Saskatchewan.

277

the bromide content of the halite (Wardlaw and Schwerdtner, 1966) also signify that precipitation occurred from a very small volume of brine. This is inconsistent with any significant filling of the basin with brine (Brodylo and Spencer, 1987), Based upon the height of the earlier Winnipegosis buildups above the basin floor, the basal brine-pan salts in the basin would have formed in a depression at least 100 m deep. During Lower Prairie Evaporite deposition, therefore, there would have been a substantial elevation head capable of causing groundwater inflow into the basin. The distribution of evaporite facies in the Elk Point Basin reflects a progressive southeasterly movement and an increasing concentration of the seawater that entered the basin through its northern barrier (Fig. 3). Carbonates adjacent to the Presqu'ile Barrier pass laterally into sulfates and, in the distal parts of the basin, into halites (Grayson et al., 1964; McCamis and Griffith, 1967; Bebout and Maiklem, 1973). Assuming that Middle Devonian seawater had more or less the same relative proportions of major ions as at present, it must be supposed that in the distal parts of the basin, in excess of 1000 km from the barrier, the brines were incapable of precipitating any significant amount of gypsum. They would have been depleted in calcium. Yet around the earlier pinnacle reefs are envelopes of anhydrite (Fig. 6) of equivalent thickness to the reefs and of unknown lateral extent (Wardlaw and Reinson, 1971; Bebout and Maiklem, 1973; Corrigan, 1975; Kendall,

2-2-34-7W3

1975). Based upon a seismic interpretation, Corrigan (1975) suggests that the anhydrite envelopes in central Alberta extend away from the edge of reefs for up to "several thousand feet". This is not inconsistent with the situation in Saskatchewan where wells drilled in exploration programs for Middle Devonian Winnipegosis reefs repeatedly intersected marginal anhydrite rather than the reefal carbonate itself. This suggest that a substantial part of the seismic anomaly identified as a Winnipegosis reef buildup is in fact formed by the anhydrite envelope. I suggest that this "anomalous" anhydrite is the result of brine mixing and indirectly points to the coeval dolomitization of carbonates underlying the evaporite basin. Anhydrite surrounding Middle Devonian carbonate buildups has previously been identified as a replacement of the marginal parts of the reefal (or mound) carbonates by calcium sulfate--replacement having taken place by reactions between the carbonates and concentrated brines (Bebout and Maiklem, 1973; Corrigan, 1975). The occasional preservation of pseudomorphs after gypsum crusts (Fig. 7A, previously figured but misidentified as replaced halite crystals by MeijerDrees, 1986, fig. 50e,f), however, clearly establishes that at least some of the anhydrite represents a primary precipitate rather than a replacement of a pre-existing carbonate. Furthermore, the anhydrite envelopes interfinger with brine-pan halite away from the reefs (Fig. 6) suggesting that calcium sulfate and halite were deposited simulta-

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278

neously and that both are younger than the carbonate buildups they surround--basin-floor brine-pan halite and carbonate buildups containing a varied marine fauna cannot have been contemporaneous. The rare occurrence of fragments of unreplaced corals within the anhydrite are in-

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terpreted to be fragments of the older carbonate buildup. These broke off and moved downslope to rest and become incorporated into the marginal evaporitic environment which ultimately produced the anhydrite envelopes. Margins of these clasts are sharp and there is no evidence for any sulfate

2 C m

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Fig. 7. Structures developed within Whitkow Anhydrite. A. Steeply dipping crusts of former gypsum crystals pseudomorphed by anhydrite, and enclosed within beds of nodular mosaic anhydrite; Pheas KR Punnichy 6-13-28-17W2 well (south Saskatchewan), 3495 ft (1065.3 m) B. Nodular mosaic anhydrite with numerous broken fragments of dolomitic pisolites; Pheas KR Punnichy 6-13-28-17W2 well (south Saskatchewan), 4264 ft (1299.7 m).

279 replacement--many clasts still retain primary intraskeletal porosity, even though the coral skeleton has been dolomitized. The main characters of the anhydrite have been well described by Wardlaw and Reinson (1971), Bebout and Maiklem (1973), and Corrigan (1975). It is composed predominantly of mosaic, nodularmosaic, distorted mosaic and distorted nodular mosaic anhydrite (Fig. 7) with gradations into massive and bedded mosaic anhydrite (terminology of Maiklem et al., 1969). Original characteristics have apparently been changed substantially by dehydration of the original gypsum to anhydrite, and the flowage and distortion which probably accompanied this alteration. The anhydrite is commonly cloudy with dolomite inclusions and contains abundant dolomite fragments. It is these features that have led previous authors to conclude that the anhydrite is replacive. However, the nature of most of the dolomite fragments does not support this view. Many anhydrites contain numerous individual carbonate pisolites or fragments of pisolitic gravel with a dolomicrite matrix (Fig. 7B). Adjacent fragments commonly appear to be capable of fitting together and the impression is given that they have been separated and fractured by growth of displacive calcium sulfate. The pisolites display remnants of an original radial crystal texture and appear to have been similar to those described by Risacher and Eugster (1979) from hot-spring deposits adjacent to saline lakes in the Andean A1tiplano of Bolivia. Here they are similarly interpreted to have been deposited by springs, with precipitation occurring during rapid degassing of carbon dioxide. Other parts of the anhydrite envelopes contain abundant fragments of thinly-bedded to laminar fine-grained dolomite. This material was apparently soft and plastic at the time of calcium sulfate emplacement because the laminations are commonly disturbed. Occasionally, the form of the anhydrite nodules within the laminar carbonate indicates that they were originally large displacive gypsum crystals (Fig. 8). The depositional environment of the laminar carbonates cannot be established with any certainty but it clearly was not part of a carbonate reef or mud mound. Its

2cm Fig. 8. Anhydrite after gypsum crystals (note lozenge-shaped outlines of some "nodules") that displaced finely-laminated anhydriticdolomitemudstones; Ceepee Keppel Forest 8-3 well (8-3-40-14W3; western Saskatchewan),4260 ft (1298.4 m). characteristics are, however, consistent with deposition on a carbonate mudflat, perhaps downflow from marginal spring travertines and pisolitic gravels. In the few wells that have intersected the margins of the carbonate buildups, but particularly in those cored through the top of the carbonate mounds, the anhydrite of the envelope is underlain by laminar and pisolitic dolomites (Fig. 9) that have been previously identified as of algal origin (Machielse, 1972). The close association between laminar carbonates and pisolites (with radial internal structure and therefore of

280

2~ Fig. 9. Pisolitic and laminated dolomites at top of Winnipegosis buildups. A. Pisolitic dolomite overlain and underlain disconformably by finely laminar dolomite incorporating a lightcoloured layer (at top of specimen) with ghosts of vertical acicular crystal fabric (probably a carbonate sinter); Banff et al Langham 12-30-38-7(W3) well, south Saskatchewan, 3544 ft (1080.2 m). B. Nearly vertical laminated dolomite representing flowstone or travertine deposit; Pheas Banff et al Asquith 10-29-36-9(W3)well, south Saskatchewan, 3989 ft (1188.1 m).

non-algal origin), the occasional preservation within the laminar dolomite of columnar crystal textures, and the presence of steep dips (sometimes near-vertical; Fig. 9B) of the laminated carbonates over several tens of metres in some wells suggest that this carbonate also represents the deposits of subaerial springs. Although algae may have been present and some of the laminar dolomites may represent stromatolites, the sequence as a whole is better considered as a travertine that coated the former walls of the Winnipegosis buildups. The occurrence of (1) presumed travertine deposits adjacent to, and on top of, the former carbonate buildups, (2) pisolitic gravels that also record deposition from springs, and (3) soft laminar carbonate muds, which are interpreted to be downflow mudflats with carbonate precipitation occurring in response to evaporation of groundwater (and analogous to the formation of present-day playa carbonates--Hardie et al., 1978), provides evidence for the entrance into the evaporite basin of a water containing abundant

calcium. This water entered the basin through the porous and permeable carbonates of the earlier carbonate reefs and mudmounds which, after the basin had become desiccated, would have stood as isolated positive features. This water entered the basin at the time halites were forming in the basin and so, in the vicinity of the carbonate reefs, would have mixed with a marine-derived brine essentially depleted of calcium and unable to precipitate any further quantity of gypsum. Where the two brines mixed, however, calcium sulfate--in the form of displacive gypsum--grew within the flanking spring carbonate deposits, commonly to the extent that calcium sulfate is now the dominant rock component. Groundwater flow into the basin was focussed through the Winnipegosis buildups because the basin floor was lined by an aquitard composed of a thin bituminous dolomite which forms a highly radioactive "black shale", and an overlying (and equally impermeable) unit of laminated carbonates and calcium sulfates with organic films between laminae--the Ratner Member of Wardlaw and Reinson (1971). These units are absent from localities occupied by the carbonate buildups (Kendall, 1975) although the buildups may marginally interfinger with the bituminous dolomite unit. Fig. 10 interprets diagrammatically the environment in the neighbourhood of a Winnipegosis carbonate buildup at the beginning of Prairie Evaporite deposition (Whitkow Member). Calcium-rich groundwaters preferentially moved through the buildups and entered the basin as springs. They precipitated pisolitic gravels and travertines, presumably as a consequence of both rapid carbon dioxide degassing, and the initial stages of evaporation. Upon coming into contact with, and mixing with, the concentrated seawater brine of the basin floor, large amounts of gypsum were formed from their interaction, and now form the anomalous anhydrite envelopes around the carbonate buildups in the distal part of the Elk Point Basin. I suggest that the calcium-bearing water that entered the basin through the carbonate buildups, and which was responsible for the formation of the anhydrite envelopes (waters derived originally from those contained within formations beneath

281

Travertine and Plsolltlc Gravels

\

Displacive and Replaclve Gypsum Sabkha Flats

Salt Flats

WHITKOW ANHYDRITE, WHITKOW HALITE Fig. 10. Diagrammatic interpretation of environment proximal to an exposed Winnipegosis carbonate buildup during deposition of the Lower Prairie Evaporite. Groundwaters issuing from springs on the exposed buildup flow into the desiccated basin (black arrows) initially precipitating travertines and pisolitic gravels. As these waters mix with marine-derived brines (white arrow) they react together and precipitate gypsum. Blocks and fragments of the older carbonate buildup occasionally fell onto the surface of the gypsum-precipitating area and became incorporated. Such material has given rise to an interpretation that the Whitkow Anhydrite has replaced the marginal parts of the Winnipegosis buildups.

the Elk Point Basin or which were transiting these formations) may have obtained its calcium via coeval dolomitization of carbonates underlying the evaporite basin. This dolomitization occurred when an artesian flow of formation-water was induced by the desiccation of the evaporite basin. The reverse interpretation--that dolomitization of the underlying carbonates occurred by a process of brine reflux during formation of the Prairie Evaporite--appears to be a less viable alternative. The presence of laterally persistent units on the floor of the evaporite basin that would have acted as aquitards would have confined reflux to areas beneath the carbonate buildups--locations for which there is evidence for water-movement in the opposite direction (presence of spring-deposited carbonates). Discussion

Significance to the origin of potash deposits Bittern salts in the Prairie Evaporite, like many other potash deposits, are composed of potassium-

and potassium-magnesium chlorides (Holter, 1969) and lack the magnesium sulfate minerals that would be expected from simple seawater evaporation. There have been several attempts to explain such a "sulfate deficiency" in evaporite sequences but most have been judged inappropriate. The most widely held interpretations have been that (1) sulfate is removed from a brine by bacterial reduction (Borchert, 1940; Sonnenfeld, 1984), and (2) that sulfate may be eliminated from brines by a reaction with calcium bicarbonate introduced by fluvial influx into the basin (Lotze, 1938). Hite (1985), however, provides calculations and evidence that rates of bacterial reduction are insufficient to eliminate sulfate from the marine input. The quantity of fluvial influx required to eliminate marine-sourced sulfate is so large as to make this possibility untenable also. Garrett (1970) calculated that it would require a river the size of the Mississippi to enter the Middle Devonian Elk Point Basin in order to deplete the brine of its sulfate. Such a fluvial input would dilute the brines in the basin to such an extent that no evaporites would have formed.

282

Braitch (1971) suggested that dolomitization of pre-existing calcium carbonate by magnesium sulfate containing brines would eliminate sulfate by causing precipitation of gypsum as a byproduct: Mg 2+ + SO2 - + 2CaCO 3 + 2 H 2 0 = CaMg(CO3)

2 + CaSO 4 • 2H20

Hite (1985), in attempting to explain potash salt mineralogy in the Paradox Basin, invoked penecontemporaneous dolomitization within a deep and stratified brine-filled basin. The brine-mixing model proposed here to explain the anomalous distribution of anhydrite around carbonate buildups in the Elk Point Basin can be considered as a further variant of the explanation suggested by Braitch (1971), but with a separation between the locales of dol0mitization and sulfate elimination. Dolomitization occurred outside the evaporite basin and generated a calcium-enriched brine. It was the calcium of this brine that reacted with the remaining sulfate of the marine brine within the evaporite basin. Continued brine interaction and precipitation of gypsum may have significantly depleted the marine brine of its contained sulfate to the extent that magnesium sulfate minerals would not form upon further brine evaporation. Unfortunately for this hypothesis, the potash salt horizons of the Prairie Evaporite are younger than the period when brine-mixing can be inferred to have occurred. The oldest potash-bearing horizon was deposited after the carbonate buildups had been completely buried by halite (Holter, 1969). If brine-mixing was responsible for the sulfate depletion found in Upper Prairie Evaporite sections, then it must be further supposed either that (1) brine-mixing continued after the carbonate buildups had been buried--perhaps brine mixing occurred at the margin of the basin, or (2) the sulfate-impoverished brines were stored within porosity in the halite until conditions were suitable for brine concentration to the potash depositional stage. Application to other basins

The model of brine mixing within large, deep and desiccated evaporite basins should be applica-

ble to other basins than the Middle Devonian Elk Point Basin of western Canada. However, to date, only a single example can be reported in support of this claim. Perhaps the closest parallel to the Elk Point Basin is the Upper Silurian Michigan Basin, in which the Salina evaporites were deposited between and above Niagaran carbonate buildups. Gill (1977), in particular, has drawn attention to this similarity. Many stratigraphic sections and diagrams drawn between reef and off-reef locations (Budros and Briggs, 1977, fig. 18; Huh et al., 1977, fig. 2; Nurmi and Friedman, 1977, figs. 6 and 15; Sears and Lucia, 1980, fig. 2) illustrate a lateral passage from anhydrite adjacent to the buildups into halite in distal locations--a transition particularly well developed at the A2 Evaporite horizon. This is an analogous situation to the anhydrite envelopes of the Elk Point Basin. No explanation has been offered for this localized development of anhydrite around carbonate buildups and their presence may also be an indication that groundwaters flowed into the Michigan Basin through the Niagaran carbonate buildups. In support, laminar carbonates are developed at the top of the buildups, and are commonly identified as being of algal origin and termed the "stromatolite cap" or "supratidal island" facies. These closely resemble the carbonate spring deposits on Winnipegosis buildups and have been identified as a "caliche crust" by at least one other author (Bay, 1983). The greater part of the dolomitization of Niagaran carbonate buildups (and other carbonates beneath the Salina evaporites) has usually been attributed to brine reflux (Gill, 1977; Sears and Lucia, 1980). However, the distribution of dolomite led Jodry (1969) to propose that dolomitization occurred by reaction with upward-moving fluids that were focussed through the buildups. He also suggested that these waters were driven by compaction. The possibility that dolomitization may have occurred by upward moving basinal brines drawn into the evacuated Michigan Basin by an elevation head is a hypothesis that should be given more attention.

283

Conclusions

References

It is p r o p o s e d that the a n o m a l o u s p o s i t i o n of d e v e l o p m e n t s of massive a n h y d r i t e a r o u n d W i n nipegosis c a r b o n a t e b u i l d u p s in the distal p a r t s of the M i d d l e D e v o n i a n Elk P o i n t Basin (which previously h a d been i n t e r p r e t e d as m a r g i n a l replacem e n t of the c a r b o n a t e b u i l d u p s ) r e p r e s e n t m a r g i n a l c a r b o n a t e spring a n d g r o u n d w a t e r d e p o s i t s s u b s t a n t i a l l y m o d i f i e d b y the g r o w t h of displacive gypsum. This g y p s u m f o r m e d as a result of the m i x i n g of the c o n c e n t r a t e d m a r i n e - d e r i v e d , b u t c a l c i u m - d e p l e t e d , b r i n e in the Elk P o i n t Basin with a n o t h e r brine, c o n t a i n i n g calcium, which ent e r e d the b a s i n p r e f e r e n t i a l l y t h r o u g h the c a r b o n a t e b u i l d u p s a n d initially p r e c i p i t a t e d the spring and groundwater carbonates. T h e regional d e v e l o p m e n t of d o l o m i t e in the L o w e r Palaeozoic a n d M i d d l e D e v o n i a n c a r b o n ates (including the c a r b o n a t e b u i l d u p s ) b e n e a t h the Prairie E v a p o r i t e ( a n d its a b s e n c e f r o m carb o n a t e s a b o v e the Prairie) s t r o n g l y suggest t h a t d o l o m i t i z a t i o n o c c u r r e d at the s a m e time as e v a p o r i t e d e p o s i t i o n . W a t e r s that h a d c a u s e d such d o l o m i t i z a t i o n w o u l d be e x p e c t e d to b e e n r i c h e d in calcium (not b a l a n c e d b y sulfate o r b i c a r b o n ate) a n d w o u l d b e suitable c a n d i d a t e s (if they h a d e n t e r e d the Elk P o i n t e v a p o r i t e b a s i n ) to have r e a c t e d with m a r i n e - d e r i v e d brines to cause gypsum precipitation. Large e v a p o r i t e b a s i n s will b e c o m e desiccated. T h e c r e a t i o n of an elevation h e a d b e t w e e n the b r i n e level on the floor of the e m p t i e d b a s i n a n d sealevel ( a n d g r o u n d w a t e r levels) in the a r e a surr o u n d i n g the b a s i n will i n d u c e m o v e m e n t of these waters into the basin. It is suggested that this h y d r o d y n a m i c drive was r e s p o n s i b l e for the m o v e m e n t of f o r m a t i o n waters w i t h i n the c a r b o n a t e s b e n e a t h the Elk P o i n t b a s i n a n d that this c a u s e d their d o l o m i t i z a t i o n . Brine m i x i n g which caused g y p s u m p r e c i p i t a tion m a y also have b e e n r e s p o n s i b l e for the further m o d i f i c a t i o n of the m a r i n e b r i n e s a n d ultim a t e l y for the p r e c i p i t a t i o n o f s u l f a t e - i m p o v e r ished p o t a s h salt deposits. However, this scenerio requires c o n t i n u e d b r i n e m i x i n g elsewhere or b r i n e - s t o r a g e within the halite porosity.

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