SEQUENCE STRATIGRAPHY OF THE MIDDLE DEVONIAN WINNIPEGOSISCARBONATE-PRAIRIE EVAPORITE TRANSITION, SOUTHERN ELK POINT BASIN

1Jisuo Jin and 2J<atherine M. Bergman'Depanmetuof Earth Sciences. University of Western Ontario, Ontario, Canada, N6A5B7"Depanment of Geology, University of Regina, Regina, Saskatchewan. Canada. S4S OA2

ABSTRACT: 'TheMiddle DevonianWinnipegosis reefs in the southernSaskatchewanportion of the Elk Point Basin contain extensivevadose diagenetic features, such as dissolution breccia, and cavities/caves filled by microbialite, pisolite and anhydrite. Basinal faciesadjacentto reefbuildupsarecharacterized by theRatnerlaminite, whichconsistsof threebrining-upward successionsoflaterally continuous,laminated dolomite and anhydrite. 'The basal cycle starts with a relatively thick unit of anhydrite-free. millimeter-scale dololaminite,changingupwardinto interlarninated carbonateand anhydrite. andendingwithenterolithic, nodulartomosaic anhydrite. Subsequentcyclesgenerallylack the dololaminiteof the basal cycle. 'TheRatnerlaminitegrades upward into the bedded to massive mosaic anhydriteof theWhitkow Member (lower Prairie Evaporite) in areas adjacent to reefs. Depositionof the Ratner laminite and the Whitkow Anhydrite isinterpretedas geneticallyrelated to the vadose diageneticprocesses, when the Elk Point Basin becamerestricted 'Thecarbonatelaminitein the basal Ratner was accumulated when seepage of fresh marine water through the barrier kept pace with the rate of evaporation,preventing a completedrawdownanddesiccationof thebasin. Precipitation of thelaminatedcarbonatewas stimulatedby vadosediagenesisof the carbonatebuildups and by microbialactivity. Each Ratnerbrining-upward successionrepresents a progressivedrawdownwhen therate of basin brine evaporationexceededseepage of marine waterinto thebasin. Marine water seeping through the reefs was enriched incalciumcations by Mg" -Ca'r exchangewith the limestone (dolomitization) and by dissolutionof reef rocks, and was responsible for theprecipitation of calcium sulphate in areas adjacent to the carbonatereefs throughbrine mixingprocesses. During vadose diagenesis of theWinnipegosis reefs and depositionof the Ratner laminite and WhitkowAnhydrite. brine level in the barred Elk Point Basinwas controlledby therate of seepage of marinewaterthroughthebarrier.whichin turn wascontrolledby eustaticsea levelchangesin the openocean. 'Thebasal anhydrite-free dololaminite of the Ratner represents a Falling Stage System Tract when evaporative drawdown was largelycompensated by seepage of marine water into the basin. 'Theinterlarninated carbonate and anhydriteof the middle and upper Ratner areinterpretedas a LowstandSystemTract associatedwithevaporative drawdown and increasedcyanobacterial activityunder near-desiccationconditions whenevaporationexceededseepage. 'TheWhitkowAnhydrite representsaTransgressive SystemTract depositedduringsea levelrise in the open ocean that led to an increased rate of seepage, higher basin brine level and diminishedmicrobial influence on beddingstructures.

INTRODUCTION

TheMiddleDevonian Elk PointBasinextends fromsouthernNorthwest Territories to northwestern NorthDakotaandfromwestern Albertato southernManitoba (Fig. 1). On a regionalscale,it is divided into the Saskatchewan Sub-basin, CentralAlberta Basin and Northern Alberta Basin. In theSaskatchewan Sub-basin, the Middle Devonian Elk PointGroup largely comprises a thick carbonate-evaporitesuccession of the Ashern, Winnipegosis andPrairieEvaporiteformations (Fig. 2). This is underlain by a major erosionalsurfacethat truncates theUpperOrdovician toLowerSilurianstrata.

Devonian sedimentation in the Saskatchewan Sub-basinbegan with deposition of the Ashern Formation followinginundation of the MeadowLakeEscarpment (Fig. I), a linearfeature that defines the northern depositional edge of thePaleozoic Williston Basin(Williams 1984; HaidlI989). TheAshern Formation consistsof barren to poorly fossiliferous,argillaceous dolomudstone with localized anhydrite in thebasal portion, and was deposited in a restricted marineenvironment during the early stage of marine transgression.Following Asherndeposition, continued relativesea-level riseresulted in deposition of the Winnipegosis Formation and its .age equivalent deposits on a broad carbonate shelf thatextended across theElkPointBasinfrom southern Northwest

Carbonates and Evaporites, v, 14, no. I, 1999,p. 64-83.

Territories to northwestern North Dakota and from westernAlberta to well beyond the present eastern erosional edge(Norris et at. 1982; Williams 1984;Campbell 1992). StrataoftheLowerWinnipegosis Member(Fig.2) weredeposited in acarbonate ramp setting. Later the basin became decoupledinto a series of platforms and basins,probably as a result ofcombined basin subsidence and sea levelrise. Patchreefs orclusters of patchreefsdeveloped throughout thedeeperpart ofElk Point Basin, especially in the Saskatchewan Sub-basinand the Northern Alberta Sub-basin. These deposits havebeenvariably referred toaspinnaclereefs,carbonate mounds,orbanksbecauseof theirgeneral lackof frame-building coralsand stromatoporoids, particularly in the lower and middleparts (Ehrets and Kissling 1987; Campbell 1992). Theplatform carbonates and the basinalpinnaclereefs comprisetheUpperWinnipegosis Member. Inbothmarginal platformsand basinal areas of the Saskatchewan Sub-basin, manypinnaclereefsattainthicknesses of 57-63m (Martindale et at.1991), and largerUpperWinnipegosis banksreached heightsof 76-91m (e.g.,Wells5-34-45-18W2 and 12-19-36-23W2).Earlier reports of thicker carbonate deposits over 100 m(Holter1969; Reinson andWardlaw 1972)probably includedtheLowerWinnipegosis or Ratnermembers.

In basinalareas between thereefs, deposition of organic-richlaminite with interbeds of reef-derived detritus was coevalwith that of the Upper Winnipegosis deposits. This unit,

IIN AND BERGMAN

NWT//

~ Western platform

~ Platformmargin

B:l Presquile Reef

IiiWJ Hay River Platform

o Basin/pinnaclereefs

~ Quill Lake Bank

Figure 1. Isopach mapof Winnipegosis Formation andequivalent deposits showing major elements ofMiddle Devonian ElkPointBasin. Contour interval is 150feet. Square A: northern study area, Quill Lake Bank(see Fig. 3for details); Square B:southern study area, Tableland andHitchcock reefcomplexes (Modifiedfrom Ehrets andKissling 1987).

informally namedthe Brightholme Member by Stoakes et aI.1987, is regarded as the source rock for oil produced fromWinnipegosis reefs in southern Saskatchewan (Osadetz et aI.1990).

Fonnation (Reinson and Wardlaw 1972), despite that theRatner apparently has nogenetic relationship with the growthof theWinnipegosis reefssimply because of its predominantanhydrite content In this study, the Ratner is treated as aseparate "formation" andis referred toas theRatnerlaminite.

In basinal areas and on top of low-relief pinnacle reefs, asuccession of fmely laminated carbonate and anhydrite is Despite many previous studies, the spatial and temporaIpresent. sandwiched between the Winnipegosis carbonate! relationships between various facies of the UpperBrightholme shale and the massive anhydrite/halite of the Winnipegosis carbonate and lowerPrairieEvaporite remainPrairieEvaporite (Fig. 2). In current stratigraphic usage, the poorly understood. This is reflected by the questionableRatner has been treated as a member of the Winnipegosis assignment of theRatnerlaminite either to the Winnipegosis

65

CARBONATE-EVAPORITE TRANSITION OF MIDDLE DEVONIAN ELKPOINT BASIN

LITHOSTRATIGRAPHY CHRONOSTRATIGRAPHY

reef basin reef basin

§!d'§t:IJ§I:lI§Prairie §1;1:§iII;§IIII§U;

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o , i Winnipegosis '3'-'- ~ Lower EI

.r:. ! rn Winnipegosis +=+-

Ashern Formation Ashern

Figure 2. Comparison of lithostratigraphic and chronostratigraphic relationships between Winnipegosis carbonate andvadolite, Brightholme organic shale, Ratner laminite andWhitkow anhydrite.

Formation or Prairie Evaporite (Jones 1%5; Holter 1%9;Kendall 1975). Furthermore, there has been considerabledebate about the depositional environment of the Ratnerlaminite. The problems largely stem from the fact thatthereare no modem analogues for a giant intracratonic salina liketheElk PointBasin. Shearman andFuller(1969) andFullerandPorter(1%9) interpreted the laminite, which consists offinely laminated carbonate and iD.eihiffiinate<I carbonate andanhydrite, as intertidal sediments analogues to modemsabkha algal mats of the Trucial Coast This interpretationimplies a rapid, complete, sea-level drawdown after thetermination of theWinnipegosis reefdevelopment tocreate amudflaton thefloorof theElkPointBasin. Lateral continuityof some individual laminae however, has been cited asevidence for a deep-water origin of the Ratner laminite(Kendall 1975,1992). DavisandLudlam (1973) suggested adeep-water depositional regime witha stratified water columnand anoxic bottom environment not only for the carbonatelaminite but also for the bedded anhydrite and halite of thePrairie Evaporite based on a comparison of the MidQIeDevonian Elk Point laminite withmodem turbidite deposits.A partial evaporative water-level drawdown of at least 30meters was postulated by Maiklem (1971) for the post-

66

WinnipegosisElkPointBasin basedonthepresence ofvadosediagenetic features (e.g., internal silt, anhydrite cement,pisolites, andbreccia) in theKegRiverFormation in northernAlberta (equivalent of the Winnipegosis Formation ofSaskatchewan). Maiklem's (1971) interpretation was alsobased on a mathematic model of a barred basin floodedperiodically through seepage of fresh marine water.

Thispaperpresents a detailed study of the Quill Lake Bankarea(squares A in Fig. 1)withadditional datafrom thedistalpart (square B) of the Saskatchewan Sub-basin forcomparison. The study is basedon detailed examination of115 cores and their corresponding wireline logs. Sequencestratigraphic techniques wereused to determine the temporalrelationships ofvarious carbonate andevaporite facies. Theserelationships areusedto further interpret howfluctuations ineustatic sea level and basinal brine table controlled thetransition of the Elk Point Basin from a normal marinecarbonate depositional system to a giantintracratonic salina.

LOWER WINNIPEGOSIS MEMBER

The Lower Winnipegosis Member is characterized by

JIN AND BERGMAN

10 9 8 7 6 5 4 3 2 1 28 27 26 25 24 23W3

20 19

34

33

32

31

30

Upper Winnipegosiso-300 ft thickness

_ Upper Winnipegosis.''E'' > 300 ft thickness

Figure 3. QuillLake Bankstudy areashowing distribution of Upper Winnipegosis reefs. Three cross sections wereconstructedacross the study area, A-A', B-B' and C-C'.

relatively homogeneous lithology of crinoidal dolomudstoneand dolowackestone and uniform thickness throughout thebasin(Figs. 3,4,5, and6). Locally, oncolitic bedsarepresentnear the top of the member. These lithological andstratigraphical features indicate a relatively shallow marinedepositional environment on a carbonate ramp setting.Vadosediagenetic features are rare,although small, sporadic,anhydrite-filled cavities havebeenobserved in somecores.

UPPER WINNIPEGOSIS MEMBER

The platform carbonate and basinal patch reefsof the UpperWinnipegosis Member have thicknesses of 50-90m (Figs. 3,4,5, and 6). In the northern part of the Saskatchewan Sub­basin, the buildups are composed mainly of carbonate mudderived from calcareous green algae (e.g., codiaceans) andmicrobes (e.g.• Renalcis), with subordinate fragments of

67

crinoids andbrachiopods. Thereisa generallackof coralandstromatoporoid frame builders andthis hasresulted in the useof suchnamesas carbonate mounds or banksfor thedeposits,eventhough theseare truereefstructures highabovethebasinfloor, somewithsteepslopes(Reinson andWardlaw 1972). Inthe southern part of the Saskatchewan Sub-basin, however,coral-stromatoporoid framestone is common in the top partsof relatively largepinnacle reefs (Martindale et al. 1991).

MostUpperWinnpegosis reefshavebeenalteredbyextensiveand intense vadose diagenesis, especially in the top parts.Vadose diagenetic features include pisolite, calcrete,microbialitic breccia, cavities and caves filled primarily bydolomitic internal silt, pisoids and anhydrite, pervasiveanhydrite cement, and hematite-stained microbialite andliesegangcn banding (Jin er al. 1997). Penetration of thevadose zone varies from aboutone meter to over ten meters

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JIN AND BERGMAN

Well 12-30-38-7W3

3515ft [1071.4m)

Light to medium gray mosaic anhydrite.

Medium and dark gray mosaic anhydnte.

Reddish brown. pisolitic and rrucrobiolific dolomudstorie andco'coockstone:Pisolitic beds 10-40 mm thick. with psoids mUltiple pisolos rangingfrom 5-40 mm in diameter; large pisoios commonly showdesiccation cracks and shrinkage wnnkling.

Light to medium brown. microbiolific and pisolitic oolornudstoneand ooiopcckstone. with opening dissolution partings(5-25 mm thick) tilled by anhydrite.

Darkbrown colornuostone.Light to medium gray mosaic anhydrite.

Microbialitic dolomudstone with thin-bedded. reploclve anhydrite.

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Pisolites and Microbialitic Breccia

below the preserved reeftops (e.g..Well 12-3D-38-7W3, Fig.7). Extensive anhydrite cement and cavity fill indicate astrong influence ofmarine phreatic water supersaturated withrespect to CaS0

4, whereas caverns, caves, and hematite

staining/liesegangen banding in the microbialite-rich zoneare evidence ofmeteoric vadose processes.

Meredith 1983), the Mississippian vadose pisoids of southernManitoba (Ahmed and Last 1991), and the Permian reefpisoids of New Mexico and Texas (Dunham 1969b; Estebanand Pray 1983). The Winnipegosis pisoids and other similarpisolitic deposits of various ages (Permian, Jurassic,Cretaceous, and Quaternary) from many parts of the worldwere grouped as vadolite by Peryt (1983). In theSaskatchewan Sub-basin, the thickness ofpisolitic zone variesform tens ofcentimeters (e.g., Well B7-29-17-20W2) tomore

Large coated grains are common inthe top ten meters of the than ten meters below the top ofthe Upper Winnipegosis reefsUpper Winnipegosis Member. These coated grains are (e.g..Wells 12-3D-38-7W3 and 24-35-12W3; Figs. 7 and 8).interpreted asmicrobial pisoids because of their similarity to The pisolitic zone is thickest and most widespread onornearthe Quaternary bacterial pisoids intravertine deposits of Italy the top of large Upper Winnipegosis banks and is variably(Folk and Chafetz 1983) and southeastern Idaho (Chafetz and present also in pinnacle reefs in the southern partof the sub-

71

CARBONATE-EVAPORITE TRANSITION OF MIDDLEDEVONIAN ELK. POINT BASIN

Well 2-4-35-12W3

4195ft [1278.7m]

Blockyanhydrite and dolomite

Dolomite with replacive anhydrite nodules

Densely packed pisoids with anhydrite cement

Large pisoids and multiple pisoidsperched in anhydrite cement

Large. multiple, or calcrete-like pisoidswith minor microbialitic partingsand anhydrite cement

WC/)

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irregular dolomitic streaks

Dolomudstone with layered and nodular anhydrite,interbedded laminated dolomite

4255 ft (1296.9 mj

Figure 8. Litholog of cored interval in Well 24-35-12W3. The anhydrite andpisolite in the Upper Winnipegosis reefareinterpreted as deposits in vertically stacked caves andvadose zones.

basin (Perrin 1982; Martindale et al. 1991). A petrograph­ically and stratigraphically comparable pisolite cap is alsoknown in the upper Keg River Formation in the NorthernAlberta Sub-basin (Bebout and Maildem 1973; Corrigan1975). MostWinnipegosis pisoids are relatively largein size,varying from 4 mm to 10 mm in diameter, although small,ooid-sized or extremely large coated grains (with single ormultiple pisoids up to 7 em diameter) are alsopresent (Fig~9.3). Inversely graded pisolites, characterized by small-scalecoarsening-upward successions up to 10 em thick, are

common and best preserved in Wells 12-30-38-7W3 (depth1079.3 m) and6-15-36-25W2 (depth1073.2 m). The pisoidsmayalso occuraspockets of various sizes(commonly 10-30em thick), bounded and subdivided by thin layers of finelylaminated microbialite. The nucleiare mostly composed offme micrite, with minor fine anhydrite laths (probably aspartial cement), but some nuclei are composed ofstromatoporoid or red-algal fragments. The number ofcortices arehighly inconsistent, varying from twoor three toover twenty. The coatings, though crudely concentric,

72

JINANDBERGMAN

Figure 9. Vadose diageneticfeatures of theUpper Winnipegosis reefs. I, depth 1283.4m(4210.5ft): single andmultiplepisoidsin dark-grey, blocky anhydrite cement. Noteextensive cracks (filled by anhydrite), probablyformedduring desiccation stage,Upper Winnipegosis Member, We1l2-4-35-12W3, depth 1283.4m. 2, pisolitic calcrete withanhydrite cement in irregular­shaped cavities. Notedripstone structures, Upper Winnipegosis Member, 2-4-35-12W3, depth 1285.2 m.3, giant coated grainsin light-grey anhydrite cement. Note multiple stages of coating, desiccation cracks, and exfoliation of cortices as a result ofdisplacive anhydrite growth between layers. Upper Winnipegosis Member. Well 15-21-35-19W2, depth 992.7 m. 4, denselypackedpisoids; multiple stage ofcoating anddesiccation and leaching areindicatedbyrecoatedpisoliticfragments andpisoidaggregate. Note prominent undercoating of the large pisoidaggregate, Upper Winnipegosis Member, Well 12-30-38-7W3,depth 1079 m. S,leached and broken pisolite. Note scattered fragments of cortices of large pisoids. Upper WinnipegosisMember, 15-21-35-19W2, depth 993 m.6, stromatactis cavityfilledbywhite dolomite and anhydrite. Notemicrobialitic bands(dark) interbedded withwhite dolomite, We1l2-19-4-8W2, depth 2437m. Scale bar =3 em.

commonly show a high degree of irregularity, giving thepisoids an irregular or polygonal shape. The presence ofdrapestone-like or pendent-like undercoatings (Figs. 9.2 and9.4) indicates that vertical percolation of meteoric waterplayedan important role in theformation ofpisolitic deposits.Sharply angularmicrobiaIitic fragments are also common inthepisoliticpocketsandoftenshowpost-brecciation recoating(Fig. 9.5). Most large pisoids have nuclei formed fromaggregates of small pisoids, which is a clear indication ofmultiple-stage coating (Fig. 9.4). Radial fractures andtruncated cortices in someof theselargegrainsmayhavebeentheresultof leaching anddesiccation shrinkage. MicrobiaIiticbreccia, comprised mainly of fragments of laminatedcarbonate, commonly occur in association with pisolite and

73

even form the nuclei of some pisoids. Pisolite and brecciaassociation is well developed in Wells B7-29-17-20W2 and14-8-324W3 (depths 1227.7 and 1227.1 m respectively).

Thecortices are composed of alternating lightanddark layers(Figs. 9.3, 9.4, and 9.5; see also Jin et al. 1997). The darklayers are rich in clusters or strands of fine micrites,resembling the"bacteria shrubs" in modemtravertine pisoids(Folk and Chafetz 1983). Radially arranged dark strands(probably of microbial origin) are present in some pisoids.Most pisoids have a fairly even coating around the entiresurface, indicating certain amount of movement and rollingduring accretion. However, the irregular to polygonal shapeof the relatively large grains, some with preferential

CARBONATE-EVAPORITE 'TRANSITION OF MIDDLEDEVONIAN ELK POINT BASIN

downward accretion, is good evidence that movement androlling was minimalas the size of pisoids increased. Someextremely largecoatedgrains (over 10cm in diameter) showa regular spherical curvature and uniform coating (Well 15­21-35-19W2, depths992-993 m), that resembles the uniformcoatings of "inorganic pisoliths" described by Folk andChafetz (1983) from the Quaternary travertines of Italy. Theunusually largeWinnipegosis pisoids, mayhavehada similarorigin as stalagmite.

Cavities and Caves

Most pinnacle reefs and large carbonate banks commonlycontain cavities and caves in their top ten to fifteen meters.Dissolution cavitiesmay penetrate 52 m belowthe topof theWinnipegosis Formation in the southern Saskatchewan Sub­basin (e.g., Well 6-304-8W2). These dissolution featureshave been variously filled by anhydrite, white dolomite,microbialite, pisoids,or somecombination of theseelements.Relatively small cavities of irregular, polygonal orstromatactis types, varying from a few millimeters to a fewcentimeters in diameter, are filled by white to bluish grey,clean anhydrite (Fig. 9). Large cavities and caves, withvertical spansvaryingfromtensofcentimeters to threemetersin cores, are filled by clean white dolomite or bluish grey,blocky anhydrite, commonly withreddish brownmicrobialitepartings and crusts (e.g., Wells 12-3Q-38-7W3 and 6-30-4­8W2). The walls of large cavities and caverns often showmicrobialitic coating. Internal fills of subhorizontal to slightlytilteddolomitic microbialite or laminated dolomite arepresentin some large cavities, resembling the "crystal silt" in thePermian reefs of New Mexico (Dunham 1969a) and the"internal silt" in the Keg River Formation of Alberta(Maiklem 1971). In the Saskatchewan Sub-basin,microbialite andanhydritefills aremostcommonly associatedwith relatively large carbonate banks, whereas dolomite fillsare common in small pinnacle reefs.

both lithology and stratigraphic succession, the presence ofmassive stromatoporoids and clusters of well-preservedbrachiopods in the overlying carbonate indicates that theanhydrite and pisolite were probably deposited in verticallystacked, multiple stagecavesand vadosezonesduringevap­orativedrawdown.

Anhydrite Cementation

The top part of UpperWinnipegosis reefs is characterized bypervasive anhydrite cement Dissolution cavities, varyingfrom millimeters to centimeters in diameter, are often filledcompletely by clean anhydrite. Most pisoids containextremely fineanhydrite lathsdispersed inboththenucleiandcortices. Extensive anhydrite cement is one of the moststriking features of the Winnipegosis pisolites. In Well 2-4­35-12W5, theanhydrite grewto suchanextentthatthepisoidsnow appear to be floating in an anhydrite matrix (Fig. 9).What appears to be blocky anhydrite cement in handspecimens iscomposed of bundles of acicularor lathcrystals.Some of the anhydrite bundles show a chevron-like pattern(probably gypsum pseudomorph), growing into interstitialcavities from the pisoid surfaces. This indicates that theanhydrite cementwasformed afteraperiodofmeteoric vadosediagenesis, probably during a rise in basin brine level whenthe cavities, caves and porous pisolitic zone becamesubmerged.

BRIGHTHOLME MEMBER

The Brightholme Memberis a dark-brown to black,organic­rich shale that was deposited in basinal areas betweenpinnacle reefs. This unitcontains clastsof the Winnipegosiscarbonates at thebaseandthininterbeds of skeletal debrisandhas been interpreted as coeval deposits of the UpperWinnipegosis reefs (Jinet al. 1997).

RATNER LAMINITE

Basinal Facies

The thickness and areal distribution of the Ratner is closelyrelated to the sizeof the UpperWinnipegosis reefs. In areasadjacent to the large carbonate banks, the Ratner is thickestand most laterally continuous (Fig. 4). On the basis of apreliminary study, Jin and Bergman (1998a) recognized threefacies of the Ratner laminite in the Quill Lake Bank area.TheseRatnerfacies provide crucial information on the waterdepthand otherparameters of the depositional environment

During vadose diagenesis, carbonate dissolution wasstimulated andaccelerated by microbial activity and meteoricwater percolation. In Well 4-32-4-8W2, for example, acombined actionof downward leaching and microbial growthin voids and cracks of the Upper Winnipegosis created apaleosol-type profile: within about 30 cm of section, thevadose profile changes from light-grey, fossiliferouscarbonate bed rock upward to fresh carbonate fragment, toweathered, brown-mottled fragments in a reddish brown,microbialitic mudstonematrix. In wellsof the adjacent area(e.g., Wells 8-31-4-8W2 and 11-21-4-8W2), the correlativevadose zones contain reddish-brown liesegangen banding,whichwasmostlikelyformed bypercolating meteoric wateratthephreaticwatertableduringsubaerial exposure of thereefs.

In basinal areas adjacent to carbonate banks, the Ratner ischaracterized by threesuccessions (heredesignated as cycles)of flat-lying, laterally continuous, thinly laminated carbonate

In We1l2-4-35-12W3, a 15 m-thick anhydrite-pisolite unit is andanhydrite(Fig.4). Cycle1startswitha thickbasal unitofdeveloped in the fossiliferous UpperWinnipegosis carbonate light-brown carbonate laminite (e.g., Well 1-15-48-17W2;(Fig.8). Although thepisolitic unitappears tobesimilartothe Figs. 5, 10,and II) and in placesoverlying the BrightholmeQuill Lake Marker Beds of Reinson and Wardlaw (1972) in Member. Thisbasalunitlacksanyanhydrite laminae butvery

74

JIN AND BERGMAN

Well 1-15-48-17W2 (Ratner Type Section)

Mosaic anhydrite

Mosaic anhydrite with irregular dolomitic streaks

1639 ft (499.6 rn]

Nodular dolowackstoneWith biogenic fragments andabundant vugs

1719 ft 524 m

Dolomitic limestone, finely laminated:laminae planar, 1-2 mm thick with dark-grey partings:anhydrite in open partings occurs only near top

LowerWinnipegosisMember

Dark-grey bedded-nodular anhydrite interbedded yvithlight-grey calcitized enterolithic anhydnte; ooloiornrutepartings near base and top

Bedded to nodular anhydrite bounded bycrinkly to contorted dololaminite

Dololaminite with anhydrite in fenestrae and partings

Dolomitic limestone. laminated: laminae planar tosligr,tiywavy, 2-3 rrrn thick. light brown. withdark-grey partings: anhydrite-filled fenestrae andopen portings; blocky anhydrite near top

Planar to slightly wavy ooioiornone w~th finely layeredanhydrite in fenestrae and open partings

1664 ft (507.2 m)

1703ft (519.1 m)

Dark-greyto black. organic-rich ~udstone,

finely laminated, with common light-browndolomudstone intraclasts

en'<Ii°c0)0(])+=0.0'e Ec~.- °5LL

(]):!:::c'E.Q

Figure 10. Litholog of core interval inWelll-1548-17W2 (Ratner type section). Three shallowing upward (brining upward)successions are identified.

fine, isolated anhydrite laths may bepresent. Atthetype well pyritized) anhydrite laths. This supports the interpretation(1-15-48-17W2), the basal Ratner laminite shows a that thebasal laminite wasformed inrelatively deep, stratifiedgradational contact withthe topof theBrightholme Member water with anoxic bottom conditions (Davies and Ludlam(Fig. ILl). Thebasal Ratner has a shaly appearance like the 1973; Kendall 1975). It further indicates that theBrightholme, but the Ratner laminite contains no skeletal Winnipegosis reefs had been well cemented when depositionfragments but may have fine, dark-colored (probably of theRatner laminite began. Within Cycle I, thebasal unit

75

CARBONATE-EVAPORITE 1RANSITIONOF MIDDLE DEVONIAN ELKPOINT BASIN

Figure 11.Facies variation of theRatnerMember. 1-3. three typical lithologies of the Ratner in the type section (1-15-48­17W2): 1. brownish buff. finely laminated dolomudstone of the Ratner Member overlying dark-grey, organic-rich. wavy­laminated dolomudstone of the Brightholme Member. depth 532.8 m (in cycle 1); 2, dark-grey. bedded nodular anhydriteinterbedded withlight-grey enteroluhic anhydrite. depth 523.4m (in cycle 2);3 ,finelylaminated, weakly dolomitized mudstone,withanhydrite in openpartings andfenestrae, depth 521.1 m (in cycle 3).4. syndepositional tilting of interlaminated dolomiteand anhydrite associated with slumping and contortion. reef-slope facies, Well 6-15-36-25W2. depth 10773 m: S. mottledanhydrite andwhitedolomite. withinterlaminated dolomite andanhydrite partly preserved. reef-top facies. Well 8-5474W3.depth 7375 m. 6. whitedolomite laminae contorted by dark-grey, irregularly laminated, displacive anhydrite. basinal facies.Welll-22-37-1W3. depth 1168.7 m. Scale bar =3 em.

grades upward into planar interlaminated carbonate andanhydrite, through contorted and enterolithic anhydrite, tonodular andmosaic anhydrite. As shown in Fig. 10, Cycle 2andCycle 3 aresimilar toCycle 1 in lithological succession,different only in lacking the essentially anhydrite-free basalunitofCycle 1.

76

Reef-slope Facies

In We1l6-15-36-25W2 (Figs. 4 and12), a single Ramer cyclecomprising 4.5 m of interlaminated carbonate and anhydriteis present, overlying 30 m of theUpperWinnipegosis. Thelaminae arestrongly contorted andsome intervals aretilted at

JIN AND BERGMAN

Well 6-15-36-25W2cored intorval 3518-3549 ft (1072.3-1081.7 m)

Nodular to mosaic anhydrite, light brown to reddish brown,with Irregular dolomitic streaks, dololaminite fragments,incipient Plsoids, and halite packets (near top)

Bedded nodular to mosaic anhydrite, reddish brown,with irregular dolomitic partings and streaks

___ Light grey incipient pisolite and brecciated calcrete

interbedded anhydrite and dolomite, reddish brown, thin-beddedto laminated, dipping at angles up to 30 degrees

Dolomudstone, light brown, finely laminated, dipPing to contorted,with thin anhydrite partings and large displacive anhydrite nodules

Figure 12. Reef-slopefacies of theRatner Member in We1l6-15-36-25W2. Refer to Figure 11.4 for tilted laminite.

20-25degrees.. The tilted bedsare regarded as depositionalbut not the result of drilling deviation because the beds varyfrom strongly tiled to essentially horizontal within a fewmeters of section. The immediately overlying nodular tomosaic anhydrite contains pockets ofpisoids. Thesemayhavebeentravertine deposits on theslopeof a carbonate banknearbasinal watertable ti.e., about 30 m above basinfloor).

Reef-top Facies

In Well 8-5-47-4W3 (Figs. 6 and 13), three cycles of theRatnerlaminite restdirectly above arelatively low(30mhigh)pinnacle reef. The upper part of the reefconsists of nodulardolomudstone with irregular to subhorizontal microbialiticpartings, suggesting a drowned pinnacle. Thereef-top faciesis similar to thebasinal facies, exceptthatcycle 2 begins witha thick unitofdolomudstone mottled byanhydrite nodules andlaths. This unit was deposited initially as thinly laminatedmudstone with finegypsum/anhydrite partings, as shown bytheremnants of laminite preserved within. Themottling wasprobably theresultofbothsediment disturbance between fair­weather and storm wave base (estimated at 15-30 m depth)andsubsequent displacive gypsum/anhydrite growth nearthebasinal water table. Using the paleo-relief of the drownedpinnacle as a reference, the undisturbed laminated carbonateof thebasinal Ratner musthavebeen deposited between45-60m of water depth.

77

In the distal part of the Saskatchewan Sub-basin (betweenRegina andtheCanada/USA border, square B inFig.1)southof the Quill Lake Bank, the Ratner has been poorlyunderstood. Arecent survey ofover50coresfrom theareabyJinandBergman (l998b),however, showsthatall three faciesof theRatner described above can berecognized. Comparedwith the Ratner facies in the Quill Lake Bank area, theseauthors recognized a number of distinct features in the"southern Ratner": 1)a much darker greycolorbecause of itsgreater proportion of organic-rich partings, 2) ubiquitouscrinkly laminae (cryptalgal structures), 3) considerably fewenterolithic anhydrite beds, 4) morecommon calcitization ofanhydrite, whereby anhydrite laminae werealtered to coarse­grained white dolomite, 5) less common occurrences ofstrongly contorted, interlaminated dolomite and anhydritecaused bydisplacive anhydrite growth, and6) predominanceof single brining-upward succession. Most of thesecharacteristics, especially the darker-grey color and generallack of multiple cycles, were interpreted as the result ofdeposition of the laminite in a deeper-water setting in thesouthern part of the Saskatchewan Sub-basin, where morefrequent and prolonged plankton blooms and fewerdesiccation episodes would have prevailed. This, togetherwith a greater height of theWinnipegosis reefsformed inpre­Ratner times, indicates a consistent trend of southeastwardtilting and deepening of the Saskatchewan Sub-basin.

CARBONAlE-EVAPORIlE TRANSITION OF MIDDLEDEVONIAN ELK POINT BASIN

Well 8-5-47-4W3

2351 ft (716.6m)

Mosaic anhydrite, with subtonzontol to high-angle dolomitic partings

721.2 mBuff-coloured, finely laminated coiornuostone: laminae may beplanar, crinkly.or wavy. with interlaminated anhydrite near top.Bedded mosaic anhydrite unit in middle.

723.8 mme.wavy to contorted laminae of dolornuostone with

nodular to bedded nodular, displacive anhydrite.

Fine, planar to wavy laminated dolomudstane intorlaminated withdark gray anhydrite.

Light gray dolomudstone mottled by reddish brown anhydritelaths and blebs, interbedded with two 30 em-thick units offinely interlaminated dolomite and anhydrite near top.

730.4 m

Planar to wavy dololaminite with fine anhydrite laminaein fenestrae and open partings;Anhydrite·free laminated dolomudstone unit in middle.

Dolomudstone. buff-coloured, non-fossiliferous,. with dark gray, fine, crinkly to slighl1y inclined cryptalgal structures.

2473 ft [753.8 rn]

Figure 13. Reef-topfaciesoftheRatnerMember (We1l8-5-47-4W3) overlying a drowned pinnacle reef Notedevelopment ofmottled dolomite and anhydrite (see alsoFigure 115).

WHITKOW ANHYDRITE

Like the Ramerlaminite, the thickness and arealdistributionof theWhitkow anhydrite showa closerelationship tothesizeof the Upper Winnipegosis buildups. In the SaskatchewanSub-basin, preferential accumulation of relatively largeandthickanhydrite wedges on south or southeast slopes of majorcarbonate buildups wereftrstnotedby Reinson andWardlaw(1972) and is shown on the cross sections across the QuillLake Bank (Figs. 4 and 5). In other parts of the Elk PointBasin(Fig. 1), largeanhydrite wedges are alsopreferentially'developed on the southeast side of the Presquile Barriercomplex in northern Alberta and against the south side of

78

large carbonate banks on the Meadow Lake Escarpment(Holter 1969; Klinspor 1969; Bebout and Maiklem 1973;Corrigan 1975). Oneof themoststriking characteristicsof theevaporite successions in giantepicratonic evaporite basins isthe unusual thickness of anhydrite, particularly in theDevonian ElkPointBasinand thePermian Zechstein BasinofEurope. Chemical models of staticevaporation of sea water(Braitsch 1971; Harvie et al. 1980; Horitaet al, 1996) predictthatgypsum, precipitated at brine concentration of 1.8 timesofnormalseawater(McCaffrey et al. 1987), wouldconverttoonly2.9mofbasalanhydrite ina 100mthickanhydrite-halitesuccession. This anhydrite/halite ratio is far below theobserved ratios near carbonate buildups in the Elk Point

lIN ANDBERGMAN

Basin. In the Saskatchewan Sub-basin, the Whitkowanhydriteattains thicknesses of 72.8 m (Well6-2-37-2IW2)to 74.4m (Well7-29-37-18W2)on thesouthflank of theQuillLake Bank(seealso Reinsonand Wardlaw 1972),makingupapproximately one third of theanhydrite-halite succession ofthe Prairie EvaporitePormation (maximum thickness of 218m). In this area, anhydrite units over 50 m thick have beenrecordedin manywells,includinga 29 mcoreof an estimated68 m thick Whitkow anhydrite (Well 9-29-30-IW3). Inbasinal areas away from carbonate reefs, the Whitkowanhydrite thins rapidly, and the Ramer laminite may bedirectly overlain by Prairie halite. In wells 20-30 km awayfrom theedgesof relatively largecarbonate banks(e.g.,Wells5-29-47-3W3 and 8-5-47-4W3), only about 5-8 m of theWhitkowanhydrite is present. In basinal areasfromReginatothe U.S. border, mosaic anhydrite rarely exceeds 10 m inthickness, even in areas immediately against the slope ofpinnaclereefs(e.g.,Well 16-15-2-9W2 in theTablelandpatchreefcomplex). TheWhitkowanhydrite is similarlythinon theslopesof large marginalplatforms of the Saskatchewan Sub­basin. In the Hidrogas Regina well (7-29-17-20W2), forexample, only a thin (about I m thick) unit of mosaicanhydrite is present between the pisolite cap of the UpperWinnipegosis carbonateand the Prairie halite.

SEQUENCE STRATIGRAPHIC ANALYSIS

Lower Sequence Boundary (SBl)

The vadolite zone in the Upper Winnipegosis Member wasassociated mainly with carbonatedissolution by percolatingmeteoric water. In areas of carbonate reefs, therefore, thelowersequenceboundary is drawnat the baseof the vadolitezone (see Fig. 2) and is analogous to an unconformityassociated with stream rejuvenation and down-cutting in asiliciclastic sequence(SBI). The Brightholme/Ratner contactin the basinal facies and similarUpper Winnipegosis/Ramercontacts in the reef-top and reef-slope Ramer facies aregradational in core. The unconformable sequenceboundaryin high-relief reefs passes laterally into a conformableboundary at the base of the Ramerin areas of low-reliefreefsand basinalareas (Fig. 2).

Falling Stage System Tract

TheRamerlaminiteis interpreted as genetically relatedto thevadosediagenetic processes of thereefsandformstheFallingStage and Lowstand Systems Tract deposits. The basallaminated carbonate of Ramer Cycle 1 is largely anhydrite­free,gradingupward intofinelyinterlaminated or interbeddedcarbonate and anhydrite, and then into contorted toenterolithic beddedanhydrite. The gradational aspect of the

DEPOSITION OF RATNER CYCLE 1 IN THE SASKATCHEWAN SUB-BASIN

A. Falling Stage System TractSASKATCHEWAN SUB-BASIN

rain.."'"

B. Lowstand System Tract

Ei555I Lower r:;::r:;:::Il UpperI:5:5:2l Winnipegosis t::r=rl Winnipegosis

~ Laminated~ carbonate

.."'.,

~ Interlaminated~ carbonate/anhydrite

Evaporativedrawdown

@t~l Mosaic ~ Vadose,.,«:. anhydrite ~ zone

Figure 14. Sequence stratigraphic interpretation of the depositional environment for the Ratner laminite and its geneticrelationship to the vadose diagenesis of Winnipegosis reefs.

79

CARBONATE-EVAPORITE TRANSITION OF MIDDLE DEVONIAN ELK POINT BASIN

PALEOHYDROLOGIC MODEL FOR THE WHITKOW ANHYDRITE

Transgressive System Tract '.

evaporation

Presquilebarrier

rain

SASKATCHEWANSUB-BASIN

;. ••• + .: ••..............-1-.:

.,.· -". ·,.· .... ·f.· .•• .;. •• "-+ ••• .j. •

.j. +. ,.'

.: ::.-t', :::t'· :..',"t'·:·

• I j • + ' I • +.•• ~ • +•••.j. ,j, .j. .

~Lower~ Winnipegosis

jZ7=Z;i UpperEZ3 Winnipegosis

~ Ratner~Iaminite

~Whitkow

~ anhydriteIIII§IIH

IUI~ll!1

Whitkowhalite

Figure 15. Sequence stratigraphic interpretation of thedepositional environment for theWhitkow anhydrite: a transgressivesystems tract.

Transgressive System Tract

indicatethat these beds were formed subaqueously (Kendall1975),although these are not interpretedhere as deep-waterturbidites as suggestedby Daviesand Ludlam (1973). Largetepee structures or megapolygons, typical of frequentlydesiccated modemcoastalsalinasin South Australia(Warren1982,1983;Fergusonetal. 1982),aregenerallyabsentor veryrare in theRatner, indicating that thebasin floorwasprobablyneverdry for prolongedperiodsof time. The interlaminatedcarbonateand anhydriteshow littledisruptionand reworkingby wavesor storms,probablybecausethe watermasswas tooshallow to generate significant waves or storms, In a giantsalina, a low-energy regime could prevail in very shallowwaterconditionsfor lackof influencefrom open oceanstormwaves. Under this type of depositional environment. a waterbody 5 m deep couldaccommodate accumulation of laterallycontinuous algal mats and interbedded anhydrite layers,provided that the floor of the Saskatchewan Sub-basin wasrelatively flat (Fig. 14). Under near-desiccation conditions,growth of gypsum clusters between microbial mats wouldcause intense contortion of laminae,as typical of the middleand upper Ratner.

Lowstand System Tract

laminated carbonate and anhydrite is evidence for the post­Winnipegosis age when the carbonate reef environmentterminated andanevaporitedepositional systemwasinitiated.Thisrelativelythickunitwasestimatedtohavebeendepositedat 45-60 m of water depths (Fig. 14) and suggests that theinitialdrawdownwas a slowprocesswhenthe rate of seepagekept pace withthe rate of evaporation and that the basinbrineconcentration factor was below 3.8 required for anhydriteprecipitation (McCaffreyet al. 1987).

The upperpartof RatnerCycle 1resembles Cycle2 andCycle3 having crinkly carbonate laminae and contorted anhydritelaminae. These laminae are most likely to have formed bydisplaciveanhydritegrowthbetweenmicrobial mats becauseof their similarity to the mat-bounded nodular anhydritedescribed from modem sabkhas of the Persian Gulf(Shearmanand Fuller 1%9). The Elk Point basin, however,was an isolatedbasin not connectedto the open ocean at thetime the Ratner was deposited. The modem Trucial Coastsabkha,as a depositional model, probablycannotbe directlyapplied to the Ratner laminite. The planar interlaminatedcarbonateand anhydriteof the Ratnermay have been formedfrom microbial mats at slightlydeeperand hypersaline water The Whitkow Anhydrite and its basinward halite areconditions. Lateral continuity of the carbonate-anhydrite characteristically massiveand havean averagethickness of46laminae and bedded enterolithic anhydrite in basinal areas m (below the Shell Lake Marker). Because of the large

80

JIN ANDBERGMAN

quantity of sea waterneededto form the thickanhydrite andhalite bodies (approximately 85 times thick column of seawater),a higherrate of seepagethrough thePresquile Barriermust have occurred. This was most likelythe resultof a risein sea levelof the open ocean (Fig. 15).

Spatialrelationships betweenthe Whitkow anhydrite and theUpper Winnipegosis carbonate suggest that accumulation ofthick anhydrite wedges was influenced by the movement ofmarine phreatic water through the carbonate. Movement ofthe phreatic water was probably in a south to southeasterlydirection becauseof slight tilting of thebasinto the southeast(Figs. 14 and IS). The marine phreatic water woulddolomitize the limestone reef by Mg"-Ca" cation exchange(molar concentration of Mg++ is about five times moreabundant than Ca" in modem sea water). This caIcium­enriched phreatic water, when flowing out of the reef andmixing with basin brine, would stimulate precipitation ofcalciumsulphate (mostlikelyin the formof gypsum becauseof its lower nucleation energy than anhydrite) throughcommonion effect, that is, the solubility of calcium sulphatedecreases through addition of Ca" or S04"" (Braitseh 1971).Thisprocessof sea-water alteration through mixing calcium­enriched phreatic water with basinal brine (Raup 1982;Kendall 1989) explains the unusually high anhydrite-haliteratio and preferential accumulation of anhydrite wedges onsouthor southeastern slopesof carbonate reefsintheElkPointBasin (Figs. 4 and 5).

SUMMARY

The stratigraphic relationships among the Winnipegosisvadolite, the Ratner laminiteand the Whitkow anhydrite arecrucialfor understanding the transition of theElkPointBasinfrom a normal marinecarbonate reef environment to a giantsalina in the Middle Devonian. In the present study, thecarbonate-evaporite transition is not viewed as a simplepro­cess of progressive basin brine drawdown producing a car­.bonate-anhydrite-halite-sylvinite succession. Usingsequencestratigraphic concept, the relationship between the brining­upward successions of the Ratner laminite and eustatic sealevelchangecanbeinferred, andaFallingStageSystem Tractand a Lowstand System Tractare established. A thickunitofanhydrite-free carbonate laminite in the Basal Ratner wasaccumulated whenseepageof fresh marine waterthrough thePresquilebarrierreef kept pace with the rate of evaporation,preventing complete drawdown and desiccation of the basin.Upward increase in both the number and thickness ofanhydriteinterbeds in the Ratnercyclessuggests progressivedrawdown whenthe rateof basinbrineevaporation exceededseepage of fresh marine water into the basin through thePresquile barrier complex. In the sequence stratigraphiccontext, theWhitkow anhydrite is regarded as a TrangressiveSystem Tract, not as deposits formed during progressivedrawdown and increased concentration of basin brine.Followingdeposition of the Ratner laminite, a moderate risein sea level in the open ocean led to increased seepage of

81

marine water through the barrierreef and in-basin pinnaclereefs was enrichedin calcium cationsby dolomitization anddissolution of the reef carbonate and was responsible for theprecipitation of calcium sulphate against the carbonate reefs.Preferential accumulation of anhydrite wedges on the southand southeast sides of the carbonate banks suggests that theflow of seepage water was primarily to the southeast Thisreflects the input of fresh marine water from the northwestbeyondthePresquile barriercomplex and thedevelopment ofhydrostatic pressure created by a higher rate of brineevaporation on the south sideof in-basin banks.

ACKNOWLEDGMENTS

The study was funded by a Williston Basin Research Grantfrom Vista Energy of Regina to Jin and Bergman and aNSERC lORResearch Grantto BergmanandJin, MikeBlairand AngelaRicci provided valuable assistance in core datacollection. Saskatchewan Energy and Mines generouslyprovided access to its core lab at no charge.

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Received: July 15,1998Accepted: November 11,1998