DEEP-BASINIDEEP-WATER CARBONATE-EVAPORITE DEPOSITION OF A SALINE GIANT:LOCH MACUMBER(VISEAN), ATLANTIC CANADA

PaulE. Schenk',PeterH. vonBitter2, and Ryo Matsumoto'

IDalhousie University, Halifax, Canada B3H4J12Royal Ontario Museum and University ofToronto , Toronto, Canada M5S2C6

"Universlty ofTokyo, Hongo, Tokyo, Japan

ABSTRACT: The Carboniferous saline giant of Atlantic Canada is approximately 500 m thick and extends over an area of 250 x 103 km',It consists of three lithologies: 1. a relatively thin (usually less than 5 m) extensive carbonate sheet containing buildups, 2. an overlying thick(up to 400 m) evaporite complex of sulphates and chlorides, and 3. wedge-shaped units of conglomerate, breccia, and sandstone (up to 200m). The latter interfinger with the preceding lithologies but locally may underlie the carbonate sheet. The basal carbonate occurs mainly asfine, laminar couplets of alternating peloidal grainstones and bituminous films; these pass laterally into peloidal marlstones and mounds.Laminites andmarlstones have incipient brecciation, recumbent folds, siliciclastic and carbonate turbidites, rubble, and olistostromes.Fossils are rare. They are restricted in space to the basaldecimeter and in diversity to conodonts and crustaceans. A Nereites ichnofaunaoccurs at one locality. Mounds consist mainly of turbid fans of fascicular-optic calcite and botryoids of laminated calcite. Their fauna showslow diversity but high density of some species; tube worms and microbial growths are noteworthy. The overlying evaporites are mainlyanhydrite with lateral and vertical transition into halite and potash. Siliciclastic sediments thicken toward boundary faults. Both the baseand top of the giant are unconformities, the former abrupt, the latter karsdc,

The basal carbonate accumulated subaqueously in deep, physically and chemically stratified water. Initially, the desert floor ofthe complex rift 'basin was below ambient sea level. After catastrophic submergence, basal waters were at first dysaerobic and restrictedmarine, but changed to anoxic and pene- to hypersaline as the water column became stratified. Its sedimentary record corresponds to a giant,meromictic, saline lake - Loch Macumber. The laminite records either seasonal changes in rainfall and temperature, or episodic stormspunctuating normal chemical sedimentation. Sulphate-reducing bacteria fed on terrestrial organic matter preserved in anoxic bottom water,and precipitated peloidal calcite. Concentrations of bacteria over hot springs may also have supported chemosynthetic organic communitiesand precipitated petroliferous and sulphide-rich mounds. Siliciclastic rocks increase toward basin margins. Locally steep depositionalsurfaces caused mass movements of sediment down basin margins. A long term increase in salinity resulted in precipitation of initiallycorroded and subsequently euhedral crystals of gypsum. Thick sulphates and chlorides followed. Sedimentary rock of Loch Macumberrecords topographic filling of a deep rift basin. Isostatic analysis suggests thatboth depth of the subaerial desert basin and initial water depthafter submergence were considerably less than the preserved thickness of the saline giant. Its record corresponds closely to that expectedfrom Schmaltz's model of deep-basin/deep-water carbonate-evaporite deposition.

INTRODUCTION

Saline giants consist of thick, extensive, evaporitebodies and genetically related sedimentary rocks. Their ori­gin, such as that of the Messinian of the Mediterranean, re­mainsa controversy (Dietz andWoodhouse, 1988; Hsu, 1988;Schmalz, 1991; Friedman, 1991). Were they deposited indeepor shallow water(Schmalz, 1969; Hsu, 1972, respectively)?Were theythe resultof rare, catastrophic events or common,actualistic happenings (Benson and Bied, 1991; ShearmanandFuller, 1969, respectively)?

In semi-arid regions, evaporites commonly form to­day in shallow water within either topographically deeporshallow basins. In contrast, the lithology and fauna of someancientsalinegiantssuggestthat waterdepth wasdeep,im­plying significant relief. Examples are controversial but in­cludePleistocene Lake Lissan of the Jordan Rift (Katz andothers, 1977), the Permian Delaware Basin (Anderson andKirkland, 1966), and the PermianZechstein of western Eu­rope (Fiichtbauer and Peryt, 1980). Controversy centers onthe absence of a modern example(Warren, 1989).

Formorethan20years Schenk (1967a through 1986)usedmodern marineanalogues to interpret theCarboniferous

Carbonates and Evaporites, v.9, no. 2, 1994, p. 187-210

saline giantof Atlantic Canadaas an example ofdeep-basin!shallow-waterdeposition. Does adeep-basin/deep-watermodelsuch as thatproposed bySchmalz (1969) predictthedatabet­ter?He suggested sucha model on theoretical grounds, andapplied it to the Mediterranean Sea,Piano del Sale (Ethio­pia), theGulfof Mexico, and theZechstein, Michigan, Mid­land,andElkPointbasins. Morerecently he defended itsap­plication to the Mediterranean Evaporite (Schmalz, 1991).This paper testshis model on stratigraphic, sedimentologic,andfaunal data from Atlantic Canada's salinegiant.

Loch Macumber is the name given for the deposi­tional basinofthissalinegiant(Schenk andothers, 1992, andin press). It represents a unique, short-term episode of theMaritimes Basin(Fig. 1).Thisbasinexisted from LateDevo­nian through Permian time as several strike-slip subbasinscreated during dextral shearing between Laurasia andGondwana (Belt, 1968; Lefort, 1989). The term"loch" con­notes sediments withbothmarineand lacustrine aspects. Thelowermost decimeter of a thin basalcarbonate contains a re­stricted marinefauna; however, thesedimentary facies of thegiantislacustrine (Schenk, 1984, Schenkandothers, in press).Theword"loch"is Scottish for a lake or a restricted, narrow,cutoff arm of the sea; it is highlysuitable for New Scotland(Nova Scotia). "Macumber" isthenameoftheextensive, basal

DEEP-WATER CARBONATE-EVAPORITES OF A SALINEGIANT,CANADA

carbonate formationin NovaScotia and NewBrunswick; adridiculum; an 'umber' coloris a dark,dusky, brownish shadereflecting the deep-water, restricted settingof the earlyphaseofLoch Macumber,

STRATIGRAPHY AND GEOLOGIC SETTING

From Late Devonian to Permian time, tens of kmsof fluvial and lacustrinesediments as well as minor amountsof restricted marine and volcaniclastic rocksfilledsubbasinsof the Maritimes Basin(Giblingandothers,1992). This sedi­mentaryrecord has three distinctphases(Fig.2).

Middle Devonian To Early VISean (HortooAnd AnguilIeGroups)

Duringthis timetheMaritimes Basinwas intermon­tane and mainlysubaerial. Thick fanglomerates and interca­latedlavascharacterizedescarpment margins. Axial, braidedriver systems and shallow, ephemeral, saline lakes occupied

basin centers (Belt, 1968; Hamblin and Rust, 1989). New­foundland lakes were exceptional in being deep and steep­walled. There, turbidites grade laterally and vertically intosubaqueous, incoherent slides anddebris flows (Knight, 1983).Local horstssupplied sedimentto the intervening basins.Thearea was 100 southof the equator(Roy and Robertson, 1968;van der Zwan and others, 1985). The climate was at leastseasonallysemi-arid, assuggested byextensive evaporite strataand redbeds.

Middle VlSian (Wmdsor and Codroy Groups)

Strata are cyclic, each cycleconsisting usuallyof astratigraphicallyupward succession ofa restricted marinecar­bonate, evaporite, and redbed(Fig.3). Thecarbonates recordrepeated incursions of the last vestiges of the Rheic Oceaninto the Maritimes Basin. Cycles combineintofivesequences(Giles, 1981; Schenkand others, 1992).

Sequence one is the Carboniferous saline giant of

KM

(:·Wi·;;.::;\;1i%::ii CARBONIFEROUS

o 150I

Windsor andCodroy Groups-

Figure 1. Maritimes Basin ofAtlantic Canada showing present extent ofCarboniferous strata andexposures oftheWindror (maritimes) andCodroy (Newfoundland) Groups. Numbers refer to localities in text.

188

SCHENK, VON BITIER, AND MATSUMOTO

Figure 2. Stratigraphic column ofmajorMiddle Devonianand Carboniferous formations in the Maritimes Basin. Thetext simplifies them into three phases. Vertical and slantedorientation offormational namesindicate lateral andpartialequivalency, respectively.

vi Grantrnire ego _...o~ Spouts "":l ~o~... Falls- ~z~ . Cheverie -"l " ~ ~0~'<f Friars Cove:3 eo; 8 ~

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Figure 3. Generalized lithic column for the Windsor Groupshowing times and extents of submergences that generatedfive sequences (after Giles. 1981). Symbols/orlithologies areblack. carbonate.. blank. calcium sulphate.. cubic, halite; dot­ted. siliciclastic rocks.

undivided

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AtlanticCanada. In central NovaScotia, it is approximately500 m thick and comprises half the total thickness of theWindsor Group. Its areal extent exceeds 250 x 103 km2

(Boehner, 19800; Schenk and others, 1992). This giant de­posit consists of three distinctivelithologic suites: 1. a rela­tively thin (usuallyless than 5 m but up to 50 m) extensivecarbonate blanket containing buildups (the Macumber For­mation and adjoiningformations of the MaritimeProvinces;the Ship Coveand lowerBig Coveformations of Newfound­land); 2. an overlying thick (up to 400 m) evaporite complexof sulphatesand chlorides; and 3. wedge-shaped unitsofcon­glomerate, breccia,and sandstone (up to 200 m thick). Thelatter interfinger with the preceding suites but locally mayunderlie the carbonate blanket. Where present, thesesiliciclastics usuallyrest with angular unconformity on olderunits. Where theyare absent, the baseon which the blanketrests is a paraconformity representing MiddleTournaisian toMiddleVisean time(Geldsetzer, 1978). Theseunconformitiesdefine the base of sequenceone. The upperboundaryof thesequenceis a disconformity involving karstic dissolution ofevaporites (Gilesand Boehner, 1979).

Above the saline giant, the sequences lWOto fiveofthe WindsorGroup contrast with sequence one in lithology,

thickness, extent,and fauna(Fig.3). EachConsistsofrepeated,laterally extensive cycles usuallyof shallow marine carbon­ates changing upwardsto sabkhacarbonates and evaporites,and to fine-grained redbeds (Schenk,1969). Each cycle is aconformable succession ofgenetically relatedbedsor bedsetsboundedbymarine-flooding surfaces, i.e., a parasequence (VanWagoner and others, 1990).

Late Carboniferous

Depositional environments weremainlyfluvial andlacustrine. Fluvialpaleocurrent trendsare uniformly fromthesouthwest, ignoreprevious topographic highs, and suggestasource area outside Atlantic Canada (Fralick and Schenk,1981; Gibling and others, 1992). Lakes became large, cool­producing swamps, suggesting a more temperate, uniformlywann and humid climate.

BASIN STAGES OF LOCH MACUMBER

To test the applicability of the deep-basin/deep-wa­ter hypothesis to Loch Macumber, we consider its depositsunder headings of the Schmalz model's five stages. Table 1summarizes themainfeatures ofthemodelandcompares themto observations from Loch Macumber.

189

DEEP-WATER CARBONATE-EVAPORITES OF A SALINEGIANT. CANADA

Table 1. Schmalz modelof deep-water deep-basin evaporitedeposition compared to general set­ting and stagesof Loch MacumberModel.

SCHMALZ MODEL LOCH MACUMBER

LOCATION 20-30N and S latitude 15 S

topographic deep; area up to 5x106 krrr'; topographic deep; area 0.3 x 106

depth up to 2 km;restricted km2 thickness(depth?) up to 0.6sediment-starved rift or basinconnected km; restricted sediment-starved rift

SETTING to open sea by shallow sill(e.g.• basinsconnected to last vestiges ofNorwegianfjords, the Mediterranean Rheic Ocean (e.g. Scottish lochs;

Sea) East Africa;Great Basin)

evap'n. > runoff+ ppt'n. evap'n > runoff+ pptnCLlMATE hot, dry hot, dry

seasonally wet seasonally wet

shoaling upwards mainly CaS04and shoalingupwards mainly CaSa4

NaCIwith basaland interlaminated, and NaCIwith basal andSUCCESSION sapropeiio, mineralized carbonate interlaminated, sapropelic,

commonlydolomitized mineralized carbonate commonlydolomitized

densitystratification thus> 20 m; laminations thusINITIAL WATER sedimentthickness=water depth so densitystratifiedthus> 20 m;

DEPTH severalhundreds to severalthousands of min. 240 m for basal carbonate ofm. sequence 1

petroleum, naturalgas, asphalt seeps, natural gas;ECONOMICS base-metal sulphides PbS, ZnS, res, U. MnOz, BaS04•

SrS04

STAGES

5 salt flat withkarsting; salt flat with karsting

Terminal or meromictic "lake"

4 3. bittern salts 3. potash

Permanent 2. thick "clean"NaCI 2. 300 m "clean"NaCI

Evaporite 1. thick "clean"CaS04 1. 300 m "clean"CaS04

sulphatization ofupper dolomiticcarbonates;

3 hiatus;early evaporitesfrom surface siltymicrolaminae with little

Ephemeral dissolveat depth; dolomitization; bitumen;thinner crinklylaminae;

Evaporite abioticwith little organicmatter; silo and transjtionto permanentwithcalc. turbidites, slumps gypsummolds

meromictic, stronglyreducingwith~S; black clay=sulphide-richonly pelagicfauna,anaerobicbenthos; sapropel.or transitionto ephemeral

2 dark- coloured,organic- and with alternatingfinelysulphide-rich sapropels; bitumen-rich interlaminated clean, dolomitic

Euxiniclaminae of transitional phase to carbonate (ephemeral evaporite)

ephemeralevaporite could be due to and siltysapropel (weteuxinic)wet seasons becomingthinnerupward with silo

and calc. turbidites, slumps; tufamounds (cyanobacterial) over

hydrothermal springs

sudden flooding; alluvial fans ring basins;suddenevaporationat surfacecreates dense flooding; bottom laminae = calcsiltywater wat sinks,carryingoxygento turbidites; basaldecimetreofbottom where benthonicfauna may profundal carbonateshave burrows,

1flourish; minoramountsof carbonate conodonts;ppted micrite;

mayprecipitate; salinityof down-welling laminations becomefinerupward,brinesincrease; dense,basal brines (becoming meromictic)

Formative "dissolve" carbonate;water mass profundal fu.ies. :: bio-pptedbecomesdensity-stratified (meromictic); limestone;profunda!~=bio-ppted limestones; no evaporitesbasjn-mariW =prodeltaicsiliciclastics; hasin .tnariin = prodeltaic

no significant precipitation of evaporites. marlstone, siliciclastics;debrisflows;

190

SCHENK, VON BITTER, AND MATSUMOTO

Formative Stage: Siliciclastics, Burrowed Carbonate

Observations» Angularrelationships between strataof sequence one and underlying rocks are complex. Alongbasinmargins, the interveningsurface is a nonconformity orangularunconformity. Toward basincenters, theunconformitychanges to a disconformity and laterally a paraconformity.Furthermore, an angulardiscordance occurs in placesbetweenthe attitudesof the strata of sequenceone and the underlyingunconformity. In the Gays RiverMine(Fig.1)geopetal struc­tures in the basalcarbonatemoundof sequence one are hori­zontalwhereas theunderlying uncooformitydipssteeply. Thesedips average70 degreesover short horizontal distancesof 12m, and 45 degreesoverat least300m (Hatt, 1978). Basementrocksareof theMegumamassif. The moundis wedge-shaped,passing basinwardinto thin, laminatedcarbonate beneathal­most 400 m of evaporites.

Initial sediments of the salinegiant consistof threelithologies: 1) conglomerates and breccias; 2) the lowermostlithozoneof theMacumber/Ship Coveformations and 3) car­bonatemounds: Mixedassociations ofcalcareous, usuallyfine­grainedsiliciclastic sedimentslocallymakeup thebasal unitsof sequence one. Their description and interpretation are inthe next section.

The conglomerates and breccias thicken towardes­carpmentmargins (Fig.4a). Grain size,shape,and lithologyof clasts depend on the distance from the main border faultand the natureofnearbyoutcrops(Fralickand Schenk,1981).Theseclastsarepoorlysorted,disorganized, and in grain sup­port often with a calcareous mud matrix; however, they arecommonly in matrixsupportin upperpartsof the units.Largeblocks of basement rock occur within the carbonate in thearea of the GaysRiverMine.Thecontactbetween theseboul­der rudites and overlying carbonate mudstone of the basallevelsof the carbonateblanket is abruptand striking.

The lowermost carbonatelithozoneis a thinlybed­ded limestonethat reappearsas thin zoneswithinstrataoftheoverlying EuxinicStage. Small recumbent folds with usuallya basalthrustzoneand upwarddecrease in amplitudeare com­mon. Three, gradational, fossiliferous lithologies character­ize the basal lithozone. The basal part is only a few ems inthicknessand consistsof graded,thickly laminatedcarbonatemudstone. Locallythe laminae wedgeoutagainstslightbase­ment highs (Fig. 4b). In Newfoundland this unit includesagrey-green micriticandarenaceous limestone (Dix, 1981) thatcontains abradedOrdovicianconodonts. The middle part ismuch thickerand is made up of bioturbated mudstone. Bur­rows are horizontaland decrease in abundance stratigraphi­cally upward. The top lithology is a discontinuously lami­nated mudstonewith the inclusionof somepeloids (Fig.4c).Fossils occur mainly in the basal lithozone and all the con­odontsin sequence oneoccurhere(von BitterandPlint-Geberl,1982). Smallcarapaces ofthecrustaceans Tealliocaris (Deweyand Fahraeus, 1982) and Bellarocaris (Fong, 1972) are 10-

callyabundanton Portau PortPeninsula,westernNewfound­land (Fig. 1). There, the brachiopod Martinia occurs at astratigraphically anomalous level in basal strata of the se­quence; elsewhere, this genus is present only in uppermostcarbonate strata of the WmdsorGroup. Composita-like bra­chiopods may bepresent(Kirkham, 1978)and ostracodes areof limitedvariety(Dewey, 1991). Thin, laminatedcrusts andbotryoids commonly encrustthefossils. Thesesuccessivecoat­ingsoffascicular-optic calciteare an importantcomponentofsequence one. Their thickness corresponds with the size oftheir substratum: micronsthickoverpeloidsin grainstones oftheEuxinic stage (seebelow); millimeters thick overclumpsof thesepeloidsor over fossils; centimetersthick on the topsof basement boulders; and decametres thick over basementhighs.

The largestencrustations formmicrobial biocement­stone mounds either directly on or close to basement (Fig.4d). The latter occur as stacked,compound moundsembed­ded in alternating thinlystratified, graded arenitesand blackshale (see next stage). In commonwith smaller crusts, mostof the mounds consist of coarsely crystalline to micritic ce­ments with calcite peloidsboth between and within succes­sivecementcoatings.Bothcementsand peloidsare presum­ablytheresultofmicrobial activity(seebelow), thus, the termbiocementstone is appropriate. Basalparts of the moundsarefossiliferous but the fauna is restrictedin diversity but not innumberof individuals. The mostenvironmentally significantfossils occur in Newfoundland and New Brunswick. Theseare straight to slightlycurved tubes up to 20 em long and 3em wide(vonBitterand others, 1992).They occurwith nestsof nearly monospecific brachiopods in Newfoundland (Dixand James, 1987, 1989a,b). Slender,widelyspaced,verticalstrandsofmicrobially coated,trepostome bryozoaare presenton Port au Port Peninsula(Dix and James, 1987, 1989a,b).The homeomorphic equivalent in Nova Scotia is the tabulatecoralCladochonus (Gilesand others, 1979).It occursas frag­ile,erect,spaced, twig-like colonies withpipe-shapedcoralliteshaving thick peripheral stereozones of laminar or reticulatesclerenchyme. The upper parts of these mounds and the en­tiretyofothersarealmostunfossiliferous andconsistofmassesof botryoidal biocementstone (Schenk and Hatt, 1984). Be­tween the botryoids, a variety of materials fill Stromatactisand irregular voids: usually blocky calcite or geopetal silt,commonly sulphidesand sulphates,and rarely asphalt.Eco­nomicgrade sphalerite,galena,celestite,and barite occur inNova Scotia; sub-economic deposits are present in NewBrunswick and Newfoundland.

lnierpretaiions-WatersofLoch Macumberenteredsubaerial basins that had ruggedtopographicrelief. At GaysRiver the attitudesof the basal unconformity and overlyingstrataof sequenceone are in original (depositional) position.Thus,Loch Macumberhad steeplysloping,submerged wallsandreliefofat least400 m. In Newfoundland this topographywasa continuation of that of theTournaisianbasins (Knight,1983). Ubiquitous indications of downslope movements also

191

DEEP-WATER CARBONATE-EVAPORITES OF A SALINEGIANT, CANADA

Figure 4. a. Ordovician limestone boulders are in matrix-support within Carboniferous limy muds. upperpart of basalconglomerate. The contactwithoverlying carbonate laminite is sharpaTfil abrupt. Scalein inches. Boswarlos, western New­foundland. b. Inclined basal contact between underlying Horton and lowermost Macumber luhozone. The Horton is aburrowed. red(dark) and green (light) mottled, siliciclastic mud. ThisbasalpartoftheMacwnber lithozone consists ofsharp­based,graded,onlapping laminae ofcarbonate mudstone. Width ofviewis35 em. SB1 core, centralNovaScotia. c. Contactbetween themiddleand toplithologies of thelowermost Macumber lithozone. Middle unitshows horizontal burrows ina veryfinely laminated carbonate mudstone. The top lithozone is mottled and discontinuously laminated. Width ofviewis 3.5em.SB1 core, centralNova Scotia. d. Carbonate mounds in thinlystratified hostrock. Themounds are1'IUJSsive biocememstones.The enclosing strataare laterally continuous, sharply based(with solemarks), graded. peloidalgrainstones alternating withdark. carbonaceous, limy shales. The central moundis 2 metershigh. Big Cove.western Newfoundland

192

SCHENK, VON BITTER, AND MATSUMOTO

suggeststeepdepositional slopes(seealso next section).

Streamflowand later subaqueous debris flows and!or rock falls spread clasts of the basal conglomerates overalluvial fans. Conglomerates occur mainly along presumedfault-linescarps,the locationof which maydefinemaximumwater depth in adjacent half-grabens (Schenkand others, inpress). After flooding of the basins (and birth of LochMacumber), subaerialparts of the area retainedconsiderabletopographic relief (Fig. 5). The escarpment walls remainedhigh enoughforcontinuederosionofuplandsurfaces. Coarsedebrisflows continuedtocharacterize theseboundaries. Rockfalls contributed basementblocksto be embedded in the car­bonatesediments.

Initial submergence of the basins was catastrophic(Schenk, 1967a; Geldsetzer, 1977, 1978; Kirkham, 1978;Giles,1981;Boehner, 1984).Thereis no indication ofa trans­gressivephase. Only on Port au Port Peninsula is there evi­dence of reworkingof underlying strata; abradedconodontsof Ordovician age in the basal few centimeters of the lime­stone are residual. Instead, the basal boundary of sequenceone marks a plummetin energylevel,from bouldery redbedsor local debris flows to laminated, dark-colored carbonatemudstone. The carbonate ooze draped an extensive area.

Progradation is unlikely because thecarbonateformation con­tains the fauna of only a singleconodont zone.Furthermoreequallyextensive butmuchthickerevaporitic rocksbounditsupper stratigraphiclimit The floors of Tournaisian to earlyVISean desert basins were probably considerably belowpre­vailing sea level to be flooded to depths of several hundredmeters (see next stage). Tectonic breaching of a presumedbarrier or a eustatic rise of sea level may have caused thecatastrophe (Kirkham, 1978 and Geldsetzer, 1978, respec­tively).

The basal carbonate lithozone was a pelagic ooze(Fig. 5). Circulation was poordue to irregular bottomtopog­raphy and constrictions between basins. Depositional slopescausedslumping and sliding that generated turbidites of thelowermost lithology. Impoverished benthic fauna of initialwatersofLochMacumber showslowoxygen,dysaerobic con­ditions. Episodicbottom currents may have brought enoughoxygen to basinal areas so that some fauna could survive.Upward decrease in fossil activityand increase in laterallycontinuous films of bitumenreflect the growing hostilitytolifeand thearrivalofdensity stratification in LochMacumber.Repetitions of this lithology in higher sections show short­time returns to a marginally oxygenated environment

modified from Schmalz. f 969

1. FORMA TIVE

t

193

Figure 5. Formative stage showingplanandsection views ofhalf-grabenwith escarpment detachment marginon right and platform ramp on left.Lower rightdetails general environ­ment. Walers ofLoch Macumber areslightly stratifiedas indicatedbybrinedensities (glee). Warm. nutrient-richwaters emptying directly from base­mentfractures into the loch supportchemosynthetic IIoases" andprecipi­tate tufamounds oversprings. Modi­fied from Cohen (1989) for LakeTanganyika and Schmalz (1969).

DEEP-WATER CARBONATE-EVAPORITES OF A SALINE GIANT, CANADA

A notable exception to thisgeneral hostility tolife is thecon­centration of organisms in oasis-like buildups (Fig. 5). ThecoralCladachonus is similarin morphology and associationstothemodem deep-water ahermatypic corals Madrepora andSolenosmilia (cf. Stanley and Cairns, 1988). The calcareoustubesin these buildups in Newfoundland and NewBrunswickareprobable vestimentiferean worm tubes that today occur inchemosynthetic communities overhotsprings (vonBitterandothers, 1990, 1992). Bacteria using~S and/orCR. from thesprings mayhavesupported thecommunities. Thesebacteriaprobably inducedprecipitation of fibrous calciteoverclasts,as wellas thebiocementstone that comprises at least the up­per parts of the mounds (seenext section). Such localized,warm, chemically-rich waters would havemaintained chemo­synthetic communities in a generallyhostileenvironment. Thereason for this hostility to life may have variedin time andspace, i.e., absence of light during the Formative and laterstages; high turbidity neardeltas (marlstone facies); absenceofoxygen duringthe EuxinicStage; and high salinityduringtheEphemeral Stage. Hotbasinalbrines issuingfrom springswould haveconcentrated sulphides, sulphates, and hydrocar­bonsin themounds (Russell, 1981; Ravenhurst and Zentilli,1987; von Bitterand others, 1992; Savard, 1992). The gen­eral setting of high relief, and active faulting and vulcanism

Figure 6. a. Soft.jet-blackcarbon-rich clayzone in carbon­ate laminite. Themainpartofthe zone isfree ofbothcarbon­ate and silt. Sub-mm thick. flat carbonate laminae occurmainlyat the bottombut also to a lesserextentat the top ofthelayer. Laminae consistofalternate coupletsofgrey weath­ering, bitumen-free carbonate and thinner, bituminous, silty

194

would increase the likelihood ofsuchsprings (seeDiscussionbelow).

Euxinic Stage: Bituminous Laminite

Observations- A largeportion of carbonate rockofsequence one maycorrespond to this stage; consequently thefollowing description andinterpretation aremoredetailed thanforsubsequent ones. Thisstage is critical to theSchmalz model.

Above the lowermost lithozone, a thin carbon-richlayer underlies carbonate laminites that make up the lowerthirdof the carbonate blanket. This thin (1 to 3 em) soft jet­black, carbon-rich clayis a marker-zone in centraland east­ern mainlandNovaScotia, and on thePortau PortPeninsulaof western Newfoundland (Figs. 1 and 00). Sub-mm thick,flatcarbonate laminaeoccurmainlyat thebottom butalsoatthetopof the layer. The interior is free ofbothcarbonate andsilt Lithologically it isreminiscent of thin, widespread blackshales in Carboniferous cyclothems of theAmerican Midwest(Schenk, 1967b). Stratigraphically thecarbon-rich zonesepa­ratestheunderlying, bioturbated, thicklyand discontinuouslylaminated mudstone from much thicker, overlying,unfossiliferous, thinlylaminated grainstones.

micro-laminae. Note incipientbrecciation ofindividual lami­naewithfractures notextending intobounding layers. Widthof viewis 3.5 em. SBl core. central Nova Scotia. b.Peloidalpebbles in bituminous laminite. Carbonate lamina showin­verse grading ofpeloidschanging upwardfrompackstone tograinstone. The peloids are well-sorted and spherical.Isopachous, fibroaxial calcite encrusts peloidsand may en­capsulate several toform spheroidal pebblesup to two em indiameter. Smaller onesoccurisolatedinpeloidalgrainstone.White areas in pebbles and grainstone are replacive anhy­drite. The bituminous layers ofthe couplets areblack.moreor lessplanarand laterally continuous. Width ofviewis 1.2em.SB1 core. central NovaScotia.

SCHENK, VON BITTER, ANDMATSUMOTO

These overlying grainstones are thinly laminatedwith siltymicro-laminae (Fig. 6b). The grainstones are greyweathering, bitumen-free, and peloidal. The micro-laminaeare dark-colored, bituminous, sulphide-rich, and clayey. Car­bonatelaminaebecome thinner stratigraphically upwards (10mm to sub-mm) witha decrease in rangeof thickness.

The carbonate laminae of thesecouplets are mainlypeloidal grainstones although stratigraphically lower lami­nae may each grade upward from peloidal packstone tograinstone(Fig.6b). The peloids arewell-sorted and spheri­cal, in places with lobateoutlinesor polygonal packing. In­ternallytheyhavea grumulous texture withpyrite framboidsand rare filaments. Mineralogyof thecements gradesupwardfromcalcite to dolomite toreplaciveanhydrite (Schenk, 1984).Isopachous, fibroaxial calciteencrustsbasal peloids and mayencapsulate severalto form spheroidal pebbles up to two cmin diameter(Fig.6b).Smalleronesoccurisolated in peloidalgrainstone. Largestones arepolygonally packedin a single,distinctive, pebbly laminae. This layer is continuous over a420 m wideexposure at Boswarlos, western Newfoundland.An identical layer at a similar stratigraphic level occursthroughout the Antigonish subbasin and the GaysRiver areaof eastern and central mainland Nova Scotia, respectively,approximately 500 km fromBoswarlos (Fig. 1).

The bituminous layers of the couplets are black toreddishbrown in color, and almostplanar and laterallycon­tinuous in form, Carbonate occursonly as angular chips ofthe grainstonelaminae. Quartzsilt,clayminerals, framboidaliron sulphides, bitumen,and microbial filaments occuronlyin theselayers. Bulksamplesthat includecarbonate laminaerange from4 to6% freecarbon(McMahon, 1988). The florawas terrestrial (P; G. McMahon, pers. comm. 1987). Thelaminiteis thesource rockforhydrocarbons in adjacentreser­voir rock (ibid.).

Evidenceofsoft-rockdeformation iscommon. Smallrecumbentfolds arefrequent Angularclastsoflaminaerangewidely in diameter. At the microscopic scale, listricfaultsin­cipientlydislocate individualmm-thick carbonate layersintotablets (Fig. 7a). In the field, pebbleto cobble sized, angularblocks offinely laminatedcarbonateandmolded clastsofgreensandstone form decimetre-thick layersof breccia on CodroyIsland, Newfoundland. These laterallycontinuous layers oc­cur with graded, siliciclastic sandstone containing Boumasequences and solemarks (Fig. 7b).Their silty, dark greyup­permostpartscontain theNereites ichnogenus Spiroraphe (Fig.7c). Rubble of similar blocks of laminated carbonate formstratiform, tabularto shoestringbodies upto several metersinthickness (pembroke breccia - Fig. 2).

Marly correlatives of the carbonate laminite occurmainly in twoareas (Fig. 1):eastern CapeBreton(facies lIbof Geldsetzer, 1977 or Frenchvale-Lake Enon facies ofKirkham, 1978)and Port au Port Peninsula, Newfoundland(Big Cove Formation of Dix and James, 1989a, especially

near Boswarlos). Dark grey to black, very thinly stratified,peloidal packstones or grainstones alternate with dark-col­oredcalcisiltites or shale.The grainstonelayersare lensyorlaterally continuous, with sharp, lower contactsoften show­ing groove and bounce marks, and gradational upper con­tacts. Deformed intervals up to two m in thickness containrecumbent folds and slumpballs; theseintervalsalternatewithundeformed, marlstone strata (Fig. 7d).

Calcareous siliciclastic sediments are also strati­graphic equivalents to the laminite (Fig. 2). In Nova ScotiatheMeaghers GrantFormation is fine-grained in texture,200m in thickness, and interfingers laterally with both theMacumber Formation and the overlying evaporite complex(Boehner, 1984). In Newfoundland, the Big CoveFonnationis coarser-grained, thinner(50 m), and laterallyequivalent tothe Ship CoveFormation (Dix and James, 1989a). The BigCoveFormationcontainscarbonate moundswithin alternat­ing, thinly stratified, graded, calcareous arenites and darkshales.

Interpretation.- The thin, widespread black clayrecords two significant events in Loch Macumber. First, itmarksanepisode of reducingconditions distinct from thoseofoverlying bituminous microlaminae. If rates of depositionarecomparable, theepisode wasmuchlonger.Theabsence ofcarbonate clastsand quartz silt also suggests a more intensereduction. Both the permanence and intensityofbottom-wa­ter oxygen depletion maybe directly dependenton the depthofwaterbeneaththepycnocline following rapid submergence(Wignall, 1991). As is true for thin, black shales of theMidcontinent, this similar, uniquezone in sequence onemayalsorecordmaximumwaterdepth (Schenk,1967b). Boththezoneand the pebbly laminaoccurdiscontinuously overa dis­tanceofapproximately 500 km, suggesting greatwaterdepth(see Discussion). Second, densitystratification becameper­manent in the waters of Loch Macumber. Only below thiszonedotracesofaerobic infaunaoccur; onlyabove it doeven,varve-like, fine laminaepredominate. Thus, from this blackclayonwards, Loch Macumber wasessentially a giant, deep,meromictic, saline lake.

Laminatedsediments arenatural products of rhyth­micsedimentaccumulation bysettlingin any stratified waterbodysubject to cyclic changes (Davies and Ludlam, 1973).Thesecycles must havebeen climatic, involving changesinprecipitation and temperature (Fig. 8). The changes maybemoreor less regular(making seasonalnon-glacial varves) orquiteirregular(forming tempestites). In the former case, sa­linegiantsprobably developed underhumid,tropical and sub­tropical climateswherevegetation was lush,rainfalland run­offwereabundantbut seasonally distributed, and whereperi­ods of drought wereextremebut of relatively short duration(Schmalz, 1971). In the later case, evaporation overmodemsalinelakesis normally continuous so that chemicalprecipi­tation is continuous throughout each year. Storm-flood in­fluxes ofsiliciclastic sediment (mainly mud)puctuate thecon-

195

DEEP-WATER CARBONATE-EVAPORITES OF A SALINE GIANT, CANADA

Figure 7. a. Two listric faults (arrows) cutting bituminous layerin carbonate laminite. White spots in bitumen are grains ofquartz silt. In under-andoverlying grainstones, peloidsare dark grey in color, their isopachous calcite cements are lightgrey. andreplacive anhydrite is white. Width of viewis3 em. SBI core, central NovaScotia. b.Basal siliciclastic sandstonewithoverlying carbonate breccia. The sandstone has a sharp basewitha cross-section ofaflute cast. Grain sizedecreasesupward from coarse to fine sand. The breccia consists of angular flat fragments ofPeloidal carbonate laminite and rare,plastically deformed. green, muddy sandstone (arrow). Lens cap is 65 em wide. Codroy Island. c. Nereites ichnogenusSpiroraphe in thin,silty. darkgrey shale. Coinis2.5 cmindiameter. Codroy Island. d. Recumbent isoclinal folds andslumpbreccia between laterally continuous. marlyfaciesofthelaminite. Clasts in thebreccia areofisolated remnants offold nosesin a deformed matrix. Basal layer(arrow) maybe a translation zone. Hammer is 33 em long. Boswarlos.

stant rain of chemicalsediment(Hardie and others, 1978). Inthe caseofLoch Macumber, thickness of couplets is not con­stant so that storm eventswhich mayor may not be seasonalare probably responsible for laminations. Wedoplan testsforperiodicity in the loch's laminites.

Dry, warm conditionscould occur, thereforeeitherduring summersor generallyas the usualenvironmentof thesaline lake.Twoprocesses wouldforce carbonateproduction.Both operate now in the Dead and Red Sea, respectively(Friedman, 1972).

First, the carbonatecouldhaveprecipitated episodi­cally from the water mass as "whitings" due in part to in­creased temperature and blooms of cyanobacteria. The car­bonate matrix of peloidal packstones in basal laminae mayrepresent such direct precipitation, possibly as transitional

196

intercalations of carbonate mudstonethat dominates the for­mativestage.Thus, the peloidsmay be productsof spontane­ousor bacterially inducednucleation and lumpingofMg-cal­cite in the saline water column(Tsien, 1985;Chafetz, 1986;Sun and Wright, 1989; Reid and others, 1990; James andChoquette, 1990).Thesettingwas equatorialso thatevapora­tionofsurfacewaters duringarid timescouldcausesuchspon­taneous, inorganic precipitation. This process, although op­erating in analogous locations today (see Discussion) is per­haps less likelythan the following.

Second, calcite could be a by-product of bacterialreduction of sulphate, as in the following equation:

SCHENK, VON BITTER, ANDMATSUMOTO

modified trom Schmalz, 1969

2. EUXINIC

Mixolimnion

- - - - - M;;-im~li;;ni;

GlidingBrecciation

TufaMounds

Figure 8. Euxinic Stage using alter­nating l1Iet (left) and dry (right) sea­sons toformlaminite couplets (black).Underlying sediments (dot and dashpatterns) record theFormative Stage(Fig. 8). Warm watersfrombasementfractures opening directly intostrati­fied Loch Macumber produce tufamounds. Modified from Schmalz(1969).

Anaerobic bacteriamayhavescavenged sulphate firstin solution (formingcalcareous, biogenic ooze) and later assalinityincreased,gypsumcrystals(forming gypsum molds­see next section).They would feedon the bituminouslayerspreservedin the anoxicenvironment. The calcitewouldpre­cipitateas micritic aggregates withincoccoid bacterial clumpsto form peloids.'The hydrogen sulphidecouldcombinewithferrousions (or other reducedmetals)to produceframboidalsulphides within thepeloids. Bottool waters ofLochMacumbermust havebeenrich in sulphate:verticalpersistency of lami­naesuggests a permanentlystratified waterbodyprobably dueto salinity stratification; molds of gypsum crystals are com­monin upperpartsofthe laminite; inplaces, individual peloidsare suspended in anhydrite; and hundreds of meters of cal­ciumsulphategradationallyQverlie thelaminite. Furthermore,peloidsshow no sign of gfft\ritative settlingfromsuspension,suchasgradedbedding.Instead,thesuspension ofcompoundpeloidalpebbles in peloidal grainstones and their polygonal.packing as laterally continuous sheets show in situ growth.The following association suggests the activity of sulphate­reducing bacteria: bituminous films, carbonate clots, pyriticframboids and cubes, and tracesof bothcoccoid and ftlamen­tous remains.

Bacteriamayalsoberesponsible forthe upwardpro­gression through a lamina from peloidal packstone tograinstone. Cements in the latter were first calcite, thenreplacive anhydrite. Initially, increased evaporation wouldgenerateheavy, downwelling brinesorindividual gypsum crys­tals settlingfrom the surfaceto increasethe sulphatecontentof bottom waters. There, anaerobic bacteria would be mostactivenextto theirfoodsupply- thebituminous, microlaminarsubstrata. Theywould reduce the sulphate, feed on the bitu­men, and precipitate by-products of calcite and metallicsulphides. An individual colonycouldform a peloidalclotofcarbonate and possibly calcite rim cement between peloids.In places, bacterial activity may have been great enough toconstruct compound peloidal pebbles coated with radial fi­brous calcite. Perhaps this cement was originallyaragonite,as is trueforcarbonatecrusts in theRedSea(Friedman, 1972;Stoffers and Botz, 1990).Gypsum couldpersisteither whencarbonatecovered the bituminoussubstrata or whenthe vol-

197

umeof incominggypsum crystals outpaced the rate of bacte­rial reduction. The gypsum wouldconvertto replacive anhy­drite afterburial.The productof such a processis visiblenotonlyat the scaleof individual laminaebutalso throughoutthethickness of the formation (Schenk, 1984). In conclusion, awann (or a warming of the) water mass and higher rates ofevaporation could havecausedprecipitation of the carbonatelaminae by either or both of the two processes (see Discus­sion).

Wet times couldbe long or short termed,as duringrainyseasonsor heavyrainstorms, respectively. Runofffromthe platform margin would have created a brackish waterwedge. This would reduce surface salinity, increase densitystratification, and washfine-grain siliciclastics and terrestrialplant material into the loch. Deposition of fine-grainsiliciclastics wouldbebothlocaland basin-wide. Locally, del­tas wouldbeactivewith deposition of marlstonealong deltamargins. Maskingof carbonatewouldinhibit early cementa­tion so that sedimentwas unstableon delta fronts.Slumping,perhaps initiated by earthquakes from border faults, gener­atedolistostromes as wastrue in the steep-walled, deep-waterprecursor basin in Newfoundland (Knight, 1983). Theseunderflows coulderode previously deposited, profunda! car­bonatecrusts to form debris flows as at Codroy Island. Dur­ingmassive storms, unchannelled sheet-floods carrying coarsesediment from the surrounding land could generate denseunderflows and so thick, siliceous, sandstoneturbidites, alsoat Codroy Island. More usual, basin-wide deposition ofsiliciclastic clay and silt may bedue to overflows and near­surface flows from the deltas. Overflows or interflows couldissueas turbidplumesfromthe deltas.Their clayand finesiltwould settle toward the bottom of Loch Macumber. Iron inthereducedbottom waterscombined with hydrogen sulphideproduced by the putative bacteria to precipitate sulphideframboids. Thus,microlaminaerich in sulphide,bitumen,andsiliciclastics would form in the profunda! zone of LochMacumber. If they are seasonal pulses, they fonn varves; ifthey record storm-floodings, they are chronostratigraphicevents.

After deposition, in situ precipitation of fibroaxial

DEEP-WATER CARBONATE-EVAPORITES OF A SALINEGIANT,CANADA

calcitecement, possiblybacterially induced, coatedindividualpeloidsandgroupsofpeloids to produce syndepositional, mm­thickcrusts or"micro-hardgrounds". Thisprocess was a small­scale version of more concentrated precipitation that builtbiocementstone mounds (seebelow). Foundering and limiteddownslope gliding on slippery, underlying, sapropelic lami­naewould incipiently brecciate thecrusts. After shallowburialthesefluidlayers wouldhavehadhighporefluid pressures topromote gliding.The release of pressure by dewatering anddegassing would havefurther broken thethin,delicate, brittlecrusts. Elsewhere, downslope mass movement ofthelaminiteproduced slumpstructures. Thesiliceous sandstones onCodroyIslandareturbidites. TheassociaiOONereites ichnofacies typify:1. highly unstable, slowly accreting pelagic muds,2. mostlyquiet. oxygenated conditions, 3. occasional influxes of bot­tom currents or turbidity currents, and 4. bathyal to abyssalwater depths (Freyand Pemberton, 1984). Their occurrencein theanoxic environment ofsequence oneisprobablyoppor­tunistic immediately following turbidite deposition.

Waters of Loch Macumber became progressivelymoresalineso that even anaerobic organisms declined. Up­warddecrease in numberandsizeofcompoundpeloidal lumpspresumably reflects a decrease in bacterial activity (Folk andChafetz, 1983). As notedabove, the abundance of bitwnenalso decreases upward. Locally, bacterial activity probablycontinued over hot springs to precipite biocementstone in atleastupperpartsof the mounds. This process may haveper­sisted through much of the following stage, until evaporitecovered the mounds.

Ephemeral Evaporite Stage: Carbonate Laminite

Observations.- Siltcontent ofthe laminite increasesupward as the carbonate laminae decrease in thickness. Thecarbonate is mainlydolomite so that the colorof weatheredrockisbrown. Thelaminae becomecrinklywithsharppointed,often broken anticlines. Bitumen is absent and peloids arerare. Laminae fold around vugs whose shapes range fromround in lowerstratigraphic levels to angular in higherpor­tions (Fig.9). The contactbetween dolostone and overlying,massive, impureanhydrite is gradational overseveral meters(Schenk, 1984).

Primary fluidinclusions in moundcalciteare all oftheliquidvaportype, without daughter products. Salinity fig­ured out by freezing-point depression ranges. from 15 to 21equivalent percentNaCI. Homogenization temperatures rangefrom 65 to 1870 C and yetthereisnoevidence ofboiling (vonBitterand others, 1990).

Interpretation.- Decrease incarbonateproduction andpreservation of sulphatecouldbedue to increases in aridityand/orEh (Fig. 10).Increased aridity would decrease vegeta­tion, reduce runoff, and so starve carbonate-precipitating,sulphate-reducing bacteria in the loch. Greater erosion bywinds couldconceivably bring siltsand claysinto the loch;

198

however, the laminae are still distinct, suggesting episodicaccumulation ofsiliciclastics. Higherevaporation ratesmightlessenthevertical salinity gradientand/orreduce waterdepthso that bottom waters became less reducing. An increase inEh could decrease the effectiveness of the bacteria and stillallow runofffrom thelandto transportthesiliciclastics. Any­how, decreases in carbonate and bitumen, and increases inpreserved sulphate reflect declines in anaerobic bacteriaand/or increases in salinity.

Theshapes oftheangularvugsare thoseofgypsumcrystals, including swallow-tail twins. Stratigraphically lower,rounded vugs probably held corroded gypsum crystals. Theupward trend from rounded to euhedral gypsum may reflectincreasing saturation of bottom waters with respect to cal­ciumsulphate. Bacteria mayhavescavenged the sulphatetoproduce the molds.

Plastic deformation ofdolostonelaminaearoundvugsis due to differential compaction. Conceivably the roundedvugs formed duringearlydiagenesis. but therangetoangularvugssuggests a common origin. Presumably thelaminaewerenotcemented when thegypsum crystals weredeposited. Be­cause the laminae show someevidence ofearlycementationat the water/sediment interface, the sulphate is the sameageas the carbonate. Their protection from lithostatic pressureandconversion toanhydrite maybeduetotheirsurrounding,earlycemented carbonate host Thepresence ofgypsum crys­tals signals transition to thepermanentevaporite stage.

According to the study of fluid inclusions, moundcalciteprecipitated from a heavy brine.Thehighest-tempera­tureinclusions require a waterdepthofat least100meters forthesefluids notto have boiled (vonBitterandothers, 1990).

Permanent Evaporite Stage: Sulphate, Chlorides

Observations.- Microdolomiteandanhydritebecomeprogressively moreabundant in the upperhalfof thecarbon-

Figure9. Molds instratification surface ofcarbonate laminite.Euhedral molds are swallow-tail twins of gypsum crystals.Roundedmoldsmay be partiallydissolved gypsum masses.Coin is2.5 em in diameter. Cheverie, NovaScotia.

SCHENK, VON BITTER, AND MATSUMOTO

EARLY

3. Ephemeral.

Schmalz, 1969

LATE

Gypsum MoldsIn Crinkly Laminite

Life Sparse Or Absent

Figure 10. EphemeralEvaporiteStageshowing early (left) and late (right)development. Theblackprofundalpat­tern records thepreviousEuxinic Stage(Fig. 8). Most euhedral andswallow­tail twins of gypsum dissolve beforereaching the bottom of LochMacumber. Springs may continue toform tufa mounds. Modified fromSchmalz (1969).

ate layer. At Gays River, the upper third of the 4.6-m-thickMacwnberFormationis almost entirelyblocky or microcrys­talline anhydrite (Schenk, 1984).Anhydrite occursas down­ward narrowing fingers that cut and embaycarbonate struc­tures and textures (Fig. 11). Anhydrite occupies areas for­merly filled with carbonate in the following temporal order:first, blockyintergranularcalcite;then,dolomitic rims aroundpeloids;next, the peloidsthemselves; and finally, bituminouslaminae. Where almost complete, the anhydrite structureranges from laminated to mainly thinly stratifiedwith rem­nant bituminousfilms (ibid.).

Abovethis transitionallithozone the main mass ofanhydrite underlies and in part interfmgers laterallywith ha­lite and locally, potash. Borate and bora-silicate minerals areabundant in places.The evaporitebodyis 160to 300 m thickin NovaScotia(Boehner, 1986a).It has a pseudomosaic struc­ture and microcrystalline toblockytexture. Cycles occur,eachconsistingof a basal 6 to 10m ofcarbonate-free sulphate,andan upper0.5 mof carbonate-richanhydriteor anhydrite-richcarbonate. The latter are laminated or very thinly stratified,unfossiliferous, carbonate mudstone, containing peloids orcoatedgrains in places(Schenk, 1984). Embayedcontactsarecommon between anhydrite and carbonate. The halite has amaximumthicknessof300 rn,is colorbandedand verythinlystratified with minor amounts of siltstone and anhydrite(Evans, 1965;Boehner,19800). At the PugwashMine, haliteis alsocyclicwith fivedistinct zonesdividedby three litholo­gies: anhydrite layers (up to 30 m in thickness), threesiliciclastic breccias, and three carnallite breccias (Evans,1965).Deformation has been intenseand hasdestroyed origi­nal depositional textures. Regionally, the amountof brominein halite is low (20 to 60 ppm.); "secondary"halite containsvalues of up to 195 ppm (Baar, 1965). Bromine content ofhalite in Nova Scotia and New Brunswick increasesstratigraphically upward. A distinctive bromine peak in thelowerpart of halite sectionscoincides with the start of majorpotash deposition (Boehner, 1986b).

Interpretation» Wholesale replacement of carbon­ate by sulphate is transitional into the permanent evaporitestage (Fig. 12). Bottom brine must have become saturated

199

with respect to calcium sulphate. Conceivably, denser,sulphate-rich brinespercolateddownwardbygravityflowandmixedwith less dense, interstitialbrines in equilibriumwiththe carbonate.Alternatively upwelling, hot, CaC~-rich basi­nal brines could mix with the interstitial brine. Such mixingcorrosion could cause replacement (Schenk, 1985). A thirdalternative involves formation ofthereplacivebrineafterburialbydehydration ofoverlying gypsumto anhydrite(see Discus­sionbelow).

Diageneticand shallow marine textures and struc­tures typical of the evaporites in overlying sequences of the

Figure 11. Anhydrite (light colour) partiallyreplaces bitumi­nouscarbonate lamina. Notespherulitic texture ofanhydritepossibly after carbonate peloids. Width of view is 35 em.SBl core.

DEEP-WATER CARBONATE-EVAPORITES OF A SALINE GIANT, CANADA

modified from Schmalz, 1969

EvaporitesDisplace Brines

Sulphate - Rich

4. PERMANENT~

EVAPORITIC

Freshening

Sulphate

Figure 12. Permanent Evaporite Stagewith gypsum crystals reaching bottomofLochMacumber. andraftsofhaliteforming at the surface. Episodic fresh­ening ofsurface waters result in layersof sulphate within carbonate and lay­ers ofhalite within sulphate deposits.Evaporites plug fractures so that tufamounds cease to form. Modified fromSchmalz (1969).

Surface

Oxidizing

With Brine

Pools And

Karsting

5. TERMINAL

BRINE POOLS SALT FLAT

Evaporites

Macumber

modified from Schmalz. 1£69

Figure 13. Terminal Stage showingevaporites filling topographic relief.Subaerial surface becomes karstic, ter­minating Sequence 1. Modified fromSchmalz (1969).

Windsor Group are absent in sequence one (Schenk, 1969,1984). Ratherthaninterpreting theanhydritebody in sequenceoneasa deep-water precipitate, Schenk (1984) concluded onmicroscopic grounds that all of the anhydrite is secondary ­i.e.,sulphate massivelyreplaceda shallow-water tointertidal,carbonate precursor. Remnants of the forerunner wouldbevisible aspartiallydissolved layers. Although somedissolu­tion is evident, the scaleis daunting. Rather, almost all of thesulphate mustbe a directprecipitate.

Toprecipitate gypsum, thebrinemust havebeenatleast 4.5 times the concentration of sea-water (Sonnenfeld,1991). It shouldcontain less than four ppm oxygen and beeffectually anaerobic (Kinsman and others, 197~). However,onlyepisodic, minor vestiges of peloidal and laminated car­bonate remain. If sulphate-reducing bacteria are responsibleforcarbonate production, thecycles of mainlyanhydrite andthin,laminated, peloidalcarbonate suggestalternation ofpre­dominantly oxidizing and minor reducing environments(Friedman, 1972).

Accessory minerals associated with the evaporitebody may be significant to environmental reconstruction.Celestite andbaritearecommon lacustrine authigenic miner-

200

als (Picard and High, 1972); these are locally of economicgrade in Nova Scotia. Borate minerals are commonplace inthemainevaporite body; theyareuniqueto salinelakedepos­its (ibid.).

Brinereaches saturation for halitewhen its salinityreaches 10to 11 times the concentration of sea-water. Haliteprecipitates more readily in calm. open waters or in waterswith dissolved organic compounds, than in stratified or agi­tatedwaters (Sonnenfeld. 1991).

The halitebody shows repetitive cycles of thickha­lite and thin sulphate and minor siliciclastic layers. Thesecycles record episodic freshening of the brine, probably re­flecting times when water level rose, rather than when thebasin floor was exposed. The freshening could have a tec­tonic, eustatic. or climatic cause. Tectonic activity was rela­tively lowduringtheV~ in the Maritimes Basin. Deposi­tionof the evaporites was probably toorapid to reflect eitherrepeated tectonism or eustasy (seeDiscussion). The associa­tion with siliciclastic siltstones suggests increased runoff. Thus,thecycles probably are duetowetter times,eitherofseasonalor shorter-termed, storm-induced duration. Their boundingsurfaces mightbe equivalent to marine-flooding surfaces in

SCHENK, VON BITTER, AND MATSUMOTO

termsofsequence stratigraphy. Hso,theevaporitic cycles couldpossibly be lacustrineparasequences.

Terminal Stage: Karstic Salt Flat

Observations- The deeply buried, uppermost unitof sequenceone consistsof gypswn (not anhydrite) and silt­stone. Thicknesses vary considerably, reaching a maximwnof 100m (Giles, 1981).An unconformity separatesthis zonefrom the first of 12parasequences of sequence two (ibid.).

Interpretation.- Hydration of the anhydrite faciesoccurredduringVisean time (Boehner, 19800). Sequence oneculminatedin arid-flatsedimentation, subaerial exposure, andkarstification (Fig. 13) (Gilesand Boehner, 1979). Thus, theupper boundary of sequence one is a karstic disconformityrepresenting an extensive lacuna (Giles, 1981; Boehner,1986a).

DISCUSSION

Water Depth

Waterdepth is criticalin distinguishing between thetwocompetingmodels for deposition of saline giants. Waterdepth is mostdifficultto assess quantitatively in sedimentaryrocks. Estimates for Loch Macwnber range from shallowsubtidal(Schenk,1967a,1984),possibly deep (Evans,1970),more than 100m (vonBitterand others, 1992), lessthan 250m (Geldsetzer, 1978), and between 200and 400 m (Boehner,1984).This sectionconsidersseveral directand indirectlinesofevidence that togethersuggests that waterdepthcouldhavebeen severalhundredsof meters,possibly more than 400 m..

Perhaps the strongest indirect evidencefor deeperwater is the widespread nature of a single blanketof uniquelithology. The basalcarbonateblanketof sequence one is usu­ally less than 5 m in thicknessbut extendsover an area of atleast 150 x 1()3 km-, These formations always underlie theevaporitecomplexthat extends over250 x 1()3 km'; thus, thecarbonateunit probablyalso coversthis area (Fig. 1). More­over, thelaminiteisuniform in both lithologyand stratigraphicrelationsdespitethe existenceof highlandsand horsts beforeand after its deposition. The present areal extent is only aremnant; erosion from Permian to Early Jurassic time alonehas removed 1 to 3 krn of Carboniferous strata. That is, thelaminite likely extended as a thin blanketover onshore andoffshore AtlanticCanada (R. 1. Ryan, pers.comm. 1991). In­dividual bedssuch as the black clayand pebbly laminae arealso widespread. The depositional environment of carbonateooze was quiet and anoxic, interrupted episodically by set­tling of sapropelic ooze. Rate of sedimentation would havebeen greatest if the couplets wereannual varves; an averagethickness ofeightmeterscontainsseveralthousandmm-thickcouplets. This restricted environment must have been uni­form over a very large area and persisted for thousands ofyears.Such uniformity is easiestto achievein deep water.

Four factors ensured preservation of such a wide­spread,thin layer. First, deposition in a deeptopographic lowconsiderablybeneath regional base-level wouldhaveincreasedchancesof preservation. The present structural relief of 400m nearGays River is alsoVisean depositional relief; the floorof thisbasinhas probably always beenbeneathsea-level. Sec­ond, exceptionally high rates of deposition possibleonly byprecipitation of overlying, thick evaporites would have pre­served the deep-water succession. Third,preservation oflami­nated sediments is particularly good where stratification ofthe water bodypersists for a long time (Daviesand Ludlam,1973). Fourth, continuation of an at least seasonal, arid cli­mateduring the lacuna following deposition of sequence oneprevented severedissolution oftheprotective evaporiuc cover.

Strataof sequence onerecorda differentbathymetryof the MaritimesBasin than thosedepositedbeforeand afterit. Older Carboniferous strata in Nova Scotia and NewBrunswick showlocalderivation, short transport, and mainlysubaerial or shallowlacustrinedeposition withinisolatedfault­boundbasins (Fralickand Schenk, 1981;Martel, 1992). Sig­nificantly, Newfoundland lakes were deep and steep-walledduring deposition of time-equivalent rocks (Knight, 1983).Later parasequences of sequences two through five of theMaritimeWindsorGroupoverlaidsurfacesof low reliefandgentleslope{Schenk, 1969; Boehner19800). In contrast, strataof sequence one do not show obvious parasequences but doexhibitremarkable continuity(Fig. 3). Changes in sedimen­tary stylebefore, during,and after sequenceone aredramatic:respectively, 1)frommainlysubaerialdeposition to subsalineto mainly subaerial, 2) from local derivation to regional toprovincial, 3) fromcyclic successions toprogressive shoalingto cyclic, and 4) from rugged paleotopography to ruggedbathymetry to subdued paleotopography. These changes re­sultedfrombasinfillingandtopographic levellingbythethickevaporites of sequence one (Boehner, 1984).

Downslope movement shown by synsedimentaryfolds, slump zones, incipient brecciation, debris flows, andturbidites are common (see Formativeand Euxinic Stages).Suchdeposits require someinitial slopeand accommodationspace downslope. Similar evidence of syndepositionaldownslope movementoccursin siliciclastics conformablyandgradationally beneathsequence one in the area of Codroy is­land,western Newfoundland. Overtwokm ofsuchsedimentscontainincoherent slides,subaqueous mudflows, debrisflows,and turbidites - all deposited in a deep, restricted lake withsteepsides (Knight, 1983).

Mound organisms probably dependent on chemo­synthesis than on photosynthesis suggestdeposition beneaththe euphotic zone (von Bitter and others, 1990, 1992).Thisvaries in thickness but mayaverage80 m with a maximumof220 m in clear water. The absence of micritization also sug­gests deposition beneath the photic limit (Lees and others.1985). Theequatorial settingof the MaritimesBasinsuggestsmaximum depth of light penetration; water depth probably

201

DEEP-WATER CARBONATE-EVAPORITES OF A SALINEGIANT. CANADA

exceeded 200 m.

Buildups similar in age and structureto thosein se­quence one show four faunal assemblages related to waterdepth (cf.Leesand others, 1985;Lees and Miller, 1985). In­terpretedwater depths for theseassemblages range fromlessthan 120 to more than 300 m. Mounds containing these as­semblages grade downwardinto deeperwater wherebasinalmuds correspond to the laminite facies of sequenceone. Al­thoughrestricted in variety, fauna in basal Windsormoundscorresponds to assemblages indicative of waterdepthsof 280to more than 300 m.

Nereites ichnogenus Spiroraphe that occurs ineuxinic laminites at CodroyIsland may bean opportunisticfauna. Turbidity currents could have carried the fauna intothe anoxic.deep-water environment. The coralCladachonusis similar to modern deep-water ahermatypic corals.

Homogenization temperatures of fluid inclusionsrange from65 to 1870 C. Waterdepthwasat least 100metersto preventboiling of the fluid in these inclusions (von Bitterand others, 1990).

Stagesof The Model

How well does the deep-basin/deep-water hypoth­esis predict observations of Loch Macwnber? McCutcheon(1982) gave preliminary support to the model as applied tosequence one in southern New Brunswick. Table 1 showsinsummarythat the variousstagesof themodelagreewiththoseof sequence one, especially if we consider transitions.

In theFormativeStage,thepaleoenvironment ofthebasallithozone was poorlyventilated. However. it was morehabitable for life than those of overlying units. Densitycur­rents that deposited-basal silty turbidites could have resultedfromdisplacive, densebrines.Reappearances of the lithologyin upper parts of sections show episodic freshening of thewater body. Difficulty in correlating these episodesbetweendifferent segments of thelochemphasizes itscomplex bathym­etry.

Carbonatemoundsin sequence one are similarpet­rographically to calcareous tufa deposits of the Great Basinand elsewhere (Schenk and others, in press). Both theMaritimes and Greatbasinswere/are due to extensional fault­ing. Tufa moundsare low-temperature hydrothermal, bacte­rial-induced buildupsprecipitated overvents. Suchventsmayhavebeenthelociforchemosynthetic communities in Wmdsormounds (vonBitter and others, 1990). Somemoundsas wellas the adjacentbasal laminite are petroliferous and sulphide­rich. Mineralizingfluidswerehot salinebrinessimilar isoto­pically and chemically to basinal brines (Ravenhurst andZentilli, 1987). Hot springs beneathLoch Macumbercouldmixmetalliferous,CaC~-rich,MgS0

4-poor basinal brineswith

nonmarine or hybrid surface waters. Bacteria concentrated

over the springs would support chemosynthetic fauna andprecipitate tufa.Sulphidemineralization couldbeeither rela­tivelyearly. or late if subsurface plumbing persisted. Evapo­ration of the loch's water would produce thick deposits ofMgSO4-poor evaporites (Hardie, 1991).

Conceivably the extensive black clay layer couldrecord the entire Euxinic Stage; the overlying bitumen-car­bonate laminae wouldthen be transitional into the Ephem­eral Stage. We have not yet establishedcontinuous correla­tion of the clay; it may representonly local extremes in an­oxia.On the otherhand, the alternatingwetand dry episodescould correspond to rapid alternations of Schmalz's WetEuxinic (Terminal Stage III) and Ephemeral Stage. Thus,duringrainy times,runoffwouldcarry siliciclastics and plantmaterial to the Loch; deltas would rejuvenate; siliciclasticsand marlstones would deposit at deltas;slumpsfrom prodeltaslopes woulddevelop turbidites and debris flows; a fresh- orbrackish-water wedgewould inhibit carbonateprecipitationfrom the surface; and sapropel would form on the bottomofthe loch. During dry periods. water levelwouldfall; salinitywouldincrease; sulphate-reducing bacteriawouldtransformsulphate to clottedcarbonate mud;andpeloidal carbonatelami­nae wouldform on the bottomof the loch.

The EphemeralEvaporitic Stage shows a decreasein bitumen content and the occurrence of gypsum molds inthe carbonatelaminite. This increasein salinityprobablyre­flects a decrease in waterlevel,due either to a changein theclimatetowardgreateraridityor to restrictionin the influxofnormalmarine waters.

Halite precipitated in different parts of the basinshould show different brominecontents because of the den­sitystratification characteristic ofdeepbasins,e.g.•75 ppm atthesurfaceand 150ppmin the basincenter (Schmalz,1969).Thus haliteprecipitated fromhighlyconcentratedbrineoccu­pying the deeperparts of the basin, oc surfacehalite that re­crystallized at depth.wouldbecharacterizedbyhighamountsof solid-solution bromine. Values of 160 to 195 ppm in "sec­ondary" halite of sequence one could also have precipitatedfrom shrinking brines covering only deeper pans of thesubbasins (Baar, 1965). Presumably these values would in­crease stratigraphically'upward. as is true in sequence one(Boehner, 1986b). Thus,brominecontentcanbe usedfocbothdeepand shallowwatermodels. Boehner (19800) concludedthat the halite localized in a shrinking basin next to contem­poraneous. subaerially exposed anhydrite. Proof ofcontemporaneity is difficult given the duration of subaerialexposure beforedeposition of sequencetwo.

Either tectonic closingof the inlet or eustatic dropin sealevelcouldhavestopped inflowof sea-water. Tectonismsupports alternative twoofSchmalz'sTerminal Stage. Eustasycontrolled Dinantian cycles in Britain (Ramsbottom, 1973);Windsor sequences conform broadly to these cycles (Giles,1981).If eustatic sea-level dropped, we might expectan in-

202

SCHENK, VON BITTER, AND MATSUMOTO

fluxof siliciclastics. However, their absence in sequence onecouldbe due to two reasons: first,aridity mayhaveprecludedrunoff; second, deposition of thickevaporites mayhavefilledtopographic relief. Potash occurs in increasing amounts innorthwesterly located subbasins (Giles, 1981), supporting bothcircumstances. Their combination could have led to hydra­tion of anhydrite, karstification of the surface, and lengthyemergence.

The general conditions of location, setting, climate,succession, and economic products predicted by the modelapplyto Loch Macumber. Initialwaterdepth,however needsdiscussion.

Isostatic Adjustment

According to the deep-water model, the sedimen­tary thickness of sequence one equates with original waterdepthin LochMacumber. Thusmaximum water depthwouldbe approximately 400 m. The Schmalz model, however doesnot consider isostatic lowering of the floor of the basin inresponse to the weight of overlying water and sediment. Cal­culationofboththeoriginaldepthofwater and theminimumtopographic reliefofthedry basinbefore submergence is pos­sibleif the area wasin isostatic equilibrium.

This would be so if the time for deposition of se­quence one was longenough for viscous response of the sub­stratum. The succession is without obvious lacunas, whichare common in overlying sequences of the Windsor Group.Two roughapproximations ofthe leastamount oftimefollow.First,iftheWindsorian epochlastedfor4 Ma(Mamet, 1970),sequence onecould havebeen deposited ina conservative5~Ka. Second, if thepreservation rateofevaporitic rockrangedfrom .1 to .4 cm/a (Barnettand Straw, 1983), the 400 m ofevaporites in sequence onewouldrequire 100to 400 Ka. Re­laxation time is the half-life of the process from one stateofisostatic equilibrium toanother. Crittenden (1963) calculatedthe relaxation time for Pleistocene Lake Bonneville in theBasinand RangeProvince (Walcott, 1970). Before shrinkingto its presentsize, this lakewas 112 x l(Pkm'in areaand 100m in maximum water depth. Removal ofwater has decreasedthe load on the crust so that the areahas risen isostatically.Elevations of raisedbeaches showthe amount of this uplift.The relaxation time for the observed 64 m uplift of LakeBonneville cannotbegreater than4 ka. If thisnumber were5ka, the entire uplift would take 20 ka, a fifth the least timeavailable for sequence one.Moreover, movement alongbor­der faults of Loch Macumber would help rapid adjustment.Furthermore, the upperboundary of sequence one is a majorlacuna representing a presumed long span of time. Thus,enough time is probably available during deposition of se­quence one for the areaofLochMacumber to reach isostaticequilibrium.

Calculations of isostatic adjustment dependalsoonthe specific gravity of the rock deposited in the basin. Cal-

cium sulphate is now the major lithology of sequence one;was it originally anhydrite or gypsum? This question is sig­nificant because gypsum is24 percent lighterandoccupies 63percent more space than anhydrite. The initial sulphate inLoch Macumber was gypsum because: 1. the underlying car­bonate laminite (Ephemeral Evaporite Stage) contains moldsofgypsum, notanhydrite; 2. theanhydrite does not showpri­maryor diagenetic features that are common in immediatelyoverlying sequences but instead hasa vaguely nodular, meta­morphic habit, probably the result of recrystallization afterdehydration (thehalitehasalso recrystallized); 3.gypsum pre­cipitates before anhydrite even in a solution supersaturatedfor halite because gypsum nucleates easierand grows fasterthananhydrite (Braitsch, 1964); 4. transformation from gyp­sum to anhydrite occurs at shallow depths (450 m) and lowtemperatures (600C) inareasofhighheatflow andbrinesatu­ration (Hosler, 1979) - l.e., the assumed conditions beneathLoch Macwnber; 5. heated water of dehydration, sealed inpart byoverlying chloride had important effects: a. probablyoverpressured the sulphate layer to encourage flowage andensure loss of primary sedimentary features; b. passedstratigraphically downward to replace upperparts of the car­bonate with anhydrite, c. hydrofractured underlyingsiliciclastics andfilled voidspaces withanhydrite; d. perhapsremobilized Pb-Zn-Ba (Ravenhurst and Zentilli, 1987); ande. passed upward where possible to help in hydration andkarstification of the uppermost parts of sequence one. Note,however, that upwelling basinal brinesmayhavebeen impor­tant.

Wemayestimateminimum originalwater depth andminimum subaerial depth of the basin floor before submer­gence by twodifferent ways. Table 2 provides a perhaps morerealistic sedimentologic model but theformula that follows ismore concise. Forbothestimates, the assumptions are: 1. thebasinwasin isostatic equilibrium at thebeginning andendofsequence one;2. the surrounding topography was flat; 3. sub­sidences ofthe initiallysubaerial basinfloor weredueto load­ingfirstbybrine(notetheunderlying redbeds andbasaldeep­water carbonate laminite), and subsequently by precipitationof evaporitic rock; 4. the thickness of this rock is now ap­proximately 400 m,ofwhich 200m isanhydrite and200m ischloride; however, theanhydrite probably formed at depthbydehydration of gypsum. The 200 m of anhydrite represents320 m of original gypsum. If so, the original thickness ofgypsum andchloride wasapproximately 520m;5. thesemin­erals form bulk evaporitic rock without individual order ofcrystallization or stratigraphic position (important for thetable); and6. compaction of theevaporitic rockwas minimal.

For theequation, let:R = reliefof the subaerial basin,in metersD =depth of waterin the submerged basin,in metersT= thickness ofevaporitic sediments = 520 mrm=density of mantle= 3.3glee

rs=density ofevaporites =(2.3gypSllll + 2.4halitd/2 =2.3 glee

203

DEEP-WATER CARBONATE-EVAPORITES OF A SALINEGIANT, CANADA

Table 2. Fortyeight reiteratesteps to fill a 160m deep, subaerial basin(step 1) with fast brine (step7) and finally 523 m ofevaporite rocks (step48). Assumptions are in the text. Note that we consider the evaporitic minerals as bulk rock, withoutindividual orderof crystallization or stratigraphic position.

Start With A Dry Basin 160 Meters Deep; Fill With Brine; Subside; Reiterate

STEPS 1 2 3 4 5 6ACCOMOD~ SPACE 160 53 18 6 2 1

FILL WIm BRINE 160 53 18 5.9 2 1SUBSIDENCE (BRINE) 53 18 6 2 1 0

FLOOR=WATERDEPm 213 231 237 239 240 240

160 =depth ofair-filled basincatastrophically floodedwith brine ofS.G. 1.1

Precipitate Evaporites; Subside; Fill With Brine; Subside; ReiterateTo A Total Of520 m ofGypsum + Halite = 400 m Anhydrite + Halite

STEP 7 8 9 10 11 12 13 14 15 16 17 18 19SUBSIDENCE 87 29 10 3 1 0 48 16 5 2 1

FILL WITH EVAPORITE 240 131FILL WIlB BRINE 87 29 10 3 1 0 48 16 5 2 1

FLOORDEPTB 240 327 356 366 369 370 371 371 418 434 439 441 442TOTAL EVAPORITES 240 371

TOTAL BRINE 0 87 116 126 129 130 131 0 48 63 69 70 71

brine/mantle = 1.1/3.3

evap/mantle =(2.3-1.1 )/3.3

240 m.= depth ofbrine-filled basinafter subsidencedue to weight ofwater (step7)

precipitationof 240 m of evaporite causesfurthersubsidenceof240 x (2.3-1.1 )13.3 = 87 m. (step 8)

87 m is flooded causing furthersubsidenceof87 x 1.1/3.3= 29 m (step 9) reiterating 'till no further subsidencedue to weight of brine (step 13); then fill water depthwith evaporitecausingfurthersubmergence (step 15)

20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 370 26 9 3 1 0 14 5 21 1 0 8 3 1 0

71 39 21 I0 26 9 3 1 0 14 5 2 1 0 8 3 1 0

442 442 468 476 479 480 480 480 494 499 501 501 501 501 509 512 512 513442 480 501

71 0 26 35 37 38 39 0 14 19 20 21 21 0 8 10 11 11

38 39 40 41 42 43 44 45 46 47 484 1 0 2 1 0 1 0

11 6 34 1 . 0 2 1 0 1 0

513 517 518 519 519 521 522 522 522 523 523513 519 522

0 4 5 6 0 2 3 3 0 1 2

204

SCHENK, VON BITTER, ANDMATSUMOTO

rw =density of brine =1.1glcc

D = r. rm ~ rS ] X T= [3.3 ~ 2.3 ] x520 =240Lr -r 3.3-1.1

m w

[rm - r, ] [ 3.3 - 1.1 ] x240 =160

R= r xD= 3.3m

Thus 400 m of evaporite(now half anhydrite,halfchloride) could be held in a basin with an initial, minimalwater depth of 240 m. Before submergence the floor of thedesertbasin wasat least 160m beneath the flat , surroundinglandscape. Becausethe water was initiallymarine, this floormust havebeen at least 160 in beneath sea level. Accommo­dationspacemorethan tripledaftercombined initial submer­gence (150 percent) and subsequent rock fill (over 200 per­cent). These estimates do not considereither the basal car­bonateor the locallythick, coevalsiliciclastic sediments. Be­cause the surrounding landscape was not flat but had suffi­cientrelieftofeed debristhroughout deposition oftheWmdsor/Codroygroups,original topographic reliefwasprobably con­siderablymorethan 200 m.

Ifweviolateassumption5, table2 suggests that pre­cipitationof gypswn could have startedin a maximumwaterdepth of 240 m (step 6) and ended shortly before step 14.Precipitation of halitewould then start near a maximumwa­ter depth of 70 m (step 20) and end when the basin was fulland in isostatic equilibriwn (step 48). The table shows sim­plified,unnaturalcycles, each consisting ofstep-wise loadingfirst by increments of brine and then by undifferentiated,evaporitic rock.Thus waterdepthsaremaximabutat leastforhalite, are well within a general maximum of near 150 m(Tucker and Cann, 1986;Sonnenfeld, 1991).

Localbase level (the elevation of the water surfaceofLochMacumber) wouldcontrolsubaerial erosion. Thismusthave changed radically during time because of: 1. seasonalchanges in evaporation of, and run-in into restricted waterbodiesin an at leastseasonally arid, tropicalsetting; 2. tortu­ous, fault-controlled connections both between subbasins ofLoch Macwnberand the adjacent sea; 3. probable step-like,faultadjustments to isostatic loading,coupledwithearly tim­ing of maximwn adjustments.

Analogues

The Great Basin of the southwestern United Statesis a closeanaloguetothe MaritimesBasin.It isa blockfaultedterrain approximately 500 x 1()3 km2 in area, centeredin Ne­vadabutextending intoadjacent states (Morrison, 1991). DeathValley, a dextrally faulted, strike-slip basin is one of its mostspectacular components (Shelton,1966). Its subaerial, desertfloor nowreaches86 m belowsea level; escarpment marginstowerover 1600 m abovethe floor; valley width is approxi­mately8 km and length is over 150Ian. Alluvial fans fringeborder faults; playasand dry lake bottoms occupy its central

205

areas. Some three Ian of sediment underlie the floor (Huntand Mabet, 1966).Flooding during the Early or Late Pleis­tocene time createda lake at least 180 m-deep, Lake Manly.This event also produced another lake approximately 300 mdeep in adjacentPanamint Valley (Smith and Street-Perrott,1983). The Colorado Riverconnected thesesalinelakestothesea.To the north, ancientLake Lahontan occupied a collec­tion of flooded grabens and half-grabens that are attractiveanalogues to strike-slip segments of Loch Macumber.

Lake Lahontan was the second largestpluvial lakein the westernhemisphere. It inundatedalmost22 x 1()3 km2

of the Basin and Range Province west of Lake Bonneville(Russell, 1885). The flooded area was a complex system ofnarrow intermontanevalleys withinterveninghorstsforminglongpeninsulasand islands. The tectonic settingwas similartoLochMacumber withextensivemovementalongsteepfaultsto create grabens and horsts and active vulcanism. Modemremnantsincludesalinelakessuchas PyramidLake,Nevada.Itsmaximumwaterdepthis now 102m (Galatand Jacobsen,1985) and 13,000yearsago was 276 m (Benson and Thomp­son, 1987). Today, "whitings" precipitate lime mud to theprofundal zone. Such events correlate with several factors:blooms of cyanobacteria; high temperatures of surface wa­ters; and influx of calcium-charged waters from geothermalspringsand streams(GalatandJacobsen, 1985).Asteamgey­ser and several hot springs are now active in the lake.Biocementstone tufa mounds up to 100m high tower overthesesubaqueous geothermal springs. Their shapes, structures,and textures are similar to thoseof Loch Macwnber (Schenkand others, 1993). Moundsand tufacrustssimilar to thoseofLochMacumber alsooccurto the southwest at MonoLake. Itoccupies a half-graben and is now40 m deep but in the LatePleistocene exceeded 300 m (in Smith and Street-Perrott,1983). Metal-rich, stagnantbottom waters are possible epi­sodic, ephemeral ore-forming solutions (Maest and others,1987). Profundal sediments include finely laminated, prob­ably varved, cyanobacterial sapropel (Newton and Stines,1991). Other strike-slip basin analogues include LakeTanganyika, the Dead Sea, and the Rhine Graben (Schenkand others, in press).

CONCLUSIONS

Loch Macwnber was deep enough that a stratifiedwaterbodycouldestablish itself. Slopes weresufficiently steepand extensive as to allow turbidite deposition. The loch waslargeenoughto allowundisturbed, quietwater,pelagicdepo­sitionofooze. Thebasincenterhad laminarorganic-rich bedsofprecipitated carbonateassociated with mass flows and tur­bidites. Evaporites replaced the volume of water to fill thetopographic depression and end the life of Loch Macumber.

"Scientific theory predicts and is proven wrong ifthe prediction is not verified (Hsu, 1988)". He applied thisstatementto the deep-basin/deep-water hypothesis and con­cluded that the model "is worthless because it does not ex-

DEEP-WATER CARBONATE-EVAPORITES OF A SALINEGIANT, CANADA

DIX, G. R, AND JAMES, N. :P., 1987, Late Mississippian

206

plain factsother than those that led to formulation of thehy­pothesis".

Schmalz'shypothesispredicts thelithologies, stratig­raphy, and interpreted depositional environments of LochMacumber.

ACKNOWLEDGEMENTS

R E. Renaut and W. M Last introduced P. S. tomodernlacustrinesedimentation. M. S. Newton and S. Stineencouraged and aided his trip to saline lakes of NevadaandCalifornia. We thank C. Beaumontfor confirming calcula­tions on isostaticadjustments, M. Gibling for improvinganearly draft of the manuscript, and p.. Sonnenfeld and G. MFriedmanfor helpful suggestions in review. P.. S. Giles,R C.Boehner, and D. B. Clarke aidedwith material.The NaturalSciences and Engineering Research Councilof Canada andthe Development Fund of Dalhousie University funded thisstudyof Loch Macumber.

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Received: March 4, 1994Accepted: May24, 1994

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