evaporitic subtidal stromatolites …web.gps.caltech.edu/~grotz/ewexternalfiles/pope et...

JOURNAL OF SEDIMENTARY RESEARCH, VOL. 70, NO. 5, SEPTEMBER, 2000, P. 1139–1151Copyright ! 2000, SEPM (Society for Sedimentary Geology) 1073-130X/00/070-1139/$03.00

EVAPORITIC SUBTIDAL STROMATOLITES PRODUCED BY IN SITU PRECIPITATION: TEXTURES,FACIES ASSOCIATIONS, AND TEMPORAL SIGNIFICANCE

MICHAEL C. POPE1*, JOHN P. GROTZINGER1, AND B. CHARLOTTE SCHREIBER21 Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, U.S.A.

2 Department of Geology, 130 Rankin Science Building, Appalachian State University, Boone, North Carolina 28760, U.S.A.

ABSTRACT: The transition between carbonate platforms or isolatedcarbonate buildups and overlying evaporites commonly is marked byassemblages of stromatolites and interlaminated carbonates and evap-orites. Stromatolites display lamination textures that vary from peloi-dal and discontinuous on a scale of a millimeter to a few centimeters,to isopachous and continuously laminated on a scale of a centimeterto a few meters. The isopachous lamination texture may be composedof either: (1) micritic or radial-fibrous calcite, or (2) dolomite. Iso-pachous stromatolitic laminae are remarkably uniform, varying littlein thickness over a given lateral distance, in contrast to stromatolitesformed of peloidal laminae, which show marked variation in thicknessover an equivalent lateral distance. These isopachous textures are un-common on most open-marine carbonate platforms and apparently de-veloped in transitional carbonate-to-evaporite settings because of in-creasing temperature, salinity, and anoxia related to water stratifica-tion, which would have created ecologic restriction and an opportunityfor stromatolite growth. Stromatolites with isopachous lamination arehere interpreted to have formed as a result of in situ precipitation ofsea-floor-encrusting calcite and possibly dolomite, whereas the stro-matolites composed of peloidal, discontinuous lamination are inferredto have formed by trapping and binding of loose carbonate sedimentin microbial mats. While the presence of microbes in almost all near-surface environments nullifies use of the term ‘‘abiotic’’ to describemost precipitated minerals, we interpret growth of the isopachous stro-matolites to have been dominated by chemogenic precipitation in theabsence of microbial mats, and the growth of peloidal stromatolites tohave been controlled by sedimentation in the presence of microbialmats.These transitional stromatolite facies are best developed atop Pro-

terozoic and Paleozoic carbonate platforms that underlie major evap-orite successions. However, inspection of Jurassic and younger evap-orite basins, such as the Messinian of the Mediterranean region, showsthat stromatolites with thin, isopachous lamination and radial-fibroustextures, though present, are rare. Instead, these facies may have beenreplaced by stromatolites with peloidal, clastic textures and by low-diversity diatomaceous and coccolith mudstones. Accumulation of themudstones would have imposed two important effects: (1) Productionof coccoliths would have helped extract calcium carbonate from sea-water, thus lowering the growth potential for precipitation of sea-floor-encrusting stromatolites. (2) Settling of both coccoliths and diatomswould have created a sediment flux to the sea floor, which would haveserved to impede growth of precipitated stromatolites because ofsmothering of growing crystals.

INTRODUCTION

The transition between open-marine carbonate platforms or isolated car-bonate buildups and overlying or interfingering evaporites represents adrastic chemical shift in depositional conditions across a basin. Althoughconditions leading to evaporite precipitation can be generated locally in thetidal flats and lagoons of semiarid to arid settings where circulation is

* Present address: Department of Geology, Washington State University, Pullman,Washington 99164, U.S.A.; [email protected]

restricted, volumetrically large evaporite deposits are associated with theisolation or partial isolation of entire sedimentary basins from the world’soceans. As a general pattern, and as a result of increasing salinities, car-bonates formed of open-marine faunas and floras are replaced by increas-ingly restricted facies, which culminate in the deposition of calcium sulfateand halite evaporites (Fig. 1). Commonly this transition zone is relativelysharp and is defined by an unusual stromatolite facies in shallow subtidalwaters and organic-rich, interstratified laminated carbonate and evaporitein deeper subtidal waters. These stromatolites occur at the tops of manycarbonate platforms or buildups immediately preceding evaporites (Fig. 1).The lowermost stromatolites have irregular lamination that downlaps andpinches out at the margins of domes. Laminae may show internal peloidaltextures and possible relict filament molds, and clastic grains and peloidalmuds commonly fill depressions between zones. The lower stromatolitesare overlain by a second generation of stromatolites that are unusual in thattheir lamination has several remarkable properties including fine-scale(commonly !! 1 mm), isopachous geometry (thickness constant as mea-sured normal to layering), extreme lateral continuity, and high degree ofuniformity (internal texture does not vary significantly). These propertiesgive rise to stromatolite forms that display extremely high degrees of ‘‘in-heritance’’ in which stromatolite geometry changes little between succes-sive laminae. In addition, these isopachously laminated stromatolites differfrom most other stromatolites in that they commonly lack evidence forinfilling of topographic depressions with clastic carbonate sediments, in-cluding stromatolite fragments, peloids, or other detrital grains—even mi-critic fills are relatively uncommon. Desiccation features and microbialcomponents such as filament molds or casts are not present.As discussed recently in the literature, many stromatolites are likely to

have formed in response to in situ precipitation of calcite and or aragoniteas crusts on the sea floor (Grotzinger and Read 1983; Hofmann and Jackson1987; Sami and James 1996; Kah and Knoll 1996; Sumner 1997; Bartleyet al. in press). However, most of the examples that are cited in thesestudies are Mesoproterozoic and older, formed during a time in earth his-tory when sea-floor precipitation may have been widespread in unrestrictedmarine environments (Grotzinger and Kasting 1993; Sumner and Grotzin-ger 1996). Stromatolites with chemically precipitated textures declined dur-ing Mesoproterozoic time (Grotzinger 1990; Bartley et al. in press) andgenerally are absent in younger rocks, with notable exceptions that maycorrespond to major changes in the local and possibly global compositionof seawater (Grotzinger and Knoll 1995). Consequently, the appearance ofunusual, chemically precipitated stromatolites at times of environmentalcrisis may be analogous to the resurgence of stromatolites as disaster formsfollowing episodes of mass extinction (Schubert and Bottjer 1992). Indeed,in some cases the two effects may exhibit a cause-and-effect relationship,with environmental crisis leading to mass extinction (e.g., Knoll et al.1996). However, it is also possible that widespread subtidal stromatolitefacies may develop as a result of environmental stress on a basinwide scalethat is not coincident with global mass extinction.Whether or not the chemically precipitated stromatolites are related to

extinctions, it is likely that stromatolite texture may be an important guideto the record of environmental change, thus motivating the present study.The goal of this paper is to document the stratigraphic setting and texturesassociated with stromatolites that immediately predate deposition of volu-metrically significant evaporites, for several different basins ranging in age

1140 M.C. POPE ET AL.

FIG. 1.—Generalized schematic of carbonate-to-evaporite transition emphasizing thestratigraphic position of subtidal precipitatedcarbonate fabrics forming just prior to the onsetof calcium sulfate and/or halite evaporitedeposition.

from Paleoproterozoic through Miocene, in an attempt to identify the mech-anisms of stromatolite accretion and the significance of their textural andmorphologic variability. Furthermore, given their stratigraphic setting,composition, and texture, these stromatolitic facies are susceptible to dia-genetic dissolution and the creation of secondary porosity to form reservoirrocks for hydrocarbons.

NOMENCLATURE

We adopt the nongenetic definition of stromatolites recommended bySemikhatov et al. (1979): a stromatolite is ‘‘an attached, laminated, lithifiedsedimentary growth structure, accretionary away from a point or limitedsurface of initiation’’. This definition provides a concise statement of thebasic geometric and textural properties of all stromatolites, while at thesame time allowing for multiple or even indeterminate origins. Acceptingthis as a general definition, it then becomes possible to evaluate objectivelythe various processes that may influence stromatolite development, on acase-by-case basis (Grotzinger and Knoll 1999).Another important issue concerns the degree to which stromatolites are

laminated. That stromatolites be laminated is inherent in the definition;exactly how well laminated they are is another matter. This parameter iscritical for the current paper but unfortunately has not been standardized;consequently, an illustrated distinction is made here for the purpose of thedescriptions that follow. At one end of the spectrum are thrombolites,which differ from stromatolites in having clotted rather than laminatedtextures (Fig. 2A). Kennard and James (1986) have established that stro-matolites and thrombolites may be intergradational. Stromatolites gradefrom relatively crudely laminated, such as the modern stromatolites foundin Shark Bay (Fig. 2B), through relatively well laminated, such as in typicalProterozoic stromatolites (Fig. 2C), to very finely and isopachously lami-nated, such as the stromatolites discussed in this paper (Fig. 2D).

CARBONATE-TO-EVAPORITE TRANSITIONSExamples of carbonate platforms and reefs overlain by evaporites that

contain isopachous, thinly laminated stromatolites, fibrous marine cements,and interlaminated carbonates and evaporites are discussed below, and theircharacteristics are listed in Table 1.

Paleoproterozoic Athapuscow Basin, Northwest Territories, Canada

The Pethei Group was deposited on a gently dipping carbonate platformin the Athapuscow Basin that developed during convergence between theSlave Craton and Taltson–Thelon Orogen (Hoffman 1968, 1969, 1981;Sami and James 1993, 1994). The uppermost 10 m of inner-platform facies(Fig. 3) of the upper part of the Pethei Group (Hearne Formation) is com-posed of three distinctive facies, in ascending order: (1) dendriticallybranching tufa; (2) irregularly laminated stromatolites displaying flat todomal morphologies; and (3) isopachous, evenly laminated stromatolites.More detailed descriptions and interpretations of these facies are presentedin Pope and Grotzinger (in press). The uppermost facies immediately un-derlying evaporite collapse breccias is discussed below.Dolomitic stromatolites (" 3 m thick) with even, isopachous, and very

thin laminae form the uppermost bed of the Hearne and are sharply overlainby evaporite collapse breccias of the Stark Formation. Exhumed beddingplanes yield three-dimensional exposures of these stromatolites, whichshow large, smooth composite domes with up to 40 cm of synoptic reliefbetween the tops of the domes and their intervening troughs. Individualdomes (Fig. 4A) are asymmetric, with a steep nearly vertical side and amore gently sloping side. Superimposed upon the more gently sloping sideare irregular smaller-scale bumps and ridges, which similarly are expressedby even, uniform laminae with isopachous geometry. Individual laminae(0.4 to 1 mm thick) are isopachous (Fig. 4B) and composed predominantlyof dolomicrite (crystal size ! 10 #m diameter). Internally, laminae mayshow grading from fine dolosparite into dolomicrite (Fig. 4C). The dolos-parite layers commonly contain interstitial secondary silica. Stylolites lo-cally distort the original regular shape of laminae. Stable-isotope values ofthese isopachous stromatolites are relatively heavy (" $4‰ %18OPDB;&3‰ %13CPDB) compared to the rest of the Pethei Group ($9 to $6‰%18OPDB and 0 to &2‰ %13CPDB; Hotinski and Kump 1997; Hotinski per-sonal communication 1997; Whittaker et al. 1998).These unique facies in the upper part of the Pethei Group are interpreted

to reflect deposition in a setting that became increasingly hypersaline, cul-minating with in situ chemical precipitation of finely crystalline carbonatedirectly from seawater to form stromatolites (Sami and James 1996; Popeand Grotzinger in press). The lack of subaerial exposure surfaces, mud-

1141EVAPORITIC SUBTIDAL STROMATOLITES PRODUCED BY IN SITU PRECIPITATION

FIG. 2.—Slabs illustrating the spectrum of stromatolite lamination textures utilized in this paper. A) Nonlaminated thrombolitic fabric, Upper Cambrian Petit JardinFormation, Nova Scotia. Scale bar is 2 cm. B) Modern stromatolite with crude lamination, Shark Bay, Western Australia. Top of penknife in lower right corner is 2 cmlong. C) Discontinuous well-laminated stromatolite, Paleoproterozoic Rocknest Formation. Scale bar is " 3 cm. D) Very thinly and isopachously laminated stromatolite,Neoarchean Malmani Formation, Transvaal South Africa. Scale bar is approximately 2 cm.

cracks, flat-pebble conglomerate, troughs filled with clastic carbonate, andassociated intertidal facies indicate that deposition occurred subaqueously.The steep inclination of isopachous, evenly laminated stromatolites sug-gests that deposition during this unit was shallow enough to be influencedby wave-generated or wind-generated currents, but not so much so as toproduce either elongation or fragmentation of stromatolites. Increasinglyheavy stable isotopes in the uppermost Pethei Group indicate onset of evap-oritic conditions immediately prior to deposition of the Stark Formation.The overlying Stark Formation is interpreted to represent an evaporite

collapse breccia, judging by its chaotic bedding, in situ brecciation, brecciageometry, local derivation of clasts, and abundance of evaporite molds andcasts (Hoffman et al. 1977; Badham and Stanworth 1977; Stanworth andBadham, 1984; Pope and Grotzinger 1997). Large blocks, up to 1.5 kmlong and 40 m thick, with a preserved stratigraphy of interbedded wave-rippled carbonates and siliciclastics, and red shale containing evaporitemolds, indicate that the Stark was deposited in a restricted shallow, subtidalenvironment with only sporadic subaerial exposure. Chaotic, brecciatedbedding resulted from dissolution of evaporites and foundering of overlyingsediments. Silicified pipes with halite pseudomorphs at the ends of manylarge blocks may be evidence of salt or brine diapirism during collapse (cf.Badham and Stanworth 1977). The large size of clasts floating in brecciatedmatrix and platform geometry suggests that the original salt thickness wasa few tens of meters to possibly a few hundreds of meters. The uniqueinner-platform carbonates are subtidal and interpreted to have formed dur-ing deposition of a Transgressive System Tract (TST) whereas evaporites

of the overlying Stark Formation were deposited during the late TST andsubsequent Highstand System Tract (HST) (Pope and Grotzinger in press).

Terminal Proterozoic–Early Cambrian Huqf Supergroup, Oman

Terminal Proterozoic–Cambrian sediments of the Huqf Supergroup inOman consist of interbedded clastics, carbonates, and evaporites (Gorin etal. 1982; Wright et al. 1990). These rocks were deposited in a structurallycomplex rift (?) setting of alternating basement highs and intervening lows(Gorin et al. 1982; Mattes and Conway Morris 1990) probably producedby contemporaneous wrench faulting related to displacement along theNajd fault system (Husseini and Husseini 1990). Deposition occurred instratified basins with anoxic bottom waters (Mattes and Conway Morris1990; Amthor et al. 1997).In the subsurface, platform carbonates of the Buah Formation (up to 600

m thick, upper Huqf Supergroup) are overlain by evaporites of the AraGroup, which contain carbonate ‘‘stringers’’ (Gorin et al. 1982), the lowerone of which is thickest (a few hundred meters) and is designated as theBirba Formation. It is currently unclear if the Birba is a separate carbonateplatform, originally enclosed in evaporites, or if it is a stratigraphic (down-dip?) equivalent to the upper Buah. In outcrop, the upper Buah comprisesstratiform, linked domal and columnar stromatolites that contain desicca-tion cracks and teepee structures, which are indicative of supratidal depo-sition (Gorin et al. 1982; Wright et al. 1990). The Birba Formation containsa variety of stromatolitic and thrombolitic facies, as well as thinly laminated


TABLE 1.—Carbonate-To-Evaporite Transitions with Unique Carbonate Fabrics.

Location, Age Units Carbonate Fabric(s)EvaporiteComposition References

Great Slave Lake, CanadaPaleoproterozoic (1.9–1.8Ga)

Pethei Grp (c)Stark Fm. (e)

Tufa, isopachously laminated stromatolite at con-tact in shallow water; Fibrous marine cementsthroughout platform

Halite '' gypsum (no an-hydrite)

Badham and Stanworth, 1977; Hoff-man et al., 1977; Stanworth andBadham, 1984; Sami and James,1996; Pope and Grotzinger inpress

Oman, Vendian (570–543Ma)

Buah Fm. (c) AraFm. (e)

Fibrous marine cements throughout platform andisolated thrombolitic bioherms; Isopachouslylaminated stromatolites at contact; tufa-likecrusts in carbonates within evaporites

Halite '' gypsum Mattes and Conway-Morris, 1990;Al-Majerby and Nash, 1986; J.Amathor, personal communication,1998

Michigan Basin, Silurian("400 Ma)

Guelph Fm. (c)Salina Fm. (e)

Fibrous marine cements within pinnacle reefs; Iso-pachously laminated stromatolites (travertine-like coatings) cap reefs; Calcite laminites in in-terpinnacle reef areas

Halite '' anhydrite, potashsalts

Petta, 1980; Sarg, 1986; Huh et al.,1977

Western North America;Middle Devonian ("385Ma)

Winnepegosis (c) Stromatolites cap reefs, fibrous marine cements inreefs and carbonate-evaporite laminites betweenreefs in deeper-water settings

Halite '' anhydrite Davies and Ludlum, 1973; Kendalland Harwood, 1991; Campbell,1992

Sverdrup Basin, Arctic Can-ada

Nansen (c)Otto Fiord (e)

Carbonate-evaporite laminites in basinal setting,fibrous marine cements throughout shelf-marginreefs and buildups

Halite ( anhydrite Davies and Nassichuk, 1980

Zechstein Basin, England,Late Permian ("260 Ma)

Middle Magnesian Lime-stone (c)

Hartlepool Anhydrite (e)

Fibrous marine cements throughout reef complex;Isopachously laminated stromatolites (CrinklyBeds) in bioherm capping reef complex; Lami-nar coatings within bioherm, neptunian dikesand cavities; Laminites in basinal setting be-tween buildups

Basin wide: Halite '' an-hydrite

Locally: Anhydrite '' ha-lite

Smith, 1980a, 1980b, 1981, 1995

Mediterranean Messinian(Middle Miocene)

Terminal Carbonate Com-plex (c)

Thinly laminated stromatolites at transition fromcarbonate to evaporite; fibrous marine cementswithin reefs underlying stromatolites

Halite '' gypsum Estaban, 1979; 1996; Feldmann andMcKenzie, 1997

Abu Shaar Complex, Egypt;Messinian (Middle Mio-cene)

Ruidais Fm. (c)Kareem Fm. (e)

Fibrous marine cements throughout reef complex;Thinly laminated stromatolites with fibrous ce-ments on toe of slope; Unique pisoids with dis-tinctive fibrous fabric

Anhydrite ' gypsum ' ha-lite

Coniglio et al., 1988

c ( carbonate.e ( evaporite.

limestone and dolostone with intraclastic interbeds composed of fragmentsof laminated carbonate. A marine origin for the carbonates is shown by thepresence of abundant fossils of Cloudina (Mattes and Conway Morris 1990)and the elevated bromine concentrations within enclosing evaporites(Schreiber 1997). In the subsurface, the transition zone between the car-bonate ‘‘stringers’’ and overlying evaporites of the Ara Formation may bemarked by intervals up to a few meters thick of stromatolitic dolostonewith isopachous, very thin laminae (Fig. 5), or sea-floor-encrusting crystalfans of dolomitized aragonite. These facies pass gradationally into over-lying anhydrite facies. The Ara evaporite is composed mainly of halite withvery little gypsum and some anhydrite, which formed from marine watersduring either a sealevel lowstand (Mattes and Conway Morris 1990) or byenhanced evaporation during a highstand (J. Amthor, personal communi-cation 1998); the latter interpretation is supported by the fact that the is-opachously laminated stromatolites shown in Figure 5 overlies a karst sur-face, indicating that the facies belongs to a TST. By analogy to Mioceneand modern evaporites (Schreiber and Hsu 1980) the Huqf evaporites areinterpreted to have formed in a very short period (! 20 to 250 kyr; J.Amthor, personal communication 1998).

Silurian Michigan Basin

Upper Silurian marine carbonates and evaporites were deposited in theintracratonic Michigan Basin. The basin margins are marked by a gentlydipping carbonate platform that developed coral–stromatoporoid biohermsalong the platform margin whereas isolated pinnacle reefs of similar com-position formed seaward of the platform (Fig. 6). The pinnacle reefs andcorresponding shelf shoals grew quickly during a relative sea-level high-stand, keeping pace with any sea-level fluctuations and/or basinal subsi-dence. Though there were high-frequency drawdown events during high-stand development (e.g., Nurmi and Friedman 1977) there was probablyno significant long-term relative sea-level drop until after the evaporiteunits were deposited within the basin (Sarg 1986). Platform-margin bioh-

erms and pinnacle reefs contain abundant fibrous marine cements that filloriginal porosity in early-formed voids and cavities. Additionally, herring-bone calcite cement, an unusual marine precipitate that is prevalent in Ar-chean carbonates, but thereafter occurs only rarely, fills voids and neptuniandikes in the pinnacle reefs (Lehmann 1978; Sumner and Grotzinger 1996).Increasing restriction led to the demise of open-marine organisms, and

conditions became sufficiently restricted so that deposits composed of rarerestricted-marine fauna and irregularly laminated micritic stromatolites cov-er the tops of the reefs and the outer platform (Huh et al. 1977; Briggs etal. 1980; Petta 1980; Sarg 1986). Irregularly laminated stromatolites areexpressed as couplets of micrite laminae (0.05 mm thick) separated by 0.1to 1 mm thick dolospar (Gill 1985) that form simple, laterally linked hem-ispheroids (Logan et al. 1964), with the laminae pinching out at marginsof each structure (Fig. 7A). Irregularly laminated stromatolites are overlainby stromatolites with isopachous fine lamination (0.4–1.0 mm thick), com-monly with radial fibrous texture (Fig. 7B). Laminae are defined by darkermicritic inclusions. The isopachously laminated stromatolites generate com-plex morphologies that contrast with the simple domal forms of the un-derlying micritic stromatolites; because of their remarkable degree of in-heritance, small perturbations can be propagated outward for many laminaebefore their relief is damped (Fig. 7A, B). The contact between the lowerreef horizons and overlying stromatolites is sharp, as is the contact betweenthese stromatolites and overlying evaporites (Sarg 1986).The isopachously laminated stromatolites developed during or immedi-

ately prior to the deposition of evaporites in surrounding basins (Sarg 1986;Huh et al. 1977; Petta 1980). The lack of mudcracks, subaerial exposurefeatures or vadose features in both types of stromatolites capping the Si-lurian bioherms indicates that they formed in a subtidal setting and mayhave formed in restricted anoxic waters (Gill 1985; Sarg 1986). However,erosion of the thinly laminated stromatolites capping the Silurian pinnaclereefs may indicate they were subaerially exposed before or during depo-sition of later basinal evaporites (Huh et al. 1977) or that subtidal erosionoccurred (Sarg 1986).


FIG. 3.—Regional cross section of the Paleoproterozoic Pethei carbonate platform and overlying Stark Formation evaporite-collapse breccia. Isopachously laminatedstromatolites described here occur in the uppermost Pethei (gray shading) immediately preceding the evaporites.

In somewhat deeper water, between the pinnacle reefs, extremely uni-form laminites (100 to 1000 mm thick) make up the coeval basin deposits.These laminites are composed of couplets of micritic carbonate and organicmatter and grade up into calcite–anhydrite laminites, then anhydrite, andfinally halite (A1) of the Salina Group (Briggs et al. 1980). The calcitecrystals in the laminae are very fine (" 10 #m) but coarsen in evaporiteunits (Briggs et al. 1980). Carbonate laminites occurring within the evap-orites may record short-lived marine influxes, which led to decreases insalinity in the evaporitic basin (Briggs et al. 1980).The subtidal stromatolites capping the pinnacle reefs are interpreted to

have formed coevally with the basinal laminites, and thus form a TST orHST as depositional conditions became restricted. The overlying A1 evap-orites formed during the late HST. This interpretation contrasts with thatof Briggs et al. (1980), who interpreted the basinal laminites to be a LST.The key difference is that the interpretation here recognizes that evaporitedeposition may occur despite rising base level.

Permian Zechstein Basin

Upper Permian rocks of northeast England, northern continental Europe,and parts of Greenland are dominated by carbonates and evaporites thatfilled the Zechstein Basin (Peryt 1987; Smith 1980a, 1980b, 1995; Perytand Kovalevich 1997). In northeast England carbonate platform facies (Fig.8) of the Middle Magnesian Limestone (Ford Formation) consist of well-developed bryozoan–marine cement reefs (' 100 m thick) that pass land-ward into oolitic grainstone and packstone, and basinward into deeper-water carbonate rudstones with talus blocks up to 5 m across (Smith

1980a). The reef is capped by a biostrome (Hesleden Dene) of stratiformand domal stromatolites at least 28 m thick (Smith 1980a, 1995). TheHesleden Dene stromatolite biostrome is overlain by 20 m of oolites, whichare capped in the subsurface by interbedded halite and anhydrite.Stromatolites in the lower part of the reef-capping biostrome (‘‘Crinkly

Bed’’) are very thinly and evenly laminated (Fig. 9), with isopachous ge-ometry, and are continuous across the outcrop for over 5 km (Smith 1981;Kitson 1982). The laminae may show gentle doming with relief up to 1.3m (Smith and Francis 1967; Kitson 1982) and resultant domes are oval inplan view (Fig. 9) with the longer axes aligned NW–SE (Smith 1981; 1995,his figure 3.49). Stromatolites in the bioherm commonly occur in bedsdipping 30–65) (Smith 1980a, 1980b). Other associated facies include do-lomitic fibrous marine cements after a Mg-calcite or aragonite precursorand irregular but very thinly laminated pisoliths in the basal part of thebiostrome. The pisolith facies is interpreted to have formed from inorganicprocesses resulting in travertine-like textures (Smith 1995). Many lowerand middle Zechstein reef complexes in Poland and Germany also aredominated by bryozoans capped by stromatolites with many of the featuresdiscussed here (cf. Peryt and Piatkowski 1977; Paul 1980).The thin, isopachously laminated stromatolites, abundant marine ce-

ments, and unique pisoliths formed in response to increased salinity duringdeposition (Smith 1995). Thinly laminated stromatolites of the HesledenDene biostrome developed subtidally on a subaerially exposed unconfor-mity during development of a TST (Smith 1980b). These stromatolites maylocally be partly equivalent with the lower Hartlepool Anhydrite (Mawson,personal communication 1998) and are regionally correlative with calciumsulfate evaporite and carbonate laminites (Tucker 1991).


FIG. 4.—Isopachous, thinly laminated dolomitic stromatolites of uppermost Pethei Group. A) Plan-view cross section of a single dome in outcrop. B) Side-view crosssection of laminae in polished slab. Scale bar is 1 cm. C) Photomicrograph of laminae. Scale bar is " 25 mm. Note upward divergence of peak due to isopachous growthnormal to the depositional surface.

Miocene Mediterranean Region

Gulf of Suez (Middle Miocene).—Platform carbonates and overlyingevaporites of the Gulf of Suez and Red Sea area developed on upliftedbasement blocks formed by rifting (Aissaoui et al. 1986). The Abu Shaarcomplex (Fig. 10) is one of several well-exposed Middle Miocene carbon-ate platforms that characteristically are dolomitized and overlain by anevaporite-collapse breccia (Monty et al. 1987; El-Haddad et al. 1984; Jameset al. 1988; Burchette 1988; Purser 1998; Purser and Plaziat 1998). How-ever, in the subsurface many of these carbonate platforms are encased inevaporite and are productive petroleum reservoirs (Aissaoui et al. 1986).Inner-platform carbonates consist of bioclastic wackestones and pack-

stones with rare stromatolitic beds (Monty et al. 1987) that pass laterallyinto reefs composed of Porites and other massive corals developed alongthe platform margin. Fibrous marine cements after Mg calcite and aragoniteare abundant throughout this carbonate platform (Aissaoui et al. 1986).Stromatolites with simple domal geometries cap the carbonate platform.An unconformity on top of the carbonate platform is correlative downdipwith a thin slope facies (! 3 m thick) consisting of pisoliths with radialfibrous fabrics interbedded and interfingering with laminated stromatolitesand rare ahermatypic corals (El-Haddad et al. 1984; Aissaoui et al. 1986;Monty et al. 1987; James et al. 1988; Burchette 1988; Purser and Plaziat1998). These stromatolites develop on an unconformity and are overlainby brecciated stromatolites, evaporites, or evaporite-collapse breccia (Purs-er and Plaziat 1998).The stromatolites form low, broad domes (1 m high, 10 m diameter)

composed of smaller domes (4–20 cm diameter). Although the stromato-lites are not described in detail, they have isopachous (Fig. 11A), very thin(0.1–0.3 mm), dolomicritic laminae with a fibrous crystalline texture (Fig.11B; Coniglio et al. 1988; Purser and Plaziat 1998). These laminae havebeen interpreted to represent alternating cyanobacteria-rich and cyanobac-teria-poor environmental events (Monty et al. 1987). We suggest, however,that the accretion mechanism may have been one of in situ precipitationrather than trapping and binding; microbes may still have been involved,but their role was probably limited to catalysis of precipitation in that theremarkably smooth lamination and fibrous crystalline fabric shown in Fig-ure 11 is not consistent with the presence of an active mat (cf. Bartley etal. in press). It is not known how abundant these isopachously laminatedstromatolites are relative to stromatolites with other lamination textures onthis platform.The occurrence of surfaces interpreted to be marine hardgrounds and

lack of desiccation features suggest that stromatolites of the Abu Shaarcomplex formed in a subtidal setting. This interpretation is supported bythe observation that at least some of the stromatolites formed in downdiplocations; however, the interpretation of water depths currently is contro-

versial because of structural complications (cf. James et al. 1988; El-Had-dad et al. 1984). These thinly laminated stromatolites formed above anunconformity during a TST.Spain (Late Miocene).—Carbonates and evaporites formed during the

Late Miocene salinity crisis are intimately interbedded in the Mediterraneanregion (Hsu et al. 1977). Commonly, massive Porites reefs indicating openmarine conditions pass upward into an unusual Porites /coralline algal as-semblage and then into stromatolitic facies indicative of increasing restric-tion immediately prior to evaporite precipitation within the basin (Esteban1979; Rouchy and Saint-Martin 1992; Martin and Braga 1994; Feldmannand McKenzie 1997). The Terminal Carbonate Complex (Esteban 1979) isthe last occurrence of marine carbonates in the western Mediterranean andis locally equivalent to, or may have just preceded, precipitation of theupper evaporites in the basin (Esteban 1979; Rouchy and Martin 1992).This carbonate complex contains some Porites patch reefs but, because ofincreasing seawater salinity at the time of deposition, is dominated by sub-tidal to intertidal stromatolitic and thrombolitic facies (Esteban 1979, 1996;Montenat et al. 1987; Rouchy et al. 1986; Rouchy and Martin 1992; Martinand Braga 1994; Feldmann and McKenzie 1997). Stromatolites with con-oform geometry are present in the basal part of the Terminal CarbonateComplex and have been interpreted as foreslope deposits (Feldmann andMackenzie 1997), consistent with older but more widespread occurrencesof conoform stromatolites, which typically occurred in subtidal settings(Grotzinger 1989).The stromatolites commonly are thinly laminated (0.7–1.0 mm thick),

and are composed of alternating dolomicrite and dolomicrospar (Dabrio etal. 1981; Feldmann and McKenzie 1997). The occurrence of very fine-grained, fabric-retentive dolomicrite suggests that dolomite may have pre-cipitated as a primary mineral in the increasingly saline conditions leadingup to the Messinian salinity crisis (Feldmann and McKenzie 1997). Do-lomitic, laminar to domal stromatolites occur within the overlying gypsumbeds or alternating with them (Lonergan and Schreiber 1993). These stro-matolites developed during a TST are bounded by subaerial unconformitiesand are thought to be coeval with evaporite formation in the basin (Esteban1996).Interestingly, laminae that compose stromatolites of the Terminal Car-

bonate Complex do not have the great lateral continuity, isopachous ge-ometry, or radial-fibrous texture commonly observed at the tops of car-bonate platforms underlying other major evaporites. Although thin lami-nation is locally preserved, all stromatolites described or illustrated in theliterature or observed by us (B.C. Schreiber, unpublished data) show dis-continuous laminae, and in some cases preserve peloidal textures (Feld-mann 1995; his figure 4.25). On the basis of lamination geometry andinternal texture these stromatolites are interpreted to result from trapping


FIG. 5.—Core of stromatolitic Birba carbonates contained within evaporites of theterminal Proterozoic Ara Formation, Huqf Supergroup, Oman. Note upward diver-gence of dome due to isopachous growth normal to the depositional surface. Plughole is 3 cm diameter. Published with permission of Petroleum Development Oman.

and binding of clastic carbonate by microbial mats (Feldmann 1995), apoint with which we agree. Therefore, this appears to be a significant de-parture from Precambrian and Paleozoic evaporite basins, where stromat-olites with textures consistent with growth by in situ precipitation occur inaddition to stromatolites with textures consistent with microbial trappingand binding.

DISCUSSION

Stratigraphic Distribution of Stromatolites and Related Facies

The carbonate-to-evaporite transitions discussed in this paper comprisethree distinctive facies that are not common in open marine settings: (1)

stromatolites with isopachous, very thin laminae of uniform thickness, witheither micritic or radial-fibrous internal texture, commonly dolomitized,that formed in shallow subtidal conditions and often are associated withdistinctive pisolith units; (2) carbonate and evaporite laminites depositedin deeper-water settings; and (3) fibrous marine cements formed along themargins of the carbonate platform. These characteristics help constrain theconditions for formation of these unique carbonate fabrics.The stromatolites, fibrous cements, and carbonate–evaporite laminites

evidently formed in highly restricted environments independent of tectonicsetting (e.g., rift, foredeep, intracratonic basin), and form draping strataatop major carbonate platforms just prior to precipitation of thick succes-sions of calcium sulfate and halite evaporites. Although the underlyingplatform carbonates may contain mudcracks, teepee structures, and/or pa-leosols, which indicate deposition in very shallow water with multiple ep-isodes of subaerial exposure, the stromatolites and related facies describedin this paper all formed subtidally with no evidence of subaerial exposure.These stromatolites are interpreted to have formed contemporaneously withassociated deep-water, organic-rich carbonate–evaporite laminites, both ofwhich are overlain by subaqueous evaporites or evaporite-collapse breccias.Stromatolitic facies are characteristically separated from underlying plat-form carbonates by disconformities. This relationship, along with the ap-parent synchroneity of the shallow-water stromatolites and deeper-waterlaminites, suggests these units formed during relative sea-level rise as TSTor early HST deposits.The directly overlying evaporites that blanket the shallow marine plat-

form and pinnacle reefs likely were produced by increased evaporation,restricted circulation, or higher-frequency sea-level falls during the subse-quent HST. This does not imply that all the evaporites in these basinsformed during highstand, because it is highly likely that the thick basin-center evaporites formed during local or global sea-level lowstands (cf.Tucker 1991). We suggest, however, that the facies association of subtidalstromatolites, seafloor-encrusting marine cement, and carbonate-sulfate/ha-lite laminites indicates that evaporite deposition began during relative sea-level rise in most cases and that the facies association comprises a TST.As water chemistry became more restricted during these carbonate-to-

evaporite transitions, chemical processes became dominant over biologicalprocesses. We suggest that this does not just apply to the precipitation anddeposition of calcium sulfate and halite but is applicable to carbonate pre-cipitates as well. In the examples presented here, the stromatolites withisopachous, very thin lamination textures are interpreted as carbonate evap-orites, with the dominant growth process being in situ precipitation of cal-cium carbonate, or possibly primary dolomite. The highly restricted marinesettings in which these stromatolites developed were quite different fromthe open marine waters following certain Phanerozoic mass extinctions inwhich opportunistic microbes formed stromatolites with irregular lamina-tion across equally broad areas (cf. Schubert and Bottjer 1992).

Fibrous Marine Cements

Fibrous calcite and/or aragonite cements, which commonly are dolomi-tized, occur in all the examples cited above. In the Pethei Group they occuras precipitates developed directly on the seafloor (Sami and James 1996),whereas they fill pore spaces and voids in the remaining examples. In manyof these cases these cements were dolomitized early and preserve fine pet-rographic textures. Commonly these cements constitute a large part of plat-form margins or isolated reefs, suggesting that these units are cementstones.These radial fibrous cements are morphologically similar to fibrous calciteand aragonite cements that are interpreted to have formed in warm, CaCO3-saturated marine environments (e.g., Morse and He 1993; Wilson and Dick-son 1996). Thus, the fibrous marine cements in these pre-evaporitic car-bonates further substantiate the warm, oversaturated nature of the precipi-tating fluid.


FIG. 6.—Regional cross section of Middle toUpper Silurian rocks on northwestern edge ofthe Michigan Basin (adapted from Nurmi 1978).Precipitated stromatolites occur at the tops ofpinnacle reefs and along the platform (bank)margin.

Carbonate–Evaporite Laminites

Thinly laminated carbonate and evaporite developed in deeper water dur-ing most of the transitions outlined above. The laminites consist of organic-rich layers interlaminated with carbonate, overlain by interlaminated car-bonate and evaporites and eventually evaporites alone. The laminites canbe correlated for many tens to hundreds of kilometers laterally (Dean et al.1975; Davies and Ludlam 1973). In many basins these laminites formedduring a sea-level rise immediately preceding basin restriction and evapo-rite precipitation (e.g., western Canadian Basin, Campbell 1992). Many ofthese laminites, though in many cases less than 10 m thick, commonly areeconomically important because they are the source rocks for many car-bonate platforms or reefs surrounded by evaporites (cf. Middle and LateDevonian, western Canada basin; Michigan Basin, Zechstein, Sverdrup Ba-sin, Arctic Canada).Though many of the laminites were originally interpreted to be stro-

matolites formed in sabkhas, they are now known to have formed subtidallyin a euxinic setting (Davies and Ludlam 1973). Widespread lateral corre-lation of the laminites suggests that the carbonate mud in this unit formedby in situ precipitation (Davies and Ludlum 1973). If the thinly laminatedstromatolites developed synchronously with the laminites, then precipita-tion of carbonate mud was occurring throughout these basins immediatelyprior to evaporite precipitation.

Stromatolite Texture: In Situ Precipitation Versus Trapping andBinding

Studies of the lamination textures in ancient stromatolites provide evi-dence for growth of stromatolites through accretion of loose sediment (mi-crite, grains) and in situ mineral precipitation. Although much evidence hasbeen supplied for the involvement of loose sediment in forming lamination(summarized in Semikhatov et al 1979), it has become increasingly clearover the last decade that in situ mineral precipitation is indeed an importantaccretion mechanism in ancient stromatolites (Grotzinger and Read 1983;Grotzinger 1986; Hofmann and Jackson 1987; Kah and Knoll 1996; Knolland Semikhatov in press; Bartley et al. in press; Pope and Grotzinger inpress). Stromatolite laminae that form by in situ precipitation require both

an increase in the calcium carbonate saturation of seawater and a decreasein the flux of loose, clastic carbonate sediment to the site of deposition(Grotzinger 1990; Grotzinger and Knoll 1999). In some remarkably wellpreserved stromatolites of late Archean age it can be observed that the onlycomponents that constitute the stromatolite were microbial mats, early ma-rine cement, and later porosity-occluding burial cement; sedimentary par-ticles are completely absent (Sumner 1997). Stromatolites with very thinand/or isopachous lamination are regarded to have formed hard, synsedi-mentary crusts directly on the sea floor (Grotzinger and Read 1983; Hof-mann and Jackson 1987; Grotzinger and Knoll 1995; Sami and James 1996;Kah and Knoll 1996; Grotzinger and Rothman 1996; Sumner 1997; Bartleyet al. in press; Pope and Grotzinger in press). Although bacteria may playa role in catalyzing mineral precipitation (Buczynski and Chafetz 1991;Vasconcelos et al. 1995), it is clear in several cases that mineral precipi-tation did not template microbial mats and so the texture and morphologyof these thinly laminated stromatolites is considered to be the result ofchemical processes dominating over biological processes (Hofmann andJackson 1987; Bartley et al. in press; Pope and Grotzinger in press; Grot-zinger and Knoll 1999).The distribution of stromatolites with isopachous lamina textures and

self-replicating morphologies indicative of in situ precipitation is time-de-pendent; stromatolites with precipitated textures are common in Archeanand Paleoproterozoic carbonates, declined through the Mesoproterozoic,and are rare to absent in the Neoproterozoic and Phanerozoic (Grotzinger1989; 1990; Grotzinger and Knoll 1995, 1999). Consequently, the recur-rence of these sorts of stromatolites in Phanerozoic carbonates is significant,and may thus provide clues to changes in processes and environments (cf.Grotzinger and Knoll 1995). In terms of the examples discussed here, it isintriguing to note that the Paleozoic carbonates apparently contain a higherproportion of precipitated stromatolites than younger occurrences. For ex-ample, the Middle Miocene of Egypt seems to contain only patchy devel-opment of precipitated stromatolites, and none at all are known from theUpper Miocene carbonates of Spain and elsewhere. Although our data baseof younger examples is limited, it is worth pointing out that this changemay coincide with the first appearance of Jurassic calcareous phytoplank-ton. Marine stromatolites of post-Triassic age and younger may only rarely


FIG. 7.—Pinnacle-encrusting dolomitic stromatolites immediately beneath evaporites, Michigan Basin. A) Photo of isopachously laminated stromatolite atop a pinnaclereef. Note upward divergence of domes due to isopachous growth normal to the depositional surface. Pan-Am Well 1–21, Depth ( 6695* (2041 m); Scale bar is 2.5 cm.B) Photo of core along the platform margin (Shell Cross 1–28) showing stromatolites formed of alternating, even laminae (black arrow) and more irregular laminae (whitearrows). Scale bar is " 2.5 cm. Note how irregular laminae exhibit greater discontinuity as well as thin microlenses that infill depressions and onlap topography (layersabove white arrows).

show evidence of in situ precipitation for at least two reasons. (1) Theadvent of calcareous phytoplankton would have resulted in increased ex-traction of calcium carbonate from the oceans, thus decreasing carbonatesaturation levels and lowering the potential for development of in situ stro-matolite precipitation. (2) The settling of coccoliths (and diatoms) wouldhave impeded in situ mineral growth on precipitating stromatolites bysmothering growing crystals and thus forcing constant renucleation. Con-sequently, the growth of stromatolites at the tops of the Miocene platformsin the Mediterranean region may have been controlled by a balance insediment fluxes, of pelagic as well as benthic origin, and is discussed fur-ther below.Most stromatolites are interpreted to be the remains of trapping and

binding of clastic carbonate by microbial mats (see summary in Grotzingerand Knoll 1999). In terms of process, the upper cyanobacterial layer withina mat affects the development of layering and lamina growth in stromat-olites in several important ways. Loose sediment deposited on the uppersurface of the mat is tethered in place by the upward propagation of cy-anobacterial sheaths through the sediment layer (Gebelein 1974). It is read-ily apparent that, physically, the microbiota must compete with the influxof sedimentary detritus in order to populate the depositional interface atdensities sufficient to maintain a coherent mat. Under conditions of rela-tively small sediment influx all constituents of the mat community arecapable of rising through a given sediment layer (Thompson et al. 1995).Primary producers are displaced first, followed by an assemblage of con-sumers, degraders, and anaerobic photobacteria (Seong-Joo and Golubic1999). If a relatively higher sedimentation rate is sustained, then the pro-portion of filamentous cyanobacteria in mats increases relative to coccoidforms, because the gliding motility of filamentous forms provides a selec-tive advantage (Thompson et al. 1995). Logically, as the sedimentation rateincreases past some (currently unknown) critical value, the sediment-sta-bilizing effect should drop off dramatically because sediment accumulationsimply outpaces the maximum possible microbial response. The key pointis that in natural systems there will be specific response times and scalesfor both microbial and sedimentation processes and the growth of stro-matolites will clearly be sensitive to how these processes balance. The end-

member products of these interactions are clear (Monty 1976). In the ab-sence of sedimentation, mats decay and stromatolites are not formed, be-cause of a lack of building material. On the other hand, stromatolites donot develop in the presence of critically high sediment fluxes because matgrowth is not sustainable.For stromatolites growing by accretion of sediment settling from sus-

pension in restricted basins, it is possible that during increasingly evaporiticconditions sediment fluxes became high enough to eventually smother matsand prevent the growth of stromatolites. Evidence for this facies substitu-tion is supplied by the work of Sprovieri et al. (1996) and Sprovieri et al.(in press), who show that pelagic sediment accumulation rates may havebeen as high as 1m/ky just prior to calcium sulfate precipitation. Conse-quently, the rarity of post-Triassic stromatolites (Miocene in particular)formed by in situ precipitation can be explained by smothering by pelagicsediment; on the other hand, it may have promoted growth of stromatolitesformed by accretion of loose sediment, only to eventually impede thatprocess as well once sediment accumulation rates became critically high.Hence, the restriction of marine stromatolites with precipitated textures maybe a consequence of the evolution of pelagic organisms, which, in turn,would have changed the physical environment by modifying sedimentationregimes.

Reservoir Potential of Seafloor-Encrusting Precipitates

Stromatolites that grew by in situ precipitation and related facies formas continuous, relatively impermeable deposits on the margins, upperslopes, and as caps of associated reefs of many ancient basins. Such de-posits apparently occur at the early onset of evaporative precipitation whereseawater concentrations exclude normal marine biota but have not yetreached the stage of gypsum precipitation (equivalent to the Cenozoic‘‘evaporative carbonates’’ of Decima et al. 1988). While this facies appearsto be a sedimentological oddity, it is actually more common than previouslysupposed; it seems, however, to have formed predominantly prior to themid-Mesozoic, although it exists locally in rare younger sites. Recognitionof the stratigraphic position of these seafloor precipitates in the rock record


FIG. 8.—Schematic cross section of MagnesianLimestone facies in Yorkshire Province ofEngland (adapted from Smith 1980b).

FIG. 9.—Isopachous, thinly laminated dolomitic stromatolites of Magnesian Limestone. A) Plan view of growth surface, lower (Crinkly Bed) Hesleden Dene biostrome,Upper Permian, England. The two small domes have a very symmetric shape. B) Photomicrograph of Hesleden Dene stromatolites. Note upward divergence of stromatoliticlaminae. Coin is " 2 cm diameter. (Photographs courtesy of D. Smith.)


FIG. 10.—Schematic cross section of MiddleMiocene Abu Shaar complex, Egypt. (Adaptedfrom James et al. 1988.)

FIG. 11.—Isopachously laminated dolomitic stromatolites on the eastern end of Abu Shaar complex. A) Thin-section photograph. Scale bar is 2 cm. B) Photomicrographof stromatolites with thin, isopachous lamination and showing palimpsest palisades textures. Stromatolites are coated with botryoidal cements that precipitated on seafloor(arrows). Scale bar is " 1 cm. (Photographs courtesy of Mario Coniglio.)

is particularly significant, because these deposits are the partial host forsome petroleum reservoirs.Porosity develops in the isopachous stromatolites for two reasons: (1)

These precipitates begin their existence as aragonite and/or high Mg calcite.These two minerals are metastable and as such may simply invert to calcite,but they also are readily replaced by evaporites, and/or are dolomitized.(2) As noted earlier, such sea-floor precipitates develop on the shelf andupper slope just at the environmental transition between normal open-ma-rine conditions and a passage into a hypersaline sea. Many evaporativewater bodies, however, become partially to completely cut off from theopen ocean, and general basin levels are lowered during evaporite accu-mulation (drawdown). Because of their physical position on the shelvesand upper slopes of the desiccating basins, the carbonate precipitates areespecially vulnerable to alteration. Migrating ground waters from adjacentterranes during drawdown, and also migrating basinal pore waters afterburial, are focused on these metastable deposits, almost guaranteeing theiralteration. In both the Ara Formation (of Oman) and the uppermost NiagaraFormation (Michigan Basin) these isopachous cements are dolomitized andhave developed marked bedding-parallel porosity, sufficient to become res-ervoir rocks.

CONCLUSIONS

For major restricted basins throughout Earth history, increased evapo-ration produced waters that were oversaturated with respect to calciumcarbonate and subsequently had higher salinities and temperatures. ForPhanerozoic basins, the higher salinities and elevated temperatures led toa decrease in macrobiota, further raising the saturation level.Stimulated by the high saturation levels, carbonate precipitation occurred

along the sediment–water interface and formed the thinly laminated, iso-pachous stromatolites, along with fibrous marine cements that filled voidswithin reefs. Concomitant precipitation of carbonate in the water columnproduced laminites that accumulated in anoxic deep waters and enhancedpreservation of organic matter.In the final stages, evaporite precipitation progressed until carbonate pro-

duction ceased and evaporites blanketed the entire carbonate platform. Inthis interpretation, the thinly laminated stromatolites and the carbonate inthe deep-water laminites formed as primary precipitates during times ofTST or HST deposition and are in essence evaporite deposits.Thinly laminated, isopachous stromatolites are considered to have a

largely abiotic origin, in that as part of the evaporite sequence, the inorganic


process of evaporative seawater concentration was critical for their growth.While microbes were almost certainly present on the growth crystals, whatrole they played in shaping the isopachous lamina morphology is presentlyunclear. They may have helped catalyze crystal precipitation by their lifeand death processes, or intermittent growth of biofilms may have impededthe highly uniform growth of these laminae.

ACKNOWLEDGMENTS

This research was supported by National Aeronautics and Space Agency GrantNAG5-6722 to JPG. We thank Paul Hoffman for supplying us with field maps andaerial photos of the East Arm and discussions concerning the development of theunique stromatolites discussed here. Bill Padgham, Mike Beauregard, and Mike Pol-lock of DIAND are thanked for their hospitality, field support, and expediting. MarioConiglio and Denys Smith graciously provided photographs and are thanked forthorough reviews of an earlier version of this manuscript. JSR reviewers Jack Farm-er, John Stolz, and Pam Reid provided complete and thoughtful reviews of thismanuscript. John Southard’s editorial handling of the manuscript is greatly appre-ciated.

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Received 16 October 1998; accepted 20 June 1999.

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