EVAPORITIC SUBTIDAL STROMATOLITES PRODUCED BY IN SITU PRECIPITATION: TEXTURES,
FACIES ASSOCIATIONS, AND TEMPORAL SIGNIFICANCE
MICHAEL C. POPE1*, JOHN P. GROTZINGER1, AND B. CHARLOTTE SCHREIBER 2
1
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 isolated
carbonate buildups and overlying evaporites commonly is marked by
assemblages of stromatolites and interlaminated carbonates and evaporites. Stromatolites display lamination textures that vary from peloidal and discontinuous on a scale of a millimeter to a few centimeters,
to isopachous and continuously laminated on a scale of a centimeter
to a few meters. The isopachous lamination texture may be composed
of either: (1) micritic or radial-fibrous calcite, or (2) dolomite. Isopachous stromatolitic laminae are remarkably uniform, varying little
in thickness over a given lateral distance, in contrast to stromatolites
formed of peloidal laminae, which show marked variation in thickness
over an equivalent lateral distance. These isopachous textures are uncommon on most open-marine carbonate platforms and apparently developed in transitional carbonate-to-evaporite settings because of increasing temperature, salinity, and anoxia related to water stratification, which would have created ecologic restriction and an opportunity
for stromatolite growth. Stromatolites with isopachous lamination are
here interpreted to have formed as a result of in situ precipitation of
sea-floor-encrusting calcite and possibly dolomite, whereas the stromatolites composed of peloidal, discontinuous lamination are inferred
to have formed by trapping and binding of loose carbonate sediment
in microbial mats. While the presence of microbes in almost all nearsurface environments nullifies use of the term ‘‘abiotic’’ to describe
most precipitated minerals, we interpret growth of the isopachous stromatolites to have been dominated by chemogenic precipitation in the
absence of microbial mats, and the growth of peloidal stromatolites to
have been controlled by sedimentation in the presence of microbial
mats.
These transitional stromatolite facies are best developed atop Proterozoic and Paleozoic carbonate platforms that underlie major evaporite successions. However, inspection of Jurassic and younger evaporite basins, such as the Messinian of the Mediterranean region, shows
that stromatolites with thin, isopachous lamination and radial-fibrous
textures, though present, are rare. Instead, these facies may have been
replaced by stromatolites with peloidal, clastic textures and by lowdiversity diatomaceous and coccolith mudstones. Accumulation of the
mudstones would have imposed two important effects: (1) Production
of coccoliths would have helped extract calcium carbonate from seawater, thus lowering the growth potential for precipitation of sea-floorencrusting stromatolites. (2) Settling of both coccoliths and diatoms
would have created a sediment flux to the sea floor, which would have
served to impede growth of precipitated stromatolites because of
smothering of growing crystals.
INTRODUCTION
The transition between open-marine carbonate platforms or isolated carbonate buildups and overlying or interfingering evaporites represents a
drastic chemical shift in depositional conditions across a basin. Although
conditions leading to evaporite precipitation can be generated locally in the
tidal 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]
JOURNAL OF SEDIMENTARY RESEARCH, VOL. 70, NO. 5, SEPTEMBER, 2000, P. 1139–1151
Copyright ! 2000, SEPM (Society for Sedimentary Geology) 1073-130X/00/070-1139/$03.00
restricted, volumetrically large evaporite deposits are associated with the
isolation or partial isolation of entire sedimentary basins from the world’s
oceans. As a general pattern, and as a result of increasing salinities, carbonates formed of open-marine faunas and floras are replaced by increasingly restricted facies, which culminate in the deposition of calcium sulfate
and halite evaporites (Fig. 1). Commonly this transition zone is relatively
sharp and is defined by an unusual stromatolite facies in shallow subtidal
waters and organic-rich, interstratified laminated carbonate and evaporite
in deeper subtidal waters. These stromatolites occur at the tops of many
carbonate platforms or buildups immediately preceding evaporites (Fig. 1).
The lowermost stromatolites have irregular lamination that downlaps and
pinches out at the margins of domes. Laminae may show internal peloidal
textures and possible relict filament molds, and clastic grains and peloidal
muds commonly fill depressions between zones. The lower stromatolites
are overlain by a second generation of stromatolites that are unusual in that
their lamination has several remarkable properties including fine-scale
(commonly !! 1 mm), isopachous geometry (thickness constant as measured normal to layering), extreme lateral continuity, and high degree of
uniformity (internal texture does not vary significantly). These properties
give rise to stromatolite forms that display extremely high degrees of ‘‘inheritance’’ in which stromatolite geometry changes little between successive laminae. In addition, these isopachously laminated stromatolites differ
from most other stromatolites in that they commonly lack evidence for
infilling of topographic depressions with clastic carbonate sediments, including stromatolite fragments, peloids, or other detrital grains—even micritic fills are relatively uncommon. Desiccation features and microbial
components 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 aragonite
as crusts on the sea floor (Grotzinger and Read 1983; Hofmann and Jackson
1987; Sami and James 1996; Kah and Knoll 1996; Sumner 1997; Bartley
et al. in press). However, most of the examples that are cited in these
studies are Mesoproterozoic and older, formed during a time in earth history when sea-floor precipitation may have been widespread in unrestricted
marine environments (Grotzinger and Kasting 1993; Sumner and Grotzinger 1996). Stromatolites with chemically precipitated textures declined during Mesoproterozoic time (Grotzinger 1990; Bartley et al. in press) and
generally are absent in younger rocks, with notable exceptions that may
correspond to major changes in the local and possibly global composition
of seawater (Grotzinger and Knoll 1995). Consequently, the appearance of
unusual, chemically precipitated stromatolites at times of environmental
crisis may be analogous to the resurgence of stromatolites as disaster forms
following 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 stromatolite
facies may develop as a result of environmental stress on a basinwide scale
that 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 guide
to the record of environmental change, thus motivating the present study.
The goal of this paper is to document the stratigraphic setting and textures
associated with stromatolites that immediately predate deposition of volumetrically significant evaporites, for several different basins ranging in age
1140
M.C. POPE ET AL.
FIG. 1.—Generalized schematic of carbonateto-evaporite transition emphasizing the
stratigraphic position of subtidal precipitated
carbonate fabrics forming just prior to the onset
of calcium sulfate and/or halite evaporite
deposition.
from Paleoproterozoic through Miocene, in an attempt to identify the mechanisms of stromatolite accretion and the significance of their textural and
morphologic variability. Furthermore, given their stratigraphic setting,
composition, and texture, these stromatolitic facies are susceptible to diagenetic dissolution and the creation of secondary porosity to form reservoir
rocks for hydrocarbons.
NOMENCLATURE
We adopt the nongenetic definition of stromatolites recommended by
Semikhatov et al. (1979): a stromatolite is ‘‘an attached, laminated, lithified
sedimentary growth structure, accretionary away from a point or limited
surface of initiation’’. This definition provides a concise statement of the
basic geometric and textural properties of all stromatolites, while at the
same time allowing for multiple or even indeterminate origins. Accepting
this as a general definition, it then becomes possible to evaluate objectively
the various processes that may influence stromatolite development, on a
case-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 is
critical for the current paper but unfortunately has not been standardized;
consequently, an illustrated distinction is made here for the purpose of the
descriptions that follow. At one end of the spectrum are thrombolites,
which differ from stromatolites in having clotted rather than laminated
textures (Fig. 2A). Kennard and James (1986) have established that stromatolites and thrombolites may be intergradational. Stromatolites grade
from relatively crudely laminated, such as the modern stromatolites found
in Shark Bay (Fig. 2B), through relatively well laminated, such as in typical
Proterozoic stromatolites (Fig. 2C), to very finely and isopachously laminated, such as the stromatolites discussed in this paper (Fig. 2D).
CARBONATE-TO-EVAPORITE TRANSITIONS
Examples 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 their
characteristics are listed in Table 1.
Paleoproterozoic Athapuscow Basin, Northwest Territories, Canada
The Pethei Group was deposited on a gently dipping carbonate platform
in the Athapuscow Basin that developed during convergence between the
Slave 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 composed of three distinctive facies, in ascending order: (1) dendritically
branching tufa; (2) irregularly laminated stromatolites displaying flat to
domal morphologies; and (3) isopachous, evenly laminated stromatolites.
More detailed descriptions and interpretations of these facies are presented
in Pope and Grotzinger (in press). The uppermost facies immediately underlying 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 overlain
by evaporite collapse breccias of the Stark Formation. Exhumed bedding
planes yield three-dimensional exposures of these stromatolites, which
show large, smooth composite domes with up to 40 cm of synoptic relief
between the tops of the domes and their intervening troughs. Individual
domes (Fig. 4A) are asymmetric, with a steep nearly vertical side and a
more gently sloping side. Superimposed upon the more gently sloping side
are irregular smaller-scale bumps and ridges, which similarly are expressed
by even, uniform laminae with isopachous geometry. Individual laminae
(0.4 to 1 mm thick) are isopachous (Fig. 4B) and composed predominantly
of dolomicrite (crystal size ! 10 #m diameter). Internally, laminae may
show grading from fine dolosparite into dolomicrite (Fig. 4C). The dolosparite layers commonly contain interstitial secondary silica. Stylolites locally distort the original regular shape of laminae. Stable-isotope values of
these 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 personal 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, culminating with in situ chemical precipitation of finely crystalline carbonate
directly from seawater to form stromatolites (Sami and James 1996; Pope
and Grotzinger in press). The lack of subaerial exposure surfaces, mud-
EVAPORITIC SUBTIDAL STROMATOLITES PRODUCED BY IN SITU PRECIPITATION
1141
FIG. 2.—Slabs illustrating the spectrum of stromatolite lamination textures utilized in this paper. A) Nonlaminated thrombolitic fabric, Upper Cambrian Petit Jardin
Formation, 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 cm
long. 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, and
associated intertidal facies indicate that deposition occurred subaqueously.
The steep inclination of isopachous, evenly laminated stromatolites suggests that deposition during this unit was shallow enough to be influenced
by wave-generated or wind-generated currents, but not so much so as to
produce either elongation or fragmentation of stromatolites. Increasingly
heavy stable isotopes in the uppermost Pethei Group indicate onset of evaporitic 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, breccia
geometry, local derivation of clasts, and abundance of evaporite molds and
casts (Hoffman et al. 1977; Badham and Stanworth 1977; Stanworth and
Badham, 1984; Pope and Grotzinger 1997). Large blocks, up to 1.5 km
long and 40 m thick, with a preserved stratigraphy of interbedded waverippled carbonates and siliciclastics, and red shale containing evaporite
molds, indicate that the Stark was deposited in a restricted shallow, subtidal
environment with only sporadic subaerial exposure. Chaotic, brecciated
bedding resulted from dissolution of evaporites and foundering of overlying
sediments. Silicified pipes with halite pseudomorphs at the ends of many
large blocks may be evidence of salt or brine diapirism during collapse (cf.
Badham and Stanworth 1977). The large size of clasts floating in brecciated
matrix and platform geometry suggests that the original salt thickness was
a few tens of meters to possibly a few hundreds of meters. The unique
inner-platform carbonates are subtidal and interpreted to have formed during deposition of a Transgressive System Tract (TST) whereas evaporites
of the overlying Stark Formation were deposited during the late TST and
subsequent Highstand System Tract (HST) (Pope and Grotzinger in press).
Terminal Proterozoic–Early Cambrian Huqf Supergroup, Oman
Terminal Proterozoic–Cambrian sediments of the Huqf Supergroup in
Oman consist of interbedded clastics, carbonates, and evaporites (Gorin et
al. 1982; Wright et al. 1990). These rocks were deposited in a structurally
complex rift (?) setting of alternating basement highs and intervening lows
(Gorin et al. 1982; Mattes and Conway Morris 1990) probably produced
by contemporaneous wrench faulting related to displacement along the
Najd fault system (Husseini and Husseini 1990). Deposition occurred in
stratified basins with anoxic bottom waters (Mattes and Conway Morris
1990; 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 Ara
Group, which contain carbonate ‘‘stringers’’ (Gorin et al. 1982), the lower
one of which is thickest (a few hundred meters) and is designated as the
Birba Formation. It is currently unclear if the Birba is a separate carbonate
platform, originally enclosed in evaporites, or if it is a stratigraphic (downdip?) equivalent to the upper Buah. In outcrop, the upper Buah comprises
stratiform, linked domal and columnar stromatolites that contain desiccation cracks and teepee structures, which are indicative of supratidal deposition (Gorin et al. 1982; Wright et al. 1990). The Birba Formation contains
a variety of stromatolitic and thrombolitic facies, as well as thinly laminated
1142
M.C. POPE ET AL.
TABLE 1.—Carbonate-To-Evaporite Transitions with Unique Carbonate Fabrics.
Evaporite
Composition
Location, Age
Units
Great Slave Lake, Canada
Paleoproterozoic (1.9–1.8
Ga)
Pethei Grp (c)
Stark Fm. (e)
Tufa, isopachously laminated stromatolite at contact in shallow water; Fibrous marine cements
throughout platform
Halite '' gypsum (no anhydrite)
Oman, Vendian (570–543
Ma)
Buah Fm. (c) Ara
Fm. (e)
Halite '' gypsum
Michigan Basin, Silurian
("400 Ma)
Guelph Fm. (c)
Salina Fm. (e)
Western North America;
Middle Devonian ("385
Ma)
Sverdrup Basin, Arctic Canada
Winnepegosis (c)
Zechstein Basin, England,
Late Permian ("260 Ma)
Middle Magnesian Limestone (c)
Hartlepool Anhydrite (e)
Mediterranean Messinian
(Middle Miocene)
Terminal Carbonate Complex (c)
Abu Shaar Complex, Egypt;
Messinian (Middle Miocene)
Ruidais Fm. (c)
Kareem Fm. (e)
Fibrous marine cements throughout platform and
isolated thrombolitic bioherms; Isopachously
laminated stromatolites at contact; tufa-like
crusts in carbonates within evaporites
Fibrous marine cements within pinnacle reefs; Isopachously laminated stromatolites (travertinelike coatings) cap reefs; Calcite laminites in interpinnacle reef areas
Stromatolites cap reefs, fibrous marine cements in
reefs and carbonate-evaporite laminites between
reefs in deeper-water settings
Carbonate-evaporite laminites in basinal setting,
fibrous marine cements throughout shelf-margin
reefs and buildups
Fibrous marine cements throughout reef complex;
Isopachously laminated stromatolites (Crinkly
Beds) in bioherm capping reef complex; Laminar coatings within bioherm, neptunian dikes
and cavities; Laminites in basinal setting between buildups
Thinly laminated stromatolites at transition from
carbonate to evaporite; fibrous marine cements
within reefs underlying stromatolites
Fibrous marine cements throughout reef complex;
Thinly laminated stromatolites with fibrous cements on toe of slope; Unique pisoids with distinctive fibrous fabric
Nansen (c)
Otto Fiord (e)
Carbonate Fabric(s)
Halite '' anhydrite, potash
salts
Halite '' anhydrite
Halite ( anhydrite
References
Badham and Stanworth, 1977; Hoffman et al., 1977; Stanworth and
Badham, 1984; Sami and James,
1996; Pope and Grotzinger in
press
Mattes and Conway-Morris, 1990;
Al-Majerby and Nash, 1986; J.
Amathor, personal communication,
1998
Petta, 1980; Sarg, 1986; Huh et al.,
1977
Davies and Ludlum, 1973; Kendall
and Harwood, 1991; Campbell,
1992
Davies and Nassichuk, 1980
Basin wide: Halite '' anhydrite
Locally: Anhydrite '' halite
Smith, 1980a, 1980b, 1981, 1995
Halite '' gypsum
Estaban, 1979; 1996; Feldmann and
McKenzie, 1997
Anhydrite ' gypsum ' halite
Coniglio et al., 1988
c ( carbonate.
e ( evaporite.
limestone and dolostone with intraclastic interbeds composed of fragments
of laminated carbonate. A marine origin for the carbonates is shown by the
presence 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 carbonate ‘‘stringers’’ and overlying evaporites of the Ara Formation may be
marked by intervals up to a few meters thick of stromatolitic dolostone
with isopachous, very thin laminae (Fig. 5), or sea-floor-encrusting crystal
fans of dolomitized aragonite. These facies pass gradationally into overlying anhydrite facies. The Ara evaporite is composed mainly of halite with
very little gypsum and some anhydrite, which formed from marine waters
during either a sealevel lowstand (Mattes and Conway Morris 1990) or by
enhanced evaporation during a highstand (J. Amthor, personal communication 1998); the latter interpretation is supported by the fact that the isopachously laminated stromatolites shown in Figure 5 overlies a karst surface, indicating that the facies belongs to a TST. By analogy to Miocene
and modern evaporites (Schreiber and Hsü 1980) the Huqf evaporites are
interpreted 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 the
intracratonic Michigan Basin. The basin margins are marked by a gently
dipping carbonate platform that developed coral–stromatoporoid bioherms
along the platform margin whereas isolated pinnacle reefs of similar composition formed seaward of the platform (Fig. 6). The pinnacle reefs and
corresponding shelf shoals grew quickly during a relative sea-level highstand, keeping pace with any sea-level fluctuations and/or basinal subsidence. Though there were high-frequency drawdown events during highstand development (e.g., Nurmi and Friedman 1977) there was probably
no significant long-term relative sea-level drop until after the evaporite
units were deposited within the basin (Sarg 1986). Platform-margin bioh-
erms and pinnacle reefs contain abundant fibrous marine cements that fill
original porosity in early-formed voids and cavities. Additionally, herringbone calcite cement, an unusual marine precipitate that is prevalent in Archean carbonates, but thereafter occurs only rarely, fills voids and neptunian
dikes 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 rare
restricted-marine fauna and irregularly laminated micritic stromatolites cover the tops of the reefs and the outer platform (Huh et al. 1977; Briggs et
al. 1980; Petta 1980; Sarg 1986). Irregularly laminated stromatolites are
expressed as couplets of micrite laminae (0.05 mm thick) separated by 0.1
to 1 mm thick dolospar (Gill 1985) that form simple, laterally linked hemispheroids (Logan et al. 1964), with the laminae pinching out at margins
of each structure (Fig. 7A). Irregularly laminated stromatolites are overlain
by stromatolites with isopachous fine lamination (0.4–1.0 mm thick), commonly with radial fibrous texture (Fig. 7B). Laminae are defined by darker
micritic inclusions. The isopachously laminated stromatolites generate complex morphologies that contrast with the simple domal forms of the underlying micritic stromatolites; because of their remarkable degree of inheritance, small perturbations can be propagated outward for many laminae
before their relief is damped (Fig. 7A, B). The contact between the lower
reef horizons and overlying stromatolites is sharp, as is the contact between
these stromatolites and overlying evaporites (Sarg 1986).
The isopachously laminated stromatolites developed during or immediately prior to the deposition of evaporites in surrounding basins (Sarg 1986;
Huh et al. 1977; Petta 1980). The lack of mudcracks, subaerial exposure
features or vadose features in both types of stromatolites capping the Silurian bioherms indicates that they formed in a subtidal setting and may
have formed in restricted anoxic waters (Gill 1985; Sarg 1986). However,
erosion of the thinly laminated stromatolites capping the Silurian pinnacle
reefs may indicate they were subaerially exposed before or during deposition of later basinal evaporites (Huh et al. 1977) or that subtidal erosion
occurred (Sarg 1986).
EVAPORITIC SUBTIDAL STROMATOLITES PRODUCED BY IN SITU PRECIPITATION
1143
FIG. 3.—Regional cross section of the Paleoproterozoic Pethei carbonate platform and overlying Stark Formation evaporite-collapse breccia. Isopachously laminated
stromatolites described here occur in the uppermost Pethei (gray shading) immediately preceding the evaporites.
In somewhat deeper water, between the pinnacle reefs, extremely uniform laminites (100 to 1000 mm thick) make up the coeval basin deposits.
These laminites are composed of couplets of micritic carbonate and organic
matter and grade up into calcite–anhydrite laminites, then anhydrite, and
finally halite (A1) of the Salina Group (Briggs et al. 1980). The calcite
crystals in the laminae are very fine (" 10 #m) but coarsen in evaporite
units (Briggs et al. 1980). Carbonate laminites occurring within the evaporites may record short-lived marine influxes, which led to decreases in
salinity 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 or
HST as depositional conditions became restricted. The overlying A1 evaporites formed during the late HST. This interpretation contrasts with that
of Briggs et al. (1980), who interpreted the basinal laminites to be a LST.
The key difference is that the interpretation here recognizes that evaporite
deposition 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 that
filled the Zechstein Basin (Peryt 1987; Smith 1980a, 1980b, 1995; Peryt
and Kovalevich 1997). In northeast England carbonate platform facies (Fig.
8) of the Middle Magnesian Limestone (Ford Formation) consist of welldeveloped bryozoan–marine cement reefs (' 100 m thick) that pass landward into oolitic grainstone and packstone, and basinward into deeperwater carbonate rudstones with talus blocks up to 5 m across (Smith
1980a). The reef is capped by a biostrome (Hesleden Dene) of stratiform
and domal stromatolites at least 28 m thick (Smith 1980a, 1995). The
Hesleden Dene stromatolite biostrome is overlain by 20 m of oolites, which
are 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 geometry, and are continuous across the outcrop for over 5 km (Smith 1981;
Kitson 1982). The laminae may show gentle doming with relief up to 1.3
m (Smith and Francis 1967; Kitson 1982) and resultant domes are oval in
plan view (Fig. 9) with the longer axes aligned NW–SE (Smith 1981; 1995,
his figure 3.49). Stromatolites in the bioherm commonly occur in beds
dipping 30–65) (Smith 1980a, 1980b). Other associated facies include dolomitic fibrous marine cements after a Mg-calcite or aragonite precursor
and irregular but very thinly laminated pisoliths in the basal part of the
biostrome. The pisolith facies is interpreted to have formed from inorganic
processes resulting in travertine-like textures (Smith 1995). Many lower
and middle Zechstein reef complexes in Poland and Germany also are
dominated by bryozoans capped by stromatolites with many of the features
discussed here (cf. Peryt and Piatkowski 1977; Paul 1980).
The thin, isopachously laminated stromatolites, abundant marine cements, and unique pisoliths formed in response to increased salinity during
deposition (Smith 1995). Thinly laminated stromatolites of the Hesleden
Dene biostrome developed subtidally on a subaerially exposed unconformity during development of a TST (Smith 1980b). These stromatolites may
locally be partly equivalent with the lower Hartlepool Anhydrite (Mawson,
personal communication 1998) and are regionally correlative with calcium
sulfate evaporite and carbonate laminites (Tucker 1991).
1144
M.C. POPE ET AL.
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 cross
section 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 growth
normal to the depositional surface.
Miocene Mediterranean Region
Gulf of Suez (Middle Miocene).—Platform carbonates and overlying
evaporites of the Gulf of Suez and Red Sea area developed on uplifted
basement blocks formed by rifting (Aissaoui et al. 1986). The Abu Shaar
complex (Fig. 10) is one of several well-exposed Middle Miocene carbonate platforms that characteristically are dolomitized and overlain by an
evaporite-collapse breccia (Monty et al. 1987; El-Haddad et al. 1984; James
et al. 1988; Burchette 1988; Purser 1998; Purser and Plaziat 1998). However, in the subsurface many of these carbonate platforms are encased in
evaporite and are productive petroleum reservoirs (Aissaoui et al. 1986).
Inner-platform carbonates consist of bioclastic wackestones and packstones with rare stromatolitic beds (Monty et al. 1987) that pass laterally
into reefs composed of Porites and other massive corals developed along
the platform margin. Fibrous marine cements after Mg calcite and aragonite
are 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 downdip
with a thin slope facies (! 3 m thick) consisting of pisoliths with radial
fibrous fabrics interbedded and interfingering with laminated stromatolites
and rare ahermatypic corals (El-Haddad et al. 1984; Aissaoui et al. 1986;
Monty et al. 1987; James et al. 1988; Burchette 1988; Purser and Plaziat
1998). These stromatolites develop on an unconformity and are overlain
by brecciated stromatolites, evaporites, or evaporite-collapse breccia (Purser 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 stromatolites 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 have
been interpreted to represent alternating cyanobacteria-rich and cyanobacteria-poor environmental events (Monty et al. 1987). We suggest, however,
that the accretion mechanism may have been one of in situ precipitation
rather than trapping and binding; microbes may still have been involved,
but their role was probably limited to catalysis of precipitation in that the
remarkably smooth lamination and fibrous crystalline fabric shown in Figure 11 is not consistent with the presence of an active mat (cf. Bartley et
al. in press). It is not known how abundant these isopachously laminated
stromatolites are relative to stromatolites with other lamination textures on
this platform.
The occurrence of surfaces interpreted to be marine hardgrounds and
lack of desiccation features suggest that stromatolites of the Abu Shaar
complex formed in a subtidal setting. This interpretation is supported by
the observation that at least some of the stromatolites formed in downdip
locations; however, the interpretation of water depths currently is contro-
versial because of structural complications (cf. James et al. 1988; El-Haddad et al. 1984). These thinly laminated stromatolites formed above an
unconformity during a TST.
Spain (Late Miocene).—Carbonates and evaporites formed during the
Late Miocene salinity crisis are intimately interbedded in the Mediterranean
region (Hsü et al. 1977). Commonly, massive Porites reefs indicating open
marine conditions pass upward into an unusual Porites/coralline algal assemblage and then into stromatolitic facies indicative of increasing restriction immediately prior to evaporite precipitation within the basin (Esteban
1979; Rouchy and Saint-Martin 1992; Martin and Braga 1994; Feldmann
and McKenzie 1997). The Terminal Carbonate Complex (Esteban 1979) is
the last occurrence of marine carbonates in the western Mediterranean and
is locally equivalent to, or may have just preceded, precipitation of the
upper evaporites in the basin (Esteban 1979; Rouchy and Martin 1992).
This carbonate complex contains some Porites patch reefs but, because of
increasing seawater salinity at the time of deposition, is dominated by subtidal to intertidal stromatolitic and thrombolitic facies (Esteban 1979, 1996;
Montenat et al. 1987; Rouchy et al. 1986; Rouchy and Martin 1992; Martin
and Braga 1994; Feldmann and McKenzie 1997). Stromatolites with conoform geometry are present in the basal part of the Terminal Carbonate
Complex and have been interpreted as foreslope deposits (Feldmann and
Mackenzie 1997), consistent with older but more widespread occurrences
of 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 et
al. 1981; Feldmann and McKenzie 1997). The occurrence of very finegrained, fabric-retentive dolomicrite suggests that dolomite may have precipitated as a primary mineral in the increasingly saline conditions leading
up to the Messinian salinity crisis (Feldmann and McKenzie 1997). Dolomitic, laminar to domal stromatolites occur within the overlying gypsum
beds or alternating with them (Lonergan and Schreiber 1993). These stromatolites developed during a TST are bounded by subaerial unconformities
and are thought to be coeval with evaporite formation in the basin (Esteban
1996).
Interestingly, laminae that compose stromatolites of the Terminal Carbonate Complex do not have the great lateral continuity, isopachous geometry, or radial-fibrous texture commonly observed at the tops of carbonate platforms underlying other major evaporites. Although thin lamination is locally preserved, all stromatolites described or illustrated in the
literature or observed by us (B.C. Schreiber, unpublished data) show discontinuous laminae, and in some cases preserve peloidal textures (Feldmann 1995; his figure 4.25). On the basis of lamination geometry and
internal texture these stromatolites are interpreted to result from trapping
EVAPORITIC SUBTIDAL STROMATOLITES PRODUCED BY IN SITU PRECIPITATION
FIG. 5.—Core of stromatolitic Birba carbonates contained within evaporites of the
terminal Proterozoic Ara Formation, Huqf Supergroup, Oman. Note upward divergence of dome due to isopachous growth normal to the depositional surface. Plug
hole is 3 cm diameter. Published with permission of Petroleum Development Oman.
and binding of clastic carbonate by microbial mats (Feldmann 1995), a
point with which we agree. Therefore, this appears to be a significant departure from Precambrian and Paleozoic evaporite basins, where stromatolites with textures consistent with growth by in situ precipitation occur in
addition to stromatolites with textures consistent with microbial trapping
and binding.
DISCUSSION
Stratigraphic Distribution of Stromatolites and Related Facies
The carbonate-to-evaporite transitions discussed in this paper comprise
three distinctive facies that are not common in open marine settings: (1)
1145
stromatolites with isopachous, very thin laminae of uniform thickness, with
either micritic or radial-fibrous internal texture, commonly dolomitized,
that formed in shallow subtidal conditions and often are associated with
distinctive pisolith units; (2) carbonate and evaporite laminites deposited
in deeper-water settings; and (3) fibrous marine cements formed along the
margins of the carbonate platform. These characteristics help constrain the
conditions for formation of these unique carbonate fabrics.
The stromatolites, fibrous cements, and carbonate–evaporite laminites
evidently formed in highly restricted environments independent of tectonic
setting (e.g., rift, foredeep, intracratonic basin), and form draping strata
atop major carbonate platforms just prior to precipitation of thick successions of calcium sulfate and halite evaporites. Although the underlying
platform carbonates may contain mudcracks, teepee structures, and/or paleosols, which indicate deposition in very shallow water with multiple episodes of subaerial exposure, the stromatolites and related facies described
in this paper all formed subtidally with no evidence of subaerial exposure.
These stromatolites are interpreted to have formed contemporaneously with
associated deep-water, organic-rich carbonate–evaporite laminites, both of
which are overlain by subaqueous evaporites or evaporite-collapse breccias.
Stromatolitic facies are characteristically separated from underlying platform carbonates by disconformities. This relationship, along with the apparent synchroneity of the shallow-water stromatolites and deeper-water
laminites, suggests these units formed during relative sea-level rise as TST
or early HST deposits.
The directly overlying evaporites that blanket the shallow marine platform and pinnacle reefs likely were produced by increased evaporation,
restricted circulation, or higher-frequency sea-level falls during the subsequent HST. This does not imply that all the evaporites in these basins
formed during highstand, because it is highly likely that the thick basincenter evaporites formed during local or global sea-level lowstands (cf.
Tucker 1991). We suggest, however, that the facies association of subtidal
stromatolites, seafloor-encrusting marine cement, and carbonate-sulfate/halite laminites indicates that evaporite deposition began during relative sealevel rise in most cases and that the facies association comprises a TST.
As water chemistry became more restricted during these carbonate-toevaporite transitions, chemical processes became dominant over biological
processes. We suggest that this does not just apply to the precipitation and
deposition of calcium sulfate and halite but is applicable to carbonate precipitates as well. In the examples presented here, the stromatolites with
isopachous, very thin lamination textures are interpreted as carbonate evaporites, with the dominant growth process being in situ precipitation of calcium carbonate, or possibly primary dolomite. The highly restricted marine
settings in which these stromatolites developed were quite different from
the open marine waters following certain Phanerozoic mass extinctions in
which opportunistic microbes formed stromatolites with irregular lamination across equally broad areas (cf. Schubert and Bottjer 1992).
Fibrous Marine Cements
Fibrous calcite and/or aragonite cements, which commonly are dolomitized, occur in all the examples cited above. In the Pethei Group they occur
as precipitates developed directly on the seafloor (Sami and James 1996),
whereas they fill pore spaces and voids in the remaining examples. In many
of these cases these cements were dolomitized early and preserve fine petrographic textures. Commonly these cements constitute a large part of platform margins or isolated reefs, suggesting that these units are cementstones.
These radial fibrous cements are morphologically similar to fibrous calcite
and aragonite cements that are interpreted to have formed in warm, CaCO3saturated marine environments (e.g., Morse and He 1993; Wilson and Dickson 1996). Thus, the fibrous marine cements in these pre-evaporitic carbonates further substantiate the warm, oversaturated nature of the precipitating fluid.
1146
M.C. POPE ET AL.
FIG. 6.—Regional cross section of Middle to
Upper Silurian rocks on northwestern edge of
the Michigan Basin (adapted from Nurmi 1978).
Precipitated stromatolites occur at the tops of
pinnacle reefs and along the platform (bank)
margin.
Carbonate–Evaporite Laminites
Thinly laminated carbonate and evaporite developed in deeper water during most of the transitions outlined above. The laminites consist of organicrich layers interlaminated with carbonate, overlain by interlaminated carbonate and evaporites and eventually evaporites alone. The laminites can
be correlated for many tens to hundreds of kilometers laterally (Dean et al.
1975; Davies and Ludlam 1973). In many basins these laminites formed
during a sea-level rise immediately preceding basin restriction and evaporite precipitation (e.g., western Canadian Basin, Campbell 1992). Many of
these laminites, though in many cases less than 10 m thick, commonly are
economically important because they are the source rocks for many carbonate platforms or reefs surrounded by evaporites (cf. Middle and Late
Devonian, western Canada basin; Michigan Basin, Zechstein, Sverdrup Basin, Arctic Canada).
Though many of the laminites were originally interpreted to be stromatolites formed in sabkhas, they are now known to have formed subtidally
in a euxinic setting (Davies and Ludlam 1973). Widespread lateral correlation of the laminites suggests that the carbonate mud in this unit formed
by in situ precipitation (Davies and Ludlum 1973). If the thinly laminated
stromatolites developed synchronously with the laminites, then precipitation of carbonate mud was occurring throughout these basins immediately
prior to evaporite precipitation.
Stromatolite Texture: In Situ Precipitation Versus Trapping and
Binding
Studies of the lamination textures in ancient stromatolites provide evidence for growth of stromatolites through accretion of loose sediment (micrite, grains) and in situ mineral precipitation. Although much evidence has
been supplied for the involvement of loose sediment in forming lamination
(summarized in Semikhatov et al 1979), it has become increasingly clear
over the last decade that in situ mineral precipitation is indeed an important
accretion mechanism in ancient stromatolites (Grotzinger and Read 1983;
Grotzinger 1986; Hofmann and Jackson 1987; Kah and Knoll 1996; Knoll
and Semikhatov in press; Bartley et al. in press; Pope and Grotzinger in
press). Stromatolite laminae that form by in situ precipitation require both
an increase in the calcium carbonate saturation of seawater and a decrease
in the flux of loose, clastic carbonate sediment to the site of deposition
(Grotzinger 1990; Grotzinger and Knoll 1999). In some remarkably well
preserved stromatolites of late Archean age it can be observed that the only
components that constitute the stromatolite were microbial mats, early marine cement, and later porosity-occluding burial cement; sedimentary particles are completely absent (Sumner 1997). Stromatolites with very thin
and/or isopachous lamination are regarded to have formed hard, synsedimentary crusts directly on the sea floor (Grotzinger and Read 1983; Hofmann and Jackson 1987; Grotzinger and Knoll 1995; Sami and James 1996;
Kah and Knoll 1996; Grotzinger and Rothman 1996; Sumner 1997; Bartley
et al. in press; Pope and Grotzinger in press). Although bacteria may play
a role in catalyzing mineral precipitation (Buczynski and Chafetz 1991;
Vasconcelos et al. 1995), it is clear in several cases that mineral precipitation did not template microbial mats and so the texture and morphology
of these thinly laminated stromatolites is considered to be the result of
chemical processes dominating over biological processes (Hofmann and
Jackson 1987; Bartley et al. in press; Pope and Grotzinger in press; Grotzinger and Knoll 1999).
The distribution of stromatolites with isopachous lamina textures and
self-replicating morphologies indicative of in situ precipitation is time-dependent; stromatolites with precipitated textures are common in Archean
and Paleoproterozoic carbonates, declined through the Mesoproterozoic,
and are rare to absent in the Neoproterozoic and Phanerozoic (Grotzinger
1989; 1990; Grotzinger and Knoll 1995, 1999). Consequently, the recurrence 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 is
intriguing to note that the Paleozoic carbonates apparently contain a higher
proportion of precipitated stromatolites than younger occurrences. For example, the Middle Miocene of Egypt seems to contain only patchy development of precipitated stromatolites, and none at all are known from the
Upper Miocene carbonates of Spain and elsewhere. Although our data base
of younger examples is limited, it is worth pointing out that this change
may coincide with the first appearance of Jurassic calcareous phytoplankton. Marine stromatolites of post-Triassic age and younger may only rarely
EVAPORITIC SUBTIDAL STROMATOLITES PRODUCED BY IN SITU PRECIPITATION
1147
FIG. 7.—Pinnacle-encrusting dolomitic stromatolites immediately beneath evaporites, Michigan Basin. A) Photo of isopachously laminated stromatolite atop a pinnacle
reef. 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 (white
arrows). Scale bar is " 2.5 cm. Note how irregular laminae exhibit greater discontinuity as well as thin microlenses that infill depressions and onlap topography (layers
above white arrows).
show evidence of in situ precipitation for at least two reasons. (1) The
advent of calcareous phytoplankton would have resulted in increased extraction of calcium carbonate from the oceans, thus decreasing carbonate
saturation levels and lowering the potential for development of in situ stromatolite precipitation. (2) The settling of coccoliths (and diatoms) would
have impeded in situ mineral growth on precipitating stromatolites by
smothering growing crystals and thus forcing constant renucleation. Consequently, the growth of stromatolites at the tops of the Miocene platforms
in the Mediterranean region may have been controlled by a balance in
sediment fluxes, of pelagic as well as benthic origin, and is discussed further below.
Most stromatolites are interpreted to be the remains of trapping and
binding of clastic carbonate by microbial mats (see summary in Grotzinger
and Knoll 1999). In terms of process, the upper cyanobacterial layer within
a mat affects the development of layering and lamina growth in stromatolites in several important ways. Loose sediment deposited on the upper
surface of the mat is tethered in place by the upward propagation of cyanobacterial sheaths through the sediment layer (Gebelein 1974). It is readily apparent that, physically, the microbiota must compete with the influx
of sedimentary detritus in order to populate the depositional interface at
densities sufficient to maintain a coherent mat. Under conditions of relatively small sediment influx all constituents of the mat community are
capable of rising through a given sediment layer (Thompson et al. 1995).
Primary producers are displaced first, followed by an assemblage of consumers, degraders, and anaerobic photobacteria (Seong-Joo and Golubic
1999). If a relatively higher sedimentation rate is sustained, then the proportion of filamentous cyanobacteria in mats increases relative to coccoid
forms, because the gliding motility of filamentous forms provides a selective advantage (Thompson et al. 1995). Logically, as the sedimentation rate
increases past some (currently unknown) critical value, the sediment-stabilizing effect should drop off dramatically because sediment accumulation
simply outpaces the maximum possible microbial response. The key point
is that in natural systems there will be specific response times and scales
for both microbial and sedimentation processes and the growth of stromatolites will clearly be sensitive to how these processes balance. The end-
member products of these interactions are clear (Monty 1976). In the absence of sedimentation, mats decay and stromatolites are not formed, because of a lack of building material. On the other hand, stromatolites do
not develop in the presence of critically high sediment fluxes because mat
growth is not sustainable.
For stromatolites growing by accretion of sediment settling from suspension in restricted basins, it is possible that during increasingly evaporitic
conditions sediment fluxes became high enough to eventually smother mats
and prevent the growth of stromatolites. Evidence for this facies substitution is supplied by the work of Sprovieri et al. (1996) and Sprovieri et al.
(in press), who show that pelagic sediment accumulation rates may have
been as high as 1m/ky just prior to calcium sulfate precipitation. Consequently, the rarity of post-Triassic stromatolites (Miocene in particular)
formed by in situ precipitation can be explained by smothering by pelagic
sediment; on the other hand, it may have promoted growth of stromatolites
formed by accretion of loose sediment, only to eventually impede that
process as well once sediment accumulation rates became critically high.
Hence, the restriction of marine stromatolites with precipitated textures may
be a consequence of the evolution of pelagic organisms, which, in turn,
would have changed the physical environment by modifying sedimentation
regimes.
Reservoir Potential of Seafloor-Encrusting Precipitates
Stromatolites that grew by in situ precipitation and related facies form
as continuous, relatively impermeable deposits on the margins, upper
slopes, and as caps of associated reefs of many ancient basins. Such deposits apparently occur at the early onset of evaporative precipitation where
seawater concentrations exclude normal marine biota but have not yet
reached the stage of gypsum precipitation (equivalent to the Cenozoic
‘‘evaporative carbonates’’ of Decima et al. 1988). While this facies appears
to be a sedimentological oddity, it is actually more common than previously
supposed; it seems, however, to have formed predominantly prior to the
mid-Mesozoic, although it exists locally in rare younger sites. Recognition
of the stratigraphic position of these seafloor precipitates in the rock record
1148
M.C. POPE ET AL.
FIG. 8.—Schematic cross section of Magnesian
Limestone facies in Yorkshire Province of
England (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 stromatolitic
laminae. Coin is " 2 cm diameter. (Photographs courtesy of D. Smith.)
EVAPORITIC SUBTIDAL STROMATOLITES PRODUCED BY IN SITU PRECIPITATION
1149
FIG. 10.—Schematic cross section of Middle
Miocene Abu Shaar complex, Egypt. (Adapted
from James et al. 1988.)
is particularly significant, because these deposits are the partial host for
some 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 and
upper slope just at the environmental transition between normal open-marine conditions and a passage into a hypersaline sea. Many evaporative
water bodies, however, become partially to completely cut off from the
open ocean, and general basin levels are lowered during evaporite accumulation (drawdown). Because of their physical position on the shelves
and upper slopes of the desiccating basins, the carbonate precipitates are
especially vulnerable to alteration. Migrating ground waters from adjacent
terranes during drawdown, and also migrating basinal pore waters after
burial, are focused on these metastable deposits, almost guaranteeing their
alteration. In both the Ara Formation (of Oman) and the uppermost Niagara
Formation (Michigan Basin) these isopachous cements are dolomitized and
have developed marked bedding-parallel porosity, sufficient to become reservoir rocks.
CONCLUSIONS
For major restricted basins throughout Earth history, increased evaporation produced waters that were oversaturated with respect to calcium
carbonate and subsequently had higher salinities and temperatures. For
Phanerozoic basins, the higher salinities and elevated temperatures led to
a 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, isopachous stromatolites, along with fibrous marine cements that filled voids
within reefs. Concomitant precipitation of carbonate in the water column
produced laminites that accumulated in anoxic deep waters and enhanced
preservation of organic matter.
In the final stages, evaporite precipitation progressed until carbonate production ceased and evaporites blanketed the entire carbonate platform. In
this interpretation, the thinly laminated stromatolites and the carbonate in
the deep-water laminites formed as primary precipitates during times of
TST 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
FIG. 11.—Isopachously laminated dolomitic stromatolites on the eastern end of Abu Shaar complex. A) Thin-section photograph. Scale bar is 2 cm. B) Photomicrograph
of 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.)
1150
M.C. POPE ET AL.
process of evaporative seawater concentration was critical for their growth.
While microbes were almost certainly present on the growth crystals, what
role they played in shaping the isopachous lamina morphology is presently
unclear. They may have helped catalyze crystal precipitation by their life
and death processes, or intermittent growth of biofilms may have impeded
the highly uniform growth of these laminae.
ACKNOWLEDGMENTS
This research was supported by National Aeronautics and Space Agency Grant
NAG5-6722 to JPG. We thank Paul Hoffman for supplying us with field maps and
aerial photos of the East Arm and discussions concerning the development of the
unique stromatolites discussed here. Bill Padgham, Mike Beauregard, and Mike Pollock of DIAND are thanked for their hospitality, field support, and expediting. Mario
Coniglio and Denys Smith graciously provided photographs and are thanked for
thorough reviews of an earlier version of this manuscript. JSR reviewers Jack Farmer, John Stolz, and Pam Reid provided complete and thoughtful reviews of this
manuscript. John Southard’s editorial handling of the manuscript is greatly appreciated.
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