Crystal Fan-Dominated Precipitates

Flat-laminated and domal carbonate beds, interpreted as originally aragonitic seafloor precipitates, are conspicuous features of Angmaat outcrops (Kah et al., 2001; Turner, 2009). Where preserved as dolomite, precipitated laminae are mm scale with mm- to cm-scale, convex-upward microdomal surfaces (Fig. 7A). Laminae consist, for the most part, of interlocking dolomite anhedra up to ∼80 µm in maximum dimension capped by thin drapes of dolomitic microspar that commonly preserve ghosts of upward-directed crystal terminations. Even in these relatively coarse-grained dolostones, UV fluorescence highlights organic inclusions that define acicular needles and sprays of crystalline carbonate that grew perpendicular to the substrate. Minute pyrite crystals (or iron oxides formed by diagenetic pyrite oxidation) are common within microspar laminae (black minerals in Figs. 7B–C, F). Dolospar laminae also have irregular clear patches that impart a clotted microtexture to the carbonate.

FIGURE 7—

Crystal fan-dominated precipitate facies. A) Overview showing silicified region (brown) above diagenetically altered carbonate. B–C, F) Fine acicular microfabrics reflecting original carbonate precipitates, preserved by early diagenetic silicification; note dissolution voids in B and zonation in B–C and F, reflecting variations in the incorporation of organic matter during crystal growth. D) Possible microfossils (endolithic cyanobacteria?) oriented parallel to crystal growth axes. E) Originally aragonitic crystals, with square terminations. All illustrated specimens are reposited in Harvard University Paleobotanical Collection 64840; England finder coordinates are given in parentheses for thin sections and microfossils. Scale bars  =  250 µm for all images except D, where the bar  =  2 µm.

Early diagenetic chert further clarifies the origin of textures observed in encompassing carbonate. The principal fabric elements are mm-scale crystal fans, originally deposited as seafloor carbonate precipitates. The majority of precipitate textures consist of acicular crystals up to 2 mm long but no more than a few µm thick (Figs. 7B–C). In some places, however, thicker (∼30 µm) crystals show square terminations indicative of original aragonite (Fig. 7E). Discontinuous mm-scale lamination, visible to the naked eye, is resolved in thin section as zoned precipitates, pigmented more or less strongly by included organic matter (Figs. 7A, F). Similar precipitates form today in local environments of extreme oversaturation, for example, calcitic precipitates formed in leachate pipe systems in Florida (Maliva et al., 2000). More continuous surfaces show a granular texture, with relatively abundant 1–5 µm pyrite crystals; these are interpreted as exposure surfaces that underwent micritization prior to silica emplacement. Light patches in the chert are largely irregular dissolution features, now filled by silica and dolomite euhedra, and often surfaced by tiny pyrite crystals (Figs. 7B–C).

Fabric elements preserved in chert map fairly straightforwardly onto textures apparent in unsilicified carbonate of this facies. The mm-scale laminae so prominent in unsilicified carbonate correspond to the micritized exposure surfaces observed in the chert (Fig. 7A). Crystal fans, well-preserved in chert, have been recrystallized completely to dolospar in carbonate phases, with dissolution features preserved as clear irregular patches within dolospar layers. Microfossils are rare in this facies—the only possible candidate fossils are isolated, hollow organic filamentous structures oriented parallel to the long axes of crystals that could be the remains of endolithic cyanobacteria (Fig. 7D).

Thin-Filament Tufted Mat

In outcrop, vertically striped black and white chert is commonly associated with cm-scale tufted microbialites (Fig. 8), and interbedded with precipitate textures. Tufts are interpreted to reflect synsedimentary cementation that conferred structural rigidity onto vertical tufts of cyanobacterial filaments; and chert similarly reflects interaction of microbial behavior elements and synsedimentary cement. In thin section, the black regions of variegated chert nodules are resolved as dense populations of thin filaments and associated biofilm; the white regions are cement-filled voids (Fig. 9).

FIGURE 8—

Field views of thin filament-tufted mat facies. A) Tufted mats draped by white micrite in lower part and partially silicified tufts in upper part of image. B) Silicified tufts—black chert marks organic-rich tufts of filaments, whereas white chert is predominantly original void space preserved by early mineralization. Scale bars  =  2 cm.

FIGURE 9—

Thin filament-tufted mats in thin section (G11). A) Silicified (above) and unsilicified fabrics. B–C) Higher-resolution views of silificied mat fabric, showing oriented thin filamentous microfossils and primary void spaces, some originally filled with fans of precipitated carbonate (arrow in C). Scale bar in A  =  4.5 mm; scale bars in B and C  =  400 µm.

Microfossils in this facies comprise a single population of thin cylindrical filaments (Figs. 9B–C). Most filaments show evidence of partial decay and collapse, yielding micron-thick, commonly fragmented individuals. In well-preserved regions, filaments are preserved as 2–3 µm hollow, filamentous tubes ascribed to Siphonophycus robustum (Schopf) Knoll et al. (1991) and interpreted as the extracellular sheaths of Phormidium-like cyanobacteria. The spatial orientation of S. robustum populations is distinctive: vertical bundles of filaments form tent-pole structures from which sheets of filaments extend outward and attach to adjacent vertical elements, forming convex downward laminae that isolate primary void space (Figs. 9B–C). Voids are generally subrounded and commonly contain silicified splays of acicular carbonate that formed prior to silicification (arrow in Fig. 9C). The voids closely resemble gas bubbles trapped in modern mats (Bosak et al., 2010; Mata et al., 2012). Some samples also contain mm- to cm-scale voids that crosscut mat fabrics and are interpreted as dissolution features. Void structures within tufted thin-filament mats were filled during diagenesis by an isopachous lining of chalcedony and, in their centers, by large twinned dolomite crystals. Within Angmaat chert, gas bubbles and dissolution features are readily distinguished, but in associated carbonate, differentiation of these structures is more difficult.

The fabric of the thin-filament tufted mats resembles that described by Sumner (1997) from cuspate stromatolites in the 2.52 Ga Gamohaan and Frisco formations, South Africa. Microfossils are not preserved in the Archean examples, but preserved petrofabrics suggest that filamentous microorganisms formed vertical supports that were cemented penecontemporaneously with mat growth. Perhaps slightly flocculent mats draped across the vertical supports, forming highly porous constructions filled by synsedimentary cement. The spatial scale of the Archean structures is much larger—cm- to m-scale cusps as opposed to the mm-scale features observed in Angmaat chert—although similar but larger coniform structures are preserved in deeper-water facies of the Angmaat Formation. Archean coniform structures similarly formed at depth during the drowning of an extensive carbonate platform—a setting far different from the peritidal environments discussed here. Nonetheless, the two examples appear to reflect common processes of population growth and penecontemporaneous cementation.

The thin section illustrated in Figure 9A shows that where these mats were not silicified, compaction commonly resulted in an irregular fabric whose origin would be difficult to interpret in the absence of chert preservation: the fabric is dominated by dark stringers interpreted as compacted mats and lighter clots interpreted as void-filling precipitates. A thin layer at the base of the thin section composed of uniform, isopachous cements indicates that seafloor precipitates and thin filament tufted mats formed in close environmental proximity.

Laminated Precipitates and Coccoid-Rich Mats

Stratiform laminites occur in carbonate successions through the Proterozoic record and are usually interpreted as flat mats that trapped and bound or precipitated carbonate sediments. Consistent with this larger record, stratiform laminites are common in the Angmaat succession, consisting of subtly undulating mm-scale microspar laminae, with finer-grained, and hence darker, microspar-defining lamina surfaces (Fig. 10A). Silicified samples present a more nuanced picture of this facies, with mat layers (Fig. 10B) interlaminated with a combination of finely laminated precipitate structures (Fig. 10C) and fine intraclastic microbreccia beds. The mat layers commonly contain undulating to billowy organic matter; with denser organic accumulations corresponding to the dark microspar surfaces observed in unsilicifed portions of the unit. Early diagenetic pyrite is common, especially in organic-rich laminae, but microfossil preservation is typically poor. Precipitate layers up to about 300 µm thick consist of 5–10 µm layers defined by slight organic pigmentation along their surfaces. In carbonate, it is impossible to tell which laminae formed as mats and which originated as precipitate horizons. Intraclastic microbreccia beds are locally common in this facies, and are associated with cement-filled voids formed by dissolution. These features are recorded in nonsilicified facies as detrital layers and sparry patches, both visible in thin section (Fig. 10A).

FIGURE 10—

Laminated precipitates and coccoid-rich mat facies (G13). A) Interspersed silicified and unsilicified lithologies, showing characteristic fabric development. B) Higher-resolution image showing stacked, organic-rich mat layers preserved in chert. C) Higher-resolution image showing layered precipitates preserved in chert. Scale bar in A  =  5 mm; scale bars in B and C  =  175 µm.

Where microfossils are preserved, microbial populations are almost exclusively coccoidal, with Eoentophysalis belcherensis Hofmann (1976) as the dominant builder (Fig. 4B) and Gloeodiniopsis species locally abundant as mat dwellers (Figs. 4I–L). The association between laminated precipitates and entophysalid cyanobacteria has been noted previously, both in the Angmaat Formation and elsewhere (Knoll and Sergeev, 1995; Sergeev et al., 1995; Kah and Knoll, 1996). Either coccoid populations were favored on the substrate provided by seafloor precipitates, or filamentous populations were inhibited. Locally, this facies also contains rare isolated individuals of the stalked cyanobacterium Polybessurus (Figs. 5A–B) and endolithic cyanobacteria (Fig. 4A). Within a single thin section, stratiform precipitates and coccoid mats can be interbedded with mats formed by fine filamentous cyanobacteria that trapped gas bubbles preserved as cement-filled primary void space (Fig. 11). Thus, these two biofacies/microfabrics must have been contiguous on the Angmaat platform.

FIGURE 11—

A) Interlaminated thin filament mats (TFM) with abundant primary void space and laminated precipitate/coccoidal mats (LP/CM) that display few if any primary voids. Carbonate microspar preserved within many voids in black chert (34). B) Vertical tuft of filamentous microfossils, outlining void space (noted by arrow in A  =  34, K53/2). Scale bar in A  =  3.5 mm; scale bar in B  =  15 µm.

Filament-Dominated Mats

Two distinct filamentous mat assemblages can be differentiated within Angmaat chert units. The first, interlaminated with the stratiform precipitates and Eoentophysalis mats described in the previous section (Fig. 11A), consists of thin (2–3 µm cross-sectional diameter) filamentous sheaths assigned to Siphonophycus robustum (Fig. 4C). The filaments are commonly oriented vertically and are associated with abundant extracellular polymeric substances, or EPS; coccoidal cyanobacteria occur among filaments in the mat assemblage as do the stalk-forming cyanobacterium Polybessurus and the filamentous red alga Bangiomorpha Butterfield (2000). Filaments commonly line primary, mm-scale globular voids filled by early diagenetic cement. Most of the voids appear to have formed as gas bubbles preserved by penecontemporaneous calcification of entrapping filaments. In some cases, voids remain wholly or partially filled by early microspar cement (Fig. 11A). Evidently these cements occluded porosity, limiting the subsequent incursion of silicifying fluids. Microspar fill is also conspicuous in surrounding carbonate, forming irregular clots lined by highly compacted, commonly organic-darkened mat dolomicrite (lower part of Fig. 11A).

A much different filament assemblage occurs in thicker (mm-scale lamination) undulating (and less frequently tufted) mats that only occasionally preserve primary void spaces (Fig. 12). The primary mat builder in these mats is Siphonophycus inornatum Zhang (1981), ∼6 µm filaments that form densely interwoven populations (Fig. 3A). A subset of these mats also contains thicker (up to 25 µm cross-sectional diameter) and more deeply pigmented sheaths assigned to Siphonophycus capitaneum Nyberg and Schopf (1984) (Figs. 2, 3C), and a smaller subset also contains much larger sheaths that contained multiple trichomes, assigned to Eomicrocoleus crassus Horodyski and Donaldson (1980) (Figs. 3D–E). On the scale of individual thin sections, assemblage composition (1, 2, or 3 filamentous taxa) appears to be consistent within individual laminae, but varies vertically from one layer to the next. Filament orientation also varies from one lamina to another; populations can be oriented horizontally or vertically, and commonly alternate repeatedly within a single thin section. Vertically oriented filaments occasionally result in cm-scale tufts with substantial void space (Fig. 12B). Thick silicified mats without trapped bubbles or dissolution features invariably contain predominantly horizontally oriented laminae of these three mat-building filaments.

FIGURE 12—

A–B) Filament-dominated mat facies (A  =  DAC-10 and B  =  G4). Note arenitic carbonate beneath mat accumulation in A. Scale bars  =  5 mm.

Co-occurring, but numerically subordinate coccoidal populations include Gloeodiniopsis, Polybessurus, Sphaerophycus medium Horodyski and Donaldson (1981), the simple spheroids assigned to two species of Myxococcoides, and unnamed coccoidal microfossils from which elongate tubes emerge (Figs. 3F, 4H, 5F). Less commonly, S. inornatum mats also contain isolated clusters of Eogloeocapsa. Fossil preservation is variable through these mats; laminated EPS with few or no discrete microfossils intergrade with expanded mats containing exceptionally well-preserved microfossils. In all cases, the dark S. capitaneum sheaths preserve differentially well. This differential preservation can be observed, as well, in associated unsilicified carbonate, where mat fabrics comprised of vertically oriented filaments of S. capitaneum are occasionally preserved via in situ mineralization of filament sheaths (Fig. 2E; see also Kah and Riding, 2007).

Modern microbial mats at Laguna Mormona, in Baja California (Horodyski et al., 1975), provide a useful framework for interpreting filamentous microbialites in the Angmaat succession. Microbial mats within peritidal environments at Laguna Mormona have undulating 1–3 mm laminae similar to those in Angmaat chert (fig. 5 in Horodyski et al., 1975) built by a community of filamentous cyanobacteria that includes large Microcoleus chthonoplastes, Lyngbya aestuarii (with 20–25 µm sheaths), and smaller filamentous populations. Similar to the Angmaat assemblage, the sheaths of L. aestuarii preserve differentially well at Laguna Mormona and become the most apparent taxon at depth within the mats, regardless of their proportional abundance in surface communities. And, as in the Angmaat Formation, filamentous mats at Laguna Mormona occur in close spatial proximity to Entophysalis mats that (1) are associated with local seafloor aragonite precipitation and (2) have a relatively low preservation potential (Horodyski and vonder Haar, 1975).

Eogloeocapsa-Rich Laminae

Within thin sections of flat-laminated filamentous mats, a few laminae are texturally and paleobiologically distinct. These layers commonly contain poorly preserved Eoentophysalis populations, but they are dominated by large colonial coccoids assigned to Eogloeocapsa bella Golovenok and Belova (1984) (Figs. 6A–C, 13) and interpreted in terms of the extant cyanobacterial genus Gloeocapsa. Eogloeocapsa-rich laminae differ from filamentous mat layers in the same thin section by their cumulate texture and an absence of internal lamination (Fig. 13). Although clearly distinguished in chert nodules, filamentous and Eogloeocapsa-rich layers are difficult to tell apart in unsilicified carbonate beds.

FIGURE 13—

Thin section DAC-5, showing interlayering of filamentous mats built by Siphonophycus inornatum, thin filament-mats, an Eogloeocapsa accumulation, and a thin grainstone; note position of Bangiomorpha within the S. inornatum mat interval. Bar  =  5 mm.

Microbes and Microbialites along the Angmaat Platform

Petrographic variations suggest that the principal petrofabrics identified in Angmaat chert can be ordered along a gradient of decreasing environmental restriction and subaerial exposure, identified on the basis of variations in seafloor carbonate precipitates and evidence for post-depositional dissolution (Fig. 14). This ordering may be simplistic, as petrofabric generation might easily vary along axes other than water depth, but it finds support in the distribution of microfossil populations within the formation. Restricted facies with abundant evidence for subaerial exposure also have the lowest diversity of microfossils, whereas petrofabrics interpreted as less frequently exposed (or more persistently wet) harbor a larger number of taxa. Many taxa occur in more than one petrofabric, and when placed in the spatial order suggested by petrofabric, all but one show contiguous distributions (Fig. 14). Kah and Knoll (1996) also noted that the number of chert samples represented by the main fabric types varied spatially within the basin, again indicating a broad-scale covariance between microfossil assemblages and depositional texture preserved in Angmaat chert. To a first approximation, then, microfossil assemblages map one-to-one onto preserved depositional textures, supporting the hypothesis that microbialite petrofabric reflects, at least in part, microbial community composition.

FIGURE 14—

Diagram showing the distribution of microfossil taxa, seafloor precipitates, preserved gas bubbles, and dissolution features among the five microfacies recognized in Angmaat samples. Thicker lines indicate principal mat builders, as inferred from the density and orientation of microfossils within these populations. Thinner lines indicate populations that lived in mats but did not play a major role in their construction.

Microbial populations influence petrofabric development in a number of ways. Mat-building populations can trap and bind fine particulate carbonate, leading to stromatolite accretion when accompanied by cementation. Microbially induced precipitation can also govern carbonate accumulation in the absence of trapping and binding, and this appears to be the case in our Angmaat samples (Kah and Knoll, 1996). More specific features of microbial populations can also influence petrofabric: did the organisms produce extracellular sheaths or envelopes that resist decay and act as nucleation sites for carbonate precipitation? Were filament populations oriented vertically or horizontally (a reflection of microbial behavior that influences the spatial pattern of precipitation)? Did mats have sufficient cohesion to trap gas bubbles and retain them until precipitation occurred?

Of course, physical setting will also influence petrofabric by governing the degree of carbonate oversaturation, imposing shear stress on mat communities, and introducing traction- or suspension-load sediment particles. In most stromatolites, observed petrofabric will reflect both both biological and physical processes, as well as later diagenesis. The Angmaat microbialites are exceptional in that early diagenetic chert allows us to distinguish primary fabrics formed by interacting biological and physical processes from the more common petrofabrics that additionally reflect carbonate diagenesis.

The principal fabric elements in the Angmaat samples include tabular to acicular seafloor precipitates, fine-grained precipitated carbonate that preserves filament orientation, cement-filled primary void space generated by gas bubbles, and cement-filled secondary void space produced by dissolution. Clastic microbreccia beds occur in association with precipitate-rich laminae, and thin sandstones occur locally beneath some of the most diverse mats (Fig. 12A); detrital carbonate, however, appears to have played little role in Angmaat microbialite accretion.

The Stratigraphic Distribution of Angmaat Petrofrabrics

Primary voids preserved in Angmaat samples are particularly interesting because comparable petrofabrics are relatively abundant in Mesoproterozoic carbonate rocks (Figs. 15A–C). The formation and preservation of these voids require a source of biogenic gas (Bosak et al., 2010), a mat sufficiently cohesive to trap gas bubbles (Mata et al., 2012), carbonate precipitation on a time scale sufficiently rapid to frame the voids before elimination by compaction or diffusion, and an absence of traction- or suspended-load sedimentation that would compromise mat development, bubble formation and retention, and rapid cementation.

FIGURE 15—

Distinctive mat petrofabrics comparable to those of the Angmaat Formation preserved in chert within mid-Proterozoic carbonate accumulations from A) northwestern Greenland; B, D, F) Siberia; C) China; and E) Australia. Scale bars in A–C  =  5 mm; scale bars in D–F  =  100 µm.

As pointed out by Bosak et al. (2010), oxygenic photosynthesis is the only form of photoautotrophy that produces gas (O₂) as a by-product. The presence of at least moderately oxidizing surface environments in the Mesoproterozoic era (Kah and Bartley, 2011) makes it likely that cyanobacteria dominated primary production in Angmaat mat communities, and beautifully preserved cyanobacterial fossils corroborate the view that gas bubbles likely originated via oxygenic photosynthesis. Within the Angmaat setting, only filament-dominated mats (and mixed filamentous-coccoid mats) contain primary void fabrics, suggesting that mat communities lacking filamentous populations may not have had the structural coherence required to trap and retain gas bubbles.

Microbial mats dominated by filamentous cyanobacteria occur commonly in Neoproterozoic successions that lack preserved bubble textures (e.g., Knoll et al., 1991; Cao and Yin, 2011). Thus, the additional key to primary void preservation lies in penecontemporaneous mineral precipitation. As found in other Paleo- and Mesoproterozoic carbonate successions (Knoll and Semikhatov, 1988; Bartley et al., 2000; Grotzinger and James, 2000; Kah et al., 2012), (originally) aragonitic crystal fans indicate that the Angmaat basin was at least episodically restricted to the point at which carbonate readily nucleated in or on the shallow seafloor.

Preservation of primary void textures requires that thin mats buoyed by oxygen bubbles also acted as nucleation sites for carbonate precipitation. Local carbonate nucleation could reflect high abundances of Ca²⁺ absorbed onto cyanobacterial sheaths and biofilm polymers (e.g., Pentecost and Riding, 1986; Dupraz et al., 2009). OH⁻ generated by carbon dioxide–concentrating mechanisms in cyanobacteria might also have facilitated carbonate precipitation, as it does in some modern freshwater environments (Merz, 1992; Kah and Riding, 2007). Microbial sulfate reduction has also been linked empirically to carbonate precipitation within mats; whether dissimilatory sulfate reduction increases or decreases the probability of carbonate precipitation depends on the substrate being metabolized (Gallagher et al., 2012) and the fate of H₂S generated by this process (Canfield and Farquhar, 2009). Under favorable circumstances, the net result is an increase in pH, facilitating carbonate precipitation (Aloisi, 2008). Microbial iron reduction provides another heterotrophic metabolism that would increase carbonate oversaturation within mats (Fischer and Knoll, 2009). Thus, within Proterozoic mat communities, a variety of microbial processes could have contributed to penecontemporaneous carbonate cementation (Aloisi, 2008).

That seafloor carbonate precipitates and preserved gas bubble textures are widespread in peritidal microbialites of late Paleoproterozoic and Mesoproterozoic age (Fig. 15), but are uncommon in younger Neoproterozoic successions (e.g., Knoll and Swett, 1990) prompts the question of what was different in younger coastal environments. Filamentous cyanobacteria and anaerobic bacterial metabolisms didn't go away, suggesting that the most likely shift was in the carbonate saturation state of seawater. This has previously been proposed as a principal driver of secular change in Proterozoic stromatolite morphology (Grotzinger and Knoll, 1999) and changes in the distribution of abiotic carbonate fabrics, such as seafloor precipitates (Kah and Knoll, 1996) and molar-tooth microspar (Pollock et al., 2006), and it has also been used to explain fundamental shifts in the marine C-isotope record (Bartley and Kah, 2004). What governed such changes, however, has proven elusive. Declining CO₃²⁻ is consistent with a Neoproterozoic expansion in gypsum and anhydrite deposition (Kah et al., 2001), and might reflect a general decrease in DIC associated with high rates of organic carbon burial, inferred from carbonate C isotopic abundances (Knoll et al., 1986; Bartley and Kah, 2004; Halverson et al., 2005). Whatever the causation, the sedimentary record indicates that carbonate fabrics like those in the Angmaat succession, which record precipitation mostly on and within benthic mats and the shallow substrate, were replaced in part by carbonate fabrics that refect a greater abundance of material precipitated as whitings from the water column (Kah and Knoll, 1996), largely restricting the petrofabrics described here to older successions.

Microbialite Fabric Preservation in Angmaat Carbonate

As described earlier in this paper, microbialite fabrics in Angmaat carbonate can be interpreted in light of biological and physical processes inferred from early diagenetic chert. With the exception of filament cast and molds, direct fossil evidence of mat-building communities is not preserved in unsilicified Angmaat carbonate units. Nonetheless, digenesis has not obliterated fabrics associated with seafloor precipitates, tufts formed by locally vertical filament growth, trapping of gas bubbles by filamentous mat populations, and undulating mats built by filamentous cyanobacteria. That is, diagenesis has operated on a template provided by depositional fabrics, and so microbialite textures preserved in carbonate facies do reflect microbial mat composition, at least in part. As noted above, biological inferences drawn from Angmaat petrology owe much to the physical circumstances of their formation, namely, penecontemporaneous carbonate precipitation from highly oversaturated waters. More generally, however, under favorable circumstances, petrographic textures imparted by trapping and binding, coccoidal versus filamentous mat construction, filament orientation, and mat-builder diversity may all leave a petrographic signature in Proterozoic stromatolites (see also Walter et al., 1988; Knoll and Semikhatov, 1998; and references therein).