Geo.Cro.2-3-61-KB.pdf 73 � Robert Riding AB STRA CT Authigenic seafl oor carbonate crusts include fenestrate microbialite, thrombolite, and four types here designated: Fine-grained Crust, Sparry Crust, Hybrid Sparry Fine-grained Crust, and Sparry Crust plus Coarse Grains. Each of the latter four types includes at least some layered examples that have generally been regarded as stromatolites. Recognition and interpretation of these various deposits assists understanding of stromatolite development. Sparry Crust is common in the Late Archaean-Mesoproterozoic. It includes botryoidal fans and other crystal pseudomorphs, microdigitate stromatolite, dendrite, isopachous laminite, and herringbone calcite. Although differing in primary mineralogy and bedform, these are all characterized by coarse sparry, commonly radial fi brous, fabric and appear light coloured in thin-section. They have commonly been referred to as seafl oor cement, although they formed at the open sediment-water interface rather than as void-fi lls. Two of them in particular, isopachous laminite and micro- digitate “tufa”, typically form isopachous layers with good vertical inheritance and have been regarded as stro- matolites. In contrast to Sparry Crust, Fine-grained Crust has fi ne-grained (micritic, clotted, peloidal, fi lamentous) microfabric that appears dark in thin-section, and irregular uneven layering with relatively poor inheritance. Mixed crusts, composed of millimetric alternations of Sparry and Fine-grained crust, are here termed Hybrid Sparry Fine- grained Crust. Sparry Crust with coarse allochthonous grains – here termed Sparry Crust plus Coarse Grains – includes some examples that have been given formal stromatolite names, e.g., Gongylina and Omachtenia. Sparry, Hybrid, and Fine-grained crusts are common components of Precambrian stromatolites. Their relative abun- dances change through time. Archaean stromatolite fabrics are commonly obscured by recrystallization, but their preserved lamina arrangements suggest that many of them could be composed mainly of Sparry or Hybrid crust. During the Palaeoproterozoic-Mesoproterozoic, Sparry Crust fabrics were common in peritidal stromatolites, where- as Hybrid Crust appears to have dominated large subtidal domes and columns. Fine-grained Crust may not have become generally abundant until the Neoproterozoic, when it commonly formed both stromatolites and throm bolites. Phanerozoic normal marine stromatolites are also typically composed of Fine-grained Crust. Present-day analogues of Sparry Crust fabrics occur in some speleothem, hot spring, and splash-zone marine crusts, and of Fine-grained Crust in lithifi ed microbial mats. Light-dark millimetric alternations of sparry and fi ne-grained crust that characterize Hybrid Crust have analogues in freshwater stromatolites. Taken together, these com parisons suggest that some Precambrian stromatolites are abiogenic, some microbial, and others are intimate hybrid mixtures of the two, and that – preservation permitting – these varieties can be distinguished using microfabric and lamina criteria. Keywor ds: Archaean, carbonate, microbial, Proterozoic, stromatolite, thrombolite Department of Earth and Planetary Sciences, University of Tennessee, 1412 Circle Drive, Knoxville, TN 37996, USA; (riding@Cardiff.ac.uk) Abiogenic, microbial and hybrid authigenic carbonate crusts: components of Precambrian stromatolites Geologia Croatica 61/2–3 73–103 14 Figs. 1 Tab. Zagreb 2008 Geologia CroaticaGeologia Croatica � Geologia Croatica Geologia Croatica 61/2–3 74 1. INTRODUCTION Stromatolites (KALKOWSKY, 1908) are often carbonate in composition and characteristically exhibit decimetric domical and columnar morphologies (HOFMANN, 1969). Based on present-day analogues (WALCOTT, 1914; BLACK, 1933; LOGAN 1961), they have long been regarded essentially as lithifi ed microbial mats (AWRAMIK & MARGULIS, 1974). However, morphological similarities between stromatolites and a variety of other geological deposits and structures have confused their recognition and generated debate about how the term “stromatolite” should be defi ned (SEMIKHATOV et al., 1979; RIDING, 1999; AWRAMIK & GREY, 2005). Uncertainty about KALKOWSKY’s (1908) view of stro- matolites has been created by a defi nition made by KRUM- BEIN (1983, p. 499) and wrongly attributed to Kalkowsky: “stromatolites are organogenic, laminated, calcareous rock structures, the origin of which is clearly related to microscopic life, which in itself must not be fossilised”. Although KAL- KOWSKY (1908) did not write this statement (see RIDING, 1999, p. 323), it has been repeated as if it were a literal trans- lation from his paper (e.g., GINSBURG, 1991, p. 25; FELD- MANN & MCKENZIE, 1998, p. 201; GROTZINGER & KNOLL, 1999, p. 316; McLOUGHLIN et al., 2008, p. 96). Compounding this mistake, the somewhat awkward wording employed by KRUMBEIN (1983) (use of “must not” rather than “need not”) has been cited not only as “paradoxical”, and “confusion” to be avoided, but also as an example of the defi ciencies of such genetic defi nitions (GROTZINGER & KNOLL, 1999, p. 316; McLOUGHLIN et al., 2008, p. 96). In his 1908 paper Ernst Kalkowsky did not provide a specifi c defi nition of stromatolite, apart from repeatedly emphasizing that they he regarded them as laminated organic structures. He thought that the life forms involved were “niedrig orga- nisierte planzliche Organismen” (simple plantlike organisms, KALKOWSKY, 1908, p. 125). It is reasonable to conclude that he regarded stromatolites essentially as laminated micro- bial deposits (RIDING, 1999). During the century since KALKOWSKY (1908) intro- duced the term “stromatolith” (stromatolite), particular prob- lems have centred on confi dent discrimination between lithifi - ed microbial mats and a variety of other geological deposits that can have broadly similar appearances, such as invertebrate skeletons, diagenetic concretions, deformation structures, and sub-aqueous abiogenic precipitates. The variety of these dif- fi culties has been reduced as understanding of fossils and carbonate sediments has progressed. For example, it is now unusual for invertebrate skeletons or diagenetic concretions to be mistaken for lithifi ed microbial mats, although confusion between deformed soft sediment and microbial domes was suggested relatively recently (LOWE, 1994). However, re- search (e.g., GROTZINGER 1989a, b) incrementally focused attention on the diffi culty of discriminating between lithifi ed microbial mats and sparry sub-aqueous authigenic carbonate crusts. This continuing problem (PERRY et al., 2007) can ari- se for several reasons. Firstly, essentially abiogenic seafl oor crusts and lithifi ed mic robial mats can both create layered, often domical, structures of broadly similar appearance. Sec- ond ly, processes that drive seafl oor precipitation and microbial calcifi cation are not necessarily mutually exclusive, and their products may be intimately associated, raising the possibility that, in addition to lithifi ed microbial mat and sparry crust end -members, there are deposits that represent complex mixtures of both. Thirdly, scarcity of present-day analogues for sparry seafl oor crusts (GROTZINGER & JAMES, 2000, p. 9) has hindered their recognition as distinct deposits. The need to distinguish these components has been recently em- phasized. PERRY et al. (2007, p. 169) noted that “microbially constructed stromatolites should not … be confused with abiotic, chemically precipitated carbonate crusts”. POPE et al. (2000, p. 1139) regarded “iso pachous 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 sedi mentation in the presence of microbial mats”, and suggested that “thinly laminated iso- pachous stromatolites are considered to have a largely abiotic origin” (idem, p. 1149). Here I explore this suggestion that microfabric details and lamina arrangement can be used to discriminate between an cient abiogenic deposits and those made by microbial mats, by reviewing published details of Precambrian authigenic carbonate crusts and their possible present-day analogues. In addition to stromatolites, Precambrian authigenic sub- aq ue ous carbonate crusts include botryoidal crystal fans, dendrite, herringbone calcite, fenestrate microbialite, and thrombolite. Since these are often intimately associated with stromatolites and share similar components with them, I in- clude them here. But the focus is stromatolites, and three ge- neralizations arise from this overview. Firstly, Precambrian stromatolites basically consist of one or both of two com po- nents: fi ne-grained carbonate and sparry carbonate. Secondly, com parisons with present-day analogues suggest that Fine- grained Crust is lithifi ed microbial mat, and that Sparry Crust is essen tially abiogenic. Thirdly, Precambrian stromatolites generally consist of one of these components (Fine-grained Crust, Spar ry Crust) or of millimetric alternations of both of them – Hybrid Crust. Tracing the secular distribution of these deposits reveals that Hybrid Crusts were very important in stromatolite formation during the Palaeoproterozoic and Me- so proterozoic. They are major components of large stroma- tolite domes that dominate subtidal facies of extensive Prote- rozoic carbonate platforms such as the ~1.9 Ga Pethei Group (SAMI & JAMES, 1996) and ~1.0 Ga Burovaya Formation (PETROV & SEMI KHATOV, 2001, fi g. 6). The combination of microbial growth and abiogenic precipitation in Hybrid Crusts may have promot ed rapid accretion of these large, locally decametric, stroma tolites. Some Archaean stromatolites are equally large, e.g., in the Campbellrand-Malmani platform of South Africa (BEUK ES, 1987) and at Steep Rock, Ontario (WILKS & NISBET, 1985). These examples are more diffi - cult to interpret because discrimination between Sparry and Fine-grained crust relies on microfabric details that are readily obscured by poor preservation in old stromatolites. In the Campbellrand, large elongate stromatolite domes that are ma- Geologia CroaticaRobert Riding: Abiogenic, microbial and hybrid authigenic carbonate crusts: components of Precambrian stromatolites 75 jor components of extensive platform carbonates (SUMNER & GROTZINGER, 2004, fi gs. 2,10) have an overall appear- ance of smooth even lamination penetrated by crystal pseudo- morphs (SUMNER & GROTZINGER, 2004, fi g. 11a) con- sistent with essentially abiogenic precipitation. However, these “Boetsap laminae” contain both sparry and micro crys- talline layers, and the latter could be interpreted as detrital silt or as microbial mat pre cipitate (SUMNER & GROTZ INGER, 2004, fi g. 3). If the micro crystalline layers are silt that was not microbially trap ped, and the sparry fabrics are abiogenic crusts, then these large domes would be essentially abiogenic structures; but if they are mat precipitate then these deposits are Hybrid Crusts. Discrimination between Sparry and Hybrid crusts therefore focuses attention on whether such large Ar- chaean domes are hybrid combinations of mats and abio genic crusts, similar to those of the Pethei and Burovaya reefs, or are Sparry Crusts – possibly with detrital carbonate – and therefore essentially abiogenic? Proterozoic stromatolite de- vel opment is more readily interpreted due to better overall preservation. Hybrid Crust dominated subtidal stromatolites during the early-mid Proterozoic, and Sparry Crust pro g- ressively declined (GROTZ INGER & KASTING, 1993, p. 235; KAH & KNOLL, 1996, p. 81). By the Neoproterozoic, Fine-grained Crust stromatolites (and thrombolites) had pro- bably surpassed Hybrid Crust de posits in abundance. This suggests that, whereas present-day microbial mats may pro- vide analogues for most Phane rozoic stromatolites, their rele- vance is diminished in examples older than ~1000 Ma. These considerations lead to a liberal view of the term “stromatolite” as broadly encompassing laminated authigenic crusts formed at the sediment-water interface in springs, riv- ers, lakes and seas. These characteristically can exhibit both large and small domical and columnar morphologies. 2. PRECAMBRIAN CARBONATE CRUSTS Research into Precambrian stromatolites has revealed not only fi ne-grained lithifi ed microbial mats (e.g., VOLOGDIN, 1962; WALTER, 1972; KOMAR, 1976) but also dis tinct ive sparry fabrics. Radial spar is the dominant component of the small digitate stromatolites recognized by DONALDSON (1963) and described as microdigitate tufa by HOFFMAN (1975). These were given names such as Pseudogymnosolen (CAO & LIANG, 1974) and Asperia (SEMIKHATOV, 1978) and came to be generally termed microdigitate stromatolites. Larg er stromatolites often exhibit sparry layers that alternate with fi ner ones (HOFMANN, 1969, p. 4, 16, fi g. 13), as in Conophyton (KOMAR et al., 1965) (Fig. 1) and in botryoidal sparry crust fabrics of digitate stromatolites that BERTRAND- SAR FATI (1972) compared with calcifi ed cyanobacterial co lonies. At the same time, evidence of early lithifi cation was noticed in the localization of stromatolitic carbonates (SERY- BRYAKOV & SEMIKHATOV, 1974), and in the support re quired by high-relief coniform stromatolites (GEBELEIN, 1976; DONALDSON, 1976). John Grotzinger’s research, beginning with the ~1.9 Ga Rocknest platform, was seminal in focusing attention on these seafl oor precipitates. Recognition of the primary aragonite mine ra logy of microdigitate stromatolites (GROTZINGER & READ, 1983) led to interpretation of large crystal botryoids as originally aragonite rather than gypsum, and to the sug gest- i on that long-term decline in deposits such as microdigitate stromatolites could refl ect progressive reduction in seawater carbonate saturation (GROTZINGER, 1989a, p. 96, fi g. 15). GROT ZINGER (1989b, p. 11) listed “substrate-parallel la- yers of neomorphic fi brous cement”, “radial fi brous fabics … that constitute microdigitate stromatolites”, and “conoform stro ma tolites” as evidence for “in situ carbonate production”. The outcome was increased recognition of seafl oor sparry crusts. The superposed radial fi brous botryoid fabrics of Me- so proterozoic Tarioufetia and Tungussia, fi rst thought to be cal cifi ed cyanobacteria (BERTRAND-SARFATI, 1972), were com pared with aragonite cements (FAIRCHILD et al., 1990, p. 61). In addition to botryoidal fans and microdigitate stro ma- tolites, isopachous laminite (JACKSON, 1989), den drites (SAMI & JAMES, 1996, fi g. 6a), and herringbone calcite (GROTZINGER & KASTING, 1993), were distinguished, especially in Palaeoproterozoic and Archaean carbonates. As a result, GROTZINGER & JAMES (2000, p. 7) were able to sum marize Precambrian marine “abiotic precipitates” as: (i) decimetric to metric radial fans (after aragonite), (ii) mic ro- digitate stromatolites, (iii) isopachous millimetric la minites, (iv) isopachous layers of herringbone calcite, and (v) dendrites (“dendritic tufa”). GROTZINGER & KNOLL (1999, p. 329– 330) cited “petrographic evidence not only for early lithi fi - cation, but also for direct growth of encrusting marine cement directly on the growing stromatolite, particularly for stro ma- tolites of Mesoproterozoic and older ages”. Fi gu re 1: Conophyton. Stag Arrow Formation, Manganese Group, Bangemall Basin, Western Australia, ~1050–1100 Ma. Width of view, 5.5cm. Photograph courtesy of Kath Grey. Geologia Croatica Geologia Croatica 61/2–3 76 Sparry crusts occur thinly interlayered with fi ne-grained crust in coniform stromatolites (e.g., WALTER, 1972), and, for example, in Palaeoproterozoic Pethei stromatolites des- cribed by SAMI & JAMES (1996), in latest Mesoproterozoic and early Neoproterozoic Baicalia lacera described by KNOLL & SEMIKHATOV (1998) and PETROV & SEMIKHATOV (2001), and in the ~800 Little Dal “lamelliform elements” described by AITKEN (1989) and TURNER et al. (2000a). Thus, the main components of Precambrian subaqueous carbonate crusts recognized here are sparry and fi ne-grained precipitates, and hybrid mixtures of the two. All three of these may incorporate allochthonous grains. Discrimination of mud- and silt-grade allochthonous grains is diffi cult, but coarse grains can be recognized. These components occur in fi ve main combinations (Fig. 2): Fine-grained Crust, Sparry Crust, Hybrid Sparry Fine-grained Crust, Sparry Crust with Coarse Grains, and Fine-grained Crust with Coarse Grains. All of these include at least some deposits that have been generally regarded as stromatolites. Fine-grained Crust with Coarse Grains is common in Neogene coarse-grained stroma- tolites, such as Lee Stocking Island and some Shark Bay co- lumns, but does not appear to be common in the Precambrian. Two additional seafl oor crust categories that are locally com- mon during certain periods in the Precambrian, but which do not contain stromatolites, are fenestrate microbialite and throm bolite. Accordingly, the categories of seafl oor carbonate crust discussed here include Fine-grained Crust, Sparry Crust, Hybrid Sparry Fine-grained Crust, Sparry Crust with Coarse Grains, fenestrate microbialite and thrombolite (Table 1). These are outlined below. Fi gu re 2: Authigenic sparry and fi ne-grained carbonate crust recognized here together with Hybrid Crust and coarse-grained admixtures. All crust categories in boxes (Fine-grained, Sparry, Hybrid, Sparry + Coarse Grains, and Fine-grained + Coarse Grains) all include at least some deposits that have been generally regarded as stromatolites. Note that fi ne-grained crust can include fi ne allochthonous grains as well as fi ne-grained in situ precipitate. Table 1: Categories of subaqueous authigenic carbonate crust common in Precambrian carbonates. Fine-grained Crust, Sparry Crust, Hybrid Sparry Fine-grained Crust and Sparry Crust with Coarse Grains all contain examples generally regarded as stromatolites. The following general interpretations, based on present-day analogues, are suggested: Fine- grained Crust, fenestrate microbialite and thrombolite represent lithifi ed microbial mat; Sparry Crust and Sparry Crust with Coarse Grains are essentially abiogenic precipitates. Hybrid Sparry Fine-grained Crust results from submillimetric to millimetric alternations of Sparry (abiogenic) and Fine-grained (lithifi ed microbial mat) crust. 1. FINE-GRAINED CRUST 2. SPARRY CRUST Botryoidal fans and crystal pseudomorphs; Radial fi brous microbotryoids Microdigitate stromatolite; Dendrite Isopachous laminite Herringbone calcite 3. HYBRID SPARRY FINE-GRAINED CRUST Microcrystalline-peloidal carbonate Conophyton Baicalia lacera Laminar fi brous crusts and micritic peloidal laminae Clotted-bushy-peloidal micrite Filamentous Boetsap laminae: microspar crusts of uncertain origin 4. SPARRY CRUST PLUS GRAINS Herringbone Calcite with coarse grains Radial fi brous crust with silt and sand grains (e.g., Gongylina, Omachtenia) Crystal fans with coarse grains 5. FENESTRATE MICROBIALITE 6. THROMBOLITE Geologia CroaticaRobert Riding: Abiogenic, microbial and hybrid authigenic carbonate crusts: components of Precambrian stromatolites 77 2.1. Fine-grained Crust Fine-grained (micritic, clotted, peloidal, fi lamentous) micro- fabrics and irregular uneven layering with relatively poor inhe- ritance are typical of microbial stromatolites (e.g., MONTY, 1976). These fabrics occur interleaved with other deposits in Hybrid Crusts, but on their own they also constitute the do- minant components of many stromatolitic domes, columns and layers, as well as thrombolites. They may contain fenes- trae and incorporate allochthonous grains. In older examples the micritic fabrics have often aggraded to microspar. Fine- gra ined Subaqueous Crust described here is regarded as the pro duct of lithifi ed microbial mats, and therefore as an es sen- tially biotic deposit (see Present-day Analogues). Proterozoic Fine-grained Crust, together with including Hybrid Crust, is fi gured extensively by VOLOGDIN (1962). Latest Ediacaran examples are fi gured by SCHMITT (1979, pl. 16, fi gs. 3, 4; pl. 22, fi g. 2) from the Anti-Atlas Mountains, Morocco. Neoproterozoic examples (some of which are fe- nestral) with “streaky” microstructure occur in the ~800 Ma Bitter Springs Formation of Central Australia (WALTER, 1972, pls. 2, 3, 23, 25) and in the Little Dal reefs (e.g., TURN- ER et al., 2000a, fi g. 15f). In the Little Dal, these locally also exhibit fi lamentous fabrics (e.g., AITKEN, 1989, fi g. 10; TURN ER et al., 2000a, fi g. 8 e, f, g). JEFFERSON & YOUNG (1989, fi g. 5a) show stromatolites underlying the Little Dal Group which have “clotted/grumous microfabric”. GROTZ- INGER & KNOLL (1999, fi g. 3f) fi gure fi lament moulds in “micritic stromatolite laminae” from the Neoproterozoic Cher- nya Rechka Formation, Siberia. RIDING & SHARMA (1998) found clotted microfabrics “irregular masses of micrite bound- ed by microspar and sparite” to be the common in late Palaeo- proterozoic Vempalle stromatolites from southern India. In RIDING & SHARMA (1998), they dominate ex am ples of both poorly (idem, fi g. 2) and evenly (idem, fi gs. 3, 4) lami- nated forms in which sparry layers or fenestrae occupy a relatively minor volume of the structure. In general, however, fi ne-grained stromatolites – as opposed to Hybrid Crusts – appear relatively scarce in the Palaeo pro terozoic, but this requires further verifi cation. 2.2. Sparry Crust Sparry Crust includes stromatolitic deposits (microdigitate stro- matolite, isopachous laminite); large and small botryoidal fans as well as related crystal pseudomorphs; extensive her ring- bone calcite beds; and rarely recorded large den dri tes. 2.2.1. Botryoidal fans and crystal pseudomorphs These centimetric to metric pseudomorphs after crystals that formed at the sediment-water interface occur as layers and beds, commonly draped by fi ne-grained carbonate. Typic- ally they are conical and fan-shaped with a convex upper sur- face (Fig. 3). They range from isolated skeletal crystals and widely spaced “fanning pseudomorphs” to extensive beds of juxtaposed botryoids of upwardly diverging radial crystal fans, e.g., in the Late Archaean Campbellrand-Malmani plat- form of South Africa (SUMNER & GROTZINGER, 2000, fi gs. 3a, 5; SUMNER & GROTZINGER, 2004, fi g. 7). The original mineralogy of these “giant botryoids” (GROTZ INGER & KASTING, 1993, p. 234) has been inter- preted as gypsum (BERTRAND-SARFATI, 1976; HARDIE, 2003) or aragonite (SUMNER & GROTZINGER, 1996a, p. 120). They also occur in the ~3.45 Ga Warrawoona Group ~50 km west of Marble Bar, Western Australia (HOFMANN et al., 1999, p. 1257); the ~2.8 Ga Steep Rock carbonate platform, Ontario; the ~2.6 Ga Carawine Dolomite, Western Australia; the ~2.9 Ga Uchi Greenstone Belt of Ontario (SUMNER & GROTZINGER, 2000, p. 139), and as “Coxco needle” fans in the ~1.64 Ga McArthur Group of the Northern Territory, Australia (WINEFIELD, 2000). At Steep Rock they may form the structure that WALCOTT (1912) named Atiko- kania (HOF MANN, 1971; SUMNER & GROTZ INGER, 2000, p. 134). Fans up to 1.6m high that lack detrital sediment Fi gu re 3: Laterally juxtaposed botryoidal radial fans with convex internal bands. Late Archaean Campbellrand-Malmani platform, South Africa. Width of fi eld ~12 cm. Geologia Croatica Geologia Croatica 61/2–3 78 indicate they “did not grow within the sediment” (SUMNER, 2002, p. 109). SUMNER & GROTZ ING ER (2000, p. 139) regarded “all the pseudomorphs as replacing aragonite with the exception of morphologically distinct gypsum pseudo- morphs from the 2.6 Ga Carawine Dolomite (SIMONSON et al., 1993)” (SUM NER, 2002, p. 109). BERTRAND-SARFATI (1976) interpreted Camp bel- lrand-Malmai crystal rosettes as pseudomorphs after gyp- sum. MARTIN et al. (1980) considered similar “radiating crystal structures” associated with stromatolitic domes, in the ~2.6 Ga Cheshire Formation of the Belingwe Greenstone Belt, ~150 km east of Bulawayo, Zimbabwe, to be replace- ments of aragonite or gypsum. GROTZINGER & KASTING (1993, p. 235) cited “prolifi c precipitation of aragonite as giant botryoids up to 1 m in radius” as evidence that Archaean seawater was signifi cantly oversaturated for CaCO3. Well exposed fan beds occur in the Cambellrand-Malmani platform (SUMNER & GROTZINGER, 1996a, fi g., 4; 2000, fi g. 3). SUMNER & GROTZINGER (2000, p. 131–133) described cyclic sequences (approximately similar in age to those des- cribed by MARTIN et al., 1980) in Huntsman Quarry, ~50 km NNE of Bulawayo with layers of crystal fans after aragonite, herringbone calcite, large domical stromatolites (described by MACGREGOR, 1941), and fenestrate microbialite. They concurred with MAC GREGOR (1941) that the environment was probably subtidal. HARDIE (2003) restated BERTRAND- SARFATI’s (1976) interpretation that Late Archaean fans were after gypsum. SUMNER (2004) countered this with petrographic and trace element data that support their primary aragonite mineralogy (see also SUMNER & GROTZINGER, 2000, p. 1137–1139). Radial fi brous microbotryoids. Small (~< 1 mm) radial fi brous botryoids, reminiscent of far larger “giant” botryoids (e.g., GROTZINGER & KASTING, 1993), form tussocky microfabric in some Mesoproterozoic stromatolites (BERT- RAND-SARFATI, 1976). Similar but more irregular “fi brous precipitate masses” are principal components of some Pala eo- proterozoic stromatolites (SAMI & JAMES, 1996, fi g. 7c, d). Tussocky microstructure (“microstructure en touffes”) occurs as superposed radial fi brous botryoid fabrics in digitate Mesoproterozoic stromatolites such as Tarioufetia hemi sphe- rica, Tungussia globulosa, Tungussia cumata and Serizia ra- dians in NW Africa (BERTRAND-SARFATI, 1972, p. 94, 103, 105, 131, fi g. 26c, pls. 23–26). The columns are elongate to irregular and up to 8cm in diameter and 30 cm in height. The botryoids are sub-millimetric to millimetric and arranged from isolated irregularly superposed hemispheroids to laterally amalgamated layers of lenses. Locally botryoids are inter- layered with micrite (idem, pl. 25, fi g. 2) and scattered detrital quartz grains (idem, pl. 26, fi g. 1). BERT RAND-SARFATI (1976, p. 253, fi g. 2a) refi gured T. globulosa and described “micro struc ture en touffes” as “tussocks” commonly inter- layered by spa rite cement, a dark fi lm, or detrital quartz. She compared them with present-day calcifi ed colonies of Rivu- laria, but it was sub sequently noted that they are “strikingly similar to … originally aragonitic cements” (FAIRCHILD et al., 1990, p. 63). In these Atar examples intercalated micrite and quartz layers appear to be minor components and so they are here classed as essentially Sparry rather than Hybrid Crust. But in the Pethei Group, where they also create digitate colum- nar stromatolites, fi brous precipitate masses are associated with clotted micrite cores and voids fi lled by detrital micrite (SAMI & JAMES, 1996, fi gs. 7c, d, 8h) and they can be re- garded as Hybrid Crust. SAMI & JAMES (1996, p. 218) noted that “digitate stromatolite heads composed of clustered fi brous cement fans formed a rigid framework analogous to Paleozoic reef fabrics”. 2.2.2. Microdigitate stromatolites and dendrite Microdigitate stromatolites are small stubby digitate lami- nated columns, typically <5 mm wide and <20 mm high, closely packed in extensive layered sheets that can dominate the shallow parts of peritidal cycles (HOFFMAN, 1975, p. 262), especially in the early-mid Proterozoic (Fig. 4). The laminae show good inheritance and may be traced through adjacent columns, and individual columns can exhibit radial fi brous fabric (HOFMANN & JACKSON, 1987, p. 964). In the ~2.1 Ga Denault Formation of Labrador, DONALD- SON (1963, p. 12, pls. 4–5) noticed very small “digitate stro- matolites”, “branching, fi nger-like structures 1 to 5 mm in Fi gu re 4: Microdigitate stromato- lites, silicifi ed after carbonate. Wumishan Formation, Mesoproter- ozoic, ~25 km north of Beijing. Width of view ~25 cm. Note well- developed overall layering, and large size variation of individual digitate forms. Geologia CroaticaRobert Riding: Abiogenic, microbial and hybrid authigenic carbonate crusts: components of Precambrian stromatolites 79 diameter and less than 2 cm in height” that show regular la- yering and “correspondence of lamination thickness at co- incident levels”. HOFFMAN (1975, p. 262) recognized that similar deposits were important components of the shallow parts of Rocknest peritidal cycles and described them as “tiny arborescent stromatolites that resemble structures in modern algal tufa. Where silicifi ed, microscopic fi lament molds are preserved in the stromatolites”. He compared them with “crusts of calcareous tufa” in “brackish algal marshes, such as those in the Bahamas” described by SHINN et al. (1969). GROTZ- INGER & READ (1983, p. 712, fi g. 1f) subsequently termed these Rocknest deposits “cryptalgal tufas”, describing them as “cement laminae” that “commonly form discrete, tiny co- lumnar structures (microdigitate stromatolites), 1–10 mm wide and with 0.1–5 mm relief”. They followed HOFFMAN (1975) in interpreting them as tidal fl at deposits but suggested that the environment was semiarid rather than humid, adding “cement crusts appear to have formed by precipitation of ara- gonite as sheet-like tufa layers and microdigitate stromatolites within mats on surfaces of tidal fl ats or shallow, evaporitic ponds” (GROTZINGER & READ, 1983, p. 712). “Digitate stromatolites”, “calcareous tufa” and “cryp talgal tufas” noted by DONALDSON (1963), HOFFMAN (1975), and GROTZINGER & READ (1983) have also since then va- riously been termed microdigitate tufa, microdigitate stroma- tolites, ministromatolites (HOFMANN & JACKSON, 1987), and tidal fl at tufa (GROTZINGER & KNOLL, 1999, fi g. 4a), as well as being assigned formal names (e.g., Pseudo gymno- solen CAO & LIANG, 1974; Asperia SEMIKHATOV, 1978). GREY & THORNE (1985) regarded them as biogenic, but GROTZINGER (1986a, p. 842) considered that “the tufas are, in essence, evaporites”, and suggested that they refl ect “micro- bially infl uenced inorganic calcifi cation (although it is possible that they are entirely abiotic in origin)” (GROTZ ING ER, 1986b). HOFMANN & JACKSON (1987) compared 1.9 Ga examples from the Belcher Supergroup in Hudson Bay with those described by DONALDSON (1963) from the De nault Formation, and discussed a variety of possible inter pre tations. They compared the radial fi brous fabric with “che mo genic car- bonate crusts” including pisoids, aragonite ce ments and spe- leothems (idem, p. 969) and concluded “chemical pre ci pitation played a signifi cant role in the formation of the ra dial-fi brous fabric here described. Whether the precipitation was biologically mediated, or occurred within or on microbial mats is less clear” (idem, p. 970). GROTZ INGER (1989b, p. 11) described them as “microbial tufa”, but subsequently they have often been re- garded as inorganic. SAMI & JAMES (1994, p. 116) described them as cement laminae up to 2 cm thick con sisting of mic- ro digitate “stalks” separated by thinner mi crite layers, and GROTZ INGER & KNOLL (1999, p. 347) wrote that mic ro- digitate stromatolites are “pure precipitate structures”. Microdigitate stromatolites are common in late Archaean and early Proterozoic carbonates (RAABEN, 1980, 2005; LI- ANG et al., 1984, 1985; GREY & THORNE, 1985, p. 193– 194, fi g. 12; CAO, 1991; SHARMA & SHUKLA, 1998), and are also well-known in the Mesoproterozoic (RAABEN, 1980; KAH & KNOLL, 1996). Dendrite. Closely spaced subvertical dendrites, often 3– 5 cm in height and ~0.5 cm wide, form layers and irregular higher-than-wide mounds 50 cm or more in width that con- stitute beds up to ~3m thick; individual dendrites consist of micritic stalks and branches coated by fi brous spar (POPE & GROTZINGER, 2000, p.106, fi g. 5). These dendrites broadly resemble microdigitate stroma- tolites, but are larger and less well bedded. The Hearne For- mation at the top of the Pethei Group remains the only des- cribed occurrence. They may have fi rst been fi gured by SAMI & JAMES (1996, fi g. 6a), and are shown as “dendritically branching tufa” by GROTZINGER & KNOLL (1999, fi g. 6c; see also GROTZINGER & JAMES, 2000, fi g. 5e). POPE & GROTZINGER (2000, p. 106–110) described them in detail. They considered the dendrites to be “chemically precipitated structures” formed “in a manner similar to laboratory depo- sition of zinc and copper dendrites” (POPE & GROTZINGER, 2000, table 1, p. 109). The dendrites are overlain by irregularly laminated stromatolites and then by isopachous laminites. 2.2.3. Isopachous Laminite Isopachous Laminite forms stromatolites composed of even, laterally continuous, radial fi brous layers that grew “normal to the stromatolite surface, regardless of local curvature” (GROTZ- INGER & KNOLL, 1999, fi g. 6a, b; POPE & GROTZING ER, 2000, p. 113) (Fig. 5). These stromatolites can form thin (e.g., 3–5 m) but extensive beds (JACKSON, 1989, p. 70) within shallowing sequences, associated with transition to evaporite conditions (POPE et al., 2000, p. 1140). In addition to smooth domical morphologies (e.g., GROTZINGER & KNOLL, 1999, fi g. 3a), isopachous laminites can exhibit peaked crests (JACKSON, 1989, fi gs. 6, 13; SUMNER & GROTZINGER, 2004, fi g. 4a) and angular asymmetry (POPE et al., 2000, fi gs. 2d, 4, 7a, 9b; POPE & GROTZINGER, 2000, fi g. 8). POPE et al. (2000, p. 1142) found that “stromatolites with isopach- ous fi ne lamination” commonly have “radial fi brous texture”. “Isopachous, evenly laminated stromatolites”, described in detail from the uppermost Pethei Group, consist of dolomi- crite and fi ne dolosparite (POPE & GROTZINGER 2000, p. 112–113). In the ~2.6 Ga Cheshire Formation of the Belingwe Gre- enstone Belt, ~150 km ESE of Bulawayo, MARTIN et al. (1980, fi gs. 10, 12, p. 348, table 2) recognized “crinkle lami- nation” “with good inheritance” and synoptic relief up to 10 cm, forming metric beds, which they compared with Stra- tifera?. SUMNER & GROTZINGER (2000 p. 128) des cribed these as “crinkly laminite facies” overlying pseudo morph fans and “composed of sub-millimeter to millimeter-thick micro sparitic laminae that have a constant thickness normal to la yering”. JACKSON (1989, p. 70, fi gs. 6, 13) described “un usual, 5 m thick, ridged or peaked stromatolites” inter- preted to form a laterally continuous subtidal bioherm in the 1.89 Ga Cowles Lake reef south of Coronation Gulf, Canada, and added “the laminations show very strong inheritance and have a maximum synoptic relief of about 1 m”. GROTZ- INGER (1989b, p. 11) commented that these “show textural evidence for having been produced by in situ carbonate pro- Geologia Croatica Geologia Croatica 61/2–3 80 duction”. GROTZINGER & KNOLL (1995, p. 581) noted that “stroma tolites formed by direct precipitation on the sea fl oor are a conspicuous feature of Archean and Proterozoic carbonates. Isopachous sparitic, fi brous and micritic layering, generally devoid of clastic carbonate, is the characteristic microstructure. These structures were fully lithifi ed as they accreted”. TURNER et al. (2000a, p. 189, fi g. 12g) described “ce- ment -rich stromatolites” from the Neoproterozoic Little Dal of north-west Canada that form “a uniform veneer of domal stromatolites” (idem, fi g. 11). They consist of millimetric “cement-rich grumous layers alternating with thin fi lms (ca. 100 µm) of micrite”; “laminae are even and regular, and show a high degree of inheritance” (idem, p. 189). POPE et al. (2000) fi gured isopachous thinly laminated stromatolites from the ~2.55 Ga Malmani Fm of the Transvaal (idem, fi g. 2d), ~1.9 Ga uppermost Pethei Group (idem, fi g. 4), and Late Per- mian Zechstein deposits of NE England (idem, fi g. 9). They interpreted these to have formed by carbonate pre cipitation at the sediment-water interface, stimulated by high saturation levels (idem, p. 1149), and concluded, “thinly laminated iso- pachous 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” (POPE et al. 2000, p. 1149–1150). Nonetheless, SUMNER & GROTZINGER (2004, p. 7, fi g. 4a) fi gured the same isopachously laminated stroma tolites of the Neo archa- ean Cambellrand-Malmani platform and sug gested that they formed by precipitation or trapping within microbial mats. Late Miocene-Pliocene (~6–4 Ma) lacustrine Furnace Creek stromatolites in Death Valley, Cali fornia, have isopachous laminae composed of radiating crystal fans interpreted to be “indicative of predominantly abiotic pre cipitation” (COR- SETTI & STORRIE-LOMBARDI, 2003, fi g. 1). 2.2.4. Herringbone Calcite Herringbone calcite occurs as void-fi lling cement but also, especially in the Late Archaean, has formed extensive deci- metric to metric massive sheet-like seafl oor crusts. It is char- acterized by distinctive delicate serrated or crenulated banding formed by light and dark couplets ~<1 mm in thickness (Fig. 6), and is thought to be derived from a Mg-calcite precursor (SUMNER & GROTZINGER, 1996b). Herringbone calcite occurs as a cement in Palaeozoic reef and stromatactis cavities (e.g., KREBS, 1969; LEHMANN, 1978; MCGOVNEY, 1989; DE WET et al., 2004) and has been variously named (SUMNER & GROTZINGER, 1996b). GROTZINGER & KASTING (1993, fi g. 1) recognized her- ringbone calcite beds as a feature of Late Archaean carbonate sedimentation. In the Campbellrand-Malmani platform, for example (SUMNER & GROTZINGER, 1996b), it is laterally extensive, forming decimetric beds traceable over 140 x 50 km in the deep subtidal transgressive Gamohaan Formation (SUMNER & GROTZINGER, 1996b, p. 420; SUMNER, 2002, fi g. 2c). It is also closely associated with “plumose micro bialites” and in “grainstone-precipitate” beds (SUM- NER, 1997a, table 1, p. 464, 470), where it can be “repetitively interbedded with clastic carbonate on a centimetre scale” (SUMNER & GROTZINGER, 1996b, p. 420). It both fi lls voids and isopachously encrusts seafl oor surfaces (SUMNER & GROTZINGER, 1996b, p. 420). GANDIN & WRIGHT (2007, p. 301) interpreted some Gamohaan herringbone cal- cite as a replacement of a precursor sediment “likely to have been a gypsum-mush”, whereas SUMNER & GROTZINGER (1996a, b) suggested that herringbone calcite refl ects anoxic conditions with low [Fe2+] seawater values that inhibited cal- cite precipitation. Herringbone calcite ~2.6 Ga in age occurs in Huntsman Quarry, ~50 km NNE of Bulawayo, in decimetric layers associated with crystal fans, domical stro ma tolites, and “fenestrated microbialites” (SUMNER & GROTZ INGER, 2000, fi g. 9). SUMNER (2002, fi g. 2b) term ed her ringbone calcite “serrate, fi brous marine cement”. 2.3. Hybrid Sparry Fine-grained Crust Hybrid Crust comprises alternations of light-dark layers. For example, BERTRAND-SARFATI (1972, p. 25–26) noted Fi gu re 5: Peritidal isopachous laminite composed of fi brous dolomite. Society Cliff s Formation, Bylot Supergroup, ~1.2 Ga, White Bay, south of Bylot Island northern Baffi n Island. Width of view, ~7cm. Note 1–2 mm very even layering and good inheritance. Photograph courtesy of Linda Kah. Geologia CroaticaRobert Riding: Abiogenic, microbial and hybrid authigenic carbonate crusts: components of Precambrian stromatolites 81 light-dark couplets (“doublet: couche claire et couche som bre”) in some Atar and other African Proterozoic stromatolites. These include some members of the Crustophycaceae and Lopatinellaceae of VOLOGDIN (1962, p. 195–226), Zonalia and Arca microstructures of KOMAR (1989, pl. 3), and spar- micrite couplets of SAMI & JAMES (1996). The dark layers show a variety of micrite and/or microspar fabrics, including dense, peloidal, bushy, clotted, and/or fi lamentous. The light layers are sparry carbonate, often radial fi brous in form. Frequency curves of light and dark layer thickness have been used to routinely compare stromatolites with this microstruc- ture (e.g., KOMAR et al., 1965; BERTRAND-SARFATI, 1972, p. 25–26). In English, this general fabric has variously been termed “ribboned” and “striated” (HOFMANN, 1969, p. 16, fi g. 13), “streaky” (WALTER, 1972, p. 12), and “fi lm” (BERT- RAND-SARFATI, 1976, p. 253). Three main types of Hybrid Crust are recognized here, based on fi ne-grained dark layer microfabric: microcrystalline- peloidal, clotted-bushy-peloidal, and fi lamentous. In addition, layer defi nition, thickness and evenness vary; typically from thinner (≤ 1 mm), better defi ned and more even, to thicker (≥ 1 mm), less well defi ned and less even. Layer defi nition, thick- ness and evenness appear generally to progressively decrease from microcrystalline-peloidal, through clotted-bushy-pelo- idal, to fi lamentous fabrics. However, fabric preservation com pli cates recognition of these sub-types, particularly of fi la mentous microfabric. For example, KNOLL & SEMI - KHATOV (1998, p. 410) found that fi lmy microstructure in early Neoproterozoic Baicalia lacera “intergrades with a distinctly fi lamentous microstructure”. In contrast, late Me- so proterozoic B. lacera shows distinctive micritic fi lms but only “rare ghosts of fi laments” (PETROV & SEMIKHATOV, 2001, p. 270, fi g. 6). Whether well-preserved Baicalia la- cera consistently exhibits fi lamentous microfabric remains to be determined. The categories and examples distinguished here are based on relatively well-preserved Proterozoic ex- am ples. They require further description, comparison and clarifi cation. 2.3.1. Microcrystalline-peloidal carbonate Conophyton. Laterally persistent, interleaved sub-millimet- ric to millimetric dark-light layers are common in Protero- zoic coniform stromatolites (VOLOGDIN, 1962, pls. 24–25; KO MAR et al., 1965; BERTRAND-SARFATI, 1972, pl. 11(4); WALTER, 1972, p. 103–112) (Figs. 7, 8). The layer- ing ranges from uneven with irregular thicknesses to even and regular. Thickness and confi guration of these bands are among the features used to distinguish Conophyton species (CLOUD & SEMIKHATOV, 1969, fi g. 2), and KOMAR et al. (1965) re cognized general long term increase in dark rela- tive to light laminae in Conophyton through the Riphean. It was suggested that dark laminae “represent originally alga- rich layers” (KOMAR et al., 1965, p. 67), and WALTER (1972, p. 86) commented that “presumably the pale laminae originally had less organic matter”. Some coniform stromat- olites show co arse sparry layers whose lateral variation in thickness sug gests that they refl ect recrystallization in addi- tion to their primary character (e.g., VOLOGDIN, 1962, pl. 32, fi g. 3, pl. 72, fi g. 1; WALTER, 1972, pl. 10, fi g. 2). Fi gu re 6: Herringbone calcite seafl oor crust. Neoarchaean, Campbellrand- Malmani platform, South Africa. Note delicate sub-millimetric uneven, crenulated layering. Fi gu re 7: Conophyton garganicum, Middle Riphean, Russia. Stratigraphic unit and locality not known; specimen donated to Geological Survey of Canada by M.A. Semikhatov. Width of view 8 mm. Note laterally persistent, relatively even, submillimetric interleaved layering of thin fi ne-grained and light-coloured sparry layers, and good inheritance. Photograph courtesy of Hans Hofmann. Geologia Croatica Geologia Croatica 61/2–3 82 Inzeria lindina. BERTRAND-SARFATI (1972, p. 155, fi g. 58, pl. 22(2, 3)) described repeated millimetric alternations of dolomicrite-microsparite with dolospar in decimetric stub- bily branched Inzeria lindina from the late? Proterozoic of Lindi, Zaire (now Congo). She suggested that these might in- clude seasonal rhythmicity. Baicalia. The bands are often laterally persistent and can occur on steep-sided coniform (e.g., WALTER, 1972, pl. 5, fi gs. 3, 4) and other (e.g., Baicalia lacera, PETROV & SEMI- KHATOV, 2001, fi g. 5a, b) stromatolites. The fi ne-grained material ranges from micrite to microspar and can be quite heterogeneous, including calcifi ed fi laments, clots, peloids and bush-like structures, as well as spongy “vermiform” fabrics. The spar is typically radial-fi brous or blocky. The laminae are submillimetric to millimetric, occasionally cen- timetric (e.g. WALTER, 1972, pl. 12, fi g. 1) and range from isopachous with good inheritance to irregular and discon ti- nuous. Proportions of light and dark bands range from predo- minantly dark to ~50% light. Where dark layers predominate and sparry bands are thin and few, spar can occur in irregular fenestrae. In some cases, fi lamentous fabrics are well devel- oped, as in the case of Baicalia lacera which forms distinctive “platy” dark-light alternations (KNOLL & SEMIKHATOV, 1998; PETROV & SEMIKHATOV, 2001, fi g. 6) (Fig. 9) (see Filamentous, below). BERTRAND-SARFATI (1972, p. 112, pl. 13) described distinctive light-dark layers in Mesoproterozoic Baicalia mau- ritanica from Atar. The dark layers are thin, generally <0.1 mm, whereas the light layers range up to 0.75 mm (fi g. 37). The dark layers are themselves composed of up to 5 or 6 thin leaves (“feuillets”) with a platy appearance (idem, pl. 13(1–3)) that she compared with that of Baicalia lacera (idem, p. 113). Tungussia nodosa. BERTRAND-SARFATI (1972, p. 99, p. 187, pl. 19(1–3)) described millimetric micrite-micro spar (and locally sparite) alternations as “microstructure en tapis” (“micritic mat” in BERTRAND-SARFATI, 1976, p. 256) in Tungussia nodosa and T. aff. nodosa from Atar. Laminar fi brous crust and micritic peloidal laminae. In the Pethei platform SAMI & JAMES (1994, p. 116; 1996, p. 216, fi g. 6d) recognized “wavy microbialite” as a major component of peritidal facies. It consists of “stromatolites composed of cement laminae, 1–2 mm thick, separated by thin (< 1 mm), dark micritic surfaces and lenticular peloid grainstone laminae” (SAMI & JAMES, 1994, p. 116). The laminae are laterally persistent, smoothly undulose and iso- pachous, with good inheritance and intervening troughs are occupied by fi ne sand. Laminar fi brous crusts can be inter- bedded with detrital grains (SAMI & JAMES, 1996, fi g. 5f) (and see Gongylina, below). They attributed accretion to “com bination of cement precipitation and binding of peloids by smooth microbial mats” and interpreted it to form in lower intertidal and shallow subtidal conditions. SAMI & JAMES (1994, p. 116) considered fenestral microbial lami- nite to be a low energy, upper intertidal to supratidal equi- valent of laminar fi brous crust, consisting of “thin (1–2 mm), Fi gu re 9: Hybrid Crust composed of submillimetric dark-light alterna- tions. Baicalia lacera. Burovaya Fm, Turukhansk, Siberia, latest Mesoprot- erozoic (~1020 Ma). The delicate, gently curved thin, even and persistent appearance of the dark layers, characteristic of “platy” Baicalia lacera microstructure, may be fi lamentous in detail (KNOLL & SEMIKHATOV, 1998). Reprinted from PETROV & SEMIKHATOV (2001, fi g. 6a), with permission from Elsevier. Fi gu re 8: Hybrid Crust microfabrics (detail of Fig. 7, Conophyton garganicum). Sparry and fi ne-grained layers can be interpreted as Hybrid Crust composed of thin layers of lithifi ed microbial mat (fi ne-grained) separated by thicker layers of surfi cial crust and/or early cavity fi ll (sparry) (cf. Monty & Hardie, 1976, fi g. 2b, see Analogues, below). However, neomorphic spar aggradation cannot be ruled out. Geologia CroaticaRobert Riding: Abiogenic, microbial and hybrid authigenic carbonate crusts: components of Precambrian stromatolites 83 irregular to continuous, micrite and spar laminae with thin (< 1 mm), dark, clay-rich drapes” and irregular spar-fi lled fenestrae. The clotted microfabric “may represent either pelo ids or micrite-cement”. Based on SAMI & JAMES’ (1994, p. 116) inter pre tation of combined “cement” preci- pitation and mat bound peloids it could be regarded as a hybrid of isopachous, es sentially abiogenic, laminite and detrital carbonate, but if the peloids are in place microbial precipitates then this would essentially be a hybrid abio ge- nic-biotic precipitated crust. 2.3.2. Clotted-bushy-peloidal micrite Clotted-peloidal-bushy micrite forms laminae interlayered with seafl oor precipitate and detrital micrite. The irregular micrite aggregates are often interspersed with microspar and spar, giving the lamina a relatively light appearance in thin- section. In addition to abundant synsedimentary “cement”-like precipitate, SAMI & JAMES (1996, p. 213) recognized pelo- idal clotted micrite as “a signifi cant component (10–45%) of most stromatolitic laminae”. The clots consist of poorly de- fi ned 50–100 µm peloids and together with fi brous and blocky spar form laminae in “prone stromatolitic laminite” (SAMI & JAMES, 1996, fi g. 7a, b). “Prone microbial laminite” is a major component of Pethei carbonates where it contributes signifi cantly to large elongate stromatolite domes (SAMI & JAMES, 1993, p. 405, table 1). Clotted-peloidal-bushy micrite aggregates can be very irregular, but locally distinctive vertically elongate shrub-like structures occur that are similar to present-day calcifi ed cy- anobacterial sheaths (KAH & RIDING, 2007). The shrubs are typically separated by spar, giving the layers a broadly fl oc- culent or palisade-like appearance. In the ~1200 Ma Society Cliffs Formation, laminae with shrubs form submillimetric laminae within isopachous laminite (KAH & RIDING, 2007). The shrubs consist of fi ne microspar and are up to 600 μm high and 200 μm wide and have irregular margins and tend show vertical orientation on sloping surfaces. They closely resemble the calcifi ed thick irregular sheaths of present-day oscillatoriacean cyanobacteria (see RIDING & VORONOVA, 1982). The Society Cliffs shrubs are associated with calcifi ed fi laments that are currently the oldest examples of sheath- calcifi ed cyanobacteria (KAH & RIDING, 2007). Somewhat similar fabrics have been fi gured from latest Precambrian stromatolites as Vesicularia (VOLOGDIN, 1962, pl. 39) and as “vermiform microstructure” in Madiganites mawsoni from Central Australia (WALTER, 1972, pl. 1, fi gs. 1,2) of Late Cambrian age (LINDSAY et al., 2005; see also BERTRAND- SARFATI, 1976, p. 255), but these do not appear to be ele- ments of hybrid deposits. 2.3.3. Filamentous Tangled to prostrate Girvanella-like fi laments within spar- microspar cement form relatively persistent platy to curved fl occulent submillimetric to millimetric layers. The fi laments may be constant diameter tubes with thin even-thickness walls, conforming to the calcifi ed cyanobacterial sheaths of Girvanella (see RIDING, 1977a) but more commonly are less regular and less distinctly tubiform. They are tangled and irregular, often prostrate, and interspersed among microspar- spar. Filamentous microstucture with Girvanella tubules, but without well developed interleaved sparite layers, occurs in some Phanerozoic oncoid cortices (e.g., GARWOOD & GOODYEAR, 1924; BIDDLE, 1983). In Proterozoic stro- matolites, layers of fi lamentous microstructure are commonly interleaved with millimetric sparite layers. Some of these exhibit a distinctively striated “fi lmy” or “streaky” microstruc- ture, as in Baicalia lacera, Tungussia confusa and other forms (KNOLL & SEMIKHATOV, 1998, table 1). Microstructure with fi lament moulds was termed Cana- liphorida and Filiformita by KOMAR (1976, 1989; see also BERTRAND-SARFATI et al., 1994, p. 182, fi g. 18). AITKEN (1989) recognized “dendriform” and “lamelliform” fabrics as framework components of stromatolitic bioherms in ~835 Ma Little Dal Group of NW Canada. He remarked that these fa- brics “are not typically stromatolitic” and that “sediment trap- ping may not have been the dominant process in their for- mation” (idem, p. 15). He described them as “cellular” and containing tubular and Renalcis-like structures (idem, fi gs. 10–13). Subsequently, TURNER et al. (1993, 2000a, fi g. 10b; 2000b) compared the tubules with Girvanella and noted that the lamelliform fabric consists of alternating dark layers of “calcimicrobial fi laments” and lighter “more cement-rich” areas. Little Dal “hollow tubules with micritic walls” are fi gured by BATTEN et al. (2004, fi g. 9b). KNOLL & SEMIKHATOV (1998, p. 410, fi gs. 3, 4) des- cribed well-preserved “fi lmy or platy” microstructure in early Neoproterozoic Baicalia lacera stromatolites from the Cher- naya Rechka Formation, Igarka, Siberia. They found it to be associated with “a distinctly fi lamentous microstructure” in which “laminae comprising densely interwoven to scattered, vertically or subhorizontally oriented fi laments are in ter spers- ed with layers of spongy or dense microspar”. They interpreted the 8–10 µm tubes as “sheaths of LPP-type (Lyngbya, Phor- midium, Plectonema) cyanobacteria and preserved as drusy microspar encrustations” (KNOLL & SEMIKHATOV, 1998, p. 411). Similar Baicalia lacera fabrics in the ~1Ga Burovaya Formation of west-central Siberia locally contain calcifi ed tubes resembling Siphonophycus (PETROV & SEMI KHAT- OV, 2001, p. 270). AITKEN (1989, p. 15–16) described “dendriform” and “lamelliform elements” as important components of Little Dal reefs in the Mackenzie Mountains. He regarded both as stromatolites with “unusual” or “unique” characteristics: thin- walled tubes and Renalcis-like objects in dendriform element, and a reticulate “ladder-rung” arrangement that “may be form- ed by a meshwork of tubes” in lamelliform element. Den- driform and lamelliform elements look quite similar in two of his illustrations (idem, fi gs. 10, 13). TURNER et al. (2000a, p. 185, 188) related these elements to growth stages in the reefs, with dendriform most common in Stage III and lamel- liform in Stage IV. Their illustrations of dendriform elements Geologia Croatica Geologia Croatica 61/2–3 84 (TURNER et al., 2000a, fi g. 8a, b, e) show irregular closely- spaced centimetric digitate stromatolites with laminar-reti- culate cores and marginal Renalcis-like clots. They des cribe lamelliform elements as commonly steeply sloping (45–70°) and containing dark fi lamentous and more cement-rich light layers (TURNER et al., 2000a, fi g. 10b). These resemble the distinctive “fi lmy” microstructure of similar age Baicalia lacera (PETROV & SEMIKHATOV, 2001, fi gs. 5b, 6a) which also has steeply dipping laminae and, as noted above, quite possibly fi lamentous microstructure too. In the examples cited above, layers of fi lamentous fabric are generally interleaved with lighter, sparry, layers. If the sparry layers were lacking then the deposit would be in di- stinguishable from “skeletal stromatolite” (RIDING, 1977b) and “porostromate stromatolite” (MONTY, 1981). In addition to the Little Dal and Chernaya Rechka examples, calcifi ed fi laments reminiscent of Girvanella are relatively widespread elsewhere in the Neoproterozoic, e.g., in the ~750–700 Ma Draken Fm (SWETT & KNOLL, 1985; KNOLL et al., 1993), ~725–675 Ma Svanbergfgellet Fm (RAABEN, 1969), and ~700 Ma Upper Eleonore Bay Supergroup, Greenland (BERT- RAND-SARFATI & CABY, 1976) (all references in KNOLL & SEMIKHATOV, 1998, p. 413). However, Mesoproterozoic examples reported from the ~1200 Ma Society Cliffs Fm are currently the oldest known Girvanella-like calcifi ed fi laments, and are associated with micritic bush-like structures also interpreted as calcified cyanobacteria (KAH & RIDING, 2007). 2.3.4. Boetsap laminae: microspar crusts Well-developed relatively even lamination described from the Neoarchaean Campbellrand-Malmani platform of South Africa as Boetsap lamination (SUMNER & GROTZINGER, 2004) also represent a type of Hybrid Sparry-Microcrystalline Crust, but is diffi cult to interpret due to uncertainty regarding the origin of the layers, which appear to be entirely microspar, with no sign of clotted or peloidal fabric. The main question is whether the microspar is entirely primary, or includes altered micrite. Giant elongate domes in the Campbellrand-Malmani platform are dominated by millimetric layers of fi ne-grained dolomite (red-brown) and calcite (grey) that SUMNER & GROTZINGER (2004, p. 14–16) termed “Boetsap lami na- tion”. They distinguished two main, equally abundant, com- ponents: (i) dark microcrystalline dolomite, varying 1–3 mm in thickness along a single lamina, commonly with peaked upper surfaces, (ii) thin (<1mm) uniform layers of light micro- crystalline calcite and dolomite, showing a vertical fabric in thicker laminae. They interpreted the thicker layers with varied thickness as fi ne clastic carbonate, and the thinner uniform layers with vertical fabric as precipitated laminae. SUMNER & GROTZINGER (2004, p. 22) commented that “apparent paucity of micrite suggests that spontaneous pre- cipitation of carbonate, i.e., whitings, was not common across seaward sides of the platform”, and noted that “micrite beds were not observed in shallow subtidal depositional environ- ments” and “most intertidal to deep subtidal stromatolites and microbialites contain fi brous calcite cements”. But they also emphasized that extensive recrystallization made it diffi cult to interpret the fi ne-grained components (idem, p. 6, 8). At the Boetsap section (SUMNER & GROTZINGER, 2004, fi g. 3) they estimated that elongate stromatolites are dominated by “microcrystalline” and “precipitated” fabrics, in which micro- crystalline represents microspar to silt-sized crystals that “could have been either transported silt-sized carbonate or carbonate precipitated within microbial mats”, and preci- pitated represents “cement-like crystal textures”. They con- cluded that some elongate stromatolite mounds contain “a signifi cant component of clastic carbonate” whereas others, especially those better preserved, have “more precipitated tex- tures” (idem, p. 16). Boetsap laminae differ from isopachous laminite mainly by the presence of dark-light layering. Interpretation of Boetsap laminae presents problems to similar those of laminar fi brous crusts and micritic peloidal laminae (the key components of SAMI & JAMES’ (1994, p. 116) “wavy microbialite”). Both are hybrid deposits in which the origin of the fi ne-grained carbonate requires clarifi cation. Boetsap lamination is regular but includes discontinuous la- yers (SUMNER & GROTZINGER, 2004, fi g. 11a) and the microspar to silt-sized crystals could represent detrital grains or microbial mat precipitate (SUMNER & GROTZINGER, 2004, fi g. 3). Until these possibilities are resolved it is not possible to tell whether Boetsap laminae, and therefore the “Giant” domes of which they are an important component (SUMNER & GROTZINGER, 2004, p. 14, 16) are essentially Sparry or Hybrid crust. 2.4. Sparry Crust plus Coarse Grains Abiogenic precipitates both cement and surfi cially veneer particulate carbonate. These hybrid “grainy crusts” are most distinctive where the grains are coarse. In the Precambrian, examples of grains incorporated in seafl oor herringbone cal - cite and in radial fi brous carbonate crusts have been des cribed, and some have been given stromatolite names, e.g., Gongy- lina. 2.4.1. Herringbone Calcite with coarse grains Herringbone calcite associated with grainstone forms laterally persistent centimetric to decimetric layers with scours and cross-lamination; the grainstone occurs as basal graded units or fi lls troughs between herringbone calcite domes (SUMNER, 1997a). In centimetric to decimetric “grainstone-precipitate” cyc- les in the subtidal Gamohaan and Frisco formations of the Campbellrand-Malmani platform, basal grainstones pass up into “precipitate-rich” beds (SUMNER, 2002, fi g. 2c), or grainstones fi ll troughs between stromatolitic “precipitate” domes (SUMNER, 1997a, table 1, p. 464–466). Grainstone beds have basal scours and contain wave ripples. Stromatolites are poorly laminated and dominated by herringbone calcite, which is also present between grains. Synsedimentary lithi- fi cation is refl ected in vertical ripple propagation. Geologia CroaticaRobert Riding: Abiogenic, microbial and hybrid authigenic carbonate crusts: components of Precambrian stromatolites 85 2.4.2. Radial fi brous crusts with silt and sand grains (e.g., Gongylina, Omachtenia) Alternating submillimetric layers of particulate carbonate and radial fi brous crusts, infl uenced by synsedimentary scouring and micro-crosslamination, create distinctive dark-light well laminated rippled microstructures in laterally persistent deci- metric to metric beds (KNOLL & SEMIKHATOV, 1998, p. 414–418). The pseudocolumnar to stratiform deposits formed by these grainy crusts form have been given form names within groups such as Gongylina KOMAR, 1966 (Fig. 10) and Omachtenia NUZHNOV, 1967. Omachtenia omachtensis with muddy to silty sediment, and Gongylina differenciata with silt and sand, are regarded as characteristic of the Mesoproterozoic (KNOLL & SEMI- KHATOV, 1998, p. 418). HOFMANN (1969, table 13, p. 38) recognized that stratiform Gongylina “appears to be nothing more than a form dependent on the periodic infl ux of sand- or silt-sized material”. KNOLL & SEMIKHATOV (1998, p. 417–418, fi g. 11) agreed, and extended this inter pretation to include Omachtenia omachtensis. They ruled out both trap- ping/binding and precipitation “by actively photosynthesizing mats”, and regarded these deposits as “mechanically emplaced sediments” encrusted by thin veneers of “cements”. They interpreted them as alternations of grains and seafl oor pre- cipitates on peritidal fl ats locally associated with “micro di- gitate precipitates”. Nonetheless, they con sidered that micro- bial mats appeared to have “covered and stabilized event beds and provided sites for the nucleation of carbonate crystals after degradation (KNOLL & SEMI KHATOV, 1998, p. 418). A variant of this mixed deposit is where grainy sediment accumulated lateral to domes, as in Rocknest isopachous la- minites where “precipitated laminae pinch out in adjacent depression, fi lled by both precipitated laminae and peloidal grains” (GROTZINGER & KNOLL, 1999, fi g. 3a). SAMI & JAMES (1994, fi g. 6) noted “ooid grainstone with thin micro- bial laminae draping climbing ripples” in Pethei shallowing cycles, and laminar fi brous crusts interbedded with detrital grains (SAMI & JAMES, 1996, fi g. 5f). Whereas grainy her- ringbone calcite deposits are centimetric-decimetric, in Gon- gylina and Omachtenia the radiaxial carbonate and grainy layers are both submillimetric. These deposits evidently form ed by alternation of Sparry Crust precipitation with sand and silt infl ux. They are likely to fi nd analogues, albeit in shallower water, in present-day hot-spring travertines and cave fl ow- stone. 2.4.3. Crystal fans with coarse grains Crystal pseudomorph fans also can be interbedded with cross- stratifi ed grainstones, e.g., Cheshire Fm., Zimbabwe (SUM NER, 2002, fi g. 1c). 2.5. Fenestrate Microbialite These are thin beds of net-like masses of thin curved wispy dark layers, often rounded and contorted, that defi ne millimetric to centimetric lensoid to irregular areas of light-coloured cement that includes radial, sparry and herringbone calcite fabrics (Fig. 11). The network is commonly structured by thinner dark layers draped from thicker dark subvertical “supports”. These distinctive deposits were recognized and described in detail from the Campbellrand-Malmani platform where they form thin (decimetric, SUMNER, 1997a, p. 462; SUM- NER & GROTZINGER, 2004, fi g. 12) but laterally very ex- tensive (SUMNER, 1997b, p. 315) beds. SUMNER (1997a, b) interpreted them as delicate convoluted microbial mats forming open networks (SUMNER & GROTZINGER, 2004, p. 16), with the thicker supports and laminated drapes being due to different microbial communities; the delicate wispy sheets being encrusted by calcite as they grew. Varieties have been termed tented, cuspate, irregular columnar and plumose (SUMNER, 1997b) and, as a whole, fenestrate microbialites (SUMNER, 2000). They are typically closely associated with herringbone calcite, that preferentially veneers the vertical “supports”, together with bladed and blocky calcite cements (SUMNER, 1997b, p. 313). KERANS & DONALDSON (1989, p.85, fi g. 6) described “massive accumulation of concave-upward, dish- or bowl- shaped algal plates ranging in size from 0.1 to 2 m” in the Dismal Lakes Group, and termed them “cyanobacterial plate bioherms” (idem, fi g. 3). The plates, a few millimetres thick and a few centimetres long, are veneered by a few millimetres of “isopachous fi brous cement crust” (idem, fi g. 6b). These show some resemblance to fenestrate microbialites, but lack the net-like organization. In Campbellrand-Malmani car bo- nates, SUMNER (1997a, p. 458, fi g. 7) recognized “fi lmy laminae” draping over “supports creating complex microbial structures with complex voids” that combine to form cuspate, planar laminated, irregular columnar and contorted laminated structures, cemented and coated by herringbone calcite, in deep subtidal environments. SUMNER (1997b, p. 313) interpreted fenestrate micro- bialite as delicate thin microbial mats that provided irregular substrates for herringbone calcite cements that “precipitated contemporaneously with microbial growth”, and recognized that these deposits graded into herringbone calcite beds (SUM- NER, 1997b, fi g. 7) that precipitated directly on the seafl oor. She suggested that present-day mineral encrusted fl oating sub- strate-attached mats may help understand fenestrate micro- bialites (SUMNER, 1997b, p. 311). Fi gu re 10: Diagrammatic representation of Gongylina: submillimetric interlayers of Sparry Crust and coarse grains (after KNOLL & SEMIKHATOV, 1998, fi g. 11). Geologia Croatica Geologia Croatica 61/2–3 86 SUMNER (2000) reported fenestrate microbialites from greenstone belt carbonates at Steep Rock, Ontario (~2.8 Ga) and Huntsman, Zimbabwe (~2.8 Ga) and regarded fenestrate microbialite essentially as “laminated mat encased in … fi b- rous marine cement” (SUMNER, 2002, fi g. 2b). SUMNER & GROTZINGER (2004, p. 16–17) described spatial distri bu- tions of varieties of these deposits across the Campbellrand- Malmani platform, where they are most abundant in deeper water facies. They are associated with layers of “contorted laminated mat” (idem, fi g. 12) and may grade into less dis- tinctive by grossly similar “fenestral laminite” that in turn is associated with isopachous laminite (idem, fi g. 9). BART LEY et al. (2007, p. 216) reported, but did not fi gure, cuspate micro- bialite from the ~1300–1000 Ma Avzyan Fm of the southern Urals. Fenestrate microbialites appear to lack fi lamentous mic- rofabric (see SUMNER, 1997b, fi g. 5) but show some broad resemblance to “dendriform” and “lamelliform” Little Dal microfabrics. Both essentially consist of thin wispy netlike layers that defi ne cement fi lled voids, but fenestrate micro- bialite microstructure is generally signifi cantly (~10–100 x) coarser than that of dendriform and lamelliform fabric (cf. SUMNER, 1997b, fi gs. 8, 10 with AITKEN, 1989 fi gs. 10, 13). Nonetheless, the fi ne structure of lamelliform “ladder- rung” fabric (AITKEN, 1989 fi g. 12) and laminated mat (SUM NER, 1997b, fi g. 5a) is not dissimilar. 2.6. Thrombolite The dense, peloidal, clotted and/or fi lamentous, micritic and microspar microfabrics typical of Fine-grained Crust occurs in thrombolites as well as stromatolites. Thrombolites are dis- tinguished by their lack of well-developed layering, and by their macroscopic patchy or clotted fabrics – typically milli- metric to centimetric irregular dark masses (clots) in a lighter coloured matrix (AITKEN, 1967). They form beds and mo - unds, sometimes in association with stromatolites, and in the Proterozoic are rich in cement and/or fi laments. Calcifi ed microbial thrombolites (RIDING, 2000, p. 192) are well known in the Early Palaeozoic (PRATT & JAMES, 1982; KENNARD & JAMES, 1986). The earliest reported thrombolites are in the ~1.9 Ga Rocknest Formation and are suggested to have formed “through the inorganic encrustation of probable microbial communities by marine cements” (KAH & GROTZINGER, 1992, p. 305). No Mesoproterozoic thrombolites have been reported, but there are several reports from the Neoproterozoic. AITKEN & NARBONNE (1989) described thrombolites from the ~800 Ma Little Dal Group and the Ediacaran Bluefl ower Formation of northwest Canada. In the lower two-thirds of the Little Dal reefs “dendriform” and “lamelliform” stromatolites (AITKEN, 1989) are inter- layed with thrombolitic deposits with fi lamentous, clotted and spongy “cellular” fabrics (TURNER et al., 1993; 2000a, fi gs. 6e, 8h,i) comparable with those of Cambro-Ordovician throm- bolitic bioherms (TURNER et al., 1997, p. 441, 449; BATTEN et al., 2004). Thrombolites also occur in the latest Neopro- terozoic of Oman (MATTES & CONWAY MORRIS, 1990) and Namibia (GROTZINGER et al., 2000, 2005; JOHNSON & GROTZINGER, 2006). Coincidence of relatively widespread Neoproterozoic de- velopment of thrombolites with the development of fi la men- tous fabric supports the view (KENNARD & JAMES, 1986) that these thrombolites refl ect microbial calcifi cation. BAT- TEN et al. (2004, p. 264, fi g. 10) suggested that Neoproterozoic and Early Palaeozoic thrombolites generally developed in relatively deeper water than associated stromatolites. 3. DISCUSSION 3.1. PRESENT-DAY ANALOGUES The early search for present-day analogues for ancient stromat- olites led from freshwater tufa (WALCOTT, 1914; RODDY, 1915) to marginal marine domes (BLACK, 1933) and columns (LOGAN, 1961). These discoveries strongly supported KAL - KOWSKY’s (1908) inference that stromatolites are essentially microbial deposits. They stimulated widespread studies of lith- ifi ed microbial mats, but optimism that such examples provide appropriate analogues for all ancient marine stromatolites diminished as studies of Precambrian examples advanced (e.g., SEREBRYAKOV, 1976, p. 633; GROTZINGER & KNOLL, 1999, p. 314). Although there may be no present-day examples that closely resemble the very large domes and columns of the Late Archaean and early-mid Proterozoic, nonetheless there are smaller examples in diverse environments that appear to contain comparable fab rics. Fi gu re 11: Fenestrate microbialite. Late Archaean Campbellrand- Malmani platform, South Africa. Width of fi eld ~ 16 cm. Meshwork of rounded or angular to lunate fenestrae (white), generally <1cm in size, defi ned by thin curved dark layers, and crudely stacked in elongate, erect to sloping, masses a few centimeters in length. Geologia CroaticaRobert Riding: Abiogenic, microbial and hybrid authigenic carbonate crusts: components of Precambrian stromatolites 87 3.1.1. Fine-grained Crusts Fine-grained Crusts typically contain complex, dominantly fi ne-grained, carbonate microfabrics that refl ect precipitation in intimate association with organic matter, especially cell material and the extracellular polymeric substances that they produce, in microbial mats as a result of synsedimentary calci- fi cation associated with processes such as oxygenic photosyn- thesis and bacterial sulphate reduction (e.g., TRICHET & DÉFARGE, 1995; VISSCHER et al., 1998, 2000; REID et al., 2000; RIDING, 2000, table 1; ARP et al., 2003; KUHL et al., 2003; DUPRAZ et al., 2004; DUPRAZ & VISSCHER, 2005; BAUMGARTNER et al., 2006; KREMER et al., 2008). These fi ne-grained microfabrics range from dense, through clotted, to peloidal and fi lamentous (RIDING, 2000, fi gs. 6, 7). Indi- vidual, or associations of a few, micrite grains have been attri- buted to calcifi cation of bacterial cells after death (MAURIN & NOËL, 1977; KRUMBEIN, 1979; FOLK, 1993) and during life (THOMPSON & FERRIS, 1990). Clotted (grumous) microfabrics have commonly been linked with microbial processes (KAISIN, 1925; PIA, 1927, p. 36; HOFMANN, 1969, p. 40; BERTRAND-SARFATI, 1976; MONTY, 1976, fi g. 27, 1981, p. 2). Peloids – micritic aggregates of uncertain origin (MCKEE & GUTSCHICK, 1969) – include in place precipitates that have been variously interpreted as essentially abiogenic cements (MACINTYRE, 1984, 1985) and as bacte- rial aggregates (CHAFETZ, 1986). Associations of clotted and peloidal micrite develop in microbial organic matter (MONTY, 1976, p. 229, fi g. 27e; ZANKL, 1993), including decaying sponges (REITNER et al., 2000), and are commonly preserved in fossil sponges (MOCK & PALMER, 1991; WARNKE, 1995). They have also been interpreted as prod- ucts of calcifi ed bacterial biofi lm (RIDING, 2002). Although the presence of heterotrophic bacteria has been suggested to lead to cyanobacterial calcifi cation (PENTECOST, 1991, p. 6; CHAFETZ & BUCZYNSKI, 1992) this may in part be related to the experimental growth medium used (ARP et al., 2002). Furthermore, such degraded sheaths are likely to be irregular in form and encrusted by carbonate to varying de grees, whereas fossils such as Girvanella exhibit regular tube morphology in which wall-thickness remains constant in individual specimens, suggesting in vivo sheath impregnation (RIDING, 1977a, 2006). Such sheath calcifi cation is linked to photosynthetic carbon uptake (GOLUBIC, 1973; PENTE- COST, 1987, p. 134) particularly of HCO3 -. Some cyanobac- terial sheaths are tubular and others are irregularly digitate and often show vertical orientation that creates a bush-like appear- ance (RIDING & VORONOVA, 1982). These diverse exam- ples indicate that a wide range of fi ne-grained clotted-peloi- dal-shrub-like and fi lamentous fabrics, often co-occurring, characterize lithifi ed microbial mats. 3.1.2. Sparry Crusts Speleothem. Cave carbonate precipitates include a wide array of deposits that include sparry subaqueous crusts, e.g., phre- atic pool deposits (FAIRCHILD et al., 2007, fi g. 7.1b), and extensive fl owstone (e.g., BURNS et al., 1999, p. 499) that can also incorporate allochthonous grains. Speleothem calcite exhibits palisade calcite (KENDALL & BROUGHTON, 1978). KENDALL & IANNACE (2001, fi g. 6c) fi gured stro- matolitic crusts from a Pleistocene rimstone dam from Sor - rento, Italy, and also laminated dendrite crystals (idem, fi g. 8). They suggested (KENDALL & IANNACE, 2001, p. 695) that these might assist interpretation of similar lamination in fresh- water stromatolites such as described by FREYTET & VER - RECCHIA (1999), and also the sub-millimetric micrite- mic rospar laminae typical of the problematic stromatolite-like structure Archaeolithoporella. Travertine. Hot spring travertines can include crystalline crusts and shrub-like fabrics (CHAFETZ & FOLK, 1984; GUO & RIDING, 1992; RIDING, 2000, p. 196; PENTECOST, 2005, pl. 8c) that resemble some Precambrian isopachous laminites, and dendritic fabrics, as well as alternations of sparry crusts and allochthonous grains such as in Gongylina and Omachtenia. Calcrete. Laminar calcretes include stromatolitic fabrics with sub-millimetric light-dark bands (READ, 1976, pl. 3). Some have been termed lichen stromatolites (KLAPPA, 1979) and terrestrial stromatolites (WRIGHT, 1989), and can in clu- de diverse fabrics (see references in RIDING, 2000, p. 196). Alkaline lake crusts. GROTZINGER & JAMES (2000, p. 9) noted the scarcity of present-day analogues of seafl oor precipitated calcite and aragonite. They suggested that partial analogues may exist in non-marine thermal springs and also in alkaline lakes such as those of Pyramid Lake, Nevada (e.g., BENSON, 1994). KAZMIERCZAK & KEMPE (2006, fi g. 3, p. 124) illustrate partially silicifi ed aragonite stromatolitic crusts from alkaline lakes of Niuafo’ou Island, Tonga, that have with laminated, arborescent and tussock fabrics. They compared them with Proterozoic and also Palaeozoic ex am- ples. The stromatolites contain cyanobacterial remains (KAZ- MIERCZAK & KEMPE, 2006, fi g. 2) but do not appear to be precipitating at present (idem, p. 124). Marine evaporative splash crusts. Intertidal-supratidal carbonate crusts have been termed “pelagosite”, after the Italian name Pelagosa for the Croatian island Palagruža (see PALACHE et al., 1951), and “coniatolite” (PURSER & LO- REAU, 1973). These can be well-developed along evaporitive shorelines and intertidal radial-fi brous aragonite crusts up to 3cm thick on beach rock in the southern Persian Gulf (PURS- ER & LOREAU, 1973) form through repeated immersion and evaporation of slightly hypersaline seawater. Such in du- rated crusts have been termed “marine cements” and com- pared with travertine and Great Salt Lake cements (ALSHAR- HAN & KENDALL, 2003, pl. 2, p. 230, 237). Locally they are coated by cyanobacteria (ALSHARHAN & KENDALL, 2003 p. 214) and may therefore provide examples of Hybrid Crusts. HOFMANN & JACKSON (1987, p. 969) compared Proterozoic microdigitate stromatolite fabrics with the mic- rostructure of the carbonate crusts described by PURSER & LOREAU (1973, p. 368). MONTANARI et al. (2007) des crib- ed pelagosite from Palagruža and Hvar, Croatia, as “microstro- ma tolite” and interpreted the light-dark laminae as annual layers. Geologia Croatica Geologia Croatica 61/2–3 88 Hypersaline stromatolites. In the marginal marine Seb- kha el Melah of SE Tunisia, 5500 BP stromatolites that form- ed on beachrock and serpulid bioherms at the margins of a restricted lagoon have clotted and radial fi brous aragonitic microfabrics (DAVAUD et al., 1994, fi gs. 9, 10c, d). Metric stromatolitic domes composed of aragonite also occur in the present-day Great Salt Lake (EARDLEY, 1938; CAROZZI, 1962; HALLEY, 1976) and in Late Pleistocene Lake Lisan deposits of the Dead Sea (BUCHBINDER, 1981). Beachrock. Aragonite cements are lithifying components in both beachrock and stromatolites near Lee Stocking Island, Exuma Cays (WHITTLE et al., 1993). Beachrock at San Sal- vador Island, while differing in morphology from Stocking Island stromatolites (REID & BROWNE, 1991; MAC IN TY- RE et al., 1996) exhibits similar fenestral layering (KINDLER & BAIN, 1993, fi g. 4b, p. 245). Microbial infl uences on beach- rock formation (KRUMBEIN, 1979) suggest a con nec tion with the formation of coarse-grained near beach stro matolites (e.g., Stocking Island and Highborne Cay, Bahamas) that should be explored. Subtidal marine “cement” crusts. Research into marine lithifi cation during the 1960’s and 1970’s revealed thick fi b- rous calcite crusts, for example in Late Palaeozoic reefs (OTTE & PARKS, 1963), that were subsequently compared with Holocene submarine cements (SCHROEDER, 1972; JAMES et al., 1976) and, in some cases, interpreted to have been precipitated directly on the seafl oor (e.g., MAZZULLO & CYS, 1979, p. 918). These were often referred to as “ce- ments” and this terminology has commonly been applied to similar Precambrian sparry seafl oor precipitates. Present-day examples are typically subtidal botryoidal crusts of aragonite and Mg-calcite. Well-documented examples from the Belize fore-reef are restricted to millimetric to centimetric cavities (JAMES & GINSBURG, 1979, p. 117, fi gs. 6–5), and in some cases aragonite cement is intimately associated with peloidal silt (idem, fi gs. 6–15d, 6–17d). 3.1.3. Hybrid Crusts Freshwater tufa. Partial analogues for Proterozoic stromato- litic Hybrid Crust are likely to exist in present-day evaporitic and freshwater carbonates. Freshwater “tufa stromatolite” (RIDING, 2000, p. 191) is characterized by light-dark banded cyanobacterial deposits that commonly consists of fi lamen- tous, shrub-like and coarse spar fabrics (e.g., PIA, 1933, p. 41–42; STIRN, 1964; IRION & MÜLLER, 1968; GOLUBIC, 1973; MONTY, 1976, fi g. 7; PENTECOST, 1995; FREYTET & PLET, 1996; KANO et al., 2003; ANDREWS, 2005; PEN TECOST, 2005, pl. 14c, d). These intimate associations of sheath impregnation and encrustation (e.g., RIDING, 1977a; MONTY & MAS, 1981, fi g. 18b) preserve seasonal variations in microbial growth and associated precipitation. BERTRAND-SARFATI (1972, p. 29, 169, 188; 1976, p. 253) compared light-dark “fi lm” layering in Mesoproterozoic Atar stromatolites with present-day cyanobacterial mats from Andros Island (MONTY, 1965) and suggested that, for exam- ple in some Conophyton and Inzeria specimens, they may be seasonal (BERTRAND-SARFATI, 1972, pl. 11(4), pl. 22(2)) or even virtually daily (BERTRAND-SARFATI, 1976, p. 253). In this context, it is relevant to compare BERTRAND-SAR- FATI’s (1976, fi g. 1b) Mesoproterozoic fi lm microstructure with superposed layers of Schizothrix (MONTY & HARDIE, 1976, fi g. 2b) in present-day Andros mats. Similarly, BERT- RAND-SARFATI et al. (1994, p. 178–184) compared “alter- nating micrite-microsparite laminae” and fi lamentous and tussocky fabrics in Palaeogene fl uvio-lacustine stromatolites from France, with similar fabrics in Proterozoic stromatolites. In Late Pleistocene and Holocene marginal stromatolites of East African Rift lakes, CASANOVA (1994, fi g. 10a) describ- ed “doublets” composed of “light-coloured sparitic laminae and dark micritic laminae” as the “most frequent micro struc- ture observed in lacustrine stromatolites”. Although light-dark bands are widespread and often dis- tinct in freshwater stromatolites, their interpretation may not be straightforward (PENTECOST, 2005, p. 38–40). MONTY (1976, p. 199–208) described the complexity of layering in Andros and also fl uviatile mats. He noted that Andros mats essentially show alternations of “whitish calcareous layers and brownish organic ones”, but emphasized their complexity, that can include layers that develop within mats (MONTY, 1976, p. 199, 204). Fluviatile Rivularia shows both broad seasonal bands that relate to inorganic precipitation and fi ner bands thought to relate to photosynthetic activity (PENTE- COST, 1987, p. 125). As a result, the winter bands can be more heavily calcifi ed and light-coloured (PENTECOST, 1987, fi g. 6b; PENTECOST & SPIRO, 1990, p. 18). Similarly, in fl uviatile tufas, IRION & MÜLLER (1968, fi g. 3) recog- nized light sparry winter layers, and commented “as the algae do not grow in winter, pure layers of sinter are formed during this period” (idem, p. 165). On the other hand, in seasonal couplets from Lake Manyara and Lake Natron, Tanzania, CASANOVA (1994) interpreted the thinner (5–900 µm) orga- nic rich micritic layers as forming during the dry season, and thicker (20–1500 µm) sparitic layers, with numerous erect ~1 µm diameter fi laments, representing rainy season growth of fi lamentous cyanobacteria (CASANOVA, 1994, p. 212–213, fi gs. 10, 11). Thus, in the fl uviatile tufas the light bands may be relatively inorganic sparry precipitates, whereas in the Lake Natron example the sparry layers represent rapid growth of erect cyanobacteria. In fact this latter case may also apply to some fl uviatile tufa too; e.g., IRION & MÜLLER (1968, fi g. 4) show “dark layers … deposited during the winter” and “white layers, formed during the summer”. Not surprisingly, therefore, there has been debate concern- ing controls on lamina formation in fl uviatile tufas (KANO et al., 2003, p. 259; ANDREWS & BRASIER, 2005, p. 413; ANDREWS, 2005; PENTECOST, 2005, table 3). KANO et al. (2003, p. 255) report reversed seasonal patterns at different sites: dense winter and porous summer laminae at one, and dense summer and porous winter laminae at another. In con- trast, many studies report denser/micritic winter-spring layers and more porous/sparry summer layers at both North Ame ric- an (e.g., CHAFETZ et al., 1991) and European (e.g., JANS- Geologia CroaticaRobert Riding: Abiogenic, microbial and hybrid authigenic carbonate crusts: components of Precambrian stromatolites 89 SEN et al., 1999, fi g. 2d; and other references in AND REWS & BRASIER, 2005, p. 413). In addition to depositional pro- cesses, diagenetic effects are also probably important (ARP et al., 2001; ANDREWS & BRASIER, 2005, p. 419; PEN TE- COST, 2005, fi gs. 8, 9). For example, fossil tufas in Belgium possess “more sparry calcite laminae than the Recent preci pi- tates” that have preferentially developed at particular horizons (JANSSEN et al., 1999, fi g. 5). These studies suggest that interpretation of Hybrid Crust in ancient stromatolites will not be simple, although it remains possible that they too, in some cases, may be seasonal. Travertine shrubs. The likelihood that microdigitate stro matolites are essentially inorganic has long been con si- dered (GROTZINGER, 1986B; HOFMANN & JACKSON, 1987). However, based on similar structures (shrub travertine) in present-day hot spring travertines, they too may have combined inorganic and microbial components (CHAFETZ & FOLK, 1984; GUO & RIDING, 1994). These shrubs are typically a few millimetres to centimetres in size, but larger examples up to 8 cm long (CHAFETZ & FOLK, 1984, p. 305, fi g. 8) resemble dendrites described from the upper Pethei Group by POPE & GROTZINGER (2000). Cave crusts. Sparry cave crusts can be interleaved with fi ne-grained layers, e.g. fi ne dark laminae in cave popcorn, some of which may be microbial (THRAILKILL 1976; MELIM et al., 2001) in which case they could be regarded as Hybrid Crusts. Nonetheless, “much cave popcorn contains thick layers of clear calcite or aragonite with no indication of organic involvement” (THRAILKILL, 1976, fi gs. 7–12, p. 83). COX et al. (1989) described cyanobacterial speleothem as subaerial stromatolite. Marine evaporitive splash crusts. Present-day Abu Dha- bi crusts formed through repeated immersion and evaporation of slightly hypersaline seawater are ephemerally coated by cyanobacteria (ALSHARNAN & KENDALL, 2003 p. 214) possibly develop interlayered sparry and microbial fabrics, and alternations of pellet micrite and fi brous aragonite layers occur in sub-Recent Dead Sea stromatolites (DRUCKMAN, 1981, fi g. 6). Evaporite stromatolites. Microbial mats in hypersaline environments can be colonized and be encrusted by evaporite minerals (e.g. KENDALL & SKIPWITH, 1968; GERDES et al., 1993, pl. 13) and involved in the development of stro ma- tolitic structures (AREF, 1998, fi gs. 4a, 6b). These can contain calcifi ed microbial fi laments and show well-defi ned even la mi- nation (ROUCHY & MONTY, 1981, fi gs. 7, 9; 2000, fi g. 1). Miocene marine stromatolite. CONIGLIO et al. (1988, p. 102, 105, fi gs. 3, 7) described a mid-Miocene reefal plat- form veneered by a 1m thick dolomitized deep-water “stro- matolite” bed, forming domes up to 10 m across, composed of micropeloidal and homogeneous mudstone that locally grades to fi brous fabric that they compared with calcitized aragonite cement. 3.1.4. Fenestrate microbialite SUMNER (1997b, p. 311) envisaged that plumose and simi- lar Archaean fenestrate fabrics originated by synsedimentary lithifi cation of vertically tufted microbial fi lms, as in hot springs (e.g., WALTER et al., 1976) and in the pinnacle, colum- nar and lift-off mats of ice-covered lakes in Antarctica (WHAR- TON 1994, fi g. 3). Comparisons could also be suggested with cool and hot spring travertine fabrics, especially those with rounded millimetric to centimetric voids formed by precipita- tion on water and bubble surfaces; these (e.g., REIS, 1926, p. 181; GUO & RIDING, 1998, fi gs. 4, 5). GANDIN & WRIGHT (2007) interpreted Campbellrand-Malmani fenestrate fabrics as products of synsedimentary deformation of organic fi la- ments “exerted by the growth of evaporite nodules, during the coalescence of enterolithic folds”. 3.1.5. Thrombolite Neoproterozoic thrombolites have been compared with Early Palaeozoic examples (TURNER et al., 1997), but present-day analogues of these types of thrombolite have not been confi - dently recognized. LAVAL et al. (2000) suggested that fabrics within freshwater tufa mounds from Pavilion Lake, British Columbia, might be analogous with those of Cambrian throm- bolitic reefs containing Epiphyton and Girvanella. This brief and very incomplete overview suggests that diverse partial analogues of Sparry and Hybrid crust deposits may be found in Quaternary evaporitic, alkaline lake, and fresh water environments. None of these present-day deposits is known to create sparry crusts on the scale observed in the Precambrian, e.g., in metric domes and cones. Nonetheless, some should provide analogues for small crusts, and in parti- cular for their microfabrics. 3.2. RECOGNITION AND INTERPRETATION OF PRECAMBRIAN STROMATOLITIC CRUSTS Abiogenic precipitated stromatolites. Awareness of the wide- spread existence of seafl oor precipitates that could be confused with lithifi ed microbial mats emerged gradually from studies of Proterozoic, and subsequently Archaean, stromatolites in the 1980’s (e.g., KERANS, 1982; GROTZINGER, 1986a). This research led to critical reassessment of the nature and signifi cance of Precambrian authigenic seafl oor carbonate crusts. GROTZINGER & READ (1983) described microdig- itate stromatolites as “cement laminae”, and GROTZINGER (1986b) considered the possibility that they were “entirely abiotic”. GROTZINGER (1989b, p. 11) drew attention to “the direct precipitation of stromatolitic laminae” and GROTZ- INGER & ROTHMAN (1996, p. 424) suggested that the growth of large Early Proterozoic stromatolites (JACKSON, 1989, fi g. 13) could “be accounted for exclusively by abiotic mechanisms, particularly where growth by precipitation is thought to be important”. GROTZINGER & KNOLL (1999, p. 343) noted that “the growth of abiotic marine crusts might substitute for mats and create the same end result” and GROTZ- INGER & JAMES (2000, p. 7) commented “abiotic precipi- tates are morphologically and mineralogically identical to marine cements of Phanerozoic age … with the striking differ- ence that they do not simply fi ll voids but are widespread as direct precipitates on the sea fl oor itself”. These abiogenic precipitates were commonly referred to as seafl oor cements, Geologia Croatica Geologia Croatica 61/2–3 90 and this usage continued even after GROTZINGER & KNOLL (1995, p. 579) pointed out that seafl oor crusts/encrustations should be distinguished from “true cements which bind sedi- ment particles and line voids” (e.g., KAH & KNOLL, 1996, p. 79; POPE et al., 2000, p. 1145). These investigations led to realization that abiogenic sea- fl oor precipitates were not only associated with stromatolites but also, in some cases, included them. Thus, GROTZINGER & JAMES (2000, p. 7, fi g. 5) summarized “sea-fl oor encrust- ing precipitates” as including microdigitate stromatolites, large crystal fans, isopachous laminites, herringbone calcite, and dendritic tufa. Inclusion of isopachous laminites implied that abiogenic seafl oor crusts had not only formed micro di- gitate stromatolites on peritidal fl ats, but were also responsible for larger subtidal stromatolites that included Palaeoproterozoic (JACKSON, 1989, fi gs. 6, 13; GROTZINGER & ROTH- MAN, 1996, fi g. 1b; GROTZINGER & KNOLL, 1999, fi g. 3a; POPE et al., 2000, fi g. 4; POPE & GROTZINGER, 2000, fi g. 8) and late Archaean (GROTZINGER & KNOLL, 1995, fi g. 1b; POPE et al., 2000, fi g. 2d; SUMNER & GROTZ ING- ER, 2004, fi g. 4a) examples. As a result, POPE et al. (2000, p. 1149) considered “thinly laminated, isopachous stroma- tolites” “to have a largely abiotic origin”. Fine-grained and Sparry crust. The outline of previous research presented here suggests that three principal categories of well-preserved stromatolites can be recognized in the Pro- terozoic: Fine-grained, Sparry and Hybrid crust. Although no present-day large subaqueous domes and cones with compa- rable structure are known, smaller present-day deposits can guide interpretation by providing partial analogues on two levels: microfabric and lamina structure. Precambrian Fine- grained Crust stromatolites resemble present-day lithifi ed microbial mats; in addition they conform to the great majority of Phanerozoic normal marine stromatolites. In contrast, Spar ry Crust stromatolites have fabrics and structures that resem ble present-day speleothem fl owstone and hot-spring traver tine crystalline crust. This suggests that Sparry Crust stromatolites are essentially abiogenic aqueous precipitates, in the sense that their formation does not require biotic pro cesses and that they do not typically contain organically generated fabrics. Sparry and Fine-grained carbonate stromatolites can broadly resemble one another in stratiform to domical and columnar morphologies, but are generally distinct in fabric and lamina arrangement. Fine-grained stromatolites have micritic and microspar microfabrics and their layering is relatively uneven to discontinuous and usually shows poor inheritance. Sparry stromatolites have coarsely crystalline, equant spar or radial- fi brous, microfabrics and their layering is even to isopachous, and laterally persistent layers with good inheritance. Hybrid Crust. Since Fine-grained and Sparry stro ma- tolites differ in fabric and detailed structure and, as interpreted here, differ in origin (microbial as opposed to abiogenic) it could well be argued that they need not be grouped together as stromatolites. However, the gap between Fine-grained and Sparry crust stromatolites is bridged by Hybrid Crust stroma- tolites, which typically consist of millimetric alternations of Fi gu re 12: Interpretive summary of Precambrian authigenic crusts. Principal components: Sparry Crust (essentially abiogenic precipitate), Fine-grained Crust (lithifi ed microbial mat), and allochthonous grains. Intermediates: Hybrid Crust, Coarse Grained Crust, and Coarse Grained Mat. Apart from allochthonous grains, each of the other fi ve components and intermediates has at some time been regarded as containing examples of stromatolites. Examples of these stromatolitic deposits are indicated in red. Geologia CroaticaRobert Riding: Abiogenic, microbial and hybrid authigenic carbonate crusts: components of Precambrian stromatolites 91 Sparry and Fine-grained crust. These alternations are inter- preted here as more-or-less regular, possibly seasonal, fl uct- uations in microbial accretion and abiogenic precipitation. If this is correct then they refl ect a relatively balanced mix of abiotic and biotic processes. In the early-mid Proterozoic, Hybrid Crust does not merely provide a link between Fine- grained and Sparry crusts, but – in many subtidal carbonate platform environments – it appears to supercede them in ab- undance. For example, giant decametric subtidal domes of the Palaeoproterozoic Pethei Group (SAMI & JAMES, 1996), and Mesoproterozoic Burovaya Formation (PETROV & SEMI KHATOV, 2001) are composed of Hybrid Crusts, and one of the most distinctive stromatolites, Conophyton, which locally forms decametric cones, also often has a Hybrid Crust composition (see WALTER, 1972). Awareness of the importance of authigenic carbonate crusts in association with Precambrian stromatolites was pre- saged by recognition of the role of synsedimentary lithifi cation in the formation of high-relief coniform stromatolites (DO- NALDSON, 1976; GEBELEIN, 1976). Conophyton and simi- lar forms were already known to commonly contain distinctive streaky microstructure (KOMAR et al., 1965; CLOUD & SEMIKHATOV, 1969, fi g. 2). Clearer understanding of the signifi cance of these fabrics came from KERANS’ (1982) (see GROTZINGER, 1989b, p. 10) suggestion that “cement crusts were precipitated on microbial laminae while stro ma- tolites were growing”. Similarly, GROTZINGER & KNOLL (1999, p. 329–330) later suggested that “the growth of abio- genic marine crusts might substitute for mats and create the same end result”. It thus appears that some stromatolites, such as some forms of Conophyton, persistently had a dual abio- genic and microbial origin in which the fi ne-grained layers are essentially organic in origin (KOMAR et al., 1965, p. 67; see WALTER, 1972, p. 86) and the precipitated spar, as KE- RENS (1982) suggested, is essentially inorganic. Sub se qu- ently, SAMI & JAMES (1994, p. 120) suggested that spar- micrite couplets refl ect alternation of “cement precipitation and microbial mat growth”. There is therefore a need to distinguish not only between what PERRY et al. (2007, p. 169) regarded as “microbially constructed stromatolites” and “abiotic, chemically precipitated carbonate crusts”, but also between these and Hybrid Crust stromatolites. Crust discrimination. Against earlier expectation (e.g., GROTZINGER & ROTHMAN, 1996; GROTZINGER & KNOLL, 1995, 1999) it now seems possible to apply details of fabric and lamina arrangement criteria to the recognition of Fine-grained, Hybrid, and Sparry crusts (Fig. 12). These cri- teria draw on observations developed by GROTZINGER & READ (1983) in their recognition of the nature of micro di- gitate stromatolites, and by GROTZINGER (1989b, p. 11) when he drew attention to “the direct precipitation of stro- matolitic laminae”. Seafl oor encrusting precipitates typically consist of fans and layers of elongated fi brous crystals or dendrites (GROTZINGER & JAMES, 2000, p. 7, fi g. 5). Simi- larly, POPE et al. (2000, p. 1142) found that “stromatolites with isopachous fi ne lamination” commonly have “radial fi brous texture”. POPE et al. (2000) interpreted “isopachous stromatolites to have been dominated by chemogenic pre- cipitation in the absence of microbial mats, and the growth of peloidal stromatolites to have been controlled by sedimentation in the presence of microbial mats” (idem, p. 1139), and added “thinly laminated isopachous stromatolites are considered to have a largely abiotic origin” (idem, p. 1149). Thus, whereas lithifi ed microbial mats are characterized by micritic (clotted- peloidal-bushy), and sometimes grainy, fabrics and uneven to irregular layering, crystalline seafl oor precipitated crusts are characterized by sparry/radial-fi brous fabrics and more even and regular layering. Hybrid Crusts consist of millimetric alternations of these fabrics, in layers that are more regular than those usually present in Fine-grained stromatolites, and less regular than those of Sparry Crust stromatolites (Fig. 12). If these generalizations are valid, they signal an advance to- wards the Holy Grail of stromatolite studies – confi dent dis- crimination between abiogenic and microbial deposits. At the same time this recognizes Hybrid Crust as a key component of early-mid Proterozoic stromatolites. Realization of the existence of Hybrid Crust raises questions concerning its role in “giant” stromatolite formation, which may be signifi cant, as well as the nature of Archaean stromatolites – specifi cally the relative importance of Sparry, Hybrid and Fine-grained crusts in their formation. Sparry Crust with subordinate Fine-grained Crust. Hyb rid Crust as defi ned here generally exhibits relatively regular alternations of Sparry and Fine-grained crust. But in some cases the proportions of Sparry and Fine-grained crust are less balanced, as in the ~1200 Ma Society Cliffs Formation (KAH & RIDING, 2007, p. 799) where fi ne-grained calcifi ed cyanobacterial crust layers are subordinate to Sparry Crust. This raises questions, apart from terminological ones. For ex- ample, is there an overriding control on Hybrid Crust de vel- opment? In fl uviatile and lacustrine tufa stromatolites, dark- light layers appear to refl ect seasonal controls on microbial growth and carbonate precipitation, and this might also apply to Precambrian Hybrid Crusts (see Analogues, Hybrid Crusts, Freshwater tufa, above). However, if subaqueous colonization of Sparry Crust by microbial mat were intermittent, in res- ponse to environmental factors operating on different and less regular time-scales, such as changes in water depth or salinity, then irregular alternations could be produced. These could include rare layers of Fine-grained Crust within Sparry Crust, and vice versa. Further exploration of these possibilities and their controlling factors is required. 3.3. GIANT STROMATOLITES The volumetric importance of stromatolites in the construc- tion of Precambrian carbonate platforms has long been empha- sized (e.g., HOFFMAN, 1969; GROTZINGER, 1990, p. 96) and the sizes of individual domes and cones can be remark- able. KERENS & DONALDSON (1989, p. 84, fi gs. 4,5c) described upward transition from conical to domal stromato- lites in the Dismal Lakes Group, with cones up to 6 m diam- eter and 12 m in synoptic relief, and domes up to 40 m in Geologia Croatica Geologia Croatica 61/2–3 92 diameter and 10–15 m in synoptic relief. Whereas such large cones appear to be relatively rare, metric to decametric domes are locally important subtidal components of Precambrian carbonate platform (SUMNER & GROTZINGER, 2004, p. 16). Archaean examples include Steep Rock (e.g., NISBET & WILKS, 1989), Campbellrand-Malmani (e.g., YOUNG, 1932; TRUSWELL & ERIKSSON, 1973, p.6; ERIKSSON, 1977; BEUKES, 1987), and Carawine (e.g., MURPHY & SUMNER, 2008). Palaeoproterozoic examples include the Whalen Group, Wyoming (HOFMANN & SNYDER, 1985, p. 843) (now regarded as probably correlative with the lower Nash Form Fm., and therefore ~2.1 Ga, BEKKER et al., 2003, p. 311), Pethei Group (e.g., HOFFMAN, 1969), Rocknest (GROTZ- INGER, 1986b, p. 833) and Beechey Fm (PELECHATY & GROTZINGER, 1989, fi g. 9). A late Mesoproterozoic exam- ple is the Burovaya Fm (PETROV & SEMIKHATOV, 2001). Neoproterozoic examples include Little Dal reefs (AITKEN, 1989), Boot Inlet Fm (NARBONNE et al., 2000), and Noon- day Dolomite (CLOUD et al., 1974; CORSETTI & GROTZ- INGER, 2005). In the latest Archaean Campbellrand-Malmani platform, elongate mounds up to 10m across and 40m or more in length (BEUKES, 1987, p. 9; SUMNER & GROTZINGER, 2004, fi gs. 10, 14) contain occasional pseudomorph fans, and grain- stone and “cement” layers, but their principal constituents are “Boetsap-style lamellae” consisting of darker fi nely crys tal- line and lighter coarse sparry layers (SUMNER & GROTZ- INGER, 2004, p. 14, fi g. 11). Archaean and Palaeoproterozoic “giant mounds” are commonly steep-sided, elongate – pre- sumably in response to current infl uence – and associated with decimetric fans and crusts (GROTZINGER, 1986b, p. 833; SUMNER & GROTZINGER, 2004, p. 16). GROTZ- INGER (1986b, p. 833) described Rocknest stromatolitic mo- unds “5–40 m wide and with up to 4m of synoptic relief” locally “encrusted with layers of bladed, isopachous marine cement which may compose up to 50% of the bioherm”. At the Groot Boetsap River section, 45 km WNW of Warrenton, South Africa, a 135 m section of Cambrellrand-Malmani car- bonates shows elongate stromatolite mounds up to 10 m wide, 40 m long and 2.5m relief dominated by crinkled lamination with good inheritance (TRUSWELL & ERICKSSON, 1973, p. 6, fi g. 3) (Fig. 13). Where well-preserved microstructures are documented, these large domes often appear to be characterized by Sparry or Hybrid Crusts. SAMI & JAMES (1996, p. 217) emphasized the importance of “spar-micrite couplets” in Pethei subtidal stromatolites, and in the late Mesoproterozoic Burovaya PET- ROV & SEMIKHATOV (2001, fi g. 6, p. 269) noted that “ce- ment-based microstructures” interlayered with clotted micrite create parallel lamination that “is remarkable in the giant dome facies for its smoothness and lateral extent”. It seems reasonable to infer that, in addition to providing increased strength and stability, a signifi cant abiogenic Sparry Crust component enhanced stromatolite accretion, contributing to their size and relief. Conversely, it appears possible that few if any of the impressively large stromatolites that dominate the shallow subtidal areas of Proterozoic carbonate platforms was solely composed of lithifi ed microbial mat. At the present-day, coarse grained agglutinated stroma- tolites (RIDING, 1991, p. 30) can have metric dimensions, as at Lee Stocking Island (DILL et al., 1986), but none is known that compares in size with the largest Precambrian domes. Nonetheless, there are Phanerozoic examples where stro ma- tolite size has increased with evaporative conditions. For example, metric domes occur in association with gypsum de- posits in the mid-Miocene of the eastern Ukraine (PERYT et al., 2004, fi g. 4). Dolomitized laminar crusts, usually regarded as stromatolites and often associated with early marine ce- ments, form large reefal masses in the Late Permian Zechstein carbonate-evaporite cycles of northern Europe (PAUL, 1995), and POPE et al. (2000, p. 1143) drew attention to the similar age “very thinly and evenly laminated” metric stromatolites associated with evaporites in the Zechstein Basin of north- east England. Fi gu re 13: Stromatolite domes, tens of metres in extent, elongated in direction of view. Late Archaean Campbellrand-Malmani platform, dry bed of Groot Boetsap River, South Africa. Note generally even layers and good inheritance, even at this scale. Persons at upper left indicate size. Geologia CroaticaRobert Riding: Abiogenic, microbial and hybrid authigenic carbonate crusts: components of Precambrian stromatolites 93 3.4. NATURE OF ARCHAEAN STROMATOLITES Whether giant Archaean domes, such as those in Steep Rock and Campbellrand-Malmani carbonates, are also largely Hyb - rid Crusts remains uncertain. Their relatively even layering and good inheritance suggest that they are likely to be Hybrid and/or Sparry crusts. However, their microfabric preservation is generally poor and even in the relatively well-preserved Cambellrand-Malmani carbonates the nature of the Boetsap laminae that are major components of the large domes is unclear. SUMNER & GROTZINGER (2004, p. 16) concluded that some elongate stromatolite mounds contain “a signifi cant component of clastic carbonate” whereas others, especially those better preserved, have “more precipitated textures”. But whether this was microbially mediated or essentially abio- genic is uncertain. Thus, Campbellrand-Malmani giant domes may have been Hybrid Crusts; but there is also the possibility that they are more completely abiogenic. Coniform stromatolites in the Warrawoona Group (~3.45 Ga) of Western Australia show fi ne continuous laminae (LO- WE, 1980, 1983) and sparry microfabrics (HOFMANN et al., 1999, fi g. 3), although these could well be secondary (HOF- MANN et al., 1999, p. 1259). The origins of Pilbara stro ma- tolites have been debated (e.g., LOWE, 1994, 1995; BUICK et al., 1995). HOFMANN et al. (1999, p. 1260–1261) argued that examples ~50 km west of Marble Bar should be regarded as having “a biogenic component” based on features such as greater uniformity of laminae in the columns than in inter- vening areas, second-order corrugation that appear to have accreted upward, continuity of non-isopachous laminae, exten- sive regular development, steep slopes – often >40° and up to 75° – not known to be formed abiogenically. ALLWOOD et al. (2006, p. 717) supported a biogenic origin, including in their reasoning the diffi culty of accounting abiogenically for both the conical shape and the non-isopachous layering which has produced parallel-sided pseudocolumns, and also the mo- re variable interspace laminae. Furthermore, the only known present-day analogues for coniform stromatolites are struc- tures formed by the infl uence of “vertically motile” microbes in hot springs such as Yellowstone (WALTER et al., 1976; ALLWOOD et al., 2006, suppl. notes, p. 16). Thus, although some of these Pilbara structures superfi cially resemble iso- pachous laminites, they could differ from them in signifi cant details: specifi cally conical form and non-isopachous laminae with near vertical rather than upward expanding margins to the pseudocolumns. Some Pilbara stromatolites show well la- mi nated interspaces (see HOFMANN, 2000, fi g. 3b), sug gest- ing that these as well as the cones were seafl oor crusts. Per- haps the outstanding question is whether coniform structures with vertical margins really cannot be produced by abiogenic precipitation. 3.5. SECULAR CHANGES AND CONTROLS GROTZINGER & KASTING (1993, p. 235, fi gs. 1, 2) pointed out that “massive, thick beds of marine cements”, common in the Late Archaean, gave way to “microdigitate stromatolites (tidal-fl at marine cement crusts)” in the Palaeoproterozoic, and to “micritic whitings” in the Neoproterozoic. They argued that “prolifi c precipitation of aragonite as giant botryoids up to 1 m in radius and magnesian calcite as stratigraphic sheets up to several meters thick” in the Archaean refl ected elevated over-saturation for CaCO3 that subsequently declined over geological time (GROTZINGER & KASTING, 1993, 235– 236). Subsequent research provided further details of this sig nifi cant long-term trend (GROTZINGER & KNOLL, 1995; KAH & KNOLL, 1996; SAMI & JAMES, 1996; SUM NER & GROTZINGER, 1996a, b). On an even larger time-scale, JAMES et al. (1998, JSR) suggested that carbonate sedimen- tation was respectively dominated by massive seafl oor precip- itates (Archaean-Palaeoproterozoic), molar-tooth mudstones and grainstones (Meso-Neoproterozoic), and burrowed and fossiliferous limestones (Phanerozoic). In this context it seems possible that the early-mid Pro- terozoic importance of Hybrid Crust stromatolites coincided with long-term transition from dominance of Sparry Crusts on Archaean seafl oors to the rise to prominence of Fine-grain- ed stromatolites and thrombolites in the Neoproterozoic. It may even be speculated that conditions favouring abiogenic Sparry Crust precipitation in the late Archaean tended to in- hibit microbial growth and substrate colonization, and that Hybrid Crusts developed as these conditions gradually be- came more favourable to microbial growth. Perhaps condi- tions that alternately favoured microbial growth and abiogenic precipitation fl uctuated at relatively regular intervals, perhaps even seasonally. As Sparry and Hybrid crust stromatolites dec- lined, Fine-grained Crust stromatolites, together with throm- bolites, increased and probably become dominant during the Neoproterozoic. From the mid-Mesoproterozoic onward they locally contain conspicuous – presumably cyanobacterial – fi lamentous fabrics. The key long-term secular control on Sparry Crust devel- opment during the Archaean and Proterozoic has long been suggested to be seawater chemistry and its effect on carbonate nucleation and precipitation (GROTZINGER, 1990; GROTZ- INGER & KASTING, 1993; SUMNER & GROTZINGER, 1996a). Hybrid Crust development can be integrated with this view. As seawater carbonate saturation declined, Sparry Crusts declined and Fine-grained Crusts increased, and during this long transition Hybrid Crusts were volumetrically abundant. In addition, cyanobacterial sheath calcifi cation could refl ect induction of CO2-concentrating mechanisms in response to declining atmospheric CO2 level, and this may have been primarily responsible for the mid-Proterozoic appearance of widespread fi lamentous microbial fabrics in stromatolites and thrombolites (RIDING, 2006; KAH & RIDING, 2007). Thus, long-term patterns of stromatolite and thrombolite fabric development may be intimately related to large-scale changes in ocean-atmosphere composition. Conditions of Sparry Crust formation, especially rapid accumulation, may have tended to inhibit microbial growth and colonization. As these conditions reduced, Sparry and Fine-grained crusts may increasingly have interacted to develop Hybrid Crust. Lithifi ed microbial mat stromatolites Geologia Croatica Geologia Croatica 61/2–3 94 may therefore have antecedents in Hybrid Crusts that formed in environments of intense seafl oor carbonate precipitation. Certainly it appears that many stromatolites older than ~1000 Ma differ from present-day normal marine stromatolites cha- racterized by Fine-grained Crust. Conversely, marine Sparry Crusts, as both stromatolitic and other deposits, have been generally scarce since the Mesoproterozoic (SUMNER & GROTZINGER, 2004, p. 2). However, they redeveloped bri- efl y in Cap Carbonates associated with rapid Neoproterozoic deglaciation events (e.g., GROTZINGER & JAMES, 2000, fi g. 7; SUMNER, 2002; NOGUEIRA et al., 2003) and also at times during the Phanerozoic when “massive carbonate pre- cipitation was favored” (GROTZINGER & KNOLL, 1995, p. 578). POPE et al. (2000, p. 1139) suggested that isopach- ously laminated stromatolites “are dominated by chemogenic precipitation in the absence of microbial mats” and are “best developed atop Proterozoic and Paleozoic carbonate platforms that underlie major evaporite successions”. Among several examples, they cited coatings on reefs in the Silurian Michigan Basin, and also Late Permian crinkly stromatolites noted by SMITH (1981) from the Zechstein Basin of northern Europe (POPE et al., 2000, table 1, fi gs. 7, 9). They described the Mi- chigan isopachous stromatolites as commonly having “radial fi brous texture” (POPE et al., 2000, p. 1142) which suggests that they are Sparry Crust, but the precise nature of the “crink- ly” Zechstein stromatolites remains uncertain. 4. SUMMARY Seafl oor carbonate crusts. Petrographic classifi cations emerg- ing from the “carbonate revolution” of the 1950’s (e.g., FOLK, 1959; DUNHAM, 1962) were primarily focused on Phanero- zoic marine examples. Extensive research since then has shown that Precambrian seafl oor carbonate crusts com prise a wide variety of deposits that accreted at the sediment-water interface at depths ranging from intertidal to deep subtidal. They occur as irregular sheets and also as domes and columns, some of which are decametric in scale. Based on the informa- tion reviewed here, six categories can be recognized (Table 1) of which four (Fine-grained Crust, Sparry Crust, Hybrid Crust, Sparry Crust plus Grains) include at least some examples that have been regarded as stromatolites. Interpretations based on partial present-day analogues suggest that Fine-grained Crust is lithifi ed microbial mat, Sparry Crust is essentially abiogenic precipitate, Hybrid Crust is a mixture in which microbial mat and abiogenic crusts alternate, and Sparry Crust plus Grains forms where relatively large grains are incorporated into abio- genic crust (Fig. 14). Fine-grained Crust is dominated by micritic and micro- sparitic (dense, clotted, peloidal, fi lamentous) microfabrics. These may contain fenestrae and incorporate allochthonous grains. In older examples micritic fabrics have often aggraded to microspar. It forms diverse stratiform, domical and colum- nar stromatolites with relatively uneven to discontinuous la- yers that usually show poor inheritance. It is also a key com- ponent of thrombolite. In the Proterozoic, Fine-grained Crust is interleaved with Sparry Crust to form Hybrid Crust. In ad- dition, it is the dominant components of many, usually rela- tively small (typically centimetric–decimetric) stromatolitic domes, columns and layers. Palaeoproterozoic examples of these are less well-known, possibly due to poor fabric pre ser va- tion. Fine-grained Crust thrombolites are relatively wide spread in the Neoproterozoic. Present-day analogues of Fine-gra ined Crust are diverse as lithifi ed microbial mats in non -marine and marine environments (see Analogues). On this basis, Pro- terozoic Fine-grained Crust is interpreted as an essentially bio- tic deposit resulting from in situ microbial mediation of car- bonate precipitation, locally augmented by incorporation of allochthonous grains. Sparry Crust has coarsely crystalline, often radial-fi b- rous, microfabric. Examples include large and small radial bo- tryoids and crystal pseudomorphs, microdigitate stromatolitic “tufa”, dendrite, isopachous laminite, and herringbone calcite. These variously form domes, vertical crystal growths, and ex- tensive layers. At least two categories, microdigitate “tufa” and isopachous laminite, create structures that have been ge- nerally regarded as stromatolites. These Sparry Crust stro ma- tolites are characterized by even, often isopachous, laterally persistent layers with good inheritance, and have been most widely recognized in the Palaeoproterozoic and Meso pro te- rozoic, with microdigitate forms occupying peritidal envi ron- ments and isopachous laminite relatively deeper water facies. Large subaqueous Sparry Crust domes, comparable with tho- se of the Proterozoic, are not known at the present-day. None- theless, there is a wide variety of potential present-day ana- logues for Sparry Crust fabrics in smaller scale deposits, e.g., speleothem fl owstone, and hot-spring travertine crystalline crust (see Analogues). These analogues suggest that Sparry Crust is essentially abiogenic, in the sense that its formation does not require biotic processes and that it does not typically contain organically generated fabrics. Nonetheless, it can incorporate and veneer organisms and organic material. Fi gu re 14: Key features of crust discrimination, based on microfabric (fi ne-grained or sparry) and layer arrangement (irregular, poor inherit- ance; even, good inheritance). Geologia CroaticaRobert Riding: Abiogenic, microbial and hybrid authigenic carbonate crusts: components of Precambrian stromatolites 95 Sparry Crust has often been termed “seafl oor cement”, although it has been pointed out that this confl icts with the ge- neral usage of cement as precipitate between grains and with- in voids (GROTZINGER & KNOLL, 1995, p. 579). It has also been referred as “seafl oor crusts” and “inorganic crusts” (GROTZINGER & KNOLL, 1995, p. 578–579), “encrusting beds” (of bladed and herringbone calcite) and “microbialites coated by cements” (SUMNER & GROTZINGER, 1996a), “abiotic marine crusts” (GROTZINGER & KNOLL, 1999, p. 343), “encrusting marine cement directly on the growing stro- matolite” (GROTZINGER & KNOLL, 1999, p. 329–330), “seafl oor-encrusting marine cement” (POPE et al. 2000, p. 1145), and “seafl oor encrustations” (SUMNER, 2002). Of the four general categories of subaqueous Sparry Crust recognized here, based on Precambrian examples (Table 1), three include stromatolitic deposits: (i) Botryoidal fans and crystal pseudo- morphs include small radial fi brous millimetric microbotryoids that build Tarioufetia and Tungussia (BERTRAND-SARFATI, 1972); (ii) microdigitate stromatolites: small laminated col- umns of radial crystals (GROTZINGER & READ, 1983; HOF MANN & JACKSON, 1987); (iii) isopachous laminite (JACKSON, 1989, SUMNER & GROTZINGER, 2004). In addition, thin (millimetric and submillimetric) Sparry Crust interlayered with Fine-grained Crust is an integral component of Hybrid Crust (see below). Hybrid Crust typically consists of light-dark, often mil- li metric, alternations of Sparry and Fine-grained crust. It bu- ilds stromatolites with well-developed even, although not usu- ally isopachous, layering that is laterally quite persistent with generally good inheritance. This layering is therefore inter- mediate in regularity between that for Sparry Crust stro ma- tolites and Fine-grained Crust stromatolites. Hybrid Crust appears to be a major component of Palaeoproterozoic and Mesoproterozoic stromatolites, which can include very large domical and conical examples. For example, metric to deca- metric stromatolite domes of Hybrid Crust are prominent components of some Palaeoproterozoic (e.g., Pethei Group) and Meso pro terozoic (e.g., Burovaya Formation) shallow subtidal carbo nate platform sequences. Hybrid Crust exhibits a variety of Fine-grained Crust microfabrics, and fi lamentous microfabric is locally common in Hybrid Crusts from the mid -Proterozoic onward. Light-dark millimetric alternations typical of Hybrid Crust have long been recognized in many Precambrian stro matolites (e.g., VOLOGDIN, 1962; HOF- MANN, 1969, fi g. 13), particularly in coniform examples (e.g., KOMAR et al., 1965; WALTER, 1972, pls. 5, 6, 10, 12). For example, BERT RAND-SARFATI et al. (1994) noted that microstructure consisting of alternations of micrite-microspar laminae “is one of the most frequently found in Proterozoic stromatolites”. In the Pethei Group, SAMI & JAMES (1994, p. 113; 1996, p. 217) emphasized the widespread importance of “spar-micrite couplets”, which are generally 1–2 mm thick, and comprise two broad groups: “laminar fi brous crusts” inter- layered with micritic laminae (idem, fi g. 6d) and “clotted micrite pre ci pi tates arranged in vertical pillars and surrounded by fi brous and blocky precipitates” (idem, fi g. 7a,b). Laminar fi brous crusts are here regarded as a possible hybrid form of iso pa chous la minite, and clotted micrite pillars are grouped with “bushy” microfabrics. KNOLL & SEMIKHATOV (1998), BART LEY et al. (2000) and PETROV & SEMIKHATOV (2001) also drew attention to the contrast between fi brous and mic ritic syn se dimentary precipitates in Proterozoic stro ma- tolites. Large subaqueous Hybrid Crust domes, comparable with those of the Proterozoic, are not known at the present-day, but potential analogues for their fabrics occur in freshwater stro- matolites (see Analogues). These examples tend to support the SAMI & JAMES’ (1994, p. 120) suggestion that spar- micrite couplets refl ect alternation of “cement precipitation and microbial mat growth”. Furthermore, layered alternations could refl ect secular changes that in some cases may be sea- sonal (BERTRAND-SARFATI, 1972, pl. 11(4), pl. 22(2)). In present-day fl uviatile examples, the dark – microbial – layers often have fi lamentous fabric produced by fi lamentous cyano- bacteria. In the Proterozoic, Hybrid Crust appears to be res- ponsible for some of the largest stromatolites known, with decametric dimensions. The size of these deposits might rapid accretion amplifi ed by combined effects of abiogenic pre- cipitation and microbial growth. However, interpretation of Hybrid Crust can be complicated by poor preservation that hinders discrimination between detrital and microbial micrite, and blocky and radial spar (SAMI & JAMES, 1994, p. 120). In particular, it can be diffi cult to decide (i) whether fi ne-gra- ined carbonate represents primary silt- or micrite-grade ma- terial; (ii) whether it is detrital or precipitated, (iii) the extent to which spar is void-fi lling or precipitated directly on the sea- fl oor, and (iv) the precise origins of putative microbial fabrics that may be clotted, peloidal, bush-like fabrics or fi la men tous. Three broad categories of “spar-micrite couplet” are re- cog nized here, according to the dominant fi ne-grained fabric: (i) relatively dense, but also peloidal, microcrystalline car- bonate with either generally even and extensive laminae or uneven and discontinuous laminae, (ii) clotted-bushy-peloidal micrite, (iii) fi lamentous. These categories can intergrade and co-mingle, both in terms of components and of laminar even- ness and continuity. Their discrimination is highly dependent on the quality of fabric preservation; micrite may be present locally (e.g., Pethei, SAMI & JAMES, 1996, p. 203) but even so is often converted to microcrystalline spar (SAMI & JAM ES 1996, p. 210) and in other cases is absent or hard to recog nize in others (e.g., Campbellrand-Malmani, SUMNER & GROTZ- INGER, 2004, p. 8). These categories should be regard ed as preliminary generalizations, and the examples selected require further comparison and probably subdivision. Sparry Crust plus Grains. Intercalation of Sparry Crust with allochthonous grains may prove to be a common deposit, but it has relatively rarely been reported. Nonetheless, stro- matolitic examples from the Mesoproterozoic, in which radial fi brous sparry crust is interleaved with draped layers of silt and sand, have been given formal names, e.g., Gongylina, Omachtenia (SEMIKHATOV & KNOLL, 1998, fi gs. 9–11). These alternations of Sparry Crust precipitation with grainy sedimentation probably have partial analogues in hot-spring travertine and cave fl owstone (see Analogues). Geologia Croatica Geologia Croatica 61/2–3 96 Abiogenic and biogenic stromatolites. Awareness of the existence of abiogenic stromatolites, emphasized by POPE et al. (2000), has led to uncertainties regarding stromatolite de- fi nition and interpretation and has made it diffi cult to assess their signifi cance as indicators of both early life and environ- ments. As CORSETTI & STORRIE-LOMBARDI (2003, p. 649) noted, “it has been underappreciated that inorganic pro- cesses can produce stromatolites”, and PERRY et al. (2007, p. 169) recognized the need to discriminate between microbial stromatolites and abiotic carbonate crusts. POPE et al. (2000, p. 1149) showed the way forward by regarding “thinly lami- nated isopachous stromatolites” as largely abiotic. The over- view presented here suggests that Precambrian stromatolites include not only essentially abiogenic (Sparry Crust) and lithi- fi ed microbial mat (Fine-grained Crust) examples, but also intimate mixtures of the two (Hybrid Crust). Nonetheless, des- pite these complexities it seems likely that many Proterozoic stromatolites retain suffi cient structural and fabric information to be distinguished as either Sparry, Hybrid or Microcrystalline crust. This may also apply to well-preserved Archaean stro- matolites. If this is correct then it should be possible to use these distinguishing features to further elucidate the history of Precambrian stromatolites and increase understanding of their signifi cance as environmental and biological indicators of past life and conditions. ACKNOWLEDGEMENT Kath Grey, Hans Hofmann and Linda Kah each generously provided stromatolite photographs. Stimulating discussions with Linda Kah helped to motivate concept development. Jam es Wheeley and Dave Wright kindly made very helpful reviews. This article owes its existence to the persistence of Tonći Grgasović. Alisa Martek provided expert editorial ad vice. Participation in the Zagreb International Fossil Algae Sympo- sium was supported by the The Royal Society of London. To all, my grateful thanks. REFERENCES AITKEN, J.D. (1967): Classifi cation and environmental signifi cance of cryptalgal limestones and dolomites, with illustrations from the Cambrian and Ordovician of southwestern Alberta. – J. Sediment. Petrol., 37, 1163–1178. AITKEN, J.D. (1989): Giant “algal” reefs, Middle/Upper Proterozoic Little Dal Group (>770, <1200 Ma), Mackenzie Mountains, N.W.T., Canada. – In: GELDSETZER, H.H.J., JAMES, N.P. & TEBBUTT, G.E. (eds): Reefs, Canada and Adjacent Area. Canadian Soc. Petrol. Geol. Memoir, 13, 13–23. AITKEN, J.D. & NARBONNE, G.M. (1989): Two occurrences of Pre- cambrian thrombolites from the Mackenzie Mountains, northwest- ern Canada. – Palaios, 4, 384–388. ALLWOOD, A.C., WALTER, M.R., KAMBER, B.S., MARSHALL, C.P. & BURCH, I.W. (2006): Stromatolite reef from the Early Ar- chaean era of Australia. – Nature, 441, 714–718. ALSHARHAN, A.S. & KENDALL, C.G.St.C. (2003): Holocene coast- al carbonates and evaporites of the southern Arabian Gulf and their ancient analogues. – Earth-Sci. Rev., 61, 191–243. ANDREWS, J.E. (2005): Palaeoclimatic records from stable isotopes in riverine tufas: synthesis and review. – Earth-Sci. Rev., 75, 85–104. ANDREWS, J.E. & BRASIER, A.T. (2005): Seasonal records of climat- ic change in annually laminated tufas: short review and future pros- pects. – J. Quaternary Sci., 20, 411–421. AREF, M.A.M. (1998): Holocene stromatolites and microbial laminites associated with lenticular gypsum in a marine-dominated environ- ment, Ras El Shetan area, Gulf of Aqaba, Egypt. – Sedimentology, 45, 245–262. ARP, G., REIMER, A. & REITNER, J. (2002): Calcifi cation of cyano- bacterial fi laments: Girvanella and the origin of lower Paleozoic lime mud: comment. – Geology, 30, 579–580. ARP, G., REIMER, A. & REITNER, J. (2003): Microbialite formation in seawater of increased alkalinity, Satonda Crater Lake. – J. Sedi- ment. Res., 73, 105–127. ARP, G., WEDEMEYER, N. & REITNER, J. (2001): Fluvial tufa for- mation in a hard-water creek (Deinschwanger Bach, Franconian Alb, Germany). – Facies, 44, 1–22. AWRAMIK, S.M. & GREY, K. (2005): Stromatolites: biogenicity, bio- signatures, and bioconfusion. – Proceedings of SPIE 5906, 590601– 590609. AWRAMIK, S.M. & MARGULIS, L. (1974): Defi nition of stromatolite. – Stromatolite Newsletter, 2, 1–5. BARTLEY, J.K., KAH, L.C., MCWILLIAMS, J.L. & STAGNER, A.F. (2007): Carbon isotope chemostratigraphy of the Middle Riphean type section (Avzyan Formation, southern Urals): signal recovery in a fold-and-thrust belt. – Chem. Geol., 237, 211–232. BARTLEY, J.K., KNOLL, A.H., GROTZINGER, J.P. & SERGEEV, V.N. (2000): Lithifi cation and fabric genesis in precipitated stroma- tolites and associated peritidal carbonates, Mesoproterozoic Billya- kh Group, Siberia. – In: GROTZINGER, J.P. & JAMES, N.P. (eds): Carbonate sedimentation and diagenesis in the evolving Precam- brian world. SEPM Spec. Publ. 67, 59–73. BATTEN, K.L., NARBONNE, G.M. & JAMES, N.P. (2004): Paleoen- vironments and growth of early Neoproterozoic calcimicrobial reefs: platformal Little Dal Group, northwestern Canada. – Precam- brian Res., 133, 249–269. BAUMGARTNER, L.K., REID, R.P., DUPRAZ, C., DECHO, A.W., BUCKLEY, D.H., SPEAR, J.R., PRZEKOP, K.M. & VISSCHER, P.T. (2006): Sulfate reducing bacteria in microbial mats: changing paradigms, new discoveries. – Sediment. Geol., 185, 131–145. BEKKER, A., KARHU, J.A., ERIKSSON, K.A. & KAUFMAN, A.J. (2003): Chemostratigraphy of Paleoproterozoic carbonate succes- sions of the Wyoming craton: tectonic forcing of biogeochemical change?– Precambrian Res., 120, 279–325. BENSON, L.V. (1994): Carbonate deposition, Pyramid Lake subbasin, Nevada: 1. Sequence of formation and elevational distribution of carbonate deposits (tufas). – Palaeo., Palaeo., Palaeo., 109, 55–87. BERTRAND-SARFATI, J. (1972): Stromatolites columnaires du Pré- cambrien supérieur du Sahara Nord-Occidental. – CNRS, Paris, Centre de Recherches sur les Zones Arides, Géologie, 14, xxxvii+ 245 p. BERTRAND-SARFATI, J. (1976): Pseudomorphoses de gypse en ro- settes dans un calcaire cryptalgo-laminaire du Précambrien inférieur (Système du Transvaal, Afrique du Sud). – Bull. Soc. Géol. France, Suppl., 1976(3), 99–102. BERTRAND-SARFATI, J. & CABY, R. (1976): Carbonates et stroma- tolites du sommet du Groupe d’Éleonore Bay (Précambrien termi- nal) au Canning Land (Groenland oriental). – Bulletin Grønlands Geologiske Undersøgelse, 119, 51 p. BERTRAND-SARFATI, J., FREYTET, P. & PLAZIAT, J.C. (1994): Microstructures in Tertiary nonmarine stromatolites (France). Com- Geologia CroaticaRobert Riding: Abiogenic, microbial and hybrid authigenic carbonate crusts: components of Precambrian stromatolites 97 parison with Proterozoic. – In: BERTRAND-SARFATI, J. & MON- TY, C. (eds): Phanerozoic stromatolites II. Kluwer, Dordrecht, 155– 191. BEUKES, N.J. (1987): Facies relations, depositional environments and diagenesis in a major early Proterozoic stromatolitic carbonate plat- form to basinal sequence, Campbellrand Subgroup, Transvaal Su- pergroup, Southern Africa. – Sediment. Geol., 54, 1–46. BIDDLE, K.T. (1983): Girvanella oncoids from Middle to Upper Trias- sic allochthonous boulders of the Dolomite Alps, northern Italy. – In: PERYT, T.M. (ed.): Coated grains. Springer, Berlin, 390–397. BLACK, M. (1933): The algal sedimentation of Andros Island Bahamas. – Philos. T. Roy. Soc. London, Ser. B: Biol. Sci., 222, 165–192. BUCHBINDER, B. (1981): Morphology, microfabric and origin of stro- matolites of the Pleistocene precursor of the Dead Sea, Israel. – In: MONTY, C. (ed.): Phanerozoic stromatolites. Springer, Berlin, 181–196. BUICK, R., GROVES, D.I. & DUNLOP, J.S.R. (1995): Abiological or- igin of described stromatolites older than 3.2 Ga: comment and re- ply. Comment. – Geology, 23, 191. BURNS, S.J., MATTER, A., FRANK, N. & MANGINI, A. (1999): Spe- leothem-based climatic record from northern Oman. – Geology, 26, 499–502. CAO, R. (1991): Origin and order of cyclic growth pattern in mat-min- istromatolite bioherms from the Proterozoic Wumishan Formation, North China. – Precambrian Res., 52, 167–178. CAO, R. & LIANG, Y. (1974): On the classifi cation and correlation of the Sinian System of China, based on a study of algae and stroma- tolites. – Nanjing Inst. Geol. Palaeont. Memoir, 5, 1–26. CAROZZI, A.V. (1962): Observations on algal biostromes in the Great Salt Lake, Utah. – J. Geol., 70, 246–252. CASANOVA, J. (1994): Stromatolites from the East African Rift, a syn- opsis. – In: BERTRAND-SARFATI, J. & MONTY, C. (eds): Phan- erozoic Stromatolites II. Kluwer, Dordrecht, 193–226. CHAFETZ, H.S. (1986): Marine peloids; a product of bacterially in- duced precipitation of calcite. – J. Sediment. Petrol., 56, 812–817. CHAFETZ, H.S. & BUCZYNSKI, C. (1992): Bacterially induced lithi- fi cation of microbial mats. – Palaios, 7, 277–293. CHAFETZ, H.S. & FOLK, R.L. (1984): Travertines; depositional mor- phology and the bacterially constructed constituents. – J. Sediment. Petrol., 54, 289–316. CHAFETZ, H.S., UTECH, N.M. & FITZMAURICE, S.P. (1991): Dif- ferences in the delta 18O and delta 13C signatures of seasonal lami- nae comprising travertine stromatolites. – J. Sediment. Res., 61, 1015–1028. CLOUD, P. & SEMIKHATOV, M.A. (1969): Proterozoic stromatolite zonation. – Am. J. Sci., 267, 1017–1061. CLOUD, P., WRIGHT, L.A., WILLIAMS, E.G., DIEHL, P.E. & WAL- TER, M.R. (1974): Giant stromatolites and associated vertical tubes from the upper Proterozoic Noonday Dolomite, Death Valley re- gion, eastern California. – Geol. Soc. Am. Bull., 85, 1869–1882. CONIGLIO, M., JAMES, N.P. & AĎSSAOUI, D.M. (1988): Dolomiti- zation of Miocene carbonates, Gulf of Suez, Egypt. – J. Sediment. Petrol., 58, 100–119. CORSETTI, F.A. & GROTZINGER, J.P. (2005): Origin of tube structures in Neoproterozoic post-glacial cap carbonates: example from Noon- day Dolomite, Death Valley, United States. – Palaios, 20, 348–362. CORSETTI, F.A. & STORRIE-LOMBARDI, M.C. (2003): Lossless compression of stromatolite images: a biogenicity index?– Astro- biology, 3, 649–655. COX, G., JAMES, J.M., LEGGETT, K.E.A. & OSBORNE, R.A.L. (1989): Cyanobacterially deposited speleothems: subaerial stromatolites. – Geomicrobiol. J., 7, 245–252. DAVAUD, E., STRASSER, A. & JEDOUI, Y. (1994): Stromatolite and serpulid bioherms in a Holocene restricted lagoon (Sabkha el Melah, southeastern Tunisia). – In: BERTRAND-SARFATI, J. & MONTY, C. (eds): Phanerozoic stromatolites II. Kluwer, Dordrecht, 131–151. DE WET, C.B., FREY, H.M., GASWIRTH, S.B., MORA, C.I., RAH- NIS, M. & BRUNO, C.R. (2004): Origin of meter-scale submarine cavities and herringbone calcite cement in a Cambrian microbial reef, Ledger Formation (U.S.A.). – J. Sediment. Res., 74, 914–923. DILL, R.F., SHINN, E.A., JONES, A.T., KELLY, K. & STEINEN, R.P. (1986): Giant subtidal stromatolites forming in normal salinity wa- ters. – Nature, 324, 55–58. DONALDSON, J.A. (1963): Stromatolites in the Denault Formation, Marion Lake, coast of Labrador, Newfoundland. – Geol. Surv. Can- ada Bull., 102, 33 p. DONALDSON, J.A. (1976): Paleoecology of Conophyton and associ- ated stromatolites in the Precambrian Dismal Lakes and Rae groups, Canada. – In: WALTER, M.R. (ed.): Stromatolites. Developments in Sedimentology 20. Elsevier, Amsterdam, 523–534. DRUCKMAN, Y. (1981): Sub-Recent manganese-bearing stromatolites along shorelines of the Dead Sea. – In: MONTY, C. (ed.): Phanero- zoic stromatolites. Springer, Berlin, 197–208. DUNHAM, R.J. (1962): Classifi cation of carbonate rocks according to depositional texture. – In: HAM, W.E. (ed.): Classifi cation of car- bonate rocks. AAPG Memoir, 1, 108–121. DUPRAZ, C. & VISSCHER, P.T. (2005): Microbial lithifi cation in ma- rine stromatolites and hypersaline mats. – Trends Microbiol., 13, 429–438. DUPRAZ, C., VISSCHER, P.T., BAUMGARTNER, L.K. & REID, R. P. (2004): Microbe-mineral interactions : early carbonate precipita- tion in a hypersaline lake (Eleuthera Island, Bahamas). – Sedimen- tology, 51, 745–765. EARDLEY, A.J. (1938): Sediments of the Great Salt Lake, Utah. – AAPG Bull., 22, 1305–1411. ERIKSSON, K.A. (1977): Tidal fl at and subtidal sedimentation in the 2250 M.Y. Malmani Dolomite, Transvaal, South Africa. – Sedi- ment. Geol., 18, 223–244. FAIRCHILD, I.J., FRISIA, S., BORSATO, A. & TOOTH, A.F. (2007): Speleothems. – In: NASH, D.J. & MCLAREN, S.J. (eds): Geochem- ical sediments and landscapes. Blackwells, Oxford, 200–245. FAIRCHILD, I.J., MARSHALL, J.D. & BERTRAND-SARFATI, J. (1990): Stratigraphic shifts in carbon isotopes from Proterozoic stromatolitic carbonates (Mauritania): infl uences of primary min- eralogy and diagenesis. – Am. J. Sci., 290A, 46–79. FELDMANN, M. & MCKENZIE, J.A. (1998): Stromatolite-thrombol- ite associations in a modern environment, Lee Stocking Island, Ba- hamas. – Palaios, 13, 201–212. FOLK, R.L (1993): SEM imaging of bacteria and nannobacteria in car- bonate sediments and rocks. – J. Sediment. Petrol., 63, 990–999. FOLK, R.L. (1959): Practical petrographic classifi cation of limestones. AAPG Bull., 43, 1–38. FREYTET, P. & PLET, A. (1996): Modern freshwater microbial carbon- ates: the Phormidium stromatolites (tufa-travertine) of southeastern Burgundy (Paris Basin, France). – Facies, 34, 219–237. FREYTET, P. & VERRECCHIA, E.P. (1999): Calcitic radial palisade fabric in freshwater stromatolites: diagenetic and recrystallized fea- ture or physicochemical sinter crust?– Sediment. Geol., 126, 97– 102. Geologia Croatica Geologia Croatica 61/2–3 98 GANDIN, A. & WRIGHT, D.T. (2007): Evidence of vanished evaporites in Neoarchaean carbonates. – In: SCHREIBER, B.C., LUGLI, S. & BABEL, M. (eds): Evaporites Through Space and Time. Geo- logical Society, London, Spec. Publ. 285, 285–308. GARWOOD, E.J. & GOODYEAR, E. (1924): The Lower Carbonifer- ous succession in the Settle district and along the line of the Craven faults. – Quart. J. Geol. Soc. London, 80, 184–273. GEBELEIN, C.D. (1976): The effects of the physical, chemical and bio- logical evolution of the Earth. – In: WALTER M.R. (ed.): Stroma- tolites. Developments in Sedimentology 20. Elsevier, Amsterdam, 499–515. GERDES, G., CLAES, M., DUNAJTSCHIK-PIEWAK, K., RIEGE, H., KRUMBEIN, W.E. & REINECK, H-E. (1993): Contribution of microbial mats to sedimentary surface structures. – Facies 29, 61– 74. GINSBURG, R.N. (1991): Controversies about stromatolites: vices and virtues. – In: MULLER, D.W., MCKENZIE, J.A., & WEISSERT, H. (eds): Controversies in Modern Geology; Evolution of Geolog- ical Theories in Sedimentology, Earth History and Tectonics. Aca- demic Press, London, 25–36. GOLUBIC, S. (1973): The relationship between blue-green algae and carbonate deposits. – In: CARR, N. & WHITTON, B.A. (eds): The Biology of Blue-Green Algae. Blackwell, Oxford, 434–472. GREY, K. & THORNE, A.M. (1985): Biostratigraphic signifi cance of stromatolites in upward shallowing sequences of the early Protero- zoic Duck Creek Dolomite, Western Australia. – Precambrian Res., 29, 183–206. GROTZINGER, J.P. (1986a): Evolution of Early Proterozoic passive- margin carbonate platform, Rocknest Formation, Wopmay Orogen, Northwest Territories, Canada. – J. Sediment. Petrol., 56, 831–847. GROTZINGER, J.P. (1986b): Cyclicity and paleoenvironmental dynam- ics, Rocknest platform, northwest Canada. – Geol. Soc. Am. Bull., 97, 1208–1231. GROTZINGER, J.P. (1989a): Facies and evolution of Precambrian car- bonate depositional systems: emergence of the modern platform archetype. – In: CREVELLO, P.D., WILSON, J.L., SARG, J.F., & READ, J.F. (eds): Controls on carbonate platform and basin devel- opment. SEPM Spec. Publ., 44, 79–106. GROTZINGER, J.P. (1989b): Introduction to Precambrian reefs. – In: GELDSETZER, H.H.J., JAMES, N.P. & TEBBUTT, G.E (eds): Reefs, Canada and adjacent areas. Canad. Soc. Petrol. Geol. Mem., 13, 9–12. GROTZINGER, J.P. (1990): Geochemical model for Proterozoic stro- matolite decline. – Am. J. Sci., 290-A, 80–103. GROTZINGER, J., ADAMS, E.W. & SCHRÖDER, S. (2005): Micro- bial-metazoan reefs of the terminal Proterozoic Nama Group (c. 550-543 Ma), Namibia. – Geol. Mag., 142, 499–517. GROTZINGER, J.P. & JAMES, N.P. (2000): Precambrian carbonates: evolution of understanding. – In: GROTZINGER, J.P. & JAMES, N.P. (eds): Carbonate sedimentation and diagenesis in the evolving Precambrian world. SEPM Spec. Publ., 67, 3–20. GROTZINGER, J.P. & KASTING, J.F. (1993): New constraints on Pre- cambrian ocean composition. – J. Geol., 101, 235–243. GROTZINGER, J.P. & KNOLL, A.H. (1999): Stromatolites in Precambri- an carbonates: evolutionary mileposts or environmental dipsticks?– Annu. Rev. Earth Pl. Sc., 27, 313–358. GROTZINGER, J.P. & READ, J.F. (1983): Evidence for primary arago- nite precipitation, lower Proterozoic (1.9–Ga) Rocknest Dolomite, Wopmay Orogen, Northwest Canada. – Geology, 11, 710–713. GROTZINGER, J.P. & ROTHMAN, D.R. (1996): An abiotic model for stromatolite morphogenesis. – Nature, 383, 423–425. GROTZINGER, J.P., WATTERS, W.A. & KNOLL, A.H. (2000): Calci- fi ed metazoans in thrombolite-stromatolite reefs of the terminal Pro- terozoic Nama Group, Namibia. – Paleobiology, 26, 334–359. GUO, L. & RIDING, R. (1992): Microbial micritic carbonates in upper- most Permian reefs, Sichuan Basin, southern China: some similar- ities with Recent travertines. – Sedimentology, 39, 37–53. GUO, L. & RIDING, R. (1994): Origin and diagenesis of Quaternary travertine shrub facies, Rapolano Terme, central Italy. – Sedimen- tology, 41, 499–520. GUO, L., & RIDING, R. (1998): Hot-spring travertine facies and se- quences, Late Pleistocene, Rapolano Terme, Italy. – Sedimentology, 45, 163–180. HALLEY, R.B. (1976): Textural variation within Great Salt Lake algal mounds. – In: WALTER, M.R. (ed.): Stromatolites. Developments in Sedimentology 20. Elsevier, Amsterdam, 435–445. HARDIE, L.A. (2003): Secular variations in Precambrian seawater che- mistry and the timing of Precambrian aragonite seas and calcite seas. – Geology, 31, 785–788. HOFMANN, H.J. (1969): Attributes of stromatolites. – Geol. Surv. Can- ada Paper, 69–39, 58 p. HOFMANN, H.J. (1971): Precambrian fossils, pseudofossils, and prob- lematica in Canada. – Geol. Surv. Canada Bull., 189, 146 p. HOFMANN, H.J. (2000): Archean stromatolites as microbial archives. – In: RIDING, R.E. & AWRAMIK, S.M. (eds): Microbial Sedi- ments. Springer, Berlin, 315–327. HOFMANN, H.J. & JACKSON, J.D. (1987): Proterozoic ministroma- tolites with radial fi brous fabric. – Sedimentology, 34, 963–971. HOFMANN, H.J. & SNYDER, G.L. (1985): Archean stromatolites from the Hartville Uplift, eastern Wyoming. – Geol. Soc. Am. Bull., 96, 842–849. HOFMANN, H.J., GREY, K., HICKMAN, A.H. & THORPE, R.I. (1999): Origin of 3.45 Ga coniform stromatolites in Warrawoona Group, Western Australia. – Geol. Soc.Am. Bull., 111, 1256–1262. HOFFMAN, P.F. (1969): Proterozoic paleocurrents and depositional his- tory of the East Arm Fold Belt, Great Slave Lake, Northwest Ter- ritories. – Can. J. Earth Sci., 6, 441–462. HOFFMAN, P.F. (1975): Shoaling-upward shale-to-dolomite cycles in the Rocknest Formation (lower Proterozoic), Northwest Territories, Canada. – In: GINSBURG, R.N. (ed.): Tidal Deposits. Springer, Berlin, 257–265. IRION, G. & MÜLLER, G. (1968): Mineralogy, petrology and chemical composition of some calcareous tufa from the Schwäbische Alb, Germany. – In: MÜLLER, G. & FRIEDMAN, G.M. (eds): Recent Developments in Carbonate Sedimentology in Central Europe. Spring- er, Berlin, 157–171. JACKSON, M.J. (1989): Lower Proterozoic Cowles Lake foredeep reef, N.W.T., Canada. – In: GELDSETZER, H.H.J., JAMES, N.P. & TEBBUTT, G.E. (eds): Reefs, Canada and Adjacent Area. Can. Soc. Petr. Geol. Memoir, 13, 64–71. JAMES, N.P. & GINSBURG, R.N. (1979): Petrography of limestones from the wall and fore-reef. – In: JAMES, N.P. & GINSBURG, R.N. (eds): The seaward margin of Belize barrier and atoll reefs. IAS Spec. Publ. 3. Blackwell, Oxford, 111–152. JAMES, N.P., GINSBURG, R.N., MARSZALEK, D.S. & CHOQUET- TE, P.W. (1976): Facies and fabric specifi city of early subsea ce- ments in shallow Belize (British Honduras) reefs. – J. Sediment. Petrol., 46, 523–544. Geologia CroaticaRobert Riding: Abiogenic, microbial and hybrid authigenic carbonate crusts: components of Precambrian stromatolites 99 JAMES, N.P., NARBONNE, G.M. & SHERMAN, A.G. (1998): Molar- tooth carbonates: shallow subtidal facies of the mid- to late Prot- erozoic. – J. Sed. Res., 68/5, 716-722. JANSSEN, A., SWENNEN, R., PODOOR, N. & KEPPENS, E. (1999): Biological and diagenetic infl uence in Recent and fossil tufa depos- its from Belgium. – Sediment. Geol., 126, 75–95. JEFFERSON, C.W. & YOUNG, G.M. (1989): Late Proterozoic orange- weathering stromatolite biostrome, Mackenzie Mountains and west- ern Arctic Canada. – In: GELDSETZER, H.H.J., JAMES, N.P. & TEBBUTT, G.E. (eds): Reefs, Canada and Adjacent Area. – Can. Soc. Petrol. Geol. Memoir, 13, 72–80. JOHNSON, J. & GROTZINGER, J.P. (2006): Effect of sedimentation on stromatolite reef growth and morphology, Ediacaran Omkyk Member (Nama group), Namibia. – S. Afr. J. Geol., 109, 87–96. KAH, L.C. & GROTZINGER, J.P. (1992): Early Proterozoic (1.9 Ga) thrombolites of the Rocknest Formation, Northwest Territories, Canada. – Palaios, 7, 305–315. KAH, L.C. & KNOLL, A.H. (1996): Microbenthic distribution of Prot- erozoic tidal fl ats: environmental and taphonomic considerations. – Geology, 24, 79–82. KAH, L.C. & RIDING, R. (2007): Mesoproterozoic carbon dioxide levels inferred from calcifi ed cyanobacteria. – Geology, 35, 799–802. KAISIN, F. (1925): Les calcaires oolithiques de l’étage viséen. – Ann. Soc. Sci. Bruxelles, 44, 365. KALKOWSKY, E. (1908): Oolith und Stromatolith im norddeutschen Buntsandstein. – Zeitschr. Deutsc. geol. Gesellsch., 60, 68–125. KANO, A., MATSUOKA, J., KOJO, T. & FUJII, H. (2003): Origin of annual laminations in tufa deposits, southwest Japan. – Palaeo., Pa- laeo., Palaeo., 191, 243–262. KAZMIERCZAK, J. & KEMPE, S. (2006): Genuine modern analogues of Precambrian stromatolites from caldera lakes of Niuafo‘ou Is- land, Tonga. – Naturwissenschaften, 93, 119–126. KENDALL, A.C. & BROUGHTON, P. (1978): Origin of fabrics in spe- leothems composed of columnar calcite crystals. – J. Sediment. Petrol., 48, 519–538. KENDALL, A.C. & IANNACE, A. (2001); “Sediment”-cement relation- ships in a Pleistocene speleothem from Italy: a possible analogue for “replacement” cements and Archaeolithoporella in reefs. – Sed- imentology, 48, 681–698. KENDALL, C.G.St.C. & SKIPWITH, P.A. d’E. (1968): Recent algal mats of a Persian Gulf lagoon. – J. Sediment. Petrol., 38, 1040– 1058. KENNARD, J.M. & JAMES, N.P. (1986): Thrombolites and stromato- lites; two distinct types of microbial structures. – Palaios, 1, 492– 503. KERANS, C. (1982): Sedimentology and stratigraphy of the Dismal Lakes Group, Proterozoic, Northwest Territories. – Unpublished PhD thesis, Carleton University, Ottawa, Canada. KERANS, C. & DONALDSON, J.A. (1989): Deepwater conical stro- matolite reef, Sulky Formation (Dismal Lakes Group), middle Pro- terozoic, N.W.T. – In: GELDSETZER, H.H.J., JAMES, N.P. & TEBBUTT, G.E. (eds): Reefs, Canada and Adjacent Area. Can. Soc. Petrol. Geol. Memoir, 13, 81–88. KINDLER, P. & BAIN, R.J. (1993): Submerged Upper Holocene beach- rock on San Salvador Island, Bahamas: implications for recent sea- level history. – Geol. Rundsch., 82, 241–247. KLAPPA, C.F. (1979): Lichen stromatolites: criterion for subaerial ex- posure and a mechanism for the formation of laminar calcretes (cal- iche). – J. Sediment. Petrol., 49, 387–400. KNOLL, A.H. & SEMIKHATOV, M.A. (1998): The genesis and time distribution of two distinctive Proterozoic stromatolite microstruc- tures. – Palaios, 13, 408–422. KNOLL, A.H., FAIRCHILD, I.J. & SWETT, K. (1993): Calcifi ed mi- crobes in Neoproterozoic carbonates; implications for our under- standing of the Proterozoic/ Cambrian transition. – Palaios, 8, 512– 525. KOMAR, V.A. (1966): Stromatolites of the upper Precambrian sediments of the north of the Siberian Platform, and their stratigraphic sig- nifi cance. – Nauka, Moscow, 122 p. [In Russian]. KOMAR, V.A. (1976): Classifi cation of stromatolites according to mic- rostructure. – In: Palaeontology of the Precambrian and early Cam- brian, All Union Symposium, Novosibirsk, USSR, p. 41–43. [In Rus sian]. KOMAR, V.A. (1989): Classifi cation of the microstructures of the Up- per Precambrian stromatolites. – Himalayan Geology, 13, 229–238. KOMAR, V.A., RAABEN, M.E. & SEMIKHATOV, M.A. (1965): Cono- phyton in the Riphean of the USSR and their stratigraphic impor- tance. – Trudy Geological Institute, Leningrad, 131, 72 p. [In Rus- sian]. KREBS, W. (1969): Early void-fi lling cementation in Devonian fore-reef limestones (Germany). – Sedimentology, 12, 279–299. KREMER, B., KAZMIERCZAK, J. & STAL, L.J. (2008): Calcium car- bonate precipitation in cyanobacterial mats from sandy tidal fl ats of the North Sea. – Geobiology, 6, 46–56. KRUMBEIN, W.E. (1979): Photolithotrophic and chemoorganotrophic activity of bacteria and algae as related to beachrock formation and degradation (Gulf of Aqaba, Sinai). – Geomicrobiol. J., 1, 139– 203. KRUMBEIN, W.E. (1983): Stromatolites – the challenge of a term in space and time. – Precambrian Res., 20, 493–531. KUHL, M., FENCHEL, T. & KAZMIERCZAK, K. (2003): Growth, structure and calcifi cation potential of an artifi cial cyanobacterial mat. – In: KRUMBEIN, W.E., PATERSON, D. & ZAVARZIN, G. (eds): Fossil and recent biofi lms, a natural history of life on Earth. Kluwer, Dordrecht, 77–102. LAVAL, B., CADY, S.L., POLLACK, J.C., MCKAY, C.P., BIRD, J.S., GROTZINGER, J.P., FORD, D.C. & BOHM, H.R. (2000): Modern freshwater microbialite analogues for ancient dendritic reef struc- tures. – Nature, 407, 626–629. LEHMANN, P. (1978): Deposition, porosity evolution, and diagenesis of the Pipe Creek Jr. reef (Silurian), Grant County, Indiana. – Un- published MS thesis, University of Wisconsin, Madison, USA, 234 p. LIANG, Y., CAO, R.-J., ZHANG, L., QIU, S., XIAO, Z., CAO, R.-G., DUAN, J., DU, R., BU, J. & GAO, Z. (1984): Pseudogymnoso- lenaceae of the Late Precambrian of China. – Scientia Sinica, B27 (5), 534–546. LIANG, Y., ZHU, S., ZHANG, L., CAO, R., GAO, Z. & BU, D. (1985): Stromatolite assemblages of the late Precambrian in China. – Pre- cambrian Res., 29, 15–32. LINDSAY, J.F., KRUSE, P.D., GREEN, O.R., HAWKINS, E., BRA- SIER, M.D., CARTLIDGE, J. & CORFIELD, R.M. (2005): The Neoproterozoic-Cambrian record in Australia: a stable isotope stu- dy. – Precambrian Res., 143, 113–133. LOGAN, B.W. (1961): Cryptozoon and associated stromatolites from the Recent, Shark Bay, Western Australia. – J. Geol., 69, 517–533. LOWE, D.R. (1980): Stromatolites 3,400–3,500 Myr old from the Ar- chean of Western Australia. – Nature, 284, 441–443. Geologia Croatica Geologia Croatica 61/2–3 100 LOWE, D.R. (1983): Restricted shallow-water sedimentation of early Archean stromatolitic and avaporitic strata of the Strelley Pool Chert, Pilbara Block, Western Australia. – Precambrian Res., 19, 239–283. LOWE, D.R. (1994): Abiological origin of described stromatolites older than 3.2 Ga. – Geology, 22, 387–390. LOWE, D.R. (1995): Abiological origin of described stromatolites older than 3.2 Ga: comment and reply. Reply. – Geology, 23, 191–192. MACGREGOR, A.M. (1941): A pre-Cambrian algal limestone in south- ern Rhodesia. – Trans. Geol. Soc. S. Africa, 43 (1940), 9–16. MACINTYRE, I.G. (1984): Extensive submarine lithifi cation in a cave in the Belize Barrier Reef Platform. – J. Sediment. Petrol., 54, 221– 235. MACINTYRE, I.G. (1985): Submarine cements – the peloidal question. – In: SCHNEIDERMANN, N. & HARRIS, P.M. (eds): Carbonate cements. SEPM Spec. Publ. 36, 109–116. MACINTYRE, I.G., REID, R.P. & STENECK, R.S. (1996): Growth his- tory of stromatolites in a Holocene fringing reef, Stocking Island, Bahamas. – J. Sediment. Res., 66, 231–242. MARTIN, A., NISBET, E.G. & BICKLE, M.J. (1980): Archaean stro- matolites of the Belingwe greenstone belt, Zimbabwe (Rhodesia). – Precambrian Res., 13, 337–362. MATTES, B.W. & CONWAY MORRIS, S. (1990): Carbonate/evaporite deposition in the late Precambrian-early Cambrian Ara Formation of southern Oman. – In: ROBERTSON, A.H.F., SEARLE, M.P. & RIES, A.C. (eds): The geology and tectonics of the Oman region. Geol. Soc. London, Spec. Publ., 49, 617–636. MAURIN, A.F. & NOËL, D. (1977): A possible bacterial origin for Fa- mennian micrites. – In: FLÜGEL, E. (ed.): Fossil Algae, Recent results and developments. Springer, Berlin, 136–142, MAZZULLO, S.J. & CYS, J.M (1979): Marine aragonite sea-fl oor growths and cements in Permian phylloid algal mounds, Sacramen- to Mountains, New Mexico. – J. Sediment. Res., 49, 917–936. MCGOVNEY, J.E. (1989): Thornton reef, Silurian, northeastern Illinois. – In: GELDSETZER, H.H.J., JAMES, N.P. & TEBBUTT, G.E (eds): Reefs, Canada and Adjacent Area. Can. Soc. Petrol. Geol. Memoir, 13, 330–338. MCKEE, E.D. & GUTSCHICK, R.C. (1969): Analysis of lithology. – In: MCKEE, E.D. & GUTSCHICK, R.C. (eds): History of the Redwall Limestone of Northern Arizona. – Geol. Soc. Am. Mem., 114, 97– 124. MCLOUGHLIN, N., WILSON, L.A., & BRASIER, M.D. (2008): Growth of synthetic stromatolites and wrinkle structures in the absence of microbes – implications for the early fossil record. – Geobiology, 6, 95–105. MELIM, L.A., SHINGLMAN, K.M., BOSTON, P.J., NORTHUP, D.E., SPILDE, M.N. & QUEEN, J.M. (2001): Evidence for microbial involvement in pool fi nger precipitation, Hidden Cave, New Mex- ico. – Geomicrobiol. J., 18, 311–329. MOCK, S.E. & PALMER, T.J. (1991): Preservation of siliceous spong- es in the Jurassic of southern England and northern France. – J. Geol. Soc. London, 148, 681–689. MONTANARI, A., BICE, D., DRUSCHEL, G., MARIANI, S., MAR- SHALL, C., OLCOTT, A., SHARP, W., TIGUE, T. & VUČETIĆ, M. (2007): Rediscovering pelagosite: a Mediterranean “microstro- matolite” recording recent climate cycles. – Geophys. Res. Ab- stracts, 9, 01555. MONTY, C. (1965): Recent algal stromatolites in the Windward lagoon, Andros Island, Bahamas. – Ann. Soc. Géol. Belgique, 88, 269– 276. MONTY, C.L.V. (1976): The origin and development of cryptalgal fab- rics. – In: WALTER, M.R. (ed.): Stromatolites. Developments in Sedimentology 20. Elsevier, Amsterdam, 193–249. MONTY, C.L.V. (1981): Spongiostromate vs. porostromate stromatolites and oncolites. – In: MONTY, C.L.V. (ed.): Phanerozoic stromato- lites. Springer, Berlin, 1–4. MONTY, C.L.V. & HARDIE, L.A. (1976): The geological signifi cance of the freshwater blue-green algal calcareous marsh. – In: WAL- TER, M.R. (ed.): Stromatolites. Developments in Sedimentology 20. Elsevier, Amsterdam, 447–477. MONTY, C. & MAS, J.R. (1981): Lower Cretaceous (Wealden) blue- green algal deposits of the province of Valencia, eastern Spain. – In: MONTY, C. (ed): Phanerozoic stromatolites. Springer, Berlin, 85– 120. MURPHY, M.A. & SUMNER, D.Y. (2008): Variations in Neoarchean microbialite morphologies: clues to controls on microbialite mor- phologies through time. – Sedimentology, 55/5, 1189-1202. NARBONNE, G.M., JAMES, N.P., RAINBIRD, R.H. & MORIN, J. (2000): Early Neoproterozoic (Tonian) patch reef complexes, Vic- toria Island, Arctic Canada. – In: GROTZINGER J.P. & JAMES N.P. (eds): Carbonate sedimentation and diagenesis in the evolving Precambrian world. SEPM Spec. Publ. 67, 163–177. NISBET, E.G. & WILKS, M.E. (1989): Archaean stromatolite reef at Steep Rock Lake, Atikokan, northwestern Ontario. – In: GELDSETZER, H.H.J., JAMES, N.P. & TEBBUTT, G.E. (eds): Reefs, Canada and Adjacent Area. Can. Soc. Petrol. Geol. Memoir, 13, 89–92. NOGUEIRA, A.C.R., RICCOMINI, C., SIAL, A.N., MOURA, C.A.V. & FAIRCHILD, T.R. (2003): Soft-sediment deformation at the base of the Neoproterozoic Puga cap carbonate (southwestern Amazon craton, Brazil): confi rmation of rapid icehouse to greenhouse tran- sition in snowball Earth. – Geology, 31, 613–616. NUZHNOV, S.V. (1967): Riphean deposits of the south-eastern margin of the Siberian Platform. – Nauka, Moscow, 160 p. [In Russian]. OTTE, C., Jr. & PARKS, J.M., Jr. (1963): Fabric studies of Virgil and Wolfcamp bioherms, New Mexico. – J. Geol., 71, 380–396. PALACHE, C., BERMAN, H. & FRONDEL, C. (1951): The system of mineralogy of James Dwight Dana and Edward Salisbury Dana, Yale University 1937–1892, volume II: halides, nitrates, borates, carbonates, sulfates, phosphates, arsenates, tungstates, molybdates, etc., 7th edition. – John Wiley and Sons, New York, 183 p. PAUL, J. (1995): Stromatolite reefs of the Upper Permian Zechstein Ba- sin (Central Europe). – Facies, 32, 28–31. PELECHATY, S.M & GROTZINGER, J.P. (1989): Stromatolite bio- herms of a 1.9 Ga foreland basin carbonate ramp, Beechey Forma- tion, Kilohigok Basin, Northwest Territories. – In: GELDSETZER, H.H.J., JAMES, N.P. & TEBBUTT, G.E. (eds): Reefs, Canada and Adjacent Area. Can. Soc. Petrol. Geol. Memoir, 13, 93–104. PENTECOST, A. (1987): Growth and calcifi cation of the freshwater cy- anobacterium Rivularia haematites. – P. Roy. Soc., London, B 232, 125–136. PENTECOST, A. (1991): Calcifi cation processes in algae and bacteria. – In: RIDING R. (ed.): Calcareous algae and stromatolites. Spring- er, Berlin, 3–20. PENTECOST, A. (1995): Signifi cance of the mineralizing bioniche in a Lyngbya (cyanobacterium) travertine. – Geomicrobiol. J., 13, 213– 222. PENTECOST, A. (2005): Travertine. – Springer, Berlin, 445 p. PENTECOST, A. & SPIRO, B. (1990): Stable carbon and oxygen iso- tope composition of calcites associated with modern freshwater cy- anobacteria and algae. – Geomicrobiol. J., 8, 17–26. Geologia CroaticaRobert Riding: Abiogenic, microbial and hybrid authigenic carbonate crusts: components of Precambrian stromatolites 101 PERRY, R.S., MCLOUGHLIN, N., LYNNE, B.Y., SEPHTON, M.A., OLIVER, J.D., PERRY, C.C., CAMPBELL, K., ENGEL, M.H., FARM ER, J.D., BRASIER, M.D. & STALEY, J.T. (2007): Defi n- ing biominerals and organominerals: direct and indirect indicators of life. – Sediment. Geol., 201, 157–179. PERYT, T.M., PERYT, D., JASIONOWSKI, M., POBEREZHSKYY, A.V. & DURAKIEWICZ, T. (2004): Post-evaporitic restricted dep- osition in the Middle Miocene Chokrakian-Karaganian of east Cri- mea (Ukraine). – Sediment. Geol., 170, 21–36. PETROV, P.YU. & SEMIKHATOV, M. A. (2001): Sequence organiza- tion and growth patterns of late Mesoproterozoic stromatolite reefs: an example from the Burovaya Formation, Turukhansk Uplift, Si- beria. – Precambrian Res., 111, 257–281. PIA, J. (1927): Thallophyta. – In HIRMER, M.: Handbuch der Paläo- botanik. Oldenbourg, Munich, 1, 31–136. PIA, J. (1933): Die rezenten Kalksteine. – Mineralogische und Petrolo- gische Mitteilungen, Neue Folge, Ergänzungsband, 1–420. POPE, M.C. & GROTZINGER, J.P. (2000): Controls on fabric develop- ment and morphology of tufas and stromatolites, uppermost Pethei Group (1.8 Ga), Great Slave Lake, northwest Canada. – In: GROTZ- INGER, J.P. & JAMES, N.P. (eds): Carbonate sedimentation and diagenesis in the evolving Precambrian world. SEPM Spec. Publ., 67, 103–121. POPE, M.C., GROTZINGER, J.P., & SCHREIBER, B.C. (2000): Evap- oritic subtidal stromatolities produced by in situ precipitation: tex- tures, facies associations, and temporal signifi cance. – J. Sediment. Res., 70, 1139–1151. PRATT, B.R. & JAMES, N.P. (1982): Cryptalgal-metazoan bioherms of early Ordovician age in the St. George Group, western Newfound- land. – Sedimentology, 29, 543–569. PURSER, B.H. & LOREAU, J.P. (1973): Aragonitic supratidal encrus- tations on the Trucial Coast, Persian Gulf. – In: PURSER, B.H. (ed.): The Persian Gulf. Springer, Berlin, 343–376. RAABEN, M.E. (1980): Microstromatolites – a characteristic element of the lower Proterozoic stromatolite assemblage. – Doklady Aka- demia Nauk SSSR, 250, 734–737 [In Russian]. RAABEN, M.E. (2005): Archean and Proterozoic ministromatolites: tax- onomic composition of successive assemblages. – Stratigr. Geol. Correlation, 13, 367–379. READ, J.F. (1976): Calcretes and their distinction from stromatolites. – In: WALTER, M.R. (ed.): Stromatolites. Developments in Sedi- mentology 20. Elsevier, Amsterdam, 55–71. REID, R.P. & BROWNE, K.M. (1991): Intertidal stromatolites in a fring- ing Holocene reef complex, Bahamas. – Geology, 19, 15–18. REID, R.P., VISSCHER, P.T., DECHO, A.W., STOLZ, J.F., BEBOUT, B.M., DUPRAZ, C., MACINTYRE, I.G., PAERL, H.W., PINCK- NEY, J.L., PRUFERT-BEBOUT, L., STEPPE, T.F. & DESMA- RAIS, D.J. (2000): The role of microbes in accretion, lamination and early lithifi cation of modern marine stromatolites. – Nature, 406, 989–992. REIS, O.M. (1926): Zusammenfassung über die im Ries südlich von Nördlingen auftretenden Süβwasserkalke und ihre Entstehung. – Jahresberichte und Mitteilungen des oberrheinischen geologischen Vereins, Neue Folge 14, 176–190. REITNER, J., THIEL, V., ZANKL, H., MICHAELIS, W., WÖRHEIDE, G. & GAUTRET, P. (2000): Organic and biogeochemical patterns in cryptic microbialites. – In: RIDING, R.E. & AWRAMIK, S.M. (eds): Microbial sediments. Springer, Berlin, 149–160. RIDING, R. (1977a): Calcifi ed Plectonema (blue-green algae), a Recent example of Girvanella from Aldabra Atoll. – Palaeontology, 20, 33–46. RIDING, R. (1977b): Skeletal stromatolites. – In: FLÜGEL, E. (ed.): Fos- sil algae, recent results and developments. Springer, Berlin, 57– 60. RIDING, R. (1991): Classifi cation of microbial carbonates. – In: RID- ING, R. (ed.): Calcareous Algae and Stromatolites. Springer, Berlin, 21–51. RIDING, R. (1999): The term stromatolite: towards an essential defi ni- tion. – Lethaia, 32, 321–330. RIDING, R. (2000): Microbial carbonates: the geological record of cal- cifi ed bacterial-algal mats and biofi lms. – Sedimentology, 47 (Sup- pl. 1), 179–214. RIDING, R. (2002): Biofi lm architecture of Phanerozoic cryptic carbon- ate marine veneers. – Geology, 30, 31–34. RIDING, R. (2006): Cyanobacterial calcifi cation, carbon dioxide con- centrating mechanisms, and Proterozoic-Cambrian changes in at- mospheric composition. – Geobiology, 4, 299–316. RIDING, R. & SHARMA, M. (1998): Late Palaeoproterozoic (~1800– 1600 Ma) stromatolites, Cuddapah Basin, southern India: cyano- bacterial or other bacterial microfabrics?– Precambrian Res., 92, 21–35. RIDING, R. & VORONOVA, L. (1982): Recent freshwater oscillato- riacean analogue of the Lower Palaeozoic calcareous alga Angulo- cellularia. – Lethaia, 15, 105–114. RODDY, H.J. (1915): Concretions in streams formed by the agency of blue-green algae and related plants. – P. Am. Philos. Soc., 54, 246– 258. ROUCHY, J.M. & MONTY, C.L. (1981): Stromatolites and cryptalgal laminites associated with Messinian gypsum of Cyprus. – In: MON- TY, C. (ed): Phanerozoic stromatolites. Springer, Berlin, 155–180. ROUCHY, J.M. & MONTY, C. (2000): Gypsum microbial stromatolites: Neogene and modern examples. – In: RIDING, R.E. & AWRAMIK, S.M. (eds): Microbial sediments. Springer, Berlin, 209–216. SAMI, T.T. & JAMES, N.P. (1993): Evolution of an early Proterozoic foreland basin carbonate platform, lower Pethei Group, Great Slave Lake, northwest Canada. – Sedimentology, 40, 403–430. SAMI, T.T. & JAMES, N.P. (1994): Peritidal carbonate platform growth and cyclicity in an early Proterozoic foreland basin, upper Pethei Group, northwest Canada. – J. Sediment. Res., B64, 111–131. SAMI, T.T. & JAMES, N.P. (1996): Synsedimentary cements as Paleo- proterozoic platform building blocks, Pethei Group, northwestern Canada. – J. Sediment. Res., 66, 209–222. SCHMITT, M. (1979): The section of Tiout (Precambrian/Cambrian boundary beds, Ant-Atlas, Morocco): stromatolites and their bios- tratigraphy. – Arb. Paläont. Inst. Würzburg, 2, 188 p. (Dissertation, Julius-Maximilians-Universität, Würzburg, Germany). SCHROEDER, J.H. (1972): Fabrics and sequences of submarine carbon- ate cements in Holocene Bermuda cup reefs. – Int. J. Earth Sci., 61, 708–730. SEMIKHATOV, M.A. (1978): Aphebian assemblage of stromatolites: general characteristics and comparison with the Riphean ones. – In: Lower boundary of the Riphean and stromatolites of the Aphe- bian. Trudy Geol. Inst. Acad. Nauk SSSR, 312, 148–158 [In Rus- sian]. SEMIKHATOV, M.A., GEBELEIN, C.D., CLOUD, P., AWRAMIK, S.M. & BENMORE, W.C. (1979): Stromatolite morphogenesis – progress and problems. – Can. Jour. Earth Sci., 16, 992–1015. SEREBRYAKOV, S.N. (1976): Biotic and abiotic factors controlling the morphology of Riphean stromatolites. – In: WALTER M.R. (ed.): Stromatolites. Developments in Sedimentology 20. Elsevier, Am- sterdam, 321–336. Geologia Croatica Geologia Croatica 61/2–3 102 SEREBRYAKOV, S.N. & SEMIKHATOV, M.A. (1974): Riphean and Recent stromatolites: a comparison. – Am. J. Sci., 274, 556–574. SHARMA, M. & SHUKLA, M. (1998): Microstructure and microfabric studies of Palaeoproterozoic small digitate stromatolites (ministro- matolites) from the Vempalle Formation, Cuddapah Supergroup, India. – J. Palaeont. Soc. India, 43, 89–100. SHINN, E.A., LLOYD, R.M. & GINSBURG, R.N. (1969): Anatomy of a modern carbonate tidal-fl at, Andros island, Bahamas. – J. Sedi- ment. Petrol., 39, 1202–1228. SIMONSON, B.M., SCHUBEL, K.A. & HASSLER, S.W. (1993): Car- bonate sedimentology of the early Precambrian Hamersley Group of Western Australia. – Precambrian Res., 60, 287–335. STIRN, A. (1964): Kalktuffvorkommen und Kalktufftypen der Schwä- bische Alb. – Abhandlung zur Karst und Hohlenkunde, Reihe E, 1–92. SUMNER, D.Y. (1997a): Carbonate precipitation and oxygen stratifi ca- tion in late Archean seawater as deduced from facies and stratigra- phy of the Gamohaan and Frisco formations, Transvaal Supergroup, South Africa. – Am. J. Sci., 297, 455–487. SUMNER, D.Y. (1997b): Late Archean calcite-microbe interactions: two morphologically distinct microbial communities that affected cal- cite nucleation differently. – Palaios, 12, 302–318. SUMNER, D.Y. (2000): Microbial vs environmental infl uences on the morphology of late Archean fenestrate microbialites. – In: RIDING, R.E. & AWRAMIK, S.M. (eds): Microbial Sediments. Springer, Berlin, 307–314. SUMNER, D.Y. (2002): Decimeter-thick encrustations of calcite and aragonite on the sea fl oor and implications for Neoarchean and Ne- oproterozoic ocean chemistry. – In: ALTERMANN, W. & CORC- ORAN, P.L. (eds): Precambrian sedimentary environments: a mod- ern approach to ancient depositional systems. I.A.S. Spec. Publ. 33, 107–120. SUMNER, D.Y. (2004): Secular variations in Precambrian seawater che- mistry and the timing of Precambrian aragonite seas and calcite seas: Comment. – Geology: Online Forum, p. e56. SUMNER, D.Y. & GROTZINGER, J.P. (1996a): Were kinetics of Arche- an calcium carbonate precipitation related to oxygen concentration. – Geology, 24, 119–122. SUMNER, D.Y. & GROTZINGER, J.P. (1996b): Herringbone calcite: petrography and environmental signifi cance. – J. Sediment. Res., 66, 419–429. SUMNER, D.Y. & GROTZINGER, J.P. (2000): Late Archean aragonite precipitation: petrography, facies associations, and environmental signifi cance. – In: GROTZINGER, J.P. & JAMES, N.P. (eds): Car- bonate sedimentation and diagenesis in the evolving Precambrian world. SEPM Spec. Publ., 67, 123–144. SUMNER, D.Y. & GROTZINGER, J.P. (2004): Implications for Neoar- chaean ocean chemistry from preimary carbonate mineralogy of the Campbellrand-Malmani platform, South Africa. – Sedimentology, 51, 1–27. SWETT, K. & KNOLL, A.H. (1985): Stromatolitic bioherms and micro- phytolites from the late Proterozoic Draken Conglomerate Forma- tion, Spitsbergen. – Precambrian Res., 28, 327–347. THOMPSON, J.B. & FERRIS, F.G. (1990): Cyanobacterial precipita- tion of gypsum, calcite, and magnesite from natural alkaline lake water. – Geology, 18, 995–998. THRAILKILL, J. (1976): Speleothems. – In: WALTER, M.R. (ed.): Stro- matolites. Developments in Sedimentology 20, Elsevier, Amster- dam, 73–86. TRICHET, J. & DÉFARGE, C. (1995): Non-biologically supported or- ganomineralization. – Bull. Inst. Océanogr. Monaco, Num. Spéc. 14, 203–236. TRUSWELL, J.F. & ERIKSSON, K.A. (1973): Stromatolitic associa- tions and their palaeo-environmental signifi cance: a re-appraisal of a lower Proterozoic locality from the northen Cape Province, South Africa. – Sediment. Geol., 10, 1–23. TURNER, E.C., JAMES, N.P. & NARBONNE, G.M. (1997): Growth dynamics of Neoproterozoic calcimicrobial reefs, Mackenzie mo- un tains, northwest Canada. – J. Sediment. Res., 67, 437–450. TURNER, E.C., NARBONNE, G.M. & JAMES, N.P. (1993): Neopro- terozoic reef microstructures from the Little Dal Group, northwest- ern Canada. – Geology, 21, 259–262. TURNER, E.C., NARBONNE, G.M. & JAMES, N.P. (2000a): Frame- work composition of early Neoproterozoic calcimicrobial reefs and associated microbialites, Mackenzie Mountains, N.W.T., Canada. – In: GROTZINGER, J.P. & JAMES, N.P. (eds): Carbonate sedi- mentation and diagenesis in the evolving Precambrian world. SEPM Spec. Publ. 67, 179–205. TURNER, E.C., JAMES, N.P. & NARBONNE, G.M. (2000b): Tapho- nomic control on microstructure in early Neoproterozoic reefal stro- matolites and thrombolites. – Palaios, 15, 87–111. VISSCHER, P.T, REID, R.P., BEBOUT, R.M., HOEFT, S.E., MACIN- TYRE, I.G. & THOMPSON Jr., J.R. (1998): Formation of lithifi ed micritic laminae in modern marine stromatolites (Bahamas): the role of sulfur cycling. – Am. Mineral., 83, 1482–1493. VISSCHER, P.T, REID, R.P. & BEBOUT, R.M. (2000): Microscale ob- servations of sulfate reduction: correlation of microbial activity with micritic lithifi ed laminae in modern marine stromatolites. – Geol- ogy, 28, 919–922. VOLOGDIN, A.G. (1962): The oldest algae of the USSR. – Academy of Sciences, Moscow, 656 p. [In Russian]. WALCOTT, C.D. (1912): Notes on fossils from limestone of Steeprock series, Ontario. – Geol. Surv. Canada Mem., 28, 16–23. WALCOTT, C.D. (1914): Cambrian geology and paleontology III. Pre- cambrian Algonkian algal fl ora. – Smithsonian Miscellaneous Col- lection, 64, 77–156. WALTER, M.R. (1972): Stromatolites and the biostratigraphy of the Aus- tralian Precambrian and Cambrian. – Spec. Pap. Palaeontology, 11, 190 p. WALTER, M.R. (ed.) (1976): Stromatolites. Developments in Sedimen- tology 20. Elsevier, Amsterdam, 790 p. WALTER, M.R., BAULD, J. & BROCK, T.D. (1976): Microbiology and morphogenesis of columnar stromatolites (Conophyton, Vacerrilla) from hot springs in Yellowstone National Park. – In: WALTER, M.R. (ed.): Stromatolites. Developments in Sedimentology 20. Elsevier, Amsterdam, 273–310. WARNKE, K. (1995): Calcifi cation processes of siliceous sponges in Viséan limestones (counties Sligo and Leitrim, northwestern Ire- land). – Facies, 33, 215–227. WHARTON, R.A., Jr., (1994): Stromatolitic mats in Antarctic lakes. – In: BERTRAND-SARFATI, J. & MONTY, C. (eds): Phanerozoic stro- matolites II. Kluwer, Dordrecht, 53–70. WHITTLE, G.L., KENDALL, C.G.ST.C, DILL, R.F. & ROUCH, L. (1993): Carbonate cement fabrics displayed: a traverse across the margin of the Bahamas platform near Lee Stocking Island in the Exuma Cays. – Marine Geol., 110, 213–243. WILKS, M.E. & NISBET, E.G. (1985): Archaean stromatolites from the Steep Rock Group, northwestern Ontario, Canada. – Can. J. Earth Sci., 22, 792–799. Geologia CroaticaRobert Riding: Abiogenic, microbial and hybrid authigenic carbonate crusts: components of Precambrian stromatolites 103 WINEFIELD, P.R. (2000): Development of late Paleoproterozoic arago- nite seafl oor cements in the McArthur Group, northern Australia. – In: GROTZINGER, J.P. & JAMES, N.P. (eds): Carbonate sedi- mentation and diagenesis in the evolving Precambrian world. SEPM Spec. Publ. 67, 145–159. WRIGHT, V.P. (1989): Terrestrial stromatolites and laminar calcretes; a review. – Sediment. Geol., 65, 1–13. YOUNG, R.B. (1932): The occurrence of stromatolitic or algal lime- stones in the Campbell Rand Series, Griqualand West. – Transac- tions Geological Society South Africa, 35, 19–36. ZANKL, H. (1993): The origin of high-Mg-calcite microbialites in cryp- tic habitats of Caribbean coral reefs – their dependence on light and turbulence. – Facies, 29, 55–60. Manuscript received July 6, 2008 Revised manuscript accepted September 3, 2008