Stromatolite-like Structures Research Articles

Microstratigraphic geochemical characteristics of ferromanganese crust from central pacific: Implication for the role of Fe and Mn in REY enrichment

Rare earth elements plus yttrium (REY), well-known critical metals for many new and emerging technologies and as proxies in geological events, are one type of the most important high-tech metals in ferromanganese (Fe-Mn) crusts. The REY(III) are accumulated in Fe-Mn crusts through adsorption by Fe-Mn (oxyhydr)oxides. However, negative correlations between Mn and REY in some Fe-Mn crusts reveal a controversial role of Mn oxides in REY enrichment, which remains equivocal. In this study, texture, structure, mineralogy, and microstratigraphic geochemistry with the growth of a Fe-Mn crust (MP2D32A) from the Line Islands archipelago were analyzed using multiple microanalysis methods. The Fe-Mn crust is characterized by laminar structures, stromatolite-like structures, and micronodules that are primarily composed of low-crystalline Fe-vernadite and ferrihydrite. Throughout the profiles of MP2D32A, the contents of elements are more or less interdependent. Analysis of elemental correlations from the MP2D32A samples and 847 collected data sets from non-phosphatized hydrogenetic Fe-Mn crusts shows Fe plays a positive role and Mn plays multiple roles in REY accumulation during Fe-Mn crust formation. The REY content increases initially and then decreases with increasing Mn content and Mn/Fe ratio and the turning points (i.e., highest REY contents) mainly occur at an Mn content of ∼18–26% and an Mn/Fe content ratio of ∼0.8. These fundamental results provide novel insights into understanding the influence of Fe and Mn in REY accumulation in the Fe-Mn crusts and are very significant in enhancing our understanding of the REY geochemical processes in the ocean.

Stromatolite-like Structures Within Microbially Laminated Sandstones of the Paleoarchean Moodies Group, Barberton Greenstone Belt, South Africa.

We report abundant small calcareous mounds associated with fossilized kerogenous microbial mats in tidal-facies sandstones of the predominantly siliciclastic Moodies Group (ca. 3.22 Ga) of the Barberton Greenstone Belt (BGB), South Africa and Eswatini. Most of the bulbous, internally microlaminated mounds are several centimeters in diameter and formed at the sediment-water interface contemporaneously with sedimentation. They originally consisted of Fe-Mg-Mn carbonate, which is now largely silicified; subtle internal compositional laminations are composed of organic matter and sericite. Their presence for >6 km along strike, their restriction to the inferred photic zone, and the internal structure suggest that mineral precipitation was induced by photosynthetic microorganisms. Similar calcareous mounds in this unit also occur within and on top of fluid-escape conduits, suggesting that carbonate precipitation may either have occurred abiogenically or involved chemotrophic metabolism(s) utilizing the oxidation of organic matter, methane, or hydrogen, the latter possibly generated by serpentinization of underlying ultramafic rocks. Alternatively or additionally, carbonate may have precipitated abiotically where heated subsurface fluids, sourced by the intrusion of a major Moodies-age sill, reached the tidal flats. In summary, precipitation mechanisms may have been variable; the calcareous mounds may represent "hybrid carbonates" that may have originated from the small-scale overlap of bioinduced and abiotic processes in space and time. Significantly, the widespread occurrence of these stromatolite-like structures in a fully siliciclastic, high-energy tidal setting broadens search criteria in the search for life on Mars while their possible hybrid origin challenges our ability to unambiguously identify a biogenic component.

Silica botryoids from chemically oscillating reactions and as Precambrian environmental proxies

Abstract In this petrographic and geochemical study, we differentiated diverse quartz botryoids, including circular-concentric, twinned, columnar, wavy, and stromatolite-like structures versus synchronous biotic patterns of similar geometry and size dimensions (filamentous traits and stromatolites) in Precambrian cherts of Barberton, South Africa, and Gunflint, Canada. The botryoidal habits explored retained self-similar patterns of radially aligned acicular quartz with concentric laminae, which are not documented in biologically built stromatolites. These ancient fractals and their composition imitate those in chemically oscillating reactions, implying that the precipitation of botryoids was fueled by abiotic diagenetic degradation of organic matter and subsequently metamorphosed into chert.

Mechanisms shaping the gypsum stromatolite-like structures in the Salar de Llamara (Atacama Desert, Chile)

The explanation of the origin of microbialites and specifically stromatolitic structures is a problem of high relevance for decoding past sedimentary environments and deciphering the biogenicity of the oldest plausible remnants of life. We have investigated the morphogenesis of gypsum stromatolite-like structures currently growing in shallow ponds (puquíos) in the Salar de Llamara (Atacama Desert, Northern Chile). The crystal size, aspect ratio, and orientation distributions of gypsum crystals within the structures have been quantified and show indications for episodic nucleation and competitive growth of millimetric to centimetric selenite crystals into a radial, branched, and loosely cemented aggregate. The morphogenetical process is explained by the existence of a stable vertical salinity gradient in the ponds. Due to the non-linear dependency of gypsum solubility as a function of sodium chloride concentration, the salinity gradient produces undersaturated solutions, which dissolve gypsum crystals. This dissolution happens at a certain depth, narrowing the lower part of the structures, and producing their stromatolite-like morphology. We have tested this novel mechanism experimentally, simulating the effective dissolution of gypsum crystals in stratified ponds, thus providing a purely abiotic mechanism for these stromatolite-like structures.

MICROFACIES OF LATE PLEISTOCENE TRAVERTINE DEPOSITS IN JORDAN

The Late Pleistocene travertine outcrops from Deir Alla, Suwayma, and Az Zara were investigated, and their microfacies were identified. The microfacies of the Deir Alla travertines include micrite and spar groundmass, shrubs, crystalline crusts, a stromatolite-like structure, peloids, and cements. Shrub travertine includes spar calcite-coated stems with probably microbial micritic clumps. The crystalline crust travertine displays an alternation of micrite and sparite laminae. The micritic laminae are dark-coloured. Bundles of radial spar crystals are associated locally with micritic groundmass. The crystalline crust developed where biogenic activity is limited. Peloidal microfacies are less than 0.25 mm in diameter, cryptocrystalline, pale-dark green in colour, elliptical to spherical in shape, and usually associated with microorganisms. The microfacies of the Suwayma and Az Zara travertines include crystalline calcite rhombs and other composite scalenohedral crystals. They occur as small anhedral-subhedral crystals, monocrystalline to some polycrystalline, corroded, subrounded, and mainly coated with iron oxide and/or clay minerals. Peloids, ooids, and oncoids are common. They are dark-green coloured, cryptocrystalline to microcrystalline carbonates of spherical and ellipsoidal shape with less than 1 mm in diameter. Rich flora travertines include reed and paper-thin rafts with leaf impressions encrusted on moss cushions. The flora observed in the upper part of the Suwayma section was identified as charophyte oospores (gyrogonites). A few grains of quartz are present as small subhedral-euhedral crystals, monocrystalline, corroded, rounded, and mainly coated with iron oxide. The iron is irregularly distributed among the laminae and voids and is occasionally replaced by carbonates. The described macrophyte encrustation structures probably represent algae, cyanobacteria, or bryophytes. All samples of micrite and spar calcite appear as groundmass.

Critical metal enrichment mechanism of deep-sea hydrogenetic nodules: Insights from mineralogy and element mobility

The micro-/nano-textural features, major and trace element compositions of ferromanganese nodule from the Marcus Wake Seamount Group of the Western Pacific Ocean were documented in this study. The ferromanganese nodules genetically comprise a core and three types of layers. The core is made up of porous quartz and locally Fe-Mn oxides. Layer close to the core (layer I), mantle (II) and rim (III) are featured by a massive dense layer with stromatolite-like structure, laminated and detrital and dendritic rim, respectively. These layers contain mainly birnessite, goethite and quartz. The Mn/Fe ratios (1.0–4.1, mean 1.9) demonstrate a hydrogenetic genesis for these nodules. The nodules have moderate enrichments (1000–10,000 ppm) of Na, Mg, Al, P, K, Ti, Co, Ni and Cu, with minor enrichments (100–1000 ppm) of V, Zn, As, Sr, Y, Zr, Nb, Mo, Ba and Pb, whereas other elements (Li, Sc, Cr, Ge, Se, Rb, Sn, Cs, Hf, W, Bi, Th and U) have concentrations of several tens of ppm. Based on the growth rate calculated by Co concentration, layer I has the lowest mean growth rate (1.8 mm/Myr), followed by layer II (2.3 mm/Myr) and then layer III (2.7 mm/Myr). Integrating the previous marine nodule studies in the South China Sea, we suggest that the massive dense layers were possibly resulted from a slow growth rate. The conduits (of various scales) may have promoted the post-depositional processes, as they enlarged the surface area for fluid dissolution and reprecipitation. Nickel enrichment is associated with birnessite and promoted by the post-depositional processes, as evidenced by the higher Ni content in the early-formed layer I and its decreasing trend from core to rim. Cobalt may have associated with Mn3+ and has less effect by post-depositional accumulation. The Fe content, which may have related to goethite (Fe3+O(OH)), shows good correlations with the REY (REE + Y) contents. This correlation demonstrates the scavenging of REYs by hydrothermal Fe- (oxyhydr)oxides. REYs are likely released during the post-deposition, as the REY contents are decreasing from the core to rim and negatively correlated with the accumulation time. The similarity of REY distribution pattern between hydrothermal nodule and goethite-bearing (and Mn-oxides free) banded iron formation (BIF) may suggest hydrothermal Fe may have linked to REY enrichments in deep-sea sediments. These micro-/nano-sized conduits are well-developed in either type I, II or III layer, which likely served as fluid circulating conduits and facilitate post-depositional processes.

Microbial diversity and biosignatures of amorphous silica deposits in orthoquartzite caves

Chemical mobility of crystalline and amorphous SiO2 plays a fundamental role in several geochemical and biological processes, with silicate minerals being the most abundant components of the Earth’s crust. Although the oldest evidences of life on Earth are fossilized in microcrystalline silica deposits, little is known about the functional role that bacteria can exert on silica mobility at non-thermal and neutral pH conditions. Here, a microbial influence on silica mobilization event occurring in the Earth’s largest orthoquartzite cave is described. Transition from the pristine orthoquartzite to amorphous silica opaline precipitates in the form of stromatolite-like structures is documented through mineralogical, microscopic and geochemical analyses showing an increase of metals and other bioessential elements accompanied by permineralized bacterial cells and ultrastructures. Illumina sequencing of the 16S rRNA gene describes the bacterial diversity characterizing the consecutive amorphization steps to provide clues on the biogeochemical factors playing a role in the silica solubilization and precipitation processes. These results show that both quartz weathering and silica mobility are affected by chemotrophic bacterial communities, providing insights for the understanding of the silica cycle in the subsurface.

Characterization of Pustular Mats and Related Rivularia-Rich Laminations in Oncoids From the Laguna Negra Lake (Argentina).

Stromatolites are organo-sedimentary structures that represent some of the oldest records of the early biosphere on Earth. Cyanobacteria are considered as a main component of the microbial mats that are supposed to produce stromatolite-like structures. Understanding the role of cyanobacteria and associated microorganisms on the mineralization processes is critical to better understand what can be preserved in the laminated structure of stromatolites. Laguna Negra (Catamarca, Argentina), a high-altitude hypersaline lake where stromatolites are currently formed, is considered as an analog environment of early Earth. This study aimed at characterizing carbonate precipitation within microbial mats and associated oncoids in Laguna Negra. In particular, we focused on carbonated black pustular mats. By combining Confocal Laser Scanning Microscopy, Scanning Electron Microscopy, Laser Microdissection and Whole Genome Amplification, Cloning and Sanger sequencing, and Focused Ion Beam milling for Transmission Electron Microscopy, we showed that carbonate precipitation did not directly initiate on the sheaths of cyanobacterial Rivularia, which dominate in the mat. It occurred via organo-mineralization processes within a large EPS matrix excreted by the diverse microbial consortium associated with Rivularia where diatoms and anoxygenic phototrophic bacteria were particularly abundant. By structuring a large microbial consortium, Rivularia should then favor the formation of organic-rich laminations of carbonates that can be preserved in stromatolites. By using Fourier Transform Infrared spectroscopy and Synchrotron-based deep UV fluorescence imaging, we compared laminations rich in structures resembling Rivularia to putatively chemically-precipitated laminations in oncoids associated with the mats. We showed that they presented a different mineralogy jointly with a higher content in organic remnants, hence providing some criteria of biogenicity to be searched for in the fossil record.

Stratified Bacterial Diversity along Physico-chemical Gradients in High-Altitude Modern Stromatolites

At an altitude of 3,570 m, the volcanic lake Socompa in the Argentinean Andes is presently the highest site where actively forming stromatolite-like structures have been reported. Interestingly, pigment and microsensor analyses performed through the different layers of the stromatolites (50 mm-deep) showed steep vertical gradients of light and oxygen, hydrogen sulfide and pH in the porewater. Given the relatively good characterization of these physico-chemical gradients, the aim of this follow-up work was to specifically address how the bacterial diversity stratified along the top six layers of the stromatolites which seems the most metabolically important and diversified zone of the whole microbial community. We herein discussed how, in only 7 mm, a drastic succession of metabolic adaptations occurred: i.e., microbial communities shift from a UV-high/oxic world to an IR-low/anoxic/high H2S environment which force stratification and metabolic specialization of the bacterial community, thus, modulating the chemical faces of the Socompa stromatolites. The oxic zone was dominated by Deinococcus sp. at top surface (0.3 mm), followed by a second layer of Coleofasciculus sp. (0.3 to ∼2 mm). Sequences from anoxygenic phototrophic Alphaproteobacteria, along with an increasing diversity of phyla including Bacteroidetes, Spirochaetes were found at middle layers 3 and 4. Deeper layers (5–7 mm) were mostly occupied by sulfate reducers of Deltaproteobacteria, Bacteroidetes and Firmicutes, next to a high diversity and equitable community of rare, unclassified and candidate phyla. This analysis showed how microbial communities stratified in a physicochemical vertical profile and according to the light source. It also gives an insight of which bacterial metabolic capabilities might operate and produce a microbial cooperative strategy to thrive in one of the most extreme environments on Earth.

The stratigraphic architecture and depositional environments of non-marine carbonates from barremian-aptian pre-salt strata of the Brazilian continental margin

The stratigraphic architecture and depositional environments of non-marine carbonates from barremian-aptian pre-salt strata of the Brazilian continental margin

Calcareous thrombolitic crust on Late Quaternary beachrocks in Kuwait, Arabian Gulf

Thinly bedded beachrocks exposed within the intertidal zone on the northern coast of Kuwait are coated with a calcareous microbialite crust that varies in thickness from a few millimetres to several centimetres. A macromorphological investigation revealed the occurrence of two primary crust types: a thick fenestral crust and a thin massive crust. The latter includes compound laminite/columnar crusts and planar laminite crusts. The thick fenestral crust is restricted to the lower part of the intertidal zone, whereas the compound laminite/columnar and the planar laminite crusts dominate the middle and upper parts of the intertidal zone, respectively. Micro- and nanoscopic examinations have revealed that these crusts are composed of thrombolitic microbialite, in which the thrombolite clots consist of aragonitic nanoglobules that nucleate in association with extracellular polymeric substances (EPS) and microbial fossils. It is suggested that these crusts have been constructed during two phases. The first phase involved the prevalence of organomineralisation processes, which formed the thrombolitic microbialite, while the second phase was dominated by physiochemical processes, which led to the precipitation of fibrous aragonite. The interlamination of dark microbialitic laminae with light-coloured isopachous fibrous aragonite results in the development of stromatolite-like structure. These results may provide support for the importance of the combination of biogenic and abiogenic processes in the formation of Precambrian stromatolites.

Zircon SHRIMP dating confirms a Palaeoarchaean supracrustal terrain in the southeastern Kaapvaal Craton, southern Africa

We report SHRIMP zircon ages for felsic volcanic rocks of the early Archaean Nondweni greenstone belt (NGB) located in northern KwaZulu-Natal Province, South Africa. The NGB is part of a chain of greenstone belt remnants that occurs south of the well-known Barberton greenstone belt (BGB). Two samples of felsic volcanic rocks of the basal Toggekry Formation yielded zircon ages of ~3.53Ga, whereas significantly younger ages of ~3.41Ga were obtained for two samples of felsic rocks from near the top of the Witkop Formation. The latter date now constrains the age of stromatolite-like structures in cherts associated with the felsic rocks, which probably represent some of the oldest preserved evidence of life on Earth. The zircon ages also indicate that the NGB correlates well chronologically with the BGB and is one of the oldest volcano-sedimentary successions in the southeastern Kaapvaal Craton. Geochemical data for the felsic NGB rocks suggest formation by partial melting or crystal fractionation of a mafic source that was contaminated by older crust, possibly in a back-arc environment. Initial εNd values for NGB rocks range from −1.2 to +0.3 and suggest involvement of pre-greenstone continental crust that was possibly extensive in the southeastern part of the Kaapvaal Craton during Palaeoarchaean times.

Microbes in caves: agents of calcite corrosion and precipitation

Abstract Diverse biogenic and abiogenic processes produce calcite speleothems. From a biogenic perspective, cave microbes mediate a wide range of destructive and constructive processes that collectively influence the growth of calcite speleothems and their internal fabrics. Destructive processes include substrate breakdown by dissolution, boring and residue micrite production, whereas constructive processes include microbe calcification, trapping and binding of detrital particles to substrates, and microbial induced calcite precipitation. Biogenesis can be established from: (1) the presence of mineralized microbes; (2) fabrics, such as stromatolite-like structures, that can be attributed to microbial activity; and/or (3) geochemical proxies (carbon and oxygen isotopes, lipid biomarkers) considered indicative of microbe activity. Such criteria have, for example, been used to demonstrate microbial involvement in the formation of pool fingers, stalactites/stalagmites, cave pisoliths and moonmilk. Nevertheless, absolute proof of microbial biogenesis in calcitic speleothems is commonly difficult because taphonomic processes and/or diagenetic processes commonly mask evidence of microbial activity. The assumption that calcitic speleothems are abiogenic, which has been tacitly assumed in many studies, is dangerous as there is clear evidence that microbes thrive in most caves and can directly and indirectly influence calcite precipitation in many different ways.

ドーム型層状シアノバクテリア・マットの形成：堆積物被覆実験によるストロマトライト様構造

ドーム型層状シアノバクテリア・マットの形成：堆積物被覆実験によるストロマトライト様構造

Evaluation of Early Archean Volcaniclastic and Volcanic Flow Rocks as Possible Sites for Carbonaceous Fossil Microbes

Sedimentary rocks have traditionally been the focus of the search for Archean microfossils; the Earth's oldest fossil bacteria are associated with carbonaceous matter in sedimentary cherts in greenstone belts in the eastern Pilbara block of Western Australia and Barberton greenstone belt of South Africa. Reports of possible fossils in a martian meteorite composed of igneous rock and the discovery of modern bacteria associated with basalts have stimulated a new look at Archean volcanic rocks as possible sites for fossil microbes. This study examines silicified volcaniclastic rocks, near-surface altered volcanic flow rocks, and associated stromatolite- like structures from the Archean Barberton greenstone belt to evaluate their potential for the preservation of carbonaceous fossils. Detrital carbonaceous particles are widely admixed with current-deposited debris. Carbonaceous matter is also present in altered volcanic flow rocks as sparse particles in silica veins that appear to be fed by overlying carbonaceous chert layers. Neither microfossils nor mat-like material was identified in the altered volcanic rocks or adjacent stromatolite-like structures. Ancient volcanic flow and volcaniclastic rocks are not promising sites for carbonaceous fossil preservation.

The Büdöskút Olistolith, an exotic limestone block from the Bükk mountains (NE-Hungary)

The present paper presents results of stratigraphical, sedimentological and micropalaeontological investigations of an olistolith from the Budoskut area in the Bukk Mts (NW Hungary). The olistolith consists of Norian sub- and peritidal platform carbonates (Budoskut lime-stone). During early diagenesis dissolution cavities originated which where filled by ostracodal mudstones. Other cavities first were wallpapered by evinospongial crusts and later filled either by ostracodal mudstones or by brick-red haematitic material (weathered bauxite). The evinospongial crusts show stromatolite-like structures and their forming therefore is supposed to be organically (microbially) induced.

ABSTRACT: Carbonate Production, Relative Sea Level Fluctuations and Slope Geometry: Case Histories from the Middle-Triassic of the Italian Dolomites

The understanding of carbonate platform reservoirs can be enhanced through the analysis of outcropping counterparts, such as the ones provided by the study platforms (Cernera and Latemar), particularly rich in both marine cements and microbial boundstones. The birth of both coeval platforms was matched with an anoxic event and with the deposition of a potential source horizon, rich in marine organic matter. The evolution of the short lived Cernera Platform was controlled by a very fast subsidence, badly compensated by its aggrading evolution. The lengthening slopes rapidly steeped up, while the cementation intensity increased. The carbonate production was unable to support the fast platform volume increase and the slope deposits therefore became thinner and thinner, forcing the margin to retrograde; the platform eventually drowned and it was covered by condensed pelagic facies and deep water stromatolite-like structures. The Latemar slope experienced a similar lengthening and steeping up evolution; this carbonate system however grew in a comparatively less subsiding area and it was therefore able to keep and eventually catch up the relative sea level increase. While the platform core shallowed from subtidal environments to cyclic emersions, in the adjacent slopes loose bioclastic and micritic sediments gave place to breccia rich in carbonate cements and microbial boundstones. The subsidence then slowed down considerably enabling the platform to laterally prograde, but no margin are presently preserved from this phase. Both platforms were then at least partially sealed by volcanic and terrigenous deposits and incompletely affected by a permeability enhancing dolomitization.

Sedimentology and geochemistry of fluvio‐lacustrine tufa deposits controlled by evaporite solution subsidence in the central Ebro Depression, NE Spain

The Urrea de Jalón tufa deposits constitute the 20‐ to 50‐m‐thick caprock (0·3 km2) of an isolated mesa. They disconformably overlie horizontal strata of the Tertiary Ebro Basin (NE Spain), which contains a thick succession of lacustrine gypsum and marls, followed by limestones, marls and, locally, fluvial sandstones and mudstones. The tufa deposits show a complex, large‐scale framework of basin‐like structures with centripetal dips that decrease progressively from the base to the top of the tufa succession, and beds that thicken towards the centre of the structure (cumulative wedge‐out systems). These geometries reveal that the tufa deposits were affected by differential synsedimentary subsidence. Distinct onlapping depressions reflect time migration of the subsiding areas. The studied carbonates are composed mostly of low‐Mg calcite, with minor quartz. Some samples have anomalously high contents of Fe, Mn and Ba that may exceed 1% (goethite, haematite and barite are present). Carbonate facies are: (a) macrophyte encrustation deposits; (b) bryophyte build‐ups; (c) oncolite and coated grain rudstones; (d) non‐concentric stromatolite‐like structures; (e) massive or bioturbated biomicrites; and (f) green and grey marls. Facies a and c show a great variety of microbial‐related forms. These facies can be arranged in dm‐ to 2‐m‐thick vertical associations representing: (i) fluvial–paludal sequences with bryophyte growths; (ii) pond‐influenced fluvial sequences; and (iii) lacustrine–palustrine sequences. The Urrea de Jalón tufa deposits formed in a fluvio‐lacustrine environment that received little alluvial sediment supply. Isotope compositions (δ13C and δ18O) reveal meteoric signatures and accord with such a hydrologically open system of fresh waters. The Fe, Mn and Ba contents suggest an additional supply of mineralized waters that could be related to springs. These would have been discharge points in the Ebro Depression of a regional aquifer of the Iberian Ranges. Rising groundwater caused the solution of the underlying evaporites and the synsedimentary subsidence of the tufa deposits.

Epiphytic Overgrowth of Charophyte Thalli by Stromatolite-like Structures and Fungi in the Lower Cretaceous of the Iberian Ranges (Spain)

The Lower Cretaceous (113 million years old) lacustrine deposits of Las Hoyas(Iberian Ranges, Cuenca, Spain) yield an epiphytic assemblage formed by thecharophyte species Clavatoraxis diaz-romeraliiMartín-Closas & Diéguez overgrown by a muddy, laminatedstromatolite-like structure. Fossil charophyte thalli coated with biogeniclaminations were previously unknown from the fossil record. Within thisstromatolite-like structure, filaments are found which correspond to thalli ofanother charophyte, Palaeonitella vermicularisMartín-Closas & Diéguez, and to septate hyphae of fungi(Ascomycetes). The distribution pattern of the biogenic lamination and itsselective growth only on the strongest thalli ofClavatoraxis diaz-romeralii suggests that it occurredwhen these macrophytes were still alive, standing upright on the lake bottom.From this point of view the stromatolite-like structures are reminiscent ofextant overgrowths of charophyte thalli by epiphytic, filamentouscyanobacteria, eucaryotic algae and fungi. These assemblages appear to developafter eutrophication of nutrient-poor environments, which are more suitablefor charophyte growth.

Sedimentary geology and stromatolites of the Middle Proterozoic Belt Supergroup, Glacier National Park, Montana

Abstract The Belt Supergroup is a thick, dominantly fine-grained sequence of Middle Proterozoic strata occurring in western Montana, northern Idaho, and parts of Washington state, Alberta, and British Columbia. The sequence in Glacier National Park is located along the northeastern part of present exposures of the Belt Supergroup; it is ∼ 2.9 km thick, extremely well exposed, and for the most part structurally simple. Although it was subjected to lowermost greenschist-facies metamorphism, primary sedimentary structures are exceptionally well preserved. Subtidal, intertidal, alluvial and possibly deltaic depositional environments appear to be represented in the Belt sequence in Glacier National Park. The lowermost unit, the Altyn Limestone, is not entirely exposed in the park. A partial section, ∼ 150 m thick, consists of impure dolostones deposited largely in shallow subtidal and intertidal settings. This carbonate unit is overlain by terrigenous strata of the Appekunny and Grinnell Argillites. The Appekunny Argillite is ∼ 700 m thick, consists largely of green-colored, fine-grained terrigenous material and appears to have been deposited predominantly in offshore and/or deltaic settings. The overlying Grinnell Argillite is 605 m thick and consists of red-colored terrigenous material deposited largely on an alluvial plain. The overlying Siyeh Limestone is 780 m thick and consists largely of impure dolostones and dolomitic limestones deposited in shallow subtidal and intertidal settings. Overlying the Siyeh Limestone is the 385 m thick Snowslip Formation, which consists of slightly dolomitic, predominantly fine-grained terrigenous strata deposited largely in intertidal settings. The overlying Shepard Formation is not exposed in its entirety in the central part of Glacier National Park. A 270 m thick section, which excludes the uppermost part of the formation, consists of impure dolostones and argillites, and appears to have been deposited in subtidal and intertidal settings. Stromatolites are abundant, diverse and well preserved in Glacier National Park, with mound-shaped forms and columnar forms of the group Baicalia occurring in the Altyn Limestone and Siyeh Limestone, and mound-shaped stromatolite-like structures occurring in the Snowslip and Shepard Formations. Particularly prominent is a 24–32 m thick stromatolite unit in the upper Siyeh Limestone, which contains Baicalia and Conophyton and appears to represent a prograding stromatolite reef, with Baicalia originating in a moderate-energy reef-front setting, and Conophyton originating in a lower energy back-reef setting. Individual units in these cycles can be correlated for 90 km. Many of the Conophyton in these cycles are inclined, probably as a result of gentle wave action, and the direction of inclination is relatively constant for 90 km, with the axes trending SW-SSW and plunging 30–60° SW.