Stromatolite Morphology Research Articles

The relationship between morphology, microstructure, and microbiota in three vertically intergrading stromatolites from the Gunflint Iron Formation

The microstructure in three silicified morphologically distinct and vertically intergrading stromatolites from the Aphebian (~2 Ga old) Gunflint Iron Formation of Ontario, Canada was found to be different for each stromatolite morphology studied; each in turn has a characteristic microbial composition. Non-columnar stromatolites that comprise the middle zone of the intergrading sequence contain a microstructure characterized by relatively thin laminae composed of a cyanophytic community dominated by the coccoid Huroniospora Barghoorn while the columnar stromatolites of the lower (small, frequently divergent branching columns) and upper zone (large, infrequently α- to β-parallel branching columns) have thicker laminae differing in shape and continuity, which were built by a filamentous cyanophyte-dominated community. 'Microstromatolites' referable to Frutexites Maslov are found in the middle non-columnar and occasionally in the lower parts of the upper columnar zone.In the stromatolites thus far examined from the Gunflint formation, the microbiota, microstructure, and macrostructure are all interrelated, the composition of the microbiota, the nature of the microstructure, and the resulting gross morphology of the stromatolite presumably were influenced or controlled by the microbiota.

First report of well-preserved precambrian microfossils in Brazil (Paraopeba formation, Bambuí Group, near Brasilia)

Thin sections of black chert from two 'localities at the base of the upper Precambrian Paraopeba Formation (Bambui Group) near Brasilia in south-central Brazil contain abundant microfossils. Although supposed Precambrian rnicrofossils have been recorded previously from Precambrian rocks (summarized in FAIRCHILD DARDENNE, in prep.), the following preliminary study marks the first time that well-preserved, morphologically diverse rnicrofossils have been reported from indisputably Precambrian rocks in Brazil. These rnicrofossils occur in silicified, branched, columnar stromatolites from 3040 km north of Unai, Minas Gerais, about 100 km to the northwest in silicified, flat-Iaminated dolostone 30 km west-southwest of Sao Gabriel, Goias. Solitary colonial forms predominate at both localities, with the Sao Gabriel assemblege being the better preserved, more diverse, more abundant assemblage. These discoveries are important because (1) They provide a standard of comparison for past future rnicrofossil fmds in the Brazilian Precambrian; (2) They may further elucidate the relationships between biological content columnar stromatolite morphology, thereby facilitating insight into the possible reasons for the apparent of stromatolites; (3) Together with stromatolite data (see FAIRCHILD DARDENNE, in prep.), they may help us to better date and correlate the Bambu! Group make better paleoenvironrnental paleogeographical interpretations of these strata. On a broader scale, the Unai Sao Gabriel rnicrofloras represent additional valuable evidence for deciphering the early evolution of the algae for testing Precambrian biostratigraphic schemes (see SCHOPF, 1977).

Phormidium Hendersonii Howe: Identity And Significance of a Modern Stromatolite Building Microorganism

ABSTRACT Populations of a modern, stromatolite-building microorganism have been studied in the subtropical Atlantic and the Caribbean area. In four geographically distinct populations, growing under different ecological conditions (exposed to protected, subtidal to intertidal) the same nocti-diurnal rhythm in filament orientation and growth dynamics was maintained. This resulted in the formation of characteristic finely laminated stromatolitic domes. High degree of similarity and overlap in morphometric properties leads to the conclusion that all populations studied belong to the same microbial species. An historic confusion concerning the taxonomic identity of this microorganism has now been resolved and its valid name established: Phormidium hendersonii Howe 1918 (Oscillatoriaceae, Cyanophyta). Stromatolite morphology in this case is determined by the biological (genetic) properties of the organism irrespective of differences in environmental conditions within the range of its distribution. This conclusion bears relevance to the interpretation of ancient stromatolitic structures.

The biostratigraphic potential of Precambrian stromatolites

The morphology of stromatolites is controlled partly by the biological composition of algal mats and partly by the direct action of environmental factors. Both trapping of detrital grains and precipitation of carbonate in algal mats are important processes in stromatolite formation. Precambrian stromatolites were probably not subject to the environmental restrictions of Recent stromatolites. Stromatolites can be classified on the basis of a number of morphological features, some of which relate directly to the biological characteristics of algal mats (lamina shape and microstructure) and some of which have at least partial environmental control (e.g., column shape, branching style). The taxonomic significance of characters may vary in different taxa. A binomial (but not biological) form nomenclature is applied, using “groups” (analogous to genera) and “forms” (analogous to species). The biostratigraphic potential of Precambrian stromatolites, theoretically possible from what is now understood of the controls of Recent stromatolite morphology, can only be tested empirically by determining the time ranges of defined taxa. This relies heavily on other means of dating and correlation. Initial results of stromatolite biostratigraphy in Australia proved encouraging and appeared to confirm the applicability of the biostratigraphic scheme worked by Russian geologists. However, none of the stromatolite assemblages described from Australia is identical to any from the U.S.S.R.: none contains all of the elements described from the Russian assemblages, and most of the common elements are comparable only at group level. New data suggest that the time ranges of several groups are longer than previously accepted, and caution is therefore urged in using group level identifications alone as a basis of correlation. Identity of forms is a better criterion for correlation, but most forms have a limited geographic distribution. Further systematic description and identification of stromatolites from rock sequences of all ages on all continents are needed to establish the actual time ranges of taxa and assemblages of taxa, and hence their biostratigraphic applicability. Stromatolite assemblages presently known from various Proterozoic basins of Australia are discussed. Pre-Riphean assemblages include groups previously known only from the Riphean, as well as groups not found in the younger rocks. Partial equivalents of each of the subdivisions of the Riphean occur in a number of basins, but their stromatolite assemblages are not yet well known. Late Riphean equivalents have the best documented assemblages and the groups present are generally consistent with their time ranges determined in the U.S.S.R. However, the Burra Group (Adelaidean) of South Australia lacks typical Late Riphean stromatolites and was originally correlated with the Middle Riphean. New radiometric and stromatolite data suggest that deposition of the whole Adelaidean sequence commenced during the Late Riphean.

Biostratigraphic usefulness of stromatolitic precambrian microbiotas: A preliminary analysis

Diverse, cellularly preserved microbial communities are now known from stromatolitic sediments of at least twenty-eight Precambrian formations. These fossiliferous deposits, principally cherts and cherty portions of carbonate units, range in age from Early Proterozoic (Transvaal Dolomite, ca. 2250 Ma old) to Vendian (Chichkan Formation, ca. 650 Ma old) and include units from Australia, India, Canada, South Africa, Greenland, the United States and the Soviet Union. More than three-quarters of these microbiotas have been discovered since 1970. Although few, therefore, have as yet been studied in detail, virtually all of the assemblages are known to be dominated by prokaryotic (bacterial and blue-green algal) microorganisms and to contain three major categories of microfossils: spheroidal unicells, cylindrical tube-like sheaths, and cellular trichomic filaments. Analyses of data now available (including measurements of more than 7800 fossil unicells) indicate that each of these three types of microfossils exhibited a gradual, but marked, increase in mean diameter and size range during the Proterozoic and that taxonomic diversity apparently also increased, especially beginning about 1400 Ma ago. Thus, it now seems evident that (i) the microbial components of Proterozoic stomatolitic assemblages have varied systematically as a function of geologic age and that (ii) such communities are both more abundant and more widespread than had previously been recognized. These observations augur well for the future use of such assemblages in Precambrian biostratigraphy. At present, however, data are sufficient to warrant the provisional establishment of only a few microfossil-based subdivisions of the Proterozoic. Such zones, necessarily relatively long-ranging, are here tentatively defined; it is of interest to note that boundaries between certain of these microfossil-based subdivisions appear to coincide, at least approximately, with previously suggested stromatolite-based boundaries. To some extent, therefore, results of this study seem consistent with, and may be supportive of, the concept of stromatolite-based biostratigraphy. At the same time, however, the study seems to indicate that stromatolites of markedly differing age, whether of similar or of dissimilar morphology, were probably formed by distinctly differing microbiotas. Data are as yet insufficient to indicate whether differing types of coetaneous, stratigraphically useful, stromatolites were formed by differing microbial communities and two what extent the “evolution” of stromatolite morphology was a result of the biologic evolution of stromatolite-building microorganisms. There is thus continued need for investigation of the potential biostratigraphic usefulness of stromatolitic microbiotas and, especially, for more effective integration of results of such studies with those available from studies of stromatolites without preserved microbiotas and from studies of the acritarchs preserved in Proterozoic shales.

Palaeoecological interpretations of South Australian Precambrian stromatolites

Abstract Despite the palaeoecological emphasis of recent stromatolite research outside the USSR, controversy still exists regarding the environmental restriction of these organosedimentary structures and some analogies with modern occurrences have been maintained too rigidly. Palaeo‐environments should be determined where possible by a combination of all biological and sedimentological evidence. In South Australia, Precambrian carbonate rocks contain widespread stromatolites. The absence of independent biological environmental indicators necessitates a reliance on general stratigraphic and sedimentological data for the interpretation of regional palaeogeography and local environmental conditions. The Skillogalee Dolomite contains one predominant form of stromatolite almost throughout its extent, and it chiefly grew as biostromes in littoral and lagoonal settings on an extensive, level, carbonate‐depositing platform, probably of restricted access to the open sea. The interspace and associated sediments show evidence of varying energy conditions, degree of oxidation and possibly salinity, but the same stromatolite persists, although there is some modification of morphology in the higher energy environments. The Umbera‐tana Group has basinal shale and siltstone in its lower part, but during a subsequent widespread regression, the marginal parts of the basin were shallowed sufficiently to allow the growth of stromatolites, probably in water depths ranging from shallow subtidal to low supratidal. In addition, in the northern central part of the basin, offshore banks of shallow water limestone may have been related to areas of diapiric activity. The stromatolites of the Umberatana Group most often occur as bioherms of varying sizes intimately associated with ooid and intraclast grainstones, cross‐bedded sandy limestone, and, in marginal areas, penecontemporaneous dolomite. Different forms may occur in sequences of seemingly indistinguishable litho‐facies. These observations strengthen the view that stromatolite morphology is at least partly biologically controlled, the defined taxa being largely independent of the local environment.

Prokaryotic algae associated with Australian proterozoic stromatolites.

Five instances of association between distinctive stromatolites and blue-green algal nannofossils are recorded from a 100-m sequence of carbonate rocks about 1.6 x 10(9) years old, along the south side of Paradise Creek, northwestern Queensland, Australia. No eukaryotes were identified in any of these systematically limited assemblages, although they are known from rocks as old as 1.3 x 10(9) years in eastern California. Thus, eukaryotes may not have appeared until after 1.6 x 10(9) years ago (but before 1.3 x 10(9) years ago). The associations observed would also be consistent with (but do not prove) a biotic influence on stromatolite morphology. As is usual among prePaleozoic forms described, the morphology of the nannofossils is very similar to living forms, displaying marked evolutionary conservatism. Primary orientation of stromatolitic laminae and columns is not invariably convex upward, as conventionally believed, but convex away from and parallel to the initial point or surface of attachment, which may be horizontal or even downward beneath overhangs.

Conical-Columnar Stromatolites and Subtidal Environment: ABSTRACT

Several distinct varieties of stromatolites are present in dolostones of the Proterozoic Dismal Lakes Group, Great Bear Lake region, Northwest Territories. Comparison of the stromatolite types with respect to abundance of associated sedimentary structures (cross-laminations, ripple marks, oolites, desiccation cracks, evaporite casts, intraformational conglomerates) supports the concept that stromatolite morphology is closely related to environment. As in modern analogues, water turbulence appears to be a particularly significant determinant of morphology. On the basis of textures, associated sedimentary structures, and comparison with modern algal stromatolites, most stromatolites of the Dismal Lakes Group appear to have formed in either supratidal or intertidal environments. However, conical-columnar stromatolites (Conophyton), for which a modern analogue is lacking, are confined largely to a prominent dolostone unit in which sedimentary structures indicative of turbulence are almost totally absent. The paucity of such structures, coupled with consideration of the stromatolite morphology during growth, suggests that conical-columnar stromatolites may be characteristic of a subtidal environment. Maintenance of a vast field of conical surfaces in an intertidal or supratidal environment without reduction or fragmentation of the apices eems unlikely. The relation of the conical-columnar stromatolites to flat-bedded subjacent strata renders interpretation of origin by deformation untenable, and continuity of lamination within and between the columns refutes a diagenetic origin. The Dismal Lakes dolostone unit consisting mainly of conical-columnar stromatolites is interpreted as a Proterozoic subtidal algal reef of unusual persistence in space and time. End_of_Article - Last_Page 614------------