Robert V. Burne

Microbialites; organosedimentary deposits of benthic microbial communities

Microbialites are organosedimentary deposits formed from interaction between benthic microbial communities (BMCs) and detrital or chemical sediments. Processes involved in the formation of calcareous microbialites include trapping and binding of detrital sediment (forming microbial boundstones), inorganic calcification (forming microbial tufa), and biologically influenced calcification (forming microbial framestones). Microbialites contrast with other biological sediments in that they are generally not composed of skeletal remains. -from Authors

Saline lake charophytes and their geological significance

ABSTRACT Charophytes are found in some ephemeral saline lakes in Australia. Healthy green charophytes, bearing oogonia, are frequently observed at salinities ca. 1-2 times that of seawater. Field observations of salinity tolerance are confirmed by experiments which demonstrate photosynthetic 14C-carbon dioxide fixation at these salinities. The lakes containing these charophytes have various hydrological settings but are all influenced by inputs of continental groundwater of seasonally varying salinity. Different evaporite minerals are deposited depending upon local desiccation-evaporation balances. In coastal lagoons adjacent to the Coorong, charophytes are found associated with calcite, aragonite and protodolomite while in the continental lakes around northern Spencer Gulf t e observed associations are with gypsum and halite. Lake deposits containing charophyte oogonia and discoidal gypsum provide a recent analog for some ancient evaporite units which are not adequately explained by the arid, coastal marine evaporite model based on the present-day Persian Gulf sabkhas. For example the Purbeckian evaporites of Dorset (England) and the Tertiary evaporites of the Paris Basin (France) both contain charophyte and evaporite-bearing units which are similar to the deposits of these Recent Australian lakes. We suggest that sediments containing associations of charophyte oogonia and evaporites formed as a result of seasonally varying salinity within lakes of the semi-arid, Mediterranean-type, climatic zone.

Correlation and environmental setting of the Skomer Volcanic Group, Pembrokeshire

Abstract: The Skomer Volcanic Group (previously thought to be of Lower Ordovician age) grades upward and laterally into sediments containing Lower Silurian faunas. Ostracods of late Ordovician or Silurian age have been found on Midland Island in beds interstratified with the volcanics; the presence of Eocoelia hemisphaerica proved an early Upper Llandovery (C 1-2 ) age for the higher parts of the group. The group is overlain unconformably by rocks of the Coralliferous Series (C 6 to Wenlock). Though most of the lavas are basic, each flow covering a wide area, some rhyolite domes of considerable relief provided the source of much of the sediment. The sediments in the lower parts of the group are predominantly terrestrial, and those in the upper parts are paralic. The paralic sequence is interrupted by several basic lavas, which temporarily re-established emergent conditions. In this upper part, each flow is followed by a transgressive sequence, the successive environments being distinguished by sedimentary and faunal characteristics.

The Modern Thrombolites of Lake Clifton, Western Australia

Thrombolites and stromatolites are microbialites with contrasting internal structures. The decline of stromatolites at the end of the Proterozoic and the rise of thrombolites during the Cambrian have been related to the evolution of burrowing and grazing metazoans, and it has been suggested that the thrombolites were the result of metazoan activity disrupting the original stromatolitic laminae. Some maintain that the thrombolitic structure is intrinsic, and due to penecontemporaneous mineralisation associated with coccoid-dominated BMCs. However, it is recognised that thrombolitic fabrics are complex, show a great variation, and may have originated in several ways. The interpretation of the genesis of thrombolites has been limited by the absence of well-documented modern examples. Thrombolitic and stromatolitic microbialites are presently forming in Lake Clifton, a marine-derived coastal lake in southwestern Australia. The thrombolites are by far the most predominant form of microbialite in Lake Clifton, with small stromatolites restricted to certain upper shore areas. The thrombolites exhibit a wide range of external morphologies including conical, domical, discoidal and tabular formations which vary considerably in size, as well as more irregular and columnar structures up to 1.3 m high. Many of the tabular and domical forms have coalesced to form an extensive reef-like formation over 6 km long. As documented by other studies of modern microbialites, external morphology appears to be primarily the product of the environmental setting. In Lake Clifton, seasonal fluctuations in water depth, regional variations in sedimentation rates and the effects of prevailing winds and currents are major controlling factors. By contrast, the internal morphology of the various thrombolites is remarkably similar, composed of a framework of aragonitic mesoclots. The aragonite is precipitated as a consequence of microenvironmental chemical changes induced by the metabolic activity of principally filamentous cyanobacteria. The Lake Clifton thrombolites represent an important modern analogue for understanding fossil thrombolites. The mesoclots appear to be a growth fabric and are not the result of disruption of pre-existing stromatolitic laminae. Moreover, Lake Clifton provides an important example of the coexistence between both stromatolitic and thrombolitic microbialites and an abundant and diverse metazoan fauna as well as demonstrating an apparent inability of this fauna to markedly influence microbialite development. Thus apart from changing chemical conditions, competition for space (and the resultant lack of suitable habitats) was probably a major factor restricting thrombolite distribution throughout the Phanerozoic following the middle Ordovician.

Stevensite in the modern thrombolites of Lake Clifton, Western Australia: A missing link in microbialite mineralization?

Microbialites form the earliest macroscopic evidence of life, and have always been important in particular aquatic ecosystems. They demonstrate the remarkable ability of microorganisms to provide the foundation for structures that can rival coral reefs in size. Microbialites are generally assumed to form by microbial trapping and binding of detrital grains, by carbonate organomineralization of microbial biofi lms, or by inorganic mineralization around microbial templates. Here we present a signifi cant discovery that modern thrombolitic microbialites in Lake Clifton, Western Australia, gain their initial structural rigidity from biofi lm mineralization by the trioctahedral smectite mineral stevensite. This nucleates in and around microbial fi lament walls when biological processes suppress carbon and Ca activities, leaving Mg to bind with silica and form a microporous framework that replaces and infi lls the fi lament web. After microbial materials are entombed, local carbon and Ca activities rise suffi ciently for aragonite microcrystals to grow within the stevensite matrix and perhaps replace it entirely, with eradication of biogenic textural features. This may explain why many ancient microbialite carbonates lack clear evidence for biogenicity. Stevensite may provide the missing link between microbial organomineralization and subsequent abiotic calcifi cation.

Deterministic KPZ model for stromatolite laminae

The deterministic variant of the Kardar–Parisi–Zhang equation for the evolution of a growing interface is used to model patterning produced by successive laminations in certain stromatolites. Algebraic solutions of the model together with numerical simulations are employed to fit model parameters consistent with digital recordings of individual stromatolite laminae. Numerical values for model parameters, related to lateral growth, vertical growth and surface diffusion, provide a set of indices which may prove useful for classifying different stromatolites.

Lithification of Peritidal Carbonates by Continental Brines at Fisherman Bay, South Australia, to Form a Megapolygon/Spelean Limestone Association

ABSTRACT Groundwater springs in the supratidal zone at Fisherman Bay, South Australia, are the sites of aragonite precipitation and cementation of Holocene marine carbonates. Lithification, which commenced less than 3000 years BP and is still active, has formed a cavernous limestone containing megapolygons, tepees, and speleothems including pisoliths, floe aragonite, and aragonitic pool deposits. The emerging waters are from a continental source and have evolved from low alkalinity waters of Pleistocene sand and clay coastal plain aquifers which have passed through an underlying Tertiary marine carbonate aquifer. The resulting waters have high PCO2, total carbonate. Ca, and sulphate concentrations. They are close to saturation with respect to aragonite, and their mMg2+/mCa2+ ratios approach or exceed the critical aragonite precipitation value. Aragonite is precipitated from these waters when they approach the surface and evolve CO2. Features which, taken together. may diagnose ancient examples of this process include: 1) primary aragonitic cements with high mSr2+/mCa2+ values; 2) nonmarine 34S values in associated gypsum; 3) two superimposed networks of surface polygons, one delineated by extensional boundaries, the other by tepees; 4) high-water vadose-zone isopachous grain cements; 5) interconnected, speleothem-lined cavities; and 6) the presence of evaporites only in surface sediments. Possible ancient examples of peritidal carbonate lithification by saline spring waters rising from confined aquifers are recognized in descriptions of carbonates from West Texas, Lombardy, and the Atlas Mountains. The areal extent of each of these deposits suggests that the process may be a geologically important feature, and its products may be diagnostic of semi-arid or arid-zone paralic sedimentation.

Microatoll microbialites of Lake Clifton, Western Australia: Morphological analogues ofCryptozoön proliferum Hall, the first formally-named stromatolite

SummaryThe Linnaean nameCryptozoön proliferumHall was proposed in 1883 for a previously undescribed life-form preserved in spectacular exposures of Cambrian limestones in New York State, USA. It is now recognised that these are exposures of stromatolitic microbialites, laminated organosedimentary structures formed from interaction between a benthic microbial community (BMC) and the environment. Microbialites are neither fossil organisms nor trace fossils. These complex structures are the products of dissipative, self-organising systems involving a BMC, the external environment and the accreting microbialite. Functionally analogous BMCs of different species compositions may build similar structures in similar environments in quite separate periods. The type exposures ofCryptozoön proliferum show objects composed of complex, concentric rings, up to a metre in diameter, that have grown laterally without any restriction other than that provided by neighbouring structures. They are not the relicts of domes truncated by penecontemporaneous erosion or Pleistocene glaciation, but depositional forms in which upward growth was restricted. Analogous modern structures occur on a reef platform along the north east shore of hyposaline Lake Clifton, Western Australia. These are tabular thrombolitic microbialites that vary lakeward across the reef platform from low, compound structures to discrete, concentric structures up to 50 cm high. The Lake Clifton forms are, in turn, morphological analogues of microatolls found on coral reef platforms. Coral microatolls are coral colonies with flat, dead tops and living perimeters in which upward growth is constrained by the sea surface. In shallow water they form circular rims of laterally growing coral around a dead centre. In deeper water they form coral heads that develop flat tops on reaching sea level. It is concluded that both the tabular microbialites of Lake Clifton and the type exposures ofCryptozoön proliferum are analogous to coral microatolls in both form and origin-structures that have been able to grow laterally, but in which upward growth is restricted by subaerial exposure. Similar microatoll microbialites have been described from other modern environments, including Great Salt Lake, Utah, USA and Stocking Island, Exuma Cays, Bahamas. Ancient examples may include some of the “tufa” deposits of the Basal Purbeck Formation in Dorset, UK, as well as the coalesced domal bioherms of the Upper Cambrian Arrinthrunga Formation of the Georgina Basin, Central Australia, and the “washbowl” structures described from the Båtsfjord Formation of the Varanger Peninsula, north Norway. Progress towards a reliable interpretation of ancient microbialites depends on an understanding of modern environments in which analogous structures are forming. This study of microatolls has demonstrated that other sessile life forms may create colonial ecomorphs that, used cautiously, can serve as analogues for understanding the factors controlling the growth and form of microbialites. The surprising lack of pre-Pleistocene examples of microatolls recorded to date has simply been due to their lack of recognition in the geological record. They occur in sequences from the Proterozoic onwards, and provide powerful environmental indicators of ancient reef platforms on which biological growth was adjusted to contemporary sea level.

Iron mineralization of peritidal carbonate sediments by continental groundwaters, fisherman bay, South Australia

Abstract High concentrations of goethite, limonite and iron sulphides and lesser concentrations of hematite and ferromanganese oxides are forming in Holocene carbonate sediments in the peritidal zone of Fisherman Bay, a hypersaline marine embayment on the semi-arid northeast shore of Spencer Gulf, South Australia. The iron and manganese are precipitating from seawards-flowing saline groundwaters as they emerge from red-bed aquifer systems of the coastal plain. Iron concentrations in these groundwaters are as high as 67 ppm and they produce Fe2O3 concentrations of 30 to 70% w/w in a lens-shaped deposit of ferric oxyhydroxides located near the interface of the marine carbonates and underlying red-bed sediments. The Fe in the lens occurs as a limonite and goethite cement binding detrital quartz grains together with a minor proportion of haematitic concretions. The mineralogy and textures of the Fisherman Bay iron lens are similar in some respects to those of the oxide facies of some minette ironstones (Sandy, Clayey, Oolitic, Shallow-Inland-Sea Iron Formations) and it is forming at a rate comparable to those of the more rapidly formed deposits. However, the iron-enriched sediments are of limited extent, geochemically they are more akin to the volcanically associated Lahn-Dill ironstones and, in general, marine-hosted ironstones lack red-bed associations.

Sedimentological and Geobiological Studies of Intertidal Cyanobacterial Mats in North-Eastern Spencer Gulf, South Australia

Stratiform stromatolites, which occur in some modern intertidal and lacustrine environments, are organo-sedimentary structures commonly produced by the sediment-trapping activities of the cyanobacteria (blue-green algae). Stromatolites were prominent in analogous Proterozoic environments and are often associated with sulfide mineral deposits (Mendlsohn, 1976). It has been suggested that this association implies a role for the biological activities of stromatolitic ecosystems in the formation of some base metal sulfide deposits (Renfro, 1974; Trudinger and Mendelsohn, 1976). This suggestion appears feasible since the accumulated organic matter in the stromatolites would have been a potential source of energy for sulfate-reducing bacteria and, furthermore, fractionation during bacterial sulfate reduction could account for particular sulfur isotope distribution patterns in some deposits.