stromatolites near the permian triassic boundary in chongyang hubei province south china a geobiological window into palaeo oceanic fluctuations following the end permian extinction abstract stromatolites just after the end permian extinction are exceptionally well developed in the chongyang area of hubei province south china the stromatolites include endolithic coccoidal microbes and exhibit three growth forms columnar layered pseudo columnar and columnar each located in different stratigraphic horizons the columnar type is the most conspicuous and provides key information on the mode of their construction in this paper we describe the mega to microscopic features of these stromatolites together with interpretations of their construction the columnar type typically comprises alternations of lighter and darker laminae with intercalations of dense micritic laminae the lighter laminae are composed of fine to coarse calcite cement and fine dolomite whereas the darker laminae are composed of micrite and microspar endolithic coccoids are sporadic in these laminae in contrast the dense micritic laminae consist of micrite with abundant endolithic coccoids accentuating the laminae of stromatolites the columnar stromatolites were formed under deteriorating conditions after the end permian mass extinction as follows 1 the alternating development of lighter and darker laminae due to the colonization of different microbial communities in lower energy shallow to deep subtidal settings 2 carbonate precipitation within each microbial community through microbial metabolic activity and degradation of extracellular polymeric substances eps leading to the alternations of lighter and darker laminae 3 episodic interruption of the growth of the lighter and darker laminae by endolithic coccoidal activity and 4 high levels of endolithic activity accompanied by the precipitation of carbonate within boreholes to produce the dense micritic laminae in contrast the columnar layered and pseudo columnar types comprise alternations of lighter and darker laminae composed mainly of fine to coarse calcite cement fine dolomite micrite peloids and microspar with sporadic endolithic coccoids the dense micritic laminae are absent or rare in these stromatolites these types were essentially produced by the repetition of processes 1 and 2 documented in the columnar type the finely alternating laminae of these stromatolites are therefore the product of the repeated initiation and cessation of growth of relevant microbial communities in response to changing environmental conditions the stromatolites were formed immediately after the end permian extinction and persisted intermittently throughout the early triassic the chongyang stromatolites are undoubtedly some of the best developed stromatolites near the permian triassic boundary and retain a high resolution record of microbial successions and palaeo oceanic perturbations immediately following the end permian extinction

trapping and binding a review of the factors controlling the development of fossil agglutinated microbialites and their distribution in space and time trapping and binding of allochthonous grains by benthic microbial communities has been considered a fundamental process of microbialite accretion since its discovery in popular shallow marine modern examples bahamas and shark bay however agglutinated textures are rare in fossil microbialites and thus the role of trapping and binding has been debated in the last four decades recently renewed attention on this subject has produced new findings of fossil agglutinated microbialites those mainly formed by trapping and binding and analogous to modern examples but they are still few and geologically recent mainly post paleozoic when compared to the 3 5 gyr long record of microbialites in order to better understand this discrepancy between modern and fossil examples an extensive literature review is presented here providing the first thorough database of agglutinated microbialites which shows that all of them are formed in shallow marine environments and most under tidal influence in addition a lower cretaceous example is described including very diverse microbialites each of them formed in a particular paleoenvironment some of these microbialites developed in grainy settings but only those formed in marginal marine tide influenced environments accreted mainly by trapping and binding the surrounding grains being analogous of modern agglutinated microbialites and matching the environmental pattern observed in the literature database the combination of the literature review with the case study allows to discuss the factors that control and enhance trapping and binding a occurrence of grains in the microbialite environment b frequent currents that mobilize the grains and supply them onto the microbialite surface c high concentration and diversity of electrolytes in the water to increase the adhesiveness of the extracellular polymeric substances eps of the microbialite surface and d a caco3 saturation state not high enough to promote early and strong carbonate precipitation within eps which would eventually decrease its availability to adhere grains therefore this review shows that the keys to solve the trapping and binding debate may be environmental because the conjuction of these hydrodynamic and hydrochemical parameters is preferentially achieved in shallow marine settings and especially in those influenced by tides at least since mesozoic times this explains the limited environmental and stratigraphic distribution of microbialites mainly formed by trapping and binding and opens new ways to look geologically and microbiologically at this process so often cited and yet so rare

microbial deposits in the aftermath of the end permian mass extinction a diverging case from the mineral mountains utah usa the lower triassic mineral mountains area utah usa preserves diversified smithian and spathian reefs and bioaccumulations that contain fenestral microbialites and various benthic and pelagic organisms ecological and environmental changes during the early triassic are commonly assumed to be associated with numerous perturbations productivity changes acidifica tion redox changes hypercapnia eustatism and temperature changes post dating the permian triassic mass extinction new data acquired in the mineral mountains sediments provide evidence to decipher the relationships between depositional environments and the growth and distribution of microbial structures these data also help to understand better the controlling factors acting upon sedimentation and community turnovers through the smithian early spathian the studied section records a large scale depositional sequence during the dienerian spathian interval during the transgression depositional environments evolved from a coastal bay with continental deposits to intertidal fenestral microbial limestones shallow subtidal marine sponge microbial reefs to deep subtidal mud dominated limestones storm induced deposits microbialite sponge reefs and shallow subtidal deposits indicate the regression three microbialite associations occur in ascending order i a red beds microbialite association deposited in low energy hypersaline supratidal conditions where microbialites consist of microbial mats and poorly preserved microbially induced sedimentary structure ii a smithian microbialite association formed in moderate to high energy tidal conditions where microbialites include stromatolites and associated carbonate grains oncoids ooids and peloids and iii a spathian microbialite association developed in low energy offshore conditions that is preserved as multiple decimetre thick isolated domes and coalescent domes data indicate that the morphologies of the three microbialite associations are controlled primarily by accommodation hydrodynamics bathymetry and grain supply this study suggests that microbial constructions are controlled by changes between trapping and binding versus precipitation processes in variable hydrodynamic conditions due to the presence of numerous metazoans associated with microbialites throughout the smithian increase in accommodation and spathian decrease in accommodation the commonly assumed anachronistic character of the early triassic microbialites and the traditional view of prolonged deleterious conditions during the early triassic time interval is questioned

giant stromatolites of the eocene green river formation colorado usa the eocene green river formation in the sand wash basin colorado united states contains the largest known lacustrine columnar stromatolites spectacular columns as much as 5 5 m high with diameters of 7 m occur over a broad area the stromatolites are composed of laterally continuous centimeter thick layers that can be traced from the base to the top of the column synoptic relief thus these stromatolites stood as much as 5 5 m above the lake floor the layers consist of one to several different kinds of microbialites which makes these large columns even more unusual the giant columns were the result of a combination of factors including the lake transgressing a flooded woodland in situ tree stumps providing elevated substrates above the lake floor an abundant supply of calcium rich spring and surface water that mixed with saline alkaline lake water and the subsidence rate of the nearshore lake environment not exceeding the rate at which the stromatolites grew

giant holocene freshwater microbialites laguna bacalar quintana roo mexico with more than 10 km of total length holocene microbialites in laguna bacalar mexico belong to the largest freshwater microbialite occurrences microbialites include domes ledges and oncolites domal forms can grow to diameters and heights of 3 m microbialites are composed of low magnesium calcite which is to a large extent precipitated due to the metabolic activity of the cyanobacteria homeothrix and leptolyngbya and associated diatoms photosynthesis removes carbon dioxide and triggers carbonate precipitation also an elevated carbonate concentration in lagoon waters derived from dissolution of cenozoic limestone in a karst system supports carbonate precipitation trapping and binding of detrital grains is also observed but is not as common as precipitation bacalar microbialites are largely thrombolitic however stromatolitic sections occur as well the bulk of bacalar microbialites probably formed in the late holocene ca 1 kyr bp until present according to 14c dating microbialites accreted 9 to 8 cal kyr bp however these ages may be too old as a result of a strong hard water effect this effect is seen in 14c ages of living bivalve and gastropod mollusc shells from bacalar lagoon which are 8 to 7 cal kyr bp the modern associated fauna of microbialites is characterized by low diversity and high abundance of the bivalve mollusc dreissena sp and the gastropod pomacea sp the abundant grazing gastropods presumably hamper modern microbialite formation a comparison of bacalar microbialites with other modern microbialite occurrences worldwide shows only a few patterns sizes shapes microbial taxa mineralogy type of accretion and settings including water properties of microbialite occurrences exhibit high variability a trend may be seen in the grazing metazoa which are rare to absent in the marine and brackish examples but apparently present in all the freshwater occurrences of microbialites also freshwater examples are usually characterized by elevated concentrations of carbonate and or calcium ions in the surrounding waters

microbial carbonates the geological record of the calcified bacterial algal mats and biofilms summary deposits produced by microbial growth and metabolism have been important components of carbonate sediments since the archaean geologically best known in seas and lakes microbial carbonates are also important at the present day in fluviatile spring cave and soil environments the principal organisms involved are bacteria particularly cyanobacteria small algae and fungi that participate in the growth of microbial biofilms and mats grain trapping is locally important but the key process is precipitation producing reefal accumulations of calcified microbes and enhancing mat accretion and preservation various metabolic processes such as photosynthetic uptake of co2 and or hco3 by cyanobacteria and ammonification denitrification and sulphate reduction by other bacteria can increase alkalinity and stimulate carbonate precipitation extracellular polymeric substances widely produced by microbes for attachment and protection are important in providing nucleation sites and facilitating sediment trapping microbial carbonate microfabrics are heterogeneous they commonly incorporate trapped particles and in situ algae and invertebrates and crystals form around bacterial cells but the main component is dense clotted or peloidal micrite resulting from calcification of bacterial cells sheaths and biofilm and from phytoplankton stimulated whiting nucleation interpretation of these texturally convergent and often inscrutable fabrics is a challenge conspicuous accumulations are large domes and columns with laminated stromatolite clotted thrombolite and other macrofabrics which may be either agglutinated or mainly composed of calcified or spar encrusted microbes stromatolite lamination appears to be primary but clotted thrombolite fabrics can be primary or secondary microbial precipitation also contributes to hot spring travertine cold spring mound calcrete cave crust and coated grain deposits as well as influencing carbonate cementation recrystallization and replacement microbial carbonate is biologically stimulated but also requires favourable saturation state in ambient water and thus relies uniquely on a combination of biotic and abiotic factors this overriding environmental control is seen at the present day by the localization of microbial carbonates in calcareous streams and springs and in shallow tropical seas and in the past by temporal variation in abundance of marine microbial carbonates patterns of cyanobacterial calcification and microbial dome formation through time appear to reflect fluctuations in seawater chemistry stromatolites appeared at 3450 ma and were generally diverse and abundant from 2800 to 1000 ma inception of a proterozoic decline variously identified at 2000 1000 and 675 ma has been attributed to eukaryote competition and or reduced lithification thrombolites and dendrolites mainly formed by calcified cyanobacteria became important early in the palaeozoic and reappeared in the late devonian microbial carbonates retained importance through much of the mesozoic became scarcer in marine environments in the cenozoic but locally re emerged as large agglutinated domes possibly reflecting increased algal involvement and thick micritic reef crusts in the late neogene famous modern examples at shark bay and lee stocking island are composite coarse agglutinated domes and columns with complex bacterial algal mats occurring in environments that are both stressed and current swept products of mat evolution ecological refugia sites of enhanced early lithification or all three

climatic and tectonic controls on carbonate deposition in syn rift siliciclastic fluvial systems a case of microbialites and associated facies in the late jurassic this work provides new insights to assess the factors controlling carbonate deposition in the siliciclastic fluvial systems of rift basins sedimentological and stable isotope data of microbialites and associated carbonate facies along with regional geological information are shown to reveal the influence of climate and tectonics on the occurrence and attributes of carbonate deposits in these settings the vega formation a 150 m thick lower kimmeridgian siliciclastic fluvial sequence in asturias province northern spain constitutes a candidate for this approach this unit includes varied facies stromatolites rudstones packstones and wackestones containing oncoids intraclasts charophytes and shell bioclasts marlstones and polygenic calcareous conglomerates that formed in a low gradient fluvial lacustrine system consisting of shallow low sinuosity oncoid bearing channels and pools within marshy areas with sporadic coarse alluvial deposition the sedimentological attributes indicate common erosion by channel overflow and rapid lateral changes of subenvironments caused by water discharge variations the carbonate fluvial lacustrine system developed near uplifted marine jurassic rocks the occurrence of the system was conditioned by normal faults active during the deposition of the unit that favoured i springs of hco3 ca rich water from a rhaetian sinemurian carbonate rock aquifer and ii carbonate deposition in areas partially isolated from the adjacent siliciclastic fluvial system the microbialite 13c and 18o values support deposition in a hydrologically open system fed by ambient temperature meteoric water with riparian vegetation three types of lamination in the stromatolites and oncoids reflect distinct morphological types of cyanobacterial communities the textural pattern of lamination parallels 13c and 18o changes suggesting short term cycles of precipitation and temperature a moderately to strongly contrasted seasonal and or pluriannual precipitation regime is inferred from the cyclic 13c pattern of the lamination and from the discontinuous and asymmetrical growth of oncoids thus the isotopic and sedimentological attributes of the carbonate deposits were linked to short term climate changes associated with semi arid conditions consistent with the studied climatic zone

microbe mineral interactions early carbonate precipitation in a hypersaline lake eleuthera island bahamas microbialites benthic microbial carbonate deposits were discovered in a hypersaline alkaline lake on eleuthera island bahamas from the edge towards the centre of the lake four main zones of precipitation could be distinguished 1 millimetre sized clumps of mg calcite on a thin microbial mat 2 thicker and continuous carbonate crusts with columnar morphologies 3 isolated patches of carbonate crust separated by a dark non calcified gelatinous mat and 4 a dark microbial mat without precipitation in thin section the precipitate displayed a micropeloidal structure characterized by micritic micropeloids strong autofluorescence surrounded by microspar and spar cement no fluorescence observations using scanning electron microscopy sem equipped with a cryotransfer system indicate that micrite nucleation is initiated within a polymer biofilm that embeds microbial communities these extracellular polymeric substances eps are progressively replaced with high mg calcite discontinuous eps calcification generates a micropeloidal structure of the micrite possibly resulting from the presence of clusters of coccoid or remnants of filamentous bacteria at high magnification the microstructure of the initial precipitate consists of 200 500 nm spheres no precipitation is observed in or on the sheaths of cyanobacteria and only a negligible amount of precipitation is directly associated with the well organized and active filamentous cyanobacteria in deeper layers of the mat indicating that carbonate precipitation is not associated with co2 uptake during photosynthesis instead the precipitation occurs at the uppermost layer of the mat which is composed of eps empty filamentous bacteria and coccoids gloeocapsa spp two dimensional mapping of sulphate reduction shows high activity in close association with the carbonate precipitate at the top of the microbial mat in combination these findings suggest that net precipitation of calcium carbonate results from a temporal and spatial decoupling of the various microbial metabolic processes responsible for caco3 precipitation and dissolution theoretically partial degradation of eps by aerobic heterotrophs or uv fuels sulphate reducing activity which increases alkalinity in microdomains inducing caco3 precipitation this degradation could also be responsible for eps decarboxylation which eliminates ca 2 binding capacity of the eps and releases ca 2 ions that were originally bound by carboxyl groups at the end of these processes the eps biofilm is calcified and exhibits a micritic micropeloidal structure the eps free precipitate subsequently serves as a substrate for physico chemical precipitation of spar cement from the alkaline water of the lake the micropeloidal structure has an intimate mixture of

interplay between biotic and environmental conditions in pre salt messinian microbialites of the western mediterranean upper miocene mallorca spain microbial buildups that predate the messinian salinity crisis including one of the few phanerozoic examples of giant microbialites crop out upon the island of mallorca w mediterranean sedimentological and geochemical data from microbialites and associated deposits both essentially dolomitic indicate that they grew in shallow marine conditions but relatively restricted from the open marine realm two microbialite bearing sequences occur both consisting of subtidal to supratidal deposits periods of restriction led to evaporative and hypersaline conditions that favored the development of microbialites and local precipitation of sulfates by contrast ephemeral periods marked by more open conditions were devoid of microbialites and allowed production of bioclastic deposits microbialites range from decameter to decimeter scale reflecting differences in accommodation space despite contrasting sizes all microbialite bodies record similar mesostructure evolution through time from thrombolites to stromatolites with a sharp transition between these endmembers the change from subtidal to shallower more restricted and saline intertidal environments triggered biotic substitution of thrombolite generating microbial communities to stromatolite generating ones furthermore a wide variety of microstructures from agglutinated to micritic with fossilized microbes indicates that two main accretion processes occurred microbially influenced primary dolomite precipitation and grain trapping and binding which were controlled by the interaction between microbes and changes in environmental conditions e g grain supply hydrodynamics and hydrochemistry therefore the diversity of macro meso and microstructures of these microbialites was caused by a complex interplay between depositional biotic and hydrological parameters which offers useful insights for the palaeoecological interpretation of other examples at any scale and throughout geological time

response of a late miocene mediterranean reef platform to high frequency eustasy the upper miocene reef complex of mallorca a third order depositional sequence shows how carbonate deposition responds to high frequency sea level cycles this reef complex displays a fractal architecture of hierarchically stacked high frequency units from fourth to seventh order the building blocks are sigmoidal units bounded by unconformities not parasequences these sigmodds are stacked in sets of sigmoids the sets are stacked in cosets and these in turn are stacked in sets of cosets all these accretionary units have characteristics of depositional sequences but differ from the model developed by exxon and from other carbonate sequence stratigraphy models these departures result from changes in carbonate production in relation to sea level fluctuations which determined the changes in accommodation in the absence of significant subsidence all these high frequency sequences are composed of four systems tracts 1 a lowstand systems tract formed during the early rise of a sea level cycle and composed of progradational reef to open shelf facies thinning toward the basin 2 an aggrading systems tract formed during sea level rise and composed of thick lagoon to open shelf deposits without landward shift of facies 3 a highstand systems tract formed during the high part of a sea level cycle and composed of progradational reef to open shelf facies thinning toward the basin with lagoon beds thin or absent and 4 an offlapping systems tract formed during sea level fall and composed of thin downlapping reef facies passing into a condensed open shelf section which correlates with an erosion surface on the platform top two bounding surfaces can be recognized by lithofacies changes between these systems tracts the erosion surface on top of the offlapping systems tract and the downlap surface at its base both surfaces merge basinward into a condensed section the sequence boundary which corresponds to the lowest point of the sea level cycle is the erosion surface landward and within the correlative condensed section basinward

fluorescent bands in massive corals result from terrestrial fulvic acid inputs to nearshore zone the discovery of yellow green fluorescent bands in the skeletons of massive corals from inshore waters of the great barrier reef and the temporal and spatial correlations of these bands with adjacent river flow1 prompted an investigation into the substances responsible for the phenomenon we have determined that all regions of the coral skeleton contain fulvic acids2 3 which impart a blue background fluorescence the yellow green fluorescent bands found only in inshore coral appear to result from enhanced concentrations of low relative molecular mass 10 000 fulvic acids from adjacent river input these compounds are ubiquitous and the yellow green banding phenomenon can be expected to occur in massive corals located near to significant river systems in the widely distributed genus porites4 the great age of the largest colonies1 means that extremely long highly resolvable records of coastal runoff exist throughout tropical nearshore seas

activators of photoluminescence in calcite evidence from high resolution laser excited luminescence spectroscopy abstract laser excited luminescence spectroscopy of a red algal biogenic calcite and a synthetic mn calcite can make the distinction between organic and trace element activators of photoluminescence organic activated photoluminescence in biogenic calcite is characterized by significant peak shifts and increasing intensity with shorter wavelength excitation and by significant decreases in intensity after heating to 400 c in contrast mn activated photoluminescence shows no peak shift greatest intensity under green excitation and limited changes after heating examination of samples with a high sensitivity spectrometer using several wavelengths of exciting light is necessary for identification of photoluminescence activators

reconstructing sea level change from the internal architecture of stromatolite reefs an example from the mesoproterozoic sulky formation dismal lakes group arctic canada the mesoproterozoic dismal lakes group arctic canada contains a relatively thin yet regionally extensive stromatolitic reef complex that developed subtidally during a major transgression shoaled to sea level and was overlain by intertidal to supratidal carbonate and evaporite strata the september lake reef complex exhibits a complex internal architecture that records the interaction between stromatolite growth and changes in accommodation space derived from both higher order 4th or 5th order parasequence scale changes in sea level and the variable bathymetry of the sea floor reef growth which was initiated during three sea level cycles records progressive marine transgression over depositional lows that were formed during pre reef subaerial exposure and erosion of the underlying strata a fourth sea level cycle represented by spectacular coniform stromatolites with 10 m of synoptic relief marks a more dramatic rise in sea level and establishment of the main reef complex aggradation and ev

evaporitic subtidal stromatolites produced by in situ precipitation textures facies associations and temporal significance abstract the transition between carbonate platforms or isolated carbonate buildups and overlying evaporites commonly is marked by assemblages of stromatolites and interlaminated carbonates and evaporites stromatolites display lamination textures that vary from peloidal and discontinuous on a scale of a millimeter to a few centimeters to isopachous and continuously laminated on a scale of a centimeter to a few meters the isopachous lamination texture may be composed of either 1 micritic or radial fibrous calcite or 2 dolomite isopachous stromatolitic laminae are remarkably uniform varying little in thickness over a given lateral distance in contrast to stromatolites formed of peloidal laminae which show marked variation in thickness over an equivalent lateral distance these isopachous textures are uncommon on most open marine carbonate platforms and apparently developed in transitional carbonate to evaporite settings because of increasing temperature salinity and anoxia related to water stratification which would have created ecologic restriction and an opportunity for stromatolite growth stromatolites with isopachous lamination are here interpreted to have formed as a result of in situ precipitation of sea floor encrusting calcite and possibly dolomite whereas the stromatolites composed of peloidal discontinuous lamination are inferred to have formed by trapping and binding of loose carbonate sediment in microbial mats while the presence of microbes in almost all near surface environments nullifies use of the term abiotic to describe most precipitated minerals we interpret growth of the isopachous 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 sedimentation in the presence of microbial mats these transitional stromatolite facies are best developed atop proterozoic and paleozoic carbonate platforms that underlie major evaporite successions however inspection of jurassic and younger evaporite basins such as the messinian of the mediterranean region shows that stromatolites with thin isopachous lamination and radial fibrous textures though present are rare instead these facies may have been replaced by stromatolites with peloidal clastic textures and by low diversity diatomaceous and coccolith mudstones accumulation of the mudstones would have imposed two important effects 1 production of coccoliths would have helped extract calcium carbonate from seawater thus lowering the growth potential for precipitation of sea floor encrusting stromatolites 2 settling of both coccoliths and diatoms would have created a sediment flux to the sea floor which would have served to impede growth of precipitated stromatolites because of smothering of growing crystals

characteristics distribution and morphogenesis of subtidal microbial systems in shark bay australia abstract the distribution nature and extent of microbial deposits in hamelin pool shark bay have been investigated and mapped with emphasis on the occurrence external morphologies internal fabrics constructional mechanisms microbial communities growth rates and sediment associations in the intertidal and previously little researched subtidal zone detailed georeferenced substrate mapping revealed extensive subtidal microbial deposits occupying approximately 300 km 2 of the total holocene 1400 km 2 area of hamelin pool the microbial pavement covers 227 km 2 of the subtidal substrate that together with columnar structures reveals a subtidal microbial habitat which occupies an area 10 times larger than the area of the intertidal deposits microbial carbonate is composed of aragonite 80 98 that reveals high positive values of 13 c 4 46 to 5 88 and 18 o 3 06 to 3 88 as a characteristic of the highly evaporative environment with extensive microbial activity oldest dated heads are 1915 and 1680 14 c years bp and the overall system was deposited in two stages the first between 2000 and 1200 and the last from 900 years bp to the present slow growth rates vary from less than 0 1 mm year to 0 5 mm year different internal fabrics were constructed according to their position in relation to the littoral zone by distinct microbial communities and lateral fabric relations have been established evidence of shallowing upward fabric sequences of microbial origin reflects relative falling sea levels during the late holocene and is likely useful in ancient environmental interpretation a sequence of events and mechanisms are described emphasizing differences between the stromatolitic thrombolitic and cryptomicrobial deposits in shark bay the new substrate map and depositional history for this distinctive and peculiar microbial habitat establish the significance of subtidal structures and emphasize the geoscientific importance of hamelin pool especially with respect to early life studies and ancient analogues for understanding microbial activity deposit characteristics fenestral fabrics and distribution

stromatolite decline a reconsideration the prevailing concept of phanerozoic stromatolite decline is considered too simplistic changes in stromatolite character and proportion to other lithologies after the precambrian can be explained as responses to substrate competition and changing sedimentological conditions caused by the evolution of skeletal metazoans grazing pressure was likely insignificant as a limiting factor

evolution of an early proterozoic foreland basin carbonate platform lower pethei group great slave lake north west canada the taltheilei utsingi mclean and blanchet formations form a 175 390 m thick carbonate platform to basin succession in the lower part of the paleoproterozoic pethei group preserved in the eastern arm of great slave lake carbonates accumulated along the south east margin of the slave craton within a foredeep formed during the collision of the slave and churchill cratons the rocks include eight predominantly microbial carbonate facies that comprise five facies associations representing 1 shallow water rimmed shelf 2 shallow water open shelf 3 shallow water ramp 4 upper slope and deep ramp and 5 lower slope and basin plain environments microbialite facies grew by organically mediated precipitation of spar and micritic cement and trapping and binding of lime mud these wholly subtidal facies typically reflect progressive shallowing and changing geometry of the lower pethei sea floor from ramp to open shelf to shallow rimmed shelf with associated slope and basin plain deposition repeated relative sea level changes influenced platform growth this resulted in five shallowing upward packages each separated by an incipient drowning event of varying magnitude antecedent topography and the size of the preceding drowning event strongly influenced the initial growth of each interval this repeated pattern is attributed to interaction between a the inherent tendency of microbial carbonates to aggrade vertically b changing sedimentation rates and c readjustments of relative base level the lower pethei succession is one of few paleoproterozoic examples of carbonate platform growth within a foreland basin it has 1 a low gradient profile 2 extensive slope and basin plain carbonate production and sedimentation 3 no ooids 4 minor terrigenous clastic sediments and 4 a mobile submergent shelf rim lacking substantial carbonate sand shoals

topographic and sea level controls on oolite microbialite coralgal reef sequences the terminal carbonate complex of southeast spain the terminal carbonate complex of southeast spain is a miocene messinian unit of oolite microbialite and coralgal reefs deposited in association with glacioeustasy and evaporitic drawdown the relationship between paleotopography and sea level history is useful for prediction of microbialite and oolite reservoir facies in the subsurface four sequences record sea level change with minimum amplitudes of 32 77 m 105 253 ft sequences commonly have local basal stromatolites overlain by local thrombolites ooid grainstone volcaniclastic rich planar bedded ooid grainstone and fenestral ooid grainstone at low substrate positions thrombolite boundstones are thicker and laterally more continuous than at higher positions at intermediate substrate positions relative to sea level history sequences have a build and fill architecture characterized by a relief building phase and a relief filling phase with thin sequences draping paleotopography microbialites dominate during the relative sea level rises and build topographic relief oolites dominate during the relative sea level falls and fill topographic relief at higher substrate positions close to highstand sequences thicken and yield stratigraphic character that is inconsistent with a build and fill model apparently the build and fill model requires an intermediate elevation substrate position and nonoptimal carbonate productivity during rapid sea level change sequences progressively show increasing diversity and more normal marine organisms possibly caused by decreasing aridity lithofacies of the la molata area show evidence of more restricted conditions compared to the la rellana ricardillo area lithofacies likely because la molata was in an embayment these results show that distribution of oolite microbialite and reef facies are predictable given known interaction among sea level paleotopography of the depositional surface substrate and coastline configuration

giant subtidal stromatolites forming in normal salinity waters we report here the discovery of giant lithified subtidal columnar stromatolites 2 m high growing in 7 8 m of clear oceanic water in current swept channels between the exuma islands on the eastern bahama bank they grow by trapping ooid and pelletal carbonate sand and synsedimentary precipitation of carbonate cement within a field of giant megaripples the discovery is important to geologists and biologists because similar organo sedimentary structures built by a combination of cementation and the trapping of sediment by microbes were the dominant fossil types during the precambrian stromatolites are thought to have been responsible for the production of free oxygen and thus the evolution of animal life1 2 until the discovery of small lithified subtidal columnar stromatolites in the bahamas3 the only subtidal marine examples known to be living while undergoing lithification were in the hypersaline waters of hamelin pool at shark bay western australia4 7 shark bay stromatolites range from intertidal to the shallow subtidal with the larger columns reaching 1 m in height the shark bay stromatolites have strongly influenced geological interpretation by analogy many ancient stromatolites have been considered to have grown in intertidal and or hypersaline conditions8 although hypersalinity was not a necessity for growth during the precambrian because grazing metazoan life had not then evolved

sedimentology of tufa facies and continental microbialites from the palaeogene of mallorca island spain abstract the middle eocene and oligocene basins in northwestern half of mallorca are filled by detrital rocks conglomerates calcarenites and lutites and carbonates deposited in alluvial fluvial palustrine and lacustrine settings the sediment supplied to these continental basins was transported by alkaline bicarbonate rich waters as a consequence these non marine carbonate deposits display extensive developments of diverse kinds of organosedimentary facies the sedimentary record from three localities peguera alaro and sineu enabled one to characterize well preserved eocene tufa deposits as well as eocene and oligocene stromatolites and oncolites a total of ten carbonate facies are described and interpreted the facies related to tufa deposits are diverse types of boundstones rudstones and packstones of bryophytes in situ stems s l phytoclasts etc and associated oncolite rudstones and bioclastic mudstones to packstones these facies deposited in littoral lacustrine and fluvio lacustrine environments oncolites and stromatolites present a great variety of shapes and sizes oncolites from centimetres to several meters in diameter that somehow were conditioned by environmental conditions within shallow lacustrine and fluvial settings three types of lamination apparently dependent on the type of microorganisms responsible for calcite precipitation are distinguished fan shaped and bush shaped grouped filaments and isolated to loosely grouped filaments the 13c and 18o composition of these deposits agrees with carbonate precipitation in freshwater hydrologically open systems with similar carbon sources throughout the eocene and oligocene however eocene and oligocene samples are set apart by the 18o oligocene facies present lower 18o values which might indicate deposition from isotopically less evolved waters perhaps under slightly greater precipitation evaporation conditions tufa facies have the highest 13c composition and show some positive covariation of 13c and 18o primarily reflecting the effects of physical and biological co2 outgassing and maybe also minor evaporation

origins of microbial microstructures in the neoproterozoic beck spring dolomite variations in microbial community and timing of lithification abstract microbialite microstructures form at the scale of microbial processes and can preserve evidence of growth degradation and lithification in microbial communities understanding how these processes are preserved in microstructures will provide insights into microbial ecosystems and how microbial communities have evolved through time microbialites in the neoproterozoic beck spring dolomite southern california u s a contain microstructures that record variations in the morphology of microbial communities as well as the timing of degradation versus lithification in the communities these processes are represented by five end member microbial microstructures 1 distinct laminated microstructure 2 diffuse laminated microstructure 3 distinct clotted microstructure 4 diffuse and regular clotted microstructure and 5 diffuse and irregular clotted microstructure these microstructures are distinguished by fabrics defined by organic inclusions in distinct microstructures organic inclusions sharply define microbial features such as laminae that are laterally continuous for millimeters or 50 200 micron diameter microdots the contacts between distinct microstructures are always sharp in diffuse microstructures zones of organic inclusions lack distinct boundaries neighboring laminae and clots grade into each other within tens of microns and boundaries between microbial structures and encasing cements are indistinct diffuse microstructures grade into each other and into distinct microstructures in a systematic way that suggests that differences are due to variations in the timing of mineralization relative to growth and degradation of microbial communities sharply defined textures in distinct microstructures suggest that lithification started during growth of the microbial community and the close spatial association but lack of intergrading between distinct laminated and distinct clotted microstructures suggests that they formed from two distinct microbial communities that lived in the same environment by contrast diffuse microstructures formed where lithification occurred after the onset of decay and degradation of the primary growth structure the continuum from distinct microbial textures through more diffuse and irregular textures documents variations in the timing of lithification relative to growth and decay of the microbial community thus where petrographic preservation is extremely good such as in the beck spring dolomite sub millimeter scale microbial microstructures record a gradation between growth and degraded microbial structures depending on the relative timing of lithification and heterotrophic decay

sedimentology of ancient coastal wetlands insights from a cretaceous multifaceted depositional system coastal wetlands are prominent modern environments widely studied in geomorphology and ecology but the term coastal wetland is still barely used for the sedimentological classification of ancient deposits the depositional system studied here leza formation cameros basin early cretaceous n spain includes diverse carbonate and clastic facies deposited at the sea land transition and is an illustrative example of the wide array of sedimentary environments that may occur in coastal wetlands systems the studied system was composed mainly of carbonate water bodies whose salinity ranged from fresh to brackish and near marine and which had variable input of clastic material due to their lateral connection with alluvial fans in addition the system also included carbonate water bodies with stronger marine influence tide influenced oolitic areas and relatively restricted evaporative settings the deposits of all these environments occur alternating with each other throughout a unit 30 280 m thick and they range from continental to marine conditions in a relatively small area approximately 10 km 3 30 km thus this sedimentological study of the leza fm provides an ideal opportunity to investigate challenging ancient deposits with both continental and marine features comparison with other modern and ancient coastal settings allows the conclusion that coastal wetland may be the most accurate sedimentological classification for the leza fm since it was not part of a major coastal system e g delta or estuary a series of general sedimentological characteristics of coastal wetland deposits are gathered from the cretaceous case study and from the modern and ancient examples examined these characteristics include predominance of shallow water facies common subaerial exposure and edaphic features great variety of interrelated continental transitional and marine environments with contrasting hydrodynamic and hydrochemical conditions and low diversity biotic communities including both continental and marine fossils as well as fossils of ambiguous affinities

oligo miocene lacustrine microbial and metazoan buildups from the limagne basin french massif central the limagne basin french massif central is an extensive continental lacustrine system accommodating microbial and metazoan buildups from chattian to aquitanian age a description of these buildups and their associated biotic components in grand gandaillat and crechy quarries provides insights into their spatio temporal distribution patterns flats cauliflowers domes cones and coalescent columnar morphologies have been identified with a main laminated mesofabric and laminated columnar filamentous and caddisfly coated microfabrics two low gradient margin models emerged based on the changes in the distribution morphology and size of the microbial and metazoan rich deposits through time the first model applies to the chattian deposits in the grand gandaillat quarry and the second to the aquitanian deposits in the crechy quarry this study emphasizes the heterogeneity of marginal lacustrine carbonate siliciclastic cycles the lacustrine palustrine cycles of the limagne basin record i lacustrine deposits composed of microbial and metazoan buildups and organic matter rich marls indicating periods of high accommodation and ii palustrine deposits composed of mudstone and clayey paleosoils indicative of periods of low accommodation the cycles differ with thin and symmetrical deposits that dominate the chattian cycles whereas thicker and asymmetrical deposits mark the aquitanian cycles in addition the chattian buildups are exclusively microbial and only a few centimeters thick whereas the aquitanian ones are multiple meters in height and are composed of both microbes and metazoans climatic tectonism volcanic and local parameters are involved in the deposition of buildups and their organization in each different cycle climate may control the long term arid humid sedimentary succession tectonic may explain the difference in the deposit thickness between two cycles of different ages and localizations as well as the specific distribution of the buildups throughout the basin volcanism may control the chemical lacustrine conditions and local physiography may impact the soil removal ability of the lake margin

stromatolite reef crusts early cretaceous spain bacterial origin of in situ precipitated peloid microspar peloidal crusts are significant components of early cretaceous aptian reef carbonates in eastern spain the crusts form steep sided laminated deposits on coral and other skeletal surfaces their microfabric consists almost entirely of silt sized peloids in fenestral microspar matrix this microfabric contrasts with more poorly sorted and generally finer grained detrital wackestone packstone fabrics of the adjacent reef matrix scarcity of incorporated grains indicates that the crusts did not trap many particles it is proposed that the crusts are stromatolites and that peloids and inter peloid space were created concurrently by bacterial degradation of organic matter as they developed inter peloid voids were protected from infiltration of extraneous sediment by the organic rich exterior surface of the stromatolite even spacing of the peloids within microspar may reflect self organization of bacterial colonies in the decaying organic matrix compressed and partly amalgamated peloids marginal to burrows in the stromatolites suggest that the peloid fabrics were initially only partially lithified the grainstone like peloid fabric is therefore interpreted as having formed in situ by very early diagenetic processes driven by heterotrophic bacteria

late mississippian thrombolite bioherms from the pitkin formation of northern arkansas two types of bioherms occur in the pitkin formation chesterian of northern arkansas they are composed of a complex series of ovoid thrombolite heads intergrown with various other algal and bryozoan biolithites and are distinguished from each other on the basis of geometry and faunal and floral content type i mounds have sharp lateral margins and interfinger with horizontally bedded contemporaneous flanking strata owing to having expanded and contracted in size during upward accretion they are interpreted as having had synoptic relief of 3 m or less during growth type ii mounds occur slightly higher stratigraphically to the east of type i mounds and formed in deeper water farther out on the ozark shelf they are interpreted as having had synoptic relief of 6 m or more during growth their margins are similar to those in type i mounds but owing to higher synoptic relief they interfinger with sediments derived from erosion higher on the mound

microbialites of the neoproterozoic beck spring dolomite southern california ancient microbialites reflect interactions between microbial communities and environmental conditions however evaluating the relative roles of microbial community processes and environmental influences on microbialite morphology and internal fabric in the rock record can be challenging the neoproterozoic beck spring dolomite preserves diverse microbialites and thus provides an opportunity to explore the factors that influenced microbialite development locally stromatolitic thrombolitic and composite microbialites are abundant in subtidal to upper intertidal carbonates in the beck spring dolomite thrombolitic and composite microbialites have not been recognized previously in this unit but compose much of the newly defined thrombolitic member stratigraphic relationships demonstrate that these three types of microbialites formed in close spatial and temporal association in subtidal to intertidal environments the relative proportions and distributions of stromatolitic and thrombolitic microbialites vary with depositional environment stromatolitic microbialites dominate in deeper intertidal to subtidal facies whereas thrombolitic textures are more abundant in upper intertidal facies composite microbialites composed of intermingled clotted and laminated textures formed in all environments but are most abundant in intertidal facies the broad environmental distribution of stromatolitic thrombolitic and composite microbialites and the intermingling of textures suggest that laminated and clotted textures reflect diverse microbial community morphologies rather than environmental variations furthermore the ca 750 ma age of thrombolitic microbialites in the beck spring dolomite requires that they formed without the influence of calcimicrobes or metazoans colonizing and grazing the microbial mat surface thus these thrombolites provide further evidence that the biostratigraphic distribution of thrombolites cannot be uniquely attributed to evolution of calcifying and grazing organisms in the earliest cambrian and that older microbial communities were capable of producing clotted textures

biosedimentological features of major microbe metazoan transitions mmts from precambrian to cenozoic abstract biotic activities are involved in almost all sedimentation processes throughout the evolutionary history of life on our planet however deep time organism induced sedimentation and biosedimentary records remain unclear in terms of lithologic types strata stacking patterns and possible controlling factors we document biosedimentary features of major transitions from microbe dominated switching to metazoan dominated biosedimentary systems based on the global distributions of both microbial and metazoan carbonates through precambrian to phanerozoic times with emphasis on sedimentary records from china the compilation of 150 and 180 well documented metazoan and microbial reefs respectively from china reveals that metazoan reefs proliferated during the middle ordovician middle devonian and middle permian whereas microbial reefs were well developed during the cambrian late devonian and early middle triassic plus a moderate development during the early silurian these stratigraphic abundances of metazoan and microbial carbonates of china generally match the global patterns the updated variation trends of microbial and metazoan carbonates throughout the late precambrian and phanerozoic reveal that there were five major microbe metazoan transitions mmts the late ediacaran the cambrian and the aftermaths of the mass extinctions of the end ordovician late devonian and end permian the late ediacaran mmt began with microbe dominated oceans with occasional occurrences of metazoans the presence of cloudina dominated reefs in the latest ediacaran marks the completion of the switching of this microbe dominated depositional system into a metazoan dominated system the cambrian saw the expansion of skeletal microbes i e epiphyton renalcis in the oceans and the stratigraphic successions yield the most diverse biosedimentary deposits and or structures of the entire phanerozoic the cambrian mmt was the longest microbial metazoan alternation period and is marked by two metazoan occurrence peaks marked by dominance of abundant archaeocyath buildups during its epoch 2 and by maceriate and lithistid sponge reefs during the late furongian epoch the early silurian in china saw the deposition of a thick suite of organic rich black shales followed by alternations of microbe rich sediments oil shales and metazoan bearing deposits which are replaced by microbial and metazoan reefs during the late early silurian the late devonian mmt started during the late frasnian and persisted into the early mississippian and thus extended slightly longer than the aftermath of the frasnian famennian extinction interval alternating occurrences of microbial and metazoan reefs characterize this late devonian mmt almost all microbe mediated sediments structures observed in the cambrian mmt reoccurred in the aftermath of the end permian mass extinction during the early middle triassic mmt suggesting high similarities between those two mmts cambrian and early middle triassic mmts also share comparable carbon and sulfur isotopic perturbations warming regimes and generally oxygen deficient seawaters some of these environmental and climatic extremes may also occur during other mmts but they usually did not occur synchronously most mmts seem to have undergone four developmental stages they initiated as microbe dominated successions stage a and then were characterized by alternations of microbe dominated and of metazoan bearing or bioturbated successions stage b both microbial and metazoan reefs co occurred during stage c and a dominance of metazoan reefs marks the development of stage d ediacaran and cambrian mmts seem to have undergone the first three development stages whereas the three post extinction mmts experienced the full set of stages a d corresponding to metazoan survival initial recovery and full recovery the majority of volatile rich large igneous provinces lips coupled with intensive acidification events anoxia and global warming regimes took place during the mesozoic cenozoic however microbe dominated sediments were only widely deposited during the early triassic and greatly declined after that time therefore it seems that microbial abundance in mmts may not be directly related to these extreme lip events this is probably because a primary source of food for the metazoans might have shifted to phytoplankton e g coccoliths dinoflagellates and radiolarians in the marine waters since the triassic certainly the pre mesozoic oceans were not dominated by phytoplankton perturbations in the carbon isotope record characterize all mmts and thus may be reliable proxies indicating mmt biosedimentary systems

steep rock lake sedimentology and geochemistry of an archean carbonate platform abstract the 2800 ma mosher carbonate formation at steep rock lake in south central canada is one of earth s oldest limestone deposits it is both thick up to 500 m and relatively well preserved and was one of the first precambrian successions to be noted for evidence of early life it continues to provide information on the antiquity of such processes as photosynthesis the carbonate contains a variety of fabrics and facies from apparently fine grained bedded limestones with domal and columnar stromatolites atikokania radial fans and sheet cracks to closely packed metric scale hybrid domes of originally aragonite seafloor crust interlayered with fenestral fabric here we review previous work provide new observations concerning these sediments and their geochemistry and discuss the inception overall development and ultimate demise of the steep rock platform we divide the mosher carbonate into the lower hogarth member and upper elbow point member these units constitute a shallow water carbonate platform succession the hogarth member overlies eroded 3000 ma tonalite and is at least 120 m thick thin horizons of domal pseudocolumnar columnar and fenestral stratiform stromatolites and occasional layers of atikokania radial crystal fans occur within relatively fine grained thin bedded limestone with iron carbonate horizons in the upper part meter scale domes occur locally with sheet cracks and stromatactis the irregular laminae that build the small domal and columnar stromatolites are interpreted to be lithified mats and biofilms presumably cyanobacteria dominated fine grained well bedded facies of the lower hogarth member and its varied association of small stromatolites is interpreted to have accumulated in a relatively protected inner platform environment numerous very thin horizons of iron carbonate in the lower and upper parts of the hogarth member suggest temporally limited influence of iron rich offshore waters vertical trends in fe mn ba sr ce and 13 c probably reflect progressive restriction of circulation with at least one abrupt but short lived change back to more open seawater conditions the upper hogarth member with large banded domes and fenestral crusts and stromatolites is interpreted as back margin facies the overlying elbow point member is at least 70 m thick and appears to be entirely composed of innumerable juxtaposed elongate smooth and thickly layered domes lacking intervening sediment each dome is typically several meters across and composed of centimetric alternations of cuspate fenestral fabric possibly microbial and crystal fan fabric that is probably abiotic seafloor aragonite precipitate adjacent cuspate fenestral and crystal fan fabric show significant differences in concentration of 13 c sr ba fe and mn this giant dome facies is interpreted as a platform margin deposit in which alternating calcite aragonite mineralogies within each dome reflect a laterally fluctuating offshore redox boundary negative ce anomalies positive gd anomalies and reduced positive eu anomalies relative to laterally correlative deep water iron formation occur in the majority of crystal fan samples and in some cuspate fenestral fabric samples ca carbonate precipitation at the margin of an anoxic iron rich sea would have been promoted by oxygenic removal of iron from seawater otherwise fe carbonate would have been favored over ca carbonate precipitation the mosher carbonate formation is interpreted as an early marine oxygen oasis a shallow platform that favored cyanobacterial productivity was sufficiently isolated from open marine circulation for the seawater to become relatively oxygenated and where carbonate sediment aggradation was a positive feedback that sustained these shallow water conditions limestone accumulation terminated as transgressive suboxic and anoxic seawater deposited first manganese oxide rich and then manganese oxide poor iron formation sediments

origin and significance of lamination in lower cretaceous stromatolites and proposal for a quantitative approach stromatolite lamination is typically defined as alternation of dark and light laminae study of lower cretaceous stromatolites fromthe leza fm n spain supports this statement but recognises additional complexities in lamination that have implications for interpreting accretion processes these stromatolites are partial analogues of present day coarse grained carbonate stromatolites in the bahamas and shark bay australia that mainly form by trapping and binding carbonate sand the leza examples contain both grain rich and micrite rich laminae with scarce particles suggesting that they accreted both by trapping and not trapping grains lamination in modern and ancient coarse grained stromatolites is commonly defined by thinmicritic crusts that formed during interruptions in accretion and separate contiguous grainy laminae repetitive lamination leza examples also contain these thin hiatal crusts and locally showrepetitive lamination but their conspicuous macroscopic lamination is defined by thicker alternating grain rich and micrite rich laminae alternating lamination this indicates that although hiatuses in accretion occurred change in accretion process was the main cause of macroscopic lamination these differences in accretion processes and lamination styles between leza examples and modern coarse grained stromatolites may reflect differences in their environmental settings modern examples occur in shallow marine tidal environments whereas leza fm coarse grained stromatolites developed in ideinfluencedwater bodies in coastal wetlands that experienced fluctuations inwater salinity and hydrochemistry analysis of lamina thickness in these cretaceous stromatolites and similar published examples provides a quantitative approach to the processes that underlie stromatolite lamination

sedimentology and temporal distribution of microbial mats from brejo do espinho rio de janeiro brazil abstract this study covered an annual cycle of the growth of microbial mats in brejo do espinho which is a small lagoon in the state of rio de janeiro located near lagoa vermelha two developmental stages of the microbial mats were observed throughout the study period these stages presented laminations with colours that varied between yellow green and reddish brown hues their microfabric comprised peloids and micritic lumps with a predominance of magnesian calcite and evidence of dolomite the mats also contained other minerals such as gypsum and halite energy dispersive x ray spectrometer analysis also indicated the presence of a silicate associated with the organic matter and sometimes with carbonates the first stage was characterised by a gelatinous texture low mineral content and sparse populations of cyanobacteria that were located only in permanently flooded areas of the brejo do espinho this structure occurred during the drought period and the beginning of the flood period the second stage exhibited peloids and larger micritic lumps that sometimes formed thin carbonate layers and occurred in the infra and mesocoastal regions of the lagoon these structures were observed only during the lagoon drought period quantitatively most represented cyanobacteria were aphanothece nageli 1849 and chroococccus nageli 1849 in both stages the microbial mats of brejo do espinho developed only during the dry season until the start of the lagoon flooding period which is characterised by shallow water low turbidity high salinity stable substrate and virtually no terrigenous input conditions in the flooding season microbial mats were replaced by an intense expansion of charophyceae algae thus there is a seasonal development of microbial mats linked to extreme hydrologic conditions that limit the growth of other algae during certain periods at brejo do espinho

dolomite occurrence evolution and economically important associations abstract dolomite is not a simple mineral it can form as a primary precipitate a diagenetic replacement or as a hydrothermal metamorphic phase all that it requires is permeability a mechanism that facilitates fluid flow and a sufficient supply of magnesium dolomite can form in lakes on or beneath the shallow seafloor in zones of brine reflux and in early to late burial settings it may form from seawater from continental waters from the mixing of basinal brines the mixing of hypersaline brine with seawater or the mixing of seawater with meteoric water or via the cooling of basinal brines bacterial metabolism may aid the process of precipitation in settings where sulfate reducing species flourish and microbial action may control primary precipitation in some hypersaline anoxic lake settings dolomite is a metastable mineral early formed crystals can be replaced by later more stable phases with such replacements repeated a number of times during burial and metamorphism each new phase is formed by the partial or complete dissolution of an earlier dolomite this continual re equilibration during burial detracts from the ability of trace elements to indicate depositional conditions and resets the oxygen isotope signature of the dolomite at progressively higher temperatures because subsurface dolomite evolves via dissolution and reprecipitation a bed of dolomite can retain or create porosity and permeability to much greater burial depths and into higher temperature realms than a limestone counterpart dolomitization also creates new crystals with new rhomb growth following the dissolution of less stable precursors repetition of this process without complete pore cementation can generate intercrystalline porosity a number of times in the rock s burial history intercrystalline porosity is a highly interconnected style of porosity that gives dolomite reservoirs their good fluid storage capacity and efficient drainage the fact that many dolomite reservoirs formed via brine reflux means that they sit beneath an evaporite seal in both platform and basinwide evaporite settings the same association of evaporites sulfate source and entrained hydrocarbons means that burial conditions are also suitable for thermochemical sulfate reduction and the precipitation of base metals this tends to occur at higher temperatures 60 c 80 c and so the resulting dolomites tend to be ferroan and consist of saddle shaped crystals

depositional depth of laminated carbonate deposits insights from the lower cretaceous valdeprado formation cameros basin northern spain the lower cretaceous berriasian valdeprado formation cameros basin northern spain contains more than 900 m of laminated carbonates and pseudomorphs after sulfates traditionally many sedimentary packages of different ages and lithologies have been interpreted as deep water deposits based essentially on the abundance of laminations and the absence of subaerial exposure features in contrast the valdeprado formation provides an example of a shallow water deposit dominated by laminations with scarce evidence of subaerial exposure and gives criteria to solve the challenge of distinguishing shallow water and deep water ancient laminated deposits the two most abundant facies all along the valdeprado formation are a parallel laminated limestone formed by alternating carbonate mudstone and calcite and quartz pseudomorphs after displacive gypsum and b graded laminated limestone consisting of quartz mica ostracodes and pseudomorphs after detrital gypsum grains at the base which changes gradually upwards to carbonate mudstone parallel laminated limestone and graded laminated limestone could have been deposited in either deep or shallow environments as a result of salinity fluctuations driven by alternation of flooding and evaporation and by sediment resuspension processes respectively subaerial exposure features such as desiccation mudcracks are scarce in most of the succession except in a few meter scale stratigraphic intervals where they are very abundant interestingly in these intervals desiccation cracks are present at the tops of several successive laminae up to 25 mudcracked laminae per meter of deposit indicating that at least during those periods of time deposition occurred in shallow water bodies that were desiccated frequently in the upper part of the stratigraphic section parallel laminated and graded laminated limestones are associated with current ripple and wave ripple cross laminated arenites and ostracode mudstone to wackestone with centimeter size pseudomorphs after lenticular gypsum and abundant desiccation mudcracks and tepees which also suggest sedimentation in shallow water environments moreover the laminated carbonates display continuous parallel layering and the same facies along the 40 km long outcropping area these deposits are directly interbedded with and pass laterally to siliciclastic sandy muddy flat deposits in the western area of the basin without clinoforms slump structures or slide masses in between all of these features suggest deposition in shallow perennial carbonate sulfate water bodies and their peripheral mudflats developed in a flat bottomed basin with no marked gradients

early triassic stromatolites as post mass extinction disaster forms aftermaths of mass extinctions have been thought to be characterized by relaxation of ecological constraints accompanied by increased prominence of opportunistic generalists such taxa termed disaster forms have been shown to increase dramatically in range and abundance after several mass extinction events the cambrian ordovician stromatolite decline in normal marine level bottom environments has been explained as a direct or indirect consequence of increases in ecological constraints such as greater levels of predation and or bioturbation of microbial communities caused by early paleozoic benthic invertebrate evolution and diversification thus one would predict that in post ordovician strata stromatolites might appear in normal marine level bottom environments as disaster forms in the aftermaths of mass extinction particularly devastating to the benthic biota such as during early triassic time mounded stromatolites are present in two beds up to 1 5 m thick of the lower triassic spathian virgin limestone member moenkopi formation in the southwestern spring mountains of nevada stromatolites from level bottom normal marine subtidal environments have also been described from other lower triassic strata in north america europe and asia these stromatolites unusual in level bottom normal marine settings may have developed locally during the long aftermath 4 5 m y of the permian triassic mass extinction because of partial relaxation of the ecological constraintsmore that typically restricted them from unstressed subtidal normal marine level bottom environments less

microbial mediated pre salt carbonate deposition during the messinian salinity crisis calcare di base fm southern italy abstract a multi scale analysis of sedimentary carbonate facies and post sedimentary diagenetic features of the calcare di base formation the precursor to evaporites in upper messinian successions of northern calabria and central sicily has revealed their microbial bio mediated origin massive to laminated microbial boundstones represent the most common sedimentary facies forming flat to low relief cm to m scale stromatolitic and thrombolitic bodies the fabric of the micrite varies from peloidal to aphanitic and almost always preserves filamentous bacteria which characterized the original microbial mat the mat was dominated by sulphur oxidizing bacteria belonging to the thiotrichaceae but there is evidence for a more complex community with sulphate and or nitrate reducing bacteria all being responsible for the mediation of the carbonate precipitation microbial boundstones are rich in pseudomorphs of ca sulphate and halite which formed during the deposition of the microbial carbonate layers of primary gypsum are interbedded locally with carbonates suggesting the presence of restricted marine conditions the stable o and c isotopic composition of the carbonates that vary from dolomite to aragonite and calcite suggests a complex interplay between arid to humid climatic changes expressed cyclic interbedding of the carbonate with marl marlstone later diagenetic events mainly consist of phreatic meteoric recrystallization and cementation although considered as diachronous the microbial carbonates can be mapped out over a distance of more than 500 km across southern italy this indicates near constant environmental conditions upon the central mediterranean shelf at the beginning of the salinity crisis deposition of the extensive subaqueous microbial deposit that largely comprises the calcare di base is envisaged to have taken place across a shallow to moderate depth platform with local slopes into deeper water areas where some resedimentation occurred

microbial domes and megaoncoids in miocene reefs in the mahakam delta in east kalimantan indonesia abstract coral patch reefs in the miocene mahakam delta in east kalimantan borneo indonesia developed in shallow marine turbid waters these reefs grew in delta front to prodelta environments under fluctuating siliciclastic input langhian patch reefs are well exposed in limestone quarries of the air putih area near the city of samarinda and include two different types of microbial carbonates in two sections 2 km apart in the northern section microbialites occur as low relief domes up to 2 m wide and 0 5 m high the domes show internal lamination and coralline algae and encrusting foraminifers interlayered with microbial micrite they developed around large coral fragments at the transition from reef deposits to fine grained siliciclastics microbialites in the southern section formed decimetre to metre scale nodules here termed megaoncoids around nuclei of large coral fragments microbial micrite with laminated to digitate fabrics intergrew with coralline algae and encrusting foraminifers to form the thick covers of megaoncoids small nodules were bound together in larger ones which are usually strongly asymmetric internally megaoncoids accumulated in a carbonate breccia laterally changing into coral boundstones in both sections microbialites grew on the flanks of shallow water low relief patch reefs in poorly illuminated low energy conditions flat domed microbialites formed on a nearly horizontal stable seafloor seawards of the patch reef the megaoncoids in the southern section formed as a result of downslope displacement of coral fragments coated by microbialite coralline algal encrusting foraminifer crusts the slope of the patch reef s flank favoured the falling and rolling of encrusted corals with the continued growth of microbial crusts on all sides of nodules in contrast to quaternary and recent occurrences of microbialites in indo pacific and caribbean reefs the samarinda microbialites are not components of the reef framework nor formed in deep water instead they developed in shallow turbid waters of the miocene mahakam delta in periods of arrested siliciclastic input

microbially influenced formation of mg calcite and ca dolomite in the presence of exopolymeric substances produced by sulphate reducing bacteria some species of sulphate reducing bacteria srb are known to mediate the formation of dolomite and mg calcite however their exact role in the mineralization process remains elusive here we present the result of a laboratory experiment that was designed to test whether formation of carbonate minerals by srb can occur in the absence of living cells through passive mineralization of their exopolymeric substances eps srb capable of mediating dolomite were cultivated in the laboratory allowing them to secrete eps microbial activity within the cultures was subsequently inhibited with antibiotics only after this step ca2 and mg2 were added to the solution and carbonate minerals could form mg calcite and disordered ca dolomite precipitated in association with eps the mol of mg2 in the crystals increased with longer incubation times this result demonstrates that organic compounds produced by srb can mediate the formation of ca mg carbonates in the absence of an active metabolism

diversity and factors controlling widespread occurrence of syn rift ladinian microbialites in the western tethys triassic catalan basin ne spain abstract the fault block carbonate ramps of the ladinian middle triassic catalan basin hosted a wide variety of microbial deposits which formed during a syn rift stage stratigraphical and sedimentological analyses of these microbialites allow us to reconstruct two depositional models for such microbial deposits moreover new insights into the interplay between the intrinsic and extrinsic factors that controlled the widespread development of these microbialites are provided stromatolites ooidal muddy microbial laminites and thrombolites were analyzed on the basis of their geometry lamination and textural attributes these microbialites are distributed over two transgressive regressive sequences and coexisted during the fassanian early ladinian regressive stage later stromatolites and ooidal muddy microbial laminites developed during the longobardian late ladinian transgressive stage three types of lamina couplets reflect distinct accretion processes linked to water chemistry sediment supply and hydrodynamic conditions thrombolite textures were produced by accretionary binding and encrusting processes microbially mediated and pervasive cementation the widespread occurrence of microbial deposits is explained as a consequence of a biogeochemical cascade of events e g anoxic dysoxic seawater conditions volcanic activity upwelling of alkaline waters nutrient concentrations and microbial blooms which promoted favorable conditions for microbial growth in the tethys during the ladinian furthermore in the catalan basin syn rift fault induced subsidence and local water energy gradients exerted an essential role in the distribution of the three types of microbialites through space and time thrombolites were prone to grow in the hanging wall block of a half graben subtidal conditions whereas stromatolites and ooidal muddy microbial laminites flourished preferably in a foot wall block setting intertidal shallow subtidal conditions however the fact that thrombolites exhibit abundant botryoidal and isopachous fibrous marine cements corrosion and significant microbial evidence allow us to hypothesize about the relationship between thrombolite occurrence and hydrothermal fault controlled fluid circulation during the triassic rifting the two depositional models here proposed constitute a step forward the understanding of the platform to basin microbialite heterogeneity during the ladinian furthermore this work sheds new light on the mechanisms that likely promoted microbialite development during a period of major ecological restructuration and complex oceanographic conditions

nanosized authigenic magnetite and hematite particles in mature paleosol phyllosilicates new evidence for a magnetic enhancement mechanism in loess sequences of china

facies relations depositional environments and diagenesis in a major early proterozoic stromatolitic carbonate platform to basinal sequence campbellrand subgroup transvaal supergroup southern africa the campbellrand subgroup and its correlative the malmani subgroup of the transvaal supergroup represent a major 2300 2600 ma old carbonate buildup it is on average between 1500 and 1700 m thick and covers an area of approximately 190 000 km2 on the kaapvaal craton the original depository probably extended across the entire 600 000 km2 surface area of the craton and beyond the buildup consists essentially of two major lithofacies assemblages namely a basinal non stromatolitic laminated carbonate and shale sequence with minor chert iron formation and mafic tuff interbeds off the craton to the west and a shallow water stromatolitic carbonate sequence on the craton proper growth faults along the edge of the craton controlled the platform margin defined by columnar stromatolite oolite and carbonate arenite shoal deposits these are flanked basinward by reef like giant subtidal stromatolitic mounds elongated perpendicular to the platform margin and grading into slope deposits consisting of laminated carbonates with interbeds of chaotic dolomite breccia and conophyton stromatolites platform lagoonal deposits consist of dark grey laminoid fenestrate stratified stromatolites interfingering to the interior of the craton with intertidal to supratidal light grey cherty dolomite units with a variety of structures such as domal and stratified stromatolites laminoid fenestrae tepee structures and flat pebble conglomerates the buildup consisted primarily of limestone which was replaced by early diagenetic dolomite the chemical composition of the dolomite is related to depofacies platform margin and lagoonal dolomites are manganese rich basinal dolomites are iron rich and intertidal to supratidal deposits are virtually free of iron and manganeses indications are that the carbonate buildup developed from an early carbonate ramp stage into a mature rimmed carbonate shelf eventually the carbonate shelf was drowned by a major transgression followed by the deposition of the kuruman iron formation

messinian stromatolite thrombolite associations santa pola se spain an analogue for the palaeozoic stromatolite thrombolite associations are the dominant facies forming large portions of the santa pola carbonate platform se spain during deposition of the terminal carbonate complex tcc the tcc the last period of marine sedimentation in the western mediterranean associated with the messinian salinity crisis comprises a ne sw trending thrombolite reef with occasionally interlayered stromatolite horizons and a predominantply stromatolite and oolite facies in the back reef area the stromatolites are mainly dome shaped but fine columnar or wavy undulose forms can occur the stromatolites form huge bioherms extending tens to hundreds of metres they are finely laminated with alternating layers of dolomicrite and dolomicrospar the dolomicrite layers appear to be a primary dolomite precipitate whereas the dolomite crystals in the dolomicrospar layers apparently formed around a meta stable nuclei which was subsequently dissolved or degraded the low content of sand sized particles in the stromatolitic layers indicates formation under low energy conditions possibly on a tidal flat as reported from other areas in the western mediterranean deposition of the tcc at santa pola was apparently cyclic whereby stromatolites generally terminate each depositional cycle subtidal conophyton stromatolites possibly the only known occurrence younger than palaeozoic are however found on the reef slope at the base of the first tcc depositional cycle the dolomitic nature of the unadulterated stromatolitic laminations and the association of stromatolites and thrombolites as platform builders were a common feature in the early palaeozoic but are unusual in post ordovician carbonate facies we propose that the conditions during tcc deposition were very restricted possibly reflecting an environment similar to that of the early palaeozoic

a theoretical and experimental comparison of the anisotropies of magnetic susceptibility and remanence in rocks and minerals summary susceptibility thermo remanent magnetization trm and isothermal remanent magnetization irm anisotropy ellipsoids have been determined for several rock samples the results indicate that the ellipsoid of initial susceptibility is less anisotropic than the trm and low field irm ellipsoids which are found experimentally to be of identical shape this suggests that palaeomagnetic data for anisotropic rocks may be corrected by using the anisotropy ellipsoid determined from magnetically non destructive low field irm measurements such irm measurements can also be used to obtain anisotropy axes of samples which are inherently anisotropic but which have a susceptibility which is too weak to be accurately measured the results for a series of artificial anisotropic samples containing magnetite particles of different sizes in the range 0 2 90 m were very similar to those for the rocks in contrast a comparison of the susceptibility and irm ellipsoids for anisotropic samples containing particles from a magnetic tape gave very different results in accordance with theory such results imply that susceptibility and irm ellipsoids could be used to determine whether anisotropic rocks contain uniaxial single domain particles magnetization confined to the easy axis or whether the particles are essentially multidomain

microbial fossilization in carbonate sediments a result of the bacterial surface involvement in dolomite precipitation recent dolomitic sediment samples from lagoa vermelha brazil were examined microscopically to study the process of bacterial fossilization in carbonate sediments bacteria like bodies were intimately associated with carbonate mineral surfaces and coatings on the former demonstrate the calcification of single bacterial cells the bacterial fossilization process in lagoa vermelha sediments was simulated in the laboratory by cultivation of mixed and pure cultures of sulphate reducing bacteria which were isolated from the lagoa vermelha sediments these cultures produced carbonate minerals that were studied to provide insight into the initiation of the fossilization process in mixed culture experiments bacterial colonies became calcified whereas in pure culture experiments single bacterial cells were associated with dolomite surfaces dolomite nucleated exclusively in bacterial colonies intimately associated with extracellular organic matter and bacterial cells electrophoretic mobility measurements of the bacterial cells in electrolyte solutions demonstrated the specific adsorption of ca 2 and mg 2 onto the cell surfaces indicating the role of the bacterial surface in carbonate nucleation and bacterial fossilization the affinity of the cells for mg 2 was related to the capability of the strains to mediate dolomite formation combined with sulphate uptake which dissociates the mgso 4 o ion pair and increases the mg 2 availability the concentration of mg 2 ions in the microenvironment around the cells where the conditions are favourable for dolomite precipitation may be the key to overcome the kinetic barrier to dolomite formation these results demonstrate that bacterial fossilization is a consequence of the cell surface involvement in carbonate precipitation implying that fossilized bacterial bodies can be used as a tool to recognize microbially mediated carbonates

biosedimentology of a silurian thrombolite reef with meter scale growth framework cavities a completely dolomitized silurian reef at maumee ohio u s a is uncommon among early paleozoic reefs in that its framework consists of about 95 thrombolite boundstone and 5 corals the reef lithified syndepositonally as evidenced by centimeter to meter scale growth framework cavities within it that are completely filled with internal sediment the largest cavities 2 3 5 m in diameter are prob ably the largest ever recorded in a thrombolite reef four main types of components are distinguished in the reef frame work 1 dense bushy mesoclots 2 dense rounded to subrounded mesoclots 3 saccate microstructures and 4 crusts mostly struc tureless but also containing dense bushy mesoclots the dense bushy components are abundant rounded to sub rounded mesoclots less so and the remaining components are rare the dense bushy and rounded to subrounded mesoclots are interpreted to represent the calcified re mains of what were initially either aggregates of the probable filamen tous cyanobacterum angulocellularia or aggregates of renalcid calci microbes or both and the saccate mesoclots as the calcimicrobe ren alcis sensu stricto it is speculated that syndepositonal lithification of the reef framework was accomplished by impregnation of unlithified protomesoclots by micrite to create mesoclots along with the formation of micritic cement around and between mesoclots that served to fuse together successive generations of mesoclots the direction of branching of bushy mesoclots in the reef indicates that unlithified protomesoclots were able to grow upward laterally and downward this ability along with syndepositional lithification to create mesoclots and to cement them together enabled the protome soclots and mesoclots to create reef knobs and eventually to roof over spaces that were centimeters to three or more meters in width and height to create growth framework cavities features indicative of rap id deposition including graded bedding poor sorting of bioclasts and textural inversion are common in internal sediment in meter size cav ities these features suggest that storms possibly hurricanes were at least as important and probably more so than everyday wave en ergy in transporting sediment into all cavities in the reef regardless of size

thrombolite fabrics and origins influences of diverse microbial and metazoan processes on cambrian thrombolite variability in the great basin california and nevada thrombolites are a common component of carbonate buildups throughout the phanerozoic that are usually described as microbialites with an internally clotted texture a wide range of thrombolite textures have been observed and attributed to diverse processes leading to difficulty interpreting thrombolites as a group interpreting thrombolitic textures in terms of ancient microbial and metazoan ecosystems requires understanding diverse processes specifically those due to microbial growth and metazoan activity many of these processes are reflected in thrombolites in the cambrian carrara bonanza king highland peak and nopah formations great basin california usa they comprise eight thrombolite classes based on variable arrangements and combinations of depositional and diagenetic components four thrombolite classes hemispherical microdigitate bushy coalescent columnar and massive fenestrated contain distinct mesoscale microbial growth structures that can be distinguished from surrounding detrital sediments and diagenetic features by contrast mottled thrombolites have mesostructures that dominantly reflect post depositional processes including bioturbation mottled thrombolites are not bioturbated stromatolites but rather formed from disruption of an originally clotted growth structure three thrombolite classes arborescent digitate amoeboid and massive contain more cryptic textures all eight of the thrombolite classes in this study formed in similar cambrian depositional environments marine passive margin overall this suite of thrombolites demonstrates that thrombolites are diverse in both internal fabrics and origin and that clotted and patchy microbialite fabrics form from a range of processes the diversity of textures and their origins demonstrate that thrombolites should not be used to interpret a particular ecological evolutionary or environmental shift without first identifying the microbial growth structure and distinguishing it from other depositional post depositional and diagenetic components furthermore thrombolites by definition are fundamentally different from stromatolites and dendrolites in which the laminae and dendroids reflect a primary growth structure because clotted textures in thrombolites do not always reflect a primary microbial growth structure this article is protected by copyright all rights reserved

cryptalgal metazoan bioherms of early ordovician age in the st george group western newfoundland bioherms are common in the st george group a sequence of shallow water carbonate rocks deposited on the western continental shelf of iapetus ocean they range from small heads and metre sized mounds to extensive banks and complexes many metres thick and hundreds of metres in lateral extent the cores of these bioherms are principally composed of thrombolites unlaminated branching columnar stromatolites structures quite distinct from laminated stromatolites which are common in intertidal beds associated with thrombolites is a diverse fauna of burrowing invertebrates trilobites nautiloids pelmatozoans brachiopods gastropods rostroconchs and archaeoscyphiid sponges on the basis of framework building components three main bioherm types are distinguished 1 thrombolite mounds 2 thrombolite lichenaria or sponge mounds and 3 thrombolite lichenaria renalcis reef complexes the framework of the last is the most complex with abundant cavities and a demonstrably uneven growth surface of thrombolites corals and free standing renalcis heads walls and roofs some cavities were active sediment conduits while others were protected their roofs draped with renalcis and their walls coated by cryptalgal laminites these bioherms possess the attributes of shallow water ecologic reefs they span a critical time gap in the development of reefs the transition period from algal dominated bioherms of the precambrian and cambrian to the metazoan dominated bioherms of the middle ordovician and remaining phanerozoic

the meaning of stromatolites stromatolites document microbial interactions with sediments and flowing water throughout recorded earth history and have the potential to illuminate the long term history of life and environments modern stromatolites however provide analogs to only a small subset of the structures preserved in archean and proterozoic carbonates thus interpretations of secular trends in the shapes and textures of ancient columnar stromatolites require nonuniformitarian scale dependent models of microbial responses to nutrient availability seawater chemistry influx of sediment grains shear and burial models that integrate stromatolite scales macroscopic organization and shapes could also help test the biogenicity of the oldest stromatolites and other structures whose petrographic fabrics do not preserve direct evidence of microbial activity an improved understanding of stromatolite morphogenesis in the presence of oxygenic and anoxygenic microbial mats may illuminate the diversity of microbial metabolisms tha

glacioeustatic fluctuations the mechanism linking stable isotope events and sequence stratigraphy from the early oligocene to middle miocene one of the most difficult challenges of sequence stratigraphy is the establishment of synchronism between events observed in widely separated basins problems arise because the resolution of the best stratigraphic methods is not good enough to establish the synchronism of similar aged events on a global scale unless a common mechanism affecting global eustasy is assumed such as variations in the ice volume there is no a priori reason to expect that sequences of similar age in widely separated basins are indeed synchronous the stable oxygen isotope composition of marine carbonates is a proxy for sea level which has been underutilized in sequence stratigraphy identification of isotope events is based on d sup 18 o data from dsdp and odp sites 522 529 563 608 and 747 drilled in the atlantic and indian oceans these records were used to define oligocene and miocene oxygen isotope zones in addition we present isotope data from petrobras well a drilled in the campos basin brazil ages of isotope events correspond well with the ages of sequence boundaries and maximum flooding surfaces because of the good correlation between the isotope and sequence stratigraphic records we reconfirm that ice volume change is the common mechanism driving sea level fluctuations from the oligocenemore to present less

implications for neoarchaean ocean chemistry from primary carbonate mineralogy of the campbellrand malmani platform south africa the precipitation of calcite and aragonite as encrustations directly on the seafloor was an important platform building process during deposition of the 2560 2520 ma campbellrand malmani carbonate platform south africa aragonite fans and fibrous coatings are common in unrestricted shallow subtidal to intertidal facies they are also present in restricted facies but are absent from deep subtidal facies decimetre thick fibrous calcite encrustations are present to abundant in all depositional environments except the deepest slope and basinal facies the proportion of the rock composed of carbonate that precipitated as encrustations or in primary voids ranges from 0 to 65 depending on the facies subtidal facies commonly contain 20 35 in situ precipitated carbonate demonstrating that neoarchaean sea water was supersaturated with respect to aragonite carbonate crystal growth rates were rapid compared with sediment influx rates and the dynamics of carbonate precipitation were different from those in younger carbonate platforms the abundance of aragonite pseudomorphs suggests that sea water ph was neutral to alkaline whereas the paucity of micrite suggests the presence of inhibitors to calcite and aragonite nucleation in the mixed zone of the oceans

microbial deposits in upper miocene carbonates mallorca spain the santanyi limestone a 30 35 m thick upper miocene limestone succession cropping out in mallorca contains abundant microbialite deposits the shape microstructure and texture of which was controlled by environmental factors depth energy and salinity three main types of microbialites are distinguished 1 domed dnos and stratiform mostly undulate unos non oolitic stromatolites 2 undulate oolitic laminites uol and 3 domed oolitic thrombolites dots based on lithofacies associations and occurrence of microbialite types the santanyi limestone succession is subdivided into five stratigraphic units i to v separated by sharp surfaces within units ii iii and v the vertical evolution of microbialites was induced by changes in accommodation space depth 1 intertidal very shallow subtidal conditions at the base were induced by flooding over a wide area 2 continued sea level rise caused submergence to subtidal conditions and 3 a significant bathymetric decrease created the sharp surface bounding these units in units ii and iii nos accumulated in variable energy and depth conditions as buildups with thick somewhat discontinuous and mostly non isopachous lamination surrounded by oolitic grainstones with wave and current structures and oolitic intraclasts in contrast thin and generally regular and smooth lamination of nos in unit v suggests along with the absence of oolite grainstones and macrobiota calm and restricted maybe more saline conditions uol consisting of oolitic layers separated by thin micritic laminae developed adjacent to nos in units ii and iii and to dot at the lower part of unit iii in shallow water and low energy conditions both ooids and micrite laminae have evidence for biogenesis micritized ooids containing microbial remains are common micritic laminae in uol and the dark micritic laminae in nos are thought to represent bacterially enhanced calcite precipitation and lithification during periods of low sedimentation oolitic thrombolites containing macrobiota are only present in unit iii they represent deeper and open marine conditions affected by high energy events in which microbially mediated precipitation favoured microbialite accretion and lithification

deposition and early alteration of evaporites summary the depositional settings for primary and early diagenetic evaporite deposits generally fall into three categories marginal mixed shallow subaqueous and subaerial shallow and deep subaqueous these three environmental groupings hold for both marine and nonmarine settings although the details of continental evaporites may be far more complex than in most marine fed water bodies the primary evaporite morphologies from many continental playa hypersaline marine and marine marginal depositional settings are reasonably well understood because of the numerous detailed studies of recent holocene and cenozoic deposits that serve as models for sedimentary interpretation the sedimentological features that develop in deeper water settings are inferred based on examination of unaltered cenozoic deposits each environmental setting develops characteristic depositional features and patterns and one facies grades into the next because there may be significant physiochemical changes in water composition during deposition as well as sudden change s in both water depth and basinal circulation description and interpretation of evaporative rocks should not be based on mineralogy alone but on the distinct sedimentary characteristics of each part of a deposit in cases where sediments still reflect their primary mineralogy and morphology most of the environments can now be recognized however geochemical studies are commonly required to determine the source s of the original water the determinative geochemical techniques that presently serve as a support to sedimentologic study include studies of fluid inclusions bromine content of chloride salts and the stable isotopes of strontium sulphur carbon and oxygen only after these and perhaps other chemical analyses are considered can the full depositional history of a region be reasonably unravelled

geobiology of microbial carbonates metazoan and seawater saturation state influences on secular trends during the phanerozoic abstract microbial carbonates are long ranging essentially bacterial aquatic sediments their calcification is dependent on ambient water chemistry and their growth is influenced by competition with other organisms such as metazoans in this paper these relationships are examined by comparing the geological record of microbial carbonates with metazoan history and secular variations in caco3 saturation state of seawater marine abundance data show that microbial carbonates episodically declined during the phanerozoic eon past 545 myr from a peak 500 myr ago this abundance trend is generally inverse to that of marine metazoan taxonomic diversity supporting the view that metazoan competition has progressively limited the formation of microbial carbonates lack of empirical values concerning variables such as seawater ionic composition atmospheric partial pressure of co2 and ph currently restricts calculation of caco3 saturation state for the phanerozoic as a whole to the use of modeled values these data together with palaeotemperature data from oxygen isotope analyses allow calculation of seawater caco3 saturation trends microbial carbonate abundance shows broad positive correspondence with calculated seawater saturation state for caco3 minerals during the interval 150 545 myr ago consistent with the likelihood that seawater chemistry has influenced the calcification and therefore accretion and preservation of microbial carbonates these comparisons suggest that both metazoan influence and seawater saturation state have combined to determine the broad pattern of marine microbial carbonate abundance throughout much of the phanerozoic in contrast for the major part of the precambrian it would seem reasonable to expect that seawater saturation state together with microbial evolution was the principal factor determining microbial carbonate development interrelationships such as these with feedbacks influencing organisms sediments and the environment are central to geobiology

carbonate stromatolites from a messinian hypersaline setting in the caltanissetta basin sicily petrographic evidence of microbial activity and related stable isotope and rare earth element signatures lower messinian stromatolites of the calcare di base formation at sutera in sicily record periods of low sea level strong evaporation and elevated salinity thought to be associated with the onset of the messinian salinity crisis overlying aragonitic limestones were precipitated in normal to slightly evaporative conditions occasionally influenced by an influx of meteoric water evidence of bacterial involvement in carbonate formation is recorded in three dolomite rich stromatolite beds in the lower portion of the section that contain low domes with irregular crinkly millimetre scale lamination and small fenestrae the dominant microfabrics are i peloidal and clotted dolomicrite with calcite filled fenestrae ii dolomicrite with bacterium like filaments and pores partially filled by calcite or black amorphous matter and iii micrite in which fenestrae alternate with dark thin wispy micrite the filaments resemble beggiatoa like sulphur bacteria under scanning electron microscopy the filaments consist of spherical aggregates of dolomite interpreted to result from calcification of bacterial microcolonies the dolomite crystals are commonly arranged as rounded grains that appear to be incorporated or absorbed into developing crystal faces biofilm like remains occur in voids between the filaments the dolomite consistently shows negative d 13 c values down to 11ae3 and very positive d 18 o mean value 7ae9 that suggest formation as primary precipitate with a substantial contribution of organic co2 very negative d 13 c values down to 31ae6 of early diagenetic calcite associated with the dolomite suggest contribution of co2 originating by anaerobic methane oxidation the shale normalized rare earth element patterns of sutera stromatolites show features similar to those in present day microbial mats with enrichment in light rare earth elements and m type tetrad effects enrichment around pr coupled to a decline around nd and a peak around sm and eu taken together the petrography and geochemistry of the sutera stromatolites provide diverse and compelling evidence for microbial influence on carbonate precipitation

structure and evolution of a messinian mixed carbonate siliciclastic platform the role of evaporites sorbas basin south east spain the sorbas member is a late messinian complex sedimentary system that formed immediately following deposition of the messinian evaporites in the sorbas basin south east spain this work describes the sequence architecture and facies organization of a continuous kilometre long alluvial fan to open platform transect near the village of cariatiz in the north east of the basin the post evaporitic cariatiz platform was a mixed carbonate siliciclastic system composed of four intermediate frequency fifth order depositional sequences depositional sequences 1 to 4 arranged in an overall prograding trend the intense fracturing and brecciation of these deposits is attributed to the deformation and dissolution of an evaporite body measuring several tens of metres in thickness the four sequences display significant spatial temporal variability in both architecture and facies distribution with two main phases i depositional sequences 1 and 2 are ooid and oobioclastic dominated and show normal marine faunas and ii depositional sequences 3 and 4 show a higher siliciclastic contribution and are microbialite dominated these important changes are interpreted as modifications of the primary controlling factors following an initial 70 m drowning possibly linked to increased oceanic input depositional sequences 1 to 3 were controlled mainly by eustatic variations and inherited topography their progradation destabilized the evaporite body near the end of the depositional sequence 2 period during the second phase depositional sequences 3 and 4 recorded a progressive restriction of the sorbas basin related to a 30 to 40 m fall in water level that was driven mainly by regional factors these regional factors were dissolution and gravity induced deformation of the evaporites and correlative evaporative fluid circulation associated with the contrasted arid humid regional climate that respectively controlled sequence geometry and fluctuating water salinity which caused a microbialite bloom