microbially induced cementation of carbonate sands are micritic meniscus cements good indicators of vadose diagenesis characteristic fabrics such as micrite envelopes calcified filaments and micritic grain to grain bridges are observed in a modern subtidal firmground wood cay bahamas and in a variety of firm and hardgrounds of lower cretaceous and upper jurassic platform carbonates swiss and french jura mountains their similarity to microbial fabrics described in grapestones and in intertidal to continental vadose environments suggests that microbial activity played an important role in the initial stabilization and cementation of carbonate sands meniscus type cements to distinguish them from vadose meniscus cements which clearly formed in subtidal environments are related to filament calcification trapping of percolating micrite and microbially induced carbonate formation such meniscus type cements are commonly micritic but meniscus shaped precipitation of fibrous aragonite or sparitic calcite around organic filaments is also observed therefore an interpretation of vadose early diagenesis should not be based on meniscus cements alone similarly subtidally formed filamentous structures can strongly resemble alveolar septal structures and be interpreted incorrectly as related to subaerial exposure

combined flow processes and sedimentary structures on the shoreface of the wave dominated grande riviere de la baleine delta abstract shoreface sediments exposed in raised deltaic deposits along the hudson bay coast are characterized by a thick succession of predominantly parallel laminated and small scale cross laminated fine sand ripple forms where preserved include both combined flow and wave ripple geometries hummocky cross stratification is present in lower parts of the section similar facies have been found in cores from the present day shoreface measurements of waves currents and suspended sediment concentrations in 10 m water depth conducted over a 15 day period in the fall of 1999 captured four different meteorological periods these included an intense four day storm which sustained significant wave heights over 3 m for 15 hours and a storm surge of over 1 m height fair weather conditions produce weak alongshore tidal flows but oscillatory motions generated by small local waves predominate during storms orbital motions dominate the flow but a strong inertial flow generated by wind forcing is also present comparison with a published combined flow bedform stability diagram suggests that the predominant bedforms at 10 m depth would be small 2 d wave dominated ripples and small weakly asymmetrical 3 d ripples under fair weather conditions and large 3 d ripples and upper plane bed under storm wave conditions the predominantly interbedded parallel laminated and small scale cross laminated facies is therefore interpreted to be characteristic of the upper shoreface where the seabed experiences intense oscillatory motions under all storms and even some fair weather conditions combined flow ripples would form during storm events because of the stronger wind driven mean residual current hummocky cross stratification forms in deeper water during storms where the ratio of maximum orbital velocity to mean residual current velocity remains for longer periods in the large 3 d ripple field during the waning phase of storms a numerical bed predictor model using the 15 day data set as input also suggests bimodal conditions fluctuating between wave ripples and upper plane bed at 10 m water depth this model further indicates that combined flow ripples would form rarely but tends to underestimate the threshold of sediment suspension

sabkhas slumping and compaction at mufulira zambia algal structures such as stromatolites have long been recognized as closely associated with the cupriferous dolomitic shales of the katanga supergroup of zaire and zambia discovery of bioherms and biostromes in the predominantly arenaceous deposits of the mufulira copper mine indicates a similar origin with the more common copper deposits in dolomitic shale and their barren gaps due to bioherms renfro 1974 recognized the worldwide association of sabkhas algal mats and stratiform copper deposits his interpretation of sabkha environments in cyclic depositional sequences at mufulira is corroborated by detailed features in the mine recognition of abundant carbonized debris of algal mat from 0 5 m down to microscopic size in the marine sediments of mufulira indicates an accumulation of decomposing vegetable debris that is more than sufficient to promote anoxic conditions in the bottom waters of the lagoons with resulting precipitation of copper and iron sulfides contemporaneously with sedimentation compaction of algal material explains the 80 percent contraction of sun cracked mud flakes also compaction by roughly two thirds of the algal rich graywacke and the c orebody accounts for the rapid sagging of overlying sabkha and terrestrial sediments which was followed by a second marine invasion allowing deposition of the b orebody and central graywacke similar rapid compaction of the latter under its cover of sabkha and red bed sediment caused further sagging marine invasion and deposition of the a orebody and graywacke vertically above the c and b graywackes although renfro s hypothesis of mineralization by ascending groundwaters may account for some initial precipitation of sulfides beneath algal mats the bulk of the preserved mineralization at mufulira exhibits features more compatible with syngenetic deposition of sulfides associated with decaying transported algal material in marine lagoons

microbial mat control on siliciclastic precambrian sequence stratigraphic architecture examples from india abstract an attempt has been made to understand within a cause and effect framework the sequence building pattern in proterozoic time that witnessed non uniformiteranean microbial mat growth and epeiric sea development marine or marginal marine successions in three neoproterozoic formations the sonia sandstone in western india and the sirbu shale and upper bhander sandstone in central india have been examined here for this purpose the strikingly common feature of all three formations is vertical stacking of highstand systems tracts without intervention of any significant transgressive deposits in one instance only the transgressive systems tract is represented by a thin granular transgressive lag and in all other cases the evidence of transgression is simply marked by the presence of marine flooding surfaces the absence of transgressive strata relates to the low sea floor gradients which facilitated rapid transgressions combined with a generally low sediment supply aggradation under normal regressive highstand conditions in spite of the low sediment supply was promoted by the prolific growth of microbial mats which reduced the effects of wave and current reworking by organic binding of clastic particles

algal mats load structures and synsedimentary sulfides in revett quartzites of montana and idaho carbonate rich patches in quartzites of the revett formation are due to algal mats in marine arenites of about a 1 3 b y age some occur in growth positions on bedding planes some on slumped foresets but mostly they were disrupted by faster currents depositing heavy minerals as lag sediments discovery of stromatolites in the revett formation confirms the algal growths algal debris provided the food source for anaerobes which produced h 2 s for sulfide precipitation of copper silver lead zinc and iron in the zonal sequence typical of stratiform syngenetic sulfides at the troy mine these sulfide zones cross the strata sulfides vary in grain size and aggregation and are classified into original fine disseminations reworked aggregates formed during dewatering and infilling of fractures and vein openings by lateral secretion the relationship of fine sulfide disseminations to heavy minerals and to algal carbonate indicates deposition of minute sulfide grains with the sediment a quartzite pillow which contained 24 sagged and folded layers each layer with three sublayers of first heavy minerals and then clean quartzite both overlain by sulfide rich quartzite was examined and revealed the previously poorly understood mechanism of pillow growth contemporaneous with sedimentation and in this example also with sulfide precipitation better mineralization in arenites is ascribed not to permeability but to stronger currents depositing sand and causing increased mixing of the upper metal bearing waters with the lower anoxic water in which hydrogen sulfide precipitates selectively first copper and silver as minute sulfide grains the sulfides are incorporated into the suspended sediment which is then sorted by waxing and waning currents according to grain size and specific gravity into the varying proportions especially observed in the tripartite laminae of the 24 layers of the quartzite pillow described in detail

wrinkle structures microbially mediated sedimentary structures common in subtidal siliciclastic settings at the proterozoic phanerozoic transition sedimentary structures produced by microbial activity are well known from ancient carbonate facies but little is known about equivalent microbial structures in ancient siliciclastic facies wrinkle marks and kinneyia ripples are closely related sedimentary structures that are commonly found in ancient siliciclastics and they may represent microbial activity to evaluate this possibility these structures were studied in vendian cambrian strata of north america and compared to structures formed by modern microbial mat communities in redfish bay texas striking similarities in sedimentologic petrographic and morphologic characteristics of modern and ancient occurrences suggest that the enigmatic but widespread ancient structures could have been formed by microbial processes the relative abundance of these structures in siliciclastic facies of this interval further suggests that prior to the onset of relatively intense bioturbation in the ordovician microbial mats could have played a significant role in the evolution and diversification of early life in a broad variety of sea floor environments

the possible role of benthic microbial mats during the formation of carbonaceous shales in shallow mid proterozoic basins a considerable portion of the upper member of the mid proterozoic newland formation meagher county montana consists of carbonaceous silty shales striped shale facies this type of shale facies is common in proterozoic basins and is host to several major base metal deposits the striped shales were deposited in a subtidal setting basinward of carbonates characterized by cryptalgal laminites mudcracks and flat pebble conglomerates the carbonaceous silty shales are considered remnants of benthic microbial mats irregular internal laminae patterns of particle trapping mechanical deformation during penecontemporaneous soft sediment deformation and filamentous microbiota provide evidence for this interpretation the dolomitic clayey shale contains graded silt clay couplets and these are interpreted as storm layers modern subtidal microbial mats can only survive under special conditions but in the proterozoic it is suggested that benthic microbial mats colonized the shallow seafloor during periods of low sediment input leading to the formation of carbonaceous shales

wrinkle structures a critical review abstract in this paper a variety of so called wrinkle structures is reviewed in an attempt to help distinguish between crinkly decorations arising from physical processes that acted on siliciclastic bedding surfaces and true microbially induced wrinkle structures two types of small scale microbially induced sedimentary structures are prominent due to their distinct geometry and mode of occurrence 1 elephant skin textures characterized by reticulate patterns of sharp crested ridges forming mm to cm scale polygons occurring on argillite or argillaceous veneers above fine grained sandstone and likely reflecting growth structures of microbial mats 2 kinneyia structures characterized by mm scale flat topped winding ridges and intervening troughs and pits sometimes resembling small scale interference ripples kinneyia structures usually occur on upper surfaces of siltstone sandstone beds themselves frequently event deposits and are thought to have formed beneath microbial mats additionally more linear variations of mat growth structures partly resembling small scale petees may be developed finally some wrinkly structures resulting from tractional mat deformation or mat slumping are occasionally preserved these may appear as arcuate belts of non penetrative small scale folds or as wrinkled bulges on otherwise flat surfaces wrinkle structures as indicators for the former presence of mats gain in importance if other mat related structures are additionally observed in the same clastic succession e g sand chips sandy intraclasts or spindle shaped or sinuously curved to circular sand cracks frequently combined in networks furthermore appropriate lithologies and facies are required for instance if compared with the distribution of modern cohesive microbial mats laminated siltstone argillite with intercalated siltstone sandstone beds representing event deposits in tidal flat successions would be compatible with microbial mat development within a variety of physically induced small scale wrinkly structures miniature load structures may above all be misinterpreted as microbially induced wrinkle structures due to their similar size and appearance and their comparatively frequent occurrence

pyrite and marcasite coated grains in the ordovician winnipeg formation canada an intertwined record of surface conditions stratigraphic condensation geochemical reworking and microbial activity sulfide coated grains are enigmatic sedimentary particles that occur in sediments ranging in age from archean to jurassic although their oolitic and pisoidal texture suggests energetic and agitated conditions their mineralogy seems to require a reducing and stagnant environment because of this conflict they have long been considered diagenetic replacements of precursor iron oxide chamosite or carbonate ooids coated iron sulfide cis grains from the ordovician winnipeg formation of saskatchewan canada formed as primary diagenetic precipitates under shallow burial conditions involving intermittent episodes of tractional reworking grain abrasion reburial and precipitation of further concentric laminae these iron sulfide grains are found in shallow marine sandstones with indications of episodic wave reworking and indicate sufficient organic matter burial to promote reducing pore waters and iron sulfide formation horizons with strong enrichment of iron sulfide grains up to 40 indicate negative net sedimentation to facilitate geochemical reworking of iron cis grains formed during intervals of maximum sediment starvation when organic matter was enriched in surface sediments and rare storm waves intermittently reworked the surface sediment alternating laminae of pyrite and marcasite within these grains point to substantial fluctuations in pore water ph driven by intermittent oxidation of previously formed iron sulfides filamentous and coccoid structures within cortical laminae suggest microbial involvement in lamina formation through providing a growth medium for sulfate reducing bacteria and potentially a template for mineral growth coated iron sulfide grains of the winnipeg formation preserve a combined record of condensation seafloor reworking fluctuating chemical conditions in the surface sediment and high surface productivity this information rich record is made even more valuable by the fact that the host sediment consists largely of heavily bioturbated monotonous sandstones that could not provide these insights on their own merit

microbially induced sedimentary structures a new category within the classification of primary sedimentary structures abstract cyanobacterial films and mats syndepositonally influence erosion deposition and deformation of sediments the biomass levels surface morphologies and microbial mats stabilize depositional surfaces and shelter the sediment against erosion or degassing growing microbial mats dredge grains from their substrate upwards whereas cyanobacterial filaments that are oriented perpendicular to the mat surface reach into the supernatant water and baffle trap and bind suspended particles these and similar biotic physical interactions are reflected in syndepositional formation of microbially induced sedimentary structures we distinguish structures on bedding planes leveled bedding surfaces wrinkle structures microbial mat chips erosional remnants and pockets multidirectional ripple marks and mat curls and internal bedding structures sponge pore fabrics gas domes fenestrae structures sinoidal laminae oriented grains benthic ooids biolaminites mat layer bound grain sizes we propose to place this group of microbially mediated structures as a fifth category bedding modified by microbial mats and biofilms in pettijohn and potter s 1964 existing classification of primary sedimentary structures

possible indicators of microbial mat deposits in shales and sandstones examples from the mid proterozoic belt supergroup montana u s a abstract it has been suspected for some time that microbial mats probably colonized sediment surfaces in many terrigenous clastic sedimentary environments during the proterozoic however domination of mat morphology by depositional processes post depositional compaction and poor potential for cellular preservation of mat building organisms make their positive identification a formidable challenge within terrigenous clastics of the mid proterozoic belt supergroup a variety of sedimentary structures and textural features have been observed that can be interpreted as the result of microbial colonization of sediment surfaces among these are a domal buildups resembling stromatolites in carbonates b cohesive behaviour of laminae during soft sediment deformation erosion and transport c wavy crinkly character of laminae d bed surfaces with pustular wrinkled appearance e rippled patches on otherwise smooth surfaces f laminae with mica enrichment and or randomly oriented micas g irregular curved wrinkled impressions on bedding planes h uparched laminae near mud cracks resembling growth ridges of polygonal stromatolites and i lamina specific distribution of certain early diagenetic minerals dolomite ferroan carbonates pyrite although in none of the described examples can it irrefutably be proven that they are microbial mat deposits the observed features are consistent with such an interpretation and should be considered indicators of possible microbial mat presence in other proterozoic sequences

neoproterozoic early cambrian biota and ancient niche a synthesis from molecular markers and palynomorphs from bikaner nagaur basin western india abstract the time frame of present study is neoproterozoic early cambrian an enigmatic and consequential time in the history of earth the present study elucidates the terminal proterozoic biota and palaeoenvironment by characterizing the soluble and insoluble fractions of organic matter extracted from sediment core samples recovered from marwar supergroup bikaner nagaur basin western india the normal alkanes range from c15 to c33 wherein c19 to c21n alkanes predominate and carbon preference index ranges from 1 12 to 1 19 pristane phytane ratio varies between 0 9 and 1 2 the kerogen matter occurs as homogenous laminae and discrete organic remains which are mostly derived from microbial mat triterpenoids include cheilanthanes and hopanes which are confirming evidences of microbial input algal presence has also been confirmed from palynological data green algae evolved to be a major biota during the neoproterozoic early cambrian time and this is validated by the appreciable abundance of c29 steranes in the bitumen fraction the amorphous pieces of kerogen residues lack any visibly distinct internal features however a very few simple ornamented acritarchs like micrhystridium sp lophosphaeridium sp and leiosphaeridia sp are observed in the organic residues micro ftir spectroscopy of the amorphous organic particles reveals the presence of long chain aliphatic components and unusually high c c aromatic ring stretching band the organic remains were deposited in restricted marine water and elevated saline condition as indicated by several salinity biomarkers the present study clearly demonstrates a microbial mat dominated niche with significant eukaryotic green algae that had been capable of surviving in the high salinity of the terminal proterozoic oceans

rb sr isotope dating of neoproterozoic malani group magmatism from southwest rajasthan india evidence of younger pan african thermal event by 40ar 39ar studies abstract this paper reports rb sr isotope ages of the neoproterozoic volcanics and associated granitoids of the trans aravalli belt of northwestern india all these rocks along with the earlier reported 779 10 ma old felsic volcanics from diri and gurapratap singh of pali district rajasthan constitute the malani group the study indicates that different rock suites belonging to the malani group represent a polyphase igneous activity which spanned for about 100 ma ranging from 780 to 680 ma the granitoids of the malani group i e peraluminous jalore type and peralkaline siwana type were emplaced around 730 and 700 ma ago respectively these plutonic suites represent two different magmatic episodes within a short time interval the initial sr ratios of these granitoids suggest lower crustal derivation of the magma the peralkaline granitoids and the associated peralkaline rhyolites pantellerites are coeval and cogenetic the ultrapotassic rhyolite exposed at manihari of pali district represents the youngest magmatic activity at 681 20 ma having a very high initial sr ratio of 0 7135 0 0033 the high initial sr ratio of these rocks may be due to incorporation of radiogenic 87 sr from the country rock by assimilation or fusion into the residual fraction of the magma in the crust which gave rise to other differentiated rocks of the group 40 ar 39 ar studies of two jalore granite samples indicate presence of post crystallisation thermal disturbance between 500550 ma ago the timing of this thermal overprinting on the malani rocks is related to the widespread pan african thermo tectonic event which is witnessed and magmatically manifested in different part of the indian shield

sequence stratigraphy of the precambrian abstract the method of sequence stratigraphy requires the application of the same workflow and principles irrespective of the age of strata under analysis in that respect its application to precambrian successions is similar to the approach used for phanerozoic case studies differences however are recorded in terms of the preservation potential and the amount of data available for analysis the rates and intensities of the allogenic controls on sedimentation the environmental conditions and related physical processes and the evolution of competing groups of organisms and associated biogenic processes the combined effect of these contrasting aspects accounts for differences in the architecture of depositional sequences particularly with respect to the relative contributions of various systems tracts to the makeup of a sequence the application of sequence stratigraphy to precambrian basins has considerably enlarged the perspective on the fundamental principles governing the processes of sedimentary basin formation and the mechanisms controlling stratigraphic cyclicity in the rock record these first order principles are perhaps the most important contribution of precambrian research to sequence stratigraphy at the broader scale of earth s geological history the tectonic regimes governing the formation and evolution of sedimentary basins are shown to have been much more erratic in terms of nature and rates than originally inferred solely from the study of the phanerozoic record this provides important clues with respect to the criteria that should be involved in the hierarchy system of classification of stratigraphic sequences and bounding surfaces

microbial mat structures in profile the neoproterozoic sonia sandstone rajasthan india abstract ubiquitous microorganisms especially cyanobacteria preferably grow on the sediment surface thereby producing microbial mats in the absence of grazers and bioturbators microbial mat is a unique feature of the proterozoic most of the papers so far published described a wide variety of bed surface microbial mat structures with rare illustrations from sections perpendicular to bedding nonetheless bed surface exposures are relatively rare in rock records this limitation of bed surface exposures in rock records suggest that a study of microbial mats in bed across sections is needed the 60 m thick coastal marine interval of the sonia sandstone formation is bounded between two terrestrial intervals a transgressive lag at the base and an unconformity at the top and has been chosen for exploration of microbial mat structures in bed across sections a wide variety of microbial mat induced structures in bed across sections are preserved within the coastal interval of the sonia sandstone though many of these structures are similar in some aspects with bed surface structures some of those presented here are new the palaeogeographic range of these microbial structures extends from supralittoral to neritic diagenetic alterations of microbial mats produce pyrite and those zones are suitable for the preservation of microbial remains sem and edax analyses show fossil preservation of filamentous microbial remains that confirm the presence of microbial mats within the coastal interval of the sonia sandstone effects of proliferation of microbial mats in the siliciclastic depositional setting are numerous the mat cover on sediment surfaces hinders reworking and or erosion of the sediments thereby increases the net sedimentation rate successive deposition and preservation of thick microbial mat layer under reducing environments should have a great potential for hydrocarbon production and preservation and therefore these proterozoic formations could be a target for exploration

the origin of eddy heat fluxes in the northern california upwelling regime observed eddy heat fluxes i e covariances between heat content and horizontal currents are analyzed with moored data from the coastal ocean dynamics experiment it is demonstrated that much of each sampled covariance arises from the separate correlations of currents and temperature with the upwelling variability which is attributed to the particular warming and cooling processes present thus a large part of the observed eddy heat fluxes is wind driven the remainder is consistent with being largely statistical noise in the observed covariance between those parts of the flow and temperature which are not correlated with the upwelling this also suggests that the wind driven part alone might be a better estimate of the ensemble average eddy heat flux

microbial mat mediated structures in the ediacaran sonia sandstone rajasthan india and their implications for proterozoic sedimentation abstract a strong influence of microbial mats on the physics of neoproterozoic sedimentation is explored within a 60 m thick stratigraphic interval within the sonia sandstone jodhpur group western india this marine interval is bounded by two terrestrial units its base being marked by a transgressive lag and the top by an unconformity progradation from upper neritic above fair weather wave base to supralittoral settings was later terminated by a transgression deposits of lower shoreface upper shoreface transition thus overlie supralittoral aeolian sandstones the basal contact of the former deposits being marked by another transgressive lag a wide spectrum and abundant examples of microbial mat or mat derived structures supports unusual cohesiveness within granular sand deposited in a high energy marine environment the cohesiveness is manifested in abundant preservation of several delicate primary structures and also in their replication in overlying beds even after they were subjected to high energy currents a low rate of sedimentation and severely restricted sediment reworking resulted from prolific mat growth and consequent depletion in the sediment budget was manifested in bedform migration and evolution these factors could also have influenced the sequence building pattern in a distinctive way as is already reported from several meso and neoproterozoic successions ubiquitous mat growth thus had the potential to impart significant distinctions to proterozoic clastic sedimentary successions

a model for the evolution of brines in salt from the lower salado formation southeastern new mexico fluid inclusions were collected from a bedded salt horizon in the lower permian salado formation in the delaware basin southeastern new mexico the sampling horizon at a depth of approximately 645 meters consists primarily of recrystallized halite with thin layers of anhydrite other trace minerals dispersed throughout the salt include quartz polyhalite gypsum k feldspar magnesite and clays large fluid inclusions up to several mm on an edge are common in the halite in addition bands of microscopic 10 m fluid inclusions are present as primary chevron structures in fragments of unrecrystallized salt we sampled 109 large inclusions by individual extraction of the fluids which were analyzed for ca mg k na cl br and so4 the chemistry of the inclusion fluids and the associated mineralogy suggest that these brines represent permian seawater that has undergone evaporation and subsequent modification by diagenetic reactions dominated by the alteration of calcium sulfate to polyhalite and magnesite formation the range of fluid inclusion compositions suggests a significant departure from a simple seawater evaporation model other brines from the same horizon in the salado formation were sampled and analyzed for the same elements as the fluid inclusions and differed significantly from them primarily by the depletion of mg relative to k the association of these brines with argillaceous and or anhydritic halite containing a suite of authigenic minerals quartz magnesite and mg rich clays suggests that these are intergranular brines with compositions determined over a much longer time scale than that required by the fluid inclusions the principal reactions affecting intergranular brine chemistry are dehydration of gypsum dewatering of detrital clays and uptake of mg during clay diagenesis overall the observed variation in brine compositions implies that if large scale hydrologie circulation is occurring in the salado halite the time scale is limited by the rate required for low temperature silicate diagenesis

microbial signatures in peritidal siliciclastic sediments a catalogue a catalogue of microbial structural signatures is presented based upon the coupling of fundamental biogeochemical microbial processes and local morphogenetic determinants it summarizes a collection of sedimentary structures obtained from two modern siliciclastic peritidal environments in different climatic zones temperate humid mellum island southern north sea subtropical arid coast of southern tunisia textural geometries reveal a high structural diversity but their determinants are primarily based upon six major parameters 1 intrinsic biofactors structural diversification of sedimentary microbial films and mats inherent in the organisms i e their construction morphology growth taxis and behaviour and local abundance of specific morphotypes most prominent are the ensheathed filamentous cyanobacteria microcoleus chthonoplastes and lyngbya aestuarii and the sheathless filamentous cyanobacterium oscillatoria limosa 2 biological response to physical disturbances sediment supply erosion and fracturing of surface layers resulting from desiccation cause growth responses of biofilms and microbial mats 3 trapping binding effects physicobiological processes give rise to grain orientations and wavy to lenticular lamina lamina specific grain arrangements and sucrose calcium carbonate accumulations 4 secondary physical deformation of biogenic build ups mechanical stresses acting upon sediments overgrown and biostabilized by biofilms and mats produce erosional and overthrust structures 5 post burial processes textural fabrics that evolve from mechanical effects of gas formation from decaying mats and features related to the formation of authigenic minerals calcium carbonates calcium sulphates pyrite 6 bioturbation and grazing post depositional structures such as skolithos type dwellings traces of burrowing insects gastropod grazing traces and faecal pellets in synopsis the catalogue firstly comprises a sound set of ubiquitous signatures this uniformity in architectural characteristics is attributed to the presence and local dominance of certain microbes throughout the different settings the catalogue secondly documents signatures that are extremely sensitive to tidal position hydrodynamic regime and overall climatic conditions these kinds of signature indicate narrow facies zones which often coincide with the activity or dominance zones of certain organisms an overview of structures of microbial origin from the fossil record underlines the potential of many of the signatures included in this catalogue to become fossilized and provide strong indicators of former siliciclastic tidal settings

a new window into early archean life microbial mats in earth s oldest siliciclastic tidal deposits 3 2 ga moodies group south africa newly discovered sedimentary structures produced by ancient microbial mats in early archean sandstones of the 3 2 ga moodies group south africa differ fundamentally in appearance and genesis from early archean stromatolites and bacterial cell fossils preserved in chert wrinkle structures desiccation cracks and roll up structures record the previous existence of microbial mats that effectively stabilized sediment on the earliest known siliciclastic tidal flats in thin section the sedimentary structures reveal carpet like laminated fabrics characteristic of microbial mats negative d 13 c isotope ratios of 220 1 to 221 5 6 0 2 are consistent with a biological origin for the carbon preserved in laminae the biogenicity of the sedimentary structures in the moodies group is substantiated by comparative studies on identical mat related features from similar tidal habitats throughout earth history including the present day this study suggests that siliciclastic tidal flat settings have been the habitat of thriving microbial ecosystems for at least 3 2 billion years independently of controversial silicified microfossils and stromatolites the newly detected microbially induced sedimentary structures in sandstone support the presence of bacterial life in the early archean

extensive microbial mats and their influences on the erosional and depositional dynamics of a siliciclastic cold water environment lower arenigian montagne noire france abstract lower arenigian ordovician rocks of the montagne noire france represent a shallow marine environment of a high latitude position along the northern margin of gondwana within the weakly metamorphosed siliciclastics six depositional units were recognized 1 outer shelf 2 foreshore zone below wave base 3 foreshore zone above wave base 4 sand bars of shoreface zone 5 intertidal deposits and 6 lagoonal zone wrinkle structures and various other phenomena were mediated by coherent cyano bacterial mats the structures can be related to the different facies zones of the paleoenvironment whereas the muddy outer shelf and the high energetic sand bars were not overgrown by any mat constructing microbial populations fine sands of the foreshore zone the tidal flats and lagoonal areas were widely colonized restriction of mats due to competition of space by endobenthic macroorganisms or by grazers was low the microbially induced structures are composed of organic material pyrite clay minerals illite chamsonite chlorite and chert the minerals precipitated in situ during degradation of the organic layers by the activity of heterotrophic bacteria at low temperatures because the microorganisms formed a dense organic carpet covering extensive areas of the ancient sea bottom they influenced significantly the erosional and depositional dynamics of the sedimentary system of the local arenigian biostabilization counteracted erosion and baffling trapping and binding enriched mineral particles additionally the in situ formed minerals contributed to the total amount of sediment the biotic influence lead to increased accumulation of sediment within the depositional area the study shows that microbial mats of great extension occur within phanerozoic siliciclastics of cold paleoclimate zones and that preservation of the mat fabrics was possible the significant mats influenced the local sedimentary system an aspect scarcely taken into account for such paleoenvironments