palaeoenvironments and faunal recovery after the end permian mass extinction abstract strata of the lower triassic werfen formation dolomites northern italy record deposition in a wide range of palaeoenvironments from peritidal to outer ramp settings this enables accurate assessment of faunal recovery following the end permian mass extinction by eliminating facies related effects extinction of the shelly taxa appears to be rapid and occurs in the latest permian tesero oolite horizon disappearance of the various trace fossil taxa appears to be more gradual following the extinction the lower and middle griesbachian are characterised by laminated sediments containing low diversity bedding plane assemblages ichnological geochemical and sedimentological data show evidence of widespread low oxygen conditions normally oxygenated waters return in the late griesbachian and there is a moderate increase in tiering above and below the surface macrofaunal and ichnofossil diversity decrease markedly during the smithian with the appearance of more terrigenous strata possibly reflecting an increased run off and more brackish conditions complete recovery begins in the basal spathian with an increase in tiering above and below the substrate crinoids and ammonoids appear for the first time in the werfen formation and the ichnogenus rhizocorallium re appears global correlation is hindered by poor biostratigraphy in the dolomites region however on a broad scale it appears as though many of the palaeoenvironmental changes are repeated worldwide current available data suggest that the re appearance of some trace fossil taxa e g diplocraterion rhizocorallium may be globally synchronous

braided fluvial to marine transition the basal lower cambrian wood canyon formation southern marble mountains mojave desert california abstract the lower cambrian wood canyon formation recognized over a 39 000 km2 belt through the southern great basin and mojave desert was examined in the southern marble mountains eastern mojave desert southeastern california there it crops out as a well exposed 90 130 m thick feldspathic sandstone nonconformably overlying proterozoic basement previous investigators have suggested a tide dominated nearshore shallow marine origin for the entire unit in the eastern mojave based on broad scale stratigraphic trends and localized fossil occurrences in the upper member of the formation in contrast to a regional approach the present study uses a detailed lithofacies analysis in a smaller study area to suggest that a fluvial to marine transition occurs within these strata the middle and upper members of the wood canyon formation are recognized in the southern marble mountains and are divisible into six distinct lithofacies that possess gradational contacts where exposed cross stratified sandstone facies a mediumpebble conglomerate facies b planar stratified sandstone facies c sandstone and thin mudstone facies d mudstone with intercalated sandstone facies e and horizontally laminated siltstone facies f facies a d contained within the middle member comprise the bulk of wood canyon sediments and record the depositional characteristics of a transition zone from a distal alluvial braid plain to a tidally influenced fluvial dominated braid delta complex cross stratification in the braid plain sediments displays a strongly unimodal pal ocurrent pattern suggestive of downstream migrating sinuous crested megaripples braid delta sediments however show a more dispersed paleocurrent pattern and contain a sparse trace fossil assemblage a change from fluvial dominated to marine dominated processes is recorded in upper member rocks facies e and f facies e contains many of the traits characteristic of a low to mid tidal flat sequence such as coarsely interlayered bedding flaser and lenticular bedding microripple marks and locally abundant rusophycus and planolites trace fossils facies f which dominantly consists of horizontally laminated siltstone is interpreted as the product of an estuarine lagoon that developed behind and within a discontinuous barrier of stranded braid bars

fault reactivation analysis using microearthquake clustering based on signal to noise weighted waveform similarity the cluster formation of about 2000 induced microearthquakes mostly m l 2 is studied using a waveform similarity technique based on cross correlation and a subsequent equivalence class approach all events were detected within two separated but neighbouring seismic volumes close to the geothermal powerplants near landau and insheim in the upper rhine graben sw germany between 2006 and 2013 besides different sensors sampling rates and individual data gaps mainly low signal to noise ratios snr of the recordings at most station sites provide a complication for the determination of a precise waveform similarity analysis of the microseismic events in this area to include a large number of events for such an analysis a newly developed weighting approach was implemented in the waveform similarity analysis which directly considers the individual snrs across the whole seismic network the application to both seismic volumes leads to event clusters with high waveform similarities within short seconds to hours and long months to years time periods covering two magnitude ranges the estimated relative hypocenter locations are spatially concentrated for each single cluster and mirror the orientations of mapped faults as well as interpreted rupture planes determined from fault plane solutions depending on the waveform cross correlation coefficient threshold clusters can be resolved in space to as little as one dominant wavelength the interpretation of these observations implies recurring fault reactivations by fluid injection with very similar faulting mechanisms during different time periods between 2006 and 2013

evolutionary paleoecology of the earliest echinoderms helicoplacoids and the cambrian substrate revolution bioturbation in neritic siliciclastic settings during the proterozoic phanerozoic transition increased in depth and intensity causing a change in substrates from the matgrounds characteristic of the proterozoic to the mixgrounds characteristic of the phanerozoic this change in bioturbation increased the water content of surficial layers of sediment and blurred the sediment water interface leading to the first appearance of a mixed layer development of a mixed layer throughout neritic environments would have had a strong impact on any benthic metazoans particularly sessile suspension feeders that were well adapted for survival on relatively unbioturbated proterozoic substrates the impact of this substrate transition on benthic metazoans has been termed the cambrian substrate revolution the unusual early cambrian helicoplacoid echinoderms were well adapted for survival on typical proterozoic style substrates the examination of new helicoplacoid specimens collected during this study combined with extensive study of the rocks in which they are preserved indicate that helicoplacoids lived as sediment stickers on a muddy substrate that underwent only low to moderate levels of strictly horizontal bioturbation and did not have a mixed layer the significant increase of bioturbation through the cambrian in neritic siliciclastic settings is likely to have led to the extinction of the helicoplacoids other similarly adapted sessile suspension feeding echinoderms may have also been driven to extinction by the effects of the cambrian substrate revolution the co existence during the cambrian of organisms adapted to the variety of substrates characteristic of this transitional period may also have contributed to the high degree of perceived morphological disparity during the cambrian explosion

depositional facies of high energy beach to offshore sequence comparison with low energy sequence a study of the nearshore zone and continental shelf of a high energy coast was made in the ventura port hueneme area of california during the summer of 1974 to investigate depositional facies characteristics and compare them with similar facies from a low energy coastal setting analysis of box cores can cores and vibracores permits recognition of three principal zones 1 nearshore facies backshore to 9 m water depth made up primarily of parallel ripple and cross bedded clean sand bioturbation is only locally significant 2 transition facies 9 to 18 m water depth a zone of fine sand and silty sand characterized by an increase in biogenic over physical sedimentary structures that are commonly preserved as laminated to burrowed beds wave ripple bedding and parallel lamination are important structures in this facies 3 offshore facies 18 m water depth sandy silt is the primary texture and bioturbation is the dominant sedimentary structure remnant parallel lamination is the only physical sedimentary structure present comparison of the results of this study with a previous description of a low energy beach to offshore sequence at sapelo island georgia indicates that the two areas do not differ greatly in their facies or overall vertical sequence of sedimentary structures the principal difference is in the thickness of the three facies the california facies are significantly thicker than their georgia counterparts this difference is in direct response to the role of higher wave energy on the california coast

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 unusual sedimentary rock record of the early triassic a case study from the southwestern united states abstract sedimentary systems as well as the earth s biota changed dramatically following the end permian mass extinction this global sedimentological transformation is reflected in the widespread occurrence of flat pebble conglomerates subtidal wrinkle structures microbialites and carbonate seafloor fans in various lower triassic sections as a case study a facies analysis of the lower triassic virgin limestone member of the moenkopi formation and the middle and upper members of the union wash formation southwestern united states was conducted and all unusual facies were documented within a palaeoenvironmental framework the occurrence of the flat pebble conglomerates and wrinkle structures is significant because these reflect a long term reduction in vertical bioturbation in the case of the subtidal microbialites and carbonate seafloor fans their formation is linked to an increase in alkalinity the presence of these facies in the uppermost lower triassic strata of the southwestern united states indicates that reduced levels of infaunal bioturbation as well as and perhaps a consequence of unusual ocean chemistry extended for 5 6 million years after the end permian mass extinction suggesting that depositional environments as well as the biota were significantly affected by the extinction event

sea level changes an integrated approach sea level changes an integrated approach in october 1985 sepm sponsored a four day conference entitled sea level changes an integrated approach the purpose of the conference was to provide a forum for an interdisciplinary exchange of ideas on sea level changes and to provide an opportunity for integrating various types of evidence in approaching unresolved issues the conference was successful in bringing together scientists from industry academia and government representing all of the major geosciences disciplines presentations of many new papers plus significant releases of data that were previously held proprietary provided fertile ground for discussion this much cited volume represents the best of the material presented at the conference includes the early vail chart

palaeoecology and sedimentology of the dysaerobic bedford fauna late devonian ohio and kentucky usa abstract oxygen deficient biofacies models rely on lithologic and paleontologic attributes to identify distinctive biofacies interpreted to reflect levels of oxygenation in anaerobic dysaerobic and aerobic parts of a stratified water column this study of the bedford fauna from the bedford shale of ohio and kentucky and from adjacent black shale units reports faunal distributions different from those predicted by the accepted models this study suggests that although oxygenation was an important factor that determined the taxonomic makeup of the fauna bacterially mediated nutrient recycling and substrate characteristics were more important than oxygenation in determining faunal distribution in the dysaerobic zone

lower triassic storm dominated ramp sequence in northern hungary an example of evolution from homoclinal through distally steepened ramp to middle triassic flat topped platform

trace fossils and the aftermath of the permo triassic mass extinction evidence from northern italy abstract the duration of early triassic anoxia in the carbonate ramp settings of the werfen formation dolomites northern italy has been constrained using ichnofabric and sedimentological data this includes fine lamination and abundant pyrite framboids commonly reworked into lags by storm events the anoxic event spanned the entire griesbachian stage and extended from deep waters into extraordinarily shallow water settings evidence for oxygen deficiency can be found in mid and inner ramp facies the extent and duration of this event may at least partially explain the magnitude of the permo triassic mass extinction normal benthic conditions returned rapidly in the dienerian stage and pervasively bioturbated distal ramp facies testify to the ventilation of the deeper water areas of the region in post griesbachian times a post mass extinction radiation is not particularly well displayed in either trace fossil or body fossil diversity compilations from the dolomites moderate low levels of trace fossil diversity are maintained throughout the post griesbachian werfen formation whilst the body fossil data shows a moderate increase from the mid smithian stage there is thus a curious lag period spanning the dienerian to mid smithian when environmental conditions were apparently normal but the marine fauna failed to show any recovery

trends in depth and extent of bioturbation in cambrian carbonate marine environments western united states cambrian carbonate strata of the great basin deposited below fairweather wave base but above mean storm wave base inner shelf and below mean storm wave base but above maximum storm wave base middle shelf were examined for extent and depth of bioturbation data indicate that initial extensive colonization of marine infaunal habitats occurred between tommotian and atdabanian time throughout the remaining 65 m y of cambrian time bioturbation increased although depth of bioturbation remained relatively constant at less than 6 cm during this time the amount of bioturbation in the inner shelf was greater than that in the middle shelf

proliferation of early triassic wrinkle structures implications for environmental stress following the end permian mass extinction lower triassic strata in the western united states and northern italy have yielded the first reported occurrence of microbially mediated wrinkle structures in shallow subtidal siliciclastic paleoenvironments since the cambrian we hypothesize that wrinkle structures formed under reduced infaunalization conditions during the aftermath of the end permian mass extinction these wrinkle structures occur in siliciclastic sedimentary rocks in association with hummocky cross stratification and in some cases trace fossils such as asteriacites rhizocorallium planolites and gyrochorte that indicate a subtidal paleoenvironment wrinkle structures were found in the campil member in the werfen formation of northern italy and in the virgin limestone member of the moenkopi formation and the upper member of the thaynes formation exposed in the western united states the presence of wrinkle structures is likely related to a unique taphonomic window that allowed for the preservation of these delicate features on storm dominated siliciclastic shelves wrinkle structures commonly formed in a variety of subtidal paleoenvironments during the proterozoic cambrian but thereafter became restricted to intertidal supratidal and deep sea environments this restriction has been attributed to the increase in infaunalization of metazoans following the cambrian radiation the proliferation of wrinkle structures in subtidal settings during the early triassic suggests that infaunal bioturbation was reduced after the end permian mass extinction and that this reduction lasted for millions of years

late early triassic microbial reefs of the western united states a description and model for their deposition in the aftermath of the end permian mass extinction abstract research in the western united states to reinvestigate and characterize the previously reported stromatolites of the latest early triassic spathian virgin limestone member of the moenkopi formation has shown that these microbial build ups formed patch reef mounds detailed field analysis of these reef mounds and the surrounding beds has established that they attained 1 2 m of topographic relief above the seafloor a petrographic study of these microbial mounds illustrates that they consisted of microbial fabrics as well as voids with early marine cements and formed in the absence of in situ metazoans these reef mounds formed during the aftermath of the end permian mass extinction that spanned the early triassic due to the devastation of marine metazoans at this extinction the early triassic has often been deemed a reef gap because no metazoan reefs are found at this time whereas colonial metazoan reefs are globally absent from lower triassic strata microbial build ups similar to these described from the western united states have been found in south china armenia southern turkey and greenland a regional early permian to late early triassic unconformity exists in the study region however analysis of these microbial reefs within a global context suggests that conditions favouring microbial reef development must have existed as long as 4 8 m y after the end permian mass extinction a depositional model of these microbial mounds indicates that flooding of the shelf with anoxic and or co2 rich deep waters may have fostered their occurrence

anoxia as a cause of the permian triassic mass extinction facies evidence from northern italy and the western united states facies faunal and geochemical evidence from the permian triassic boundary sediments of the dolomites and idaho indicates a major anoxic event in the earliest triassic in both regions the basal beds consist of finely laminated micrites with common syngenetic pyrite the only fauna consists of occasional bedding plane assemblages of lingula or clacaria a typical lower dysaerobic assemblage this is a level where previous studies have shown a major negative carbon isotope excursion and a cerium anomaly in the dolomites the pyritic micrite directly overlies strata containing a diverse and typically permian marine fauna of algae foraminifers including fusulinids and articulate brachiopods implying an abrupt extinction in contradiction to many previous views sequence stratigraphic analysis of the dolomite boundary sediments reveals a minor sequence boundary in the late permian followed by extremely rapid transgression leading to the development of the relatively deep water pyritic micrite a maximum flooding surface at the permo triassic boundary a further pulsed deepening in the lower griesbachian recorded in both the dolomites and idaho lead to the widespread establishment of dysaerobic facies it is clear that most of the extinctions occurred at the erathem boundary although the subsequent failure of the marine benthos to fill the empty ecospace in the ensuing griesbachian may have been due to the widespread development of dysaerobic conditions

paleoecology of the marine endobenthos endobenthic animals which reside within the sea bottom include stationary suspension feeders mobile deposit feeders and both stationary and mobile carnivores their activities especially with regard to dwelling feeding walking crawling and resting nesting are recorded as trace fossils abundance diversity and density of some kinds of trace fossils allow interpretation of the population strategies of the trace makers in terms of opportunistic r selected and equilibrium k selected strategies opportunistic ichnotaxa tend to be faciesbreaking traces which are highly localized in low diversity high density trace fossil associations in rocks representing environmental extremes e g variable salinities harsh temperatures low oxygen levels or shifting substrates equilibrium ichnotaxa usually are restricted to particular sedimentary facies and are characteristic of high diversity low dominance trace fossil associations in sediments reflecting stable predictable environmental conditions the most important environmental factors influencing the composition of trace fossil assemblages in marine settings are bathymetry substrate oxygen and hydrodynamic energy the four factors are closely interrelated because as water depth increases there is a general decrease in sediment grain size and hydrodynamic energy of the depositional environment as depth below the water sediment interface increases the firmness of the sediment due to compaction and dewatering increases and the oxygen content of interstitial waters drops drastically marine ichnofacies are largely substrate controlled soupgrounds are water saturated incompetent substrates typified by highly compressed and usually unidentifiable burrows softgrounds commonly contain numerous distinctive burrows and are zoned bathymetrically by the skolithos cruziana zoophycos and nereites ichnofacies firmgrounds are characterized by stiff compacted sediments in which traces of the glossifungites ichnofacies are excavated hardgrounds are cemented substrates in which bioerosion traces of the trypanites ichnofacies are bored woodgrounds are woody materials that have been exposed to the sea and bored by bivalves which produce characteristic traces of the teredolites ichnofacies tiering of endobenthic communities is common and is related to substrate preference of the burrowers and oxygen stratification of interstitial waters

unusual intraclastic limestones in lower triassic carbonates and their bearing on the aftermath of the end permian mass extinction flat pebble conglomerates were a common carbonate facies in cambrian to early ordovician open marine settings but they become extremely rare in these environments after this time however the early triassic witnessed an anachronistic reappearance of flat pebbles together with other intraclast types in a range of carbonate depositional settings in south china flat pebble conglomerates are encountered in storm dominated platform carbonates to deep basinal settings while prefossilized bivalve intraclasts and flat pebbles are common in mid ramp facies of northern italy the emplacement mechanisms of the intraclast bearing beds appear to have been diverse and to have included basinal turbidity flows and storm generated hyperconcentrated flows true storm beds deposited under combined flow conditions are rare the cause of the widespread early lithification implied by the early triassic intraclasts appears to have been twofold suppression of bioturbation allowing the preservation of thin beds and rapid submarine lithification both features appear to be a response to the widespread development of benthic dysoxia anoxia during and following the end permian mass extinction this event appears to have temporarily recreated the conditions that pertained in cambro ordovician shelf seas flat pebble conglomerates may therefore constitute a proxy indicator of stressed environmental conditions associated with global anoxic dysoxic events

early triassic calcimicrobial mounds and biostromes of the nanpanjiang basin south china stanley s model 1988 proposed that a permiantype reef fauna survived the early triassic reefless period in yet undiscovered refuges in the ancestral pacific ocean and then reappeared as the builders of middle triassic reefs flugel 1994 in contrast suggested that the middle triassic reefbuilding fauna are not holdovers from the permian but bear only a superficial resemblance to permian fossils recent publications reflect a consensus that the early triassic lacked reefs and reef mounds although some authors acknowledged the possibility that reef like structures may have existed in poorly explored areas such as south china and russia senowbari daryan et al 1993 flugel 1994 it is shown herein that calcimicrobial framework mounds and biostromes existed in the early triassic in the nanpanjiang basin of south china organic framework in this paper refers to a three dimensional biologically constructed network of calcified fossils that forms a rigid mass and supports an interconnected network of internal cavities that were open during accumulation although the calcimicrobial frameworks discussed herein are much smaller than frameworks of modern metazoan reefs the term framestone is more appropriate for these fabrics than other reef rock terms such as bindstone or bafflestone embry and klovan 1971 these calcimicrobial mounds and biostromes are important as the first organic framestones documented from the early triassic

trace fossil model for reconstruction of paleo oxygenation in bottom waters recognition of fluctuations in the degree of paleo oxygenation of bottom waters recorded in fine grained pelagic strata is important for interpretation of paleoceanographic and paleoclimatologic conditions general sedimentary fabric composition of trace fossil assemblages and burrow size and crosscutting relationships have been incorporated into a trace fossil tiering model that permits detailed reconstruction of changes in paleo oxygenation of bottom waters applications of this model to the miocene monterey formation california and the cretaceous niobrara formation colorado indicate that the ichnologic approach is more sensitive to both magnitude and rates of change in oxygenation levels compared to macrobenthic body fossil information

p and s wave tomography of the mantle beneath the united states mantle seismic structure beneath the united states spanning from the active western plate margin to the passive eastern margin was imaged with teleseismic p and s wave traveltime tomography including usarray data up to may 2014 to mitigate artifacts from crustal structure 5 40 s rayleigh wave phase velocities were used to create a 3 d starting model major features of the final p and s models include two distinct low velocity anomalies at depths of 60 300 km beneath the central and northern appalachians and passive margin the central appalachian low velocity anomaly coincides with eocene basaltic magmatism and the northern anomaly is located along the cretaceous track of the great meteor hot spot at depths of 300 700 km beneath the central and eastern u s large high velocity anomalies are inferred to be remnants of the farallon slab that subducted prior to 40 ma during the laramide orogeny

earth s earliest microbial mats in a siliciclastic marine environment 2 9 ga mozaan group south africa this study provides evidence for the existence of filamentous cyano bacteria forming sediment stabilizing mats in shelf environments at 2 9 ga the oldest known occurrence of microbial mats in siliciclastic rocks the mesoarchean mozaan group south africa features fine grained quartzites of an ancient shallow shelf environment these sandstones contain wrinkle structures which in thin section reveal filamentous textures forming carpet like microbial mat fabrics the textures resemble the trichomes of modern cyanobacteria chloroflexi or sulfur oxidizing proteobacteria mineralogical geochemical and isotopic analyses acre consistent with a biological origin of the filament like textures carbon filaments with biogenic isotopic signatures 1 3 c 24 2 0 5 are closely associated with hematite goethite and chert minerals which may derive from the former presence of oxygen within the microbial mats detrital quartz zircon and rutile in the mats could indicate baffling trapping and binding of the bacterial communities

microbially originated wrinkle structures on sandstone and their stratigraphic context palaeoproterozoic koldaha shale central india abstract wrinkle structures on sandstone beds are abundant and present in diverse forms in the palaeoproterozoic koldaha shale vindhyan basin central india the inferred palaeogeography of sedimentation ranges from offshore marine to terrestrial the wrinkled sandstones belonging to the relatively deeper marine part good preservation of ultramicroscopic microbial elements within these sandstones strongly supports a microbial mat origin of the wrinkles which was facilitated by development of an oxygen depleted condition immediately beneath the sediment water interface as depicted in early pyrite growth later impermeable shale encasing the sandstone beds arrested further scope for destruction of the microbial elements trapping of fine detritus by microbial filaments is documented by the invariable occurrence of a silty clay veneer on the wrinkled surfaces and by its characteristic texture wrinkle morphology has been classified and an attempt has been made to understand the origin of each of the varieties and to visualize the processes that acted upon the microbial mats to produce the wrinkle structures in their specific palaeogeographic context the general palaeogeographic bias in occurrence of the sandy wrinkles is well reflected in their concentration in the lower part of the highstand systems tracts hst s one superposed above the other comprising the koldaha shale the smaller scale palaeogeographic control is indicated by compartmentalization of two wrinkle varieties in the upper and the basal intervals of the hst s and overlapping occurrence of other varieties in the medial interval with apparent preference for either the upper or the lower levels

the paleoenvironmental distribution of phanerozoic wrinkle structures abstract wrinkle structures are microbially induced sedimentary structures that are known to have formed in ancient marine siliciclastic environments this study reviews the sedimentary context and paleoenvironmental distribution of these sedimentary structures throughout three distinct intervals of the phanerozoic the cambrian the post cambrian paleozoic and the lower triassic during these times wrinkle structures are found primarily within two sedimentary environments storm dominated subtidal environments and the intertidal zone subtidal occurrences of wrinkle structures during the post ordovician phanerozoic only appear to occur during intervals of environmental stress while earlier examples from the cambrian and lowermost ordovician likely formed prior to significant increases in extent and depth of bioturbation that hallmark most of the phanerozoic intertidal examples occur regardless of severe environmental stress and may have formed under low levels of bioturbation due to inherent vagaries of a marginal marine setting these environmental preferences appear to be conservative throughout the phanerozoic and may very well extend throughout earth s history

mat related sedimentary structures in neoproterozoic peritidal passive margin deposits of the west african craton anti atlas morocco abstract proterozoic inliers in the central anti atlas mountains expose predominantly siliciclastic sedimentary successions deposited in peritidal zones along the neoproterozoic continental margin of the west african craton wac the low grade metamorphic and modestly deformed sediments contain a wealth of sedimentary structures related to the former presence and activities of microbial mats and respective physicobiological processes the well preserved structures include wrinkle structures erosion marks microbial sand chips spindle shaped and subcircular microbial shrinkage cracks and possibly gas domes and cabbage head structures thin sections exhibit mat fragments and dispersed grains of hematite limonite after pyrite in fine grained quartzitic storm deposits post storm layers frequently consist of matrix supported sand sized to silt sized grains and are overlain by argillaceous veneers including isolated silt sized grains and black carbonaceous laminae the muddy veneers are considered to represent compacted stacks of microbial mats biolaminites which colonized and biostabilized storm and post storm layers and thus prevented them from eroding in the absence of grazing and burrowing organisms and at suitable depositional and hydrodynamic conditions it may be expected that proterozoic microbial mats extensively grew from the supratidal to the intertidal zones and occasionally e g behind protective barriers in the subtidal zone and beyond mat related structures however need specific conditions for their formation and preservation wrinkle structures erosion marks and microbial sand chips require tractional currents and soon deposition and burial respectively such conditions are preferably met in intertidal and supratidal zones spindle shaped and subcircular cracks require mat shrinkage due to either desiccation or syneresis crack propagation implies progressive shrinkage while superposition of crack generations indicates repeated alternation between mat exposure and flooding respective conditions prevail in the upper intertidal zone gas domes and cabbage head structures are related to the production of gas from decaying organic material beneath a sealing cohesive mat they may be some of the first features formed during genesis of petee structures in the intertidal to lower supratidal zones mat related structures may serve as sensitive facies indicators once their modes of formation are revealed

environmental and diagenetic variations in carbonate associated sulfate an investigation of cas in the lower triassic of the western usa an integrated stable isotope elemental and petrographic analysis of early triassic spathian carbonates and evaporites along a proximal to deep environmental transect reveals significant variations in 34s composition of carbonate associated sulfate cas the variations in the 34s of cas are strongly correlated with the ca mg composition of carbonates suggesting that the variations are driven by the degree of dolomitization the 34s of dolostones and evaporites are similar to one another and exhibit lower 34s values than limestones from all localities three hypotheses may explain the differences in 34s between proximal dolostones evaporites and inner middle shelf limestones 1 limestones experienced anaerobic sulfate reduction and subsequent incorporation of 34s enriched sulfate into cas during diagenesis while dolostones did not this is unlikely because of the lack of correlation between 34scas and toc as well as other indicators of diagenesis 2 dolomitization controlled the 34scas in proximal paleoenvironments where the source of the 34s depleted fluids was either continentally derived or the result of rayleigh distillation during evaporite formation and 3 a 34s depth gradient existed during the early triassic such that limestones formed in distal waters are more enriched in 34s versus evaporites and dolostones formed in proximal settings we do not favor this hypothesis because the strong correlation between ca mg and 34scas implies that dolomitization controls the 34scas in these samples results from subtidal well preserved non dolomitized limestones suggest that the 34s of spathian seawater sulfate may have been heavier than previously suggested from analyses of evaporite deposits alone