phosphate rich deposits associated with the mio pliocene unconformity in south east australia phosphates are present on the surface of the mio pliocene unconformity in the otway port phillip and gippsland basins of south east australia the phosphates occur as lenticular lag deposits and include reworked phosphatic intraclasts vertebrate bone and teeth in situ phosphatized burrows are also found in sediments of late miocene and early pliocene age the phosphatic intraclasts on the unconformity are interpreted as reworked phosphatized burrows derived from latest miocene sediments 6 to 5 ma the phosphatization of these intraclasts is temporally related to the unconformity the timing of phosphogenesis coincides with a period of transgression across the south east australian margin following late miocene uplift this transgression is responsible for initial marine erosion of the underlying miocene sequence creation of a period of very slow sedimentation that was favourable to phosphate formation and subsequent deposition of the latest miocene through to pliocene sediments the continental weathering of the uplifted highlands adjacent to the sedimentary basins global phosphorus enrichment in the late miocene oceans and localized upwelling may all have contributed to phosphatization in south eastern australia

sedimentology and c isotope geochemistry of the sturtian cap carbonate south australia abstract a combined sedimentary and isotopic analysis of the sturtian aged tindelpina cap carbonate in south australia has revealed a strong relationship between sedimentary facies and 13 c carb depositional water depths for the cap carbonate are difficult to constrain however a lack of diagnostic shallow water structures and the inferred magnitude of post glacial transgression several 100 m indicates a relatively deep water environment for each key facies the shallowest facies peloidal dolomite has 13 c values ranging between 3 6 and 0 3 in comparison to deeper water laminated limestones which have lighter values ranging between 5 5 and 3 5 equivalent calcareous shales deposited at basinal depths show even more 13 c depletion with values ranging between 6 7 and 3 7 the average difference in 13 c between the shallowest and deepest cap facies is up to 3 6 and may be an approximation of the seawater 13 c depth gradient at the time of deposition we interpret this high 13 c depth gradient as the result of prolonged physical ocean stratification both during and after deglaciation variations in the rates of upwelling mixing under this stratified regime may have significantly influenced the climate as well as controlling the precipitation and isotopic composition of the cap carbonate

c isotopic stratification in a neoproterozoic postglacial ocean we report c isotopic data from the cap carbonate overlying the terminal proterozoic nantuo diamictites in south china the c isotopic measurements reveal a large c isotopic gradient of 3 along paleoenvironmental transect from shelf to deep basinal sedimentary facies the c isotopic stratification may attribute to the effect of biological pumping in a postglacial ocean our results confirm that the sea level rise was rapid and that atmospheric co2 was significantly high immediately following deglaciation

the timing and environment of tepee formation in a marinoan cap carbonate thin carbonate units capping marinoan age glacial diamictites contain atypical tepee like structures that have been interpreted as evidence of seafloor dolomite cementation at parachilna gorge in the adelaide fold thrust belt a marinoan age cap dolomite contains paired tepee and growth fault structures that demonstrate the tepees grew 3 5 7 1 m below the sediment water interface in the early diagenetic zone although generally much larger such tepees and their associated sheet veins have an external morphology and internal structure identical to that of inter tidal tepee structures formed through expansive crystallisation on the other hand cap carbonate hosted tepees lack the size chaotic internal bedding carbonate crust and concretionary morphologies and expected carbon isotope geochemistry to be cold seep structures expansive crystallisation during early diagenesis seems the most likely means of forming tepees in cap carbonates the only cement common to all cap carbonate tepees is microcrystalline dolomite the crystallisation of this mineral phase drove both tepee growth and the fluid overpressure fracturing that formed the sheet veins the same microcrystalline dolomite is the component of cap carbonates routinely selected for isotopic analysis with the resulting 13c values assumed to be representative of coeval marine bicarbonate however the parachilna gorge tepees suggest that the microcrystalline dolomite contains both primary and early diagenetic components their separation is not achieved by current analytical methods

palaeomagnetism and magnetic anisotropy of late neoproterozoic strata south australia implications for the palaeolatitude of late cryogenian glaciation cap carbonate and the ediacaran system abstract the ediacaran global stratotype section and point gssp is defined near the base of the nuccaleena formation the cap carbonate overlying the terminal cryogenian glaciogenic elatina formation at a basin wide sequence boundary in the adelaide geosyncline south australia previous palaeomagnetic data for 205 specimens from the elatina formation yielded a structurally corrected mean declination d 208 3 and inclination i 12 9 95 4 2 palaeolatitude 6 5 2 2 and pole position at latitude p 47 3 s longitude p 359 3 e dp 2 1 and dm 4 2 new palaeomagnetic data for 141 specimens from the nuccaleena formation yield a structurally corrected mean direction of d 208 3 i 34 9 95 3 4 19 2 2 3 2 2 and pole position at p 32 3 s p 350 8 e dp 2 2 and dm 3 9 there is a substantial difference in inclination between the elatina and nuccaleena formations despite their juxtaposition positive fold tests for both formations seem to rule out remagnetisation of one or both formations fold tests on tepee like structures in the nuccaleena formation accord with the magnetisation having been acquired at or near the time the structures formed during early diagenesis magnetisation therefore variably lags deposition causing the three identified magnetostratigraphic horizons to appear highly bedding transgressive whereas local correlations may seem satisfactory regional correlations are not nevertheless the number and duration of polarity chrons at various localities appear valid implying that the nuccaleena formation accumulated over an interval of 10 5 years the question of compaction related inclination shallowing was addressed by examining the anisotropy of remanence using a superconducting 12 t magnet the effective flattening factors f determined were nuccaleena dolostones n 4 f 0 98 0 02 elatina tidal rhythmites n 6 f 0 97 0 01 elatina arenites n 9 f 0 92 0 02 the mean elatina inclination is corrected to 14 0 palaeolatitude of 7 1 and the mean nuccaleena inclination to 35 4 palaeolatitude of 19 6 inclination shallowing also was investigated using the elongation inclination method the distributions of directions for the elatina were somewhat skewed with respect to bedding possibly invalidating use of this method nevertheless for the elatina the method yields an inclination of 19 palaeolatitude of 10 for the nuccaleena the corrected inclination is 41 9 palaeolatitude of 24 these corrections do not materially alter the near equatorial palaeolatitude for the elatina formation which is supported by data for other neoproterozoic strata in southern australia however the substantial increase in palaeolatitude between the late cryogenian and the early ediacaran is maintained the sequence boundary below the nuccaleena formation is a disconformity to low angle unconformity that in places cuts deeply into the 1500 m thick late cryogenian glaciogenic succession and may represent a significant time gap marked by rapid polar wander such a hiatus may be an important factor in the juxtaposition of glacial deposits and cap carbonates

dolomite formation and biogeochemical cycles in the phanerozoic summary based on present knowledge of the purely chemical controls on the kinetics of massive dolomite formation the abundance and distribution of dolomite throughout the phanerozoic remains an enigma sometimes referred to as the dolomite problem comparing dolomite abundance to secular variation in seawater chemistry indicates that some changes in seawater chemistry are more likely to have resulted from extensive dolomitization rather than to have caused it the recently formulated microbial dolomite model provides the opportunity to view the geological history of dolomite formation from a new perspective a biogeochemical approach to the dolomite problem reveals a plausible connection between phanerozoic geochemical cycles and dolomite formation in particular periods of more extensive dolomitization broadly correlate with diverse indicators of decreased oxygen levels in the atmosphere and oceans lowered oxygen levels would have fostered a more active community of anaerobic microbes including sulphate reducing bacteria which in turn could have led to more extensive dolomitization of marine carbonates

characteristics of deep marine dolomite abstract deep marine sediment has not been subjected to the influences of meteoric waters deep burial or tectonic stress that have profoundly altered most rocks available to us in continental exposures hence it preserves the record of formation and early development of its dolomite evaluation of the dsdp x ray and smear slide data files indicates that dolomite is a common component in deep marine sediments it averages 1 and is found in all sampled ocean basins throughout post jurassic time dolomite abundance is not related to the abundance of any other detrital authigenic or alteration mineral except aragonite petrographic examination of 48 samples suggests that deep marine dolomite crystal size and appearance are similar to the size and appearance in modern supratidal deposits do omite does not increase in abundance or in crystal size downhole nor does crystal size increase with increasing age the dolomite is commonly nonstoichiometric avg 56 caco3 and poorly ordered globally it does not change in stoichiometry or order with depth or age dolomite in deep marine sediments does not originate from unusual or regionally important conditions it entered the sediments early most probably as a chemical precipitate from pore waters an uncertain amount estimated to be 10 is detrital

subtidal carbonate cycles implications for allocyclic vs autocyclic controls thick successions of repetitive metre scale subtidal carbonate cycles require that short term sedimentation rates approximate long term accumulation rates in order to maintain submergence below peritidal depths intrinsic processes such as storm and wave reworking and redistribution may act to inhibit aggradation into the zone of optimal carbonate production the presence of subtidal vs peritidal carbonate cycles may be a function of energy regime that in turn may relate to platform morphology the ultimate control on stacked subtidal cycles appears to be eustasy acting in concert with intrinsic processes such as storm and wave reworking

extreme winds and waves in the aftermath of a neoproterozoic glaciation the most severe excursions in the earth s climatic history are thought to be associated with proterozoic glaciations according to the snowball earth hypothesis the marinoan glaciation which ended about 635 million years ago involved global or nearly global ice cover at the termination of this glacial period rapid melting of continental ice sheets must have caused a large rise in sea level here we show that sediments deposited during this sea level rise contain remarkable structures that we interpret as giant wave ripples these structures occur at homologous stratigraphic levels in australia brazil canada namibia and svalbard our hydrodynamic analysis of these structures suggests maximum wave periods of 21 to 30 seconds significantly longer than those typical for today s oceans the reconstructed wave conditions could only have been generated under sustained high wind velocities exceeding 20 metres per second in fetch unlimited ocean basins we propose that these extraordinary wind and wave conditions were characteristic of the climatic transit and provide observational targets for atmospheric circulation models

low latitude and multiple geomagnetic reversals in the neoproterozoic puga cap carbonate amazon craton palaeomagnetic study of the carbonates that ubiquitously cap glacial deposits may constrain the latitudinal extent of neoproterozoic glaciations and the duration of the greenhouse recovery we present the first palaeomagnetic data on the neoproterozoic cap carbonates covering the amazon craton which are folded along the paraguay belt samples collected at deformed beds along the paraguay belt present a single polarity secondary magnetization acquired by the end of the brasiliano orogeny 540 520 ma in the cratonic area a dual polarity component was isolated in dolostones at the base of the sequence the presence of a stratabound reversal stratigraphy along with high unblocking temperatures strongly suggest that this magnetization is primary this result implies a low palaeolatitude 22 6 5 for the amazon block just after deposition of puga diamictites in addition the presence of multiple reversals across the first 20 m of the cap carbonate sequence suggests that their sedimentation must have spanned hundreds of thousands of years at least

enigmatic chambered structures in cryogenian reefs the oldest sponge grade organisms abstract previously undescribed chambered structures are common and widespread in the cryogenian post sturtian glacial carbonates of the oodnaminta reef complex adelaide geosyncline south australia the rasthof berg aukas formation namibia and the gauss formation namibia these carbonate structures have millimetre to centimetre scale chambers separated by well defined and generally thin micritic walls chamber walls now consist of dolomite but were probably originally aragonitic the chambers may have a lobate polygonal or dendritic morphology and are often further divided into smaller chambers chambered structures occur as reefal growth frameworks as cavity fillings in neptunean dykes and growth cavities and as intercolumnar material within stromatolite frameworks in the oodnaminta reef complex these structures are only present in the sub photic deep water framework these structures probably represent the calcified remains of an organism or community of organisms that was globally distributed and widespread for a significant time period following the sturtian glaciation no precisely analogous structures have been previously described from modern or ancient settings but the complexity and degree of organization suggests a significant evolutionary advance over older proterozoic fossils the closest morphological analogues for the structures are a some types of reef dwelling sponges and b some complex microbialites from archaean and paleoproterozoic carbonates the structures lack spicules and ostia found in sponges ruling out a true poriferan origin however it is plausible that they are proto sponges sponge grade organisms or complex microbial precursors to sponge grade organisms whatever their affinity we suggest these structures record a significant evolutionary event on the path towards organic complexity

characterization and interpretation of discontinuity surfaces in a jurassic ramp setting high atlas morocco discontinuity surfaces are widely recognized but often poorly understood features of epeiric carbonate settings in sedimentary systems these features often represent hiatus surfaces below biostratigraphic resolution and may represent a considerable portion of the time contained in the sediment record from an applied perspective discontinuities may represent horizontal flow barriers and result in reservoir compartmentalization here a total of 80 condensed surfaces s1 firmgrounds s2 and hardgrounds s3 from a jurassic middle and upper bajocian assoul formation ramp setting of the high atlas in morocco are carefully documented with respect to their morphology their secondary impregnation by fe and mn oxides and phosphates and their palaeoecological record a statistical frequency distribution of two surfaces of the s1 type 1 1 surfaces of the s2 type and 0 4 surfaces of the s3 type per 10 section metres is observed along a 220 m long carbonate succession based on two stratigraphically and spatially separated study windows and correlative sections the stratigraphic frequency distribution the lateral extent and the nature of facies change across discontinuities are documented in a quantitative manner specific features of the study site include the considerable stratigraphic thickness of the assoul formation and the conspicuous absence of subaerial exposure related features based on the data presented here firmground and hardground surfaces are best interpreted as maximum regression related features relative sea level lowstand results in a lowered wave base and wave orbitals and currents result in sea floor omission and lithification care must be taken to avoid overly simplistic interpretations as differences in bathymetry and carbonate facies result in marked changes in discontinuity characteristics in proximal distal transects the data shown here are of significance for those concerned with the interpretation of shoal water carbonate environments and are instrumental in the building of more realistic carbonate reservoir flow models

stratigraphy sedimentology and isotopic geochemistry of australian neoproterozoic postglacial cap dolostones deglaciation delta 13 c excursions and carbonate precipitation abstract thin d13c excursions in the geologic record they are interpreted to record a brief postglacial anomaly in ocean chemistry in australia the marinoan varanger equivalent 600 ma cap dolostone units exhibit many of the characteristics displayed by such deposits elsewhere around the globe including fine grained dolomitic mineralogy lateral persistenc at basinal scales thin laminae graded beds intervals with abundant marine cements crystal fans and a distinctive negative d13c isotopic signature of up to 5 pdb on the basis of these features the australian examples are interpreted here to be deeper water deposits below storm wave base resulting from an anomalous flux of inorganic carbonate to the sea floor during postglacial transgression detailed isotopic analysis of australian cap dolostones indicates d13c values ranging from 1 to 5 and gen rally becoming more depleted in 13c upsection trace element data indicate some diagenetic stabilization but textural evidence and the presence of similar profiles in different basins argue against a pervasive recrystallization event the range in d13c values between sections as well as more negative 13c values appear to correlate with greater paleobathymetry within basins this implies either 1 only partial preservation of the complete oceanic variation of d13c or 2 precipitation of this peculiar facies owes its genesis to basinal specific oceanographic processes such as proximi y of a given section to a postglacial upwelling zone in the later case d13c values would not represent whole ocean values stratigraphic constraints and paleoenvironmental interpretations suggest that much of this excursion lies within the geologically brief period of postglacial transgression this implies that the cause s of d13c variation likely operated on time scales significantly less than the residence time of carbon in the oceans 105 yr the continent wide and perhaps global nature of cap dolostones indicates that large volumes of carbonate were precipitated during the postglacial transgressive period such large scale carbonate precipitation over the hypothesized short time interval of postglacial transgression must have caused or have been the product of profound changes in the carbon cycle and global climate at tha time similar evidence for transgression and increased carbonate deposition in the holocene are attributed to the changing basin shape and ph of the oceans the coral reef hypothesis of berger 1982 cap dolostones may record a proterozoic equivalent of this process with the substitution of abiotic carbonate precipitation for skeletal precipitation by reef organisms

the paragenetic history of a marinoan cap carbonate abstract carbonates that cap late neoproterozoic glacial diamictites contain large atypical tepee like structures that have been interpreted as evidence for seafloor dolomite cementation the nuccaleena formation in the adelaide fold thrust belt is the stratotype marinoan age cap dolostone at parachilna gorge the nuccaleena formation contains paired cogenetic tepees and growth faults that demonstrate these structures grew well below the sediment water interface these tepees and their associated sheet veins were the result of pore fluid overpressure driven by expansive early diagenetic dolomite cementation fluid overpressure resulted in two forces 1 macroscale lateral compression that overcame the lithostatic weight to buckle sediments upward forming the tepee structure and 2 macro to microscale extension that overcame the lithologic tensile strength to form sheet veins the physical processes of fluid overpressure within the parachilna gorge cap carbonate are in many ways analogous to the fluid overpressure dynamics in mud volcanoes and were sufficiently high to generate veins extending 17 m below the cap carbonate overpressure driven dewatering removed pore fluid clay and organic matter from the dolomicrospar matrix into dewatering structures and fabrics the latter are hierarchically arranged from small linear clay fabrics to clay lined dendritic arrays to clay choked curvilinear veins to large sheet veins packets of red clay laminae were deposited when pore fluids vented to the sea floor the early diagenetic chemical evolution of the parachilna gorge cap carbonate was equally dynamic in which an initial reducing trend was supplanted by an oxidation trend brought about by tepee sheet vein deformation once established the sheet vein network also reinvigorated diffusional processes that likely enhanced early diagenetic cementation of the cap dolostone the detailed paragenetic history also suggests that calcite not dolomite was the initial carbonate precipitated from the post glacial adelaidean sea the nuccaleena formation exhibits the dolomicrospar isotopic signatures and tepee like structures that distinguish marinoan age cap carbonates at parachilna gorge these typical features are a product of early diagenesis suggesting that other marinoan age cap carbonates may also be early diagenetic in origin

an assessment of ocean margin anaerobic processes on oceanic alkalinity budget 1 recent interest in the ocean s capacity to absorb atmospheric co2 and buffer the accompanying ocean acidification has prompted discussions on the magnitude of ocean margin alkalinity production via anaerobic processes however available estimates are largely based on gross reaction rates or misconceptions regarding reaction stoichiometry in this paper we argue that net alkalinity gain does not result from the internal cycling of nitrogen and sulfur species or from the reduction of metal oxides instead only the processes that involve permanent loss of anaerobic remineralization products i e nitrogen gas from net denitrification and reduced sulfur i e pyrite burial from net sulfate reduction could contribute to this anaerobic alkalinity production our revised estimate of net alkalinity production from anaerobic processes is on the order of 4 5 tmol yr 1 in global ocean margins that include both continental shelves and oxygen minimum zones significantly smaller than the previously estimated rate of 16 31 tmol yr 1 in addition pyrite burial in coastal habitats salt marshes mangroves and seagrass meadows may contribute another 0 1 1 1 tmol yr 1 although their long term effect is not yet clear under current changing climate conditions and rising sea levels finally we propose that these alkalinity production reactions can be viewed as charge transfer processes in which negative charges of nitrate and sulfate ions are converted to those of bicarbonate along with a net loss of these oxidative anions

intense fracturing and fracture sealing induced by mineral growth in porous rocks article i nfo mineral precipitation in the pores of a rock may exert a force which is called crystallization pressure this process has been studied experimentally and results bring a new look on the way fractures may develop and seal in natural systems cylindrical core samples of porous limestone and sandstone were left for several weeks in contact with an aqueous solution saturated with sodium chloride at 30 or 45 c under axial normal stress in the range 0 02 0 26 mpa the fluid was allowed to rise in the core samples by capillary forces up to a controlled height where evaporation and precipitation occurred the uniaxial deformation of the samples was measured using high resolution displacement sensors the samples were characterized using computed x ray tomography allowing therefore imaging in 3d the intensity and localization of the damage two kinds of damage could be observed firstly small rock fragments were peeled from the sample surface secondly

late proterozoic submarine canyons of adelaide geosyncline south australia a well exposed series of major basin slope submarine canyons of late proterozoic age occur within the mixed carbonate and siliclastic wonoka formation of the adelaide supergroup adelaide geosyncline flinders ranges south australia they were cut to depths of about 1 5 km into an underlying deltaic and basin slope progradational sequence which includes the brachina formation and the abc range quartzite the canyons occur in close association with local synclinal structures an unfolded section across one canyon patsy springs canyon shows steep almost vertical canyon walls two axial channels and a tributary canyon the canyon filling sequence includes canyon wall and axial debris flow material axial channel conglomerates ripple cross laminated sands and silts hin and thick bedded turbidite sands and a series of spectacular sand filled megachannels which markedly truncate underlying units lenticular olistostromes of micritic carbonate clast breccias of carbonate platform derivation occur throughout canyon fill and immediately overlying strata the sedimentary sequence preserved in the canyons demonstrates an episodic process of canyon backfilling by onlap of an aggrading submarine fan sequence backfilled canyons were finally buried by progradation of a complex unit comprising interdeltaic carbonate platform delta and basin and slope environments evidence from the wonoka formation canyons suggests that initial canyon incision was triggered by a significant fall in relative sea level which allowed fluvial systems to deliver their bed loa s directly to the contemporary basin slope a subsequent relative rise in sea level was required during latter stages of canyon filling to account for delta and basin slope progradation of the wonoka formation

compilation and time series analysis of a marine carbonate δ18o δ13c 87sr 86sr and δ34s database through earth history abstract the sr s o and c isotope database of marine carbonates contains over 55 000 published isotope values of low mg calcite from diagenetically little altered phanerozoic fossil shells as well as samples of whole rocks and calcite cements of ordovician to archean age carbon and oxygen isotope data for the shell material are divided into habitat subsets high mid low latitude and deep sea and whole rock data are separated by mineralogy into calcite dolomite subsets trend correlation wavelet and spectral analyses on gaussian filtered isotope records were applied to detect and quantify similarities and patterns in temporal records with the following results 1 oxygen isotope trends from the high latitude and deep sea habitats are almost indistinguishable through the last 115 ma consistent with the existence of the oceanic conveyor belt throughout this interval 2 all oxygen isotope habitat records show a strong coherent 30 45 ma 38 ma cyclicity throughout the cretaceous and the cenozoic 3 up to 70 of the multi million year variability in the 18o record of the last 115 ma can be simulated by the following equation 18 o 0 64 sin 2 t 120 ma 0 9 x sin 2 t 38 3 ma 1 1 with x ranging from 0 4 0 6 for the low high latitude and deep sea habitats to 0 8 for the mid latitude realm 1 a 120 20 ma cycle occurs in the paleozoic and neoproterozoic 18o record consistent with paleoclimate variability as interpreted from sedimentological and faunal records 2 the offset of 13c values between deep water and high latitude vs surficial habitats at lower latitudes is consistent with the operation of a biological pump in the oceans since at least the cretaceous 3 sr and s isotope records exhibit a 60 70 ma cyclicity throughout the phanerozoic

detrital remanent magnetization in haematite bearing neoproterozoic puga cap dolostone amazon craton a rock magnetic and sem study summary neoproterozoic cap dolostones of the araras group mato grosso brazil deposited on top of glacial diamictites puga formation 630 ma yielded a dual polarity component providing al ow palaeolatitude for the amazon craton because palaeomagnetic negative directions are very close to the present day dipolar field a rock magnetic and sem study was conducted in order to check the primary nature of the remanent magnetization and to identify the nature and origin of the remanence carriers results show that detrital specular haematite grain size 10 m is the main magnetic carrier in the araras dolostones and was likely inherited from the underlying puga diamictites in some sites a mixture of haematite and magnetite carries the remanence of both polarities the primary nature of the magnetization is supported by the detrital character of the magnetic carriers in addition the remanence anisotropy is weak pointing to the lack of significant shallowing due to compaction these results reinforce the low palaeolatitude of the amazon craton at the time of the marinoan ice age

iridium anomaly from the acraman impact ejecta horizon impacts can produce sedimentary iridium peaks the observation of anomalously high iridium concentrations at the cretaceous tertiary k t boundary1 triggered the still unresolved debate concerning the origin of sedimentary ir anomalies several hypotheses have been presented including meteorite impacts1 2 and intense volcanism3 4 here we present data from the acraman impact ejecta horizon south australia5 which conclusively links high sedimentary iridium concentrations to a major meteoroid impact structure and its widely dispersed ejecta an ir anomaly up to 2 parts per 109 p p b ir as well as au pt pd ru and cr anomalies is present within the ejecta from the acraman impact event preserved in late precambrian 600 myr bp shales within the adelaide geosyncline the highest ir and cr concentrations within the ejecta are associated with the coarsergrained clasts of acid volcanics boulder to medium sand size suggesting that most of the ir is carried by these fragments the target rocks at the impact site are comparable acid volcanics that have very low ir au pt pd ru and cr concentrations indicating a meteoritic origin for the anomalous ir levels within the ejecta to our knowledge this is the first example in which an accepted widely dispersed impact ejecta blanket itself uniquely linked to a major terrestrial impact structure5 6 has been shown to be anomalously high in cosmogenic siderophile elements this finding strongly supports the hypothesis that ir anomalies in sediments can be produced by terrestrial meteorite impact events

tepees in peritidal carbonates origin via earthquake induced deformation with example from the middle cambrian of western canada abstract tepee structures are common in many ancient carbonate tidal flat successions and because analogues are known from tropical coastal areas they are well established as evidence for synsedimentary lithification of subaerially exposed carbonate sediment existing explanations involving desiccation thermal expansion and contraction fluctuating groundwater pore pressure or diagenetic force of crystallization do not adequately account for the style and degree of deformation seen in some examples such as those in the middle cambrian cathedral formation of the southern rocky mountains western canada these tepees provide evidence for essentially instantaneous events causing brittle failure of decimetre thick crusts of ooidal and pisoidal grainstone followed by their dilation rupturing separation tilting and shingling they can be explained as breccias generated by sporadic episodes of strong ground motion from earthquakes that emanated from a syndepositionally active fault system nearby variably directed tensile and compressive stresses broke crusts shifted them upwards and shuffled them laterally subsequent microbial encrustation and vigorous flushing by seawater in the resulting shelter cavities led to rapid stabilization by cementation recognition of the role of seismically induced deformation in explaining some tepee structures furnishes a proxy for synsedimentary tectonism and adds another member to the family of seismites which reflect a range of rheological properties of unlithified and lithifying sediments

a late holocene slip rate for the central north anatolian fault at tahtakopru turkey from cosmogenic 10be geochronology implications for fault loading and strain release rates 1 measurement of a stream offset and cosmogenic dating 10be of the alluvial surface into which the stream incised yield a preferred late holocene slip rate of 18 6 3 5 3 3 mm a 1 for the central part of the north anatolian fault naf at tahtakopru turkey use of variable cosmogenic production rate vpr models yields a slightly slower rate of 16 4 6 4 4 5 mm a 1 the offset drainage karanlik dere which flows southward almost perpendicular to the east west trace of the naf at the site is displaced right laterally by 55 10 m with no vertical displacement a 10be age of 3 ka 3 5 ka vpr from the top of a boulder on the best preserved part of the incised alluvial surface provides the most reliable maximum age for the onset of incision 11 10be ages from cobbles collected from the cultivated surface to the south yield younger ages consistent with exhumation erosion and mechanical mixing during plowing our 18 6 3 5 3 3 mm a 1 rate is similar to other geologic slip rates measured along the naf all of which cluster between 15 and 20 mm a 1 over a wide range of 103 105 timescales all of these geological rates however are slower than the 25 2 mm a 1 short term rate of elastic strain accumulation measured geodetically this disparity suggests the possibility that the naf is experiencing a strain transient in which the lower crust beneath the fault is deforming faster than its long term rate

diagenetic replacement controlled by force of crystallization many diagenetic replacements in sedimentary rocks occur along thin solution films between the authigenic and host phases rather than conventionally invoking bulk pore water undersaturation for host phase dissolution we propose that many diagenetic replacements occur by a force of crystallization controlled replacement mechanism whereby nonhydrostatic stresses resulting from authigenic crystal growth are principally responsible for host phase dissolution this model explains diagenetic features such as the restriction of host phase dissolution to authigenic host crystal contacts and euhedral authigenic crystal faces in planar contact with unreplaced host phases as well as the relative replacement tendencies of some common authigenic minerals

the sea level fingerprint of a snowball earth deglaciation cap dolostones are thought to represent deposition from seas transgressing over formerly glaciated continental margins during marinoan snowball deglaciation nevertheless facies associations within some cap dolostones indicate that an episode of regional regression punctuated these transgressive sequence tracts to date inferences of sea level change during and after the marinoan snowball deglaciation have been interpreted using simple qualitative arguments in the present study we explore the full spatio temporal variability of sea level change during snowball deglaciation and its aftermath using a gravitationally self consistent theory that accounts for the deformational gravitational and rotational perturbations to sea level on a viscoelastic earth model the theory is applied to model marinoan snowball deglaciation on a generalized ediacaran paleogeography with a synthetic continental ice sheet distribution we find that sea level change following a synchronous rapid 2 kyr collapse of snowball ice cover will exhibit significant geographic variability including site specific histories that are characterized by syn deglacial sea level fall or stillstand moreover some sites that experience syn deglacial transgression will continue to experience transgression in the post deglacial phase taken together these results suggest that sea level change recorded by strata capping snowball glaciogenic units may reflect a more complicated trajectory than previously thought including deposition that was not limited to the deglaciation phase these simulations as well as others that explore the response to asynchronous melting and deglaciation phases of longer duration demonstrate that an episode of regional regression interrupting a cap dolostone transgressive sequence tract may reflect one of several processes or their combination 1 near field adjustment associated with rapid local melting during an otherwise global hiatus in deglaciation 2 post glacial uplift of sites during a period of slowing deglaciation and 3 a transition at some sites from a sea level fall dominated by post glacial uplift to a phase of sea level rise due to eustasy and peripheral bulge subsidence throughout an extended order 50 kyr or greater snowball deglaciation

synsedimentary diagenesis in a cryogenian reef complex ubiquitous marine dolomite precipitation abstract analysis of the synsedimentary diagenetic phases from a pervasively dolomitised cryogenian 650 ma reef complex south australia reveals a fundamentally different style of marine diagenesis to that of phanerozoic carbonates textural evidence from dolomitised and undolomitised lithologies of the oodnaminta reef complex indicates that depositional components were precipitated initially as calcium carbonate aragonite and calcite dissolution textures and preserved fabrics in ooids indicate an originally aragonitic mineralogy early fibrous marine cements that have well preserved fibrous crystallographic fabrics with a length fast optical character are likely to have precipitated as high magnesium calcite however following this very early phase of diagenesis the dominant marine process occurring in these reefs was dolomite precipitation this involved the dolomitisation of earlier depositional and diagenetic phases as well as the precipitation of fibrous dolomite cements these fibrous marine cements include the newly described radial slow fascicular slow radiaxial slow and rhombic dolomite cements the cements have optical properties chemical zonation and cathodoluminescent characteristics indicating that they were direct marine precipitates dolomite precipitation during marine diagenesis in this reef complex suggests that the oceans of the cryogenian were chemically different to those of the phanerozoic abundant dolomite formation appears to be linked to anoxic magnesium rich seawater during this time marine dolomite precipitation under these conditions may explain the abundance of dolomite in the precambrian sedimentary record

combined flow origin of edgewise intraclast conglomerates sellick hill formation lower cambrian south australia unusually thick coarse grained edgewise intraclast conglomerates occur at eight or more horizons within subtidal nodular and ribbon bedded wackestones and packstones of the lower cambrian sellick hill formation south australia the intraclast beds are flat based and laterally discontinuous forming bar like structures that must have exhibited bathymetric relief of as much as 1 m the internal fabrics of these beds are variable thinner beds are dominated by flat lying intraclasts thicker beds contain both chaotic randomly oriented steeply inclined intraclasts and clusters of fan shaped vertically stacked edgewise intraclasts the sellick hill formation intraclast conglomerates are inferred to have been formed by intense storm generated combined flows on a broad subtidal carbonate ramp superimposition of wave induced oscillatory motions on geostrophic bottom flows during large storms generates short lived but exceptionally high instantaneous shear stresses in the bottom boundary layer entrainment of the relatively large intraclasts occurs through sliding rather than pivoting edgewise fabrics are a product of asymmetric acceleration and deceleration of intraclasts during passage of waves and the chaotic nature of collisions between intraclasts moving within the boundary layer collisions between intraclasts impart a rotating moment causing intraclasts to tip up during maximum fluid shear stress lodgement or packing of clasts in vertical or steeply inclined positions occurs within scours where intraclasts can wedge between other vertically inclined clasts or where intraclasts are pinned in steep orientations by collisions with shallowly inclined intraclasts differential erosional resistance of the intraclast deposits probably led to the development of sharp lateral changes in thickness the sellick hill formation intraclast conglomerates record erosion and reworking of subtidal subfairweather wave base environments by exceptionally intense and presumably rare storm flows the intraclast horizons represent a substantial loss in stratigraphic resolution due to widespread erosion of the ramp

controls on rock weathering rates by reaction induced hierarchical fracturing abstract weathering of rocks takes place through complex processes involving an intimate coupling between mechanical disintegration of the rocks and interactions between the exposed rock surface the hydrosphere and the biosphere it is still poorly understood how this coupling affects weathering rates and patterns produced by weathering here we show that the observed spheroidal weathering patterns in basaltic intrusions dolerites in the karoo basin in south africa are created through volume expanding reactions which set up stresses that not only lead to spalling of spheroidal layers of rock but also drive a large scale hierarchical fracturing process by which the dolerite is continuously undergoing domain division the fracture processes generates new reactive surface area in a self accelerating manner thus providing a first order control on the total weathering rate

the cryogenian balcanoona reef complexes of the northern flinders ranges implications for neoproterozoic ocean chemistry abstract the cryogenian balcanoona reef complexes of south australia consist of several dolomite platforms with well defined backreef reef margin and slope basinal facies the mature platforms have prograded large distances into the basin 15 km and have high relief margins over 1 km of relief at the platform margin backreef facies are dominantly ooidal peloidal grainstones with abundant tepees and marine cemented sheet cavities the reef margin facies consists of upper stromatolitic boundstones and where present deeper water non stromatolitic chambered and clotted boundstones the deepwater chambered and clotted boundstones grew on near vertical escarpments below 150 m paleo water depths and consist of a complex array of marine cemented cavernous and calcimicrobial material the strongly prograding platforms range from accretionary low angle stromatolitic boundstone margins through to more mature escarpment margins the mature escarpment margins 200 m high escarpment are characterized by the development of both stromatolitic and non stromatolitic boundstones the slope basin facies of the mature escarpment margins consists of coarse debris deposits talus debrites and allochthonous blocks proximal to the platforms and calcareous dolomitic mudrocks distal to the platforms the development of an accretionary escarpment margin together with the large proportion of deepwater boundstone suggests that these cryogenian reefs were fundamentally different from phanerozoic reefal systems the enormous volume of deepwater boundstone in these reefs may be due to a severely anoxic water column that prevailed during this period of the cryogenian anoxia led to greater degrees of carbonate saturation at depth that enabled the development of deepwater boundstones with escarpment margins a community of anoxic adapted non photosynthetic organisms that were perhaps similar in their ecology to phanerozoic coelobiontic reefal communities may have constructed the enigmatic deepwater boundstone just prior to the final termination of the reef the system was subject to large scale erosion and margin collapse producing a major regional unconformity following this the reefs briefly re grew before final and complete cessation of the reef complexes the large scale margin collapse event may have been triggered by sea level fluctuation and or changes in ocean chemistry associated with late cryogenian oxygenation

exceptional basal body preservation in some early triassic conodont elements from oman abstract conodont elements are composed of two main parts the crown and the basal body the basal body of a conodont element is a laminated structure that is less densely mineralized than the crown and as a result is rarely preserved elements with preserved basal bodies are found only in exceptional cases in the fossil record to date no s or m element with preserved basal body has ever been documented for post devonian conodonts which had raised the question of an evolutionary trend towards unmineralized basal bodies in conodonts here we report the discovery in smithian early triassic rocks of oman of neospathodid conodont elements whose basal body is partly preserved we demonstrate the presence of basal body in all elements including s and m elements of novispathodus and most likely of all gondolelloideans thereby suggesting that the absence of basal bodies in post devonian conodonts was due to a preservational bias only we further demonstrate that the morphology and extent of the basal bodies in novispathodus are in agreement with previous predictions and do not contradict a cyclostome like functional model of the apparatus based on a review of the occurrences of basal bodies in the literature we suggest a general pattern for the morphology of the basal bodies in conodont elements that may reflect the mechanical constraints associated with feeding motions

mineral growth in rocks kinetic rheological models of replacement vein formation and syntectonic crystallization abstract we model three cases of coupling between mineral growth kinetics and mechanical response of the rock i dispersed spherical crystals growing by replacement in a hydrostatically stressed elastic rock ii growth of veins or vein networks accommodated by viscous relaxation of surrounding rock and iii syntectonic crystallization in a rock undergoing bulk pure shear such models for the microscopic environment of mineral growth together with additional assumptions or knowledge about rheological behavior and aggregate geometry provide refined estimates of the behavior of a macroscopic volume element which could be combined with geochemical reaction transport models crucial in the models are the various consequences pressure solution creep fracturing of the local stress that is necessarily generated by mineral growth in rocks other than in pores in the first model the dispersed spherical crystals of mineral a are assumed to grow within a spherical volume of rock consisting of mineral b the mineralized zone mz itself embedded in elastic rock the macroscopic stress in the mz and the far field stress in the surrounding rock are uniform and hydrostatic mineral growth of the a crystals is driven by supersaturation with respect to mineral a is accommodated by replacement of b grains and leads to an expansion of the mz described by an infinitesimal strain the radial growth rate of a spherical crystal of mineral a with replacement of mineral b is da dt k a r tln a k b v 0 b k b v 0 b k a v 0 a where ka and kb are kinetic constants r is the gas constant t the temperature in kelvin and v0a and v0b are specific volumes reference saturation states of mineral a a 1 and host mineral b b 1 are specified at the far field hydrostatic stress 0 the microscopic environment of each crystal of a is modeled by a representative volume element rve consisting of a sphere of mineral a embedded in a spherical shell of mineral b in each rve stress is neither uniform nor hydrostatic the model links the local microscopic stress with the macroscopic stress in the mz and surrounding rock the second model refers to veins that make room for themselves by growing not to veins that form by cementation of previously opened or opening cracks if the growth of mineral a as propagating veins or as an interconnected vein network takes place by deformation and expansion of the mz with the deformation itself driven by the stress arising from the supersaturation driven growth the surrounding rock undergoes deformation and is treated as viscous the rate of growth in vein width w is dw dt 2 k a rtln a sv 0 a 1 m where m 16 kav0ac2 3b3 is the medium viscosity c is the vein radius and each vein is treated as centered in a spherical rve of radius b s is an effective tensile stress required for vein propagation for a vein network in which veins surround equant polyhedra of rock of radius b we set c b and drop the term in s for simplicity veins may also widen by replacement the ratio of vein widening by expansion to that by replacement is dw dt expansion dw dt replacement 3 b 16 3 b 16 k a v 0 a k b v 0 b k b v 0 b k a v 0 a both mechanisms of accommodation contribute equally when 3b 16kbv0b at higher viscosity replacement is dominant the incipient growth of a cylindrical crystal with circular cross section in a medium deforming in pure shear simulates syntectonic crystallization both dissolution and deformation of the host accommodate growth in the model the crystal tends to grow faster in the direction of maximum rate of extension in this direction the host mineral may either dissolve to accommodate growth or precipitate to form a pressure shadow accommodation by host dissolution is greatest in the direction of maximum rate of shortening

carbon sulfur and oxygen isotope evidence for a strong depth gradient and oceanic oxidation after the ediacaran hankalchough glaciation in order to understand spatial variations of stable isotope geochemistry in the quruqtagh basin northwestern china in the aftermath of an ediacaran glaciation we analyzed carbonate carbon isotopes 13ccarb carbonate oxygen isotopes 18ocarb carbonate associated sulfate sulfur 34scas and oxygen isotopes 18ocas and pyrite sulfur isotopes 34spy of a cap dolostone atop the ediacaran hankalchough glacial diamictite at four sections the four studied sections ykg mk h and zbs represent an onshore offshore transect in the quruqtagh basin our data show a strong paleobathymetry dependent isotopic gradient from the onshore to offshore sections 13ccarb values decrease from 2 to 16 vpdb whereas 18ocarb values increase from 4 to 1 vpdb both 34scas and 34spy show stratigraphic variations in the two onshore sections mk and ykg but are more stable in the two offshore sections h and zbs 18ocas values of onshore samples are consistent with terrestrial oxidative weathering of pyrite we propose that following the hankalchough glaciation seawater in the quruqtagh basin was characterized by a strong isotopic gradient the isotopic data may be interpreted using a three component mixing model that involves three reservoirs deep basin water surface water and terrestrial weathering input in this model the negative 13ccarb values in the offshore sections are related to the upwelling of deep basin water where anaerobic oxidation of dissolved organic carbon resulted in 13c depleted dic whereas sulfur isotope variations are strongly controlled by surface water sulfate and terrestrial weathering input derived from oxidative weathering of pyrite the new data provide evidence for the oceanic oxidation following the hankalchough glaciation

crystallization pressure versus crack seal as the mechanism for banded veins banded fibrous veins are often pointed to as evidence for episodic crack opening driven by oscillations in fluid pressure or bulk strain advances in understanding the geochemistry of precipitation data on veins and experiments suggest that pressure due to growing crystals may be an alternate explanation for many observations on these types of veins we propose that some veins originate at sites of precipitation and then propagate due to the pressure exerted by the crystal growth as materials precipitate the vein walls are pushed apart the resulting veins have shapes typical for mode i cracks except that mechanically crystallization pressure replaces the role of internal fluid pressure in their propagation a nonzero remote differential stress serves to align veins mechanical and geochemical considerations suggest that this process will be most important in fine grained rocks such as greenschist grade pelites where diffusion from sites of dissolution rather than advection is the dominant mass transport process veins may owe their orientation to tectonism but their initiation and growth are due to processes that supersaturate the pore fluid veins formed by this mechanism involve cracking to the extent that precipitation forces an original flaw to extend during precipitation cyclic quartz mica bands may indicate geochemical self organization at the vein wall driven by pressure solution enhanced supersaturation in the pore fluid and nonlinear precipitation kinetics at the vein wall

neoproterozoic marine dolomite hardgrounds and their relationship to cap dolomites abstract neoproterozoic cap dolomites are unusual and distinctive marker units that occur after large cryogenian glaciations several thin dolomite units that have features resembling cap dolomites occur within the cryogenian ediacaran succession of the adelaide geosyncline while each of these dolomite units has distinctive features they also share many similarities with cap dolomites including stratigraphic thickness ranging from several centimetres to several metres impure dolomicrite composition graded beds with a sharp top that become more dolomite rich upwards and evidence of volume expansion in the form of sheet cracks tepees or other soft sediment deformation structures the carbon isotopic compositions for these thin dolomite horizons have values ranging from 5 to 2 per mil vpdb which overlap those commonly found in both sturtian and marinoan cap dolomites the generally negative carbon isotope compositions of these dolomites and cap dolomites is most easily explained by mixing of marine and authigenic carbonate sedimentological evidence indicates the graded beds within cap dolomites and other dolomite units are a form of submarine hardground or firmground where dolomicrite is precipitated authigenically at or near the seafloor during periods of little or no sedimentation dolomite replacement of clay and other silicates is an important process in the formation of these dolomicrite firmgrounds volume expansion structures are a product of this near surface dolomicrite precipitation brought about by the high degrees of dolomite supersaturation with a consequent large force of crystallization and unlimited marine carbonate source for the precipitating dolomite we suggest that this deepwater authigenic dolomicrite precipitation is a result of the dominantly anoxic nature of cryogenian early ediacaran oceans and caused by alkalinity producing anoxic bacterial reactions when sedimentation rates slow sufficiently e g during transgressions thin dolomicrite horizons are produced in this hypothesis cap dolomites may largely be the result of sediment starvation combined with a possible increase in carbonate saturation during and following the post glacial transgressions associated with the sturtian and marinoan glacial events this scenario explains most of the sedimentological features of these unusual carbonates the widespread development of such authigenic carbonate would have profound implications for the interpretation of precambrian marine carbon isotopic compositions

storm dominated mixed carbonate siliciclastic shelf sequence displaying cycles of hummocky cross stratification late proterozoic wonoka formation south australia abstract the regressive late proterozoic wonoka formation of the adelaide geosyncline in south australia contains a thick mixed carbonate siliciclastic interval of middle to inner shelf origin the shelf was storm dominated and displays abundant hummocky cross stratification hcs of varying scales and other wave formed sedimentary structures some parts of this sequence are notably cyclic two kinds of cycles are described here the first average cycle 8 m thick consists of the alternation of silty and argillaccous limestone with fine grained hummocky cross stratified calcareous sandstone climatic cyclicity e g milankovitch cycles involving alternate wet and dry periods or minor sealevel fluctuations along a low gradient shoreline are considered the most likely cause of the observed sediment cyclicity carbonate deposition predominated during dry periods or high sealevel followed by influxes of sand from deltaic sources during wet periods or times of low sealevel the prevalence of hcs in the sandstones is most likely due to much slower rates of lithification in comparison to the limestones which give evidence of rapid lithification on the seafloor the second type of cycle average cycle 5 10 m thick is characterised by the cyclic appearence of red hummocky cross stratified glauconitic and phosphatic limestones within a sequence dominated by grey green stylonodular limestone tempestites the most likely origin of the red limestone involves contributions of volcanic ash fall material the resulting sediment did not undergo the rapid seafloor lithification of the associated grey green limestones and thus was involved in considerable storm wave reworking before deposition the micritic shelf carbonates of the wonoka formation are considered to represent direct inorganic precipitation into the middle shelf environment rather than reworking of detrital material from shallow carbonate banks

late neoproterozoic cap carbonates mackenzie mountains northwestern canada precipitation and global glacial meltdown the 3 27 m thick cap carbonate overlying marinoan ice brook formation glacigene sediments and keele formation carbonate and terrigenous clastic rocks consists of two distinctive stratigraphic units a lower splintery buff weathering microcrystalline dolostone of extensive lateral uniformity comprises mm laminated peloidal sediment with local low angle hummocky like cross stratification micro ridges and synsedimentary tepees all elongated per pendicular to depositional strike this dolostone is unconformably overlain by an upper limestone that exhibits pro nounced facies variation from inboard peloidal lime grainstone and mudstone to shelf edge cementstone to outboard lime wackestone and mudstone calcite cementstones range from isolated crystal fans in laminated limestone to huge decimetre scale crystal arrays to hemispherical and elongate crystal stromatolites wholly composed of acicular crystals that form decametre scale reeflike structures crystal stromatolites are precipitates and replaced microbiolites that con structed biostromes and bioherms like those on many flat topped reef rimmed platforms the calcite crystals have all the physical and chemical attributes of neomorphosed aragonite this aragonite extensively replaced sediment and microbiolite just below the sea floor and grew upward into the overlying water column such interpreted massive synsedimentary replacement is rare in geological history and attests to the highly saturated state of the immediate postglacial ocean all sediment is interpreted to have been caco3 originally low and constant 18 o values reflect diagenetic modification of these carbonates although chemical attributes such as sr and c isotopes in some lithologies are near pristine lower dolostones virtually identical to most other coeval marinoan caps worldwide were part of a global precipitation event of remarkable similarity upper limestones are a more local phenomenon deposited during sea level rise in an aragonitic sea returning to equilibrium after global glaciation low 87 sr 86 sr ratios and varying 13 c values with carbonate sedimentary facies indicate that both units must have formed relatively rapidly prior to significant flu vioglacial runoff or that the influence of this runoff on the chemistry of seawater along continental shelves was mini mal the cap carbonate is thus interpreted to have formed in two steps 1 during initial marine ice melting accompanied by oceanic overturn and upwelling preceding continental margin rebound and 2 during initial stages of sea level rise accompanying continental deglaciation while confirming brief but extensive carbonate precipitation from an ocean highly perturbed by global glaciation the rocks also suggest that this event did not permanently affect either late neoproterozoic ocean chemistry or the contained marine biosphere 1262

low latitude glaciation in the neoproterozoic of oman although earth is widely believed to have undergone a series of extreme low latitude snowball glaciations during the neoproterozoic ca 1000 543 ma only one reliable paleomagnetic result from elatina south australia places glacial rocks close to the equator we report new paleomagnetic data from the neoproterozoic huqf supergroup of oman that pass fold and reversal tests and yield a paleopole at 52 3 s 074 4 e n 25 sites 95 7 3 this paleopole places the muscat region of oman at a latitude of 13 in the late neoproterozoic and provides the first direct evidence that both glacial and overlying cap carbonate units were deposited in the tropics the presence of glacial interglacial cyclicity within the huqf supergroup indicates that areas close to the equator may have been largely free of ice at the time of deposition a result that is inconsistent with the classic snowball earth model our result precludes the possibility that contrasting lithologies mark a phase of rapid plate motion and provides the first evidence for low latitude glaciation in arabia a series of magnetic reversals in the fiq tillite and the overlying hadash dolomite in northern and central oman correlates well with a similar sequence in the mirbat formation in southern oman and indicates that recovery from glacial conditions took place over long time scales possibly 105 106 yr

a review of the origin and setting of tepees and their associated fabrics carbonate hardgrounds often occur at the surface of shallow subtidal to supratidal lacustrine and subaerial carbonate shelf sediments these are commonly disrupted and brecciated when the surface area of these crusts increases in the subtidal environment megapolygons form when cementation of the matrix causes the surface area of the hardgrounds to expand similar megapolygons form in the supratidal lacustrine and subaerial settings when repeated incremental fracturing and fracture fill by sediment and or cement also causes the area of the hardgrounds to expand the arched up antiform margins of expansion megapolygons are known as tepees the types of tepees found in the geological record include 1 submarine tepees which form in shallow carbonate saturated waters where fractured and bedded marine grainstones are bound by isopachous marine phreatic acicular and micritic cements the surfaces of these brecciated crusts have undergone diagenesis and are bored unlike tepees listed below they contain no vadose pisolites or gravity cements 2 peritidal and lacustrine tepees are formed of crusts characterized by fenestral pisolitic and laminar algal fabrics this similarity in fabric makes these tepees of different origins difficult to separate peritidal tepees occur where the marine phreatic lens is close to the sediment surface and the climate is tropical they are associated with fractured and bedded tidal flat carbonates their fracture fills contain geopetal asymmetric travertines of marine vadose origin and or marine phreatic travertines and or terra rossa sediments the senile form of these peritidal tepees are cut by labyrinthic dissolution cavities filled by the same material lacustrine tepees form in the margins of shallow salinas where periodic groundwater resurgence is common they include groundwater tepees which form over evaporitic boxwork carbonates and extrusion tepees which also form where periodic groundwater resurgence occurs at the margins of shallow salinas but the dominant sediment type is carbonate mud these latter tepee crusts are coated and crosscut by laminated micrite the laminae extend from the fractures downward into the underlying dolomitic micrite below the crust both peritidal and lacustrine tepees form where crusts experience alternating phreatic and vadose conditions in time intervals of days to years cement morphologies reflect this and the crusts often contain gravitational meniscus vadose cements as well as phreatic isopachous cement rinds 3 caliche tepees which are developed within soil profiles in a continental setting they are formed by laminar crusts which contain pisolites and fractures filled by micritic laminae microspar spar and terra rossa most of the cements are gravitational and or meniscoid in ancient carbonates when their cementation and diagenetic fabric can be interpreted tepee structures can be used as environmental indicators they can also be used to trace the evolution of the depositional and hydrological setting

the snowball earth aftermath exploring the limits of continental weathering processes article i nfo carbonates capping neoproterozoic glacial deposits contain peculiar sedimentological features and geochemical anomalies ascribed to extraordinary environmental conditions in the snowball earth aftermath it is commonly assumed that post snowball climate dominated by co2 partial pressures several hundred times greater than modern levels would be characterized by extreme temperatures a vigorous hydrological cycle and associated high continental weathering rates however the climate in the aftermath of a global glaciation has never been rigorously modelled here we use a hierarchy of numerical models from an atmospheric general circulation model to a mechanistic model describing continental weathering processes to explore characteristics of the earth system during the supergreenhouse climate following a snowball glaciation these models suggest that the hydrological cycle intensifies only moderately in response to the elevated greenhouse indeed constraints imposed by the surface energy budget sharply limit global mean evaporation once the temperature has warmed sufficiently that the evaporation approaches the total absorbed solar radiation even at 400 times the present day pressure of atmospheric co2 continental runoff is only 1 2 times the modern runoff under these conditions and accounting for the grinding of the continental surface by the ice sheet during the snowball event the simulated maximum discharge of dissolved elements from continental weathering into the ocean is approximately 10 times greater than the modern flux consequently it takes millions of years for the silicate weathering cycle to reduce post snowball co2 levels to background neoproterozoic levels regarding the origin of the cap dolostones we show that continental weathering alone does not supply enough cations during the snowball melting phase to account for their observed volume

neoproterozoic aragonite dolomite seas widespread marine dolomite precipitation in cryogenian reef complexes the extraordinary abundance of dolomite in the proterozoic challenges our understanding of precambrian marine environments here we show that synsedimentary marine dolomite precipitation was pervasive within cryogenian reef complexes from the adelaide fold belt south australia although these reefs are composed of dolomite textural evidence indicates an originally aragonitic mineralogy for depositional components in common with many other neoproterozoic carbonates allochthonous slope debris from the reefs invariably contains both limestone and dolomite clasts indicating synsedimentary dolomitization in the reefs we describe several new forms of fibrous marine dolomite cement from the reefs that have a length slow optical character these fascicular slow radial slow and rhombic dolomite cements have finely preserved cathodoluminescent growth zones and optical characteristics that indicate they originally precipitated as dolomite rather than replacing calcite or aragonite cements abundant early marine dolomite precipitation implies a radically different seawater chemistry for the cryogenian perhaps these aragonite dolomite seas are associated with extreme neoproterozoic glacial events and or ocean anoxia

submarine origin for the neoproterozoic wonoka canyons south australia abstract an examination of the deeply incised ediacaran wonoka canyons in the adelaide geosyncline most recently interpreted as subaerial valleys demonstrates their submarine origin and confirms them as some of the best examples of ancient outcropping submarine canyons in the world the entire canyon fill succession is interpreted to be of deep water below wave base origin consisting of calcareous shale and siltstone together with a variety of mass flow deposits including turbidites grain flows and debris flows the canyon fill lacks definitive shallow water structures e g mud cracks fenestral fabrics or wave ripples at all stratigraphic levels canyon lining carbonate crusts that have previously been interpreted as non marine calcretes or tufas and used to suggest a non marine origin for the canyons are argued to be of deep water marine microbial origin extremely negative carbon isotope values from the canyon fill and canyon lining crusts have a primary marine origin previously interpreted deepening upward trends in the canyon fill used as evidence of a subaerial erosion episode followed by drowning are suggested to be fining upward trends caused by the transition from canyon cutting to canyon filling with the majority of the fill being of deep water slope origin the basal conglomeratic canyon fill sediments represent the last vestiges of the high energy deep water canyon erosion environment in which the incisions formed a deep water origin for the canyons is consistent with all previous stratigraphic observations of the wonoka canyons including the conspicuous lack of regional unconformities in the lower wonoka formation and their emanation from the deep water facies of the wonoka formation a submarine canyon origin also removes the need for extreme 1 km relative sea level fluctuation and associated problems i e an enclosed basin with messinian style evaporative drawdown or thermal uplift above a migrating mantle plume required by the subaerial valley hypotheses

sequence stratigraphy and the interpretation of neoproterozoic earth history abstract the application of sequence stratigraphy to neoproterozoic successions is important for improving the resolution of time correlation within individual sedimentary basins and potentially at a global scale the methodology is illustrated in this paper by reference to two contrasting examples from the flinders ranges adelaide geosyncline of south australia where the younger part of the neoproterozoic to earliest cambrian succession 770 ma to 540 ma has been divided into thirteen unconformity bounded depositional sequences one of the most prominent sequence boundaries at or near the base of the wonoka formation is characterized by a series of buried canyons as much as 1 km deep high resolution sequence stratigraphic studies at umberatana syncline continue to support the view that the canyons were cut subaerially and filled by fluvial and shallow water sediments in contrast to the wonoka canyons sequence boundaries interpreted at the base of the nuccaleena formation seacliff sandstone and near the top of the abc range quartzite are relatively subtle with only limited evidence for erosion and valley incision four sequence boundaries at the level of the sturtian and marinoan varanger glacial deposits and in the vicinity of the precambrian cambrian boundary are thought to correlate with surfaces in the amadeus basin of central australia other prominent sequence boundaries including the wonoka canyons and surfaces within the upper part of the wonoka formation and at the base of the ediacara member of the rawnsley quartzite correspond with a relatively condensed section in the amadeus basin and their lateral persistence beyond the adelaide geosyncline is therefore difficult to evaluate given the lack of precision in biostratigraphy and isotope geochemistry in neoproterozoic rocks and in a marked departure from phanerozoic practice we recommend placement of a terminal proterozoic gssp at a sequence boundary a prime candidate in australia is the sequence boundary at the base of the nuccaleena formation seacliff sandstone immediately above the marinoan glacial rocks in the adelaide geosyncline and its likely correlative at or near the base of the gaylad sandstone in the amadeus basin

the elatina glaciation late cryogenian marinoan epoch south australia sedimentary facies and palaeoenvironments abstract the late cryogenian elatina glaciation in south australia of marinoan age is named after the elatina formation of the glaciogenic yerelina subgroup which covers 200 000 km2 in the adelaide geosyncline and on the cratonic stuart shelf and is up to 1500 m thick the elatina glaciation is marked by numerous facies that are like those of phanerozoic glaciations basal diamictite displaying glacitectonites with penetrative deformation of preglacial beds indicating grounded ice or scouring by icebergs glaciomarine diamictites containing numerous faceted and striated clasts up to several metres across of intrabasinal and extrabasinal origin laminated siltstone and mudstone containing scattered ice rafted dropstones in outer marine shelf environments sandstones deposited in fluvial deltaic and inner marine shelf settings tidalites deposited in estuaries and on tidal deltas during interstadial rise of sea level with cyclic tidal rhythmites recording the annual oscillation of sea level and displaying wave generated ripple marks which together indicate long lived and extensive open seas siltstone containing acicular crystal pseudomorphs implying the formation of evaporite minerals in littoral deposits permafrost regolith of frost shattered quartzite breccia 20 m deep displaying large scale periglacial structures including primary sand wedges 3 m deep that indicate a frigid strongly seasonal climate near sea level periglacial aeolian sandstone covering 25 000 km2 and containing primary sand wedges near its base these deposits record a spectrum of settings ranging from permafrost regolith and periglacial sand sheet on the stuart shelf in the west through fluvial deltaic and inner marine shelf in the western and central parts of the adelaide geosyncline to outer marine shelf in the north and southeast the widespread and persistent rainout of fine grained sediment and ice rafted debris indicates that the sea was not frozen over during the elatina glaciation no direct age determination is available for the elatina glaciation and only maximum and minimum age limits of 640 and 580 ma respectively can at present be applied high quality palaeomagnetic data for red beds from the elatina formation supported by positive fold tests on soft sediment slump folds that demonstrate the early acquisition of magnetic remanence indicate deposition within 10 of the palaeoequator the yerelina subgroup in the adelaide geosyncline and on the stuart shelf is disconformably to unconformably overlain by the nuccaleena formation cap carbonate that marks the early ediacaran post glacial marine transgression the presence in near equatorial palaeolatitudes of glaciomarine deposition grounded ice and permafrost near sea level a strongly seasonal periglacial climate and widespread open seas implies a paradoxical palaeoclimate and palaeogeographic setting for the elatina glaciation the strong evidence for a non actualistic late cryogenian glacial climate in south australia has been a stimulus to worldwide multidisciplinary research on cryogenian glaciogenic successions

facies dependent δ13c variation from a cryogenian platform margin south australia evidence for stratified neoproterozoic oceans abstract analysis of a cryogenian interglacial platform margin in the adelaide geosyncline reveals a strong carbonate 13 c facies relationship detailed chronostratigraphic correlation between sections ranging from shallow platform facies to deep basinal facies indicates the presence of a carbon isotopic gradient of between 8 and 11 in time equivalent strata shallow water back reef facies have 13 c values up to 8 2 while equivalent basinal sediments have 13 c values between 3 and 0 allochthonous blocks that have been transported from the platform margin into basinal environments retain their heavy 13 c values 4 9 and are surrounded by basinal calcareous shales with light 13 c values ave 0 8 the regional and stratigraphic consistency of these 13 c trends suggests a primary marine origin we interpret this 13 c facies correlation to be the result of ocean stratification stagnation that significantly reduced the rate of deep ocean ventilation and produced a large deep water organically derived carbon reservoir we suggest that stratification may have been a persistent feature of the global ocean throughout much of neoproterozoic time ocean stratification may explain many of the unusual features that characterise the sedimentary record of this era including large scale 13 c variation extreme climatic fluctuations and the presence of cap carbonates a highly variable climatic regime would be expected with the development of a large deep water carbon reservoir small changes in ocean circulation could rapidly transfer or remove large volumes of carbon to and from the surface ocean and atmospheric reservoirs leading to intense greenhouse or icehouse conditions respectively

an organodiagenetic model for marinoan age cap carbonates abstract the nuccaleena formation cap carbonate is the global stratotype for the marinoan glaciation and ediacaran gssp a designation that emphasises the importance of cap carbonates to our current understanding of global neoproterozoic biogeochemistry sedimentology and stratigraphy however to date there is no agreed depositional model for cap carbonates and there remains minimal detailed paragenetic data for developing process based depositional models here an early diagenetic organodiagenetic dolomite cementation model for marinoan cap carbonate formation is hypothesised and explored it is demonstrated how this process based model can explain the three main atypical sedimentary structures that are the basis for correlation of marinoan age cap carbonates giant tepee like structures sheet veins and tubestones these features are interpreted to be products of fluid overpressure deformation induced by organodiagenetic expansive cementation giant tepee like structures the hallmark of marinoan cap carbonates conform to a simple structural analysis that is consistent with the predicted stress field induced by expansive cementation driven fluid overpressure the undeformed to highly deformed gradient of deformation features in cap carbonates is consistent with the range of rheologies in a shallowly buried cementing carbonate modification of matrix textures and bedding surfaces also conforms to the range of expected textures within this model overall it is hypothesised that the aggradational condensed section architecture of cap carbonates was the primary control over the generation of these atypical sedimentary features the available paragenetic analysis suggests that the geochemical data particularly the c isotopic data can be reinterpreted as supportive of the organodiagenetic model and that the neoproterozoic ocean was perhaps similar to that of today the proposed model indicates that the current criteria for the placement of the ediacaran gssp are non unique and potentially non isochronous perhaps the most important aspect of this model is that it is testable and is a call for focused research on the much overlooked paragenesis of cap carbonates

globally synchronous marinoan deglaciation indicated by u pb geochronology of the cottons breccia tasmania australia u pb zircon data from the uppermost cottons breccia representing the marinoan glacial postglacial transition on king island tasmania provide the first direct age constraint on the cryogenian ediacaran boundary in australia zircons in four samples from the topmost meter of the cottons breccia dated by sensitive high resolution ion microprobe exhibit two modes ca 660 ma and ca 635 ma the younger component predominates in the uppermost sample a possibly volcanolithic dolomitic sandstone apparently lacking glacially transported debris in the transition to cap carbonate chemical abrasion thermal ionization mass spectrometry ca tims u pb dating of euhedral zircons from that sample yields a weighted mean age of 636 41 0 45 ma equivalence to published tims ash bed dates from cryogenian ediacaran transitional strata in namibia 635 51 0 82 ma within glacial deposit and china 635 23 0 84 ma 2 m above glacial deposit supports correlation of those strata to the australian type sections and globally synchronous deglaciation at the end of the cryogenian period

testing models for post glacial cap dolostone deposition nuccaleena formation south australia sedimentologically and geochemically distinctive carbonate sequences consistently drape the glacial deposits associated with the younger cryogenian ice age the presence of ice rafted debris in the basal dolostone implies that at least the lower portion of the cap sequence records deglaciation an isochronous model proposes that cap dolostones were deposited synchronously around the world regardless of water depth whilst a diachronous model proposes that deposition tracked glacioeustatic flooding during deglaciation the nuccaleena formation of the adelaide rift complex arc south australia exhibits many of the unique sedimentary features observed in other younger cryogenian cap dolostones around the world some bedforms are the product of wind driven oscillatory flow thus constraining post glacial dolostone deposition to 400 m these sedimentary features are absent in the deepest basinal facies in the northern arc suggesting that this region was below storm wave base even at the glacial sea level lowstand in the north flinders ranges there is a distinct relationship between lateral facies variability in the pre syn and post glacial sediments and the axes of 50 km scale structural folds the northern fold limbs are characterised by basinal facies whilst the southern limbs are associated with upper slope facies we interpret the abrupt facies transitions to reflect lower slope to upper slope shelf breaks across a series of linked south facing half grabens the majority of cap dolostone carbon isotope records show monotonic declines in 13c of 2 with starting points between 0 5 and 3 5 an isochronous model implies a 3 0 lateral gradient from platform to lower slope that varies dramatically on a short spatial scale and non systematically with palaeobathymetry if pre and syn glacial facies are used to infer palaeobathymetry and cap dolostones are deposited diachronously as sea level rises during deglaciation then 13c values becomes progressively lighter with time implying that cap dolostone deposition tracked the glacioeustatic sea level rise over a series of half grabens that deepened to the north the carbon isotope dataset cannot rule out uniquely isochronous or diachronous models given the high frequency spatial variability of 13c values temperature cannot be the dominant control on the isotopic variability of the cap dolostone

problematic fossils in the late neoproterozoic wonoka formation south australia abstract complex biogenic structures are described from the upper wonoka formation late neoproterozoic of the adelaide geosyncline south australia the structures are preserved within silty limestones and calcareous siltstones as dark bedding plane markings defining branching arrays of parallel elongate curved elements the fossils bear at least a superficial resemblance to the putative trace fossil palaeopascichnus normally preserved in siliciclastic sediments and reported from a number of late neoproterozoic sections worldwide the material from the wonoka formation is not a trace fossil and seems most likely to represent a surface encrusting alga the nature of palaeopascichnus needs to be carefully reassessed in the light of the possibility that the wonoka fossils represent a different preservational window into the same phenomenon the thin ca 2 m but widespread nature of the fossil horizon makes it a useful stratigraphic marker within the wonoka formation with the potential to aid in regional correlation within the adelaide geosyncline