calcium isotopes as a record of the marine calcium cycle versus carbonate diagenesis during the late ediacaran abstract calcium isotope ratios in ancient carbonate rocks can provide insight into the global marine calcium cycle as well as local conditions during carbonate mineral precipitation and diagenesis we compare two extraction techniques for the separation of calcium from other ions before 44ca analysis using an automated ion chromatograph and using manual gravity columns the two techniques produce the same 44ca within error 2 we present 31 44ca analyses of carbonate rocks from the nama group namibia which record a negative shift in 44ca of 0 35 between 550 and 547 ma from 1 25 to 1 60 followed by persistently low 44ca 1 48 0 06 between 547 and 539 ma very low 44ca

cap carbonate platform facies model noonday formation se california the neoproterozoic outcrop belt of the death valley region california preserves an oblique cross section of the noonday formation a mixed carbonate siliciclastic platform that hosts distinctive basal ediacaran cap carbonate affiliated sedimentary structures stromatolite textures and 13 c carb values the noonday platform encompasses two depositional sequences that reveal two cycles of relative sea level change within strata conventionally considered to record a single rapid postglacial sea level rise in updip localities facies of the first depositional sequence record the transition from a carbonate ramp to a stromatolite bearing tubestone textured reef rimmed platform downdip localities seaward of the reefal escarpment variably preserve a thin and condensed onlapping foreslope wedge base level fall exposed the reef crest to karstic dissolution and propagated submarine incised valleys into the seaward margin of the reef overlying strata record the backfilling of a submarine incised valley and reestablishment of a back stepping carbonate dominated ramp prior to a second subaerial exposure event that defines the contact between the noonday and johnnie formations we address the relative contributions of syndepositional tectonism and recovery from low latitude deglaciation in dictating noonday platform architecture and the intra noonday formation sequence boundary noonday formation deposition coincided with extension of the laurentian margin during disaggregation of the rodinian supercontinent within this framework previous work has suggested that the intra noonday formation sequence boundary records growth faulting that reinforced differential topography uplifting reef rimmed horsts exposing the reef crest to karstic dissolution and downdropping grabens however we trace the intra noonday formation sequence boundary seaward of the reef crest and demonstrate that for a time wave base was situated downdip of the reef escarpment on putatively downdropped fault blocks thus if the noonday margin were undergoing extension then the creation of the intra noonday formation sequence boundary required a concomitant decrease in accommodation due perhaps to postglacial isostatic uplift attendant with low latitude deglaciation we speculate that noonday formation sequence architecture records 1 immediate deglacial flooding 2 shoaling and exposure due to isostatic rebound induced by either a hiatus in meltwater flux or rapid ice sheet collapse against a background of global deglaciation and 3 resumed flooding following complete deglaciation as rift related tectonism could amplify or counter glacial isostasy inferences of the amplitude of local postglacial sea level change will require robust estimates of syndepositional extension across the noonday margin

predatorial borings in late precambrian mineralized exoskeletons the late precambrian tube forming cloudina the earliest known animal to produce a mineralized exoskeleton shows evidence of having been attacked by shell boring organisms of more than 500 tubes from shaanxi province china 2 7 have rounded holes 40 to 400 micrometers in diameter the relation between the size of the holes and the width of the bored tubes suggests that the attacking organism was a predator selecting its prey for size if true this would be the oldest case of predation in the fossil record and would support the hypothesis that selection pressures from predation was a significant factor in the evolution of animal skeletons around the precambrian cambrian boundary

exceptionally preserved microbially induced sedimentary structures from the ediacaran post glacial successions in the kimberley region northwestern australia abstract a set of exceptionally preserved microbially induced sedimentary structures miss is described from the post glacial yurabi formation of late ediacaran age in the kimberley region northwestern australia these miss comprise oriented grains bamboo leaf like cracks polygonal cracks worm like cracks sinuous cracks gas domes pyrite concentrated laminae and micrite laminae most of the yurabi miss are morphologically comparable with their ancient and modern counterparts micrometre or nannometre scale bacteria or biofilm like inorganic objects are detected from most miss and are interpreted as bacterial bodies or remains that have been replaced with inorganic minerals upon fossilization oriented grains indirectly support microbial control on their accumulation both pyrite conformable laminae and micrite laminae suggest degradation and decay of original microbial mats upon diagenesis sinuous cracks were likely generated in a soft sediment deformation process whereas other sand cracks bamboo leaf like cracks worm like cracks polygonal cracks could have formed from sediment surface layer microbial organics shrinkage upon desiccation gas domes may have formed as the result of anoxic redox reactions and being filled with gases and sediments transported from below two proxies bedding plane coverage percentage and accumulative thickness of single miss bearing beds are developed to semi quantitatively evaluate miss abundance on bedding plane and vertical outcrop respectively the yurabi miss have substratum preference to fine sandstone facies and intertidal settings the yurabi formation yields the most abundant and diverse post glacial miss assemblages during the late ediacaran worldwide both kimberley and global fossil records indicate that miss may have proliferated in shallow marine ecosystems soon after the retreat of ediacaran gaskiers glaciation during late neoproterozoic

ediacarian sponge spicule clusters from southwestern mongolia and the origins of the cambrian fauna carbon and strontium isotopic data are used to show that the earliest sponge spicule clusters and associated phosphatic sediments with anabarites from southwestern mongolia are of ediacarian age spicule morphologies include bundles of oxeas arranged in three dimensional quadrules linked together at junctions by tetracts pentacts hexacts or polyactines all are referred to the phylum porifera class hexactinellida these sponge spicules provide the oldest remains that can be assigned without question to an extant phylum and also the first firm evidence for filter feeding and metazoan silica biomineralization in the fossil record it is suggested that siliceous and phosphatic members of the cambrian fauna may have had their origins in eutrophic and outer shelf facies of the late proterozoic

digital reconstruction and stratigraphic evolution of a microbial dominated isolated carbonate platform terminal proterozoic nama group namibia abstract a terminal proterozoic ca 550 ma isolated carbonate platform developed downdip on a carbonate ramp of the nama group of southern namibia the stratigraphic evolution of the platform was reconstructed from an extensive dataset that was compiled by using digital surveying technologies and traditional outcrop observations digital field geology techniques enabled rapid and accurate mapping over the 10 km outcrop belt with spatial and geological information captured at scales ranging from centimeters to kilometers furthermore these digital geology techniques made the reconstruction and interpretation of the development of the carbonate platform considerably more efficient and quantitative the carbonate platform deposits encompass three accommodation cycles in which each subsequent cycle experienced progressively greater influence of a long term accommodation increase aggradation and progradation during the first cycle produced a flat topped sheet like platform deposits of the first cycle show a coarsening upward motif grading from dominantly columnar stromatolitic thrombolites and interbedded mudstone grainstone at the base to dominantly massive and cross bedded dolostones toward the top the second cycle features aggradation and is typified by the formation of a distinct margin and development of a bucket shaped geometry the margin contained thrombolite mounds and domes whereas columnar stromatolitic thrombolites dominated the platform interior the third and last cycle records a gradual deepening trend with initial aggradation and formation of well bedded thin deposits in the interior and microbial mounds at the margins while the interior drowned the platform margins kept up with rising sea level and complex pinnacle reefs formed which contained fused and coalesced thrombolite mounds flanked by bioclastic grainstones and collapse breccias the top of the unit consists of a set of isolated large thrombolite mounds flanked by shale and is interpreted to indicate the final stage of the carbonate platform before drowning the overall geometric evolution and spatial scale of this platform from flat topped to bucket shaped with elevated margins is recorded in many proterozoic and phanerozoic isolated carbonate platforms this study clearly illustrates that accommodation changes are a fundamental control on the development of carbonate platforms regardless of whether they are formed from microbial or metazoan components or are of precambrian or phanerozoic age significantly the architectural elements and spatial dimensions are comparable throughout earth history

microbial metazoan reefs of the terminal proterozoic nama group c 550 543 ma namibia thrombolite and stromatolite reefs occur at several stratigraphic levels within the terminal proterozoic nama group c 550 543 ma of central and southern namibia the reefs form integral parts of several carbonate platforms within the nama group including the kuibis platform of the northern nama basin zaris subbasin and huns platform witputs subbasin of the southern nama basin the reefs are composed of both thrombolites and stromatolites that form laterally continuous biostromes isolated patch reefs and isolated pinnacle reefs ranging in scale from a metre to several kilometres in width in the majority of cases the reefs occur stratigraphically as an integral facies within the transgressive systems tracts of sequences making up the kuibis and huns platforms this suggests that a regime of increasing accommodation was required to form well developed reefs though reefs also occur sporadically in highstand systems tract settings within a given transgressive systems tract a regime of increasing accommodation through time favours the transition from sheet like biostromal geometries to more isolated patch and pinnacle biohermal geometries similarly increasing accommodation in space such as a transect down depositional dip shows a similar transition from more sheet like geometries in updip positions to more isolated geometries in downdip positions reefal facies consist of thrombolitic domes columns and mounds with well developed internal clotted textures in addition to stromatolitic domes columns and mounds with crudely to moderately well developed internal lamination stromatolites are better developed in conditions of relatively low accommodation and updip locations under conditions of higher current velocities and greater sediment influx thrombolites are better developed in conditions of relatively high accommodation and low sediment influx both types of microbialites are intimately associated with the first calcifying metazoan organisms which may have attached themselves to the sediment surface or otherwise lived within sheltered depressions within the rough topography created by ecologically complex mats the appearance of thrombolitic textures during terminal proterozoic time is consistent with colonization of cyanobacterial mats by higher algae and metazoans which would have been an important process in generating clotted textures fabrics in the nama thrombolites are well preserved and show evidence of thrombolitic mesoclots being overgrown by fibrous marine carbonate interpreted as former aragonite this was followed by emplacement of geopetal micrite fills and precipitation of dolomite as an isopachous rim cement followed by occlusion of remaining porosity by blocky calcite spar

incipiently drowned facies within a cyclic peritidal ramp sequence early ordovician chepultepec interval virginia appalachians the chepultepec interval 145 to 260 m thick in virginia although dominated by peritidal carbonates contains thick subtidal sequences that formed during repeated incipient drowning of the early ordovician ramp cyclic peritidal sequences make up the lower member up to 150 m thick and upper member up to 85 m thick subtidal limestone and bioherms make up the middle member up to 110 m thick and pass landward into cyclic peritidal facies average long term subsidence rates were up to 4 cm 1 000 yr mature passive margin rates and ramp gradients commonly were extremely low 2 cm km computer modeling helped define mechanisms involved in formation of the stratigraphic sequence the peritidal sequences are upward shallowing cycles average period 140 000 to 200 000 yr of pellet and skeletal limestone pellet grainstone thrombolites intraclast grainstone and laminite caps these resulted from periodic sea level oscillations of several metres amplitude and possible slow long teim third order sea level fall superimposed on long term subsidence tidal flats were shifted far westward after each sea level rise but had sufficient time to prograde back across the ramp during sea level fall the middle number contains repeated sequences of subtidal skeletal limestone and pellet limestone with storm beds and burrowed units formed below fair weather wave base local rippled pellet limestone and thrombolite bioherms subwave base to wave agitated shoals that may have erosional tops overlain by thin grainstone sheets tidal flat facies are absent these sequences formed by periodic sea level rise more than 12 to 17 m amplitude which caused incipient drowning of the platform and migration of tidal flats several hundred kilometres back onto the platform these flats were unable to prograde onto the outer ramp during regression because of insufficient time and because seaward retreat of the shoreline outpaced progradation of tidal flats this left them abandoned on the inner ramp similar sequences reflecting incipient drowning are widespread in the geological record

extinction of cloudina and namacalathus at the precambrian cambrian boundary in oman biostratigraphic carbon isotope and u pb zircon geochronological data from the ara group of oman indicate an abrupt last appearance of cloudina and namacalathus coincident with a large magnitude but short lived negative excursion in the carbon isotope composition of seawater that is globally coincident with the precambrian cambrian boundary u pb zircon age data from an intercalated ash bed directly define this negative excursion to be at 542 0 0 3 ma consistent with previous age constraints from siberia and namibia combined with the global biostratigraphic record these new data strengthen hypotheses invoking mass extinction within terminal proterozoic ecosystems at or near the precambrian cambrian boundary

the metamorphic history of the damara orogen based on k ar data of detrital white micas from the nama group namibia abstract detrital white micas from molasse sediments of the nama group were dated by the k ar method the kuibis subgroup and the underlying pre damara sinclair sequence contain detrital muscovite with ages of 1100 1000 ma and thus cannot be derived from the damara orogen but prove to have originated from a basement probably affected by the kibaran orogeny muscovite size fractions of 250 100 m from the upper nama group upper schwarzrand and fish river subgroups yielded k ar ages 670 570 ma because these sediments are regarded as molasse series of the damara orogen these k ar ages can be interpreted as cooling ages of a first early tectono thermal event in the damara orogen a later damaran metamorphic overprint which peaked at about 530 ma is not documented in the detrital content of the nama group at the present level of erosion because detrital micas with k ar ages over 600 ma have been found up to the uppermost fish river subgroup it can be concluded that at least two tectono metamorphic events followed by uplift and erosion have taken place during the development of the damara orogen thermal alterations of the nama sediments were detected by means of illite crystallinity and k ar dating of clay size fractions these data as well as the almost exclusive occurrence of the 2m mica polytype in these size fractions point to a diagenetic to very low grade metamorphic alteration of the area investigated at about 530 500 ma and younger the deposition of the upper nama group is younger than the 570 ma detrital white micas but occurred before the 530 500 ma thermal alteration of the nama deposits the deposition of the lower nama group probably took place between 635 and 570 ma

structure and diversity of oldest sponge microbe reefs lower cambrian aldan river siberia the oldest sponge reef is a small early cambrian bioherm at the base of the tommotian stage 535 540 ma in southeast siberia the mainly archaeocyath construction may be a response to turbid conditions cambrocyathellus bowls fused to create a rigid cavernous frame colonized by cryptic archaeolynthus and calcified microbes renalcis in addition to these constructors and binders other reef guilds are present bafflers other archaeocyaths spiculate sponges and hyoliths and dwellers hyoliths mollusks and many others this is the oldest known reef possessing an open skeletal frame structure built by animals and a mixed animal autotroph composition it provides a blueprint for younger phanerozoic reefs

quasi planar laminated sandstone beds of the lower cretaceous bootlegger member north central montana evidence of combined flow sedimentation abstract lower shoreface to shallow shelf strata of the bootlegger member of the lower cretaceous blackleaf formation are characterized by interbedded sandstone and mudstone sandstone beds are characterized by a peculiar planar lamination showing a subtle although perceptible undulation spacing to height ratios of the undulation are generally 100 or more typically the undulation shows no evidence of lateral accretion but only vertical aggradation and as a result most beds consist of a single laminaset aspects of quasi planar laminated beds indicate single event storm sedimentation and paleocurrent data indicate offshore sediment transport by its sedimentary characteristics and its similarity with a bed configuration generated in un experimental wave duct quasi planar lamination is produced by high energy combined flows this style of stratification should be common in the shallow marine stratigraphic record and its recognition should aid in interpreting high energy combined flow depositional events

experiments on bed configurations in fine sands under bidirectional purely oscillatory flow and the origin of hummocky cross stratification abstract we made experiments on bed configurations in bidirectional purely oscillatory flows at a wide range of oscillation periods t and maximum orbital velocities um in two closed flow ducts 0 1 m and 0 4 m wide the motivation was that understanding of bed configurations in oscillatory flows with long periods and high velocities during storm induced deposition in the shallow ocean is needed for interpreting the stratification in such deposits the ripples produced were classified geometrically as two dimensional 2d or three dimensional 3d in plan view geometry and dynamically as oscillatory current ripples dependent upon the patterns of flow and sediment transport specific to the existence of the oscillation itself and reversing current ripples dependent on he existence of the current in one direction or the other during the oscillation runs in the small duct with effectively 0 19 mm and 0 30 mm sand showed the familiar small 2d oscillatory current ripples at short t and low um passing into much larger ripples with spacing up to 2 m at periods of 6 20 s and velocities up to 1 m s with no apparent limit on size with increasing t and um even in such a narrow duct there was some evidence of 3d geometry of the large ripples runs in the large duct with effectively 0 11 mm sand at a single period of about 9 5 s showed a progression from small 2d oscillatory current ripples through small 3d reversing current ripples to large round crested 3d oscillatory current ripples with prominent superimposed 3d to 2d reversing current ripples the large 3d ripples which increased in spacing to 1 3 m at the h ghest attainable um of 1 m s shifted in position and changed in size with time seemingly at random causing substantial local and temporary erosion and deposition at a given point on the bed there was no net aggradation during the runs but a deposit was synthesized by taking a time series of sidewall bed profiles during a run in which large 3d ripples developed from an initially plane bed and changed irregularly in size and position as they grew toward equilibrium and then plotting the profiles one above the previous with a constant upward increment the resulting stratification is strikingly similar to much hummocky cross stratification in ancient sandstones this suggests that some hummocky cross stratification is generated during sediment fallout from strong purely oscillatory flows at moderate to long oscillation periods as large 3d oscillatory current bed forms develop from a planar bed during strong but waning flow

mesoproterozoic deep water reefs from borden peninsula arctic canada a superbly exposed stromatolite reef complex occurs in the victor bay formation near strathcona river on northern baffin island individual reefs are up to 130 m thick and nearly 1 km in length and their development was clearly related to their position in the facies spectrum and to sea level dynamics in the first sea level cycle metre scale reefs grew amongst mid ramp calcarenites and outer ramp shales during slow sea level rise a 25 m thick oblate reef tract separating mid ramp and outer ramp facies formed during the highstand the greatest period of reef growth was during the second sea level cycle pinnacle reefs nucleated on the karsted upper surface of the oblate reef tract and aggraded rapidly in response to rising sea level producing structures with more than 75 m of depositional relief a gradual symmetrical succession of stromatolite growth forms from stratiform to cylindrical columns to conical columns and then back through cylindrical columns to stratiform is mirrored by evidence in offreef deposits for deepening to a maximum flooding surface and then shallowing the tops of these high standing reefs were karsted during the following regression while dolomite cryptodomes grew as sheets on their submerged flanks and as progradational tongues extending basinward of the reefs continued sea level fall resulted in subaerial exposure of the entire reef complex and the extensive formation of surface and subsurface karst these proterozoic slope buildups are similar to phanerozoic deep water reefs in size shape prevalence of synsedimentary lithification presence of neptunian dykes and in their well developed vertical zonation of reefbuilders however they differ in being constructed exclusively by stromatolites rather than being mud mounds with small skeletal elements and in lacking halos of perireefal sand and gravel sized calcareous debris their responses to changes in sea level were strikingly similar to those shown by their younger counterparts and suggest that sequence stratigraphic concepts derived from studies of phanerozoic reefs can also be applied to the proterozoic

environmental covariation of metazoans and microbialites in the lower ordovician boat harbour formation newfoundland abstract an antagonistic view of the relationship between microbialites and metazoans has long been inferred in part because of the large scale anticorrelation of these two groups through geologic time the nexus of this relationship occurs in the early paleozoic era stromatolites declined in abundance as complex animals and algae diversified but thrombolites a type of microbialite little known before the proterozoic cambrian boundary proliferated for the first time well preserved parasequences in the basal portion of the lower ordovician boat harbour formation western newfoundland contain a succession of stromatolites and thrombolites that permit an investigation into the role metazoans played in shaping the nature and abundance of microbialites in early paleozoic carbonate seas sessile benthic animals colonized thrombolite surfaces but are nearly absent from stromatolites bioturbation rarely co occurred with microbialites but is widespread in clastic carbonates that lack microbialites our results thus support the hypothesis of ecological antagonism between microbial communities and motile benthic animals but also demonstrate biological facilitation between thrombolites and both sessile benthic animals and nekton

carbonate ramp depositional systems abstract the classification tectonic settings stratigraphy and early diagenesis of carbonate ramp systems are reviewed carbonate ramps are common in all geological periods but were dominant at times when reef constructing organisms were absent or inhibited ramps can be subdivided into inner mid and outer ramp environments the mid ramp zone extends from fair weather wave base to normal storm wave base although the water depths which these boundaries represent vary an additional outer slope environment occurs on distally steepened ramps as with siliciclastic shelves a range of wave storm and tide dominated ramps can be recognised and this forms the most convenient basis for ramp classification the carbonate productivity profile of ramps differs from that of rimmed shelves with the inner ramp showing lower production rates than comparable shallow water facies on rimmed shelves the zone of greatest organic carbonate sediment production appears to have shifted from the mid ramp to the inner ramp since the late jurassic carbonate ramps occur in most types of sedimentary basin but are best developed where subsidence is flexural and gradients are slight over large areas as in foreland and cratonic interior basins and along passive margins the featureless depositional profiles of many ramps means that sequence geometries are best observed on regional seismic lines some show low angle sigmoidal or shingled clinoforms suggesting that ramps may seldom be homoclinal but possess subtle slope geometries which reflect depositional environments because of their low angle slopes ramps respond differently to rimmed shelves during relative sea level changes although results seem to be strongly dependent on the rate of relative sea level change during a minor fall shallow ramp facies belts will simply shift basinwards in a forced regression in contrast the whole surface of a steep sloped flat topped rimmed shelf may be exposed so that sediment production ceases or is drastically reduced during a major fall shallow ramp bounded basins may empty completely conversely ramps also flood gradually whereas rimmed shelves do so more rapidly homoclinal ramps develop no resedimented lowstand deposits rimmed shelves and distally steepened ramps in contrast may develop lowstand talus or turbiditic wedges distally steepened ramps may behave more like homoclinal ramps during minor base level falls and like rimmed shelves during major base level falls many ramps consist of layered successions of several ramp sequences stacked one upon the other ramp stacks of this sort may show gross vertical accretion but individual ramp sequences seldom appear to develop in a keep up style apart from minor organic buildups as with many rimmed shelves steepening of the outer ramp margin due to tectonism slope inheritance or differential sedimentation may promote the development of a distally steepened ramp or rimmed shelf a wide variety of organisms have constructed buildups in mid and outer ramp environments isolated buildups may seed early in ramp development accrete to wave base or sea level and continue growth by stacking through successive ramp sequences so that depositional and diagenetic features within them are in concert with those of the shallow ramp the location of isolated buildups on ramps is governed by tectonism halokinesis antecedent topography or by the subtle slope geometry of the previous ramp sequence diagenesis on ramps shows some major variations compared with diagenesis on steep sloped flat topped carbonate platforms ramp bounded basins may form prolific petroleum sourcing and reservoiring systems and offer a range of subtle stratigraphic play types and lateral facies variations which determine reservoir quality and distribution isolated buildups in the mid and outer ramp environments represent one of the commonest petroleum reservoirs in ramp systems and tend to have their foundations in transgressive systems tracts grainstone and packstone reservoirs are widespread and range from shoreline carbonate sandbodies to major detached shoal complexes or shoals over offshore highs grainstone sandbodies occur in both highstand systems tracts and in prograding lowstand wedges

microbial communities and their primary to early diagenetic mineral phases the record from neoproterozoic microbialites of qarn alam oman

sequence stratigraphy and sedimentology of the neoproterozoic kuibis and schwarzrand subgroups nama group southwestern namibia abstract the kuibis and schwarzrand subgroups of the nama group form a succession of shallow marine and minor fluvial sedimentary rocks that is exposed over much of central and southern namibia ediacaran type body fossils and the stratigraphic carbon isotope variability indicate a vendian age for much of these strata the precambrian cambrian boundary is in the uppermost part of the schwarzrand subgroup radiometric dating of abundant volcanic ash beds which span much of the vendian and extend into the cambrian part of the section could make the nama group the best calibrated reference section for vendian chronostratigraphy here we describe the sedimentology and sequence stratigraphy of exposures of the kuibis and schwarzrand subgroups in southwestern namibia we place tighter environmental constraints on the paleontology and geochemistry and we identify seven depositional sequences the boundaries of the depositional sequences correspond to the more important disconformities in the kuibis and schwarzrand subgroups their identification is crucial for defining rates of organism evolution and isotopic differentiation

chambered structures from the ediacaran dengying formation yunnan china comparison with the cryogenian analogues and their microbial interpretation enigmatic chambered structures have been reported forming reef frames in cryogenian interglacial carbonates prior to the commonly acknowledged microbial metazoan reefs at the terminal ediacaran and interpreted as fossils of possible sponge grade organisms a better constraint on the affinity of these structures is partly hindered by few analogues in other time periods this study describes similar structures from peritidal dolostones of the ediacaran denying formation from yunnan china samples were investigated using optical microscopy and three dimensional 3 d reconstruction based on grinding tomography the dengying chambered structures are comparable with cryogenian structures in basic construction but are not frame building and show variations in overall shape and inhabiting facies two dimensional 2 d cross sections show that thin homogeneous micritic laminae are the basic building blocks of the chamber walls thick walls represent parallel accretion of these laminae and thin walls developed from the angular growth of a single lamina or merging of multiple laminae in 3 d space the laminae primarily correspond to continuous surfaces which sometimes contain sub circular holes while a few represent filamentous elements connected to the surfaces the morphological features and growth pattern of the dengying chambered structures indicate that they are likely to be calcified microbial constructions rather than skeletal remains of basic metazoans however aside from the cryogenian and dengying examples comparable chambered constructions with laminae based architecture are yet unknown from other fossil or extant microbialites further work investigating related structures is needed to determine the microbial consortia and controlling environmental factors that produced these chambered structures

cloudina microbial reef resilience to substrate instability in a cadomian retro arc basin of the iberian peninsula abstract the navalpino anticline of the central iberian zone provides a unique case study of well exposed late ediacaran reefs drone images show their spatial arrangement as tens of shelly microbial patch biohermal and complex reefs encased in a dominantly shaly succession the latter was deposited on a perturbed block platform located in a cadomian retro arc basin fringing west gondwana reefs display the traditional core flank inter reef subdivision and developed on generally low energy clear water substrates between normal and storm wave base cores consist of millimetre sized irregular mesoclots of dense microbial microsparite locally interrupted by laminated stromatolitic fabrics containing abundant sinotubulites and cloudina shells the last with upright preservation basement breakdown perturbations related to fissuring hydrothermal influence and slope apron breccia deposition led to nucleation and development of patch reefs on unstable substrates their ecological succession is characterized by stabilization pioneer to colonization phases the latter represented by cloudina shells preserved upright and encased in microbial crusts pointing to their active role as pioneer frame building taxa

neoproterozoic reef microstructures from the little dal group northwestern canada basinal facies in the neoproterozoic little dal group contain large reefs up to 300 m in height and 8 km in diameter in contrast to the stromatolitic composition typical of earlier proterozoic buildups little dal reefs consist mainly of three different calcareous microfossils a tubule thread microfossil analogous to the calcimicrobe girvanella in paleozoic reefs formed laminar reticulate structures like those in ordovician bioherms a clotted to saccate microfossil comparable morphologically to paleozoic renalcis constructed botryoidal to encrusting masses an enigmatic component of multicellular appearance that has attributes of an early calcified metaphyte exhibits the encrusting sheetlike habit characteristic of middle to late paleozoic algae these reefs represent an intermediate stage between proterozoic stromatolitic buildups and paleozoic metazoan calcimicrobial reefs and biogenic mounds

shell structure and distribution of cloudina a potential index fossil for the terminal proterozoic cloudina bearing biosparites and biomicrites in the lower part of the nama group namibia contain a wide morphological diversity of shell fragments that can all be attributed to the two named species c hartmannae and c riemkeae the curved to sinuous tubular shells of cloudina were multi layered each shell layer was 8 to 50 micrometers thick and in the form of a slightly flaring tube with one end open and the other closed growth appears to have been periodic with successive shell layers forming within older layers each added layer was slightly elevated from the previous layer at the proximal end and was asymmetrically placed within the older layer so that only a portion of the new shell layer was fused to the previous layer this type of growth left a relatively large unminerialized area between the shell layers which was often partially or fully occluded by early marine cements the thin shell layers exhibit both plastic and brittle deformation and were likely formed of a rigid caco3 impregnated organic rich material often the shell layers are preferentially dolomitized suggesting an original mineralogy of high magnesian calcite both species in the nama group formed thickets or perhaps bioherms and this sedentary and gregarious habit suggests that cloudina was probably a filter feeding metazoan of at least a cnidarian grade of organization the unusual shell structure of cloudina gives rise to a characteristic suite of taphonomic and diagenetic features that can be used to identify cloudina bearing deposits within the nama group and in other terminal proterozoic deposits around the world species of cloudina occur in limestones from brazil spain china and oman in sequences consistent with a latest proterozoic age assignment in addition supposed lower cambrian pre trilobitic shelly fossils from northwest mexico and the white inyo mountains in california and nevada including sinotubulites nevadatubulus and wyattia are all either closely related to or con generic with cloudina hence it is probable that these outcrops are latest proterozoic in age and that cloudina or cloudina like organisms were widely distributed at that time it is possible moreover to suggest that metazoan biomineralization occurred on a global scale by the latest proterozoic at the same time that evidence for complex multicellularity and locomotion in animals appears in siliciclastic ediacaran rocks in the form of body and trace fossils

a composite reference section for terminal proterozoic strata of southern namibia integrated sequence stratigraphic and chemostratigraphic data yield a framework for correlations of stratigraphic units in the terminal proterozoic to cambrian witvlei and nama groups of namibia coupled with precise u pb zircon age constraints these correlations make it possible to construct a composite reference section for use in calibrating terminal proterozoic chronostratigraphy the namibian reference section starts with two distinct glacial horizons and extends up to within 1 million years of the proterozoic cambrian boundary the two glacial horizons may represent each of two distinct varanger age glaciations better known from the north atlantic region from the higher of the two glacial horizons up the composite stratigraphy preserves one of the thickest and most complete available records of carbon isotope variability in post varanger terminal proterozoic seawater four carbon isotope chemostratigraphic intervals are recognized 1 a postglacial negative 13 c excursion npg interval 2 a rising interval pr interval of increasing positive delta 13 c values 3 a falling interval pf interval characterized by decreasing positive 13 c and culminating in near zero or negative values and 4 an interval of moderately positive relatively invariant 13 c values i interval that extends up to the unconformity that contains the proterozoic cambrian boundary each of these chemostratigraphic intervals can be recognized in widely separated correlative sections around the world by comparing sediment accumulation rate in the radiometrically calibrated namibian stratigraphy with sediment accumulation rates in correlative sections in arctic canada and oman a maximum age of 564 ma is estimated for the end of the younger varanger glaciation 25 m y younger than previous estimates

the late early cambrian microbial reefs immediately after the demise of archaeocyathan reefs hunan province south china abstract the archaeocyath bearing reefs that first appeared in south china in the atdabanian were last seen in the lower toyonian e g in the tianheban and jindingshan formations the upper toyonian lower amgan qingxudong formation of hunan province examined in this study includes large sized reefs formed solely by calcimicrobes of epiphyton kordephyton girvanella hedstroemia and renalcis during the middle and upper cambrian small scale stromatolite reefs were formed the qingxudong reefs are therefore interpreted to be the most well developed microbial reefs of the post archaeocyath bearing reefs in the late early cambrian the transition from archaeocyath bearing to purely microbial reefs was caused by a decline in the diversity of metazoans and was accompanied by a reduction in the abundance of reefs possibly related in part to late early cambrian regression the late early cambrian reefs of south china contain a well preserved record of reef succession and provide important insights into the factors that led to the demise of archaeocyath bearing reefs and the subsequent long lasting hindrances to the development of skeletal dominated reefs during the middle and late cambrian

rare earth elements in the terminal ediacaran bambui group carbonate rocks brazil evidence for high seawater alkalinity during rise of early animals abstract rare earth elements plus yttrium rey mass fractions of ancient carbonate rocks are used to track changes in chemistry of past seawater here we investigate rey patterns in two carbonate sections from the ediacaran bambui group sao francisco basin brazil which comprise its two lowermost transgressive regressive second order sedimentary sequences shale normalised distributions vary with the sequence stratigraphy framework in the basal 2nd order sequence carbonate samples from the basal sequence transgressive systems tract display light rey lrey distributions slightly depleted to enriched that reflect input of freshwater possibly in a post glacial episode upwards carbonate rocks from the early highstand systems tract ehst yielded lrey enriched distributions which progressively turns into lrey shale normalized depleted distributions on samples from the late highstand systems tract lhst this portion of the sequence also displays y positive anomaly in some cases carbonate samples from the upper second order sequence do not display coherent patterns ce ce values 1 in most samples throughout the two sections suggest permanent anoxia of seawater the rey change from the ehst to lhst in the basal sequence marks an important paleoenvironmental overturn in the basin with increasing alkalinity in seawater driving rey fractionation and lrey depletion confinement of the basin in the inner areas of west gondwana due the uplift of marginal neoproterozoic orogens probably changed the weathering style of source areas to more congruent thus delivering a higher ionic influx to a restricted setting increasing alkalinity during lhst cloudina sp fragments were reported in this stage of the bambui group and in carbonate rocks with high sr mass fractions in other west gondwana basins supporting the hypothesis that the high alkalinity of seawater during late ediacaran may have driven the appearance of the first biomineralizing organisms

did anoxia terminate ediacaran benthic communities evidence from early diagenesis abstract the ediacaran oceanic redox landscape was heterogeneous where many basins had a shallow and highly dynamic chemocline above anoxic ferruginous or euxinic or low oxygen manganous waters seawater mmg ca ratio was also high promoting early diagenetic dolomitisation how the benthos responded to these conditions is fundamental to understanding their ecological dynamics here we utilise redox sensitive elements in early marine carbonate cements to investigate possible water column redox controls on the distribution and growth of the oldest metazoan communities skeletal communities in the zaris sub basin of the nama group namibia 550 547 ma grew in shallow waters where fine grained carbonate sediment often shows evidence of early dolomitisation mid ramp cloudina reefs are composed of open highly porous structures that formed multiple successive assemblages each assemblage is terminated by thin reef cements show a paragenetic sequence from synsedimentary to early marine cement and final burial which we infer were precipitated under dynamic redox conditions first acicular pseudomorphed aragonite cement formed under oxic conditions low fe and mn next the presence of iron rich dolomicrite sediment often associated with a recrystallised ferroan dolomite crust suggests that originally aragonitic or calcitic sediment and a high mg precursor cement were preferentially dolomitised dolomitisation may have been enhanced either via upwelling of deeper water anoxic ferruginous seawater or by later fluid remobilisation from adjacent shales a following mn rich calcite cement is inferred to be early marine due to its inclusion rich fibrous form and well preserved cl zonation the final blocky cement precipitated under oxic conditions probably during shallow burial the cements likely record a general shallow to deeper water transect from oxic shallow waters to low oxygen manganous waters then to oxic shallow burial conditions we hypothesize that cloudina reef communities were terminated by episodic sediment incursions during short term transgressive cycles possibly accompanied by upwelled anoxic ferruginous and dolomitising waters although the timing for this is poorly constrained more generally such incursions may have terminated ediacaran benthic communities that grew close to the chemocline

proterozoic modular biomineralized metazoan from the nama group namibia we describe a proterozoic fully biomineralized metazoan from the omkyk member 549 million years before the present of the northern nama group namibia namapoikia rietoogensis gen et sp nov is up to 1 meter in diameter and bears a complex and robust biomineralized skeleton it probably represents a cnidarian or poriferan namapoikia encrusts perpendicular to the walls of vertical synsedimentary fissures in microbial reefs this finding implies that large modular metazoans with biologically controlled mineralization appeared some 15 million years earlier than previously documented

exceptional preservation of fossils in an upper proterozoic shale late proterozoic organisms must have been diverse and widely distributed but in general their fossil record is both taxonomically and environmentally limited exceptional preservation of proterozoic fossils is not unknown but it is usually associated with silicified carbonates from restricted peritidal or playa lake environments1 2 we report here an exceptionally well preserved and distinctive assemblage of late proterozoic fossils from subtidal marine shales in addition to the sphaeromorphic acritarchs and cyanobacterial sheaths routinely preserved in proterozoic rocks this assemblage includes multicellular algae seaweeds a diverse assortment of morphologically complex protistan vesicles and probable heterotrophic bacteria thus it provides one of the clearest and most taxonomically varied views of proterozoic life yet reported

growth dynamics of neoproterozoic calcimicrobial reefs mackenzie mountains northwest canada abstract neoproterozoic buildups of the little dal group grew in a deep water epicratonic basin these kilometer scale reefs display aggradational and progradational geometries comparable to those described from phanerozoic reefs four phases of reef growth correspond to four regional shale to carbonate packages in laterally equivalent off reef strata the lower shaly part of each package is interpreted to reflect transgressive to highstand deposition the upper carbonate part of each package reflects carbonate precipitation in the water column as a result of postulated basin restriction and increased salinity during sea level lowstand reefs nucleated at the beginning of the first major transgressive event reefs typically aggraded during transgressive intervals but could also prograde or contract likely depending on the rate of relative sea level rise versus reef growth rate reefs prograded during regressions probably owing to reduction of accommodation space they shed talus at lowstand in response to increased erosion and or progradation over unstable substrates reef growth ended with return of shallow water conditions the overall architecture of aggradation and progradation demonstrates that the neoproterozoic calcimicrobial reef ecosystem was capable of responding to environmental changes in the same way as the more extensively studied ecologically complex and faunally diverse buildups of the phanerozoi

stromatolites in precambrian carbonates evolutionary mileposts or environmental dipsticks stromatolites are attached lithified sedimentary growth structures accretionary away from a point or limited surface of initiation though the accretion process is commonly regarded to result from the sediment trapping or precipitation inducing activities of microbial mats little evidence of this process is preserved in most precambrian stromatolites the successful study and interpretation of stromatolites requires a process based approach oriented toward deconvolving the replacement textures of ancient stromatolites the effects of diagenetic recrystallization first must be accounted for followed by analysis of lamination textures and deduction of possible accretion mechanisms accretion hypotheses can be tested using numerical simulations based on modern stromatolite growth processes application of this approach has shown that stromatolites were originally formed largely through in situ precipitation of laminae during archean and older proterozoic times but that younger proterozoic stromatolites grew largely through the accretion of carbonate sediments most likely through the physical process of microbial trapping and binding this trend most likely reflects long term evolution of the earth s environment rather than microbial communities

observations of neutral circulation at mid latitudes during the equinox transition study measurements of ion drift velocity made by the millstone hill incoherent scatter radar have been used to calculate the meridional neutral wind velocity during the september 17 24 1984 period strong daytime southward neutral surges were observed during the magnetically disturbed days of september 19 and 23 in contrast to the small daytime winds observed as expected during the magnetically quiet days the surge on september 19 was also seen at arecibo in addition two approaches have been used to calculate the meridional wind component from the radar derived height of the f layer electron density peak hmf2 results confirm the wind surges particularly when the strong electric fields measured during the disturbed days are included in the calculations the magnitudes of the surges are uncertain however due to the poorly known o o collision cross section and neutral composition the two approaches for the f layer peak wind calculations are applied to the radar derived hmf2 as a function of latitude to reveal the large north south extent as well as the temporal variation of the southward daytime surges

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

the nama foreland basin sedimentation major unconformity bounded sequences and multisided active margin advance abstract the nama foreland basin developed in neoproterozoic times as a peripheral foreland basin between the kalahari craton in the east and the partly contemporaneous damara and gariep orogenic belts in the north and west the foreland basin is subdivided into three basins from north to south the zaris witputs and vanrhynsdorp basins deposits of the nama group proper fill the first two basins whereas the third contains the vanrhynsdorp group which differs in many aspects from the nama group only during its latest phase of development did the foreland basin form one continuous depository the stratigraphic sequences in all three basins are suggested to be genetically related to orogenesis flexural bulging and sea level changes that affected the basin throughout its history palaeocurrent studies show that clastic sediments were initially derived from the craton and transported west and southwestwards in fluvial environments along the entire length of the basin as mountain belts emerged along the interactive northern and western orogenic margins transport directions switched to southerly and southeasterly foredeep flysch and ramp carbonates grade upwards into reddish conglomeratic molasse derived entirely from these orogens three major unconformity bounded sequences composite sequences probably of second order cyclicity can be recognised the uppermost composite sequence cs3 comprises the fish river subgroup whereas the lower composite sequence cs1 is composed of the flaminkberg formation and the kwanous kuibis and lower and middle schwarzrand subgroups cs2 comprises the knersvlakte brandkop and upper schwarzrand subgroups cs1 is composed of transgressive and highstand sequence sets onlapping the craton and truncated by a type 1 unconformity at the base of the nomtsas formation cs2 contains a conformable lowstand flysch delta partly derived from cs1 deposits eroded on flexural bulges as well as an orogenic source in the northwest cs3 is entirely composed of orogen derived molasse onlapping a basal type 2 unconformity and overstepping older sequences in a cratonward direction a number of intrabasinal unconformities is ascribed to flexural bulging in response to tectonic and sedimentary loading the age of the nama and vanrhynsdorp groups is estimated at between 600 ma and 530 ma as determined by the age of the basement the age of detrital micas and the age of subsequent thrust and fold tectonic events that vary in vergence from southeast to northeast which affected nama and vanrhynsdorp rocks along the active basin margins marginal tectonogenesis of the nama foreland basin resulted from northwesterly subduction of the kalahari plate underneath the congo and south american plates through a process of sinistral transpression along the western kalahari margin leading to sequential closure of the proto atlantic adamastor ocean from north to south

sequence stratigraphy and carbonate siliciclastic mixing in a terminal proterozoic foreland basin urusis formation nama group namibia abstract superb three dimensional exposures of mixed carbonate and siliciclastic strata of the terminal proterozoic urusis formation in namibia make it possible to reconstruct cross basin facies relations and high resolution sequence stratigraphic architecture in a tectonically active foreland basin six siliciclastic facies associations are represented coastal plain upper shoreface middle shoreface lower shoreface storm influenced shelf and pebble conglomerate siliciclastic shoreface facies pass seaward into and interfinger with facies of an open carbonate shelf four carbonate facies associations are present mid shelf shelf crest outer shelf and slope facies are arranged hierarchically into three scales of unconformity bounded sequences small scale sequences are one to tens of meters thick and span a few thousand years they consist of shelf carbonate with or without shoreface siliciclastic facies near the bottom medium scale sequences are tens of meters thick and span a few hundred thousand years they consist of shoreface siliciclastic facies in their lower parts which grade upward and pass seaward into shelf carbonate large scale sequences are tens to hundreds of meters thick and span 1 to 2 million years they are identified by widespread surfaces of exposure abrupt seaward shifts in shoreface sandstone patterns of facies progradation and retrogradation and shoreline onlap by medium scale sequences patterns of carbonate siliciclastic mixing distinguish tectonic from eustatic controls on the evolution of large scale sequences characteristics of eustatically controlled large scale sequences include 1 basal unconformities and shoreface sandstone that extend across the shelf to the seaward margin 2 retrograde carbonate and siliciclastic facies belts that onlap the shoreline together symmetrically during transgression and 3 upper shoreface sandstone that progrades seaward during highstand in contrast tectonically controlled sequences feature 1 basal erosion surfaces and upper shoreface sandstone that are restricted to near the landward margin and pass seaward into zones of maximum flooding and 2 asymmetric stratigraphic development characterized by landward progradation of carbonate from the seaward margin coincident with backstepping and onlap of the shoreline by siliciclastic facies a two phase tectonic model is proposed to account for the stratigraphic asymmetry of tectonically controlled sequences increased flexural bending during periods of active thrust loading caused submergence of the seaward margin and uplift of the landward margin rebound between thrusting episodes flattened the basin gradient and submerged the landward margin causing expansion of carbonate facies from the seaward margin and simultaneous transgression of the landward margin although the two phase model should apply to single lithology successions deposited in active foreland basins the mixing of carbonate and siliciclastic facies provides a particularly sensitive record of tectonic forcing the sensitivity may be sufficient for medium and small scale sequences to record higher frequency variations in flexural warping

evolution of early proterozoic passive margin carbonate platform rocknest formation wopmay orogen northwest territories canada abstract the superbly exposed rocknest formation wopmay orogen northwest territories canada is an early proterozoic 1 89 ga carbonate platform that formed along the eastern passive margin of slave craton archean the palinspastically restored shelf sequence is an eastward thinning prism 0 1 100 m thick extending for over 220 km parallel to strike and over 200 km perpendicular to strike it contains from west to east slope outer shelf shoal complex and inner shelf facies slope facies are carbonate rhythmites megabreccias and platy rhythmite breccias outer shelf facies include stromatolitic reefal boundstone and backreef ooid intraclast grainstore shoal complex facies are shallowing upward cycles of tufa cryptalgalaminite laminated dolosiltite lutite overlain by tepees b eccias and pisolite these formed a paleotopographic high restricting an eastern lagoon from western ocean inner shelf facies are dominantly asymmetric shallowing upward cycles of lagoonal mixed carbonates and siliciclastics that grade into intertidal stromatolitic dolomites cyclic facies pass eastward into noncyclic siliciclastics of central and eastern lagoon the rocknest lagoon spanned the width of slave craton and was bounded by a western carbonate shoreline shoal complex and by a tectonically active eastern siliciclastic shoreline the rocknest carbonate shelf developed on an underlying siliciclastic shelf an initial ramp rapidly evolved into a progradational accretionary rim and shoal complex followed by vertical aggradation of the rim concurrently the shoal complex underwent numerous west to east progradational events to produce many shallowing upward cycles in the shelf interior cyclic sedimentation was interrupted during three events of incipient shelf drowning associated with local backstepping of the rim and noncyclic sedimentation over most of the shelf probably caused by fourth order eustatic sea level oscillations the shelf was terminally drowned and overlain by deep water synorogenic siliciclastics during attempted subduction of the margin carbonates capping the shelf are stromatolitic patch re fs which overlie a possible unconformity suggesting emergence of the shelf prior to drowning