isotopic stratigraphy suggests neoproterozoic ages and laurentian ancestry for high grade marbles from the north central norwegian caledonides carbon and strontium isotope stratigraphy has been applied to constrain the depositional ages of high grade marble sequences in the ofoten district of the north central norwegian caledonides two marble formations hosted by diverse schists from the bogen group all previously correlated over long distances with a late ordovician early silurian low grade fossiliferous succession have been studied for carbon oxygen and strontium isotopes the least altered 87 sr 86 sr ratios ranging between 0 7062 and 0 7068 and the best preserved 13 c values falling between 5 0 and 6 5 obtained from two marble formations are consistent with a seawater composition in the time interval 700 600 ma the results obtained do not support the previously proposed correlation of the bogen group with an ordovician silurian lithostratigraphic succession further north the apparent depositional ages suggest that the tectonostratigraphic succession studied is inverted and that the tectonostratigraphy of the region requires revision the neoproterozoic depositional ages combined with the palaeogeographic position of baltica imply that carbonates were initially accumulated in seas on a continental shelf probably laurentia and were tectonically transported onto baltica during early silurian scandian collision at c 425 ma prospecting for new dolomite marble deposits of the hekkelstrand type and carbonate hosted manganese iron ores should be restricted to 700 600 ma sequences in the uppermost allochthon of the norwegian caledonides

the history of the walls boundary fault shetland the northward continuation of the great glen fault from scotland interpretations of the walls boundary fault usually take into account only a small part of the previously published evidence and some even present unsupported speculative interpretations in particular geological accounts rarely mention the geophysical evidence available and geophysical papers take little account of the geology of the area this paper reviews all published evidence geological and marine geophysical evidence show that the fault passes from close to fair isle through shetland and continues to the northeast that a 65 km dextral offset occurred in jurassic times on a gouge filled fracture and that prior to this a net sinistral displacement of the order of 100 km had occurred including a mylonite producing offset of unknown sense and distance in carboniferous times

saltation and wind flow interaction in a variable slope wind tunnel abstract in a 15 m long horizontal wind tunnel we studied the internal boundary layer over fixed arrays with roughness z0 similar to the roughness of saltation of a sand bed utilizing turbulence spires the logarithmic overlap layer is at least 75 mm thick some 5 m downwind of the entry this permits a precise determination of u from wind profile data over a sand bed in a variable slope wind tunnel with a 6 m long working section we studied how slope grain size and friction speed influence bed roughness owen 1964 proposed that z 0 c u 2 2 g where c is a constant but we found that c depends in a complex manner on grain size and friction speed c increases from very low values near the threshold to an apparently limiting value at large friction speeds for an almost uniform 125 m sand the limiting value of c is 0 06 the limiting value decreases however with particle size for a uniform 544 m sand c only approaches 0 03 for friction speeds as high as 1 m s with the measured values of friction speed corrected for the effects of slope our results appear to show that the limiting values of c do not depend on the slope angle even though the slope affects the individual shapes of particle trajectory c is perhaps independent of slope angle because the roughness height and therefore c is more directly related to the amount of momentum extracted from the air by the particles i e the shear stress rather than to the manner of extraction particle trajectory shapes

700 ma rift event in the blue ridge province of virginia a unique time constraint on pre iapetan rifting of laurentia discovery of compositionally distinctive rhyolites produced from a peralkaline volcanic center within the late neoproterozoic robertson river batholith and which are intercalated with terrestrial deposits of the previously undated mechum river formation indicates that rift related sedimentation and magmatism were ongoing at 702 705 ma about 130 m y before the breakup of laurentia recognition of late neoproterozoic age rhyolites in central virginia indicates that rift related magmatism was more widespread along the incipient eastern present coordinates margin of laurentia than previously thought neoproterozoic rifting in the blue ridge province of virginia appears to have been part of a broad scale process that affected most if not all of the preserved margins of laurentia the association of similar extension related terrestrial deposits and compositionally distinctive felsic rocks elsewhere may provide a unique marker for testing proposed predrift configurations of the late neoproterozoic supercontinent rodinia

a chemostratigraphic overview of the late cryogenian interglacial sequence in the adelaide fold thrust belt south australia within the neoproterozoic sequence of the adelaide fold thrust belt south australia two of the most severe ice ages in earth history sturtian and marinoan are recorded in the glacigenic rocks which mark the base and top of the 4 5 km thick umberatana group the focus of the present chemostratigraphic study rock units from three drillholes and ten measured outcrop sections located in the southern and central flinders ranges and on the stuart shelf were sampled for isotopic analysis of their co existing calcite 13ccal dolomite 13cdol and kerogen 13corg strontium and sulphur isotopic analyses were also run on selected samples the sample sites are in tectonic domains where regional metamorphism has been insufficient to significantly alter the isotopic composition of the kerogen the resulting data when integrated with the lithostratigraphy of the umberatana group yield one of the most complete records of secular variation in the c isotope signature of the late cryogenian ocean between 750 and 680 ma its composite 13ccarb curve begins with a sharp positive excursion 3 to 1 5 in the dolomitic tindelpina shale which caps sturtian glacial diamictites wilyerpa formation then follows a steady climb through the shoaling upward tapley hill formation reaching 4 in the brighton limestone and culminating in a plateau of 13c enrichment within the platformal carbonates of the etina formation 8 to 10 the upper part of the temporal trend shows a decline to 6 to 7 in the stromatolite reef facies of the enorama shale and a dramatic drop to 8 to 9 5 followed by a recovery to 3 in the youngest carbonate unit trezona formation beneath the marinoan glacigenic elatina formation the latter is capped by a thin dolostone the nuccaleena formation which records a consistent up section decrease in 13c 1 to 3 superimposed on this overall pattern of c isotopic variation are second and third order fluctuations attributable to eustatic sea level change pyritic sulphur in the lower tapley hill formation 34s 9 to 40 is distinctly heavier than that in the underlying wilyerpa formation 4 to 20 minimum 87sr 86sr values obtained from the brighton limestone 0 7071 and etina formation 0 7076 are consistent with their late cryogenian age this sturtian to marinoan interglacial succession is more complete than equivalent sequences elsewhere and thus provides a useful reference for global chemostratigraphic correlation notable features of its c isotope curve are the strong post wilyerpa positive excursion also seen in key sections from svalbard and western mongolia the etina plateau of 13c enrichment which is remarkably similar to maxima in the terminal proterozoic c isotope records of western mongolia north western canada and brazil and the spectacular depletion of 13c in the lower trezona relative to similar carbonate facies in the underlying etina formation and enorama shale a preglacial offset in 13ccarb values of such proportions 14 is almost unprecedented except perhaps for that recently documented in the upper ombaatjie formation beneath tillites of the ghaub formation otavi group namibia it represents a key piece of evidence in the case presented by the proponents of a neoproterozoic snowball earth

carbon and oxygen isotope systematics of a paleoproterozoic cap carbonate sequence from the sausar group central india abstract well preserved sedimentological features of the paleoproterozoic age sausar group of central india indicate glaciogenic origin for the diamictite in the lower part of the unit deglaciation was responsible for a marine transgression and the deposition of proximal glaciomarine facies and a cap carbonate followed by fine clastics with manganese ore analyses of 13 c and 18 o contents in the carbonate unit overlying the diamictite together with sr and ba contents and ree data indicate the preservation of primary geochemical signatures the average 13 c carb content ranges between 3 1 and 0 1 v pdb having peak negative 13 c excursions of up to 7 4 v pdb similar to paleoproterozoic and neoproterozoic cap carbonates elsewhere high sr and ba contents indicate the presence of aragonite and barite precursors similar to well studied neoproterozoic cap carbonates a single continuous carbonate unit shows 13 c excursions from 4 4 to 2 6 v pdb the lithological association and chemostratigraphy are comparable with the carbonate tillite association of the huronian supergroup of canada the snowy pass supergroup of the usa the transvaal supergroup of south africa and the turee creek group of australia

new evidence that the lower cambrian leny limestone at callander perthshire belongs to the dalradian supergroup and a reassessment of the exotic status of the highland border complex

strontium and carbon isotope geochemistry applied to dating of carbonate sedimentation an example from high grade rocks of the norwegian caledonides isotope stratigraphy has been applied to an undated high grade carbonate sequence from the norwegian caledonides in order to constrain the depositional age sokumfjell group marbles of the beiarn nappe complex in the uppermost allochthon of the nordland caledonides were subjected to several phases of deformation culminating in amphibolite facies metamorphism the ca 1000 m thick marble sequence studied is composed of five different lithofacies lithofacies 1 4 are calcite marbles and lithofacies 5 is dolomite marble all the calcite marble lithofacies exhibit throughout rhythmic bedding and textural grading on different scales suggesting that carbonate material was deposited in a pelagic turbidite system 123 samples were analysed for major and trace elements and acid soluble mg and ca of which 42 samples of calcite marbles and four samples of dolomite marbles were selected for 13 c 18 oa nd 87 sr 86 sr analyses all selected calcite marbles have high sr contents averaging 2000 ppm they are characterised by 87 sr 86 sr ratios ranging between 0 70642 and 0 70999 and 13 c values ranging between 1 3 and 8 4 v pdb 18 o values are highly variable 14 1 24 8 v smow the best preserved samples selected on the basis of mn sr and rb concentration and mn sr and mg ca ratios yield average 87 sr 86 sr values of 0 70669 0 70697 0 70727 0 70749 and 0 70737 for lithofacies 1 2 3 4 and 5 respectively post sedimentary alterations including amphibolite facies metamorphism resulted in only a slight increase of 87 sr 86 sr values in the marbles studied apparently due to aragonite precursor high sr concentration and a very early and fast diagenetic alteration caused by fluids isotopically comparable with that of seawater projection of these values along with 13 c measurements onto 87 sr 86 sr and 13 c calibration curves for the secular variations of seawater suggests that the lithofacies studied were apparently deposited between 635 and 600 ma 2001 elsevier science b v all rights reserved

a major perturbation of the carbon cycle before the ghaub glaciation neoproterozoic in namibia prelude to snowball earth 1 a large 11 15 negative shift in 13c is observed in shallow water carbonates directly beneath neoproterozoic glacial deposits or correlative disconformity in northwest namibia ascribed to a snowball earth reproducibility and stratigraphic concordance of this anomaly in 16 sections across the ancient continental shelf support a primary origin and field relations show it predates the fall in sea level associated with the ghaub glaciation we crudely estimate the duration of the isotopic shift as 0 6 106 years from a simple thermal subsidence model similar or larger 13c anomalies are found directly beneath neoproterozoic glacial units in australia canada china scotland and svalbard after considering conventional interpretations for negative 13c anomalies we conclude that a prolonged methane release to the atmosphere is most consistent with the timescale magnitude and geological context of the anomaly in namibia counterintuitively an anomalous methane flux that is sustained for 100s kyr may be consistent with a snowball glaciation

pre sturtian 800 730ma depositional age of carbonates in sedimentary sequences hosting stratiform iron ores in the uppermost allochthon of the norwegian caledonides a chemostratigraphic approach abstract carbon and strontium isotope chemostratigraphy 52 13ccarb and 18o and 50 87sr 86sr analyses of carbonate components in whole rock samples was applied for constraining an apparent depositional age of the carbonate protolith to amphibolite grade calcite marbles occurring in siliciclastic sedimentary sequences hosting iron formations the dunderlandsdalen type iron ores of previously unknown age in the rodingsfjallet nappe complex of the rana region nordland norway the least altered 87sr 86sr 0 70676 and 13c 2 5 to 5 6 values of the marbles dunderland marble 2a in the hanging wall sequence of the stensundtjern iron formation in the rana region are consistent with seawater composition in the time interval 800 730 ma hence the middle cryogenian pre sturtian the least altered sr and c isotopic values obtained from two other marbles units marble 3 and 4 of the rodningsfjallet nappe complex are consistent with the late cryogenian c 660 and 670 700 ma respectively the ages obtained provide the first insight into the depositional time of sediment hosted iron formations of the uppermost allochthon in the north central norwegian caledonides the middle cryogenian age of marbles associated with iron formations in the rana region and those located c 250 km to the north the hafjellet iron ore horizon share similar 87sr 86sr and 13c ratios and hence similar chemostratigraphic ages these iron formations were originally accumulated outside of baltica on a glacially influenced carbonate siliciclastic shelf apparently on a margin of an unknown microcontinent the scandinavian dunderlandsdalen and the hafjellet iron ores of middle cryogenian age 800 730 ma were accumulated in an open marine environment distant from volcanic centres and hence represent an outstanding exception to other reported neoproterozoic iron formations which were all accumulated in volcanically active continental rift settings

a vendian cambrian boundary succession from the northwestern margin of the siberian platform stratigraphy palaeontology chemostratigraphy and correlation siberia contains several key reference sections for studies of biological and environmental evolution across the proterozoic phanerozoic transition the platonovskaya formation exposed in the turukhansk region of western siberia is an uppermost proterozoic to cambrian succession whose trace and body fossils place broad limits on the age of deposition but do not permit detailed correlation with boundary successions elsewhere in contrast a striking negative carbon isotopic excursion in the lower part of the platonovskaya formation permits precise chemostratigraphic correlation with upper most yudomian successions in siberia and possibly worldwide in addition to providing a tool for correlation the isotopic excursion preserved in the platonovskaya and contemporaneous successions documents a major biogeochemical event likely involving the world ocean the excursion coincides with the palaeontological breakpoint between ediacaran and cambrian style assemblages suggesting a role for biogeochemical change in evolutionary events near the proterozoic cambrian boundary

50 myr recovery from the largest negative δ13c excursion in the ediacaran ocean sedimentary rocks deposited during the ediacaran period 630 542 ma contain carbonates whose carbon isotopic ratios show a marked negative excursion consisting of a precipitous drop from 5 to 12 followed by a sub linear recovery to positive 13c values isotopic ages u pb and thermal subsidence modelling are combined to constrain the excursion in time and indicate an onset at 600 ma and duration of recovery of approximately 50 myr the excursion is widely recognized in oman and has potential correlatives in ediacaran strata elsewhere and may thus represent a characteristic feature of the ediacaran period the amplitude of this carbon isotope excursion far exceeds those of other neoproterozoic anomalies the isotopic trend of negative excursion and long term recovery spanned at least one short lived glacial episode at 580 ma but appears unrelated to glaciation which indicates that negative anomalies in the neoproterozoic marine carbon isotope record are not directly or uniquely linked to ice ages

late precambrian marbles of unusual carbon isotope composition southeastern british columbia carbon and oxygen isotope analyses were made of both carbonates and graphites from several precambrian metamorphic samples from british columbia the carbon isotope data are unusual in that the 13c values of many marbles are very high up to 9 9 and the most positive values occur in the sillimanite zone rocks the 13c values of graphite are also relatively high and the 13c fractionations between calcite and graphite suggest that 1 the rocks attained and retained carbon isotope equilibrium during metamorphism and 2 the temperature of graphite equilibration in marbles from the sillimanite zone is 650 690 c in agreement with temperatures estimated from mineralogical phase equilibria possible precursor carbonates for the 13c rich marbles in british columbia include 1 precambrian carbonate secreting organisms such as algae that participate in reactions with large kinetic isotope effects 2 carbonates that exchanged carbon isotopes with a large reservoir of organic material prior to metamorphism

carbon isotope variability across the ediacaran yangtze platform in south china implications for a large surface to deep ocean δ13c gradient new isotope data obtained from relatively conformable carbonate rich strata of the ediacaran yangtze platform in south china reveal substantial 13 c variability in platform sections four negative 13 c anomalies with a nadir down to 8 pdb are present in the interval between the cap carbonate level 635 ma and the precambrian cambrian boundary 542 ma while in slope and basinal sections 13 c values are negative through the entire doushantuo formation 635 551 ma if these 13 c values are close to their primary seawater signature they imply a strong 10 surface to deep ocean 13 c gradient that is consistent with long term deep ocean anoxia and the presence of a large dissolved organic carbon doc reservoir the two prominent negative 13 c excursions within the doushantuo formation above the cap carbonate level are associated with shoaling and local exposure of the platform the anomalies may thus record remineralization of a large oceanic doc pool via sulfate reduction that transferred 13 c depleted carbon from the oceanic doc reservoir to the surface ocean during regression inconsistencies in ediacaran 13 c profiles globally and variations in south china in particular highlight the need for further evaluation of local departures in 13 c from an inferred average seawater signature

the stratigraphic expression of a large negative carbon isotope excursion from the ediacaran johnnie formation death valley the rainstorm member of the ediacaran johnnie formation of the southern basin and range us records a large negative excursion in carbon isotope ratios of carbonate strata 13 c vpdb 6 the character of the excursion raises fundamental questions about whether this isotopic pattern is accurately capturing the time series behavior of marine dissolved inorganic carbon dic or is a product of diagenesis to explore this issue we examined the expression of this isotopic excursion within the johnnie oolite a 2 m thick marker bed which records the highest rate of change in 13 c sedimentologically the oolite unit is thought to be time transgressive if the isotopic excursion reflects time series behavior of the carbon cycle its expression in the oolite across the basin should systematically align the sections according to their slight diachroneity detailed carbon and oxygen isotopic stratigraphy of the oolite at seven different locations indicates the magnitude of the excursion at the base of the oolite is spatially variable such that after a palinspastic reconstruction the sections align along a systematic north to south gradient in the isotopic data the oolite preserved in the old dad mountains the most southerly section measured is an outlier to this trend and the most difficult to ordinate accurately in the reconstruction showing the largest isotopic range between the onset of carbonate deposition and top of the oolite several hypotheses are congruent with these data but the sum of observations is best explained by a scenario wherein the oolite is time transgressive and deposited in a north to south manner throughout the onset of the isotopic excursion if correct this implies that the stratigraphic 13 c pattern reflects time series behavior of marine dic

87sr 86sr chemostratigraphy of neoproterozoic dalradian limestones of scotland and ireland constraints on depositional ages and time scales new calcite 87 sr 86 sr data for 47 limestones from the metamorphosed and deformed neoproterozoic cambrian dalradian supergroup of scotland and ireland are used to identify secular trends in seawater 87 sr 86 sr through the dalradian succession and to constrain its depositional age dalradian limestones commonly have sr 1000 ppm indicating primary aragonite and marine diagenesis low mn mn sr 18 o and trace element data indicate that many 87 sr 86 sr ratios are unaltered since diagenesis despite greenschist to amphibolite facies metamorphism consistent with the documented behaviour of sr and o during metamorphic fluid rock interaction thus the 87 sr 86 sr data are interpreted largely to reflect 87 sr 86 sr of coeval seawater currently available data show that neoproterozoic seawater 87 sr 86 sr rose from c 0 7052 at 850 900 ma to c 0 7085 or higher in the latest neoproterozoic temporal changes at c 800 ma and c 600 ma bracket the range in 87 sr 86 sr values of calcite in grampian appin and lowest argyll group c 0 7064 0 7072 and middle and uppermost argyll group c 0 7082 0 7095 limestones consistent with a rise in seawater 87 sr 86 sr around 600 ma 87 sr 86 sr data are consistent with the sedimentary affinity of the islay subgroup with the underlying appin group and with a possible time interval between deposition of islay and easdale subgroup rocks they indicate that the dalradian as a whole is younger than c 800 ma

global c cycle perturbations recorded in marbles a record of neoproterozoic earth history within the dalradian succession of the shetland islands scotland abstract the shetland islands in scotland contain a c 12 km thick mostly siliciclastic neoproterozoic succession that experienced deformation and greenschist to amphibolite facies metamorphism during the ordovician silurian caledonian orogeny correlating these rocks with neoproterozoic successions elsewhere has remained problematic however four marble units punctuate the succession these are in stratigraphic order the weisdale whiteness girlsta and laxfirth limestones two of which record hallmark c isotopic excursions diagnostic of neoproterozoic time the whiteness limestone is lithologically similar to the 635 ma marinoan post glacial cap carbonate sequence and displays a 13 c carbonate profile in which values decline from a range of 0 to 2 to a nadir of 5 and then rise towards and oscillate around 0 the girlsta limestone is a 700 900 m thick marble unit with values of 9 to 11 and is interpreted as recording the late neoproterozoic shuram wonoka event these correlations permit linking the shetland succession to neoproterozoic sections elsewhere and also show that rift related rocks in shetland post date the shuram wonoka event post c 600 ma pre c 550 ma thereby supporting the concept of a diachronous phase of extensional tectonism from c 700 ma in east central laurentia to near the close of the proterozoic in northeastern laurentia prior to the opening of the iapetus ocean

isotope stratigraphy of the ediacarian neoproterozoic iii of the adelaide rift complex australia and the overprint of water column stratification abstract this study establishes a high resolution 13c chemostratigraphic framework for the type ediacarian terminal proterozoic section in the adelaide rift complex a framework that contributes to the emerging global chronostratigraphic synthesis of a period that saw momentous changes in the earth s surface environment only 15 m of the 3 5 km thick type section consists of shallow water carbonates of the kind that have formed the basis of most previous studies of neoproterozoic chemostratigraphy however the organic 13corg record reveals a surprising richness of information and shows excellent utility in correlation between sections in the adelaide rift complex lateral variations in 13corg profiles correspond to expected diachronous patterns of basin fill with the addition of 87sr 86sr data correlation can also be made to the officer and amadeus basins consistent with known lithostratigraphic and sequence stratigraphic constraints sublittoral micritic carbonates of the upper nuccaleena formation cap dolostone and lower and middle wonoka formation show strong basinwide 13c depletion 13ccarb 3 to 10 despite compelling evidence for preservation of depositional 13ccarb such as good textural preservation evidence for sea floor lithification and in the wonoka formation pristine 87sr 86sr and low mn sr associated with the depleted carbonates are rare subaqueous evaporite relatively heavy 13corg and anomalously 13c depleted organic carbon of benthic microbial mat origin and a common cause salinity stratification of the water column is postulated the indicated palaeogeographic setting an evaporative mediterranean like basin tenuously connected to the world ocean is entirely consistent with previous suggestions of one or more messinian type desiccation events to explain very deep 1 km incised valleys or canyons in the adelaide rift complex and officer basin at horizons within the wonoka formation and correlates positive excursions in 13corg during basin stratification may in part be due to 12c depletion of surface waters by photosynthetic carbon fixation despite the proposed semi isolation of australian ediacarian basins from the world ocean major 13c excursions which can to some extent be differentiated by the monotonic rise in ediacarian marine 87sr 86sr appear to be correlatable to the upper windermere supergroup northwest canada

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

neoproterozoic chemostratigraphy and correlation of the port askaig glaciation dalradian supergroup of scotland the 13 c of preglacial carbonates beneath the glaciogenic port askaig formation falls from 5 to negative values while least altered 87 sr 86 sr values are close to 0 7067 postglacial cap carbonates of the bonahaven formation begin with negative 13 c carb values jumping sharply to 11 7 near the top which we correlate with excursions above the elbobreen wilsonbreen glaciations of svalbard and putative sturtian not varanger glaciations elsewhere dated at c 720 ma much of the succeeding terminal neoproterozoic is missing in ne svalbard and east greenland raising doubts about their use for global sr and c isotope curves in the neoproterozoic

geochemical preservation potential of high grade calcite marble versus dolomite marble implication for isotope chemostratigraphy the neoproterozoic hekkelstrand marble from the north central norwegian catedonides contains a calcite marble unit hosting a commercially exploited white dolomite marble deposit which underwent polyphase amphibolite facies scandian metamorphism major and trace element geochemistry delta 18 o delta 13 c and sr 87 sr 86 ratios have been used to define isotopic differences between interbedded calcite and dolomite marbles and to assess their post depositional isotopic resetting a high strontium concentration in the calcite marble up to 1200 ppm suggests an aragonitic lime mudstone precursor and its least altered delta 13 c 4 parts per thousand v pdb delta 18 o 27 parts per thousand v smow and sr 87 sr 86 0 70615 to 0 70624 ratios are consistent with 700 600 ma seawater the dolomite marbles contain minor relics of the calcite aragonite precursor despite pervasive dolomitisation the average mn 44 ppm and fe 1041 ppm contents of the dolomite marbles are comparable with those measured in the host calcite marbles and much lower than those reported from non metamorphosed dolostones of a similar neoproterozoic age the dolomite marbles have an average sr content of 115 ppm an average mg ca ratio of 0 56 is lower than that for stoichiometric dolomite 0 62 and is controlled mainly by remnant inclusions of precursor calcite sr 87 sr 86 ratios range between 0 70725 and 0 71034 the least altered delta 13 c of 6 parts per thousand and delta 18 o of 30 parts per thousand suggest a high degree of isotopic preservation of the dolomite the dolomite marbles are isotopically heavier both delta 18 o and delta 13 c by 2 parts per thousand on average compared with the calcite marble thus suggesting a pre metamorphic lower temperature isotopic equilibrium the dolomite was apparently originally precipitated from 600 to 700 ma seawater dominated fluids the sr 87 sr 86 ratio was modified by post depositional processes the high delta 13 c and delta 18 o values and the low mn content of the bulk dolomite marbles are not consistent with common meteoric diagenesis or metamorphic alteration the highest sr 87 sr 86 ratio of 0 70868 suggests that the earlier dolomite was apparently modified by ca 560 ma seawater or by meteoric water considerably buffered by ca 560 ma limestones the most significant geochemical and isotopic modification of the dolomite took place during metamorphism this has been observed across the dolomite marble silicate rock contacts over a limited distance of lt 0 5 in where c and 0 isotope systems were reset by up to 4 5 parts per thousand and 9 7 parts per thousand respectively the sr 87 sr 86 ratio increased by up to 0 0012 whereas the fe mn and sr concentrations have increased by factors of similar to 80 similar to 15 and similar to 1 5 respectively the dolomite marbles despite amphibolitc facies polyphase metamorphic alterations have preserved high delta 18 o and delta 13 c values which can be utilised for chronostratigraphic purposes whereas sr 87 sr 86 ratios provide insights into the composition of the post depositional fluids

structurally focused fluid flow during orogenesis the islay anticline sw highlands scotland abstract displacement of isotopic compositions at boundary layers across strata of contrasting composition is commonly used to investigate hydrothermal fluid flow during orogeny this study investigates whether hydrothermal fluid flow was focused along the islay anticline islay sw highlands of scotland as shown in the axial zone of the neighbouring ardrishaig anticline four localities from the limb to the axial plane of the islay anticline were investigated for isotopic homogenization of metacarbonate units to silicate values at mull of oa on the limb of the anticline metacarbonate samples show limited isotopic resetting and the fluid flux is estimated to be 3 m 2 within the axial zone of the islay anticline metacarbonate units from port a9 chotain and bagh an da dhoruis show complete isotopic homogenization to silicate values indicating higher fluid fluxes fluid flow was enhanced along localized parasitic folds such as at port an t sruthain where metacarbonates have been isotopically reset and there are abundant quartz carbonate veins that precipitated during d 1 d 2 deformation metamorphic fluid flow was higher in the axial zone of the islay anticline and in localized antiformal structures fluid fluxes are estimated to be considerably lower than at the neighbouring ardrishaig anticline

two from donegal neoproterozoic glacial episodes on the northeast margin of laurentia two glacial episodes the sturtian ca 720 ma and marinoan varangerian ca 590 ma characterize many neoproterozoic successions worldwide in contrast northeast laurentia is typically considered as containing only the latter in scotland and ireland this is represented by the port askaig tillite here we present evidence ice rafted debris for a second younger neoproterozoic glacial event in the dalradian southern highland group of donegal ireland these deposits overlie strata correlative to ca 595 ma rocks in scotland making this heretofore unrecognized glacial episode a marinoan varangerian equivalent in that the port askaig occurs 3 5 km stratigraphically below the southern highland group rocks it is most likely sturtian not marinoan varangerian in age thus the dalradian succession in the irish caledonides contains strata representing both the sturtian and marinoan varanger glaciations

self formed waterfall plunge pools in homogeneous rock waterfalls are ubiquitous and their upstream propagation can set the pace of landscape evolution yet no experimental studies have examined waterfall plunge pool erosion in homogeneous rock we performed laboratory experiments using synthetic foam as a bedrock simulant to produce self formed waterfall plunge pools via particle impact abrasion plunge pool vertical incision exceeded lateral erosion by approximately tenfold until pools deepened to the point that the supplied sediment could not be evacuated and deposition armored the pool bedrock floor lateral erosion of plunge pool sidewalls continued after sediment deposition but primarily at the downstream pool wall which might lead to undermining of the plunge pool lip sediment evacuation and continued vertical pool floor incision in natural streams undercutting of the upstream pool wall was absent and our results suggest that vertical drilling of successive plunge pools is a more efficient waterfall retreat mechanism than the classic model of headwall undercutting and collapse in homogeneous rock

the shuram wonoka event recorded in a high grade metamorphic terrane insight from the scandinavian caledonides an approximately 250 m thick polydeformed and polymetamorphosed isotopically unusual variegated marble locally termed the leivset marble shows a great lateral extent in the scandinavian caledonides its extremely 13c depleted primary nature 7 9 1 2 on average n 93 makes the leivset marble unique this together with a high sr content up to 8740 ppm that buffered 87sr 86sr ratios between 0 70802 and 0 70872 suggests correlation with the worldwide shuram wonoka isotopic event occurring within the 600 550 ma time interval during the ediacaran vendian period despite a high grade deformation and metamorphism the leivset marble has retained its original carbon and strontium isotope ratios a combination of the variegated colour with unusually low 13ccarb can potentially be used for stratigraphic correlations in high grade non fossiliferous marble dominated terranes across the caledonian orogenic belt in baltica and laurentia isotope chemostratigraphy has identified a prominent cryptic stratigraphic discontinuity and suggests that the ediacaran leivset marble was tectonically juxtaposed above low grade llandovery age fossiliferous marbles during the scandian orogeny

identifying structural trend with fractal dimension and topography a digital elevation model dem derived from light imaging detection and ranging lidar altimetry data is used to investigate the fractal characteristics and structural trends of a topographic surface the study area is located on the southeastern rim of the sudbury basin a meteorite impact feature in ontario canada visual interpretation and statistical analysis of fractal dimension d indicate that the geological units in the study area are characterized by statistically different distributions of d in addition the landscape exhibits two predominant aspect directions 127 and 290 or se and nw high values of d correspond to these predominant aspect directions topographic ridgelines exhibit a ne sw trend which is perpendicular to that of the highest fractal dimensions and the predominant aspect directions it is suggested that these ridgelines are topographic expressions of tectonic fabrics present in the various geological units genesis of the tectonic fabric is related to the nw se directed shortening of the sudbury basin impact crater

glacial trinity neoproterozoic earth history within the british irish caledonides two distinct neoproterozoic glacial episodes are known for the dalradian supergroup in the british irish caledonides the port askaig formation and the inishowen loch na cille ice rafted debris ird beds here we describe a third intermediate between those two the stralinchy reelan formations composed of diamictite and ird developed directly above these rocks is the cranford limestone which consists of a basal 1 6 m thick tan gray dolostone overlain by a variably developed but as much as 340 m thick sequence of thin bedded limestone and dolostone this unit exhibits a c isotopic trend that begins negative in the basal dolostone reaches a nadir of 7 and then rises to 0 2 these characteristics match strikingly those of marinoan style cap carbonates consequently we interpret the port askaig formation the stralinchy reelan units and the inishowan loch na cille beds as equivalents of the ca 700 ma sturtian the 635 ma marinoan and the ca 580 ma gaskiers glacials respectively two additional observations are noteworthy carbonate rocks below the port askaig formation record a 13c decline to 6 that implies that such downturns may occur in both pre sturtian and pre marinoan strata in addition the bonahaven dolomite is not a cap carbonate to the port askaig formation but exhibits a 13c rise to 12 which we correlate with the inferred global keele peak these data further document the utility of neoproterozoic glacial cap carbonate sequences in global correlations and denote the base of the cranford linestone as the cryogenian ediacaran boundary