frictional melting processes and the generation of shock veins in terrestrial impact structures evidence from the steen river impact structure alberta canada abstract shock produced melt within crystalline basement rocks of the steen river impact structure sris are observed as thin 1 510 m wide interlocking networks of dark veins which cut across and displace host rock minerals solid state phase transformations such as ferro pargasite to an almandine andradite majorite garnet and amorphization of quartz and feldspar are observed in zones adjacent to comparatively wider 50 500 m sections of the shock veins shock pressure estimates based on the coupled substitution of na ti4 and si4 for divalent cations al3 and cr3 in garnet 14 19 gpa and the pressure required for plagioclase ab62 83 amorphization at elevated temperature 14 20 gpa are not appreciably different from those recorded by deformation effects observed in non veined regions of the bulk rock 14 20 gpa this spatial distribution is the result of an elevated temperature gradient experienced by host rock minerals in contact with larger volumes of impact generated melt and large deviatoric stresses experienced by minerals along vein margins micrometer size equant crystals of almandine pyrope majorite garnet define the shock vein matrix consistent with rapid quench 100 200 ms at 7 5 10 gpa crystallization of the vein occurred during a 0 1 0 15 s shock pressure pulse majoritic garnet formed during shock compression by solid state transformation of pargasite along shock vein margins is observed in tem bright field images as nanometer size gouge particles produced at strain rates in the supersonic field 106 108 these crystals are embedded in vesiculated glass and this texture is interpreted as continued movement and heating along slip planes during pressure release the deformation of high pressure minerals formed during shock compression may be the first evidence of oscillatory slip in natural shock veins which accounts for the production of friction melt via shear when little or no appreciable displacement is observed our observations of the mineralogy chemistry and microtextures of shock veins within crystalline rocks of the sris allow us to propose a model for shock vein formation by shear induced friction melting during shock compression

sustained low marine sulfate concentrations from the neoproterozoic to the cambrian insights from carbonates of northwestern mexico and eastern california abstract stratigraphic carbonate carbon isotope trends are similar for correlative ediacaran and cambrian carbonates of sonora mexico and death valley california in contrast the sulfur isotope compositions of both carbonate associated sulfate cas and pyrite in the two regions exhibit unique trends with high degrees of stratigraphic variability we have established that the sulfur records are coeval using 13c chemostratigraphy and biostratigraphic markers where present over short stratigraphic intervals 34scas variability is consistent with regionally low marine sulfate concentrations during this period values of 34s 34scas 34spyr range from 5 8 to 27 1 and average 11 consistent with limited net fractionation during bacterial sulfate reduction which is additional evidence for low sulfate concentrations modeling based on these regional sulfur isotope trends suggests sustained low sulfate conditions throughout the neoproterozoic and well into the cambrian with concentrations of 2 mm or lower when all of the available sulfate proxy data from our work and previously published studies are considered most neoproterozoic and cambrian successions exhibit trends consistent with low seawater sulfate the persistent and complete disagreement in 34ssulfate among multiple basins was briefly interrupted 580 million years ago coincident with the onset of the wonoka shuram carbon isotope anomaly and again near the termination of series 3 of the cambrian characteristics generally unrecognized in older rock units during these two intervals similar stratigraphic trends in 34scas are recorded globally whereas absolute values remain distinct among individual basins however these periods of broad isotopic trend agreement coincide with large magnitude sulfur isotope excursions which also point to low seawater sulfate concentrations therefore although brief intervals of isotopic homogeneity exist the neoproterozoic and cambrian ocean must have been dominated by low sulfate throughout ultimately the recognition of persistently low sulfate well into the paleozoic raises questions about the relationships between sulfate concentration in seawater and its primary controls including ocean oxygenation and its influence on metazoan evolution

mesoproterozoic carbonate systems in the borden basin nunavut existing stratigraphic nomenclature lithologic descriptions and geological interpretations for an economically important mesoproterozoic dolostone in the milne inlet graben borden basin nunavut do not adequately portray its unusual facies or their spatio temporal configuration four new stratigraphic units are introduced to replace this dolostone formerly known as the society cliffs formation in the southeastern half of the graben the iqqittuq formation represents a distally steepened ramp that grades northwestward into deep water mudstone that is indistinguishable from that of the underlying arctic bay formation the overlying angmaat formation represents a rimmed restricted peritidal platform that grades northwestward across a tepee cortoid shoal barrier into the unusual deep water dolo laminite of the nanisivik formation deep water carbonate mounds up to hundreds of metres thick and kilometres in areal dimensions belong to the ikpiarjuk formation these mounds are geometrically equivalent

constraints on meso to neoproterozoic seawater from ancient evaporite deposits abstract marine sulfate so 4 2 is intimately connected to the global carbon and oxygen cycles through its important role as an electron acceptor for the microbial respiration of organic carbon the biogeochemical feedbacks within the sulfur carbon and oxygen cycles may have changed through time reflecting changes in the concentration of sulfate in the oceans unfortunately there is much uncertainty about the size of the marine sulfate reservoir throughout earth history in particular conflicting estimates for marine sulfate exist during the latest neoproterozoic an interval of time associated with striking changes in earth system evolution and oxidation published interpretations of fluid inclusion chemistry place sulfate greater than 16 mmol kg whereas other interpretations of carbonate associated sulfate data suggest concentrations less than 2 mmol kg calcium isotope ratios in evaporite successions provide an independent method for deriving semi quantitative constraints on sulfate concentrations as well as other properties of seawater chemistry here the calcium isotope behavior of bedded sulfate evaporites from 1050 ma baffin and bylot islands nunavut canada 830 ma officer basin western australia and 545 ma south oman salt basin sultanate of oman are examined in combination with other geological observations the results suggest relatively low millimolal level sulfate in the latest mesoproterozoic and a more specific range of 6 10 mmol kg sulfate during the latest neoproterozoic these new constraints suggest that previous interpretations of sulfate concentrations and seawater chemistry need to be revised opening up new possible solution spaces for the major ion composition of seawater

cross section restoration of salt related deformation best practices and potential pitfalls abstract cross section restoration typically assumes plane strain deformation and area conservation constraints that are usually invalid for salt because of its characteristic three dimensional flow and possible dissolution thus restoration of salt related deformation provides added challenges and uncertainty in this review paper we summarize the historical development of ideas methods and applications of restoration in salt basins while most published restorations do not maintain salt area constraints on its variation range from arbitrary assumptions to quantitatively incorporating isostatic calculations we illustrate several scenarios in which the presence of salt adds ambiguity to restoration primarily because it can hide deformation diapirs can widen during extension and narrow during shortening translating overburden can move into salt and drive allochthonous advance secondary minibasin subsidence can be accommodated at both shallow and deep salt levels and allochthonous salt can record evacuation of deeper salt although we caution against using restoration to test and validate small scale details of interpretations we emphasize that sequential restoration remains an essential tool in structural and basin analyses however because of the uncertainties a regional three dimensional approach and sound geological reasoning are critical for deriving meaningful and useful results from cross section restoration of salt structures

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

nature of the connection between the northern and southern zechstein basins across the mid north sea high abstract the seismic expression of a salt filled channel which cuts across the mid north sea high in quadrant 37 is described with features interpreted as being produced by salt edge dissolution forming both eastern and western margins of the channel the apparent half graben nature of the channel is shown to be only superficial and due to complex faulting associated with but not defining its western margin the shallower faulting here is a mesozoic to early tertiary growth fault related to local dissolution of zechstein salt the dissolution effect appears in turn to have been localized by the presence of a deeper fault that was already downthrown to the east in zechstein times when it seems to have limited the eastward progradation of zechstein shelf carbonates and anhydrites and had probably ceased to move significantly before the onset of the late cimmerian erosional phase the origin of this arcuate fault is tentatively ascribed to subsidence around a granite batholith zechstein salt originally spread some distance to the east and west of the channel it was dissolved from the edges inwards mainly before the late cretaceous giving rise to a thicker mesozoic sequence on parts of the flanks of the channel than in the middle besides providing an interesting structural case history the features described have implications regarding zechstein sedimentation reservoir potential the tectonic history of the north sea and the nature of the mid north sea high itself

depositional facies and platform architecture of microbialite dominated carbonate reservoirs ediacaran cambrian ara group sultanate of oman intrasalt carbonates of the ediacaran cambrian ara group constitute a significant reservoir element of the intrasalt stringer play in oman in which dolomitic carbonates are encased in salt at depths of 3 to 7 km 1 9 to 4 3 mi these reservoir carbonates have significant microbial influences although ara group reservoirs are mostly latest precambrian the models developed here may be applicable to younger microbially dominated carbonate reservoirs in basins of higher salinity when higher organisms are excluded in lacustrine settings where calcified invertebrates are not a significant source of carbonate or after periods of mass extinction before faunal recovery a broad range of carbonate facies provides the context in which to understand the origin of the microbialite dominated reservoirs developed across both ramp and rimmed shelf profiles major facies associations include carbonate evaporite transition zone deep ramp and slope subtidal microbialites clastic textured carbonates and restricted peritidal carbonates microbialites are subdivisible into a number of facies that all have significance in terms of understanding environmental history as well as reservoir properties and that help in predicting the location of reservoir fairways microbially influenced facies include shallow subtidal thrombolites with massive clotted textures and very high initial porosities 50 shallow subtidal pustular laminites with cm scale variability of lamina morphology deeper subtidal crinkly laminites that show mm scale variability of lamina morphology and intertidal tufted laminates that show mm to cm scale tufted textures other reservoir facies are more conventional grainy carbonates including ripple cross stratified grainstone packstone hummocky cross stratified grainstone packstone flat pebble conglomerate ooid and intraclast grainstone packstone and cloudina grainstone packstone these facies are almost invariably dolomitized and all have moderate to excellent reservoir quality these facies comprise carbonate platforms broken up during salt tectonics that range up to 160 m 525 ft in thickness and extended laterally prior to halokinesis for tens to over 50 km 31 mi the distribution of reservoir facies follows sequence stratigraphic predictions with microbialites occurring in every accommodation profile late highstand and early transgressive systems tracts favor greater lateral extent of thrombolite build ups whereas later transgressive to early highstand system tracts favor greater lateral discontinuity and compartmentalization of buildup reservoir facies pustular laminites occur in close association with thrombolite buildups but form laterally extensive sheets in late transgressive to late highstand periods crinkly laminites form during late transgressive to early highstand systems tracts and may represent maximum flooding intervals when the flux of carbonate sediment was greatly reduced allowing pelagically derived organics to accumulate

upper proterozoic evaporites in the amadeus basin central australia and their role in basin tectonics the amadeus basin an isolated intracratonic basin in the center of the australian continent contains an upper proterozoic to mid paleozoic stratigraphic succession of shallow marine sediments that in places exceeds 14 km in thickness the gillen member of the bitter springs formation which occur toward the base of the upper proterozoic succession includes evaporites which are among the world9s earliest 0 8 to 0 7 ga because of their age the evaporites have been cited in discussions of sea water chemistry and have been the focus of scrutiny for early life forms in spite of their importance the evaporites are poorly known particularly from the viewpoint of their depositional and tectonic setting in an attempt to rectify this deficiency more than 6 000 km of seismic data were analyzed in conjunction with a field and well log study of the unit the late proterozoic amadeus basin appears to have consisted of two major poorly circulated anoxic sub basins which perhaps opened to the ocean to the southeast through the adelaide geosyncline data relating to facies are limited but suggest that deposition of the evaporites was cyclic and followed the patterns identified in other major evaporite basins the carbonates and sulfates being closer to the basin margins and later stage halite and possibly potassium salts being toward the basin center the evaporites were deposited in a shallow marine setting at the time of a relative sea level high stand the apparent sea level high may relate to basin dynamics whereas the cyclicity of the evaporites may be due to eustatic sea level controls acting on the barrier to allow intermittent inflow of sea water salt tectonism began shortly after the evaporites were deposited and continued throughout basin development consequently most of the major anticlinal structures have salt cores the geometry of the salt structures suggests that during their growth the mean strain rate was 10 16 s 1 a rate typical of large salt structures growth on these salt structures has played an important role in controlling later sedimentation particularly during the cambrian

the major ion composition of cenozoic seawater the past 36 million years from fluid inclusions in marine halite fluid inclusions from ten cenozoic eocene miocene marine halites are used to quantify the major ion composition mg2 ca2 k na so42 and cl of seawater over the past 36 my criteria used to determine a seawater origin of the halites include 1 stratigraphic sedimentologic and paleontologic observations 2 br in halite 3 34s of sulfate minerals 4 87sr 86sr of carbonates and sulfates and 5 fluid inclusion brine compositions and evaporation paths which must overlap from geographically separated basins of the same age to confirm a global seawater chemical signal changes in the major ion chemistry of cenozoic seawater record the end of a systematic long term 150 my shift from the ca2 rich mg2 and so42 poor seawater of the mesozoic cacl2 seas to the mgso4 seas with higher mg2 and so42 ca2 of the cenozoic the major ion composition of cenozoic seawater is calculated for the eocene oligocene 36 34 ma serravallian tortonian 13 5 11 8 ma and the messinian 6 5 ma assuming chlorinity 565 mmolal salinity and the k concentration 11 mmolal are constant and the same as in modern seawater fluid inclusions from cenozoic marine halites show that the concentrations of mg2 and so42 have increased in seawater over the past 36 my and the concentration of ca2 has decreased mg2 concentrations increased from 36 mmolal in eocene oligocene seawater 36 34 ma to 55 mmolal in modern seawater the mg2 ca2 ratio of seawater has risen from 2 3 at the end of the eocene to 3 4 and 4 0 respectively at 13 5 to 11 8 ma and 6 to 5 ma and to 5 in modern seawater eocene oligocene seawater 36 34 ma has estimated ranges of so42 14 23 mmolal and ca2 11 20 mmolal if the ca2 so42 product is assumed to be the same as in modern seawater 300 mmolal2 eocene oligocene seawater had ca2 16 mmolal and so42 19 mmolal the same estimates of ca2 and so42 for serravallian tortonian seawater 13 5 11 8 ma are so42 19 27 mmolal and ca2 8 16 mmolal and so42 24 mmolal and ca2 13 mmolal if the ca2 so42 product is equal to that in modern seawater messinian seawater has an estimated range of so42 21 29 mmolal and ca2 7 15 mmolal with so42 26 mmolal and ca2 12 mmolal assuming the ca2 so42 product is equal to that in modern seawater regardless of the estimation procedure so42 shows progressively increasing concentrations from 36 ma to the present values which are the highest of the cenozoic

mesozoic dissolution tectonics on the west central shelf uk central north sea abstract 3 d seismic mapping of the upper jurassic kimmeridge clay formation on the west central shelf in the central north sea reveals a complex fault array which is constrained by seismic interpretation and well control to be of late jurassic early cretaceous age fault shapes in plan view range from linear to circular linear fault lengths are 200 300 m to 5 km the strongly curved and circular faults range in diameter from 100 1000 m fault trends are apparently random and display no correlation in location or trend with basement sub zechstein structures there is however a strong link between this fault pattern and the structure of the top zechstein top salt surface linear faults occur at the edges of elongate salt walls and the circular faults lie directly above structures which have been interpreted here as tall steep sided salt chimneys the salt chimneys are present only in the thick elongate minibasins of triassic sediment which lie between the salt walls it is argued that salt dissolution controls the timing location orientation and shape of the late jurassic early cretaceous faults a model is provided to account for the development of both salt walls and chimneys we suggest that early triassic karstification of the zechstein evaporites led to development of an array of circular collapse features during the ensuing episode of triassic halokinesis which led to minibasin subsidence and salt wall growth salt passively intruded the circular collapse features within the subsiding minibasins to form narrow salt chimneys the resulting array of salt walls and chimneys was subject to dissolution during subsequent subaerial exposure and the late jurassic marine transgression of the basin creating the observed fault array prior to sealing of the salt from circulating groundwater by compaction of the upper jurassic and lower cretaceous shales which blanket the area

paleoproterozoic stark formation athapuscow basin northwest canada record of cratonic scale salinity crisis abstract the paleoproterozoic stark formation a thick breccia unit formed during the transition from marine to non marine foreland basin fill in the athapuscow basin northwest territories canada four types of breccia occur in the stark formation 1 bedded breccias at the base of the unit overlying the basinal facies of the underlying pethei group are interpreted as debris flows 2 massive chaotic breccias occur in the lower part of the stark formation throughout the basin and these are interpreted to be evaporite collapse breccias 3 siltstone and mudstone breccia occurs in the upper part of the stark formation and is also interpreted as evaporite collapse breccia though much less evaporite was involved in its formation than the underlying chaotic breccia and 4 laminar accretionary breccia consisting of laminar carbonate clasts commonly with flat bottoms and convex up upper surface geometries these clasts occur in lenticular pods and are interpreted to form in caves following dissolution of evaporites or carbonates the widespread presence and abundance of evaporite pseudomorphs especially halite in all breccia types the stratigraphic restriction of the stark formation and irregular filling above this unit indicates that all but the bedded breccia formed by the dissolution and collapse of evaporites chaotic breccia directly above underlying pethei group shallow water and deep water carbonates indicates the evaporates formed across the platform and adjacent deep basin foundered megaclasts up to 1 5 km long and 40 m thick in basinal settings suggest cumulative evaporite thicknesses on the order of a few tens to a few hundreds of meters carbonate clasts in all the breccias contain abundant wave ripples planar laminations stromatolites and ooids these structures along with a paucity of subaerial exposure features indicates nearly all carbonate deposition in the stark was subaqueous thus by inference the evaporites in the stark formation were also partially subaqueous displacive halite pseudomorphs are common throughout all facies in the stark formation silicified and dolomite filled halite pseudomorphs are common along the contact between the evaporite collapse breccia and underlying carbonate platform pethei group and surrounding large clasts in basinal settings silicified hopper crystal casts within deep water rhythmites of the uppermost pethei group indicate precipitation of halite over deep waters gypsum pseudomorphs are exceedingly rare and anhydrite pseudomorphs were not observed the dominance of halite features in this basin and two penecontemporaneous basins kilihigok basin and wopmay orogen surrounding the slave craton suggests that relative to the present day seawater at 1 9 to 1 8 ga was oversaturated with respect to calcium carbonate had higher hco3 concentration and possibly was depleted in so42 the dearth of extensive thick bedded marine gypsum in the archean and early paleoproterozoic can be explained by low atmospheric oxygen content prior to 2 ga in conjunction with increased carbonate saturation before 1 6 ga

salt tectonics in the norwegian danish basin a study is presented of salt tectonics in a 28 000 km2 area in the norwegian danish basin of the north sea the basin is mainly of permian to early jurassic age and contains sediments which locally reach 8 km in thickness zechstein upper permian salt occurs widely in the basin and has given rise to widespread and intense salt tectonics using seismic reflection sections with a total length of 14 000 km as well as borehole data 86 salt structures have been mapped within the study area the floor of the basin that is the base of the zechstein salt and the faults affecting this floor have also been mapped it is therefore possible to investigate the association of faults and salt structures and hence to ascertain to what extent faulting of the basin floor may have initiated salt movement various lines of investigation and reasoning have been developed to arrive at an estimate of the number of structures for which salt movement is likely to have been initiated by faulting of the basin floor it is found that only for a limited number of salt structures probably about five out of a total of 86 structures the salt movement is likely to have been initiated by faulting the very large majority of the salt structures originated independent of faulting the mechanism responsible for the initiation of salt movement in this majority has also been studied it is found that these salt structures can be divided into two groups one group comprises about 37 salt structures which are randomly distributed nearly all of them are situated where the present basin floor is relatively flat and at its deepest they are thought to be the result of classical halokinesis that is the autonomous upward movement under the influence of gravity of mobile salt at locations where the sedimentary overburden at the time of initial movement was locally less heavy a second group comprises about 43 individual structures which are aligned along nine so called trend lines it is first shown that these trend lines are independent of faults and fault trends in the basin floor the origin of these trends is then discussed by considering two alternative hypotheses for the mechanism responsible for the initiation of salt movement along these trend lines the first hypothesis assumes that salt ridges in the subsurface are initially pushed up by a prograding sedimentary overburden the second hypothesis assumes that initial ridges of mobilized salt are formed along a palaeo depth contour which migrates up the flank of the subsiding basin both hypotheses assume that the salt ridges thus formed can subsequently develop into individual salt structures which produce the observed pattern of trend lines a critical analysis of these two alternative hypotheses leads to the conclusion that the second one involving the concept of a palaeo depth contour which migrates up flank is the more acceptable the mechanism involved may be characterized as a special type of halokinesis namely as halokinesis constrained to operate along a line

ediacaran growth of the marine sulfate reservoir abstract the sulfur isotope record as preserved in sedimentary sulfate and sulfide minerals reflects the progressive oxidation of earth s surface environment the latest proterozoic to early paleozoic marine sulfur isotope record is distinguished by unusually high 34 s values in marine sulfates and sulfides geological considerations and sulfur isotopic data indicate that the 34 s enrichment of the marine sulfate reservoir was the result of a rapid increase in the net fractionation between sedimentary sulfate and pyrite 34 s without a compensatory decline in the fractional burial rate of pyrite f pyr a simple one box model in which 34 s is linked non linearly to marine so 4 is used to explore the response in sedimentary sulfur isotope records 34 s sulf and 34 s pyr to an increasing marine sulfate reservoir the model results when compared with a compilation of existing 34 s sulf and 34 s pyr data suggest that the marine sulfate reservoir grew rapidly to roughly phanerozoic levels in the middle ediacaran period before the gaskiers glaciation and prior to the onset of the shuram wonoka negative 13 c anomaly it is further hypothesized that p o 2 levels rose prior to the onset of this extreme perturbation to the global carbon cycle permitting the oxidation of a previously euxinic deep ocean and appearance of the first ediacaran fauna by ca 575 ma despite the enormous demand on oxidants implied by this large magnitude and long duration carbon isotope anomaly

3d seismic analysis of circular evaporite dissolution structures eastern mediterranean buried circular collapse structures above a tabular evaporitic body are recorded by recently acquired 3d seismic data on the levant basin and continental margin offshore israel eastern mediterranean the structures formed during the pliocene as buried messinian late miocene evaporites underwent extensive dissolution in a submarine deep water setting three dimensional seismic analysis is used to describe the detailed morphology of the structures and the associated overburden allowing the reconstruction of their origin and development it is proposed that evaporite dissolution led to the collapse of the weakly lithified overburden and this deformed with a series of concentric extensional faults from the structural analysis of the overburden the estimated maximum duration of the dissolution event is 0 75 1 ma the mechanism proposed for the creation of the circular collapse structures is subjacent dissolution of the more soluble evaporites in the messinian evaporites as a result of focused vertical fluid flow at the base of the evaporitic series rapid release of overpressured fluids as for example during an earthquake is thought to have initiated the focused fluid flow which impinged on the evaporitic seal to the point where dissolution occurred creating the localized circular collapse structures in the overburden

seismic facies of the permian section of the barents shelf analysis and interpretation abstract the palaeozoic succession of the barents shelf has been investigated with data from only a few wells however a considerable amount of high quality seismic data complements the sporadic well control permitting an extensive seismic study of the three major intervals distinguished in the permian section the lowermost interval i consists of an evaporitic sequence both vertical and horizontal gradations of the seismic facies are recognized and interpreted in terms of basin evolution in the centre of the basin a lower unit characterized by high amplitude parallel reflections grades upwards to a mounded reflection free upper unit the mounds are interpreted as salt pillows overlying a lower unit with lithology dominated by anhydrite a rather thick evaporite interval in basinal setting passes laterally to a mixed carbonaceous anhydritic section on the margins and basinal highs the thickness distribution and the areal extent of the seismic facies provide information on the configuration of the evaporite basin on syn sedimentary tectonism and on evaporite diagenesis as a result of renewed transgression the overlying interval ii was deposited in open marine conditions on a shallow shelf belts and patches of seismic mounds interpreted as carbonate build ups are widely spread on the basinal highs and on the basin margins depositional environments are inferred from the distribution of build ups during the progressive drowning of the shelf a back stepping belt of seismic mounds were deposited on the southern basin margin this transgressive episode which constitutes interval iii culminated in the generalized deposition of deep water facies

mid neoproterozoic biostratigraphy and isotope stratigraphy in australia abstract palynomorph and stromatolite assemblages from mid neoproterozoic strata of australia appear highly significant when collated with results obtained from isotope stratigraphy and allow correlation across the centralian superbasin and adelaide rift complex the majority of palynomorphs from the lower mid neoproterozoic 830 ma have limited biostratigraphic value although a few apparently have restricted ranges carbonates at this stratigraphic level contain the distinctive stromatolite acaciella australica and other taxa of the acaciella australica stromatolite assemblage these carbonates have 13ccarb values of between 4 and 0 and associated anhydrite has 87sr 86sr ratios of 0 7069 these depleted 13ccarb values are immediately preceded by values of 6 1 in the western officer and amadeus basins of the centralian superbasin similar c and sr isotopic features are seen in the similarly aged upper shaler supergroup of canada upper mid neoproterozoic 760 ma carbonates contain the distinctive stromatolite baicalia burra and other stromatolites of the baicalia burra stromatolite assemblage siliciclastic horizons in the same interval contain the acritarch cerebrosphaera buickii and have 13ccarb values of between 0 and 8 the highest mid neoproterozoic 13ccarb values of between 6 7 and 8 1 occur within the stratigraphic range of c buickii in australia in the western officer basin and adelaide rift complex the mid neoproterozoic 13ccarb peaks are preceded by values of ca 0 the apparently restricted range of c buickii in australia suggests the need for reinterpretation of the stratigraphic position of its occurrence in the svanbergfjellet formation akademikerbreen group spitsbergen which was previously considered a post sturtian glacial deposit

early cambrian seawater chemistry from fluid inclusions in halite from siberian evaporites abstract thirty nine samples of lower cambrian marine sedimentary halite with zoned primary fluid inclusions were analyzed using the umca method introduced by petrichenko 1973 petrichenko o i 1973 metody doslidzhennya u mineralakh galogennykh porid naukova dumka kyiv 91 pp in order to determine the contents of k mg 2 so 4 2 and ca 2 ions and to interpret the composition of parent seawater the analyzed samples were from the giant salt basin of eastern siberia where five major phases of salt deposition can be distinguished in the late vendian danilovo and early cambrian usolye belsk angara and litvintsevo basins our samples are all from early cambrian basins although the largest data set comes from the angara basin the results indicate that early cambrian parent seawater was similar in all studied basins and was characterized by lower mg 2 and so 4 2 and higher ca 2 concentrations relative to modern seawater the concentration of k in early cambrian seawater was similar to that of modern seawater accordingly the seawater in the entire early cambrian was ca 2 rich and so 4 2 poor the change of composition of seawater from so 4 2 rich ca 2 poor in the latest neoproterozoic to ca 2 rich so 4 2 poor recorded throughout the early cambrian probably occurred within a relatively short time span during the earliest cambrian nemakit daldynian time and caused a rise in ca 2 concentrations in the shelf seas leading to the onset of biocalcification and then the cambrian explosion

physical modeling of structures formed by salt withdrawal implications for deformation caused by salt dissolution by creating 15 physical models we investigated deformation above subsiding tabular salt salt walls and salt stocks dry quartz sand simulated a brittle sedimentary roof above viscous silicone representing salt the modeled diapiric walls had linear planforms and rectangular semicircular triangular or leaning cross sectional shapes the stock was cylindrical in models where the source layer or allochthonous salt sheet was initially tabular a gentle flat bottomed syncline bounded by monoclinal flexures formed above a linear zone where the silicone was locally removed above all subsiding diapirs the deformed roof was bounded by an inner zone of steep convex upward reverse faults and an outer zone of normal faults above subsiding diapiric walls extensional and contractional zones were balanced above the subsiding salt stock conical concentric fault zones comprised inner reverse faults and outer normal faults sediments were added both before prekinematic and during synkinematic salt withdrawal in entirely prekinematic roofs reverse fault zones and normal fault zones both widened with time reverse faults propagated upward from the corners of the withdrawing diapirs new reverse faults formed in the footwalls of reverse faults each nearer the center of the deepening roof trough conversely new normal faults formed successively outward from the sagging trough synkinematic deposition retarded faulting but the pattern of inner reverse and outer normal faults was repeated however reverse faults formed successively outward whereas normal faults formed inward new conceptual models suggest that salt dissolution forms similar structures to those physically modeled for salt withdrawal the appropriate physical models resemble natural dissolution structures above tabular salt extension alone above diapirs is not caused merely by salt withdrawal or dissolution but by regional extension or active diapirism

a depositional model for the terminal neoproterozoic early cambrian ara group evaporites in south oman six evaporite carbonate sequences are recognized in the terminal neoproterozoic early cambrian ara group in the subsurface of oman individual sequences consist of a lower evaporitic part that formed mainly during a lowstand systems tract overlying platform carbonates contain minor amounts of evaporites and represent transgressive and highstand systems tracts detailed sedimentological and geochemical investigation of the evaporites allowed reconstruction of the depositional environment source of brines and basin evolution at the beginning of the evaporative phase prograding succession a shallow water carbonate ramp gradually evolved into a series of shallow sulphate and halite salinas minor amounts of highly soluble salts locally record the last stage of basin desiccation this gradual increase in salinity contrasts sharply with the ensuing retrograding succession in which two corrosion surfaces separate shallow water halite from shallow water sulphate and shallow water sulphate from relatively deeper water carbonate respectively these surfaces record repeated flooding of the basin dissolution of evaporites and stepwise reduction in salinity final flooding led to submergence of the basin and the establishment of an open water carbonate ramp marine fossils in carbonates and bromine geochemistry of halite indicate a dominantly marine origin for the brines the ara group sequences represent a time of relatively stable arid climate in a tectonically active basin strong subsidence allowed accommodation of evaporites with a cumulative thickness of several kilometres while tectonic barriers simultaneously provided the required restricted conditions subsidence allowed evaporites to blanket basinal and platform areas the study suggests a deep basin shallow water model for the evaporites

the effect of gravitational compaction on estimation of vertical salt structure growth abstract the growth history of salt structures is reflected in the variation of the present thickness of the overlying and surrounding sediments in relation to the regional normal a method for estimating the growth history of salt structures in the pillow and post diapiric phase including the effect of differential syn and post depositional compaction is described the assumption that stratigraphic thinning and established structural relief is a result of vertical salt movement alone is inadequate a quantification of the true vertical movement of the salt crest necessitates a correction for compaction and differential basement subsidence the on structure thickness is compared to the regional normal for the time stratigraphic interval under consideration on the basis of the dominant porosity reducing process the minimum and maximum net growth can be calculated the true net growth is assumed to be contained within these limits probably closest to the minimum net growth two shallow salt diapirs in the danish basin exemplify the relationship between the observed stratigraphic thinning and structural relief and the differential compaction differential basement subsidence and vertical growth of the salt surface the average minimum and maximum net growth of the salt surface at the batum and mors salt diapirs in the post diapiric phase are calculated the minimum and maximum net growth values for the batum salt diapir are 731 m and 1303 m respectively and for the mors salt diapir 276 m and 996 m respectively thus emphasizing the importance of including the effect of compaction in the calculations of salt structure net growth the implications for storage of hydrocarbons radioactive and chemical wastes as well as for petroleum exploration are emphasized

dynamic changes in sulfate sulfur isotopes preceding the ediacaran shuram excursion large excursions in 13 c and 34 s are found in sedimentary rocks from the ediacaran period that may provide detailed mechanistic information about oxidation of earth s surface however poor stratigraphic resolution and diagenetic concerns have thus far limited the interpretation of these records here we present a high resolution record of carbon and sulfur isotopes from the khufai formation leading up to and including the onset of the shuram carbon isotope excursion we document large coherent excursions in the sulfur isotope composition and concentration of carbonate associated sulfate cas that occur both independently and synchronously with the carbon isotope excursion isotopic changes appear decoupled from major stratigraphic surfaces and facies changes suggesting regional or global processes rather than local controls our data suggest that very low marine sulfate concentrations are maintained at least through the middle khufai formation and require that the burial fraction of pyrite and the fractionation factor between sulfate and pyrite necessarily change through deposition reconciliation of simultaneous up section increases in marine sulfate concentration and 34 s cas requires the introduction of strongly 34 s enriched sulfate possibly from weathering of cryogenian and earlier ediacaran 34 s enriched pyrite our analysis of the onset of the shuram carbon isotope excursion observed in stratigraphic and lithologic context is not consistent with diagenetic or authigenic formation mechanisms instead we observe a contemporaneous negative excursion in sulfate 34 s suggesting linked primary perturbations to the carbon and sulfur isotope systems this work further constrains the size isotopic composition and potential input fluxes of the ediacaran marine sulfate reservoir placing mechanistic constraints on possible drivers of extreme isotopic perturbations during this critical period in earth history

new insights into the genesis of the miocene collapse structures of the island of gozo malta central mediterranean sea the large palaeosinkholes located in the nw of gozo central mediterranean sea malta offer excellent exposures that provide information on the geometry and kinematics of large karst related collapse structures detailed geological analysis of these peculiar palaeosinkholes indicates that deep seated evaporite dissolution is the most feasible hypothesis to explain their formation according to the following evidence 1 several structures have been formed by progressive foundering of cylindrical blocks with limited internal deformation as revealed by the synsedimentary subsidence recorded by their miocene sedimentary fill this subsidence mechanism is more compatible with interstratal dissolution of evaporites than karstification and cave development in limestone formations 2 the dimensions and deformation style of the palaeosinkholes are similar to those of other collapse structures related to deep seated dissolution of salt bearing evaporites 3 the arcuate monocline associated with some of these collapse structures is also a characteristic feature of subsidence related to dissolution of evaporites however no major evaporite formations have been documented so far in the subsurface of the malta platform supplementary material detailed descriptions of the collapse structures of the island of gozo malta central mediterranean sea are available at www geolsoc org uk sup18808

seismic interpretation of carbonate depositional environments most seismic stratigraphic or seismic lithologic interpretations have been applied to clastic depositional systems because of their unique sedimentologic and mineralogic characteristics carbonate rocks are more difficult to study using seismic data increased knowledge of carbonate deposits and their petrophysical parameters and higher quality geophysical tools now permit a better understanding of carbonate rocks using seismic images in this paper seismic facies of carbonate rocks are characterized step by step from the basin to the supratidal environment 1 pelagic deposits shales and micritic limestone layers exhibit continuous parallel reflections with an apparently high frequency chalk deposits display continuous high amplitude reflections at the top and base with an internal reflection free zone 2 talus deposits are characterized by discontinuous oblique reflections of high amplitude carbonate debris flow generates chaotic reflections with hummocky surfaces channels exhibit erosional truncations 3 reef barriers are mound shaped biogenic deposits that display marginal onlapping reflections overlying reflections drape the reefs and underlying reflections exhibit pull up or pull down effects hyperbolic diffractions also may occur 4 platform border sands are mound shaped oblique reflections of moderately high amplitude 5 inner shelf strata are characterized by parallel continuous reflections typically displaying low frequency patch reefs may be distinguished within the inner shelf system mound shape drape of overlying reflections velocity anomalies and spurious events 6 where dolomitized the intertidal to supratidal facies exhibit a marbled zone a practically reflection free zone with a few discontinuous reflections diagenetic events such as paleokarst zones may be delineated using special analyses involving amplitude offset variations the study of carbonate depositional environments and petrophysical characteristics provides a more complete geologic insight including relative changes in sea level within a sedimentary basin such studies are fundamental in analyzing carbonate reservoirs and their paleogeographic settings

chemical composition of seawater in neoproterozoic results of fluid inclusion study of halite from salt range pakistan and amadeus basin australia abstract data on chemical composition of brines in primary inclusions of marine halites and on mineralogy of marine evaporites and carbonates lead to the conclusion that during the phanerozoic two long term cycles of chemical composition of seawater existed during each of those cycles seawater dominantly a na k mg ca cl ca rich type changed to a na k mg cl so 4 so 4 rich type recrystallised halite from the uppermost neoproterozoic salt range formation ca 545 ma in pakistan contains solitary inclusions indicating so 4 rich brines this supports the concept derived from the study on primary fluid inclusions from the neoproterozoic ara formation of oman so 4 rich seawater existed during latest neoproterozoic time ca 545 ma in contrast samples of recrystallised halite from the bitter springs formation 840 830 ma in australia contain inclusion brines that are entirely ca rich indicating that basin brines and seawater were ca rich during deposition of central australian evaporites these combined data supported by the timing of aragonite and calcite seas suggest that during the proterozoic significant oscillations of the chemical composition of marine brines and seawater occurred which are similar to those known to exist during the phanerozoic it is suggested that ca rich seawater dominated for a substantial period of time more than 200 ma at 650 ma this was replaced by so 4 rich seawater finally returning to ca rich seawater at 530 ma

evolution of the nw margin of the north permian basin uk north sea mapping of depositional zones within the zechstein supergroup using semi regional 2d and 3dseismic data constrains the geological evolution of the northwestern margin of the north permian basinin uk offshore quadrants 14 15 19 20 and 21 four depositional zones are identified on the basis of mappable seismic facies which are assumed to vary with halite content well control characterizes the most reflective seismic facies as an interbedded carbonate and anhydrite shelf succession which grades laterally through clastic rich sequences to the least reflective a thick halite basinal equivalent the trendsof the depositional zones illustrate the geometry of the north permian basin and may have been locally controlled by basement lineaments within the north permian basin a n s trending axis devil s hole high characterized by thick reflective facies separates the central north sea from the forth approaches basin basement fault blocks control facies type and thickness along the northwest margin of the north permian basin in the moray firth area permian tilt of the zechstein shelf related to regional flexure played a role in precipitating gravity driven tectonics within the seismically reflective facies on the east flank of the devil s hole high this permian phase of salt tectonics resulted in numerous intra zechstein structures which can be interpreted as rafted and folded zones of interbedded halite carbonate sulphate and clastics the rafts initially slid downdip towards the main halite basin then their evolution became dominated by differential subsidence of the interbeds which forced salt layers into adjacent passive diapirs this episode predates deposition of the triassic smith bank shales which rest upon the zechstein with local angular unconformity the spatial distribution of zechstein facies influenced the tectonostratigraphy of the triassic jurassic and cretaceous systems boundaries between zechstein facies correspond to the limit of triassic halokinesis on the west central shelf and the limit of detachment of jurassic and lower cretaceous thin skinned tectonics in moray firth half graben

superheavy pyrite δ34spyr δ34scas in the terminal proterozoic nama group southern namibia a consequence of low seawater sulfate at the dawn of animal life sulfur isotope analysis 34 s of well preserved carbonates spanning an 10 ma interval of the terminal proterozoic nama group reveals that disseminated pyrite is consistently enriched in 34 s relative to coeval seawater sulfate as preserved in carbonate associated sulfate cas this observation is not consistent with the current paradigm for interpreting the geologic record of sulfur isotopes which assumes that pyrite 34 s 34 s pyr will be equal to or less than co occurring cas 34 s 34 s cas due to the kinetic isotope effect of bacterial sulfate reduction bsr that favors the lighter isotope of sulfur 32 s during sulfur oxygen bond breakage although the precise mechanism of pyrite sulfur isotope enrichment is debatable our combined observations of extremely 34 s enriched pyrite low bulk rock concentrations of sulfur from cas and pyrite and high frequency fluctuations in 34 s cas and 34 s pyr throughout the nama group carbonates point to very low concentrations of sulfate in portions of the terminal proterozoic ocean the additional occurrence of 34 s enriched pyrite in contemporaneous terminal proterozoic sections from poland and canada reveal that low seawater sulfate may have been widespread in the oceans at this time however the absence of such extremely 34s enriched pyrite from well preserved coeval carbonate sections in oman suggests that such conditions were not globally uniform low geographically varied concentrations of marine sulfate in terminal proterozoic time are consistent with elevated geographically varied concentrations of reactive nonpyritized iron in marine shales recently reported for this interval suggesting that high dissolved fe ii and low o 2 persisted in a range of marine facies as late as 543 ma tens of millions of years after the origin of animals yet prior to their major diversification in early cambrian time

silicate versus carbonate weathering in iceland new insights from ca isotopes to trace sources of ca in icelandic rivers we report elemental and ca isotope data for rivers high and low temperature groundwater basalt hydrothermal calcite including iceland spar and stilbite and heulandite which are two types of zeolites commonly formed during low grade metamorphism of basalt in agreement with previous research we find that rivers have higher 44 40ca values than basalt with a maximum difference of 0 40 this difference may reflect isotope fractionation in the weathering zone i e preferential uptake of 40ca during clay mineral formation adsorption and other geochemical processes that cycle ca however calcite 44 40ca values are also up to 0 40 higher than bedrock values and on a diagram of 44 40ca versus sr ca nearly all waters plot within a plausible mixing domain bounded by the measured compositions of basalt and calcite with glacial rivers plotting closer to calcite than non glacial rivers calcite and heulandite form during hydrothermal alteration of basalt in the deep lava pile and often occur together in metabasalts now exposed at the surface because heulandite 44 40ca values are 1 2 lower than basalt we suggest that 40ca uptake by heudlandite explains the relatively high 44 40ca values of calcite and that calcite weathering in turn elevates riverine 44 40ca values high mechanical erosion rates are known to facilitate the exposure and weathering of calcite which explains the isotopic contrast between glacial and non glacial watersheds using a mixing model we find that calcite weathering provides 0 65 of the ca in non glacial rivers and 25 90 of the ca in glacial rivers with silicate weathering providing the remainder icelandic hydrothermal calcite contains mantle carbon noting that zeolite facies metamorphism and hydrothermal fluid circulation are ubiquitous characteristics of basaltic eruptions and assuming that hydrothermal calcite in other basaltic settings also contains mantle carbon we suggest that the contribution of basalt weathering to long term co2 drawdown and climate regulation may be less significant than previously realized

salt related deformations in the deep antalya basin preliminary results of the mac gan cruise abstract seismic investigation of the deep antalya basin shows the contribution of halokinesis of messinian salt in shaping the shallow morphological setting of the area the causes of such salt related deformations are difficult to firmly establish the plio quaternary overburden 200 250 ms seems too little to be responsible for the extensive diapirism so widespread in the antalya basin extensional tectonics may therefore be involved to some degree there are indications that the messinian of the deep antalya basin bears two separate salt layers in this case a significant overburden in the form of the sum of plio quaternary sediments and the younger evaporites may be present with the older salt layer responsible for most of the observed doming

seismic response to collapse structures in the southern north sea abstract the presence of probable salt dissolution associated collapse structures in the north sea based on the evidence of contemporary seismic data was discussed by lohmann 1972 since then little attention has been paid to them in north sea literature although many oil companies will have studies on file this paper describes and illustrates a selection of such features in the area from recent seismic data and suggest a broad classification for them as they are seen on the seismic section based on mode of origin reference will also be made to certain types of carbonate dissolution and collapse effects

large carbonate associated sulfate isotopic variability between brachiopods micrite and other sedimentary components in late ordovician strata carbonate associated sulfate cas is trace sulfate incorporated into carbonate minerals during their precipitation its sulfur isotopic composition is often assumed to track that of seawater sulfate and inform global carbon and oxygen budgets through earth s history however many cas sulfur isotope records based on bulk rock samples are noisy to determine the source of bulk rock cas variability we extracted cas from different internal sedimentary components micro drilled from well preserved late ordovician and early silurian age limestones from anticosti island quebec canada mixtures of these components whose sulfur isotopic compositions vary by nearly 25 can explain the bulk rock cas range large isotopic variability of sedimentary micrite cas 34 s depleted from seawater by up to 15 is consistent with pore fluid sulfide oxidation during early diagenesis specimens recrystallized during burial diagenesis have cas 34 s enriched by up to 9 from hirnantian seawater consistent with microbial sulfate reduction in a confined aquifer in contrast to the other variable components brachiopods with well preserved secondary layer fibrous calcite a phase independently known to be the best preserved sedimentary component in these strata have a more homogeneous isotopic composition these specimens indicate that seawater sulfate remained close to about 25 v cdt through hirnantian end ordovician events including glaciation mass extinction carbon isotope excursion and pyrite sulfur isotope excursion the textural relationships between our samples and their cas isotope ratios highlight the role of diagenetic biogeochemical processes in setting the isotopic composition of cas

cenozoic evolution of the eastern danish north sea this paper provides a review of recent high resolution and conventional seismic investigations in the eastern danish north sea and describes their implications for the development of the eastern north sea basin the results comprise detailed time structure maps of four major unconformities in the eastern danish north sea the top chalk surface mid paleocene near top oligocene the mid miocene unconformity and base quaternary the maps show that the eastern danish north sea has been affected by faulting and salt diapirism throughout the cenozoic carbonate mounds erosional valleys and pockmark or karst like structures were identified at the top of the upper cretaceous danian chalk group strike parallel erosional features and depositional geometries observed at near top oligocene and at the mid miocene unconformity indicate that these major sequence boundaries can be attributed to large scale lateral changes in sediment supply directions increases in sediment flux to the southeastern north sea at the eocene oligocene transition and in the post middle miocene appear to correlate with similar events world wide and with long term 18o increases indicating forcing by global factors i e eustasy and climate stratal geometries observed on the seismic data indicate that the so called neogene uplift of the eastern danish north sea may have been hundreds of metres less than previously suggested it is argued that late cenozoic uplift of the basin margin and of mountain peaks in southern norway may have been caused entirely by isostatic uplift of the crust in response to accelerated late cenozoic denudation and dissection of topography created in the paleogene the late cenozoic periods of accelerated denudation and incision rates were most likely driven by climatic deterioration and long term eustatic lowering rather than active late cenozoic tectonics the cause of which is conjectural a series of shallow thrust structures and an associated system of deep buried valleys were mapped thrust faulting most likely occurred in response to gravitational loading at the margin of an advancing ice sheet and it was followed by deep incision due to subglacial melt water erosion probably during the elsterian glaciation