linear coupling of carbon and strontium isotopes in rotliegend sandstone north sea evidence for cross formational fluid flow isotopic analyses of carbon and strontium in diagenetic dolomite and ankerite present in the eolian lower permian rotliegend sandstone southern north sea show an excellent linear correlation between delta sup 13 c and sup 87 sr sup 86 sr r 0 93 suggesting that the sources of these elements were interlinked the isotopic data indicate that late permian zechstein seas flooded the basin and displaced the interstitial meteoric rotliegent pore fluids producing early dolomite with predominantly marine bicarbonate delta sup 13 c minus 15 per thousand and strontium sup 87 sr sup 86 sr 0 7076 isotopic signatures the isotopic values of this dolomite form a trend to merge with the diagenetically later ankerite which has lower sup 13 c sup 12 c delta sup 13 c minus 4 per thousand and more radiogenic strontium sup 87 sr sup 86 sr 0 7112 the obvious source of low delta sup 13 c carbon was thermal decarboxylation of organic matter in the underlying upper carboniferous mudstones the authors also suggest that the increasing quantities of radiogenic strontium sup 87 sr sup 86 sr 0 720 were released when organic acids dissolved silicates feldspar in the same underlying mudstones the upward movement of fluid or diffusion of ions across formation boundaries therefore progressively became the dominant control on carbonate precipitation in the rotliegend sandstone

reservoir characteristics and their effects on hydrocarbon accumulation in lacustrine turbidites in the jiyang super depression bohai bay basin china abstract a great difference exists between the hydrocarbon charging characteristics of different tertiary lacustrine turbidites in the jiyang super depression of the bohai bay basin east china based on wireline log data core observation and thin section analyses this study presents detailed reservoir property data and their controlling effects from several case studies and discusses the geological factors that govern the hydrocarbon accumulation in turbidite reservoirs the lacustrine fluxoturbidite bodies investigated are typically distributed in an area of 0 5 10 km2 with a thickness of 5 20 m the sandstones of the tertiary turbidites in the jiyang super depression have been strongly altered diagenetically by mechanical compaction cementation and mineral dissolution the effect of compaction caused the porosity to decrease drastically with the burial depths especially during the early diagenesis when the porosity was reduced by over 15 the effect of cementation and mineral dissolution during the late stage diagenesis is dominated by carbonate cementation in sandstones high carbonate cement content is usually associated with low porosity and permeability carbonate dissolution secondary porosity zone and primary calcite dissolution is believed to be related to thermal maturation of organic matter and clay mineral reactions in the surrounding shales and mudstone two stages of carbonate cementation were identified the precipitation from pore water during sedimentation and secondary precipitation in sandstones from the organic acid dissolved carbonate minerals from source rocks petrophysical properties have controlled hydrocarbon accumulation in turbidite sandstones high porosity and permeability sandstones have high oil saturation and are excellent producing reservoirs it is also noticed that interstitial matter content affects the oil bearing property to some degree there are three essential elements for high oil bearing turbidite reservoirs excellent pore types low carbonate cement 15 and average permeability 10 md

thin section and s e m textural criteria for the recognition of cement dissolution porosity in sandstones cement porosity relationships are described from the lower triassic sherwood sandstone group and the middle jurassic ravenscar group in the united kingdom calcite cemented sandstones display a variety of replacement textures with preferential replacement of grains and of overgrowth faces with high free surface energy dolomite and siderite cemented sandstones display similar textures but replacement is less specific and euhedral overgrowth surfaces are commonly embayed by carbonates examination of the more porous sandstones with the scanning electron microscope reveals a range of pitting and embayment textures in authigenic overgrowths and in detrital grains these range from small v shaped notches and pits through regular and irregular shaped embayments into large depressions these textures appear to be morphologically similar to the quartz surfaces seen in thin sections of carbonate cemented sandstones and are interpreted to have been formed by the dissolution of pore filling and grain replacive authigenic carbonates this is confirmed by examination of experimentally exhumed overgrowth surfaces from carbonate cemented sandstones these textures indicate that part of the intergranular porosity in these sediments is secondary in origin and has been generated by the dissolution of carbonate cements the identification of such textures may lead to a more confident interpretation of the nature of intergranular porosity in the subsurface

extreme 13ccarb enrichment in ca 2 0 ga magnesite stromatolite dolomite red beds association in a global context a case for the world wide signal enhanced by a local environment abstract the palaeoproterozoic positive excursion of 13 c carb is now considered as three positive shifts of 13 c carb separated by returns to 0 which all occurred between 2 40 and 2 06 ma this isotopic event is unique in terms of both duration 300 ma and 13 c enrichment up to 18 the mechanism responsible for one of the most significant carbon isotopic shifts in earth history remains highly debatable to date 13 c of 10 to 15 cannot be balanced by organic carbon burial forg as there is no geological evidence for an enhanced corg accumulation prior to or synchronous with the excursion instead termination of these excursions is followed by formation of a vast reservoir of 13 c depleted organic material 45 at shunga and by one of the earliest known oil generation episodes at 2 0 ga none of the three positive excursions of 13 c carb is followed by a negative isotopic shift significantly below 0 as has always been observed in younger isotopic events reflecting an overturn of a major marine carbon reservoirs this may indicate that forg was constant implying that the mechanism involved in the production of corg was different onset of intensive methane cycling resulting in c change is another possibility the majority of sampled 13 c carb rich localities represents shallow water stromatolitic dolostones red beds and evaporites formed in restricted intracratonic basins and may not reflect global 13 c carb values closely spaced drill core samples n 73 of stromatolitic dolostones from the 1980 27 ma tulomozerskaya formation in the onega palaeobasin russian karelia have been analysed for 13 c carb and 18 o carb in order to demonstrate that different processes were involved in the formation of 13 c carb rich carbonates the 800 m thick magnesite stromatolite dolomite red beds succession formed in a complex combination of environments on the karelian craton peritidal shallow marine low energy protected bights barred basins evaporative ephemeral ponds coastal sabkhas and playa lakes the carbonate rocks exhibit extreme 13 c enrichment with 13 c values ranging from 5 7 to 17 2 vs v pdb mean 9 9 2 3 and 18 o from 18 6 to 26 0 vs v smow mean 22 0 1 6 the tulomozerskaya isotopic excursion is characteristic of the global 2 4 2 06 ga positive shifts of carbonate 13 c 12 c although it reveals the greatest enrichment in 13 c known from this interval an external basin s is considered to have provided an enhanced corg burial and global seawater enrichment in 13 c the global background value for the isotopic shift at tulomozero time ca 2 0 ga is roughly estimated at around 5 an explosion of stromatolite forming microbial communities in shallow water basins evaporative and partly restricted environments high bioproductivity enhanced uptake of 12 c and pene contemporaneous recycling of organic material in cyanobacterial mats with the production and consequent loss of co2 and ch4 are believed to be additional local factors which may have enhanced 13 c from 5 up to 17 such factors should be taken into account when interpreting carbon isotopic data and attempting to discriminate between the local enrichment in 13 c and globally enhanced 13 c values we propose that many previously reported 13 c values from other localities where environmental interpretations are not available or have not been taken into account may not represent the global 13 c values

chemistry and environments of dolomitization a reappraisal abstract dolomitization of calcium carbonate can best be expressed by mass transfer reactions that allow for volume gain preservation or loss during the replacement process experimental data as well as textures and porosities of natural dolomites indicate that these reactions must include co 3 2 and or hco 3 supplied by the solution to the reaction site since dolomite formation is thermodynamically favoured in solutions of a low ca 2 mg 2 ratios b low ca 2 co 3 2 or ca 2 hco 3 ratios and c high temperatures the thermodynamic stability for the system calcite dolomite water is best represented in a diagram with these three parameters as axes kinetic considerations favour dolomitization under the same conditions and additionally at low as well as at high salinities if thermodynamic and kinetic considerations are combined the following conditions and environments are considered chemically conducive to dolomitization 1 environments of any salinity above thermodynamic and kinetic saturation with respect to dolomite i e freshwater seawater mixing zones normal saline to hypersaline subtidal environments hypersaline supratidal environments schizohaline environments 2 alkaline environments i e those under the influence of bacterial reduction and or fermentation processes or with high input of alkaline continental groundwaters and 3 many environments with temperatures greater than about 50 c subsurface and hydrothermal environments whether or not massive replacive dolostones are formed in these environments depends on a sufficient supply of magnesium and thus on hydrologic parameters most massive dolostones particularly those consisting of shallowing upward cycles and capped by regional unconformities have been interpreted to be formed according to either the freshwater seawater mixing model or the sabkha with reflux model however close examination of natural mixing zones and exposed evaporitic environments reveals that the amounts of dolomite formed are small and texturally different from the massive replacive dolostones commonly inferred to have been formed in these environments many shallowing upward sequences are devoid of dolomite it is therefore suggested that massive replacive dolomitization during exposure is rare if not impossible rather only small quantities of dolomite cement or replacement are formed which may act as nuclei for later subsurface dolomitization alternatively large scale dolomitization may take place in shallow subtidal environments of moderate to strong hypersalinity the integration of stratigraphic petrographic geochemical and hydrological parameters suggests that the only environments capable of forming massive replacive dolostones on a large scale are shallow hypersaline subtidal environments and certain subsurface environments

large scale hydrocarbon driven sand injection in the paleogene of the north sea the detailed study of many diagenetic carbonates concretions cemented bioturbation burrows etc and dykes of a large scale sandstone injection complex which are preserved in a cored early eocene section recovered from the south viking graben north sea provides evidence for the dyke emplacement by gas driven sand injection and coeval methane seepage the morphology chemistry of fluid inclusions isotopic signature d 13 c as low as 35x and petrographic characteristics of the diagenetic carbonates reveal gradual carbonate precipitation with a contribution of carbon derived from methane oxidation stratigraphic and cross cutting relationships reveal that sand injection post dates the formation of the diagenetic carbonates and therefore the beginning of hydrocarbon migration fluorescing fluid inclusions carbonate cement isotopic signature d 13 c as low as 27x and petrographic observations of the injected sandstones indicate the persistence of hydrocarbons including methane after the injection process gas chromatography of fluid inclusions confirms the presence of methane in both diagenetic carbonates and injected sandstones carbonate cements we suggest that gas influx in the eocene deep water sandstones increased pore fluid pressures during early burial and decreased the grain framework stability inducing liquefaction and sand injection gas involvement in sand injection has been widely invoked to explain the large scale injection complexes recently recognised in the paleogene of the north sea but had previously never been proven large scale injection structures and hydrocarbon seepage appear linked in several localities with obvious great economic significance d 2005 elsevier b v all rights reserved

the structural and diagenetic evolution of injected sandstones examples from the kimmeridgian of ne scotland injected sandstones occurring in the kimmeridgian of ne scotland along the bounding great glen and helmsdale faults formed when basinal fluids moved upward along the fault zones fluidizing oxfordian sands encountered at shallow depth and injecting them into overlying kimmeridgian strata the orientation of dykes in addition to coeval faults and fractures was controlled by a stress state related to dextral strike slip along the bounding fault zones diagenetic studies of cements allow the reconstruction of the fluid flow history the origin of deformation bands in sandstone dykes and sills was related to the contraction of the host rocks against dyke and sill walls following the initial stage of fluidized flow and these deformation bands are the earliest diagenetic imprint early non ferroan calcite precipitated in injection structures at temperatures between 70 and 100 c indicating that it precipitated from relatively hot basinal fluids that drove injection coeval calcite filled fractures show similar temperatures suggesting that relatively hot fluids were responsible for calcite precipitation in any permeable pathway created by dextral simple shear along the faults during progressive burial percolating sea water was responsible for completely cementing the still relatively porous injected sandstones with a second generation of ferroan calcite which contains fluid inclusions with homogenization temperatures below 50 c during this phase depositional host sandstones were also cemented

organic maturation thermal history and hydrocarbon generation in the orcadian basin scotland the devonian orcadian basin contains thick sequences of organic rich lacustrine sediments of interest as hydrocarbon source rocks throughout the basin the range of organic maturation is large spore colours vary from yellow orange to black and vitrinite reflectance for type iii kerogens ranges from 0 7 to 10 5 maximum palaeotemperatures are estimated to range from those typical of moderate burial diagenesis 100 c to those characteristic of greenschist facies metamorphism 400 c the highest maturities are related to contact metamorphism by devonian plutons and it is postulated that a large pluton underlies much of caithness high and low maturity regions are often juxtaposed across major faults which were active during variscan inversion regionally the lowest maturity is about 0 8 r o implying uplift and erosion of 2 3 km of cover basin wide this cover was probably a sequence of late devonian and carboniferous rocks removed largely during variscan uplift geothermal gradients were high but variable during devonian extension this together with the contact metamorphic affects of the devonian plutons resulted in early maturation metamorphism of large areas of the basin most of the onshore sequence entered the oil window during devonian and carboniferous times significant generation of hydrocarbons from orcadian lacustrine facies offshore during subsequent mesozoic burial will have occurred only in those areas which escaped maturation during the palaeozoic

physical modelling of sand injectites abstract sand injectites are structures that result from intrusion of fluidized sand into fractures we have studied them in the tampen spur area of the north sea and have reproduced them experimentally by driving compressed air through layers of sand glass microspheres and silica powder the silica powder was cohesive and capable of hydraulic fracturing whereas the sand and glass microspheres were almost non cohesive and therefore able to fluidize the models were dynamically similar to their natural counterparts for as long as equilibrium was static when the processes became dynamic so that inertial forces were significant the scaling was approximate and the corresponding reynolds numbers differed the experimental apparatus was a square box 1 m 1 m wide resting on a grid of fluid diffusers during the experiments the fluid pressure increased until it attained and surpassed the weight of overburden flat lying hydraulic fractures containing air formed within cohesive and least permeable layers heterogeneities in material properties and layer thicknesses were responsible for localizing fracture networks when any one network broke through to the surface rapid flow of air through the fractures fluidized the underlying mobile materials and even depleted some of the layers some of the fluidized material extruded at the surface through vents forming volcanoes and sheets the remainder lodged at depth forming sand injectites or laccoliths conical sand injectites formed preferentially where layers had high resistance to bending laccoliths formed nearer the surface where overlying layers had low resistance to bending the experimental sand injectites were broadly similar to those in the tampen spur area of the north sea as well as other areas

development of basins in the inner moray firth and the north sea by crustal extension and dextral displacement of the great glen fault abstract reflection seismic data provide evidence that mesozoic dextral movements along the great glen fault line have had an important influence on the development of the inner moray firth basin geophysical evidence further indicates that deep structure beneath the inner basin is dissimilar to that beneath the outer part and viking and central grabens in the north sea tectonic development of the inner basin can nevertheless be fitted into a pattern of north sea extensional movements which led to the formation of the graben system with which the major north sea hydrocarbon resources are associated

distribution chemistry isotopic composition and origin of diagenetic carbonates magnus sandstone north sea abstract diagenetic ferroan carbonates grew in the upper jurassic reservoir sandstones of the magnus oilfield in porewaters which differed in composition across the field these porewaters remained compositionally different and stratified for at least 35 m y variations in carbonate chemistry across the field are attributable to these porewater variations which resulted from displacement of marine depositional water from the crest of the field by meteoric water during late cimmerian subaerial exposure original depositional facies and detrital mineralogy strongly influenced diagenetic carbonate distribution rare diagenetic calcite occurs as discrete rhombic crystals diagenetically late magnesian siderites have developed throughout the reservoir sandstone and are commonly intimately associated with altered detrital biotite grains poikilotopic ankerite cement postdates calcite and siderite and occurs only adjacent to mudstones and in thin sandstones within mudstones three compositional growth zones in siderite crystals are observed across the field from crest to flank in all three wells studied a similar trend of compositional evolution through time is observed in both two and three zoned rhombs first formed siderite is relatively magnesian intermediate zones are more ferroan and outer zones are at least as magnesian as the first stage these individual grain variations overprint a fieldwide variation where siderite is more ferroan in the crestal samples up to 87 mol fe mn and more magnesian downdip up to 58 mol mg this reflects the greater influence of relatively fe rich meteoric derived water in the crest and the greater influence of marine derived mg rich porewater downdip ankerite shows a similar variation in fe and mg abunda ces across the field crest and flank maximum 25 and 17 mol fe mn respectively 27 and 43 tool mg and developed due to release of mg fe ca and hco3 ions from mudstones into adjacent sandstones following dissolution of detrital minerals and organic decarboxylation reactions both siderite and ankerite have lower 18o at the crest of the oilfield than downdip respectively siderite 16 0 and 17 6 smow ankerite 17 7 and 21 4 these differences in 18o reflect the retention during burial diagenesis of a larger component of meteoric water in the crest of the field below the unconformity whereas downdip porefluid contained a larger marine derived component strong organic influence on 13c 8 0 to 14 6 pdb for magnesian siderite 7 7 to 13 6 for ankerite closed system 34s values up to 15 7 for late cubic pyrite and stratified 18o from crest to flank of the field argue against large scale porewater movements diagenetic porewater stratification is strongly supported by the parallel but distinct geochemical fingerprints of siderite and ankerite cements from the crest to the flank of the field

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

isotopic variations in dolomite concretions from the monterey formation california abstract isotopic analysis of dolomite concretions from the miocene monterey formation of california yields a covariance of 18o and 18c within every concretion analyzed the total range for all concretions in 18o smow is 27 3 to 33 6 the total range in 13c pdb is 10 2 to 10 8 variations of up to 3 6 in oxygen and 12 2 in carbon are encountered within a single concretion decreasing 18o is interpreted in terms of increasing temperatures of formation as the concretion grows during burial the claypool and kaplan 1974 model can be successfully used to interpret carbon isotope variations as the result of progressive concretion growth in the zones of microbial sulfate reduction microbial methane production and thermocatalytic decarboxylation monterey dolomite concretions are too rich in carbonate to have originated by simple cementation of initially porous sediment much of the originally diatomaceous sediment appears to have been replaced by dolomite isotopic zonation within one concretion is found to be complex suggesting nucleation at several sites which then grew together complex growth patterns may result from local variations in lithology and porosity during precipitation multiple sampling of several concretions at a single outcrop yields a more complete early diagenetic history than by sampling only one concretion isotopic diagenetic profiles inferred from the dolomite concretions are similar to those seen in deep sea drilling project cores where deep sea dolomitization is occurring today single samples from a concretion cannot reveal the range or the trend of the diagenetic reactions that have occurred multiple sampling of diagenetic carbonate concretions is clearly essential in determining the early diagenetic history from isotopic measurements

on steady homogeneous sand water flows in a vertical conduit in this paper an idealized model of the steady state phase of the flow in a vertical conduit leading to a sand volcano eruption is developed from first principles the model assumes that a sand water mixture flows upwards driven by an overpressure at the base of a vertical cylindrical conduit or a two dimensional fracture and opposed by gravity viscous resistance and turbulent drag the conditions for flow are analysed in detail and the mechanisms controlling the eruption rates are studied quantitatively the flow predictions are in accordance with our observations of analogous vigorous sand eruptions at deepwater oil fields for sufficiently high flow velocities u 10ut and small sand fractions s 0 2 the flow may be well mixed and homogeneous if these conditions are not met the flow may either become two phase or does not develop combining geological considerations with the steady homogeneous model it is possible to predict the behaviour of the vigorous quasi steady stage of a sand volcano eruption it is shown that based on the average density of the overlying sediments there are a range of overpressures for which sand volcanoes may develop

the formation of carbonate cements in the forth and balmoral fields northern north sea a case for biodegradation carbonate cementation and oil leakage during early burial

pyritic and baritic burrows and microbial filaments in postglacial lacustrine clays in the northern baltic sea abstract holocene postglacial lacustrine clays in the northern baltic sea were studied for ichnofossils petrography and microscale elemental and 34 s 32 s composition to understand the impact of benthos produced organic matrices on the early diagenesis of sediments x radiographs of these clays display intense burrow mottling with discrete palaeophycus mycellia9 and rare arenicolites small burrow like and irregularly shaped concretions were separated from these clays the burrow like concretions are composed of framboidal pyrite and poorly crystalline fes 2 cement they formed within worm produced burrows by the decomposition of mucous coatings on the burrow walls by sulphate reduction 34 s cdt composition of the framboids indicates saline inflows from the north sea as the sulphate source the irregular concretions are microcrystalline pyritic masses that formed in pore spaces with less reactive organic compounds their 34 s cdt values reflect precipitation from the ambient porewater sulphate by sulphate reduction the burrow like and irregular concretions are microcrystalline barite in the lower parts of the clays mycellia9 are mineral replaced filaments produced by large sulphur oxidizing bacteria or mycelial fungi they concentrate in monosulphide bands in the lower substrate levels but are associated with organic patches upward this study demonstrates that substantial amounts of pyrite formed within organic matrices close to the sediment water interface this differs from previous studies that proposed a deeper burial origin and allochthonous sulphur source supplementary material sulphur isotope data are available at http www geolsoc org uk sup18426

shear wave velocity and attenuation structure for the shallow crust of the southern korean peninsula from short period rayleigh waves abstract we analyzed the short period rayleigh waves from the first crustal scale seismic refraction experiment in the korean peninsula kcrust2002 to determine the shear wave velocity and attenuation structure of the uppermost 1 km of the crust in different tectonic zones of the korean peninsula and to examine if this can be related to the surface geology of the study area the experiment was conducted with two large explosive sources along a 300 km long profile in 2002 the seismic traces recorded on 170 vertical component 2 hz portable seismometers show distinct rayleigh waves in the period range between 0 2 s and 1 2 s which are easily recognizable up to 30 60 km from the sources the seismic profiles which traverse three tectonic regions gyeonggi massif okcheon fold belt and yeongnam massif were divided into five subsections based on tectonic boundaries as well as lithology group and phase velocities for the five subsections obtained by a continuous wavelet transform method and a slant stack method respectively were inverted for the shear wave models we obtained shear wave velocity models up to a depth of 1 0 km overall the shear wave velocity of the okcheon fold belt is lower than that of the gyeonggi and yeongnam massifs by 0 4 km s in the shallowmost 0 2 km and by 0 2 km s at depths below 0 2 km attenuation coefficients determined from the decay of the fundamental mode rayleigh waves were used to obtain the shear wave attenuation structures for three subsections one for each of the three different tectonic regions we obtained an average value of q 1 in the upper 0 5 km for each subsection q 1 for the okcheon fold belt 0 026 is approximately three times larger than q 1 for the massif areas 0 008 the low shear wave velocity in the okcheon fold belt is consistent with the high attenuation in this region

seismic signature of the lake mien impact structure southern sweden the eugeno s seismic profile iv in southern sweden crossed lake mien a confirmed heavily eroded 121 ma impact structure the lake mien structure has a pre erosion rim diameter of about 9 km it is

strontium isotopic compositions and potassium and rubidium contents of formation waters in sedimentary basins clues to the origin of the solutes abstract strontium isotopic compositions of formation waters in sedimentary basins relate primarily to sr from dissolution replacement of alkali feldspar minerals dissolution recrystallization precipitation of marine carbonate and sulfate minerals and illitization formation waters in sedimentary basins are often depleted in k but enriched in rb relative to evaporated seawater and have k rb ratios commonly between 200 and 2000 the k and rb characteristics of formation waters suggest that deep burial conversion of smectite to illite has a major influence on the evolutionary history of these solutes

the geochemistry of concretions from the kimmeridge clay formation of southern and eastern england concretions from the kimmeridge clay formation are of three types calcareous concretions septarian calcareous concretions and pyrite calcite concretions and nodules which occur within different mudstone facies isotopic and chemical analysis of the concretionary carbonates indicate growth in the fe reduction sulphate reduction and decarboxylation zones the septarian concretions show a long and complex history with early initiation of growth and development spanning several phases of burial each often resulting in the formation of septaria growth apparently ceased in the transitional zone between the sulphate reduction and the methanogenesis zones very early growth in the fe reduction zones is also seen in one sample the non septarian concretions began growth later within the sulphate reduction zone and have had a simpler burial history while the pyrite calcite concretions show carbonate cementation in the sulphate reduction methanogenesis transition zone a ferroan dolomite calcite septarian nodule with decarboxylation zone characteristics also occurs development of concretions appears to be indirectly controlled by the sedimentation rate and depositional environment the latter determining the organic matter input to the sediments calcareous concretions predominate in swell areas and during periods of low sedimentation rate in the basins with poor organic matter preservation and deposition of calcareous mudstones pyrite calcite concretions occur in organic rich mudstones deposited under higher sedimentation rates in the basins while the ferroan dolomite nodule grew under very high sedimentation rates

thickness and spatial distributions of clastic dykes northwest sacramento valley california abstract the geometry and distribution of the clastic dykes of the ono district north sacramento valley are examined within stream sections five traverses along dry stream beds provide good exposure allowing the spacing thickness and geometry of the dykes to be recorded the spatial and thickness distribution of the dykes are considered using cumulative frequency plots allowing a visual estimation of a best fit distribution dyke thickness conforms best to a log normal distribution there is also a characteristic minimum dyke thickness in a traverse and this is attributed to the minimum aperture that a fluid with sand clasts is able to exploit dyke spacing however shows a good correlation with a power law distribution for four traverses suggesting that there is a mechanistic control on the spatial distribution plotting dyke thickness against minimum dyke spacing reveals that thin dykes do not generally intrude in isolation unlike veins and igneous dykes clastic dykes continue to provide preferential pathways for fluid flow subsequent to their intrusion thus inhibiting intrusion in the area surrounding a pre existing dyke a combination of this process and dyke branching provides the best model for the observed spatial and thickness distribution of clastic dykes seen in the ono district california

seismic refraction studies in the basement of the vredefort structure abstract the 30 mgal positive gravity anomaly in the centre of the basement of the vredefort structure has been a feature of discussion for a number of years to supply constraints on the depth and nature of this body refraction seismic measurements were made on a n s line across the basement anomaly the results support a three fold subdivision of the uppermost structure on the seismic profile granitic gneisses in the north dioritic gneisses in the central sector over the gravity anomaly and carboniferous karoo strata in the southern part of the basement throughout the central section of the refraction profile rocks with p wave velocity of 6 40 6 43 km s dominate the profile whilst to the north and south of this central section basement velocities are all less than 6 0 km s computer travel time simulations of the basic model were carried out using 2 d ray tracing

in situ seismic measurements in borehole lb 08a in the bosumtwi impact structure ghana preliminary interpretation in order to assist in the interpretation of previous seismic refraction and reflection surveys a vertical seismic profile was acquired in the lake bosumtwi ghana hard rock core hole lb 08a no seismic reflections are seen in the up going wave field obtained and this observation is consistent with the lack of reflectivity observed in the corresponding 2 d surface seismic profile obtained in earlier studies direct down going p waves were found both in the cased sediment column from a depth of 73 m to 239 m below the lake surface and in the open hole hard rock section from a depth of 239 m to 451 m of lb 08a analysis of the observed travel times indicates a nearly constant p wave velocity of 1520 m s through the soft lacustrine sediments in the hard rock however the p wave velocity rapidly increases by nearly 30 from 2600 m s to 3340 m s these values are in good agreement with the gross velocity structure obtained in the earlier joint inversion of seismic reflection and refraction data these values are low relative to those expected for the metasedimentary protoliths an observation that has been made at other young impact structures of similar size the low velocities together with the fact that they increase so rapidly is suggestive of a decreasing density of fractures and microcracks with depth consequently the seismic velocity trend may provide a proxy measure of damage and hence the decay of the shock pressure from the impact point validation of this requires additional detailed studies of the porosity structure in the core

mechanisms and controls on the formation of sand intrusions sandstone intrusions are found in all sedimentary environments but have been reported most commonly from deep water settings they also appear to be more frequently developed in tectonically active settings where applied tectonic stresses facilitate development of high fluid pressures within the sediments a variety of mechanisms have been cited as triggers for clastic intrusions these include seismicity induced liquefaction application of tectonic stresses excess pore fluid pressures generated by deposition related processes and the influx of an overpressured fluid from deeper within the basin into a shallow sand body the formation of sandstone dykes and sills is investigated here by considering them as natural hydraulic fractures when the seal on an unconsolidated overpressured sand body fails the resulting steep hydraulic gradient may cause the sand to fluidize the fluidized slurry can then inject along pre existing or new fractures to form clastic intrusions the scale and the geometry of an intrusive complex is governed by the stress state depth and pre existing joints or faults within the sedimentary succession as well as the nature of the host sediments for the simplest tectonic setting where the maximum stress in a basin is vertical gravitational loading small irregular intrusions commonly result in the formation of sills at shallow depths within a few metres of the surface whereas at greater depth dykes and sills forming clastic intrusion networks are more typical a simple relationship is derived to calculate the maximum burial depth at which a dyke sill complex forms as a function of the source bed to sill height the bulk density of the surrounding sediments and the ratio of the vertical to horizontal effective stresses k 0 when applied to three examples of large scale dyke sill complexes developed within paleocene and eocene deep water reservoir sand bodies of the north sea maximum burial depths in a range of 375 to c 500 m 450 700 m and 550 850 m are estimated for intrusion of each of the three complexes

oxygen and carbon isotopic composition of marine carbonate concretions an overview abstract compilation of stable isotope data 18o and 13c for marine calcite siderite and dolomite concretions from a large number of studies reveals consistent and unanticipated isotopic trends although many of the dolomites have 18o values that are compatible with precipitation from unaltered seawater at low temperatures most of the calcites and siderites are far too depleted in 12o for such an origin this is surprising as most of the concretions appear to have formed very early in the diagenetic history of the sediments often within several meters of the sediment water interface the most likely cause of the anomalously depleted oxygen values is a decrease in pore water 18o values during early diagenesis a result perhaps of a significant influx of meteoric water into the marine shelf environment or water mineral interaction e g precipitation of 18o enriched minerals the carbon isotopic field for dolomite is quite different from that for calcite and siderite highly positive 13c values are fairly common only for dolomites this indicates that while dolomite concretions often form in sediments where methanogenesis is rapid precipitation of calcite and siderite concretions is restricted to sediments where either n methanogenesis takes place or where methanogenesis is not extensive enough to generate highly positive pore fluid 13c values this variation in carbon isotopic composition among the different minerals suggests that the rates of organic carbon oxidation and sedimentation may control the mineralogy of carbonate concretions high organic carbon oxidation rates favor dolomite and lower oxidation rates favor calcite or siderite

a note on sand dyke orientations abstract the techniques of the statistics of directional data are applied to the orientations of sand dykes and bedding in strata unaffected by significant penetrative tectonic strain or compaction from the records of the orientations of both bedding and sand dykes in 578 individual cases it is shown that the mean bedding dyke dihedral angle for each group of dykes recorded in the literature ranged from 87 7 to 89 6 the hypothesis is accepted that these dykes tended to form perpendicular to bedding this has beneficial implications for their use as strain markers in compaction studies and in tectonic strain analysis

sulfur isotopes and paragenesis of sulfide minerals in the silurian waldron shale southern indiana a diverse assemblage of sulfide morphologies and a wide range of sulfur isotope compositions indicate that precipitation of sulfide minerals was an ongoing process during both early and later stages of diagenesis in the silurian waldron shale in indiana five distinct forms of sulfide minerals have been identified in samples from seven locations in southern indiana 1 pyritized brachiopod shells 2 pyritized burrows 3 pyrite cubes 4 prismatic marcasite and 5 cockscomb marcasite petrographic evidence is used to determine a probable sequence of sulfide precipitation isotope compositions range from about 40 to 40 and each sulfide morphology exhibits a distinctive range of 34s values these results are consistent with a model in which sulfate reduction and subsequent precipitation of sulfide minerals occurred in a system that became progressively more closed with respect to sulfate the sequence of sulfide mineralization reflects the chemical evolution of pore waters during diagenesis of the waldron shale redox conditions in the sediment were initially aerobic to dysaerobic sulfate reduction occurring only in reducing microniches where brachiopod shells and then organic walled burrows were pyritized sediments became more fully reducing with burial when euhedral sulfides were formed the transition from pyrite to marcasite precipitation suggests a decrease in pore water ph during later diagenesis and may also indicate fluctuations in redox conditions

sulphur isotope geochemistry of pyrite from the upper cretaceous marshybank formation western interior basin abstract variations in the texture and sulphur isotopic composition of pyrite in the upper cretaceous marshybank formation are linked to depositional environment abundant framboidal and lesser euhedral pyrite precipitated in offshore marine rocks moderate quantities of pyrite also crystallized in brackish coastal plain rocks however in contrast to marine pyrite coastal plain pyrite is dominantly euhedral in texture reflecting direct precipitation from a porewater with a relatively low dissolved sulphide concentration in the marine rocks pyrite 34s values range from 35 7 to 27 4 avg 4 8 canyon diablo troilite cdt pyrite within carbonate concretions hosted in these marine rocks has a similar isotopic composition 49 8 to 10 6 cdt however isotopic values are often highly variable within individual concretions as a result of the heterogeneous nature of sulphate reduction and pyrite formation within marine sediments pyrite in coastal plain rocks is characterized by relatively high 34s values 4 2 to 35 5 avg 13 2 cdt while the overlying sideritized conglomerates have the lowest 34s values reported for cretaceous rocks from the western interior basin of north america 49 8 to 41 7 cdt very low 34s values which are only observed in the marine rocks are indicative of microbial sulphate reduction and pyrite formation in a sulphate replete i e open system higher 34s values up to 18 which were obtained for both the marine and coastal plain rocks are indicative of progressive pyrite crystallization in a sulphate limited i e closed system such conditions are expected in marine sediments as burial occurs and in brackish i e low sulphate sediments pyrite with very high 34s values 18 is common in the coastal plain rocks these high values are the result of influx of 34s enriched residual sulphide derived from overlying marine units a minor amount of 34s enriched pyrite is also present within and proximal to marine sedimentary unit h which was deposited at the same time as the coastal plain rocks we hypothesize that porewater containing 34s enriched residual sulphide flowed from the coastal plain and along unit h into the marine sediments thus producing anomalously high 34s pyrite

a classification of dyke fracture geometry with examples from precambrian dyke swarms in the vestfold hills antarctica a classification of dilational fracture geometry is proposed it applies to structures that can be characterized by the combination of a fracture system and a dilation vector field the classification is illustrated with examples of igneous dykes pegmatites and pseudotachylites segmentation in the form of offsets jogs or bifurcations is common to most fracture systems four basic types of dilational fracture systems are distinguished on the basis of the geometry of segmentation these are irregular braided en echelon and zigzag zigzag fracture systems are further differentiated they consist of newly formed obliquely dilated fractures or alternatively of pre existing planes in the host rock that are reactivated by oblique dilation this paper is the first to actually report an igneous zigzag dyke involving newly formed fractures rotation of the regional stress field in the direction of propagation leads to en echelon segmentation braided fracture systems reflect high local stress intensities probably related to propagation rate two possibilities exist for the formation of zigzag dykes consisting of newly formed fractures that are obliquely dilated they may form by extreme interaction of tensile fracture segments in a regional stress field with a low differential stress alternatively they may form in a regional stress field with high differential stress through the propagation of shear fractures segmentation of dykes characterized by offsets is common such segmentation can be the result of protrusions of the fracture termination offsets also occur where a dyke cuts an older planar structure such offsets are the result of the local inhibition of fracture propagation the resulting apparent offset can lead to a misinterpretation of relative age apophyses form as a result of the dilation of a segmented fracture system

on folding and shear zone development a mesoscale structural study on the transition between two different tectonic styles abstract an example of coeval development of ductile shear zones and folds in schistose terrains is documented by detailed mapping and structural analysis folds and shear zones were formed under retrogressive conditions after the peak of the regional hercynian metamorphism and both affect a previous regional schistosity a gradual change in structural style on a regional scale is evidenced by the disappearance in higher grade domains of late folds and the preponderance of shear zones geometrical analysis reveals that the bulk structural pattern is compatible with a non coaxial strain history structural variation reflects a gradual change in mechanical behaviour of schists which show a marked variation in crystallinity at the time of deformation

porewater sulphur geochemistry and fossil preservation during phosphate diagenesis in a lower cretaceous shelf mudstone lower cretaceous mudstones exposed at speeton in north yorkshire uk contain lobsters and burrows preserved in diagenetic phosphate concretions isotopic compositions of sulphur in both diagenetic sulphide and structural sulphate in diagenetic phosphate have been measured in an attempt to constrain diagenetic porewater chemistry the occurrence of phosphatized and pyritized lobsters and similarly preserved burrows allows a detailed comparison of these microenvironments with the host sediments host sediments are extensively bioturbated and characterized by very light sulphide isotopic compositions mean sulphide 34s 48 3 3 1 n 19 and sulphate isotopic compositions that are lighter than lower cretaceous seawater sulphate mean sulphate 34s 8 7 3 2 1 n 19 these isotopic values can be explained by the action of bioturbating macrofauna the introduction of oxygen in ventilating seawater results in the oxidation of early formed isotopically light pyrite resulting in porewater sulphate that is enriched in 32s subsequent pyrite formation via bacteriogenic reduction of isotopically light porewater sulphate leads to the formation of isotopically light pyrite whilst residual porewater sulphate apparently remains relatively enriched in the 32s isotope sulphur isotopic values for the burrows are very different sulphide isotopic compositions average 34 4 0 4 1 n 3 and sulphate isotopic compositions average 14 4 6 6 1 n 4 these isotopic compositions are the result of rapid development of closed system conditions in burrows resulting in the build up of acidity necessary for phosphate precipitation and coprecipitation of isotopically light sulphate formed by oxidation of isotopically light sulphide surrounding the burrows lobster shell isotopic compositions fall between these two groups on the basis of their isotopic compositions some lobsters appear to have died close to the sediment water interface whilst others appear to have been buried in burrows prior to death

geophysical aspects of meteorite impact craters in eroded shield environment with special emphasis on electric resistivity abstract the traditional geophysical methods of magnetics and gravity as applied to meteorite impact crater studies are shortly reviewed their optimal resolution power with respect to crater size and pre existing structures is discussed the morphology of some typical impact craters is illustrated to show the character of the post impact deformation a new geophysical method in impact crater studies the in situ measurement of electrical resistivity is introduced and illustrated with case studies from simple and complex craters it is concluded that the resistivity method represents a useful tool for mapping the extent of authigenic brecciation in crater structures and their surroundings the method constitutes in combination with detailed elevation data gravity and magnetic measurements a powerful tool for constraining the classification of impact craters in crystalline shield environments the resistivity measurements have revealed a complex brecciation pattern in large central uplift structures consisting of a central less brecciated core surrounded by a ring of more intense brecciation typical impact brecciated crystalline rocks have resistivities between 2000 and 8000 ohm m the crater structures treated in this paper are bjorko dellen siljan and uppland all located in sweden

investigating the crustal structure of a strike slip step over zone along the great glen fault strike slip step overs and bends are regions of anomalous deformation that may yield clues to the mechanical behavior of the earth s crust experimental reprocessing of a marine deep seismic reflection profile cutting across an 35 km wide right stepping step over developed on the great glen fault system north of the inner moray firth basin east of northwest highlands scotland reveals a restricted zone of prominent reflections and diffractions beginning at 7 8 km depth continuing as deep as the interpreted moho discontinuity at 26 27 km geological interpretation of this zone suggests a concentration of possible diffractors marking sharp structural disruption underlain by a 12 km thick layer of subhorizontal and moderately dipping reflectors that dominates the lower crust in the step over region the known kinematic history of the great glen fault system together with the observed direction of the step over implies that a zone of contraction would have formed within the step over during early caledonian sinistral strike slip the seismic reflection structure can be interpreted as developing by deformation associated with contraction and possible block rotation between the two fault segments such that the diffractive zone represents steep structure in a near vertical zone of strike slip which passes deeper into a layer of low angle dipping thrusts or shear zones the results of this study are consistent with a concept of mechanical detachment in the middle crust as documented for areas of contraction along active strike slip faults

disequilibrium trace element partitioning in jurassic sparry calcite cements implications for crystal growth mechanisms during diagenesis sparry calcite cements from middle jurassic limestones in central england contain complex non concentric zoning patterns produced by a combination of sector zonation and a sector specific lamellar or polygonal zonation electron microprobe analyses of individual zones within crystals document 1 a sympathetic covariation of fe mg and sr between two non equivalent sectors and 2 episodic further increase of sr contents coupled with exclusion of fe mg and mn within the more enriched sectors comparison of results from three different localities demonstrates the regional consistency of this pattern although the magnitude of the differential trace element partitioning varies from combined petrographic and geochemical data the trace element variation in the crystals can be shown to bear no simple relationship with either 1 the chemical evolution of the 9bulk9 precipitating porefluid or 2 the relative overall growth rates of the different crystal faces previous sector zoning models based on crystal surface configurations and internal structure are unable to account directly for the geochemical patterns observed in this study differences between growth mechanisms on the crystal faces of the studied spars are likely to have exerted a significant influence on trace element partitioning in particular they offer a plausible explanation for the geometric and compositional characteristics of the sector specific episodic zoning on the most enriched crystal faces in general crystal morphology and composition will be intimately linked by the growth mechanisms operating on different faces and by their response to changes in the diagenetic environment the validity of diagenetic interpretations from bulk geochemical analysis of sparry calcite cements thus rests upon a detailed analysis of any zonation that they contain

experimental study of the influence of oil wet calcite cements on oil migration and implications for clastic reservoirs abstract the influence of oil wet calcite cements on oil migration in clastic reservoirs is of significant interest in the field of petroleum exploration however the microscopic migration of oil in microscopic pore spaces with oil wet calcite cements has not been directly observed microscopic observations of oil migration were conducted in a glass micromodel at normal temperature and pressure to investigate the influence mechanism of oil wet calcite cements on oil migration in pore spaces the calcites were precipitated as cements prior to the oil migration a two dimensional glass micromodel with two interconnected spaces with the same pore throat structures was used in the experimental study a new method to create firmly adhering calcites was used to precipitate equal amounts of calcites in the two interconnected spaces the oil migration was recorded in real time under a microscope throughout the experiment the experimental results show that oil migration occurs faster in spaces with than without oil wet calcites during the oil migration in the pore throat spaces the oil migrates towards the surfaces of the oil wet calcites when the oil is in contact with the calcites the presence of oil wet calcites significantly reduces the oil breakthrough pressure the electrostatic attraction between the carboxylic groups and calcites and the intermolecular forces between the carboxylic groups and organic molecules in the oil cause the oil migration towards the surfaces of the oil wet calcites the experimental results indicate that oil preferentially migrates into clastic reservoirs that possess appropriate amounts of oil wet calcite cements precipitated prior to oil emplacement

the lake bosumtwi impact crater ghana the 1 m y old bosumtwi crater ghana has a nearly circular shape with a rim diameter of 11 km north south and 10 km east west it is surrounded by a circular depression and an outer ridge of diameter 20 km polymict breccias averaging at least 20 m thick with clasts as much as 5 m long occur on the outer ridge and the crater rim shows in situ shattered rock patches of suevite have been found in the circular depression north and south of the crater analogy with better known craters suggests that bosumtwi has a central uplift rising to 200 m beneath the lake floor an aeromagnetic anomaly of amplitude 50 nanotesla nt over the northern half of the lake is interpreted as due to a layer of magnetized fallback breccia beneath the lake sediments the normal polarity of the breccia shows that the crater was formed during the normal jaramillo event of 0 97 to 0 85 m y ago which agrees with the magnetic stratigraphy of the related ivory coast microtektites a regional gravity survey indicates a negative bouguer anomaly over the crater there is some geochemical evidence that the meteorite was an iron and its mass and energy are suggested as about 108 tons and 3 1019 joules or 7 3 103 megatons

clastic dike swarms and the structural evolution of the foreland fold and thrust belt of the southern andes clastic dike swarms are intruded into a cenozoic foreland fold and thrust belt in southern chile the dike material consists of sandstone and conglomerate with clasts as much as 25 cm in diameter brecciated wall rocks are also found in the clastic matrix the dikes often exceed 50 cm in width and hundred of metres in length in outcrop and are oriented overwhelmingly transverse to fold axes and parallel to cross joints the dike swarms are concentrated near the toe of major thrusts where they are found injected into the hanging wall a regional axial planar spaced cleavage is developed in the clastic dikes and sills clearly postdating injection of clastic material dike injection postdates at least the initiation of extension fractures the cross joint set within the swarms the dike intrusions represent as much as 20 extension parallel to fold axes in addition to several percent extension represented regionally by the cross joints the structural features of the dike swarms and their geometric relationship to other structures suggest rapid single phase injection the arrival of the front edge of a thrust on an already highly overpressured sedimentary sequence would cause an instantaneous increase in the overburden pressure and therefore could result in injection of a dense slurry of wet coarse sediments along preexisting planes of weakness field relationships of mesoscopic structures suggest the following evolution cross joint development incipient cleavage development initiation of thrusting and buckling clastic dike injection regional spaced cleavage development and associated late stage tightening of folds

a complex subglacial clastic dyke swarm solheimajokull southern iceland abstract a complex network of clastic dykes dissects loosely consolidated holocene sediments along the east flank of the proglacial braid plain of solheimajokull southern iceland the dykes comprise downward bifurcating intrusions up to 0 5 m thick and several metres in length and are intruded into glaciogenic deposits sandy gravel gravelly sand interlaminated silt and sand and diamicton the dykes were sourced from a clast poor sandy diamicton interpreted as a subglacial till and were intruded downwards beneath solheimajokull glacier during a previous phase of advance as the glacier advanced southwards it loaded the sediment column resulting in the intrusion of dykes with a consistent south southwest dip with rare northward dipping examples the dyke fills are characterised by laminated sediment with laminae oriented parallel to the dyke margins and comprise interlaminated clay silty clay silt sand sandy gravel and diamicton in some dykes high concentrations of pebble to boulder sized clasts occur in association with rotated pods of the laminated sediment the laminae are thought to have evolved by a slow long lived intrusion process that involved the repeated fracture and expansion of the host sediments followed by viscous smearing on of subglacial material onto the dyke walls rather than rapid injection of fluidised sediment

geometry and tectonic significance of albian sedimentary dykes in the sisteron area se france abstract sedimentary dykes in the sisteron area have penetrated pre existing fracture sets in late aptian and early albian strata dynamic characteristics of such fractures are revealed by the bifurcation and the branching of dykes three dimensional statistical analysis of the initial orientation of two vertical conjugate sets of dykes indicates their mechanics of formation as well as the principal compressional stress direction during the development of the sill dyke regime in the sisteron area another set of sandstone dykes present in the area suggests syn sedimentary and extensional faulting tectonic activity before the area became involved in compressional tectonics several significant dyke forms clearly show the sense of shear along fractures filled with sand and enable the stress pattern during dyke formation in the area to be recognized

in situ measurements of shear wave velocity in sediments with higher mode rayleigh waves a seismic survey was carried out on a tidal flat in the sw netherlands in order to determine shear wave velocities in sediments by means of higher mode rayleigh waves the dispersion properties of these rayleigh waves were measured in the 2 d amplitude spectrum or f k spectrum and resulted in phase velocities for six different modes as a function of frequency 5 30 hz these observed phase velocities were inverted for a nine layer model for the shear wave velocity to a depth of 50 m

geochemistry and origin of organic rich sediment veins in fractured granitic basement helmsdale sutherlandshire uk abstract black sediment veins up to 2 cm width penetrate the caledonian helmsdale granite in the vicinity of the helmsdale fault onshore moray firth the black colour and geochemistry of the veins reflect a high content of organic carbon both devonian and jurassic shales are conceivable available sources but sterane compositions relate the organic matter to the jurassic shales a content of extractable organic matter higher than in the shales suggests that the carbon in the veins represents oil rather than mechanically mobilized shale the oil was present during sediment vein emplacement the veins were emplaced forcefully which may reflect high fluid pressure associated with post jurassic movement on the helmsdale fault

structural geology of easter ross scotland implications for movement on the great glen fault zone the integration of a recently drilled exploration well sited near the town of tain ross shire with a seismic database and onshore exposures allows a new interpretation of the structure in the easter ross peninsula ne scotland it is now evident that previous interpretations which highlighted the importance of ne sw striking lineaments need some modification instead numerous n s to nne ssw trending folds and faults affect devonian sequences and show evidence of compressional reactivation of structures which had previously experienced extension these inversion structures only appear to be dissected by ne sw and e w striking faults and extensional fractures in areas adjacent to the great glen fault zone integration of the onshore data with results from adjacent offshore areas suggests that the peninsula has undergone a complex structural history since the early devonian it appears to have been characterized by early devonian probably rift related extensional faulting and subsequent mid late devonian regional thermal subsidence the restriction of similar structural relations to devonian sequences on offshore seismic lines from adjacent areas suggests that the contractional and inversion structures which similarly affect the late palaeozoic sequences in onshore areas were probably related to permo carboniferous structural events rather than younger deformation further integration of offshore data with the occurrence of jurassic sequences on the coast suggests that the peninsula may be considered an integral part of the inner moray firth basin and experienced a similar mesozoic subsidence history to offshore areas before undergoing regional uplift and dissection due to reactivation of the great glen fault zone during the cenozoic there is no evidence to suggest that this fault zone had any significant control on basin development sedimentary sequences or structural geometries during palaeozoic or mesozoic periods of extension and it probably only had minor effect during permo carboniferous inversion similarily its role in tectonic events after the early cretaceous appears to have been limited to local deformation with little evidence for any large scale strike slip offset along its length the unusual occurrence of jurassic coastal exposures in easter ross appears to represent fault bounded slices of sediment downfaulted to accommodate the formation of a negative flower structure in response to limited dextral strike slip movement the results suggest that the great glen fault zone was a strike slip fault system effectively locked during phases of extension in the inner moray firth but was reactivated during phases of inversion when extensional slip vectors sub paralleled the lineament

palaeomagnetism and the siljan impact structure central sweden summary the siljan impact structure diameter c 52 km located in central sweden is considered the largest meteorite crater in western europe the time of impact has been estimated to be 368 ma from ar ar dating of an impact melt a palaeomagnetic study of rocks from inside and outside the impact structure has been carried out with the aim of identifying natural remanent magnetizations nrms related to the impact event primary magnetizations are partly preserved within the impact area however the directions of these original magnetizations are disturbed due to tectonic movements related to the impact three different directions of magnetizations not recognized elsewhere were identified within the central part of the structure these three magnetizations are defined in order of increasing coercivity hc and blocking temperature tb the origin of a high hc chemical remanent magnetization crm c of late palaeozoic time carried by haematite can be interpreted as due to oxidation caused by circulating meteoric water in cracks and fractures caused by the impact another component b identified in a lower temperature range 40d 580dc is probably carried by magnetite in pseudotachylite this magnetization is considered as a thermal remanent magnetization while in other rocks its pre tectonic character and occurrence also in the low coercivity range implies a shock remanent origin the palaeomagnetic pole for this remanence corresponds to a silurian devonian age which gives an age of impact which is not very diffirent from that estimated by radiometric data a third low hc and tb component a is interpreted to be a crm carried by slowly growing goethite the direction of this crm implies it to be a recent post impact magnetization

the nature and significance of sand intrusions in a hydrocarbon rich fold and thrust belt eastern carpathians bend zone romania sand intrusions are commonly associated with hydrocarbon bearing clastic reservoirs and they can host a significant part of the reserves they modify the depositional sandbody architecture and can have an impact on the petroleum system elements their significance and complex architecture is difficult to assess in the subsurface due to limited resolution of seismic or core data outcrop analogues are key to better understand these complex reservoirs in the eastern carpathians bend zone the oligocene and lower miocene succession offer spectacular exposures of deep water siliciclastic rocks modified by remobilization and sand intrusion processes these rocks are also present in producing fields in this area sills can be as thick as the depositional sandstones c 4 m and sill dominated outcrops can provide a net gross net sandstone to shale ratio of about 35 50 we quantified their architecture in a study of the timing and mechanism of emplacement their relationship to regional tectonics and their impact on reservoir connectivity the intrusions are interpreted to be syn tectonic related to the major mid miocene compression in this part of the carpathian mountain belt the intrusions lead to a large increase in vertical and horizontal connectivity our results will help to improve reservoir characterization and production in these reservoirs

fluid inclusion petrography abstract a procedure of fluid inclusion studies is proposed with emphasis on the criteria of selecting fluid inclusions for detailed microthermometry and spectroscopic analysis an overview of descriptive and genetic classifications of fluid inclusions in single crystals and in massive rocks is given with the intention of further differentiating the commonly used terms primary and secondary fluid inclusions some principles of fluid inclusion modification are explained cathodoluminescence cl studies of quartz with the optical high power cl microscope and the electron microprobe provided with a cl detector are an important help in fluid petrography cl textures are subdivided in primary growth textures and a wide variety of secondary microtextures which are in part induced by fluid inclusions the latter is grouped in textures indicative of local lower crystal order increasing defect structures and microtextures indicative of local quartz healing reduction of the defect structures microtextures showing the genetic relationship between fluid inclusions and the host mineral provide information about the possible post entrapment changes of fluid inclusions and therewith testify their geological relevance

construction of component maps from aeromagnetic total field anomaly maps1 total field anomalies as defined from normal aeromagnetic surveys give good approximations of the anomalous components along the direction of the main geomagnetic field which is generally much larger than the anomalous field using the relations between vertical and horizontal components of the field the total field anomaly is related to any vertical or horizontal component and the corresponding horizontal and vertical derivatives an example from the siljan impact structure indicates that such directional filters may be applied to extract useful information from magnetic maps

seismic velocities and q factors in the uppermost crust beneath the sveka profile in finland the sveka deep seismic sounding profile in central finland was shot in 1981 to study the crustal structure of the fennoscandian shield in the contact zone between the archaean and svecokarelian provinces apart from p and s waves well developed rayleigh surface waves r waves with periods of 0 5 2 0 s were recorded from all the shot points in the frequency band 0 5 2 5 hz the r waves were clearly observed on the entire 325 km long profile amplitudes of p s and r waves corrected for charge size were used to study the attenuation of waves and q factors from the records of body and r waves the distributions of seismic velocities and quality factors were determined using the dynamic ray tracing method 2 d modelling and the reflectivity method 1 d modelling in the 2 d model of the uppermost crust the p wave velocities vary from 5 8 5 9 km s at the top to 6 2 6 3 km s at a depth of 6 km the velocity ratio solvpvs varies with depth and distance from 1 67 to 1 74 reaching its maximum value of 1 8 2 0 in the uppermost 200 500 m and values of 1 76 1 80 in the uppermost 1 km the quality factors qp and qs vary in the uppermost 1 km from 20 to 120 reaching a higher value of 140 in the marginal zone of the archaean basement in the depth range of 1 2 km q values of 80 400 were found in the depth range 2 6 km q values of 300 800 were found for the svecokarelian province and 200 400 for the archaean the low values of vs and q in the uppermost 1 km are thought to be the result of rock fracturing the crack density was estimated for the uppermost crust along the profile for the sveka profile we found values of 0 35 0 54 in the uppermost 200 m 0 10 0 32 at a depth of 0 2 1 0 km and 0 1 for depths of 1 4 km the comparison of our seismic data with results of theoretical and laboratory investigations implies that the uppermost crustal rocks are saturated the zones of strong attenuation of r waves coincide with faults and schist zones which are well known from surface geology

selective crystallization and precipitation of authigenic pyrite during diagenesis in uranium reservoir sandbodies in ordos basin abstract the occurrence characteristics of authigenic pyrite during diagenesis in uranium reservoir sandbodies of zhiluo formation in ordos basin are investigated based on outcrop area investigations and drilled cores observations the pyrite is closely related to carbonaceous debris microscopically the host components for pyrite consist of carbonaceous debris clay minerals biotite the earlier pyrite and ilmenite by emphasizing on 34s values via la icp ms as well as observing through optical microscope and scanning electron microscope equipped with energy dispersive spectroscope three kinds of distribution pattern of pyrite are classified as i periphery ii infilling iii the combined form of the above two in situ micrometer scale sulfur isotope analyses of pyrite demonstrated that broad scale isotopes are heterogeneous from 52 5 to 35 4 of 34s and sulfur is derived from more than a single source significant differences of host components effects on authigenic pyrite formation are discussed the carbonaceous debris can supply the energy for the bacterial sulfate reduction process and the biotite can provide the source of iron for pyrite precipitation the ferrous ion can be adsorbed on the surface of the clay minerals meanwhile the authigenic pyrite is distributed in or around the earlier pyrite and ilmenite by the indirect adsorption of ferrous ion and the ilmenite may also provide the source of iron futhermore the uranium minerals are mainly associated with the authigenic pyrite relevant to carbonaceous debris and clay minerals which indicates there exists constraints of authigenic pyrite during diagenesis on the uranium mineralization

origin and timing of sand injection petroleum migration and diagenesis in tertiary reservoirs south viking graben north sea petrographic fluid inclusion and carbon and oxygen stable isotope studies of tertiary injectite reservoirs in the south viking graben of the north sea allow an understanding of the origin and timing of sand injection petroleum migration and diagenesis injection from shallowly 400 m 1300 ft buried paleocene and eocene unconsolidated sandstones occurred at the end of the eocene probably in response to earthquake activity liquid oil was already present in the parent sands prior to injection and leaked from the injectites to the seabed upward migrating oil and basinal brines mixed with downward invading mixed meteoric marine pore fluids in the injectites causing extensive biodegradation of the oil biodegradation of oil provided the driver for early carbonate cementation in injectites causing diminished reservoir quality however early carbonate cementation also sealed off the injectites as potential escape routes for petroleum from the underlying parent sands oil and gas continued to migrate into the reservoir parent sands upon increased burial causing a mixing of high api oil with the early charged extensively biodegraded low api oil the study of early diagenetic imprints reveals the evolution of injectite reservoirs which forms the basis for understanding how to explore and develop them

the origin of carbonate cements in the hildasay reservoir cambo field faroe shetland basin clumped isotopic analysis and implications for reservoir performance abstract the early eocene paralic sandstones of the hildasay member of the flett formation form the major oil bearing reservoir in the cambo field located in the faroe shetland basin the sandstones locally contain calcite cemented intervals that vary in thickness from decimetre to over 1 m a calcite cemented interval from well 204 10a 5 has been analysed petrographically and using clumped isotopes to determine its mode of formation and potential lateral extent petrographic analysis shows the cemented interval to consist of ferroan calcite with a consistent dull red cathodoluminescence suggesting a single phase of precipitation the centre of the cemented interval comprises a finer grained unit with detrital kerogen and early sphaerosiderite while the rest comprises homogeneous porosity occluding ferroan calcite the early sphaerosiderite in the centre is replaced by ferroan calcite with a high 13 carbon isotopic signature 13 cvpdb 7 53 suggesting that it formed during anaerobic methanogenesis or fermentation samples from the intermediate zone have a lower 13 carbon isotopic composition 13 cvpdb 0 72 to 3 68 in comparison the outer margin of the cemented unit has an even lower 13 carbon isotopic signature 13 cvpdb 15 5 to 15 9 more typical of a strong aerobic oxidation source the oxygen isotopic signature of the cements is similar 18 ovpdb 10 9 to 12 2 analysis of the clumped isotopes suggest that the ferroan calcite formed at 40 50 c from pore waters that were predominantly meteoric in origin 18 ovsmow 4 8 to 5 7 burial history modelling would suggest that the cements formed at a depth of 500 1000 m meteoric water was likely to have been introduced during the formation of the mid lutetian unconformity 45mya approximately 10 mya after the sandstones were deposited the model proposed is that ferroan calcite precipitation was initiated in the fine lag deposits that contained the kerogen and sphaerosiderite and then grew outwards if the model is correct the cemented units should be restricted to this facies and consequently of limited lateral extent consequently it is likely that the cemented intervals will have a limited impact upon the reservoir performance and are unlikely to act as major barriers to fluid flow

the development of cavities and clastic infills along fault related fractures in tertiary basalts on the ne atlantic margin field based geological observations have revealed the hitherto unrecognised development of post magmatic brittle deformation structures cutting tertiary volcanic rocks in the faroe islands these faults and fractures are characteristically associated with different styles of clastic sedimentary infill including 1 0 3 1 0 m thick clastic units infilling open fractures formed along pre existing steeply dipping to sub vertical faults 2 0 1 0 6 m thick sub horizontal clastic units displaying internal features consistent with deposition from flowing water passing through complex open subterranean cavity systems within fractured basalts 3 anastomosing mm scale and planar dm scale clastic intrusion features mobilised and emplaced during transient fault related overpressuring events along pre existing fractures cutting the surrounding volcanic units the infill features provide evidence for the existence of sustained open cavities in the sub surface the clastic materials are commonly internally affected by later fault related deformation and lack mineralisation unlike all preceding faulting episodes in the faroes region perhaps reflecting their near surface development we believe structures equivalent to these features may occur widely in other parts of the ne atlantic margin particularly along the outer arcs of gentle regional scale fold hinges the uncemented fracture hosted clastic infills potentially represent important fluid migration pathways within the otherwise low permeability cenozoic volcanic sequences of the ne atlantic region

facies and flow regimes of sandstone hosted columnar intrusions insights from the pipes of kodachrome basin state park sandstone hosted intrusions are found in a wide variety of environments but remain poorly understood compared with their mudstone hosted counterparts in particular they remain largely unrecognized in the subsurface in part because they are problematic to image in seismic data this study reports on the facies and fluid flow associated with a 20 000 km3 sandstone intrusion province in utah usa forming a small portion of this intrusion province the intrusions cropping out in kodachrome basin state park display a very wide array of facies and morphologies factors which would make their identification in core a significant challenge remobilized sediment is shown to have been injected at least 200 m vertically from its source with flow prolonged enough to concentrate heavy minerals in placer style deposits at the pipe margins evidence for lateral pipe migration and for associated broader fluidization regions is also presented a new approach to estimating flow parameters in injectites is implemented herein and indicates that previous work has overestimated velocities and flow reynolds numbers by up to two orders of magnitude flow modelling suggests turbulent flow in the pipes that is consistent with field observations of erosive margins and chaotic internal structures post emplacement these pipes remained as long term fluid conduits as revealed by their diagenetic history focussing and facilitating flow of extraformational fluids despite the relatively high porosity and permeability of the aeolian host strata

the upper jurassic boulder beds and related deposits a fault controlled submarine slope ne scotland during the late jurassic kimmeridgian rifting in the north sea large scale normal faults developed to form relatively deep marine basins the north western margin of one of these basins defined by the helmsdale fault is described to show the nature of sedimentation associated with a submarine fault scarp subsidence to the se of the helmsdale fault led to a fault scarp down which various sediment mass flows moved including debris flow rockfall high and low concentration turbidity currents and possible storm surge wash over deposits redeposition by soft sediment sliding was important as a slope accretion process deep channels were incised into the slope and infilled by coarse grained elastics contemporaneous small scale submarine fans located farther off in the present north sea may have developed as a result of slope bypassing sediment flows down such channels the succession is characterized by extreme sediment heterogeneity compared to many deep water environments and it may be part of an ancient fan delta the main influences upon slope accretion were the synsedimentary movements on the helmsdale fault and the nature of the available sediment input such that the succession represents structurally controlled and sedimentologically modified slope development

fluid inclusions in sedimentary and diagenetic systems abstract some of the major problems in sedimentary geology can be solved by using fluid inclusions in sedimentary and diagenetic minerals important fluids in the sedimentary realm include atmospheric gases fresh water of meteoric origin lake water seawater mixed water evaporated water formation waters deep in basins oil and natural gas preserving a record of the distribution and composition of these fluids from the past should contribute significantly to studies of paleoclimate and global change research is essential for improving understanding of diagenetic systems and provides useful information in petroleum geology applications of fluid inclusions to sedimentary systems are not without their complexities some fluid inclusions exposed to natural conditions of increasing temperature may be altered by thermal reequilibration which results in stretching or leakage and refilling of some fluid inclusions similarly overheating in the laboratory can also cause reequilibration of fluid inclusions so fluid inclusions from the sedimentary realm must be handled carefully and protected from overheating natural overheating of fluid inclusions must be evaluated through analysis of the most finely discriminated events of fluid inclusion entrapment fluid inclusion assemblages fia consistency in homogenization temperatures within a fluid inclusion assemblage consisting of variably sized and shaped inclusions is the hallmark of a data set that has not been altered through thermal reequilibration in contrast fluid inclusion assemblages yielding variable data may have been altered through thermal reequilibration if a fluid inclusion assemblage has not been altered by thermal reequilibration its fluid inclusions may be useful as geothermometers for low and high temperature systems or useful as geobarometers applicable throughout the sedimentary realm if a fluid inclusion assemblage has been altered partially by thermal reequilibration techniques for distinguishing between altered and unaltered fluid inclusions may be applied in studies of global change fluid inclusions can be used as sensitive indicators of paleotemperature of surface environments fluid inclusions also preserve microsamples of ancient seawater and atmosphere the analysis of which could figure prominently into discussions of past changes in chemistry of the atmosphere and oceans in petroleum geology fluid inclusions have proven to be useful indicators of migration pathways of hydrocarbons they can delineate the evolution of the chemistry of hydrocarbons and they remain important in understanding the thermal history of basins and relating fluid migration events to evolution of reservoir systems in studies of diagenesis fluid inclusions can be the most definitive record most diagenetic systems are closely linked to temperature and salinity of the fluid thus fluid inclusions are sensitive indicators of diagenetic environments

3 d palinspastic restoration of normal faults in the inner moray firth implications for extensional basin development abstract balanced cross section techniques and the construction of a restored section permit 2 dimensional palinspastic restorations to be made in both compressional and extensional terraines in 3 dimensions an equivalent restoration can be made by assuming conservation of bedding plane area and considering the volume of a stratigraphic interval rather than its cross sectional area extensional basins displaying upper crustal listric normal faulting are particularly amenable to this approach computerised 3 d restorations have been made of the inner moray firth basin offshore scotland this basin is not isostatically compensated and was produced by 7 8 post triassic extension of which 2 5 3 is post jurassic above a detachment surface at 20 25 km depth close to the base of the crust limited lower crustal thinning and lithospheric stretching has affected the eastern part of the basin but this can account for no more than half of the measured upper crustal extension some of this shallow extension is probably coupled by low angle faults or shear zones into major zones of lithospheric stretching such as the north sea grabens where it may help account for discrepancies between estimates of lithospheric thinning and upper crustal extension

fossil seep structures of the monterey bay region and tectonic structural controls on fluid flow in an active transform margin abstract the subsurface plumbing system of ancient seeps of hydrocarbon rich gases and fluids is partially preserved by seep deposits in miocene rocks of the coast ranges and on the seafloor of the monterey bay region in central california the fossil seep related deposits are of two types authigenic carbonates and intrusions of oil saturated sandstones both types occur in fractured and faulted siliceous mudstone and porcelanite layers of the organic rich miocene santa cruz mudstone they are associated with tectonic structures indicating deformation and are located in the proximity of faults the morphology of the seep carbonates is characterized by strong asymmetries and by holes pipes and other types of fluid conduits with orientations parallel to the directions of the local fracture and fault sets thus suggesting structural control over pathways of fluid migration the dominant direction of the conduits in the seep carbonates and of the intrusive dikes northeast southwest differs from the dominant orientation of the tectonic structures of the san andreas fault system northwest southeast previous interpretations of these ancient seep related deposits suggested that lateral changes of the thickness of the cenozoic marine host sediments favored fluid migration from basin depocenters located offshore toward basin margins different driving forces for fluids have been hypothesized including hydrocarbon generation differential compaction and tectonic compression most of the fossil seeps are located on structural highs of the basement and adjacent to offshore basins that form elongated depressions oriented northwest southeast previously unidentified structures that offset the basement along northeast southwest trends cross setting faults paralleling the orientations of the conduits of the seep carbonates and the dikes of oil saturated sandstones were also recognized tectonic deformation block rotations documented by local clockwise rotations of paleomagnetic directions in the coastal ranges and cross faults were probably produced during the late miocene and pliocene to accommodate the shear stress between conjugate faults during eastward shifting of the activity of the san andreas fault system comparison with previous studies on ancient seep deposits regional geology and offshore tectonic data suggest that the fluids that sourced the seeps were driven by episodes of inverse tectonics during which previously extensional depositional basins were involved in episodes of tectonic compression tectonic deformation created cross faults in rotated blocks which could have acted as important conduits for the flow of hydrocarbon rich fluids and gases and fostered the injection of liquefied sands

effect of meteoric water flow on calcite cementation in the middle jurassic oseberg formation well 30 3 2 veslefrikk field norwegian north sea abstract the middle jurassic delta front sandstones of the oseberg formation in well 30 3 2 in the veslefrikk oilfield contain abundant calcite cemented intervals with thicknesses of up to 2 15 m most of the calcite cement is characterized by 18 o pdb values in the range 5 to 10 and by low strontium concentrations mostly in the range 100 500 ppm the low strontium concentration in the studied calcite cement can probably best be explained by the precipitation of calcite during meteoric water flushing 18 o pdb values of 5 to 10 have also been considered characteristic of calcite cement precipitated from meteoric water and ubiquitous authigenic kaolinite and feldspar dissolution in the studied sandstones suggests that extensive meteoric water flushing of the oseberg formation has taken place during and soon after deposition a few of the studied calcite cemented intervals contain a calcite cement with a much higher strontium content 750 1430 ppm and more negative 18 o pdb values 11 1 to 14 3 these calcite cemented intervals are thought to have formed at greater depth after meteoric water flushing had terminated the calcite cement with the highest strontium concentration and the most negative 18 o values occurs within calcite cemented intervals with sharp boundaries the calcite cement with a low strontium concentration and intermediate 18 o values is found within calcite cemented intervals with gradational boundaries characterized by a transition zone of calcite cemented specks in uncemented sandstone this difference in cementation pattern can be explained by the effect of pore water flow rates on the nucleation and growth of calcite cement

explosion generated short period surface wave dispersion and noise studies along linear seismic arrays in southern sweden summary the dispersion of short period fundamental mode rayleigh waves rg was measured along linear arrays on the baltic shield the data used is a selected set of 176 records along three refraction profiles in southern sweden the phase velocity determined in the 0 2 3 5 hz band could be grouped into a number of dispersion regions each regional dispersion showed a high internal consistency and the slowness as function of frequency was almost linear in the 1 3 hz band linear regression was used to separate the effects of the medium from the experimental uncertainties systematic studies of the noise to signal ratio showed that the standard deviation of the dispersion within each region and the above frequency band essentially reflected the lateral heterogeneities from the dispersion shear velocity models were inverted down to about 2 3 km in one case down to 6 km the s wave velocity was weakly constrained to a non decreasing function of depth with decreasing gradient the p wave velocity was constrained using recent results on poisson s constant ranging from 0 28 to 0 25 in the 0 2 km interval the density was constrained using geological knowledge of the area using these constraints the s wave velocity increased rapidly with depth in the first few hundred metres of the crust the models could be naturally grouped into three sets with similar character in close agreement with the large scale surface geology the highest velocities were found in the east in the smiland varmland granitic belt and the smallest in the northwest in the sveconorwegian crust

paleogeographic significance of oriented calcareous concretions in the triassic katberg formation south africa abstract elongated calcareous concretions in sandstones of the fluvial katberg formation beaufort group south africa display a distinct preferred orientation that coincides with the regional paleoslope as deduced from cross stratification and primary current lineations these concretions formed after deposition of the sandstone but before the strata were affected by tectonic movement presumably by groundwater flowing down the regional paleoslope similar sandstone hosted concretions in other non marine successions could potentially yield valuable information on paleo groundwater flow and by inference depositional paleoslopes

low mg calcite marine cement in cretaceous turbidites origin spatial distribution and relationship to seawater chemistry lower cretaceous hauterivian bioclastic sandstone turbidites in the scapa member north sea basin were extensively cemented by low mg calcite spars initially as rim cements and subsequently as concretions five petrographically distinct cement stages form a consistent paragenetic sequence across the scapa field the dominant and pervasive second cement stage accounts for the majority of concretions and is the focus of this study stable isotope characterization of the cement is hampered by the presence of calcitic bioclasts and of later cements in sponge spicule moulds throughout the concretions nevertheless trends from whole rock data augmented by cement separates from synlithification fractures indicate an early calcite 18o value of 0 5 to 1 5 pdb as such the calcite probably precipitated from marine pore fluids shortly after turbidite deposition carbon isotopes 13c 0 to 2 pdb and petrographic data indicate that calcite formed as a consequence of bioclastic aragonite dissolution textural integrity of calcitic nannoplankton in the sandstones demonstrates that pore fluids remained at or above calcite saturation as expected for a mineral controlled transformation electron probe microanalyses demonstrate that early calcite cement contains 2 mol mgco3 despite its marine parentage production of this cement is ascribed to a combination of an elevated aragonite saturation depth and a lowered marine mg2 ca2 ratio in early cretaceous calcite seas relative to modern oceans scapa cement compositions concur with published models in suggesting that hauterivian ocean water had a mg2 ca2 ratio of 1 this is also supported by consideration of the spatial distribution of early calcite cement in terms of concretion growth kinetics in contrast to the dominant early cement late stage ferroan 18o depleted calcites were sourced outwith the scapa member and precipitated after 1 2 km of burial our results emphasize that bioclast dissolution and low mg calcite cementation in sandstone reservoirs should not automatically be regarded as evidence for uplift and meteoric diagenesis

large scale conical sandstone intrusions and polygonal fault systems in tranche 6 faroe shetland basin abstract kilometre scale sandstone intrusions are important components of many deep water clastic systems they result from the remobilisation and injection of sand during early stages of burial and are commonly found in fine grained mudrocks containing a pervasive polygonal fault network this paper uses three dimensional seismic data from the faroe shetland basin on the uk atlantic margin calibrated to nearby wells to examine the relationship between an extensive system of large downward tapering conical sandstone intrusions to the pervasive polygonal fault network present within the surrounding encasing host mudstones the intrusions were injected upwards during the late miocene through eocene oligocene claystones with a pre existing polygonal fault network to form conical bodies up to 300 m high within the 700 m thick claystone sequence this study shows that polygonal faults and the discordant limbs of conical sandstone intrusions form two distinct dip populations with little overlap that have average dips of 58 and 26 respectively polygonal faults display decreasing dip with increasing depth however conical sandstone intrusion dip is uniform throughout the eocene oligocene succession these observations lead us to conclude that the geometry of conical sandstone intrusions is not greatly affected by the presence of a pre existing polygonal fault network nor is it affected by increased overburden pressure with depth therefore the 10 50 dip range displayed by the limbs of conical sandstone intrusions observed within the study area is a fundamental property of conical sandstone intrusions the presence of a widespread region of conical sandstone intrusions in the faroe shetland basin has important implications for vertical fluid migration in the region the eocene oligocene claystones in the region have very low permeability but the presence of large scale conical sandstone intrusions and any possible feeder systems is likely to provide high permeability fluid conduits through this sequence in effect the otherwise good regional caprock provided by the eocene oligocene claystone sequence is breached by a network of sandstone intrusions possibly resulting in reduced hydrocarbon trapping potential beneath the seal

water rock interaction in tertiary sandstones san joaquin basin california u s a diagenetic controls on water composition three water types are recognized among petroleum associated waters of the southern san joaquin basin 1 meteoric water 2 diagenetically modified connate water and 3 mixed meteoric modified connate water the diagenetically modified connate marine water can be further subdivided into three types 1 fully mg depleted mg 1 mole of the sum of na k ca and mg 2 partially mg depleted and 3 ca enriched enrichment in ca which is the sharpest chemical divide found for the diagenetically modified connate water is the result of feldspar albitization the ca enriched waters are also rich in total dissolved al up to 6 mg l 1 and exhibit lighter 18o values than other diagenetically modified connate waters the total dissolved al abundance in these waters may be due to the presence of organic complexing agents simple and aromatic organic acids and phenolics the lighter 18o values are probably due to both albitization and the lack of substantial clay diagenesis illitization of smectite

diagenetic history of eocene wilcox sandstones south central texas abstract sandstones and shales of the lower eocene wilcox group extend from outcrop to depths greater than 5 km where temperatures exceed 200 c eight authigenic minerals have been identified quartz kaolinite calcite ankerite and albitized plagioclase are volumetrically significant whereas chlorite illite albite overgrowths and dolomite are present only in trace amounts quartz kaolinite and ankerite cements are present over the entire depth range sampled 1528 to 4446 m calcite cement occurs only in sandstones shallower than 3300 m albitized plagioclase is the predominant feldspar in samples deeper than 2600 m whereas detrital potassium feldspar has been completely dissolved out of sandstones deeper than about 3200 m no systematic variations in the chemical composition of calcite with depth no zoning within calcite or ankerite and no compositional differences between pore filling and grain replacing calcite or ankerite were found calcite cements have 87 sr 86 sr ratios generally less than 0 708 whereas ankerite cements are significantly enriched in 87sr the oxygen isotopic composition of carbonate cement ranges between 9 and 14 pdb for calcite and between 8 and 12 for ankerite with no systematic variation with depth calculated 18o values for authigenic quartz and kaolinite range from 22 to 31 smow and from 14 to 17 respectively volumetrically important amounts of authigenic quartz and kaolinite precipitated during early burial before calcite cementation secondary porosity generation or smectite to illite transformation in encasing shales the isotopic composition of strontium in calcite cement shows most of it to be of eocene or older marine carbonate origin ankerite cementation followed the generation of secondary porosity and occurred after the products of organic maturation and shale diagenesis plagioclase albitization or potassium feldspar dissolution significantly affected the sandstone pore water system

a combined chemical isotopic and microstructural study of pyrite from roll front uranium deposits lake eyre basin south australia abstract the common sulfide mineral pyrite is abundant throughout sedimentary uranium systems at pepegoona pepegoona west and pannikan lake eyre basin south australia combined chemical isotopic and microstructural analysis of pyrite indicates variation in fluid composition sulfur source and precipitation conditions during a protracted mineralization event the results show the significant role played by pyrite as a metal scavenger and monitor of fluid changes in low temperature hydrothermal systems in situ micrometer scale sulfur isotope analyses of pyrite demonstrated broad scale isotopic heterogeneity 34 s 43 9 to 32 4 vcdt indicative of complex multi faceted pyrite evolution and sulfur derived from more than a single source preserved textures support this assertion and indicate a genetic model involving more than one phase of pyrite formation authigenic pyrite underwent prolonged evolution and recrystallization evidenced by a genetic relationship between archetypal framboidal aggregates and pyrite euhedra secondary hydrothermal pyrite commonly displays hyper enrichment of several trace elements mn co ni as se mo sb w and tl in ore bearing horizons hydrothermal fluids of magmatic and meteoric origins supplied metals to the system but the geochemical signature of pyrite suggests a dominantly granitic source and also the influence of mafic rock types irregular variation in 34 s coupled with oscillatory trace element zonation in secondary pyrite is interpreted in terms of continuous variations in fluid composition and cycles of diagenetic recrystallization a late stage oxidizing fluid may have mobilized selenium from pre existing pyrite subsequent restoration of reduced conditions within the aquifer caused ongoing pyrite re crystallization and precipitation of selenium as native selenium these results provide the first qualitative constraints on the formation mechanisms of the uranium deposits at beverley north insights into depositional conditions and sources of both sulfide and uranium mineralization and an improved understanding of pyrite geochemistry can also underpin an effective vector for uranium exploration at beverley north and other sedimentary systems of the lake eyre basin as well as in comparable geological environments elsewhere

shallow subsurface growth and burial alteration of middle devonian calcite concretions abstract organic rich shale hosted septarian and nonseptarian calcite concretions in the middle devonian givetian hamilton group of central new york state formed early in the shallow subsurface as indicated by detailed petrography sem x radiographic lithologic geochemical and isotopic analyses 13ccarb values suggest that carbon was derived from microbial breakdown of organic matter in the sulfate reduction zone at subsurface depths of less than 10 m however strongly depleted 18o 7 to 10 pdb suggests much greater burial depths the sup 18o data appear to represent an altered isotopic signature as indicated by their narrow depleted range and petrographic evidence for recrystallization of concretion carbonate evidence of primary porosity in concretions during growth indicates that early stage diagenetic alteration occurred in an open marine phreatic environment that homogenized isotopic and geochemical signatures of metastable carbonate minerals the association of septarian calcite and bitumen along with the depleted 18o values suggests that late stage diagenesis was a result of migration of warm basin derived connate waters through the open system preburial condensation of sediments in younger stratigraphic units within the hamilton group possibly caused by increased winnowing bioturbation and erosional events during shallowing of the hamilton sea has generated stratigraphic variations in concretion lithology and shape within any given unit however variation in characteristics of concretions remains a primary function of their timing of growth versus burial compaction unlike cenozoic carbonate concretions in organic rich marine sediments that possess a dolomitic matrix no dolomite concretions were discovered in the hamilton group preliminary data suggest that restriction of calcite as the host mineral for early to middle paleozoic concretions may be controlled by elevated sulfate concentrations that inhibited dolomite precipitation and or primary secular variations in carbonate mineralogy

the concretions of the bearreraig sandstone formation geometry and geochemistry the sandbodies of the bearreraig sandstone formation inner hebrides uk are cemented by two generations of calcite the first generation an inhomogeneous ferroan calcite 0 05 3 28 mol feco3 formed during sulphate reduction 13c 24 to 32 o pdb in marine porewaters 18o of cement from 1 to 4 o pdb at very shallow burial depths a few centimetres these cements are rare but form millimetre scale clusters of crystals which acted as nuclei to the later concretionary cements the second generation of cements are more homogeneous ferroan calcites mean 1 58 mol feco3 which evolve to progressively higher fe mg ratios they are sourced by shell dissolution 13c of cement from 1 to 3 o pdb into meteoric 18o of cement from 6 to 10 o pdb or mixed marine meteoric waters 18o of cement from 4 to 6 o smow these were introduced into the formation either during bathonian times as a freshwater lens or subsequent to partial inversion by confined aquifer flow corroded feldspars within the concretions suggest that an interval of at least 8 ma separated the deposition of the sediments from the onset of concretion growth abundant concretions are preferentially developed at certain horizons within the sandbodies where the early generation of ferroan calcite cements provided nuclei the latter formed close to the sediment water interface the concentration of cement within the sediment being related to sedimentation rate the relatively high concentrations of the first generation of cement upon which the concretionary horizons are nucleated formed during periods of minimal sedimentation

sand injectites from source to emplacement an example from the miocene age frimmersdorf seam garzweiler open cast mine lower rhine embayment

saucer shaped intrusions occurrences emplacement and implications abstract magmatic intrusions and fluidized injected sands represent the two main types of sheet intrusions in the earth s crust in this paper we show that both intrusion types often display a saucer like geometry as revealed by 3d seismic imaging and field observations saucer shaped intrusions are fairly common in sedimentary basins as for example offshore in the norwegian and north sea basins and onshore in the karoo basin of south africa the formation of the saucer geometry is controlled by the low viscosity of the injected fluid and by the interaction between a growing shallow hydraulic fracture and overburden deformations statistics gathered from observations and modelling show a linear relationship between the depth of emplacement and the size of the saucer shaped intrusions we anticipate future cross disciplinary studies aiming to discover other occurrences of saucer shaped intrusions and to identify the physical processes controlling the development of this fundamental geometry

a new variscan deformation map of england and wales collections of bedding orientations from subareas of 550 km 2 are used to define bedding fabrics from the variscan fold belt and foreland in england and wales the bedding fabrics constructed from data taken from published geological maps 1 50 000 scale are analysed in terms of the scheidegger watson orientation tensor the calculated eigenvalues of the orientation tensor define the average structural trends including the best fit fold axis for each subarea the eigenvectors of the tensor allow the original sedimentary fabrics to be distinguished from those modified by deformation the progressive tectonic modification of these fabrics leads to the concept of fabric evolution paths and the extent of the modification is expressed by a deformation parameter although it is emphasised that this parameter is not solely a function of finite strain magnitude its variation is represented on a map which brings out different degrees of variscan tectonic modification of bedding fabrics this map delineates a variscan bedding fabric front corresponding to the northern boundary of a zone of pervasive and strong deformation in the foreland this new analysis allows the variation of variscan trends to be mapped and the significance of these trends to be quantified in terms of deformation magnitude 1999 elsevier science b v all rights reserved

a geochemical framework for the application of stable sulphur isotopes to fossil pyritization pyrite formation is associated with fossils where the organic material decays by sulphate reduction or where the carbonate skeleton acts either as a nucleation substrate or induces iron sulphide precipitation by dissolution these requirements define specific combinations of porewater chemistry and saturation state with respect to carbonates and iron sulphides a review of modern marine sediments suggests that near surface porewaters are always over saturated or close to saturation with iron sulphides but are alternately rich in either dissolved iron or dissolved sulphide however significant variations in carbonate saturation state may occur with porewaters being undersaturated in the early stages of sulphate reduction but subsequently becoming oversaturated until the later stages of methanogenesis here undersaturation may return if there is a substantial input of carbon dioxide and if little alkalinity has arisen from iron reduction organic matter pyritization requires that soft tissue material decays by sulphate reduction to release dissolved sulphide with dissolved iron supplied by the surrounding porewaters the modern sediment studies therefore suggest that organic matter pyritization can occur only during the earliest stages of sulphate reduction where there are iron rich porewaters which are saturated with iron sulphides pyrite replacement of carbonate shell material requires porewater undersaturation with respect to carbonates and analogy with modern sediments suggests that these conditions occur either during the earliest stages of sulphate reduction or when methanogenesis produces undersaturation between these periods porewaters are saturated with respect to carbonates and pyrite of different morphologies can precipitate on carbonate shells sulphur isotope data provide support for these associations of porewater chemistry and style of fossil pyritization the best described examples are for soft tissue pyritization in beecher9s trilobite bed ordovician and the hunsruck slate devonian where the isotopic data are consistent with rapid early pyritization in iron rich porewaters more data are needed on shell pyritization with the range of isotopic compositions indicating formation through all stages of diagenesis