de glauconiarum origine the glauconitic facies is widespread on present day continental shelves from 50 s to 65 n and at water depths between 50 and 500 m and is in particularly great abundance on the upper slope and outer shelf between 200 and 300 m it is also common in many ancient rocks of post late precambrian age it occurs as sand to pebble sized essentially green particles granular facies but also as a surface coating on particles and hardgrounds and as a diffuse impregnation film and diffuse facies we suggest the replacement of the term glauconite which has been interchangeably used to designate a morphological form and a specific mineral by glaucony facies and glauconitic smectite and glauconitic mica as end members of the glauconitic mineral family the widely accepted model of burst and hower for glauconitization requires a degraded micaceous 2 1 layer lattice structure parent clay mineral however detailed analysis of numerous samples of recent glaucony reveals that such a parent substrate is exceptional the model therefore requires modification generally the parent material is carbonate particles argillaceous kaolinitic faecal pellets infillings of foraminiferal tests various mineral grains and rock fragments that pass gradually into the commonly occurring green grains we show that the process of glauconitization is achieved by de novo authigenic growth of automorphous crystallites in the pores of the substrate accompanied by progressive alteration and replacement of the substrate it is this two fold evolution that causes the verdissement of granular substrates macrofossils and hardgrounds the authigenic mineral is an iron rich and potassium poor glauconitic smectite while new smectites are growing into the remaining pore space the earlier smectites are modified by incorporation of potassium producing decreasingly expandable minerals with a non expandable glauconitic mica as the end member this mineralogical diversity of the glauconitic mineral family explains the highly variable physical and chemical properties of glaucony four categories nascent little evolved evolved and highly evolved glaucony are distinguished glauconitization appears to be controlled by a delicate balance between degree of physical confinement of a particle and the amount of ionic exchange between the micro environment and ambient open marine sea water the optimum conditions for glauconitization are those of semi confinement as a result the interior of a grain is more glauconitized than its less confined periphery similarly for identical substrate types large grains 500 m provide more favourable substrates for glauconitization than lesser confined small grains on a larger scale the formation of glaucony is governed by the availability of iron and potassium and the balance between detrital influx and winnowing low accumulation rates expose grains to the open marine environment for sufficiently long times 105 106 years for highly evolved glaucony

microtextures geochemistry and geochronology of authigenic xenotime constraining the cementation history of a palaeoproterozoic metasedimentary sequence an 26 m thick unit of phosphatic sandstone and black shale the phosphatic unit in the palaeoproterozoic mount barren group of south western australia contains abundant authigenic xenotime crystals showing well preserved diagenetic textures despite extensive regional deformation and thermal metamorphism the peak of which occurred at 1205 ma the phosphatic unit was preserved as a low strain envelope because of its pre compaction carbonate and phosphate cementation in situ u pb geochronology of xenotime reveals four discrete age populations at 1693 4 1645 3 1578 10 and 1481 21 ma when integrated with petrography the age data place a timeframe on i sediment deposition ii phosphogenesis iii diagenetic cement infilling iv diagenetic pyrite formation v secondary porosity generation vi hydrocarbon migration vii burial compaction and viii hydrothermal alteration up until peak thermal metamorphism xenotime growth at 1693 ma occurred prior to compaction whereas xenotime growth at 1645 ma occurred during burial xenotime growth at 1580 ma and at 1480 ma appears to be the far field record of thermotectonic events associated with intracontinental extension and magmatism recorded elsewhere in australia geochemical analysis integrated with geochronology shows a systematic increase in mree hree in xenotime crystals with decreasing age and with increasing stratigraphic depth coupled with a decrease in xenotime abundance and age with depth it suggests that i the main focus of porosity infilling was at the top of the phosphatic unit and progressed downwards over the 200 myr period of porosity infilling and ii the changes in xenotime ree chemistry may be due to an influx of mree from increasing amounts of dissolved apatite or changes with respect to ree solubility in the physiochemical nature of the fluids with burial depth

the paleoproterozoic maclellan deposit and related au ag occurrences lynn lake greenstone belt manitoba an emerging structurally controlled gold camp abstract the paleoproterozoic era is important for au ag mineralization including deposits in the trans hudson orogen of central canada the association between au ag deposits and regional structures that formed during this era has seen markedly less research than archean counterparts the maclellan deposit and related au ag occurrences occur within the northern part of the paleoproterozoic lynn lake greenstone belt in northwest manitoba field relationships combined with a detailed characterization of the host rocks alteration and structure along with mass balance calculations indicate that the host rocks were largely derived from a common amphibole plagioclase schist protolith originally a porphyritic basalt that was variably deformed and altered to initially biotite quartz during d2 deformation and subsequently to cr bearing chlorite carbonate amphibole quartz during the late stages of d2 and d3 gold silver mineralization is associated with these alteration events and consists predominantly of native au aurostibite ausb2 au in arsenopyrite and ag in galena these can be separated into two distinct au mineralizing events 1 early disseminated arsenopyrite aligned with d2 fabrics that contains structurally bound au and 2 late fracture filling native au aurostibite and au rich rims and fractures on and within arsenopyrite the highest concentrations of au and ag occur where e w trending dextral d2 shear zones intersect nne ssw trending brittle faults d4 suggesting late remobilization of precious metals by hydrothermal fluids into dilatant zones the structural control on formation and distribution of au ag deposits along the maclellan shear zone is similar to that observed along the regionally significant johnson shear zone which occurs in the southern portion of the lynn lake greenstone belt suggesting similar processes operated within both portions of the belt integrating detailed geologic and structural mapping petrography geochemistry and alteration demonstrates that these au ag occurrences formed as a result of structurally controlled mineralizing systems associated with major shear zones rather than being primarily syngenetic in origin i e seafloor exhalative as previously suggested these major paleoproterozoic shear zones display similar characteristics and relationships to well documented structural breaks in archean terranes e g porcupine destor deformation zone and larder lake cadillac deformation zone suggesting that at least in terms of the lynn lake region structural and mineral deposit forming processes may have occurred on comparable spatial scales to those in archean systems

high resolution sequence stratigraphy of clastic shelves iii applications to reservoir geology abstract high frequency clastic shelf sequences contain porous deposits consisting of coastal to shallow water sandstones and conglomerates which may represent very good reservoirs and therefore critical targets for exploration and production as well as for fluid hydrocarbon water or co2 storage the study of these reservoirs following a sequence stratigraphic approach is essential to predict their features and distribution within systems tracts forming high frequency sequences which in turn compose higher rank systems tracts and sequences a critical aspect to improve the exploitability of the reservoirs is their heterogeneity which directly affects the volume of the reservoirs and determine internal compartmentalization the heterogeneity can be primary related to depositional processes influencing grain size and facies distribution and secondary related to diagenesis in particular the primary heterogeneity is referred to the facies variability from distal to proximal settings to the accumulation of condensed shell beds and to facies characteristics of bedsets whereas the secondary heterogeneity derives from diagenetic processes at sequence stratigraphic surfaces and facies contacts and within sandbodies both the distribution of reservoirs and their heterogeneity can be predicted following a high resolution sequence stratigraphic approach ultimately the integration of subsurface data with the analysis of outcrop analogs is essential to better understand the features of clastic shelf reservoirs and to develop 3d models of fluid migration

muddy and dolomitic rip up clasts in triassic fluvial sandstones origin and impact on potential reservoir properties argana basin morocco abstract the significance of rip up clasts as sandstone framework grains is frequently neglected in the literature being considered as accessory components in bulk sandstone composition however this study highlights the great value of muddy and dolomitic rip up clast occurrence as a information source about low preservation potential from floodplain deposits and b key element controlling host sandstone diagenetic evolution and thus ultimate reservoir quality high resolution petrographic analysis on triassic fluvial sandstones from argana basin t6 and t7 t8 units highlights the significance of different types of rip up clasts as intrabasinal framework components of continental sediments from arid climates on the basis of their composition and ductility three main types are distinguished a muddy rip up clasts b dolomitic muddy rip up clasts and c dolomite crystalline rip up clasts spatial distribution of different types is strongly facies related according to grain size origin of rip up clasts is related to erosion of coeval phreatic dolocretes in different development stages and associated muddy floodplain sediments cloudy cores with abundant inclusions and clear outer rims of dolomite crystals suggest a first replacive and a subsequent displacive growth respectively dolomite crystals are almost stoichiometric this composition is very similar to that of early sandstone dolomite cement supporting phreatic dolocretes as dolomite origin in both situations sandstone diagenesis is dominated by mechanical compaction and dolomite cementation a direct correlation exists between 1 muddy rip up clast abundance and early reduction of primary porosity by compaction with irreversible loss of intergranular volume igv and 2 occurrence of dolomitic rip up clasts and dolomite cement nucleation in host sandstone occluding adjacent pores but preserving igv both processes affect reservoir quality by generation of vertical and 3d fluid flow baffles and barriers that compartmentalize the reservoir these findings may provide quantitative useful data for the better understanding of reservoir quality in analogous hydrocarbon bearing basins such as the bay of fundy nova scotia canada

equatorial aridity and climatic oscillations during the early carboniferous southern britain evidence from palaeosols palaeokarsts and styles of meteoric diagenesis shows that the early carboniferous climate of southern britain was dominantly seasonally semi arid this semi aridity occurred while the region was in an equatorial palaeolatitudinal position the cause of the seasonal aridity was possibly the deflection of easterly winds into a major low pressure zone set up over gondwana during the southern hemisphere summer

sandstone hosted thorium bitumen mineralization in the northwest irish basin low grade sandstone hosted thorium mineralization is recorded in the carboniferous northwest irish basin in the republic of ireland and northern ireland the thorium is concentrated within bitumen nodules which are replacive within the sandstones a source of thorium was available in a caledonian granite in the basin watershed and the bitumen was a product of hydrocarbons generated within the basin the thorium occurs as thorite inclusions in the bitumen and the development of the nodules was enhanced by the polymerization of fluid hydrocarbons by irradiation from the thorium thorium mobility may have been enhanced by the formation of organometallic complexes mineralization probably therefore occurred during hydrocarbon migration dated as late carboniferous to permian

a new geochemical classification of sedimentary environments because of the relative constancy of ph in most subaqueous sediments and the general lack of measurability of eh these parameters are not practically useful for classifying sedimentary environments in their place a new classification is offered which derived from studies of modern sediments is based on the presence or absence of dissolved oxygen and dissolved sulfide in the sediments at the time of authigenic mineral formation sedimentary environments are first of all divided into oxic and anoxic depending upon the presence or absence of measurable dissolved oxygen anoxic environments in turn are divided into sulfidic and non sulfidic depending upon the presence of measurable dissolved sulfide anoxic nonsulfidic environments are further divided into postoxic or resulting from oxygen removal without sulfate reduction weakly reducing and methanic or resulting from complete sulfate reduction with consequent methane formation strongly reducing the environments are shown to succeed one another during early diagenesis in the order oxic post oxic sulfidic methanic iron and manganese minerals characteristic and stable in each environment are listed and discussed so that they may be used to distinguish the environments when studying the ancient geological record

high resolution sequence stratigraphy applied to reservoir zonation and characterisation and its impact on production performance shallow marine fluvial downstream and lacustrine carbonate settings abstract high resolution sequence stratigraphy hrss is a data driven methodology that refers to the stratigraphic refinement achieved below seismic resolution hrss follows the standard sequence stratigraphy methodology and nomenclature which are well established for the shallow marine to downstream reaches of the fluvial systems still nomenclature must be flexible to some extent when applied to lacustrine carbonate settings a high resolution stratigraphic sequence is a cycle of change in stacking patterns of facies facies associations architectural elements or depositional systems bound by the recurrence of the same sequence stratigraphic surface in the rock record that is observed below the resolution of a third order sequence criteria that allow its identification include i typical internal transgressive regressive t r pattern ii vertical recurrence of the signature of stacking patterns at each considered hierarchy iii trends non random distribution of the vertical arrangement of stacking patterns that determine high frequency sequences as the basis for defining and building the systems tracts of immediately higher order sequences iv mappability of the stacking patterns and their respective bounding stratigraphic surfaces at each considered hierarchy reservoir zonation and characterisation is an essential application of hrss in the petroleum industry hrss highlights the reservoir spatial distribution and heterogeneities which are the stratigraphical essence of 3d geological and fluid flow models such models must be sufficiently robust to forecast production performance to guide reservoir management to optimise production and to increase the ultimate recovery factor in greenfields hrss identifies the best reservoirs for production in brownfields it guides an increase in production that marks a new phase of rejuvenation hrss rationalises and optimises hydrocarbon production and hence is an indispensable methodology for reservoir geology

87sr86sr isotopic evolution of lower carboniferous seawater dinantian of western europe abstract the chemical and isotopic evolution of ocean water mirrors the dynamics of the earth system the original seawater signature can be measured only indirectly for the palaeozoic samples with the greatest utility for such an approach are the secondary layers of articulate brachiopods for a correct interpretation of isotope data samples that have suffered alteration by diagenetic processes must be avoided trace element analysis is frequently utilized as an analytical tool the present study deals with the delineation of a detailed 87 sr 86 sr curve for the dinantian this in turn enables us to search for causative factors of the observed isotopic oscillations a total of 175 lower carboniferous dinantian brachiopod shells from a variety of western european locations have been analysed for their 87 sr 86 sr ratios for 152 of these shells quantitative geochemical tests on diagenetic alteration based on trace element repartitioning during recrystallisation have been performed the concentrations of ca mg sr mn and fe were routinely measured by inductively coupled plasma atomic emission spectroscopy icp aes analysing the diluted phosphoric acid that remained after carbonate dissolution for stable isotope gas preparation in addition 71 samples have been analysed by proton induced x ray emission spectroscopy micro pixe despite large differences in the analysed masses 3 6 mg for icp aes as low as 10 ng for pixe both methods yield comparable data for the sr and mn contents any discrepancies can be related to inhomogeneities of the shells e g punctae of these shells 73 have sr and mn contents of 600 ppm and the sr isotope record of the dinantian seawater is characterised by a decline in 87 sr 86 sr ratio from 0 7082 at the devonian carboniferous transition to 0 7076 in the mid visean superimposed on this trend are higher order fluctuations with a periodicity in the ma range the dinantian seawater curve may potentially serve as a geochronological and correlation tool particularly for the hastarian to lower chadian interval where the attainable resolution is 1 ma this is better than the resolution available by biostratigraphy the higher order wiggles for the most part can be generated by changing the riverine flux of sr or its isotopic ratio within reasonable geological scenarios some spikes however are too large and too short to be explained by the above phenomena these apparent spikes may reflect instead the fact that their duration has been underestimated due to unreliable geochronology and or the presence of unrecognised hiatuses

the significance of hiatus beds in shallow water mudstones an example from the middle jurassic of switzerland abstract limestone beds formed in nearly carbonate free shallow water mudstones by discontinuous sedimentation and erosion are called hiatus beds anaerobic oxidation of organic matter by microbes provided excess alkalinity inducing carbonate precipitation a multiphase history of such beds is documented from the swiss jurassic by several cementation and dissolution phases four cement types occur micrite as earliest cement 13c 10 to 20o oo stellate calcite between micrite cemented parts 13c 5 to 10o oo fibrous calcite cement in dissolution cavities 13c 30o oo and blocky calcite in remaining pores 13c 5o oo except for the late blocky cement all cements contain pyrite indicating carbonate precipitation within the sulfate reduction zone after early cementation by micrite the beds moved relatively upwards into a shallower geochemical zone and some even to the seafloor because of erosion cavities formed during reburial by dissolution in the upper part of the sulfate reduction zone and in the upper part of the methanogenic zone strongly reduced sedimentation rates provided the requisite stable geochemical conditions for at least several thousands of years which permitted precipitation and dissolution of carbonate by biochemical processes and diffusion this happened on short lived swells caused by differential subsidence and rotation along listric faults when basement structures became reactivated during the extensional stress regime from opening of the tethys during the jurassic and cretaceous breakup of pangea the shelf area increased and differential subsidence on these newly formed shelves was the main factor responsible for the observed post paleozoic maximum in hiatus beds and hiatus concretions

carbonate cementation in a sequence stratigraphic framework upper cretaceous sandstones book cliffs utah colorado abstract three macroscopic diagenetic features can be recognized in the sandstones of the upper cretaceous desert member of the blackhawk formation and castlegate sandstone of the mesaverde group exposed in the book cliffs utah each of which have distinctive form geometry and stratigraphic distribution diagenetic alterations are 1 leached zones whitecaps up to 10 m thick beneath coal beds 2 large up to 8 m concretionary carbonate cemented bodies in amalgamated shoreface and thin fluvial sandstones and 3 thin up to 2 m laterally extensive carbonate cemented horizons beneath major marine flooding surfaces each feature has distinct petrographic and geochemical signatures and formed through discrete diagenetic processes large isolated carbonate cemented bodies are composed of ferroan dolomite most of which precipitated during early diagenesis field and petrographic data coupled with stable isotope data early cements 13c 2 5 to 3 4o oo vpdb 18o 7 8 to 12 0o oo vpdb 87sr 86sr 0 7078 later cements 13c 3 1 to 5 7o oo vpdb 18o 12 0 to 15 1o oo vpdb 87sr 86sr 0 7093 suggest precipitation from meteoric fluids input into sediments during times of relative sea level fall the source of carbonate for the dolomite cement was dissolution of detrital dolomite from beneath coals by organic acids and subsequent mobilization by meteoric fluids carbonate precipitation in laterally extensive cement horizons appears to have started as a result of hiatus in sediment accumulation during marine flooding events relative sea level rise cement precipitation in these horizons continued through sediment burial as a result of organic matter oxidation reactions in overlying organic rich mudstones the results of this study show a link between sedimentation related to changes in relative sea level and diagenesis leading to the potential for the development of process based predictive models of early diagenesis in depositional successions

distribution of diagenetic alterations in fluvial deltaic and shallow marine sandstones within a sequence stratigraphic framework evidence from the mullaghmore formation carboniferous nw ireland abstract the distribution of diagenetic alterations in the fluvial deltaic and shallow marine arkosic to subarkosic sandstones average q72f26l02 of the mullaghmore formation carboniferous nw ireland can be predicted within a sequence stratigraphic framework eogenetic calcite 18opdb 13 3 to 6 5 13cpdb 3 0 to 3 4 and 87sr 86sr 0 706721 to 0 709227 and ferron dolomite feco3 8 12 mol 18opdb 14 2 to 7 8 13cpdb 1 4 to 1 0 and 87sr 86sr 0 709051 to 0 709167 occur in bioclast rich transgressive lag deposits at parasequence boundaries and transgressive surfaces and in wave influenced deltaic highstand systems tract hst deposits mesogenetic illite chlorite baroque dolomite feco3 16 mol 18opdb 14 2 to 12 7 13cpdb 3 8 to 1 0 quartz and calcite 18opdb 15 7 to 12 5 13cpdb 5 8 to 3 7 and 87sr 86sr 0 709016 to 0 709122 were formed mainly in the bioclast poor deposits which were not pervasively cemented by carbonates during near surface eodiagenesis these deposits include fluvial incised valley sandstones of lowstand systems tract lst and fluvial dominated deltaic sandstones of transgressive systems tract tst and hst illite is the dominant diagenetic clay mineral in the fluvial incised valley sandstones of lst possibly because of simultaneous albitization of k feldspars conversely chlorite dominates in the fluvial dominated deltaic sandstones of tst and hst because of the presence of suitable precursor clays the integration of diagenesis into sequence stratigraphic framework of clastic sequences should improve the ability to predict the spatial and temporal distribution of diagenetic alterations and related reservoir quality modifications of sandstone deposits

albitization of detrital plagioclase in triassic reservoir sandstones from the snorre field norwegian north sea abstract detrital plagioclase an12 an28 in triassic reservoir sandstones of the lunde formation in the snorre field offshore norway has been partially to completely albitized during burial diagenesis depths of about 2 500 3 000 m 75 100 c present temperature calcite caco3 99 mole and kaolinite were formed as by products of plagioclase albitization sodium rich detrital plagioclase an2 an11 are usually unalbitized detrital k feldspar is fresh or rarely partly albitized differences in albitization behavior of detrital plagioclase and k feldspar are shown to be related to the chemistry of formation waters reactivity of kaolinite and temperature the diagenetically formed albite pseudomorphs are very pure ab 9 well ordered low albite dark luminescent vacuolized mostly untwinned and comprised of numerous euhedral albite crystals some of the microporosity within the albite pseudomorphs is caused by volume reduction during albitization of the detrital plagioclase the amount of albite formed in the pseudomorphs is related to the molar proportion of albite component in the detrital unstable ca rich plagioclase the microporosity formed during albitization is proportional to the anorthite component which is believed to have dissolved and resulted primarily in the formation of calcite and kaolinite the albitization of detrital plagioclase in the studied sandstones can thus be explained without assuming the consumption of significant amounts of na from pore solutions

porosity permeability and textural heterogeneity of reservoir sandstones from the lower cretaceous putaohua member of yaojia formation weixing oilfield songliao basin northeast china abstract with the granulometric analyses mineral composition x ray diffraction xrd analyses porosity and permeability measurements scanning electron microscopy sem observations and mercury porosimetry measurements of the reservoir sandstones from four wells in the weixing oilfield songliao basin ne china the porosity permeability and pore structural heterogeneity features of the putaohua member sandstone of the lower cretaceous yaojia formation have been investigated in this paper the sandstones are mostly characterized by a similar deposition in continental detrital environments with quartz and feldspar taking the majority 97 and argillaceous minerals illite kaolinite and chlorite the minority 3 however their granule size sorting index porosity and permeability vary significantly in contrast to the generality that high porosity permeability often corresponds to large grain sizes and sorting indexes the porosity and permeability here show no good correlations to the grain size and sorting index suggesting the presence of a remarkable textural heterogeneity in terms of sequential change in permeability along the well depth cyclic sedimentation modes have been identified in some sub layers despite the porosity data showing little response to those scenarios this may be ascribed to the sandstone s peculiar intergranular pore structures in which the cementation occurred in pore lining and filling manners to different extents depending on their depositional environments the distributary channel facies tends to have lowest detrital clay content and rather open packed texture in pore lining cementation manner whereas sand sheet facies has pore filling manner the hg porosimetry analytical data are in fairly good agreement with the heterogeneous pore structures observed by sem large sized pores and channels connecting them though only accounting for a low percentage of the total porosity are believed to contribute mostly to the effective permeability in the sandstone reservoir such textural heterogeneities should be closely associated with the dynamic changing sedimentary facies of sandstones in the continental environment

diagenetic alterations related to marine transgression and regression in fluvial and shallow marine sandstones of the triassic buntsandstein and keuper sequence the paris basin france abstract the distribution of diagenetic alterations in triassic fluvio deltaic quartzarenitic to sublitharenitic lowstand systems tract lst sandstones of the gres a voltzia formation anastomosing fluvial quartzarenitic transgressive systems tract tst sandstones of the gres a roseaux formation and shallow marine quartzarenitic to sublitharenitic tst sandstones of the gres coquiller formation the paris basin france can be linked to transgression and regression events and thus to the sequence stratigraphic context near surface eogenetic alterations which display a fairly systematic link to the depositional facies and sequence stratigraphic framework include i cementation by meteoric water calcite 18 o 8 9 and 13 c 9 1 in the fluvio deltaic lst sandstones ii cementation by mixed marine meteoric calcite 18 o 5 3 to 2 6 and 13 c 3 9 to 1 3 and dolomite 18 o 4 6 to 2 6 and 13 c 2 9 to 2 3 in the foreshore tst sandstones and below parasequence boundaries pb and transgressive surface ts and in the shoreface tst sandstones below maximum flooding surfaces mfs being facilitated by the presence of carbonate bioclasts iii dissolution of detrital silicates and precipitation of k feldspar overgrowths and kaolinite particularly in the fluvio deltaic lst sandstones owing to effective meteoric water circulation and vi formation of autochthonous glauconite which is increases in abundance towards the top of the fluvio deltaic lst sandstones and along ts and in the shoreface tst sandstones by alteration of micas owing to the flux of seawaters into the sandstones during transgression whereas parautochthonous glauconite is restricted to the ts sandstones owing to marine reworking mesogenetic alterations which include cementation by quartz overgrowths and illite display fairly systematic link to fluvio deltaic lst sandstones this study has revealed that linking of diagenesis to transgression and regression events enables a better understanding of the parameters that control the spatial and temporal distribution of diagenetic alterations in sandstones and of their impact on reservoir quality evolution

spatial distribution of carbon and oxygen isotopes in laterally extensive carbonate cemented layers implications for mode of growth and subsurface identification abstract carbonate concretions and laterally extensive cemented layers are common features in the prairie canyon member of the mancos shale at its type locality in western colorado stable isotope and petrographic data were obtained from these cemented zones in an attempt to determine 1 how the cemented layers form 2 whether or not they can be chemically correlated and 3 if it is possible to differentiate them from concretions in the subsurface the cement in both the layers and concretions consists of ferroan dolomite and lesser amounts of stoichiometric dolomite and ankerite samples for stable isotope analysis were drilled from 10 grids with a typical grid spacing of 10 cm10 cm gridded samples were obtained from two continuous cemented layers at several locations along the layers 2 3 m to 1 5 km apart on the same layer and from isolated and stratabound concretions taken together all the cements regardless of spatial position show a remarkably linear correlation r2 0 98 between 13c and 18o systematic center to edge decreases in both 13c and 18o occur in each of the cemented zones and are persistent over lateral intervals of at least 1 5 km the cores of the cemented zones have positive 13c values up to 10o oo pdb which result from incorporation of 13c enriched carbon during methanogenesis the edges of the cemented zones have negative 13c values down to 9o oo pdb which suggests incorporation of 13c depleted carbon from thermocatalytic decarboxylation reactions oxygen isotope paleothermometry indicates that precipitation may have started at moderate temperatures 25 c and ended at temperatures as high as 92 c the spatial distribution of the isotopic data in the cemented layers can be explained as resulting from the coalescence of stratabound concretions followed by growth above and below the coalesced concretions such growth may have occurred in a complex manner in which early cements precipitated throughout the thickness of the layer but were more abundant in the core zone given the lateral persistence of the pattern of isotopic variation in the cemented layers geochemical correlation of the layers may be possible in the subsurface in addition because laterally persistent cemented layers in the prairie canyon member exhibit patterns of isotopic variation different from concretions layers have a more asymmetrical vertical distribution it should be possible to distinguish between the two styles of cementation from subsurface core data alone

verdine and glaucony facies from surficial sediments of the eastern continental margin of india the present paper reports the first mineralogical description of green grains verdine and glaucony from sediments of the eastern continental margin of india only 24 of the 82 sediment samples studied at depths between 18 and 247 m contain green grains and their percentages in the coarse fraction 125 250 m range from 6 to 40 of these 13 samples from the inner shelf 18 70 m contain irregular dark green grains 6 samples from the outer shelf 70 125 m contain dark green pellets and a few dark green internal moulds of skeletal materials and 5 samples from the continental slope 160 247 m contain dark green moulds and infillings of planktonic and benthic foraminifers the green grains found at depths between 18 and 125 m consist of poorly crystalline phyllite v with very little detrital clay at 170 m depth odinite young phyllite v occurs and further seaward between 200 and 247 m glauconitic smectite is present all verdine and glaucony grains studied show an early stage of evolution and the colour does not reflect their evolution the verdine and glaucony from the slope sediments may correspond to the last glacial maximum and the outer shelf verdine facies formed during the subsequent transgression the inner shelf green grains may represent the present day formation of verdine

intergranular pressure solution in four quartzose sandstones abstract cathodoluminescence petrography and scanning electron microscopy reveal that several geologic variables significantly influenced intergranular pressure solution and quartz cementation in the bromide ordovician oklahoma upper minnelusa permian wyoming nugget jurassic utah and tuscarora silurian pennsylvania sandstones a linear relationship exists between mean grain size and intergranular pressure solution with finer grained samples having experienced more intergranular pressure solution than coarser grained ones samples that display pervasive illite grain coatings have also experienced more intergranular pressure solution than those with no or poorly developed grain coatings sorting and limited framework grain compositional variability have exerted little influence o intergranular pressure solution silica budgets indicate that in the bromide upper minnelusa and nugget sandstones more quartz has been dissolved by intergranular pressure solution than is present as cement these sandstones have exported silica during burial diagenesis in the tuscarora the amount of quartz dissolved by intergranular pressure solution is about equal to the amount of quartz cement present implying approximate mass balance however petrographic evidence suggests that most of the quartz cement in at least three of these sandstones predates most intergranular pressure solution implying that they acted as silica importers during shallow diagenesis and silica exporters during deeper diagenesis intergranular pressure solution has played varying roles in determining the ultimate porosity of these four sandstones in the nugget intergranular pressure solution was the single most important diagenetic process that influenced porosity evolution in the bromide intergranular pressure solution was slightly more important than mechanical compaction and considerably more important than cementation in the tuscarora intergranular pressure solution was slightly more important than mechanical compaction and of about equal importance to quartz cementation in the upper minnelusa intergranular pressure solution was less important than both mechanical compaction and gypsum anhydrite cementation

systematic sequence scale controls on carbonate cementation in a siliciclastic sedimentary basin examples from upper cretaceous shallow marine deposits of utah and coloradao usa whilst the relationship between stratigraphic development and carbonate cementation within siliciclastic succession has been documented through a number case studies these studies have been generally restricted to observations upon individual sequences and or limited sub surface data in this paper long term 5 million years large scale 200 km stratigraphic controls on carbonate cementation patterns are documented from the upper cretaceous panther tongue member blackhawk formation and castlegate sandstone exposed in the book cliffs in utah and colorado usa together these comprise eight progradational wedges of sandstones which interfinger with the mancos shale deposited within the western interior seaway foreland basin petrographic analyses of ferroan dolomite cement bodies within these sandstone wedges show that the ferroan dolomite cements are all early relative to burial diagenesis within the host sandstones stable isotope analyses indicates that a significant meteoric component was present in precipitating fluids and this is consistent with the observation that cements are always present down dip of sequence boundaries and or leached whitecaps beneath coals in addition the lateral distribution of cement bodies increases consistently up succession from less than 5 km in extent in the older sequences to 30 km in extent in the youngest sequences these changes in distribution are in response to the increased progradation and increased and more aerially extensive sequence boundary development in younger sequences the implications of these data are that whilst localized spatial patterns of diagenesis and in particular carbonate cementation are predictable and controlled by the nature and presence of individual stratal surfaces systematic diagenetic alteration patterns are also present at the sedimentary basin scale and controlled by the nature of larger scale stratigraphic development and basin evolution this evolution may be driven by eustatic shifts or through tectonic or climatic driven base level shifts these observations allow an improved insight into the basin scale processes that control the macroscopic diagenetic properties of sedimentary successions and sub surface hydrocarbon reservoirs

dolomitization associated with transgressive surfaces a mid cretaceous example abstract increased seawater circulation within a carbonate platform generated through transgression and flooding of the platform is proposed here as a cause of the common selective dolomitization of calcitic silts in the upper part of limestone sequences of the barremian aptian urgonian platform of se france which underwent selective porosity creation as a result of karstification during an aptian relative sea level fall parts of this porosity are infilled by microcrystalline dolomite and some of the host limestone is partially to completely dolomitized the microcrystalline dolomite is considered to have formed by dolomitization of calcitic silt internal sediment within the karstic cavities as a result of the pumping of seawater through the upper layers of the platform during subsequent transgression the calcitic silt provided abundant nucleation sites for the dolomitization seawater dolomitization following subaerial exposure of limestone is likely to be a common phenomenon if there is sufficient meteoric diagenesis and porosity production and a prolonged period of submarine erosion winnowing and condensed sequence formation enhancing the amount of circulation of seawater as seafloor sediment is lithified

firmgrounds key surfaces in the recognition of parasequences in the aptian lower greensand group isle of wight southern england aptian lower greensand group exposures in the cliffs of the isle of wight southern england display a consistent coarsening up cyclicity on the scale of centimetres to tens of metres that reflects the bed bed set parasequence parasequence set and sequence hierarchy these coarsening up cycles are most commonly recognized at the scale of parasequences 20 cm to 10 m thick genetically related groups of which form parasequence sets both parasequences and parasequence sets contain the succession of biofacies that culminate in firmground development numerous episodes of erosion deposition and colonization are recorded reflecting multiple erosion bypass events the increase in mean grain size through each cycle is reflected by changes in physical sedimentary structures ichnofauna or bioturbational fabric fossil fauna and diagenesis interbedded mudstones siltstones and sandstones in the lower beds of each cycle display a variety of structures ranging from low angle hummocky or tabular cross strata sandstone filled erosional gutters and planar lamination the cleaner sandstones found in the upper parts to each cycle are often completely bioturbated with only rare stratification and pebble plant debris filled scours preserved bioturbational fabrics change upward through each cycle from small subhorizontal mud or sandstone filled burrows to large branching clay filled or cemented burrow systems the top surface of each cycle is marked by a fossil epifauna indicative of firm to hard substrate conditions concentrations of bivalves brachiopods bryozoa crinoids and corals are preferentially cemented by iron oxide carbonate or phosphate such cements were early and thus utilized by firm or hard substrate dwellers this fossiliferous cemented sandstone is overlain by a flooding surface marked by the mudstone and silt rich sandstones at the base of the next cycle together the fauna and ichnofauna in each cycle represent the gradual development of firm substrate conditions culminating in the diverse firmground fauna preserved at the top of each cycle the fauna and changing substrate conditions reflect the hiatuses developed during successive episodes of marine flooding high species diversity is matched by complex patterns of taphonomic feedback in the mature firmground faunas that mark major flooding surfaces increasing faunal maturity allows recognition of a hierarchy of hiatuses this hierarchy is analogous to the parasequence parasequence set division the stratigraphic condensation of firmgrounds can be used to empirically define the condensed section the thickness of sediment between firmgrounds being a function of sediment supply and water depth accommodation space

the sedimentary record of carboniferous rivers continuing influence of land plant evolution on alluvial processes and palaeozoic ecosystems abstract evidence from modern rivers and the deep time geological record attests to the fundamental importance of plant life for the construction of physical habitats within fluvial environments data from an extensive literature review and original fieldwork demonstrates that many landforms and geomorphic features of modern river systems appear in the palaeozoic stratigraphic record once terrestrial vegetation had adopted certain evolutionary advances for example stable point bars are associated with the onset of rooted plants in the siluro devonian and avulsive and anabranching fluvial systems become common at the same time as extensive arborescent vegetation in the carboniferous in this paper we demonstrate a correlation between the diversification of physical fluvial environments and the expansion of terrestrial fauna and flora with an emphasis on the culmination of these trends within carboniferous alluvial systems many extrinsic factors have been considered as possible controls on the evolutionary timelines of terrestrialization for organisms however a fundamental prerequisite for achieving terrestrial biodiversity was the variety of physical habitats especially riparian systems available for newly evolved organisms in association with abundant lowland meandering systems the widespread appearance across carboniferous alluvial plains of fixed channel and anabranching reaches created further physical landforms for colonization and would have promoted increasingly complex hyporheic flow regimes furthermore the associated increase in arborescent vegetation and supply of large woody debris to inland and coastal rivers would have created a wealth of microhabitats for continental organisms we argue that the expanding extent and diversity of physical alluvial niches during the palaeozoic is an underappreciated driver of the terrestrialization of early continental life the study of the deep time fossil and stratigraphic record also illustrates that vegetation is a fundamental prerequisite for the creation of biogeomorphic alluvial landforms and physical habitats and microhabitats

linking diagenesis to sequence stratigraphy in fluvial and shallow marine sandstones evidence from the cambrian ordovician lower sandstone unit in southwestern sinai egypt abstract by integrating diagenesis and sequence stratigraphy the distribution of diagenetic alterations and their impact on reservoir quality was investigated within a sequence stratigraphic framework using the fluvial and shallow marine sandstones in the cambrian ordovician succession of southwest sinai the perographic and geochemical analysis of the studied sandstone revealed that the eogenetic alterations display fairly systematic spatial and temporal distribution patterns within the lowstand system tract and transgressive system tract as well as along the sequence stratigraphic surfaces i e sequence boundaries transgressive surfaces and parasequence boundaries during relative sea level fall percolation of meteoric waters through sandstones of the lst and below sequence and parasequence boundaries resulted in extensive dissolution of detrital grains and formation of kaolinite authigenic k feldspar and feldspar overgrowths as well as formation of mechanical infiltrated clays around the detrital grains during relative sea level rise invasion of marine water into the sandstones as a consequence of landward migration of the shoreline as well as low sedimentation rates encountered in the tst resulted in the formation of glauconite apatite and pyrite development of pseudomatrix which was formed by mechanical compaction of mud intraclasts is mostly abundant along transgressive surfaces and parasequence boundaries of the tst and is related to the abundance of mud intraclasts in the transgressive lag deposits the types and extent of eogenetic alterations have an important impact on the distribution of the mesogenetic alterations including the formation of quartz overgrowths and dickite distribution of mesogenetic quartz overgrowths in the sandstones was controlled by the distribution of mechanically infiltrated clays and the presence of eogenetic cement sandstones that remained poorly cemented during eodiagenesis and that have thin or discontinuous infiltrated clay rims around the detrital grains were cemented during mesodiagenesis by quartz the absence of extensive eogenetic cements in the sandstones suggested that the partial deterioration of porosity was mainly due to mechanical compaction partial transformation of kaolinite to dickite which indicates neomorphic change to a better ordered and more stable crystal structure at the elevated temperatures during mesodiagenesis is partially a function of distribution of kaolinite during eodiagenesis the conceptual model developed in this study shows the diagenetic evolutionary pathways in the reservoir sandstones within a sequence stratigraphic context which in turn provides some insights into the controls on reservoir heterogeneity

holocene cemented beach deposits in belize abstract two types of cemented beach deposits occur on reef islands off the coast of belize these are 1 intertidal beachrock that is dominantly cemented by marine aragonite and high magnesium calcite cements and 2 supratidal cayrock that is cemented mainly by vadose low magnesium calcite cements besides differences in position relative to present sea level and resulting early diagenesic features beachrock and cayrock can be distinguished on the basis of differences in composition texture geographical position and age whereas the composition of beachrock is similar to that of the adjacent marginal reef sediments cayrock is enriched in benthic foraminifera intertidal beachrock is moderately to well sorted and well cemented while supratidal cayrock is very well sorted poorly cemented and friable beachrock occurs preferentially on windward beaches of sand shingle gays on the middle and southern barrier reefs and on the isolated platforms glovers and lighthouse reefs cayrock only occurs on larger mangrove sand gays of the isolated platforms turneffe islands lighthouse reef and the northern barrier reef 14c dating of ten whole rock and mollusk shell samples produced calibrated dates between ad 345 and ad 1435 for beachrock and between bc 1085 and ad 1190 for cayrock the large scale distribution of beachrock in belize supports the contention that physical processes such as water agitation rather than biological processes control beachrock formation and distribution only on windward sides of cays that are close to the reef crest where large amounts of seawater flush the beaches considerable amounts of cements can be precipitated to produce beachrock cayrock forms due to cementation in the vadose zone and is only preserved on larger stable mangrove sand cays

significance of hypoburrow nodule formation associated with large biogenic sedimentary structures in open marine bay siliciclastics of the upper eocene birket qarun formation wadi el hitan fayum egypt abstract unusually large biogenic sedimentary structures from the shallow quiescent marine siliciclastics of the upper eocene birket qarun formation in the fayum area of egypt display pronounced concretion formation around the trace fossils the structures are massive and vary morphologically forming branched pillars up to dm scale vertical up to 180 cm height amphora like masses and 3 d box work maze bioturbation mainly thalassinoides attributable to the glossifungites ichnofacies mediated and modified the physical and chemical microenvironments influencing early diagenesis i e burrows promote the precipitation of pervasive calcite dominated cement the inferred paragenesis combined with the negative light carbon and oxygen stable isotopic values of the bulk calcite 13cpdb from 0 94 to 4 98 and 18opdb from 4 63 to 7 22 and bulk dolomite 13cpdb from 2 05 to 8 23 and 18opdb from 1 41 to 11 20 imply that the pore water carbon was derived directly from seawater and dissolution of metastable carbonate which was mediated by bacterial decomposition of organic matter and mixing of meteoric ground water thereby the carbonate cement precipitated mostly under eodiagenetic conditions near the sediment water interface

albitized microcline grains of post depositional and probable detrital origins in brottum formation sandstones upper proterozoic sparagmite region of southern norway untwinned albite ab 99 4 an 0 4 or 0 2 grains forming during diagenesis and very low grade metamorphism are common in sandstones of the brottum formation upper proterozoic in the sparagmite region of southern norway these untwinned albite grains are usually clouded by vacuoles and tiny inclusions which are mostly made of carbonaceous material although some albite grains also contain some combination of carbonates quartz phengite chlorite pyrite halite and anatase the presence of microcline grains replaced partly by albite indicates that the albite grains are pseudomorphs after microcline chessboard albite which occurs in the brottum formation is suggested to be detrital in origin

granulitic xenoliths from the french massif central petrology sr and nd isotope systematics and model age estimates the protoliths of granulitic xenoliths found in tertiary quaternary alkali volcanics from the french massif central are shown to have been principally immature sediments and calc alkaline intrusives the association may indicate formation of the crust in an island arc or continental margin environment pressure and temperature estimates indicate that they have undergone intermediate to high pressure granulite facies metamorphism 9 15 kbar followed by uplift to lower pressures 4 9 kbar rb sr and sm nd isotopic data and ree abundances are presented for both meta igneous and meta sedimentary suites esr present day values for the granulitic xenoliths range from 2 7 to 325 with end values between 0 2 and 12 0 there are no isochron relationships within the sr and nd data but possible mixing lines in the rb sr system may indicate sedimentation at 700 1000 myr t nd dm model ages indicate a proterozoic 800 2100 myr origin for the protoliths of the meta igneous samples and a similar provenance age for the meta sediments 1986 the geological society

stratigraphic control on laterally persistent cementation book cliffs utah field data from the upper cretaceous blackhawk formation in the book clitfs utah colorado demonstrate the presence of laterally extensive carbonate cements beneath major marine flooding surfaces the occurrence of firmgrounds compaction around concretions and high minus cement porosities indicate that cementation occurred during early diagenesis it is proposed that the early cementation associated with these marine flooding surfaces occurred as a result of reduced sediment accumulation rates or non depositional hiatuses linked to abrupt deepening during marine flooding these reduced sediment accumulation rates extended sediment residence time within early diagenetic zones leading to enhanced early cementation in shallow marine sandstones and mudstones