are there large bathymetric highs near 73 n 139 w in the arctic basin comment on acoustic backscattering from the basin and margins of the arctic ocean by i dyer et al

the geological history of the ballydeenlea chalk breccia county kerry ireland the ballydeenlea chalk breccia co kerry occurs as an outlier surrounded by namurian rocks and is composed dominantly of namurian mudstone clasts with cretaceous campanian chalk acting as a matrix a new model is proposed for the formation of the deposit in which it is considered to be the remnant of a suite of sediments deposited at the foot of a submarine fault scarp active during the campanian in this model namurian mudstones comprised the active scarp and were redeposited by rockfall events these were mixed with unlithified chalk ooze derived from slumping from the upthrown side of the fault an organic maturation study of the ballydeenlea chalk breccia indicates a maximum burial depth of between 1 and 1 5 km suggesting uplift and erosion during the early tertiary

reconstructing fluid expulsion and migration north of the variscan orogen northern england abstract lower carboniferous platform carbonates in northern england host lead zinc fluorite barite deposits that have previously been classified as mississippi valley type mvt the controls on the distribution of low temperature particularly mvt minerals have long been debated but the interplay of basin evolution and fluid movement is often neglected early carboniferous sedimentation on the derbyshire platform took place within a back arc extensional regime north of the northward migrating variscan orogenic front which increasingly influenced fluid movement on the platform throughout the carboniferous it is the interplay of fluid movement cementation and variscan tectonism that is the focus of this paper which aims to reconstruct fluid movement within a basin moving from an extensional to a compressional regime in particular it demonstrates the variability of fluid flux during post rift subsidence and basin inversion and the corresponding influences of this upon mineral distribution during early burial of the derbyshire platform pore filling cements were precipitated from meteoric porewaters driven downdip from the east under topographic drive these cements occluded most interparticle porosity and therefore permeability became fracture controlled the waning effects of extensional tectonism during post rift subsidence permitted intermittent expulsion of small volumes of trace metal charged carbonate saturated fluids from overpressured contemporaneous clastic basins adjacent to the derbyshire platform overall however waning tectonism meant that the volume of fluids released during this period remained minor with the onset of the variscan orogeny and resultant compressional tectonism and basin inversion caledonian basement fault systems were reactivated channeling large volumes of trace metal charged fluids from the basins onto the derbyshire platform establishing an east west component to fluid flow the reconstruction of fluid expulsion and migration within this tectonically active regime demonstrates variations in fluid source and migration pathways during basin evolution consequently fluid movement is complex with the platform receiving fluids from a variety of sources this highlights the importance of understanding both the temporal and the spatial variations in cement chemistry when reconstructing flow the results of this study have relevance to other mineralized carbonate platforms in extensional and compressional settings worldwide and especially to sedimentary basins that host mvt deposits and hydrocarbons where it is crucial to understand the timing and mechanisms of diagenetic cementation and hydrocarbon emplacement

acoustic backscattering from the basin and margins of the arctic ocean sound waves created by high energy explosives were used to measure reverberation and backscattering in the arctic ocean from an ice camp in the beaufort sea signals were backscattered from the continental margin and other major features of the basin an acoustic array was used to analyze the signals in azimuth based on these data we constructed charts of normalized backscatter level which can be compared with known topography of the arctic ocean resolution of this remote sensing technique for our experimental conditions analysis frequency 9 hz averaging time 20 s array size 600 600 m is about 8 in azimuth 15 km in radial extent and 1 km in depth we obtained interpretable signals out to 2700 km for 1000 km or less the backscatter charts provide continuous coverage of the margins and major submerged features strong returns from about 73 2 n 139 0 w indicate one or more prominent features that many contemporary topographic charts do not show but which we believe to be real other strong returns are evident from the northwind escarpment and from the continental slopes of alaska and the canadian archipelago via a backscatter model we estimate the roughness product rms height times correlation radius of these features to be about 4500 m2 on average

global vegetation dynamics and latitudinal temperature gradients during the mid to late miocene 15 97 5 33 ma a 617 site palaeobotanical dataset for the mid to late miocene is presented this dataset is internally consistent and provides a comprehensive overview of vegetational change from 15 97 to 5 33 ma the palaeobotanical dataset has been translated into the biome4 classification scheme to enable direct comparison with climate model outputs the vegetation change throughout the langhian seravallian tortonian and messinian is discussed the data shows that the langhian which includes the end of the mid miocene climatic optimum represents a world significantly warmer than today the high northern latitudes were characterised by cool temperate forests the mid latitudes by warm temperate mixed forests the tropics by tropical evergreen broadleaf forests and antarctica by tundra shrub vegetation cool temperate mixed forest existed during the seravallian in the high northern latitudes a reduction in warm temperate mixed forests in the mid latitudes and a loss of tundra on antarctica was noted tortonian vegetation distribution indicates that further cooling had occurred since the seravallian the major changes in vegetation include the first evidence for cold taiga forest in the high northern latitudes and a further reduction of warm temperate mixed forests by the messinian this cooling trend had eliminated warm temperate mixed forests from the western usa and australia and had formed mid latitude deserts despite the cooling trend throughout the mid to late miocene the vegetation distribution of all four reconstructed stages reflect the vegetation of a world warmer than the pre industrial conditions the latitudinal distribution of bioclimatic zones suggests that the latitudinal temperature gradient for the langhian would have been significantly shallower than at present and has gradually but asymmetrically become more modern towards the end of the miocene first the southern hemisphere distribution of bioclimatic zones became more modern probably due to the climatic effects of a fully glaciated antarctica the northern hemisphere bioclimatic zone gradient continued to be shallower than modern throughout the miocene and slowly became more modern by the messinian

palaeoenvironment palaeoclimate and age of the brassington formation miocene of derbyshire uk the brassington formation is the most extensive miocene sedimentary succession onshore in the uk because of its unique position at the margin of nw europe the pollen from this lithostratigraphical unit provides evidence on the development of vegetation affected by north atlantic currents and hypothesized atmospheric circulation changes during the middle to late miocene climate cooling palynostratigraphy suggests that the uppermost kenslow member of the brassington formation is not coeval previously all occurrences of the kenslow member were assumed to be contemporary the oldest pollen assemblage is from the more southern bees nest pit which represents a subtropical conifer dominated forest of late serravallian age c 12 ma a younger assemblage was observed from the more northern kenslow top pit this indicates that a subtropical mixed forest was present during the early tortonian 11 6 9 ma the shift from a conifer dominated to a mixed forest was related to precipitation although the total precipitation did not change between the two assemblages the younger assemblage had more uniform rainfall throughout the year the diachronous nature of the kenslow member means that the depositional model of the brassington formation needs revising and this will have implications for neogene to recent uplift of the southern pennines supplementary material all valid formally defined palynomorph taxa below the generic level that are mentioned in this paper with full author citations are available at http www geolsoc org uk sup18889

burial dolomitization of the woo dale limestones formation lower carboniferous derbyshire england dolomite occurs in open shelf packstones and grainstones of the vincent house member of the woo dale limestones formation lower carboniferous in the wye valley area east of buxton derbyshire central england two stages of dolomitization can be recognized stage 1 consists of limpid non luminescent dolomite lacking detectable iron and manganese stage 2 comprises cloudy inclusion rich orange red luminescing dolomite with significant iron and manganese the contact between the two stages is irregular and suggests a hiatus between dolomite growth stages dolostones lacking any unreplaced limestone may contain up to 35 calcite cement rilling intercrystal porosity this cement shows a characteristic zonal sequence and is normally in optical continuity with the surrounding dolomite crystals in some cases precementation compaction of the dolomite mosaic caused fracturing of the crystals sherds of dolomite on the floors of pore spaces then provided the nucleus for growth of calcite cement the petrographic character and stratigraphic distribution of the dolomite suggest that dolomitization occurred at depth during burial of the woo dale limestones in the late carboniferous dolomitizing solutions expelled from basinal shales moved up dip along permeable limestones or along the sub carboniferous unconformity fluids responsible for the generation of stage 2 dolomite may have been contaminated by ions such as iron and manganese released from the alteration of volcanic rocks which occur beneath the carboniferous limestone in the wye valley area

selective and reversible carbonate silica replacements in lower cretaceous carbonate bearing turbidites of the eastern alps in the diagenetic history of calcareous sandstones silicacementation and silicification may be followed by carbonatecementation and replacement and vice versa and the change over from one to the other may occur more than once this is well illustrated by calcareous and siliciclastic turbidites of the gault formation aptian to albian of the eastern alps which have been interpreted as deep sea trench plain and deep sea fan deposits in these turbidites silicification selectively affects ooids and a few other biogenic carbonate fragments rich in organic matter algae and bryozoans which form a small fraction of the bulk sediment the type and sequence of diagenetic changes are largely controlled by host rock composition and may vary vertically within individual beds as a result of compositional grading in the carbonate turbidites silicification follows widespread calcite cementation the process is slow resulting in relatively coarsely crystalline replacement quartz in ooids with quartz nuclei rim quartz forms mostly monocrystalline overgrowths by outward replacement of the concentrically laminated carbonate cortex this type of silicification is often incomplete leaving parts of the ooid cortices unaffected in quartz arenites and sublitharenites silicification precedes calcite cementation there the process is rapid forming microcrystalline quartz even if the ooid nucleus consists of quartz a syntaxial overgrowth does not normally form the replacement quartz is almost always polycrystalline late stage diagenetic calcite and dolomite which develop euhedral crystal shapes and cut across grain boundaries may replace the earlier secondary rim quartz of the ooids as well as other minerals possible sources of the silica are pressure solution of quartz dissolution of opaline silica of radiolarian tests and of sponge spicules and feldspar in the host bed in a number of examples an increase in the degree of silicification can be observed towards the lower bedding planes of individual turbidites requiring an additional external source of silica which seems to necessitate cross formational flow of pore solutions silicification in both the carbonate and the siliciclastic turbidites probably took place at about the same time in the carbonate turbidites it was preceded however by calcite cementation which significantly reduced porosity and permeability before silicification took place the greater degree of alteration experienced by the gault turbidites of the falknis and tasna nappes which are more internal structural units of the alps compared to the flysch zone of the eastern alps is reflected by the growth of quartz beards and spikes from the ooids in the direction perpendicular to maximum stress this is the only case observed where the rim quartz of the ooids grows beyond the original grain boundaries

meteoric water participation in the genesis of jurassic cherts in the subbetic of southern spain a significant indicator of penecontemporaneous emergence abstract in the eastern sector of the betic cordillera outcrops of the internal subbetic display chert lenses in the upper parts of oolitic limestones camarena formation mid jurassic the stratigraphy indicates a sedimentary record related to synsedimentary tectonics tilting the sediments were deposited in different tectonic blocks at varying depths the camarena formation is mainly made up of shallow platform oolitic limestones and was deposited on a pelagic swell a shallow isolated platform that developed far from continental areas the chert lenses appear in those stratigraphic units deposited in the lowermost sunken fault blocks which display biosiliceous facies overlying the camarena formation the chert mainly consists of megaquartz formed by replacement of the oolitic host rocks without an opaline precursor the part of the host that has been most affected by the silicification is the carbonate cement the petrological features indicate a slow replacement from solutions which were poor in silica and cations comparing the textures and fabric of the cherts and their host rocks it is deduced that the silicification took place after compaction cementation fracturing and local dissolution of the oolitic limestones the stable isotopic data 18o 13c obtained from the chert oolitic host rock and overlain biosiliceous rocks display significant differences the host rocks camarena formation and overlying biosiliceous rocks have isotopic signatures that are typical of marine environments or shallow marine diagenesis however the chert isotopic values are interpreted to suggest meteoric water participation in the silicification process the timing of the chertification is suggested to have been between the bathonian beginning of the deposition of the biosiliceous source facies and the aptian first appearance of this chert as clasts in breccia beds in palaeogeographic terms the optimum timing would have been during emergence i e between the bathonian and tithonian the exposed areas could have acted as recharge areas of meteoric water into the regional aquifer system the involvement of meteoric water in the silicification processes gives new support to the hypothesis that pelagic swells were subjected to episodic exposure

stable isotopes of nonsulphide zn pb ores in britain and ireland fluid characteristics and palaeoclimatic variability stable isotope c o analysis was applied to smithsonites and cerussites from zn pb nonsulphide ores from britain and ireland to determine the nature of the fluids responsible for the precipitation of secondary carbonates and any relationship with palaeoclimatic conditions the carbon isotope compositions of the irish smithsonites show a major contribution of organic carbon whereas those of both zn and pb carbonates in the uk deposits indicate contribution of organic carbon and host rock carbonates 18ovsmow values for cerussite from both areas are about 13 lower than those of smithsonites uk smithsonite 18ovsmow values are more variable than those for cerussites and the irish smithsonites have unusually high values isoscape maps indicate that 18o values for both carbonates are higher in the west and decrease toward the east this trend correlates with a general decrease in the oxygen isotope compositions of rainwater moving eastward and suggests that oxidation of sulphides occurred at temperatures of about 15 20 c under climatic conditions warmer than today supplementary material a comprehensive overview of the investigated sites detailed data for the samples and sem images are available at https doi org 10 6084 m9 figshare c 4579535

isostatic compensation of quaternary vertical crustal motions coupling between uplift of britain and subsidence beneath the north sea new data from offshore and onshore regions confirm the view that the quaternary uplift of britain and subsidence beneath the north sea are coupled mediated by lower crustal flow induced by the lateral pressure gradient caused by climate driven surface processes most measured onshore uplift rates in both upland and lowland localities are only applicable since the mid pleistocene revolution beforehand generally lower rates prevailed earlier oligocene pliocene phases of uplift and subsidence had typically even lower rates furthermore the western margin of the north sea depocentre was 200 km farther from the modern coastline of north east england in the earliest cenozoic complex isostatic adjustments occurred in response to the magmatism associated with the iceland mantle plume the contemporaneous land surface relative to the present day rock column in northern england was typically 1500 m od however this activity died out during the palaeogene the main effect of these events on the modern isostatic configuration was via the emplacement of thick mafic underplating which now constricts the mobile lower crustal layer explaining the late cenozoic ultra stability of ireland the evidence enables interpretations of the present day topography of britain as static caused by the iceland mantle plume or actively developing in response to plate motions or plate boundary forces to be excluded

sedimentological evaluation of general circulation model simulations for the greenhouse earth cretaceous and jurassic case studies abstract conceptual climate models based on the workings of the present day climate system provided a first order approach to ancient climate systems they are potentially very subjective in character their main drawback was that they involved the relocation of continents beneath a stable atmospheric circulation modelled upon that of the present general circulation models gcms use the laws of physics and an understanding of past geography to simulate climatic responses they are objective in character however they require super computers to handle vast numbers of calculations nonetheless it is now possible to compare results from different gcms for a range of times and over a wide range of parameterisations gcms are currently producing simulated climate predictions which compare favourably with the distributions of climatically sensitive facies e g coals evaporites and palaeosols they have been used effectively in the prediction of oceanic upwelling sites and the distribution of petroleum source rocks and phosphorites parameterisation is the main weakness in gcms e g sea surface temperature orography cloud behaviour sensitivity experiments can be run on gcms which simulate the effects of milankovitch forcing and thus provide insights into possible patterns of climate change both globally and locally i e provide predictions that can be evaluated against the rock record future use of gcms could be in the forward modelling of sequence stratigraphic evolution and in the prediction of the diagenetic characteristics of reservoir units in frontier exploration areas the sedimentary record provides the only way that gcms may themselves be evaluated and this is important because these same gcms are being used currently to predict possible changes in future climate

calcretes from a palaeosinkhole in jurassic palaeokarst subbetic southern spain abstract calcretes have been recognized in the bottom of a palaeokarstic sinkhole etched into early jurassic carbonates from the subbetic southern spain these are 3 to 10 cm thick and they show an internal structure in which three units can be distinguished the lower unit is composed of alternate irregular bands of unfossiliferous micrite and sparry calcite in the micrite several features point out the activity of endolithic microorganisms as fungi and bacteria the middle unit which has a laminated structure with palisade calcite spar and some vadose silt is interpreted as a speleothem and finally the upper unit shows calcrete pisoids and rhizocretions the isotope ratio analysis gives results in which 13 c ranges from 10 1 to 10 9 and 18 o ranges from 5 7 to 7 0 these values are very similar to those from palaeosols of other ages and localities which is an important argument favouring emersion stages with continental deposition in an island with an arid or semi arid climate nevertheless these calcretes outcrop in association with a palaeokarstic relief reflecting wet climate before the calcrete formation

tectonic control in the boundaries of the genetic units an example in the dogger of the external zones of the betic cordillera province of murcia and almeria spain abstract i have studied the evolution of a genetic unit of the dogger to the north of velez rubio external zones of the betic cordilleras southeast spain this unit at its lower and upper boundaries is limited by sedimentary breaks of the uppermost toarcian and intra upper bathonian respectively starting from the analysis and distribution of facies rich in ostracods oncolites oolites crinoids peloids pelagic pelecypods or deposited particles on diverse drawn columns the lateral and vertical evolution in this section of the basin for this period of time can be deduced this permits the differentiation between systems tracts lowstand systems tract lst transgressive systems tract tst and highstand systems tract hst with a slightly homogeneous distribution which can be related to an uneven morphology of the substratum different factors that controlled the evolution of the basin during this period of time have been analysed from this we can deduce that the role of tectonics could be dominant as contrasted with that of eustasy an anomalous evolution toward the top of the highstand systems tract is indicated in this unusually thick succession some evidence of retrogradation can be deduced this could not be explained eustatically but it could certainly be explained tectonically

reply to are there large bathymetric highs near 73 n 139 w in the arctic basin comment on acoustic backscattering from the basin and margins of the arctic ocean by i dyer et al

a tortonian late miocene 11 61 7 25 ma global vegetation reconstruction for the tortonian age of the miocene epoch 11 6 7 25 ma we present a global palaeobotanical and palaeoecologically based vegetation dataset combined with a best fit late miocene climate vegetation model experiment to create an advanced global data model hybrid biome reconstruction this new palaeoecological database and global vegetation reconstruction can be used both for the purposes of validating future palaeoclimate model simulations as well as a land cover dataset to initialise palaeoclimate modelling experiments our tortonian reconstruction shows significant changes in the distribution of vegetation compared to modern natural vegetation for example in contrast to the modern scenario in the northern hemisphere boreal forests reached 80 n and temperate forests were present above 60 n warm temperate forests covered much of europe coastal north america and south east asia our reconstruction shows a spread of temperate savanna in central usa the middle east and on the tibetan plateau evidence for arid deserts is sparse with the exception of the atacama region south america areas that exhibit arid desert today in the tortonian were instead covered by shrublands grasslands savannas and woodlands the extent of tropical forests in south america was likely reduced but expanded in the indian sub continent and east africa this pattern of global vegetation in the late miocene suggests a warmer and wetter world which is supported by the pattern of climate anomalies predicted by our best fit palaeoclimate vegetation model experiment global mean annual temperature may have been as much as 4 5 c higher than present day with many regions experiencing higher than modern amounts of precipitation over the annual cycle the pattern of temperature and precipitation change reconstructed palaeobotanically and predicted within our climate model experiment infers a global forcing agent on tortonian climate e g such as elevated concentrations of greenhouse gases to explain the observed and modelled climate anomalies this is in contrast to current proxy records of tortonian atmospheric co2 which range from last glacial maximum to mid 20th century levels

dolomite occurrence evolution and economically important associations abstract dolomite is not a simple mineral it can form as a primary precipitate a diagenetic replacement or as a hydrothermal metamorphic phase all that it requires is permeability a mechanism that facilitates fluid flow and a sufficient supply of magnesium dolomite can form in lakes on or beneath the shallow seafloor in zones of brine reflux and in early to late burial settings it may form from seawater from continental waters from the mixing of basinal brines the mixing of hypersaline brine with seawater or the mixing of seawater with meteoric water or via the cooling of basinal brines bacterial metabolism may aid the process of precipitation in settings where sulfate reducing species flourish and microbial action may control primary precipitation in some hypersaline anoxic lake settings dolomite is a metastable mineral early formed crystals can be replaced by later more stable phases with such replacements repeated a number of times during burial and metamorphism each new phase is formed by the partial or complete dissolution of an earlier dolomite this continual re equilibration during burial detracts from the ability of trace elements to indicate depositional conditions and resets the oxygen isotope signature of the dolomite at progressively higher temperatures because subsurface dolomite evolves via dissolution and reprecipitation a bed of dolomite can retain or create porosity and permeability to much greater burial depths and into higher temperature realms than a limestone counterpart dolomitization also creates new crystals with new rhomb growth following the dissolution of less stable precursors repetition of this process without complete pore cementation can generate intercrystalline porosity a number of times in the rock s burial history intercrystalline porosity is a highly interconnected style of porosity that gives dolomite reservoirs their good fluid storage capacity and efficient drainage the fact that many dolomite reservoirs formed via brine reflux means that they sit beneath an evaporite seal in both platform and basinwide evaporite settings the same association of evaporites sulfate source and entrained hydrocarbons means that burial conditions are also suitable for thermochemical sulfate reduction and the precipitation of base metals this tends to occur at higher temperatures 60 c 80 c and so the resulting dolomites tend to be ferroan and consist of saddle shaped crystals

late cenozoic uplift history of the peak district central england inferred from dated cave deposits and integrated with regional drainage development a review and synthesis abstract the white peak upland in the peak district region of central england extends over an area of 100 km2 locally exceeding 400 m above sea level formed in extensively karstified limestone of early carboniferous age historically regarded as palaeo karst following the availability of u series dates since the 1980s it has become apparent that the superimposed cave levels in this region formed during the succession of pleistocene interglacials their differences in height reflecting downcutting by the river derwent and its tributaries in response to regional uplift however quantitative interpretation is complicated because of effects of glaciation most importantly glacial diversion of rivers in adjoining lowland regions making the determination of uplift from fluvial incision particularly difficult the derwent terrace dataset is also complicated by lithologically controlled knickpoints which can remain localized or propagate upstream depending on individual details cave fluvial and glacial datasets for this region are integrated to quantify the uplift history establishing the quaternary uplift history of a wide region of britain uplift of 100 m since the mid pleistocene revolution and by 300 400 m since the miocene is thus inferred these amounts exceed the uplift deduced over corresponding timescales in surrounding regions indicating that the quaternary uplift has slightly accentuated the overall domal form of this carboniferous limestone inlier the analysis supports the previous assignment of the wragby glaciation in the surrounding lowland regions to mis 8 and constrains the most recent glaciation within the peak district represented by the bakewell till also to mis 8 overall this work provides an important case study demonstrating integrated use of multi disciplinary datasets to quantify late cenozoic landscape development

diagenetic trends of the pleistocene calcareous ridges mersa matruh area egypt abstract ooids are a major constituent of the pleistocene carbonates along the northwestern coastal zone of egypt the mersa matruh area was selected for a detailed diagenetic study of these rocks 18 o 13 c and sr values were determined on whole rock samples on manually separated ooids and calcite cements plerographic examination shows that these rocks are partly consolidated ooid grainstones with low mg calcite cement in granular voids oxygen isotopic and sr values for the whole rock 0 5 vs pdb and 9300 ppm respectively are consistent with data on limestones of the same age in contrast oxygen isotopic values for ooids ave 0 5 vs pdb are heavier relative to values for low mg calcite cements ave 1 4 vs pdb and sr contents are higher for ooids 10 000 ppm in addition 13 c values are identical for the whole rock and ooids ave 4 5 vs pdb and heavier relative to 13 c values of the calcite cements ranging from 1 4 to 3 8 vs pdb these data may suggest that calcite cements are formed in non diluted pore water under relatively high rock water ratios

diagenetic trends in the siliceous facies of the monterey shale in the santa maria region california x ray diffraction and oxygen isotopic analyses of outcrop and subsurface samples of siliceous rocks were used to reconstruct thermal and diagenetic histories of the miocene monterey shale near santa maria california within many stratigraphic sections soft porous diatomaceous rocks change gradationally to underlying hard and brittle chert porcellanite and siliceous shale the accompanying silica mineral zones are in descending stratigraphic order 1 biogenic silica opal a 2 cristobalitic silica opal ct and 3 microcrystalline quartz boundaries between silica mineral zones and stratigraphic horizons are often discordant within the opal ct zone the d 101 spacing of opal ct decreases in a smooth non linear fashion from about 4 10 a to 4 04 a in the santa maria valley and bradley oil field areas the thicknesses of the opal ct zones are greater and the present thermal gradients less than in the adjacent orcutt oil field thin opal ct zones at shallow maximum burial depths apparently correlate with higher thermal gradients using present thermal gradients and reconstructed maximum burial depths from well data in the santa maria region the ranges in temperatures for the top and base of the opal ct zone are 38 54 c and 55 110 c respectively the temperature difference between these two boundaries ranges from 17 to 60 c in comparison temperature ranges for these two boundaries computed from oxygen isotopic compositions of opal ct and quartz extrapolated experimental quartz water fractionations and assuming o18 0 o for the isotopic composition of the equilibrating fluid are 18 56 c and 31 80 c for the top and base of the opal ct zone respectively the temperature difference between these boundaries is 11 36 c using this method thermal gradients and sedimentation rates strongly influence rates of silica transformations reconstructed thermal and diagenetic histories of siliceous rocks of the monterey shale at four well sites in the santa maria region demonstrate that most silica conversions probably occurred during the last 3 4 myr in response to accelerated rates of sedimentation and therefore burial heating during the pliocene

fluid expulsion from overpressured basins implications for pb zn mineralisation and dolomitisation of the east midlands platform northern england abstract our understanding of burial diagenesis within carbonates is often limited by poor constraints on available fluid volumes and geochemistry however regional stratigraphic and burial history data are often readily available using these data to develop numerical models which couple sedimentological and hydrological basin evolution we estimate the volumes and geochemistry of fluids that were available to drive dolomitisation and pb ore genesis within the carboniferous derbyshire platform of northern england current conceptual models of these processes invoke tectonic release of burial induced overpressure developed within adjacent dinantian basins as a drive for reactive fluid flow to the platform our simulations show that compaction driven flow may lead to a supply of fluids that is more complex in its temporal evolution than may be expected spatial variations in the rate of fluid expulsion from different sediments lead to a staggered delivery of fluids from different sources rapid fluid expulsion within deeper sediments leads to a downwards decreasing pressure gradient that subsequently draws down fluid from within overlying sediments thus early fluid supply to the platform is sourced from the deep basins while later fluid supply descends from above the platform as well as from the sides we suggest that such a flow development may have important implications for the relative timing and distribution of a sequence of diagenetic products within the platform this hypothesis is tested using volume estimates from our simulations we conclude that although this staggered fluid supply model may be applicable generically it appears that it is only valid for explaining mineralisation in the derbyshire platform fluid volumes supplied to the platform are insufficient to explain dolomitisation our simulations are supported by a sensitivity analysis that identifies that compaction driven flow in this system is strongly controlled by the rate of burial and sediment permeabilities within the compacting basins

a reassessment of the brassington formation miocene of derbyshire uk and a review of related hypogene karst suffosion processes the brassington formation of the pennines is the most extensive onshore miocene succession in the uk it is preserved as outliers in lower carboniferous limestone during the cenozoic central england underwent uplift with erosion of post mississippian strata from the pennine axis in the peak district the brassington formation is hence significant in reconstructing cenozoic geological history it is non marine derived from triassic sandstone and of mid late miocene age the c 60 outliers occur in three clusters over c 220 km2 they are remnants of a sedimentary prism at least 75 m thick and with a volume of c 10 km3 suffosion of bedrock conditioned by aggressive precursor fluids of hypogenic origin was the major control on subsidence the calculated volume of the fills in the bees nest and green clay outliers is 3 7 5 0 106 m3 referenced to a sub miocene surface at c 450 m od in pre subsidence times the original volume of this subsidence complex was c 21 8 106 m3 and the aggregated volume of the late neogene white peak palaeokarst was at least 0 66 km3 subsidence was concomitant with pliocene uplift of the pennine axis suggesting suffosion accommodation of 3 106 m3 per km2 or 244 m3 per annum

dolomitization and chertification of the early eocene rus formation in abu dhabi united arab emirates abstract the early eocene carbonates of the rus formation in the jabal hafit area near al ain united arab emirates were sampled to study their diagenetic history these carbonates were cyclically deposited in a shallow water environment alternating with marls or marly limestones and are characterized by extensive dolomitization and chertification with associated glauconitization the cyclic nature of sedimentation suggests that deposition was eustatically driven the diagenetic history of the limestones proceeded through the following path 1 marine cementation of allochems concomitant with formation of micrite envelopes 2 micritization of peloids ooids and skeletal grains 3 neomorphism of formerly high mg calcite and aragonite grains to low mg calcite 4 partial to complete dolomitization 5 partial chertification of both calcite and dolomite 6 glauconitization of chert 7 fracturing during shallow burial and 8 precipitation of late coarse calcite spar or mosaic and saddle baroque dolomite spar filling voids and fractures porosity is virtually nil in these rocks except for infrequent unfilled fractures vugs and oomolds due primarily to cementation and chertification biogenic grains are abundant in the limestones with nummulites sp alveolinid and miliolid foraminifera and crinoid ossicles and other echinoderm fragments being most common the tests of nummulites sp are commonly replaced by length fast chalcedony fibers which are occasionally later replaced by glauconite spherulitic and zebraic chalcedony and more rarely megaquartz occur as void filling cements and fringing chalcedonic fibers frequently rim the tests of skeletal grains all except the fringing chalcedonic fibers were subject to glauconitization prior to the chertification process is an extensive dolomitization period rhombic and sucrosic dolomite partially to completely replaced calcite at a relatively early diagenetic stage silicification caused the replacement of calcite by chert at a much more rapid rate than that of dolomite which was found floating within a chertified matrix fractures occurred both early and late in the rus and are usually filled by a late coarse mosaic calcite spar or coarse mosaic and saddle dolomite spar in completely dolomitized samples

speleogenetic evidence from ogof draenen for a pre devensian glaciation in the brecon beacons south wales uk the british isles have been affected by as many as 30 glaciations during the quaternary however the evidence for pre devensian glaciations in upland regions is scarce understanding the extent and timing of earlier upland glaciations is essential for modelling the long term evolution and sensitivity of the british ice sheet caves being protected from surface erosion and weathering can preserve evidence of earlier glaciations in the form of speleothem and sediment archives the 70 km long ogof draenen cave system in south wales uk contains multiple cave levels related to changes in the surface topography and drainage during the past 0 5 ma the cave contains evidence of massive influxes of sediment that were sufficient to choke the cave and alter the underground drainage analysis of the cave sediments passage morphology and geometry suggests the cave once acted as a subterranean glacial spill way before being overridden by ice speleothem u series data demonstrate that this sediment influx occurred before marine isotope state mis 9 probably during the anglian glaciation mis 12 evidence from ogof draenen indicates the impact of subsequent glaciations on the landscape evolution of the region was minimal and that much of the surface topography dates from the anglian

nodular chert from the arbuckle group slick hills sw oklahoma a combined field petrographic and isotopic study nodular chert from the middle and upper arbuckle group early ordovician in the slick hills sw oklahoma was formed by selective replacement of grainstones burrow fillings algal structures and evaporite nodules chert nodules are dominantly microquartz with minor fibrous quartz both quartzine and chalcedony megaquartz and microflamboyant quartz lepisphere textures of an opal ct precursor are preserved in many especially in finely crystalline chert nodules the 18o values of microquartz chert range from 23 4 to 28 80 00 smow significantly lower than those of cenozoic and mesozoic microquartz chert formed both in the deep sea and from near surface sea water the 18o values of chert decrease with increasing quartz crystal size silicification in the arbuckle group occurred during early diagenesis with the timing constrained by the relative temporal relationships among silicification burial compaction and early dolomite stabilization silica for initial chert nucleation may have been derived from both dissolution of sponge spicules and silica enriched sea water chert nucleation appears to have been controlled by the porosity permeability and organic matter content of precursor sediments this conclusion is based on the fact that chert selectively replaced both porous grainstones and burrows and algal structures enriched in organic matter growth of chert probably occurred by a maturation process from opal a to opal ct to quartz as indicated by the presence of opal ct precursor textures in many chert nodules although field and petrographic evidence argues for an early marine origin for chert in the arbuckle group the light 18o values are inconsistent with this origin meteoric resetting of the 18o values of the chert during exposure of the carbonate platform best explains the light 18o values because i the 18o values of chert nodules decrease with decreasing 18o values of host limestones and ii chert nodules from early dolomite which underwent more extensive meteoric modification than associated limestones have lighter 18o values than chert nodules from limestones increasing recrystallization of chert nodules by meteoric water resulted in progressive 18o depletion and quartz crystal enlargement

the use of burial diagenetic calcite cements to determine the controls upon hydrocarbon emplacement and mineralization on a carbonate platform derbyshire england

chert occurrences in carbonate turbidites examples from the upper jurassic of the betic mountains southern spain in upper jurassic carbonate turbidites of the betic mountains southern spain chert occurs in three morphologies bedded chert nodular chert and mottled chert the last refers to a weak dispersed and selective silification which gives a speckled appearance to the rock the three types of chert are formed by replacement of limestones and are associated with different calcareous facies turbidite packstones of saccocoma and peloids and turbidite lime mudstones of pelagic material contain bedded and nodular cherts the silicification textures are mainly micro and cryptocrystalline quartz with local chalcedonic quartz both length fast and length slow which is more common in the packstones only mottled chert is produced where calcareous breccia beds are silicified mottled chert consists of micro and cryptocrystalline quartz length slow chalcedonic quartz and mosaics or individual crystals of euhedral megaquartz beds and nodules are the result of early diagenetic silicification with silica derived from the calcitization and dissolution of radiolarians and subordinately sponge spicules whereas mottled chert is the consequence of later silicification in a probably mg rich environment early silicification is mainly confined to turbidite beds and only rarely occurs in the interbedded pelagic limestones turbidite sedimentation favours silicification because rapid burial of the transported siliceous tests prevents silica from the dissolution of tests passing into overlying sea water a silica rich interstitial fluid develops in the turbidite layer and this migrates to more permeable zones giving rise to bedded and nodular chert