the stable isotopic composition of modern soil carbonate and its relationship to climate the oxygen isotopic composition of modern soil carbonate is well correlated with the isotopic composition of local meteoric water the carbon isotopic cycle for co 2 in soils can be described in terms of the proportion of biomass using the c 4 photosynthetic pathway and the co 2 respiration rate of the soil at low soil respiration rates significant atmospheric co 2 mixing can occur in general the carbon isotopic composition of soil carbonate is related to the proportion of c 4 biomass present in soil but soils that freeze to the depth of carbonate formation often have a significant atmospheric component this suggests that freezing of the soil solution should be considered as another important mechanism for soil carbonate formation because of these relationships the isotopic composition of soil carbonate may be a paleoclimatic and paleoecologic indicator in cases in which diagenetic alteration has not occurred

groundwater dolocretes from the upper triassic of the paris basin france a case study of an arid continental diagenetic facies thick dolomite cemented horizons dolocretes occur within a fluvial sandstone mudstone sequence of late triassic age in the western part of the paris basin france two types of dolomites can be distinguished a nodular dolomitic beds less than a few metres thick which formed within mottled overbank siltstones and mudstones and b massive dolomite up to 16 m thick which occurs in coarse grained channel sandstones and conglomerates the majority of the dolomite consists of a finely crystalline groundmass of dolomicrospar and less commonly dolomicrite glaebules irregular spar filled cracks spheroidal dolomite silicification and vuggy porosity are locally abundant in the massive dolomite in contrast biologically induced micromorphological features such as rhizocretions and alveolar septal fabrics were observed in the thin nodular dolomite beds the dolomite is near stoichiometric well ordered and non ferroan 18o values range from 7 7 to 0 4 o pdb and 18o values range from 5 1 to 1 8 0 pdb and no obvious difference in the stable isotopic composition between both types of dolomites was observed sr isotope ratios range from 0 7101 to 0 7126 and are invariably higher than the contemporary triassic sea water a vadose pedogenic origin for the thin dolocrete layers is indicated by the occurrence of rhizocretions and other biological structures several features however argue against a pedogenic origin for the massive carbonates most notably the absence of biologically induced structures the occurrence in coarse grained channel and not overbank deposits and the great thickness these units are thus interpreted as groundwater in origin phreatic calcretes of quaternary age widespread in inland australia are regarded as a modern analogue for the triassic paris basin dolocretes petrographic observations argue in favour of primary proto dolomite precipitation although early diagenetic replacement of calcite by proto dolomite cannot be ruled out strontium and carbon isotope data of early diagenetic dolocrete cements and oxygen isotope data of early diagenetic silica indicate an entirely non marine continental origin for the groundwaters the poorly ordered and non stoichiometric protodolomite probably underwent stabilization upon further burial resulting in a near stoichiometric well ordered dolomite that clearly lacks evidence for pervasive recrystallization

groundwater dolocretes in a lake marginal environment an alternative model for dolocrete formation in continental settings danian of the provence basin france in the long continental history of the provence basin which extended from santonian to oligocene times a major period of palaeosol development occurred in the danian dolocretes developed within floodplain silts and partly from palustrine limestones dolocretes are overlain by pedogenic facies calcretes and palustrine limestones gradational lower and upper limits of profiles succession of nodular coalescent and massive horizons the epigenesis of quartz by dolomite the unimodal crystal size and the euhedral dolomite fabric as well as the absence of biogenic structures and vadose cements show that dolocretes formed in the phreatic zone the exclusive occurrence of dolocretes around the palaeolake or playa suggests that dolocrete formed by the mixing of groundwaters and lake brines which infiltrated the phreatic zone during periods of strong evaporation and lake level lowering the term halo dolocrete is proposed to describe this type of dolocrete deposit subsequent alteration of the dolocrete includes leaching of the central core of dolomite crystals and calcitization calcitization was either fabric destructive type i or fabric preserving type ii and took place during very early diagenesis i e concomitant with calcrete formation or palustrine limestone deposition fabric destructive calcitization is attributed to a drop in the lake level when the upper part of the dolocrete was subjected to vadose zone processes whereas the fabric preserving calcitization resulted from reactions with dilute lake and groundwaters during rise in lake level

palaeoenvironment of braided fluvial systems in different tectonic realms of the triassic sherwood sandstone group uk abstract fluvial successions comprising the fills of sedimentary basins occur in a variety of tectonic realms related to extensional compressional and strike slip settings as well as on slowly subsiding passive basin margins a major rifting phase affected nw europe during the triassic and resulted in the generation of numerous sedimentary basins in the uk much of the fill of these basins is represented by fluvial and aeolian successions of the sherwood sandstone group additionally regions that experienced slow rates of mesozoic subsidence unrelated to triassic rifting also acted as sites of accumulation of the sherwood sandstone group one well exposed example being the eastern england shelf the fluvial depositional architecture of deposits of the sherwood sandstone group of the eastern england shelf a shelf edge basin is compared with similar fluvial deposits of the st bees sandstone formation eastern irish sea basin a half graben the two studied successions represent the preserved deposits of braided fluvial systems that were influenced by common allogenic factors climate sediment source delivery style differences in preserved sedimentary style principally reflect their different tectonics settings analysis of lithofacies and architectural elements demonstrates that both studied successions are characterized by amalgamated channel fill elements that are recorded predominantly by downstream accreting sandy barforms the different tectonic settings in which the two braided fluvial systems accumulated exerted a dominant control on preserved sedimentary style and long term preservation potential on the eastern england shelf the vertical stacking of pebbly units and the general absence of fine grained units reflect a slow rate of sediment accommodation generation 18 19 4 m myr in this shelf edge basin successive fluvial cycles repeatedly reworked the uppermost parts of earlier fluvial deposits such that only the lowermost channel lags tend to be preserved by contrast in the eastern irish sea basin of west cumbria the rate of sediment accommodation generation was substantially greater 119 m myr such that space was available to preserve complete fluvial cycles including silty drape units that cap the channelized deposits

mercia mudstone formation caprock to carbon capture and storage sites petrology and petrophysical characteristics porous rocks in the subsurface are now used for carbon capture and storage ccs to help ameliorate the effects of greenhouse gas emissions these porous reservoir rocks require a caprock to retain the co 2 it is important to characterize caprock quality and its stability in the presence of elevated partial pressures of co 2 lower triassic sandstones are common in the uk and nw europe and are being considered for future ccs projects the caprock to these sandstones is the middle and upper triassic mercia mudstone group we have studied the mercia mudstone using mineralogy petrology and mercury injection porosimetry to assess its caprock quality detrital minerals are dominated by quartz k feldspar illite and chlorite diagenetic pore filling minerals are dominated by calcite dolomite and gypsum in samples with abundant clay minerals there are only small quantities of pore filling diagenetic cements porosity is broadly uniform for both clay rich and clay poor samples the cleaner clay poor samples had their initial pore spaces filled with early diagenetic pore filling carbonates and gypsum despite the broadly uniform porosity mean pore throat diameter displays a strong inverse correlation with clay content whereas threshold capillary entry pressure shows a strong positive correlation with clay content the more clay rich samples represent much better caprock than the coarser grained samples that contain abundant pore filling cement the samples could support potential column heights of supercritical co 2 between 70 and 540 m and have calculated permeabilities between 10 20 and 10 19 m 2 leakage would occur only on geological time scales assuming that these samples are representative of the whole caprock because caprock quality correlates with illite content and illite will be relatively immune to elevated partial pressures of co 2 the mercia mudstone probably represents a durable caprock for future lower triassic sandstone ccs projects

diagenetic effects on porosity permeability relationships in red beds of the lower triassic bunter sandstone formation in the north german basin abstract carbonate and anhydrite cement clay clasts and inter granular clay are the main components that reduce reservoir quality in the studied bunter sandstone formation the impacts of these parameters on porosity and permeability are determined by combining petrographic mineral quantification with conventional core analysis of samples from the danish part of the north german basin the depositional environments are considered because they largely control the distribution of cements clays and grain sizes the lateral variability of depositional environments is defined by the position in the basin and the proximity to the source areas the stratigraphic distribution of depositional environments is related both to local topography and to climate because high aridity promoted aeolian deposition the bunter sandstone formation has high porosity and permeability in most of the sandstone intervals in the northern north german basin the reservoir quality is good as long as the cements and clays are present as confined bodies that leave the remaining pore spaces available for flow in contrast inter granular clay and pervasive cementation hinder virtually all flow through the sandstone the ephemeral fluvial deposits have an average porosity and permeability of 20 3 and 810 md respectively and the values are 24 6 and 807 md for the aeolian sandstones excluding the unconsolidated aeolian sands which presumably have higher porosity and permeability the aeolian sandstones of the volpriehausen member have very good reservoir quality since they have a thickness of about 25 m are laterally continuous are largely clay free and the cement occurs in small amounts the sandstones of the solling member consist mainly of ephemeral fluvial deposits which generally have good reservoir quality however some intervals have high contents of inter granular clays or pervasive carbonate anhydrite or halite cement and these components reduce the permeability significantly the lateral distribution of the ephemeral fluvial sandstones is variable and therefore difficult to predict when planning a geothermal exploration well thus the volpriehausen member is the preferred target

depositional environments architecture and controls of early cretaceous non marine successions in the northwestern part of kyongsang basin korea abstract in the northwestern part of kyongsang basin southeast korea non marine successions of interbedded sandstone and mudstone are divided into successive stratigraphic units on the basis of facies assemblages and sandstone architecture parts of the successions sinpyong anpyong and jotap units formed in southward or southeastward draining fluvial systems are documented in detail in terms of macroform scale to stratigraphic level architecture for the assessment of fluvial styles and evolution of non marine depositional environments the sinpyong anpyong unit consists of thick sandstone bodies 2 47 m thick thin sandstone bodies generally the jotap unit overlying the sinpyong anpyong unit comprises thick sandstone bodies 2 10 m thick interbedded sandstone siltstone bodies and siltstone dominated bodies the thick sandstone bodies are dominated by the deposits of small bars and dunes with common lenticular beds of purple siltstone and internal scour surfaces suggestive of bedform dominated channels with ephemeral discharges the channels are estimated at generally a few meters in depth and a few tens to hundreds of meters in width the interbedded sandstone siltstone bodies comprise thin sheet like and lenticular sandstone beds along with the marked change in fluvial styles the successions show systematic variations in the proportion and connectedness of channel sandstone bodies and the distribution of floodplain lake facies the basal and uppermost parts of the sinpyong anpyong unit are characterized by the sourceward expansion of distal poorly drained floodplain shallow lake facies and the low proportion and connectedness of channel bodies following these gray mudstone dominated sequences the middle part of sinpyong anpyong unit and jotap unit show increases in the proportion connectedness and grain size of channel bodies and basinward expansion of proximal well drained floodplain facies such a stratigraphic architecture can be attributed to the fluctuations in the ratio of accommodation space sediment supply regulated by repeated basin subsidence

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

the stratigraphic and geochemical record of playa cycles monitoring a pangaean monsoon like system triassic middle keuper s germany abstract the combination of high resolution stratigraphy with high resolution geochemistry allowed monitoring the cyclicity of a playa system in two dimensions time and space the playa cycles of the middle upper triassic steinmergel keuper located in southern germany were studied both in terms of lateral facies shift variations and long term cyclic patterns the lateral variation was expressed by investigating a time equivalent transect that spanned the kieselsandstein horizon within a 130 km distance from marginal to mid playa position whereas the long term cyclic pattern was investigated in core malschenberg where a time record of ca 5 106 years was encountered mudstone dolomite cycles with dolomites representing the wet and mudstones the dry phases reflect cyclic playa lake level changes indicating varying intensities of monsoon like circulation moisture transport from the tethys ocean into the german basin is implied by 18o depletion trends of isochronous dolomite beds from the margin to mid playa position suggesting dominantly rayleigh like circulation patterns distinct covariance of 18o 13c is used as indicator of lake level stands the k al curve with inverse covariation compared to 18o reflects fluctuating weathering conditions on the fan and the vindelician high a hierarchic cycle stacking pattern composed of metre scale to tens of metre cycles is interpreted to be a result of changing pangaean monsoon like intensities in the milankovitch frequency band spectral analysis shows that the power spectra are dominated by precession and eccentricity

occurrences and genesis of calcrete and dolocrete in the mio pleistocene fluviatile sequence in kuwait northeast arabian peninsula abstract sedimentologic and petrographic investigation of the subsurface mio pleistocene clastic sequence of the kuwait group in umm ghudair water field southwest of kuwait revealed the occurrence of successive cycles of calcrete profiles the calcrete host sediment kuwait group is mainly represented by a sequence of muddy sands interbedded with mud and sand layers with occasional lenses of gravels individual calcrete profile has vertical gradational maturity pattern which is manifested by the occurrence of well differentiated horizons arranged from bottom to top as follows non calcretized host sediment incipient calcrete nodular calcrete concretionary calcrete massive laminar calcrete abundance of organosedimentary structure mainly plant roots and diversity of calcite crystal fabrics suggest that the studied calcrete profiles were developed within two stages of calcretization a pedogenic stage followed by a phreatic stage the variation in the mineralogical composition between umm ghudair calcrete profiles and the dolocrete profiles that occur within the same clastic host sediment but at distal location within the kuwait group flood plain about 80 km to the northeast is mainly attributed to differences in the chemical composition of the groundwater and its flow pattern it is suggested that the hydrogeological and hydrogeochemical settings may play a significant role in the development of cyclic pedogenic phreatic and polygenetic calcretes and dolocretes within the same sedimentary basin alternation of tectonically stable and unstable periods may be responsible for the cyclicity of calcretization dolocretization

large scale cycle architecture in continental strata hornelen basin devonian norway abstract nine large scale stratigraphic cycles each about 100 m thick along the northern margin of the hornelen basin western norway record systematic expansions and contractions of alluvial fan braidplain and lake facies tracts the braidplain facies tract occupies the basin center from time to time expanding toward the margins and constitutes the deposits of axial or longitudinal drainage from an eastern source the alluvial fan facies tract which comprises the deposits of high gradient transverse drainages occupies a narrow belt close to the fault bounded basin margin the lake facies tract is between the alluvial fan and braidplain facies tracts physical correlation of strata shows that the large scale cycles are symmetric in all facies tracts the symmetry of the large scale cycles in which the base level rise and fall hemicycles of each cycle are of approximately the same thickness indicates that sediment accumulated in all environments during the entire base level cycle absence of regional unconformities sequence boundaries also indicates approximately continuous sediment accumulation in the basin during base level cycles the base level fall to rise turnaround is represented by an interval of strata rather than by a sequence boundary at the maximum expansion of the alluvial fan facies tract condensed sections are also absent they are represented mainly by intervals of lake strata at base level rise to fall turnarounds strata in the base level fall hemicycle decreasing accommodation accumulated as the braidplain and alluvial fan facies tracts expanded from the basin center and the northern basin margin respectively expansions of these two facies tracts filled the basin while the lake facies tract contracted strata in the base level rise hemicycle accumulated as the alluvial fan and braidplain facies tracts contracted sourceward respectively toward the northern and eastern basin margins during times of increasing accommodation this sourceward retreat of the alluvial fan and braidplain facies tracts was coincident with expansion of the lake facies tract at the rise to fall turnarounds alluvial fan and braidplain sediments are stored near their respective sources toward the basin margins and the centers of mass of these facies tracts are at basin margin positions during base level fall time the centers of mass of the alluvial fan and braidplain facies tracts migrate basinward the alluvial fan and braidplain facies tracts usually expand toward each other simultaneously and then move away from each other simultaneously this in phase relationship is replaced by a reciprocal stacking pattern when the two facies tracts abut each other near base level fall to rise turnarounds when the two facies tracts are in contact the higher gradient alluvial fan facies tract initially blocks expansion of the braidplain as the alluvial fan facies tract retreats toward the basin margin and fan margin gradients are reduced however the braidplain expands across former fan margins in the reciprocal configuration the two facies tracts move in tandem in the same direction this reciprocal pattern plus changes in aggradation to progradation ratio of the alluvial fan facies tract suggest that alluvial fan morphology changes during base level cycles large scale cycle stacking patterns show significant changes in basin fill architecture through time including syndepositional structural changes accompanied by changes in stratal geometries a change from ephemeral to permanent lakes permanent increase in alluvial fan gradients and permanent reduction of alluvial fan radii and volume at the northern basin margin consideration of these changes and the largely in phase relationships between facies tracts sourced by two separate sediment supplies suggest that an interplay of climate self regulatory and tectonic factors controlled sedimentary accumulation within the hornelen basin

laramide tectonics and humid alluvial fan sedimentation ne uinta uplift utah and wyoming abstract laramide uplift and erosion of the uinta mountains are recorded in a 10 km long outcrop of the lower eocene wasatch formation on the utah wyoming border this 750 m thick package of interbedded sandstones and conglomerates is dominated by a coarsening then fining upward megasequence 650 m thick that records the growth and abandonment of a humid alluvial fun system during a major cycle of uplift and unroofing of the thrust bounded northern flank of the uinta mountains grain size thickness and lateral extent of channel complex deposits increase upward in the lower 400 m of the sequence reflecting construction and northward progradation of the fan grain size and channel complex thickness decrease upward in the upper 250 m of the sequence reflecting gradual reduction of both sedime t yield and sediment caliber during postorogenic lowering of source area relief within the megasequence coarsening up sequences 10 100 m thick built mainly of channel complex deposits reflect progradation of fan lobes punctuated by periodic fan head avulsions these medium scale sequences in turn comprise small scale fining up cycles 1 10 m thick that reflect such fluvial processes on the fan as bar building discrete flood events and the filling and lateral migration of braided stream channels both the medium scale and small scale sequences are commonly underlain by paleosols evidence of debris flows or other mass movement processes is conspicuously lacking above the main megasequence a second thinner megasequence containing lacustrine mudstones and wave reworked conglomerates indicates that the fan persisted as a locus of coarse clastic deposition during the first of a series of lacustrine transgressions that began as ratios of sediment flux to subsidence rate decreased toward the end of the early eocene partial inundation of the fan during this transgression transformed it into a fan delta

paleosol and dolocrete associated clay mineral occurrences in siliciclastic red sediments of the late miocene komisini formation of the tuzgolu basin in central turkey abstract the late miocene was characterized by high global temperatures and partly to nearly complete desiccation of the mediterranean sea throughout the latter part of the messinian age the messinian salinity crisis the late miocene marine mediterranean sea area has been widely studied from different perspectives but terrestrial paleoenvironmental and paleoclimatic records from the mediterranean area are highly limited in their spatial range to this end the paleosols dolocrete and fluvial sedimentary rocks of the komisini formation in the northwestern part of the tuzgolu basin are described in detail six lithofacies were identified as a clast supported gravel b massive pebbly sandstone c massive mudstone d disrupted mud e disrupted conglomerate and f disrupted matrix supported conglomerate the siliciclastic facies were interpreted as braidplain and the distal part of braidplain deposits is associated with an ophiolitic area while the disrupted mud and disrupted conglomerate were identified as three types of paleosols inceptisols aridisol and vertisols the calcretes occur in the paleosols in nodular powder and massive forms clay minerals include smectite chlorite palygorskite illite and serpentine gypsum smectite and chlorite are dominant minerals at the base and middle part of the section whereas palygorskite and illite appear as the prevailing clay minerals in the upper sections smectite illite and palygorskite precipitated in a vadose zone from strongly evaporated alkaline water rich in si ca and mg and poor in al at increased temperatures and at an advanced stage of dolocrete and paleosol formation k si mg al fe and ca are required for smectite illite palygorskite and calcification given the increase in ba cr ni and co these cations might have originated from ophiolite carbonate and volcanic units as a consequence the vertical distribution of clay mineral types suggests the variations of paleoclimatic conditions during the deposition and the relationships among the type of host rock paleosol and dolocrete which corresponds to nearly complete desiccation of the mediterranean sea

the sedimentary facies of a late bathonian regressive episode the kilmaluag and skudiburgh formations of the great estuarine group inner hebrides scotland the kilmaluag and skudiburgh formations of the bathonian great estuarine group represent a unique palaeoenvironment within the british jurassic the kilmaluag formation is divided into two distinctive facies the clastic facies of n skye comprises calcareous mudstones and shales with subordinate sandstones and argillaceous limestones the depositional setting was one of shallow ephemeral lagoons closed from the sea sublittoral lagoon mudflat and channel environments are identified and fossil faunas indicate low salinity the argillaceous limestone facies of southern skye eigg and muck formed in a similar depositional setting although sandstones are not present alternations of carbonate rocks some dolomitic and calcareous shales probably reflect palaeoclimatic fluctuations between humid and arid periods the overlying skudiburgh formation is interpreted as being alluvial floodplain channel and overbank deposits are identified by the presence of mottled mudstones sands and lensoidal silts syndepositional carbonate nodules are interpreted as calcrete which occasionally coalesced to form extensive hardpan calcrete horizons the vertical facies sequence reflects a late bathonian regression the coastal plain terrestrial environments of the skudiburgh formation were transgressed during the callovian stage an event which ended paralic deposition in the hebridean jurassic

climate of the supercontinent pangea numerous climate models predict that the geography of the supercontinent pangea was conducive to the establishment of a megamonsoonal circulation in general geologic evidence supports the hypothesis of a megamonsoon that reached maximum strength in the triassic pangea in the late carboniferous had widespread peat formation in what is now central and eastern north america and europe and relatively dry conditions on the colorado plateau the equatorial region of the continent became drier through the end of the carboniferous by the permian the equatorial region of pangea was dry and indicators of aridity and rainfall seasonality became more widespread wind directions from colorado plateau eolian sandstones are consistent with an increasing influence of monsoonal circulation at this time in the triassic climate in the colorado plateau region became relatively wet though still seasonal and the few eolian sandstones indicate a major shift in wind direction at that time in addition sedimentation

diagenesis of carbonate cements in permo triassic sandstones from the iberian range spain evidence from chemical composition and stable isotopes petrographic chemical and stable isotope investigations have been made on calcite dolomite and fe dolomite ankerite from permian triassic clastic sequences from the iberian range spain calcite and dolomite cements have precipitated before the fe dolomite ankerite calcite however has also formed by calcitization of dolomite due to uplift the negative 13cpdb values 9 51 to 2 13 in these carbonates suggest derivation of carbon from oxidation of organic matter in the early carbonates and from decarboxylation of organic matter in the late fe dolomite ankerite early diagenetic dolomitic cements have heavier 13c 3 54 higher mn mn fe and mn mn mg and lower fe fe mg ratios than later diagenetic dolomitic cements the latters are also characterized by lighter 18opdb 10 64 than the early cements due to increase in temperature and to pore water mineral reactions

dawsonite cement in the triassic lam formation shabwa basin yemen a natural analogue for a potential mineral product of subsurface co2 storage for greenhouse gas reduction abstract the origin and permeability effects of carbonate cement blamed for poor reservoir quality in the upper triassic turbidite sandstones of the lam formation in yemen have been investigated using petrography x ray diffraction stable isotopes fluid inclusions and thermodynamic modelling in order of growth in the sandstone the carbonate cements include ferroan calcite dawsonite naalco3 oh 2 ferroan dolomite and siderite ferroan dolomite is the most abundant cement there is an inverse correlation between permeability and total carbonate cement volume supporting the earlier assertion that carbonate cements are the main control on reservoir quality in these sandstones ferroan calcite replaced and was sourced by marine limestone rock fragments that are abundant in the turbidite sandstone the later cements were probably also at least partly sourced from the marine limestone rock fragments stable isotope data from ferroan calcite ferroan dolomite and siderite show that diagenesis occurred in formation water with oxygen isotopes typical of closed system diagenesis and that only minor organic derived carbon dioxide was involved in diagenesis dawsonite while not common as a diagenetic mineral in sandstones has recently been suspected of being a routine consequence of co2 storage in the subsurface for greenhouse gas reduction in the lam formation dawsonite found at concentrations of up to 8 vol of the rock grew at the expense of detrital feldspar minerals under conditions of elevated partial pressure of carbon dioxide isotopic data show that dawsonite grew in the presence of carbon dioxide with a 13c value of about 4 to 2 this is distinctly different from the carbon isotope ratio of the gas phase carbon dioxide now present in the sandstone 27 which had an organic source rock origin the dawsonite 13c data probably represent a mixture between carbonate from detrital marine limestone rock fragments and carbon dioxide from a moderately negative source possibly of magmatic or basement origins dawsonite may thus be the result of a mass influx of deep e g magmatic carbon dioxide that pushed the lam formation into the dawsonite stability field at the expense of albite

seismic stratigraphic subdivision of the triassic succession in the central north sea integrating seismic reflection and well data the triassic sedimentary succession in the central north sea has been investigated to establish a broader understanding of the triassic period from the combined interpretation of seismic reflection data and well data the triassic succession has been subdivided into four seismic units where unit boundaries are characterized by regional seismic amplitude anomalies reflecting changes in gross sedimentary facies or rock properties a successful correlation between sedimentary facies interpreted within the well sections and distinct seismic reflection patterns allowed a thorough mapping of the gross palaeoenvironment throughout the triassic the method presented of subdividing a continental sedimentary succession into seismic units should be applicable elsewhere in other basins the main source area during the triassic was scandinavia to the north and sediment transportation was mainly along north south and ne sw trending lineaments which are at present located onshore southern norway and in the asta graben and the varnes graben offshore an uplifted skagerrak graben area acted as source area in the early and early middle triassic with sediment dispersal to the south and sw high relief existed for a longer period in western scandinavia than in eastern scandinavia which supports an asymmetric shape of the scandinavian mountains during the triassic accommodation space in the early triassic was mainly controlled by the relief inherited from a late carboniferous permian rift phase although thermally induced regional subsidence continued in the middle and late triassic creation of local accommodation space was mainly limited to halokinesis including redistribution and withdrawal of salt from the subsurface the upper triassic succession is eroded across the western and central parts of the study area although the upper triassic unit is preserved in synforms adjacent to salt structures in the western part of the study area dry playa conditions prevailed during the early triassic although fluvial systems supplied long transported sandy detritus southeastwards in the late early triassic more sandy detritus was transported into the sedimentary basin in the middle and late triassic concurrently with a gradually wetter climate supplementary material uninterpreted seismic sections are available at www geolsoc org uk sup18726

modeling the growth of laccoliths and large mafic sills role of magma body forces 1 the use of elastic plate theory to model the emplacement of laccoliths and large mafic sills has been debated for nearly 40 years these intrusions typically attain a horizontal width that is large relative to the emplacement depth provided that large scale plasticity and or heterogeneity is not observed in the overlying host rock it should then be valid to approximate its deformation based on analysis of a thin elastic plate with effective properties that are the consequence of interaction among heterogeneities that are small relative to the size of the intrusion but the predictions that are usually cited from elastic plate theory are characterized by bell shaped geometry in contrast to the flat topped steep sided geometry typical of many laccoliths or the nearly uniform thickness typical of large mafic sills this fact has motivated several alternate explanations of laccolith and large mafic sill emplacement nonetheless elastic plate theory should be revisited in light of the fact that previous elastic plate based predictions have in general not taken into account an appropriate fracture propagation condition fluid flow in the growing intrusion and importantly the influence of the weight of the magma on intrusion growth we present a model for the growth of circular intrusions that accounts for all of these factors the model predicts the appropriate geometry for both laccoliths and large mafic sills the predicted thickness to length relationships are also consistent with field data hence while it may sometimes be appropriate there is in general no fundamental need to appeal to large scale rock plasticity in order to explain observed intrusion geometries and it may in fact be appropriate to understand the growth of laccoliths and large sills in light of a single underlying mechanical model

sedimentary facies and the context of dolocrete in the lower triassic sherwood sandstone group corrib field west of ireland abstract the lower triassic sherwood sandstone group ssg reservoir in the corrib field is predominantly fine to medium grained fluvial sandstone with minor aeolian and playa sediments that formed under semi arid to arid conditions there is no evidence of roots burrows or organic rich soils in the ssg suggesting an environment with sparse vegetation in the catchment and sedimentation from a predominantly braided sand dominated river dolomite is the main porosity occluding cement

calcretes in semi arid alluvial systems formative pathways and sinks late quaternary deposits in gujarat western india show an abundant development of calcretes three major sinks of carbonate in the alluvial deposits are recognized 1 groundwater calcretes 2 pedogenic calcretes and 3 calcrete conglomerates groundwater calcretes originate from carbonate saturated waters travelling preferentially along stratification planes pedogenic calcretes form through soil forming processes typically in extra channel areas calcrete conglomerates occur as ribbons sheets and lenses due to the reworking of both pedogenic as well as groundwater calcretes as a result a pathway of calcretization develops that has the route groundwater calcrete to pedogenic calcrete to calcrete conglomerate the formation of pedogenic calcretes over sediments containing groundwater calcretes demonstrates that 1 apart from aeolian dust river waters are also a major source of carbonate and 2 pedogenic carbonates may attain large sizes at accelerated rates due to the presence of pre existing groundwater calcretes consequently the maturity of a soil may be overestimated if determined by following established morphogenetic sequences

the character and genesis of calcrete in late quaternary alluvial deposits gujarat western india and its bearing on the interpretation of ancient climates late quaternary deposits in mainland gujarat contain sediments deposited in subhumid and semi arid climates the 30 35 m succession shows the presence of vertisols at the base and a red bed horizon ferric calcisol when pedogenic in origin that roughly bisects the succession a widespread development of calcretes is observed throughout the succession the various varieties of calcrete include pedogenic calcrete groundwater calcrete calcrete conglomerate transported calcrete and rhizogenic calcrete pedogenic calcrete nodules associated with vertisols and the red soil show marked differences in morphology dimensions and the distribution of microscopic features these differences arise due to contrasting climate controlled physicochemical environs under which they formed pedogenic calcrete nodules associated with vertisols acquire large 5 10 cm dimensions and are characterised by either showing the presence of a nucleus of soil matter or showing a dense micritic groundmass cut by thick sparitic veins in contrast calcrete nodules that formed in the red soil are 3 cm in size and do not exhibit dense networks of sparitic veins calcretes associated with vertisols and the ferric calcisol also exhibit differences in the morphology of rhizoliths these differences also show up in the distribution of microfabrics of calcretes grain coats are present in rhizogenic and pedogenic calcretes but absent in groundwater calcretes and less profuse in hydromorphic soil calcretes clotted micrite is present in all types except groundwater calcretes sparitic veins however are observed in each type but are relatively less developed in groundwater calcretes a similar distribution of displacive and replacive textures is also seen although some grains in groundwater calcretes showed signs of corrosion the vertisol associated calcretes represent a subhumid 500 700 mm climate whereas the red soil calcretes suggest a semi arid 100 500 mm climate calcretes from the vertisol association show a range in 13c composition constrained between 9 and 5 whereas the red soil calcretes exhibit the whole spectrum of values from 9 to 1 based on mineralogical associations calcretes usually taken to reflect semi arid episodes in the earth s history may be classified further calcretes when associated with sepiolite palygorskite suggest an arid climate mean annual rainfall 50 100 mm when associated with smectite haematite and the absence of hydromorphism a semi arid climate mean annual rainfall 100 500 mm and when found in smectitic vertisols subhumid climates mean annual rainfall 500 700 mm

dolomitized calcrete in the middle devonian winnipegosis carbonate mounds subsurface of south central saskatchewan canada dolomitized calcrete is a widespread diagenetic feature in the uppermost portions of the middle devonian winnipegosis mud mounds in the subsurface of south central saskatchewan individual calcrete profiles are composed of 2 to 5 horizons including in descending order laminar crust massive pisolitic breccia chalky and transitional horizons the calcrete profiles are interpreted to be pedogenic in origin formed by diagenetic alteration of the host carbonate deposits during subaerial exposure of the mud mounds this interpretation is based on the occurrence of an orderly set of well differentiated horizons disconformities vadose pisoids micritic stringers circumgranular cracks linkage coatings and gradational contacts between the calcrete and underlying host carbonate rocks lack of root related structures such as rhizoconcretion and alveolar septal fabric in the winnipegosis calcrete suggests a limited influence of macrophytes on the formation of calcrete the occurrence of up to three discrete pedogenic calcrete profiles in a single winnipegosis calcrete succession indicates the mounds underwent three periods of subaerial exposure that resulted from drops of water level in the elk point basin of saskatchewan during middle devonian time d 2004 elsevier b v all rights reserved

mesozoic climates general circulation models and the rock record abstract 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 tend to require powerful computers to handle vast numbers of calculations nevertheless it is now possible to compare results from different gcms for a range of times and over a wide range of parameterisations for the past present and future e g in terms of predictions of surface air temperature surface moisture precipitation etc gcms are currently producing simulated climate predictions for the mesozoic which compare favourably with the distributions of climatically sensitive facies e g coals evaporites and palaeosols they can be used effectively in the prediction of oceanic upwelling sites and the distribution of petroleum source rocks and phosphorites models also produce evaluations of other parameters that do not leave a geological record e g cloud cover snow cover and equivocal phenomena such as storminess parameterisation of sub grid scale processes is the main weakness in gcms e g land surfaces convection cloud behaviour and model output for continental interiors is still too cold in winter by comparison with palaeontological data the sedimentary and palaeontological record provides an important way that gcms may themselves be evaluated and this is important because the same gcms are being used currently to predict possible changes in future climate the mesozoic earth was by comparison with the present an alien world as we illustrate here by reference to late triassic late jurassic and late cretaceous simulations dense forests grew close to both poles but experienced months long daylight in warm summers and months long darkness in cold snowy winters ocean depths were warm 8 c or more to the ocean floor and reefs with corals grew 10 of latitude further north and south than at the present time the whole earth was warmer than now by 6 c or more giving more atmospheric humidity and a greatly enhanced hydrological cycle much of the rainfall was predominantly convective in character often focused over the oceans and leaving major desert expanses on the continental areas polar ice sheets are unlikely to have been present because of the high summer temperatures achieved the model indicates extensive sea ice in the nearly enclosed arctic seaway through a large portion of the year during the late cretaceous and the possibility of sea ice in adjacent parts of the midwest seaway over north america the triassic world was a predominantly warm world the model output for evaporation and precipitation conforming well with the known distributions of evaporites calcretes and other climatically sensitive facies for that time the message from the geological record is clear through the phanerozoic earth s climate has changed significantly both on a variety of time scales and over a range of climatic states usually baldly referred to as greenhouse and icehouse although these terms disguise more subtle states between these extremes any notion that the climate can remain constant for the convenience of one species of anthropoid is a delusion although the recent rate of climatic change is exceptional

late devonian rifting in the central north sea evidence from altered felsic volcanic rocks in the embla oil field abstract in the embla oil field on the northern flank of the mid north sea high the central north sea multiple quartz porphyric volcanic beds at ca 4600 m depth form part of a volcano sedimentary interval above the caledonian basement as interpreted from seismic data zircon u pb laser ablation icpms date one bed to 374 3 ma indicating that the volcanic rocks and interbedded sediments are early famennian and correlate to the buchan formation the volcanic rocks have been extensively clay and carbonate altered in a near surface environment but high field strength element data show that the protoliths were alkali rhyolites yielding intra plate signatures in tectonic discrimination diagrams famennian quartz porphyric volcanic rocks have also been reported from well a17 1 on the southern flank of the mid north sea high the famennian volcanism on the northern and southern flanks testify to an active magmatic environment in the central north sea in the early famennian supporting the existence of a late devonian proto central graben rift extending northwards into the central north sea the rift is likely an early example of strain localisation to a zone of reduced crustal strength along the caledonian suture between avalonia and baltica

challenging the sheetflood myth the role of water table controlled sabkha deposits in redefining the depositional model for the ormskirk sandstone formation lower triassic east irish sea basin

triassic sand supply to the slyne basin offshore western ireland new insights from a multi proxy provenance approach to assess provenance data and derive basin infill models with a higher level of certainty it is vital to understand how minerals used in sedimentary provenance behave in the sedimentary system a multi proxy approach helps ensure rigour in how provenance data are interpreted u pb geochronology of refractory zircon and relatively less stable apatite and pb isotopic analysis of labile k feldspar are employed in this study to reassess the provenance of triassic sandstones in the slyne basin offshore western ireland this approach aims at reducing the potential bias in each method producing a complementary dataset u pb zircon and apatite geochronology yields archean paleoproterozoic ages caledonian ages and previously unrecognized permo triassic ages pb isotopic analysis of k feldspar does not identify the permo triassic as a source the detection of permo triassic aged detritus may suggest that volcanism was more widespread than previously recognized these results support the hypothesis of a triassic drainage divide between the basins offshore western ireland and those onshore britain and in the irish sea sand supply was dominantly from the north with significant input from the flanks of the basin and previously unrecorded sources such as permian aged rocks playing an important role the provenance signal is consistent and homogenized throughout the sampled sequence indicating that the drainage system was long lived supplementary material details of laboratory and sample preparation data tables and summary pie charts for each sample are available at https doi org 10 6084 m9 figshare c 4571096

geology mineralogy and geochemistry of late miocene paleosol and calcrete in the western part of the central anatolian volcanic province cavp turkey abstract the late miocene was characterized by high global temperatures and partly to nearly complete desiccation of the mediterranean sea throughout the latter part of the messinian age the messinian salinity crisis the non marine paleoenvironmental and paleoclimatic records from the mediterranean area are highly limited in their spatial range late miocene river flood plain sediments in the central anatolian volcanic province cavp of central turkey consist of siliciclastic and pyroclastic sediments with paleosols and calcrete five lithofacies were identified as a massive conglomerate b trough cross bedded sandstone c massive mudstone d disrupted mud e and disrupted matrix supported conglomerate the paleosols were composed predominantly of smectite and illite with smaller amounts of feldspar quartz opal a pyroxene and amphibole and the calcretes were composed mainly of calcite with smaller amounts of feldspar quartz and opal a and minor amounts of smectite chlorite and illite authigenic clay minerals and micritic calcite coating on the detrital grains and soil peds and pendant cement in the calcrete and paleosol horizons were evidence of a vadose zone in contrast the large equant spar drusy mosaics were formed in phreatic environments the weathering of ignimbrite and marble led to the depletion of sio2 cao al2o3 fe2o3 tio2 and k2o through the precipitation of smectite and illite in the paleosols and cao in the form of micrite and calcite in the calcrete horizons field observations mineralogy geochemistry and the results of 18o and 13c isotopic analysis reveal that alternating wet and dry periods resulted in an upward increase in precipitation of authigenic smectite and illite within the late miocene sections under the influence of geochemical and ph fluctuations in phreatic or vadose water within the river flood plain paleoenvironment

diagenetic formation of ferrimagnetic iron sulphide minerals in rapidly deposited marine sediments south island new zealand abstract detailed magnetostratigraphic studies of late neogene siliciclastic sediments of the awatere group south island new zealand 41 45 s 174 05 e have revealed a wide range of palaeomagnetic behaviour examination of rock magnetic properties was undertaken using conventional palaeomagnetic techniques and thermomagnetic x ray diffraction and electron microprobe analyses these analyses indicate that the ferrimagnetic iron sulphide minerals greigite and pyrrhotite are responsible for a stable and intense magnetic remanence in fine grained sediments whereas titanomagnetite is the only remanence bearing mineral identified in coarser grained sediments which are less strongly and less stably magnetised than the fine grained sediments detrital titanomagnetite grains are likely to have undergone dissolution during early diagenesis as a result of iron sulphide formation which occurs commonly in rapidly deposited anoxic sediments that support active sulphate reduction and h 2 s formation preservation of greigite and pyrrhotite is inferred to result from the arrest of the pyritisation process probably due to the low permeability of the fine grained sediments and consumption of available h 2 s before full reaction to pyrite occurred relative palaeomagnetic instability and weak remanence intensities in coarser grained sediments is likely to be due to low titanomagnetite concentrations resulting from titanomagnetite dissolution

calcretes related to phreatophytic vegetation from the middle triassic otter sandstone of south west england carbonate cementation in the triassic otter sandstone of budleigh salterton sw england occurs in two distinct forms large vertical concentrically zoned cyclinders and thin subhorizontal sheets the former represent rhizocretions of the tap roots of phreatophytic plants which colonized bars and abandoned channels on a large braidplain the sheets represent cementation around ancient water tables the precipitation of the rhizocretions took place at least in part during the life of the plants and 13c and 18o values support the view that evapo transpiration induced carbonate precipitation palaeosol profiles are rare in the otter sandstone reflecting the geomorphological instability of the braidplain surfaces in contrast rhizocretionary and sheet calcretes which formed several metres below the active depositional surface are abundant because they had a high preservation potential

paleoclimate controls on stratigraphic repetition of chemical and siliciclastic rocks climate is a primary control on sediment flux from continental sources into sedimentary systems in warm climates siliciclastic input is greatest under highly seasonal rainfall nonseasonal conditions favor formation of end member chemical rocks perennially wet climates are conducive to coal formation whereas dry climates produce carbonates and or evaporites stratigraphic repetition of siliciclastic and chemical rocks therefore appears to be related to paleoclimate cycles as well as to transgressive regressive events and tectonics

an abandoned channel fill with exquisitely preserved plants in redbeds of the clear fork formation texas usa an early permian water dependent habitat on the arid plains of pangea abstract a well preserved plant assemblage at the colwell creek pond locality of leonardian kungurian age provides an opportunity to evaluate taphonomic conditions in a dryland alluvial setting a narrow channel body incised to 5 m depth through red paleo vertisols contains 2 m of varicolored laminated mudstone with graded layers and plant material x ray diffraction analysis of individual laminae indicates the presence of chlorite illite kaolinite and mixed layer clay with hematite in red and gray layers and goethite in yellow brown laminae no carbonate was identified and the total organic carbon content is minimal the fine sediment accumulated in a shallow abandoned channel from suspension and gentle underflows probably linked to seasonal inflow and analysis of lamina thickness suggests that standing water may have persisted for up to a few millennia the preservation of lamination is attributed to a lack of bioturbation possibly linked to a paucity of subsurface oxygen low productivity elevated salinity rapid deposition or a combination of these factors minimal bioturbation may also reflect the limited use of freshwater ecospace during the early permian clay rich paleo vertisols complete the fill with drab root traces that indicate growth of vegetation in a strongly seasonal setting abundant plant material in the laminated beds includes branches of walchian conifers the possible cycadophyte taeniopteris spp and the comioid possible peltasperm auritifolia waggoneri they were derived from an adjacent riparian zone and preserved as 3d goethite petrifactions much of the foliage shows evidence of arthropod herbivory although a humid climatic episode cannot be ruled out the exceptional abundance and preservation of the plants probably reflects the persistence of an oxbow lake on a relatively arid alluvial plain where riparian plants experienced periodic moisture stress but had access to groundwater nearly year round rapid burial in standing water the lack of bioturbation in the laminated sediments and early biomineralization probably explain the exceptional preservation of the plant remains

controls on the stratigraphic development of the triassic fundy basin nova scotia implications for the tectonostratigraphic evolution of triassic atlantic rift basins abstract the late triassic synrift succession of the minas sub basin bay of fundy nova scotia includes a fluvio alluvial fan dominated unit lower and middle wolfville formation a transitional fluvio playa unit upper wolfville formation and an upper playa dominated package blomidon formation we describe the facies development in the 240 m thick upper wolfville formation examine vertical facies stacking patterns and develop a depositional model for this succession the unit comprises channelized and unconfined fluvial deposits interbedded with aeolian and playa sediments that grade upward into playa deposits the succession comprises progradational and retrogradational fluvial packages and thin aggradational playa claystones comparison with 11 other atlantic margin continental triassic rift basins allows an assessment of the controls that govern rift basin development the transition from fluvial to playa or lacustrine conditions occurs in all basins at different times the lack of synchronicity indicates that global climate change was not a controlling factor we suggest that the fluvial to lacustrine or playa transition was due to a decrease in source area relief related to a decline in regional tectonic activity and that the basin was hydrologically closed throughout sedimentation this tectonostratigraphic model may be appropriate for other triassic basins developed along the atlantic margin

carbon isotope stratigraphy using carbonate cements in the triassic sherwood sandstone group corrib field west of ireland abstract carbon stable isotopes from carbonate minerals mainly dolomite from six wells from the lower triassic sherwood sandstones of the corrib gas field slyne basin west of ireland allow stratigraphic correlation the results also provide information on palaeoenvironmental change during the deposition of these continental redbed sedimentary rocks the triassic reservoir rocks have been buried to 4000 m and heated to 165 c and now contain methane rich gas although the oxygen isotopic signal has been at least partially reset during burial and heating a primary carbon isotopic signal appears to have survived diagenesis the carbon isotope ratio varies from 3 2 to 2 1 all six wells show similar stratigraphic changes when all the carbon isotope data are plotted relative to a major playa horizon 13 c increases from about 3 at the base of the sherwood to about 2 170 m above the base 13 c then decreases to about 2 for the next 70 m and remains steady for the following 50 m the top 20 m of the sherwood contains carbonate with a 13 c values decreasing to about 3 the occurrence of a stratigraphically correlatable carbon isotope pattern implies that the primary evolution signal has been preserved the change in 13 c correlates with indicators of aridity and biological stress such that the highest 13 c values are in sedimentary rocks deposited in a playa lake arid times these rocks contain the greatest quantity of dolomite cement conversely the lowest 13 c values correspond to sedimentary rocks deposited from well developed rivers relatively humid times from the lowest quantity of dolomite cement the same carbon isotope evolution has been found in another well in the slyne basin and in belgium suggesting that the palaeoenvironmental isotope signal in the triassic sedimentary rocks of the corrib field may have a regional significance

petrography and diagenesis of cavity fill dolocretes kuwait abstract cavity fill dolocretes within the palaeokarst zone of the middle eocene chertified dolomicrite dammam formation are exposed within a quarry located on the top of al ahmadi ridge in southern kuwait they are characterised by the abundance of zoned and spherulitic dolomite these dolocretes are formed by massive precipitation of primary dolomite into cavity fill siliciclastics within phreatic groundwater conditions during arid to semi arid period successive stages of karstification and dolocretisation are responsible for the development of two types of cavity fill dolocretes namely mature and immature dolocretes the association of diagenetic alunites with the cavity fill dolocretes suggests that hydrocarbon seepage may have played a role in the development of these dolocretes it is suggested that the occurrence of similar diagenetic setting within carbonate sequence may potentially affect their reservoir characteristics

the control of sedimentary facies by climate during phases of crustal extension examples from the triassic of onshore and offshore england and northern ireland crustal extension controls the tectonic accommodation space available for sediments in rift settings and may be defined by the structural and depositional geometry of sedimentary successions observed on seismic data and the rate of subsidence through time as represented by the accommodation of sediment the characteristic features of each are dependant on three variables the time taken for deposition the interplay between tectonics and eustasy and the lithology thus facies of the succession observed the sherwood sandstone group has been considered to represent a syn rift phase of fluvial deposition throughout europe with the overlying mercia mudstone group interpreted as the succeeding phase of deposition in an evaporitic seaway during post rift thermal subsidence more recently however there has been the recognition that it is the mercia mst gp which is seen to thicken markedly into faults imaged on seismic data rather than the sherwood sandstone group this work demonstrates the mercia mst gp to be a syn rift phase of deposition with the fine grained nature of the sedimentary record at this time controlled by the prevailing arid climate such conditions were not conducive to the large scale and rapid movement of sediments from the hinterlands raised by relative footwall uplift thus the sediments are fine grained the minor thickening of the sherwood sst gp into faults is interpreted to be a combination of minor extension in the early triassic superimposed on thermal subsidence inherited from the important regional phase of extension in the early permian analysis of the timing of fault growth indicates a larger proportion of fault controlled synsedimentary movement occurred during the mid to late triassic mercia mst gp rather than the early triassic sherwood sst gp

diagenesis and reservoir quality of the sherwood sandstone triassic corrib field slyne basin west of ireland abstract the triassic sherwood sandstone in the corrib field slyne basin west of ireland on the european continental margin is a dry gas reservoir with a mercia mudstone top seal petrographic analysis combined with x ray diffraction stable isotope fluid inclusion and core analysis have been used to assess timing of growth and origin of mineral cements whether sandstone diagenesis involved mass flux the controls on reservoir quality and how reservoir quality is likely to vary in more deeply buried sections depositional and early diagenetic characteristics of the sherwood in corrib are typical of a semi arid fluvial environment containing groundwater of meteoric origin early diagenesis included the development of copious dolomite cement in the form of dolocrete as well as the formation of abundant clay while less volumetrically important grain coating haematite and k feldspar cement also grew burial diagenesis witnessed the initial growth of minor chlorite and albite quartz overgrowths and ankerite followed and are the most important burial cements growing over a temperature interval between 100 and 1165 c albite commenced growth at about 90 c quartz cement at 100 c and ankerite at 110 c these cements reached the zenith of their development at 105 110 c for albite 125 130 c for quartz and 135 145 c for ankerite siderite and anhydrite are relatively minor late stage cements the formation water has been consistently highly saline during burial and in terms of mineral cement derived oxygen stable isotope values is likely to be a diagenetically evolved version of the initial depositional water suggesting long term stagnation of the pore fluids the diagenetic reactions that formed burial cements all seem to be essentially isochemical including the ankerite that has carbon isotope characteristics of the cannibalisation of dolocrete reservoir quality is mainly controlled by early diagenetic dolomite and clay although grain size is also important because only sandstones with 200 m grain size have 50 md permeability both shallower and deeper sections than corrib may have similar porosity and permeability since temperature dependent diagenetic controls on reservoir quality e g quartz and illite cement are peripheral

distinguishing early groundwater alteration effects from pedogenesis in ancient alluvial basins examples from the palaeogene of southern portugal colour mottling and horizons of secondary carbonates are common in ancient alluvial sequences and are normally interpreted as pedogenic features they have been used to assess palaeoclimates soil drainage conditions and deposition rates palaeogene alluvial deposits in the sado and lisbon basins of portugal exhibit prominent colour variations and mottle patterns as well as carbonate accumulations both at the bases of fining upwards cyclothems and as thick units up to 20 m capping alluvial megasequences however these colour and carbonate features are interpreted as the products of shallow saline reducing groundwaters unrelated to pedogenesis such non pedogenic products are easily mistaken for soil formed ones and criteria for differentiating the two are reviewed to assist interpretations in other alluvial deposits key criteria are thickness gradational tops and bases absence of soil horizon features occurrence in coarser alluvium and prevalence of hydromorphic colour and mottling patterns

monoclinal bending of strata over laccolithic intrusions abstract sedimentary strata on top of some laccolithic intrusions are nearly horizontal and little deformed but are bent into steeply dipping monoclinal flexures over the peripheries of these intrusions this form of bending is not explained by previous theories of laccolithic intrusion which predict either horizontal undeformed strata over the center and faulted strata around the periphery or strata bent continuously into a dome however a slight generalization of these theories accomodates the observed form and contains the previous forms as special cases a critical assumption is that the strength of contacts within a multilayered overburden is overcome locally by layer parallel shear if this strength is less than the strength of the layers themselves then layers over the center remain bonded together and display negligible bending whereas layers over the periphery slip over one another and are readily bent into a monoclinal flexure

evolution of calcrete in palaeodrainages of the lake napperby area central australia abstract carbonate lithofacies developed along the trunk valley of palaeodrainage channels as well as those adjacent to gypsite facies in the lake napperby drainage basin in central australia show wide ranges of sedimentary characteristics reflecting significant variation in depositional environments sedimentological and geochemical characterization of the calcrete lithofacies in this drainage area indicate that whereas variations in bedrock morphology are important in overall distribution of the phreatic and vadose calcrete varieties faulting and basin adjustments since the early quaternary may also have played a significant role in redistribution of the hydrologic zones and hence the evolution of these calcretes accordingly phreatic calcretes which initially precipitated from laterally moving groundwaters along the palaeodrainage channels undergo systematic mineralogical changes due to direct evaporative precipitation and diagenetic displacement replacement processes mainly dolomitization and silicification under present hydro chemical conditions within the zone of grounwater fluctuations however lateral and downward shifts in groundwater flow due to excessive volumetric expansion and reduction in effective permeability of calcrete may ultimately result in isolation of the calcrete like those observed from the eastern margins of the lake the evolution of the vadose calcretes is thereafter topographically controlled and relates to mobilization and redistribution of earlier deposited calcretes under present meteorological conditions