sedimentary dynamics of the kimmeridge clay tempests and earthquakes storms are suggested to be an important influence upon the formation of epicontinental black shales controlling both their distribution and facies types a variety of storm produced event beds are described from the kimmeridge clay of dorset these include horizons of graded rip up clasts silt laminae thin graded mud horizons and shell pavements biostratinomy and population dynamics indicate that the pavements represent brief benthic colonization events in a predominantly anaerobic dysaerobic environment differential near surface lithification has also affected the sediments leading to the formation of flow phenomena these include loop structures microfolding and brecciation earthquake induced fissuring also occurs at a number of horizons the kimmeridge clay shows well developed cyclically alternating organic rich shales and mudstones on a decimetre scale a feedback mechanism of storm induced benthic oxygenation and temperature stratified inhibition of storm mixing may account for the abrupt nature of the cyclicity subsidence and sedimentation rate were probably important factors controlling the thickness and organic richness of the sediments whilst the influence of storms was important in controlling the distribution and facies types of these important source rocks the model proposed for the kimmeridge clay may be of general application to many epicontinental black shales in the geological record

pervasive dolomitization with subsequent hydrothermal alteration in the clarke lake gas field middle devonian slave point formation british columbia canada the clarke lake gas field in british columbia canada is hosted in pervasively dolomitized middle devonian carbonates of the slave point formation the clarke lake field consists mostly of pervasive matrix dolomite and some saddle dolomite the latter varying in volume from about zero in limestones to normally 20 40 locally up to 80 in dolostones over any given 10 m 33 ft core interval some of the saddle dolomite is replacive some is cement and both varieties are associated with dissolution porosity and recrystallized matrix dolomite the major objective of this study is to identify the causes and timing of matrix and saddle dolomite formation specifically whether these dolomites are hydrothermal a comprehensive petrographic and geochemical examination indicates that pervasive matrix dolomitization was accomplished by long distance migration of halite saturated brines during the late devonian to mississippian fluid inclusion homogenization temperatures suggest about 150 uncorrected to 190c corrected at the time of matrix dolomitization these temperatures differ markedly from most published work on the dolomitized devonian reefs in the alberta basin south of the peace river arch where pervasive matrix dolomitization was accomplished by advection of slightly modified seawater at temperatures of about 60 80c and where no hydrothermal influence was ever present the saddle dolomites at clarke lake are not cogenetic with matrix dolomite and are not the product of hydrothermal dolomitization sensu stricto instead they formed through the hydrothermal alteration of matrix dolomite by way of invasion of a gypsum saturated brine during periods of extremely high heat flow and regional plate margin tectonics in the late devonian to mississippian fluid inclusion homogenization temperatures suggest that hydrothermal alteration occurred between 230 uncorrected and 267c corrected which is significantly higher than the maximum temperature of about 190c attained by the slave point formation during burial the sources of the halite and gypsum saturated brines are middle devonian evaporite depositional environments roughly 200 km 124 mi south and or east of clarke lake near the peace river arch

reworked early diagenetic concretions and the bioerosional origin of a regional discontinuity within british jurassic marine mudstones the coinstone is a well known hiatus concretion level in the lower lias lower jurassic upper sinemurian of dorset southern england it has long been recognized as a layer of bored and encrusted early diagenetic clay hosted septarian concretions coincident with a biostratigraphic gap of three ammonite subzones several different types of concretion of variable complexity can be distinguished of which two probably derived from slightly different stratigraphic levels have been juxtaposed by condensation at the erosion surface diagenetic and biological processes occurring before during and after exhumation on the jurassic sea floor can be recognized the relative timing of these events can be distinguished suggesting that initial concretion consolidation the first generation of septarian cracking and the precipitation of the first generation of crack lining calcite preceded exhumation these therefore probably took place at an early stage at shallow burial depths within the accumulating sediment pile the early calcite is brown uv fluorescent and inclusion rich and is similar to the first calcite generations seen in many other clay hosted septarian concretions a generally early diagnetic origin of this material is thus inferred observations on crack textures and geometries and the interactions of the post exhumation fauna of encrusters borers and burrowers lend support to previous suggestions that initial cracking in some septarian concretions took place in a stiff rather than a fully rigid concretion body possibly given coherence by initial growth of some organic substance that was only later replaced by the calcite cements seen in most such concretions today the burrowing activities of a benthic fauna in muds cause resuspension of sediment and facilitate erosion even in the absence of high energy physical processes regional stratigraphic gaps may be formed as a consequence of sea level rises or falls or as a response to sediment supply reduction independent of sea level change such major episodes of biologically mediated erosion in mudstone sequences may be of more general importance than has hitherto been recognized

the continental margin of the mesozoic tethys in the western alps abstract most of the structural units of the western alps were derived from the european continental margin of the ligurian ocean a segment of the mesozoic tethys ocean their mesozoic palaeotectonic and stratigraphic evolution bears witness of the following main stages 1 deposition of the triassic platform carbonates essentially prerift but nevertheless bearing the imprint of some extensional movements 2 the liassic middle jurassic rifting corresponds to the creation of a horst and graben system with especially tilted blocks and hence a shoal and basin palaeogeography but without a preliminary doming event 3 the late jurassic early cretaceous opening and spreading of the ligurian ocean began with a general collapse of the continental margin thermal subsidence latest middle jurassic and earliest late jurassic in this paper emphasis is given to the refting derived structures more especially to the major tilted crustal blocks a few tens of kilometres wide which can be either reconstructed or directly observed the rifting stage lasted roughly 40 m y with the alternation of extensional tectonic phases and of relatively quiet periods during the extensional phases movements along fault planes and related episodes of tilting were followed by sudden and rapid episodes of subsidence the latter may be interpreted as resulting from the isostatic and thermal readjustment that follows a phase of stretching of the crust and of the lithosphere the tectonic evolution of the margin continued but decreased during the late jurassic early cretaceous oceanspreading stage tectonic activity resumed in the late cretaceous this probably resulting from the beginning of contraction of both the ocean and the continental margin leading progressively to the continental collision in the tertiary

faunal response to transgressive regressive cycles example from the jurassic of western india abstract upper callovian to oxfordian shelf sediments at ler in the kachchh basin western india are interpreted in terms of transgressive regressive cycles the transgressive phases are represented by thin layers of reworked and bored concretions sometimes in association with skeletal concentrations the regressive phases are documented by much thicker units of largely fine grained sediments the benthic fauna of transgressive and regressive phases differes markedly and thus mirrors the sedimentary cycles during transgressive phases non sedimentation produced hard substrates colonized mainly by byssate cemented or pedicle attached suspension feeding epifaunal species whilst sediment input during regressive phases lead to soft substrate conditions characterized by infaunal deposit and suspension feeders as well as by epifaunal opportunists whereas the composition of the regressive fauna remains fairly constant through time that of successive transgressive faunas often differs drastically this is possibly due to a greater environmental sensitivity of epifaunal taxa as compared to infaunal taxa

interrelationships between porosity and other geotechnical properties of slowly deposited fine grained marine surface sediments wetzel a 1990 interrelationships between porosity and other geotechnical properties of slowly deposited finegrained marine surface sediments mar geol 92 105 113 the pore volume expressed as porosity or void ratio of fine grained marine deposits at the sediment surface 0 2 cm is closely related to other geotechnical properties such as liquid limit plastic limit compression index and specific surface area equations have been formulated for the interdependencies between 1 average porosity at the sediment surface n and liquid limit ll in 100 0 0378ll 0 43 0 0378ll 1 43 2 liquid limit ll and specific surface area sg ll 1 01 88 46 5 if ll 50 and 3 compression index c and void ratio at a stress of 0 01 mpa eo e o 3 352 exp col from the relationship between geotechnical properties and initial porosity the state of consolidation of surface sediments and the influence of bioturbation often lead to an increase in porosity 5 10 and compressibility up to 10 and a decrease in the overconsolidation ratio from 2 3 to about 1 at the sediment surface the interrelationship between initial porosity and physical properties can be explained by the dependence of the geotechnical properties on the electrostatic forces between the particles and the adsorbed water around them

pore water evolution during sediment burial from isotopic and mineral chemistry of calcite dolomite and siderite concretions abstract coal measures often contain concretions segregations of diagenetic minerals originally formed within unconsolidated sediments three different types calcite pyrite dolomite pyrite and siderite occurring spatially quite close together in the central pennine region of england vary widely in carbon isotope composition 10 35 13c 21 49 and in major cation chemistry ca mg fe mn within some siderite concretions very high mn fe ratios were found in central subsamples these were also most enriched in 13c the fe mg ratio decreases systematically from centre to edge early shallow to deeper later precipitation the calcite pyrite and dolomite pyrite concretions developed completely prior to significant burial both have high mn fe ratios but negative 13c values calcite 21 49 dolomite 8 67 to 10 48 all of these patterns can be equated precisely with theories of pore water evolution developed on the basis of geochemical investigations of modem sediments microbial processes sulphate reduction methanogenesis contributed significantly as did thermal decarboxylation to siderite precipitated at considerable burial depth mn iv and fe iii acted differentially as oxidants producing co2 and increasing alkalinity the interplay of fresh and marine depositional waters is seen most obviously in the presence or absence of sulphate reduction this controlled mineral type iron sulphide or carbonate as well as isotopic and mineral chemistry

microbial processes controls on the shape and composition of carbonate concretions abstract shape size and mineralogical chemical compositions of carbonate concretions vary considerably the questions to be answered are why do they form at all what processes are involved why do they so often form as regular spheroids why do they sometimes form laterally extensive bands what controls whether the mineral composition is calcite dolomite rhodochrosite or siderite or mixtures of them the questions are addressed and the answers illustrated by selected examples carbonate precipitation is the result of the sedimentary system exploiting the potential energy provided by juxtaposing oxidised detrital input with reducing agents organic matter microbes take advantage of the energy source and are believed to be involved in most of the various processes pore waters in steadily accumulated sediments show a depth related succession of chemical and carbon stable isotope compositions this results from degradation of organic matter proceeding via an orderly succession of processes during burial each mediated by a specific microbial group aerobic oxidation using dissolved oxygen occurs first followed by mn iv reduced to mn ii as the oxidant fe iii reduction and sulphate reduction are probably more significant methanogenesis is the final deepest microbial process the common product of all these reactions is co 2 or dissolved hco 3 which can result in carbonate supersaturation the specific process controls the chemical and isotopic compositions of pore water and leads to a characteristic mineralogy of any carbonate precipitated however the processes may not proceed successively and localised micro environments occur recent work on siderite concretions in very young salt marsh sediments norfolk u k has shown that microbial populations are not homogeneously distributed and that these localise and control concretionary growth concretions in ancient rocks can be described in terms of the same processes observed in modern sediments laterally extensive siderite bands in bullhouse quarry westphalian u k resulted from interaction of two sets of complementary processes shallow manganese and iron reduction coupled with deeper methanogenesis and decarboxylation the model is supported by a mass and charge balance calculation similar complementary processes formed spheroidal concretions gammon shale usa but there is evidence that the outer shell of the concretion formed before the inner core and in the absence of a core the shell was crushed by burial compaction the spheroidal form resulted from diffusion of reactive components controlled by the relative rates of the contributory processes calcite or dolomite analogues of the siderite concretions are formed by similar processes but inhibition of carbonate precipitation makes it impossible to use an equilibrium mass balance model kimmeridge bay uk dolomite ledges are marine analogues to the bullhouse examples for the pyrite rimmed calcite jet rock concretions early jurassic yorkshire u k the rate of production of sulphide from sulphate reduction locally exceeded availability of fe ii to precipitate pyrite outward diffusion of sulphide produced the characteristic spheroidal shape of the concretion because of their developmental pattern bedded concretions are more likely to preserve a sequence of diagenetic cements which record the history of pore water evolution however all represent an expression in ancient rocks of microbiological processes similar to those operating today

sea level changes an integrated approach sea level changes an integrated approach in october 1985 sepm sponsored a four day conference entitled sea level changes an integrated approach the purpose of the conference was to provide a forum for an interdisciplinary exchange of ideas on sea level changes and to provide an opportunity for integrating various types of evidence in approaching unresolved issues the conference was successful in bringing together scientists from industry academia and government representing all of the major geosciences disciplines presentations of many new papers plus significant releases of data that were previously held proprietary provided fertile ground for discussion this much cited volume represents the best of the material presented at the conference includes the early vail chart

origin and timing of siderite and calcite concretions in late palaeogene non to marginal marine facies of the te kuiti group new zealand abstract the late eocene earliest miocene te kuiti group represents an overall transgressive sequence of formations ranging upwards from non marine coal measure facies through marginal marine low energy shoreline siliciclastic sediments to fully marine mixed siliciclastic carbonate deposits concretionary structures are common in the lowest formations of the group being dominantly sideritic in the non marine to brackish sediment facies waikato coal measures and glen afton claystone and calcitic in the more marine influenced sediments rotowaro siltstone the geochemistry of the carbonate cements within these early diagenetic concretions records the change from carbonate precipitation from purely meteoric fluids to precipitation from marine fluids and a shifting source of carbonate carbon across this transition siderite precipitation in the waikato coal measures concretions and hardpans was initiated and often largely completed in the methanogenic zone within the upper 20 m of the sediment pile over a period possibly lasting 100 300 ka precipitation continued at a reduced rate to burial depths of 300 m or more when septarian cracking occurred in some siderite concretions and was healed by calcite vein cements that probably derived carbonate from the oxidation of methane ascending from the maturing coal seams directly below siderite precipitation in the concretions and hardpans of the overlying brackish marine glen afton claystone was completed in the topmost few metres of sediment possibly a few 10 ka stable isotope values near zero support a marine carbonate carbon source suggested to be from remobilised shell material as evidenced by the common occurrence of shell casts in the formation and the high degree of mg and ca substitution in the siderites the overlying marginal marine to low energy marine rotowaro siltstone contains calcite concretions precipitated from pore waters enriched in bicarbonate derived from the sulphate reduction zone largely within the upper 8 m of sediment and possibly growing over a few 100 ka concretion development continued at a reduced rate to burial depths of several tens of metres and in some cases septarian cracks developed during the later stages of concretion growth the precipitation of septarian vein calcites with more depleted 18 o values and more enriched 13 c signatures than central concretions cements may be due to increasing burial temperatures and the progressive introduction of carbonate sourced from shell dissolution

chemical model for the origin of minor limestone shale cycles by anaerobic methane oxidation chemical and isotopic studies of jurassic carbonate concretions indicate an origin by anaerobic methane oxidation concretion growth occurred within the top 1 m of sediment in a thin zone where methane was consumed to stimulate a late rejuvenated phase of sulfate reduction similar chemical and isotopic characteristics are shown by diagenetic carbonate in several limestone shale sequences and an origin by anaerobic methane oxidation is proposed for these sequences also the evolution of isolated concretions into diagenetic limestones depends on the extent of and depth variations in carbonate supersaturation arising from anaerobic methane oxidation and the persistence of a reduced sedimentation rate or a depositional hiatus diagenetic limestones formed in this way can often be recognized by their association with a later pyrite phase that is more abundant and has more positive sup 34 s values than the adjacent shales

limestone concretion growth documented by trace fossil relations a concretionary limestone bed and subjacent black laminated shales examined within the upper part of the jurassic posidonienschiefer in the holzmaden area of southern germany contain trace fossils that are derived exclusively from an overlying bioturbated green claystone bed that reflects a short episode of oxygenation the character and systematic distribution of trace fossil assemblages which include chondrites thalassinoides and rhizocorallium provide a record of depth and timing of syngenetic concretionary limestone growth that is more precise than growth histories provided by more traditional physical and geochemical criteria ichnologic evidence indicates that concretion growth began at isolated nuclei at or just below the sediment water interface just prior to or contemporaneous with the onset of the oxygenation episode concretion growth progressed downward and laterally from these nuclei during and after minor compaction but was essentially complete before the termination of the oxygenation event at a depth less than 1 m below the sea floor the temporal association of this concretion growth with the oxygenation episode implies a genetic relation that warrants further investigation of the significance of bottom water oxygenation fluctuations for the formation of other concretionary limestones in the stratigraphic record

dolomite reservoirs porosity evolution and reservoir characteristics systematic analyses of the published record of dolomite reservoirs worldwide reveal that the majority of hydrocarbon producing dolomite reservoirs occurs in 1 peritidal dominated carbonate 2 subtidal carbonate associated with evaporitic tidal flat lagoon 3 subtidal carbonate associated with basinal evaporite and 4 nonevaporitic carbonate sequence associated with topographic high unconformity platform margin buildup or fault fracture reservoir characteristics vary greatly from one dolomite type to another depending upon the original sediment fabric the mechanism by which dolomite was formed and the extent to which early formed dolomite was modified by postdolomitization diagenetic processes e g karstification fracturing and burial corrosion this paper discusses the origin of dolomite porosity and demonstrates the porosity evolution and reservoir characteristics of different dolomite types

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

application of an integrated wave equation datuming scheme to overthrust data a case history from the chinese foothills an integrated wave equation datuming scheme improves the imaging quality of seismic data from overthrust areas it can be regarded as integrated because upward layer replacement is included in this scheme data are downward continued to a nonplanar datum such as the base of the weathering layer followed by upward continuation from the nonplanar datum to a final planar datum using a one way extrapolator when compared with a kirchhoff integral this method can deal better with the strong lateral velocity variation within the near surface after a test on synthetic data the scheme is applied successfully to real 2d overthrust data acquired in the qi lian foothills western china compared with results using static corrections integrated wave equation datuming results lead to better reconstruction of the diffractions and reflections more reliable migration velocity analyses and stronger stack and final depth images

characterizing the shale gas resource potential of devonian mississippian strata in the western canada sedimentary basin application of an integrated formation evaluation devonian mississippian strata in the northwestern region of the western canada sedimentary basin wcsb were investigated for shale gas potential in the subsurface thermally mature strata of the besa river horn river muskwa and fort simpson formations attain thicknesses of more than 1 km 0 6 mi encompassing an area of approximately 125 000 km2 48 300 mi2 and represent an enormous potential gas resource total gas capacity estimates range between 60 and 600 bcf section of particular exploration interest are shales and mudrocks of the horn river formation including the laterally equivalent lower besa river mudrocks muskwa formation and upper besa river formation which yield total organic carbon toc contents of up to 5 7 wt fort simpson shales seldom have toc contents above 1 wt horn river and muskwa formations have excellent shale gas potential in a region between longitudes 122w and 123w and latitudes 59n and 60n national topographic system nts 94o08 to 94o15 in this area which covers an areal extent of 6250 km2 2404 mi2 average toc contents are higher 3 wt as determined by wire line log calibrations and have a stratal thickness of more than 200 m 656 ft gas capacities are estimated to be between 100 and 240 bcf section and possibly greater than 400 tcf gas in place a substantial percentage of the gas capacity is free gas caused by high reservoir temperatures and pressures muskwa shales have adsorbed gas capacities ranging between 0 3 and 0 5 cm3 g 9 6 16 scf t at reservoir temperatures of 60 80c 140 176f whereas besa river mudrocks and shales have low adsorbed gas capacities of less than 0 01 cm3 g 0 32 scf t liard basin region because reservoir temperatures exceed 130c 266f potential free gas capacities range from 1 2 to 9 5 cm3 g 38 4 to 304 scf t when total pore volumes 0 4 6 9 are saturated with gas the mineralogy has a major influence on total gas capacity carbonate rich samples indicative of adjacent carbonate platform and embayment successions commonly have lower organic carbon content and porosity and corresponding lower gas capacity 1 toc and 1 porosity seaward of the carbonate slave point edge muskwa and lower besa river mudrocks can be both silica and toc rich up to 92 quartz and 5 wt toc and most favorable for shale gas reservoir exploration because of possible fracture enhancement of the brittle organic and siliceous rich facies however an inverse relation between silica and porosity in some regions implies that zones with the best propensity for fracture completion may not provide optimal gas capacity and a balance between favorable reservoir characteristics needs to be sought

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

authigenic carbonates derived from oxidized methane vented from the makran accretionary prism off pakistan we report the first discovery and sampling of a methane hydrogen sulfide rich cold seep from the makran accretionary prism off pakistan arabian sea a variety of cm to m scale pockmarks and gas seepage structures were identified from underwater tv photo sled profiles crossing the oxygen minimum zone omz related to highresolution seismic parasound records from the seeps isotopically light bacterially formed methane is emanating that is partly oxidized to hco3 in the bacterial sulfate reduction zone this results in the precipitation of irregular dark gray to black crusts of indurated authigenic carbonates mainly cryptocrystalline magnesian calcite and ca rich dolomite near the sediment seawater interface within the omz downcore the crusts grade into hemipelagic carbonate poor silty clays with transitional lithologies in between similar to the pockmark carbonates from the oregon margin and in the north sea the authigenic carbonates are extremely depleted in 13c 13ccarbonate 40 this suggests that they were derived from bacterial methane 13cmethane 77 8 that was oxidized under anaerobic conditions the authigenic carbonates are associated with white fluffy mats of chemoautotrophic h2s oxidizing bacteria small pockmarks appear to be concentrated at small scale escarpments suggesting focussed fault controlled pore fluid expulsion due to the tectonic dewatering and degassing of the accretionary prism whereas diffuse discharge of pore fluids is inferred from the widespread occurrence of tiny gas bubble tubes

carbonates and relative changes in sea level abstract in the geologic record some of the most accurate gauges of changes in sea level are the sediment type geometry and diagenesis of carbonate shelves and platforms this is because carbonates frequently occur at or very near sea level and are usually less compacted than siliciclastics world wide changes in relative sea level the sum of eustatic sea level changes sedimentation and crustal movements have occurred repeatedly and cyclicly through geologic time producing characteristic responses in carbonates 1 1 relative rises in sea level usually caused by the cumulative effect of tectonic subsidence and eustatic rise may result in the following 1 1 a drowned carbonate reefs or platforms here carbonate growth potential is exceeded by relative sea level rise and is characterized by shallow water sediments overlain by hardgrounds and or deep water sediments some of which may be condensed sequences 1 2 b platforms where only the fast growing rim and patches of the interior are able to match sea level rise while the remainder of the platform is drowned temporarily 1 3 c platforms which keep up and maintain a flat top at sea level and contain shallow water sediments whose thickness at the least matches the height of the sea level rise if terrigenous supply is limited prograding sheets of shelf carbonate occur frequently capped by supratidal evaporites with prograding shelf margin carbonate clinoforms and turbidites if terrigenous supply is high the shelf carbonates encroach on deltaics 2 2 relative drops in sea level caused by crustal uplift or by subsidence being outpaced by a eustatic drop in sea level cause karst and soil development over shelves and platforms deposition of deep water evaporites in adjacent semi enclosed basins and in open marine basins the deposition of deltaic and aeolian clastics that bypassed the shelf falls are accompanied by platform wide fresh water diagenesis during relative sea level rises marine diagenesis is common in the subtidal portions of the shoaling upward carbonates and fresh water diagenesis and dolomitization and sulphate deposition is common in their intertidal and supratidal portions the stratigraphic significance of these reponses to relative sea level change is that many are tied to eustatic events and so are predictable within a basin of deposition

precursor and ambient rock paleothermometry to assess the thermicity of burial dolomitization in the southern cantabrian zone northern spain paleozoic rocks in the cantabrian zone and the variscan foreland fold and thrust belt on the iberian peninsula have been affected by a sequence of diagenetic to epizonal thermal events late to post variscan hot fluid circulation caused a large scale burial dolomitization and ore mineralization mostly in cambrian and a lower to middle carboniferous carbonate sucessions the goal of this study is to analyze and compare the temperatures experienced by the carbonate precursor rocks as well as the under and over lying siliciclastic ambient rocks to gain a better understanding of the thermicity of dolomitization these temperatures are evaluated based on published paleothermal datasets combined with new data obtained from rock eval pyrolysis and vitrinite reflectance analysis of carboniferous rocks rich in organic matter the overall results indicate that reworking of detrital sediments in synorogenic ambient siliciclastics results in an anomalously high thermal maturity recorded by bulk rock techniques such as illite crystallinity and rock eval pyrolysis in situ vr derived paleotemperatures recorded by ambient siliciclastic rocks appear to be higher compared to cai derived temperatures for carbonate precursor rocks this variation in thermal maturity is likely related to the analytical techniques used to obtain cai and vr data and the empirical equations applied to calculate corresponding paleotemperatures conodont fragments were not as sensitive compared to vitrinite and the color alteration process could have suffered from hydrothermal alteration a secondary cause might be a different response to mechanical deformation between siliciclastic and carbonate units during the variscan and post variscan geodynamic evolution of the study area rigid precursor carbonate units experienced fluid circulation mainly along distinct and spaced fracture zones creating fracture related dolomite geobodies and ore mineralization soft ambient siliciclastic rocks experienced more diffuse fluid circulation and heat dissipation the different paleothermometry datasets compiled for the study area indicate that the fluids circulating during late to post variscan times with associated fracture related dolomitization and ore mineralization in carbonate precursors are hydrothermal the highest paleotemperatures were recorded in ambient and precursor rocks in the highly tectonized northern part of the study area where several thrusts and faults allowed intense fluid circulation positive temperature anomalies within the precursor carbonates correlate well with the occurrence of dolomite geobodies and ore mineral deposits such anomalies could thus be used as an exploration tool for hydrothermal dolomite bodies in analog sub surface settings

an oxygen minimum palaeoceanographic signal from early toarcian cavity fills moderately deep marine toarcian sedimentary rocks of the medianes nappe alpine front ranges switzerland comprise calcarenites and lime mudstones they were deposited under storm and fairweather conditions in the dysaerobic zone near the storm wave base these remnants of the northern margin of the tethyan ocean show facies where primary sediment 9deep water9 stromatolitic micrite was reworked during early diagenesis stromatactis like cavities were formed by carbonate dissolution near the sediment water interface suboxic early diagenetic conditions are indicated by corroded pyrite and newly formed glauconitic oncoids carbon and oxygen stable isotope data indicate cavity stabilization and cementation above the sulphate reducing zone oceanographic conditions during the time of cavity development are reconstructed from the carbon and oxygen isotope composition of cavity cements of bulk sediment and of associated ammonites 13 c values of up to 4 are related to the oceanic anoxic event of the early toarcian and oxygen isotope values from cements and ammonites define a shallow toarcian thermocline

3d geological characterization of the hontomin co2 storage site spain multidisciplinary approach from seismic well log and regional data abstract the first spanish technological development plant for co2 storage is currently under development in hontomin spain in a fractured carbonate reservoir the subsurface 3d geological structures of the hontomin site were interpreted using well log and 3d seismic reflection data a shallow low velocity zone affects the wave propagation and decreases the coherency of the underlying seismic reflections deteriorating the quality of the seismic data and thus preventing a straightforward seismic interpretation in order to provide a fully constrained model a geologically supervised interpretation was carried out in particular a conceptual geological model was derived from an exhaustive well logging analysis this conceptual model was then improved throughout a detailed seismic facies analysis on selected seismic sections crossing the seismic wells and in consistency with the regional geology leading to the interpretation of the entire 3d seismic volume this procedure allowed characterizing nine main geological levels and four main fault sets thus the stratigraphic sequence of the area and the geometries of the subsurface structures were defined the resulting depth converted 3d geological model allowed us to estimate a maximum co2 storage capacity of 5 85 mt this work provides a 3d geological model of the hontomin subsurface which is a challenging case study of co2 storage in a complex fractured carbonate reservoir

exploring for dolomitized slave point carbonates in northeastern british columbia in northeastern british columbia a number of significant gas fields have been discovered in dolomitized slave point carbonates located along the edge of the mid devonian barrier reef system reflection seismic techniques have been used extensively in exploring for these fields a common exploration practice has been to detect the reef edge on seismic data and then to drill on isochron thins and structural highs close to this edge the remaining reserves however are proving more difficult to find and this exploration technique does not always guarantee that the slave point formation is dolomitized at this location as with many other complex play types combining geologic concepts with geophysical exploration techniques can improve the explorationist s success ratio in this example a geologic model the dorag dolomitization model provides a possible explanation for the localization of the slave point dolomites in this model dolomitizing solutions are created by mixing fresh phreatic water with sal