the resolving power of cross borehole tomography the data collected in a cross borehole tomography problem is inherently incomplete since the boreholes do not completely surround the media under study the ability of the experiment to resolve structure is therefore limited by poor experimental geometry we use the method of backus and gilbert to investigate the ability of this technique to resolve variations in acoustic velocity and attenuation between the two boreholes both integrals that appear in the backus gilbert formulation can be performed analytically thus expediting the calculations the resolving kernels have significant horizontal width demonstrating that the horizontal position of structures can be only poorly resolved

diagenesis and formation water chemistry of triassic reservoir sandstones from southern tunisia the fluvial triassic reservoir subarkoses and arkoses 2409 5 2519 45 m of the el borma oilfield southern tunisia were subjected to cementation by haematite anatase infiltrated clays kaolinite and k feldspar at shallow burial depths from meteoric waters subsequently basinal brines controlled the diagenetic evolution of the sandstones and resulted initially in the precipitation of quartz overgrowths magnesian siderite minor ferroan magnesite and anhydrite the enrichment of siderite in 12c isotope 13cpdb 14 5 to 9 results from derivation of carbon from the thermal decarboxylation of organic matter during further burial the precipitation of dickite and pervasive transformation of kaolinite into dickite occurred followed by the formation of microcrystalline k feldspar and quartz chlorite and illite prior to the emplacement of oil present day formation waters are na ca cl brines evolved by the evaporation of seawater and water mineral interaction and are in equilibrium with the deep burial 3 1 km minerals these waters are suggested to be derived from the underlying silurian and devonian dolomitic mudstones

triggered slip along the san andreas fault after the 8 july 1986 north palm springs earthquake in addition to minor surface cracks in the region of the 8 july 1986 north palm springs earthquake minor aseismic surficial rupture occurred along three segments of the san andreas fault 44 to 86 km southeast of the epicenter data from a creepmeter and a tiltmeter at one locality suggest that triggered slip occurred coseismically beneath the instruments but took 33 hr to propagate to the surface that slippage occurred coseismically at depth favors mechanisms for triggered slip that involve dynamic or static strain changes rather than creep migrating from the source region the distribution of slip along the san andreas fault associated with the north palm springs earthquake differed significantly from that recorded after the moderate 1968 borrego mountain california and 1979 imperial valley california earthquakes during these earthquakes triggered slip occurred along the san andreas fault in the durmid hill area and in the mecca hills triggered slip associated with the north palm springs earthquake occurred in these two areas again but also extended farther northwest into the indio hills where as much as 9 mm of dextral slip occurred in the mecca hills surface cracks in 1986 appeared over a shorter fault length than in previous events and the dextral displacement was smaller with maximum values of only 2 to 3 mm on durmid hill surface cracks in 1986 were localized along a 200 m long stretch of the fault spanning the mecca beach creepmeter and extending about 150 m to the southeast right lateral displacements on surface cracks in this area were 1 4 to 2 0 mm smaller than those observed in previous events although the mechanism of triggered aseismic slip is poorly understood examination of displacement rates for the past several decades to centuries may indicate whether the aseismic slip rate is constant or represents accelerating premonitory failiure of the southernmost san andreas fault

the critical slip distance for seismic faulting experimentally based friction laws1 2 have been found to predict virtually the entire range of observed behaviour of natural faults3 4 these laws contain a critical slip distance l which plays a key role in determining the degree of fault instability the size of the zone of earthquake nucleation the frictional breakdown width and the proportion of pre and post seismic slip to co seismic slip in laboratory measurements l is found to be about 10 5m but modelling results show that it must be about 10 2m if natural earthquake behaviour is to be simulated the discovery that fault surfaces are fractal over the scale range 10 5 105 refs 5 6 even for faults with large net slip has confused the problem of scaling this parameter from laboratory experiments to natural faults because fractal surfaces have no characteristic length here i show that geometrically unmated fractal surfaces when in contact under a normal load develop a characteristic length in their contact because long wavelength apertures close under load whereas short wavelength apertures may remain open this critical distance may be identified with l and calculations based on fault topography data show that at seismogenic depths it will be in the range anticipated from the earthquake modelling studies

petrology chemistry and diagenesis of calcite concretions in silurian shales from central sweden calcite concretions in the lower silurian rastrites shale from central sweden were formed within 1 m depth of the seafloor micrite 1 4 m was formed initially but recrystallized into microspar 4 50 m and subsequently transformed into pseudospar 50 150 m skeletal calcite in the concretions and in the host shale has been replaced by microspar which is chemically similar to that occurring in the concretion body the 13cpdb values mostly between 2 and 2 suggest that carbonates were mainly derived from seawater some of the carbon within calcite was derived from the oxidation of organic matter recrystallization has resulted in increasingly negative 18 opdb values 7 to 4 3 whereas the 13c was not changed significantly

siderite concretions from nonmarine shales westphalian a of the pennines england controls on their growth and composition back scattered electron microscopy has been used to examine the microstructure of nonmarine shale hosted siderite concretions the concretions are composed of 50 100 micrometer zoned crystallites which exhibit no noticeable center to edge variation within any individual concretion this indicates that siderite crystallites nucleated at virtually the same time across the entire concretion and that the concretions did not grow by radial addition of siderite layers around a central nucleus further siderite precipitation took place by crystal growth onto the nuclei the total proportion of siderite in any part of the concretion bears no simple relationship to the porosity of the enclosing shale at the time of precipitation and growth by passive precipitation in pore space is unlikely integration of microprobe data with bulk mineral chemical and stable isotope data suggests that the siderite crystallites are composed of an fe mn rich end member with a delta 13 c value of approximately 10 per mil and a mg ca rich end member with a delta 13 c value of approximately 0 per mil to 5 per mil the mineral chemical and stable isotope compositions of these concretions resulted from microbially mediated processes operating close 10 m to the sediment water interface during methanogenesis methanogenesis can generate low delta 13 c as well as high delta 13 c carbonate cements hence deep burial diagenetic reactions such as decarboxylation of organic matter need not be invoked to generate solutes for siderite precipitation

isotopic composition of displacive fibrous calcite veins reversals in pore water composition trends during burial diagenesis abstract stable isotope and petrographic analyses of diagenetic calcite cone in cone and beef veins from british jurassic and lower cretaceous shales have been used to determine the environment of precipitation of fibrous calcites successive growth took place by antitaxial displacive addition at the vein margins away from primary sedimentary laminations or early diagenetic concretions carbon and oxygen isotopic ratios 13c 0 and 18 4 to 11 indicate a relatively late dingenetic origin for the veins after tens or probably hundreds of meters of burial and after cessation of bacterial activity nd considerable modification of the oxygen isotopic composition of the pore water vein growth was discontinuous and took place in waters of changing isotopic and trace element composition changes are not unidirectional and are unlikely to result from the simple evolution of a single conhate pore water reversals in isotopic trends indicate that precipitation took place during periods of renewed lateral groundwater flow tapping different sources of bicarbonate bearing solution

carbon oxygen and sulphur isotope variations in concretions from the upper lias of n e england carbon oxygen and sulphur isotope data for transects across two pyrite bearmg carbonate concretions and their host sediments from the upper lias of n e england show symmetrical zonation 13cpdb values of the calcite cement 12 9 to 15 4 indicate that most of it originated from organic matter by bacterial reduction of sulphate augmented with marine and to a lesser extent fermentation derived carbonate organic carbon 13cpdb 26 1 to 37 0 reflects the admixture of allochtho nous terrestrial organic matter with marine material and the selective preservation of isotopically light organic material through microbiological degradation two phases of pyrite are present in each concretion the earlier framboidal pyrite formed throughout the sediment prior to concretionary growth and has 34scd values of 22 to 26 indicating formation by open system sulphate reduction the later euhedral phase is more abundant and reaches values of 2 5 to 5 5 at concretion margins this phase of sulphate reduction provided the carbonate source for concretionary growth and occurred in a partially closed system the 13c and 34s data are consistent with mineralogical and chemical evidence which suggest that both concretions formed close to the sediment surface the 18o values of the calcite in one concretion 18opdb 2 3 to 4 8 indicate precipitation in pore waters whose temperature and isotopic composition was close to that of overlying seawater the other concretion is isotopically much lighter 18opdb 8 9 to 9 9 and large 18o differences between concretions in closely spaced horizons imply that local factors control the isotopic composition of pore waters

formation of siderite mg calcite iron sulphide concretions in intertidal marsh and sandflat sediments north norfolk england concretions cemented mainly by siderite mg calcite and iron monosulphide are common in late holocene marsh and sandflat sediments on parts of the north norfolk coast field experiments have shown that the concretions are actively forming in reduced sediments in which sulphate reducing bacteria are active 13c values ranging from 3 to 11 8 mean 5 9 0 suggest that the carbonate in the concretions is derived partly from marine sources and partly from microbial degradation of organic matter 18o values ranged from 6 4 to 0 8 mean 1 0 suggesting that carbonate precipitated in porewaters ranging from pure sea water to sea water diluted with meteoric water chemical analysis of porewaters showed no evidence of significant sulphate depletion at the depth of concretion formation some concretions have formed around fragments of wood or metal but others contain no apparent nucleus in field experiments siderite fes and mg calcite were precipitated around several different nuclei within a period of six months we suggest that siderite may form wherever the rate of iron reduction exceeds the rate of sulphate reduction such that insufficient dissolved sulphide is available to precipitate all the available dissolved ferrous iron

the hunghae formation se korea miocene debris aprons in a back arc intraslope basin during early to middle miocene times a sudden opening of the ulleung tsushima back arc basin in the east sea sea of japan led to the development of intraslope basins along the rifted southwestern margin southeast korea abrupt subsidence resulted in the deposition of the 200 m thick hunghae formation middle miocene a sand mudstone sequence that can be divided into five facies facies i sand and mudstone couplet and ii coarse sand are turbiditic in origin as evidenced by massive graded crudely layered and parallel laminated sand beds facies iii homogeneous mudstone is characterized by various lignite and plant fragments clastic and biogenic grains that are randomly oriented suggestive of hemipelagic deposition facies iv chaotic deposit is characterized by the disruption of beds the presence of isolated siltstone blocks or balls and large clasts in the muddy matrix indicative of retrogressive rockfall and slide slump facies v conglomerate is of debris flow origin as evidenced by clast and matrix supported features floating large clasts and absence of traction structures individual facies are organized into two types of facies association 1 homogeneous mudstone facies iii associated randomly with the rest facies i ii iv and v indicative of hemipelagic and episodic sediment gravity flow processes respectively 2 conglomerate facies v coarse sand facies ii and sand mudstone couplet facies i representing the flow transformation from debris flow to high and low concentration turbidity currents these facies associations are similar in many respects to modern and ancient debris or slope aprons found elsewhere numerous isolated slide slump blocks wedged conglomerates with armoured mudstone balls discontinuous lignite containing sand mudstone beds chaotic structure and growth faults suggest that the deposition occurred on a steep slope intraslope basin off coalescing fan deltas mainly by unchannellized sediment gravity flows ancient deposits with irregular facies sequences can be viewed as debris apron systems which provide alternatives to submarine fan models in many clastic basins with a line rather than point source

isotope geochemistry of upper permian early diagenetic calcite concretions implications for late permian waters and surface temperatures in continental gondwana abstract this paper presents comprehensive geochemical and isotopic studies on calcite concretions from upper permian freshwater continental deposits that were laid down at 55 s in the interior of southern gondwana the shales hosting the concretions are partially bioturbated and calcareous the concretions are predominantly spheroidal and contain abundant septarian cracks the maximum content of cement carbonate in concretion matrix varies from 65 at the centre to 15 at the edges ostracod remains are exclusively preserved within the concretions septarian calcite occurs in dull and bright bands with three distinct generations of vein fills brown euhedral calcite with saw tooth habit coarse fibrous white calcite and minor fibrous white calcite with sutured termination at the centre of the veins early formed cement both in concretions and septarian veins is magnesium rich whereas later formed carbonates are pure calcite carbon and oxygen isotope ratios pdb of the carbonates are as follows host shales 13 c 4 363 to 0 777 18 o 12 732 to 17 118 concretion cements 13 c 0 256 to 5 378 18 o 9 882 to 9 336 and vein fills 13 c 7 053 to 1 094 18 o 8 280 to 18 241 13 c and 18 o ratios are depleted from the centre of concretions to the periphery as well as from the centre of veins to the tip near surface cementation as suggested by textural evidence and maximum burial temperatures of 50 70 c established from organic matter and clay mineral composition allow independent estimates of surface palaeotemperatures from 18 o ratios 18 o in the range of 12 636 to 8 989 smow is derived for the meteoric palaeowaters from which average annual surface temperatures of 5 8 c is inferred 18 o of early formed cements also yields a mean annual surface temperature of 2 6 c considering the continentality of the palaeolatitudes of northern malawi during the late permian we contend that mean annual surface palaeotemperatures may have been as high as 10 c similar to those found in modern continental temperate climates this is the first independent palaeotemperature data from the interiors of southern gondwana

microbially mediated fossil bearing carbonate concretions and their significance for palaeoenvironmental reconstructions a multi proxy organic and inorganic geochemical appraisal carbonate concretions are widespread within the geological record however the lack of recent known analogues creates a need for novel approaches to unravel the major microbial players involved in concretion formation and establish their suitability as palaeoenvironmental recorders here we used a combination of geochemical and geological techniques to study two pyritiferous calcite concretions and compared the results with their host sediment toarcian posidonia shale 183 ma sw germany the c 13 depleted nature of the concretion bodies with average values of delta c 13 carb 14 8 delta c 13 org 32 4 and delta c 13 n alkanes 34 9 indicates that sulphate reducing bacteria srb played a major role in the concretion growth and preservation of the nucleus via the rapid decomposition of organic matter om however rock eval analyses from both concretions revealed elevated hydrogen indices hi in the body and low hi values at the rim these values suggest that most of the microbial activity did not occur in the concretion body but rather at the rim and at the surface of the nuclei which generally supports the exceptional preservation of om in carbonate concretions furthermore enrichment in euhedral pyrite in the concretion rims suggests they were formed through increased activities of iron reducing fer bacteria coupled to a decrease of srb activity leading towards conditions more favourable to the direct precipitation of pyrite despite low delta c 13 values the known lipid biomarkers such as acyclic extended isoprenoids or 3 beta methyl hopanes did not reveal evidence of an active methane cycling the present study emphasises the crucial role of carbonate concretion in om preservation and highlights their great potential as palaeoenvironmental recorders c 2016 elsevier b v all rights reserved

siderite concretions indicators of early diagenesis in the gammon shale cretaceous the upper cretaceous gammon shale in the northern great plains contains abundant siderite concretions that formed during the early compactional history of the fine grained marine sediments the relative depth of siderite precipitation can be estimated on the basis of textural mineralogical and isotopic evidence siderite concretions that formed at shallow depths in the sediment column contain a large volume percent carbonate 75 85 preserve uncompacted sediment textures and have oxygen isotope ratios similar to those of carbonates precipitated in equilibrium with seawater delta 18 o nearly equal e 0 to 2 per thousand pdb in contrast lower volume percent carbonate values 55 75 and relative decreases in 18 o 16 o values characterize concretions that formed longer after deposition and or at greater depths in the accumulating sediments both volume percent carbonate and delta 18 o values are strongly correlated with independently determined sediment accumulation rates concretions in rapidly deposited sediments formed at shallow depths 200 m later in the burial history the gammon served as both source rock and reservoir for accumulations of biogenic methane and like the siderite concretions the methane was generated during early diagenesis both siderite precipitation and methane accumulation require interstitial pore waters that have sufficiently high bicarbonate activities and that are depleted in dissolved sulfur and free oxygen because of chemical requirements siderite did not precipitate until after the onset of methane generation therefore if the depth of siderite formation is known the minimum depth for the onset of methane accumulation can be inferred the presence of abundant pyrite in slowly accumulated gammon sediments suggests by analogy to data from recent sediments that the depth of siderite precipitation and methane accumulation was controlled by the sedimentation rate in slowly accumulating sediments most organic matter was probably consumed by aerobic organisms and sulfate reducing anaerobic bacteria whereas in areas of more rapid deposition large quantities of organic matter were available for methane generation

assessment of p and s wave energy radiated from very small shear tensile seismic events in a deep south african mine 1 we discuss requirements for reliable estimates of radiated seismic energy in s and p phases and derive ratios of s to p radiated energy es ep of 539 seismic events with local magnitudes 5 23 ml 2 41 subdecimeter size recorded by high frequency acoustic emission ae sensors of the jaguars seismic network in the mponeng deep gold mine south africa the analyzed events are aftershocks of a mw1 9 earthquake and the recording ae sensors are located within about 40 m of the events a shear tensile model is used to simulate the radiation pattern of p and s phases from a family of rupture models ranging from pure shear to pure tensile failure the calculations include correction factors for energy estimates associated with given source receiver geometries and expected focal mechanism with possible tensile component synthetic calculations are used to assess the effects of limited observed frequency band and attenuation on the estimated es ep ratios the model calculations provide guidelines on when different approximations may be used the obtained es ep ratios for the analyzed events are relatively low median value 5 for the full range of model parameters tested suggesting that significant number of the events display a tensile component events with very small ratios e g 1 may reflect enhanced p radiation associated with rock damage in the source volumes

the direct current response of electrically conducting fractures excited by a grounded current source abstracthydraulic fracture stimulation of low permeability reservoir rocks is an established and cross cutting technology for enhancing hydrocarbon production in sedimentary formations and increasing heat exchange in crystalline geothermal systems whereas the primary measure of success is the ability to keep the newly generated fractures sufficiently open long term reservoir management requires a knowledge of the spatial extent morphology and distribution of the fractures knowledge primarily informed by microseismic and ground deformation monitoring to minimize the uncertainty associated with interpreting such data we investigate through numerical simulation the usefulness of direct current dc resistivity data for characterizing subsurface fractures with elevated electrical conductivity by considering a geophysical experiment consisting of a grounded current source deployed in a steel cased borehole in doing so the casing efficiently energizes the fractures with steady current finite element

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

calcite cemented concretions in shallow marine and fluvial sandstones of the birket qarun formation late eocene el faiyum depression egypt field petrographic and geochemical studies implications for formation conditions abstract this work deals with field petrographic stable oxygen and carbon isotopic and elemental composition studies of pervasively calcite cemented concretions within shallow marine and fluvial host sandstones of the upper eocene birket qarun formation of el faiyum depression egypt in order to decipher their conditions of formation the studied concretions have been classified into two types type 1 and type 2 type 1 concretions occur in the fluvial sandstones deposited at the northern scarp of birket qarun some of these concretions are confined within the host sandstones while others appear as erosional remnants of the host sandstones they show oblate spheroidal to flattened ellipsoidal shapes and have variable diameters ranging from 0 5 to 3 m geometrically they are bedding parallel concretions with spacing ranges from 0 3 to 1 m petrographically the rocks of type 1 concretions are attributed to quartzarenite cemented by sparry calcite that exhibits poikilitopic texture these calcite cements are characterized by small ranges of 18o 8 36 to 9 73 pdb and 13c 11 01 to 15 04 pdb values they were originated from light root respired carbon and media of meteoric pore water geometry of these concretions is interpreted to correspond to fluid flow during early diagenetic processes type 2 concretions are recorded in the shallow marine sandstones deposited at gabal el lahun they are confined within the host rocks these concretions exhibit spheroidal to ellipsoidal shapes and range from 0 3 to 1 m in diameter geometrically the individuals of these concretions occur as scattered strata bound and continuously cemented layers microscopically the rocks of type 2 concretions are mainly belonged to quartzarenites cemented by micritic and microsparitic calcites these calcites have small ranges of 18o 3 4 to 4 60 pdb and 13c 0 78 to 1 67 pdb values the calcite cements of type 2 concretions are interpreted to be a result of dissolution of marine carbonate shells at shallow burial depth in slightly modified marine pore water geometry of these concretions was controlled by the spatial distribution of carbonate shells in their host sandstones this study imports an idea that variation of depositional environments of the host clastic rocks of concretions plays an important role in the difference of modes of formation of their concretions also caution should be made to simplify and generalize the conditions of formation of concretions in host clastic rocks deposited in one sedimentary basin of similar age

the internal structure of carbonate concretions in mudrocks a critical evaluation of the conventional concentric model of concretion growth abstract carbonate concretions have traditionally been viewed as forming concentrically through the progressive addition of carbonate to the outer edge of the growing concretion this conventional model of concretion growth is based mainly upon center to edge textural and geochemical trends that are consistent with concentric growth however a number of recent studies of concretions in mudrocks have documented complex zonation that is not compatible with the conventional concentric model in some cases complexly zoned concretions contain late stage cements that precipitated throughout the concretion body none of the center to edge relationships used to support the conventional concentric model exclude more complex modes of origin as concretions that fit the conventional model may be the exception rather than the rule high resolution petrographic analysis of concretions should always be used to characterize internal structure and mode of growth prior to interpretation of geochemical and textural data

on two characteristic frequencies of acceleration spectra patch corner frequency and fmax abstract we examine two issues related to the radiation of tectonic earthquake sources i the complexity of the shape of observed source spectra and ii the effect of recording site characteristics on the cut off frequency f max of acceleration spectra beyond which spectral amplitudes diminish sharply we compute the far field spectra of the 1979 imperial valley and the 1984 morgan hill earthquakes from the slip functions that were inferred from detailed inversion studies of strong motion and teleseismic data of these two california strike slip events these spectra along with data from the 1983 central japan sea earthquake sequence main shock and aftershocks demonstrate the existence of the so called patch corner frequency which is related to the scale length of heterogeneities of the fault plane such a frequency is a distinct feature of the source spectrum of the specific barrier model of papageorgiou and aki by analyzing accelerograms of the san fernando earthquake of 1971 we found that f max determined from records at soil stations is nearly the same as that determined from records at hard rock stations

distinct sedimentary processes reflected in the isotopic signatures of dolomitic concretions in the miocene pohang basin southwestern east sea abstract dolomitic concretions in diatomaceous hemipelagic sediments of the miocene pohang basin in the southwestern east sea sea of japan preserve distinct signals of two independent sedimentary processes which controlled the extents of isotopic compositions variable 18o 9 1 to 0 7 and high 13c 3 1 to 17 9 values suggest that the concretions formed in the methanogenic zone with alteration of the residual mid miocene seawater by volcanogenic sediments remarkable 18o and 13c values show a strong linear relationship indicating that distinctly independent depositional processes operated during the formation of the concretions the degree of methanogenesis was enhanced during rapid hemipelagic sedimentation of organic rich particles resulting in higher 13c values and the effect of volcaniclastics was diluted maintaining the original properties of ambient mid miocene seawater in contrast lower 18o and 87sr 86sr values characterize the effect of volcaniclastic sediments that were transported by intermittent gravity flows and interacted with mid miocene seawater the input of volcaniclastic sediment probably degraded the role of methanogenesis by lowering the contents of organic matter and thereby decreased the 13c values within the concretions isotopic signals recorded within the concretions highlight understanding of the depositional environment and evolution of the pore water chemistry

the diagenetic history of some septarian concretions from the kimmeridge clay england abstract large septarian concretions from the kimmeridge clay up to 1 2 m in diameter have centres comprising anhedral calcite microspar passing into margins of radiating fibrous calcite microspar with a pyrite rich zone at the transition septarian veins formed and were lined with brown calcite synchronously with fibrous matrix growth with white calcite precipitated in septarian cavities after concretion growth ceased septarian veins filled only with white calcite formed later at the same time as the outermost calcite microspar crystals were enlarged the concretions were buried in the late jurassic to about 130 m and in the late cretaceous to about 550 m with uplift between oxygen isotopes show that the concretion grew throughout the first burial with septarian veins forming from about 30 m depth onwards later septarian veins formed between about 200 and 500 m during the second burial carbon isotopes show that the compact inner matrix grew in the sulphate reduction zone the end of which is marked by the pyrite enriched zone dissolving shells and possibly minor methanogenic carbonate slowly diluted sulphate reduction zone carbonate during deeper burial during early concretion growth mg and sr were depleted in the pore water during later stages of the first burial mg sr mn and fe all increased especially after concretion growth ceased during the second burial fe mn and mg decreased as calcite precipitated implying relatively closed systems for these elements synchronous formation of septarian fractures and fibrous calcite matrix shows that the kimmeridge clay became overpressured during the later stages of both burials

fault plane solutions for the 1984 morgan hill california earthquake sequence evidence for the state of stress on the calaveras fault fault plane solutions for 946 aftershocks of the april 24 1984 morgan hill california m6 2 earthquake reveal a pattern of complex faulting within a 10 km wide zone surrounding the calaveras fault the fault plane solutions fall into three groups strike slip mechanisms located along the calaveras fault with north northwest striking dextral slip planes nearly parallel to the fault strike slip mechanisms located northeast of the calaveras fault with north striking dextral slip planes and reverse mechanisms located southwest of the calaveras fault with northeast or southwest dipping slip planes the average azimuth of p axes for aftershocks located on the calaveras fault is n10 1 e in contrast the average azimuths of p axes for aftershocks northeast and southwest of the calaveras fault are n49 7 e and n37 3 e respectively by assuming that the earthquakes occur on preexisting cracks in response to a uniformly oriented regional stress field we are able to infer from the observed average slip directions an orientation for 1 the axis of maximum horizontal compression 63 80 from the local strike of the calaveras fault such a high angle orientation of 1 is incompatible with the classical andersonian model of strike slip faulting but is consistent with an alternate model in which a weak calaveras fault is driven by plate motion slightly convergent with respect to the san andreas fault system in central california if this inferred high angle uniform stress field is combined with the changes in the static stress field calculated with elastic dislocation theory for the 1984 morgan hill main shock the resulting stress field predicts most of the observed spatial pattern of the aftershocks and their fault plane solutions the orientations of the inferred aftershock slip planes are generally consstent with the fracture orientations predicted by the coulomb failure criterion areas with predicted increases in the coulomb failure function of only a few tenths of a megapascal correspond to areas of intense aftershock activity the invariance of focal mechanisms in the vicinity of the south end of the main shock rupture before and after the main shock suggests that the magnitude of compressive stress in this area is at least 10 mpa in addition to explaining a preponderance of the seismic observations of the aftershock sequence the presence of a predominantly fault normal compressive regional stress field accounts for the contemporary development of folds reverse faults and topographic relief observed throughout the central california coast ranges

strong ground motion from the michoacan mexico earthquake the network of strong motion accelerographs in mexico includes instruments that were installed under an international cooperative research program in sites selected for the high potenial of a large earthquake the 19 september 1985 earthquake magnitude 8 1 occurred in a seismic gap where an earthquake was expected as a result there is an excellent descripton of the ground motions that caused the disaster

injection clastic dykes in the lower oxford clay jurassic of central england relationship to compaction and concretion formation an injection dyke of fine grained sandstone derived from the kellaways sand formation intrudes overlying organic rich shales and shell beds of the lower oxford clay the dyke shows cross cutting relationships with early carbonate concretions and fills uncompacted kosmoceratid ammonite shells both within the concretions and surrounding shales internally the dyke displays flow like features and the walls show lobate flow structures clasts of uncompacted lower oxford clay and fragments of pyrite rich concretions occur within the sandstone intrusions the sandstone of the dyke was cemented by calcite identical to that precipitated in septarian cracks in the concretions this cementation took place prior to final compaction of the oxford clay the dyke has a sub parallel relationship to the nearby tinwell marholm fault suggesting that the dyke may be related to local tectonic events during the middle jurassic

inversion based interpretation of borehole sonic measurements using semianalytical spatial sensitivity functions abstractone of the major challenges in the interpretation of sonic logs is the presence of bed boundary shoulder bed effects which can decrease the accuracy of the estimated elastic properties due to spatial averaging caused by the sonic receiver array across thin beds a reliable approach to account for shoulder bed effects in the interpretation of sonic logs is the implementation of forward modeling and inversion techniques we have developed an efficient inversion based interpretation method for the estimation of in situ rock formation elastic properties from sonic logs acquired in vertical wells the method uses semianalytical spatial sensitivity functions for fast forward modeling of sonic slowness logs and estimates formation elastic properties by iteratively matching the available logs with numerical simulations due to the intrinsic vertical resolution limits of sonic logs the inversion method requires predefined bed boundaries using other high resolution well logs we first developed a workfl

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

geochemical constraints on the origins of calcite cements and their impacts on reservoir heterogeneities a case study on tight oil sandstones of the upper triassic yanchang formation southwestern ordos basin china calcite cementation has been identified as an active process in the upper triassic yanchang formation throughout its burial history and as a major diagenetic factor causing strong reservoir heterogeneities the origins of calcite cements and their relevance to reservoir heterogeneities were investigated using a suite of petrographic and geochemical methods including optical microscopy with fluorescence and cathodoluminescence scanning and backscattered electron microscopy with energy dispersive spectrometry x ray diffraction x ray fluorescence electron probe microanalysis quantitative evaluation of minerals by scanning electron microscopy fluid inclusion analysis and carbon and oxygen stable isotope analyses the sandstones are compositionally immature with relatively high amounts of volcanic rock fragments the two generations of calcite cements are ca i and ca ii the ca i calcites are distributed along the interface of sandstone and mudstone units and were formed during the late triassic to early jurassic at formation temperatures of approximately 90 c the ca ii calcite mainly developed in the lower part of the fining upward sandstone units and was formed in the late jurassic at higher temperatures of approximately 110 c the origins of calcite cements were constrained by geochemical and isotope measurements fluid inclusion homogenization temperature and in situ element analysis the ca i calcite cement originated from dissolution of the lacustrine depositional carbonates in the interbedded mudstones and reprecipitation in the adjacent sandstones the ca ii calcite was mainly related to organic matter decarboxylation with ca2 having been provided internally by volcanic fragment alteration and plagioclase dissolution calcite cementation had caused strong reservoir heterogeneities in the yanchang formation tight sandstones the ca i calcite cementation destroyed reservoir properties along the interface of sandstones and mudstones the lower parts of the fining upward sandstone units were tightly cemented by ca ii calcite although they originally had high porosity and permeability the middle upper parts of the fining upward sandstone units contain less calcite cements and thus have better preserved reservoir pores because of oil emplacement inhibiting the calcite cementation processes

corner frequency ratios of p and s waves and strain drops of earthquakes recorded by a tight network around the karadere segment of the north anatolian fault zone evidence for non classical source processes

pore fluid evolution distribution and water rock interactions of carbonate cements in red bed sandstone reservoirs in the dongying depression china the compositions distribution and its interaction with rocks of the evolving pore fluids controls the distribution of carbonate cements and reservoir storage spaces the reservoir quality of the red bed sandstone reservoirs in the dongying depression was investigated by an integrated and systematic analysis including carbonate cement petrology mineralogy carbon and oxygen isotope ratios and fluid inclusions the investigation was also facilitated by probing the mineral origins precipitation mechanisms pore fluid evolution and distribution and water rock interaction of carbonate cements and their influences on reservoir quality diagenetic evolving fluids in the interbedded mudstones are the main source for the precipitation of calcite cements that completely fill the intergranular volume cfiv calcite with heavier oxygen and carbon isotopes the ferro carbonate cements in the reservoir sandstone are enriched in lighter carbon and oxygen isotopes in addition to the cations released by the conversion of clay minerals in reservoirs products of organic acid decarboxylation and the associated feldspar dissolution process provide important sources for such carbonate cementation the carbon isotopes of co2 and the oxygen isotopic composition of fluids equilibrated with the cfiv calcite ferro calcite dolomite and ankerite cements indicate that the pore in the red bed reservoirs experienced high salinity fluids which evolved from the early formed interbedded mudstones through organic acid input and to organic acid decarboxylation pore fluids from nearby mudstones migrated from the edge to the centre of sandbodies causing strong calcite cementation along the sandbody boundaries and forming tight cementation zones pore fluids associated with organic co2 and acids and organic acid decarboxylation are mainly distributed in the internal portion of sandbodies causing feldspar dissolution and precipitation of ferro carbonate cements the distribution of pore fluids caused the zonal distribution of carbonate cements in sandbodies during different periods this may be advantageous to preserve the porosity of reservoirs as exemplified by the distribution of high quality reservoirs in the red bed sandbodies

characterization of fault zones there are currently three major competing views on the essential geometrical mechanical and mathematical nature of faults the standard view is that faults are possibly segmented and heterogeneous euclidean zones in a continuum solid the continuum euclidean view is supported by seismic gravity and electromagnetic imaging studies by successful modeling of observed seismic radiation geodetic data and changes in seismicity patterns by detailed field studies of earthquake rupture zones and exhumed faults and by recent high resolution hypocenter distributions along several faults the second view focuses on granular aspects of fault structures and deformation fields the granular view is supported by observations of rock particles in fault zone gouge by studies of block rotations and the mosaic structure of the lithosphere which includes the overall geometry of plate tectonics by concentration of deformation signals along block boundaries by correlation of seismicity patterns on scales several times larger than those compatible with a continuum framework and by strongly heterogeneous wave propagation effects on the earth s surface the third view is that faults are fractal objects with rough surfaces and branching geometry the fractal view is supported by some statistical analysis of regional hypocenter locations by long range correlation of various measurements in geophysical boreholes by the fact that observed power law statistics of earthquakes are compatible with an underlying scale invariant geometrical structure by geometrical analysis of fault traces at the earth s surface and by measurements of joint and fault surfaces topography

controls on reservoir heterogeneity of tight sand oil reservoirs in upper triassic yanchang formation in longdong area southwest ordos basin china implications for reservoir quality prediction and oil accumulation abstract compared to conventional reservoirs pore structure and diagenetic alterations of unconventional tight sand oil reservoirs are highly heterogeneous the upper triassic yanchang formation is a major tight oil bearing formation in the ordos basin providing an opportunity to study the factors that control reservoir heterogeneity and the heterogeneity of oil accumulation in tight oil sandstones the chang 8 tight oil sandstone in the study area is comprised of fine to medium grained moderately to well sorted lithic arkose and feldspathic litharenite the reservoir quality is extremely heterogeneous due to large heterogeneities in the depositional facies pore structures and diagenetic alterations small throat size is believed to be responsible for the ultra low permeability in tight oil reservoirs most reservoirs with good reservoir quality larger pore throat size lower pore throat radius ratio and well pore connectivity were deposited in high energy environments such as distributary channels and mouth bars for a given depositional facies reservoir quality varies with the bedding structures massive or parallel bedded sandstones are more favorable for the development of porosity and permeability sweet zones for oil charging and accumulation than cross bedded sandstones authigenic chlorite rim cementation and dissolution of unstable detrital grains are two major diagenetic processes that preserve porosity and permeability sweet zones in oil bearing intervals nevertheless chlorite rims cannot effectively preserve porosity permeability when the chlorite content is greater than a threshold value of 7 and compaction played a minor role in porosity destruction in the situation intensive cementation of pore lining chlorites significantly reduces reservoir permeability by obstructing the pore throats and reducing their connectivity stratigraphically sandstones within 1 m from adjacent sandstone mudstone contacts are usually tightly cemented carbonate cement 10 with low porosity and permeability lower than 10 and 0 1 md respectively the carbonate cement most likely originates from external sources probably derived from the surrounding mudstone most late carbonate cements filled the previously dissolved intra feldspar pores and the residual intergranular pores and finally formed the tight reservoirs the petrophysical properties significantly control the fluid flow capability and the oil charging accumulation capability of the chang 8 tight sandstones oil layers usually have oil saturation greater than 40 a pore throat radius of less than 0 4 m is not effective for producible oil to flow and the cut off of porosity and permeability for the net pay are 7 and 0 1 md respectively

growth mechanisms and geochemistry of carbonate concretions from the cambrian wheeler formation utah usa carbonate concretions provide unique records of ancient biogeochemical processes in marine sediments typically they form in organic rich mudstones where a significant fraction of the bicarbonate required for carbonate precipitation is supplied from the decomposition of organic matter in the sediments as a result carbonates that comprise concretions are usually characterized by broad ranges in 13c and include values that are significantly depleted relative to seawater this article reports results from a physical petrographic and geochemical analysis of 238 concretions from the wheeler formation cambrian series 3 utah usa which are unusual in several respects most prominently they formed in organic poor mudstones total organic carbon 0 1 to 0 5 and are characterized by a narrow range of 13c that onlaps the range of contemporaneous seawater values subtle centre to edge trends in 13c demonstrate that concretion precipitation was initiated by local chemical gradients set up by microbial activity in the sediments but was sustained during growth by a large pool of inorganic bicarbonate probably derived from alkaline bottom waters the large inorganic pool appears to have been important in facilitating rapid precipitation of the concretion matrix which occurred via both displacive and replacive carbonate precipitation during early diagenesis stable isotope data from cogenetic pyrite 34s and silica 18o phases provide insight into the evolution of biogeochemical processes during concretion growth and suggest that concretions were formed almost entirely during sulphate reduction with only minor modification thereafter concretions of the wheeler formation appear to represent an end member system of concretion formation in which rapid growth was promoted by ions supplied from sea water as such they offer insight into the spectrum of processes that may influence the growth of carbonate concretions in marine sediments

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

early neogene biochemostratigraphy of pohang basin a paleoceanographic response to the early opening of the sea of japan east sea abstract the location of the pohang basin near the korea strait in the southwest of the sea of japan east sea makes this area appropriate for providing a record of paleoenvironmental biotic changes associated with the early neogene 16 5 ma opening of this gateway to the pacific ocean stable isotopic and planktonic foraminiferal records are presented that assist with the understanding of paleoenvironmental changes in the sea of japan resulting from the opening of the korea strait the oldest sediments of early neogene age of the pohang basin overlie cretaceous basement and are early miocene 16 5 ma shallow estuarine facies containing a benthic foraminiferal assemblage dominated by ammonia beccarii l the oldest early neogene planktonic foraminiferal assemblages in the basin are 16 5 ma in age latest early miocene foraminiferal zone n8 the migration of these planktonic assemblages to the sea of japan at that time appears to have resulted from the initial opening of the korea strait the overlying early neogene marine sequence of the pohang basin extends from 16 5 ma zone n8 through 14 ma zone n10 change in the oxygen isotopic record of the pohang basin sequence suggests strong local paleoenvironmental control related to the early opening of the korea strait early middle miocene isotopic temperatures of planktonic foraminifera are relatively cool at 15 ma at a time when global temperatures were high in the middle low latitude regions it was not until 14 8 ma that isotopic temperatures of planktonic foraminifera increased markedly this distinct warming is inferred to reflect the major intrusion of the kuroshio current into the sea of japan probably as a result of further critical opening and deepening of the korea strait at this time planktonic foraminifera increased in abundance reflecting expanding oceanic influence a cooling that followed at 14 5 ma when the strait was well open is unlikely to reflect local tectonic control on the paleoceanography but global cooling during the early middle miocene associated with the expansion of the east antarctic ice sheet

isotopic constraints on growth conditions of multiphase calcite pyrite barite concretions in carboniferous mudstones carbonate concretions in the lower carboniferous caton shale formation contain diagenetic pyrite calcite and barite in the concretion matrix or in different generations of septarian fissures pyrite was formed by sulphate reduction throughout the sediment before concretionary growth then continued to form mainly in the concretion centres the septarian calcites show a continuous isotopic trend from 1 3 c 28 7 pdb and 1 8 o 1 6 pdb through to 1 3 c 6 9 pdb and 1 8 o 14 6 pdb this trend arises from 1 a carbonate source initially from sulphate reduction to which was added increasing contributions of methanogenic carbonate and 2 burial temperature effects or the addition of isotopically light oxygen from meteoric water the concretionary matrix carbonates must have at least partially predated the earliest septarian cements and thus used the same carbonate sources consequently their isotopic composition 1 3 c 12 0 to 10 1 pdb and 1 8 o 5 7 to 5 6 pdb can only result from mixing a carbonate cement derived from sulphate reduction with cements containing increasing proportions of carbonate from methanogenesis and directly or indirectly also from skeletal carbonate concretionary growth was therefore pervasive with cements being added progressively throughout the concretion body during growth the concretions contain barite in the concretion matrix and in septarian fissures barite in the earlier matrix phase has an isotopic composition 1 4 s 24 8 cdt and 1 8 o 16 4 smow indicating formation from near surface sulphate depleted porewaters barites in the later septarian phase have unusual isotopic compositions 3 4 s 6 to 11 cdt and 1 8 o 8 to 11 smow which require the late addition of isotopically light sulphate to the porewaters either from anoxic sulphide oxidation using ferric iron or from sulphate dissolved in meteoric water carbon isotope and biomarker data indicate that oil trapped within septarian fissures was derived from the maturation of kerogen in the enclosing sediments

in situ stress orientation near the san andreas fault preliminary results to 2 1 km depth from the cajon pass scientific drillhole stress induced wellbore breakouts were analyzed in the cajon pass scientific drillhole 4 3km ne of the san andreas fault to determine the orientation of the principal horizontal stresses using borehole televiewer digitized data in the depth interval 1750 2115 m the mean orientation of the maximum horizontal stress is 073 the scatter in the orientation of breakouts is as much as 37 and variations occur with wavelengths of few hundred meters in this interval reflecting either true changes in stress orientation with depth or the variability of rock anisotropy all the measured orientations however result in left lateral shear stress resolved onto planes parallel to the san andreas fault contrary to its post miocene right lateral slip of 300 km the stress orientation is consistent however with reported left lateral displacement across the cleghorn fault 3 km northeast of and possibly subparallel to the san andreas near cajon pass since the pleistocene the cajon pass stress orientation may reflect the current state of stress across the san andreas fault in which case it should change prior to right lateral slip on this fault or a shallow local perturbation to the san andreas stress field in which case it should rotate couterclockwise with depth determination of the stress orientations at greater depths in the cajon pass drillhole will better sample the near field of the san andreas fault and help resolve this question

evidence for and implications of self healing pulses of slip in earthquake rupture dislocation time histories of models derived from waveforms of seven earthquakes are discussed in each model dislocation rise times the duration of slip for a given point on the fault are found to be short compared to the overall duration of the earthquake 10 however in many crack like numerical models of dynamic rupture the slip duration at a given point is comparable to the overall duration of the rupture i e slip at a given point continues until information is received that the rupture has stopped propagating alternative explanations for the discrepancy between the short slip durations used to model waveforms and the long slip durations inferred from dynamic crack models are 1 the dislocation models are unable to resolve the relatively slow parts of earthquake slip and have seriously underestimated the dislocations for these earthquakes 2 earthquakes are composed of a sequence of small dimension short duration events that are separated by locked regions barriers 3 rupture occurs in a narrow self healing pulse of slip that travels along the fault surface evidence is discussed that suggests that slip durations are indeed short and that the self healing slip pulse model is the most appropriate explanation a qualitative model is presented that produces self healing slip pulses the key feature of the model is the assumption that friction on the fault surface is inversely related to the local slip velocity the model has the following features high static strength of materials kilobar range low static stress drops in the range of tens of bars and relatively low frictional stress during slip less than several hundreds of bars it is suggested that the reason that the average dislocation scales with fault length is because large amplitude slip pulses are difficult to stop and hence tend to propagate large distances this model may explain why seismicity and ambient stress are low along fault segments that have experienced large earthquakes it also qualitatively explains why the recurrence time for large earthquakes may be irregular

isotopic evidence for origin of the moeraki septarian concretions new zealand abstract oxygen isotopic analyses show that the moeraki septarian concretions formed in marine porewaters at shallow depths carbon isotopic analyses indicate that porewaters were influenced by sulfate reduction and methanogenic bacterial action carbon becomes progressively lighter toward the concretion margins at a calculated rate of depletion of 1 to 3 ma for 13cpdb in a two meter concretion trace element and isotopic data demonstrate that the initial septaria form contemporaneously with or soon after the rim of the concretion septarian vein fill of yellow spar calcite and rare very late dolomite are both interpreted as forming in mixed meteoric marine porewaters probably related to the regressive sea level cycles during the tertiary after deposition of the moeraki formation the calculated 18osmow values for the mixed porewater from which the late calcites and dolomite could have precipitated are 5 and 8 respectively assuming a maximum crystallization temperature of 40 c the observed decrease in the mn fe ratio and change in oxygen isotopic values with growth of the yellow spar stage is consistent with a change in the redox conditions and the gradual alteration of the mixed meteoric marine water chemistry

the stress heat flow paradox and thermal results from cajon pass conventional friction models predict a substantial thermal anomaly associated with active traces of strike slip faults but no such anomaly is observed from over 100 heat flow determinations along 1 000 km of the san andreas fault the cajon pass well is being drilled to bring deep heat flow and stress data to bear on this paradox preliminary heat flows to 1 7 km in the neighboring arkoma well indicated a positive anomaly of about 30 as would be expected if frictional resistance of the fault were 50 mpa or so but the pliocene and pleistocene history of the transverse ranges particularly high rates of uplift and erosion could also explain the anomaly preliminary stress results from cajon pass and a new interpretation of regional data by mark d zoback and colleagues suggests that the maximum compressive stress near the fault is almost normal to the trace and hence the resolved shear stress is low and the fault weak although this is consistent with long standing evidence from low heat flow vexing questions remain regarding the physics of strike slip faulting the heat flow data show large variability with depth probably from three dimensional structure and an overall decrease from over 90 mw m in the upper kilometer to less than 80 mw m in the lower 300 m with no evidence of advective heat transfer whether this decreasing trend is significant and whether it continues downward to the background value 70 mw m less the radiogenic heat from overlying rocks as expected for a weak fault must be verified with data from greater depth

revised stratigraphy and reinterpretation of the miocene pohang basinfill se korea sequence development in response to tectonism and eustasy in a back arc basin margin abstract the miocene pohang basin is a pull apart basin formed along the eastern continental margin of korea ecmk during the back arc opening of the east sea sea of japan the basin is filled by more than 1 km thick nonmarine to deep marine strata these strata show extreme vertical and lateral lithofacies changes and have caused decades long controversies on their nature and stratigraphy previous sedimentological studies suggest that the basinfill was deposited by a series of contemporaneously developed depositional systems including fan delta prodelta slope apron and basin plain detailed mapping and magnetotelluric surveying show however that the basinfill is composed of several packages of strata sequences that are bounded by distinct and laterally persistent stratigraphic discontinuities sequence boundaries this suggests that the depositional systems in the pohang basin developed sequentially rather than contemporaneously six packages of strata are identified in the basin a nonmarine to shallow marine transgressive sequence sequence 1 a gilbert type delta conglomerate sequence 2 and alternations of submarine conglomerates and hemipelagic mudstones sequences 3 6 the conglomerates and hemipelagic mudstones of the latter four sequences are interpreted to represent lowstand depositional systems slope apron submarine fan and high gradient delta and condensed intervals respectively compilation of geochronologic paleomagnetic and biostratigraphic data suggests that sequence 1 formed during the gradual subsidence of the ecmk prior to 17 ma whereas sequence 2 formed in response to abrupt downfaulting of the pohang basin at about 17 ma both sequences are interpreted to have developed in response to the early miocene back arc opening tectonism of the east sea on the other hand sequences 3 6 formed between 17 and about 10 5 ma the pohang basin was subject to only minor tectonism during this period and could record global sea level fluctuations we suggest that the four alternations of conglomerates lowstand systems and hemipelagic mudstones condensed intervals resulted most probably from the 3rd order glacioeustatic cycles during the middle miocene this finding implies that the signatures of global sea level fluctuations can be deciphered from a tectonically active sedimentary basin if the timing of regional tectonic development is well constrained and the global sea level chart of haq et al haq b w hardenbol j vail p r 1987 chronology of fluctuating sea levels since the triassic science 235 1156 1167 haq b u hardenbol j vail p r 1988 mesozoic and cenozoic chronostratigraphy and eustatic cycles in wilgus c k hastings b s posamentier h van wagoner j ross c a kendall c g s c eds sea level changes an integrated approach soc econ paleont mineral spec publ 42 pp 71 108 may serve as a guide to basinfill interpretation even in tectonically active sedimentary basins

oxygen isotope composition of diagenetic calcite in organic rich rocks evidence for 18o depletion in marine anaerobic pore water upper cretaceous organic rich carbonates in israel contain benthonic foraminifera and varying amounts of early diagenetic infilling of sparry calcite the sparry calcite has oxygen isotope values down to 9 5 pdb that are significantly lower than those of the coexisting skeletal calcite averaging 2 pdb low 18o values are very common in carbonates that are associated with organic matter these occurrences are conventionally explained as being due to carbonate formation under conditions of low salinity and or elevated temperature on the basis of petrographic data and local geologic evidence however none of the conventional explanations can be applied in this case we advance an alternative model that relates the light oxygen compositions of the diagenetic calcite to depletion of 18o in pore water of normal salinity the proposed operating mechanism is a direct consequence of organic matter decomposition in the sulfate reducing zone being inherent to reducing marine environments this model can be applied to many organic rich rock associations

stable oxygen and carbon isotopes of carbonate concretions of the miocene yeonil group in the pohang basin korea types of concretions and formation condition abstract carbonate concretions in the miocene sedimentary rocks of the yeonil group in the pohang basin korea were investigated in terms of stable oxygen and carbon isotope composition to delineate the origin and associated diagenetic environment for their formation carbonate concretions are widely distributed in all the sedimentary rocks in the pohang basin showing that the calcitic concretions are preserved within the mass flow deposits and the dolomitic ones mostly in the hemipelagic siliceous rocks diatomites concretions can be classified into four different types on the basis of the stable isotopic signatures each of which represents its own geochemical range type i concretions are calcitic and are composed of micrite to microspar they occur in the conglomerates and sandstones which were deposited by mass flows debris flow to turbidity current it shows relatively lower 18 o 14 0 to 9 3 and 13 c 19 6 to 8 4 values these concretions grew in a sulfate reducing zone under the influence of residual ambient seawater which had been significantly modified by volcanogenic sediments type ii concretions are also calcitic composed mostly of micrite with minor microspar and found in the sandstones these concretions are characterized by relatively high 18 o 1 8 to 2 4 and variable 13 c 17 3 to 0 4 values these isotopic signatures reflect that type ii concretions formed from just beneath the sediment water interface down to the sulfate reducing zone through the early stage of methanogenesis type iii concretions are also calcitic and composed largely of micrite with a minor contribution of microspar they are observed in hemipelagic mudrocks which were deposited under the influence of mass flows they are characterized by intermediate to high 18 o 4 6 to 1 6 and high 13 c 1 3 to 8 8 values these concretions grew in a methanogenic zone by residual ambient seawater and or seawater slightly modified by reaction with volcanogenic sediments type iv concretions are dolomite with calcite inclusion and occur in hemipelagic siliceous rocks these concretions are mostly composed of micrite and characterized by variable 18 o 9 1 to 0 7 and high 13 c 3 1 to 17 9 values suggesting formation in the methanogenic zone although the residual ambient seawater is slightly modified by volcanogenic sediments the same type of the concretions is widely distributed throughout the basin and always shows its own distinctive stable isotopic signature this means that the formation of the given type depends upon the lithology and composition of host sediments that are closely related to the depositional process of the fan delta systems regardless of their localities further the different types of concretions are also found at the different but closely spaced stratigraphic levels in the same locality displaying the distinctive diagenetic conditions for each type such preservation of the unique diagenetic signatures in individual type of concretion suggests that the concretions formed in a completely closed diagenetic system therefore caution should be made to simplify and generalize the diagenetic condition for the formation of any concretions in a large sedimentary basin

septarian carbonate concretions in the permian rio do rasto formation birth growth and implications for the early diagenetic history of southwestern gondwana succession abstract between the late carboniferous and early triassic the southwestern gondwana supercontinent was characterized by the development of a huge intracratonic basin a large confined epeiric sea and the accumulation of a transgressive regressive sequence were formed by continuous subsidence related to tectonic effects caused by the sanrafaelic orogeny and the consequent generation of accommodation space the permian rio do rasto formation documents the last progradational cycle related to the complete continentalization of this epeiric sea the basal member of the rio do rasto formation serrinha is believed to have been deposited in a shallow epicontinental water body subjected to storms and influenced by episodic deltaic incursions one of the most remarkable characteristics of the serrinha member is the presence of carbonate concretions hosted in mudstones and very fine sandstones here we combine sedimentological and petrographic descriptions coupled with geochemical and stable carbon and oxygen isotopic data to elucidate the nature of these carbonate concretions the non deformed internal structure decreasing proportion of carbonate cements relative to detrital grains toward the concretion edges core to rim isotopic variations and perhaps most importantly the preservation of a well developed cardhouse fabric support an early diagenetic origin for these structures at shallow burial depths of tens of meters stable isotope analyses of micritic calcite cements and calcites filling the septarian fractures reveal major negative excursions in both 18 o and 13 c values oxygen isotope ratios obtained for the micritic calcite cements vary between 12 1 and 2 6 the calcite filling septarian fractures also exhibit negative values of 18 o 14 2 to 13 8 with an average of 14 the 13 c values of micritic calcite cements range from 5 0 0 2 the carbon isotopic data from the calcite filling septarian fractures are also negative 4 4 to 3 3 the 18 o signatures suggest that the early diagenetic carbonate concretions precipitated in a shallow freshwater environment rather than in a marine setting the 13 c values suggest that the carbon isotopes were derived from a source with slightly depleted 13 c supporting at least a partial organogenic contribution with weak sulfate reduction rates typical of freshwater systems sedimentological analysis shows that the epicontinental water body in which the serrinha member was deposited was constantly supplied by rivers and meteoric waters which suggests that an enormous freshwater basin with restricted marine connections to the panthalassa ocean once existed

early diagenetic concretions associated with intrastratal shrinkage cracks in an upper proterozoic dolomite tasmania australia the upper proterozoic irby siltstone contains an unnamed 127 m thick member predominantly composed of fine grained dolomite and dolomitic mudstone deposited below fair weather wave base in a quiet marine shelf environment storm deposited layers consisting of planar to ripple laminated dolosiltite beds with erosional bases constitute a minor more proximal facies the quiet water sediments contain an abundance of intrastratal shrinkage cracks and small 5 50 mm early diagenetic nodules that in places have coalesced to form essentially continuous diagenetic layering shrinkage cracks appearing as narrow spar filled veinlets subperpendicular to bedding are most abundant in silt free lutites conversely concretion development was favored and shrinkage cracks somewhat inhibited where minor primary silty laminae are present most of the shrinkage cracks appear considerably deformed by sedimentary compaction but many particularly those in concretion rich beds were surrounded by zones of early cementation that developed into tabular bedding subperpendicular concretions that resisted compaction transport of ions to concretion sites was evidently facilitated along the cracks rare small clastic dykes may be sediment filled synaeresis cracks which probably formed by synaeresis in very early diagenesis a phase of tectonism imposed a cleavage steeply inclined to bedding a bulk plane strain and a preferred orientation of the diagenetic structures in the cleavage direction

genesis and distribution pattern of carbonate cements in lacustrine deep water gravity flow sandstone reservoirs in the third member of the shahejie formation in the dongying sag jiyang depression eastern china abstract the lacustrine deep water gravity flow sandstone reservoirs in the third member of the shahejie formation are the main exploration target for hydrocarbons in the dongying sag eastern china carbonate cementation is responsible for much of the porosity and permeability reduction in the lacustrine deep water gravity flow sandstone reservoirs the sandstones are mainly lithic arkose with an average framework composition of q43f33l24 the carbonate cements are dominated by calcite ferroan calcite ankerite and a small amount of dolomite the calcite and ferroan calcite are mainly poikilotopic blocky crystals while the dolomite and ankerite are mainly euhedral rhombohedra crystals filling intergranular and intragranular pores the relatively positive 13c values 2 to 3 9 of the carbonate cements in the sandstone reflect a mainly inorganically sourced carbon from 32 ma to 25 ma the pore water was rich in bicarbonate and ca2 due to carbonate dissolution in mudstone and which were transported with the pore water from mudstone to sandstone via advection and precipitated calcite cementation in thinly bedded sandstones and some high permeability zones in the middle of medium to thick sandstone beds from 12 ma to present abundant ca2 fe3 fe2 mg2 and bicarbonate had been transported from mudstone to sandstone via diffusion to form tight ferroan calcite cementation in the upper and lower parts of the medium to thick bedded sandstones ankerite is mainly distributed in the reservoirs associated with oil migration or charge because change of fe3 to fe2 from oil charge may supply sufficient fe2 for ankerite precipitation the center of sandstone beds 0 6 m is with potential of high quality reservoirs in the research area carbonate cementation appears to be an important factor that controls the accumulation of oil in deep water gravity flow sandstone reservoirs in the study area

constraints on water transport and alteration in the oceanic crust from the isotopic composition of pore water the oxygen isotopic compositions of pore fluids from sediments of older oceanic crust commonly decrease with depth increases in calcium ion concentrations always accompany the 18o 16o decreases these changes are attributed principally to the alteration of basalts or alteration of volcanic ash in the sediments oxygen isotope material balance calculations suggest that 5 20 of the upper kilometer of basalt is altered at the time of subduction of the oceanic crust this corresponds to a flux of 18o into the oceanic crust of 2 3 1015 moles per million years evaluation of the oxygen and hydrogen isotope data from the pore fluids also suggests that the principal process of mass transfer in the sediments of older oceanic crust is diffusion convection of water in the basalts of the oceanic crust appears to continue until subduction

the cajon pass scientific drilling experiment overview of phase 1 the cajon pass scientific drilling project is a broad interdisciplinary experiment involving over two dozen principal scientists phase i of drilling coring and downhole experimentation began dec 8 1986 and ended april 2 1987 with the hole at a depth of 2115 m 82 m of core recovered and a wide range of downhole experiments successfully completed in this paper we briefly outline the scientific motivation for the project and provide an overview of the scientific program we also indicate some of the varied research areas in the earth sciences where data obtained in this experiment will provide unique and important insight into active in situ processes regional geologic structure and rock and fluid composition and origin

controls and evolution of fan delta systems in the miocene pohang basin se korea abstract the miocene pohang basin southeastern korea occurs in the eastern part of the ne sw trending yangsan fault which has experienced right lateral strike slip deformation since the eocene during the miocene however the strike slip movement was negligible and instead was under extensional regime large scale drainage systems were developed at the junction between the strike slip zone and the oblique transfer faults acting as major sources for coarse grained sediments high rate of sediment supply and rapid subsidence of the hanging wall resulted in the progradation of large scale gilbert type fan deltas doumsan duksung and gohyun systems which all show a radial distributional pattern along the steeply inclined footwall scarps created by ne sw trending faults the drainage systems were relatively small here low rate of sediment supply formed scree apron type malgol and yugye systems and steep faced slope type maesan system fan deltas based on variations in sedimentary facies and distribution architecture and overlapping patterns the fan delta systems in the pohang basin can be divided into four stages of evolution during the early stage early miocene alluvial fans prograded into shallow marine setting in the doumsan and duksung systems high rate of sediment supply and rapid rise of relative sea level resulted in the transformation of shoal water type to gilbert type foresets in the malgol and maesan systems rapid sea level rise accompanied with low rate of sediment supply caused transformation of alluvial fans to scree apron type and steep faced slope type fan deltas respectively in the second stage subsidence of the hanging wall formed large scale truncation surfaces in the doumsan and duksung fan deltas coarse grained sediments were continuously supplied in the doumsan system whereas only fine grained sediments were deposited in the duksung system in the malgol system sediment supply was almost terminated in this stage in the third stage middle miocene sediment supply rate of the doumsan fan delta decreased abruptly and the system was overlain by fine grained gilbert type deposits the duksung fan delta was overlain by the maesan and gohyun systems the maesan system prograded further basinward during the last stage late miocene sediment supply of the doumsan and maesan systems was also terminated and the entire basin was draped by fine grained biogenic materials mainly diatoms

origin of low δ18o pre compactional ferroan carbonates in the marine sto formation middle jurassic offshore nw norway siderite delta 18 o pdb 9 3 to 5 7 parts per thousand and fe dolomite ankerite delta 18 o pdb 13 7 to 7 7 parts per thousand occur as texturally early diagenetic pre compactional ce

the source of pedogenic carbonate associated with gold calcrete anomalies in the western gawler craton south australia abstract elevated au concentrations above bedrock mineralization are commonly closely associated with calcrete in arid residual and semi residual soils in australia the origin of australian calcrete has been argued for many years but there have been very few published studies calcrete from au deposits and prospects and elsewhere in the gawler craton south australia were studied for their geochemical composition and sr and c isotope ratios by comparing carbonate 87 sr 86 sr ratios with underlying weathered rock and bedrock it is demonstrated that many samples have overwhelmingly atmospheric sr 94 and by inference ca 98 rather than a local bedrock or soil mineral origin the isotopic composition of calcretes at the challenger gold deposit lie on a mixing trend of decreasingly marine sr from the nullarbor plain to tarcoola 300 km to the east the 13 c pdb values 7 5 to 0 1 suggest that the carbonate c in calcrete has a mixed origin derived from c3 and c4 plants a model is presented to explain how the au ca association may form in calcrete at challenger based on these new sr and c isotope data and in which the role of plants and atmospherically deposited sr and ca combine with pre existing au in the soil the implication for mineral exploration is that since most of the ca is derived from marine sources it is not a pre requisite that autochthonous soil minerals derived from underlying weathered bedrock supply ca for calcrete to form and for the au ca association to occur thus the independence of the ca with respect to underlying lithology reaffirms the usefulness of calcrete as a sampling medium for au exploration

focal mechanisms and the state of stress on the san andreas fault in southern california focal mechanisms have been determined from p wave first motion polarities for 138 small to moderate 2 6 m 4 3 earthquakes that occurred within 10 km of the surface trace of the san andreas fault in southern california between 1978 and 1985 on the basis of these mechanisms the southern san andreas fault has been divided into five segments with different stress regimes earthquakes in the fort tejon segment show oblique reverse sup on east west and northwest striking faults the mojave segment has earthquakes with oblique reverse and right lateral strikesup motion on northwest strikes the san bernardino segment has normal faulting earthquakes on north south striking planes while the banning segment has reverse strike sup and normal faulting events all occurring in the same area the earthquakes in the indio segment show strike slip and oblique normal faulting on northwest to north south striking planes these focal mechanism data have been inverted to determine how the stresses acting on the san andreas fault in southern california vary with position along strike of the fault one of the principal stresses is vertical in all of the regions the vertical stress is the minimum principal stress in fort tejon and mojave the intermediate principal stress in banning and indio and the maximum principal stress in san bernardino the orientations of the horizontal principal stresses also vary between the regions the trend of the maximum horizontal stress rotates over 35 from n15 w at fort tejon to n20 at indio except for the san bernardino segment the trend of the maximum horizontal stress is at a constant angle of about 65 to the local strike of the san andreas fault implying a weak fault the largest change in the present stress state occurs at the end of the rupture zone of the 1857 fort tejon earthquake it appears that the 1857 rupture ended when it propagated into an area of low stress amplitude possibly caused by the 15 angle between the strikes of the san jacinto and san andreas faults the strong correlation between present state of stress and segmentation in previous earthquakes suggests that the stress state is important in controlling rupture propagation

methane derived high mg calcite submarine cement in holocene nodules from the fraser delta british columbia canada carbonate nodules and slabs in late holocene shelly terrigenous deposits of the modern fraser river delta 49 n are formed close to the seafloor by precipitation from saline pore waters of mainly fibrous to bladed crystals of high mg 10 20 mol mgco3 calcite cement as coalescing isopachous crusts on grains previous reports that the cement is low mg calcite are not supported by this study highly negative 13c values of 7 to 59 for the cements indicate that the bulk of their carbonate carbon was derived from the microbiological degradation of organic matter in the deltaic deposits during shallow burial in particular the production of biogenic methane ch4 by anaerobic bacterial fermentation its upward migration chemical or biological oxidation to co2 and neutralization in the near surface sediment and diffusion to microenvironments relatively enriched in organic components are a possible set of conditions influencing the process and sites of carbonate cementation methane derived mg calcite appears also to be the major submarine cement in several other modern occurrences of lithified shallow water terrigenous sands and muds at non tropical latitudes

rupture mechanism and interface separation in foam rubber models of earthquakes a possible solution to the heat flow paradox and the paradox of large overthrusts abstract spontaneous stick slip along the interface between stressed foam rubber blocks is a simple analog of earthquake rupture and stick slip results from this model are used to elucidate the role of normal stress variations along the interface in the stick slip process observations indicate significant normal interface vibrations and separation during slip suggesting that dynamic changes in normal stress rather than a drop in the coefficient of friction may control stick slip as suggested for example by tolstoi oden and martins and brune and co workers observations of particle trajectories indicate that stick slip shear motion is associated with various degrees of fault separation for an asymmetric model the motion is consistent with slipping motion of the type suggested by schallamach and price for a symmetric model the motion is similar to that suggested by comninou and dundurs if interface waves of this type involving separation during slip occur in earthquakes they may be a solution to the heat flow paradox since a major part of the slip occurs during separation and during low normal stress thus frictional heat generation is reduced normal interface vibrations during stick slip may explain the high corner frequency of p wave spectra and the generally high levels of p wave spectra beyond the corner frequency schallamach comninou type waves are consistent with the partial stress drop abrupt locking self healing models of brune and heaton

the mechanical paradox of large overthrusts the mechanical paradox of large overthrusts originates with a fallacious assumption in the conceptual model upon which mechanical analyses of overthrust faulting have been based the notion that the maximum width of a sheet of rock that may be displaced along an overthrust fault can be determined by using the mohr coulomb failure criterion to compare the strength of the rocks with the stress required to overcome the total frictional resistance to slip and the total cohesion or adhesion over the entire fault surface tacitly assumes that slip is initiated and occurs simultaneously over the entire fault surface this unstated and untenable assumption which is incompatible with what is known about real faults is the real explanation for the mechanical paradox a realistic model for overthrust faulting must be compatible with the nature of real overthrust faults and particularly with the way in which displacements including earthquake displacements occur on real faults most large overthrusts are discrete shear surfaces within a coherent mass of rock that is physically continuous around their ends total displacement clearly varies from place to place over these discrete shear surfaces and moreover must be the cumulative result of many relatively small incremental displacements that also varied from place to place over the finite part of the fault surface on which they occurred each increment began as a local shear failure and propagated along the fault as a smeared out somigliana dislocation at a velocity that was small considering the time required for propagation over the total area of the fault during any individual displacement event whether a megathrust earthquake or a creep event only a small part of the fault surface was in relative motion at any one time fault displacements may appear to occur simultaneously across laboratory specimens that are very small relative to the velocity of propagation of displacement because the transit time is so short when the linear scale is expanded over six orders of magnitude to encompass an entire surface of a large overthrust fault however the transit time is much longer because the velocity of propagation is very small relative to the area of the fault surface and the displacement clearly involves only a very small part of the fault surface at any one time the shape and size of overthrust faults result from the propagation of dislocations and this process is controlled by the strength heterogeneity and anisotropy of the rock mass as well as by variations in the regional stress field but not by the frictional resistance to sliding integrated over the entire fault surface

methane derived authigenic carbonates of mid cretaceous age in southern tibet types of carbonate concretions carbon sources and formation processes abstract methane derived authigenic carbonates with distinctive structures and morphologies have been documented worldwide but they are rarely found from ancient strata in the eastern tethys ocean the methane derived authigenic carbonates found in southern tibet are developed in calcareous or silty shales of mid cretaceous age in the xigaze forearc basin and in the tethyan himalaya tectonic zone the morphology mineralogy elemental geochemistry and composition of carbon and oxygen isotopes of these carbonates are studied in detail the carbonates have nodular tubular and tabular morphologies they are primarily composed of carbonate cement that binds and partly replaces host sediment grains host siliciclastic sediments are composed mainly of quartz and plagioclase feldspar a few foraminifers and framboidal or subhedral to euhedral pyrite carbonate cements dominantly are micritic calcite with minor contribution of dolomite nodular concretions are characterized by depleted 13c values commonly ranging from 30 to 5 the 13c values show a gradual decrease from the periphery to the center and the cao sio2 fe2o3 al2o3 k2o and tio2 contents generally show a gradual change these features indicate that the nodular concretions grew from an early formed center toward the periphery and that the carbon source of the nodular concretions was derived from a mixture of methane methanogenic co2 and seawater dissolved inorganic carbon the tubular concretions are characterized by 13c values of 8 85 to 3 47 in the shangba section and 27 37 to 23 85 in the upper gamba section unlike the nodular concretions the tubular concretions show central conduits which are possible pathways of methane rich fluids suggesting that the cementation of tubular concretions begins at the periphery and proceeds inward moreover the tubular concretions show morphological similarity with the methane derived carbonate chimneys pipes and slabs reported in present day cold seep settings we suggest that the carbon source of the tubular concretions was derived from a mixture of seawater dissolved inorganic carbon and oxidized methane formed by released hydrate the tabular concretions are characterized by 13c values of 21 87 to 6 67 in the xiege sections these depleted 13c values suggest that the carbon of the tabular concretions was derived at least in part from aom the tabular concretions are characterized by 13c values of 28 81 to 12 99 in the gamba section according to the 13c values and field observation we infer that their carbon source was more likely to be a mixture of the oxidized methane formed by released hydrate and seawater dissolved inorganic carbon

dynamic stress drop and rupture dynamics of the october 15 1979 imperial valley california earthquake abstract a dynamic faulting model is constructed to determine the dynamic stress drop and relative strength distribution for the october 15 1989 imperial valley earthquake and to study the dynamics of rupture propagation for this event reproduction of the kinematic quantities of the strike slip rate strike slip final offset rupture time and rupture duration bu varying the dynamic parameters of dynamic stress drop and yield strength is attempted the fault plane is discretized in 1 5 km square grids the model incorporates first order corrections for the effect of the free surface and material property heterogeneity on the calculation of rupture at depth the strike slip particle velocities final offsets and rupture durations of the kinematic model are matched within about 40 everywhere below about 5 km except in the hypocentral region and in a high displacement region 20 km north of the epicenter the strike slip rate of the dynamic model is half and rupture duration double those of the kinematic model in the epicentral region the rupture durations and strike slip final offsets are two thirds those of the kinematic model in a high stress drop region beneath the el centro crossing array two slip concentrations correspond to stress drop concentrations one of about 80 bar at 20 km from the epicenter and at 8 km depth and the other of about 70 bar at 30 km from the epicenter and 10 km depth most of the dynamic stress drop occurs between the depths of 9 and 11 km averaging about 60 bar stress drops in the 7 9 km range vary between 25 and 35 bar while stress drops in the 5 7 km depth range vary between 10 and 15 bar this depth dependence of stress drop was a consistent feature across nearly 20 km of the fault stress drop appeared to be well correlated with rock type negative stress drop was found in the sediment regions and the regions at depth while positive stress drop was found in the metasediment regions between the depths of 5 and 11 km yield strength and rupture velocity are inversely related high yield strength is found in regions of low rupture velocity and vice versa rupture was found to terminate at the northern end of the fault by collision with a high strength barrier a region of supersonic rupture velocity was found to occur in a region of high stress drop and low fracture strength the kinematic rupture velocity at the northern end of the fault was found to be unreasonably low results show that this region reaches its peak stress before the kinematic break time yield strength increased with depth in the seismogenetic zone until the base of the zone was reached below which it tapered off slightly

geochemistry and diagenesis of stratabound calcite cement layers within the rannoch formation of the brent group murchison field north viking graben northern north sea abstract up to four calcite cemented horizons doggers form impermeable barriers to fluid flow within the middle jurassic rannoch formation and are correlatable across the murchison field calcite precipitated during early diagenesis within high porosity permeability sandstones at the top of coarsening shoaling upward shoreface cycles calcite 13c and 18o compositions range from 4 1 to 13 4 pdb and 6 6 to 16 7 pdb respectively sr concentrations of up to 1334 ppm are consistent with marine carbonate sources probably shell fragments but no viable intraformational carbonate source has been identified in the murchison field area initial 87sr 86sr compositions 0 71109 0 71266 are higher than middle jurassic seawater 0 7073 and consistent with precipitation from modified porewaters containing significant proportions of continentally derived meteoric fluids enriched in 87sr as a result of basement weathering or percolation through hinterland soils unconsolidated detritus an internal source of 87sr is not considered viable in view of the high proportion up to 25 clastic constituents of unaltered detrital alkali feldspar and mica within the rannoch formation geochemical and isotope data indicate correlations between increasing 18o composition and increasing iron and magnesium content within calcite calcium concentrations decrease with increasing 18o for calcites geochemical data trends can be interpreted differently in terms of either static or evolving 18o porewater models static 18o porewater models using jurassic early cretaceous meteoric water 18 o 6 smow predict cement precipitation temperatures of 17 77 c however 13c compositions are more depleted than those typical of carbon derived from shell debris and correlation between decreasing 18o and decreasing 13c suggests modification of a meteoric derived porewater system during burial via contribution of low 13c carbon derived from a deep basinal source however it is difficult to constrain accurately the degree to which porewater 18o may have evolved during burial the degree to which porewater 18o could have evolved from that of a jurassic early cretaceous meteoric water can be loosely constrained as from 18 o 6 smow to 18 o 2 smow if the temperature at which the most 18o depleted cements precipitated is limited by present day bottom hole temperatures 110 c for wells currently at maximum burial depth

rapid formation of gigantic spherical dolomite concretion in marine sediments abstract spherical carbonate concretions are present in sedimentary strata of varying geological ages worldwide recent studies reveal that calcium carbonate concretions form very rapidly around dead organisms after burial in the seabed however the formation mechanism of spherical dolomite concretions in marine sediments particularly the carbon source and the reason for their spherical shape are still moderately known this study aims to elucidate their formation process and diagenetic evolution through the characterization of the structure mineralogical composition and geochemistry in and around a concretion here detailed studies were conducted on a gigantic dolomite concretion approximately 170 cm in diameter which formed in tuffaceous fine sandstone of the morozaki group in chita peninsula japan the ca and mg distributions in and around the concretion show that it rapidly formed by outward diffusion of bicarbonate from the carbon source in its center the 13c values ranging from 4 4 to 7 5 and large volume of dolomite cement indicate that the dolomite concretion formed at shallow depth from methanogenic organic matter decomposition during rapid sedimentation heulandite occurred only in the surrounding rock matrix comprising altered volcanic glasses because of high temperature during deep burial up to 2 4 km depth this gigantic dolomite concretion properly preserves its evolution and changes in the superimposed post depositional environment this study shows that gaining a better understanding of spherical dolomite concretions can potentially help reveal the burial process of sediments during early diagenesis

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

detection and quantification of three dimensional hydraulic fractures with horizontal borehole resistivity measurements this paper investigates the suitability of low frequency borehole resistivity measurements for detecting and appraising hydraulic fractures the effects of the borehole logging tool s transmitter receiver spacing orientation and operating frequency as well as the fracture s shape cross sectional area dip and electrical conductivity contrast with the embedding formation are quantified using an fast fourier transform fft accelerated integral equation solver simulation results for a logging tool that consists of one transmitter and two receivers in a homogeneous shale formation of 1 3 s m conductivity show the following 1 longer transmitter receiver spacings can differentiate larger fractures while shorter transmitter receiver spacings can detect smaller fractures specifically the tool can differentiate fractures up to 1000 m 2 in area if the receivers are 18 and 19 2 m away from the transmitter but this long spacing configuration cannot detect fractures smaller than 10 m 2 in area and requires the effective electrical conductivity of the fracture to be larger than 100 s m in comparison the tool can detect fractures as small as 1 m 2 in area if the receivers are 1 2 and 1 5 m away from the transmitter even if the effective electrical conductivity of the fracture is only 10 s m but this short spacing configuration cannot distinguish fractures larger than 10 m 2 in area 2 coaxial measurements are sensitive to the fracture cross sectional area but cannot differentiate fractures of the same area with different cross sectional shapes or dips transverse copolarized measurements can discern axially symmetric fractures from asymmetric ones for example in long spacing configuration they can differentiate elliptical fractures with an aspect ratio of 8 from circular or square fractures if their areas are within 1 m 2 to 1000 m 2 cross polarized measurements can quantify fracture dip and become more sensitive as the dip angle increases for example in long spacing configuration they can detect 15 dipping fractures that are larger than 100 m 2 in area but cannot distinguish them while they can detect and distinguish 60 dipping fractures if their areas are in the range of 100 1000 m 2 3 the measurements are mostly insensitive to the operating frequency in the 10 hz 1 khz range in the short spacing configuration and depend moderately on the operating frequency in the long spacing configuration 4 the relative strength of the measured signals is proportional to the fracture effective electrical conductivity when the fracture is more than ten times conductive compared to the embedding rock formation 5 short spacing measurements exhibit higher resolution the tool can differentiate two fractures as long as they are more than 1 m 4 m away from each other in the short spacing long spacing configuration 6 in complex fracture networks not only major branches but intermediate and small branches contribute to the shape and magnitude of measured signals the contributions depend mainly on the area and position of the branches

mudrock hosted carbonate concretions a review of growth mechanisms and their influence on chemical and isotopic composition existing interpretations of cement textures and isotopic compositions may significantly under estimate the depth and duration of concretionary growth minus cement porosities can commonly under estimate depths of concretionary growth for some or all of the following reasons i cements might not passively replace host sediment porosity ii non cement carbonate phases such as replaced bioclastic carbonate can be significant iii sediment compaction models over estimate rates of porosity loss at shallow carbon isotope data from modern sediments show that dissolved carbonate in the methanogenic zone has a continuum of values from 30 to 15 and thus overlaps 13 c depleted values normally considered characteristic of sulphate reduction many concretions previously thought to have grown entirely during sulphate reduction may therefore have continued cementation during methanogenesis indicating a deeper and more prolonged cementation history the necessary carbonate supersaturation for concretionary growth could either occur throughout the porewaters the equilibrium model or be generated in situ by organic matter decay the local equilibrium model or created where external fluids are introduced the fluid mixing model

hydraulic fracturing in situ stress measurements to 2 1 km depth at cajon pass california stress measurements to 2 1 km reveal stress changes with depth that cannot be explained by an elastic response to uniform crustal strain the data at about 1 km depth suggest that the stress is limited by the frictional strength of rock and is perturbed at greater depths by faults which intersect the borehole the stress data indicate that there is little or no right lateral shear stress acting on planes parallel to the san andreas fault

the association of gold with calcrete abstract the discovery that au accumulates in calcrete pedogenic carbonate or caliche was made in 1987 by csiro calcrete is a general term describing accumulation of alkaline earth metals in soils of arid and semi arid terrains around the world the principal constituent of calcrete is calcite while au is a noble metal calcrete has been a significant tool in a number of au deposit discoveries so understanding the mechanisms by which these diametrically different components come together is valuable for enhancing future discovery numerous laboratory experiments case histories and exploration models have been published most from australia yet we do not fully understand the mechanisms involved it is timely therefore twenty five years on since the first publication of this phenomenon to review this highly unusual but economically important association critical to any review on au in calcrete is to first consider calcretes themselves the nature of a particular calcrete where it has formed and mode of formation is relevant to how where and why au accumulates within it this review commences with a background nomenclature history classification and some examples of calcrete types found near au deposits how calcretes form their origins and the role of biota is considered their locations in the regolith and landscape as well as exploration models for au in calcrete are discussed a section on the chemistry of au in calcretes details what we know about possible mechanisms of formation and considers what laboratory experiments on microorganisms and abiotic experiments tell us following on is a summary of practical aspects of identifying collecting and analysing samples for exploration purposes selected mineral exploration case histories are described and how they fit into models of exploration and different regolith settings concluding sections include a summary and implications of this accumulated knowledge to discovering au deposits

depositional environment of the lower cretaceous upper albian winton formation at isisford central west queensland australia inferred from sandstone concretions abstract numerous vertebrate and plant fossils have been found in ex situ sandstone concretions near isisford in central west queensland since the mid 1990s these concretions are found in the lower cretaceous portion upper albian 100 5 102 2 ma of the winton formation the lower most winton formation is thought to have formed in a fluvial channel or flood basin setting proximal to the eromanga sea but due to the scarcity of good exposures the local depositional environment at isisford has not been ascertained minimal compression of vertebrate and plant fossils a lack of grain suturing predominantly cement supported fabric and fractures running through calcite cement as well as fossil bone and framework grains indicates that concretions formed during early diagenesis pre compactional or syndepositional calcite stable isotope 18ovpdb values range from 12 25 to 4 indicating mixed marine and meteoric pore waters and 13cvpdb values range from 5 3 to 4 1 indicative of both sulfate reduction and methanogenesis of organic material including decaying vertebrate soft tissues in the burial environment the mixed marine and freshwater signature suggests a marginal marine setting possibly deltaic or estuarine connected to the regressive epicontinental eromanga seaway at around 102 100 ma this is not inconsistent with the lithology from nearby cores coupled with isisford fossil vertebrate ecology personal observation our research demonstrates the utility of investigating ex situ concretions to refine paleoenvironments at localities where little or no outcrop is available and traditional facies analysis is impractical

bayesian inference gibbs sampler and uncertainty estimation in geophysical inversion the posterior probability density function ppd m d obs of earth model m where d obs are the measured data describes the solution of a geophysical inverse problem when a bayesian inference model is used to describe the problem in many applications the ppd is neither analytically tractable nor easily approximated and simple analytic expressions for the mean and variance of the ppd are not available since the complete description of the ppd is impossible in the highly multi dimensional model space of many geophysical applications several measures such as the highest posterior density regions marginal ppd and several orders of moments are often used to describe the solutions calculation of such quantities requires evaluation of multidimensional integrals a faster alternative to enumeration and blind monte carlo integration is importance sampling which may be useful in several applications thus how to draw samples of m from the ppd becomes an important aspect of geophysical inversion such that importance sampling can be used in the evaluation of these multi dimensional integrals importance sampling can be carried out most efficiently by a gibbs sampler gs we also introduce a method which we called parallel gibbs sampler pgs based on genetic algorithms ga and show numerically that the results from the two samplers are nearly identical we first investigate the performance of enumeration and several sampling based techniques such as a gs pgs and several multiple maximum a posteriori map algorithms for a simple geophysical problem of inversion of resistivity sounding data several non linear optimization methods based on simulated annealing sa ga and some of their variants can be devised which can be made to reach very close to the maximum of the ppd such map estimation algorithms also sample different points in the model space by repeating these map inversions several times it is possible to sample adequately the most significant portion s of the ppd and all these models can be used to construct the marginal ppd mean covariance etc we observe that the gs and pgs results are identical and indistinguishable from the enumeration scheme multiple map algorithms slightly underestimate the posterior variances although the correlation values obtained by all the methods agree very well multiple map estimation required 0 3 of the computational effort of enumeration and 40 of the effort of a gs or pgs for this problem next we apply gs to the inversion of a marine seismic data set to quantify uncertainties in the derived model given the prior distribution determined from several common midpoint gathers