paleo seawater ree compositions and microbial signatures preserved in laminae of lower triassic ooids abstract signals indicative of paleo seawater rare earth element ree composition and microbial activity were identified in marine ooids from the lower triassic of south china using in situ laser ablation inductively coupled plasma mass spectroscopy la icp ms together with petrographic and isotope geochemical techniques quantitative trace element profiles were generated across successive growth laminae in the cortices of selected ooids the shale normalized ree and yttrium y data of these layers show 1 light rare earth element depletion mean la sn yb sn 0 36 0 14 2 positive la anomalies mean la la sn 2 02 1 25 and 3 high ratios of y ho mean 53 12 which are similar to modern seawater compositions and bahamian ooids these findings imply that ooids in the study samples faithfully record early triassic paleo seawater chemical signals such as ree y distributions petrographic observations revealed dark colored layers with intense fluorescence containing structures that resemble microbial filaments or extracellular polymeric substances these layers also possess more enriched ree compositions and higher concentrations of nutrient like elements e g zn and ba than adjacent recrystallized light colored laminae most ooid laminae contain negative ce anomalies indicative of a well oxygenated environment of formation but some laminae have minimal negative anomalies suggesting the influence of suboxic porewater conditions the present study provides a new perspective on the role of microbes in ooid genesis and on the utility of ooids as proxies for paleo seawater chemistry

tomographic imaging by reflected and refracted arrivals at the north sea we discuss some processing steps of a marine 3 d data set from the oseberg field north sea we compare the prestack depth migrated images obtained by the velocity fields provided by different tools velocity spectra reflection tomography and joint tomographic inversion of reflected and refracted arrivals the last ones are definitely better we also produced a synthetic example by modeling the estimated earth structure and the actual recording geometry and we reached similar conclusions the correlation between reflected and refracted signals may be unclear for later arrivals because of their reciprocal interference and multiple reflections we adopt a technique based on a surgical mute in the p domain which allows coupling the signals coming from the same elastic interface

development of cleavage in limestones of a fold thrust belt in eastern new york abstract tectonic cleavage has developed in lower devonian limestones of the hudson valley fold thrust belt in new york state morphology and distribution of cleavage in these limestones is controlled by the amount of clay quartz matrix present and by strain only limestones with greater than 10 clay quartz matrix developed widespread cleavage x ray diffraction analyses indicate that the clay matrix was altered during deformation the width of the 001 illite peak in cleaved lime wackestone is less than that in uncleaved lime wackestone from the same stratigraphic level a minimum of 10 clay is necessary to provide interconnectivity between sites of dissolution and the free fluid system preliminary analyses of bulk chemistry indicate that calcite is removed from the local rock system during cleavage development supporting proposals that circulation of fluid through the rock plays a major role in the development of cleavage textural evidence suggests that both pressure solution and free face dissolution contribute to the removal of ions at grain boundaries cross cutting relations indicate that cleavage was initiated early during the development of the fold thrust belt but continued to develop during the late stages of folding

paleolatitude distribution of phanerozoic marine ooids and cements opdyke b n and wilkinson b h 1990 paleolatitude distribution of phanerozoic marine ooids and cements palaeogeogr palaeoclimatol palaeoecol 78 135 148 data on 493 phanerozoic marine ooid and cement occurrences indicate that the dominance of calcite versus aragonite in tropical marine settings has changed in response to variation in atmospheric co2 and or oceanic temperature gradient holocene ooid and cement precipitation occurs over similar latitudes with means centered around 24 and 28 respectively aragonite and calcite also display roughly comparable distributions with average occurrences between 25 and 28 surface seawater saturation values requisite for ooid cement carbonate precipitation are at least 3 8 rs for aragonite and 3 4 a s for calcite ancient ooid cement occurrences vary in space and time with depositional zones generally closer to the equator during continental emergence greatest extent correlates with periods of maximum transgression aragonite formation is favored in more equatorial localities than calcite when cement ooid distributions are narrow and continents are emergent similarity of latitude distribution of marine ooids cement and biogenic carbonate suggests that physicochemical processes that control levels of carbonate saturation were more important in predicating sites of limestone accumulation in phanerozoic seas than biological processes related to net productivity of various carbonate platform communities continental position and tropical shelf area available for carbonate accumulation dictates the relative abundance of shallow water inorganic carbonate precipitates in space and time

an oscillating trend in phanerozoic non skeletal carbonate mineralogy textural and compositional data on ancient ooids and cements suggest the existence of an oscillating phanerozoic temporal trend in non skeletal carbonate mineralogy the trend shows good correlation with eustatic climatic oscillations proposed by others the most likely mechanism appears to be plate tectonically influenced oscillations in pco2 the vapour pressure of co2 rather than change in oceanic mg ca ratio

perturbations of strain and rotation in a nodular slate abstract strain is estimated at the hinge and in one limb of an anticline by the measurement of phyllosilicate preferred orientation in the matrix of a nodular calcareous slate at three dimensional grid points we calculate an average strain for each and also the average shapes of the calcite cemented nodules which we explain by a strain distinct from that of the matrix acting on originally oblate spheroidal concretions cleavage folia show symptoms of stress solution and the nodular concretions are capped by strain shadows almost free of cleavage the nodules were stiffer than the matrix during most of the deformation history near the concretions the strain is too inhomogeneous at a small scale to allow the resolution of details of the strain field the local strain we take to be the compound of the average and a perturbing strain the strain perturbations are not random but statistically share their intermediate principal axis these axes are less tightly clustered in the hinge than in the limb the axes of local rotations that are concomitant with the perturbing strains also have only a slight preferred orientation in the hinge but they are statistically aligned in the limb there all axes lie near the cleavage great circle and they cluster near the fold axis the regularity of the rotation axes indicates a history of simple flexural shear in the limb with the local rotations caused by retardations or accelerations of the shear rate near stiff nodules comparisons between the mean strain in hinge and limb strained nodule shapes strain perturbations and perturbing rotations allow recognition of several distinct episodes of strain 1 40 compaction 2 9 homogeneous bedding parallel contraction 3 buckling and 4 a further 33 bedding parallel contraction in the hinge and 37 approximately bedding normal contraction in the limb volume loss was greatest near the concretions both during compaction and later by stress solution the limb lost 13 more volume than the hinge

clay fabrics in relation to the burial history of shales quantitative appraisal of compaction strain is essential for the study of the burial history of shales in sedimentary basins the results of a preliminary fabric analysis of westphalian and zechstein shales in the campine basin belgium show that clay fabric analysis using an x ray pole figure goniometer is suitable for this purpose clay fabrics in the range studied are independent of depth and therefore cannot be used as depth indicators this suggests that in the early stages of the burial history a stable clay fabric has to develop which will basically remain unchanged during the subsequent burial history the degree of clay particle preferred orientation not only reflects the compaction strain but is also determined by mineralogical parameters the presence of non platy particles and the relative concentrations of the different clay minerals this degree of preferred orientation furthermore determines the degree of fissility of the shales these mineralogical factors limit the use of clay fabrics as truly quantitative strain markers their use as semi quantitative strain markers remains advantageous mainly because of the common occurrence of clay fabrics in the geological record moreover the relative ease of measurement and the possibility of distinguishing compaction from tectonic strains favour the use of clay fabrics in the quantitative strain analysis of argillaceous rocks

microbial mats as bioreactors populations processes and products microbial mats are dynamic and complex ecosystems exhibiting spatial and temporal heterogeneity the physical chemical environment is typified by steep gradients and distinct microenvironments these microenvironments support a great diversity of species with a wide range of metabolic processes these processes often result in coupled reactions and biogeochemical cycles and produce important end products such as trace gases and mineral precipitates the latter can impact the composition and character of the sediment imparting a biosignature these biosignatures can be preserved in the rock record and are useful in the interpretation of fossil record on earth and possibly as an indication of life on other planetary bodies the modern marine stromatolites of the exuma cays bahamas provide an ideal system for studying the populations processes and products in a microbial ecosystem using a multidisciplinary approach in order to acquire redox energy microbial populations need to carry out metabolic reactions at rates faster than the equivalent chemical abiotic reactions as such microbes can be viewed as bioreactors that preferably oxidize carbon to co2 to maximize the energy yield the study of the microbial role in carbonate sedimentation and lithification in these stromatolites provides a picture of microbial mats as bioreactors producing a biosignature

strain factorization by identical finite increments and its use abstract progressive deformation of layered rocks up to their present finite strain and bedding attitudes is simulated by an identical increment path model a deformation matrix connecting the initial and deformed states is formulated with the conceptual help of a hypothetical three step deformation path two sequential non coaxial rigid body rotations bring an original horizontal bed to a pre strain attitude a pure shear follows the rotations thus the deformation matrix is calculated by premultiplying the matrices representing the three stages of deformation sixteen identical tectonic deformation increments are then obtained by four times sequentially extracting the square root of that deformation matrix the method has been applied to the campbell block a late jurassic fault bounded turbidite sliver in the northern sierra nevada california the block was deformed into a train of folds during the nevadan orogeny fourteen oriented samples were collected for march strain measurements based on the strains and bedding attitudes the block was divided into 11 deformation bands of three types by applying the proposed model to the campbell block it was found that the deformation style of the block consists of a non coaxial deformation path the regional strain is accommodated among the several deformation bands by strain partitioning the collective deformation indicated by this model is transpressional

geochemical evidence for widespread euxinia in the later cambrian ocean it has been suggested that the cambrian ocean was oxygen deficient but physical evidence for widespread anoxia has been lacking gill et al present sulphur isotope data from cambrian rocks at six different locations around the world and find a positive sulphur isotope excursion in phase with a large excursion in the marine carbon isotope record which is thought to be indicative of a global carbon cycle perturbation at the time a prolonged period of anoxia during the cambrian may explain the previously enigmatic peculiarities seen in the fossil record it has been suggested that the cambrian ocean was oxygen deficient but physical evidence for widespread anoxia has been lacking now sulphur isotope data from cambrian rocks at six different locations around the world is presented with the finding of a positive sulphur isotope excursion in phase with a large excursion in the marine carbon isotope record which is thought to be indicative of a global carbon cycle perturbation at the time with the help of a box model these isotope shifts support the idea of large scale anoxic and sulphidic conditions in the later cambrian ocean widespread anoxia in the ocean is frequently invoked as a primary driver of mass extinction as well as a long term inhibitor of evolutionary radiation on early earth in recent biogeochemical studies it has been hypothesized that oxygen deficiency was widespread in subsurface water masses of later cambrian oceans1 2 possibly influencing evolutionary events during this time1 2 3 physical evidence of widespread anoxia in cambrian oceans has remained elusive and thus its potential relationship to the palaeontological record remains largely unexplored here we present sulphur isotope records from six globally distributed stratigraphic sections of later cambrian marine rocks about 499 million years old we find a positive sulphur isotope excursion in phase with the steptoean positive carbon isotope excursion spice a large and rapid excursion in the marine carbon isotope record which is thought to be indicative of a global carbon cycle perturbation4 5 numerical box modelling of the paired carbon sulphur isotope data indicates that these isotope shifts reflect transient increases in the burial of organic carbon and pyrite sulphur in sediments deposited under large scale anoxic and sulphidic euxinic conditions independently molybdenum abundances in a coeval black shale point convincingly to the transient spread of anoxia these results identify the spice interval as the best characterized ocean anoxic event in the pre mesozoic ocean and an extreme example of oxygen deficiency in the later cambrian ocean thus a redox structure similar to those in proterozoic oceans6 7 8 may have persisted or returned in the oceans of the early phanerozoic eon indeed the environmental challenges presented by widespread anoxia may have been a prevalent if not dominant influence on animal evolution in cambrian oceans

a comparison of the strain of crinoid columnals with that of their enclosing silty and shaly matrix on the appalachian plateau new york abstract a pole figure goniometer was used to measure the preferred orientation of the basal planes of chlorite in 87 samples from the upper devonian clastic rocks of the appalachian plateau new york interpretation of the preferred orientation according to the theory of march yields an estimate of how much an element of the sediment has changed shape since deposition this deformation can be interpreted according to three distinct models for the strain history all begin with compaction followed by one of three types of tectonic strain 1 a plane strain that conserves bedding plane area by compensating for horizontal shortening with horizontal elongation at right angles 2 one that uniaxially shortens a horizontal line normal to the fold axes without compensating elongation and 3 a plane strain again which conserves the area in the vertical plane containing a shortened horizontal direction by stretching it vertically for lack of suitable names we will simply call the three total strains type b u and v from the initials of the distinguishing features of their tectonic increment the horizontal strain components estimated from preferred orientation are compared at a number of localities with those implicit in the principal horizontal diameters of the elliptical outlines of deformed crinoid columnals found on bedding planes expressed for constant bedding plane area or type b total strain the strain in the direction of greatest elongation measured by preferred orientation was found to range from no recorded strain to 0 19 with a median of 0 04 as a rule however the more intensive strains were recorded in shales with a fine grained phyllosilicate content exceeding 85 by volume in which the mean was 0 07 deformed crinoid columnals in the same set of samples have elongations parallel to their long axes that range from 0 04 to 0 10 with a median and average of 0 07 the discrepancy between the two sets of strain measures at least in the coarser and more permeable rock types may either be real and caused by easier flushing of dissolved ions or may be caused by imperfect strain recording in coarse matrix materials

new distribution coefficient for the incorporation of strontium into dolomite and its implications for the formation of ancient dolomites late tertiary dolomites form the bahamas exhibit a strong inverse correlation between sr and mol mgco sub 3 it is proposed that this relation is a result of a distribution coefficient dependent on the concentration of ca in dolomite because these dolomites precipitated from water with a sr sup 2 ca sup 2 ratio similar to that of seawater a distribution coefficient d sub sr sup dolomite of 0 0118 is calculated for sr in stoichiometric dolomite this coefficient increases by about 0 0039 for every additional mol caco sub 3 in nonstoichiometric dolomite these data have three important implication first the new d sub sr sup dolomite is substantially lower than previously suggested second a relation has been established among the sr sup 2 ca sup 2 ratio of dolomite forming waters the major element composition of dolomites and the sr content of dolomites allowing the determination of the sr sup 2 ca sup 2 ratio of fluids involved in the formation of ancient dolomites third some published data sets on dolomites show a similar relation between sr content and major element composition suggesting that these dolomites may also have formed from seawater other data sets deviate from the correlation defined by little bahama bank dolomites indicating either dolomitization in fluids other than seawater or changes ofmore the sr sup 2 ca sup 2 ratio of seawater through time less

mapping bathymetry and depositional facies on great bahama bank satellite imagery and an extensive set of water depth measurements have been used to map and critically evaluate the magnitude and patterns of bathymetry across great bahama bank descriptions of previously collected sediment samples were combined with satellite imagery to map and refine the interpreted distribution of surficial carbonate sediments depositional facies data reveal that 60 of great bahama bank lies in 5 m or less of water the deep portion occurs mainly in a generally east west trending area in the southern portion of the platform the re evaluation of the facies reveals that great bahama bank is essentially a very grainy platform with muddier accumulations primarily in the lee of andros island this area of great bahama bank also experiences currents related to an excursion of the florida current onto the platform top possibly enhancing sediment mud production through the generation of whitings sediment equivalents to mudstones wackestones and mud rich packstones cover 8 5 and 14 respectively of the platform top whereas sediment equivalents to mud poor packstones grainstones and rudstones account for 20 45 and 3 of the surface area boundstones reefs were not specifically mapped in this study due to the resolution of the mapping there is a poor relationship between the occurrence of the depositional texture and water depth in that the grainier sediment types are abundant across the full range of water depths the most abrupt lateral facies changes portrayed on the facies maps are observed leeward of islands areas which also hold the highest diversity in facies type the majority of the islands on the platform align with the north west south east strike of the platform margin and these islands in turn exert control on the shape and orientation of facies belts that develop in proximity to them for this reason regions of the platform that contain principal islands host facies belts that align with the principal axis of the platform whereas for regions lacking islands the facies belts adopt an east west trend consistent with prevailing winds and currents there is a clear trend that the wide southern portion of the platform hosts the most continuous expanses of grain rich sediments

calcite and aragonite precipitation from seawater solutions of various salinities precipitation rates and overgrowth compositions abstract the reaction rate and composition of calcite and aragonite overgrowths precipitated from seawater solutions of various salinities i e s 5 15 25 35 44 were determined at 25 c and 10 2 5 atm co2 partial pressure using a constant disequilibrium seeded technique the rate data were fitted to an empirical rate law of the form log r n c or a 1 log k where n is the empirical reaction order and k is the rate constant calcite precipitation rates in seawater solutions do not vary appreciably as a result of salinity variations over the range investigated while those for aragonite show an increase in going from the higher i e s 35 44 to the lower i e s 5 15 25 salinity range this study also confirms previously published findings that above a given saturation state i e c 2 6 aragonite precipitates more rapidly than calcite at 25 c the incorporation of sr2 in aragonite and mg2 in calcite overgrowths are independent of the precipitation rate the partition coefficient of sr2 in aragonite is approximately equal to unity and is unaffected by salinity variations between 5 and 44 however the mg2 partition coefficient in calcite increases with decreasing salinity of the parent seawater solutions possibly as a result of variations in the sulfate content of the solutions and solids the experimental results were discussed in the context of a number of geological environments

microbes and ooids abstract the bahamian archipelago is one of the few locations in the world where ooid formation is actively occurring ooid cortices from six locations in the region were incrementally dissolved and analyzed for 14c 18o 13c mg ca and sr ca ratios ooids were examined under sem after each step in the incremental analyses to characterize the nature of dissolution radiocarbon dating indicates that ooids presently at the sediment water interface began forming across the archipelago between 1000 and 2800 yr bp and continue to form today the ooids have the same pattern of microboring alteration across the region the surface and outer cortex of the ooids are punctuated with unfilled microborings whereas the inner cortex contains two morphologies of aragonite cement filling the microborings the two morphologies of cement form in association with two different species of cyanobacteria one is solentia sp the other is interpreted to be hyella sp the chemistry of ooids from across the region is remarkably similar 18o and 13c values for all samples vary directly having a slope of approximately 1 the outer cortex has low 18o and 13c values of 3 4 and 0 2 respectively whereas the 18o and 13c values of the inner cortex are high with values of 1 9 and 6 8 respectively the presence of aragonite cement in microborings in the inner cortex increases the overall isotopic composition of both oxygen and carbon in the ooid causing it to appear close to equilibrium with seawater the isotopic variation in 18o and 13c within the cortex can be characterized as a mixing line between the low values in the unaltered ooid laminae and the aragonite cement in the microborings the most exterior portion of the ooid has very high mg ca values and is interpreted as an amorphous calcium carbonate acc coating there are two other phases in the cortex both being aragonite the outer cortex has a higher mg ca ratio and lower sr ca ratio than the inner cortex this difference in chemistry is a result of the presence of aragonite cement in the inner cortex stable isotopic and trace element results coupled with sem investigations indicate that microbes do not play a role in ooid formation but instead alter the texture and chemistry of ooids after they have formed a new model of ooid formation is proposed whereby a veneer of acc precipitates on an ooid while it is at the sediment water interface the active phase this veneer of acc later recrystallizes to aragonite needles and a new cortex layer is formed the stationary phase observations from this study lead to a deeper understanding of the chemical processes involved in ooid genesis which allow a better understanding of paleoenvironments hosting ooid formation

hydraulic control of pore water geochemistry within the oxic suboxic zone of a permeable sediment the geochemical behavior of the top 70 cm of permeable reef flat sediments on checker reef oahu hawai i was examined using spatial and temporal changes in pore water o2 no no nh and n2o concentrations 221 324 the shallow depth of the sediment water interface relative to the height of waves propagating across the reef creates an environment in which pore water mixing and transport are dominated by wave induced mechanisms dissolved oxygen penetrated 15 50 cm in the four sites studied the depth integrated concentration of dissolved oxygen was greater under larger wave conditions and in more permeable sediments which suggests that dissolved oxygen may be a good indicator of the relationship between sediment metabolism and the physical nature of the sediment environment the absence of any subsurface n2o maxima limited temporal variability in pore water n2o concentrations and vertically coherent profiles of o2 no no and n2o suggest that suboxic processes are typically 22 32 restricted to spatial scales that are smaller than for oxic processes in these sediments the geochemical response of the sediments to a natural perturbation was monitored by examining the composition of pore waters immediately following a large wave event and for several weeks thereafter significant changes in the depth integrated concentrations of o2 no no and nh were observed following the perturbation however only changes in no 22 1

geochemical evidence of microbial activity within ooids ooid formation remains elusive despite their importance as palaeoclimatic indicators and important contributors to global carbonate budget based on stable isotopes nutrient and elemental analyses on solid components and ooidal leachates this study supports the notion of microbial involvement in the development of ooids from great bahama bank carbon and nitrogen isotopic analyses on organic fractions identified geochemical signatures of microbial activity the 13c values for organic carbon in the bulk 11 9 to 16 9 intercrystalline intracrystalline 11 9 to 16 7 and intracrystalline phases 12 4 to 17 7 were similar and except for the more enriched values of ooids from butterfly beach were within the range of photosynthesisers the 15n values for the bulk 0 5 to 0 2 intercrystalline intracrystalline 0 3 to 0 7 and intracrystalline organic matter 0 3 to 1 7 showed a narrow range consistent with nitrogen fixation while positive 15n and 18o values of the leached from the ooids provided evidence of denitrification the carbonate associated sulphate 34scas of the bulk sediments 19 2 to 19 6 and 34s of the leachates 16 6 to 18 3 provided weak indication of sulphate reduction suggesting either that high concentrations of isotopically enriched s are overriding bio signatures of sulphate reduction or that microbes are preferentially using as an electron acceptor in contrast the elevated sulphate concentrations of the leachates suggest the occurrence of microbial sulphide oxidation within ooids the high mg ca of the leachates and scanning electron microscope analyses provide putative evidence of amorphous calcium carbonate and a formative role in caco3 precipitation together these findings indicate that a redox dependent microbial consortium may influence caco3 precipitation in the form of ooid accretion cementation and micritization it is also inferred that ooid deposits are not suitable indicators of palaeoclimate because ooids are affected throughout their life by a complex chain of abiotic and biological processes which can lead to large geochemical offsets

global warming and the end permian extinction event proxy and modeling perspectives abstract the mass extinction event that occurred at the close of the permian period 252 million years ago represents the most severe biodiversity loss in the ocean of the phanerozoic the links between the global carbon cycle climate change and mass extinction are complex and involve a whole range of often inter related geochemical biological ecologic and climatic factors it has become widely accepted that the end permian mass extinction was associated with a global warming event because the age of the siberian trap eruption a potentially massive source of carbon dioxide coincides within error with the extinction event however geologic data that are in support of this global warming event are relatively sparse the chain of events and the causal relationship between the eruption of siberian traps and mass extinction is not well established global warming in particular has only been reported from limited proxy data and climate models for which the pco2 in the atmosphere just before and during the end permian extinction event is poorly known this study critically assesses both the proxy climate data and the existing paleoclimate simulations with the goals of assessing our current understanding of the link between mass extinction and climate change and providing some guidance for future studies proxies indicate that prior to the end permian extinction event tropical sea surface temperatures ranged from 22 to 25 c and possible pco2 values ranged from 500 to 4000 ppm before the extinction event during the peak extinction tropical temperatures rose up to 30 c while pco2 perhaps increased up to 8000 ppm climate models that use different pre event pco2 values show similar amount of co2 doubling to replicate the observed carbon isotope excursions we find that the temperature anomaly during the end permian extinction is consistent with 1 5 doublings of atmospheric pco2 and that the implied climate sensitivity is 5 6 c within the upper range of values produced by climate models

appalachian stress study 2 analysis of devonian shale core some implications for the nature of contemporary stress variations and alleghanian deformation in devonian rocks detailed stress measurements in three boreholes penetrating horizontally bedded devonian silt stones sandstones and limestones above a prominent salt decollement in the appalachian plateau of western new york have revealed variations in horizontal stress magnitudes which correlate with lithologic and stratigraphic units in all wells high differential stress levels up to 20 mpa were found in shales of very high clay content contrary to the proposition that such materials have negligible long term strength elastic modulus data show that stiffer beds generally host higher stress levels and suggest that sand shale stress contrasts result in large part from elastic shortening of the section in response to regional ene compression no correlation between stress and poisson s ratio was found however a major systematic drop in stress level within the generally massive shales which occurs across a group of sand beds near the base of the rhinestreet formation appears to be of different origin the stress offset corresponds to the top of a section which we conclude on the basis of local and regional total strain data derived from chlorite fabric measurements once hosted abnormally high pore pressures the total strain data also suggest the entire section above the salt has been uniformly shortened during alleghanian compression to explain the stress discontinuity two kinematic patterns for alleghanian deformation of the devonian section are proposed both involving abnormal pore pressure development in the sub rhinestreet section in response to limited drainage of fluid drainage of this paleo overpressure is the best available explanation of the stress offset although an additional remnant stress component must also be present to satisfy the stress data precisely

a linear model for velocity anomalies variations of seismic interval velocities within the cable length cause anomalies in the stacking velocity analyses utilizing the approximation of rectilinear ray propagation i e supposing that the velocity changes cause time delays only it is shown that the stacking velocity anomalies are linearly related to the interval velocity variations in particular the stacking velocity anomaly is calculated when the interval velocity of an intermediate layer undergoes a stepwise variation the amplitude of the anomaly increases with the ratio between horizon depth and cable length from the forward model a program for the inversion is derived in order to identify lateral changes of interval velocities from unsmoothed stacking velocity analyses some examples of the application of this technique to synthetic and real data are presented

signatures of biologically influenced caco3 and mg fe silicate precipitation in hot springs case study from the ruidian geothermal area western yunnan province china hot springs at gongxiaoshe and zhuyuan maximum temperatures of 73 to 84 c respectively are characterized by deposits formed of calcite aragonite non crystalline si mg fe deposits and minor amounts of barite and gypsum the deposits at gongxiaoshe are formed largely of alternating calcite and aragonite laminae whereas those at zhuyuan are formed largely of calcite the calcite is in the form of i pseudodendrites that grew as sub crystals stacked upon each other and ii unattached euhedral and incompletely formed dodecahedral and rhombohedral crystals amorphous caco3 formed of nanoparticles 1 m long is common in some of the zhuyuan deposits but minor in the gongxiaoshe deposits the morphologically diverse arrays of aragonite crystals that lie parallel to bedding were not nucleated on a growth surface many substrates in these deposits are covered with reticulate coatings that are formed largely of si and mg with minor fe and micro granular coatings that are formed largely of si and fe biofilms with their extracellular polymeric substances and microbes are common at both springs the compositionally and crystallographically diverse precipitates at these two springs are attributed to a biologically influenced model with precipitation taking place in micro domains that developed in the extracellular polymeric substances according to this model precipitation varied at the micron scale influenced by the elemental concentrations that developed in the hydrogel of extracellular polymeric substances critically the very low preservation potential of the extracellular polymeric substance and its formative microbes means that the precipitates will rapidly lose evidence of their biotic origin the compositional diversity of the precipitates the crystallographic diversity of the calcite and aragonite with numerous incompletely formed crystals and local concentrations of si mg and fe may however serve as proxies of that biologically influenced precipitation

global perturbation of the marine calcium cycle during the permian triassic transition a negative shift in the calcium isotopic composition of marine carbonate rocks spanning the end permian extinction horizon in south china has been used to argue for an ocean acidification event coincident with mass extinction this interpretation has proven controversial both because the excursion has not been demonstrated across multiple widely separated localities and because modeling results of coupled carbon and calcium isotope records illustrate that calcium cycle imbalances alone cannot account for the full magnitude of the isotope excursion here we further test potential controls on the permian triassic calcium isotope record by measuring calcium isotope ratios from shallow marine carbonate successions spanning the permian triassic boundary in turkey italy and oman all measured sections display negative shifts in 44 40ca of up to 0 6 consistency in the direction magnitude and timing of the calcium isotope excursion across these widely separated localities implies a primary and global 44 40ca signature based on the results of a coupled box model of the geological carbon and calcium cycles we interpret the excursion to reflect a series of consequences arising from volcanic co2 release including a temporary decrease in seawater 44 40ca due to short lived ocean acidification and a more protracted increase in calcium isotope fractionation associated with a shift toward more primary aragonite in the sediment and potentially subsequently elevated carbonate saturation states caused by the persistence of elevated co2 delivery from volcanism locally changing balances between aragonite and calcite production are sufficient to account for the calcium isotope excursions but this effect alone does not explain the globally observed negative excursion in the 13c values of carbonate sediments and organic matter as well only a carbon release event and related geochemical consequences are consistent both with calcium and carbon isotope data the carbon release scenario can also account for oxygen isotope evidence for dramatic and protracted global warming as well as paleontological evidence for the preferential extinction of marine animals most susceptible to acidification warming and anoxia

the widespread occurrence of coupled carbonate dissolution reprecipitation in surface sediments on the bahamas bank using two complimentary approaches pore water advection diffusion reaction modeling and stable isotope mass balance calculations we show that carbonate dissolution reprecipitation occurs on early diagenetic time scales across a broad range of sediments on the great bahamas bank the input of oxygen into the sediments which strongly controls sediment carbonate dissolution has two major sources belowground input by seagrasses that is seagrass o 2 pumping and permeability driven advective pore water exchange the relative importance of these o 2 delivery mechanisms depends on both seagrass density and on how bottom water flow interacts with the seagrass canopy and leads to this advective exchange dissolution appears to involve the preferential dissolution of high mg calcite and the rates of dissolution increase linearly with increasing seagrass density isotopic evidence of dissolution reprecipitation is consistent with the occurrence of ostwald ripening as the mechanism of reprecipitation in which smaller crystals dissolve and then reprecipitate as larger crystals with little or no change in mineralogy estimates of the aerially integrated dissolution flux on the bahamas bank suggest that carbonate dissolution is an important loss term in the budget of shallow water carbonate sediments and that on bank carbonate dissolution rather than offshore transport may represent an important sink for gross shallow water carbonate production dissolution in carbonate bank and bay sediments may also be a significant alkalinity source to the surface ocean and should be considered in global alkalinity carbonate budget finally coupled dissolution reprecipitation may have a major impact on the stable isotope composition of carbonate sediments that are ultimately preserved in the rock record these processes may therefore need to be considered for example when using carbon isotope records to obtain information on the operation of the global carbon cycle during the phanerozoic

the role of salt in fold and thrust belts davis d m and engelder t 1985 the role of salt in fold and thrust belts in n l carter and s uycda editors collision tectonics deformation of continental lithosphere tectonophysics 119 67 88 the style of deformation in thin skinned fold and thrust belts is critically dependent upon the resistance to sliding along the detachment between the mass of deforming sediments and the underlying rocks evaporites can provide an extremely weak horizon within which a basal detachment can form and along which only a relatively small shear traction can be supported fold and thrust belts that form atop a salt layer such as the appalachian plateau the franklin mountains in northwestern canada and the jura of the alps among others share several readily observable characteristics as predicted by a simple mechanical model for fold and thrust belts a detachment in salt permits a thrust belt to have an extremely narrow cross sectional taper in addition predicted orientations of the principal stress axes over a salt dtcollement are consistent with the commonly observed lack of a consistently dominant vergence direction of structures within the thrust belt other common attributes of salt basal thin skinned deformation include the presence of several widely but regularly spaced folds and abrupt changes in deformational style at the edge of the salt basin

relations between geomorphic form and sedimentologic stratigraphic variability holocene ooid sand shoal lily bank bahamas although the linkages among surface sediments geomorphic forms and hydrodynamics in holocene ooid tidal sand shoals have been evaluated recently how these factors are reflected in the geomorphic evolution and stratigraphic record of shoals is less constrained yet such understanding is essential to developing meaningful predictive conceptual models of three dimensional architecture of ancient reservoir analogs integrating remote sensing imagery high frequency seismic data and core characterization from lily bank a modern tidally dominated bahamian ooid shoal in which sedimentologic processes are well documented reveals the stratigraphic record of geomorphic change an irregular gently dipping rocky surface interpreted as the top pleistocene with no pronounced topographic high underlies the holocene oolitic succession a 6 m 20 ft thick poorly sorted gravelly muddy sand with few ooids overlies this basal surface this lower interval is overlain by sand with an upward increase in proportion of ooids sorting and grain size the uppermost unit present only under active bar forms is well sorted oolitic medium sand with accretionary foresets sediments vary stratigraphically and geomorphically the lower unit is finer and less well sorted than the upper units and in the oolitic upper unit sediment size and sorting on bar crests are distinct from bar flanks collectively these results suggest that a marked antecedent bump is not necessary for occurrence of ooid shoals and that the stratigraphic record of analogous ooid shoal systems may preserve clues of geomorphic position as well as geobody size and orientation

a neogene seawater sulfur isotope age curve from calcareous pelagic microfossils abstract until now our knowledge of the sulfur isotopic composition of seawater through geologic time has depended on stable isotopic analysis of sulfate from evaporites owing to the sporadic occurrence of evaporites through time the secular sulfur isotope age curve contains many gaps with little or no data in order to fill in some of these gaps particularly the neogene we have analyzed the sulfur isotopic composition of carbonate associated sulfate in carbonate tests of planktonic foraminifera other investigators have shown that sulfate may occur in biogenic calcites either lattice bound as micro fluid inclusions in adsorbed phases or as protein polysaccharides whatever the origin the sulfur isotopic composition of this sulfate appears to be representative of that of the water in which the organism lived as shown by results on recent calcareous foraminifera and macrofossils using this approach for study of miocene to recent pelagic marine sediments supplemented by new data for miocene marine evaporites from the gulf of suez we have found that the 34 s of seawater has decreased about 2 5 over the past 25 m y and that most of the decrease has occurred over the past 5 m y parallelling a decrease in the 13 c of dissolved oceanic bicarbonate from the same interval sedimentary redox models based on isotope records suggest that organic carbon and sulfide burial have both decreased over the past 5 m y alternatively an increase in weathering rates over the past 5 m y would not require a decrease in organic carbon or sulfide burial as long as the isotopic effect of the increased river input exceeds the isotopic effect of the burial of the reduced species in either case the net result would be a decrease in atmospheric p o 2

the pliensbachian and tithonian extinction events the claim by raup and sepkoski1 that an approximately 26 myr mass extinction periodicity can be recognized on a global scale at family level over the past 250 myr has excited much attention and provoked speculation about an ultimate extraterrestrial cause2 10 while the purported periodicity can be challenged on grounds of both the inaccuracy of the geological timescale11 and the method of extinction analysis12 there can be no doubting the reality of at least some of the 12 proposed extinction events such as those at or near the end of the permian triassic and cretaceous others are less widely acknowledged further analysis by raup and sepkoski13 confirms the importance of two of these events within the jurassic at the end of the pliensbachian and tithonian stages i demonstrate here that marine invertebrate mass extinctions at approximately these times occurred on a regional not a global scale they can be related to severe reductions in habitat area provoked either by regression of epicontinental sea or by widespread anoxia

the relationship of phyllosilicate orientation x ray diffraction intensity ratios and c b fissility ratios in metasedimentary rocks of the helvetic zone of the swiss alps and the caledonides of jamtland central western sweden phyllosilicate preferred orientations x ray diaraction intensity ratios and c b fissility ratios were determined from 23 metasedimentary rocks from the helvetic zone of the swiss alps and from the caledonides of jamtland sweden the relationships between these parameters of phyllosilicate orientation depend strongly on the lithology and the cleavage morphology diaerences in orientation direction and in degree of preferred orientation reflect both deformation intensities and lithologies whereas polyphase deformed fine grained samples generally have phyllosilicates that are oriented parallel to cleavage showing high orientation intensities phyllosilicates in less deformed and or coarse grained rocks are oriented parallel to bedding with low orientation intensities weak orientation intensities in samples with phyllosilicates that are oriented parallel to bedding reflect compaction strain the higher strain in samples with micas and chlorites that are parallel to cleavage is a combination of compaction and tectonic strain due to the development of crenulation cleavage 2000 elsevier science ltd all rights reserved

active ooid growth driven by sediment transport in a high energy shoal little ambergris cay turks and caicos islands ooids are a common component of carbonate successions of all ages and present significant potential as paleoenvironmental proxies if the mechanisms that control their formation and growth can be understood quantitatively there are a number of hypotheses about the controls on ooid growth each offering different ideas on where and how ooids accrete and what role if any sediment transport and abrasion might play these hypotheses have not been well tested in the field largely due to the inherent challenges of tracking individual grains over long timescales this study presents a detailed field test of ooid growth hypotheses on little ambergris cay in the turks and caicos islands british overseas territories this field site is characterized by westward net sediment transport from waves driven by persistent easterly trade winds this configuration makes it possible to track changes in ooid properties along their transport path as a proxy for changes in time ooid size shape and radiocarbon age were compared along this path to determine in which environments ooids are growing or abrading ooid surface textures petrographic fabrics stable isotope compositions 13 c 18 o and 34 s lipid geochemistry and genetic data were compared to characterize mechanisms of precipitation and degradation and to determine the relative contributions of abiotic e g abiotic precipitation physical abrasion and biologically influenced processes e g biologically mediated precipitation fabric destruction through microbial microboring and micritization to grain size and character a convergence of evidence shows that active ooid growth occurs along the transport path in a high energy shoal environment characterized by frequent suspended load transport median ooid size increases by more than 100 m and bulk radiocarbon ages decrease by 360 yr westward along the 20 km length of the shoal crest lipid and 16s rrna data highlight a spatial disconnect between the environments with the most extensive biofilm colonization and environments with active ooid growth stable isotope compositions are indistinguishable among samples and are consistent with abiotic precipitation of aragonite from seawater westward increases in ooid sphericity and the abundance of well polished ooids illustrate that ooids experience subequal amounts of growth and abrasion in favor of net growth as they are transported along the shoal crest overall these results demonstrate that in the ambergris system the mechanism of ooid growth is dominantly abiotic and the loci of ooid growth is determined by both carbonate saturation and sediment transport mode microbes play a largely destructive rather than constructive role in ooid size and fabric

secular variation in abiotic marine carbonates constraints on phanerozoic atmospheric carbon dioxide contents and oceanic mg ca ratios abiotic marine carbonates have been precipitated as ooids and synsedimentary cement within shallow marine carbonate sequences throughout the phanerozoic these components exhibit variable abundances and compositions that temporally correlate with estimates of global eustasy with rising sea level aragonite and acicular magnesium calcite precipitation gave way to the formation of equant calcite and possibly dolomite similarly the abundance of ooids was limited by positions of global sea level and associated changes in ocean chemistry modern abiotic marine carbonates also exhibit variable abundances and compositions as a function of changing fluid chemistry with latitude and depth chemical data on ambient marine waters suggest that carbonate ion concentrations and rates of precipitation presently control the nature of marine cement and ooids using data from modern oceans to calculate the requisite atmospheric pco 2 levels necessary to form this range of phases in ancient seas suggests that atmos

strain analysis of the ninemile fault zone western montana insights into multiply deformed regions abstract by means of detailed field mapping and a systematic strain analysis we investigated the development of the ninemile fault in western montana the fault has been considered to be either a mesozoic or mid tertiary strike slip fault or a late eocene early oligocene normal fault the rock of the study area experienced a mesozoic to early tertiary ne sw shortening expressed by thrusts folds and pervasive cleavage we estimated total strain by measuring preferred orientation of chlorite grains in samples from within and near the ninemile fault zone and by calculating the strain according to the march theory our goal was to explain how strain accumulated by successive superposition of strains during the two deformation events onto an early compaction strain in all samples the minimum elongation direction maximum shortening is nearly vertical the maximum elongation directions of all the samples outside the ninemile shear zone are subperpendicular to the strike of the shear zone whereas in the shear zone they are either parallel to the strike of the shear zone or form an angle of about 45 the systematic variation in the principal strain directions is the result of two deformational events expressed by local structures created by early ne sw shortening and later ne sw extension the observed strain requires that the ninemile fault a normal fault during the tertiary had earlier been either a thrust or a zone of contractional shear deformation along which there was fault perpendicular shortening and fault parallel extension although the average value of maximum elongation is greater within the ninemile shear zone than outside it varies significantly inside the fault zone suggesting a highly inhomogeneous strain distribution created by a complex deformation history

extreme ocean anoxia during the late cryogenian recorded in reefal carbonates of southern australia abstract the neoproterozoic was a time of great change in the earth s surface and marine environments including extensive climate variability the widespread oxygenation of the oceans and the accompanying rise of animal life however the timing of ocean oxygenation remains uncertain particularly in regard to cryogenian seas which were disrupted by large periods of global glaciation interglacial cryogenian reef complexes in the northern adelaide fold belt of australia contain abundant primary marine dolomite cements these cements have well preserved textural and growth zonation indicating they preserve their original marine chemistry and can be used as geochemical proxies for late cryogenian paleooceanography analysis of marine cements from peritidal nearshore facies shallow platformal facies and deep framework facies of the reef complexes reveals significant geochemical gradients with paleo depth while nearshore cements have low fe contents and commonly contain iron oxide inclusions the shallow and deep cements have very high fe concentrations chalcophile elements cu cd pb zn etc are most abundant in the nearshore cements whereas rare earth elements are found in highest concentrations in the deeper facies cements rare earth element profiles are unusual with shallow and deep facies having convex profiles with negligible ce ce anomalies and positive eu eu anomalies being constrained by sedimentology this carbonate geochemistry provides a window into interglacial cryogenian ocean chemistry and structure the marine cements reveal pronounced chemical stratification in this late cryogenian ocean a thin veneer of oxic surface waters existed at the ocean surface in peritidal facies with increasingly anoxic and fe rich seawater at depth the distribution of strongly chalcophile elements like cd and cu across the chemocline suggests that although ferruginous deeper anoxic waters probably contained some dissolved sulphide these conditions describe a ferro sulfidic ocean and encompass some of the most extreme anoxia yet documented during the late precambrian a return to archean like ocean conditions at this time suggests large scale disruption of the ocean system during the neoproterozoic

experimental evidence that ooid size reflects a dynamic equilibrium between rapid precipitation and abrasion rates ooids are enigmatic concentrically coated carbonate sand grains that reflect a fundamental mode of carbonate sedimentation and inorganic product of the carbon cycle trends in their composition and size are thought to record changes in seawater chemistry over earth history substantial debate persists concerning the roles of physical chemical and microbial processes in their growth including whether carbonate precipitation on ooid surfaces is driven by seawater chemistry or microbial activity and what role if any sediment transport and abrasion play to test these ideas we developed an approach to study ooids in the laboratory employing sediment transport stages and seawater chemistry similar to natural environments ooid abrasion and precipitation rates in the experiments were four orders of magnitude faster than radiocarbon net growth rates of natural ooids implying that ooids approach a stable size representing a dynamic equilibrium between precipitation and abrasion results demonstrate that the physical environment is as important as seawater chemistry in controlling ooid growth and more generally that sediment transport plays a significant role in chemical sedimentary systems

continental flood basalt weathering as a trigger for neoproterozoic snowball earth abstract atmospheric co2 levels and global climate are regulated on geological timescales by the silicate weathering feedback however this thermostat has failed multiple times in earth s history most spectacularly during the cryogenian c 720 635 ma snowball earth episodes the unique middle neoproterozoic paleogeography of a rifting low latitude supercontinent likely favored a globally cool climate due to the influence of the silicate weathering feedback and planetary albedo under these primed conditions the emplacement and weathering of extensive continental flood basalt provinces may have provided the final trigger for runaway global glaciation weathering of continental flood basalts may have also contributed to the characteristically high carbon isotope ratios 13 c of neoproterozoic seawater due to their elevated p contents in order to test these hypotheses we have compiled new and previously published neoproterozoic nd isotope data from mudstones in northern rodinia north america australia svalbard and south china and sr isotope data from carbonate rocks the nd isotope data are used to model the mafic detrital input into sedimentary basins in northern rodinia the results reveal a dominant contribution from continental flood basalt weathering during the ca 130 m y preceding the onset of cryogenian glaciation followed by a precipitous decline afterwards these data are mirrored by the sr isotope record which reflects the importance of chemical weathering of continental flood basalts on solute fluxes to the early middle neoproterozoic ocean including a pulse of unradiogenic sr input into the oceans just prior to the onset of cyrogenian glaciation hence our new data support the hypotheses that elevated rates of flood basalt weathering contributed to both the high average 13 c of seawater in the neoproterozoic and to the initiation of the first sturtian snowball earth

phyllosilicate preferred orientation in relation to strain path determination in the lower paleozoic stavelot venn massif ardennes belgium abstract phyllosilicate preferred orientation measured by means of an x ray pole figure goniometer allows a quantitative exploration of the slaty cleavage development the relationship between phyllosilicate preferred orientation and strain as demonstrated in the march model furthermore enables the application of phyllosilicate pole figures in a strain determination in slate belts both applications are illustrated in this paper in which the results are presented of a crystallographic fabric analysis of argillites in the southern domain of the lower paleozoic stavelot venn massif belgium in these argillites the development of a phyllosilicate preferred orientation is primarily defined by the low grade metamorphism and the deformation history the metamorphism causing recrystallisation to be active during cleavage development assures the development of a high degree of preferred orientation on the other hand the deformation history defines the type of phyllosilicate pole figure pattern based on the distinction of different pole figure patterns a possible strain path for the argillites in the southern domain of the stavelot venn massif is proposed an axially symmetric pole figure pattern indicates a caledonian cleavage development in a pure flattening regime the variscan cleavage development on the other hand is characterised by an additional extensional component in the cleavage plane if the primary phyllosilicate fabric due to a pre deformational compaction or a caledonian cleavage development underwent a complete reorientation caused by syntectonic recrystallisation a typical orthorhombic pole figure pattern is formed otherwise a transition pole figure pattern develops reflecting the superposition of the variscan cleavage development on a pre existing caledonian cleavage fabric the general deformation history of the southern domain of the stavelot venn massif differs substantially from that described for the northern domain contrary to the northern domain the southern domain seems to have undergone considerable variscan deformation

the correlation between low tectonic stress and the appalachian basin quiet zone abstract in order to assess the nature of the appalachian basin quiet zone abqz a region of low seismic activity a minimum horizontal stress profile from sonic logs was calibrated using hydraulic fracture stress measurements in a well from mckean county pa the calibrated minimum horizontal stress profile that best fits with the hydraulic fracture stress data was established using an extended eaton s model that treats rock as a transverse isotropic material with a vertical symmetry axis tiv to achieve a best fit several assumptions are introduced 1 a constant tectonically induced elastic strain of ehmax 3 3 10 4 2 an anisotropic velocity model annie approximation for shales and 3 a bimodal pore pressure distribution with abnormally high pore pressure in the marcellus gas shale other calibration methods assume an isotropic medium a constant tectonic strain gradient corresponding to zero stress at ground surface or constraints from sidewall core data to estimate stiffness constants by testing four calibration methods we find that stress profiles are affected by multiple factors including the ratio of maximum tectonically induced elastic strain over minimum tectonic strain pore pressure in the marcellus biot s coefficients and elastic properties sensitivity analysis was conducted to analyze the influence of these parameters our stress profile is most influenced by pore pressure and tectonically induced elastic strain while other factors had much smaller effects tectonically included strain in the abqz is insufficient to achieve the critical stress necessary to induce frictional slip on favorably oriented faults in the sedimentary cover which presumably speaks to the lack of earthquakes in the basement of the abqz however induced earthquakes from deep disposal wells suggest that basement at the edge of the abqz may be decoupled from the sedimentary cover in which case the cover has relaxed through a time dependent deformation mechanism such as pressure solution creep

preservation of elemental and isotopic source identification of sedimentary organic matter the amount and type of organic matter in the sediments of lakes and oceans contribute to their paleoenvironmental and paleoclimatological records only a small fraction of the initial aquatic organic matter survives destruction and alteration during sinking and sedimentation selective degradation modifies the character of the surviving small fraction of organic matter which becomes incorporated in bottom sediments organic matter alterations can continue to sub bottom depths of hundreds of meters corresponding to millions of years source and paleoenvironmental information nonetheless remains preserved in the molecular elemental and isotopic compositions of organic matter c n and 13c values of total organic matter in particular appear to retain paleoenvironmental information for multi myr time periods

cathodoluminescent bimineralic ooids from the pleistocene of the florida continental shelf a bored and encrusted late pleistocene ooid grainstone was recovered from the seafloor at a depth of approximately 40 m on the outer continental shelf of eastern florida ooid cortices are dominantly bimineralic generally consisting of inner layers of radial magnesian calcite and outer layers of tangential aragonite ooid nuclei are dominantly rounded cryptocrystalline grains although quartz grains and a variety of skeletal grains also occur as nuclei ooids are partially cemented by blocky calcite and interparticle porosity is partially filled by micrite radial cortex layers are composed of brightly cathodoluminescent magnesian calcite having a composition of approximately 12 mol mgco3 and 1000 ppm strontium the iron and manganese concentrations in radial cortex layers are generally in the range of 500 1000 ppm and 100 250 ppm respectively tangential cortex layers are composed of noncathodoluminescent aragonite containing approximately 11 500 ppm strontium and less than 0 5 mol mgco3 iron concentrations in tangential cortex layers are generally in the range of 150 400 ppm and manganese concentrations are generally below the detection limit of 100 ppm echinoderm skeletal fragments which are present as accessory grains are composed of brightly cathodoluminescent magnesian calcite some ooid nuclei and the thin outer edges of some blocky calcite cement are cathodoluminescent micrite matrix and the bulk of blocky calcite cement are noncathodoluminescent ooids do not exhibit textural evidence of recrystallization the ooid grainstone underwent an episode of meteoric diagenesis but ooid cortices were not affected by the event we propose a previously unrecognized process by which the magnesian calcite cortex layers underwent diagenetic alteration in oxygen depleted seawater during this diagenesis magnesium was lost and manganese was incorporated without apparent textural alteration and without mineralogical stabilization thus we suggest that cathodoluminescence may result from diagenetic alteration on the sea floor

the stable carbon isotopic composition of organic material in platform derived sediments implications for reconstructing the global carbon cycle in pelagic carbonate sediments the degree to which the 13c values of inorganic and organic fractions co vary has been used to interpret rates of production burial and decomposition of organic carbon this relationship is relatively consistent through time permitting estimates of organic carbon production and preservation however as the majority of pelagic sediments older than 200 myr have been subducted carbonate sediments deposited in epeiric seas and platforms are often substituted for pelagic carbonates in analyses of ancient global carbon cycling there are well known pitfalls to using shallow marine carbonate materials including diagenesis semi isolation of depositional environments and input of different types of sediments with varying inorganic 13c 13cinorganic values which can obscure any global signatures one method used to assess whether global changes in 13c are accurately represented by 13cinorganic records is to examine variations in the 13c of co occurring organic material 13corganic if a 13corganic record co varies with a co occurring 13cinorganic record it is argued that the signals must be related to variations in the global carbon cycle this assumption has been investigated by analysing the isotopic composition of the organic carbon preserved in the uppermost 150 m of periplatform sediments recovered during odp leg 166 from the western margin of great bahama bank the 13corganic values measured in this study were compared to previously published 13cinorganic records measured on identical samples thus allowing a study of the correlation between the two records through time these analyses showed that the correlation coefficient between 13cinorganic and 13corganic increased from the proximal location site 1005 r2 0 1 to the distal site site 1006 r2 0 63 the importance of platform derived carbonate and organic material at the proximal location site 1005 is reflected in the absence of a co variation between inorganic and organic 13c records which exhibit no correlation on the platform itself in contrast the co variance in 13c values at the basinal location site 1006 is explained by a two point mixing model which demonstrates the importance of both pelagic and platform derived carbonate and organic carbon in generating the positive correlation between the organic and inorganic 13c values this results in a correlation between 13cinorganic and 13corganic records at site 1006 that is unrelated to global carbon cycling such data question the applicability of using 13corganic values to support the ability of 13cinorganic values to record global carbon cycling in carbonates recovered from environments where multiple sources of carbonate and organic carbon contribute to the bulk 13c signal

interactions between tidal flows and ooid shoals northern bahamas abstract oolitic strata are common through the geologic record ooids generally form in high energy environments but it remains unclear how ooids remain in such systems and form geomorphic features such as bars and shoals integrating remote sensing hydrodynamic bathymetric granulometric and field observations of modern tidal systems in the bahamas provides insight into this fundamental question oolitic tidal sands in the northern abacos display a geomorphic pattern in which bedrock islands restrict and focus tidal flow down a main channel within the channel a shallow shoal separates an ebb dominated subchannel from a flood dominated subchannel tidal velocities in these subchannels can exceed 1 m s enough to transport the oolitic sediments hydrodynamics and bathymetry in these subchannels produce a net circular hydrodynamic pattern around the shoal the spin cycle allowing the sands to remain in motion without being transported out of the ooid factory this general pattern is apparent in several other ooid shoal complexes this concept provides integrated insights into the physical influences impacting the formation suspension transport and deposition of ooids and the resulting geomorphic forms these results represent first steps toward developing more comprehensive and predictive analogs of spatial heterogeneity in ancient tidally dominated oolitic shoals

an overview of biomineralization processes and the problem of the vital effect biomineralization links soft organic tissues which are compositionally akin to the atmosphere and oceans with the hard materials of the solid earth it provides organisms with skeletons and shells while they are alive and when they die these are deposited as sediment in environments from river plains to the deep ocean floor it is also these hard resistant products of life which are mainly responsible for the earth s fossil record consequently biomineralization involves biologists chemists and geologists in interdisciplinary studies at one of the interfaces between earth and life leadbeater and riding 1986 biomineralization refers to the processes by which organisms form minerals the control exerted by many organisms over mineral formation distinguishes these processes from abiotic mineralization the latter was the primary focus of earth scientists over the last century but the emergence of biogeochemistry and the urgency of understanding the past and future evolution of the earth are moving biological mineralization to the forefront of various fields of science including the earth sciences the growth in biogeochemistry has led to a number of new exciting research areas where the distinctions between the biological chemical and earth sciences disciplines melt away of the wonderful topics that are receiving renewed attention the study of biomineral formation is perhaps the most fascinating truly at the interface of earth and life biomineralization is a discipline that is certain to see major advancements as a new generation of scientists brings cross disciplinary training and new experimental and computational methods to the most daunting problems it is however by no means a new field the first book on biomineralization was published in 1924 in german by w j schmidt schmidt 1924 and the subject has continued to intrigue a dedicated community of scientists for many years until the early 1980s the field was known as

the late cambrian spice δ13c event and the sauk ii sauk iii regression new evidence from laurentian basins in utah iowa and newfoundland carbon isotope data from upper cambrian sections in three laurentian basins in northern utah central iowa and western newfoundland record a large positive d 13 c excursion spice event of up to 1 5 peak d 13 c ratios are well dated by trilobite collections to the middle of the steptoean stage dunderbergia zone and occur during maximum regression associated with formation of the sauk ii sauk iii subsequence boundary on the north american craton max imum regression was marked by an influx of quartz sand into carbon ate platform settings in all three widely separated basins in northern utah this quartz sand formed a thick sequence known as the worm creek quartzite which marks a conspicuous interruption of carbonate deposition during the middle to late cambrian in the region in west ern newfoundland the thickness of the quartz sand unit is much re duced but still marks a brief shutdown of the carbonate factory that is unique to the cambrian shelf succession of the area in the central iowa area of the cratonic interior an upward shallowing carbonate succession culminates in cross stratified trilobite grainstones at the peak of the spice in dunderbergia zone time and the lowest point on the relative sea level curve is associated with the occurrence of coarse quartz sand derived from the encroaching shoreface although it is difficult to determine precisely the departure from baseline d 13 c that marks the beginning of the spice excursion in the stratigraphic successions analyzed our results are consistent with a rise and subsequent fall in d 13 c tracking a major regressive transgressive event recorded across northern laurentia the correlation of a major d 13 c excursion with regression is similar to that described for the late ordovician for which the pattern has been attributed to either in creased carbonate relative to terrigenous weathering rates as ice sheets covered up organic matter containing silicates at high latitudes or high productivity and organic carbon burial driven by oceanic overturn the lack of known steptoean age ice sheets that could have affected the ratio of carbonate to silicate weathering rates suggests that organic carbon burial was the likely cause of the spice event we suggest that increased weathering and erosion rates during relative sea level fall sauk ii iii increased the burial fraction of organic carbon in an expanded region of fine grained siliciclastic deposits in shelf and upper slope environments during the steptoean

change in dominant mechanisms for phyllosilicate preferred orientation during cleavage development in the kitakami slates of ne japan abstract selected slates from the kitakami mountains of ne japan were studied by means of finite strain measurement x ray texture goniometry optical microscopy back scattered sem and x ray fluorescence and diffraction analyses finite strain recorded by radiolarians in slates although not equivalent to bulk strain shows a positive correlation with strength of phyllosilicate basal plane fabrics however there is a possible difference between the fabric pattern and radiolarian strain ellipsoid shape as well as a difference in fabric pattern between chlorite and illite mica these relationships between phyllosilicate fabrics and finite strains suggest that strain estimation from phyllosilicate fabrics based on the march model may be unreliable unless their relationship is established in each case optical and back scattered sem studies revealed a change in microstructures in accordance with increasing fabric strength in slates with weak fabric strength cleavage domains are dark to opaque in appearance and characterized by a concentration of insoluble residues such as carbonaceous matter sphene ilmenite rutile epidote and apatite as well as phyllosilicates phyllosilicates in cleavage domains are oriented subparallel to cleavage whereas those in intercleavage domains are variously oriented as fabric strength increases cleavage domains become transparent and composed mainly of phyllosilicates phyllosilicates become finer grained and more uniform in grain size they are well oriented in both cleavage and intercleavage domains as fabric strength further increases phyllosilicates tend to coarsen these lines of microstructural evidence suggest that the dominant mechanism for phyllosilicate preferred orientation changes as cleavage develops from pressure solution transfer through syntectonic crystallization recrystallization and possibly to oriented grain growth because fabric strength increases with increasing illite crystallinity metamorphic grade mainly temperature must be one of the major factors which control the dominance of phyllosilicate preferred orientation mechanisms

oxygen isotope trends and seawater temperature changes across the late cambrian steptoean positive carbon isotope excursion spice event the globally recognized late cambrian steptoean positive c isotope excursion spice is characterized by a 3 5 positive 13 c shift spanning org and pyrite we analyzed 18 o values of apatitic inarticulate brachiopods from three upper cambrian successions across laurentia to evaluate paleotemperatures during the spice 18 o values range from 12 5 to 16 5 estimated seawater temperatures associated with the spice are unreasonably warm suggesting that the brachiopod 18 o values were altered during early diagenesis despite this all three localities show similar trends with respect to the spice 13 c curve suggesting that the brachiopod apatite preserves a record of relative 18 o and temperature changes the trends include relatively high 18 o values at the onset of the spice decreasing and lowest values during the main event and an increase in values at the end of the event the higher 18 o values during the global extinction at the onset of the spice suggests seawater cooling and supports earlier hypotheses of upwelling of cool waters onto the shallow shelf decreasing and low 18 o values coincident with the rising limb of the spice support the hypothesis that seawater warming and associated reduced thermohaline circulation rates contributed to decreased dissolved o 2 concentrations which enhanced the preservation burial of c org causing the positive 13 c shift

the relationship of strain and preferred orientation of phyllosilicate grains in rocks a review abstract the calculation of strain from the preferred orientation of platy phyllosilicate grains taking strain to be homogeneous is surprisingly often successful from a pragmatic point of view although numerous theoretical objections to this procedure can be and have been raised for the calculation to succeed the original distribution of mica or clay mineral grains must be known or assumed the usual assumption that the starting distribution was random and therefore nearly uniform implies that the strain history of pelitic sediments is taken to start when the watery sediment was last stirred by waves or bioturbation or if it was laid down in still anoxic waters from the moment of first deposition it therefore includes compaction as part of the strain history which is usually neglected by structural geologists originally random distribution of clay flake orientations has been observed in recently deposited sediments in many instances and has been theoretically explained in various ways exceptions probably exist but their frequency has not been estimated biologically disturbed sediments may be brought nearer to a state of random orientation of clay flakes than they were at deposition with the exception of whole beds or concretions in which pores are early filled with cement mudrocks are almost always compacted by the weight of later deposits the strain so induced is axially symmetric about the vertical and thus about the pole of the bedding planes compaction acts on soft unconsolidated rocks but may continue with increasing overburden on rocks strengthened by cementation and by increasing interlocking of grains tectonic deformation may start early before the rock is fully compacted or it may begin only much later and the mechanisms of deformation increase in complexity with the increasing temperature and hydrostatic stress of deeper burial up to a certain level of mild metamorphism however phyllosilicate grains seem to be rather permanent if not as mineral species then at least by preservation of their constituent sheets of silica tetrahedra given this permanence the mechanisms of rock deformation whether intergranular slip cataclasis or dissolution and reprecipitation and various intragranular processes in the non phyllosilicate matrix hardly matter as far as the recording of strain is concerned even a certain amount of stress solution at especially heavily loaded points of the phyllosilicate grains themselves does not destroy their value as strain markers as long as the dissolved material is disposed of mainly by crystallographically continuous overgrowth on existing grains the same holds for local dissolution of phyllosilicates in portions of old grains that have become thermodynamically unstable by bending widespread irregular or random nucleation and growth of newly nucleated phyllosilicate grains at intermediate or high grades of metamorphism can reduce the reliability of the strain record and so can domainal structure of the deformed rock if both the volume fraction of phyllosilicate grains and the intensity of total strain differ sharply from one domain to another if distinct domains are not equant and have themselves a strong preferred orientation of shape nevertheless rocks containing an anastomosing network of zones rich in phyllosilicate grains as slates and other deformed mudrocks commonly do have in many cases been found to have average preferred orientations of mica or chlorite which if used to estimate the strain give results similar to those obtained from independent strain markers in the same rocks thus the most significant verification of the usefulness of strain estimates by measurement of preferred orientation is the mutual confirmation with independently determined strain the limit to such verification is the rarity of strain markers so that the coexistence of two independent ones is exceptionally rare for this reason however it is important to make full use of the orientation of phyllosilicate grains where they are available the alternative to have no strain measurements at all causes structural geologists to miss potential information

zebra textures in carbonate rocks fractures produced by the force of crystallization during mineral replacement abstract zebra textures are enigmatic banded fabrics that occur in many carbonate hosted ore deposits dolomite hydrocarbon reservoirs and carbonate successions globally they consist of a variety of minerals and are characterised by parallel light and dark bands that occur at a millimetre to centimetre scale based on petrological evidence there is general consensus that the dark bands formed by replacement of the carbonate host rock historically more contention surrounds the origin of the light bands but the dominant view is that these are mineral filled cavities which is supported by overwhelming textural evidence overall the feature common to all versions of zebra textures is mineral replacement of the original carbonate host we suggest that mineral replacement and the force of crystallization in association with open space generation is a viable mechanism for the development of zebra cavity systems dissolution and open space generation in either evaporites or carbonates adjacent to the site of replacement reactions is necessary to remove the confining pressure from the rock and to allow the development of fractures the pressure of the growing replacement crystals within the carbonate pervasively splits the carbonate apart producing thin strips of carbonate surrounded by open space the fractures may then be subject to dissolution and are later filled by cements very regular stratabound zebra textures as found in ore deposits like cadjebut australia and san vicente peru may be related to stratabound dissolution of evaporites or carbonates whereas irregularly distributed zebra textures are more likely to be associated with irregular carbonate dissolution

vertical movements of the crust case histories from the northern appalachian basin evidence of former deep burial of ordovician to devonian strata of the northern appalachian basin has been obtained from various techniques of study including fluid inclusion homogenization temperatures 18 o and vitrinite reflectance diagenetic minerals indicate paleotemperatures of 100 200 c maximum depths of burial were calculated from the estimated paleotemperatures a gradient of 26 c km was assumed silurian strata of the basin are interpreted to have reached maximum burial depths of 5 0 km devonian strata in the catskill mountains had former burial depths of 6 5 km lower ordovician carbonate sequences were buried to 7 km middle ordovician strata had paleodepths of 5 km and devonian carbonate strata had paleodepths from 4 5 to 5 km if these strata were buried deeper than previously thought unexpectedly large amounts of uplift and erosion ranging from 4 3 to 7 km must also have taken place to bring these strata to the present land surface the occurrence of such large scale vertical movements of the crust and lithosphere must be recognized in paleogeographic reconstructions such drastic changes represent isostatic unroofing with widespread implications for paleogeography of a kind unrecognized at present

the role of pore water circulation during the deformation of foreland fold and thrust belts volume loss strain accompanying pressure solution of calcite occurred within both the umbria marches apennines of italy and the appalachian mountains of western new york data from strain markers show that volume loss strain was greater within the shallow portions of the apennines than within the appalachians within the deeper portions of both fold and thrust belts strain was nearly volume constant calcite solubility data suggest that downward circulation of meteoric water is necessary for the 35 volume loss strain of the limestones within the apennines strain at a depth of about 1 km was volume constant and is interpretated as indicative of restricted pore fluid circulation in the appalachians calcite comprises less than 1 of the clastic rocks and a 10 volume loss of this calcite may occur during circulation of connate or dehydration water derived from dewatering of the shales but in an environment that restricts the circulation of meteoric water here the volume of calcite removed 0 1 of the total rock is so small that circulation of meteoric water is not necessary for strain by pressure solution

loading paths to joint propagation during a tectonic cycle an example from the appalachian plateau u s a abstract based on the timing of joint propagation during the history of burial lithification deformation and denudation of clastic rocks within sedimentary basins four types of joints may be distinguished tectonic hydraulic unloading and release tectonic and hydraulic joints form at depth prior to uplift in response to abnormal fluid pressures whereas unloading and release joints form near the surface in response to thermal elastic contraction accompanying erosion and uplift tectonic joints are distinguished from hydraulic joints in that tectonic compaction is a mechanism for achieving abnormal pore pressures leading to the propagation of the former whereas compaction by overburden loading leads to the abnormal pore pressures in the latter case the orientation of unloading joints is controlled by either a residual or contemporary tectonic stress whereas the orientation of release joints is controlled by a rock fabric examples of some of these joints are found within the devonian catskill delta of the appalachian plateau new york during the alleghanian orogeny tectonic joints cross fold joints formed under abnormal pore pressure as indicated by the observation that joints propagated in the siltstones before they developed in shales and by the cross cutting relationships of folds cleavage and joints this sequence is compatible with oil company hydraulic fracture data which show that the least principal stress within sandstone layers is less than that in the intercalated shale layers plumose structures indicate that the joints within siltstones propagated as discontinuous rupture events each of which affected less than a meter of bed length the discontinuous rupturing is compatible with models for natural hydraulic fracturing release joints strike joints post date the alleghanian orogeny as indicated by abutting relationships within the deeper parts of the devonian clastic section unloading joints are orthogonal to the contemporary tectonic stress field

trends in oolite dolomitization across the neoproterozoic cambrian boundary a case study from death valley california abstract changes in dolomite fabric within the neoproterozoic cambrian stratigraphic succession in death valley california preserve evidence for a first order biogeochemical change in early diagenetic environments through the neoproterozoic cambrian transition oolite forms the primary focus of the study so that the general paleoenvironmental setting and presumed susceptibility to early diagenesis and fluid flow can be kept relatively constant across geologic time two key generalizations can be made 1 late neoproterozoic dolomitization in death valley occurred very early in the paragenetic sequence was volumetrically significant and preserved fine detail of the original aragonite or calcite precursor and 2 early cambrian dolomitization in death valley occurred late in the paragenetic sequence with burial was commonly incomplete and obliterated details of the calcite or aragonite precursor early diagenetic dolomitization of any kind was absent the dolomitization fabrics in death valley parallel the presumed pattern of seawater mg ca with respect to mimetic dolomitization in the paleozoic low mg ca little no mimetic dolomitization and cenozoic higher mg ca more common mimetic dolomitization however the volume of neoproterozoic mimetic dolomitization is immense compared to the cenozoic examples we speculate that a fundamental shift in the early diagenetic environment occurred in late neoproterozoic time perhaps in association with secular variation in the sulfate concentration of seawater and the advent and proliferation of penetrative bioturbation that destroyed sediment capping microbial mats an oscillatory trend in the fabric and volume of dolomite through the phanerozoic has been suggested but the termination of volumetrically significant mimetic dolomitization across the neoproterozoic cambrian transition a critical time in earth history for many reasons dwarfs the oscillatory trends noted in the phanerozoic

facies specific calcitic and bimineralic ooids from middle and upper cambrian platform carbonates western newfoundland canada abstract oolites in the port au port group consist of intercalated gray oolite deposited on the shallow subtidal mobile fringe of ooid sand shoals and brown oolite which accumulated on intertidal sand flats along the shoal crests gray oolite is composed of finely and coarsely preserved radial ooids that were precipitated and deposited in situ and whose fabric was controlled in large part by suspension versus bedload transport brown oolite is composed of concentric ooids or superficial radial ooids that were either formed and deposited in situ or swept onto the sand flats from the mobile fringe the present fabric of the ooids is the result of protracted burial diagenesis cortices of finely and coarsely preserved radial ooids excluding radial concentric ones and superficial radial ooids were mg calcite originally they changed to calcite still retaining fine fabric and subsequently to a variety of cortical fabrics during shallow burial and altered to blocky fabrics during deep burial concentric ooids and radial concentric ooids were bimineralic originally composed of aragonite now micrite and blocky calcite and mg calcite now radial fibrous calcite primary aragonitic laminae were partially or completely dissolved and crushed during shallow burial resulting voids were filled with blocky calcite during deep burial correlation of original ooid mineralogy fabric and environment of formation indicates that local physical conditions such as turbulence influenced ooid formation in cambrian time this correlation is absent in modern deposits suggesting that more widespread chemical and organic controls now prevail

loading configurations and driving mechanisms for joints based on the griffith energy balance concept abstract using the griffith energy balance concept to model joint propagation in the brittle crust two laboratory loading configurations serve as appropriate analogs for in situ conditions the dead weight load and the fixed grips load the distinction between these loading configurations is based largely on whether or not a loaded boundary moves as a joint grows during displacement of a loaded boundary the energy necessary for joint propagation comes from work by the dead weight i e a remote stress when the loaded boundary remains stationary as if held by rigid grips the energy for joint propagation develops upon release of elastic strain energy within the rock mass these two generic loading configurations serve as models for four common natural loading configurations a joint normal load a thermoelastic load a fluid load and an axial load each loading configuration triggers a different joint driving mechanism each of which is the release of energy through elastic strain and or work the four mechanisms for energy release are joint normal stretching elastic contraction poroelastic contraction under either a constant fluid drive or fluid decompression and axial shortening respectively geological circumstances favoring each of the joint driving mechanisms are as follows the release of work under joint normal stretching occurs whenever layer parallel extension keeps pace with slow or subcritical joint propagation under fixed grips a substantial crack normal tensile stress can accumulate by thermoelastic contraction until joint propagation is driven by the release of elastic strain energy within the earth the rate of joint propagation dictates which of these two driving mechanisms operates with faster propagation driven by release of strain energy like a dead weight load acting to separate the joint walls pore fluid exerts a traction on the interior of some joints joint propagation under fluid loading may be driven by a release of elastic strain energy and or work by the pore fluid during poroelastic contraction again propagation velocity dictates the driving mechanism with critical to supercritical joint propagation accompanying fluid decompression and subcritical crack propagation taking place under a constant fluid drive during axial shortening a jointing process called axial splitting may occur following wing crack growth from an initial crack tilted relative to the maximum compressive stress here the joint driving mechanism may be either the work of a remote stress i e a dead weight or the release of elastic strain energy i e fixed grips

formation of regional cross fold joints in the northern appalachian plateau abstract fold axis parallel elongation associated with the development of arcuate fold and thrust belts is proposed as a causal mechanism for syn orogenic cross fold joints such a mechanism can be coupled with other joint propagation models providing a widely applicable resolution to the enigmatic origin of regional systematic joints the fold axis parallel elongation model is compatible with kinematic indicators as recorded by a sequence of cross fold joints and related deformational fabrics in the central and northern appalachian plateau modeling of an arcuate tectonic boundary subjected to normal compressive loading demonstrates that tangential tensile stress can be large enough to initiate cross fold joints on the convex side of the tectonic boundary simulated stress trajectories from boundary element modeling bear a strong resemblance to the stress trajectories inferred from the regional cross fold joint patterns in the central and northern appalachian plateau modeling also displays a cratonward decrease in both the tangential stress and the tangential strain such a decrease is consistent with the deformation styles observed in the central and northern appalachian plateau

chemical control of carbonate phases implications from upper pennsylvanian calcite aragonite ooids of southeastern kansas abstract ooids in grainstones from the spring hill member of the pennsylvanian plattsburg limestone in southeastern kansas were precipitated as both aragonite and calcite either phase predominated in different outcrops in different units within an individual outcrop in different grains within an individual sample and in different portions of an individual ooid cortex selective diagenetic dissolution of aragonite cortices gave rise to a variety of fabrics reported from many other politic limestones these include oomoldic porosity sparry cortices strings of spastolithic deformed and elephantine trunk to tail connected ooids and displaced nuclei these and other reported occurrences of fabrics which record the former presence of aragonite are not evenly distributed through phanerozoic time rather they predominate during times of general continental emergence while this distribution supports the postulations of mackenzie and pigott 1981 and sandberg 1983 that the dominant mineralogy of ancient marine limestones may have corresponded closely to positions of global sea level fabrics of the plattsburg limestone demonstrate that evaluating changes in ooid compositions with geologic time is not a simple either aragonite or calcite proposition

correlation between abnormal pore pressure and tectonic jointing in the devonian catskill delta using the preferred orientation of chlorite we measured vertical compaction in 53 samples from the devonian catskill delta in central new york state the marine part of this delta contains three levels based on different amounts of vertical compaction these levels correspond roughly to 1 black shales at the base 2 prodelta turbidites and 3 a cap of shallow water sediments including abundant storm washed shell hashes deposited within the wave base the cap is normally compacted whereas the lower two levels are undercompacted tectonic cross fold joints that propagated during the late paleozoic alleghanian orogeny are restricted to the deeper undercompacted levels of the catskill delta whereas unloading cross fold joints pervade the cap the correlation between undercompaction and the distribution of tectonic joints indicates that abnormal fluid pressure was a key mechanism during the propagation of these joints

the tonian beck spring dolomite marine dolomitization in a shallow anoxic sea abstract the reason for the abundance of dolomite lithologies in earth s early geological record compared to modern environments remains contentious this study provides new insight into this precambrian dolomite problem by revisiting one of the most controversial dolomite localities the beck spring dolomite of death valley usa consistent with some previous studies petrographic evidence indicates that although the beck spring dolomite now consists almost entirely of dolomite it was originally precipitated largely as aragonite and high mg calcite depositional constituents microbialites and ooids were likely originally aragonitic and early marine length fast cements now dolomite are suggested to have precipitated as high mg calcite then replaced syntaxially by dolomite based on petrographic and geochemical evidence we suggest that marine dolomitization was the dominant synsedimentary diagenetic process in the unit and for the most part involved syntaxial and mimetic replacement further a length slow fibrous dolomite generation was precipitated during the later stages of marine diagenesis as a primary marine dolomite cement this is indicated by the length slow crystallographic structure of the cement and from its preserved geochemical and cathodoluminescence growth zonation this new evidence for tonian marine dolomite precipitation reinforces the idea of precambrian marine environmental conditions including the chemical composition of seawater promoting dolomite formation at this time the trace metal geochemical composition of well preserved marine components especially dolomite marine cements reveals information about redox conditions in this tonian shallow seawater in terms of rare earth element geochemistry the beck spring dolomite has no significant ce anomaly and a ubiquitous positive eu anomaly consistent with widespread oceanic anoxia during deposition furthermore the relatively low levels of iron and chalcophile elements co cu pb and zn in marine components compared to other neoproterozoic carbonates suggest euxinic conditions both anoxic and sulphidic prevailed during deposition of the beck spring dolomite

tectonic controls of phanerozoic sedimentary rock cycling the cyclic nature of the phanerozoic sedimentary rock distribution carbon sulphur coupling and material transfer among sedimentary reservoirs appears to be controlled by tectonic factors the distribution of preserved sedimentary mass in terms of rock mass remaining u geologic age shows a minimum c 300 350 ma ago which separates two subcycles of erosion and deposition of sedimentary rocks the older subcycle was interrupted because of the major continental collisions of the devonian and late carboniferous these collisions resulted in a reduction of outcrop areas of rocks of the older cycle relative to their masses leading to a decline in the probability of destruction and an increase in half life of these older sediments a strong correlation exists between the long term cyclicity in the phanerozoic global sea level curve and the distribution of carbon and sulphur among their major exogenic reservoirs this correlation is related to two principal tectonic modes of the phanerozoic oscillatory and submergent it is postulated that the submergent mode of active plate convergence obduction and subduction of sediments large ridge volume and high sea level gave rise to low erosion and sedimentation rates less restricted environments of carbonate deposition and relatively high atmospheric co 2 levels high temperatures resulting from an increased rate of production of co 2 from diagenetic and metamorphic reactions at subduction zones as sea level rose carbon was transferred from the sedimentary reservoir of reduced organic carbon to that of oxidized inorganic carbon in limestones whereas sulphur moved from the oxidized sulphate reservoir to the reduced sulphide reservoir as sea level fell reservoir transfers were opposite to those above culminating in the oscillatory mode of generally elevated continental interiors these reservoir transfers are consistent with secular changes observed in the distribution of 13 c and 34 s in sedimentary materials during the phanerozoic petrographic examination of phanerozoic oolite formations shows that ooids with preserved calcitic relict textures are characteristic of pre carboniferous carbonate rocks whereas ooids with relict textures indicative of initial aragonite mineralogy are dominant in rocks of younger age these changes in ooid mineralogy may be interpreted as reflecting changes in co 2 levels of the ocean atmosphere system consistent with the above tectonic considerations atmospheric co 2 levels were higher prior to carboniferous time favouring formation of calcitic ooids and skeletal parts after the carboniferous co 2 levels fell and aragonite and mg calcites of greater than 8 mol mg increased in abundance as precipitates

compaction fabrics of pelites experimental consolidation of kaolinite and implications for analysis of strain in slate abstract compaction of clay and shale results in large reductions in volume as pore water is expelled preferred orientation of the platy minerals increases with compaction strain and loss of porosity according to the march owens model this relationship has been studied quantitatively by experimentally consolidating kaolinite clay from slurries and analyzing the resulting fabrics with the x ray pole figure goniometer and scanning electron microscope sem initial porosity corresponds to the onset of the strain recorded by the preferred orientation and the values of 0 78 for dispersed slurries and 0 76 for flocculated slurries reflect the electrostatic forces between the clay platelets initial porosities of recently deposited fine silt and clay are in the range of 0 60 0 90 and are a function of grain size and mineralogy loss of this initial porosity has a large effect on the subsequent development of slaty cleavage matrix methods were used to model deformation paths for slates in the welsh slate belt preferred orientation of mica and ellipsoidal shapes of reduction spots were simulated for one locality by loss of a 0 60 initial porosity a 6 tilt of the beds and horizontal shortening involving plane strain strain determinations for shales and slates should include the large reduction in volume

tracking the burial and tectonic history of devonian shale of the appalachian basin by analysis of joint intersection style the most pervasive systematic joints hosted by devonian black shale of the appalachian plateau include east northeast joints and younger northwest striking cross fold joints both sets were driven exclusively by fluid pressure generated as a consequence of hydrocarbon related maturation supplemented by subsequent tectonic compaction during the alleghanian tectonic cycle in the more deeply buried proximal region of the catskill delta joints of both sets cross cut however in stratigraphically equivalent black shale of the distal shallower region of the delta to the west 25 of the joint intersections are crosscutting whereas 75 of the intersections are defined by east northeast joints curving into and abutting cross fold joints east northeast joints in the distal shale propagated early but were neither as long nor as pervasive as similarly oriented joints in the more deeply buried proximal delta deposits however these same joints are much better developed more closely spaced stratigraphically lower in the distal shale succession where essentially all intersections are crosscutting it is likely that relatively high contact stress an effective stress on east northeast joints in the more deeply buried parts of the delta enabled the unimpeded transmission of elastic stress concentration at the tip of cross fold joints across the older joints in the distal delta exhumation of the devonian shale sequence following establishment of the contemporary stress field in middle tertiary time to within 1 km of the earth s surface resulted in renewed propagation of east northeast joints the propagation path of the reactivated joints curved into and terminated against cross fold joints at angles of 60 90 the curving to termination intersection style is a manifestation of crack tip stress field blunting as a consequence of slippage along the interface of cross fold joints enabled by exhumation induced relaxation of contact stress on these latter joints it is noteworthy that the orientation of the densely formed east northeast joints parallel to the maximum horizontal stress direction of the contemporary lithospheric stress field likely imparts a meaningful permeability anisotropy to these hydrocarbon source rocks

contrasting roles of detrital and authigenic phyllosilicates during slaty cleavage development eight samples from a prograde slate sequence from central wales ranging from the diagenetic zone to the epizone were studied by x ray texture goniometry sem and tem x ray pole figure data imply that in the four lower grade samples the crystallographic preferred orientations of both mica and chlorite change from parallel to bedding to intermediate to bedding and cleavage to parallel to cleavage orientation with increasing grade there is no difference in the degree of preferred orientation between mica and chlorite in individual samples the sample with an intermediate preferred orientation has the lowest degree of preferred orientation in the four higher grade samples the preferred orientations of mica are primarily in the cleavage orientation whereas those of chlorite are largely parallel to bedding observations by sem and tem show that the samples have two populations of phyllosilicates 1 large tens of pm chlorite mica stacks of detrital origin dominated by chlorite in a matrix of 2 authigenic fine grained tens of nm mica and chlorite with dominant illite muscovite preferred orientation measurements of chlorite primarily reflect the chlorite in the stacks however preferred orientation measurements of mica are dominated by the mica in the stacks for four lower grade samples but by mica in the fine grained matrix with 001 parallel to cleavage for the four higher grade samples these observations collectively show that 1 mechanical rotation of detrital grains was the dominant reorientation mechanism of cleavage formation in the lower grade samples 2 in the higher grade samples the preferred orientation of matrix mica and chlorite transformed from bedding parallel to cleavage parallel primarily by dissolution and neo crystallization copyright 1996 elsevier science ltd

pencil structure and strain in weakly deformed mudstone and siltstone abstract weakly deformed mudstone and siltstone middle ordovician knobs formation of the appalachian valley and ridge province south western virginia u s a show strain dependent transitions between bedding fissility pencil structure and cleavage pencil structures are associated with a bulk inhomogeneous shortening deformation where minimum principal strain z ranges between 9 and 26 shortening assuming a plane strain and constant volume where strains are less bedding fissility dominates pencil fragments are defined by intersecting fracture sets subparallel to the pre existing bedding fissility and cleavage their long axes are both parallel to the bedding cleavage intersection and to the inferred y axis of the tectonic strain ellipsoid pencil development is considered to result from fracturing along both fabric anisotropies during weathering and post tectonic stress relaxation pencils show variations in size and shape depending on lithology grain size and composition degree of initial clay preferred orientation degree of cleavage development type of cleavage total bulk strain and degree of strain homogeneity their shape l w however is a direct measure of total y z strain since strain determinations from chlorite pressure fringes on framboidal pyrite within the pencil fragments give y z 0 913 0 019 l w pencil structure is therefore a potential strain marker in weakly deformed rocks

roof sequence response to emplacement of the wills mountain duplex the roles of forethrusting and scales of deformation abstract this paper focuses on the behavior of a roof sequence in the appalachian plateau of west virginia u s a and emplacement of the wills mountain duplex with 17 5 km of displacement unlike the plateau along strike in pennsylvania and new york where forethrusting was previously documented this roof sequence lacks an underlying salt dominated roof decollement kinematic analyses reveal that the roof sequence in the west virginian plateau accommodated about two thirds of the 17 5 km of shortening by the adjacent wills mountain duplex as a forethrusting kinematic response the remaining shortening imbalance of about 5 km between the duplexes and younger roof sequence rocks is accommodated by additional forethrusting further into the foreland and local compensation this kinematic response matches that along strike in the central appalachians despite the loss of the salt decollement we interpret that an ordovician shale dominated formation was sufficiently weak to substitute for the salt horizon thus a weak mechanical unit rather than specifically a salt decollement is a necessary prerequisite for forethrusting a contributing factor to forethrusting may be the subvertical front of the wills mountain duplex which inhibited other responses by the roof sequence mesoscale and smaller processes including grain to grain pressure solution twinning and cleavage formation account for over 75 of the shortening in the roof sequence and if ignored would result in an erroneous interpretation of backthrusting or local compensation this result suggests that failure to consider all deformation scales could lead to incorrect kinematic conclusions in other tectonic systems

recurrent early triassic ocean anoxia the early triassic record from the smithian stratotype shows that the organic carbon isotope record from northwest pangea closely corresponds to major fluctuations in the inorganic carbon records from the tethys indicating truly global perturbations of the carbon cycle occurred during this time geochemical proxies for anoxia are strongly correlated with carbon isotopes whereby negative shifts in 13 c org are associated with shifts to more anoxic to euxinic conditions and positive shifts are related to return to more oxic conditions rather than by a delayed or prolonged recovery the early triassic is better characterized by a series of aborted biotic recoveries related to shifts back to ocean anoxia potentially driven by recurrent volcanism

the basement versus the no basement hypotheses for folding within the appalachian plateau detachment sheet within the appalachian plateau detachment sheet of southwestern pennsylvania tectonic thickening is a central element to the growth of anticlines in a silurian devonian section containing a three tiered mechanical stratigraphy that is a basal detachment zone a lower imbrication zone and an upper wedge zone the detachment zone is predominantly within disturbed shale of the silurian vernon formation which sits above a disturbed surface on the lockport dolomite large scale anticline growth is in part a consequence of small scale blind imbrication of the more competent mechanical layers in the lower portion of the detachment sheet the imbrication zone here salt within the silurian syracuse formation hosts secondary detachment responsible for imbrication and the development of triangle zones in the core of the anticlines some fold amplification is also accomplished by extensive smaller scale thrust wedging and concomitant tectonic thickening of the less competent devonian section in the upper portion of the detachment sheet the wedge zone these spatially periodic anticlines are separated by horizontally bedded synclines characterized by the absence of thrust wedging and fault imbrication across all three zones tectonic thickening continued behind the foreland propagation of the detachment tip line within the vernon shale as indicated by a systematic hinterland increase in the cross sectional width and amplitude of the anticlines on the basis of seismic images each detachment sheet anticline is situated above prominent periodically spaced pre alleghanian structures in the footwall of the detachment sheet these footwall structures arise from a combination of thrust imbrication at the depth of the ordovician trenton group and high angle basement involved faulting at the depth of the cambrian gatesburg formation some of the high angle fault displacement is a consequence of alleghanian inversion on faults associated with the extensional rome trough and other basement structures developed during the late proterozoic rifting of the eastern margin of laurentia evidence for tectonic inversion is present in seismic reflection images that show buttress anticlines in the cambro ordovician carbonate section the superposition of detachment sheet anticlines above footwall structures strongly suggests that the foreland transport was disrupted by topographic bumps on the base of the detachment zone bending of the detachment sheet over these irregularities may have promoted the spatially periodic collapse and concomitant tectonic thickening in the over riding sheet as it was pushed laterally toward the foreland a strain hardening of the detachment zone part of the detachment sheet at the topographic irregularities permitted the growth of double sided tapered wedges that is each anticline the overall structural profile of the detachment sheet is consistent with the growth of a coulomb wedge whose low basal friction is interrupted by patches of strain hardening

three dimensional finite strain from crinoid ossicles abstract randomly oriented crinoid ossicles are useful markers for the determination of three dimensional finite strain two techniques are presented both make use of established methods to measure the two dimensional strain ellipses on three surfaces which are then combined to calculate the shape and orientation of the strain ellipsoid first ossicles positioned such that a thin section cuts across the cylinder appear as randomly oriented ellipses prior to deformation and can be analyzed using standard rf methods second ossicles positioned such that the section cuts lengthwise through the cylinder appear as rectangular or sub rectangular shapes with orthogonal geometries in the undeformed state measurements of angular shear strain on two or more such markers are analyzed using a non linear least squares solution to the breddin graph allowing determination of the best fit strain ellipse both methods are applied to an echinoderm grainstone from the central helvetic nappes of switzerland the results are internally consistent and compatible with those from other strain analysis techniques

primary and tectonic fabric intensities in mudrocks abstract in this paper we present a summary of quantitative measurements of the preferred orientation of phyllosilicate grains in a variety of tectonically weakly deformed to undeformed fine grained pelitic rocks we also briefly examine a small amount of data on clay fabrics in deep sea cores and compare them to fabrics in deformed equivalents from the mariposa formation and calaveras complex sierra nevada california the measured fabric ellipsoids support the following conclusions 1 a bedding parallel foliation forms in pelitic rocks during compaction that in our samples is associated with shortening between 3 and 74 2 the amount of compaction is in part controlled by the quartz feldspar content interbedded sandy layers undergo little to no compaction and do not develop bedding parallel foliations and 3 it is impossible to determine precisely the amounts of compaction horizontal shortening and later tectonic strain in deformed samples because of the variability of initial compactions our data and results presented by others indicate that strain paths in pelitic rocks always include early compaction followed by an episode of simultaneous compaction and plane strain in accretionary wedges and by a variety of other types of strain in other settings

a global marine sedimentary response to the end permian mass extinction examples from southern turkey and the western united states the end permian mass extinction greatly diminished marine diversity and brought about a whole scale restructuring of marine ecosystems these ecosystem changes also profoundly affected the sedimentary record data presented here attained through facies analyses of strata deposited during the immediate aftermath of the end permian mass extinction southern turkey and at the close of the early triassic southwestern united states in combination with a literature review show that sedimentary systems were profoundly affected by 1 a reduction in biotic diversity and abundance and 2 long term environmental fluctuations that resulted from the end permian crisis lower triassic strata display widespread microbialite and carbonate seafloor fan development and contain indicators of suppressed infaunal bioturbation such as flat pebble conglomerates and wrinkle structures facies considered unusual in post cambrian subtidal deposits our observations suggest that depositional systems too respond to biotic crises and that certain facies may act as barometers of ecologic and environmental change independent of fossil assemblage analyses close investigation of facies changes during other critical times in earth history may serve as an important tool in interpreting the ecology of metazoans and their environment

kinematics and strain distribution of a thrust related fold system in the lewis thrust plate northwestern montana u s a abstract in order to understand interactions between motion along thrusts and the associated style of deformation and strain distribution in their hangingwalls geologic mapping and strain measurements were conducted in an excellently exposed thrust related fold system in the lewis thrust plate northwestern montana this system consists of 1 an e directed basal thrust the gunsight thrust that has a flat ramp geometry and a slip of about 3 6 km 2 an e verging asymmetric anticline with its nearly vertical forelimb truncated by the basal thrust from below 3 a 4 km wide fold belt the frontal fold complex that lies directly in front of the e verging anticline 4 a w directed bedding parallel fault the mount thompson fault that bounds the top of the frontal fold belt and separates it from the undeformed to broadly folded strata above and 5 regionally developed w dipping spaced cleavage although the overall geometry of the thrust related fold system differs from any previously documented fault related folds the e verging anticline itself resembles geometrically a rich type fault bend fold the observed initial cut off and fold interlimb angles of this anticline however cannot be explained by cross section balancing models for the development of either a fault bend fold or a fault propagation fold possible origins for the e verging anticline include 1 the fold initiated as an open fault bend fold and tightened only later during its emplacement along the basal thrust and 2 the fold started as either a fault bend or a fault propagation fold but simultaneous or subsequent volume change incompatible with any balanced cross section models altered its shape strain in the thrust related fold system was determined by the preferred orientation of mica and chlorite grains the direction and magnitude of the post compaction strain varies from place to place strains in the foreclimb of the hangingwall anticline imply bedding parallel thinning at some localities and thickening at others this inhomogeneity may be caused by the development of thrusts and folds strain in the backlimb of the hangingwall anticline implies bedding parallel stretching in the thrust transport direction this could be the effect of bending as the e verging anticline was tightened and transported across the basal thrust ramp strain measured next to the gunsight thrust again indicates locally varying shortening and extension in the transport direction perhaps in response to inhomogeneous friction on the fault or else to a history of alternating strain hardening and softening in the basal thrust zone

correlation between magnetic anisotropy and fabric for devonian shales on the appalachian plateau the magnetic anisotropy of devonian black shale samples was measured from two cores drilled in the appalachian plateau the mineralogy of the shales is predominantly clay with small quantities of quartz and minor amounts of opaques and chlorite magnetite is the predominant ferromagnetic mineral present in the samples the magnetic fabric was measured at both room temperature and liquid nitrogen temperature and is dominated by a well defined bedding vertical compaction and a lesser defined magnetic lineation measurements of the anisotropy of magnetic susceptibility ams at liquid nitrogen temperature which enhances the paramagnetic contribution in the rock showed a strong increase in both the bulk susceptibility and susceptibility differences this increase suggests that the ams is controlled by the paramagnetic minerals particularly the clays and chlorite strain was measured from the orientation of basal planes of the chlorite crystals by texture goniometry good correlations have been found 1 between the orientation of the magnetic lineation and the long axes of the chlorite crystals and 2 between the degree of magnetic foliation and the amount of vertical compaction the magnetic lineation also agrees well with the direction of seismic anisotropy over the plateau the anisotropy of the anhysteretic remanence which expresses the anisotropy due to the ferromagnetic component in the rocks shows a weaker correlation with the amount of vertical compaction a weak magnetic lineation suggests that the magnetite grains were aligned during a deformation phase which post dates the main alleghanian orogeny the magnetic anisotropy of the devonian shales mirrors the compaction and tectonic fabric on the appalachian plateau

stable isotope compositions of fluid inclusions in biogenic carbonates abstract measurements have been made of hydrogen carbon and oxygen isotope compositions of inclusion waters and co2 extracted from eleven species of modern marine and freshwater skeletal carbonates the samples were collected in environments of highly contrasting temperatures and isotopic compositions of ambient waters inclusion waters were extracted in vacuum by thermal decrepitation of samples that were previously treated with h2o2 to remove organic matter water extractions were quantitative above 200 c for aragonite species and above 350 400 c for calcite species amounts of water liberated ranged from 0 6 to 2 2 and were generally very reproducible within a species but varied strongly from one species to another except for red algae and corals the 18o values of the shells are in accord with crystallization of carbonate at or near equilibrium with marine water of 18o near 0 the inclusion waters however are not in oxygen isotope equilibrium with ambient water and have high 18o values of 6 to 18 these high 18o values do not result from partial exchange between water and either the host carbonate or small amounts of co2 released during decrepitation d values of inclusion waters range from 80 to 10 and are sensitive to the presence of small amounts of organic matter the data for each species define a distinct field in d 18o space that is controlled by a vital fractionation effect stable isotope compositions of inclusion waters can be explained by metabolic reactions that incorporate relatively 18o rich o2 that is dissolved in the water and used by the organism in respiration thus inclusion waters in shells probably represent remnants of metabolic fluids produced by the mantle epithelium the stable isotope compositions of such waters most likely result from varying metabolic rates that are specific to each species as well as to formation temperature and the isotopic composition of ambient waters inclusion fluids in biogenic carbonates constitute an isotopic reservoir that has heretofore been unrecognized the 18o and d values of the inclusion waters are very different from those of meteoric magmatic and seawaters because these trapped fluids are released by the shells during heating they could play a role in burial diagenesis burial of significant amounts of biogenic carbonates could liberate enough water to control diagenetic or metamorphic reactions in some cases in the absence of other types of fluids the participation of inclusion waters in such reactions should be easily recognized by their distinctive isotopic compositions

appalachian stress study 3 regional scale stress variations and their relation to structure and contemporary tectonics using commercial hydrofracture data to supplement published stress data we map regional variations in least horizontal stress magnitude sh and orientation within the paleozoic sediments of the appalachian plateau systematic trends revealed by the data are examined for correlations with structure the study area includes a prominent salt detachment and tectonic history the dominant stress component present at all depths including basement throughout the region is ene oriented compression and is most likely of tectonic origin no deviations from this rule are found that are statistically significant near surface 600 m stresses are consistently greater than the overburden with strike slip regime conditions prevailing below however where the salina salt beds are present thrust regime conditions extend deeper reaching the salt in most places where data are available limited to where the salt is shallower than 1 5 km at the salt a discontinuity in the trend of sh with depth marks the abrupt transition to the underlying strike slip regime characterized by least stress ratios i e rh sh sv in the range 0 5 0 7 and extending to basement the comparatively high stress found above the salt may reflect a difference between the strain histories of the overlying and underlying sections facilitated by mechanical decoupling by the salt and perhaps includes a component of stress remnant from nw oriented alleghanian shortening of the upper section the lower subsalt stress regime extends beyond the eastern salt margin and displays remarkable uniformity over distances of hundreds of kilometers this is particularly evident across ohio where an rh value of 0 6 0 05 appears valid down to basement north of ohio values increase markedly whereas to the south of the salt pinch out in west virginia and eastern kentucky values as low as 0 35 are found in devonian shales at 1 1 5 km depth although values deeper in the section may be higher the northward increasing trend can be reasonably well modeled as the effect of persisting lithospheric flexure due to the pleistocene ice sheet however flexure was much greater in the past and as devonian shales in south ohio west virginia are currently only marginally stable a modest increase over the present flexure would be expected to cause shear failure this suggests that the very low stress in the shales of south ohio west virginia is of different origin and more recent than the pleistocene

a global carbon isotope excursion spice during the late cambrian relation to trilobite extinctions organic matter burial and sea level abstract the steptoean positive carbon isotope excursion spice marks a global oceanographic event that confirms intercontinental correlations between different biogeographic realms based on agnostids and other blue water trilobites the spice excursion is documented from sections in laurentia kazakhstan china and australia where it begins with the mass extinction at the base of the pterocephaliid biomere steptoean stage in laurentia and at coeval extinction horizons in gondwana and perigondwana terranes the peak of spice 5 coincided with a time of maximum regression in laurentia spice is similar in this regard to excursions that coincide with glacio eustatic falls such as in the late ordovician a plausible scenario involves the transformation of ocean circulation between two states which led to enhanced coastal upwelling and benthic extinctions the lack of evidence for glaciation indicates that the coeval sea level fall sauk ii sauk iii event resulted from tectonic or hydrologic changes that remain poorly understood at this time