electron microprobe study of mg distribution in recent mg calcites and recrystallized equivalents from the pleistocene and tertiary abstract the effect of early freshwater diagenesis on mg distributions in former mg calcite cements was determined by electron microprobe analysis of two major groups of samples 1 mg calcites in recent limestones not yet exposed to freshwater and 2 tertiary recent mg calcites and recrystallized equivalents which have undergone freshwater exposure pristine mg calcites are characterized by a uniform mg concentration in contrast former mg calcites from the miocene of enewetak atoll most of which are now radiaxial prismatic calcites have a much greater variation in mg content and traverses across the calcites reveal definite trends mg content of the calcite is inversely correlated with abundance of inclusions some of the cloudy growth layers are due to infestation by endolithic organi ms fungi the small variation in mg content in original mg calcite cements is a valuable starting point to be aware of when interpreting mg trends in recrystallized equivalents mg content can be related to inclusion abundance in recrystallized former mg calcites but not in pristine mg calcites therefore mg trends in altered materials most likely reflect dissolution reprecipitation processes not original mg trends a likely cause for the low mg content associated with inclusion rich growth layers is relatively high rates of dissolution and reprecipitation in these relatively permeable layers in some cases the increased permeability is the result of infestation by endoliths such organisms may cause the cloudiness observed in some other radiaxial calcites

palaeomagnetic and rock magnetic reliability criteria in ophiolitic rocks a case study from the palaeozoic ballantrae ophiolite scotland abstract palaeomagnetic studies of the ballantrae ophiolite southwest scotland are assessed in the light of palaeomagnetic and rock magnetic reliability criteria the remanence directions are palaeomagnetically reliable but have only limited significance for apparent polar wander path construction rock magnetic reliability criteria are found to be ambiguous when treated in isolation original grain size effects are detected in magnetic properties across three sections through pillow lavas we propose that a systematic study of spatial magnetic property variations within pillow lavas can provide an additional field test in the assessment of reliability thermomagnetic analyses and microscopic observations suggest three remanence carrying magnetic phases exist within the extrusive layer of the ophiolite these are almost pure magnetite hematite and titanomaghemite palaeomagnetic rock magnetic and geological observations are best satisfied by a pre obduction magnetisation age this study links the remagnetisation of an ultra oceanic conglomerate to local hydrothermal circulation rather than to a pervasive remagnetisation event as previously suggested

magnesium isotope composition of sabkha porewater and related sub recent stoichiometric dolomites abu dhabi uae abstract earliest marine diagenetic sabkha type dolomites are widespread in earth s geological record potentially these carbonates may act as archives of past seawater magnesium isotope 26mg ratios at present however the fractionation of magnesium isotopes 26mg between seawater here evaporated marine porewater and sabkha dolomite is not constrained in order to explore 26mgdol mg porewater we make use of actualistic sabkha type dolomite precipitation in the gulf region trucial coast united arab emirates this paper documents and discusses the first detailed sabkha 26mg data set of mg bearing solids including stoichiometric dolomites degree of ordering 0 9 mean 26mgdol 0 79 0 41 2 n 17 and related marine pore waters the presence of dolomite crystals with a broad range of ordering in shallow cores is documented using x ray diffraction and scanning electron microscopy but individual crystals are too small

understanding how volume affects the mobility of dry debris flows 1 the prediction of the runout length l of large dry debris flows has long been the subject of a considerable research effort primarily due to the obvious concern caused by their destructive power one seemingly well established feature is the increase of the mobility of a rock avalanche defined as the ratio of the runout distance to the fall height with its volume v the physical nature of this lubrication mechanism remains however controversial in this paper we analyse field data and discrete numerical simulations of granular flows and demonstrate the geometrical origin of the apparent enhancement of the mobility with the volume we evidence the intertwined role of volume and topography and show the existence of two contributions in the runout defining two flow regimes one dominated by sliding in which the runout is independent of v and another dominated by spreading in which the runout is strongly dependent on v in the light of these results the search of a volume dependent lubrication mechanism appears to be an ill posed problem

classification of dolomite rock textures abstract dolomite rock textures can be classified according to crystal size distribution and crystal boundary shape the classification scheme presented here is largely descriptive but carries genetic implications because size distribution is controlled by both nucleation and growth kinetics and crystal boundary shape is controlled by growth kinetics size distributions are classified as unimodal or polymodal crystal boundary shapes are classified as planar or nonplanar if the evidence permits a complete classification includes a description of recognizable allochems matrix and void filling allochems and preexisting cements may be unreplaced partially replaced replaced mimically or replaced nonmimically allochems may be dissolved leaving molds matrix can be unreplace partially replaced or replaced by a unimodal or polymodal size dolomite unimodal size distributions generally indicated a single nucleation event on a unimodal substrate polymodal sizes can be formed by multiple nucleation events on a unimodal or polymodal substrate or differential nucleation on an originally polymodal substrate planar crystal boundaries develop when crystals undergo faceted growth and nonplanar boundaries develop when crystals undergo nonfaceted growth nonplanar boundaries are characteristic of growth at elevated temperature 50 c and or high supersaturation both planar and nonplanar dolomite can form as a cement replacement of caco3 or neomorphism of a precursor dolomite

a direct comparison of the properties of crm and vrm in the low temperature oxidation of magnetite summary we report results of a collaborative investigation of the experimental properties of chemical remanent magnetization crm acquired when nearly single domain size magnetite undergoes low temperature oxidation to maghemite and of viscous remanent magnetization vrm acquired by the daughter phase maghemite magnetite grains 100 200 nm in diameter were oxidized for varying lengths of time at a temperature of 220 c the acquisition of crm in fields of 0 2 0 5 and 1 oe was monitored at frequent intervals during heating runs lasting 30 hr zero field decay of this remanence was then monitored for periods of 160 430 hr subsequently vrm acquisition and decay were measured for similar time periods for the daughter phase maghemite z 0 96 the principal results are as follows

statistically learning archean carbonate diagenesis abstract geochemical data are often very noisy due to the large natural heterogeneity of the geological materials as well as oversimplification of rock classification this has serious repercussions on the precision of our knowledge of the deep past as we often rely solely on geochemical proxies to investigate the geological evolution of archean and proterozoic environments here statistical learning procedures were applied to achieve unbiased classification of neoarchean stromatolitic dolostone textures on the basis of the distribution of their trace elements and rare earth elements ree investigated through laser ablation induced coupled plasma mass spectrometry la icp ms multivariate statistical analyses and supervised statistical learning have revealed that different dolomite fabrics thought as products of aggrading diagenesis and recrystallization are in fact chemically indistinguishable the diagenetic processes that cause the re crystallization of dolomite and the consequent change of textures is not affecting the distribution of major and trace elements inherited by the depositional environment or during early stages of diagenesis at the same time the algorithm has revealed that an optically homogeneous microcrystalline dolomite sample may in fact be geochemically inhomogeneous because of processes of ripening and recrystallization occurred at an early stage of marine diagenesis and that have contributed to element mobilization statistical learning has succeeded in recognizing chemofacies which not always overlap with dolomite textures and fabrics highlighting the importance of crystallographic and diagenetic studies before any study of carbonates as geochemical proxies

subsolidus evolution and alteration of titanomagnetite in ocean ridge basalts from deep sea drilling project ocean drilling program hole 504b9 leg 83 implications for the timing of magnetization magnetic minerals in six samples of oceanic basalts of the transition zone and upper sheeted dikes from deep sea drilling project ocean drilling program dsdp odp hole 504b leg 83 were studied by methods of rock magnetism and transmission electron microscopy tem tem observations showed that the magnetic mineral in these basalts is end member magnetite tmo of extremely fine grain size 30 100 nm primarily in the range of pseudosingle domain magnetite consistent with the rock magnetic properties including hysteresis parameters curie temperature and low temperature measurements verwey transition magnetite formed by two different processes 1 oxidation exsolution true exsolution and hydrothermal alteration and 2 oxidation exsolution a second stage of oxidation exsolution and hydrothermal alteration the primary titanomagnetite tm60 70 that crystallized from the melt thus evolved to end member magnetite coexisting with titanite sphene kassite ulvospinel tm 87 and ilmenite on a submicroscopic scale on the basis of the formation mechanisms of the magnetic carrier the primary titanomagnetite tm 60 with curie temperature of 180 c did not acquire thermoremanent magnetization trm in these basalts instead the ti bearing magnetite tm 10 20 that formed as oxidized or exsolved lamellae acquired its first thermal chemical remanent magnetization crm at 500 400 c during subsolidus cooling upon the onset of hydrothermal alteration the recrystallized end member magnetite acquired a second crm the natural remanent magnetization of the basalts from the transition zone and upper sheeted dikes is therefore characteristic of crms that were acquired when titanomagnetite altered in part to magnetite during subsolidus cooling and hydrothermal alteration close to the ridge axis

the angle of reach as a mobility index for small and large landslides the relations between the angle of reach fahrboschung and other indexes expressing the mobility of landslides and vertical drop horizontal reach and volume of landslide mass have been analyzed by means of simplified plots and regression equations results for 204 landslides of all sizes show that whatever the mechanism of motion all kinds of landslides experience a continuous reduction of the angle of reach with volume increase this reduction starts from the smallest sizes scattering in this relation is mostly due to mechanisms of motion and to both obstacles and topographic constraints on the path this synthesis indicates that small landslides less than 0 5 106 m3 in relative terms display excesses of travel distance similar to large landslides since most small landslides are not expected to develop any change in the mechanism of progression while moving the decrease in the reach angle with volume suggests that scale effects should be taken into account the angle of reach is found to be

experimental observations of water like behavior of initially fluidized dam break granular flows and their relevance for the propagation of ash rich pyroclastic flows the physics of ash rich pyroclastic flows were investigated through laboratory dam break experiments using both granular material and water flows of glass beads of 60 90 m in diameter generated from the release of initially fluidized slightly expanded 2 5 4 5 columns behave as their inertial water counterparts for most of their emplacement for a range of initial column height to length ratios of 0 5 3 both types of flows propagate in three stages controlled by the time scale of column free fall h0 g 1 2 where h0 denotes column height and g denotes gravitational acceleration flows first accelerate as the column collapses transition to a second constant velocity phase occurs at a time t h0 g 1 2 1 5 the flow velocity is then u equation image gh0 1 2 larger than that for dry initially nonfluidized granular flows transition to a last third phase occurs at t h0 g 1 2 4 granular flow behavior then departs from that of water flows as the former steadily decelerates and the front position varies as t1 3 as in dry flows motion ceases at t h0 g 1 2 6 5 with normalized runout x h0 5 5 6 the equivalent behavior of water and highly concentrated granular flows up to the end of the second phase indicates a similar overall bulk resistance although mechanisms of energy dissipation in both cases would be different interstitial air particle viscous interactions can be dominant and generate pore fluid pressure sufficient to confer a fluid inertial behavior to the dense granular flows before they enter a granular frictional regime at late stages efficient gas particle interactions in dense ash rich pyroclastic flows may promote a water like behavior during most of their propagation

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

a comparison of pyroclastic flow and debris avalanche mobility many pyroclastic flows have runout distances longer than expected from basic frictional arguments in these cases the ratio of the height descended h and the runout l are as small as 0 2 a feature which has been attributed to fluidization however significant fluidization by gases is now considered unlikely during emplacement of debris avalanches where h l and volume are inversely related we determine whether pyroclastic flows are more mobile than debris avalanches of the same volume and if different emplacement mechanisms are needed to describe their runout h l and volume data were compiled for 34 nonvolcanic and 40 volcanic debris avalanche deposits as well as 15 pyroclastic flow deposits systematic differences in the volume versus h l plots were explored by statistical analysis results show an inverse relationship between h l and volume for both nonvolcanic and volcanic debris avalanche deposits as expected and also for pyroclastic flow deposits regression lines for plots of pyroclastic flow and volcanic debris avalanche data are statistically indistinguishable and the non volcanic debris avalanche line is parallel to the other two we argue that the nonvolcanic and volcanic debris avalanche differences are due to material properties rather than emplacement mechanisms therefore since pyroclastic flows and volcanic debris avalanches have indistinguishable h l versus volume it is unlikely that pyroclastic flows have a significantly different primary emplacement mechanism based on the data in hand we conclude that it is unnecessary to invoke gas fluidization to explain the mobility of the types of pyroclastic flows examined or the excess runout of large volume debris avalanches

submarine cementation in a high energy platform reef one tree reef southern great barrier reef three major framework facies algal pavement coral heads and branching corals show varying degrees of cementation the widest distribution and greatest variety of submarine cements occur within those facies or subfacies that have extensive encrustation by coralline algae these are found beneath the windward margin and within the patch reef drilled both mg calcite and aragonite cements exhibit diverse fabrics and textures mg calcite in the dominant cement and is present as both an interskeletal and intraskeletal infill whereas aragonite cement is exclusively intraskeletal interskeletal cements are only developed within those facies capable of trapping significant quantities of fine grained internal sediment lithified crusts with either a laminated or columnar morphology are a prominent cavity infill within many of the reef framework facies organic influence during formation peloids which are the major component of the lithified crusts as well as many other cavity infillings are a particular form of mg calcite micrite and have nucleated within the interstitial waters of the reef microcrystalline rim cements around the peloids have often cemented them into a coherent mass that resembles detrital lime mudstones modified journal abstract

paleomagnetism of the spences bridge group and northward displacement of the intermontane belt british columbia a second look the spences bridge group is a mid cretaceous 104 ma volcanic succession in the southern intermontane belt of the canadian cordillera 50 5 n 121 w it comprises the pimainus formation mafic to felsic lava volcaniclastic and interbedded epiclastic rocks and the overlying spius formation andesitic lava flows including previous work we have 55 sites distributed among 15 localities representing most of the 3000 m thickness forty seven sites 286 oriented cores 457 specimens mainly andesites yielded acceptable data the beds are gently to moderately tilted partly due to synvolcanic deformation samples taken from a sequence of flows at any one locality have with one exception well grouped magnetization directions polarities are all normal as expected for rocks laid down in the cretaceous normal superchron declinations always are clockwise of that expected of cratonic north america indicating 60 rotation of the spences bridge group as a whole however declinations differ from locality to locality implying relative interlocality rotations about vertical axes hence inclination only analysis has been used to estimate dispersion mean inclination and paleolatitude minimum dispersion for the spius formation 27 sites was achieved after 80 untilting but the changes between 80 and 100 untilting are insignificant indicating that magnetization was acquired predominantly before tilting by contrast minimum dispersion for the pimainus formation 20 sites was achieved after 50 untilting indicating that magnetization was acquired after synvolcanic tilting when buried beneath the overlying spius formation polished thin section studies show that magnetite in the pimainus formation has undergone extensive low temperature hydrothermal alteration whereas magnetite in the spius formation shows little alteration throughout the range of tilt correction from 0 to 100 the mean inclinations of both formations were less than expected from observations obtained from mid cretaceous rocks of cratonic north america the best estimate of paleolatitude from the spius formation is 50 8 5 0 p 0 05 which is 9 5 5 7 less than would be expected had the rocks had been rigidly attached to north america this corresponds to displacement from the south of 1100 600 km displacement between the northern intermontane belt and craton probably was accommodated along major strike slip faults northern rocky mountain trench finlay pinchi etc in the south our results require a major dextral fault the intra quesnellia fault to be situated during late cretaceous or paleocene time within or marginal to the omineca belt along which about 1000 km of dextral motion occurred this could be a southern extension of the pinchi fault whose trace is now obscured by eocene extension and tectonic denudation the results also indicate that the largest tectonic discontinuity in the canadian cordillera occurs not to the east of the intermontane belt as commonly assumed but to the west because the displacements relative to cratonic north america observed from the intermontane belt are only about one third of those observed from the coast belt

temporal evolution of a carbonate coastal system abu dhabi united arab emirates abstract the southern shore of the persian arabian gulf comprises a low angle carbonate evaporite ramp system that provides a rare recent analogue for ancient epeiric settings including many carbonate petroleum reservoirs during the 1960s several studies documented the distribution of naturally occurring shallow marine facies in the shallow marine environments proximal to abu dhabi island subsequent coastal engineering projects have significantly modified the coastline and sublittoral zone many sedimentary environments have been lost with marine sites now sited beneath buildings parks or roads the data from the earlier studies are used as a pre anthropogenic base line in order to document and assess the temporal changes in the characteristics and distribution of carbonate sedimentary facies as observed today the locations of earlier sample sites were revisited in order to represent the complete range of shallow marine sedimentary environments the shallow subtidal environments adjacent to abu dhabi island continue to be areas of active carbonate sedimentation temporal changes in facies belt geometries and occurrence are assessed changes are recorded for most of the investigated sedimentary settings both in terms of sedimentary environment distribution and sediment composition offshore reefs and their associated back reef lagoons are no longer present ooid shoal and delta complexes are much reduced in their aerial extent now being replaced with shoreface facies or dredged channels significant dredging activity has increased tidal flow to lagoon environments thereby increasing connectivity to open marine conditions however land reclamation has substantially reduced the volume of the area of the lagoon environments despite these changes many facies display surprising resilience to anthropogenic activities with no discernible change in sedimentary facies or biotic distributions understanding these modifications and their associated driving mechanisms allows discrimination between anthropogenic and naturally occurring environmental perturbations this in turn develops our understanding of the temporal evolution of shallow marine carbonate facies under natural and anthropogenic influences

numerical modeling of self channeling granular flows and of their levee channel deposits when not laterally confined in valleys pyroclastic flows create their own channel along the slope by selecting a given flowing width furthermore the lobe shaped deposits display a very specific morphology with high parallel lateral levees a numerical model based on saint venant equations and the empirical variable friction coefficient proposed by pouliquen and forterre 2002 is used to simulate unconfined granular flow over an inclined plane with a constant supply numerical simulations successfully reproduce the self channeling of the granular lobe and the levee channel morphology in the deposits without having to take into account mixture concepts or polydispersity numerical simulations suggest that the quasi static shoulders bordering the flow are created behind the front of the granular material by the rotation of the velocity field due to the balance between gravity the two dimensional pressure gradient and friction for a simplified hydrostatic model competition between the decreasing friction coefficient and increasing surface gradient as the thickness decreases seems to play a key role in the dynamics of unconfined flows the description of the other disregarded components of the stress tensor would be expected to change the balance of forces the front s shape appears to be constant during propagation the width of the flowing channel and the velocity of the material within it are almost steady and uniform numerical results suggest that measurement of the width and thickness of the central channel morphology in deposits in the field provides an estimate of the velocity and thickness during emplacement citation mangeney a f bouchut n thomas j p vilotte and m o bristeau 2007 numerical modeling of self channeling granular flows and of their levee channel deposits

burial cementation is it important a case study stuart city trend south central texas the stuart city trend is a shelf edge buildup of lower cretaceous bioclastic and reefal carbonates that is currently buried to depths of between 3 300 and 5 000 m 11 000 and 16 000 ft compaction and cementation have generally reduced rock porosities to less than 9 sediments were cemented in the marine environment by finely crystalline bladed isopachous cement and volumetrically important 14 volume coarse to very coarsely crystalline fibrous to bladed isopachous mg calcite cement these cements have been neomorphically altered end page 536 to low mg calcites forming an unusual radiaxial texture observed in the coarse to very coarsely crystalline bladed calcite cement evidence of their marine origin consists of a relative 1 mole mg memory a marine like isotopic character dgr18o approx 2 5 and dgr13c approx 2 0 and early relative timing of precipitation diagenetic alteration of these carbonate sediments by the interaction with meteoric water in lenses that formed within topographic highs along the shelf margin changed the initial marine chemical isotopic and textural character of the sediments secondary porosity formation mineral stabilization aggrading neomorphism and equant spar calcite cementation are the important products of meteoric diagenesis the equant spar calcite cements make up approximately 16 by volume of the limestones studied they are iron and manganese poor the majority have a dgr13c composition which falls in the range of modern marine carbonates i e 0 9 to 3 5 pmil the dgr18o compositions range from 1 3 to 6 6 pmil relative to the pdb standard oxygen stable isotopic and petrographic data suggest that over 50 of the equant spar calcite cements were formed in a near surface meteoric environment a large percentage of the remaining equant spar calcite cements formed at shallow burial depths in a water limited system where mass transfer was dominated by diffusional processes thermally induced dgr18o depletion of the equant spar calcites indicating significant fluid flow was of minimal importance pyrobitumen pore fillings and inclusions in the outer 1 0 mm thick rims of the very coarsely crystalline equant spar calcite cements indicate that only minor amounts of cementation have occurred since the introduction of hydrocarbons deep burial diagenesis i e post hydrocarbon migration consisted of the precipitation of minor amounts of galena fluorite and sr rich equant spar calcites these diagenetic products can be directly related to the present day formation water the chemical isotopic and textural characteristics of the stuart city trend limestones contain the imprints of their initial marine composition and shallow diagenetic alterations in a hydrodynamic system burial diagenesis has not significantly altered these limestones fault and fracture control on the movement of formation waters in this system determine the location and intensity of late stage diagenetic events end of article last page 537

burial dissolution of micrite in middle east carbonate reservoirs jurassic cretaceous keys for recognition and timing abstract this paper discusses burial diagenesis and especially a burial dissolution phase and its effects on micrite microtexture in reservoirs in the middle east three microporous reservoirs were selected the cenomanian mishrif formation in iraq the kimmeridgian tithonian arab d and the barremian kharaib formations both in abu dhabi staining cathodoluminescence and scanning electron microscopy were used for petrographic observations in the selected reservoirs three typical micrite microtextures are distinguished micro rhombic compact anhedral and rounded the chronology of diagenesis based on petrographic observations shows that mineralogical stabilisation through high magnesian calcite replacement by low magnesian calcite led to the formation of micro rhombic micrites non reservoir facies such as seals present compact anhedral micrites correlation between these micrites and high aluminium content suggests alumino silicate minerals clays as a catalyst for pressure dissolution and thus for reservoir degradation through cementation micrites from the upper part of oil reservoirs are dissolved during burial diagenesis dissolution is responsible for rounded shape and for reduction of crystal size it also implies pore diameter and pore throat size enlargement comparison of porosity and permeability with scanning electron microscope data confirms that rounded micrites are better reservoirs than micro rhombic micrites

submarine lithification on windward reef slopes capricorn bunker group southern great barrier reef abstract windward reef slopes of the capricorn bunker group southern great barrier reef are the sites of extensive submarine lithification both crystalline aragonite and bladed mg calcite spar and microcrystalline micrite and peloids cements occur within skeletal pores and chambers whereas interskeletal cements are mainly microcrystalline these cements are considered to be contemporaneous with reef growth and have developed in an environment characterized by slow accretion rates continuous precipitation of aragonite cement has in some places been inhibited by the formation of mg calcite micrite apparent dissolution of some of the mg calcite cements was observed but the processes involved are unknown

a two fluid model for avalanche and debris flows geophysical mass flows debris flows avalanches landslides can contain o 10 6 10 10 m 3 or more of material often a mixture of soil and rocks with a significant quantity of interstitial fluid these flows can be tens of meters in depth and hundreds of meters in length the range of scales and the rheology of this mixture presents significant modelling and computational challenges this paper describes a depth averaged thin layer model of geophysical mass flows containing a mixture of solid material and fluid the model is derived from a two phase or two fluid system of equations commonly used in engineering research phenomenological modelling and depth averaging combine to yield a tractable set of equations a hyperbolic system that describes the motion of the two constituent phases if the fluid inertia is small a reduced model system that is easier to solve may be derived

rudists as gregarious sediment dwellers not reef builders on cretaceous carbonate platforms abstract it has sometimes been claimed that rudist bivalves competitively displaced corals from reef frameworks during the cretaceous this hypothesis combines two assertions 1 that the autecology of rudists was convergent with that of reef building corals and 2 that rudist formations commonly developed as reefs we dispute both assertions and thus reject the hypothesis of competitive displacement we argue instead that mobile sediments rather than frameworks dominated the margins and tops of most of the extensive carbonate platforms of the period and that it was on these non reefal biotopes that the rudists flourished definitions of reefs tend to combine two major elements 1 a robust biogenic framework with accompanying sedimentary and diagenetic components and 2 topographical relief such definitions are clearly rooted in recent coral reefs in which endosymbiotic zooxanthellae permit the extensive growth of colonial coral frameworks in shallow but relatively nutrient poor waters coralline algae and cementation may contribute significantly to the growth of rigid structures and topography is largely the legacy of pleistocene changes in sea level in rudist formations in contrast individual rudist congregations are volumetrically limited relative to sediment they are often loosely constructed and they evidently showed little if any original relief tabular and small lenticular units predominate these differences in structure and palaeoenvironmental situation between rudist and modern coralgal associations reflect the different autecologies of the constituent organisms the clonal growth of corals predisposes them to the development of frameworks projecting above the sediment surface herein termed superstratal growth by contrast the aclonal development of rudists was better suited to the opportunistic occupation of a variety of temporarily available substrata by large numbers of individuals in particular elevator rudists in which the entire commissure exhibited upward growth evidently grew with their shells largely embedded in and supported by the ambient sediment herein termed constratal growth moreover the tolerances and growth responses of rudists to such factors as water turbidity nutrients and current regime were quite different from those of the majority of reef building corals despite repeated assertions in the literature that rudists possessed zooxanthellae only a few species show any evidence for such a symbiosis and other evidence suggests that most lacked them in view of these differences in their preferred biotopes competition between rudists and corals is doubtful even though members of both groups co occur in many areas the relative decline of coral frameworks in the cretaceous was thus probably independently caused

flow of variably fluidized granular masses across three dimensional terrain 1 coulomb mixture theory rock avalanches debris flows and related phenomena consist of grain fluid mixtures that move across three dimensional terrain in all these phenomena the same basic forces govern motion but differing mixture compositions initial conditions and boundary conditions yield varied dynamics and deposits to predict motion of diverse grain fluid masses from initiation to deposition we develop a depth averaged three dimensional mathematical model that accounts explicitly for solid and fluid phase forces and interactions model input consists of initial conditions path topography basal and internal friction angles of solid grains viscosity of pore fluid mixture density and a mixture diffusivity that controls pore pressure dissipation because these properties are constrained by independent measurements the model requires little or no calibration and yields readily testable predictions in the limit of vanishing coulomb friction due to persistent high fluid pressure the model equations describe motion of viscous floods and in the limit of vanishing fluid stress they describe one phase granular avalanches analysis of intermediate phenomena such as debris flows and pyroclastic flows requires use of the full mixture equations which can simulate interaction of high friction surge fronts with more fluid debris that follows special numerical methods described in the companion paper are necessary to solve the full equations but exact analytical solutions of simplified equations provide critical insight an analytical solution for translational motion of a coulomb mixture accelerating from rest and descending a uniform slope demonstrates that steady flow can occur only asymptotically a solution for the asymptotic limit of steady flow in a rectangular channel explains why shear may be concentrated in narrow marginal bands that border a plug of translating debris solutions for static equilibrium of source areas describe conditions of incipient slope instability and other static solutions show that nonuniform distributions of pore fluid pressure produce bluntly tapered vertical profiles at the margins of deposits simplified equations and solutions may apply in additional situations identified by a scaling analysis assessment of dimensionless scaling parameters also reveals that miniature laboratory experiments poorly simulate the dynamics of full scale flows in which fluid effects are significant therefore large geophysical flows can exhibit dynamics not evident at laboratory scales

magnetic properties and opaque mineralogy of rocks from selected seafloor hydrothermal sites at oceanic ridges magnetic properties natural remanent magnetization nrm susceptibility curie point temperature tc saturation isothermal remanent magnetization irms and koenigsberger ratio q and opaque mineralogy were determined for basalts diabases gabbros peridotites and serpentinites collected by dredging and submersible from the rift valley at five hydrothermal sites 15 n 17 n 23 n the snake pit hydrothermal field 26 n the tag hydrothermal field on the mid atlantic ridge and 42 n the sea cliff hydrothermal field on the northern gorda ridge the magnetomineralogy is interpreted in terms of deuteric hydrothermal metasomatic and ambient seawater alteration evidence for unequivocal magnetic mineral modification by hydrothermal action is present only in a small percentage of extrusive basalts but is pervasive in diabases gabbros and ultramafic rocks this lithologiec distribution suggests a progressive increase in alteration intensity with depth in the oceanic crust and upper mantle from minor low temperature alteration in young surface basalts to more pervasive high temperature alteration in diabases gabbros and peridotites the studies reveal distinct correlations between magnetization intensity thermomagnetic behavior and magnetic mineralogy grain size style and intensity of alteration and rock type basalts have the highest nrm intensities and the lowest curie points 105 294 c basalts with nrm irms ratios less than 10 2 may have been remagnetized and have nrm intensities and q values much lower one third and one half respectively than basalts with nrm irms ratios of 10 2 or greater in the oceanic lithospheric suite examined remanent q 1 magnetization is present in basalts and induced magnetization q 1 occurs in metabasalts and the other rock types layer 2a basalts are the source of median valley magnetic anomalies the magnetic source may shift from the surface to deeper horizons with progressive seafloor aging and enhanced deuteric oxidation in layer 2b dikes and layer 3 gabbros and the formation of magnetite in serpentinized peridotites this shift is influenced by fluctuations in the curie isotherm related to the duration of active magma chambers and convective hydrothermal cells the magnetic and mineralogie properties of oceanic rocks determined show the effects of a wide range of alteration processes that are variously related to depth in oceanic lithosphere magmatic cooling history and hydrothermal circulation

classification and description of dolomitic fabrics of rocks from the floridan aquifer u s a abstract biozones of the lake city formation avon park formation and ocala limestone are characterized by interbedded massive fossiliferous carbonate rocks wackestones to grainstones and thinly bedded peloidal and carbonaceous rocks mudstones and wackestones several horizons have been partly or completely dolomitized a wide range of early to late stages of dolomitic fabrics are recognized the fabrics are classified descriptively as equigranular unimodal or inequigranular multimodal fabrics composed of crystals two processes of dolomitization are suggested a homogeneous dolomitization resulting in a single stage development of microtextured groundmass crystals

nonskeletal peloidal precipitates in upper triassic reefs yukon territory canada abstract peloidal sediments and crusts form up to 75 percent of framework limestones in upper triassic reefs in the yukon territory the peloids are spherical to irregular in shape 20 250 m in size and are commonly surrounded by one or more rims of sparry calcite the peloidal sediments occur in confined cavities and in open spaces between reef framebuilders the peloidal crusts generally coat calcareous sponges and tabulozoans internal evidence suggests that the peloids are not detrital grains or fecal pellets and comparisons with mg calcite peloids and peloidal crusts in modern environments suggest that they are precipitates possibly resulting from bacterial activity

mixed siliciclastic and carbonate sediments a proposed first order textural and compositional classification sediments that are composed of mixtures of siliciclastic and carbonate material occur in both the modern and the ancient to date there is no commonly used classification of this type of sediment a descriptive first order classification system is proposed here that is applicable to most mixed sediments and their lithified equivalents the sediments are composed of four components 1 siliciclastic sand sand sized quartz feldspar etc 2 mud mixtures of silt and clay 3 allochems carbonate grains such as peloids ooids bioclasts and intraclasts 20 m in size and carbonate mud or micrite 20 m in size these components define a tetrahedra whose subdivision produces eight general classes of mixed sediment the name of a class reflects both the dominant grain type and the most abundant antithetic component for example a rock that contains greater than 50 siliciclastic material of which most is sand sized and subordinate allochems is termed an allochemic sandstone other rock names include micritic sandstone allochemic and micritic mudstone sandy or muddy allochem limestone and sandy or muddy micrite prefixes and adjectives can be added to these terms to include dolomitic sediments conglomeratic textures and biogenic constituents

the petrographic description of carbonate facies are we all speaking the same language both academia and industry require consistent and repeatable carbonate lithofacies classifications as a primary input to the construction of depositional diagenetic and reservoir models and the dunham system has long been held to satisfy this requirement however ambiguities in the petrographic description of carbonates are widespread this study investigates the classification of carbonate lithofacies across academia and industry at a wide range of experience levels in order to quantitatively assess reproducibility some 241 volunteers with over 4200 years of combined experience examined a range of synthetic rock textures and natural lithologies and assigned textures as they saw appropriate the results of the study identify the situations where classification ambiguities and inconsistencies are most common the dunham classification system was proved to be the scheme of choice with 89 of the classifications using some form of the dunham system however all of the 24 samples yielded a wide variability in assigned texture with between 22 and 131 different names being assigned to a single specimen the most common causes of inconsistency are errors in assessing the mode of support mistakes in estimating the size and volume of grains within the lithology and confusion as to how to classify lithologies in which more than one texture is present the textures of the modified dunham classification system are redefined in order to clarify any classification criteria that have been identified as points of confusion detailed classification guidelines are offered in order to minimize the possibility of misidentification or confusion it is suggested that the term bafflestone is redundant and should be removed from the classification system the adoption of these guidelines will increase confidence reliability and value in the petrographic classification of carbonate lithologies thereby enhancing communication and facilitating the development of more realistically constrained depositional diagenetic and reservoir models

particle velocity fields and depositional processes in laboratory ash flows with implications for the sedimentation of dense pyroclastic flows we conducted laboratory experiments on dam break flows of sub 250 m volcanic ash generated by the release of gas fluidized and variably non expanded to expanded up to 35 beds in order to gain insights into the internal kinematics of pyroclastic flows the flows were typically several cm thick and had frontal speeds of up to 2 m s 1 high speed videos taken through the transparent sidewall of the 3 m long channel were analyzed with a particle tracking algorithm providing a spatial and temporal description of transport and sedimentation the flows deposited progressively as they traveled down the flume being consumed by sedimentation until they ran out of volume deposition commenced 5 20 cm rearward of the flow front and for a given expansion proceeded at a rate independent of distance from the lock gate deposit aggradation velocities were equal to those inferred beneath quasi static bed collapse tests of the same ash at the same initial expansions implying that shear rates of up to 300 s 1 have no measurable effect on aggradation rate the initially non expanded and just fluidized flow deposited progressively at a rate indicative of an expansion of a few percent perhaps due to shear induced reynolds dilation during initial slumping the fronts of the flows slid across the flume floor on very thin basal shear layers but once deposition commenced a no slip condition was established at the depositional interface within the flows the trajectory of the constituent particles was linear and sub horizontal the velocities of the particles increased with height above the depositional interface reached a maximum then declined slightly towards the flow surface perhaps due to air drag at a given location the velocity profiles were translated upwards as the deposit aggraded the results show that even cm thin poorly expanded flows of ash deposit progressively as inferred for many pyroclastic flows the change from frontal slip to rearward no slip conditions at the bases of the laboratory flows are qualitatively consistent with some textural features of pyroclastic flow deposits

the preservation potential of environmentally diagnostic sedimentary structures from a coastal sabkha abstract the abu dhabi sabkha constitutes the landward portion of the shallow sloping carbonate ramp situated on the northwestern coast of the united arab emirates uae the sabkha in this region is composed of a complex assortment of carbonate organic and evaporitic facies that constitute a regressive sabkha sequence the surface features of the sabkha can be grouped into distinct laterally continuous facies associations that are directly correlatable with elevation above present day sea level previous investigations have suggested the employment of the abu dhabi sabkha as a modern depositional analogue to ancient carbonate evaporite formations however a quantitative analysis of the relationship between sabkha surface and subsurface characteristics has not previously been undertaken this study describes in detail the sedimentary structures and textures seen at the sabkha surface and compares them to the holocene age facies retained in the shallow subsurface of the 36 sedimentary structures observed at the sabkha surface only seven 19 are still identifiable in the early burial realm the majority of the surface structures were lost during early burial as a result of microbial mat decomposition destructive evaporite precipitation and compaction the loss of diagnostic sedimentary features during early burial as a result of fabric destructive processes has considerable implications for the recognition and interpretation of ancient sabkha sequences documenting these processes from a range of modern sedimentary environments will aid in the identification of those diagnostic features that may be reliably employed during paleoenvironmental reconstructions

the descriptive nomenclature and classification of growth fabrics in fossil scleractinian reefs abstract framework sensu stricto is a concept originally relating to modern reefs and is defined by structural ecological and sedimentological criteria however in the geological literature the term framework has come to be applied to almost any organically influenced in situ accumulation including non calcareous non rigid facies associated with sea grasses this drastic departure from the original meaning is misleading and scientifically unhelpful the concept of wave resistance is integral to that of framework but in the geological record it is not easy to evaluate and the term may be of little comparative use between reef environments it is suggested that the term growth fabric be generally applied as a descriptive term for the presence of aggregated in situ corals or other organisms in growth position within a facies the term growth fabric should be used as an alternative to framework as a purely descriptive term the term framework should be restricted to its original definition and used only where the criteria for framework can be clearly demonstrated two end member types of scleractinian growth fabric genesis are recognised termed superstratal and constratal terms adapted from gili e masse j p skelton p w 1995 rudists as gregarious sediment dwellers not reef builders on cretaceous carbonate platforms palaeogeogr palaeoclimatol palaeoecol 118 245 267 in a superstratal growth fabric the constituent organisms collectively projected decimetres to metres above the substratum creating positive topographic relief in contrast constratal growth is where vertical organic accretion occurred at a similar rate to sediment accumulation hence the majority of the skeletal material remained continuously embedded in the sediments and only a small proportion of the skeleton projected centimetres above the sediment surface the architectural palaeoecological sedimentological and diagenetic implications of different types of growth fabric genesis is discussed a review of the classification of reef fabrics suggests that there is currently no adequate system to describe fossil scleractinian growth fabrics the most commonly used classification of reefal fabrics is that of embry and klovan 1971 embry a f klovan j e 1971 a late devonian reef tract on northeastern banks island northwest territories bull can pet geol 33 730 781 there are a number of shortcomings in this scheme which may be grouped into three categories 1 the interpretative nature of the classification 2 problems in interpreting biological effect from form and 3 insufficient categories to adequately describe mesozoic and cenozoic growth fabrics moreover there appears to be a lack of a standardised nomenclature for growth fabrics which has hindered meaningful comparisons of scleractinian growth fabrics through time and space a descriptive system based on a modification and expansion of the embry and klovan system 1971 is proposed and a revised nomenclature for growth fabrics presented the system is designed to be flexible in its application it can be used simply to describe a growth fabric or through the use of genetic and non genetic modifiers to imply types of reef building processes and growth fabric heterogeneity although the concepts and terminology discussed in this paper relate to scleractinian growth fabric they are equally applicable to fabrics comprising other organisms

alteration processes at deep sea drilling project ocean drilling program hole 504b at the costa rica rift implications for magnetization of oceanic crust magnetics properties and oxide petrography results are presented from the most recent penetration at hole 504b during ocean drilling program leg 111 our results combined with those from previous studies show abrupt first order changes in magnetic properties at alteration boundaries within the 504b crustal section changes in style and degree of alteration fall near the boundaries of the three well defined lithologic units the extrusive basalts the transition zone and the sheeted dike complex this postemplacement alteration heavily influences magnetic properties and is observed to change in both style and degree with depth within each lithologic unit our results indicate that at 504b the extrusive crustal section deeper than 600 m below seafloor has become magnetically less stable due to postemplacement reheating and alteration the upper more permeable basalts above this critical depth have not experienced this reheating within the sheeted dike complex the subsolidus cooling rate and the degree of hydrothermal alteration of the opaque minerals both decrease with depth the changes in alteration of the oxide minerals occur in parallel with the decrease in bulk permeability associated with increasing depth overall these observations suggest that the effective penetration of water into layer 2c decreased with increasing depth and resulted in a lower rate of convective cooling magnetic properties of rock samples from the sheeted dike complex suggest that as a result of this gradient in hydrothermal alteration the upper dikes have become magnetically more stable than the lower dikes a review of all magnetic properties indicate that the dike section at 504b carries a lower remanent magnetization than its intrinsic rock magnetic properties and mineralogy would predict we suggest that this lower remanent magnetization is a result of the long and complex thermal and alteration history which involves the acquisition of magnetic components in different directions despite a magnetization which is lower than expected it appears that the sheeted dike complex at hole 504b is capable of making a substantial contribution to the overlying marine magnetic anomaly because of the systematic decrease in hydrothermal alteration of magnetic minerals the ability of the dike section to contribute decreases as a function of depth

a revised classification of limestones abstract the most widely used classifications of limestones are now thirty years old and our appreciation of the diagenetic effects on limestone textures is now much greater a revision of the classifications of dunham 1962 and embry and klovan 1971 is offered and new diagenetic categories are proposed the confusing term lime mudstone is replaced by calcimudstone two categories of diagenetic textures are recognized non obliterative and obliterative the former contains limestones which are almost wholly composed of diagenetic cement cementstones and those whose fabric is the result of intergranular pressure solution condensed and fitted grainstones obliterative categories include limestones and dolomites whose original fabric has been destroyed sub categories include sparstones or dolosparstones crystals 10 m in size and microsparstones dolomicrosparstones crystals 4 10 m in size and dolomicrostones crystals m in size many diagenetic processes can produce fabrics which mimic depositional ones and these are reviewed

age dependent three dimensional magnetic modeling of the north pacific and north atlantic oceanic crust at intermediate wavelengths three dimensional magnetic modeling of the north atlantic and northeast pacific is performed at intermediate wavelengths using three models for the acquisition of a natural remanent magnetization it is shown that a remanent magnetization which is dependent on the crustal age is the dominant source for the intermediate wavelength pattern in both basins however a pure thermoremanent magnetization of layer 2 alone is insufficient to model the intensity and shape of the observed magnetic anomalies at satellite altitude we conclude that the best fitting magnetization model for both basins is a combination of a chemical remanent magnetization in the altered upper crust and a thermoviscous remanent magnetization of the slowly cooling lower crust and upper mantle the north pacific requires a bulk magnetization which is 50 higher than that of the north atlantic in order to fit the magsat field geological processes associated with a faster spreading rate such as a faster hydrothermal alteration and the growth of a thicker gabbro layer at the expense of a weakly magnetized sheeted dike layer are plausible explanations for the higher north pacific magnetization the lineated positive magnetic anomaly observed over the north atlantic spreading center is not well reproduced by our models this anomaly is likely due to a highly magnetized body along or in the vicinity of the spreading center this highly magnetic body could be an unstable serpentinized lens of crustal material younger than 20 m y which develops within the zone cooled by hydrothermal circulation

syngenetic formation of grainstones and pisolites from fenestral carbonates in peritidal settings abstract experiments with unconsolidated muds and petrofabric studies of some ancient peritidal carbonate rocks suggest that intense fenestral alteration of micritic sediments can lead to the in situ formation of grainstones of diagenetic origin intermediate stages in this process have been recognized in many ancient rocks the contemporaneous diagenetic modification of such grainstones commonly results in the formation of pisoliths and associated features that mimic those found in marine pisolites as well as in pisolitic caliche deposits of either marine or meteoric origin

effects of hydrothermal alteration on the magnetization of the oligocene carpenter ridge tuff bachelor caldera san juan mountains colorado intracaldera oligocene carpenter ridge tuff fills the bachelor caldera in the central san juan caldera complex and hosts mineral deposits of the creede mineral district the carpenter ridge tuff and unaltered portions of its intracaldera bachelor mountain member have strong high coercivity reverse magnetizations with average magnetic susceptibility ms and natural remanent magnetization nrm of 6 10 3 volume si and 8 a m respectively oxide phenocrysts in these rocks are titanomagnetite and ilmenite however magnetization appears to be controlled by microcrystic titanomaghemite based on thermal demagnetization unblocking temperatures and curie temperatures between 580 c and 620 c much of the intracaldera tuff was affected by potassic metasomatism a type of hydrothermal alteration characterized by addition of k and loss of ca and na between 27 3 ma and 25 1 ma potassic metasomatism resulted in the replacement of original feldspars by potassium feldspar and quartz oxidation of the original oxide phenocrysts to hematite and rutile and consequent suppression of ms and nrm by a factor of 5 also present however are metasomatized rocks that have high magnetizations even though their original oxide phenocrysts were destroyed values of ms and nrm are similar to those of outflow tuff such rocks are suspected of containing secondary magnetite on the basis of 1 curie and thermal unblocking temperatures at and below 580 c indicating that magnetization is carried by magnetite but that primary microcrysts of maghemite were destroyed and 2 observation in one sample of magnetite rimming cores of hematite rutile the presence of secondary magnetite in metasomatized rocks requires a local shift to more reducing fluid chemistry although we are unable to determine whether this shift occurred during potassic metasomatism or during later alteration or ore deposition these altered rocks may record a change from a pervasive alteration by alkaline oxidizing fluids to a vein controlled alteration dominated by more reduced fluids in which magnetite was stable

delayed magnetization of the deeper kilometer of oceanic crust at ocean drilling project site 504 the fe ti magnetic carriers of the dike rich second kilometer of oceanic crust at ocean drilling program site 504 are shown to be secondary in origin that is their formation is likely to considerably postdate initial igneous crustal formation an important consequence of the delay in formation of carriers is that the polarity of their magnetization has a significant probability of opposing that of the overlying little altered extrusives if this is the case the linear magnetic anomalies in the area have sources crust which at least locally is magnetized with successive layers of opposite polarities the magnitude of the contribution of deeper layers to anomalies will depend both on their thickness and average magnetization which are not as yet well known two types of secondary magnetite smi and smii separated spatially and by the steps leading to their formation are recognized in the dikes of the second kilometer of the crust at site 504 both were formed in conditions of hydrothermal greenschist metamorphism since all newly formed oceanic crust is thought to experience this type of alteration at an early stage polarity reversal with depth as a result of the delayed formation of magnetic carriers is likely to be widespread

the role of cementation in the diagenetic history of devonian reefs western canada devonian givetian and frasnian reef reservoirs in alberta and british columbia contain 60 of the conventional recoverable oil and 20 of the recoverable gas in the western canada sedimentary basin although the depositional history of these reefs is well understood it is the diagenetic overprint that is often responsible for their reservoir quality regardless of differences in size depositional history and age most reefs have been subjected to diagenesis in essentially three environments 1 submarine marine to hypersaline pore waters 2 subaerial fresh to brackish pore waters and 3 subsurface below phreatic aquifers saline to brackish pore waters primary porosity and permeability are altered by the overlapping processes of cementation and solution vadose and or phreatic that occur early in the diagenetic history in reef interiors these subaerial processes produce stratified reservoirs with impermeable barriers cemented beds to vertical flow e g golden spike swan hills judy creek submarine cementation is rare in most reefs but can be locally pervasive resulting in occlusion of bank margin and fore reef porosity absence of significant subsurface cementation in many reefs e g redwater golden spike aids in preservation of the reservoirs formed during earlier diagenesis

paleomagnetism of the upper cretaceous strata of mount tatlow evidence for 3000 km of northward displacement of the eastern coast belt british columbia this study contributes the first paleomagnetic data from dated cretaceous units of the coast belt 51 3 n 123 8 w in which paleohorizontal bedding surfaces can be identified with certainty we sampled 38 sites through a 1 5 km stratigraphic interval of cenomanian silverquick sedimentary strata and cenomanian to campanian powell creek volcanics in the tatlow syncline in the tyaughton methow basin magnetizations at 21 sites have high unblocking temperatures tub and pass the tilt test precision k 5 before untilting maximum k 48 after 100 untilting magnetic minerals of volcanics show original textures mean declinations in volcanic and sedimentary rocks are statistically identical and polarity is consistently normal the mean inclination 55 4 3 4 probability p 0 05 of this pretilting magnetization is 20 1 4 0 shallower than that expected if the rocks were deposited in their present position relative to cratonic north america field and laboratory observations rule out other causes for these anomalous shallow inclinations these rocks therefore have been displaced northward by 3000 500 km relative to cratonic north america since mid cretaceous time this displacement is almost 3 times greater than that estimated from mid cretaceous volcanics in the now adjacent intermontane belt evidently the two belts were not amalgamated until latest cretaceous or early tertiary time these results are inconsistent with all previous geologically based tectonic reconstructions of the cordillera magnetizations at eight sites in silverquick and powell creek beds have low tub and directional characteristics similar to those in three younger diorite intrusions reversed and normal polarities occur the rocks are commonly highly altered and maximum precision occurs after 40 untilting the magnetizations therefore are interpreted as overprints of latest cretaceous age acquired during folding

effects of pore pressure in pyroclastic flows numerical simulation and experimental validation pyroclastic flows are mixtures of volcanic gases and particles that can be very hazardous owing to their fluid like behavior one possible mechanism to explain this behavior is the reduction of particles friction due to the internal gas pore pressure to verify this hypothesis we present a numerical model of a granular flow with high initial pore pressure that decreases with time as the gas particle mixture propagates first we validate the model by reproducing laboratory experiments then the numerical code is applied to pyroclastic flows of lascar volcano 1993 eruption chile the simulation reproduces the runout and the morphological features of the deposits with lateral levees a central channel and a lobate front our results support the hypothesis of the role of gas pore pressure in pyroclastic flows and explain both the fluid like behavior of the flows and the formation of lateral levees

magnetization of the la palma seamount series implications for seamount paleopoles paleopoles determined from seamount magnetic anomalies constitute the major data source for the pacific apparent polar wander path but relatively little is known about the processes of remanence acquisition in seamounts since magnetic anomalies reflect both natural remanence nrm and the induced field it is important first to assess whether the nrm is likely to represent an original field direction and second to constrain the magnitude of the induced component to this end we present paleomagnetic data from an uplifted subaerially exposed section through a seamount on la palma canary islands the pliocene seamount series of la palma comprises a 6 km sequence of alkalic extrusives and intrusives which includes all lithologies likely to be volumetrically important in seamounts the structural tilt of the seamount series allows separation of early thermal or chemical remanence from magnetization components acquired after tilting e g viscous remanence the nrm provides a poor indication of the original magnetization direction although the characteristic magnetization of many la palma samples is compatible with the original pretilt direction hydrothermal alteration has resulted in the production of ti poor magnetite and an increasing contribution of hematite with increasing degree of alteration more importantly well defined magnetization directions which deviate from any reasonable geomagnetic direction at la palma can be attributed to hydrothermal alteration in a different polarity than prevalent during the original magnetization based on a comparison of the magnitude of low stability components of magnetization and laboratory acquisition of viscous remanence and previous estimates of the induced magnetization we conclude that viscous and induced magnetization probably account for 15 25 of the total magnetization of seamounts the resulting paleopole bias is a function of the polarity and paleolatitude of the seamount and ranges from 4o to 16 o for cretaceous seamounts in the pacific

micritic cement in microborings is not necessarily a shallow water indicator abstract intragranular micritic cement has been observed in microborings from deep sea sedimentary particles pteropod shells depths 528 m to 871 m it is particularly conspicuous in depositional environments such as the blake plateau such cementation may be concurrent and analogous to the formation of intergranular deep sea cement it can occur at such energy levels where particle to particle contacts do not persist for sufficient time to allow intergranular cementation

the characteristics and significance of secondary magnetite in a profile through the dike component of the troodos cyprus ophiolite secondary magnetite sm is the dominant fe ti oxide over a 3 4 km thick vertical interval in a profile through the troodos cyprus ophiolite sm is important in dikes and flows where dike density exceeds about 30 within the transition from extrusives to sheeted dikes and remains important until dike density falls below about 30 in the transition from sheeted dikes to plutonics the association of sm with high dike density suggests that it is a product of the thermal metamorphism of hydrothermally metamorphosed dikes and flows the most common occurrence of sm is at the site of primary titanomagnetites that had been altered earlier through deuteric low temperature and regional hydrothermal alteration processes sm often closely pseudomorphs the form of primary magnetite pm grains but it can be distinguished from pm using a series of criteria sm is low in tio2 and other impurities estimated and observed curie temperatures are undistinguishable which combined with optical evidence indicates tha

the magnetization of oceanic crust contribution to knowledge from the troodos cyprus ophiolite an extensive study of a segment of the troodos cyprus ophiolite using both outcrop and drill core samples and extending from the sediment extrusive interface through sheeted dikes to cumulate ultramafics has allowed a number of key questions regarding the magnetization of oceanic crust to be addressed these include the number of strongly magnetized intervals with depth their lateral variability and controls on their occurrence comparison has also been made with the section in ocean drilling program odp hole 504b and a reinterpretation of its constructional setting is offered two strongly magnetized intervals occur in the area studied the upper is in the extrusive sequence extends on average from 0 2 to 0 6 km depth and has a thickness of 0 4 km here magnetization is dominated by remanence the lower interval extends from the lowest level at which flows occur with dikes average depth 0 9 km into the sheeted complex average depth 1 2 km and has a thickness of 0 3 km here magnetization is dominantly induced no other strongly magnetized intervals occur in the section the extent of dike intrusion is closely related to the position of the lower limit of the highremanence layer and to the occurrence of the high induced magnetization layer in both cases the replacement of primary magnetite which can carry a strong remanence by magnetically soft secondary magnetite appears to be the controlling process comparison of the troodos and hole 504b magnetization profiles shows close similarity in the upper remanence dominated magnetic interval the absence of the deeper interval of high induced magnetization in the hole 504b profile is interpreted as meaning that sheeted dikes have not been penetrated by the drill hole

packstones and floatstones ambiguous textures and origins in need of critical appraisal abstract the depositional textural classification of allochthonous limestones by dunham 1962 and its modifications by embry iii and klovan 1971 are widely used by carbonate petrologists during petrographic field studies and core examinations the origin and textures of packstones and floatstones used in this classification scheme need re appraisal towards their unified and unequivocal usage by carbonate petrologists in academia and the petroleum industry such usage will help avoiding inadequate interpretations of the depositional sequence stratigraphic or reservoir quality distribution models based on examination of jurassic cretaceous and holocene carbonate deposits from united arab emirates and oman it is suggested here that packstones should not be used to interpret energy of depositional environments because the majority are originally deposited as grainstones i e free of mud matrix thus sand and mud sized particles are expected to be hydraulically separated i e not deposited simultaneously therefore packstones might be originally grainstones that have incorporated variable amounts of mud matrix by various post depositional processes primarily bioturbation and or compaction deformation of peloids known as pseudomatrix dickinson 1970 we further observed that the 2 mm allochems in floatstones float in grainstones and packstones and not in mud wackestone matrix

accumulation of bank top sediment on the western slope of great bahama bank rapid progradation of a carbonate megabank high resolution seismic profiles and submersible observations along the leeward slope of western great bahama bank show large scale export of bank top sediment and rapid progradation of the slope during the holocene a wedge shaped sequence up to 90 m thick is present along most of the slope and consists of predominantly aragonite mud derived from the bank since flooding of the platform 6 8 ka total sediment volume of the slope sequence is 40 80 that of holocene sediment currently retained on the bank maximum rates of vertical accumulation and lateral progradation are 11 15 m ka and 80 110 m ka respectively 10 to 100 times greater than previously known for periplatform muds slope deposition of exported mud during sea level highs appears to have been a major mechanism for the westward progradation of great bahama bank throughout the quaternary this may provide a critical modern analogue for ancient progradational margins

the perfect debris flow aggregated results from 28 large scale experiments 1 aggregation of data collected in 28 controlled experiments reveals reproducible debris flow behavior that provides a clear target for model tests in each experiment 10 m3 of unsorted water saturated sediment composed mostly of sand and gravel discharged from behind a gate descended a steep 95 m flume and formed a deposit on a nearly horizontal runout surface experiment subsets were distinguished by differing basal boundary conditions 1 versus 16 mm roughness heights and sediment mud contents 1 versus 7 percent dry weight sensor measurements of evolving flow thicknesses basal normal stresses and basal pore fluid pressures demonstrate that debris flows in all subsets developed dilated coarse grained high friction snouts followed by bodies of nearly liquefied finer grained debris mud enhanced flow mobility by maintaining high pore pressures in flow bodies and bed roughness reduced flow speeds but not distances of flow runout roughness had these effects because it promoted debris agitation and grain size segregation and thereby aided growth of lateral levees that channelized flow grain size segregation also contributed to development of ubiquitous roll waves which had diverse amplitudes exhibiting fractal number size distributions despite the influence of these waves and other sources of dispersion the aggregated data have well defined patterns that help constrain individual terms in a depth averaged debris flow model the patterns imply that local flow resistance evolved together with global flow dynamics contradicting the hypothesis that any consistent rheology applied we infer that new evolution equations not new rheologies are needed to explain how characteristic debris flow behavior emerges from the interactions of debris constituents

extensive submarine lithification in a cave in the belize barrier reef platform abstract extensive deposits of submarine cements discovered in a submerged pleistocene cave in the outer barrier reef platform off belize central america occur in an unusual setting on and within countless calcareous projections formed by serpulid worms on the ceiling of the cave the cements are predominantly dense submicrocrystalline and microcrystalline to porous chalky or submicrosucrosic magnesium calcite approximately 15 mole mgco3 and they contain minor amounts of acicular aragonite which generally forms a lining inside the aragonitic serpulid tubes radiocarbon dates three year artificial substrate experiments and evidence of the ceiling s original relief indicate a low rate of accumulation for the serpulid cement projections the characteristic 20 60 m peloidal texture associated with magnesium calcite submarine cements is well developed here and there within the projections these peloids are physiochemical precipitates having formed during the development of the submarine cement deposits which accumulated preferentially on substrates that have a slow rate of accumulation and that experience little or no disturbance of their sediment cover artificial substrates placed on the ceiling of the cave for three years revealed that the initial stage of precipitation of magnesium calcite from seawater is in the form of a submicrocrystalline magnesium calcite dust

can the collapse of a fly ash heap develop into an air fluidized flow reanalysis of the jupille accident 1961 a fly ash heap collapse occurred in jupille liege belgium in 1961 the subsequent flow of fly ash reached a surprisingly long runout and had catastrophic consequences its unprecedented degree of fluidization attracted scientific attention as drillings and direct observations revealed no water saturated zone at the base of the deposits scientists assumed an air fluidization mechanism which appeared consistent with the properties of the material in this paper the air fluidization assumption is tested based on two dimensional numerical simulations the numerical model has been developed so as to focus on the most prominent processes governing the flow with parameters constrained by their physical interpretation results are compared to accurate field observations and are presented for different stages in the model enhancement so as to provide a base for a discussion of the relative influence of pore pressure dissipation and pore pressure generation these results show that the apparently high diffusion coefficient that characterizes the dissipation of air pore pressures is in fact sufficiently low for an important degree of fluidization to be maintained during a flow of hundreds of meters

viscous magnetization at 300 k in a profile through troodos type oceanic crust abstract this study examines the question of the relative importance of viscous magnetism vm at 300 k to the total magnetization of older oceanic crust a section through the troodos cyprus ophiolite now recognized as a good proxy for in situ oceanic crust has been used as the source of samples the section extends downwards continuously from little altered submarine extrusives through greenschist facies sheeted dikes to mafic and variously serpentinized ultramafic cumulate intrusives the principal result of the investigation is that vm is not expected to be the dominant magnetization and is often relatively negligible at all crustal levels vm acquisition varies irregularly with depth with predicted maximum values equivalent to about one third of the total magnetization in the extrusives and sheeted dikes the contribution of vm in two samples of serpentinized ultramafics is insignificant vm acquisition does not show any simple relationships with primary lithology alteration history or magnetic properties or history the occurrence of two stage vm growth mechanisms is widespread the second stage typically has an onset time of about 103 min and an acquisition rate three times that of the initial growth mechanism the physical origin of this two stage mechanism and its significance in predicting vm growth over geologic time intervals are considered to be important problems for future work

constructional features of troodos type oceanic crust relationships between dike density alteration magnetization and ore body distribution and their implications for in situ oceanic crust quantitative relationships are demonstrated between dike density and hydrothermal alteration magnetization location of volcanic massive sulfide vms ore bodies and a number of other properties in a 40 by 15 km segment of the extrusive series of the troodos ophiolite low temperature alteration extends from the sediment extrusive interface to close to the 25 dike density surface while the onset of greenstone type alteration is closely associated with the 50 dike density surface these associations are explained in terms of the regional scale variations in the dike density contour surfaces following the form of the top of the gabbroic layer of the ophiolite an upper magnetic zone characterized by stable high remanence terminates just below the 25 dike density surface a deeper magnetic zone characterized by high induced magnetization straddles the top of the sheeted complex volcanic massive sulfide bodies are concentrated in a narrow depth range centered about 0 1 km above the 25 dike density surface a model for the postridge crest evolution of ore bodies accounts for this optimum depth range for preservation physical property changes with dike density and the depth distribution of vms ore bodies and prospects for the ophiolite as a whole support this model similar relationships appear to be present in the samail macquarie island newfoundland and chilean ophiolites and icelandic crust as a result of relationships found in the troodos ophiolite an alternative is proposed to the current lithologic profile for ocean drilling program hole 504b

biofilm related calcification sediment trapping and constructive micrite envelopes a criterion for the recognition of ancient grass bed environments micrite envelopes are a common feature in carbonate sediments and are typically associated with the micrite filling of borings produced by microendolithic organisms these are referred to as destructive micrite envelopes and have long been recognized as reflecting an important early diagenetic process recent analysis of sediments collected from back reef environments at discovery bay north jamaica however has demonstrated envelope formation on the surfaces of carbonate grains clearly distinct from the micrite filling of microborings such constructive envelopes occur almost exclusively in sediments from grass bed environments and are always intimately associated with biofilms comprising abundant mucilage cyanobacteria bacteria and diatoms it is suggested that these envelopes represent a product of both biologically mediated micrite precipitation occurring within the biofilm mucilage and around the biofilm components i e cyanobacteria and diatoms and associated trapping of carbonate mud and fine grained sediment their recognition only within grass bed sediments may enable their use as a diagnostic feature of grass bed environments or vegetation stabilized substrates in the rock record

pore fluid pressure and internal kinematics of gravitational laboratory air particle flows insights into the emplacement dynamics of pyroclastic flows 1 the emplacement dynamics of pyroclastic flows were investigated through noninvasive measurements of the pore fluid pressure in laboratory air particle flows generated from the release of fluidized and nonfluidized granular columns analyses of high speed videos allowed for correlation of the pressure signal with the flow structure the flows consisted of a sliding head that caused underpressure relative to the ambient followed by a body that generated overpressure and at the base of which a deposit aggraded for initially fluidized flows overpressure in the body derived from advection of the pore pressure generated in the initial column and decreased by diffusion during propagation relatively slow diffusion caused the pore pressure in the thinner flow to be larger than lithostatic at early stages furthermore partial auto fluidization revealed in initially nonfluidized flows also occurred and contributed to maintain high pore pressure whereas dilation or contraction of the air particle mixture with associated drag and or pore volume variation transiently led the pressure to decrease or increase respectively the combination of all these processes resulted in long lived high pore fluid pressure in the body of the flows during most of their emplacement in the case of the initially fluidized and slightly expanded 3 4 flows at least 70 100 of the weight of the particles was supported by pore pressure which is consistent with their inertial fluid like behavior dense pyroclastic flows on subhorizontal slopes are expected to propagate as inertial fluidized gas particle mixtures consisting of a sliding head possibly entraining basement derived clasts and of a gradually depositing body

magnetic properties of dikes from the oceanic upper crustal section recovery of dredge samples of upper crustal dikes from the steep north wall of the blanco fracture zone west blanco deep allows an evaluation of this rock type as a possible contributor to the magnetic anomaly source layer magnetic data from these dike sill samples indicate that the majority of the rocks meet the essential criteria as a contributor to the source layer including a high magnetic intensity 2 7 a m a ratio of remanent to induced magnetization that is greater than 1 and a relative stability against large scale viscous remanent magnetization acquisition over long geological time periods however because the natural remanent magnetization intensities of the dike samples are an order of magnitude lower than the extrusive rocks within the axial zone the presence of abundant near surface dikes would lower the net magnetization of a crustal section and may be responsible for the axial magnetic low that has been observed on medium to fast spreading centers if the magnetization of the dikes does not change with time due to low temperature oxidation then the decreasing ratio of extrusive intrusive contribution as the pillow basalts undergo systematic oxidation will cause the magnetic layer to appear to thicken and the polarity transition zones to widen as the crust ages and moves off axis

expanded reef rock textural classification and the geologic history of bryozoan reefs the wide range of colony forms assumed by bryozoans results in a greater variety of bryozoan reef carbonate rock types than can be accommodated within present classifications consequently the currently used textural classification is expanded to include several new reef rock categories branchstone cruststone globstone lettucestone shellstone immediately applicable to bryozoan reefs those in the early paleozoic were small mounds largely cruststones and bindstones accompanied later by globstones and lettucestones a very few similar mounds live today others many larger in the mid and late paleozoic were branchstones and bafflestones in some cases so sediment rich as to appear mostly as floatstones with interspersed rudstones to micstones

peloidal fabrics in upper jurassic reefal limestones weald basin southern england abstract coraliferous limestones from the corallian group upper jurassic of subsurface of the weald basin southern england record the localized development of superimposed patch reefs and banks the reefal limestones exhibit a variety of peloidal fabrics in particular well sorted peloids 10 60 m in diameter with a well developed euhedral rim of fibrous sparry calcite around a micritic anhedral nucleus they occur in confined intraskeletal pores corals molluscs and borings growth cavities between framebuilders and as stromatolitic crusts the peloids in interskeletal cavities and as crusts resemble microbial precipitates and merge into spongiostromate fabrics the distribution of these peloidal fabrics seems to be facies related and is best developed in zones which had relatively low framework growth rates low sedimentation rates and moderate energy levels the peloids are identical in form and occurrence to those in some ancient and quaternary reefs the association of the peloids with stromatolitic coatings and spongiostromate fabrics leads us to interpret the peloids as microbial precipitates either by direct precipitation or by induction the abundance of microbial carbonate in these reefal limestones places them in an intermediate category between frame built reefs and microbial mud mounds

internal precipitation of microcrystalline carbonate a fundamental problem for sedimentologists abstract recent studies of submarine lithification have shown that microcrystalline carbonate or carbonate mud traditionally thought to represent ooze deposited in quiet water commonly forms as an internal precipitate below the sediment water interface this internally precipitated micrite alters the mud grain ratio of pre existing sedimentary deposits and invalidates classifications that relate the amount of micrite matrix in limestones to the degree of water energy associated with the environment of deposition

geology of continental shelf onslow bay north carolina as revealed by submarine outcrops lithologic and stratigraphic data from rocks dredged from the continental shelf off onslow bay north carolina provide surface control for seismic studies of the southeastern united states continental margin and help to explain the distribution of potentially economic phosphate rich sediments on this shelf outcropping miocene rocks in this area indicate that the region has long been a positive geologic feature and has received relatively little pliocene and pleistocene sedimentation leached molluscan moldic calcareous quartz sandstones of late oligocene to early miocene age belgrade formation and middle to late miocene age pungo river formation crop out in southwestern and northeastern onslow bay respectively these two areas border a band of highly phosphatic su ficial sediments probably derived from unlithified phosphatic units of the pungo river formation lower pleistocene calcarenites that correlate with the waccamaw formation crop out on the sea floor near cape fear which bounds onslow bay on the south a core on frying pan shoals off cape fear after passing through pleistocene coquina calcareous quartz sandstone and oolitic sand penetrated upper pliocene calcarenites of the bear bluff formation and middle miocene phosphatic argillaceous sandstones of the pungo river formation samples from this core hole show that phosphate increases toward the top of the unindurated pungo river formation section indicating that this formation is probably the source of the high phosphate concentrations in the surficial sediments near cape fear wher the bear bluff calcarenites also crop out upper pleistocene units include oolitic limestones adjacent to the outer shelf calcareous quartz sandstones around cape fear and molluscan coquinas near cape lookout which bounds onslow bay on the north the outer shelf is blanketed by submarine lithified algal limestones and sandstones of holocene age

fully automated carbonate petrography using deep convolutional neural networks abstract carbonate rocks are important archives of past ocean conditions as well as hosts of economic resources such as hydrocarbons water and minerals geologists typically perform compositional analysis of grain matrix cement and pore types in order to interpret depositional environments diagenetic modification and reservoir quality of carbonate strata such information can be obtained primarily from petrographic analysis a task that is costly labor intensive and requires in depth knowledge of carbonate petrology and micropaleontology recent studies have leveraged machine learning based image analysis including deep convolutional neural networks dcnn to automate description classification and interpretation of thin sections subsurface core images and seismic facies which would accelerate data acquisition and reproducibility for these tasks in carbonate rocks this approach has been applied primarily to recognize carbonate lithofacies and no attempt has been made to individually identify and quantify various types of carbonate grains matrix and cement in this study the applicability and performance of dcnn based object detection and image classification approaches are assessed with respect to carbonate compositional analysis the training data comprised of more than 13 000 individually labelled objects from nearly 4000 carbonate petrographic images the dataset is grouped into six and nine different classes for the image classification and object detection tasks respectively even with a small and relatively imbalanced training set the dcnn was able to achieve an f1 score of 92 for image classification and 84 mean precision for object detection by combining one cycle policy class weight and label mixup smoothing methods this study highlights the inefficiency of image classification as an approach to replicating human description and classification of carbonate petrography by contrast dcnn based object detection appears capable of approaching human speed and accuracy in the area of carbonate petrography because it is able to individually locate and identify different carbonate components with greater cost efficiency speed and reproducibility than conventional human petrographic analysis

relation between dry granular flow regimes and morphology of deposits formation of levees in pyroclastic deposits experiments on dry granular matter flowing down an inclined plane are performed in order to study the dynamics of dense pyroclastic flows the plane is rough and always wider than the flow focusing this study on the case of laterally unconfined free boundary flows we found that several flow regimes exist depending on the input flux and on the inclination of the plane each flow regime corresponds to a particular morphology of the associated deposit in one of these regimes the flow reaches a steady state and the deposit exhibits a levee channel morphology similar to those observed on small pyroclastic flow deposits the levees result from the combination between lateral static zones on each border of the flow and the drainage of the central part of the flow after the supply stops particle segregation features are created during the flow corresponding to those observed on the deposits of pyroclastic flows moreover the measurements of the deposit morphology thickness of the channel height of the levees width of the deposit are quantitatively related to parameters of the dynamics of the flow flux velocity height of the flow leading to a way of studying the flow dynamics from only measurements of the deposit some attempts to make extensions to natural cases are discussed through experiments introducing the polydispersity of the particle sizes and the particle segregation process