the mechanism of surtseyan volcanism during eruption the vent of a surtseyan volcano is occupied by a highly mobile slurry of tephra hyaloclastite and water as magma ascends rapidly through the slurry mixing occurs due to velocity shear acceleration of the fluid fluid contacts and mass incorporation consequent expansion of the admixed water causes the jetting and continuous up rush activity that characterizes surtseyan volcanism there is no evidence of the widely accepted fuel coolant interaction mechanism

the dykes and structural setting of the volcanic front in the lesser antilles island arc the orientations of dykes from many of the islands of the lesser antilles island arc have been mapped most of these dykes can be interpreted in terms of local or regional swarms derived from specific volcanoes of known age with distinct preferred orientations dykes are known from all cenozoic epochs except the palaeocene but are most common in pliocene miocene and oligocene rocks a majority of the sampled dykes are basaltic intrude volcaniclastic host rocks and show a preference for widths of 1 1 25 m locally dyke swarms dilate their hosts by up to 9 over hundreds of metres and up to 2 over distances of kilometres the azimuths of dykes of all ages show a general ne sw preferred orientation with a second nw se mode particularly in the miocene rocks of martinique the regional setting for these minor intrusions is a volcanic front above a subduction zone composed of three segments saba montserrat guadeloupe martinique st lucia grenada the spacing of volcanic centres along this front is interpreted in terms of rising plumes of basaltic magma spaced about 30 km apart this magma is normally intercepted at crustal depths by dioritic plutons and andesitic dacitic magma generated there plumes which intersect transverse fracture systems or which migrate along the front can avoid these crustal traps throughout its history the volcanic front as a whole has migrated episodically towards the backarc at an average velocity of about 1 km ma the local direction of plate convergence is negatively correlated with the local preferred orientation of dykes the dominant ne sw azimuth mode corresponds closely to the direction of faulting in the sedimentary cover of the backarc and the inferred tectonic fabric of the oceanic crust on which the arc is founded a generalised model of the regional stress field that controls dyke intrusion outside of the immediate vicinity of central volcanic vents is proposed in which the maximum horizontal stress parallels the volcanic front except in the northern segment where subduction of the barracuda rise perturbs the stress field there is also evidence of specific temporal changes in the stress field that are probably due to large scale plate kinematics

structure and composition of organic reefs and carbonate mud mounds concepts and categories defined here as essentially in place calcareous deposits created by sessile organisms organic reefs are diverse and complex structures with a long geological history their classification has been the subject of fierce debate often characterized by reliance on subjective features such as wave resistance and qualitative attempts to discriminate between first and second class reefs in contrast emphasis is here placed on the objective characteristic of the type of sedimentary support which largely determines the sedimentary composition of the deposit constructional and depositional processes result in three principal sedimentary components matrix m essentially in place skeletons s and cavity cement c whose proportions can be represented on msc triangular plots separately or together these components also provide the structural support for the reef on these compositional and structural bases three main categories of organic reef are recognized 1 matrix supported reefs agglutinated microbial reefs cluster reefs segment reefs 2 skeleton supported reefs frame reefs 3 cement supported reefs cement reefs agglutinated microbial reefs possess laminated clotted or aphanitic fabrics created by microbial trapping of particulate sediment in place skeletons and large primary cavities are rare early cementation may provide added support topographic relief is limited by the need for currents to provide sediment to accreting surfaces cluster reefs skeletal reefs in which essentially in place skeletons are adjacent but not in contact resulting in matrix support characterized by relatively high matrix skeleton ratios and low volumes of extra skeletal early cement sediment trapping is an important corollary of skeletal growth and cluster reef organisms are tolerant of loose sediment absence of framework limits the topographic relief that cluster reefs can attain relative to spatial extent and may permit bedding to develop within the reef close cluster reefs have skeletons up to 1 unit distance apart spaced cluster reefs have skeletons more than 1 and up to 2 unit distances apart with increasing separation of skeletons they grade to level bottom communities segment reefs matrix supported reefs in which skeletons are adjacent and may be in contact but are mostly disarticulated and mainly parauthochtonous matrix abundance is high and early cement relatively low moderate relief can develop in response to intense on reef sediment production frame reefs skeletal reefs in which essentially in place skeletons including calcified microbes are in contact characterized by relatively high skeleton matrix ratio skeletal support enables them to raise themselves above the substrate independently of cementation and particulate sedimentation simultaneously by creating partly open shelter cavities skeletal support may facilitate early cementation both relief and early lithification promote marginal talus formation skeletal shape and orientation distinguish conical stick like dendritic domical and laminar frames each of these may be open or filled open frame reefs

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

sector collapse at kick em jenny submarine volcano lesser antilles numerical simulation and landslide behaviour kick em jenny volcano is the only known active submarine volcano in the lesser antilles it lies within a horseshoe shaped structure open to the west northwest toward the deep grenada basin a detailed bathymetric survey of the basin slope at kick em jenny and resulting high resolution digital elevation model allowed the identification of a major submarine landslide deposit this deposit is thought to result from a single sector collapse event at kick em jenny and to be linked to the formation of the horseshoe shaped structure we estimated the volume and the leading edge runout of the landslide to be ca 4 4 km3 and 14 km respectively we modelled a sector collapse event of a proto kick em jenny volcano using volcflow a finite difference code based on depth integrated mass and momentum equations our models show that the landslide can be simulated by either a coulomb type rheology with low basal friction angles 5 5 6 5 and a significant internal friction angle above 17 5 or with better results by a bingham rheology with low bingham kinematic viscosity 0 b 30 m2 s and high shear strength 130 180 m2 s2 the models and the short runout distance suggest that the landslide travelled as a stiff cohesive flow affected by minimal granular disaggregation and slumping on a non lubricated surface the main submarine landslide deposit can therefore be considered as a submarine mass slide deposit that behaved like a slump

microbial carbonates the geological record of the calcified bacterial algal mats and biofilms summary deposits produced by microbial growth and metabolism have been important components of carbonate sediments since the archaean geologically best known in seas and lakes microbial carbonates are also important at the present day in fluviatile spring cave and soil environments the principal organisms involved are bacteria particularly cyanobacteria small algae and fungi that participate in the growth of microbial biofilms and mats grain trapping is locally important but the key process is precipitation producing reefal accumulations of calcified microbes and enhancing mat accretion and preservation various metabolic processes such as photosynthetic uptake of co2 and or hco3 by cyanobacteria and ammonification denitrification and sulphate reduction by other bacteria can increase alkalinity and stimulate carbonate precipitation extracellular polymeric substances widely produced by microbes for attachment and protection are important in providing nucleation sites and facilitating sediment trapping microbial carbonate microfabrics are heterogeneous they commonly incorporate trapped particles and in situ algae and invertebrates and crystals form around bacterial cells but the main component is dense clotted or peloidal micrite resulting from calcification of bacterial cells sheaths and biofilm and from phytoplankton stimulated whiting nucleation interpretation of these texturally convergent and often inscrutable fabrics is a challenge conspicuous accumulations are large domes and columns with laminated stromatolite clotted thrombolite and other macrofabrics which may be either agglutinated or mainly composed of calcified or spar encrusted microbes stromatolite lamination appears to be primary but clotted thrombolite fabrics can be primary or secondary microbial precipitation also contributes to hot spring travertine cold spring mound calcrete cave crust and coated grain deposits as well as influencing carbonate cementation recrystallization and replacement microbial carbonate is biologically stimulated but also requires favourable saturation state in ambient water and thus relies uniquely on a combination of biotic and abiotic factors this overriding environmental control is seen at the present day by the localization of microbial carbonates in calcareous streams and springs and in shallow tropical seas and in the past by temporal variation in abundance of marine microbial carbonates patterns of cyanobacterial calcification and microbial dome formation through time appear to reflect fluctuations in seawater chemistry stromatolites appeared at 3450 ma and were generally diverse and abundant from 2800 to 1000 ma inception of a proterozoic decline variously identified at 2000 1000 and 675 ma has been attributed to eukaryote competition and or reduced lithification thrombolites and dendrolites mainly formed by calcified cyanobacteria became important early in the palaeozoic and reappeared in the late devonian microbial carbonates retained importance through much of the mesozoic became scarcer in marine environments in the cenozoic but locally re emerged as large agglutinated domes possibly reflecting increased algal involvement and thick micritic reef crusts in the late neogene famous modern examples at shark bay and lee stocking island are composite coarse agglutinated domes and columns with complex bacterial algal mats occurring in environments that are both stressed and current swept products of mat evolution ecological refugia sites of enhanced early lithification or all three

upper permian main dolomite microbial carbonates as potential source rocks for hydrocarbons w poland abstract various microbialite lithofacies were common in the shallow water environments which flourished under highly saline and arid climate conditions in the polish part of the zechstein upper permian main dolomite ca2 carbonates data came from detailed analysis of 78 cores from wells located at the southern and northern margins of the main dolomite basin in west poland microbialite lithofacies are represented by columnar planar and domal stromatolites clotted thrombolites and biolaminites developed in high to low energy environments within the upper slope lower parts of oolitic barrier shoal restricted lagoon and tidal flat and tidal channel zones in barrier environments ooids were predominant whereas in lagoonal settings microbial oncoids and peloids were common in basinal settings laminated dolomudstones formed which are regarded as hemipelagic and are partly pelagic microbial in origin microbial communities coexisted with algae and developed mainly within shallow marine environments microbialites are built of cyanobacterial filaments and organo mineral coccoidal forms possibly of bacterial origin organic geochemistry data confirm that organic matter was transformed by cyanobacteria and bacteria which may have played a main role in hydrocarbon generation this is confirmed by c29 to c35 hopanes pr ph ratio below 1 0 and the presence of gammacerane as typical for ca2 profiles the role and occurrence of algae as confirmed by c29 steranes in hydrocarbon generation was possibly minor thus it is concluded that the main dolomite microbialites could be the likely source rocks for hydrocarbons with calculated original toc values from 0 8 to ca 2 0 wt these units also form the reservoir rocks with porosities reaching 20

magma ascent rates from amphibole breakdown an experimental study applied to the 1980 1986 mount st helens eruptions recent 1980 1986 mount st helens dacites contain the phenocryst assemblage plagioclase amphibole low ca pyroxene magnetite ilmenite and rare high ca pyroxene which indicates that they all originated from an 8 km deep reservoir at 900 20 c with xh2o 0 67 in fluid according to experimental data iron titanium oxide phenocryst compositions indicate that all post may 18 dacitic magmas erupted at 900 20 c except for the final lava extrusion in october 1986 the magma reservoir may have cooled to 866 c by october 1986 amphiboles in the post may 18 1980 magma contain one or more amphibole populations characterized by reaction rims of different thicknesses the development of the amphibole reaction rims in these rocks is a response to water loss from the coexisting melt during an approximately adiabatic ascent from a deep reservoir constant p and t and isothermal decompression experiments show that during a 900 c constant rate decompression from 8 km to the surface no reaction rim develops on amphibole in 4 days a 10 m rim develops in 10 days and a 35 m rim develops in 20 days these experimental data and histograms of rim widths in 1980 1986 mount st helens dacites show that post may 18 eruptions are composed in large part of magma represented by a population of thin rimmed amphiboles magma which ascended from the deep 8 km reservoir in 6 to 10 days the remainder of each sample consists of magma containing amphiboles with reaction rims ranging from 14 to 60 m magma which apparently spent from 8 to 25 days along the conduit margins before being mixed thoroughly millimeter scale into the erupting magma the mixing in a viscous slowly ascending dacite may be enhanced by its flow through partially crystallized magma emplaced earlier and by the evolution and loss of a large vesicle population the experimental calibration of amphibole reaction rim width versus decompression time yields average ascent velocities for post may 18 dacites of about 15 30 m h for magma represented by the thick rimmed amphiboles and from 35 to 50 m hr for magma represented by the thin rimmed crystals an ascent rate of 66 m h is indicated for the may 18 1980 eruption which contains amphiboles with no reaction rims the volume of endogenous dome growth which preceded extrusion of magma newly derived from the deep source region suggests that the effective conduit volume beneath mount st helens in 1981 1982 was equivalent to a cylinder 8 km long and 8 9 m in radius

regional setting and role of meteoric water in dolomite formation and diagenesis in an evaporite basin studies in the zechstein permian deposits of poland the zechstein basin of poland was an area of widespread cyclical deposition of carbonates and evaporites during late permian time the zechstein shelves along both the northern and the southern margins of the basin were sites of shallow water sedimentation during the formation of the main dolomite and platy dolomite two widespread carbonate units these units consist of oolitic peloidal skeletal micritic and evaporitic carbonates formed in depositional settings ranging from open marine to coastal lagoonal sabkha and salina although originally deposited as limestones the main dolomite and platy dolomite are inferred to have been completely replaced by dolomite through very early stage essentially penecontemporaneous reflux of hypersaline brines the dolomites of the two basin margins however have very different petrographic and isotopic characteristics many northern shelf dolomites show early stage calcitization dedolomitization and even in some cases evidence of a subsequent redolomitization event these northern shelf samples also have a broad range of carbon and oxygen isotopic ratios up to 12 0 for oxygen samples from the southern shelf on the other hand are petrographically much simpler they do not show complex calcitization and redolomitization patterns likewise their isotopic values are much more tightly clustered with only about a 5 0 range of oxygen isotopic ratios the differences between dolomites of the same age from the northern and southern margins are best explained by regional variations in river water influx during episodic exposure events associated with regional or global sea level fluctuations the distribution of clastic terrigenous materials and palaeokarstic features indicate that areas of the northern shelf had extensive river input an influx largely lacking on the southern shelf early formed dolomites appear to have been calcitized during sea level lowstands through the infiltration of meteoric fluids into the evaporitic dolomites created during the previous highstand in some cases redolomitization occurred when meteoric fluids were again replaced by hypersaline brines during subsequent sea level highstands although repeated sea level fluctuations are clearly evident in these strata it is likely that associated climatic changes rainfall variations also played a role in forming these complex diagenetic patterns age equivalent strata from texas and new mexico from sites at much lower palaeolatitudes show no such alteration patterns samples from greenland slightly higher palaeolatitudes show even more intense diagenetic alteration during depositional cycles thus the examination of patterns of diagenesis may be useful in interpreting ancient palaeolatitudinally sensitive climate patterns

hydrothermal venting and mineralization in the crater of kick em jenny submarine volcano grenada lesser antilles kick em jenny is a frequently erupting shallow submarine volcano located 7 5 km off the northern coast of grenada in the lesser antilles subduction zone focused and diffuse hydrothermal venting is taking place mainly within a small 70 110 m depression within the 300 m diameter crater of the volcano at depths of about 265 m much of the crater is blanketed with a layer of fine grained tephra that has undergone hydrothermal alteration clear fluids and gas are being discharged near the center of the depression from mound like vents at a maximum temperature of 180 c the gas consists of 93 96 co2 with trace amounts of methane and hydrogen gas flux measurements of individual bubble streams range from 10 to 100 kg of co2 per day diffuse venting with temperatures 5 35 c above ambient occurs throughout the depression and over large areas of the main crater these zones are colonized by reddish yellow bacteria with the production of fe oxyhydroxides as surface coatings fragile spires up to several meters in height and elongated mounds up to tens of centimeters thick a high resolution photomosaic of the inner crater depression shows fluid flow patterns descending the sides of the depression toward the crater floor we suggest that the negatively buoyant fluid flow is the result of phase separation of hydrothermal fluids at kick em jenny generating a dense saline component that does not rise despite its elevated temperature

sequence stratigraphy of carbonate evaporite basins models and application to the upper permian zechstein of northeast england and adjoining north sea the successions of carbonate evaporite basins which are periodically cut off from the world ocean are considered in terms of their sequence stratigraphy two basin types are distinguished those where incomplete drawdown takes place and those showing complete drawdown and wholesale basin desiccation it is argued that following relative sea level falls sequences begin with the evaporites precipitated as marginal lowstand gypsum wedges in the first basin type and as lowstand basin centre halite fills in the second the evaporites pass up into carbonate sediments which are mainly deposited on shallow water platforms around the basin during transgressive and highstand systems tracts the concepts and models developed are applied to the zechstein upper permian strata of northeast england and adjoining north sea seven sequences are proposed with some containing thin parasequences the sequence approach to carbonate evaporite basins which considers both basin margin and basin centre facies gives a more encompassing perspective of sedimentation than the traditional cycle concept with its emphasis on the basin centre

a magmatic geohygrometer application to mount st helens and other dacitic magmas compositions of andesitic to dacitic melts in equilibrium with plagioclase orthopyroxene and clinopyroxene have been experimentally calibrated as a geohygrometer application to melson s data on melt inclusions and matrix glasses from the 1980 1982 cycle of eruptions of mount st helens reveals systematic dewatering trends ca 4 1980 to 1 1982 h 20 in subliquidus intratelluric melt in pyroclastics and 2 3 1980 to 1 1982 in subliquidus melt in dome building dacites before each pyroclastic event intratelluric crystallization achieved melt h 20 contents of ca 6 at a depth of at least 8 km a necessary condition for crystallization of amphibole dewatering trends indicate that individual magma blobs or ribbons then ascended to successively higher levels over the 3 yr cycle from a minimum depth of 4 km in 1980 to 0 35 km in 1982 and pyroclastically erupted comparison of mount st helens compositions to worldwide andesites and dacites suggests that near liquidus arc magmas that crystallize plagioclase and pyroxenes contain l 2 h 2 0 on average whereas magmas in continental margins contain appreciably more h2o

what makes hydromagmatic eruptions violent some insights from the keanakāko i ash kilauea volcano hawai i abstract volcanic eruptions at the summit of kilauea volcano hawai i are of two dramatically contrasting types 1 benign lava flows and lava fountains and 2 violent mostly prehistoric eruptions that dispersed tephra over hundreds of square kilometers the violence of the latter eruptions has been attributed to mixing of water and magma within a wet summit caldera however magma injection into water at other volcanoes does not consistently produce widespread tephras to identify other factors that may have contributed to the violence of these eruptions we sampled tephra from the keanak ko i ash the most recent large hydromagmatic deposit and measured vesicularity bubble number density and dissolved volatile content of juvenile matrix glass to constrain magma ascent rate and degree of degassing at the time of quenching bubble number densities 9 10 4 1 10 7 cm 3 of tephra fragments exceed those of most historically erupted kilauean tephras 3 10 3 1 8 10 5 cm 3 and suggest exceptionally high magma effusion rates dissolved sulfur average 330 ppm and water 0 15 0 45 wt concentrations exceed equilibrium saturation values at 1 atm pressure 100 150 ppm and 0 09 respectively suggesting that clasts quenched before equilibrating to atmospheric pressure we interpret these results to suggest rapid magma injection into a wet crater perhaps similar to continuous uprush jets at surtsey estimates of reynolds number suggest that the erupting magma was turbulent and would have mixed with surrounding water in vortices ranging downward in size to centimeters such fine scale mixing would have ensured rapid heat exchange and extensive magma fragmentation maximizing the violence of these eruptions

evolution of high ca high sr c series basalts from grenada lesser antilles the effects of intra crustal contamination basaltic lavas from grenada lesser antilles may be divided into two series on the basis of cao mgo relations the c series roughly corresponds to the ankaramites or high sr series of the earlier literature and may be distinguished from the m series basalts by higher cao at a given mgo and by their strongly augite phyric nature compositional variation within the c series is dominated by fractional crystallization of augite and plagioclase but this was accompanied by assimilation of sedimentary material from the island arc crust the assimilation fractional crystallization process is well illustrated by an increase in 206pb 204pb from 18 6 to 19 4 with fractionation and correlated increases in nd sr zr sm and rb ba the most primitive members of the c series can not be generated from erupted m series near primary basalts but instead show evidence for an origin by olivine fractionation from high mg basalts never seen at the surface these appear to have been derived from a source similar to that of morb but enriched in 87sr and other hydrophile elements by dehydration of subducted lithosphere and in ree and radiogenic pb by a small component of subducted sediment

permian and triassic rifting in northwest europe the permo triassic roughly coincided with the life span of pangaea so far as europe is concerned pangaea s creation began in the early carboniferous visean when the northward drifting megacontinent gondwana began to collide with the iberian portion of the slower moving laurussia while proto tethys was subducted beneath the southern margin of central europe creation was complete by the end of the carboniferous or early in the permian with the final development of the variscan orogenic belt which trends from brittany eastward through central europe and with the addition of western siberia along the line of the ural orogen ziegler 1989 despite or perhaps because of its bulk pangaea was not a stable megacontinent no sooner had it formed than it tried to break apart again the disintegration of pangaea had already started before the end of the triassic with the westerly extension of tethys between iberia and africa though not yet underlain by oceanic crust and by early in the jurassic rifting was taking place between africa and the americas in the newly forming central atlantic ocean ziegler 1988 indeed e w extensional movements within a proto atlantic ocean possibly began as early as the late carboniferous haszeldine russell 1987 whilst mid permian extension is well documented in east greenland surlyk et al 1984 these extensional movements may have propagated southward to initiate fracturing in the viking and central grabens of the north sea along which the late permian zechstein sea was to break into the subsiding rotliegend basins and towards the central atlantic where a shallow seaway eventually developed early in the jurassic thus in the northern half of pangaea the continued existence of the former laurussia was already at risk in the permian although crustal separation in the north atlantic which possibly started in the rockall trough in the early cretaceous was finally achieved along the line of the reykjanes ridge only in the paleocene these major events on the periphery of what is now europe separately and jointly were factors that probably controlled a whole sequence of tectonic events within the continent which in turn controlled its patterns of mostly terrestrial sedimentation climatically the northward drift of laurussia had carried nw europe from a region of equatorial rain forest during the later carboniferous to the latitudes of a trade wind desert like the modern sahara in the permian the late permian basins of upper rotliegend and zechstein deposition were arid rotliegend sediments are characterized by dune sand and the saline mudstones of a semipermanent desert lake and the zechstein by shallow water carbonates and anhydrite and deeper water halite during the triassic however brackish water fluvial and lacustrine sediments occupied the basinal areas although even here halite horizons e g rrt muschelkalk and keuper in the north sea area associated with local transgressions from tethys testify that arid conditions were never far away

characteristics distribution and morphogenesis of subtidal microbial systems in shark bay australia abstract the distribution nature and extent of microbial deposits in hamelin pool shark bay have been investigated and mapped with emphasis on the occurrence external morphologies internal fabrics constructional mechanisms microbial communities growth rates and sediment associations in the intertidal and previously little researched subtidal zone detailed georeferenced substrate mapping revealed extensive subtidal microbial deposits occupying approximately 300 km 2 of the total holocene 1400 km 2 area of hamelin pool the microbial pavement covers 227 km 2 of the subtidal substrate that together with columnar structures reveals a subtidal microbial habitat which occupies an area 10 times larger than the area of the intertidal deposits microbial carbonate is composed of aragonite 80 98 that reveals high positive values of 13 c 4 46 to 5 88 and 18 o 3 06 to 3 88 as a characteristic of the highly evaporative environment with extensive microbial activity oldest dated heads are 1915 and 1680 14 c years bp and the overall system was deposited in two stages the first between 2000 and 1200 and the last from 900 years bp to the present slow growth rates vary from less than 0 1 mm year to 0 5 mm year different internal fabrics were constructed according to their position in relation to the littoral zone by distinct microbial communities and lateral fabric relations have been established evidence of shallowing upward fabric sequences of microbial origin reflects relative falling sea levels during the late holocene and is likely useful in ancient environmental interpretation a sequence of events and mechanisms are described emphasizing differences between the stromatolitic thrombolitic and cryptomicrobial deposits in shark bay the new substrate map and depositional history for this distinctive and peculiar microbial habitat establish the significance of subtidal structures and emphasize the geoscientific importance of hamelin pool especially with respect to early life studies and ancient analogues for understanding microbial activity deposit characteristics fenestral fabrics and distribution

the liquid composition and crystallization history of the 1979 soufriere magma st vincent w i abstract electron microprobe analyses of glassy melt inclusions in phenocrysts of tephra lava and scoria samples of the basaltic andesite 1979 eruption of the soufriere volcano st vincent reveal that the magma is the product of fractional crystallization of alkalic high alumina olivine basalt magma using differentiation trends defined by melt inclusion matrix glass analyses we have constructed a quantitative model of the crystallization history of the 1979 magma sulfur analyses of melt inclusions in selected phenocrysts and estimates of volatile contents by difference suggest that the soufriere volcanic system was not closed with respect to volatiles before the 1979 eruption the volatile content of rare primitive melt inclusions in olivine phenocrysts ranges from two to eight weight percent the sulfur content of these inclusions ranges from 320 to 560 ppm and generally decreases with increasing silica content the volatile content of common melt inclusions in plagioclase orthopyroxene and clinopyroxene airfall phenocrysts on the other hand is probably less than two weight percent and sulfur contents are below the detection limit of 50 ppm these melt inclusions contain appreciably lower volatile content than the most primitive olivine melt inclusions suggesting that they may be samples of largely degassed magma the low volatile content of most soufriere 1979 eruption products inferred from microprobe data is consistent with the vulcanian style and small magnitude of the 1979 eruption which probably involved mainly phreatomagmatic disruption of magma rising and cooling beneath the 1971 lava island rhyodacitic glass of rare banded pumices is observed to be the product of fusion of crustal xenoliths

upper permian carbonates at the northern edge of the zechstein basin utsira high norwegian north sea abstract the first detailed description of zechstein carbonates and their diagenesis along the northern margin of the northern zechstein basin is based on core material from four wells on utsira high in the norwegian sector of the north sea it provides a generic link to the better known southern zechstein basin and northwards to the late permian east greenland basin the zechstein succession consists of shale carbonate cycles and anhydrite only occurs in the uppermost preserved cycle believed to correlate to zechstein 4 the eleven carbonate facies reflect deposition in shallow shelf and arid shoreline environments and resemble zechstein 2 and 3 carbonate facies from the southern zechstein basin the carbonates are pervasively dolomitized by an early texture preserving dolomite characterized by 18o values between 6 and 2 4 similar to dolomites in the southern zechstein basin and 13c values between 0 7 and 5 which is significantly lower than time equivalent dolomites from the southern zechstein basin later subaerial exposure and fresh water infiltration during the late permian or early triassic caused replacement of the upper part of the dolomite by crystalline fabric destructive calcite dedolomite the dedolomitized zone is up to 23 m thick and calcite crystals are characterized by moderately negative 18o values in the range 8 to 5 5 characteristic for meteoric water the utsira high carbonates thus share a depositional history with the classical zechstein carbonates whereas their diagenesis seems to have more in common with the time equivalent succession in east greenland early leaching and dolomitization enhanced porosity whereas subsequent late permian early triassic subaerial exposure and widespread dedolomitization had profound negative impact on reservoir properties of the zechstein carbonates

carbonate ramp depositional systems abstract the classification tectonic settings stratigraphy and early diagenesis of carbonate ramp systems are reviewed carbonate ramps are common in all geological periods but were dominant at times when reef constructing organisms were absent or inhibited ramps can be subdivided into inner mid and outer ramp environments the mid ramp zone extends from fair weather wave base to normal storm wave base although the water depths which these boundaries represent vary an additional outer slope environment occurs on distally steepened ramps as with siliciclastic shelves a range of wave storm and tide dominated ramps can be recognised and this forms the most convenient basis for ramp classification the carbonate productivity profile of ramps differs from that of rimmed shelves with the inner ramp showing lower production rates than comparable shallow water facies on rimmed shelves the zone of greatest organic carbonate sediment production appears to have shifted from the mid ramp to the inner ramp since the late jurassic carbonate ramps occur in most types of sedimentary basin but are best developed where subsidence is flexural and gradients are slight over large areas as in foreland and cratonic interior basins and along passive margins the featureless depositional profiles of many ramps means that sequence geometries are best observed on regional seismic lines some show low angle sigmoidal or shingled clinoforms suggesting that ramps may seldom be homoclinal but possess subtle slope geometries which reflect depositional environments because of their low angle slopes ramps respond differently to rimmed shelves during relative sea level changes although results seem to be strongly dependent on the rate of relative sea level change during a minor fall shallow ramp facies belts will simply shift basinwards in a forced regression in contrast the whole surface of a steep sloped flat topped rimmed shelf may be exposed so that sediment production ceases or is drastically reduced during a major fall shallow ramp bounded basins may empty completely conversely ramps also flood gradually whereas rimmed shelves do so more rapidly homoclinal ramps develop no resedimented lowstand deposits rimmed shelves and distally steepened ramps in contrast may develop lowstand talus or turbiditic wedges distally steepened ramps may behave more like homoclinal ramps during minor base level falls and like rimmed shelves during major base level falls many ramps consist of layered successions of several ramp sequences stacked one upon the other ramp stacks of this sort may show gross vertical accretion but individual ramp sequences seldom appear to develop in a keep up style apart from minor organic buildups as with many rimmed shelves steepening of the outer ramp margin due to tectonism slope inheritance or differential sedimentation may promote the development of a distally steepened ramp or rimmed shelf a wide variety of organisms have constructed buildups in mid and outer ramp environments isolated buildups may seed early in ramp development accrete to wave base or sea level and continue growth by stacking through successive ramp sequences so that depositional and diagenetic features within them are in concert with those of the shallow ramp the location of isolated buildups on ramps is governed by tectonism halokinesis antecedent topography or by the subtle slope geometry of the previous ramp sequence diagenesis on ramps shows some major variations compared with diagenesis on steep sloped flat topped carbonate platforms ramp bounded basins may form prolific petroleum sourcing and reservoiring systems and offer a range of subtle stratigraphic play types and lateral facies variations which determine reservoir quality and distribution isolated buildups in the mid and outer ramp environments represent one of the commonest petroleum reservoirs in ramp systems and tend to have their foundations in transgressive systems tracts grainstone and packstone reservoirs are widespread and range from shoreline carbonate sandbodies to major detached shoal complexes or shoals over offshore highs grainstone sandbodies occur in both highstand systems tracts and in prograding lowstand wedges

a review of volcanic island evolution and magma production rate an example from a cenozoic island arc in the caribbean different stages of volcanic island evolution are evident in the cenozoic lesser antilles arc developed near the eastern edge of the caribbean plate the early stage of seamount evolution is represented by the kick em jenny submarine volcano near the southern end of the arc the volcanically active islands known as the volcanic caribbees are composed of a series of composite volcanoes and are examples of an emergent volcanic island stage those islands that were formerly active known as the limestone caribbees provide evidence that an erosional to submergent stage occurred in the northern part of the island arc during the oligocene and miocene epochs growth of the islands and underlying crust is dependent upon the magma production rate along the arc which has for the last 100 ka ranged from less than 1 km 3 to 40 km 3 these differences in magma production and volcanic rock composition along the arc are attributed to the obliquity of plate convergence and the plate convergence rates

palaeokarst influenced depositional and diagenetic patterns in upper permian carbonates and evaporites karstryggen area central east greenland the karstryggen area of eastern greenland represents the western edge of sedimentation in the jameson land basin an arm of the northern zechstein seaway upper permian strata of this area were deposited as two major sequences the first marine incursion transgressed largely peneplaned lower permian strata and deposited thin paralic conglomerates sandstones and shales the huledal formation followed by a thick package of carbonates and evaporites the karstryggen formation although the karstryggen formation represents the transgressive maximum of this sequence it contains only marginal or restricted marine strata including micritic stromatolitic and peloidal carbonates and thick but localized bedded gypsum deposits these lithofacies indicate that relatively arid climates prevailed in this basin as in most of the zechstein region a major regression associated with a change to a more humid climate terminated karstryggen sedimentation pre existing evaporites and carbonates underwent diagenetic alteration including widespread calcitization and dissolution of gypsum more importantly topographic relief in excess of 120 m was generated by fluvial drainage systems and karstic sinkholes a second marine incursion accompanied by a return to a semi arid climate drowned this high relief topography producing a complex sequence of strata the wegener halvo formation in which sedimentation was greatly influenced by the rugged underlying terrain marine cemented algal molluscan grainstones draped pre existing palaeotopography during the initial stages of flooding continued drowning led to differential sedimentation on highs and in lows oolitic and bryozoan brachiopod grainstones formed as shoals on the crests of most prominences whereas shales conglomeratic debris flows evaporites or oolitic turbidites were deposited in the lows more restricted sedimentation took place in the westernmost areas which lay closest to the mainland shoreline and were situated to the west of a palaeotopographic ridge there oolitic stromatolitic and evaporitic strata were deposited under hypersaline conditions indicative of a return to more arid climatic conditions three subcycles mark smaller scale relative changes of sea level that occurred during deposition of the wegener halvo formation they are delimited by regional surfaces with moderate relief 5 20 m developed during subaerial exposure widespread diagenetic changes including leaching of aragonitic grains dissolution collapse brecciation of evaporites and meteoric calcite cementation occurred in association with these smaller scale sequence boundaries again reflecting climatic oscillations relative sea level fluctuations coupled with regional climate changes played a dominant role in determining both depositional and diagenetic relations in these strata these features undoubtedly extend into subsurface parts of this basin as well as into yet unexplored areas of the northern zechstein basin and barents shelf and may have economic significance for the localization of hydrocarbons

mechanism of the 1979 explosive eruption of soufriere volcano st vincent abstract the vulcanian explosive phase of the soufriere of st vincent in 1979 occurred when magma penetrated the water logged 1971 1972 lava island in the center of the crater lake the 13 day period of explosive and volcanoseismic activity ceased when the crater lake had dried up and subsequent activity was restricted to quiet lava extrusion the explosive phase produced 15 5 105m3 dense rock equivalent d r e of new magma and 22 106m3 d r e of lithic material from the 1971 1972 island whereas the following extrusive phase produced a dome of volume 47 106m6 considerations of the energetics of the explosive eruption indicate that the available heat content of the new magma 5 8 1016j was not sufficient to account for the observed phenomena including evaporation of the 25 106m3 crater lake and that additional 8 1016 j of heat remaining in the 1971 1972 lava mass are required to drive the eruption plumes field relations the high proportion of lithic material and the energy relations of the eruption are consistent with a phreato magmatic eruption mechanism involving a three way interaction of the 1971 1972 lava mass crater lake water and magma as a consequence of net veining and thermally induced hydraulic fracturing of the water logged 1971 1972 lava island by new magma in 1979

petrogenesis of the basalt andesite dacite association of grenada lesser antilles island arc revisited the basalt andesite dacite association of grenada is produced by fractional crystallization of primary magmas that contain about 15 weight percent mgo and 1 2 weight percent h2o the compositions of primary magmas indicate that they were last in equilibrium with the mantle at depths greater than about 65 km indicating that they must ascend rapidly after they segregate from residual mantle it is inferred that mantle wedge diapirism typically leads to moderately large degrees of batch partial melting in the formation of primary arc magmas 10 30 the trace element geochemical diversity of primary magmas mainly reflects variable degrees of source region enrichment by fluids derived from the subducted slab the degree to which source regions are enriched is likely related to mantle wedge dynamics water pressure suppresses plagioclase crystallization in derivative magmas leading to development of the calc alkaline rather than tholeiitic differentiation trend amphibole crystallization typically occurs only after melt mgo content decreases below about 4 weight percent the island arc crust acts as a density filter as arc crust thickens parental basaltic magmas will have a greater tendency to stagnate at mid crustal levels the observation that basalts predominate in young island arcs while andesites and dacites predominate in more mature arcs is explained by this hydrostatic control the thicker the arc crust the more magmas must differentiate by crystal segregation to become buoyant enough to erupt at the surface

oceanic crust of the grenada basin in the southern lesser antilles arc platform seismic refraction data permit the southern lesser antilles arc and surrounding regions to be divided by the velocity of their basement we propose that high velocity basement of the arc platform beneath the grenadine islands and below a part of the tobago trough forearc basin is oceanic and continuous and was originally connected with oceanic crust of the grenada basin low velocity basements of the tobago terrane and the arc platform from st vincent north lie south and north respectively of the high velocity basement of the arc platform an oceanic origin of this high velocity crust in the grenadines is argued to be more plausible than an origin as unroofed lower arc crust the segment of probable oceanic crust in the arc platform was greatly uplifted during development of the present island arc mainly in late neogene time relative to the grenada basin and tobago trough accepting the proposition of shallow oceanic crust in the grenadines early middle eocene and possibly older pillow basalts of mayreau the oldest rock unit of the southern lesser antilles arc platform may be an exposure of such basement major and minor element compositions of mayreau basalt are indicative of a spreading rather than arc origin the stratigraphy of the pillow basalts indicates extrusion in an open marine environment distant or shielded from sources of arc or continental sediment followed by a period of pelagic sedimentation above the carbonate compensation depth the eocene basalt and pelagic cover formed a relatively deep floor of a marine basin in which arc derived turbidites and pelagic sediments accumulated over the succeeding 25 30 ma such basalts thus indicate a probable spreading origin of the grenada basin and an age of cessation of spreading in the region of mayreau in eocene time the configuration of the eocene basin and the direction of spreading however are unknowns regional structural relationships imply the spreading was probably backarc an origin also hinted at by ce zr ratios of mayreau basalt although the aves ridge probably was the remnant arc the existence and position of an early middle eocene frontal arc are unresolved the early paleogene tectonic architecture of the southeastern caribbean differed substantially from that of the neogene

reconstructing a high paleolatitude mesozoic paleoenvironment from a truncated and deeply buried regolith norwegian north sea abstract the paleoregolith in this study was unconformably overlain by lower cretaceous valanginian marine deposits of the asgard formation and buried below two kilometers of sediments on the utsira high norwegian north sea the upwards intensification of chemical alteration manifested in the replacement of feldspars by clays leaching of base cations and reduced mechanical rock strength in addition to the presence of terrestrial biotitic features suggested weathering during subaerial exposure of the utsira high during the mesozoic the paleoregolith can be divided into three weathering facies altered coherent rock saprock and saprolite whereas the absence of true soil horizons suggested erosional truncation of the regolith before burial possible remnants of the eroded soil were identified as pedogenic mud aggregates in the overlying asgard formation bearing similar compositional characteristics as the saprolite clay mineralogy bulk geochemical composition micromorphology and depth of biotic features were used to reconstruct the paleoenvironmental conditions that prevailed during the formation of the regolith in the mesozoic north sea our results point to a regolith formation under periodically water restricted conditions promoting the co existence of interstratified illite smectite beidellite and kaolinite deep presence of endomycorrhizae the fungal extension of the root structure and precipitation of iron oxide nodules application of recently developed geochemical climofunctions indicated that the regolith formed in a mesic 10 12 c and subhumid 1157 1406 mm yr environment and under strongly acidic soil conditions ph 5 2 5 6

dolomite calcitization and cement zonation related to uplift of the raisby formation zechstein carbonate northeast england abstract the raisby formation is the basal zechstein carbonate of northeast england it has undergone a complex diagenetic history during burial including multiphase dolomitization and replacement by sulphates upon uplift meteoric derived fluids have dissolved this replacive sulphate and precipitated calcite cements in the resultant porosity study of these cements by cathodoluminescence and trace element geochemistry shows that they are zoned and different cathodoluminescence zones correlate well with iron and manganese geochemical changes demonstrating a gradual increase in oxygenation of the precipitating fluids through time major diagenetic processes during cement growth include precipitation of anhydrite fluorite and barite calcitization of dolomite and internal sedimentation relating these processes to cement zonation allows this complex diagenesis to be explained in terms of uplift under the influence of meteoric derived groundwaters of constantly changing geochemical and electrochemical characteristics

petrology and eruption styles of kick em jenny submarine volcano lesser antilles island arc basaltic magmas erupted from the kick em jenny submarine volcano have assimilated amphibole bearing crustal material the mgo content of the basalts ranges from 12 to 4 wt and some of the more evolved members of the suite may have eventually become saturated with amphibole as well as olivine plagioclase clinopyroxene and titanomagnetite major element and trace element compositions are consistent with processes dominated by assimilation fractional crystallization analysis of glassy melt inclusions mgo 2 9 wt in olivine phenocrysts suggests that pre eruption melt water contents may have been as high as 5 wt glass compositions are interpreted to reflect eruption related loss of water submersible observations and dredge sampling of the volcano have revealed two styles of eruption explosive tephra producing eruptions and non explosive dome forming lava eruptions the contrasting styles of eruption are thought to be due principally to the rate and extent of eruption related degassing of 5 wt juvenile h2o although magma interactions with external water may also play a role in explosive eruptions

30 years in the life of an active submarine volcano a time lapse bathymetry study of the kick em jenny volcano lesser antilles effective monitoring is an essential part of identifying and mitigating volcanic hazards in the submarine environment this is more difficult than onshore because observations are typically limited to land based seismic networks and infrequent shipboard surveys since the first recorded eruption in 1939 the kick em jenny kej volcano located 8km off northern grenada has been the source of 13 episodes of t phase signals these distinctive seismic signals often coincident with heightened body wave seismicity are interpreted as extrusive eruptions they have occurred with a recurrence interval of around a decade yet direct confirmation of volcanism has been rare by conducting new bathymetric surveys in 2016 and 2017 and reprocessing 4 legacy datasets spanning 30 years we present a clearer picture of the development of kej through time processed grids with a cell size of 5m and vertical precision on the order of 1 4m allow us to correlate t phase episodes with morphological changes at the volcano s edifice in the time period of observation 7 09x106 m3 of material has been added through constructive volcanism yet 5 times this amount has been lost through landslides limited recent magma production suggests that kej may be susceptible to larger eruptions with longer repeat times than have occurred during the study interval behavior more similar to sub aerial volcanism in the arc than previously thought t phase signals at kej have a varied origin and are unlikely to be solely the result of extrusive submarine eruptions our results confirm the value of repeat swath bathymetry surveys in assessing submarine volcanic hazards

dolomitization and calcitization of the devonian grosmont formation northern alberta abstract the grosmont formation upper devonian comprises a platform carbonate sequence which extends over 100 000 sq km in the subsurface of alberta and contains extensive areas of calcitized dedolomitized dolomite reservoir rocks hydrocarbon content of the grosmont formation is estimated to be 50 109 m3 of 7 api gravity oil with bitumen recovery rates as high as 70 percent using thermally enhanced recovery environments of deposition in the grosmont formation range from subtidal and intertidal to supratidal early stratigraphically controlled and later cross cutting dolomites are observed dissolution fabrics are generally related to the uppermost dolomitized units of the grosmont formation below an unconformity which overlies paleozoic rocks calcitization textures are found in limestone and dolomite units and do not bear any relationship to the unconformity petrographic examination shows that calcitization of dolomite occurs in different forms large 0 25 0 5 mm dolomite rhombs observed floating in micrite or at calcitic micrite dolomite contacts have cores replaced by calcite dolomite rhombs that are not calcitized contain cloudy cores with abundant calcite inclusions in some samples fragments of dolomite rhombs are observed with the cores dissolved away indicating that part of the calcitization process involves dissolution a complete range from dolomitic rocks that show no evidence of calcitization to calcitic rocks with the textures of sucrosic dolomites are observed late calcite cements filling the remaining available pore space are also related to the calcitization process there is a strong tendency for dolomites from stratigraphic units to be grouped by their isotopic compositions three phases of dolomitization are interpreted in the grosmont formation an early sabkha dolomite upper grosmont 3 dolomites formed during reflux of hypersaline brines upper grosmont 2 and dolomites related to waters expelled from shales during burial lower grosmont further dolomites in each unit tend to be depleted in the heavy isotopes of both carbon and oxygen with increasing degree of dissolution and or calcitization the geological and geochemical evidence indicates that calcitization was early and related to dolomitization probably with a later stage that took place during uplift and the entry of meteoric water along the unconformity

origin of disconformity dedolomite in the martin formation late devonian northern arizona abstract calcitized dolomite dedolomite occurs along a prominent disconformity in the upper martin dolomite of north central arizona petrography stable isotopes and field associations were used in an effort to constrain the origin and timing of the dedolomite 18 o 16 o and 13 c 12 c ratios of the dedolomite show significant depletions relative to unaltered dolomite above and below the disconformity this depletion trend and dedolomitization are interpreted to have resulted from a subaerial exposure and an influx of 18 o depleted meteoric water or b descending groundwater from paleoaquifers in the lower pennsylvanian sedimentary features consistent with a subaerial exposure include detrital filled desiccation cracks and an accumulation of iron rich residual clays features suggestive of dedolomitization by paleogroundwater include calcite replaced evaporite nodules and the great volume of water inferred to have been associated with extensive continental lower pennsylvanian karstification of overlying younger carbonate units

the influence of structural inheritance and multiphase extension on rift development the northern north sea the northern north sea rift evolved through multiple rift phases within a highly heterogeneous crystalline basement the geometry and evolution of syn rift depocentres during this multiphase evolution and the mechanisms and extent to which they were influenced by pre existing structural heterogeneities remain elusive particularly at the regional scale using an extensive database of borehole constrained 2d seismic reflection data we examine how the physiography of the northern north sea rift evolved throughout late permian early triassic rp1 and late jurassic early cretaceous rp2 rift phases and assess the influence of basement structures related to the caledonian orogeny and subsequent devonian extension during rp1 the location of major depocentres the stord and east shetland basins was controlled by favorably oriented devonian shear zones rp2 shows a diminished influence from structural heterogeneities activity localises along the viking sogn graben system and the east shetland basin with negligible activity in the stord basin and horda platform the utsira high and the devonian lomre shear zone form the eastern barrier to rift activity during rp2 towards the end of rp2 rift activity migrated northwards as extension related to opening of the proto north atlantic becomes the dominant regional stress as rift activity in the northern north sea decreases through documenting the evolving syn rift depocentres of the northern north sea rift we show how structural heterogeneities and prior rift phases influence regional rift physiography and kinematics controlling the segmentation of depocentres as well as the locations styles and magnitude of fault activity and reactivation during subsequent events

the origin of common dolomite fabrics clues from the pliocene abstract characteristic features common to dolomites of all ages are found in pliocene dolomites from the netherlands antilles these features include 1 pseudomorphic replacement 2 selective replacement 3 fossil moldic porosity 4 cloudy centered clear rimmed dolomite 5 sucrosic dolomite and 6 dolomite cement the development of a particular feature is controlled by 1 mineralogy of the material being replaced 2 the degree of saturation of the bulk solution with respect to the mineral being replaced and 3 the availability of dolomite nucleation sites low mg calcite lmc cement and fossils were often neither replaced by dolomite nor dissolved but aragonite and high mg calcite hmc were both replaced and dissolved the resistence of lmc to dolomitization was due to solutions saturated with respect to lmc and the lack of nucleation sites cryptocrystalline lmc was more readily replaced by dolomite because it provides more nucleation sites cloudy centered clear rimmed dolomite and sucrosic dolomite formed in sediments with abundant lmc and few nucleation sites fossil moldic porosity developed concurrent with pseudomorphic replacement suggesting that the dolomitizing solution was undersaturated with respect to the mineral being replaced dolomite cement commonly lines pores and fills intraparticle pore space dolomite cement occasionally has inclusions of a precursor lmc cement dolomite cement crystals are larger than replacement crystals because there are fewer nucleation sites for the cement crystals

deep submarine pyroclastic eruptions theory and predicted landforms and deposits submarine pyroclastic eruptions at depths greater than a few hundred meters are generally considered to be rare or absent because the pressure of the overlying water column is sufficient to suppress juvenile gas exsolution so that magmatic disruption and pyroclastic activity do not occur consideration of detailed models of the ascent and eruption of magma in a range of sea floor environments shows however that significant pyroclastic activity can occur even at depths in excess of 3000 m in order to document and illustrate the full range of submarine eruption styles we model several possible scenarios for the ascent and eruption of magma feeding submarine eruptions 1 no gas exsolution 2 gas exsolution but no magma disruption 3 gas exsolution magma disruption and hawaiian style fountaining 4 volatile content builds up in the magma reservoir leading to hawaiian eruptions resulting from foam collapse 5 magma volatile content insufficient to cause fragmentation normally but low rise speed results in strombolian activity and 6 volatile content builds up in the top of a dike leading to vulcanian eruptions we also examine the role of bulk interaction steam explosivity and contact surface steam explosivity as processes contributing to volcaniclastic formation in these environments we concur with most earlier workers that for magma compositions typical of spreading centers and their vicinities the most likely circumstance is the quiet effusion of magma with minor gas exsolution and the production of somewhat vesicular pillow lavas or sheet flows depending on effusion rate the amounts by which magma would overshoot the vent in these types of eruptions would be insufficient to cause any magma disruption the most likely mechanism of production of pyroclastic deposits in this environment is strombolian activity due to the localized concentration of volatiles in magma that has a low rise rate magmatic gas collects by bubble coalescence and ascends in large isolated bubbles which disrupt the magma surface in the vent producing localized blocks bombs and pyroclastic deposits another possible mode of occurrence of pyroclastic deposits results from vulcanian eruptions these deposits being characterized by the dominance of angular blocks of country rocks deposited in the vicinity of a crater should be easily distinguishable from strombolian and hawaiian eruptions however we stress that a special case of the hawaiian eruption style is likely to occur in the submarine environment if magmatic gas buildup occurs in a magma reservoir by the upward drift of gas bubbles in this case a layer of foam will build up at the top of the reservoir in a sufficient concentration to exceed the volatile content necessary for disruption and hawaiian style activity the deposits and landforms are predicted to be somewhat different from those of a typical primary magmatic volatile induced hawaiian eruption specifically typical pyroclast sizes might be smaller fountain heights may exceed those expected for the purely magmatic hawaiian case cooling of descending pyroclasts would be more efficient leading to different types of proximal deposits and runout distances for density flows would be greater potentially leading to submarine pyroclastic deposits surrounding vents out to distances of tens of meters to a kilometer in addition flows emerging after the evacuation of the foam layer would tend to be very depleted in volatiles and thus extremely poor in vesicles relative to typical flows associated with hawaiian style eruptions in the primary magmatic gas case we examine several cases of reported submarine volcaniclastic deposits found at depths as great as 3000 m and conclude that submarine hawaiian and strombolian eruptions are much more common than previously suspected at mid ocean ridges furthermore the latter stages of development of volcanic edifices seamounts formed in submarine environments are excellent candidates for a wide range of submarine pyroclastic activity due not just to the effects of decreasing water depth but also to 1 the presence of a summit magma reservoir which favors the buildup of magmatic foams enhancing hawaiian style activity and episodic dike emplacement which favors strombolian style eruptions and 2 the common occurrence of alkalic basalts the co2 contents of which favor submarine explosive eruptions at depths greater than tholeiitic basalts these models and predictions can be tested with future sampling and analysis programs and we provide a checklist of key observations to help distinguish among the eruption styles

genesis of evaporite associated platform dolomites case study of the main dolomite zechstein upper permian leba elevation northern poland dolomitization of the zechstein late permian main dolomite carbonates of northern poland was penecontemporaneous and or very early diagenetic well ordered stoichiometric dolomites are associated with the basinal facies the platform dolomitesare relatively poorly ordered and usually non stoichiometric most samples are highly enriched in 13c as in other zechstein carbonates 6 o values show large variations from 5 l to 7 4 there is an isotope zonation of the examined dolomites the isotope signature indicates that dolomites formed from variable solutions of meteoric water seawater and evaporitic brines of possible marine or continental origin once initiated dolomitization proceeded despite the evolution of dolomitizing brines this evolution explains the occurrence of lagoonal dolomites with common evidence for dissolution in the lower part of sections compared with well developed rhombohedra in the upper part crystal zoning suggests the initiation of dolomite growth in hypersaline water and progressive dilution by fresh water there is isotopic evidence for migration of continental waters into the basin presumably following sea level fall at the end of the deposition of the main dolomite influence of fresh water on syndepositional dolomitization well established in the main dolomite strongly suggests that similar relationships may be characteristic for other evaporite associated dolomites as well

shallow burial dolomite cement a major component of many ancient sucrosic dolomites dolomite cement is a significant and widespread component of phanerozoic sucrosic dolomites cements in dolomites that were never deeply buried are limpid have planar faces non saddle forms often distinct zonation in cathodoluminescence and form syntaxial overgrowths on crystals facing pores five samples of sucrosic dolomites interpreted as having had mostly limemudstone or wackestone precursors in four carbonate aquifers provide insights into the abundance of planar cements in sucrosic dolomites such cement comprises 11 to 45 32 mean of peritidal to sub tidal dolomites on an outcrop in the edwards aquifer early cretaceous of central texas 19 to 33 25 mean of ramp dolomites in the hawthorn group oligomiocene and 50 to 70 in shelf dolomites of the avon park formation eocene in the upper floridan aquifer of sub surface peninsular florida 18 to 45 32 mean of sub tidal shelf dolomites in quarry sections of the burlington keokuk formation early mississippian in south eastern iowa and 18 to 76 50 mean in shallow cores and outcrops of outer shelf dolomites from the gambier limestone oligo miocene of south australia backstripping the cement phases revealed by cathodoluminescence colour photomicrographs documents the effects of cements on textural coarsening pore space reduction induration and general maturation of these dolomites most pre holocene dolomites are multiphase crystalline rocks composed of i seed crystals or cores ii crystal cortices that concentrically enlarged the cores and iii free space syntaxial precipitates of limpid cement around the crystals remaining caco3 grains and micrite can be replaced by dolomite but typically they are dissolved between stages ii and iii creating systems of intercrystal and mouldic pores typical of sucrosic dolomites networks of cement overgrowths aided by water filled pore systems under hydrostatic to lithostatic pressure are judged to slow or prevent compaction in sucrosic dolomites it can be argued that cortex growth involves both replacement of caco3 particles and microcementation of their interparticle pores this interpretation and the abundance of cements in so many dolomites would obviate the controversy over the volumetrics of replacement dolomitization limpid planar and syntaxial dolomite cements of early diagenetic origin are interpreted to have precipitated from clear pore waters at low temperatures 30 to 35 c and shallow burial depths 100 m in water saturated networks of dolomite silt and sand cements in many dolomites in island and continental aquifer systems appear to result from event driven processes related to sea level highstands cementation events can follow replacement

tectonic and paleogeographic controls on development of the early middle ordovician shanganning carbonate platform ordos basin north china lower middle ordovician platform carbonates are important oil and gas reservoir rocks in the ordos basin of the north china craton ncc in recent years numerous cores have been recovered from these units but the depositional environments and facies architecture of this large carbonate platform system have not yet been reported in detail this study integrates sedimentological and magnetic fabric data collected from outcrops and cores to document variation in microfacies and facies associations within the shanganning carbonate platform scp infer paleowind directions and the paleogeographic location of the scp and evaluate tectonic and paleogeographic controls on platform development this analysis documented 25 microfacies belonging to 7 microfacies fabric and sediment types carbonate mudstone wackestone packstone grainstone evaporite bindstone framestone siliciclastics and dolostone microfacies can be grouped into facies associations that represent tidal flat open platform restricted platform evaporite platform platform margin slope and deep basinal depositional environments these environments are mainly distributed zonally around the l shaped qingyang paleo uplift on the western and southern margins of the scp anisotropy of magnetic susceptibility measurements at five platform margin locales yielded paleocurrent directions of all approximately 318 348 51 81 42 72 80 110 and 54 84 from paleo north after correction for a 120 150 clockwise rotation of the ncc since the ordovician the depositional architecture of the scp depended on a combination of tectonic and paleogeographic wind influences local tectonic movements controlled the spatial distribution of facies associations around the qingyang paleo uplift which was characterized by open restricted and evaporite platform facies in the platform interior and mainly slope and basinal facies on the outer platform margin to the west and south of the qingyang paleo uplift vertical tectonic movements resulted in cyclic submergence and emergence of the platform over its approximately 20 m y long formation history paleo trade winds influenced aspects of facies zonation across the platform including growth of metazoan reefs on the windward platform margin and formation of oolitic grainstone and microbial reefs on the leeward platform margin the facies model for the scp developed in this study should provide a useful reference framework for future carbonate oil and gas exploration in lower middle ordovician units of the ordos basin furthermore this study makes innovative use of magnetic anisotropy data in combination with standard petrographic core and field data to discern facies patterns and controls on a paleo carbonate platform which is an integrated methodology that may serve as a model for future studies of other carbonate platforms

mesozoic regional tectonics and south viking graben formation evidence for localized thin skinned detachments during rift development and inversion abstract a regional thin skinned model of south viking graben development is presented in which zechstein marginal evaporites and interpreted salt have acted as a key de collement horizon along which mechanical decoupling of post salt triassic and younger strata from pre salt basement has occurred the western margin of the south viking graben defines an asymmetrical late jurassic half graben controlled by episodic extension along a nne ssw trending eastward dipping fault system an initial minor phase of rifting in the triassic is suggested by the slight westward thickening and associated divergence of interpreted triassic sequences at depth the scarcity absence of upper triassic and lower jurassic sequences is believed to result from a subsequent phase of uplift which affected the region in the mid cimmerian attributed to the development of a thermal dome at the future site of a jurassic rift triple junction to the southwest the onset of large scale extension from the late callovian onwards marked the major phase of half graben formation and created a regional hanging wall tilt which locally caused gravity gliding of post salt section to the wnw with resultant low angle extensional faulting upslope and salt cored buckling downslope the late jurassic half graben was subsequently tightened by a phase of thin skinned inversion in the latest jurassic early cretaceous associated with back steepening of the western margin fault system with resulting reactivation of the extensional detachment system the effects of this inversion have been noted in many parts of the northern north sea within the south viking graben shortening estimates vary between 14 and 18 and are expressed on seismic data by localized hanging wall folds and the back steepening of faults along the western margin fault complex of the graben together with out of graben thrusting up the hanging wall dip slope variation in both the extension and inversion mechanisms appears to be controlled by interpreted nw se transfer systems which compartmentalize the graben into three subgraben within which differential zechstein basin development occurred

reconnaissance de structures tectoniques et volcaniques sous marines de l arc recent des petites antilles kick em jenny qualibou montagne pelee nordouest de la guadeloupe abstract a multibeam bathymetric survey seacarib 1 was carried out in april 1985 on board the r v conrad lamont doherty geological observatory off the western coasts of several islands of the lesser antilles complementing a previous survey of the same type arcante 2 thermosite 1980 devoted to adjoining areas in guadeloupe martinique and st lucia it allowed us to precisely map submarine volcanoes in a marginal setting behind the recent arc the main volcanic axis of the archipelago emplaced during the neogene the kick em jenny volcano south of the grenadines is the only active submarine volcano of the lesser antilles it is a small and very young cone probably only tens or hundreds of years old which was hydrothermally active during the survey the directeur volcano guadeloupe is the largest submerged volcanic centre thus far detected in the lesser antilles having been formed between 3 3 and 2 9 ma during the 1980 survey two other large volcanoes were discovered off southern basse terre guadeloupe vieux fort volcano 2 5 2 0 ma and in front of fort de france bay martinique schoelcher volcano older than 4 2 ma several other much smaller volcanoes have been mapped off the same coasts particularly the bouillante seapeaks guadeloupe moreover large tectonic structures cut the recent arc transversally and obliquely the well known qualibou depression on the island of st lucia was interpreted in various ways neither the hypothesis of a large caldera the western side of which collapsed offshore nor the suggestion of a huge rotational gravity slide are confirmed the depression seems more likely to have been controlled by tectonic factors shaping a complex graben structure the seaward extension appears to be restricted to a northern channel the soufriere channel the st pierre fault in martinique is a large transverse feature extending seaward down to some 2000 m depth to the south it bounds the mount pelee active volcanic system and the escarpment of the fault forms a dam against subaqueous pyroclastic flows running down from the volcano these flows are responsible for the unusually smooth morphology of the submarine flanks the zombi fault of northwest guadeloupe is part of a first order feature extending along at least 130 km from montserrat in the north to marie galante in the east this curved fracture cuts across the northern central part of the lesser antilles passing progressively from a subparallel to a transverse orientation with respect to the axis of the island arc these types of surveys are important because they provide better understanding of the structure and recent geological evolution of the recent volcanic arc they are also a useful complementary tool for geothermal and seismological studies

nature and origin of collapse breccias in the zechstein of ne england local observations with cross border petroleum exploration and production significance across the north sea

diagenetic history and hydrocarbon potential of upper permian carbonate buildups wegener halvo area jameson land basin east greenland 1 the upper permian of jameson land includes two carbonate sequences the karstryggen and wegener halvo formations the karstryggen formation contains hypersaline carbonates and localized evaporites that were heavily weathered and dissected prior to deposition of the overlying strata the overlying wegener halvo formation represents an abrupt and extensive marine inundation over the underlying karstified karstryggen surface bryozoan brachiopod algal cement buildups of the wegener halvo formation are localized on karstic highs and show up to 150 m of depositional relief in situ mound core deposits are flanked by allodapic limestones which pass laterally into intermound calcareous shales the diagenetic histories of the core and flank facies are very different core facies porosity was intially obliterated by marine cements but repeated meteoric exposure altered unstable core facies constituents this alteration produced extensive secondary porosity through grain and cement leaching with local collapse brecciation flank strata however underwent little sea floor diagenesis and low permeability and mineralogically stable grain composition protected these strata from meteoric alteration early cementation and stabilization of core strata led to minimal burial diagenetic porosity loss uncemented flank beds however underwent profound mechanical and chemical compaction during this stage thus at the time of hydrocarbon generation distal flank beds had less than 2 porosity coarse upper flank beds had 5 10 remnant primary porosity and core facies deposits had 8 12 secondary pore space hydrocarbons generated from surrounding marine shales charged many of the bioherms with oil subsequent fracturing and hydrothermal fluid flow however flushed hydrocarbons and filled pores with ferroan calcite barite fluorite galena and baroque dolomite this heating and flushing is thought to have been especially intense in the wegener halvo region thus more basinal areas may still have reservoirs containing significant oil in equivalent upper permian limestones if as is likely the sea level changes affecting the greenland permian were eustatic then this study may provide significant clues to porosity development throughout the largely unexplored northern zechstein basin and the arctic basin of the barent sea this study also provides some important connections to the probably time equivalent guadalupian carbonate reservoir rocks of west texas new mexico and wyoming

submarine hydrothermal venting related to volcanism in the lesser antilles evidence from ferromanganese precipitates 1 radiogenic isotope compositions sr nd pb hf and os of sediment hosted seafloor ferromanganese crusts and sediments incrusted with ferromanganese oxyhydroxides from the lesser antilles island arc were measured to distinguish between hydrogenous seawater derived and hydrothermal metal sources the ages of the precipitates range between recent last few thousand years and a few 100 kyr as deduced from 10be and co concentrations evidence from the presence of bladed todorokite and nontronite together with the major element and ree composition suggests that a significant proportion of these sediment hosted precipitates formed at relatively low temperatures from a mixture of seawater and hydrothermal fluids associated with island arc volcanism the radiogenic isotope compositions of all metals mentioned above except pb show large differences in hydrothermal versus hydrogenous contributions over space and time in contrast to precipitates of high temperature fluids which mainly scavenge their ree contents from seawater the crusts of this study show 143nd 144nd of up to 0 512817 nd 3 5 this is close to the signature of the nearby island arc rocks and far above the expected local seawater ratio of 0 51209 nd 10 7 these crusts also show high 176hf 177hf up to 0 283102 low 87sr 86sr up to 0 7069 and low 187os 188os up to 0 16 compared with local seawater as expected from hydrothermal island arc derived metal contributions in contrast the pb isotope signatures of the crusts cannot be explained by mixing between seawater and hydrothermal sources it is suggested that pb was either removed from the ascending fluids within the sediment column before they reached seawater or the temperatures were too low to leach significant amounts of pb from the rocks or sediments external sources such as saharan dust particulate inputs from the orinoco river or even incongruent release of pb isotopes from the island arc rock derived particles must have contributed to the observed pb isotope variability our results suggest that submarine hydrothermalism originating from intraoceanic island arc volcanism creates distinct geochemical environments for the dispersion of hydrothermal fluids and may be an important mechanism to supply metals of hydrothermal origin to seawater

the hazards of eruptions through lakes and seawater abstract eruptions through crater lakes or shallow seawater referred to here as subaqueous eruptions present hazards from hydromagmatic explosions such as base surges lahars and tsunamis which may not exist at volcanoes on dry land we have systematically compiled information from eruptions through surface water in order to understand the circumstances under which these hazards occur and what disastrous effects they have caused in the past subaqueous eruptions represent only 8 of all recorded eruptions but have produced about 20 of all fatalities associated with volcanic activity in historical time excluding eruptions that have resulted in about a hundred deaths or less lahars have killed people in the largest number of historical subaqueous eruptions 8 followed by pyroclastic flows excluding base surges 5 tsunamis 4 and base surges 2 subaqueous eruptions have produced lahars primarily on high 1000 m steep sided volcanoes containing small

carbonate stromatolites from a messinian hypersaline setting in the caltanissetta basin sicily petrographic evidence of microbial activity and related stable isotope and rare earth element signatures lower messinian stromatolites of the calcare di base formation at sutera in sicily record periods of low sea level strong evaporation and elevated salinity thought to be associated with the onset of the messinian salinity crisis overlying aragonitic limestones were precipitated in normal to slightly evaporative conditions occasionally influenced by an influx of meteoric water evidence of bacterial involvement in carbonate formation is recorded in three dolomite rich stromatolite beds in the lower portion of the section that contain low domes with irregular crinkly millimetre scale lamination and small fenestrae the dominant microfabrics are i peloidal and clotted dolomicrite with calcite filled fenestrae ii dolomicrite with bacterium like filaments and pores partially filled by calcite or black amorphous matter and iii micrite in which fenestrae alternate with dark thin wispy micrite the filaments resemble beggiatoa like sulphur bacteria under scanning electron microscopy the filaments consist of spherical aggregates of dolomite interpreted to result from calcification of bacterial microcolonies the dolomite crystals are commonly arranged as rounded grains that appear to be incorporated or absorbed into developing crystal faces biofilm like remains occur in voids between the filaments the dolomite consistently shows negative d 13 c values down to 11ae3 and very positive d 18 o mean value 7ae9 that suggest formation as primary precipitate with a substantial contribution of organic co2 very negative d 13 c values down to 31ae6 of early diagenetic calcite associated with the dolomite suggest contribution of co2 originating by anaerobic methane oxidation the shale normalized rare earth element patterns of sutera stromatolites show features similar to those in present day microbial mats with enrichment in light rare earth elements and m type tetrad effects enrichment around pr coupled to a decline around nd and a peak around sm and eu taken together the petrography and geochemistry of the sutera stromatolites provide diverse and compelling evidence for microbial influence on carbonate precipitation

mixed evaporative and meteoric water dolomitization isotope study of the zechstein limestone upper permian southwestern poland abstract dolomites of the zechstein limestone in western poland have high 13c values characteristic of the zechstein carbonates and there is some variation in 18o profiles dolomites are mostly stoichiometric and well ordered and are characterized by variable crystal textures the area studied underwent constant burial since the late permian and the isotopic curves despite their complexity show distinct regularities and signals of meteoric water input that in most cases could occur only during the permian moldic porosity interpreted as originated shortly after dolomitization commonly occurs stratigraphic patterns and facies analyses suggest that dolomitization phases occurred at the top of each eustatically controlled subcycle of the zechstein limestone in association to subaerial exposures most of the dolomites therefore originated during the permian a significant proportion of the dolomites originated via meteoric marine mixing during deposition of the zechstein limestone the mixing mechanism was closely related to sea level falls conventionally evaporite related dolomites are believed to have originated in a sabkha environment and or by the reflux mechanism the zechstein limestone case suggests that the mixing of evaporative and meteoric water may be responsible for dolomitization of other evaporate related carbonates