metre scale cyclicity in permian ramp carbonates of equatorial pangea venezuelan andes implications for sedimentation under tropical pangea conditions abstract palmarito strata in the mid permian of the venezuelan andes show three different types of metre scale cycle type a cycles are mixed clastic carbonate shallowing upward peritidal cycles the upper boundaries of the cycles are exposure surfaces with calcrete this kind of cycle shows a significant degree of randomness in its thickness pattern and this is attributed to an autocyclic origin through tidal flat progradation and variations in carbonate productivity and clastic input to the depositional area type b cycles consist entirely of shallow subtidal facies with a shallowing upward arrangement and an alternation between heterozoan and photozoan assemblages type c cycles were deposited in a middle to outer ramp setting and are characterized by a heterozoan assemblage and shallowing upward trends that includes spiculitic wackestone packstone passing up into neomorphic bioclastic wackestone or bioclastic crinoidal bryozoan wackestone packstone all cycles are interpreted as 4th 5th order 104 to 105 years duration and some can be linked to milankovitch orbital rhythms notably short eccentricity 100 000 years overall the origin of the palmarito cycles was a complex combination of autocyclic and allocyclic controls however the evidence suggests that cycles in the lower part of the succession were dominated more by autocyclic processes whereas those in the upper part were more the result of allocyclic controls climate driven processes such as migration of the intertropical convergence one and the waxing and waning of polar ice caps had significant effects on deposition of the palmarito succession and the cycles therein controlling the clastic sediment supply nutrient availability and thus carbonate production and sea level change and these factors determined the vertical stacking pattern of the cycles

continuity and facies heterogeneities of shallow carbonate ramp cycles sinemurian lower jurassic north east spain the lateral continuity and facies heterogeneities of metre scale cycles in a greenhouse lower jurassic sinemurian carbonate ramp from the northern iberian basin spain was evaluated from extensive field analysis carried out on a well exposed 12 km long outcrop eleven high frequency continuous cycles and their bounding surfaces are traceable laterally through the entire outcrop however three of these cycles are found to split laterally into discontinuous cycles of more limited distribution up to 3 to 5 km of lateral extent the continuous and discontinuous cycles have a similar field expression in one dimensional logs as a consequence the number of cycles that can be differentiated is variable along the logged sections i e from 11 to 16 cycles have variable facies heterogeneities and sedimentary trends depending on the environment of formation shallowing upward and symmetrical cycles occur in protected lagoon tidal flat areas and in the open marine high energy domain these cycles show significant facies heterogeneities which were controlled mainly by lateral migration of a mosaic of facies over an irregular topography deepening upward and aggradational cycles are generated in low energy sub wave base open marine areas facies are laterally homogeneous reflecting low potential for carbonate accumulation and inability to fill the created accommodation space in this low relief and relatively deep area cycle boundaries are generated by stages of rapid accommodation gain involving the flooding of the carbonate ramp they are more likely to originate from regional tectonic pulses related to the extensional tectonics operating in the northern iberian basin rather than greenhouse low amplitude eustacy discontinuous cycles tend to occur in thickened areas and are interpreted as originating from the infill of wedge shaped accommodation space resulting from differential subsidence i e local tectonic pulses in conclusion where thickness variations occur in extensional settings lateral continuity of cycles should not be expected in less well exposed or in one dimensional sections and in wells it would not be possible to distinguish continuous from discontinuous cycles or to understand such two dimensional heterogeneities identification of unique cycle forming mechanisms or attempting cyclostratigraphic long distance correlation of cycles is unrealistic without a detailed analysis of the architecture of cycles in laterally continuous outcrops

holocene carbonate sedimentation on the west eucla shelf great australian bight a shaved shelf the southern continental margin of australia is a cool water carbonate sedimentary province located in a high energy swell dominated oceanographic setting a vibrocore transect of 14c dated sediments across the centre of the eucla shelf is the first record of holocene shelf deposition in the great australian bight much of the seafloor shallower than 70 m water depth the base of wave abrasion is bare cenozoic limestone in some places encrusted by late pleistocene coral rich limestone that is cemented by high magnesium calcite 12 mole mgco3 the areally extensive 100 km wide hard bored substrate supports an epibiota of coralline algae minor bryozoans and soft algae or is covered by patches of holocene sediment up to 1 5 m thick generally a basal bivalve lag 3 ka overlain by quartzose bioclastic palimpsest sand this pattern of active carbonate production but little accretion on the wave swept mid to inner shelf is similar to that on other parts of the southern australian continental margin the term shaved shelf is proposed for this style of carbonate platform formed by alternating periods of sediment accretion cementation and erosion the palimpsest sand is typically rich in bivalves coralline algae and locally detrital dolomite outer shelf holocene sediment below the base of wave abrasion but inboard of the shelf edge is a metre thick unit of fine microbioclastic muddy sand with minor delicate bryozoans overlying a 9 13 ka rhodolith gravel some of this outer shelf sediment appears to have been resedimented the shelf edge is a sandy and rocky seafloor with active bryozoan growth and sediment production the holocene sediments are enriched in coralline algal particles and conspicuous large foraminifers cf marginopora and depleted in bryozoans as compared to coeval deposits on the lacepede and otway shelves off southeastern australia these differences are interpreted to reflect warmer waters of the leeuwin current and prevalent downwelling in this area as opposed to the general upwelling and colder waters in the east

structure and elasticity of serpentine at high pressure abstract serpentines occur in the subduction zone settings both along the slab and within the mantle wedge they are candidates for transporting water in to the deep earth their presence is manifested by serpentine mud volcanoes high electrical conductivities magnetic and seismic anomalies using theoretical methods we predict a pressure induced structural transformations in serpentine the transformations are related to the behavior of the silicate framework and misfit between octahedral and tetrahedral layers as the structure is compressed the octahedral layer and tetrahedral layers are compressed at different rates at 7 gpa the misfit between the layers vanishes this causes non linear pressure dependence of tetrahedral rotational angle this is also manifested by the onset of anomalous pressure dependence of the elastic constants c 11 c 33 c 12 c 13 beyond 7 gpa the misfit between the layers grows again reaching extremum at 22 gpa this is also manifested by discontinuity in average si o bond length volume of tetrahedron and re orientation of hydroxyl vector the symmetry of the crystal structure however remains unaffected evidence of pressure induced hydrogen bonding is absent in serpentine as evident from reduction of o h bond length upon compression results of compression for the low pressure regime p k 0 79 gpa k 0 12 and k 0 2 where k is the bulk modulus prime indicates pressure derivatives and o refers to zero pressure our best estimates of k 0 k 0 and the gruneisen parameter at 300 k and zero pressure based on our results are 61 gpa 17 and 0 77 respectively at low pressures serpentine structure is anisotropic with c 11 2 4 c 33 the pressure derivative of elastic constants c ij p are such that around 22 gpa c 11 c 33 an elastic instability c 66 50 gpa is also noted the elastic constant tensor reveals large acoustic anisotropy 41 in v p and seismic wave velocities that are significantly higher than those inferred from experiments on serpentinites

subtidal carbonate cycles implications for allocyclic vs autocyclic controls thick successions of repetitive metre scale subtidal carbonate cycles require that short term sedimentation rates approximate long term accumulation rates in order to maintain submergence below peritidal depths intrinsic processes such as storm and wave reworking and redistribution may act to inhibit aggradation into the zone of optimal carbonate production the presence of subtidal vs peritidal carbonate cycles may be a function of energy regime that in turn may relate to platform morphology the ultimate control on stacked subtidal cycles appears to be eustasy acting in concert with intrinsic processes such as storm and wave reworking

role of temperature and nutrients in extinction of turritelline gastropods cenozoic of the northwestern atlantic and northeastern pacific abstract turritelline gastropods mesogastropoda turritelline are common to abundant components in many fossil cretaceous cenozoic and recent marine benthic communities but the environmental factors affecting their evolution and extinction are poorly understood this is in large part because today they occur most abundantly in cooler waters associated with upwelling or other high nutrient conditions but thermal tolerances and preferences are unclear for extinct turritelline species turritellines were diverse and abundant throughout the new world for much of the cenozoic beginning in the early neogene through both extinction and range contraction however they started to disappear from higher latitudes on both atlantic and pacific coasts of north and central america the highest rate of extinction took place in the western atlantic in the plio pleistocene when almost all turritelline species in this area became extinct preliminary comparison of the available stratigraphic record suggests that this paleobiogeographic pattern is not an artefact of the record this biogeographic shift coincides with a postulated regional mass extinction among all marine mollusks in the western atlantic at around the same time causes for turritelline extinction however appear to be more complex than the decline in temperature that is cited most often as the probable cause of the western atlantic molluskan extinction event neogene turritellines in the western atlantic appear to have been adversely affected by collapse and or major reorganization of oceanic productivity associated with the formation of the isthmus of panama nutrients thus may have been more important than temperature change in controlling extinction probability in this group

facies analysis and palaeoenvironmental interpretation of the late oligocene attard member lower coralline limestone formation malta the oligocene represents a key interval during which coralline algae became dominant on carbonate ramps and luxuriant coral reefs emerged on a global scale so far few studies have considered the impact that these early reefs had on ramp development consequently this study aimed at presenting a high resolution analysis of the attard member of the lower coralline limestone formation late oligocene malta in order to decipher the internal and external factors controlling the architecture of a typical late oligocene platform excellent exposures of the lower coralline limestone formation occurring along continuous outcrops adjacent to the victoria lines fault reveal in detail the three dimensional distribution of the reef associated facies a total of four sedimentary facies have been recognized and are grouped into two depositional environments that correspond to the inner and middle carbonate ramp the inner ramp was characterized by a very high energy shallow water setting influenced by tide and wave processes this setting passed downslope into an inner ramp depositional environment which was colonized by seagrass and interfingered with adjacent areas containing scattered corals the middle ramp lithofacies were deposited in the oligophotic zone the sediments being generated from combined in situ production and sediments swept from the shallower inner ramp by currents compositional characteristics and facies distributions of the attard ramp are more similar to the miocene ramps than to those of the eocene an important factor controlling this similarity may be the expansion of the seagrass colonization within the euphotic zone this expansion may have commenced in the late oligocene and was associated with a concomitant reduction in the aerial extent of the larger benthonic foraminifera facies stacking pattern analysis shows that the depositional units parasequences at the study section are arranged into transgressive regressive facies cycles this cyclicity is superimposed on the overall regressive phase recorded by the attard succession furthermore a minor highstand correlated with the ru4 ch1 sequence and subsequent minor lowstand ch2 sequence have been recognized the biota assemblages of the attard member suggest that carbonate sedimentation took place in subtropical waters and oligotrophic to slightly mesotrophic conditions the apparent low capacity of corals to form wave resistant reef structures is considered to have been a significant factor affecting substrate stability at this time the resulting lack of resistant mid ramp reef frameworks left this zone exposed to wave and storm activity thereby encouraging the widespread development of coralline algal associations dominated by rhodoliths

seismic velocities anisotropy and shear wave splitting of antigorite serpentinites and tectonic implications for subduction zones 1 antigorite the high temperature ht form of serpentinite is believed to play a critical role in various geological processes of subduction zones we have measured p and s wave velocities vp and vs anisotropy and shear wave splitting of 17 serpentinite samples containing 90 antigorite at pressures up to 650 mpa the new results combined with data for low temperature lt lizardite and or chrysolite reveal distinct effects of lt and ht serpentinization on the seismic properties of mantle rocks at 600 mpa vp 5 10 and 6 68 km s vs 2 32 and 3 67 km s and vp vs 2 15 and 1 81 for pure lt and ht serpentinites respectively above the crack closure pressure 150 mpa the velocity ratio of antigorite serpentinites displays little dependence on pressure or temperature serpentine contents within subduction zones and forearc mantle wedges where temperature is 300 c should be at least twice that of previous estimates based on lt serpentinization the presence of seismic anisotropy high pressure fluids or partial melt is also needed to interpret ht serpentinized mantle with vp 1 81 the intrinsic anisotropy of the serpentinites 3 8 16 9 with an average value of 10 5 for vp and 3 6 18 3 with an average value of 10 4 for vs is caused by dislocation creep induced lattice preferred orientation of antigorite three distinct patterns of seismic anisotropy correspond to three types of antigorite fabrics s l and ls tectonites formed by three categories of strain geometry i e coaxial flattening coaxial constriction and simple shear respectively our results are thought to provide a new explanation for various anisotropic patterns of subduction systems observed worldwide

numerical simulations of subduction zones effect of slab dehydration on the mantle wedge dynamics abstract in oceanic subduction zones dehydration of slab s minerals may favor asthenospheric flow in the mantle wedge by decreasing rocks strength this should enhance the upper plate base reheating and markedly alter its thermal structure to quantify this phenomenon we model slab subduction within a viscous mantle dehydration hydration processes and the rock strength dependence on water content we use accurate phase diagrams for a h2o satured mantle peridotite and a gabbroic crust to determine at each time step the amount of water released or absorbed by each unit of rock transition phases are supposed to be not metastable water is released from the oceanic crust and from the altered peridotite portion of the slab dehydration of the subducting lithosphere occurs first at 60 75 km depth when the crust is eclogitized and second deeper around 105 km when serpentine and chlorite in serpentinite layer below the crust become unstable for high convergence rates because of cold p t t paths in the slab serpentine can be transformed into the hydrated phase a and water is recycled by the slab until great depth however in all investigated cases the released water is sufficient to hydrate by dissolution the whole mantle wedge until 217 55 km away from the trench and as it goes up to form hydrated minerals in the overlying lithosphere over a significant volume the convergence rate slightly shifts dehydration fronts location and consequently widens or reduces the hydrated mantle wedge note that for the dynamic corner flow modelled here with a non newtonian rheology the slab surface is significantly warmer than for an isoviscous analytical corner flow model yielding plausible crust melting we assume a strength reduction associated to hydration larger for rocks containing nominally hydrous minerals than for rocks with only dissolved water the rock strength thus becomes quite uniform at the base of the hydrated upper plate and in the wedge this results in cooler temperatures along the slab top but in an enhanced corner flow effect on the upper plate thermal structure for large hydration strength reductions a strong thermal erosion of the overlapping lithosphere develops in less than 15 myr due to convective destabilization this weakens drastically the upper plate at a distance from 110 to 220 km away from the trench the geometry of the eroded region could correspond to the low velocity zone observed below the arc region

the origin of high frequency platform carbonate cycles and third order sequences lower ordovician el paso gp west texas constraints from outcrop data and stratigraphic modeling abstract the passive margin succession of the diablo platform lower ordovician west texas is represented by the second order sauk c supersequence set consisting of a basal transgressive clastic unit the bliss sandstone above the breakup unconformity marking the second order basal lowstand transgressive phase overlain by 750 m of drift related shallow marine platform carbonates the el paso group recording the second order highstand due to late paleozoic structuring of the gondwanan passive margin present exposures in texas are in an updip shelf position and lack internal stratal geometries across depositional strike so sequences and systems tracts are identified solely by the vertical stacking patterns of depositional subfacies and higher frequency fifth order cycles accommodation plots fischer plots of high frequency cycles gauge systematic shifts in third order accommodation of two complete third order sequences each about 2 m y long and 60 140 m thick within the el paso group this is expressed in the vertical succession of cycle types systematic changes in cycle thickness and variations in subfacies as revealed by histograms of subfacies types tied to fischer plots a complete el paso shelf sequence contains a thin lowstand systems tract of quartz arenite a thick transgressive systems tract dominated by upward thickening thrombolitic subtidal cycles and a highstand systems tract marked by dolomitic thinning upward peritidal cycles containing quartz sand the el paso third order depositional sequences have been correlated by biostrat graphy and cycle stacking pattern from the franklin mountains southeast to the diablo arch northeast into the ardmore basin and thence to the appalachian basin hence the third order sequences appear to be eustatic based on biostratigraphic correlation of accommodation plots around the periphery of gondwana time series analyses suggest that a strict allocyclic milankovitch driven glacio eustatic mechanism alone is insufficient to account for the higher frequency fifth order and lower frequency third order accommodation cycles we used a two dimensional forward computer simulation to compare allocyclic and autocyclic models for high frequency cycles the allocyclic simulation superimposes a deterministic milankovitch glacio eustatic driver on a lower frequency third order accommodation cycle and generates a two dimensional synthetic cross section composed of high frequency peritidal and subtidal cycles stacked into third order sequences the autocyclic simulation is generated by a ginsburg autocyclic mechanism for high frequency cyclicity triggered by third order accommodation changes the autocyclic simulation quantitatively replicates an autocyclic mechanism for fifth order cycle development linked to onshore sediment transport and seaward progradation of tidal flats the two dimensional internal facies architecture of a third order sequence and the high frequency cycle stacking patterns are simulated in both scenarios important differences in patterns of cycles and subfacies stacking in the simulations shed light on the interpretation of the diablo platform cycles and sequences as an alternative or a supplement to autocyclicity or milankovitch forced glacio eustasy we have studied stochastic models for driving high frequency changes in relative sea level these intriguing results maintain essentially all the features observed in the rock record our smoothed sto hastic simulations suggest that the cycle problem is much more complex than presently realized

initiating intermediate depth earthquakes insights from a hp lt ophiolite from corsica abstract the hypocentres of intermediate depth earthquakes have been shown to overlap with the regions in subducting slabs that contain high abundances of hydrous minerals this relationship was initially revealed using geophysical and numerical modelling and until recently has lacked corroboration from direct field based research we evaluated the relationship of the coincidence of intermediate depth earthquakes with hydrous minerals in the slab by undertaking detailed geochemical analyses of blueschist to lawsonite to eclogite facies pseudotachylytes and their hostrocks located within an exhumed ophiolite the eocene schistes lustres complex in corsica these units comprise incompletely metamorphosed metagabbro and peridotite the wallrocks of the pseudotachylytes contain variable amounts of hydrous minerals tremolite mg hornblende glaucophane in the metagabbro and serpentine tremolite and chlorite in the peridotite back scatter electron images show that the hydrous minerals entrained in the melt undergo fusion rather than dehydration vesicular and h 2 o rich melt veins are observed cross cutting partially molten pseudotachylyte fault veins and show evidence of h 2 o exsolution during melt solidification the crystallisation products of these melts indicate formation under high temperature high pressure conditions 1400 1700 c 1 5 gpa the peridotite hosted pseudotachylytes crystallised olivine orthopyroxene and diopside which are surrounded by interstitial al and h 2 o rich glass the metagabbro pseudotachylyte is dominated by al rich omphacite ilmenite and high fe anorthite xrf bulk analyses of the wallrock of the pseudotachylyte and electron microprobe analyses of the pseudotachylyte matrix entrained survivor clasts and the crystallisation products show that near total disequilibrium melting took place the peridotite hosted pseudotachylyte composition is skewed strongly towards chlorite however the preservation of delicate dendritic diopside and olivine hopper crystals suggests that the pseudotachylyte is unaltered indicating that preferential fusion of chlorite took place the metagabbro hosted pseudotachylyte matrix composition is very similar to the bulk wallrock composition but slightly skewed by the preferential melting of mg hornblende and tremolite not all the pseudotachylytes are hydrous as the h 2 o content of the melts is highly variable the metagabbro pseudotachylyte ranges from 0 to 4 wt and the peridotite pseudotachylyte ranges from 0 to 14 wt the range in h 2 o content of the pseudotachylytes has lead us to conclude that the localised dehydration of hydrous minerals may be a second order factor in initiating intermediate depth seismicity however we have observed that the pseudotachylytes with the most chaotic vein networks thickest fault veins and most comminuted material have the highest abundances of hydrous wallrock minerals possibly owing to repeated fluid ingress in between pseudotachylyte generating events this implies that free fluids enhance pseudotachylyte generation and possibly seismicity but are not a first order requirement microtextural and geochemical results from this study suggest that the presence of abundant h 2 o rich minerals in the slab exerts a strong rheological control during high strain rate deformation facilitating thermally triggered localising shear instabilities these field based observations allow us to explore the assumption of the causal link between slab hydration and earthquake nucleation and offer fresh insight into the debate of how intermediate depth earthquakes take place

sedimentology and sequence stratigraphy of a pronounced early ordovician sea level fall on baltica the bjorkasholmen formation in norway and sweden abstract the bjorkasholmen formation consists of interbedded carbonates shales and glauconitic beds and is characterized by heavy bioturbation and few preserved sedimentary structures the unit shows five facies shale glauconitic packstone and three predominantly mud dominated carbonate facies carbonates and shales are arranged in small scale deepening upward cycles a minimum of fourteen of these small scale cycles are recognized in the bjorkasholmen formation they are arranged in stacks of 3 to 5 forming a total of four medium scale cycles separated by decimeter thick shale units based on the predominance of mud rich facies the succession is interpreted to have been deposited in an overall tranquil setting during one mayor sea level fall and subsequent initial rise of third order time estimates suggest that the 14 small scale cycles fall into the milankovitch band of precessional forcing and the overriding medium scale cycles likely represent short eccentricity the sequence stratigraphic interpretation shows that the bjorkasholmen formation is characterized by falling stage lowstand and initial transgressive systems tracts consequently the contact between the bjorkasholmen and the underlying alum shale formation represents the basal surface of forced regression the maximum regressive surface is defined by a hiatus in the oland sections and by shallow marine packstones within mud rich distal ramp carbonates in norway the top of the bjorkasholmen formation represents a flooding surface at the base of the transgressive systems tract a comparison of time equivalent successions worldwide suggests that the bjorkasholmen formation represents a tectonically enhanced lowstand with two overriding short term milankovitch eustatic signals although deposition of the bjorkasholmen formation coincides with the initiation of a foreland basin in the caledonides of norway it remains unclear how these tectonic movements may have lead to the widespread bjorkasholmen lowstand during the early ordovician it is suggested in this study that a combination of compressional forces from avalonia and the caledonian margin may have acted in concert to produce an uplift of larger parts of the baltica plate for a time span of approximately 0 5 myr

controls on a shallow water hemipelagic carbonate system adjacent to a siliciclastic margin example from late turonian of central europe abstract this study interprets the controls on cyclic and secular changes in a shallow water hemipelagic system of the bohemian cretaceous basin using i spatial and temporal changes in facies and geometries of the hemipelagic succession and ii correlation of the hemipelagic cycles with the coeval nearshore siliciclastic sequences three orders of hemipelagic rhythms characterized by changes in lithology between foram dominated limestones 70 caco3 and mudstones 3 are recognized couplets first order bundles and second order bundles the hemipelagic bundles are interpreted to reflect changes in siliciclastic flux that followed transgressive regressive movements of the adjacent shoreline upward muddying intervals correspond to regressive trends whereas downward muddying intervals are interpreted as due to transgressions the upward muddying and downward muddying intervals of the second order hemipelagic bundles display offlapping and onlapping internal geometries respectively suggesting that the transgressive regressive cycles were coupled with changes in potential accommodation in the hemipelagic system these relationships may point to sea level forcing of the second order hemipelagic bundles finally spectral analyses of gamma ray signatures of the hemipelagic strata suggest that the second order hemipelagic bundles were driven primarily by the milankovitch cycles of long eccentricity c 400 kyr whereas the first order hemipelagic bundles record an interplay of the cycles of short eccentricity c 100 kyr and obliquity c 40 kyr thus the transgressive regressive and possibly sea level cycles acted as mediators of milankovitch driven changes in insolation the secular onset of carbonate dominated conditions in the distal part of the basin does not follow any secular transgressive trend but coincides with a major change in differential subsidence uplift pattern of the study area it is interpreted to reflect an increase in the background carbonate production in the basin due to a tectonically and or climatically induced change in circulation and associated acceleration of water mass exchange with the pelagic carbonate factory of northwestern europe in summary processes related to relative sea level changes and transgressive regressive shoreline movements are capable of producing hemipelagic rhythms but it is the interplay of these processes with background oceanographic conditions regional circulation patterns open marine trophic resource levels etc that controls the actual character of the hemipelagic rhythms e g whether carbonate or clastic dominated thus changes in hemipelagic conditions as expressed in the rock record are rarely proportional to either changes in sea level and terrigenous input alone or changes in the background oceanography alone sequence stratigraphic analysis of hemipelagic strata must consider the spatial and temporal variability in the local oceanographic conditions that result from a combination of the above processes

fluctuations in the trophic resource continuum a factor in global diversity cycles the trophic resource continuum trc in euphotic zones of the world s oceans is the spectrum of conditions from the richest runoff and upwelling areas to the most nutrient deficient subtropical seas that spectral range has likely varied through geologic history as more efficient phytoplankton taxa evolved as oceanic circulation rates changed and in response to oceanic mixing events reduced rates of oceanic turnover should produce regionally intensified eutrophy and oligotrophy that is trc expansion because oceanic waters as a whole are relatively nutrient rich increased rates of oceanic turnover should prevent extremely eutrophic and extremely oligotrophic conditions from developing causing trc contraction global biotic diversities in the oceans are strongly influenced by communities in oligotrophic environments nutrient deficient waters are more stable and more transparent than richer waters permitting higher degrees of specialization sparse nutrient supplies necessitate smaller phytoplankton and longer more complex food chains preservation potential of oligotrophic biotas is also greater because calcareous taxa are more prevalent and because calcium carbonate is less apt to be bioeroded or dissolved fluctuations in the trc should profoundly affect diversity of habitats within the euphotic zone of the oceans the evolutionary response to a trc expansion diversification would occur in a geologic time frame of millions of years while evolutionary responses to a trc contraction including extinction could occur in a biological time frame of generations a simple model indicates that a reduction in the trophic resource continuum from expansion during a major marine transgression down to modern levels could result in loss of 40 of the euphotic habitats how fluctuations in the trc control diversity of euphotic habitats in the oceans provides an hypothesis by which to interpret global taxonomic diversity cycles as well as taxonomically selective extinctions changes in species dominance and some morphologic trends

an epeiric ramp low energy cool water carbonate facies in a tertiary inland sea murray basin south australia the murray supergroup records temperate water carbonate deposition within a shallow mesotrophic oligo miocene inland sea protected from high energy waves and swells of the open ocean by a granitic archipelago at its southern margin rocks are very well preserved and exposed in nearly continuous outcrop along the river murray in south australia most facies are rich in carbonate silt contain a background assemblage of gastropods especially turritellids and infaunal bivalves and are packaged on a decimetre scale defined by firmground and hardground omission surfaces bioturbation is pervasive and overprinted resulting in rare preservation of physical sedimentary structures facies are grouped into four associations large foraminiferan bryozoan echinoid bryozoan mollusc and clay facies interpreted to represent shallow water 50 m deposition under progressively higher trophic resource levels from low mesotrophy to eutrophy and restricted marine conditions from relatively offshore to nearshore regions a large scale shift from high to low mesotrophic conditions within lower miocene strata reflects a change in climate from wet to seasonally dry conditions and highlights the influence terrestrially derived nutrients had upon this shallow land locked sea overall low trophic resource levels during periods of seasonally dry climate resulted in a deepening of the euphotic zone a widespread proliferation of foraminiferan photozoan fauna and a relatively high carbonate productivity inshore heterozoan facies became progressively muddier and restricted towards the shoreline in contrast periods of wet climate led to rising trophic resource levels resulting in a shallowing of the euphotic zone a decrease in epifaunal and seagrass cover and widespread development of a mostly heterozoan biota dominated by infaunal echinoids rates of carbonate production and accumulation were relatively low the murray basin is best described as an epeiric ramp wide facies belts developed in a shallow sea on a low angled slope reaching many hundreds of kilometres in length grainy shoal and back barrier facies were absent internally generated waves impinged the sea floor in offshore regions and because of friction along a wide and shallow sea floor created a low energy expanse of waters across the proximal ramp storms were the dominating depositional process capable of disrupting the entire sea floor

late pleistocene and holocene cool water carbonates of the western mediterranean sea post glacial neritic cool water carbonates of the western mediterranean sea were examined by means of hydroacoustic data sediment surface sampling and vibrocoring to unravel geometries and to reconstruct sedimentary evolution in response to the last sea level rise the analysed areas located on the alboran ridge in the bay of oran and at the southern shelf of the island of mallorca are microtidal and bathed by oligotrophic to weakly mesotrophic waters seasonal water temperature varies between 13 c and 27 c echosounder profiles show that the bay of oran and the southern shelf of mallorca are distally steepened ramps while the alboran ridge forms a steep flanked rugged plateau around the alboran island in the three areas an up to 10 m thick post glacial sediment cover overlies an unconformity in oran and mallorca stacked lowstand wedges occur in water depths of 120 to 130 m on the alboran ridge and in the bay of oran highstand wedges occur at 35 to 40 m up to 5 m long cores of upper pleistocene to holocene successions were recovered in water depths between 40 and 81 m deposits contain more than 80 carbonate with mixed carbonate volcaniclastics in the lower part of some cores in alboran the carbonates consist of up to 53 of aragonite and up to 83 of high magnesium calcite radiocarbon dating of bivalve shells coralline algae and serpulid tubes indicates that deposits are as old as 12 400 cal yr bp the carbonate factories in the three areas are dominated mostly by red algae but some intervals in the cores are richer in bivalves a facies rich in the gastropod turritella reflecting elevated surface productivity is restricted to the mallorca shelf rhodoliths occur at the sediment surface in most areas at water depths shallower than 70 m they form a 10 to 20 cm thick veneer overlying rhodolith poor bioclastic sediments which nonetheless contain abundant red algal debris this rhodolith layer has been developing for the past 800 to 1000 years similar layers at different positions in the cores are interpreted as reflecting in situ growth of rhodoliths at times of reduced net sedimentation sedimentary successions in the cores record the post glacial sea level rise and the degree of sediment exposure to bottom currents deepening upward trends in the successions are either reflected by shallow to deep facies transitions or by a corresponding change of depth indicative red algae there are only weak downcore variations of carbonate mineralogy which indicate that no dissolution or high magnesium to low magnesium calcite neomorphism occurs in the shallow subsurface these new data support the approach of using the recent facies distribution for interpretation of past cool water low energy microtidal carbonate depositional systems hydroacoustic data show that previous pleistocene transgressive and highstand inner ramp deposits and wedges were removed during sea level lowstands and accumulated downslope as stacked lowstand wedges this suggests that under conditions of high amplitude sea level fluctuations the stratigraphic record of similar cool water carbonates may be biased

thermal dehydration reactions characterised by combined measurements of electrical conductivity and elastic wave velocities combined laboratory measurements of seismic velocities and electrical conductivity as a function ofpt and drainage conditions have been performed on various rocks containing hydrous minerals this paper presents experimental results for evaporite rocks containing gypsum caso4 2h2o and carnallite kmgcl3 6h2o and for serpentinite the experiments on the evaporite rocks were carried out in a triaxial cell in the range 20 130 c and 5 0 24 0 mpa confining pressure the measurements on serpentinite covered the range 20 750 c at pressures of up to 200 mpa and were performed in a cubic pressure apparatus the thermally induced onset of dehydration was indicated by the pronounced discontinuous behaviour of conductivity corresponding to a marked drop in the elastic wave velocities the respective poisson ratios were inversely correlated with permeability and seem to be a sensitive parameter to describe the reaction related variations of pore space the seismic properties can be satisfactory modelled by using the self consistent approximation of o connell and budiansky 1 illustrating the effects of variations in pore space and saturation the dehydration behaviour of the various hydrous rock types was found to be different because the changes in the rock physical properties are closely linked to the internally created pore fluid to the changes in the porosity and pore geometry and to the resulting pore pressure progressive thermal dehydration reactions induce an opening of pore space accompanied by lowered saturation conditions whereby the rate of fluid release depends mainly on temperature and also on the drainage conditions of the system our findings as reflected by the variations in the geophysical parameters may be of importance for the interpretation of natural dehydration processes in the crust caused by prograde metamorphism

environmental covariation of metazoans and microbialites in the lower ordovician boat harbour formation newfoundland abstract an antagonistic view of the relationship between microbialites and metazoans has long been inferred in part because of the large scale anticorrelation of these two groups through geologic time the nexus of this relationship occurs in the early paleozoic era stromatolites declined in abundance as complex animals and algae diversified but thrombolites a type of microbialite little known before the proterozoic cambrian boundary proliferated for the first time well preserved parasequences in the basal portion of the lower ordovician boat harbour formation western newfoundland contain a succession of stromatolites and thrombolites that permit an investigation into the role metazoans played in shaping the nature and abundance of microbialites in early paleozoic carbonate seas sessile benthic animals colonized thrombolite surfaces but are nearly absent from stromatolites bioturbation rarely co occurred with microbialites but is widespread in clastic carbonates that lack microbialites our results thus support the hypothesis of ecological antagonism between microbial communities and motile benthic animals but also demonstrate biological facilitation between thrombolites and both sessile benthic animals and nekton

low wave velocity and high electrical conductivity layer of serpentine a compilation serpentine is formed by hydrothermal metasomatism and may penetrate into the deep crust or mantle of the earth it is considered to be the water resource of the mantle owing to its high water content and high temperature decomposition 600 900 c in some geological settings it has low wave velocity lower than that of the surrounding rocks and high electrical conductivity higher than that of the surrounding rocks which are the universal physical and geological properties of serpentine found across continents on earth studying the properties of this layer in particular the wave velocity and electrical conductivity contributes to a more in depth understanding of the mid lower crust or mantle of the earth it is also helpful for exploring the mechanism of seismicity owing to the relationship of this layer with the earthquake center location in subduction zones the stability of serpentine is a factor affecting seismicity in this study the wave velocity and electrical conductivity data for serpentine together with temperature and pressure have been collated from previous studies some of these studies provide more comprehensive results and others provide new conclusions overall the findings showed that a low wave velocity and high electrical conductivity layer can be found in serpentine under suitable geological settings details of the driving mechanism behind seismicity in subduction zones are also shown

stratigraphic sedimentology of tertiary cool water limestones se australia abstract mid tertiary limestones exposed as sea cliffs along the coast of victoria southeastern australia accumulated on the inner part of a cool water distally steepened ramp or open shelf all sediments are bioclastic dominated by particles of bryozoans echinoderms benthic foraminifers brachiopods and molluscs shallow grainy facies also contain coralline algae quartz and glauconite while deep muddy facies include ahermatypic corals ostracodes sponge spicules planktic foraminifers and terrigenous clays the fundamental depositional unit is a meter scale subtidal cycle or rhythm shallow shelf cycles are shallowing upward cross bedded or burrowed grainstone capped by a marine cemented hardground or quartz granule lag while mid shelf cycles are upward coarsening and thickening proximal to distal tempestites such cycles are interpreted to form by eustatically driven climatically controlled oscillations in abrasion wave base and swell wave base deep shelf rhythms are interbedded bryozoan marl and calcareous clay thought to be generated by climatically influenced fluctuations in carbonate productivity and terrigenous dilution lowstand systems tracts are depositional or erosional sequence boundaries are complex multigeneration mostly submarine surfaces condensed cycle boundaries that can be traced offshore into multiple omission surfaces and conformable contacts basinward facies shifts result in deposition of condensed lowstand wedges of stacked shallowing upward grainstone cycles transgressive systems tracts are thick bryozoan marl calcareous clay rhythms associated with facies backstepping and abrupt deepening such rhythmites typically grade stratigraphically upward into tempestite cycles highstand systems tracts are highly progradational wedge shaped rock bodies of shallowing upward grainy cycles these limestones provide the critical link of stratigraphy between modern cool water carbonate shelf deposits and the older mesozoic and paleozoic record of similar sediments

upper mantle structure in the south central chilean subduction zone 30 to 36 s 1 regional p and s wave travel time data were used to obtain three dimensional seismic tomography models for vp vs and vp vs above the subducting slab in central chile and argentina in this region there is an abrupt change from a normal subduction geometry south of 33 s to a flat subduction geometry to the north we find low vp low vs and high vp vs ratios in the southern half of our study area directly beneath the modern active volcanic arc which we interpret as localized pockets of melt in the northern half of our study area above where the subducting nazca plate flattens at 100 km depth we find low vp high vs and low vp vs ratios these unusual results point to a lack of melt or hydrated mineralogies such as serpentine both of which are characterized by high vp vs values the only mantle rocks that have low vp vs and high vs are mg rich compositions such as dehydrated serpentinite or orthopyroxenite we suggest that significant portions of the mantle overlying the flat slab consist of orthopyroxenite formed by a transient fluxing of silica rich fluids such fluids may have come from sediments that were subducted during the initiation of flat subduction at this latitude at 10 ma this would imply that the hydration of mantle material above a flat slab can be a transient phenomenon which leaves little residual free water behind but significantly alters the mantle chemistry

oligo miocene seagrass influenced carbonate sedimentation along a temperate marine palaeoarchipelago padthaway ridge south australia oligo miocene carbonates associated with the padthaway ridge form the southern margin of the murray basin south australia the carbonates are a thin somewhat condensed succession of echinoid and bryozoan rich limestones that record accumulation in the complex of islands and seaways and progressive burial of the ridge through time the rocks are grainy to muddy bioclastic packstones grainstones and floatstones composed of infaunal echinoderms bryozoans coralline algae and benthic foraminifera with lesser contributions from molluscs and serpulid worms locally as much as half of these skeletal components are fe stained relict grains that imbue the lithologies with a conspicuous yellow to orange hue this variably lithified succession is partitioned into metre scale firmground bounded and hardground bounded beds textured by extensive thalassinoides burrows dominant lithologies are interpreted as temperate seagrass facies limestones contain attributes indicative of both seagrass dominated palaeoenvironments and carbonate production and accumulation on unconsolidated barren sandflat palaeoenvironments together these two depositional systems are thought to have generated a single multigenerational amalgamated facies recording sedimentation within a complex temperate seagrass environment limestones overlying the padthaway ridge reflect a gradually warming climate increasing water temperature and decreasing nutrient content within the framework of a ridge gradually being buried in sediment this succession from cool temperate to warm temperate to subtropical through time permits recognition of the relative influence of changing oceanography on a seagrass dominated shallow inter island sea floor criteria are proposed herein to enable future recognition of similar temperate seagrass facies in cenozoic limestones elsewhere

a proposed integrated multi signature model for peritidal cycles in carbonates abstract a new integrated multi signature model for meter scale peritidal carbonate cycles presented here is relevant to the interpretation of these cycles in the stratigraphic record meter scale shallowing upward micro facies cycles are the most common type of peritidal parasequence peritidal parasequences are a fundamental component of carbonate sequence stratigraphic models however the existence of meter scale cycles in the geological record has been questioned owing to the failure to prove statistically the existence of these cycles using data on micro facies and stacking pattern taken from previous studies nevertheless the shallowing upwards micro facies interpretive template remains widely used to identify these cycles and parasequences we propose that the sedimentary signatures of meter scale peritidal cycles vary systematically during the various phases of an underlying 3rd order relative sea level cycle although during specific phases of a 3rd order relative sea level cycle meter scale peritidal cycles may be characterized by shallowing upward units e g early transgressive systems tract during other periods the stratigraphic signal of cyclicity may be more cryptic and complex causing them to be overlooked e g late transgressive systems tract and highstand systems tract we also propose that there is a half cycle phase shift in periods of deposition and the formation of drowning surfaces relative to 4th to 5th order relative sea level cycles over the course of a 3rd order relative sea level cycle these points have ramifications for defining parasequences and using stacking patterns to determine peritidal cycle periodicity specifically parasequences defined using flooding surfaces identify sedimentary units that inconsistently partition 4th to 5th order relative sea level cycles at different stages of a 3rd order relative sea level cycle we propose testable new physical stratigraphic signatures for identifying meter scale cycles that have consistent genetic significance each cycle recording a relative sea level cycle characterized by a 4th to 5th order relative sea level rise and fall meter scale peritidal cycles are recorded by an ordered pattern of systematically varying micro facies signatures related to their position within a 3rd order relative sea level cycle the integrated multi signature model of meter scale peritidal carbonate cycles we propose here may assist in explaining the apparently contradictory evidence both for and against their existence in the geological record as a result of the detailed consideration in this paper of the nature of parasequences and their variations with 3rd order relative sea level changes a redefinition of parasequence is required and one is presented here

coseismic dehydration of serpentinite evidence from high velocity friction experiments abstract high velocity friction hvf experiments on serpentinite under conditions equivalent to large amounts of earthquake slip produced large volumes of water vapor derived from the frictional heating induced dehydration of serpentinite fourier transform infrared ftir and thermogravimetry tg differential scanning calorimetry dsc microspectroscopic analyses show that the water in the slip zone and its bounding zones was released due to dehydration during the hvf experiments our experimental results demonstrate that i the run product consists of 10 wt molten materials with abundant vesicles and 90 wt fine grained clasts ii both serpentine and olivine minerals were melted during high velocity slip 1 1 1 3 m s iii rapid serpentine dehydration occurred in a zone of up to 3 mm wide including the slip zone and its bounding zones accompanying frictional melting during high velocity slip and iv no distinct dehydration or frictional melting occurred during low velocity slip

ongoing cenomanian turonian heterozoan carbonate production in the neritic settings of peru abstract the present paper reports on the sedimentological and geochemical record of albian turonian neritic carbonates from the eastern subequatorial pacific domain in peru the focus is on one of the most extreme carbon cycle perturbations of the phanerozoic the oceanic anoxic event 2 late cenomanian early turonian thanks to the very expanded and well exposed sections in peru the oae2 interval was sampled at high temporal resolution for both bulk micrite and bulk organic matter carbon isotopes despite the scarcity of significant amounts of organic matter or evidence for oxygen deficiency the 13c curve matches well with global published high resolution data for coeval successions such as those reported from the english chalk and the portland 1 core biostratigraphic data and the detailed sequence stratigraphic interpretation of these sections are combined with the carbon isotope chemostratigraphy documented here applying the characteristic peak and trough chemostratigraphic terminology for oae2 a c the following main environmental and carbon isotope stratigraphic features are observed from the late albian to the early middle turonian in peru i an albian to early late cenomanian heterozoan ramp recording the pre oae2 13c excursions specifically the mid cenomanian event ii a late cenomanian trough of 13c values b showing a progressive deepening leading to the short lived establishment of middle ramp type sedimentation iii a late cenomanian to early turonian 13c plateau c characterised by benthonic inner ramp sedimentation during a sea level highstand phase iv a recovery of 13c values at the end of oae2 associated to a trophic change increased influx of argillaceous facies and reduced carbonate production v a early to middle turonian fluctuating 13c curve linked to a maximum flooding phase in the mammites nodosoides zone and carbonate production during the collignoniceras woollgari zone the data shown here are particularly relevant as they come from very expanded neritic sections in the sub equatorial eastern pacific many of the features recognized share important similarities with tethyan and atlantic sections whilst the ramp system as such did not suffer from a carbonate crisis during oae2 as recorded for instance in mexico and tibet

holocene sediment dynamics on a cool water carbonate shelf otway southeastern australia the otway shelf is covered by cool ocean waters and veneered by bryozoan dominated carbonate sediments radiocarbon dating and stratigraphy of shelf vibracores and slope gravity cores document late pleistocene holocene deposition shelf sediments of the late pleistocene highstand 60 26 ky are rare either never having been deposited or having been removed during the following sea level fall 26 17 ky during the subsequent lowstand the shelf was exposed facies shifted basinward and beach dune complexes were constructed near the shelf edge the deep shelf was characterized by nondeposition and hardground formation and the shelf margin became locally erosional upper slope bryozoan sponge assemblages continued to grow actively and lower slope foraminifera and nannofossil ooze was increasingly enriched in hemipelagic terrigenous mud swept off the wide shelf coarse shelf debris and lowstand dune sands were erosively reworked and transported onto the upper slope and redistributed to deep slope aprons during early transgression 17 10 ky this event locally terminated shelf edge bryozoan and sponge growth continued erosive shoreface retreat formed a bored and cemented ravinement surface mantled by bivalves and lithoclasts across the open shelf base level rise reduced the amount of terrigenous material reaching the continental slope slower transgressive flooding from 10 to 6 5 ky allowed infilling of shallow embayments on the shelf and outer shelf sediment accumulation began by 7 ky sea level stabilized near its present level by 6 5 ky and the area is now partitioned into three zones by an energetic wave and swell regimen 1 inboard protected embayments and or beach dune complexes with high sediment accumulation rates about 100 cm ky 2 an open shelf where although sediment production is active accumulation is patchy 3 5 cm ky to nonexistent within the zone of wave abrasion and minimal about 23 cm ky between maximum abrasion depth and swell wave base and 3 a deep shelf edge and upper slope where sediment is accumulating at rates of 2 50 cm ky interpreted deep water 100 m bryozoan sponge mounds are growing at rates of 105 cm ky accumulation decreases progressively down slope with deep slope pelagic carbonate sedimentation at rates of 5 cm ky the late quaternary shelf record resembles that of flat topped warm water platforms with holocene sediment overlying pleistocene tertiary limestone but for different reasons much of this high energy shelf is still within lag depth more than 10 ky after flooding the steady state deep water shelf margin bryozoan factory has been largely unaffected by 50 100 m eustatic changes in sea level and has contributed to slow but continuous platform progradation the slow growth potential uniform profile of sediment production and distribution and inability of constituent organisms to construct rigid frameworks favor maintenance of a shallow ramp profile and makes the cool water carbonate system an excellent modern analog for interpretation of many ancient ramp successions

demise and recovery of antillean shallow marine carbonate factories adjacent to active submarine volcanoes lutetian bartonian limestones st bartholomew french west indies abstract among other parameters volcanic activity adjacent to shallow marine environments influences the development of ecosystems and their carbonate producing communities volcaniclastic sediment influx in particular has potential to cause rapid and drastic environmental changes affecting biological systems in their composition and activity and ultimately leading to changes to and breaks in carbonate sedimentation such sedimentary breaks that form in response to volcanic processes are rarely described in detail despite the common occurrence of carbonate platforms adjacent to active volcanoes both in the recent and past geological record the island of st bartholomew st barths french west indies exposes sections of middle eocene limestones intercalated with thick volcaniclastic deposits and lavas theses carbonates provide an example of a low latitude tropical platform where non framework building biota were important if not dominant sediment contributors the carbonate system records the repeated collapse and renewal of carbonate production as a result of episodic volcaniclastic material input the discontinuous nature of the carbonate sedimentation is reflected in contrasted depositional systems across sedimentary surfaces and gradational contacts the studied eocene deposits provide a sedimentary record of how volcanic events impacted warm water carbonate factories both in their disturbance demise and recovery

electrical conductivity measurements on synthetic olivines and on olivine enstatite and diopside from dreiser weiher eifel germany under defined thermodynamic activities as a function of temperature and pressure abstract results of electrical conductivity measurments on synthetic olivines of the system mg 2 sio 4 fe 2 sio 4 and on minerals of dreiser weiher peridotite nodule ib 8 eifel germany are discussed in relation to the measuring procedure and to the variation of thermodynamic parameters the measurements were performed in solid state high pressure vessels between 340 and 1100 c and at a pressure of 10 kbar it is shown that for ternary olivines and for pyroxenes the control of two further variables like the chemical activities of the components is needed besides temperature and pressure control the experimental set up for the control of chemical activities and oxygen partial pressure is shown from the slopes of the lines of log against 1 t the activation energies were calculated though in most cases the same oxygen fugacity o 2 is applied the results reveal different values for synthetic and natural samples since the chemical activities of sio 2 are different

the role of nutrient availability in bioerosion consequences to carbonate buildups abstract zooxanthellate organisms which are among the major carbonate producers on coral reefs are highly adapted to nutrient deficient conditions and tend to be outcompeted by filamentous or fleshy algae if nutrients are abundant reef dwelling bioeroding organisms on the other hand seem to increase in abundance with increasing availability of nutrient and food resources maximum rates of calcium carbonate production in a reef system are comparable in magnitude to maximum rates of bioerosion the dynamic interplay between accretion destruction of coral reefs is therefore likely to be strongly influenced by nutrient availability nutrient availability may also influence development of carbonate facies in reef systems that develop in oligotrophic waters skeletal sands and framework should dominate carbonate buildups in systems that develop in mesotrophic waters enhanced bioerosion should reduce much of the skeletal material to carbonate muds increasing the prevalence of mud rich facies eutrophic conditions should result in bioeroded hardgrounds and carbonate depositional hiatuses when carbonate accumulation rates fail to equal or exceed rates of local relative sea level rise reefs drown bored and lithified pavements with crusts of ferromanganese oxides phosphates or glauconite often separate shallow reef from overlying deeper water deposits nondeposition bioerosion and evidence for low redox potentials at the sediment water interface all implicate excess nutrient availability as a factor in reef drowning some carbonate depositional hiatuses and bioerosional surfaces are local phenomena evidence of responses to local increases in nutrient availability resulting from submergence of soils runoff or topographic upwelling others appear to reflect regional or worldwide events because many carbonate producing organisms have life history strategies specialized to nutrient deficient conditions they are unable to compete when nutrients become plentiful events that triggered ocean turnover or sharp increases in the rate of deep ocean circulation would have increased sea surface nutrient availability worldwide such eutrophication could have caused widespread extinctions in reef communities because eutrophication promotes bioerosion details of the fossil records of those events are probably lost in bioerosional hiatuses

lower carboniferous ramp sedimentation of the central alborz basin northern iran integrated sedimentological and rock magnetic studies the lower carboniferous mobarak formation in the alborz basin northern iran was deposited along the northeastern margin of gondwana in a carbonate ramp setting this paper focuses on the tournaisian stratigraphic interval of this formation that crops out at the jaban section in the southwestern central alborz basin the following facies associations representing different ramp palaeoenvironments have been identified 1 mudstone wackestone outer ramp facies 2 crinoidal to skeletal grainstone packstone mid ramp facies 3 peloidal to crinoidal grainstone packstone inner ramp facies and 4 coastal facies which include a variety of microbial laminated to oncoidal grainstones and mudstones with evaporitic pseudomorphs this ramp profile was affected by frequent storms that were responsible for the formation of several skeletal to non skeletal shoals in the distal mid ramp to the most proximal inner ramp areas the development of the skeletal to non skeletal shoals along the sea side of the ramp formed a semi enclosed lagoon sensitive to the influence of both high tides and storm surges the magnetic susceptibility xin of all the samples was measured and compared with that of the facies from which the sample was taken there is a clear link between xin and the facies the average xin values were higher for the distal facies than for the proximal facies the xin profile of this lower carboniferous carbonate sequence reflects stratigraphic variations in response to relative changes in sea level and the input of detrital materials in the context of the sequence stratigraphic framework the average xin values for lowstand and transgressive systems tract deposits are higher than for the highstand systems tract deposits the clear link between xin and facies indicates at least a partly preserved primary xin signal related to the detrital inputs however to obtain a better understanding of the nature and origin of the minerals carrying the xin we performed hyster esis measurements on selected samples it appears that the xin signal is mainly carried by low coercivity ferromagnetic minerals such as magnetite with a mixture of relatively coarse grains detrital fraction and ultra fine grains probably formed during diagenesis

double seismic zone and dehydration embrittlement of the subducting slab 1 dehydration embrittlement of metamorphosed oceanic crust and mantle in the subducting slab may be responsible for the occurrence of intermediate depth earthquakes we explore the possibility that this hypothesis can explain the morphology of the double seismic zones observed in northeast japan southwest japan northeast taiwan northern chile cape mendocino and eastern aleutians we calculate transient temperature structures of slabs based on geologically estimated subduction histories of these regions we then determine dehydration loci of metamorphosed oceanic crust and serpentinized mantle using experimentally derived phase diagrams the depth range of the dehydration loci of metamorphosed oceanic crust and serpentine is dependent on slab age the dehydration loci of serpentine produce a double layered structure because the upper dehydration loci of serpentine are mostly located in the wedge mantle above the slab we regard the upper plane seismicity representing dehydration embrittlement in the oceanic crust and we fix the slab geometry so that the upper plane seismicity is just below the upper surface of the slab we find that the lower plane seismicity is located at the lower dehydration loci of serpentine which indicates that the morphology of the double seismic zones is consistent with the dehydration embrittlement

chondrites a trace fossil indicator of anoxia in sediments the trace fossil chondrites a highly branched burrow system of unknown endobenthic deposit feeders occurs in all types of sediment including those deposited under anaerobic conditions in some cases such as the jurassic posidonienschiefer formation of germany chondrites occurs in black laminated carbonaceous sediment that was deposited in chemically reducing conditions in other cases such as numerous oxic clastic and carbonate units throughout the geologic column chondrites typically represents the last trace fossil in a biotutbation sequence this indicates that the burrow system was produced deep within the sediment in the anaerobic zone below the surficial oxidized zone that was characterized by freely circulating and oxidizing pore waters

brachiopods in epeiric seas as monitors of secular changes in ocean chemistry a miocene example from the murray basin south australia abstract early and middle miocene cool water carbonates from the murray basin south australia preserve an excellent stable isotope record of ocean climate change these variably fossiliferous heterozoan deposits accumulated on a low energy mesotrophic centripetal epeiric ramp during a gradual shift in climate from cool wet conditions and abundant continent derived nutrients to a seasonal arid climate with reduced delivery of trophic resources to the marine realm temporal trends in 13c and 18o from unaltered brachiopods record an epeiric sea response to this warming the globally recognized middle miocene monterey event 17 to 13 5 ma dominates the carbon isotope record albeit with higher 0 5 than open ocean 13c values such higher 13c values are attributed to an increase in benthic carbonate production that accompanied climate change and the relatively short seawater mixing times characteristic of epeiric sea systems the murray basin oxygen isotope curve contains lower 18o values 2 0 lower than those of the deep sea record this difference is ascribed to the warmer seawater temperatures 17 to 22 c that prevailed across the miocene murray basin these results show that the isotope chemistry of epeiric sea brachiopods can be a reliable gauge of regional and global environmental evolution although diagenetic overprinting from meteoric cement filled punctae and local forcing factors introduce noise that mutes isotopic signals the open ocean secular record is clearly discernible

deposition of a saline giant in the mississippian windsor group nova scotia and the nascent late paleozoic ice age abstract saline giants are vast marine evaporite deposits that currently have no modern analogues and remain one of the most enigmatic of chemical sedimentary rocks the mississippian windsor group ca 345 ma maritimes basin atlantic canada is a saline giant that consists of two evaporite rich sedimentary sequences that are subdivided into five subzones sequence 1 is composed almost entirely of thick halite belonging to subzone a osagean sequence 2 is in unconformable contact and comprised of stacked carbonate evaporite peritidal cycles of subzones b through e meramecian subzone b the focus of research herein documents the transition from wholly evaporitic to open marine conditions and thus preserves an exceptional window into the processes forming saline giants lithofacies stacking patterns in subzone b reveal that higher order fluctuations in relative sea level produced nine stacked parasequences interpreted to reflect high frequency glacioeustatic oscillations during the onset of the late paleozoic ice age each parasequence reflects progradation of intertidal and sabkha sediments over subtidal carbonate and evaporite deposits dissimilarities in cycle composition between sub basins imply the development of contrasting brine chemistries from differing recharge rates with the open ocean what the windsor group shows is that evaporite type is ostensibly linked to the amplitude and frequency of sea level rise and fall during deposition true saline giants like the basinwide evaporites of sequence 1 apparently require low amplitude long frequency changes in sea level to promote the development of stable brine pools that are only periodically recharged with seawater by contrast the high amplitude short frequency glacioeustatic variability in sea level that controlled the accumulation of peritidal evaporites in subzone b produce smaller subeconomic deposits with more complex facies relationships

pot and gutter casts from the chapel island formation southeast newfoundland abstract member 2 of the chapel island formation of southeast newfoundland contains a diverse suite of erosional structures known as pot and gutter casts these are particularly abundant in a lithofacies deposited in a nearshore zone characterized by erosion and sediment bypass during storm emplacement of sands the steep to overhanging walls of both pot and gutter casts imply rapid filling during the same storm events that created these depressions erosion is thought to have taken place under strong offshore directed unidirectional flow storm surge or other relaxation flow as with many sandstone tempestites the initial stages of the storm events were dominated by strong and in this case highly erosive unidirectional flow and the latter stages by oscillatory flow as evidenced by wave diagnostic lamination in the sandy infills pot casts have geometries and soles indicating vortex flow similar to that which forms potholes in bedrock in glaciated regions gutter casts that lead into potholes do so at their edge a requirement for generating rotary currents within the depression pot casts are commonly tilted relative to paleohorizontal and their direction of plunge is either upcurrent or downcurrent variation in geometry of the erosional structures from the chapel island formation and from erosional structures in general can be accounted for by the following substrate type diagenetic history of substrate and flow parameters such as pattern of water motion velocity and intermittency of flow

fining upwards sedimentary sequences generated in seagrass beds shallow marine clastic and carbonate environments of southeast florida contain fining upwards sedimentary sequences being generated by beds of the seagrass thalassia testudinum lateral seaward migration of flute shaped sand pockets storm blowouts in the grass platform produces a lenticular fining upwards package 10 50 m in width 20 100 m in length and 0 3 2 m in thickness migration and vegetative restabilization of recurring blowouts on a grass platform can produce a composite sequence containing numerous complete or stacked incomplete fining upwards packages a complete fining upwards sequence consists of 1 an erosional base 2 a coarse basal shell layer representing the winnowed lag of the eroded preexisting grass stabilized platform 3 a ripple laminated well sorted medium grained sand zone representing the mobile sand on the leeward slope of the blowout and 4 a shelly bioturbated sand or silty sand having a strongly bimodal texture modes at 100 and 500 mu m in which the fine mode dominates upwards this upper portion records increased trapping of storm suspensions and decreased bedload transport as seagrasses are reestablished the mollusks chione cancellata and codakia orbicularis characterize the grass stabilized zone the distinctive textural sequence and molluscan assemblage serve to differentiate these biogenically induced fining upwards sequences from others in the geological record

a genetic classification of carbonate platforms based on their basinal and tectonic settings in the cenozoic abstract this paper reassesses the usefulness of the morphological classification of carbonate platforms into rimmed shelves and ramps whilst the existing classifications have value in describing platform margin morphology at any one time the terms rimmed shelf and ramp are less successful at categorising the entire morphology and stratigraphy of carbonate platforms research on cenozoic carbonate platforms from a range of different tectono stratigraphic settings indicates that the basinal and tectonic setting of a platform can be used to erect a first order genetic classification of carbonate platforms the basinal and tectonic setting of carbonate platforms is shown to control their occurrence the overall 3 d platform morphology the large scale stratigraphic features and depositional sequences climate ocean chemistry and biological evolution control grain types facies and some elements of platform margins but not the larger scale features considered in this new classification from a review of well exposed outcropping and seismically imaged cenozoic platforms it is proposed that eight types of carbonate platform can currently be recognised and characterised based on their basinal and tectonic setting fault block salt diapir subsiding margin offshore bank volcanic pedestal thrust top delta top and foreland margin carbonate platforms these eight types are described using information from cenozoic platforms worldwide and the controls on their development are discussed many platform types e g subsiding margin offshore bank salt diapir thrust top and foreland margin are typical of particular classes of sedimentary basins others e g fault block volcanic pedestal and delta top are more widespread in their occurrence and occur in a range of basin types the value of this classification is that it is genetic rather than morphological the classification reflects the entire morphology and large scale stratigraphy of the platform and the controls on its development in addition the platform models can be used to understand the details of less well exposed or seismically imaged platforms so that they can be characterised and understood in terms of tectono sedimentary processes conversely the classification also provides valuable information on basin evolution as carbonate platforms house information on palaeoenvironments sea level change and are sensitive recorders of the tectonic environment

deformation of a weak subducted slab and variation of seismicity with depth oceanic lithosphere is assumed to possess platelike properties whether it is lying at the earth s surface or descending deep into the mantle yet the geometries of subducting slabs indicate significant deformation as they descend through the mantle1 4 suggesting that lithospheric plates might be so weakened during subduction as to act not as a rigid solid but as a viscous fluid numerical and laboratory experiments have shown that fluid slabs can indeed take on realistic profiles5 7 however it has not been clear whether a fluid or weak model of slab dynamics can account for deep earthquakes which are usually ascribed to the deformation of a rigid or strong slab here we show using numerical simulations of slab evolution that a weak slab model is in fact consistent with seismic observations assuming that earthquakes occur at a rate proportional to deformation rate we reproduce the observed variation with depth of seismicity rate and focal mechanisms and the cessation of seismicity at 670 km depth provided that sinking material encounters resistance at depth here modelled as a viscosity jump at 670 km the pattern of seismicity can be explained by any mechanism for deep earthquakes in which the rate of seismicity is proportional to strain rate in the slab

high pressure elasticity of serpentine and seismic properties of the hydrated mantle wedge 1 a subset of the single crystal elastic moduli of natural antigorite has been measured using brillouin scattering at high pressure up to 9 gpa aggregate properties and axial compressibilities are in good agreement with equation of state results from x ray diffraction stiffness along the c axis increases becoming close to those within the silicate layer near 7 gpa a slight discontinuity in the evolution of the elastic moduli near 7 gpa is associated with a phase transition raman spectroscopy shows that the transition does not occur in subducting slabs except in the core of the coldest slabs where serpentine may be subducted to depths of about 200 km varying temperature and pressure has limited effects on the interpretation of seismic velocities principally because of the limited depth range of serpentine stability mostly above 100 km depth and also due to the compensating effects of pressure and temperature that maintain velocity variations well within the uncertainties and statistical variability of seismological studies serpentinites are excellent candidates for explaining low velocities in the hydrated mantle wedge inversion of the moho and thin anisotropic low velocity layers at the plate interface serpentinite layers provide an alternative explanation to fluid saturated oceanic crust for explaining these phenomena and account for a transition to an aseismic decoupled plate interface

are the lower planes of double seismic zones caused by serpentine dehydration in subducting oceanic mantle in a number of subduction zones earthquakes at 50 200 km depth define two dipping planes separated by 20 40 km that appear to merge downdip upper plane earthquakes are inferred to occur within the subducting oceanic crust whereas lower plane earthquakes occur in the subducting oceanic mantle beneath northeast japan the results of a new finite element heat transfer model suggest that the lower seismic plane cuts across isotherms at a shallow angle lower plane earthquakes occur at 550 800 c at 100 km depth and at 350 600 c at 160 km depth these conditions coincide with the dehydration reaction antigorite serpentine forsterite enstatite h 2 o which suggests that lower plane earthquakes may be triggered by dehydration embrittlement which in turn suggests that the subducting oceanic mantle is partially hydrated serpentinization may occur in the trench outer rise region where faulting may promote infiltration of seawater several tens of kilometers into the oceanic lithosphere if this hypothesis is correct current subduction zone h 2 o budgets may significantly underestimate the amount of bound h 2 o entering the subduction factory

trace fossil model for reconstruction of paleo oxygenation in bottom waters recognition of fluctuations in the degree of paleo oxygenation of bottom waters recorded in fine grained pelagic strata is important for interpretation of paleoceanographic and paleoclimatologic conditions general sedimentary fabric composition of trace fossil assemblages and burrow size and crosscutting relationships have been incorporated into a trace fossil tiering model that permits detailed reconstruction of changes in paleo oxygenation of bottom waters applications of this model to the miocene monterey formation california and the cretaceous niobrara formation colorado indicate that the ichnologic approach is more sensitive to both magnitude and rates of change in oxygenation levels compared to macrobenthic body fossil information

orbital forcing recorded in subtidal cycles from a lower miocene siliciclastic carbonate ramp system central italy metre scale siliciclastic carbonate cycles are the basic depositional motif of the lower miocene guadagnolo formation outcropping in the central apennines the mechanisms which formed the mixed lithology cycles are still a matter of debate the mixed siliciclastic carbonate system discussed in this paper provides a new case study to illustrate the role of orbital forcing in controlling the facies evolution and cyclic stacking of small scale sequences deposited on the outer sector of a ramp two sections are discussed that display mixed siliciclastics and carbonates arranged in upward shallowing cycles each cycle shows an upward decrease in the terrigenous input and a parallel increase in benthic fauna time series analyses indicate the cyclic carbonate terrigenous pattern to be largely controlled by orbital forcing in the milankovitch frequency band coupling of climate change and sea level fluctuations in tune with orbital cycles are proposed as driving mechanisms

generic types of stratigraphic cycles controlled by eustasy thought experiments imply that stratigraphic cycles controlled directly by eustasy include keep up catch up catch down foreshortened or truncated and give up variants different patterns of thick ness and facies being governed by the interplay of sedimentation rate and accommodation space most examples of these types are in addition base cutout cycles wherever lowstands of sea level dropped below the top of the underlying cycle overall thicknesses of multiple cycles at any locale are controlled ultimately by net subsidence rate accordingly even keep up and catch up cycles can faithfully record full accommodation only where tectonic subsidence during cycle formation paced eustatic fluctuation paradoxically however only condensed cycles that fortuitously compensate for syncycle tectonic subsidence can accurately record the eustatic amplitude as adjusted for water loading

seismic wave velocity and anisotropy of serpentinized peridotite in the oman ophiolite shallow seismic measurements in harzburgite from the oman ophiolite performed in a zone where the maximum horizontal anisotropy is expected vertical foliation and horizontal lineation point to a dominant dependence of seismic properties on fracturing optical microscopy studies show that microcracks are guided by the serpentine lizardite penetrative network oriented subparallel to the harzburgite foliation and subperpendicular to the mineral lineation and that serpentine lizardite vein filling has a maximum concentration of 001 planes parallel to the veins walls the calculated elastic properties of the oriented alteration veins filled with serpentine in an anisotropic matrix formed by oriented crystals of olivine and orthopyroxene are compared with seismic velocities measured on hand specimens laboratory ultrasonic data indicate that open microcracks are closed at 100 mpa pressure e g j geophys res 65 1960 1083 and proc odp sci results leg 118 1990 227 above this pressure laboratory measurements and modeling show that p compressional and s shear wave velocities are mainly controlled by the mineral preferred orientation veins scaled with serpentine are effective in slightly lowering p and s velocities and increasing anisotropy the penetrative lizardite network does not affect directly the geometry of seismic anisotropy but contributes indirectly in the fact that this network controls the microcrack orientations comparison between seismic measurements of peridotite and gabbro in the same conditions suggest that p and s waves anisotropies are a possible discriminating factor between the two lithologies in the suboceanic lithosphere c 2003 published by elsevier b v

deepening upward subtidal cycles murray basin south australia abstract decimeter scale deepening upward cycles are the basic depositional motif of oligocene miocene murray supergroup limestones in the murray basin southeastern australia cycles formed in this large intracratonic basin on a centripetal temperate water epeiric ramp they reflect generation by trophic resource influenced carbonate production under mostly transgressive conditions listed from the base up each cycle is ideally composed of five parts part a biotically depauperate carbonates reflecting relatively shallow water restricted variably stressed highly mesotrophic environments part b increasingly biotically diverse limestones recording progressively more physical energy and less mesotrophic conditions upward om1 a conspicuous hardground to firmground surface formed during late transgression to stillstand during which wave sweeping and reworking contributed to omission and lithification part c relatively biotically diverse epifauna dominant sediments that were highly abraded during periods of condensed sedimentation under marginally oligotrophic conditions and om2 cycle boundary a rarely conspicuous surface representing arrested sedimentation and variable cementation as trophic resources increased and conditions for carbonate production deteriorated ten cycle types are grouped into four major styles clay cycles mollusc cycles echinoid cycles and bryozoan cycles these are interpreted to form a lithological continuum from inner restricted terrigenous locales to outer open marine bryozoan colonized environments deepening upward cycles were profoundly sensitive to autogenic factors such as nutrient influx terrigenous content and turbidity hydrodynamic energy water temperature salinity and water depth manifested in this intracratonic setting although these cycles share a similar sedimentological motif and hydrodynamic energy based cycle capping process with open shelf epicratonic shallowing upward subtidal cycles of tertiary age they formed in a very different way reflecting the distinctive intracratonic environment in which they formed murray basin cycles preserve a record of relative sea level rise whereas open shelf cycles accumulated mostly during relative sea level fall

an inverted continental moho and serpentinization of the forearc mantle volatiles that are transported by subducting lithospheric plates to depths greater than 100 km are thought to induce partial melting in the overlying mantle wedge resulting in arc magmatism and the addition of significant quantities of material to the overlying lithosphere asthenospheric flow and upwelling within the wedge produce increased lithospheric temperatures in this back arc region but the forearc mantle in the corner of the wedge is thought to be significantly cooler here we explore the structure of the mantle wedge in the southern cascadia subduction zone using scattered teleseismic waves recorded on a dense portable array of broadband seismometers we find very low shear wave velocities in the cold forearc mantle indicated by the exceptional occurrence of an inverted continental moho which reverts to normal polarity seaward of the cascade arc this observation provides compelling evidence for a highly hydrated and serpentinized forearc region consistent with thermal and petrological models of the forearc mantle wedge this serpentinized material is thought to have low strength and may therefore control the down dip rupture limit of great thrust earthquakes as well as the nature of large scale flow in the mantle wedge

variability of zonal currents in the eastern equatorial indian ocean on seasonal to interannual time scales this study examines equatorial zonal current variations in the upper layers of eastern indian ocean in relation to variations in the indian ocean dipole iod the analysis utilizes data from the research moored array for african asian australian monsoon analysis and prediction rama and the european centre for medium range weather forecasts ocean reanalysis system 4 ecmwf oras4 surface currents are characterized by semiannual eastward flowing wyrtki jets along the equator in boreal spring and fall forced by westerly monsoon transition winds the fall jet intensifies during negative iod niod events when westerlies are anomalously strong but significantly weakens during positive iod piod events when westerlies are anomalously weak as zonal wind stress weakens during piod events sea surface height becomes unusually low in the eastern basin and high in the west setting up an anomalous pressure force that drives increased eastward transport in the thermocline opposite tendencies are evident during niod events in response to intensified equatorial westerlies current transport adjustments to anomalous zonal wind forcing during iod events extend into the following year consistent with the cycling of equatorial wave energy around the basin a surface layer mass budget calculation for the eastern sea surface temperature sst pole of the iod indicates upwelling of 2 9 0 7 sv during normal periods increasing by 40 50 during piod events and reducing effectively to zero during niod events iod related variations in wyrtki jet and thermocline transports are major influences on these upwelling rates and associated water mass transformations which vary consistently with sst changes

composite ichnofabrics and tiering of burrows infaunal communities in marine environments typically are tiered that is different taxa live at different depths within the sediment tiered suites of biogenic structures yield complex biogenic sedimentary fabrics ichnofabrics with the traces of deep burrowing organisms overprinted on those of shallow burrowing organisms careful analysis of crosscutting relationships of burrows in such composite ichnofabrics allows reconstruction of the tiered nature of fossil endobenthic communities it is important to recognize that the best preserved and most prominently displayed trace fossils in most assemblages usually represent the deepest tier thus they were farther removed from the sea floor and therefore less indicative of actual sea floor conditions than the more poorly preserved traces of the shallower tiers on which the deeper traces are juxtaposed

stratigraphic completeness tiered trace fossils provide a measuring stick trace fossil associations commonly display a characteristic vertical zonation tiering within the substrate when burrows of a particular tier become truncated by turbidites tempestites or other results of catastrophic events they provide a simple but quantitative caliper for estimating stratigraphic incompleteness the minimum amount of penecontemporaneous sea floor erosion can be determined by types and sequences of trace fossils preserved below event deposits

punctuated aggradational cycles a general hypothesis of episodic stratigraphic accumulation according to the hypothesis of punctuated aggradational cycles presented here the stratigraphic record consists of small scale 1 5 m thick shallowing upward cycles pacs separated by surfaces marked by abrupt change to deeper facies this motif pervasive in time and environment is produced by relatively long periods tens of thousands of years of base level stability punctuated by geologically instantaneous relative base level rises of at least basin wide extent as basin wide lithologic time stratigraphic units pacs are fundamental to all aspects of stratigraphic analysis including correlation paleoenvironmental interpretation and paleogeographic reconstruction of several possible allogenic mechanisms for a pervasively cyclic stratigraphic record glacial eustasy driven by orbital perturbations is preferred the pac hypothesis is presented as a comprehensive model to be tested and modified by examination of the stratigraphic record from an episodic perspective

study of the mechanism of the influence of water on elastic wave propagation in amphibolites and some metamorphic rocks at high pressures and temperatures an experimental study of the compressional and shear wave velocities vp and vs of amphibolite granite pyroxenite serpentinite and obsidian at water pressures up to 500 mpa and temperatures up to 800 c has been conducted the experiments were carried out in a gas apparatus with internal heating ultrasonic pulse transmission was used for measurement of elastic wave velocities the functions of vp and vs vs temperature for amphibolite obtained under water pressure differ sharply from those for dry conditions with a minimum at 650 c the change in elastic properties of rocks corresponds with both an increase and a decrease in microfracturing an increase in temperature above 650 c results in a decrease in microfracturing as a result of the closing of fractures among specific features of the rocks which underwent changes at 705 c we must mention the appearance of acid volcanic glass mostly in areas of sericitization at 800 c the glass like phase is frequently attained it is readily visible and reaches 0 5 1 0 of the total volume therefore variation of fluid regime and temperature in the earth s crust allows for the existence of both high velocity and low velocity layers within rocks of the same composition in regions with a high temperature gradient

seismic consequences of warm versus cool subduction metamorphism examples from southwest and northeast japan warm and cool subduction zones exhibit differences in seismicity seismic structure and arc magmatism which reflect differences in metamorphic reactions occurring in subducting oceanic crust in southwest japan arc volcanism is sparse and intraslab earthquakes extend to 65 kilometers depth in northeast japan arc volcanism is more common and intraslab earthquakes reach 200 kilometers depth thermal petrologic models predict that oceanic crust subducting beneath southwest japan is 300 to 500 c warmer than beneath northeast japan resulting in shallower eclogite transformation and slab dehydration reactions and possible slab melting

measurement of elastic wave velocities in granulite and amphibolite having identical h2o free bulk compositions up to 850 c at 1 gpa abstract simultaneous measurements of compressional and shear wave velocities v p and v s in two rocks with identical chemistry but with different mineral assemblages were carried out from room temperature to 850 c at 1 gpa one of the rocks is granulite and the other is amphibolite formed from the granulite by addition of h 2 o both v p and v s in the granulite and amphibolite decreased linearly with increasing temperature and there were obvious differences between the elastic wave velocities in the granulite and amphibolite for example the values of v p and v s at 700 c are respectively 7 75 km s 1 and 4 37 km s 1 for granulite and 7 40 km s 1 and 4 21 km s 1 for amphibolite the low velocity layer lies between the descending lithosphere and the overlying asthenosphere beneath the japanese island arc and the ratio of velocity drop for the low velocity layer compared to the surroundings is approximately 5 6 if the observed velocity drop were caused by a phase change i e granulite h 2 o amphibolite the 5 6 velocity drop could indeed be explained by an appropriate phase change e g anhydrous peridotite h 2 o amphibole peridotite

mixed siliciclastic carbonate upward deepening cycles of the upper cambrian inner detrital belt of laurentia abstract high resolution sedimentological and biostratigraphic data recently recovered from upper cambrian strata in the northern rocky mountain and central appalachian regions reveal that meter scale cycles of very different character developed synchronously during deposition of the basal subzones of the ibexian series in both the carbonate facies belt and the distal part of the inner detrital mixed siliciclastic carbonate facies belt of the laurentian paleocontinent a typical mixed siliciclastic carbonate cycle in the snowy range formation of wyoming and montana consists of the following lithofacies in ascending order shale shale with very thin grainstone interbeds grainstone with subordinate thin shale interbeds flat pebble conglomerate and in some cycles thrombolitic bioherms internal transitions between the constituent lithofacies are mostly gradational but cycle boundaries are sharp with shale directly overlying flat pebble conglomerate and or thrombolitic boundstone we postulate that these mixed siliciclastic carbonate cycles developed in response to different depositional dynamics than meter scale cycles developed in carbonate belt facies specifically cycles in the carbonate belt were produced largely by temporal variation in accommodation space and associated filling of that space through progradation of the carbonate system and aggradation to sea level in contrast cyclic deposition within the inner detrital belt was controlled by variations in terrigenous sediment input and resulting effects on carbonate productivity in this case enhanced delivery of siliciclastic mud by rivers was triggered by regression and or a shift to more humid climatic conditions an upward increase of carbonate in the main part of the cycles reflects enhanced carbonate production in response to the reduction of terrigenous clay supply during shoreline retreat widespread cementation of fine grainstone substrates led to deposition of extensive beds of flat pebble conglomerate possibly in association with elevated sea surface temperatures and enhanced storm intensity thrombolites formed at times of maximum transgression as microbial communities thrived in the absence of turbidity colonizing the irregular surfaces of flat pebble conglomerate beds we thus interpret these wholly subtidal cycles to record full changes in paleobathymetry with the bulk of the cycles recording upward deepening rather than shoaling associated with regression comparison with age equivalent shoaling cycles of the conococheague formation of the carbonate belt of the appalachian mountains highlights the different depositional dynamics in the mixed siliciclastic carbonate systems and underscores the peril in extrapolation of depositional models derived from carbonate systems to mixed systems regardless of age or paleogeographic setting