shelf to basin temperate skeletal carbonate sediments three kings plateau new zealand abstract three kings plateau is 10 000 km2 in area less than 500 m deep and includes the mainland shelf and submarine ridges and banks centered on three kings islands 34 s off northernmost new zealand the plateau receives little input of terrigenous sediment and lies in a zone of active nutrient upwelling and vigorous wind wave and tidal current activity as a consequence surficial sediments on and about the margins of the plateau are dominantly pure skeletal carbonate sands and gravels which lack mud to depths of 1500 to 3000 m plateau top sediments include a bryozoan calcareous red algal barnacle lithofacies on coarse and rocky substrates and a bivalve bryozoan lithofacies associated with particulate bottoms plateau edge sediments include a bryozoan ahermatypic co al serpulid lithofacies in areas of rugged topography and a bivalve gastropod lithofacies on sandy substrates both grading down slope into a planktic foraminiferal lithofacies by 1500 to 3000 m depth depending on locality the carbonates are dominated by a high mg calcite and or low mg calcite mineralogy with only subordinate aragonite active skeletal production is presently restricted to only about 10 percent of the plateau area principally in regions of rough sea floor topography on submarine cliff faces on bank tops and in in shore portions of the mainland shelf about these areas modern carbonates occur down to depths of 50 to 200 m below which the sediments are mainly relict and include rare glauconite redeposition of plateau derived skeletal carbonates into surrounding basins was especially active during the last glacial low sea level s it is inferred that this is the ultimate fate of the bulk of the shallow marine sediment and that the plateau top itself preserves only a thin laterally discontinuous mixed age and highly condensed sequence of quaternary temperate skeletal carbonates

characterization of the natural fracture system of the eagle ford formation val verde county texas the eagle ford formation has attracted considerable industry attention as a self sourced unconventional shale reservoir the productive interval in the eagle ford formation is the transgressive systems tract which contains parasequences whose lithologic content varies upward with increasing proportions of limestones optimum success in both exploration and production depends on the adequate characterization of fracture systems as a function of lithology the outcrops present along us highway 90 in val verde and terrell counties texas provide considerable insight into the regional natural fracture system of the eagle ford formation fracture orientation analysis reveals two sets of conjugate hybrid shear fractures and two sets of regional fractures abutting relationships suggest that hybrid shear fractures formed first followed by the thoroughgoing northeast striking fracture set and finally by a northwest striking set which tends to be confined to individual mechanical units the orientation of these fractures suggests that they formed during post laramide stress relaxation and progressive exhumation spacing frequency distribution analysis of the fracture population reveals a mature hypersaturated fracture system that likely formed at depth by overburden load and or fluid pressure near maximum burial our results indicate that the eagle ford formation displays a well developed fracture network regionally distributed in the val verde basin and likely present in the productive eagle ford play these observations provide evidence for pathways and vertical connectivity for potential fluid pathways throughout the eagle ford formation

geomechanical modeling of hydraulic fracturing why mechanical stratigraphy stress state and pre existing structure matter the increasing exploration and production in unconventional resource plays in the past decade has been accompanied by a greater need for understanding the effectiveness of multistage hydraulic fracturing programs particularly in long 1500 m or 5000 ft subhorizontal boreholes laterals traditional analytical analysis techniques for estimating the size and orientation of fractures induced by fluid injection typically result in predictions of relatively long and planar extension mode i bi wing fractures which may not be representative of natural systems although these traditional approaches offer the advantage of rapid analysis neglect of key features of the natural system e g realistic mechanical stratigraphy pre existing natural faults and fractures and heterogeneity of in situ stresses may render results unrealistic for planning executing and interpreting multimillion dollar hydraulic stimulation programs numerical geomechanical modeling provides a means of including key aspects of natural complexity in simulations of hydraulic fracturing in this study we present the results of two dimensional finite element modeling of fluid injection induced rock deformation that combines a coupled stress pore pressure analysis with a continuum damage mechanics based constitutive relationship the models include both the natural mechanical stratigraphic variability as well as the in situ stress state anisotropy and permit tracking of the temporal and spatial development of shear and tensile permanent strains that develop in response to fluid injection our results show that simple long planar fractures are unlikely to be induced in most mechanically layered natural systems under typical in situ stress conditions analyses that assume this type of fracture geometry may significantly overestimate the reach of hydraulically induced fractures and or effectively stimulated rock volume

textural and compositional evidence for magma mixing and its mechanism abu volcano group southwestern japan quaternary basalts andesites and dacites from the abu monogenetic volcano group sw japan composed of more than 40 monogenetic volcanoes show two distinct chemical trends especially on the feo mgo vs sio2 diagram one trend is characterized by feo mgo enrichment with a slight increase in sio2 content fe type trend whereas the other shows a marked sio2 enrichment with relatively constant feo mgo ratios si type trend the fe type trend is explained by fractional crystallization with subtraction of olivine and augite from a primitive alkali basalt magma rocks of the si type trend are characterized by partially melted or resorbed quartz and sodic plagioclase phenocrysts and or fine grained basaltic inclusions they are most likely products of mixing of a primitive alkali basalt magma containing olivine phenocrysts with a dacite magma containing quartz sodic plagioclase and hornblende phenocrysts petrographic variation as well as chemical variation from basalt to dacite of the si type trend is accounted for by various mixing ratios of basalt and dacite magmas pargasitic hornblende and clinopyroxene phenocrysts in andesite and dacite may have crystallized from basaltic magma during magma mixing olivine and spinel and quartz sodic plagioclase and common hornblende had crystallized in basaltic and dacitic magmas respectively before the mixing within a lava flow the abundance of basaltic inclusions decreases from the area near the eruptive vent towards the perimeter of the flow and the number of resorbed phenocrysts varies inversely suggesting zonation in the magma chamber

a model of correlated episodicity in magnetic field reversals climate and mass extinctions correlated periodicities of mass extinctions climate magnetic field reversals and other geological phenomena may best be explained by cycles of activity within the mantle and core we propose that these cycles are due to variations in the thickness of the thermal boundary layer at the base of the mantle as the layer alternates between two phases of activity in the quiescent phase little flow occurs and the layer thickens with time by thermal diffusion the active phase begins when the thickening layer becomes dynamically unstable hot material erupts from the layer causing it to become thinner as the layer thins the rate of energy supply to the geodynamo and hence its reversal frequency increases the hot material rises to the surface where it causes widespread basaltic volcanic eruptions these eruptions release large amounts of co 2 and sulfates which have a pronounced effect on the climate and biota

petrogenesis of a basalt rhyolite tephra from the west central afar ethiopia the cindery tuff is an unusual tephra fall deposit that contains evidence for the mixing of basaltic and rhyolitic liquids prior to eruption it contains clear rhyolitic glass shards together with brown basaltic glass spheres and a broadly bimodal phenocryst assemblage brown glasses are ferrobasaltic in composition and are similar to the voluminous pliocene tholeiites of the surrounding west central afar volcanic field both are enriched in the light rare earth and incompatible elements and possess higher 87sr 86sr and lower 143nd 144nd than morb rhyolitic glasses are subalkaline and compared to the basaltic glasses are strongly depleted in the compatible elements and enriched in the incompatible elements both glass types have similar incompatible element and isotopic ratios and with the rhyolite glass showing a 2 fold parallel enrichment in rare earth element abundances over the basaltic glass these observations suggest that the two glasses are genetically related

origin of crystal poor differentiated magmas insights from thermal gradient experiments crystal poor differentiated magmas are commonly erupted from shallow thermally zoned magma chambers in order to constrain the origin of these magmas we have experimentally investigated crystallization differentiation and crystal melt separation in presence of a thermal gradient experiments have been designed taking advantage of the innate temperature gradient of the piston cylinder apparatus and carried out on a phonolitic system at 0 3 gpa and temperature ranging from 1 050 to 800 c crystallization degree and melt composition in experimental products vary as a function of the temperature gradient in particular melt composition differentiates from tephri phonolite starting material to phonolite moving from the hotter glassy zone t 1 050 c towards the cooler heterogeneously crystallized zone t 900 c of the charge the heterogeneously crystallized zone is made up of 1 a crystal rich mushy region crystallinity 30 vol 2 a rigid crystal framework crystallinity 80 vol and 3 glassy belts of phonolitic glass at the top thermal gradient experiments picture crystallization differentiation and crystal melt separation processes occurring in a thermally zoned environment and reveal that relatively large volumes of crystal poor melt glassy belts can originate as a consequence of the instability and collapse of the rigid crystal framework analogously in thermally zoned magma chambers the development and collapse of a solidification front may represent the controlling mechanism originating large volumes of crystal poor differentiated magmas

evidence for two stage mixing in magmatic inclusions and rhyolitic lava domes on niijima island japan abstract fine grained inclusions are found in the achiyama and the mukaiyama rhyolitic lava domes on niijima island japan the existence of fine grained rims in the inclusions indicates that these inclusions are a quenched mafic magma incorporated into their rhyolite host the chemical composition and the phenocryst assemblage of the inclusions in the achiyama dome are identical with those of the effusive basaltic lava on niijima island wakago basalt the inclusions of the mukaiyama dome are dacitic and have a disequilibrium phenocryst assemblage olivine clinopyroxene orthopyroxene hornblende biotite plagioclase quartz and opaque minerals whole rock compositions lie on a mixing line between the host rhyolite and wakago basalt the dacitic magma is therefore a product of mixing of the basaltic and rhyolitic magmas the occurrence of two different inclusions basaltic and dacitic in two different domes can be explained by a two stage magma mixing model the dacite is formed in the first stage by mixing in a vertically stratified magma chamber caused by an interfacial instability between rhyolitic and basaltic layers a small amount of rhyolitic magma is entrained into the lower basaltic magma layer and two magmas can mix essentially in a liquid state the mixed dacitic magma is formed in the lower layer the second stage is mixing caused by forced convection during the ascent through a conduit just before eruption the stably stratified silicic and mafic magmas in a magma chamber can turn over and efficiently mix because of the formation of an unstable flow in a conduit under this condition the system reaches thermal equilibrium immediately after the mafic magma is disaggregated and dispersed in the silicic host the mafic magma solidifies when the silicic endmember is predominant in the mixture hence the equilibrium temperature is below the solidus of the mafic magma mafic inclusions are formed at this stage whether the inclusions are basaltic or dacitic depends on whether the final ascent through a conduit eruption occurred before or after the formation of the layer of a mixed dacitic magma in a magma chamber

the offshore quaternary sediment bodies of the english channel and its western approaches after a short review of present day surficial sediments on the sea floor of the english channel and its western approaches this paper summarises recent results on the present quaternary sandy accumulations tidal bank systems based on sea floor imaging very high resolution seismic surveys and a few piston cores the english channel a typical funnel shaped tidal shelf is presently starved of terrigenous sediments most of mobile sediments at the sea floor are autochthonous either skeletal carbonates or siliciclastic sands reworked from old littoral deposits or incised valley fills three zones are considered 1 the central and western central channel which is a bedload parting zone characterised by a pebbly lag making up the sole of the quaternary sediments throughout the shelf area 2 the eastern channel a flood dominated area characterised by medium to fine siliciclastic sands mainly of aeolian periglacial origin 3 the western approaches an ebb dominated area with a polygenic sediment cover characterised by coarse carbonate rich gravelly to muddy sands of marine to glaciomarine origin with a diachronous faunal content these sediments are sourced by reworking of a former littoral barrier or incised valley fills the seismic stratigraphy of quaternary sediment accumulations 10 50 m points to the predominant control of tidal dynamics together with the general interplay between the channel morphology and the sea level story the thickest deposits in the eastern channel and western approaches are built up into bank systems whereas the interpretation of the active eastern channel banks finds a general agreement the moribund banks in the western approaches i e the southern celtic banks remains under debate owing to the lack of long cores in our hypothesis both systems were built up during the last post glacial sea level rise the celtic banks would represent the tidal transgressive systems tract whereas the eastern channel banks correspond to the highstand systems tract the architecture of both bank systems is studied from two examples the bassure de baas bank in the eastern channel and the kaiser bank in the western approaches both examples show transgressive features the bassure de baas exhibits from base to top an estuarine mouth barrier a wave erosion surface a shoreface bank and finally a classic tidal bank by contrast the kaiser bank consists of the offshore reworking of fully marine deposits its base would correspond to the amalgamation of the last glacial sea level fall with a strong tidal ravinement surface from base to top the bank underwent tidal bar accretion very large dune field aggradation perched channels incision storm erosion and deposition

surficial cool water carbonate sediments on the otway continental margin southeastern australia abstract the otway continental margin of southern australia is a swell dominated open cool water carbonate platform it is partitioned into a series of depth related zones of biological sediment production and accumulation by three hydrodynamic interfaces 1 abrasion depth 70 m 2 swell wavebase 130 m and 3 storm wavebase 250 m the otway shelf profile drops steeply from the strandline the shallow shelf lies largely between 30 and 70 m and is a high energy zone of carbonate sediment production intensive destructive diagenesis and erosion the seafloor is characterized by widespread exhumed limestone substrates which host dense encrusting assemblages of molluscs sponges bryozoans and red algae loose sediment accumulation is patchy and consists of a thin quartz rich palimpsest gravel veneer the middle shelf 70 to 130 m is a zone of swell wave shoaling and production and accumulation of coarse mega rippled 2 d dunes bryozoan sands deep shelf areas 130 to 180 m accumulate finer bioclastic sands foraminiferal echinoid bryozoan that are intensely bioturbated and episodically reworked by storms grains are variably fragmented and marginally bored at the shelf edge and top of slope 180 to 350 m nutrient rich upwelling currents support laterally extensive aphotic biostromal hermal bryozoan sponge coral communities which generate and accumulate thick successions of muddy skeletal sand sediments of the upper slope 350 to 500 m are a bioturbated mixture of periplatform bioclastic debris and pelleted foraminiferal nannofossil mud turbidites and resedimentation features are common bioturbation and shelf derived skeletal content decrease progressively downslope and pelagic muds dominate below 500 m lower slope areas are cross cut by gullies and have low accumulation rates the base of slope is characterized by sediment apron deposits consisting of interbedded shelf derived coarse grained turbidites and pelagic ooze

origin and evolution of a peraluminous silicic ignimbrite suite the violet town volcanics the violet town volcanics are a 373 ma old comagmatic s type volcanic sequence mainly comprising crystal rich intracaldera ignimbrites rock types vary from rhyolites to rhyodacites all containing magmatic cordierite and garnet phenocrysts variation in the suite is primarily due to fractionation of early crystallized quartz plagioclase and biotite plus minor accessory phases in a high level magma chamber prior to eruption early magmatic crystallization occurred at around 4 kb and 850 c with melt water contents between 2 8 and 4 wt this high temperature markedly water undersaturated restite poor granitic magma was generated by partial melting reactions involving biotite breakdown in a dominantly quartzofeldspathic source terrain leaving a granulite facies residue

soret separation of mid ocean ridge basalt magma chemical differentiation of an initially homogeneous mid ocean ridge basalt morb liquid has been experimentally observed in a temperature gradient above the liquidus the magnitude of this effect in producing differences in composition is comparable to that of crystal liquid fractionation for a given temperature difference however the chemical changes produced by the two processes soret and crystal liquid fractionation are different soret separations resemble those observed in a third process silicate liquid immiscibility this similarity is a reflection of the fact that the soret separable components are the same network former network modifier structural components which segregate during silicate liquid immiscibility

significance of 87 sr 86 sr ratios in the merensky cyclic unit of the bushveld complex in a number of mafic layered intrusions layers with platinum group element pge mineralization coincide with breaks in cumulus mineral composition and isotopic variation trends through the layered sequence1 2 for example in the bushveld complex in south africa abrupt increases in initial 87sr 86sr ratios occur at the ug 2 and merensky pegmatoid or reef horizons both of which are exploited for pges we report here the results of a detailed sr isotope study across the merensky unit and its immediate foot and hangingwall rocks which when considered together with new ideas on processes operating in magma chambers3 5 suggest that magma mixing and post cumulus infiltration of liquids displaced from below were important during crystallization of the merensky pegmatoid

a proposed model for the formation of reversely zoned plutons based on a study of the lacorne complex superior province quebec the lacorne complex is a calc alkaline mass of archean age located in the abitibi region of quebec the complex can be subdivided into three zones the inner zone varies in composition from hornblende biotite gabbro breccia to monzonite with all rocks containing 5 modal quartz the outer zone rocks contain more than 5 modal quartz and range in composition from biotite hornblende quartz diorite to granodiorite the third zone consisting of muscovite biotite garnet leucomonzogranite is exposed only in the northwestern portion of the complex the transition between each zone is relatively abrupt but continuous based on 400 modal estimates of stained slabs and 107 whole rock and trace element analyses it is demonstrated that both the inner zone and the outer zone rocks are reversely zoned mafic cores are surrounded by more felsic borders various methods that could explain the observed geochemical trends are considered the data indicate that crystal liquid equilibria were maintained during cry

evidence for magma mixing processes within the critical and lower zones of the northwestern bushveld complex south africa abstract during deposition of the lower lz lower critical lcz and upper critical ucz zones the course of crystallization was dominated by repeated emplacement of primitive mafic magma evidence for this is provided by textural chemical and isotopic data emplacement of fresh magma into resident residual liquids produced reversal in fractionation trends i e progressive increase in mg mg fe 2 ratios and enrichment of cr in orthopyroxenes through intervals of 350 m in the lz 100 200 m in the lcz and 10 80 m within the lower part of the ucz reversals culminated with the deposition of olivine cumulates upon pyroxenite within the lz and lcz but a change in the style of reversals led ultimately to ultramafic cumulates with or without olivine resting directly upon anorthositic cumulates at the top of the ucz merensky and bastard units these incursions of fresh mafic liquid led to deposition of ultramafic rocks at the base of units constituting the ucz whereas the cumulus feldspar bearing rocks at the tops of such units crystallized from hybrid liquids resulting from the mixing of the new and the resident residual liquids there is a general decoupling of trends of cryptic variation of the feldspar and orthopyroxene populations in the ucz wide variations of initial sr isotope ratios exist throughout the entire section within individual units and within individual members of units isotopic disequilibrium also exists between feldspar and pyroxene fractions of some samples and between individual feldspar grains of different textural habit the first appearance of cumulus feldspar bearing cumulates of the ucz is presaged by the appearance of small partially resorbed plagioclase inclusions within the orthopyroxene grains of pyroxenites at the top of the lcz this texture is conspicuous throughout the lowermost 400 m of the ucz but is still sporadically developed within the bastard unit at the top of the ucz anorthositic rocks of the lower part of the ucz are held to have been derived from residual liquids evolved during crystallization of earlier cumulates but anorthosites with a main zone isotopic signature cap the merensky and bastard units

rare earth element evidence concerning the origin of voluminous mid tertiary rhyolitic ignimbrites and related volcanic rocks sierra madre occidental chihuahua mexico abstract the mid tertiary volcanic sequence of the central sierra madre occidental in chihuahua mexico is about one kilometer thick and is composed predominantly of rhyolitic ignimbrites basaltic andesite to dacitic lavas are interbedded with the rhyolites but they are of minor volumetric importance rare earth element ree data are used to constrain a crustal anatexis model for the origin of the voluminous ignimbrites and to test a fractional crystallization model the ree patterns indicate that if the rhyolites were formed by direct crustal anatexis the residue from partial melting could contain no more than a few percent garnet or about 20 hornblende this eliminates residues with the mineralogy of amphibolite eclogite or garnet granulite but melting of a garnet free granulite source is permitted the crustal anatexis model is difficult to evaluate critically because of a lack of knowledge concerning the mid tertiary geothermal gradient and the composition of the crust beneath the sierra madre occidental in contrast the fractional crystallization model can be tested rigorously rayleigh fractionation calculations are used to closely model ree patterns in the basaltic andesite to rhyolite series the minerals involved are those occurring as phenocryst phases and the mineral proportions were generated by leastsquares major element calculations the results of the calculations are consistent with the hypothesis that the voluminous rhyolites originated by plagioclase dominated crystal fractionation

major element variations in basalts and diabases from the north arm mountain massif bay of islands ophiolite implications for magma chamber processes at mid ocean ridges abstract the major element compositions of aphyric diabases from the north arm mountain massif range from primitive basalts to highly evolved ferrobasalts from modeling of these compositional variations it is clear that the diabases have undergone highly variable 10 to 90 increments of crystallization at low pressures the dominant minerals inferred to have crystallized from the liquids that formed these diabases are olivine plagioclase and clinopyroxene the same minerals that dominate the underlying cumulate gabbroic and ultramafic rocks the projection of these compositions onto pseudo liquidus phase diagrams suggests that the primary magma s from which this north arm suite was produced were generated by melting at 10 25 kbar or greater pressure there is no indication in the major element variations of these diabases of the operation of high pressure crystal factionation processes which is not surprising because low pressure fractionation processes will effectively mask evidence for earlier high pressure processes it is suggested that the tapping of intercumulus liquid from crystal mush areas and the tapping of discrete fractionated magmas within the roof zone of magma chambers is a method of producing a wide variety of magma compositions from a single magma chamber in which the majority of the chamber may be actively convecting and therefore remain relatively uniform in composition

heterozoan carbonates in subtropical to tropical settings in the present and past water temperature has received considerable attention as steering factor for the genesis of different types of marine carbonate sediments however parameters other than temperature also strongly influence ecosystems and consequently the carbonate grain associations in the resulting carbonate rock among those factors are biological evolution water energy substrate water chemistry light penetration trophic conditions co2 concentrations and mg ca ratios in the seawater increased nutrient levels in warm water settings for example lead to heterotrophic dominated associations that are characteristic of temperate to cool water carbonates failure to recognize the influence of such environmental factors that shift the grain associations towards heterotrophic communities in low latitudes can lead to misinterpretation of climatic conditions in the past modern analogues of low latitude heterozoan carbonates help to recognize and understand past occurrences of heterozoan warm water carbonates careful analysis of such sediments therefore is required in order to achieve robust reconstructions of past climate

small scale variations and subtle zoning in granitoid plutons the liberty hill pluton south carolina u s a abstract imperfect flow separation and remixing of crystals and melt produced chemical mineral and textural variability on the scale of several centimeters to meters in coarse grained granitoids of the liberty hill pluton this is believed to develop during emplacement because the resulting layering conforms to the geometry of the pluton and is better developed toward the margins mineral specific equilibrium and fractional crystallization regimes superimposed on the flow separation and remixing produced more complex compositional variations than expected from such granitoid cumulate processes alone the process is chemically comparable to restite unmixing but it is difficult to demonstrate that many minerals involved in the flow are restites local compositional inhomogeneities also formed by deformation and disaggregation of enclaves depending on whether the enclaves are interpreted as xenoliths chilled mafic magma or high melting point restite clots they represent contamination by mechanical assimilation magma mingling and mixing or restite unmixing marginal granitoids distinguished from the central granitoids by pink feldspar biotite muscovite rather than white feldspar amphibole biotite are more evolved but overlap the compositional range of the central granitoids mineralogic and textural similarity and a gradational contact between the two granitoids indicate they are related and that the pluton s subtle reverse zoning was produced by a continuous in situ process flow separation and remixing operating more efficiently and on a larger scale may have produced the more evolved composition of the marginal granitoids however proximity to the contact is more important than rock composition in determining mineralogy there is slight geochemical evidence for contamination of the marginal granitoids by the wall rocks compared to central granitoids they have small lree depletions and hree enrichments which more nearly reflects the lree depletion and hree enrichment of the wall rocks cross cutting contacts and slightly differing compositional trends indicate a later and distinct evolution for the finer grained biotite granitoids they have silica enrichment incompatible element enrichment rb u th compatible element depletion major elements sr ba depleted hree and less frequently lree contents as compared to the central granitoids the fine grained granitoids could be a contemporaneous and unrelated magma or fractionally crystallized from the coarse grained magma compositional mineralogic and textural differences among the later finer grained granitoids indicate either multiple magmas periodic sampling and intrusion of successive residual melt fractions or flow separation and remixing during emplacement

the 1883 eruption of krakatau the 1883 eruption of krakatau was a modest ignimbrite forming event the deposits are primarily coarse grained dacitic non welded ignimbrite large explosions produced pyroclastic flows that entered the sea generating destructive tsunami grain size studies of the ignimbrite suggest that these explosions were not driven by magma seawater interaction the total bulk volume of pyroclastic deposits including co ignimbrite ash is estimated to be 18 21 km3

rollover in volcanic crater lakes a possible cause for lake nyos type disasters abstract seemingly stably stratified fluids that is a heavier layer of fluid underlying an upper layer of lighter fluid can overturn if there is a heat flux through the system such events are termed rollover in the engineering literature occurring for instance in liquid natural gas tanks they take place as well in lakes and ponds in all such cases the stratification starts off with the heavier more dense fluid underlying lighter convection driven mixing at the stratification interface due to the heat flux as well as other processes serves to minimize with time the stabilizing density differences which may eventually cause the stratification to invert if gas has been contained under pressure in the lower layer this gas may explosively vent from the fluid as it suddenly rises to the top such as occurs in liquid natural gas tanks where rollover is a hazard these processes are quantifiable and are applied here to various scenarios that might refer to volcanic crater lakes whose lower layers are charged with volatiles provided herein are some examples of calculated conditions and calculated time of evolution leading to rollover with attendant release of gases from supersaturated ascending fluids these calculations suggest rollover could occur in volcanic lakes the august 1986 gas release at lake nyos is employed as an example these estimates were made taking note that water is commonly supersaturated in co2

petrology of the younger andesites and dacites of iztaccihuatl volcano mexico i disequilibrium phenocryst assemblages as indicators of magma chamber processes

control of mechanical stratigraphy on bed restricted jointing and normal faulting eagle ford formation south central texas outcrops of the middle eagle ford formation in south central texas reveal well developed joint networks in subhorizontal competent carbonate chalk beds and less well developed networks in interlayered incompetent calcareous mudrock beds northeast striking bed perpendicular joints in competent beds have the longest trace lengths and are abutted by northwest striking joints all observed joints terminate vertically in incompetent beds normal faults are common but less abundant than joints dominantly dip north northwest or southeast and are abutted by the joint sets and thus predated jointing the faults cut multiple competent and incompetent beds providing vertical connectivity across mechanical layering products of hybrid and shear failure the dip of these faults is steep through competent beds and moderate through incompetent beds resulting in refracted fault profiles with dilation and calcite precipitation along steep segments fluid inclusions in fault zone calcite commonly contain liquid hydrocarbons rare two phase fluid inclusions homogenized between about 1 40 and 58 c and 2 90 and 100 c suggesting trapping of aqueous fluids at elevated temperatures and depths on the order of 2 km 6562 ft fluid inclusion and stable isotope geochemistry analyses suggest that faults transmitted externally derived fluids these faults likely formed at depths equivalent to portions of the present day oil and gas production from the eagle ford play in south texas faults connect across layering and provide pathways for vertical fluid movement within the eagle ford formation in contrast to vertically restricted joints that produce bed parallel fracture permeability these observations elucidate natural fractures and induced hydraulic fracturing within the eagle ford formation

mafic ultramafic sills new insights from m and s shaped mineral and whole rock compositional profiles the origin of mafic and ultramafic sills exhibiting different wholerock compositional profiles e g i c d m and s shaped profiles remains controversial we have addressed this issue by revisiting three 100 m thick siberian dolerite sills vavukansky kuz movsky and vilyuysky that display remarkable internal differentiation the vavukansky sill has an m shaped profile with prominent basal and top reversals showing inward increases in whole rock mgo mg number 100mg mg fe and normative an content 100an an ab followed by the layered and upper border series with inward decreases in these indices the kuz movsky and vilyuysky sills both show s shaped profiles similar to the vavukansky sill but lack a top reversal these whole rock m and s shaped profiles are accompanied by similar profiles in mineral compositions plagioclase and to a lesser extent olivine show systematic inward increases in an content and mg number respectively across basal and top reversals these compositional trends are followed by inward decreases in these ratios in the interiors of the vavukansky and kuz movsky sills currently accepted models attribute whole rock m and s shaped compositional profiles to crystal settling compositional convection or compaction operating in closed systems our observations challenge these traditional interpretations because variations in mineral compositions observed in marginal reversals cannot result from closed system fractionation we suggest instead that initially the sills evolved as open systems that were slowly inflated by magmas that became gradually more primitive with time the inflation was accompanied by in situ crystallization that preserved the preceding fractionation history of the injected magmas by forming basal and top reversals with minerals becoming more primitive inwards this process culminated with rapid inflation of the sills to their current size owing to a major influx of primitive magma subsequently magma flow through the sills ceased and they evolved as closed systems by fractional crystallization this resulted in the layered and upper border series with minerals becoming more evolved inwards this model can be extended to explain other compositional profiles and petrological features in mafic and ultramafic sills

the petrogenesis of an i type volcanic plutonic suite the st marys porphyrite tasmania the devonian i type st marys porphyrite 388 1ma comprises two petrographically similar units an 800 m thick pyroclastic sheet compositionally dacite and rhyolite and a subvolcanic feeder dyke the pyroclastics are crystal rich and contain in order of decreasing abundance plagioclase quartz biotite augite hypersthene and sanidine phenocrysts in an aphanitic groundmass

the geochemistry of the blackstairs unit of the leinster granite ireland the blackstairs unit is the southernmost of the five units which comprise the leinster granite four granite types are recognized which in order of decreasing age are graiguenamanagh granite porphyritic microcline granite non porphyritic granite and type 1 granite using whole rock geochemical data in conjunction with field and petrographic evidence a model is proposed whereby partial melting of a lower palaeozoic immature sedimentary source produced a parental granitic melt which crystallized without further differentiation to form the non porphyritic granites of the northeastern end of the pluton at the sw end this parental melt fractionated to form the porphyritic microcline granite leaving the melt depleted in k 2 o which then crystallized to form a slightly more acidic non porphyritic granite than that at the ne end a mixture of restite minerals plus melt from this partial melting process crystallized forming the graiguenamanagh granite fractionation of thorium into restite allanite in the graiguenamanagh granite accounts for the low th and th u levels which characterize the non porphyritic granite and the leinster granite in general a high degree of partial melting of similar source material produced the low volume type 1 granite magmas which were subsequently intruded along faults a similar petrogenesis is envisaged for the whole of the leinster granite

origin of a cool water oligo miocene deep shelf limestone eucla platform southern australia the abrakurrie limestone is an areally extensive bryozoan rich unit within the eucla platform a tertiary carbonate shelf which caps the central part of the southern australian continental margin the onshore portion the topic of this study has been exposed since middle miocene time and lies beneath the nullarbor plain the rocks are fine sand to granule sized calcarenites composed of bryozoans bivalves benthic foraminifera and echinoids with lesser numbers of brachiopods solitary corals and serpulids they conspicuously lack significant numbers of planktonic foraminifera and coralline algae most bryozoan remains are from delicate branching cyclostomes although delicate branching robust branching foliose fenestrate multilaminar encrusting and free living cheilostomes are variably abundant in specific units the poorly lithified sequence is punctuated by well cemented layers with erosional tops which are interpreted as hardgrounds the limestone is interpreted as a cool water deep shelf deposit which accumulated in water depths generally greater than 50 m on the inner part of the eucla platform a model which involves deposition and cementation on a carbonate shelf swept by open ocean swells is proposed to explain the style of sedimentation the shelf is envisaged as partitioned by the depth of the zone of wave abrasion sediments were produced throughout but accumulated only below this depth when the seafloor was above this depth it was an environment of cementation and erosion the vertical sequence correlates in a general way with the global sea level model for the mid cenozoic while accumulation rates for southern australian carbonates are similar to rates of cool water carbonate deposition elsewhere c 2 5 cm kyr 1 the rate of abrakurrie accumulation is much less suggesting that significant time periods are represented by the hardgrounds

shocked minerals at the k t boundary explosive volcanism as a source abstract azimuthal symmetry of p waves first motion up and diminished s wave amplitude indicates an explosion to be the source of the seismicity of the 18 may 1980 eruption at mt st helens explosion coupling efficiencies and scaling from underground mining excavation and nuclear explosions imply that peak pressures could have been as high as 1000 kbar which is more than sufficient for the formation of shocked minerals in addition high load rates as well as contamination may lower phase transition pressures a region of formation of shocked minerals surrounding the magma chamber is indicated to have a thickness on the order of 100 m the outer portions of this region should be sufficiently cool so that shocked features are not annealed hence the source of shocked minerals should be the country rock surrounding the magma chamber it is unlikely any of the magmatic material would be able to retain shocked features due its high temperature the explosive mechanism may be quench supersaturation which theoretically can yield shock producing pressures but chemical processes should not be dismissed as an explosive source tektite distributions show no iridium or mass extinction association industrial and present volcanic atmospheric discharge are known to have worldwide distribution the clays present at the k t boundary may have had a volcanic origin and it is now known that the deccan traps which have been suggested by others as a source of the iridium lie athwart the k t boundary as there is common intimacy between silicic and mafic volcanism both should be anticipated at the k t boundary and their global appearance there may be a manifestation of a surge in whole mantle convection in short there is a definite possibility shocked minerals at the k t boundary have a volcanic source

geochemistry and origin of the regan intrusive suite and other granitoids in the northeastern slave province northwest canadian shield the regan intrusive suite of about 100 plutons of tonalite granodiorite and quartz diorite intruded the yellowknife supergroup and migmatite terrain in the northwest slave structural province 2 59 ga ago rare earth element ree trace element and major element analyses from 39 representative whole rocks from the suite suggest it was derived by batch melting of the crust producing a parental magma of tonalitic or granodioritic composition by analysing ree from different parts of a zoned pluton it was concluded that ree distribution was controlled by early separation of quartz diorite from the parent magma by flow differentiation and that the bulk of the ree were contained in early cumulate accessory apatite and monazite the residual magma was further fractionated in pipelike magma chambers during ascent into more leucocratic rocks chondrite normalized ree patterns of single lithology plutons are similar to lithologies in zoned plutons and it is proposed they initially segregated during ascent

geochemical trends in the ackley granite southeast newfoundland their relevance to magmatic metallogenic processes in high silica granitoid systems the ackley granite occupying an area of approximately 2700 km2 was intruded at about 355 ma across the boundary between the avalon and gander tectonic terranes of southeast newfoundland it is dominated by k feldspar megacristic and equigranular biotite bearing granite with subordinate granodiorite but 10 separate phases have been recognized also including hornblende and muscovite bearing varieties small aplite pegmatite type molybdenite deposits occur in a southwest lobe and cassiterite wolframite bearing quartz topaz greisens are found as steeply dipping veins and pods within a south central lobe of the pluton a geochemical study of 357 rock samples randomly distributed on a grid of 2 km spacing shows systematic geographic variation in the concentrations of a range of elements the concentrations of silica and some other elements show a relatively abrupt change approximately 10 15 km west of the projected boundary between the avalon and gander terranes indicating that the different terranes ha

fault zone processes and fluid history in austin chalk southwest texas faults are commonly the largest fractures in a fracture network forming the backbone of the permeability system in low permeability rocks faults in outcrops of the austin chalk an important oil and gas reservoir in south texas louisiana and westernmost mississippi exemplify deformation and fluid histories of subseismic scale faults in mechanically layered chalk dominated strata exposures of near horizontal austin chalk in southwest texas contain northwest and southeast dipping normal faults with exposed heights of meters to tens of meters and displacements of centimeters to decimeters local fault dips are steep to vertical through chalk and limestone beds and moderate through marl mud rock and clay rich ash beds producing refracted fault profiles failure surface characteristics indicate various failure modes including tensile hybrid shear and locally compactive shear behavior depending on the host lithology dilational zones associated with steep fault segments host calcite veins crack seal textures in fault zone veins record repeated reactivation of refracted fault profiles fluid inclusion and stable isotope geochemistry analyses of fault veins indicate formation at depths of 1 4 to 2 9 km 4593 to 9514 ft and possibly of 4 2 km 13 780 ft fluids include locally sourced saline waters and externally sourced waters and oil in rocks that did not reach oil maturation conditions inclusions of 30 api to greater than 50 api oil are found in veins 30 km 19 mi updip from the eagle ford formation oil zone these results illustrate the migration of oil and other fluids within subseismic scale austin chalk faults and fundamental control of lithology on fault zone architecture

a multidisciplinary approach in determining the maceral kerogen type and mineralogical composition of upper cretaceous eagle ford formation impact on pore development and pore size distribution abstract maceral kerogen or organic matter type and mineralogical composition with the associated porosity of the upper cretaceous eagle ford formation karnes county texas was investigated in karnes county the calcareous mudstone of the eagle ford formation was deposited in an open marine outer shelf to upper slope environment to adequately characterise all components of this mudrock a multidisciplinary approach was implemented to analyse 26 core samples from a single producing well that completed within the eagle ford formation analyses included organic petrology organic geochemistry light and electron microscopy and spectroscopy pore size distribution analyses and x ray diffraction the bitumen reflectance ranges between 1 94 and 2 08 calculated vr o 1 60 1 69 indicating the mudrock is in the condensate rich gas window the toc content ranges between 1 7 and 7 2 with an average of 3 9 mineral composition is dominated in descending order by calcite clay quartz and plagioclase calcite varies between 32 and 87 with an average of 57 porosity measured by helium pycnometry ranges between 3 5 and 10 3 with an average of 7 5 organic petrology coupled with backscattered and field emission scanning electron microscopy indicates matrix bitumen as the most significant contributor to the toc content and porosity matrix bitumen contains significant meso and microporosity and is enhanced by the secondary cracking of the bitumen an increase in bioclastic mostly foraminifera and siliciclastic components correlates with lower toc content due to the lower volume of organic rich matrix peloids fecal pellets are calcareous organic rich and lack terrigenous sediment indicating a zooplankton source pelagic deposition of peloids is interpreted as a primary mechanism of amorphinite deposition which is converted to hydrocarbons and matrix bitumen the mixed siliciclastic calcareous matrix that surrounds the peloids is comprised of matrix bitumen and dispersed organics from land plants vitrinite fusinite and inertodetrinite the siliciclastic component increases in the middle of the eagle ford and marks the boundary maximum flooding surface between the lower tst and upper hst section of the formation the inorganic and organic components suggest a mixed terrigenous and marine source with both inorganic and organic sediments being derived from the proximal ouachita thrust belt and from pelagic organisms within the overlying water column the lower tst eagle ford formation contains more micro and mesopores than the upper hst eagle ford formation due to higher toc content in the lower eagle formation these results illustrate that a multidisciplinary approach is necessary to identify how pore network are distributed within mudrocks and how this distribution is controlled by organic inorganic deposition and hydrocarbon generation processes

bioclastic carbonate sedimentation on a high latitude tide dominated shelf northeast orkney islands scotland abstract shell sands and gravels cover much of the shallow orkneys shelf at 59 n accumulating locally into 30 m high banks at rates up to 540 g m2 yr 67 cm l 000 yr overall the orkney shelf sedimentation rate is approximately 10 cm l 000 yr compared with 3 cm 1 000 yr for the entire scottish continental shelf major sandbanks are located off headlands that produce circulation loops in the tidal flow regional sandwave orientations reveal a clockwise transport of sediment around the islands probably resulting from storm wave reinforcement of the tidal asymmetry combined with the net inflow of atlantic water into the north sea carbonate production is high from the modiolus epifauna modiolus shell gravels commonly containing glycymeris pass laterally into comminuted shell sands within the euphotic zone down to 40 m dead shells are weakened by echinoid biting algal boring and limpet grazing boring by fungi and clionid sponges and grazing by chitons are common but not depth restricted sediments contain 89 95 carbonate on the level bottom offshore but 94 99 in sandwaves mean values for the main skeletal components are bivalves 46 barnacles 18 bryozoans 11 and serpulids 7 calcareous algal gravels occur in sheltered areas less than 20 m deep bryozoa typify lower energy offshore environments while more durable barnacle and serpulid debris is concentrated in sandwave fields the sediments are dominantly calcitic and have a high preservation potential

low sea level tidal origin of celtic sea sand banks evidence from numerical modelling of m2 tidal streams abstract numerical modelling of m 2 tidal streams in the celtic sea with a sea level lowered by 100 m indicates tidal currents about twice as strong as those at present such currents would have been sufficient to generate and maintain the linear tidal sand banks now found in that region as moribund bedforms furthermore an indicated clockwise rotation of the major axes of the tidal ellipses at a sea level lowered by 100 m with respect to the present day is in the correct sense to further the comparison with modern examples of actively maintained linear tidal sand banks

convective fractionation a mechanism to provide cryptic zoning macrosegregation layering crescumulates banded tuffs and explosive volcanism in igneous processes a large array of igneous and volcanic features has characteristics that are recognized in other disciplines as conclusive and direct evidence of convection in stratified and or solidifying melts e g macroscopic segregation cryptic variation zoning in magma chambers mineral layering in mafic intrusives crescumulates fingering in the vertical and horizontal banding in pyroclastics rollover with attendant flashing of volatiles explosive volcanism etc some quantitative and qualitative aspects of convection in solidifying and or stratified melts e g mineral layer widths such as are observed in the skaergaards are examined to show consistency with field evidence convective fractionation does not possess the physical implausibilities of gravitational segregation crystal settling neither is the field evidence as ambiguous if interpreted in terms of convective fractionation which can explain amongst other things heavier material overlying lighter convective fractionation may operate on larger scales in the interior of planets

soret convection and rheology viscous dissipation arguments for whole mantle convection abstract excluding phase changes associated with rearrangement of crystal structure with depth i e pressure computations of soret convection for mantle situations indicate that a chemically layered mantle within the earth cannot be maintained for more than several tens of millions of years this result is conservative that is high viscosities etc were employed to determine the longest incubation times lifetime of the layering possible under accepted values of the physical properties of the mantle the layering is removed by roll over there is a possibility that sparging of continental material from the mantle could lead to recurrent stratification and roll over although each layer was assigned various viscosities in determining maximum time scales the short duration of the layering in conjunction with mantle rheology which some workers eliminate by dismissing viscous dissipation supports conjectures that the mantle may assume an effective viscosity of approximately constant value a review of the pathologies of numerical procedures employed in assessing mantle rheologies is provided so that the effects of viscous dissipation may be correctly discerned from numerical artifacts the potential of soret convection in the mantle leads to the possibility of isotope and chemical fractionation across core mantle and mantle crust boundaries the isotope fractionation effect cannot be corrected by the usual corrections that attend mass spectroscopy

eagle ford shale prospecting with 3d seismic data within a tectonic and depositional system framework the south texas eagle ford shale formation in is an important emerging shale play in the united states more than 1510 wells have either been drilled or permitted in the play what has emerged is a well defined downdip gas play that transitions rapidly updip into less well defined wet gas and oil fairways with initial gas well rates exceeding 17 million cubic feet per day and initial oil well rates in excess of 1000 barrels of oil per day common in the expanding play exploration companies are pursuing methods to optimize drilling plans limited well and core data describing the eagle ford require the use of 3d seismic to help characterize reservoir quality variations avoid drilling hazards and help predict sweet spots modern long offset full azimuth 3d is an essential tool for building a geophysical description of the shale between wells anisotropic time imaging feeding both acoustic and elastic inversions is helping operators gain better insight into the reser voir s variability the following discussion highlights components of an ongoing effort to extract value from seismic data in shale prospecting unfortunately limited well data for calibration as in many immature shale plays remains the primary constraint on confident linkage between the actual reservoir quality and seismic measurements

effects of volatiles on mixing in calc alkaline magma systems the intimate mixing between different magmas of disparate densities characteristic of some calc alkaline magma systems is explained by a new mechanism involving the emplacement of a layer of wet undersaturated mafic magma at the base of a magma chamber containing more differentiated magma turbulent transfer of heat between the mafic magma and the overlying magma leads to crystallization and exsolution of volatiles in the lower layer for an initial water content of a few per cent the bulk density of the mafic magma can become equal to that of the overlying magma causing overturning and intimate mixing of the magmas the mechanism is most probable in the low pressure environment of a high level magma chamber

composition gaps critical crystallinity and fractional crystallization in orogenic calc alkaline magmatic systems the recognition of a three way correlation between magmatic sio2 content critical crystallinity and the size magnitude of crystal fractionation generated composition gaps in calc alkaline magmatic systems suggests an important control of magmatic critical crystallinity on the formation of such composition gaps to explain this correlation it is proposed that fractionation generated composition gaps are caused by 1 simultaneous interior i e non substrate crystallization and vigorous chamberwide convection which leads to progessive crystal suspension 2 cessation of convection when the percentage of suspended crystals reaches the critical crystallinity of the magma and 3 eventual buoyancy driven crystal liquid segregation producing a discrete body of fractionated magma which is separated from the initial magma by a composition gap this mechanism implies that many if not most magma bodies are characterized by interior crystallization and vigorous convection conditions which are not universally agreed upon at present given that such conditions characterize natural magma bodies fractional crystallization through crystal settling in low velocity boundary layers should be an important mechanism of fractional crystallization in a crystallizing and convecting body of magma composition gap formation should represent one endmember of a complete spectrum of possible evolutionary paths governed by the relative rates of crystal settling and crystal retention as a given volcanic plumbing system matures with time average settling retention ratios within individual magma bodies should increase due to higher average wall rock temperatures it follows that within a given volcanic center early stage volcanism should be more likely to display fractionation generated composition gaps than later stage volcanism such a temporal evolution has been documented at at least two aleutian calc alkaline volcanic centers

a model for mixing basaltic and dacitic magmas as deduced from experimental data it has been demonstrated experimentally that basaltic and dacitic magmas can be easily mixed to form both banded dacite and homogeneous andesite in less than a few hours the presence of phenocrysts larger than 0 5 mm increased considerably the efficiency of mixing flow patterns in the experimental system were visualized using pt spheres which indicated that convection occurs in basalt melt but not in dacite melt the reynolds numbers of the basaltic and dacitic melts in the experimental system were calculated to be about 10 3 and 10 6 respectively mixing proceeds initially by mechanical mixing of the two magmas in a large scale but later by coupling interfacial convection and mutual diffusion thus depending on the depth where vesiculation and following disruption of the magma occurs banded pumice homogeneous pumice and homogeneous andesite lava are erupted the observed textures of mixed rocks of plinian type eruption and the limiting occurrence of banded pumice are satisfactorily accounted for on this model

the petrology of tambora volcano indonesia a model for the 1815 eruption abstract tambora is an active volcano in the east sunda arc and is well known for its catastrophic eruption in april 1815 its lavas are of unusual moderately undersaturated k2o rich types ranging from ne trachybasalt to ne trachyandesite they evolved under conditions of high oxygen fugacity about two log units below the m h buffer and crystallised leucite at low pressures the products of the 1815 eruption are black glassy biotite bearing ne trachyandesites together with scoria pumice and tuff of the same composition the parent trachybasalt liquid probably contained about 3 wt h2o and the trachyandesites nearly 6 wt the model presented here suggests that the 1815 eruption which is known to have followed a lengthy period of inactivity was the result of gradual cooling of a hydrous magma in a closed high level magma chamber emplaced at depths between 1 5 and 4 5 km eruption followed the exsolution of a high pressure aqueous fluid phase formed during cooling and crystallisation of the magma second boiling by the time eruption took place overpressures of about 4 5 kbar had been generated and temperatures in the magma chamber ranged from about 850 c to 700 c the catastrophic eruption took place after failure of the roof and the discharge of the vapour phase may have reached velocities of 650 m s 1 calculations indicate that the volume of magma involved in the eruption was of the order of 33 km3 and this may have evolved about 1 2 1027 ergs of energy during the eruption a blast equivalent to 30 000 megatons of tnt

a model for temporal changes in the faunal composition of shell gravels during a transgression on the continental shelf around the british isles abstract the faunal composition of many of the cool temperate holocene biogenic carbonates on the continental shelf around the british isles reflects the composition of the living fauna of the source area although the relative proportions of particular faunal components may differ radiocarbon dating confirms the presence of material of various ages older debris is frequently discoloured and heavily bored more recent components are unstained and contain fewer borings faunas can be grouped into hard substrate and soft substrate associations a model is proposed for the sedimentological and faunal development of a tidal current dominated temperate continental shelf for several stages from low to high sea level for a shelf with a abundant sand supply and b sparse sand supply in a at the start of transgression the available siliciclastic sand is initially formed into hummocky cross stratified sands as sea level rises tidal currents develop and a suite of bedforms is formed as sea level continues to rise these migrate shorewards sand banks and sand waves have a low faunal density and diversity following cessation of sand supply the sediments gradually coarsen with the accumulation of bioclastic debris in b hard substrate faunas dominate throughout as sea level rises sand patches with a diverse fauna develop on the outer shelf following cessation of the sand supply the sediments become coarser faunal diversity is likely to be greatest on continental shelves with minimal clastic sediment supply faunal assemblages from abundant sand shelves can be distinguished by the presence of a much higher proportion of infaunal species