the fluid dynamics of a basaltic magma chamber replenished by influx of hot dense ultrabasic magma this paper describes a fluid dynamical investigation of the influx of hot dense ultrabasic magma into a reservoir containing lighter fractionated basaltic magma this situation is compared with that which develops when hot salty water is introduced under cold fresh water theoretical and empirical models for salt water systems are adapted to develop a model for magmatic systems a feature of the model is that the ultrabasic melt does not immediately mix with the basalt but spreads out over the floor of the chamber forming an independent layer a non turbulent interface forms between this layer and the overlying magma layer across which heat and mass are transferred by the process of molecular diffusion both layers convect vigorously as heat is transferred to the upper layer at a rate which greatly exceeds the heat lost to the surrounding country rock the convection continues until the two layers have almost the same temperature the compositions of the layers remain distinct due to the low diffusivity of mass compared to heat the temperatures of the layers as functions of time and their cooling rate depend on their viscosities their thermal properties the density difference between the layers and their thicknesses for a layer of ultrabasic melt 18 mgo a few tens of metres thick at the base of a basaltic 10 mgo magma chamber a few kilometres thick the temperature of the layers will become nearly identical over a period of between a few months and a few years during this time the turbulent convective velocities in the ultrabasic layer are far larger than the settling velocity of olivines which crystallise within the layer during cooling olivines only settle after the two layers have nearly reached thermal equilibrium at this stage residual basaltic melt segregates as the olivines sediment in the lower layer depending on its density the released basalt can either mix convectively with the overlying basalt layer or can continue as a separate layer the model provides an explanation for large scale cyclic layering in basic and ultrabasic intrusions the model also suggests reasons for the restriction of erupted basaltic liquids to compositions with mgo 10 and the formation of some quench textures in layered igneous rocks

sedimentological characteristics and depositional processes of sediment gravity flows in rift basins the palaeogene dongying and shahejie formations bohai bay basin china abstract sediment gravity flow deposits are common particularly in sandy formations but their origin has been a matter of debate and there is no consensus about the classification of such deposits however sediment gravity flow sandstones are economically important and have the potential to meet a growing demand in oil and gas exploration so there is a drive to better understand them this study focuses on sediment gravity flow deposits identified from well cores in palaeogene deposits from the liaodong bay depression in bohai bay basin china we classify the sediment gravity flow deposits into eight lithofacies using lithological characteristics grain size and sedimentary structures and interpret the associated depositional processes based on the scale spatial distribution and contact relationships of sediment gravity flow deposits we defined six types of lithofacies associations las that reflect transformation processes and depositional morphology la1 unconfined proximal breccia deposits la2 confined channel deposits la3 braided channel lobe deposits la4 unconfined lobe deposits la5 distal sheet deposits and la6 non channelized sheet deposits finally we established three depositional models that reflect the sedimentological characteristics and depositional processes of sediment gravity flow deposits 1 slope apron gravel rich depositional model which involves cohesive debris flows deposited as la1 and dilute turbidity currents deposited as la5 2 non channelized surge like turbidity current depositional model which mainly comprises sandy slumping suspended load dominated turbidity currents and dilute turbidity currents deposited as la5 and la6 and 3 channelized subaqueous fan depositional model which consists of non cohesive bedload dominated turbidity currents suspended load dominated turbidity currents and dilute turbidity currents deposited as la2 la5 originating from sustained extrabasinal turbidity currents hyperpycnal flow the depositional models may be applicable to oil and gas exploration and production from sediment gravity flow systems in similar lacustrine depositional environments elsewhere

diagenetic origin for quartz pebble conglomerates the occurrence of quartz pebble conglomerates qpc in the rock record increases backward through time from the tertiary through the precambrian the positive correlation between qpc abundance and age is valid both for numbers of reported qpc and for qpc as a percentage of all conglomerate and at both the era and the period level qpc are usually interpreted as being due to intense chemical weathering protracted transport or sediment recycling but none of these can account for the age distribution of qpc which is the opposite of the global mass age distribution for sedimentary rocks precambrian and tertiary conglomerates with similar sources and sedimentology have vastly different clast populations nonquartzose clasts being much more abundant in the younger rocks comparison of the petrology of qpc and polymict conglomerates shows that qpc have consistently higher proportions of diagenetic secondary matrix and pressure solved grain contacts we conclude that diagenetic factors play an important role in qpc formation by preferentially destroying less durable clasts

on estimating crystal shape for crystal size distribution analysis abstract the accurate determination of three dimensional crystal habit long intermediate short axis is a requirement to enable correct stereological conversion of 2d crystal size measurements to true 3d crystal size distributions csds in this contribution we introduce a database and spreadsheet program which provides an objective estimate of true crystal habit from raw 2d measurements the database compares the sample s 2d measurements with 2d shape curves for random sections through 703 different habits the output gives the five best match curves and corresponding crystal habits based on a least squares fit between sample and database the minimum sample size required to give good shape estimates from 2d data is tested using random runs of sections through known shapes with different population densities at minimum 75 crystal sections are required to robustly determine crystal habit for csd measurements if crystals are tabular in shape with more acicular shapes requiring a minimum of 250 sections suggesting a sample size of 250 sections to be used for shape determination in natural examples where the true 3d shape is unknown the crystal habit of the best match shape can then be used to provide a more robust csd analysis which in turn can be used to investigate magmatic processes with more certainty

deep water facies processes and models a review and classification scheme for modern and ancient sediments abstract a review of previous work on modern and ancient deep water facies processes and models is presented with a new classification scheme involving 40 distinct facies related to 15 conceptually distinct facies groups these facies are fixed points in a spectrum of facies generated in a process continuum from resedimentation processes through semi permanent bottom currents to pelagic settling in essence the scheme is descriptive of the sedimentary attributes of sediments although it is designed to aid interpretation of possible sediment transport deposition processes the classification scheme is three tier with facies classes groups and constituent facies and is hierarchical to allow flexibility in its use there are seven facies classes with classes a e defined largely on the basis of grain size differences class f on the basis of internal organization and class g on composition the facies classes are class a gravels muddy gravels gravelly muds and pebbly sands with 5 gravel grade class b sands with 80 sand grade and 80 mud 40 silt and 0 20 sand class e muds and clays with 95 mud grade for the purpose of large scale mapping or reconnaissance fieldwork either the level of facies classes or groups may be appropriate whereas for more detailed sedimentology the more detailed facies level will be necessary

groundmass crystallization of mount st helens dacite 1980 1986 a tool for interpreting shallow magmatic processes the 1980 1986 eruption of mount st helens volcano provides an unprecedented opportunity to observe the evolution of a silicic magma system over a short time scale groundmass plagioclase size measurements are coupled with measured changes in matrix glass plagioclase and fe ti oxide chemistry to document increasing groundmass crystallinity and thus to better constrain proposed physical models of the post may 18 1980 magmatic reservoir measurements of plagioclase microlite and microphenocryst sizes demonstrate that relatively rapid growth approximately 10 9 cm s of groundmass plagioclase occurred immediately subsequent to may 18 relatively rapid plagioclase growth continued through the end of 1980 at an average rate of 3x10 11 cm s plagioclase growth rates then decreased to 1x10 11 cm s through 1986 changes in groundmass crystallinity are reflected in changes in both matrix glass and plagioclase microphenocryst rim chemistry although the matrix glass composition appears to have remained approximately constant from 1981 1986 after a rapid compositional change from may 18 until the end of 1980 plagioclase microphenocrysts show increasingly more complex zoning patterns with time microphenocryst core compositions are commonly positively correlated with crystal size both of these observations indicate continuous groundmass plagioclase growth through 1986 magmatic temperatures estimated from fe ti oxide pairs are approximately constant through 1981 at eruption temperatures of 930 c and at log fo2 of 10 8 by 1985 1986 oxide temperatures decreased to 870 c chemical and textural changes can be explained by 1 rapid degassing and crystallization in response to the intrusion of magma into a shallow 4 5 km reservoir toward the end of the may 18 1980 eruption 2 continued crystallization at a much reduced rate through 1986 due to slow cooling of the shallow magma reservoir growth rates and consequent chemical changes appear to decrease at the end of 1980 this is coincident with the change in eruption style from explosive eruptions sometimes followed by dome growth to solely extrusive dome building events and can be explained by the expected viscosity increase of both degassing and increasing crystallinity the model of twostage crystallization of magma in a shallow reservoir is consistent with conclusions from gas studies casadevall et al 1983 gerlach and casadevall 1986 a b patterns of crater deformation chadwkck et al 1988 and post 1980 seismicity endo et al 1990 although it does not explain the experimental data of hill and rutherford 1989 on the growth rate of amphibole reaction rims textural measurements on mount st helens dacite can also be used to evaluate crystallization kinetics in silicic magmas systems for which experimental data is almost non existent plagioclase growth rates are 5 10 times slower than estimated plagioclase growth rates in basaltic systems a result consistent with the higher viscosity of a more silicic melt furthermore patterns of textural change both average crystal size and number density are similar to those observed during the 1984 mauna loa eruption by lipman and banks 1987 suggesting that the only modification to the crystallization behavior of plagioclase required in extrapolation from basaltic systems is a moderate decrease in rates such that the rate of crystallization scales with the melt viscosity

zirconolite allanite and hoegbomite in a marble skarn from the bergell contact aureole implications for mobility of ti zr and ree zirconolite allanite and hoegbomite are present as accessory phases in a metasomatically altered spinel calcite marble from the contact with the bergell intrusives switzerland italy textural relationships indicate a step wise alteration of spinel to 1 hoegbomite or corundum magnetite 2 margarite and 3 chlorite replacement of spinel by hoegbomite can be described by the substitution 1 94 mg2 fe2 zn2 mn2 ca2 ti4 0 12 oh where al3 and fe3 are held constant the average composition of the bergell hoegbomites is given by the formula fe 0 97 2 mg0 69zn0 04ti0 17al3 94fe 0 06 3 o7 98 oh 0 02 and seems to be imposed by the composition of pre existing spinel during the first two steps of spinel alteration calcite was replaced by anorthite phlogopite and the rare earth element ree bearing minerals zirconolite allanite and sphene were formed allanites have characteristic chondrite normalized ree patterns with enrichment in the light ree the zirconolite patterns show a marked increase in concentration from la to ce followed by an almost constant section sphene lacks detectable la and its ree patterns vary from grain to grain contemporaneous formation of phlogopite ree bearing minerals and hoegbomite during replacement of the spinel calcite marble indicates that the metamorphic fluid introduced potassium along with ree and other high valence cations ti4 zr4 u4 th4a3804265 nb5a3804265 y3a3804265 possibly as polynuclear complexes the abundance of fluorine bearing phlogopite and fluor apatite as well as their close association with ree bearing minerals and hoegbomite suggests f and po 4 3 as likely ligands for complexing of the above mentioned elements

heterogeneity and lithotype distribution in ancient deep sea canyons point lobos deep sea canyon as a reservoir analogue abstract an evolution and history of filling is proposed for an exceptionally exposed ancient deep sea canyon on a paleocene oblique slip tectonic margin which on a number of scales reveals successive phases of canyon activity the quantitative methods adopted for this study make it of direct use to modellers as an example of reservoir heterogeneity in an ancient canyon fill where facies distribution from boreholes can be scaled up to reconstruct the reservoir using the methods outlined in this paper the point lobos submarine canyon near carmel in california provides a complete cross section of an ancient canyon with a fill which displays a whole range of channel morphologies and laterally extensive coverage of the internal architecture of associated conglomerate packages and related debris flows this paper presents quantitative documentation of the canyon fill sediments and canyon wide fill packages on scales which vary from bed to bed analysis reflecting processes in operation during individual events to canyon wide analysis reflecting the overall evolution of the canyon the northern and southern canyon margins are both exposed and the paleocene fill onlaps the subvertical canyon wall the canyon was incised into cretaceous granodiorite the fill comprises five thick sequences which correspond to five successive phases of sediment deposition within the canyon each sequence typically consists of resedimented conglomerates that are stacked and channelised with a vertical architecture which resembles that of subaerial braided stream deposits these are overlain by channelised turbidite sandstones interbedded with intraformational conglomerates and mud chip breccias these in turn are overlain by mudstones and shales which are commonly slumped and disturbed published classification schemes that show the range of deep water facies were found insufficient to describe the point lobos canyon fill methods were developed for recording the lithologic proportions of the fill twenty three lithotypes were defined which show the variation in the sedimentary units and a range of processes are proposed to explain each lithotype these methods produced a quantitative database of lithologic proportions for the canyon fill over 80 of the fill comprises sandstone and conglomerate conglomerate is more common near the canyon margins mud and silt comprise the other 20 of the canyon fill predominantly as laterally extensive slumped packages which cap the coarser sequences the distribution of these lithotypes within the fill was found to have a cyclicity with five cycles of sedimentation recorded and lateral transport of material from the canyon walls a primary feature

mixing and mingling in the evolution of andesite dacite magmas evidence from co magmatic plutonic enclaves taupo volcanic zone new zealand abstract the southeastern side of the taupo volcanic zone new zealand is marked by a line of andesite dacite low silica rhyolite complexes co magmatic plutonic enclaves occur within the lavas of the four youngest complexes white island motuhora whale island edgecumbe and tauhara the enclaves range from coarse grained gabbros diorites granodiorites and a syenite to finer grained dolerites and microdiorites the more mafic types are generally porphyritic with large phenocrysts of plagioclase usually with extensive sieve textures in the cores and corroded margins most of these enclaves including the coarser grained plutonic examples contain glass and many are miarolitic diorites and microdiorites dolerites predominate at white island motuhora and edgecumbe many are porphyritic enclaves at tauhara are more variable those collected from hipaua dome include a range from microdiorites to quartz microdiorites and those from rubbish tip dome include microdiorites a granodiorite and a syenite most enclaves show textural evidence for disequilibrium with multiple populations of plagioclase and pyroxene they also show considerable textural variation even within a thin section with coarse grained gabbros diorites intimately mixed with finer grained dolerites microdiorites geochemically and isotopically most enclaves have a similar composition with their host lavas although some have lower silica contents enclaves at motuhora and tauhara are isotopically more variable indicating multiple sources and a more complex petrogenesis most diorite microdiorite enclaves are interpreted to represent parts of a crystal mush formed during fractionation of andesite dacite magma and entrained during later rise of magma to the surface the granodiorite from rubbish tip dome tauhara probably represents part of a silicic magma chamber within the crust that fed the host low silica rhyolite lava dome variability within the enclaves indicates the complexity likely to occur within the magma chambers beneath a typical volcanic arc

textural analysis of magmatic enclaves from the kameni islands santorini greece abstract the kameni islands have been the focus of historic volcanic activity on santorini greece the islands comprise a series of dacite lavas the erupted compositions of which have remained approximately constant for the last 2200 years the dacite lava flows of nea kameni contain a variety of mafic enclaves distinguished by differences in texture composition and mechanical behaviour the mafic enclaves represent chilled fragments of mafic magma that crystallised rapidly on contact with the cooler more silicic host magma the absence of chilled margins on the enclaves suggests that the replenishing magma initially formed a dense layer at the base of the chamber groundmass textures are interpreted as an indication of the degree of undercooling between the enclaves and the host groundmass textural information obtained from the enclaves has been used to determine the relative volumes of replenishing magmas injected into the host magma before eruption which indicates that the volume of erupted magma is directly proportional to the volume of replenishing magma emplaced in the chamber prior to eruption which thus may act as a potential eruption trigger the variety of replenishing magmas implies the existence of a complex conduit and chamber system beneath santorini volcano andesitic enclaves with linear crystal size distributions are thought to have originated as an aphyric andesitic melt expelled from a crystal mush by filter pressing disequilibrium phenocryst assemblages in the lavas and phenocryst bearing enclaves provide evidence for the entrainment and recycling of older phenocryst populations

vertical and lateral organization of a carbonate deep water slope marginal to a submarine fan system miocene southern turkey organization is recognized in the forereef deep water slope submarine fan system of the burdigalian langhian kaplankaya formation a basinwards transition from a prograding shelfal reef complex through forereef talus deep water slope and laterally encroaching bypass deep water clastic system is described although the deep water slope makes up the bulk of the succession considerable thickness variations occur between the reef and deep water clastic complexes these are controlled by sea floor topography carbonate foreslope gradient and degree of mass wasting off the platform and foreslope the vertical and lateral heterogeneity of the kaplankaya deep water slope system is described from a number of localities along a 40 km long and up to 3 km wide exposed section of the northern margin of the miocene adana basin a foreland basin setting resulting from thrust sheet loading from the north during the tauride orogeny detailed field mapping is supplemented with vertical sedimentary logs photomosaics palaeontological and petrological data to investigate stratal variation diagnostic architectural elements controls on slope progradation differential timing of basinward encroachment of the reefal complex and lateral onlap of the deep water clastic system onto the slope three dimensional models are presented showing the vertical and lateral facies associations in different parts of the deep water slope system and provide a basis for architectural prediction of geometry and relative position in such environments

stratigraphic evolution from shoreface to shelf indenting channel depositional systems during transgression insights from the lower pliocene sua member of the basal upper onzole formation borbon basin northwest ecuador abstract shelf indenting canyons and their tributary systems are fairly common constituents of quaternary shelves of active continental margins but they have been rarely reported from older successions recognition of these prominent geomorphologic features in the ancient record has important implications not only for a proper understanding of shoreface to shelf depositional systems but also from a petroleum exploration standpoint as they represent efficient conduits for moving coarse grained river and nearshore borne sediments to the adjacent slope even during periods of relative rise in sea level coastal exposures of the lower pliocene sua member in the surroundings of sua northwest ecuador preserve the unusual juxtaposition of incising submarine channels onto nearshore deposits this succession accumulated along a narrow active continental margin during tectonically induced transgression and affords a rare opportunity to evaluate the stratigraphic evolution of such systems from an outcrop perspective a comprehensive facies characterization combined with application of sequence stratigraphic concepts has led to definition of the following physical surfaces and stratal units in ascending order i a polygenetic regionally extensive erosional surface resulting from the superposition of the wave ravinement surface onto the previous subaerial sequence boundary sb wrs ii a nearshore sand prone lithofacies succession comprising a condensed basal shellbed deepening upwards through lower shoreface bioturbated silty sandstones into inner shelf sandy mudstones iii two steep u shaped erosional features turbidite shelf entrenchment surfaces interpreted as shelf channels deeply incised into the subjacent nearshore sediments and marking an abrupt deepening of facies iv a thick fining upward sedimentary succession laid down within the confines of the channels by high and low density turbidity currents and including both bed load traction and suspended load deposits the overall fining and thinning upward character exhibited by the infill of these channels is thought to reflect decreasing flow energies and is consistent with the gradual cut off of clastic influx to their upper reaches in response to progressive detachment from an adjacent coastal source during relative rise in sea level based on detailed analysis of facies and a sequence stratigraphic interpretation of outcrop data this study contributes to extend the existing sequence stratigraphic schemes further attesting that shelf sediment bypass and deep water sedimentation can take place at sea levels other than lowstand

the record of magma chamber processes in plagioclase phenocrysts at thera volcano aegean volcanic arc greece lavas and pyroclastic rocks throughout the volcanic stratigraphy of the tertiary quaternary volcanic complex of thera in the aegean island arc display inhomogenous plagioclase populations and phenocryst resorption textures interpreted as indicative of magma mixing plagioclase zoning characteristics studied by nomarski and laser interferometry techniques establish three main categories of plagioclase i inherited plagioclase nucleated in endmember prior to initial mixing event ii in situ plagioclase nucleated in mixed or hybrid magma and iii xenocrystic plagioclase nomarski contrast images and linearized compositional zoning profiles reveal striking differences between calcic and sodic plagioclases depending on the composition of the lava in which they are hosted these differences reflect the contrasting effects of changes in physical chemical parameters in basic vis a vis more acidic melts during magma mixing and or influx of new magma into the subvolcanic magma chamber as well as the influence of magma chamber dynamics on plagioclase equilibration variations in bulk major and trace element abundances of thera volcanic products reflect the dominant overprint of crystal fractionation but decoupling between major and trace element fractionation models and variations in incompatible trace element distributions are all indicative of magma mixing processes consistent with compositional and textural zoning in plagioclases

benthic foraminifera of a miocene canyon and fan abstract we present benthic foraminiferal assemblage data from an exhumed miocene canyon and fan system from the tabernas basin se spain the presence of good indicator taxa and unique assemblages occupying specific environments allows the distinction of slope canyon and fan environments within the tabernas basin by foraminiferal assemblages alone five assemblages are defined on the basis of the occurrence of the indicator taxa primary control on the distribution of these assemblages is consistent with trends of physical disturbance and consequent defaunation barren samples which are predominantly found in high energy parts of the proximal canyon are recognized as representing recently defaunated substrates i e early successional assemblages high diversity assemblages containing a high abundance of agglutinated taxa are recognised in the undisturbed slope sequences as being the regional equilibrium climax fauna intermediate between these end members are assemblages with low diversity dominated by calcareous taxa typically found in the relatively low energy canyon and fan environments which are recognized as representing the middle phases of the ecological succession two further assemblages a low diversity assemblage typified by cassidulina laevigata and bulimina costata and a very low diversity assemblage dominated 10 of all benthic tests by globobulimina spp are restricted to low energy parts of the canyon and fan and are absent from the proximal canyon and slope the composition of these assemblages indicate that nutrient supply oxygenation is a secondary control on the palaeoecology of the canyon system a conceptual model for the recolonisation of defaunated substrates in el buho canyon is proposed in which either an oligotrophic climax assemblage or a eutrophic climax assemblage can be achieved at the completion of recolonisation of defaunated substrates depending on environmental conditions

field expressions of the transformation of debris flows into turbidity currents with examples from the polish carpathians and the french maritime alps various transformation mechanisms can generate turbidity currents from subaqueous debris flows different transformation mechanisms have been described and interpreted in the past from laboratory experiments and from deposits but the two approaches have not generally been linked this has made the genetic interpretation and comparison of deposits difficult in this paper a generic classification scheme of debrite turbidite couplets is proposed based on transformation mechanisms inferred from laboratory experiments five different flow types called a e herein and their resulting deposits are detailed but they are all part of a continuous spectrum and a mixture of types is likely to be found in the field type a flows are strong dense debris flows that undergo little transformation their deposit will be a debrite overlain by a thin turbidite which is separated from it by a clear grain size break type b flows are weaker and can develop waves at the debris flow turbidity current interface the deposit will be a debrite with a wavy top overlain by a turbidite that is thicker than for type a flows for type c flows the interfacial waves will grow so much that the debris flow disintegrates into separate parts the deposit will consist of debrite lenses encased in a turbidite type d flows will undergo even more mixing than type c flows so that the debrite parts will be mixed their deposit will be a turbidite with laterally varying areas of debrite characteristics near the bed type e flows will be so transformed that the debris flow character has disappeared and the flow is a turbidity current with high sediment concentration the deposit will be largely turbiditic the flow types and deposits will be illustrated with some examples from two field areas the polish carpathians and the french maritime alps

caldera formation on santorini and the physiography of the islands in the late bronze age the caldera of santorini is a composite structure with a subsidence history extending over 100 ka or more geomorphological mapping shows that the present day caldera wall is a complex assemblage of cliff surfaces of different ages and that collapse at santorini has repeatedly exhumed earlier caldera cliffs and unconformities cliffs bounding the southern southeastern and northwestern rims of the caldera are morphologically fresh and probably formed during or soon after the minoan eruption in the late bronze age the well scalloped shape of these cliffs is attributed to large scale rotational landslip around the margins of the minoan caldera the deposit from one landslip is preserved subaerially minoan landslips in southeast santorini detached along the basement unconformity exposing a cliff of the prevolcanic island the caldera wall in the north northeast and east preserves evidence for three generations of cliff those of minoan age and two earlier generations of caldera wall the two early calderas can be dated relative to a well established statigraphy of lavas and tuffs the presence of in situ minoan tephra plastered onto the present day caldera wall provides evidence that these ancient caldera cliffs had already been exhumed prior to the minoan eruption field relationships permit reconstruction of the physiography of bronze age santorini immediately before the minoan eruption the reconstruction differs from some previously published versions and is believed to be the most accurate to date bronze age sa ntorini had a large flooded caldera formed 21 ka ago this caldera must have acted as an excellent harbour for the bronze age inhabitants of the island the 3 6 ka minoan eruption deepened and widened the extant caldera the volume of minoan collapse 25 km3 is in good agreement with published estimates for the volume of discharged magma if between 5 and 8 km3 of minoan ignimbrite ponded as intracaldera tuff

effects of antecedent topography on reefal carbonate deposition early middle miocene of the adana basin s turkey abstract early middle miocene reefal limestones and other shallow marine sedimentary sequences were important components of neogene basins especially the adana basin in southern turkey the most important controlling factors of sedimentation were antecedent topography and substratum type clastic input and sea level changes of different scales the effects of these factors can be observed on the southwest extension of the adana basin miocene sediments unconformably overlie the older limestones ophiolite and ophiolitic melange lithological variations and structural properties of pre miocene rocks caused an irregular sea floor topography development that was initially filled by local terrestrial conglomerates during the early miocene transgression these terrestrial clastics transitionally passed into shallow marine clastics and also new pebbly conglomerate sandstone bearing deposits observed in local depressions of pre miocene topography remaining low lying areas were filled by claystones and sandy clayey limestones at the same time reef limestone deposition developed over the higher areas where a hard substratum was available for red algae and other reef builders large shelled organisms in the clastics created a suitable substratum for successive reef level development the carbonate deposition shifted landwards due to continuous transgression intermediate sea level changes causing backstepping reef development small sea level changes in the study area controlled the energy of the environment lithological variations in the clastics and lateral facies changes in each reef body

crystal size distribution csd in rocks and the kinetics and dynamics of crystallization ii makaopuhi lava lake crystal size distribution csd theory has been applied to drill core samples from makaopuhi lava lake kilauea volcano hawaii plagioclase and fe ti oxide size distribution spectra were measured and population densities n were calculated and analyzed using a steady state crystal population balance equation n n0 exp l g slopes on ln n versus crystal size l plots determine the parameter g a product of average crystal growth rate g and average crystal growth time the intercept is j g where j is nucleation rate known temperature depth distributions for the lava lake provide an estimate of effective growth time allowing nucleation and growth rates to be determined that are independent of any kinetic model plagioclase growth rates decrease with increasing crystallinity 9 9 5 4 10 11 cm s as do plagioclase nucleation rates 33 9 1 6 10 3 cm3 s ilmenite growth and nucleation rates also decrease with increasing crystallinity 4 9 3 4 10 10 cm s and 15 2 2 10 3 cm3 s respectively magnetite growth and nucleation rates are also estimated from the one sample collected below the magnetite liquidus g 2 9 10 10 cm s j 7 6 10 2 cm3 s moments of the population density function were used to examine the change in crystallization rates with time preliminary results suggest that total crystal volume increases approximately linearly with time after 50 crystallization a more complete set of samples is needed for material with 50 crystals to define the entire crystallization history comparisons of calculated crystallization rates with experimental data suggests that crystallization in the lava lake occurred at very small values of undercooling this interpretation is consistent with proposed thermal models of magmatic cooling where heat loss is balanced by latent heat production to maintain equilibrium cooling

volcanism and the dynamics of open magma chambers the repeated eruption of many volcanoes is now accounted for by events taking place in a subterranean magma chamber which is open in the sense that the inflow of material to the chamber from below leads to a magma pressure p which exceeds the lithostatic pressure pl by an amount greater than the tensile strength of the chamber walls before the contents of the magma chamber have reached thermodynamic and geochemical equilibrium this is the condition which must be satisfied when an eruption occurs we have now derived a relationship giving the critical volume of added material v required to cause an eruption as a function of magma compressibility and the volume by which the chamber expands

paleogeography and sedimentology of upper cretaceous turbidites san diego california upper cretaceous campanian and maestrichtian marine strata of the rosario group in the san diego area include the point loma formation and overlying cabrillo formation these units contain six facies associations 1 shelf and lagoonal sandstone 2 slope and basin plain mudstone 3 outer fan lobe sandstone 4 middle fan channel fill sandstone 5 middle and inner fan interchannel and channel margin thin bedded turbidites and mudstone and 6 inner fan channel fill conglomerate and sandstone the facies associations define a deep sea fan deposited by westward flowing sediment gravity flows that transported sediments derived chiefly from batholithic and prebatholithic metamorphic rocks of the peninsular ranges the sedimentary basin initially deepened abruptly partly aided by eustatic sea level rise the fan then prograded westward into the basin with a retrogradational phase recorded in the uppermost part of the sequence which is erosionally truncated by transgressive lower eocene conglomerate the fan was deposited along the eastern edge of a forearc basin similar to that of the great valley sequence in northern california the western part of the fan which probably contained mostly outer fan lobe and associated basin plain deposits appears to have been truncated by late cenozoic strike slip faulting associated with the san andreas fault system the fan and remnants of the western part of the basin and associated subduction complex may be present on the northwest in the channel islands region or still farther north

cenozoic sedimentation and paleotectonics of north central new mexico implications for initiation and evolution of the rio grande rift the rio grande rift is one of the major late cenozoic continental rifts of the world sharing most geophysical geochemical and geological characteristics with other rifts cenozoic evolution of the rift was synchronous with lithospheric plate interactions along and under the western north american margin paleocene eocene laramide primarily amagmatic compression related to flat slab subduction oligocene intermediate to silicic volcanism related to collapse of the slab miocene to present rifting related to complex plate interactions overprinted on the previous history cenozoic paleogeographic and paleotectonic characteristics are consistent with a passive mantle mode of rifting north central new mexico provides a unique opportunity to constrain models for rift initiation and evolution it is one of the few locations within any rift where excellently exposed pre rift and syn rift basin fill has been studied thoroughly enough to allow detailed paleogeographic reconstruction for almost the entire cenozoic cenozoic paleogeography for the study area is summarized as follows 1 eocene 58 37 ma a single amagmatic sedimentary basin el rito galisteo trended northwest southeast with laramide basement uplifts on three sides 2 early to late oligocene 37 28 ma intermediate magmatism with volcaniclastic aprons derived from the san juan and ortiz cerrillos volcanic fields and residual laramide uplifts 3 late oligocene early miocene 28 21 ma initiation of bimodal volcanism with widely dispersed volcaniclastic aprons derived from the predominantly silicic san juan and latir volcanic fields 4 early to middle miocene 21 15 ma continued volcaniclastic dispersal from silicic volcanic centers concurrent with initiation of block faulting to form half grabens internal drainage and erosion of phanerozoic strata and precambrian basement 5 middle to late miocene 15 8 ma continued deepening of half grabens widespread exposure of precambrian terranes formation of complex depositional environments in basin centers and continued bimodal volcanism 6 late miocene to present 8 0 ma concentration of extension in central grabens linked by accommodation zones major bimodal volcanism for example taos plateau and jemez mountains regional uplift and integration of rio grande drainage dispersal patterns petrofacies analysis k ar ages and chemical analyses of volcanic clasts provide details concerning three primary volcanic centers 1 san juan mountains 27 29 m y high k andesite and rhyodacite 2 latir volcanic field 25 28 m y high k andesite and rhyolite and 3 the previously unrecognized servilleta plaza center 22 23 m y latite high k andesite and rhyodacite which may have been a southern extension of the latir field these three volcanic centers provided detritus to the following units respectively 1 esquibel mbr of los pinos fm upper abiquiu fm middle picuris fm bishops lodge mbr of tesuque fm and volcaniclastic units in the northern albuquerque basin zia and abiquiu 2 cordito mbr of los pinos fm and uppermost abiquiu fm and 3 chama el rito mbr of tesuque fm and upper picuris fm use of petrofacies 1 esquibel 2 cordito and 3 plaza simplifies the chaos of stratigraphic nomenclature and promotes regional correlations of poorly exposed units

magma dynamics beneath kameni volcano thera greece as revealed by crystal size and shape measurements size distributions of plagioclase crystals in series of recent porphyritic dacite lavas from kameni volcano greece can be modelled by mixing two populations of crystals each with overlapping linear crystal size distributions csd termed microlites and megacrysts the magmas bearing the microlites and megacrysts started to crystallise 6 13 and 24 96 years respectively before each eruption the dates of initiation of crystallisation of the megacrysts indicate that they are left overs of earlier injections of new magma into a shallow chamber some magma remains after each eruption and continues to crystallise new magma with few or no crystals is then introduced and the microlites crystallise from the mixed magma eruption followed 6 13 years after mixing such a model would suggest that some porphyritic magmas are products of a shallow magma chamber that is never completely emptied just topped up from time to time

irregular plate boundary controls on foreland basin sedimentation miocene kahramanmaras foreland basin se turkey abstract the northern movement of the arabian plate led to the development of a compressional regime in the south eastern part of turkey after the late cretaceous the collision of the arabian plate with the anatolide taurides platform and subsequent development of the kahramanmaras foreland basin at the beginning of the miocene evolved as a result of this movement an irregular plate geometry or promontory of the arabian plate caused partitioning of the plate margin precollision collision postcollision this irregular plate boundary was delimited by western and northern boundary faults and an interior basin fault the nw oriented small wedge top basins on the overlying anatolide taurides platform obliquely cut through the foredeep basin the region across the wedge top basin on the irregular promontory of the underlying arabian plate contains different sedimentation depocentres small fining upward submarine fan deposits including pebbly channel deposits and coarsening upward lobe sequences are located at the centre of the irregular part of the plate in the tanir region and overlie planktic foraminifera bearing claystones these submarine fan sediments pass outboard into distal interbedded turbidite claystones and siltstones in a downdip direction s se the boundary fault intersection of the promontory led to the development of a submarine slope environment with irregular sea floor topography in the firniz area 4 5 km south east of the tanir region the slump deposits sandy debrites and fine grained thin bedded turbidites filled this region structural alignments and seismicity associated with the plate margin were the main controlling factors on the geometry of the depocentre sediment quantity sediment input sedimentary facies local sea level changes and post sedimentary deformation in the kahramanmaras foreland basin

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

experimental and textural constraints on mafic enclave formation in volcanic rocks abstract we have used experiments and textural analysis to investigate the process of enclave formation during magma mixing at southwest trident volcano alaska andesite enclaves are present throughout the four dacite lava flows produced by the eruption and resemble mafic enclaves commonly found in other volcanic rocks our experiments replicate the pressure temperature time path taken by enclave forming andesite magma as it is engulfed in dacite during magma mixing pressure and temperature information for the andesite and dacite are from coombs et al contrib mineral petrol 140 2000 99 118 the andesite was annealed at 1000 c and then cooled to 890 c at rates of 110 c h 1 10 c h 1 and 2 c h 1 once cooled to 890 c andesite was held at this lower temperature from times ranging from 1 to 40 h the andesite that was cooled at the slower rates of 2 c h 1 and 10 c h 1 most resembles enclave groundmass texturally and compositionally based on simple thermal calculations these rates are more consistent with cooling of the andesite groundmass below an andesite dacite interface than with cooling of enclave sized spheres if enclaves do crystallize as spheres post crystallization disaggregation must occur calculations using the melts algorithm ghiorso and sack contrib mineral petrol 119 1995 197 212 show that for incoming andesite to become less dense than the dacite 34 volume of its groundmass must crystallize to undergo 18 volume vesiculation these values are similar to those determined for southwest trident enclaves thus such crystallization may lead to flotation of enclaves and be a viable mechanism for enclave formation and dispersal the residual melt in the cooling experiments did not evolve to rhyolitic compositions such as seen in natural enclaves due to a lack of a decompression step in the experiments decompression experiments on southwest trident dacite suggest an average ascent rate for the eruption of 2 3 mpa h 1 an andesite experiment that was cooled and then decompressed at this rate contains melt that matches that of the natural enclaves it is apparent that decompression ascent induced crystallization occurs in enclaves but not in the form of microlites as happens in the dacite host due either to insufficient residence time at chamber temperatures or to the pre existing microphenocrysts which act as sites for new growth

mineral disequilibrium in lavas explained by convective self mixing in open magma chambers characteristic features of many porphyritic andesite and dacite lavas are that they are rich in crystals and display a range of disequilibrium features including reversely zoned crystals resorption surfaces wide ranges of mineral compositions and minerals which are not in equilibrium with the surrounding rock matrix these features are often interpreted as evidence of the mixing of magmas of contrasting composition temperature and origin1 2 here however we propose that such features can also be caused by convection within a magma body with a single composition that is heated from below and cooled from above we describe petrological observations of andesite lava erupted at the soufriere hills volcano montserrat which indicate a heating event and the intermingling of crystals that have very different thermal histories we present experimental data on a representative groundmass composition of this lava which indicate that it is difficult to explain the calcic compositions of plagioclase overgrowth rims and microphenocrysts unless parts of the magma were at temperatures much higher than the inferred average temperature the concept of convective self mixing allows us to explain the occurrence of compositions of minerals that apparently cannot coexist under equilibrium conditions