paragonite hornblende assemblages and their petrological significance an example from the austroalpine schneeberg complex southern tyrol italy paragonite bearing amphibolites occur interbedded with a garbenschist micaschist sequence in the austroalpine schneeberg complex southern tyrol the mineral assemblage mainly comprises paragonite mg hornblende tschermakite quartz plagioclase biotite ankerite ti phase garnet muscovite equilibrium p t conditions for this assemblage are 550 600 c and 8 10 kbar estimated from garnet amphibole plagioclase ilmenite rutile and si contents of phengitic muscovites in the vicinity of amphibole paragonite is replaced by symplectitic chlorite plagioclase margarite biotite assemblages muscovite in the vicinity of amphibole reacts to form plagioclase biotite margarite symplectites the reaction of white mica hornblende is the result of decompression during uplift of the schneeberg complex the breakdown of paragonite hornblende is a water consuming reaction and therefore it is controlled by the availability of fluid on the retrogressive p t path paragonite hornblende is a high temperature equivalent of the common blueschist assemblage paragonite glaucophane in ca bearing systems and represents restricted p t conditions just below omphacite stability in a mafic bulk system while paragonite glaucophane breakdown to chlorite albite marks the blueschist greenschist transition the paragonite hornblende breakdown observed in schneeberg complex rocks is indicative of a transition from epidote amphibolite facies to greenschist facies conditions at a flatter p t gradient of the metamorphic path compared to subduction zone environments ar ar dating of paragonite yields an age of 84 5 1 ma corroborating an eoalpine high pressure metamorphic event within the austroalpine unit west of the tauern window eclogites that occur in the otztal crystalline basement south of the schneeberg complex are thought to be associated with this eoalpine metamorphic event

comments and reply on implications of magmatic epidote bearing plutons on crustal evolution in the accreted terranes of northwestern north america and magmatic epidote and its petrologic significance

magmatic epidote and its petrologic significance epidote is a major magmatic mineral in tonalite and granodiorite in a belt coextensive with the mesozoic accreted terranes between northern california and southeastern alaska textural and chemical evidence indicates that epidote crystallized as a relatively late but magmatic mineral that formed through reaction with hornblende in the presence of a melt phase the observed relations concur with experimental data on crystallization of epidote from synthetic granodiorite at 8 kbar total pressure plutonic rocks bearing magmatic epidote must have formed under moderately high pressures corresponding to lower crustal depths under fairly oxidizing conditions

epidote in calcalkaline magmas an experimental study of stability phase relationships and the role of epidote in magmatic evolution experiments on tonalite and granodiorite were performed at conditions ranging from 2 1 to 18 kbar and 550 to 850 c to establish the magmatic stability field of epidote as a function of p t and f02 at water saturated conditions and f02 buffered by nno epidote has a wide magmatic stability field in tonalite at 13 kbar this field extends from the wet solidus at 630 to 790 0c the low pressure intersection of the magmatic epidote crystallization reaction and the h20 saturated tonalite solidus and hence the minimum pressure for magmatic epidote occurs at about 5 kbar the clapeyron slopes of epidote melting reactions are moderately positive in p t space at pressures below the intersection of the epidote melting and the gamet in reactions at 13 kbar 790 c at this intersection epidote reaches its maximal thermal stability 790 c in tonalite h20 in the presence of garnet that is above 14 kbar epidote melting reactions have steep negative clapeyron slopes in p t space at p t conditions near the low pressure intersection of the epidote melting reaction with the water saturated solidus experiments were also performed with f02 buffered by hm at these oxidizing conditions the magmatic stability field of epidote is enlarged down to a pressure of about 3 kbar in granodiorite the stability of epidote has been investigated at pressures from 7 5 to 18 kbar with respect to the tonalite the epidote out reaction is located almost at the same p t conditions at 10 kbar but the maximum thermal stability is about 50 c lower toward higher pressures although few experiments indicate that epidote stability does not increase dramatically with decreasing water activity at present state it is not possible to predict melting and crystallization behavior of epidote at waterundersaturated conditions modal abundances of epidote are low 05 vol in the presence of major 40 vol melt fractions thus epidote in calc alkaline magmatic rocks is not likely to affect major elements strongly by fractional crystallization

cenozoic ice volume history development of a composite oxygen isotope record to view cenozoic ice volume and ocean temperature history a composite 18 o time series was constructed for tropical shallow dwelling planktonic foraminifers and for deep water benthic foraminifers we conclude that cenozoic benthic 18 o primarily reflect considerable deep ocean temperature variation and that tropical planktonic 18 o afford the best record of cenozoic ice volume the planktonic composite indicates the presence of a significant ice budget for the past 40 m y the difference between the benthic and planktonic 18 o composites suggests that low latitude production of warm saline bottom water dominated tertiary thermohaline circulation

coesite and pure pyrope in high grade blueschists of the western alps a first record and some consequences a pyrope quartzite originally described by vialon 1966 from the dora maira massif was resampled and reinvestigated garnet up to 25 cm in size phengite kyanite talc and rutile are in textural equilibrium in an undeformed matrix of polygonal quartz the garnet is a pyrope almandine solid solution with 90 to 98 mol mg end member it contains inclusions of coesite which has partially inverted to quartz resulting in a typical radial cracking of the host garnet around the inclusions several lines of evidence show that coesite crystallised under nearly static pressure conditions and that the whole matrix has once been coesite

melting and subsolidus reactions in the system k2o cao al2o3 sio2 h2o corrections and additional experimental data melting relationships in the system k2o cao al2o3 sio2 h2o have been reinvestigated using schreinemakers analysis and hydrothermal experiments the reaction sanidine muscovite zoisite quartz vapor melt has been bracketed at 10 15 and 20 kbars and 670 680 680 690 and 690 700 c respectively and it marks the lowest solidus temperatures in the system investigated

origin of spectacular fields of submarine sediment waves around volcanic islands understanding how large eruptions and landslides are recorded by seafloor morphology and deposits on volcanic island flanks is important for reconstruction of volcanic island history and geohazard assessment spectacular fields of bedforms have been recognised recently on submerged flanks of volcanic islands at multiple locations worldwide these fields of bedforms can extend over 50 km and individual bedforms can be 3 km in length and 150 m in height the origin of these bedform fields however is poorly understood here we show that bedforms result from eruption fed supercritical density flows turbidity currents in some locations but most likely rotational landslides at other locations general criteria are provided for distinguishing between submarine bedforms formed by eruptions and landslides and emphasise a need for high resolution seismic datasets to prevent ambiguity bedforms associated with rotational landslides have a narrower source with a distinct headscarp they are more laterally confined and internal bedform structure does not suggest upslope migration of each bedform eruption fed density currents produce wide fields of bedforms which extend radially from the caldera internal layers imaged by detailed seismic data show that these bedforms migrated up slope indicating that the flows that produced them were froude supercritical due to the low density contrast between interstitial fluid and sediment the extent and dimensions of submarine eruption fed bedforms is much greater than those produced by pyroclastic density currents on land

petrology of amphibolite facies mafic and ultramafic rocks from the catalina schist southern california metasomatism and migmatization in a subduction zone metamorphic setting the catalina schist of southern california is a subduction zone metamorphic terrane it consists of three tectonic units of amphibolite high p greenschist and blueschist facies rocks that are structurally juxtaposed across faults forming an apparent inverted metamorphic gradient migmatitic and non migmatitic metabasite blocks surrounded by a meta ultramafic matrix comprise the upper part of the catalina amphibolite unit fluid rock interaction at high p high t conditions caused partial melting of migmatitic blocks metasomatic exchange between metabasite blocks and ultramafic rocks infiltration of silica into ultramafic rocks and loss of an albitic component from nonmigmatitic clinopyroxene bearing metabasite blocks partial melting took place at an estimated p 8 11 kbar and t 640 750 c at high h2o activity the melting reaction probably involved plagioclase quartz trondhjemitic melts were produced and are preserved as leucocratic regions in migmatitic blocks and as pegmatitic dikes that cut ultramafic rocks the metasomatic and melting processes reflected in these rocks could be analogous to those proposed for fluid and melt transfer of components from a subducting slab to the mantle wedge aqueous fluids rather than melts seem to have accomplished the bulk of mass transfer within the mafic and ultramafic complex

alpine metamorphism of calcareous metasediments in the western hohe tauern tyrol mineral equilibria in cohs fluids during tertiary regional metamorphism in the western hohe tauern reaching maximum p t conditions around 6 kb 550 c in calcareous metasediments reaction of pyrite to pyrrhotite is suggested by regional distribution and textural relations in rocks without graphite pyrite is common at all metamorphic grades in graphite bearing rocks however the dominant fe sulfide is pyrite at lower grade and pyrrhotite at higher grade furthermore in graphite bearing high grade rocks pyrite is restricted to assemblages with mg rich silicates several factors control pyrite pyrrhotite relations increase of temperature is most effective by increase of pyrrhotite vs pyrite stability field shift of silicate sulfide reactions toward the stability field of pyrrhotite creation of sulfur free fluids from devolatilization reactions and increase in the proportions of sulfur bearing fluid species presence of graphite also favours progress of pyrite to pyrrhotite reaction as shown by different f s 2 stabilities and changes in the amount of minerals and fluid during metamorphic heating of graphite bearing and graphite free assemblages an opposite effect is shown by assemblages with low fe contents in fe mg silicates due to the enlarged stability field of such minerals with increasing mg and f content another inhibition of pyrite to pyrrhotite reaction is suggested to be due to relatively high sulfur contents of h2o rich infiltrating fluids

magmatic muscovite in felsitic parts of rhyolites from southwest germany in felsitic parts of rhyolites from the permian volcanism in southwest germany muscovite phenocrysts of magmatic origin were observed they occur besides two other light mica components of secondary formation a permian tuff from the saarnahe area also contains this primary muscovite type

submarine pyroclastic deposits formed at the soufriere hills volcano montserrat 1995 2003 what happens when pyroclastic flows enter the ocean the soufriere hills volcano montserrat west indies has undergone a series of dome growth and collapse events since the eruption began in 1995 over 90 of the pyroclastic material produced has been deposited into the ocean sampling of these submarine deposits reveals that the pyroclastic flows mix rapidly and violently with the water as they enter the sea the coarse components pebbles to boulders are deposited proximally from dense basal slurries to form steep sided near linear ridges that intercalate to form a submarine fan the finer ash grade components are mixed into the overlying water column to form turbidity currents that flow over distances 30 km from the source the total volume of pyroclastic material off the east coast of montserrat exceeds 280 106 m3 with 65 deposited in proximal lobes and 35 deposited as distal turbidites

the saturation behaviour of pumice and some sedimentological implications abstract caldera forming explosive eruptions from rhyolitic volcanoes typically produce huge quantities of pyroclastic material blanketing the landscape and triggering long term voluminous and widespread sedimentary responses pumice is a major component of such eruptions but its behaviour in the sedimentary environment is little understood its most notable feature is its very low density which frequently enables pumice to float on water for long periods of time before eventually waterlogging enough to sink once fully saturated pumice behaves in a broadly similar way to normal clastic material in terms of its hydrodynamic behaviour modified by its lower density however relatively little is known about its earlier behaviour in the sedimentary environment and in particular the manner and rate by which initially dry pumice absorbs water in this paper we develop a mathematical model which predicts that the time required for a pumice clast to saturate is proportional to the square of its radius tests on samples of real pumice accord with predicted trends and match lacustrine pumiceous facies observed in the field the ability to predict the time required for large pumice clasts to sink may provide a tool for estimating the timescales over which certain rapid post eruptive sedimentary processes operate

depositional processes of primary to reworked volcaniclastics on an alluvial plain an example from the lower pliocene ohta tephra bed of the tokai group central japan abstract a change from primary to reworked volcaniclastic deposits in the lower pliocene ohta tephra bed has been identified using facies analysis the ohta tephra bed is a widespread marker tephra distributed over a distance of more than 130 km the source volcano of this tephra is inferred to lie 50 to 100 km north of the most proximal ohta tephra bed the ohta tephra bed shows a change in volcaniclastic facies in distal areas eight sedimentary facies are recognized these are interpreted as pyroclastic fall pyroclastic flow and reworked mudflow flood flow channel fill swamp or backswamp lacustrine and ephemeral pond deposits in underlying and overlying non volcaniclastic beds four sedimentary facies are recognized indicating a fluvio lacustrine depositional system on an alluvial plain from the successions of volcaniclastic sedimentary facies deposition of the ohta tephra bed can be divided into stages i to vi each depositional stage is intimately related to volcanic activity stage i is the first eruption during which only the northern part of the study area was covered with fallout ash the second eruption stage of stage ii involved fallout ash distributed over all the study area these two fallout ash beds are derived from precursor eruptions stage iii reflects a period of volcanic quiescence and development of widespread lacustrine conditions the fallout ash may have destroyed vegetation and prompted greater discharge to form the large lacustrine deposits of stage iii a pyroclastic flow stage in stage iv is associated with an explosive eruption and resulted in pyroclastic flow deposits reaching the northern part of the study area stage v is characterized by mudflow deposits widespread mudflow deposits also originated from high water and clastics discharge on the unvegetated surface mudflows extend more than 180 km from the source volcano abundant supply of volcaniclastic material and high water and clastic discharge enhanced a rapid progradation of alluvial plains final reworking occurred during stage vi when volcaniclastic sediments were deposited from a fluvio lacustrine system the reworking system comprises initial mudflows followed by a normal fluvial system normal stream flow the mudflow deposits of stage v are traceable with roughly constant thickness but the volcaniclastic content of the deposits decreases in distal areas the fluvial system deposits of stage vi are thicker and have less abundant volcaniclastics in distal areas fluvial systems were braided streams promoted by volcanic eruption while they were meandering streams during the pre eruption period some eruption induced braided streams remained for a while following the cessation of volcanic activity

quenching of steam charged pumice implications for submarine pyroclastic volcanism abstract huge quantities of silicic pumice have been deposited in intra oceanic convergent margin settings throughout earth s history the association of submarine silicic calderas with thick proximal accumulations of pumice lapilli suggests that these pyroclasts were deposited as a direct result of submarine eruptions yet when first erupted these highly vesicular gas filled clasts had densities significantly less than seawater experiments carried out 1 atm on heated pumice samples whose vesicles were charged with steam the dominant component of magmatic volatiles show that buoyancy of freshly erupted submarine pumice is transient upon quenching the phase change of steam to liquid water creates strong negative pore pressures within the pumice vesicles that accelerate the absorption of surrounding water generating high density pumice and promoting rapid clast sinking variations in the physical properties of steam with temperature and pressure have important implications for submarine pyroclastic eruptions firstly highly vesicular pumice can be deposited on the seafloor at temperatures elevated significantly above ambient if they are erupted at sufficient depths to remain wholly submarine 200 m and either the fluid in which they cool contains heated water and or they only absorb sufficient water to sink secondly the rapid increase in density of the eruption column caused by condensation and the transition from buoyant gas filled to denser water saturated pumice lapilli together with turbulent mixing with the surrounding seawater favour collapse and transport of pyroclasts in water supported gravity currents finally this mixing of the ejecta with seawater and the ease of water ingestion into permeable pumice clasts suggest that water supported transport mechanisms can operate as primary dispersal processes in explosive submarine eruptions

melting and subsolidus reactions in the system k2o cao mgo al2o3 sio2 h2o experiments and petrologic application experiments with synthetic starting materials of muscovite phlogopite zoisite kyanite and quartz were performed in the pressure temperature range 10 25 kbar 640 780 c under water excess conditions the reaction muscovite zoisite quartz vapor liquid kyanite was bracketed at 10 5 kbar 689 700 c 15 5 kbar 709 731 c and 20 kbar 734 745 c the equivalent reaction in the mg bearing system muscovitess zoisite quartz vapor liquid kyanite phlogopitess lies at the same temperature around 10 kbar and approximately 10 c higher around 20 kbar compared with the mg free reaction at slightly higher temperatures formation of melt and tremolitess was reversibly observed from the assemblage phlogopitess zoisite kyanite quartz around 10 5 kbar 690 710 c 15 5 kbar 720 750 c and 20 5 kbar 745 760 c in the subsolidus region the reaction muscovitess talcss tremolitess phlogopitess zoisite quartz vapor were located in the range 700 c 16 7 19 0 kbar and 740 c 19 7 20 8 kbar from these data a wedge shaped stability field of phlogopitess zoisite quartz appears with a high p t termination around 21 kbar 755 c muscovite tremolite talc or kyanite comes in at higher pressures these phase relations are in qualitative accord with petrographic observations from high pressure metamorphic areas formation and crystallization of melts in rocks of a wide compositional range involving zoisite epidote has been ascribed to relatively high pressures and is consistent with experimentally determined stability fields in the simplified kcmash system

resedimentation of cold pumiceous ignimbrite into water facies transformations simulated in flume experiments deposits and transport processes resulting from the resedimentation of cold unconsolidated ignimbrite into water were simulated by flume experiments the ignimbrite sample used was poorly sorted 2 4 3 fine ash rich 2000 kg m 3 and pumice 500 to ca 1300 kg m 3 as a result of the binding forces of the ash matrix the experiments involved resedimentation from a steep front onto the floor with or without an initial ramp of the water filled tank under both still and wave generated conditions larger discrete collapse events were induced by oversteepening the sample front and by undercutting from wave action the mass of the collapse and proportion of pore space water strongly influenced the style of resedimentation and the deposits initial collapse events were from the top of the steep front and fell onto the floor the largest densest clasts were deposited as a lithic lag in a proximal sediment wedge or rolled down to a break in slope fine ash was transported in dilute turbidity currents and coarse unsaturated pumice clasts floated off moderate collapse events generated high density turbidity currents trapping pumice in the flow causing them to saturate these low density pumice clasts were easily remobilized by wave activity and passing currents and accumulated on the gentle slope at the bottom of the resedimented deposit large collapse events slumped producing poorly sorted mounds similar in texture to the original starting material as the matrix of the ignimbrite sample became saturated with water moderate and large collapse events generated debrisflows and slurries that deposited massive poorly sorted deposits furthermore once more gentle slopes were established between the sample and deposit small cascading grainflows deposited lithic clasts on the upper slopes and levees of pumice at the terminus of low relief ash channels the experiments show that excluding large collapse events and debrisflows resedimenting ignimbrite in water is effective at segregating low density pumice clasts from dense lithic clasts and fine ash experiments using fine ash poor ignimbrite and well sorted quartz sand for comparison formed an inherently unstable initial steep front that immediately collapsed by continuous grain avalanches the grainflow deposits had textures similar to the fines poor starting material

syneruptive and intereruptive lithofacies in lacustrine environments the cretaceous beolkeum member wido island korea abstract the cretaceous beolkeum member was deposited in a lacustrine environment affected by explosive volcanism and a number of syneruptive lithofacies are intercalated with lacustrine mudstones the syneruptive lithofacies were formed by primary volcanic processes and resedimented depositional processes during and after eruptions and exhibit unique depositional features which can be grouped into four syneruptive lithofacies assemblages sla 1 to sla 4 from bottom to top primary syneruptive lithofacies are represented by welded massive lapilli tuff in sla 1 and a normally graded lapilli tuff in sla 2 in sla 1 sustained pyroclastic density currents were able to displace lake water and rarely interacted with lake water resulting in the formation of a welded texture and columnar joints in sla 2 unsteady pyroclastic density currents rapidly disintegrated mixed with ambient water and transformed into high density turbidity currents depositing thick normally graded lapilli tuff resedimented syneruptive lithofacies were primarily controlled by volcaniclastic sediment supply after eruptions when volcaniclastic sediments were continuously supplied into the lake by debris flows a volcaniclastic fan can be formed sla 1 resulting in a coarsening upward trend and progradational geometry in case of relatively small amounts of volcaniclastic sediment supply turbidity currents would be a main depositional process depositing a series of normally graded tuff on the primary syneruptive lithofacies sla 2 showing a fining upward trend in sla 3 and sla 4 there are only a few cm thick normally graded tuff reflecting minor volcaniclastic sediment supply however overlying normally graded sandstones in sla 3 showing a coarsening upward trend and a thick normally graded sandstone in sla 4 suggest favorable conditions for the generation of the sediment gravity flows probably due to an increase in volcaniclastic sediment supply after eruptions

effects of water depth on pumice formation in submarine domes at sumisu izu bonin arc western pacific domes of the sumisu volcanic complex western pacific having summits at ocean depths of 1100 600 245 and 95 m are mantled with compositionally identical rhyolitic pumice that has similar vesicularity but that varies systematically in distribution size and surface texture suggesting that facies and morphology can serve as useful indicators of eruption depth at depths 500 m the pumice formed a thick carapace on dense rhyolite and disintegrated by quench fracture and mechanical failure into a jumble of giant meters to tens of meters polyhedral blocks with smooth curviplanar surfaces that display a single quenched margin vesiculation was arrested on eruption in seawater in all but the interior of the thickest carapace at 500 m depths the pumice occurs as an apron of blocky giant and smaller rough textured clasts enclosed by quenched margins and pockmarked by coarse cm vesicles no carapace pumice occurs and the summit is composed of craggy dense dome rock these shallower water pumice clasts resemble those spalled from historic submarine dome forming eruptions that buoyed to the sea surface we interpret spalling to result from vent derived weak volatile driven explosions that take place at water depths 500 m our study shows that an increase in hydrostatic pressures over a range of 12 mpa reduces volatile driven explosivity for subaqueous rhyolitic dome forming eruptions but does not affect vesicularity we conclude that meter size highly vesicular pumice is diagnostic of subaqueous dome eruptions in water depths of at least 1300 m and its morphology can be used to distinguish between explosive and effusive origins

an oxygen buffer for some peraluminous granites and metamorphic rocks the mineral assemblage biotite garnet muscovite magnetite quartz and its subsets are common in many peraluminous granites schists and gneisses if the biotite and garnet are reasonably iron rich then the system is a useful buffer for oxygen fugacity available thermochemical data indicate that in temperature oxygen fugacity space the buffer curve is located between the hematite magnetite curve and the quartz magnetite fayalite curve in a region that previously had no buffer curve applicable to peraluminous rocks the buffer equation partially taking into account the departures of biotite garnet and muscovite compositions from their end member formulae is logfo2 10 29 262847 t 0 148 p l r atog x3l 3log y 3log ff 2log 4lni 4log xgi 1l 650 t examples of applications are given for three peraluminous granitic rocks and three pelitic schists the calculated oxygen fugacity values corrected for mineral compositions generally agree with published estimated values based on the compositions of coexisting iron and titanium oxide phases

quantifying time scales of pedogenic calcrete formation using u series disequilibria abstract time scales of pedogenic calcrete development are quantified by subsampling carbonate from within a mature stage v pedogenic calcrete profile from southeast spain and dating the material by u series disequilibria the location of the earliest and latest cements can be estimated by comparing previous studies of calcrete morphological development with micromorphological analysis of the study profile carbonate was sampled and dated from three locations within the profile 1 below the lower surface of clasts within the hardpan representing the earliest cement present 207 11 ka 2 from the centre of cement filled pores within the hardpan reflecting the final plugging of the calcrete hardpan 155 9 ka and 3 from the laminar calcrete overlying the hardpan representing the latest cement 112 15 ka these results show that the hardpan took between 73 and 31 ka to form whilst the mature stage v profile took between 121 and 69 ka to form this is the first time that rates of mature calcrete development have been established by direct radiometric dating of the authigenic carbonate the technique is appropriate for dating mature calcretes in dryland regions worldwide and offers the opportunity of increasing our understanding of the spatial and temporal variability in rates of pedogenic calcrete development

miocene fluvial systems and palynofloras at the southwestern tip of africa implications for regional and global fluctuations in climate and ecosystems abstract high amplitude climate fluctuations have been inferred from marine isotope data in the early neogene but few well documented terrestrial records exist from this era to gauge the effects of these high latitude events on continental climates and ecosystems the extensive three dimensional exposures of miocene fluvial and fluvio lacustrine sediments in the rondeberg clay pit near cape town provide a unique window on this era palaeomagnetic data suggests that the deposits accumulated over a period of ilex type euphorb type rhamnaceae and mesothermic podocarpus type palynofloras suggests a humid subtropical tropical climate however abundant charcoal charred in situ tree stumps overall poor preservation of organics evidence for upward drying lacustrine successions and an appreciable fynbos presence point to cyclical periods of drought we suggest that these climate fluctuations may have been influenced by the orbital pacing seen in the marine isotope record of the earlier miocene pointing to a high latitude link with mid latitude terrestrial climate patterns earlier studies of pollen spectra from the nearby slightly older noordhoek deposits show cyclical alternations from tropical to cooler climates and more recent biogeochemical work has shown dramatic coincident fluctuations in depositional temperature these vegetation changes were previously correlated with major global events embracing the entire neogene from the oligo miocene late oligocene to early miocene to the pliocene we offer a different interpretation suggesting that the deposits represent a much shorter time interval in the earlier miocene and that these climate fluctuations may have been influenced by orbital forcing evinced in the marine isotope record along the northern west coast the arrisdrift vertebrate fossil assemblage in early middle miocene terrace deposits of the orange river indicate a tropical climate but possibly less humid than in the south with more open vegetation patterns the presence of pedogenic calcretes and gypcretes in the deposits suggests periodic extremes of aridity not seen in south the current pronounced north aridity gradient from humid temperate to hyper arid may have had its inception in the earlier miocene

late pleistocene stratigraphy of iodp site u1396 and compiled chronology offshore of south and south west montserrat lesser antilles marine sediments around volcanic islands contain an archive of volcaniclastic deposits which can be used to reconstruct the volcanic history of an area such records hold many advantages over often incomplete terrestrial data sets this includes the potential for precise and continuous dating of intervening sediment packages which allow a correlatable and temporally constrained stratigraphic framework to be constructed across multiple marine sediment cores here we discuss a marine record of eruptive and mass wasting events spanning 250 ka offshore of montserrat using new data from iodp expedition 340 as well as previously collected cores by using a combination of high resolution oxygen isotope stratigraphy ams radiocarbon dating biostratigraphy of foraminifera and calcareous nannofossils and clast componentry we identify five major events at soufriere hills volcano since 250 ka lateral correlations of these events across sediment cores collected offshore of the south and south west of montserrat have improved our understanding of the timing extent and associations between events in this area correlations reveal that powerful and potentially erosive density currents traveled at least 33 km offshore and demonstrate that marine deposits produced by eruption fed and mass wasting events on volcanic islands are heterogeneous in their spatial distribution thus multiple drilling coring sites are needed to reconstruct the full chronostratigraphy of volcanic islands this multidisciplinary study will be vital to interpreting the chaotic records of submarine landslides at other sites drilled during expedition 340 and provides a framework that can be applied to the stratigraphic analysis of sediments surrounding other volcanic islands

geochemistry and palaeoclimatic reconstruction of a palaeosol sequence at langebaanweg south africa abstract neogene langebaanweg lbw palaeosol sediment sequences at the west coast of south africa are well known for their well preserved rich palaeontological heritage the palaeosols developed on estuarine marine coastal alluvial deposits stacked on each other ranging in age from the mid miocene to quaternary elemental mobility and redistribution based on major and minor element geochemistry of 11 horizons of the 18 m sequences were studied to evaluate the degree of chemical weathering a major process of soil formation and to reveal important palaeoenvironments in which the palaeosols developed elemental geochemistry was carried out on 2 50 mean wt two wi 1 and wi 2 of the four weathering indices applied to evaluate weathering intensity proved unsuitable for use in an alluvial palaeosol sediment sequences such as lbw chemical index of alteration cia and chemical index of weathering ciw values were relatively more consistent in distribution pattern across the pedocomplex but caution should be applied when using them in similar cases the pedogenic carbonate horizons on average had the lowest values for both cia and ciw 1 12 2 60 suggesting more advanced weathering plots of geochemical ratios quantifying pedogenic processes against depth show striking variations among and within the palaeosol and sediment sections soil formation was more advanced in the mid miocene and pliocene than in the quaternary suggesting a more humid and warmer climate than present we evaluated selected geochemical climofunctions and conclude that more data are obviously needed for a more robust calibration of the models in langebaanweg palaeosols

submarine deposits from pumiceous pyroclastic density currents traveling over water an outstanding example from offshore montserrat iodp 340 pyroclastic density currents have been observed to both enter the sea and to travel over water for tens of kilometers here we identified a 1 2 m thick stratified pumice lapilli ash cored at site u1396 offshore montserrat integrated ocean drilling program iodp expedition 340 as being the first deposit to provide evidence that it was formed by submarine deposition from pumice rich pyroclastic density currents that traveled above the water surface the age of the submarine deposit is ca 4 ma and its magma source is similar to those for much younger soufriere hills deposits indicating that the island experienced large magnitude subaerial caldera forming explosive eruptions much earlier than recorded in land deposits the deposit s combined sedimentological characteristics are incompatible with deposition from a submarine eruption pyroclastic fall over water or a submarine seafloor hugging turbidity current derived from a subaerial pyroclastic density current that entered water at the shoreline the stratified pumice lapilli ash unit can be subdivided into at least three depositional units with the lowermost one being clast supported the unit contains grains in five separate size modes and has a 12 phi range particles are chiefly subrounded pumice clasts lithic clasts crystal fragments and glass shards pumice clasts are very poorly segregated from other particle types and lithic clasts occur throughout the deposit fine particles are weakly density graded we interpret the unit to record multiple closely spaced 2 d hot pyroclastic density currents that flowed over the ocean releasing pyroclasts onto the water surface and settling of the various pyroclasts into the water column our settling and hot and cold flotation experiments show that waterlogging of pumice clasts at the water surface would have been immediate the overall poor hydraulic sorting of the deposit resulted from mixing of particles from multiple pulses of vertical settling in the water column attesting to complex sedimentation slow settling particles were deposited on the seafloor together with faster descending particles that were delivered at the water surface by subsequent pyroclastic flows the final sediment pulses were eventually deflected upon their arrival on the seafloor and were deposited in laterally continuous facies this study emphasizes the interaction between products of explosive volcanism and the ocean and discusses sedimentological complexities and hydrodynamics associated with particle delivery to water

high pressure metamorphism in mafic schist from northern vermont the only confirmed occurrence of glaucophane and omphacite in the appalachian mountains of new england is in mafic schist from tillotson peak 44 48 72 33 north central vermont mineral assemblages observed all with epidote sphene magnetite phengitic muscovite carbonate calcite or dolomite apatite sulfide are 1 actinolite glaucophane chlorite garnet quartz 2 actinolite glaucophane chlorite albite garnet 3 actinolite glaucophane omphacite garnet 4 glaucophane chlorite garnet quartz 5 glaucophane omphacite quartz 6 albite glaucophane chlorite quartz and 7 albite paragonite chlorite quartz electron microprobe investigations show that plagioclase chlorite sphene white mica carbonate and magnetite grains are relatively homogeneous however amphibole pyroxene epidote and garnet grains are zoned indicating incomplete equilibration and changing physical conditions during metamorphism glaucophane in assemblage 6 has the general formula rim composition na 1 9 m4 ca 0 1 mg 1 8fe 1 2 2 al 1 0 vi fe 0 3 3 si 7 9 al 0 1 iv o 22 oh 2 glaucophane in assemblages 1 to 5 is somewhat poorer in the glaucophane endmember and richer in actinolite ca rich amphibole grains are zoned extensively with barroisite cores and actinolite rims omphacite grains in assemblages 3 and 5 have the general formula rim composition na 0 5 ca 0 5 mg 0 4 fe 0 1 2 al 0 4 vi fe 0 1 3 si 2o 6 in spite of the significant zoning the partitioning of elements among the minerals at their grain margins suggests that all seven assemblages above may coexist at the same physical conditions of metamorphism in mafic rocks with a relatively small range in bulk composition correlation of mineral compositions and assemblages with mineral stability data suggests high pressure facies series metamorphism at about 9 2 kb and 450 100 c with p h2o p total it is proposed that the metamorphism occurred during the ordovician

characteristics for the recognition of pliocene and early pleistocene marker tephras in central japan abstarct numerous widespread tephra layers of late pleistocene and holocene age have been known since the early 1970s and greatly contribute to palaeoenvironmental reconstruction in the japan islands and adjacent seas however the mapping of older tephra deposits has developed more slowly recent progress in describing many tephras occurring in the pliocene and early pleistocene uplifted marine and terrestrial deposits has resulted both from developments in the characterization of their component phases their radiometric and geomagnetic dating and from an enhanced knowledge of their distribution this study has constructed tephra correlations using the trace element composition of volcanic glass determined by icp aes analysis consequently many widespread tephra layers of pliocene and early pleistocene age have been identified and can now be correlated extensively in central japan this paper catalogs these tephras and provides the fundamental criteria for the establishment of a regional pliocene pleistocene chronostratigraphy in central japan

emplacement processes of submarine volcaniclastic deposits iodp site c0011 nankai trough during iodp nantroseize expedition 322 four packages of tuffaceous sandstones tst 1 2 3a 3b were recovered within a moderately lithified and bioturbated silty claystone succession in the late miocene 7 6 to 9 1 ma upper part of the middle shikoku basin deposits to assess the emplacement processes of the tuffaceous sandstones we investigate modal and geochemical compositions of 24 thin sections that reveal systematic vertical changes within each bed tst 1 2 and 3b are single beds whereas tst 3a is composed of at least two beds suggesting several rapidly succeeding sedimentation events the beds are density graded such that low density pyroclasts including pumice lapilli are enriched at the top whereas dense lithic components and minerals are enriched at the bottom the volcanic glass particles pumice and shards that are the dominant modal constituents of each sandstone bed have homogeneous geochemical compositions in each bed moreover tst 1 2 and 3a glass compositions overlap completely but tst 3b glass has a different composition as is analogously observed for the mineral compositions this unique multistage approach of sedimentological and geochemical methods facilitated the detailed investigation of distal volcano derived probably tsunamogenic turbidites in order to contribute to the distinction between primary and secondary induced mass flows we interpret that all tuffaceous sandstones were emplaced by turbidity currents that were formed during major explosive volcanic eruptions however while tst 1 2 and 3a turbidity currents formed by the entry into the ocean of voluminous pyroclastic flows erupted at a volcano on mainland japan tst 3b was emplaced by a turbidity current formed by a shallow submarine or subaerial volcanic eruption at the izu bonin arc where it collided with japan these results regarding distal turbidites encourage the revisiting of older marine deposits in the scope of hazard evaluation through past events especially in regions near to volcanic sources

geothermometry geobarometry and the al2sio5 triple point at mt moosilauke new hampshire the distribution of aluminum silicate minerals in pelitic schists constrains pressure and temperature conditions during the acadian orogeny to have been approximately those of the al 2 sio 5 invariant point thus providing an excellent opportunity to evaluate the internal consistency of a variety of published geothermometers and geobarometers with the al 2 sio 5 invariant point of holdaway 1971 four fluid independent equilibria yield p t estimates consistent with holdaway 1971 two fluid dependent equilibria yield internally consistent but unrealistically high temperatures our data are consistent with the conclusions that 1 only pyrope grossular mixing in the quaternary garnet system is significantly non ideal and 2 the activity coefficient for the anorthite component in plagioclase is approximately 2 at triple point p t conditions modified journal abstract

the upper thermal stability of chlorite quartz an experimental study in the system mgo al2o3 sio2 h2o experiments up to water pressures of 21 kbar have been undertaken to bracket the reactions chlorite quartz talc kyanite h2o chlorite quartz talc cordierite h2o and talc kyanite quartz cordierite h2o by reversed runs in the system mgo al2o3 sio2 h2o mash these reaction curves intersect at an invariant point ip1 at ph2o 6 4 0 2 kbar and a temperature of 624 4 c the curve of the chlorite quartz breakdown to talc kyanite h2o at water pressures above 6 4 kbar shows a negative dp dt with the slope decreasing with rising pressure whereas the slope of the breakdown curve to talc cordierite h2o at water pressures is clearly positive the composition of the chlorite solid solution reacting with quartz has been estimated to be approximately mg4 85al1 15 al1 15si2 85o10 oh 8 over the entire pressure range investigated the composition of the talc solid solution forming by the breakdown of chlorite quartz appears to be mg2 94al0 06 al0 06si3 94o10 oh 2 at ph2o 2kbar with increasing pressure the al content of talc decreases reaching a value of about 0 06 atoms per formula unit at p h2o 21 kbar as a consequence of the new experimental data the existing phase topologies of the mash system and k2o mash system have been revised for example the invariant point ip1 and the univariant reaction curve kyanite talc h2o chlorite cordierite are stable for this reason the development of medium to high temperature metamorphic rocks compositionally approximating the mash system must be reconsidered the whiteschists from sar e sang afghanistan are treated as an example the application of the present experimental data to metamorphic rocks of more normal composition requires the examination of the influence of further components this leads to the conclusion that the introduction of fe2 into magnesian chlorite extends its stability field in the presence of quartz by 10 15 c in comparison with pure mg chlorite

facies architecture of a continental below wave base volcaniclastic basin the ohanapecosh formation ancestral cascades arc washington usa the 800 m thick oligocene ohanapecosh formation records voluminous sedimentation of volcanic clasts in the ancestral cascades arc washington state usa most volcaniclastic beds are dominated by angular pumice clasts and fiamme of andesitic composition now entirely devitrified and altered all beds are laterally continuous and have uniform thickness fine sandstone and mudstone beds have features typical of deposits from low density turbidity currents and suspension settling erosion surfaces cross beds and evidence of bi directional oscillatory currents i e wave ripples and swaley and hummocky cross stratification are almost entirely absent we infer that the setting was subaqueous and below wave base the abundance of angular pumice clasts crystals and dense volcanic clasts and the extreme thickness of several facies suggest they were derived from magmatic volatile driven explosive eruptions the extremely thick beds are ungraded or weakly graded and lack evidence of hot emplacement suggesting deposition from subaqueous water supported high concentration volcaniclastic density currents some of the thickest beds contain coarse rounded dense clasts at their base and are interbedded with accretionary lapilli bearing mudstone these beds are interpreted to be deposits from subaqueous density currents fed by subaerial pyroclastic flows that crossed the shoreline shallow basaltic intrusions and mafic volcanic breccia composed of scoria lapilli indicate the presence of intra basinal scoria cones that may have been partly subaerial the range in facies in the ohanapecosh formation is typical of below wave base continental lacustrine basins that form in proximity to active volcanic arcs and includes eruption fed and resedimented facies extreme instantaneous aggradation rates are related directly to explosive eruptions and sediment pathways reflect the locations of active volcanoes in contrast to conventional sedimentation processes acting in non volcanic environments

source mechanisms of volcanic tsunamis volcanic tsunamis are generated by a variety of mechanisms including volcano tectonic earthquakes slope instabilities pyroclastic flows underwater explosions shock waves and caldera collapse in this review we focus on the lessons that can be learnt from past events and address the influence of parameters such as volume flux of mass flows explosion energy or duration of caldera collapse on tsunami generation the diversity of waves in terms of amplitude period form dispersion etc poses difficulties for integration and harmonization of sources to be used for numerical models and probabilistic tsunami hazard maps in many cases monitoring and warning of volcanic tsunamis remain challenging further technical and scientific developments being necessary and must be coupled with policies of population preparedness

late mesozoic and tertiary sediment supply to the eastern cape basin se atlantic and palaeo drainage systems in southwestern africa isopach asymmetry and sediment component changes in dsdp cores from the se atlantic orange basin support the hypothesis of major drainage system changes in sw africa during late cretaceous cenozoic time this involved alternations in the use of the 28 s modern orange river and 31 s modern olifants river exit points across the western escarpment by rivers carrying run off from the upper orange vaal catchment areas as well as radical re organizations of internal drainage geometry it is postulated that during late cretaceous times the 28 s exit was used with the middle orange river following a course in the interior well to the south up to 150 km of its modern channel sediment discharge rates from this river were relatively high at least 10 106 m3 yr 1 and resulted in rapid advancement of the continental margin sediment prism west of the mouth by large scale slumping the palaeogene orange vaal river exit was via the 31 s escarpment crossing and during the later part of this period the cape canyon was cut across the continental shelf and slope a significant reduction in sediment discharge to 2 0 106 m3 yr 1 suggests that the lower tertiary climate for sw africa was drier than that of late cretaceous times however aridity did not commence until late miocene times when the orange vaal discharge had switched back to the 28 s exit modern sediment discharge rates 6 5 106 m3 yr 1 are relatively high and reflect soil erosion caused by agricultural activity the two major alterations in exit point of the orange vaal late cretaceous early tertiary and late oligocene early miocene are related to periods of low sea level which promoted river capture adjacent to the western escarpment an additional factor in the first course change may have been the disruption of the middle orange channel by late cretaceous igneous intrusions less important internal reorganizations of the drainage system are postulated in late miocene pleistocene times economic implications for offshore diamond distribution are briefly mentioned

river history and tectonics the analysis of crustal deformation by tectonic processes has gained much from the clues offered by drainage geometry and river behaviour while the interpretation of channel patterns and sequences benefits from information on earth movements before or during their development the interplay between the two strands operates at many scales themes which have already benefited from it include the possible role of mantle plumes in the breakup of gondwana the cenozoic development of drainage systems in africa and australia himalayan uplift in response to erosion alternating episodes of uplift and subsidence in the mississippi delta buckling of the indian lithospheric plate and changes in stream pattern and sinuosity along individual alluvial channels subject to localized deformation developments in remote sensing isotopic dating and numerical modelling are starting to yield quantitative analyses of such effects to the benefit of geodymamics as well as fluvial hydrology

epidote phenocrysts in dacitic dikes boulder county colorado epidote ps21 crystallized early as elongate phenocrysts in late cretaceous rhyodacitic dikes in the vicinity of ward boulder county colorado other unusual phenocryst phases are garnet gr17 24 and muscovite in a xenolith containing kyanite corundum biotite and plagioclase magmatic garnet grew as a rim around xenocrystic pyrope rich py37 garnet the xenolith was derived from a granulite facies zone not represented at the present day erosion surface which is composed of upper amphibolite facies cordierite and sillimanite bearing gneisses the dike magmas were fed not from an immediately underlying batholith but from a magma chamber at a depth corresponding to a pressure of 8 13 kilobars phenocrysts cystallized in the temperature range 800 to 700 c under h2o and o2 activities greater than normal for silicic magmas this occurrence shows convincingly not only that epidote can be magmatic but that it is a possible early crystallization phase in silicic magmas

the zambezi delta mozambique channel east africa high resolution dating combining bio orbital and seismic stratigraphies to determine climate palaeoprecipitation and tectonic controls on a passive margin abstract the zambezi delta draining the southern african plateau and the southern part of the east african rift is one of a the largest delta of africa with a long lasting history starting during early cretaceous with more than 12 km of sediments deposited the zambezi delta is therefore a unique archive of the past topographic evolution of southern and eastern africa and their related deformations but also of the climate changes global and regional consequences of local topographic growths understanding this archive supposes to get a high resolution dating of the sediments our two objectives are here 1 to construct an age model of the zambezi cenozoic delta using a combination of biostratigraphy orbital stratigraphy and sequence stratigraphy and 2 to determine the palaeoprecipitation variations of the zambezi catchment from the oligocene to present day in a known tectonic framework the neogene sequences were dated at high resolution assuming that the third order sequences are of eustatic origin and record long term eccentricity cycles the sequences were correlated in ages on the calculated earth orbital solutions of laskar for the time intervals provided by the biostratigraphy nannofossils planktonic foraminifers the palaeoprecipitation record was based on the definition of a humidity index based on pollen analysis and associated botanical associations the late oligocene was a quite wet period getting dryer in the uppermost chattian the base tortonian 11 ma was a humid period the messinian was a dry period with a slight increase of the humidity during the zanclean and a sharp increase around the zanclean piacenzian boundary the zambezi delta has recorded the uplifts of the southern african plateau around 85 ma and around 25 ma and those of the southward migration of the east african rift since 5 5 ma

water settling and resedimentation of submarine rhyolitic pumice at yali eastern aegean greece abstract the yali pumice breccia is a very thick 150 m quaternary succession of submarine pumice that has been uplifted and exposed in the southern part of yali island in the eastern aegean greece the pumice breccia comprises moderately to well sorted 0 03 3 m thick beds of loose pumice clasts that are poor in fine matrix predominantly the larger pumice clasts cobbles and boulders are prismatic with quenched margins and internal polyhedral joints in comparison the smaller pumice clasts are polyhedral angular to subrounded blocky pebbles and granules these smaller clasts lack quenched margins and have sharp curviplanar surfaces that cross cut vesicle boundaries we interpret the larger pumice clasts to be the products of spalling and explosive fragmentation of a small extrusion of submarine pumiceous lava the smaller pumice clasts were generated by a combination of 1 passive and explosive disintegration of the larger pumice clasts and 2 phreatomagmatic explosions phreatomagmatic explosions also generated a minor component of non or poorly vesicular juvenile and basement derived lithic clasts the yali pumice breccia includes four major facies cobble boulder pebble mixed cobble dominant and mixed pebble dominant the cobble boulder facies occurs in well sorted massive tabular beds of large 64 mm 1 5 m pumice up to 3 m thick that lack both lithic clasts and fine matrix this facies has textural and lithological features consistent with deposition of large pumice clasts by means of water settling from suspension the three remaining facies are comparatively less well sorted although still fines poor and include rare lithic clasts they exhibit massive or internally diffusely stratified wedging 0 1 2 m thick beds of coarser grained medium pebble to cobble pumice clasts that onlap or are separated by wedge shaped thinner

mihi breccia a stack of lacustrine sediments and subaqueous pyroclastic flows within the taupo volcanic zone new zealand abstract the taupo volcanic zone tvz new zealand encompasses a wide variety of arc related strata although most of its small volume non caldera forming eruptions are poorly exposed and extensively hydrothermally altered the mihi breccia is a stratigraphic sequence consisting of interbedded rhyolitic pyroclastic flows and lacustrine sediments with eruption ages of 281 18 to at least 239 6 ka uncertainties at 2 in contrast to other small volume rhyolitic eruptions within the tvz mihi breccia is relatively well exposed within the paeroa fault block and contains minimal hydrothermal alteration pyroclastic flow characteristics and textures include 1 prismatically jointed juvenile clasts 2 lack of welding 3 abundant ash rich matrix 4 lack of fiamme and eutaxitic textures 5 lack of thermal oxidation colors 6 lack of cooling joints 7 exclusive lacustrine sediment lithic clasts and 8 interbedding with lacustrine sediments all indicating that mihi breccia strata originated in a paleo lake system this ephemeral paleo lake system is inferred to have lasted for 50 kyr based on mihi breccia age constraints and referred to as huka lake mihi breccia pyroclastic flow juvenile clast geochemistry and petrography correspond with similar aged 264 8 263 10 and 247 4 ka intra caldera rhyolite domes filling the reporoa caldera source of the 281 ka kaingaroa formation ignimbrite these exposed intra caldera rhyolite domes as well as geophysically inferred subsurface domes are proposed to be source vents for the mihi breccia pyroclastic flows soft sediment deformation associated with mihi breccia strata indicates either seismic shock rapid sediment loading during pyroclastic flow emplacement or both thus the mihi breccia reflects a prolonged series of subaqueous rhyolite dome building and associated pyroclastic flows accompanied by seismic activity emplaced into a large paleo lake system within the tvz

implications of magmatic epidote bearing plutons on crystal evolution in the accreted terranes of northwestern north america magmatic epidote in a plutonic rock signifies that the rock probably crystallized at pressures of at least 8 kbar 25 30 km depth in western north america undeformed late cretaceous plutons containing magmatic epidote occur within a mobile belt extending from northern california to southeastern alaska this belt has been interpreted as consisting of accreted terranes because the present crustal thickness within this belt is about 30 km the plutons imply a crustal thickness at the time of emplacement of 55 60 km comparable to that in the central andes the plutons further imply paleogeothermal gradients in the host country rock at the time of intrusion of not more than 20 c km and may be closer to 12 c km such cool thick crust suggests that subduction and piling of thrust sheets were probably the principal processes of crustal accretion in western north america the implications regarding the depth of crystallization of the plutons can be used to estimate the minimum rate of regional uplift it is on the order of 1 mm yr the rapid uplift of the mobile belt by and large did not affect the craton

vesiculation and fragmentation history in a submarine scoria cone forming eruption an example from nishiizu izu peninsula japan an uplifted 50 m thick half dissected submarine emplaced below wave base scoria cone occurs as dipping beds in coastal outcrops at nishiizu on the izu peninsula in japan concentrically outward dipping weakly stratified ungraded framework supported thin to very thick beds consist of brown coarse tuff to scoria lapilli tuff with outsized fluidal bombs throughout accessory lithic clasts chiefly occur in the lowermost visible beds scoria bombs have quenched margins weak bread crust textures and their vesicle number densities decrease inward which is indicative of fast surface cooling composite textures in the scoria bombs indicate recycling and agglutination of quenched and semi molten pyroclasts at the submarine vent in contrast to weak concentric gradations in vesicle size distribution in the bombs lapilli have asymmetrical gradients in vesicle size distribution indicating that they are fragments of coarser quenched lumps three grain size modes characterise the nishiizu brown scoria with coarse magma lumps ejected during magmatic fragmentation and quench jointed upon contact with seawater to be subsequently fragmented into lapilli and coarse ash by various styles of fragmentation where seawater plays a critical role the cone was constructed by slow moving fallout fed granular flow creep fed directly by suspension settling focused at the crater rim but extending onto the cone flanks with only minor resedimentation by granular flows nishiizu deposits yield an exceptional record of eruption and sedimentation dynamics during submarine cone building activity and in this study we compare their vesiculation and fragmentation mechanisms with those of potential subaerial analogues

palynological and lithological evidence for the miocene palaeoenvironment in the saldanha region south africa abstract palynological and lithological studies were carried out on a section of clays of the elandsfontyn formation in the langebaanweg area the pollen assemblages suggest a middle miocene age for these clays which is supported by fossil faunal evidence in the overlying gravel member of the late miocene saldanha formation the pollen sequence indicates the retrogressive succession of subtropical tropical gallery forest to dominant palm vegetation and finally to marsh these changes are clearly related to the lithological evidence for the progressive lateral northward migration of the palaeo berg river it is possible that the middle miocene global high sea level cycle may have influenced these palaeoenvironmental changes

influence of convective sedimentation on the formation of widespread tephra fall layers in the deep sea the formation of widespread volcanic ash fall layers in deep sea sediments was investigated experimentally to examine the settling behavior of tephra 20 180 m diameter as it travels from the atmosphere into water using a fallout mass flux rate that was constrained by measurements of distal fallout from the 1980 eruption of mount st helens 0 2 g cm 2 hr the experiments show that particle settling in the water column is dramatically accelerated by the formation of diffuse vertical gravity currents the currents form as a result of convective instabilities that develop in a surface boundary layer when the local particle concentration becomes large at the air water interface the settling velocity of particles drops abruptly and the concentration of particles increases because of the differential in mass flux above and below the fluid surface the implication of the experiments is that deposition of distal tephra fall layers in deep sea sediments may be dominantly controlled by diffuse vertical gravity currents as opposed to passive settling of individual particles through the water column this process greatly reduces the residence time of fine ash in the ocean and diminishes the role of ocean currents in influencing the distribution patterns of individual tephra layers support for this mechanism comes from observations of greatly accelerated tephra settling rates in the south china sea following the 1991 eruption of mount pinatubo in the philippines

fallout of pyroclastic debris from submarine volcanic eruptions volcanic fallout deposits on land being widespread and accessible for study have received much attention and have revealed a great deal about subaerial eruption mechanisms in contrast virtually nothing is known about equivalent deposits produced by submarine volcanoes despite the probable abundance of such material in today s oceans and in accreted volcanic arc terrains many submarine deposits may form by the fallout of debris to the sea floor downcurrent from the umbrella region of submarine eruption columns experiments on water saturated pumice and pieces of rock lithics show that particles settling to the sea floor at terminal velocities of 10 to 50 centimeters per second will display conspicuous bimodality of particle diameters pieces of pumice may be five to ten times as large as codeposited lithic fragments similar material erupted into the air and deposited on land displays less well developed bimodality pumice diameters are generally two to three times as large as associated lithics submarine fallout deposits are therefore distinctive and may be used to indicate a subaqueous origin for some of the great thicknesses of nonfossiliferous volcanic debris contained in ancient volcanic terrains worldwide whose environment of deposition has been uncertain

an experimental determination of the effect of bulk composition on phase relationships in metasediments at near solidus conditions sediments of terrigenous origin represent a conspicuouspart of the cover of the oceanic and continental crust taylor mclennan 1985 wedepohl 1991 pelitesand greywackes are involved in a variety of processes from deep mass transfer in subduction environments e g chopin 1984 smith 1984 sobolev shatsky 1990 reinecke 1991 responsible for dehydration and fluidrelease during dehydration schmidt etal 2004 to partialmelting in continental collision zones which is at theorigin of intracrustal differentiation clemens vielzeuf 1987 the evolution of metapelites and metagreywackescan be reconstructed if the dependence of phase relation ships on pressure temperature and bulk composition iswell known and under h

the behaviour of subaerially produced pyroclastic flows in a subaqueous environment evidence from the roseau eruption dominica west indies abstract the roseau tephra 30 000 yrs b p with its source on the island of dominica represents the largest eruption in the caribbean in the last 200 000 years pyroclastic flows generated by the eruption flowed offshore into the grenada basin giving rise to a widespread layer of sediment gravity flow deposits on land the flow deposits are valley ponded and their granulometric characteristics allow them to be classified as weakly fines depleted ignimbrites offshore the characteristics of the sediment gravity flow deposits are consistent with deposition by numerous small scale events rather than a single large scale event a comparison of the on land and offshore deposits reveals an increase in the crystal lithic ratio in the latter this change can be best explained by the fragmentation of hot pyroclastic material as the subaerial flows mixed with water mixing and fragmentation occurred not at the air water interface but as flows descended the steep submarine slopes of the island of dominica

rare element enriched s type ash flow tuffs containing phenocrysts of muscovite andalusite and sillimanite southeastern peru ash flow tuffs of neogene age exposed over 2 500 km 2 in the macusani region of southeastern peru are the volcanic equivalent of s type granites the strongly peraluminous tuffs contain phenocrysts of andalusite sillimanite and muscovite and have high 87 sr 86 sr i 0 7258 and 0 7226 and 18 o 11 elevated concentrations of li cs be sn b and other minor elements compare with those in tin granites mineral phase relations and composition are indicative of low magmatic temperatures and oxygen fugacities and high a hf a h 2 o the chemical isotopic and mineralogical features and regional geologic relations are consistent with models of magma generation involving the incorporation of large amounts of pelitic rock

international geomagnetic reference field the eleventh generation the eleventh generation of the international geomagnetic reference field igrf was adopted in december 2009 by the international association of geomagnetism and aeronomy working group v mod it updates the previous igrf generation with a definitive main field model for epoch 2005 0 a main field model for epoch 2010 0 and a linear predictive secular variation model for 2010 0 2015 0 in this note the equations defining the igrf model are provided along with the spherical harmonic coefficients for the eleventh generation maps of the magnetic declination inclination and total intensity for epoch 2010 0 and their predicted rates of change for 2010 0 2015 0 are presented the recent evolution of the south atlantic anomaly and magnetic pole positions are also examined

characteristics of submarine pumice rich density current deposits sourced from turbulent mixing of subaerial pyroclastic flows at the shoreline field and experimental assessment this study investigates the types of subaqueous deposits that occur when hot pyroclastic flows turbulently mix with water at the shoreline through field studies of the znp marine tephra in japan and flume experiments where hot tephra sample interacted with water the znp is a very thick pumice rich density current deposit that was sourced from subaerial pyroclastic flows entering the japan sea in the pliocene notable characteristics are well developed grain size and density grading lithic rich base pumice rich middle and ash rich top preponderance of sedimentary lithic clasts picked up from the seafloor during transport fine ash depletion in coarse facies and presence of curviplanar pumice clasts flume experiments provide a framework for interpreting the origin and proximity to source of the znp tephra on contact of hot tephra sample with water steam explosions produced a gas supported pyroclastic density current that advanced over the water while a water supported density current was produced on the tank floor from the base of a turbulent mixing zone experimental deposits comprise proximal lithic breccia medial pumice breccia and distal fine ash experiments undertaken with cold water saturated slurries of tephra sample and water did not produce proximal lithic breccias but a medial basal lithic breccia beneath an upper pumice breccia results suggest the characteristics and variations in znp facies were strongly controlled by turbulent mixing and quenching proximity to the shoreline and depositional setting within the basin presence of abundant curviplanar pumice clasts in submarine breccias reflects brittle fracture and dismembering that can occur during fragmentation at the vent or during quenching subsequent transport in water supported pumiceous density currents preserves the fragmental textures careful study is needed to distinguish the products of subaerial versus subaqueous eruptions

comments and reply on implications of magmatic epidote bearing plutons on crustal evolution in the accreted terranes of northwestern north america and magmatic epidote and its petrologic significance comment

evolution of cenozoic seaways in the circum antarctic region abstract a complete circum antarctic seaway did not open until both the south tasman rise cleared the oates land coast of east antarctica and drake passage opened between the southern tip of south america and the northern end of the antarctic peninsula major plate motions based on dated seafloor spreading anomalies and distinct fracture zone lineations constrain the age of the opening of a seaway between the south tasman rise and antarctica as very close to the eocene oligocene boundary with an unrestricted opening deeper than 2000 m dating from 32 ma timing of the opening of drake passage is more circumstantial because the exact motions of certain micro continental fragments are not known the motion of africa with respect to south america as well as the motion of east antarctica with respect to africa are well constrained for the cenozoic these major plate motions are used with the reasonable assumption of no cenozoic motion of the antarctic peninsula with respect to east antarctica to constrain the location of the antarctic peninsula with respect to the southern tip of south america for the critical period of late eocene to late oligocene uncertainty of motion of the south georgia and south orkney microcontinents and other possible continental fragments make an exact time for opening of drake passage difficult to ascertain even so the early oligocene position of the antarctic peninsula with respect to south america requires a through going deep water seaway to have been open at drake passage prior to 28 ma even given the unconstrained motion of various high standing crustal fragments in the scotia sea with reasonable assumptions concerning motion of the crustal fragments in the western and central scotia sea it is likely that drake passage or passage through powell basin was open to deep water circulation by 31 2 ma

comments and reply on implications of magmatic epidote bearing plutons on crustal evolution in the accreted terranes of northwestern north america and magmatic epidote and its petrologic significance reply

fluid heterogeneities and hornblende stability in interlayered graphitic and nongraphitic schists tauern window eastern alps the assemblage hornblende white mica occurs in graphite free schists at two localities in the southwest corner of the tauern window eastern alps in interbedded graphitic layers 1 mm to 1 m thick however hornblende is typically replaced by pseudomorphs of biotite plagioclase epidote chlorite staurolite in the presence of white mica garnets adjacent to these pseudomorphs have pronounced growth discontinuities near their rims in contrast to the continuously zoned garnets in nongraphitic layers these observations imply that reactions of the type hbl white mica gar bio plag epid chl staur h2o occurred in the graphitic samples but that hbl white mica remained stable in graphite free layers calculation of the equilibrium constants for solid phases in five dehydration equilibria at locality 1 indicates thata h2o in the nongraphitic layers was 6 to 11 times greater thana h2o in the graphitic layers similar calculations involving six dehydration equilibria at locality 2 show no difference ina h2o between layers at the conditions of final equilibration initial differences in fluid composition maintained between the graphitic and nongraphitic layers caused the hbl white mica reaction to occur at differentp t conditions in different horizons of the schists these data indicate that systematic differences in fluid composition were generated during metamorphism of the interlayered graphitic and non graphitic schists but were subsequently homogenized at locality 2 the heterogeneities could initially have been produced while the rocks were in thep t field of co2 h2o immiscibility development of a penetrative layer parallel shear foliation at this time would have prevented subsequent mixing of the fluids across layers after temperatures exceeded the consolute temperature in the co2 h2o system late stage homogenization of fluids at locality 2 is thought to reflect loss of the buffer capacity of the mineral assemblage in response to total consumption of hornblende

morphology and surface sedimentary features of point bars in welsh gravel bed rivers two examples of point bars in single channel gravel bed rivers in wales illustrate some of the morphological and surface sedimentary features observed during a wider examination of 40 sites of recent less than 100 years channel sedimentation the point bars consist of predominantly sandy gravel bar platforms overlain by discontinuous sand bodies cut bank exposures show generally massive sandy gravels abruptly underlying a silt sand unit morphologically the bars are platformlike with primary inner attached or secondary medial chutes this relief together with the effects of seasonal vegetation determines the locations of finer grained non gravel material the frequency with which deposits produced by this type of channel sedimentation may be encountered in quaternary and pre quaternary successions is briefly discussed

behaviour of particle laden flows into the ocean experimental simulation and geological implications the behaviour of subaerial particle laden gravity currents e g pyroclastic flows lahars debris flows sediment bearing floods and jokulhlaups flowing into the sea has been simulated with analogue experiments flows of either saline solution simple suspensions of silicon carbide sic in water or complex suspensions of sic and plastic particles in methanol were released down a slope into a tank of water the excess momentum between subaerial and subaqueous flow is dissipated by a surface wave at relatively low density contrasts between the tank water and the saline or simple suspensions the flow mixture enters the water and forms a turbulent cloud involving extensive entrainment of water the cloud then collapses gravitationally to form an underwater gravity current which progresses along the tank floor at higher density contrasts the subaerial flow develops directly into a subaqueous flow the flow slows and thickens in response to the reduced density contrast which is driving motion and then continues in the typical gravity current manner complex suspensions become dense flows along the tank floor or buoyant flows along the water surface if the mixtures are sufficiently denser or lighter than water respectively flows of initially intermediate density are strongly influenced by the internal stratification of the subaerial flow material from the particulate depleted upper sections of the subaerial flow becomes a buoyant gravity current along the water surface whereas material from the particulate enriched lower sections forms a dense flow along the tank floor sedimentation from the dense flow results in a reduction in bulk density until the mixture attains buoyancy lifts off and becomes a secondary buoyant flow along the water surface jokulhlaups lahars and debris flows are typically much denser than seawater and thus will usually form dense flows along the seabed after sufficient sedimentation the freshwater particulate mixture can lift off to form a buoyant flow at the sea surface leading to a decoupling of the fine and coarse particles flood waters with low particulate concentrations 2 may form buoyant flows immediately upon entering the ocean subaerial pyroclastic flows develop a pronounced internal stratification during subaerial run out and thus a flow splitting behaviour is probable which agrees with evidence for sea surface and underwater flows from historic eruptions of krakatau and mont pelee a pyroclastic flow with a bulk density closer to that of sea water may form a turbulent cloud resulting in the deposition of much of the pyroclasts close to the shore dense subaqueous pyroclastic flows will eventually lift off and form secondary buoyant flows either before or after the transformation to a water supported nature

experimental constraints on phase relations in subducted continental crust synthesis piston cylinder experiments were carried out in the range 2 0 4 5 gpa and 680 1 050 c to investigate phase relations in subducted continental crust a model composition kcmash has been used because all major ultrahigh pressure uhp minerals of the whole range of rock types typical for continental crust can be reproduced within this system the combination of experimental results with phase petrologic constraints permits construction of a uhp petrogenetic grid the phase relations demonstrate that the most important uhp paragenesis consists of coesite kyanite phengite clinopyroxene and garnet in subducted continental crust below 700 c talc is stable instead of garnet as most of these minerals are also stable at much lower pressure and temperature conditions it is thus not easy to recognize uhp metamorphism in subducted crust a general feature however is the absence of feldspars at h2o saturated conditions plagioclase is never stable at uhp conditions but k feldspar can occur in h2o undersaturated rocks mineral compositions in the experiments are fully buffered by coexisting phases the si content of phengite and biotite increase with increasing pressure at 4 0 gpa 780 c biotite contains 3 28 si per formula unit which is most probably caused by solid solution of biotite with talc above 800 c the caal2sio6 component in clinopyroxene buffered with kyanite coesite and a mg phase increases with increasing temperature providing a tool to distinguish between cold and hot eclogites up to 10 ca eskolaite ca0 5 0 5alsi2o6 in clinopyroxene has been found at the highest temperature and pressure investigated 900 c 4 5 gpa garnet buffered with coesite kyanite and clinopyroxene displays an increase of grossular component with increasing pressure for a given temperature although the investigated system represents a simplification with respect to natural rocks it helps to constrain general features of subducted continental crust the observed phase relations and phase compositions demonstrate that at pressures 3 0 gpa and temperatures 800 c continental crust can retain significant amounts of h2o 1 wt whereas k free mafic or ultramafic rocks are dry at these conditions uhp parageneses are only preserved if the whole exhumation path is situated within the stability field of phengite i e if there is cooling during exhumation or if the whole exhumation occurred at t 700 c in contrast break down of phengite and concomitant partial melting in terranes that show isothermal decompression may lead to a complete recrystallization of the subducted crust during exhumation the density of uhp rocks can be estimated on the basis of the established phase relations pelitic rocks are likely to have a density close to mantle rocks 3 3 g cm3 because of significant amounts of dense garnet and kyanite whereas granitic rocks are less dense 3 0 g cm3 hence subducted average continental crust is most probably buoyant with respect to mantle rocks and tends to get exhumed as soon as it is detached from the down going slab electronic supplementary material to this paper can be obtained by using the springer link server located at http dx doi org 10 1007 s00410 001 0336 3

miocene climate and vegetation changes in the cape peninsula south africa evidence from biogeochemistry and palynology organic material from the noordhoek area on the western margin of the cape peninsula south africa was obtained from a 50 m long drill core dominated by fluvio lacustrine siliciclastic sediments the aim of this study is to constrain fluctuations in climate and the decline of tropical vegetation elements along the southwestern coast and the cape peninsula of south africa during the late cenozoic phase when the benguela upwelling system was established the approach was to combine palynological biogeochemical tetraether lipids and stable isotope c n studies of the organic bearing record from the noordhoek area on the western margin of the cape peninsula bulk c and n isotope data of sediment organic matter point to a predominantly c3 higher plant source vegetation mean annual air temperature mat from the analyses of tetraether lipids mbt cbt index was compared with palynomorphs from partly unpublished data of a previously drilled core adjacent to the study site the palynomorphs are of subtropical affinities and suggest that an open riparian forest would have existed in the early to middle miocene of the southwestern coast of south africa together these data sources allow vegetation and climate reconstructions of subtropical conditions during the early to middle miocene which comprised fluctuating open riparian forest and swamp vegetation temperatures rose in the middle miocene and were higher than those of the present day

the stability and composition of phengitic muscovite and associated phases from 5 5 to 11 gpa implications for deeply subducted sediments abstract the stability and composition of phengitic muscovite was investigated from 5 5 11 gpa 700 1150 c in synthesis experiments performed in a multianvil apparatus starting materials consisted of natural minerals with a bulk composition similar to that of a k deficient intermediate dioctahedral trioctahedral mica phengitic muscovite was found to be stable from 5 5 11 gpa at 900 c phengite melting occurs between 1075 1150 c at 7 8 gpa and 1000 1050 c at 10 gpa at 10 11 gpa 800 c octahedral cation deficient ocd muscovite and k hollandite are observed rather than phengite the average phengite content of muscovite is positively correlated with pressure ranging from 3 65 si pfu at 5 5 gpa to 3 81 si pfu at 11 gpa the maximum phengite content of stable muscovite appears to be 3 80 3 85 si pfu most phengite examined exhibits at least minor solid solution towards phlogopite averaging 2 04 0 06 2 octahedral cations pfu the hydrous phases phengite lawsonite topaz oh and mg pumpellyite occur between 6 8 gpa 700 900 c with increasing pressure lawsonite and mg pumpellyite dehydrate to form garnet between 8 9 gpa with increasing temperature mg pumpellyite lawsonite and topaz oh devolatilize to form garnet and kyanite between 900 1000 c at 7 8 gpa phengitic muscovite and topaz oh would be stable in hydrous sediments in cool mature subduction zones to depths exceeding 360 km while lawsonite and mg pumpellyite would be stable to 240 300 km allowing the transport of h2o contained in these phases deep into the upper mantle in warmer subduction zones the dehydration of mg pumpellyite lawsonite and topaz oh as well as the melting of phengite would result in fluid release at depths of 180 240 km the presence of phengite at far greater depths than the zone of melt generation beneath arcs 100 150 km requires a mechanism such as the partitioning of k be b ba and rb into migrating fluids rather than the simple dehydration of phengite beneath arcs in order to provide for the transfer of these slab signature elements from phengite into arc magmas

unlocking the ice house oligocene miocene oxygen isotopes eustasy and margin erosion oxygen isotope records and glaciomarine sediments indicate at least an intermittent presence of large continental ice sheets on antarctica since the earliest oligocene circa 35 ma the growth and decay of ice sheets during the oligocene to modern ice house world caused glacioeustatic sea level changes the early eocene was an ice free greenhouse world but it is not clear if ice sheets existed during the middle to late eocene doubt house world benthic foraminiferal 18o records place limits on the history of glaciation suggesting the presence of ice sheets at least intermittently since the earliest oligocene the best indicator of ice growth is a coeval increase in global benthic and western equatorial planktonic 18o records although planktonic isotope records from the western equatorial regions are limited subtropical planktonic foraminifera may also record such ice volume changes it is difficult to apply these established principles to the cenozoic 18o record because of the lack of adequate data and problems in stratigraphic correlations that obscure isotope events we improved oligocene to miocene correlations of 18o records and erected eight oxygen isotope zones oi1 oi2 mi1 mi6 benthic foraminiferal 18o increases which are associated with the bases of zones oil circa 35 8 ma oi2 circa 32 5 ma and mil circa 23 5 ma can be linked with 18o increases in subtropical planktonic foraminifera and with intervals of glacial sedimentation on or near antarctica our new correlations of middle miocene benthic and western equatorial planktonic 18o records show remarkable agreement in timing and amplitude we interpret benthic planktonic covariance to reflect substantial ice volume increases near the bases of zones mi2 circa 16 1 ma mi3 circa 13 6 ma and possibly mi5 circa 11 3 ma possible glacioeustatic lowerings are associated with the 18o increases which culminated with the bases of zone mi4 circa 12 6 ma and mi6 circa 9 6 ma although low latitude planktonic 18o records are required to test this these inferred glacioeustatic lowerings can be linked to seismic and rock disconformities for example we link 12 oligocene early late miocene inferred glacioeustatic lowerings with 12 of the sequence boundaries inferred eustatic lowerings of haq et al 1987

entrance of hot pyroclastic flows into the sea experimental observations the entrance of hot pyroclastic flows into water has been observed in a series of experiments in which shooting granular flows of hot ignimbrite ash 403 c of bulk density near that of water run down a smooth chute and enter a water filled tank at an angle of 26 flows of relatively cool ash 250 c mixing across the water surface over some distance from shore generates steam explosions forming fountains of wet and dry ash and convectively rising fine ash plumes steam explosions increase in strength and lateral extent towards higher ash temperatures and mass fluxes the explosions generate water waves that remain driven by massive fountain fallout across some distance from shore the ash fountains feed pyroclastic surges which advance down tank over water at high speeds underwater plumes of sediment falling from the ash fountains and surges drop coarse load onto the floor whereas the finer load forms a turbidity current high temperature and poor size sorting of pyroclastic flows are key parameters that determine the processes of interaction with water and the associated hazards poor sorting allows for the formation of ash cloud surges moving over water even from cool flows high temperatures cause littoral explosions and facilitate extensive mass transport over water all flows generate tsunami waves by different mechanisms

sedimentology of the krakatau 1883 submarine pyroclastic deposits the majority of tephra generated during the paroxysmal 1883 eruption of krakatau volcano indonesia was deposited in the sea within a 15 km radius of the caldera two syneruptive pyroclastic facies have been recovered in scuba cores which sampled the 1883 subaqueous pyroclastic deposit the most commonly recovered facies is a massive textured poorly sorted mixture of pumice and lithic lapilli to block sized fragments set in a silty to sandy ash matrix this facies is indistinguishable from the 1883 subaerial pyroclastic flow deposits preserved on the krakatau islands on the basis of grain size and component abundances a less common facies consists of well sorted planarlaminated to low angle cross bedded vitric enriched silty ash entrance of subaerial pyroclastic flows into the sea resulted in subaqueous deposition of the massive facies primarily by deceleration and sinking of highly concentrated deflated components of pyroclastic flows as they traveled over water the basal component of the deposit suggests no mixing with seawater as inferred from retention of the fine ash fraction high temperature of emplacement and lack of traction structures and no significant hydraulic sorting of components the laminated facies was most likely deposited from low concentration pyroclastic density currents generated by shear along the boundary between the submarine pyroclastic flows and seawater the krakatau deposits are the first well documented example of true submarine pyroclastic flow deposition from a modern eruption and thus constitute an important analog for the interpretation of ancient sequences where subaqueous deposition has been inferred based on the facies characteristics of encapsulating sedimentary sequences

regional and global context of the late cenozoic langebaanweg lbw palaeontological site west coast of south africa the palaeontological site of langebaanweg lbw is internationally renowned for its prolific diverse and exceptionally well preserved mio pliocene vertebrate faunas the site is located on the southern west coast of south africa which represents a passive intraplate trailing edge setting the southern african subcontinent is also removed from glacial influence and has experienced no cenozoic volcanic activity rates of vertical crustal motion are consequently low and late cenozoic shoreline datums at lbw chiefly reflect glacio eustatic sea level history the primary aim of this study is to clarify the chronology as well as the regional and global context of lbw and to review previous work on these aspects lbw is ideally situated to document the complex interactions of ocean atmosphere and land and their respective influence on climate evolution given its location near the coast and mix of marine estuarine and terrestrial faunas and depositional settings this paper also provides a background to the study of the vast existing faunal collections and a guide to undiscovered fossil deposits towards these ends the first detailed geological topographic maps of the site and surrounds accompanied by a summary stratigraphic column are provided virtual geological modelling using a subsurface database has clarified the spatial and temporal relationships of sedimentary facies as well as their depositional settings the geological and palaeontological record at lbw tracks and documents the major regional and global climatic oceanographic events of the late cenozoic during the oligocene drawdown in sea levels the landscape was etched by river incision fluctuating sea levels of the neogene periodically reversed the trend from erosion to deposition preserving contemporary faunas and floras in the oligocene palaeovalleys earlier miocene pollen from fluvial facies indicates a humid sub tropical climate reflecting a warm southern atlantic ocean the abrupt late middle miocene global cooling monterey excursion coincided with intensified cold upwelling in the benguela current and extensive phosphate authigenesis a globally documented early pliocene highstand possibly related to the shoaling of the isthmus of panama reached 90 m above sea level asl implying extensive melting of the cryosphere palaeomagnetic data in tandem with global sea level reconstructions suggested an age of 5 15 0 1 ma for the faunas and a correlation with the earlier part of this transgression a subtropical c3 vegetation is indicated by the faunas and floras but with a significant contribution by sclerophytic fynbos pointing to a cooler and more seasonal climate than in the miocene a mid pliocene highstand to 50 m asl truncated the early pliocene succession at lbw and the globally documented late pliocene highstand to 30 m asl saw the atlantic shoreline approaching lbw for the last time with the progressive climatic cooling and instability of the terminal pliocene culminating in the growth of the arctic ice cap strengthening southerly winds driven by a tighter coiled south atlantic anticyclone deposited extensive coastal dune fields over the region

laramide crustal thickening event in the rocky mountain foreland and great plains the rocky mountain foreland and great plains of the western united states were formerly part of a continental platform adjusted by erosion and deposition in cambrian through jurassic time to near mean sea level and therefore to a near uniform crustal thickness of approximately 33 km today the region stands at regional elevations up to 2 km isostatically supported by a crust exceeding 50 km in thickness reasonable estimates of tertiary sedimentation and laramide strain do not account for more than 15 of the implied thickening however from approximately 70 40 m y b p this region was underlain by a horizontally subducting slab of farallon plate lithosphere moving northeast this slab could have been the cause of the thickening finite difference thermal models with specified kinematics show only a temporary cooling of the base of the north american lithosphere by this slab the excess weight of the slab would have depressed the region plate bending calculations show a quantitative agreement of predicted depression with upper cretaceous isopachs since depression by this slab lasted until the eocene at least the latest cretaceous regression was probably caused by a laramide crustal thickening event the farallon slab might have caused crustal thickening in two ways its excess weight would have drawn in ductile lower crust from surrounding regions however calculations show that this effect is too slow too local and too reversible to explain most of the crustal thickening therefore it seems likely that ductile lower crust was transported from sw to ne by shear stresses which the farallon plate exerted on the base of the north american lithosphere a preliminary finite element calculation based on this hypothesis shows the correct general pattern of crustal thickening an unexpected but encouraging result is that predicted principal compression directions are orthogonal to many laramide basement uplifts

analysis of geological hiatus surfaces across africa in the cenozoic and implications for the timescales of convectively maintained topography geological maps are a powerful but underutilized tool for constraining geodynamic processes and models unraveling the cenozoic elevation history of africa and distinguishing between competing upli

products of neptunian eruptions a common pyroclastic facies in subaqueous volcanic successions comprises massive to graded very thick several to tens of meters laterally extensive several kilometers beds of nonwelded pumice lapilli with volumes ranging to tens of cubic kilometers this facies may be overlain by laminated ash or bimodal ash and giant 1 m pumice clasts and underlain by coarse lithic breccia the association is inferred to be the typical product of sustained magmatic volatile driven explosive eruptions from vents at water depths of 1300 200 m we propose the term neptunian for such eruptions and their products the eruption column rapidly mixes with the surrounding water cools increases in density and collapses while remaining under water lithic clasts that are too heavy to be entrained in the column are deposited close to the source forming a neptunian lithic breccia pumice lapilli are rapidly waterlogged and form the dominant component in the collapsing column and in eruption fed water supported density currents neptunian density currents hot buoyant giant pumice clasts continue to rise and may reach the water surface before being waterlogged and settling along with temporarily suspended ash forming neptunian suspension deposits eruption magnitude fragmentation mechanisms and juvenile pyroclast characteristics especially vesicularity are very similar in neptunian and plinian style eruptions but column behavior differs primarily because of the contrasting physical properties of the ambient fluid water versus air

high resolution ice volume estimates for the early miocene evidence for a dynamic ice sheet in antarctica ice volume estimates for the early miocene 23 16 ma ats were determined by applying d 18 o to sea level calibrations to high resolution d 18 o records from odp sites 1090 and 1218 these calibrated records indicate that ice volume ranged between 50 and 125 of the present day east antarctic ice sheet eais during most of the early miocene 23 17 ma maximum icevolume occurred at each of the early miocene isotopic events i e mi events concomitant with bottom water temperatures generally between 1 and 2 8c rapid bb 1 myr and high amplitude ice volume changes also occurred intermittently during this period with some fluctuations ranging from a fully glaciated east antarctic continent to a partial collapse of the ice sheet equivalent to a 50 70 reduction in the eais these large scale ice volume changes often occurred at b100 kyr time scales suggesting an orbitally driven dynamic eais existed during the early miocene in contrast the calibrated d 18 o record from site 1090 indicates significantly less ice volume 25 70 of the present day eais was present between 17 and 16 ma these results are supported by numerical climate ice sheet modeling studies that show increased orbitally driven ice volume variability with elevated levels of atmospheric co2 the distribution of bottom water masses during the miocene was likely significantly different from today owing to variability in the production of deep water near the antarctic continent i e proto antarctic bottom water proto aabw weakening of proto aabw is suggested to have occurred during the early miocene especially during glacial minima resulting in it becoming more spatially restricted to the southern ocean and becoming entrained into a warmer deep water mass increased production of a warmer bottom water mass is also considered to have been important in the lighter isotopic values observed in the atlantic and indian oceans these changes are supported by evidence of a large dynamic eais based on strata cored on the antarctic margin and require a reevaluation of the view that antarctic ice volume was significantly reduced for most of the early miocene previous estimates have been constrained by the lack of deep water isotopic records proximal to antarctica and unduly influenced by the expansion of warmer deep water into most of the ocean basins at this time d 2005 elsevier b v all rights reserved

orbitally paced climate evolution during the middle miocene monterey carbon isotope excursion one of the most enigmatic features of cenozoic long term climate evolution is the long lasting positive carbon isotope excursion or monterey excursion which started during a period of global warmth after 16 9 ma and ended at similar to 13 5 ma approximately 400 kyr after major expansion of the antarctic ice sheet we present high resolution 1 9 kyr astronomically tuned climate proxy records in two complete sedimentary successions from the northwestern and southeastern pacific odp sites 1146 and 1237 which shed new light on the middle miocene carbon isotope excursion and associated climatic transition over the interval 17 1 12 7 ma we recog nize three distinct climate phases with different imprints of orbital variations into the climatic signals 1146 and 123 7 delta 18 o delta 13 c 1237 xrt fe fraction 63 pm 1 climate optimum prior to 14 7 ma characterized by minimum ice volume and prominent 100 and 400 kyr variability 2 long term cooling from 14 7 to 13 9 ma principally driven by obliquity and culminating with rapid cryosphere expansion and global cooling at the onset of the last and most pronounced delta 13 c increase 3 icehouse mode after 13 9 ma with distinct 100 kyr variability and improved ventilation of the deep pacific the monterey carbon isotope excursion 16 9 13 5 ma consists overall of nine 400 kyr cycles which show high coherence with the long eccentricity period superposed on these low frequency oscillations are high frequency variations 100 kyr which closely track the amplitude modulation of the short eccentricity period in contrast to delta 13 c the delta 18 o signal additionally shows significant power in the 41 kyr band and the 1 2 myr amplitude modulation of the obliquity cycle is clearly imprinted in the 1146 delta 18 o signal our results suggest that eccentricity was a prime pacemaker of middle miocene climate evolution through the modulation of long term carbon budgets and that obliquity paced changes in high latitude seasonality favored the transition into the icehouse climate c 2007 elsevier b v all rights reserved

rapid erosion of the southern african plateau as it climbs over a mantle superswell we present new sedimentary flux data confirming that a large pulse of erosion affected the southern african plateau in the late cretaceous and is likely to be related to a major uplift episode of the plateau this short phase of erosion i e less than 30 myr in duration has commonly been difficult to reconcile with a mantle origin for the plateau anomalous uplift given its size the rise of the african superplume is likely to have lasted much longer here we demonstrate by using a simple model for fluvial erosion that tilting of the continent as it rides over a wide dynamic topography high cannot only cause rapid uplift of the plateau but also trigger continent wide drainage reorganization leading to substantial denudation in a relatively short amount of time the amplitude and short duration of the sedimentary pulse are best reproduced by assuming a strong erodibility contrast between the karoo sedimentary and volcanic rocks and the underlying basement we also present a new compilation of paleoclimate indicators that shows a transition from arid to very humid conditions approximately at the onset of the documented erosional pulse suggesting that climate may have also played a role in triggering the denudation the diachronism of the sedimentary flux between the eastern and western margins of the plateau and the temporal and geographic coincidence between the uplift and kimberlite eruptions are however better explained by our tilt hypothesis driven by the migration of the continent over a fixed source of mantle upwelling

sea surface temperature fluctuations during the holocene off the south coast of africa implications for terrestrial climate and rainfall models of southern african palaeoclimate implicate surface atmospheric circulation anoma lies as forcing large scale changes during the late quaternary the available proxy data are insufficient to test the models since they provide information about temperature and rainfall rather than surface circulation a conceptual model is proposed which links coastal ocean temperatures with atmospheric circulation and allows a history of surface circulation to be inferred from sea temperature data a holocene sea surface temperature sst record was constructed by measuring the oxygen isotope composition of marine mollusc shells preserved in an archaeological cave deposit on the coast of the eastern agulhas bank southern africa radiocarbon dating of individual shells allowed definition of the timing and timespan of events in the record by serially sampling along the growth axis of each shell information was obtained about intra annual variability as well as millennial scale trends during the early holocene th