origin of plagiogranites in oceanic complexes a case study of the nicoya and santa elena terranes costa rica abstract the origin of minor leucocratic intrusions known as plagiogranites in oceanic complexes dominated by basaltic compositions have been debated in the literature here we use well preserved plagiogranites within the contrasting santa elena ophiolite and the nicoya complex nw costa rica to investigate the origin and age of these leucocratic intrusions magmatic zircons of plagiogranites of the santa elena ophiolite and the nicoya complex yield weighted mean shrimp 206 pb 238 u ages of 125 3 2 0 ma and 90 9 2 0 ma to 88 5 2 0 ma n 5 respectively these ages record the main magmatic phases of formation of these intrusions preserved in each complex relatively flat unfractionated chondrite normalized ree patterns of plagiogranites from both complexes mimic signatures of related mafic rocks which is consistent with formation via fractional crystallization from a mafic source as opposed to partial melting of a mafic basalt amphibolite protolith in the case of the nicoya complex modelling suggests that the nicoya plagiogranites are consistent as 10 15 residual liquids after fractional crystallization from its mafic parental source trace element systematics of the plagiogranites are consistent with an oceanic arc origin for santa elena and a mostly mid ocean ridge to oceanic plateau origin for nicoya

geomorphology and palaeoclimatology of some upper cretaceous palaeosols in central india abstract the late cretaceous maastrichtian lameta formation exposed around jabalpur in central india comprises a pedogenically modified fluvial succession accumulation of early diagenetic calcite cement during penecontemporaneous pedogenesis has transformed the fluvial sandstones into calcareous sandstones the microscopic features of the palaeosol material include floating grain fabric root traces laminated micrite microspar micrite envelopes and corona structures illuvial clay envelopes poikilotopic spars and minor amounts of siliceous grain and void cutans the palaeosol profiles of the lameta formation can be classified into eleven different types nine of which are comparable to present day mature aridisols and two to inceptisols nine to thirteen palaeosol profiles are present in the individual sections of the lameta formation within the study area each profile comprises two to four distinct pedohorizons arranged in vertical succession there are six types of pedohorizons present in the area of study namely partly altered hosts light coloured and poorly cemented horizons horizons with root traces and root related alterations horizons of subspherical glaebules horizons of platy fused glaebules and subhorizontal root traces and horizons with solution features they show many characteristics similar to the present day soil horizons for example those termed c bk k a and e by pedologists the palaeosol characteristics show correspondence to the nature of fluvial channel migration and the depth of the groundwater table controlled by the geomorphic setting a shallow groundwater table favoured the development of rootmat bearing and gleyed horizons horizons with vertical root traces and glaebules occur where the groundwater table was low mature and thin palaeosols developed in those parts of the lameta fluvial tract that were characterized by stable channels and wide floodplains in contrast thin and comparatively immature soils occurred in the parts where laterally migrating channels were operative the characteristics of the lameta palaeosols suggest that a hot and semiarid climate prevailed for most of the time represented by the lameta formation but short term wet spells occurred during the initial and the terminal phases of the lameta accumulation the low latitudinal palaeoposition of india and the absence of the palaeo monsoon circulation might have produced the hot and semiarid climatic setting during this time

double beam imaging mapping lower mantle heterogeneities using combinations of source and receiver arrays we present a new technique for imaging spatially distributed heterogeneities using information from combinations of source and receiver arrays the method is based on the single scattering assumption and is closely related to the double beam method of kruger et al 1993 1995 1996 in that it exploits amplitude delay time slowness and azimuth information in two arrays simultaneously a crucial step in the method is the application of static time corrections for a chosen reference phase here pcp one type of image is obtained as a spatial likelihood distribution for prescribed source and receiver array slowness and azimuth values and delay times with respect to a reference phase this allows the determination of the spatial origin of coherent phases in the p coda in a complementary approach we perform a double beam stack migration according to the theoretical slowness and azimuth values for candidate scatterers distributed over a three dimensional 3 d grid the scattering strength is expressed as the resulting beam power beam amplitude or semblance in a time window determined from the theoretical delay time with respect to the reference phase while likelihood mapping provides an image reflecting the probability of a region to explain observed kinematic phase properties by single scattering double beam migration provides information about the scattering strength the method is applied to nuclear explosion sources in eastern kazakhstan recorded at the yellowknife array in northern canada we identify several scattering volumes within the lower mantle below the arctic producing individual anomalous lower mantle phases in the p coda these anomalies range in depth from the core mantle boundary cmb up to 500 km into the lowermost mantle and are mainly located under the eurasian side of the arctic the joint interpretation of these results with the results of previous studies suggests a connection of these anomalies to the cenozoic and mesozoic subduction of the pacific plate and the kula plate the determination of the nature of such 3 d anomalies necessitates further array studies with better azimuthal coverage

geochemistry and petrogenesis of intrusive and extrusive ophiolitic plagiogranites central anatolian crystalline complex turkey plagiogranites associated with the sarikaraman ophiolite of the central anatolian crystalline complex turkey closely resemble other plagiogranites from supra subduction zone type ophiolites of neotethys the ophiolite is remarkable in displaying a higher proportion of the plagiogranite suite ca 10 by volume than is usually associated with such bodies the sarikaraman plagiogranites are represented by intrusive sheets and netvein trondhjemites largely developed at the top of the upper gabbros and as multiphase dykes within the sheeted dyke complex the plagiogranite dykes are considered to feed extrusive silicified rhyolites associated with the basaltic lavas in the volcanic section of the ophiolite field relations suggest that the trondhjemites were probably generated from the roof section of a dynamic and evolving gabbroic magma chamber both the deep seated trondhjemites and the volcanic rhyolites constitute the sarikaraman plagiogranite suite geochemically there is complete overlap between the intrusive trondhjemites and extrusive rhyolites which are characterised by morb normalized low hfs element contents with small negative nb ta anomalies and variably enhanced lil element abundances unlike other plagiogranites however the sarikaraman suite is not characterized by consistently low k2o contents a feature that reflects the variable mobilization of the lil elements under lower greenschist facies conditions the ree are uniformly enriched relative to the basic components of the complex but have similar normalized patterns exhibiting mild light ree depletion in terms of their origin the initial or most primitive plagiogranite melts could have been generated by either fractional crystallization 70 85 of clinopyroxene feldspar amphibole or partial melting 5 15 batch melting of a gabbroic source material although only the first process can produce most of the range of the plagiogranite compositions as a group the plagiogranites exhibit some degree of internal variation which can be generated by further fractionation largely dominated by feldspar with minor apatite and amphibole

neoproterozoic s type granites in the western jiangnan orogenic belt south china implications for petrogenesis and geodynamic significance abstract the neoproterozoic jiangnan orogenic belt in south china was formed by the amalgamation of the yangtze and cathaysia blocks however the timing of its formation is still debated in this study we present whole rock geochemistry u pb zircon ages lu hf and nd isotope systematics on the neoproterozoic yexijiang and baomuping granites occurring in the western segment of the jiangnan orogenic belt these granites mark the initiation of post collision extension environment zircon u pb age data identify 807 805 ma emplacement age for these granites they have low and variable ree concentrations 47 115 ppm and show moderate to highly fractionated ree patterns la sm n 2 83 3 78 with low gd yb n 1 34 2 31 and negative eu anomalies eu 0 34 0 57 geochemical features and presence of peraluminous minerals classify them as s type granites and p t parameters indicate low to moderate temperature 730 791 c and pressure middle crustal level conditions these granites show consistent negative whole rock nd t 5 7 to 6 6 and magmatic zircon hf t values from 0 3 to 3 8 while inherited zircon grains show variable hf t values 2 1 to 13 0 and two stage hf model ages 1 80 to 2 88 ga in particular one neoproterozoic 899 ma inherited zircon grain displays depleted hf hf t 5 4 and tdm2 1 34 ga isotopic characteristic these lines of evidence indicate a sedimentary source for yexijiang and baomuping granites possibly the archean to neoproterozoic crustal rocks and emplacement in a post collision tectonic environment triggered by mafic magma underplating during the amalgamation of the yangtze and cathaysia blocks we infer that convergence in the jiangnan orogenic belt occurred during 830 800 ma therefore unrelated to the global grenvillian orogeny

sedimentology and stable isotopes from a lacustrine to palustrine limestone deposited in an arid setting climatic and tectonic factors miocene pliocene opache formation atacama desert chile abstract field relations petrography gis and geochemistry of a late miocene pliocene limestone the opache formation calama basin atacama desert chile shows that depositional facies diagenetic histories and isotopes differ from eastern to western parts of the basin we recognize two facies lacustrine characterized by ostracod diatom mudstone and wackestone and palustrine characterized by both shallow water and pedogenic features gis analysis of the basin topography reveals an intrabasinal low in the eastern area and steeper hydrological gradients to the west with the calama constriction restricting flow between east and west petrography and geochemical results confirm that the calama constriction influenced depositional parameters ponding groundwater to the east forming a shallow lake and creating palustrine conditions to the west our analyses indicate that lake waters were less enriched in 13c or 18o than palustrine waters even though petrologic studies show that the lacustrine carbonates have a complex diagenetic history derived from groundwater rich in dissolved minerals this seemingly contrary observation can be reconciled if an important source of water for the western carbonates was sourced from the pacific ocean and was less evolved whereas eastern limestones precipitated from water which had become depleted in 18o during transport from the east today moisture from the pacific very rarely if ever reaches inland as far as calama indicating that during the late miocene mid pliocene breakdown in the temperature inversion due to weakened or absent upwelling in the cold coastal pacific current allowed moisture from the pacific to reach inland this changing atmospheric circulation pattern may have also affected upper level airflow that today blocks eastern sourced moisture from crossing the andes allowing it to reach farther westward into the atacama region during the late miocene pliocene together these moisture sources were sufficient to produce carbonate rich palustrine and lacustrine environments in the calama basin

formation and paleoenvironment of rhizoliths of shiyang river basin tengeri desert nw china abstract rhizoliths found in bed of the late quaternary paleolake zhuyezhe minqin basin central tengeri desert nw china were studied vegetation coverage is present at some locations in the paleolake area although much of the bed area is covered with moving dunes rhizoliths are occasionally eroded out and exposed to the air the morphology of the rhizoliths resembles singular or branching carbonated tubules which are hollow in their central part the rhizoliths are black to grey and are broken and scattered randomly on the sand surface of the lake bed the lacustrine deposits are black greenish silt or silty clay with a large quantity of white petite lake snail shells the possible plant types sources and formation conditions of carbon dioxide and calcium the sedimentary and diagenetic environments and the process of rhizolith formation were discussed via examining rhizoliths macromorphology studying the micromorphology and mineralogy by microscopy cathodoluminescence and scanning electron microscope and studying the chemical compositions of cementing minerals and fragments by energy dispersive x ray spectra the original roots of the rhizoliths belong to hydrophytes such as typha latifolia scirpus maritimus and carex stenophylla lake snails and previous pollen data indicate that the rhizoliths formed in a sedimentary environment of shallow fresh water lake like marginal or palustrine areas during the holocene shallow lake water disturbance by desert wind above the loose sandy sediment or soil was favoring the rhizoliths formation a continuous supply of oxygen through water disturbance led to complete oxidation of roots and producing carbon dioxide dissolution of co2 in water and so carbonic acid production resulted in minerals weathering such as feldspar and primary carbonate particles and the release of k and ca2 ions at presence of co2 and ca2 saturated in the pore water around the roots of hydrophytes within the sediments or soil carbonate precipitation occurred around the root channel and led to rhizoliths formation as tubules the petrifaction process therefore has happened in margin of the paleolake with shallow water and weak redox condition which implies the suitability of rhizolith for reconstructing paleoenvironment

scattering of seismic waves simulated by finite difference modelling in random media application to the gravberg 1 well sweden we generate surface and vsp synthetic seismograms using finite difference modelling of the elastic wave equation in self similar media the elastic model is determined by analyses of the sonic log from the deep gravberg 1 borehole in central sweden the upper 1200 m is highly fractured and the velocities are best described by a log normal distribution rather than a gaussian distribution analyses of this interval after removing the deterministic trend and assuming a self similar gaussian distribution of the random component give a standard deviation of 370 m s a correlation distance of 45 m and a hurst number of 0 18 these values and others from deeper levels are used to generate the 2 d elastic model and the synthetic seismograms are compared with real data synthetic surface seismic data show a poor qualitative match when compared to real data synthetic vsp data match the real vsp better qualitatively in general the synthetic data show considerably more scattering effects than the real data possible explanations for this discrepancy include 1 intrinsic attenuation has been ignored 2 a gaussian distribution of the random component was assumed or 3 the heterogeneities have a preferred orientation the poor match implies that the method for extracting the 2 d or 3 d velocity variations in the uppermost crust from 1 d sonic log data need to be studied further

an analytical model to predict dune erosion due to wave impact an analytical model is developed to calculate recession distance and eroded volume for coastal dunes during severe storms the transport relationship used in the model is based on wave impact theory where individual swash waves hitting the dune face induce the erosion combining this relationship with the sediment volume conservation equation describes the response of the dune to high waves and water levels four different data sets on dune erosion originating from the laboratory and the field were employed to validate the model and to determine the value of an empirical transport coefficient appearing in the analytical solutions the time evolution of dune recession observed in the different data sets was well described by the model but the empirical coefficient showed some variation between cases especially for the field data in practical applications of the model it is recommended to use a range of coefficient values to include an uncertainty estimate of calculated quantities such as recession distance and eroded volume

beach and dune erosion during storm surges this paper presents the set up and results of an extensive research programme concerning the erosion of coastal dunes during storm surges a large number of two dimensional and three dimensional mobile bed model tests has been carried out to investigate the process of dune erosion the state of art after a series of small scale tests is summarized attention is focussed on large scale tests carried out in the delta flume with random waves up to 2 m significant height sediment concentration and orbital velocity measurements are discussed the large scale tests have confirmed the validity of a modelling technique based on the dimensionless fall velocity parameter h tw the model results are being applied to check the safety of existing coastal dunes as a water retaining structure that has to protect the major part of the netherlands from inundation during storm surges

megarhizoliths in pleistocene aeolian deposits from gran canaria spain ichnological and palaeoenvironmental significance the pleistocene dune field of tufia located on the east of gran canaria spain contains different stratigraphic levels of indurated pillar like structures that are interpreted as megarhizoliths the megarhizoliths occur at the top of different aeolian sets and reach 31 5 cm in diameter and over 1 m in height these scattered free standing vertical cylindrical to slightly conical columns usually appear as hollow cylinders displaying elliptical cross sections aligned with the prevailing wind on the leeward side of some specimens the external wall shows a tail of rock matrix resembling a sort of wind shadow these tails and other remains of the associated rock matrix show a texture composed of long horizontal parallel cylinders orientated with the wind internally the most complete structures show five concentrically arranged zones zone a is a central pore corresponding to the cavity originally occupied by the root zones b and c which include alveolar and laminated carbonate textures indicating that carbonate precipitation was mostly induced by the roots and their associated microorganisms and zones d and e consisting mostly of aeolian sands in d the sand grains show thin micritic coatings whereas in e vadose aragonite cements can be seen on the grain surface suggesting a less biogenic influence in their formation the degree of cementation and the time of the precipitation of carbonate around the roots controlled the preservation of these zones thus in some cases zones b c and or d are not preserved cylinders are up to 30 the diameter of the root that nucleated them the presence of the megarhizoliths at the top of the aeolianite beds indicates that aeolian sedimentation halted several times allowing soil formation and plant colonisation during slightly more humid periods the occurrence of megarhizoliths is further proof of the alternation of arid and slightly more humid climates in the north atlantic during the last glacial period it is also noted that they may be misinterpreted as animal trace fossils or tree trunk casts resulting in incorrect ichnological or palaeoenvironmental interpretations

calcic paleosols of the plio pleistocene camp rice and palomas formations southern rio grande rift usa abstract seventy four separate paleosols are recognized in the plio pleistocene camp rice and palomas formations of the southern rio grande rift seven types of profiles are present a bw bk c a bt bk c bw bk c bt bk c bt k c bk c and ak calcic and petrocalcic horizons bk and k commonly overlain by argillic horizons bt dominate most paleosol profiles greater pore size and enhanced permeability typical of sandy and silty parent material favored the development of argillic horizons however micrite is the dominant form of calcium carbonate in bk and k horizons regardless of parent material grain size non calcified a horizons are typically lighter in color than adjacent subsurface pedogenic intervals due to their oxidized nature or lower clay content they are only rarely preserved probably due to erosion prior to final burial calcified root mats ak are not directly associated with other pedogenic horizons abundant horizontal roots suggest root mats formed under high water table conditions restriction of ak horizons to the distal piedmont lithofacies supports their development as spring or pond deposits near the toes of alluvial fans individual pedogenic horizons can often be traced hundreds of meters to kilometers normal and parallel to a basin s axis paleosol maturity and preservation potential are greatest for symmetrical basins especially where pedogenesis was far removed from active channels within the axial fluvial system or within distal piedmont settings of asymmetrical basins

tectonic and petrogenetic implications of major and rare earth element chemistry of troodos glasses cyprus chilled pillow margins dike margins and interpillow hyaloclastites from the extrusive sequences of the troodos ophiolite cyprus reveal two compositional groups of glasses 1 andesites to dacitic andesites containing plagioclase ca rich pyroxene and ca poor pyroxene as phenocrysts occur in a stratigraphically low position and 2 basaltic andesites containing olivine and ca rich pyroxene phenocrysts occur in an upper position in the lava sequence the glasses contain between 52 and 62 wt sio2 and have mg numbers between 65 and 27 the troodos glasses are very similar to basaltic andesite or boninitic glasses from the mariana trench and together these two groups define a trend intermediate between typical calcalkaline and tholeiite evolutions the troodos glasses are inferred by difference to contain 3 4 5 wt h2o which is taken to be of primary origin and to reflect the ocean depth of extrusion and quenching based on normative projections in the basalt phase diagram there are indications that elevated water pressures during formation of the phenocryst assemblages may have shifted the isobaric pseudo quaternary peritectic towards silica quantitative crystal fractionation models fail to explain the observed chemical variation it is suggested that the chemical variation may result from high pressure melts equilibrated to lower pressure 2 and 3 phase cotectic relations some mariana and the troodos glasses have similar ree patterns with relatively low abundances depletion in lree and flattening in the hree region the close similarities between the troodos and the mariana glasses point to an identical tectonic and petrogenetic control on their origin

observations of sand bar evolution on a natural beach waves currents and the location of the seafloor were measured on a barred beach for about 2 months at nine locations along a cross shore transect extending 255 m from 1 to 4 m water depth the seafloor location was measured nearly continuously even in the surf zone during storms with sonar altimeters mounted on fixed frames the crest of a sand bar initially located about 60 m from the shoreline moved 130 m offshore primarily when the offshore significant wave height exceeded about 2 m with 1 5 m of erosion near the initial location and 1 m of accretion at the final location an energetics type sediment transport model driven by locally measured near bottom currents predicts the observed offshore bar migration but not the slow onshore migration observed during low energy wave conditions the predicted offshore bar migration is driven primarily by cross shore gradients in predicted suspended sediment transport associated with quasi steady near bottom offshore flows these strong 50 cm s currents intensified near the bar crest by wave breaking are predicted to cause erosion on the shoreward slope of the bar and deposition on the seaward side the feedback amoung morphology waves circulation and sediment transport thus forces offshore bar migration during storms

modelling storm impacts on beaches dunes and barrier islands a new nearshore numerical model approach to assess the natural coastal response during time varying storm and hurricane conditions including dune erosion overwash and breaching is validated with a series of analytical laboratory and field test cases innovations include a non stationary wave driver with directional spreading to account for wave group generated surf and swash motions and an avalanching mechanism providing a smooth and robust solution for slumping of sand during dune erosion the model performs well in different situations including dune erosion overwash and breaching with specific emphasis on swash dynamics avalanching and 2dh effects these situations are all modelled using a standard set of parameter settings the results show the importance of infragravity waves in extending the reach of the resolved processes to the dune front the simple approach to account for slumping of the dune face by avalanching makes the model easily applicable in two dimensions and applying the same settings good results are obtained both for dune erosion and breaching

geochemistry of calcic horizons in relation to hillslope processes southern india abstract the study concerns chemical and mineralogical properties of calcic horizons in relation to their physical environment such as position in the soil profile and the terrain the carbonates were calcitic in upslope positions but dolomite was abundant in valley sites the content of fluorine increased downslope the distribution of the composition in the terrain indicates a sequential precipitation of calcite dolomite and fluorite from soil water and groundwater as the water flows downslope to discharge areas carbonates in vertisols formed from the more basic charnockites were of the low mg calcite type while dolomite occurred only in entisols and alfisols formed from biotite hornblende gneisses manganese in calcic horizons increased as the groundwater level was approached black intercalations in laminated petrocalcic horizons contained elevated concentrations of mn k al and si this may indicate periods of higher soil moisture and clay formation the presence of low mg calcitic carbonates overlying dolomitic ones in valley sites is likely to be due to deposition of calcite rich material from upslope the amount of soil eroded from upslope positions was between 0 2 and 2 m in four investigated profiles

early carbonate diagenetic fabrics in the rhizosphere of late pleistocene aeolian sediments analysis of a range of aeolianite deposits from the western mediterranean roquetas in southern spain and campo de tiro in mallorca and the bahamas crab cay on san salvador island has shown that biota can be important in the diagenesis of deposits in the vadose zone and this may aid in the interpretation of palaeoenvironments particularly where pedogenic calcretes are absent this paper looks at how the growth of roots into dune sediments and the associated profusion of micro fauna and micro flora can have on a local scale a profound affect on the early diagenetic and sediment fabric these fabrics are affected at three levels 1 at the site of the former root 2 directly surrounding the root the peritrophic zone and 3 from the peritrophic zone to a few tens of centimetres from the former root

phase relations of peraluminous granitic rocks and their petrogenetic implications peraluminous granitic rocks are magmatic rocks that contain quartz po tassic feldspar and or sodic plagioclase and one or more aluminum rich minerals thus these rocks contain more aluminum than could be accom modated by feldspar in a cipw normative calculation and show normative corundum peraluminous granites are defined by their chemical nature but their recognition is often based on petrography this discrepancy thus may lead to inconsistency although peraluminous granitic rocks are com monly true granite streckeisen 1973 they range from tonalite to granite of special interest are strongly peraluminous as defi ned below granite granodiorite or in rare instances trondhjemite these rocks contain alumi nous minerals almandinic garnet cordierite tourmaline muscovite that are thought to be stable in silicate magmas only if these magmas are themselves peraluminous the origin of peraluminous granite and related intrusive rocks is of much current interest many of these form shallow level intrusions and they can reach enormous sizes examples are the batholiths in the lachlan fold belt in southeastern australia white chappell 1983 the cornub ian batholith in england exley stone 1966 the south mountain batholith in nova scotia mckenzie clarke 1975 the manaslu batho lith in the himalayas lefort 1981 much of the idaho batholith

diagenetic controls on the quality of the middle permian lucaogou formation tight reservoir southeastern junggar basin northwestern china abstract the middle permian lucaogou formation in the southeastern part of the junggar basin in china is a typical tight oil reservoir with low to medium porosity average 11 2 and ultra low permeability average 0 01 md here we investigate lucaogu formation reservoir diagenesis and its impact on reservoir quality to effectively explore appraise and develop this complex tight oil reservoir core and thin section analyses scanning electron microscopy x ray diffraction xrd electron probe analyses fluid inclusions isotopic analysis pressure controlled mercury injection rate controlled mercury injection and micro nanometer x ray computer tomography ct imaging were performed on core samples from the lucaogou tight reservoir the sweet spots in the lucaogu formation are composed of siltstone tuffaceous siltstone and silty dolomite which underwent two stages of carbonate cementation primarily calcite dolomite and ankerite clay mineral alteration and dissolution and one stage of quartz cementation the major authigenic mineral cements derived from the original formation water clay mineral alteration dissolution of acid soluble components and the compaction and drainage of the interbedded mudstone are also a probable source of ions it is likely that oil emplacement occurred after quartz cementation and the first stage of carbonate cementation and feldspar dissolution but before the second stage of calcite cementation and feldspar dissolution organic acids produced during hydrocarbon generation contributed to the dissolution of the feldspar and tuffaceous components locally preventing a decrease in porosity by concurrent cementation and compaction carbonate cementation decreased the porosity and permeability of the silty dolomite substantially in the siltstone decreased permeability is largely attributed to cementation of clay minerals the pore structure of the sweet spot is controlled not only by the various cementation events but also by other geological factors such as heterogeneous dissolution of the tuffaceous component

stratigraphy and sedimentology at bir sahara egypt environments climate change and the middle paleolithic abstract bir sahara situated in northeast africa contains a set of sedimentary sequences that imply episodic changes in climate and environment during the middle and late pleistocene some of the stratigraphic contexts are associated with middle paleolithic artifact assemblages the artifact assemblages are typically in sands that underlie deposits composed of high amounts of carbonate or fine clastics muds indicative of expanding lakes and wetter climates the wetter climates may have provided landscapes that were periodically inhabitable by pleistocene hominids

freshwater limestone in an arid rift basin a goldilocks effect abstract the olduvai basin 3 s situated just west of the east african rift system in northern tanzania contains a two million year record of paleoclimate and paleoenvironmental change as well as a rich archive of vertebrate paleontology including hominins milankovitch precession cycles 20 kyr regulate the mean annual precipitation 250 700 mm yr and the 2500 mm yr evapotranspiration results in a negative hydrologic balance despite persistent aridity extensive deposits of freshwater limestone punctuate the stratigraphic record between 2 and 1 ma lake olduvai occupied the basin and its sediments are a proxy for climate driven cycles three limestones 1 84 1 80 and 1 36 ma in age which formed within lake margin floodplains adjacent to the lake were studied to determine their depositional environment using field relations sedimentary structures fossils petrography and stable isotope and major element geochemistry the three limestones are similar in that they contain peloidal micrite siliciclastic detrital grains and rhizoconcretions abundant faunal remains gastropods ostracods charaphytes and fish indicate that ponded water was relatively fresh and alkaline geochemical and stable isotope data indicate two types of groundwater sourced carbonate producing waters a deeper sourced fluid that was enriched in iron and manganese due to extended water rock interactions and a shallower groundwater that traveled through alluvial fan deposits regional faults tapped the deeper groundwater producing carbonate at spring sites while seeps associated with basinward changes in alluvial fan slopes drew on shallower groundwater sources isotope compositions indicate that fault related waters experienced some evaporation as water moved away from the spring sites while compositions of the seep related carbonate remained relatively constant pedogenic alteration and meteoric calcite cementation affected the carbonate when the spring and seep sites dried out secondary strontium rich dolomite precipitated within the limestones during burial under paleo lake olduvai sediments and fluids during periods of lake expansion integrating these data within the geological context of regional paleoclimatic and local environmental change indicates that the freshwater carbonates formed periodically when the conditions were just right i e a goldilocks effect carbonates near the basin center formed from groundwater flowing under a hydraulic head from faults or fractures during the falling limbs of milankovitch cycles when the lake was in recession carbonates near the basin margin formed from an increased rate of groundwater seepage that occurred only on the rising limbs of cycles in both contexts the continuous flow of groundwater with surface evaporation and co2 degassing optimizes the conditions for limestone formation in this arid environment these results help explain the formation of freshwater limestones in the rift basin link the carbonates to specific portions of milankovitch cycles and document that localized sources of potable water were present for use by hominins in olduvai gorge during an important period of human evolution

70 m y history 139 69 ma for the caribbean large igneous province it is commonly accepted that large igneous provinces are formed through catastrophic volcanic events occurring over vast areas within a few million years at the initiation of hotspots mantle plumes new 40ar 39ar ages 111 139 ma and geochemical results from the nicoya peninsula costa rica extend the age range of volcanism in the caribbean large igneous province to 70 m y 69 139 ma our results are not consistent with the formation of this vast igneous province through a single plume head at the initiation of a mantle plume such as the galapagos instead we propose that multiple oceanic intraplate igneous structures such as plateaus and hotspot tracks were accumulated through the subduction process the igneous structures could be remnants of the earlier history of the galapagos hotspot making it one of the oldest active hotspots on earth alternatively they could have been derived from several spatially distinct mantle melting events that sampled similar source material e g oceanic lithosphere of similar age

rip currents mega cusps and eroding dunes abstract dune erosion is shown to occur at the embayment of beach mega cusps o 200 m alongshore that are associated with rip currents the beach is the narrowest at the embayment of the mega cusps allowing the swash of large storm waves coincident with high tides to reach the toe of the dune to undercut the dune and to cause dune erosion field measurements of dune beach and rip current morphology are acquired along an 18 km shoreline in southern monterey bay california this section of the bay consists of a sandy shoreline backed by extensive dunes rising to heights exceeding 40 m there is a large increase in wave height going from small wave heights in the shadow of a headland to the center of the bay where convergence of waves owing to refraction over the monterey bay submarine canyon results in larger wave heights the large alongshore gradient in wave height results in a concomitant alongshore gradient in morphodynamic scale the strongly refracted waves and narrow bay aperture result in near normal wave incidence resulting in well developed persistent rip currents along the entire shoreline the alongshore variations of the cuspate shoreline are found significantly correlated with the alongshore variations in rip spacing at 95 confidence the alongshore variations of the volume of dune erosion are found significantly correlated with alongshore variations of the cuspate shoreline at 95 confidence therefore it is concluded the mega cusps are associated with rip currents and that the location of dune erosion is associated with the embayment of the mega cusp

crustal structure across the pacific margin of nicaragua evidence for ophiolitic basement and a shallow mantle sliver seismic wide angle measurements across the pacific margin of nicaragua were carried out using ocean bottom hydrophones and land stations recording marine airgun shots the structure and the p wave velocity of the subducting cocos and overriding caribbean plates were determined by modelling wide angle data and further constrained by coincident seismic reflection borehole and gravity data the oceanic crust of the cocos plate is 5 5 km thick with a thin pelagic sediment cover the plate boundary can be traced to 40 km depth and is generally similar to configurations derived earlier from the local seismic network a major feature of the upper plate is an 80 km wide high velocity high density rock unit reaching from the front of the margin to about the middle of the shelf this wedge shaped unit is 15 km thick beneath the shelf edge and is composed of a 5 km thick upper part with velocities increasing from 3 5 km s 1 near the trench to 5 2 km s 1 at the shelf break overlying a 10 km thick lower part with velocities of 6 0 km s 1 analysis of the gravity field requires densities of 2 6 2 7 g cm 3 for the upper part and 2 9 g cm 3 for the lower part of this unit these velocities and densities suggest that the margin wedge is composed of ophiolitic rock similar to those sampled offshore guatemala and exposed on the nicoya peninsula costa rica the velocity structure of this ophiolitic unit ends about 50 km offshore landward the basement underneath the sandino basin is formed by older igneous rock drilled beneath upper cretaceous sedimentary rock onshore nicaragua beneath the ophiolitic basement we find a sliver with velocities typical of mantle material that begins at depths of 12 18 km and coincides with the down dip limit of the seismogenic zone mantle densities are required for the sliver to match the gravity data in a tectonic reconstruction the suture of an oceanic plateau on the farallon plate against the chortis block in upper cretaceous time is suggested suturing left the former trench and margin in deep water consistent with the late cretaceous to palaeocene deep water sediment of the rivas formation at the base of the sandino basin suture of the farallon plate and chortis block might have initiated the strike slip movement along the motagua polochic fault system the development of a new subduction zone in eocene oligocene times left the ophiolitic basement and a sliver of oceanic mantle attached to the chortis block and shifted the volcanic arc about 70 km southwestwards close to its present position

physico chemical environment of pedogenic carbonate formation in devonian vertic palaeosols central appalachians usa the morphology and geochemistry of pedogenic carbonate found in vertic claystone palaeosols in the devonian catskill formation in central pennsylvania preserve a record of the physical and chemical environment of carbonate precipitation the carbonate is characterized by three distinct petrographic generations pedogenic rhizoliths and nodules are the earliest precipitated generation and typically consist of dull red brown luminescent micrite clear equant calcite spar cement fills voids in the centres of rhizoliths as well as circumgranular cracks and septarian voids in nodules early spar cements are nonluminescent to dull luminescent whereas later spar cements exhibit bright yellow orange luminescence late stage pedogenic fractures are always occluded with very bright yellow orange luminescent spar cements the incorporation of progressively higher concentrations of mn up to 34 000 ppm into successively younger calcite spar cements without concomitant increases in fe suggests carbonate precipitation from an evolving meteoric water in which mn2 became increasingly mobile over time the increased mobility is possibly due to decreasing eh resulting from oxidation of organic matter after rapid soil burial on the floodplain the amount of fe2 available for incorporation into calcite was limited because most iron was immobile having been earlier oxidized and bound to the palaeosol clay matrix as a poorly crystallized ferric oxide or oxyhydroxide mineral carbon isotope compositions of pedogenic carbonate correlate with the inferred depth of carbonate precipitation rhizoliths preserved below the lowest stratigraphic occurrences of pedogenic slickensides are consistently depleted in 3c relative to nodules which formed stratigraphically higher within the zone of active soil shrink and swell processes nodular carbonate precipitated in proximity to deep cracks in the soil is enriched due to increased gas exchange with isotopically heavy atmospheric c02 accordingly rhizolith compositions will most accurately estimate palaeoatmospheric levels of c02 the use of nodule compositions may result in overestimation of pco by as much as 30

seismic stratigraphy and structure of the caribbean igneous province multichannel seismic reflection profiles reveal the tectonic and stratigraphic complexity of the caribbean province a smooth reflector b forms the top of widespread cretaceous volcanic flows drilled during dsdp leg 15 and odp leg 165 in addition we also show other seismic evidence of volcanism sills and dipping horizons the southern parts of the venezuela and colombia basins are underlain by rough basement that shows many characteristics of oceanic crust several thick volcanic plateaus separated by deep basins with thin crust comprise the caribbean volcanic province in the venezuela basin only normal oceanic crust is present but in other basins this crust is underlain by underplated volcanic material the crust of the venezuela basin is less than 5 km thick and was probably formed in the pacific during the jurassic refraction data indicate that a thin layer 2v is formed by original oceanic crust overlain by a thin volcanic layer an intra basement reflection sub b marks the top of original oceanic crust that is sandwiched between an upper volcanic layer and lower underplated material this underplated layer forms a very thick layer 3v beneath the beata and nicaragua volcanic plateaus the upper part of layer 3v is gabbroic and outcrops on the beata ridge the thickening occurs mainly in the high velocity lower part of layer 3v and is attributed to the presence of magnesian rocks picrites or ultramafic cumulates a highly reflective horizon r is located at the top of this layer we present a model where the galapagos plume is 2500 km wide but has narrow heads that formed the caribbean volcanic plateaus separated by deep basins the caribbean igneous province is compared to similar volcanic features in the indian ocean kerguelen plateau and in the western pacific ocean ontong java plateau nauru and pigafetta basins the thick plateaus of the caribbean province resemble the kerguelen and ontong java plateaus whereas the deep basins present some similarities with the nauru and pigafetta basins the caribbean igneous province is younger 90 75 ma than the indian and pacific plateaus 110 130 ma however several geological evidences from land and geophysical offshore observations indicate that the onset of volcanic activity in the caribbean province could be as old as 115 ma

modelling infragravity motions on a rip channel beach a non linear shallow water wave model operating on the time scale of wave groups is compared with measurements of infragravity motions on a rip channel beach to verify the model concepts and assess the model performance the measurements were obtained during the rip current experiment ripex in concert with the steep beach experiment sbe performed at sand city monterey bay ca during the spring of 2001 the nearshore bathymetry was made up of shore connected shoals incised by relatively narrow rip channels spaced approximately 125 m apart the comparison considers a 20 day period during which significant changes in both the offshore wave climate and nearshore bathymetry occurred the temporal variation in infragravity conditions during the experiment is strong with computational results typically explaining 70 to 80 of the observed infragravity motions within the nearshore in contrast to the temporal variation the alongshore spatial variation in infragravity intensity during the experiment is generally weak even though the underlying bathymetry shows strong depth variations model computations suggest preferential coupling between the computed edge wave motions and the quasi periodic bathymetry is present a prerequisite for strong spatial variability however the infragravity field is dominated by cross shore infragravity motions which are only weakly coupled to the quasi periodic bathymetry resulting in a weak alongshore variability of the total infragravity motions

a classification of rhizogenic root formed calcretes with examples from the upper jurassic lower cretaceous of spain and upper cretaceous of southern france reply abstract studies of fluvial sequences from the upper jurassic and lower cretaceous of the cameros basin spain and the upper cretaceous of the aquitaine basin france have revealed large numbers of calcrete profiles which have formed wholly or largely through the action of plant root calcification those from spain have formed by the calcification of root mats whereas those from france resulted from the accumulation of calcified root cells microcodium these root related rhizogenic calcretes developed in stages allowing developmental time series chronosequences to be identified contrasting with existing models for calcrete profile growth they provide unique examples where vegetation has been the dominant control on profile development and probably indicate specific vegetation types and palaeoecological settings the role of root activity in forming some quaternary calcretes is also discussed

transient fluctuation in paleoclimate at the end of permian new constraints from paleosol carbonates in the erlongkou section chongqing southwestern china abstract paleosol carbonates were found in the erlongkou section at the top of the changhsing formation in chongqing southwestern china detailed petrology study suggested that these paleosol carbonates have typical pedogenic structures and fabrics conodont biostratigraphy volcanic ash zircon dating and stratigraphic carbon isotopes indicated that the paleosol carbonates formed at the end of permian 252 3 1 0 ma geochemical results indicate two different provenances for the paleosol i e the micrite of the changhsing formation and volcanic ash the calcretes and carbonates in the paleosol are pedogenic calcites that precipitated under well drained conditions and they are free of carbonate debris from the bedrocks atmospheric co2 concentrations pco2 estimated based on the model of cerling 1999 suggested that there was greenhouse paleoclimates with a transient fluctuation in pco2 at the end of the permian

shrinkage and sediment supply control on multiple calcrete profile development a case study from the maastrichtian of central india multiple calcrete profiles of the mottled nodular beds form part of an extensive dinosaur bearing infratrappean regolith in central india the profiles have clear variations in the distribution of carbonate morphologies powdery nodular and sheet platy calcretes sheet calcrete is however distinctive and resembles to some extent a k horizon these calcretes are interpreted as having formed in the soil vadose zone profile development was governed mainly by the pattern and depth of shrinkage it is possible that the mature stage 3 to incipient stage 4 calcrete sheets of these shrinkage related profiles formed quite rapidly and certainly in no more than about 400 000 years prominent shrinkage cracking appears to have favoured quite rapid calcrete formation and associated rhizocretion formation was also likely to have been rapid a combination of climatic conditions carbonate availability and volcanism induced sediment starved conditions resulted in the development of a stack of fourteen calcrete profiles sediment starvation allowed most of the stratigraphic interval to be affected to some degree by calcareous pedogenesis this contrasts with other phanerozoic calcrete stacks where nodular carbonate typically occurs in discrete horizons caused by relatively higher sediment supply

a note on the three dimensional shear stress distribution in a surf zone the three dimensional time mean shear stress distribution in a surf zone is analyzed thereby determining the vertical distribution of the driving forces due to wave breaking it is found that on a uniform coast the longshore current can be described as driven by a surface shear stress equal to the cross shore gradient in the shear component of the radiation stress the near surface shear stress is found to be in the direction of wave propagation having a magnitude determined by the dissipation of wave energy it can be determined as the shear force acting on the waves from the surface rollers of spilling breakers or broken waves

use of micromorphology for palaeoenvironmental interpretation of complex alluvial palaeosols an example from the mill creek formation albian southwestern alberta canada abstract field observations are often not sufficient for process based interpretations of palaeosols particularly where they form parts of thick aggradational pedocomplexes within alluvial successions under such conditions micromorphology provides genetic temporal and spatial information on soil forming processes that is critical to an understanding of past environmental conditions thick alluvial successions of the albian mill creek formation contain abundant evidence of pedogenesis but few well developed palaeosol profiles and therefore provide an ideal case study in which to demonstrate the usefulness of micromorphological data for palaeoenvironmental interpretation the micromorphological features of greatest interpretive value are types of clay coatings and ferruginous segregations structure and fabric papules evidence of bioactivity and ferruginous concretions provide information on geomorphic surface stability and assist in reconstructing temporal changes in drainage conditions while individual features can provide some palaeoenvironmental information the relationships of features to one another and assemblages of features provides additional information when analysed hierarchically to establish a sequence of sedimentologic and pedogenic events a common recurring sequence of palaeoenvironmental events subject to local variations can be recognized throughout the mill creek formation the presence of illuvial clay requires that water percolated through the soil and that the soil periodically dried out so that the translocated clay was retained dark reddish clay coatings indicate clay illuviation under freely drained conditions while pale yellow and silty clay coatings suggest that phases of free drainage alternated with phases of poorly drained or saturated soil conditions the presence of iron depletion coatings iron nodules and quasiferrans indicates that these units were at least periodically saturated and the occurrence of multiple overlapping phases within single thin sections demonstrates that redox conditions fluctuated strongly suggesting development in the vadose zone recent soils containing similar assemblages of features develop under warm temperate seasonal climates alternating phases of well drained and saturated conditions on the mill creek floodplains are attributed to changing sediment supply and local palaeogeomorphology rather than to any major regional climate change this type of process based micromorphological analysis should have broad application in other complex pedogenically modified alluvial successions and similar studies would lead to a more detailed understanding of ancient palaeoenvironments

the distribution and discrimination of shallow authigenic carbonate in the pliocene pleistocene palomas basin southern rio grande rift nine types of authigenic carbonate are present in the pliocene pleistocene alluvial fan and axial fluvial sediment of the palomas half graben in the southern rio grande rift pedogenic and other vadose carbonate includes 1 pedogenic carbonate of stage ii and stage iii morphology underlying bw and bt horizons 2 mudstones in which the carbonate nodules may be pedogenic or the result of shallow groundwater invasion of the vadose zone 3 pedogenic calcic nodules and tubules in eolian sand and 4 gully bed cement of proximal fan conglomerates formed by infiltration and evaporation of surface runoff shallow groundwater carbonate exists as 5 thin 30 50 cm massive beds with an upper fringe of nodules and tubules precipitated at the water table and in the capillary fringe 6 thick 1 5 3 m massive beds deposited by lateral flow of groundwater or at springs and 7 thin 30 cm calcified root mats associated with near surface water saturated sediment or springs phreatic spar cements 8 occupy the interstices of conglomerates and sandstones and locally exist as 9 oriented concretions groundwater carbonates are best developed near the toes of the large hanging wall derived alluvial fans whereas phreatic cement preferentially exists in footwall derived alluvial fan conglomerates pedogenic carbonate is distinguished from groundwater carbonate by the association with other diagnostic paleosol horizons a predominantly vertical arrangement of root traces peds desiccation cracks and calcic tubules and by gradational contacts the 13 c and 18 o values are similar among pedogenic and shallow groundwater carbonate although locally nodules in eolian sand and gully bed cement have higher 18 o values perhaps due to the effects of evaporation some phreatic cements may be distinguished from pedogenic and shallow groundwater carbonate by lower values of 13 c and 18 o authigenic carbonate in footwall derived alluvial fan sediment has consistently higher values of 13 c than that in hanging wall derived sediment which may reflect differences in vegetative type and or density on either side of the basin

large scale dune erosion tests to study the influence of wave periods article i nfo large scale physical model tests were performed to quantify the effects of the wave period on dune erosion attention was focussed on 2d cross shore effects in a situation with sandy dunes and extreme water levels and wave conditions besides profile measurements detailed measurements in time and space of water pressure flow velocities and sediment concentrations were performed in the near near shore area it was concluded that a longer wave period leads to a larger dune erosion volume and to a larger landward retreat of the dune face tests with double peaked wave spectra showed that the influence of the spectral shape on dune erosion was best represented by the tm 1 0 spectral mean wave period better than the peak wave period tp the effect of the wave period on dune erosion was implemented in a dune erosion prediction method that estimates erosion volumes during normative storm conditions for the dutch coast more details of the measurements and additional analyses of physical processes are described in an accompanying paper by van thiel de vries et al van thiel de vries j s m van gent m r a reniers a j h m and walstra d j r submitted for publication analysis of dune erosion processes in large scale flume experiments in this volume of coastal engineering

multiproxy studies of early miocene pedogenic calcretes in the santa cruz formation of southern patagonia argentina indicate the existence of a temperate warm vegetation adapted to a fluctuating water table abstract the lower miocene santa cruz formation in southern patagonia austral basin argentina contains several horizons of pedogenic calcretes which record 17 5 myr old vegetation adapted to a shallow and fluctuating water table at paleolatitude of 56 s to reconstruct the paleoenvironment paleoclimate and paleoecosystem we performed a multiproxy study of the calcretes examining abiotic and biotic components the calcretes exhibit a variety of morphologies horizontal and vertical rhizoliths laminar structure nodules massive crusts microfabrics beta predominant over alpha microfabrics and 18o and 13c values that fluctuate within each morphotype and throughout the analysed interval microfossils and phytoliths in the host material of the calcretes indicate fluctuating terrestrial freshwater and marine conditions and record an ecosystem dominated by herbaceous plants and arboreal elements in association with a typical coastal santacrucian vertebrate fauna we propose that the calcretes developed in soils in a coastal fluvial setting during pauses in floodplain aggradation that typically lasted between 8 25 ka and 400 ka years variable sedimentation rates in different parts of the coastal fluvial floodplain the fine texture of the host sediment and the influence of a fluctuating water table also influenced the formation of the calcrete a high water table in low relief areas of the floodplain created the conditions necessary to form a horizontally extended rhizolithic system that jointly with the biotic proxy can be correlated with a radicular pattern similar to the arboreal elements from coastal settings abiotic and biotic proxies of the studied interval attest to environmental fluctuations recorded at different scales that took place under temperate warm and subhumid climates with a marked rainfall seasonality with a slight increase in the aridity towards the top of the studied interval under these conditions a subtropical fauna and a c3 dominated ecosystem developed coincident with the onset of the mid miocene climatic optimum in patagonia

primary siliceous rhizoliths from loop road hot springs north island new zealand abstract siliceous rhizoliths and rhizocretions which developed in diatomaceous sediments at loop road hot springs on the north island of new zealand started to form while the plants were alive or shortly after their demise the porous and solid laminae in the rhizoliths and rhizocretions incorporate well preserved filamentous microbes spores and diatoms the silica needed for the formation of these laminae came from the hydrothermal fluids that permeate through the sediments silica precipitation was triggered by degradation of the plant tissue which caused a local increase in co2 and a decrease in ph during the initial phase of rhizolith development the plant tissues acted as a template for silica precipitation the silica precipitated during these early phases provided the sites for later more extensive silica precipitation

vertical flow structure during sandy duck observations and modeling abstract observations of the vertical distribution of time averaged cross shore and alongshore flows during the sandy duck field experiment are compared with model predictions to assess the parameters governing the flow behaviour the measurements were obtained with a vertical stack of eight two component current meters with the lowest and highest sensor at respectively o 0 1 and o 2 7 m above the bed observations under breaking wave conditions within the surfzone show that the maximum return flow velocities occur in the lower part of the water column consistent with laboratory observations under non breaking conditions outside the surfzone the maximum return flow velocities are observed closer to the water surface again in line with laboratory results analogous to previous observations the measured longshore current velocity profiles are logarithmic under non breaking conditions and become more depth uniform under breaking conditions the model description of the vertical structure of the flow includes the presence of wind stresses wave stresses pressure gradients turbulent eddy viscosity and a wave boundary layer the model utilizes parabolic shape functions to describe the vertical distribution of the turbulent eddy viscosity in the middle layer and within the bottom boundary layer eddy viscosity is enhanced in regions where turbulence is produced i e near the surface in the case of breaking waves and within the bottom boundary layer estimates of the wave breaking induced turbulent eddy viscosity and bottom friction are obtained by minimizing the model measurement discrepancies predictions utilizing calibrated expressions for both the turbulent eddy viscosity and bottom friction are in general agreement with the observations provided the wave transformation and associated mass flux are modeled correctly and a parabolic eddy viscosity distribution is used using a piecewise constant eddy viscosity distribution generally results in a degrading of the agreement between measurements and model results

morphodynamic modelling of rip channel growth the morphological evolution of a rip channel system due to normally incident waves has been investigated using two different approaches the first approach is a state of the art commercial 2dh coastal area model whereas the second approach is a linear stability analysis the results of the two different approaches have been compared and both models predict rip channel spacings and initial growth rates of similar magnitudes

the juchatengo complex an upper level ophiolite assemblage of late paleozoic age in oaxaca southern mexico the juchatengo complex jc suite is located between the proterozoic oaxacan complex to the north and the xolapa complex to the south and was amalgamated by late paleozoic magmatism it consists of mafic and sedimentary rocks that have oceanic affinities with internal pseudostratigraphic structural and metamorphic characteristics which resemble a typical upper level ophiolite assemblage new u pb zircon and previous hornblende k ar analyses yield ages of ca 291 313 ma u pb for plagiogranites and ca 282 277 ma for tonalites intruding the entire sequence including pelagic sediments at the top with a maximum deposition age of ca 278 ma and noteworthy local provenance these data constrain the age of the jc to the late pennsylvanian early permian period hf isotopic analyses obtained from zircons in the jc plagiogranite and tonalite show that they come from a similar primitive mantle source 176hf 177hf 0 282539 0 283091 hf t 3 2 to 15 0 hf t values from near 0 to 2 8 in the tonalites indicate a contribution from the continental crust trace elements and ree patterns in whole rock and zircons point to a primitive mantle source for differentiated mafic plagiogranite dykes and tonalitic plutons geochronological and geochemical data address the generation of new oceanic crust above the subduction zone probably in a backarc setting in this tectonic scenario the jc ophiolite originated due to the convergence of the paleo pacific plate below the already integrated oaxacan and acatlan complexes in western pangea the dextral displacement places the deformation in a transtensional regime during the late paleozoic age

the origin of oceanic plagiogranites from the karmoy ophiolite western norway both field relationships and geochemical characteristics indicate two suites of plagiogranitic and related rocks coexisting in the higher parts of the karmoy ophiolite of western norway the plutonic zone of this ophiolite can be subdivided into three complexes the east karmoy igneous complex the visnes high level complex and the veavagen igneous complex and plagiogranitic rocks are well developed in the first two of these within the east karmoy igneous complex plagiogranites are associated with high temperature pre basic dyke shear zones rare earth element modelling indicates that these plagiogranites were derived by anatexis of amphibolite hydrated diabase assuming a starting material consisting of 40 hornblende and 60 plagioclase and that batch melting occurred within the stability field of hornblende in comparison plagiogranite occurs in a number of bodies in the upper part of the visnes high level complex and forms a sandwich horizon together with biotite diorites and epidosites between a roof assemblage of dykes microgabbros and magnetite gabbros and a floor assemblage of layered and non layered gabbros the r e e modelling of the petrogenesis of this series of plagiogranites indicates that they were derived by filter pressing of a differentiated interstitial liquid to the vari textured gabbros although the distribution of highly hygromagmatophile elements such as k rb ba etc cannot be explained satisfactorily by this model alone depletion in these elements appears to be an autometasomatic effect

an isotopic study of a fluvial lacustrine sequence the plio pleistocene koobi fora sequence east africa stable isotopic analyses of plio pleistocene and modern sediments in the fluvial lacustrine system occupying the turkana basin east africa provide constraints on the paleoenvironmental and diagenetic histories of the pliocene through the recent sediments in the basin the 13c values for carbonates in lacustrine sediments range from 15 to 22 relative to pdb depending on the varying proportions of co2 from the atmospheric reservoir and from various metabolic sources the 18o values of carbonates in lacustrine sediments indicate that the isotopic composition of paleolake water varied by over 10 from the pliocene to the present the 13c values for pedogenic carbonates record paleoccologic variations and suggest that c4 plants did not become well established in the preserved depositional parts of the basin until about 1 8 myr ago the 18o values pedogenic carbonates suggest a range of over 10 for the isotopic composition of soil water during this interval they also suggest a period of major climatic instability from about 3 4 to 3 1 myr and at about 1 8 myr together the 13c and 18o values of pedogenic carbonates indicate that the present conditions are as arid and hot as any that had prevailed during deposition of these plio pleistocene sediments

petrography micromorphology and genesis of holocene pedogenic calcrete in al jabal al akhdar sultanate of oman abstract holocene pedogenic calcrete nodules occur within quaternary terra rossa soil that fills shallow phytokarst cavities in the permian triassic carbonate sequence of al jabal al akhdar in the sultanate of oman they are preferentially clustered around plant roots and are almost wholly formed of calcite holocene pedogenic calcrete nodules developed within a mediterranean like climate that is characterized by intermittent wet and dry seasons petrographic examination revealed that the host terra rossa soil is partially calcitized and that the calcrete nodules consist of smaller coalesced diffused and or discrete granular nodules the latter consist of several types of rhizoliths floating in a groundmass formed of micrite and microspar as well as spar to a lesser extent several textures of calcrete groundmass are recognized the most common is partially neomorphosed micrite three types of rhizoliths are recognized root moulds root casts and petrified calcified root tissues the latter were developed by mimic replacement of the tissue cells by cytomorphic calcite sem investigation of the calcite forming the calcrete nodules revealed that it consists of closely stacked sheets formed of calcareous nanoglobules agglutinated by eps and entombed calcareous microbial fossils which indicate a biogenic origin the calcareous nature of the bedrock climatic conditions and prevalence of biological activities of microorganisms associated with the plant root system in the host soil have significantly controlled the development and texture of these nodules the relative abundance of scalenohedral calcite within the calcrete groundmass is attributed to the impact of mg ions and organic compounds it has been concluded that the studied pedogenic calcrete is a type of freshwater terrestrial thrombolitic microbialite this study may help improve the understanding of the palaeoenvironmental conditions of analogue fossil calcretes

a plate tectonic model for the mesozoic and early cenozoic history of the caribbean plate abstract we present a model in which the caribbean plate is an intra american feature formed along the caribbean spreading center as opposed to the current model that considers the caribbean plate as a far travelled crustal segment that formed in the pacific region paleomagnetic data which cover an age range from jurassic through paleocene indicate the ophiolite complexes in costa rica and panama formed in an equatorial position the ophiolites did not fundamentally change their position relative to south america since their origin basaltic rocks of the lower part of the ophiolites are of mid ocean ridge type suggesting formation at a spreading center they are interpreted to have formed as part of the proto caribbean crust at a spreading axis in an intra american position during jurassic and early cretaceous times the upper part of the ophiolites is mainly built up by island arc and intraplate basalts the island arc basalts evolved at the central american landbridge which started in the middle cretaceous the intraplate basalts are related to the caribbean plateau basalt which thickened and stiffened the caribbean crust in middle cretaceous to probably campanian times changes in relative plate motions along the middle american convergent plate margin are reflected by orientations and time sequences of paleostress tensors calculated from fault slip data in southern mexico and costa rica kinematic indicators from mylonite zones display sinistral movement along secondary shear zones within the southern mexican crustal complexes and along the northern boundary of the chortis block in the motagua polochic fault system this reflects the eastward drift of the chortis block since the beginning of the cenozoic at this time the chortis block became part of the caribbean plate which moved more than 1000 km towards the east relative to the north and south american plates during the cenozoic

diagenetic carbonate in miocene sandstone reservoir san joaquin basin california stevens arkosic sandstones deposited about 7 ma in a deep marine environment form the north coles levee reservoir dolomite and calcite cements occur as zones averaging 1 ft thick and having less than 300 ft of lateral continuity based on dgr18o measurements and estimated pore fluids between 0 and 2 8 the dolomite formed at 30 40 c an early calcite cement formed at 50 60 c and a late pore filling and plagioclase replacing calcite formed at 70 80 c corresponding to these temperature estimates dolomite formed in the zone of methanogenesis dgr13c 4 to 8 and calcites in the zone of decarboxylation dgr13c 3 to 12 dolomite has between 4 and 12 mole fe and calcites contain up to 8 mole o fe mg mn with a characteristic fe mn mg ratio of about 3 1 the latest calcite is the most ca rich due in part to ca sourced from plagioclase dissolution calcite cements in nonmarine sections have markedly high mn contents to 5 mole compared with the marine section at north coles levee the volume of cement systematically decreases from about 40 20 over the cementation temperature interval suggesting progressive compaction of the sandstones present day mean porosity of the reservoir 15 appears to be largely a result of mechanical compaction a very late dolomite cement with high fe content 13 mole and light carbon dgr13c 14 is found in the cleavage flakes of crushed biotite the dolomite is calculated to have formed from a fluid with a dgr18o of 0 9 at present reservoir temperatures of 105 c suggesting precipitation in the past 20 years during release of the gas cap and injection of isotopically light water calcite also forms thick scales in production tubing primarily as a result of pressure drops from 4 900 to 400 psi in the tubing string the calcites have high mg contents 7 mole due to the composition of the injection water calculated distribution coefficients for fe and mn are similar to published values but mg is two times higher than published values at 90 c similar results are obtained for late calcite cements a time temperature plot for north coles levee constrains the timing of dolomite cementation between 5 and 6 ma and the calcite cementation between 4 and 2 5 ma plagioclase dissolution and hydrocarbon emplacement occurred within the past few million years suggesting that the presence of the hydrocarbons and associated byproducts of kerogen maturation enhance porosity despite the narrow window for diagenesis minimum fluid fluxes of only 0 02 cm3 yr per 100 cm3 rock are required to account for the volume of dissolved plagioclase

calcrete as a source of heterogeneity in triassic fluvial sandstone aquifers otter sandstone formation sw england carbonate is not generally considered as a potential source of large scale heterogeneity in permo triassic sandstone aquifers this study shows that carbonate in the form of early diagenetic calcrete forms an abundant component of the triassic otter sandstone formation in south devon three types of calcrete are described from the outcrop of this fluvial sandstone dominated aquifer rhizocretions calcrete sheets and calcrete conglomerates data obtained from core plugs show that calcrete reduces sandstone permeability to less than 10 15 m2 and porosity to less than 12 calcrete can be sufficiently abundant to produce a significant up to 30 reduction in the total effective porosity of the aquifer moreover calcrete conglomerates can form laterally extensive up to 1 km low permeability sheets that will represent major baffles to vertical flow

the formation of sio2 rich melts within the deep oceanic crust by hydrous partial melting of gabbros small amounts of felsic evolved plutonic rocks often called oceanic plagiogranites always occur as veins or small stocks within the gabbroic section of the oceanic crust four major models are under debate to explain the formation of these rocks 1 late stage differentiation of a parental morb melt 2 partial melting of gabbroic rocks 3 immiscibility in an evolved tholeiitic liquid and 4 assimilation and partial melting of previously altered dikes recent experimental data in hydrous morb type systems are used to evaluate the petrogenesis of oceanic plagiogranites within the deep oceanic crust experiments show that tio2 is a key parameter for the discrimination between different processes tio2 is relatively low in melts generated by anatexis of gabbros which is a consequence of the low tio2 contents of the protolith due to the depleted nature of typical cumulate gabbros formed in the oceanic crust on the other hand tio2 is relatively high in those melts generated by morb differentiation or liquid immiscibility since the tio2 content of many oceanic plagiogranites is far below that expected in case of a generation by simple morb differentiation or immiscibility these rocks may be regarded as products of anatexis this may indicate that partial melting processes triggered by water rich fluids are more common in the deep oceanic crust than believed up to now at slow spreading ridges seawater may be transported via high temperature shear zones deeply into the crust and thus made available for melting processes

petrology and geochemistry of rhizoliths from plio pleistocene fluvial and marginal lacustrine deposits east lake turkana kenya abstract recent studies of rhizoliths from the koobi fora formation plio pleistocene of east lake turkana kenya indicate that their shapes vary with the depositional environment of their host sediment vertical rhizoliths are associated with channel bar and overbank deposits of fluvial origin these rhizoliths represent the tap roots of phreatophytes that lived in well drained upland settings horizontal rhizoliths predominate in facies ascribed to beaches lagoons and floodplains associated with ancient lake turkana they are produced by water emergent aquatic macrophytes and coastal grasses that grow in saturated or poorly drained soil environments the rhizoliths consist of sparry and micritic calcite cements whose petrography and geochemistry reflect differences in diagenesis between the two types of root systems the vertical roots contain little organic debris and have poorly developed meniscus and microstalactitic cements these attributes reflect precipitation of the calcite in well oxygenated well drained vadose conditions in contrast the cements of the horizontal root systems lack textures indicative of vadose conditions contain abundant clay and plant debris and display mn concentrations as high as 4 5 cation percent these features confirm that the horizontal rhizoliths developed in chemically reduced water saturated conditions bacterial decay of the plant debris maintained reduced eh conditions while also supply ng abundant mn2 to the pore solutions groundwater that had moved through volcanic strata may have also contributed mn2 the mn may have been incorporated in the calcite cements through a combined process of chemisorption and ionic substitution of mn2 as well as by inclusion of manganese oxides

generation of plagiogranite by amphibolite anatexis in oceanic shear zones field geochronological and rare earth element ree evidence obtained from the fournier oceanic fragment of the canadian appalachians indicates that its plagiogranite component was generated by the anatexis of amphibolite and not by the fractionation of basic magma we propose that the process occurred in two stages first gabbro was plastically deformed at high temperature to form dry low angle shear zones that subsequently evolved to amphibolite via the addition of light ree enriched hydrothermal solutions second the amphibolite underwent partial melting during shear to yield a migmatite comprising bands of plagiogranite alternating with amphibolite restite the plagiogranite locally coalesced to form pods dikes and lenses that injected the surrounding undeformed gabbro we attribute the development of the fournier plagiogranite to dynamothermal processes occurring in proximity to a spreading center due to asthenosphere induced shear within ocean layer 3 this serves as an important illustration of the dynamic nature of metamorphism and melting that can occur in the ocean crust

paleoenvironmental interpretation of lake margin deposits using δ13c and δ18o results from early pleistocene carbonate rhizoliths olduvai gorge tanzania isotope analyses of lake margin rhizoliths from paleo lake olduvai ca 1 75 ma form the basis of a new model proposed here for interpreting stable isotope values of phreatic rhizolith carbonates average 18o and 13c values from rhizoliths formed in transgressive lacustrine waxy claystones are relatively low low 18o averages 5 5 reflect meteoric water values consistent with increased precipitation during wet periods that would increase the outflow of fresh water from subsurface seeps and shift a brackish groundwater zone lakeward low 13c averages 4 1 could indicate little atmospheric exchange high plant decay and or increased groundwater low 13c flow higher averages in 18o 3 6 and 13c 2 0 occur during dry periods and lake regressions earthy claystones when hydraulic head is reduced the lake recedes and water within the wetlands is subjected to intense evaporation and gas exchange with the atmosphere the isotope ratios of the rhizoliths from lowermost bed ii change in response to groundwater hydrology on milankovitch time scales but the isotopes also provide evidence of shorter term decadal to centennial scale climate fluctuations the orbitally driven climate changes are recorded faithfully by lithologic variations and stable isotope patterns

pseudobreccias revealed as calcrete mottling and bioturbation in the late dinantian of the southern lake district uk two types of pseudobreccia one with grey and the other with brown mottle fabrics occur in shoaling upward cycles of the urswick limestone formation of asbian late dinantian carboniferous age in the southern lake district uk the grey mottle pseudobreccia occurs in cycle base packstones and developed after backfilling and abandonment of thalassinoides burrow systems burrow infills consist of a fine to coarse crystalline microspar that has dull brown to moderate orange colours under cathodoluminescence mottling formed when an early diagenetic aerobic decay clock operating on buried organic material was stopped and sediment entered the sulphate reduction zone this probably occurred during progradation of grainstone shoal facies after which there was initial exposure to meteoric water microspar calcites then formed rapidly as a result of aragonite stabilization the precipitation of the main meteoric cements and aragonite bioclast dissolution post date this stabilisation event the brown mottle pseudobreccia fabrics are intimately associated with rhizocretions and calcrete which developed beneath palaeokarstic surfaces capping cycle top grainstones and post date all depositional fabrics although they may also follow primary depositional heterogeneities such as burrows they consist of coarse inclusion rich microspar calcites that are always very dull to non luminescent under cathodoluminescence sometimes with some thin bright zones these are interpreted as capillary rise and pedogenic calcrete precipitates the d 18 o values 5 to 8 pdb and the d 13 c values 2 to 3 pdb of the pseudobreccias are lower than the estimated d 18 o values 3 to 1 pdb and d 13 c values of 2 to 4 pdb of normal marine calcite precipitated from late dinantian sea water reflecting the influence of meteoric waters and the input of organic carbon

upper pennsylvanian permian naco group paleosols north central arizona field and isotopic evidence deposits of the upper naco group upper pennsylvanian permian north central arizona include features consistent with multiple episodes of soil formation the preserved paleosols are developed on shallowing upward regressive carbonate cycles that accumulated on the margins of a shallow shelf platform field evidence of paleosols consist of a monopodial taproot like and branching root and rootlet trace fossils b cylindrical carbonate nodules rhizoliths c subspherical carbonate nodules that occur as discrete entities in the paleosol and d an accumulation of red clay and quartz silt petrographic evidence consists of a plasmic fabrics analogous to those found in some modern soils cf brewer 1976 b microscopic carbonate nodule accumulations c soil nodule spar crystallaria and d alveolar septal structures paleosol profiles occur as a lithified in situ partly eroded clay rich horizons that commonly grade into unmodified parent material at depth below the disconformity and b reworked pedogenic carbonate nodule horizons oxygen and carbon stable isotope ratios of pedogenic carbonate are depleted relative to local marine carbonate this suggests that isotopically light soil co2 and meteoric water were present during pedogenic carbonate precipitation the depleted stable isotope ratios are consistent with an interpretation of soil carbonate precipitated under near surface conditions the presence of well developed calcareous rhizoliths and soil nodules in a red bed matrix and desiccation like crystallaria in host rhizoliths may qualitatively signify seasonally dry paleoclimatic conditions alteration of the underlying limestone units and development of multiple albeit truncated paleosols record a cyclical shift between marine and terrestrial paleoenvironmental conditions near the pennsylvanian permian boundary 280 m y ago

depositional and compositional controls on sandstone diagenesis the tetori group middle jurassic early cretaceous central japan abstract the tetori group is a representative middle jurassic to lower cretaceous deposit in the northern central part of honshu japan the tetori group is divided into the kuzuryu itoshiro and akaiwa subgroups with decreasing age it consists mainly of a marine sequence in the lower part kuzuryu subgroup and non marine sequence in the upper part akaiwa subgroup with transitional sequence in the middle part itoshiro subgroup diagenesis of tetori group sandstones comprises compaction cementation and replacement early diagenetic events include mechanical compaction and pore filling cementation of calcite siderite dolomite and formation of kaolinite late diagenesis includes formation of quartz cement and albitization of feldspar and formation of illite chlorite calcite ankerite and pyrite among the diagenetic minerals carbonate forms the major cement in tetori group sandstones calcite of each subgroup is subdivided into two or three types based on cathodoluminescence tetori group sandstones of marine fresh water and mixed depositional environments are characterized by distinct chemical compositions of early diagenetic calcites late diagenesis seems to have been mainly influenced by detrital compositions of the sequence burial temperatures and partially hydrothermal processes especially late diagenetic carbonate and authigenic clay minerals seem to have formed mainly from ions released from clay mineral transformations in the adjacent shales based on the illite crystallinity ic the kuzuryu subgroup exhibits higher diagenetic grades than the overlying itoshiro and akaiwa subgroups indicating that the diagenesis of the tetori group was mainly affected by depth of burial

mineralogy and origin of rhizoliths on the margins of saline alkaline lake bogoria kenya rift valley abstract a wide range of rhizoliths occurs around the margins of lake bogoria kenya these include root casts moulds tubules rhizocretions and permineralised root systems these rhizoliths are variably composed of opaline silica calcite zeolites mainly analcime fluorite and possibly fluorapatite either alone or in combinations some rhizoliths are infilled moulds with detrital silicate grains most rhizoliths are in situ showing both vertical and horizontal orientations reworked rhizoliths have been concentrated locally to form dense rhizolites hot spring fluids concentrated by evapotranspiration and capillary evaporation have provided most of the silica for the permineralisation of the plant tissues precipitation involved the growth of silica nanospheres and microspheres that coalesced into homogeneous masses calcite rhizoliths formed following evaporative concentration evapotranspiration and or co2 degassing of ca bearing runoff water that infiltrated the sediment or by mixing of runoff with saline alkaline groundwater fluorite precipitated in areas where mixing of hot spring and meteoric waters occurred or possibly where hot spring fluids came into contact with pre existing calcite zeolitic rhizoliths formed during a prolonged period of aridity when capillary rise and evaporative pumping brought saline alkaline waters into contact with detrital silicate minerals around roots

using petrography and geochemistry to determine the origin and formation mechanism of calcitic plant molds rhizolith or tufa rhizoliths root casts and rhizocretions are prolific in the geologic record and their morphology and isotopic signatures can be important indicators of paleoenvironment and paleoclimate in a semiarid playa in nevada the calcite plant encrustations preserved at the playa surface resemble rhizoliths morphologically and would seem to indicate pedogenesis and root activity the features are preserved as short thin upright and or overturned tubes that often exhibit an apron at the base branching is rare and the tubes can be filled with sediment or calcite or left open however detailed macroscopic microscopic and geochemical analyses of these features reveal that they are instead macrophyte stem encrustations that form in standing water while these features form in a manner similar to paludal tufa they do not resemble typical tufa deposits e g barrages mounds cascades instead they mimic rhizoliths this study describes the macroscopic and microscopic characteristics of these features and distinguishes them from rhizoliths so that they can be accurately identified in the geologic record furthermore this study identifies a more appropriate method of interpreting the stable isotope values of these tufa like plant molds these types of carbonates are faithful recorders of ambient aquatic conditions e g geochemistry as well as local hydrologic parameters e g water depth when the playa surface is flooded but their geological application is restricted to a partial annual signal e g plant growing season presence of standing water on the playa it is important to correctly identify these types of plant encrustations in the geologic record since 1 their presence indicates a shallow standing body of water rather than a paleosol and 2 their isotopic composition records not only an ambient aquatic signal but an extremely restricted climatic signal that should not be interpreted with models for pedogenic carbonates

paleosols in shallow marine carbonate sequences abstract shallow water carbonate sequences in the geological record commonly exhibit numerous exposure surfaces reflecting emersion of sediment due to progradation processes and sealevel oscillations some ancient platform sequences contain hundreds of such surfaces associated with paleosols fossil soils typically calcretes a wide range of macro and microstructures are associated with these calcretes which are reviewed profile types and stages in profile development can be defined the nature of the exposure surface and associated paleosols can be used to identify the nature of the prevailing climate although hydromorphic effects related to marine flooding of the platforms can modify the paleosols a potentially important use of paleosols is to define the nature of sequence boundaries several studies have identified sets of paleosols with varying maturities reflecting changes in rates of accommodation space creation during sealevel rises and falls

calcretes from a late quaternary interfluve in the ganga plains india carbonate types and isotopic systems in a monsoonal setting abstract calcretes are abundant in late quaternary channel and floodplain strata of the southern ganga plains in a key section at kalpi pedogenic carbonates nodules rhizoconcretions and powdery carbonate are present within aggradational floodplain deposits where they correspond to relatively high monsoonal precipitation and river discharge in contrast groundwater carbonate has cemented degradational surfaces discontinuities which correspond with periods of relatively low precipitation mixed groundwater and pedogenic calcretes are present in the deposits of small interfluve channels and reworked nodules line degradational surfaces and locally fill channels most carbonates show alpha fabrics that include floating textures shrinkage crack fills and grain coatings the predominance of alpha fabrics is unexpected but is characteristic of calcretes across dryland and seasonal parts of northern india where soil formation led to only weakly developed or poorly preserved beta fabrics interpretation of 13c and 18o values of bulk and microdrilled calcrete samples suggests relatively little variation in precipitation and vegetation types through the sampled interval at kalpi floodplain deposits were vegetated with a mixture of c4 and c3 plants predominantly c4 with a higher proportion of c3 plants associated with channel deposits this apparent lack of variation is surprising because the sampled interval represents at least 60 000 years of marine isotope stages 3 5 during which climate models suggest that asia experienced radical fluctuations in monsoon intensity and precipitation some of the apparent lack of variation may be explained by preferential preservation of aggradational strata that represent relatively active monsoonal periods as well as by the mixing of drier floodplain c4 and riparian c3 vegetation however local departures from the regionally based climate model cannot be ruled out a modest upsection increase in c4 plants may represent increased aridity and lower atmospheric co2 isotopic analysis of organic matter from floodplain pedogenic nodules suggests a higher c3 plant contribution than carbonate based data would suggest the preserved organic matter may reflect the annual average biomass in the soil whereas carbonate formation may have taken place mainly during the drier season when respiration of c4 plants was more important in interfluve settings such as kalpi seasonality may strongly affect the c3 c4 system with preferential preservation of only part of the biomass

the caribbean and farallon plates connected constraints from stratigraphy and paleomagnetism of the nicoya peninsula costa rica plate kinematic reconstructions play an essential role in our understanding of global geodynamics but become increasingly difficult to constrain back in geological time due to the subduction of oceanic lithosphere here we attempt to kinematically reconstruct the cretaceous and older plate tectonic history of the caribbean plate within the mesozoic panthalassa paleo pacific ocean to this end we present new paleomagnetic data from jurassic and cretaceous oceanic sedimentary and volcanic large igneous province related rocks of the nicoya peninsula and murcielago islands of northwestern costa rica we use these data in combination with constraints from marine magnetic anomalies to infer the age of the lithospheric basement seismic tomography to locate deep mantle plume generation zones and general kinematic feasibility to test different reconstruction scenarios connecting the caribbean plate to the farallon plate as restored from pacific spreading records our resulting reconstruction implies that the western caribbean subduction zone initiated around 100 ma in an intraoceanic setting breaking up oceanic lithosphere of at least 70 myr old

crustal structure of the middle american trench off costa rica from wide angle seismic data off costa rica seismic wide angle experiments were made using marine seismic sources of high shot density and ocean bottom hydrophones obh and land stations for recording three profiles perpendicular to the middle american trench mat and one parallel to it are presented here the coverage from closely spaced obh and structural constraints from the coincident near vertical reflection seismic survey provided detailed velocity depth distributions over 4 0 km s velocities were found directly underlying the thick continental margin sediment in a margin wedge that extends from below the coastal area seaward within 10 km from the mat axis the interpretation of the wide angle data required a low velocity zone lvz between the upper and lower plate indicating its nature as subducted and underplated sediment the velocity depth distributions support the hypothesis that the continental margin wedge off costa rica is most likely an offshore extension of the nicoya complex ophiolitic rocks which are exposed along the coast the subduction of the sediments seems very efficient and the accretionary prism appears to be confined to the lower slope

unravelling late pleistocene and holocene landscape dynamics the upper guadalentin basin se spain abstract landscapes in se spain have developed in response to tectonics climate fluctuations and more recently human activity fluvial and colluvial sediments such as river terraces and slope deposits found in the valleys reflect a complex interplay between landscape forming processes investigating these sediment archives we reconstructed landscape evolution for the upper guadalentin basin se spain placing recent erosion processes in a landscape evolution context palaeo lake sediments dated between 17 and 13 8 ka evidence that a late glacial lake existed in the area probably formed by a sudden blockage of the guadalentin river differences in relative height above the floodplain and age between the river terraces of parts of the guadalentin river indicate that they have not been in equilibrium in the late quaternary deposition of river terraces along the upstream part of the river is recorded at 13 and 9 5 ka whereas no evidence of deposition is found for that period along the lower part of the river there episodes of sedimentation occur at 7 5 5 ka 3 4 1 6 0 7 and 0 4 ka this discrepancy is explained by the palaeo lake and its influence on erosion and sedimentation processes through base level changes combining these processes we propose a schematic model of late pleistocene and holocene landscape evolution from the model we can conclude that i the influence of the palaeo lake on deposition and erosion processes both upstream and downstream is evident and for the younger river terraces that ii episodes of deposition seem to coincide in time but iii episodes of erosion in between terrace level aggradation do not coincide and neither do terrace level heights and iv no evidence of deposition is found for the upstream part of the river correlation of erosion and sedimentation episodes with climate change and human impact is discussed although some correlations can be made there is strong evidence that climate was not the main driver of landscape processes we suggest that internal dynamics and local processes are more important drivers for landscape dynamics in the upper guadalentin basin than external and regional factors

origin of the central american ophiolites evidence from paleomagnetic results the ophiolite complexes exposed along the pacific coast of costa rica and western panama evolved from jurassic ocean floor to the island arc of the tertiary to the present we attribute late cretaceous volcanism to the caribbean sill event that thickened the oceanic crust to form an oceanic plateau tectonostratigraphic units formed by late cretaceous nappe thrusting and block rotation rotated blocks as revealed by paleomagnetic data are covered by neoautochthonous sedimentary and volcanic rocks collision with the cocos ridge in the late tertiary fragmented and rotated the ophiolite complexes south of the costa rica fracture zone the oceanic basement formed in the jurassic period in an equatorial position its latitudinal drift path as deduced from paleomagnetic inclination data matches that of the south american continent and differs from that of the farallon phoenix plates we favor formation of the ophiolite complexes in a caribbean inter american position as opposed to a distant origin in the pacific region

an island arc origin of plagiogranites at oytag western kunlun orogen northwest china shrimp zircon u pb chronology elemental and sr nd hf isotopic geochemistry and paleozoic tectonic implications abstract this paper presents detailed shrimp zircon u pb chronology major and trace element and sr nd hf isotope geochemistry of three intrusive plagiogranite bodies formed in an island arc setting within the oytag ophiolite suite of the oytag kuda suture zone shrimp zircon u pb dating of two of these intrusions shows that they were emplaced into the oytag volcanic rocks at 337 5 4 1 ma and 327 7 4 9 ma and that the younger intrusion contains inherited zircons ranging from 435 3 to 480 8 ma both intrusions are composed of low al trondhjemite and tonalite the rocks have low k 2 o and sr and high y contents with low sr y ratios they have relatively low rare earth element ree concentrations and are depleted in light ree lree and relatively enriched in heavy ree hree with marked negative eu anomalies in the chondrite normalized ree patterns they have initial 87 sr 86 sr ratios of 0 7048 to 0 7068 and e nd t values of 6 2 to 7 6 which are lower than those of typical morb in situ hf isotopic compositions of zircons with early carboniferous age from both intrusions are characterized by positive initial e hf values ranging from 12 5 to 19 5 similar to typical morb the inherited ordovician zircons in the younger intrusion have initial e hf values of 1 9 to 3 1 similar to zircons from ordovician granitoids in the region detailed elemental and isotopic data suggest that the oytag plagiogranites were formed in an oceanic island arc setting by differentiation of arc tholeiitic magmas that formed in response to the intra oceanic subduction the inherited zircons are interpreted as the xenocrysts captured from the incipient oceanic crust that contains relicts of discrete fragments of continental crust with ordovician zircons through which the plagiogranite was emplaced the oytag ophiolite suite might be the remnants of an ancient island arc the basement of which was the incipient oceanic crust the oytag kuda suture zone is suggested to be a multiple suture zone that represents the remnants of both the proto tethyan and paleo tethyan oceans the western kunlun orogen was therefore not a long lived active continental margin during the early and late paleozoic times it most likely underwent at least two orogenic cycles and thus might represent a multiple orogenic belt

palaeoenvironments of the dinosaur bearing lameta beds maastrichtian narmada valley central india abstract the maastrichtian lameta beds of central india are intimately associated with the deccan lavas and are critical to determining upper cretaceous palaeoenvironments and palaeogeography of the area in the type jabalpur sub region four mappable units of the lameta beds are recognised the basal green sandstone is interpreted as a braided stream deposit the lower limestone characterised by brecciation and shrinkage cracks is interpreted as a sub aerially exposed palustrine flat with calcrete formation occurring on topographic highs of low relief plains the overlying mottled nodular beds exhibit a variable range of calcrete fabrics and morphologies including circum granular and linear cracks root casts and nodules these are interpreted as pedogenically modified sheet wash deposits of a semiarid alluvial plain the upper sandstone is a sheet flood deposit again pedogenically modified before arrival of the basal lava flows overall the lameta beds are considered to represent a regionally extensive maastrichtian regolith carbon and oxygen isotope analyses of calcareous components are entirely consistent with soil zone environments the 13c values are low typically 8 to 9 pdb demonstrating a strong input of carbon from the decay of terrestrial land plants calcrete 18o values are variable 5 to 10 pdb consistent with precipitation from meteoric water some of which was evaporatively modified in pools on the alluvial palustrine flat the lameta beds are well known for sauropod nesting sites and sedimentological analyses of these sites suggest that the animals selected topographic highs usually in marly or sandy soft sediment multiple nests with similar egg types probably indicate colonial nesting isotopic analyses of eggshell carbonate agree with earlier work suggesting that the sauropods ate a c3 plant food