high resolution numerical modeling of stress distribution in visco elasto plastic subducting slabs abstract we employ high resolution 2d thermomechanical modelling with complex elasto visco plastic rheology to analyse stresses in subducting and overriding plates the model contains a dynamic subduction channel defined as a few km thick zone of specific weak visco plastic rheology between the slab and the overriding plate our high resolution model corresponds to the zoom in of the large scale thermomechanical model of the central andes with the 4 fold resolution increase initial model configuration material temperature and stress distributions was interpolated from the parent model boundary conditions including slab pull and push velocities as well as velocity of the overriding plate were also re mapped from the large scale model we demonstrate that for shallow dipping overridden slabs the dominant stress is unbending stress at 50 100 km depth this stress reaches 1 1 5 gpa and strongly prevails over the slab bending stress near the trench which is limited by the frictional plastic yield unbending creates double 5 10 km distance between maximums compression zone in the upper part of the slab and a zone of extension some 20 30 km below pressure in these zones differs from the lithostatic pressure to as much as plus minus 0 8 gpa details of the distribution and magnitude of bending and unbending stresses in the slab are mainly controlled by the overriding velocity and direction of the slab pull force overpressure in the subduction channel in all models does not exceed 100 mpa large over and under pressures in the slab do not however lead to significant more than 10 km offsets of main dehydration reactions in crust and mantle of the slab

p s velocity and vp vs ratio beneath the toba caldera complex northern sumatra from local earthquake tomography summary in this paper we investigate the crustal and uppermost mantle structure beneath toba caldera which is known as the location of one of the largest cenozoic eruptions on earth the most recent event occurred 74 000 yr bp and had a significant global impact on climate and the biosphere in this study we revise data on local seismicity in the toba area recorded by a temporary passcal network in 1995 we applied the newest version of the lotos 07 algorithm which includes absolute source location optimization of the starting 1 d velocity model and iterative tomographic inversion for 3 d seismicp s or thevp vs ratio and source parameters special attention is paid to verification of the obtained results beneath the toba caldera and other volcanoes of the arc we observe relatively moderate for volcanic areas negative p and s velocity anomalies that reach 18 per cent in the uppermost layer 10 12 per cent in the lower crust and about 7 per cent in the uppermost mantle much stronger contrasts are observed for the vp vs ratio that is a possible indicator of dominant effect of melting in origin of seismic anomalies at a depth of 5 km beneath active volcanoes we observe small patterns 7 15 km size with a high vp vs ratio that might be an image of actual magmatic chambers filled with partially molten material feeding the volcanoes in the mantle wedge we observe a vertical anomaly with low p and s velocities and a high vp vs ratio that link the cluster of events at 120 140 km depth with toba caldera this may be an image of ascending fluids and melts released from the subducted slab due to phase transitions however taking into account poor vertical resolution these results should be interpreted with prudence although the results show clear signatures that are quite typical for volcanic areas low velocity and high vp vs ratio beneath volcanoes we do not observe any specific features in seismic structure that could characterize toba as a super volcano

creating realistic models based on combined forward modeling and tomographic inversion of seismic profiling data amplitudes and shapes of seismic patterns derived from tomographic images often are strongly biased with respect to real structures in the earth in particular tomography usually provides continuous velocity distributions whereas major velocity changes in the earth often occur on first order interfaces we propose an approach that constructs a realistic structure of the earth that combines forward modeling and tomographic inversion fm ti using available a priori information we first construct a synthetic model with realistic patterns then we compute synthetic times and invert them using the same tomographic code and the same parameters as in the case of observed data processing we compare the reconstruction result with the tomographicimage of observed data inversion if a discrepancy is observed we correct the synthetic model and repeat the fm ti process after several trials we obtain similar results of synthetic and observed data inversion in this case the derived synthetic model adequately represents the real structure of the earth in a working scheme of this approach we three authors used two different synthetic models with a realistic setup one of us created models but the other two performed the reconstruction with no knowledge of the models we discovered that the synthetic models derived by fm ti were closer to the true model than the tomographic inversion result our reconstruction results from modeling marine data acquired in the musicians seamount province in the pacific ocean indicate the capacity and limitations of fm ti

conjunctive use of spectral gamma ray logs and clay mineralogy in defining late jurassic early cretaceous palaeoclimate change dorset u k abstract detrital clay mineralogy is controlled by weathered source rock climate transport and deposition that in turn influence the spectral gamma ray sgr response of resultant sediments whilst a palaeoclimate signal in clay mineralogy has been established in some ancient successions the sgr response remains contentious largely because the data sets have yet to be collected at the same or appropriate vertical scales to allow comparison in addition the influence of organic matter on sgr is not always considered here we present clay mineralogical total organic carbon toc and sgr analyses from the late jurassic and early cretaceous of the wessex basin a period of previously documented palaeoclimate change the aim of this paper is to estimate the sensitivity of sgr as palaeoclimatic tool sgr and clay mineral data having been collected at the same sample points making this one of the most rigorous comparison of clay mineral and sgr to date overall the correlation between high thorium potassium or thorium uranium and kaolinite associated with a well established palaeoclimate change shows that elevated thorium may be used as a proxy for humid palaeoweathering as suggested by few previous studies

relationship between diagenesis and reservoir quality in sandstones of the garn formation haltenbanken mid norwegian continental shelf a quantitative model is proposed for the dependence of porosity and permeability on diagenesis in subarkosic arenites of the middle jurassic garn formation porosity variation is controlled by the combined effects of compaction and quartz cementation in general more porosity has been lost by compaction than by quartz cementation but compactional porosity loss appears to have occurred relatively early in the burial history before burial below 2 km below the sea floor thus progressive reduction of the total porosity with increasing burial below 2 km results mainly from quartz cementation permeability is a function of the abundance of intergranular macroporosity which is interpreted to be a measure of effective porosity at depths less than 3 5 km below the sea fl or intergranular macroporosity generally comprises more than 50 percent of the total porosity but at greater depth as total porosity decreases below about 16 percent the ratio of effective to ineffective porosity drops below one this depth corresponds to the onset of extensive illitization and grain dissolution which results in major reorganization of the pore system generation of secondary porosity by feldspar dissolution appears not to have been beneficial to reservoir quality in the garn formation but it is regarded as an intrinsic part of the overall process of porosity destruction with increasing diagenesis

late jurassic palaeoclimatic change from clay mineralogy and gamma ray spectrometry of the kimmeridge clay dorset uk abstract the late jurassic was a time of increasing aridity in nw europe here a new clay mineral dataset is presented from a 600 m thick composite core through the kimmeridge clay formation southern england clay mineral assemblages comprise mainly illite and kaolinite with minor randomly interstratified illite smectite mixed layer clays sem observations indicate that clay minerals are mainly detrital except in silty strata of late tithonian age which contain abundant pore filling kaolinite aggregates th k ratios determined from gamma ray spectrometry mirror palaeoclimatically significant variations in kaolinite illite ratios with notable exception where diagenetic kaolinite occurs comparison with age equivalent strata identifies regional cyclic variations in clay minerals at the 100 300 m scale small scale 10 20 m variations in clay minerals and th k also occur it is proposed that the region experienced progressively intense humidity through the kimmeridgian followed by a return to more arid conditions during the early tithonian following a mid tithonian peak in aridity the hudlestoni event9 more humid climatic conditions returned prior to the development of latest tithonian intense aridity late jurassic climate was apparently subject to synchronously cyclic changes across a broad area of the laurasian continent such changes could not have resulted from mountain building or continental rotations supplementary material clay data are available at http www geolsoc org uk sup18376

origin of clay rich beds in turonian chalks from lower saxony germany a rare earth element study abstract the mineralogy and ree geochemistry of thin clay rich beds from turonian chalks of lower saxony germany has been studied two types of clay rich beds are present the first is of volcanic origin and is characterised by 1 a ree signature similar to marine argillized volcanic ash bands and 2 a silicate mineralogy dominated by smectite with minor corrensite the second type is of continental origin and is characterised by 1 a ree signature similar to continental shale and 2 a silicate mineralogy dominated by smectite with minor heulandite clinoptilolite the use and misuse of these widespread clay rich horizons in the turonian stratigraphy of lower saxony is discussed

the milankovitch fingerprint on the distribution and thickness of bedding parallel veins beef in source rocks bed parallel mineralized fractures are common in source rocks and generally consist in mm to cm thick veins developed parallel to bedding known as beef or bedding parallel veins considering they can form a dense network of mechanical discontinuities the prediction of beef distribution is a major issue impacting shale reservoir production beef distribution is predominantly controlled by the lithological characteristics of source rocks and we here decipher the relation between mineralogical and chemical proxies controlled by orbital parameters and distribution of the beef along a late jurassic section of the well known vaca muerta formation source rock in the neuquen basin using multiple proxies collected along the beef rich huncal section we show that milankovitch cycles rule the mineralogical evolution and beef distribution in these organic rich mudrocks cycles inferred from the statistical treatment of sedimentary magnetic susceptibility elemental and mineralogical ratios biogenic total organic carbon and diagenetic beef distribution and thickness signals revealed indeed the influence of an astroclimatic fingerprint in sediments and on processes controlling mineralized fracture generation and distribution the astroclimatic memory recorded in many source rocks worldwide is therefore envisaged as a suitable proxy for the prediction of mineralized fracture distribution

palaeoclimatic interpretation of clay minerals in marine deposits an outlook from the continental origin abstract as distribution of clay minerals in modern oceans seems to be controlled by contemporary climates marine clays in ancient deposits have been widely used to reconstruct palaeoclimates in this review we examine this question mainly from the continental origin of the clay minerals and show the difficulties of retrospective analysis of the influence of climate on common marine deposits we examine successively how soils develop and behave are reworked and palaeoweatherings are preserved focusing the review on kaolinitic soils indicative of tropical wet climates several stages intervene between the time of clay mineral development in soils and their final deposition in the basin 1 the persistence over geological times of huge amounts of kaolinitic palaeoweathering materials in the landscapes may seriously alter the palaeoclimatic signal of kaolinite in the sedimentary record although the kaolinite deposited in recent sediments of the world oceans is consistent with wet climatic zones this is mainly because the major kaolinitic and bauxitic palaeosurfaces coincide with the tropical areas where kaolinite is still developing at present 2 possible development of deep kaolinitic profiles at high latitudes and cold conditions under co 2 enriched palaeoatmospheres or even by unusual acidic weathering linked with dry climates is misleading palaeoenvironments for palaeoclimatic reconstruction 3 erosion and transport processes always cloud the signal to some extent because frequently erosion products from the bedrock and different soil horizons will be mixed and major drainage systems may include soils of different climatic zones 4 the arrival of the soil clay assemblages in the basin inevitably lags against their formation on the continent and may occur several million years after formation of the soils on the continent and climate may have considerably changed at that time it is pointed out that sedimentologists and palaeoclimatologists should consider the complexity of the relations between soil and sedimentary clay minerals soil formation rates are slow and therefore the resolution of the palaeoclimatic record in marine clay may not be closer than 1 or 2 ma at present the palaeoclimatic interpretations of marine clay assemblages are yielding at best nothing more than rather broad palaeoclimatic information this approach cannot achieve the same degree of resolution as other techniques such as isotope or microfossil studies clay mineral assemblages may provide integrated records of overall climatic impacts whereas other techniques are more likely to reveal local or temporary climates

structure and tectonics of the erosional convergent margin off antofagasta north chile 23 30 s subduction erosion has dominated the evolution of the north chile convergent continental margin since at least the mesozoic we investigate the structure of the antofagasta 23 s sector of this margin along a transect using coincident wide angle and near vertical seismic profiling and gravity data a 2 d velocity field of the overriding and subducting plates was obtained using joint refraction and reflection travel time tomography a velocity derived density distribution was used to model marine gravity data and substantiate the velocity model the gravity and velocity models imply that the overriding plate is mainly made of arc type igneous basement the upper plate is constructed of two main rock bodies separated by a subhorizontal layer defined by a velocity inversion the top coincident with a reflection in near vertical seismic images the seismic boundary is interpreted as a detachment separating an upper extended domain with large scale normal faulting from a lower domain apparently undergoing a different type of deformation velocity derived porosity indicates that the front of the margin is probably fluid saturated and disaggregated by fracturation as a consequence of frontal subduction erosion fluids carried into the subduction channel within slope debris filling underthrusting grabens reduce basal friction and probably induce hydrofracturing and basal erosion along the underside of the overriding plate at depths greater than 20 km porosity and density values imply that most fluids have been exhausted and the lower part of the upper plate is structurally coherent and little fractured the change in physical properties leads to increased mechanical coupling along the plate boundary and occurs at the updip limit of the distribution of aftershocks of the 1995 antofagasta earthquake m w 8 0 defining the seismogenic zone

a paleoceanographic approach to the kimmeridge clay formation the upper jurassic lower cretaceous kimmeridge clay formation kcf is northern europe s premier source rock and can be understood using a new but relatively simple oceanographic model this explains or accommodates most current observations about the kcf and its depositional environment and draws upon paleogeographic paleoclimatic geochemical sedimentological and paleontological evidence

isotopic evidence for late jurassic early cretaceous climate change abstract strontium oxygen and carbon isotope ratios have been determined from late jurassic early cretaceous belemnites from the volga basin russia and kawhia harbour new zealand 87sr 86sr ratios derived from well preserved belemnites from the volga basin support a middle tithonian age derived from the analysis of the endemic ammonite fauna the kawhia harbour section records a gradual rise in 87sr 86sr values and in comparison with the published 87sr 86sr curve suggests that the lower part of the section is latest oxfordian in age whilst the upper part of the section correlates well with the biostratigraphic correlation suggestion of an early middle tithonian age although the published strontium calibration curve shows a degree of scatter our new data confirm the uniform rise in 87sr 86sr values from the late jurassic into the early cretaceous such an increase may result from either a decrease in mid oceanic ridge spreading and or an increase in weathering rates and flux of radiogenic strontium although a eustatic drop in sea level and concurrent western cordillera uplift suggests that weathering may have been the controlling factor of late jurassic seawater strontium isotope composition palaeotemperatures derived from the well preserved belemnite 18ocarb values from the volga basin indicate that the middle volgian late kimmeridgian was warm 14 20 c followed by a slight cooling and a subsequent gradual increase to the jurassic cretaceous boundary the 18ocarb values from new zealand located at a palaeolatitude of 80 s if interpreted in terms of palaeotemperature indicate a high degree of variability such variability may not be related to palaeotemperature but to changes in oceanic chemistry resulting from the formation and dissolution of an ice sheet and or snow during the oxfordian kimmeridgian carbon isotope trends for the late jurassic show a fall in values from the oxfordian with lowest values occurring in the early middle tithonian before rising but without reaching values obtained in the oxfordian the overall low 13ccarb may be related to a global increase in continental weathering and or upwelling of cooler oceanic water enriched in oxidised organic carbon 12c enriched

late jurassic early cretaceous climate change record in clay minerals of the norwegian greenland seaway abstract the humid climate of northwestern europe during the late jurassic early cretaceous was punctuated by a widespread arid phase there are few well constrained and complete sections that document this climate shift from semi humid to arid and back to more humid conditions detrital clay minerals formed in soils and derived through weathering may be used as palaeoclimatic proxies and here clay mineral assemblages from several well constrained sections from the norwegian continental shelf are presented as a n s transect of the norwegian greenland seaway palaeoclimate signals in most of the clay mineral sections are complicated by diagenetic overprinting yet towards the boreal realm of the norwegian greenland seaway there is a discernible palaeoclimatic signal in the clay mineralogy of well 16 3 4 at 30 n kaolinite is known to have reappeared in the clay mineralogical record in the upper part of calpionellid zone b early mid berriasian however we find an absence of kaolinite at 40 n in well 16 3 4 from the north sea until the late berriasian suggesting that initiation of the first humid phase in the early cretaceous norwegian greenland seaway was diachronous

geological record of fluid flow and seismogenesis along an erosive subducting plate boundary a fossil erosive subduction channel the shear zone marking the plate boundary preserved in the northern apennines of italy is identified the fossil zone records the presence of two decollements which seem to have been simultaneously active at the top and base of the subduction channel in modern subduction zones the onset of seismic activity is believed to occur at 150 c however in the fossil channel the onset seems to have occurred at colder temperatures tectonic erosion of the overriding plate by the downgoing slab is believed to occur at half the earth s subduction zones1 2 in situ investigation of the geological processes at active erosive margins is extremely difficult owing to the deep marine environment and the net loss of forearc crust to deeper levels in the subduction zone until now a fossil erosive subduction channel the shear zone marking the plate boundary3 has not been recognized in the field so that seismic observations have provided the only information on plate boundary processes at erosive margins here we show that a fossil erosive margin is preserved in the northern apennines of italy it formed during the tertiary transition from oceanic subduction to continental collision and was preserved by the late deactivation and fossilization of the plate boundary the outcropping erosive subduction channel is 500 m thick it is representative of the first 5 km of depth with its deeper portions reaching 150 c the fossil zone records several surprises two decollements were simultaneously active at the top and base of the subduction channel both deeper basal erosion and near surface frontal erosion occurred at shallow depths extension was a key deformation component within this erosive convergent plate boundary and slip occurred without an observable fluid pressure cycle at depths greater than about 3 km a fluid cycle is clearly shown by the development of veins and the alternation of fast co seismic and slow inter seismic slip in the deepest portions of the outcropping subduction channel extension is finally overprinted by compressional structures in modern subduction zones the onset of seismic activity is believed to occur at 150 c but in the fossil channel the onset occurred at cooler palaeo temperatures

fault activity and sedimentation in a marine rift basin upper jurassic wessex basin uk shallow marine carbonates and siliciclastics of the corallian formation oxfordian early kimmeridgian accumulated on and around an intrabasinal high in the extensional wessex basin four sequences can be recognized sequences 1 3 accumulated under conditions of thermal subsidence on a ramp type margin the initial sequence was siliciclastic highstand sedimentation in this sequence reflects the supply of sandy mud from a recently emergent intrabasinal high during transgression and regression this muddy sediment was reworked into cleaner sandstone bodies by landward or basinward migrating zones of shoreface erosion carbonates dominate the second and third sequences when rising sea level increased the area of carbonate production and reduced siliciclastic input oolite bodies developed as both transgressive barrier bars and highstand sheets the forth sequence formed during the activation of major normal faults this caused the breakdown of the ramp system and patterns of sediment accumulation were strongly controlled by tectonic subsidence patterns

mudstone lithofacies in the kimmeridge clay formation wessex basin southern england implications for the origin and controls of the distribution of mudstones abstract lithofacies analysis has been undertaken on a suite of apparently monotonous organic rich mudstones from the upper jurassic kimmeridge clay formation kcf in the wessex basin southern england using a combination of hand specimen description whole rock geochemistry optical petrography and electron optical petrography five lithofacies have been identified these are clay rich mudstones silt rich mudstones nannoplankton rich mudstones laminated mudstones and concretionary carbonates these facies can be distinguished from one another on the basis of physical sedimentary structures and the relative proportions of allochthonous autochthonous and diagenetic components in each all five of the kcf lithofacies studied were deposited in a marine shelf environment below fair weather wave base but in relatively shallow water close to storm wave base deposition occurred in regions where sand and other coarse clastic allochthonous debris were not being supplied and the bottom waters varied from oxic to anoxic the main factors influencing the development of the five facies were distance from source area primary productivity bottom water anoxia absolute bathymetry clastic dilution and local sediment accumulation rates specifically the clay rich mudstones were deposited in regions farthest down the sediment transport paths commonly in areas by passed by the main sediment supply and beneath surface waters of relatively low primary productivity in contrast the silt rich mudstones were deposited in more proximal areas closer to the source of the sediment in regions where surface productivity and sediment accumulation rates were much higher the nannoplankton rich mudstones accumulated beneath regions where productivity in the surface water layers was fairly high dominated by coccolithophoroid production and the supply of clastic sediment was insufficient to significantly dilute the autochthonous fraction of the sediment unlike the other facies laminated mudstones were deposited in areas where the bottom waters were anoxic surface productivity was enhanced and local sediment accumulation rates were high the concretionary carbonates were precipitated by diagenetic processes in areas where sediment accumulation rates were very low these facies descriptions have enabled a detailed sedimentological study of the kimmeridge clay formation to relate depositional and diagenetic processes directly to the facies present thereby allowing classical facies analyses to be undertaken on apparently homogeneous mudstone successions

palaeoclimatic significance of changes in clay mineralogy across the jurassic cretaceous boundary in england and france abstract clay mineralogical changes across the jurassic cretaceous boundary from the northern to the southern part of western europe show similar patterns in the mudrocks exposed on the yorkshire coast the paralic sediments of the dorset wessex basin and the jura and vocontian trough of s e france this comprises a change from smectite poor sediments in the jurassic with correspondingly abundant illite and kaolinite to sediments with abundant smectite in the jurassic cretaceous boundary beds depleted in kaolinite this change occurs in the oxfordian in the south to latest volgian in the north of the study area kaolinite reappears in abundance in the late ryazanian or stratigraphic equivalent in all the locations studied comparison with other climatic indicators and other areas suggest that these variations can be ascribed to a phase of latest jurassic earliest cretaceous aridity the onset of which appears to be diachronous across w europe possible causes for the climatic change towards aridity are examined the orographic effect of mountains or plateaus to the east of the study area is considered most likely

accretion and erosion in subduction zones the role of fluids convergence occurs today along about 60 000 km of plate boundaries parsons 1981 in this paper we consider frontal subduction which occurs along about 40 000 km of convergent plate boundaries using the kine matic solution of de mets et al 1990 and including 0 5 km 2 yr1 of marginal basin openings otsuki 1989 we find that 2 95 km 2 per year disappear in frontal subduction at an average rate of 7 3 cm yr the upper overriding plate is significantly modified not only through the addition of a large amount of magmatic material markhinin 1968 but also through tectonic processes which result either in outgrowth of the margin through addit ion of sediments silver 1969 or in recession through frontal and subcrustal erosion which appears to be linked to sediment subduction coats 1962 gilluly 1963 the various phenomena observed must in some way be related to variations in mechanical coupling between the two plates uyeda kanamori 1979 had proposed that the transition from erosion to accretion results from increasing mechanical coupling the marianas trench was presented as typical of an crosional margin and the chile trench of an accreting margin ruff kanamori 1980 and kana mori 1986 showed that the magnitude of the subduction earthquakes increases with the convergence velocity and the inverse of the age of the subducted lithosphere this correlation was qualitatively explained by the control of age on the d ip of the platc and of convergence velocity on the

nonlinear variations of the physical properties along the southern ecuador subduction channel results from depth migrated seismic data abstract we use two high quality pre stack depth migrated multichannel seismic profiles acquired to quantify physical properties variations of underthrust sediments along the first 30 km of subduction off the erosional southern ecuadorian margin seismic data show three zones along the subduction channel referred to as zones i ii and iii characterized by distinct velocity and velocity derived physical properties which are in agreement with values estimated from experimental results of deformation in granular media these three zones result from transformational changes of underthrust sediments governed by fundamentally different physical processes that control their mechanical behavior at increasing confining pressures based on our observations and its comparison with experimental results we argue that the transformations undergone by underthrust sediments as they dip into the subduction zone are the following within zone i progressively increasing velocity and decreasing velocity derived porosity indicates continuous sediment compaction which must be accompanied by effective fluid drainage along the decollement and or across the accretionary wedge the underthrust material is here unconsolidated from a mechanical point of view laboratory experiments indicate that the dominant processes at this range of pressures are grain rolling particle rotation and frictional slip at grain contacts within zone ii velocity and porosity remains constant for 16 km sis 72 and 12 km sis 18 this suggests undrained conditions resulting in growing fluid overpressure at the subduction channel grain deformation is similar to zone i within zone iii velocity increases and porosity falls rapidly indicating sediment compaction and subsequent release of over pressured fluids where grain deformation is likely to be elastic this might be the dominant process until the grains attain their crushing strength resulting in granular cataclasis and eventually in the collapse of the system we suggest that over pressured fluid release may induce hydrofracturation and it is likely to increase inter plate coupling down from zone iii

depositional conditions and organic matter preservation pathways in an epicontinental environment the upper jurassic kashpir oil shales volga basin russia abstract the middle volgian kashpir oil shales formation located on the russian platform is a lateral equivalent of the north sea and west siberian petroleum source rocks in the volga basin this formation is 6 m thick and shows alternations of marls and black shales the organic carbon content is often higher than 1 although bioturbation and benthos are abundant even in the black shales in the marls highly degraded organic matter om dominates while aliphatic sulphur rich om is dominant in the black shales the combination of sedimentological and geochemical studies allowed to determine that the redox conditions of the sediment regularly fluctuated from oxic to anoxic under relatively productive waters while climatic conditions were increasingly arid it is proposed that om deposition occurred in relation to the increasing aridity through the combination of recurrent disruption of salinity stratification and aeolian supply of iron which both promoted phytoplankton productivity

environmental controls on clay mineralogy of an early jurassic mudrock blue lias formation southern england we present a dataset including clay mineralogy gamma ray spectrometry organic matter content and magnetic susceptibility of the hettangian to lowest sinemurian successions of dorset and somerset southern uk blue lias formation bristol channel and wessex basins in both areas the clay assemblages comprise predominantly detrital illite kaolinite and illite smectite mixed layers clays probably originated from the erosion of the hercynian massifs the relative proportions of kaolinite and illite being modulated by arid humid climatic fluctuations the organic matter om content types ii to iv ranges up to 12 in both areas a clear stratigraphical trend in clay mineral assemblages is apparent in somerset whereas in dorset sharp contrasts between adjacent horizons and a greater dilution by carbonate mask the long term evolution correlations between both areas based on similar vertical trends in clay mineral abundance support the suggestion of a hiatus within the angulata zone of the dorset succession as expected the kaolinite illite ratio correlates with the th k ratio deduced from gamma ray spectrometry however significant departures from the correlation occur in om rich intervals suggesting that th may be partly adsorbed on to om particles surprisingly high magnetic susceptibility correlates with abundant kaolinite not with fe rich clays indicating either that kaolinite is accompanied by a soil inherited magnetisable phase possibly iron oxide or that illite rich rocks are more strongly diluted by carbonate than are kaolinite rich strata

the influence of a sudden climatic change on marine deposition in the kimmeridgian of northwest europe a sudden change from humid style to semi arid style deposition markedly affected the accumulation of the upper kimmeridge clay in southern england many of the changes appear to be related to a change in sedimentation rate at this time thus softground faunas are replaced by firmground faunas diagenetic dolostones formed in the methanogenic zone are replaced by sulphate reduction zone carbonate nodules and depositional gradients recorded by lateral biofacies changes becomes steeper the evidence available is in accord with a decline in offshore sedimentation rates during this interval other changes such as a decline in kaolinite abundance were more directly controlled by the drying out of the hinterland similar changes elsewhere in the marine geological record could be used as climatic indicators the climatic change is part of a wider northern hemisphere dry event which affect a broad area in the late jurassic the kimmeridge clay of southern england was one of the last depositional environments to be influenced by the climatic change at this latitude

palaeoecology and sedimentology across a jurassic fault scarp ne scotland a detailed palaeoecological and sedimentological analysis of the kimmeridgian coastal outcrops around helmsdale northeast scotland has been related for the first time to a detailed biostratigraphical scheme allowing the depositional history of this important area adjacent to some north sea oilfields to be elucidated a broad shallow marine sub tropical shelf was disrupted by the onset of normal faulting along the helmsdale fault in earliest kimmeridgian times initially water depths remained shallow on the hanging wall reflected by a normal benthic fauna but a rapidly steepening southeast directed palaeoslope produced frequent sediment sliding shoreface sands developed immediately to the west of the fault and were transferred to the hanging wall via a fault transfer zone where they now constitute the allt na cuile formation continued rapid subsidence during the remainder of the kimmeridgian and the portlandian produced deep poorly oxygenated to anoxic benthic conditions on the hanging wall in which a combination of rock slide debris flow turbidity current deposition alternated with hemipelagite deposition to produce the helmsdale boulder bed formation the eathie locality cromarty previously regarded as a deeper more distal setting than helmsdale contains a slightly more diverse benthic fauna implying shallower water the deepest waters were immediately adjacent to the fault scarp as predicted by the standard half graben depositional model

a sequence stratigraphy of the kimmeridgian and bolonian stages late jurassic wessex weald basin southern england a sequence stratigraphic framework for the late jurassic kimmeridgian and bolonian stages of the wessex weald basin southern england is proposed based on the integration of sedimentological geophysical and geochemical data the nerc funded rapid global geological events rgge boreholes of swanworth quarry 1 and 2 and metherhills 1 are used as reference sections eleven complete depositional sequences and their component systems tracts are recognized within the kimmeridge clay formation bounded by 12 sequence boundaries km1 12 seventy four boreholes have been used in this study during the kimmeridgian a major transgression associated with tectonic movements led to a marked change in basin geometry major deepening of the sea during the late kimmeridgian eudoxus zone led to a change from unconformities of basinwide extent to those confined to the basin margins the progressive deepening of the basin towards a highstand in the mid bolonian wheatleyensis to pectinatus zones was associated with a broad scale change in sedimentary faunal and geochemical characters the layer cake basin architecture and thickening of the kimmeridge clay formation towards the basin centre suggests a dominantly aggradational system the partially enclosed nature of the kimmeridgian and bolonian seas generated basin dynamics more analogous to those of modern day meromictic lakes consequently sequence stratigraphic approaches derived from continental shelf margins are inappropriate in epicontinental basins of this type

comparison of clay mineral stratigraphy to other proxy palaeoclimate indicators in the mesozoic of nw europe this paper reviews the opportunities and pitfalls associated with using clay mineralogical analysis in palaeoclimatic reconstructions following this conjunctive methods of improving the reliability of clay mineralogical analysis are reviewed the mesozoic succession of nw europe is employed as a case study this demonstrates the relationship between clay mineralogy and palaeoclimate proxy analyses may be integrated with clay mineralogical analysis to provide an assessment of aridity humidity contrasts in the hinterland climate as an example the abundance of kaolinite through the mesozoic shows that while interpretations may be difficult the mesozoic climate of nw europe was subject to great changes in rates of continental precipitation we may compare sedimentological facies mineralogy geochemistry indicators of palaeoprecipitation with palaeotemperature estimates the integration of clay mineralogical analyses with other sedimentological proxy indicators of palaeoclimate allows differentiation of palaeoclimatic effects from those of sea level and tectonic change we may also observe how widespread palaeoclimate changes were whether they were diachronous or synchronous how climate sea level and tectonics interact to control sedimentary facies and what palaeoclimate indicators are reliable

continental humid and arid zones during the jurassic and cretaceous abstract the global distribution of climatically significant criteria is surveyed for six successive time intervals hettangian toarcian aalenian callovian oxfordian tithonian berriasian barremian aptian cenomanian and turonian maastrichtian the criteria are primarily mineralogical evaporites coals bauxites ironstones and kaolinite but include sedimentological and palaeontological aspects of facies e g aeolian sands fossil ferns and xerophytes fresh water invertebrates in early and mid jurassic times an arid zone can be distinguished in low to mid latitudes in western pangaea with humid zones elsewhere in the late jurassic this arid zone spread northwards to embrace much of southern eurasia there followed in the early cretaceous a sharp reversal to more humid conditions over a large area extending from the eastern margin of north america to the middle east a major consequence of which was the widespread replacement of carbonates by coarse siliciclastic facies by late cretaceous times humid conditions were well established also in north america and the south atlantic margins and the tropical arid zone had contracted to a minimum the 87 sr 86 sr ratio of marine carbonates provides an independent monitor of continental runoff which is related to a area and b extent of aridity changes in the ratio confirm that taking sea level stand into account the late cretaceous was appreciably more humid than the late jurassic these climatic changes are related primarily to the progressive breakup of pangaea in the late jurassic and cretaceous which together with sea level rise in the mid to late cretaceous caused a significant increase in maritime influence this implies that monsoonal winds probably had a major role in promoting higher precipitation rates a puzzle remains as to why there was a spread of aridity in the late jurassic finally some implications for terrestrial plant evolution are noted

nannofossil carbonate fluxes during the early cretaceous phytoplankton response to nutrification episodes atmospheric co2 and anoxia 1 greenhouse episodes during the valanginian and aptian correlate with major perturbations in the c cycle and in marine ecosystems carbonate crises and widespread deposition of corg rich black shales quantitative analyses of nannofossil micrite were conducted on continuous pelagic sections from the southern alps northern italy where high resolution integrated stratigraphy allows precise dating of early cretaceous geological events rock forming calcareous nannofloras were quantified in smear slides and thin sections to obtain relative and absolute abundances and paleofluxes that are interpreted as the response of calcareous phytoplankton to global changes in the ocean atmosphere system increased rates of volcanism during the formation of ontong java and manihiki plateaus and the parana etendeka large igneous province lip are proposed to have caused the geological responses associated with early aptian oceanic anoxic event oae 1a and the valanginian event respectively calcareous nannofloras reacted to the new conditions of higher pco2 and fertility by drastically reducing calcification the valanginian event is marked by a 65 reduction in nannofossil paleofluxes that would correspond to a 2 3 times increase in pco2 during formation of the parana endenteka lip a 90 reduction in nannofossil paleofluxes which occurred in a 1 5 myr long interval leading into oae1a is interpreted as the result of a 3 6 times increase in pco2 produced by emplacement of the giant ontong java and manihiki plateaus high pco2 was balanced back by an accelerated biological pump during the valanginian episode but not during oae1a suggesting persisting high levels of pco2 in the late aptian and or the inability of calcareous phytoplankton to absorb excess pco2 above threshold values

finding a realistic velocity distribution based on iterating forward modelling and tomographic inversion tomography is like a photograph that was taken by a camera with blurred and defective lenses that deform the shapes and colours of objects reporting quantitative parameters derived from tomographic inversion is not always adequate because tomographic results are often strongly biased to quantify the results of tomographic inversion we propose a forward modelling and tomographic inversion fm ti approach that aims to find a more realistic solution than conventional tomographic inversion the fm ti scheme is based on the assumption that if two tomograms derived from the inversion of observed and synthetic data are identical the synthetic structure may appear to be closer to the real unknown structure in the ground than the inversion result however the manual design of the synthetic velocity distribution is usually time consuming and ambiguous in this study we propose an approach that automatically searches for a probabilistic model in this approach a synthetic model is iteratively updated while taking into account the bias of the model in previous stages of the fm ti performance here we present an example of synthetic modelling and real data processing for an active source refraction data set corresponding to a marine profile across the subduction zone in chile at about 32 s latitude a key feature of the model is a low velocity channel above the subducted oceanic crust which was defined in the synthetic model and expected in the real case the conventional first arrival traveltime tomography was barely able to resolve this channel however after several iterations of the fm ti modelling we succeeded in reconstructing this channel clearly in the paper we briefly discuss the nature of this low velocity subduction channel and we compare the results with other studies

teleseismic tomography reveals no signature of the dead sea transform in the upper mantle structure abstract we present results of a tomographic inversion of teleseismic data recorded at 48 stations of a temporary network which was installed in the area of the dead sea transform dst and operated for 1 yr in the framework of the multidisciplinary desert project the 3366 teleseismic p and pkp phases from 135 events were hand picked and corrected for surface topography and crustal thickness the inversion shows pronounced low velocity anomalies in the crust beneath the dst which are consistent with recent results from local source tomography these anomalies are likely related to the young sediments and fractured rocks in the fault zone the deeper the retrieved anomalies are quite weak most prominent is the high velocity strip like anomaly striking se nw we attribute this anomaly to the inherited heterogeneity of lithospheric structure with a possible contribution by the shallow precambrian basement east of the dst and to lower crustal heterogeneity reported in this region by other seismic studies we do not observe reliable signature of the dst in the upper mantle structure some weak indications of low velocity anomalies in the upper mantle beneath the dst may well result from the down smearing of the strong upper crustal anomalies we also see very little topography of the lithosphere asthenosphere boundary beneath the dst which would generate significant horizontal velocity variations these results are consistent with predictions from a recent thermo mechanical model of the dst our tomographic model provides some indication of hot mantle flow from the deeper upper mantle rooted in the region of the red sea however resolution tests show that this anomaly may well be beyond resolution of the model

geomorphic change detection using historic maps and dem differencing the temporal dimension of geospatial analysis abstract the ability to develop spatially distributed models of topographic change is presenting new capabilities in geomorphic research high resolution maps of elevation change indicate locations processes and rates of geomorphic change and provide a means of calibrating temporal simulation models methods of geomorphic change detection gcd based on gridded models may be applied to a wide range of time periods by utilizing cartometric remote sensing or ground based topographic survey data to measure volumetric change advantages and limitations of historical dem reconstruction methods are reviewed with a focus on coupling them with subsequent dems to construct dems of difference dod which can be created by subtracting one elevation model from another to map erosion deposition and volumetric change the period of dod analysis can be extended to several decades if accurate historical dems can be generated by extracting topographic data from historical data and selecting areas where geomorphic change has been substantial the challenge is to recognize and minimize uncertainties in data that are particularly elusive with early topographic data this paper reviews potential sources of error in digitized topographic maps and dems although the paper is primarily a review of methods three brief examples are presented at the end to demonstrate gcd using dods constructed from data extending over periods ranging from 70 to 90 years

distribution of clay minerals in early jurassic peritethyan seas palaeoclimatic significance inferred from multiproxy comparisons article i nfo a set of published unpublished and new clay mineral data from 60 european and mediterranean localities allows us to test the reliability of clay minerals as palaeoclimatic proxies for the pliensbachian toarcian period early jurassic by reconstructing spatial and temporal variations of detrital fluxes at the ammonite biochronozone resolution in order to discuss their palaeoclimatic meaning a compilation of low latitude belemnite 18 o 13 c mg ca and 87 sr 86 sr values is presented for the first time for the whole pliensbachian toarcian period once diagenetic and authigenic biases have been identified and ruled out kaolinite content variation is considered as a reliable palaeoclimatic proxy for the early jurassic major kaolinite enrichments occur during times of low 18 o high mg ca and increasing 87 sr 86 sr implying warm climates and efficient runoffs during the davoei falciferum and bifrons zones conversely cooler and drier times such as the late pliensbachian or the late toarcian are characterized by low hydrolysis of landmasses and correspond to kaolinite depleted intervals secondary factors as modifications of sources or hydrothermalism may sporadically disturb the palaeoclimatic signal e g in the bakony area during the late pliensbachian in addition a spatial comparison of clay assemblages displays significant kaolinite enrichments towards northern parts of the peritethyan realm probably related to the latitudinal zonation of hydrolyzing conditions this implies enhanced runoffs on northern continental landmasses that reworked kaolinite rich

calcareous nannofossils and mesozoic oceanic anoxic events abstract twenty five years ago mesozoic oceanic anoxic events oaes were documented and formalised as intervals of widespread to global deposition of organic matter the toarcian early aptian oae1a and latest cenomanian oae2 oaes are truly global in nature commonly carbonate poor and typically represented by organic carbon rich black shales in some areas these oaes are also characterised by abundant radiolarian sands and silts they are associated with negative and positive excursions in the 87sr 86sr record in addition to large global carbon isotope anomalies in carbonate and or organic matter caused by a major perturbation of the global carbon budget increased rates of volcanism during the formation of the ontong java and manihiki and caribbean plates and the karoo ferrar traps are believed to have caused the geological responses associated with oae1a oae2 and the toarcian oae respectively excess volcanogenic co2 in the atmosphere most probably turned the climate into a greenhouse mode accelerating continental weathering and increasing nutrient content in oceanic surface waters via river run off higher fertility in the global ocean was also probably triggered directly by submarine igneous events that introduced enormous quantities of biolimiting metals within hydrothermal plumes because mesozoic oaes are often represented by carbonate poor sediments quantitative studies of calcareous nannofossils have been applied to explore a the causes and effects of igneous tectonic events and climate changes relative to nannofloral increases and crises as well as b dissolution events and c diagenetic modifications characterization of calcareous nannofloras in oae intervals can improve our understanding of the marine ecosystem and biological processes such as photosynthesis biological pump and biomineralisation carbonate pump that affect the organic and inorganic carbon cycle as well as adsorption of atmospheric co2 in the oceans types and rates of nannoplankton production and evolution are interpreted to trace the impact of major palaeoceanographic and palaeoclimatic events in selected sections it has been documented that calcareous nannofloras rapidly reacted to new conditions of fertility and higher pco2 by drastically reducing calcification as in the modern oceans during oaes the increase of nutrients and atmospheric co2 induced higher abundances of nannoplankton producing small placoliths and inhibited the deep photic zone nannoconids and schizosphaerellids similarly to the nannoconid crisis preceding deposition of the early aptian oae1a black shales a schizosphaerellid crisis is detected prior to the toarcian oae both oaes are further characterised by a rapid nannofloral speciation beginning approximately 1 5 myr before the oae but without extinctions global changes during the latest cenomanian oae 2 exerted different influences on calcareous nannoplankton that experienced a turnover due to most extreme environmental conditions this event in fact was a time of extinctions followed by originations within calcareous nannofossils precise timing of the events before during and after oae1a oae2 and the toarcian oae indicates that they were intervals of enhanced oceanic productivity and that anoxia dysoxia post dated biotic changes

p and s velocity anomalies in the upper mantle beneath europe from tomographic inversion of isc data we present a new tomographic model for p and s velocity anomalies beneath europe 30 n 55 n 5 w 40 e extending in depth up to 700 km and constrained by inversion of data from the international seismological center isc catalogue the algorithm uses the traveltimes from events located in the study area recorded by all available worldwide stations as well as times from teleseismic events recorded by european stations the events from the isc catalogue have been relocated and combined into composite events all the traveltimes were corrected for crustal structure using the reference model eucrust 07 the resulting velocity anomalies show similar large scale patterns as observed in previous studies but have a higher resolution which allows detection of some features in more detail for example it is now possible to assess the depth extension of the small slow velocity body beneath the eifel region and eger graben the p and the s model show a good consistency in the uppermost 200 km below most of the european area and in some parts even in the deeper layers e g beneath the apennines and the hellenic arc the new model provides clear images of some principal features which were previously detected in a limited number of studies while the comparison between p and s velocity anomalies provides novel constraints to address on their nature e g the gap in the adriatic plate subducted below the central southern apennines in this paper we pay special attention to testing the reliability of the results the random noise effect is evaluated using a test with independent inversion of two data subsets with odd even events the spatial resolution is estimated using different checkerboard tests furthermore we present a synthetic model with realistic patterns which reproduces after performing forward and inverse modelling the same shape and amplitudes of the anomalies as in the case of the real data inversion in this case the parameters of the model can be used to assess the amplitudes of p and s anomalies that is critical for evaluation of other petrophysical parameters temperature density composition etc in the upper mantle 2009 the authors journal compilation 2009 ras