precambrian ophiolites cambrian island arc and variscan suture in the south carpathian balkan region a late proterozoic ophiolite in the south carpathian balkan region has a total surface outcrop area of 400 km 2 it is a complete ophiolitic section that includes mid ocean ridge basalts indicating that it was formed in a large ocean the oceanic crust is unconformably overlain by an ensimatic island arc of cambrian age the ophiolite island arc assemblage was obducted over the moesian platform during early ordovician time both sequences are now incorporated in a continental block as a result of intracontinental convergence which took place in variscan time the ophiolite is situated along the thracian suture which divides the moesian platform and the thracian continental fragment this suture is part of a larger one referred to as the south european suture that is situated between the thracian massif and the adriatic plate to the southwest and the moesian platform bohemian and central french massif to the northeast

significance of some geotechnical properties of continental slope and rise sediments off northeast newfoundland shear strength measurements carried out on the 0 20 cm interval of continental slope and continental rise surficial sediments yielded mean values per station that ranged from 3 7 to 8 9 kpa the factors controlling the bathymetric variation of sediment shear strength appear to be sediment texture which is controlled locally by sediment spillover on the upper slope increased deposition of fines silt and clay sized particles from suspension and greater bioturbation activity on the middle slope and bottom current sediment reworking on the base of slope and upper rise sediments deposited on the downslope side of the present presumed location of the western boundary undercurrent wbu axis have comparatively high shear strength gradients that appear to denote the increased proportion of fine sediment deposition during early to middle holocene time when the wbu axis may have been located further upslope than at present

spatial variability in selected geotechnical property measurements from three sedimentary provinces in the venezuela basin abstract relatively undisturbed surficial sediment samples from three sedimentary provinces pelagic turbidite dominated and hemipelagic in the venezuela basin were examined for spatial variability in water content and undrained shear strength a miniature vane shear apparatus boring method which minimized sample disturbance was used to determine variability of sediment shear strength directly within individual box core samples the variability of water content and sediment shear strength was also determined from subsamples 61 mm id cores collected from replicate box cores at each province sediments at the turbidite site yielded a higher average lateral coefficient of variation in water content and sediment shear strength both methods of measurement than sediments from the pelagic and hemipelagic sites vertical coefficients of variation were consistently higher than lateral coefficients of variations for all measured parameters between box core variability in sediment shear strength was less than within box core variability reflecting differences in measuring techniques and sample disturbance rather than actual differences biological activity primarily compaction by burrowing polychaetes maintains steep gradients of decreasing water content and increasing shear strength in the upper 100 200 mm of sediments at all three sites the biologically controlled gradients and the occasional deposition of turbidites results in much higher vertical compared to lateral variability of some sediment geotechnical properties on scales of tens of millimeters within box cores and kilometers between box cores collected at the same site

segmentation and coalescence of cenozoic carbonate platforms northwestern great bahama bank seismic profiles over the northwestern great bahama bank reveal that it is formed by the coalescence of three smaller platforms and significant lateral progradation this conclusion is contrary to previous models which assumed that the bank is the result of continuous accumulation on one huge buildup in late cretaceous to early tertiary time a north south trending depression the straits of andros separated an eastern platform andros bank from a western platform bimini bank initially the straits of andros had dimensions similar to the modem tongue of the ocean but was progressively filled from east to west in early to middle tertiary time a second depression the bimini embayment formed within bimini bank by folding it had a maximum depth of about 470 m this depression was also filled from east to west in addition prograding systems built the western margin of bimini bank more than 25 km westward into the straits of florida

combining a computer simulation and eustatic events to date seismic sequence boundaries a case study of the neogene of the bahamas abstract this paper demonstrates that the principles of seismic sequence stratigraphy and the techniques of computer simulation can be combined to date the sedimentary section as an example it uses the neogene section of the western shelf of the straits of andros in the bahamas to this end type i second order seismic sequence boundaries were identified on a seismic cross section from the region these were used as brackets to correlate enclosed third order events with an eustatic sea level curve the interpreted section was then reproduced using a computer simulation program the simulation successfully reproduced the onlapping geometries of the shelf margin identified on the seismic section the early neogene fill of the straits of andros is assumed to have had an uncomplicated and uniform tectonic setting this simplified the inputs to the simulation which focused on capturing the sedimentary response to the eustatic changes inputs included a constant rate of subsidence of 0 009 m ka a uniform linearly increasing rate of carbonate accumulation from 28 5 ma to 11 5 ma and the sea level cycle chart of haq et al 1987 updated for the neogene with the absolute ages provided by berggren et al 1995 the subsequent graphical output shows a good match to the interpreted seismic this suggests that the amplitude and ages of the sea level changes as shown in the eustatic chart can be used to date the sequence boundaries for the neogene of the straits of andros it also establishes that for areas of known subsidence simulation techniques can be used to date seismic sections where biostratigraphic data are poor

metamorphosed and dismembered ophiolite suites in the basement units of the eastern alps abstract the pre alpine basement of the eastern alps contains several ultramafic mafic rock complexes four of which have been investigated although polyphase deformation and metamorphism obscures much of the primary features field petrographical and geochemical evidence favours ophiolitic origins in mid ocean ridge and back arc tectonic settings the stubach complex belongs to the penninic basement of the tauern window and is interpreted as an obducted back arc ophiolite with ultramafic cumulates gabbro and minor amounts of basalt which shows the geochemistry of subduction related magma generation the rating complex belongs to a group of strongly dismembered basement slices carrying the palaeozoic austroalpine sequence of the northern greywacke zone basaltic rocks are accompanied by former metamorphic peridotite containing chromite concentrations nearly all members of an ophiolite suite are present in the speik complex in the austro alpine crystalline basement the source rocks are metamorphic peridotite harzburgite with chromite occurrences cumulate ultramafic and mafic rocks isotropic gabbro basalt and or dolerite and oceanic sediments the plankogel complex also part of the austro alpine crystalline basement represents a tectonic melange blocks of serpentinite and amphibolite derived from metamorphic peridotites and mid ocean ridge basalts together with marble and quartzite are contained in a micaschist matrix admixed is a rock assemblage with alkali basaltic amphibolites interpreted as a seamount environment the ritting plankogel and possibly parts of the speik complexes contain relict high pressure minerals and therefore went through a stage of subduction the inferred ages of all complexes is late precambrian to early palaeozoic the ophiolite suites have their distinct characteristics and contrasting tectonic settings and are therefore considered to be genetically unrelated to each other

testing 3d computer simulation of carbonate platform growth with repro the miocene llucmajor carbonate platform mallorca abstract repro a 3d computer based simulation program for reef and carbonate platform growth was tested using published data from the late miocene llucmajor carbonate platform in mallorca the device central to the program is a depth dependent logistic growth function for carbonate growth combined with linear diffusion so accounting for the lateral spreading of reefs evolution of the carbonate platform was simulated using a composite sea level curve consisting of superposed sine functions with different frequencies and amplitudes overall carbonate platform shape and internal geometries as produced by the program match fairly well with the outcrop data thus we demonstrate that repro has the potential to model lateral facies changes and lateral variations in the depositional geometries of carbonate platforms in three dimensions

steep microbial boundstone dominated plaform margins abstract seaward progradation of several kilometers has been documented mostly for leeward margin low angle carbonate slope systems with a dominant platform top sediment source however steep and high relief margins fronting deep basins can also prograde and as such are somewhat perplexing characteristics of two prograding carboniferous examples provide a model which may apply elsewhere 1 outcrops in asturias northern spain serve as important analogs for 2 hydrocarbon reservoirs in steep sided isolated platforms of the north caspian basin kazakhstan such as tengiz seismic and well data from tengiz corroborate outcrop patterns for slope development showing progradation of up to 5 and more than 10 km respectively despite the high relief up to 600 m and steep 20 35 nature of these margins the two examples share a highly productive microbial boundstone factory extending from the platform break down the slope to nearly 300 m or more depth and a lower slope dominated by mega breccias and grain flow deposits derived from the margin and slope itself the broad depth range of microbial boundstone increases the potential for production during both lowstands and high stands of sea level and thereby facilitates progradation independent from platform top derived sediment rapid in situ lithification of the boundstone provides stability to the steep slopes but also leads to readjustment through shearing and avalanching what controls the microbial cement boundstone formation remains a debate but its presence is a key factor controlling the progradational geometry of these and possibly other margins this new model of slope shedding has implications for slope readjustment processes and resulting architecture sequence stratigraphic interpretation reservoir characterization and reservoir modeling especially the isotropic character of microbial boundstone will reduce the potential for coherent seismic reflections to develop and possibly invoke under certain stress regimes shattering and fracturing thereby generating significant non matrix permeability key considerations are the contrasts with the bahamian high stand shedding depositional model slope progradation rates that range from 450 to 1500 m my and net growth rates of in situ boundstone of 1000 m my comparable to or higher than accretion rates for metazoan skeletal reef growth

geodynamic model for the palaeozoic crustal consolidation of western and central europe abstract the crystalline basement of western and central europe consists of a mosaic of crustal elements which were consolidated during pre grenvillian the grenvillian dalslandian morarian cadomian caledonian and hercynian orogenic cycles contemporaneous with the caledonian suturing of the precambrian laurentia greenland and fennosarmatian shields a number of gondwana derived cadomian micro cratons were accreted to the southern margin of laurasia following the late caledonian paroxysm the devonian and early carboniferous evolution of europe was dominated by continued northward subduction of the proto tethys plate at an arc trench system parallelling the southern margin of laurasia the accretion of additional gondwana derived continental fragments back arc rifting and a sinistral translation of major proportions between the laurentia greenland and the fennosarmatian sub plates the acadian and bretonian orogenies were of the pacific type the visean collision of gondwana with laurasia marked the onset of the himalayan type variscan orogeny during which collision related compressive stresses overpowered the devonian early carboniferous back arc rift systems and caused the development reactivation of a subduction zones the central armorican and saxothuringian successor basins became folded and destroyed during the latest visean whilst the variscan foredeep became scooped out in part by nappes involving the basement during the late westphalian major crustal shortening during the variscan diastrophism was accompanied by the anatectic remobilisation of subducted crustal and upper mantle material and a widespread syn and late orogenic magmatism the latest carboniferous early permian reorientation of the convergence direction between gondwana and laurasia induced the development of a complex wrench fault system transecting the variscan fold belt and an extensive post orogenic volcanism the hypotheses summarised here require confirmation by further palaeomagnetic and radiometric data and faunal analyses

understanding the pre variscan and variscan basement components of the central tauern window eastern alps austria constraints from single zircon u pb geochronology new single grain and within grain u pb zircon ages from the central tauern window help sorting out the time dimension among the various variscan and pre variscan basement components that were strongly overprinted by alpine orogeny single grain isotope dilution id tims u pb zircon geochronology of three basisamphibolit samples yield protolith formation ages of 351 2 349 1 and 343 1 ma laser ablation icp ms and id tims u pb detrital zircon dating of the biotitporphyroblastenschiefer constrained the maximum time of sedimentation to between 362 6 ma and 368 17 ma paragneisses from the zwolferzug yield maximum sedimentation ages from 345 5 ma ion microprobe data to 358 10 ma zircons from gabbroic clasts and detrital zircons from a meta agglomerate from the habach phyllite give an upper intercept age of 536 8 ma and a near concordant age of 506 9 ma respectively hence apart from the habach phyllite the maximum sedimentation ages of the metasediments investigated range from upper devonian to lower carboniferous consequently the basisamphibolit the biotitporphyroblastenschiefer and the paragneisses of the zwolferzug form parts of the variscan basement series the basisamphibolit 351 343 ma is distinct both in space and time of formation from the zwolferzug garnet amphibolite c 486 ma which forms part of the pre variscan basement

modelling weathering processes at the catchment scale the witch numerical model a numerical model of chemical weathering in soil horizons and underlying bedrock witch has been coupled to a numerical model of water and carbon cycles in forest ecosystems aspects to simulate the concentration of major species within the soil horizons and the stream of the strengbach granitic watershed located in the vosges mountains france for the first time simulations of solute concentrations in soil layers and in the catchment river have been performed on a seasonal basis the model is able to reproduce the concentrations of most major species within the soil horizons as well as catching the first order seasonal fluctuations of aqueous calcium magnesium and silica concentrations however the witch model underestimates concentrations of mg2 and silica at the spring of the catchment stream and significantly underestimates ca2 concentration the deficit in calculated calcium can be compensated for by dissolution of trace apatite disseminated in the bedrock however the resulting increased ca2 release yields important smectite precipitation in the deepest model layer in contact with the bedrock and subsequent removal of large amount of silica and magnesium from solution in contrast the model accurately accounts for the concentrations of major species ca mg and silica measured in the catchment stream when precipitation of clay minerals is not allowed the model underestimation of mg2 and h4sio4 concentrations when precipitation of well crystallized smectites is allowed strongly suggests that precipitation of well crystallized clay minerals is overestimated and that more soluble poorly crystallized and amorphous materials may be forming in agreement with observations on other watersheds draining granitic rocks this study indicates that highly soluble trace calcic phases control the aqueous calcium budget in the strengbach watershed

the paradox of drowned reefs and carbonate platforms shallow water carbonate platforms and reefs are drowned when tectonic subsidence or rising sea level outpaces carbonate accumulation and benthonic carbonate production ceases drowned platforms are common in the geologic record but they present a paradox if one considers rates of processes involved during the early holocene when sea level rose at rates of 6 000 to 10 000 m yr mm 1 000 yr most reefs and platforms were outpaced by the rising sea during the late holocene with sea level rising 500 to 3 000 m yr in the atlantic caribbean area reefs and platforms started to recover built to sea level and prograded seaward 1 000 m yr is thus a conservative estimate of the average growth potential of modern reefs and platforms independently accumulation rates of prograding platforms in the geologic record suggest growth potential in excess of several hundred microns per year the growth potential of 1 000 m yr exceeds any relative rise of sea level caused by long term processes in the geologic record newly formed ocean crust subsides at a maximum of 250 m yr basin subsidence averages 10 to 100 m yr and sea level rises due to increased sea floor spreading amount to less than 10 m yr rapid pulses of relative rise of sea level or reduction of benthic growth by deterioration of the environment remain the only plausible explanations of drowning the geologic record shows examples of both of these processes global mass extinctions of reefs and platforms occurred in the middle cretaceous eustatic rise due to submarine volcanism or desiccation of a small ocean basin and the late devonian global crisis of ocean environment extraterrestrial cause drowning controlled by regional tectonics prevailed in the jurassic and early cretaceous of the tethyan realm and the drowning of mesozoic platforms in the western north atlantic seems to have been dictated by plate tectonic drift to higher latitudes

tectonometamorphic evolution of the koralm complex eastern alps constraints from microstructures and textures of the plattengneis shear zone abstract lattice preferred orientations lpo of quartz have been investigated along a south north oriented section across the plattengneis of the koralm complex eastern alps the plattengneis forms an important shear zone within the austroalpine nappe complex of the eastern alps which has developed during the cretaceous evolution of the alpine orogen the quartz c axes form small circle distributions in the southernmost parts of the koralm complex which represent the uppermost structural level of the plattengneis further to the north two maxima between the y and z directions of the finite strain can be interpreted in terms of preferred slip on the rhomb planes these fabrics continuously grade into type i and type ii crossed girdle distributions in a northward direction a strong maximum near the y axis with the tendency to be distributed along a single girdle with three corresponding maxima of a axes near the margin of the pole figure can be observed in the central and northern parts such lpo are characteristic for both high grade metamorphic conditions and high finite strain the microstructures display that the deformation within the plattengneis shear zone was synmetamorphic a continuous increase of peak temperatures and pressure from approximately 550 c to approximately 750 c from the south to the central parts can be inferred from geothermometric calculations the temperatures then decrease to approximately 650 c from the central parts to the north the related pressures increase from 8 to 16 kbar and then decrease to 10 kbar the lpo changes that have been observed in the study area are best interpreted in terms of temperature dependence of the activation of glide systems within quartz aggregates the temperature and pressure evolution may indicate that the central parts of the koralm complex have been exhumed by larger amounts than the northern and southern parts this is also documented by the lpo evolution therefore we assume that the plattengneis shear zone formed during the exhumation of the koralm complex and is related to the exhumation of high pressure units in the footwall of this shear zone accordingly the kinematics of the plattengneis shear zone is rather extensional than thrust related the implications for the structural evolution of the eastern alps are shortly discussed

tectonics and plate tectonics model for the variscan belt of europe abstract a plate tectonics model is presented to explain the tectonometamorphic characteristics of the european variscides after the closing of two oceanic domains by two sided subduction 500 420 ma and obduction 420 380 ma collision of the european and african continental plates occurred we propose that the subsequent complex intracontinental deformation 380 290 ma is the result of a double subduction of the continental lithosphere accompanied by crust mantle decollement this mechanism explains the progressive crustal thickening and migration of the deformation through time from the sutures toward the external parts of the variscan belt accounting for this model and for the relationships between the european variscides and the other paleozoic peri atlantic belts caledonides appalachian mauritanides and morocco we infer the relative positions of africa america and europe between the silurian and the permian

concepts of the evolution of the austroalpine basement complex eastern alps during the caledonian variscan cycle the austroalpine basement complex has a complicated pre alpidic history which begins with the caledonian era in the late precambrian and early paleozoic a magmatic sedimentary rock sequence is formed presumably in an island arc or active continental margin environment subduction with eclogite formation is followed by collision high grade metamorphism and anatexis in the ordovician this caledonian basement is preserved in parts of the austroalpine crystalline mass the post caledonian deposits are mainly shelf type sediments with intercalated volcanics although there is evidence for an oceanic basin to the south the variscan facies zones are arranged in sw ne direction oblique to the alpidic trend in a first stage of variscan orogeny in the carboniferous south east vergent decollement nappes syntectonic flysch deposits and granitoids are formed along with regional metamorphism this is followed by a second stage in the permian with north west vergent thrusting renewed granite formation and metamorphism the variscan nappe pile is today exposed in a deeper level in the west or northwest than in the east or southeast

from platform to basin the evolution of a paleocene carbonate margin eastern desert egypt in this study progradation and the subsequent retrogradation of a late paleocene isolated carbonate platform galala mountains eastern desert egypt is demonstrated by variations of distinct facies associations from the platform margin in the north to the hemipelagic basin in the south a combination of a sea level drop and tectonic uplift at around 59 ma calcareous nannofossil biozone np5 favored the initiation of the carbonate platform from this time onwards the facies distribution along the platform basin transect can be subdivided into five facies belts comprising nine different facies associations their internal relationships and specific depositional settings are strongly coupled with the maastrichtian paleocene seafloor topography which resulted from local tectonic movements patch reefs and reef debris were deposited at the platform margin and the horizontally bedded limestones on the upper slope slumps and debris flows were stored on the lower slope in the subhorizontal toe of slope facies belt mass flow deposits pass into calciturbidites further southwards in the basin only hemipelagic marls were deposited between 59 and 56 2 ma np5 np8 the overall carbonate platform system prograded in several pulses distinct changes in facies associations from 56 2 to 55 5 ma np9 resulted from rotational block movements they led to increased subsidence at the platform margin and a coeval uplift in the toe of slope areas this resulted in the retrogradation of the carbonate platform furthermore the patch reef and reef debris facies associations were substituted by the larger foraminifera shoal association the retrogradation is also documented by a significant decrease in slump and debris flow deposits on the slope and calciturbidites at the toe of slope

distribution of variscan i and s type granites in the eastern alps a possible clue to unravel pre alpine basement structures abstract meta granitoid rocks with variscan intrusion ages have been investigated in the three large austroalpine basement massifs of the schladminger tauern the seckau bosenstein tauern and the semmering raabalpen the first two massifs are dominated by calc alkaline i type granitoids mainly granodiorites tonalites and are thus similar to most other intra alpine variscan granitoid massifs while the semmering raabalpen area is one of the rare examples of an intra alpine s type batholith this confirms previous concepts according to which the semmering raabalpen area is part of a separate variscan terrane within the austroalpine basement it is suggested that this basement terrane already had some affinities to the thick mature crust that is typical for the internal moldanubian zone of the variscides the widespread occurrence of carboniferous i type plutons suggests the presence of a less evolved or newly underplated variscan lower crust in most other parts of the eastern alps this kind of crust was probably typical for the former southern flank of the variscan orogen

osmium isotope constraints on contrasting sources and prolonged melting in the proterozoic upper mantle evidence from ophiolitic ru os sulfides and ru os ir alloys this study presents the first extensive data set of os isotopic compositions of primary ru os ir alloys and ru os sulfides derived from two neoproterozoic ophiolite type ultramafic massifs kunar in northern taimyr russia and kraubath in eastern alps austria the study employed a multi technique approach and utilized a number of analytical techniques including hydroseparation electron microprobe analysis negative thermal ionization mass spectrometry n tims and laser ablation attached to multiple collector inductively coupled plasma mass spectrometry la mc icp ms the results identify two different os isotope sources subchondritic and suprachondritic for bedrock ru os sulfides at kraubath whereas detrital ru os ir alloys at kunar are characterized by unradiogenic 187os 188os values indicative of a chondritic or subchondritic mantle source of platinum group elements pge the unradiogenic 187os 188os values yield a very wide range of 187os 188os 0 1094 0 0003 to 0 1241 0 0004 n 19 n tims data which is almost identical to the os isotope composition of bedrock laurite ru os s2 in podiform chromitite within an ophiolitic mantle section at kraubath 0 11249 0 00062 to 0 12437 0 00051 n 17 n tims and la mc icp ms data these results are consistent with a model in which a prolonged history of melting events of parent ultramafic source rocks took place in the oceanic upper mantle in contrast erlichmanite and laurite from banded chromitite within the transition zone at kraubath show high 187os 188os 0 13080 to 0 13212 values indicative of a suprachondritic source of pge this signature might be interpreted as either evidence of an enriched mantle source or an indication of a radiogenic crustal component which was introduced during a subduction related event the os isotope results obtained support the conclusion that the re os system represented by pgm has remained unchanged from the time of formation of the pgm until now despite later thermal events which occurred in the vicinity of ophiolite type complexes

a comparison of shear strength in calcareous and siliciclastic marine sediments abstract more than 2000 measurements of shear strength versus depth in fine grained marine sediments were collected from the literature and our own experiments in order to study the relationship between carbonate content and strength behavior of marine sediments depositional environments include slope rise and abyssal plain with most data originating from the slopes the depth range is restricted to 6 m subbottom depth in order to minimize diagenetic effects contour plots of shear strength versus depth for calcareous and siliciclastic sediments clearly reveal different trends whereas shear strength in siliciclastic sediments shows low variability and increases only gradually with depth the shear strength in the calcareous population is highly variable and increases rapidly with depth the difference in strength behavior of the two sediment groups has implications for the stability of submarine slopes previous research has shown that carbonate sediments build up steeper and more variable slopes than siliciclastic sediments this difference in slope angle can be explained at least in part by the higher internal strength of the carbonate sediments

permian high temperature metamorphism in the western alps nw italy during the late palaeozoic lithospheric thinning in part of the alpine realm caused high temperature low to medium pressure metamorphism and partial melting in the lower crust permian metamorphism and magmatism has extensively been recorded and dated in the central eastern and southern alps however permian metamorphic ages in the western alps so far are constrained by very few and sparsely distributed data the present study fills this gap we present u pb ages of metamorphic zircon from several adria derived continental units now situated in the western alps defining a range between 286 and 266 ma trace element thermometry yields temperatures of 580 890 c from ti in zircon and 630 850 c from zr in rutile for permian metamorphic rims these temperature estimates together with preserved mineral assemblages garnet prismatic sillimanite biotite plagioclase quartz k feldspar rutile define pervasive upper amphibolite to granulite facies conditions for permian metamorphism u pb ages from this study are similar to permian ages reported for the ivrea zone in the southern alps and austroalpine units in the central and eastern alps regional comparison across the former adriatic and european margin reveals a complex pattern of ages reported from late palaeozoic magmatic and metamorphic rocks and relics thereof two late variscan age groups 330 and 300 ma are followed seamlessly by a broad range of permian ages 300 250 ma the former are associated with late orogenic collapse in samples from this study these are weakly represented clearly dominant is the permian group which is related to crustal thinning hinting to a possible initiation of continental rifting along a passive margin

carbonate ramp depositional systems abstract the classification tectonic settings stratigraphy and early diagenesis of carbonate ramp systems are reviewed carbonate ramps are common in all geological periods but were dominant at times when reef constructing organisms were absent or inhibited ramps can be subdivided into inner mid and outer ramp environments the mid ramp zone extends from fair weather wave base to normal storm wave base although the water depths which these boundaries represent vary an additional outer slope environment occurs on distally steepened ramps as with siliciclastic shelves a range of wave storm and tide dominated ramps can be recognised and this forms the most convenient basis for ramp classification the carbonate productivity profile of ramps differs from that of rimmed shelves with the inner ramp showing lower production rates than comparable shallow water facies on rimmed shelves the zone of greatest organic carbonate sediment production appears to have shifted from the mid ramp to the inner ramp since the late jurassic carbonate ramps occur in most types of sedimentary basin but are best developed where subsidence is flexural and gradients are slight over large areas as in foreland and cratonic interior basins and along passive margins the featureless depositional profiles of many ramps means that sequence geometries are best observed on regional seismic lines some show low angle sigmoidal or shingled clinoforms suggesting that ramps may seldom be homoclinal but possess subtle slope geometries which reflect depositional environments because of their low angle slopes ramps respond differently to rimmed shelves during relative sea level changes although results seem to be strongly dependent on the rate of relative sea level change during a minor fall shallow ramp facies belts will simply shift basinwards in a forced regression in contrast the whole surface of a steep sloped flat topped rimmed shelf may be exposed so that sediment production ceases or is drastically reduced during a major fall shallow ramp bounded basins may empty completely conversely ramps also flood gradually whereas rimmed shelves do so more rapidly homoclinal ramps develop no resedimented lowstand deposits rimmed shelves and distally steepened ramps in contrast may develop lowstand talus or turbiditic wedges distally steepened ramps may behave more like homoclinal ramps during minor base level falls and like rimmed shelves during major base level falls many ramps consist of layered successions of several ramp sequences stacked one upon the other ramp stacks of this sort may show gross vertical accretion but individual ramp sequences seldom appear to develop in a keep up style apart from minor organic buildups as with many rimmed shelves steepening of the outer ramp margin due to tectonism slope inheritance or differential sedimentation may promote the development of a distally steepened ramp or rimmed shelf a wide variety of organisms have constructed buildups in mid and outer ramp environments isolated buildups may seed early in ramp development accrete to wave base or sea level and continue growth by stacking through successive ramp sequences so that depositional and diagenetic features within them are in concert with those of the shallow ramp the location of isolated buildups on ramps is governed by tectonism halokinesis antecedent topography or by the subtle slope geometry of the previous ramp sequence diagenesis on ramps shows some major variations compared with diagenesis on steep sloped flat topped carbonate platforms ramp bounded basins may form prolific petroleum sourcing and reservoiring systems and offer a range of subtle stratigraphic play types and lateral facies variations which determine reservoir quality and distribution isolated buildups in the mid and outer ramp environments represent one of the commonest petroleum reservoirs in ramp systems and tend to have their foundations in transgressive systems tracts grainstone and packstone reservoirs are widespread and range from shoreline carbonate sandbodies to major detached shoal complexes or shoals over offshore highs grainstone sandbodies occur in both highstand systems tracts and in prograding lowstand wedges

gradual carbonate to clastic transition and provenance of quartz along the nw trento plateau southern alps italy abstract over the drowned site of the nw trento carbonate platform a gradual transition from early cretaceous pelagic carbonate to more siliciclastic sedimentation during the early tertiary is denoted by mineralogical and geochemical trends decreasing oxygen isotope ratios of quartz in the matrix from 34 33 to 18 47 smow suggest a mainly biogenic origin for the silica with insignificant terrigenous quartz input during the early cretaceous and only minor introduction of terrestrial quartz during the late cretaceous and early tertiary the insignificance of terrigenous input during the late cretaceous and early tertiary is consistent with existing depositional models that involve bypassing of the nw trento plateau margin the bulk of the terrigenous deposition being funnelled into the adjacent lombardian flysch basin bernoulli and winkler 1990 during the eoalpine tectonic phase doglioni and bosellini 1987

quantifying the geometry and sediment fabric of linear slopes examples from the tertiary of italy southern alps and gargano promontory the geometries and sediment characteristics of two well exposed submarine slope successions of ancient carbonate platforms were studied to analyse the relationship between inclinations of linear slopes and sediment fabric the two examples are an oligocene platform to slope succession exposed on the brione mountain in the southern alps and an eocene slope tobasin succession exposed on the saraceno mountain in the gargano promontory the planar slope of the brione mountain outcrop is inclined up to 20 25j and consists of fine to coarse sand sized skeletal packstones and grainstones and that of the saraceno mountain outcrop has a modal angle of 15 22 5j and comprises very fine to coarse sand sized poorly washed skeletal packstones grainstones and rudstones the good agreement between the inclinations of the linear slope profiles and angles of repose published in the literature implies that these slopes are at or very close to the angle of repose the general rule that in systems built to the angle of repose slope angle is directly correlated to sediment fabric can be applied to linear slopes d 2002 elsevier science b v all rights reserved

footwall uplift in an orogenic wedge the tauern window in the eastern alps of europe abstract in the tauern window tw the deepest structural units of the eastern alps are exposed in a 30 160 km antiform european basement and cover occur under nappes of oceanic penninic units and continental origin austroalpine nappes the tertiary structural history of the tw is related to a rapid uplift of 20 30 km since oligocene times structural mapping revealed 1 detachment and folding of the cover units and stacking of orthogneiss sheets 2 constrictional folding of the entire duplex with amplitudes and half wavelengths of several kilometres 3 backthrusting and backfolding along the northern margin of the tw and 4 displacement along steep dip divergent strike slip faults the strain history is complex and related to all these deformation phases the most uniform structural element is a stretching parallel to the long axis of the tw and its major fold axes strain did not contribute to tw uplift because ductile n s shortening was compensated by e w extension restoration of the tw shows that the exposed structures are not sufficient to explain the tw uplift by analogy with the engadine window and the western alps where seismic data are available a deep seated duplex is suspected the tw uplift is best explained in a dextral transpressive regime

carbonate platform flanks slope angle and sediment fabric more than 20 examples of fossil carbonate platform systems were compared for slope angle and sediment fabric plots of slope angle versus sediment fabric show that grainy non cohesive mud free sediments build steeper slopes than muddy cohesive sediments examples near the end members of grainy and muddy carbonate platform flanks are found in the triassic of the dolomites in northern italy and in the bahamas respectively they document the flank geometry and the processes readjusting the slope profile once the limiting slope angle is exceeded the grainy flank sediments in the dolomites modified by shearing and avalanching produce straight slope profiles with declivities up to 35 whereas the muddy bahamian flank sediments modified by large scale creep and rotational to translational sliding and slumping produce a concave upwards slope profile inclined at less than 4 the comparison between slope angle and sediment fabric indicates that the physical behaviour of sediments in the gravity field angle of shearing and mode of readjustment processes is linked to the composition of the slope sediment among the variables such as sea level subsidence climate plate motion and oceanographic setting windward leeward sediment fabric is suggested to be a major if not the major control on slope angle and slope curvature of carbonate platform flanks besides the recently documented tendency of carbonate sediments to build steeper slopes than siliciclastics this proposed relation sheds new light on the analysis and quantification of the variables influencing the geometry and depositional evolution of carbonate systems furthermore it provides an opportunity to deduce sediment composition from seismic lines and predict lithology prior to drilling

automicrite in a nummulite bank from the monte saraceno southern italy evidence for synsedimentary cementation following introduction of the term nummulite bank there has been debate regarding interpretation of these types of deposits as autochthonous automicrite or allochthonous detrital micrite these banks are made up of large foraminifera and ill defined fine grained components the fine grained components consist mainly of micrites the recognition of automicrite has deep implications for the synsedimentary cementation and stabilization of the bank in order to distinguish between automicrite and detrital micrite the nanomorphology geochemistry and organic matter remains in the microfacies of a nummulite bank in the middle eocene of monte saraceno gargano southern italy were analysed optical and scanning electron microscope investigations showed that the micrites have been recrystallized to aggrading microsparite epifluorescence observations on selected micrite microsparite areas with peloidal texture revealed the presence of organic matter scanning electron microscope analyses on epifluorescent micrites showed that the microbial peloids have smaller crystal sizes than those in organic matter depleted areas the geochemical characterization of extracted organic matter performed through the functional group analyses by fourier transform infrared spectroscopy shows strong prevalence of the aromatic fraction over the aliphatic and carboxylic ones these characteristics of organic compounds indicate both their thermal maturation and their likely derivation from degradation of bacterial communities the local presence of peloidal anti gravity textures bright epifluorescence and organic molecules in clotted peloidal areas suggest that the metabolic activity of microbial communities could have induced precipitation of these micrites and consequently the syndepositional cementation of the nummulite bank this type of cementation can rapidly stabilize sediments and promote the depositional bank geometry

categorizing the morphologic variability of siliciclastic passive continental margins the basic morphology of the continental shelf slope and rise on passive continental margins has many variations in the modern ocean environment through using a satellite derived bathymetric data set of the global ocean we have developed a classification for the shape of modern siliciclastic dominated passive margins the classification subdivides 50 margins into five distinct shape categories based on each margin s pattern of sea floor slopes as a function of depth by comparing these shape categories with the underlying stratal architecture sediment input and present degree of canyon incision of the margins we find apparent correlations between these factors and basic margin shape in general gently sloped margins tend to occur in regions with high sediment input few modern canyons and unstable substrates higher gradient margins tend to have lower sediment input more modern canyons and a variety of subsurface architectures these results suggest two important conclusions 1 morphology differences among modern siliciclastic passive margins can be objectively and systematically categorized and 2 these shape differences seem to be related to aspects of the modern sedimentary environment despite the fact that the margins have evolved over geologic time

out of our depth on the impossibility of fathoming eustasy from the stratigraphic record the evolution of a sedimentary basin s gross morphology depends upon 1 basement movement 2 sediment accumulation and compaction and 3 variations of eustasy with time thus if an accurate history of eustasy could be derived it would aid in our understanding of basin stratigraphy and help in the prediction of sediment geometry from described areas techniques which attempt to determine the magnitude of eustatic sea level excursions include the measurement of 1 the amount of sedimentary onlap onto the continental margins with and without the use of hypsometric curves 2 the thickness of marine sedimentary cycles and the elevation and distance between indicators of old strandlines 3 the perturbations on individual thermo tectonic subsidence curves and stacked crustal subsidence curves 4 the variations in deep ocean oxygen isotopes found in sediments and 5 the size of variables used in graphical and numerical simulations of basin fill in terms of tectonic behavior rates of sediment accumulation and eustasy which invert the problem to date a combination of the use of relative sea level charts derived from sediment onlap of the continental margin with dimensioning by oxygen isotopes responding to glacial events offers the best potential for relative tectono eustatic sea level curves but even this method can not produce a unique solution for absolute eustatic variations mathematical modelling shows that at best it is possible to obtain only the sum of tectonic basement subsidence and sea level variations from the above methods and at worst not even that simple a combination thus every proposed scheme to measure eustatic sea level excursions assumes some behavior for two of the three underlying processes tectonic movement of the basement sedimentary accumulation and eustatic sea level each scheme then determines the third process relative to the assumed model behavior of the other two it would seem that like king canute we cannot command the sea though we can still use undifferentiated relative tectono eustatic sea level curves to generate a family of solutions which are the product of a variety of absolute sea level curves and tectonic models each solution has then to be assessed in terms of geologic setting and the hypotheses it generates

evolution of late neoproterozoic to early paleozoic tectonic elements in central and southeast european alpine mountain belts review and synthesis abstract late neoproterozoic to early paleozoic cadomian tectonic elements are widespread in the southeastern alpine mediterranean mountain belts with discontinuous exposure extending from the alps to the menderes massif in turkey the sequences include voluminous plutonic island and continental arc successions some ophiolites of variable late neoproterozoic and cambrian to early ordovician ages high to medium grade metamorphic sequences and subordinate metasediments geological and geochronological data suggest that these cadomian elements partly experienced tectonothermal activity including metamorphism and magmatism between ca 650 and 600 ma followed by further i and s type plutonism in an andean type continental margin setting ca between 570 and 520 ma subsequently a major rift zone developed which resulted in continental stretching and subsequent formation of a back arc basin which is dated as late cambrian rifting is followed by oceanic spreading which commenced ca at the cambrian ordovician boundary possibly in a back arc setting this tectonic scenario suggests that alpine mediterranean cadomian tectonic elements were accreted to gondwana during early cadomian events ca 600 650 ma subsequently they were likely part of a long lasting outer subduction zone of gondwana at the margin of a panthalassa type ocean during late neoproterozoic iii and cambrian due to back arc spreading continental pieces started to split off from gondwana ca at the cambrian ordovician boundary

origin and emplacement of ophiolites ever since their recognition ophiolites have been a source of controversy although all workers have agreed upon their importance a number of conflicting schools of thought have become unified in the interpretation of ophiolite complexes as fragments of oceanic crust and mantle formed at spreading centers an expanded version of the penrose ophiolite definition is proposed herein called the ophiolite association taking into account rocks which bear upon the origin and significance of ophiolite complexes an ideal ophiolite association includes the following units from bottom to top 1 a crystalline basement and shallow water sedimentary sequence 2 a tectonic unit of thrust slices of continental margin and rise and abyssal sediments and or melange 3 a metamorphic unit as much as a few hundred meters thick generally with higher grade rocks over lower grade ones 4 ultramafic tectonite unit composed dominantly of multiply deformed peridotite dunite and minor chromitite 5 a cumulate complex ultramafic at the base grading to mafic or intermediate at the top 6 a noncumulus unit of varitextured gabbro and minor trondhjemite 7 a sheeted dike complex 8 an extrusive section of massive and pillowed flows and intercalated sediments 9 an abyssal or bathyal sediment sequence which may include radiolarian chert red pelagic limestone metalliferous sediments breccias and or pyroclastic deposits and 10 postemplacement deposits of laterite reef limestone or shallow marine or subaerial sediments this expanded ophiolite association allows greater precision in inferring original origin and emplacement some ophiolite complexes particularly in the tethys herein designated tethyan type exhibit units 1 2 3 and 4 whereas others particularly in the cordilleran belt herein termed cordilleran type lack units 1 2 3 and in places 4 the emplacement of the former probably occurred during collision of a passive margin with a subduction zone or incipient subduction zone emplacement of the cordilleran type generally is not clear cut the nature of the spreading center which produced a given ophiolite is indicated more clearly by the presence or absence of units 5 9 than by chemistry or mineralogy some ophiolite complexes lack complete sections and may have formed along transform faults others contain volcanic pyroclastic deposits instead of pelagic sedimentary sequence and probably formed within or near an arc several ophiolite complexes may have originated by conversion of a transform fault into a subduction zone such conversion may result from normal plate evolution or from major plate reorganization major times of ophiolite emplacements appear to correspond with global plate reorganizations

the iapetus model a plate tectonic concept for the variscan belt of europe abstract a plate tectonic model is presented explaining the evolution of the variscan belt of europe it regards the iapetus ocean and its branch the tornquist sea as the only ocean involved in the caledonian variscan and alleghenian mega orogen the iapetus and its marginal basins disappeared through oblique subduction and collision in the following sequence 1 1 northeastern iapetus including the tornquist sea caledonian orogeny 2 2 central iapetus acadian orogeny 3 3 adjacent marginal basins variscan orogeny and 4 4 southwestern iapetus alleghenian orogeny the area of the variscides was part of the continental margin of gondwana made up of ridges and ensialic basins no ocean lay within the variscan area during the palaeozoic although patches of oceanic crust probably existed in rift basins e g the lizard ophiolite after the silurian early devonian closure of northwestern and central iapetus the variscan area was surrounded by cratons on three sides in a manner similar to that around the modern mediterranean sea the subsequent variscan orogeny was essentially an ensialic thrusting and folding induced by a further approach of gondwana toward laurentia after the variscan collision gondwana rotated clockwise to the west by moving along a strike slip zone this rotation during the carboniferous opened the palaeo tethys in the east and caused the alleghenian collision in the west the middle ordovician to permian palaeogeography is presented on a series of maps modelling tectonic displacement and plate motions avalonia is interpreted as the ordovician volcanic island arc of gondwana separated from mainland gondwana by an oceanic gulf or a back arc basin the iberian arc may have originated by a continental escape of iberia from the center of collision into the oceanic gulf the variscan austro alpine and southern alpine units are today located near their earliest early carboniferous position

accumulation of bank top sediment on the western slope of great bahama bank rapid progradation of a carbonate megabank high resolution seismic profiles and submersible observations along the leeward slope of western great bahama bank show large scale export of bank top sediment and rapid progradation of the slope during the holocene a wedge shaped sequence up to 90 m thick is present along most of the slope and consists of predominantly aragonite mud derived from the bank since flooding of the platform 6 8 ka total sediment volume of the slope sequence is 40 80 that of holocene sediment currently retained on the bank maximum rates of vertical accumulation and lateral progradation are 11 15 m ka and 80 110 m ka respectively 10 to 100 times greater than previously known for periplatform muds slope deposition of exported mud during sea level highs appears to have been a major mechanism for the westward progradation of great bahama bank throughout the quaternary this may provide a critical modern analogue for ancient progradational margins

origin of eclogites from the austroalpine otztal basement tirol austria geochemistry and smnd vs rbsr isotope systematics geochemical and combined sm nd vs rb sr isotope data from the otztal metabasite suite point to production of gabbros and basalts in an extensional regime in late cambrian times the source of these magmas and cumulates had a chemical and isotopic composition similar to morb the time of intrusion is constrained by sm nd mineral isochrons clinopyroxeneplagioclase of relictic gabbro occurrences within the eclogite amphibolite series in the time span between 530 and 521 ma subsequently the basic eclogite precursors suffered three metamorphic stages the imprint of the caledonian event is unclear during the hercynian cycle the basic magmatic rocks were transformed into eclogites at surprisingly high p t conditions of 27 kbar 730 c the time of this subduction event is given by mineral isochrons garnet clinopyroxene zoisite whole rock as 370 340 ma with a most probable time span for the pressure peak between 360 and 350 ma during subsequent exhumation and pressure release the eclogites were partly transformed into amphibolites alpine overprint during a greenschist facies grade metamorphism caused further and partly intense retrograde alterations in the metabasic suite

diversity of platinum group mineral assemblages in banded and podiform chromitite from the kraubath ultramafic massif austria evidence for an ophiolitic transition zone we report highly unusual platinum group mineral pgm assemblages from geologically distinct chromitites banded and podiform of the kraubath massif the largest dismembered mantle relict in the eastern alps the banded chromitite has a pronounced enrichment of pt and pd relative to the more refractory platinum group elements pges of the ipge group os ir ru similar to crustal sections of ophiolites on the contrary the podiform chromitite displays a negatively sloping chondrite normalised pge pattern typical of ophiolitic podiform chromitite the chemical composition of chromite varies from cr 73 77 in the banded type to 81 86 in the podiform chromitite thirteen different pgms and one gold rich mineral are first observed in the banded chromitite the dominant pgm is sperrylite 53 of all pgms which occurs in polyphase assemblages with an unnamed pt base metal bm alloy and pd rich minerals such as stibiopalladinite mayakite mertieite ii unnamed pd rh as and pd pt as sb minerals this banded type also contains pge sulphides about 7 represented by a wide compositional range of the laurite erlichmanite series and irarsite 8 os ir alloy geversite an unnamed pt pd bi cu phase and tetrauricupride are present in minor amounts by contrast the podiform chromitite which yielded 21 different pgms is dominated by laurite 43 of all pgms which occurs in complex polyphase assemblages with pge alloys ir os os ir pt fe pge sulphides kashinite bowieite cuproiridsite cuprorhodsite unnamed fe cu ir rh 2s4 braggite unnamed bm ir and bm rh sulphides and pd telluride keithconnite a variety of pge sulpharsenides 33 including irarsite hollingworthite platarsite ruarsite and a number of intermediate species have been identified whereas sperrylite and stibiopalladinite are subordinate 2 the occurrence of such a wide variety of pgms from only two 2 5 kg chromitite samples is highly unusual for an ophiolitic environment our novel sample treatment allowed to identify primary pgm assemblages containing all six pges in both laurite dominated podiform chromitite as well as in uncommon sperrylite dominated banded chromitite we suggest that the geologically geochemically and mineralogically distinct banded chromitite from kraubath characterises the transition zone of an ophiolite closely above the mantle section hosting podiform chromitite rather than being representative of the crustal cumulate pile

the fluvial geochemistry and denudation rate of the guayana shield in venezuela colombia and brazil abstract the guayana shield is composed of early to mid precambrian igneous and metamorphic basement rocks with a quartzitic platform cover the complete absence of limestones and evaporites allows a clear chemical expression in the stream data of the primary weathering of the basement in a humid tropical environment total erosion rates are extremely slow 10 m m y with equal contributions from the dissolved and suspended loads however the former is largely silica with ratios of si to total cation equivalents si tz ranging to in excess of three weathering is complete to kaolinite and gibbsite i e the environment is one of active laterisation with a penetration rate of the weathering front into the fresh substrate about twice the denudation rate in basins of relatively homogeneous lithology rb sr isochrons constructed from the river data agree with the whole rock ages from the drainages thus all the common refractory rb containing minerals k feldspar mica are completely dissolved the thick lateritic regolith that is accumulating as a result of this intense weathering is a common relict feature on other southern hemisphere shields in the absence of active tectonics or greatly accelerated mechanical erosion the weathering rates of these basement rocks must be quite insensitive to environmental change

carbonate sand bodies along contrasting shallow bank margins facing open seaways in northern bahamas high resolution seismic profiling bottom sampling and aerial and space imagery show that widely contrasting carbonate sand bodies are present along the shallow windward leeward and tide dominated margins of little and great bahama banks which face open seaways the northwest providence channel and the northern straits of florida this high diversity in sand bodies and their associated bank margins is due to variations in the level duration and magnitude of the physical energy flux across the edges of the platforms in the bahamas the dominant winds waves and currents are from the east hence easterly facing margins are windward westerly facing margins are leeward where strong tidal currents are present because of adjacent basin shape the windward and leeward effects are diminished thus bank margin orientation relative to the climatic energy patterns appears to assert the major controls upon sedimentation antecedent topography in the form of islands or subtidal rock ridges creates energy barriers which can control the direction and volume of sediment flux on or off the banks along windward margins energy barriers such as large islands not only block the bankward transport of sands but appear to augment offbank transport by setting up seaward flows in contrast along leeward margins energy barriers prevent the vigorous offbank sand transport that normally occurs when these margins are open and water flow is unrestricted in addition sand body geometry and grain type are controlled by bank margin orientation along windward open margins skeletal sands accumulate in the lee of low relief mostly relict reefs these margins are generally sediment barren having only a sporadic thin veneer of sand along leeward open margins a broad 10 10 km thick 12 m bankward thinning blanket of sand rich in nonskeletal components peloids composite grains lies on top of rock ridges and reefs large asymmetric sand waves superimposed on this sand body indicate net offbank sand transport along the tide dominated margin large broad oolitic sand lobes have migrated onto adjacent lagoonal muddy sands and indicate net bankward transport we suggest the following classification of modern shallow carbonate bank margins 1 windward open 2 windward protected 3 leeward open 4 leeward protected 5 tide dominated this classification seems to fit many ancient analogs and may be useful in paleoenvironmental reconstructions

the simulation of the sedimentary fill of basins an interactive computer simulation which erects models of sedimentary geometries by filling in a two dimensional basin from both sides with a combination of clastic sediment and or in situ and transported carbonate sediments is described the simulation program is implemented in c using graphical plotting functions data entry including the initial basin configuration the local tectonic behavior the sea level curves the amount and source direction of clastic sediment and the growth rates of carbonates as a function of water depth is performed interactively geometries of clastic and carbonate sediments evolve through time and respond to tectonic movement eustasy and sedimentation clastic modeling includes sedimentary bypass and erosion and sedimentation in alluvial and coastal plains marine shelf basin slope and basin floor settings carbonate modeling includes progradation the development of hard grounds downslope aprons keep up catch up back step and drowned reef settings as well as lagoonal and epeiric settings also included in the model are extensional vertical faulting of the basin sediment compaction and isostatic response to sediment loading sediment geometries are plotted on a graphics terminal as they are computed so the user can immediately view the results based on these observations parameters can be changed and the program rerun until the user is satisfied with the resultant geometry this paper includes output showing the simulation s sensitivity to eustatic sea level variations and variations in sedimentation and subsidence rates also shown are outputs demonstrating the modeling of some examples from the geologic section in the permian basin of west texas and the upper permian of the sichuan basin

geochemistry and origin of eclogites from the type locality koralpe and saualpe eastern alps austria abstract in the koralpe and saualpe polycrystalline basement which is now a part of the austroalpine nappe system eclogites and metagabbros occur intercalated in gneisses and kyanite bearing schists we report here the chemical as well as sr and o isotopic composition of such eclogites and one metagabbro a low ti kyanite bearing and a high ti kyanite free group can be distinguished which may be related by igneous differentiation the low ti group which comprises eclogites and the metagabbro has chemical characteristics indicating formation of the igneous precursors as cumulates by crystallization of pyroxene and plagioclase from a mafic magma in contrast the protoliths of the high ti eclogites appear to be basaltic liquids with normal to enriched type morb affinities which are likely to have formed at an oceanic ridge their sr and o isotope relationships strongly suggest that they have suffered both a phase of low temperature and a phase of high temperature hydrothermal alteration by reaction with seawater

offbank transport of carbonate sands along open leeward bank margins northern bahamas abstract vigorous offbank transport of carbonate sands occurs along the west facing leeward open margins of little and great bahama bank large offbank oriented sand waves thick sand bodies covering reefs and shallow water sands mostly non skeletal obtained from adjacent deep flanks all demonstrate the existence of this shallow to deep pathway of sediment transport a variety of data suggests that transport occurs during storms and not during normal tidal current fluctuations an abundance of composite grains as well as vertical sequences of submarine cemented horizons seen via seismic profiling within the marginal sand bodies indicate cyclic periods of relative quiescence followed by moments of intense sediment movement current meter data obtained during six weeks of low wind activity show that critical threshold velocities for sand transport are only barely exceeded during portions of the tidal cycle these flows coupled with higher frequency bottom currents generated by surface gravity waves winnow the sands but do not provide significant net offbank transport offbank sand transport provides material to the shallow and deep bank margins enabling them to expand during sea level highstands each sea level fluctuation is likely to produce within the shallow margins a vertical sequence of basal reefs covered by a thick non skeletal calcarenite capped by a subaerially exposed diagenetically altered surface shallow water sands carried off the marginal escarpment help to construct deep flanks and large sediment drifts

variation in middle late miocene sedimentation rates in the northern south china sea and its regional geological implications abstract a complex suite of geological information from deep sea drilling cores can provide insights into the cenozoic evolution of the south china sea scs therefore the thick sediment sequence obtained during international ocean discovery program iodp expedition 368 at site u1501 in the northern scs provides excellent materials to investigate the paleoceanographic and geological evolution the generally well preserved calcareous nannofossil assemblages allowed us to establish a high resolution biostratigraphic framework and revealed dramatic fluctuations in sedimentation rates during the middle miocene through earliest pliocene ca 15 5 ma the sedimentation rate was high during the middle miocene decreased dramatically before the middle late miocene boundary and essentially ceased afterwards with resumption of deposition close to the late miocene pliocene boundary sediment source analysis with diffuse reflectance spectra shows that biogenic carbonate accounts for approximately half of the bulk sediments and that the dilution effect by terrigenous sediments partially controlled the carbonate content whereas the high sedimentation rates were fueled by both sources during the middle miocene the apparent 1 5 myr long hiatus 10 5 9 ma corresponds to the globally distributed carbonate crash which was truncated in this region by the regionally important dongsha movement the accompanying decrease in carbonate content and terrigenous materials may indicate decreased biogenic carbonate export to deep waters during the carbonate crash possibly resulting from the progressive constriction of the indonesian seaway the restricted distribution of discoaster hamatus at this site suggests that the dongsha movement commenced at 9 5 ma younger than previously proposed

testing a seismic interpretation of great bahama bank with a computer simulation sixty million years of carbonate deposition were simulated to test an interpretation of platform development based on seismic data and limited well control from northwestern great bahama bank seismic profiles of the northwestern great bahama bank document the lateral growth potential of isolated platforms that were welded together by progradation to form the modern bank the mechanism we proposed responsible for an evolution from aggradation to progradation was sediment overproduction on the platform the excess of which was transported offbank and which caused a decrease in accommodation space on the marginal slope progradation occurred in pulses that were interpreted to be the result of third order sea level fluctuations to evaluate the proposed mechanism 15 input p rameters were used to model the platform evolution the simulation program which uses empirical relationships to model basin fill successfully reproduced the geometries seen on the seismic lines indicating that the proposed interplay of mechanisms could have built the observed platform architecture the simulation demonstrated in particular that in a setting like the bahamas a basin must be substantially filled before progradation can take place and that sea level changes can drive the pulses of progradation this implies that laterally stacked sequences often contain the record of sea level changes and therefore have potential use in sequence stratigraphy the simulation can also be used to estimate the quantitative importance of individual factors controlling aggradation and progradation in the bahamas we show how close the balance between aggradation and progradation is and how small changes in the rate of subsidence or accumulation can cause immediate switches from aggradation to progradation in particular we show that the rate of subsidence exerts the major control on the timing of progradation more so than basin width carbonate production rates similar to modern rates were required to produce the necessary sediment input for progradation which suggests that carbonate production has been consistently high since the early tertiary repeated exposure and erosion however have decreased the overall accumulation rate the simula ion also suggests that the asymmetric progradation in the bahamas was only possible where there were extreme differences between windward and leeward conditions with a maximum sediment input of 10 from the windward side

ocean floor hydrothermal metamorphism in the limousin ophiolites western french massif central evidence of a rare preserved variscan oceanic marker the limousin ophiolite is located at the suture zone between two major thrust sheets in the western french massif central this ophiolitic section comprises mantle harzburgite mantle dunite wehrlites troctolites and layered gabbros it has recorded a static metamorphic event transforming the gabbros into undeformed amphibolites and the magmatic ultramafites into serpentinites and or pargasite bearing chloritites with various thermobarometric methods it is possible to show that the different varieties of amphibole have registered low p c 0 2 gpa conditions with temperature ranging from high t late magmatic conditions to greenschist zeolite metamorphic facies the abundance of undeformed metamorphic rocks which is typical of the lower oceanic crust the occurrence of ca al mg metasomatism illustrated by the growth of ca al silicates in veins or replacing the primary magmatic minerals the p t conditions of the metamorphism and the numerous similarities with oceanic crustal rocks from ocean drilling program and worldwide ophiolites are the main arguments for an ocean floor hydrothermal metamorphism in the vicinity of a palaeo ridge among the west european variscan ophiolites the limousin ophiolites constitute an extremely rare occurrence that has not been involved in any hp subduction related or mp orogenic metamorphism as observed in other ophiolite occurrences i e france spain and germany

early palaeozoic tectonothermal events in basement complexes of the eastern greywacke zone eastern alps evidence from u pb zircon data field observations and u pb zircon data yield evidence that the kaintaleck complex represents a distinct crystalline basement unit of the eastern greywacke zone and contrasts partly with other pre alpine basement complexes of the eastern alps the age data indicate possible late archean rock formation several magmatic and metamorphic events in the early palaeozoic ca 520 490 and ca 400 360 ma and low grade metamorphic overprint in the cretaceous zircons from a garnet gneiss layer in a plagioclase gneiss and amphibolite sequence yield an upper intercept age of ca 2 55 ga which is interpreted as a possible protolith age the lower intercept age at 514 ma represents either 1 a magmatic event or 2 a metamorphic overprint within amphibolite facies conditions magmatic zircons derived from granitic orthogneiss boulders of the kalwang conglomerate which covers the crystalline basement crystallised at ca 500 ma the lower discordia intercepts of the orthogneiss boulders mark the cretaceous low grade metamorphic overprint the lower intercept age of paragneiss zircons from another slice of the kaintaleck complex suggests a likely devonian metamorphic event at ca 390 400 ma the new data demonstrate that the kaintaleck complex experienced several stages of tectonothermal evolutions which are in contrast to the evolution of the main mass of the austroalpine basement the data constrain linkages of this part of the austroalpine basement to the cordillere ligerienne and cadomian block within west european variscides

sedimentation on a lower jurassic carbonate platform flank geometry sediment fabric and related depositional structures djebel bou dahar high atlas morocco abstract along the southern margin of the high atlas morocco carbonate platforms of lower jurassic age developed in a transtensional basin the south atlas trough complete exposure and preservation of the platform margin flank and basin of the djebel bou dahar allowed the subdivision of the slope into an upper mid and lower slope based on geometry sediment fabric and depositional structures at ca 50 m below the platform margin coral algal patch reefs and ooid sand shoals grade into the depositional slope the straight upper slope showing discontinuous bedding at angles up to 29 extends down to ca 250 m and consists mainly of poorly sorted mixtures of sand and gravel almost devoid of mud avalanching rockfall and shearing of the coarse gravelly sediment are the dominant processes readjusting the upper slope the middle and lower part of the slope profile extending down to ca 500 m below the margin is separated from the upper slope by a distinct slope break slope angles range from 19 near the slope break down to 4 8 where the slope merges with the basin plain the irregularly bedded mid slope is characterized by scouring and deposition of muddy sands and non cohesive gravelly debris flows sedimentation on the dominantly muddy concave upwards lower slope can be separated into evenly bedded shales muddy sands and mud free gravel sand mixtures deposited as debris sheets the results of this study emphasize the relationship between sediment fabric and the slope geometry of a carbonate platform flank the overall geometry internal heterogeneity and depositional structures are suggested to be strongly related to the original character of the sediment supplied to the djebel bou dahar platform flank

the variscan belt in central europe main structures geodynamic implications open questions abstract the variscan belt of europe originated from the confrontation of northern europe and gondwana with intervening pre variscan blocks cadomian and older armorica tepla barrandean moravo silesian though a strike slip component cannot be excluded geological evidence suggests a subduction collision process newly discovered features include basaltic rocks with morb affinities high pressure metamorphic rocks trench deposits and large scale allochthonism which also affects the pre variscan basement and has often led to an inversion of the metamorphic isogrades large scale thrusting is evident at all crustal levels and is also observable by geophysical methods the structure of the variscan crust is best described as horizontal tectonic layering late ordovician through carboniferous convergence closed one rhenohercynian and one saxo thuringian basin to the north and at least one mediterranean basin to the south of the pre variscan blocks subduction occurred from the north as well as from the south down under the dorsal core region the exact timing and the driving mechanism of this bilateral activity remains a problem likewise uncertain are the exact limination of the pre variscan blocks and the existence and width of oceanic areas in the saxothuringian zone and in the south facing part of the moldanubian zone there is good evidence of early palaeozoic rifting and of late ordovician through carboniferous plate convergence palaeomagnetic and palaeoclimatic constraints however suggest only a limited extent of oceanic basins in these areas while they appear to allow a wider ocean at the rhenohercynian saxothuringian boundary which is relatively unobtrusive from the geological point of view research during the past decade has produced a wealth of information on the variscan fold belt in europe the resulting geodynamic models however are extremely diverging evidently it is not the time yet for definitive statements the present paper intends to review the state of knowledge to deduce such basic items of a geodynamic model as appear reasonably well documented and to point out the most urgent open questions stress will be laid on central europe which offers a complete cross section through the northern flank of the variscan belt and where we have carried out most of our studies during the last years

depositional bias and environmental change important factors in sequence stratigraphy abstract differences among depositional systems here called depositional bias strongly influence sequence patterns siliciclastics and shallow water carbonates for instance shed most of their sediment during opposite phases of a sea level cycle lowstand shedding and highstand shedding respectively furthermore the two systems generate their own system specific relief on the sea floor disperse their sediment load along different avenues and differ in the way they are deactivated reefs and carbonate platforms can be drowned whereas siliciclastic deposition can be shut off and renewed at any depth as a consequence of these differences pronounced unconformities drowning unconformities develop where carbonate platforms are terminated and buried by siliciclastics the siliciclastic to carbonate transition tends to be more gradual drowned platforms and drowning unconformities appeared world wide in great abundance in the miocene cretaceous valanginian turonian jurassic toarcian and devonian frasnian famennian examples of drowning unconformities interpreted as sequence boundaries include those of the early cretaceous platforms off new jersey and off morocco the mid cretaceous unconformity in the gulf of mexico and miocene unconformities on top of reefs in the far east the eustatic cycles postulated from sequence stratigraphy are a very unlikely cause for the mass drownings of reefs and platforms either the rates of rise are an order of magnitude lower than the growth potential of platforms or cycle amplitudes are too small or the cycles are too short and subsidence too slow to remove platforms permanently from the photic zone the critical element in the mass drowning seems to be environmental stress that reduces the growth potential of carbonate systems thus drowning unconformities demonstrate the importance of environmental change as a control on sequence development other examples of dominantly environment controlled sequences include slope deposits shaped by shifting currents e g florida and blake plateau and basin fills that were controlled by changes in sediment input related to tectonic alteration of the drainage in the hinterland e g gulf of mexico environmental change must be considered as a third independent factor that competes with eustatic and tectonically driven regional changes of sea level for control of sequences the definition of sequences and sequence boundaries should be broad enough to include the possibility of non sea level controls

a sm nd isotopic study of 500 ma old oceanic crust in the variscan belt of western europe the chamrousse ophiolite complex western alps france we report the results of a sm nd isotopic study of whole rock samples from the chamrousse ophiolite complex in the variscan basement of the western alps france our data point to a 497 24 2 ma age in excellent agreement with u pb results menot et al 1984 on subordinate plagiogranites strong isotopic heterogeneity and time evolution of magmas from relatively less depleted nd 5 to very depleted nd 9 sources are also implied available data favour a mixing model between n morb and e morb type end members but a third component with distinct supra subduction zone affinity is also required these results preclude any tectonic setting such as intracontinental rift ensialic marginal basin or major ocean ridge but substantiate an oceanic back arc environment they also demonstrate that the well documented upper cambrian lower ordovician extensional phase here reached a true oceanic stage

compositional changes in sigmoidal carbonate clinoforms late tithonian eastern sardinia italy insights from quantitative microfacies analyses quantitative analysis of sediment composition was performed on a kilometre wide section of upper tithonian low relief up to 70 m gently inclined 3 to 15 sigmoidal carbonate clinoforms eastern sardinia to identify changes in sediment composition along the slope and across the studied succession these changes may reflect modifications of the carbonate factory and of processes responsible for sediment transport point count analysis of carbonate microfacies q mode r mode cluster analysis and spearman s rank provided a composition based classification of microfacies and highlighted relationships among sediment components the studied clinoforms are mainly composed of non skeletal grains 70 such as peloids and lithoclasts together with micrite and cements and only a limited contribution from coated grains 2 among skeletal grains 28 the greatest contribution derives from a coral stromatoporoid encruster reef that provided 15 of the components crinoids brachiopods and other along slope thriving biota provided nearly 5 of the allochems whilst fragments of molluscs gastropods bivalves and diceratids from the backreef sourced another 2 the contribution of platform interior biota is negligible 1 the association of composition based facies varies along the slope the upper slope beds consist of coral stromatoporoid grainstone to rudstone the middle slope deposits are dominated by encruster lithoclast grainstone and packstone at the lower slope peloidal lithoclastic packstone as well as brachiopod crinoidal wackestone prevail also the association of skeletal grains changes along the slope the encruster frame builder association typifies the upper slope whilst encrusters characterize the middle slope sediments in the lower slope encrusters are equally represented as the brachiopod crinoid association along slope compositional changes evidence a scarce downslope transport of frame builders and a progressive enrichment in along slope thriving biota quantitative analysis of microfacies allowed the sigmoidal clinoforms to be grouped into six sets each set gathers sigmoids with a similar sediment composition coated grains are dominant in the first set whilst they are lacking in the overlying sets reflecting a change in the carbonate factory other major compositional changes among the sets concern the relative amounts of peloids micrite frame builders corals and stromatoporoids and encrusters the contribution of peloids varies inversely to that of cements and micrite as evidenced in the third and fifth sets which respectively record the highest occurrence of peloids or cement and micrite variations in the amount of frame builders and encrusters are instead non linear high percentages of both frame builders and encrusters as recorded in the second and fifth sets are related to low amounts of peloids and lithoclasts that probably reflect episodes of reduced background sedimentation this study demonstrates that quantitative analysis of carbonate microfacies represents a powerful tool that can improve the reconstruction of the stacking pattern in carbonate slope successions both in outcrop and in subsurface settings

genesis of limestone megabreccias and their significance in carbonate sequence stratigraphic models a review a re appraisal of the factors controlling the deposition of carbonate megabreccias and their sequence stratigraphic significance is presented based on an examination of the mechanisms for instigating carbonate seafloor slope instability and a review of the depositional settings of these distinctive rock deposits limestone megabreccias have classically been interpreted as formed by the catastrophic collapse of high angle metastable oversteepened carbonate platform margins however a review of megabreccia occurrence shows that these deposits formed on a broad range of carbonate slope angles and that metastable oversteepened slopes are not necessary for their genesis a re evaluation constructed from first principles of mechanisms promoting submarine gravitational instability indicates that pore water overpressure of confined aquifer horizons beneath the seafloor rather than slope oversteepening is the critical control on megabreccia deposition catastrophic release of overpressure may initiate gravitational instability of previously kinematically stable slopes the relative importance of endogenic processes intrinsically linked to the depositional system and exogenic processes operating independently in generating megabreccias is assessed two particularly important mechanisms in generating megabreccias are endogenic processes causing overpressure at discrete hydrologically confined horizons beneath the seafloor during relative sea level falls and increases in stress as pore fluid drains from the sediment when the platform top becomes subaerially exposed during relative lowstands of sea level the increased likelihood of megabreccia genesis during relative sea level falls is supported by an empirical trend identified from a review of the stratigraphic distribution of documented megabreccias exogenic causes of gravitational instability with inherently random periodicity especially seismicity may account for the small number of documented megabreccias that do not conform to this underlying trend the proposed endogenic pattern favouring megabreccia deposition during relative sea level falls is important in a sequence stratigraphic context megabreccia deposits may build volumetrically significant toe of slope wedges and aprons during relative lowstands of sea level partially compensating for the decrease loss in the supply of fresh granular sediment as the platform top carbonate factory contracts lowstand wedges and aprons in carbonate systems therefore generally differ in composition from equivalent highstand deposits in being composed not of calciturbidite deposits but of megabreccias cannibalized from the upper slope

neoproterozoic to early palaeozoic magmatic evolution in the gondwana derived austroalpine basement to the south of the tauern window eastern alps in the austroalpine basement to the south of the tauern window distinct suites of metabasites occur with orthogneisses in pre early ordovician units tholeiitic and alkaline within plate basalt type metabasites are associated with acid meta porphyroids in the post early ordovician thurntaler phyllite group according to their correlated trace element abundances metabasite zircons crystallized with their host rocks protolith pb pb zircon ages whole rock ta yb th yb and oxygen sr nd isotope data define two principal evolution lines an older evolution at elevated th yb typical of subduction related magmatism started by 590 ma n morb type and 550 530 ma volcanic arc basalt type basic suites which mainly involved depleted mantle sources it finished with mainly crustal source 470 450 ma acid magmatites an other evolution line by tholeiitic and 430 ma alkaline within plate basalt type suites in both pre and post early ordovician units is characterized by an intraplate mantle metasomatism and enrichment trend along multicomponent sources these magmatic evolution lines can be related to a plate tectonic scenario that involved terranes in a progressively mature neoproterozoic to ordovician active margin and a subsequent palaeo tethys passive margin along the north gondwanan periphery