origin of highly elongate calcite cemented concretions in some italian coastal beach and dune sands abstract highly elongate vertical and horizontal calcite cemented concretions in pleistocene shallow marine and coastal eolian sandstones are mixtures of terrigenous sand and carbonate biogenic grains cemented to various degrees by low mg calcite petrographic and stable carbon and oxygen isotopic data support the earlier hypothesis of mazzanti and parea 1979 that vertically oriented concretions formed by the precipitation of vadose meteoric water percolating downward toward the water table and that horizontal concretions were precipitated from phreatic groundwater flowing toward the coast many vertical concretions started growth as rhizocretions composed of micrite but enlarged and coalesced when equant sparry calcite was precipitated from percolating vadose water horizontal concretions cemented entirely by subequant calcite spar have coalesced into nearly completely cemented beds decimeter scale domains of curved horizontal concretion elements reflect the influence of adjacent phreatic flow domains each with their own particular flow paths aragonite and high mg calcite shells in the sands were the main source of carbon in calcite cement isotopically depleted 13c values of bulk samples 6 to 8 5o oo v pdb however require the input of a light carbon source probably soil co2 18o values of bulk samples are compatible with cementation chiefly by meteoric water at temperatures between 7 and 22 c the values are typical of air and inferentially groundwater temperatures today

influence of host rock texture on the morphology of carbonate concretions in a meteoric diagenetic environment abstract the integrated stratigraphic petrographic geochemical study of quaternary marine terraces that have undergone well constrained sea level and water table oscillation s over geologically short time periods can provide important clues on diagenetic processes at the marine continental interface the punta di i cani marine terrace quaternary saint florent corsica unconformably overlies the well cemented low permeability arenites of the miocene monte s angelo formation and is erosively overlain by unlithified high permeability holocene continental sediments the terrace is composed of a retrogradational fining upward succession from fluvial pebbly sandstone to beach and shoreface deposits the entire thickness of the terrace hosts concretions made of low mg calcite concretion geometry is linked to the sedimentological facies of the host sediment i massive fluvial pebbly sandstone at the base of the terrace contains knobby subhorizontal concretions somewhat resembling turbidite flute casts ii very low angle cross bedded well sorted beach deposits contain pipy subhorizontal concretions iii trough crossbedded upper shoreface deposits in the upper part of the terrace contain a subtle diagenetic lineation on the laminae locally obliterated by a patchy isotropic cementation overall the degree of cementation decreases from the bottom to the top of the terrace cement chemistry and morphology as well as stable carbon and oxygen isotopic signatures indicate that the concretions formed by precipitation of meteoric phreatic water flowing toward the coast elongate concretions are invariably parallel to the topographic gradient consistent with the notion that the growth of concretions was controlled by advection smectitic vadose cement is locally present its crosscutting relationships with calcite cement point to periodic oscillations of the water table the results of this study i show that elongate concretions can be used as interstitial paleoflow indicators and ii suggest that concretion geometry in an advection dominated diagenetic environment can depend on the small scale permeability distribution as suggested by the fact that each type of concretion is invariably associated with a specific sedimentary facies

concretion morphologies reflecting diagenetic and epigenetic pathways abstract due to complex physicochemical processes concretions develop variable but distinctive morphologies reflecting the state of the sediment in subsequent stages of diagenesis and epigenesis they deserve a study and systematization in their own right at the same time concretional morphologies illustrate the effect of hierarchic self organization which is a basic element also in biological evolution

the internal structure of carbonate concretions in mudrocks a critical evaluation of the conventional concentric model of concretion growth abstract carbonate concretions have traditionally been viewed as forming concentrically through the progressive addition of carbonate to the outer edge of the growing concretion this conventional model of concretion growth is based mainly upon center to edge textural and geochemical trends that are consistent with concentric growth however a number of recent studies of concretions in mudrocks have documented complex zonation that is not compatible with the conventional concentric model in some cases complexly zoned concretions contain late stage cements that precipitated throughout the concretion body none of the center to edge relationships used to support the conventional concentric model exclude more complex modes of origin as concretions that fit the conventional model may be the exception rather than the rule high resolution petrographic analysis of concretions should always be used to characterize internal structure and mode of growth prior to interpretation of geochemical and textural data

concretion morphology classification and genesis abstract a discussion of the most relevant morphological features of concretionary bodies and the different classifications and the criteria involved in these classifications is presented together with suggestions for improvements to the various classification schemes the meaning of syngenetic diagenetic and epigenetic related to the relative timing and environment of growth of concretionary bodies is also reviewed and discussed the replacement of mixed morphogenetic classifications which lead to conflicting results by classification categories based on textural features is proposed because the identification of the genetic environment of a concretionary body tells little about its history i recommend defining growth patterns in which the successive steps associated with changes in composition and or texture and the development of new structures are recorded a new path is presented that accounts for the contradiction between textural and isotope features which suggests respectively syngenetic and diagenetic or even epigenetic signatures this new path is characterized by the preservation of large porosities over very long time spans and down to depths that are somewhat greater than expected due to the inhibition of normal compaction caused by early overpressuring this overpressuring is the result of the development of hydraulic seals since approximately the syngenetic phase times the concepts of force of crystallization and displacive growth are also reviewed it is being suggested to discard some controversial interpretations of their actual importance for true concretionary displacive growth under epigenetic conditions in accordance with other authors the conclusion is reached that displacive growth is only possible if the shear strength of the host is very low and the stress field within the host is almost hydrostatic a new model the symcompactional concretionary growth model is proposed which explains how a concretionary body generally a nodule can grow while its host thins down by compaction this overpressured undercompacted model will be useful for the interpretation of an assemblage of features such as injection dikes hydraulic breccias cone in cone structures among others that are thought to be representative of the former presence of overpressured horizons these overpressured horizons could serve as detachment planes in paleotectonically active basins seals that could have controlled the movement of brines and hydrocarbons during the diagenetic evolution of the basin can also be assessed this is relevant for the identification of maturation conditions the potential development of secondary fracture permeability due to hydraulic fracturing in buried areas of the basin can also be evaluated based on the identification of formerly overpressured horizons in outcrops the origin of concretions is generally a geological puzzle clifton 1957

oh defects in multiple doped orthoenstatite at 4 8 gpa filling the gap between pure and natural systems oh defects in orthoenstatite were studied experimentally between 4 and 8 gpa at 1150 c in the system cao mgo al2o3 sio2 cr2o3 na2o leading to phase assemblages enstatite forsterite diopside garnet in enstatite coexisting with garnet total oh is negatively correlated with pressure conversely in al poor systems without garnet total oh is positively correlated with pressure and both trends intersect around 8 gpa and 1000 wt ppm h2o ir spectra of enstatite reveal several pressure sensitive features such as 1 the absorbance of the absorption band at 3687 cm 1 2 the band position near 3400 cm 1 and 3 the ratio a3240 3570 a3240 3730 and their application as geobarometer in natural samples are evaluated for garnet bearing phase assemblages the band ratio a3240 3570 a3240 3730 in orthoenstatite defines a pressure trend in between that observed in the pure system mgo sio2 h2o and that found in orthopyroxenes from natural mantle peridotites suggesting that the application of ir spectra as proxy for pressure is justified

low magnesian calcite marine cement in middle ordovician hardgrounds from kirkfield ontario abstract middle ordovician trentonian biosparites from the bobcaygeon and verulam formations near lake simcoe ontario exhibit numerous features demonstrating lithification near the sediment water interface these include planar erosion surfaces which truncate shells and intraclasts undercut hummocks on upper surfaces vertical borings which cross cut fossil debris and early cement and a diverse fauna of encrusting organisms which grew attached to lithified surfaces a marine origin for early cement is indicated by the development of hardgrounds within a transgressire regionally onlapping sequence the occurrence of dozens of such units within normal marine sediments and a lack of features recording early oxidation and or partial dissolution during exposure to meteoric water marine cement from four of these units examined in thin section by sem and by cathodoluminescence consists of clear zoned syntaxial overgrowths on echinoderm debris and equant spar on trilobite bryozoan and brachiopod fragments as such this cement 1 is morphologically identical to meteoric phreatic low magnesian calcite cement from calcitized pleistocene sequences 2 is interpreted as having been precipitated as low magnesian calcite 3 is anomalous in habit and inferred composition in comparison with holocene marine cement and 4 demonstrates that generalizations which relate cement morphology and composition to environmental water chemistry in modern diagenetic systems may not be valid when applied to more ancient carbonate sequences

relationship between oriented calcite concretions and permeability correlation structure in an alluvial aquifer sierra ladrones formation new mexico abstract fluvial deposits of the sierra ladrones formation pliocene pleistocene contain abundant elongate calcite concretions that are interpreted to have precipitated in the saturated zone parallel to the direction of groundwater flow the orientation of these concretions is coincident with the orientation of the permeability correlation structure as determined from architectural element mapping and permeameter measurements in cases where the paleo groundwater flow closely corresponds with the direction of fluvial deposition measurement of concretion orientations in alluvial rocks and sediments may provide a means of rapidly estimating the major axis of the permeability correlation structure over large areas

quantitative analysis of trace oh in garnet and pyroxenes to calibrate infrared ir spectroscopy for quantitative analysis of trace structural oh in specific minerals we have determined concentrations of h in pure separates of mantle derived pyrope garnet 56 6 ppm h 2o by weight augite 268 8 ppm h 2o and enstatite 217 11 ppm h 2o by manometry after heating the samples and extracting h 2 gas under vacuum ir spectroscopy confirmed the presence of intrinsic oh in these samples prior to extraction and indicated between 86 and 100 removal of h during the extraction procedure the integral specific absorption coefficients of 1 39 0 14 l 7 09 0 32 and 15 6 0 94 ppm h 2o cm 2 for pyrope augite and enstatite respectively allow precise spectroscopic determination of the oh content of upper mantle garnets and pyroxenes to concentration levels of a few parts per million uncertainties in accuracy depend on mineral composition and characteristics of the oh absorption spectrum and are estimated to range between 10 and 50

subsolidus orthopyroxene clinopyroxene systematics in the system cao mgo sio2 to 60 kb a re evaluation of the regular solution model on the basis of an increased set of experimental data covering the miscibility gap between orthopyroxene and clinopyroxene from 2 to 60 kb with a temperature range of 850 1 500 c we present a new version of a regular solution model the model with two independent regular solutions for opx and cpx is capable of reproducing experimental data in the system cao mgo sio2 over a large range of temperatures and pressures it is qualitatively in agreement with observed stability regions for an fe free low ca pyroxene termed pigeonite the model is constrained by and thus consistent with calorimetric measurements on pyroxenes the simple form of equations provides a good starting point for the development of more reliable thermometers based on the miscibility gap

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

oriented carbonate concretions in a paleoaquifer insights into geologic controls on fluid flow oriented carbonate concretions appear to provide a powerful new tool in the evaluation of flow through heterogeneous aquifers calcite cement forms elongate bodies of indurated rock that have distinct and mappable spatial orientations we have mapped 200 of these concretion orientations as well as the hydrogeologic characteristics of the host sediments in outcrops of the pliocene pleistocene sierra ladrones formation in central new mexico the average azimuthal concretion orientations is 150 which is consistent with the southeasterly groundwater flow suggested by past and present basin geometry using a first order approximation the dimensionless transverse velocity is estimated from the orientation data the spatial correlation structure of the dimensionless transverse velocity is distinctly different in the two units investigated however both are consistent with existing stochastic models the differences between the units and the similarities with theoretical models suggest that concretion orientations can be used to study how fluids flow through heterogeneous systems over scales ranging from centimeters to tens of kilometers

application of the trace element and isotope geochemistry of strontium to studies of carbonate diagenesis carbonate rocks and natural waters exhibit a wide range in the concentration and isotopic composition of strontium this wide range and the quantifiable covariation of these parameters can provide diagnostic tools for understanding processes of fluid rock interaction careful consideration of the uncertainties associated with trace element partitioning sample heterogeneity and fluid rock interaction mechanisms is required to advance the application of the trace element and isotope geochemistry of strontium to studies of diagenesis goundwater evolution ancient seawater chemistry and isotope stratigraphy a principal uncertainty involved in the application of sr concentration variations to carbonate systems is the large range of experimental and empirical results for trace element partitioning of sr between mineral and solution this variation may be a function of precipitation rate mineral stoichiometry crystal growth mechanism fluid composition and temperature calcite and dolomite in ancient limestones commonly have significantly lower sr concentrations 20 70 p p m than would be expected from published trace element distribution coefficient values and sr ca ratios of most modern sedimentary pore waters this discrepancy probably reflects the uncertainties associated with determining distribution coefficient values as techniques improve for the analytical measurement and theoretical modelling of sr concentration and isotopic variations the petrological analysis of carbonate samples becomes increasingly important the presence of even small percentages of non carbonate phases with high rb concentrations and high 87 sr86 sr values such as clay minerals can have significant effects on the measured 87 sr 86 sr values of carbonate rocks due to the decay of 87rb to 87 sr for example a permian marine limestone with 50 p p m sr and 1 p p m rb will have a present day 87 sr 86 sr value that is 2 10 4 higher than its original value this difference is an order of magnitude greater than the analytical uncertainty and illustrates the importance of assessing the need for and accuracy of such corrections a quantitative evaluation of the effects of water rock interaction on sr concentrations and isotope compositions in carbonates strengthens the application of these geochemical tracers geochemical modelling that combines the use of trace elements and isotopes can be used to distinguish between different mechanisms of water rock interaction including diffusive and advective transport of diagenetic constituents in meteoric pore fluids during the recrystallization of carbonate minerals quantitative modelling may also be used to construct diagnostic fluid rock interaction trends that are independent of distribution coefficient values and to distinguish between mixing of mineral end members and fluid rock interaction

synchronization of north pacific and greenland climates preceded abrupt deglacial warming some proposed mechanisms for transmission of major climate change events between the north pacific and north atlantic predict opposing patterns of variations others suggest synchronization resolving this conflict has implications for regulation of poleward heat transport and global climate change new multidecadal resolution foraminiferal oxygen isotope records from the gulf of alaska goa reveal sudden shifts between intervals of synchroneity and asynchroneity with the north greenland ice core project ngrip 18 o record over the past 18 000 years synchronization of these regions occurred 15 500 to 11 000 years ago just prior to and throughout the most abrupt climate transitions of the last 20 000 years suggesting that dynamic coupling of north pacific and north atlantic climates may lead to critical transitions in earth s climate system

neogene to quaternary rifting and inversion in corsica retreat and collision in the western mediterranean 1 stratigraphic structural and fission track data have been integrated in order to unravel the neogene to quaternary tectonic evolution of northeastern corsica results indicate that during early to middle miocene the study area was affected by a rapid cooling at low temperatures accompanied by exhumation and rifting basin inversion occurred during the tortonian again followed by extension in the messinian to early pliocene compressional deformations are finally recorded by early pliocene to quaternary deposits alternating rifting and inversion events are recorded not only in northeastern corsica but also in many peri tyrrhenian regions this alternation can be explained in terms of intraplate propagation of the compressional stress related to the africa europe convergence and to a discontinuous slab retreat along the apennine subduction system a pause in the slab retreat implies a break of the back arc extension in the upper plate and it allows a temporary propagation of africa europe compression

systematic sequence scale controls on carbonate cementation in a siliciclastic sedimentary basin examples from upper cretaceous shallow marine deposits of utah and coloradao usa whilst the relationship between stratigraphic development and carbonate cementation within siliciclastic succession has been documented through a number case studies these studies have been generally restricted to observations upon individual sequences and or limited sub surface data in this paper long term 5 million years large scale 200 km stratigraphic controls on carbonate cementation patterns are documented from the upper cretaceous panther tongue member blackhawk formation and castlegate sandstone exposed in the book cliffs in utah and colorado usa together these comprise eight progradational wedges of sandstones which interfinger with the mancos shale deposited within the western interior seaway foreland basin petrographic analyses of ferroan dolomite cement bodies within these sandstone wedges show that the ferroan dolomite cements are all early relative to burial diagenesis within the host sandstones stable isotope analyses indicates that a significant meteoric component was present in precipitating fluids and this is consistent with the observation that cements are always present down dip of sequence boundaries and or leached whitecaps beneath coals in addition the lateral distribution of cement bodies increases consistently up succession from less than 5 km in extent in the older sequences to 30 km in extent in the youngest sequences these changes in distribution are in response to the increased progradation and increased and more aerially extensive sequence boundary development in younger sequences the implications of these data are that whilst localized spatial patterns of diagenesis and in particular carbonate cementation are predictable and controlled by the nature and presence of individual stratal surfaces systematic diagenetic alteration patterns are also present at the sedimentary basin scale and controlled by the nature of larger scale stratigraphic development and basin evolution this evolution may be driven by eustatic shifts or through tectonic or climatic driven base level shifts these observations allow an improved insight into the basin scale processes that control the macroscopic diagenetic properties of sedimentary successions and sub surface hydrocarbon reservoirs

concretions in glacial sediments at seglvatnet norway abstract concretions in pleistocene glacial sediments at seglvatnet norway include a wide variety of forms the simplest of which apparently reflects relatively uniform growth rates in different directions the preferred orientation of others may be associated with water movement through the sediments concretion shape generally is influenced by the grain size and bedding of the enveloping sediments some concretions may have formed during or soon after deposition of the sediments others are forming at present

oriented concretions ionian coast italy evidence of groundwater flow direction abstract highly elongate aligned calcite cemented concretions are present in exposed shallow marine pleistocene sands and gravels along the basilicata coast of the ionian sea southern italy concretions are shaped like rods pencils and cigars 0 5 3 cm in diameter and thin blades 031 cm thick and 1 5 cm wide most have aspect ratios greater than 5 and some reach 30 most concretions are in parallel laminated beds but a few are in cross stratified and ripple cross laminated beds concretions cut across foresets and inclined laminae the standard deviations of long axes at each of six localities with more than eight measurements are less than 7 the standard deviation of the orientation of concretions from 30 localities over a 400 km2 area is less than 17 previous interpretations for elongate carbonate concretions in sandstones include growth in the direction of groundwater flow growth parallel with current oriented grains and or paleocurrents and selective cementation in the troughs of wave ripples the basilicata concretions are oriented perpendicular to the local coastline and parallel with the direction of present day groundwater flow the groundwater flow direction was probably the same during the pleistocene the evidence indicates that the concretions grew parallel with groundwater flow and were not influenced by sedimentary structures or grain fabric the isotopic values 13c and 18o range from 10 to 1 pdb and from 5 to 3 pdb respectively and textures of calcite cement in the concretions indicate that the calcite was precipitated from meteoric groundwater nannobacteria may have mediated precipitation of calcite

paleogeographic significance of oriented calcareous concretions in the triassic katberg formation south africa abstract elongated calcareous concretions in sandstones of the fluvial katberg formation beaufort group south africa display a distinct preferred orientation that coincides with the regional paleoslope as deduced from cross stratification and primary current lineations these concretions formed after deposition of the sandstone but before the strata were affected by tectonic movement presumably by groundwater flowing down the regional paleoslope similar sandstone hosted concretions in other non marine successions could potentially yield valuable information on paleo groundwater flow and by inference depositional paleoslopes

mudrock hosted carbonate concretions a review of growth mechanisms and their influence on chemical and isotopic composition existing interpretations of cement textures and isotopic compositions may significantly under estimate the depth and duration of concretionary growth minus cement porosities can commonly under estimate depths of concretionary growth for some or all of the following reasons i cements might not passively replace host sediment porosity ii non cement carbonate phases such as replaced bioclastic carbonate can be significant iii sediment compaction models over estimate rates of porosity loss at shallow carbon isotope data from modern sediments show that dissolved carbonate in the methanogenic zone has a continuum of values from 30 to 15 and thus overlaps 13 c depleted values normally considered characteristic of sulphate reduction many concretions previously thought to have grown entirely during sulphate reduction may therefore have continued cementation during methanogenesis indicating a deeper and more prolonged cementation history the necessary carbonate supersaturation for concretionary growth could either occur throughout the porewaters the equilibrium model or be generated in situ by organic matter decay the local equilibrium model or created where external fluids are introduced the fluid mixing model

isotopic composition of precipitation in italy a first overall map abstract composite monthly samples of atmospheric precipitation were collected over different periods of time ranging from one to seven years at 77 different locations throughout italy these samples were measured for their oxygen and hydrogen isotopic composition to obtain basic information for hydrological and meteorological studies in this area on the basis of the results obtained a first map of the isotopic composition of precipitations in italy has been drawn up the main features of this map are the following 1 no isotopic latitudinal gradient has been found along the tyrrhenian coast from sicily to the italian french border despite the considerable range of latitude 2 a minor latitudinal gradient is found in the southeasternmost section of the country apulia partially favored by the local morphology 3 a marked shadow effect of the apennines is apparent along the southern section of the po valley and along the central section of the adriatic coast 4 the contribution of water vapor from the northernmost section of the adriatic sea affects the eastern and central sections of the po plain 5 the shadow effect of the alps is considerably smaller than expected 6 the isotopic vertical gradients calculated in nine different areas and based on groups of two to five different stations vary but are essentially close to about 0 2 100 m 7 when mean monthly temperature values were available their relationship with the isotopic composition of precipitation was found to be on average very poor 8 the relationship between mean 18o and mean d calculated for the collection locations shows shifts of both the slope and the deuterium excess when compared to the global meteoric water line these shifts are different for northern central and southern italy 9 in a few cases anomalously low monthly values suggest the existence of pronounced amount effects

follow the water connecting a co2 reservoir and bleached sandstone to iron rich concretions in the navajo sandstone of south central utah usa concretions with thick iron oxide cemented rinds and lightly cemented iron poor sandstone cores are abundant within the navajo sandstone near the southeast fl ank of the escalante anticline utah united states previous workers suggested the spheroidal concretions as analogs for martian blueberries hematite concretions and linked the origin of concretions and bleaching of the navajo sandstone to the buoyant rise of hydrocarbons toward anticlinal crests we measured azimuths of 163 pipe like pipy joint associated concretions and those of 58 comet tails that extend from some spherical concretions both data sets have vector magnitudes 90 and subparallel southeast directed resultant vectors our data indicate that groundwater transported and deposited iron over a fl ow path that spanned more than 100 km from the anticlinal crest to the colorado river buoyant fl uids could not have transported iron from bleached rock to sites of concretion growth however because iron moved downdip we propose that iron was transported southeastward by reducing groundwater that fl owed through a co 2 and methane charged reservoir within the escalante anticline iron rich carbonates precipitated during co 2 degassing after methane was fl ushed from the aquifer microaerophiles oxidized fe 2 in concretions dissolving the carbonates and forming the iron oxide rich rinds and iron poor cores these concretions defi ne the fl ow direction and geochemical evolution of a paleoaquifer this approach could be used to follow the water on other planets

apatite fission track analysis of neogene exhumation in northern corsica france apatite fission track analyses along a w e orientated transect across northern corsica indicate an important episode of crustal exhumation in late early miocene time samples taken from the alpine orogenic wedge from the adjacent foreland basin and from the crystalline basement complex flooring the basin are completely reset this implies that a 2 0 2 3 km thick crustal section made of thrust sheets and or autochthonous foreland deposits has been removed by erosion since early miocene time a geometric projection of this lost cover towards the west indicates that all of northern corsica was covered either by alpine nappes or middle eocene foreland deposits fission track ages are the same across the main boundary fault system separating the alpine orogenic wedge and the foreland indicating the absence of significant differential vertical displacement between upper and lower plates during neogene unroofing