structural traverse across the sierra madre oriental fold thrust belt in east central mexico the exposed structural geometry of the sierra madre oriental foreland fold thrust belt is quantified for an 130 km long strip along the moctezuma river 21 n lat east central mexico the strip extends from the belt s leading edge in the east to its most internal continuous outcrops in the west where the cordilleran structures disappear beneath cenozoic volcanic rocks the style of deformation is controlled mainly by the geometry and lithology of two major cretaceous carbonate banks el doctor and valles san luis potosi the long dimensions of which parallel the cordilleran structural trend the exposed shortening is concentrated on the platform margins 5 km on the eastern edge of the el doctor bank and 10 to 12 km on either edge of the valles san luis potosi platform and is characterized by thrust and fold nappes the thrust faults cut across the platform margin carbonates on 20 inclined tectonic ramps that were partly steepened by imbrication or by subsequent folding but pass in the overlying mechanically weak rocks nearly parallel to bedding the interiors of the platforms appear relatively undeformed whereas the regions between the two platforms zimapan basin and between the eastern edge of the valles san luis potosi platform and the foreland are composed of flexural slip folds resulting from decoupling along several incompetent layers the exposed bulk shortening measures 41 4 km or 25 for the entire traverse retrodeformable sections of the subsurface structure down to basement result in a bulk shortening of 90 1 km or 40 the subsurface deformation model for the western part of the traverse is thin skinned and has geometry similar to that on the surface the model is very speculative in the eastern part where the areal shortening calculated for thin skinned models is much larger than is the exposed linear shortening balance was attained by inferring a blind thrust on an upper detachment in the late cretaceous which has a shortening of 20 to 30 km it could also be attained however by intrabasement decoupling in a depth of 10 to 20 km deformation is brittle in the eastern part of the belt east of the valles san luis potosi platform whereas igneous intrusion controlled higher ductility characterizes the western part of the foreland fold thrust belt

magnesium isotope composition of sabkha porewater and related sub recent stoichiometric dolomites abu dhabi uae abstract earliest marine diagenetic sabkha type dolomites are widespread in earth s geological record potentially these carbonates may act as archives of past seawater magnesium isotope 26mg ratios at present however the fractionation of magnesium isotopes 26mg between seawater here evaporated marine porewater and sabkha dolomite is not constrained in order to explore 26mgdol mg porewater we make use of actualistic sabkha type dolomite precipitation in the gulf region trucial coast united arab emirates this paper documents and discusses the first detailed sabkha 26mg data set of mg bearing solids including stoichiometric dolomites degree of ordering 0 9 mean 26mgdol 0 79 0 41 2 n 17 and related marine pore waters the presence of dolomite crystals with a broad range of ordering in shallow cores is documented using x ray diffraction and scanning electron microscopy but individual crystals are too small

large scale intertidal polygonal features of the abu dhabi coastline this study provides the first quantitative description of modern giant polygons these large scale up to 166 m diameter sedimentological features are described here in detail from the intertidal zone of the arabian gulf near abu dhabi these features form by the displacive carbonate cementation of carbonate sand below a 3 to 5 cm thick cover of unconsolidated sediment the oxygen isotopic composition of cements is consistent with precipitation from evaporated sea water cementation proceeds via a poorly consolidated layer that grades downward into an 8 to 14 cm thick well cemented grainstone the margins of this expanding hardground form the overlapping borders of the polygons tepee structures at the borders of polygons are only occasionally preserved as supporting sediment is removed by strong tidal currents and high energy events such as storms that erode the unsupported tepees these observations have clear implications for the use of tepees in the interpretation of ancient polygons and their associated environments when preserved in the sedimentary record tepee structures are interpreted to indicate lower energy depositional environments than that observed in this study

geometric analysis of fold development in overthrust terranes abstract fault bend folding fault propagation folding and detachment or decollement folding are three distinct scenarios for fold thrust interaction in overthrust terranes simple kink hinge models are used to determine the geometric associations implicit in each scenario bedding maintains constant thickness in the models except in the forelimb of the fold the forelimb is allowed to thicken or thin without limit the models address individual folds and the calculated fold geometries are balanced structures each mode of fold thrust interaction has a distinct set of geometric relationships final fold geometry is adequate in itself to discern many fault bend folds this is not the case for fault propagation and detachment folds these two fold forms have very similar geometric relationships some knowledge of the nature of the underlying thrust or decollement zone is usually needed to distinguish between them the geometry of a fold is altered in a predictable fashion by transport through an upper ramp hinge and by fault parallel shearing of the structure the shearing results in a tighter fold whereas transport through the ramp hinge produces a broader fold the viability of the geometric analysis technique is demonstrated through its application to a pair of detachment folds from the canadian cordillera the geometric analysis is also used to evaluate cross sections through subsurface structures in an example from the turner valley oil field the analysis indicates how the interpretation should be altered so as to balance the cross section the analysis reveals hidden assumptions and specific inconsistencies in structural interpretations

lithologic and structural controls on natural fracture distribution and behavior within the lisburne group northeastern brooks range and north slope subsurface alaska the carboniferous lisburne group of northern alaska has been deformed into a variety of map scale structures in both compressional and extensional structural settings thus providing a series of natural experiments for observing the formation distribution and behavior of fractures in this thick carbonate unit two fracture sets dominate the lisburne group carbonates of the north slope subsurface and the nearby northeastern brooks range fold and thrust belt north northwest striking regional extension fractures probably formed in front of the northeastern brooks range fold and thrust belt in the north slope subsurface this fracture set overprints east northeast striking fractures related to earlier extensional deformation in contrast in the fold and thrust belt the north northwest striking fracture set is overprinted by younger east northeast striking fractures related to subsequent contractional deformation lithology is the primary control on the fracture density of both sets in mildly deformed lisburne copyright 1997 the american association of petroleum geologists all rights reserved 1manuscript received february 20 1996 revised manuscript received november 25 1996 final acceptance june 5 1997 2geophysical institute university of alaska fairbanks fairbanks alaska 99775 3sandia national laboratories ms 0705 albuquerque new mexico 87185 4department of petroleum natural gas engineering new mexico institute of mining technology socorro new mexico 87801 and sandla national laboratories ms 0705 albuquerque new mexico 87185 this study was supported by a department of energy subcontract administered by sandia national laboratories additional support was provided by arco alaska bp alaska chevron exxon mobil and japan national oil company we would like to thank arco alaska and bp alaska for giving us permission to view selected lisburne group cores w wallace for helpful discussions on deformational styles of detachment folds a j mansure for help in interpreting the interference tests and w wallace k biddle w belfield n hurley and j kelley for helpful reviews of the manuscript

bed by bed fold growth by kink band migration sant llorenc de morunys eastern pyrenees ahstract rowth strata deposited over and against the flank of the sant llorenc de morunys fold during its final stages of deformation have been mapped at high resolution as the basis for unraveling the kinematics of fold growth we use restoration techniques based on normal balancing assumptions to decipher the detailed kinematic history of folding the progressive restorations as well as balanced forward modeling show that the last few hundred meters of fold growth were dominated by kink band migration of a sort that is typical of much faultrelated folding the kink band migration has produced complex anticlinal hinge zone geometry including segmented fold hinges linked by disconformities and unconformities which has direct and detailed explanation in terms of fluctuations in deposition rate relative to curved hinge kink band migration rate large fluctuations in the convolution of non steady sedimentation and deformation are demonstrated although the absolute fluctuations in deformation and sedimentation are unknown at a length scale of 100 m kink band migration with little or no deposition is interspersed with sedimentation with little or no deformation at the length scale of 500 m deposition ranges from 200 to 50 of uplift 0 1997 elsevier science ltd all rights reserved

kinematic analysis of a large scale leading edge fold lost river range idaho abstract the mahogany creek buck creek structure within the lost river range idaho is a large scale leading edge fold which developed in late paleozoic outer shelf stratigraphy during the sevier orogeny incremental strain histories determined from antitaxial fibrous pressure shadows are used to quantify temporal variations in the magnitude and orientation of elongation as a function of structural position around the leading edge of this northward and eastward propagating blind thrust plane strain is indicated by coaxial up dip extension on cleavage planes on the backlimb of the structure the bluebird mountain formation a calcareous sandstone has deformed by flexural flow whereas the forelimb of the structure records spin through a fixed steeply plunging incremental extension direction the flat limb ahead of the anticline exhibits top to the foreland simple shear in contrast the underlying surrett canyon formation a thick bedded limestone has experienced flexural flow localized within thin zones on both limbs of the anticline samples from the anticlinal hinge zone suggests kinematic partitioning with pin lines which were variably distributed within each unit and which were temporally transient finite elongations are greatest in the hinge and above the foreland flat in the bluebird mountain formation but are negligible in the surrett canyon formation except within the mechanically active interbeds of the hanging wall and within the footwall adjacent to the fault along the northernmost exposures strain histories in the hanging wall do not vary significantly along strike while the footwall varies from undeformed in the south to penetratively deformed in the north thus the strain data are consistent with self similar along strike fold development in the hanging wall during strike parallel ramp propagation forward thrust propagation may have been blunted by a combination of fold accommodated shortening and footwall deformation to the north once the fold had tightened and fold shortening required greater differential stress forward fault propagation resumed transporting the anticline onto the upper flat with little change in hanging wall fold geometry or strain distribution

rheological controls on the shapes of single layer folds abstract information about rheology can potentially be gained from analyzing the shapes of folds in isolated buckled layers we employ two dimensional finite element models of incompressible flow in power law viscous fluids to investigate this we first show that the shape of the initial perturbation has relatively little effect on the final shape of the folds when the buckling instability is high we find that the most significant factor affecting the shapes of single layer folds is the stress exponent n l in the flow law of the layer the hinges of outer arcs become sharper as n l increases and the limbs become relatively longer and straighter these differences can be expressed quantitatively by defining a curvature index ki which has a value of 0 for a fold formed by circular arcs and 1 for a chevron fold results show a dependence of ki on n l that is well defined for l h 10 with ki increasing with n l data for experimentally produced folds are consistent with the numerical results limb dip ki and l h can all be readily measured on natural folds and provide a basis for comparing the shapes of natural and computer simulated folds the data for small folds in siltstone layers in shales in the central appalachians are consistent with highly non linear flow of the stiff siltstone layers during buckling

the magnesium isotope δ26mg signature of dolomites abstract dolomite precipitation models and kinetics are debated and complicated due to the complex and temporally fluctuating fluid chemistry and different diagenetic environments using well established isotope systems 18 o 13 c 87 sr 86 sr fluid inclusions and elemental data as well as a detailed sedimentological and petrographic data set we established the precipitation environment and subsequent diagenetic pathways of a series of proterozoic to pleistocene syn depositional marine evaporative sabkha dolomites syn depositional non marine evaporative lacustrine and palustrine dolomites altered marine mixing zone dolomites and late diagenetic hydrothermal dolomites these data form the prerequisite for a systematic investigation of dolomite magnesium isotope ratios 26 mg dol dolomite 26 mg ratios documented here range from 2 49 to 0 45 26 mg mean 1 75 1 08 n 42 the isotopically most depleted end member is represented by earliest diagenetic marine evaporative sabkha dolomites 2 11 0 54 2 n 14 in comparing ancient compositions to modern ones some of the variation is probably due to alteration altered marine 1 41 0 64 2 n 4 and earliest diagenetic lacustrine and palustrine dolomites 1 25 0 86 2 n 14 are less negative than sabkha dolomites but not distinct in composition various hydrothermal dolomites are characterized by a comparatively wide range of 26 mg ratios with values of 1 44 1 33 2 n 10 by using fluid inclusion data and clumped isotope thermometry 47 to represent temperature of precipitation for hydrothermal dolomites there is no correlation between fluid temperature 100 to 180 c and dolomite mg isotope signature r 2 0 14 nor is there a correlation between 26 mg dol and 18 o dol magnesium isotope values of different dolomite types are affected by a complex array of different mg sources and sinks dissolution precipitation and non equilibrium fractionation processes and overprinted during diagenetic resetting further progress on the use of 26 mg dol as a proxy will require new theoretical and experimental data for 26 mg fluid dol that includes dehydration effects of the free mg aquo ion versus fluid temperature in ancient diagenetic systems complex variables must be considered these include fluid chemistry and physical properties mg sources and sinks temporal changes during precipitation and post precipitation processes including open and closed system geochemical exchange with ambient fluids all of these factors complicate the application of 26 mg dol as proxy for their depositional or diagenetic environments nevertheless the data shown here also indicate that 26 mg dol can in principle be interpreted within a detailed framework of understanding

geometry and controls on fracturing in a natural fault bend fold rosario field maracaibo basin venezuela the rosario oil field lies between the perija mountain front and lake maracaibo and produces from fractured cretaceous carbonates and tertiary clastics we interpret the structure as a detached fault bend fold which ramps through cretaceous cogollo and la luna carbonates and flattens into an upper detachment at the base of the upper cretaceous colon shale the structural relief formed primarily during the mid miocene and younger seismic and well control on the three dimensional geometry illustrates the effects of 1 lithology and displacement variation on fold geometry 2 an oblique footwall ramp on hangingwall faulting and 3 fold curvature on fracturing and hydrocarbon production fold geometry at different structural levels is strongly controlled by lithology stiff cogollo and la luna carbonates exhibit kink style folding above the upper fault bend the weak colon shale decouples the faulted carbonates from the concentrically folded tertiary clastics regions of enhanced faulting and fracturing of cretaceous carbonates are a function of structural position we observe normal faults in the hangingwall where the strike of the footwall ramp changes from n20 degrees e to n65 degrees e fold curvature highlights fold hinges yet distributed faulting is seismically imaged in the forelimb suggesting that rocks fracture as they migrate through the ramp upper flatmore fault bend production rates are higher near the forelimb relative to the flat crestal region less

lithification of peritidal carbonates by continental brines at fisherman bay south australia to form a megapolygon spelean limestone association abstract groundwater springs in the supratidal zone at fisherman bay south australia are the sites of aragonite precipitation and cementation of holocene marine carbonates lithification which commenced less than 3000 years bp and is still active has formed a cavernous limestone containing megapolygons tepees and speleothems including pisoliths floe aragonite and aragonitic pool deposits the emerging waters are from a continental source and have evolved from low alkalinity waters of pleistocene sand and clay coastal plain aquifers which have passed through an underlying tertiary marine carbonate aquifer the resulting waters have high pco2 total carbonate ca and sulphate concentrations they are close to saturation with respect to aragonite and their mmg2 mca2 ratios approach or exceed the critical aragonite precipitation value aragonite is precipitated from these waters when they approach the surface and evolve co2 features which taken together may diagnose ancient examples of this process include 1 primary aragonitic cements with high msr2 mca2 values 2 nonmarine 34s values in associated gypsum 3 two superimposed networks of surface polygons one delineated by extensional boundaries the other by tepees 4 high water vadose zone isopachous grain cements 5 interconnected speleothem lined cavities and 6 the presence of evaporites only in surface sediments possible ancient examples of peritidal carbonate lithification by saline spring waters rising from confined aquifers are recognized in descriptions of carbonates from west texas lombardy and the atlas mountains the areal extent of each of these deposits suggests that the process may be a geologically important feature and its products may be diagnostic of semi arid or arid zone paralic sedimentation

sabkha facies and the preservation of a falling stage systems tract at the sauk ii iii supersequence boundary in the late cambrian eilean dubh formation nw scotland the durness group of northwest scotland represents a nearly one kilometer thick succession of mid cambrian to lower middle ordovician carbonates these rocks crop out in a narrow belt along the foreland of the caledonian orogen and in the moine thrust zone stretching some 170 km from loch eriboll southwestwards to the isle of skye the cambrian parts of the succession represent deposition on a paleo southeast facing low latitude passively subsiding continental margin situated at around 20 s carbonate deposition in scotland formed part of the extensive great american carbonate bank the sauk ii iii supersequence boundary occurs above an interval of thin meter scale shallowing upward peritidal cycles containing intraclast dolorudstones stromatolites tepees siliciclastic sand and surface karst evidence of former evaporites in the succession comprises pseudomorphs of nodular and bedded anhydrite crystal shaped calcite filled vugs representing growth of sulfate dissolution breccias and local halite casts these features represent sabkha facies arranged into cycles that constitute a preserved falling stage systems tract fsst the top of the fsst is marked by a pronounced karst surface of sandstone filled fissures the section preserves a rare snapshot of a seldom preserved environment on the cambrian laurentian margin its preservation during a time of falling relative sea level under slow tectonic subsidence is discussed

a quantitative analysis of recent arid coastal sedimentary facies from the arabian gulf coastline of abu dhabi united arab emirates abstract this study presents the results of the first quantitative analysis of surface sediments along multiple transects of the arabian gulf shoreline of abu dhabi united arab emirates the natural coastline of this low angle carbonate ramp depositional system is being anthropogenically modified at an unprecedented rate as this system is much employed as a recent analogue for many ancient depositional sequences it is imperative that sedimentological characteristics of natural facies occurrences are accurately recorded for employment in future applications we establish distinct relationships between the geomorphic setting and sediment characteristics siliciclastic grains admixed to this system are naturally sourced from the pliocene ghayathi formation and from aeolian sediment influx during dust storms this natural source of siliciclastic detritus is being increasingly superseded by siliciclastic material derived from anthropogenic activities evaporite minerals are similarly being contributed from anthropogenic sources the occurrence of mud to silt grade sediments is primarily controlled by the presence of a sediment binding microbial mat that prevents the removal of fine material during high energy events similarly the presence of a halite crust in the upper tidal flats and supratidal zone prevents aeolian deflation this study did not establish any clear relationship between coastal geomorphic setting and the bioclastic components present in the sediments this finding has major implications for the application of bioclasts in fine tuning palaeoenvironmental interpretation of ancient carbonate ramp coastal systems

progressive evolution of a fault related fold pair from growth strata geometries sant llorenc de morunys se pyrenees abstract new structural stratigraphical mapping constrains the three dimensional kinematics and mechanisms of eocene oligocene growth folding at sant llorenc de morunys ne ebro basin spain a 1 km wide sub vertical panel of syntectonic alluvial gravels passes southwards via a highly asymmetrical growth fold pair to shallowlydipping strata the axial surface of the anticline comprises either continuous or en echelon segments while that of the syncline is concave and usually continuous while converging upwards the axial surfaces do not define growth triangles principal and subsidiary growth unconformities and thickness changes occur across both axial surfaces and the common limb dips within the common limb decrease up stratigraphy and up dip mesostructures indicate that internal deformation was ongoing during folding at all stratigraphical levels and concentration of cleavage in the syncline indicates that this hinge was essentially fixed sequential restoration of three profiles shows that folds amplified principally by limb rotation but incorporated minor passive hinge migration particle movement vectors generated by section restoration are arcuate about a hinterland pinpoint a new trishear model of fault propagation folding involving non rigid limb rotation reproduces the rounded hinge forms thickening geometries and limb dip variations observed simple kink band migration models fixed axis and constant thickness theories do not replicate these features

temporal evolution of a carbonate coastal system abu dhabi united arab emirates abstract the southern shore of the persian arabian gulf comprises a low angle carbonate evaporite ramp system that provides a rare recent analogue for ancient epeiric settings including many carbonate petroleum reservoirs during the 1960s several studies documented the distribution of naturally occurring shallow marine facies in the shallow marine environments proximal to abu dhabi island subsequent coastal engineering projects have significantly modified the coastline and sublittoral zone many sedimentary environments have been lost with marine sites now sited beneath buildings parks or roads the data from the earlier studies are used as a pre anthropogenic base line in order to document and assess the temporal changes in the characteristics and distribution of carbonate sedimentary facies as observed today the locations of earlier sample sites were revisited in order to represent the complete range of shallow marine sedimentary environments the shallow subtidal environments adjacent to abu dhabi island continue to be areas of active carbonate sedimentation temporal changes in facies belt geometries and occurrence are assessed changes are recorded for most of the investigated sedimentary settings both in terms of sedimentary environment distribution and sediment composition offshore reefs and their associated back reef lagoons are no longer present ooid shoal and delta complexes are much reduced in their aerial extent now being replaced with shoreface facies or dredged channels significant dredging activity has increased tidal flow to lagoon environments thereby increasing connectivity to open marine conditions however land reclamation has substantially reduced the volume of the area of the lagoon environments despite these changes many facies display surprising resilience to anthropogenic activities with no discernible change in sedimentary facies or biotic distributions understanding these modifications and their associated driving mechanisms allows discrimination between anthropogenic and naturally occurring environmental perturbations this in turn develops our understanding of the temporal evolution of shallow marine carbonate facies under natural and anthropogenic influences

bedding plane slip in initial stages of fault related folding abstract frictional slip along bedding planes contributes to fault related folding of layered rocks we use numerical experiments to investigate the deformation of frictional bedding planes near dipping faults under layerparallel contraction and extension within the numerical experiments contraction boundary conditions produce asymmetric anticlines and extension produces asymmetric synclines the fold shape may be used to infer dip of the underlying fault in situations where the fault may not be observable additionally sense of slip along bedding planes may indicate proximity to the fault tip under uniform remote tectonic strain fault slip induces deformation in both the hangingwall and the footwall at depths as shallow as 1 km there is no significant difference between fold amplitudes in the hangingwall and the footwall this result is contrary to many kinematic models currently in use kinematic models of fault related folds commonly include the development of flat ramp and flat ramp flat fault geometries which may be attributed to initial ramp thrusting and later flat development our mechanical models show that fault flats may be produced from fault ramps due to slip along frictional bedding planes near the thrust fault tips our numerical experiments also evaluate joint initiation joints perpendicular to bedding are promoted in extensional environments we compare the results of the mechanical model to kinematic models of fault propagation folds and conclude that mechanical models offer important insights to better understand the folding process

stratigraphic controls on eformation patterns in fault related folds a detachment fold example from the sierra madre oriental northeast mexico abstract deformation patterns are described by the style intensity and distribution of structures throughout a volume of rock in fault related folds presently observed deformation patterns are controlled by structural factors like kinematic history environmental factors like burial history and stratigraphic factors like layer thickness and competence to demonstrate the role of stratigraphy in controlling fold related deformation patterns we document the stratigraphic variability of mesostructures at one structural position on a map scale detachment fold unique assemblages of faults fractures and cleavages in certain stratigraphic intervals define a mechanical stratigraphy comprised of several units each exhibiting a characteristic deformation response to the folding process the varying response of each mechanical unit suggests that folding mechanisms such as orthogonal flexure flexural slip or flexural flow were sequentially or coevally operative in each of the units using the simple theoretical strain distributions associated with each folding mechanism and the fact that each mechanical unit accommodates fold related strain by developing a characteristic set of mesostructures we are able to predict general deformation patterns elsewhere in the fold the significance of this approach is that it recognizes that even in a single structure fold kinematics and deformation response may be dramatically different in various parts of a stratigraphic section because of this the quality and character of reservoir rocks in anticlinal traps may change markedly over short stratigraphic distances predictions of fold related deformation based solely on fold kinematics are unlikely to resolve these variations

numerical modeling of trishear fault propagation folding in contrast to kink band migration modeling methods trishear numerical models produce fault propagation folds with smooth profiles and rounded hinges modeled fold hinges tighten and converge downward within a triangular zone of distributed deformation which is focused on the fault tip such features have been reported from field studies and are also seen in analogue models of compressional deformation however apart from its initial application to laramide folds little quantitative work has been undertaken on trishear fault propagation folding in other settings in addition no study has been undertaken into the growth strata which might be associated with such structures this paper uses an equivalent velocity description of the geometric model of trishear together with models of erosion and sedimentation to investigate trishear fault propagation folding of both pregrowth and growth strata the trishear model is generalized to include a variety of fault propagation to slip ratios and fault propagation from a flat decollement the models show continuous rotation of the forelimb with the characteristic development of cumulative wedges within growth strata when total slip on a structure is high the model predicts overturned pregrowth and growth strata during the initial stages of deformation beds in the forelimb thicken but later thin when they become steep or overturned the effect of variations in fault propagation to slip ratios on two dimensional finite strain in the models is assessed by the use of initially circular strain markers high fault propagation to slip p s ratios lead to narrow zones of high finite strain while lower p s ratios lead to more ductile deformation and broader zones of high strain in all cases hanging wall anticlines and footwall synclines originate as early ductile folds which are later cut by the propagating fault modeled structures are compared with natural examples

impact of diagenesis and low grade metamorphosis on isotope δ26mg δ13c δ18o and 87sr 86sr and elemental ca mg mn fe and sr signatures of triassic sabkha dolomites abstract dolomite is a common rock forming mineral in the geological record but its value as archive of ancient seawater 26 mg signatures and their variations in time is at present underexplored unknown factors include the sensitivity of 26 mg ratio to processes in the diagenetic and low grade metamorphic domain this paper documents and discusses the first detailed 26 mg data set from early diagenetic and burial dolomites samples come from the upper triassic hauptdolomit dolomia principale the dolomites italy and include coeval dolomicrites that underwent differential burial diagenesis and low grade metamorphosis in a temperature range between about 100 and more than 350 c magnesium isotope data are complemented by dolomite 13 c 18 o and 87 sr 86 sr isotope ratios as well as ca mg mn fe and sr elemental abundances and data on the dolomite crystal structure as indicated by dolomicrite 87 sr 86 sr ratios and petrographic evidence sabkha calcian d1 dolomites precipitated from evaporated seawater and stabilized at an early marine pore water diagenetic stage to d2 dolomites analysed here with increasing burial temperature dolomite 26 mg ratio scatter in the data set decreases with increasing mg ca ratio and degree of order specifically 26 mg ratio variability is reduced from 0 36 2 at burial temperatures beneath 100 c to about 0 14 2 at temperatures in excess of 350 c respectively with mean 26 mg values ranging constantly near 1 9 this suggests that at least for the rock buffered system investigated here dolomicrite 26 mg proxy data are conservative and preserve near pristine values even at elevated burial temperatures at present the main element of uncertainty is the mg isotope fractionation factor between evaporated seawater and dolomite a possible solution to this problem includes the compilation of data from modern sabkha environments including pore water and calcian dolomite 26 mg isotope signatures

strain analysis and fold shape in a limestone layer and implications for layer rheology abstract folds are developed in thin limestone layers within slates of the mckay formation exposed to the east of the rocky mountain trench british columbia canada they possess geometrical characteristics expected of development by buckling strain in the profile plane of a selected fold is similar to that predicted by tangential longitudinal strain except that magnitudes are too low for the observed curvature this is attributed to inhomogeneity of strain on the scale of measurement largely because of pressure solution material removed by pressure solution from the inner arc of the fold appears to form veins perpendicular to the hinge a direction of tectonic stretching bedding parallel stylolites developed diagenetically prior to tectonism layer parallel shortening during the initiation of buckling was less than 20 and probably less than 10 the mean arclength thickness ratio is 6 5 and 7 1 with a dispersion of 0 48 and 0 37 for local and regional populations of 29 and 212 folds respectively application of buckling theory to this data suggests that folding followed a non linear flow law the viscosity contrast between limestone and slate would be higher and the power law exponent lower if initial irregularities in the layers were in the form of a constant amplitude spectrum rather than one of white roughness the data do not allow a choice of initial amplitude spectrum to be made nor do they closely constrain estimates of n the power law exponent and viscosity contrast deformation in the limestone layers was accommodated by intracrystalline flow twin gliding pressure solution and extensional veining the last two linked by diffusive mass transfer the first two dominated deformation in the profile plane of the fold and the last in association with fracturing allowed for extension parallel to the hinge experimental and theoretical considerations suggest that deformation by a combination of these processes should be non linear the non linear flow law deduced from buckling analysis is consistent with expectations based on observations of active deformation mechanisms

the kinematics of break thrust folds abstract the kinematics of asymmetric open to close often overturned thrust related folds are commonly explained with fault propagation or detachment fold models field observations of thrust related folds exposed in tennessee virginia wyoming and montana that exhibit this geometry indicate that models of fault propagation and detachment folds do not adequately describe the kinematics of these structures existing models of fault propagation and detachment folding employ migrating kink shaped hinges and relate fold geometry entirely to fault geometry slip and to the thickness of the basal detachment layers the models exclude the relation between fold shape and amplitude and they do not predict a correlation between competent layer thickness and fold wavelength as expected from buckling theory thrust related folds examined in this study exhibit unique distributions of fabrics in the hinges and forelimbs which demonstrate a lack of hinge migration furthermore the ratios of dominant member thickness to fold wavelenght agree with those expected for folds controlled by a single layer and they are within the expected range for folds with multiple evenly spaced harmonically deforming layers our analyses suggest these folds evolved from an initial brief stage of sinusoidal buckling to a later stage of fixed hinge kinking and thrusting thrust related folds which exhibit a geometry and mesofabric distribution suggesting this type of evolution are defined as break thrust folds

the preservation potential of environmentally diagnostic sedimentary structures from a coastal sabkha abstract the abu dhabi sabkha constitutes the landward portion of the shallow sloping carbonate ramp situated on the northwestern coast of the united arab emirates uae the sabkha in this region is composed of a complex assortment of carbonate organic and evaporitic facies that constitute a regressive sabkha sequence the surface features of the sabkha can be grouped into distinct laterally continuous facies associations that are directly correlatable with elevation above present day sea level previous investigations have suggested the employment of the abu dhabi sabkha as a modern depositional analogue to ancient carbonate evaporite formations however a quantitative analysis of the relationship between sabkha surface and subsurface characteristics has not previously been undertaken this study describes in detail the sedimentary structures and textures seen at the sabkha surface and compares them to the holocene age facies retained in the shallow subsurface of the 36 sedimentary structures observed at the sabkha surface only seven 19 are still identifiable in the early burial realm the majority of the surface structures were lost during early burial as a result of microbial mat decomposition destructive evaporite precipitation and compaction the loss of diagnostic sedimentary features during early burial as a result of fabric destructive processes has considerable implications for the recognition and interpretation of ancient sabkha sequences documenting these processes from a range of modern sedimentary environments will aid in the identification of those diagnostic features that may be reliably employed during paleoenvironmental reconstructions

effect of initial fault geometry on the development of fixed hinge fault propagation folds abstract this paper describes how a model of fixed hinge basement involved fault propagation folds may be adapted to apply to thin skinned thrust faults to generate footwall synclines fixed hinge fault propagation folding assumes that the fold axial surfaces diverge upwards fold hinges are fixed in the rock the fault propagated through the forelimb thickness changes occur in the forelimb and the forelimb progressively rotates with increasing displacement on the underlying fault the original model for fixed hinge fault propagation folds was developed for the case of a planar fault in basement with a tip line that was at the interface between basement and the overlying sedimentary cover rocks the two geometries applicable to thin skinned thrusts are for the cases where a fixed hinge fault propagation fold develops above an initial bedding parallel detachment and an initial fault ramp of constant dip which flattens down dip into a bedding parallel detachment

giant polygons in the triassic salt of cheshire england a thermal contraction model for their origin abstract the triassic salt in cheshire england has large polygonal patterns on bedding surfaces the polygons are bounded by deep v shaped fissures the fissures have a banded internal structure made up of individual units of pure secondary salt and clastic rich halite the salt was deposited from shallow brines in an arid environment a thermal contraction model is proposed here for the formation of the polygons calculations of the depth of penetration of thermal waves and the net contraction of salt beds indicate that annual temperature fluctuations would produce fissures of the observed depth and width it is suggested that thermal contraction is the most important factor in the initiation of all polygonal patterns on salt surfaces and that the polygon form that eventually develops is de endent on the thickness of the salt bed and the availability of brines to the salt surface

kink detachment fold in the southwest montana fold and thrust belt the hossfeldt anticline in the southwest montana thrust belt is characterized by a kink geometry and probably overlies a thrust detachment at depth the mesofabric distribution in the limbs documents that the eastern overturned limb has undergone most of the rotation and internal deformation during folding leaving the gently dipping western limb virtually undeformed the anticline exhibits unique mesofabrics in its hinge region that require a pinned anticlinal hinge during its evolution the half wavelength of the hossfeldt anticline eustis syncline pair coincides with that predicted from buckling theory if one considers the massive carbonates of the paleozoic section as a competent beam although the geometry and mesofabric distribution of the hossfeldt anticline satisfy the geometric requirements of either a fault propagation fold or a detachment kink fold the buckling wavelength strongly suggests that its origin was as a kink buckle fold above a flat detachment rather than as a fault propagation fold above a thrust ramp

tepees modern southern australia and ancient permian texas and new mexico a comparison abstract tepee structures are forming today about the edges of a number of holocene groundwater lakes coastal salinas in the coastal zone of south australia tepees form by the groundwater induced deformation of an extensive sheet of fenestral limestone known as a veneer boundstone underlying the veneer boundstone is an extremely porous aquifer known as a boxwork boundstone areas of groundwater resurgence in these salinas are characterized by an association of tepee structures boxwork boundstones and stromatolites the tepees in these salinas are termed groundwater or boxwork tepees to emphasize the importance of resurging groundwater in their formation preliminary studies of tepee structures in the back reef strata of guadalupian age in the guadalupe mountains of west texas and new mexico has shown that these tepees are also groundwater tepees such tepee structures in the guadalupe mountains are often associated with remnants of an underlying boxwork boundstone and or an underlying sandy palaeoaquifer they are tepees which formed in zones of groundwater resurgence at a time when the nearby reef was a subaerially exposed barrier limestone sheets composed of vadose pisolites were deformed coincident with tepee formation this association implies that vadose pisolites like tepees formed in areas of periodic groundwater resurgence characterized by a complex schizohaline hydrological regime

the flexural slip mechanism abstract detailed field studies of turbiditic sequences from south georgia south atlantic north devon england and cardigan bay wales show that flexural slip occurs on discrete movement horizons between rock packets in which the beds have welded contacts stair stepping displacements of sedimentary dykes and of early quartz veins show that the movement horizons generally have a decimetre to metre spacing and are marked by bedding parallel quartz veins these veins are from 1 mm to several cm thick and can be used to identify movement horizons in the absence of displaced markers they consist of several sheets of quartz fibres which each carry a slickenfibre lineation and which together preserve a record of the displacements on an individual surface complex slickenfibre patterns and departures from ideal behaviour in which slip occurs orthogonal to the fold hinge probably result from changes in slip vector on the limbs of growing non cylindrical folds movement horizons show many of the features associated with large scale thrusting such as ramps duplexes and imbricate structures and the shear sense given by fibre steps on these surfaces and by duplexes within stratigraphically restricted packages changes across fold hinges chevron folds are thought to develop mainly by flexural flow in the early stages with flexural slip becoming dominant later as the beds become lithified new slip surfaces are generated as the dip of the fold limbs increases

flexural shear in a periclinal fold from the irish variscides abstract periclinal folds occur in a structural culmination in the centre of the irish variscan fold belt the culmination marks a zone of enhanced shortening which is associated with a locally attenuated stratigraphy and an anomalous deformation sequence with respect to the remainder of the orogen originally vertical burrows have been used along with cleavage fold relations to study both the three dimensional evolution of the periclinal folding and the temporal relationship between folding and cleavage as well as cleavage forming flattening strains the burrows record along strike shear related to the propagation of the periclinal folds a comparison with strain measurements made outside the culmination zone shows that the galley head area suffered higher strains integration of field data with published analyses of plasticine modelling of periclinal folds yields models for the propagation of a pericline and for the geometry of its lateral terminations

part 1 regional studies and epithermal deposits northern nevada is one of the earth s premier gold and silver producing regions and gold and silver are mined from a wide range of deposit types table 1 current production largely comes from carlin type sedimentary rock hosted disseminated gold deposits christensen 1995 teal and jackson 1997 bettles 2002 many of which lie along linear trends fig 1 roberts 1960 1966 berger and bagby 1991 the small area of the carlin trend approximately 8 x 65 km is north america s most prolific gold mining district producing nearly 125 t 4 moz of gold annually since large scale mining began in 1965 with the opening of the carlin mine the carlin trend has produced more than 1555 t 50 moz of gold during 2001 gold production from the entire state of nevada was nearly 253 t 8 13 moz which accounted for approximately 76 percent of the united states and 10 percent of the world production price and meeuwig 2002 in 2001 only the countries of south africa and australia produced more gold than the state of nevada in addition nevada produced 543 t 17 4 moz of silver in 2001 about 31 percent of the united states production and 3 percent of the world production in 1996 the mineral resources program of the u s geological survey began a multidisciplinary project to investigate the origin of gold deposits in northern nevada this project was undertaken in collaboration with the nevada bureau of mines and geology the ralph j roberts center for research in economic geology at the university of nevada reno several other universities and numerous mining companies active in the region in addition several universities and mining companies were conducting research on the origin of gold deposits in northern nevada this research built on more than a century of studies by the

antecedent aeolian dune topographic control on carbonate and evaporite facies middle jurassic todilto member wanakah formation ghost ranch new mexico usa

late quaternary sea level changes of the persian gulf abstract late quaternary reflooding of the persian gulf climaxed with the mid holocene highstand previously variously dated between 6 and 3 4 ka examination of the stratigraphic and paleoenvironmental context of a mid holocene whale beaching allows us to accurately constrain the timing of the transgressive highstand and regressive phases of the mid to late holocene sea level highstand in the persian gulf mid holocene transgression of the gulf surpassed today s sea level by 7100 6890 cal yr bp attaining a highstand of 1 m above current sea level shortly after 5290 4570 cal yr bp before falling back to current levels by 1440 1170 cal yr bp the cetacean beached into an intertidal hardground pond during the transgressive phase 5300 4960 cal yr bp with continued transgression interring the skeleton in shallow subtidal sediments subsequent relative sea level fall produced a forced regression with consequent progradation of the coastal system these new ages refine previously reported timings for the mid to late holocene sea level highstand published for other regions by so doing they allow us to constrain the timing of this correlatable global eustatic event more accurately

a review of the origin and setting of tepees and their associated fabrics carbonate hardgrounds often occur at the surface of shallow subtidal to supratidal lacustrine and subaerial carbonate shelf sediments these are commonly disrupted and brecciated when the surface area of these crusts increases in the subtidal environment megapolygons form when cementation of the matrix causes the surface area of the hardgrounds to expand similar megapolygons form in the supratidal lacustrine and subaerial settings when repeated incremental fracturing and fracture fill by sediment and or cement also causes the area of the hardgrounds to expand the arched up antiform margins of expansion megapolygons are known as tepees the types of tepees found in the geological record include 1 submarine tepees which form in shallow carbonate saturated waters where fractured and bedded marine grainstones are bound by isopachous marine phreatic acicular and micritic cements the surfaces of these brecciated crusts have undergone diagenesis and are bored unlike tepees listed below they contain no vadose pisolites or gravity cements 2 peritidal and lacustrine tepees are formed of crusts characterized by fenestral pisolitic and laminar algal fabrics this similarity in fabric makes these tepees of different origins difficult to separate peritidal tepees occur where the marine phreatic lens is close to the sediment surface and the climate is tropical they are associated with fractured and bedded tidal flat carbonates their fracture fills contain geopetal asymmetric travertines of marine vadose origin and or marine phreatic travertines and or terra rossa sediments the senile form of these peritidal tepees are cut by labyrinthic dissolution cavities filled by the same material lacustrine tepees form in the margins of shallow salinas where periodic groundwater resurgence is common they include groundwater tepees which form over evaporitic boxwork carbonates and extrusion tepees which also form where periodic groundwater resurgence occurs at the margins of shallow salinas but the dominant sediment type is carbonate mud these latter tepee crusts are coated and crosscut by laminated micrite the laminae extend from the fractures downward into the underlying dolomitic micrite below the crust both peritidal and lacustrine tepees form where crusts experience alternating phreatic and vadose conditions in time intervals of days to years cement morphologies reflect this and the crusts often contain gravitational meniscus vadose cements as well as phreatic isopachous cement rinds 3 caliche tepees which are developed within soil profiles in a continental setting they are formed by laminar crusts which contain pisolites and fractures filled by micritic laminae microspar spar and terra rossa most of the cements are gravitational and or meniscoid in ancient carbonates when their cementation and diagenetic fabric can be interpreted tepee structures can be used as environmental indicators they can also be used to trace the evolution of the depositional and hydrological setting

reply to the comment on first records of syn diagenetic non tectonic folding in quaternary thermogene travertines caused by hydrothermal incremental veining by billi et alii abstract in our previous paper billi et al 2017 using field geological observations u th dating and stable isotope analyses we studied two deposits of pleistocene thermogene travertines from tuscany in central italy we concluded our study 1 warning that the common stratigraphic concept of travertine being a sedimentary succession with age younging from bottom to top is not always correct 2 demonstrating that caco3 mineralization and veins can develop within the travertines after their formation with this syn diagenetic process being able to modify the continuous bottom up age evolution and 3 showing that this post depositional mineralization veining process can not only modify the temporal succession but also deform and change the initial depositional travertine structure and its petrophysical properties these conclusions could potentially make the interpretation of a travertine series more difficult than commonly thought alcicek et alii questioned our conclusions claiming that the travertine structures that we observed in tuscany and interpreted as post depositional features should have been interpreted in analogy to similar structures from travertines elsewhere as primary structures although we recognize as already thoroughly stated in billi et al 2017 that the travertine depositional post depositional processes generally require further studies we reaffirm the validity of our original interpretation at least for the structures analyzed in our previous paper we therefore counter all criticisms by alcicek et alii and conclude by indicating the way forward to further explore the depositional and post depositional processes of thermogene travertines

giant polygons in the realmonte mine agrigento sicily evidence for the desiccation of a messinian halite basin abstract the messinian realmonte salt deposit consists of a 400 600 m thick halite succession that can be divided into four main lith ologic units base to top a cumulates of plates settled out from a stratified water column b plate cumulates in a shallowing upward sequence containing six kainite layers near the top of the unit c cu mulates of skeletal hoppers with chevron overgrowths precipitated from a nonstratified water body and d skeletal halite and anhydrite spectacular vertical fissures cut through the upper part of unit b at its boundary with salt unit c these fissures are spaced at intervals of up to 5 m apart extend down to a depth of 6 m and are filled by red mud the salt beds affected by the fissures are commonly upturned buckled and are truncated and onlapped by the succeeding flat lying halite beds of unit c additionally the upturned layers are cut by vertical dissolution pipes that extend down to as much as a depth of 4 m below the fissured surface to explain the observed superimposed structures we propose two mechanisms both induced by the desiccation of the evaporite basin 1 when the salt surface was exposed the halite layers were buck led broke into polygonal crusts and formed tepee structures the buckling was caused by a net volume increase due to thermal expan sion and what is more important precipitation of new halite induced by evaporative pumping of brines moving up from the groundwater table the buckled salt layers were then affected by meteoric dissolu tion which created vertical pipes 2 the tepee structures and the associated zone of dissolution pipes are further crosscut by a new network of large polygons these new polygonal sets are outlined by deep fissures in which red silt collected blown by the wind or carried by episodes of surface flooding this type of deep contraction crack is known to be produced by volumetric changes induced by annual temperature fluctuations the documentation of contraction polygons and other exposure fea tures represents the first report of desiccation events during the de position of the sicilian salt no geological data are yet available to settle the questions if the salt of sicily is correlatable with the halite present beneath the floor of the mediterranean and if the desiccation of the realmonte salt basin could have been induced by a local tectonic causes uplift of the basin floor by thrust activity b simple evaporitic drawdown c basinwide drop of the mediterranean sea level or d complex interaction among these factors

holocene coastal carbonates and evaporites of the southern arabian gulf and their ancient analogues abstract the holocene sediments of the coast of the united arab emirates in the southeastern arabian gulf are frequently cited in the literature as type examples for analogous assemblages of carbonates evaporites and siliciclastics throughout the geologic record this paper is intended as a convenient single source for the description of sediments of this region providing information on how to reach the classic localities and some of the analogs the holocene sediments of the region accumulate over an area that is 500 km long and up to 60 km wide the sediments collecting offshore are predominantly pelecypod sands mixed with lime and argillaceous mud with these latter fine sediments increasing as the water deepens the pelecypod rich sediments also collect east of abu dhabi island both in the deeper tidal channels between the barrier island lagoons and in deeper portions of the protected lagoons west of abu dhabi island the shallow water margin is the site of coral reefs and coralgal sands whereas to the east oolites accumulate on the tidal deltas of channels located between barrier islands grapestones accumulate to the lee of the reefs and the oolite shoals where cementation becomes more common they are particularly common on the less protected shallow water margins of the lagoons west of abu dhabi island pelleted lime muds accumulate in the lagoons in the lee of the barrier islands of the eastern abu dhabi lining the inner shores of the protected lagoons of abu dhabi and on other islands to the west are cyano bacterial mats and mangrove swamps landward of these a prograding north facing shoreline is formed by supratidal salt flats sabkhas in which evaporite minerals are accumulating this paper describes the localities associated with 1 the mangrove swamps of the west side of the al dhabaiya peninsula 2 the indurated cemented carbonate crusts cyanobacterial flats and sabkha evaporites on the shore of the khor al bazam south of qanatir island 3 the reef and oolitic sand flats on the coast just east of jebel dhana and 4 the marine travertine and aragonite coats associated with the beach sediments in a small bay south of jebel dhana and 5 the sabkha mutti between jebel barakah and al sila similar sedimentological associations of carbonate and evaporites to those of the holocene of the united arab emirates are to be found in the tertiary and mesozoic sedimentary rocks of the immediate subsurface in the arabian gulf other analogs to this setting include the paleozoic carbonates of the western usa europe and asia mesozoic carbonates of the gulf of mexico europe and middle east and tertiary sedimentary rocks in the middle east

dolomite formation within microbial mats in the coastal sabkha of abu dhabi united arab emirates microbial mediation is the only demonstrated mechanism to precipitate dolomite under earth surface conditions a link between microbial activity and dolomite formation in the sabkha of abu dhabi has until now not been evaluated even though this environment is cited frequently as the type analogue for many ancient evaporitic sequences such an evaluation is the purpose of this study which is based on a geochemical and petrographic investigation of three sites located on the coastal sabkha of abu dhabi along a transect from the intertidal to the supratidal zone this investigation revealed a close association between microbial mats and dolomite suggesting that microbes are involved in the mineralization process observations using scanning electron microscopy equipped with a cryotransfer system indicate that authigenic dolomite precipitates within the exopolymeric substances constituting the microbial mats in current models microbial dolomite precipitation is linked to an active microbial activity that sustains high ph and alkalinity and decreased sulphate concentrations in pore waters such models can be applied to the sabkha environment to explain dolomite formation within microbial mats present at the surface of the intertidal zone by contrast these models cannot be applied to the supratidal zone where abundant dolomite is present within buried mats that no longer show signs of intensive microbial activity as no abiotic mechanism is known to form dolomite at earth surface conditions two different hypotheses can reconcile this result in a first scenario all of the dolomite present in the supratidal zone formed in the past when the mats were active at the surface in a second scenario dolomite formation continues within the buried and inactive mats in order to explain dolomite formation in the absence of active microbial metabolisms a revised microbial model is proposed in which the mineral template properties of exopolymeric substances play a crucial role

holocene marine hardground formation in the arabian gulf shoreline stabilisation sea level and early diagenesis in the coastal sabkha of abu dhabi abstract this study provides the first comparison between a seaward and a landward section of the same diachronous hardground surface observed in the coastal sabkha of abu dhabi this hardground is described here in terms of its mode of formation its diagenetic environment and its impact on shoreline stabilisation during transgression the hardground is exposed in the intertidal zone and buried by a late holocene prograding succession of carbonates evaporites and microbial sediments in the supratidal zone the hardground itself is composed of bioclastic grains primarily of aragonitic composition that originate from intertidal depositional environments aeolian silt to sand sized quartz grains are also observed lithification occurred through the precipitation of pore filling aragonite high mg calcite and dolomite cements from sea and interstitial water that was marked by high salinities and temperatures as confirmed by stable isotope analyses high mg calcite and non stoichiometric dolomite are also observed as secondary recrystallisation products the formation of these two mineral phases as recrystallisation products was possibly microbially mediated lithification progressed in two phases the older phase of which is marked by higher amounts of non stoichiometric dolomite and high magnesium calcite as compared to the younger phase transgressive reworking of precursor siliciclastic sands was inhibited by the development of transgressive pore filling gypsum cements in the supratidal zone

late holocene to recent aragonite cemented transgressive lag deposits in the abu dhabi lagoon and intertidal sabkha modern cemented intervals beachrock firmgrounds to hardgrounds and concretionary layers form in the lagoon and intertidal sabkha of abu dhabi seafloor lithification actively occurs in open current swept channels in lowlying areas between ooid shoals in the intertidal zone of the middle lagoon some centimetres beneath the inner lagoonal seafloor i e within the sediment column and at the sediment surface the intertidal sabkha the concept of concretionary sub hardgrounds i e laminar cementation of sediments formed within the sediment column beneath the shallow redox boundary is introduced and discussed based on calibrated radiocarbon ages seafloor lithification commenced during the middle to late holocene ca 9000 cal yr bp and proceeds to the present day lithification occurs in the context of the actualistic relative sea level rise shifting the coastline landward across the extremely low angle carbonate ramp the cemented intervals are interpreted as parasequence boundaries in the sense of marine flooding surfaces but in most cases the sedimentary cover overlying the transgressive surface has not yet been deposited aragonite micritic calcite and less commonly gypsum cements lithify the firmground hardground intervals cements are described and placed into context with their depositional and marine diagenetic environments and characterized by means of scanning electron microscope petrography cathodoluminescence microscopy and raman spectroscopy the morphology of aragonitic cements changes from needleshaped forms in lithified decapod burrows of the outer lagoon ooidal shoals to complex columnar lath and platy crystals in the inner lagoon precipitation experiments provide first tentative evidence for the parameters that induce changes in aragonite cement morphology data shown here shed light on ancient formerly aragonite cemented seafloors now altered to diagenetic calcites but also document the complexity of highly dynamic near coastal depositional environments

morphology and geometry of duplexes formed during flexural slip folding abstract bedding parallel slip on the limbs of chevron style folds during flexural slip folding results occasionally in the formation of small scale duplexes the duplexes described here from upper carboniferous turbidites in sw england generally range in thickness from 0 3 to 30 cm and are up to 2 4 m long they have smooth nearly flat roofs and both the internal thrusts and the slickenfibres on them are commonly oblique to the mean transport direction as indicated by slickenfibres on the floor and roof thrusts the morphology of the duplexes suggests that they developed between active floor and roof thrusts and they show a characteristic lack of hangingwall anticlines on the link thrusts their shear sense always corresponds to that required by flexural slip and the total bedding parallel displacement across each is commensurate with that predicted by the flexural slip model all thrust surfaces in the duplexes are marked by quartz fibre veins a feature which distinguishes them from soft sediment and other pre folding duplexes and carry slickenfibres whose mean orientation and complex variations in slip direction on different fibre sheets on the same slip surface are identical to those on flexural slip movement horizons from the same fold limb the duplexes form either as a result of resistance to thrust propagation by local facies or bed thickness changes or develop as transfer structures between the tips of movement horizons propagating along adjacent slip surfaces late stage duplexes develop from imbricated fibre veins and also form on slip surfaces oblique to the axial planes of major folds