tidal modeling of an ancient tide dominated seaway part 1 model validation and application to global early cretaceous aptian tides abstract the utility of deep time global paleotidal modeling is evaluated with a series of modeling experiments focused on the early cretaceous aptian the results from a series of paleobathymetry sensitivity experiments indicate that despite paleogeographic and paleobathymetric uncertainty appropriate use of global paleotidal models complements and enhances stratigraphic and sedimentological methods before being applied to global paleotidal modeling the imperial college ocean model icom is validated for the computation of modern global tides the icom results only deviate slightly from a state of the art published model and were achieved with considerably less computational expense and without data assimilation the results from global paleotidal modeling of the early cretaceous aptian were assessed by comparison with the published geological record icom predicts high mesotidal to macrotidal ranges on the arabian platform around india especially to the north and west along the pacific coast between north and south america northeast of australia and around southeast asia the model predicts low microtidal ranges in the proto south atlantic ocean and weddell sea a further validation test assessed the ability of the model to predict the dominant tidal constituents diurnal or semidiurnal in the aptian lower greensand seaway of southeastern england and northern france during the aptian this region was connected to the boreal proto atlantic and neotethys oceans and now preserves superbly documented tidal deposits the model predicts low microtidal ranges in the proto central atlantic ocean and boreal ocean suggesting that they had little impact on tides in the lower greensand seaway higher tidal amplitudes especially of diurnal once daily tides are predicted in the northwestern neotethys ocean suggesting that this ocean was an important source of tidal energy to the lower greensand seaway sensitivity tests to paleogeographic and paleobathymetric uncertainty indicate that the model predictions are robust the prediction of predominantly diurnal tides is supported by published analyses of large dune scale deposits in the lower greensand of southeast england

sandbox modelling of thrust wedges with fluid assisted detachments abstract we have used granular materials to model the development of thrust wedges where migrating pore fluids assisted in the formation of detachments the governing equations yield practical scales for linear dimensions stresses and time using compressed air as a pore fluid models a few centimetres thick were deformed in about half an hour model materials were sands of three different grain sizes and a loess they had suitable values of density permeability cohesion and internal friction fluid flow obeyed darcy s law at yield the materials satisfied a coulomb criterion for effective stresses models with various sequences of layers were submitted to horizontal shortening in a rectangular box compressed air entered through a sieve at the base the fluid pressure was uniform over the basal boundary in models made from a single material the style of deformation depended on the fluid pressure for no fluid flow the thrust wedge was short and high the surface slope attained large angles 30 and internal structures were mainly forethrusts for fluid pressures approaching lithostatic values thrust wedges were longer and lower and surface slopes attained smaller angles in models containing basal layers of small permeability detachments formed beneath them and the structural style was dominated by interacting forethrusts and backthrusts in multilayered models thin skinned detachments formed beneath less permeable layers in the sequence to understand how fluid flow controlled the first stages of detachment we calculated ideal vertical profiles of fluid pressure vertical normal stress effective stress and horizontal shear stress for multilayered models in the undeformed state the profiles are segmented because material properties vary from layer to layer sharp drops in shear strength occur at positions where detachments were observed in the sandbox models we infer that detachments resulted from large fluid pressures beneath relatively impermeable layers

an emplacement model for allochthonous salt sheets with implications toward subsalt exploration a model for allochthonous salt sheet emplacement is preserved to explain overthrusting of thick sediment columns above these sheets this model proposes that initial emplacement either as extrusive sheets or horizontal intrusions near the sea floor sometimes yields to shearing along thrust fault zones immediately beneath the salt sheets anomalously high pore fluid pressures drive shearing beneath the sheets by reducing the effective normal stress thereby facilitating lateral movement of the sheets and their overlying sediments several observations support this model 1 stratigraphic sections above salt repeated below salt 2 thick sediment escarpments near the toes of some salt sheets 3 compressional features in front of some salt sheets 4 low velocity zones near the base of some salt sheets 5 lithologically distinct zones beneath many salt sheets and 6 overpressured sediments below many salt sheets the mode of salt sheet emplacement has significant implications toward subsalt hydrocarbon exploitation our model is consistent with the following implications 1 subsalt strata may be younger and originate from a more landward depositional environment than otherwise expected 2 the overpressured shear zone may be a path for hydrocarbon migration a seal or a reservoir 3 one may be able to estimate the hydrocarbonmore column height of a nearby reservoir based on the detection of hydrocarbons in the shear zone and 4 hydrocarbon reservoirs trapped against shear zones may have significantly different up dip limits and depletion patterns than expected less

three dimensional to two dimensional cross strata transition in the lower pleistocene catanzaro tidal strait transgressive succession southern italy sandstone tidal cross strata are the predominant sedimentary feature of strait fill stratigraphic successions however although widely described in numerous studies tidal strait fill two dimensional and three dimensional cross strata have rarely been reported to occur in discrete intervals which are laterally adjacent or vertically stacked and the meaning of this stratigraphic architecture has not yet been fully investigated understanding of the processes responsible for changes in the internal features of modern and ancient tidal bedforms is essential in order to predict lateral and vertical heterogeneities in analogous reservoir strata this facies based study aims to interpret the three dimensional to two dimensional cross strata transition observed in the lower pleistocene mixed siliciclastic bioclastic sandstone filling the catanzaro strait in southern italy during a continuous phase of tectonically driven marine transgression tidal cross strata disappear in the uppermost interval of the studied succession where mudstone strata prevail this stratigraphic trend is interpreted as the evidence of an important change in the tidal strait hydrodynamics due to a phase of relative sea level rise at the beginning of the transgression three dimensional tidal dunes migrated throughout the ca 3 to 4 km wide and ca 30 km long wnw ese oriented catanzaro strait due to strong tidal currents amplified through the seaway and flowing in semi diurnal phase opposition as the intermediate phase of transgression enlarged the seaway width the tidal current strength decreased as tidal water exchange occurred over a larger cross sectional area the progressive reduction of the bed shear stress modified three dimensional tidal dunes into an extensive two dimensional bedform field at the end of the transgression the further widening of the catanzaro strait into a ca 10 to 12 km wide marine passageway changed the tidally dominated strait into a non tidal open shelf the results of this research suggest the presence of a critical cross sectional area in the narrowest strait centre zone which controls the activation and deactivation of tidal current amplification along a marine seaway

numerical simulations of tidal and wind driven circulation in the cretaceous interior seaway of north america twenty two numerical experiments using a multi layer numerical model of turbulent flow in shallow seas better define fluid circulation and sediment transport paths in the cretaceous interior seaway of north america each experiment consists of a different combination of paleogeography paleobathymetry coriolis acceleration boundary tidal amplitudes wind stresses and bed friction the paleogeographies and paleobathymetries represent three seaway configurations a seaway of intermediate size and depth 200 m during a transgressive peak t5 in late albian time a seaway of maximum size and depth 400 m during peak transgression t6 in early turonian time and a seaway of minimum size and depth 100 m during a peak regression r8 in the campanian values for the other key model parameters are m 2 co oscillating boundary tides of 0 1 to 0 2 m range at the arctic ocean boundary and 0 5 to 1 m range at the proto gulf of mexico boundary coriolis accelerations corresponding to 30 n 45 n and 60 n latitude chezy friction factors ranging from 31 to 70 m s 1 and average winter winds and two winter storms computed by the community climate model at the national center for atmospheric research for paleogeographic conditions during the late albian results of the numerical experiments and comparison of these results with geologic observations allow the following conclusions 1 circulation in the seaway was generally storm dominated 2 typical winter storms crossing the seaway from west to east generated 0 3 m s 1 shore parallel geostrophic currents on the shelves north of arizona at first flowing weakly to the north but later during the storm flowing strongly to the south 3 extreme storms could have produced 0 8 m s 1 currents over the shelves and 0 3 m s 1 currents in 200 m of water 4 co oscillating tides propagated into the seaway as progressive kelvin waves and therefore tidal ranges in the seaway were mesotidal to macrofdal along the southeastern margin but microtidal everywhere else 5 significant deep water wave heights and periods of the fully developed wave field are predicted to have been 5 to 6 m high and 10 s respectively the northwestern shore of the seaway would have experienced these storm waves approaching from the north and thus net sediment drift should have been to the south limited available field data support these conclusions

a nonhydrostatic finite element model for three dimensional stratified oceanic flows part ii model validation a three dimensional nonhydrostatic ocean model that utilizes finite element discretizations on structured or unstructured meshes has been formulated in the current part of this work a first step in the validation of this model is performed due to the inherent strengths of the modeling approach in being able to fully simulate three dimensional flows in arbitrary domains it has been decided here to focus attention on a single well studied collection of problems as such the flow of stratified fluid past an isolated gaussian seamount is considered it is shown that at moderate resolutions the model is relatively insensitive to the use of an increasingly fine mesh in addition qualitative as well as rough quantitative comparisons are made between results from the current model and previous studies carried out using alternate numerical models close agreement is demonstrated both in the eddy shedding and flow trapping structure of the flow as well as in the generation of internal trapped lee waves finally time periodic forced flow is examined where resonantly generated trapped waves on the seamount are shown to be produced the current model predicts resonant interactions occurring at parameter values consistent with previous numerical as well as linearized analytical studies

subsalt pressure prediction from multicomponent seismics and more predrill pore pressure predictions are vitally important in deepwater arenas where extreme depth and drilling window conditions typical of current prospects and developments require expensive rigs and highly trained personnel to successfully plan and execute complex wellbores

use of tidal circulation modeling in paleogeographical studies an example from the tertiary of the alpine perimeter tidal conditions in ancient shallow marine basins as revealed by preserved sedimentary deposits reflect the strong influence of the paleobathymetry use of a tidal circulation model can test existing paleobathymetric reconstructions meso and macrotidal conditions are believed to have existed throughout most of the miocene molasse france and switzerland seaway tidal simulations using external tidal ranges of 2 and 4 m entering from the south mediterranean fail to generate mesotidal conditions in the majority of the seaway because of a narrow constriction in the present alpes du dauphine and drome areas meso tidal conditions are simulated in the rhone alp part of the seaway only after the postulated paleobathymetry is widened in the alpes du dauphine area this change allows tidal penetration from the mediterranean despite this widening the new simulations fail to generate mesotidal conditions in the swiss part of the seaway mesotidal conditions are simulated in switzerland with the addition of a second tidal input from eastern switzerland favoring an interpretation that the swiss part of the seaway was connected with either the eastern mediterranean or the indo pacific ocean the revised paleobathymetry brackets uplift events that affected the basin margin

physical modelling of sand injectites abstract sand injectites are structures that result from intrusion of fluidized sand into fractures we have studied them in the tampen spur area of the north sea and have reproduced them experimentally by driving compressed air through layers of sand glass microspheres and silica powder the silica powder was cohesive and capable of hydraulic fracturing whereas the sand and glass microspheres were almost non cohesive and therefore able to fluidize the models were dynamically similar to their natural counterparts for as long as equilibrium was static when the processes became dynamic so that inertial forces were significant the scaling was approximate and the corresponding reynolds numbers differed the experimental apparatus was a square box 1 m 1 m wide resting on a grid of fluid diffusers during the experiments the fluid pressure increased until it attained and surpassed the weight of overburden flat lying hydraulic fractures containing air formed within cohesive and least permeable layers heterogeneities in material properties and layer thicknesses were responsible for localizing fracture networks when any one network broke through to the surface rapid flow of air through the fractures fluidized the underlying mobile materials and even depleted some of the layers some of the fluidized material extruded at the surface through vents forming volcanoes and sheets the remainder lodged at depth forming sand injectites or laccoliths conical sand injectites formed preferentially where layers had high resistance to bending laccoliths formed nearer the surface where overlying layers had low resistance to bending the experimental sand injectites were broadly similar to those in the tampen spur area of the north sea as well as other areas

fluid pressure and effective stress in sandbox models abstract we have done a series of simple tests on sandpacks involving upward flow of compressed air through the pores and its effect on the yield strength the ultimate objective is to model deformation coupled with fluid flow in sedimentary basins for all tests we used a single batch of fontainebleau sand sieved to a grain size between 0 200 and 0 315 mm poured into a cylindrical container and then fluidized the density of this sand was 1 585 g cm3 irrespective of sand thickness the lithostatic pressure was proportional to the thickness of the sandpack a yield envelope was obtained by shearing the sandpack horizontally compressed air entered the base of the sandpack and flowed upwards through the pore spaces for 69 measurements on air flow without shearing a plot of discharge velocity versus gradient of fluid pressure is close to a straight line verifying darcy s law and yielding an intrinsic permeability of about 1 7 darcy for the sand for 72 tests on simultaneous shearing and fluid flow the estimated effective stress lithostatic stress minus estimated pore fluid pressure ranged from 0 to 1600 pa and the pore fluid ratio between air pressure and lithostatic pressure from 0 0 to 1 0 a plot of shear stress versus effective stress at failure is almost linear verifying terzaghi s principle of effective stress the best fit slope coefficient of internal friction is about 0 55 and the intercept on the shear stress axis cohesion is less than 85 pa the tests show that it is feasible to use compressed air within sandpacks as a means of modelling deformation coupled with fluid flow the next step will be to build sandbox models of layered sequences and to investigate detachments caused by abnormal fluid pressures

sedimentologic palaeoecologic taphonomic and ichnologic criteria for high resolution sequence analysis a practical guide for the identification and interpretation of discontinuities in shelf deposits abstract we classify stratigraphic bounding surfaces defined by sedimentologic palaeoecologic or taphonomic and ichnologic criteria our case study refers to a particularly well preserved outcrop of miocene sedimentary rocks on the pacific coast of central costa rica these rocks mainly fine sandstones accumulated in a tropical shallow marine strongly subsiding sand swamped forearc coastal embayment sedimentary bounding surfaces or lithofacies bounding surfaces can be subdivided into erosional and non erosional boundaries between depositional systems in our case study erosional boundaries angular unconformities and erosional disconformities were produced by block tilting tectonic reactivation of cliffs or wave and tidal ravinement during sea level rises and are interpreted as sequence boundaries and bases of parasequence sets respectively non erosional transitions delimit genetically closely related depositional systems sharp contacts are considered as bases of parasequence sets diffuse transitions are interpreted as parasequences palaeoecologically and taphonomically defined fossiliferous bounding surfaces or layers mostly occur within depositional systems they seldom coincide with erosional lithofacies bounding surfaces two types can be distinguished autochthonous and allochthonous fossil concentrations autochthonous fossil concentrations can be grouped into layers with epifaunal and semi infaunal molluscs in growth position layers with infaunal bivalve associations in growth position and coprolith concentrations the majority of these layers is developed as concretion horizons autochthonous fossil concentrations invariably signal a relative rise of sea level and are interpreted as bases of parasequences or parasequence sets allochthonous fossil concentrations can be grouped into fossil concentrations produced by non turbulent shelf currents driftwood concentrations tempestite fossil concentrations fossil concentrations produced by near shore turbulent flow regimes and fossil concentrations produced by tidal currents they are only seldom developed as concretion horizons the appearance of allochthonous fossil concentrations in certain intervals of a given section and disappearance in others is clearly a signal of shallowing or deepening seas fossil concentrations produced by non turbulent shelf currents and driftwood concentrations can be interpreted as bases of parasequences ichnofabrics can be used as an additional tool for the identification and interpretation of bounding surfaces erosional boundaries may be marked by well known features such as gastrochaenolites sequence boundaries may be preceded by thalassinoides boxworks strongly thickened by concretionary growth at non erosional boundaries however trace fossils commonly fail to mark the crucial turning point yet they seem to record environmental changes for instance a coming transgression far earlier than any sedimentary phenomenon or body fossil after prolonged swamping with sand during a relative sea level lowstand they explode in diversity during the following transgression before this transgression produces its own typical sediment equally after the dumping of organic matter during the transgression trace making organisms announce the coming highstand by reworking their habitat to the degree of total destratification before the highstand deposits proper force them to keep up with aggradation which results in the typical highstand selection of deep tier structures thus the degree of reworking by bioturbation seems to be a very good and easily recognizable indicator of the internal architecture of parasequences and parasequence sets

tidal modeling of an ancient tide dominated seaway part 2 the aptian lower greensand seaway of northwest europe abstract the cretaceous lower greensand deposits of southeastern england and northern france contain abundant evidence of diurnal once daily paleotides with tidal influence increasing into the late aptian the imperial college ocean model icom is used herein to determine the amplitude origin astronomical or co oscillating and characteristics diurnal or semidiurnal of the tides in the shallow marine lower greensand seaway where these sediments were deposited a global ocean paleotidal model of the aptian predicts very low tidal ranges microtidal in the proto central atlantic ocean and boreal ocean with higher tidal amplitudes especially of the diurnal constituents predicted in the northwestern neotethys ocean this suggests that the neotethys ocean was the only significant source of oceanic tides to the lower greensand seaway herein the results of the global ocean paleotidal model are used to supply boundary conditions to a higher resolution regional paleotidal model of the lower greensand seaway the regional paleotidal model shows that the tidal range of the lower greensand seaway is largely controlled by the width of the connections to the open ocean as sea levels rose during the aptian the oceanic connections widened resulting in reduced frictional damping of the oceanic tidal wave coupled with funneling shoaling and coriolis effects this led to tidal amplification sensitivity tests using a variety of paleogeography and paleobathymetry scenarios suggest that the model predictions are robust the global to regional scale paleotidal modeling presented here strongly suggests that there was a connection through france in the aube region of the paris basin from the neotethys ocean to southeastern england however there are no time equivalent deposits in this region to validate this suggestion it is herein argued that that this is due either to postdepositional erosion or to low sediment supply coupled with strong tidal currents sweeping the seafloor to exposed bedrock

tidal circulation in an ancient epicontinental sea the early jurassic laurasian seaway we model tides and associated bed shear stress in the early jurassic laurasian seaway of northwest europe sensitivity tests with different water depths highlight those regions that are least affected by paleobathymetric uncertainty results show that although the vast seaway was largely microtidal the tides were still capable of affecting sediment transport more than 2000 km from the open ocean flow constriction associated with shallow platforms and straits produced elevated bed shear stresses a direct proxy for the entrainment and transport of sediment that were decoupled from tidal range and were capable of transporting sand areas of increased bed shear stress broadly correlate with published geological data varying the water depth in the basin provides an insight into the effects of an idealized transgression increasing the water level leads to a net reduction in the bed shear stress at the shelf edge but promotes further penetration of tidal energy into the basin interior although it is still focused into areas of flow constriction further drowning of the seaway eventually widens the straits to the point that they fail to constrict the flow and bed shear stress is reduced increasing water level counterintuitively leads to an increase in bed shear stress in a small number of locations

neoproterozoic earth moon dynamics rework of the 900 ma big cottonwood canyon tidal laminae the tidal rhythmites in the proterozoic big cottonwood formation bcc of utah the neoproterozoic elatina formation of the flinders range of southern australia and the lower pennsylvanian mansfield formation of indiana indicate that the rate of retreat of the lunar orbit has been approximately constant since the late precambrian and closely matches the present epoch value determined from apollo lasers examination of a new bcc core leads to a slightly revised length of day lod and modified values for other earth moon dynamical parameters thus implying the length of day 900 million years ago reported earlier by sonett et al 1996 should be modified to 18 9 hrs

melting and subsolidus reactions in the system k2o cao mgo al2o3 sio2 h2o experiments and petrologic application experiments with synthetic starting materials of muscovite phlogopite zoisite kyanite and quartz were performed in the pressure temperature range 10 25 kbar 640 780 c under water excess conditions the reaction muscovite zoisite quartz vapor liquid kyanite was bracketed at 10 5 kbar 689 700 c 15 5 kbar 709 731 c and 20 kbar 734 745 c the equivalent reaction in the mg bearing system muscovitess zoisite quartz vapor liquid kyanite phlogopitess lies at the same temperature around 10 kbar and approximately 10 c higher around 20 kbar compared with the mg free reaction at slightly higher temperatures formation of melt and tremolitess was reversibly observed from the assemblage phlogopitess zoisite kyanite quartz around 10 5 kbar 690 710 c 15 5 kbar 720 750 c and 20 5 kbar 745 760 c in the subsolidus region the reaction muscovitess talcss tremolitess phlogopitess zoisite quartz vapor were located in the range 700 c 16 7 19 0 kbar and 740 c 19 7 20 8 kbar from these data a wedge shaped stability field of phlogopitess zoisite quartz appears with a high p t termination around 21 kbar 755 c muscovite tremolite talc or kyanite comes in at higher pressures these phase relations are in qualitative accord with petrographic observations from high pressure metamorphic areas formation and crystallization of melts in rocks of a wide compositional range involving zoisite epidote has been ascribed to relatively high pressures and is consistent with experimentally determined stability fields in the simplified kcmash system

spatial and temporal evolution of coastal depositional systems and regional depositional process regimes campanian western interior seaway u s a abstract this paper provides a critical review and regional synthesis of late cretaceous shallow marine deposits along part of the western margin of the western interior seaway of north america which contains the most extensively documented outcrop based studies of siliciclastic coastal depositional systems in the world the results of this synthesis are presented in the form of paleogeographic maps covering present day new mexico utah colorado and wyoming usa for five timeslices in the campanian these maps are used to evaluate the spatial and temporal evolution of regional depositional process regimes along a large 1000 m stretch of coastline the evolution of regional depositional process regimes is linked to tectonic and paleoceanographic controls on the western interior seaway which enables the results of this synthesis to be applied to prediction of depositional process regimes in other less intensively studied basins six gross depositional environments have been mapped for each timeslice 1 alluvial to coastal plain sandstones 2 coastal plain coals mudstones and sandstones 3 shoreline sandstones 4 marine mudstones 5 gravity flow siltstones and sandstones and 6 marine marls and chalk shoreline sandstones in each timeslice are interpreted in further detail using documented evidence for the three principal classes of depositional process wave tidal and fluvial and published reconstructions of coastal morphology which is widely considered to reflect depositional process regime based on these interpretations shoreline sandstones are assigned to five categories of depositional process regime 1 regressive wave dominated shorefaces and delta fronts 2 regressive river dominated delta fronts 3 regressive mixed tide and wave influenced delta fronts 4 regressive tide dominated delta fronts and 5 transgressive barrier islands back barrier lagoons and estuaries the accuracy of and uncertainty in classification of depositional process regime are critically evaluated additionally stratal thickness and sediment routing pathways have been interpreted in order to assess the impact of tectonic and paleoceanographic controls on spatial and temporal changes in all of the evaluated shallow marine successions thin tide influenced intervals were deposited in back barrier and lagoon systems associated with net transgressive shorefaces and estuaries however it is notable that all preserved tide dominated and tide influenced regressive deltaic systems are located along the northern margin of the utah bight a tectonically related embayment that had a pronounced expression during the middle campanian the southern margin of the embayment and coastlines to the north are conspicuously more wave dominated which supports their exposure to a larger wave fetch in contrast the northern margin of the embayment was relatively wave protected it is concluded that tidal range was amplified due to resonance of the principal semidiurnal tide in the strongly embayed geometry of the middle campanian utah bight

a three dimensional model of diurnal and semidiurnal tides on the european shelf a three dimensional hydrodynamic model of the continental shelf is used to examine the sensitivity of computed diurnal k1 and o1 and semidiurnal m2 s2 and n2 tidal elevations and currents to changes in eddy viscosity profile friction coefficient and water depth the influence of the tide generating potential is also considered the model uses a standard finite difference grid in the horizontal with a functional approach in the vertical thereby giving a continuous tidal current profile from sea surface to seabed the magnitude of the vertical eddy viscosity depends upon the flow field comparisons are made between model computed profiles and harmonic analyses of 278 current time series at a range of locations in varying water depths with particular emphasis on locations where there are current measurements at a number of points in the vertical a detailed examination of these comparisons shows that frictional effects in the near bed region are particularly important in determining near bed shear and that an accurate specification of water depth is critical for tidal current magnitude the spatial distribution of computed current ellipses shows that the semidiurnal currents are strongest in shallow water with the diurnal currents exhibiting an intensification in the shelf edge region the inclusion of the equilibrium tide is particularly important for the correct determination of the magnitude of the k1 tidal current as well as k1 elevations in the north sea the detailed comparisons show that the magnitude of the tidal currents is correctly determined to within 5 cm s 1 at 220 locations with the model accurately to within 2 cm s 1 reproducing the vertical variation the most sensitive test of a three dimensional model of the tidal current at most locations

early to mid cretaceous tectonics and unconformities of the wessex basin southern england sediment distribution patterns time subsidence plots seismic data and outcrop analysis of the lower cretaceous ryazanian albian of the wessex basin southern england suggest that a number of unconformable horizons exist and that there is no single late cimmerian unconformity surface a tectonic change from areally restricted deposition in the weald and channel sub basins to extensive sedimentation across the whole of southern england began in the mid aptian this change ended with transgression of basement highs in the early albian of the dorset area and later albian of the devon and london platform basin margin areas its onset can be linked to the austrian tectonic phase of europe the onset of marine deposition in the aptian is coincident with periodic transgression of the margins of the wessex basin throughout the aptian albian mid late aptian and early mid aibian transgressions were preceded by periods of erosion the aptian albian lower greensand is characterized by transgressive regressive phases suggesting a short term controlling process third order eustatic changes or intra plate mechanisms tectonic changes in the mid aptian include uplift followed by widespread subsidence this change coincides with a time of plate reorganization in the european and north atlantic areas

earth s variable rotation recent improvements in geodetic data and practical meteorology have advanced research on fluctuations in the earth9s rotation theinterpretation of these fluctuations is inextricably linked with studies of the dynamics of the earth moon system and dynamical processes in the liquid metallic core of the earth where the geomagnetic field originates other parts of the earth9s interior and the hydrosphere and atmosphere fluctuations in the length of the day occurring on decadal time scales have implications for the topography of the core mantle boundary and the electrical magnetic and other properties of the core and lower mantle investigations of more rapid fluctuations bear on meteorological studies of interannual seasonal and intraseasonal variations in the general circulation of the atmosphere and the response of the oceans to such variations

experiments on shortening of a 4 layer model of the continental lithosphere davy ph and cobbold p r 1991 experiments on shortening of a 4layer model of the continental lithosphere in p r cobbold editor experimental and numerical modelling of continental deformation tecronophysics 188 l 25 we present an experimental method for modelhng lithospheric deformation the method is based on a simplified profile of lithosphere rheology where only four layers are considered brittle crust ductile crust brittle mantle and ductile mantle the strengths of the four layers calculated from laboratory determined flow laws depend strongly on geothenns and slightly on strain rates and deformation regimes the rheological profiles of the lithosphere have been recreated in experimental models built with analog materials sand silicone putty and golden syrup which flow under their own weight during lithospheric shortening the style of deformation depends on the relative strengths of the four layers especially those in the mantle of special importance is the existence or non existence of a brittle mantle the initial stages of compression often produce flexures or buckles without local isostatic compensation buckling exerts a strong control on the localisation of thrust faults in later stages the thrusts tend to form at the inflexion points of buckles the style of crustal thickening depends on the relative thicknesses and resistances of the lithospheric layers in 2 layer lithospheric models without resistant mantle shortening was concentrated upon a small number of thrusts widely spaced in 3 layer models shortening was concentrated into a large number of reverse faults more closely spaced and more steeply dipping than before in clayer models a single thrust formed in the brittle mantle motion on this thrust dragged the crustal layers down forming an asymmetric crustal root with steep imbricate slices

thrust wedges and fluid overpressures sandbox models involving pore fluids the well known model for the critical taper of an accretionary wedge includes overpressure as a first order parameter fluid overpressures reduce frictional resistance at the base of a wedge but they also act as body forces on all material particles of the wedge in addition to that of gravity by means of sandbox modeling many workers have tried to verify the predictions of the critical taper model but few of them have so far incorporated true fluid pressures we have used scaled experiments in which compressed air flows through sand packs so as to model the deformation of overpressured wedges a new apparatus provides for a horizontally varying fluid pressure for example a linear variation as in the critical taper model we have done three series of experiments involving horizontal shortening of homogeneous or multilayered sand models for various gradients of fluid pressure as predicted by the critical taper model the apical angle of the resulting wedge depends on the overpressure gradient in homogeneous sand at a high overpressure gradient deformation becomes diffuse and looks ductile in multilayered models detachments form beneath layers of low permeability so that thrusts propagate rapidly toward the undeformed foreland the efficiency of a detachment and its ability to propagate depend not only on the fluid pressure but also on the permeability ratios between the various layers

the neoproterozoic noonday formation death valley region california discussion petterson et al 2011 1 have made an important contribution to our understanding of the noonday formation and its association with neoproterozoic glacial events and they present some thought provoking insights into the depositional history implied by these rocks their work in the eastern

the structural evolution of the english channel area the structural evolution of the english channel area is controlled by structure and particularly by the pre existing cadomian and variscan crustal discontinuities which have been reactivated repeatedly in post variscan times they controlled the crustal subsidence that produced basin development in the mesozoic prior to the sea floor spreading in the north atlantic region they were then reactivated during the cenozoic compression and basin inversion the english channel development is ascribed to mid tertiary differential uplift oligocene to miocene during late tertiary to quaternary times the channel displays characteristics of a tectonically controlled fluvial basin periodically invaded by the sea at the lithospheric scale the channel can be considered as an active intraplate area influenced by the nw se alpine push the nw se atlantic ridge push and glacial rebound stresses copyright 2003 john wiley sons ltd

the role of tidal velocity asymmetries in the deposition of silty tidal rhythmites carboniferous eastern interior coal basin u s a abstract laminated to thin bedded siltstones directly overlie carboniferous coals at several localities in the eastern interior coal basin because lamina thicknesses show cyclical variations the siltstones have been termed tidal rhythmites the rhythmites commonly show a repetitive thick thin pairing of the laminae and these two lamina rhythmites have been interpreted as the result of the asymmetry of flood and ebb velocities that affected sedimentation in a diurnal paleotidal system alternatively the pairing has been interpreted to be related to a pronounced difference in height between successive high tides in a semidiurnal system this explanation suggests deposition in a mixed predominately semidiurnal system with a marked diurnal inequality in addition to the two lamina rhythmite described originally subsequent examination of drill cores indicates the occurrence of relatively rare more complex three and four lamina rhythmites a variety of mathematical techniques are used herein to analyze lamina thickness data extracted from these rhythmites processing techniques include 1 comparing the linear relationship between the thicknesses of the thinner and thicker laminae and comparing these relationships to modern tidal systems 2 analyses of thickness periodicities and comparison with modern neap spring tidal cycles and 3 extraction of lamina thickness inequalities and comparison to the diurnal inequalities that occur in semidiurnal tidal systems these tests indicate that reasonable conclusions can be made for the type of originating paleotidal system for the cases analyzed herein the paleotidal signature suggests a mixed predominantly semidiurnal system with a marked diurnal inequality

thermobarometry of garnet clinopyroxene hornblende granulites from the kapuskasing structural zone amphibole mixing properties and standard state properties for the end members tremolite tschermakite pargasite ferrotremolite ferrotschermakite and ferropargasite have been refined through analysis of relevant phase equilibrium experiments coexisting cummingtonite hornblende pairs and natural mineral parageneses when merged with the mineral properties given by r g berman these amphibole properties allow the calculation of numerous equilibria suitable for thermobarometry in a wide range of bulk compositions at amphibolite to granulite metamorphic grades the method was applied to texturally equilibrated garnet pyroxene hornblende gneisses and compared both to results based on anhydrous assemblages and to those for garnet biotite gneisses from the kapuskasing structural zone thermometer resetting is apparent only in garnet biotite gneisses whereas touching garnet pyroxene pairs in mafic gneiss show little evidence of retrograde fe mg exchange computed metamorphic temperatures garnet clinopyrox

deep marine tidal bottom currents and their reworked sands in modern and ancient submarine canyons abstract submarine canyons provide a unique setting for tidal processes to operate from shallow marine to deep marine environments in modern canyons current meter measurements at varying water depths 46 4200 m show a close correlation between the timing of up and down canyon currents and the timing of semi diurnal tides these tidal bottom currents in submarine canyons commonly attain maximum velocities of 25 50 cm s based on core and outcrop studies of modern and ancient deep marine deposits it is proposed here that sand mud rhythmites double mud layers climbing ripples mud draped ripples alternation of parallel and cross laminae sigmoidal cross bedding with mud drapes internal erosional surfaces lenticular bedding and flaser bedding can be used to interpret deposits of deep marine tidal currents this approach is an alternative to the conventional approach in which most deep water traction structures e g climbing ripples and cross bedding would be attributed to deposition from turbidity currents underwater photographs show active mass flows i e slides slumps grain flows and debris flows in modern canyons box cores taken from modern submarine canyons e g la jolla california and conventional cores and outcrops of ancient canyon fill facies oua iboe pliocene nigeria and the annot sandstone eocene oligocene se france contain deposits of both tidal processes and mass flows this facies association in the rock record can be used as a criterion for recognizing submarine canyon settings in a channel mouth environment deep marine tidal deposits are likely to develop elongate bars that are aligned parallel to the channel axis within the channel whereas turbidites are more likely to develop depositional lobes that are aligned perpendicular to channel axis turbidite depositional lobes are much larger than the channel width whereas tidal sand bars are much smaller than the channel width therefore the wrong use of a turbidite lobe model with sheet geometry in lieu of a tidal bar model with bar geometry will result in an unrealistic overestimation of sandstone reservoirs in deep water exploration

radial gravitational gliding on passive margins abstract gravitational gliding of uppermost sediments down a passive margin is possible if there is a basal layer of evaporite or other soft material to allow detachment in examples from the gulf of mexico and the brazilian margin gliding has produced three main structural domains an uppermost domain of downdip extension an intermediate domain of rigid gliding and a lowermost domain of downdip contraction domain boundaries are established by changes in slope in this paper we examine three kinds of gravitational gliding depending on the paths followed by material particles in ideal parallel gliding particle paths are parallel straight lines trending downslope this should occur where the margin is perfectly straight in ideal radial gliding particle paths are radii of a circle and the margin is shaped like a circular cone natural margins will not have ideal shapes but divergent gliding will tend to occur off coastal salients convergent gliding off coastal re entrants a simple kinematic model based on ductile behaviour illustrates some essential features of radial gliding changes in radius during divergent gliding produce strike parallel extension during convergent gliding they produce strike parallel contraction vertical strains also differ divergent gliding produces an uppermost domain of strong vertical thinning balanced by extensions in all horizontal directions similarly convergent gliding produces a lowermost domain of strong vertical thickening balanced by contractions in all horizontal directions these deformed states cannot be restored by simple techniques based on section balancing we have done three experiments using analogue materials sand to model the brittle behaviour of sediments silicone putty to model the ductile behaviour of basal layers of evaporite the experiments were properly scaled to account for gravitational forces experiment i reproduced convergent gliding above a basement with a conical upper surface strike parallel contraction was responsible for radial folds on the lower slopes the lowermost domain had a complex fold pattern indicating contraction in all horizontal directions experiment 2 reproduced divergent gliding above a basement with a pyramidal upper surface strike parallel extension produced radial rifts on the upper slopes the uppermost domain had a complex polygonal pattern of normal faults indicating extension in all horizontal directions experiment 3 also reproduced divergent gliding but due to non uniform sediment thickness we have examined the many seismic profiles both downdip and alongstrike available for the brazilian margin alongstrike profiles show structures which we attribute to strike parallel strains around the santos re entrant syn sedimentary growthfolds indicate strike parallel contraction increasing downslope this argues for a mechanism of convergent gliding in contrast around the campos salient normal growth faults and associated salt rollers indicate strike parallel extension decreasing downslope this argues for a mechanism of divergent gliding the pattern of oil fields in the campos area including the giant marlim field seems to be closely controlled by divergent gravitational gliding and associated processes radial gliding should occur on other passive margins as well especially where the coastline is sinuous structures around the western gulf of of mexico seem to indicate convergent gliding at the scale of the entire gulf structures at a smaller scale around the mississippi delta seem to indicate divergent gliding

metamorphic amphiboles composition and coexistence amphiboles have long been key players in metamorphic petrology the concept of facies was developed for rocks of basaltic composition eskola 1920 1939 fyfe et al 1958 and within this concept amphiboles had a leading role the first appearance of amphiboles the presence of amphiboles of certain compositions or last appearance of amphiboles that are replaced by pyroxene bearing assemblages mark a number of major facies boundaries the transition from greenschist to amphibolite facies is marked in part by hornblende replacing actinolite and the granulite facies is defined by the breakdown of hornblende to orthopyroxene bearing assemblages the blueschist facies is defined by the appearance of glaucophane while the transition from blueschist facies to eclogite facies conditions is indicated by the breakdown of glaucophane to omphacitic pyroxene bearing assemblages references to amphibole names actinolite glaucophane or rocks characterized by certain amphiboles amphibolite hornblende figure prominently in six facies names turner 1981 liou et al 1985 oh et al 1988 evans 1990 peacock 1993 a large part of the literature that is devoted to metamorphic petrology of amphiboles concerns reactions which produce amphiboles from more hydrated mafic igneous rocks or lead to the destruction of amphiboles as they break down to form anhydrous or nearly anhydrous mineral assemblages consequently amphiboles can be thought of as occupying the wide intermediate portion of p t xfluid space in which metamorphic rocks are found the number of citations of the published amphibole nomenclature serves as a yardstick meterstick of the interest in amphiboles and a minimum estimate of the amount of literature devoted to this mineral two versions of the nomenclature of amphiboles have appeared 1978 and 1997 leake 1978 leake et al 1997 and they have been cited a over 2300 times in the past 28 years this works out to a minimum

the miocene sommieres basin se france bioclastic carbonates in a tide dominated depositional system abstract the miocene sommieres basin in se france is a semi enclosed depression that was connected to the mediterranean sea by a flooded paleo incised valley and then filled by a suite of sediments comprising carbonate grains coming from temperate factories that were largely deposited in tidal dominated paleoenvironments the strata are partitioned into two sequences that reflect repeated flooding of the incised valley system one of several similar situations in this region of france the carbonate grains are mostly bioclasts namely from barnacles bryozoans coralline algae encrusting branching and rhodoliths echinoids and benthic foraminifers large and small with ostracods sponge spicules and planktic foraminifers prominent in muddy facies particles were produced by shallow water carbonate factories on hard substrates valley walls in particular associated with subaqueous dunes and in deeper water basinal settings each depositional sequence is underlain by an eroded and bored hard surface that is progressively overlain by tst subaqueous tidal dunes or storm deposits that grade up in one case into hst marls the hst of the upper sequence has been removed by erosion the lower sequence is ebb tide dominated whereas the upper sequence is flood tide dominated the succession is interpreted to represent a tst whose tidal currents were focused by the narrow valley and a hst that reflected flooding of the overbanks this stratigraphic and depositional motif is comparable to that in other spatially separated neogene paleovalleys that are filled with tide dominated clastic carbonates in the region together with other recently documented similar systems these limestones constitute an important new group of carbonate sand bodies in the carbonate depositional realm

the role of salt in fold and thrust belts davis d m and engelder t 1985 the role of salt in fold and thrust belts in n l carter and s uycda editors collision tectonics deformation of continental lithosphere tectonophysics 119 67 88 the style of deformation in thin skinned fold and thrust belts is critically dependent upon the resistance to sliding along the detachment between the mass of deforming sediments and the underlying rocks evaporites can provide an extremely weak horizon within which a basal detachment can form and along which only a relatively small shear traction can be supported fold and thrust belts that form atop a salt layer such as the appalachian plateau the franklin mountains in northwestern canada and the jura of the alps among others share several readily observable characteristics as predicted by a simple mechanical model for fold and thrust belts a detachment in salt permits a thrust belt to have an extremely narrow cross sectional taper in addition predicted orientations of the principal stress axes over a salt dtcollement are consistent with the commonly observed lack of a consistently dominant vergence direction of structures within the thrust belt other common attributes of salt basal thin skinned deformation include the presence of several widely but regularly spaced folds and abrupt changes in deformational style at the edge of the salt basin

lower cretaceous stage durations combining radiometric data and orbital chronology towards a more stable relative time scale abstract we propose an alternative calibration of lower cretaceous stage durations constrained by direct absolute dating of each stage combined with orbital chronology ten glauconitic horizons sampled in the vocontian basin se france from the base of the lower hauterivian to upper albian yielded k ar ages from 123 3 1 7 ma to 96 9 1 4 ma respectively the relative duration of each stage has been derived by cyclostratigraphy previously obtained in the south east france and central and south italy basins using the gl o standard from the albian cenomanian boundary at 95 3 ma as the anchor point a cyclostratigraphic age for each stage boundaries has been extrapolated and thus compared with the k ar ages this shows a very well defined linear correlation which demonstrates the robustness of the proposed durations of the lower cretaceous stages the estimated durations are 5 3 0 4 my 5 1 0 3 my 6 8 0 4 my and 11 6 0 2 my for the hauterivian barremian aptian and albian stages respectively it also shows that glauconite minerals are powerful radiochronometric tools when precisely stratigraphically defined and carefully selected moreover the large discrepancy of the estimated aptian duration of more than 6 my between the most recent published time scale and this study highlights the problem of the radiometric calibration of the m 0 magnetic chron finally the stage durations and boundary ages proposed here bring strong constraints towards the calibration of the lower cretaceous time scale such accurate temporal calibration is required before any relationship between major biological crises and magmatic emplacement for instance could be further investigated

modelling tidal current induced bed shear stress and palaeocirculation in an epicontinental seaway the bohemian cretaceous basin central europe lower to middle turonian deposits within the bohemian cretaceous basin central europe consist of coarse grained deltaic sandstones passing distally into fine grained offshore sediments dune scale cross beds superimposed on delta front clinoforms indicate a vigorous basinal palaeocirculation capable of transporting coarse grained sand across the entire depth range of the clinoforms ca 35 m bi directional alongshore oriented trough cross set axes silt drapes and reactivation surfaces indicate tidal activity however the bohemian cretaceous basin at this time was over a thousand kilometres from the shelf break and separated from the open ocean by a series of small islands the presence of tidally influenced deposits in a setting where co oscillating tides are likely to have been damped down by seabed friction and blocked by emergent land masses is problematic the imperial college ocean model a fully hydrodynamic unstructured mesh finite element model is used to test the hypothesis that tidal circulation in this isolated region was capable of generating the observed grain size distributions bedform types and palaeocurrent orientations the model is first validated for the prediction of bed shear stress magnitudes and sediment transport pathways against the present day north european shelf seas that surround the british isles the model predicts a microtidal to mesotidal regime for the bohemian cretaceous basin across a range of sensitivity tests with elevated tidal ranges in local embayments funnelling associated with straits increases tidal current velocities generating bed shear stresses that were capable of forming the sedimentary structures observed in the field the model also predicts instantaneous bi directional currents with orientations comparable with those measured in the field overall the imperial college ocean model predicts a vigorous tide driven palaeocirculation within the bohemian cretaceous basin that would indisputably have influenced sediment dispersal and facies distributions palaeocurrent vectors and sediment transport pathways however vary markedly in the different sensitivity tests accurate modelling of these parameters in this instance requires greater palaeogeographic certainty than can be extracted from the available rock record

contact zone interaction of metabasites with metapelites amphibolite facies mineral assemblages chemical profiles and their origin the puolankajarvi formation finland abstract the amphibolite facies iron rich metapelites garnet staurolite mica schists of the early proterozoic puolankajarvi formation are intercalated with metamorphosed basic rocks which vary from iron rich amphibolites to magnesium rich layered sills and serpentinites garnet cummingtonite hornblende assemblages are abundant at the contacts of the metapelites with amphibolites while garnet chlorite garnet gedrite chlorite garnet gedrite cummingtonite chlorite and chlorite cummingtonite hornblende assemblages occur at their contacts with the layered magnesium rich sill the bulk composition profiles of these contacts normalised with respect to chromium and or aluminium show conspicuous enrichment in fe s ga and co and depletion in si k ba and rb some elements e g mg mn ni p zr and hf are depleted or enriched in only small amounts while others e g ca na sr and the rare earth elements may show both enrichment and depletion in the course of a profile although early infiltration during the hydration of metabasites and tectonic reworking may have produced some interaction of the basites with the pelites the major metasomatic event is considered to have bile elements the composition of the metapelites for example approached that of magnesium rich ultramafites through potassium depletion while the ultramafites became markedly enriched in iron to conserve the volume silica was depleted from the contact which underwent iron enrichment

enhanced subsalt exploration utilizing the basal salt shear model abstract the basal salt shear model entails an initial salt sheet emplacement stage with salt extruding or intruding near the sea floor subsequent sediment loading upon the sheet drives salt withdrawal and suprasalt sediment deformation salt withdrawal occurs via pure shear within the salt sheet as sediments thicken over the salt sheet overpressures develop in a subsalt transition zone these anomalously high pore fluid pressures facilitate simple shearing beneath the sheets by reducing the effective normal stress thereby allowing lateral movement of the sheets and their overlying sediments with minimal force evidence supporting the basal salt shear model includes 1 lithologically distinct transition zones beneath salt sheets 2 overpressures in these transition zones 3 stratigraphic sections above salt repeated below salt 4 compressional features in front of salt sheets 5 thick sediment escarpments near salt sheet toes and 6 low velocity zones near salt sheet bases characteristics of the basal salt shear model significantly impact subsalt hydrocarbon exploration the overpressured shear zone may be a path for hydrocarbon migration a seal or a reservoir depending upon its local characteristics one may be able to estimate the hydrocarbon column height of a nearby reservoir in contact with the shear zone based on the detection of hydrocarbons in the shear zone also hydrocarbon reservoirs trapped against shear zones may have larger hydrocarbon columns and different updip limits than expected furthermore this model implies that subsalt reservoir sands may be younger and originate from a more basinward depositional environment than otherwise expected finally geophysicists should consider anomalous velocities near the base of salt sheets when depth migrating seismic data

evolution and hydrocarbon prospectivity of the brittany basin western approaches trough offshore north west france abstract the faulted preservation of a complete jurassic cretaceous succession in the brittany basin is primarily the result of early cretaceous rifting throughout the western approaches trough jurassic and cenozoic strike slip deformation has also contributed to the distribution of sedimentary successions now observed consequently the brittany basin has a different burial uplift history to the adjacent basins of the western approaches making the lias lower jurassic a mature source rock reservoir rocks are present but the unique deformation history has resulted in the late formation and possible breaching of structural traps the sedimentary record from the brittany basin allows us to observe the depositional effects of tectonic events occurring elsewhere in the western approaches trough these suggest middle jurassic early cretaceous late cretaceous and cenozoic uplift events each have different structural effects the origin of the most prominent intra mesozoic unconformity throughout the western approaches trough the early mid cretaceous is discussed with reference to causes of juxtaposed uplift and renewed rifting

the role of pore fluid overpressure in the substrates of advancing salt sheets ice glaciers and critical state wedges critical state wedges ice glaciers and salt sheets have many geometric and mechanical similarities each has a tapering geometry and moves along a basal detachment their motions result from the combined effects of internal deformation and basal sliding wedge deformation and geometry basal conditions and overpressure pore fluid pressure less hydrostatic pore fluid pressure development within the substrate interact with each other in this mechanically coupled system however the nature of this interaction is poorly understood in order to investigate this coupled system we have developed two dimensional poromechanical finite element models with porous fluid flow in sediments we have simulated the advance of a salt sheet wedge across poroelastic sediments in this study we emphasize that our results have applications beyond salt wedges to both critical state wedges and ice glaciers overpressure develops within the substrate over time during the advance of the wedge the magnitude of the overpressure influences the wedge geometry and the wedge advance rate lower overpressure results in a thicker and steeper wedge geometry and a slower advance rate while higher overpressure favors a thinner wider and more flattened wedge geometry and a faster advance rate this study provides key insights into the links between wedge geometry basal shear stress and overpressure in substrates

sequence stratigraphic analysis of the aptian albian lower greensand in southern england abstract facies analysis and its use in sequence stratigraphy is described using the lower greensand of southern england as an example and the resulting interpretations are outlined this work forms an independent test of sequence stratigraphic techniques and the exxon cycle chart sequence stratigraphy is a very powerful tool in the analysis of problematic successions although some modification is made here in emphasizing the use of the transgressive surface changing palaeoenvironmental conditions and the effect that palaeoceanography has on the recognition of sequence boundaries are also discussed broad agreement is found between these results and the cycle chart although changing tidal conditions alter the criteria available for analysis of the aptian albian lower greensand succession comparison between this study and sequence stratigraphic analyses conducted in the more condensed type sections of england and france shows that further sequence boundaries are recognizable

a three dimensional model of the northwest european continental shelf with application to the m4 tide abstract the solution of the fully nonlinear three dimensional hydrodynamic equations in polar coordinates for tides is developed using a finite difference grid in the horizontal and the galerkin method in the vertical using this method a three dimensional model of the northwest european continental shelf is used to investigate the spatial distribution of the m4 tidal elevations and currants over the shelf computed m4 currents are compared with observations and found to he in good agreement in the celtic sea although there are significant errors in the north sea the reasons for these discrepancies and the spatial and depth variations in the m4 currants are discussed in detail

singular equilibria in paragonite blueschists amphibolites and eclogites chemographic constraints from high pressure mafic rocks of the introduction nevado filabride complex betic rif mountain belts se spain the relevance of singular equilibria has been largely and from worldwide high pressure terrains have been used to unravel recognized in chemical systems of interest for the earth phase relationships between amphibole garnet clinopyroxene and sciences after the early experiments of konovalov on albite in the system na2o cao feo mgo al2o3 sio2 h2o azeotropic or congruent transformations ricci 1951 ncfmash at quartz paragonite clinozoisiteand waterexhaustively discussed the fundamentals of singularities saturated conditions extensive analysis of element partitioning two types of singularities were recognized the first between amphibole and clinopyroxene suggests that the fe mg called type m and most relevant is associated with azeopartition coefficient is low at low temperature and high pressure tropic points and the second called type k is related to conditions high temperature blueschists low temperature eclogites the intersection of solvi with compositional loops as and a progressive increase is achieved by an increase in temperature occurs in the alkali feldspar system at low pressures or by a decrease in pressure or by a decrease in the sodium content morse 1970 examples and discussion of both type m of the clinopyroxene mass balance analysis of paragonite eclogites and type k singular points in igneous systems have been and amphibolites suggests that at least two singular reactions are given by morse 1980 e g see chapters 16 and 18 the possible in ncfmash clinopyroxene paragonite quartz widespread occurrence of singularities in igneous systems amphibole albite clinozoisite water garnet absent and is obviously related to the extremely large compositional amphibole clinozoisite garnet clinopyroxene paragonite range of silicate liquids the well known concept of quartz water albite absent albite absent singular equithe thermal barrier or thermal divide implies the librium is relevant for the blueschist eclogite transition whereas occurrence of singular equilibria and their importance garnet absent equilibrium may control the eclogite to amphibolite in the petrogenesis of granites e g modelled in the binary transition a complex arrangement of divariant reactions singular system kalsilite silica lindsley 1966 or basalts e g in reactions and conjugate non degenerate reactions arises in topologies the binary system forsterite silica or in the ternary system at the blueschist eclogite amphibolite transitions schreinemakers forsterite silica diopside or forsterite silica anorthite analysis in p t diagrams and mixed variable t xm xc and presnall et al 1979 has been widely discussed p xm xc diagrams suggest that complex pressure temperature in contrast the significance and implications of singular paths are not mandatory if a recurrent appearance and disappearance point nets at subsolidus conditions have been largely of similar phase assemblages is observed in paragonite bearing mafic undervalued although korzhinskii 1963 analysed azeorocks the p t location of the singular curve net is discussed in tropic transformations in subsolidus open systems only the context of nevado filabride complex mafic rocks recently have applications to metamorphic assemblages

gulf of mexico subsalt deformation zones weak formation integrity rather than anomalous high pressure zones abstract a study of pressure gradient profiles mudweights and drilling report data from subsalt wells in the gulf of mexico indicates that reported shale deformation zones underneath salt are not anomalously high pressure zones but rather low formation integrity zones pore pressure gradients were calculated from shale transit time and resistivity logs and supplemented by detailed drilling reports and available wireline pressure test data for 23 subsalt wells located in the shelf and deepwater areas of the central gulf of mexico the study indicates those pore pressure gradients beneath salt sheets remain constant from deformation zones into the deeper subsalt section and are moderate to high 16 5 20 kilopascals meter kpa m 14 17 pounds gal mud weight ppg but not anomalously high for the depth and age of the subsalt section unusually low shale resistivities found in sections beneath salt indicate higher pore pressure gradients but sonic values from the same intervals do not corroborate the higher pressures suggesting that the low shale resistivities may be caused instead by highly saline formation brines lost circulation and well flow problems encountered when drilling out of salt indicate that some subsalt deformation zones have low formation integrity results of this study do not support current subsalt exploration models that evoke hydrocarbon bearing fluids migrating upward to subsalt reservoirs through an undercompacted high pressure shale deformation zone instead some subsalt sections appear to be unstable zones of lower fracture strength with implications for both sealing capacity and subsalt well design new models are needed to predict pressure and fracture gradient in subsalt deformation zones to evaluate migration and seal risk and ensure better design of subsalt wells

allochthonous salt structure and stratigraphy of the north eastern gulf of mexico ii structure the kinematic evolution of allochthonous salt in the north eastern gulf of mexico proceeds in three stages 1 since its middle jurassic deposition the louann salt was loaded by sediments causing episodic basinward movement of salt ultimately leading to large concentrations of salt masses in a slope environment by the end of the lower cretaceous 2 a regime of starved sedimentation during late cretaceous and early oligocene is responsible for the stabilization of these early salt accumulations 3 with renewed rapid accumulation of sediments during the neogene and pliocene extensive allochthonous salt tongues and sheets formed by gravity spreading within the younger sediments of the slope

sandbox experiments on gravitational spreading and gliding in the presence of fluid overpressures whereas in previous analogue experiments on gravitational spreading and gliding detachment occurred on a ductile layer we have used a relatively new technique of injecting compressed air into sand packs so as to simulate the effects of fluid overpressures in sedimentary strata and to trigger slope instabilities in our experiments the governing equations yield scales for dimensions stresses and fluid pressure however the more transitory phenomena of production and decrease of overpressure cannot be suitably scaled by using layers of differing permeability we are able to produce sharp detachments in models made of sand alone the experiments involve gravity spreading or gravity gliding in gravity spreading propagation of the detachment and of extensional deformation depends on the fluid pressure for medium values of fluid overpressure normal faults are closely spaced numerous and bound rotated blocks they propagate progressively toward the back of the model for the highest pressures the deformation propagates very fast and faults bound non rotated blocks which slide on an efficient basal detachment fault dips are also controlled by fluid pressure and by frictional resistance at the base to model gravitational gliding required an apparatus with a more complex system of air injection we did a series of experiments using injection windows of various lengths and compared the results with predictions from a quasi 3d analytical model of sliding in contrast with predictions for an infinite slope sliding depends on 1 the fluid overpressure on the detachment 2 the fluid overpressure in the body of the sliding sheet and 3 the shape of the detachment surface in particular we show that frictional resistance at the lower edge is a primary control on the dynamics of gliding

a two dimensional tidal model for the mediterranean sea the tidal propagation in the mediterranean sea is described through a high resolution two dimensional hydrodynamic model forced by the equilibrium tide and the incoming tide at the strait of gibraltar the four most significant constituents m2 s2 k1 and o1 are included in the model good agreement with a set of 63 coastal gauges is achieved the significance of the equilibrium tide and the forcing at the open boundary is investigated the incoming wave from the strait of gibraltar is important in tuning the tides in the whole of the mediterranean for the north aegean sea the solution without the forcing at gibraltar results in a doubling of the amplitudes of the semidiurnal tides the estimated energy dissipation due to bottom friction is 8 8 108 w the major area of dissipation is the gulf of gabes energy fluxes through the major straits are calculated

lower cretaceous tides revealed by cross bedding with mud drapes the sand dominated folkestone beds widely exposed around the weald of south east england are of upper aptian to lower albian lower cretaceous age and range in thickness up to 84 m refs 1 3 their content of ammonites bivalves and sponges indicates a marine origin while their relative coarseness and the incorporation of drifted timber point to a shallow water deposition near land one striking sedimentary facies is formed by medium to coarse grained cross bedded sands in which several foreset and or bottomset surfaces support mud drapes although it was suggested4 that this facies may have been formed by the storm induced movement of sand waves i now consider that the facies is best explained by the migration of sand waves under the influence of strongly asymmetrical tides and that the mud drapes are formed at slack water a model of this process has been described elsewhere5 and i now describe measurements of a section which supports my earlier conjecture6that the separation of the mud drapes is systematically related to the phase of the spring neap tidal cycle at the time of deposition although criteria for recognizing tidal deposits have long been available it has not previously been possible to analyse a series of ancient sediments with a view to characterizing the tidal regime

physical modelling of chemical compaction overpressure development hydraulic fracturing and thrust detachments in organic rich source rock abstract geological evidence for overpressure is common worldwide especially in petroleum rich sedimentary basins as a result of an increasing emphasis on unconventional resources new data are becoming available for source rocks abnormally high values of pore fluid pressure are especially common within mature source rock probably as a result of chemical compaction and increases in volume during hydrocarbon generation to investigate processes of chemical compaction overpressure development and hydraulic fracturing we have developed new techniques of physical modelling in a closed system during the early stages of our work we built and deformed models in a small rectangular box 40 40 10 cm which rested on an electric flatbed heater but more recently in order to accommodate large amounts of horizontal shortening we used a wider box 77 75 10 cm models consisted of horizontal layers of two materials 1 a mixture of equal initial volumes of silica powder and beeswax micro spheres representing source rock and 2 pure silica powder representing overburden by submerging these materials in water we avoided the high surface tensions which otherwise develop within pores containing both air and liquids also we were able to measure pore fluid pressure in a model well during heating the basal temperature of the model surpassed the melting point of beeswax 62 c reaching a maximum of 90 c to investigate tectonic contexts of compression or extension we used a piston to apply horizontal displacements in experiments where the piston was static rapid melting led to vertical compaction of the source layer under the weight of overburden and to high fluid overpressure lithostatic or greater cross sections of the models after cooling revealed that molten wax had migrated through pore space and into open hydraulic fractures sills most of these sills were horizontal and their roofs bulged upwards as far as the free surface presumably in response to internal overpressure and loss of strength of the mixture we also found that sills were less numerous towards the sides of the box presumably as a result of boundary effects in other experiments in which the piston moved inward causing compression of the model sills also formed however these were thicker than in static models and some of them were subject to folding or faulting for experiments in which we imposed some horizontal shortening before the wax had started to melt fore thrusts and back thrusts developed across all of the layers near the piston producing a high angle prism in contrast as soon as the wax melted overpressure developed within the source layer and a basal detachment appeared beneath it as a result thin skinned thrusts propagated further into the model producing a low angle prism in some experiments bodies of wax formed imbricate zones within the source layer thus in these experiments it was the transformation from solid wax to liquid wax which led to chemical compaction overpressure development and hydraulic fracturing all within a closed system according to the measurements of overpressure load transfer was the main mechanism but volume changes also contributed producing supra lithostatic overpressure and therefore tensile failure of the mixture

mid cretaceous onlap history of the market weighton structural high northeast england the mid cretaceous sediments of northeast england were deposited at the western margin of the southern north sea basin with sedimentation occurring in a range of tectonic settings detailed analysis of the areal distribution and sedimentary facies of aptian to earliest cenomanian sediments has allowed the pattern of onlap onto the market weighton structural high and changes in relative sea level to be documented successive onlap episodes during the early aptian late aptian and early albian culminated in the final flooding of the structure during the late albian varicosum subzone sea level curves generated from coastal onlap patterns are difficult to relate to published global sea level curves due to the high frequency of the fluctuations in relative sea level observed despite this detailed correlation and analysis of sedimentological events suggest that even the most expanded basinal succession is relatively incomplete this study has also shown that the change from dominantly syn tectonic to dominantly post tectonic sedimentation style occurred in the late early albian

precambrian length of day and the validity of tidal rhythmite paleotidal values paleotidal records obtained from sedimentary tidal rhythmites may be systematically abbreviated and so may give incorrect paleotidal and paleorotational values the validity of determined values including past length of day l o d can be assessed by testing for internal self consistency through application of the laws of celestial mechanics three independent values obtained from the 620 ma elatina reynella rhythmites in south australia 14 1 sidereal months year 401 sidereal days year and 19 5 years for the lunar nodal period when employed in different equations that make allowance for lunar and solar tidal effects each give a lunar semimajor axis in the range of 96 5 96 9 of the present figure such self consistency strongly supports the validity of the derived l o d of 21 9 hours at 620 ma the validity of the estimated l o d of 20 9 hours at 900 ma revised value big cottonwood rhythmites utah and of 17 1 18 9 hours at 2 5 ga weeli wolli rhythmites western australia cannot be assessed in that way because each data set has only one directly determined value the derived mean rate of lunar retreat of 2 16 cm year since 620 ma averts a close approach of the moon at least since 3 ga and a lower rate of retreat seems likely during the proterozoic

a three dimensional model of the principal tides on the european shelf abstract a three dimensional hydrodynamic model of the continental shelf is used to examine the spatial distribution both elevations and currents of 28 of the major tides in the region tidal constituents in the diurnal and semi diurnal band together with higher harmonics and long period tides are used to illustrate the magnitude and spatial variability of the tides over the shelf comparisons are made between model computed profiles and harmonic analyses of 278 current time series at a range of locations in varying water depths with particular emphasis on locations where there are current measurements at a number of points in the vertical observed and computed tidal elevation amplitudes and phases are also compared at up to 257 shore based and off shore tide gauges these detailed comparisons are used to determine any bias in the model and the accuracy of predicted elevations and currents derived with the model results of these comparisons show that although the diurnal and semi diurnal tidal elevations and currents can be accurately reproduced using a 12 km grid the error in the higher harmonics is significantly larger the fact that the model contains all the non linear terms advection quadratic friction wave drift time varying viscosity necessary to generate these harmonics together with a full set of tidal harmonics in order to have the correct level of friction suggests that potentially it should reproduce these constituents however a rigorous comparison was found to be very difficult because of the rapid variation of the higher harmonics in shallow water which cannot be adequately resolved in the model and possible inaccuracy in the measurements of the small currents associated with the higher harmonics

shelf sheet sand reservoir of the lower cretaceous greensand north celtic sea basin offshore ireland core and log data show that the marine early to middle albian a sand of the aptian to lower cenomanian greensand gault interval north celtic sea basin offshore ireland was deposited as an approximately tabular sand body in shelf water depths the a sand is the major reservoir interval at kinsale head and ballycotton gas fields the reservoir sandstone is bioturbated variably glauconitic shell rich and least muddy toward its provenance in a local area of the irish massif and finer grained southeastward into the basin thickness and coarseness of the a sand are related in part to distance from a narrow area of the paleoshore bathymetric control of sand thickness was superimposed on proximal distal trends specifically thick intervals are inferred to have be n deposited in shelf lows and thin zones were deposited over bathymetric highs the a sand was not deposited as a ridge sand and positive relief on the depositional sand body appears to have been minor deposition of the greensand occurred during thermal subsidence of the north celtic sea basin following early cretaceous rifting overall late early cretaceous to late cretaceous transgression was interrupted by progradation of the a sand probably caused by a relative drop in sea level forced regression the a sand sheet consists of several units within the central depositional basin three slightly coarsening upward units up to 15 m thick probably are the consequence of high frequency fluctuations in sea level during progradation the top of the a sand consists of a several m ter thick very glauconitic muddy sandstone to sandy mudstone the upper unit accumulated in deepening water following the maximum lowstand

measurements of friction coefficients and cohesion for faulting and fault reactivation in laboratory models using sand and sand mixtures abstract simple laboratory tests have been made on dry quartz sand and on mixtures of sand and clay and sand and cement tests were made on both low and high density specimens of each mixture as determined by physical handling techniques the tests included measurements of density estimates of the coefficient of internal friction based on fault orientation and estimates of the friction coefficients and cohesion values for fault initiation and reactivation in a shear frame apparatus which was also used to test artificial fault zones made from pure clay glass microspheres and wheat flour the results suggest that density is determined more by handling technique than by composition and that friction coefficients and perhaps cohesion both increase with increasing density the shear testing revealed a consistent drop in cohesion for fault reactivation in all densely packed mixtures tested although small this drop may be significant for some types of laboratory models

late proterozoic and paleozoic tides retreat of the moon and rotation of the earth the tidal rhythmites in the proterozoic big cottonwood formation utah united states the neoproterozoic elatina formation of the flinders range southern australia and the lower pennsylvanian pottsville formation alabama united states and mansfield formation indiana united states indicate that the rate of retreat of the lunar orbit is dxi dt approximately k2 sin 2delta where xi is the earth moon radius vector k2 is the tidal love number and delta is the tidal lag angle and that this rate has been approximately constant since the late precambrian when the contribution to tidal friction from the sun is taken into account these data imply that the length of the terrestrial day 900 million years ago was approximately18 hours

allochthonous salt structure and stratigraphy of the north eastern gulf of mexico part ii structure abstract the kinematic evolution of allochthonous salt in the north eastern gulf of mexico proceeds in three stages 1 since its middle jurassic deposition the louann salt was loaded by sediments causing episodic basinward movement of salt ultimately leading to large concentrations of salt masses in a slope environment by the end of the lower cretaceous 2 a regime of starved sedimentation during late cretaceous and early oligocene is responsible for the stabilization of these early salt accumulations 3 with renewed rapid accumulation of sediments during the neogene and pliocene extensive allochthonous salt tongues and sheets formed by gravity spreading within the younger sediments of the slope autochthonous salt allochthonous salt and detached allochthonous salt are typical stages of evolution major down to the basin growth faults separate allochthonous salt sheets from their original feeder stocks extension along down to the basin master growth faults is compensated mainly by salt withdrawal and partly by basinward shortening

salt tectonics driven by sediment progradation part ii radial spreading of sedimentary lobes prograding above salt experimental models indicate that sedimentary lobes deposited above mobile evaporites deform gravitationally by spreading outward radially radial spreading forms both concentric and radial families of thin skinned grabens that are then pierced diapirically by salt ridges the overburden blocks located between the ridges become subsiding depocenters later continued spreading after depletion of the salt layer forces salt ridges and diapirs to fall forming rapidly subsiding bathymetric lows that channel or trap younger sediments spreading causes large horizontal overburden movements whose distribution and orientation change when the locus of regional deposition shifts

large sea small tides the late carboniferous seaway of nw europe debate surrounds the extent of tidal influence in palaeozoic shallow epi continental seas in the absence of analogical reasoning numerical modelling provides a quantitative means of investigating tidality in the geological record herein a new finite element model tested for accuracy on the present day mediterranean is used to predict an exceedingly low tidal range cm in the epi continental seaway that covered much of nw europe during the late carboniferous this small bulge may have been amplified to c 1 m in estuaries leading to the localized deposition of cyclic rhythmites agreeing with geological observations extremely low tidal ranges in ancient epi continental seas may be one mechanism to prevent water body mixing enhancing stratification and promoting anoxia

european cretaceous shorelines stage by stage abstract inferred shorelines are presented for the twelve cretaceous stages for the area of europe and adjacent seas contained between approximately 35 and 65 n and between 15 w and 25 e there is a brief stage by stage commentary and a bibliography of source materials

palaeotidal speeds and ranges estimated from cross bedding sets with mud drapes tidally formed cross bedding sets in which mud drapes are spaced out among sandy foresets occur in cretaceous1 4 eocene5 and holocene6 sands deposited subtidally by strongly asymmetrical sand waves a striking feature of some sets4 6 is that the horizontal drape spacing measured in the direction of sand wave advance varies cyclically on a time scale related to the spring neap tidal cycle as indicated by the numbers of drapes each drape forms part of a distinctive lithological couplet consisting of a group of sandy foresets recording avalanching on the steeper side of the sand wave followed by the drape itself typically divided into upper and lower parts by a centrally placed silt sand parting the drapes have been assigned to slack water but the groups of sandy foresets to episodes of vigorous flow the combination suggesting a quantitative model based on the tide here i show how the peak speeds and ranges of ancient tides can be estimated by combining an empirical bedload transport function with appropriate measurements from such cross bedding sets

recognition and documentation of depositional sequences from outcrop an example from the aptian and albian on the eastern margin of the wessex basin stratigraphic data relating to the aptian and albian sediments around folkestone in southeast england on the eastern margin of the wessex basin are plotted graphically to illustrate biostratigraphic gaps and condensed intervals these are then used to suggest candidate sequence boundaries notated lg1 4 and gl 3 which are named after the lower greensand and gault the biostratigraphic subzones apparently unrepresented in the area are lg1 obsoletus and bodei lg2 gracite and debile lg3 schrammeni gl bulliensis candidate sequence boundaries g2 cristatum subzone and g3 auritus subzone are erosion surfaces correlating with probable biostratigraphic gaps elsewhere in the basin the age of lg4 is uncertain but must lie within the mammillatum superzone all candidate sequence boundaries correspond closely to those predicted by recent global sea level cycle charts and additionally there is a close match between predicted maximum flooding surfaces and the occurrence of condensed intervals most of the candidate sequence boundaries could have been formed as a result of rapid and at least regionally extensive relative sea level falls but the sedimentological contexts of lg3 and gl suggest that these may have been generated by transgression related sediment condensation the minor sequence boundary in the loricatus zone intermedius subzone illustrated in the cycle charts cannot be unambiguously recognized at outcrop in the folkestone area additional sequence boundaries not shown in the cycle charts may also be present in the wessex basin

chlorine titanium and barium rich biotites factors controlling biotite composition and the implications for garnet biotite geothermometry barium cl and ti rich biotite occurs together with garnet plagioclase and amphibole within narrow shear zones in 1800 ma old noritic granulites in the flakstadoy basic complex lofoten north norway the granulite facies assemblage plagioclase clinopyroxene orthopyroxene biotite and ilmenite was replaced by an amphibolite facies mineral assemblage including ba cl and ti rich biotite during ductile deformation biotite shows complex compositional variations with respect to the contents of ba k cl ti al fe mg and si there are correlations between si aliv k ba and cl and between alvi and ti titanium and cl are uncorrelated the fe and mg are correlated to both cl and ti multivariate analysis shows that most of the compositional variation of biotite can be described by two exchange reactions this indicates that most of the variation in biotite composition was controlled by two chemical variables of the system the content of the first exchange component ti1 0 fe0 6 al 1 1 vi mg 0 8 in biotite can be related to the original distribution of ti bearing minerals in the igneous protolith the content of the second exchange component al 0 4 iv fe0 8 ba0 5 cl1 0 si 0 4 mg 1 0 k 0 5 oh 1 0 is related to compositional variations of an externally derived ba and cl bearing fluid in equilibrium with biotite the initially low cl content of the externally derived fluid was increasing during bioite forming reactions because oh was preferentially incorporated relative to cl into biotite continued hydration chloridisation reactions resulted in a gradual consumption of the free fluid phase resulting in local fluid absent conditions the composition of biotite reflects the composition of the last fluid in equilibrium with the mineral i e the composition of the fluid immediately before the grain boundaries were fluid undersaturated thus the variations in biotite composition reflect how the fluid was gradually consumed throughout the shear zone rock the correlations between fe mg ba k and cl can be attributed to differences between the structure of the crystal lattices and the sizes of the cation sites of oh phlogopite and cl annite the dependency of the fe mg ratios of biotite on the cl and ti content has a strong effect on the fe mg partitioning between biotite and garnet the relationship between lnkd x ti bt and x cl bt can be expressed by the regression equation lnk d 1 82 2 60x ti bt 5 67x cl bt

mechanism of thin skinned detachment in the amazon fan assessing the importance of fluid overpressure and hydrocarbon generation abstract since about 10 ma a wedge of clastic sediment up to 10 km in thickness has accumulated near the mouth of the amazon river forming the amazon fan thin skinned extensional and contractional structures root into a common basal detachment hundreds of kilometers long extensional faults have formed landward of the prograding delta slope and contractional folds and faults have formed seaward of it by plan view restoration we show that extension and contraction balance we therefore infer that slope instability has provided the driving force for thin skinned deformation on seismic data the detachment appears as a sharp surface within late cretaceous strata however there is no evidence at that level for soft layers such as evaporites or muds abnormal fluid pressure provides a more reasonable mechanism for the detachment at depths of several thousand meters deep wells have encountered fluid overpressures amongst the possible causes of abnormal fluid pressures are compaction and hydrocarbon generation although compaction may have contributed to fluid overpressures encountered in wells we argue that hydrocarbon generation provides the best explanation for detachment in cretaceous rocks which were old and lithified when it occurred in particular the broadly triangular area of detachment coincides with an inferred pod of cenomanian turonian source rock which is interpreted to have been in the gas window during detached deformation we have developed new techniques of sandbox modelling to investigate the geometries of fault patterns produced by fluid flow through porous media for the amazon fan we have been able to reproduce the structural pattern on the assumption that fluid overpressures existed over a broadly triangular area beneath the detachment

firmgrounds key surfaces in the recognition of parasequences in the aptian lower greensand group isle of wight southern england aptian lower greensand group exposures in the cliffs of the isle of wight southern england display a consistent coarsening up cyclicity on the scale of centimetres to tens of metres that reflects the bed bed set parasequence parasequence set and sequence hierarchy these coarsening up cycles are most commonly recognized at the scale of parasequences 20 cm to 10 m thick genetically related groups of which form parasequence sets both parasequences and parasequence sets contain the succession of biofacies that culminate in firmground development numerous episodes of erosion deposition and colonization are recorded reflecting multiple erosion bypass events the increase in mean grain size through each cycle is reflected by changes in physical sedimentary structures ichnofauna or bioturbational fabric fossil fauna and diagenesis interbedded mudstones siltstones and sandstones in the lower beds of each cycle display a variety of structures ranging from low angle hummocky or tabular cross strata sandstone filled erosional gutters and planar lamination the cleaner sandstones found in the upper parts to each cycle are often completely bioturbated with only rare stratification and pebble plant debris filled scours preserved bioturbational fabrics change upward through each cycle from small subhorizontal mud or sandstone filled burrows to large branching clay filled or cemented burrow systems the top surface of each cycle is marked by a fossil epifauna indicative of firm to hard substrate conditions concentrations of bivalves brachiopods bryozoa crinoids and corals are preferentially cemented by iron oxide carbonate or phosphate such cements were early and thus utilized by firm or hard substrate dwellers this fossiliferous cemented sandstone is overlain by a flooding surface marked by the mudstone and silt rich sandstones at the base of the next cycle together the fauna and ichnofauna in each cycle represent the gradual development of firm substrate conditions culminating in the diverse firmground fauna preserved at the top of each cycle the fauna and changing substrate conditions reflect the hiatuses developed during successive episodes of marine flooding high species diversity is matched by complex patterns of taphonomic feedback in the mature firmground faunas that mark major flooding surfaces increasing faunal maturity allows recognition of a hierarchy of hiatuses this hierarchy is analogous to the parasequence parasequence set division the stratigraphic condensation of firmgrounds can be used to empirically define the condensed section the thickness of sediment between firmgrounds being a function of sediment supply and water depth accommodation space

variscan structure and its influence on post carboniferous basin development western approaches basin sw uk continental shelf reflection seismic records from the western approaches basin sw uk continental shelf reveal sse dipping reflection packages within the variscan basement these have been mapped and are believed to originate from variscan thrusts orthogonal transfer faults are apparent as nnw trending zones of discontinuity structural trends in the permian mesozoic western approaches basin parallel those of the underlying basement an end variscan structural high in the northern part of the basin was progressively onlapped from the south during permian times this high may have been formed by thrust driven tectonic thickening of the crust during late jurassic early cretaceous times variscan thrusts were reactivated in the western part of the basin melville basin to form a half graben however in the east st mary9s basin the basin is more symmetrical and mesozoic reactivation of variscan thrusts cannot be unequivocally demonstrated

transgressive changes from tidal estuarine to marine embayment depositional systems the lower cretaceous woburn sands of southern england and comparison with holocene analogs the lower cretaceous woburn sands lower greensand group in southern england constitutes one of the most intensively studied tidal sandstone outcrops for sedimentological and reservoir analog studies most recent workers have interpreted the whole 30 60 m 100 200 ft thick succession around leighton buzzard as representing an ancient tide dominated estuary however unequivocal estuary characteristics are limited to the lowermost part about 15 20 m 50 66 ft we suggest that a significant portion of the woburn sands and most of the middle part was formed in a tide dominated marine embayment hence the vertical facies change from the lower to middle part of the woburn sands is interpreted as a change from 1 a narrow estuary to 2 a broad marine embayment the wash embayment in eastern england is a striking modern analog it receives most of its sediments and waters from marine sources and is largely filled with nondiluted seawater moreover the holocene transgressive history of the wash is remarkably similar to the transgressive evolution of the woburn sands early estuarine sequence models predict landward translation of facies zones along the valley thalweg during transgression but eventual facies translation in the strike direction has not been fully documented or discussed an embayment facies that is not commonly confined in the early incised valley can occur vertically between the estuarine valley fill and marine shelf deposits and is probably underrepresented in current models however this segment of the transgression comprising along strike bay expansion and the development of a broad marine embayment may be more important than previously thought

physical modelling of sub salt gliding due to fluid overpressure in underlying sedimentary strata abstract the gulf of mexico and the offshore campos and santos basins of brazil provide good examples of sub salt detachments by shearing in the presence of overpressure we have developed new physical models of such situations including layers of ductile silicone putty which simulated an evaporite and fluid overpressure in porous brittle layers we studied two configurations in which the maximal overpressure was either beneath a single ductile layer of silicone simulating salt or between two such layers of silicone simulating an evaporite sequence the results of our analogue modelling showed that detachment faults appeared and persisted at the bases of the ductile layers even though some ductile deformation occurred simultaneously within these layers for models where maximal overpressure was between two ductile layers detachment occurred mainly at the base of the upper silicone layer on comparing our models with structures in the gulf of mexico or offshore brazil we find strong similarities which lead us to suspect that detachment is indeed possible at the base of an evaporite sequence in nature this conclusion has strong implications not only for the understanding of petroleum systems but also for better management of petroleum production in sedimentary basins

between scylla and charybdis part 2 the sedimentary dynamics of the ancient early pleistocene messina strait central mediterranean based on its modern analogue abstract the 3 km wide messina strait divides sicily from the italian peninsula representing one of the most impressive examples of tide dominated marine passageway of the central mediterranean sea the tidal hydrodynamics governing the strait results from a semi diurnal phase inversion between two interlinked wider basins i e the tyrrhenian sea to the north and the ionian sea to the south due to the presence of a nodal tidal point in the centre of the strait this condition associated with coastal narrowing causes continuous water exchanges and a local phenomenon of tidal amplification producing collinear tidal currents up to 3 m s 1 in velocity in the point of tidal maxima of the strait centre currents follow bi directional patterns across the strait decelerating towards either opposite peripheries because of the progressive enlargement of the cross sectional areas tides exert a relevant influence on the strait bottom shaping characteristic erosional features governing the main sediment transport routes and promoting the migration of distinctive bedforms the textural and dimensional attributes of these bottom features vary according to the current velocity trends and main directions indicating a specific partition of the strait in individual sedimentary zones in the early pleistocene analogous conditions established across the same area and huge quantities of siliciclastic and bioclastic deposits accumulated in the ancient messina strait under a similar tidal hydrodynamics the 250 m thick strait fill succession which is magnificently exposed across the two opposite margins i e scylla and charybdis of the modern passageway represents a crucial correlative element in supporting the reconstruction of the sedimentary dynamics of the past strait based on the observation of the modern system in this work which represents the second part of a first article focused on the modern messina strait previous stratigraphic data are reviewed and integrated with new sedimentological observations carried out in both onshore and offshore key areas in particular outcrop analysis allows distinguishing a number of facies associations which are described and interpreted by taking into account the correspondent modern depositional strait setting the results of this study highlight a mutual correlation between the depositional zones detected in the modern messina strait and the individual facies associations recognised in the onshore margins the strait centre zone represented a bedload parting sector in the ancient system this zone which is one of the most difficult environment to detect in ancient straits due to a condensed sedimentary record is suggested to coincide with a structural palaeo high exposed in the southern sector of the investigated onshore area and continuing offshore in the modern strait this tectonic horst symmetrically separated two opposite highly depositional zones to the north and to the south hosting gravel and sand size siliciclastic and bioclastic deposits exhibiting diffuse motifs of large scale tidal cross stratification and a specific vertical facies stacking these characteristic tidal deposits which are believed to record the migration of several generations of tidal dunes forming extensive bedform fields in the strait reflect a tidal asymmetry between flood and ebb currents tidal asymmetry is referable to the oceanographic setting observable nowadays in the messina strait and deriving from a marked density driven water mass stratification between the two opposite flows in the ancient system tidal dunes transited down current to highly bioturbated fines including sporadic small scale tidal cross strata representing the distal periphery of the early pleistocene strait here bedforms followed migration pathways not always reflecting mutually evasive net gross sediment transport routes due to local conditions of current rotation occurring during each tidal phase inversion during these stages similar to slack water periods currents were weak enough to allow fine sediments to be settled from suspension reproducing a signature typical of other tide dominated coastal systems e g estuaries tidal flats etc these sedimentary zones of the ancient messina strait were laterally bounded by tectonically active margins where fan deltas clastic aprons and carbonate wedges developed under the constant interplay between mass transport processes and tidal currents the resulting architectural and sedimentary features observable in outcrop reveal impressive similarities with the morpho bathymetric features nowadays documented in the correspondent strait margin modern environments these considerations allow deciphering a general palaeo environmental reconstruction of the ancient strait which although suffering of limitations due to a partial preservation of the whole rock record discloses sedimentary processes and depositional environments of the ancient messina strait the process based analysis of this field based case study is crucial in predicting long term environmental changes and facies migration pathways and improves the state of knowledge on tidal straits which are the less understood among the most common tidal depositional systems

record of latest barremian cenomanian environmental change in tectonically controlled depressions from the jura burgundy threshold jura mountains eastern france and western switzerland abstract the area of the western jura mountains constitutes the former jura burgundy threshold between the tethys ocean and the epicontinental paris basin sea during the barremian the area was covered by a shallow water urgonian carbonate platform tectonic processes influenced the architecture of the urgonian platform and were notably responsible for the formation of fault related depressions on top of the urgonian series which were subsequently transformed into incised valleys and then to marine depocenters their sedimentary infills are mostly represented by the perte du rhone formation and record stepwise environmental change on the innermost platform which was strongly influenced by the nearby landmass from the latest barremian onward sedimentation progressively changed from i predominantly oncoidal and detrital charophyte rich marly limestones to ii heterozoan marly limestones and marls followed by iii detrital sediments interspersed with numerous phosphatic conglomerates these changes are linked with the onset of the latest barremian taxy episode and with the following oceanic anoxic episodes oae s 1a to d in the neighbouring basins the sedimentary changes are associated with sedimentary sequences which formed during 2nd order transgressions and which express the effect of sea level fall followed by drowning events the presence of a rich ammonite fauna allowed a precise biostratigraphical dating of these events during the latest barremian to cenomanian time interval