comparative stratigraphy and subsidence history of mesozoic rift basins of north atlantic mesozoic rift basins around the north atlantic all possess many of the following upper jurassic to lower cretaceous oxfordian to albian stratigraphic elements 1 an early oxfordian hiatus 2 backfill deposits in upper jurassic paleovalleys 3 multiple tithonian transgressive regressive sequences 4 early berriasian regressive sequences 5 a sub valanginian unconformity overlain by coarse siliciclastic sandstones and conglomerates 6 valanginian to hauterivian fining upward sequences 7 condensed barremian sequences and 8 prominent aptian unconformities more local features include abrupt kimmeridgian deepening lusitanian basin portugal albian deepening cantabrian and aquitaine basins northern spain and southern france and late albian early cenom nian source rejuvenation cantabrian basin northern iberia decompaction and backstripping of sediments from well and outcrop sections in six basins also reveal broad similarities in trends of rates of apparent basement subsidence relatively high rates 50 100 m m y characterize onset of rift phases in the triassic and the kimmeridgian locally in the lusitanian basin of portugal kimmeridgian rates reach about 250 m m y in the cantabrian and aquitaine basins a third phase of rapid subsidence characterizes the aptian albian interval after the main phase of continental breakup around iberia this phase may be due to transtensional stresses as iberia continued to rotate relative to europe tensional intraplate stresses or gravity driven partial collapse of the new continental marg n the north atlantic mesozoic basins share many features because they had a common origin controlled by intracontinental rifting and subsequent separation of europe and north america pulses of extension generally are reflected in the stratigraphy of all of the basins superimposed on this record are the effects of eustatic sea level variations the main difference between the canadian jeanne d arc basin and iberian basins is the significantly greater early and middle jurassic basement subsidence rate in the former about 40 m m y vs 0 10 m m y this difference may result from prolonged extension on the canadian side perhaps due to northward transfer of shear stress from left lateral transform motion between the grand banks and northwestern africa

the shelf sand plume model a critique in recent years a new model for deposition of sand bodies in a shelf environment has appeared this model known as the shelf sand plume model is hypothesized to result from storm driven currents that are deflected around a deltaic headland stripping sand from the headland and redepositing it in a downcurrent plume on the inner shelf the modern analogue for this model is considered to be an arcuate shelf sand body located off the damietta branch of the nile delta however the distribution of older deltaic and shoreline sands probably controls the arcuate outline of the sand body the present current system has certainly reworked these sands into ridges and large scale bedforms but is not responsible for the overall outline of the sand body grain size range and distribution of sand on the shelf demonstrate that the source of sand in the nile shelf sand body is not the modern damietta headland as postulated by the shelf sandplume model in our view the shelf sand plume model is presently unsubstantiated and has orginated as a misapplication of the original nile example as a geological model the shelf sand plume model lacks a set of observable consistently applicable criteria the only common denominator to the model is the plume geometry of a sand body located off a deltaic promontory however workers postulating the existence of shelf sand plumes have neither clearly established a plume geometry nor shown the juxtaposition of these bodies with respect to coeval deltaic headlands in their outcrop or subsurface examples the model does not provide criteria to distinguish a shelf sand plume from other classes of shelf sand bodies notably sand ridges and storm generated sheet like sands its application to the rock record should be re evaluated

actinide series disequilibrium as a tool to establish the chronology of deep sea hydrothermal activity this paper describes the different radiochronological methods used to date geologically recent i e 400 000 years deep sea hydrothermal deposits as well as the basic conditions necessary to obtain reliable dates the limitations of the different techniques also are described using measurements of 210pbpb 228th228ra 230th234u 231pa235u and 228ra226ra the authors have undertaken an exhaustive chronological study of the hydrothermal deposits along the east pacific rise the mid atlantic ridge and in some back arc basins the objectives of this study were to obtain regional chronologies and to establish a general synthesis on the evolution of the hydrothermal processes at the scale of the mid oceanic ridge system some results obtained by other authors are included in this synthesis the dependence of the general trends of temporal development of the hydrothermal chimneys edifices and fields on their tectonic settings is discussed this study demonstrates that hydrothermal activity does not represent a regular input of matter to the ocean and that its pulsed character must be taken into account in all modeling attempts chemical biological and tectonic affected by hydrothermal processes in areas of rapid spreading like the east pacific rise recent and fossil deposits are spatially separated by contrast at the slow spreading mid atlantic ridge fossil and present activity are found in the same location

an ensemble kalman filter for the time dependent analysis of the geomagnetic field we present the application of the ensemble kalman filter to a three dimensional convection driven model of the geodynamo our implementation rests on a suitably modified version of the parallel data assimilation framework of nerger and hiller 2013 we resort to closed loop experiments for validation purposes using a dynamo model of intermediate resolution observations for these experiments consist of spectral coefficients describing the surface poloidal magnetic field with arbitrary truncation our synthetic tests demonstrate the efficacy and adaptivity of the method provided the ensemble comprises o 500 members in which case the typical spin up time we find for our system is o 1000 years in case of a poor resolution of the observations we find that the knowledge of the full covariance matrix describing the uncertainty affecting the spectral coefficients as opposed to its sole diagonal results in a much better estimate of the internal structure of the dynamo

evidence of an imbricate crustal thrust belt in the southern british variscides contributions of southwestern approaches traverse swat deep seismic reflection profiling recorded through the english channel and the celtic sea the southwestern approaches traverse swat seismic reflection profiles recorded through the celtic sea and the english channel provide unexpected data concerning notably the deep structure of the irish and south british variscan crust one of the most significant results is the recognition of prominent deep southerly dipping reflectors regarded as large scale foreland directed variscan thrusts which constitute in the southern innermost zones a crustal stacking wedge the resulting overthickened crust may have induced the petrogenesis of the cornubian granitic batholith by an anatectic melting process the roots of this crustal duplex are not imaged on the swat lines because of their subsequent assimilation by the more recent layering of the present day lower crust upward the deep crustal ramps are assumed to flatten out and to join a roof thrust which acts as a major midcrustal decoupling zone not revealed by the swat profiles and which underlies the thick devonian allochthonous high strained units of cornwall northward this shallow northerly verging shear zone is overstepped by the tintagel antithetic back thrusts which are closely related to deeper northward dipping basement faults considered as caledonian features southward the imbricate crustal thrust unit is overridden by the lizard ophiolitic suture which appears as a 8 km thick gently southerly dipping sheared zone crosscuttting the entire crust and rooting deep beneath the present day moho its hanging wall is constituted by the channel cadomian block characterized by an almost seismically featureless upper crust which is not involved in the main variscan thrust stacking northward the imbricate thrust unit is delineated by a moderately southerly dipping ramp which penetrates straight down into the upper crust without any evidence of an intervening flat lying decollement level this frontal ramp emerges along a broadly 110 n trending discontinuous thrust front located above the northern downfaulted margins of the munster ireland and pembroke wales devonian basins the swat deep seismic data relate the overall variscan crustal shortening undergone by the south british domain to an oblique converging process involving a continental shelf pinched between the rigid allochthonous channel block and the northern autochthonous foreland this crustal shortening occurred during a carboniferous intracontinental collisional context subsequent to the closure and the obduction of a small oceanic crust floored basin lizard

bottom up control of geomagnetic secular variation by the earth s inner core temporal changes in the earth s magnetic field known as geomagnetic secular variation occur most prominently at low latitudes in the atlantic hemisphere that is from 90 degrees east to 90 degrees east whereas in the pacific hemisphere there is comparatively little activity this is a consequence of the geographical localization of intense westward drifting equatorial magnetic flux patches at the core surface despite successes in explaining the morphology of the geomagnetic field numerical models of the geodynamo have so far failed to account systematically for this striking pattern of geomagnetic secular variation here we show that it can be reproduced provided that two mechanisms relying on the inner core are jointly considered first gravitational coupling aligns the inner core with the mantle forcing the flow of liquid metal in the outer core into a giant westward drifting sheet like gyre the resulting shear concentrates azimuthal magnetic flux at low latitudes close to the core mantle boundary where it is expelled by core convection and subsequently transported westward second differential inner core growth fastest below indonesia causes an asymmetric buoyancy release in the outer core which in turn distorts the gyre forcing it to become eccentric in agreement with recent core flow inversions this bottom up heterogeneous driving of core convection dominates top down driving from mantle thermal heterogeneities and localizes magnetic variations in a longitudinal sector centred beneath the atlantic where the eccentric gyre reaches the core surface to match the observed pattern of geomagnetic secular variation the solid material forming the inner core must now be in a state of differential growth rather than one of growth and melting induced by convective translation

microfacies diagenesis and hydrocarbon potential of the neoproterozoic cap carbonate of the southern amazon craton abstract the low latitude marinoan glaciation 635 ma record in the southern amazon craton has been one of the most important sites with strong evidence for the neoproterozoic snowball earth hypothesis in central brazil the 50 m thick carbonate succession of the post marinoan cap carbonate exposed in the tangara da serra region central brazil represents a complementary section of the classical occurrence of the puga cap carbonate in mirassol d oeste region that overlies glaciogenic deposits marking the well preserved cryogenian ediacaran boundary in south america the 8 m thick cap dolostone that represents the mirassol d oeste formation consists of pinkish microcrystalline peloidal dolomudstone exhibiting even parallel to low angle lamination locally tubestone and megaripple bedded dolopackstone the cap dolostone is succeeded upsection by the cap cementstone limestone basal guia formation comprising laminated lime mudstone rich in silt sized terrigenous grains quartz and subordinately bituminous shale dolomitic marls intraformational breccias and abundant calcite crystal fans and crusts crystal fans occur in isolated and decimetre scale arrays and are considered aragonite pseudomorphs primary precipitation of dolomite crystal fans and micrite and the formation of tubestone are related to depositional processes the cap dolostone was affected by the neomorphism of the dolomicrite development of vug and intercrystalline porosity euhedral dolomite precipitation synsedimentary fracturing calcite and quartz precipitation chemical compaction fe oxide substitution and precipitation the diagenetic processes of the cap cementstone limestone were pyrite precipitation calcite cementation neomorphism of micrite and crystal fan partial crystal fan dissolution dolomitization chemical compaction fracturing ferrous dolomite and calcite precipitation pyrite pseudomorph and micrite replacement with fe oxide and replacement of rhombohedral dolomite by mn oxides hydrocarbons are rare and its migration represents the last diagenetic event being found 1 in fractures and interpeloidal vug and intercrystalline pores 2 with low permeability and connectivity of the cap dolostone and 2 in intercrystalline pores fractures and rare pores in crystal fans of the cap cementstone limestone during the post marinoan sea level rise occurred the establishment of a shallow to moderately deep microbially induced dolomitic platform locally influenced by storm action the progressive transgression led the implantation of a deep anoxic and caco3 oversaturated aragonitic platform the similarities of cap carbonate microfacies among the post marinoan succession worldwide reflects an extraordinary and non recurring global precipitation event in geological history in contrast besides the majority of cap carbonate have the same textural and compositional framework the diagenetic history seems to be regional and completely dependent on the tectonic thermal and burial history of the depositional basins where they were deposited

geomagnetic forecasts driven by thermal wind dynamics in the earth s core there exists a fundamental as well as practical interest in being able to accurately forecast the future evolution of earth s magnetic field at decadal to secular ranges this work enables such forecasts by combining geomagnetic data with an earth like numerical model of a convection driven fluid dynamo the underlying data assimilation framework builds on recent progress in inverse geodynamo modelling a method which estimates an internal dynamic structure for earth s core from a snapshot of the magnetic field and its instantaneous rate of change at the surface and takes advantage of linear relationships and long range correlations between observed and hidden state variables here the method is further evolved into a single epoch ensemble kalman filter in order to initialise at a given epoch an ensemble of states compatible with the observations and representative of the uncertainties in the estimation of hidden quantities the ensemble dynamics obtained by subsequent numerical integration of the prognostic model equations are found to be governed by a thermal wind balance or equilibrium between buoyancy forces the coriolis force and the pressure gradient the resulting core fluid flow pattern is a quasi steady eccentric gyre organised in a column parallel to earth s rotation axis in equilibrium with a longitudinal hemispheric convective density anomaly pattern the flow provides induction for the magnetic field which also undergoes a realistic amount of diffusion predictions of the present magnetic field from data taken within the past century show that the ensemble has an average retaining good consistency with the true geomagnetic evolution and an acceptable spread well representative of prediction errors up to at least a secular range the predictability of the geodynamo thus appears to significantly exceed previous theoretical expectations based on the chaotic divergence of ensemble members the assimilation generally outperforms the linear mathematical extrapolations from a 30 yr prediction range onwards with a 40 per cent improvement in earth surface error at a secular range the geomagnetic axial dipole decay observed over the past two centuries is predicted to continue at a similar pace in the next century with a further loss of 1 1 0 3 t by year 2115 the focal or minimum intensity point of the south atlantic geomagnetic anomaly is predicted to enter the south pacific region in the next century with the anomaly itself further deepening and widening by year 2065 the minimum intensity is predicted to decrease by 1 46 0 4 t at the earth surface and the focal point to move 12 8 1 4 deg westwards with a slight northward component this corresponds to a drift rate of 0 26 deg yr 1 similar to the typical geomagnetic westward drift observed over the past four centuries the same drift rate is also predicted until 2115 with a further but more uncertain intensity decrease

a reduced stochastic model of core surface dynamics based on geodynamo simulations we make use of recent geodynamo simulations to propose a reduced stochastic model of the dynamics at the surface of earth s core on decadal and longer periods this model replicates the most energetic eigen directions of the geodynamo computation towards shorter timescales it proposes a compensation for weaknesses of these simulations this model furthermore accounts for the signature in the geomagnetic secular variation of errors of representativeness associated with unresolved processes we incorporate the reduced stochastic model into a geomagnetic data assimilation algorithm an augmented state ensemble kalman filter and apply it to re analyze magnetic field changes over the period 1880 2015 errors of representativeness appear to be responsible for an important fraction of the observed changes in the secular variation as it is the case in the dynamo simulation recovered core surface motions are primarily symmetric with respect to the equator we observe the persistence of the eccentric westward gyre over the whole studied era and vortices that partly follow isocontours of the radial magnetic field at the core surface our flow models provide a good fit to decadal changes in the length of day and predict its interannual variations over 1940 2005 the largest core flow acceleration patterns are found in an equatorial belt below 10 in latitude and are associated with non axisymmetric features no systematic longitudinal drift of acceleration patterns is found even over the past decades where satellite data are available the acceleration of the high latitude westward jet in the pacific hemisphere is during the satellite era a factor 5 smaller than previously reported and its structure shows some evidence for equatorial asymmetry the era 1 of continuous satellite records provides enhanced contrast on the rapid core flow variations the proposed assimilation algorithm offers the prospect of evaluating earth likeness of geodynamo simulations

progradational sequences in miocene shoreline deposits southeastern caliente range california abstract an exceptionally well exposed marine nonmarine transition in middle miocene strata exists in the southeastern caliente range california about 50 individual progradational sequences form a succession that ranges in thickness from approximately 1000 m where predominantly nonmarine to more than 2500 m where predominantly marine paleogeographic evidence in basalt flows near the top of the succession and in overlying fluvial deposits indicates that these middle miocene strata were deposited across a north northwest trending shoreline a complete progradational sequence typically is several meters to a few tens of meters thick and includes strata that represent three intertonguing stratigraphic units individual sequences generally rest on a thin gravel deposit interpreted as a transgressive lag on an erosional surface the gravel is overlain by structureless siltstone or fine grained sandstone deposited at water depths where the rate of faunal mixing exceeded that of production of structures by physical processes these rocks grade upward into bedded fine sandstone deposited closer to shore where physical processes exceeded bioturbation crossbedded lenses of coarse sand or fine gravel in the upper part of this facies suggest the presence of fairly long period surface waves the bedded fine sandstone is sharply ove lain by a cross bedded coarse sandstone facies that is interpreted as a combined offshore bar rip channel surf zone assemblage cross strata dip dominantly offshore suggesting substantial deposition from rip currents a secondary shoreparallel mode of cross strata direction suggests longshore currents produced by surface waves from the northwest the crossbedded coarse grained sandstone grades upward into planar bedded medium grained sandstone that is interpreted as a beach foreshore this facies grades upward through structureless medium grained sandstone into nonmarine or lagoonal red and green mudstone of the caliente formation the middle miocene succession was deposited in a subsiding basin that was otherwise remarkably stable tectonically the position of the strand line differed no more than a few kilometers through a period of 1 to 3 m y the average duration of the transgressive regressive cycles a few tens of thousands of years together with their distribution in groups of three or four in the lower two thirds of the succession is consistent with the pattern of long term climatic cycles produced by periodicity of the earth s solar orbit and may be related to eustatic sea level changes attendant to the development of the antarctic ice cap changes in the pattern of progradation in the upper part of the succession and nearby basaltic eruptions may have been precursors to the onset of movement along th san andreas fault in this area 12 14 m y ago

a review of mesozoic climates there is overwhelming evidence based on the distribution of distinctive sediments and fossils and oxygen isotope data that the climate of the mesozoic world was appreciably more equable than that of today with no polar ice caps but precise quantitative data are not available except for an episode of late cretaceous cooling there is no good documentation of any significant change in global temperature distributions through the era the distribution of coals and evaporites together with other criteria indicates a pattern of humid arid and zones appreciably different in important respects from that of today during the triassic and jurassic western pangaea in low to middle latitudes was largely arid but in the early cretaceous the lands on the margin of the newly opening central atlantic and western tethys experienced a humid climate by late cretaceous times arid zones had become very restricted in extent because of insufficient suitable data attempts at climatic modelling have had only modest success and only to a limited extent can the major long term changes in climate between the permian and the present be explained in terms of changing geography the most probable explanation of mesozoic equability is an increased atmospheric co 2 content a number of enigmas remain such as the existence of flourishing forests in polar palaeolatitudes whereas for the late cenozoic short term climatic changes can be related successfully to variations in the geometry and mechanics of the earth sun system there is a long way to go before comparable success can be claimed for the mesozoic

sedimentology and fractal based analysis of permeability data john henry member straight cliffs formation upper cretaceous utah u s a interpretation of depositional environments combined with field measurement of permeability for a portion of the upper cretaceous straight cliffs formation near escalante utah provides new results for understanding and modeling facies dependent perme ability variations offshore transition lower shoreface upper shore face and foreshore environments are interpreted for part of the john henry member on the basis of outcrop investigation using a newly designed drillhole minipermeameter probe permeability was measured for two of the facies within this unit lower shoreface bioturbated sand stone and upper shoreface cross bedded sandstone approximately 500 permeability measurements at a sample spacing of 15 cm were made along four vertical profiles and three horizontal transects o na6m 3 21 m outcrop permeability ranges from 41 to 1 675 millidarcies md in the bio turbated sandstone facies which is massive bedded moderately to well sorted and very fine to fine grained in contrast permeability ranges from 336 to 5 531 md in the moderately to moderately well sorted fine to medium grained cross bedded sandstone facies a high degree of variability in permeability of the cross bedded facies is caused by small scale variations in grain size and structure related to depositional processes the geometric mean permeability in the bio turbated sandstone is 253 md compared with 1 395 md in the cross bedded facies while the facies dependent differences in permeability k are ap parent and related to depositional and biological processes fractal based statistical analysis of the horizontal ln k increments yields near ly identical results for the bioturbated facies and the cross bedded fa cies possibly suggesting an underlying statistical commonality in the formation of both facies increment distributions from both facies ap pear similar with peaking around the mean ln k increments from the smaller vertical data set appear similar also but with a variance approximately 3 7 times larger than the horizontal value variance scaling analyses of horizontal and vertical data both yield a hurst co efficient near 0 26 which is characteristic of negative spatial correla tion of the increments the methodology developed herein offers a po tential high resolution alternative to existing methods for understand ing and characterizing subsurface properties

a combined length of day series spanning 1832 1997 lunar97 abstract universal time ut measurements taken by the techniques of lunar occultation optical astrometry lunar laser ranging and very long baseline interferometry have been combined using a kalman filter to produce a smoothed length of day lod series lunar97 spanning 1832 5 1997 5 at yearly intervals decadal lod variations having amplitudes as large as a few milliseconds ms are clearly evident in this newly determined series as they have been in previously determined series comparing the lunar97 lod series with other available series shows differences that are generally less than the uncertainty in the determination of the lunar97 lod values which is about 0 6 ms during the early 19th century improving by more than an order of magnitude to about 0 03 ms during the late 20th century decadal lod variations are therefore a robust feature of the lunar occultation optical astrometric and space geodetic measurements of the earth s rotation that have been taken since 1832

time average and time dependent parts of core flow abstract we infer fluid motions below the core mantle boundary by inverting geomagnetic secular variation data over a 150 years assuming helical geostrophic flow we obtain snapshot images of core flow at 5 year intervals which we combine to give time average and time dependent parts of the motion over this time interval the most prominent time average flow structure is a large anti cyclonic vortex in the southern hemisphere beneath the atlantic and indian oceans the time average zonal core flow outside the inner core tangent cylinder is significantly westward in the southern hemisphere but nearly zero in the northern hemisphere westward polar vortices occur inside the tangent cylinder in both hemispheres particularly in the north in terms of mantle versus core origins mantle driving appears to be responsible for the mid latitude asymmetry in the zonal core flow whereas core driving appears to be responsible for the flow at high latitudes we also compare changes in the core s angular momentum calculated from our time dependent core flow with changes in the mantle s angular momentum derived from decade scale length of day variations and find adequate agreement a fit of our time dependent core flow to a torsional oscillations model yields dominant periods of 110 and 53 years

earth s core internal dynamics 1840 2010 imaged by inverse geodynamo modelling inverse geodynamo modelling seeks to estimate the dynamic state of earth s core from geo magnetic data and the statistical information brought by a prior self consistent 3 d numerical model of the geodynamo the method rests on the use of least squares inversions under constraints and estimates hidden quantities by taking advantage of linear relations and long range statistical correlations with the magnetic observations the data together with their error statistics are provided by geomagnetic field models cov obs gufm sat q3 and cm4 covering epochs 1840 2010 the prior numerical model is the recently published coupled earth dynamo the output of which presents a high degree of morphological semblance to the geomagnetic field while reproducing the main features of its secular variation an analysis of the inversion misfits to the data shows that the prior model generally accounts well for the main field and secular variation data within their specified errors throughout the investigated time period inverted core flows are confirmed to be mainly organized in columns parallel to the earth s rotation axis a previously observed giant eccentric columnar gyre of westward drifting flow is found to be present from epoch 1870 onwards and its structure is shown to slowly rotate westwards at a rate up to 0 1 yr 1 confirming an earlier prediction based on direct numerical modelling temporal variations in the axisymmetric part of the gyre accurately account for observed variations of the length of the day in recent epochs inverted magnetic structures support a mechanism of azimuthal flux expulsion by convective columns to explain the origin of low latitude magnetic flux patches existing beneath the atlantic the 1840 2010 time average of the inverted density anomaly field has a longitudinal hemispheric structure with most of the buoyancy in the eastern hemisphere consistent with rapid surges of convective columns imaged in this hemisphere with earlier proposals of a faster inner core freezing there and with a possible east to west convective translation of the solid inner core the typical timescales of flow variation observed in the inversions are two to three times shorter than those naturally produced by a direct simulation of the prior model underlining its limits in fully rendering the earth s core short timescale dynamics

exploratory flow duct experiments on combined flow bed configurations and some implications for interpreting storm event stratification abstract experiments were conducted to define equilibrium combined flow bed configurations developed under a wide range of oscillatory uo and unidirectional uu velocity components with a constant oscillation period of 8 5 sec and median grain size of 0 09 mm in the range of flows studied uo 0 0 80 m sec uu 0 0 25 m sec the bed phases are no movement small 2d ripples small 3d ripples large 3d ripples and plane bed small 2d ripples are stable at low uo and uu by increasing either velocity component the plan view bed configuration becomes progressively less regular as small 3d ripples develop small 3d ripples are stable at low to moderate uo and a wide r nge of uu under purely oscillatory flow as uo increases beyond about 0 40 m sec large scale hummocky bed forms become stable when a unidirectional component is applied these bed forms quickly develop a downstream asymmetry and generally move in the direction of uu the dip angles of the lee flanks increase with increasing uu although they remain less than the angle of repose at high oscillatory velocities uo 0 70 0 80 m sec plane bed is the stable bed phase when uu 0 05 m sec much hummocky cross stratification shows a preferred dip direction of the coset laminae our experiments support the idea that such cross stratification is produced by combined flow the large 3d ripples produced at high oscillatory velocities and small to moderate unidirectional velocities would presumably generate what might be called anisotropic hummocky cross stratification if we had been able to aggrade the bed during the runs this would be the case even in dominantly oscillatory flow the experiments show that even a weak unidirectional current of several centimeters per second superimposed upon a strong oscillatory flow results in bed forms that move predominantly in a single direction and that leave sets of laminae that dip predominantly in that direction in runs at large uo the purely oscillatory large scale bed forms were quickly planed off as uu was increased from zero but a series of almost stationary gentle undulations on the bed surface persisted for indefinitely long times where the original topographic highs were situated even at the highest unidirectional flow investigated 0 25 m sec this suggests a possible origin for the gentle undulation observed in cretaceous parallel laminated shallow marine sandstones described by arnott 1987 1988 with spacings up to a few meters and heights not much more than a centimeter

quantitative determination of tertiary sediment supply to the north sea basin sediment supply comprises a major family of regime variables that influence geomorphic surface grade spatial and temporal changes in total sediment supply and sediment texture gravel sand mud ratio will cause reconfiguration of depositional and erosional profiles potentially creating or influencing the sequence stratigraphic framework of the basin sequence grain volume which is defined as the volume of sedimentary grains in an individual stratigraphic sequence total sequence volume minus cement and porosity volume has been quantitatively determined for each of 16 genetic stratigraphic sequences in the north sea basin rates of total sediment supply to the basin in both time and space were then calculated sand grain volume and sand mud ratio were also calculated for each sequence these data define four principal episodes of tertiary sediment supply the most significant episode occurred in the late paleocene and was followed by secondary episodes in the eocene and oligocene a fourth neogene episode extends through the present all episodes correlate to source terrain tectonic pulses related to evolution of the north atlantic basin to intraplate stress changes associated with successive phases of the alpine orogeny or to the late cenozoic epeirogenic uplift of scandinavia the major episodes in turn contain secondary sequence to sequence variations that correspond to changes in spatial or temporal values of one or both of the supply regime variables again most changes closely reflect details within the histories of the principal tectonic phases the history of changing source area relief and resulting topographic grades and related changes in sediment yield into the basin was a principal control on north sea cenozoic sequence development source basin relief in turn was largely determined by regional tectonism

dynamics of large and small scale bedforms on a macrotidal shoreface under shoaling and breaking waves abstract bedform dimensions bed position changes near bed velocities and suspended sand concentrations are analyzed from measurements at a single location on a macro tidal beach in the south west of england the study was conducted in 0 5 2 25 m water depth under both swell and wind generated storm waves with both weak and strong currents present bed positions and suspended sand concentrations were measured using the 3 transducers of a multi frequency acoustic backscatter sensor with 5 mm vertical resolution two distinct bed types based on wavelength were observed each with two or more subtypes possible i small scale bedforms 20 cm which include two dimensional 5 forms and three dimensional vortex 6 forms small scale forms were dominant under non breaking conditions while the large scale forms occurred under both non breaking and breaking waves the large scale forms dominate under breaking conditions both types but particularly low steepness forms were highly mobile with maximum horizontal migration rates of 5 cm min 1 large upto 15 cm and rapid upto 3 0 cm min 1 changes in vertical bed elevation were also observed in association with the development and migration of large scale forms large scale bedforms were also highly variable spatially often being interspersed with smaller scale forms under decaying flow regimes and with areas of flat bed under increasing regimes observations suggest these forms are present even under high energy surf zone conditions wave shields 1 models for predicting ripple dimensions did not perform well in this environment this lack of agreement reflects the complex hydrodynamic regimes associated with random grouped shoaling and breaking waves together with the presence of currents often at large angles to the waves as well as the rapid rates of change in the wave forcing associated with tidal cycle oscillations in this macrotidal environment suspended sediment concentrations and transport rates are particularly sensitive to the bedforms present and also to bed position changes associated with ripple migration estimates of transport rates are subject to potentially large errors upto 30 without compensation for bed elevation changes relative to sensor position

the tectonic evolution of the north celtic sea and cardigan bay basins with special reference to basin inversion abstract the morphology and structural evolution of the north celtic sea basin ncsb and cardigan bay basin cbb are in large part controlled by reactivation of basement features of caledonian and possibly variscan origin from earliest triassic until maastrichtian times sedimentation was almost continuous initially in a rapidly subsiding rift system and finally in a progressive passive sag phase at the onset of the alpine orogeny both basins experienced spectacular inversion along readily identifiable axes the inversion takes the form of a compressive reactivation and reversal of the pre existing growth faults and a broad regional upwarping of the cretaceous depocentre the amount of inversion varies depending on the location within the basin but values in excess of 1000 m are possible by utilising regional seismic data the timing of inversion can be estimated as occurring between maastrichtian and eocene times which correlates with the so called laramide pulse of the alpine orogeny

forced regressions in a sequence stratigraphic framework concepts examples and exploration significance sequence stratigraphic concepts suggest that stratal geometries develop and are largely controlled by changes in relative sea level on the shelf lowstand deposits which form during falls and subsequent stillstands of relative sea level can be recognized by the presence of an unconformity at the base the isolated and basinward position relative to the previous shoreline and the abrupt seaward translation of shallow water and shoreline facies into the basin across an unconformity surface this seaward translation of facies and shoreline regression in response to relative sea level lowering is termed a forced regression a forced regression is independent of variations of sediment flux and is in contrast with normal regressions that occur in response to excess sediment flux relative to space available on the shelf i e accommodation forced regressions commonly are associated with a zone of sedimentary bypass subaerial exposure and possible fluvial erosion between the newly formed and preceding shorelines certain shelf sands previously interpreted as offshore or mid shelf sand bodies thus can be reinterpreted as stranded lowstand shorelines associated with forced regressions this alternative interpretation has economic significance insofar as it suggests different subsurface correlations and reservoir geometries with the potential for development of new play types and enhanced recovery in o der fields examples of forced regression can be observed at a variety of scales and ages several such examples include the modern east coulee fan delta and the lower cretaceous viking formation in alberta canada the quaternary rhone delta and the quaternary hudson valley system

geomagnetic evidence for fluid upwelling at the core mantle boundary summary previous studies both geomagnetic and seismic have been unable to show conclusively whether or not there is fluid upwelling at the core mantle boundary here a new method is developed in which an attempt is made to invert geomagnetic secular variation data measured at the earth s surface for a frozen flux purely toroidal core mantle boundary cmb velocity field under the assumption that the mantle is electrically insulating and flux is frozen in at the cmb these data have previously been inverted for the core mantle boundary radial secular variation from which the appropriate fit between model and data is known two different main field models were used to assess the effect of uncertainty in its radial component at the cmb the conclusions were the same in both cases frozen flux purely toroidal motions provide a poor fit a statistical test allows very firm rejection of the hypothesis that the residuals are not significantly larger whereas there is no statistical difference between the residuals of inversions for radial secular variation and frozen flux velocity fields at the cmb if upwelling and down welling is included the inherent non uniqueness in the velocity field obtained is not of concern since only their statistical properties are utilized and no physical significance is attached to the flows obtained

celtic sea western approaches area an overview abstract a mesozoic tectonic and a pre quaternary geological map of the celtic sea western approaches area is presented the rift and wrench induced mesozoic basins of the celtic western approaches came into evidence during the permo triassic and remained active until the aptian crustal separation in the bay of biscay compressional intra plate stresses induced their inversion during the cenozoic

crustal structure of the celtic sea and western approaches from gravity data and deep seismic profiles constraints on the formation of continental basins a free air anomaly map of the celtic sea and the western approaches is presented including all available marine gravimetric data and bouguer anomalies on land n60 features are predominant in the area they correspond to the n60 so called caledonian grain which played a major role in the variscan orogeny and in the formation of the permo triassic sedimentary basins the fundamental new point which comes out from this gravimetric map is the existence of a closely spaced pattern of n105 features which we regard as late carboniferous transpressional structures in the eastern pan of the celtic sea thrusts are in the same n105 direction west of 6 w longitude however folds and thrusts trend preferentially in a n60 direction following the older grain there the n105 lineaments may be regarded as dextral shear structures with a possible compressive component the hercynian shortening of the celtic sea domain located between the variscan front and the lizard suture resulted from the displacement along the n160 direction of the two adjacent rigid domains north of the variscan front the n105 and n135 features were possibly active in more recent times then both the n60 and n105 trends controlled the formation of permo triassic basins the n105 directions being transfer faults two dimensional gravity modelling of the swat profiles in the celtic sea shows that the base of the reflective lower crust may be associated with the moho discontinuity everywhere in the celtic sea except below the saint george s channel basin the lower crust does not change significantly in depth and thickness beneath the permo triassic basins none of the available models simply satisfies such conditions they all require at least a modification of the geometry of the lower crust after the formation of the celtic sea basins nevertheless if the simple shear model can be ruled out mainly from structural arguments the uniform stretching and to a lesser degree the thermomechanical models seem to be more relevant models however two additional conditions are necessary 1 the preservation of crustal seismic features formed before the creation of basins must be explained and 2 a significant portion of the volume of the stretched lower crust about 40 of this volume for the north celtic sea basin must be added below or perhaps partly above the lower crust by lateral flow of the crust or at the expense of the upper mantle

rapidly changing styles of subsidence revealed by high resolution mudstone allostratigraphy coniacian of sweetgrass arch area southern alberta and northern montana marine mudstone of the coniacian muskiki member was deposited by storms on a shallow marine ramp across southern alberta and northern montana the rocks range from the c crassus crassus zone of the upper lower coniacian to the s depressus zone of the upper coniacian however upper coniacian strata are only a few metres thick at kevin montana and appear to be thin to absent beneath parts of the adjacent alberta plains probably because of minimal contemporaneous subsidence the muskiki member is partitioned into three packages by subtle bevelling unconformities cut during regional tectonic tilting the lower package indicates flexural subsidence in the nw whereas the middle package indicates subsidence in the sw the rotation of the axis of subsidence may indicate an along strike shift of the region of active thickening in the orogenic wedge the upper package is dominated by a nw trending region of stratal thickening forming a trough shaped rock body 100 km wide 250 km long and up to 50 m thick

cretaceous albian turonian foraminiferal biostratigraphy and paleontology of northern montana and southern alberta abstract in albian turonian time the interior of north america was flooded by a seaway extending from the present gulf of mexico to the arctic ocean transgressive phase greenhorn cyclothem detailed studies of this interval north and south of the international border have usually not been integrated the present foraminiferal biostratigraphic study includes a 38 000 km2 area straddling the alberta montana border from the lewis thrust in the west to the sweetgrass hills in the east including the sweetgrass arch stratigraphic cross sections and isopach maps of 6 albian early turonian stratigraphic units prepared from 57 surface and subsurface sections demonstrate that sedimentation was controlled primarily by 1 spasmodic volcanism to the west 2 tectonic activity coincident with the present location of the sweetgrass arch the occurrence of the late cenomanian planktonic foraminifer rotalipora cushmani in association with three other keeled species suggests an east west marine connection between the eastern pacific and the western interior at the maximum greenhorn transgression this is consistent with the facies model described by kauffman 1967 1969 1977 the paleogeographic model developed in the present study reported gastropod paleo biogeographic data and reevaluation of pelecypod and ammonite paleo zoogeographic interpretations

turonian to santonian paleoenvironmental changes in the cretaceous western interior sea the carlile and niobrara formations in southern alberta and southwestern saskatchewan canada abstract the upper cretaceous upper colorado group including the carlile and niobrara formations reveals a complex and dynamic marine basin system in southern alberta and southwestern saskatchewan an integrated study of the lower offshore to shelf environment including sedimentology geochemistry petrophysics and micropaleontology demonstrates how sea level fluctuations played a major role in the facies distribution and paleogeography of the interior seaway paleogeographic maps depicting seven time slices covering the late turonian to santonian illustrate general lithologies and their distribution in the north central part of the interior seaway of north america these maps are based on data from southern alberta and southwestern saskatchewan integrated with studies of time equivalent strata in neighboring regions in western canada and northwestern u s the interior seaway reached its maximum extent during the peak transgression of the greenhorn cycle early turonian and the basin was gradually infilled starting in the late turonian with the carlile formation the boundary between the carlile and niobrara formations is an angular unconformity where up to 20 m of the upper carlile formation was eroded at this time late late turonian the interior seaway was a narrow strait of a few hundred kilometers with its basin axis in eastern alberta marginal marine sandstones were deposited in western alberta cardium formation and central saskatchewan north central montana bowdoin sandstone the overlying transgressive part of the niobrara cycle shows multiple dramatic basin changes with two examples of relative lowstand and complete basin drainage implied by the verger member coniacian these rapid lowstand events occurred within periods of marine mudstone deposition which indicate shelfal to offshore conditions during deposition of the medicine hat member early to middle santonian changes from a shaly to a sandy shelf took place with deposition of thirteen sandstones in the vicinity of the bow island arch of southern alberta during lowstand periods with low accommodation the bow island arch affected sedimentation whereas during high accommodation the arch had no influence a subtle syncline in the northwest part of the study area is the primary reason for deposition of the main gas producing sandstone of the medicine hat member the maximum extension of the interior seaway during the niobrara cycle occurred in the late santonian with the deposition of the first white specks member

wave formed coarse grained ripples and their relationship to hummocky cross stratification abstract wave formed coarse grained ripples cgr form in water depths ranging from 3 to 160 m on transgressive surfaces in upper and lower shoreface sediments and in finer grained offshore transition deposits cgr have crest spacings of 0 25 3 0 m with amplitudes of 5 35 cm internal stratification is rarely preserved but where present crossbeds dip en echelon away from the ripple crest and individual laminae feather out away from the crest waves that form cgr commonly have wavelengths on the order of 2 5 m with periods of 8 to 14 sec these are not large storm events especially on a geological scale cgr commonly occur in close association laterally and vertically with hummocky cross stratification it is postulated that if sediment is medium to very coarse grained sand or pebbly sand then two dimensional cgr will be formed from oscillatory or oscillatory dominant flow if however sediment is very fine to fine grained sand then three dimensional hummocky cross stratification will be formed the wave conditions under which cgr form are similar to those which would form hummocky cross stratification it is only the grain size that is different reported and calculated wave conditions for hummocky cross stratification are almost identical to those for cgr modern cgr crests trend parallel to the local shoreline and bathymetry as such ancient ripple crests are a useful tool for basin reconstruction

recognition of facies bounding surfaces and stratigraphic patterns in foreland ramp successions an example from the upper devonian appalachian basin u s a abstract a detailed stratigraphic study of the upper devonian lock haven formation in the appalachian basin provides new interpretations applicable to understanding sedimentation and stratigraphic architecture in the foreland ramp setting the common occurrence of graded beds hummocky cross stratification and shell lags indicates that storm processes played an important role in deposition gradational based coarsening upward shoreface sequences of interbedded sandstone and mudstone formed during gradual fall in relative sea level in contrast sharp based regressive sequences consist of laminated sandstone facies overlying a submarine erosion surface that developed in response to more rapid fall in relative sea level transgressive sequences become finer grained upward from conglomerate lag which was deposited on a basal transgressive surface to heterolithic facies and mudstone facies a combination of eustasy and tectonic subsidence of the foreland ramp produced changes of relative sea level these fluctuations provided a means for transporting sand onto the ramp and for producing repetition of stratigraphic patterns in the lock haven formation the upper devonian clastic succession which includes both the lock haven formation and the catskill formation represents northwestward progradation of shoreline to nonmarine environments as sediment supply to the foreland ramp exceeded the rate of formation of accommodation compared to passive margin shelves thick fine grained successions such as the lock haven formation are more likely to form in foreland basins where tectonic subsidence provides a mechanism for creating accommodation sediment influx from uplifted areas and absence of a slope break on the foreland ramp results in a higher rate of fine grained clastic sedimentation than occurs on shelves of passive margins

rates controls and sand body geometries of transgressive regressive cycles cretaceous moosebar and gates formations british columbia the moosebar and gates formations lower cretaceous in northeastern british columbia consist of seven transgressive regressive sequence nonmarine sediments are thickest in the south and pinch out northward the sequences are laterally persistent extending east from the foothills for 240 km 150 mi the orientation of maximum transgressive and regressive limits in five cycles occurs within an east west trending zone 30 40 km 19 25 mi wide a recurring northward bulging sediment depocenter near the alberta british columbia border was fed by a series of stacked northerly flowing rivers the rivers were a major source of sediment for gravel beaches to the north and east isopach maps of marine sandstone and conglomerate show a series of west northwest east southeast o iented sand bodies 60 70 km 37 43 mi wide parallel with the shorelines each coastline prograded northward as a shoreline attached strand plain and a series of arcuate or cuspate deltas gates formation marine nonmarine cyclicity is a series of fourth order cycles superimposed on a major third order cycle the cyclicity and lateral extent of the sequences can be explained as a result of tectonic thrusting and loading superimposed on the trend of the peace river arch the influence of eustatic sea level variations and sedimentologic switching of point sources had minimal effect coastal retreat ranged from 437 to 875 m 1 000 years 1 433 to 2 780 ft 1 000 years coastal progradation ranged from 218 to 437 m 1 000 years 175 to 1 433 ft 1 000 years the duration of several transgressive regressive cycles was 103 000 275 000 years

is there evidence for geostrophic currents preserved in the sedimentary record of inner to middle shelf deposits abstract unidirectional oscillatory and combined flow paleocurrent data from many ancient wave dominated coastlines indicate that sediment transport was directed offshore at angles of 70 to 90 from the inferred local paleoshoreline solemarks and parting lineations from turbidite like storm beds and solemarks from hummocky beds indicate that flows transporting sediment offshore were orthogonal to the local shoreline the shore normal orientations of solemarks parting lineation and asymmetric ripples on hummocky cross stratification and their orthogonal relationship with respect to wave ripple crest orientation indicate that the dominant direction of wave approach at the sediment water interface was also orthogonal to the shoreline the formation of hummocky cross stratification by an al ng shore flow component superimposed on oscillatory wave motion is not supported by paleocurrent data rather much of the evidence from the ancient record suggests that hummocky cross stratification forms under high energy oscillatory flow with a weak shore normal combined flow component the orientation of wave ripple crests from the lower shoreface to offshore transitional zone of wave dominated coasts can be used to approximate the local paleoshoreline trend the paleocurrent data from the ancient examples suggest a strong predominance of offshore directed sediment transport as opposed to the nearly shore parallel orientation of currents observed in modern geostrophic flows

new insights on faults in the peace river arch region northwest alberta based on existing well log data and refined trend surface analysis this paper presents the results of structural mapping in the peace river arch region obtained by applying a refined trend surface analysis to existing well log data maps generated with the new approach allow for accurate location of formation top offsets and recognition of faults associated with small metre scale offsets consequently new faults were identified in the mesozoic strata and faults previously recognized as only offsetting paleozoic strata were found to extend into the cretaceous strata but with much smaller formation top offsets this provided direct evidence for re activation of the dawson creek graben complex dcgc during the cretaceous an additional structural feature named clear river graben was recognized to have affected permian to jurassic strata relationships among previously interpreted faults were clarified and the structural controls on the dcgc were more accurately evaluated

shelf sandstones in the mowry shale evidence for deposition during cretaceous sea level falls two previously undescribed sandstone beds in the lower cretaceous mowry shale in north central wyoming form elongate retrogradational bodies meters thick and tens of kilometers in length that are encased in shale the sandstones were deposited under different depth and oxygen conditions than were the shales the sands are extensively bioturbated and contain wave induced features oscillation ripples and hummocky stratification whereas the surrounding muds contain few biogenic structures and lack evidence of wave motion mowry sandstone units are interpreted to have been deposited by storms in shallow 10 to 30 m oxygenated water the shales that encompass these sandstones were deposited largely from turbid bottom flows in dysaerobic and anaerobic water a drop in sea level is postulated to have promoted deposition of each sandstone bed on sections of the shelf where muds had accumulated previously below wave base in oxygen poor water during low water level this region was exposed to well oxygenated water and to the effects of storms subsequent sea level rise shifted the locus of sand deposition wave base and the intersection of oxygen boundaries with the seafloor in a shoreward direction the retrogradational geometry of each sand body is explained by the shoreward migration of sand deposition during sea level rise

preservation and grain size trends of holocene wave dominated facies successions in eastern japan implications for high resolution sequence stratigraphic analysis vertical grain size trends provide significant information for the sequence stratigraphic analysis of siliciclastic wave dominated facies successions this paper discusses the relationships between the preservation timing and the grain size trends of holocene wave dom inated successions in two contrasting coastal plains the kujukuri and sendai using high resolution ams 14 c dating depositional facies and grain size modal grain size is proposed as a good criterion for eval uating qualitative changes of depositional depth the kujukuri and sendai successions are both fining upward to coarsening upward wave dominated successions that have developed upon ravinement surfaces the fining upward part of the kujukuri succession formed in regressive deposits during a highstand period whereas that of the sendai formed during transgression a difference in the timing of the generation of upward fining is due to the varying preservation potential of transgressive shoreface deposits on the ravi nement surface in the sendai area the potential was much higher than in the kujukuri area because the sendai ravinement surface had re ceived greater sediment supply from the adjacent rivers during trans gression a differing rate of tectonic uplift between the areas was not critical to the preservation potential because of the short time scale the successions show bimodal grain size distributions in the basal parts and unimodal distributions elsewhere the bimodality is consid ered to be the result of mixing of a coarse transgressive lag with finer sediment derived from external sources the finer mode forms a con sistent trend with unimodal grain sizes in overlying deposits the trend is of a simple upward coarsening in the kujukuri and an upward fining to coarsening in the sendai showing distinctive correlation with chang es of depositional depth thus the vertical trend of the modal grain size and not mean or median is best for evaluating changes of de positional depth in ancient wave dominated facies successions

development of shoreline shelf sand bodies in a cretaceous epeiric sea deposit abstract the 350 m thick spirit river formation lower cretaceous of west central alberta comprises a clastic wedge which prograded into an epeiric sea within the clastic wedge eight major transgressive and regressive sequences of sedimentation from 30 to 50 m thick have been identified cores from four sequences in the central pan of the study area reveal that the sediments consist of fine grained well sorted sandstone with mostly low angle to horizontal laminations and some palaeophycus burrows conglomerate units up to 10 m thick occur locally the sequences are capped by carbonaceous mudstone and or coal each sequence is interpreted as consisting of shoreface beach deposits with conglomeratic fluvial channel and beach deposits overlain by backswamp lagoon sediments these s quences have sharp sandy bases rather than shaly gradational ones because of a shallow depths and high wave energies and b preservation of basal transgressive deposits on a large scale the shoreline sequences can each be correlated to marine sequences which grade from shale at the bases to fine sandstone at the tops the upper pans of each marine sequence form shoreline attached sheet sandstones each coarsening upward sequence is bounded by marine shale laid down during a transgression when coarser sediment was trapped in nonmarine or marginal marine areas some sequences break up seaward into two coarsening upward units because of minor transgressions within the marine portions of some thicker sequences 5 to 10 m thick coarsening upward units can be defined which are believed to reflect fluctuations in sediment supply the correlation of these units is an application of a technique called event correlation which outlines approximate time surfaces these correlations reveal a series of imbricated seaward sloping surfaces of deposition within each major sequence sedimentation in the epeiric seaway proceeded by seaward accretion of evenly sloping bodies of sediment with no development of a shelf slope basin configuration the gradual slope formed because of a shallow depths controlled by the stable tectonic setting and rapid sediment input and b high wave energy levels which distributed sediment almost uniformly over the ea bottom

erratum contributions to the geomagnetic secular variation from a reanalysis of core surface dynamics and assimilation of ground and satellite magnetic measurements inference of core surface magnetic and velocity field changes

thrust loads and foreland basin evolution cretaceous western united states two dimensional modeling of loading during the formation of the idaho wyoming thrust belt shows that regional isostatic compensation by flexure of an elastic lithosphere is sufficient to control the formation of a foreland basin the flexural rigidity of the lithosphere is inferred to have been approximately 1023 nm 1030 dyne cm on the basis of palinspastic comparison of predicted downwarping due to the thrust plate loads to the shape of the sedimentary wedge on the west side of the cretaceous western interior seaway erosion of part of the uplifted thrust plates redistributed the load depositing it farther to the east thereby causing subsidence over a much wider area than could have been accomplished only by the loading by thrust plates paleotopography after major cretaceous thrust events was calculated the resulting mountainous terrain gentle alluvial plain and flat sea floor correspond well with the topography of the modern foreland thrust belt and basin system in the andes of south america and to paleogeographic reconstructions in the western united states thrust belt topography is controlled by the subsurface geometry of the thrust faults particularly the positions of ramp zones and by isostatic subsidence

storm deposition in marine sand sheets wall creek member frontier formation powder river basin wyoming abstract the turonian cretaceous wall creek member frontier formation and equivalent turner sandy member carlile shale powder river basin wyoming were deposited on the western margin of the north american seaway the seaway occupied the foreland of the sevier orogenic belt the wall creek turner interval is up to 67 m thick and consists of shale sandstone and minor conglomerate and bentonite deposition occurred during rising sea level following an approximately 90 ma lowstand sandstone and mudstone of the wall creek were deposited in thin shelf sand sheets 6 15 m thick up to approximately 120 km from equivalent shorelines shelf sand sheets dip gently seaward core and outcrop show shelf sandstone of the wall creek in the powder river basin to consist of burrowed to bioturbated medium scale cross beds near horizontal laminated beds and ripple laminated beds reactivation surfaces and mud drapes are common the lack of consistent periodicity across cross sets in foreset angle and thickness and unperiodic spacing of reactivation surfaces mud drapes and burrowed intervals suggest that tides were not an important transport process over most of the depositional area the dominance of storm currents over tides is also indicated by variable bed thickness structure type grain size and degree of burrowing vertically much of the mud separating beds and draping foresets likely represents sedimentation from river floods following storm events p

accounting for magnetic diffusion in core flow inversions from geomagnetic secular variation summary we use numerical dynamos to investigate the possible role of magnetic diffusion at the top of the core we find that the contribution of radial magnetic diffusion to the secular variation is correlated with that of tangential magnetic diffusion for a wide range of control parameters the correlation between the two diffusive terms is interpreted in terms of the variation in the strength of poloidal flow along a columnar flow tube the amplitude ratio of the two diffusive terms is used to estimate the probable contribution of radial magnetic diffusion to the secular variation at earth like conditions we then apply a model where radial magnetic diffusion is proportional to tangential diffusion to core flow inversions of geomagnetic secular variation data we find that including magnetic diffusion does not change dramatically the global flow but some significant local variations appear in the non frozen flux core flow models termed diffusive the hemispherical dichotomy between the active atlantic and quiet pacific is weaker a cyclonic vortex below north america emerges and the vortex below asia is stronger our results have several important geophysical implications first our diffusive flow models contain some flow activity at low latitudes in the pacific suggesting a local balance between magnetic field advection and diffusion in that region second the cyclone below north america in our diffusive flows reconciles the difference between mantle driven thermal wind predictions and frozen flux core flow models and is consistent with the prominent intense magnetic flux patch below north america in geomagnetic field models finally we hypothesize that magnetic diffusion near the core surface plays a larger role in the geomagnetic secular variation than usually assumed

sequence stratigraphy of sixth order 41 k y pliocene pleistocene cyclothems wanganui basin new zealand a case for the regressive systems tract this study is based on a late pliocene and early pleistocene approximately 2 6 1 7 ma succession about 1 km thick of 20 sixth order 41 k y duration cyclothems of shelf origin exposed in the rangitikei river valley in the eastern part of wanganui basin the cyclothems correlate with 18 o isotope stages 100 58 and each 41 k y glacialinterglacial stage couplet is represented by an individual depositional sequence comprising transgressive highstand and regressive systems tracts unlike most examples inferred from the stratigraphic record these systems tracts were deposited during phases of known sea level cycles indicated by the contemporary oxygen isotope ice volume curve because of the high rate of subsidence of wanganui basin glacioeustatic sea level falls during most cycles were not of sufficient magnitude to expose the outer shelf thus the rangitikei section provides an exceptional example of regressive strata deposited landward of the contemporary shelf break simple one dimensional modeling shows that moderate to high rates of basin subsidence 1 2 mm yr and low rates of sedimentation 0 2 mm yr during transgressions combined to produce an accommodation surplus at the relative highstand this surplus accommodation was infilled during the late highstand and ensuing fall partly by aggradational highstand systems tract shelf siltstone and chiefly by strongly progradational shoreface sediments of the regressive systems tract rangitikei regressive systems tracts are distinguished from forced regressive systems tracts sensu hunt and tucker 1992 by their different stratal geometry by definition forced regressive systems tracts display an erosional contact with the underlying highstand systems tracts and typically occur as a series of downstepped disjunct shoreline wedges stranded on the shelf and or slope in contrast regressive systems tracts exhibit a gradational lower contact above which parasequences are stacked in a strongly progradational pattern terminated by the superjacent sequence boundary cyclothems display two types of motif termed rangitikei dt depositional transgression and rangitikei nt nondepositional transgression which include the following architectural elements in ascending stratigraphic order 1 a basal sequence boundary that is coincident with either the transgressive surface of erosion which displays small scale up to 50 cm erosional relief and may be penetrated by the ichnofossil ophiomorpha or its deeper water correlative conformity 2 either a thick 5 30 m transgressive systems tract comprising a deepening upward nearshore to inner shelf mixed carbonate siliciclastic lithofacies succession depositional transgression or a thin 2 m transgressive systems tract comprising condensed fossiliferous facies deposited on the sediment starved offshore shelf nondepositional transgression 3 a sharp downlap surface separating condensed fossiliferous facies of the transgressive systems tract from terrigenous siltstone of the superjacent highstand systems tract 4 a highstand systems tract comprising a 10 20 m thick interval of aggradational shelf siltstone and 5 a thick up to 45 m progradational inner shelf to shoreface lithofacies assemblage ascribed to the regressive systems tract condensed shell beds are associated with intrasequence and sequence bounding discontinuities and together with the sedimentological and stratal characteristics of the sequences are important indicators of stratigraphic architecture four types of shell bed are associated with surfaces formed by four different types of stratal termination onlap backlap downlap and flooding surface shell beds cf kidwell 1991 are associated respectively with the transgressive surface of erosion apparent truncation at the top of the transgressive systems tract the downlap surface and local marine flooding surfaces a fifth shell bed type termed a compound shell bed forms in offshore environments where the downlap surface converges with the sequence boundary and elements of both the downlap and the backlap shell beds become mixed or superposed the shell beds mark zones of stratal attenuation and can be used as surrogates for seismic discontinuities when applying sequence stratigraphic concepts at outcrop scale

the south celtic sea bristol channel basin origin deformation and inversion history abstract the south celtic sea bristol channel basin is a relatively narrow and elongated sedimentary trough located within the northern part of the hercynian fold belt and filled with permo triassic redbeds to jurassic marine sediments its present configuration is the result of a three fold sequence of tectonic phases initiated during the late palaeozoic the origin of the basin appears to be related to localised extensional reactivation of hercynian thrust zones during the permian and early triassic followed by regional subsidence during the late triassic and jurassic uplift of the basin margins together with dextral strike slip movements in an east west sense and large scale erosion during the late jurassic to early cretaceous resulted in the present elongated configuration of the lower mesozoic basin regional subsidence from aptian time onwards resulted in widespread deposition of a relatively thin section of wealden elastics and upper cretaceous chalk unconformably overlying folded pre cretaceous strata sedimentation was interrupted during the early tertiary by a series of tectonic events that included emplacement of the lundy igneous mass during the early eocene and mild basin inversion during the oligo miocene inversion was accompanied by dextral reactivation of the nw se oriented sticklepath fault zone the total amount of uplift probably not exceeding 350 m reflects the rather stable configuration of the underlying lithospheric crust as indicated by seismic reflection and refraction data

sedimentation and basin fill history of the neogene clastic succession exposed in the southeastern fold belt of the bengal basin bangladesh a high resolution sequence stratigraphic approach abstract the tertiary basin fill history of the bengal basin suffers from oversimplification the interpretation of the sedimentary history of the basin should be consistent with the evolution of its three geo tectonic provinces namely western northeastern and eastern each province has its own basin generation and sediment fill history related mainly to the indo burmese and subordinately to the indo tibetan plate convergence this paper is mainly concerned with facies and facies sequence analysis of the neogene clastic succession within the subduction related active margin setting oblique convergence in the southeastern fold belt of the bengal basin detailed fieldwork was carried out in the sitapahar anticline of the rangamati area and the mirinja anticline of the lama area the study shows that the exposed neogene succession represents an overall basinward progradation from deep marine through shallow marine to continental fluvial environments based on regionally correlatable erosion surfaces the entire succession 3000 m thick has been grouped into three composite sequences c b and a from oldest to youngest composite sequence c begins with deep water base of slope clastics overlain by thick slope mud that passes upward into shallow marine and nearshore clastics composite sequence b characteristically depicts tide dominated open marine to coastal depositional systems with evidence of cyclic marine regression and transgression repetitive occurrence of incised channel tidal inlet tidal ridge shoal tidal flat and other tidal deposits is separated by shelfal mudstone most of the sandbodies contain a full spectrum of tide generated structures e g herringbone cross bedding bundle structure mud couplet bipolar cross lamination with reactivation surfaces tidal bedding storm activities appear to have played a subordinate role in the mid and inner shelf region rizocorallium rosselia planolites and zoophycos are the dominant ichnofacies within the shelfal mudstone this paralic sedimentation of neogene succession in the study area can serve as a good point of reference for tide dominated regressive shelf depositional systems the top of the composite sequence b is marked by a pronounced erosion surface indicating the final phase of marine regression followed by the gradual establishment of continental fluvial depositional systems represented by composite sequence a in this composite sequence stacked channel bars of low sinuosity braided rivers gradually pass upsequence into high sinuosity meandering river deposits a sequence stratigraphic approach has been adopted to interpret the basin fill history with respect to relative sea level changes and to subdivide the rock record into several sequences and units systems tracts and parasequences based on identified bounding discontinuities such as transgressive erosion surface tes regressive erosion surface res marine flooding surface mfs and incised valley floor ivf this approach provides new insight for both exploration and exploitation strategy for hydrocarbon plays that may prove vital to the oil companies engaged in exploration activities in the bengal basin it is strongly recommended here that the traditional lithostratigraphic classification of this part of the basin which is based on the assam stratigraphy be abandoned or at least revised a tentative allostratigraphic scheme is presented and it is suggested that to formalize the scheme further study both surface and subsurface is needed

on core surface flows inferred from satellite magnetic data abstract satellite data allows the magnetic field produced by the dynamo within the earth s core to be imaged with much more accuracy than previously possible with only ground based data changes in this magnetic field can in turn be used to make some inferences about the core surface flow responsible for them in this paper we investigate the improvement brought to core flow computation by new satellite data based core magnetic field models it is shown that the main limitation now encountered is no longer the now high accuracy of those models but the non modelled secular variation produced by interaction of the non resolvable small scales of the core flow with the core field and by interaction of the partly resolvable large scales of the core flow with the small scales of the core field unfortunately masked by the crustal field we show how this non modelled secular variation can be taken into account to recover the largest scales of the core flow in a consistent way we also investigate the uncertainties this introduces in core flows computed with the help of the frozen flux and tangentially geostrophic assumptions it turns out that flows with much more medium and small scales than previously thought are needed to explain the satellite data based core magnetic field models it also turns out that a significant fraction of this flow unfortunately happens to be non recoverable being either non resolvable because too small scale or invisible because in the kernel of the inverse method even though it produces the detectable non modelled secular variation applying this to the magsat 1980 to orsted 2000 field changes leads us to conclude that a flow involving at least strong retrograde vortices below the atlantic hemisphere some less resolved prograde vortices below the pacific hemisphere and some poorly resolved and partly non resolvable polar vortices is needed to explain the 1980 2000 satellite era average secular variation the characteristics of the fraction of the secular variation left unexplained by this flow are also discussed

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

climatic cyclicity and terrigenous sediment influx to the early turonian greenhorn sea southern utah abstract rhythmically alternating beds of limestone and shale in pelagic deposits of the cenomanian turonian north american western interior seaway have been widely attributed to climatic cyclicity in the milankovitch band whether these rhythmic beds record cycles of terrigenous dilution or of productivity remains controversial we present results of a study of early turonian strata of the tropic shale and tununk member of the mancos shale in southern utah aimed at testing the dilution model in a prodeltaic environment within 130 km of the paleoshoreline the study transect consisted of four localities spaced along a 110 km onshore offshore transect in each of the four sections we studied a stratigraphic interval characterized by beds of limestone or marlstone that alternate with beds of marly shale or calcareous shale along the transect content of quartz sand and coarse silt in each of the beds generally decreases and content of calcium carbonate increases in each section quartz sand and coarse silt content varies antithetically with calcium carbonate content these observations suggest that the limestone marlstone beds accumulated during times when rates of terrigenous sediment transport to offshore sites were relatively slow conversely the shaly beds were deposited during times of enhanced terrigenous sediment flux these ob ervations support the hypothesis that limestone shale bedding rhythms record wet and dry phases of climate cycles in utah the limestone marlstone beds contain smaller proportions of mixed layer illite smectite clays than do the shaly beds climatic cyclicity apparently led to cyclic variations in the proportions of clay minerals supplied to the prodeltaic environments or to variation in the loci of accumulation of different clay minerals analysis of intensity of bioturbation and preservation of fecal pellets also indicates that the limestone marlstone beds accumulated under more oxic benthic conditions than did the shaly beds detailed examination of the alternating beds of limestone marlstone and shale in southern utah indicates that the deposits are characterized by a distinctive succession of facies these deposits are better interpreted as bedding cycles composed of three gradational lithologies than as bedding rhythms composed of two alternating lithologies data on grain size and calcium carbonate content suggest that the bedding cycles correspond to shoaling upward parasequences that developed in response to small scale changes in relative sea level our study suggests that climatically modulated changes in fresh water discharge and terrigenous sediment supply were mainly responsible for these sea level changes and for cyclic variations in benthic oxygenation in prodeltaic environments of the earl turonian western interior seaway

combined tide and wave influence on sedimentation patterns in the upper jurassic swift formation south eastern alberta in south eastern alberta the oxfordian swift formation comprises two unconformity bounded sequences sequence 1 consists of the shale member it unconformably overlies bathonian calcareous shale of the rierdon formation and consists dominantly of shale with uncommon hum mocky cross stratified siltstone the shale member comprises up to four parasequences that show a subtle upward fining stacking pattern suggesting overall deepening in a storm dominated fully marine basin deposition of the shale member was terminated by a fall in relative sea level which in turn initiated sequence 2 during lowstand a network of north east to south west trending meandering channels incised the top of sequence 1 despite the preserved morphology of these channels observations in core suggest that lowstand deposits are absent these strata were most likely thoroughly reworked by ravinement processes associated with the ensuing transgression and are recorded only by a thin chert pebble lag also associated with transgression were significant changes in the physical and ecological conditions in the local swift basin these changes were related to modifications in basin configuration and the development of a low energy strait which probably formed as a result of uplift of the sweetgrass arch within this newly configured basin brackish water conditions prevailed and sedimentation was dominated by suspension deposition of mud bed load sediment composed mostly of very fine and fine sand made up only a small part of the total sediment flux into the area and was transported principally by low energy combined flows with variable speed microtidal currents and a low energy oscillatory component related to storm waves sand was generally deposited in the form of small discontinuous sand ridges that developed locally throughout the study area it is these features that form the principal hydrocarbon reservoir bodies in the swift formation in the study area after initiation ridges built upwards and migrated laterally in response to transport and preferential local deposition of bed load sediment although common near the unconformity along the top of sequence 1 sand ridges also occur at several higher stratigraphic levels in sequence 2 these latter ridges however are not associated with regionally correlatable discontinuities but instead were initiated by local bed irregularities possibly related to breaking internal waves and were supplied with new sediment transported into the study area the origin of these sand ridges is therefore not related to changes in relative sea level but instead to intrabasinal processes in a low energy strait

lithology dependent diagenetic control of reservoir properties of conglomerates falher member elmworth field alberta conglomerates in the falher member of the lower cretaceous spirit river formation of alberta are the reservoir rocks for the giant 2 3 tcf or 5 8 1010 m3 elmworth gas field three types of conglomerates are present a unimodal granules or pebbles lacking any matrix b bimodal grain supported a framework of pebbles with fine to medium sand in the interstices and c bimodal sand supported pebbles floating in fine to medium sandstone the distribution of these three types is controlled by the original depositional environment the granules and pebbles are composed of chert and silicified sedimentary and volcanic rock fragments the sand grains are dominantly quartz during diagenesis very small 01 0 1 mm drusy quartz crystals form on the cherts and rock fragments whereas quartz grains undergo heavy quartz overgrowth formation because of differences in nucleation and growth of quartz crystals as a consequence most unimodal conglomerates low in quartz have not experienced major reduction of porosity and permeability bimodal conglomerates both sand and pebble supported have lost significant amounts because of cementation of the quartz rich matrix streaks and patches of kaolinite and calcite cements have also reduced porosity and permeability locally gas production from falher wells therefore de ends not only on the amount of conglomerate but also on the proportion of unimodal and bimodal types

tectonic subsidence flexure and global changes of sea level tectonic models for the evolution of passive continental margins predict that following rifting sediments should progressively onlap basement at the edge of a margin as the lithosphere cools and increases its flexural rigidity with age the pattern of modelled onlap is strikingly similar to that used by vail and colleagues to estimate sea level rise through geological time this similarity suggests that major portions of stratigraphical sequences at margins may have a tectonic rather than eustatic control the patterns of onlap used by vail and colleagues may be widespread however because several widely separated passive margins rifted at similar times but they are unlikely to be worldwide

four storm event beds and the tropical cyclones that produced them a numerical hindcast a three dimensional numerical prediction system for storm sedimentation has been constructed to compute a cyclonic wind field coastal circulation storm waves generated over the continental shelf the combined effects of steady currents and waves on the benthic boundary layer both suspended and bed load transport of sediment and conservation of the sea floor it has been used to hindcast the oceanographic and sedimentological responses of the western gulf of mexico to four historical tropical cyclones in order to investigate the effects of coastal configuration and storm variability on event bed genesis the simulations reveal several common responses to these storms 1 onshore flow to the fight of the storm track generally transports fine sediment landward 2 off shore flow to the left transports coarser sediment seaward and 3 for an observer facing the coast right to left along shelf flow transports finer sediment in deep water and coarser sediment in shallow water the main source of sandy sediment on the inner shelf is the shoreface which is eroded by currents driven by wind stress geostrophy and offshore flows associated with collapse of the coastal setup coastal geometry is the dominant factor in determining sedimentation patterns along the coast in front of each storm the volume of sediment transported obliquely onshore or offshore is a function of shelf gradient and coastal configuration steeper gradients constrain flow to a more longshore pattern concave coastlines promote greater shoreface erosion because of increased coastal setup none of the simulations reveals the vertical current structure predicted by the geostrophic model for continental shelf circulation because the calculated depth of the wind mixed layer exceeded local water depth everywhere over the shelf thus in these experiments most of the continental shelf fell within the friction dominated zone in which the upper and lower boundary layers overlap resulting in vertically uniform along shelf flow

barriers barrier platforms and spillover deposits in st george s bay newfoundland paraglacial sedimentation on the flanks of a deep coastal basin abstract st george s bay and adjacent coastal areas of southwest newfoundland contain thick sequences of late quaternary glaciogenic sediments and have been subject to relative sea level fluctuations over a rang of 70 m during the past 13 5 ka glaciogenic sediments have been mobilized by littoral processes to form two types of deposits these are 1 the prograded barrier beach and spit at stephenville and flat island respectively which are associated with large subaqueous sandy platforms and 2 a wedge of well stratified sand and gravel on the landward flank of the shallow sill which extends across st george s bay at an average depth of about 25 m the subaerial barriers and spits are composed of gravel beach ridges overlain locally by thin aeolian deposits with freshwater and salt marsh peats in swales these barriers developed during the last several thousand years as relative sea level approached its present position from an early postglacial minimum of approximately 25 m the prerequisite for beach ridge formation was the deposition of large sandy subaqueous platforms which appear on shallow seismic reflection records as clinoform wedges prograded into the deep basin of inner st george s bay the barrier platform at stephenville is constructed across a deep submarine valley the flat island barrier platform sits on the flank of a glacially overdeepened trough partly filled by glacial and postglacial sediments and envelops a delta formed during the early postglacial low stand of relative sea level the platform and barrier together constitute a thick coastal sequence coarsening up from sandy mud at the base to gravel at the top the wedge of well stratified coarse sediment up to 50 m thick situated on the landward flank of the shallow sill overlies older glaciogenic sediments and passes laterally into postglacial basin muds steep foreset reflections dipping 10 20 are evident on shallow seismic reflection records suggesting that the wedge was formed by spillover of sediment entrained by wave and current action into the relatively deep basins of the inner bay active gravel ripples testify to the continued mobility of coarse sediment on the sill the formation of the spillover deposits barriers and platforms demonstrates the potential for glacially overdeepened basins to host thick heterogeneous coarse grained coastal and inner shelf deposits

variability modes in core flows inverted from geomagnetic field models the flow of liquid metal inside the earth s core produces the geomagnetic field and its time variations understanding the variability of those deep currents is crucial to improve the forecast of geomagnetic field variations which affect human spacial and aeronautic activities moreover it may provide relevant information on the core dynamics the main goal of this study is to extract and characterize the leading variability modes of core flows over centennial periods and to assess their statistical robustness to this end we use flows that we invert from two geomagnetic field models gufm1 and cov obs and apply principal component analysis and singular value decomposition of coupled fields the quasi geostrophic qg flows inverted from both geomagnetic field models show similar features however cov obs has a less energetic mean and larger time variability the statistical significance of flow components is tested from analyses performed on subareas of the whole domain bootstrapping methods are also used to extract significant flow features required by both gufm1 and cov obs three main empirical circulation modes emerge simultaneously constrained by both geomagnetic field models and expected to be robust against the particular a priori used to build them large scale qg dynamics mode 1 exhibits three large vortices at medium high latitudes with opposite circulation under the atlantic and the pacific hemispheres mode 2 interestingly accounts for most of the variations of the earth s core angular momentum in this mode the regions close to the tangent cylinder and to the equator are correlated and oscillate with a period between 80 and 90 years each of these two modes is energetic enough to alter the mean flow sometimes reinforcing the eccentric gyre and other times breaking it up into smaller circulations the three main circulation modes added together to the mean flow account for about 70 of the flows variability 90 of the root mean square total velocities and 95 of the secular variation induced by the total flows direct physical interpretation of the computed modes is not straightforward nonethe less similarities found between the two first modes and time spatial features identified in different studies of core dynamics suggest that our approach can help to pinpoint the relevant physical processes inside the core on centennial timescales

global tectonics since the breakup of pangea 180 million years ago evolution maps and lithospheric budget abstract pangea the earth s youngest single supercontinent broke up 180 million years ago tectonic plates were subsequently formed by dispersal of the continental fragments and accretion of new oceanic lithosphere the configurations of all the major lithospheric plates at 0 20 65 95 140 180 ma bp are reconstructed on six globes of the earth each with a radius of 10 cm it appears that plate boundaries maintain a remarkably close fit on model globes of constant radii if the reconstructions include the recovery of subducted spreading patterns this is illustrated with maps in equatorial orthographic oblique orthographic and transverse hammer aitoff projections the snug fit of the tectonic plates at every tested time since the breakup of pangea 180 ma bp is consistent with the theory of plate tectonics on a non expanding earth and contradicts rapidly expanding earth models the areas of oceanic lithosphere produced and consumed during the past 180 ma bp are estimated from surface measurements of the globes reconstructed on the basis of particular assumptions these measurements suggest a consistent increase in the production rate of oceanic lithosphere during the past 140 ma it was decided to revise the assumptions and see if alternative reconstructions of the ancient spreading patterns on the floors of the tethys and eo pacific oceans could avoid implying an increase of lithospheric production rates with time this appeared to be possible the revised maps suggest that ophiolites older than 180 ma bp may have been obducted in cenozoic collision zones of the himalayas andes rockies and the western part of the banda arc timor new guinea estimates of the ocean floor production and consumption budget appear to be quite similar for both map series and only the possible ranges are summarized here time averaged for the past 180 ma world wide production and consumption of oceanic lithosphere appears to have varied between 2 6 and 3 5 km2 a 1 at most the mean of world wide spreading velocities ranges between 2 4 and 3 5 cm a 1 and world wide means of subduction velocities lie between 4 2 and 6 3 cm a 1 more accurate estimates are possible on the basis of particular assumptions and have been specified for the various time intervals distinguished i e the neogene 0 20 ma bp palaeogene 20 65 ma bp upper cretaceous 65 95 ma bp lower cretaceous 95 140 ma bp and middle and upper jurassic 140 180 ma bp

magnetic boundary layers in numerical dynamos with heterogeneous outer boundary heat flux abstract it has been proposed that magnetic flux expulsion due to outer core fluid upwellings may affect the geomagnetic secular variation on the core mantle boundary bloxham 1986 in this process intense horizontal field lines are concentrated below the outer boundary introducing small radial length scales and consequently strong radial diffusion here we explore such magnetic boundary layers in numerical dynamo simulations with heterogeneous outer boundary heat flux inferred from a tomographic model of lower mantle seismic shear waves velocity anomalies our scheme associates magnetic boundary layers to peak horizontal magnetic fields at the top of the shell in our models mean magnetic boundary layer thickness ranges 200 400 km and decreases with increasing magnetic reynolds number extrapolation or interpolation to earth s core conditions based on total core flow amplitude or its poloidal part gives mean magnetic boundary layer thickness of 220 and 260 330 km respectively we find magnetic boundary layers associated with the azimuthal field at the equatorial region whereas magnetic boundary layers associated with the meridional field are found at mid latitudes negative outer boundary heat flux anomalies yield preferred locations of expulsion of azimuthal field below africa and the pacific while positive outer boundary heat flux anomalies yield preferred locations of expulsion of meridional field below the americas and east asia furthermore we find a tendency of the azimuthal field to low latitudes of the northern hemisphere our results suggest that the local diffusion time is on the order of several kyr and the local magnetic reynolds number is on the order of 10 both much smaller than classical estimates

sedimentary balance and sand stock availability along a littoral system the case of the western gulf of lions littoral prism france investigated by very high resolution seismic very high resolution seismic data was acquired along the microtidal wave dominated littoral zone of the western gulf of lions se france with a view to estimate the volume of sand present in the upper shoreface under conditions of stable high sea level an upper sand unit is thus identified at the top of the shoreface bound at its base by a hard floor made of beach rock layers or geological substrate this upper sand unit usu is mobile and represents the available sand stock used for internal exchanges and beach supply estimate of the usu volume takes place within each sedimentary compartment and cell defined along the littoral zone two main results are obtained from this quantification 1 the distribution of the sand stock fits with the littoral drift pattern updrift source zones being characterized by low volume usu while downdrift sink areas are characterized by much larger sand reservoirs this distribution pattern is verified at all scales the regional the compartment and the cell scales 2 the volume of the upper shoreface sand reservoir seems to match the adjacent beach behaviour wherever the beach evolution trend is negative the stock is depleted and on the contrary the beaches associated to voluminous reservoirs do not suffer erosion

influence of the ancestral sweetgrass arch on sedimentation of the lower cretaceous bootlegger member north central montana abstract in north central montana the bootlegger member of the upper lower cretaceous blackleaf formation is well exposed near great falls and at one locality in the sweetgrass hills in both areas bootlegger strata comprise four stratigraphic successions in the great falls study area each succession consists of an upward coarsening shoaling package of interbedded very fine sandstone and shale grading upward into fine to medium sandstone abruptly overlain by a transgressive erosion surface and capped in many places by a conglomeratic transgressive lag deposit the stacking of these successions is attributed to episodic changes of relative sea level probably related to episodic reactivation of the ancestral sweetgrass arch during episodes of arch uplift accommodation space was reduced a d caused the bootlegger shoreline to prograde eastward subsequently because of arch subsidence and or the ongoing basinwide eustatic rise the shoreline migrated westward erosion associated with transgressive shoreface retreat was most intense near the crest of the arch as a result in the western study area which lies above the arch crest successions are thin and typically consist only of inner shelf strata lithofacies 1 strata deposited in shallower environments were removed mainly by later transgressive erosion by contrast in the eastern area farther from the arch crest the upward shoaling part of each succession is more completely preserved at the end of bootlegger time however movement of the arch either ceased or simply could no longer keep pace with the eustatic ri e consequently the shoreline continued to transgress westward and only fossiliferous silt and mud accumulated in a distal shelf environment this marked a eustatic drowning and termination of rhythmic bootlegger sedimentation

sequence stratigraphic analysis of individual depositional successions effects of marine nonmarine sediment partitioning and longitudinal sediment transport mannville group alberta foreland basin canada in the falher member of the mannville group aptian albian of western canada two shoreline successions contain the reservoir conglomerates for the giant elmworth gas field the falher b succession has a basal sheetlike shoreface unit of hummocky cross stratified sandstone that thins seaward and terminates about 30 km north seaward of the landward limit of the transgression another 25 km farther basinward the succession shows a 20 30 m thick sandstone unattached to the prograding shoreface and an overlying coarsening upward shoreface succession with thin muds and coals interpreted as back barrier deposits these basinward facies are the results of a relative sea level fall and the early stage of the subsequent rise in the upper falher a succession immediately andward south of the barriers fluvial valleys were incised into nonmarine mudstones and coals during the base level fall as relative sea level subsequently rose in nonmarine areas the valleys were filled by estuarine and fluvial sands then a widespread sheet of fine grained nonmarine sediment was deposited at the same time the shoreline migrated back across the shelf as it reached the original shorezone structurally controlled reworking of underlying deposits successively generated three gravelly barrier islands superimposed on the sandy shoreface succession the conglomeratic reservoirs all rest above the unconformities in the transgressive depositional system because this sequence is essentially a set of linked facies and not a composite of stacked individual facies successions it is affected by sediment partitioning between facies during relative sea level rise little marine sedimentation occurred because sediment was trapped mainly in nonmarine areas conversely during sea level fall most deposition occurred in marine areas because of the absence of nonmarine accommodation westward alongshore toward the thrust belt no falling or lowstand sea level succession developed instead a wide regressive shoreface sandstone with a transgressive cap occurs subsidence rates were higher in this area and relative sea level appears always to have risen but at varying rates the surface under the transgressive facies changes from a type 1 unconformity in eastern lower subsidence areas to a type 2 unconformity in western higher subsidence areas any two dimensional sequence stratigraphic model therefore is inadequate to describe the lateral variation of the sequence and distribution of shoreface sandstones because the subsidence gradient was not parallel to the direction of shoreface progradation

tectonic influence on deposition and erosion in a ramp setting upper cretaceous cardium formation alberta foreland basin this paper presents the results of a combined outcrop and subsurface investigation of the upper cretaceous cardium formation in northwestern alberta and adjacent british columbia canada more than 100 measured core and outcrop sections combined with more than 1100 well logs were used to examine stratigraphic relationships in an area of over 45 000 km2 the cardium consists of stacked regressive transgressive cycles deposited during shoreline progradation along the western margin of a foreland basin occupied by the western interior seaway in general these cycles consist of shoaling upward successions highstand systems tracts composed of shoreface sandstones and conglomerates and heterolithic offshore deposits capped by transgressive erosion surfaces linear incised lowstand coarse grained shoreface deposits are present at some stratigraphic levels a single thick locally more than 20 m thick in the study area nonmarine wedge that overlies shoreface sandstones is interpreted to represent deposition during rising sea level rather than accumulation behind a prograding strandline preservation of the incised lowstand shoreface deposits is associated with topographic breaks in places trends of isopachs facies transitions erosional bevels on transgressive erosion surfaces synsedimentary faults and modern production trends can be directly superimposed and closely correspond to basement structural trends this suggests that basement structures were being remobilized during cardium deposition possibly in response to thrusting in the orogen c belt

transgressive depositional systems of the mississippi delta plain a model for barrier shoreline and shelf sand development abstract depositional sequences generated in the mississippi river delta plain consist of a regressive and a transgressive component the transgressive component has been considerably less studied but accounts for the majority of the surface area on the lower mississippi river delta plain and up to 50 percent of the total sequence thickness in shallow water deltas the development and preservation of transgressive depositional systems in abandoned delta complexes follows the process of transgressive submergence in which the horizontal component of reworking occurs during shoreface retreat combined with a vertical component of submergence acting to preserve the sequence the evolution of transgressive depositional systems in each of the abandoned holocene mississippi river delta complex s can be summarized in a three stage model beginning with stage 1 an erosional headland and flanking barriers in this stage regressive sand deposits contained within abandoned deltaic headlands are reworked by the eroding shoreface and dispersed longshore into contiguous flanking barriers enclosing restricted interdistributary bays submergence of the delta plain during relative sea level rise generates an intradeltaic lagoon separating the former stage 1 sand body from the shoreline forming stage 2 a transgressive barrier island arc the landward migrating barrier island arc is unable to keep pace with relative sea level rise and the retreating mainland shoreline resulting in submergence and the formation of stage 3 an inner shelf shoal following submergence the former barrier island arc sand body continues to be reworked into a marine sand body on the inner continental shelf during stage 3 this sequence of coastal evolution provides direct evidence of barrier island formation with each stage producing a distinctive stratigraphic signature the current sea level rise models of shoreface retreat and in place drowning developed for the u s atlantic continental shelf do not adequately explain either the morphology or the stratigraphy of transgressive mississippi river delta sand bodies current models of mississippi deltaic stratigraphy emphasize the deep water artificially maintained balize delta which differs considerably from the shallow water shelf phase delta complexes that are the primary depositional constituents of he holocene mississippi river delta plain

a time varying 3 d displacement model of the 5 9 year westward motion and its applications for the global navigation satellite system positions and velocities

a model for migrating shelf bar sandstones in upper mancos shale campanian northwestern colorado the duffy mountain sandstone was deposited by migrating submarine shelf bars that are now completely encased in marine mudstones of the mancos shale upper cretaceous the shelf bars were deposited at least 10 mi 16 km offshore and stood approximately 100 to 150 ft 30 to 46 m above a shallow shelf the bars were never emergent but were shallow enough for fair weather waves to form ripples along their crests these elongate sand bodies trend northeast southwest approximately parallel to the shoreline the sediment was probably derived from reworking of transgressed littoral deposits in southwestern wyoming four subenvironments are recognized 1 landward flank of the shelf bar the back bar 2 the axial crest of central bar 3 a seaward facing ramp subfacies and 4 the shelf upon which the shelf bars migrated two major processes were active during deposition 1 fair weather longshore southwesterly currents and 2 storm erosion and westerly across the shelf bar sediment transport during fair weather sand waves transported sediment along the central bar while the back bar and shelf were being bioturbated storms eroded the seaward face of the bar and portions of the central bar creating hummocky erosion surfaces eroded sediment transported across the bar was deposited in coarsening upward spillover lobes in the back bar after the storm waned sediments deposited in t e back bar were bioturbated and sand waves were deposited on the eroded central bar migration of sand waves onto the eroded face of the bar formed the ramp facies as the shelf bar migrated the central bar facies climbed stratigraphically over the back bar facies the direction of shelf bar migration was probably controlled by the interaction of fair weather and storm sediment transport