magnetic fabrics of the miocene ignimbrites from west cameroon implications for pyroclastic flow source and sedimentation abstract the miocene ignimbrites of mounts bambouto and bamenda located in the central part of cameroon volcanic line are generally made of welded and non welded massive lapilli tuff and lithic breccias these discontinuous deposits cover a total area of 180 km 2 with thickness ranging from 25 to 200 m the different facies contain several lithic fragments of mainly trachytic nature the devitrified matrix of the welded ignimbrites is constituted by sanidine anorthoclase quartz plagioclase clinopyroxene biotite fe ti oxides and devitrified fiammes anisotropy of magnetic susceptibility ams is used to characterize magnetic fabrics and to provide an estimate of flow direction of each ignimbrite sheet magnetic mineralogy results from different flow units show that titanomagnetite titanohematite maghemite and goethite with grain size ranging from coarse md to very fine sp are the main magnetic carriers of these ignimbrites inferred transport directions based on the ams data and field indicators show that bambouto caldera is the source of main pyroclastic deposits of mount bambouto in southwestern mount bamenda santa mbu caldera or bambouto caldera constituted the probable emission center of mbengwi bamenda and mbu ignimbrite sheets whereas magnetic fabrics of bambili sabga and big babanki ignimbrites demonstrate that these deposits were emitted from a northeastern source namely oku vent in mount oku a small number of subvertical ams fabrics correspond to rocks possibly modified by an elutriation process

world s highest tides hypertidal coastal systems in north america south america and europe abstract hypertidal systems can be defined as areas where spring tides have ranges greater than 6 m these very high tidal ranges results in unique patterns of sedimentation within hypertidal estuaries such systems are not common but they do occur on a number of continents this report will discuss six areas that have the highest tides in the world north america hypertidal systems occur within cook inlet in alaska usa leaf basin in ungava bay quebec province canada and the bay of fundy nova scotia and new brunswick canada in south america the straits of magellan and associated atlantic coastal settings exhibit hypertidal conditions european hypertidal systems include bristol channel and severn estuary in southwest england and the gulf of st malo in normandy france these six areas have the highest tides in the world and spring tidal ranges that regularly exceed 10 m all the six areas can be divided into intertidal sedimentological zones zone 1 is the outermost zone and contains longitudinal bars zone 2 exhibits laterally extensive sand flats zone 3 includes the innermost extent of tides and estuarine point bars annual and neap spring cycles have been documented in zone 3 and are probably the most indicative features of hypertidal systems the north american systems occur in high latitude cold climates where winter ice can have a minor or major impact on the development of sedimentary facies conversely the european and patagonia systems have climates minimal ice formation

porosity and hydrocarbon distribution in the north sea chalk reservoirs abstract since 1970 more than 25 billion barrels of hydrocarbons in place at reservoir conditions have been discovered in the north sea chalk reservoirs for accurate development drilling satisfactory reserve estimate and optimal recovery a precise description of the porosity and hydrocarbon distribution has proved necessary because such reservoirs were unknown it took some time to fully understand their local and regional variation based on a 12 year experience of the north sea chalks and the data from 180 wells a basic geological model was established and tested on the other fields in the area and is described in this paper thickness variations are due to differential diagenesis as well as depositional factors porosity generally decreases downsection and towards the flank as a result of the progressive invasion of rock by oil and subsequent inhibition of chemical compaction layering in porosity reflects differences in initial porosity hydrocarbon saturation varies with structural elevation and rock type i e depositional facies mineral content differential behaviour to early and late diagenesis pore and pore throat size and morphology tortuosity this report is divided into two parts development of the model and comments on the individual fields

reply to wolff and turbeville s comment on a reappraisal of ignimbrite emplacement progressive aggradation and particulate to non particulate flow transitions during emplacement of high grade ignimbrite by mj branney and p kokelaar branney and kokelaar 1992 emphasized that many features of pyroclastic flow deposits largely record the last processes involved in their formation so wolff and turbeville s glib reminder of this philosophy at the end of their comment is quite unnecessary many structures in high grade ignimbrites record non particulate flow that occurred some considerable time after particulate aggradation ceased this is not in question and we pointed this out in our paper it is the earlier deposition and deformation history of rheomorphic ignimbrites that is at issue this is the aspect that bears more widely on the nature of pyroclastic flows and related eruptive phenomena in another paper branney and kokelaar 1994 we have documented evidence for hotstate remobilization and deformation of some stationary rheomorphic ignimbrites by post emplacement disturbance caldera collapse however in branney and kokelaar 1992 we provided evidence that shows that for most rheomorphic ignimbrites it is inappropriate to assume a twofold flow history of hot remobilization after an initial flow came to rest progressive aggradation and agglutination provide the most straightforward explanation for many of the markedly irregular vertical variations in welding intensity characteristic of high grade ignimbrites just as progressive aggradation best accounts for vertical variations in sedimentary lithofacies and or in chemical composition in other ignimbrites

structural evolution of the feda graben area a new model the pre cretaceous basin evolution of the feda graben area in the vicinity of the norwegian danish basin has been reconstructed utilizing geological and structural interpretation the analysis reveals that the basin was faulted at its borders prior to the salt deposition in the late permian salt movement was initiated in late triassic and thick triassic and lower jurassic pods were deposited in the graben area due to this movement salt pillows were developing along the feda graben bordering faults until middle jurassic when the pillows were collapsed salt diapirs within the study area preferentially occupy the crest of the feda graben and their occurrence is controlled by the underlying faulted topography the diapirs were fed by salt from the central and southern parts of the basin and were developed by different processes i e upbuilding downbuilding various raft structures were developed in the graben area hanging wall while some uplift occurred in the footwall during mesozoic rifting the feda graben area experienced rifting from late jurassic to early cretaceous the most pronounced subsidence episode related with this rifting in the feda graben area took place along the eastern bounding gert fault the mesozoic rifting event is marked by a major unconformity on the seismic sections throughout the study area furthermore the region experienced basin inversion in late cretaceous the effects of inversion are more pronounced in the western part and along the gert fault the inversion phenomenon can be properly understood only when considered together with the geometry of the late jurassic half graben due to some inconsistencies in the previously proposed models for the development of the feda graben a new conceptual model has been constructed

paleoflow directions of a subaqueous lahar deposit around the miocene keserűs hill lava dome complex north hungary as constrained by photo statistics and anisotropy of magnetic susceptibility ams abstract a twofold fabric analysis by using photo statistics on rock surfaces and low field anisotropy of magnetic susceptibility ams is presented making it possible to infer paleoflow directions which in turn helps to constrain the primary volcanic geomorphology of a deeply eroded mid miocene field keser s hill lava dome group visegrad mountains north hungary the analyses were carried out on the ram hill pumiceous sandstone rhps that based on its petrographic features is considered as a subaqueously emplaced laharic deposit the observed anisotropic frequencies of clast a axis azimuths the good agreement of fabric directions obtained for all and most elongated samples elongation 2 5 the good clustering of the declinations of the k1 susceptibilities largely corresponding with the result of image analysis allow to infer reliable flow directions the obtained large scale paleoflow paths show a quasi radial pattern around the central part of the visegrad mountains which quantitatively confirms the previous hypothesis on the volcanic structure namely a central edifice topped syn eruptive topography the rhps is characterized by a clast fabric revealed by photostatistics on cutted rock surfaces considering the lapilli sized fraction as strong as in subaerial pdc deposits but by a significantly weaker magnetic fabric the weak magnetic fabric is supposed to be the result of rock heterogeneity thus the weakening effect of abundant lithic clasts with strong internal magnetic susceptibility anisotropy different from the shape anisotropy of the clasts and the flow related magnetic fabric of the fine grained matrix

the geology and geophysics of the ekofisk field waterflood abstract successful waterflooding of ekofisk field will enhance recovery of the total oil in place producing an extra 170 mm barrels of oil monitoring matching and predicting the reservoir performance will be performed with a complex reservoir model of 7700 cells the chalk reservoir sequence at ekofisk is multilayered with each layer exhibiting distinct thickness porosity permeability and water saturation trends the upper part of the tor formation is one of those layers with imbibition characteristics suitable for waterflooding overlying the tor formation is the ekofisk tight zone a thin layer of low but variable permeability and porosity which generally acts as an hydraulic barrier this geological and geophysical model provides a data set for the required complex reservoir model

influences of storm erosion and deposition on rhythmites of the upper wenchang formation upper ordovician around tonglu zhejiang province china abstract tonglu rhythmites upper ordovician in zhejiang province east central china display three orders of cyclicity in sandstone and mudstone layer thickness millimeter thick alternations of sandstone and mudstone laminae are ascribed to single tidal cycles centimeter thick alternations of sand dominated layers sdls and mud dominated layers mdls are interpreted to be related to alternation of storm and calm weather conditions with a periodicity longer than that forced by neap spring tidal cyclicity the sdls are interpreted as storm deposits on the basis of presence of scour structures abundant intraformational mud pebbles oscillation ripples and thinning upward trends in the sandstone laminae a third meter thick cycle of variations in sandstone lamina thickness is interpreted as a reflection of cross shore changes in coastal dynamics and water depth in the subtidal intertidal environment storm waves usually considered to be random destructive factors to normal cyclic deposits are here highlighted as effective agents of sediment transport and deposition of the sand dominated layers this study aims at improving our facies level understanding of the genesis and preservation of storm related tidal flat rhythmites on open coasts and highlights the fact that storm related facies can mimic the cyclicity that is commonly ascribed to neap spring tidal variation

triassic evaporites and plio quaternary diapirism in western greece wide triassic evaporite outcrops run parallel to the strike of hellenide thrust faults in western greece they crop out along the leading edges of thrust sheets emplaced as part of the foreland migrating fold and thrust belt the evaporites acted as the plane of decollement for thrust sheet emplacement over much of the external hellenides plio quaternary diapirs occur locally within the broad outcrops of triassic evaporite they are the result of local mobilization of evaporite after its emplacement their rise may have been initially triggered by hellenide thrust sheet loading and subsequently accentuated by plio quaternary subsidence geophysical records and onland exposures show that diapiric rise controlled local sedimentation patterns above the sites of earlier hellenide thrusts the subsequent intrusion of diapirs into these cover sediments caused localized extensional structures to develop this is best seen in se zakinthos where strain hardened conjugate faults developed in friable pliocene sandstones early in the intrusion history principal strains calculated from these sets demonstrate patterns of extension sub orthogonal to intrusion margins they are comparable with patterns known from other field and experimental examples but contrast with some recent studies on diapir related strains

magnetic structural and geochronologic evidence bearing on volcanic sources and oligocene deformation of ash flow tuffs northeast nevada magnetic properties of mid tertiary volcanic rocks west of jiggs in northeast nevada were investigated for the purposes of interpreting igneous structural and tectonic processes in this part of the basin and range province anisotropy of magnetic susceptibility ams patterns reflect flow fabrics and suggest previously unknown sources for these ash flow tuffs paleomagnetic and structural evidence suggest counterclockwise relative rotation of the southern part of the area with respect to the north new stratigraphic chemical and k ar isotopic data support these interpretations dacite to rhyolite ash flow tuffs of the indian well formation were subdivided into two main units the lower and predominant tuff of jiggs 35 8 37 0 ma and the unconformably overlying but areally restricted tuff of hackwood 30 8 ma the jiggs unit has two polarities whereas the hackwood has only a reversed polarity remanence together these units with tilt correction define a pole 92 e 87 n dp 6 dm 8 for approximately 30 37 ma this pole is concordant with coeval north american reference poles indicating that this zone of approximately 30 km ns extent has not undergone significant vertical axis rotation relative to the north american reference andesite lavas of normal magnetic polarity and of 32 5 ma age characterize the diamond hills immediately to the south we interpret this region from both structural evidence and discordant paleomagnetic direction to have rotated approximately 25 counterclockwise relative to the indian well volcanic units to the north the apparent rotation of the diamond hills is possibly the result of drag on the left lateral garcia fault which limits the diamond hills on the southwest analysis of ams data suggests by patterns of the k1 axes two distinct sources for the jiggs unit a northern buried source and a central partially buried source lithologic evidence consistent with proximal vent facies is found near the latter source an unusual pattern in the ams k3 axes suggested as resulting from the tilt correction applied to initial dip is consistent with the interpreted flow directions tight age constraints can be placed on the deformation of the main ash flows of the jiggs unit in the 35 8 to 30 8 ma interval because the diamond hills unit is slightly younger and appears to have been affected by the same deformative episode as the jiggs the regional deformation affecting both units can be more narrowly confined to the 32 5 30 8 ma interval this deformation is inferred to coincide with the major early tertiary episode of crustal extension in this region of the basin and range

sunspot periods in the late precambrian glacial climate and solar planetary relations cycles conspicuous in late precambrian 680 myr glacial varves in australia reveal strong climatic periods of about 11 22 145 and 290 yr and a weaker period near 90 yr the 11 22 and 90 yr cycles equate with sunspot periods and the longer cycles with solar and climatic periods as indicated by tree ring studies the varve cycles provide clear evidence of 11 and 22 yr climatic periods in the geological record and may carry important implications for solar physics and planetary science

partitioning of transgressive deposits in the southeastern yellow sea a sequence stratigraphic interpretation abstract the last glacio eustatic sea level fluctuation in the southeastern part of the yellow sea formed a number of unconformity bounded sedimentary sequences these sequences comprise 1 regressive to transgressive incision fills unit c 2 transgressive sand ridges and swales unit b and 3 transgressive to highstand muds lower mud unit a2 upper mud unit a1 the sequence boundary between unit c and the lower unit comprises erosional surface which can be traced for the entire study area the even and sharp boundary between the incision fills unit c and the sand ridges unit b represents a transgressive surface of erosion the depositional boundary between the lower mud unit a2 and the upper mud unit a1 suggests a change in oceanographic conditions the boundary is confined landward and correlated offshore with the top of units b and c that is exposed on the seafloor these transgressive deposits show partitioning controlled by currents tides and sediment supply as well as physiography of the seafloor

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

influence of the changing lunar orbit on the astronomical frequencies of pre quaternary insolation patterns the sensitivity of the frequencies of the earth s orbital elements involved in the astronomical theory of paleoclimates obliquity and climatic precession to the earth moon distance and consequently to the length of the day and to the dynamical ellipticity of the earth is investigated for the last billions of years but the new frequencies have been computed for the last 440 million years only a time interval over which data and a model of the earth were available moreover we consider only the impact of the changing precessional parameter assuming that the fundamental frequencies of the planetary motion are steady at the geological time scale under such hypotheses the shortening of the earth moon distance and of the length of the day back in time induces a shortening of the fundamental astronomical periods for obliquity and precession a shortening that is larger for longer periods

sedimentology and diagenesis of resedimented and rhythmically bedded chalks from the eldfisk field north sea central graben 1 chalk reservoirs in the eldfisk field comprise autochthonous chalks of the hod formation overlain by mixed allochthonous and autochthonous chalks of the tor and ekofisk formations core material contains previously undescribed fabrics which are of importance in determining the sedimentological and diagenetic history of north sea chalk reservoirs resedimentation within the tor and ekofisk formations was dominated by slumping evidence of deposition from liquefied flows is scant sediment filled fractures indicate early overpressuring of the sequence in response to burial beneath tertiary mudrocks within the hydrocarbon zone the hod and ekofisk formations contain small scale about 7 5 cm rhythmic alternations of oil stained and unstained layers related to variation in sedimentation rate and or early diagenesis below the hydrocarbon zone and in impermeable intervals within the hydrocarbon zone small scale alternations are replaced by decimeter scale interbedding of clay rich and clay poor chalks more typical of autochthonous chalk facies and a product of more advanced chemical compaction these fabrics suggest that porosity reduction is closely related to lower hydrocarbon saturation in turn largely a function of clay content early overpressuring and structural position within the field

magnetic properties and emplacement of the bishop tuff california anisotropy of magnetic susceptibility ams and characteristic remanence were measured for 45 sites in the 0 76 ma bishop tuff eastern california thirty three sites were sampled in three stratigraphic sections two in owens gorge south of long valley caldera and the third in the adobe lobe north of long valley the remaining 12 sites are widely distributed but of limited stratigraphic extent weakly indurated highly porous to dense welded ash flow tuffs were sampled saturation magnetization vs temperature experiments indicate two principal iron oxide phases low ti magnetites with 525 570 c curie temperatures and maghemite with 610 640 c curie temperatures af demagnetization spectra of isothermal remanent magnetizations are indicative of magnetite maghemite predominantly in the multidomain to pseudo single domain size ranges remeasurement of ams after application of saturating direct fields indicates that randomly oriented single domain grains are also present the degree of anisotropy is only a few percent typical of tuffs the ams ellipsoids are oblate with kmin axes normal to subhorizontal foliation and kmax axes regionally aligned with published source vents for 12 of 16 locality means kmax axes plunge sourceward confirming previous observations regarding flow sense topographic control on flow emplacement is indicated by the distribution of tuff deposits and by flow directions inferred from kmax axes deposition east of the benton range occurred by flow around the south end of the range and through two gaps benton notch and chidago gap flow down mammoth pass of the sierra nevada is also evident at least some of the adobe lobe in the northeast flowed around the west end of glass mountain eastward flow directions in the upper owens gorge and southeast directions in the lower owens gorge are parallel to the present canyon suggesting that the present drainage has been established along the pre bishop paleodrainage characteristic remanence directions from 45 sites 267 samples yield an overall mean of d 348 i 53 for the bishop tuff a correlation is found in two of the three profiles between density and remanence inclination a mean remanence direction based on 13 localities together with data from uncompacted xenoliths and data from the ash fall tuff at lake tecopa is d 353 i 54 k 172 95 2 9 n 15

imbrication flow direction and possible source areas of the pumice flow tuffs near bend oregon u s a abstract imbrication indicating flow and source direction occurs in three pleistocene or upper pliocene pumice flow tuffs exposed in a 700 km2 area on the east flank of the cascade range near bend oregon and shows the location of previously unknown source vents of these tuffs the imbrication is formed by inclined elongate and or flat pumice or lithic fragments and locally by elongate plagioclase crystals imbrication is best developed within the lower zones of individual flow units the pumiceous top zones also locally show imbrication directions parallel to that in the lower zones moreover the areal pattern of size distribution of lithic and pumice fragments in the flows is concordant with the flow direction pattern indicated by imbrication the upper pumice flow shows a fan shaped pattern of flow directions indicated by imbrication which points to a western source a possible vent about 20 km west of bend in the highland near broken top volcano is marked by many silicic domes and basaltic cinder cones where there is a 6 8 mgal negative bouguer gravity anomaly in contrast imbrication in the middle and lower pumice flows indicates flow from a source southwest of bend vents in this direction are not obvious possible buried vents are located about 30 km and 45 km southwest of bend near sitkum butte and lookout mountain respectively

late ordovician glaciation under high atmospheric co2 a coupled model analysis the analysis of the geologic record has revealed a question concerning how the late ordovician glaciation could have occurred simultaneously with high co2 levels 10 18x sensitivity studies using a coupled atmosphere ocean sea ice model show that it is possible to maintain a permanent snow cover which corresponds to 60 of all the glacial deposits found on gondwana under 10x co2 levels warm fall cool spring orbital parameters a 4 5 reduction in solar luminosity a length of day of 21 5 hours and an enhanced snow sea ice albedo of 0 3 a cold summer orbit experiment with 10x co2 and a reduced snow sea ice albedo of 0 1 also sustains a permanent albeit less extensive snow cover the geographic configuration of the late ordovician results in an up to 42 increase in the global ocean poleward heat transport in the southern hemisphere relative to present day and a significant asymmetry relative to the equator

on the structural evolution of the central trough in the norwegian and danish sectors of the north sea abstract the central trough of the north sea is not a simple rift graben it is an elongated area of regional subsidence which was initiated in mid cretaceous times and continued to subside through to the late tertiary its form is not representative of pre mid cretaceous tectonics in late permian times the north sea was divided into a northern and southern zechstein basin by the e w trending mid north sea ringkobing fyn high the latter was dissected by a narrow graben trending nnw through the tail end graben and the sogne basin the feda graben was a minor basin on the northern flank of the mid north sea high at this time this structural configuration persisted until end middle jurassic times when a new wnw trend separated the tail end graben from the sogne basin right lateral wrench movement on this new trend caused excessive subsudence in the tail end and feda grabens while the sogne basin became inactive upper jurassic subsidence trends continued during the early cretaceous causing the deposition of large thicknesses of sediments in local areas along the trend from mid cretaceous times the regional subsidence of the central trough was dominant but significant structural inversions occurred in those areas of maximum early cretaceous and late jurassic subsidence

tidal sedimentology and estuarine deposition of the proterozoic big cottonwood formation utah abstract the mesoproterozoic to neoproterozoic big cottonwood formation of north central utah contains some of the oldest known 900 ma examples of cyclic tidal rhythmites despite mild metamorphic overprinting there is excellent preservation of sedimentary structures the thick formation 4 8 km has been previously interpreted as a shallow water intracratonic basin deposit five distinct facies are recognized two quartz arenite facies are dominated by dune meter scale cross bedding recording westward flow but distinguished by different large scale geometries 1 thick tabular bodies 10 20 m thick by 600 m long and 2 channeled beds 0 3 0 3 m thick by hundreds to thousands of meters long channeled beds have scoured bases and coarse grained lags or rip up clasts stacked channel beds form upward fining successions up to 50 m thick three distinct argillite facies contain different structures and cyclicities and are also characterized by color dark laminated argillites contain abundant heterolithic rhythmites with thick couplets up to 1 cm per lamina syneresis cracks flame structures internal truncation scours intraformational blocks clasts and diagenetic pyrite the other two argillite facies commonly occur overlying channelized quartzites in large scale upward fining successions a transitional argillite composed of thin intertidal beds grades upward into mud cracked argillite of massive to weakly bedded intertidal to supratidal beds characterized by wave ripples and abundant mud cracks important diagnostic tidal features recognized in the big cottonwood formation include 1 heterolithic tidal rhythmites 2 current ripples with crests rounded by backflow 3 sigmoidal bundles and 4 abundant clay draped reactivation surfaces other structures that corroborate the tidal interpretation include 1 flaser bedding 2 mud cracks and 3 mud draped wave ripples the sedimentary structures and genetic sequence relationships recognized in this study suggest deposition in a tide dominated estuary the laminated argillites with heterolithic rhythmites indicate subtidal deposition in tidal channels sand filled tidal channels are represented in stacked quartzite beds that grade to transitional argillite and thin successions of mud cracked argillite thicker units of mud cracked argillite may represent deposition in tidal flats with periodic exposure tabular sheet quartzites suggest deposition as sand sheets near the mouth of the estuary our documentation of tidal rhythmites and estuarine deposition suggests previously unrecognized mesoproterozoic to neoproterozoic tidal deposition in north central utah

evidence of deposition from individual tides and of tidal cycles from the francis creek shale host rock to the mazon creek biota westphalian d pennsylvanian northeastern illinois abstract a silt clayrock sequence in the francis creek shale pennsylvanian westphalian d of northeastern illinois is characterized by 1 the presence of numerous clayrock paired bands and 2 cyclic variations in thickness of silt between and within these clayrock pairs the sequence is interpreted to result from tidal influence on inner estuarine sediments the clayrocks representing deposition from suspension at times of tidal slack the interbedded silts and fine sands representing deposition from bottom traction during ebb and flood tidal flow the cyclic variations in thickness is interpreted to represent lunar neap spring forcing of tidal range in a dominantly diurnal tidal system the association of tidal characteristics provides a means whereby the rate of sedimentation can be quantified at about 1 0 m yr of compacted sediment during times of active deposition such a high rate of sedimentation would account for the rapid burial and preservation of soft bodied organisms as indicated by the associated mazon creek biota similar rates in similar settings i e tidal estuaries have recently been reported although the depositional setting of many westphalian sequences was favorable to the preservation of tidal features few examples have been recorded it is suggested that evidence of westphalian tides may have been misinterpreted or overlooked in field studies elsewhere

panthalassa paleotidal resonance and a global paleocean seiche various paleotidal periods can be extracted from analyses of cyclic tidal rhythmites which are common in upper carboniferous 300 ma rocks of the eastern and midcontinental united states these periods provide details of the paleotidal regime that controlled sedimentation during deposition of the rhythmites in addition to shorter term diurnal semidiurnal periods and longer term neap spring tidal periods some rhythmites have unusual intermediate periods of approximately 50 hours these intermediate periods are not readily explainable by any known completely tidal period it is hypothesized that this 50 hour period could be the product of a basinal resonance or seiche herein estimates are made regarding the required basin depths and widths necessary to produce a seiche of this period these estimates are based upon paleogeographic reconstructions of the global paleocean panthalassa that surrounded pangea these reconstructions provide basinal paleowidth estimates subsequently paleodepths can be extrapolated from the depth to width relationships in modern oceans combining such fine scale tidal rhythmite periodicities with large scale paleogeographic information provides many useful constraints for modeling of pangean tidal systems forward modeling of global tides based upon very large scale paleogeographic controls when combined with reverse modeling based upon fine scale detailed rhythmite period analysis allows detailed calibration of the paleotidal systems response on the basis of available fine and large scale information the unusual 50 hour period that occurs within cyclic rhythmites is most likely related to a global paleocean seiche

modelling of chalk diagenesis eldfisk field norwegian north sea using whole rock and laser ablation stable isotopic data laser ablation techniques permit the determination of the stable isotopic ratios of finely crystalline calcite cements in chalks for the first time modelling of fluid rock interaction using whole rock and laser ablation stable isotopic data indicates that carbonate mineral diagenesis in the eldfisk field consisted largely of the dissolution and reprecipitation of calcite with little associated loss of porosity cementation by calcite derived from stylolites apparently occurred throughout the eldfisk field chalk but had only a subsidiary effect on whole rock isotopic ratios oxygen isotopic data indicates a pore water temperature of 50 80 c during the bulk of chalk recrystallization increases in whole rock 13 c values with depth are likely the result of bacterial methanogenesis during chalk recrystallization

interaction between bottom currents and slope failure in the late cretaceous of the southern danish central graben north sea the nw european chalk group was deposited in a deep epicontinental sea traditionally conceived as a quiet depositional setting that was only affected by redeposition along structural highs the chalk sea was however locally affected by powerful bottom currents during some periods three dimensional seismic reflection data from the southern central graben show abundant intra chalk morphological elements concentrated at two stratigraphic levels within the turonian campanian sequence both levels are topped by extensive unconformities the geometry of the elements produced by alongslope processes is interpreted as caused by se directed bottom currents flowing between the bo jens and adda ridges the highest current intensities led to the formation of the two unconformities judged by the occurrence of the largest current formed channels and drifts at these two stratigraphic levels this sea floor topography was locally modified by large scale downslope mass transport stratigraphical coincidence of the largest slumps and bottom current erosion suggests a coupling between downslope and alongslope processes current erosion impinged upon the slopes during periods of highest current speeds destabilizing the slope sediments and triggering slumping the smoothing of the bathymetry following local relaxation of the tectonic inversion accounted for the coeval deactivation of alongslope and downslope processes during the succeeding maastrichtian deposition

structural style and early stage growth of inversion structures 3d seismic insights from the egersund basin offshore norway abstract high quality three dimensional 3d seismic reflection and borehole data from the egersund basin offshore norway are used to characterise the structural style and determine the timing of growth of inversion related anticlines adjacent to a segmented normal fault system two thick skinned normal faults which offset permian clastics and evaporites delineate the north eastern margin of the basin these faults strike nnw sse have up to 1900 m of displacement and are separated by an ese dipping c 10 km wide relay ramp both of these faults display exclusively normal separation at all structural levels and tip out upwards into the upper part of the lower cretaceous succession at relatively shallow structural levels in the hangingwalls of these faults a series of open low amplitude fault parallel anticlines are developed these anticlines which are asymmetric and verge towards the footwalls of the adjacent faults are interpreted to have formed in response to mild inversion of the egersund basin the amplitude of and apparent shortening associated with the anticlines vary along strike and these variations mimic the along strike variations in throw observed on the adjacent fault segments we suggest that this relationship can be explained by along strike changes in the propensity of the normal faults to reactivate during shortening wider damage zones and lower angles of internal friction coupled with higher pore fluids pressures at the fault centre mean that reactivation is easier at this location than at the fault tips or in the undeformed country rock seismic stratigraphic analysis of growth strata indicate that the folds initiated in the latest turonian to earliest coniacian c 88 6 ma and santonian c 82 6 ma the control on this c 6 myr diachroneity in the initiation of fold growth is not clear but it may be related to strain partitioning during the early stages of shortening anticline growth ceased in the maastrichtian and the inversion event is therefore interpreted to have lasted at least c 20 myr this study indicates that 3d seismic reflection data is a key tool to investigate the role that normal fault segmentation can play in controlling the structural style and timing of inversion in sedimentary basins furthermore our results highlight the impact that this structural style variability may have on the development of structural and stratigraphic hydrocarbon traps in weakly inverted rifts

modeling of cyclic tidal rhythmites based on a range of diurnal to semidiurnal tidal station data abstract where individual tides deposit appreciable accumulations of sediment cyclic tidal rhythmites can form that exhibit semimonthly tidal cyclicities a number of modern analogs and ancient counterparts indicate the importance of such features for paleoenvironmental analyses in this study predicted high low tidal station data are used to simulate current speeds which in turn are used to simulate sedimentation of tidal rhythmites this modeling has been done to depict the patterns encoded in cyclic tidal rhythmites in order to delineate the spectrum of different tidal regimes the simulations are based upon a series of macrotidal diurnal to semidiurnal tidal systems depending upon thresholds of relative tidal current speed only portions of semimonthly tidal cycles will be preserved within cyclic tidal rhythmites

geometry of allochthonous chalk group members central trough north sea abstract the temporal and spatial distribution of the hydrocarbon bearing late cretaceous and early tertiary allochthonous chalk is of economic importance examination of cores from the uk sector of the central trough has confirmed the correlation of major allochthonous chalk units to wireline log responses dependent on high porosity allochthonous units were recognized from wireline log coverage of over 100 exploration wells in the central trough area correlation indicated extensive apparent lateral continuity of allochthonous sediment packages enabling the establishment of an event stratigraphy a minimum of nineteen major redeposition events were recognized within the late cretaceous maastrichtian and early palaeocene danian chalks essentially synchronous sourcing and redeposition occurred on both margins of the central trough but with a marked dominance of activity along the eastern margin in terms of incorporated sediment volume the described core material was placed in a regional context by means of the event stratigraphy and confirmed a previously proposed relationship of depositional rock type and transport distance this study recognizes five principal mass flow complexes named after their dominant sediment source areas jaeren north hidra south hidra lindesnes and auk the jaeren north hidra and auk flow complexes are here documented for the first time as are other less extensive deposits

mt pelee martinique may 8 and 20 1902 pyroclastic flows and surges abstract the distribution stratigraphic relationships and fragmental components of the may 8 and 20 1902 pyroclastic flows from mt pelee martinique together with eyewitness accounts suggest the following explanation for those eruptions the eruptions were vertically directed magmatic perhaps initiated phreatically and contained abundant juvenile lithics from congealed magma of the dome and neck this resulted in a two part eruption column having 1 a dense lithic charged part which collapsed into the crater and flowed out of a pre existing notch in its side giving rise to pyrochlastic flows and 2 a magmatically derived column containing gases juvenile vitric material and crystals which largely by passed the neck and dome and escaped into the atmosphere all of the energy of the flows was apparently focused through the notch they emerged fully turbulent and flowed down riviere blanche gravity segregation of large and abundant fragments soon resulted in a dense high concentration poorly fluidized block and ash flow confined to the valley leaving above a fully turbulent high energy ash cloud surge as the ash cloud surge moved down the mountain it continued to expand outward the process of gravity segregation continued as the ash cloud surge expanded resulting in secondary block and ash underflows toward st pierre the secondary block and ash flows developed on a gently sloping upland surface 100 m or more above the valley of riviere blanche the turbulent fragment depleted surges above the secondary block and ash flows maintained sufficient energy to devastate the landscape outward to about 3000 m including st pierre the surges refracted around obstacles and in one place moved up a small valley in a direction opposite to the main flows

the lunar orbit in the late precambrian and the elatina sandstone laminae the discovery of layered sediments of the elatina formation of south australia has attracted intense speculation regarding their origin1 2 the formation 10 m layer of graded sandstone consists of a pattern of light bands of periodically varying thickness3 a sequence of dark bands separates on average 11 6 light bands the spacing of these dark bands exhibits a rich and complex spectrum previously the periodicity has been attributed to the sunspot cycle and unassigned but putative solar periods prompted by a letter from g e williams summarizing new field data from the adelaide region south australia which imply that the elatina varves may be tidal we re examined the dark band spectrum and propose a luni solar tidal interaction model as the source of the laminae

seismic stratigraphy of the chalk group in the norwegian central graben north sea abstract the late cretaceous to early palaeogene chalk group in the norwegian central graben contains prolific hydrocarbon reservoirs that have been producing at high rate for more than forty years based on integration of regionally extensive 3d seismic data and numerous wells this paper describes the seismic development of the chalk group with particular focus to the syndepositional geomorphological features produced by the activity of bottom currents and large scale gravity flows using standard seismic stratigraphic interpretation techniques eight seismic sequence boundaries are identified which in turn define seven seismic stratigraphic sequences characterized by different seismic facies and well log signatures the seismic sequences can be grouped into three sequence groups that reflect the general tectono stratigraphic evolution of the chalk depositional system in the norwegian central graben during the late cretaceous uplift of the lindesnes ridge and the albuskjell anticline due to inversion tectonic and halokinesis modified the physiography of the norwegian central graben basin and affected the style of chalk sedimentation by influencing the location and importance of gravity flows in addition bottom currents sea level fluctuations and environment changes significantly influenced the chalk depositional system creating regional unconformities and influencing the input of terrigenous material in the chalk epeiric sea

synchroneity of climatic change and extinctions in the late triassic although the late triassic was a time of widespread aridity evidence exists for a significant increase in rainfall during the middle to late carnian upper triassic playa lake sediments were interrupted by late carnian fluviatile sandstones with erosive bases and high kao linite illite ratios there was also an increase in the clastic component of marine sequences during this interval middle and upper carnian marine carbonates show an extreme depletion in 13 c values consistent with increased fresh water influx large scale karstic phenomena in limestone areas subaerially exposed during the late triassic are a further indication of increased rainfall important faunal and floral changes occurred during the carnian norian interval marine invertebrate turnover was greatest at the lower middle carnian boundary and terrestrial extinctions were concentrated at the carnian norian boundary the cause of this carnian pluvial episode may have been related to the rifting of pangea through disruption of atmospheric and oceanic circulation patterns eustatic changes or the effects of volcanism associated with rifting a change in surface ocean temperature salinity or ph or habitat loss may have caused the decline of many shallow marine invertebrates at the start of the middle carnian a return to arid conditions at the carnian norian boundary would account for the turnover among terrestrial vertebrates and plants

flow directions in ash flow tuffs a comparison of geological and magnetic susceptibility measurements tshirege member upper bandelier tuff valles caldera new mexico usa this study concludes that the elongation axis k 1 of the ellipsoid of anisotropic magnetic susceptibility ams is a suitable proxy for flow axis in ashflow tuffs 153 oriented samples 176 specimens were studied from 18 sites in the 1 1 ma tshirege member of the bandelier tuff these sites are distributed around the valles caldera at distances of 5 25 km outside of the rim k 1 axes correlate well with postulated radial flow axes at 13 sites k 1 also agrees with measured geological flow indicators mainly imbricated larger clasts at 7 sites at 2 of the 5 sites where significant disagreement is seen between theoretical radial flow directions and measuredk 1 axes thek 1 axes correspond well with geological flow indicators indicating that the divergence of flow from the predicted radial flow pattern is real two major topographic buttresses are suggested as the cause of flow divergence for the tshirege ash flows the san pedro buttress northwest of the caldera and the san miguel buttress in the southeast in situk 1 axes plunge about 7 toward the source at two thirds of the sites therefore the plunge ofk 1 is a plausible in situ indicator for thedirection of flow multiple flow zones in sections of several meters thickness indicate changes of flow direction that are both rapid and large during ash flow emplacement these observations raisre the question of how best to represent mean flow directions in ash flow sheets by eigenvector methods by vector sum methods or by modes a method for measuring imbrication of larger clasts using apparent dips in vertical joints is outlined imbrication determined in this way at one third of the sites dips toward the source i e up flow the minimum k 3 axis of the ams ellipsoid correlates with the flow foliation rather than with the larger clast imbrication the flow axes of ash flows correspond with thek 1 axes not with the declination ofk 3 axes as suggested by some authors initial dip of the sampled ash flows is not large and does not affect the paleomagnetic remanence direction which is reversed with a mean ofd 173 5 i 38 4 95 3 4 n 18 this mean is not different at the 95 confidence level from that of earlier workers the mean pole at 098 0 e 74 8 n a 95 3 3 n 18 is about 15 far sided relative to the expected time averaged geomagnetic pole suggesting a history of emplacement too short to adequately average secular variation

identifying ash flow sources with directional data an application to the kizilkaya ignimbrite central anatolia clast shape preferred orientation or anisotropy of magnetic susceptibility ams data are increasingly used to constrain the source location of ash flow tuffs little attention has been paid to processing of these directional data for the accurate and objective identification of the sources this topic is the focus of this paper as applied to locating the source of the 5 ma kizilkaya ignimbrite central turkey first geological criteria allowing a rough identification of source vent location are discussed second ams data are processed to more accurately locate the source of this widespread deposit hillhouse and wells 1991 present a method which consists of determining the areal overlapping density function of the ams error fans defined by the kmax confidence ellipse applications to the kizilkaya ignimbrite show various possible source positions however a unique anomaly occurs in every model suggesting a source in the derinkuyu basin another method is proposed here and is named the average radial angular difference arad function it consists of calculating the average angular difference of lines that join a given source point to the sampling sites with the lineations corresponding to the ams data interpreted in terms of kinematic axes application with the data obtained at the 44 ams sampling sites shows that the function decreases to 35 in the derinkuyu basin if the analysis is narrowed to the presumed 12 most proximal sites the function provides values of 20 in the central part of the derinkuyu basin the identified source area occurs near the source zone of older miocene ignimbrites and coincides with a gravity anomaly and with a basaltic shadow zone

apogean perigean signals encoded in tidal flats at the fluvio estuarine transition of glacier creek turnagain arm alaska implications for ancient tidal rhythmites turnagain arm is a macrotidal fjord style estuary glacier creek is a small glacially fed stream which enters the estuary tangentially near girdwood alaska trenches and daily sedimentation measurements were made in a mudflat along the fluvio estuarine transition of glacier creek during several summers since 2003 each year the flats appear to erode during the winter and then accrete vertically in the spring and summer in each of the years studied tidal laminae in vertically thickening and thinning laminae bundles were deposited by twice daily tides in neap spring tidal cycles in 2004 bundles of thickening and thinning laminae couplets were noted in trenches cut into the flats five laminae bundles alternated between thicker and thinner bundles corresponding to the perigean high spring and apogean low spring tides well preserved apogean perigean cycles have rarely been documented in modern tidal flat sediments at this location vertical accretion of tidal rhythmites with well developed neap spring cyclicity is possible because of the near complete removal of the flat from the previous year which creates accommodation space for vertical accretion without significant reworking macrotidal conditions no reworking by infaunal invertebrates protection from the main tidal channel by a gravel bar and protection from storm waves and fluvial erosion by a recess in the sedge marsh that surrounds the flats all aid in preservation of rhythmites during aggradation the position of the flats relative to tidal range allows for accumulation of complete spring cycles and incomplete neap cycles in the summer of 2004 apogee and perigee were closely aligned with the new and full moons resulting in successive strong perigee and apogee tides which probably aided in the accumulation of successive thick thin spring cycles encoding the apogean and perigean tidal cycle the apogean perigean signal was not observed in subsequent years

evolution of the lunar orbit with temperature and frequency dependent dissipation the coupled thermal dynamical evolution of the earth moon system is modeled the lunar orbit is assumed noncircular and inclined to earth s equator plane solid planet dissipation in the earth and moon is assumed to occur by temperature and frequency dependent solid friction the dissipative response of earth s oceans is represented as an equilibrium ocean with constant phase lag subsolidus convection cools the earth and moon removing primordial radioactive and in the moon tidal heating a model without oceanic dissipation results in the moon moving out to about 50 re earth radii in less than about 1 5 b y thereafter dynamic evolution slows considerably because the earth has cooled and become less dissipative a model that includes oceanic dissipation shows that about 700 m y of large oceanic dissipation equivalent phase lag equals 1 11 is required to move the moon to 60 re in one interpretation of this result consistent with variable ocean basin configuration several episodes of large oceanic dissipation have occurred since the end of the archean with a cumulative time span of about 700 m y in our model about 90 of the lunar angular momentum received from the earth is the result of solid friction uncertainties in the thermal evolution and dissipation law preclude an unequivocal partitioning of tidal dissipation into solid earth and oceanic parts our model likely underestimates solid earth dissipation however and so emphasizes the proposition that solid earth tidal friction might have been the main energy sink in the earth moon system and that oceanic dissipation may have been relatively much less important than solid earth dissipation over geologic time

secular effects of oceanic tidal dissipation on the moon s orbit and the earth s rotation the earth and moon are considered as a two body system in gravitational isolation from the sun and other planets the lunar orbit is taken as circular and the solid earth is assumed to be a rigid sphere with no tidal deformation so that there are no precessional torques other than those arising from the tidally deformed ocean numerical solutions to laplace s tidal equations with dissipation by linear bottom friction are used to obtain ocean wide distributions of tidal amplitude for two idealized continentalities a single circular continent spherical cap centered at the north pole and a single spherical cap centered on the equator calculations are made for two different values of the frictional resistance coefficient thus giving rise to four sets of solutions the computed tidal amplitudes are used to calculate the oceanic tidal torque which in turn is used to integrate the orbital equations backward in time for solution of the two body problem the coriolis parameter and the tidal frequencies change with time thus requiring that the tidal equations be solved several times during the course of each orbital integration in this manner the earth s rotational and the moon s orbital histories are determined on a geologic time scale for each of the four models the calculated position of the lunar orbit at 4 5 billion years ago is found to range from 38 to 53 earth radii in the four models and corresponds to a sidereal month of 330 to 550 hours the sidereal day would have been 12 to 18 hours with a relative inclination of 3 to 22 between the terrestrial and lunar poles these results are in sharp contrast to those from previous studies of the earth moon system most of which indicated a roche limit approach of the two bodies roughly 1 to 2 billion years ago and presented therefore a time scale difficulty in theories of lunar origin this contrast arises mainly from the fact that previous modelers avoided solution of laplace s tidal equations by prescribing a constant frictional phase lag angle between the angular position of the moon and the major axis of the second degree harmonic of the tidally deformed surface of the earth the amount of phase lag was established from the present astronomically deduced rate of tidal dissipation but this precluded dynamic variations in tidal torque over geologic time which are critical for determination of the orbital time scale the present rate of oceanic tidal dissipation is evidently anomalously high because of the near resonance of the oceanic response in both frequency and shape to the tidal forcing for the earth and moon to have evolved to their present separation from a distance of less than about 35 earth radii solely on the basis of oceanic tidal dissipation would have required the m2 response to have been near resonance for a significant portion of geologic history a rather implausible scenario the calculations reported here show that on the contrary frictional coupling between the earth and moon was much weaker than at present throughout most of the orbital history and consequently the moon s closest approach could not have been as small as 35 earth radii although idealized continentalities were used in these calculations the general conclusion of a weaker frictional coupling in the past is believed to be relatively insensitive to the continental configuration this is because at faster paleorotation the semidiurnal tidal frequencies would be resonant only with smaller scale normal modes of the ocean rather than with global scale modes as at present the time scale difficulty heretofore present in models of orbital evolution is thereby eliminated it should be noted that the orbital history developed here supports the theory of binary planetary accretion for lunar origin as opposed to fission or tidal capture

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

the calcareous nannofossil biostratigraphic framework of the late maastrichtian danian north sea chalk abstract the calcareous nannofossil biostratigraphy of cored maastrichtian tor formation and danian ekofisk fm chalk sections from 10 selected wells of the greater ekofisk area central north sea was studied wells come from the edda tor se tor ekofisk and eldfisk fields among others more than 1000 m of cores were examined and about 500 samples were selected and analysed most of them quantitatively the main emphasis was placed on the danian sequence and on the k t boundary biostratigraphy provided detailed subdivision for the danian sediments intra danian hiatuses were detected and consistently recorded at the regional scale extensive mass movement was observed highlighting the complex depositional history during this time interval intervals of cretaceous chalk were found embedded in the danian sequence well 2 5 7 redeposited by slides as discrete chalk packages similarly lower danian intervals were recognised as transported into younger danian sediments well 2 4 b19a an attempt to reconstruct a regional stratigraphic picture for the area north of edda field including ekofisk se tor and tor fields has been made this suggests the following salient features the k t boundary of three wells of the northern sector of the studied area tor field is preceded by a prediscosphaera stoveri acme suggesting the presence of a rather complete maastrichtian sequence below the boundary above the k t boundary a few tens of centimetres of generally clay enriched pelagic chalk of the basal danian d1 biantholithus sparsus zone are present in most of the studied wells above this a condensed section often marked by hard ground levels or clay rich intervals was detected in almost all the wells the sedimentary expression of upper d1 above cruciplacolithus primus fad d2 and d3 time interval is lacking or extremely reduced in some cases sediments whose age is near the early late paleocene boundary were found unconformably overlying pelagic chalk referred to d5 d6 biozones

buried pockmarks on the top chalk surface of the danish north sea and their potential significance for interpreting palaeocirculation patterns three dimensional 3d seismic data from the southern danish north sea were used to analyse the morphology and spatial distribution of depressions in the danian chalk deposit previously these depressions were either interpreted as karst structures or pockmarks the observed depressions occur in an interval from 25 ms below to 12 ms above the top chalk surface three types of depressions were differentiated based on their plan view geometry and their degree of symmetry type 1 comprising sub circular and symmetrical depressions is the dominant group ca 70 type 2 elongated and symmetrical depressions represents only a small fraction ca 5 the elongated and asymmetrical depressions of type 3 compose ca 25 in cross section each depression type can be either characterised by a v or a u shape the maximum size of the depressions ranges from 50 to 580 m with an average internal depth of 10 m we interpret the depressions as pockmarks formed by the expulsion of biogenic or thermogenic fluids at the danian seafloor likely the initial form of the pockmarks has been circular type 1 and was subsequently modified for types 2 and 3 to an elongated form by currents the long axis of the pockmarks is interpreted to represent the effective current direction the inferred direction is sub parallel to the palaeobathymetric contours the 3d seismic interpretation of pockmarks presented in this paper contributes to the understanding of fluid migration and palaeocirculation patterns during the sedimentation of the terminal chalk group in the southern danish north sea

flow directions of carboniferous ignimbrites southern new england orogen australia using anisotropy of magnetic susceptibility abstract anisotropy of magnetic susceptibility ams is used to define regional flow patterns and to locate possible source vents of carboniferous ignimbrites in the rocky creek region northern tamworth belt southern new england orogen the major axes of ams fabrics from 115 sites are oriented in mainly eastward or northeastward directions and the fabrics can be classified into three groups normal inverse and intermediate comparisons between flow directions indicated by ams fabrics and the alignment of feldspar laths in oriented thin sections from major ignimbrite units indicate a moderate to good level of agreement the contoured intersections of flow lineations indicate a number of possible source regions such as paleo valleys which controlled the dispersal of particular flows when all trends from contoured intersections of flow lineations are combined two major trends emerge ne sw and e w these trends converge within an area approximately 20 km west of the mooki thrust at the overthrust margin of the orogen this convergent area is proposed as a possible major volcanic source region that may have existed during the late carboniferous because of westward overthrusting with a magnitude greater than 20 km during the late permian to early triassic this region is now probably located beneath carboniferous forearc basin rocks of the tamworth belt

particle fabric in a small type 2 ignimbrite flow unit laacher see germany and implications for emplacement dynamics ignimbrites of the 13 ka upper laacher see tephra were deposited from small highly concentrated moderately fluidized pyroclastic flows their unconsolidated nature and the prominence of accidental devonian slate fragments make these ignimbrites ideal for clast fabric studies the upper flow unit of ignimbrite m14 has characteristics typical of a type 2 ignimbrite layer 2a and the lower part of layer 2b of the flow unit have strong upstream inclined a p fabrics a p means long particle axes parallel to flow direction only clasts with a b axial ratios of 2 5 or greater preserve good a p fabrics whereas the a b planes of flat fragments dip upstream irrespective of axial ratio the a axis fabric becomes weaker flatter and more girdle like in the upper half of layer 2b at one locality the a axis fabric appears to rotate 40 up through the flow unit suggesting either shear decoupling of different levels in the moving flow or unsteadiness effects in a flow depositing progressively at its base the existence of similarly strong a p fabrics in layer 2a and the lower half of layer 2b appears inconsistent with the common interpretation that ignimbrite flow units are emplaced as a plug of essentially non shearing material layer 2b on a thin shear layer layer 2a and that the entire flow freezes en masse to form the deposit the data suggest that if the flow froze en masse it was shearing pervasively through at least half its thickness another possibility is that the flow unit aggraded progressively from the base up and that the fabrics record the integrated history of shear directions and intensities immediately above the bed throughout the duration of deposition

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

tidal bar cyclicity record in the lower permian the rio bonito formation parana basin southern brazil abstract tidal bars register gravitational interactions between the earth moon and sun and allow inferences about astronomical positioning through the cyclic variation of sandstone layer thickness throughout depositional events measurements of sandstone layers thickness in a tidal bar of the rio bonito formation parana basin southern brazil and further spectral analysis allowed the definition of the tide patterns imprinted on rock layers the study was based on a remarkable tidal bar outcrop located in the candiota paleovalley in the southern basin candiota town brazil with sedimentary characteristics that suggest to a tidally dominated estuary depositional environment the tidal bar shows a medium to large scale cross stratification reactivation surfaces bidirectional structures and tidal bundles the spectral analysis resulted in a significant frequency peak 0 0329 which corresponds to a period of 30 45 layers per cycle six of these cycles are recognizable in the tidal bar by its cyclic thickening thinning of layers that range from 1 5 to 11 cm this cyclic pattern is interpreted as being produced by the neap spring tide cycle our study also allows us to infer that the south parana basin permian tide had a semi diurnal pattern with a neap spring tide period of 14 9 days comparison with previous studies points to small changes in the duration of the neap spring tide cycle in the last 900 ma 15 2 days in the cambrian 14 9 days in the early permian and 14 7 days today explained by the distance and acceleration of the movement of the moon s revolution and decrease in the speed of rotation of the earth

the record of tidal cycles in mixed silici bioclastic deposits examples from small plio pleistocene peripheral basins of the microtidal central mediterranean sea the pliocene pleistocene peripheral marine basins of the mediterranean sea in southern italy from basilicata and western calabria to northern and eastern sicily represent tectonically formed coastal embayments and narrow straits here units of cross stratified mixed silici bioclastic sand 25 to 80 m thick record strong tidal currents the central mediterranean sea has had a microtidal range of ca 35 cm and the local amplification of the tidal wave is attributed to tides enhanced in some of the bays and to the out of phase reversal of the tidal prism in narrow straits linking the tyrrhenian and ionian basins the siliciclastic sediment was generated by local bedrock erosion whereas the bioclastic sediment was derived from the contemporaneous foramol type cool water carbonate factories the cross strata sets represent small to medium sized 10 to 60 cm thick two dimensional dunes with mainly unidirectional foreset dip directions these tidalites differ from the classical tidal rhythmites deposited in mud bearing siliciclastic environments firstly the foreset strata lack mud drapes and instead show segregation of siliciclastic and bioclastic sand into alternating strata secondly the thickness variation of the successive silici bioclastic strata couplets measured over accretion intervals of 2 to 3 m and analysed statistically reveal only the shortest term diurnal and semi diurnal tidal cycles thirdly the record of diurnal and semi diurnal tidal cycles is included within the pattern of neap spring cycles differences between these sediments and classical tidal rhythmites are attributed to the specific palaeogeographic setting of a microtidal sea with the tidal currents locally enhanced in peripheral basins it is suggested that this particular facies of mud free silici bioclastic arenite rhythmites in the stratigraphic record might indicate a specific type of depositional sub tidal environment of straits and embayments and the shortest term tidal cycles

chalk fields along the lindesnes ridge eldfisk abstract eldfisk field lies on the lindesnes ridge northwest of the valhall and hod fields and to the southeast of edda in block 2 7 of the norwegian north sea the field has a complex geological history and reservoir geometry seismic reflection data is strongly gas affected and degraded over the majority of the field the geological model of the field was based exclusively on well data and the model was inadequate to explain some of the production characteristics or to identify additional prospective well locations in 1982 a programme of vertical seismic profiling vsp was started in the northern part of the field the seismic data provided by vsp have improved and confirmed the geological model provided some explanations for production variances and identified further areas of interest

solar signature in sedimentary cycles from the late precambrian elatina formation australia climatic cyclicity is recorded by regular variations in the thickness of varves in the elatina formation a late precambrian periglacial lake deposit in south australia earlier conclusions that the climatic cycles reflect solar variability are strongly supported by a complexity of periods revealed through study of drill cores of the varved sequence the new data promise to contribute significantly to our understanding of solar processes

late jurassic half graben control on the siting and structure of hydrocarbon accumulations uk norwegian central graben abstract the late jurassic structure of the central graben is discussed both in terms of the extensional features developed at this time and their subsequent controls on younger inversion structures the regional kinematics of the north sea rift suggest that during the late jurassic the central graben opened approximately orthogonally to the main nnw sse fault trend within the basin some evidence that triassic extensional faults were reactivated at this time is seen towards the basin margins and a significant triassic extensional history is inferred the late jurassic basin comprises the axial c 100 km of the central graben s l the basin axis is offset progressively to the se by both transfer zones and discrete transfer faults footwall uplift of the basin margins during extension provided the source for the upper jurassic reservoir sands encountered by exploration drilling these sands were probably preferentially fed into the basin at transfer zones in the margins salt structures are ubiquitous within the central graben and their siting is everywhere controlled by the underlying fault block topography salt structures form preferentially towards the crests of half graben or buttressed against their boundary faults inversion structures in the norwegian sector reactivated with a reverse sense of displacement late jurassic extensional faults a model is proposed which shows the two main inversion structures feda graben norway tail end graben denmark to be detached on antithetic reverse faults riding in the salt these faults terminate at tip folds within major salt walls the geometry of the inversion structures can be understood only when considered together with the geometry of the underlying late jurassic half graben

modelling of cyclic tidal rhythmites carboniferous of indiana and kansas precambrian of utah usa as a basis for reconstruction of intertidal positioning and palaeotidal regimes analyses are presented for a variety of ancient cyclic tidal rhythmites which exhibit well developed neap spring tidal periods many such rhythmites were formed within the upper intertidal zone and exhibit truncated cycles that contain relatively few discrete lamina per neap spring cycle in such cases it can be difficult to determine if the originated palaeotidal system was diurnal or semidiurnal based on sedimentological controls observed in modern analogues the development of cyclic tidal rhythmites can be modelled by use of predicted tidal height information the modelling used a 19 year long series of tidal heights which were subsequently used to approximate tidal velocities and rhythmite sedimentation modelling was based on a range of diurnal to semidiurnal regimes and comparisons were performed at a number of levels within the intertidal zone this modelling produced a long series of simulated laminae thickness series which could be cross correlated with ancient laminae thickness series measured directly from ancient rhythmites this approach involved 1 6 106 comparisons of each ancient rhythmite series to the series simulated from the predictive tidal height information the high correlations derived for such comparisons indicate that reasonable approximations to the palaeotidal systems can be made in some cases it is possible to determine the diurnal or semidiurnal nature of the originating palaeotidal system

the origin of neap spring tidal cycles abstract the origin of oceanic tides is a basic concept taught in most introductory college level sedimentology geology oceanography and astronomy courses tides are typically explained in the context of the equilibrium tidal theory model yet this model does not take into account real tides in many parts of the world not only does the equilibrium tidal model fail to explicate amphidromic circulation it also does not explain diurnal tides in low latitude positions it likewise fails to explain the existence of tide dominated areas where neap spring cycles are synchronized with the 27 32 day orbital cycle of the moon tropical month rather than with the more familiar 29 52 day cycle of lunar phases synodic month both types of neap spring cycles can be recognized in the rock record a complete explanation of the origin of tides should include a discussion of dynamic tidal theory in the dynamic tidal model tides resulting from the motions of the moon in its orbit around the earth and the earth in its orbit around the sun are modeled as products of the combined effects of a series of phantom satellites the movement of each of these satellites relative to the earth s equator creates its own tidal wave that moves around an amphidromic point each of these waves is referred to as a tidal constituent the geometries of the ocean basins determine which of these constituents are amplified thus the tide raising potential for any locality on earth can be conceptualized as the result of a series of tidal constituents specific to that region a better understanding of tidal cycles opens up remarkable opportunities for research on tidal deposits with implications for among other things a more complete understanding of the tidal dynamics responsible for sediment transport and deposition changes in earth moon distance through time and the possible influences tidal cycles may exert on organisms

reliability of lunar orbital periods extracted from ancient cyclic tidal rhythmites abstract ancient tidal periods extracted from the geologic record are useful to constrain and calibrate theoretical models of the evolution of the earth moon system analysis of rocks that contain such tidal periodicities termed cyclic rhythmites have tended to concentrate on long series and have focused on the more obvious periods for example neap spring periods have been the most widely discussed such information can be reliably extracted from bedform thickness series measured within laterally accreted tidal bundles and vertically accreted tidal laminae other lunar orbital data is also encoded in cyclic rhythmites however this additional information has been much less extensively applied to earth moon models herein short segments of well formed cyclic rhythmites are examined to determine how reliably the various tidal periods can be extracted via harmonic analysis rhythmite series analyzed here exhibit well formed neap spring tidal periods and also exhibit a prominent fortnightly inequality or a variation in the height of successive spring tides this inequality is related to the elliptical nature of the lunar orbit anomalistic month and it is demonstrated that this period can also be reliably extracted from cyclic rhythmites tropical periods which reflect the changing lunar declination that occurs during a tropical month have also been reported from cyclic rhythmites unlike synodic and anomalistic periods the apparent tropical periods extracted from some cyclic rhythmites do not always accurately correspond to the expected values in only a few cases can this period be reliably extracted from short segments of cyclic tidal rhythmite data this period is commonly only poorly encoded in rhythmites because 1 it tends to be masked by higher amplitude synodic periods and 2 it is not well expressed within predominantly semidiurnal tidal systems samples selected from the proterozoic and phanerozoic tend to be more common during specific intervals of geologic time these intervals were characterized by glacio eustatic flux and in some cases by unique continental configurations understanding these basic controls on rhythmite occurrences should aid in the discovery of additional examples

eruption transport and deposition of ignimbrite a case study from mexico abstract the acatlan ignimbrite covers 150 km 2 around a vent situated 40 km southwest of guadalajara mexico the upper and main ignimbrite flow unit is compositionally zoned and has white or pink rhyolitic pumice in its lower part and black andesite in its upper these parts are believed to have been erupted continuously from the same magma chamber this zonation has been important in correlating an extremely coarse lithic rich deposit a co ignimbrite lag fall deposit with this ignimbrite the co ignimbrite lag fall deposit indicates that the flow unit formed by continuous collapse of an explosive eruption column and enables the source vent to be located the vertical compositional grading of the flow unit also provides a means of testing the various proposed mechanisms of ignimbrite emplacement turbulent vs high concentration flow the preservation of compositional zonation and other features in the deposit indicate that flow was not turbulent details of the grain size distribution indicate that laminar flow was important early in the flow history when collapsing material from the eruption column was superimposed in stratigraphical sequence initial high gas flow rates associated with flow segregation and formation at the site of column collapse rapidly decayed away from vent and later in its history the flow degassed to a semi rigid plug overriding a sheared basal layer

calculating earth moon system parameters from sub yearly tidal deposit records an example from the carboniferous tradewater formation abstract the secular evolution of the earth moon system remains poorly constrained largely because there are few continuous tidal rhythmite or foreset bundle sequences that preserve deposition over more than several neap spring tidal cycles deposits recording less than one year of deposition do not facilitate direct calculation of past lunar distance directly from kepler s laws but may prove necessary to reconstruct lunar orbital evolution because they are far more common than longer records a method is demonstrated to make use of shorter tidal deposit sequences by utilizing conservation of angular momentum between the earth and moon and estimating the solar component of tidal deposition while assuming a constant moment of inertia for earth since the proterozoic the precision and accuracy of spectral estimates obtained from short records are considered as are the limitations of subsequent calculations of earth moon parameters the late carboniferous abbott sandstone of the tradewater formation in the illinois basin preserves just over 6 apparently continuous neap spring cycles in its semidiurnal deposits the quality of these data as assessed via sedimentological evidence and statistical time series properties produces spectral estimates that are likely within at least 5 of the actual underlying periodicity 90 accuracy to test the usefulness of such records we assessed the possible scenarios of 90 and 95 accuracy at 90 accuracy the error bounds on earth moon parameter estimates become rather large and render individual data sets to be of limited use at 95 accuracy very general inferences about the evolution of the earth moon system may be made calculated mean lunar orbital distance at 315 ma is 3 798 10 8 m with error bounds of 0 086 10 8 m and 0 046 10 8 m we conclude that short sequences of cyclic tidal deposits offer rather limited resolution of lunar distance estimates utilizing multiple sub yearly data of similar age in ensemble may prove necessary if further investigations of secular changes in earth moon parameters are to proceed

flow behavior of large scale pyroclastic flows evidence obtained from petrofabric analysis the grain orientations within the matrix of two large scale welded two small scale nonwelded and two nonwelded low aspect ratio pyroclastic flow deposits are measured to analyze flow behavior preferred grain alignments are especially apparent in the middle part of layer 2 of each deposit preferred grain alignments do not vary laterally within a 10 m interval the grain alignments obtained are radial from the source caldera especially in proximal to medial and plateau forming facies of pyroclastic flow deposits grain alignments are controlled by valley channel directions for the valley ponded facies of pyroclastic flow deposits especially at medial to distal locations such local topographic factors strongly affect the data for high aspect ratio and smallscale deposits and weakly affect the data for widespread low aspect ratio pyroclastic flow deposits this work suggests that grain alignment analysis should be used with care when attempting to determine the location of an unknown source

comment on a reappraisal of ignimbrite emplacement progressive aggradation and particulate to non particulate flow transitions during emplacement of high grade ignimbrite by mj branney and p kokelaar

ekofisk first of the giant oil fields in western europe discovery of the giant ekofisk field in block 2 4 in the norwegian part of the north sea in 1969 was a major turning point in the exploration for petroleum in western europe since that time the north sea has proven to be one of the best areas for exploration anywhere in the world current production is 1 5 million barrels of oil per day and north sea proven reserves total 18 billion barrels with estimates of ultimate reserves as high as 40 billion barrels ekofisk is located in the central graben in the southern part of the norwegian sector of the north sea although several periods of tectonism have affected this area it has remained an intercratonic basin since devonian time the main elements of the tectonic fabric were established during the caledonian and hercynian orogenies and later remained as controlling features for facies and sediment distribution a stratigraphic history reveals petrography of the main chalk group environment of deposition and diagenetic history of the area a section outlines the preservation of porosity which has allowed the ekofisk and tor formations to retain an average porosity of 30 to 40 percent which would not have been possible under normal circumstances it is appropriate to state that the anomalously high porosity in ekofisk field is probably due to a combination of 1 overpressuring of the reservoir 2 magnesium rich pore fluids and 3 early introduction of hydrocarbons the six greater ekofisk fields now being developed were all located during the 1960s by reconnaissance seismic work the history of the geophysical exploration is outlined and a study has been made on source rock analysis and geopressures also outlined is the three phase greater ekofisk development program