the front speed of intrusions into a continuously stratified medium this paper examines intrusive boussinesq gravity currents propagating into a continuously stratified fluid we develop a model based on energy arguments to predict the front speed of such an intrusive gravity current from a lock release we find that the depth at which the intrusion occurs which corresponds to the level of neutral buoyancy i e the depth where the intrusion density equals the stratified fluid density affects the front speed the maximum speeds occur when the intrusion travels along the top and bottom boundaries and the minimum speed occurs at mid depth experiments and numerical simulations were conducted to compare to the theoretically predicted values and good agreement was found

changing depth distribution of hiatuses during the cenozoic the differential effects of climate change sea level and water mass circulation on deposition erosion of marine sediments can be constrained from the distribution of unconformities in the world s oceans i identified temporal and depth patterns of hiatuses hiatus events from a large and chronologically well constrained stratigraphic database of deep sea sediments the paleogene is characterized by few several million year long hiatuses the most significant cenozoic hiatus event spans most of the paleocene the neogene is characterized by short frequent hiatus events nearly synchronous in shallow and deep water sediments epoch boundaries are characterized by peaks in deep water hiatuses possibly caused by an increased circulation of corrosive bottom water and sediment dissolution the plio pleistocene is characterized by a gradual decrease in the frequency of hiatuses future studies will focus on the regional significance of the hiatus events and their possible causes

increasing restriction of the egyptian shelf during the early eocene new insights from a southern tethyan carbonate platform the evolution of the isolated galala carbonate platform has been studied intensively with respect to the paleocene eocene thermal maximum petm and the strong climatic variability from the late paleocene to the early eocene in this study we compare the results of different approaches which deal with the early palaeogene evolution of the egyptian shelf including new data from the galala platform microfacies analyses along a platform to basin transect reveal a sedimentological response to a massive tectonic uplift along the syrian arc fold belt in the early eocene this uplift triggered the restriction of the egyptian shelf by modulating and weakening tethyan onshore currents from the north the increasing deposition of quartz and proliferating gastropods green algae and radiolaria which indicate elevated nutrient levels reflect an intensified eutrophication of shallow marine platforms and deeper marine shelf environments strongly depleted carbon isotope ratios in the sediments of the galala succession strongly suggest the presence of restricted conditions throughout the early eocene we assume that a strong climatic divergence with arid conditions on the shelf and humid conditions at the african hinterland triggered intensified chemical weathering at the nubian arabian craton the enhanced riverine transport of terrestrial organic material to the north african shelf caused the increased availability of trophic resources and a strong negative shift of carbon isotope ratios between np10 and np14a the recovery to open ocean conditions is linked to the termination of tectonic uplift along the syrian arc fold belt in the latest early eocene and to stronger ocean currents caused by increasing latitudinal temperature gradients additionally the post petm carbon isotope evolution at the galala platform indicates at least two further negative carbon isotope excursions which reflect hyperthermal events on a global scale the associated reorganization of shallow marine assemblages as described for example for the larger foraminifera turnover at the petm boundary event is however missing we assume that the absence of a foraminiferal reorganization in this region can be linked to an increasing robustness of the major early eocene platform organisms larger benthic foraminifera and their adaption to unfavorable conditions and repeated environmental shifts 2011 elsevier b v

climatic and environmental changes across the early eocene climatic optimum at mid waipara river canterbury basin new zealand abstract the cretaceous paleogene marine sedimentary succession exposed in the banks of the middle reaches of the waipara river referred to as mid waipara north canterbury new zealand has been the subject of several high profile studies of paleogene paleoclimate over the past decade it is one of relatively few sections globally where a multi proxy approach is possible due to the good preservation of microfossils and organic biomarkers the eocene section is also well dated by magnetostratigraphy and biostratigraphy based on planktic foraminifera calcareous nannofossils and dinoflagellate cysts dinocysts here we build on this previous work and undertake a comprehensive analysis of paleontological and geochemical indicators of climatic and environmental changes through the early middle eocene part of the section with particular focus on the early eocene climatic optimum eeco 53 26 49 14 ma we correlate a 33 5 m thick interval with the eeco based on biostratigraphy magnetostratigraphy tex86 paleothermometry and bulk carbonate 13c our new sea surface temperature sst record based on tex86 agrees with a previous lower resolution record based on tex86 and planktic foraminiferal 18o and mg ca ratios the eeco interval in this section extends from the upper part of the new zealand waipawan stage to the mangaorapan heretaungan stage boundary at 49 27 ma the eeco onset is not exposed but the termination is well constrained by a fall in sst and shift to more positive 13c values six negative carbon isotope excursions cies are recognised within the eeco and are tentatively correlated with cies j k m o q t and c22nh4 in the global 13c compilation the cies are associated with warmer ssts indicating that they represent hyperthermals the bayspar tex86 calibration indicates sst increased by as much as 12 c from the early eocene 55 ma to the eeco where sst peaked at 35 c sst gradually declined from mid eeco 51 ma into the middle eocene the marked warming in the early eeco is associated with the highest abundance of warm water taxa in calcareous nannofossil and dinocyst assemblages the highest proportion of planktic foraminifera and a coeval long term shift to abundant angiosperm vegetation primarily driven by a rise in casuarinaceae there is good agreement between tex86 and marine microfossil based proxies for temperature providing confidence that both approaches are useful guides to past water temperature warm water marine taxa are most abundant in the eeco but are not dominant comparison of the abundance of nannofossil warm water taxa between mid waipara and a low latitude site on shatsky rise suggests the latitudinal temperature gradient between mid and low latitudes in the eeco was greater than the tex86 proxy implies there is no clear evidence for enhanced sedimentation rates associated with the eeco in contrast to evidence from the nearby mead stream section superabundant homotryblium a euryhaline dinocyst in the early and middle eeco suggests elevated salinity and or stratified surface waters and there is no clear evidence of increased surface productivity associated with the eeco declining sst in the late eeco 50 ma corresponds with an increase in cool water taxa and terrigenous material this article highlights the importance of combining well calibrated paleontological and geochemical records to better constrain and understand past warm climate states

magnetostratigraphic tests of sequence stratigraphic correlations from the southern california paleogene abstract several sequence stratigraphic analyses have been conducted on the eocene oligocene rocks of the san diego region and of the western transverse ranges in southern california recently developed magnetic stratigraphy and 40ar 39ar dates as well as previously neglected biostratigraphic data provide a high resolution chronostratigraphic framework to test correlation of these sequence stratigraphic interpretations to the global onlap offlap chart haq et al 1987 1988 hardenbol et al 1998 in the san diego area previous stratigraphic analyses miscorrelated nearly all sequences and sequence boundaries to the global time scale some correlations are off by as much as ten million years in the western transverse ranges three contradictory sequence stratigraphic analyses are evaluated the revised dates of the sequence boundaries show poor correlation with the onlap offlap chart of haq et al 1987 1988 or hardenbol et al 1998 and some units are misdated by as much as six million years in both regions few sequences were correctly predicted by the onlap offlap chart suggesting that tectonic controls were more important than eustatic controls in this tectonically active region the errors in so many sequence stratigraphic correlations when tested by high resolution chronostratigraphic data should caution geologists about using sequence stratigraphic age assignments uncritically sequence stratigraphic correlations are only as good as the chronostratigraphy on which they are based

middle to late miocene canyon cutting on the new jersey continental slope biostratigraphic and seismic stratigraphic evidence we have identified and dated a major miocene erosional surface m1 on the new jersey continental slope this surface was penetrated at deep sea drilling project dsdp site 612 which was drilled near the thalweg of a buried v shaped canyon biostratigraphic data at site 612 firmly constrain the age of strata above the buried canyon surface as zones cn7 nn9 and n16 lowermost upper miocene the upper miocene surface at site 612 lies above lowermost oligocene strata because of coalesced unconformities we traced the m1 erosional surface to the cost b 3 well where upper middle miocene strata underlie it biostratigraphic studies of other new jersey continental slope boreholes asp 14 asp 15 suggest that elsewhere the sediments immediately below the m1 surface encompass the globorotalia fohsi robusta zone zone n12 earliest n13 middle middle miocene the best age estimate is that m1 was eroded between 11 5 and 10 0 ma this erosional event apparently correlates with a similar event on the irish and florida continental margins and with oxygen isotope evidence for a glacio eustatic lowering

mechanisms of hydrovolcanic pyroclast formation grain size scanning electron microscopy and experimental studies pyroclasts produced by explosive magma water interactions are of various sizes and shapes data from analysis of over 200 samples of hydrovolcanic ash are interpreted by comparison with experimentally produced ash grain size and scanning electron microscopy sem reveal information on the formation of hydrovolcanic pyroclasts strombolian explosions result from limited water interaction with magma and the pyroclasts produced are dominantly centimeter sized with increasing water interaction hydrovolcanism increases in explosivity to surtseyan and vulcanian activity these eruptions produce millimeter to micron sized pyroclasts the abundance of fine ash 63 m diameter increases from 5 to over 30 percent as water interaction reaches an explosive maximum this maximum occurs with interactions of virtually equal volumes of melt and water five dominant pyroclast shape types determined by sem result from hydrovolcanic fragmentation 1 blocky and equant 2 vesicular and irregular with smooth surfaces 3 moss like and convoluted 4 spherical or drop like and 5 plate like types 1 and 2 dominate pyroclasts greater than 100 m in diameter types 3 and 4 are typical of fine ash type 5 pyroclasts characterize ash less than 100 m in diameter resulting from hydrovolcanic fragmentation after strong vesiculation fragmentation mechanisms observed in experimental melt water interactions result from vapor film generation expansion and collapse fragments of congealed melt are products of several alternative mechanisms including stress wave cavitation detonation waves and fluid instability mixing all result in rapid heat transfer these mechanisms can explain the five observed pyroclast shapes stress wave fracturing cavitation of the melt results from high pressure and temperature gradients at the melt water interface simultaneous brittle fracture and quenching produces type 1 pyroclasts type 2 develops smooth fused surfaces due to turbulent mixing with water after fracture and before quenching fluid instabilities promote turbulent mixing of melt and water and produce fine ash this kind of fragmentation occurs during high energy explosions the increased melt surface area due to fine fragmentation promotes high efficiency heat exchange between the melt and water shapes of resulting pyroclasts are determined by maximum surface area type 3 and surface tension effects type 4 type 5 pyroclasts result from nearly simultaneous vesicle burst and melt water fragmentation

intraplate stresses and dynamical aspects of rifted basins abstract intraplate stresses play a important role during the syn and post rift evolution of sedimentary basins deviations from long term thermal patterns of post rift subsidence as well as the development of unconformities within rifted basins reflect to a large extent the temporal fluctuations of the regional stress fields in the lithosphere it appears that incorporation of intraplate stresses in dynamical models for basin development together with a quantification of the role of petrological changes in the crust lithosphere system can explain some of the observed shortcomings of thermal models of post rift basin subsidence late neogene accelerations in tectonic subsidence in the deeper parts of rifted basins in the northern atlantic and mediterranean regions could reflect a change in the level of intraplate stress over large areas of the globe non thermal contributions to the subsidence record affect estimates of crustal extension and consequently influence the depth at which hydrocarbon source rocks enter the maturity window stress induced changes in basement topography have also important consequences for the budget of fluid flow in the deeper parts of the sediment fill of the rifted basins as well as on the dynamics of diapirism

depositional mechanics and sequences of base surges songaksan tuff ring cheju island korea the songaksan mount in the southwestern part of cheju island korea is a taalian tuff ring produced by phreatomagmatic explosions at an aquifer a detailed analysis of proximal to distal facies changes reveals that the tuff ring sequence can be represented by 21 sedimentary facies one lateral facies sequence lfs and three vertical facies sequences vfs the vfs 1 and 2 are representative of facies relationships in horizontal near vent deposits the vfs 1 comprises scour fill bedded tuff inversely graded tuff massive tuff and laminated tuff from base to top the vfs 2 is a variant of the vfs 1 replaced by an inversely graded lapilli tuff unit at the base the sequences suggest traction carpet suspension and minor traction sedimentation from a high concentration near vent base surge the lfs 1 and the vfs 3 are distilled from outward dipping flank deposits both sequences begin with disorganized lapilli tuff followed successively by stratified lapilli tuff dune bedded lapilli tuff very thinbedded tuff and accretionary lapilli they are suggestive of waning base surge which decreases in particle concentration suspended load fall out rate and flow regimes with an increase in traction and sorting these facies sequences suggest that a base surge experiences flow transformation with its flow characters changing with time and space a near vent base surge is turbulent uniformly mixed and highly concentrated and produces scour fill bedded tuff as capacity decreases the surge transforms into a dense and laminar underflow and a dilute and turbulent upper part gravity transformation depositing inversely graded massive and normally graded lapilli tuff ensuing loss of sediment load and mixing of ambient air result in flow dilution surface transformation stratified and dune bedded units are produced by tractional processes of turbulent and low concentration surge further dilution causes deceleration and cooling and results in precipitation of moistened ash and accretionary lapilli from suspension

paleogene geochronology an integrated approach geochronology is the conceptual division of continuous time as measured geochronometry by the progression in an ordinal series of events this is best achieved by an approach which integrates four independent data sets magnetostratigraphy seafloor spreading magnetic lineation patterns biostratigraphy and isotopic ages this integrated approach results in an ordinal framework which can measure time with greater resolution though perhaps less accuracy than a radiometric approach alone a comparative analysis of two recently proposed paleogene geochronologic time scales is presented

intrusive gravity currents between two stably stratified fluids we present an experimental and numerical study of one stratified fluid propagating into another the two fluids are initially at rest in a horizontal channel and are separated by a vertical gate which is removed to start the flow we consider the case in which the two fluids have the same mean densities but have different constant non zero buoyancy frequencies in this case the fluid with the smaller buoyancy frequency flows into the other fluid along the mid depth of the channel in the form of an intrusion and two counter flowing gravity currents of the fluid with the larger buoyancy frequency flow along the top and bottom boundaries of the channel working from the available potential energy of the system and measurements of the intrusion thickness we develop an energy model to describe the speed of the intrusion in terms of the ratio of the two buoyancy frequencies we examine the role of the stratification within the intrusion and the two gravity currents and show that this stratification plays an important role in the internal structure of the flow but has only a secondary effect on the speeds of the exchange flows

cenozoic unconformities and depositional supersequences of north atlantic continental margins testing the vail model integrated outcrop borehole and seismic reflection stratigraphy from the u s and irish margins of the north atlantic basin reveals a framework of cenozoic depositional supersequences and interregional unconformities that resembles the vail depositional model paleo bathymetric and paleoceanographic analyses of associated microfossil assemblages indicate a genetic link between the depositional framework and the relative position of sea level

a benthic foraminiferal record of paleocene sea level and trophic redox conditions at gebel aweina egypt we quantitatively analyzed benthic foraminiferal assemblages in order to reveal biotic and paleoenvironmental trends in the paleocene section of gebel aweina egypt on the southern tethyan shelf the first correspondence axis ranks taxa along an empirically derived bathymetric gradient with neritic taxa e g discorbis duwi and haplophragmoides walteri opposed to bathyal taxa e g nuttallides truempyi and gavelinella beccari ormis the estimated paleodepth as reflected by the various assemblages was mostly 200 m but varied between 50 m p3a np4 and 300 m p4 n p8 the resultant four relative sea level cycles appear to be at least of regional extent and may in part be eustatically controlled the second correspondence axis reflects trends in trophic and redox development low food high oxygen conditions prevailed during biochrons plc p3b np4 np5 and late p4 np8 when assemblages were mostly dominated by cibicidoides spp most oligotrophic conditions coincided with maximum paleodepth intermediate trophic levels were established during middle p4 np6 and early p5 np9 from the level of the latest paleocene benthic extinction event bee middle p5 and np9 onwards until biochron p6a np10 assemblages dominated by buliminids marked increased trophic levels and reduced ventilation probably resulting from enhanced surface productivity enhanced surface productivity along the southern tethyan margin started during biochron p4 and at least on some shelves intensified after the bee increasing productivity during the latest paleocene contrasts with global trends this may be expected during climatic warming and an expanding trophic resource continuum leading to regionally intensified eutrophy and oligotrophy high productivity through upwelling along the southern tethyan provides support for gcm predictions of intense wind stress in this region during the warm late to latest paleocene 1998 elsevier science b v

oligocene glacio eustasy and erosion on the margins of the north atlantic oligocene foraminiferal 18 o records suggest the development of ice caps inferred glacio eustatic falls at ca 36 31 and 25 ma biostratigraphic analyses of wells from the united states east coast and irish continental margins consistently show that upper oligocene sediments overlie a diachronous erosional surface underlain by lower oligocene to eocene strata at the minimum the hiatus extends between ca 34 and 30 ma we speculate that erosion during a glacio eustatic fall near the early late oligocene boundary ca 32 31 ma developed 1 an unconformity on the margins 2 numerous canyons noted in seismic profiles from the margins of the north atlantic and 3 a coastal offlap event using 18 o data we apply a model for eustatic changes and margin response that explains the relationships of sea level unconformities and coastal onlap offlap events

sequence stratigraphy of miocene pliocene carbonate siliciclastic shelf deposits in the eastern mediterranean margin israel effects of eustasy and tectonics abstract mio pliocene marine sediments comprise a discontinuous succession of carbonates and mixed carbonate siliciclastics on the neogene shelf which coincides with the judean foothills area on the slope present mediterranean coastal plain the succession forms a wedge up to 1500 m thick of mostly fine grained siliciclastics the facies and architecture of the succession were influenced both by sea level fluctuation and by the geodynamic change from thermally subsiding passive margins of the arabo african plate to a complex pattern of vertical movements that were associated with the breakup of the plate along the red sea rift and dead sea transform and the convergence of the african arabian and eurasian plates the miocene eustatic succession is highly interrupted by vertical uplift tectonics while the pliocene eustatic succession is well represented despite the limited subsidence rates during most of the early and middle miocene the shelf was tectonically uplifted above sea level except for a short period of flooding in early middle miocene times n8 n9 when sea level rise outstripped the uplift trend and a carbonate platform formed the combination of tectonic uplift and sea level fall prior to late miocene times about 11 m y ago resulted in the incision of a deep beer sheva canyon about 280 m below the shelf plane late miocene sandy marlstones of the bet eshel cycle filled the beer sheva canyon and spilled over it they display a shallowing upward trend resulting in progradational clinoform stacking of the highstand systems tract and channelled debris flows of the forced regression systems tract the messinian desiccation event left the shelf emerged while depositing evaporites on the slope and in the basin the degree of erosion during the messinian was relatively minor because of arid conditions and limited runoff the pliocene shelf shows a very low subsidence rate and compared to the miocene succession is less affected by vertical uplifts the pliocene deposits are divided into six sequences their relative onlap curve largely corresponds to the sequence stratigraphic scheme in sicily and the gulf of mexico the two lowermost sequences i and ii are composed of two sandy units the lower forms an incised valley fill of shoreface deposits in the beer sheva area and offshore sand deposits in distal shelf areas besor although largely eroded sequences i and ii display maximum coastal onlap of all pliocene cycles in the beer sheva canyon area sequence iii consists of mixed carbonate siliciclastic grainstones of shoreface foreshore deposits in the proximal area and lower shoreface deposits in more distal areas sequences iv and v may be considered as a single 3rd order cycle where iv represents regressive lowstand shale and sand deposits and v represents prograding highstand sandy grainstone deposits sequence vi consists of marginal marine to coastal plain marlstones and sands deposits and is largely truncated by the ahuzam conglomerate representing a major uplift phase of latest pliocene to early pleistocene age and marking the permanent emergence of the shelf

tertiary oxygen isotope synthesis sea level history and continental margin erosion tertiary benthic and planktonic foraminiferal oxygen isotope records are correlated to a standard geomagnetic polarity time scale making use of improved chronostratigraphic control and additional oligocene isotope data synchronous changes in both benthic and planktonic 18o values which occurred in the oligocene to miocene 36 5 2 ma are interpreted in part to represent ice growth and decay the inferred ice growth events correlate with erosion on passive continental margins as interpreted from seismic and chronostratigraphic records this association is consistent with a link between oligocene to miocene erosional events and rapid 15 m m y glacioeustatic lowerings of about 50 m high benthic foraminiferal 18o values suggest the presence of continental ice sheets during much of the oligocene to recent 36 0 ma substantially ice free conditions probably existed throughout the paleocene and eocene 66 36 ma the mechanisms and rates of sea level change apparently were different between the early and late tertiary with glacioeustatic changes restricted to the past 36 m y pre oligocene erosion on passive continental margins was caused by eustatic lowerings resulting from global spreading rate changes we apply a model which suggests that large areas of the continental shelves were subaerially exposed during such tectonoeustatic lowstands stimulating slope failure and submarine erosion the different mechanisms and rates of eustatic change may have caused contrasting erosional patterns between the early and late tertiary on passive continental margins this speculation needs to be confirmed by examination of data from several passive margins

plate reorganization a cause of rapid late neogene subsidence and sedimentation around the north atlantic subsidence analysis of wells in the central north sea and labrador grand banks and oft the west greenland scotian shelf and united states atlantic margin shows distinct quantitative stratigraphic correlation patterns of circum north atlantic sites a significant departure from the overall decrease in subsidence for the pliocene occurs in many wells when the rate is found to have increased one or more orders of magnitude from oligocence miocene rates wells were selected along transects from shore to basin to find if relative basin position is influenced by differential basin subsidence although stratigraphic resolution is not detailed more basinward sites experienced up to four times larger subsidence rates in the late neogene than in the oligocene miocene with a peak in the pliocene wells at the basin edge experienced much less subsidence or showed uplift the observations are consistent with a rapid change of intraplate stress at the cause for this observed transition in neogene subsidence major reorganizations of spreading direction and rate occurred during the pliocene along the entire atlantic spreading system possibly in conjunction with more global changes in plate motions we propose that the associated changes in intraplate stress caused the excess margin subsidence relative uplift along basin edges is consistent with this mechanism of relative movement and may explain apparent eustatic changes in sea level

sequence stratigraphy and tectonosedimentary history of the upper jurassic of the eastern paris basin lower and middle oxfordian northeastern france in the present study the difficulty which lies in the identification of sea level fall discontinuities in deep depositional environments led the authors to use the transgressive surfaces i e the most noticeable surfaces in the present case to determine depositional cycles four 3rd order lower and middle oxfordian cycles were identified s1 s2 s3 and s4 in the eastern paris basin these four cycles can be organised into two lower frequency cycles so i and so ii which comprise the s1 s2 and the s3 s4 cycles respectively during the time intervals of the s1 s2 and s3 cycles sedimentation occurred along a southward dipping carbonate siliciclastic ramp prograding from the northern ardennes area the s4 cycle shows the development of a reefal distally steepened ramp which subsequently evolved into a flat topped platform as a result of the compensation infill of the available accommodation space by the carbonate production during a climatic warming in this case reef growth isopach and facies maps suggest synsedimentary activities of hercynian faults coevals with the floodings of the so i and so ii cycles one of these events generated a diachronism of the maximum flooding plicatilis zone during the so ii cycle between the northwestern and southeastern parts of the studied area the depositional patterns found in the eastern paris basin and the swiss jura show great similarities for the early middle oxfordian both regions were probably connected and recorded the same tectonosedimentary evolution in contrast a tectonic control certainly generated differences between the sequence stratigraphic framework of the eastern paris basin and the eustatic chart

eocene eustatic sea level changes evidence from western sinai egypt abstract a major horizon of reworked material lies within the thebes formation near the top of the upper part of the lower eocene planktonic foraminiferal zone p9 the thebes formation is well exposed in western sinai and this reworked horizon lag deposit has been found to be widely distributed and coeval with similar deposits in israel and jordan when examined in detail it is found to consist of concentrated phosphatic nodules coprolites phosphatic grains and polished benthic and planktonic foraminifera the thebes formation has generally been described as a deep water basinal deposit however the presence of this lag deposit indicates an important period of shallow water deposition before the return to deeper water conditions in which the upper 20 m of this formation and the beds of the overlying darat formation were laid down a second but less pronounced phase of shallowing both in terms of concentration of lag deposits and its geographic distribution occurred at the top of the middle eocene khaboba formation top of zone p14 the shallowings are attributed to eustatic sea level falls and not to tectonic movements the eocene period in sinai was one of general tectonic stability and the effect of movements associated with the syrian arc folds are not detectable south of hammam faraun the two horizons of reworked material coincide stratigraphically with two global low sea level stands

tectonics and global change inferences from the late cenozoic subsidence and uplift patterns in the atlantic mediterranean region summary quantitative subsidence analysis for a number of rifted basins in the northern atlantic mediterranean region provides evidence for rapid phases of plioquaternary subsidence the observed acceleration in tectonic subsidence occurs after a phase of general quiescence in subsidence in these basins and deviates from predictions of stretching models the latter indicate a decay of subsidence with time after mesozoic tertiary basin formatioh and a slow tectonic subsidence in plio quaternary times a possible explanation for the observed patterns of anomalous subsidence could be an increase in the level of intraplate compression in the northern atlantic region intraplate stress changes in the plio quaternary are related to the dynamics of african eurasian collision processes and a reorganization of spreading directions in the atlantic possibly reflecting a plate reorganization of global nature it seems that the plio quaternary record reflects a period of increased levels of neotectonic activity interplaying with periods of de glaciation stress induced topography in the onshore parts of continental margins coupled with the stress induced subsidence in the offshore deeper parts of the basins could have contributed to recent phases of uplift in fennoscandia augmenting the uplift induced by glacial unloading estimates of ice thicknesses are directly inferred from the observed uplift ignoring other driving mechanisms whereas topography plays a crucial role in the dynamics of glaciation it is therefore important to quantdy the interplay of rapid tectonic uplift and subsidence phases with climatic effects during the plio quaternary

sequence stratigraphy eustasy or tectonic imprint sequence stratigraphy is viewed by many geologists as the expression in the stratigraphic record of the history of sea level changes sequence boundaries are its most distinctive elements and are used for global correlations until now correlations between unconformities and postulated lowerings of sea level have been seen primarily in terms of cause and effect relationships the postulated fall providing an explanation for the presence of an unconformity they have not been truly perceived in terms of stratigraphic correlations which would imply the establishment of synchrony between the cause and the effect to decipher the presence of a global sea level signal in the stratigraphic record the synchrony of sea level related unconformities in widely separated basins must be established which can be achieved by comparing the degree of overlap between hiatuses in different stratigraphic sections outcrop sections wells and the deep sea record reveal an apparently worldwide unconformity ies across the lower middle eocene boundary characterized by intensive reworking and or relatively rapid shallowing in some areas e g cyrenaica ne libya california egypt the lower middle eocene intervals in sections on the north atlantic margins have been correlated through magnetobiostratigraphy and the durations of the hiatuses have been precisely determined this record is compared with that in sections in other basins where shallowing and intensive reworking occur this study shows that there are two groups of unconformities and associated hiatuses around the lower middle eocene boundary the older hiatuses in the latest early eocene are generally short even on the shelf 1 m y the younger hiatuses in the early middle eocene are somewhat longer 2 m y the stratigraphic record around the lower middle eocene boundary suggests that there may be two mechanisms acting simultaneously to produce unconformities unconformities of a given age on the shelf may result from a global fall in sea level while their stratigraphically correlative unconformities on the slope and rise may result from tectonic instability unconformities related to the third order cycles in sea level changes in vail s model primarily may reflect tectonoeustasy

modelling intrusions through quiescent and moving ambients volcanic eruptions commonly produce buoyant ash laden plumes that rise through the stratified atmosphere on reaching their level of neutral buoyancy these plumes cease rising and transition to horizontally spreading intrusions such intrusions occur widely in density stratified fluid environments and in this paper we develop a shallow layer model that governs their motion we couple this dynamical model to a model for particle transport and sedimentation to predict both the time dependent distribution of ash within volcanic intrusions and the flux of ash that falls towards the ground in an otherwise quiescent atmosphere the intrusions spread axisymmetrically we find that the buoyancy inertial scalings previously identified for continuously supplied axisymmetric intrusions are not realised by solutions of the governing equations by calculating asymptotic solutions to our model we show that the flow is not self similar but is instead time dependent only in a narrow region at the front of the intrusion this non self similar behaviour results in the radius of the intrusion growing with time t as t3 4 rather than t2 3 as suggested previously we also identify a transition to drag dominated flow which is described by a similarity solution with radial growth now proportional to t5 9 in the presence of an ambient wind intrusions are not axisymmetric instead they are predominantly advected downstream while at the same time spreading laterally and thinning vertically due to persistent buoyancy forces we show that close to the source this lateral spreading is in a buoyancy inertial regime whereas far downwind the horizontal buoyancy forces that drive the spreading are balanced by drag our results emphasise the important role of buoyancy driven spreading even at large distances from the source in the formation of the flowing thin horizontally extensive layers of ash that form in the atmosphere as a result of volcanic eruptions

intraplate stresses a new tectonic mechanism for fluctuations of relative sea level stress variations in the lithosphere of a few hundred bars can explain the major part of the seismostratigraphic record at passive margins and intracratonic basins however to induce apparent sea level fluctuations with magnitudes greater than 50 m changes in stress level of more than 1 kbar are required these must be related to major reorganizations at convergent plate boundaries fragmentation of plates or collision processes specific rapid falls in the vail et al curves can be associated quantitatively with particular plate tectonic reorganizations of lithospheric stress fields the seismostratigraphic record may provide a new source of information on paleo stress fields to be correlated with results of independent numerical modeling of intraplate stresses

phreatomagmatic eruptions on the ontong java plateau an aptian 40ar 39ar age for volcaniclastic rocks at odp site 1184

rift related jurassic basaltic phreatomagmatic volcanism in the central transantarctic mountains precursory stage to flood basalt effusion the middle jurassic kirkpatrick flood basalts and comagmatic ferrar intrusions in the transantarctic mountains represent a major pulse of tholeiitic magmatism related to early stages in the breakup of gondwana a record of the volcano tectonic events leading to formation of this continental flood basalt province is provided by strata underlying and only slightly predating the kirkpatrick lavas in the central transantarctic mountains the lavas rest on widespread 7500 km2 tholeiitic pyroclastic deposits of the prebble formation the prebble formation is dominated by lahar deposits and is an unusual example of a regionally developed basaltic lahar field related partly fault controlled pyroclastic intrusions cut underlying strata and vents are represented by the preserved flanks of two small tephra cones associated with a volcanic neck lahar and air fall deposits typically contain 50 60 accidental lithic fragments and sand grains derived from underlying triassic lower jurassic strata in the upper part of the beacon supergroup juvenile basaltic ash and fine lapilli consist of nonvesicular to scoriaceous tachylite sideromelane and palagonite and have characteristics indicating derivation from hydrovolcanic eruptions the abundance of accidental debris from underlying beacon strata points to explosive phreatomagmatic interaction of basaltic magma with wet sediment and groundwater which appears to have occurred in particular where rising magma intersected upper beacon sand aquifers composite clasts in the lahar deposits exhibit complex peperitic textures formed during fine scale intermixing of basaltic magma with wet sand and record steps in subsurface fuel coolant interactions leading to explosive eruption the widespread sustained phreatomagmatic activity is inferred to have occurred in a groundwater rich topographic basin linked to an evolving jurassic rift zone in the transantarctic mountains coeval basaltic phreatomagmatic deposits of the mawson and exposure hill formations which underlie exposures of the kirkpatrick basalt up to 1500 km to the north along strike in victoria land appear to represent other parts of a regional extension related middle jurassic phreatomagmatic province which developed immediately prior to rapid outpouring of the flood basalts this is consistent with models which assign an important role to lithospheric stretching in the generation of flood basalt provinces

pre emergent construction of a lacustrine basaltic volcano pahvant butte utah usa the subaqueous phases of an eruption initiated approximately 85 m beneath the surface of pleistocene lake bonneville produced a broad mound of tephra a variety of distinctive lithofacies allows reconstruction of the eruptive and depositional processes active prior to emergence of the volcano above lake level at the base of the volcano and very near inferred vent sites are fines poor well bedded broadly scoured beds of sideromelane tephra having local very low angle cross stratification m1 lithofacies these beds grade upward into lithofacies m3 which shows progressively better developed dunes and cross stratification upsection to its uppermost exposure approximately 10 m below syneruptive lake level both lithofacies were emplaced largely by traction from relatively dilute sediment gravity flows generated during eruption intercalated lithofacies are weakly bedded tuff and breccia m2 and nearly structureless units with coarse basal layers above strongly erosional contacts m4 the former combines products of deposition from direct fall and moderate concentration sediment gravity flows and the latter from progressively aggrading high concentration sediment gravity flows early in the eruption subaqueous tephra jetting from phreatomagmatic explosions discontinuously fed inhomogeneous unsteady dilute density currents which produced the m1 lithofacies near the vent dunes and crossbeds which are better developed upward in m3 resulted from interaction between sediment gravity flows and surface waves triggered as the explosion generated pressure waves and eruption jets impinged upon and occasionally breached the surface intermingling of a tephra emplaced after brief transport by tephra jets within a gaseous milieu and b laterally flowing tephra formed lithofacies m2 along vent margins during parts of the eruption in which episodes of continuous uprush produced localized water exclusion zones above a vent m4 comprises mass flow deposits formed by disruption and remobilization of mound tephra intermittent explosive magma water interactions occurred from the outset of the pahvant eruption with condensation entrainment of water and lateral flow marking the transformation from eruptive to sedimentary processes leading to deposition of the mound lithofacies

astronomical calibration of the middle eocene contessa highway section gubbio italy abstract the eocene climatic system experienced an important transition from warm paleocene greenhouse to icehouse oligocene conditions this transition could first appear as a long term cooling trend but at an up close look this period is a complex combination of climatic events and for most of them causes and consequences are still not fully characterized in this context a study has been carried out on the middle eocene sedimentary succession of the contessa highway section central italy which is proposed as the global stratotype section and point gssp for the lutetian bartonian boundary at the top of the chron 19n with an astronomically calibrated age of 41 23 ma through a cyclostratigraphic analysis of the rhythmic sedimentary alternations and combination with the results of time series analysis of the proxy record we provide an orbital tuning of the middle eocene and astronomical calibration of the bio magnetostratigraphic events particularly for the c19n c18r chron boundary recognized at the contessa highway section

evolution of the gouge particle size distribution a markov model the hydrologic and mechanical properties of faults are determined by their internal structures and zoning due to fracturing grain breakage and diagenesis the proposed model in this study is a first step in the development of a more complete model of fault internal structure it describes the evolution of the gouge particle size distribution psd with shear induced grain breakage within a small assumed uniform volume element a markov formalism is applied for binary breakage within each time step over the many time steps constituting the evolution the particle size strain and stress dependent breaking probability is constructed based on physical arguments and data on natural and simulated gouge psds the model is calibrated by using the results of on ottawa sand psd experiments a perfect fit is obtained between experimental and numerically simulated psds for a range of normal effective stress nes results of the numerical simulations capture the bimodal form of gouge psd and also clearly define two different breakage mechanisms the gouge development behavior makes a dramatic transition as the normal effective stress exceeds the grain crushing stress of the gouge mineral for a more complete intrafault structure analysis the interplay of diagenesis and the mechanics of grain breakage can be integrated into a single mechano chemical model of the type presented here the model proposed here has great potential for predicting the complex roles of faults as seals or fluid conduits

comment on sea level control on facies architecture in the cenomanian coniacian apulian margin western tethys a record of glacio eustatic fluctuations during the cretaceous greenhouse by s galeotti g rusciadelli m sprovieri l lanci a gaudio and s pekar palaeogeography palaeoclimatology palaeoecology 276 2009 196 205 abstract galeotti et al 2009 galeotti s rusciadelli g sprovieri m lanci l gaudio a pekar s a record of glacio eustatic fluctuations during the cretaceous greenhouse palaeogeogr palaeoclimatol palaeoecol 276 196 205 hereafter getal09 use the sequence stratigraphic record of a cenomanian coniacian base of slope succession exposed in the morrone mountain of central italy to calibrate the sea level history inferred from the adjacent lazio abruzzi carbonate platform the episodes of sea level fall recorded by this carbonate platform and slope in the western tethys are said to coincide with episodes of global cooling and sea level fall recorded in the new jersey margin miller k g sugarman p j browning j v kominz m a hernandez j c olsson r k wright j d feigenson m d van sickel w 2003 late cretaceous chronology of large rapid sea level changes glacioeustasy during the greenhouse world geology 31 585 588 on the basis of this correlation getal09 claim that the late cretaceous sea level history of the lazio abruzzi carbonate platform was controlled by glacio eustatic fluctuations this conclusion conflicts with a wealth of published data supporting the view that the major unconformities in the cretaceous carbonate platforms of central southern italy were controlled by regional tectonics we will show how the sea level history of the lazio abruzzi carbonate platform proposed by getal09 and its correlation with the base of slope section hinge on a misrepresentation of the chronostratigraphic resolution attainable with biostratigraphy in shallow water carbonate facies we will also show how contrary to what claimed by getal09 the age model for their base of slope section does not support the correlation with the new jersey margin sequences finally we will argue that in upper cretaceous carbonate platforms the integration of bio and chemostratigraphy can greatly improve stratigraphic resolution and chronostratigraphic correlation this is a prerequisite for using the shallow water record to test the hypothesis of global glacio eustatic fluctuations in the cretaceous greenhouse

new zealand and australian cenozoic sedimentary cycles and global sea level changes a global sea level history determined by vail et al consists of 23 sea level cycles between the base of the cenozoic and the pliocene pleistocene boundary 65 to 1 8 ma we have examined the relation between this global sea level history and the new zealand and australian continental margin shallow marine sedimentary record sedimentary cycles in the australian cenozoic marginal marine sequences are bounded by unconformities of various durations the tectonic stability and aridity of the western and southwestern margin of australia during the cenozoic produced a sedimentary record dominated by hiatuses this contrasts with the relatively complete sequences of the tectonically more active southeastern marginal basins the four major sedimentary cycles in the australian cenozoic paleocene to early eocene middle to late eocene latest oligocene to late middle miocene and latest miocene to quaternary correlate with the supercycles ta tb tc td and te of vail et al in new zealand most of the cenozoic stages represent classic sedimentary cycles bounded by unconformities or correlative conformities formed as a result of large rapid eustatic sea level changes the marine tertiary sequence in new zealand consists of 23 stages of the 18 stage boundaries between the end of the paleocene 53 ma and the end of the miocene 5 ma 16 appear to correlate with the boundaries of eustatic sea level cycles eustatic sea level lowstands are well recorded as unconformities in new zealand as a result of its unique tectonic setting during the cenozoic for most of the early cenozoic until the middle oligocene 30 ma new zealand was generally subsiding after the middle oligocene this region began to be uplifted as the pacific australian plate boundary migrated onto new zealand climaxing in the latest cenozoic late pliocene quaternary thus exposing a nearly complete sequence of marine cenozoic strata well suited for the study of sedimentary cycles unconformities which form as a result of eustatic sea level changes represent a very useful correlation tool especially on continental margins with a sufficiently high terrigenous sediment supply and can be used to supplement paleontologic correlations their ultimate usefulness for correlation depends on the rapidity of the sea level changes or the speed at which sedimentary facies change in response to sea level changes if sea level changes rapidly 10 m 1 000 years stratigraphic resolution will be high 105 years but resolution decreases as the speed of sea level change decreases unconformities may well prove to be a vital tool linking the classic land based sections around the world with deep sea sections

relative sea level changes in baltoscandia in the cumbrian and early ordovician the predominance of tectonic factors and the absence of large scale eustatic fluctuations abstract fluctuations in sea depth within a magnitude 20 100 m and a duration of 1 10 m y are often explained by rapid eustatic changes so called third order eustatic events considerable influence of regional tectonics on relative sea level changes has been demonstrated by many authors but because of uncertainties in the timing of short events in widely separated regions the problem of separating tectonic and eustatic factors still remains unsolved in this paper a new and simple approach is used to reveal the presence or absence of eustatic events we consider the st petersburg area and north estonia in the north eastern region of the east baltic from the late early cambrian until the middle of the tremadoc early ordovician deposition was extremely slow and the sea bed remained for a long time in a well defined peritidal zone in a water depth 10 m under such environmental conditions a sea level rise of 10 m would result in marked changes in the character of faunas and sedimentation in the time interval considered here significant sea deepening in the north eastern baltic region occurred only twice and its magnitude did not exceed 10 20 m a fall of sea level by 10 m would result in complete regression in the peritidal zone this situation also occurred in region however the preservation of a sequence of unconsolidated sands which is only a few tens of metres thick and includes all the main stratigraphic subdivisions on a regional scale indicates that the crustal surface reached a very low altitude 10 20 m above sea level these data show that in the late late cambrian to the middle of the tremadoc over a period of 40 m y long eustatic sea level changes did not exceed 10 20 m this limits the magnitude of several third order cycles eustatic events with duration of a few million years which have been proposed previously for the epoch of the transition from the cambrian to the ordovician in the late early cambrian to the late cambrian transgressions and regressions with a magnitude of 50 150 m took place in southern sweden and lithuania since these phenomena occurred when there were no comparable eustatic sea level changes they must be associated with regional tectonic movements some were rapid and could be easily misinterpreted as indications of third order eustatic changes it is probable that some of the other eustatic events that have been proposed for the phanerozoic were actually not of eustatic but of tectonic origin such rapid tectonic movements with magnitude of 50 100 m in cratonic areas can be caused by changes in the forces in the lithospheric layer with a laterally variable thickness and by phase transitions in the mafic lower crust depending on the spatial distribution of vertical crustal movements both these mechanisms could have been operating in the east baltic and southern sweden in the cambrian

intrusive gravity currents in a stratified ambient shallow water theory and numerical results the intrusion of a fixed volume of fluid which is released from rest and then propagates horizontally at the neutral buoyancy level in a vertically stratified ambient fluid is investigated the density change is linear in a restricted layer or over the full depth of the container and locks of both rectangular and cylindrical shapes are considered a closed one layer shallow water inviscid formulation is used to obtain solutions of the initial value problem similarity solutions for the large time developed motion and an approximate box model are also presented the results are corroborated by numerical solutions of the full two dimensional navier stokes equations and comparisons with previously published experiments

cenozoic global sea level sequences and the new jersey transect results from coastal plain and continental slope drilling the new jersey sea level transect was designed to evaluate the relationships among global sea level eustatic change unconformity bounded sequences and variations in subsidence sediment supply and climate on a passive continental margin by sampling and dating cenozoic strata from coastal plain and continental slope locations we show that sequence boundaries correlate within 0 5 myr regionally onshore offshore and interregionally new jersey alabama bahamas implicating a global cause sequence boundaries correlate with 18o increases for at least the past 42 myr consistent with an ice volume glacioeustatic control although a causal relationship is not required because of uncertainties in ages and correlations evidence for a causal connection is provided by preliminary miocene data from slope site 904 that directly link 18o increases with sequence boundaries we conclude that variation in the size of ice sheets has been a primary control on the formation of sequence boundaries since 42 ma we speculate that prior to this the growth and decay of small ice sheets caused small amplitude sea level changes 20 m in this supposedly ice free world because eocene sequence boundaries also appear to correlate with minor 18o increases subsidence estimates backstripping indicate amplitudes of short term million year scale lowerings that are consistent with estimates derived from 18o studies 25 50 m in the oligocene middle miocene and 10 20 m in the eocene and a long term lowering of 150 200 m over the past 65 myr consistent with estimates derived from volume changes on mid ocean ridges although our results are consistent with the general number and timing of paleocene to middle miocene sequences published by workers at exxon production research company our estimates of sea level amplitudes are substantially lower than theirs lithofacies patterns within sequences follow repetitive predictable patterns 1 coastal plain sequences consist of basal transgressive sands overlain by regressive highstand silts and quartz sands and 2 although slope lithofacies variations are subdued reworked sediments constitute lowstand deposits causing the strongest most extensive seismic reflections despite a primary eustatic control on sequence boundaries new jersey sequences were also influenced by changes in tectonics sediment supply and climate during the early to middle eocene low siliciclastic and high pelagic input associated with warm climates resulted in widespread carbonate deposition and thin sequences late middle eocene and earliest oligocene cooling events curtailed carbonate deposition in the coastal plain and slope respectively resulting in a switch to siliciclastic sedimentation in onshore areas oligocene sequences are thin owing to low siliciclastic and pelagic input and their distribution is patchy reflecting migration or progradation of depocenters in contrast miocene onshore sequences are thicker reflecting increased sediment supply and they are more complete downdip owing to simple tectonics we conclude that the new jersey margin provides a natural laboratory for unraveling complex interactions of eustasy tectonics changes in sediment supply and climate change

sedimentary basin deformation an incremental stress approach abstract a key component of sedimentary basin evolution is the spatial distribution and temporal variation of stress and deformation the many deformation processes poroelasticity fracturing irreversible nonlinear viscosity and pressure solution are inextricably bound in a tightly coupled network which in turn is coupled to a myriad of basin diagenetic thermal and hydrologic processes in the approach presented here the various deformation processes are integrated through an incremental stress approach together with mass momentum and energy conservation this approach yields a complete fully coupled basin model that captures basin and fault phenomena that are beyond the scope of simpler or decoupled models many of the most interesting basin phenomena are not only dependent on multiple coupled processes but also are fundamentally three dimensional to address this three dimensional complexity we have developed a numerical simulator using a moving adapting accreting finite element grid which is allowed to deform and to grow and adapt with the addition of sediment to capture smaller sedimentary features as a result our fully coupled comprehensive model allows one to solve a number of key problems in basin and fault dynamics these include compaction fractured reservoir and compartment genesis and dynamics examples illustrating these applications are presented for idealized systems and the piceance basin colorado and the permian basin west texas the incremental stress rheology is found to be a powerful formalism for integrating basin hydrology diagenesis and mechanics

failure memory and cyclic fault movement faulting is typically a two timescale process that is rapid failure and slow chemical healing once failed a rock has a long memory until slow chemical processes have time to re establish the grain grain contacts underlying rock competency the memory of rock failure can be captured by a sufficiently rich textural model and the texture must be coevolved with rock stress and deformation to yield a self consistent model of strain hardening weakening fault narrowing and earthquake cyclicity a model based on incremental stress rheology and rock texture dynamics is introduced that emphasizes the interplay of rock competency porosity and other texture variables with stress and strain the deformation mechanisms taken into consideration are poroelasticity and viscosity the rheology equations are strongly coupled to the evolution equations of rock texture and pore fluid flow the model is used to gain an understanding of several oscillatory modes of fault movement the roles of rock competency fluid pressure and continuous deformation in these oscillations is illustrated for various conditions the approach is shown to be a natural starting point for a theory of the three dimensional multiprocess dynamics of fault nucleation growth morphology reactivation and continuous versus seismic behavior

on a tectonic mechanism for regional sealevel variations abstract no satisfactory tectonic explanations have yet been offered for the apparent sealevel fluctuations of about 1 cm 1000 years and a magnitude of up to a few hundred meters that have been proposed by vail et al 1 we propose a mechanism that does appear to be able to explain these changes if horizontal stresses of the order of a few kilobars occur in the lithosphere and if changes in these stress fields occur on geological time scales the proposed model is one of an interaction between these stresses and the deflections of the lithosphere caused by sedimentary loading apparent sealevel changes of up to 100 m can be produced at the flanks of the sedimentary basins by this interaction the mechanism is most effective for young margins that are subject to rapid sediment loading by its nature the tectonic model can explain contemporaneous fluctuations in apparent sealevel in neighbouring depositional environments in principle it implies the possibility of regional correlations in different basinal settings

paleogene calcareous nannoplankton biostratigraphy of northwestern europe abstract the classic paleogene stages thanetian cuisian lutetian bartonian ludian and stampian defined in the epicontinental deposits of the anglo paris basin s are placed within a standard biostratigraphic framework based on an analysis of their calcareous nannoplankton floras relatively precise biostratigraphic correlations can be made with some chronostratigraphic units e g lutetian while biostratigraphic resolution of others e g ludian is limited important hiatuses are recognized between certain chronostratigraphic units only the base of the lutetian late np14 and bartonian mid np16 stages can be precisely determined in terms of calcareous nannoplankton zones with the recent study of the magnetic polarity history of the paleogene of southern england and recent advances in magnetobiostratigraphic correlations in deep sea cores it is possible to interpret the paleogene history of the anglo paris basin s in a synoptic global framework in which paleomagnetic stratigraphy provides the chronologic and eustatic variations the graphic units of northwestern europe within rather than at the boundaries of i e at dynamic element redefinition of the boundaries of the standard paleogene chronostratigraphic unconformities eustatically controlled depositional cycles would result in more precise correlations in terms of standard calcareous nannoplankton biostratigraphy

intrusive gravity currents from finite length locks in a uniformly stratified fluid gravity currents intruding into a uniformly stratified ambient are examined in a series of finite volume full depth lock release laboratory experiments and in numerical simulations previous studies have focused on gravity currents which are denser than fluid at the bottom of the ambient or on symmetric cases in which the intrusion is the average of the ambient density here we vary the density of the intrusion between these two extremes after an initial adjustment the intrusions and the internal waves they generate travel at a constant speed for small departures from symmetry the intrusion speed depends weakly upon density relative to the ambient fluid density however the internal wave speed approximately doubles as the waves change from having a mode 2 structure when generated by symmetric intrusions to having a mode 1 structure when generated by intrusions propagating near the bottom in the latter circumstance the interactions between the intrusion and internal waves reflected from the lock end of the tank are sufficiently strong and so the intrusion stops propagating before reaching the end of the tank these observations are corroborated by the analysis of two dimensional numerical simulations of the experimental conditions these reveal a significant transfer of available potential energy to the ambient in asymmetric circumstances

eocene oligocene sea level changes on the new jersey coastal plain linked to the deep sea record we use magnetostratigraphy and sr isotope stratigraphy to improve stratigraphic control for the eocene to oligocene of the new jersey coastal plain acgs4 borehole magnetostratigraphy in many cases is complicated in outcrop sections of shallow water

problems in chronostratigraphy stages series unit and boundary stratotypes global stratotype section and point and tarnished golden spikes abstract stratigraphy is a magnificent subject magnificent in its breadth and scope magnificent in its importance to so many lines of human endeavor it is essentially that branch of geology which deals with the arrangement the distribution and the chronological succession of rock strata and other associated rock bodies with respect to any or all of the various characters properties and attributes which rocks may possess hedberg 1958 p 1881 chronostratigraphy the temporal ordering of geologic strata is at the heart of earth history in reviewing chronostratigraphic theory from d orbigny to hedberg we see that concepts and practices have evolved under pressure of constant improvements in time control and data quality such stimuli are naturally felt more strongly in the youngest part of the stratigraphic record where dating is most precise and the biostratigraphic data are most abundant and well preserved it is not surprising therefore to find that chronostratigraphy is conceptually more advanced in the cenozoic erathem than in the mesozoic and paleozoic erathems where biostratigraphy and chronostratigraphy are widely confused evidently out of necessity taking the five inter series boundaries in the cenozoic erathem as case histories we review their current status in relation to the lower boundary of the lowest standard stage above the boundary in light of the hierarchical principle recommended in the international stratigraphic guide hedberg 1976 salvador 1994 in each instance what should be a simple nested relationship turns out to be complex and difficult and in the cases of the proposed global stratotype section and point gssps for the eocene oligocene series boundary and the paleocene eocene series boundary there are discrepancies so large with regard to the base of the rupelian and ypresian standard stages respectively that some essential relaxation of the hierarchic rule seems to be required if the historic value of these long known stages is not to be violated since stage boundaries should not be defined a posteriori to fit series boundaries we propose here to reconsider the role of the standard stage as the obligate elemental subdivision in a rigidly hierarchical chronostratigraphic scale and suggest that series and subseries should be formalized and their boundaries defined by gssps we do not suggest that stages be abandoned on the contrary we propose to integrate them in a two step hierarchical chronostratigraphic framework so as to use them in complementary fashion with series and subseries however the boundary concept is more appropriate for stage than the gssp we recognize the need for precise and globally valid chronostratigraphic correlations based on gssps in attempts to understand the earth system while at the same time we emphasize the need for conceptual continuity with regard to the central place that the stage has played in chronostratigraphy over several decades

accretionary lapilli bearing pyroclastic rocks at odp leg 192 site 1184 a record of subaerial phreatomagmatic eruptions on the ontong java plateau

two models global sea level change and sequence stratigraphic architecture abstract two different conceptual models underlie the application of sequence stratigraphy by exxon stratigraphers and later researchers one the global sea level model gsm relates to presumed sea level behaviour through time the other the sequence stratigraphic model ssm relates to the stratigraphic record produced during a single cycle of sea level change though the two models are inter related they are logically distinct and it is important to test them separately a summary is presented of the nature of the two models and of the nomenclature that is used in their description it is concluded a that the global sea level model comprises an assembly of local relative sea level events which are widely recognisable within their parent sedimentary basin and b that the sequence stratigraphic model is robust and its application is therefore an insightful way to approach the interpretation of sedimentary rocks

carbonate deposition and sequence stratigraphy of the terminal cambrian grand cycle in the southern appalachians u s a abstract the cessation of shale and carbonate deposition of the conasauga group grand cycles middle to upper cambrian and the establishment of the widespread peritidal carbonate deposition of the knox group upper cambrian to lower ordovician represent a prominent change in sedimentation along the early paleozoic passive continental margin in the southern appalachians to evaluate the causes for this change this study focuses on the maynardville formation which is the uppermost carbonate unit of the conasauga group the maynardville consists of 1 a lower subtidal facies succession which is underlain by the nolichucky shale and resembles the rest of the conasauga group carbonate deposits and 2 an upper peritidal facies succession which is conformably overlain by similar peritidal deposits of the copper ridge dolomite knox group deposition of shale and subtidal carbonate took place in deep ramp nolichucky to shallow ramp subtidal shoal and lagoonal settings lower maynardville the carbonate ramp was westward sloping toward the conasauga intrashelf shale basin to the east the ramp was linked to a broad semiarid carbonate tidal flat encompassing a variety of peritidal environments upper maynardville and copper ridge the maynardville is a shallowing upward succession that formed by carbonate platform aggradation and basinward progradation the transition between the maynardville and the copper ridge dolomite is within a conformable peritidal carbonate succession that contains common siliciclastic sand size detritus this interval is interpreted as a sequence boundary correlative with the craton wide late steptoean dresbachian franconian or sauk ii sauk iii unconformity the change from a conasauga to a knox depositional style may be related to distinct stages in passive margin evolution the end of grand cycle deposition in the early late cambrian is coincident with the cessation of tectonic activity along extensional features an intracratonic graben and other fault systems and marks the transition into a mature passive margin setting the mature margin was characterized by decreased rates of thermal subsidence which coupled with the infilling of the conasauga intrashelf basin favored shallow water carbonate deposition the final stabilization of the margin is reflected in the deposition of the thick peritidal carbonate strata of the knox group

recent volcanic history of the veidivotn fissure swarm southern iceland an approach to volcanic risk assessment abstract the recent volcanic history of the southwestern part of the veidivotn fissure swarm southern iceland provides a basis for assessment of volcanic risk in an area of large hydropower potential local tephrastratigraphy and regional tephrochronology provide relative and absolute dating of individual eruptions as well as information on the volume and distribution of the products formed in each eruption three large eruptions took place in this area in 1480 a d 900 a d and 150 a d respectively each eruption produced approx 1 km3 dre of basaltic and minor amounts of silicic lava and tephra on fissures up to 42 km long no evidence is found of smaller eruptions during this period the estimated eruption frequency one eruption every 600 800 years implies that this part of the veidivotn fissure swarm is inactive for long periods between relatively large volcanic events a change in the mode of eruption from effusive to explosive took place during this period the hazards posed by this area include far reaching lava flows widespread heavy tephra fall with thicknesses in excess of 2 m at distances of 10 km and damming of a large glacial river with the consequent formation of unstable lakes a volcano tectonic model which explains the observed eruption frequency and provides a basis for a long term monitoring program is proposed eruptions on the veidivotn fissure swarm are interpreted as corollaries of rifting episodes initiated in the bardarbunga central volcano volcano tectonic episodes affect the fissure swarm at an average interval of 100 years minor episodes are limited to the central volcano and adjacents parts of the fissure swarm during the less frequent major episodes rifting and volcanic activity extends to the extreme southwestern part of the fissure swarm seismic monitoring of the bardarbunga central volcano could provide an early warning of renewed activity on the veidivotn fissure swarm a major rifting episode resulting in eruption on its southwestern part can be expected during the next 100 to 300 years

the missing link in the evolutionary origin of the foraminiferal genus hantkenina and the problem of the lower middle eocene boundary the initial appearance of the planktonic foraminiferal genus hantkenina has been used for about fifty years to recognize the base of the lutetian and middle eocene however probably as a result of incomplete stratigraphic records discrepant ranges of hantkenina have been reported by various investigators at many eocene sections here we report the first complete evolutionary transition from clavigerinella to hantkenina from the northwestern tethyan deep water section at holzhausl salzburg austria a newly discovered species hantkenina nov sp is the link between clavigerinella caucasica and hantkenina mexicana this finding unequivocally heralds the initial entry of hantkenina which is correlated to the upper part of calcareous nannoplankton subzone np15b sullivania gigas subzone to be defined this indicates a mismatch of 4 5 m y between the base of the lutetian at the type locality which has been placed within subzone np14b and the first appearance datum of hantkenina consequently the first occurrence of hantkenina can no longer be used as a marker for the base of the middle eocene

time varying underflow into a continuous stratification with bottom slope results are presented from a laboratory investigation of a continuous discharge gravity current moving down an inclined plane into a linearly stratified fluid the density of the inflow decreasing linearly with time initially larger and finally smaller than the bottom ambient density the inflowing water was observed to follow both underflowing and intrusive flow regimes hence during the time in which the inflow was denser than the water in the stratified reservoir an underflow was observed to descend down the sloping bottom with a speed that was consistent with that given by the theory for a buoyancy conserving gravity current on gentle slopes however the continuous decrease of the density at the source shortly lead to an unstable density distribution within the initial underflow which then collapsed into an intrusion that traveled as a horizontal gravity intrusion scaling arguments were used to identify both the position and time to the breakup of the underflow to the end of the experiment multiple intrusions were established successively at different depths in between the initial underflow and the surface buoyant plume

integrated stratigraphy magneto bio and chronostratigraphy and geochronology of the palaeogene pelagic succession of the umbria marche basin central italy

capelinhos 1957 1958 faial azores deposits formed by an emergent surtseyan eruption the 1957 1958 eruption of capelinhos faial island azores involved three periods of surtseyan hydromagmatic activity two in 1957 and one in 1958 deposits from this eruption are exposed both in sea cliffs cut into the flanks of the tuff cone and more distally 1 km from the vent five lithofacies are identified lithofacies i is composed of even thickness beds with laterally continuous internal stratigraphy and is interpreted to have been formed by fallout lithofacies ii consists of beds with internally discontinuous lenses and has sand wave structures that increase in abundance toward the outer margins of the tuff cone this lithofacies is interpreted as having been deposited from pyroclastic surges lithofacies iii is composed of mantle bedded deposits with laterally discontinuous internal stratigraphy this lithofacies is interpreted to have been formed by hybrid processes where fallout of tephra occurred simultaneously with pyroclastic surges in the outer flanks of the tuff cone lithofacies iii grades laterally into fallout beds of lithofacies i lithofacies iv consists of alternating beds of coarse ash aggregates and non aggregated fine ash and is particularly well developed in distal regions some of this facies was formed by fallout alternating beds also occur plastered against obstacles up to 2 km from the vent indicating an origin from wet pyroclastic surges the orientation of plastered tephra indicates that the surges were deflected by topography as they decelerated the distinction between surge and fallout in distal regions is uncertain because wind drifted fallout and decelerating surge clouds can generate similar deposits lithofacies v consists of scoria lapilli beds interpreted to be fallout from hawaiian style fire fountaining in the later stages of the eruption juvenile pyroclasts within hydromagmatic deposits are predominantly poorly vesicular 25 60 of clasts 30 vesicles however on both micro and macroscopic scales there is a wide range in clast vesicularity up to 70 vesicles indicating that although fragmentation was predominantly hydromagmatic vesiculation and magmatic volatile driven fragmentation operated simultaneously

high rate sea level change during the mesozoic new approaches to an old problem abstract it is generally assumed that the effects of tectonism sediment accumulation and compaction and eustasy on accommodation change cannot be untangled on a regional scale hence most reconstructions of past eustatic change have focused on global correlations here this approach is questioned in the time domain of few my and less global cycle correlation is often unreliable and the effects of isostasy on sea level show strong provincial variations in contrast on a regional scale cycle correlation is more precise and regional tectonism has predictable limitations these considerations form the base for an assessment of the poorly understood causes of high rate sea level change in the mesozoic three well documented regional semi quantitative sea level curves are chosen here for reference jurassic of britain cretaceous of russia albian of oman the absolute numbers of amplitudes are open to question but the order of magnitude rates of several meters to tens of meters per my is a robust property of all three curves even when considering large error bars variable mechanisms that cause these regressive transgressive cycles are considered but rejected with exception of the controversial issue of glacio eustasy or unknown mechanisms therefore orbital forcing of glacio eustasy is assessed freezing melting cycles of higher altitude ice shields can explain high rate sea level change with moderate amplitudes but conclusive evidence is buried beneath 3 km of east antarctic ice thus although several ambiguities of global sea level correlation are avoided by the regional approach the causes of high rate mesozoic sea level change remain poorly understood

unlocking the ice house oligocene miocene oxygen isotopes eustasy and margin erosion oxygen isotope records and glaciomarine sediments indicate at least an intermittent presence of large continental ice sheets on antarctica since the earliest oligocene circa 35 ma the growth and decay of ice sheets during the oligocene to modern ice house world caused glacioeustatic sea level changes the early eocene was an ice free greenhouse world but it is not clear if ice sheets existed during the middle to late eocene doubt house world benthic foraminiferal 18o records place limits on the history of glaciation suggesting the presence of ice sheets at least intermittently since the earliest oligocene the best indicator of ice growth is a coeval increase in global benthic and western equatorial planktonic 18o records although planktonic isotope records from the western equatorial regions are limited subtropical planktonic foraminifera may also record such ice volume changes it is difficult to apply these established principles to the cenozoic 18o record because of the lack of adequate data and problems in stratigraphic correlations that obscure isotope events we improved oligocene to miocene correlations of 18o records and erected eight oxygen isotope zones oi1 oi2 mi1 mi6 benthic foraminiferal 18o increases which are associated with the bases of zones oil circa 35 8 ma oi2 circa 32 5 ma and mil circa 23 5 ma can be linked with 18o increases in subtropical planktonic foraminifera and with intervals of glacial sedimentation on or near antarctica our new correlations of middle miocene benthic and western equatorial planktonic 18o records show remarkable agreement in timing and amplitude we interpret benthic planktonic covariance to reflect substantial ice volume increases near the bases of zones mi2 circa 16 1 ma mi3 circa 13 6 ma and possibly mi5 circa 11 3 ma possible glacioeustatic lowerings are associated with the 18o increases which culminated with the bases of zone mi4 circa 12 6 ma and mi6 circa 9 6 ma although low latitude planktonic 18o records are required to test this these inferred glacioeustatic lowerings can be linked to seismic and rock disconformities for example we link 12 oligocene early late miocene inferred glacioeustatic lowerings with 12 of the sequence boundaries inferred eustatic lowerings of haq et al 1987

northwestern european paleogene magnetostratigraphy biostratigraphy and paleogeography calcareous nannofossil evidence global calcareous nannofossil biostratigraphy is used to correlate the paleogene chronostratigraphic units of northwestern europe and standard nannofossil biozones such biostratigraphic correlations show that the lithostratigraphic sequence is discontinuous and allows interpretation of the paleogeography of northwestern europe during the paleogene and evaluation of the influence of global sea level eustatic changes on northwestern european stratigraphy a well documented unconformity occurs near the lower middle eocene boundary in the paris basin this unconformity which also can be traced in the london hampshire basin and in belgium correlates with a coastal offlap event presumed sea level of vail et al on the basis of magnetostratigraphic biostratigraphic correlations the duration of the hiatus associated with this unconformity can be estimated to be 0 6 to 1 5 m y in the hampshire basin and 2 to 4 m y in the paris basin

pre quaternary sea level changes while the paramount importance of global sea level changes has been widely accepted by quaternary geologists throughout this century it is only within the last decade that such changes in the rest of the phanerozoic have received much attention there are two principal reasons for this arousal of interest first increased exploration of the stratigraphic record across the earth both on land and under the sea and significant improvement in biostratigraphic correlation have drawn attention to what appear to have been globally synchronous events second oceanographic research has demonstrated the importance of tectonically controlled changes in the cubic capacity of the ocean basins notably variations in ridge volume which provide a ready mechanism for sea level changes even at nonglacial times the adjective eustatic was first proposed for global changes of sea level by suess 1906 in his great treatise the face of the earth the corresponding noun is eustasy not eustacy as it is commonly misspelt the unconvinced reader need only think of ecstasy eustatic sea level changes can be inferred as suess recognized by a plotting the temporal spread of marine sediments over the continents and b estimating the depositional depth variations in marine stratal sequences which can be correlated across and between continents to these methods can be added seismic stratigraphy in which biostratigraphically age determined seismic sequences on the con tinental margins are analyzed in terms of cycles of coastal onlap and offiap when the results of these methods are broadly in accord confidence in a eustatic interpretation is increased but the range of confidence can vary considerably it is widely accepted that there was an exceptionally high sea

quantifying stratigraphic completeness a probabilistic approach using paleomagnetic data although sedimentary hiati of variable magnitude are abundant in most stratigraphic sections quantification of the degree of completeness for individual sections has proven difficult a new method of estimating stratigraphic completeness is developed for sections that preserve a detrital remanent magnetic reversal history because the timing of geomagnetic reversals is independent of sedimentation events the distribution of reversals in a stratigraphic section can be used to evaluate completeness the most direct method in which sections are sampled at the top and base of each stratigraphic unit evaluates the frequency of reversals within units versus at unit contacts the more complete a section the greater the probability that reversals occurred during depositional events and are recorded as within unit reversals the less complete a section the greater the probability that reversals occurred during depositional hiati and are recorded as unit contact reversals monte carlo simulations suggest that

immense vent complex marks flood basalt eruption in a wet failed rift coombs hills antarctica large lava effusions can have impressive explosive antecedents although our picture of flood basalt is overwhelmingly effusive phreatomagmatic eruptions have preceded quiet effusion of some flood basalts and reflect the same influence of vent architecture and hydrology on eruptive style as seen for small volume eruptions the scale of phreatomagmatic deposits associated with flood basalts can be huge at coombs hills a vast but otherwise typical phreatomagmatic vent complex is exposed over more than 25 km 2 and its features are interpreted to reflect processes of tephra jet eruptions with diatreme development similar vent complexes are probably the source of laharic deposits reported elsewhere in the transantarctic mountains and in the karoo province of south africa

hydrothermal minerals and alteration rates at surtsey volcano iceland the volcanic island surtsey off the south coast of iceland was created by volcanic activity in 1963 1967 core from a 181 m deep hole extending 123 m below sea level shows the results of 12 yr of hydrothermal alteration of basaltic tephra the primary cause of heating of the tephra and of development of the hydrothermal system was the intrusion of dikes below sea level at present the hottest part of the hole at a maximum temperature of 150 c is cooling at 0 9 c per year palagonitization of sideromelane glass a dominant constituent of the tephra is an important alteration process that is strongly temperature dependent the rate doubling for every 12 c increase at 60 c 90 is palagonitized above 120 c olivine crystals are replaced on their edges by nontronite the thickness of clay doubles for each 8 c increase ten hydrothermal minerals have crystallized in the tephra at 25 to 150 c the dominant species are smectite nontronite analcite phillipsite and tobermorite the primary clay species of palagonite is probably nontronite other minerals are halite opal calcite chabazite xonotlite anhydrite and gypsum no major differences in mineral occurrence are noted above and below sea level but phillipsite and tobermorite tend to grow larger below sea level even at the same temperature analcite appears at lower temperature 55 c above sea level than below sea level 75 c anhydrite is most abundant deep in the hole where inflowing cool sea water precipitated sulfate due to reduced sulfate solubility at higher temperatures

northwest african continental margin history of sediment accumulation landslide deposits and hiatuses as revealed by drilling the madeira abyssal plain 1 odp drill sites in the madeira abyssal plain reveal sequences of organic rich turbidites derived from the northwest african margin in which each turbidite has a volume of tens to hundreds of cubic kilometers the frequency of turbidite emplacement has been combined with core and seismic data to show the volume of redeposited sediment the basin began to fill about 22 ma with numerous small turbidites up to 100 per million years each with volumes of a few cubic kilometers the total volume of turbidites deposited increased between 16 and 11 ma as did their individual volumes and then declined to 7 ma at 7 ma there was a dramatic increase in the amount of turbidite input to 768 km3 myr and a rise in the average volume of each unit to 59 km3 these high values have been maintained to the present day the variations in the amount of redeposited sediment most likely reflect the rates of sedimentation on the northwest african margin since high sedimentation leads to oversteepening of the slopes and eventual mass wasting the dramatic changes at about 7 ma may be due to a large increase in upwelling off northwest africa caused by circulation changes associated with increased glaciation of the poles up to 20 of sediment may be remobilized by landslides with each event leaving a hiatus each of these hiatuses extends over an average area of 4800 km2 and represents removal of sediment layers several tens of meters thick and of several hundred thousand years duration

magnetic and calcareous nannofossil stratigraphy of the lower palaeogene formations of the hampshire and london basins a magnetostratigraphic study of the lower palaeogene formations exposed in the hampshire and london basins southern uk has led to the identification of eight stratigraphically distinct normal polarity magnetozones within an otherwise reverse polarity sedimentary sequence which spans the late palaeocene to middle eocene a complementary study of the calcareous nannofossil biostratigraphy of these sections and related sections elsewhere in nw europe allows a correlation of this succession of normal polarity magnetozones with the biostratigraphically calibrated geomagnetic polarity timescale based on marine magnetic anomalies it is concluded that the normal polarity chrons corresponding to anomalies 20 21 and 23 are recorded within the huntingbridge earnley and wittering divisions of the bracklesham group respectively that anomalies 24a and 24b are represented within divisions d to b2 of the london clay and anomaly 26 is represented in the lower part of the thanet beds an additional normal polarity magnetozone within the oldhaven beds is believed to represent a short normal polarity event between anomalies 24b and 25 anomalies 22 and 25 appear not to be represented in the uk lower palaeogene succession because of major unconformities at the early middle eocene boundary within the wittering division of the bracklesham group and within the late palaeocene respectively the former appears to correlate with a widespread erosional event in the oceans identified from seismic records vail et al 1979 and interpreted as reflecting a major eustatic sea level fall

shallow seated controls on styles of explosive basaltic volcanism a case study from new zealand abstract the pyroclastic deposits of many basaltic volcanic centres show abrupt transitions between contrasting eruptive styles e g hawaiian versus strombolian or dry magmatic versus wet phreatomagmatic these transitions are controlled dominantly by variations in degassing patterns magma ascent rates and degrees of interaction with external water we use crater hill a 29 ka explosive effusive monogenetic centre in the auckland volcanic field new zealand as a case study of the transitions between these end member eruptive styles the crater hill eruption took place from at least 4 vents spaced along a nne trending 600 m long fissure that is contained entirely within a tuff ring generated during the earliest eruption phases early explosive phases at crater hill were characterised by eruption from multiple unstable and short lived vents later dominantly extrusive volcanism took place from a more stable point source most of the crater hill pyroclastic deposits were formed in 3 phreatomagmatic p and 4 dry magmatic m episodes forming in turn the outer tuff ring and maar crater p1 m1 p2 and scoria cone 1 m2 m4 this activity was followed by formation of a lava shield and scoria cone 2 purely wet activity is represented by the bulk of p1 and p2 and purely dry activity by much of m2 m4 however m1 and parts of m2 and m4 show evidence for simultaneous eruptions of differing style from adjacent vents and rapid variations in the extent and timing of magma water interaction at each vent the nature of the wall rock lithics and these rapid variations in inferred water magma ratios imply interaction was occurring mostly at depths of 80 m and the vesicularity patterns in juvenile clasts from these and the p beds imply that rapid degassing occurred at these shallow levels we suggest that abrupt transitions between eruptive styles in time and space at crater hill were linked to changes in the local magma supply rate and patterns and vigour of degassing during the final metres of ascent