paleogeography of the late cretaceous quadersandstein of central europe abstract thick sequences of cross bedded quartzose sandstones indicate paleogeographic circumstances favorable for their accumulation post variscan peneplanation of central europe relatively rapid subsidence of a level bottom where transport and deposition were ruled by non tidal and tidal currents and continuous uplift of two islands which yielded deeply weathered detrital material primarily derived from plutonic rocks the relatively narrow depositional area represented a connection between the extensive epicontinental sea in northern europe and the oceanic basin of the tethys the fluctuating share of non tidal and tidal currents was detected from rose diagrams constructed for three different stages

excess argon in mount st helens plagioclase as a recorder of magmatic processes excess argon in plagioclase crystals from young 4000 ybp tephra layers from mount st helens washington illustrates the importance of argon isotopes in understanding magmatic processes 40ar 39ar step heating identifies two distinct argon reservoirs in the plagioclase a high cl k low ca k reservoir with atmospheric 40ar 36ar and a low cl k higher ca k reservoir with variable 40ar 36ar the first is probably glass whereas the second is true plagioclase felsic dacite plagioclases have little or no excess argon but those from more mafic layers have significantly higher 40ar 36ar indicating a non atmospheric source these variations are seen within a layer that contains both mafic and felsic dacite and are inconsistent with either xenocrystic or restitic origin for plagioclase the magma chamber exhibits long term open system degassing behavior punctuated by short term fluctuations from influx of high 40ar 36ar basalt the relative timing between mixing and eruption can affect the amount of excess argon recorded in plagioclase

u series constraints on intraplate basaltic magmatism intraplate magmatism represents approximately one tenth of the flux of magma to the earth s surface sleep 1990 this type of magmatism has received considerable attention from petrologists and geochemists as it generally exhibits a wider range of chemical compositions than the more uniform mid ocean ridge basalts hence it is rather paradoxical that our understanding of intraplate magmatism is rather poor in this chapter we review the insights that have been gained from using u series measurements combined with other chemical and isotopic constraints to better understand the sources and processes related to intraplate volcanism several unique constraints can be obtained from measurement of u series disequilibria in basalts first u series fractionation can tell us about the residual phases present during melting as small differences in partitioning behavior between the nuclides will induce distinct signatures second as has been shown by the earlier work of allegre and condomines 1982 th isotope ratios can be used to infer the th u ratio of the mantle source providing another useful probe for mapping mantle heterogeneities lastly as detailed below the time dependence of u series fractionation during melting and melt migration can place constraints on several rate dependent parameters such as the melt production rate and melt velocities an important feature of hotspot magmatism is that in many cases the timing of hotspot activity seems to be decoupled from the motion of the lithospheric plate this observation which has been the basis for proposing the existence of mantle plumes suggests that magmas erupted at hotspots should reveal something about the nature of the deeper mantle understanding the processes of hotspot magmatism should also tell us about the nature of convective motion responsible for hotspots in the following section we first review some of the outstanding issues that need to be resolved to better understand intraplate magmatism we then

transgressive sand bodies on the maresme continental shelf western mediterranean sea abstract three large scale elongate asymmetrical sand bodies lie beyond the shoreface of the straight sandy coast of maresme barcelona in the northwestern mediterranean sea they are found on the continental shelf at water depths of 15 30 m 35 80 m and 95 113 m they are typically 12 20 m thick 1000 2000 m wide and trend parallel or subparallel to the coast for a distance of up to 24 km slopes on the steep offshore side vary between 3 and 20 while the shoreward slopes are gentler the shallowest sand body is modern in dynamic equilibrium with present conditions the other two however are relict bodies built up during the last postglacial rise in sea level development of these sand bodies occurs in the shoreface environment and in a space time evolution they show a punctuated but nearly continuous nature in the way that their preservation during the holocene transgression occurs only in specific locations on the continental shelf they begin growing on pleistocene scarps with offshore slopes and their growth pattern is mostly controlled by the sediment supply flow distribution and rate of sea level rise the formation of these sandy bodies is related to winter storms and southwesterly flows which cause large volumes of sediment to be transported downshelf and offshore by downwelling bottom currents generated by coastal set up the result is sand movement onto and across the sand body surface down to water depths of 15 20 m the thick sandy lenses of the maresme shelf are interpreted as topographically induced lee side sand bodies and not primarily as sand ridges in the classic sense the occurrence of these bodies on a low energy shelf may be significant for the interpretation of the geological record

prolonged residence times for the youngest rhyolites associated with long valley caldera 230th 238u ion microprobe dating of young zircons abstract we describe a new ion microprobe method for dating magmatic zircon growth that is based on in situ measurement of the magnitude of238u 230th disequilibrium our results support independent inferences that zircon can remain suspended for long periods 100 ka in the convecting potions of the magma from which it crystallizes because the crystallization ages date when the magma cooled to its zircon saturation temperature even when the zircons have long magmatic residence times 238u 230th zircon dating can be used to constrain the thermochemical evolution of silicic magmas 238u 230th ages have been determined for individual zircons from rhyolites associated with the long valley magmatic system of eastern california the samples are from deer mountain an 115 3 ka low silica moat rhyolite and from the coarsely porphyritic low silica rhyolite of south deadman dome one of the 0 6 ka inyo domes previous investigations have suggested that the two lavas were derived from the same magma reservoir a few of the zircon model ages calculated with respect to the isotopic characteristics of the whole rocks are within error of that for eruption of the deer mountain rhyolite however the majority of zircons from both lavas cluster around an age of 230 ka this common interval of zircon nucleation and growth for petrologically similar lavas suggests that the younger inyo dome lava may have tapped the same magma body from which the deer mountain rhyolite erupted more than 100 ka before on the other hand most of the zircon model ages are younger than previous episodes of silicic volcanism in the long valley caldera suggesting that the rhyolites may have been generated during development of a silicic upper crustal magma chamber in the western portion of long valley caldera zircon saturation temperatures for the rhyolites studied 795 810 c are the same as those obtained from coexisting fe ti oxides 809 4 c showing that the magma cooled to

an integrated approach to coastal erosion problems in northern tuscany italy littoral morphological evolution and cell distribution abstract occupation of the coast has significantly increased in recent decades mostly due to a greater demand for recreation and tourism today erosion threatens many human made structures and activities requiring an integrated approach for the understanding of coastal dynamics and identification of alternatives to associated problems this study investigates a 64 km long coastal physiographic unit in the northern microtidal littoral of tuscany italy vertical aerial photographs and direct field surveys were used to retrieve changes in shoreline position over 1938 1997 and 1997 2005 time intervals significant beach accretion was observed during the first period updrift of carrara 84 m and viareggio 280 m harbours and at marina di pietrasanta 100 m whereas severe erosion occurred downcoast of carrara harbour 130 m at marina dei ronchi and on the northern side of arno river mouth 400 m similar trends were observed between 1997 and 2005 beach slope between the 1997 shoreline position and the closure depth correlated well with the distribution of erosion accretion patterns from the 1938 1997 period slopes were lower in eroded areas than at sites under accretion longshore distribution of erosion accretion patterns was controlled by coastal compartmentalisation three of the main littoral cells were mostly formed by natural limits i e punta bianca promontory marina di pietrasanta the arno river mouth and the port of livorno several sub cells were created within these cells due to the introduction of human made structures such as carrara and viareggio harbours which formed artificial fixed limits that allowed the transport of sediments exclusively fines in one direction only results will help improve the understanding of coastal processes and manage littoral sediment transport in a sustainable manner this will reduce the need for structural interventions such as breakwaters and groynes which in the past decades prevented coastal retreat at local scale but shifted erosion downdrift leading to degradation of the investigated area and requiring continuous maintenance

experiments on non channelized turbidity currents and their deposits abstract large scale experiments on non channelized turbidity currents show that a wide flow opening angle forms and a rapid dilution of the current with distance takes place the thickness of the deposit decreases radially away from the source resulting in tongue shaped isopachs the mean grain diameter also decreases with distance while the sorting improves with increasing distance the following succession of bedforms was observed non deposition parallel lamination ripples parallel lamination this corresponds to the b c and d division of the bouma sequence for turbidites the experiments are in good agreement with the models presented by bouma mutti and ricci lucchi and walker for classical turbidites in the depositional lobes of submarine fans

implications of pre eruptive magmatic histories of zircons for u pb geochronology of silicic extrusions zircons in silicic magmas begin to crystallize 10 s to 100 s of thousands of years ka prior to their eruption u pb and u th dating for zircons from 32 young extrusions indicate a range of mean pre eruption ages from 33 to 247 ka with a median of 92 ka explosive large volume widespread eruptions show a narrower range and a younger median of 70 ka these results indicate that the difference between initial zircon growth and eruption age measured by other e g 40ar 39ar techniques is in excess of the achievable analytical uncertainty for eruptions 30 million years ma and likely for eruptions of up to 300 ma protracted pre eruption zircon growth is well established by an increasing number of studies on magma process time scales but this source of eruption age bias has been largely ignored in chronostratigraphic applications of high precision u pb data until intrinsic properties of zircons or their host deposits enable a priori estimates of pre eruption age bias it is prudent to expand the uncertainties attending all u pb zircon ages interpreted as deposition ages in e g chronostratigraphic studies by 0 200 ka

late pleistocene granodiorite beneath crater lake caldera oregon dated by ion microprobe variably melted granodiorite blocks ejected during the holocene caldera forming eruption of mount mazama were plucked from the walls of the climactic magma chamber at 5 km depth ion microprobe u pb dating of zircons from two unmelted granodiorite blocks with shrimp rg sensitive high resolution ion microprobe reverse geometry gives a nominal 238 u 206 pb age of 100 78 80 ka or 174 89 115 ka when adjusted for an initial 230 th deficit shrimp rg u th measurements on a subset of the zircons yield a 230 th 238 u isochron age of 112 24 ka considered to be the best estimate of the time of solidification of the pluton these results suggest that the granodiorite is related to andesite and dacite of mount mazama and not to magmas of the climactic eruption the unexposed granodiorite has an area of at least 28 km 2 this young shallow pluton was emplaced in virtually the same location where a similarly large magma body accumulated and powered violent explosive eruptions 7700 yr ago resulting in collapse of crater lake caldera

a review of cenozoic evolution of the southeastern united states atlantic coast north of the georgia trough abstract from late eocene or oligocene to the present the pattern of cyclic sediment deposition on the atlantic coastal plain of the southeastern united states is the product of 1 a relatively stable trailing edge continental margin with gentle downwarping 2 deposition of siliciclastics by major appalachian mountain and piedmont draining rivers and shelf generated carbonates 3 a humid temperate to subtropical climate 4 a mesotidal coast with moderate wave energy and periodic tropical storms 5 dominant longshore drift towards the southwest 6 a history of sea level change that was moderately submergent from late cretaceous through late eocene or oligocene and mildly emergent during the neogene intermittent fluctuations in sea level up to 100 m in the neogene subdivide the stratigraphy as unconformities during the late oligocene through holocene but are less pronounced in the paleogene each of these six factors has influenced the geomorphology sedimentation stratigraphy petrology and geometry of the cenozoic sedimentary units and the diversity of surficial quaternary coastal deposits during the quaternary sea level change would have been produced by glacial interglacial cycles the late eocene oligocene to early miocene and the late miocene transition experienced a major sea level decline influenced by ice cap formation in addition to climatic cycle induced sea level fluctuations

storm surge versus turbidite origin of the coniacian to santonian sediments in the eastern part of the bohemian cretaceous basin a flyschoid facies occurs in the b ezno formation coniacian up to santonian of the eastern marginal part of the bohemian cretaceous basin according tojerzykiewicz 1970 1971 this facies is of deep water turbidite origin he based his interpretation on the study of sedimentary structures new palaeogeographic reconstruction of lithofacies relations investigation of deep drillings and detailed sedimentological analysis however allowed new results concerning the sedimentary environments and depositional mechanism of this facies according to the new model presented here this flyschoid facies was deposited in a shallow marine environment clayey sedimentation was the natural response to normal environmental conditions whereas sandstone intercalations reflect anomalous conditions during heavy storms during these events sands were transported basinwards up to distances of 25 km with normal bottom currents as well as turbidity currents acted as transporting mechanisms this flyschoid facies corresponds to other examples of clayey sequences with sandy storm layers

age and correlation of emerged pliocene and pleistocene deposits u s atlantic coastal plain abstract paleontologic and paleomagnetic investigations were conducted on several hundred pliocene and pleistocene marine samples from five regions of the emerged atlantic coastal plain 1 the delmarva peninsula 2 eastern virginia 3 central and northern north carolina 4 southern north carolina and northeastern south carolina and 5 the charleston area south carolina molluscan and ostracode interval and assemblage zonations which are the primary means of regional correlation have been calibrated using planktic biochronologic paleomagnetic radiometric and amino acid recemization data these multiple dating criteria were used to determine the age and where possible the duration of marine transgressive regressive sequences a correlation chart illustrates the age relationships of 27 formations from five regions one important conclusion is some of the yorktown formation of virginia and north carolina including the duplin formation and some of the raysor of south carolina are late pliocene in age the late pliocene chowan river formation of north carolina is older than the early pleistocene waccamaw formation of south carolina which in turn may be older than the james city formation of north carolina during the last 1 0 million years multiple marine transgressions occurred in each region but the age of these middle and late pleistocene formations often may differ from one area to the next a significant result of the study is the evidence for the lack of time equivalence of formations in the five different regions that is the sequence of marine transgressions in one region does not necessarily correspond to that in another this appears to be the result of differing subsidence and uplift histories the patchiness of the depositional record and the limitations of the dating techniques in light of the rapidity and frequency of sea level fluctuations

amino acid racemization geochronology of reworked quaternary mollusks on u s atlantic coast beaches implications for chronostratigraphy taphonomy and coastal sediment transport approximately 200 shells primarily mercenaria from 21 beach sites between new jersey and florida have been analyzed for the extent of racemization epimerization of their fossilized amino acids the greatest concentration of sites is in north carolina in regions least affected by human modification of natural beach sediment transport processes these results can be used to estimate the frequency of age mixing of pleistocene shell material in holocene depositional environments selected shells have also been dated by 14c conventional and or ams to provide direct calibration of the amino acid epimerization ratios shell taphonomic characteristics particularly color can be qualitatively related to apparent ages data for shell fragmentation combined with amino acid age estimates provide insights into probable transport distances of reworked shells shells with apparent pleistocene ages have epimerization values equivalent to those seen in onshore exposures of pleistocene coastal units amino acid epimerization measurements are a comparatively rapid and inexpensive chronologic tool for use in taphonomic and sediment shell transport studies paired amino acid 14c analyses on single shells provide insights into geochemical alteration of shells and permit modelling of the residence time of shells on beaches greater numbers of analyses at each site are needed before statistically valid estimates of age mixing can be obtained but the apparent distribution of pleistocene shells on atlantic coast beaches appears to be related to the distribution of pleistocene units in the shoreface and inner shelf and the thickness of holocene sedimentary cover in specific areas these observations provide useful criteria for the evaluation of possible age mixing of shells collected at outcrops of pleistocene units in the coastal plain

from sediment to granite timescales of anatexis in the upper crust abstract granite formation is the culmination of a sequence of events initiated by prograde heating of the protolith and followed by formation of a grain boundary melt melt segregation into a vein network ascent of the melt through the network and finally crystallisation of the melt experimental constraints on the formation of a crustal melt from the incongruent melting of muscovite combined with geochemical studies of anatexis in the high himalaya allow the timescales required for each of these processes to be assessed discordant temperatures determined from monazite and zircon thermometry for himalayan anatectic granites indicate that at least for some intrusives the melt was undersaturated in lree implying that melts have probably been extracted in less than 10 ka experimental studies suggest that some himalayan melts are also undersaturated in zr implying segregation may have occurred within 100 years such short timescales confirm that deformation driven mechanisms are important in extracting these melts from their source the transport distances of himalayan granitic melts of 10 km may be achieved by the ascent of magma through dykes in about 1 day at such rates even the largest granite could theoretically be emplaced in 10 years crystallisation of himalayan melts involves much longer periods if emplaced as thin sheets 100 m wide a timescale of 500 years is required compared with 30 ka for single stage intrusion of the larger laccoliths for composite sheet complexes magma crystallisation rather than melt ascent comprises the rate determining step on the emplacement of the intrusion the overall timescales of melt segregation and emplacement for many orogenic granites are therefore less than 10 ka and possibly less than 1 ka in contrast the timescale required for prograde heating of the protolith is 1 ma since the melt production rate is determined by heat flow into the protolith and not by reaction kinetics for any geologically significant period we conclude that heat flow determined by both the mechanism of heating and the thermal diffusivities of crustal rocks provides the overall rate determining step of crustal anatexis

sequence architecture during the holocene transgression an example from the great barrier reef shelf australia abstract the application of sequence stratigraphic analysis to post glacial sediments requires the correct identification of sequence boundaries flooding surfaces and systems tracts the nature of coastal sediments deposited during sea level cycles is complex and misinterpretation is easy particularly in ancient sequences for which eustasy is inferred rather than demonstrated a large core seismic and radiocarbon database from cleveland bay in tropical queensland allows detailed analysis of the systems tracts and parasequences developed during the late holocene despite the large database a unique sequence stratigraphic interpretation is not possible because oscillations in the sea level curve apparently occurred at a frequency higher than the resolving power of radiocarbon dating our best estimate is that the final stages of the holocene sea level rise comprise four parasequences each deposited during episodic slowdowns in sea level rise and or a pulse of rapid sediment supply sea level pauses probably occurred at 12 10 m below modern levels at 8 5 ka bp followed by a fall to ca 17 m at 8 1 ka bp a very rapid transgression to ca 10 m at 7 9 ka bp a further transgression to ca 5 m at 6 8 ka bp and a final rise to the holocene highstand at 1 65 m at 5 5 ka bp the stratigraphic record of these sea level rises is complex occurs within the modern shore connected sedimentary wedge and comprises parasequences associated with shorelines at 11 m p11 17 m p17 10 m p10 5 m p5 and 1 65 m p 1 7 each parasequence comprises in ascending order onlapping estuarine shoreline sediment including mangrove mud and beach sand and progradational shallow bay muddy sand across the bay units up to and including the lower parts of the shallow bay mud correspond to the transgressive systems tract tst of the last 18 ka glacial interglacial cycle the particular parasequence represented being dependent upon location especially depth bay mud above this level corresponds to the post 5 5 ka bp highstand systems tract hst which comprises sand cheniers on the coastal plain and bioturbated muddy sand in the shallow bay the junction between the tst and the hst lies within the bay mud and corresponds approximately to the local flooding surface for the p 1 7 parasequence deposited at the 1 65 m shoreline offshore terrigenous tst strata do not occur over most of the middle and outer shelf which instead is veneered with a thin mid cycle shellbed the lower part of this shellbed is time equivalent to the later part of the tst and the upper part is time equivalent to the hst offshore the maximum flooding horizon mfh and peak eustatic sea level horizon pesh of the late pleistocene holocene sea level cycle lie at some level within the mid cycle shellbed nearshore the mfh and pesh lie within the shore connected sediment wedge at levels near the tst hst boundary however at no locality do either the mfh and pesh necessarily correspond to a physical boundary within the stratigraphy

late pleistocene granodiorite source for recycled zircon and phenocrysts in rhyodacite lava at crater lake oregon abstract rhyodacite tephra and three lavas erupted 27 ka interpreted to be early leaks from the climactic magma chamber of mount mazama contain ubiquitous resorbed crystals antecrysts that were recycled from young granodiorite and related plutonic rocks of the same magmatic system the shallow composite pluton is represented by blocks ejected in the 7 7 ka climactic eruption that formed crater lake caldera plagioclase crystals in both rhyodacite and granodiorites commonly have cores with crystallographically oriented fe oxide needles exsolved at subsolidus conditions at least 80 of plagioclase crystals in the rhyodacite are antecrysts derived from plutonic rocks other crystals in the rhyodacite notably zircon also were recycled sims 238 u 230 th dating indicates that zircons in 4 granodiorite blocks crystallized at various times between 20 ka and 300 ka with concentrations of analyses near 50 70 110 and 200 ka that correspond to periods of dacitic volcanism dated by k ar u th ages of zircon from a rhyodacite sample yield similar results no analyzed zircons from the granodiorite or rhyodacite are pre quaternary zircon minimum ages in blocks from different locations around the caldera reflect ages of nearby volcanic vents and may map the distribution of intrusions within a composite pluton survival of zircon in zircon undersaturated hydrous magma and of fe oxide needles in plagioclase suggests that little time elapsed from entrainment of antecrysts to the 27 ka eruption of the rhyodacite the 27 ka rhyodacite is an example of young silicic magma that preserved unstable antecrysts from a known source early during growth of a large high level magma chamber in contrast the voluminous 7 7 ka climactic rhyodacite pumice is virtually lacking in zircon indicating dissolution of any granodioritic debris in the intervening period mineralogical evidence of assimilation may be destroyed in hot vigorously growing silicic magma bodies such as ultimately produced the climactic eruption of mount mazama

post caldera volcanism in situ measurement of u pb age and oxygen isotope ratio in pleistocene zircons from yellowstone caldera the yellowstone plateau volcanic field the site of some of the largest known silicic volcanic eruptions is the present location of ne migrating hotspot volcanic activity most volcanic rocks in the yellowstone caldera 0 6 ma which formed in response to the climactic eruption of 1000 km 3 of lava creek tuff lct have unusually low oxygen isotope ratios ion microprobe analysis of both u pb age and n 18 o in zircons from these low n 18 o lavas reveals evidence of complex inheritance and remelting a majority of analyzed zircons from low n 18 o lavas erupted inside the yellowstone caldera have cores that range in age from 2 4 to 0 7 ma significantly older than their eruption ages 0 5 0 4 ma these ages and the high n 18 o cores indicate that these lavas are largely derived from nearly total remelting of normal n 18 o huckleberry ridge tuff hrt and other pre lct volcanic rocks a post hrt low n 18 o lava shows similar inheritance of hrt age zircons the recycling of volcanic rocks by shallow remelting can change the water content and eruptive potential of magma this newly proposed mechanism of intracaldera volcanism is best studied by combining in situ analysis of oxygen and u pb isotope ratios of individual crystals fl 2001 elsevier science b v all rights reserved

sediment transport in the new york bight north american atlantic shelf vincent c e swift d j p and hillard b 1981 sediment transport in the new york bight north american atlantic shelf mar geol 42 369 398 aanderaa current meter data collected during the course of noaa s new york bight project are sufficiently extensive in time 135000 observation hours and space 41 stations to permit an estimate of the sediment transport in this area the analysis is based on mean flow observations the role of wave oscillatory currents in resuspending sediment is considered but not explicitly included in the sediment transport calculations an estimate of the potential sediment transport rate qb at each station for three different threshold velocities uth 12 16 and 20 cm s 1 were made using a bagnoldtype transport equation qb const uc uth 3 for uc uth qb 0 for uc uth where uc is the mean current speed comparison was made between the results from this simple equation and other bedload and suspended load transport rates which include wave data for a limited subset of the bight data this comparison indicates that the bagnold results are reasonable for the along isobath transport estimates but are not reliable for the cross isobath transport in the cross isobath direction waves appear to play a dominant role in determining the transport rate the comparison also indicates that the bagnold approach tends to overestimate transport in deep water in shallow water resuspension of sand and rotor pumping of current meters both may become important and sediment transport calculations may not be accurate fine sediment suspended transport is approximately double the potential bedload transport for fine sand 0 02 cm but does not dominate the total transport as is believed to occur on the washington shelf smith and hopkins 1972 the data show that the new york bight is a storm dominated shelf with the general southwestward drift of water 6 cm s 1 modulated by more intense storm induced flows the transport calculations indicate that the bulk of sediment transport occurs during the winter months when wind stress is at a maximum and the water column is well mixed two major transport provinces are present a the hudson shelf valley and b the rest of the shelf the shelf valley acts as a sink and a barrier to the continuity of the mainly southwestward transport of sediment 104 g cm 1 yr 1 along the shelf within the valley the potential transport of material is of the order of l05 g cm 1 yr 1 along the valley axis progressive vector transport paths for fine sediment indicate that removal of sediment out of the ends of the valley can occur rapidly however consider ation of the finesediment budget for the whole hudson shelf valley and of the attenua tion of wave activity with depth show that these results are not inconsistent with the presence of high mud concentrations in the valley up valley sediment rates predominate during the period for which data are available

generalities on static equilibrium bays abstract previous methods of determining the curved waterlines of bays which form on the coasts around the world have involved the logarithmic spiral this has been applied to unstable bays which could erode back to a limit termed static equilibrium for which the constant in the spiral could be related to the obliquity of the wave crests to the control line joining the upcoast headland to the downcoast extremity of the bay it is shown that this applies mainly to the shadow zone behind the upcoast headland and not to the full bay periphery and does not have a fixed centre a new method is proposed utilising arcs from the point of wave diffraction to the shoreline whose lengths and angles to the wave crest line are related to those of the control line data and curves are presented for the static equilibrium shape by which the stability can be tested they thus predict the waterline for the case of no sediment supply either from littoral drift or rivers debouching into the bay an indentation ratio can also be derived to test stability

cardium formation u cretaceous at seebe alberta storm transported sandstones and conglomerates in shallow marine depositional environments below fair weather wave base as a result of detailed mapping at seebe alberta we have defined five coarsening upward sequences in the turanian cardium formation the sequences begin with bioturbated mudstones and coarsen int

crystallisation ages in coeval silicic magma bodies 238u 230th disequilibrium evidence from the rotoiti and earthquake flat eruption deposits taupo volcanic zone new zealand abstract the timescales over which moderate to large bodies of silicic magma are generated and stored are addressed here by studies of two geographically adjacent successive eruption deposits in the taupo volcanic zone new zealand the earlier caldera forming rotoiti eruption 100 km3 magma at okataina volcano was followed within months at most by the earthquake flat eruption 10 km3 magma from nearby kapenga volcano both generated non welded ignimbrite and coeval widespread fall deposits the rotoiti and earthquake flat deposits are both crystal rich high silica rhyolites with sparse glass bearing granitoid fragments also occurring in rotoiti lag breccias generated during caldera collapse here we report 238u 230th disequilibrium data on whole rocks and mineral separates from representative rotoiti and earthquake flat pumices and the co eruptive rotoiti granitoid fragments using tims and in situ zircon analyses by sims multiple grain zircon controlled crystallisation ages measured by tims from the rotoiti pumice range from 69 3 ka 350 ka with a pronounced peak at 70 90 ka the weighted mean of isochrons is 83 14 ka in accord with the tims data one glass bearing rotoiti granitoid clast yielded an age of 57 8 ka by tims controlled by th rich phases that however are not apparently present in the juvenile pumices another glass bearing rotoiti granitoid yielded sims zircon model ages peaking at 60 90 ka having a similar age distribution to the pumice age data from pumices are consistent with a published 64 4 ka eruptive age now modified to 62 2 ka but chemical and or mineralogical data imply that the granitoid lithics are not largely crystalline rotoiti rhyolite but instead represent contemporaneous partly molten intrusions reflecting different sources in their chemistries and mineralogies similarly although the earthquake flat eruption immediately followed and probably was triggered by the rotoiti event age data from juvenile material are significantly different a multiple grain zircon controlled crystallisation age measured by tims from a representative pumice is 173 5 ka while sims model ages range from 70 26 34 ka to 350 ka with a peak at 105 ka these age data coupled with previously published geochemical and isotopic data show that the rotoiti and earthquake flat deposits were erupted from independent unconnected magma bodies

evolution of peralkaline silicic complexes lessons from the extrusive rocks abstract young 4 wt and the rhyolitic varieties evolve to temperatures 4 10 6 pa s result in rapid growth of volcanic edifices highly efficient crystal melt separation and the ubiquitous development of compositionally zoned caps to reservoirs no peralkaline equivalent of the monotonous intermediate ignimbrites found in some calc alkaline systems has yet been found

mineral resource assessments for the continental shelf of georgia as a part of the continental margins studies program seismic interpretation of the phosphatic miocene age hawthorne group abstract phosphate bearing strata have long been recognized in the shallow continental shelf of the southeastern atlantic including off the coast of georgia prior to the continental margins studies program these strata were poorly defined both geographically and stratigraphically the strata of interest are believed to be a part of the miocene hawthorne group which serves as the confining unit for the floridan aquifer system this aquifer system is the major source of fresh water for coastal georgia south carolina and florida in the event that economic phosphate deposits are found to occur it will be imperative to define the relationship of these phosphate deposits to the aquifer the middle miocene strata can easily be distinguished in seismic profiles characteristic continuous strongly banded parallel reflectors can be traced throughout the study area the general trend of the middle miocene strata is a southward thickening and increasing depth the tybee phosphorite member is approximately 10 m thick in chatham county georgia lack of lithologic information precludes a delineation of its extent and thickness seaward but evidence does support its presence on the inner shelf

morphological response of a sandy shoreline to a natural obstacle at sa mesa longa beach italy abstract a case study is presented of a comprehensive analysis of the shoreline stability of a salient primary response in the lee of an emergent natural reef at sa mesa longa beach sardinia italy the analysis combines sources of information at different levels of detail and accuracy field surveys conducted with a gps unit remotely sensed imagery geometrical and shape predictive formulas and simulations with a 2d wave current sediment transport numerical model the morphological simulation results for the primary response agree qualitatively with field observations and predictions from empirical formulas the accuracy of a simplified wave energy model in defining the littoral sediment cell compared to the more detailed results of the morphological model is also investigated although the analysis has some limitations as highlighted in the paper the results appear to be promising in terms of applying the analysis to fragile morphologies in the lee of natural reefs in the mediterranean sea

palynological foraminiferal and aminostratigraphic studies of quaternary sediments from the u s middle atlantic upper continental slope continental shelf and coastal plain abstract a pollen climate index curve provides the basic stratigraphic subdivision of the 305 m cored quaternary section of amcor 6021c located on the upper continental slope off new jersey u s a cold and warm intervals from top to bottom were assigned to marine oxygen isotope stages 1 through 13 at a sub bottom depth of about 220 m foraminiferal data indicate a possible hiatus and if so sediments below this may be as old as isotope stage 16 or older aminostratigraphic studies yielded results that are consistent with either interpretation benthic foraminifers are dominated by elphidium excavatum and islandiella norcrossi the planktic foraminiferal record provided no independent means of subdividing the section the stratigraphic distribution in 6021c of six species of quercus oak and aminozone determinations were applied to dating in terms of presumed oxygen isotope stages of other offshore core hole samples and some onshore quaternary deposits of delaware stratigraphic information from amcor sites 6021 and 6009 correlated to seismic reflection profiles that cross the shelf slope break indicate that the thickest accumulations of terrigenous quaternary sediments are located along the outer continental shelf upper continental slope offshore the northeastern united states

lower silurian distal shelf storm induced turbidites in the welsh borders sediments tool marks and trace fossils the upper llandovery hughley shales of shropshire are interpreted as storm induced distal shelf turbidites they comprise a sequence of calcareous sandstones interbedded with purple mudstones the trace fossils body fossils and palaeogeography indicate a deep shelf position for their deposition the thin sandstone units are interpreted as having been deposited from density current bedload and suspension storm surge ebb traction currents arising from intermittent major offshore storms are considered to have initiated the density currents a deepened storm wave base allowed transportation of fine sand in suspension out onto the distal shelf internally the resultant sands are differentiated from distal fan turbidites by multi directional bedload currents which are shown most clearly by spectacular tool marks on sandstone bases the tools are clearly identifiable in many cases as indigenous benthonic organisms such as horn corals coral colonies crinoids brachiopods and trilobites the sandstone beds were later colonized by burrowers and other track producing organisms the trace fossils include diplocraterion or corophioides palaeophycus skolithos chondrites scolicia rusophycus cruziana and diplichnires cruziana facies assemblage a new ichno species walcottia devilsdingli is established for small burrows with regular lateral appendage marks at certain points

granite magma formation transport and emplacement in the earth s crust the origin of granites was once a question solely for petrologists and geochemists but in recent years a consensus has emerged that recognizes the essential role of deformation in the segregation transport and emplacement of silica rich melts in the continental crust accepted petrological models are being questioned either because they require unrealistic rheological behaviours of rocks and magmas or because they do not satisfactorily explain the available structural or geophysical data provided flow is continuous mechanical considerations suggest that far from being geologically sluggish granite magmatism is a rapid dynamic process operating at timescales of or 100 000 years irrespective of tectonic setting

migration of the pee dee river system inferred from ancestral paleochannels underlying the south carolina grand strand and long bay inner shelf several generations of the ancestral pee dee river system have been mapped beneath the south carolina grand strand coastline and adjacent long bay inner shelf deep boreholes onshore and high resolution seismic reflection data offshore allow for reconstruction of these paleochannels which formed during glacial lowstands when the pee dee river system incised subaerially exposed coastal plain and continental shelf strata paleochannel groups representing different generations of the system decrease in age to the southwest where the modern pee dee river merges with several coastal plain tributaries at winyah bay the southern terminus of long bay positions of the successive generational groups record a regional southwestward migration of the river system that may have initiated during the late pliocene the migration was primarily driven by barrier island deposition resulting from the interaction of fluvial and shoreline processes during eustatic highstands structurally driven subsurface paleotopography associated with the mid carolina platform high has also indirectly assisted in forcing this migration these results provide a better understanding of the evolution of the region and help explain the lack of mobile sediment on the long bay inner shelf migration of the river system caused a profound change in sediment supply during the late pleistocene the abundant fluvial source that once fed sand rich barrier islands was cut off and replaced with a limited source supplied by erosion and reworking of former coastal deposits exposed at the shore and on the inner shelf

residence resorption and recycling of zircons in devils kitchen rhyolite coso volcanic field california zircons from the devils kitchen rhyolite in the pleistocene coso volcanic field california have been analyzed by in situ pb u ion microprobe shrimp rg and by detailed cathodoluminescence imaging the zircons yield common pb corrected and disequilibrium corrected pb u ages that predate a previously reported k ar sanidine age by up to 200 kyr and the range of ages exhibited by the zircons is also approximately 200 kyr cathodoluminescence imaging indicates that zircons formed in contrasting environments most zircons are euhedral and a majority of the zircons are weakly zoned but many also have anhedral embayed cores with euhedral overgrowths and multiple internal surfaces that are truncated by later crystal zones concentrations of u and th vary by two orders of magnitude within the zircon population and by 10 20 times between zones within some zircon crystals indicating that zircons were transferred between contrasting chemical environments a zircon saturation temperature of 750 c overlaps within error a previously reported phenocryst equilibration temperature of 740 25 c textures in zircons indicative of repeated dissolution and subsequent regrowth are probably caused by punctuated heating by mafic magma input into rhyolite the overall span of ages and large variation in u and th concentrations combined with calculated zircon saturation temperatures and resorption times are most compatible with crystallization in magma bodies that were emplaced piecemeal in the crust at coso over 200 kyr prior to eruption and that were periodically rejuvenated or melted by subsequent basaltic injections

comagmatic a type granophyre and rhyolite from the alid volcanic center eritrea northeast africa vermicular to cuneiform barker 1970 typically the granophyric blocks within late pleistocene pyroclastic flow ejecta from the alid volcanic center northeast africa are the rapidly groundmass feldspars radiate off pre existing feldspar crystallized intrusive equivalent of pumice from the pyroclastic flow phenocrysts with which they are in optical continuity phenocryst compositions and geochemical characteristics of the dunham 1965 some workers prefer the term pumice and granophyre are virtually identical silicate melt inclusions micrographic for highly regular interlocking arand other geochemical and geological constraints reveal those processes rangements of quartz and feldspar as in the coarser leading to development of the granophyric texture rhyolitic agraphic granite fenn 1986 lentz fowler 1992 type magma with 2 6 wt dissolved h2o and a temperature though schloemer 1964 has shown that some graphic near 870 c was intruded to within 2 4 km of the surface causing textures can grow by replacement see also augustithis deformation and structural doming of shallow marine and subaerial 1973 most workers agree that phenocryst bearing strata eruptions of crystal poor rhyolite from this shallow magma granophyres typically form by rapid and simultaneous chamber caused degassing which forced undercooling and consequent crystallization of quartz and feldspar from a melt smith granophyric crystallization of some of the magma remaining in the 1974 such crystallization is generally believed to be due intrusion the most recent eruption from alid excavated the cryto pronounced undercooling of the silicate liquid vogt stallized granitic wall of the magma chamber bringing the grano1930 dunham 1965 not necessarily at eutectic temphyric clasts to the surface peratures or compositions fenn 1986 london et al 1989 lentz fowler 1992 granophyric textures are common in epizonal granitic bodies particularly those associated with volcanic rocks buddington 1959 dunham 1965 on occasion they erupt as comagmatic ejecta in pyroclastic deposits e g

beach morphologies induced by breakwaters with different orientations abstract a desired outcome in the construction of a detached emerged breakwater is the formation of an accretionary salient in its lee to augment the beach improve beach amenity and provide an additional buffer from storm waves the extent to which this salient forms and its morphology are strongly controlled by the breakwater geometry with respect to the original shoreline sediment availability and local wave climate the purpose of this paper is to identify how breakwater geometry and orientation of gaps between individual breakwaters alter the direction of waves entering the gaps and change the asymmetry of the salients four distinct breakwater sites along the emilia romagna coastline in northern italy were chosen for a detailed field and desktop study comprising three dimensional topographic and bathymetric surveys sediment sampling lidar flights and historical shoreline mapping the orientations of the shorelines at these four sites range over 43 resulting in different exposures to the dominant waves the oblique orientations of the gaps between individual breakwater segments at three of the four sites effectively create a gap window between breakwaters favoring the exposure of short period waves from the north and diminishing the effect of longer waves from the dominant east salients can be symmetrical despite an acute angle of approach of the dominant deep water waves where refraction is enhanced by offshore topography and breakwaters are parallel to the shore waves approaching normal to the gap window undergo less diffraction due to their shorter length relative to the gap window width and undergo less attenuation by breaking and bottom friction if they are locally generated and have short periods greater breaking wave energy on the gap facing slope of the salient can create shoreline and morphological asymmetry the implication is that breakwater orientations can be designed or altered to selectively dampen or facilitate wave energy to enhance sediment transport in a desired direction provided that breakwaters are not too far offshore and sediment availability is not restricted to affect salient formation adjusting exposure via gap orientation can create morphologies that cannot be inferred from process dominant conditions

storm and tide dominated shorelines in cretaceous moosebar lower gates interval outcrop equivalents of deep basin gas trap in western canada the moosebar and gates formations crop out in the deformed foothills belt south of fort st john british columbia they permit examination of the depositional environments of sandstones and conglomerates that are gas bearing in the equivalent wilrich falher interval of the deep basin of alberta and british columbia the moosebar gates interval consists of several upward coarsening and marine to nonmarine sequences moosebar bioturbated shales pass upward into turbidites and offshore storm deposits flow directions indicate a north dipping paleoslope a second coarsening upward sequence at the top of the moosebar and base of the gates terminates in a lenticular fluvial conglomerate up to 30 m thick which is overlain by coals carbonaceous mudstones and sandstones above the carbonaceous zone a major lateral facies change is observed fluvial conglomerates in the south grade into beach conglomerates northward the upper part of the beach conglomerate consists of exposed three dimensional storm berms at least 500 m long up to 1 5 m high with wavelengths of 12 to 24 m the berms trend east west seaward of the conglomerate beach are herringbone cross bedded tidal sandstones with a marine trace fauna graded sharp based conglomerates also are present probably washed out to sea by storms farther north at the same stratigraphic level is a series of partly overlapping conglomerate bars up to 5 m thick and 40 to 50 m across the tops of these bars are covered with storm formed symmetric conglomeratic dunes details of the orientation and geometry of these conglomerate bodies along with their internal structure and pebble fabrics suggest that four major environments fluvial beach offshore conglomerate bars and offshore graded storm conglomerates should be identifiable and predictable in the subsurface

processes controlling the characteristics of the surficial sand sheet u s atlantic outer continental shelf abstract knebel h j 1981 processes controlling the characteristics of the surficial sand sheet u s atlantic outer continental shelf mar geol 42 349 368 a review of recent data on the velocity of bottom currents the frequency of bottom sediment movement the kinds and amounts of suspended sediments in near bottom waters and the acoustic and sedimentary features of subbottom strata indicates that the characteristics of the ubiquitous sand sheet on the atlantic outer continental shelf of the united states have been controlled by a variety of past and present processes although these processes collectively have had a widespread effect on the characteristics of the sand sheet the relative importance of each process changes geographically on georges bank late pleistocene glaciations along with modern tidal currents and the regional circulation pattern have played a dominant role on the middle atlantic shelf ancestral rivers former near shore processes and modern wind and wave generated currents are important factors on the south atlantic shelf the sediments reflect subaerial weathering erosion or nondeposition over or near hardgrounds and the production of biogenic carbonate other processes such as the movement of water masses bioturbation and bottom fishing probably have affected the sediments in all areas a knowledge of the various factors affecting the sand sheet is fundamental to an understanding of its general geologic history and to the paleoenvironmental interpretation of ancient sand strata

uranium series accessory crystal dating of magmatic processes complex and protracted crystallization histories over geologic timescales are recorded in accessory minerals e g zircon allanite although magmatic crystallization was traditionally assumed to occur essentially instantaneously for the purposes of interpreting mineral geochronometers with low absolute time resolution for ancient samples it emerged relatively recently that magmatic crystallization can occur over extended durations this discovery arose from applying high spatial resolution accessory mineral dating techniques for uranium series isotopes to young volcanic and cognate plutonic rocks the emerging pattern from these studies is that individual crystals and crystal populations record crystallization episodes lasting from 1 000 to many hundreds of thousands of years accessory mineral dating of volcanic rocks and cognate plutonic xenoliths opens new research avenues for crystal age fingerprinting that correlates pyroclastic deposits lavas and plutonic rocks by using characteristic age dis

reversed responses within a segmented detached breakwater the tuscany coast italy a case study abstract a multiple detached breakwater system was designed to protect an extended length of shoreline in front of the presidential villa located in the gombo region tuscany italy the purpose of this empirical field study was to examine the impact of the surface piercing segmented breakwater on the subaqueous morphology and to follow the effect of the breakwaters on the beach shoreline mapping was performed on rectified stereo plotted air photos and was also based on topographic field surveying bathymetric mapping was based on interpolation of profile lines extending to 10 m water depth building the breakwaters in a downstream order caused the trapping of the longshore driven sediments mainly at the southernmost breakwaters that faced the longshore sediment supply the longshore down current direction controlled the hierarchy of the beach response two relatively coherent behavioral domains were found to exist 1 the permanent tombolo stage of segments 1 3 and 2 the no sinuosity response of the beach opposite the northern segment no 5 the segment no 4 in between did not exhibit a coherent behaviour indicating a drastic reversal in the sedimentary regime the three southernmost tombolos facing the longshore current became the main sediment trap causing a lee side erosional effect to emerge within the protection scheme of the segmented detached breakwaters the oblique incident waves enter through the gaps and maintain in the inshore the depleted longshore drift causing the shoreline configuration in the lee of the northern breakwaters to develop into a prograding log spiral bay

constraints on the nature of the subvolcanic reservoir at south sister volcano oregon from u series dating combined with sub crystal trace element analysis of plagioclase and zircon abstract we present sub crystal scale 238u 230th zircon ages and 238u 230th 226ra plagioclase ages of bulk mineral separates from the holocene 2 0 2 3 ka eruptions of the rock mesa rm and devil s hills dh rhyolites at south sister volcano oregon we link these age data with sub crystal trace element analyses of zircon and plagioclase to provide insight into the subvolcanic system at south sister as an example of a small volume continental arc volcano our results document the presence of coeval yet physically distinct regions within the magma reservoir and constrain the timescales over which these heterogeneities existed zircons from the rm and dh dominantly record ages from 20 to 80 ka with some grains recording ages 350 ka whereas plagioclase records 230th 226ra ages of 2 3 6 8 ka rm and 4 0 9 6 ka dh 3 and a 238u 230th age of 10 34 ka dh 3 we interpret zircons with ages 50 ka within a heterogeneous magmatic system and coexisted as physically distinct dominantly liquid bodies prior to eruption thus we favor a model where rhyolites are generated in independent batches by accumulation of evolved liquids in a heterogeneous largely crystalline reservoir similarities in crystal age and chemical data to that at other young silicic systems e g mount st helens okataina caldera complex suggest that this model may be more generally applicable to silicic magmas

the extent of u series disequilibria produced during partial melting of the lower crust with implications for the formation of the mount st helens dacites the extent to which u series disequilibria can be produced during partial melting of mafic lower crust is quantified using a simple batch melting model and both experimental and theoretical partition coefficients for u th and ra we show that partial melting of mafic lower crust can only produce small disequilibria between 238u 230th and 226ra crystallisation of basalt and mixing between young basalt and crustal derived melts will have a similar or smaller effect consequently u series disequilibrium in arc andesites and dacites can generally only be an inherited feature derived from a mantle parent unless the timescales of silicic magma production within the crust are short compared to the half life of 226ra our results have profound implications for several recent models of silicic magma production by thermal incubation and partial melting of the lower crust we show that the 226ra excess observed in most arc andesites and dacites requires extremely rapid differentiation and or the involvement of mantle derived basalts less than a few thousand years old application to mount st helens suggests that crystallisation of young mantle derived magma is likely to be the dominant process in the formation of these dacites

age intercalibration of 40ar 39ar sanidine and chemically distinct u pb zircon populations from the alder creek rhyolite quaternary geochronology standard abstract we report results from a 40 ar 39 ar sanidine and ca tims 238 u 206 pb zircon dating study of eruption and crystal residence timescales of the alder creek rhyolite acr california extruded during the cobb mountain normal polarity subchron c1r 2n a 40 ar 39 ar acr sanidine date of 1 1850 0 0016 ma 2 external uncertainty determined relative to the astronomically dated a1 tephra sanidine is interpreted as the acr eruption age this age is supported by ca tims u pb zircon dating guided by la icpms trace element analyses titanium in zircon tizr thermometry and cathodoluminescence cl imaging using these data two compositionally distinct zircon populations were revealed pre acr group b zircon exhibit oscillatory zoning large positive ce and negative eu anomalies high incompatible trace element contents tizr temperatures of 650 c 750 c and th u disequilibrium corrected 238 u 206 pb dates of 1 38 1 24 ma these crystals are interpreted as antecrysts inherited from earlier intrusives in the geysers cobb mountain magma source region acr related group a zircon present as discrete grains and overgrowths on group b zircon display less intense cl with diffuse zoning and have less pronounced positive ce and negative eu anomalies lower incompatible trace element contents higher tizr temperatures that range up to 840 c and significantly younger dates the youngest group a dates yield a weighted mean of 1 1978 0 0046 ma 2 including systematic uncertainties that is interpreted as the mean age of zircon crystallization prior to eruption the 13 5 ka offset between the 40 ar 39 ar and 238 u 206 pb dates can be attributed to zircon magma residence time recognition of a young population of acr zircon is consistent with the 40 ar 39 ar eruption age which coincides with the astronomical age estimate for the cobb mountain subchron determined by correlating the oxygen isotope record of the giant piston core md972143 to the la93 1 1 orbital solution on the basis of independent radio isotopic and orbital forcing results we propose the refined age of 1 1850 0 0016 ma 2 external uncertainty for the quaternary acr 40 ar 39 ar sanidine standard

the formation of sedimentary strata in an allochthonous shelf environment the washington continental shelf nittrouer c a and sternberg r w 1981 the formation of sedimentary strata in an allochthonous shelf environment the washington continental shelf mar geol 42 201 232 models of fine scale 120 m shelf regions the inner shelf sand and mid shelf sandy silt are the predominant accretionary deposits on the shelf important sedimento logical observations obtained by boxcoring the upper 1 2 m of these deposits are a progressive decrease in grain size with distance from the columbia b downward coarsening within the seabed c loss of distinct sedimentary structure homogenization of sediment with distance from the columbia non dimensional parameters based on sediment transport and benthic biological studies can be used to relate rates of sediment mixing re working to rates of accumulation these parameters have the potential for quantitative prediction of strata formation from measurements of active processes physical and biological mixing can be assumed to remain relatively constant in an alongshelf direction on the washington shelf and accumulation rate is known from pb 210 geochronology to decrease away from the columbia together these factors tend to increase the ratio of mixing to accumulation with distance from the columbia and predict the sedimentological observations listed above

timescales of crustal magma reservoir processes insights from u series crystal ages the dynamic processes operating within crustal magma reservoirs control many aspects of the chemical composition of erupted magmas and crystals in volcanic rocks provide a temporally constrained archive of these changing environments in this review i compile 238 u 230 th ages of accessory phases and 238 u 230 th 226 ra ages of bulk mineral separates of major phases these data document that crystals in individual samples can have ages spanning most of the history of a volcanic centre age populations for accessory phases show protracted pre eruptive crystal residence times but few crystals predate magmatic activity at a given centre these data have been interpreted in the context of residence times of the host magmas or timescales of the storage of crystals within a largely crystalline portion of the reservoir system in contrast less than half of the bulk separate 238 u 230 th 226 ra ages for major phases are more than 10 kyr older than the eruption many of these apparently conflicting observations of ages of major and accessory phases can be reconciled within the context of a model where a crystal mush was remobilized during processes leading to eruption overall the compiled data show that crystals contain rich archives of magmatic processes in crustal reservoirs especially when combined with other crystal scale geochemical data

chronology and evolution of caldera forming and post caldera magma systems at okataina volcano new zealand from zircon u th model age spectra u th disequilibrium model age data are presented for zircons from four young eruptive units from okataina volcano new zealand these data highlight contrasts in the time scales over which eruptible silicic magma bodies are generated and accumulated below a highly active rhyolite volcano prior to and following the 61 ka caldera forming rotoiti eruption 80 120 km 3 magma the rotoiti event was followed by 12 explosive eruptions of the mangaone subgroup between 45 and 30 ka a change in eruptive styles between 30 and 25 ka brackets the major oruanui eruption of taupo volcano 80 km away subsequently eight rhyolitic lava dominated and two basaltic explosive eruptions occurred between 25 ka and 1886 ce we present 238 u 230 th zircon model age data determined by secondary ion mass spectrometry sims and thermal ionization mass spectrometry tims from the rotoiti ngamotu unit b 45 ka mangaone unit i 33 ka and rotorua 15 4 ka eruptive units we couple these data with published and new compositional information to trace magma crystallization and storage patterns population density curves of sims model ages from zircons in the two pumice types biotite free and biotite bearing from the rotoiti eruption differ zircons from the former yield a model age peak coincident with eruption age whereas zircons from the latter show a peak at 70 90 ka and little variation in tims model age values from different crystal size fractions concentration weighted means of model ages from the two pumices however are the same within 1 sd error and their sr isotopic values overlap at 2 sd precision suggesting that they are genetically linked model age spectra for the ngamotu mangaone and rotorua pumices are complex indicating recycling of crystals from multiple older populations that largely pre date the 61 ka caldera forming eruption superimposed on the older age spectra are variably developed younger pre eruptive suites of ages reflecting varying amounts of crystallization prior to each eruption a lack of commonality in these younger peaks in the post caldera eruptive rocks along with compositional and isotopic differences between and sometimes within the eruptive units collectively precludes their origin from a single melt bearing mush system okataina contrasts with taupo where comparable age eruption deposits have simpler age spectra consistent with larger scale crystallization cycles and thermal events in the magma chambers there when compared with other caldera related silicic systems for which suitable data are available okataina differs in lacking a simple pre eruptive history prior to its caldera forming event and having a complex non coherent magmatic history of post caldera eruptions

uranium series coral ages from the us atlantic coastal plain the 80 ka problem revisited abstract uranium series coral ages for emergent units from the passive continental margin us atlantic coastal plain acp suggest sea level above present levels at the end of marine oxygen isotope stage mis 5 contradicting age elevation relations based on marine isotopic or coral reef models of ice equivalent sea level we have reexamined this problem by obtaining high precision 230th 238u and 231pa 235u thermal ionization mass spectrometric ages for recently collected and carefully cleaned acp corals many in situ we recognize samples that show no evidence for diagenesis on the basis of uranium isotopic composition and age concordance combining new and earlier data among those ages close to or within the age range of mis 5 over 85 cluster between 65 and 85 ka bp of the corals that we have analyzed those that show the least evidence for diagenesis on the basis of uranium isotopic composition and age concordance have ages between 80 and 85 ka bp consistent with a mis 5a correlation the units from which these samples have been collected are all emergent and have elevations within 3 5 m of those few units where early stage 5 125 000 ka bp coral ages have been obtained the acp appears to record an unusual history of relative sea level throughout mis 5 a history that is also apparent in the dated coral record for bermuda we speculate that this history is related to the regional near to intermediate field effects of ancestral laurentide ice sheets on last interglacial shorelines of the western north atlantic

the hudson shelf valley its role in shelf sediment transport abstract freeland g l stanley d j swift d j p and lambert d n 1981 the hudson shelf valley its role in shelf sediment transport mar geol 42 399 427 the hudson shelf valley was deeply incised by pleistocene lowstands of the sea during the holocene transgression fluvial estuarine and estuary mouth depositional environments were displaced landward and sedimentation in these environments partially filled the valley holocene sediment thickness in the seaward half of the valley is about the same as on the shelf to either side but in the landward half of the valley 22 m of sediment is present versus 5 10 m on the adjacent shelf sediment volume calculations in the upper shelf valley show that there is about three times the infill volume east of the thalweg as compared to the infill to the west this configuration indicates that the valley has served as a sediment trap and records the westward migration of the thalweg during the holocene most of the infill was the result of littoral drift which transported sand westward along the south shore of long island during the holocene transgression modern sediment transport as calculated from current meter observations and inferred from bedform observations is mainly south westward along the regional trend of the isobaths and across the shelf valley therefore infill on the eastern side of the shelf valley is continuing at present observations of the regional distribution of sediment types indicate that the asymmetrical valley fill is part of a pattern of bottom response to flow the former drainage divides on both sides of the shelf valley are surfaced by coarse lag deposits on slopes facing northeast into the direction from which the major storm flows come while the down current sides are mantled with fine sand

theoretical correlations and lateral discontinuities in the quaternary aminostratigraphic record of the u s atlantic coastal plain abstract aminostratigraphic correlations of emergent quaternary deposits along the u s atlantic coastal plain have employed independent radiometric data regional temperature history models and assumptions regarding the nature of the preserved late quaternary sea level record on this passive margin a substantial aminostratigraphic offset is required if regional aminozones are rigorously constrained by all available th u data new insights regarding the relation of this offset to subsurface stratigraphy in the cape fear region of southeastern north carolina can explain these conflicts as consequences of the highly incomplete post cretaceous depositional record of the region southward projection of theoretical aminostratigraphic correlation trends suggests that stage 5 correlative marine units are rarely preserved on the emergent portion of the coastal plain between cape lookout and central south carolina and that samples of this age would be most frequently found in this region only as fragmentary and or reworked deposits on the inner shelf or in the subsurface of modern barrier islands if this hypothesis is correct then the accuracy of several th u coral dates from the south carolina coastal plain must be questioned along with sea level tectonic and paleoclimatic conclusions derived from these dates

the effects of magma replenishment processes on 238u 230th disequilibrium short lived isotope data are increasingly used to investigate magma chamber residence times arc lavas have excellent potential as tracers of magma evolution because their magmas are often out of u series isotope equilibrium and some arc magmas have yielded surprisingly old pre eruption u series ages from mineral separates a model is presented to examine the behaviour of u th isotopes for a range of conditions between closed system and steady state magmatic systems magmas can be maintained out of secular equilibrium for long periods of time by periodic mixing events with new magma batches the latter results in oscillations in 230th 232th consistent with the relatively constant values observed in lavas of different ages from some centres the effects of periodic reinjection of magma and mixing of old and young crystals on u series mineral isochrons are also considered mixtures of crystals produced by either replenished or closed system processes can readily result in linear arrays in many cases it may be valid to interpret these as minimum average residence times for the crystals whereas the liquid residence times calculated from u series data for periodically replenished systems are maximum values

the georgia embayment continental shelf stratigraphy of a submergence forty vibracores taken across the continental shelf and in proposed drilling lease areas of the southeast georgia embayment are used to document the unconsolidated shelf sediment cover the holocene pleistocene sediment veneer is thin generally less than 4 m thick lagoon sediments deposited during the last regression shelf emergence or the holocene transgression shelf submergence occur in patches on the inner and central shelf because essentially only late pleistocene and holocene mollusk shells are present in the shelf sediment cover it is believed that most of the carbonate fraction was removed by subaerial leaching during low sea level stands aided by mechanical abrasion and biological degradation during the regressive transgressive cycle during each transgression or submergence the surficial sand sheet is recharged with a new biogenic carbonate fraction along with the addition of small amounts of clastic sediments derived from overrun estuaries and erosion of underlying tertiary sediments the stratigraphy based on the vibracores supports the concept of cross shelf migration of the shore face barrier island systems in response to rising sea level sedimentologic and paleontologic analyses also indicate that extensive in place mixing of shelf sediments may have occurred an important factor to consider in evaluation of the fate of particulate pollutants the establishment of the time frame of such mixing should be given high priority in future studies

bedforms of the canadian eastern seaboard a comparison with global occurrences this paper presents a compilation of sedimentary bedforms identified from acoustic and seismic records collected over the southeastern canadian continental shelf in order to make the compilation it was found necessary to firstly review and synthesise the various terminologies used to describe the bedforms found and thereafter to relate the subsequent terminology to any diagnostic generic characteristic identifiable from the records sand waves are considered to be those forms that are composite in nature and are generated solely by the action of superimposed megaripples these latter features are identifiable adopting the classification of dalrymple et al 1978 which proves adequate for the acoustic differentiation of 2 d and 3 d types though we observed a greater coherence in bedform spacing of the 2 d types than reported hitherto sand ridges were adequately defined and recognised using the description of swift et al 1978 with respect to storm generated ridges and that of off 1963 and others with respect to tidal current ridges such ridges appear not to be uniquely described by ratios of their metrics the bedforms on the southeastern canadian continental shelf are abundant and wide spread virtually all types of bedforms recognisable on the u s shelf are present to the north the majority of these bedforms appear to be storm generated and are considered intermittently active many of the sand waves s s have no measureable relief though retaining their distinctive spatial geometry both active and moribund sand ridges were found of these ridges both tidal and storm generated types were identified the storm generated ridges were further broken down into shoreface connected ridges and offshore ridges a metrical analysis of the various bedforms show that a differentiation into stable bed phases can be made using height length ratios provided the bedforms are active and fully developed there is some overlap between megaripples and small sand waves and indeed between 2 d and 3 d megaripples yet the distinction is good between sand waves and sand ridges even moribund types sand ribbons in general can be confused with moribund sand waves and megaripples a detailed morphological examination is required to distinguish each feature data from sand ridges of the eastern canadian shelf combined with data from similar features found globally shows that ridge heights tend to cluster into three distinctive modes these modes are at heights of 7 13 and 27 m the first mode is correlated with storm generated ridges the second with tidal current ridges and the third possibly with submerged barrier islands

ion microprobe analysis of 231pa 235u and an appraisal of protactinium partitioning in igneous zircon ion microprobe u pa measurements in zircon crystals from a holocene rhyolite salton buttes california are utilized to assess the partitioning of pa and u between zircon and melt the relative sensitivity factor rsf for ion microprobe u pa zircon analysis was calibrated on a natural high u secular equilibrium zircon from buff peak nevada zircon crystals from salton buttes rhyolite show evidence for excess 231 pa with a weighted average 231 pa 235 u activity ratio of 1 9 from this ratio model zircon melt partition coefficient ratios d pa d u 0 9 2 2 are obtained assuming plausible limits for 231 pa 235 u melt and correcting for the age of zircon crystallization 15 ka these values roughly fit lattice strain models for tetravalent cations but are more than one order of magnitude lower than predictions for pentavalent pa based on this appraisal of pa partitioning initial 231 pa disequilibrium caused by zircon melt fractionation alone can produce minor discordance or excess 207 pb in late cenozoic zircon but is insignificant for older u pb zircon ages

extreme u th disequilibrium in rift related basalts rhyolites and granophyric granite and the timescale of rhyolite generation intrusion and crystallization at alid volcanic center eritrea received september 20 2005 accepted june 14 2006 advance access publication july 14 2006rhyolite pumices and co erupted granophyric granite xenolithsyield evidence for rapid magma generation and crystallization priorto their eruption at 15 2 2 9 ka at the alid volcanic center inthe danikil depression eritrea whole rock u and th isotopicanalyses show

timescales of magmatic differentiation from alkali basalt to trachyte within the harrat rahat volcanic field kingdom of saudi arabia a fundamental goal of igneous petrology is to quantify the duration of time required to produce evolved magmas following influx of basalt into the crust however in many cases complex field relations and or the presence of a long lived magmatic system make it difficult to assess how basaltic inputs relate to more evolved magmas therefore precluding calculation of meaningful timescales here we present field relations geochemistry 40ar 39ar ages and 36cl ages for volcanic rocks from the harrat rahat volcanic field saudi arabia these data document a systematic and repeated temporal progression from alkali basalt to trachyte for the youngest eruptives from 150 to 17 ka the following eruptive sequence occurred four times 1 alkali basalt 2 hawaiite mugearite or benmoreite and 3 trachyte we interpret each eruptive sequence to result from injection of basalt into the crust and its subsequent differentiation and eruption of progressively evolved magmas we use the interval time between successive eruptions within a given sequence to calculate the duration of time required to produce trachyte from alkali basalt differentiation from alkali basalt to intermediate compositions hawaiite mugearite benmoreite took 2 kyr on average differentiation from intermediate compositions to trachyte took a maximum of 6 6 3 5 to 22 5 1 6 kyr thus the total duration of differentiation was 9 to 25 kyr timescales presented here are insensitive to processes evoked to drive differentiation because they are based solely on the ages and compositions of eruptive products from a system characterized by a simple repeated differentiation sequence

aminostratigraphic results from cape lookout n c and their relation to the preserved quaternary marine record of se north carolina abstract aminostratigraphic data were obtained for mulinia lateralis samples from closely spaced drillholes on the cape lookout north carolina barrier islands two major aminozones are recognized in the subsurface section based upon d alloisoleucine l isoleucine a i values these major aminozones can each be subdivided into two additional aminozones based upon direct comparisons with mulinia a i data from other north carolina localities correlation of the cape lookout mulinia aminostratigraphy with u series calibrated a i data in n c indicates that the sampled units represent deposition during the middle to late pleistocene the four cape lookout aminozones may be assigned to late and early stage 5 or possibly stages 5 and 7 a portion of the interval during stages 17 19 and at least one interglacial in stages 25 31 of the oxygen isotope record based on correlation to calibrated sites and kinetic model extrapolation the aminostratigraphic data obtained from the cape lookout barrier islands and nearby areas indicate that there are significant differences in the extent of preservation of the pleistocene sedimentary record to the southwest of cape lookout compared with that to the north northeast all four of the recognized cape lookout aminozones are present in the subsurface section to the north northeast of cape lookout in contrast the two late pleistocene aminozones are almost completely absent in the subsurface of the barrier islands and in onslow bay to the southwest of cape lookout these aminostratigraphic interpretations are consistent with the incomplete stratigraphic record recognized by previous investigators for the cape lookout onslow bay area the calibrated kinetic model age estimates for the cape lookout aminostratigraphic data now permit quantification of these stratigraphic gaps the detailed aminostratigraphic results from cape lookout also have significant consequences for regional aminostratigraphic correlations on the atlantic coastal plain and provide valuable information with which to test contrasting models proposed for regional correlation within the context of the preserved stratigraphic record

systematic mineralogical diversity in a type granitic intrusions control of magmatic source and geological processes the origin and causes of mineralogical diversity of a type granites are debated the series of a type granite plutons with distinct mineralogical differences emplaced along an upper paleozoic crustal scale shear zone in the cobequid highlands nova scotia provide an opportunity to examine the origin of different a type plutons in a similar tectonic setting based on the ferromagnesian minerals present the plutons are classified into sodic granites with sodic amphibole calcic granites with calcic amphibole and biotite granites sodic and calcic granites occur exclusively in complex intrusions with subequal amounts of gabbro in the eastern shear zone whereas plutons in the western shear zone with lesser gabbro are solely biotite granites trace elements and radiogenic isotopes show that the three granite types have different sources intensive parameters including temperature pressure and water in melt contents were estimated from mineralogical and geochemical data modeling of these geochemical data suggests that the biotite and calcic granites were derived by 20 40 partial melting of intracrustal feldspathic rocks whereas the sodic granites are extreme fractionates 90 of coeval mafic magma we propose that supply of upper paleozoic mafic magma probably related to regional extension and decompression melting beneath the magdalen basin created a deep crustal hot zone in the eastern cobequid highlands and extreme fractionation of underplated mafic sills produced the sodic granites heat transfer from crystallizing mafic magma induced partial melting of the surrounding crust creating batches of biotite and calcic granitic melts in different depths fractionated and crustally derived melts segregated along crustal scale faults constructing the complex plutons in the east melting of the crust was further facilitated by the release of water from the crustal rocks upon heating in the eastern shear zone water was released predominantly by magmatic rocks and in lesser amounts compared to the west where neoproterozoic sedimentary and volcaniclastic rocks are more abundant the volatile rich granitic melts in the western part of the shear zone were crystallized rapidly stabilizing only biotite this study demonstrates that the mineralogical variations in a type granites arise from rather similar magma compositions but they are important petrogenetic indicators of varying sources specific magmatic processes and emplacement conditions

architecture and morphology of the outer segment of a mixed tide and wave dominated incised valley revealed by hr seismic reflection profiling the paleo charente river france abstract we present new single channel high resolution seismic reflection profiles ground truthed by vibrocore data dedicated to the outer segment of an incised valley connected to the charente river french atlantic coast the present day charente is located about 50 km northward of the well known gironde estuary those river mouths are comparable in terms of marine hydrodynamic parameters but strongly differ by their water discharge and catchment area seismic data are processed and interpreted to develop a three dimensional seismic stratigraphic framework for the charente incised valley fill the channel network of the drowned charente valley is evidenced for the first time and shows a seaward distributary pattern which is likely influenced by neotectonic control incision depth and width of the charente and gironde incised valleys are similar supporting the idea that correlation between valley width and modern hydrology is poor the internal geometry of the charente valley fill shows in seismic lines high to middle angle dipping reflectors at the base and a top of sequence with an intervening low angle seismic unit this pattern is associated with a landward migration of the seismic unit depocentres sedimentary facies of the main seismic units suggests an upward and landward shift from estuarine mixed sands and muds to estuary mouth massive sands topped by fine marine sands the described sedimentary and seismic characters and the holocene age of the valley fill near the present day charente river mouth suggest that the drowned charente valley infill mainly corresponds to a single transgressive sequence emplaced during the last sea level rise beyond the local interest the dense seismic grid recorded across the charente drowned valley gives a seismic validation for the model of large valley fill proposed by ashley and sheridan ashley g m sheridan r e 1994 depositional model for valley fills on a passive continental margin in r w dalrymple r j boyd b a zaitlin eds incised valley systems origin and sedimentary sequences sepm soc sediment geol spec publ tulsa vol 51 pp 285 301 we propose that high low high internal reflection pattern succession represents the seismic sandwich in reference to the ashley and sheridan ashley g m sheridan r e 1994 depositional model for valley fills on a passive continental margin in r w dalrymple r j boyd b a zaitlin eds incised valley systems origin and sedimentary sequences sepm soc sediment geol spec publ tulsa vol 51 pp 285 301 sedimentary sandwich such seismic pattern is also recognized in the gironde valley mar geol 175 2001 183

updrift river mouth migration on cuspate deltas two examples from the coast of tuscany italy the configuration of the arno and ombrone deltas italy is evaluated the terminal courses of the rivers forming these deltas face the dominant wave orthogonal this configuration came about during rapid delta progradation as a consequence of increased river sediment input due to widespread deforestation within the catchment area a prevailing phenomenon from the early middle ages to the xviii century the rivers attained and maintained their directions as the result of the increased accretion rate on the less exposed downdrift side of the delta on the more exposed updrift side delta growth caused the shoreline to gradually rotate so that it directly faced the waves approaching the coast here due to the lower refraction wave energy per shore unit increased whilst accretion rate decreased this rotation of the shoreline induced longshore inversion on the updrift side whereas today cusp erosion is restoring the original drift direction

some remarks on u th mineral ages from igneous rocks with prolonged crystallisation histories abstract mineral isochron dating is a frequently used geochronological tool one of its assumptions is that the minerals grow over a time period that is small compared to the half life of the radiogenic isotope system used in recent years increasing analytical precision has promoted the use of the short lived u series isotope system in order to date young crystallisation events three whole rock zircon u th isochrons from the 26 5 ka oruanui eruption in the taupo volcanic zone new zealand yield pre eruptive model ages of 5 5 0 8 ka 9 7 1 7 ka and 12 3 0 8 ka for the sub 63 m 63 125 m and 125 250 m zircon size fractions respectively this suggests that in this case the assumption of instantaneous crystal growth breaks down instead the u th data may be explained by continuous zircon growth over a period of 90 ka however cathodoluminescence shows that crystals are typically composed of an euhedral core surrounded by a sector zoned euhedral rim and the u th data can also be modelled by mixing an older 27 ka model age population of zircon crystals with a young zircon rim that formed shortly prior to eruption of the oruanui rhyolite this indicates that detailed petrographic studies are critical for deciphering the histories of prolonged crystallisation in the magmatic environment it is concluded that conventional u series mineral isochrons may underestimate the age of the onset of crystallisation by more than an order of magnitude in future microanalytical techniques will lead to significant advances in the understanding of crystallisation processes and timescales

late cenozoic marine deposition in the united states atlantic coastal plain related to tectonism and global climate abstract major hiatuses in upper cenozoic marine deposits in the united states atlantic coastal plain are recognized on the basis of molluscan faunal changes at erosional unconformities these hiatuses generally coincided with periods of global cooling and ice sheet formation such hiatuses provide information to supplement global climatic data major hiatuses are recognized within the early miocene 23 20 m y ago at the end of the middle miocene 11 10 m y ago at the end of the late miocene 6 5 5 m y ago at the end of the early pliocene 4 0 2 5 m y ago at the end of the late pliocene 1 9 or 1 8 m y ago within the pleistocene 1 1 0 5 m y ago and several times within the last 0 4 m y estimates of the amount of water contained in ice sheets at different times in the pliocene and pleistocene facilitate calculation of probable minimum sea levels on the coastal plain during different high stands of the sea the altitudes of dated shoreline deposits in the atlantic coastal plain show that the amount of uplift in the cape fear area has averaged at least 1 3 cm per 1000 years since the beginning of pliocene time the coastal plain of georgia has apparently experienced relatively little vertical deformation during this same time

crystal and magma residence at kilauea volcano hawaii 230th 226ra dating of the 1955 east rift eruption abstract previous estimates of crustal storage time of magmas at kilauea volcano hawaii range from a few years to a few thousand years leading to considerable uncertainty in the time scales of processes of magmatic storage and differentiation we present a new approach for determining minimum magma residence times which involves dating phenocrysts in a magma using 226ra 230th disequilibria and apply this approach to the early phase of the 1955 east rift eruption at kilauea when fractionation of ra from ba a proxy for initial ra in the crystals during crystal growth is considered along with the effects of inclusions in the minerals the data are consistent with co precipitation of plagioclase and clinopyroxene from a melt represented by the groundmass at a mean age of 1000 300 400 a unless a significant fraction 30 of the crystals are remnants from an earlier batch of evolved magma in the system these data constrain the minimum magmatic residence time to be 550 yr considerably longer than most previous estimates of storage time at kilauea as well as those for some other basaltic systems for the temperature interval of augite plagioclase growth in the early 1955 magma a maximum constant cooling rate of 0 1 c yr 1 10 5 c h is derived from the minimum magmatic residence time of 550 yr the total magma storage time would be 2500 yr if this cooling rate applied to the entire thermal history of the magma although a more complex cooling history where cooling rates were more rapid early in the storage history is permissive of a total residence time which is not much longer than 550 yr the disparate estimates of magma residence at kilauea may reflect the uncertainties in the methods of estimation in addition to true variations in storage time for different batches of magma more work is necessary in order to determine whether a long residence time is characteristic of rift zone lavas and or of kilauean lavas in general

evidence for the hudson river as the dominant source of sand on the us atlantic shelf previous attempts to determine the origin of sand on the us atlantic shelf by standard heavy mineral analysis have concluded that local rivers and coastal erosion were probable sources1 here a new approach to determine the history of surficial sand dispersal is developed using the elemental composition of detrital fe ti oxide grains primarily ilmenite instead of heavy mineral abundances sands from the entire middle atlantic shelf latitude 35 n to 41 n have been interpreted as coming from specific drainage basins1 2 more than half of the sand samples collected as far south as north carolina can be traced to the hudson river and other new england sources the dominance of the hudson river is cause for rethinking sources of shelf sediment south of new jersey these findings suggest that the hudson river may have rivalled the sediment discharge of much larger rivers like the mississippi or amazon rivers at least during brief glacial melting events

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