an integrated hypothesis for regional patterns of shoreline change along the northern north carolina outer banks usa combining analyses of plan view shoreline change and shoreline curvature with existing nearshore geologic and bathymetric data and the results of a recent theoretical large scale shoreline evolution model that couples geologic framework to alongshore sediment transport we propose an integrated explanation for persistent patterns of shoreline change observed on the northern outer banks of north carolina usa concentrated sources of coarse grained sediment derived from relict fluvial stratigraphy or densely grouped relict inlet channels excavated from the shoreface may both enable persistence of nearshore bathymetric anomolies and control multi km scale undulations in shoreline curvature which in turn affect gradients in wave driven alongshore sediment transport that drive long term shoreline change

high angle wave instability and emergent shoreline shapes 2 wave climate analysis and comparisons to nature 1 recent research has revealed that the plan view evolution of a coast due to gradients in alongshore sediment transport is highly dependant upon the angles at which waves approach the shore giving rise to an instability in shoreline shape that can generate different types of naturally occurring coastal landforms including capes flying spits and alongshore sand waves this instability merely requires that alongshore sediment flux is maximized for a given deepwater wave angle a maximum that occurs between 35 and 50 for several common alongshore sediment transport formulae here we introduce metrics that sum over records of wave data to quantify the long term stability of wave climates and to investigate how wave climates change along a coast for long point a flying spit on the north shore of lake erie canada wave climate metrics suggest that unstable waves have shaped the spit and furthermore that smaller scale alongshore sand waves occur along the spit at the same locations where the wave climate becomes unstable a shoreline aligned along the trend of the carolina capes united states would be dominated by high angle waves numerical simulations driven by a comparable wave climate develop a similarly shaped cuspate coast local wave climates along these simulated capes and the carolina capes show similar trends shoreline reorientation and shadowing from neighboring capes causes most of the coast to experience locally stable wave climates despite regional instability

modeling large scale shoreline change caused by complex bathymetry in low angle wave climates abstract coastlines where waves consistently approach at highly oblique angles experience anti diffusional behavior causing perturbations to grow seaward and form sand waves capes and spits coasts where waves approach at low offshore angles experience the opposite perturbations diffuse and the coastline remains or becomes smooth in this paper by coupling a 2 d large scale coastline evolution model to a spectral wave model we show that anti diffusional behavior is also possible in low angle wave climates if the nearshore wave field is altered by complex bathymetry in model simulations low angle waves refract over local shoals creating a convergence in alongshore sediment flux behind the shoals that coalesces into small minor capes depending on wave height and period shoreline features take 80 400 years to reach an equilibrium cross shore relief of 1 1 5 km over an alongshore distance of 20 km the modeled equilibrium time scale is consistent with analytically determined characteristic shoreline diffusion time scales and the modeled cross shore relief and aspect ratio are similar to observed minor capes along the u s atlantic coast where offshore bathymetric anomalies have previously been linked to shoreline change patterns better understanding of the links among nearshore bathymetry wave transformation and alongshore sediment transport is critical to understanding shoreline change patterns at the local level and how they fit into broader regional scale behavior

regional seismic stratigraphy and controls on the quaternary evolution of the cape hatteras region of the atlantic passive margin usa abstract seismic and core data combined with amino acid racemization and strontium isotope age data enable the definition of the quaternary stratigraphic framework and recognition of geologic controls on the development of the modern coastal system of north carolina u s a seven regionally continuous high amplitude reflections are defined which bound six seismic stratigraphic units consisting of multiple regionally discontinuous depositional sequences and parasequence sets and enable an understanding of the evolution of this margin data reveal the progressive eastward progradation and aggradation of the quaternary shelf the early pleistocene inner shelf occurs at a depth of ca 20 40 m beneath the western part of the modern estuarine system pamlico sound a mid to outer shelf lowstand terrace also early pleistocene with shelf sand ridge deposits comprising parasequence sets within a transgressive systems tract occurs at a deeper level ca 45 70 m beneath the modern barrier island system the outer banks and northern pamlico sound seismic and foraminiferal paleoenvironmental data from cores indicate the occurrence of lowstand strandplain shoreline deposits on the early to middle pleistocene shelf middle to late pleistocene deposits occur above a prominent unconformity and marine flooding surface that truncates underlying units and contain numerous filled fluvial valleys that are incised into the early and middle pleistocene deposits the stratigraphic framework suggests margin progradation and aggradation modified by an increase in the magnitude of sea level fluctuations during the middle to late pleistocene expressed as falling stage lowstand transgressive and highstand systems tracts thick stratigraphic sequences occur within the middle pleistocene section suggesting the occurrence of high capacity fluvial point sources debouching into the area from the west and north furthermore the antecedent topography plays a significant role in the evolution of the geomorphology and stratigraphy of this marginal system

formation of coastline features by large scale instabilities induced by high angle waves along shore sediment transport that is driven by waves is generally assumed to smooth a coastline this assumption is valid for small angles between the wave crest lines and the shore as has been demonstrated in shoreline models but when the angle between the waves and the shoreline is sufficiently large small perturbations to a straight shoreline will grow here we use a numerical model to investigate the implications of this instability mechanism for large scale morphology over long timescales our simulations show growth of coastline perturbations that interact with each other to produce large scale features that resemble various kinds of natural landforms including the capes and cuspate forelands observed along the carolina coast of southeastern north america wind and wave data from this area support our hypothesis that such an instability mechanism could be responsible for the formation of shoreline features at spatial scales up to hundreds of kilometres and temporal scales up to millennia

large scale dynamics of sandy coastlines diffusivity and instability 1 the dynamics of small amplitude perturbations of an otherwise rectilinear coastline due to the wave driven alongshore sediment transport is examined at large time and length scales years and kilometers a linear stability analysis is performed by using an extended one line shoreline model with two main improvements 1 the curvature of the coastline features is accounted for and 2 the coastline features are assumed to extend offshore as a bathymetric perturbation up to a finite distance for high incidence angles instability is found in accordance with ashton et al 2001 however it is seen that instability is inhibited by high waves with long periods and gently sloping shorefaces so that in this case the coastline may be stable for any angle similarly there is no instability if the bathymetric perturbation is confined very close to the coast it is found that the traditional linearized one line model larson et al 1987 tends to overpredict the coastline diffusivity the overprediction is small for the conditions leading to a stable coastline and for moderate incidence angles but can be very dramatic for the conditions favoring instability an interesting finding is that high angle waves instability has a dominant wavelength at the linear regime which is in the order of 4 15 km one to two orders of magnitude larger than the length scale of surf zone rhythmic features intriguingly this is roughly the same range of the wavelength of some observed shoreline sand waves and in particular those observed along the dutch coast a model application to this coast is presented

non hydrostatic numerical modelling of nearshore wave transformation over shore oblique sandbars abstract observations from high resolution bathymetric surveys on the coast of north carolina indicate areas with shore parallel depth contours and other areas with shore oblique sandbars these shore oblique bars are large scale morphological features that extend up to 1000 m from shore and previous studies have shown that they are spatially correlated with higher shoreline change rates and induce erosional hotspots to investigate the influence the oblique sandbar morphology has on nearshore wave transformation and energy distribution a non hydrostatic wave model is used with the bathymetry corresponding to two areas with shore parallel depth contours and two areas with shore oblique sandbars the model is applied with a horizontal resolution of 5 m and a boundary condition representative of the directional spectrum of a typical southeast swell event an idealized approach with relatively short alongshore model domains and cyclic lateral boundary conditions is adopted due to limited availability of high resolution bathymetry however this approach elucidates the influence of shore oblique sandbars on the nearshore hydrodynamics the numerical results show that higher alongshore variability in depth due to the shore oblique bars drive large alongshore changes in cross shore wave energy flux and cause spatial variability in dissipation in the nearshore region the simulations indicate that alongshore variations in wave energy caused by the shore oblique bars results in a 50 75 reduction in wave setup and a 25 40 decrease in alongshore velocity averaged across the surf zone for the conditions investigated the results suggest that wave driven hydrodynamics can influence nearshore sediment dynamics to create erosional zones compared to areas with shore parallel bathymetric contours since the morphology of shore oblique sandbars are an important control on wave energy that impacts the beach

the effect of meltwater plumes on the melting of a vertical glacier face abstractfreshwater produced by the surface melting of ice sheets is commonly discharged into ocean fjords from the bottom of deep fjord terminating glaciers the discharge of the freshwater forms upwelling plumes in front of the glacier calving face this study simulates the meltwater plumes emanated into an unstratified environment using a nonhydrostatic ocean model with an unstructured mesh and subgrid scale mixing calibrated by comparison to established plume theory the presence of an ice face reduces the entrainment of seawater into the meltwater plumes so the plumes remain attached to the ice front in contrast to previous simple models ice melting increases with height above the discharge also in contrast to some simple models and the authors speculate that this overcutting may contribute to the tendency of icebergs to topple inwards toward the ice face upon calving the overall melt rate is found to increase with discharge flux only up to a critical value which depends on the channel size

high angle wave instability and emergent shoreline shapes 1 modeling of sand waves flying spits and capes 1 contrary to traditional findings the deepwater angle of wave approach strongly affects plan view coastal evolution giving rise to an antidiffusional high wave angle instability for sufficiently oblique deepwater waves with angles between wave crests and the shoreline trend larger than the value that maximizes alongshore sediment transport 45 a one contour line numerical model shows that a predominance of high angle waves can cause a shoreline to self organize into regular quasiperiodic shapes similar to those found along many natural coasts at scales ranging from kilometers to hundreds of kilometers the numerical model has been updated from a previous version to include a formulation for the widening of an overly thin barrier by the process of barrier overwash which is assumed to maintain a minimum barrier width systematic analysis shows that the wave climate determines the form of coastal response for nearly symmetric wave climates small net alongshore sediment transport cuspate coasts develop that exhibit increasing relative cross shore amplitude and pointier tips as the proportion of high angle waves is increased for asymmetrical wave climates shoreline features migrate in the downdrift direction either as subtle alongshore sand waves or as offshore extending flying spits depending on the proportion of high angle waves numerical analyses further show that the rate that the alongshore scale of model features increases through merging follows a diffusional temporal scale over several orders of magnitude a rate that is insensitive to the proportion of high angle waves the proportion of high angle waves determines the offshore versus alongshore aspect ratio of self organized shoreline undulations

late neogene and quaternary evolution of the northern albemarle embayment mid atlantic continental margin usa seismic surveys in the eastern albemarle sound adjacent tributaries and the inner continental shelf define the regional geologic framework and provide insight into the sedimentary evolution of the northern north carolina coastal system lithoand chronostratigraphic data are derived from eight drill sites on the outer banks barrier islands and the mobil 1 well in eastern albemarle sound within the study area parallel bedded gently dipping miocene beds occur at 95 to n160 m below sea level m bsl and are overlain by a southward thickening pliocene unit characterized by steeply inclined southward prograding beds the lower pliocene unit consists of three seismic sequences the 55 60 m thick quaternary section unconformably overlies the pliocene unit and consists of 18 seismic sequences exhibiting numerous incised channel fill facies shallow stratigraphy b40 m bsl is dominated by complex fill patterns within the incised paleo roanoke river valley radiocarbon and amino acid racemization aar ages indicate that the valley fill is latest pleistocene to holocene in age at least six distinct valley fill units are identified in the seismic data cores in the valley fill contain a 3 6 m thick basal fluvial channel deposit that is overlain by a 15 m thick unit of interlaminated muds and sands of brackish water origin that exhibit increasing marine influence upwards organic materials within the interlaminated deposits have ages of 13 11 cal ka the interlaminated deposits within the valley are overlain by several units that comprise shallow marine sediments bay mouth and shoreface environments that consist of silty fine to medium grained sands containing open neritic foraminifera suggesting that this area lacked a fronting barrier island system and was an open embayment from 10 ka to 4 5 ka seismic data show that initial infilling of the paleo roanoke river valley occurred from the north and west during the late pleistocene and early holocene later infilling

discordant iron rich ultramafic pegmatites in the bushveld complex and their relationship to iron rich intercumulus and residual liquids the iron rich ultramafic pegmatites comprise a suite of coarse grained rocks that form discordant bodies within the layered sequence of the bushveld complex these pegmatites which are considerably more abundant than is generally realized provide evidence for the differentiation of iron rich residual melts the pegmatites are composed largely of iron rich olivine and clinopyroxene together with ti magnetite and ilmenite feldspar is characteristically absent but paradoxically the pegmatites preferentially replace anorthositic cumulates two subgroups are recognized olivine clinopyroxene pegmatite and fe ti oxide pegmatite with increased stratigraphic height the pegmatites become richer in fe ti oxides thus olivine clinopyroxene pegmatite is prevalent in the upper critical and lower main zones whereas fe ti oxide pegmatite is restricted to the upper main and upper zones zoned pegmatite with a core of fe ti oxide pegmatite is transitional between the two subgroups new whole rock and electron microprobe analyses of olivine clinopyroxene pegmatite from the upper critical and lower main zones provide convincing evidence that their composition is directly related to height cryptic compositional variations are analogous to those displayed by the layered cumulates but for a given height the pegmatites are always more evolved compositions of clinopyroxene in the pegmatites reflect a near linear relationship with height whereas cumulus pyroxenes display upward iron enrichment trends complicated by replenishment and reaction with trapped intercumulus liquid the olivine clinopyroxene pegmatite formed by magmatic replacement of earlier formed cumulates in response to infiltration of iron rich melts suitably differentiated melts comprised intercumulus and residual liquids derived from thick anorthosite layers the absence of feldspar although not fully understood is attributed to an immiscible relationship between dense iron rich melts and light silica alkali rich liquids the latter infiltrated upward to be reincorporated into the resident magma the iron rich melts however drained down into the crystallizing cumulate pile channelling along early formed fractures and joints was significant locally resulting in huge pipe like bodies of pegmatite the iron rich melts became increasingly differentiated with height partly in response to the fractional crystallization of more evolved cumulates the olivine clinopyroxene pegmatites are related to infiltration of fe ti oxide rich silicate melt whereas fe ti oxide pegmatite is ascribed to fe ti oxide liquid as originally argued by bateman 1951 the bushveld complex followed the fenner trend of almost uninterrupted iron enrichment evidence of pronounced iron enrichment is however manifested in the discordant iron rich ultramafic pegmatites several thousands of metres below the height at which iron rich cumulates are observed

post deposition structures in the bushveld complex mafic sequence the stratigraphy of the bushveld complex is characterized by laterally continuous horizontal layering at the interface of some of these layers there are phenomena analogous to flame load cast ball and pillow and liquid escape structures of sedimentary rocks three plastic deformation styles recognized in the complex are i vertical ii vertical followed by horizontal and iii vertical fluid movement with brittle failure these post deposition structures are used to estimate viscosity and density of adjacent layers during consolidation the distribution of the structures through the mafic sequence provides some information on magma movement and rates of crystal accumulation

potholes of the merensky reef at brakspruit shaft rustenburg platinum mines primary disturbances in the magmatic stratigraphy potholes are locations where parts of the magmatic stratigraphy are missing in the foot wall sequence of the merensky reef in general potholes are circular to elliptical in plan and funnel shaped in cross section within narrow limits most potholes reveal the same three dimensional orientation to the strike and dip of the magmatic layering the updip pothole flank is steep to vertical the downdip flank is horizontal the pothole bottom generally coincides with one of the marker horizons of the footwall stratigraphy and is conformable with layering where the merensky reef fills potholes it is more variable in thickness than it is at its normal stratigraphic level in the potholes it changes over short distances from an unusually thin chromite rich contact phase of the reef to thick unlayered lenses of harzburgite pegmatite in deep seated parts of the potholes when the bastard reef is observed in potholes it sometimes grades into massive graphite rich transgressive gabbroic pegmatite the marker horizons in the anorthositic footwall units around potholes are often intensely mineralized with stratiform and crosscutting sulfides such marginal footwall mineralization is absent in the undisturbed footwall sequence away from potholes potholes are traditionally considered to be resorption pits due to the influx of a new hot primitive magma at the merensky reef level or to represent scars of a late magmatic fumarole activity the present study suggests that cumulate resorption played a minor role only potholes are sites of nondeposition where the missing rock units did not crystallize they formed where locally high concentrations of dissolved c h o s volatiles lowered the liquidus temperature of the magma and suppressed the crystallization of cumulus plagioclase potholes seem to be concentrated above anticlines of the metasedimentary basement rocks in the footwall of the bushveld complex volatiles derived by pyrometamorphism were focused upward into the overlying melt above such topographic highs and were accumulated at the level of the merensky reef

geological characteristics and spatial distribution of paleo inlet channels beneath the outer banks barrier islands north carolina usa abstract nearly 200 km of high resolution ground penetrating radar gpr data were acquired along the outer banks barrier island system of north carolina usa gpr data combined with lithofacies and biofacies data reveal multiple depositional facies including inlet channel flood tide delta overwash peat and inner shelf previously undocumented paleo inlet channels constitute a significant portion of the shallow geologic framework between oregon inlet and cape hatteras gpr data reveal the complex stratigraphy associated with multiple sequences of cut and fill within inlet channels two types of paleochannels non migrating and migrating were classified based on geometry and fill patterns sediments and foraminifera collected from vibracores were correlated to gpr data to define the regional shallow stratigraphic framework channel fill facies are characterized by clinoform packages sometimes bounded by erosional surfaces indicating variable sediment transport directions from the ocean and sound sides channels are incised into older flood tide delta deposits corresponding to older inlet activity when barriers existed further seaward flood tide delta deposits are capped with marsh peat and overwash units migrating inlet facies occur under the widest portions of the island whereas narrow portions of the island are underlain by the non migrating inlet facies or flood tide delta overwash facies this geologic geomorphic relationship is attributed to the successional stage of island evolution during transgression and sediment transport processes associated with the different inlet types the radar facies lithofacies and biofacies provide a comprehensive dataset that will permit more precise identification of barrier island facies in the geologic record

reconstructing and understanding the impacts of storms and surges southern north sea coastal barriers are ubiquitous globally and provide a vital protective role to valuable landforms habitats and communities located to landward they are however vulnerable to extreme water levels and storm wave impacts a detailed record of sub annual to annual decadal and centennial rates of shoreline retreat in frontages characterised by both high 3 m and low 1 m dunes is established for a barrier island on the uk east coast for four storms 2006 2013 we match still water levels and peak significant wave heights against shoreline change at high levels of spatial densification the results suggest that at least in the short term shoreline retreat of typically 5 8 m is primarily driven by individual events separated by varying periods of barrier stasis over decadal timescales significant inter decadal changes can be seen in both barrier onshore retreat rates and in barrier extension rates alongshore whilst the alongshore variability in barrier migration seen in the short term remains at the decadal scale shoreline change at the centennial stage shows little alongshore variability between a region of barrier retreat at 1 15 m a 1 and one of barrier extension a data mining approach synchronising all the variables that drive shoreline change still water level timing of high spring tides and peak significant wave heights is an essential requirement for validating models that predict future shoreline responses under changing sea level and storminess

sorted bed forms as self organized patterns 1 model development 1 here we present a modified version of the exploratory numerical model originally presented by murray and thieler 2004 and use it to further investigate the development of sorted bed forms on the inner continental shelf the new version of the model is based on widely accepted parameterizations for hydrodynamics and sediment transport that are more detailed and empirically tested than the parameterizations used in the original model the shallow water wave equations are replaced by the general equations and changes in water depth related to morphological development are accounted for this model reproduces the main results of the previous one supporting the hypothesis that sorted bed forms are a self organized pattern and develop as a result of a coupling between bed composition and sediment flux the sensitivity of the model predictions evaluated in terms of the geometry of the sorted bed forms and their growth rate with respect to external forcing variables and internal model parameterizations shows increasing decreasing sorted bed form spacing height with increasing wave height and period the opposite occurs for increasing water depths more complicated behavior is observed when the ratio fine to coarse sediment diameter or the percentage of fine and coarse material is varied the shape of the suspended sediment and current profiles and the formulation used to predict the geometry of wave generated ripples can significantly affect or even inhibit growth rates of sorted bed forms results from a sensitivity analysis show that there are deficiencies in the description of small scale processes that limit the ability to quantitatively predict sorted bed form evolution highlighting gaps in our knowledge of sediment transport over mixed grain beds that need to be filled

a fluid dynamic model for the potholes of the merensky reef the cyclic unit that hosts the merensky reef is attributed to the entry of a new pulse of bronzite saturated magma into a chamber containing a melt that was crystallizing plagioclase mixing of these magmas produced a hybrid magma that was superheated with respect to both bronzite and plagioclase this magma melted the plagioclase cumulates that formed the floor of the chamber and because the products of erosion were light they were swept away by turbulent convection and replaced by hot uncontaminated magma allowing erosion to continue thermochemical erosion leading to the release of a light fluid has been modeled in a series of experiments in which a block of ice was melted by an overlying salt solution two features of the experiments are noteworthy first melting produced a pitted texture on the surface of the ice similar to the dimpled contact between the merensky reef and the underlying plagioclase cumulates and second small holes in the ice rapidly expanded into larger holes which were circular in plan and rounded in section the similarity between these structures and the potholes suggests that they formed by similar processes it is proposed that the erosion in the larger potholes was arrested at olivine rich layers because the products of thermochemical erosion were dense and could not convect away from the melting surface

influence of fluvial processes on the quaternary geologic framework of the continental shelf north carolina usa abstract digital single channel high resolution seismic reflection profiles were acquired from the insular continental shelf of north carolina usa along a data grid extending from oregon inlet northward 48 km to duck north carolina and from the nearshore zone seaward approximately 28 km total surveyed area 1334 km 2 these data were processed and interpreted to delineate principal reflecting horizons and develop a three dimensional seismic stratigraphic framework for the continental shelf that was compared to stratigraphic data from the shoreward back barrier estuarine and barrier island system six principal reflecting horizons designated r 0 through r 5 were present within the upper 60 m of the shelf stratigraphic succession three dimensional mapping of reflector r 1 demonstrated its origin from fluvial incision of the continental shelf during an episode or episodes of lowered sea level fluvial processes during development of reflector r 1 were responsible for extensive reworking and re deposition of sediment throughout most of the northern half of the study area five seismic stratigraphic units designated s 1 through s 5 were tentatively correlated with depositional sequences previously identified from the north carolina back barrier estuarine and barrier island system these five stratigraphic units span the quaternary period s 1 early holocene s 2 51 78 ka s 3 330 530 ka s 4 1 1 1 8 ma s 5 earliest pleistocene unit s 1 is composed of fine grained fluvial estuarine sediment that back filled incised streams during early holocene sea level rise the four other stratigraphic units s 2 s 5 display tabular depositional geometries low total relief and thicken toward the east southeast as their basal reflectors dip gently between 0 41 m km 1 0 02 and 0 54 m km 1 0 03 knowledge of the three dimensional subsurface stratigraphic architecture of the continental shelf enhances understanding of the development of shelf depositional successions and provides a framework for development of better quaternary sea level data especially offshore north carolina where such data are sparse

decoupling processes and scales of shoreline morphodynamics abstract behavior of coastal systems on time scales ranging from single storm events to years and decades is controlled by both small scale sediment transport processes and large scale geologic oceanographic and morphologic processes improved understanding of coastal behavior at multiple time scales is required for refining models that predict potential erosion hazards and for coastal management planning and decision making here we investigate the primary controls on shoreline response along a geologically variable barrier island on time scales resolving extreme storms and decadal variations over a period of nearly one century an empirical orthogonal function analysis is applied to a time series of shoreline positions at fire island ny to identify patterns of shoreline variance along the length of the island we establish that there are separable patterns of shoreline behavior that represent response to oceanographic forcing as well as patterns that are not explained by this forcing the dominant shoreline behavior occurs over large length scales in the form of alternating episodes of shoreline retreat and advance presumably in response to storms cycles two secondary responses include long term response that is correlated to known geologic variations of the island and the other reflects geomorphic patterns with medium length scale our study also includes the response to hurricane sandy and a period of post storm recovery it was expected that the impacts from hurricane sandy would disrupt long term trends and spatial patterns we found that the response to sandy at fire island is not notable or distinguishable from several other large storms of the prior decade

the influence of shelf indenting canyons and infralittoral prograding wedges on coastal morphology the carchuna system in southern spain abstract submarine canyons and progradational sedimentary bodies are common features of continental margins worldwide few studies have analyzed the relationship between the coastal morphology and the morphology and sedimentary characteristics of the adjacent shelf in wave dominated coasts or the potential role played by shelf indenting canyons as inducers of sediment deposition in shallow waters this work presents a study that correlates the coastal morphology with the characteristics of the adjacent offshore shallow shelf in an area with a complex inner bathymetry the carchuna system spain the study area is characterized by large scale non rhythmic cuspate features in the coastline whereas the inner bathymetry exhibits a shallow water infralittoral prograding wedge ipw and a shelf indenting canyon that has tributaries located a short distance from the coastline using detailed bathymetric measurements observational data and wave propagation modeling the influence of the different bathymetric elements was quantified and a mechanism proposed for the generation of the carchuna ipw the wave propagation results show that the canyon is the main source of the alongshore modulation of nearshore waves and that it plays a major role in the generation of the large scale cuspate features in the shoreline additionally these results suggest that the long term coastal erosion associated with extreme storms yielded sufficient sediment which when subsequently transported eastward the dominant littoral drift direction formed the ipw once formed the ipw modified the wave propagation patterns and contributed to the development and reinforcement of the shoreline features

deposition of cohesive sediment from turbulent plumes gravity currents and turbidity currents models for the deposition of cohesive sediment from turbulent plumes or buoyant jets gravity currents and turbidity currents are provided in this paper the cohesive sediment is made up of small particles that aggregate together to form larger flocs which are in turn broken up by turbulent shear the equilibrium mean floc size and thus the equilibrium mean fall speed is a function of the turbulent dissipation rate and the sediment concentration the flows are modeled by using integral and box models with dissipation related to bulk flow properties for plumes it is shown that there is a well defined equilibrium fall speed at the virtual origin and that the fall speed changes relatively slowly in the momentum dominated part of the flow within one jet length or so of the source if the flocs are assumed to adjust instantaneously to their equilibrium size an integral model for a turbulent plume carrying cohesive sediment can be described in terms of two parameters the angle between the plume an

controls on coastal dune morphology shoreline erosion and barrier island response to extreme storms abstract the response of a barrier island to an extreme storm depends in part on the surge elevation relative to the height and extent of the foredunes which can exhibit considerable variability alongshore while it is recognized that alongshore variations in dune height and width direct barrier island response to storm surge the underlying causes of the alongshore variation remain poorly understood this study examines the alongshore variation in dune morphology along a 11 km stretch of santa rosa island in northwest florida and relates the variation in morphology to the response of the island during hurricane ivan and historic and storm related rates of shoreline erosion the morphology of the foredune and backbarrier dunes was characterized before and after hurricane ivan using empirical orthogonal function eof analysis and related through canonical correlation analysis cca the height and extent of the foredune and the presence and relative location of the backbarrier dunes varied alongshore at discrete length scales of 750 1450 and 4550 m that are statistically significant at the 95 confidence level cospectral analysis suggests that the variation in dune morphology is correlated with transverse ridges on the inner shelf the backbarrier cuspate headlands and the historical and storm related trends in shoreline change sections of the coast with little to no dune development before hurricane ivan were observed in the narrowest portions of the island between headlands west of the transverse ridges overwash penetration tended to be larger in these areas and island breaching was common leaving the surface close to the watertable and covered by a lag of shell and gravel in contrast large foredunes and the backbarrier dunes were observed at the widest sections of the island the cuspate headlands and at crest of the transverse ridges due to the large dunes and the presence of the backbarrier dunes these areas experienced less overwash penetration and most of the sediment from the beachface and dunes was deposited within the upper shoreface it is argued that this sediment is returned to the beachface through nearshore bar migration following the storm and that the areas with larger foredunes and backbarrier dunes have smaller rates of historical shoreline erosion compared to areas with smaller dunes and greater transfer of sediment to the washover terrace since the recovery of the dunes will vary depending on the availability of sediment from the washover and beachface it is further argued that the alongshore pattern of dune morphology and the response of the island to the next extreme storm is forced by the transverse ridges and island width through alongshore variations in storm surge and overwash gradients respectively these findings may be particularly important for coastal managers involved in the repair and rebuilding of coastal infrastructure that was damaged or destroyed during hurricane ivan

sedimentology and morphodynamics of a barrier island shoreface related to engineering concerns outer banks nc usa abstract forty nine vibracores were collected from a barrier island shoreface following 12 4 years of biweekly profile surveying the sedimentologic architecture of the shoreface was linked to time series elevation change and profile shape to determine relationships between morphodynamics facies development erosional processes profile closeout and cross shore transport the modern shoreface mass which erosionally overlies a tidal inlet associated complex attains a maximum thickness of 3 4 m below the beach to middle profile before pinching out seaward between 9 and 12 m depth concave erosional surfaces overlain by cross stratified fine to medium sand and gravel make up most of the lower half of the shoreface prism below the beach through middle profile reflecting longshore trough incision and subsequent current dominated aggradation at the landward margin gravel rich laminae record episodic seaward progradation of beach surfaces over coarse inner trough settings seaward a parallel laminated fine sand facies dominates the upper part of the prism recording intermittent shoal zone buildup including trough filling under high velocity plane bed conditions similarly a bioturbated parallel laminated fine to very fine sand facies makes up the entire prism below the outermost lower ramp sector again indicating buildup under high velocity conditions however accretion of the lower ramp results from major storms that cause trough scour along landward locations and simultaneous displacement of fine sand onto the lower ramp conversely lower ramp erosion typically occurs during less energetic conditions as sediment is slowly returned shoreward causing inner shoreface buildup close spacing of major storms during some years led to net progradation of the shoreface the upper and lower limits of surveyed elevation change ule and lle repeatedly develop similar limit profile shapes over shoreface accretion erosion cycles the ule reflects accretion maxima resulting from beach bar and lower ramp buildup the lle and lower sedimentologic limit lls along the inner 250 to 300 m of the active shoreface are a product of storm trough scour down to a maximum depth of 5 5 m below the lower ramp facies the lle and lls are primarily products of less energetic wave erosion down to 5 m shoreward and 9 m offshore depths the lle closely matches the lls documenting that 90 of the shoreface prism was reworked during the 12 4 year period whereas actual ages for erosional events indicate a potential of 2 to 4 years for complete reworking of the shoreface mass textural distribution indicates net long term transport direction and loci of deposition for different sized material the coarsest material is concentrated at landwardmost locations and well sorted fine to very fine sand at seaward locations medium sand to gravel tends to remain within the trough zone even during extreme storm events the ule and lle also represent the upper and lower limits for profile closure events a location of about 4 5 m depth at 300 to 350 m from shoreline marks the boundary between inner profile and lower ramp associated closure events the boundary between trough associated and lower ramp facies and the lower ramp morphologic break all of which correspond to the juncture between longshore current and shoaling wave dominated zones the lower ramp zone of closure is a zone of seaward decreasing storm transport in which fine to very fine sand is the typical bedload material

hardbottom morphology and relationship to the geologic framework mid atlantic continental shelf abstract high energy sediment starved continental shelves of the mid atlantic region have abundant hardbottoms that extend from the shoreface to the shelf edge because of the thin and irregularly distributed holocene sand sheet shelf morphology is determined mainly by outcropping tertiary and pleistocene stratigraphic units each unit and combination of units produces different hardbottom morphologies that depend upon the geometry and spatial relationships of the units lithology and patterns of stratification and subsequent weathering and erosion hardbottoms vary in surface relief from smooth flat surfaces to scarped surfaces with up to 10 m of relief the morphology ranges from sloping and stepped erosional ramps to vertical and undercut scarps with associated broad rubble ramps hardbottoms associated with each of the different gently dipping tertiary depositional sequences have distinctive morphologies hardbottoms developed on pleistocene units unconformably overlie the tertiary sequences as flat lying marine carbonates or cut into them as channel systems backfilled with fluvial and estuarine sediments initial dissection of hardbottoms produced highly convoluted surfaces that resulted from subaerial weathering stream erosion and karst formation during sea level lowstands during subsequent sea level highstands these rimary morphologies were greatly modified through the interaction of bioerosion and storms understanding continental shelf hardbottoms is critical for interpreting the sedimentology and stratigraphy of the atlantic coastal plain and for reconstructing paleoceanographic conditions for the following reasons 1 they are an extensive part of the stratigraphic record on shelves that are not actively subsiding and have small volumes of terrigenous input with low sediment accumulation rates 2 they are important stages in the formation of major stratigraphic unconformities condensed sections and sequence boundaries 3 they support diverse biological communities that produce primary carbonate sediments and are rapidly degraded and modified by bioerosion and physical processes supplying abundant new sediment to the continental shelf

the halogen geochemistry of the bushveld complex republic of south africa implications for chalcophile element distribution in the lower and critical zones solidification of the interstitial liquid the stratigraphic distribution halogen bearing minerals especially apatite are minor but ubiof s cu and the pge in the critical zone cannot readily be quitous phases throughout the bushveld complex interstitial apatite explained either by precipitation of sulfide as a cotectic phase or as is near end member chlorapatite below the merensky reef lower a function of trapped liquid abundance evidence from potholes and and critical zones and has increasingly fluorian compositions with the pge rich driekop pipe of the bushveld complex imply that increasing structural height above the reef main and upper zones migrating cl rich fluids mobilized the base and precious metal cl f variations in biotite are more limited owing to crystal chemical sulfides we suggest that the distribution of sulfide minerals and controls on halogen substitution but are also consistent with a the chalcophile elements in the lower and critical zones reflects a decrease in the cl f ratio with structural height in the complex general process of vapor refining and chromatographic separation of a detailed section of the upper lower zone to the critical zone is these elements during the evolution and migration of a metalliferous characterized by an upward decrease in sulfide mode from 0 01 cl rich fluid phase 0 1 to trace 0 001 cu tends to correlate with other incompatible elements in most samples whereas the platinum group elements pge can behave independently particularly in the critical zone the decrease in the cl f ratio of apatite in the

isotope and trace element patterns below the merensky reef bushveld complex south africa evidence for fluids abstract mineralisation in the merensky reef of the bushveld complex has recently been attributed to the ingress of fluids possibly via channels beneath depressions in the footwall of the reef referred to as potholes in this study samples from immediately below three potholes and from the same horizon a norite under normal reef have been analysed for oxygen and strontium isotopes chlorine and noble metals to investigate this possibility results from one pothole are interpreted as retaining the primary magmatic oxygen isotope signature with average 18o of plagioclase and orthopyroxene being 7 1 0 1 and 6 6 0 1 respectively and imply a 18 o melt of 6 8 interaction with an early magmatic fluid cannot be detected from the oxygen isotope patterns and therefore the importance of this process cannot be assessed results from this oxygen isotope study of the western lobe of the bushveld complex are practically identical to those obtained for the laterally equivalent horizons in the eastern lobe a second pothole shows more variability with the norite under the pothole reef showing secondary alteration of plagioclase to paragonite and epidote and orthopyroxene to serpentine and chlorite fresh plagioclase from these altered samples is only slightly depleted in 18o compared with normal reef however measured whole rock 18o values are up to 1 lighter than those predicted from mineral modes and suggest that the secondary assemblage contains a component depleted in 18o which could have been the result of exchange with a low 18o fluid this fluid interaction was probably a late stage relatively low temperature event secondary alteration of the footwall was accompanied by introduction of chlorine au contents also appear to be slightly enriched in the footwall under pothole reef

geochemical and mineralogical evidence for the occurrence of at least three distinct magma types in the famous region bulk rock major and trace element variations in selected basalts from the famous area in conjunction with a detailed study of the chemical compositions of phenocryst minerals and associated melt inclusions are used to place constraints on the genetic relationship among the various lava types the distribution of nio in olivine and cr spinel phenocrysts distinguishes the picritic basalts plagioclase phyric basalts and plagioclase pyroxene basalts from the olivine basalts for a given mg mg fe2 atomic ratio of the mineral the nio content of these phenocrysts in the former three basalt types is low relative to that in the phenocrysts in the olivine basalts the zr nb ratio of the lavas similarly distinguishes the olivine basalts from the plagioclase phyric and plagioclase pyroxene basalts and in addition distinguishes the picritic basalts from the other basalt types these differences indicate that the different magma groups could not have been processed through the same magma chamber and preclude any direct inter relationship via open or closed system fractional crystallization

concentration of platinum group elements by magmatic fluids in layered intrusions evidence leading to a model which supports the formation of pt pd reefs in a layered intrusion by the action of magmatic fluids is presented as well as some of the constraints imposed on such a model the stillwater complex is used as an example of this model within the stillwater complex no laterally continuous sulfide enriched cumulates occur above the lowermost parts of the ultramafic series until one reaches the platiniferous j m reef this sulfide poor stratigraphic interval beneath the j m reef is characterized by unusually cl rich apatite although reported sulfide modes and whole rock sulfur contents in the cumulates below the j m reef are approximately an order of magnitude lower than that expected for a sulfide saturated magma sulfide saturation prior to the formation of the j m reef is suggested by relatively high pd and pt concentrations reported in the cumulates below the j m reef as well as by the presence of sulfide as inclusions in silicate and oxide cumulus minerals hence it is suggested that the footwall cumulates to the j m reef were originally sulfide saturated but later lost sulfur to a fluid phase exsolving from inter cumulus liquid sporadic sulfide bearing pegmatoids in some of these rocks are additional evidence for late loss of sulfur these observations as well as the association of the j m reef at the first significant stratigraphic break in the cl cl f ratio of apatite and the relatively high solubility of pt and pd as aqueous cl complexes are consistent with the formation of the j m reef by exsolution of cl and volatile rich fluids from footwall intercumulus liquids which leached and transported ore components both the pge and s upward to the level of the reef using published whole rock platinum group element pge concentrations for the ultramafic and lower banded series the cumulates below the j m reef must have been enriched by a factor of at most 1 24 for pt and 1 75 for pd if these rocks were to be the source for the metals in the j m reef model raleigh fractionation calculations suggest that the cumulates beneath the j m reef could have had the required metal concentrations if during the crystallization of a parent magma containing no more than 15 ppb of either metal the average bulk solid liquid distribution coefficients for pt and pd were no more than 5 these values are consistent with assumption of sulfide saturation prior to the formation of the j m reef during solidification of intercumulus liquid the separation and loss of a cl rich fluid will deplete the residual assemblage of s and pge if 0 1 to 0 5 wt percent fluid containing 5 percent s evolved during solidification of the footwall cumulates the amount of fluid expected to be exsolved by the crystallization of approximately 2 to 10 percent fluid saturated intercumulus liquid the fluid liquid distribution coefficients for pt and pd would need to be almost as large as the sulfide liquid silicate liquid distribution coefficients to remove significant amounts of pt and pd much smaller distribution coefficients are needed if sulfide is completely lost to fluid during degassing the latter possibility is consistent with the low s contents of the footwall cumulates beneath the j m reef numerical models of the degassing of intercumulus liquids suggest that thick cumulus sequences can act as chromatographic columns to separate the pge and s during the degassing of intercumulus silicate liquids the pge being enriched at a sulfide dissolution front as upward migrating sulfide undersaturated fluids resorb cumulus sulfides zones of pge and s enrichment can occur where degassing fronts encounter stratigraphic physical chemical discontinuities such as changes in the fluid saturation temperature of the intercumulus silicate liquids

observations and 3d hydrodynamics based modeling of decadal scale shoreline change along the outer banks north carolina abstract long term decadal scale shoreline change is an important parameter for quantifying the stability of coastal systems the decadal scale coastal change is controlled by processes that occur on short time scales such as storms and long term processes such as prevailing waves the ability to predict decadal scale shoreline change is not well established and the fundamental physical processes controlling this change are not well understood here we investigate the processes that create large scale long term shoreline change along the outer banks of north carolina an uninterrupted 60 km stretch of coastline using both observations and a numerical modeling approach shoreline positions for a 24 yr period were derived from aerial photographs of the outer banks analysis of the shoreline position data showed that although variable the shoreline eroded an average of 1 5 m yr throughout this period the modeling approach uses a three dimensional hydrodynamics based numerical model coupled to a spectral wave model and simulates the full 24 yr time period on a spatial grid running on a short second scale time step to compute the sediment transport patterns the observations and the model results show similar magnitudes o 105 m3 yr and patterns of alongshore sediment fluxes both the observed and the modeled alongshore sediment transport rates have more rapid changes at the north of our section due to continuously curving coastline and possible effects of alongshore variations in shelf bathymetry the southern section with a relatively uniform orientation on the other hand has less rapid transport rate changes alongshore gradients of the modeled sediment fluxes are translated into shoreline change rates that have agreement in some locations but vary in others differences between observations and model results are potentially influenced by geologic framework processes not included in the model both the observations and the model results show higher rates of erosion 1 m yr averaged over the northern half of the section as compared to the southern half where the observed and modeled averaged net shoreline changes are smaller

how does underlying geology affect coastline change an initial modeling investigation 1 conceptual geologic framework models predict that areas of the coast underlain by a weak or very fine grained lithology retreat landward faster than adjacent more resistant or coarser grained segments these apparently commonsense predictions seem inconsistent with observations that sandy coastlines underlain by heterogeneous lithology remain fairly smooth we have developed a numerical model to explore the interaction between shoreface lithology effects and alongshore transport processes on scales greater than years and kilometers shoreline indentations or sections of the coast that are slightly landward of adjacent sections tend to form in the model where the shoreface weathers more rapidly or is composed of finer grained material however alongshore transport tends to fill in these indentations slowing the weathering rates there while increasing them in adjacent areas where mobile sediment is preferentially removed over the long term this leads to an alongshore uniform retreat rate subtle shoreline undulations reflecting the heterogeneous geologic framework exist in the model but their amplitude reaches a steady state rather than increasing with time compositional heterogeneities determine the morphology of the coast while weathering rate heterogeneities control the thickness of sediment on the shoreface thus indentations can form in sections composed of harder less erodible rocks if they are finer grained

petrologic evolution of partially molten cumulate the atok section of the bushveld complex the postcumulus evolution of a portion of the bushveld complex that includes the merensky reef is inferred from the study of a continuous 56 m drill core the core penetrated the basal orthopyroxenites of the merensky and bastard units the massive anorthosites overlying the two pyroxenites and about 10 m of norite underlying the merensky pyroxenite detailed profiles of major minor and rare earth element ree contents of clinopyroxene and orthopyroxene were determined by electron and ion probe good correlations exist between textural and lithological variations and the ree contents of the pyroxenes specifically enrichments in pyroxene ree abundances are observed in the basal pyroxenites of the merensky and bastard units relative to the underlying and overlying rocks in the pyroxenites the nd content of clinopyroxene is typically over 12 ppm and reaches nearly 40 ppm 90 chondrite and nd yb ratio is in the range 8 to 25 these extreme enrichments in ree are not accompanied by large variations in major element contents computations of the compaction parameters relevant to the conditions of crystallization of the bushveld complex combined with a consideration of cooling history confirm the importance of compaction as a post cumulus process this analysis indicates that the geochemical variability is a result of redistribution of interstitial melt driven by compaction and cannot reflect variations in the initial porosity of the accumulating crystal pile a model for the development of the atok section is developed the merensky anorthosite is interpreted to have served as a barrier to the upward porous flow of late stage hydrous and incompatible element enriched melt which was thus trapped in the underlying merensky pyroxenite the flow was driven by compaction of the 350 meter thick section of predominantly norite beneath the anorthosite the introduction and accumulation of this melt in the pyroxenite and subsequent cooling resulted in partial dissolution recrystallization and ree enrichment of the rock forming minerals and in the formation of the main lithologic features of the merensky pyroxenite further upward percolation of the interstitial melt through the merensky anorthosite was restricted to channels due to the relatively impermeable nature of the cemented anorthosite this melt accumulated in and metasomatized the bastard pyroxenite in the same manner as the merensky pyroxenite having again been trapped by the overlying bastard anorthosite

coastline responses to changing storm patterns 1 researchers and coastal managers are pondering how accelerated sea level rise and possibly intensified storms will affect shorelines these issues are most often investigated in a cross shore profile framework fostering the implicit assumption that coastline responses will be approximately uniform in the alongshore direction however experiments with a recently developed numerical model of coastline change on a large spatial domain suggest that the shoreline responses to climate change could be highly variable as storm and wave climates change large scale coastline shapes are likely to shift causing areas of greatly accelerated coastal erosion to alternate with areas of considerable shoreline accretion on complex shaped coastlines including cuspate cape and spit coastlines the alongshore variation in shoreline retreat rates could be an order of magnitude higher than the baseline retreat rate expected from sea level rise alone

structure of a horizontal sediment laden momentum jet abstractpartially treated wastewater discharge from marine outfall diffusers often contains inorganic and organic solids that may settle close to the source and give rise to sludge banks that affect benthic ecology an experimental and theoretical investigation of the structure and settling mechanisms of a horizontal sediment momentum jet in stagnant ambient has been carried out the cross sectional particle concentration distribution of a horizontal sediment jet is measured for the first time bottom deposition measurements are made for a total of 41 experiments of momentum jets laden with sand particles or synthetic spherical glass particles the sediment jet can be characterised by a sedimentation length scale lm defined by the jet momentum flux and particle settling velocity the measured longitudinal sediment deposition rate can be described by a semiempirical log normal distribution based on lm in the instantaneous particle distribution the sediment fallout is concentrated in a central region and

sediment rich meltwater plumes and ice proximal fans at the margins of modern and ancient tidewater glaciers observations and modelling turbid meltwater plumes and ice proximal fans occur where subglacial streams reach the grounded marine margins of modern and ancient tidewater glaciers however the spacing and temporal stability of these subglacial channels is poorly understood this has significant implications for understanding the geometry and distribution of quaternary and ancient ice proximal fans that can form important aquifers and hydrocarbon reservoirs remote sensing and numerical modelling techniques are applied to the 200 km long marine margin of a svalbard ice cap austfonna to quantify turbid meltwater plume distribution and predict its temporal stability results are combined with observations from geophysical data close to the modern ice front to refine existing depositional models for ice proximal fans plumes are spaced ca 3 km apart and their distribution along the ice front is stable over decades numerical modelling also predicts the drainage pattern and meltwater discharge beneath the ice cap modelled water routing patterns are in reasonable agreement with satellite mapped plume locations however glacial retreat of several kilometres over the past 40 years has limited build up of significant ice proximal fans a single fan and moraine ridge is noted from marine geophysical surveys closer to the ice front there are smaller recessional moraines and polygonal sediment lobes but no identifiable fans schematic models of ice proximal deposits represent varying glacier terminus stability i stable terminus where meltwater sedimentation produces an ice proximal fan ii quasi stable terminus where glacier readvance pushes or thrusts up ice proximal deposits into a morainal bank and iii retreating terminus with short still stands allowing only small sediment lobes to build up at melt stream portals these modern investigations are complemented with outcrop and subsurface observations and numerical modelling of an ancient ordovician glacial system thick turbidite successions and large fans in the late ordovician suggest either high magnitude events or sustained high discharge consistent with a relatively mild palaeo glacial setting for the former north african ice sheet

the upper critical zone of the bushveld complex and the origin of merensky type ores the mg number mg mg fe atomic ratio of bronzite in each unit at the top of the critical zone of the bushveld complex decreases upward in a manner consistent with fractional crystallization the base of each unit is marked by an increase in the mg number suggesting that crystallization of fresh magma is responsible for the initiation of the unit bulk trace and major element concentrations in pegmatoids of the merensky reef which itself forms the basal part of one of a series of these cyclic units are similar to those in medium grained feldspathic bronzitite the base of each unit is also marked by pockets or a layer of sulfide with a higher ni cu and much higher pt cu ratio than the sulfides of the remainder of the unit thus the pt tenor alternates from high to low three times upward through the three units these data indicate strongly that the pt tenor was established with the stratigraphy of the rocks and are inconsistent with models calling upon upward migration of hydrothermal or magmatic fluids to enrich the sulfides in platinum group elements there are no features in the rocks to indicate that fluids could have been channeled to flow laterally for the tens to hundreds of kilometers occupied by continuous merensky reef a model based on that of campbell et al 1983 in which small amounts of sulfides form during injection of fresh magma as a turbulent plume and then settle after becoming thoroughly mixed with a large mass of magma accounts well for all of our observations and many other features of the upper part of the critical zone

sedimentation from turbulent jets and plumes theoretical models are developed for the sedimentation from the margins of a particle laden axisymmetric turbulent buoyant plume in a still environment and for an axisymmetric turbulent momentum jet the models assume that the mass of each individual size fraction of sediment carried in a parcel of fluid decreases exponentially with time for relatively coarse particles the fallout models predict that the sediment deposition beyond a distance r on the ground expressed in log units should decay linearly with distance away from the vent for the momentum jet and should decrease with r1 3 for the buoyant plume the exponential decay constant j is proportional to the terminal fall velocity vt of the particles in both cases and inversely proportional to the square root of the initial momentum flux m0 for the jet fallout jj vtmo 1 2 and to the third power of the initial buoyancy flux fo for the plume fallout jp vtfo 1 3 smaller particles are affected by reentrainment caused by the turbulent eddies sweeping ambient fluid back into the plume or jet and thus reincorporating some particles that were released from the flow at greater heights this is taken into account by introducing a reentrainment coefficient into the theoretical models with the assumption that the coefficient has a constant value for a plume of given strength in new experiments fallout occurs from the margins of particle laden fresh water buoyant jets and plumes in a tank of salty water and sedimentation is measured on the tank floor two experiments were weakly affected by reentrainment and show excellent agreement with the simple theory for smaller particles and increasingly buoyant plumes and strong jets particle reentrainment is important the experimental data are fitted by the new reentrainment theory confirming that values of the reentrainment coefficient are approximately constant for a given flow a settling number is defined as the ratio of the characteristic velocity of the jet or plume to the particle settling velocity for 1 reentrainment seems to reach an equilibrium state for which the reentrainment coefficient is a constant of value 0 1 for jets and 0 4 for plumes irrespective of flow strength or particle size the plume experiments indicate that the value of the reentrainment coefficient is strongly dependent on plume strength and particle size for slightly less than 1 the general principles of sedimentation from turbulent plumes and jets are applied to the fallout of pumice from volcanic eruption columns and of metalliferous particles from black smokers on the ocean floor for volcanic eruptions the results provide an explanation for the near vent overthickening of tephra fall deposits and imply that lithic and pumice fragments from small lapilli up to at least 1 m diameter blocks are efficiently reentrained into eruption columns the size of particles reentrained in hydrothermal plumes is predicted to vary from less than 100 m in weakly buoyant plumes up to over 1000 m in megaplumes

process signatures in regional patterns of shoreline change on annual to decadal time scales gradients in wave driven alongshore sediment transport influence the morphologies of sediment covered coastlines on a range of spatial and temporal scales affecting accretion and erosion patterns relevant to human development recent theoretical findings predict that a correlation between shoreline change and shoreline curvature results from patterns of alongshore sediment flux the sign positive or negative of that correlation depends on whether high or low angle waves dominated the wave climate using lidar surveys of the northern north carolina coast from 1996 2005 to document shoreline change and quantify alongshore patterns of erosion and deposition we isolate these signals diagnostic of alongshore transport processes our analyses show a persistent significant negative correlation between shoreline position change and shoreline curvature consistent with a low angle dominated incident wave climate over the last decade at large spatial scales convex seaward promontories have eroded landward while concave seaward bays have aggraded seaward resulting in an apparent diffusion of alongshore morphological features

reversing storm hotspots on sandy beaches spatial and temporal characteristics coastal erosion hotspots are defined as sections of coast that exhibit significantly higher rates of erosion than adjacent areas this paper describes the spatial and temporal characteristics of a recently identified type of coastal erosion hotspot which forms in response to storms on uninterrupted sandy coasts largely free from human intervention these are referred to here as reversing storm hotspots because the erosion is reversed by accretion of a similar magnitude to the storm induced erosion the accretion occurs within a few days or weeks of fair weather after the storm reversing storm hotspots observed here on two us east coast beaches have a longshore length averaging 3 86 km a cross shore excursion magnitude of erosion or accretion averaging 15 4 m and a time scale of days to weeks associated with individual storm events these spatial and temporal scales clearly distinguish reversing storm hotspots from previously described forms of longshore variability in erosion including those attributed to several types of shoreline undulations and hotspots associated with long term shoreline change

deposition from particle laden round turbulent horizontal buoyant jets in stationary and coflowing receiving fluids results are presented from a series of laboratory experiments investigating the characteristic features of particle laden round turbulent buoyant jets discharged horizontally into stationary and coflowing receiving fluids for the volumetric source concentrations of particles tested 0 1 the presence of the particle load was found to have no significant influence on mean buoyant jet trajectories deposition patterns on the bed of the receiving water container indicated the existence of two separate sedimentation processes for discharges into stationary or coflowing ambients namely 1 a relatively concentrated narrow band of particle accumulation associated with near source fallout from the buoyant jet margins and 2 a broader and more disperse downstream depositional fan associated with particle fallout from the radially expanding surface gravity current formed by the impingement of the buoyant jet with the free surface of the receiving fluid scaling arguments have been developed and applie

modern sedimentation on the shoreface and inner continental shelf at wrightsville beach north carolina u s a abstract the geologic framework and surficial morphology of the shoreface and inner continental shelf off the wrightsville beach north carolina barrier island were mapped using high resolution sidescan sonar bathymetric and seismic reflection surveying techniques a suite of over 200 diver vibracores and extensive seafloor observations by divers the inner shelf is a sediment starved active surface of marine erosion modern sediments where present form a patchy veneer over tertiary and quaternary units the lithology of the underlying units exerts a primary control on the distribution texture and composition of surficial sediments as well as inner shelf bathymetry the shoreface is dominated by a linear cross shore morphology of rippled scour depressions rsds extending from just seaward of the surf zone onto the inner shelf on the upper shoreface the rsds are incised up to 1 m below surrounding areas of fine sand and have an asymmetric cross section that is steeper sided to the north on the inner shelf the rsds have a similar but more subdued cross sectional profile the depressions are floored primarily by shell hash and quartz gravel vibracore data show a thick up to 1 5 m sequence of rsd sediments that unconformably overlies ancient coastal lithosomes in this sediment starved inner shelf setting rippled scour depressions probably form initially on preexisting coarse sediment substrates such as modern lag deposits of paleofluvial channel lithosomes or ancient tidal inlet thalwegs interannual observations of seafloor morphologic change and the longer term record contained in vibracores suggest that the present seafloor morphology is either relatively stable or represents a recurring preferential morphologic state to which the seafloor returns after storm induced perturbations the apparent stability is interpreted to be the result of interactions at several scales that contribute to a repeating self reinforcing pattern of forcing and sedimentary response which ultimately causes the rsds to be maintained as sediment starved bedforms responding to both along shore and across shore flows sediment accumulation from over 30 years of extensive beach nourishment at wrightsville beach appears to have exceeded the local shoreface accommodation space resulting in the leaking of beach and shoreface sediment to the inner shelf a macroscopically identifiable beach nourishment sediment on the shoreface and inner shelf was used to identify the decadal scale pattern of sediment dispersal the nourishment sediment is present in a seaward thinning wedge that extends from the beach over a kilometer onto the inner shelf to waters depths of 14 m this wedge is best developed offshore of the shoreline segment that has received the greatest volume of beach nourishment

regional trends in the platinum group mineralogy of the critical zone of the bushveld complex south africa a wealth of quantitative data on the types volume distribution and mode of occurrence of the platinum group minerals residing in the merensky reef the ug 2 chromitite layer and platreef has been obtained over a twelve year period with the result that general platinum group mineral types for unexplored areas can perhaps now be predicted the platinum group minerals of the ug 2 and overlying merensky layers although locally different in composition mode of occurrence and size show close ties when viewed on a regional scale especially with regard to their pt pd sulfide and pt fe alloy contents thus if in one particular area the merensky reef contains pt pd sulfides as the predominant platinum group minerals then it can be safely inferred that pt pd sulfides will be a significant component of the ug 2 layer in that area the occurrence of pt fe alloy also follows this principle the distribution of pt fe alloy is in fact a major factor in differentiating the ores in the vicinity north and south of the pilansberg alkaline complex from those of the same layers farther along the arcuate strike northeastern and southeastern horns of the western bushveld and has led to an overall concept relating platinum group mineral types to postulated centers of increased volatile activity or perhaps feeder sites of the complex the mineralogy of reef disturbances such as potholes is discussed the mode of occurrence of the platinum group minerals varies between base metal sulfide silicate and chromite associations and the size of platinum group minerals in the various reefs is given solid solution or lattice substitutions of platinum group elements in the base metal sulfides account for the very important latent fraction of the total platinum group elements as distinct from platinum group minerals of the bushveld complex rhodium in the merensky reef occurs almost wholly in solid solution within pentlandite whereas rh minerals occur in the ug 2 layer chromite tends to host microscopic to submicroscopic inclusions of laurite and does not appear to take ru into the crystal lattice the probability of solid solution of platinum group elements in silicate and other oxide phases is discussed

geologic control in the nearshore shore oblique sandbars and shoreline erosional hotspots mid atlantic bight usa abstract beach and nearshore morphology defined primarily by slope and sandbar development is very dynamic and is largely controlled by waves currents and regional sediment characteristics results presented here challenge this long established concept and suggest that underlying framework geology may also exert a first order control on nearshore morphology by influencing the stability and or persistent re establishment of large scale sandbar morphology and position as well as surface sediment characteristics repeated sub bottom chirp and swath bathymetry surveys of the nearshore 2 10 m depths covering over 56 km of the north carolina outer banks and southeastern virginia indicate the following 1 development of shore oblique sandbars adjacent to large gravel outcrops that are surface exposures of the underlying geologic strata 2 identical re development or sustained maintenance of large scale sandbar morphology and position before and after very energetic conditions 3 vertical and horizontal heterogeneity of lithology and grain size and a minimum volume of sand ranging from 0 to 1 5 m thick and 4 close spatial alignment between the location of outcrops shore oblique bars and shoreline erosional hotspots a hypothesis is proposed from these findings that links framework geology to bar morphodynamics and sorted bedforms and ultimately erosional hotspots sediment transport and shoreline evolution models based solely on waves and currents and an assumption of unlimited and uniform sediment may be inadequate in similar heterogeneous sand limited regions

modeling glacial meltwater plume dynamics and sedimentation in high latitude fjords 1 a model sedplume has been developed to simulate marine sediment deposited by glacial meltwater plumes emerging from tidewater glaciers turbid meltwater emerging from beneath a glacier into a fjord rises as a buoyant forced plume due to density contrasts with the ambient fjord water sedplume assumes that meltwater discharge flows at a constant rate for long enough periods that the plume reaches a steady state entrainment of ambient fluid into the turbulent plume is assumed to occur at a rate proportional to the local velocity of the plume plume motion is considered in two dimensions one horizontal dimension perpendicular to the glacier front and the vertical dimension an integral model is formulated for the conservation equations of volume momentum buoyancy and sediment flux along the path of a turbulent plume injected into stably stratified ambient fluid sedimentation occurs from the plume when the radial component of the sediment fall velocity exceeds the entrainment velocity when the plume reaches the surface it is treated as a radially spreading surface gravity current for which exact solutions exist for the sediment deposition rate flocculation of silt and clay particles is modeled using empirical measurements of particle settling velocities in fjords to adjust the settling velocity of fine grained sediments sedplume has been applied to mcbride inlet alaska a temperate glaciated fjord where the majority of sedimentation originates from meltwater sources sedplume produces rates and patterns of sedimentation in good agreement with observations with calculated peak ice proximal annual sedimentation rates of approximately 22 m yr 1

calculating shoreline erosion potential using nearshore stratigraphy and sediment volume outer banks north carolina 1 despite the acknowledged influence of coastal geological framework on the behavior of beaches and barrier islands and a wealth of geological and bathymetric observations from the inner shelf quantitatively connecting those observations to shoreline behavior has been difficult nearshore geologic and morphologic variability described by recent research is not well represented by conventional geologic parameters such as mean grain size and shoreface slope used in most shoreline change models we propose that total nearshore sediment volume as calculated to a continuous seismic reflection surface provides a flexible and robust metric for use in the prediction of shoreline change this method of determining the volume of sediment in the nearshore accounts for three dimensional sandbar morphologies and heterogeneous seafloor sediments the decadal scale shoreline change rate for northeastern north carolina is significantly correlated to the volume of sediment in the nearshore when a geologically defined base is used in volume determinations suggesting that the shallow stratigraphic framework of transgressive coasts is an important influence on decadal shoreline behavior nearshore sediment volume was overestimated when an arbitrary depth constant baseline was used and was not correlated to decadal shoreline change this implies that a volume metric which accounts for both framework geology and variable seafloor morphology better represents the geologic character of the shoreface and may help to improve existing models of shoreline change an empirical model of regional shoreline erosion potential demonstrates the importance of incorporating nearshore sediment volume shallow framework geology and surface morphology when predicting seasonal to decadal shoreline evolution

particle recycling in the tag hydrothermal plume abstract analysis of th isotope distributions in the tag hydrothermal plume yields evidence for pronounced 234 th 230 th isotope fractionation this fractionation can be attributed to a combination of four processes 1 particle ageing 2 differential scavenging of 234 th and 230 th 3 differential focusing of 234 th and 230 th and 4 recycling of old particles we present arguments for the importance of the last of these processes particle recycling particles in the tag hydrothermal plume exhibit grain sizes which fall in the range 1 10 m applying the range of densities reported for amorphous iron oxyhydroxide and iron oxide mineral phases stokes law predicts that these particles should achieve terminal settling velocities ranging between 4 and 8 m d 1 for coarse grained 5 m radius particles and 0 2 0 3 m d 1 for fine grained particles 1 m radius these velocities have been used in a physical model of re entrainment and recycling in neutrally buoyant plumes our results indicate that particle recycling and re entrainment in hydrothermal plumes may be important over length scales of 1 10 km the model predicts that the proportion of particles re entrained is independent of particle size but increases with plume strength for plumes with volumetric fluxes greater than 10 3 m 3 s 1 the proportion of particles re entrained exceeds 50 in agreement with previous calculations based upon 230 th distributions sampling strategies in the past have typically concentrated upon processes operating within 1 km of active vent fields and will need to be revised accordingly in the future

influence of inherited geologic framework on barrier shoreface morphology and dynamics abstract passive margin coastlines with limited sand supplies such as much of the u s atlantic margin are significantly influenced by the geologic framework of older stratigraphic units that occur beneath and seaward of the shoreface many u s east coast barrier islands are perched barriers in which the underlying pre modern sediments determine the morphology of the shoreface and strongly influence modern beach dynamics and composition perched barriers consist of varible layers of beach sand on top of older eroding stratigraphic units with highly variable compositions and geometries along many parts of the coastal system stratigraphically controlled bathymetric features on the inner shelf modify waves and currents and thereby effect patterns of sediment erosion transport and deposition on the adjacent shoreface it is essential to understand this geologic framework before attempting to model the large scale behavior of these types of coastal systems in north carolina most shoreline features are controlled by the pre holocene stratigraphic framework of the shoreface the beaches are perched on top of pre existing pleistocene tertiary and cretaceous sediments the surficial geology of the coastal zone is subdivided into two distinct provinces resulting in different stratigraphic controls of the shoreface north of cape lookout the geological framework consists of a quaternary sequence that fills a regional depositional basin called the albemarle embayment the coastal zone south of cape lookout is dominated by tertiary and cretaceous units that crop out across the coastal plain and continental shelf with very thin quaternary units only locally preserved superimposed upon this regional stratigraphy is an ancient drainage system resulting in a series of fluvial valleys filled with younger coastal sediments separated by large interfluve areas of older stratigraphic units this results in a coastal system in which the shoreface is either nonheadland or headland dominated respectively headland dominated shorefaces are further divided into subaerial and submarine categories nonheadland dominated shorefaces are further divided into those influenced primarily by transgressive or regressive processes or channel dominated depositional processes i e inlet migration or stream valley fill examples of each of these six types of shorefaces are presented to demonstrate the control that the geologic framework exhibits on shoreface morphologies and processes

sorted bed forms as self organized patterns 2 complex forcing scenarios 1 we employ a numerical model to study the development of sorted bed forms under a variety of hydrodynamic and sedimentary conditions results indicate that increased variability in wave height decreases the growth rate of the features and can potentially give rise to complicated a priori unpredictable behavior this happens because the system responds to a change in wave characteristics by attempting to self organize into a patterned seabed of different geometry and spacing the new wavelength might not have enough time to emerge before a new change in wave characteristics occurs leading to less regular seabed configurations the new seabed configuration is also highly dependent on the preexisting morphology which further limits the possibility of predicting future behavior for the same reasons variability in the mean current magnitude and direction slows down the growth of features and causes patterns to develop that differ from classical sorted bed forms spatial variability in grain size distribution and different types of net sediment aggradation degradation can also result in the development of sorted bed forms characterized by a less regular shape numerical simulations qualitatively agree with observed geometry spacing and height of sorted bed forms also in agreement with observations is that at shallower depths sorted bed forms are more likely to be affected by changes in the forcing conditions which might also explain why in shallow waters sorted bed forms are described as ephemeral features finally simulations indicate that the different sorted bed form shapes and patterns observed in the field might not necessarily be related to diverse physical mechanisms instead variations in sorted bed form characteristics may result from variations in local hydrodynamic and or sedimentary conditions

shape and distribution analysis of merensky reef potholing northam platinum mine western bushveld complex implications for pothole formation and growth syn magmatic removal of the cumulate pile during the formation of the bushveld complex resulted in potholes erosion progressed downward in the cumulate pile resulting in a series of steep transgressive contacts between locally conformable potholed reefs in the regional pothole sub facies of the swartklip facies in the western limb of the bushveld complex the deepest of these potholes third order or fwp2 potholing occurs where the base of the merensky cyclic unit transgresses the upper pseudo reef chromitite marker horizon the base of a fwp2 pothole on northam platinum mine consists of an unconformable stringer merensky chromitite overlain by a medium grained poikilitic orthopyroxenite and underlain by either a pegmatitic harzburgite or the medium grained lower pseudo reef anorthosite detailed shape and distribution analysis of fwp2 potholes reveals underlying patterns in their shape and distribution which in turn suggest a structural control the ratio between pothole short vs long axes is 0 624 n 1 385 although the ratio increases from 0 48 to 0 61 in the long axis range 10 to 60 m then decreases from 0 61 to 0 57 from 61 to 100 m increasing again from 0 57 to 0 61 from 101 to 400 m suggesting that there is not a simple relationship between pothole shape and size shape circularity eccentricity and dendricity analysis of a subset of 638 potholes indicates that potholes with long axes 100 m have an average normalized shape that is bilobate and elongate on a 120 orientation the average aspect ratio short axis length divided by long axis length of potholes is highest for potholes with long axis lengths 100 m and lowest for potholes with long axis lengths between 35 and 60 m the most common long axis orientation for potholes with long axis lengths 100 m fractal analysis indicates that the distribution of pothole centers is controlled neither by a single nor several interacting fractal dimensions autocorrelation fry analysis of the distribution of pothole centers shows recurring pothole distribution trends at 038 070 and 110 for potholes over the full range of long axis lengths while the trends of 008 and 152 occur in potholes with long axes lengths between 60 and 100 m chi squared x2 analysis of the locations of pothole centers suggests that the distribution of small potholes is highly non uniform but becomes exponentially more uniform with increasing pothole size the model which best fits the observed shape and distribution analysis is a combination of protracted independent growth and nearest neighbor merging along specific orientations for instance the clustered distribution of original pothole centers resulted in merged potholes with long axes lengths of up to 60 m exhibiting short vs long axes ratios of 0 61 preferred orientations of 150 and alignment along 010 and 150 trends further independent growth allowed for merging of similar sized and smaller neighboring potholes generating potholes with long axes of up to 100 m in length a preferred long axis orientation of 150 and alignment along 010 040 075 and 150 subsequent preferential merging occurred along a 120 trend thereby preserving a bilobate form this implies that while pothole initiation and enlargement may be driven by a top down i e possibly thermomechanical process an underlying linear or structural catalyst control is revealed in changes in pothole shape during enlargement and furthermore in the preferred trends along which potholes merged over a considerable period possibly concomitant with adjustment of major structures in the footwall to the bushveld complex and pulses into the magma chamber

the petrogenesis of merensky reef potholes at the western platinum mine bushveld complex sr isotopic evidence for synmagmatic deformation potholes represent areas where the normally planar pge rich merensky reef of the upper critical zone of the bushveld complex transgresses its footwall such geometric relationships being unusual in layered intrusions the recognition of vertical dykes of merensky pyroxenite in the footwall suggests downward collapse of crystal mush into pull apart sites resulting from tensional deformation due to the loading effects of major new magma additions in contrast crosscutting anorthosite veins display physical and isotopic evidence of upward emplacement the merensky reef and its footwall have distinct initial sr isotope ratios r 0 0 7066 and 0 7066 respectively which may be used to constrain these processes related to pothole formation merensky reef in potholes r 0 0 7069 0 7078 shows no isotopic evidence of assimilation of or reaction with footwall material discrete discordant replacement bodies of anorthosite extend from the footwall lithologies to cross cut the merensky reef and its hanging wall the initial sr isotope ratio in these replaced rocks is totally reset to footwall values r 0 0 7066 and immediately adjacent stratiform lithologies are slightly modified towards footwall values in contrast neptunian pyroxenitic merensky dykes cross cutting the footwall lithologies with a large surface area to volume ratio and low sr content do not display footwall like sr isotope initial ratios r 0 0 7077 and thus show no evidence for assimilation of or reaction with footwall material furthermore pegmatoidal replacement pyroxenite replacement pegmatoid at the base of the merensky reef within potholes has a high initial ratio r 0 0 7071 and so models of pervasive metasomatism by footwall material are not applicable this isotopic evidence indicates that there was no active interaction of footwall material with the overlying magma during or after the formation of merensky reef potholes a basic tenet of existing pothole formation hypotheses involving footwall mass transfer in contrast the isotopic data are entirely consistent with an extensional model for pothole formation with the more radiogenic merensky magma migrating laterally to fill extensional zones in the footwall layers

linking framework geology and nearshore morphology correlation of paleo channels with shore oblique sandbars and gravel outcrops abstract this work describes a newly discovered spatial correlation between paleo channels and nearshore morphology along the barrier islands of northeastern north carolina and southeastern virginia u s mid atlantic coast these nearshore morphological features include regions of shore oblique sandbars and coincident gravel outcrops that previously have been linked to erosional hotspots at the shoreline these new findings have direct bearing on both nearshore morphodynamics and shoreline management paleo channels were identified in the chirp seismic record and compared to shore oblique sandbar locations discerned from bathymetric data and corroborated by gravel outcrops depicted in the side scan record both graphical correlation and statistical cross correlation analyses revealed a significant spatial relationship between these features with higher statistical correlation for channels 500 m wide seismic records also indicate that the gravel outcrops seen on the updrift flanks of the shore oblique bars are surface expressions of the underlying geology we hypothesize that these outcrops are composed of relict channel fill sediments that interact with the hydrodynamic regime to produce sorted bedforms the bathymetric anomalies then serve to alter incident wave energy and affect shoreline behavior the spatial and temporal stability of these features over several years and through a variety of wave conditions suggests some degree of underlying geologic control while the mechanisms responsible for this relationship remain speculative these results may bridge the gap between studies focusing on framework geology and its influence on shoreline change and those that investigate bar morphodynamics and the initiation of sorted bedforms

sedimentation from buoyant jets an integral model is developed to describe sedimentation from a turbulent buoyant jet injected at an angle into a stationary uniform ambient fluid entrainment is modeled using the standard entrainment assumption and sediment is assumed to fall from the jet where the outward component of the fall velocity normal to the jet boundary exceeds the inward entrainment velocity when appropriately scaled by source momentum and buoyancy fluxes turbulent buoyant jets may be described in terms of a single parameter the angle 0 between the flow and the horizontal at the virtual origin which is close to the actual source for large initial densimetric froude numbers including sedimentation in the model introduces a further parameter ws which is the ratio of the fall speed of the sedimenting particles to a typical entrainment velocity and so ws is a nondimensional fall speed an important result is that this ratio is independent of the source speed so that the proportion of the sediment load deposited nea

the merensky cyclic unit and its impact on footwall cumulates below normal and regional pothole reef types in the western bushveld complex the merensky reef of the bushveld complex occurs in its highest stratigraphic position as a heterogeneous pegmatitic feldspathic melanorite bounded by two narrow chromitite stringers at the base of the merensky cyclic unit mcu in the swartklip facies of the rustenburg layered suite the occurrence of widespread thermal and mechanical erosion termed potholing has led to the subdivision of the merensky reef into normal reef and regional pothole reef sub facies the transition between the two sub facies occurs where the mcu transgresses the lower chromitite stringer of the normal merensky reef and cuts down into the underlying cumulate lithologies in the regional pothole reef at the northam platinum mine several economic reef types are identified where the merensky reef becomes conformable to cumulate layering in particular to the footwall marker np2 reef type and the upper pseudoreef p2 reef type the normal merensky reef as well as the p2 and np2 reefs contains economic platinum group element pge grades and includes the lower portion of the mcu melanorite and the merensky chromitite whole rock geochemistry indicates that this package is compositionally identical in normal p2 and np2 reefs suggesting that the base of the mcu is a relatively homogeneous drape over both normal and regional pothole reef regions however the lower sections of the three reefs are variables depending on the depth of transgression of the mcu in the normal and p2 reef types transgression by the mcu was arrested within harzburgites melanorites and norites resulting in coarse pegmatitic textures in the immediate footwall units for the np2 reef transgression by the mcu was arrested within leucocratic rocks and resulted in the formation of troctolites below the merensky chromitite these troctolites are characterised by a coupled relationship between olivine and sulphides and by changes in major element chemistry and pge contents relative to equivalent units in the footwall of the normal reef along with micro textural relationships these features suggest that troctolization of leucocratic cumulates in the np2 reef beneath the merensky chromitite was a result of a reactive infiltration of a chromite saturated melt and an immiscible sulphide liquid from the overlying mcu rather than a significant fluid flux from below in all reef types the concentration of s defines symmetrical peaks centred on the merensky chromitite and chromitites from pre existing cyclic units in normal and p2 reefs whereas pge concentrations define asymmetrical peaks with higher pge contents in reconstituted footwall rocks relative to the mcu melanorite this signature is attributable to a magmatic model of pge collection followed by deposition towards the base of the mcu and within reconstituted footwall rocks the continuity of the asymmetrical magmatic pge signature between the normal reef and regional pothole reef sub facies indicates that pge mineralization inherent to the merensky magma occurred as a drape over a variably eroded and subsequent texturally and geochemically reworked or reconstituted footwall

noble metal enrichment processes in the merensky reef bushveld complex we have analysed sulphides silicates and chromites of the merensky introduction reef for platinum group elements pges re and au using laser the enrichment of noble metals in layered gabbroic ablation inductively coupled plasma mass spectrometry and synthetic intrusions to form stratiform merensky type ore horizons pyrrhotite standards annealed with known quantities of noble metals is an extremely efficient yet poorly understood process os ir and ru reside in solid solution in pyrrhotite and pentlandite metal enrichment must be related to the formation of rh and part of the reef s pd in pentlandite whereas pt au re magmatic layering as the ore horizons form an integral and some pd form discrete phases olivine and chromite often part of the magmatic stratigraphies of their host insuspected to carry os ir and ru are pge free all phases analysed trusions metal concentration is also related to the prescontain noble metals as discrete micro inclusions with diameters ence of magmatic sulphides because the noble metals are either dissolved in base metal sulphides or occur as typically 100 nm inclusions in sulphides commonly have the discrete noble mineral phases intergrown with sulphides element combinations os ir pt and pt pd au inclusions in vermaak hendriks 1976 ballhaus ryan 1995 olivine and chromite are dominated by pt au pd few inclusion ballhaus ulmer 1995 spectra can be related to discrete noble metal phases and few there is considerable controversy as to where the inclusions have formed by sub solidus exsolution rather some metals came from and how metal enrichment worked pge inclusions notably those in olivine and chromite are earlyfor a stratiform platinum group element pge deposit magmatic nuggets trapped when their host phases crystallized we inside the magmatic stratigraphy of a layered intrusion suggest that the silicate melt layer that preceded the merensky reef there are two possibilities either the metals were derived was pge oversaturated at early cumulus times experiments from the melt column above the ore horizon or they came combined with available sulphide silicate partition coefficients sugfrom the magma equivalent to the cumulate package gest that a silicate melt in equilibrium with a sulphide melt underlying the ore owing to the large density contrast containing the pge spectrum of the merensky ore would indeed be between silicate and sulphide melt a popular view is that oversaturated with respect to the least soluble noble metals sulphide the metals were enriched by gravitational settling of melt apparently played little role in enriching the noble metals in sulphide melt i e out of the magma column above the the merensky reef rather its role was to immobilize a pre existing ore campbell et al 1983 barnes naldrett 1985 in situ stratiform pge anomaly in the liquid stratified magma naldrett et al 1987 on the other hand it is well chamber established that gravitational crystal settling plays little role in producing magmatic layering and magmatic stratigraphies mcbirney noyes 1979 morse 1986 campbell 1987 and this cannot remain without consequence for stratiform noble metal deposits a third alternative is noble metal enrichment by late

cumulative versus transient shoreline change dependencies on temporal and spatial scale using shoreline change measurements of two oceanside reaches of the north carolina outer banks usa we explore an existing premise that shoreline change on a sandy coast is a self affine signal wherein patterns of change are scale invariant wavelet analysis confirms that the mean variance spectral power of shoreline change can be approximated by a power law at alongshore scales from tens of meters up to 4 8 km however the possibility of a power law relationship does not necessarily reveal a unifying scale free dominant process and deviations from power law scaling at scales of kilometers to tens of kilometers may suggest further insights into shoreline change processes specifically the maximum of the variance in shoreline change and the scale at which that maximum occurs both increase when shoreline change is measured over longer time scales this suggests a temporal control on the magnitude of change possible at a given spatial scale and by extension that aggregation of shoreline change over time is an important component of large scale shifts in shoreline position we also find a consistent difference in variance magnitude between the two survey reaches at large spatial scales which may be related to differences in oceanographic forcing conditions or may involve hydrodynamic interactions with nearshore geologic bathymetric structures overall the findings suggest that shoreline change at small spatial scales less than kilometers does not represent a peak in the shoreline change signal and that change at larger spatial scales dominates the signal emphasizing the need for studies that target long term large scale shoreline change

sedimentation from gravity currents generated by turbulent plumes sedimentation from radially spreading gravity currents generated at the top of ascending sediment laden plumes is described by a model which assumes that sediment is dispersed homogeneously by turbulence in the gravity current resulting in an exponential decrease in the concentration of sediment with time as particles settle out of the lower boundary of the current for radial spreading this model predicts a gaussian distribution of sediment accumulation away from the source with an exponential constant b which depends on flow rate q and particle settling velocity v b nv q in the experiments described sedimentation occurs from gravity currents generated by ascent of buoyant particle laden plumes of fresh water in a tank of salty water the sediment accumulation shows close agreement with the theoretical model and the gaussian decay constant b can be determined from a maximum in the accumulated mass of sediment per unit distance and from the slope of the line in s s0 br2 where r is the radial distance s is the sediment mass flux per unit area and s0 is the value of s at r 0 data from the dispersal of volcanic ejecta from a large c 24 km high plinian eruption column in the azores also show good agreement with the theory confirming that it is general and independent of scale and the nature of the fluid the experimental data also show a change in sedimentation behaviour at distances from the source corresponding to the corner of the plume where it diverts into a lateral gravity current and there is an abrupt decrease in vertical velocity sedimentation of coarse grain sizes between the source and the corner occurs from the inclined plume margins and does not behave as predicted by the theoretical model

petrogenesis of the merensky reef in the rustenburg section of the bushveld complex a petrogenetic model for the merensky reef in the rustenburg section of the bushveld complex has been developed based on detailed field and petrographic observations and electron microprobe data the model maintains that the reef formed by reaction of hydrous melt and a partially molten cumulate assemblage the model is devised to account for several key observations 1 although the dominant rock type in the rusterburg sections is pegmatoidal feldspathic pyroxenite there is a continous range of reef lithology from pyroxenite to pegmatoidal harzburgite and dunite and small amounts of olivine are present in nearly all pegmatoids 2 the pegmatoid is usually bounded above and below by chromitite seams and the basal chromitite separated from underlying norite by a centimeter thick layer of anorthosite the thicknesses of the two layers exhibit a well defined positive correlation 3 inclusion of pyroxenite identical to the hanging wall and of leuconorite identical to the footwall are present in the pegmatoid the leuconorite inclusions are surrounded by thin anorthosite and chromitite layers in the same sequence as that at the base of the reef 4 chromite in seams adjacent to plagioclase rich rocks is characterized by higher mg mg fe and al r3 and lower cr r3 than that in seams adjacent to pyroxene rich rocks similar variations in mineral compositions are observed across individual chromitite seams where the underlying and overlying rock types differ the chromite compositional variations cannot be rationalized in terms of either fractional crystallization or reequilibration with surrounding silicates it is proposed that the present reef was originally a melt rich horizon in norite immediately overlain by relatively crystallized pyroxenite magmatic vapor generated by crystallization of intercumulus melt migrated upward through fractures in the cumulate pile below the protoreef the melt rich protoreef became hydrated because fractures were unable to propagate through it and because the melt itself was water undersaturated hydration of the intercumulus melt was accompanied by melting and the hydration melting front migrated downward into the footwall and upward into the hanging wall in the footwall melting resulted first in the dissolution of orthopyroxene and then of plagioclase with continued hydration chromite was stabilized as melt alumina content increased the regular variations in chromite compositions reflect the original gradients in melt composition at the hydration front the stratigraphic sequence downward through the base of the reef or pegmatoid melt chromitite anorthosite norite represents the sequence of stable mineral assemblages across the hydration melting front the sequence is shown to be consistent with knowledge gained from experiments on melting of hydrous mafic systems at crustal pressures with cooling the hydrated mixture from partial melting of norite footwall and more mafic hanging wall crystallized in the sequence chromite olivine pyroxene plagioclase with peritectic loss of some olivine calculations of mass balance indicate that a significant proportion of the melt was lost from the melt rich horizon variations in the development of the pegmatoid and associated lithologies and amount of modal olivine in the pegmatoids along the strike of the merensky reef resulted because the processes of hydration melting and melt loss operated to varying extents

nearshore shore oblique bars gravel outcrops and their correlation to shoreline change abstract this study demonstrates the physical concurrence of shore oblique bars and gravel outcrops in the surf zone along the northern outer banks of north carolina these subaqueous features are spatially correlated with shoreline change at a range of temporal and spatial scales previous studies have noted the existence of beach surf zone interactions but in general relationships between nearshore geological features and coastal change are poorly understood these new findings should be considered when exploring coastal zone dynamics and developing predictive engineering models the surf zone and nearshore region of the outer banks is predominantly planar and sandy but there are several discrete regions with shore oblique bars and interspersed gravel outcrops these bar fields have relief up to 3 m are several kilometers wide and were relatively stationary over a 1 5 year survey period however the shoreward component of the bar field does exhibit change during this time frame all gravel outcrops observed in the study region a 40 km longshore length were located adjacent to a shore oblique bar in a trough that had width and length similar to that of the associated bar seismic surveys show that the outcrops are part of a gravel stratum underlying the active surface sand layer cross correlation analyses demonstrate high correlation of monthly and multi decadal shoreline change rates with the adjacent surf zone bathymetry and sediment distribution regionally areas with shore oblique bars and gravel outcrops are correlated with on shore areas of high short term shoreline variability and high long term shoreline change rates the major peaks in long term shoreline erosion are onshore of shore oblique bars but not all areas with high rates of long term shoreline change are associated with shore oblique bars and troughs

micropaleontologic record of late pliocene and quaternary paleoenvironments in the northern albemarle embayment north carolina u s a abstract micropaleontological data provide a strong actualistic basis for detailed interpretations of quaternary paleoenvironmental change the 90 m thick quaternary record of the albemarle embayment in the mid atlantic coastal plain of the usa provides an excellent opportunity to use such an approach in a region where the details of quaternary environmental change are poorly known the foraminiferal record in nine cores from the northern outer banks east of albemarle sound north carolina indicates the deposition of subhorizontal mostly open marine early to late pleistocene units unconformably upon a basement of late pliocene reduced oxygen fine grained shelf basin deposits pollen data record several warm cool fluctuations within the early to mid pleistocene deposits diatom data indicate that some fresh and brackish water units occur within the generally open marine pleistocene succession a channel cut by the paleo roanoke river during the last glacial sea level lowstand occurs in the northern part of the study area pollen indicates that the basal fluvial valley fill accumulated in cooler than modern climate conditions in the latest pleistocene overlying silts and muds accumulated under cool climatic estuarine conditions according to diatom and pollen data radiocarbon ages from the estuarine deposits indicate that the bulk of these sediments accumulated during the latest pleistocene the estuarine channel fill deposits are overlain by holocene open marine sands deposited as the rising sea transgressed into the estuary approximately 8 5 to 9 0 kyr bp within the barrier island drill cores of this study fully marine sedimentation occurred throughout the holocene however immediately west of the present barrier island mid to late holocene estuarine deposits underlie the modern albemarle sound the islands that currently form a continuous barrier across the mouth of albemarle sound have a complex history of holocene construction and destruction and large portions of them may be less than 3 kyr old the barrier island sands overlie open marine sands of colington shoal in the north and to the south overlie fluvial and marine sand filling paleo roanoke tributary valleys the pleistocene sediments underlying the northern outer banks study area are mainly of open inner to mid shelf origin if as is likely sea level continues to rise a return to such environmental conditions is likely in the near future

a pilot study of geochemical and redox relationships between potholes and adjacent normal merensky reef of the bushveld complex lateral and vertical variations of f sub o 2 and carbon in whole rock and f sub o 2 fe cr mg and al variations in separated chromites are employed to elucidate the chemical relationships between the merensky reef and associated structures termed potholes these chemical distributions are utilized to develop a model relating the origin of the potholes to the platiniferous merensky reef potholes are large roughly circular and or rarely irregular depressions up to hundreds of meters wide and tens of meters deep in the merensky reef footwall filled with merensky equivalent material pothole and adjacent merensky reef samples studied herein were the footwall anorthosite the overlying basal chromitite and the platinum bearing gabbroic pegmatoidal pyroxenite three potholes were analyzed in this study intrinsic oxygen fugacity analyses demonstrate that pothole bottoms are more reduced than adjacent normal merensky reef 10 super 13 9 10 super 12 1 at 1 150 degrees c and analyses show that noncarbonate inorganic carbon concentration is up to 14 times greater in the pothole bottom cation distributions in the basal chromitite also reflect the variation in f sub o 2 for example the ratio feo super feo super mgo where feo super total iron as feo increases from pothole bottom to adjacent normal merensky reef 0 78 0 84 the carbon and intrinsic oxygen fugacity gradients in addition to the variations in spinel chemistry are all consistant with more reduced conditions within the pothole as compared to the adjacent normal merensky reef for the studied parameters geochemical gradients not only exist along stratigraphically equivalent units lateral gradients but also exist between units vertical gradients various hypotheses which address the genesis of pothole formation are reviewed in light of these discovered gradients a model of potholes acting as plutonic fumaroles releasing reducing fluid rich in carbon and or sulfur is discussed as a possible mechanism for merensky reef petrogenesis the model integrates our geochemical data with published research from the literature to present an integration of ideas for platinum group element enrichment in the merensky reef

spreading hypothesis of a particle plume abstractintegral models of single phase plumes are often closed using the entrainment hypothesis which assumes the entrainment velocity is proportional to a characteristic plume velocity but the corresponding theoretical development for modeling particle plumes has received less attention in this paper an integral model is developed by proposing a new spreading hypothesis for particle plumes in which the fluid phase spreading rate is taken as that of a single phase plume whereas the particle phase spreading rate is a function of the particle phase and fluid phase average vertical velocity the change in momentum flux of the particle and fluid phases are calculated by considering the cross sectional integrated buoyant and drag forces acting on the particles and their reaction force acting on the fluid mixing characteristics of the particle plume can then be determined the model was validated by laboratory particle plume experiments conducted for various particle sizes and initial plume to particle