formation of beach rock at siesta key florida and its influence on barrier island development seaward dipping strata of carbonate cemented shell debris located along the coast of siesta key on the gulf coast of the florida peninsula have long been interpreted to be beachrock equivalent in age to the pleistocene anastasia formation stage 5e of the east coast of florida detailed examination of thin sections along with radiometric dating and isotopic analyses demonstrates clearly that this is a holocene deposit that is not beachrock but was lithified in a meteoric environment whole rock dates dates from shells only and from cement only demonstrate that these beach deposits were in place by at least 1800 yr bp and might have been there as long ago as 4300 yr bp this means that some type of barrier island was in place at that time previous investigations have depicted siesta key as having a maximum age of 3000 yr with these deposits being located about 2 km landward of the beach deposits this suggests that the beach deposits might have been the site of the original position of siesta key these data also indicate that sea level must have been near its present position at the time that these foreshore beach deposits were deposited sometime between 1800 and 4300 yr ago this scenario indicates that sea level along this coastal reach probably reached its present level at least about 2000 yr ago

fault whispers transmission distortions on prestack seismic reflection data transmission distortions are observed on prestack seismic data at two locations in the gulf of mexico these distortions produce anomalous amplitude versus offset avo signatures the locations of the distortion zones are determined using acquisition geometry and ray tracing no obvious reflection events such as shallow gas zones are observed at the predicted locations of the distortion zones instead the distortion zones correlate with buried faults and unconformities it is postulated that the distortions are produced by velocity changes across buried faults and unconformities the distortions result from an interference pattern resulting from seismic waves arriving from different sides of the faults a simple model is developed to explain many of the characteristics of the distortion pattern

avo polarization and hodograms avo strength and polarization product an alternative approach to identifying amplitude variation with offset avo anomalies is to consider the avo polarization in the avo intercept avo gradient a b plane this method does not require deviations or separations from a background trend exhibited in traditional crossplots such as intercept gradient a b or near trace far trace n f a benefit of the hodogram or polarization method is that the wavelet is taken into consideration crossplotted intercept and gradient are polarized along a background trend for nonanomalous events and at angles different from the background trend for anomalous events this allows recognition of anomalous behavior otherwise buried in a background attributes resulting from this methodology include 1 the polarization angle 2 the polarization angle difference 3 the avo strength 4 the polarization product and 5 the linear correlation coefficient these different attributes can then be used to enhance avo interpretation synthetic modeling for a succe

beach cement incipient caco 3 cemented beachrock development in the upper intertidal zone north uist scotland abstract beachrock has only rarely been observed previously along high latitude coasts on several beaches on north uist nw scotland at over 57 n there is evidence of small scale beach cementation an area of beach cement at the back of traigh ear as well as unconsolidated sands from the beach and the adjacent traigh hornais have been studied using sem xrd and petrographical thin sections to throw light on the nature and causes of cementation unconsolidated sands from both beaches are similar in grain sizes shape and mineralogy but traigh ear grains commonly possess abundant cover of high mg calcite the beach cement on traigh ear exhibits low porosity with abundant micritic and aragonitic cements showing no evidence of vadose cementation along with traces of microorganisms these findings contrast with the vadose low mg calcite cements commonly reported from temperate zone beachrocks

beachrock cementation in alexandria egypt beachrock is found in the intertidal and sublittoral zones along the mediterranean coast of alexandria egypt it occurs as beds or patches and shows several stages of cementation and hardness the seaward dip of the beachrock is similar to that of the slope of the foreshore the fabric shows micrite and fringe cement types aragonite was not identified in the beachrock at alexandria the average concentrations of ca mg sr fe and mn in the beachrock are similar to those of the beach deposits the process of beachrock cementation in alexandria is rather recent and typically marine

formation of holocene limestone sequences by progradation cementation and erosion two examples from the bahamas abstract holocene limestones on north bimini and joulter cays bahamas reveal sedimentary structures that are directly comparable to structures in the modern beach environment a complete sequence comprises shallow subtidal intertidal and supratidal deposits holocene and equivalent modern sedimentary features are found on more or less the same topographic levels freshwater cementation phreatic and vadose takes place after accretion and progradation of the shore subsequent erosion exposes the consolidated sediments secondary cementation in the intertidal zone leads to beachrock formation continued erosion may break off blocks of beachrock and cliffs which will be incorporated into the basal part of the sequence during a new phase of progradation a freshwater cemented sequence going from shallow subtidal to supratidal and eolian deposits does not therefore necessarily imply sea level changes but can be created by a cycle of progradation cementation and erosion

magnesian calcite cements and their diagenesis dissolution and dolomitization mururoa atoll the most ubiquitous syn sedimentary cements affecting mururoa atoll are composed of magnesian calcite three main types are distinguished fibrous bladed and sparitic on the basis of petrography morphology and mgco3 concentration of the constituting crystals while peloid infills a particular form of hmc chemical precipitation also exist petrographic evidence and isotopic signatures are compatible with marine precipitation mururoa atoll was exposed several times to meteoric diagenesis resulting in varied diagenetic alterations including selective dissolution and partial dolomitization of mg calcite cements these alterations are responsible for substantial modifications of the initial cement fabrics and may introduce unconformities in the diagenetic chronology the first stage of the partial dissolution of mg calcite induces the development of chalky white friable zones within the initially crystalline hard cement layers at ultrascale this is due to the creation of micro voids along the elongate cement fibres advanced dissolution includes total disappearance of cement portions as attested to by large voids within the cement crust and or between superposed cement layers mg calcite dissolution is related to meteoric diagenesis during periods of quaternary exposure the creation of voids within mg calcite layers is due to the mechanical removal of previously altered calcium carbonate a process suggesting marine or non marine water flow probably in the vadose environment selective dolomitization of mururoa cements involves alternations of calcite and dolomite which form successive cement like rinds within primary cavities at mururoa these alternations are the result of selective dolomitization of the pre existing mg calcite cements rather than successive precipitation of calcite and dolomite selective dolomitization of mg calcite cements at mururoa indicates that a given cement succession is not necessarily a simple chronological sequence oxygen isotope values of dolomites are enriched in 186 by about 3 pdb within calcite dolomite pseudo alternations the dolomitizing fluid at mururoa seems similar to present marine water although some mixture with meteoric water is probable to favour dissolution associated with dolomitization

diagenesis of quaternary bahamian beachrock petrographic and isotopic evidence abstract carbonate beachrock is a common feature on many beaches of san salvador island bahamas these well indurated slabs of cemented beach sand are jointed normal and parallel to the shoreline and slope gently seaward constituent sediment grains are identical to those of adjacent unlithified beach sands beachrock exposed along the northern western and southern coastline of the island is commonly characterized by aragonite needle cement micritized grains and micritic envelopes which are features of early marine phreatic diagenesis these beachrocks have whole rock 13c values of 4 75 pdb and 18o values of 0 75 pdb which are identical to carbon and oxygen isotopic compositions of isolated aragonite needle cement and unlithified beach sand inclusion of recent artifacts in the rock indicates that these deposits are holocene beachrock from one locality on the western side of the island is covered with a calcrete crust indicating prolonged subaerial exposure and vadose weathering associated with the formation of organic rich soils petrographic features include low mg calcite cement evidence of neomorphism of aragonitic cement and micritic rims to low mg calcite random needle fiber calcite calcified filaments and micritic networks whole rock 13c values of these beachrocks range from 3 66 to 6 85 and 18o from 2 16 to 3 68 the wide range of 13c values in these samples is probably a function of the relative proportions of meteoric vadose to meteoric and or marine phreatic cements in individual samples isolated meteoric phreatic cements are only slightly enriched in the light isotopes relative to marine cements whereas meteoric vadose cements are significantly lighter the diagenetic features suggest the beachrock passed through marine phreatic to meteoric phreatic and meteoric vadose diagenetic environments this diagenetic sequence commonly forms in response to sea level lowering evidence of deposition in a shoreline environment and prolonged residence in meteoric phreatic and meteoric vadose environments suggests that these beachrocks may be late pleistocene

coastal forms and quaternary processes along the arid pilbara coast of northwestern australia abstract the pilbara coast is set in the most arid part of coastal australia this coast and coastal plain composed of riverine plains deltas tidal flats coastal dunes and limestone barriers relic deltas and archipelago ria shores is predominantly a terrigenous and carbonate sediment complex the coastal landforms therein formed during the quaternary through influence of ancestral landforms and fluvial and shoreline accretion coastal erosion and cementation the pre holocene features controlled development of holocene coastal forms and a variety of holocene depositional styles climate also plays a significant part in the coastal processes and in the generation of sedimentary products through high evaporation rates coupled with the limited rainfall cyclonic storms wind wind waves and the limited sediment delivery to the coastal zone this results in a distinctive range of stratigraphic sequences and chemical products as a result this arid coast is characterised by a range of features such as construction of arid zone deltas delta destruction and sediment redistribution during times of sediment depletion cyclone induced erosion and sedimentation mangroves and their associated sedimentary deposits evolution of coastal groundwater hypersalinity formation of salt flats and precipitation and cementation to form beachrocks high tidal crusts and gypsum precipitates the pilbara coastal stratigraphy and geomorphology suggests aridity was intricately involved in the sedimentation geomorphic evolution and pedogenic and diagenetic alteration of this coastal zone throughout the whole of the holocene and pleistocene

observation of undertow and turbulence in a laboratory surf zone abstract undertow and turbulence in the surf zone have been studied in a wave flume for a spilling breaker and a plunging breaker fluid velocities across a 1 on 35 sloped false bottom were measured using a fiber optic laser doppler anemometer and wave decay and set up were measured using a capacitance wave gage the characteristics of mean flow and turbulence in spilling versus plunging breakers were studied the mean flow is the organized wave induced flow defined as the phase average of the instantaneous velocity while the turbulence is taken as the deviations from the phase average it was found that under the plunging breaker turbulence levels are much higher and vertical variations of undertow and turbulence intensity are much smaller in comparison with the spilling breaker it was also found that turbulent kinetic energy is transported seaward under the spilling breaker and landward under the plunging breaker by the mean flow the study indicates that there are fundamental differences in the dynamics of turbulence between spilling and plunging breakers which can be related to the processes of wave breaking and turbulence production it is suggested that the types of beach profile produced by storm and swell waves may be the results of different relationships between mean flow and turbulence in these waves

identification of shallow gas by seismic data and avo processing example from the southwestern continental shelf of the ulleung basin east sea korea abstract this study investigates the evidence of shallow gas from a newly collected dataset comprising 2d multi channel seismic mcs single channel seismic scs chirp sub bottom profiler sbp and multi beam echo sounder mbes data from the southwestern continental shelf of the ulleung basin east sea korea various indicators of shallow gas were identified in this part the shelf including seismic chimneys acoustic flares in the water column pockmarks enhanced reflections and acoustic blanking seismic chimneys which are related to fluid leakage within the subsurface are characterized weak to high amplitude upturned or concave upward internal reflection with polarity reversal in mcs section whereas they are faint or transparent on scs section with 700 hz and chirp sbp sections with 3 5 khz frequency acoustic flares which are diagnostics of active gas venting were detected only on the chirp sbp sections there is also evidence of small scale depressions immediately below the acoustic flares these are interpreted as formed by the sudden explosion of gas enhanced reflections 3 km long concordant with stratification are observed at different levels acoustic blanking anomalies caused by absorption of acoustic energy due to the presence of gas are seen in the range 50 200 ms twt below seabed amplitude versus offset avo analysis indicates the presence of free gas ca 350 ms twt below sea surface using the mcs dataset recorded by a 600 m long streamer cable the avo cross section shows that the lower repetitive signals in the mcs data are due to the gas plume not lithological contrast our study highlights that avo analysis is an extremely useful tool for identifying free gas helping to discriminate water gas contacts and bright events among the chaotic signals on the mcs data

geophysical evidence for gas hydrates in the deep water of the south caspian basin azerbaijan new 2 d seismic reflection data from the south caspian sea offshore azerbaijan document for the first time in the deep water up to 650 m of this area the presence of gas hydrates geophysical evidence for gas hydrates consists of a shallow 300 500 m below seafloor zone of pronounced high velocity 2 100 m s as compared with the surrounding sediments 1550 1600 m s this zone appears on the seismic data as a depth limited 200 m thick layer extending down the flank of an elongate structural high and displays seismic blanking effects on the sedimentary section a strong positive polarityorc 0 123u reflector marks the top of this velocity anomaly and is interpreted as the top of the gas hydrate layer similarly a high amplitudeorc 0 11u negative polarity reflector coincides with the base of the highvelocity layer and is interpreted as the base of the hydrate zone both the top and bottom of the hydrate layer approximately parallel the seafloor bathymetry and cut discordantly across the stratigraphic section suggesting that the two reflectors are thermobaric and not stratigraphic interfaces decreasing amplitude with offset at the base of the gas hydrate layer may indicate the accumulation of free gas beneath this interface these gas hydrates fall within the hydrate stability field predicted from thermobaric modeling for the south caspian basin but typically in thinner layers than would be expected from theoretical calculations the minimum predicted water depth that allows hydrate formation is 150 m and the maximum predicted thickness of the gas hydrate stability field is 1350 m q 2001 elsevier science ltd all rights reserved

carbonate cement fabrics displayed a traverse across the margin of the bahamas platform near lee stocking island in the exuma cays abstract consolidated to friable carbonate rocks found in the lee stocking island area in the exuma cays include 1 reef rock 2 channel stromatolites 3 shallow water hardgrounds 4 beachrock rimming the islands and 5 pleistocene bedrock the most common cement fabrics observed are aragonitic fibers which include acicular fan druse and square tipped coarse fibers cementing beachrock and stromatolites and an isopachous needle fiber rim cementing hardgrounds and stromatolites less common are high mg calcite bladed textures of the reef rock and stromatolites two types of blades are present the more common stubby variety of either high mg or low mg calcite and an elongated variety of high mg calcite which was found in only three beachrock samples aragonitic micrite envelopes usually surround grains in beachrock hardgrounds and stromatolites but only in association with fibrous cement an aragonitic crust cements the surfaces of lime mud beds of the tidal channel while a high mg calcite cryptocrystalline cement occurs in all the rock types calcified algal filaments of high mg calcite from the abundant green and blue green algae in the area are a primary cement in stromatolites and a secondary cement in hardgrounds and beachrock a low mg calcite equant spar cements the pleistocene samples and is associated with meteoric diagenesis and cementation of the pleistocene surface cement precipitation coincides with the path of the cool oceanic water from exuma sound as it warms and loses co 2 and moves up onto the bank near lee stocking with the incoming tide cryptocrystalline cement is the first and commonest cement forming to the seaward while platformward fibrous cements become predominant as suggested by their crystal size and location on the shelf margin we think that the reef rock cryptocrystalline material are the fastest forming of the cements where the incoming oceanic water is more saturated with respect to calcium carbonate and undergoes the most significant warming the rate of the warming and degassing process is thought to increase in the tidal channel though the cementation rate is thought to fall slightly in response to a reduced availability of calcium carbonate on the platform interior further warming and degassing are believed to cause cement precipitation and the development of hardgrounds but these may form at a slower rate than that of the margin though this rate is still quite high cementation gradients occur from the tidal channel to the intertidal zones of 1 west norman s pond cay where cement fabric suggests a reduced calcium carbonate availability and 2 west lee stocking island where a change in mineralogy suggests a change in water chemistry thus a sequence of cement fabrics and mineralogies can be traced micritic textures occur in a more seaward position fine fibrous aragonite fibers in a more lagoonal and levee position and coarser aragonite fibers and mg calcite cements in the intertidal and supratidal position this sequence is thought to track the evolution of the water mass

two dimensional model of mud transport a two dimensional in plan finite difference model is presented which models the transport of well mixed cohesive sediments in estuaries and coastal waters a description of each of the physical processes incorporated in the model is given an erosion mechanism is not included explicitly because of the difficulties involved in a realistic formulation instead areas of net deposition throughout a tidal cycle are identified and rates of mud deposition within these areas are predicted two applications of the model to siltation studies are presented involving different modes of operation of the model in each case field data in the form of bed core samples were available so that the models could be both verified and calibrated under these circumstances the model can be used to make useful engineering predictions such as projected dredging requirements in estuaries and ports

a calibration of the bagnold beach equation abstract a simple sand trap is used to measure swash and backwash bedload transport rates on intertidal profiles data from sixty eight beach experiments are used to calculate a mean value of 12 78 kg m 4 s 2 for the calibration coefficient in the bagnold beach equation

beachrocks from the island of la palma canary islands spain beachrocks on la palma island developed on platform forming lavas of the cumbre vieja volcano some of these lavas are related to the 1585 puerto naos 1677 and 1971 echentive eruptions radiocarbon dating of the charco verde beachrock gives a conventional age of 33 330 8 490 bp while that at playa chica beach gives a calibrated age of 14 940 8 525 bp the beachrocks up to 1 5 m thick and some tens of metres wide consist of several decimetre thick horizons dipping 2 15 seaward petrographically they can be classified as rudstones and arenites with volcanic clasts as their main component the original porosity of the beachrocks was intergranular and occasionally intragranular and was partially occluded by cementation and locally by internal sediments the main cements are fibrous aragonite and micrite high magnesium calcite hmc spar aragonite peloidal hmc and microbotryoidal hmc are scarce the elemental geochemistry of these cements is consistent with a marine origin whereas the isotopic geochemistry indicates precipitation from marine waters slightly modified by meteoric waters the evolution of beach deposits and especially the beachrocks in la palma island follows three stages 1 beach deposition 2 beachrock formation and 3 beach retrogradation and or erosion the studied beachrocks prompt us to make some important considerations 1 the mean tidal range in the canary islands has not varied over the last thousand years 2 the position of the beachrocks at the present day sea level would require a combination of eustatic and isostatic movements to keep the sea level stable at the present level over the last thousand years 3 volcanic activity supplies the sediment that forms the beaches 4 a dry warm climate with a very low rainfall below 250 mm year and a high insolation rate 6 11 h day favours and favoured cement precipitation and beachrock formation by increasing the water temperature in the intertidal zone and in the inner part of the beaches 5 the presence of beachrocks in the la palma beaches prevents the total disappearance of the beaches 2003 elsevier science b v all rights reserved

another perspective on avo crossplotting principles of avo crossplotting by castagna and swan tle april 1997 raises some very interesting points and we would like to add to the discussion crossplotting the avo attributes is useful for interpreting anomalies in the context of lithology and pore fluids also crossplotting shows that there is a background trend for reflections from nonhydrocarbon related interfaces this trend which we call the fluid line comes from correlations of rock properties to relate avo anomalies to rock and pore fluid properties it is important to calibrate background nonhydrocarbon related seismic attributes to the background rock property trend once the trend is established then a quantitative interpretation of an anomaly can be made

beachrock occurrence characteristics formation mechanisms and impacts beachrocks are hard coastal sedimentary formations consisting of various beach sediments lithified through the precipitation of carbonate cements the objectives of this contribution are to a collate and review information on the reported occurrences characteristics and formation mechanisms of beachrocks and b consider their impacts on the coastal zone the analysis of the available information has shown that a beachrock formation is a global and diachronic phenomenon and b the great majority of beachrocks are found in tropical subtropical and low temperate latitude microtidal coasts the cementing agents of beachrocks are composed predominantly of the metastable carbonate phases high magnesian calcite hmc and aragonite ar appearing in a diverse crystalline morphology it has been suggested that cement precipitation in the coastal environment is controlled by i the physicochemical conditions ii the presence of organic compounds and microbes iii the magnitude and distribution of the wave energy along the coast and iv the textural characteristics of the constituent sediments various theories have been proposed to explain beachrock formation itself linking the phenomenon to either physicochemical or biological processes these theories however do not seem to be of universal validity and acceptance as each is able to explain only some of the reported occurrences the presence of beachrocks appears to affect beach morphodynamics by i locking the beach profile ii modifying the nearshore hydrodynamics iii changing the porous character of the beach and thus its response to wave forcing and iv differential bed erosion at the margins of the beachrock outcrops that can alter significantly the long and particularly the cross shore sediment transport therefore although relict submerged beachrock outcrops may provide some coastal protection by reducing the wave energy impinging onto the coastline modern beachrocks may promote offshore loss of unconsolidated beach sediments and buried beachrock outcropping finally the presence of beachrocks may have also significant ecological impacts as the indigenous mobile substrate fauna and flora of the beach is replaced by hard substrate benthic assemblages which are commonly arranged in hydrodynamically controlled zones

calcification of cyanobacterial mats in solar lake sinai pore water samples were obtained from the shallow water part of solar lake sinai where luxurious cyanobacterial mats grow these samples were analyzed for ca sup 2 mg sup 2 sr sup 2 cl sup so sub 4 sup 2 and titration alkalinity ta to determine the role of cyanobacterial growth and degradation on the calcification of the mats the data are modeled thermodynamically to predict mineral pore water equilibria the data support earlier bacterial and sedimentological studies suggesting that the degradation of the cyanobacterial mat via sulfate reduction is of major importance in the calcification process 34 references 1 figure 2 tables

formation exposure and evolution of a high latitude beachrock in the intertidal zone of the corrubedo complex ria de arousa galicia nw spain abstract the presence of beachrocks is noted on the galician shoreline at 41 n a latitude at which this phenomenon is rarely found the present study focuses on the petrological characterization and sedimentological evolution of a beachrock that was exhumed during an exceptional storm in the winter of 2001 in the intertidal zone of a high energy beach the cemented material comprises medium to coarse mixed siliciclastic carbonate sands unlike tropical beachrocks the cement consists of epitaxial low mg calcite presenting varied morphologies and textures acicular and bladed coatings and pore linings pore fillings of sparitic calcite and meniscus style cements seawater dominantly wave action plus marine and meteoric mixed waters are invoked as the main generative fluids beachrock formation and evolution are approached from a morphodynamic viewpoint considering four progressive stages 1 initial cementation in the intertidal zone 2 exposure and modelling by wave action 3 colonization and hardening and 4 disintegration preservation and burial finally the morphodynamic significance climatic implications and occurrence of this type of cementation along the north and northwest shoreline of spain are discussed

effective avo crossplot modeling a tutorial the ability to crossplot attributes from a 3 d seismic volume permits a geophysicist to identify and high grade subsets of the 3 d volume that warrant detailed inspection in the case of amplitude variation with offset avo crossplotting the seismic attributes are derived from cdp data crossplotting has become a fundamental process in avo analysis just as it is in petrophysical analysis comprehending the intricacies and selection of attributes is essential for successful avo analysis and improved seismic interpretation avo crossplotting of modeled seismic data derived from well logs with the biot gassmann equations provides a basis for understanding fluid substitution effects on avo attribute interactions when crossplotting with these model based understandings improved multi attribute interpretation processes can commence with avo crossplotting of seismic volumes

contrasting diagenesis of two carboniferous oolites from south wales a tale of climatic influence two oolites in the dinantian mississippian lower carboniferous of glamorgan sw britain were deposited in similar depositional environments but have contrasting diagenetic histories the brofiscin and gully oolites occur in the upper parts of shallowing upward sequences formed through strandplain progradation and sand shoal and barrier growth upon a southward dipping carbonate ramp the brofiscin oolite is characterized by a first generation cement of equant calcite spar preferentially located at grain contacts and forming non isopachous fringes around grains interpreted as meteoric vadose and phreatic in origin isopachous fibrous calcite fringes of marine origin are rather rare and occur only at a few horizons burial compaction was not important and porosity was occluded by poikilotopic calcite spar fitted grain grain contacts locally occur and could be the result of near surface vadose dissolution compaction syntaxial overgrowths on echinoderm debris are common pre compaction overgrowths are cloudy inclusion rich and probably of meteoric origin and post compaction overgrowths are inclusion free by contrast the gully oolite has little first generation cement however marine fibrous calcite is common in oolitic intraclasts as isopachous fringes of acicular calcite crystals closely associated with peloidal internal sediment and early equant drusy calcite spar occurs in the uppermost part of the gully beneath a prominent palaeokarst where pedogenic cements also occur the major feature of gully diagenesis is burial compaction resulting in extensive grain grain dissolution and microstylolitic grain contacts and post compaction poikilotopic spar occluded remaining porosity the brofiscin oolite is pervasively dolomitized up dip but the gully oolite for the most part only contains scattered pre compaction dolomite rhombs and late veins of baroque dolomite with less pervasive dolomitization the difference in diagenetic style of the two dinantian oolites is attributed to prevailing climate the paucity of early meteoric cements in the gully is a result of an arid climate and this is supported by the nature of the capping palaeokarst the abundant meteoric cements in the brofiscin reflect a more humid climate and effective meteoric recharge also resulted in up dip pervasive mixing zone dolomitization the style of early diagenesis in these two oolites exerted a major control on the later burial diagenesis in the brofiscin the early cements inhibited grain grain dissolution and pressure solution while these processes operated extensively in the gully oolite thus prevailing climate can influence a limestone s diagenetic history from near surface through into deep burial

marine vadose beachrock cementation by cryptocrystalline magnesian calcite maui hawaii abstract active precipitation of high magnesian cryptocrystalline calcite cement 15 5 mole mgco3 in the intertidal zone on the north shore of maui hawaii results in formation of an extensive beachrock exposure at hookipa park framework grains in the beachrock are coarse algal molluscan and intraclast bearing sands meniscus fabric is dominant in the cement indicating that most lithification occurs in the vadose zone the cryptocrystalline cement is generally brownish gold in color but very thin clear isopachous fringes of microcrystalline calcite surround darker cryptocrystalline cement especially in upper horizons of the beachrock peloidal texture is scarce in the cement but is present in a few samples silt and fine sand sized skeletal fragments have been trapped in he evolving cement resulting in cement supported fabric hollow rims of microcrystalline calcite occupy some pore spaces these hollow rims may have precipitated at the air water interface of bubbles that formed in pores and they may therefore be additional evidence of cementation in the vadose zone interstitial water samples from wells that penetrate portions of the beachrock and unconsolidated beach sands reveal temporal chemical variations that suggest the importance of co2 degassing in the formation of beachrock cement when chemical and tidal cycles fluctuate with the same period pco2 values in pore water of the intertidal zone generally bear a direct relationship to tidal stand increasing values of pco2 most often occur during semi daily rising tides and decreasing values of pco2 occur during falling tides saturation ratios with respect to calcite vary in direct proportion to pco2 saturation levels generally increase during rising tides and decrease during falling tides t ese data suggest that co2 degassing during falling tides results in precipitation of calcite cement from pore water of the beach

concepts in gravel beach dynamics abstract the dominant processes in gravel beach dynamics are reviewed highlighting some common themes which unify the various components of the gravel beach system the repercussions of which impart on how gravel beach dynamics might be understood conceptually in particular gravel beach dynamics are thought to be highly dependent on the temporal and spatial variation in grain size and the continual adjustments made by an active beach step both of which act not only as the expression of changing morphodynamic conditions but also as a controlling influence morphodynamics the notion that the exchanges on beaches between the hydrodynamics sediment transport and morphological change takes the form of reciprocal relationships which are mediated through feedback mechanisms in such a way that they cannot be thought of or studied independently is not a new one yet it appears that for the gravel beach morphodynamics must be re defined to describe conditions where variations in sediment size are thought to deserve parity rather than as merely a sequent entity or boundary condition morpho sedimentary dynamics is a phrase coined to intuit such cause and effect detailing the co evolution of morphology hydro hydraulics and sediment properties whilst acknowledging causative pluralism feedbacks and multiplier effects this is the recommended conceptual framework within which to crystallise thought and organise further research for the gravel beach essentially it increases the minimum number of parameters needed to describe the state of the gravel beach as a physical system therefore it is advised that simplicity will be most expedient in our future modelling efforts if complexity is to be adequately encapsulated

3d avo crossplotting an effective visualization technique crossplots visual representations of the relationship between two variables are used to a visually identify outliers that may bias a correlation b gain a visual sense for the strength of the correlation between variables c detect if the relationship between variables is linear or nonlinear d identify trends which may indicate multiple populations within the same data set and e detect significant departures from a background trend in other words to detect anomalies crossplotting is widely used in avo analysis because it facilitates the simultaneous and meaningful evaluation of two attributes generally common lithology units and fluid types cluster together in avo crossplot space allowing identification of background lithology trends and anomalous off trend aggregations that could be associated with hydrocarbons initially avo crossplotting typically used the intercept and gradient however in 1997 goodway et al used crossplots of elastic parameters lambda rho and mu rho to improve petrophysical discrimination of rock properties other attributes have also been used as avo anomaly indicators castagna and smith 1994 crossplotting appropriate pairs of attributes so that common lithologies and fluid types generally cluster together allows for straightforward interpretation the off trend aggregations can then be more elaborately evaluated as potential hydrocarbon indicators this is the essence of successful avo crossplot analysis and interpretation all of which is based on the premise that data that are anomalous statistically are interesting geologically this article describes attempts to extend crossplotting to three dimensions and assess any advantages that result ross and sparlin 2000 made this extension by using the intercept gradient and inlines for a 3d seismic volume as the three dimensions on a 3d crossplot our procedure begins by first visualizing different combinations of the measured well log parameters p velocity s velocity density porosity and gamma ray in two and three dimensions next observed patterns are visualized and compared in the

beach groundwater dynamics abstract an understanding of the interaction between surface and groundwater flows in the swash zone is necessary to understand beach profile evolution coastal researchers have recognized the importance of beach watertable and swash interaction to accretion and erosion above the still water level swl but the exact nature of the relationship between swash flows beach watertable flow and cross shore sediment transport is not fully understood this paper reviews research on beach groundwater dynamics and identifies research questions which will need to be answered before swash zone sediment transport can be successfully modelled after defining the principal terms relating to beach groundwater the behavior measurement and modelling of beach groundwater dynamics is described research questions related to the mechanisms of surface subsurface flow interaction are reviewed particularly infiltration exfiltration and fluidisation the implications of these mechanisms for sediment transport are discussed

sea level and diagenesis a case study on pleistocene beaches whalebone bay bermuda pleistocene fluctuations of sea level have left marine and aeolian limestones intercalated with glacial red soils on the bermuda carbonate platform atlantic 64 50 w 32 20 n successive eustatic highstands of similar amplitude drowned the tectonically stable platform and piled up similar sets of sediments up to three pleistocene beaches are stacked in shorelines sections post depositional diagenetic histories of these beaches can be linked to repeated changes in sea level and pore waters

radiocarbon evidence for the timing and rate of island development beach rock formation and phosphatization at lady elliot island queensland australia abstract lady elliot island great barrier reef is a coral cay constructed from near concentric shingle ridges composed of coral tridacna shells and bioclastic sand bound by guano derived phosphatic cement radiocarbon dating of individual tridacna samples is used to investigate the evolution of the island and place constraints on the variations in storminess of the last 3200 years a shallow platform reef existed at lady elliot since at least 6500 yrs b p shortly before 3200 yrs b p island growth by shingle progradation commenced and proceeded to the present at a rather uniform lateral rate 60 m ka 1 to leeward and 90 m ka 1 windward phosphatization may have occurred continuously or episodically at any time throughout the last 3200 years the youngest cemented ridge top is less than 770 years whereas at the base of uncemented ridges phosphatization is a modern 1950 a d process similarly low level beach rock has continued to form within the last 25 years a significant period of instability occurred less than 500 years ago during which marine erosion truncated the southwestern extremity of lady elliot island formed an eastern spit and stranded high level beach rock however the radiocarbon ages of tridacna samples from shingle ridges from lady elliot and curacoa islands are uniformly distributed kolmogorov smirnov test throughout the last 4000 years we interpret this to indicate no significant variation in ridge forming processes or fluctuation in average storminess during this interval

estimating and interpreting velocity uncertainty in migrated images and avo attributes estimating a suitable velocity field for use in prestack depth migration is inherently uncertain because of limitations on the available data and estimation techniques this uncertainty affects both the migrated depth of structures and their amplitudes in the inverted images these effects can be estimated by performing multiple migrations with a set of velocity fields and colocating features in the migrated images this lets us examine the imaging procedure s sensitivity to changes in the velocity field so we can assess both structural and amplitude uncertainties in migrated images these two types of uncertainties affect interpretation in different ways for instance with structural uncertainty interpretation we consider the change in migrated location of structures when deciding on drilling locations optimizing well trajectories or computing uncertainty in volumetric calculations with amplitude uncertainty or amplitude versus offset avo uncertainty interpretation we consider 1 uncertainty in c

holocene cemented beach deposits in belize abstract two types of cemented beach deposits occur on reef islands off the coast of belize these are 1 intertidal beachrock that is dominantly cemented by marine aragonite and high magnesium calcite cements and 2 supratidal cayrock that is cemented mainly by vadose low magnesium calcite cements besides differences in position relative to present sea level and resulting early diagenesic features beachrock and cayrock can be distinguished on the basis of differences in composition texture geographical position and age whereas the composition of beachrock is similar to that of the adjacent marginal reef sediments cayrock is enriched in benthic foraminifera intertidal beachrock is moderately to well sorted and well cemented while supratidal cayrock is very well sorted poorly cemented and friable beachrock occurs preferentially on windward beaches of sand shingle gays on the middle and southern barrier reefs and on the isolated platforms glovers and lighthouse reefs cayrock only occurs on larger mangrove sand gays of the isolated platforms turneffe islands lighthouse reef and the northern barrier reef 14c dating of ten whole rock and mollusk shell samples produced calibrated dates between ad 345 and ad 1435 for beachrock and between bc 1085 and ad 1190 for cayrock the large scale distribution of beachrock in belize supports the contention that physical processes such as water agitation rather than biological processes control beachrock formation and distribution only on windward sides of cays that are close to the reef crest where large amounts of seawater flush the beaches considerable amounts of cements can be precipitated to produce beachrock cayrock forms due to cementation in the vadose zone and is only preserved on larger stable mangrove sand cays

the avo hodogram using polarization to identify anomalies traditional methods for designing avo indicators are based on the deviation of gas sands from the wet sand shale trend in an avo crossplot such as intercept and gradient a b near and far angle n f p and s reflectivity rp rs etc these crossplots are typically constructed directly from log data such that each point represents an interface a drawback of this approach is that it neglects the seismic wavelet when a wavelet is convolved with the reflection coefficients each point on the avo crossplot becomes a series of points which typically spread across all four quadrants of the crossplot