predicting the crest height of a gravel beach the beach crest is a common morphological feature formed by the deposition of sediment carried up slope by wave swash the elevation to which waves can pile gravel is a function of the size and density of the material relative to the hydraulic components of swash velocity wave frequency and runup height two equations 17 and 21 that predict the height of the beach crest to the wave forces and the beach material are derived the first derivation compares the wave force acting to move a stone up the beach face with a weight force acting to hold the stone in place the second derivation relates the potential energy per unit area of the beach crest to the total wave energy that lifted and deposited the material above a given sea level datum as a test of the derived equations the actual crest height of one natural gravel beach was accurately estimated by both equations derived further research and testing on other natural beaches are needed to fully verify eqs 17 and 21 a comparison of other equations eqs 2 3 and 4 that predict crest height is also made the poor fit of the other equations are a result of using values for the coefficients that were established in wave tank experiments another shortcoming of eqs 3 and 4 is that they do not include variables related to the beach material hence they predict that all particle sizes from sand to boulders would be piled to the same elevation by the same wave forcing eq 2 includes a particle size variable but predicts increasingly higher beach crests for larger particles more realistic results for all three equations eqs 2 3 and 4 would be expected with field tested values for the coefficients but a range in values for each different beach would be necessary the advantage of eqs 17 and 21 is that equations are not dependent on coefficients whose values come from correlation with other data sets

uranium thorium helium dating of apatite experience with samples from different geochemical environments abstract u he dating which during its history has been very problematic might find special application in the field of hydrothermal mineralizations where other methods generally fail the mineral apatite is a potential candidate for this method in order to shed light on various aspects of apatite dating by the u th he method two samples each from different geochemical environments plutonic pneumatolytic and hydrothermal were probed isotope dilution techniques were used for concentration analyses from which 4he model dates also called helium indices after n b keevil were calculated 4he diffusion measurements were carried out for comparison of the 4he retentivities between the samples comparisons of the apatite he indices with plausible reference ages of the sampling locations in combination with results obtained from diffusion experiments show that the u th he dating method applied to apatite yields an age of cooling rather than an emplacement age the calculated closure temperatures are similar to those obtained by the fission track method 100 c our results suggest that the high temperature apatites plutonic and pneumatolytic may be more suitable for he dating than hydrothermal apatite important criteria for the dating of apatite by u th he are the presence of moderate radionuclide concentrations tens of ppm and a minimum abundance of inclusions

impact lithologies and their emplacement in the chicxulub impact crater initial results from the chicxulub scientific drilling project yaxcopoil mexico the chicxulub scientific drilling project csdp mexico produced a continuous core of material from depths of 404 to 1511 m in the yaxcopoil 1 yax 1 borehole revealing top to bottom tertiary marine sediments polymict breccias an impact melt unit and one or more blocks of cretaceous target sediments that are crosscut with impact generated dikes in a region that lies between the peak ring and final crater rim the impact melt and breccias in the yax 1 borehole are 100 m thick which is approximately 1 5 the thickness of breccias and melts exposed in the yucatan 6 exploration hole which is also thought to be located between the peak ring and final rim of the chicxulub crater the sequence and composition of impact melts and breccias are grossly similar to those in the yucatan 6 hole compared to breccias in other impact craters the chicxulub breccias are incredibly rich in silicate melt fragments up to 84 versus 30 to 50 for example in the ries the melt in the yax 1 hole was produced largely from the silicate basement lithologies that lie beneath a 3 km thick carbonate platform in the target area small amounts of immiscible molten carbonate were ejected with the silicate melt and clastic carbonate often forms the matrix of the polymict breccias the melt unit appears to have been deposited while molten but brecciated after solidification the melt fragments in the polymict breccias appear to have solidified in flight before deposition and fractured during transport and deposition

potbellies wasp waists and superparamagnetism in magnetic hysteresis because the response of a magnetic substance to an applied field depends strongly on the physical properties of the material much can be learned by monitoring that response through what is known as a magnetic hysteresis loop the measurements are rapid and quickly becoming part of the standard set of tools supporting paleomagnetic research yet the interpretation of hysteresis loops is not simple it has become apparent that although classic single domain pseudo single domain and multidomain loops described in textbooks occur in natural samples loops are frequently distorted having constricted middles wasp waisted loops or spreading middles and slouching shoulders potbellies such complicated loops are often interpreted in oversimplified ways leading to erroneous conclusions the physics of the problem have been understood for nearly half a century yet numerical simulations appropriate to geological materials are almost unavailable in this paper we discuss results of numerical simulations using the simplest of systems the single domain superparamagnetic sd sp system examination of the synthetic hysteresis loops leads to the following observations 1 wasp waisting and potbellies can easily be generated from populations of sd and sp grains 2 wasp waisting requires an sp contribution that saturates quickly resulting in a steep initial slope and potbellies require low initial slopes the sp contribution approaching saturation at higher fields the approach to saturation is dependent on volume hence the cube of grain diameter therefore there is a very strong dependence of hysteresis loop shape on the assumed threshold size 3 we were unable to generate potbellies using an sp sd threshold size as large as 30 nm and wasp waists cannot be generated using a threshold size as small as 8 nm the occurrence of both potbellies and wasp waists in natural samples is consistent with a room temperature threshold size of some 15 nm 5 4 simulations using a threshold size of 15 20 nm with populations dominated by sp grain sizes that is with a small number of sd grains produce synthetic hysteresis loops consistent with measured hysteresis loops and transmission electron microscopic observations from submarine basaltic glass 5 simulations and measurements using two populations with distinct coercivity spectra can also generate wasp waisted loops a relatively straightforward analysis of the resulting loops can distinguish the latter case from wasp waisting resulting from sp sd behavior

incomplete retention of radiation damage in zircon from sri lanka a suite of 18 zircon gemstones from placers in the highland southwestern complex sri lanka were subjected to a comprehensive study of their radiation damages and ages the investigation included x ray diffraction raman and pl spectroscopy electron microprobe pixe and hrtem analysis as well as u th he and shrimp u th pb age determinations zircon samples described in this study are virtually homogeneous they cover the range from slightly metamict to nearly amorphous generally concordant u th pb ages averaging 555 11 ma were obtained late ordovician zircon u th he ages scattering around 443 9 ma correspond reasonably well with previously determined biotite rb sr ages for rocks from the hswc slightly to moderately metamict zircon has retained the radiogenic he whereas only strongly radiation damaged zircon calculated total fluences exceeding 3 5 10 18 events g has experienced significant he loss when compared to unannealed zircon from other localities sri lanka zircon is about half as metamict as would correspond to complete damage accumulation over a 555 m y lasting self irradiation period suggesting significant annealing of the structural radiation damage insufficient consideration of this has often resulted in significant underestimation of radiation effects in zircon we suggest to estimate effective doses for sri lanka zircon by multiplying total fluences which were calculated using the zircon u th pb age by a correction factor of 0 55 this conversion may be applied to literature data as well because all gem zircon samples from sri lanka this work and previous studies seem to reveal the same general trends of property changes depending on the radiation damage the use of effective doses for sri lanka zircon contributes to more reliable quantitative estimates of radiation effects and makes possible direct comparison between natural and synthetic radiation damaged zircon

singular points at berm breakwaters scale effects rear round head and longshore transport abstract design aspects other than the profile development of the seaward side have been investigated in this paper aspects such as scale effects rear stability round head design and longshore transport have been treated here based on extensive test series on two different berm breakwater designs a first conclusion is that scale effects were not present in a 1 35 scale model compared with a 1 7 large scale model with wave heights up to 1 7 m a first design rule was assessed on the relationship between damage at the rear of a berm breakwater and the crest height wave height wave steepness and rock size tests on a berm breakwater head showed that enlarging the berm height at the crest and therefore the amount of rock in the berm was effective with regard to stability finally the onset of longshore transport due to oblique wave attack was studied and compared with literature formulae were derived for this onset of transport and also for the range of more serious transport up to longshore transport of coarse gravel

helium chronometry of apatite and titanite using nd yag laser heating we developed a technique for extracting 4he from apatite and titanite using laser heating that permits measurement of u and th and hence u th he ages on a single sample aliquot unlike direct laser heating quantitative he extraction and u th retention can be achieved when samples are indirectly laser heated to temperatures below their melting point resulting he ages are consistent with those obtained from conventional furnace heating of larger samples the major advantages of this technique include faster analysis and far lower more reproducible he blanks that permit single crystal analyses single crystal dating opens a variety of new avenues for u th he chronometry

laser ablation u th sm he dating of detrital apatite abstract laser ablation u th sm he dating of apatite la ahe is an emerging technique that presents a number of advantages over conventional whole grain methods increased analytical efficiency through the elimination of he re extractions and acid digestion increases sample throughput and facilitates larger datasets the spatially resolved selection of helium extraction locations eliminates the need for alpha ejection corrections and allows for imperfect grains with inclusions fractures unusual morphologies rounding and surface frosting or coatings to be measured these advantages allow the effective analysis of detrital samples which opens new avenues for geologic applications time resolved parent isotope concentration data collected by la icp ms enables the screening of grains for chemical zonation a commonly cited cause of over dispersion in u th sm he dates simultaneous collection of other chemical e g anion or trace element compositions or isotopic u pb age information for the grain may enhance provenance interpretations we demonstrate the accuracy and uncertainty of the measurement method using two measurement sessions we determine the age of the fish canyon tuff apatite reference material to be 27 9 0 6 ma n 45 mswd 1 5 which is in agreement with previous determinations our data reveal that zonation is a common feature in fish canyon tuff apatite and can be resolved by la ahe methods to reduce dispersion la ahe methods are ideally suited to detrital studies where a large number of measurements and a reduction in grain selection bias compared to whole grain methods should result in the sample better reproducing the source population the utility of the method is demonstrated using a modern sand from the wenatchee river washington usa previously characterized by conventional whole grain ahe methods our la ahe dataset n 74 better reproduces the ahe ages of bedrock samples within the drainage in comparison to the age distribution derived from conventional ahe dating

reverse polarity magnetized melt rocks from the cretaceous tertiary chicxulub structure yucatan peninsula mexico abstract we report paleomagnetic results for core samples of the breccia and andesitic rocks recovered from the yucatan 6 petroleos mexicanos exploratory well within the chicxulub structure about 60 km ssw from its center northern yucatan mexico a previous study has shown that the rocks studied contain high iridium levels and shocked breccia clasts and an ar ar date of 65 2 0 4 ma sharpton et al 1992 andesitic rocks are characterized by stable single component magnetizations with a mean inclination of 42 6 2 4 breccias present a complex paleomagnetic record characterized by multivectorial magnetizations with widely different initial nrm inclinations however after alternating field demagnetization well defined characteristic components with upward inclinations are defined irm acquisition experiments comparison of irm and nrm coercivity spectra and the single component magnetization of the andesitic rocks indicate the occurrence of iron rich titanomagnetites of single or pseudo single domain states as the dominant magnetic carriers mean inclinations from the andesitic rocks and most of the breccia samples give a mean inclination of about 40 to 45 indicating a reverse polarity for the characteristic magnetization that is consistent with geomagnetic chron 29r which spans the cretaceous tertiary k t boundary the inclination is also consistent with the expected value and corresponding paleolatitude for the site estimated from the reference polar wander curve for north america we suggest that the characteristic magnetizations for the andesitic and breccia rocks are the result of shock heating at the time of formation of the impact structure and that the age polarity and paleolatitude are consistent with a time at the k t boundary

three dimensional magnetic imaging of the chicxulub crater although few magnetization measurements are available for the structural elements of the chicxulub impact crater the magnetization intensities of the melt sheet upper breccia unit and central uplift are 3 4 orders of magnitude greater than the 3 to 4 km thick carbonate and evaporite stratigraphy covering the yucatan block this allows three dimensional modeling of the crater s structure by inversion using a two layer model two layers are separately inverted by dividing the crater s magnetic field expression into 40 km wavelength components the upper layer average depth 2 km models the distribution of highly magnetized zones in the crater s melt sheet the lower layer average depth 5 km represents relief on the yucatan block s basement surface and effectively maps the crater s 50 km diameter central uplift and possibly the expression of the surrounding collapsed disruption cavity fill the shallower magnetized zones consist of two generally concentric distributions at radii of 20 and 45 km these highly magnetized zones are thought to result from hydrothermal systems localized at the edge of the central uplift and the collapsed disruption cavity having produced magnetic phases during alteration of the melt sheet

stability of the seaward slope of berm breakwaters abstract an extensive research on stability of statically and dynamically stable rock slopes and gravel beaches was reanalysed and focussed on berm breakwaters berm breakwaters are initially dynamically stable under severe wave attack but become more or less statically stable after reshaping the effects on the seaward profile of wave height period storm duration spectral shape initial slope rock size rock shape and grading water depth and angle of wave attack are described in a qualitative way relationships between these variables and the profile parameters are described leading to a computer program this program was verified with independent data of berm breakwaters from various international institutes the program showed to give a good prediction of the behaviour of the seaward slope of berm breakwaters and can therefore be used as a conceptual design tool

a new formula for front slope recession of berm breakwaters the front slope stability of breakwaters with a homogeneous berm was studied in a large number of two dimensional model tests at aalborg university denmark the results are presented together with a new formula for prediction of the berm recession which is the most important parameter for describing the reshaping the formula has also been calibrated and validated against model test data from other researchers the significance of the new design formula is that it predicts berm recession much better than the existing methods especially in case of more stable structures

empirical model for probabilistic rock stability on flat beds under waves with or without currents abstract an empirical formula was developed for predicting the stability of isolated quarry rocks on relatively flat beds under waves with or without current in terms of the damage ratio the damage ratio was examined using 100 crushed stones median mass m50 293 g and mass density s 2 62 g cm3 as scale models of quarry rocks and replicas with a lower density s 1 38 g cm3 by placing them on beds of different roughness in a wave flume not only under periodic waves with periods of 2 3 5 s but also in symmetric and asymmetric oscillatory flows periods 8 12 s simulating waves without and with current in a circulating water channel comparison between hydrodynamic forces expressed in terms of three non dimensional mobility indices the maximum velocity maximum semi velocity amplitude ua and maximum acceleration relative to the friction force expressed in terms of the median coefficient of friction suggested that the damage ratio was most closely related to the ua based mobility index 2 nevertheless significant differences remained between data from the wave flume and circulating water channel tests the variation in the damage ratio which included the effects of the oscillatory velocity asymmetry oscillation period superimposed steady current mass density of stones and bottom friction was reasonably well described via the product of 2 and a function of a keulegan carpenter number the results of the field tests on quarry rocks with m50 2 04 t placed on a thin sand layer overlaying hard substrate show that the minimum stable mass is consistent with the prediction

u th he dating of apatite a potential thermochronometer abstract we found a gem quality crystal of durango fluorapatite to have a 4he content consistent with complete retention of radiogenic helium since its formation at about 31 ma isothermal heating and step heating analysis reveal 4he loss to occur systematically by volume diffusion at low temperatures the linear low temperature portion of the diffusion data yields an activation energy of 38 5 8 1 kcal mol and a frequency factor of in d 0 a 2 16 4 2 8 sec 1 corresponding to a closure temperature of 105 c 30 c cooling rate 10 c m y it appears that u th he dating of apatite might represent a useful new thermochronometer with a range similar to that of fission track dating of apatite from these results we infer that a number of the too young u th he dates reported in the literature on minerals such as zircon and magnetite may in fact represent valuable records of the low temperature thermal history of their host rocks

paleomagnetism of impact breccias from the chicxulub crater implications for ejecta emplacement and hydrothermal processes abstract results of a paleomagnetic study of 89 samples from impact breccias from the chicxulub crater are presented and used to investigate on ejecta emplacement hydrothermal and alteration processes the impactite sequence sampled in cores from yaxcopoil 1 borehole is 100 m thick and formed by six subunits characterized by petrographic scan images geochemical and textural analyses which point to different emplacement modes impact occurred within reverse polarity chron c29r therefore magnetization acquired at crater formation and shortly thereafter will present reverse polarity which is supported by paleomagnetic measurements on melt samples from yucatan 6 borehole results after step wise thermal and alternating field demagnetization document a pattern of 23 upward and 29 downward inclinations at yaxcopoil 1 around 33 and 38 respectively measurements of magnetic hysteresis and variation of low field susceptibility with temperature suggest low ti titanomagnetites and magnetite as main magnetic carriers petrographic observations indicate the presence of fine grained magnetite hematite and fe oxyhydroxides related to hydrothermal alteration processes curie points are in the range of 520 580 c the wide range of unblocking temperature spectra points to variable grain sizes of a magnetic phase that unblocks around 580 c hysteresis ratio plots indicate most samples fall in the pseudo single domain field in some samples wasp waist constrained hysteresis loops suggest magnetite and hematite analyses of vector plots and coercivity and unblocking temperature spectra do not show apparent differences within and between subunits subunits are characterized by distinct textural and compositional differences in size type and relative abundance of clasts and melt rich or carbonate rich matrix types bulk properties vary with composition and clast contents particularly within the middle suevite and brecciated melt rock subunits that show higher values while low values characterize the lower suevite carbonate rich with rare basement clasts subunit magnetization acquisition mechanisms are discussed with reference to impactite characteristics and may relate to remagnetization from hydrothermal and post impact alteration processes

influence of toe berm geometry on stability of reshaping berm breakwaters reshaping berm breakwaters have been mainly designed and built for water depth less than 20 m in such conditions a toe structure may be needed to reduce bottom settlements and increase geotechnical stability in the loose subsoil condition also a toe berm can be deployed to reduce the berm recession and increase the stability of the reshaping berm breakwater especially in deep water although many studies have been conducted to study the effects of toe berms on stability of conventional breakwaters investigations on the influence of a toe berm on stability of reshaping berm breakwaters are rare this paper presents results of an experimental work that has been carried out to investigate the influence of a toe berm on the hydraulic stability of reshaping berm breakwaters in a 2d physical test setup in a wave flume a total of 207 tests were conducted to systematically examine the effects of toe berm configuration on the reshaping of berm breakwaters with three types of armor stones the experimental program covers the influence of different geometrical parameters of the toe berm on the berm recession under various sea state conditions comparing results of berm recession of the cases having a toe structure with the cases without a toe berm shows that the amount of berm recession with toe berm is considerably less than the analogous amount for without toe structure it is observed that an increasing toe berm width and thickness both have considerable influence on recession reduction however the toe depth has a relatively larger influence on recession than the toe width and the influence is larger for higher stability numbers it is concluded that a toe berm in front of reshaping berm breakwater not only does have influence on recession due to the depth influence and the changing wave conditions but also the toe depth has a direct influence on the reshaped profile by preventing the displaced rocks to fall down to deeper parts using the test results a new formula is developed for estimation of berm recession by taking into account the influence of the toe berm configuration given an acceptable stability number ho as a design criterion for reshaping berm breakwaters the present method predicts that the recession can be reduced up to 35 by the application of a toe berm thus the main berm width can be shortened resulting in reduction of the required armor stones volume so by using a smaller stone size for the toe berm the stability of the structure is secured and also the total cost of the breakwater can be reduced

new links between the chicxulub impact structure and the cretaceous tertiary boundary the 200 km diameter chicxulub structure1 3 in northern yucatan mexico has emerged as the prime candidate for the cretaceous tertiary k t boundary impact crater3 6 concentric geophysical anomalies associated with enigmatic occurrences of upper cretaceous breccias and andesitic rocks led penfield and camargo1 to suspect that this structure was a buried impact basin more recently the discovery of shocked quartz grains in a chicxulub breccia3 and chemical similarities between chicxulub rocks and k t tektite like glasses3 6 have been advanced as evidence that the chicxulub structure is a k t impact site here we present evidence from core samples that chicxulub is indeed a k t source crater and can apparently account for all the evidence of impact distributed globally at the k t boundary without the need for simultaneous multiple impacts or comet showers shocked breccia clasts found in the cores are similar to shocked lithic fragments found worldwide in the k t boundary ejecta layer7 8 the chicxulub melt rocks that we studied contain anomalously high levels of iridium up to 13 5 parts per 109 also consistent with the iridium enriched k t boundary layer9 our best estimate of the crystallization age of these melt rocks as determined by 40ar 39ar analyses is 65 2 0 4 1 myr in good agreement with the mean plateau age of 64 98 0 05 myr recently reported10 furthermore these melt rocks acquired a remanent magnetization indicating that they cooled during an episode of reversed geomagnetic polarity the only such episode consistent with40ar 39ar constraints is chron 29r which includes the k t boundary

an experimental study on the hydraulic stability of icelandic type berm breakwaters abstract one of the key aspects of breakwater design and safety assessment is their stability evaluation in the present paper hydraulic stability of a multi layer berm breakwater mlbb has been studied through varying the parameter of eroded area to this end a 2 d experimental model has been carried out in the wave flume of tarbiat modares university using irregular waves with a jonswap spectrum a total of 150 tests have been performed to cover a wide range of sea state conditions and geometrical parameters such as wave height wave period water depth berm width berm elevation stone class i height front slope and nominal diameter of stone class i in this study a new dimensionless parameter is introduced to indicate that the wave breaking type plays a fundamental role in stability of mlbbs eventually a new formula is derived to estimate the damage of mlbbs based on different sea state conditions and various geometrical parameters comparisons are made between the estimated damage parameter of mlbbs by the proposed formula and those estimated by existing formulae the results denote that the present formula not only performs well for mlbbs experimental data but also indicates a reliable performance for some of lykke andersen s 2006 data in statically homogenous berm breakwaters hbbs outside the range of given formula furthermore an innovative approach is presented to examine the eroded area based on the berm recession and the intersection point of reshaped and initial profile from swl which performs faultlessly rather than the existing one

palaeomagnetism of the lappajarvi impact structure western finland abstract palaeomagnetic and rock magnetic results are presented for rocks from the lappajarvi meteorite impact structure western finland oriented samples were collected from the melt rocks in the central island of the crater and of the palaeoproterozoic svecofennian target rocks at the crater rim the natural remanent magnitization nrm of the melt rocks is generally weak and metastable during demagnetization the characteristic nrm component carried by pyrrhotite yields a palaeomagnetic pole latitude 55 4 n longitude 152 6 e dp 6 2 dm 8 7 dg 6 sites suggesting an age of 195 ma for the impact the results from a deep core drilled from the impact melt rock display similar palaeomagnetic directions but with slightly shallower inclinations the palaeomagnetic age differs from the 40ar 3939ar age of 77 ma for the impact assuming that the 40ar 39ar age is correct the discrepancy between palaeomagnetic and radiometric ages is best explained by post impact and post cooling tilting of the melt layer the tilting 15 may be due to isostatic rebounds along joints in the melt impact remanence has also been isolated in a few target rocks at the rim this poorly defined overprint could be a mild shock remanent magnetization superimposed on the 1 9 ga old svecofennian direction this interpretation is supported by the observation that the nrm intensity is enhanced in specimens having the impact overprint the palaeomagnetic results are consistent with earlier interpretations based on geophysical morphological mineralogical and geochemical data that the lappajarvi structure is a complex meteorite impact site less than 200 ma old

hydraulic stability of reshaping berm breakwaters using the wave momentum flux parameter abstract an empirical new berm recession formula is derived based on the assumption that the maximum wave force causing armor movement and berm recession is proportional to the maximum wave momentum flux near the structure toe this concept introduces a more physics based first principles approach to estimate the berm recession recession seems well predicted by the new formula for reshaping berm breakwaters the results from this formula show a better estimation than earlier formulae used for estimating the berm recession

armor stability of hardly or partly reshaping berm breakwaters abstract this paper deals with stability of hardly or partly reshaping berm breakwaters a simple physical argument is used to derive a new stability formula based on the assumption that the maximum wave force causing damage of armor layer is proportional to the maximum wave momentum flux near the structure toe the main goal of the present paper is to provide an estimation technique based on this physical principle to predict the deformation of the front slope in terms of the eroded area the proposed method is verified by comparison with model test data it is found that by using the maximum wave momentum flux approach the damage to the front slope eroded area can be very well predicted moreover a simple method to estimate the eroded area based on measured or calculated berm recession rec and depth of intersection of reshaped and initial profile h f is presented the performance of the simple method based on measured data of rec and h f is better than the presented stability equation based on wave momentum flux parameter when using the best prediction formulae for recession and depth of intersection the simple method and the momentum flux stability equation provide similar uncertainty

an experimental study on the parameterization of reshaped seaward profile of berm breakwaters abstract berm breakwaters are rubble mound structures in which the seaward slope of the initial profile may be reshaped to become more stable under severe wave attack the stones in the seaward slope move from the initial slope to an equilibrium profile a 2d experimental study has been carried out in a wave flume at a hydraulic laboratory of tarbiat modares university to study the effects of sea state and structural parameters on the reshaped profile parameters of such breakwaters a series of 287 tests have been performed to cover the effect of various sea state conditions such as wave height wave period number of waves and water depth at the toe of the structure and structural parameters such as berm width berm elevation above still water level and armor stone size all the tests have been done employing irregular waves with a jonswap spectrum in this paper first the reshaped profiles are schematized and then the key parameters of the reshaped seaward profiles such as step height step length and depth of intersection point of initial and reshaped profile are investigated using results of this experimental work eventually formulae that include some sea state and structural parameters are derived for estimation of the reshaped profile parameters to assess the validity of the proposed formulae comparisons are made between the estimated parameters of reshaped profiles by these formulae and earlier formulae given by other researchers the comparisons show that the estimation procedure foretells reshaping parameters well and with less scatter according to the present data and also other experimental results within the range of parameters tested

westernmost grand canyon incision testing thermochronometric resolution the timing of carving of grand canyon has been debated for over 100 years with competing endmember hypotheses advocating for either a 70 ma old or 6 ma young grand canyon several geological constraints appear to support a young canyon model but thermochronometric measures of cooling history and corresponding estimates of landscape evolution have been in debate in particular 4he 3he thermochronometric data record the distribution of radiogenic 4he from the 238u 235u and 232th decay series within an individual apatite crystal and thus are highly sensitive to the thermal history corresponding to landscape evolution however there are several complicating factors that make interpreting such data challenging in geologic scenarios involving reheating here we analyze new data that provide measures of the cooling of basement rocks at the base of westernmost grand canyon and use these data as a testbed for exploring the resolving power and limitations of 4he 3he data in general we explore a range of thermal histories and find that these data are most consistent with a young grand canyon a problem with the recovered thermal history however is that burial temperatures are under predicted based on sedimentological evidence a solution to this problem is to increase the resistance of alpha recoil damage to annealing thus modifying he diffusion kinetics allowing for higher temperatures throughout the thermal history this limitation in quantifying radiation damage and hence crystal retentivity introduces non uniqueness to interpreting time temperature paths in rocks that resided in the apatite helium partial retention zone for long durations another source of non uniqueness is due to unknown u and th distributions within crystals we show that for highly zoned with a decrease in effective u of 20 ppm over the outer 80 of the radius of the crystal the 4he 3he data could be consistent with an old canyon model to reduce this non uniqueness we obtain u and th zonation information for separate crystals from the same rock sample through la icp ms analysis the observed u and th distributions are relatively uniform and not strongly zoned thus supporting a young canyon model interpretation of the 4he 3he data furthermore we show that for the mapped zonation the difference between predicted 4he 3he data for a uniform crystal and a 3d model of the crystal are minimal highlighting that zonation is unlikely to lead us to falsely infer an old grand canyon

chicxulub crater a possible cretaceous tertiary boundary impact crater on the yucatan peninsula mexico we suggest that a buried 180 km diameter circular structure on the yucatan peninsula mexico is an impact crater its size and shape are revealed by magnetic and gravity field anomalies as well as by oil wells drilled inside and near the structure the stratigraphy of the crater includes a sequence of andesitic igneous rocks and glass interbedded with and overlain by breccias that contain evidence of shock metamorphism the andesitic rocks have chemical and isotopic compositions similar to those of tektites found in cretaceous tertiary k t ejecta a 90 m thick k t boundary breccia also containing evidence of shock metamorphism is present 50 km outside the crater s edge this breccia probably represents the crater s ejecta blanket the age of the crater is not precisely known but a k t boundary age is indicated because the crater is in a thick carbonate sequence shock produced co2 from the impact may have caused a severe greenhouse warming

accounting for long alpha particle stopping distances in u th sm he geochronology 3d modeling of diffusion zoning implantation and abrasion in apatite u th he thermochronology the helium distribution in a crystal is a function of the simultaneous processes of radiogenic production thermally activated volume diffusion and the ejection of he caused by long alpha stopping distances these processes are further complicated by zonation of u th and sm within the grain and implantation of 4he from neighboring u th sm bearing minerals we use a refined version of the 3d monte carlo diffusion code of gautheron and tassan got 2010 to simulate the interplay between ejection and diffusion with or without zonation ejection and abrasion for a suite of thermal histories we examine the phenomenon of over correction produced by the alpha ejection correction parameter ft or fzac for homogeneous or heterogeneous eu repartition by comparing the raw measured and ft or fzac corrected ages for a number of scenarios to the ejection free age aef which we define as the age that would be obtained if alpha ejection had not occurred or equivalently if the stopping distance was zero we show that the use of ft or fzac corrected ages generally reproduces the ejection free age to within typical u th he uncertainties 8 even for zoned apatites we then quantify the effect of alpha implantation on u th he ages showing that implantation from a single external source with modest relative u or th enrichment can generate as much as 50 excess he for more extreme cases where an apatite is surrounded by multiple external sources the measured age can be 300 of that determined from an isolated crystal while abrasion of the outer 20 25 m can significantly reduce the age dispersion for rapidly cooled samples slowly cooled samples can still retain 10 30 excess he the removal of the rim of the crystal reduces the thermal information from very low temperatures 40 c and introduces additional technical complications and biases and should therefore be used with caution overall we demonstrate that although zonation and implantation may not be routinely determined we now have the 3d modeling capability to fully investigate and constrain the causes of age dispersion within a sample leading to significant improvement in our ability to interpret u th he data

magnetostratigraphy of the impact breccias and post impact carbonates from borehole yaxcopoil 1 chicxulub impact crater yucatan mexico we report the magnetostratigraphy of the sedimentary sequence between the impact breccias and the post impact carbonate sequence conducted on samples recovered by yaxcopoil 1 yax 1 samples of impact breccias show reverse polarities that span up to 56 cm into the post impact carbonate lithologies we correlate these breccias to those of pemex boreholes yucatan 6 and chicxulub 1 from which we tied our magnetostratigraphy to the radiometric age from a melt sample from the yucatan 6 borehole thin section analyses of the carbonate samples showed a significant amount of dark minerals and glass shards that we identified as the magnetic carriers therefore we propose that the mechanism of magnetic acquisition within the carbonate rocks for the interval studied is detrital remanent magnetism drm with these samples we constructed the scale of geomagnetic polarities where we find two polarities within the sequence a reverse polarity event within the impact breccias and the base of the post impact carbonate sequence up to 794 07 m and a normal polarity event in the last 20 cm of the interval studied the polarities recorded in the sequence analyzed are interpreted to span from chron 29r to 29n and we propose that the reverse polarity event lies within the 29r chron the magnetostratigraphy of the sequence studied shows that the horizon at 794 11 m deep interpreted as the k t boundary lies within the geomagnetic chron 29r which contains the k t boundary

numerical modelling of wave interaction with dynamically stable structures wave interaction with dynamically stable structures is simulated by means of a numerical model based on finite amplitude shallow water wave equations the model can simulate wave motion on and inside permeable structures for dynamically stable structures including berm breakwaters reef type structures and gravel beaches a procedure is developed to simulate the natural response to wave attack this procedure is extended for instance by implementing effects of grading and effects of seawalls to increase the applicability for practical applications

trends in stability of dynamically stable breakwaters the reshaping of dynamically stable breakwaters was studied subject to variations in armour stone gradation and shape wave characteristics and duration of wave attack from head on waves in a two dimensional wave flume tests were undertaken at the coastal engineering research laboratory of queen s university kingston canada using irregular waves profiles of the structure during the various stages of reshaping were measured using a semi automatic profiler developed for this study the volume of stones and the initial berm width required for development of a stable profile along with the extent to which the toe of the structure progressed seaward were chosen as the characteristic parameters of the reshaped breakwater the results indicated that the toe width formed as a result of reshaping and the area of stones required for reshaping were dependent on the wave height gradation of the armour stone and duration of the storm the initial berm width required for reshaping was also found to be dependent on the wave height armour stone gradation percentage of rounded stones in the armour and the duration of the storm

revised age calibration points for the geomagnetic polarity time scale two of the seven cenozoic age calibration points in the widely used cande and kent s geomagnetic polarity time scale have recently been revised the rest of the five calibration points are revised here based on recently available radioisotopic dates and magnetostratigraphic data ages of the magnetic reversals are recalculated using the revised calibration points differences between the present time scale and cande and kent s revised time scale are generally less than 0 3 m y the largest up to 1 3 m y being in the early miocene

improved confidence in u th he thermochronology using the laser microprobe an example from a pleistocene leucogranite nanga parbat pakistan 1 the newly developed laser microprobe u th he thermochronometer permits for the first time the ability to generate precise u th he cooling ages for even very young 1 ma samples with a spatial resolution on the order of tens of micrometers this makes it possible to test the reproducibility of independent u th he age determinations within individual crystals further increasing the reliability of the method as an example we apply it here to a pleistocene granite from nanga parbat pakistan where previous constraints on the thermal history are consistent with rapid exhumation and cooling twenty one u th he dates determined on two monazite crystals from a single granite sample yield a mean of 748 000 years with a 95 confidence level of 19 000 years there is no discernible variation in the distribution of u th he ages in the cores of these crystals and therefore no evidence for the development of substantial diffusive loss 4he zoning over 80 of the interior of the monazite crystals during postcrystallization cooling of the granite modeling of these data suggests that cooling at a mean rate of 300 k ma would be necessary to produce the observed ages and the lack of a 4he gradient which is consistent with preexisting constraints for nanga parbat increased precision in thermochronology permits more tightly constrained exhumation models which should aid geologic interpretation

u pb and u th sm he double dating of detrital apatite by laser ablation a critical evaluation abstract paired u pb and u th sm he dating of individual detrital apatite crystals has potential as a valuable tool for constraining the high and low temperature thermal evolution of sediment source terrains here we present the results of exploratory applications of the laser ablation double dating ladd method originally developed for detrital zircon research to the widely available durango fluorapatite standard in order to evaluate the practicality of the method sixty two laser ablation analyses of a single large crystal of durango fluorapatite yielded an inverse variance weighted mean 206pb 238u date of 31 46 0 48 ma and an inverse variance weighted mean u th sm he date of 31 75 0 60 ma both of which are in good agreement with previously published conventional dates while these results are encouraging several factors suggest that ladd may be less useful for detrital apatites than for detrital zircons given widely available analytical instrumentation these principally reflect the propensity for apatites to have comparatively lower u th concentrations and thus lower radiogenic he and pb concentrations as well as high concentrations of common pb these factors contribute to substantially higher analytical imprecision for most ladd u pb dates for apatite occasionally too high for the dates to be geologically useful reasonably precise laser ablation u th sm he dating of detrital apatites requires relatively large crystal sizes 100 m in the shortest dimension with the minimum useful size increasing with decreasing u th sm he apparent age in contrast to the geological interpretation of ladd datasets for detrital zircons the interpretation of datasets for detrital apatites is less straightforward in particular researchers should consider carefully the possibility that u th sm he apparent age distributions are biased by the need to analyze only larger apatites

on front slope stability of berm breakwaters abstract the short communication presents application of the conventional van der meer stability formula for low crested breakwaters for the prediction of front slope erosion of statically stable berm breakwaters with relatively high berms the method is verified burcharth 2008 by comparison with the reshaping of a large norwegian breakwater exposed to the north sea waves as a motivation for applying the van der meer formula a discussion of design parameters related to berm breakwater stability formulae is given comparisons of front erosion predicted by the use of the van der meer formula with model test results including tests presented in sigurdarson and van der meer 2011 are discussed a proposal is presented for performance of new model tests with the purpose of developing more accurate formulae for the prediction of front slope erosion as a function of front slope relative berm height relative berm width method of armour stone placement and hydraulic parameters the formulae should cover the structure range from statically stable berm breakwaters to conventional double layer armoured breakwaters

bunte breccia of the ries continuous deposits of large impact craters the ries crater an impact structure of 26 km diameter in south germany is the largest terrestrial crater where substantial amounts of ejecta are preserved on occasion 100 m deep further the target stratigraphy is well known and it is possible to relate specific clasts and breccia lithologies to initial target depth as a consequence the continuous deposits of the ries also known as bunte breccia may be studied with exceptional field control we report field observations and laboratory analyses obtained from 560 m core materials taken at nine different locations that range from 16 to 37 km in radial distance from the impact center the objective is to relate the ries observations to ejection and to emplacement processes of large scale planetary crater deposits the observations regarding the modal stratigraphic characteristics of the bunte breccia may be summarized as follows only 600 m some 0 7 is composed of triassic clasts originating from 300 to 600 m depths lower and middle jurassic horizons approximately 300 150 m constitute some 2 3 and upper jurassic 0 150 m makes up some 31 5 in addition the bunte breccia contains tertiary materials in the form of 1 cm clasts 29 1 and as highly comminuted fine grained matrix 10 gpa constitute 0 1 weight of the entire deposit which indicates that bunte breccia was emplaced at essentially ambient temperatures when possible the above observations are quantified via linear regressions throughout the text all of these observations are consistent with if not predicted by a ballistic emplacement scenario as postulated by oberbeck and co workers primary crater ejecta are expelled ballistically and will form secondary craters in the local substrate a mixture of primary and secondary ejecta results and combines into a highly turbulent ground hugging debris surge as the final phase of ejecta emplacement total emplacement time for the bunte breccia 200 km is estimated to be of the order of 5 min only these findings are compared with cratering theory relating to a number of ejecta thickness decay models and with the so called z model addressing material flow during various stages of crater formation qualitative to fair agreement of observations and predictions results an initial crater radius of 6 5 km an excavation depth of 1650 m an excavation volume of 136 km and an associated transient cavity volume of aproximately 230 km appear to be reasonable estimates approximately 170 km of material was involved in slumping and restoration of the transient cavity for the above radius and z 2 7 the modal composition of ries ejecta with regard to preimpact target stratigraphy indicates that materials contained in the continuous deposits of large complex planetary craters are predominantly derived from depths as small as one hundredth the apparent crater diameter a number of implications are addressed regarding remote sensing of planetary surfaces and investigation of lunar and meteoritic impact breccias

aeromagnetic anomaly modeling of central zone structure and magnetic sources in the chicxulub crater abstract the analytic signal method and forward modeling in three dimensions are applied to magnetic field data over the chicxulub crater in order to determine and document the main magnetic anomaly sources and crater structure in the central zone aeromagnetic data over the structure reveal three strong well defined concentric patterns with a central 40 km diameter zone of high amplitude anomalies magnetic anomalies are interpreted to be associated with the melt sheet upper breccias and central uplift which present three to four orders of magnitude contrasts with the surrounding carbonate units the limited number of magnetic property measurements apparent wide range in the remanent intensity and susceptibility in the upper breccias and possible yet unconstrained zones of hydrothermal systems have restricted our ability to determine the characteristics and distribution of major structural elements of the chicxulub crater the amplitude of the analytic signal produces maxima over magnetization contrasts independent of the direction of magnetization interpretation of maxima location and depth distribution is used in a second stage as a priori information in the construction of an input prism assemblage magnetic configuration and its properties for three dimensional forward modeling results indicate that magnetic sources extend to a radial distance of 45 km from the center of the structure with average depths ranging between 2 and 4 km the magnetic anomaly sources in the central structural uplift zone are located in the range from 3 5 to 8 km depth with dominant contributions from an apparent large body forming the basement uplift

size and morphology of the chicxulub impact crater the chicxulub impact in mexico has been linked to the mass extinction of species at the end of the cretaceous period from seismic data collected across the offshore portion of the impact crater the diameter of the transient cavity is determined to be about 100 km this parameter is critical for constraining impact related effects on the cretaceous environment with previous estimates of the cavity diameter spanning an order of magnitude in impact energy the offshore seismic data indicate that the chicxulub crater has a multi ring basin morphology similar to large impact structures observed on other planets such as venus

a new formula for the sea state and structural parameters influencing the stability of homogeneous reshaping berm breakwaters the berm recession of a reshaping berm breakwater has a very important role for the stability of this kind of structure based on a 2d experimental modeling method in a wave flume the recession of the berm due to sea state and structural parameters has been studied irregular waves with a jonswap spectrum were used a total of 215 tests have been performed to cover the impact of sea state conditions such as wave height wave period storm duration and water depth at the toe of the structure and structural parameters such as berm elevation from still water level berm width and stone diameter on berm recession in this paper first a new dimensionless parameter is introduced to evaluate the combined effect of wave height and wave period on berm recession using results of the experimental work then a formula that includes some sea state and structural parameters is derived using the new dimensionless parameter for estimating the berm recession a comparison is made between the estimated berm recessions by this new formula and formulae given by other researchers to show the preference of using the new dimensionless parameter the comparison shows that the recession estimated by the new formula has not only a better correlation with the present experimental data but also has an improved correlation with other experimental results within the range of parameters tested outside the range of parameters tested the lykke andersen 2006 formula performed best

on berm breakwaters recession crown wall wave forces reliability abstract the paper reviews different formulae lykke andersen 2006 lykke andersen and burcharth 2010 torum 2007 and moghim et al 2011 for recession of berm breakwaters and compares calculated recession with previous and new recession test data there are differences of the results by different formulae but by and large all the formulae give reasonable results on the recession for shallow water berm breakwaters it seems that the lykke andersen 2006 formula works best for the deep water cases although the torum 2007 formula seems to work as good for the homogenous berm breakwater case the moghim et al 2011 formula was not included in the comparison for the deep water cases because the deep water cases are outside the validity of the formula results of tests on different alternative berm breakwater designs which may be considered as practical economic designs have also been included but without any comparison with any formulae simply because the formulae do not cover the specific designs wave forces from model tests on crown walls on berm breakwaters are presented and compared with an existing formula for wave forces on a crown wall on conventional rubble mound breakwaters pedersen 1995 it appears that the wave forces on a crown wall on a berm breakwater are smaller than the wave forces on a crown wall on a conventional rubble mound breakwater finally a probability of failure analysis of a site specific berm breakwater the sirevag berm breakwater norway is presented using monte carlo simulations the probability of failure analysis shows that the sirevag berm breakwater is extremely strong

rock uplift and erosion rate history of the bergell intrusion from the inversion of low temperature thermochronometric data the bergell intrusion european alps was one of the first locations where thermochronometry was used to resolve changes in erosion rate yet relating these changes to variations in climate or in local tectonics remains challenging one approach that enables changes in erosion rate to be related to changes in climate or rock uplift rate is to utilize landscape evolution models as topographic response to these forcing parameters is unique furthermore low temperature thermochronometric systems have the potential to resolve topography through time and thus topographic response we present new u th he data for samples collected across 2 km of relief from the bergell the ages range from 2 to 16 ma and define an age elevation with an apparent exhumation rate of 0 1 km myr in order to infer erosion rates we use a thermokinematic model to solve the heat equation in the crust track material points through time and predict thermochronometric data paleo topography and erosion rate are parameterized using the stream power model we find that rock uplift rates were 0 4 km myr from 25 to 20 ma and subsequently decreased to 0 05 km myr this results in a gradual decrease in erosion rate from rates of 0 4 to 0 1 km myr a recent increase in rock uplift rate at 4 ma to 0 6 km myr is required to explain the youngest ages and high topographic relief

u th he geochronology of single zircon grains of known tertiary eruption age we performed a systematic study of u th he ages of volcanic zircons of known eruption age to establish a reliable protocol for zircon he dating and to validate the resulting ages using laser he extraction and dissolution by lithium borate flux melting we analyzed many aliquots of single zircon grains separated from four rapidly cooled rocks fish canyon tuff accepted age 27 8 0 7 ma buluk tuff 16 3 0 2 ma tardree rhyolite 58 4 0 7 ma and utaosa rhyolite 2 52 0 02 ma he ages of fish canyon tuff buluk tuff and utaosa rhyolite zircons are in good agreement with their reference ages mean emission corrected ages were 28 6 1 4 1 16 1 0 8 and 2 61 0 18 ma respectively however the zircon he ages of tardree rhyolite are irreproducible and consistently too old with a mean he age of 78 8 7 0 ma spontaneous fission track densities of tardree rhyolite zircons indicate that about 80 of these zircons are strongly zoned in u with high track density i e high u content in the core and low density on the rim this causes an overestimation of the ejection correction that likely explains the erratic and excessively high he ages from these zircons assessment of the degree and style of u zonation within a zircon population is thus prudent before application of the u th he method step heating experiments indicate that for most zircons he extraction is 99 complete after 1 h at 1300 c however for as yet unknown reasons occasional grains seem to retain substantial amounts of he under these conditions

coeval 40ar 39ar ages of 65 0 million years ago from chicxulub crater melt rock and cretaceous tertiary boundary tektites 40 ar 39 ar dating of drill core samples of a glassy melt rock recovered from beneath a massive impact breccia contained within the 180 kilometer subsurface chicxulub crater in yucatan mexico has yielded well behaved incremental heating spectra with a mean plateau age of 64 98 0 05 million years ago ma the glassy melt rock of andesitic composition was obtained from core 9 1390 to 1393 meters in the chicxulub 1 well the age of the melt rock is virtually indistinguishable from 40 ar 39 ar ages obtained on tektite glass from beloc haiti and arroyo el mimbral northeastern mexico of 65 01 0 08 ma mean plateau age for beloc and 65 07 0 10 ma mean total fusion age for both sites the 40 ar 39 ar ages in conjunction with geochemical and petrological similarities strengthen the recent suggestion that the chicxulub structure is the source for the haitian and mexican tektites and is a viable candidate for the cretaceous tertiary boundary impact site

thermal metamorphic signature of an impact event in the vredefort dome south africa pseudotachylitic breccias and shock deformation features related to the 2 02 ga formation of the vredefort dome by meteorite impact were overprinted by a static metamorphic event the intensity of which decreased from granulite facies t 700 c in the center of the dome to greenschist facies t 400 c around its margins geobarometric estimates of 0 2 0 3 gpa for the metamorphic parageneses indicate some 8 11 km of erosion since the impact event the strong lateral thermal gradient implied by these p t results is attributed to the combined effects of differential uplift of mid crustal rocks heated along a pre impact geotherm and increased shock heating of the target crust toward the center of the impact structure we suggest that the exceptionally high grade of metamorphism in the center of the dome may in part reflect an elevated regional geothermal gradient of 25 c km in the target crust due to lingering thermal effects related to the 2 05 2 06 ga bushveld magmatic event

paleomagnetic and rock magnetic study of the yaxcopoil 1 impact breccia sequence chicxulub impact crater mexico results of a detailed paleomagnetic and rock magnetic study of samples of the impact breccia sequence cored in the yaxcopoil 1 yax 1 borehole between abou 800 m and 896 m are presented the yax 1 breccia sequence occurs from 794 63 m to 894 94 m and consists of rerdeposited melt rich clast size sorted fine grained suevites melt rich no clast size sorting medium grained suevites coarse suevitic melt agglomerates coarse melt rich heterogeneous suevites brecciated susevites and coarse carbonate and silicate melt suevites the low field susceptibility ranges from 0 3 to 40187 x 10 6 si and the nrm intensity ranges from 0 02 ma m up to 37510 ma m in general the nrm intensity and magnetic susceptibility present wide rangesand are positively correlated pointing to varying magnetic mineral contents and textures of the melt rich breccia sequence the vectorial composition and magnetic stability of nrm were investigated by both stepwise alternating field and thermal demagnetization in most cases characteristic single component magnetizations are observed both upward and downward inclinations are present through the sequence and we interpret the reverse magnetization as the primary component in the breccias both the clasts and matrix forming the breccia appear to have been subjected to a wide range of temperature pressure conditions and show distinct rock magnetic properties an extended interval of remanence acquisition and secondary partial or total remagnetization may explain the paleomagnetic results

badly behaved detrital u th he ages problems with he diffusion models or geological models dispersion in apatite u th he ages makes extracting thermochronological information challenging this is particularly problematic for constraining the thermal history of sedimentary basins as the total amount of heating during burial can be small and at the limits of the sensitivity of the apatite u th he system in apatite but undetectable with other thermochronometric systems here we explore the importance of the differences in predepositional thermal histories for individual crystals and the effect of radiation damage related diffusion during both the predepositional and postdepositional thermal history on the dispersion of apatite u th he ages different predepositional thermal histories can lead to a large spread of detrital ages due to radiation damage and its effect on he diffusivity the resulting ages may appear scattered and relationships between age and eu a conventional proxy for accumulated radiation damage may be nonexistent or nonsensical furthermore other variables that are expected to control he diffusivity could vary as a function of source region resulting in false conclusions about controls on he diffusivity and the applicability of the apatite u th he system in general we suggest that in some situations in which detrital apatite u th he age does not correlate with eu it is not because of problems with the thermochronometric system but problems with the assumed geological model

unam scientific drilling program of chicxulub impact structure evidence for a 300 kilometer crater diameter as part of the unam drilling program at the chicxulub structure two 700 m deep continuously cored boreholes were completed between april and july 1995 the peto unam 6 and tekax unam 7 drilling sites are 150 km and 125 km respectively sse of chicxulub puerto near the crater s center core samples from both sites show a sequence of post crater carbonates on top of a thick impact breccia pile covering the disturbed mesozoic platform rocks at unam 7 two impact breccia units were encountered 1 an upper breccia mean magnetic susceptibility is high 55 10 6 si units indicating a large component of silicate basement has been incorporated into this breccia and 2 an evaporite rich low susceptibility impact breccia similar in character to the evaporite rich breccias observed at the pemex drill sites further out the upper breccia was encountered at 226 m below the surface and is 125 m thick the lower breccia is immediately subjacent and is 240 m thick this two breccia sequence is typical of the suevite bunte breccia sequence found within other well preserved impact craters the suevitic upper unit is not present at unam 6 instead a 240 m thick evaporite rich breccia unit similar to the lower breccia at unam 7 was encountered at a depth of 280 m the absence of an upper breccia equivalent at unam 6 suggests some portion of the breccia sequence has been removed by erosion this is consistent with interpretations that place the high standing crater rim at 130 150 km from the center consequently the stratigraphic observations and magnetic susceptibiity records on the upper and lower breccias depth and thickness support a 300 km diameter crater model

on berm breakwaters stability scour overtopping abstract experimental studies on the stability of multilayer berm breakwaters have been carried out two stone mass densities have been used the results have been compared with the results of homogenous berm breakwaters and with other results of multilayer berm breakwaters an equation for the berm recession for homogenous berms has been derived including the effect of stone gradation and water depth erosion at and erosion protection of berm breakwaters have been investigated as well as wave overtopping discharges

wave loads on rubble mound breakwater crown walls abstract crown walls are primarily built to reduce wave overtopping of mound breakwaters several methods have been proposed to calculate wave loads on the crown wall e g iribarren and nogales iribarren r nogales c 1964 obras maritimas dossat ed madrid 376 pp jensen jensen o j 1984 a monograph on rubble mound breakwaters danish hydraulic institute and gunbak and gokce gunbak a r gokce t 1984 wave screen stability of rubble mound breakwaters international symposium of maritime structures in the mediterranean sea athens greece pp 2 99 2 112 in this paper a new method based on those previous results and on further experimental work using monochromatic waves is presented the application of the new method requires waves breaking on the armour layer i e only broken waves will reach the crown wall the method is extended to irregular waves via the hypothesis of equivalence introduced by saville saville t 1962 an approximation of the wave run up frequency distribution proc 8th international conference on coastal engineering mexico city and is applied to the crown walls of gijon and bilbao breakwaters in spain the comparison of the probability force distributions obtained by the present method to that measured by burcharth et al burcharth h f frigaard p berenguer j m gonzalez b uzcanga j villanueva j 1995 design of the ciervana breakwater bilbao in t telford ed proc 4th coastal structures and breakwaters chap 3 institution of civil engineers and jensen 1984 is relatively good

the dimensions of the chicxulub impact crater and impact melt sheet the chicxulub impact crater which is the principal source of impact debris in cretaceous tertiary boundary sediments is currently buried by a 1 km thick sequence of carbonate sediments because the crater is not exposed for direct scrutiny its size has remained uncertain and in particular estimates ranging from 100 to 300 km have been made on the basis of gravity and magnetic field data in this study the physical properties of the k t boundary impact ejecta and chicxulub impact melt sheet have been used to test the geophysical estimates of the impact crater s size on the basis of the measured thickness of impact ejecta in haiti and north america the calculated volume of the impact melt sheet and the chemical and isotopic composition of the melt sheet the chicxulub impact crater is inferred to be 180 km in diameter and to contain a 3 to 7 km thick melt sheet and breccia sequence within a centrally located 100 km diameter region that corresponds to the limits of the transient cavity

anomalous quartz from the roter kamm impact crater namibia evidence for post impact hydrothermal activity abstract centimeter sized quartz pebbles have been found on the rim of the roter kamm impact crater the roter kamm crater has a diameter of about 2 5 km and is situated in the namib desert swa namibia because of the sand coverage impact products breccias impact melt shocked rocks are exposed exclusively in the form of ejecta on the crater rim the quartz pebbles were found close to the main deposits of the impact breccias and show signs of wind abrasion thin sections revealed that the pebbles consist of individual quartz domains that are up to 1 mm in size under crossed nicols polarized light all individual domains show extinction almost simultaneously within 2 which is a rare phenomenon microprobe studies neutron activation analyses and x ray diffractometry confirmed that the material consists of pure quartz the quartz contains three different types of fluid inclusions primary inclusions size about 5 10 m that record the formation conditions of the quartz very small m secondary inclusions associated with the grain boundaries and late inclusions of irregular size freezing point depression measurements of the primary inclusions indicate fluid salinities between 18 3 and 19 6 wt nacl homogenization temperatures t h for the primary inclusions range from 165 to 250 c the quartz and the primary inclusions may provide evidence for a post impact phase of extensive hydrothermal activity generated by the residual heat from the kinetic energy of the impact

a simple method for in situ u th he dating this paper presents a significantly simplified method for in situ u th he dating removing the need to know any absolute concentrations this is done by calculating the normalised u th and he concentrations of a conventionally dated calibration standard from its measured th u ratio and known u th he age and scaling these concentrations to the raw u th and he signals of the sample the th u ratio of the standard can be determined from its measured 208pb 206pb ratio removing the need to use nist glass as a reference material we introduce an la icp ms based method to correct for variable ablation depths between the standard and the unknown using the strength of the ablated 29si signal finally we propose a pseudo depth profile method to assess the effects of compositional zoning on the accuracy of in situ u th he data the effectiveness of the proposed method has been demonstrated on three samples of gem quality sri lanka zircon which yield ages that are in agreement with previously published conventional u th he measurements

dating of a regional hydrothermal system induced by the 1850 ma sudbury impact event new u pb geochronologic data from the sudbury structure directly link extensive hydrothermal activity in crater fill breccias with the 1850 ma impact event and constrain complex impact induced processes to 4 m y semiconformable alteration of breccias in the onaping formation includes silicification albitization chloritization calcitization and complex feldspathization which directly underlies zn cu pb ore deposits geochronologic data demonstrate a middle to lower crustal source for the crater fill deposit that was subsequently affected by the hydrothermal event at 1848 4 3 8 1 8 ma these and other data confirm that the crater fill breccia hydrothermal system sudbury igneous complex and its sublayer ni ore bearing units were emplaced as a result of impact the ages of events associated with the sudbury structure are now more closely constrained than in any other meteorite impact structure

effects of gradation on the long shore transport processes and reshaping of rubble mound breakwaters under construction exposed to head on and oblique waves abstract breakwaters under construction are prone to undesired reshaping because their core is not designed to withstand severe or moderate wave loads while the reshaping of a finished breakwater was subject of comprehensive research few studies dealt specifically with the reshaping mechanisms of a breakwater under construction i e of the breakwater core this paper presents a set of experimental tests focused on the reshaping of rubble mound breakwater trunks and roundheads undergone during the construction phase the study accounts for oblique wave attacks and grading of the construction material better insights on how and how far such reshaping takes place are derived in the perspective of providing coastal designers and constructors with valuable information for optimising the selection of the fractions and volumes of the construction materials and for taking into account potential damages within the construction schedule and therefore additional costs

physical properties of the yaxcopoil 1 deep drill core chicxulub impact structure mexico abstract the chicxulub structure in mexico one of the largest impact structures on earth was formed 65 ma by a hypervelocity impact that led to the large mass extinction at the k pg boundary the chicxulub impact structure is well preserved but is buried beneath a sequence of carbonate sediments and thus requires drilling to obtain subsurface information the chicxulub scientific drilling program was carried out at hacienda yaxcopoil in the framework of the international continental scientific drilling program in 2001 2002 the structure was cored from 404 m down to 1511 m through three intervals 794 m of postimpact tertiary sediments a 100 m thick impactite sequence and 616 m of preimpact cretaceous rocks thought to represent a suite of megablocks physical property investigations show that the various lithologies including the impactite units and the k pg boundary layer can be characterized by their physical properties which depend on either changes in fabric or on mineralogical variations the magnetic properties show mostly dia or paramagnetic behavior with the exception of the impactite units that indicate the presence of ferromagnetic probably hydrothermally deposited magnetite and pyrrhotite the magnetic fraction contributes mainly to enhanced magnetization in the impactite lithologies and in this way to the observed magnetic anomalies the shape and orientation of the magnetic grains are varied and reflect inhomogeneous fabric development and the influence of impact related redeposition and hydrothermal activity the chicxulub impact occurred at the time of the reverse polarity geomagnetic chron 29r and this finding is consistent with the age of the k pg boundary