the fossil record of drilling predation on barnacles abstract the fossil record of drilling predation has been studied in detail for a few major invertebrate groups bivalves gastropods brachiopods and echinoids while other prey e g scaphopods serpulids decapods and barnacles have been largely neglected herein we report on drilling predation using an extensive collection of cretaceous quaternary barnacles 50 000 specimens in total 312 drill holes of predatory origin were found in eocene holocene wall and opercular plates of balanomorph and scalpellomorph barnacles the drilled specimens originated from localities in the usa jamaica panama the netherlands belgium antarctica south africa chile and venezuela suggesting that drilling predation on barnacles was a worldwide phenomenon during the cenozoic muricid gastropods are the inferred producers of the majority of the drill holes two drill holes were likely caused by octopods drilling frequencies lack major temporal trends and appear low

sand ripple characterization using an extended synthetic aperture sonar model and parallel sampling method the aim of this work is to characterize the seafloor by estimating invariant sand ripple parameters from synthetic aperture sonar sas imagery using a hierarchical bayesian framework and a known sensing geometry a method for estimating sand ripple frequency amplitude and orientation values from a single sas image as well as from sets of sas imagery over an area is presented this is accomplished through the development of an extended model for sand ripple characterization and a metropolis within gibbs sampler to estimate sand ripple frequency amplitude and orientation characteristics for multiaspect high frequency side look sonar data results are presented on synthetic and measured sas imagery that indicate the ability of the proposed method to estimate desired sand ripple characteristics

late quaternary marine mollusca as palaeoenvironmental proxies a compilation and assessment of basic numerical data for ne atlantic species found in shallow water abstract temperature salinity and depth limits are given for selected molluscs from boreal and arctic regions of the ne atlantic and adjacent shelf seas the significance of the data for palaeoenviromental analysis is briefly discussed

taphonomic processes affecting late quaternary molluscs along the coastal area of buenos aires province argentina southwestern atlantic molluscan concentrations were abundantly deposited along the coastal area of buenos aires province argentina southwestern atlantic by different sea level episodes during the late quaternary most of them represent holocene ridges highly energetic depositional environments subparallel to the modern shore where the main associations preserved are parautochthonous and others mainly autochthonous associations occurring in tidal flats seaward of the ridges or coastal lagoon deposits accumulated behind them low energetic analysis of three physical taphonomic signatures disarticulation fragmentation and abrasion of the molluscan skeletons of four bivalve species tagelus plebeius mactra isabelleana pitar rostratus brachidontes rodriguezi which exhibit different shell shape and thickness conditions habitats and life habits and represent the most widely distributed and palaeoecologically significant taxa along the area of study support the distinction of both palaeoenvironments characterized by low and high taphonomic grades respectively the taphonomic alteration breakage and abrasion of most shells seems to have been a direct consequence of the amount of reworking of the material especially during periods of exposure along an extensive beach zone more than a consequence of transport from a distant original habitat abrasion signatures are the only probably correlated with after death time span shells showing a near modern aspect and original luster being most probably younger than those with chalky or polished appearance the infaunal species analysed typically living on clays and silts or fine sandy bottoms of the infralittoral or intertidal t plebeius m isabelleana are better preserved less fragmented and abraded than shells of the epibyssate b rodriguezi which lives on hard bottoms of the intertidal and supralittoral areas more exposed to highly unstable conditions with longer episodes of exposure although these nearshore concentrations may represent a time averaging of 1000 yr b p taking into account the ecological requirements of the most constant and dominant taxa together with the taphonomic analysis of shells from parautochthonous and autochthonous associations they still represent valid indicators of past marginal marine conditions in the area but are only reliable to establish long term palaeoenvironmental variations

sedimentologic palaeoecologic taphonomic and ichnologic criteria for high resolution sequence analysis a practical guide for the identification and interpretation of discontinuities in shelf deposits abstract we classify stratigraphic bounding surfaces defined by sedimentologic palaeoecologic or taphonomic and ichnologic criteria our case study refers to a particularly well preserved outcrop of miocene sedimentary rocks on the pacific coast of central costa rica these rocks mainly fine sandstones accumulated in a tropical shallow marine strongly subsiding sand swamped forearc coastal embayment sedimentary bounding surfaces or lithofacies bounding surfaces can be subdivided into erosional and non erosional boundaries between depositional systems in our case study erosional boundaries angular unconformities and erosional disconformities were produced by block tilting tectonic reactivation of cliffs or wave and tidal ravinement during sea level rises and are interpreted as sequence boundaries and bases of parasequence sets respectively non erosional transitions delimit genetically closely related depositional systems sharp contacts are considered as bases of parasequence sets diffuse transitions are interpreted as parasequences palaeoecologically and taphonomically defined fossiliferous bounding surfaces or layers mostly occur within depositional systems they seldom coincide with erosional lithofacies bounding surfaces two types can be distinguished autochthonous and allochthonous fossil concentrations autochthonous fossil concentrations can be grouped into layers with epifaunal and semi infaunal molluscs in growth position layers with infaunal bivalve associations in growth position and coprolith concentrations the majority of these layers is developed as concretion horizons autochthonous fossil concentrations invariably signal a relative rise of sea level and are interpreted as bases of parasequences or parasequence sets allochthonous fossil concentrations can be grouped into fossil concentrations produced by non turbulent shelf currents driftwood concentrations tempestite fossil concentrations fossil concentrations produced by near shore turbulent flow regimes and fossil concentrations produced by tidal currents they are only seldom developed as concretion horizons the appearance of allochthonous fossil concentrations in certain intervals of a given section and disappearance in others is clearly a signal of shallowing or deepening seas fossil concentrations produced by non turbulent shelf currents and driftwood concentrations can be interpreted as bases of parasequences ichnofabrics can be used as an additional tool for the identification and interpretation of bounding surfaces erosional boundaries may be marked by well known features such as gastrochaenolites sequence boundaries may be preceded by thalassinoides boxworks strongly thickened by concretionary growth at non erosional boundaries however trace fossils commonly fail to mark the crucial turning point yet they seem to record environmental changes for instance a coming transgression far earlier than any sedimentary phenomenon or body fossil after prolonged swamping with sand during a relative sea level lowstand they explode in diversity during the following transgression before this transgression produces its own typical sediment equally after the dumping of organic matter during the transgression trace making organisms announce the coming highstand by reworking their habitat to the degree of total destratification before the highstand deposits proper force them to keep up with aggradation which results in the typical highstand selection of deep tier structures thus the degree of reworking by bioturbation seems to be a very good and easily recognizable indicator of the internal architecture of parasequences and parasequence sets

great earthquakes on canada s west coast a review this article is one of a series of papers published in this special issue on the theme lithoprobe parameters processes and the evolution of a continent geological survey of canada contribution 20090196 the first lithoprobe transect in 1984 across vancouver island had primary objectives to define the structure associated with subduction and constraints on the potential for great thrust earthquakes

experiments on bed configurations in fine sands under bidirectional purely oscillatory flow and the origin of hummocky cross stratification abstract we made experiments on bed configurations in bidirectional purely oscillatory flows at a wide range of oscillation periods t and maximum orbital velocities um in two closed flow ducts 0 1 m and 0 4 m wide the motivation was that understanding of bed configurations in oscillatory flows with long periods and high velocities during storm induced deposition in the shallow ocean is needed for interpreting the stratification in such deposits the ripples produced were classified geometrically as two dimensional 2d or three dimensional 3d in plan view geometry and dynamically as oscillatory current ripples dependent upon the patterns of flow and sediment transport specific to the existence of the oscillation itself and reversing current ripples dependent on he existence of the current in one direction or the other during the oscillation runs in the small duct with effectively 0 19 mm and 0 30 mm sand showed the familiar small 2d oscillatory current ripples at short t and low um passing into much larger ripples with spacing up to 2 m at periods of 6 20 s and velocities up to 1 m s with no apparent limit on size with increasing t and um even in such a narrow duct there was some evidence of 3d geometry of the large ripples runs in the large duct with effectively 0 11 mm sand at a single period of about 9 5 s showed a progression from small 2d oscillatory current ripples through small 3d reversing current ripples to large round crested 3d oscillatory current ripples with prominent superimposed 3d to 2d reversing current ripples the large 3d ripples which increased in spacing to 1 3 m at the h ghest attainable um of 1 m s shifted in position and changed in size with time seemingly at random causing substantial local and temporary erosion and deposition at a given point on the bed there was no net aggradation during the runs but a deposit was synthesized by taking a time series of sidewall bed profiles during a run in which large 3d ripples developed from an initially plane bed and changed irregularly in size and position as they grew toward equilibrium and then plotting the profiles one above the previous with a constant upward increment the resulting stratification is strikingly similar to much hummocky cross stratification in ancient sandstones this suggests that some hummocky cross stratification is generated during sediment fallout from strong purely oscillatory flows at moderate to long oscillation periods as large 3d oscillatory current bed forms develop from a planar bed during strong but waning flow

macrofaunal palaeoecology and stratigraphy in late quaternary shelf sediments off northern norway abstract in a late pleistocene holocene glaciomarine marine sequence a minimum of 115 taxa macrofossils and trace fossils are recognized on the basis of 690 samples from gravity cores from andfjorden nine and malangsdjupet one the biostratigraphy is proposed on the basis of reconstructions of autochthonous macrofaunal assemblages a total of seven zones are defined five in the pleistocene and two in the holocene the ages of which are estimated by radiocarbon datings in ascending order the zones are mixed assemblage zone barren interzone arctinula greenlandica assemblage zone before 14 000 yr b p yoldiella intermedia assemblage zone 14 000 13 000 yr b p bathyarca glacialis assemblage zozne 13 000 yr b p bathyarca glacialis kelliella miliaris interval zone 10 000 ca 7800 yr b p and kelliella miliaris assemblage zone ca 7800 present the bathyarca glacialis zone is divided into three faunules i 13 000 12 000 yr b p ii 12 200 11 100 yr b p and iii 11 100 10 000 yr b p the palaeoecology of the faunal succession is elucidated by means of the sample frequency method two major environmental events have influenced the faunal history the first was the onset of the deglaciation of the thoughs the second the intrusion of the norwegian current bringing warm saline atlantic water into the troughs at 10 000 yr b p the pleistocene fossils demonstrate an arctic macrofaunal succession from a low order stage to a high order equilibrium stage that is in principle identical to that of a modern faunal development in fjords following a disturbance causing anoxia an estimated polychaete mollusc ratio together with changes in sediment influx depositional rates and grain size of substrate indicate the presence of different energy regimes where the inner and outer reaches of andfjorden are compared about 10 000 yr b p a major faunal change took place the previous arctic fauna was replaced by a boreal fauna which developed into the modern fauna the most important factor causing the replacement and renewed faunal succession were probably the rise in temperature and salinity as well as an increase in nutrient supply brought about by the intrusion of the norwegian current a comparison with shallower shelf areas indicates a somewhat different faunal composition in the pleistocene brought about by different depositional environments however a similar major faunal change took place at 10 000 yr b p and there are fewer discrepancies among the holocene faunas regional comparison indicates that some of the phases in the development of the fauna probably are rather local while other have a more regional nature

seismic potential associated with subduction in the northwestern united states despite good evidence of present day convergence of the juan de fuca and north american plates there has been remarkably little historical seismic activity along the shallow part of the juan de fuca subduction zone although we cannot completely rule out the possibility that the plate motion is being accommodated by aseismic creep we find that the juan de fuca subduction zone shares many features with other subduction zones that have experienced great earthquakes

the influence of molluscan taxon on taphofacies development over a broad range of environments of preservation the sseti experience abstract we utilize information from a suite of molluscan species deployed in a variety of caribbean and gulf of mexico environments of preservation eop by sseti shelf and slope experimental taphonomy initiative to examine to what degree interpretation of the taphofacies is influenced by the mix of species presented to the observer the environment of preservation was the overwhelming determinant of taphonomic state at sseti sites species mix played a subsidiary role a few consistent trends were observed for example arctica islandica strombus luhuanus mytilus edulis and mercenaria mercenaria tended to be more highly dissolved than others mytilus edulis consistently was fragmented more than other species otherwise most species behaved uniquely over a range of eops significant species by eop interactions were the norm a taphofacies model is developed to examine further the role of species on the degradational state of the assemblage two metrics are described a taphonomic degradation score describes the overall degradational state of the assemblage from pristine to highly degraded and a taphonomic signature describes the relative importance of a suite of taphonomic processes model simulations show that the impact of species on degradational state is low initially because all shells are in relatively pristine condition shortly after death the impact is high at intermediate degradational states because factors inherent to the species modulate the time course of taphonomic decay the impact is low again at highly degraded assemblages as the taphonomic process brings shell condition to a common denominator of relatively intense degradation regardless of species fossil assemblages almost certainly express this continuum in some degree the consequence is that better preserved assemblages are ones in which species can affect taphofacies character and so similar eops may be judged distinctive due to differential dominance by one species or another teasing out characteristics that might forewarn the observer in this instance is an important goal simulations show that low evenness is an important warning sign

characteristic trace fossil associations in oxygen poor sedimentary environments the sedimentologic record of oxygen poor depositional environments typically includes trace fossils produced by deposit feeding organisms we propose an oxygen controlled trace fossil model in which increasing oxygen concentration of interstitial water in sediment parallels a transition from fodinichnia dominated through pascichnia dominated to domichnia dominated trace fossil associations in certain places such as parts of the late paleozoic oquirrh basin in utah this model provides a reasonable alternative to the more traditional depth controlled model of trace fossil distribution

high frequency bottom backscatter measurements in shallow water bottom backscatter measurements have been made at six shallow water sites over the frequency range 20 85 khz for bottom types including silt sand and gravel bottom samples were taken at all experimental sites and some sites are well characterized in terms of seabed acoustic parameters and roughness a slight frequency dependence was observed and scattering strength was generally found to decrease as grazing angle decreased backscatter measurements in four azimuthal directions showed no measurable anisotropy for grazing angles between 10 and 40 spatial variability of backscatter strength was measured along two tracks approximately 35 and 75 km long

taphonomy of eemian marine molluscs and acorn barnacles from eastern arkhangelsk region northern russia abstract marine eemian deposits along the pyoza river and its tributary varchuska arkhangelsk region constitute successions of muddy and sandy facies with rich macrobenthic fauna dominated by bivalves and barnacles taphonomic features formed by abrasion disarticulation dissolution fragmentation bioerosion and encrustation define taphofacies for a palaeoenvironmental model five bivalve taphofacies and three barnacle taphofacies could be distinguished both bivalves and barnacles are poorly preserved in foreshore shoreface environments as the shells were subjected to extensive transportation by currents the shells were best preserved in offshore environments where rapid episodic sedimentation enabled within habitat preservation in some cases even preservation in life position barnacles are absent from the most clay rich offshore deposits probably because of clogging of filters by turbidity and lack of suitable substrate such dissimilarities suggest that the number and distribution of taphofacies may depend on which fossil groups are used interspecific variability may exist within the individual taphofacies the barnacles for example tend to be better preserved than the mussel mytilus edulis although both are fixosessile suspension feeders this indicates that not only life habits but also intrinsic shell properties influence preservation thus taphofacies analyses should combine data on taphonomic features specific life habit and shell properties to determine overall preservation patterns in that way taphofacies analyses may form a powerful tool for palaeoenvironmental analyses of marine deposits

taphofacies models for epeiric sea environments middle paleozoic examples abstract skeletonized organic remains display various modes of occurrence which may be used to better understand the sedimentary dynamics of strata within which they occur aspects of skeleton biostratinomy and early diagenesis are distributed nonrandomly within the stratigraphic record and therefore are useful in discriminating taphonomic facies for purposes of taphonomic comparisons skeletonized marine taxa are reduced to five generally defined types massive arborescent univalved bivalved and multielement different skeletal types display different attributes which reflect specific responses to physical chemical and or biological processes the taphonomic properties of disarticulation reorientation and sorting fragmentation corrosion and abrasion skeletal dissolution and early diagenesis display patterns which are predictable with respect to a variety of inter related physico chemical parameters these can be quantified through indices which evaluate relative frequencies of occurrence sedimentary events alter the temporal and spatial homogeneity of processes which generate background taphofacies and superimpose event related properties thereby generating composite taphofacies such events produce taphonomic signature which in themselves correspond to specific environmental conditions i e water depth and event intensity indeed background and event related processes interact to generate complex but nonetheless diagnostic facies attributes useful in reconstructing paleoenvironments the comparison of standard indices among beds should allow the recognition of stratigraphic patterns and facilitate the diagnosis of systematic gradients among taphonomic properties deductively derived taphofacies models corroborated by empirical data from paleozoic strata illustrate the distribution of taphonomic properties with respect to environmental energy turbulence background sedimentation rate and sediment oxygenation these parameters also indirectly influence other less obvious factors including bioturbation and sediment chemistry trends in the distribution of taphonomic properties taphonomic gradients permit us to map otherwise unrecognized trends in these environmental parameters across a hypothetical onshore offshore transect within a generalized epeiric sea setting taphofacies models provide the quantifiable basis by which evolution of taphofacies may be gauged and then related to biotic and abiotic phenomena moreover taphofacies models evaluate the preservability of skeletal remains in different environmental settings and therefore provide information important in paleoecological studies

hydrography and mollusc faunas of the baltic and the white sea north sea seaway in the eemian abstract palynologically dated mollusc and cirriped faunas from a region extending from the north sea through the baltic and karelia to the arkhangelsk region show that the hydrography of the baltic was very different from the holocene for 2 2 5 ka in the early eemian a seaway existed from the barents to the north sea through karelia until it was severed at the present continental watershed to the north of lake onega after this the ladoga onega trough remained an arm of the baltic for several millennia the benthic faunas are comparable to the holocene but their boundaries were displaced much further into the baltic notable differences from the holocene are the absence or rarity of the macoma balthica biocoenosis and the presence of cold portlandia dominated biocoenoses in karelia in the belt sea and western baltic winter sea surface temperatures and salinity were higher than now by ca 6 c and 15 and the distinctly brackish top layer was missing at the same time cold bottom water 2 5 c with a tendency to anoxia characterised the karelian arm of the baltic water exchange through this area was inhibited by the constriction to the north of lake onega and the basin and threshold bathymetry water transport through the white sea baltic seaway was too sparse to play an active role in the north atlantic surface circulation or climatic change in the region the high salinity and temperature in the belt sea and western baltic persisted throughout the eemian and are explained by wider and deeper passage from the north sea to kattegat wider straits through denmark higher salinity in the north sea higher evaporation as well as more dispersed fresh water supply the advection of oceanic water into the baltic culminated in the early eemian before the carpinus zone and probably resulted in an oceanic climate in the baltic region while at the same time cold winters produced cold bottom water in the karelian arm

occurrence and orientation of anorbital ripples in near shore sands dimensional and of the anorbital type l0 d50 524 to 535 d0 d50 4300 to 25 000 with 7 6 cm mean wavelength in 150 mm median diameter sand the ripple crests were typically aligned perpendicular to the incident sea and swell direction specifically about 60 of the normals to the crests differ from wave direction by less than 5 and more than 90 by less than 10 during rapid changes in wave direction however ripple orientation adjustment lagged the wave direction by o 1 h ripple reorientation occurred piecewise along the crests with new crest segments sometimes bridging two or more old segments the 1 h reorientation time scale is longer by several orders of magnitude than the adjustment time based on theoretical bed load transport rate and the ripple volume as a possible explanation we suggest that a significant fraction of the transport more than 90 bypasses the ripples during orientation adjustment through large angles

geometry migration and evolution of wave orbital ripples at leo 15 observations of the temporal evolution of the geometric properties and migration of wave formed ripples are analyzed in terms of measured suspended sand profiles and water velocity measurements six weeks of bedform observations were taken at the sandy medium to coarse sized sand leo 15 site located on beach haven ridge during the late summer of 1995 with an autonomous rotary sidescan sonar during this period six tropical storms several of hurricane strength passed to the east of the study site ripples with wavelengths of up to 100 cm and with 15 cm amplitudes were observed the predominant ripples were found to be wave orbital scale ripples with ripple wavelengths equal to 3 4 of the wave orbital diameter although orbital diameters become larger than 130 cm during the maximum wave event it is unclear if a transition to non orbital scaling is occurring ripple migration is found to be directed primarily onshore at rates of up to 80 cm day suspended transport due to wave motions calculated by multiplying acoustic backscatter measurements of suspended sand concentrations by flow velocity measurements are unable to account for a sufficient amount of sand transport to force ripple migration and are in the opposite direction to ripple migration thus it is hypothesized that the onshore ripple migration is due to unobserved bedload transport or near bottom suspended transport bedload model calculations forced with measured wave velocities are able to predict the magnitude and direction of transport consistent with observed ripple migration rates sequences of ripple pattern temporal evolution are examined showing mechanisms for ripple directional change in response to changing wave direction as well as ripple wavelength adjustment and erosion due to changing wave orbital diameter and relative wave to current velocities

time sequence observations of wave formed sand ripples on an ocean shoreface analysis of an 18 day time lapse film record of shoreface ripple development with concurrent measurements of near bottom flow and surface waves provides new insight on equilibrium bedform conditions adjustment of ripple planform to variable hydrodynamics and ripple migration behaviour the study was conducted in approximately 10 m water depth 1 km off martinique beach on the atlantic coast of nova scotia canada under low energy summer wave conditions significant wave height and peak period during the study averaged 0 7 m and 8 s respectively with extremes up to 1 7 m and 11 s during passage of three weak weather disturbances six mutually exclusive ripple types have been defined 1 short wavelength regular ripples 2 variable bifurcated ripples 3 variable terminated ripples 4 short crested ripples 5 long wavelength regular ripples and 6 chaotic ripples ripple wavelength ranged from 0 07 m to 0 24 m and displayed a strong reynolds number dependence together with other published field data the results suggest a lower limit of 0 06 m for the wavelength of wave ripples in ocean shoreface environments ripple orientation ranged through 38 and responded rapidly to changes in wave approach direction but did not conform to the orientation of the adjacent shoreline ripples were observed to migrate both on and off shore with and against the wave advance direction at rates up to 0 1 m h 1 associated with net flows other than wave induced onshore asymmetry and mass transport migration mainly of ripple types 1 and 2 occurred during the peak of storm events but showed no obvious correlation with measured near bottom flow magnitude or direction ripple behaviour demonstrates equilibrium with prevailing dynamic conditions when straight crested rippie types 1 and 5 are present disequilibrium in orientation or dimensions is expressed by increasing sinuosity bifurcation and crest termination in types 2 3 4 and 6

late pleistocene in the southeastern white sea and adjacent areas arkhangelsk region russia stratigraphy and palaeoenvironments abstract an overview of the late pleistocene and holocene stratigraphy of the southeastern white sea region is presented in the paper based upon the analysis of published data new results and correlation between marine and terrestrial archives the pleistocene stratigraphic record in the southeastern white sea region begins with mikulino horizon which comprises the whole mis 5 and is composed of marine sediments sands sandy clays and clays with boreal arctic boreal and lusitanean boreal microfauna and mollusc fauna dated back at 161 77 ka the podporozhie horizon in the southeastern white sea region comprises the mis 4 71 57 ka complex of marine mostly near shore sediments the glaciation during mis 4 was localized on one hand within the kola peninsula on the other hand the glacier probably overlapped the kanin peninsula and the mezen bay however the gorlo strait was not blocked and the northward river outflow was preserved the leningrad horizon had been formed also in the marine sedimentation pattern during the mis 3 57 29 ka however the lack of data in the region allows us to make only the comparative analysis the ostashkov horizon mis 2 29 11 7 ka is composed of the deposits of glacial paragenetic series tills glaciofluvial glaciomarine and glaciolacustrine deposits but also comprises the deglaciation late glacial sedimentary complex including marginal tills the holocene stratigraphic record is subdivided into three stages 11 7 9 5 9 5 3 5 and 3 5 0 ka bp and do not match with new formal stratigraphic subdivision of the holocene series

possible tsunami along the northwestern coast of the united states inferred from indian traditions subduction of the juan de fuca and gorda plates beneath western north america presents a paradox despite the fact that there is good evidence of 3 to 4 cm yr of ongoing convergence there is a remarkable paucity of either historic or instrumentally recorded shallow subduction earthquakes steady aseismic slip along the entire cascadia subduction zone provides one explanation for this seismic quiescence however the cascadia subduction zone shares many features including temporal quiescence with other subduction zones that have experienced very large shallow subduction earthquakes heaton and kanamori 1984 yet there is no direct geologic or historical evidence presently available to confirm that great shallow subduction earthquakes have occurred along the coast of washington oregon and northern california however there are reports describing indian legends of great sea level disturbances that may be related to large nearby earthquakes in this letter we briefly review the history of exploration and settlement of this region by nonnative people and then discuss legends from indians in northern washington and northern california

principal bed states during sandyduck97 occurrence spectral anisotropy and the bed state storm cycle 1 results are presented from 70 days of nearly continuous in situ acoustic imagery of the nearshore sandy seabed in 3 m mean water depth at two locations separated by 40 m cross shore distance the bottom sediments were 150 m median diameter sand with nearly identical size distributions at the two locations five principal bed states were observed irregular ripples cross ripples linear transition ripples lunate megaripples and flat bed the linear transition and flat bed states were the most frequent together accounting for 68 of the total time bed state occurrence was a strong function of incident wave energy each bed state occurring within a relatively narrow range of sea and swell energies during the 12 major storm events spanned by the record the bed response was characterized by a repeatable bed state storm cycle involving four of the five principal states lunate megaripples did not appear repeatedly and thus may be a special case with no obvious dependence of bed state occurrence on prior bed state or on third moment measures of wave nonlinearity radial spectra from the rotary acoustic images indicate pronounced differences in the anisotropy of spatial scales for the different bed states and exhibit onshore offshore differences which are likely related to ripple migration

time averaging overcompleteness and the geological record time averaging makes events that happened at different times appear to be synchronous in the geological record it can generate overcompleteness by concentrating objects e g fossils clasts from a longer time interval into a unit that formed during a shorter interval time averaging results both from extrinsic agents e g vertical mixing condensation analytical pooling of data and the intrinsic properties of the fossilized objects preservational potential and abundance variation in intrinsic and extrinsic factors results in disharmonious time aver aging different taxa preserved together may be time averaged over different time scales because extrinsic and intrinsic factors vary in time and space the geological record is characterized by vertical and lateral variations in time averaging the importance of time averaging depends on the time aver aging threshold which is determined by the time scale of the process of interest significant time aver aging occurs when the scale of temporal mixi

source characteristics of hypothetical subduction earthquakes in the northwestern united states historic earthquake sequences on subduction zones that are similar to the cascadia subduction zone are used to hypothesize the nature of shallow subduction earthquakes that might occur in the northwestern united states based on systematic comparisons of several physical characteristics including physiography and seismicity subduction zones that are deemed most similar to the cascadia subduction zone are those in southern chile southwestern japan and colombia these zones have all experienced very large earthquake sequences and if the cascadia subduction zone is also capable of storing elastic strain energy along its greater than 1000 km length then earthquakes of very large size m w 8 1 2 must be considered circumstantial evidence is presented that suggests but does not prove that large subduction earthquakes along the cascadia subduction zone may have an average repeat time of 400 to 500 yr

time and size of a giant earthquake in cascadia inferred from japanese tsunami records of january 1700 geological evidence shows that great earthquakes have occurred in the recent prehistoric past in the cascadia subduction zone off the pacific coast of north america the most recent event or series of events is dated at about 300 years ago1 4 but the precise date and magnitude have not been determined geological investigations have not been able to distinguish a single giant earthquake from a series of great earthquakes occurring over a timespan of a decade or two4 although this information is important for the assessment of future hazard5 we have found several tsunami records in japan from ad 1700 with no indication of a local cause historical earthquake records and palaeoseismic evidence indicate the absence of a large earthquake in 1700 in south america alaska or kamchatka leaving cascadia as the most likely source of this tsunami the estimated time of the earthquake is the evening about 21 00 local time of 26 january 1700 the magnitude is estimated as 9 from the tsunami heights in which case the earthquake ruptured the entire length of the cascadia subduction zone2 these estimates are consistent with native american legends that an earthquake occurred on a winter night6

wave formed sand ripples at duck north carolina a recently developed acoustic multiple transducer array was utilized to measure small scale bed forms in the nearshore and inner shelf regions at duck north carolina two populations of wave formed ripples were observed short wave ripples swr with heights ranging from 3 mm to 2 cm and lengths ranging from 4 to 25 cm and long wave ripples lwr with heights ranging from 3 mm to 6 cm and lengths ranging from 35 to 200 cm the swr were only present sometimes and their presence or absence was determined by a critical value of the near bed mobility number the swr were highly dynamic sometimes flattening during wave groups and reforming over several incident wave periods the lwr in contrast were almost always present they were longer and lower relief than predicted by models or generally observed previously both swr and lwr were often observed to migrate shoreward but were rarely observed to migrate seaward the dimensions of the swr when they were present were predictable by the nielsen 1981 model or the wiberg and harris 1994 model to within approximately a factor of 2

significance of molluscan shell beds in sequence stratigraphy an example from the lower cretaceous mannville group of canada1 detailed study of marine shales the ostracod zone within a cretaceous third order transgressive regressive sequence in the alberta foreland basin reveals a systematic association between shell beds and parasequence scale flooding surfaces including surfaces of maximum flooding the ostracod zone a subsurface lithostratigraphic unit known as the calcareous member in outcrop consists of 10 20 m of black shale and bioturbated sandstones with many thin fossiliferous limestones parasequences shallowing up cycles 2 3 m thick were delineated within this transgressive unit based on lithology sedimentary structures degree of bioturbation dinoflagellate diversity total organic carbon and carbon sulphur ratios many flooding surfaces are firmgrounds or hardgrounds shell rich limestones occur in three different positions relative to these flooding surfaces and each has a distinctive bioclastic fabric and origin i base of parasequence shell beds bops lie on or just above flooding surfaces in the deepest water part of a parasequence they are thin up to a few centimetres graded or amalgamated skeletal packstones wackestones composed of well sorted granular shell and are interpreted as hydraulic event concentrations of exotic shell debris ii top of parasequence shell beds tops are capped by flooding surfaces at the top shallowest water part of a parasequence they typically are several decimetres thick are physically amalgamated packstones grainstones or bioturbated wackestones and contain abundant whole as well as comminuted shells these are composite multiple event concentrations of local shells iii mid sequence shell beds rest on as well as are capped by firmgrounds or hardgrounds and are intercalated between parasequences in the deepest water part of the larger sequence they are laterally extensive lime mudstones a few decimetres thick with sparse shells in various states of dissolution recrystallization and replacement these beds are terrigenous starved hiatal concentrations and record maximum flooding within the ostracod zone offshore sections of the ostracod zone typically contain several starved mid sequence shell beds underscoring the difficulty of identifying a single maximum flooding surface within a third order sequence

shell beds as tools in basin analysis the jurassic of kachchh western india skeletal concentrations are ubiquitous in bathonian oxfordian shallow water sediments of the pericratonic basins of kachchh and rajasthan western india by analysing the biofabric of the concentrations and the taphonomic signatures of individual components it is possible to distinguish between the final concentration process recorded by the biofabric and events taking place prior to final deposition recorded by taphonomic signatures the skeletal concentrations are grouped in nine genetic types ranging from fair weather wave concentrations to storm concentrations and condensed concentrations they mainly reflect biological and hydraulic processes and in some instances permit estimates of their duration and show a rough zonation along a bathymetric gradient plotted against the stratigraphic sections the skeletal concentrations show in accordance with other sedimentological data three orders of bathymetric trends the first two orders are shallowing deepening cycles which possibly correspond to eustatic changes in sea level the third order represents parasequences with a strongly asymmetric sedimentary record skeletal concentrations are a useful additional tool in basin analysis but are best used in combination with other data