resistivity and magnetic surveys in groundwater prospecting in volcanic areas case history maar of beaunit puy de dome france volcanic maar structures sometimes contain important groundwater reserves the volcanic formations of the beaunit maar have a much higher magnetization than their granite substrata the structure of the maar and its volcanoclastic infill can therefore be defined by magnetic surveying and resistivity soundings resistivities of the geological formations present in the beaunit maar are comparable to those recorded in other volcanic systems the geophysical data confirmed by drilling tests contain useful information on the hydrogeological regime of the volcanic system under investigation

groundwater prospecting and management in an agro based rural environment of crystalline terrain of india an integrated hydrogeological investigation was made to delineate the socioeconomic problems in the agro based rural environment of the crystalline terrain of guntur district andhra pradesh india the integrated investigation comprised geoelectrical ge surveys remotely recorded data and pumping tests the subsurface layer parameters obtained from the sounding interpretation are presented in the form of ge sections and a fence diagram characteristic resistivities of various layers were obtained by correlating the known lithology with the ge sounding data total transverse resistance ttr and total longitudinal conductance tlc were computed from the interpretation of the sounding data the ttr map was compared with the depth of bedrock map and the values of tlc were correlated with the thickness of geoelectrical layers and water quality lineaments were mapped from the satellite imageries hydraulic properties were determined using the standard pumping test analysis potentiality of aquifer zones was assessed after comparing the known thickness of aquifer formations with the corresponding discharge q values the integrated study helps to evolve a criterion to classify the groundwater potential zones gpz into low medium and high based on the thickness of the aquifer zone and its corresponding resistivity ttr tlc and q a management plan has been proposed for sustainable development of groundwater in the region to eliminate water scarcity and its related problems

removal of strip noise in radio echo sounding data using combined wavelet and 2 d dft filtering radio echo sounding res can be used to understand ice sheet processes englacial flow structures and bed properties making it one of the most popular tools in glaciological exploration however res data are often subject to strip noise caused by internal instrument noise and interference and or external environmental interference which can hamper measurement and interpretation for example strip noise can result in reduced power from the bed affecting the quality of ice thickness measurements and the characterization of subglacial conditions here we present a method for removing strip noise based on combined wavelet and two dimensional 2 d fourier filtering first we implement discrete wavelet decomposition on res data to obtain multi scale wavelet components then 2 d discrete fourier transform dft spectral analysis is performed on components containing the noise in the fourier domain the 2 d dft spectrum of strip noise keeps its linear features and can be removed with a targeted masking operation finally inverse wavelet transforms are performed on all wavelet components including strip removed components to restore the data with enhanced fidelity model tests and field data processing demonstrate the method removes strip noise well and incidentally can remove the strong first reflector from the ice surface thus improving the general quality of radar data

effects of triple junction migration and glacioeustatic cyclicity on evolution of upper slope morphologies offshore eel river basin northern california abstract the upper continental slope of the eel river basin is affected both by uplift and seismicity associated with the northward migrating mendocino triple junction mtj and by glacioeustatic fluctuations as a result seismically imaged slope sequences vary dramatically along strike the southern slope is dominated by the humboldt slide a stack of nine deformed sequences older undeformed sequences thicken landward while slide sequences maintain a constant travel time thickness onset of deformation was abrupt within the slide reflector amplitudes alternate high amplitude reflectors are wavy to shingled and suggest increasing deformation with depth low amplitude reflectors are sub parallel to wavy throughout high and low amplitude sequence couplets suggest repetitive deposition of contrasting lithologies a response we ascribe to late pleistocene glacioeustatic cyclicity if each couplet represents a 100 kyr late pleistocene sea level cycle then humboldt slide deformation began 450 ka this is approximately coeval with onset of deformation on the adjacent shelf 500 ka previously attributed to uplift and seismicity associated with northward encroachment of the mtj we believe migration of the mtj triggered the slide and is the cause for continuing deformation today by sediment creep along internal glide planes north of the humboldt slide we interpret an upper slope anticline as a seaward extension of the little salmon fault zone lsfz previously identified landward both faulting and folding extend to the seafloor indicating that deformation continues an adjacent buried anticline suggests that deformation has also jumped northward within the lsfz on some parts of the upper slope north of the lsfz the slope is dominated by downslope trending channels and gullies these features are v shaped vertically stacked and increase in depth downslope they are mostly infilled their physiographic expression is generally subdued by sediment draping channel fills generally consist of basal high amplitude reflectors overlain by reflectors of lower amplitudes several large channels on older surfaces are concentrated near the northern end of the seismic coverage smaller channels are more evenly distributed along the margin on younger surfaces channels also extend increasingly landward on progressively younger surfaces indicating migration in that direction with time these incisions are clearly inactive during highstands as at present slope deposition today is dominated by draping of hemipelagic sediments channels form and migrate landward through headward erosion during base level changes when both shoreface and fluvial sediment sources are more proximal to the upper slope the increasing lateral distribution of channels through time supports an increase in shoreface erosion north of the lsfz a possible explanation is a known decrease in fluvial sediment input from the north combined with mtj related uplift in the south along strike contrasts in upper slope sequence morphology in the southern eel river basin are clearly a function of both regional tectonism and proximity to sediment sources influenced by base level changes humboldt slide sequences while deformed are continuously influenced by systematic lithological variations occurring as a result of glacioeustatic cycles effects of base level changes similarly control the distribution and fill of incisions on the northern slope all of these observations reinforce previous seismic investigations of the offshore eel river basin shelf and confirm that the competing effects of tectonism and glacioeustacy on the preservation of continental margin stratigraphy can be differentiated

spontaneous potentials and resistivity surveys applied to hydrogeology in a volcanic area case history of the chaine des puys puy de dome france two geophysical methods resistivity soundings and spontaneous potentials sp are used to investigate aquifers in the quaternary volcanic formations of the chaine des puys france the interpretation of the resistivity soundings required a specific work of determination of the resistivity of the formations concerned the basement topography characterized by axial horst and graben structures and perpendicular palaeovalleys was revealed sp was developed experimentally in the field and theoretically it is shown that the sp anomalies can be considered as double layer potentials with their source on the aquifers interfaces the most significant of them is the water table which creates an sp anomaly of similar but inverted form sp anomalies due to flows in unconfined aquifers in palaeovalleys and in volcanic ranges are computed and compared with the observed ones these two methods have determined the extent and the boundary conditions of the different hydrogeological basins and have determined the major drainage axes and the groundwater flow pattern

strataform overview of its design and synthesis of its results abstract the strata formed on continental margins record the history of both marine and terrestrial events occurring on earth the strataform program was created to develop a better understanding of relationships between these events and margin stratigraphy in order to obtain the maximum information from strata they are being examined simultaneously on various temporal and spatial scales and their formation is being studied where events and accumulation are known to be active the general objectives of strataform are to add geological perspective to studies of sedimentary processes to constrain stratigraphic interpretations better and to develop numerical models relating processes and stratigraphy this research is being undertaken on margins off northern california and new jersey which provide unique opportunities for investigation of short term and long term stratigraphy respectively this volume and paper focus upon studies undertaken off northern california north of cape mendocino this area is tectonically active with a coastal mountain range narrow shelf and input of sediment primarily from the eel river recent estimates indicate that 1 to 3 10 7 t yr of mud silt and clay are supplied to the continental shelf sand accumulates on the inner shelf 70 reaches the continental slope or deeper during the previous lowstand of sea level sediment discharge was 25 greater and probably led to hyperpycnal debris and other flows transporting sediment down the slope on the eel shelf flood and storm events combine to create normally stratified fine grained layers with basal sand and much terrestrial organic debris these layers are modified through time but some can be buried and preserved subsurface flood layers seem to impact observations of acoustic backscatter on the middle shelf and subsurface fluids gas freshwater affect electromagnetic measurements on the inner shelf budgets of eel river mud accumulation for the past century indicate 20 trapped on the adjacent shelf and 20 on the upper slope 150 600 m the remaining 60 is dispersed to more distal locations over holocene time scales eel river sediment has built a small sandy subaqueous delta and a subtle bulge of muddy sediment on the middle shelf north of the river gullies have been eroded on the open slope during lowstands of sea level and have accreted upward during highstands a massive failure has formed the humboldt slide due to earthquake activity and unstable substrates possibly from excess pore pressures and gas the tectonic setting has impacted margin stratigraphy from small scales fluid expulsion pockmarks to large scales spatial variation of late cenozoic sedimentation the broad dispersal of eel sediment demonstrates the three dimensional character of the resulting stratigraphy including the large scale clinoform structure developed on the shelf upper slope the rollover point topset foreset boundary for this feature is the shelf break i e below sea level which is important information for stratigraphic interpretations of sea level change

rapid and widespread dispersal of flood sediment on the northern california margin the dispersal of flood sediment from small river systems is a poorly studied yet potentially important aspect of active continental margin sedimentation in january 1995 during a flood with a 30 yr return period the eel river northern california delivered an estimated 25 3 106 t metric tons of fine grained 62 m sediment to the ocean the flood formed a distinct layer on the sea bed that was centered on the 70 m isobath extended for 30 km along shelf and 8 km across shelf and was as thick as 8 5 cm but contained only 6 106 t of sediment thus 75 of the flood derived sediment did not form a recognizable deposit but was instead rapidly and widely dispersed over the continental margin stratigraphic models of and compilations of sediment flux to active continental margins need to take the dispersive nature of small river systems into account

environmental signal propagation in sedimentary systems across timescales abstract earth surface processes operate across erosionally dominated landscapes and deliver sediment to depositional systems that can be preserved over a range of timescales the geomorphic and stratigraphic products of this source to sink sediment transfer record signals of external environmental forcings as well as internal or autogenic dynamics of the sedimentary system here we evaluate environmental signal propagation across sediment routing systems with emphasis on sediment supply qs as the carrier of up system forcings we review experimental numerical and natural examples of source to sink sediment routing and signal propagation during three timescales 1 historic which includes measurement and monitoring of events and processes of landscape change and deposition during decades to centuries 2 centuries to several millions of years referred to as intermediate timescale and 3 deep time we discuss issues related to autogenic dynamics of sediment transport transient storage and release that can introduce noise lags and or completely mask signals of external environmental forcings we provide a set of conceptual and practical tools for evaluating sediment supply within a source to sink context which can inform interpretations of signals from the sedimentary record these tools include stratigraphic and sediment routing system characterization sediment budgets geochronology detrital mineral analysis e g thermochronology comparative analog approaches and modeling techniques to measure calculate or estimate the magnitude and frequency of external forcings compared to the characteristic response time of the sediment routing systems

floodplain evolution in a small tectonically active basin of northern california sediment cores were collected along oodplains in the navarro river basin of coastal northern california to examine the controls on oodplain evolution in a tectonically active setting sedimentary strata were subsampled for organic content bulk density and grain size measurements organic samples were analysed for 14c age which yielded net averaged sedimentation rates for all cores overbank deposition rates decreased at all study sites through time and declined in the downstream direction the ability of intermediary order streams to store sediment in oodplains decreased the ability of highest order streams to record sediment pulse events the effects of anthropogenic disturbance primarily logging on long term overbank deposition rates were minimal climatic variability by affecting sediment loading in the channel network is the principal control on oodplain evolution through the holocene a hypothetical model is proposed to explain overbank deposition rates in the navarro basin which may be extrapolated to the northern coastal california region during the late pleistocene and holocene the complexities observed in sediment storage and routing in this study imply that caution should be made when extrapolating sediment yield measurements obtained at river mouths or coastal shelves to geomorphic events within small tectonically active basins

modeling studies of the coastal circulation off northern california statistics and patterns of wintertime flow we conduct modeling studies of the coastal circulation off northern california in the vicinity of the eel river 40 6 n the site of the 1995 2000 strata formation on margins strataform marine geology observational program using a series of nested hydrostatic primitive equation models to aid in the understanding of the shelf flow field that receives and transports sediments during the winter floods the basic objectives of our numerical studies are to model the continental shelf and slope flow and to understand the dominant dynamical processes we simulate the shelf and slope flow surrounding cape mendocino during 100 days in winter 1996 1997 using a 3 km resolution model embedded in a 9 km resolution regional model of the north pacific ocean the naval research laboratory s pacific west coast pwc model the validity of the model simulations is assessed by comparison with current measurements from a strataform shelf tripod outfitted with an acoustic doppler current profiler and with satellite altimetry good agreement is found between the amplitude and time variability of the 3 km resolution modeled and observed alongshore currents on the shelf the 3 km resolution model outperforms the 9 km resolution pwc model using statistical maps of flow variables strong alongshore variability in wintertime flow is documented the role of the forcing supplied by wind and river runoff in establishing this flow asymmetry between regions north and south of cape mendocino is examined using empirical orthogonal functions the evolution of a robust anticyclonic eddy over the shelf and slope adjacent to cape mendocino is described the eddy forms when strong poleward winds weaken and reverse direction during winter storms the important role of alongshore pressure gradients in the shelf circulation is documented

late holocene sediment accumulation on the northern california shelf oceanic fluvial and anthropogenic influences the late holocene sedimentary record of the northern california continental shelf archives the combined influences of fluvial and oceanic mechanisms of land to ocean sediment flux and provides perspective on the cumulative impacts of major floods and land use change in the drainage basin during historical time piston cores collected on the shelf 50 150 m water depths off the eel and klamath rivers were analyzed to identify lithologies indicative of depositional processes over the past 5 k y and dated using 137cs and 14c methods to resolve spatial patterns in sediment accumulation rates sediment accumulation rates averaged over the past several millennia display along and across shelf gradients that reflect interactions between the shelf flow field and antecedent tectonically produced topography the highest rates 3 6 mm yr 0 4 0 8 g cm2 yr are associated with structural lows on the middle shelf whereas the lowest rates 0 2 0 6 mm yr 0 03 0 05 g cm2 yr occur at structural highs and at the shelf edge this accumulation pattern is spatially coincident with postglacial sediment thicknesses on the shelf and with depocenters that form on shorter time scales in response to oceanic flood events the sediment column exhibits an upward fining sequence consistent with increased bypass of river silt and clay as delta plains progressively filled during the holocene transgression sedimentary event beds indicative of major river floods including two probable hyperpycnal discharges of the eel river within the past 1 k y are preserved stratigraphic evidence of accelerated sediment delivery to the shelf during historical time in the form of increased burial of fluvial mud relative to fine grained sand is superimposed on this natural record this change is concordant with the regional history of forest clearing and related land use practices in the watershed after the early 1800s and appears to record the combined impacts of timber harvesting and several extreme floods after ca a d 1950

hummocky cross stratification like structures and combined flow ripples in the punta negra formation lower middle devonian argentine precordillera a turbiditic deep water or storm dominated prodelta inner shelf system abstract turbidity current and storm induced deposits may exhibit similarities in particularly when the latter is laid down by a combination of oscillatory and unidirectional flows recent progress in facies analysis helps to discriminate the sedimentary effects of oscillatory from unidirectional components of the flow on the basis of detailed analysis of sedimentary facies strata geometry and palaeocurrent data the present study reinterprets the punta negra formation pnf lower middle devonian argentine precordillera previously considered as a depositional system of deep water as a storm dominated prodeltaic shelf depositional system in the sandstone beds of the pnf planar low angle and undulating laminations with weakly asymmetric hummocky and swaley bedforms combined flow ripples accretionary hummocky cross stratification like hcs like and anisotropic hcs like suggest the action of oscillatory currents combined with unidirectional currents in forming the deposits different hypotheses on the origin of the oscillatory currents have been examined the most convincing interpretation is that the oscillatory component of the velocity is attributed to storm induced waves the palaeocurrent data indicate offshore current directions suggesting that the unidirectional flow was a gravity induced bottom current inverse grading at the base and overlying normally graded divisions of the sandstone beds testify to waxing waning behaviour of the depositional flows interbedding of sedimentary structures undulating laminations low angle and parallel laminations and combined flow ripples in the lower and intermediate divisions of the beds indicate fluctuations of flow velocity this organisation of the sedimentary structures permits association of the unidirectional component with hyperpycnal bottom currents the terrestrial origin of the hyperpycnal flows is suggested by the abundance of terrestrial plant remains the mineralogical and textural immaturity of the sandstone composition and the relative scarcity of bioturbation which was likely controlled by fresh water input and a high rate of sedimentation storm influenced hyperpycnal flows generated subaqueous channelised forms at the mouth of the river deltas which later filled with sand at the distal end of the channelised forms lobe shaped sandstone beds were deposited evolving distally into thin sandstone beds alternating with sandy mudstone

iron and macronutrients in california coastal upwelling regimes implications for diatom blooms the supply of iron relative to that of the macronutrients nitrate phosphate and silicic acid plays a critical role in allowing extensive diatom blooms to develop in coastal upwelling regimes the presence or absence of a broad continental shelf influences the supply of iron the iron input to central california upwelling waters varies spatially and can be characterized by two end member regimes one end member which includes monterey bay and extending north to pt reyes is an iron replete regime where upwelling occurs over a relatively broad continental shelf that results in waters with high concentrations of dissolved and particulate iron 10 nm entrained together with high concentrations of nitrate and silicic acid in these iron replete regions extensive blooms of large diatoms deplete macronutrient concentrations which results in correspondingly high chlorophyll a concentrations the other end member located to the south of monterey bay off the big sur coast is an iron deplete regime where upwelling is focused offshore of a narrow continental shelf upwelled waters in the big sur region are characterized by low dissolved and particulate iron concentrations 1 nm together with high concentrations of nitrate and silicic acid extremely low iron concentrations unused nitrate and silicic acid and a low abundance of large diatoms characterize surface waters in these iron deplete regions and thus represent coastal upwelling high nutrient low chlorophyll systems limited by the micronutrient iron

measuring the shallow porosity structure of sediments on the continental shelf a comparison of an electromagnetic approach with cores and acoustic backscatter we validate estimates of shallow sedimentary porosities made on the continental shelf by electromagnetic methods through a comparison with those measured directly in cores specifically we show that an electromagnetic surveying system we have used to measure sediment resistivity in a number of seafloor locations returns porosity depth profiles that are related to those measured in cores and that our data have good sensitivity to the structure within the uppermost few tens of centimeters of seafloor we then consider inferences of sediment type made by observations of acoustic backscatter in the light of our porosity measurements our results suggest that electromagnetic methods provide complementary information that can be used to improve grain type estimates at and just below the seafloor especially when combined with acoustic backscatter data because the system is pulled along the seafloor data can be quickly collected from a large area of seafloor and can be used to interpolate facies conditions between core locations

post glacial sediments of the californian shelf from cape san martin to the us mexico border abstract previous workers of the californian shelf from cape san martin to the us mexico border have assembled compilations of high resolution acoustic seismic profiling data from a number of primary sources to examine these data more effectively the region has been divided into four areas for convenience in mapping and partly in response to natural geographic divisions these data plus additional detailed profiling off san onofre south of dana point have made possible calculations of the total volume of unconsolidated post glacial sediments on this narrow active transform margin shelf the san onofre survey shows that there are several well defined buried terraces cut into the bedrock surface these occur at about 20 30 to 35 40 to 50 and 60 to 80 m below present datum dating reported in the literature suggests that the bulk of the sediments are holocene as sea level shifts from glacial to interglacial much of the sediment stored on the shelf at high stand is reworked to deeper water during the fall to the following low stand previous studies suggest that for the large deposits off the santa maria santa ynez and ventura santa clara rivers some sediment may remain after a sea level cycle the estimated total holocene past 10 000 years river discharge for the region based on records of the past century is about 130 km3 thus the total shelf deposit of ca 60 km3 represents about one half of the holocene siliciclastic contribution however the 60 km3 includes basal sediments in the three large river lenses that are probably pre holocene a more probable holocene contributed volume is about 30 km3 previous studies show that biogenic and fluvial contributions are generally little sediment from the areas of relatively high input passes to the starved areas which remain as non depositional low deposition rate or erosional surfaces the major portion of the shelf is sediment starved since only a few small ephemeral streams enter most of the cliffed coastal areas and rocky surfaces are exposed in the inner and outer shelf

tectonics sedimentation and erosion in northern california submarine geomorphology and sediment preservation potential as a result of three competing processes abstract on the continental margin of northern california north of the mendocino triple junction the geomorphic grain varies dramatically between the coast and the toe of the accretionary wedge this grain represents the composite signature of three competing processes tectonics sedimentation and erosion all three are active across the region but their rates and orientations vary spatially and temporally affecting the resultant grain and preservation potential in this rapid sedimentation environment at the toe of the accretionary wedge the relative rate of tectonic deformation exceeds the rate of sediment accumulation and erosion is largely limited to areally restricted but at times large mass wasting features in this region isobaths representing the geomorphic grain are parallel to structural highs and lows as in most forearcs and sediment accumulation is restricted to small basins located between anticlinal ridges the slope between the shelf break and the accretionary complex shows the competing effects of sediment accumulation erosion and tectonism slope perpendicular gullies are the prominent erosional features in this area especially in the region surrounding the trinity submarine canyon anticlinal ridges crop out on the seafloor although the ridge crests decrease in bathymetric expression both up and down slope on the shelf the rate of sediment accumulation 6 12 mm yr exceeds the relative rate of tectonic deformation with 1 3 mm yr of active uplift on land in this region the geomorphic grain is controlled by sediment accumulation with compressed but continuous stratigraphic sections above anticlinal highs and expanded sections in actively subsiding regions

shelf record of climatic changes in flood magnitude and frequency north coastal california continental shelf deposits off the eel river north coastal california document a recent increase in the magnitude and frequency of major hydrologic floods 10 yr recurrence interval the shelf record reveals a sudden three fold increase in sedimentation rate ca 1954 and a concomitant increase in the frequency of preserved flood beds comparison of sedimentary and river discharge records reveals that major floods after ca 1950 had a more pronounced effect on coastal sediment delivery and accumulation offshore than previous recorded events of similar magnitude and that stratigraphic preservation of flood events is highly dependent on flood frequency and net sedimentation rate we contend that this change in marine sedimentation is a response to documented climatic phenomena that have increased the frequency of major floods throughout the western united states during the past half century together with intrabasinal impacts of extreme floods in 1955 and 1964 anthropogenic increase in watershed sediment production is a probable secondary factor

a 5000 yr record of climate change in varved sediments from the oxygen minimum zone off pakistan northeastern arabian sea the upper holocene marine section from a kasten core taken from the oxygen minimum zone off karachi pakistan at water depth 700 m contains continuously laminated sediments with a sedimentation rate of 1 2 mm yr and a unique record of monsoonal climatic variability covering the past 5000 years our chronostratigraphy is based on varve counts verified by conventional and ams 14 c dating individual hemipelagic varve couplets are about 0 8 1 5 mm thick with light colored terrigenous laminae a deposited mainly during the winter monsoon alternating with dark colored laminae b rich in marine organic matter coccoliths and fish debris that reflect deposition during the high productivity season of the late summer monsoon august october precipitation and river runoff appear to control varve thickness and turbidite frequency we infer that precipitation decreased in the river watershed indicated by thinning varves after 3500 4000 yr b p this is about the time of increasing aridification in the near east and middle east as documented by decreasing nile river runoff data and lake level lowstands between turkey and northwestern india this precipitation pattern continued until today with precipitation minima about 2200 1900 yr b p 1000 yr b p and in the late middle ages 700 400 yr b p and precipitation maxima in the intervening periods as documented by spectral analysis the thickness of varve couplets responds to the average length of a 250 yr cycle a 125 yr cycle the gleissberg cycle of solar activity 95 yr and a 56 yr cycle of unknown origin higher frequency cycles are also present at 45 39 29 31 and 14 yr the sedimentary gray value also shows strong variability in the 55 yr band plus a 31 yr cycle because high frequency cyclicity in the enso band ca 3 5 and 5 yr is only weakly expressed our data do not support a straightforward interaction of the pacific enso with the monsoon driven climate system of the arabian sea

a new look at old carbon in active margin sediments recent studies suggest that as much as half of the organic carbon oc undergoing burial in the sediments of tectonically active continental margins may be the product of fossil shale weathering these estimates rely on the assumption that vascular plant detritus spends little time sequestered in intermediate reservoirs such as soils freshwater sediments and river deltas and thus only minimally contributes to the extraneously old 14c ages of total organic matter often observed on adjacent shelves here we test this paradigm by measuring the 14c and 13c values of individual higher plant wax fatty acids as well as the 13c values of extractable alkanes isolated from the eel river margin california the isotopic signatures of the long chain fatty acids indicate that vascular plant material has been sequestered for several thousand years before deposition a coupled molecular isotope mass balance used to reassess the sedimentary carbon budget indicates that the fossil component is less abundant than previously estimated with pre aged terrestrial material instead composing a considerable proportion of all organic matter if these findings are characteristic of other continental margins proximal to small mountainous rivers then the importance of petrogenic oc burial in marine sediments may need to be reevaluated 2009 the geological society of america

the use of 7be to identify event and seasonal sedimentation near the po river delta adriatic sea in october 2000 a major flood event of the po river occurred the resulting seabed deposit was initially sampled in december 2000 and most sites were reoccupied on subsequent cruises over a 3 yr period using the maximum penetration depth of the short lived radioisotope 7be half life 53 3 d the flood deposit was found to be up to 15 cm thick individual depocenters of thick strata were located immediately adjacent to the distributaries at the river mouth in relatively shallow water 30 m these flood deposit thicknesses are minimum estimates because the depth of 234th half life 24 1 d exceeds that of 7be in physically stratified flood sediment indicating that the first sediment deposited during the flood event likely originated from the river channel 30 55 of the estimated sediment load delivered to the shelf during the flood event can be accounted for using 7be penetration depths seasonal deposition also can be quantified using 7be after removing the effects of biological mixing seasonal deposition rates are a maximum of 6 cm yr 1 near the pila distributary decreasing to 2 cm yr 1 in the southern portions of the dispersal system

towards a sediment budget for the santa cruz shelf a conceptual model is presented for the northern monterey bay continental shelf in which coarse sediment moves southward along the coast in the littoral zone while fine sediment moves to the north by advection and diffusion along the midshelf data from measurements and estimates of various sediment sources and sinks show that the midshelf mudbelt is the dominant sink for fine grained sediment introduced into monterey bay the principal sources of the fine sediment are the three rivers that enter monterey bay the san lorenzo pajaro and salinas rivers accumulation rates in the midshelf mudbelt are high relative to documented yields of rivers and cliff erosion and also are high relative to other documented mud accumulations of the west coast continental shelves

shelf sedimentation on a tectonically active margin a modern sediment budget for poverty continental shelf new zealand abstract the combination of high sediment yields and the prevalence of tectonically controlled accommodation on collision margins such as that adjacent to the waipaoa river nz create the potential for these areas to contain high resolution records of natural and anthropogenic signals this study describes modern 100 yr sedimentation patterns off the waipaoa and quantifies a sediment budget for the continental shelf which is compared to long term holocene trends 210 pb and 239 240 pu geochronologies reveal three main shelf depocenters two of these depocenters are located in subsiding synclinal basins on the mid shelf landward of the actively deforming ariel and lachlan anticlines the depocenters exhibit accumulation rates ranging from 0 75 to 1 5 cm yr 1 and display both steady state and non steady state 210 pb activity profiles textural characteristics of the non steady state cores indicate the possible preservation of flood event layers the third depocenter is located near the shelf break and has accumulation rates as high as 1 0 cm yr 1 the inner shelf and central mid shelf are characterized by low uniform 210 pb activity profiles and low accumulation rates indicating that sediment is bypassing the inner shelf region and being deposited on the mid to outer shelf the modern sedimentation patterns seen in this study are similar to those for the holocene suggesting that regional tectonics are the major influence on poverty shelf sedimentation a modern bulk sediment budget estimates that 3 6 0 9 10 6 t yr 1 of sediments remains on the shelf amounting to only 25 of the 15 mt of sediments discharged from the river per year this indicates massive export of sediments from the study area to the adjacent slope or along the shelf in contrast studies of the mid late holocene sediment budget in the same area indicate that the sediment input and shelf trapping have been roughly in balance when compared with the modern budget this suggests an extraordinarily rapid shift from shelf trapping to shelf bypassing most likely driven by increasing sediment discharge in response to deforestation

lower pleistocene to present structural deformation and sequence stratigraphy of the continental shelf offshore eel river basin northern california seismic stratigraphic sequences and deformational features are mapped beneath the continental shelf of the offshore eel river basin of northern california using a closely spaced high resolution multichannel seismic grid geometries of sequences and morphologies of bounding unconformities reflect competing tectonic and glacioeustatic influences producing shifting sedimentation patterns in the offshore basin during the last v1 myr estimated timing of unconformity formation correlates generally with a deep ocean global n 18 o curve both before and after a v500kyr transition from 41 kyr to 100 kyr dominated cycles we suggest that glacioeustatic fluctuations are the dominant control on unconformity formation basinwide however regional tectonic influences on sequence development are also observed folding associated with gorda north america plate convergence ended progressively from south to north between v1 0 ma and v500 ka in response to northward migration of the mendocino triple junction mtj since v500 ka continued encroachment of the mtj produces rotation of preexisting structures uplift of the table bluff anticline tba related periods of channel incision south of the tba and generally reduced sediment preservation across the shelf mtj related uplift may also have induced formation of eel canyon over the past v1 0 myr a dominant northern sediment source becomes progressively less important a southern source probably the paleo eel river has become dominant since v750 ka seismic unconformities fall into two categories 1 irregular surfaces of limited mappable extent interpreted as incision exposure surfaces forming during relative sea level lowstands and 2 smooth laterally extensive regional unconformities interpreted as ravinement surfaces that erode lowstand surfaces in all but deeply incised areas during sea level transgressions a sequence stratigraphic model for sediment deposition and preservation on the eel river shelf predicts that preserved sediment on the eel margin is dominated by fluvially derived highstand silts and muds deposited by longshore directed currents and waves lowstand sediments are preserved only in fluvial channels infilled during late lowstand early transgression preserved transgressive sediments are likely limited to a thin veneer of well sorted coarse sediment or shell debris deposited above the transgressive ravinement future deep coring will be required to confirm these predictions 2002 elsevier science b v all rights reserved

wave modified turbidites combined flow shoreline and shelf deposits cambrian antarctica sandstone tempestite beds in the starshot formation cen tral transantarctic mountains were deposited in a range of shoreline to shelf environments detailed sedimentological analysis indicates that these beds were largely deposited by wave modified turbidity currents these currents are types of combined flows in which storm generated waves overprint flows driven by excess weight forces the interpreta tion of the tempestites of the starshot formation as wave dominated turbidites rests on multiple criteria first the beds are generally well graded and contain bouma like sequences like many turbidites the soles display abundant well developed flutes they also contain thick divisions of climbing ripple lamination the lamination however is dominated by convex up and sigmoidal foresets which are geometries identical to those produced experimentally in current dominated com bined flows in clear water finally paleocurrent data support a tur bidity current component of flow asymmetric folds in abundant con volute bedding reflect liquefaction and gravity driven movement and hence their orientations indicate the downslope direction at the time of deposition the vergence direction of these folds parallels paleocur rent readings of flute marks combined flow ripples and a number of other current generated features in the starshot event beds indicating that the flows were driven down slope by gravity the wave component of flow in these beds is indicated by the presence of small to large scale hummocky cross stratification and rare small two dimensional ripples wave modified turbidity currents differ from deep sea turbidity cur rents in that they may not be autosuspending and some proportion of the turbulence that maintains these flows comes from storm waves such currents are formed in modern shoreline environments by a com bination of storm waves and downwelling sediment laden currents they may also be formed as a result of oceanic floods events in which intense sediment laden fluvial discharge creates a hyperpycnal flow event beds in the starshot formation may have formed from such a mechanism oceanic floods are formed in rivers of small to medium size in areas of high relief commonly on active margins the starshot formation and the coeval douglas conglomerate are clastic units that formed in response to uplift associated with active tectonism sedi mentological and stratigraphic data suggest that coarse alluvial fans formed directly adjacent to a marine basin the geomorphic conditions were therefore likely conducive to rapid fluvial discharge events asso ciated with storms the abundance of current dominated combined flow ripples at the tops of many starshot beds indicates that excess weight forces were dominant throughout deposition of many of these beds

eel river margin source to sink sediment budgets revisited abstract the eel river coastal margin has been used as a representative source to sink sediment dispersal system owing to its steep high sediment yield river and the formation of sedimentary strata on its continental shelf one finding of previous studies is that the adjacent continental shelf retains only 25 of the eel river fine grained sediment less than 63 m discharged over time scales of both individual floods and the 20th century thus suggesting that the eel shelf trapping efficiency is uniquely lower than other similar systems here i provide data and analyses showing that sediment discharge relationships in the eel river have varied strongly with time and include substantial decreases in suspended sediment concentrations during the latter 20th century including these trends in margin wide sediment budgets i show that previous eel river sediment discharge rates were overestimated by a factor of two thus revised sediment budgets shown here reveal that the eel shelf retained 50 of the discharged river fine grained suspended sediment during intensively sampled events of 1995 97 and over the 20th century in light of this hypotheses about high rates of sediment export away from the primary shelf depocenter should be reevaluated

geotechnical and groundwater geophysics because of a change in emphasis from power plant siting to projects principally involving process and facility monitoring and certification of existing structures dams pipelines etc geotechnical and groundwater geophysics is in quite a healthy state after some lean years following the demise of nuclear energy construction projects the orders of magnitude jump in the computational capability of geophysicists working in these fields has overshadowed advances in instrumentation e g digital enhancement seismographs field methods e g cross borehole em and interpretive procedures the advent of powerful affordable microcomputers has enabled expansion into applications demanding finer resolution and quicker turnaround of results as a result shallow seismic reflection seismic and electromagnetic geotomography and the complementary use of surface and borehole electrical resistivity and seismic data will soon be common if not dominant methods in geotechnical and groundwater investigations fut