laboratory experiments on the formation of subaqueous depositional gullies by turbidity currents abstract gullies and furrows are shallow sub parallel features widespread on the seafloor and some lacustrine environments vast fields of linear channels of erosional origin called furrows with spacings of meters and up to hundred of meters are common in settings with relatively low seafloor slopes and have been previously ascribed to helical motion in benthic boundary layers submarine gullies are linear donwslope trending features of erosional depositional or mixed nature commonly observed in the upper continental slope submarine canyon walls and other relatively steep submarine environments while gully fields have been associated to the action of unconfined turbidity currents and other cascading dense flows their relatively large spacings ranging from several hundreds of meters to 1 2 km suggest that these features might be the result of other mechanisms different than boundary layer helical motion in this paper we propose an alternative mechanism for the initiation of gullies observed in relatively steep seabed gradients downslope of a seabed slope change such as those observed in continental margins self slope break or submarine canyon walls our working hypothesis is that gullies could be the result of a centrifugal instability that sets in interfaces of gravity currents as they flow through a bedslope transition since for these cases the centrifugal force excess is of the same order of magnitude of the driving force and therefore a potential source of flow instability we propose that gully spacing might be controlled by the magnitude of the centrifugal force scaling roughly with the radius of curvature of the transition slope break we also present experimental evidence on the formation of longitudinal features resembling slope gullies that were created in a laboratory tank after running successive supercritical depositional turbidity currents through a slope change similar to that observed in continental margins gullies in the tank developed through differential deposition only and showed spacing an order of magnitude larger that that expected from a boundary layer helical motion origin these experimental results along with the observation that gullies were not formed in the tank without a bed slope change low to high support the hypothesis of a centrifugal instability origin for longitudinal features in relatively steeper submarine settings our experimental results also support our first order approximate gully spacing rule which is in general agreement with field observations of gully spacing in upper continental slopes and submarine canyon walls

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

uplifted neogene margin southernmost cascadia mendocino triple junction region california along a 10 km length of coast north of the mendocino triple junction a neogene accretionary complex has been uplifted 2 km and tilted northward in response to the interaction of the southern juan de fuca gorda plate with the older north american and pacific plates these plate interactions were accompanied by tectonic intercalation of miocene to pliocene deposits of the southern cascadia forearc wildcat group with penetratively deformed oligocene to miocene accretionary prism deposits of the false cape terrane new name and the cretaceous to eocene coastal terrane of the franciscan complex north of cape mendocino more than 2 km of stratigraphic section of the accretionary false cape terrane crops out at beach level in an east west trending anticline and recumbent syncline the folded false cape rocks are juxtaposed with forearc deposits to the north along the subvertical russ shear zone to the south a second subvertical shear zone truncates the false cape terrane near the mouth of the bear river this north bear river shear zone interleaves rocks of the false cape and coastal terranes with additional neogene forearc deposits the bear river beds south of the north bear river shear zone the bear river beds crop out for about 5 km in a continuous folded and imbricated section that is again truncated to the south by a third subvertical shear zone this south bear river shear zone interleaves rocks of the coastal terrane the bear river beds and pliocene to pleistocene shelf deposits folding tilting and shear zone development reflect ongoing north south crustal shortening that has occurred in response to interplate coupling and wedge thickening in the deforming miocene and younger accretionary complex the strain recorded in the accreted terranes and in the forearc overlap assemblage reflects a stress regime unlike that in the cascadia fold and thrust belt north of the eel river syncline thus the deformation pattern may be unique to the triple junction area the false cape terrane is one of only two localities along the cascadia margin where the oligocene to miocene accretionary complex is exposed on land the other locality is that of the hoh melange on the southwest olympic peninsula on land exposure of accretionary rocks in these two areas is a consequence of high uplift rates focused compression and triple junction tectonism framework grain composition data for sandstones in the hoh melange and coeval continental deposits in northernmost california the oligocene to miocene weaverville formation suggest a provenance link while sandstone of the false cape terrane presently situated directly west of the weaverville formation is dissimilar given the northward component of farallon juan de fuca plate translation with respect to north america for the past 20 m y provenance links of hoh and weaverville sandstones might reflect a substantial component of northward translation between the farallon and north american plates that was accommodated within the accretionary prism

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

a study of the chemistry of pore fluids and authigenic carbonates in methane seep environments kodiak trench hydrate ridge monterey bay and eel river basin analyses of the chemical and isotopic composition of carbonates rocks recovered from methane seepage areas of the kodiak trench hydrate ridge monterey bay clam flats and the eel river basin coupled with the studies of the chemistry of the pore fluids have shown that these carbonates have grown within the sediment column geochemical profiles of pore fluids show that in deep water seeps kodiak trench 4450 m monterey bay 1000 m hydrate ridge 650 m 13c dic values are low isotopically light whereas in the eel river area 350 500 m 13c dic values are much higher isotopically heavier in all cases the 13c values indicate that processes of methane oxidation associated with sulfate reduction are dominant in the shallow sediments data on the isotopic composition of authigenic carbonates found at sites in kodiak trench eel river basin south and eel river basin north indicate a variable composition and origin in different geochemical environments some of the authigenic carbonates from the study sites show a trend in their 13c values similar to those of the pore fluids obtained in their vicinity suggesting formation at relatively shallow depths but others indicate formation at greater sediment depths the latter usually consist of high magnesium calcite or dolomite which from their high values of 13c up to 23 and 18o up to 7 5 suggest formation in the deeper horizons of the sediments in the zone of methanogenesis these observations are in agreement with observations by other workers at hydrate ridge in monterey bay and in the eel river basin

spatial variability in sedimentary processes on the eel continental slope the northern california continental margin receives episodically large quantities of sediment from the eel river which seasonally discharges to the shelf regional circulation broadcasts this sediment widely not only affecting sedimentation on the shelf but on the slope as well a suite of 60 cores has been collected to examine the processes of sediment accumulation and sediment delivery to the eel continental slope in waters 800 m deep spatial surveys of accumulation rate 0 2 1 3 g cm 2 year 1 and surficial sediment grain size 4 8 5 demonstrate that sediment is accumulating throughout the eel margin reflecting a rapid and widespread redistribution of the fluvial discharge downcore grain size profiles of heterogeneous sediments suggest that hemipelagic and other episodic sediment delivery processes may be important for the slope in areas proximal to the river in more distal areas downcore profiles reveal relatively homogeneous sediments delivered dominantly by hemipelagic processes a budget for fine grained sediment shows that this upper portion of the slope contains at a maximum 20 of the river s annual discharge a combined budget for the shelf and upper slope suggests that at least 60 of the annual sediment load is not accounted for in these areas the eel continental slope because of the large terrigenous sediment input the narrow width of the adjacent shelf and its energetic environment is a good analogue for slopes during transitional stands of sea level and for modern tectonically active settings processes typically associated with these geological settings i e supply of sediment to the slope and downslope mass movement are actively occurring 1999 elsevier science b v all rights reserved

modern accumulation rates and a sediment budget for the eel shelf a flood dominated depositional environment the northern california continental margin is periodically impacted by geologically significant storms which have a marked influence on terrigenous sediment supply flood deposition and long term accumulation of fine grained sediment on the eel shelf accumulation of eel river muds on the adjacent shelf was investigated using 210pb and 137cs geochronologies in order to understand the fate of sediment discharged by the eel river and to relate patterns of net sediment accumulation 100 yr time scale to sediment dynamics 210pb data demonstrate that modern accumulation of river mud occurs from the 50 m isobath seaward across shelf accumulation rates decrease from maximum mid shelf values of 0 6 1 7 g cm 2 yr 1 to values of 0 2 0 4 g cm 2 yr 1 at the shelf break with a spatially weighted mean of 0 5 g cm 2 yr 1 0 4 cm yr for the entire shelf 210pbxs sediment depth profiles from the region of highest accumulation rate are characterized by subsurface intervals of low and uniform activity which are produced by flood deposition in some cores particular 210pbxs activity intervals may be associated with major eel river floods of 1955 1964 and 1974 it is postulated that because of the coincidence of high river flow events and southerly winds during cyclonic winter storms net northward transport allows for preferential deposition of fine grained sediment north of the river mouth over the past 100 years fluvial sediment input combined with marine dispersal processes have produced a mid shelf depocenter evident by both the spatial distribution of 210pb accumulation rates and by clay rich flood layers partially preserved in shelf deposits a fine grained sediment budget for the dispersal system based on hydrological data and 210pb geochronologies demonstrates that a maximum of 20 3 109 kg yr of the mean annual supply of fluvial mud 14 109 kg yr is trapped on the shelf the results of this study demonstrate that 1 short term sedimentation processes associated with floods can influence sediment accumulation on longer time scales and 2 a major fraction of fine grained sediment supplied to tectonically active margins by flood prone mountainous rivers bypasses narrow continental shelves

benthic foraminifera associated with cold methane seeps on the northern california margin ecology and stable isotopic composition release of methane from large marine reservoirs has been linked to climate change as a causal mechanism and a consequence of temperature changes during the quaternary and the paleocene these inferred linkages are based primarily on variations in benthic foraminiferal stable isotope signatures few modern analog data exist however to assess the influence of methane flux on the geochemistry or faunal characteristics of benthic foraminiferal assemblages here we present analyses of the ecology and stable isotopic compositions of living rose bengal stained and dead fossil foraminifera 150 mm from cold methane seeps on the slope off of the eel river northern california 500 525 m and discuss potential applications for reconstructions of methane release in the past and present calcareous foraminiferal assemblages associated with calyptogena clam bed seeps were comprised of species that are also found in organic rich environments cosmopolitan paleoceanographically important taxa were abundant these included uvigerina bolivina chilostomella globobulimina a ndnonionella we speculate that seep foraminifera are attracted to the availability of food at cold seeps and require no adaptations beyond those needed for life in organic rich reducing environments oxygen isotopic values of the tests of living foraminiferal assemblages from seeps had a high range up to 0 69 as did carbon isotopic values up to 1 02 many living foraminiferal isotope values were within the range exhibited by the same or similar species in non seep environments carbon isotopic values of fossil foraminifera found deeper in the sediments 18 20 cm however were 4 10 u peregrina and 3 60 b subargentea more negative than living d 13 c values these results suggest that d 13 c values of foraminiferal tests reflect methane seepage and species specific differences in isotopic composition and can indicate temporal variations in seep activity a better understanding of foraminiferal ecology and stable isotopic composition will enhance paleo seep recognition and improve interpretations of climatic and paleoceanographic change 2000 elsevier science b v all rights reserved

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

impact of local morphology on sedimentation in a submarine canyon rov studies in eel canyon northern california u s a abstract despite existing paradigms about marine sedimentation some submarine canyons are receiving large amounts of sediment during the present highstand of sea level these modern examples can be used to unlock secrets of past canyon sedimentation however submarine canyons have complex morphology and consequently dramatic variations in sedimentary processes and deposits can occur over a range of spatial scales as small as meters to tens of meters operations from surface ships usually cannot place sampling devices on the canyon seabed with this level of accuracy the purpose of the present study was to investigate the variability of sedimentation over a range of scales in order to delineate accurate trends along and among channels at the head of a canyon sampling with the rov ventana facilitated detailed examination of microenvironments i e wall thalweg in eel submarine canyon the combination of intense sedimentation from nepheloid layers and gravity flows in a complicated morphologic system leads to clear distinctions between microenvironments as well as some recognizable and unifying trends in sedimentation inherent small scale variability due to canyon morphology is evident in narrow and steep channels at 1 m horizontal resolution cores exhibit consistent sediment fabric but laminae can differ between cores at the 10 m horizontal scale the fabric is not consistent broader more gently sloping channels reveal consistent sediment fabric at the 10 m scale in most cases physical stratification decreases along the thalwegs of channels as water depths increase in contrast channel walls generally exhibit elevated impacts of bioturbation and variable amounts of physical stratification radiochemical profiles 210pb 137cs and the predominance of physical stratification including discrete laminae with high sand content suggest that the northern entrants are receiving more sediment than their southern counterparts however 210pb profiles in this study demonstrate rapid accumulation of sediment everywhere in the head of eel canyon with the highest rates of accumulation 40 mm yr found in the channel thalwegs in thalwegs evidence from sedimentary structures e g erosional bases cross bedded sand layers demonstrates that gravity flows occur frequently many times each year and have been linked by other investigations to storm impacts on the adjacent continental shelf on decadal timescales these processes are temporarily depositing sediment in the head of the canyon which is removed over longer timescales probably as larger gravity flows triggered by earthquakes the radiochemical and sedimentological data collected at the base of entrant channels confirm that modern sediment is moving to deeper portions of the canyon

long term tectonic control on holocene shelf sedimentation offshore la jolla california a high resolution compressed high intensity radar pulse chirp survey reveals shore parallel variations in the holocene sediment thickness offshore la jolla california sediment thicknesses decrease from 20 m in the south near scripps canyon to zero in the north approaching torrey pines in addition to the south to north variation in sediment thickness the transgressive surface observed in seismic lines shoals from scripps canyon to the north despite these dramatic shore parallel subsurface changes the nearshore bathymetry exhibits little to no change along strike a left jog i e a constraining bend along the rose canyon fault causes local uplift in the region and appears to explain the northward shoaling of the transgressive surface the decrease in relief on the transgressive surface away from the left jog and the holocene sediment thickness variation this tectonic deformation is shore parallel and thus the accommodation can be separated into its tectonic and eustatic components

geological geochemical and microbiological heterogeneity of the seafloor around methane vents in the eel river basin offshore california marine methane vents and cold seeps are common features along continental margins worldwide serving as localized sites for methane release and colonization by microbial and chemosynthetic megafaunal communities the eel river basin erb located on the continental slope off northern california contains active methane vents and seep associated chemosynthetic biological communities cbc on the crests of anticlines in 520 m water depth seep related features on the seafloor have a patchy distribution and include active bubbling vents chemosynthetic clam beds and sulfide oxidizing bacterial mats methane sources supplying local seeps are heterogeneous on all spatial scales and support a large and diverse microbial assemblage involved in the anaerobic oxidation of methane aom to develop a comprehensive understanding of the complex biological geochemical and physical processes associated with and influencing seafloor methane seepage a multidisciplinary approach is required here we present an integrative multidisciplinary study that illustrates the diverse processes associated with seafloor methane seepage within the eel river basin and the complex interactions defining the geochemistry mineralogy and microbiology within this environment

acoustic backscatter of the 1995 flood deposit on the eel shelf abstract acoustic swath mapping and sediment box coring conducted on the continental shelf near the mouth of the eel river revealed regional variations in acoustic backscatter that can be related to the shelf sedimentology the acoustic backscatter variations observed on the shelf were unusually narrow compared to the response of similar sediment types documented in other areas however the acoustic data revealed four principal bottom types on the shelf that can be related to sedimentologic differences observed in cores the four areas are 1 low acoustic backscatter associated with the nearshore sand facies and the prodelta terraces of the eel and mad rivers composed of fine sands and coarse silts with low porosity 2 high acoustic backscatter associated with fine silts characterized by high porosity and deposited by the 1995 flood of the eel river 3 intermediate acoustic backscatter in the outer shelf muds where clayey silts are accumulating and the 1995 flood apparently had limited direct effect and 4 intermediate acoustic backscatter near the fringes of the 1995 flood deposits and in areas where the flood sediments were more disrupted by post depositional processes the highest acoustic backscatter was identified in areas where the 1995 flood sediments remained relatively intact and near the shelf surface into the summer of 1995 cores collected from these areas contained wavy or lenticular bedding the rapid deposition of the high porosity muddy layers results in better preservation of incorporated ripple forms than in areas less directly impacted by the flood deposit the high porosity muddy layers allow acoustic penetration into the sediments and result in greater acoustic backscatter from incorporated roughness elements

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

detailed investigation of continental shelf morphology using a high resolution swath sonar survey the eel margin northern california abstract the eel shelf northern california lies within an active compressional tectonic margin subject to abundant terrigenous sediment input from the eel river a recent high resolution swath sonar survey provides us with the opportunity to investigate seafloor morphology and acoustic backscatter patterns within this dynamic region our analysis of the statistical character of bathymetry demonstrates a clear separation into large and small scale morphologies at a 3 10 km scale with smaller scale morphology heavily damped relative to large scale morphology the eel shelf bathymetry is subtle but several structures can be readily discerned in a residual bathymetry formed by removal of the downslope gradient some shelf structures are evidently related to depositional processes as evidenced by correlation with 100 yr sediment accumulation rates whereas others appear related to tectonic processes as evidenced by correlation with subsurface synclines and anticlines the sidescan structure of the shelf is dominated by the low backscatter over the eel and mad river subaqueous deltas evidently associated with the sand to mud transition however contrary to usual correlations between backscatter and grain size in this situation higher backscatter is associated with the muddy sediments in addition we observe a series of shore perpendicular striations or ribbons spaced 0 2 1 0 km apart which extend northward from the eel river subaqueous delta and lie at or near the sand to mud transition some aspects of ribbon morphology suggest that they may be associated with down slope flows

the importance of site selection sediment supply and hydrodynamics a case study of submarine paleoseismology on the northern cascadia margin washington usa abstract the washington continental margin presents both a classic test of submarine paleoseismology and an opportunity to explore advancement of the field through analysis of sediment dispersal in a heterogeneous system new and archive core bathymetric backscatter and seismic reflection data from the washington cascadia margin show that during high stand conditions the northernmost canyons barkley nitinat juan de fuca jdf and to some extent quillayute are relict systems with little holocene recharge the remaining canyons quinault grays guide and willapa are recharged to a varying degrees by northward transport of columbia river derived sediment all systems are nonetheless active conduits for turbidity currents during the holocene which are weaker and more restricted than their pleistocene counterparts sedimentologic and ct analysis supported by radiocarbon ages micropaleontology and the mazama ash datum show that the holocene sedimentary sequence consists of a series of sand to mud turbidites in the active portions of all systems interbedded with hemipelagic sediment however the pleistocene astoria and nitinat fans are largely inactive in the holocene with turbidity current activity limited to the proximal parts of the main channels within active systems the turbidite record is modulated by local landsliding and growth of active folds and faults in the relict systems the turbidite record begins in the middle canyon reaches as opposed to the actively recharged systems which have a turbidite record in the more proximal canyon reaches deposits in the relict systems increase in thickness and modal grain size downstream suggesting multiple sources and or entrainment of material along the canyon walls hydrodynamic models using monte carlo model searches for best fitting flow parameters show the most likely pathways by which holocene and pleistocene currents flowed through the complex accretionary wedge to the abyssal plain model results supported by heavy mineral distributions show that the northern canyon systems barkley nitinat jdf quillayute are independent of the southern systems quinault guide grays willapa during the holocene supporting the classic confluence test balancing of sediment volumes required suggests sediment supplied to the slope canyons and abyssal channels in turbidity current events is 2 5 times greater than that supplied by recharge to the canyon heads by external sources this implies a process that is independent of recharge that scavenges the plentiful relict and hemipelagic sediment supply on the continental slope lithostratigraphic correlation and age models suggest similar sequences were deposited in most systems independently and likely synchronously the correlated turbidite sequence suggests deposition of 23 holocene and 16 post mazama turbidites in most parts of the canyon systems with only modest variation mostly attributable to evidence of bypassing and erosion the best candidate for triggering of these sequences remains great earthquakes and the sequence correlates well to onshore and other offshore paleoseismologic evidence the new data including new cores in northern oregon suggest revisions to segmented rupture boundaries several partial ruptures segment b likely extended further north to juan de fuca canyon one event was resolved into two and one washington only event was added most segment c ruptures were found to extend further northward to the latitude of astoria canyon mean recurrence times are therefore revised to 434 years for the washington coast and 340 years for the central and northern oregon coast conditional probabilities in the next 50 years also are revised to 10 17 for the washington coast and 16 22 for the central and northern oregon coast in cascadia sumatra and other locales globally core siting spatial sample density high resolution analysis of core and regional geophysics and understanding of flow dynamics are critical to capturing a paleoseismic record sedimentary records vary across turbidity current systems of varying sediment supply physiography and sediment sources variations in the record will reflect these factors yet may still present a consistent paleoseismic history this analysis highlights the importance of a thorough understanding of flow paths hydrodynamics and the interplay of sedimentation and local tectonics when testing for evidence of earthquakes in the sedimentary record of active margins

distinguishing sediment waves from slope failure deposits field examples including the humboldt slide and modelling results abstract migrating sediment waves have been reported in a variety of marine settings including submarine levee fan systems floors of fjords and other basin or continental slope environments examination of such wave fields reveals nine diagnostic characteristics when these characteristics are applied to several features previously attributed to submarine landslide deformation they suggest that the features should most likely be reinterpreted as migrating sediment wave fields sites that have been reinterpreted include the humboldt slide on the eel river margin in northern california the continental slope in the gulf of cadiz the continental shelf off the malaspina glacier in the gulf of alaska and the adriatic shelf a reassessment of all four features strongly suggests that numerous turbidity currents separated by intervals of ambient hemipelagic sedimentation deposited the wave fields over thousands of years a numerical model of hyperpycnal discharge from the eel river for example shows that under certain alongshore current conditions such events can produce turbidity currents that flow across the humboldt slide serving as the mechanism for the development of migrating sediment waves numerical experiments also demonstrate that where a series of turbidity currents flows across a rough seafloor i e numerical steps sediment waves can form and migrate upslope hemipelagic sedimentation between turbidity current events further facilitates the upslope migration of the sediment waves physical modelling of turbidity currents also confirms the formation and migration of seafloor bedforms the morphologies of sediment waves generated both numerically and physically in the laboratory bear a strong resemblance to those observed in the field including those that were previously described as submarine landslides

geology of the eel river basin and adjacent region implications for late cenozoic tectonics of the southern cascadia subduction zone and mendocino triple junction two upper cenozoic depositional sequences of principally marine strata about 4000 m thick overlie accreted basement terranes of the central and coastal belts of the franciscan complex in the onshore offshore eel river basin of northwestern california the older depositional sequence is early to middle miocene in age and represents slope basin and slope blanket deposition whereas the younger sequence late miocene to middle pleistocene in age consists largely of forearc basin deposits youthful tectonic activity related to gorda north american plate convergence indicates an active cascadia subduction zone and strong partial coupling between these plates structures of the northeastern margin of the eel river basin are principally north northwest trending east northeast dipping thrust and reverse faults that form imbricate thrust fans many faults merge downward into sole thrusts that extend to or near the gorda north american plate interface in the southern part of the region these structures have been modified by northward encroachment of the pacific plate the coastal belt fault the early tertiary accretionary suture between the franciscan central and coastal belts can be traced from arcata bay northward offshore to the southern oregon border it is tentatively extended farther northward based on aeromagnetic data to an offshore position west of cape blanco thereafter it may coincide with the offshore fulmar fault if so the franciscan coastal belt in california is correlative with the offshore fulmar terrane in oregon the cascadia subduction zone csz does not join the mendocino transform fault at the commonly depicted offshore location of the mendocino triple junction mtj instead the csz extends southeastward around the southern eel river basin and shoreward along mendocino canyon to join the petrolia shear zone similarly the mendocino fault may extend shoreward via mattole canyon and join the cooskie shear zone these two shear zones intersect onshore north of the king range and the area of their intersection is the probable location of the mtj

cadmium isotope fractionation in seawater a signature of biological activity investigations of cadmium isotope variations in the oceans may provide new insights into the factors that control the marine distribution and cycling of this element here we present the results of cd isotope and concentration analyses for 22 seawater samples from the atlantic southern pacific and arctic oceans the results reveal for the first time large and well resolved cd isotope fractionations in the marine environment the majority of the seawater samples display an inverse relationship between dissolved cd contents and isotope compositions which range from e114 110cd 3 0 5 for cd rich waters 0 8 1 0 nmol kg to e114 110cd 38 6 for surface water with a cd concentration of only 0 003 nmol kg all e114 110cd data are reported relative to the jmc cd munster standard this suggests that the cd isotope variations reflect kinetic isotope effects that are generated during closed system uptake of dissolved seawater cd by phytoplankton a few samples do not follow this trend as they exhibit extremely low cd contents 0 008 nmol kg and nearly un fractionated cd isotope compositions such complexities which are not revealed by concentration data alone require that the cd distribution at the respective sites was affected by additional processes such as water mass mixing atmospheric inputs of cd and or adsorption uniform isotope compositions of e114 110cd 3 3 0 5 1 s d were determined for seawater from 900 m depth despite of cd concentrations that display the expected increase along the global deep water pathway from the atlantic 0 3 nmol kg to the pacific ocean 0 9 nmol kg this indicates that the biomass which is remineralized in the deeper ocean is also characterized by a very constant cd isotope composition this observation is in accord with the interpretation that the cd distribution in surface waters is primarily governed by rayleigh fractionation during near quantitative uptake of dissolved seawater cd

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

controls of tectonics and sediment source locations on along strike variations in transgressive deposits on the northern california margin abstract we identify two surfaces in the shallow subsurface on the eel river margin offshore northern california a lowstand erosion surface likely formed during the last glacial maximum and an overlying surface likely formed during the most recent transgression of the shoreline the lowstand erosion surface which extends from the inner shelf to near the shelfbreak and from the eel river to trinidad head 80 km truncates underlying strata on the shelf above the surface inferred transgressive coastal and estuarine sedimentary units separate it from the transgressive surface on the shelf early in the transgression eel river sediment was likely both transported down the eel canyon and dispersed on the slope allowing transgressive coastal sediment from the smaller mad river to accumulate in a recognizable deposit on the shelf the location of coastal mad river sediment accumulation was controlled by the location of the paleo mad river throughout the remainder of the transgression dispersed sediment from the eel river accumulated an average of 20 m of onlapping shelf deposits the distribution and thickness of these transgressive marine units was strongly modified by northwest southeast trending folds thick sediment packages accumulated over structural lows in the lowstand surface the thinnest sediment accumulations 0 10 m were deposited over structural highs along faults and uplifting anticlines the eel margin an active margin with steep high sediment load streams has developed a thick transgressive systems tract on this margin sediment accumulates as rapidly as the processes of uplift and downwarp locally create and destroy accommodation space sequence stratigraphic models of tectonically active margins should account for variations in accommodation space along margins as well as across them

the high frequency backscattering angular response of gassy sediments model data comparison from the eel river margin california a model for the high frequency backscatter angular response of gassy sediments is proposed for the interface backscatter contribution we adopted the model developed by jackson et al j acoust soc am 79 1410 1422 1986 but added modifications to accommodate gas bubbles the model parameters that are affected by gas content are the density ratio the sound speed ratio and the loss parameter for the volume backscatter contribution we developed a model based on the presence and distribution of gas in the sediment we treat the bubbles as individual discrete scatterers that sum to the total bubble contribution this total bubble contribution is then added to the volume contribution of other scatters the presence of gas affects both the interface and the volume contribution of the backscatter angular response in a complex way that is dependent on both grain size and water depth the backscatter response of fine grained gassy sediments is dominated by the volume contribution while that of coarser grained gassy sediments is affected by both volume and interface contributions in deep water the interface backscatter is only slightly affected by the presence of gas while the volume scattering is strongly affected in shallow water the interface backscatter is severely reduced in the presence of gas while the volume backscatter is only slightly increased multibeam data acquired offshore northern california at 95 khz provides raw measurements for the backscatter as a function of grazing angle these raw backscatter measurements are then reduced to scattering strength for comparison with the results of the proposed model the analysis of core samples at various locations provides local measurements of physical properties and gas content in the sediments that when compared to the model show general agreement

regional variability of slope stability application to the eel margin california abstract relative values of downslope driving forces and sediment resisting forces determine the locations of submarine slope failures both of these vary regionally and their impact can be addressed when the data are organized in a geographic information system gis the study area on the continental margin near the eel river provides an excellent opportunity to apply gis spatial analysis techniques for evaluation of slope stability in this area swath bathymetric mapping shows seafloor morphology and distribution of slope steepness in fine detail and sediment analysis of over 70 box cores delineates the variability of sediment density near the seafloor surface based on the results of ten geotechnical studies of submarine study areas we developed an algorithm that relates surface sediment density to the shear strength appropriate to the type of cyclic loading produced by an earthquake strength and stress normalization procedures provide results that are conceptually independent of subbottom depth results at depth are rigorously applicable if sediment lithology does not vary significantly and consolidation state can be estimated otherwise the method applies only to shallow seated slope failure regional density slope and level of anticipated seismic shaking information were combined in a gis framework to yield a map that illustrates the relative stability of slopes in the face of seismically induced failure when a measure of predicted relative slope stability is draped on an oblique view of swath bathymetry a variation in this slope stability is observed on an otherwise smooth slope along the mid slope region north of a plunging anticline the section of slope containing diffuse pockmarked gullies has a lower measure of stability than a separate section containing gullies that have sharper boundaries and somewhat steeper sides such an association suggests that our slope stability analysis relates to the stability of the gully sides the remainder of the study area shows few obvious indications of slope instability except for a feature that has become known as the humboldt slide but it is too deep seated to be amenable to the slope stability prediction techniques presented herein in general few slope failures have been mapped in the eel margin study area despite the high level of seismicity the relatively high rates of sediment accumulation and the extent of gas charging observed by others

seafloor structural geomorphic evolution of the accretionary frontal wedge in response to seamount subduction poverty indentation new zealand abstract high quality swath bathymetry and seismic reflection data reveal that the steep forearc slope along the northern sector of the obliquely convergent hikurangi subduction zone is characteristic of non accretionary and tectonically eroding continental margins with reduced sediment supply in the trench relative to further south and the presence of subducting seamount relief on the hikurangi plateau these seamounts influence the subduction process and the structurally driven geomorphic development of the over riding margin of the australian plate frontal wedge the origin of the poverty indentation on the inboard trench slope is attributed to multiple seamount impacts over the last c 1 myr period accompanied by canyon incision thrust fault propagation into the trench fill and numerous large scale gravitational collapse structures with multiple debris flow and avalanche deposits ranging in downslope length from a few hundred metres to more than 40 km the indentation is directly offshore of the waipaoa river which is currently estimated to have a high sediment yield into the marine system the poverty canyon stretches 70 km from the continental shelf edge directly offshore from the waipaoa to the trench floor incising into the axis of the indentation the sediment delivered to the margin from the waipaoa catchment and elsewhere during sea level high stands including the holocene has remained largely trapped in a large depocentre on the poverty shelf while during low stand cycles sediment bypassed the shelf to develop a prograding clinoform sequence out onto the upper slope the formation of the indentation and the development of the upper branches of the poverty canyon system has led to the progressive removal of a substantial part of this prograding wedge by mass movements and gully incision sediment has also accumulated in the head of the poverty canyon and episodic mass flows contribute significantly to continued modification of the indentation by driving canyon incision and triggering instability in the adjacent slopes prograding clinoforms lying seaward of active faults beneath the shelf and overlying a buried inactive thrust system beneath the upper slope reveal a history of deformation accompanied by the creation of accommodation space the middle to lower poverty canyon represents a structural transition zone within the indentation coincident with the indentation axis the lower to mid slope south of the canyon conforms more closely to a classic accretionary slope deformation style with a series of east facing thrust propagated asymmetric anticlines separated by early stage slope basins north of the canyon system seamount impact has resulted in frontal tectonic erosion associated with the development of an over steepened lower to mid slope margin fault reactivation and structural inversion and over printing

subsurface gas offshore of northern california and its link to submarine geomorphology abstract the northern california continental margin contains evidence of abundant subsurface gas and numerous seafloor features that suggest a causative link between gas expulsion and geomorphology analyses of seismic reflection sidescan sonar and high resolution multibeam bathymetric data show that the occurrence of subbottom gas and the migration processes beneath the shelf differ from those beneath the slope subsurface gas inferred from enhanced reflectors and other geophysical indicators is spatially variable and related more to total depth and stratigraphy than to underlying structure with the exception of one band of gas that follows the regional structural trend shallow depressions on the seafloor pockmarks are used to infer expulsion sites the largest zone of acoustically impenetrable subsurface gas occurs between water depths of 100 m and 300 m where expulsion features are rare the upper slope water depths 400 600 m has a high concentration of pockmarks diameter 10 20 m in contrast to a near absence of pockmarks at water depths shallower than 400 m of nearly 4000 pockmarks observed on sidescan sonar records more than 95 are located in water depths deeper than 400 m bottom simulating reflectors bsrs on some seismic reflection profiles indicate the possible presence of gas hydrate we find that gas and pore fluid migration in the offshore eel river basin is 1 correlated to surface morphology 2 a contributor to seabed roughness 3 a significant mode of sediment redistribution on the upper slope and 4 potentially a factor in large slope failures

estimating river sediment discharge to the ocean application to the eel margin northern california abstract hydrotrend a synthetic river discharge numerical model can simulate the natural variability daily annually and longer in the flux of water and sediment delivered to the coastal ocean the numerical model is designed to make discharge predictions based on climate even though field measurements of river flow are not available provided that appropriate assumptions are made regarding past climate the model can predict how a river may have behaved in the geological past sediment load is determined from a river s rating coefficients when applied to the flood dominated eel river northern california hydrotrend captures the intermonthly and interannual variability of discharge and loads over a century or more hydrotrend is applied to the ice age paleo eel river when sea level was lower and climate much cooler and wetter the paleo eel river at 18 000 year bp would see its hinterland runoff increase but not the magnitude of the peak discharges the post dicted sediment load of the paleo eel river was 25 greater than today s although basin precipitation was 45 greater and the hinterland area 4 greater through sea level lowering a method is provided to obtain sediment discharge rating coefficients for unmonitored modern rivers and paleo rivers the relationship q s g 1 2 h 3 2 a 1 2 provides for an estimate of a river s sediment load q s to within a factor of 3 over 7 orders of magnitude from knowledge of basin relief h and basin area a rating coefficients are determined from this estimate of q s and the power law rating curve q s aq b 1 where q is discharge