OS23B-1304 1340h
The Holocene (7.250 14C yrs BP) Storegga Slide, Mid-Norwegian Margin: The Modern Analogue To Paleo High-Latitude Mega Slides
The frequency and timing of mega scale (> 2000 km3) submarine slides identified on the Mid-Norwegian margin clearly indicate that this downslope process seems to be associated with the frequency and timing of shelf-edge glaciations, inititated at c. 1.1 Ma and repeted shelf edge glaciations from 0.5 Ma. Most of these paleo-slides have been released within the present Storegga Slide depression or in the nearby North Sea Fan area. The Storegga Slide is found to have affected an area of ca. 90.000 km2 and a sediment volume of 2500-3200 km3 have been displaced. The large volume of sediments displaced, or mobilised, created a pattern of morphological and topographical features, reflecting the erosional, compressional and depositional slide processes within the Storegga area. From the seafloor texture it has furthermore been shown that a considerable difference in type of source material, length of transportation and degree of disintegration have taken place. The morphological characteristics and the available chronological data show that the Storegga Slide represent one main retrogressive slide event dated to 7.250+/-250 14C yrs BP (8.100+/-250 cal. yrs BP). Based on the detailed morphological mapping a slide generation map for the Storegga Slide event has been generated. The morphological analysis give furthermore an important contribution to the bulk material properties (rheology) characterising the area. Statistical analyses, which will give a good approach to the rheological parameters of the failed sediments and provide a basis for a more complete failure and flow model of the Storegga Slide, show a regression coefficient as high as 0.9 by correlating the run-out distance and the slide area. As an analogue to the recent Storegga Slide event, two paleo mega slides and their associated escarpment, representing the two most extreme slide escarpment settings found within the Storegga Slide, have been studied in some detail. The stratigraphical setting and the slide development of these escarpment features will be presented and discussed.
OS23B-1305 1340h
Reconstruction of the pre-slide stratigraphy in the Storegga Slide scar, mid-Norwegian shelf
As a contribution to understanding the giant Storegga Slide a reconstruction of the pre-slide stratigraphy has been performed. The isopachs between six interpreted seismic horizons from an extensive set of commercial 2D-seismic sections were interpolated to "rebuild" the pre-slide stratigraphy. The reconstructed surfaces indicate that a total of about 3500 km3 of sediment was removed from the main slide area. An additional 440 km3 of slide masses rests on top of slide slip surface in the NE part of the slide scar. There are also other regions where slide masses remain or where they have only been slightly displaced including an approx. 30 km x 80 km region of compression. The level of the reconstructed sea floor compares well with the thickness of eroded sediments calculated from geotechnical measurements.
OS23B-1306 1340h
Mass-Transport and Relative Sea Level Change in a Carboniferous Deglacial Succession: Jejenes Fm, Quebrada las Lajas, San Juan, Argentina
Quebrada las Lajas, near San Juan, Argentina, preserves a Carboniferous deglacial/postglacial succession in a highly confined paleofjord setting. The sedimentary succession can be divided into four distinct stages. The first stage is synglacial in origin, and is characterized by two related phases; a shallow-water phase, stage 1a, and a deeper-water phase, stage 1b. Stage 1a is characterized proximally by both disorganized and stratified diamictites, fluvial and subaqueous glacial outwash fans, and distally by disorganized and stratified remobilized diamictites, and interbedded varves with rare dropstones. Stage 1b records a relative water deepening. It is characterized by siltstones interbedded with sheet sandstones and conglomerates, as well as channelized sandstones and conglomerates. Rare dropstones are present in the basal portion of this phase. Stage two records a glacioeustatic marine transgression, and a shut-off or slow-down of the clastic conduit in the paleovalley. This stage is characterized primarily by marine shales with plentiful organic matter preserved, including tree trunks and leaves, interbedded with rare, thin turbidite sandstones and conglomerates. The third stage records progressive infilling of the accommodation space created in the paleovalley due to glacial overdeepening and the glacioeustatic marine transgression. Stage three is characterized by thick-bedded turbidite sandstones and associated shales. The fourth and final stage records a fan-delta paleovalley fill and is characterized by coarse turbiditic sandstones and conglomerates. All stages exhibit mass-transport related deposits, but stage 1b exhibits the most widespread mass-transport, with a wide range of processes represented, from completely mixed debris-flow deposits to coherently slumped and rafted blocks. The scale of these mass-transport deposits (MTDs) ranges from a few meters to over 50 m thick, and up to hundreds of meters wide and long. Most of the large MTDs in this stage occurred at the transition to stage 2, implicating increased relative sea-level as a possible trigger for the slope failures. Additionally, stage three preserves several thrust faults with large offsets relative to the scale of the sedimentary succession (tens to over 100 m vertically and laterally) that are interpreted as the downdip slide-scar ramp zones of MTDs. The incredible abundance of small to large-scale mass-transport deposits at the top of stage 1b may be related to the transgression at this level, combined with post-glacial rebound-generated seismicity. The abundance and size of MTDs in stages 1b and 3 provide fundamental controls on the locus of sedimentation throughout the paleovalley.
OS23B-1307 1340h
A Potential Link between Fluid Expulsion and Slope Stability: Geochemical Anomalies Measured in the Gas Blowouts along the U.S. Atlantic Margin Provide New Constraints on their Formation
Geochemical, bathymetric and AUV based surveys conducted aboard the R/V Cape Hatteras in July 2004 provided new constraints on the formation of large-scale gas blowout features located along the U.S. Atlantic margin. These features, believed to be formed by gas expulsion processes, are ~4km long, ~1km wide and up to 50m deep. The stratal geometry of these features and their location on the shelf-edge has led us to hypothesize that they may indicate incipient slope failure. Interpretation from our chirp seismic reflection data, collected in 2000, showed gas generally was trapped under a thin veneer (several tens of meters) of deltaic sediments, but may be venting along the landward wall of the blowouts. New geochemical data indicate significant methane anomalies above both the seaward and landward walls of the blowouts and reveals that these features are actively venting fluids at the seafloor. Using a METSr sensor mounted on the WHOI Seabed AUV, we observed methane concentrations ranging from 50-100nM in the water column directly above the inner and outer walls, whereas typical methane concentrations in seawater are expected to be 2-4nM. Some of these methane hot spots were also associated with salinity anomalies. Additionally, pore fluids squeezed from a series of piston cores in the blowout region show relatively high alkalinity values (>4-15mM), with a near absence of hydrogen sulfide. These initial results are particularly intriguing since high alkalinity concentrations are commonly associated with high sulfide concentrations. We speculate that there may be a flux of CO2 into the sediments that may be responsible for the high alkalinity and low sulfide. In addition to our geochemical studies, we collected a full suite of bottom photographs, gravity cores, and high resolution bathymetry. Visualization of these data in three dimensions, along with methane concentration profiles, chirp reflection, and sidescan-sonar data has enabled us to build a relatively comprehensive picture of the blowout features. There are strong spatial correlations between trapped gas and the overlying shelf-edge delta deposit, as well as with relatively high methane concentrations in the water column, and indications of inner wall venting in the chirp profiles. Nevertheless, a distinct spatial correlation between the occurrence of biological communities and the fluid expulsion sites was not observed. The active fluid expulsion we measured is consistent with our geophysical observations, and supports our hypothesis that there is a link between upslope fluid migration, downslope creep, and potential slope failure.
OS23B-1308 1340h
Overpressure, Low Effective Stress, and Slope Failure in the Ursa Region, Deep-Water Gulf of Mexico
Slope failures are associated with overpressured pore fluids and low effective stresses in the Quaternary strata of the Ursa Region, deep-water Gulf of Mexico. At Ursa, a permeable turbidite sandstone (the Blue Unit) is overlain by a low-permeability mudstone. Overpressure in the mudstone, measured with a pore pressure penetrometer (piezoprobe), begin within a few meters of the seafloor and extend 250-450 meters down to the Blue Unit. The overpressure ratio ($\lambda$$\ast$$=$(Pp$-$Phydrostatic)$\backslash$(Sv$-$Phydrostatic), where Sv is the overburden stress, Pp is pore pressure, and Phydrostatic is the hydrostatic pressure) ranges from 0.8 where the overburden is thin to 0.4 where the overburden is thick. Detachment surfaces, mapped with high resolution 3D seismic data, are associated with zones where effective stresses are low. Four subsurface slumps were mapped and are oriented generally northwest-southeast. Slump surface areas are less than 250 km$^{2}$ and maximum scarp-wall height on the largest slide is $\sim$120 meters. We interpret that asymmetric loading of the Blue Unit by low-permeable mudstone has driven fluids to where overburden is thin, decreased effective stress, and generated slope instability.
OS23B-1309 1340h
Stable Isotope Anomalies and Low Chloride Concentrations in Pore Water of CH$_{4}$-Rich Sediments at the Tanegashima Mud Volcano, Japan
Pore water from sediments collected at a Tanegashima mud volcano was analyzed for $\delta$$^{13}$C (PDB) of dissolved CH$_{4}$ together with other chemical components, Cl$^{-}$ and SO$_{4}$$^{2-}$, and the $\delta$$^{18}$O and $\delta$D (SMOW). The Tanegashima mud volcanoes are located at the water depths from 1400 m to 1800 m, off Tanegashima island between Ryukyu trench and Ryukyu arc of Japan. It is situated at the end of south-western convergent plate boundary on Nankai-trough, which forms a part of Philippine Sea plate subducting under Eurasian plate. This cruise was conducted as a part of the JNOC (Japan National Oil Corporation) geochemical survey by R/V Hakurei-maru II. The concentrations of CH$_{4}$ were generally higher than 100 micro-mol/kg. Its highest concentration (715 micro-mol/kg) was found in the crest core of a mud volcano. The $\delta$$^{13}$C values ranged from -32 to -50 $\permil$. C$_{2}$H$_{6}$ was detected only in the pore waters collected from the vicinity of the crest of the mud volcano. The highest $\delta$$^{13}$C (around -22 $\permil$) and low C1/C2 concentration ratios (less than 100) were measured at the crest site, supporting the thermogenical production of methane. Other geochemical anomalies were also observed in the crest pore water. The concentrations of Cl$^{-}$ in the pore water at this site were extremely depleted to a minimum of 350 mmol/kg. The Cl$^{-}$ anomaly has not been previously reported for pore water from mud volcanoes around Japan. An endmember of isotopic composition of the fluid is estimated to be +12 $\permil$ for $\delta$$^{18}$O and -40 $\permil$ for $\delta$D. From these results we conclude that the most likely process to reduce pore water salinity is primarily the mixing of clay mineral dehydration water with seawater. The thermogenic methane found in the crest pore waters of the Tanegashima mud volcano may be brought from the depths of sediments due to the migration of fluid evolved by mineral the dehydration process.
OS23B-1310 1340h
Modeling submarine slope failure in a gas hydrate stability zone: An example from offshore Fiordland, southwest New Zealand.
Gas hydrates are ice-like species consisting of natural gas (usually methane) enclosed in a regular, stabilizing framework of water molecules. They have been found to be a significant constituent of seafloor sediment on many continental shelf-slope environments around the world. Gas hydrate is stable only within a limited temperature and pressure regime, and dissociation in response to a change in the physical environment can liberate excess gas and elevate the local pore fluid pressure in the sediment. This effect of sediment weakening is interpreted to be a significant contributing factor to a submarine landslide that has been seismically imaged off the southwest coast of New Zealand. Data show a distinct and continuous bottom-simulating reflection (BSR) below the continental shelf from water depths of $\sim$1650 m to $\sim$700 m where it intersects the seafloor. Additionally, the outcrop of the BSR on the seafloor corresponds with an apparent landslide scarp. The geometry of the submarine landslide is well controlled in two dimensions, but the geotechnical characteristics of the material are not constrained, except by interpolation from other work. Representative soil strength parameters have been applied to both limit-equilibrium and finite-element methods of slope stability analysis with respect to the Mohr-Coulomb failure criterion to develop an understanding of the relative sensitivity of the feature to model parameters. Excess pore fluid pressure (suprahydrostatic) has been modeled with realistic material properties of: internal angle of friction, bulk soil unit weight, and cohesion, to show the considerable effect it has on stability. Permeability and cohesion in the overlying sediment have also been modeled so that their relative significance with regards to stability can be gauged.
OS23B-1311 1340h
Gas Hydrate Dissociation Triggering Mass Wasting on the Costa Rica Continental Margin
The continental margin off Costa Rica is an area of intense deformation and slope instability as recent mapping and sampling efforts have demonstrated. Several processes are simultaneously modifying the overall stability of the continental slope. Convergence-related seamount subduction and subduction erosion are the primary reasons for the large scale removal of slope sediments and consequent collapse of the lower continental slope. A second type of slope failures and slides of smaller scale was found to be related to the dissociation of methane hydrate near the sea floor. Several small scale failures of this type were discovered and sampled during recent expeditions of the SFB574 research group in Kiel. One exceptionally well imaged slide NE of Jaco Scar on the upper continental slope was surveyed and sampled in detail to evaluate a possible connection to shallow BSR occurence in this area. The slide was discovered on stacked wide-angle reflection seismic data acquired in 1999. In this area of the Costa Rica margin slope sediments are characterized by the occurrence of laterally continuous BSRs forming a laterally continous patch, where the BSR can be traced and mapped over several tens of kilometers. At the landward termination of the patch the BSR is converging towards the seafloor at a depth of about 580 m - coincident with the local minimum water depth for hydrate stability. The headwalls of a series of slides are located at this depth, suggesting a close genetic relationship with hydrate breakdown.. Data from ongoing sedimentological, geochemical and geotechnical studies on core material recovered immediately in front of the headwall of the slide will be presented. This data together with geophysical information such as Parasound and bathymetric data is utilized to constrain the mechanics, geometry and mass balance of these slides.
http://www.sfb574.geomar.de/B2_Description/index.html
OS23B-1312 1340h
A Detailed Near-bottom Survey of Large Gas Blowout Structures Along the US Atlantic Shelf Break Using the Autonomous Underwater Vehicle (AUV) SeaBED
The autonomous underwater vehicle (AUV) SeaBED was deployed along the edge of the Virginia/North Carolina continental shelf in July 2004 to investigate a series of crack-like features. Prior investigations indicated these are large (up to 4km long, ~50m deep) gas blowout structures. In addition, the features were carefully mapped with a multibeam bathymetric sonar and sampled for geochemical and sedimentological analysis. The AUV completed 16 successful dives and collected microbathymetric, temperature, salinity, and methane concentration data, along with approximately 36,000 photographs while maintaining an altitude of approximately 3m above the seafloor. The AUV was deployed without seafloor transponders. Instead, the vehicle's navigation was based on a doppler inertial system. The accuracy of the navigation was tested by comparing seafloor depth, measured by the vehicle, with the shipboard swath bathymetry, and by computing the differences in seafloor depth where the vehicle tracks cross. The mean crossover error is 2.18m, showing the vehicle's navigation is actually quite accurate. Photos taken near the crossovers will be compared to determine the exact location of the crossovers. Dissolved methane concentration data, collected using a METS sensor, show elevated concentrations throughout most of the survey area, with the largest anomalies being very localized (50-100m across). Photos, color corrected for attenuation in average seawater, are red-shifted wherever high dissolved methane concentrations are detected, providing additional evidence for seepage in those areas. No other visual evidence of methane venting, such as authigenic carbonates, chemosynthetic communities or gas bubbles, is recognized in the photos, although shells, fish and invertebrates are commonly seen. Methane concentration data at most track crossovers are inconsistent, indicating the magnitude of the anomaly may be affected by time dependent factors such as tides, currents or instrument drift. We favor the explanation of methane flux being modulated by tidal variations, in keeping with observations made at cold seeps on the western coast of North America. We will test this hypothesis by minimizing the crossover errors as a function of tidal stage.
OS23B-1313 1340h
Evidence for Large-Scale Slope Failures on Northern Gulf of Alaska Seamounts
SeaBeam 2100 multibeam bathymetric data from six seamounts in the Kodiak-Bowie seamount chain in the northern Gulf of Alaska collected during August 2004 ({\it R/V Atlantis} cruise 11-15) illuminate the complex eruptive and post-eruptive morphology of these volcanoes. Prominently expressed on the preliminary maps is evidence for large-scale slope failures on a number of these seamounts. Flank morphology suggests that these slope failures take the form of both large coherent masses and smaller debris flows. Two of the seamounts, Giacomini and Pratt, were found to have prominent head-scarp fractures cutting their summit plateaus. These scarps are over 40 meters high in places, and are in the same areas of the seamounts as the earlier slope failures, suggesting repeated failure processes over at least part of the seamounts' histories. We also see evidence for the seamounts being affected by processes on the surrounding seafloor: migration of the active section of the Mukluk deep-sea channel is undercutting the northern slope of Denson Seamount, and appears to be promoting slope failure there. Deep-sea channels are found in close proximity to a number of the seamounts and may have accelerated other slope failures. The sea channels, in addition to fan-body complexes, may also play a role in masking portions of the chaotic debris expected at the base of the failures. As we are still in the early stages of the analysis of these data, many questions remain, primarily in relation to the causes and ages of the slope failures, and the resulting effects in the near and far-fields.
OS23B-1314 1340h
Multibeam Mapping of Active Slope Instability Features: Examples from the Fraser River and Squamish River Deltas, British Columbia, Canada
Multibeam mapping of the coastal waters of British Columbia has immensly improved our ability to identify and assess submarine landslide and tsunami hazard. This paper will present analysis of high-resolution images of slope instability features from two delta slopes where recent slope failure can be documented through repetitive multibeam mapping and/or comparison with previous single-beam hydrographic soundings. Numerous mass movement features characterize the slope of the Fraser River delta, all the recent features being located at the mouths of distributary channels. Engineering works have maintained the main channel in a fixed position since the 1930's, contributing to over-steepening of the slope and development of a network of submarine channels. Repetitive multibeam mapping shows that recent slope failures have occurred in numerous locations around the main channel lobe, some at the head of a large submarine channel system and others as isolated small failures that form the headwalls of small submarine channels. The scalloped morphology and association with channels, together with volume estimates derived from repetitive multibeam mapping, indicate that these features result from shallow, small volume liquefaction failures. Smaller scale, shallow slides are present on the very shallow water slope area adjacent to the channels, raising the possibility of groundwater seepage as an influence on slope stability. The slide masses from these failures are rapidly transformed into gravity flows that carve the submarine channels. Slides and channels of a similar scale are found at the mouth of a secondary distributary channel and an abandoned distributary channel. The multibeam imagery allows discrimination between recent slide features and relict features, the latter showing infilling or reworking by bottom currents. An area of undulatory seafloor, located on the flank of the main distributary channel lobe, has been cited as a possible creep displacement feature and a potentially high risk of subsequent slope failure and tsunami generation. Repetitive mapping of this feature suggests relative movement of ridges and trenches as well as general subsidence of the lobe flank. This feature has been selected as a site for pore fluid pressure monitoring within the VENUS cabled observatory program. Recent multibeam mapping of the Squamish River delta followed a major flood in October 2003. The survey of the delta was completed in less than 12 hours on an opportunity basis. Preliminary results and comparison with maps created from more labor-intensive traditional techniques will be presented.
OS23B-1315 1340h
Pyroclastic Flow Generated Tsunami Waves Detected by CALIPSO Borehole Strainmeters at Soufriere Hills, Montserrat During Massive Dome Collapse: Numerical Simulations and Observations
The July 12-13, 2003 eruption (dome collapse plus explosions) of Soufriere Hills Volcano in Montserrat, WI, is the largest historical lava dome collapse with $\sim$120 million cubic meters of the dome lost. Pyroclastic flows entered the sea at 18:00 AST 12 July at the Tar River Valley (TRV) and continued until the early hours of 13 July. Low-amplitude tsunamis were reported at Antigua and Guadaloupe soon after the dome collapse. At the time of eruption, four CALIPSO borehole-monitoring stations were in the process of being installed, and three very-broad-band Sacks-Evertson dilatometers were operational and recorded the event at 50 sps. The strongest strain signals were recorded at the Trants site, 5 km north of the TRV entry zone, suggesting tsunami waves $>$1 m high. Debris strandlines closer to TRV recorded runup heights as much as 8 m. We test the hypothesis that the strain signal is related to tsunami waves generated by successive pyroclastic flows induced during the dome collapse. Tsunami simulation models have been generated using GEOWAVE, which uses simple physics to recreate waves generated by idealized pyroclastic flows entering the sea at TRV. Each simulation run contains surface wave amplitude gauges located in key positions to the three borehole sites. These simulated wave amplitudes and periods are compared quantitatively with the data recorded by the dilatometers and with field observations of wave runup, to elucidate the dynamics of pyroclastic flow tsunami genesis and its propagation in shallow ocean water.
OS23B-1316 1340h
Simulations of Resuspending Gravity Currents Traveling Over Slopes of Varying Angles: Conditions for Self-Sustainment
We present highly resolved two-dimensional simulations of particle-laden gravity currents and focus on the influence of particle entrainment from the bed. As turbulent motions detach particles from the bottom surface, the mass of suspended particles may increase and cause the current to accelerate. The conditions under which gravity currents may become self-accelerating through particle reentrainment are investigated as a function of slope angle, current size and particle size and concentration. The effect of container size and initial aspect ratio of the current on the evolution of the current are also considered. Applications to flows traveling over slopes of varying angle, such as turbidites spreading down the continental slope, are presented and particular attention is given to the resulting particle deposits and erosion patterns.
OS23B-1317 1340h
Trans-Terrestrial Landslides and the Impact on the Marine Environment: Big Sur Coastline, California
Recent investigation of the continental margin south of Point Sur, along the coastal area of the central Santa Lucia Mountain Range, using EM300 30 kHz multibeam bathymetry, Klein 100 kHz side-scan sonar, and 3.5 kHz seismic reflection profile data indicate that extensive landslide deposits exist along the distal part of the continental shelf and upper slope. Submersible dives using the Delta revealed that the deposits imaged in the remotely sensed data sets were composed of angular to sub-rounded boulders, cobbles and pebbles lying at a depth of 80 to 130m with very little biological growth and sediment cover suggesting a fairly young (~100 years) age for the deposits. Based on preliminary aerial examination of the deposit we estimate a volume of 1 km$^{3}$ that came to rest 2 km offshore of the Big Sur coastline. If such a deposit were to fail at one time it could produce a significant tsunami. We will present evidence that shows the distribution of the slide and a model of the possible impact wave that it could have generated.
OS23B-1318 1340h
Repeated Cycles of Flank Growth and Collapse on Olympus Mons, Mars: Comparisons to Hawaiian Volcanoes
The origin of the rough-textured aureoles that surround the immense Olympus Mons volcano on Mars is controversial. We present data from the Mars Global Surveyor (MGS) mission to demonstrate that at least two of the aureole lobes are derived from the volcano's flanks in large, likely catastrophic, mass movement events, leaving behind headwalls that constitute the basal scarp. We have identified evidence linking blocky structures in the north and northeast aureole lobes to the surface of the Olympus Mons edifice, thereby favoring a mass-movement flank-failure origin for these structures. Specifically, parallel lineations on the upper surfaces of some aureole blocks resemble the leveed or channelized lava flows that are ubiquitous on the surface of the Olympus Mons edifice. Other lineations seen on aureole block surfaces may be the remnants of faults such as those seen at the margin of the Olympus Mons edifice, near the top of the basal scarp. We have determined a plausible pre-failure geometry for the edifice by matching the volume of a reconstructed edifice section to that of the North aureole lobe. A modest ($\sim$60 km) northward expansion of the current edifice is sufficient to account for the volume of the landslide that formed the North lobe, demonstrating that repeated failure events of an edifice not greatly different in size or shape from the present one are capable of generating the observed aureole deposits. The Hawaiian Islands offer structural analogs to the Olympus Mons aureole deposits (Nuuanu slide), basal scarp faults (the Pali faults) and lower edifice benches (the Hilina slump). We suggest that a basal detachment enabled by high pore fluid pressure, as inferred for Hawaiian volcanoes, allowed flank failure and aureole emplacement at Olympus Mons. Evidence for analogous flank failure processes on Mars and Earth demonstrate that repeated failures and outward spreading are universal phenomena on volcanic constructs.