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We interpret a region of undulatory sediments adjacent to a major headwall of the Cape Fear submarine landslide system offshore of North Carolina, USA, as sediment waves rather than creep or fault-related deformation. The wave package extends 19 km upslope from the S4 landslide headwall and thickens upslope from approximately 250 to 450 m. The field of undulating sediments displays the continuity of seismic horizons, upslope-migrating crests, downslope thinning, and wave heights and lengths of approximately 26 m and approximately 1 km, respectively, which are consistent with sediment wavefields. The Western Boundary Undercurrent formed contourites on the nearby Blake Ridge and it is possible that these sediment undulations were deposited via similar mechanisms. Ocean Drilling Program (ODP) cores near the sediment undulation field suggest that the turbidity currents also may have played a role in wave formation. Although most of the 19 km long field comprises unaltered sediment waves, we observe an approximately 5 km long zone adjacent to the landslide scarp that expresses evidence of faults that offset and deform the sediment wave strata. We interpret this deformation as the result of reduction in stress following the removal of the landslide mass. Given that the Cape Fear system has generated several episodes of potentially tsunamigenic slope failure, the future stability of the system is pertinent. Redefining these undulatory sediments as sediment waves eliminates a major slope instability mechanism of the system and is important for understanding the future slope stability hazards of Cape Fear. Our analysis highlights the importance of understanding sediment waves in hybrid submarine landslide-sediment wave systems. Geological feature: Cape Fear submarine landslide sediment waves Seismic appearance: Continuous undulating horizons Alternative interpretations: Downslope creep or faults Features with similar appearance: Extensional slope failure Formation: Alongslope and downslope currents Age: Quaternary Location: Cape Fear submarine landslide complex, offshore North Carolina Seismic data: High resolution multichannel seismic data Analysis tools: Multichannel seismic data, multibeam bathymetry, sub-bottom Chirp

The Chandeleur Submarine Landslide Complex occurs on the upper Mississippi Fan of the Gulf of Mexico in approximately 1100 m of water, 200 km SE of New Orleans, Louisiana. This part of the Mississippi Fan received high sedimentation throughout the Pleistocene, causing high pore fluid pressure and abundant slope failures, though few as large as the Chandeleur. Given its large size, proximity to major coastal cities and seafloor infrastructures, we examine the Chandeleur Slide to understand what led to the initial slope failure and decipher its post-failure transport behaviour using 2D and 3D multichannel seismic surveys, high-resolution bathymetric data, and well logs. We find a large sediment mass with a translational-rotational behaviour that was displaced to the south/SE up to 40 km from the source area. The Chandeleur Slide includes extensional faulting in the headscarp area and compressional structures in the toe where confined by a natural ramp-like structure. Beneath the Chandeleur Slide, we identify a regional sand-rich unit (called the Blue Unit) that is known to be overpressured. Beneath the Blue Unit we observe an upward-migrating salt diapir. We suggest one possible scenario for the origin of the Chandeleur Slide is the combined effects of an upward-migrating salt diapir impinging on an already overpressured Blue Unit, leading to the initial failure. The initial failure was followed by retrogressive headwall retreat northward, which created the prominent scarp on the seafloor. In total, the Chandeleur Slide complex covers an area of about 1000 km 2 and contains about 300 km 3 of sediment.

The tropics are naturally vulnerable to watershed erosion. This region is rapidly growing (projected to be 50% of the global population by 2050) which exacerbates erosional issues by the subsequent land use change. The issue is particularly of interest on the many (~45,000) small tropical (2) islands, and their >115M residents, where ecotourism and sediment intolerant ecosystems such as coral reefs are the main driver of their economies. However, vulnerability to erosion and deposition is poorly quantified in these regions due to the misclassification or exclusion of small islands in coarse global analyses. We use the only vulnerability assessment method that connects watershed erosion and coastal deposition to compare locally sourced, high-resolution datasets (5 x 5 m) to satellite-collected, remotely sensed low-resolution datasets (463 x 463 m). We find that on the island scale (~52 km2) the difference in vulnerability calculated by the two methods is minor. On the watershed scale however, low-resolution datasets fail to accurately demonstrate watershed and coastal deposition vulnerability when compared to high-resolution analysis. Specifically, we find that anthropogenic development (roads and buildings) is poorly constrained at a global scale. Structures and roads are difficult to identify in heavily forested regions using satellite algorithms and the rapid, ongoing rate of development aggravates the issue. We recommend that end-users of this method obtain locally sourced anthropogenic development datasets for the best results while using low resolution datasets for the other variables. Fortunately, anthropogenic development data can be easily collected using community-based research or identified using satellite imagery by any level of user. Using high-resolution results, we identify a development trend across St. John and regions that are both high risk and possible targets for future development. Previously published modeled and measured sedimentation rates demonstrate the method is accurate when using low-resolution or high-resolution data but, anthropogenic development, watershed slope, and earthquake probability datasets should be of the highest resolution depending on the region specified.

We present a new gas hydrate system, Jackalope, in the US Gulf of Mexico ~110 km from the Louisiana coast primarily in Mississippi Canyon (MC) Blocks 127 and 128. Jackalope is a relatively shallow prospect (~1650 m water depth) and is associated with a submarine channel-levee depositional system. Several kilometers below the seafloor, an extended sub-circular salt ridge rooted into the allochthonous salt canopy creates a system of faulted anticlinal folds. Accordingly, the distribution of bottom simulating reflections mapped at ~250–300 m below seafloor (mbsf) is structurally controlled, indicating that gas tends to accumulate above the hinge of the salt ridge in sand-dominated turbidites. The Jackalope system is not a single accumulation; it comprises two primary prospects (Saint Petersburg and Columbus) and several secondary prospects. The primary prospects are defined based on seismic data by peak-leading reflections and phase reversals at the base of the gas hydrate stability zone: a strong peak-leading amplitude above it changes to trough-leading amplitude below. We interpret strong peak-leading amplitudes as gas hydrate in sand-rich intervals, which is confirmed by resistivity and gamma ray logs at the margin of the Saint Petersburg prospect. Continuous gas-water contacts below the primary prospects characterize them as permeable three-component reservoirs comprising gas hydrate above gas, and gas above water. Both prospects are located at ~230–300 mbsf occupying a total area of ~5.9 km2. Low and high estimates of in place gas at standard pressure and temperature conditions in the primary prospects range between 1.5 and 10.6 BCM. These characteristics define Jackalope as a promising, cost- and time-efficient gas hydrate system for further drilling exploration. Moreover, Jackalope demonstrates there is potential for new discoveries in the US Gulf of Mexico in particular due to expanding public 3D seismic and well log datasets.

Over half of the global population is projected to live in the tropics by 2050. Sustainable land development will be challenged by enhanced sediment erosion and deposition, which can negatively impact water quality and ecosystem services in inland and coastal waterways. Existing erosion assessments treat watersheds and coastal zones separately, but we connect them in a two-part vulnerability index to watershed erosion and coastal deposition at 0.0004° (~ 45 m) resolution throughout the tropics. We use open-source datasets and a simple, GIS-based method geared toward tropical, novice end-users. Part 1 of the index reveals a majority of the tropics is vulnerable to erosion. Vulnerability is highest where there are co-occurrences of earthquakes, steep slopes, and high precipitation such as the Caribbean and Southeast Asia. In Part 2, we assess erosion vulnerability at 4 watersheds and include their coastal systems, which can enhance or diminish vulnerability of the entire system to coastal deposition.

The base of the gas hydrate stability zone (GHSZ) is a critical interface, providing a first-order estimate of gas hydrate distribution. Sensitivity to thermobaric conditions makes its prediction challenging, particularly in the regions with dynamic pressure–temperature regime. In Green Canyon Block 955 (GC 955) in the northern Gulf of Mexico, the seismically inferred base of the GHSZ is 450 m (1476 ft) below the seafloor, which is 400 m (1312 ft) shallower than predicted by gas hydrate stability modeling using standard temperature and pressure gradient assumptions and an assumption of structure I (99.9% methane gas) gas hydrate. We use three-dimensional seismic log data and heat-flow modeling to explain the factor of the salt diapir on the observed thinning of the GHSZ. We also test the alternative hypothesis that the GHSZ base is actually consistent with the theoretical depth. The heat-flow model indicates a salt-induced temperature anomaly, reaching 8°C at the reservoir level, which is sufficient to explain the position of the base of the GHSZ. Our analyses show that overpressure does develop at GC 955, but only within an approximately 500-m (∼1640-ft)-thick sediment section above the salt top, which does not currently affect the pressure field in the GHSZ (∼1000 m [∼328 ft] above salt). Our study confirms that a salt diapir can produce a strong localized perturbation of the temperature and pressure regime and thus on the stability of gas hydrates. Based on our results, we propose a generalized evolution mechanism for similar reservoirs, driven by salt-controlled gas hydrate formation and dissociation elsewhere in the world.

Subsea hypersaline anoxic brine pools are among the most extreme habitable environments on Earth that offer clues to life on other planets. Brine is toxic to macrofauna as remotely operated vehicles commonly observe dead and preserved remains in brine pools. While brine pools are often assumed to be stable stratified systems, we show that underwater landslides can cause significant disturbances. Moreover, landslides create large-amplitude waves upon impact with the brine pool, similar to tsunami waves. We focus on the Orca Basin brine pool in the deepwater Gulf of Mexico, which contains numerous landslide deposits and blocks that originated from scarps several hundred meters above the brine pool. The impact of massive fast-moving landslides generated waves with amplitude on the order of 100 s of meters, which rival the largest known ocean waves. Brine waves can negatively affect biological communities and potentially overspill to spread hypersaline brine into surrounding basins.

Based on approximately 11,000 km of seismic reflection data collected across the South China Sea oceanic basin, we describe the sedimentary filling characteristics of the basin since its Oligocene ...

The southern Alaskan offshore margin is prone to submarine landslides and tsunami hazards due to seismically active plate boundaries and extreme sedimentation rates from glacially enhanced mountain erosion. We examine the submarine landslide potential with new shear strength measurements acquired by Integrated Ocean Drilling Program Expedition 341 on the continental slope and Surveyor Fan. These data reveal lower than expected sediment strength. Contrary to other active margins where seismic strengthening enhances slope stability, the high-sedimentation margin offshore southern Alaska behaves like a passive margin from a shear strength perspective. We interpret that seismic strengthening occurs but is offset by high sedimentation rates and overpressure. This conclusion is supported by shear strength outside of the fan that follow an active margin trend. More broadly, seismically active margins with wet-based glaciers are susceptible to submarine landslide hazards because of the combination of high sedimentation rates and earthquake shaking.

In 2017, three major hurricanes (Irma, Jose, and Maria) impacted the Northeastern Caribbean within a 2-week span. Hurricane waves can cause physical damage to coastal ecosystems, re-suspend and transport antecedent seafloor sediment, while the associated intense rainfall can yield large influxes of land-derived sediment to the coast (e.g. burial of ecosystems). To understand sedimentation provenance (terrestrial or marine) and changes induced by the hurricanes, we collected bathymetry surveys and sediment samples of Coral Bay, St. John, US Virgin Islands in August 2017, (pre-storms) and repeated it in November 2017 (post-storms). Comparison reveals morphologic seafloor changes and widespread aggradation with an average of ~25 cm of sediment deposited over a 1.28 km2 benthic zone. Despite an annual amount of precipitation between surveys, sediment yield modeling suggests watersheds contributed

A prominent paleo-landslide in the Slope Basin seaward of the Megasplay Fault in the Nankai Trough was emplaced by a relatively high-mobility mudflow based on analysis of physical properties and seismic geomorphology. Post-failure behavior of submarine landslides is a critical variable that determines amplitude of slide-generated tsunami but is historically difficult to constrain due to the wide variety of post-failure styles of submarine landslides. In recent controlled laboratory experiments, the ratio of the shear stress to yield strength (defined as the Flow Factor) controls a wide spectrum of mass movement styles from slow, retrogressive failure to rapid, liquefied flows. Here, we apply this laboratory Flow Factor approach to a natural landslide in the Nankai Trough by constraining pre-failure particle size analysis and porosity. Several mass transport deposits (MTDs), were drilled and cored at Site C0021 in the Nankai Trough during International Ocean Discovery Program (IODP) Expedition 338. The largest, MTD B, was encountered at 133–176 m below seafloor and occurred approximately 0.87 Ma. Slide volume is 2 km3, transport distance is 5 km, and average deposit thickness is 50 m (maximum 180 m). Pre-failure porosity was estimated from shallow sediments at Site C0018 (82%). Grain size distribution was measured directly from samples with the hydrometer method with the following averages: 37% clay-sized, 60% silt-sized, and 3% sand-size particles. Together, the porosity and clay fraction define the Flow Factor to be 3.5, which equates to a relatively high-mobility slide. We interpret that the landslide that created MTD B was a single event that transported the slide mass relatively rapidly as opposed to a relatively slow-moving cohesive style, landslide. This is supported by the observation in three-dimensional seismic data of a completely evacuated source area with no remnant blocks or retrogressive head scarp and an internally chaotic seismic facies with large entrained blocks. This back-analysis approach can be extended to other field settings characterized by fine-grained siliciclastics and where water content and clay percentages are known.

Las carreteras son una forma generalizada de perturbacion con potencial para afectar negativamente los procesos ecohidrologicos. Algunos de los crecimientos mas rapidos en las redes de carreteras se producen en los paises en desarrollo, particularmente en los tropicos, donde las agendas politicas a menudo se centran en fortalecer la economia, mejorar la infraestructura y reforzar la seguridad nacional., lograr la autosuficiencia y aumentar el bienestar de los ciudadanos, a menudo a expensas del medio ambiente. Revisamos lo que se sabe sobre los impactos de las carreteras en los procesos ecohidrologicos, centrandonos en los sistemas acuaticos, tanto templados como tropicales. Presentamos siete casos que representan las tendencias mas amplias del desarrollo vial y los impactos en entornos tropicales. Muchos de estos procesos dinamicos e impactos no son diferentes de los experimentados en entornos templados, aunque la magnitud de los impactos en los tropicos puede amplificarse con lluvias intensas y la falta de mejores practicas de gestion aplicadas a la construccion / mantenimiento de carreteras. Los impactos de las carreteras en entornos tropicales tambien pueden ser unicos debido a los organismos o ecosistemas afectados en particular. Delineamos unconjunto de mejores practicas para mejorar la gestion de la red vial y proporcionar recomendaciones para adoptar una agenda de investigacion y gestion vial en entornos tropicales. Es importante destacar que pedimos la incorporacion de enfoques transdisciplinarios para estudiar mas a fondo los efectos de las carreteras en los procesos ecohidrologicos en los tropicos. Tambien se debe hacer hincapie en la colaboracion con los gobiernos y los desarrolladores que estan promoviendo el desarrollo vial para ayudar a identificar los impulsores de la expansion de la carretera y los umbrales de impacto negativo, asi como los metodos de construccion y mantenimiento sostenibles de la carretera. Copyright © 2017 John Wiley & Sons, Ltd.

Tropical islands such as St. John in the U.S. Virgin Islands are naturally susceptible to terrigenous (land-based) sediment erosion due to their high-relief slopes, fast weathering rates, and intense precipitation events. Nearshore ecosystems that exist near these islands tend to thrive in static conditions, and are especially stressed by increases in terrigenous input. In the last few decades, island development and population have increased dramatically in some areas of St. John. We conducted a detailed characterization of watersheds and their sediments from ‘source to sink’ in eastern St. John. To accomplish this we combined field observations and sampling with a digital elevation model. Our research was focused on several morphologically similar embayments in eastern St. John; three impacted by anthropogenic development (Coral Harbor, Johnson Bay, and Sanders Bay) and an adjacent, virtually undeveloped bay within the Virgin Islands National Park and Virgin Islands Coral Reef National Monument...

The average depth or traveltime in which the acoustic impedance of marine muds exceeds surrounding sands is not clearly established but is important for a host of applications, such as geohazard analysis for deepwater drilling, slope stability, and prospecting for natural gas hydrates. We have established a practical depth and time for the mud-sand crossover. We have assembled the largest bulk density and compressional velocity data set for marine muds in the first 1200 m below the seafloor (mbsf) from drilling data. The marine mud data set is combined with laboratory data for sands at pressure to determine the physical property trends and crossovers in the average marine sediment column. We have used a simple lithologic model, the average marine mud and sand trends, and a Ricker wavelet to link the physical properties to marine seismic data. We found that a seismic mud-sand crossover occurs, on average, at 875 mbsf, which is approximately [Formula: see text] two-way time below the seafloor. More practically, the impedance contrast between sand layers and marine mud layers may only be sufficiently visible to 500 mbsf or approximately [Formula: see text] two-way time due to the similar acoustic impedances of muds and sands below that depth.

We describe previously undocumented but extensive gas hydrate accumulations in the mouth of Perdido Canyon in the northern Gulf of Mexico. The accumulations are located within central parts of structural domes (four-way closures) and are characterized by stacked, high-amplitude bottom simulating reflections (BSRs) that we call clustered BSRs. Seismic data from Perdido Canyon show two clustered BSRs associated with turbidite sequences within two dome folds formed from tectonic folding and salt diapir rise. The northwestern (NW) and southeastern (SE) clustered BSRs have aerial extents of ∼25 km2 and 50 km2, respectively. Well log data confirm gas hydrate occurs above the NW clustered BSR, within a 225 m-thick consistently high-resistivity interval that we interpret as gas hydrate in near-vertical fractures and turbidite sands. The SE dome is only drilled at the edge of the BSR; nevertheless, the well log data indicate that a 30 m-thick gas hydrate accumulation is present. Gas chromatographic logs in both domes suggest a gradual transition from predominantly microbial gas below the BSR (500–1000 m below seafloor (mbsf)) to thermogenic gas at 1000–2000 mbsf. Based on the well log data and seismic stratigraphic analysis, we find gas hydrate is concentrated in fractures in marine mud, as well as in the pores of submarine fan turbidities, where saturations reach as high as 75%. An estimate of the total gas hydrate-bound gas volume at standard temperature and pressure is between 0.04 and 0.17 trillion cubic meters (TCM) assuming average hydrate saturation of 5-20% in a ∼45 m thick turbidite sand unit above the Perdido Canyon BSR area. Measured BSR extent and gas volume estimates indicate that the NW and SE reservoirs are among the largest gas hydrate occurrences known in the Gulf of Mexico.

Channel-levee systems are responsible for constructing deep sea fans, among the largest sedimentary deposits on Earth. Levee height plays a key role in defining the volume and texture of the material that is deposited in the bounding levees, and thus the morphology of submarine fans. Models of channel formation and evolution generally assume that the levees aggrade in response to the cumulative overspill of turbidity flows, and that their height is controlled by these flows. In contrast, we show that levee growth in the Ursa Basin (Gulf of Mexico) is limited by the mechanical strength of the levee, not the flow behavior. While many studies document sidewall failures in levee systems, our poro-mechanical model is the first to demonstrate that collapse of levees is a large-scale, deep-seated process driven by the interaction of levee formation and high fluid pressure. Rapid deposition of a regional sand unit induced large fluid overpressure in the underlying mud, which preconditioned the system for levee failure, which then fed a large volume of sediment back into the channel-levee system. Long-lived levee failures continually reintroduced previously deposited levee material back into the channel system. This implies that a large volume of sediment is continuously recycled, which has not been previously understood. Turbidite flow models generally assume that flows progressively lose their fine-grained component due to levee overspill as they traverse the channel. In contrast, we show a mechanism by which fine-grained material can re-enter the system in large quantities, and this has significant and broad importance for models of channel and fan evolution. We also show that that levee failure introduces significant unconformities, in contrast with the common assumption that levees offer complete and high-resolution records of climate, tectonics, and sea level.

Bottom-simulating reflections (BSRs) identified in seismic data are well documented; and are commonly interpreted to indicate the presence of gas hydrates along continental margins, as well as to estimate regional volumes of gas hydrate. A BSR is defined as a reflection that sub-parallels the seafloor but is opposite in polarity and cross-cuts dipping sedimentary strata. BSRs form as a result of a strong negative acoustic impedance contrast. BSRs, however, are a diverse seismic phenomena that manifest in strikingly contrasting ways in different geological settings, and in different seismic data types. We investigate the characteristics of BSRs, using conventional and high resolution, 2D and 3D seismic data sets in three locations: the Terrebonne and Orca Basins in the Gulf of Mexico, and Blake Ridge on the US Atlantic Margin. The acquisition geometry and frequency content of the seismic data significantly impact the resultant character of BSRs, as observed with depth and amplitude maps of the BSRs. Furthermore, our amplitude maps reinforce the concept that the BSR represents a zone, over which the transition from hydrate to free gas occurs, as opposed to the conventional model of the BSR occurring at a single interface. Our results show that a BSR can be mapped in three dimensions but it is not spatially continuous, at least not at the basin scale. Rather, a BSR manifests itself as a discontinuous, or patchy, reflection and only at local scales is it continuous. We suggest the discontinuous nature of BSRs is the result of variable saturation and distribution of free gas and hydrate, acquisition geometry and frequency content of the recorded seismic data. The commonly accepted definition of a BSR should be broadened with careful consideration of these factors, to represent the uppermost extent of enhanced amplitude at the shallowest occurrence of free gas trapped by overlying hydrate-bearing sediments.

The ratio of the shear stress to yield strength, defined here as the flow factor, controls a wide spectrum of mass transport processes (slow, retrogressive failure to rapid, liquefied flows) and deposit morphologies (highly fractured and hummocky to thin and smooth) in subaqueous dam-break experiments. We control pre-failure flow factor in sediment–water slurries by systematically varying water content and silt:clay fraction. We document (1) the style and rate of sediment transport, and (2) the morphology of the entire deposit (source area to down-dip extent) for low, medium, and high flow factor slurries. A high flow factor (shear stress ≫ yield strength) generates a rapid collapse of the source area with an accelerating, long run-out flow and a prominent turbidity current. These flows remove approximately 70% of the original source area volume within 10 s from flow initiation. These flows leave behind a thin and smooth source area and deposit as a single mass that is thin, long and wide. In contrast, a medium flow factor (shear strength > yield strength) generates a slow retrogressive failure, in which individual fault blocks evacuate the source area and accumulate gradually over time into a deposit. These flows leave behind a blocky, highly fractured source area and construct short and thick deposits with a hummocky surface. Lateral levees (characteristic of these flows) focus material down-dip while inhibiting lateral spreading, thus producing an elongated deposit. When flow factor is low, approaching unity (shear stress ≈ yield strength), only a narrow zone of failure occurs and a short and thick deposit is constructed. Our experiments display characteristics similar to natural field examples of submarine failures. Our experiments suggest that a detailed analysis of deposit surface morphology from geophysical or outcrop observations can yield important clues to the flow history. In turn, this has practical implications for hazards assessments and predictions in which it is important to consider how possible future failures may behave.

Submarine geohazards threaten coastal communities and global economies. Submarine debris flows are the largest mass-wasting events observed on the Earth’s surface, comprising of up to 50 % of basin fill. Further insight can be gained into these important processes by understanding in-situ preconditioning factors that lead to slope destabilization. We examine two locations from the International Ocean Discovery Program data archive to determine how external effects on sediment properties compare between passive margins and active margins. We select representative passive margin (Amazon Fan) and active margin sites (Nankai Trough), and analyse peak shear strength, void ratio, and composition from the uppermost 100 m below seafloor. This depth corresponds to a depth range in which most submarine mass movements originate. However, it is not appropriate to directly compare shear strength and void ratio of samples from different settings due to differing stress histories, sedimentary composition, and consolidation properties. We focus on ideal locations on both margin types that have solely undergone one-dimensional burial, no diagenesis/cementation, and no unroofing. We find that active margin sediments exhibit an increase in shear strength when compared to their passive margin counterparts, while void ratio tends to be higher on active margins. We are currently conducting a focused lab program to better understand compositional effects and determine the intrinsic properties of each site to more definitively normalize the in-situ sediment profiles. Our results suggest a potential link between shear strength and margin seismicity.

A 3.4-m thick condensed section enriched in foraminifera formed the final detachment horizon of a retrogressive submarine landslide, in the Ursa Basin, northern Gulf of Mexico. The high concentration of foraminifera produces a high porosity (up to five porosity units) layer distinct from the background clay. Integrated Ocean Drilling Program Expedition 308 Site U1324 cored and logged this layer. We conducted a sedimentological analysis on 31 samples across this zone and the overlying and underlying background clay. CT images show that foraminifera are individuals dispersed within the clay, unbroken, and have retained a significant amount of intraskeletal void space. The assemblage is expected for this time interval in the Late Pleistocene (~24 kya). We interpret the layer is a result of a pause in terrigenous sedimentation. The condensed section was a preferred detachment horizon but only minimal sliding occurred before further movement ceased. One possible mechanism to explain this is the presence of foraminifera results in a dilational shear strengthening behavior, which arrested movement. Further work will be required to test this, however. On a broader scale, condensed sections with abundant microfossils, may play a key role in landslide mechanics because they can alter the shearing properties of the background material.

Recently acquired 2-D seismic data from offshore North Carolina provides images of salt diapirs and landslides in the Carolina Trough that give insight into the interaction between slope sediments and intruding salt from below. The best example of this is the Cape Fear Slide Complex in which at least two salt diapirs are surrounded by the lower headwall of the slide. Here, we present seismic images that were collected for the Eastern North American Margin Community Seismic Experiment. We describe the morphology of the slide and diapirs in order to infer rates of salt rise. We have tentatively estimated a post-slide growth rates of 517 m per million years (m/Ma). However, as the analysis continues, it is possible this estimate will change. This research provides significant insight into the interplay of salt and slope failure processes in an ocean basin setting.

The interplay between sedimentation and erosion during the late Pleistocene in the Mars-Ursa region, northern Gulf of Mexico, resulted in a complex compartmentalized reservoir. Rapid deposition, directly downdip of the Mississippi River beginning about 70 k.y., quickly filled antecedent topography in the Mars-Ursa region with a thick accumulation of sand and mud called the blue unit. This permeable reservoir was rapidly and asymmetrically buried by thick, mud-rich levees of two channel-levee systems. Both systems plunged from north to south with a steeper gradient than the underlying blue unit. Rotated channel-margin slides present in both channel-levee systems rotated low-permeability, mud-rich levee deposits beneath the sand-rich channel fill. As a result of the channel-levee systems, the east-west hydraulic connectivity of the blue unit decreases progressively from north to south until its eastern and western halves become completely separated.

Earthquakes are a primary trigger of submarine landslides, yet some of the most seismically active areas on Earth show a surprisingly low frequency of submarine landslides. Here we show that within the uppermost 100 m below seafloor (mbsf) in previously unfailed sediment, active margins have elevated shear strength by a factor of 2–3 relative to the same interval on passive margins. The elevated shear strength is seen in a global survey of undrained shear strength with depth as well as a normalized analysis that accounts for lithology and stress state. The enhanced shear strength is highest within the uppermost 10 mbsf. These results indicate that large areas of modern day slopes on active margins have enhanced slope stability, which may explain the relative paucity of landslides. These findings lend support to the seismic strengthening hypothesis that the repeated exposure to earthquake energy gradually increases shear strength by shear-induced compaction.

We clarified the interrelations between deformation and spatial variations in the physical properties and fabric of mass-transport deposits (MTDs) within the Ursa Basin in the Gulf of Mexico, based on measurements of the anisotropy of magnetic susceptibility (AMS) and porosity, and observations of the rock fabric. Drillholes sunk as part of the Integrated Ocean Drilling Program (IODP) Expedition 308 (Sites U1322, U1323, and U1324) penetrated clay-rich MTDs in the Ursa Region within the uppermost 600 m below the seafloor. The soft-sediment deformation within the Ursa MTDs is characterized by contorted sediments and a homogenized appearance. Site U1322 shows evidence of major mechanical and strain decoupling at the base of the MTDs; i.e., the intensity of the magnetic lineation, porosity, and shear strength show gradual changes in the upper parts of the MTDs, with a sharp offset in these parameters at the contact with the underlying unit. At the grain scale, these MTDs are characterized by strong shearing-induced textures (S–C structures) and bedding-oblique foliations. However, at Site U1324, MTDs show only minor variations in fabric and physical properties despite being correlatives of those at Site U1322. Site U1324 is located near the headwall areas of MTDs, whereas Site U1322 lies near the toe areas. Therefore, at Site U1324 the MTDs are close to their pre-failure condition, whereas the sediments at Site U1322 record high strain and long-distance transport. The obtained values of differential porosity and magnetic lineation (indicative of grain alignment) show a systematic relation between the sites, indicating that the physical properties of sediments in MTDs are controlled by the fabric.

[1] Clay-rich mass transport deposits (MTDs) in the Ursa Basin, Gulf of Mexico, record failures that mobilized along extensional failure planes and transformed into long runout flows. Failure proceeded retrogressively: scarp formation unloaded adjacent sediment causing extensional failure that drove successive scarp formation updip. This model is developed from three-dimensional seismic reflection data, core and log data from Integrated Ocean Drilling Project (IODP) Expedition 308, and triaxial shear experiments. MTDs are imaged seismically as low-amplitude zones above continuous, grooved, high-amplitude basal reflections and are characterized by two seismic facies. A Chaotic facies typifies the downdip interior, and a Discontinuous Stratified facies typifies the headwalls/sidewalls. The Chaotic facies contains discontinuous, high-amplitude reflections that correspond to flow-like features in amplitude maps: it has higher bulk density, resistivity, and shear strength, than bounding sediment. In contrast, the Discontinuous Stratified facies contains relatively dim reflections that abut against intact pinnacles of parallel-stratified reflections: it has only slightly higher bulk density, resistivity, and shear strength than bounding sediment, and deformation is limited. In both facies, densification is greatest at the base, resulting in a strong basal reflection. Undrained shear tests document strain weakening (sensitivity = 3). We estimate that failure at 30 meters below seafloor will occur when overpressure = 70% of the hydrostatic effective stress: under these conditions soil will liquefy and result in long runout flows.

Abstract Integrated Ocean Drilling Program (IODP) Expedition 308 in the northern Gulf of Mexico collected geotechnical and petrophysical properties of several mass transport complexes (MTCs) in the upper 600 meters below seafloor. We present a core-log-seismic integration of the largest MTC in the area. The MTC is identified in seismic data by semi-transparent to chaotic seismic facies. Close inspection of this facies in the area of Site U1324 reveals intact blocks of undeformed material. The base of the MTC is recorded in logging-whiledrilling (LWD) data as a characteristic sharp offset from highto- low values in the LWD resistivity, LWD bulk density, and sonic velocity, which is imaged as a strong reflector in seismic. The internal deposits are characterized by high LWD resistivity, LWD bulk density, and sonic velocity. At the core scale the internal deposits are composed of faulted, folded, and tilted mud with rare silt lamina. At Site U1322, core through this MTC was much more deformed than at Site U1324. This observation may record different deformation intensity and/or transport distance despite being in the same MTC unit and ~10 km apart. The characteristic signature in the resistivity log and seismic data of this MTC allows for correlation to nearby industry wells where shallow logs exist. Introduction Mass transport complexes (MTCs) are ubiquitous on continental margins where they re-shape the seafloor, transport large volumes of sediment, threaten offshore facilities, and produce tsunamis that threaten coastal communities. MTCs are generally defined as the deposits of seismically resolvable mass movement events that include deposits of slumps, slides, and debris flows1,2. MTCs are potential drilling hazards and can act as seals for hydrocarbon reservoirs. Several recent studies have focused on the 3-D seismic geomorphology of MTCs3,1,4,5,6,7. These studies and others have shown that MTCs are a major component of deepwater environments. A comprehensive study of MTCs in the Amazon Fan showed that MTCs have a high degree of consolidation relative to their burial depth8. This property of MTCs has been related to the slow penetration time of jetted conductors and suction anchor piles9. Integrated Ocean Drilling Program (IODP) Expedition 308 in the Ursa Region, northern Gulf of Mexico, cored, logged, and sampled, several prominent MTCs in the upper 600 meters (~2000 feet) below seafloor. A high-resolution 3-D seismic volume shot specifically for hazard analysis provides detailed images of the MTCs. The goal of this paper is a detailed analysis of the top, base, and internal deposits of the largest MTC by integrating core, log, and seismic data at Sites U1324 and U1322. We document lateral differences in this MTC from site-to-site. We then close by correlating this unit to two nearby industry wells. Geologic Setting The Ursa Basin lies 210 km (~125 miles) southeast of New Orleans, Louisiana (USA) on the continental slope in water depths ranging from 800-1500 meters (2600-4900 ft.) (Figure 1). IODP Expedition 308 Sites U1324, U1323, and U1322 are located in Mississippi Canyon blocks 897, 898, and 855, respectively. Sedimentation rates were very high in this area during the Late Pleistocene, averaging 10mm/yr at Site U1324 and 3.8 mm/yr at Site U132210. The Blue Unit is a ponded sand-prone unit infamous for causing shallow-water flow problems in this area11,12.

Abstract Integrated Ocean Drilling Program (IODP) Expedition 308 drilled a transect along the Ursa Basin in the eastern Gulf of Mexico for examining how sedimentation, overpressure, fluid flow, and deformation are coupled in passive margin settings. A total of eight holes were drilled in the Ursa Basin, five were cored and three were logged using either logging while drilling (LWD) or wireline techniques, to evaluate the controls on slope stability, understand the timing of sedimentation and slumping, establish the petrophysical properties of shallow sediments, and provide a better understanding of turbidite systems. Overall, the log responses vary for the different lithostratigraphic units and associated regional seismic reflectors. Significant lateral variability in physical properties has been observed on downhole profiles but the acquired data also make bed-to-bed correlation between sites possible, which is valuable for the study of sandy turbidites and studies of regional deformation. In the Ursa Basin sediments, resistivity-at-the-bit images show significant deformation of the overlying hemipelagic drape and distal turbidites that were drilled in these areas. Numerous dipping beds throughout these intervals with dips ranging from 5-55° confirm core observations. Steeply deformed beds, with dips as high as 65°, and folded and faulted beds suggest down slope remobilization as mass-transport deposits. Resistivity images also show evidence of these mass-transport deposits where steep dips and folds suggest the presence of overturned beds within a series of cyclic intervals that we interpret as a succession of sand-silt-mud lamina. Preliminary structural analyses suggest that many of the deformation features trend in an E-W direction with the majority dipping to the north. Introduction IODP Expedition 308 drilled three sites within the Ursa Basin in the eastern Gulf of Mexico for examining how sedimentation, overpressure, fluid flow, and deformation are coupled in passive margin settings [1]. The bathymetry in the Mars-Ursa region is produced by deposition of a thick wedge of Pleistocene sediments originating from the Mississippi River (Fig. 1). In the north-central Gulf of Mexico, the late Pleistocene shelf, shelf-margin, and minibasin turbidite deposits that were sourced from the Mississippi River are termed the Eastern Depositional Complex [2]. These shelf and shelf-margin deposits have been previously described described [3, 4] and the deepwater strata explored during Expedition 308 comprise one component of this system. In this overview, we examine results from wireline loging and logging while drilling data, identify changes in stratigraphy, interpret resistivity image data, and characterize deformation patterns mainly attributed to mass transport deposits (MTDs). We also identify lateral lithological varibilties using log to seismic correlations at each site. Ursa Basin IODP Expedition 308 drilled a total of eight holes in the Ursa Basin, five holes were cored and three were logged using LWD techniques. The main objectives for acquiring logging measurements in this area were to determine the physical properties of the sequences drilled, characterize the changes in stratigraphy, provide information regarding the deformation associated with MTDs, assess drilling conditions when encountering potential overpressured sand intervals, and provide the basis for linking LWD observations and seismic reflectors through time-depth conversions and synthetic seismograms.

Abstract We measured pore pressures with two pore pressure penetrometers at IODP Site U1324 in Pleistocene sediments of the Ursa Basin, Gulf of Mexico, directly offshore from the Mississippi Delta. Between the seafloor and 300 meters below the seafloor (mbsf), overpressures reach 80% of the hydrostatic effective stress (?=0.8=(u0-uh)/(?v-uh) In this interval, only low permeability mudstones are present. Beneath 300 mbsf, ?* is approximately 0.2 and the sediments are composed of interbedded mudstone, siltstone, and very fine sandstone. We interpret that the lower relative pressures beneath 300 mbsf are caused by the higher permeability of these sediments. Penetrometer deployments ranged from 30 minutes to 90 minutes, which was not enough time for the measured pressure to dissipate to the in-situ pressure. To estimate the in-situ pressure, we used two inverse time extrapolation techniques: 1/?t, and1/t. We use a theoretical soil model to show that the 1/?textrapolation provides a desirable accuracy in much shorter amount of time than the 1/t extrapolation. Introduction Overpressures, pore pressures in excess of hydrostatic pressure, have been observed in sedimentary basins around the world 1. Knowledge of the distribution of overpressure is critical to explore, drill, and produce hydrocarbons 2, 3. Overpressures drive pore water flow 4, impact large-scale structural development 5, and affect the state of stress 6. In young, cool, sedimentary basins, overpressures are generated in low permeability mudstones due to their inability to drain as they are loaded by sedimentation or deformation. Although the pressure is generated within the mudstone, it is most often measured in permeable formations adjacent to the mudstone 3. Recently, the petroleum industry has extended geotechnical techniques to measure pore pressure within mudstones through the use of pore pressure penetrometers in the borehole to depths of many hundred of meters 7–9. This exciting technique provides direct measurements of pore pressure in low permeability rocks and in some locations it has documented very high pore pressures immediately beneath the seafloor 7. When the penetrometer is pushed into the formation below the bottom of the hole (BOH), a pressure disturbance is created. The time that it takes to disipate this pressure depends on the probe diameter and the hydraulic diffusivity of the sediment. In ocean drilling, the time available for downhole tool measurements is expensive and limited. In this environment, in-situ properties must be interpreted from partial dissipation records. If detailed soil properties are available, the in-situ pressure and diffusivity of the sediment can be inferred from modeling of soil behavior for different penetrometer geometries 10–14. However, in many cases soil properties are not available, or there are insufficient resources to pursue soil modeling. In these cases, in-situ pressure is inferred from a simple extrapolation approach.