Uniaxial consolidation experiments on Mass Transport Deposit (MTD) and non-MTD core samples from Ursa Basin, Gulf of Mexico, show MTDs have a lower porosity at a given effective stress compared to adjacent non-MTD sediments; a behavior observed in additional experiments on lab remolded Ursa core and resedimented Boston Blue Clay (BBC). I hypothesize debris flow action remolded the sediment: removing its stress history through shearing action, resulting in dense sediments at shallow depth. I supplement testing this hypothesis through lab remolding of BBC (in addition to Ursa clay) due to the greater availability and knowledge of this material. Ursa MTDs record multiple submarine slope failure events within the upper 200 meters below sea floor (mbsf); the most prominent is labeled MTD-2. MTDs have lower porosity and higher bulk density than surrounding, non-MTD, sediment. Porosity ([phi]) is 52% at 125mbsf – immediately below MTD-2; whereas [phi] is 46% at 115mbsf – within MTD-2. Comparison of non-MTD samples to MTD-2 samples, and intact to remolded samples, shows a decrease in sediment compressibility (Cc) within the MTD-2 and remolded sediments. Permeability within Ursa mudstones also declines with porosity according to: log (k) = A[phi] - B. Permeability is slightly higher within MTD-2; however grain size analysis indicates lower clay content in MTD-2 versus the non-MTDs. Pre-consolidation stress interpretations from the experiments show a linear trend in both MTD and non-MTD sediments, indicating both geologic units depict the same pore pressure profile. Remolding via debris flow explains the origin of MTDs at Ursa and governs the evolution of this geologic unit to its dense, highly consolidated, state today. At some point, slope failure triggered movement of the sediment down slope in form of a debris flow. The shearing action of the debris flow weakened the sediment, reducing its ability to support the overburden. As consolidation resumed, the remolded sediment followed a new, less steep, Cc curve. Within the geologic record, a distinctive dense, shallow unit is preserved; evidence for historical slope failure. / text

Slope-channels act as conduits that transport sediments from the shelf staging area to the basin floor. The Pliocene-Pleistocene section of the Columbus Basin in the deep-water slope offshore eastern Trinidad provides an opportunity to study slope-channel morphology and evolution, as well as any association between deep-water deposits, palaeo-seafloor bathymetry, shelf sediment feeder mechanism and changes in sediment supply types and volumes. Approximately 3250 km2 of 3D seismic data allow imaging and interpretation of channels within an interval between two regional surfaces termed P30 and P40. Observations of seismic cross-sections and stratal slices reveal a number of features including channels, mud diapirs, mass transport deposits (MTDs), and faulted anticlinal ridges. Channels appear leveed and unleveed, and alternate with MTDs in a cyclic vertical succession. Nineteen channels were mapped and divided into two groups based on their degree of levee development and stratigraphic position relative to MTDs. Group 1 channels, positioned below MTDs near the base of the interval, are shallowly incised, and show limited levee development. Group 2 channels, situated above MTDs, are relatively deeply incised, and have comparatively larger, well-developed levees throughout their lengths. Morphometric data from these channel groups reveal significant variability in channel width, channel depth, meander belt width, and sinuosity downslope. This variability is associated with influences of temporally equivalent local features and regional sea-floor slope changes. Increased slope gradient causes a marked increase in sinuosity. Diapirs and anticlinal ridges confine channel paths, divert their flow, and cause post-depositional deformation of both levees and channels. Levee height decreases downslope while levee width shows considerable asymmetry, which is related to occurrences of mud diapirism and MTDs. Irregularities on the upper surface of MTDs create accommodation space that confines turbidity flows, enabling ponding of sediments and volumetrically large levee construction. This accounts for dispersion of turbidity flows below the MTD which creates a series of small channels spread over a wide area, and comparatively fewer, confined channels above the MTDs with large levees. / text

The continental margin off Nova Scotia (the Scotian margin) forms the northern edge of the North American Basin. The Cenozoic stratigraphy and geological history of the outer margin is not well known. This study examines aspects of the Upper Cretaceous-Cenozoic geological history of the outer Scotian margin addressing the following objectives: 1) determine the geological history of a large deep-water depocenter, 2) investigate processes that led to deep-water unconformity formation in the study area, 3) determine the role of deep-ocean circulation in margin evolution, 4) examine the effects of morphological heritage on subsequent depositional patterns. High quality 2-D and 3-D seismic reflection data along with lithostratigraphic and biostratigraphic data from hydrocarbon exploration wells provide the basis for this investigation. The seismic stratigraphy of a large deep-water depocenter along the western Scotian margin was broadly divided into four units. Unit 1 (Upper Cretaceous-Upper Eocene) is attributed to repeated, widespread erosion events interspersed with periods of hemipelagic and pelagic, carbonate-rich sedimentation. Unit 2 (Lower Oligocene-Middle Miocene) consists of a variety of seismic facies overprinted by dense, small-offset faults. Unit 3 (Middle Miocene-Upper Pliocene) is dominated by sediment drift deposition. Unit 4 (Upper Pliocene-present) is characterized by channel development and gravity flow deposition. The processes that led to regional seismic stratigraphic horizons were complex. Both large mass-wasting events and along-slope bottom currents contributed to the formation of unconformities in the study area. Most of the succession preserved in the depocenter belongs to seismic units 2 and 3. These deposits are mainly confined to the area seaward of the Abenaki carbonate bank and landward of shallow salt structures below the slope. Locally, however, modification of the slope profile through mass-wasting and bottom current processes greatly influenced subsequent depositional patterns. The Cenozoic geological evolution of the study area was strongly affected by northeast-to-southwest flowing bottom currents. The earliest indication of bottom current activity was in the Eocene. Upper Miocene and Pliocene sediment drifts represent >50% of the preserved stratigraphic section in the thickest part of the depocenter. It is clear that along-slope sedimentary processes were far more important in shaping the margin than previously understood.

A prominent landslide deposit in the Slope Basin seaward of the Megasplay Fault in the Nankai Trough was emplaced by a high-mobility landslide based on analysis of physical properties and seismic geomorphology. Slide acceleration is a critical variable that determines amplitude of slide-generated tsunami but is many times a variable with large uncertainty. 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, occurs at 133-176 meters below seafloor and occurred approximately 0.87 Mya. Slide volume is 2 km3, transport distance is 5 km, and average deposit thickness is 50 m (maximum 180 m). Pre-failure water content was estimated from shallow sediments at Site C0018 (82%). The average grain size distribution is 37% clay-sized, 60% silt-sized, and 3% sand-size particles as determined by hydrometer analyses of the MTD. Together, the water content and clay fraction predict a Flow Factor of 3.5, which predicts 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 slow, episodic landslide event. This is supported by the observation of a completely evacuated source area with no remnant blocks or retrogressive headscarp and an internally chaotic seismic facies with large entrained blocks. This approach can be extended to other field settings characterized by fine-grained siliciclastics and where water content and clay percentages are known.

Le but de cette thèse est d'étudier les structures sédimentaires de déformations produites par liquéfaction en subsurface des sédiments marins (soft sediment deformations) lors d'événements sismiques. Une caractérisation de différents niveaux de séismites est menée afin de mettre en relation la formation des SSDs et la tectonique régionale au moment de la déstabilisation à l'aide des directions de structures, et de discuter de l'impact de la lithologie sur la forme et la localisation des SSDs dans la pile sédimentaire. Ce travail est réalisé à partir d'observations de terrain et de mesures statistiques de la direction des structures sur des objets de dimensions variables. Le premier exemple s'est portée sur les SSDs qui affectent les dépôts du Penarth group (limite Trias-Jurassique) dans les îles Britanniques au début de la phase de dislocation de la Pangée. Le second exemple s'est appliqué à la caractérisation des parties amont de quatre grands mass transport deposits présents dans le bassin néogène de Tabernas, (cordillères Bétiques, Espagne). Cette étude a mis en évidence le lien très fort entre la déformation de surface et la tectonique et l'intérêt d'étudier ces objets pour mieux définir le contexte du bassin au moment de la déstabilisation. La lithologie du sédiment et son taux d'induration ont un impact fort sur le type de SSDs observées. Les paramètres favorisant le déclenchement de la déstabilisation gravitaire au-dessus d'un niveau liquéfié sont 1) un faible taux d'induration sédimentaire 2) un temps de liquéfaction important et des fortes magnitudes de séisme 3) l'absence de failles qui traversent le niveau liquéfié susceptibles d'ancrer la déformation. / The aim is to study the sedimentary structures of deformation produced by liquefaction in subsurface of marine sediments (soft sediment deformations) during seismic events. A characterization of different levels of seismites is carried out in order to (1) establish links between the formation of SSDs and the regional tectonics with the time of destabilization using structural directions of deformation; (2) to discuss the impact of lithology on the shape and location of deformation structures in the sedimentary pile. The characterization of SSDs is realized from: (1) field observations and (2) statistical measurements of the direction of structures on objects of variable dimensions. The first studied case concerns the characterization of the SSDs that affect the Penarth group (Triassic-Jurassic boundary) deposits in the British Isles at the beginning of the break-up phase of the Pangea. The second example present the characterization of upstream areas of four major mass transport deposits, located in the intramontane basin of Tabernas, in the Betic Cordilleras (Spain). Sediment lithology and induration rates with depth, have a strong impact on the type of observed deformation structures. The parameters that encourage the triggering of the gravity destabilization above a liquefied level are (1) a low sedimentary induration rate which does not limit the liquefaction process (2) a high liquefaction duration which requires a high magnitude of the seismic event (3) the absence of faults which cross the liquefied level and can anchor the deformation in the underlying bed by forming relief.

No description available.

Le Canal de Corse est un bassin confiné asymétrique localisé entre l’Île de Corse et l’Archipel de la Toscane, dont le flanc ouest est dominé par des processus turbiditiques et hémipélagiques et le flanc est par des mouvements en masse et des processus contouritiques. Le présent projet de doctorat a pour objectif de comprendre plus précisément les mécanismes contrôlant la formation des glissements sous-marins dans les contourites vaseuses (dépôts sédimentaires formés par les courants) pendant la période Plio-Quaternaire. Le vaste jeu de données disponible pour ce projet de doctorat inclut : la bathymétrie multifaisceaux, la sismique réflexion, les mesures géotechniques in situ, les mesures de vitesse de courant et les résultats d’un modèle hydrodynamique.Les contourites du Canal de Corse sont principalement composées de vase avec la présence de couches de sable formées par de forts courants de fond pendant les périodes de baisse du niveau marin. La croissance des dépôts contouritiques dépend de la disponibilité de sédiment fourni par le système turbiditique. Ainsi, cette croissance est lente pendant les périodes interglaciaires de haut niveau marin et rapide pendant les bas niveaux marins. Les courants contrôlent la morphologie du fond et génèrent les plastered drifts de forme convexe avec des pentes plus raides dans la partie avale, limités par une incision créée par les courants (moat). Le Pianosa Slump a été initié dans cette partie basse du plastered drift. Les moats pourraient être érodés préférentiellement pendant les périodes froides passées déclenchant ainsi certains glissements observés. Un autre facteur prédisposant l’instabilité de pente sur la Ride de Pianosa est la faiblesse d’une couche dont le comportement mécanique se caractérise par du radoucissement (perte de résistance avec le cisaillement). Cette propriété particulière est due à la présence de zéolites (produit de l’altération des roches volcaniques). Cette couche a formé la surface basale de rupture du Pianosa Slump. En conclusion, les deux principaux facteurs prédisposant la formation de glissements sous-marins sur la Ride de Pianosa sont : la morphologie du plastered drift avec une pente plus raide en aval et la couche faible composée de sédiment vaseux riche en zéolites. Le principal facteur déclenchant semble être l’érosion basale. / The Corsica Trough is an asymmetric confined basin located between the Corsica Island and the Tuscan Ar-chipelago, with the western flank dominated by turbiditic and hemipelagic processes and the eastern flank by mass transport and contouritic processes. The present PhD project aims to develop our understanding of the mechanisms that control the formation of submarine landslides within muddy contourites (sediment deposits related to bottom currents) during the Plio-Quaternary. The broad data set available for this PhD project includes: multibeam bathymetry, seismic reflection data, sediment cores, in situ geotechnical measurements, current ADCP measurements and results of a hydrodynamic model.The contourites of the Corsica Trough are mainly composed of mud with sandy layers formed by enhanced bottom currents during periods of sea level fall. The contourite drifts grow slowly during sea level high-stands and rapidly during sea level low-stands due to the high sediment availability provided by an active turbidite sys¬tem. Bottom currents control the seafloor morphology and generate plastered drifts on the slope. This is a con¬vex-shaped contourite with steep slope gradients in the lower part limited by a moat (incision created by bottom currents). The Pianosa Slump was initiated in this lower part of the plastered drift. The occurrence of continuous erosive processes during cold periods could undercut the slope and trigger submarine landslides. Another predis¬posing factor for slope instability identified is the presence of a potential weak layer with a post-peak strain soften¬ing behaviour (strength loss with increasing strain). This particular property is caused by the presence of zeolites (product of the alteration of volcanic rocks). This layer originated the basal failure surface of the Pianosa Slump.In summary, the two main factors predispose the formation of submarine landslides in the Pianosa Ridge are: the morphology of the plastered drift with steep slopes in the lower part and a potential weak layer composed of zeolitic muddy sediment. The main triggering factor seems to be undercutting by bottom currents.