Turbidity currents are turbulent particle suspensions that are the primary mechanism for transporting terrestrial sediments to the deep marine, and generate some of the largest depositional features on Earth. However the fluid-particle interactions that sustain these currents are poorly understood, principally due to the technical challenges posed by obtaining accurate velocity and density measurements, which are critical for describing flow behavior and depositional characteristics. Numerous studies have bypassed these issues by using saline density currents, but this negates the ability to link flow processes with depositional features, and it is unclear whether their density structures are representative of particle gravity flows. Consequently, numerous questions remain over the flow conditions that build up a significant part of the deep-marine geologic record. In this thesis I reports on the flow processes and depositional features of sediment-gravity currents across a broad range of particle sizes and concentrations. The technical challenges of obtaining reliable density data are overcome by running the experimental flows through a medical grade computed tomography (CT) scanner, and pair this data with three-dimensional velocity measurements using an ultrasonic Doppler velocity profiler (UDVP-3D) to get one of the first glimpses of the internal structure of turbidity currents. Unlike previous studies where flow processes are described in terms of the velocity field, this thesis demonstrates that fluid-particle interactions are controlled by momentum characteristics, and that the velocity field is determined largely by the current’s density structure. Moreover, the density structure also exerts a first order control on the morphology of their deposits.

京都大学 / 新制・課程博士 / 博士(工学) / 甲第23862号 / 工博第4949号 / 新制||工||1773(附属図書館) / 京都大学大学院工学研究科都市社会工学専攻 / (主査)教授 角 哲也, 准教授 竹門 康弘, 准教授 Kantoush Sameh / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

The evolution of landscapes and seascapes in time is the result of the constant interaction between flows and topography. Flows change topography, which in turn change the flow. This feedback causes evolution processes to be highly non-linear and complex. When full analytical derivations of the co-evolution of topography and flow are not possible without oversimplifications, as is the case in river bends, recent large topographical datasets and modern computers allow for correlations between horizontal (planview) and cross-sectional geometry of channels. Numerical analysis in the Mississippi and Trinity rivers indicate that the type of correlation between river radius of curvature and bankfull channel width depends on the migration behavior of the river. In other cases, channel topography may only have a second-order effect on its own evolution, as is the case for fully depositional turbidity currents, and the evolution of aeolian field topography may only be a function of this topography. I show that in these situations, changes in topography may be decoupled from details of the flow field and modeled very easily with a good accuracy. / text

Ancient basin-floor strata are exceptionally well exposed in the Neoproterozoic Windermere Supergroup in the southern Canadian Cordillera. Data from the Castle Creek outcrop, where strata of the upper Kaza Group crop out, and the Mt. Quanstrom outcrop, where the middle Kaza is exposed, form the main dataset for this study. The aim of this study is to describe and interpret the strata starting at the bed scale, followed by stratal element scale, lobe scale and ultimately fan scale. Strata of the Kaza Group comprise six sedimentary facies representing deposition from a variety of fluid and cohesive sediment gravity flows. These, in turn, populate seven stratal elements that are defined by their basal contact, cross-sectional geometry and internal facies distribution. The lithological characteristics of stratal elements vary little from proximal to more distal settings, but their relative abundance and stacking pattern do, which, then, forms the basis for modeling the internal architecture of lobes. Lobes typically comprise an assemblage of stratal elements, which then are systematically and predictably arranged in both space (along a single depositional transect) and time (stratigraphically upward). Lobes typically became initiated by channel avulsion. In the proximal part of the system scours up to several meters deep, several tens of meters wide are interpreted to have formed by erosion downflow of the avulsion node. Erosion also charged the flow with fine-grained sediment and on the lateral margins and downflow avulsion splays were deposited. Later flows then exploited the basin-floor topography and on the proximal basin-floor carved a feeder channel, which then fed a downflow depositional lobe. At the mouths of feeder channels flows became dispersed through a network of distributary channels that further downflow shallow and widen until eventually merging laterally in sandstone-rich terminal splays. During the lifespan of a single lobe the feeder channel remains fixed, but the distributary channel network and its associated terminal splays wander, causing them to stack and be intercalated laterally and vertically. Eventually an upstream avulsion terminates local sediment supply, causing a new lobe to be initiated elsewhere on the fan, and the process repeats.

The Fraser delta hosts a population of over 500,000 including the municipalities of Richmond and Delta and the Vancouver International Airport. The main arm of the Fraser River has been fixed in place by construction of a jetty focusing sediment deposition on the Sand Heads area. There is a history of submarine slide events at the delta crest which pose substantial risk to coastal infrastructure near the delta front. A submarine channel, characterized by prominent levee deposits, extends seaward from the Sand Heads area. In this study, sand beds in cores from levee overspill deposits are dated using excess 210Pb activity. They are interpreted as the downstream deposits of channelized turbidity currents generated by liquefied slide material. Sedimentation is characterized by sandy mud, interpreted to be deposited continuously by river plume suspension fall-out, and two distinct kinds of sand beds which represent two genetically different processes. The first type of sand bed (Facies 6) is thick, sharp based and clean, often showing classic Bouma turbidite elements including a massive sand base with laminated sands fining up to a mud top and is interpreted as the deposit from slides involving large volumes of material at the upper reaches of the tributary channels. The second type of sand bed (Facies 5) is characterized by muddy sand, has gradational contacts, and is interpreted as a low density deposit from either river generated turbidity currents or distal turbidites from smaller slide events. Facies 6 sand beds often occur as sets of 2 to 4 beds and individual bed sets have been dated to approximately the same ages of known large-scale slide events. Facies 5 sand beds occur more frequently and generally occur after periods with high flow. Sediment cores show three distinct phases of levee growth within the past 100 years approximately. A basal phase consists of very thick beds of medium sand that are interpreted to represent the early stage of channel-levee evolution when continuous overspill occurs during turbidity current events. The second stage is characterized by thick sets of frequent Facies 6 fine grained sand beds separated by less than one year of mud deposition. These sand beds are interpreted as representing a period of levee growth where channel relief is low and overspill events occur often. The third phase is characterized by thick mud intervals with less frequent fine sand beds. Phase 3 is interpreted to reflect a state when levee growth has increased channel relief to a height greater than that of the typical channelized turbidity current. In the third phase, sediment bypass is common and only larger density flows are capable of spilling onto the levees. Deposits interpreted to represent large slides have a return interval of 10 to 15 years during the past 40 years. Deposits of smaller events occur on average every four to five years. Event ages are compared to large spring floods from the Fraser River and seismic activity to determine any causal relationship. There is some relationship between ages of event beds and river flood years, but the largest sand beds do not correspond to unusually large flood years or seismic activity. It is concluded that there are likely a combination of factors which contribute to slope failure including over steepening and increased pore pressure. / Graduate / 0372 / cooper.stacey@nrcan.gc.ca

Ancient basin-floor strata are exceptionally well exposed in the Neoproterozoic Windermere Supergroup in the southern Canadian Cordillera. Data from the Castle Creek outcrop, where strata of the upper Kaza Group crop out, and the Mt. Quanstrom outcrop, where the middle Kaza is exposed, form the main dataset for this study. The aim of this study is to describe and interpret the strata starting at the bed scale, followed by stratal element scale, lobe scale and ultimately fan scale. Strata of the Kaza Group comprise six sedimentary facies representing deposition from a variety of fluid and cohesive sediment gravity flows. These, in turn, populate seven stratal elements that are defined by their basal contact, cross-sectional geometry and internal facies distribution. The lithological characteristics of stratal elements vary little from proximal to more distal settings, but their relative abundance and stacking pattern do, which, then, forms the basis for modeling the internal architecture of lobes. Lobes typically comprise an assemblage of stratal elements, which then are systematically and predictably arranged in both space (along a single depositional transect) and time (stratigraphically upward). Lobes typically became initiated by channel avulsion. In the proximal part of the system scours up to several meters deep, several tens of meters wide are interpreted to have formed by erosion downflow of the avulsion node. Erosion also charged the flow with fine-grained sediment and on the lateral margins and downflow avulsion splays were deposited. Later flows then exploited the basin-floor topography and on the proximal basin-floor carved a feeder channel, which then fed a downflow depositional lobe. At the mouths of feeder channels flows became dispersed through a network of distributary channels that further downflow shallow and widen until eventually merging laterally in sandstone-rich terminal splays. During the lifespan of a single lobe the feeder channel remains fixed, but the distributary channel network and its associated terminal splays wander, causing them to stack and be intercalated laterally and vertically. Eventually an upstream avulsion terminates local sediment supply, causing a new lobe to be initiated elsewhere on the fan, and the process repeats.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第22032号 / 理博第4536号 / 新制||理||1651(附属図書館) / 京都大学大学院理学研究科地球惑星科学専攻 / (主査)准教授 成瀬 元, 教授 生形 貴男, 准教授 堤 昭人 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

Correntes de densidade são fluxos gravitacionais gerados pela diferença de densidade entre dois fluidos. Correntes de turbidez fazem parte de uma sub-classificação das correntes de densidade, na qual o fluido mais denso tem, na sua composição, partículas em suspensão. Muitos trabalhos numéricos já estudaram a dinâmica das correntes de turbidez, mas, nenhum dos encontrados aplicou mudanças de relevo concomitantes com a simulação, causadas pela sedimentação das próprias partículas da corrente e nem alterou o relevo após a passagem de cada evento em um domínio tridimensional. O presente trabalho pretende analisar a alteração no relevo de fundo causada por uma corrente de turbidez. No código Incompact3d, as equações de Navier-Stokes, Continuidade e Transporte e Difusão são resolvidas em uma malha cartesiana tridimensional. A condição inicial adotada é a de Lock-Exchange. As simulações realizadas utilizaram Simulação Numérica Direta (DNS). O código utiliza um esquema compacto centrado de sexta ordem, em diferenças finitas, para o esquema espacial, e Adams-Bashfort de terceira ordem para o esquema temporal. A validação do código foi realizada comparando-se com trabalhos experimental e numéricos. A análise das diferentes proporções granulométricas mostrou que quanto maior é a quantidade de material grosso na condição inicial, maior será seu depósito para um dado tempo. Em consequência, mais relevante se torna a consideração da alteração do relevo de fundo. Além disso, quanto maior o fator de compactação do sedimento, maior será o erro de não considerar a atualização de fundo. Os resultados também apontaram que os erros médios ao não considerar a atualização do fundo são da ordem de 4% da massa de depósito em 20 tempos adimensionais, para os parâmetros utilizados. Ao se propagar uma corrente de turbidez sobre o depósito de outra, os erros se mostram menores. / Gravity currents are gravitational fluxes triggered by density di erence between two fluids. A sub-classification of those are turbidity currents, in which the denser fluid is composed by the lighter fluid plus suspended particles. Many papers had shown turbidity currents dynamics, although none of the papers found had applied changes in the simulated topography due to deposit during the own simulation, neither they had altered a 3D domain topography after each flux, applying the changes caused by the previous current. The present dissertation aims to analyse the turbidity current dynamics alteration caused by the influence of its own deposit, altering the topography during the very simulation. The analysis is conducted in a polidispersed turbidity current. The Incompact3d code solves Navier-Stokes, continuity and transport-di usion equation, in a tridimensional cartesian mesh. Lock-exchange was chosen to be the initial condition. Direct Numerical Simulations (DNS) are performed. Sixth order compact finite-di erence schemes are used on the spatial domain, while third order Adams-Bashfort is applied for the temporal evaluation. Comparisons with numerical and experimental papers were performed for code verification. Results showed the coarser the particles on the starting lock-exchange, the higher its deposit is, and the more the terrain will be altered. Nevertheless, the bigger the compacting factor, the bigger the error of not considering bathymetry alteration. Results also point that the average errors of not considering the update are in order of 4% on the mass deposit, after 20 dimensionless times, for the used parameters. When a current propagates over the deposit of a previous one, these errors are smaller.

Submarine slope failure releases sediments; it is an important mechanism that changes the Earth surface morphology and builds sedimentary records. I study the mechanics of submarine slope failure in sediment that dilates under shear (dilative slope failure). Dilation drops pore pressure and increases the strength of the deposit during slope failure. Dilation should be common in the clean sand and silty sand deposits on the continental shelf, making it an important mechanism in transferring sand and silt into deep sea. Flume experiments show there are two types of dilative slope failure: pure breaching and dual-mode slope failure. Pure breaching is a style of retrogressive subaqueous slope failure characterized by a relatively slow (mm/s) and steady retreat of a near vertical failure front. The retreating rate, or the erosion rate, of breaching is proportional to the coefficient of consolidation of the deposit due to an equilibrium between pore pressure drop from erosion and pore pressure dissipation. The equilibrium creates a steady state pore pressure that is less than hydrostatic and is able to keep the deposit stable during pure breaching. Dual-mode slope failure is a combination of breaching and episodic sliding; during sliding a triangular wedge of sediment falls and causes the failure front to step back at a speed much faster than that from the breaching period. The pore pressure fluctuates periodically in dual-mode slope failure. Pore pressure rises during breaching period, weakens the deposit and leads to sliding when the deposit is unstable. Sliding drops the pore pressure, stabilizes the deposit and resumes breaching. The frequency of sliding is proportional to the coefficient of consolidation of the deposit because dissipation of pore pressure causes sliding. Numerical model results show that more dilation or higher friction angle in the deposit leads to pure breaching while less dilation or lower friction angle leads to dual-mode slope failure. As a consequence, pure breaching is limited to thinner deposits and deposits have higher relative density. / text

The submarine geomorphology of Goletas Channel - Hardy Bay - Shusharti Bay is a record of environmental change, defined by sediment deposition since the late Pleistocene draped over glacially sculpted physiography. Sea level change, contiguous with waning ice extent at the termination of the Fraser Glaciation, triggered an oceanographic transition within Goletas Channel from a low energy closed embayment to a higher energy open channel environment. Morphologic evidence of lower sea level position is observed from sequence stratigraphy in Hardy Bay and suggests regression to 74 m below present. Stratigraphy also shows a correlation between sea level transgression and turbidity current flows in northwest Goletas Channel, and although triggering mechanisms remain elusive, they are likely related to reworking of glacial sediments concomitant to initial open channel conditions. Holocene sediment accumulation has been highest in southeast Goletas Channel, represented by mud with interstitial gas, and has been reworked by tidal currents into contourite structures. A combination of high-resolution multibeam bathymetry, seismic and core samples are used to study the geomorphology of the region.