H33C-1014
Siliciclastic Progradation Within a Neogene Carbonate Passive Margin - Northern Carnarvon Basin of the Northwest Shelf of Australia
Interpretations of extensive industry subsurface data (3D and 2D seismic data, wireline logs and completion reports) and results from previous studies suggest that relative sea level changes and ocean currents controlled observed variations in carbonate vs. siliciclastic sediment type and stratal architecture in the Northern Carnarvon Basin, Northwest Shelf of Australia, during the Neogene. The basin has been a site of predominantly carbonate sedimentation since the late Paleogene. However, significant (at least 30-70 km) siliciclastic progradation on top of this carbonate shelf started in the late middle Miocene. Carbonate- dominated sedimentation resumed in the late Miocene/Pliocene. Siliciclastic-rich seismic packages show dip- oriented progradation northwestward, as well as strike-oriented progradation northeastward. Clinoform heights, up to approx. 140 m, and a dip-elongated lobe morphology suggest deposition by a long-lived (approx. 6 My) deltaic system which is no longer active on this margin. Consistent northeastward switching of these interpreted delta lobes and their progradation over the preexisting carbonate shelf sediments suggest wave-dominated conditions, with strong northeastward long-shore currents, for this delta system during a time of relative base level fall. During this siliciclastic progradation event, carbonate sedimentation continued in the updrift direction southwest of the delta, but carbonates were not accumulated downdrift or northeast, where progressively younger deltaic siliciclastics filled available accommodation space. By the beginning of the Pliocene, siliciclastic sedimentation had retreated towards the Australian coast, allowing widespread carbonate production to resume; interpreted shallow-water carbonate platforms then developed over subtle topographic highs created by the underlying deltaic lobes.
H33C-1015
Changing our View of Aggradational Fluvial Systems – The Distributary Fluvial System (DFS) Paradigm
Many numerical methods exist for modeling fluvial stratigraphy, however all require development of sound conceptual models of processes and facies distributions that are formed in aggradational settings before they can reasonably capture and represent facies geometries and distributions. A review of approximately 700 modern continental sedimentary basins around the world showed that rivers in these basins are not tributary in nature; rather they form either distributary fluvial systems (DFS), commonly called megafans, fluvial fans, and alluvial fans in the literature, or axial stream systems that parallel the basin trend, with the vast majority of sedimentation in the basin occurring on the DFS (up to 95 percent). Thus, most continental sedimentary basins undergoing aggradation do not contain tributary fluvial systems. Most fluvial facies models that are currently used to develop models of fluvial stratigraphy, however, were developed through studies of degradational, tributary fluvial systems! Rivers on DFS differ from tributary rivers in many, potentially significant ways, including (1) a radial pattern of channels away from an apex (or intersection point), though many of the DFS rivers do curve to become sub-parallel to the basin strike distally; (2) channel systems commonly decrease in width and discharge (and thus cross-sectional area) distally, while tributary systems tend to increase in size downstream; (3) meanderbelts on DFS tend to have few chute and neck cutoff avulsions unless they are somehow confined (though exceptions to this trend exist), creating a different distribution of facies stacking patterns in resulting sandstone bodies; (4) floodplain deposits on DFS may be dominated by avulsion successions; (5) floodplain deposits may be more readily preserved in braided systems on DFS than in tributary systems; and (6) axial stream character may contrast with DFS stream character (these tend to be more similar to the tributary models of rivers). In order to reasonably model these aggradational fluvial systems, new facies models that consider fluvial position and DFS form in the sedimentary basin are needed.
H33C-1016
Limitations of Fluvial Analogs for the Dynamical Interpretation of Submarine Channel Systems: a Physical Modeling Case Study of Leveed Channel Formation.
Inferring process from morphology and architectures is commonly used in the interpretation of the dynamics of depositional submarine systems based on seismic and outcrop observations. The linking form to process often requires that an observed morphology is produced by a unique set of dynamics. Accordingly, submarine channel dynamics are often inferred from observed channel form through analogy with subaerial fluvial systems. For example, prograding deltaic channels bear striking morphological resemblance to leveed submarine channels that traverse continental slopes and submarine basins. However, given the different physical circumstances of these two environments, it is unclear how analogous are the relationships between channel form and flow dynamics in these two systems. To test whether the jet morphodynamics responsible for deltaic channel formation were applicable to submarine settings, we conducted a series of physical experiments in which sediment-laden flows that were considerably denser than the surrounding fluids were fully submerged in the less dense fluid. Given these dynamical conditions, we were unable to find a set of experimental conditions under which a pair of bounding levees could develop from a non-eroding sediment- laden density current undergoing sudden unconfinement. At supercritical bulk Richardson numbers, high rates of entrainment of the surrounding fluid into the sediment-laden flow led to rapid current deceleration and deposition of sediment along the axis of the flow. As bulk Richardson numbers increased, fluid entrainment decreased, allowing the current to flow further into the basin. Simultaneously, however, the rate at which the current collapsed laterally increased and as a result depositional patterns transitioned from one in which sediment was deposited parallel to the margins of the flow outlet to one in which sediment was radially distributed. As the denser flows collapsed, sediment was more broadly distributed, and consequently, relief of the deposited material was lower than observed for deposits produced by low Richardson number flows. Depositional patterns produced by dense flows differ significantly from those produced by the jet-like processes that govern deposition by deltaic channels, highlighting the fact that while such forms within subaerial and submarine environments may be similar, the underlying dynamics may be quite different. Our inability to produce self-organizing channel-levee systems for dense, depositional flows leads us to speculate that construction of leveed channels in the submarine environment may first require bed incision to provide partial confinement of the current.
H33C-1017
Predicting avulsions in river-dominated deltas
The delta lobe is a fundamental sedimentary deposit in river-dominated deltas. Delta lobes are created through a continuous cycle of lobe extension, avulsion, and abandonment, and the mechanics and timing of this cycle are poorly understood. Here we use physical experiments to quantitatively define one type of channel avulsion cycle for river-dominated deltas. The experiments simulate a river carrying a cohesive sediment mixture entering a still body of water under constant inlet and boundary conditions. We collected data on the dynamic evolution of eight separate distributary channel and river mouth bar complexes from inception to avulsion. For each complex, we recorded millimeter-scale changes in the bed and water surface topography and the cross-sectional-average channelized flow velocity at fifteen minute intervals. The cycle begins when a distributary channel extends basinward until its river mouth bar reaches a critical size and stops prograding. Then the stagnated mouth bar triggers a wave of bed aggradation moving upstream that increases cross-levee flows and bed shear stress. An avulsion occurs as a time-dependent failure of the levee at the location where the cross-levee impulse is highest (R-squared = 0.93). Cross-levee impulse is defined as the average bed shear stress multiplied by the duration of its application. The conceptualization of river delta avulsions as an impulse failure, rather than a threshold, should lead to improved stratigraphic models that can more accurately predict the sedimentology of avulsion-dominated systems.
H33C-1018
Channel Dynamics on Experimental Alluvial Fans with a Bimodal Grain Size Distribution
Alluvial fans in arid regions often display two striking patterns: a strongly bimodal grain size distribution of boulders and sand, and a network of well-defined distributary channels even in the absence of cohesive sediment. The spatial distribution of grain sizes is qualitatively linked with the channel network on the fan, with the boulders tending to line the bed and banks of channels. We hypothesize that spatial grain size sorting of a bimodal sediment supply gives rise to persistent channelization under these noncohesive conditions. Using a scaled-down bimodal grain size distribution in the lab, we build alluvial fans under sediment and water input conditions that typically produce sheet flow and ephemeral channels when a single grain size is used. The first order pattern is gradual downstream fining on the upper fan, and the formation of an abrupt boulder/sand transition and its corresponding distinct slope break in the middle fan. Superimposed on this trend is the formation and relocation of well-defined channels which form distributary networks similar to those observed in nature, and which typically require cohesive sediment mixtures to simulate experimentally. We observed the following sequence of channel creation and abandonment: (1) Spontaneous channel formation focuses flow, leading to (2) localized, rapid shoreline progradation at the channel mouth, lowering the fluvial slope and causing (3) aggradation and rapid channel backfilling, leading to (4) abandonment of the filled channel and flooding of the fan surface. Flooding activates numerous small channels and (5) eventually one of the flowpaths – typically having a substantially steeper slope than the previous channel – begins to downcut and focus the flow, leading to formation of a new dominant channel and corresponding lobe at the front of the fan. We varied the relative fraction of our two grain sizes, and also their relative mobility, in order to explore controls on fan formation and channel dynamics. We present results characterizing the co-evolution of surface grain size distribution, fan topography and channel network dynamics through time. Experiments demonstrate a characteristic timescale for avulsion and lobe switching, which we quantitatively link to sediment supply.
H33C-1019
The depositional records of two coastal lakes in south-central Chile (Lago Lanalhue and Lago Lleu Lleu, 38°S): Active forearc tectonics and climate variability
On millennial time scales, the southern Chilean active margin is not only characterized by active tectonics and subduction-related coastal deformation, but also influenced by pronounced variations in the prevailing climate conditions. Here we focus on the depositional records of two coastal lakes in the southern part of the Arauco Peninsula (38°S, Lago Lanalhue and Lago Lleu Lleu), an area very sensitive to changes in both climate and tectonics. For the present study, we used a multi-proxy approach including seismic reflection surveys, sedimentological, mineralogical, and geochemical analyses, supported by radiocarbon dating. Seismic reflection analyses reveal that Lago Lanalhue and Lago Lleu Lleu developed within former river valleys that once drained into the Pacific Ocean. During the early Holocene, the ancient rivers were dammed by rising sills due to inverse faulting and tectonic uplift, turning first into marginal-marine lagoonal systems and subsequently evolving into lakes. On the basis of sedimentological analyses and radiocarbon dating, the different stages of the lakes development have been reconstructed in consideration of the regional tectonic and climatic history. The comparison of the transitions between different stratigraphic units with contemporaneous variations in the global sea level, allowed the calculation of Holocene uplift rates. These are about twenty times higher for the upraised sills than for the lakes themselves. Therefor, we interpret the sills to be the surface expression of a blind thrust associated with a prominent inverse fault (Morguilla Fault) controlling uplift and folding of the Arauco Peninsula. Geochemical data from the lacustrine part of the sedimentary sequences reveal a continuous record of the middle to late Holocene regional climate history. The results indicate more arid conditions during the middle Holocene and more humid conditions during the late Holocene. An additional increase in climate variability is recorded during the last 2000 years. Our study demonstrates the use of lake sediments to decipher time-integrated landscape evolution by quantifying tectonic deformation and climate variability.
H33C-1020
Experiments on Subaqueous Fans Emplaced by Turbidity Currents
The passage of turbidity currents over submarine fans can locally become self-channelized. Self- channelized flows are typically sinuous and bounded by levees. The process of self-channelization remains, however, somewhat obscure. We have performed experimental work to investigate self-channelization of subaqueous fans and lobes at laboratory scale. The resulting weakly sinuous channels can be depositional, erosional, or some combination of the two. The channels elongate to the length of the reach available for their formation; in case of flume experiments, the reach is the size of the flume itself. They show both gradual shift and avulsion. Two necessary conditions for the formation of intricate channelization, in the laboratory experiments are: a) a multiplicity of grain sizes and b) turbidity currents that are insufficiently large to cover the entire area of the fan at any given time. We conducted a series of experiments to investigate the effects of changes in slope gradient and inflow concentration on the formation and evolution of channels and associated lobes. Change in slope is the main parameter to drive deposition of lobes. Different conditions were tested showing different lobe shapes and evolutional patterns. In order to validate the experiments, data are scaled to the field and compared with outcrop and seismic observations.
H33C-1021
Sedimentation and transportation of mud rock material
This research focuses on the transportation and sedimentation of mud rock slope at Niupu, southern Taiwan. The main purpose of this research is trying to identify the mechanism of erosion and sedimentation rate of the mud rock from the slope into the lake. By using regular sonar surveying and rainfall data, it is possible to identify a high erosion rate on mud rock area. The cementation of Miocene mud rock is loose and slope at 40-45 degree which is highly sensitive to the rainfall. It is also vulnerable to the erosion processes. There are some interesting characters related to the observation and surveying. The erosion processes are highly related to amounts of rainfall. When there is typhoon, tropical storm, there is high erosion. The erosion could occur when there is rainfall more than 10 mm/event and the surface of the mud rock could be incised up to 3 cm after 30 mm rainfall. The sedimentation rate could also as high as 0.5 m when there are two typhoons which attacked to this area within three months. This research also demonstrates that the high erosion on mud rock area will generate high sedimentation and high denudation rate. However the comparison of the rate of erosion and deposition is identified in this presentation. For hazard mitigation purpose, it is necessary to reduce the bared mud rock slope area to prevent the further erosion.
H33C-1022
Linking Surface Morphological Change to Subsurface Fluvial Architecture: What Imprints do big Floods Leave?
Ideas concerning the origin of alluvial deposits and their paleoenvironmental interpretation have usually resulted in two schools of thought: that such deposits are either the result of ordinary 'day-to-day' processes that acted uniformly through time, or that they are related to rare events that had a disproportionate effect on erosion and deposition rates. Despite the long running debate of gradualism and catastrophism within the Earth Sciences, there is surprisingly little quantitative data to assess what magnitude of event is represented in many fluvial sequences. This paper reports results of a unique natural 'experiment' where surface (digital elevation models obtained from digital photogrammetry) and subsurface (ground penetrating radar, GPR) data were taken immediately prior to, and after, a large (1 in 40 year) flood event that occurred in 2005 on the sand-bed, braided South Saskatchewan River, Canada. We surveyed several reaches of the river both before and after this major flood event, and collected repeat aerial surveys of the entire channel, as well as GPR surveys along identical survey lines. This allows us to examine the morphological change in the channel form during this flood, quantify the probability distributions of bed heights within the channels, and assess the amount of erosion and/or deposition represented within the subsurface architecture. Results indicate that although this high-magnitude flood had a marked geomorphic impact, the style and scale of both scour and deposition were the same as that measured during lower-magnitude, annual, floods. Hence, rather than being a reflection of either frequent or rare events, alluvial deposits in the South Saskatchewan contain the record of both but these different scale events may be virtually indistinguishable in the subsurface alluvial architecture.
H33C-1023
Role of channel-floodplain interaction in the evolution of delta distributary networks: Theory and experiment
We analyze how floodplain roughness, in the form of relict channels, affects the dynamics of delta distributary networks. Our analytical model employs coupled scaling relations for delta progradation via repeated avulsion and lobe accretion. The model predicts the evolution of channel and floodplain sedimentation rates, avulsion frequency, timescale for floodplain annealing, and number of relict channels. We assume active channels can avulse to either relict channels or to interdistributary topographic minima; only the latter increases surface roughness. Floodplain sedimentation buries relict channels and anneals the delta surface. Our model predicts a rich interplay between roughness and channel dynamics: From an initially smooth delta surface, the number of relict channels increases with time as successive avulsions roughen the delta. An increase in the number of relict channels increases the likelihood of avulsion into a relict channel, which reduces the roughening rate. This negative feedback is offset by growth of the delta surface, which causes the floodplain sedimentation rate to decrease faster than the channel sedimentation rate. A reduction in floodplain sedimentation rate increases the annealing time and avulsion frequency, thereby preserving relict channels and generating new roughness elements, respectively. We use laboratory scale deltas to test model predictions. The experimental deltas display self-formed distributary networks with relatively stable channels that avulse periodically. Experimental data support some of our predictions for floodplain-channel interaction: Roughening of the delta by channel avulsion is rapid in the initial stages of the experiments; as the floodplain becomes populated with abandoned channels, avulsions are more likely to reoccupy relict channels. A steady state is achieved—in terms of the number of distributary channels—when the timescale of channel annealing is comparable to the timescale of avulsion. Experiments support model predictions that relict channels play an important role in determining avulsion locations.
H33C-1024 INVITED
Neither fair nor balanced: distortion and bias in subsurface records of surface dynamics
The physical stratigraphic record is full of surfaces that resemble geomorphic surfaces – incised valleys, channels, and lobes, for example. Based on experiments that allow us to study the entire process from surface morphodynamics to preserved stratigraphy, we quantify the extent of this resemblance and find that it is generally deceptive. Distortion and bias in the transfer from surface to subsurface can be measured in at least two ways. The first is sampling bias: the well known tendency of the stratigraphic record to favor the most negative (lowest) parts of the topography. The strength of this effect depends on the ratio of net rate of deposition to topographic variance. The second effect we measure is distortion bias: preserved surfaces that are clearly identified with specific geomorphic features, such as incised valleys, but are systematically distorted representations of them. For example, preserved valleys are typically wider and have lower side slopes than any of the topographic valleys that produced them. Two general principles that emerge are (1) coherent surface migration increases distortion bias; and (2) the extent of stratigraphic distortion of a given preserved surface is closely related to the degree of time transgression of the surface.
H33C-1025
Evolution Dynamics of Relief and Sediment Structures During the Initial Phase of Ecosystem Evolution Within an Experimentally Constructed Catchment Landscape and Their Influence on the Shallow Subsurface
The studies are integrated in a project of the Collaborative Research Center of the German Research Foundation (DFG SFB-TRR 38) 'Structures and processes of the initial ecosystem development phase in an artificial water catchment'. The experimental catchment 'Hühnerwasser' is built up by mechanically deposited sediments reconstructing an initially emerged landscape. In order to monitor its geomorphological as well as ecofunctional evolution with time it is essential to incorporate comprehensive geophysical applications dealing with geomorphology and sedimentology of both surface as well as shallow subsurface. Digital terrain modelling on meso- to microscale is employed to reflect and analyze superficial processes and evolution trends as well as their influence on the abiotic and biotic structures and processes of the succeeding ecosystem. Especially in the case of the portrayal and analysis of subsurface characteristics, strategy needs to adapt to the special requirements of the SFB-project as the catchment system should only underlie 'natural' conditions and therefore not be altered by any implementations in the course of field investigation. Consequently, intrusive methods such as drillings or profiles are to be omitted. Methods of geophysical prospection hence constitute an indispensable tool in investigation of subsurface composition. Ground penetrating radar (GPR) and electric resistivity tomography (ERT) are applied with adequate resolution to detect the primary deposition structures within the technogenically bedded sediment. By means of this evaluation subsurface structures can be interpreted with regards to their relevance in controlling soil water movement, slope hydrology, redox interactions, root distribution etc. Furthermore, alterations of these primary structures resulting from bio- and pedoturbation processes can be monitored.
H33C-1026
Event-Based Modeling of Deep-Water Slope Channel Systems: Calibrating Experimental Results With Observed Stratigraphic Architecture
Event-based forward modeling is a rules-based approach used to construct a self-consistent 3D computer model that incorporates both erosion and deposition of a succession of stratigraphic events. This approach has been applied to model deepwater channel and lobe systems. In event-based modeling, the sedimentological processes and allogenic forcing inherently linked to the depositional record are incorporated as a set of empirical and predictive rules that control the final architecture. Rules are based on observations of numerous outcrops, high-resolution 3D seismic-reflection data, seafloor (bathymetric) data, and boreholes. The incorporation of simple rules relating autogenic processes (e.g. channel avulsion) with allogenic processes (e.g. waxing and waning flow cycles) has resulted in an efficient numerical laboratory for constructing high-resolution architectural models and exploring inputs and model response. While the implications of individual rules often can be anticipated, the interaction of multiple rules may be surprising, which coupled with very fast run times, makes event-based modeling a valuable tool. Examples of event- based models in a continental margin slope valley setting highlight the partitioning of coarse- and fine- grained sediment into channels and overbank areas, respectively. The degree and variability of channel-fill and overbank aggradation is a primary control on the resultant channel stacking pattern. Model results are analyzed and compared to stacking patterns observed and documented in natural deep-water channel systems. The objective is to test the limits of event-based modeling as an experimental platform for improving our understanding of fundamental constraints and for the construction of high-resolution stratigraphic models of natural depositional systems.
H33C-1027
Equilibrium Profile of Submarine Canyons: Theory and Inverse Analysis
This study aims to reveal theoretical profile of equilibrium geomorphology of submarine canyons. Submarine canyons distribute on submarine slopes in any regions of the world, and are conduits of turbidity currents to provide sandy sediments towards submarine fans. Thus, developmental processes of submarine canyons are closely related to that of submarine fans. Here, the equilibrium geomorphology is defined as a landmorph where rate of sedimentation or erosion is equal to zero in any locations. In other words, equilibrium profile enables sediment-laden flows (turbidity currents in this case) to by-pass completely. Although it seems impossible to attain perfect equilibrium profiles in natural environments, consideration of theoretical equilibrium profile is significant for modeling of geomorphological and stratigraphic developmens. For example, Kneller (2003) suggested that the equilibrium profile of slope (canyon) can be used as a criterion to predict regions of erosion and deposition in submarine depositioal systems. For such purposes, this study provides the first method to calculate profile of the equilibrium submarine canyons. First, we begin with the 3-equation model of turbidity current (Kostic and Parker, 2006). For the case of an equilibrium (bypass) profile, the volume transport rate per unit width of suspended sediment qs must everywhere be a) constant, b) equal to a specified upstream value qsu, and c) equal to the capacity value qse. Thus, we can define three equations in three unknowns U, H and S, which are layer-averaged values of velocity, flow height and slope inclination respectively. Those variables are the solution to which defines an equilibrium (bypass) profile. We obtained the first-order ODE that can be solved downstream from some specified upstream value Uu, along with other specified parameters (e.g. qsu). Once U is known everywhere, the elevation profile of the equilibrium submarine canyon is obtained from calculated hydraulic variables. Finally, we applied our model to conduct inverse analysis of hydraulic conditions in the submarine canyon (the Kushiro Submarine Canyon; Noda et al., 2008). Turbidity currents generally bypass submarine canyons, so that profiles of most canyons can be regarded as the semi-equilibrium state. Therefore, we calculated equilibrium profiles using any possible combinations of parameters, and compared them to the real geomorphologies. As a result, we obtained parameter sets of turbidity currents that explain geomorphology of the Kushiro Submarine Canyon best. It is estimated from inverse analysis that the average velocity of turbidity currents is 1.3 m/s, bulk-richardson number is 0.279, and grain size is about 2 phi. Our inverse analysis method is, although it is based on crude approximation, seems to be useful for rough characterization of the submarine depositional systems.
H33C-1028 INVITED
Role of Growth Faulting in the Quaternary Development of Mississippi-River Delta
We use an industry grade seismic volume and observations of present-day surface topography to resolve the influence of growth faulting on evolution of Mississippi delta in southeastern Louisiana from the Pleistocene to Recent. The volume of seismic data covers an area roughly 1400 square kilometers in size and it resolves many normal faults with displacements that can be tied to movement of Jurassic Louann Salt in the subsurface. We have defined the Quaternary activity associated with 6 of these normal faults by measuring the progressive offset of strata deposited on the delta surface over time. These measurements of fault displacement were restricted to the sedimentary section positioned 150 to 1500 m beneath the delta surface. Total vertical offsets measured within this Quaternary section range from 60 to 150 m. These fault displacements represent abrupt spatial variations in subsidence rate that are between 4 and 8 percent of the regional, long-term deposition rate. Our best estimates for the Quaternary rates of fault displacement vary between 0.1 and 1 mm/yr. Five faults can be connected to deformation of the modern delta surface. Wetland on the footwall is replaced by open water on the hanging wall of these structures. In spite of this evidence for modern surface deformation, the orientations of buried, seismically resolved channel bodies do not appear to be affected by the positions of active growth faults. We will evaluate the competition between subsidence and sedimentation patterns that leads to this style of channelized stratigraphy.
H33C-1029
Super-High-Resolution Seismic-Reflection-Based Analysis of the Late Pleistocene- Holocene Highstand-Active Newport Channel and Its Distributaries, Southern California Borderland
Super-high-resolution multibeam bathymetry (vertical resolution of 0.15-m and horizontal resolution of 1 m at 50 m survey altitude) and chirp seismic-reflection profiles (3.5 kHz; vertical resolution of 0.11 m) were collected by Monterey Bay Aquarium Research Institute's Autonomous Underwater Vehicle (AUV) in order to examine fine-scale details of the Holocene-active Newport channel and its distributaries offshore southern California. The resolution afforded by these data greatly exceeds industry-standard multichannel seismic- reflection data (typically 10 to 25 m vertical resolution). Recently acquired sparker data (100 to 4000 Hz), older Huntec deep-tow boomer (800 to 1200 Hz) and WesternGeco multichannel seismic-reflection data collected since the 1970's are available to analyze coarser-scale channel geometry and longer-term depositional evolution. As many as 48 radiocarbon ages from 16 piston cores are available in order to establish a chronostratigraphic framework for the latest Pleistocene and Holocene. Data show up to three coalescing, southwestward-trending, low-relief distributary channels at 800 m water depth in the western survey, whose features stand in sharp contrast with gentle, nearly flat topography in the eastern survey. The surveys are separated by the left-lateral Carlsbad Ridge Fault. Chirp profiles show the cut-and-fill organization of distributary channel-filling turbidites, which have an average thickness of ~ 2 m. Distributary channels are depressions < 1 km wide, with relief < 4 m from channel floor to levee top. The aim of this research is to study the stratigraphic architecture of these coalescing distributary channels and characterize their avulsion and depositional history and relationship to the main Newport channel. Additionally, the fine-scale stratigraphic architecture will be correlated with well-documented southern California allogenic influences of sedimentation, including climate, basin tectonics and sea level. This study provides insight into the finer-scale geometries and internal characteristics of deep-water channel scours and fill and the processes and controls on their formation.
H33C-1030
Extending the 1D Mass and Momentum Balance of Turbidity Currents
We have revisited and expanded the layer-averaged equations of fluid and sediment mass conservation, and momentum conservation to determine the impact that clear water entrainment, and sediment deposition and entrainment has on the dynamics of turbidity currents. These layer-averaged equations were originally used to determine conditions under which turbidity currents may self-accelerate as they entrain sediment from their beds and increase the density contrast between clear and sediment laden water: a situation termed ignition. We have modified these equations to account for the momentum loss associated with sediment entrainment and sediment deposition, and have analyzed the role that these newly-included momentum sinks play in governing the dynamics of self-accelerating turbidity currents. In addition, we use updated clear-water entrainment laws to include modern observations of this phenomenon's effect on turbidity current evolution. We performed a phase-space and physical-space analysis of these modified equations to assess the role played by these new momentum sinks in submarine flow dynamics. We found that while ignition values of depth-averaged flow velocity (Ui) and concentration (Ci) for a given flow height (ho) changed relative to the case in which these sinks were neglected, their values changed only by a small amount. The down-stream evolution of steady flows also changed with these new sinks; in many cases the flow had less rapidly decreasing flow height and less rapidly increasing velocity than was the case for which the new sinks were excluded yielding less rapid growth of sediment discharge. Finally, we determined the sensitivity of flow dynamics to the sediment entrainment function, particle size / sediment fall velocity, and fluid viscosity. We found flow dynamics to be quite sensitive to all of these factors, but most sensitive to the sediment entrainment law, highlighting the need to better understand the factors that control the rate at which sediment is entrained from the channel bed.