OS23C-1319 1340h
Sediment Flux and Fate of the Yangtze River Sediments Delivered to the East China Sea
The continental shelf of the East China Sea (ECS), together with the Yellow Sea and Bohai Sea, forms a contiguous shelf of about 0.75\times 10$^{12}$ m$^{2}$ in area. This wide( $>$ 500 km) and shallow ( $<$ 130 m) shelf also receives a large amount of terrigenous materials from two of the largest rivers in the world, the Yangtze River (Changjiang) and the Yellow River (Huanghe). The Yangtze River's annual sediment load has been about 480 million tons historically, combined with the adjacent Yellow River, equals more than 10% of the global total sediment flux to the ocean. Except for the part depositing in the river mouth, most fine sediment of the Yangtze River is believed to transport farther southward and accumulated offshore of Zhejiang and Fujian Provinces, which is commonly referred to as the "Mud belt deposit on the inner shelf of the ECS". However, for a long time, we have had no knowledge of the distribution, thickness, and sedimentary processes of this recent fine-grained deposit. In order to quantitatively define the Yangtze River's sediment transport and fate to the ocean, we conducted two geological and geophysical cruises in the summer of 2003 and 2004, with over 1000 km high-resolution Chirp sonar data, 8 gravity and box cores. The seismic profiles reveal a huge clinoform deposit off the coast of Zhejiang and Fujian Provinces, with a thickest ($\sim$ 40 m) depocenter nearshore between the 20-30 m isobaths and progressively thins offshore (less than 100 km across the shelf) reaching water depths up 60 to 70 meters in the ECS. However, this Yangtze-derived mud wedge has also been found to transport southward at least 800 km from the river mouth, extending all the way into the middle of the Taiwan Strait where it possibly meets the Taiwan rivers-derived hyperpycnal mudflow. The total volume of this mud wedge is estimated to be about 1.2\times 10$^{12}$ m$^{3}$, which is about 1.4\times 10$^{12}$ tons in total. Preliminary analysis of our seismic profiles and core data suggests that the Yangtze derived mud began to accumulate there only after the rapid sea-level rise of MWP-1B at around 11,000 yrs BP, when the sea level rose from -60 m to -40 m, together with the re-intensified Asian summer monsoon. But AMS C-14 dating suggest that major part of this mud has been transported southward along the shore since only about 7000 yrs BP by the newly formed Chinese coastal current after sea level reached its mid-Holocene highstand. This suggests that 2\times 10$^{8}$ tons of sediment could be discharged annually southward from the river to the inner shelf, mainly in the winter season, which equals to nearly 50% of the current annual Yangtze's sediment discharge. The other half has been believed to be trapped in its estuary. The existing Taiwan Warm Current in the middle shelf and Kuroshio Current in the outer-middle shelf are also believed to play critical roles in blocking any Yangtze and Yellow-derived materials from being delivered further into the deep ocean.
OS23C-1320 1340h
Sedimentary Features of the Yangtze Derived Mud Deposits in the Inner Shelf of the East China Sea
The purpose of the present study is to document the nature of the innermost shelf of the East China Sea, with particular interest in the fate of the Yangtze River sediment, which historically has discharged about 500 million tons of sediment annually. High-resolution Chirp Sonar seismic data together with a number of shallow cores reveal a broad mud wedge up to 100 km wide extending 800 km southward along the inner shelf. The mud wedge is thickest (~ 40 m) near shore between the 20m and 30m isobaths and progressively thins off shore reaching water depths up to 60 meters. Sediment accumulation rates were estimated from the vertical distribution of excess 210Pb in sediment cores collected at 8 stations along the inner shelf. Near the mouth of the Yangtze River sediment accumulation rates have historically been about 1 to 5 cm/y, due to winter resuspension and erosion events. The highest sedimentation rates in the mud wedge (~ .7 - 1.0 cm/yr) were found just south of the Yangtze estuary (30°N). Sedimentation rates generally decrease alongshore toward the south and offshore toward the east (~ .5 -.02 cm/yr). This pattern is consistent with the dispersal of Yangtze River-derived sediments by the Chinese coastal current and tidal systems. Grain size analysis of the surface sediments near the mouth of the Yangtze are dominated by sandy silts and silty/clayey sands. There is a general fining of surface sediments (silty clays) alongshore toward the south. Clay mineral assemblages of the mud wedge are composed primarily of illite (generally more than 60%), and subordinately of chlorite, kaolinite and smectite. Smectite concentrations ($<$5%) are a good indicator that the sediments are derived from the Yangtze River.
OS23C-1321 1340h
Simulating the Impact of Sediment Flux of the 2003 Flood Event of the Rhone River on the Golf of Lions, France
Sediment fluxes from flood events (return period $>$ 100yrs) tend to have an important impact on the development of strata, although their occurrence is rare. Unfortunately there are almost no field measurements that give better insight into the sediment flux distributed to, and deposited in, the ocean during such events. To get a better understanding of flood event's impact on the strata we applied two numerical models on a Rhone River flood event. A 350yrs recurrence time flood event of the Rhone River occurred in the first week of December 2003. With local accumulation of 500 mm of precipitation, the Rhone River, with an average discharge of 1,700 m$^{3}$s$^{-1}$, reached a peak discharge of 13,000 m$^{3}$s$^{-1}$ at the Beaucaire measurement station. First we applied the hydrological model {\HydroTrend} to simulate the discharge and sediment flux during the December 2003 storm. {\HydroTrend} simulates a similar hydrograph as field observations. The model predicts that 4.4 Mt of sediment flowed from the Rhone River into the Golf of Lions over the 5 day storm event. Secondly we simulated the sediment deposition in the coastal zone with a two-dimensional momentum-based jet model Plume. The model simulates the mixing and advection of river water (simulated by {\HydroTrend}) in the sea. An advection-diffusion routine uses the currents to determine the aerial extent of deposition from the surficial plume, using flocculation and settling values. The model results for the single storm event show a plume that reaches a maximum width of 5 km, flowing southwards out of the Rhone River outlet, than bending sharply west by currents. 90% of the sediment is deposited within 10 km from the river mouth, and it forms a sediment layer up to 35cm.
http://instaar.colorado.edu/deltaforce/projects/euro_strataform.html
OS23C-1322 1340h
Process Based Long Term Simulation of Shelf Stratigraphy Evolution
The Adriatic Sea has been the location of several extensive data collection programs over the past three years. The EuroStrataform and ComDelta have been supported by the U.S. Office of Naval Research (EuroStrataform) and the European Union respectively and have focused on problems of marine sedimentation and stratigraphy. The National Oceanographic Partnership Program (NOPP) has acquired and collected a wealth of hydrographic and circulation data. In addition, a number of sophisticated numerical models have been applied to physical oceanographic and marine geological investigations. The overall effort has been to develop a very rich data set concentrated within the Adriatic Sea. The existence of these data and model results has allowed us to develop and test models that relate the formation of sedimentary strata on the Italian Adriatic shelf to basic factors such as sediment supply and wind and wave forcing. The model framework follows this development from the supply of sediment to the coastal systems by the Po River and other small rivers draining the eastern slopes of the coastal Appenine Mountains to the ultimate burial of sediment beneath the zone of storm reworking. A major step has been the nesting of a detailed shelf sediment dynamics model (M3D) into the regional ROMS circulation model that has been modified at WHOI to include marine sediment transport processes. The ROMS model has been configured to extend over the entire Adriatic. The M3D model covers a subregion (100 km x 120 km) of the central Italian shelf with a more detailed grid resolution. This model also tracks the time-varying bed composition and the formation of the near-bed shelf stratigraphy. The ROMS model provides the regional circulation patterns for time periods on the order of months. These data are used as boundary conditions in the M3D model. Because the detailed M3D model is directed at representing the long term sedimentation patterns that result in the formation of shelf strata it has been necessary to develop methods to extend the relatively short time series from the ROMS model (months) to representations of processes with much longer durations (decades). Long term wind records have been analyzed to develop statistical characterizations of Scirocco and Bora wind events. The events captured during the measurement period have been compared to these long term statistics and routines have been developed to scale and extend the boundary forcing of the M3D model accordingly. Although these procedures are inherently stochastic, they provide the fundamental outline of how detailed hydrodynamic and sediment-dynamic models can be employed to simulate the development of near-bed shelf stratigraphy over geologically relevant (centuries to millennia) time intervals.
OS23C-1323 1340h
Sediment Resuspension and Transport During Bora in the Western Adriatic Coastal Current
The Western Adriatic Coastal Current (WACC) is an important agent for along-shelf transport of sediment and fresh water in the western Adriatic Sea. The WACC is driven by a combination of buoyancy forcing from the Po River (northern Adriatic) and wind forcing from northeasterly Bora winds. The large seasonal pulse of freshwater (during the winter) from the Po River influences WACC strength; however, preliminary results from current measurements and model runs indicate that the WACC responds quickly and strongly to Bora wind events, with a strengthening of the current moving southward. Along-margin sediment transport to the south is significantly increased as a result of Bora wind events, presumably because of enhanced wave resuspension and WACC velocity. Elevated sediment fluxes have been observed in both the upper water column (i.e., core of the WACC) and bottom boundary layer (BBL) during these events, which suggests that wind-driven currents may be coupled with the near-bottom transport. This study addresses the interaction of the WACC with the BBL and the impact of this interaction on sediment transport in the western Adriatic. Two benthic tripods were deployed from November 2002 to June 2003 on an across-shelf transect near the Chienti River (at 10 and 20-m water depth), in the region where WACC begins to intensify (200 km south of Po River). Continuous measurements of suspended sediment concentration and current velocity were recorded in the upper-water column and BBL to document sediment transport events. A time series of sediment fluxes and shear velocities (from currents only, u$_{*c}$; from waves and currents, u$_{*wc}$) were calculated from these data. Results show that suspended sediment concentrations near the seabed (few cmab) during Bora wind events are strongly correlated with u$_{*wc}$, which supports a previous hypothesis that wave resuspension (rather than direct fluvial input) is responsible for much of the suspended sediment available for transport southward of the Po River. In contrast, suspended sediment concentrations farther away from the bed (50 cmab) are highly correlated with u$_{*c}$, but not with u$_{*wc}$. These results suggest that WACC velocity during Bora events controls the ability of sediment to escape the wave boundary layer and be suspended farther away from the seabed. This implies that turbulence induced by currents, rather than waves, allows sediment to move higher in the water column and become available for transport by fast-moving currents generated by the WACC, thus producing strong southward sediment fluxes observed during Bora events. Specific mechanisms responsible for the vertical structure of suspended sediment and estimates of vertically integrated fluxes during these Bora events are yet to be established because of the difficulty in estimating suspended sediment concentrations throughout the water column from acoustic data; these issues are still under investigation and progress will be assessed.
OS23C-1324 1340h
Sediment Transport on Relatively Low-Energy Coastlines: Implications for Resuspension and Deposition
The western Adriatic coastline borders a relatively low-energy (as defined by wave energy) epicontinental sea. In this environment, sediment resuspension occurs primarily in response to winter storm events, as is the case in higher-energy environments. A major difference between these types of environments is that the zone in which sediment is resuspended due to active wave processes is significantly shallower in low-energy environments than in high-energy environments, resulting in different transport mechanisms. In lower-energy environments, transport appears to occur primarily in the along-shelf direction in water depths where wave resuspension is frequent, with little across-margin transport. This is likely due to resuspension by waves in shallow water where the Ekman layer may be thicker than the water depth, while in higher energy environments wave resuspension occurs in water depths where a distinct bottom Ekman layer can form. For example, on the western Adriatic continental shelf and the Ebro River margin, both relatively low-energy environments, the along-shelf sediment flux during winter experiments was one to two orders of magnitude greater than the across-shelf flux at 12-m water depth. In contrast, on the highly energetic northern California shelf distinct downwelling signatures are seen at 60-m water depth where frequent storm resuspension is observed and the across-shelf sediment flux is persistently seaward during storms and of equal or greater magnitude than the alongshelf flux. This suggests that the resulting sediment deposit from fluvial sources along low-energy coastlines would be located in shallow water and linearly distributed along the coastline with little loss of sediment to offshore environments. In higher-energy environments, fluvial sediment would be distributed farther offshore with more sediment escaping the shelf environment.
OS23C-1325 1340h
The Role of Cap de Creus Canyon in Off-Shelf Sediment Escape, Western Gulf of Lions, France
Recent studies investigating across-margin sediment transport have identified submarine canyons as an important conduit for off-shelf sediment export during the present high-stand in sea level. The Gulf of Lions, France is a bathymetrically complex region with multiple submarine canyons and significant sediment input from the Rhone and many small rivers. It has been suggested that the western Gulf of Lions is a primary outlet for sediment due to narrowing of the shelf and the general east-to-west circulation pattern. This study focuses on Cap de Creus, the westernmost canyon, which is hypothesized to be an important factor in this escape because it incises a very narrow shelf and may intercept part of the sediment transport pathway. Box cores were collected in multiple locations within the canyon and on the shelf in November 2003 and March 2004 to determine sedimentation patterns. An along-shelf transect at 50 m water depth from March shows an increase in accumulation rates ($\sim$2 to $\sim$4 mm/yr) from north to south, indicating that significant amounts of sediment are reaching the seabed near the canyon head. Analysis of cores taken within the canyon also suggests that sediment is being transported down-canyon. Cores collected at shallow depths within the canyon (150 m) reveal a distinct sand layer with excess $^{210}$Pb activities (indicating deposition $<$100 years ago) that unconformably overlies a highly-consolidated mud with only supported levels of $^{210}$Pb (deposited $>$100 years ago). This coarse material was seen in both November and March and appears to extend as deep as 400 m. Deeper within the canyon (500 m), there is a blanket of unconsolidated mud (13 cm thick) that was deposited within the last 100 years; the canyon flanks are also observed to be accumulating at varying rates. This is significant because it indicates that both coarse and fine grained sediment are moving down the canyon during the present sea level high-stand. Most cores collected deeper in the canyon reflect a shift from old, highly consolidated basal muds or coarse material to unconsolidated, younger sediments near the surface, some with a very distinct interface. One such core (at 500 m in the canyon thalweg) shows similar excess $^{210}$Pb activities in both the upper and lower layers down to the base of the core (18 cm), indicating that both were emplaced in less than one half-life (22 years). This shift in characteristics may be due to (1) two phases of a single event or (2) a shift in source over a short timescale, although the exact timing and frequency are still under investigation. Regardless of the mechanism, these preliminary data suggest that Cap de Creus is an important outlet of sediment from the continental shelf in the western Gulf of Lions.
OS23C-1326 1340h
The key to Understand Submarine Canyon Evolution
Submarine canyons are the preferential path of sediment transfer from the shelf to the deep sea, they are the key to understand the source-to-sink sedimentation and, in consequence, the shelf, slope and rise evolution. Pioneer works on submarine canyons described and proposed hypothesis to explain the formation and evolution of them. However, submarine canyons remain a matter of speculation. Our work in the Gulf of Lions (Mediterranean Sea) is based on swath bathymetry data together with sub-bottom profiles, high resolution seismic reflection profiles and cores. These data allow a detailed morphologic and stratigraphic study from the shelf to the rise through time, from 2.600.000 yrs to present. We show that two main erosive features, of very different dimensions, constitute the canyons: the axial incision and the canyon's major valley. The axial incision is interpreted as an erosive path related to the passage of hyperpycnal turbidity currents, generated up-slope by river connection. In the Gulf of Lions such currents are most likely to have formed during each Glacial Maxima (with a cyclicity of 100.000 years for the last 900.000 years and 40.000 years between 900.000 and 2.600.000 years) as both proximity of the shoreline (due to the lowstand of sea level) and high detrital sediment supply (due to glacial abrasion upstream) increased the flow of sediments delivered to the canyon heads. The axial incisions observed at the sea floor and fossil incisions observed on seismic lines, are related to equivalent conditions. The axial incision activity has a key influence on canyon evolution, it triggers mass wasting that affect the canyon's major valley (head and flanks) allowing the progressive widening and deepening of the canyon. Consequently the canyon's major valley (typically bounded by flanks of more than 700 meters in height) is the result of the axial incision activity through successive lowering of sea level. In summary: our cross-disciplinary approach (morphology, seismic stratigraphy, sedimentology and glacioeustacy) provides a regional and detailed vision of canyons. This work (i) explains the mechanism that controls the evolution of the submarine canyons and the sediment transfer from the shelf to the rise and (ii) shows the sedimentary evolution of the margin (from source to sink) in relation with sea level changes from 2.600.000 years to present.
OS23C-1327 1340h
Describing the development of submarine canyons using stream-power erosion laws
The problem of how turbidity currents erode their beds is important for understanding how canyons develop, but is complex because flow power also varies as a result of incorporation and deposition of the current's suspended load. In some canyons where the total sedimentary mass passing through the canyon has been much larger than the excavated mass, the loads of the eroding currents changed little during passage down-canyon. Canyon morphology can then potentially reveal how gradient and other factors affect erosion rate, illustrated here with two datasets from tectonically active margins. The first dataset is from Tenryu Canyon off Japan, which was entrenched by up to 1200 m by steepening of the Tokai accretionary prism. Incision depth and channel gradient S data [Soh and Tokuyama, 2002] suggest an erosion law in which incision rate E~S$^{0.8}$, which is remarkably similar to laws for detachment-limited erosion of river beds. In the second dataset, folds of the Barbados prism have created knickpoints [Huyghe et al., 2004]. Numerical modeling reveals that the knickpoints have partly smoothed out, a property of transport-limited erosion, but primarily have translated, a property of detachment-limited erosion. This mixed style of knickpoint development has also been inferred from some bedrock eroding streams on land. If scaling arguments for erosion with flow velocity apply here [Hancock et al., 1998], the inferred dependence of erosion rate on gradient implies that plucking and shear failure rather than abrasion are the main mechanisms in these channels. Hancock, G.S., Anderson, R.S., and Whipple, K.X., 1998, Beyond power: bedrock river incision process and form, in Tinkler, K.J., and Wohl, E.E., eds., Rivers over rock: Fluvial processes in bedrock channels, American Geophysical Union Monogr. 107: p. 35-60. Huyghe, P., M. Foata, E. Deville, and the Caramba Working Group, Channel profiles through the active thrust front of the southern Barbados prism, Geology, 32, 429-432, 2004. Soh, W., and H. Tokuyama, Rejuvination of submarine canyon associated with ridge subduction, Tenryu Canyon, off Tokai, central Japan, Mar. Geol., 187, 203-230, 2002.
OS23C-1328 1340h
Autocyclic Behavior of Experimental Turbidity Currents
Subaqueous turbidity currents are dilute sediment-water mixtures that are driven along the water bottom by their excess density relative to the ambient water. Turbidity currents continuously deposit and resuspend sediments as they move. If resuspension exceeds deposition, the excess density of the current grows and the current `ignites'. If deposition exceeds resuspension, the excess density is depleted and the current `dies'. The bulk flow discharge and bed slope largely control these two regimes. Deposition from a waning turbidity current falls off exponentially downstream. A continuous depositional current therefore steepens the sedimented slope it flows over. At a critical steepness, the sediment resuspension will balance deposition and the flow should shift to a bypassing regime. Laboratory experiments combined with a simple numerical model show that this behavior triggers an autocyclic mechanism that can both create and regulate deltaic slopes even while they prograde. Six flume experiments using turbidity currents generated with mixtures of sand ($D_{50}$ =110$\mu$m) and silt ($D_{50}$ =24$\mu$m) were directed at determining if turbidity currents would build a slope through deposition to a critical steepness at which the flows would bypass it. With water level fixed, deposition from a continuous experimental turbidity current repeatedly steepened a slope to a critical angle before bypassing it and depositing a sediment wedge at the slope base. Continued deposition then caused the wedge to grow back updip to the top of the slope lowering it once again below the critical angle and reinitiating a cycle of oversteepening. The critical slope was observed to vary with the bulk sediment discharge and the amount of sand in the flow. The observed autocyclic progradation is simulated with a model of a depositional turbidity current governed by a critical bypass slope that depends on the grain size and bulk discharge of the flow. Predicted critical slopes for a range of sediment discharges and grain sizes representative of field-scale turbidity currents are typically $< 8^o$. The model predicts the critical slopes measured in the experiments if the coarsest ($D_{95}$) fraction of the sand is used in the resuspension relation. Our results document a mechanism for grading deltaic slopes as they prograde by processes of sediment transport rather than mechanical failure. The process also reveals a mechanism for the delivery of bypassed sand to basin deeps, leaving deepwater turbidites typically interpreted to mark a fall in relative sea level. On an experimental scale, units in the flume deltas bounded by bypass surfaces conform to stratigraphic sequences but are not related to changes in base level.
OS23C-1329 1340h
Depositional history of the post-glacial transgression on the outer continental shelf of the Gulf of Lions (western Mediterranean)
Late Pleistocene sedimentation on the Gulf of Lions continental shelf was investigated using a combination of seismic, sedimentological and geochronological methods. According to the calibrated age data generated in the cores material, a correlation between the transgressive seismic units (and associate coastal features) and the post-glacial climatic/sea level changes was attempted. The formation of trangressive deposits was favoured during short-term cool episodes, such as Younger Dryas Event, as well as ancient sea-level positions can be assigned on the basis of the easily recognizable coastal features. The internal structure of the transgressive deposits and sedimentation processes are largely controlled by the nature of sea-level rise (deceleration or short-lived sea-level stillstands), sediment supply changes, and paleogeographic/paleoceanographic conditions related to post-glacial transgression. Sedimentological data (sedimentary facies and fossil content) and depositional ages determined on the 14C radiocarbon dating of well-preserved fossil shells allow us to establish a chronostratigraphic sequence that concerns the transgressive deposits of the Gulf of Lions. In particular, the combined use of available data permit: (1) to trace the ravinement surfaces, possibly related to mpwIA and mpwIB events, throughout the outer shelf; (2) to detect the nature of preserved sedimentary bodies that are mainly related to the deceleration of sea-level rise during the major post-glacial cold event (submerged dunes formed during Younger Dryas); (3) to describe the characteristics of the ravinement deposits and the degree of preservation of relict sediments on the shelf depositional environments. In fact, the post-Last Glacial Maximum transgressive phase occurs as complete record in the low-gradient outer shelf of the Gulf of Lions, but the deposits show a thickness that is considerably thinner with respect to the inner shelf area. They are seen as the distal part of the TST-HST margin deposits but they clearly document the effect of the sea level change on sedimentation pattern in wave-dominated shelf environment. Aloisi, J.C, Monaco, A., Thommeret, J., Thommeret Y. Geogr. Phys.Quatern., 32, 145-162, (1978). Rabineau, M., Berne, S., Ledrezen, E., Lericolais, G., Marsset, T., Rotunno, M. Mar.Petr.Geol., 15, 439-452, (1998).
OS23C-1330 1340h
Tertiary Sequences And Quaternary Glacial Sediments Of The Southern North Sea, Europe
The southern North Sea with a focus on the German exclusive economic zone was surveyed by multichannel seismic (airgun) and shallow seismic (boomer and sparker). The seismic lines, with their special focus on the shallow subsurface, complement data sets of varying quality of scientific (e.g., 2D seismic and wells of the EC Southern North Sea Project) and commercial origin (2D/3D seismic and wells of hydrocarbon exploration). The different data sets were integrated in a digital interpretation system. Interpretation was supported by numerous shallow (6 m) cores targeted at key sites. On a large scale, key reflectors are mapped below and above the prominent Middle Miocene unconformity leading to a better understanding of the prograding Eridanos delta throughout the Tertiary. Also a first subdivision of the Quaternary sequence has been achieved. The most prominent structures in the shallow subsurface are widespread ramified subglacial valleys of likely Elsterian age (~400 kyr BP), which can be up to 400 m deep and which generally have a complex infill. Another prominent feature is a shallow depression of the sea floor starting NW of Heligoland and spanning far north into Danish waters. This elongated depression ("Elbe-Urstromtal" Figge, 1980) is interpreted to be eroded by periglacial rivers running southward and westward of the ice sheet. On a regional scale, some studied 3D-seismic volumes allow the identification of more subtle structures in time slices like widespread iceberg keel scours and ice sheet scours. Also remnants of the morphology of the glacial landscapes seem to be preserved represented by contrasting seismic facies of patchy, rough moraines and more uniform planes. The planes are incised by up to 4 intersecting systems of gullies, channels, and valleys that are of supposed fluvial and subglacial origin. Recurrent seismic anomalies with exceptional high amplitudes may indicate shallow gas in subsurface sediments.
OS23C-1331 1340h
Quantifying the Bioporosity of Recent Po Delta Sediments
As part of project EuroSTRATAFORM, a series of shallow box-core samples were collected near the Po Delta in Italy. They reveal the presence of intense biogenic structures that are believed to have a strong influence on the physical properties of the sediments as they introduce another class of porosity: bioporosity (volume of biopores/total volume of the sample). Therefore, it required that they be evaluated and quantified. Two of these cores were selected for a detailed analysis. This was done primarily using 3D CATSCAN imagery (tomographic intensity) and direct physico-chemical measurements. The tomographic intensity is a complex value controlled by many factors such as the grain size, mineralogy, consolidation, water content and porosity. Two methods were used to quantify the bioporosity: an absolute and a relative bioporosity measurements both based on the use of the tomographic intensity. The relative method takes into account the variability of the sediment densities along the core, whereas the absolute method fixes the tomographic intensity based on the mean density of sediment for the whole core. Because of the evolution of the geometry of the biogenic structures, it became clear that the relative method was much better. Results have shown that the bioporosity could reach values as high a 40% and could account for more than half of the total porosity. These results suggest that significant bias on water content measurement of the matrix thus influencing estimation of physical properties like plastic and liquid limits and the liquidity index via the bias on the matrix water content measurement.
OS23C-1332 1340h
Coupled Passive Margin Stratigraphic Evolution and Fluid Flow
Continental margin processes are strongly influenced by patterns of overpressure and fluid flow within basin sediments. These patterns effect consolidation, slope stability, and the transport of heat and solutes. Numerous processes generate overpressure, but in many passive margins rapid deposition is thought to be a dominant mechanism, producing a two-way coupling between sedimentary processes and fluid flow. We study this coupling using a fully 2D margin-scale coupled stratigraphic evolution and fluid flow model. The sedimentary process model describes transport of sand and mud through coastal plain, shoreface, shelf, slope, fan, and rise depositional environments, as well as migration of internal boundaries between environments. The fluid flow model uses the finite element method (FEM) to solve for fluid flow on a dynamic grid. We explore the effects of 2D coupling for various basin geometries, sea-level histories, and sediment influx rates. We find that on a margin-scale, deposition tends to be localized in depocenters, which are initiated by sediment flux divergence across depositional environment boundaries (e.g. shoreline, shelf-break). Over timescales relevant to pore-pressure evolution, these boundaries and their associated clinoforms migrate in response to sedimentation and relative sea level change, resulting in complex dynamic patterns of overpressure. Although in some regions fluid flow is quasi-1D (vertical and forced only by local deposition), in many regions fluid flow is strongly 2D. Complex flow results from heterogeneous stratigraphy, with strong contrasts in sand and mud permeability determining preferential flow paths. On regional scales, confined horizontal sandy units, such as buried turbidite fan complexes, act as fluid flow conduits, inducing flow anisotropy. On smaller scales, interbedded sandy and muddy deposits produce anisotropic effective permeabilities with preferential horizontal flow. The complex dynamic geometries inherent to margin stratigraphic evolution have heretofore prevented effective margin-scale modeling coupled 2D stratigraphic-hydrologic system, but our computational approach introduces two innovations. A dynamic meshing technique allows for rapid meshing of arbitrary deposits while maintaining bounds on FEM conditioning and error. And a hierarchical mesh structure, which aggregates sediment layers into mesh nodes into finite elements, embeds sub-grid dynamics into the FEM model via layer averaged anisotropic permeabilities. Our results show that in regions of homogeneous lithology, quasi-1D numerical or asymptotic approximations are applicable, but that in regions with complex stratigraphy, strong anisotropy effects cannot be ignored. Our meshing and FEM approach is general and easily adapted for use with any sedimentary or backstripping model, and models including more grain sizes or processes have an even greater potential for complex 2D fluid flow. Hence our results demonstrate the viability and necessity of including 2D effects in continental margin fluid flow models.
OS23C-1333 1340h
Morphodynamic Modeling of Shoreline Transgression and Regression Driven by Changes in the Frequency and Magnitude of Coastal Storms
Sequence stratigraphy has long emphasized sea level fluctuation as a causal mechanism for transgressive-regressive cycles preserved in the ancient record. However, the surf zone is also the locus for the interplay of fluvial input with waves and currents, and several field studies have attributed modern shoreline transgression and fluvial degradation to changes in the hydrodynamic regime of the receiving basin. Here I explore quantitatively how fluctuations in the shallow-marine wave-current field might drive large-scale shoreline translation and the generation of transgressive-regressive sequences. I develop a morphodynamic model in which the repeated occurrence of floods and large coastal storms, of specified frequency and magnitude, drives long-term evolution of the sediment surface in the fluvial and shallow-marine environments. The model couples these environments across the surf zone, which collapses to a shock condition at the shoreline. The breaker height of waves associated with large coastal storms sets the height of the shoreface shock; the flux discontinuity across the surf zone drives lateral migration of the shock. I analyze the stratigraphic response to sinusoidal variations in the shallow-marine wave-current field that are superimposed on a background of steady subsidence, eustatic sea level, and fluvial input of water and sediment. For a reasonable range of wave heights and downwelling currents, variations in the magnitude and frequency of large coastal storms generate cycles of shoreline transgression and regression with stratigraphically significant amplitudes. Specifically, more frequent and larger storms increase the fraction of sediment supply partitioned to the shallow-marine environment, thereby driving shoreface transgression and fluvial degradation. Shoreface translation is most sensitive to variations in the strength of the downwelling current. Model results suggest that the phase relation between shoreline and clinoform rollover is the primary stratigraphic indicator for differentiating between sea-level-driven and wave-current-driven transgressive-regressive cycles.
OS23C-1334 1340h
Results of 2-D backstripping of the Plio-Quaternary sequence of the Golfe du Lion passive margin from the shelf to the basin: insights on the subsidence and paleo-geometries
The continental margin of the Golfe du Lion was deeply eroded during the Messinain drawdown of the western Mediterranean Sea, and has subsequently been rebuilt by sediments supplied by the Rhone and adjacent rivers. We are engaged in a study of the evolution of this margin based on sequential backstripping, which accounts for the several processes (thermal subsidence, compaction, sediment loading and tectonics ) that contribute to deformation of the preserved strata and allows us to reconstruct the true stratal geometries at various times throughout the last 5 Ma. We report on our first results of 2-D backstripping as a prelude to a full 3-D treatment that is in progress. Our approach has thus far followed these steps : 1) remove the sedimentary layer above the surface we want to restore, 2) restore offsets along faults (if necessary), 3) allow the sediments to decompact, 4) flexurally unload the underlying crust, 5) remove subsidence, 6) adjust sea level. Using ELF Aquitaine and IFREMER Golfe du Lion seismic data with a consistent interpretation of the main sedimentary units from the shelf to the basin, allows to (1) determine a quantitative estimate of the original morphology, (2) calculate the gradients of continental shelf, shelf clinoforms and continental slopes for each time slice, (3) estimate the sequential locations of the shelf edge and clinoform roll over and their water depths, (4) determine the height of clinoforms, and (5) estimate the volume of Pliocene to Quaternary sediments partitioned between the shelf to the basin, and their possible impact on post-Messinian salt tectonics. These quantitative estimates will help to better understand fundamental processes of margin building.
OS23C-1335 1340h
A Quantitative A/S Ratio for Predicting Shoreline Migration During Eustatic Base-Level Cycles
The ratio of accommodation to sediment supply (A/S) is commonly invoked in explaining shoreline response to base-level variation. We develop a quantitative shoreline-migration equation based on mass conservation at the shoreline. This relation can be recast in the form of a ratio that includes accommodation and sediment supply, along with several other effects including variation in the length of the marine deposystem, fluvial sediment storage, and spatial variation in subsidence. Ignoring the additional terms yields a quantitative A/S ratio that can be used for approximate shoreline prediction. The general shoreline equation accurately predicts observed shoreline variation in a controlled experiment for which the only externally imposed variable was base-level variation on two time scales, applied singly and then superimposed. In most field settings it would be difficult to obtain the data needed to constrain all the terms in the predictive equation. We measure the degradation of the accuracy of the predicted shoreline dynamics as we reduce the amount of data available by replacing observed time variation of successive terms in the equation with their mean values. By this measure, base level is the most important variable in predicting shoreline migration, followed in turn by sediment supply at the shoreline, geometry of the foreset, and the average subsidence rate across the foreset.
OS23C-1336 1340h
Catastrophic Flooding and the Origin of the English Channel Valley System
Late Quaternary episodes of sea-level lowering exposed large expanses of the English Channel shelf during glacial periods leading to the development of an English Channel mega-river system that integrated major north-western European rivers such as the Rhine and Seine with the Thames and flowed to the Celtic Sea continental margin. On the eastern English Channel shelf, extensive bedrock-incised valleys form an anastamosing network, however the controls on their genesis and evolution remain enigmatic due to a lack of detailed morphological data. Suggested mechanisms for valley incision include fluvial erosion in response to sea-level lowering, tidal or glacial scouring, and controversially catastrophic flooding. Here, we present new data on the detailed morphology of the northern branch of the valley system, the Northern Palaeovalley that shows clear evidence for catastrophic flooding as the cause of valley incision. By analysing a bathymetric grid derived from singlebeam sonar data compiled from UK Hydrographic Office data we have identified an assemblage of geomorphic features associated with the valley that are analogous to features indicative of catastrophic flood scouring. The valley is tens of kilometres wide and up to 50 m deep, and shows distinctive box-like cross-sections. Characteristic landforms observed include longitudinal erosional grooves, streamlined islands, inner gorges and erosionally terraced and smoothed, streamlined valley margins. These geomorphic features bear striking resemblance to landforms observed in catastrophic flood terrains in the Channeled Scabland and within outflow channels on Mars, and are difficult to explain by normal fluvial, tidal or glacial erosion mechanisms. Our results provide the first evidence indicating that the English Channel palaeovalley system was carved by catastrophic flood flows following breaching of the Dover Straits and abrupt release of freshwater from an ice-dammed lake in the present Southern North Sea region. We speculate that the English Channel flood may have been one of the most powerful flood events on Earth.
OS23C-1337 1340h
3D deflection of a lithospheric plate with a variable flexural rigidity and under a variable sedimentary load for 3D backstripping: preliminary tests
To study the stratigraphic and morphological evolution of sedimentary basins and continental margins through time, sequential backstripping is commonly used to reconstruct their past geometries. On seismic images, the present geometry of preserved strata is different from its depositional morphology due to the effects of thermal subsidence, compaction, sediment loading and tectonics. The aim of backstripping is to undo the effects of these deformations and determine improved estimates of the past bathymetry and stratigraphy at different time slices. These calculations are generally performed along 2-D cross-sections but most continental margins are highly 3-D structures. Deltaic sediment lobes, oblique structures and sediment packages, deep sea fans, etc limit the accuracy of 2-D calculations. In addition, the flexural rigidity commonly varies from the continent, across the margin and into the oceanic basin. In order to perform 3-D backstripping, flexural unloading has to be considered as the deflection of a plate with a 3-D variable sedimentary loads and a spatially variable flexural rigidity. The global form of the equation is : \nabla$^{2}$(D(\nabla$^{2}$w)) - T\nabla$^{2}$w + w = P, where D is the flexural rigidity, w is the deflection of the surface, T is the tension in the plate and P is the load. Formulating the equation in terms of the bending moments, M, leads to a set of four coupled 2nd-order equations. This theoretical problem leads, in practice, to a complex sparse matrix of the finite-difference approximation which must be solved numerically. First results will be shown (1) to estimate the validity of the approach prior to application to real data set, such as the Golfe du Lion passive margin and (2) to discuss the estimates and influence of the various parameters.