H41F-0836
Formation of levees in laboratory dry granular flows
The classical "channel-levees" deposit morphology observed after natural granular flows, such as rock avalanches or pyroclastic flows, is investigated in simple flow geometry laboratory experiments. Granular material is released on a inclined plane at a constant mass flow rate and image analyses allow to study the temporal evolution of both morphological and dynamical characteristics with respect to the values of the mass flow rate and the slope angle. Although the mass flow rate remains constant all along the experiment, the morphology of the flow evolves with time : the granular flow slowly enlarges while its thickness decreases. We show that the levees commonly observed along the sides of the deposit upon interruption of the flow, disappear for long flow durations. We demonstrate that the morphology of the deposit builds up during the flow, in the form of an underlying static layer layer, which can be deduced from surface velocity profiles, by imposing the same flow rule everywhere in the flow.
H41F-0837
A Terminating Channel-Levee System on the Middle Bengal Fan
The Bay of Bengal is fully covered by the Bengal Fan, the largest submarine fan on Earth. This fan is fed by the Ganges-Brahmaputra river system, which dewaters the Himalaya and transports huge amounts of sediment to the Bengal Basin and Bengal Shelf. From there, the sediments are transported to the deep sea fan by turbidity currents starting in a deeply incised shelf canyon. These turbidity currents build up channel-levee systems being the main architectural features on the Bengal Fan. Therefore the Bengal Fan is ideal to study not only the erosional and climatic history of the Himalayas, but also the structure of channel-levee systems to understand depositional processes on submarine fans in detail. During Cruise SO188-1, carried out in June 2006 with the German Research Vessel Sonne, a terminating channel-levee system on the middle Bengal Fan was studied by collecting multichannel seismic data, Parasound sediment echosounder data and bathymetric data. The aim of the study was to image the changing morphology and architecture of a terminating system and to test the hypothesis if a depositional lobe has developed here. The bathymetric data show that the channel terminates by bifurcating into short distributaries. However, another small channel appears south-east of the bifurcation, and a further small channel is identified to the east. Additionally, the seismic and acoustic data reveal several small completely refilled channels in the study area, which are only partially leveed. These observations indicates that the pattern of the channel-levee systems is here even more complicated than expected and that a succession of erosion and refilling has controlled the deposition. By tracking the channel to the north until its next avulsion point, the channel could be mapped by using the swath sounding system over a distance of 300 nm. This (to our knowledge) unique dataset of a surface channel on the Bengal Fan reveals significant changes of shape and relief of the channel and large variations in terms of sinuosity. Future work should parameterize these variations to try to reconstruct the properties of the turbidity currents which have built this channel-levee system.
H41F-0838
Constructional canyons built by sheet-like turbidity currents: Observations from offshore Brunei Darussalam
Submarine canyon formation and deepening are typically attributed to erosional processes. We present data from an industry-grade seismic volume located offshore Brunei Darussalam illustrating how topography typically associated with erosional processes can be produced under conditions of net sediment deposition. This data was generated via subsurface mapping in the vicinity of a shale-cored anticline on the Quaternary continental- slope. Three canyons traverse the structure at right-angles to the crest line with maximum canyon relief of 165 m. Subsurface mapping reveals that the structure is a site of net sediment deposition and defines a background sedimentation pattern that decreases gradually with distance from the shelf-edge. Profiles down canyon axes reveal local minima in deposit thickness over the anticline hinge that are associated with high downstream gradients. Deposition on ridges adjacent to canyons also displays local minima at the anticline hinge, but these minima are not correlated with gradient. A comparison of canyon axis and ridge deposition shows that somewhat higher rates of sedimentation on the ridges resulted in the preservation and growth of the submarine canyons with time. Laterally persistent seismic reflectors and depositional packages suggest that the canyon forming currents were sheet-like flows, extending for many kilometers in the strike direction. The currents drained into canyons as they approached the anticline hinge, leaving only a small supra-canyon fraction available to deposit sediment on the non-channelized zones.
H41F-0839
Experiments on Bedforms Created by Density Currents
Small-scale bedforms such as dunes and ripples formed by density underflows such as density or turbidity currents are poorly understood. The general practice is to extrapolate bedform diagrams that were obtained from subaerial rivers and laboratory open channels to the case of deep water undercurrents, assuming that different bedforms created by density or turbidity currents form under similar flow regimes as in rivers (i.e. subcritical or supercritical). However, the validity of this assumption has never been tested. We present experimental evidence that dunes (commonly associated with low Froude number flows) could be formed in critical or supercritical saline density currents. Based on our findings and measurements, we also present a scaling analysis showing that these particular bedforms could indeed be associated with what sedimentologists and fluvial geomorphologists call dunes. Our findings have important implications from a mechanistic and geologic perspective, as they imply that a wider, and not yet fully investigated and understood, spectrum of bedforms due to gravity underflows might exist.
H41F-0840
Morphology and Structure of the Algiers Deep-Sea Fan and Possible Sedimentary Record of the 2003 Boumerdès Earthquake (Maradja Project)
The Algerian margin is a Cenozoic passive margin along the plate boundary between Eurasia and Africa, presently reactivated in compression. The deformation is expressed by ESE-WNW-aligned inverse faults associated with moderate to large earthquakes. Earthquakes can generate sediment instabilities on the continental slope; for instance, strong turbidity currents are likely responsible of submarine cable breaks observed at the foot of the continental slope, after the Boumerdes earthquake in may 2003. In the Boumerdes-Algiers area, the Algiers canyon is the main sediment pathway from the shelf to the deep sea environment. Since Maradja surveys in 2003 and 2005, high-resolution bathymetric and backscatter images are available over an area of 1200 km along the margin. The morpho-bathymetric interpretation supported by seismic reflection data allowed to identify the Quaternary Algiers deep-sea fan and its main morphological features: the canyon, the main valley, the sedimentary ridge, and the distal deposits confined in mini-basins. The Algiers canyon is characterised by two tributary paths in the upper slope. The maximum incision is about 600 m deep. The canyon fed a 30 km long turbidity valley, spreading westward along the foot of the slope and bordered by the sedimentary ridge on the right side. The sedimentary ridge is covered by large sediment wave features, probably generated by turbidity overflow currents. The Algiers deep-sea fan has a complex structure resulting from the interaction between turbidity sedimentation, compressive tectonic structures and salt tectonic. In the upper part, the morphology of the Algiers canyon and fan is strongly controlled by inverse faults creating morphological highs and scarps. In the distal part, the sea floor morphology is highly deformed by salt diaprism, which control the paths of active sediment transport and the location of deposition area (mini-basins). Sediment gravity cores collected on the deep-sea fan show recent sedimentation in distinct morphological areas. The valley and the ridge present successions of thin-bedded turbidites. In the submarine valley, we collected the most recent turbidite sequences (younger than 100 years, based on 210Pb isotope analyses) in order to evaluate the impact of the 2003 event on the Algiers Fan.
H41F-0841
Sediment size of surface floodplain sediments along a large lowland river
Data on size distribution of surface sediment across a floodplain should place important constraints of modeling of floodplain deposition. Diffusive or advective models would predict that, generally, grain size should decrease away from channel banks. Variations in grain size downstream along floodplains may depend on downstream fining of river bed material, exchange rate with river banks and net deposition onto the floodplain. Here we report detailed grain size analyses taken from 17 floodplain transects along 450 km (along channel distance) reach of the middle Fly River, Papua New Guinea. Field studies have documented a systematic change in floodplain characteristics downstream from forested, more topographically elevated and topography bounded by an actively shifting mainstem channel to a downstream swamp grass, low elevation topography along which the river meanders are currently stagnant. Frequency and duration of flooding increase downstream. Flooding occurs both by overbank flows and by injections of floodwaters up tributary and tie channels connected to the mainstem. Previous studies show that about 40% of the total discharge of water passes across the floodplain, and, correspondingly, about 40% of the total load is deposited on the plain - decreasing exponentially from channel bank. We find that floodplain sediment is most sandy at the channel bank. Grain size rapidly declines away from the bank, but surprisingly two trends were also observed. A relatively short distance from the bank the surface material is finest, but with further distance from the bank (out to greater than 1 km from the 250 m wide channel) clay content decreases and silt content increases. The changes are small but repeated at most of the transects. The second trend is that bank material fines downstream, corresponding to a downstream finding bed material, but once away from the bank, there is a weak tendency for a given distance away from the bank the floodplain surface deposits to slightly coarsen downstream. We also find that sand is present (about 4%) in these surface sediments out to 1 km from the channel bank. These trends are not consistent with simple lateral transport models, and other factors, including effects of flocculation, local flow patterns, and possibly dry season wind effects may matter.