Storm-induced sediment gravity flows at the head of the Eel submarine canyon, northern California margin

10 pages, 5 figuresAs part of the STRATAFORM program, a bottom-boundary layer (BBL) tripod was deployed at 120 m depth in the northern thalweg of the Eel Canyon during winter 2000. Increases of the near-bottom suspended-sediment concentrations (SSC) recorded at the canyon head were not directly related to the Eel River discharge, but were clearly linked to the occurrence of storms. BBL measurements revealed that during intensifications of the wave orbital velocity, sediment transport at the head of the canyon occurred as sediment gravity flows directed down-canyon. Observational evidence for near-bed sediment gravity-flow transport included an increase toward the bed of the down-canyon component of wave-averaged velocity and high estimated SSC. At higher sampling frequencies (1 Hz), the current components during these events fluctuated at the same periodicity as the pressure, reflecting a clear influence of the surface-wave activity on the generation and maintenance of the sediment gravity flows. The origin of such flows is not related to the formation of fluid muds on the shelf or to intense wave-current sediment resuspension around the canyon head region. Rather, liquefaction of sediment deposited at the head of the canyon (induced by wave-load excess pore water pressures during storms) combined with elevated slopes around the canyon head appear to be the mechanisms initiating sediment transport. The resulting fluidized-sediment layer can easily be eroded, entrained into the water column, and transported down-canyon as a sediment gravity flow. Results from this study reveal that storm-induced sediment gravity flows occur periodically in the Eel Canyon head, and suggest that this kind of sediment transport process can occur in other submarine canyons more frequently than previously expected. Copyright 2004 by the American Geophysical UnionThis work has been funded by the Office of Naval Research, Marine Geology and Geophysics Program, grants N00014-95-1-0418 and N00014-99-1-0028, as part of the STRATAFORM program. P. Puig received financial support from a Fulbright scholarship provided by the Spanish Ministry of Education and CulturePeer Reviewe

Sediment deposition in a modern submarine canyon: Eel Canyon, northern California

19 pages, 9 figures, 1 tablePrevious studies on the Eel margin have shown that a substantial amount of terrigenous sediment may be rapidly transported and deposited seaward of the shelf break during the Eel River flooding season. Eel Canyon, located ∼10 km seaward of the Eel River mouth, has been investigated to determine its role in seaward escape of sediment over seasonal timescales. In order to characterize seasonal deposition, box cores were collected in upper Eel Canyon (<900 m water depth) during periods of high and low sediment discharge from the Eel River (winter and summer, respectively). Cores were collected at the beginning and end of three typical flooding seasons (January 1998 - April 2000), to assess the regularity of sediment escape to the Eel Canyon. Results showed that sediment deposition was temporally variable over the course of a year, with thick sediment deposits (up to 19 cm) being formed during the flooding season. These deposits were identified as having detectable 7Be activity, high percentage of clay and distinct physical sedimentary structures. Sediment signatures indicated sediment was composed primarily of recently derived fluvial sediment that was rapidly accreted on the seabed (on the order of days to months from when it was discharged from the river). The 3-year time series showed that sediment reached the canyon on an annual basis and extreme flooding events from the Eel River were not a necessary condition for sediment escape. Fluvial sediment was deposited in most areas of the upper canyon, following each winter flooding season, with the thickest deposits consistently observed in upper channel thalwegs of the canyon. Preferential deposition in thalwegs, along with preservation of physical structures, suggests that gravity-driven sediment flows may be important transport mechanisms in these areas. The spatial distribution of deposit thicknesses for cores collected in April 2000 was used in conjunction with a detailed swath map to construct a seasonal sediment budget for the upper Eel Canyon alone. It was found that 12.0%±3.5% of the Eel River sediment discharge can be accounted for in the upper Eel Canyon, suggesting that the canyon can act as a significant sink of Eel River sediment over seasonal timescales. © 2004 Elsevier B.V. All rights reservedThis work, as part of the STRATAFORM program, has been funded by the Office of Naval Research, Marine Geology and Geophysics Program, Grant Numbers N000149510060, N000149910028, N000149610361, N000149810503 and N00014031142Peer Reviewe

Sediment accumulation in the western Gulf of Lions, France: The role of Cap de Creus Canyon in linking shelf and slope sediment dispersal systems

Special issue Sediment Dynamics in the Gulf of Lions; the Impact of Extreme Events.-- 17 pages, 10 figures, 4 tablesPrevious work in the Gulf of Lions (western Mediterranean Sea) has suggested that significant amounts of sediment escape through the western part of this tectonically passive margin, despite it being far removed from the primary sediment source (the Rhone River, not, vert, similar160 km to the NE). The primary mechanism behind this export is hypothesized to be the interaction of a regional, southwestward sediment-transport path with a canyon deeply incising the southwestern part of the shelf, Cap de Creus Canyon. To understand the pattern of off-shelf sediment export from the western Gulf of Lions, and more specifically, the role of Cap de Creus Canyon in this transport, box cores were collected within the canyon and on the adjacent shelf during five cruises from November 2003 to April 2005. Geochronology (210Pb-derived accumulation rates), grain-size distributions, and sedimentary structures (X-radiography) were analyzed to assess temporal and spatial sedimentation patterns. Results indicate two mid-shelf depocenters (30–90 m water depth) in the northern and southern portions of the study area, separated by a zone of bypassing due to current acceleration around a headland (Cap Bear). Estimates of a sediment budget indicate that not, vert, similar6–8% of the sediment input to the Gulf is sequestered on the shelf region. Within the Cap de Creus Canyon, there is a significant spatial asymmetry in both grain size and accumulation rates. The northern flank is a modern depocenter of fine-grained sediments, while the southern flank is primarily non-depositional for mud and includes locations of apparent erosion. This suggests the influence of multiple oceanographic processes supplying sediment to the canyon: advection of nepheloid layers from the northern rim that provide a relatively continuous sediment supply (over decadal timescales), and episodic strong currents affecting the southern rim, which can scour sediment from the southern flank. The mid-depth thalweg has an ephemeral mud layer, overlying sand and consolidated mud. The mud layer appears to be flushed down canyon periodically. The canyon head contains coarse material, suggesting reworked sands may be entering. The 100-year sediment budget, based on accumulation rates for the fine-grained fraction in the upper canyon, indicates that not, vert, similar1% of the total sediment input to the Gulf is accumulating in upper Cap de Creus Canyon. However, this number may significantly underestimate the total sediment entering the canyon because water-column measurements show that sediment is likely moving through the upper canyon during major dense-water cascading events from the shelf and being deposited deeper in the canyon system. The ephemeral mud layer also indicates rapid deposition and frequent flushing of sediment through the upper canyon. Overall, this study shows that Cap de Creus Canyon is an active conduit of sediment past the shelf break, despite its location distal to the primary sediment source to the GulfWe would like to thank the Office of Naval Research for funding this work (award numbers N00014-04-1-0082, N00014-99-1-0028 and N00014-04-1-0379)Peer reviewe

Sedimentation on the Valencia Continental Shelf: Preliminary results

17 pages, 12 figuresPreliminary analysis of data collected during the course of a cooperative Spanish-United States investigation of the Valencia Shelf (western Mediterranean) reveals a storm-dominated, mud-accumulating sedimentary regime. Calcareous mud is accumulating seaward of a narrow band of shoreface sand and gravel. On the outer shelf the mud is enriched by a pelagic calcareous component. Preliminary210Pb data from vertical profiles of box cores yield nominal accumulation rates from 2.6 mm y−1 near the Ebro Delta to 1.3 mm y−1 on the southern portion of the Valencia Shelf. Storm-current winnowing has resulted in the development of a biogenic lag sand over the mid-shelf mud in the northern part of the study area. Piston cores reveal a basal Holocene sand and gravel facies similar to that presently seen on the inner shelf. Upward-fining sequences on the central and outer shelf are inferred to result from the landward shift of lithotopes during the course of the Holocene transgression. These sequences are locally repeated, perhaps as the consequence of brief, local interludes of coastal progradation. Application of a diagnostic circulation model suggests that intense, downwelling coastal flows occur during winter northeastern storms. Storm activity has induced erosional shoreface retreat during the course of the Holocene transgression and has generated by this means the basal coarse facies observed in the piston cores. In the central part of the study area seaward of the Albufera Lagoon, the mud blanket thins to a layer several centimeters thick which is draped over a thickened (10 m) basal sand. The basal sand is molded into northwest trending ridges. The data are not sufficient to determine whether these are overstepped barriers, or submarine sand ridges formed by storm flows during the shoreface retreat processPeer reviewe