HR: 16:30h
AN: H54B-03    [Abstracts]
TI: Large wave-shaped bedforms in the axial channel of Monterey Submarine Canyon: Monterey Bay, California
AU: * Paull, C K
EM: paull@mbari.org
AF: MBARI, 7700 Sandholdt Rd, Moss Landing, CA 95039, United States
AU: Normark, W R
EM: wnormark@usgs.gov
AF: USGS, 345 Middlefield Rd, Menlo Park, CA 94025, United States
AU: Ussler, W
EM: methane@mbari.org
AF: MBARI, 7700 Sandholdt Rd, Moss Landing, CA 95039, United States
AU: Caress, D W
EM: caress@mbari.org
AF: MBARI, 7700 Sandholdt Rd, Moss Landing, CA 95039, United States
AU: Keaten, R
EM: rkeaten@mbari.org
AF: MBARI, 7700 Sandholdt Rd, Moss Landing, CA 95039, United States
AU: Barry, J
EM: barry@mbari.org
AF: MBARI, 7700 Sandholdt Rd, Moss Landing, CA 95039, United States
AU: Xu, J
EM: jpx@usgs.gov
AF: USGS, 345 Middlefield Rd, Menlo Park, CA 94025, United States
AU: Smith, D
EM: douglas_smith@csumb.edu
AF: CSUMB, 100 Campus Center, Seaside, CA 93955-8001, United States
AU: Covault, J A
EM: jcovault@stanford.edu
AF: Stanford University, Dept of Geological & Environmental Sciences, Stanford, CA 94305, United States
AU: Maier, K L
EM: kmaier@pangea.stanford.edu
AF: Stanford University, Dept of Geological & Environmental Sciences, Stanford, CA 94305, United States
AB: Multibeam bathymetric data show that large wave-shaped bedforms exist on the seafloor within the axial channel of Monterey Submarine Canyon offshore northern California (Smith et al., 2006). These features have wavelengths up to 70 m, amplitudes up to 2 m, and distinct asymmetrical crests that are roughly perpendicular to the channel. Comparisons of repetitive multibeam surveys since 2004 shows that the bedforms are active features because their positions change between surveys. Three complementary studies are underway to understand the origin of these features: (1) Vibracoring - In June 2007, the ROV Ventana collected 18 vibracores up to 2 m in length along a 130-m transect in ~285 m water depth that spanned the crests of two and the flanks of three waves. Sediment in these cores is composed of one or more sequences of coarse gravel or multicolored clay-clasts that fine upward into sand. Sometimes individual gravel-clasts or clay-chips occur within sand. The internal stratigraphy of these waves shows they resemble classic gravity-flow deposits. (2) Sediment Movement - A pilot study was conducted to assess whether sediment within the canyon floor moves by traction from currents or mass transport. On February 8, 2007, three acoustic beacons were deployed in ~290 m water depth within the canyon axis using Ventana. The beacons were placed within recesses in 50-cm-high ~45 kg poured-concrete monuments. These boulder-sized monuments were buried leaving only the top of the beacon standing ~6 cm above the sediment surface. Thus, the monuments were largely entombed within the seafloor. We also placed 3 acoustic beacons mounted on trapezoidal frames at the edge of a terrace on the canyon's lower flank. On February 12th, we returned to the area and determined that all three monuments had moved ~150 m down canyon. Two trapezoidal frames were found on their sides entwined with each other 50 and 75 m down canyon from their deployment site. The third frame was never located. A sediment trap mooring downstream at 1,300 m water depth independently showed that there was a gravity flow event on February 9th. The locations of the acoustic beacons have been re-determined on 4 subsequent occasions and they have not changed as of June 28, 2007. Because the monuments moved more than one wavelength down-canyon during the February 2007 event, aliasing problems may occur when trying to track the movement of individual wave crests in the repeat mapping surveys. The movement of the buried monuments suggests that the seafloor was remobilized to more than 50-cm-depth during this sediment transport event. (3) Repeat Mapping - Repeat surveys, including AUV multibeam mapping and chirp sub-bottom profiling, are being conducted 3-4 times per year. These surveys show that the wave-shaped bedforms occur in the canyon axis down to at least 1,100 m water depth. No internal structures are seen in the chirp data. Tracking the movement of these waves is proving difficult because it depends on identifying individual waves in successive surveys. Given that initially buried boulder-sized monuments moved more than two wavelengths down-canyon during an individual event limits our confidence in the identification and tracking of individual waves. Apparently, these wave-shaped bedforms are produced during brief, discrete mass transport and/or gravity flow events.
DE: 3002 Continental shelf and slope processes (4219)
DE: 3022 Marine sediments: processes and transport
DE: 3045 Seafloor morphology, geology, and geophysics
DE: 4219 Continental shelf and slope processes (3002)
DE: 4558 Sediment transport (1862)
SC: Hydrology [H]
MN: 2007 Fall Meeting