HR: 0800h
AN: OS21A-1521    [Abstracts]
TI: Late Holocene Turbidity Currents in Monterey Canyon and Fan Channel: Implications for Interpreting Active Margin Turbidite Records
AU: * Johnson, J E
EM: jjohnson@mbari.org
AF: Monterey Bay Aquarium Research Institute, 7700 Sandholdt Rd., Moss Landing, CA 95039
AU: * Johnson, J E
EM: jjohnson@mbari.org
AF: University of New Hampshire, Department of Earth Sciences 56 College Rd. James Hall 121, Durham, NH 03824
AU: Paull, C K
EM: paull@mbari.org
AF: Monterey Bay Aquarium Research Institute, 7700 Sandholdt Rd., Moss Landing, CA 95039
AU: Normark, W
EM: wnormark@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., MS-999, Menlo Park, CA 94025
AU: Ussler, W
EM: methane@mbari.org
AF: Monterey Bay Aquarium Research Institute, 7700 Sandholdt Rd., Moss Landing, CA 95039
AB: Over the last several years, high-resolution multibeam bathymetry, ROV-guided observations and sampling, and data from canyon monitoring activities (measurements of near-seafloor currents, turbidity, pressure, temperature, and salinity) have been collected by MBARI and USGS scientists throughout the Monterey canyon-fan channel system. The upper canyon has been quite active in recent time; gravel and coarse sand, with a composition similar to the sand found on local beaches, accumulate near the canyon head and regularly move down into the canyon as episodic gravity flow events (Johnson et al., 2001; Paull et al., 2003; Xu, et al., 2004; Paull et al., 2005). Based on the known timing of these events and comparison with local earthquake, storm, and river discharge data, the most common triggers for the observed events are intense storm wave disturbance and/or random canyon wall failures; the recurrence interval for sediment transport events in the upper canyon is sub-annual. However, how frequently these events pass out of the upper canyon and extend across the fan is unknown. We are now trying to determine the characteristics, extent, and timing of the last major event that flushed the entire Monterey canyon-fan system. ROV-collected vibracore transects across the active fan channel (axial depths between 3.4 and 3.9 km) have been obtained. This tool allows the cores to be precisely located and closely spaced, enabling lateral facies interpretation at an unprecedented scale. The axial channel is characterized by laterally variable high energy deposits at least a meter thick, composed of gravel, coarse sand, and rip-up clasts. Deposits on inner bend-levees contain multiple graded sand and silt horizons (turbidite events) up to 15 cm thick, separated by hemipelagic clays. The outer bend is composed of erosionally exposed older strata. Timing of the last event is constrained with DDT_r and ¹⁴C stratigraphies. While DDT_r occurs in the uppermost clay-rich sediments, they are absent in clays that overly the uppermost sand deposit, indicating that the youngest turbidite in these cores was deposited >60 years ago. The youngest radiocarbon age on woody debris from the uppermost turbidite is 155±35 RC years B.P. Conversion to calendar years suggests that this wood grew between 1717-1891 Cal. years A.D. (2σ range). Wood samples found in turbidites represent a maximum age for the event, as the wood grew elsewhere and was transported to this environment. Thus, these age constraints are consistent with the 1906 San Andreas earthquake as a possible trigger for this event. Cores outside the axial channel are being used to constrain the recurrence interval of these full canyon--fan channel flushing events. Downcore gamma density and magnetic susceptibility measurements resolve individual turbidite deposits. Planktonic foraminifera ¹⁴C dates from underlying clays are pending. Currently available radiocarbon ages from some overbank deposits reveal multiple Holocene episodes that probably represent large canyon flushing events. Based on this data, the average recurrence for these large events is ~230 RC years.
DE: 1165 Sedimentary geochronology
DE: 3002 Continental shelf and slope processes (4219)
DE: 3022 Marine sediments: processes and transport
DE: 3080 Submergence instruments: ROV, AUV, submersibles
SC: Ocean Sciences [OS]
MN: Fall Meeting 2005