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Neogene-Quaternary Channel Incision on the Northern California Shelf:
Implications for Pronounced Shifts in Highstand vs. Lowstand Drainage in
the Southern Eel River Basin Robert L Burger 1 ((512) 232-3254; rburger@mail.utexas.edu) Craig S Fulthorpe 2 ((512) 471-0459; craig@utig.ig.utexas.edu) Jamie A Austin 2 ((512) 471-0450; jamie@utig.ig.utexas.edu) Gregory S Mountain 3 ((914) 365-8540; mountain@ldeo.columbia.edu) 1University of Texas-Austin, Department of Geological Sciences, C1140, Austin, TX 78712-1101, United States 2University of Texas-Austin, Institute for Geophysics 4412 Spicewood Springs Rd. Building 600, Austin, TX 78759-8500, United States 3Lamont-Doherty Earth Observatory, Columbia University P.O. Box 1000 61 Route 9W, Palisades, NY 10964-1000, United States The onshore-offshore Eel River Basin in Northern California is currently characterized by high sediment input to the continental shelf. This sedimentation is highly episodic, the majority occurring during large winter storm events which both increase the suspended load of the Eel River and resuspend sediment already on the shelf. Wind and currents have a general northerly orientation during initial portions of the storms, resulting in northerly sediment transport and deposition during periods of maximum sediment input. Studies of recent storm deposits indicate their location to be entirely north of the Eel River mouth, with a north-south orientation. As a result of the relatively narrow shelf width (15-30 km), much of the sediment is able to bypass the shelf, and recent studies indicate that only 20-25\% of the total modern sediment input resides on the shelf. Thus, the most important sediment transport pathways in the offshore Eel River Basin are to the north and seaward (west). Little information is available about current and sediment transport directions in the paleo-Eel River Basin. However, paleocurrent directions in deep marine through nonmarine strata exposed along the Eel River Syncline onshore consistently indicate west to northwest transport. In contrast to these observations, high-frequency multichannel seismic (MCS) data collected in the Eel River Basin in 1996 suggest periods of shelf exposure and dominantly south and southwest transport during sea level lowstands, as indicated by incised channels in the southern portion of the offshore basin. Incisions are discernable on at least eight surfaces, indicating multiple periods of relative sea level fall. These channels deepen to the south and southwest, and appear to discharge into the Eel Canyon, located just south of the study area. This repeated convergence of channels during different incision events indicates that the Eel Canyon has existed through multiple cycles of relative sea-level change. In contrast, each set of shelf channels is incised and filled in a single sea level cycle. Based on estimated uplift rates in the basin and magnitudes of glacioeustatic sea level falls, exposure of some or all of the shelf during lowstand periods is expected, and therefore the channels are likely to have been formed at least in part by fluvial processes. However, the depth of incision in the southernmost portion of the seismic grid (up to $\sim$250 ms in two-way travel time) also suggests that at least parts of the channel systems are products of submarine erosion landward of the head of the Eel Canyon. Although the full lateral extent of the channel systems has not yet been mapped seismically, nor has their fill been sampled, these lowstand incision events clearly reflect a pronounced shift in transport orientation in this basin in response to relative sea level fluctuations. |
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Last Modified: October 8, 1999
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