Late Pleistocene channel incisions in the southern Eel River Basin, northern California: implications for tectonic vs. eustatic influences on shelf sedimentation patterns

Abstract

High-resolution multichannel seismic (MCS) profiles from the southern offshore Eel River Basin off northern California show multiple, superimposed buried channels beneath the continental shelf. These channels display consistent southwest orientations. Their dendritic map pattern and low but consistent cross-shelf gradients suggest that they formed by fluvial erosion during periods of shelf exposure associated with relative sea-level lowstands. All of the channels were incised after ∼500ka, the age of the onshore–offshore Hookton Datum. Their frequency of incision exceeds that of known glacioeustatic lowstands since ∼500ka, confirming that some of the surfaces must have formed during relative lowstands initiated by local tectonic uplift.Sedimentation patterns in the Eel River Basin shift dramatically in response to relative sea-level fluctuations. Today, sediments from the Eel River move north and west on the shelf, in response mainly to large winter storm events. In contrast, fluvial incision on the exposed shelf during relative lowstands directed sediments to the southwest, most likely into the head of the Eel Canyon. Shelf channels were probably fed not only by ancestral rivers, but also by local uplifts like the Table Bluff Anticline. Increases in gradient and channel depths at the southern end of the MCS coverage indicate that canyon-forming processes modified fluvial channels as they converge towards embayments near the head of the Eel Canyon. Cyclic recurrence and consistent orientation of these channels demonstrate that the Eel Canyon has been extant through numerous relative sea level cycles, and is perhaps as old as ∼500ka.