Effects of triple junction migration and glacioeustatic cyclicity on evolution of upper slope morphologies, offshore Eel River Basin, northern California

Abstract The upper continental slope of the Eel River Basin is affected both by uplift and seismicity associated with the northward-migrating Mendocino Triple Junction (MTJ) and by glacioeustatic fluctuations. As a result, seismically imaged slope sequences vary dramatically along strike. The southern slope is dominated by the Humboldt Slide, a stack of nine deformed sequences. Older, undeformed sequences thicken landward, while slide sequences maintain a constant travel-time thickness. Onset of deformation was abrupt. Within the slide, reflector amplitudes alternate: high-amplitude reflectors are wavy to shingled, and suggest increasing deformation with depth. Low-amplitude reflectors are sub-parallel to wavy throughout. High- and low-amplitude sequence ‘couplets’ suggest repetitive deposition of contrasting lithologies, a response we ascribe to late Pleistocene glacioeustatic cyclicity. If each ‘couplet’ represents a ∼100 kyr late Pleistocene sea-level cycle, then Humboldt Slide deformation began ∼450 ka. This is approximately coeval with onset of deformation on the adjacent shelf ∼500 ka, previously attributed to uplift and seismicity associated with northward encroachment of the MTJ. We believe migration of the MTJ triggered the slide, and is the cause for continuing deformation today, by sediment creep along internal glide planes. North of the Humboldt Slide, we interpret an upper slope anticline as a seaward extension of the Little Salmon Fault Zone (LSFZ) previously identified landward. Both faulting and folding extend to the seafloor, indicating that deformation continues. An adjacent buried anticline suggests that deformation has also jumped northward within the LSFZ on some parts of the upper slope. North of the LSFZ, the slope is dominated by downslope-trending channels and gullies. These features are v-shaped, vertically stacked, and increase in depth downslope. They are mostly infilled; their physiographic expression is generally subdued by sediment draping. Channel fills generally consist of basal high-amplitude reflectors, overlain by reflectors of lower amplitudes. Several large channels on older surfaces are concentrated near the northern end of the seismic coverage; smaller channels are more evenly distributed along the margin on younger surfaces. Channels also extend increasingly landward on progressively younger surfaces, indicating migration in that direction with time. These incisions are clearly inactive during highstands, as at Present; slope deposition today is dominated by draping of hemipelagic sediments. Channels form and migrate landward through headward erosion during base-level changes, when both shoreface and fluvial sediment sources are more proximal to the upper slope. The increasing lateral distribution of channels through time supports an increase in shoreface erosion north of the LSFZ. A possible explanation is a known decrease in fluvial sediment input from the north, combined with MTJ-related uplift in the south. Along-strike contrasts in upper-slope sequence morphology in the southern Eel River Basin are clearly a function of both regional tectonism and proximity to sediment sources, influenced by base-level changes. Humboldt Slide sequences, while deformed, are continuously influenced by systematic lithological variations occurring as a result of glacioeustatic cycles. Effects of base-level changes similarly control the distribution and fill of incisions on the northern slope. All of these observations reinforce previous seismic investigations of the offshore Eel River Basin shelf, and confirm that the competing effects of tectonism and glacioeustacy on the preservation of continental margin stratigraphy can be differentiated.