Transgressive deposits along the actively deforming Eel River Margin, Northern California

Hogarth, LJ, Driscoll NW, Babcock JM, Orange DL.  2012.  Transgressive deposits along the actively deforming Eel River Margin, Northern California. Marine Geology. 303:99-114.

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accumulation, active deformation, basin, cascadia subduction zone, continental-shelf, Eel River Basin, Little Salmon Fault, Mendocino Triple junction, sea-level, sediment, sequence stratigraphy, submarine geomorphology, tectonics, transgressive deposits, variability


New high-resolution CHIRP seismic data acquired along the Eel River margin, northern California, reveal that stratal architecture and sediment thickness of the Holocene transgressive deposits are, in large part, controlled by tectonic deformation and sediment supply. A thick (>20 m) transgressive deposit is observed across the Eel margin, a forearc basin that is undergoing active folding perpendicular to the coastline at rates of mm/yr. The transgressive deposits on the Eel margin exhibit marked variations in thickness alongshore; being thickest in the Eel River Syncline and thinnest over the Eureka Anticline. The divergent character of the infill in the syncline suggests that deposition is syntectonic. Fault displacement and structural relief observed along the transgressive surface are consistent with deformation rates measured onshore. The transgressive surface is offset similar to 0.5 m across the Eureka Anticline suggesting deformation has been active since similar to 10 ka. Two distinct acoustic units have been identified within the transgressive systems tract: a basal deposit that infills relief on the transgressive surface and an upper onlapping unit. The basal deposit infills lows along the outer shelf with a maximum thickness of 10 m and appears to be controlled by the early sea-level rise (21-7 ka) of the last deglaciation. It is separated from the overlying acoustically well-laminated unit by a pronounced surface of onlap. Moving shoreward along the inner shelf (<60 m water depth) the transgressive sequence thins and becomes acoustically transparent, which suggests that the finer-grained material is bypassing the inner shelf and being sequestered on the middle to outer shelf. It is here, on the inner shelf where tectonically induced accommodation exhibits the greatest control on sediment thickness. Thus, tectonics played a greater role when sea-level rise slowed after 7 ka to rates comparable to or slower than tectonic rates (similar to 3 mm/yr). On the middle to outer shelf offshore of the Eel River, there is evidence for progradation and highstand deposition. (C) 2012 Elsevier B.V. All rights reserved.