Evolution of a trench‐slope basin within the Cascadia subduction margin: the Neogene Humboldt Basin, California

ABSTRACT The Neogene Humboldt (Eel River) Basin is located along the north‐eastern margin of the Pacific Ocean within the Cascadia subduction zone. This sedimentary basin originated near the base of the accretionary prism in post‐Eocene time. Subduction processes since that time have elevated strata in the south‐eastern portion of the basin above sea level. High‐resolution chronostratigraphic data from the onshore portion of the Humboldt Basin enable correlation of time‐equivalent lithofacies across the palaeomargin, reconstruction of slope‐basin evolution, and preliminary delineation of climatic and tectonic influence on lithological variation. Emergent basin fill is divided into five lithofacies which clearly document shoaling of the inner trench slope from deep‐water environments in early Miocene time to paralic environments in Pleistocene time. The oldest strata consist of hemipelagic mudstones and minor debris‐flow breccias deposited in a deep‐water setting during elevated sea level. These strata are overlain by glauconite‐rich, fine‐grained turbidites which heralded an increasing influx of terrigenous detritus. Water depths shoaled earlier in the eastern basin area as the palaeoshoreline prograded seaward. Turbidite deposition ceased in the eastern basin area at about 2‐2 Ma, whereas 22 km to the west, turbidite deposition continued until about 1‐8 Ma. Lithofacies at the western study site change abruptly across a middle Pleistocene unconformity from outer shelf to paralic deposits. In the east, a more complete Pleistocene section records transition from outer to inner shelf, beach and fluvial environments. The Humboldt Basin lithofacies sequence is overprinted by eustatic control of sediment source. Comparison of sediment character with palaeoceanographic conditions indicates dominance of hemipelagic facies during periods of elevated sea level in the middle Miocene and early Pliocene when depocentres were isolated from terrigenous sediment. Glauconite‐rich facies were mobilized from an upper slope setting following these periods of elevated sea level and redeposited in a deep‐marine environment. Pleistocene shoreline lithofacies display glacio‐esutatic control of depositional environment by recording several cycles of nearshore to fluvial progressions. General models of accretionary prism behaviour and trench‐slope basin evolution are compatible with the overall coarsening‐upward lithofacies sequence filling the Humboldt Basin. Early structural barriers precluded deposition of terrigenous material except from locally derived debris flows; subsequent shoaling and burial of deactivated thrust‐folds enabled turbidity flows to reach the basin floor. However, late‐stage tectonism apparently controlled the onset of coarse‐grained deposition in this sequence. Significant sand‐rich turbidite deposition began in the middle Pliocene, synchronous with tectonic uplift of the southern basin margin. Conversely, cessation of turbidite deposition in the eastern basin area in latest Pliocene time was synchronous with growth of anticlinal structures which again blocked widespread dispersal of turbidity flows. This middle Pliocene to Holocene period of crustal shortening is synchronous with continued reduction in spreading rate along the southern Juan de Fuca ridge, and probably reflects partial coupling between the subducting lithosphere and the overlying accretionary prism.