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Late Cenozoic History of Humboldt Basin, Cape Mendocino Area, California

Patricia A. McCrory

The Humboldt basin is a filled Neogene trench-slope basin within the accretionary complex immediately north of the Mendocino triple junction and adjacent to the underthrusting Juan de Fuca plate. This particular basin provides a unique opportunity for studying the transition from convergent to transform motion because of its proximity to the Mendocino triple junction. Moreover, the basin experienced almost continuous sedimentation throughout the late Neogene, and therefore yields a detailed stratigraphic record of basin deformation due to plate motion.

Basin stratigraphy has recorded deformation of the upper surface of the Juan de Fuca accretionary complex as it was uplifted from lower bathyal to littoral depths during the Neogene. Geohistory analysis of Humboldt basin is based on key Neogene surface sections and detailed micropaleontologic and sedimentologic studies that are used to evaluate age and changing climate, water depth, and depositional environment through time along a trend perpendicular to the paleomargin.

Oldest basin sediments consist mainly of hemipelagic mudstones deposited in a deep-water setting during elevated sea level 14-8 Ma. Turbidite sandstone intervals become common after about 3.5 Ma and can be divided into several discrete megasequence packages. Deposition of these megasequences was apparently influenced by climatically induced lowering of sea level and tectonic uplift of the adjacent coastal area.

Turbidite deposition was followed by slope deposition starting about 2.2 Ma. A stratigraphic section near the eastern margin of the basin contains a thick slope, shelf, beach, fluvial sequence clearly influenced by eustatic sea level events.

Humboldt basin has experienced uplift and deformation due to tectonic underthrusting and outbuilding of the accretionary prism associated with convergence of the Juan de Fuca plate. Backstripping analysis of these sections demonstrates that major tectonic uplift of Humboldt basin strata began during the early Pliocene. Fluctuations in rates of uplift and deformation are attributed to changes in rates of underthrusting and age of the downgoing Juan de Fuca plate.

AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.