The Coastal Belt of the Franciscan: youngest phase of northern California subduction

Abstract

Summary The Coastal Belt of the Franciscan complex represents the latest Cretaceous to middle Tertiary part of the accretionary prism of an arc-trench system in northern California. On the Mendocino coast, both deformed slope and trench deposits and much less deformed slope basin deposits are recognized. Deformation increases with depth in the slope deposits, towards the contact with the accretionary prism. In the accretionary prism, bands of deformed rocks from tens of metres to kilometres in width suggest major zones of shearing. Stratigraphic tops of beds face eastward, yet palaeontological ages young westward; imbricate stacking is suggested by these relationships. Stratigraphical tops and palaeontological ages indicate that the slope basin deposits are in normal stratigraphic sequence. Facies in the accretionary prism may represent trench fill, abyssal plain, feeder channel, and small fan deposits that were accreted to the leading edge of the continent and may also include slope deposits that were deformed during progressive deformation of the accretionary prism. The less deformed slope deposits may have been deposited as slope basin turbidites, small fans, and channels. South- and south-westward-directed palaeocurrent indicators and sandstone petrology suggest sediment transport from the Klamath Mountains and Oregon across the shelf and down the slope and/or along the trench axis.