Origin of conglomerate stratigraphy in the Franciscan assemblage and Great Valley sequence, northern California

In the northern California Coast Ranges, the forearc-basin Great Valley sequence of Late Jurassic and Cretaceous age contains three distinctive types of conglomerate: (1) chert-rich conglomerate of Tithonian to Valanginian age, (2) volcanic-rich conglomerate of mainly Valanginian age, and (3) mixed-clast conglomerate of Valanginian and younger age. All three types are found in stratigraphic sequence along the west side of the Sacramento Valley, but in three belts of outliers that overlie the Franciscan assemblage to the west, the oldest conglomerate present in each belt is progressively younger southwestward. This may reflect the widening of the forearc basin over time as progressively younger conglomerate types lapped onto the elevated western margin of the basin. The same three conglomerate types occur in coeval rocks of the accretionarywedge Franciscan assemblage and show a similar southwestward progression in composition, possibly due to trench deposition of conglomerate with the same sources as the Great Valley sequence, each conglomerate type being accreted successively to the west side of the accretionary wedge. The Franciscan must have underthrust the forearc basin, become deeply buried, been metamorphosed, and returned to the surface without major disruption of the distribution of conglomerate types.