Abstract: Plate-Tectonic Evolution of Sacramento Valley, California

William R. Dickinson, Raymond V. Ingersoll

The Sacramento Valley is the vestige of a late Mesozoic fore-arc basin formed between the Franciscan subduction zone and Sierran magmatic arc. Late Jurassic inception of Coast Range subduction followed arc-continent collision along the Sierran foothills suture belt. Voluminous turbidites of the Great Valley sequence accumulated within a fore-arc trough where sediment was trapped behind the accretionary Franciscan subduction complex. In west-side outcrops, dominant facies include Upper Jurassic slope deposits, Lower Cretaceous basin-plain and outer fan deposits, and Upper Cretaceous fan and slope deposits. In subcrop farther east, Upper Cretaceous depositional systems include shelf-slope assemblages and progradational deltaic wedges that reflect shoaling of the trough as t e residual fore-arc basin filled with sediment. Concurrent shoaling of the sediment dam formed by the subduction complex is suggested locally by Lower Cretaceous serpentinite protrusions and Upper Cretaceous shelf facies in the Coast Ranges. Filling of the fore-arc trough during the Cretaceous was accompanied by progressive transgression eastward across eroded igneous and metamorphic rocks of the Sierran arc roots. Such basin widening probably reflected regional isostatic downwarp induced by sedimentary loading of the ophiolitic floor of the residual fore-arc basin, as well as eastward migration of the Sierran magmatic front. By Paleogene time, a fore-arc shelf extended fully across the arc-trench gap from the shoreline of a fluviodeltaic lowland along the Sierran foothills to the trench slope break midway across the present Coast Ranges. The shelf was cut by large polyphase submarine canyons ("gorges"), which guided turbidity currents toward a remnant deep in the delta depocenter but later filled with sediment. Miocene and younger valleys and sediments were terrestrial. Neogene migration of the Mendocino triple junction triggered uplift and wrench deformation of the subduction complex as well as final bulk tilting and erosion of the west flank of the fore-arc basin.

AAPG Search and Discovery Article #90963©1978 AAPG/SEG/SEPM Pacific Section Meeting, Sacramento, California