THE SUBSURFACE COMPLEXITY OF ALLUVIUM-BUTTRESSED LANDSLIDES AT KNIGHTS VALLEY, CALIFORNIA

Knights Valley is an alluvium-filled basin located in Sonoma County, north of San Francisco Bay. The hillsides surrounding Knights Valley contain numerous large landslides ranging in surface area up to 3.8 sq. km. At the southern margin of Knights Valley, a large (0.8 sq. km.) landslide complex is apparent from interpretation of aerial imagery. This landslide complex displays a pattern of deep-seated failure high on the slope followed by secondary deep-seated failure within the lower portion of the upper landslides, producing a stepped topography with landslide unit surfaces both at higher elevations and intermediate elevations. The toes of the landslides appear to extend into the alluvial valley.

Borings drilled through one of the upper landslides revealed a relatively coherent block of displaced tuff and clastic sedimentary rock (derived from Pliocene volcanics and non-marine strata) overlying a basal shear of weak, highly plastic clay. In the downslope reaches of this upper landslide block, the basal shear overlies a matrix-supported deposit of dark brown silty clay with angular clasts of tuff. This deposit likely accumulated on a paleo-slope from debris flows or earth flows that occurred prior to emplacement of the higher deep-seated landslide. The upper landslides apparently failed during late Pleistocene time prior to deposition of most of the alluvial sediment that currently fills Knights Valley. With alluvial aggradation, over 30 meters of alluvial sediment filled the valley, buttressing the upper landslides.

During the aggradational phase, lateral migration of the nearby trunk stream may have undercut the slope, leading to failure of the lower landslides into the valley. Borings drilled through one of the lower landslides revealed displaced tuff and clastic sedimentary rock overlying Quaternary alluvial sediments that consist of rounded gravel and pebbly sand, locally fining upward to silty clay. The final stages of aggradation deposited an approximately 3-meter thick buttress of alluvial sediment at the toe of the lower landslide.

The deep-seated landslides are locally mantled with matrix-supported slope debris containing angular blocks of tuff, apparently deposited by debris flows or earth flows that failed high on the slope and appear to post-date the deep-seated landslides.