SOIL GENESIS AND LANDSLIDE DEVELOPMENT IN THE MONTEREY FORMATION, PUENTE HILLS, CALIFORNIA

Outcrops of the Miocene Monterey Formation in the Puente Hills north of Whittier, California, are associated with a history of mass wasting. Cropping out over a large portion of the southern California coast, the Monterey Formation is composed of siliceous shale, with layers of siltstone, thin sandstone, and hard dolomite. The Monterey formation is a well known petroleum source rock that has produced significant amounts of oil in California. In this study, we conducted research in the Turnbull Canyon area, located in the Puente Hills, within the La Vida Shale Member of the Monterey Formation. Soils were sampled and described along a transect across the canyon with soil pits above, in, and below landslide blocks. Munsell soil color, ped development, and horizonation were also characterized. Effects of aspect and vegetation type were also characterized. Grain size (by hydrometer) and bulk density (by the clod method) of the soils were measured in the lab. Preliminary data suggest that soils on the northern side of the canyon are generally weakly and thinly developed with poor soil structure, contributing to the concentration of landslides in that part of the canyon. Another contributing factor to the landslides in the study area of the canyon is the extensive intervals of folding and faulting within the Monterey Formation. By describing and measuring soil profiles and studying soil properties, this project investigates the role soil has played in the history of mass wasting in the area.