INVESTIGATIONS OF ACTIVELY FAILING COASTAL CLIFFS IN CEMENTED SAND DEPOSITS: WHAT OBSERVATIONS, CORRELATIONS, AND GEOLOGIC MAPPING CAN TELL US

The coastal cliffs located south of San Francisco form one of the more active geologic terrains along the central California coast due in part to their weak lithological composition. The cliffs are composed of variably, but predominantly weakly cemented sands that outcrop at the beach level and reach up to 150 meters in height. Cliff exposures extend for a distance of approximately 10 kilometers and were formed by the tectonic uplift of a near-marine depositional environment. Stratigraphy in the area shows evidence of eolian, alluvial, and marine type deposits.

Numerous studies have examined the reoccurrence of cliff failures along this stretch of coast and have concluded that most failures are linked directly to large storm and tectonic events. However to date, a comprehensive study of the detailed failure mechanisms and their relation to the site-specific lithology has not been performed.

As part of a USGS sponsored investigation on coastal landslide processes, we have undertaken a comprehensive study of a 2.5 kilometer stretch of this coastline. Detailed observations were made on a weekly basis during the 2002-2003 winter season which resulted in a complete data set of the approximate dates and sizes of failures and also identified the predominant failure mechanisms. The collected data has been correlated with mechanism signals such as off-shore buoy wave heights and daily precipitation totals. Preliminary analyses suggest that obtaining threshold values for failure from wave height and daily precipitation signals may be possible.

In addition, geologic mapping over the entire stretch was performed in order to link observed failures with lithological features. Profile maps were generated using a digital mapping system in order to outline predominant stratigraphic boundaries and bedding details. Correlations between the mapped stratigraphy, observed failure events, and potential mechanism signals show that it is possible to identify the failure mechanism for a particular stretch of cliff based predominantly on the site-specific lithology. Along the cliffs forming our study area, a clear dividing line between wave action-induced failures and groundwater seepage-induced failures was observed and related directly to the transition of weakly cemented materials to more well-cemented, paleosol-type materials.