Abstract

Colluvial deposits in hollows are the predominant source of debris flows in many mountainous areas, and the depositional history of hollows can provide insight into land-slide frequency and long-term hillslope processes. Detailed study of colluvial deposits in 20 hollows in the northern San Francisco Bay area, California, reveals diverse histories of erosion and deposition. Basal radiocarbon ages range from 1 to 29 ka and document deposition of colluvium throughout this period at different sites. Ages from multiple stratigraphic levels confirm that the deposits are cumulative and have thickened through the Holocene. Radiocarbon dates and stratigraphic observations document unconformities in hollows, reflecting incomplete evacuation of colluvium during both Pleistocene and Holocene events and providing evidence for a cycle of alternating accumulation and evacuation of colluvium. This cycle has apparently been affected by major climatic changes, and the common occurrence of basal ages at ca. 9 to 14 ka may record widespread slope instability during the Pleistocene-Holocene transition. By analogy with modern landsliding, the increased erosion may have been caused by an increased frequency of high-intensity storms during extended periods of meridional flow in the upper atmosphere. The accelerated discharge of colluvium from hillslopes may have contributed to stream aggradation in diverse parts of California.

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