FINDINGS FROM FAULT HAZARD INVESTIGATIONS ALONG THE MAACAMA FAULT FOR THE PROPOSED WILLITS HOSPITAL, WILLITS, CALIFORNIA

Fault hazard investigations for the proposed Willits hospital and adjacent facilities required a detailed trench-based study of the Maacama fault in southern Little Lake valley near Willits, California. This effort was complicated by California law, which requires interpretation of the presence or absence of faulting through the Holocene; preliminary radiocarbon dating suggested the Holocene/Pleistocene boundary is present at depths of up to 8 m beneath the site. These conditions necessitated excavation of a series of large pits to reach depths in excess of 10 m. These excavations revealed an approximately 8 m thick section of Holocene age alluvium overlying latest Pleistocene age fine-grained sediments. The fine-grained sediments consist of laterally continuous, tabular beds of predominantly clayey silt that suggest Little Lake valley was occupied by a low energy lake/shallow pond environment. Several trees were encountered in an upright, growth position near the upper part of the fine-grained section, suggesting a forest developed near the end of the lake phase. Calibrated radiocarbon age dates from these trees and charcoal samples range from 11,350 to 22,870 years. A significant depositional transition occurred at the Pleistocene-Holocene boundary within Little Lake valley, characterized by development of a higher energy fluvial system (Haehl Creek) that deposited the 8 m veneer of alluvium. The forest present in the upper lake sediments apparently died off during this transition, and the trees were truncated and buried by the onset of higher energy alluviation. Radiocarbon dates derived from charcoal samples taken from shallow alluvium range from mid-Holocene to as young as 700 years B.P. In the latest Holocene, Haehl Creek has incised through the entire Holocene alluvial section. The cause of this incision is not currently known. Active faulting associated with the main Maacama fault trace crossing the site consists of a 1-2 m wide zone of faulting and aseismic creep, with at least one approximately 15 m wide left step-over. Pleistocene lacustrine sediments are locally warped upward along the northeast side of the fault, reaching the ground surface in several places.