BIDWELL LANDSLIDE: INTERPLAY OF LANDSLIDING WITH ACTIVE FAULTING, GLACIATION, LATE-PLEISTOCENE PLUVIAL LAKES AND HYDROTHERMALLY-ALTERED VOLCANIC ROCKS

Bidwell landslide covers over 29 km² and lies in the Warner Mountains of northeastern California on the northeast extension of the active Surprise Valley fault along the western margin of the Basin & Range geologic province. This massive landslide involves Tertiary volcanic rocks and represents the catastrophic collapse of the southeastern flank of Mount Bidwell (elevation 2,511 m) at the northern end of Surprise Valley graben. Bidwell landslide created many small closed basins, containing seasonal and perennial lakes, including Lake Annie (elevation 1,484 m). Reconnaissance-level study was conducted to develop interpretive brochures of this feature on the Modoc National Forest to enhance public enjoyment of this spectacular landscape.

Several factors likely contributed to the failure. Mount Bidwell is capped by competent basaltic and andesitic flows. Underlying tuffs (welded to non-welded) are hydrothermally altered in some areas to form layers of weak, slide-prone rock. This stratigraphy is similar to the well-studied Slumgullion landslide in southwestern Colorado. Presence of unconsolidated glacial till, presently mixed with landslide debris, suggests these unstable deposits might have had a contributing role. Although lacking evidence of timing, the spatial proximity of the Surprise Valley fault suggests the possibility of coseismic failure. Four or five major seismic events have occurred during the last 18,000 years, generating earthquakes with estimated moment magnitudes of 6.8 to 7.3. Inferred chronology of events and evidence is as follows: (1) late Pleistocene glaciation (~23 Ka to 17 Ka) – well-developed glacial features in mountains to the west and till within landslide debris, (2) Bidwell landslide event, (3) late Pleistocene Lake Surprise high stand (~ 17 Ka) – lake shorelines in northern Surprise Valley, (4) further landslide movement within Bidwell landslide – fresh landforms. No glacial features imposed on Bidwell landslide topography imply failure was post-glacial. Lake Surprise shoreline cuts and reworks the toe of Bidwell landslide implying that the landslide event was pre-lake high stand. Slightly post-glacial timing of landsliding is consistent with history of major landslides at Mount Lassen, northern California. Lake sediment cores are needed to confirm this chronology.