THE CHALK MOUNTAIN LANDSLIDE COMPLEX: THE ASSOCIATION OF GEOLOGIC STRUCTURE AND DEEP-SEATED LANDSLIDING

The Chalk Mountain landslide complex is an approximately 260 hectare, dormant-mature, deep-seated, translational/rotational rock slide complex. The main scarp of the rock slide is located at the ridgetop and is defined by a 40-meter wide ridge-parallel depression. The ridgetop depression is a characteristic morphology of sackung. The Van Duzen River forms a deeply incised inner gorge at the toe of the landslide complex. Upstream and downstream of the inner gorge, the Van Duzen River meanders through broad alluvial valleys. Uplift along an unmapped splay of the Little Salmon fault and/or movement on the landslide complex has clearly reduced the width of the Van Duzen River along this reach.

Based on our recent mapping, the Chalk Mountain landslide complex is potentially in the hangingwall of an unmapped splay of the active Little Salmon fault. We tentatively call the unmapped splay the Root Creek fault. The fault overthrusts early to middle Tertiary Yager terrane over the late Tertiary to Quaternary Wildcat Group.

The landslide is within argillite, sandstone, and conglomerate of the Yager terrane, a tectonostratigraphic terrane of the Franciscan Complex, Coastal belt mélange. The Yager terrane is locally highly deformed by faulting and folding but generally contains coherent lithologic units.

A regional geologic contact and active faulting provide extensive optimally oriented discontinuities and fracture systems that allow deep rock sliding to occur under large normal forces from overburden. Consistent with proposed hypotheses for sackung development, the landslide initiates on a ridge that is being rapidly exhumed by tectonic uplift and incising rivers. This process rapidly releases buttressing forces from the flanks of the ridge. Episodic strong ground motions from earthquakes and seasonally high rainfall totals provide additional important triggering mechanisms. Our mapping in the dynamic setting of the southern Cascadia subduction zone shows preliminary association of deep-seated landslides with areas of profound geologic discontinuities and fault zones. On-going studies are focused on clarifying the distribution of deep-seated landslides in relation to the underlying geologic structure.