LANDSLIDE HAZARDS ALONG THE INTERSTATE 80 CORRIDOR ASSOCIATED WITH THE EARLY OLIGOCENE SEDIMENTARY AND VOLCANIC DEPOSITS IN THE SIERRA NEVADA

Prehistoric to active slope failure near Alta, California threatens important trans-Sierran transportation infrastructure. Eocene to Miocene sedimentary and volcanic deposits unconformably overlie Paleozoic and Mesozoic accreted basement terrains in the highlands between the Bear River and North Fork of the American River. Regional Tertiary units, best illustrated at Chalk Bluff, Nevada County, include the Eocene Ione Fm (auriferous) gravels, Oligocene rhyolitic ash-flow tuffs and interbedded smectitic sediments, and Middle to Late Miocene Mehrten Fm intermediate-composition lahars and alluvial deposits. Tuffs near I-80 were mapped as Valley Springs Fm (VS), but ⁴⁰Ar/³⁹Ar dating and stratigraphic correlations place them between 31.5 and 25.4 Ma, older than the 23 Ma KAr dates at the VS type section (Dalrymple, 1964). All Tertiary units were derived from easterly sources and accumulated in paleochannels. The Ione and smectitic sediments with interbedded tuffs were derived from upland and caldera sources in west and central Nevada, and the Mehrten from ancestral Cascade volcanoes in eastern California and western Nevada.

Widespread slope failure and foundation settling/heaving are associated with smectite clay in the Oligocene section and have occurred through a wide area of the Sierran foothills and adjacent lowlands in the Sacramento Valley. The California Geological Survey issued a Geologic Hazard Notice in 2010 warning of geohazards associated with the smectitic unit.

Slope failure in the smectitic sedimentary section is exacerbated at some locations by interbedded primary and reworked rhyolitic tuff beds. The enveloping clay causes the tuff beds to become confined aquifers. During wet periods the tuff beds with inherent microporosity fill with water resulting in excessive pore pressure. The upper surfaces of the tuff beds at the clay bed contacts become preferred landslide planes.

Preliminary landslide reconnaissance mapping shows that the I-80 roadway, UP railway, and a petroleum pipeline all traverse extensive areas of prehistoric landslide terrain. All transportation lines have suffered repeated historic failures. Better mapping and analysis of hazardous slope failure zones will help in preventative efforts along transportation routes to minimize future economic loss.