GEOLOGIC HAZARDS IN THE EARLY TERTIARY SEDIMENTS OF THE SIERRA NEVADA FOOTHILLS OF CALIFORNIA

In 2010 the California Geological Survey issued a Geologic Hazard Notice to warn engineering and building departments in affected cities and counties over concern of problematic smectitic clay fluvial sediments occurring in the Sierra Nevada foothills and adjacent areas of the Sacramento and San Joaquin Valleys. The Geohazard Notice was warranted based on preliminary data and on-going investigations that show the potential for serious engineering problems associated with this fluvial unit including landsliding and expansive soil behavior that leads to severe foundation distress.

The geologic unit in question has been variously mapped in the Sierra Nevada as the upper part of the legendary “Early Tertiary auriferous gravels,” while in the Sierra foothills and adjacent lowland valley areas the smectite-rich detrital materials superficially resemble, and are often mapped as the quartzose-kaolinitic Ione Formation sediments. Throughout the region, it has been mapped as the Valley Springs Formation where coherent rhyolitic tuffs are interbedded, yet recent age dates show most tuffs in the Sierra to be considerably older than those in Valley Springs type locality. Traditional reliance on the presence of rhyolite to mark the bottom of the Valley Springs has resulted in the mis-location of the contact between it and the underlying Ione Formation which is part of the reason some recent residential developments have been built on highly expansive smectitic clay without appropriate foundation design in the Sacramento region.

Another contributing factor is that despite careful scrutiny by numerous geotechnical professionals, the smectitic sediments are often classified as reasonably stable and suitable for use as engineered soils. ASTM soil tests often classify these sediments as having low or no expansion potential due to the occurrence of the smectite clay in a cemented granular form. The smectite clasts are stabilized largely by ephemeral amorphous silica as well as other pedogenic cements. Thus, the expansive behavior is latent, and significant soil heaving behavior can be delayed for years following its use as engineered soil. Recent investigations show that X-ray diffraction (XRD) and thin section petrography are other analytical techniques that can reliably assess the expansion potential of these sediments.