THE NATURE OF EARLY TERTIARY SOILS AND SEDIMENTS—MINERALOGY AND PETROLOGY

The mineralogy and petrology of soils and sediments in the Early Tertiary section of the Sierra Nevada foothills of Northern and Central California were influenced by global tropical and subtropical climatic regimens. Kaolinitic soils and sediments formed during the extremely warm/humid tropical climate of the Paleocene to Early Eocene, while smectite clay dominates the later Eocene to Early Oligocene soils and sediments. Micromorphological and micro-chemical investigations of both kaolinitic and smectitic sediments, large volumes of clay-rich materials were transported in these paleo fluvial systems as sand-sized aggregates derived from fluvial erosion of deeply weathered mature soils. The clay microfabric of the Early Tertiary paleosol clasts was stabilized by amorphous silica cement that precipitated in association with the intense chemical weathering in the warm/humid environment of the Early Tertiary. This pedogenic silica protected clay-rich aggregates from dispersion during fluvial transport in Early Tertiary rivers.

Today, the pedogenic silica cement that stabilizes the granular smectitic sediments creates problems for soil engineers that design for construction on the exposed smectitic soils and sediments. The silica-stabilized smectitic clay in soils engineered from Late Eocene/Early Oligocene deposits can not be characterized by conventional ASTM soil test methods. The standard soil engineering tests classify the smectitic sediments as having low expansion potential. Following placement in engineered soils for construction, the paleo silica cement in these clay materials is prone to incremental dissolution allowing the expansive clays to freely interact with cyclic soil moisture conditions. Failure to properly classify these potentially highly expansive soils has contributed to widespread cases structural damage throughout the urbanizing Sierra foothills and adjacent Sacramento Valley areas.