The Nature of Early Tertiary Soils and Sediments — Mineralogy and Petrology

This is a Power Point presentation delivered at the GSA Cordilleran Conference (Session T5: Critical Zone: Where Rock Meets Water and Life at Earth’s Surface) in Fresno CA on May 20, 2013

Summary: This presentation reports the morphology/mineralogy of the Early Tertiary fluvial sediments and their underlying paleosols from which the sediments were eroded. The unique morphologies of the clayey materials transported in the river sediments have important implications to the modern human activities occurring on outcrops of these clayey sediments.  The analytical methods and data used to characterize these sediments includes XRD, optical petrography, microprobe EDS and backscattered electron imagery, fluorescence, and others. The micromorphology and mineralogy of argillaceous material in the source rocks (i.e.., mature chemically weathered soils) is compared with equivalent materials in the overlying fluvial sediments.  A compelling  case can be made that the clay materials in the fluvial sediments were transported largely as granular sand-sized aggregates (clasts) stabilized by pedogenic silica cements.  The silica cements inhibited the clay clasts from dispersal during fluvial transport.  The occurrence of smectite clay in the form of sandy sediments in deposits that are in the path of suburban development has led to severe problems for the construction and geotechnical industries.  The standard ASTM laboratory tests used to characterize the expansion potential of soils for construction fail to detect the massive amounts of highly expansive smectite clay in these sediments.  The test method results classify these smectitic soils as granular with little dispersive clay that has expansive potential.  Later, after the smectitic sediments are placed as engineered soil for building construction, the silica cements incrementally dissolve which allows the smectite clay to become reactive (latent expansion) and cause severe soil heaving leading to significant and costly structural damage.

As a consequence, the California Geological Survey has issued a Geologic Hazard Notice for Smectite Sediments.  Another of the GSA presentations posted here addresses the engineering issues in more detail.


Power Point presentation (pdf)view here

Slide notes (pdf)…view here