CLAY DIAGENESIS AND CHEMICAL WEATHERING: A FIELD EXAMPLE FROM A DEEP SAPROLITE PROFILE, CENTRAL CALIFORNIA

While many workers have examined chemical weathering and petrogenesis in soil profiles, chemical weathering in deep saprolite remains poorly understood. Do clay minerals develop only after strong chemical weathering has taken place, or do they form relatively quickly and undergo distortion as chemical weathering continues? This study sets out to examine the relationships between chemical weathering and clay diagenesis with depth in saprolite.

Recent field efforts at an actively eroding, soil-mantled, upland hillslope in central California yielded an opportunity to excavate a 300 cm. profile of soil and saprolite to examine this relationship. The profile penetrated the scarp of a recent shallow landslide where underlying bedrock is unmetamorphosed granite. Beneath the disturbed soil column, saprolite with relict granitic structure was first encountered at 90 cm. below ground surface (bgs). Although the profile did not reach unweathered bedrock, a nearby corestone provided a sample of local, unweathered granite. Laboratory analyses of saprolite samples collected at 30 cm. depth intervals and unweathered bedrock were completed by x-ray diffraction (XRD), optical petrography, and inductively coupled plasma-optical emissions spectrometry (ICP-OES) trace metals analysis. Initial results indicate clean XRD indications of several mixed-layered clays and several non-clay weathering-resistant minerals at 300 cm. bgs. XRD analysis of shallow samples reveals a complicated pattern of non-clay minerals and incomplete mixed-layered clay diagenesis. Laboratory evidence indicates that saprolite bulk density does not vary significantly with depth in this profile, despite visual evidence of mineral leaching and XRD indications of clay diagenesis throughout the profile. Further results of ongoing laboratory work focusing on understanding the relationship between mineralogical changes with depth and mixed-layered clay development will be presented at this conference.