Elemental mass redistribution and compositional linear modeling: evidence for polygenetic development of regolith, southern California, U.S.A.

Mario Bermudez

Mario Bermudez
MS Candidate
Advisor: Dr. Gary Girty

Friday, January 17, 2020
CSL 422 – 10am

Published in 1997, an idealized model for the development of regolith from granitic parental material consists of in descending order the following four weathering zones: I, II, III, and IV.  Weathering intensity is modeled to decrease with depth, with leaching of elemental mass occurring primarily in zone I while accumulation of leached mass occurs in zones II and III.  Zone IV lies above the weathering front, and is the most weakly weathered material.  In order to evaluate the validity of such a model for a granodiorite-tonalite source in a Mediterranean (hot summer) microclimate, we collected samples from an ~6 m thick section of regolith for petrological, clay mineralogy, and geochemical study.  Above the unweathered granodiorite-tonalite basement is saprock.  Capping the saprock is a Mollisol with well developed C1 and A horizons.  Point-count data indicate that the C1 horizon is more intensely weathered than is the saprock and A horizon.  XRD data derived from the < 2 mm size fraction show that the predominate clay mineral throughout the section of regolith is kaolinite.  Compositional linear modeling and mass balance calculations reveal that statistically significant losses of Ca, Na, Mg, Mn, and P mass are focused primarily within the C1 horizon.  In contrast, within the overlying A horizon, only statistically significant losses of Fe, Mg, and Mn mass are evident.  In saprock underlying the C1 horizon, other than an increase in LOI mass, no other statistically significant changes in elemental mass are evident.  The greater intensity of weathering within the C1 horizon is at variance with the expected downward decrease in weathering intensity, a key aspect of the idealized model noted above, but can be explained as the result of the A horizon being developed within relatively fresh colluvial material spread over an already weathered section of regolith consisting of saprock and the overlying C1 horizon.  Hence, the section of studied regolith is polygenetic.