Mineralogical and weathering study of a red-brown earth formed on granodiorite

Abstract

The development of a red-brown earth has been studied by means of mineral and fabric analyses. Data from particle-size analysis and zircon distribution indicate relative uniformity of the parent material throughout the profile, with a possible minor disconformity near the bottom of the B horizon. Calculations of gains and losses of major minerals during soil formation, based on the zircon content of each soil horizon and of the rock, show a break in the progressive increase in weathering towards the surface, suggesting that the soil is in fact a two-storied profile; micromorphological evidence supports this postulate. The constancy of the qualitative mineral assemblage, however, shows that both sola were formed entirely from the underlying weathered granodiorite. The top four horizons are interpreted as being a separate soil, probably developed from a surface movement layer composed of some material from the upper horizons of the older profile with accessions from weathering granodiorite boulders. Both soils are considered to have been formed dominantly by in situ weathering of primary minerals and removal of the weathering products by leaching; clay illuviation does not appear to have been more than a very minor soil-forming process.