SOIL GENESIS ON FELSIC ROCKS IN THE ST. FRANCOIS MOUNTAINS. I. THE ROLE OF SAPROLITE AND ITS INFLUENCE ON SOIL PROPERTIES.

One soil profile from Taum Sauk Mountain and two soil profiles from Mudlick Mountain in the St. Francois Mountains of Southeast Missouri were investigated to determine if the soils evolved from the underlying rock unit and to characterize the role of saprolite on soil genesis. Taum Sauk Mountain is composed of rhyolite lava and ash flow of the Van East Group. The rock is quartz and orthoclase phenocrysts in an orthoclase groundmass. Mudlick Mountain consists of rock composed of orthoclase, oligoclase, and a minor amount of quartz phenocrysts in a groundmass of quartz and orthoclase. The shallow Aquic Hapludult formed on the summit of Taum Sauk Mountain shows no evidence of saprolite, whereas the two Typic Paleudalfs formed on Mudlick Mountain rest on a thick layer of saprolite. The presence of minerals in the host rock that weather more easily is largely responsible for the development of the saprolite on Mudlick Mountain. The very strongly acid Typic Paleudalfs have textures ranging from cobbly silt loam to cobbly sandy loam for the eluvial horizons, whereas the argillic and C horizons are generally fine or clayey-skeletal. The extremely acid Aquic Hapludults have a fine-silty texture. The clay mineralogy is mixed, with hydroxy-Al interlayered vermiculite and hydrous mica prominent in the eluvial horizons and kaolinite dominating the Bt and C horizons. The aluminum hydroxide potentials of soil solutions extracted from selected horizons are saturated or slightly supersaturated with respect to kaolinite. Elemental analysis demonstrates that the soil and saprolite originate from the same parent material and that elemental depletion is important to the formation of the saprolite. Iron oxides represent nearly all of the total Fe, and Fe is correlated highly with clay content. The rare earth elements are more abundant in the soil than in the host rock. The weathered thin loess capping is suggested as the source of the rare earth elements.