Clay Mineralogy of Soils Developed from Quaternary Deposits of the Eastern Sierra Nevada, California

Abstract

The types and abundances of clay minerals in soils formed on glacial and glaciofluvial deposits of the eastern Sierra Nevada were studied in relation to age and lithology of the soil parent material, and to variations in present climate and vegetation. The most abundant soil clay minerals are illite and halloysite. Halloysite is usually more abundant than illite in the subhu-mid northern part of the sampling traverse, whereas the converse is true in the semiarid southern part, where the persistence of kaolin minerals is striking. Montmorillonite occurs primarily in soils developed under semiarid climatic conditions from deposits derived partly from mafic or intermediate igneous rocks. The kaolin mineral most commonly associated with montmorillonite is kaolinite. Other locally abundant clay minerals include mixed-layer clays and “vermiculite.” Soil clay mineral assemblages do not vary with age of soil parent material (glacial drift); thus, time or paleoclimatic variations, or both, are not the main controlling factors of clay formation. An important corollary of this conclusion is that factors of soil and clay formation, including climate, may have been relatively constant during middle and late Quaternary weathering episodes in the eastern Sierra Nevada. Either the climate during all the weathering episodes was within the climatic limits for the stability of the observed clay mineral assemblages, or the magnitude or duration of departures from those climatic limits, or both, were too small to leave a clear signature in the soil clay mineralogy. It is concluded that factors other than age are more important in determining soil clay mineral assemblages. The abundance of kaolin minerals relative to that of illite appears to be primarily related to climate and vegetation; the persistence of kaolin minerals at the southern end of the traverse, however, may result from lithologic controls as well as climate and vegetation. The presence or absence of montmorillonite apparently also reflects a combination of lithology, climate, and vegetation.