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Mineralogy and Incipient Pedogenesis of Entisols in Anorthosite Terrane of the San Gabriel Mountains, California

Dep. of Soil and Environmental Sciences, Univ. of California, Riverside, CA 92521-0424

*Corresponding author.

ABSTRACT

Entisols on anorthosite and associated mafic dikes in the San Gabriel Mountains were examined to determine the influence of contrasting parent materials on soil mineralogy and incipient pedogenesis. Soils are shallow Xerorthents on chaparral-covered slopes of 50 to 65% gradient. The anorthosite is composed almost entirely of plagioclase and laumontite, a hydrothermally produced Ca-rich zeolite. Hornblende is the major constituent of the mafic rock, while plagioclase and laumontite are much less abundant. Soil properties are strongly influenced by the parent material. Soils on the aphanitic mafic rock have much higher silt contents than those on the coarsegrained anorthosite. Calcium dominates the exchange of soils on both lithologies, but exchangeable Mg values are higher in the mafic soil than in the soil on anorthosite. A soil pH of up to one unit higher in the anorthosite-derived soil is probably the result of the hydrolysis reaction in which the abundant, easily weathered plagioclase and zeolite weather to kaolin. Poorly crystalline kaolinite and halloysite are the dominant clay minerals in the anorthosite-derived soil. The soil developed from mafic rock contains a large proportion of well-crystallized trioctahedral smectite in addition to kaolin. The smectite is inherited from the mafic rock which has been somewhat hydrothermally altered. X-ray diffraction and optical analysis of the parent rock and soil fractions coarser than clay also detected smectite and averted its misinterpretation as pedogenic. Cation exchange capacities (CEC) that are excessively high for the amount and kinds of clay present are apparently caused by CEC contributions of smectite and zeolite in the silt and sand fractions. Slight weathering of hornblende is responsible for the citrate-bicarbonate-dithionite-extractable Fe, which is generally low, but is highest in the mafic soil and increases in A and AC horizons. The A and AC horizons of all soils exist in a quasi-stable colluvial mantle and have mineral components from both lithologic sources, whereas the mineralogy of C and Cr horizons reflect only the underlying rock. Denudation rates on the steep, chaparral-covered slopes indicate that no part of the soils, including the Cr horizons, can be older than 400 yr. Pedogenesis is inhibited by high erosion rates on this steep terrain.

Received for publication August 30, 1987.

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