Pedogenic weathering of tonalite in southern California

Abstract

Tonalite (quartz diorite) in southern California has weathered under aa mediterranean climate to depths of more than 30 m. In weathering, the rock was broken up by expansion of biotite as it weathered to vermiculite. The particle density of the resulting fragments is the same as that of the rock (2.75 g/cm 3). Biotite comprises 14% of the rock and could produce a linear extensibility of 1.8% in such a uniform, medium-grained material if all the biotite weathered to vermiculite. Measured expansion (1–2%) of the fragmented rock agrees with the predicted value. Other minerals, mostly unweathered, in the rock are: andesine, 56%; hornblende, 13%; quartz, 9%; and orthoclase, 8%. Weathering of the fragmented rock to soil was studied in three kinds of environments in four closely associated soils of a toposequence. Differences in the weathering environments are a result of differences in topographic positions of the soils and resultant differences in soil moisture regimes. A slightly leached environment occurs on hilltop and backslope positions. These position are occupied by Ustochrepts and Haploxeralfs. B horizons of these soils have base saturations of less than 90%, lack salts in soil solution, and contain little exchangeable magnesium and sodium. In this environment vermiculite and kaolinite are formed from weathered biotite. No crystalline clay minerals were detected in the other primary minerals. A high sodium environment occurs in footslope positions. Natrixeralfs occupying these positions are slightly leached in upper parts but accumulate some soluble ions, especially sodium, magnesium, and in places carbonates and amorphous silica in lower parts. In this environment biotite weathers to vermiculite and kaolinite, andesine and hornblende to montmorillonite. The third environment is a base-rich environment found in areas of subdued relief and somewhat restricted drainage. The Pelloxererts in this position have significant amounts of soluble salts and large amounts of extractable magnesium and sodium. Minor amounts of secondary carbonates and amorphous silica accumulate below their sola. In the base-rich environment biotite weathers to vermiculite and montmorillonite, andesine and hornblende to a beidellite-montmorillonite. The beidellite-montmorillonite formed from the andesine is dioctahedral. Its structural formula is (Si 7.44Al 0.56)(Al 2.62Mg 0.73Fe 0.61Ti 0.08) O 20 (OH) 4 (M + 0.95,K + 0.05) and it has a C.E.C. of 119 mequiv./100 g. The stability sequence for primary minerals from least to most stable is biotite, andesine, hornblende, quartz.