SOIL DEVELOPMENT ON A GRANITIC CATENA IN SOUTHEASTERN ARIZONA (WEATHERING).

Abstract

Chemical input-output analyses were used to evaluate the rate of rock weathering and soil development on a granitic inselberg of the Dragoon Mountains in southeastern Arizona. Soil genesis relationships were investigated through field and laboratory study of the soil profile, parent rock, microclimate and vegetation, on different catena positions and hillside aspects. Precipitation and runoff water qualities were determined, with increased summer acidity. Precipitation volume was estimated by extrapolating data from a nearby recording station. Runoff volume was estimated by the U.S.D.A.-S.C.S. Curve Number Method (1972). These data enabled the quantitative determination of cationic solution loss. The indicated decreasing order of cation mobility is calcium, sodium, potassium and magnesium. Cations of higher mobility are relatively depleted in the hillcrest soils, while transported to the footslope or beyond. Calcium, and to a lesser extent sodium, have experienced more extensive off-site removal. Differences resulting from the transformation of parent material to soil were analyzed by field morphology, X-ray analysis, laser light-scattering particle size analysis, and chemical analyses. The study site was surveyed, mapped and the spatial arrangement of soil taxa and their compositional variation were studied. Nearly all pedons on the crest or transport slope are Lithic Ustic Torriorthents; whereas footslope soils show greater development, as exemplified by the occurrence of various Haplargids, Haplustolls and Argiustolls. Weathering rates were calculated using a methodology similar to that of F. W. Barth (1961). Potassium and magnesium provided the most reasonable data of 350 and 430 Kg ha⁻¹ yr⁻¹, respectively. (equivalent to 13.5 and 16.5 mm of rock weathered per 1000 years). Two serrated projectile points were found inbedded on the clayey soil surface of an adjacent ancient pond site. These artifacts resemble another projectile point found in southeastern Arizona which has been radiocarbon dated ~7,000 yr B.P. A minimum age for the pond and immediate surrounding topography is suggested to correspond with the end of the western subhumid pluvial period. The assessment of the geochemical budget provided a valuable framework for quantification of the various processes which interactively determine the rates of weathering and soil formation.