Bedrock Weathering and Residual Soil Formation in Central Virginia

Abstract

The diversity and complexity of the geology in central Virginia are accompanied by an equally diverse and complex mantle of residual soils which characteristically show strong profile development. It was suspected that these soils would also show significant genetic relationships to the underlying bedrock. In order to substantiate this hypothesis three distinctly different profiles were chosen for textural, mineralogical, and geochemical investigations. The soils, in pedological terms, are from: (1) the Frederick series developed over the Chepultepec limestone; (2) the Iredell series developed over an unnamed hornblende metagabbro; and (3) the Lloyd series developed over the Robertson River granitic gneiss. These residual soils, developed in a temperate nonglaciated area, are quite old in a time sense, and normally show classical A, B, C, and D profile development. The studies reported here indicate that where factors, such as climate, slope, and organisms are not radically different, the bedrock plays a very important role in determining the characteristic properties of the overlying soil profile. Texturally, the limestone-derived Frederick profile is the most uniform and is very fine grained, containing approximately 40 percent clay-sized materials. The Iredell profile is relatively shallow at less than 6 ft in depth, but displays a well-developed eluvial or A horizon and a very fine, clay-rich illuvial or B horizon overlying a sandy gabbroic saprolite. The Lloyd soil, like the Iredell, exhibits a significant grain size variation down the profile, but surprisingly has no A horizon. This absence of an A horizon is attributed to improper farming methods existing over much of the Virginia Piedmont during past decades. The clay fraction of each soil contains significant amounts of quartz as well as several species of clay minerals. Both the Frederick and the Lloyd series are characterized by a single major clay species, but the Iredell mineralogy shows significant vertical changes. Illite is the major clay mineral in the Frederick series; kaolinite, in the Lloyd series; and vermiculite, illite, and montmorillonite characterized different genetic layers of the Iredell series. Variations in major element geochemistry are also reflected in the various horizons. Somewhat surprising were the relative mobilities of iron (high) and magnesium (low) in the C horizon of the Iredell soil. Weathering Potential Index values also reflect (a) the homogeneous nature of the Frederick soil, and (b) the progressive increase in weathering and soil stability up the Iredell and Lloyd profiles. The W.P.I, values for the Iredell soil show that the saprolite developed in the C horizon is chemically similar to the unaltered bedrock. The W.P.I, values of the Lloyd soil indicate a high level of maturity and stability approaching laterite in character. It is concluded that significant genetic relationships do exist between the residual soils and underlying bedrock in each of the three cases studied, and these relationships exist in a rational and predictable manner.