Investigations of the characteristics, origin, and residence time of the upland residual mantle of the Piedmont of Fairfax County, Virginia

Abstract

Undisturbed cores of upland regolith developed from a variety of crystalline rocks of the Piedmont province in Fairfax County, Va., have been obtained by using a combination of Shelby tubes, Denison sampler, and modified diamond core drilling. The core study correlated variations in chemistry, mineralogy, and texture with engineering properties throughout individual weathering profiles and contrasted these parameters among weathering profiles developed from various parent rocks. Coring sites were chosen to obtain a maximum depth of weathering on diverse lithologies. The rocks that were investigated included metapelite, metagraywacke, granite, diabase, and serpentinite. Four to twelve samples per core were selected for analysis of petrography, texture, clay mineralogy, and major-element chemistry. The number of samples was determined on the basis of (1) the thickness of the weathering profile (from about 1 m in serpentinite to more than 30 m in pelitic schist) and (2) megascopic changes in the weathering profile. Shear strength and compressibility were determined on corresponding segments of core. Standard penetration tests were performed adjacent to coring sites to evaluate in-place engineering properties. The regolith profiles on all rocks can be subdivided into soil, massive subsoil, saprolite, and weathered rock zones. Major differences in thicknesses of these zones are related to parent rock. Total regolith thickness is related to saprolite thickness. Saprolite is thickest on quartzofeldspathic metapelite, metagraywacke, and granite; thinner on diabase; and thinnest on serpentinite. Thickness of saprolite is related to rock structure and mineralogy. Geochemical changes of saprolite developed from each rock type follow predictable trends from fresh rock to soil profile, with increases in Ti, AI, Fe 3 +, and H 2 0+relative to absolute losses of Si, Fe2+, Mg, Ca, and Na. These variations are more pronounced in the weathering profiles above mafic and ultramafic rocks than in those above metagraywacke. Clay minerals in granite, schist, and metagraywacke saprolites are kaolinite, dioctahedral vermiculite, interlayered mica-vermiculite, and minor illite. Gibbsite is developed in near-surface samples of schist. Standard penetration test data for the upper 7 m of saprolite above schist, metagraywacke, and granite suggest alternations between stronger and weaker horizons that correlate with megascopic ally identified zones: soil, massive subsoil, and saprolite. The data correlate with density. Shear strength increases fairly regularly downward in the weathering profile. The engineering behavior of diabase saprolite is controlled by a dense, plastic, near-surface clay layer (montmorillonite and kaolinite) overlying rock that is weathered to a granular state (grus); the engineering properties of serpentinite are controlled by a very thin weathering profile. Similarities in regolith thickness, zonation, mineralogy, and chemistry of quartzofeldspathic rocks indicate the existence of fundamental geochemical and geomechanical controls on regolith evolution on the Piedmont upland. Data from the profiles of quartzofeldspathic regolith are used to construct a model suggesting the principal rate-control steps in the development and downwasting of the upland regolith. This model is consistent with available information about Piedmont hydrology and tectonic uplift.