Estimating the yield of crushable stone in an alluvial fan deposit by electrical resistivity methods near Stuarts Draft, Virginia

Abstract

A late Cenozoic alluvial fan complex near Stuarts Draft, Virginia, consists of three cobble and gravel units with different degrees of weathering. The youngest unit contains virtually unweathered quartzite clasts; the older two units, also derived from quartzite bedrock, range from being somewhat weathered with moderately competent clasts to very strongly weathered with cobbles that are incompetent and porous. Differences in porosity caused by these contrasts in weathering permit successful use of electrical resistivity sounding and profiling in and around an active alluvium quarry. With these methods we delineated the borders and bottom of the youngest, high resistivity gravel unit. An isopach map of this unit, based on the resistivity interpretations, indicates two paleovalleys where the minable unit is thickest. In the surveyed area, the paleovalley deposits are as much as 10 and 18 m thick and taper to 1–2 m toward the south. The weighted mean thickness of the gravel unit is 3.95 m, and the estimated volume of this unit, based on an isopach map, is about 5 × 10 6 m 3.