Silicification of Holocene Soils in Northern Monitor Valley, Nevada
1989
In Monitor Valley, Nevada, two Entisols and an Aridisol containing volcanic glass have silica cementation that is first manifest as microagglomerates and later as macroscopic (>2-mm) durinodes and weakly cemented s-matrix. The initial effect of silicification is cementation of silt and clay by opaline SiO₂, producing silt- and sand-size microagglomerates that are one source of large −1.5 MPa H₂O/clay values for the <2-mm soil fraction. In the illuvial zone, microagglomerates are nuclei for further silica cementation and eventual durinode formation. Volcanic glass is a major source of silica for cementation; the 0.05- to 0.10-mm sand fraction is commonly composed of >40% volcanic glass, of which 40 to 80% is partly weathered; greatest proportions of weathered glass occur at about the same depths as maximum pH. Laboratory measurements of water-soluble silica and 0.5 M NaOH extractable silica, as well as the −1.5 MPa H₂O/clay ratio, provide indications of incipient silicification. These values are relatively large throughout the sola but reach maxima before decreasing with increasing depth. This pattern indicates that glass weathering and relatively short-distance silica illuviation influence these recent soils.
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