Duripan Horizon and Durinodes - Soil Geography of the USA: A Diagnostic-Horizon Approach - page 144

Fig. 14.4 Genesis of duripans under three contrasting conditions
Although these soils receive minimal eolian inputs (Boettinger and Southard 1991 ),
the duripan is often polygenetic (Eghbal and Southard 1993a ). The latter authors
speculated that the polygenesis of soils was related to climatic fluctuations since the
mid-Pleistocene and to bioturbation of surface horizons. Grassland vegetation plays
an important role by forming opal phytoliths, cycling Si, and possibly influencing
mineral dissolution (Blecker et al. 2006 ).
A second kind of duric soil forms on basalt alluvium, with minor admixtures of
volcanic ash, primarily in the Great Basin (Fig. 14.4 ). The alluvium ranges from
7 to 440 ka, with stage IV silica forming on 440 ka surfaces (Harden et al. 1991 ).
The dominant soil great groups are Argidurids, Durixerolls, and Haplodurids that
have formed under Great Basin desert shrub primarily in NV, but also in OR and
WA. These soils have an aridic soil-moisture regime and a mesic soil-temperature
regime. Silicate minerals, especially plagioclase feldspars, are weathered in situ to
form opal-A and calcite (Chadwick et al. 1987 ). Microaggregates of silt and clay act
as nuclei for Si precipitation (Chadwick et al. 1989 ). These soils generally have a
pH of 8.0 to 8.5 (Table 14.2 ), at which Si is highly soluble (Cornelius et al. 2011 ).
The third kind of duric soil is derived from volcanic ash and loess; the duripan
forms in around 130 ka; and these soils occur primarily in southern ID (Fig. 14.4 ).
The dominant great groups are Haplodurids and Argidurids that have formed under
Great Basin shrub-grassland. These soils have an aridic soil-moisture regime and a