Isovolumetric Weathering of Granite in Wake County, North Carolina
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Date
2004-07-15
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Abstract
Saprolite, formed by chemical weathering of rocks near the earth's surface, holds water, serves as a parent material of soils, and is a medium for waste disposal. Saprolite formation consumes CO₂ and may stabilize atmospheric CO₂ levels. This dissertation examined the influence of joint orientation on isovolumetric weathering of saprolite developed on the Rolesville granitic batholith at Knightdale, North Carolina. Rock density (ρ[subscript s]) (μ α[subscript 0.05]) was 2.62±0.01 g cm⁻³. Mass altered per unit volume (m[subscript A/V[subscript T]) of saprolite was taken as the difference between rock density (ρ[subscript s]) and primary mineral mass remaining per unit volume (m1⁰[subscript R]/V[subscript T]). Altered mass lost per unit volume (m[subscript AL/V[subscript T]) was taken as the difference between ρ[subscript s] and bulk density (ρ[subscript b]). Altered mass retained per unit volume (m[subscript AR/V[subscript T]) was taken as (m[subscript A]/V[subscript T]) - (m[subscript AL/V[subscript T]).
Saprolite with steeply-dipping joints showed a uniformly sandy texture. The distribution (mass percent) of sand-, silt-, and clay-sized particles (μ α[subscript 0.05]) was 82.4±2.7, 10.3±1.8, and 2.3±2.5, respectively, on a whole saprolite basis, and ρ[subscript b] (μ α[subscript 0.05]) was 1.66±0.06 g cm⁻³. Saprolite with horizontally-oriented unloading joints was extensively altered and occurred between horizontal slabs of unweathered rock. The saprolite was composed of sandy layers alternating with clayey layers on the scale of approximately 1 to 2 cm. The distribution of sand-, silt-, and clay-sized particles (μ α[subscript 0.05]) in the saprolite was 50.1±10.4, 3.1±0.5, and 46.8±10.5, respectively, on a whole saprolite basis. Bulk density (μ α[subscript 0.05]) was 1.55±0.01 g cm⁻³. The mean content of sand-, silt-, and clay-sized particles in the two saprolites differed statistically at α = 0.001, and mean bulk density differed at α = 0.01.
The fine-earth fraction of saprolite with steeply-dipping joints was characterized (μ α[subscript 0.05]) by pH of 5.8±0.2, mass percent Fe2O3 of 0.21±0.09, cation exchange capacity (CEC) at pH 7.0 of 3.95±0.88 cmol⁺kg⁻¹, and percent base saturation (% BS) of 36.66±9.93. The fine-earth fraction of saprolite with horizontal joints was characterized (μ α[subscript 0.05]) by pH of 5.1±0.2, mass percent Fe2O3 of 2.68±0.28, CEC (in cmol⁺[superscript kg]⁻¹) at pH 7.0 of 8.28±0.91 and % BS of 19.73±9.22. The means of pH, mass percent Fe2O3, and CEC in the two saprolites differed statistically at α = 0.01, and the means of % BS differed at α = 0.05. The differences in mean values of individual extractable bases are not significant at α = 0.05.
Density of unweathered granite (μ α[subscript 0.05]) was 2.62±0.01 g cm⁻³. Calculated mean (μ α[subscript 0.05]) values of m[subscript A]/V[subscript T], m[subscript AL]/V[subscript T], and m[subscript AR]/V[subscript T] (all in g cm⁻³) in saprolite with steeply-dipping joints were 1.17±0.12, 0.96±0.06 and 0.21±0.05, respectively. Corresponding values in saprolite with horizontal joints were 1.85±0.15, 1.08±0.02 and 0.77±0.17, respectively. Calculated mean (μ α[subscript 0.05]) values of m[subscript AL]/m[subscript A] were 0.82±0.03 for the former saprolite and 0.58±0.06 for the latter, indicating greater leaching losses in the former. Differences in the calculated means of m[subscript A]/V[subscript T], m[subscript AR]/V[subscript T], m[subscript AR]/m[subscript A] and m[subscript AL]/m[subscript A] in the two saprolites are statistically significant at α = 0.001, and m[subscript AL]/V[subscript T] differed at α = 0.05.
Saprolite with steeply dipping joints was composed predominantly of plagioclase and potassium feldspar. Saprolite with horizontal joints contained approximately equal proportions of potassium feldspar and kaolinite (or halloysite). Nordstrandite occurred in both types of saprolite.
Saprolite was classified based on the relative proportions of (m1⁰[subscript R]/V[subscript T]) 100/ρ[subscript s], (m[subscript AR]/V[subscript T]) 100/ρ[subscript s], and (m[subscript AL]/V[subscript T]) 100/ρ[subscript s]. Saprolite with steeply-dipping joints classified as 'moderately altered, highly leached', and saprolite with horizontal joints classified as 'severely altered, moderately leached'.
Joint orientation appears to be a significant variable in saprolite formation.
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Keywords
regolith, nordstrandite, granite, weathering, saprolite, isovolumetric
Citation
Degree
PhD
Discipline
Soil Science
