Intermittency of illuviation in Dystrochrepts and Hapludults from the Piedmont and Blue Ridge Provinces of North Carolina
Rebertus, R.A.; Buol, S.W.
Geoderma 36(3/4): 277-291
ISSN/ISBN: 0016-7061 DOI: 10.1016/0016-7061(85)90008-4
Dystrochrepts and Hapludults formed from mica gneiss and schist in the Piedmont and Blue Ridge Provinces of North Carolina have a range of illuvation cutan orieintation patterns that is related to various indicators of soil development (clay content, solum thickness, and degree of primary mineral alteration). The relationship of cutan orientation patterns to soil development in these soils suggests that clay illuviation is largely intermittent in occurrence. Lessivage occurs in Typic Dystrochrepts, the soils showing the least morphological development, but because of the thinness and coarse texture of the sola the major areas of accumulation may occur beneath the cambic horizons in vertical cracks in the structurally rigid, isovolumetrically weathered saprolite. The source of clay is relatively easily weathered plagioclase. Few illuviation cutans are present in argillic horizons of fine-loamy Hapludults. Evidence of illuvial clay is lacking because biotite is being altered to sand- and silt-sized pseudomorphs of kaolinite with little production of clay-sized particles. Once the biotite is entirely kaolinized, comminution of the sand- and silt-sized kaolinite grains supplies clay for clayey argillic horizons to evolve. Soils at this stage of development have abundant illuviation cutans with strong continuous orientation. After the silt- and sand-sized kaolinite has been communited the only primary minerals remaining in any abundance to produce clay-sized particles are relatively resistant muscovite and microcline. Iron continues to concentrate as the clayey Hapludults develop, masking the anisotropy of the illuviation cutans. The increasing lack of distinctness of cutans and their apparent low degree of orientation with increasing degree of soil development indicate that illuviation has largely ceased in the most developed clayey Hapludults and the illuvial features are for the most part relic features of an earlier priod of clay production.