MICROMORPHOLOGY OF THE SOIL-SAPROLITE TRANSITION ZONE IN HAPLUDULTS OF VIRGINIA

Most residual soils occurring in the southeastern USA have formed in saprolite. Between the saprolite and the argillic horizon lies a zone that divides material undergoing pedogenesis from that undergoing geochemical processes. We examined the physical, chemical, mineralogical, and micromorphological properties and characteristics of representative soils and saprolite formed from schistose or gneissic rocks of Virginia in order to describe and define the soil-saprolite transition zone. Argillic horizons showed considerable oriented clay (3-16%), strong to moderate subangular blocky microstructure, porphyric coarse/fine related distribution patterns (c/f RDP), low (< 0.5) coarse/fine ratios separated at 20-mu-m (c/f20-mu-ratios), and many planar voids. Oriented clay was absent from C horizons and microstructure was termed rock controlled to describe the apedal nature of these horizons. The c/f RDP was chitonic or gefuric, with the c/f20-mu-ratio at least double that of the argillic horizon. Microstructure of transition horizons was moderate subangular blocky to weak subangular blocky/rock controlled. The c/f20-mu-ratios and c/f RDP were intermediate between the argillic horizons and the saprolite. Transition horizons were designated as BCt, BC, or CB depending on percent oriented clay, rates of change with depth of clay, dithionite-citrate-bicarbonate (DB) extractable Fe, and sand contents. Weathering of mica grains showed slight to almost total pellicular and parallel linear alteration to hydroxy-interlayered vermiculite (HIV), kaolinite, and an interstratified component of these minerals. Alteration of feldspar grains was generally < 25% and occurred in the form of pellicular, or parallel, or cross-linear patterns to kaolinite or gibbsite.