Genesis, mineralogy, and geochemistry of a red-black (alfisol-vertisol) complex, northeastern Queensland, Australia

A red-black complex of Udic Paleustalfs and Typic Pellusterts occurs on gently undulating metamorphic and igneous uplands, northeastern Australia. The research objectives were to elucidate factors that influence(d) the genesis of these soils, and to characterize the pedogenic dynamics in this landscape. The relationships among soils, parent rocks, and landscape were investigated. Four typical pedons representing end-member red Paleustalfs and black Pellusterts were sampled, one each formed from andesite and granodiorite gneiss. Two red-black soil boundary transitions (6 and 8 m long) were exposed, described at 0.5 m horizontal intervals, and the profiles manifesting the greatest morphological change were sampled. Chemical and physical soil properties, clay mineralogy, and soil solutions were characterized.; Parent rock has little influence on soil distribution relative to surface, subsurface, and internal soil hydrology. Field and laboratory results indicate that Alfisols are being progressively transformed into Vertisols. Along the red-black transition, Alfisols are subject to seasonal reducing conditions due to progressively poorer internal soil drainage. Active soil mixing occurs, evinced by red soil material along slickensides and engulfed in black peds at Vertisol margins. Black, smectite-rich soil material appears to be moving toward the low-activity Alfisols along shear planes, in response to Vertisol B-horizon swelling. Smectite-rich soil in the lower solum limits internal drainage. On a whole profile basis, free Fe-oxides and magnetic susceptibility decrease from Alfisol to Vertisol, whereas organic C and carbonates increase. The vertical distributions of silicate clay, organic C, Fe-oxide, and magnetic susceptibility become more uniform towards the Vertisol, due perhaps to pedoturbation. The dominant phyllosilicate in the Vertisols is Fe-rich smectite, whereas randomly interstratified kaolinite/smectite ({dollar}geq{dollar}80% kaolinite) dominates the Alfisols. The mixed-layer clay is ubiquitous along the red-black transition, decreasing relative to Fe-rich smectite towards the Vertisol. Iron is apparently conserved: Fe-oxides are reduced and Fe-rich smectites are formed. Vertical distributions of solutes indicate that Alfisols are better leached than Vertisols. The relative abundances of solutes increase along the transition, reflecting the progressive decrease in internal drainage from Alfisol to Vertisol. Unusually high Si activities in Alfisol and Vertisol surface horizons are controlled by highly soluble opaline grass phytoliths, which may contribute Si for smectite neosynthesis.