Clay mineralogy and chemistry of argillic horizons, fragipans, and paleosol B horizons of soils on a loess-thinning transect in southwestern Indiana, USA

Abstract

In southwestern Indiana Peorian loess (about 20,000 years old) thins eastward from the river valleys where it originated, and covers Sangamon paleosols developed in various parent materials. Where the loess is thick, more than about 2.5 m, the entire modern soil profile formed in loess, and where it is thin the modern soil and paleosol are contiguous, i. e., welded. The objectives of this study were to determine how the mineralogy of the argillic (Bt) horizons of modern soils changes with loess thickness, how fragipan horizons (Bx) formed in loess differ from non-fragipan horizons formed in loess, and how paleosol (Btb) horizons differ from Bt horizons and from each other. Soils were sampled on stable landscape positions along a loess-thinning transect about 160 km long. Free oxides (Al d, Fe d, Si d) were extracted with a citrate-bicarbonate-dithionite solution. Clay mineralogy was determined qualitatively by X-ray diffraction and quantitatively by thermal and chemical methods. In the loess-derived Bt horizons, kaolinite content tends to increase and smectite to decrease as loess thins. Fragipans, formed where loess is of intermediate thickness, have higher contents of smectite and free silica, greater surface area, and higher exchangeable Mg/Ca ratios than argillic horizons formed in loess. High Si d/clay and Si d/Fe d ratios are indicators of fragipan development. Paleosol Btb horizons have higher base saturation than modern Bt horizons. They also contain more kaolinite and less smectite than Bt horizons, but the paleosols developed in different materials all have similar mineralogy.