Loess Composition in Southeastern Illinois and Southwestern Indiana

Peorian loess in southeastern Illinois and southwestern Indiana varied more in composition among the Wabash, Ohio, and Mississippi river source areas than did soils developed in the upper portion of the loess. Very low magnetic susceptibility was found in the gray, basal (Iowan or Morton) Peorian loess in the Wabash valley. Carbonate content was lower in Peorian loess of the Ohio valley than in that of the Wabash valley. Peorian loess was lower in clay and higher in silt content than Farmdale or Loveland loesses. Composition of the silt fraction, other than carbonates, did not vary greatly in Peorian loess of the three major source areas. Opal phytoliths and sponge spicules were present in all calcareous Peorian loess samples, and one unidentified microfossil was found in the Wabash valley loess. Illite was the dominant clay mineral in Peorian loess of the Ohio and Wabash valleys and expandable 2:1 minerals were dominant in Mississippi river loess. A high proportion of the expandable minerals in all samples was vermiculite. Chlorite and kaolinite were comparable in the Peorian loess of the three valleys. In comparison with the Peorian, the Farmdale loess had somewhat similar magnetic susceptibility, only secondary carbonates, and greater clay content. In the silt fraction, the Farmdale had lower Fe and Mn contents, less opal phytoliths, and similar sponge spicule content as the Peorian. The Farmdale was lower in illite, equal to or slightly higher in expandable clay minerals, and somewhat higher in chlorite and kaolinite contents than the Peorian. In comparison to the overlying Farmdale, the Loveland loess had strongly developed soils and less secondary carbonates. The silt fraction was lower in K₂O and CaO contents, and lacked sponge spicules. The Farmdale and Loveland appeared to have similar clay mineral composition. The Ohio valley loess was most weathered and the five Wabash valley loess soils were least weathered as indicated by CaO/ZrO₂ molar ratios of the A horizons. Alford and Sylvan soils, developed in Peorian loess of the Wabash, Ohio, and Mississippi valleys, appear to have no real differences in clay mineral composition in the B2 horizons, although illite may be higher than expandable clay minerals in the Ohio valley loess soils. A high proportion of the expandable clay minerals in all of the B2 horizons appeared to be vermiculite. The rather marked similarity of the clay of B2 horizons of the loess soils in contrast to the varied clay mineral composition of underlying loess, suggests that the surficial loess may have varied less than that deeper in the sections.