Dark-Colored Bands in the Thick Loess of Western Iowa

Abstract

Faint to distinct dark-colored bands occur in fresh roadcuts throughout a widespread area of thick loess in western Iowa. The bands resemble very weak A-horizons of buried soils, and parallel the underlying geomorphic surfaces. In the western part of the area two dark bands are present, but in their extreme eastern distribution the upper band rises in the section and fades into the upper leached zone of the Wisconsin loess. Chemical studies show a consistent increase of about 0.08 per cent in manganese and less consistent increases of about 0.2 per cent in free iron and 0.03 per cent in organic matter in the bands compared to the overlying and underlying loess. The dark color of the bands is mainly due to the higher manganese content. Some leaching of carbonates has occurred in the loess immediately below the basal band. From the physical and chemical properties of the bands and the relationship to underlying geomorphic surfaces, the bands are believed to represent surfaces of slow or temporary non-deposition of loess. Because of the weak expression, it is questionable whether they represent true interstadial breaks in loess deposition.