Knowledge Bank

Subsampling of a large block of Wisconsinan Ashtabula Till suggests that weathering progresses outward from fractures in the root zone. In the fracture zone pore waters have precipitated iron, calcite has been partially leached, and dolomite remains constant. Illite and kaolinite phases present in samples taken from near the fracture in yellowish-brown, oxidized till do not significantly differ from gray, unoxidized till from the same horizon. However, chlorite has been altered to vermiculite in samples proximal to the fracture. Diffraction intensity ratios (illite001/kaolinite001 + chlorite002) increase near the fracture and decrease with distance away from the fracture. The concentration of goethite, the mineral responsible for the yellowish-brown color, decreases exponentially with respect to distance from the fracture. This oxidized zone has a greater sand content and lesser clay content than the unoxidized gray zone because free iron that coats the clay particles cements them into sand-size aggregates. Weathering processes in till adjacent to fractures are the same as those that form weathering zones in till landscapes.