Coarse-textured basal zones in thin loess deposits: Products of sediment mixing and/or paleoenvironmental change?

The purpose of this research was to characterize and interpret the coarse basal zones that are common in thin loess deposits that overlie coarser-textured sediment. To that end, we sampled nine pedons in northeastern Wisconsin and the Upper Peninsula of Michigan, each of which had formed in thin (≤ 55 cm) loess over sandy glacial sediment. At most of these sites, the loess became noticeably coarser near the lithologic discontinuity. The loess has a primary particle size mode in the coarse silt or fine, very fine sand fraction (≈ 30–65 μm) and a secondary mode in the fine or medium sand fraction (≈ 200–400 μm). We attribute the secondary mode to mixing of underlying sands into the loess, either during loess deposition or by post-depositional pedoturbation. In thin loess, pedoturbation processes can penetrate into the underlying sediment, facilitating mixing upward as far as 50 cm into the loess. Silt from the loess has also been mixed into the underlying sandy sediment. Alternatively, in some pedons, the loess itself coarsens with depth; the particle size mode for the loess becomes increasingly coarser with depth. This coarsening suggests that, during loess deposition, one or more of the following was occurring: (1) wind velocities were decreasing over time, (2) the textural character of the loess source area(s) were changing/decreasing, or (3) additional source areas—with finer-textured sediment—became more dominant over time. Our research demonstrates the utility of detailed particle size data for detecting and interpreting the paleoenvironmental history of loess. We also document the extent to which pedoturbation can impact the original textural characteristics of loess (or any sediment) that occurs as a thin surficial mantle.