CHEMICAL WEATHERING AND SOIL FORMATION FROM MULTIPLE PARENT MATERIALS IN A COMPLEX REGOLITH, SHALE HILLS, PENNSYLVANIA

Weathering of bedrock to create saprolite, regolith, and soil is commonly envisioned to operate on in-situ, uniform parent material. A chemical weathering front produces saprolite and eventually regolith, which then participate in hillslope creep and soil formation processes. However, many hillslopes in diverse climatic settings do not replicate these conditions. Weathering and pedogenesis are not steady process that produce hillslopes of continuously mixing and creeping soil directly from bedrock. Rather, the initial mechanical disaggregation of bedrock and saprolite can be followed by periods of transport and deposition of colluvium by multiple processes, followed by alternating periods of pedogenesis and stochastic events that disrupt the soil structure. Both the pedogenesis and the disruptive events are recorded in hillslope stratigraphy, including their soils and paleosols.

In this work, we report on the geochemical evolution of soils developing through complex colluvial stratigraphy in a swale of a small, shale-bedrock watershed in central Pennsylvania (Shale Hills CZO Experimental Watershed). We find that the bedrock-regolith contact is marked by a thin (several cm) saprolite with distinct gleyed mottles. Above this saprolite in the swales is one or more deposits of well-sorted 0.1 – 2 cm angular shale chips, interpreted as periglacial sorted talus (grèzes littés) that can exceed 2 m in thickness. We present preliminary grain size and elemental depletion profiles through these parent materials to illustrate the degree of chemical weathering and pedogenesis. The diverse transported parent materials provide significantly different textural and hydrological pathways for pedogenesis, which we contend should have a first-order feedback on weathering profile geochemistry and long-term hillslope form.