In this study we demonstrate that the geochemical weathering of the Silurian Rose Hill Shale Formation has caused mineralogical changes with depth into bedrock at the Susquehanna Shale Hills Observatory (SSHO), a forested first-order watershed in central Pennsylvania, U.S.A.. Established as a member of the Critical Zone Exploration Network, SSHO is under investigation in attempts to tie relationships between geochemistry, hydrology, ecology and geomorphology on a watershed scale. The geochemical alteration of shale under aqueous conditions induces chemical reaction fronts with depth. Chemical reaction fronts are defined in this study as depths associated with the depletion of a particular mineral or element, in comparison to the parent bedrock.
Mineral weathering fronts were identified in drilled samples from the valley floor, based on geochemical and mineralogical characterizations. Seventeen samples from a 16 meter deep drill core taken at the valley floor site were analyzed and compared to similar samples from the ridge top. A carbonate weathering front was identified at 3 m depth, an illite weathering front at 4 m depth, and a pyrite oxidation front and ferrous-ferric transition at 7 m depth in the valley floor of SSHO. Identical reaction fronts were identified at the ridge top of the SSHO catchment, although at variable depths. Ridge top carbonate weathering front is noted at 25 m, pyrite
oxidation front an ferrous-ferric transition at 25 m and illite weathering front at 0.5 m. Elemental concentration remains constant with depth, with the exception of Ca showing the highest variation. Presence of kaolinite and vermiculite is observed within the upper 4 meters in the valley floor, associated with the extent of the illite weathering front.
Holleran, M.E. (2011): A Quantitative Analysis of Deep Shale Weathering at the Shale Hills Critical Zone Observatory. Bachelor of Science, Pennsylvania State University, p. 38.
This Paper/Book acknowledges NSF CZO grant support.
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