MASS-BALANCE GEOCHEMISTRY OF SEDIMENTARY ROCK SAPROLITE: EFFECTS OF PEDOGENESIS ON VARIABLE INTERBEDDED LITHOLOGIES

Saprolite formation in sedimentary rocks is strongly influenced by primary lithologic components. We used geochemical mass-balance, coupled with detailed thin-section micromorphology, to assess these influences in a 3.4 m deep weathering profile developed on the Maryville Limestone (Middle Cambrian) in eastern Tennessee. Ribbon limestone, intraclast-rich limestone, shale, and siltstone are the dominant bedrock lithologies, and are oriented N35E, 30^o SE. Parent material uniformity and identification of immobile index element were assessed using Zr and TiO₂ cross-plots. Strain (volume change) and translocation (mass transport) calculations were performed assuming immobile Zr or Ti during weathering, and either unweathered limestone (LS), a limestone insoluble residue (LIR), or unweathered shale (SH) as the parent material, using a weighted contributions method for each saprolite parent lithology component within the profile. Greatest volume losses were shown in LS weathering and assuming immobile Zr; smallest volume losses were shown in LIR and SH parent materials, assuming immobile TiO₂. Net losses (negative translocations) for alkali, alkaline earth, redox-sensitive, and nutrient cations are highest for LS and are lower for LIR and SH parent materials. Micromorphologic evidence for weathering of limestones extends more deeply than for shale and siltstone, consistent with geochemical evidence of deeper LS weathering.

Micromorophologic observations support strain and translocation measurements, with significant illuviation of pedogenic clays and Fe/Mn oxides into biopores within limestone saprolite and in fracture pores in the shale and siltstone saprolite, commencing at 60 cm depth and extending to the base of the profile. Enrichments in Fe and Mn at depth reflect variable redox conditions associated with seasonal water table perching and localized saturation on top of the clay-enriched zone, followed by aeration upon drying. This study demonstrates the utility of combined geochemical and micromorphological methods for interpreting weathering profiles formed on heterolithic sedimentary rocks, in which the intensity of weathering may vary erratically with depth due to bedrock lithologic influences.