Abstract

Studying the chemical characteristics of Holocene sedimentary accumulations and some of the diagenetic products formed after deposition provides some clues to the history of shale beds and contributes to the understanding of the effects of overburden on compaction, dewatering, and increase in compressive strength with depth of burial. X-ray radiography was utilized extensively in examining core slabs from a fresh-water clay sequence in the Atchafalaya River Basin. From the radiographs, detailed diagenetic features such as cementation by secondary precipitated minerals, pyrite and carbonate replacement of organic fragments, and progressive formation of nodules were revealed. Selected samples were analyzed for various chemical and mineralogical constituents by means of differential thermal analysis, X-ray diffraction, X-ray fluorescence, atomic absorption spectrophotometry, Kjeldahl method for total nitrogen and wet combustion for organic carbon. The results indicated the presence of various cementing agents. The diagenetic mineral accumulations consisted of CaCO3, Fe2O3, FeCO3, and Mg and Mn compounds of unknown nature and have contributed significantly to the observed strength increase with depth. The dewatering process, commonly attributed solely to compaction resulting from overburden, may also be brought about by a gradual replacement of the pore-water space by secondary mineral accumulation. Numerous processes are responsible for the initial mineral accumulations and chemical reactions, especially of soluble organic compounds.

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