Departmental Bulletin Paper Estimating the degree of mudstone compactional thinning : Empirical relationship between mudstone compaction and geochemical composition

Izumi, Kentaro  ,  Suzuki, Ryota  ,  Inui, Mutsuko

The empirical relationship between the degree of compactional thinning and geochemical composition for fully consolidated mudstones was proposed for the first time. New data were provided by sedimentological, ichnological, and geochemical analyses of the mudstone samples from the Eocene Naharigawa Formation, Kochi Prefecture, Japan. Based on these analyses, the degree of compaction of the Naharigawa mudstones as a percentage of shortening (C_<ms>) was calculated as 83.01 %. To quantitatively estimate the empirical relationship, the C_<ms> values for various mudstones, whose geochemical compositions are available by previous studies, were further calculated and compiled. As a result of the compilation, a significant and strong negative correlation between the C_<ms> values and CaO abundances (r=−0.75;p<0.05) was recognized. Furthermore, the C_<ms> values show negative correlation between Ca concentrations normalized to terrigenous material (CaO/Al_2O_3, CaO/TiO_2) that are considered as proxies for relative contribution of non-terrigenous (i.e. biogenic) carbonates, although the recognized correlations are not statistically significant. These lines of evidence strongly suggest that the degree of physical compaction of muddy sediments is affected by carbonate contents. Although the obtained relationship may hold only for fully consolidated mudstones of the Paleogene or older ages, it is very helpful for quantitatively estimating the degree of compactional thinning for any mudstonedominated succession, as long as the major element composition of the mudstone is determined.

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