Keywords:Fine-grained clastic rocks, Compaction, Trace fossils
This study proposed the empirical relationship between the degree of compactional thinning and geochemical composition for fully consolidated fine-grained clastic rocks. New data were provided by sedimentological, ichnological, and geochemical analyses of the mudrock samples from the Eocene Naharigawa Formation, Muroto-Hanto Group, Kochi Prefecture, Japan. Based on these analyses, the degree of compaction of the Naharigawa mudstones as a percentage of shortening (Cms) was calculated as 83.01 %. To quantitatively estimate the empirical relationship, the Cms values for various fine-grained clastic rocks, 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 Cms values and CaO abundances (r = −0.75; p < 0.05) was recognized. Furthermore, the Cms values show negative correlation between Ca concentrations normalized to terrigenous material (CaO/Al2O3, CaO/TiO2) 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 fine-grained clastic sediments is affected by carbonate contents. Although the obtained relationship may hold only for fully consolidated fine-grained clastic rocks of the Paleogene or older ages, it is very helpful for quantitatively estimating the degree of compactional thinning for any fine-grained clastic rock-dominated succession, as long as the major element composition of the sediments is determined.
Furthermore, a geological example of application of the proposed empirical relationship to consolidated mudstone-dominated succession is also discussed.