[B-PT06] 地球生命史

[BPT06-05] 生痕化石の観察に基づく細粒砕屑岩の圧密の推定

*泉 賢太郎¹、鈴木 涼太²、乾 睦子³ (1.千葉大学教育学部、2.海老名市立有馬中学校、3.国士舘大学理工学部)

キーワード：細粒砕屑岩、圧密、生痕化石

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 (C_ms) was calculated as 83.01 %. To quantitatively estimate the empirical relationship, the C_ms 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 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/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.