Organic facies and geochemical aspects in Neogene neritic sediments of the Takafu syncline area of central Japan: Paleoenvironmental and sedimentological reconstructions

Abstract

Organic petrological observations of kerogen macerals and organic geochemical analyses of carbon isotopes of kerogen macerals and biomarkers were conducted on Neogene neritic sediments of the Takafu syncline area of central Japan. The Senmi, Sakainomiya and Lower Shigarami Formations in that area were deposited at the neritic provinces on the southern edge of the paleo-Japan Sea during the Late Miocene to Early Pliocene. Sedimentary organic matter in these formations was almost terrigenous in origin. Changes in kerogen maceral compositions reflect sedimentological and tectonic histories evaluated in previous studies from sedimentary facies and paleontology. It was found that carbon isotope ratios (δ 13 C) of kerogen macerals increased from −28‰ to −25‰ from the Sakainomiya to the lower part of the Lower Shigarami Formations. The cause of that increase was presumably the expansion of C 4 plants into southwest Japan. The timing was concordant with that of the expansion of C 4 plant grasslands in East Asia. The oxicity (oxic to anoxic) conditions of sea bottoms evaluated from pristane/phytane ratios varied. Particularly, in the lower part of the Senmi Formation, layers in which no steroid biomarkers could be detected were found, and had presumably formed under oxic conditions when strong biodegradation had occurred. Concentrations of regular (C 27 -C 29 ) steranes and dinosteranes were higher in the Sakainomiya and Lower Shigarami Formations. This indicates that dinoflagellates-dominant primary productions were higher at those stages. In addition, concentrations of diatomaceous biomarkers such as C 26 norsterane increased from the Lower Shigarami Formation, thus adding diatoms to the major producers. Furthermore, similar associations between the increases of δ 13 C values of kerogen macerals and concentrations of diatomaceous biomarkers were observed in the Takafu syncline area. Thus, the expansion of C 4 plants was possibly associated with the high production of diatom in the shallow-marine areas of the paleo-Japan Sea during the Neogene Period.

Kerogen morphology and geochemistry at the Permian–Triassic transition in the Meishan section, South China: Implication for paleoenvironmental variation

“…On the other hand, the lowest values of H/C in the samples that have high NFA (layers 24e-2 and 24e-3) supports that the NFA is terrigenous woody organic matter origin. Sawada (2006) confirmed that the vitrinite (wood) and NFA-rich fraction of kerogen had a relatively low H/C ratio, which agrees with the H/C results of this study. Fig.…”

“…However, this method should be used with caution, because mature amorphous organic matter (AOM) does not fluoresce (Durand, 1980 and references therein). Therefore, we examined the identification of AOMs on the basis of both the fluorescence and the morphology under transmitted light microscope as reported by Tyson (1995), Boussafir et al (1995), Tribovillard et al (2001), Sawada (2006), and Ercegovac and Kostic (2006). The Table 2 show photomicrographs of kerogens separated from sedimentary rocks and their palynofacies compositions from the Changhsing to Yinkeng Formations, respectively.…”

“…Fluorescence light microscopic observation was used to differentiate among several types of amorphous particles (Sawada and Akiyama, 1994;Sawada, 2006). However, this method should be used with caution, because mature amorphous organic matter (AOM) does not fluoresce (Durand, 1980 and references therein).…”

“…The Table 2 show photomicrographs of kerogens separated from sedimentary rocks and their palynofacies compositions from the Changhsing to Yinkeng Formations, respectively. We examined the identification of AOM on the basis of both the fluorescence and the morphology under transmitted light microscope as reported by Boussafir et al (1995), Tribovillard et al (2001), Sawada (2006), and Ercegovac and Kostic (2006): (1) Brown weakly-fluorescent amorphous organic matter (Br-WFA) corresponds to the light brown to dark brown AOM under transmitted light microscope (Fig. 3).…”

“…Palynofacies analysis was carried out as described by Sawada (2006). Briefly, wet residual particles obtained after HCl / HF treatment were siphoned and dropped onto non-fluorescent slides, and were then mounted in permanent slides for microscopic examination.…”