17:15 〜 19:15
[HCG21-P09] 白亜系久慈層群における堆積環境の復元：堆積相とパリノファシスの関係性
キーワード：堆積環境、パリノファシス、植物片、白亜紀、久慈層群
Microscopic observation of kerogen is called as palynofacies analysis. Palynofacies reflect the processes of preservation and transport of organic matter, and their composition can be used to understand the depositional environment (Tyson, 1995). For example, Carvalho et al. (2006, 2013) examined the depositional environment including the transitions with sea-level changes in sedimentary basins based on palynofacies analysis of Late Cretaceous sedimentary rocks. The Kuji Group in the northeastern part of Iwate Prefecture is subdivided into the Tamagawa Formation, Kunitan Formation, and Sawayama Formation. The Tamagawa, Kunitan and Sawayama Formations were deposited in fluvial to shallow marine, shallow marine, and fluvial environments respectively (Terui and Nagahama, 1995; Ando, 1997). Kimura et al. (2004) reported that organic matter of sedimentary rocks in the Kuji Group were immature. However, there are no examples to reconstruct depositional environments using organic matter, except paleovegetation studies using pollen and spore. In this presentation, we discuss the relationship between lithofacies and palynofacies to understand depositional environment of the Kuji Group.
The Rock-Eval analysis indicate that the organic matter in the sedimentary rocks of the Tamagawa, Kunitan and Sawayama Formations are classified as Type III kerogen with high OI values showing higher contribution of a terrigenous organic matter. The palynofacies analysis using the samples from Tamagawa Formation indicate that the percentage of NFA (non-fluorescent amorphous organic matter (AOM)) increases and those of opaque phytoclasts in the phytoclasts decreases toward the top. These results suggest that the opaque phytoclasts were decomposed into NFA due to the transition to the environment with lower energy. In fact, from sedimentary facies, the two cycles of sedimentary successions from fluvial to shallow marine environment, were assumed. In the Kunitan Formation, the average percentage of AOM is the highest among the three Formations, and the percentages of WFA (weakly fluorescent AOM) are particularly higher in the middle part of this Formation. On the other hand, although the lower percentages of phytoclasts are low, the ratios of translucent / opaque phytoclasts increase, especially in the middle part of this Formation. These results are consistent with lithofacies indicating the succession from upper shoreface to inner shelf and suggest that the middle part of the Formation was deposited in deepest environment. Kerogen in samples from the Sawayama Formation are characterized by higher percentages of AOM, especially NFA. The percentages of opaque and translucent phytoclasts are almost the equal. These results suggest that the NFA was deposited in an environment with low energy and close to the source of organic matter. It is possibly that opaque and translucent phytoclasts deposit around flood plain, assumed by the sedimentary facies, without selective preservation due to less transport process. In addition, micro-FT-IR analysis shows that the C=O and C-H peaks of opaque phytoclasts are higher in the samples of Sawayama Formations than those of the Kunitan formation. Furthermore, from SEM observation, opaque phytoclasts are thicker and have a lot of cracks compared with translucent phytoclasts. Attachment and chemical reaction of liquid to opaque phytoclasts were high in fluvial environment with rich in organic matter, while low in shallow marine environment after decomposition of them. From these results, the data from palynofacies analysis are consistent with the sedimentary facies. Especially, both of them indicate that depositional environment changed to deeper with lower energy from the Tamagawa Formation to the middle part of Kunitan Formation and became terrestrial closest to the source environments of organic matter while lower energy in the Sawayama Formation.
The Rock-Eval analysis indicate that the organic matter in the sedimentary rocks of the Tamagawa, Kunitan and Sawayama Formations are classified as Type III kerogen with high OI values showing higher contribution of a terrigenous organic matter. The palynofacies analysis using the samples from Tamagawa Formation indicate that the percentage of NFA (non-fluorescent amorphous organic matter (AOM)) increases and those of opaque phytoclasts in the phytoclasts decreases toward the top. These results suggest that the opaque phytoclasts were decomposed into NFA due to the transition to the environment with lower energy. In fact, from sedimentary facies, the two cycles of sedimentary successions from fluvial to shallow marine environment, were assumed. In the Kunitan Formation, the average percentage of AOM is the highest among the three Formations, and the percentages of WFA (weakly fluorescent AOM) are particularly higher in the middle part of this Formation. On the other hand, although the lower percentages of phytoclasts are low, the ratios of translucent / opaque phytoclasts increase, especially in the middle part of this Formation. These results are consistent with lithofacies indicating the succession from upper shoreface to inner shelf and suggest that the middle part of the Formation was deposited in deepest environment. Kerogen in samples from the Sawayama Formation are characterized by higher percentages of AOM, especially NFA. The percentages of opaque and translucent phytoclasts are almost the equal. These results suggest that the NFA was deposited in an environment with low energy and close to the source of organic matter. It is possibly that opaque and translucent phytoclasts deposit around flood plain, assumed by the sedimentary facies, without selective preservation due to less transport process. In addition, micro-FT-IR analysis shows that the C=O and C-H peaks of opaque phytoclasts are higher in the samples of Sawayama Formations than those of the Kunitan formation. Furthermore, from SEM observation, opaque phytoclasts are thicker and have a lot of cracks compared with translucent phytoclasts. Attachment and chemical reaction of liquid to opaque phytoclasts were high in fluvial environment with rich in organic matter, while low in shallow marine environment after decomposition of them. From these results, the data from palynofacies analysis are consistent with the sedimentary facies. Especially, both of them indicate that depositional environment changed to deeper with lower energy from the Tamagawa Formation to the middle part of Kunitan Formation and became terrestrial closest to the source environments of organic matter while lower energy in the Sawayama Formation.