抄録
Fossil forests from in the Early Pleistocene Kazusa Group are located on the western edge of the Kanto Plain, has been known to be composed of the deciduous coniferous tree Metasequoia. In this study, we identified tree species in the fossil forests based on fossil tree stumps in situ distributed in the riverbeds of the Asakawa and Minami-asakawa rivers in the Kasumi Formation of the Kazusa Group. The results showed that these fossil forests were dominated by Cupressaceae including large deciduous conifer such as Metasequoia, with the deciduous broad-leaved tree Fraxinus.
These fossil forest bearing layers are a mixture of very fine sand and pumice, and black to dark brown organic mud layers rich in wood, indicating a wetland environment where mud accumulated without any flow from the surrounding area.
Based on volcanic ash stratigraphy and paleomagnetic stratigraphy, the age of these fossil forests is thought to correspond to the Olduvai normal polarity stage (approximately 1.7 to 1.9 Ma).
The Asakawa riverbed was dominated by conifers including large Metasequoia trees with trunk diameters of 70 cm or more and maximum to 180 cm. In the Minami-Asakawa riverbed, a wetland forest with a different structure was formed, dominated by Metasequoia with trunks up to 90 cm in diameter and conifers from the Cupressaceae with trunks up to 40 cm or less in diameter.
Based on the macrofossil plants that occurred with fossil stumps, consisted of the deciduous broadleaved tree such as Rosaceae, Alnus, Quercus subgen. Lepidobalanus, Styrax and Juglans megacinerea, were distributed around these fossil forest. In the Early Pleistocene forests containing Metasequoia, the Echigawa fossil forest from the Gamo Formation of the Kobiwako Group constitutes a wetland forest together with Glyptostorobus, while the fossil forests in the Asakawa and Minami-asakawa riverbeds from the Kasumi Formation of the Kazusa Group contain a different species of Cupressaceae. Thus, there existed swamp forests with different constituent species and structures.
