In order to clarify the age of the early stage of the Chichibu Sedimentary Basin, we compiled data on the stratigraphy, molluscan fossils, planktonic foraminifera fossils, calcareous nannofossils, and fission-track ages of the lower layers of the basin. These data indicate that the Miyato Formation dates to ca. 16–15 Ma, representing the Middle Miocene, the Nenokami Formation to ca. 17–16 Ma, representing the late Early Miocene, and the Tomita Formation to ca. 17 Ma and the Shirasu Sandstone Member of the Ushikubi Formation to ca. 18–17 Ma, both representing the Early Miocene. The Tonogaido Breccia and Ushikubitouge Conglomerate members are locally distributed below the Shirasu Sandstone Member. Therefore, the age of the lowermost layer may be older, but the Chichibu Sedimentary Basin is considered to have been formed between ca. 18–17 Ma.

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Here, we have described 39 molluscan species from 23 families in the Lower Pleistocene Sayama Formation of the Kazusa Group, exposed in the Sayama Hills on the western margin of the Kanto Plain, and evaluated the paleoenvironment based on these assemblages. Fossils were collected from the Osawa shell bed and overlying Nakato and Shinmei localities. Most species identified are extant. The Osawa shell bed is inferred to have been deposited in an inner-bay tidal flat near a river mouth, whereas the Nakato and Shinmei fossil beds represent inner-bay tidal flats farther from the river mouth. Paleotemperature estimates from three horizons indicated slightly warmer conditions than current temperatures. Integrating previous studies, we concluded that the Sayama Formation was deposited intermittently in an inner-bay tidal flat environment throughout its history, reflecting paleoenvironmental conditions intermediate between the Bushi Formation along the Iruma River and the Kazusa Group around the Tama Hills.

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An outcrop formed by a duplex structure was discovered in the northern Chichibu Belt distributed in Minano Town, Chichibu District, Saitama Prefecture. The strata forming the outcrop belong to the Kamiyoshida Unit, formed during the Middle Jurassic, and consist of siliceous rock, siliceous mudstone and mudstone. A distinct floor thrust and a somewhat indistinct roof thrust are observed at the outcrop. Furthermore, seven horses separated by small-scale thrusts exist between the two thrusts mentioned above. At the junction of the roof thrust and the horse thrust, a curved structure indicating the direction of movement of both is observed. In addition, there are three folds with a scale of approximately 2 m. These fold structures all exhibit kink folds and are fault-propagation folds. By comprehensively examining the above facts from the perspective of displacement amount, the formation process of the duplex structure at this outcrop was clarified.

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From 2017 to 2025, surveys were conducted in Saitama, Tokyo and Yamanashi Prefectures, confirming the presence of Tripidium arundinaceum at 40 locations. The presence of all the gregarious colonies on the slopes indicates that the invasion route was likely contaminated with greening seeds. Furthermore, the species has been observed spreading in artificial locations such as roadside verges, as well as in diverse surroundings including abandoned and fallow fields, forest edges, areas near houses, and riverbanks. The trend of its distribution expanding has been identified through observations conducted over several years at specific sites. It is indicated that there is a high probability of the species invading key biodiversity areas; therefore, proactive control measures are imperative to prevent its spread. Various issues need to be resolved, including operational challenges in implementing greening guidelines for controlling the species, determining responsibility for unintentional introductions, and addressing technical and institutional challenges in control measures.

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