For the first time, fossils Isocrinida were discovered from the Miocene deposit in the Chichibu Basin, Saitama Prefecture, Central Japan. The fossils of Isocrinida are 14 specimens from the rubbly mudstone of the upper Chichibu-machi Formation in the Chichibu Basin Group exposed on the right bank of Arakawa River, Kami-kagemori, Chichibu City, on the southern margin of the Chichibu Basin. Diameter of large columns is about 10 mm. Lateral surface of internodals are stellate. Petal is lanceolate. Cirrus sockets of nodals are always five in number. Number of internodals is 7 (sometimes 8 or 9). However, since it lacks the calyx and arms that are reliable characters for species identification, all of these specimens are assigned here as ‟Isocrinus ?” sp.. 　The fossil record of ‟Isocrinus ?” sp. have been reported from the outer shallow to central bathyal zone of the early to middle Miocene in Hiroshima, Hokuriku and Gunma in Japan, suggesting the sedimentary environment of the Chichibu-machi Formation to be similar depth.

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A pumice layer, 2.6 m in thickness, ca. 200 m in depth, in the Tokorozawa core, which was drilled as an observation well for subsidence by Saitama Prefecture in 1978, is mainly composed of volcanic glass, plagioclase, green hornblende. Based on its appearance, mineral composition and upper and lower lithofacies, it is correlated to a tuff layer at a depth of 203.3 m of the GS Tokorozawa core.
Results of tephra analysis including mineral compositions and refractive indices of minerals for this pumice layer indicate that it can be correlated to a pumice layer at a depth of 80.0–89.7 m of the Higashi-yamato core and the Koyata2 tephra in the Bushi Formation based on similar heavy mineral composition and refractive indices of volcanic glass shards, hornblende etc. Previous studies revealed that these are correlated to the SGO in the Sayama Formation and the E1 tephra in the Bushi Formation. Therefore, this pumice layer is a valuable chronological marker for correlation in the subsurface of Kanto region.

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