Bulletin of the Geological Survey of Japan Top Page

Bulletin of the Geological Survey of Japan Vol.77 No.1 (2026)

Cover photograph | Table of Contents | Abstract

Cover photograph

Trace fossils in sandstone blocks used as building material at the University of Tokyo

The sandstone blocks used in buildings the University of Tokyo, including the Yasuda Auditorium and the General Library, are shallow marine deposits that contain numerous trace fossils and physical sedimentary structures. After nearly 100 years since construction of the buildings, the block surfaces have become uneven as a result of differential weathering under the influence of wind and rain. Consequently, the trace fossils are now easier to observe, similarly to those exposed in natural coastal outcrops.

Left: Exterior view of the Yasuda Auditorium. The sandstone blocks used to construct the entrance archway contain many trace fossils.
Upper right: Close-up photograph of the archway at the Yasuda Auditorium. The weathered and uneven surface shows the occurrence of dense Thalassinoides, a branching burrow-network trace fossil.
Lower right: Ophiomorpha, a tunnel-shaped trace fossil with a mud-lined burrow wall, interpreted as the dwelling trace of a benthic organism.

Table of Contents

All the pages PDF ： 77_01_full.pdf ［38MB］

Abstract

Stratigraphic and tectonic implications of the Middle Miocene Shichiyama Formation, Tottori Group, eastern Tottori Prefecture, Japan

HAJI Toshiki, KANAYAMA Kyoko, KUDO Takashi, SUGAMORI Yoshiaki, NIKI Sota and HIRATA Takafumi

To reevaluate the Neogene stratigraphy, geological mapping was undertaken in the Shichiyama area of eastern Tottori Prefecture, Japan, coupled with detailed chronological and petrological analyses. To refine depositional ages and lithological characteristics, zircon U–Pb dating and K–Ar dating on plagioclase and hornblende were conducted, together with major-element geochemical analyses. The Neogene strata in the Shichiyama area comprise the Iwami Formation (ca. 17.1–16.4 Ma) and the Shichiyama Formation (ca. 15.3–15.1 Ma). The Shichiyama Formation consists of felsic pyroclastic rocks, intrusive rocks, and lava, and unconformably overlies the Iwami Formation. The Iwami Formation consists of marine deposits, whereas the pyroclastic facies of the Shichiyama Formation indicate deposition in a terrestrial environment. These observations indicate a marine regression during the early Middle Miocene, which has been widely recognized in the San'in region and is now corroborated by geological data from the eastern Tottori region. Our findings suggest that the regression associated with widespread folding in the San'in region is not consistent with the stratigraphic and chronological data from the eastern part of the region, indicating that the regression may not have resulted from crustal shortening. To further evaluate the causes of this regression, further precise data on the timing, magnitude, and spatial extent of the Middle Miocene uplift are required.

Trace fossils in rock used as building material on the Hongo Campus of the University of Tokyo

SEIKE Koji and IKEDA Masayuki

Rock used as building material in cities can contain not only body fossils but also trace fossils. Here we report trace fossils in rock from the Upper Cretaceous Futaba Group that was used as building material on Hongo Campus, the University of Tokyo, located in Bunkyo City, Tokyo, Japan. Rock in the walls of the Yasuda Auditorium, the General Library of the University of Tokyo, and other campus buildings constructed nearly 100 years ago contain abundant trace fossils, including Thalassinoides, Ophiomorpha, Macaronichnus, Piscichnus, Scolicia, and escape structures, as well as physical sedimentary structures such as parallel laminations and trough cross-bedding.