Limestones used on the walls of the School of Science Bldg.1

  • T. Oji (Associate Professor, Department of Earth and Planetary Science)
Figure 1

Fig. 1 Sea snail shells (One of Nerineidae ?, Discovered on the south side of Area W, Science Bldg.1)

Figure 2

Fig. 2 Trace fossil of a large burrowing benthic animal (Discovered in front of the elevator on the south side of Area W, Science Bldg.1)

More than four and a half years have passed since the Department of Earth and Planetary Environmental Science moved to the current School of Science Bldg.1. I feel comfortable and am happy to be able to conduct research and education activities in this wonderful building.

Pale cream-tinged white limestones (widely called as “marble”) have been used for the exterior walls and the interior walls of the first floor of this building. On the surface of the limestones, many small rounded grains that look like a grain of rice can be seen. These grains are called oolites, which are commonly formed in coral reefs in the tropical and subtropical seas. Limestones that contain oolites have also been found from various geologic ages.

If you take a closer look, you will notice many fossils contained in the limestones. There are many corals, bivalves, and sea snail shells (Fig. 1). Trace fossils (Fig. 2) of large burrowing benthic animals, which look like a pipe, can be frequently seen, although the fossils of the benthic animals that produced the structure cannot be discovered. It is fun to discover all these various fossils on the wall while waiting for the elevator to arrive. As a part of laboratory works of the paleontological course for undergraduate students, I always show polished surface of the limestones, and explain what kind of fossils are contained there.

However, I could not figure out the origin and the age of the limestones. If we, a group of specialists in earth and planetary science, could not solve these questions, I thought it would discredit ourselves. Therefore, I have been wondering if there is a way to identify the age of the fossils on the walls since we moved into this building. Then I heard that a coral reef specialist at a different university saw the limestones of this building and regarded these rocks as the fairly recent ones in the Cenozoic Era. It is probable that there are limestones that were formed in a quite recent time but look hard. Based on the opinions like that, a faculty in our Department tried a radiocarbon isotope dating. But the radiocarbon used for the dating possesses approx. half-life 6000 years, which means that samples that are older than hundreds of thousand years cannot be measured in the dating. As anticipated, the dating of the sample could not be measured. The limestones were much older than he had expected. When we look at the fossils in detail, we notice that the cross-section of some snail shells have complicated, thick walls. Such structures are commonly discovered in a group called Nerineidae, which flourished in the Jurassic and the Cretaceous Periods. When we observed the well-polished surface more widely and more carefully, we found brown, long bullet-shaped fossils in four places. These are called “Belemnites”, fossils of squid that flourished in the Mesozoic Period. Since they became extinct at the end of the Mesozoic Period, it seems unlikely that the limestones of this building are from the Cenozoic Era.

Therefore, I decided to ask my acquaintance about these building stones, who has long been working for the facility-related team of the School of Science administration. As a result of her investigation through the general contractor, we could finally find out that the limestones are from Vilhonneur, a commune in south-western France. Through further research on the internet, I could learn that Vilhonneur is a famous town for building stone production and, the age of the rock is assigned to the Middle Jurassic Period.

It is thought that the European continent in the Middle Jurassic Period had much warmer climate than now, and numerous oolites were produced in the shallow sea. Beds of limestones of the same ages, which contain oolites, are also traceable to the United Kingdom, and the beds there are called as “Inferior Oolite” and “Great Oolite”. Large-size ammonite fossils have also been discovered there, too (some ammonite specimens from the Inferior Oolite are housed as a collection of the University Museum, the University of Tokyo.) Unfortunately, there were no ammonite fossils found on the wall of this building. But now I am relieved to know where the limestones came from. And I hope the readers of this article look at the fossils contained in the limestones of this building and imagine how the marine world looked like when it was a hundred million and tens of millions of years ago.