Bulletin of the Mizunami Fossil Museum

Bulletin of the Mizunami Fossil Museum No. 48

Strontium isotope (87Sr/86Sr) ratios for molluscan fossil fragments from the Tokorozawa observation well by Saitama Prefecture, two water wells and three surface samples were measured. Although most samples were altered due to surface water circulation and diagenesis, the highest 87Sr/86Sr ratio among samples shall be nearly equal to the original 87Sr/86Sr ratio.

Sr isotope ages for less altered samples are estimated using Look-Up Table by McArthur and Howarth (2004). It was found that there was no inconsistency in vertical relationship among the ob-tained Sr ages of the wells. However, it is not possible to evaluate the Sr ages due to lack of reliable age data. Sr isotope age of 10.5 Ma for the Tsuchishio Formation is slightly older than its geological age by diatom fossils due to alteration.

For Sr isotope ages using molluscan fossil fragments, it is necessary to analyze multiple samples in the same stratum as much as possible.