Revised age of the Sasayama Group, southwest Japan, based on ostracode and conchostracan bio-stratigraphy and zircon fission-track dating

Abstract

The age of the non-marine Cretaceous Sasayama Group, distributed in western Honshu, Japan, was reassessed based on ostracod and conchostracan bio-stratigraphy, and zircon fission-track dating. The ostracod genera Mongolocypris and Eucypris from the lower part of the Sasayama Group, combined with the conchostracan genera Yanjiestheria and Nemetheria from the upper part of the group, suggest an age range from Albian to Cenomanian. Zircon fission-track dating of the lower and upper parts of the group yields ages of 106±9, 100.9±8.4, and 96.1±7.4 Ma, also indicating a range from Albian to Cenomanian. These data indicate that deposition of the Sasayama Group began about 30 Ma later than previously thought.