Does the Himenoura Group in Amakusa, northern Ryukyu arc, involve Paleocene deposits?

Abstract

The Himenoura Group on Amakusa-Shimoshima Island, northern Ryukyu arc has been assigned to the Upper Cretaceous since the early 20th century based on molluscan fossils. However, zircon fission-track data suggested that the uppermost unit of the group contains a Paleocene succession. This hypothesis has yet to be verified. Here, we report a Campanian radiometric age from a vitric tuff in the uppermost unit of the group. We obtained the U–Pb ages of zircon crystals by laser ablation–inductively coupled plasma–mass spectrometry. The weighted mean age is 80.1±0.6_(2σ) Ma (MSWD = 0.75, n = 27), which is consistent with the age of fossils in the uppermost unit of the group on neighbouring Amakusa-Kamishima Island, where the stratigraphy of the group has been established.