Examination of the Relationship between the Ukinuno and Sakate Tephras from Sambe Volcano, Southwest Japan

Abstract

 The Sakate tephra beds in the Chugoku and Kinki districts of Japan have been correlated with the Sambe–Ukinuno (SUk) widespread marker tephras, which were erupted from Sambe volcano (ca. 20 ka). Based on geological surveys of proximal outcrops, the SUk has been divided into three tephras: the lowermost Ukinuno pumice fall deposit (Uk-pfa; normally regarded as the SUk tephra), the Midorigaoka pyroclastic flow deposit (Md-fl), and the uppermost Ukinuno ash fall deposit (Uk-fa). The Sakate tephra has been associated with the Md-fl. Moreover, previous studies have suggested that the refractive index of amphibole in the Md-fl (i.e., Sakate) samples is similar to that in the Uk-pfa samples, whereas those of the volcanic glass shards in the Md-fl samples are lower than those of the Uk-pfa samples. The petrographic properties of Uk-fa suggest that Uk-fa and Md-fl could be treated as a single tephra. However, the SiO₂ contents of the Uk-pfa volcanic glasses are slightly higher than those of Md-fl. The FeO contents of the Uk-pfa volcanic glasses are higher than those of Md-fl. This suggests that the difference in the refractive index values of the volcanic glasses is due to the FeO contents rather than the SiO₂ contents. A total of 58 elements distinguish the Md-fl volcanic glasses from Uk-pfa, particularly the light rare earth elements, although the trace element patterns are generally similar. This difference may result from fractional crystallization and/or varying magma inputs. In previous studies, the tephras correlated with the Md-fl/Uk-fa (i.e., Sakate) tephras were found to be widely distributed in the Chugoku and Kinki districts. However, those correlated with Uk-pfa were found in the southern Kinki district and immediately offshore, and at the eastern end of Shikoku Island. Uk-pfa and Md-fl/Uk-fa were erupted during the same eruptive sequence, but are clearly distinguishable from each other. The Uk-pfa and Md-fl/Uk-fa tephras can be more simply redefined as the SUk (lower) and (upper) tephras, respectively.