Re-examination of the age of brackish-water deposit in the Miocene Tottori Group, Southwest Japan

Abstract

A rapid transgression occurred at the end of the early Miocene in the eastern San’in region, Southwest Japan, owing to tectonic subsidence associated with the back-arc rifting that lead to the opening of the Sea of Japan. This study aims to provide new constraints on the date of initiation of the transgression in this region. The deposition of the early-middle Miocene Iwami Formation was contemporaneous with the transgression. The lower part of the formation in the Miyanoshita area, Tottori Prefecture, is composed of terrestrial deposits covered by brackish-water mudstones. We dated a tuff bed intercalated with the mudstone. The ²³⁸U-²⁰⁶Pb ages of zircon grains separated from the tuff were measured using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), and a weighted mean age of 17.4±0.2 Ma (95% confidence interval, MSWD = 3.7, n = 27) was obtained after excluding a Cretaceous age from one grain. This age is older than that indicated by other evidence of marine environments in the eastern San’in region. The lithofacies of the upper lower Miocene strata suggest that the Miyanoshita area was an alluvial lowland just before the rapid transgression. Therefore, we conclude that the Miyanoshita area was submerged ~0.4 My earlier than other areas because of its paleogeography.