2022 Volume 128 Issue 1 Pages 143-152
The Moriya Formation in central Nagano Prefecture, Japan, records the paleoenvironment and crustal movement in central Hon-shu during the Miocene. The lower part of the formation consists of clastic sedimentary rocks, and the upper part comprises altered volcanic rocks. Fossil foraminifera indicate that the clastic sedimentary rocks were deposited during the period of the N8 planktonic foraminiferal zone (17.0-15.1 Ma). However, no previous studies have reported ages for the overlying volcanic rocks. Here we present zircon U-Pb ages for the uppermost volcanic member of the Moriya Formation. We obtained a weighted mean age of 15.5±0.2 <sub>(2σ)</sub> Ma, indicating that Miocene volcanic activity started after 17.0 Ma and continued until ca. 15.5 Ma. This formation, including the upper volcanic rocks, can be correlated to the lower-middle Miocene Uchimura Formation, a lithostratigraphic division of a representative stratigraphic succession in the northern Fossa Magna region. The volcanic rocks of the Moriya Formation are older than most of the Shitara (Shidara) volcanic rocks (ca. 15-13 Ma) in Aichi Prefecture but may be coeval with andesitic-basaltic rocks exposed around the Aichi/Nagano prefectural boundary (the Tsugu volcanic rocks).
