2020 Volume 74 Issue 1 Pages 21-38
The Myogi area in western Gunma Prefecture shows a jagged landform of thin ridges and deep gorges bounded by precipitous cliffs up to several hundred meters high. The rock formations of the area consist of Late Miocene subaerial volcanics (the Myogi Formation), which are surrounded with the Middle Miocene marine sediments.
The Myogi Formation is composed of the Yotsuya, Ipponsugi, Nakanotake and Chosunokashira Members in ascending order. The latter two, called the upper Myogi Formation, consist mainly of alternation of ortho- and clino-pyroxene andesite lavas and andesitic volcaniclastics, over 1,800 m thick. Many synchronous andesitic to dacitic intrusions and volcano-plutonic complexes penetrate the volcanic formations. The voluminous Nakanotake M. is bounded in an extent of 7 km x 9 km, surrounded by mostly vertical faults. The member is segmented into many smaller blocks several hundred meters to one kilometer across. The blocks tilt at various angles up to 90° and are clino-unconformably overlain by the horizontal layers of the Chosunokashira M.
It is thus inferred that, after the accumulation of the thick Nakanotake M., it was segmented and depressed over 1,000 m though the collapsing and was subsequently covered unconformably by the Chosunokashira M. This Late Miocene volcanic cauldron is newly named the Myogi Caldera. The deep structure of the caldera was subsequently excavated to form the present jagged landform of the area.
