Stratigraphy of the Hayama Group in the Miura Peninsula, Japan

The late Cenozoic strata occupy the Miura Peninsula situated in the southern Kanto region. The author has been studying the geology of the Cenozoic sequence of the Miura Peninsula and the neighboring areas. The purpose of the study is to clarify the characteristics of the Cenozoic tectonics and the geologic history of the areas. The Cenozoic sequence of the peninsula consists of the Hayama, Miura, Kazusa and Sagami Groups and Alluvial sediments in ascending order. Reinvestigation was carried out by the author on the stratigraphic relationships among and within the groups, and on their tectonics in the Miura Peninsula. In the present paper, the following three themes are described; (1) the history of the geological studies of the Miura Peninsula, (2) the geologic outline of the peninsula, (3) the stratigraphy of the Hayama Group and the igneous rocks associated with the group. The Hayama Group, that is Early to Middle Miocene in age, is lithologically divided into four formations and one unit of olistostrome: namely, the Morito Formation; Abuzuru Formation including Tateishi Tuff Member that is considerd to result from pyroclastic flow deposits; Ōyama Formation; Kinugasa Muddy Olistostrome; and Yabe Formation containing Sakaguchi Tuffaceous Sandstone Member and Koyabe Tuffaceous Sandstone and Siltstone Member in ascending order. Some ultrabasic rocks are described petrographically, EPMA analysis of the minerals is shown. The author points out that the Kinugasa Muddy Olistostrome was possibly derived mostly from the Morito Formation, and partly from the Abuzuru Formation by gravity transportation of both the formations, and that most of the ultrabasic bodies and some of the basalt bodies are olistoliths respectively in the Kinugasa Muddy Olistostrome