The Neogene Chikura Group, which is widely exposed in the southeastern part of Minamiboso City, Chiba Prefecture, Japan, is made up of a thick sequence of marine sedimentary rocks deposited in a middle to upper bathyal environment. The group comprises (in ascending order) the Shirahama, Shiramazu, Mera and Hata formations. The upper Pliocene Shirahama Formation is composed mainly of red-brown volcaniclastic sandstone and the Nojimazaki Conglomerate Member. This member comprises volcaniclastic conglomerate with granules to boulders of basalt and andesite, and is characterized by pebbles of andesite, basalt, granodiorite, gabbro, sandstone, siltstone, greenish tuff, and chert. We obtained Anisian and Ladinian (Middle Triassic) radiolarians from chert pebbles, and Bajocian to Callovian (Middle Jurassic) radiolarians from a siliceous siltstone pebble. These Mesozoic pebbles were probably derived from a Mesozoic accretionary complex (present-day Kanto District) in the northwestern part of Boso Peninsula.