The Shiose-no-misaki Sandstone and Conglomerate (
SSC) in the Oga Peninsula is believed to represent a latest Eocene to early Oligocene marine succession located on the back-arc side of NE Japan. This unit is, however, isolated from adjacent units, and its stratigraphic position has remained in debate. On a sea cliff adjacent to the type locality of the
SSC, surface soil and vegetation recently slid down by heavy rain to disclose a succession that contains the
SSC and the overlying early Miocene non-marine unit correlative to the Daijima Formation. The
SSC at the new outcrop comprises scorialapilli tuff and parallel to wavy laminated fine sandstone, both of which are intruded in dike by thinly bedded pumicelapilli tuff, pumice-bearing tuff, sandstone and mudstone as observed also in the adjacent
SSC. The overlying succession is over 20 m thick and dominated by massive very coarse to coarse sandstone with a basal, normally graded cobble to granule conglomerate 1.3 m thick. Pumice lapilli tuff occurs between the conglomerate and the overlying sandstone with a thickness of 30 cm, and tuffaceous mudstone and coaly shale beds 20 to 30 cm thick occur in between the sandstone. The conglomerate and sandstone commonly contains gravels and/or sand grains of dacite welded tuff derived from the adjacent early Miocene Hokakejima Dacite (the lowest unit of the Daijima Formation). Pumice lapilli tuff in the upper unit is 20 Ma in FT age, suggesting slightly younger than the FT ages of 22-21 Ma for the Hokakejima Dacite. The pollen assemblage from the
SSC likely reflects a cool temperate climate that might have prevailed in accordance with Oligocene to Early Miocene cooling. We thus suggest the
SSC is a part of the late Eocene to early Oligocene Monzen Formation extensively distributed to the west of Cape Shiose-no-misaki.
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