Some Considerations on the Forming of the Late Pliocene

Early Pleistocene Kurotaki Formation, Distributed in the Ubara District along the Pacific Coast of the Boso Peninsula

Abstract

Concerning the mechanism of the deposition of the Kurotaki Formation in this district along the Pacific coast of the Boso peninsula, many problems still remain unsolved. There are, however, some facts and considerations clarified in this work as follows:
1) The thick sediments with colossal boulders of the Kurotaki Formation distribute only at the Boranohana and the Kurogahana areas, Ubara, Katsuura city, throughout the areas in the Boso peninsula where the Formation distributes, and are observed to be extraordinary in sediments.
2) The Formation is about 35 m thick, and contains various kinds of pebbles and boulders, such as massive medium-grained sandstone, silty sandstone, tuffaceous siltstone, chert, andesite and so forth. They came mainly from the Kiyosumi Formation below, and some of them might have come from the Anno Formation and Mesozoic and Palaeozoic Groups.3) The conglomeratic facies of the Formation in the area distribute in the direction of NE-SW. And the sedimentary cycles shown in conglomerate, coarse-grained sandstone and fine-grained sandstone, have been repeated several times. The diameter of a pebble at the lowermost horizon is larger than that at the upper horizon, and the form of the pebble at the lowermost horizon is angular or subangular, and sometimes bladed. These conglomeratic facies are deposited on the concave plane which was made by the erosion of the Kiyosumi Formation.
4) The Kurotaki unconformity seen at the Boranohana and the Kurogahana areas, has clearly been eroded at the sea-bottom by slumping or sliding.
5) From the field evidences, it is presumed that the channel was formed, and then it was filled with the conglomeratic facies of the Formation.
6) The phenomena mentioned above are regarded as the earliest appearanceof the sediments in the late Pliocene-early Pleistocene Kazusa Group.