Abstract
The stratigraphy and tectonics of the Neogene formations in the Shimonita Tectonic Zone extending along the northern margin of the Kanto Mountains are studied. The Lower Miocene formations distributed in the Shimonita Tectonic Zone are divided into the Kanohara Conglomerate, Iwayama Conglomerate and Kawai Sandstone and Mudstone Members in ascending order. The Kanohara Member is mostly composed of poorly-sorted boulder conglomerate consisting of reddish igneous rocks with Cretaceous age. The Iwayama Member is characterized by matrix-supported conglomerates having more compositional varieties of gravel than the underlying Kanohara Conglomerate Member. The Kawai Member conformably overlies the Iwayama Member and consists of sandy mudstone and sandstone. The Nukabe Formation which unconformably overlies the Shimonita Formation consists mainly of sandstone, mudstone and alternating beds of sandstone and mudstone. The Cenozoic formations in the Shimonita Tectonic Zone are separated into western and eastern area tectonically by the Sendaira-Maita Line. The formation of the western area is folded into the Shimonita Syncline, while the Nukabe Formation in the eastern area arched into open folds. On the basis of the geological, sedimentological and paleontological analysis, the lower two Members(the Kanohara and Iwayama Members) of the Shimonita Formation may have deposited in an alluvial fan environment. The Iwayama Member may have deposited in a shallow marine environment. The early Miocene shallow sea expanding in this area is designated the Paleo-Shimonita Bay.
