Miocene formations in the Takasaki district, central Japan : With special reference to the developmental process of the Niwaya Unconformity

Miocene formations in the Takasaki district are distributed along the northern margin of the Kanto Mountains. These Miocene strata unconformably overlie pre-Neogene granitic rocks, the Kanohara Conglomerate and the Kottateyama Tuff and are in fault contact with the Sanbagawa metamorphic rocks. This series is divided into the following seven units, in ascending order : Ushibuse Formation (400m+thick), Obata Formation (900m), Idozawa Formation (450m), Haratajino Formation (40m), Niwaya Formation (400m-), Haraichi Formation (550m)and Itahana Formation (1200m). The Niwaya Formation overlies the Haratajino, Idozawa and Obata Formations with partial unconformity. Miocene tectonic evolution in this area is summarized as follows. In the early Middle Miocene (zone N. 8), arkosic sand and silt were deposited in an upper～middle bathyal environment (Konda, 1980). At the time of zone N. 9, the Takasaki area underwent NE-SW shortening deformation : reverse faulting and folding. Many minor faults formed in this time show NE-SW trending σ_1. The eastern part of the study area was uplifted on account of this deformation and the crests of the folded strata were eroded to form the Niwaya Unconformity. While in the western area, sedimentation continued during this period. This unconformity seems to have progressed in association with the opening of the Japan Sea (14.9Ma : Otofuji et al., 1985). In the middle to late Middle Miocene (zone N.10～N.14), sand and silt were deposited in a lower sublittoral～upper bathyal environment. The low sediment accumulation rate indicates slow subsidence of the Takasaki district. Many normal faults were formed during (or after) this period under E-W～NW-SE extensional stress field. In the late Middle Miocene time, coarse sand and gravel filled the shallow sea to lead to land areas accreting gradually