The Hachiōji District is a quadrangle of latitudes 35°30′11.6″N to 40°11′6″N and longitudes 139°14′48.5″E to 129°48′4″E in WGS 84, located on the boundary between the Kantō Range and Kantō Plain in central Japan. Hills and uplands included in the plain and a minor contribution of mountains characterize the geomorphology of this district. The eastern flank of the Kantō Range and the Tanzawa Mountains overlie Cretaceous to Paleogene accretionary complexes in the Shimanto belt, and Miocene marine sedimentary rocks of the Aikawa, Hayato and Tanzawa Groups. The western margin hills of the Kantō Plain consist of Pliocene to Early Pleistocene marine and fluvial sediments of the Tate Formation and the Kazusa Group. They underlie Middle Pleistocene higher terrace deposits. The Pliocene and Early Pleistocene marine sediments of the Nakatsu Group are distributed along the terrace risers of the Sagami and Nakatsu Rivers. The uplands are composed of Middle Pleistocene to Holocene fluvial sediments, which are divided into the Higher, Middle, Lower and Alluvial terrace deposits, in descending order. Tephric loess up to 30-40 m thick in the Kantō Plain, called the “Kantō Loam”, overlies hills and uplands in this district. Modern alluvial fan and flood plain deposits are observed along the trunk streams.
Kobotoke and Sagamiko Groups
Accretionary complexes in the Shimanto Belt are divided into the Cretaceous Kobotoke Group and the Paleogene Sagamiko Group.
The Kobotoke Group lithologically comprises three units, which are characterized by either the coherent or the mélange units. The coherent units are composed of sandstone, shale, and interbedded sandstone and shale, named the Bomborigawa and Kosuge units.
The mélange unit is the Kobuse Unit, comprised of shale and broken beds of sandstone, with mélange including chert, tuff and basalt blocks in an argillaceous matrix. The matrix is generally phyllitic and foliated. Based on radiolarian fossils from the shale, the geologic age of the Kobotoke Group is mostly the Santonian to Campanian, with the same range between the coherent and mélange units. The Kobotoke Group was subjected to sub-greenschist facies to greenschist facies metamorphism during the Middle Eocene.
The Sagamiko Group comprises the Gongenyama and Seto units. The Gongenyama Unit is composed of conglomerate, sandstone and interbedded sandstone and shale. The Seto Unit consists of sandstone, interbedded sandstone and shale, and mélange with sandstone and tuffaceous shale blocks. The geological age of the Gongenyama Unit is the Paleocene to Middle Eocene, while the Seto Unit is from the Middle Eocene to Early Oligocene.
Tanzawa, Hayato and Aikawa Groups
The Neogene succession of the Tanzawa Mountains is divided into the Tanzawa, Hayato, and Aikawa Groups. The Tanzawa Group, dominantly basalt to andesite volcaniclastic rocks, is considered to be a product of marine volcanic activity on an accreted arc province (the Tanzawa terrane) during the latest Early Miocene to earliest Late Miocene. The Hayato and Aikawa Groups, dominated by volcaniclastic rocks, mudstone and conglomerate, are interpreted as trough basin fills, induced by collision between the Izu-Bonin
* Institute of Geology and Geoinformation
** Institute of Geology and Geoinformation (present affiliation : Faculty of Risk and Crisis Management, Chiba Institute of Science)
and Honshu Arcs during the Late Miocene to Pliocene. The Yataro Formation in the uppermost part of the Tanzawa Group, the Zike Formation in the lower part of the Hayato Group, and four formations in the Aikawa Group are distributed in the southwestern end of the Hachiōji District. These Formations display NW-SE strikes and 30-70° NE dips. Parts of them have been removed by the action the Tonoki-Aikawa and Aonohara-Susugaya faults, which trend NW-SE.
The Yataro Formation is composed of basalt to andesite lapilli tuff, tuffaceous sandstone, sandstone and mudstone, with thin beds of pumice tuff. The Zike Formation uncomformably overlies the Yataro Formation, and consists mainly of mudstone, accompanied by sandstone, tuff and conglomerate. The Aikawa Group includes the Miyagase, Funazawa, Nakatukyo and Sekirozan Formations.
The Miyagase Formation is composed mainly of basalt to andesite volcanic breccia, with dacite tuff, sandstone and mudstone. The basal portion of the Miyagase Formation is in fault contact with the Zike Formation. The Funazawa Formation rests conformably on the Miyagase Formation and is composed mainly of basalt to andesite tuff and lapilli tuff, tuffaceous sandstone and mudstone, accompanied by dacite tuff. The Nakatukyo Formation rests conformably on the Funazawa Formation, and includes volcanic breccia, tuff, and sandstone and mudstone facies. The Sekirozan Formation rests uncomformably on the Nakatsukyo Formation, and is dominated by conglomerate of deep submarine fan channel origin. The Nakatukyo and Sekirozan Formations are in fault contact with the Sagamiko Group of the Shimanto belt.
Tate Formation, Kazusa and Nakatsu Groups
Pliocene and Early Pleistocene sediments in the Hachiōji District are exposed in the western margin hills of the Kantō Plain and the uplands along the middle reach of the Sagami and the Nakatsu Rivers. The marine Tate Formation is distributed in the westernmost of the Tama Hills, unconformably overlain by the Kazusa Group. The Kazusa Group is divided into fourteen formations that underlie the Tama, Ongata, Motohachiōji, Funeda and Kasumi Hills. The Terada, Oyabe, Hirayama, Oyamada, Renokoji, Inagi and Dedana Formations in the western Tama Hills are eustasy-influenced marine and fluvial sequences, while the Tsurukawa, Kakio and Ozenji Formations are shallow marine silici-clastic sequences in the central Tama Hills. The Ongata and Motohachioji Formations in the Ongata, Motohachiōji and Funeda Hills are gravelly deposits of tributary and trunk streams. The Nakatsu Group, exposed along the terrace risers, is made of shallow and deep marine sediments divided into the Kosawa, Kanzawa, Shimizu, Otsuka and Shioda Formations.
Terrace deposits
Middle Pleistocene to Holocene fluvial terrace deposits occur in parts of the hills and in all the uplands. Middle Pleistocene higher terrace deposits of the Gotentoge, Zama Hills, and Kobiki terrace deposits crop out in the west Tama, Zama and Kobiki Hills. The Echi and Mukaihara Formations bury valleys under the Sagamino and Nakatsuhara Uplands. Late Pleistocene middle and lower terrace deposits develop strath terraces over the Sagamino and Nakatsuhara Uplands in the lower reach of the Sagami River, while fill-top and fill-strath terraces along the middle reach of Sagami, Kushi and Nakatsu Rivers. Holocene alluvial terrace deposits occur along all streams, and modern alluvial fan and riverbed deposits are found in the trunk streams of the Sagami and Tama Rivers. Thick tephric loess called the “Kantō Loam” is intercalated by numerous tephra layers, and overlies all Middle and Late Pleistocene terrace deposits.
Fig. 1 Outline of geology of the Hachiōji District
Fig. 2 Stratigraphy of the Kobotoke, Sagamiko Groups in the Shimanto belt, and the Tanzawa, Hayato and Aikawa Groups
J
O
K M (Ma)
Brunhes Matuyama Gauss Gilbert 0.78
1.07
1.77 1.95
2.58
3.22 3.11 3.04
3.58 3.33
3 2
1 J
O
Fig. 3 Stratigraphy of the Tate Formation, and the Kazusa and Nakatsu Groups
The Fukushima, Oyamada and Renkoji Formations along the Tama River are found in the Ome District.
The ages of paleo-magnetic polarity boundary are referred to Cande and Kent (1995)
10
50
100
500
(
Ⅰ
Fig. 4 Stratigraphy of the terrace deposits