Geology of Mouka-Makabe-Tsuchiura area

The Yamizo Mountains consist of four mountain blocks: the Yamizo, Torinoko, Keisoku, and Tsukuba Mountain Blocks, from north to south. The northern three mountain blocks are composed mainly of Mesozoic sedimentary rocks. The southern block, the Tsukuba Mountain Blocks, is composed of plutonic rocks and metamorphic rocks.

The sedimentary complex in the Keisoku Mountain Block is subdivided into the Kasama, Kunimiyama, Takatori, and Ayuta Units. The Kasama Unit is composed mainly of alternations of sandstone and shale with a small amount of chert and tuff layers. The Kunimiyama Unit consists of massive sandstone and alternations of sandstone and shale. The Takatori Unit characterized by the chert-clastic sequence comprising Toishi-type shale, bedded chert, siliceous shale, and clastic rocks in ascending order. The Ayuta Unit is composed mainly of alternations of sandstone and shale. These units are in fault contact with each other.

Based on the lithological and chronological characteristics, Mesozoic sedimentary complex of the Keisoku Mountain Block is correlated to those of the Tanba, Mino and Kiso areas in southwest Japan. Mesozoic strata distributed in the Keisoku Mountain Block is regarded as an accretionary wedges formed mainly by offscrape-accretion during Late Jurassic time.

Tsukuba Metamorphic Rocks and Wagakunisan Metamorphic Rocks are distributed around the Tsukuba Mountains. They suffered high temperature and low pressure (low P/T) metamorphism in Late Cretaceous – early Paleogene time. Original rocks of these metamorphic rocks are considered as the Late Jurassic – Early Cretaceous sedimentary complex. These metamorphic rocks are considered to be an eastern extension of the Ryoke Metamorphic Rocks. They consist mainly of metamorphic rocks derived from mudstone and alternating layers of mudstone and sandstone. The upper part of these metamorphic rocks is mainly derived from mudstone, sandstone, chert, and limestone. The lower part consists mainly of metamorphic rocks derived from mudstone and alternating layers of mudstone and sandstone. Mineral assemblages indicate that the metamorphic pressure of the Tsukuba Metamorphic Rocks is higher than that of the Wagakunisan Metamorphic Rocks.

The plutonic rocks consist of Late Cretaceous gabbroic rocks and Early Paleogene granitic rocks. The granitic rocks are considered to be an eastern extension of the Ryoke and San’yo Belts. The gabbroic rocks were intruded by granitic rocks, and the former were thermally metamorphosed near the contact. The K-Ar age of the gabbro is 75 Ma, which is much older than the granitic rocks dated at around 60 Ma. The Tsukubasan gabbroic complex is composed of various rocks such as olivine gabbro, hornblende gabbro, anorthosite, cortlandite, and supessartite.

The granitic rocks are classified into the Tsukuba, Inada, and Kabasan Granites, based on their intrusive relations, rock facies and the surrounding metamorphic rocks. Major facies of Tsukuba Granite are porphyritic biotite granodiorite, foliated biotite tonalite, muscovite-biotite granite, and fine-grained granites. The cordierite-bearing orbicular rock occurs in the porphyritic granodiorite. Inada Granite is composed mainly of coarse-grained hornblende-bearing biotite granite, medium-grained hornblende-biotite granodiorite, and fine-grained muscovite-biotite granodiorite. Major rock facies of Kabasan Granite are medium-grained biotite granite and fine-grained muscovite-bearing biotite granite.

The andesite lava dome occurs in the Mouka area. The andesite dikes intrude both the metamorphic and granitic rocks in the Tsukuba Mountain Blocks. The K-Ar age indicates they are Miocene volcanism. Early Miocene Motegi Formation consists of dacite pyroclastic flow deposits and andesite lava, and is distributed in northern margin of the Mouka area.

Quaternary sediments, including Sakaibayashi Formation, Tomobe Formation, Kamiiwahashi Formation, Miwa Formation, Joso Formation, terrace and gentle slope deposits, and alluvium, are widely distributed in the hills and lowland areas. The Sakaibayashi Formation consists of gravel, sand, and mud. The Tomobe Formation is composed of Middle Pleistocene marine sediments. The Miwa Formation is mainly composed of sand and gravel, corresponds to the Shimosueyoshi Formation and Kioroshi Formation in the southern Kanto district. The Joso Formation, which consists of Late Pleistocene non-marine sediments.

The terrace surfaces are divided into higher, middle I, middle II, and lower terraces. The higher terrace formed in the Middle Pleistocene, and the middle and lower terraces formed in the Late Pleistocene. The middle I and the II terrace deposits correspond to the Miwa and Joso Formations, respectively. The gentle slope deposits are distributed over mountain slope. Alluvium is distributed along the main river valley.