Geology of Mouka-Makabe-Tsuchiura area

 

Geological map

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Legend of the map

Symbols in brackets are used in gMakabeh by Miyazaki et al. (1996).

 

Quaternary

Holocene

a(a): Alluvium, Valley bottom deposits, Gravel, sand and mud

al(al): Alluvium, Natural levee deposits; Sand

 

Late Pleistocene to Holocene

ps(sl, sh): Piedmont slope and fan deposits; Boulder and sand

 

Late Pleistocene

tl: Lower terrace deposits; Gravel, sand and mud

tm2(Jo/tm3): Middle terrace deposits, Joso Formation; Sand, gravel and mud

tm1(tm2, Mi/Ki): Middle terrace deposits, Miwa Formation; Sand, mud and gravel (mainly marine)

 

Middle Pleistocene

th(th): Higher terrace deposits; Gravel, sand and mud

Sh: Kamiiwahashi Formation; Sand and mud (mainly marine)

Tb(To): Tomobe Formation; Sand, mud and gravel (mainly marine)

Sk: Sakaibayashi Formation; Gravel, sand and mud

 

Neogene

Early Miocene

Mt: Motegi Formation; Dacite pyroclastic flow deposits and andesite lava

An(An): Andesite and dacite dike and lava; Andesite and dacite

 

Late Cretaceous to Early Paleogene

Kg4(Ka4): Kabasan Granite (4); Very fine-grained biotite granodiorite

Kg3(Ka3): Kabasan Granite (3); Leucocratic fine-grained garnet-bearing muscovite biotite granite

Kg2(Ka2): Kabasan Granite (2); Fine-grained muscovite bearing biotite granite

Kg1(Ka1): Kabasan Granite (1); Medium-grained biotite granite and porphyritic (muscovite-bearing) biotite granite

In4: Inada Granite (4); Fine-grained biotite-bearing hornblende diorite

In3(In3): Inada Granite (3); Fine-grained muscovite biotite granodiorite

In2(In2): Inada Granite (2); Medium-grained hornblende-biotite granodiorite

In1(In1): Inada Granite (1); Coarse-grained hornblende bearing biotite granodiorite to granite

Ts6(Ts6): Tsukuba Granite (6); Orbicular rock (dike)

Ts5(Ts5): Tsukuba Granite (5); Fine-grained granites

Ts4(Ts4): Tsukuba Granite (4); Medium-grained muscovite-biotite granite

Ts3(Ts3): Tsukuba Granite (3); Porphyritic biotite granodiorite and coarse-grained biotite granodiorite to granite

Ts2(Ts2): Tsukuba Granite (2); Foliated biotite tonalite

Ts1(Ts1): Tsukuba Granite (1); Fine- to medium-grained biotite-hornblende diorite to tonalite

B: Tsukuba and Wagakunisan Metamorphic Rocks; Biotite zone

Cd: Tsukuba and Wagakunisan Metamorphic Rocks; Cordierite zone

S: Tsukuba and Wagakunisan Metamorphic Rocks; Sillimanite zone

Gb(Gb): Gabbroic rocks; Olivine gabbro, hornblende gabbro, anorthosite, cortlanditeand spessartite

 

Jurassic

Accretionary complex.  Yamizo Group (Ayuta Unit, Takatori Unit and Kunimiyama Unit) and protoliths of Tsukuba and Wagakunisan Metamorphic Rocks)

Js(ms): Sandstone with mudstone

Jsm(mm): Interbeded sandstone and mudstone, mudstone, siliceous mudstone and mixed rocks

Jm: Massive mudstone

Jc(mc): Chert

Jl(ml): Limestone

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Outline of geology

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.

Fig. 1 Tsukuba Metamorphic Rocks (Loc. Hirasawa)

 

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fyo 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.

Fig. 2 Gabbro (Loc. Mt. Nanatai)

 

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.

Fig. 3 Inada Granite (Loc. Inada)

 

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.

Fig. 4 Dacite pyroclastic flow deposits (Loc. Isoyama)

 

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.

Fig. 5 Tomobe Formation (Loc. Ujie)

 

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.