The geology of

Bhutan Bhutan (; dz, འབྲུག་ཡུལ་, Druk Yul ), officially the Kingdom of Bhutan,), is a landlocked country in South Asia. It is situated in the Eastern Himalayas, between China in the north and India in the south. A mountainous ...

is less well studied than many countries in Asia, together with the broader Eastern Himalayas region. Older

Paleozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ' ...

and

Precambrian The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of the ...

rocks often appear mixed together with younger sediments due to the

Himalayan orogeny The geology of the Himalayas is a record of the most dramatic and visible creations of the immense mountain range formed by plate tectonic forces and sculpted by weathering and erosion. The Himalayas, which stretch over 2400 km between th ...

Stratigraphy, tectonics & geologic history

Geologic research in Bhutan began in the southwest between 1868 and 1907, followed by regional mapping in 1961 by the Geological Survey of India. A. Gansser led much of the geologic research from the late 1960s through the 1980s. Some of the oldest rocks in Bhutan are part of the Indian Shield in the Shillong Basement—part of a

foreland basin A foreland basin is a structural basin that develops adjacent and parallel to a mountain belt. Foreland basins form because the immense mass created by crustal thickening associated with the evolution of a mountain belt causes the lithospher ...

. The Bhutanese foothills are separated from Precambrian crystalline rock by the 35 kilometer foreland, where rocks are fold and faulted newer tectonic events and covered in Quaternary sediments. The South Himalayan

molasse __NOTOC__ The term "molasse" () refers to sandstones, shales and conglomerates that form as terrestrial or shallow marine deposits in front of rising mountain chains. The molasse deposits accumulate in a foreland basin, especially on top of flys ...

is represented by the Siwalik Belt, with four kilometers of siltstone, sandstone and conglomerates in the southeast. The Damuda Belt dates to the

Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...

and

Permian The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Paleoz ...

in the

with coal seams, plant remnants and lenses of much younger

Eocene The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...

limestone included during the Himalayan orogeny. The Lesser Himalayas, to the north of the Damuda Belt contain Precambrian sedimentary rocks such as slate,

dolomite Dolomite may refer to: *Dolomite (mineral), a carbonate mineral *Dolomite (rock), also known as dolostone, a sedimentary carbonate rock *Dolomite, Alabama, United States, an unincorporated community *Dolomite, California, United States, an unincor ...

and several kilometers of

quartzite Quartzite is a hard, non- foliated metamorphic rock which was originally pure quartz sandstone.Essentials of Geology, 3rd Edition, Stephen Marshak, p 182 Sandstone is converted into quartzite through heating and pressure usually related to tect ...

. "Slices" of basement

granite Granite () is a coarse-grained (phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies undergro ...

and

gneiss Gneiss ( ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Gneiss forms at higher temperatures an ...

form

mylonite Mylonite is a fine-grained, compact metamorphic rock produced by dynamic recrystallization of the constituent minerals resulting in a reduction of the grain size of the rock. Mylonites can have many different mineralogical compositions; it is a cl ...

at the edges. These rocks are believed to be part of the Indian Shield and are one billion years old. Geologists also define the High Himalayan Crystalline rocks extending 150 kilometers from the Main Central Thrust to

Tethys Ocean The Tethys Ocean ( el, Τηθύς ''Tēthús''), also called the Tethys Sea or the Neo-Tethys, was a prehistoric ocean that covered most of the Earth during much of the Mesozoic Era and early Cenozoic Era, located between the ancient continents ...

sediments in the north.

Sillimanite Sillimanite is an aluminosilicate mineral with the chemical formula Al2SiO5. Sillimanite is named after the American chemist Benjamin Silliman (1779–1864). It was first described in 1824 for an occurrence in Chester, Connecticut. Occurrence S ...

garnet Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition. The different s ...

and

biotite Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more alumino ...

are especially common in this area. The Takhtsang monastery, carved into Takhtsang gneiss, is surrounded by rocks with calcsilicate lenses, interpreted as the "hand and foot prints" of Padmasambhva—an important founder of Bhutanese holy sites. This gneiss grades toward

migmatite Migmatite is a composite rock found in medium and high-grade metamorphic environments, commonly within Precambrian cratonic blocks. It consists of two or more constituents often layered repetitively: one layer is an older metamorphic rock tha ...

, at higher elevations such as Masang Kang mountain. Geologists have dated the rock to 500 to 400 million years ago. The Paro marbles are widespread in southwest, forming massifs, while leuocogranites are common in dikes, sills and plutons in the northeast. Tethys Ocean, Tethyan sediments outcrop along the Tibetan border. The Lingshi Basin is one example, beginning with a foundation of marble and

phyllite Phyllite ( ) is a type of foliated metamorphic rock created from slate that is further metamorphosed so that very fine grained white mica achieves a preferred orientation.Stephen Marshak ''Essentials of Geology'', 3rd ed. It is primarily compo ...

lying atop older crystalline rock, overlain by

tillite image:Geschiebemergel.JPG, Closeup of glacial till. Note that the larger grains (pebbles and gravel) in the till are completely surrounded by the matrix of finer material (silt and sand), and this characteristic, known as ''matrix support'', is d ...

and limestone. In the basin, migmatite gneiss is transgressed by calc-schist, breccia, quartzite, limestone and recrystallized fossils. The Tang Chu Basin, on the other hand, holds Devonian rocks and fossils. Young faults and fracture zones formed in the last 2.5 million years of the

Quaternary The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS). It follows the Neogene Period and spans from 2.58 million years ...

. These recent tectonic features are found in the central and east, often associated with hot springs. Research published in 1983 indicated seven glacial periods during the

Pleistocene The Pleistocene ( , often referred to as the ''Ice age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fina ...

, based on terminal

moraine A moraine is any accumulation of unconsolidated debris (regolith and rock), sometimes referred to as glacial till, that occurs in both currently and formerly glaciated regions, and that has been previously carried along by a glacier or ice shee ...

distribution.

Natural resource geology

Bhutan has small gypsum deposits in the Kuru Chu spur and limestones in the foothills of the Himalayas which are extracted locally for the cement industry. Coal seams, with 25 percent ash, are scattered throughout the Damudas region. The Tang-Chu area has high-quality Devonian slate used for roofing.

References

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