general geology of Kathmandu basin

Kathmandu basin is one of the largest depositional basins of the Lesser Himalaya in the Central Nepal. It is an oval shaped basin with area about 665sq.km.  Kathmandu basin was previously called Paleo-Kathmandu Lake which contains fluvial and lacustrine deposits also called fluvio-lacustrine deposits.

 

FORMATION OF THE BASIN:

The drainage system plays an important role in the formation of Kathmandu basin. As the drainage system is highlighted by Bagmati River flowing from the northern most part of Kathmandu to the south. During the time of formation southern part of Kathmandu uplifted due to the active thrust called Mahabharat Thrust. The further uplift of the thrust results to elevate the southern part of the Kathmandu basin. Due to which it lead to form oval basin covered by hills from all the sides. Due to the uplifted / elevated southern part, the Bagmati River flowing towards the south could not escape due to high elevation. Then the water began to accumulate on the surface along with the sediments carried by the flowage. The larger sediments carried by water were deposited towards northern part and smaller sediments like sand clay, etc were deposited on the southern part of the basin. The volume of the accumulated water became more and more as there were no outlets. But on the other hand the chemical reaction began to happen when carbon dioxide reacted with water to form carbonic acid. The carbonic acid reacted with sedimentary rocks like limestone on the southernmost part of the basin. As being the carbonate rock limestone easily reacted by carbonic acid and decomposed which results in the formation of void/holes from which water could escape. Hence in this way the accumulated water got escaped from the southern part of Kathmandu, today’s “Chovar”. 

 

 LITHOSTRATIGRAPHY

Many research works has been conducted by many geologists to study about valley sediments. Yoshida and Igarashi (1984) divided the valley sediments into 1) Lukundol Formation followed by terrace deposits like 2) Pyangaon 3) Chapagaon 4) Boregaon and younger deposits like 5) Gorkana Formation 6) Thimi Formation 7) Patan Formation in the ascending order. According to them, after the terrace deposit the valley became gradually narrower due to more upheaval of Mahabharat Thrust and depositional gap with the angular unconformity in the valley sediments.

Later Sakai (2001) and Paudel and Sakai (2008) redefined the lithostratigraphy of the valley sediments as 1) Tarebhir Formation 2) Lukundol Formation 3) Itaiti Formation 4) Kalimati Formation 5) Sunakothi Formation followed by recent gravel deposits in ascending order.

1)   Tarebhir Formation:

                                    Tarebhir formation represents the southern part of Kathmandu valley, mostly composed of cobble, pebble, and larger boulders and conglomerates of quartzite, granite, limestone, phyllite and schist. Fragments of metamorphic rocks like gneiss intrusive rocks like granite also highlight the composition of valley sediments as these are carried by the action of water from the northern most part of valley. It is separated from Tistung Formation of Phulchauki Group of Kathmandu Complex by angular unconformity. It is further divide into three parts:

i)                  Basal part:

The basal part of the Tarebhir Formation rests over the basement rock of Kathmandu valley and represents the southern end of the basin and mostly comprised of boulder, pebble and conglomerates of metasandstone, granite and phyllite.

 

ii)                Middle part:

It is mostly comprised of tourmaline granite along with pebble, cobble, metasandstone and phyllite.

 

iii)             Upper part:

The volume of argillaceous materials like mud and clay relatively increases in the upper part of Tarebhir Formation.

 

2)   Lukundol Formation:

                                             It represents the southern part of the Kathmandu basin as it overlies the Tarebhir Formation. The main lithology of this formation is dominated mostly sand and silt along with argillaceous materials like clay, mud, carbonaceous mud (kalimati) and lignite (brown coal). Lukundol formation is further divided into three members:

 

i)                  Lower part:

This part is mostly dominated by sand along with silt, clay and lignite.

 

ii)                Middle part:

It is composed of silt, sand, carboniferous mud, lignite and overlained by the gravel bed at the top.

 

iii)             Upper part:

It lies over the gravel bed of middle part consisting sand, laminated silty-sand, carbonaceous mud as well as lignite.

 

3)   Itaiti Formation:

                              Itaiti Formation overlies the Lukundol Formation and also the Tarebhir Formation at the marginal southern part. It is mostly dominated by rudaceous materials like pebble, cobble and often conglomerate. These are mostly composed of metasandstone, phyllite and quartzite. On the basis of size of the gravel of this formation it is divided into distal and proximal part. As the size of gravel bed towards the north gradually decrease than southern part.

 

4)   Kalimati Formation:

                                     Kalimati Formation lies between Itaiti and Sunakothi Formation. It is mostly comprise of black and grey, thinly laminated silt, mud and diatomaceous mud (light weight mud which can be crushed into powder form).  Kalimati Formation is also comprised the fossils of operculum (fish fossil) and shell of mollusca which strongly supports the evidence of ancient Paleo-Kathmandu lake.

 

5)   Sunakothi Formation:

                                        Sunakothi Formation lies above the Kalimati Formation and comprised mostly of sand, slit and gravel. At the basal part it consists alternating layer of sand and mud followed by gravely sand at the middle part and silt and sand at the upper part respectively.

 

 

Terrace gravel deposits:

       Pebble – cobble gravel deposit majorly highlights this unit. The sequence of gravel and conglomerate are often associated in every formation like in the unit of sand and silt and also in carbonaceous mud. This unit of gravel covers Sunakothi and Kalimati Formation and also indicates the contact relation between the formations.

 

                                     

          Conclusion:

           This concludes that, geologically Kathmandu valley is divided into five formations. From the oldest Tarebhir formation at the southernmost part to the Sanakothi formation there is difference in the deposition of the sediments and each of them are separated from others by sharp or transitional change respectively in the sediment deposition. The Sanakothi formation being the youngest among all is further covered by the recent gravel deposits. So there is long geological history of Kathmandu basin from paleo-Kathmandu lake to the recent Kathmandu valley.