Geologically State of Himachal Pradesh is Dividied into :-
-Plains, Sub Himalayan Sequence, Lesser Himalayan Sequence, Igneous Rocks, Tethyan Rocks and Greater Himalayan Crystaline Complex. Reader should not be confused it with Physiographic Division of Himachal.
Each Division represent different rock sequence, here rock sequence mean “series of rock layers (strata) that are arranged in a specific order, usually formed over time through deposition, volcanic activity, or other geological processes. Each layer in the sequence represents a particular environment, period, or type of sedimentation.
Before Proceeding furhter you need to understand that Himachal Pradesh was part of Indian Craton and rocks of Indian craton are much older than Indo- Eurasian collision.
Plain: Plains are youngest of all Geological sequence. Infact they are made of sediments Derived from the Himalaya. Intensified monsoon and vast river networks of himalaya created these Plains. These Plains are approximately 11700 Million years old. They are composed of Silt, Sand and Clay. In Himachal Pradesh you will find these plain in District Kangra, Una , Solan and Sirmaur. The Plains are separated from Sub Himalayan Sequence by Main Frontal Thrust.(M.F.T)
Sub-Himalayan Sequence: Rocks of this sequence are deposted when India was travelling toward Eurasia. The age of this sequence is rogughly vary from 60 Mya to 5 Mya. These rocks are made of sedimentaion that depostied in shallow marine environement, however as collision proceed the nature of rocks also change From Shallow marine to continental deposite. It is in this Shallow marine sequence you will find Fossils i.e Fossils of Subathu, in District Solan. This sequence is alo very rich in minerals wealth such as limestone.This seqence is also called Shivalik. ( Can you find out where Shallow marine sequence dominated or where continental ?)
Lesser Himalayan Sequence: is a middle Himalayan belt, lying between the Sub-Himalaya and the Higher Himalaya. These rocks are very old, formed long before life appeared on Earth. These rocks were deposted near continent margin i.e where Indian craton and Tethyan sea touch each other. They were deposited between the Paleoproterozoic and Cambrian. The rocks are mostly sandstone, shale, siltstone, limestone, and some augen gneiss. Most of them were changed by heat and pressure into greenschist facies rocks, while some were changed even more into amphibolite facies rocks about 11–6 million years ago. Lesser Himalayn Sequence is bounded by Main Boundary Thrust in South and Main Central Thrust in North.
Tethyan Himalayan Sequence (THS): is the northernmost belt of the Himalaya. These rocks are very old, some formed more than a billion years ago, and they were deposited along the northern edge of the Indian continent in further deep ocean. The sequence mainly consists of sedimentary rocks like sandstone, limestone, shale, and low-grade metasedimentary rocks, as well as igneous rocks like granites and granitic gneisses. It is in this sequence you will find Spiti fossils.
Igneous Rocks:- such as Dhauladhar granitie were The Dhauladhar range is primarily composed of granites that were formed by the intrusion of granitic magma into existing crustal rocks during mountain-building processes. Geological evidence suggests these granites formed around 500–470 million years ago (Late Precambrian to Cambrian period). These rocks mainly found in Kangra, Chamba, Mandi, Kullu, Lahaul Spiti and Kinnaur.
Greater Himalayan Crystaline complex: The Greater Himalayan crystalline (GHC) complex consists of high-grade metamorphic rocks (mostly gneiss and schist), which were also originally sedimentary and igneous rocks deposited on the northern margin of ancient India.
During the India-Asia collision (starting about 55-45 million years ago), immense pressure and heat at depth transformed these rocks through metamorphism and partial melting. They were then thrust upwards and emplaced over younger rock layers during Himalayan uplift.
The rocks experienced deep burial, intense deformation, and repeated episodes of metamorphism and granite intrusion, before being transported to surface levels by major faults (especially the Main Central Thrust)
