Geological formations in Kerala
A formation or geological formation is the fundamental unit of lithostratigraphy. A formation consists of a certain number of rock strata that have a comparable lithology, facies or other similar properties. Formations are not defined on the thickness of the rock strata they consist of and the thickness of different formations can therefore vary widely.
Three main geological formations are recognized in Kerala namely, the Archaeans (oldest rocks), the Warkalli beds of Tertiary Age (Upper Miocene to Pliocene) and the recent deposits (quarternary). These have North-South alignment.
(i) Dharwar formation: These occur in the Malabar area only. They are represented by garnetiferous-ferruginous quartzite, mica, talc, schist etc. and are found exposed in South-East Wayanad and North West of Gudalur.
(ii) Champion gneiss: They are seen in South and South-East Wayanad and have gold bearing veins. Rocks appear to be of post peninsular age.
(iii) Peninsular gneiss: This is one of the most widespread rock types found in Kerala. The important minerals that go to make up the rocks are quartz, Feldspars, biotite and garnite. In Cochin area, they form the most extensive rocks. The types present are biotite and hornblende gneiss. In Thiruvananthapuram area, the gneiss belongs to the peninsular suite and is made of quarts, orthoclase, mica and hornblende. Charnockite and leptynites are the most common gneisses in this area.
(iv) Charnockite: A good portion of the Western Ghats is made up of this rock. In the Travancore area the rocks are well foliated and show intrusive relationship with peninsular gneiss. They are highly garnetiferous as compared to the charnokites of North Kerala, where garnet is absent.
(v) Closepet granite: Archaean intrusions of post charnockite age are found in the Malabar region. The two intrusions of biotite granite found in Kalpetta hills and Sultan's Battery, have strong resemblance to the Dornegneiss of Hazaribagh.
(i) Basic dykes: These rocks are fresh but fractured and mylonitised. They approach dolerite in composition and are found to occur in South Malabar area. The basic dykes of Cochin area are fine to medium grained and free form olivine. The more coarse grained crystalline phases are represented by gabbros. Several exposures of gabbros are found in Cochin area.
(ii) Residual laterites: A narrow zone of lateritised rock exists to the west of crystalline rocks that constitute the eastern boundary of the State. The rock exposed on the surface in this zone is a type of laterite, which exhibits characteristics different from those of the laterite, which caps the Varkalai formation. The laterite preserves the structure of the parent rock and is less compact. Below the laterite layer is the kaolin layer, the depth of which to the undecomposed rock shows gradation.
(iii) The warkalli formation: This represents the most conspicuous sedimentary bed occurring in Varkala. They are best exposed at Varkala in the cliffs near the seashore. They consist of clayey sandstone, white and variegated clay and carbonaceous clay containing thin lenses of lignite. Most of these areas are lateritised.
(iv) Recent deposits: They are mainly developed in Northern parts of Quilon and are made up of sand and silt. The lacustrine deposits of the backwater tracts of Kerala, the mud banks of the coast of Alleppey and the marine beach deposits all along the sea shore of Kerala come under the group. From the economic point of view, the area is important as it contains valuable mineral sands.