Thread by @rapiduplift

The plateau cap layer that provides a foundation for the wind turbines (south of Satara) is an ancient hardened iron rich soil known as laterite.

Thread- Brief history of laterites of India Western Margin and their significance in understanding landscape evolution..

After Deccan volcanism ended ~ 63 mya, a vast lava field about 1 km elevation had formed. In Eocene times a phase of humid climate caused deep chemical weathering of this lava resulting in a thick weathering mantle of iron rich soil known as laterite.

Such deep prolonged weathering episodes point to a period of tectonic stability, humid weather, establishment of long lasting groundwater system to flush way mobile elements but retaining immobile iron hydroxides. The result was landscapes like the Panchgani tableland.

This laterite peneplain covering Deccan lavas today exists roughly south of the town of Satara. There is no laterite cover north of this.. Why? Erosion has cut deeper in to the lava pile north of Satara removing the youngest part of the volcanic sequence.. cross section..

The laterite surface exists at high elevations at the crest of the Western Ghats ~ 1200 m ASL. There is a lower level laterite cover in the Konkan coastal region too, at altitudes 50 m to 200 m ASL... this tells an interesting story of coastal plain evolution..

After Deccan volcanism, continental rifting resulted in blocks subsiding to the west forming a west facing cliff. Over time in the Cenozoic, erosion has caused this cliff to retreat to the east, forming in its wake a flattish coastal plain underlain by basalt and rock debris..

In Miocene times (~10 million yrs ago), another phase of humid climate caused deep weathering of coastal plain basalt and pediment (rock debris) forming a younger laterite layer covering the Konkan and Kanara plains. Pic shows a typical laterite profile - academia.edu/29910780/Evolu…

sorry , I meant to post this weathering profile of a laterite developed over a basalt host rock. The iron rich soil just below the indurated crust/cap is used to make bricks all over the western margin of India.

After the coastal laterites formed the western margin underwent uplift. The laterite surface rose to form a plateau which was cut into by invigorated streams. a deep entrenched drainage has dissected this plateau. Satellite image shows this stream network cut into laterite.

The evolution of the Konkan/Kanara coastal region from a low lying undulating surface undergoing lateritization to an uplifted dissected plateau is shown in the schematic - academia.edu/29910780/Evolu…

Moving southwards from the Deccan volcanic terrains and also going back in time, there are older laterite/bauxite weathering surfaces developed on the crests of the southern western ghats. Cross section shows multi tier Western Ghats with benches named S0, S1, S2, Gunnel 1998

The oldest surface S0 formed during deep weathering of the Indian crust in late Jurassic early Cretceous before Eastern Gondwana broke up. It is called the Gondwana surface.

Periodic uplift stripped rock cover exhuming deeper layers. At times humid weather phases resulted in repeated episodes of laterite/bauxite formation forming benches or planation surfaces at lower and lower altitudes. Surface S2 corresponds to the Panchgani laterite.

This periodic uplift, exhumation and planation and weathering, resulting in flat weathering mantles forming at successive lower levels is shown in the schematic. - mantleplumes.org/DeccanUplift.h…

This polycyclic evolution of India western margin wherein uplift alternated with stability and weathering has had significant economic impacts too. High altitude tea and coffee plantations are sustained by these ancient soil layers. Pic shows altitude control of tea and coffee.

Next time you walk on Panchgani table land or the tea and coffee plantations of south India, do think about this long and episodic history of uplift, exhumation, weathering of the Indian western margin..

end thread.