Trekking Amongst The Stratigraphy And Structure Of Lesser Himalayas In Kumaon And Gharwal

(Update November 29 2012: See my new post on the geology I saw on this trek!).

Yes.. I know Geology Map Day was last week, but although late, I have put up this map and a schematic cross section... I'm leaving on a hiking trip to the Kumaon Himalayas and have been reading up on the geology of the area. After many years of being confused about the stratigraphy and structure, I finally got some clarity on Lesser Himalayan geology reading some recent work. The paper that helped me most was:

The Kumaun and Garwhal Lesser Himalaya, India: Part 1. Structure and stratigraphy- Julian Celerier et. al. 2009 GSA Bulletin

and its companion paper

 The Kumaun and Garwhal Lesser Himalaya, India. Part 2: Thermal and deformation histories

Two other papers also were quite useful:

1) Patel, R.C. and Carter, Andrew (2009) Exhumation history of the Higher Himalayan Crystalline along Dhauliganga-Goriganga river valleys, NW India: new constraints from fission track analysis. Tectonics 28

2) Revisiting Central crystallines in Pindar and Ramganga valleys, Kumaon Hills,Uttarakhand – an expedition based case study - Geological Survey of India Mapping Report. The report describes the lithologies very near my trek route along the Ramganga river sourced from the Namik glacier, near the village of Namik.

Base camp for the trek is going to be a campsite on a ridge across the small village of Tejam. Looking at geological maps I found out that I am going to be in the Lesser Himalayas, but very just south of the Main Central Thrust which structurally juxtaposes the Greater or High Himalayas over the Lesser Himalayas. The first paper I mentioned by Celerier et.al. explains quite well the stratigraphic and structural evolution of the Lesser Himalayas.

A quick recap:



The map below shows the basic structural and stratigraphic divisions of the Gharwal and Kumaon Himalayas in the state of Uttarakhand. The Lesser Himalayas are a physiographic province juxtaposed against the Siwaliks by the Main Boundary Thrust and against the Greater or High Himalayas by the Main Central Thrust. The red line I have drawn in the upper graphic in the northeast portion of the map is where I will be trekking along the Ramganga river near the town of Munsiyari.

 Source: Celerier et. al. 2009

Through the Late Proterozoic and upto the Cenozoic the northern edge of the Indian plate was an open shelf accumulating sediment sporadically. Sediments that were later to make up the Lesser Himalayas and the Tethyan sedimentary sequence were being deposited across this shelf. These sediment sequences are termed the Lesser Himalayan Sequence (LHS) and The Tethyan Sedimentary Sequence (THS). In the map above, the Lesser Himalayas are divided into two zones; the inner LHS and the outer LHS. They represent deposition across a proximal to distal profile across the Indian shelf, the inner LHS interpreted to represent a proximal environment while the outer LHS interpreted to represent a more distal setting. Proximal means the depositional environments were closer to the Indian craton near the present day southern limit of the Himalayas and distal means they were situated in a more northerly location in the outer parts of the shelf. When the Indian plate collided with Asia this leading edge of the Indian plate along with this sediment pile deformed and was uplifted to become the Himalayan mountains.

But wait.. look at the map again. In the map the outer LHS are exposed closer to the Indian craton i.e. in a southerly depositional location and the inner LHS appear in a more northerly depositional location.

That is one mystery this paper by Celerier et.al. cleared up for me. What you see is the result of large scale south directed thrusting. The authors using detailed field mapping and absolute dating of the various stratigraphic units suggest that as the Indian plate deformed, the outer LHS i.e. the sediments of the distal shelf, were thrust over the inner LHS along a major south directed thrust ramp known as the Tons thrust (green fault in map). After this, south movement along Main Central Thrust brought the Greater Himalayan Crystallines over the LHS. This resulted in a stacking of thrust sheets, with the hanging wall of the Main Central Thrust stacked atop the hanging wall of the Tons Thrust.

Subsequent erosion has removed much of the hanging wall of the Tons thrust and the Main Central Thrust from this area leaving the inner LHS exposed in a window north of the outer LHS. The leading edge of the hanging wall of the Tons thrust has been folded into a broad synclinorium and is exposed in a more proximal location where it structurally juxtaposes the Siwaliks along the Main Boundary Thrust known locally as the Krol thrust.  A number of famous hill stations of this region including Mussoorie and Nainital are located on this folded leading edge of the hanging wall of the Tons/Krol thrust.

In order for the outer LHS to be translated south of the inner LHS requires about 50-100 km south directed movement along the Tons thrust, an interpretation supported by recent work that suggest that facies of the outer LHS are equivalent to the Tethyan Sedimentary Sequence exposed much to the north.

I've put up below a schematic cross section which explains the structural evolution of the Himalayas in this region.

 Source: Celerier et. al. 2009

So, this paper has been something of a revelation for me. I have never worked in Himalayan geology and the sheer number of faults and named stratigraphic units is confusing. There is terminological confusion historically present due to a lack of absolute age control. Faced with the structurally tangled and fossil-poor Lesser and Greater Himalayas, we just are not sure of the chronostratigraphy, meaning which rock formation is older and which is younger. That makes piecing together a coherent geological evolution of the region very difficult.   Celerier et. al's paper clears through some of this confusion by establishing a geochronology of the Lesser Himalayan sequence and consequently establishing the relationships of different rock units. They are then able to simplify terminology and reinterpret the importance of the various thrust faults and come up with a kinematic model of how and when different faults moved and juxtaposed different sequences against each other.

I'll be trekking along the red line in the image below (see also map above).

This is geographically north of the exposed outer LHS,  but geologically is the footwall of the Tons thrust (and the footwall of the Main Central Thrust!), an erosional window which exposes the inner LHS. This sequence is itself deformed into a duplex structure i.e. formations are stacked and repeated by a series of thrust faults. Reading the local stratigraphy I have come to understand that strata of the Berinag Formation (quartz arenites) and the Deobang Formation (carbonates and metamorphosed shales) are exposed along my camp site near Tejam village. The Berinag is older and has been thrust over the Deobang along the Berinag Thrust. And walking northwards towards Namik village the Main Central Thrust will bring the Greater Himalayas structurally over the Berinag Formation.

So there will be plenty for me to ponder upon and hopefull record in the field as I traverse from the inner LHS across the Berinag Formation and the Main Central Thrust into the Greater Himalayas.

I hope to fill up the above image with some geology as I walk along.

post photos when I get back in mid November!

Celerier, J., Harrison, T., Webb, A., & Yin, A. (2009). The Kumaun and Garwhal Lesser Himalaya, India: Part 1. Structure and stratigraphy Geological Society of America Bulletin, 121 (9-10), 1262-1280 DOI: 10.1130/B26344.1

Update November 29 2012: See my new post on the geologic traverse!