Field Photos- Jaisalmer: Landscapes, History, Geology, Fossils

Jaisalmer was fun!

I was staying last weekend at the pretty amazing Suryagarh hotel. It is a relatively new hotel, but built in the style of an ancient fort. The interiors are stylishly crafted, with courtyards and sunlight corridors, and absolutely beautiful elegant use of lights in the evenings...the service was impeccable and the food was great too! I sampled quite a variety of cuisines, from the classic Halwai breakfast made in the Rajasthani style, to a lunch inspired by a hodge podge of different styles from Mediterranean to Thai, to another Rajasthani style meat and rice platter dinner. The highlight for me among the myriad dishes was a palate freshener made from basil, sweet melon (Mosumbi), lime and salt. It was liquid citrus basil chutney, served chilled. Simple, yet sublime!

Pune is green and black. Trees and that volcanic beast, the Deccan Basalts, give it those hues. Jaisalmer is buff, off-white, yellow, fawn, brown, rust, red, black. These are the colors of the clays and sandstones and limestones deposited in an immense sea which covered the area from the Jurassic to the early Cenozoic. I got to see only the Jurassic section, and that too only in a hurry.

Suryagarh had organized a long drive in the outback.. I guess that's the word that comes to my mind when facing an immense desert. The Thar stretches in all directions of Jaisalmer and a couple of km off-road you begin to sense the isolation.

An Oasis with the vast Thar behind.

We visited the smallish Khaba fort. The interesting history is at the base of the fort.

These are the ruins of the Paliwal community which is said to have moved to this area from Pali, Rajasthan few hundred years ago. They built a successful sustainable agricultural society, making clever use of the limited amount of available water. Then legend has it, they fell out of favor with some powerful locals and almost overnight abandoned their homes, migrating to several larger towns in Rajasthan.

All around, the Jurassic is inescapable. These are north tilted shales and sandstones. Only a few hundred feet of strata outcrop (on the surface), but there is more than five thousand meters of sediment in the Jaisalmer basin subsurface. Some layers are oil and natural gas reservoirs which ONGC has tapped into.

The shales, sandstones and limestones are fossil rich. I could spot ammonoids, belemnites and clams. In an isolated homestead belonging to a Bhil tribal family, we met this amazing old woman who scours the countryside for fossils and then sells them in the Jaisalmer market. This is her treasure trove!

.. and the Bhil family in their abode. The Bhils occupy a proud place in Rajasthan history, being most well known for the military support they gave to Maharana Pratap is his battles against the Mughal emperor Akbar. They, as most of the tribal societies in India are, quite marginalized, sustaining themselves on small farm plots and as day laborers.

Another type of remains of this ancient Jurassic life are ichnofossils. These are tracks and trails and burrows of worms and other creatures disturbing the sea floor and preserved as impressions and casts and molds. Here is a horizontal burrow system likely made by a polychaete worm like creature. The ichnofossils of the Jaisalmer area and their paleo-ecology have been well studied by paleontologists from Pune. Check out this paper co-authored by my friend P.K Sarkar from Fergusson College, Pune.

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More aspects of the geology and the physical processes prevailing in the Jurassic seas are seen in the building stones.

Here is a shell hash with pebbles, a concentration of coarse shells of molluscs and rock fragments, deposited on a Jurassic beach or in very shallow seas where strong wave action removes the finer clay and mud particles, leaving behind a lag of clean sand. Such coarse sands are of great importance to geologists. They are often quite porous and may end up hosting petroleum. So, the basic processes that control their distribution in ancient seas are studied intensely by sedimentologists.

And this great example of deposition in a Jurassic storm. The coarser pebbly layer contains "intraclasts" which are pieces of hardened sea floor that has been ripped up by storm waves, transported and then re-deposited. I could have stood and photographed the walls of the Suryagarh hotel all day. There is so much sedimentology to be learned, all literally written in stone.

The main source of water for these remote communities are these depressions where scanty rainfall accumulates. These make for a really soothing sight in the midst of the harsher surroundings.

Groundwater is usually found at depths of few hundred feet and is often saline. Aquifers are present in the top sand, as well as the Cenozoic, Cretaceous and Jurassic strata underneath. Back in the early Holocene some 8-10 thousand years ago, Rajasthan and Jaisalmer got a lot more  rain. Groundwater got stored in these deeper strata, but over the past few thousand years, the climate became drier, and these deep aquifers don't get replenished too often. Prolonged reaction of the water with the rock increases the salinity of the water.  But there are at places shallower lenses of fresh water which do get replenished from time to time. This community got lucky and has struck potable water at twenty odd feet. Here I am at one of the wells.

Another place of historical interest we visited was an ancient cemetery. Jaisalmer had trading links with Eurasia from medieval times and scattered through the countryside are tombs for the fallen traveler and important locals.

.. and off course a trip to the desert without sand dunes?.. This is part of the Sam Sand Dune National Park..

Overall, my visit was far too short, there is just a lot of history and geology to see around Jaisalmer. I did not even have the time to visit the world famous Jaisalmer fort, which is a UNESCO heritage site.  

Hey,  I am not really complaining! Who would, if you woke up to a view like this?

Jurassic Geology- Traveling To Jaisalmer, Rajasthan

I'm traveling to the desert town of Jaisalmer in Rajasthan state this weekend. In college, my final year B.Sc. field trip went to Rajasthan.  I suffered acute appendicitis a day before our departure and missed the trip. Then some years ago, while driving from Ladakh  to Pune, I  drove through the towns of Jaipur and Udaipur but didn't spend any time there. This is going to be third  time lucky I hope. Jaisalmer is an ancient fort town and I'll  be staying at Suryagarh, a beautiful boutique hotel. I will be getting to see the fort and some other  sights and catch some of the geology too.

It is Jurassic outcrops all around the town. Jaisalmer basin is one of the important sedimentary basins of India,  with ONGC producing oil and natural gas from Mesozoic and Cenozoic reservoirs. So,  its geology is well studied.

Here is a map of the Jaisalmer basin and the larger regional context. 

 Modified from Source: N.P. Singh 2006

This basin is the south eastermost part of the Indus shelf, which made up the sea facing continental shelf of the Gondwana/ Indian plate. I  have  outlined  with dotted brown lines the depositional orientation of the sedimentary sequence. The ancient shoreline  was to the southeast  oriented roughly NE-SW.  The strata dip (tilt) towards the north-northwest. So, one finds younger and younger rocks towards the north-northwest. A few hundred feet of sediments outcrop. The rest of the sedimentary sequence which is greater than 5000 m is in the subsurface. The map also shows the major structural elements affecting the basin. These are mainly NW-SE oriented faults along which there has been block movements. The Barmer rift is a northward continuation of the Cambay trough, a linear depression that formed in the late Cretaceous-Paleocene as India broke away from Seychelles.

These are the orientations of the basin and its structures today. I like to take a step back and understand the paleo-geography. The reason is plate-tectonics.  How was India  and this basin oriented during the Jurassic? How has plate tectonics reorganized the configuration of the continents since? One gets a better understanding of the basin history and structural evolution taking this long view.

Below is a series of paleo-geographic recontructions of Gondwana and the Indian block since the late Jurassic taken from Sanker Chatterjee et al. 2013.

Fig 1 is the late Jurassic. The sea (Neo-Tethys) flooded the continents from the north. The Mesozoic, especially Jurassic onward, was a time of high sea levels with many continents experiencing marine incursions and the formation of thick sedimentary sequences. This high sea level (with smaller intermittent sea-level drops) persisted for tens of millions of years.  One  reason was that the position of the continents during this time was such that there were no continental land masses at the poles and thus there was no build up of continental ice sheets which could have caused a large global sea level drop as did happen during the Pleistocene ice ages for example. Another reason was that Pangea from early Jurassic and Gondwanaland from later Jurassic started breaking up. Continental break up eventually leads to the formation of new oceanic crust along volcanic ridges that form along the old lines  of continental separation. This new crust being hot and buoyant forms high mountain chains rising above the sea floor. This displaces sea water which floods the continents. This process was stronger beginning latest Jurassic and through the Cretaceous, accounting for the Cretaceous high seas.

Anyways, coming back to the plate tectonic fate of India. When Jurassic sediments were being deposited, Jaisalmer was located around 25 deg south of the equator and India was oriented almost E-W.  India  then separated from Africa around 165 million years ago and drifted along with Australia and Antarctic further southeastwards! The original orientation of the ancient Jurassic shoreline was then roughly E-W which I have depicted as a brown dotted line. As you can see Jurassic sea level rise deposited sediments in a wide arc covering Madagascar as well, which at that time was attached to India.

Fig 2. By early Cretaceous, India broke away from Australia and Antarctic and starts to get pulled northwards and also rotated counterclockwise. Jurassic shoreline is shown as brown dotted line.

Fig 3. Late Cretaceous. Sea floor spreading in the Indian Ocean and the Atlantic too. India now appears to be oriented more like it is today, although its axis is still slightly N-NE. It has separated from Madagascar around 90 million years ago and with Seychelles around 65 million years ago. This latter wrenching apart has formed the extentional faults and linear depressions (red dotted line)  that cut across the Rajasthan basin in a NW-SE direction including the N-S oriented southern Cambay depression. Southwards in the state of Maharashtra, the wrenching apart of India from Seychelles triggered a massive outpouring of magma (see my previous post), resulting in the Deccan Volcanic Province. Jurassic shoreline is brown dotted line.

 
Fig 4. Eocene- Sedimentation continues on the Rajasthan shelf through the Cretaceous and Early Cenozoic. Ultimately the fate of the basin and the larger Indus shelf is sealed. India crashes into Asia. The Neo-Tethys narrows and closes and the sea disappears. Sediments eroding from the rising orogenic Sind-Baluchistan mountains fill up the Indus depression. Deep below though in the sediment pile, organic matter was being cooked into oil and natural gas, a resource that ONGC is today exploiting. Jurassic shoreline is brown dotted line.

There is a further history of the formation  of the Thar desert but I'll leave that for another time.

Check out this Suryagargh hotel I'm staying in. Its looks awesome.

 Picture source: Photo Gallery Suryagarh

Pics later after my trip.  And this was a very broad geology overview. I'll write in more details of the nature of the sediments and fossils next week!