Wednesday, December 9, 2015

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!

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