Friday, November 7, 2008

More Jurassic and Cretaceous Reminiscences

Another passage about the Jurassic oolite from Simon Winchester's The Map That Changed The World.

Within the oolitic horizon there are countless variations-of color (gray, orange to ochre to pale scarlet) and fineness of texture, size of oolith, and width of banding and bedding plane.

I chanced during my journey upon a roadside quarry near the village of Northleach, and the ebullient owner happily showed me around, pointing out with delighted pride the different colors and thicknesses of his rocks, and the uses he could make of the various types.

That triggered a lot of memories for me. Not of the Jurassic but of the Cretaceous of south India. I had just started to take geology seriously and a bunch of us outdoor enthusiasts decided to go on a fossil hunting trip to the continental-marine Cretaceous sedimentary basin of southeast India.

These basins formed in the early Cretaceous as India which was part of the southern hemisphere super continent Gondwanaland broke away from Australia and Antarctica. The map below shows the paleo geography of early Cretaceous. Notice the southern hemisphere location of India at that time and that India has rifted away from Australia and Antarctica and now has a distinct eastern continental margin.

A number of NE-SW trending basins formed on this rifted continental margin of eastern India. The map below shows these eastern Mesozoic-Cenozoic basins. The Cauvery basin which I visited is the southern most of the basins.

Source: Geotimes

All these basins have offshore extensions in the Bay of Bengal. Today these Cretaceous eastern basins have acquired an economic importance with the potential of hydrocarbon deposits especially in the offshore portions of these basins. Reliance Energy recently discovered oil and natural gas from the deep water Cretaceous section of the Cauvery basin and more discoveries of oil and gas are likely.

But in those early college days my interest was palaeontology and fossils. We had discussed the trip with some palaeontologists from the graduate geology department in Pune and so had all the good fossil bearing localities on map. Every day we explored the terrain around the village of Ariyalur which was our base and is located in a rather remote rural portion of Tamil Nadu. One afternoon we got a bit frustrated trying to find a quarry. As we walked on a man on a bicycle approached. Seeing we were distant city dwellers he stopped and started chatting to us proudly in English, eager to show off his vocab. We asked him about fossils and he replied grandly:

Yes Yes, just two miles down this road, there is quarry which is notorious for fossils!

Now as far as I know these long dead Cretaceous beasts entombed in clays and marls have never harmed a human. I guess he meant famous, but maybe the man was indicating to us that the quarry owner is a psycho?

That made our day. With much merriment and anticipation we reached the quarry and found a not so notorious but like Simon Winchester's quarry owner a rather jovial and ebullient personality. He showed us around and gave us samples of claystones from the early Cretaceous littoral facies which contained plant fossils. These were in the form of delicate impressions pressed on clay and you could see clearly the structure of the leaf. A rare treasure which I promptly lost somewhere by the time I graduated.

Geology field trips often throw up these kind of special moments.

1 comment:

  1. Hi Doc,

    Thanks for re-kindling the desire to go on another trip ( Fossil Hunting). This time for real Dinosaur Eggs.