Tuesday, October 10, 2017

#Neatrock Entry For Earth Science Week

SciFri Science Club is hosting a #Neatrock challenge as part of Earth Science Week.

Here are my two entries:

Megascopic #neatrock:

This is a migmatitic gneiss from the Greater Himalayan Sequence, Darma Valley, Kumaon Himalaya. Migmatite means a mixed rock made up of a metamorphic host and a newly formed igneous rock. During continental collision, metamorphic rocks buried to great depths and subject to high temperatures may partially melt to form granite magma. The granitic melt segregates into layers. The resultant rock is composed of the original metamorphic host rock such as a gneiss (dark bands)  and granitic igneous layers (lighter bands). This migmatite formed during the Miocene.

Microscopic #neatrock:

This photomicrograph of a Late Ordovician limestone (Fernvale Limestone) from Georgia, U.S.A.  is close to my heart. It formed an important part of my PhD work.  I have stained the thin section with a Potassium Ferricyanide dye. Calcite containing minor amounts of iron (Ferroan calcite Fe+2) is stained blue. Non Ferroan calcite is unstained.  In the center of the photomicrograph is a non ferroan 'dog tooth' spar. It is a calcite crystal with a shape resembling a canine tooth of a dog.

This calcite has a pendant habit. It is hanging from the underside of a particle, in this case a piece of an echinoid shell. Such pendant crystals precipitate in a vadose zone i.e. above the water table.  In this environment, pore spaces are not completely filled with water. Rather, films of water coat grains and form drips. These drips become saturated with calcium carbonate and calcite precipitates from them.  Just like a larger and more familiar stalactite in a cave! Except that this micro-stalactite in tiny..tiny.

Development of a vadose environment indicates that sedimentation was interrupted by a large sea level fall. The sea bed got exposed to rain and a fresh water aquifer developed in the sedimentary deposits.  A tiny 'dog tooth' spar can tell us a fair bit about sedimentary basin evolution and sea level history.

Wednesday, October 4, 2017

Geo Week 2017, Pune

 Geo-Week 2017 Pune

Starting October 9th 2017, a week long geo-activity program for the public is going to be held at Raja Ravi Varma Art Gallery, Ghole Road, Pune.

It is being organized by the Center For Education and Research in Geosciences (CERG) along with Fergusson College, Pune. CERG is a citizen outreach initiative taken by students and professionals from the Pune geology community.

Take a look at the poster.

The inaugural talk by Dr. R. Shankar of Mangalore University will be on October 9th at 11.30 am . The topic is Paleoclimate Studies of Lake Sediments from South India. The venue is Raja Ravi Varma Art Gallery.

There is another lecture scheduled on October 14th  at 7.40 pm by Dr. S. N. Rajguru. The topic is Prehistoric Environment of the Mula Mutha River, Pune. This talk will be held at the Amphitheater on Fergusson College campus.

There is also a geology exhibition, art and essay competitions for school children, earth science themed film shows and a workshop on QGIS. The exhibition is at Raja Ravi Varma Art Gallery, while most of the films will be screened at the Amphitheater, Fergusson College. Check the website for schedule details.

Pune geology and science enthusiasts, share this with your friends and do stop by and support this initiative!

Geo-Week 2017, Pune

Friday, September 29, 2017

The Bay Of Bengal Once Touched Sikkim

See this satellite imagery of the Himalaya.  The Indian State of Sikkim occupies the region just east of Darjeeling.

The Siwaliks (green arrows) appear as a forested linear band forming the southernmost hilly terrain of the Himalaya. The hills abut against broad alluvial plains. Rivers traversing the Himalaya carrying enormous sediment load encounter a gentler gradient upon exiting the hilly terrain. A loss of stream power results in sediment being dumped in the channel, so much so, that rivers get chocked on their own sediment. As a result, channels split and bifurcate forming a braided river system. These rivers  also suddenly change course, abandoning their channel and carving out new ones. Such course changes may occur during floods or by tilting of the land by structural movements.  Over time, the deposits of these ever changing rivers coalesce to form cone shape aprons of sediments known as alluvial fans. These rivers like the Kosi and the Tista, which flow transverse to the mountain range, meet an axial river like the Ganga and the Brahmaputra flowing parallel to the mountain front. The axial river flows into the Bay of Bengal.

The Siwalik hills were once these type of alluvial fans.  Just as today, during Miocene and Pliocene times, sediment was being deposited in front of the rising Himalayan mountains. Beginning about half a million years ago or so, these ancient alluvial fans were crumpled up and uplifted to form the Siwalik ranges. Active alluvial fan formation shifted southwards to its present locus. This process continues. In a few million years, the present day alluvial fans deposited by rivers like the Kosi and the Teesta will be deformed into a newer mountain range south of the Siwaliks. The Himalaya are growing southwards.

How do we know that the Siwaliks were once alluvial fans? Geologists rely on analogy, comparing the Siwalik sediments with what is accumulating in the present day alluvial fans. They find a striking similarity. Siwaliks are made up of alternations of coarse gravel layers and finer sand and silt layers with characteristic bed orientations and structures like cross beds and rippled sand. The gravel layers are inferred to be the river channel deposits while the finer sand and silt layers are the river bank, levee and floodplain deposits. An important finding made throughout the length of the Siwalik ranges has been the paleo-current directions preserved in the rocks.  Geologists have measured the orientation of bedding and ripple marks and found out that rivers were flowing south and south east i.e. perpendicular to the mountain chain. There is no evidence of an axial river like the Ganga in these Siwalik sediments. The thinking is that such an axial river must have flowed much to the south of the region of deposition of Siwalik sediments.

And what about evidence of a delta? Where did these Miocene and Pliocene rivers meet the sea? The logical geographic place to look for a coast would be towards the east. And in fact, that evidence has come from the Siwalik sediments of West Bengal and Sikkim. In a really interesting paper published recently in Current Science, Suchana Taral, Nandini Kar and Tapan Chakraborty describe sedimentary structures and marine trace fossils from Middle Siwalik sediments exposed along the Gish River and its tributaries in the Tista Valley. Siwalik rocks in the central and western part of the Himalaya show current structures that indicate south flowing rivers. In this easterly location however, the sediments show evidence of being deposited in a wave influenced environment. Sedimentary structures like wave ripple laminations and hummocky-swaley stratification indicate deposition in wave dominated marine bay.  Paleo-current indicators like ripple marks preserved on sandstone surfaces show a south as well as north directed current. This suggests an environment influenced by tides and north directed waves. Associated sediments show indicators of different delta environments like distributary channels, delta mouth bar and delta flood plain deposits.

Apart from current direction indicators, the sediments contain plant fossils indicative of mangrove vegetation and brackish water environments. They also contain trace fossils i.e. impressions and burrows made by creatures moving and disturbing the sediment surface. Cylindrichnus, Chondrites, Rosselia, Taenidium, Skolithos, Planolites are some of trace fossils reported in this study. The assemblage of trace fossils is similar to those reported from marine settings.

All this suggests that during the time of deposition of these Middle Siwalik sediments in Late Miocene-Pliocene times, about 5-10 million years ago, a branch of the Bay of Bengal had invaded as far north as present day Sikkim. Rivers carrying sediment from the Himalaya were debouching them in a delta and a shallow marine bay. The Sikkim Middle Siwalik strata are ancient deformed delta and marine deposits.  

A paleo-geographic reconstruction of this eastern part of these Siwalik depositional environments in shown below.

 Source: Suchana Taral, Nandini Kar and Tapan Chakraborty 2017

The  upper graphic shows the reconstructed delta and marine depositional environment. The lower graphic shows the regional paleo-geography. The pin shows the environmental location of the study area. The yellow rose diagram shows the paleocurrent directions measured in the Siwalik sediments.

Interestingly, some earlier work by geologists has shown that in Late Miocene times the Brahmaputra was flowing along a much more easterly route towards the Bay of Bengal. They used sand thickness and sand/shale ratios from wells drilled in the delta and found lobate sand bodies, which they inferred were brought in by a large river flowing from a ENE source. Their interpretation is shown in the graphic to the left (Uddin A. and Lundberg N. 1998). At the time the Shillong Plateau did not exist. The river flowed into the Bay of Bengal from the Upper Assam valley and through the Sylhet depression in to the Bengal Basin. The uplift of the Shillong Plateau in Pleistocene times forced the Brahmaputra to turn west and wrap itself around the newly emerging uplands.

Since Pliocene times, the tremendous amount of sediment being delivered by Himalayan rivers, coupled with Pleistocene sea level fall, has caused a retreat of this arm of the Bay of Bengal southwards.

In the satellite image below, based on the location of the Sikkim Siwalik deposits and other work on the Bengal Basin paleogeography, I have drawn in brown the coastline as it would have existed 5-10 million years ago. The ancient drainage systems are shown in blue. South directed arrows shows the extent of the growth of the Bengal/Bangladesh alluvial plains and delta and the retreat of the sea since then to its present location.

Pretty amazing finding.

Monday, September 25, 2017

Evo Devo Musical Video

This is cool!

How do we develop from one cell to a complex multicellular creature? Tim Blais who runs A Capella Science has a musical video out explaining the genetic basis for this wondrous transformation. This is a field of study known as evolutionary developmental biology.

Book recommendation: Sean B. Carroll's Endless Forms Most Beautiful: The New Science of Evo Devo is a good introduction.

Tuesday, September 19, 2017

Environment Links: River Issues In India

Sharing a few interesting and informative articles I came across in the past few weeks on rivers.

Endangered Himalayan Rivers: This one is from 2012. A large number of dams are planned on the Alaknanda and Bhagirathi rivers in the state of Uttarakhand.  Parineeta Dandekar writes about the weaknesses and bias in the Environment Impact Assessment process.

Rally For Rivers Plan. Will It Help?: The Rally For Rivers campaign by the Isha Foundation is calling on creating a 1 km wide tree plantation along the river banks. This, they claim, will help rejuvenate India's dying rivers. Veena Srinivasan, Sharad Lele, Jagdish Krishnaswamy and Priyanka Jamwal with the Ashoka Trust for Research in Ecology and the Environment, Bengaluru examine their claims in detail and find them wanting.

Caution Warranted For River Linking Project: The gargantuan river linking project envisages a series of dams and canal systems to transfer water from Himalayan rain and snow fed river basins to the drier Peninsular rivers in the south. Is it worth it?

Reuter's Erroneous Reporting On The Ken-Betwa River Linking Project: Two rivers in Madhya Pradesh and Uttar Pradesh are to be linked. SANDRP clarifies that the permissions process has yet to be completed. The two states don't even have a water sharing agreement! Reuter's screwed up.

Environment Ministry Panel Reject's Uttar Pradesh's Religious Smart City Plan: I'm including this to give an example of the utter indifference to ecology and environment shown by "planners and developers". The plan is for a smart city to be built inside the Hastinapur wildlife sanctuary, along the banks of the Ganga, which would have destroyed dolphin habitat and river ecology along a 7 km stretch. How does one even come up with such ideas? Fortunately, the usually pliant Environment Ministry has balked at approving this outrageous plan.