Thursday, November 30, 2017

Remotely India: Folding At Margins Of The Vindhyan Sedimentary Basin

Remotely India # 9 (a post series about landforms and geological structures imaged by remote sensing satellites).

This is a geology rich image!


Source: Rajasthan Tourism

It shows Gagron Fort in the Jhalawar district of Rajasthan. I came across it while watching a history show on EPIC channel. Looking at the steeply dipping strata I identified them as the metamorphosed and deformed Aravalli Group sediments of early Proterozoic age.

I then checked a geological map and realized I had gotten the stratigraphy completely wrong. These steeply dipping rocks belong to the mid-late Proterozoic Vindhyan Group of sediments.

There are two distinct categories of Proterozoic basins in India. There are the mobile belts. An example of a mobile belt is the Aravalli orogenic belt. As the name suggests, these basins formed as linear depressions at the margins of Archaean cratonic blocks. They are filled with volcano-sedimentary successions, intruded by granitic bodies and subjected to intense deformation and metamorphism during convergence and collision between different cratonic blocks. They are economically important. Lead, zinc, copper, iron ore is mined from various mobile belts. The Aravalli belt formed due to the collision between the Aravalli craton and the Bundelkhand craton. Sedimentation in the Aravalli basin was initiated around 2 billion years ago. Their deformation and metamorphism has been dated to around 1.7-1.6 billion years ago.

The second category of basins are the epicratonic basins, developed as either rift basins or foreland basins within cratonic blocks. Volcanic activity is mostly restricted to the early stages of basin evolution. Sedimentary successions are sandstones, shale and limestone. Collectively these are known as the 'Purana' (ancient) basins. They show very light to no metamorphism and relatively gentle deformation. Flat lying strata form mesas and plateaus in the interior of such basins. The degree of deformation usually increase at the basin margins. In proximity to basin margins faults, sediments are often spectacularly folded. The Vindhyan Basin is a 'Purana' style basin.

I am putting up a few examples of folding in Vindhyan Basin sediments. These range in age from about 1.7 billion to 650 million years.

The first one is the Jhalawar anticline in proximity to the Mukundara Fault. Fort Gagron was built  on the steep NE dipping limb made up of the Kaimur sandstones.



Mukundara Fault is an easterly directed thrust fault. See this cross section across the Jhalawar anticline.


Source: Rajeev Bhoj, Avdhesh Nautiyal and Rajesh Sharma 2011:  Regional Structural Style of Chambal Valley Vindhyan Basin, Rajasthan, India

This second example of folding is south and east of the famous fort at Chittorgarh. Lower Vindhyan Group sediments have been folded into N-S trending tight anticlines and synclines. 


This folded zone abuts the Great Boundary Fault which structurally juxtaposes the Bundelkhand Craton with the Aravalli Craton. The fault brings into contact the Aravalli mobile belt and the Vindhyan 'Purana' basin. The Great Boundary Fault is a NW dipping thrust fault (ref).

Finally, northeast of the previous location, also along the Great Boundary Fault in the vicinity of the town of Bundi are these folded Upper Vindhyan sediments. These are the sandstones and limestones of the Rewa and Bhander Group.


And below is a geological map of the region to give some context to these structures. The Great Boundary Fault and the Mukundara Fault are orthogonal to each other, testimony to differently oriented compressive forces  affecting the Vindhyan Basin.


Source: Rajeev Bhoj, Avdhesh Nautiyal and Rajesh Sharma 2011:  Regional Structural Style of Chambal Valley Vindhyan Basin, Rajasthan, India

Three distinct structural trends can be seen in this part of the Vindhyan Basin. A NE-SW trend of the Great Boundary Fault. A N-S trend of the tight folds south and east of Chittorgarh. And a NW-SE  trend of the Mukundara Fault and associated folds. The other major structural trend in the Vindhyan Basin is the E-W trend of  Narmada rift fault zone which forms the southern boundary of the basin.

All satellite images from the Indian Remote Sensing satellite Cartosat series, accessible through ISRO's web mapping application Bhuvan.

Sunday, November 19, 2017

Geology And Homo Sapiens Habitats Pleistocene Indian Subcontinent

Came across an interesting passage from this review paper:

Environments and Cultural Change in the Indian Subcontinent: Implications for the Dispersal of Homo sapiens in the Late Pleistocene - by James Blinkhorn and Michael D. Petraglia

Yet beyond relief, the geological structure of the Indian subcontinent plays another important role in patterns of habitability in the region. The analysis of the structure of geological basins within the Indian subcontinent led Korisettar (2007) to the conclusion that the Purana basins exerted a strong influence on hominin dispersals and occupation history. Although direct precipitation within the Purana basins is lower than other regions of the subcontinent, perennial supplies of freshwater are available because of spring activity from aquifers that deliver water resources from regions that receivemuch higher monsoonal precipitation.As a result of reliable water resources and abundant raw materials for stone tool manufacture, these geological basins are thought to have acted as refugia not only for hominin populations but also for varied flora and fauna (Korisettar 2007).

The importance of such Purana basins for providing refugia is well exemplified by the recent study of fauna from the Billasurgum caves, located within the Cuddapah Basin. Here, excavations revealed the first stratified sequence to document patterns of faunal occupation spanning the late Middle Pleistocene to Late Pleistocene (Roberts et al. 2014). This study illustrated the long-term continuity of large-bodied fauna within South Asia with only a single taxon of twenty-four identified as having gone extinct across the subcontinent (Roberts et al. 2014).


The "Purana" basins are Proterozoic in age. They are scattered all over Peninsular India. A common lithology is silica cemented sandstone or quartzite which forms prominent hill ranges, ridges and escarpments with ledges, overhangs and caves. These hard quartzites would have been one source of raw material for stone tools.  The rocks are also fractured and networks of pervasive cracks allow the storage and movement of groundwater.

The map (from a different paper) below shows the distribution of Middle Paleolithic sites (red dots) in India, Arabia and Eastern Africa. I have outlined in black (very approximate!) the location of three Purana basins. V stands for Vindhyan, C for Cuddapah and B&K for Bhima and Kaladgi.

 Modified from Huw S. Groucutt et.al. 2015

This paper have lots of information about climate change, ecology and stone tool record found in India. The authors discuss the Late Acheulean (130k - 100 K) ,  Middle  Paleolithic (94k - 34 K) and the Late Paleolithic ( < 45 K). These terms refer to particular styles of stone tool manufacture.

The India skeletal fossil record is very poor. However, based on comparisons with Middle Paleolithic of Africa and Homo sapiens fossils and tool associations in SE Asia and Australia, the authors are in favor of a wave of  Homo sapiens migrating into India as early or perhaps a little earlier than 100 k ago. This was followed by a later wave around 50 k years ago.  Do changes in cultural style and tool use point to changing populations.. with an intrusive population replacing an earlier one?.. that is an intriguing question. Some recent genetic work suggests that people from these earlier migrations died out without leaving a genetic legacy in us. All non African humans have descended from migrants who left Africa between 50K-80K years ago.  I had summarized these results in an earlier post on human population continuity in India.

See also other papers from this special volume of Current Anthropology on Human Colonization of Asia In the Late Pleistocene

Open Access.

Friday, November 3, 2017

Field Photo: Sea Cliffs And Holocene Sea Level Highstand, India West Coast

All along India's coast there are indicators that 4000-6000 years ago sea level was higher than the present level, oscillating between 1-4 meters above present high tide level at different times. Since then, the sea has gradually receded to its present level. As a result, we can observe stranded beach ridges, cemented beach rock and dunes a few hundred meters inland of the present high tide mark. And we can see erosional notches on sea cliffs marking the past high tide level.

I saw these erosional notches in the sea cliffs exposed along the coast near Harnai village in Konkan.

The satellite image shows the location of the sea cliffs.


The picture below shows a sea cliff with an erosional notch (arrow) about 1.5 meters above the high tide level. This is at the Fattegad Fort near Harnai village. Also, notice the rocky platform that has formed at the current tidal level.


This notch can be traced all along the line of sea cliffs in the area. You can see it very clearly on this cliff, a little north of the previous location.


And the picture below shows a close up of the notch. Sea level must have held steady at this level for a few hundred years to have formed such a distinct erosional feature.


Why was sea level higher in the past? It has to do with the ice age and the end of the last glacial phase. The earth has been in the grips of an ice age for the past 2.6 million years. Conditions have cyclically fluctuated between colder glacial phases and warmer interglacial periods. During glacial phases,  growth of polar ice traps sea water. This causes sea levels to be lowered. During warmer interglacial phases, polar ice caps melt and raise sea levels. The last glacial phase lasted between 110 - 12 thousand  years ago.  During this time the sea level was as much as 100 meters lower than today. Large swaths of the continental shelf was land then. The earth then moved into a warmer interglacial phase. As a result of melting polar ice, the sea has been rising steadily for the past 10-11 thousand years, flooding the continental shelf, and culminating in a sea level highstand (maximum) about 4000-6000 years ago. This maximum was about 1-4 meters above the present sea level.

There is evidence scattered all along India's west and east coast (and all over the world) of this Holocene sea level high. For example, there are tidal flat deposits about 1 meter above present sea level along the Porbundar coast in Gujarat. Shells collected from these deposits give an age of about 4000 thousand years. Exposed reefs from Mithapur in Jamnagar district in Gujarat give an age of about 2100 years. Oyster reefs exposed along Saurashtra coast about 2 meters above present sea level are about 2500-3000 years old.

To the south, in Madh Island (Mumbai) and along Konkan coast, there are layers of hardened sand and pebbles, locally known as 'Karal', which occur 2-4 meters above present sea level. These sediments once formed a pebbly beach.  At Kelsi village in Konkan, there are fossil beach ridges a few hundred meters inland of the present high tide mark. I saw these on my recent visit. Along the east coast, there are 4000-6000 year old beach ridges along the Krishna-Godavari coastline. These ridges become younger towards the coast, indicating that the sea has been receding since about 4 thousand years ago.  Along the Baruva-Gandavaram coast in  Andhra Pradesh, sea cliffs have preserved a succession of erosional notches at 4.7 m, 2.3 m and 1.8 m above sea level.

All these features indicate sea level peaked about 4000-6000 thousand years ago and has been falling in fits and starts since. The exact mechanism for this recession of the sea is not well understood.

Scientists have put together data form various localities to come up with a composite sea level curve for the Holocene. The curve below has been drawn up using data from Gujarat and shows sea level rising throughout early and mid Holocene. The late Holocene has seen a lowering of seas.


Source: U.B Mathur et.al. 2004

This lowering has now been reversed and the seas are rising again globally, this time induced by anthropogenic global warming as continental glaciers melt and the ocean water expands as it gets warmer. It is estimated that sea level will rise between 0.5 - 1 meter by 2100. In centuries to come, the extent of sea level rise will depend on future warming trends and the extent of melting of the Greenland and Antarctica ice sheets. If significant portions of these ice sheets melt, sea level will rise by several meters in the next few hundred to couple of thousand years.

And what about the flat rocky platforms seen in the tidal zone below the sea cliffs? How do they form? The likely process involves "water layer leveling" combined with wave erosion. Water layer leveling means the lowering and leveling of the rock surface due to physical and chemical weathering by the action of sea water. Standing pools of water and the continuous wetting and drying conditions in the intertidal zone act to weaken the rock and create a loose surface layer which is then removed by wave action, generating a flat rocky platform.

The picture below shows a wide intertidal rocky platform from Korlai village, south of Alibag town on India's west coast.


India's Konkan coast is beautiful and has interesting geology too. Do visit if you can.

References:

1) Falling Late Holocene Sea-Level Along The Indian Coast- U.B. Mathur, D.K. Pandey, Tej Bahadur 2004

2) Quaternary Sea Level Changes Along Indian Coast - S.S Mehr 1992

Sunday, October 29, 2017

The Geology Of India In 220 Tweets

Ok, I am exaggerating.

Last week I hosted the @Geoscitweeps account and tweeted 8 stories about Indian geology. This is an earth sciences focused rotating twitter account curated by science writer Sandhya Ramesh (@sandygrains).  Geologists from all over the world have been volunteering to host the account for a week and tweet about their work. I volunteered for the week beginning October 16 and decided to broadcast some interesting stories about Indian geology.  I had written blog posts about some of the topics, but it still was a challenge to create an engaging  narrative using 20-30 tweets.

Here are the threads:

1) Does India have Cambrian age Burgess Shale type fossils?

2) The Tempo of Deccan Volcanic Eruptions

3) Deccan Lava Flows and Buddhist Caves and Rock Art

4) Piggy Back Basins and Seismic Risk of Himalaya Frontal Ranges

5) Exploring India's Fossil Sites and Paleogeography using the Paleobiology Navigator

6) Which of these Indian Island Chains is Geologically Older? Lakshadweep or Andamans?

7) Evolution of the Western Ghat Escarpment and Coastal Plain.

8) How To Discover Your Inner Geologist When You Go Trekking In The Himalaya

It was really gratifying to see the enthusiastic response by readers from all over the world... and particularly satisfying to see that a large number of Indians began following @Geoscitweeps as news spread that there was Indian geology on the menu.

More Indian geologists need to start writing and talking with the general public about their work. There is certainly an audience out there eager to hear from them. 

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.