Wednesday, January 22, 2020

Sedimentary Structures: Building Stones of Badami, Aihole And Pattadakal Temples

Is this slab in its original geological orientation (as when the sedimentary layers were deposited) or is it upside down? I'll answer this a little later, but first some background.

I recently visited the temples and rock cut monuments at Aihole, Pattadakal and Badami (6th -12 CE) in northern Karnataka and noticed some great sedimentary structures in the building stones. 

These monuments are made up of Neoproterozoic age (900-800 million year old) sandstones. Geologists have recognized using detailed sedimentological analysis that the sandstones formed mostly in a large braided river system that flowed in a northwesterly direction.

Between  roughly 1800 -800 million years ago, over the course of a billion years, the Indian continental crust sagged due to various tectonic forces to form several long lasting sedimentary basins. The Kaladgi Basin in which the Badami area sandstones were deposited is one such basin. The paleogeographic reconstruction below shows the position of the Indian continent at about one billion years ago and the location of the various sedimentary basins within it.

Source: Shilpa Patil Pillai, Kanchan Pande and Vivek S Kale: 2018: Implications of new 40Ar/39Ar age of Mallapur Intrusives on the chronology and evolution of the Kaladgi Basin, Dharwar Craton, India.

Much of this deposition took place in inland or epeiric seas that flooded the Indian continent. During intervals of sea level fall, rivers carved valleys and deposited coarse sediment. The Badami Cave sandstones are river deposits of the Kaladgi Basin. The stratigraphic column shows various sedimentary deposits of the Kaladgi Basin and their inferred environments of deposition.

Source: Shilpa Patil Pillai, Kanchan Pande and Vivek S Kale: 2018: Implications of new 40Ar/39Ar age of Mallapur Intrusives on the chronology and evolution of the Kaladgi Basin, Dharwar Craton, India.

The Badami braided river system was receiving sediment eroded from Archean age (>2.5 billion year old) rocks situated SE of the basin. These were granites, granodiorites, and low to medium grade metamorphic  rocks of the Dharwar craton (a large block of stable old continental crust). 

Land plants did not exist then. Weathered debris was moved quickly by surface flow into streams. Large sediment load, moving by traction i.e. by rolling and sliding on the stream bed, repeatedly chocked the channels, forcing bifurcation of streams and formation of braids. Very broad braided rivers formed since there were no plants to stabilize banks.  The Badami sandstones (Cave Temple Formation) are technically known as arenites. This term indicates that the rock is made up of mostly coarse sand with very little finer sized mud. Accumulation of mostly coarser sand size and pebbly particles reflects a locale of repeated high discharges and vigorous currents which winnowed away the finer sized mud.  The braided river shown below as an example is from the Canterbury Plains of New Zealand.

 Source: Braided Rivers: What's the Story?

The Badami rocks preserve a record of  various subenvironments of this paleo-river. Picture shows channel and bar deposits in outcrop.

Source: Mukhopadhyay et. al. 2018; Stratigraphic Evolution and Architecture of the Terrestrial Succession at the Base of the Neoproterozoic Badami Group, Karnataka, India.

As river channels episodically migrated sideways and the basin floor subsided to accommodate more sediment, channel deposits and adjacent sand bars got stacked to form thick 'multi-story' sandstones. Each bed tells a story of a discrete depositional episode.

The arrangement of sand layers within each bed tells us about the subenvironments in which it formed and the energy and direction of water flow during deposition. I came across many types of these internal structures. I recognized tabular cross beds, trough cross beds, planar lamination and rippled beds. Water (or wind) can move & shape sand into piles or waves. Sand grains roll along the direction of flow, then avalanche down the steeper side (lee side) of the wave forming a layer inclined (cross) to the orientation of the main sand body. Successive avalanches form a set of cross beds. The graphic shows the formation of a set of cross beds.

 Source: Dr. Diane M Burns in Teaching Sedimentary Geology in the 21st Century.

Here is an example of cross beds from near the town of Badami.

And this one is from a building stone from Pattadakal temple.

Such cross beds were built by sediment avalanching on the lee side of migrating sand bars during high flow.

This picture show trough cross bedding from near the Badami cave complex. These represent the internal structure of migrating sinuous sand dunes on a channel floor. 

See this elegant explanation by Dawn Sumner, a sedimentologist at the University of California at Davis,  of how trough cross beds form.

Email subscribers who may not be able to see the embedded video, click on this link: Trough Cross Bedding Video.

And here is a beautiful example of trough cross bedding found in a Pattadakal temple building stone.

This is planar lamination on a slab at Pattadakal. The bed is constructed of parallel layers of coarse sand. It is interpreted to have been deposited in a high flow regime from sheets of water flowing over mid channel sand bars.

Ripples on a slab at Pattadakal. This is a rare preservation of a bedding surface showing rippled sand. Erosion usually cuts off the wavy upper part. These ripples indicate migration of small sand waves in a quieter flow regime on the channel floor.

Remember, cross beds are the inclined layers that form on the lee side of a ripple or wave or dune. Here are small cross sets on the floor of Aihole rock cut temple! These represent the cross beds formed by migration of small ripples. The ripples themselves have been eroded away. Arrows indicate the direction of water flow and cross bed accretion as ripples migrated.

Okay, let's go back to my first question. Is the slab I showed in the picture geologically upside down?

Yes it is. But how to tell?

As sand avalanches down the lee slope it forms a tail at the toe of the slope resulting in cross beds which become tangential to the floor. In picture the cross beds are tangential towards the top of slab i.e. that is actually the base.

Lets see at how the cross bed contact with the top and bottom bedding plane looks in an outcrop. Here is the original depositional orientation of cross beds manifest in this outcrop near Badami caves. They show a tail or tangential contact of the cross beds with the base. Since top of cross beds are not usually preserved they show a high angle contact truncated by upper bedding plane.

This slab is upside down too! Notice again the tangential contact of the cross beds (white arrow) is towards the top, which means that must have been the base. Yellow arrow points to high angle contact with the upper bedding surface. 

Towards the top of the exposed section of sandstone around Badami I came across some truly impressive examples of cross bedding. These particular exposures were on the crags opposite the four main Badami temples. There is a narrow passage past the archaeological museum and a short climb to the top. Take a look at these beauties!

These large cross beds reminded me of the inclined beds of wind blown sand dunes. Is it possible that abandoned sand bars were sculpted by wind in to big dunes? Or does this upper level sandstone represent, as a recent study suggests, the beginning of a marine incursion in to the basin? In this scenario, deposition of sand took place in high-energy shallow waters near the shore. These cross beds represent large migrating sand waves which were eventually shaped in to beach ridges and tidal bars.

The outcrops and building stones of these monuments mostly record the processes within the Badami braided paleo-river. 900 million yrs ago a complex of channels and bars, quieter pools and rippled sand beds existed where these temples stand today.

Do visit Aihole, Pattadakal and Badami and gaze at its splendid architecture and sculptures. But spare some time to appreciate the magnificent record of our natural history that these monuments preserve. 

Quiz- Is this slab upside down or in its true depositional orientation? 😉

Until next time....

Sunday, December 22, 2019

Glacial Saraswati?: New Data On An Old Question

Did a perennial glacial fed river flow through the Indus Civilization region of what is now Haryana and Rajasthan? Previous work on the fluvial history of this region had indicated that a distributary of the glacially sourced Sutlej was flowing through a network of paleo-channels buried under the river now known as the Ghaggar until around 8,000 years ago. The Sutlej distributary system then died out, turning that river course into a smaller monsoon fed channel system.

For a more detailed history of research on this topic you can follow this link - Ghaggar /Saraswati Posts.

Recently, in November 2019, Anirban Chatterjee and colleagues published new data on deposits of grey sand in the subsurface of the Ghaggar channel and adjacent floodplains. The youngest of these deposits are 4, 500 years old. Geochemical fingerprinting points to High Himalayan granites and gneisses as their source. This likely extends the glacial phase of the Ghaggar to more recent times, until about the beginning of the urbanization of the Indus Valley Civilization (IVC).

Here is the abstract:

The legendary river Saraswati of Indian mythology has often been hypothesized to be an ancient perennial channel of the seasonal river Ghaggar that flowed through the heartland of the Bronze Age Harappan civilization in north-western India. Despite the discovery of abundant settlements along a major paleo-channel of the Ghaggar, many believed that the Harappans depended solely on monsoonal rains, because no proof existed for the river’s uninterrupted flow during the zenith of the civilization. Here, we present unequivocal evidence for the Ghaggar’s perennial past by studying temporal changes of sediment provenance along a 300 km stretch of the river basin. This is achieved using 40Ar/39Ar ages of detrital muscovite and Sr-Nd isotopic ratios of siliciclastic sediment in fluvial sequences, dated by radiocarbon and luminescence methods. We establish that during 80-20 ka and 9-4.5 ka the river was perennial and was receiving sediments from the Higher and Lesser Himalayas. The latter phase can be attributed to the reactivation of the river by the distributaries of the Sutlej. This revived perennial condition of the Ghaggar, which can be correlated with the Saraswati, likely facilitated development of the early Harappan settlements along its banks. The timing of the eventual decline of the river, which led to the collapse of the civilization, approximately coincides with the commencement of the Meghalayan Stage.

The geological work looks to be sound. The data on sediment fingerprinting overlaps with what we know about High Himalayan geochemical signatures and present day Sutlej sand composition.

I do want to comment on another sentence from the abstract (emphasis mine)-

"This revived perennial condition of the Ghaggar, which can be correlated with the Saraswati, likely facilitated development of the early Harappan settlements along its banks"

Saraswati is the name given to this river by the Vedic people. Correlation of the river's perennial phase between 9,000-4,500 years ago with Saraswati is valid only if  you can demonstrate that the Vedic people were inhabitants of this region from before 4,500 yrs ago. Geological studies cannot establish this. A combination of archeology, linguistics (cracking the Indus script would be nice!) and genetics will eventually answer that. The other scenario is that the Vedic people could have migrated into this region much later and began venerating a smaller monsoonal Ghaggar as Saraswati. Work by Liviu Giosan and colleagues suggests that stronger monsoons over the Siwaliks sustained sufficient flow in the old channels of the Ghaggar until the late IVC period (~1800-1600 B.C).

When did this river come to be called the Saraswati is still an open question.

Two recent genetics papers using ancient DNA recovered from the IVC site of Rakhigarhi and from Central Asia argue that people from the Pontic-Caspian steppes migrated into South Asia between 2000 -1500 B.C. bringing with them the Indo-Iranian branch of the Indo-European language family. These would presumably be the Vedic people.

Read these papers. They are interesting.

1) On the existence of a perennial river in the Harappan heartland.
2) The formation of human populations in South and Central Asia.
3) An Ancient Harappan Genome Lacks Ancestry from Steppe Pastoralists or Iranian Farmers.

Friday, December 20, 2019

Readings: Erectus SE Asia, Devonian Fossil Forest, Archean Iron Formations

Some selected readings:

1) New dates of Homo erectus from Ngandong Java shows late surviving populations until 117,000 to 108,000 years ago. A short clean summary by Razib Khan on SE Asian hominin diversity.

Southeast Asia during the Eemian was a hominin paradise.

Paper: Last appearance of Homo erectus at Ngandong, Java, 117,000–108,000 years ago.

2) Exquisite preservation of one of the earliest forests from the Mid Devonian ( ~385 million years ago) of New York containing a modern looking root system.

Paper - Mid-Devonian Archaeopteris Roots Signal Revolutionary Change in Earliest Fossil Forests.

Write up : The World’s Oldest Forest Has 385-Million-Year-Old Tree Roots.

3) Before around 2.3 billion years ago there was very little oxygen in the atmosphere. This was a time before the evolutionary invention of oxygenic photosynthesis wherein bacteria harvest electrons from H2O and release oxygen as a byproduct. Instead, during this time another photosynthesis pathway known as photoferrotrophy was prevalent. Here, bacteria use light and ferrous iron (Fe+2) to fix CO2 as biomass, releasing ferric iron (Fe+3) as byproduct. This ferric iron then accumulated to form large iron deposits. But these deposits lack organic matter. How to explain this if the iron was being produced from a biomass? Scientists point to a role of silica. At that time the oceans were saturated in free silica. Experimental work shows that in the presence of free silica cell surfaces repel iron hydroxides, thus creating a source of organic matter free iron deposits. This organic matter then was acted upon by methane producing microbes. The methane released kept the temperature of the earth warmer than it would have been under a dim early sun.

Fascinating story of the feedback between geology and evolution.

Photoferrotrophy, deposition of banded iron formations, and methane production in Archean oceans.

Saturday, November 30, 2019

Field Photos: Natural Arch, Lava Channel NE Of Pune

Last Saturday I went for a field trip organized by the Centre for Education and Research in Geosciences. This is an outreach effort initiated by geologists Dr. Sudha Vaddadi and Natraj Vaddadi along with the student community from various Pune colleges. They undertake these programs regularly through the  year. We explored the Deccan Plateau region northeast of Pune.

On Pune Nasik Highway we turned east at Ale Phata. Our first stop was a little past Gulunchwadi . Across the road an inclined dike intruding into older basalt flows is visible. And a groundwater seep is seen along the contact between two basalt flow units.

Such water seeps can slowly weather and remove rock eventually creating larger passages and undercuts. We saw that just a few minutes ahead. Besides a small roadside temple is a steep stairway leading you into a streambed below. There you come across a wondrous natural arch.

I am not sure I have a good explanation of how exactly this feature formed. Was there a larger waterfall cascading from the top before? At the same time, groundwater seeping along the contact between the two flow units would have eroded rock material creating large passageways which eventually coalesced. Stream flow got directed along the bed of this large tunnel. Further down cutting by the stream has lowered the level of the stream bed, leaving a stranded 'bridge'. 

Also notice in the satellite picture below that the stream makes an abrupt turn at a couple of different points along its course. Its pathways appear to be controlled by fractures.They would have provided weak zones that focused and enhanced erosion.

From this site we proceeded to Mandahol Dam. A little north of this dam is a ridge line named Mhasoba Zap. Can you spot something unusual in the topography of the ridge?

The sinuous feature is an exhumed river of lava!  It erupted between 67 and 66 million years ago.

Basalt lava is less viscous and can flow for long distances. It can follow a preexisting valley or lows in the landscape, forming a lava channel. The image below is from a USGS monitoring station that has captured a lava channel formed during a recent eruption in Hawaii. 

The view in this photo at Mhasoba Zap is looking upslope. The winding ridge which you can follow up to the isolated hill in the background is the exhumed lava channel. It stands out about 50-100 meters above the adjacent plains. 

And here is the lava channel looking downslope. It continues for a distance of about 3 kilometers 'downstream' before dying out.

The margin of the channel (white arrows) are made up of a basalt which looks a little different from the basalt in the central parts of the channel. The margin rock is reddish in color. A closer look (in the field) will tell you that it is glassy to fine grained.  Lava at the margins cools quickly. This cooled lava gets broken up because of the stresses imparted by flowing lava in the center of the channel. This gives a fragmented character to the margins. The iron in the quenched glassy matrix rusts to impart a orange red hue to the rock.

In a close up I have outlined the base of the channel in orange lines.  Dr. Sudha Vaddadi who has mapped this region when she was working with the Geological Survey of India tells me that this entire ridge is actually a lava tube. The top has been eroded away! She was able to identify the 'roof' a km away downslope.

Lava at the surface cools and solidifies quickly. That leaves a tube or a pipe through which lava is supplied from the vent across long distances. The solid crust insulates and keeps the interior hot, allowing the lava to reach long distances from its source. The photo is of a lava tube from the Reunion Islands, a site of ongoing volcanism.

Photo Credit: Nandita Wagle

Let's take a closer look at the margin rock.   It is distinctive due to the reddish color and the fractured fragmented nature of the rock.  It has also been extensively affected by secondary mineralization. Cracks are filled with (white veins) of fibrous scolecite (zeolite family) and calcite.

Blobs and lava spatter accumulates at the margins, cooling and welding together to form an 'agglomerate'. The close up shows globular masses of lava stuck together. 

In this synoptic view, almost the entire lava channel is visible.  Downslope it breaks up into distributary 'fingers'. 

This really was a fun trip. From this lava ridge we traveled south and saw stalactites at the Duryabai Temple near Wadgaon Durya and then went further south to see the famous potholes in the Kukdi river bed near Nighoj village.  I will write about these features in a later post.

In the embedded map look for Malaganga Temple, Mhasoba Zap, Wadgoan Durya and Nighoj. 

Email subscribers may not be able to see the map. Follow this Permanent Map Link.

Although the Western Ghat escarpment with its spectacular views captures a lot of attention, the Deccan plateau region to the east and northeast of Pune has a lot of interesting geology and landscapes.

Get out there and explore!

Tuesday, November 19, 2019

Kumaon Lesser Himalaya- Lessons In Mountain Building

Mountain belts like the Himalaya and the Alps formed when continental crust was squeezed, deformed and uplifted during the collision of two continental plates. The Himalaya, which is the deformed edge of the Indian continental plate is made up of different terrains. The Tethyan Himalaya is the northernmost terrain whose northern edge meets the Asian continental plate. The Greater Himalaya and the Lesser Himalaya are the two terrains successively to the south of the Tethyan Himalaya.

The Tethyan Himalaya are made up of rocks of Cambrian to Eocene in age  (542 -50 million years old) and have suffered the least burial and metamorphism. The Greater Himalaya are rocks which were buried as deep as 20-25 kilometers, suffering the highest degree of metamorphism and even partial melting. They range in age from the Neoproterozoic to the Ordovician (1000 -450 million years old). The Lesser Himalaya rocks were subjected to an intermediate level of burial and metamorphism. They span the Paleoproterozoic to Neoproterozoic in age (1840 -800 million years old).

The Siwalik ranges that occur to the south of the Lesser Himalaya are made up of sediments that were derived from the erosion of the rising Greater and Lesser Himalaya. They range in age from about 12 million to 0.5 million years.

So, exactly what happens when the crust gets caught up in such a continent-continent collision?

A recent paper by Subhadip Mandal and colleagues in Lithosphere explains the structural architecture and mechanisms of crustal deformation of the Kumaon fold and thrust belt. They propose a resolution of some long standing problems in Kumaon geology, namely the interrelationships between the different fault systems and exposed terrains.

See the map which shows part of the Kumaon and Gharwal Himalaya. Before India Asia collision, the crust between the two orange lines would have been ~575 kilometers wider! ....Squeeeze!

How did the crust get 'shortened' during India- Asia collision? In the schematic that I have drawn below, shortening of a particular length of crust takes place by folding it or by breaking it up into blocks and stacking them. The Himalaya have formed by a combination of such folding and thrust stacking.

Off course, the Himalaya is not one big fold, nor is it a stack of blocks forming a tower like the way I've drawn it. Rather, think of inclined books on a shelf. The books are inclined towards the right, or north. There are 4 books. From right to left they are the Tethyan Himalaya, Greater Himalaya, Lesser Himalaya and the Siwaliks. 

A shelf with books inclined to the right will grow by shelving from left to right. The Himalaya have grown in exactly the opposite manner. 

Imagine four books lying flat forming a chain on a bookshelf.  The rightmost book (Tethyan Himalaya) made contact with Asia and was thrust up. Then the book to its left was thrust up (Greater Himalaya), then the Lesser Himalaya and finally the Siwaliks. Deformation moved from right to left, or in the real world, from north to south.

But enough of abstraction! In the real world.. A  has been crumpled up to form B.  A in the figure below is the original disposition of rock units of the Indian plate. B shows those units as they are today,  folded and faulted after the collisional process. The Greater Himalaya is the topmost pink layer. The Lesser Himalaya layers are shown in green, blue and orange. The Siwalik ranges are fawn colored. The Tethyan Himalaya not shown in this figure. 

Source: Subhadip Mandal 2019

The structure looks like a mangled disordered heap of strata. But there is an order to this apparent chaos. As in our book analogy, these units were deformed in a sequence. The cross sections below shows the sequence of deformation and how the structural architecture evolved.

Source: Subhadip Mandal 2019

The collision of the Indian and Asian plates has been timed to around 55-50 million years ago. The first significant topography formed with the uplift of the Tethyan Himalaya between 45-35 million years ago.  The rocks that became the Greater Himalaya were buried the deepest during continental collision. They were uplifted between 23-16 million years ago. This crustal block was moved along a giant fault system known as the Main Central Thrust. In the two figures above you can see the Greater Himalaya as the pink layer overlying the Lesser Himalaya.

The Lesser Himalaya rocks which show imprints of a shallower buried state were lifted up between 16 - 4 million years ago. This rise of the rocks of  the Lesser Himalaya took place in two broad phases. In the first phase, the oldest rocks of the Lesser Himalaya were thrust up along another big fault system known as the Ramgarh-Munsiyari Thrust. In the figure, these oldest Lesser Himalaya rocks are the thick green layers immediately below the pink Greater Himalaya. Subsequently, more and more of the younger Lesser Himalaya strata got caught up in the deforming pile of rocks. Slices of the younger Lesser Himalaya were moved along thrust faults and stacked in a southerly growing fold and thrust belt.   

Initially, the Greater Himalaya and the oldest Lesser Himalaya were placed atop the younger Lesser Himalaya along very low angle faults in a manner similar to the stacked blocks I showed in the beginning of the post.  Later, the growth of the younger Lesser Himalaya lifted, tilted at steeper angles, and folded the overlying Greater Himalaya and the older Lesser Himalaya thrust sheets in a series of broad domes (anticlines) and troughs (synclines).  

The Greater Himalaya and the oldest Lesser Himalaya domes were more susceptible to erosion. As a result, these domes were removed over time, leaving behind synclinal remnants known as klippen. Isolated outcrops of Greater Himalaya and the oldest Lesser Himalaya rocks sit atop younger Lesser Himalaya at many places along the Lesser Himalaya belt. For example, the town of Almora in Uttarakhand is on a klippen of Greater Himalaya rocks. Further to the east the small town of Askot is on a klippen of the oldest Lesser Himalaya.
And finally, sediments which were being deposited in a southerly moat in front of the rising Greater and Lesser Himalaya rose to become the Siwaliks beginning around 1-0.5 million years ago. 

Mandal and colleagues work clarifies to a great extent the structure of Kumaon Himalaya and the mechanism of how fold and thrust mountain belts are constructed. I have simplified the story here. The paper has more nuanced details of the methods and techniques used to reconstruct a long and complicated process.

When you travel next across the Kumaon region, think of inclined books (thrust sheets) and their sequential uplift.