Oldest Himalaya Rocks

In Peninsular India, the most significant change in rock type occur across what is known as the Archean Proterozoic boundary.

Archean rocks, older than 2.5 billion years, are typically varieties of granite and granite gneiss. They formed when the earth was much hotter and silica rich continental crust was growing by injections of magma from the uppermost mantle and by partial melting of older mafic (silica poor) crust. At places the crust subsided by vertical movements, and lava and sediment filled the narrow depressions. These volcano-sedimentary rocks were deformed and metamorphosed to form linear schist enclaves within the granitic crust. The term granite greenstone terrain describes this rock association.

This phase of continental crust building petered out by around 2. 5 billion years ago. The thick crustal blocks or cratons became the nuclei for future continent growth. By 2 billion years ago or so in the Paleoproterozoic (the Proterozoic Eon spans from 2.5 billion years ago to 538 million years ago) , the Archean crust became the floor for several sedimentary basins. Erosion of the Archean rocks provided sediments that accumulated in these basins over the next 1 billion years, with long hiatuses punctuating pulses of sediment deposition. The names of these sites of deposition will be familiar to many readers and travelers.  Aravallis, Vindhyans, and Cuddapah, to name a few, represent this younger Proterozoic phase of crustal recycling. 

There was limited development in Peninsular India of younger sedimentary basins and as a result Archean and Proterozoic crustal sections are widely exposed all across the country.

The satellite imagery posted below shows one classic locality of the Archean Proterozoic boundary. This is from the Cuddapah Basin of South India.  

The Archean granitic terrain has a rough texture due to the bouldery nature of the landscape formed by weathering of fractured granite. Towards the east north east, the layering of sedimentary strata of the Cuddapah Basin is prominent and unmistakable.  The following graphic is a geologic log prepared to describe the succession of rock types from the Cuddapah Basin.

Source: Vivek S Kale and coworkers; Proc. Indian. Nat. Sci. Academy 2020.

Archean 'basement' and Proterozoic 'cover sequence' is a common stratigraphic motif of the Precambrian geology of India.

A few weeks ago I found an old paper from the 1970's on the sedimentology and stratigraphy of Tethyan sequences from the Kali valley area, near the Kumaon Nepal border. These are, as the name suggests, sediments deposited in the Tethyan Ocean along the  northern margin of the Indian continent. They range in age from the Proterozoic to the Mesozoic.

Here is the stratigraphic column from the paper. I have shown only the Precambrian (Archean and Proterozoic collectively make up the Precambrian) section of the column. The Central Crystallines are assigned an Archean age, while the Tethyan Sequence Martoli Formation is Early Precambrian. The name Proterozoic was not in use in the 1970's when this paper was written.

 Source: S. Kumar and coworkers; Journal of Paleontological Society of India 1977.

'Crystalline' in this context refers to the rock texture made up of large interlocking minerals formed during slow cooling of a magma or during high temperature metamorphism of a sedimentary rock.

The geologic sequence I described earlier took place along the northern margin of India too where the future Himalaya would form. At first glance the Himalaya sequence seems a replica of the geology of Precambrian Peninsular India. It records an Archean  'crystalline'  basement, succeeded by variably deformed and metamorphosed Precambrian (Proterozoic) sediments.

Except that it is wrong. There are no Archean age rocks exposed anywhere in the Himalaya. And the rock units immediately in contact with the Archean are not Early Precambrian.

When this paper was published there was precious little geochronology work done in the Himalaya. Geologists knew from sporadic absolute dating of Peninsular rocks that the granite and granite gneiss terrains are older than 2.5 billion years  (Archean).  The thick sedimentary sequences overlying the granite basement also were Precambrian as ascertained by dating intrusive granites and interbedded lava layers. The lack of any shelly fossils in them was another indicator of the Precambrian age of the cover sedimentary sequences. 

Given this familiarity with Peninsular geology, it would have been natural to assign the same chronology to a Himalaya rock sequence of high grade gneiss in contact with unfossiliferous sedimentary rocks. 

The Central Crystallines are now known as the Greater Himalaya Sequence and they are not Archean but Neoproterozoic (Late Precambrian) in age. Detailed work has shown that they represent sediments deposited roughly between 1 billion and 600 million years ago along the northern continental shelf of India. A paleogeographic reconstruction of the Himachal Himalaya by Alexander Webb and coworkers shows the original disposition of the different Himalaya rock divisions. Observe (A) that the Greater Himalaya (GHC) and the lower part of the Tethyan Himalaya (Haimanta/Martoli Formation) were deposited synchronously in adjacent areas of the continental shelf.  

 Source: Alexander Webb and coworkers; Geosphere 2011.

They look very different from each other today because they experienced different conditions during Himalaya mountain building. The Central Crystallines which began their life as marine sediments became crystalline gneisses and schists during high grade Cenozoic metamorphism 35 to 20 million years ago, while the Tethyan Sequence escaped being buried deep in the crust and retained much of their original sedimentary character. 

What about the oldest Himalaya rocks? These are Paleoproterozoic in age, dated to be about 1.9 to 1. 8 billion years old. The units Damtha, Berinag, Wangtu, Jeori and Baragaon in the Himachal Himalaya cross section are the Paleoproterozoic age rocks. They are remnants of a magmatic arc and associated basins which formed along the northern margin of India when continental blocks were colliding and suturing into an early supercontinent named Colombia.

Underneath these Paleoproterozoic rocks would have been the Archean 'basement'. But where is it now? 

We can take a step back and understand how the Himalaya are constructed. As the Indian continental crust collided and pressed into Asia, a crack or a fault initiated in the collision zone started propagating southwards, slowly splitting the Indian crust.  As India kept getting pushed under Asia along this master fault, slices of Indian crust get scraped off  and thrust upwards along subsidiary faults to form a growing mountain range. This tectonic evolution is depicted as stages B, C, D, E, F. You can also read my post Himalaya: A Critical Wedge for more details on the mechanisms of mountain building.

If as shown in the cross section, the faults that break and transport crustal sheets are located entirely within the Proterozoic and younger layers then the Archean rocks won't get incorporated into the Himalayan orogen. They lie below the basal detachment/master fault. Alternatively, this model may not be applicable everywhere in the Himalaya and there may be slivers of Archean rocks buried deep under the thrust pile, but erosion hasn't exposed them yet. 

In the geologic future, the plate tectonic engine that made the Himalaya will change track. The mountain ranges will stop growing. Erosion will wear down the Himalaya and eventually lay bare its roots. The Archean 'crystalline basement' so familiar all over Peninsular India will also be visible at the base of the gentle rolling hills that were once the mighty Himalaya.

Geology Infographics

I read a lot of technical literature on various geology topics. The papers are usually long and written in jargon filled language. It can be tough to hold your concentration and read through the paper in one sitting. What can help is a well complied figure which summarizes the ideas and the results of the study. By figure, I don't mean a graph or tabular display of data, but a graphic that presents the data with a combination of symbology, line art, text, and even images. Such infographics help in grasping the gist of the study and make reading the elaborate explanations easier (you still have to read them).

In this post I will showcase three infographics that I liked from my readings. I won't write long explanations about them, since the idea is to see if you can understand the broad findings by looking at a picture. Read the abstract of the paper to assess how effective the figure is.

1) Ediacaran Extinction and Cambrian Explosion.

The distribution through time and the changes in diversity of early complex multicellular life is depicted in this infographic. The evolutionary history of two distinct 'biotas' are tracked. The Ediacaran 'biota' is a catchall phrase that includes a diverse range of extinct large fossil organisms which may include some early animals as well. Metazoans ancestral to living animal groups are the second category. The carbon isotope curve shows two prominent deflections towards negative values, termed the 'Shuram' and "BACE" (Basal Cambrian Carbon Isotope Excursion) excursions. They are thought to indicate global environmental crises. Bookending this graphic are two diversity measures. On the left is the diversity of body fossils. On the right is the diversity of trace fossils, such as imprints, tracks, and burrows. 

Take home point. The Cambrian 'Explosion' is not about the origin of animals but their geologically rapid diversification whose roots lie a good 20 to 30 million years preceding the Cambrian events. Pulses of diversity expansion and collapse took place during that time period.

2) Dating Cave Art.

Humans have left some breathtaking artwork on the walls of caves all over the world. But how do we know when they were created? The pigments used in the drawings cannot be directly dated. One can use associated cultural artifacts to narrow down the time period. Or if lucky, mineral layers that entomb the artwork can be dated directly. This method still brackets the maximum and minimum age of the artwork. This infographic explains how artwork in a Spanish cave was dated using uranium and thorium isotopes. 

3) Angiosperm and Insect Coevolution.

I had written about this topic is detail in a previous post, but thought I'll share this infographic again. The Cretaceous was a time of great environmental shifts and changes in terrestrial biodiversity. Gymnosperms gave way to a dominance of angiosperms. The diversification of flowering plants had a large collateral impact on earth. The history of angiosperms and insect groups through the Cretaceous and Cenozoic is explained in this beautifully compiled infographic. 

If you have come across a science infographic that you particularly like, do share the link in the comments section.

Early Animals, Hominin Diets, Groundwater Governance

 A few links to interesting listening and reading.

1) Tracking the first animals on earth:  Unequivocal evidence of animals is preserved in soft sediments from about 570 million years ago. The fossil record of the Ediacaran to early Cambrian times (570 to 500 million years ago) has yielded rich information about the patterns of animal evolution. Apart from fossils, comparative genetic studies have given insights into how different animal groups are related to each other and the order of branching of these groups. Amazingly, organic molecules recovered from enigmatic fossilized taxa have been used to differentiate between animal and non-animal remains. Zoologist Matthew Cobb explains all this and much more about early animal evolution in about 30 minutes. Give it a listen! 

2) Plant-eating and meat-eating in Australopithecus: What did our ancient relatives eat? By ancient, I mean going back a million years or more. We can use isotopes of nitrogen to tease out information about diets. Carnivores have more nitrogen-15 enriched tissue than plant eaters. Carbon isotopes (C13 and C12) also yield information about the diet of herbivores. Grazers munching on grass take in more of the heavier isotope of carbon than browsers eating leaves and stems. Paleoanthropologist John Hawks discusses some recent work on nitrogen and carbon isotopes of Australopithecines and how the patterns of isotopic variation extracted from tooth enamel can be interpreted in terms of diets and life history. Fascinating stuff. 

3) Addressing Depletion in Alluvial Aquifers: Why Context Matters in Participatory Groundwater Management: India relies a lot on groundwater for agriculture. There are signs from many parts of the country of acute groundwater distress. Participatory Groundwater Management initiatives have had some success in addressing this distress. Pratik Kumar and Veena Srinivasan point out that these cooperative movements have been more successful in hard rock aquifers from different parts of the country than alluvial aquifers of northwest India. Geology matters. Aquifer properties matter. Hard rock aquifers are more sensitive to abstraction and are rapidly de-watered and recharged seasonally. Alluvial aquifers are spread over vast areas and water levels are less sensitive to abstraction. The amount you can extract doesn't vary with lowering of water level. 

People depending on hard rock aquifers experience the limitation of the resource yearly and are more willing to join cooperative initiatives to manage the resource.

I have just given a gist of the more elaborate arguments in the paper. The graphic below very neatly compares hard rock and alluvial aquifers. 

 Source: Pratik Kumar and Veena Srinivasan 2025

The paper is open access.

Lithospheric Dehumidifier

A few years ago a friend decided to demolish his old house due to extensive and expensive water damage to the ground floor. 

Could this be the explanation for the damage?

xkcd comics

A spanking new apartment building now stands at the spot of the old bungalow. There are no signs of any water damage so far. A giant lithospheric dehumidifier may have been used during construction.

There is no end to the add on and perks developers promise these days.

Oil Hunters Of India

At Independence in 1947, India had just a few operational oil wells situated in the northeastern region of the country. Most of the subcontinent's sedimentary basins remained unexplored for their hydrocarbon potential. The Oil Hunter: Journey of a Geologist for India's Oil Exploration by Dr. Shreekrishna Deshpande is a personal recollection of the immense effort undertaken by Indian geologists to re imagine these basins as hydrocarbon source and reservoir rocks. It is the story of the development of India's oil industry told with unconcealed pride.

Russia, France, and the U.S. offered personnel and technical help along the way, but the lion's share of the credit goes to geologists of the Oil and Natural Gas Corporation for their perseverance and resilience in the face of immense challenges.

Dr. Deshpande joined the ONGC in its early days in 1961 and describes vividly his field experience in remote locales all across the country, from scorching Kutch, to steep Himalaya terrain, to facing personal danger during an insurgency in Assam. There were inevitable career challenges along the way due to changing institutional structure and unrealistic political expectations. Their impact on company work culture and productivity is described in honest and direct language.

One of my favorite passages comes towards the end of the book where he explains the divergence between geologists and management in their basic understanding of exploration and discovery.

"Subsurface discoveries of oil reserves cannot be projected with certainty. The inputs to the discoveries are always deterministic, but the result is never so, and there is a strong factor of probability. Exploration efforts are to reduce the risk factor and increase the probability of discovery. Methods for direct detection of  hydrocarbons from the surface, are  yet to be evolved. This contrasts with any other industry, where the output is more predictable and proportionate to the input. 

.... As a simplistic approach the management decides the cost of discovery of one tonne of oil by dividing the amount of discovered oil by the expenditure met. It is expected by them that similar expenditure should proportionately result in additional discoveries.

When a sedimentary basin turns old and mature, the addition to the existing stock of subsurface oil becomes increasingly difficult. Non-geoscientists then blame geologists for such uncertainty. Only the high profitability in the oil industry is visible; its probabilistic nature and the risks involved are not so obvious. The stochastic nature of oil discoveries is not appreciated by non-explorationists". 

My one complaint about this book is that there is very little geology in it! Dr. Deshpande describes the geology work he and others undertook in very broad strokes. I felt that a few examples of how specific types of geologic data is useful for petroleum exploration would be illuminating for the non specialist reader. 

Let me give an example. Early in his career he is sent to Osmania University, Hyderabad, to analyze some sedimentary rock samples using Differential Thermal Analysis. He simply mentions that the results were used by ONGC in their exploration efforts, but how so? DTA is a way to understand whether the sedimentary rock was baked during burial to temperatures that are conducive for hydrocarbon formation. 

Another example, and one that involves his specialization, could have been a brief passage describing his work on limestones. What is a carbonate sedimentologist looking for in these rocks? The main reservoir of Mumbai High, India's biggest oilfield, are Miocene age limestone beds which were deposited repeatedly during phases of oscillating sea level. Among other things, exploration geologists want to know how open spaces or porosity in these rocks has evolved over time and whether its occurrence can be predicted throughout the sedimentary section. There was a geological detective story waiting to be told there. 

But these are mere quibbles. Overall, this is a very readable account of the productive and remarkable career of a pioneer exploration geologist of India.  Popular accounts of Indian geology and industry are rare. Recently, Himalaya geology expert Dr. Om N. Bhargava released his memoir, Travails and Ecstasy of a Geologist Addicted to the Himalaya, on his experiences of working in the Himalaya. Indian earth scientists are beginning to share the good work they have done with a more general audience, bringing a much needed familiarity with a lesser appreciated but critical field of study.