Saturday, April 21, 2018

Everest Summit Limestone

Most people I talk to about geology are aware that the Himalaya formed by the buckling and uplift of crust caught up in the India-Asia collision. But, I do see eyebrows raised when I tell them that the summits of some of the highest peaks are made up of marine sedimentary rocks.

The summit of Mount Everest is a fossil bearing limestone of Ordovician age.

What happened to these sediments as they got caught up in Himalayan mountain building? A recent study published in Lithosphere has teased out the deformation and metamorphic history of this limestone.

Polyphase deformation, dynamic metamorphism, and metasomatism of Mount Everest’s summit limestone, east central Himalaya, Nepal/Tibet - Travis L. Corthouts, David R. Lageson, and Colin A. Shaw

The geologists trained Nepalese Sherpa climbers to recover samples from the Everest summit. The location of the samples and the basic geological divisions of the summit is seen in the annotated photograph posted below


 Source: Travis L. Corthouts, David R. Lageson, and Colin A. Shaw 2018

The Everest region is made up of high grade metamorphic rocks of the Greater Himalayan Sequence. These are intruded by leucogranite dikes and sills. Towards the upper levels, the grade of metamorphism decreases gradationally to upper greenschist facies. The contact between the two metamorphic grades is a shear zone termed the Lhotse Shear Zone. The greenschist faces rocks are termed the Everest Series.  On top of the Everest Series is the 'Yellow Band'. This is a coarse grained marble and calc-schist. The summit limestones (Qomolangma Formation) rests on this Yellow Band. The boundary between them is a fault zone known as the Qomolangma detachment. This fault zone is a strand of the South Tibetan Detachment (STD) that puts the Tethyan Sedimentary Sequence (TSS) on top of the Greater Himalaya Sequence throughout the extent of the Himalaya.

A schematic cross section depicting this stratigraphy is shown below.


Source: Travis L. Corthouts, David R. Lageson, and Colin A. Shaw 2018

Researchers used three types of analysis to figure out the geologic history of the limestone.

a) Microfabric analysis of the samples gave the geologists clues to the deformation and stress regime experienced by the summit limestone. The limestones have been converted into a mylonite. This means that increased temperatures and pressures from faulting resulted in a new textural arrangement in which the original calcite grains of the limestone were recrystallized and deformed. New calcite crystals grew flattened and stretched along one direction, resulting in a foliated (layered) streaky appearance to the rock. This texture forms during ductile deformation in a compressive stress regime. Geologists found that near the vicinity of the Qomolangma fault, a set of dilational fractures indicating extensional forces cut across these ductile deformation textures. This indicates that the summit limestone was subjected to tensile forces and normal faulting at a later stage.

b) Titanium content of quartz and biotite from samples close to the South Summit (EV6) indicated the temperature of metamorphism. This is so because the amount of Ti incorporated in to growing crystals of quartz and biotite increases with increase in temperature of crystallization. Results indicated that the limestones at the base of the Qomolangma Formation experienced temperatures as high as 500 deg C. 

c) The age of metamorphism was estimated by dating muscovite crystals using Ar40/Ar39 technique. Muscovite crystals grew in response to the increased temperature and pressure the limestone was subjected to during Himalayan orogeny. Dates show that there were two phases of mineral growth. The first at 28 million years ago, and a younger phase at about 18 million years ago, indicating separate events of movement and heating along the Qomolangma fault zone.

The leucogranite sills and dikes, which intrude the underlying Greater Himalaya Sequence, also merit a mention. They formed by the partial melting of the crust during Himalaya orogeny.  As this magma intruded and solidified inside the Greater Himalaya Sequence, they expelled fluids with volatile elements which permeated into the overlying limestone. This caused metasomatism and crystallization of secondary minerals in the limestone. Boron, potassium, titanium and H2O were introduced into the limestone and were incorporated into minerals like muscovite, biotite and quartz. This activity is dated to about 28 million years ago based on the age of secondary muscovite in the lower parts of the summit limestone.

The sequence of geologic events is summarized in the graphic below:


Source: Travis L. Corthouts, David R. Lageson, and Colin A. Shaw 2018

And an excerpt of the conclusions from the paper-

The different fabrics and metamorphic temperatures observed between the upper and lower parts of the Qomolangma Formation are the result of distinct events that influenced the summit limestone at different times throughout Himalayan orogenesis. Fabrics seen in summit samples are the result of Eohimalayan deformation and low-grade metamorphism associated with initial thrust faulting, folding, and crustal thickening of the Tethyan Sedimentary Sequence in the Eocene. In contrast, the fabrics and elevated temperatures preserved in South Summit samples are the result of events that occurred in the late Oligocene and early Miocene, including metasomatism associated with Neohimalayan metamorphism and normal faulting on the South Tibetan detachment. This means that several significant tectonic events in Himalayan orogenesis are preserved in the Qomolangma Formation, a succession of deformed Ordovician limestone that now comprises the top of Mount Everest.

Open Access.

Wednesday, April 11, 2018

Dating Rock Art

I love it when science is explained with a well thought out and cleanly drawn illustration.

A commentary in Nature News and Views by David G. Pearce and Adelphine Bonneau presents this diagram on dating rock art.


Two recent studies on dating cave paintings from Spain are discussed.

Who were the artists?

The oldest minimum age for the paintings is 66 ka (thousand  years) leading to speculation that they might have been drawn by Neanderthals. The earliest presence of modern humans in Spain is from  40 ka. This would imply independent evolution of symbolic behavior in Neanderthals.  However, the same painting throws up a range of minimum dates from ~ 60 ka to ~ 3 ka making an exclusive link  to Neanderthals problematic.

Friday, April 6, 2018

Crisis In Indian Palaeontology

This is incredibly sad reading.

Two recent articles highlight the utter state of disarray Indian paleontology finds itself in.

Less offical importance, low budgets, low career prestige, no legal protection for fossil sites, no local fossil repositories to store collections and no national museum with an attached well funded research program.

From the article by Sanjay Kumar in Science:

With few legal protections, sites often fall victim to looting and development. And although funds are scarce for all science in India, the plight of paleontology is particularly acute. Little money is available for excavations and for acquiring and curating specimens, and the country lacks a national institution in which its natural heritage can be studied and preserved.

All of this discourages young people from entering the field. Cash-strapped universities are curtailing or axing paleontology courses, says Ashok Sahni, of Panjab University in Chandigarh, a leading figure in Indian paleontology. Sahni, best known for his finds of dinosaur nesting sites in Jabalpur and insects trapped in amber in Vastan, in Gujarat state, says he has watched waves of colleagues retire—with few young talents stepping in to replace them. "There is no critical mass of researchers left," he says. "Indian paleontology is dying."

..and Sreelatha Menon in The Wire writes about the problems in palaeontology, and more broadly, geology education:

“In well known centres of paleontological teaching and research, such as BHU, Lucknow University, Panjab University, Jadavpur University, etc., the number of palaeontologists has gone down drastically and new, prestigious educational institutions like the IISERs are not showing much interest in hiring palaeontologists,” Prasad said (IISERs: Indian Institutes of Science Education and Research). “So the country has very few palaeontologists working on large invertebrate fossils at present.”

Pratul Saraswati, a micropalaeontologist in the department of earth sciences, IIT Bombay, thinks it’s not about people not being interested in palaeontology. “If you ask me to name some micropalaeontologists other than myself, I won’t be able to give you more than five names. If you ask Prof Sahni to name some vertebrate palaeontologists, he won’t be able to name more than three or four.”

“The problem is with geology departments as a whole across the country. Except in the IITs and central universities, just one or two faculties teach all the subjects coming under geology – and that includes palaeontology,” Saraswati said. “There is no faculty for geology across India. So it is not just palaeontology but all the subjects coming under geology that are taking a beating.”

At the IITs, every subject is taught by a specialist – which is good because, according to Saraswati, “It is difficult for a non-specialist to teach palaeontology.” But in the other universities, “One or two teaching all the subjects in geology is fine till graduation. For post-graduation and research,” that will not be enough.

A couple of weeks ago I visited the Dept. of Geology at Sinhagad College of Science in Pune. One of the faculty there is working on the sequence stratigraphy of the Cretaceous deposits of the Cauvery Basin in Tamil Nadu, South India. She mentioned that many of the famous outcrops and fossil sites are being destroyed as farmlands and small villages and towns expand. This story is repeated elsewhere across India.

That really struck me hard. During my undergraduate years I had visited that area on a field trip. I saw and collected ammonoids, echnoids, molluscs, corals and plant fossils in the field and had come back with a finer appreciation of the stratigraphic and sedimentologic context in which fossils are entombed and preserved. In retrospect, we should not have collected so many fossils. But in those days we weren't taught, and neither did we introspect, about ethical issues regarding fossil collection and outcrop integrity.

India's natural history must be given more importance. It will be a real tragedy if these localities are lost to future generations.

Monday, March 19, 2018

How Old Are The Aravalli Mountains Of Rajasthan?

By the age of a mountain range I mean the time since the formation of significant topography. I don't mean the age of rocks making up the mountains. There are plenty of instances where terrains made up of rocks of a particular age have been rejuvenated and uplifted by earth movements later in time. The most spectacular example in India are the Himalaya. The oldest rocks in the Himalaya are dated to about 1.8 billion years. These, along with rocks ranging in age from more than a billion years to about 50 million years, have up thrust up during mountain building that began about 25 million years ago.

There are other less well known examples from India. The Bababudan hills in Karnataka are made up of rocks as old as 3.5 billion years. The topography though is much younger. This area is a southern extension of the Deccan plateau which has been rejuvenated during the Cenozoic. Another example are the massifs of the Nilgiri Hills in the Western Ghats. These massifs reach about 7500 feet ASL. They are made up of charnockite, a high grade metamorphic rock. This terrain was metamorphosed about 2.5 billion years ago and then again around 550 million years ago. It too has been uplifted in the more recent Cenozoic.

So, how old are the Aravalli mountains?

In a recent article in LiveMint on the ecology, geology and archaeological significance of the Aravalli mountains of Rajasthan, Ananda Banerjee writes-

"These ancient rocks are part of the oldest mountain range in the world—the Aravalli range, or “The Ridge”, as it has been more commonly known in Delhi from the days of British rule".

He quotes author Pranay Lal " “It took two billion years (from a point in time between 3.2 billion years to 1.2 billion years ago) of shoving and pushing of tectonic plates and magma outpourings to create these ancient fold mountains

This statement does not really inform us about the sequence of geological events that took place. Did the Aravalli  mountain building really take two billion years? Was it really initiated 3.2 billion years ago?

The Aravalli fold mountains are made up of layers of sediments interlayered with volcanic rocks. The deposition of these volcano-sedimentary successions took place on the sea floor. This entire pile has been subdivided into the Aravalli Super Group and the Delhi Super Group corresponding to two distinct cycles of sedimentation and orogeny.

The satellite image below shows the folded ridges of the Aravalli mountains west of the city of Udaipur


The geologic story begins, as Lal pointed out, about 3.3. to 3.2 billion years ago (1). At this time prolific 'granitic' magmatism was creating new crust. These magmas go under the name TTG, for tonalites, trondhjemites, and granodiorites. Sediments and interlayered basalt volcanic layers were deposited in contemporaneous basins. Geologists think that the tectonic setting for TTG magmatism would have been similar to a convergent plate margin, where one plate subducts or slides underneath another plate. The setting for volcanic-sedimentary deposits may have been a rifted oceanic basin.

The schematic below shows the evolution of cratons during the Archaean. Early continental nuclei were separated by oceanic basins.


Source: P.A. Cawood, C.J. Hawkesworth, and B. Dhuime - The continental record and the generation of continental crust

 These different terrains slowly sutured and welded together to form larger continental fragments. In this process the TTG  and the volcano-sedimentary deposits got metamorphosed and deformed. The result was a complicated terrain with slices of granite gneiss (metamorphosed TTG) interleaved with low to medium grade metamorphic rocks like chlorite and amphibolite schists (metamorphosed  basalt and other volcanic rocks and sediments).

The cross section below summarizes the complicated structure of the granite-greenstone belts of Rajasthan.


D.B. Guha 2008 - Tectonostratigraphy and Crustal Evolution of the Archaean Greenstone-Granulite Belt of Rajasthan

This granite-greenstone terrain (due to the presence of green colored minerals like chlorite and amphiboles) is called the Banded Gneiss Complex. It is made up of two sub terrains named the Sandamata Complex and the Mangalwar Complex. The formation of the Banded Gneiss Complex was completed by about 2.5 billion years ago when profuse granitic magmatism ended. Geologists call this craton stabilization. Initially, this terrain may have had topography. Hill ranges may have stood out in this area about 2.5 billion years ago. However, this was followed by a long period of erosion wherein the terrain was peneplained. Evidence for deep weathering of this terrain comes from paleosols (soils) which mantle parts of the Banded Gneiss Complex.

This was followed by the sagging of the granite-greenstone crust and the formation of new sedimentary basins.

The Banded Gneiss Complex forms the basement on which the Aravalli Supergroup sediments were deposited. These older rocks therefore were the sea floor at that time. No fold mountain ranges existed in this region around 2 billion years ago.

Galena (Lead Sulphide) which occurs in volcanic rocks interlayered in the lower part of the Aravalli Super Group has been dated to about 2 billion years. This is taken as roughly the start of Aravalli sedimentation. This basin lasted for about 200 million years. Granites intruding the Aravalli Supergroup have been dated to about 1. 85 billion years. These are syn-orogenic granites which form when continental fragments collide, and the deeply buried crust partially melts to generate granitic magma. Geologists think that the tectonic event responsible for this was the collision of the Aravalli craton and the Bundelkhand craton. The Aravalli orogeny and fold belt formation is thus about 1. 8 billion years old.

At this time there would have been a fold mountain range made up of crumpled up Aravalli Supergroup rocks. Subsequently, beginning around 1.7 billion years ago, another basin developed in the north and west of the older Aravalli basin. In this basin were deposited sediments and volcanic material that make up the Delhi Supergroup of rocks. Among these rocks are the resistant quartzites that make up the Delhi Ridge. The Delhi basin closed and the rocks folded and  uplifted by about 1 billion years ago. The tectonic event responsible for the Delhi orogeny is thought to be the collision between the Aravalli-Bundelkhand craton and the Marwar craton to the west. The contact between the two is the Western Margin Fault along which the Phulad Ophiolite rocks lay sandwiched. These are remnants of the oceanic crust that existed between the two continental blocks.

The Aravalli fold belt, made up of the Aravalli Supergroup and the Delhi Supergroup formed over an extended time period in two phases, the first one about 1.8 billion and the second about 1 billion years ago.

The map below shows the different geologic terrains of the Rajasthan craton


Source: Joseph Meert et.al. 2010 - Precambrian crustal evolution of Peninsular India: A 3.0 billion year odyssey

Does that mean we can say that the maximum age of the Aravalli mountains is about 1.8 billion years?

This is an intriguing question and it depends on what might seem a rather esoteric question. What is the nature of the contact between the Aravalli Supergroup rocks and the younger Delhi Supergroup rocks? The Aravalli and Delhi rocks have a sheared and faulted contact. This means that the two terrains have been moved along faults from their original positions and juxtaposed against each other. But some work suggests that their original relationship was different. Field relations and inferred contrasting folding histories (2, 3)  implies an angular unconformity between the two. That means that Aravalli Supergroup rocks were folded earlier and then over a time span of 100 million years or so, erosion wore down the Aravalli Supergroup fold mountains to a plain. The crust then sagged, and the Aravalli rocks along with the Banded Gneiss Complex became the basin floor upon which the Delhi Supergroup sediments were deposited.

If this scenario is true, then the Rajasthan fold mountain topography formed during the younger Delhi Supergroup orogeny, that is about 1 billion years ago. The rolling hills and the gentle stream gradients suggest that erosion has been wearing the mountains down and there have not been significant earth movements affecting this part of the crust since.

How does this compare with other ancient mountain ranges. The Barbeton Greenstone Belt, also known as the Makhaonjwa Mountains, on the border of South Africa and Swaziland are thought to be the oldest mountain range in the world. They are made up of 3.5 to 3.2 billion year old rocks. On the web there are top 10/9 lists of the oldest mountain ranges in the world, which include the Hammersley Range in Western Australia (3.4 billion)  and the Waterburg Mountains in South Africa (2.7 billion) (strangely they exclude the Aravallis!) But has the topography existed since the claimed age or has an old peneplain been rejuvenated in more recent times?

That is the billion year(s) question that must be asked when evaluating any "my oldest mountains are older than your oldest mountains" claim.

Saturday, March 17, 2018

Paper: Evaluating The Fossil Record Of Earliest Life

Understanding ancient life: how Martin Brasier changed the way we think about the fossil record - JONATHAN B. ANTCLIFFE, ALEXANDER G. LIU, LATHA R. MENON, DUNCAN MCILROY, NICOLA MCLOUGHLIN and DAVID WACEY

I really enjoyed reading this paper which came out in a special publication issue of the Geological Society, London, in 2017. It is a tribute to the work of Martin Brasier who made significant contributions to our understanding of early life and early animal evolution. In particular, Dr. Brasier argued for a more rigorous approach to analyzing fossils, or claimed fossils, of very early life. He developed detailed criteria for describing and interpreting enigmatic structures as either abiogenic or biogenic, and promoted the use of cutting edge imaging technology to better visualize 'fossil' structures in two and three dimensions.

An excerpt:

Crucial to our understanding of life on Earth is the ability to judge the validity of claims of very ancient fossils. Structures reported from the Apex chert (3.46 Ga) that were interpreted to occur in sedimentary rocks and to be biological in origin (Schopf & Packer 1987; Schopf 1992, 1993) were, for a decade or more, considered compelling candidates for the earliest fossils. Martin Brasier’s most important contribution to this debate was to characterize those structures in great detail and to develop a framework within which claims of the ‘oldest’ or ‘earliest’ life should be couched. In his lectures on this subject, Martin referred to the competitive tendency among palaeontologists working on early life as the MOFAOTYOF principle: My Oldest Fossils Are Older Than Your Oldest Fossils.

In particular, Brasier et al. (2002) made it clear that the burden of proof must fall on those making the claim of ancient life, not those refuting it: Ancient filamentous structures should not be accepted as being of biological origin until all possibilities of their non-biological origin have been exhausted. In particular, it is important to note that complex ‘septate’ carbonaceous structures can result from experimental hydrothermal processes. (Brasier et al. 2002, p. 80) In other words, we should assume that ancient structures resembling fossils, such as those in the Apex chert, are abiological until it can be shown beyond reasonable doubt that they are not, rather than the other way around. Brasier (2015) articulated this concept clearly:

This . . . allows palaeobiologists to set up a hypothesis which will prevail until proved false . . . Any newsworthy, and culturally challenging, interpretation must therefore be tested against a less exciting interpretation. This ‘null hypothesis’ is usually regarded as the ‘most boring explanation’. It is boring precisely because it is thought to have a higher probability of being correct. Brasier (2015, p. 9).

This could be thought of as Brasier’s razor: ‘the most boring answer is probably the correct one’.


This critical approach applies to the problem and controversies surrounding  the fossil record of the earliest animals too. A reassessment led Dr. Brasier  to retract his previous claim about the earliest sponge spicules from the Late Ediacaran ( ~ 560 million to 541 million years ago) age deposits of Mongolia.

Finally, his work on accurately characterizing the scratches, pits, holes, undulations, blobs and globules on and within sedimentary deposits has enormous implications for the search for potential fossils on other planets.

Dr. Martin Brasier died in a car accident in 2014. 

Open Access.