Assam Earthquake, Mars Rover, Shell Coiling

It has been a long break since I shared a reading list. These are some interesting articles I came across in the past couple of months.

1) How a massive earthquake in Eastern India led to the birth of modern seismology. On 12th June 1897, a massive earthquake struck the Shillong Plateau and neighboring Assam causing widespread damage. Media reports were detailed but sensational. But they did push geologists towards developing a more rigorous science of earthquake monitoring and analysis. This is a fine piece of archival research by Utsa Bose, tracing the birth of the modern science of seismology. The role of the nascent Geological Survey of India with its first generation of Indian geologists is especially noteworthy.

2) NASA’s Perseverance Sets Off-World Record for Total Distance Driven. News from NASA informs us that the Mars Perseverance Rover has now traveled further than any vehicle ever has on another world. On October 11, the rover’s odometer clocked 45.22 kilometers surpassing the 45.16 km set by the Mars Opportunity Rover.

Perseverance’s wandering has taken it to the western rim of the Jezero Crater, an area known as “Lac de Charmes”.

Around the rover are some of the oldest rocks known in the solar system. The Jezero Crater was formed by an impact about 3.9 billion years ago and is filled with surficial impact debris, as well as rocks from Mar’s interior that were thrown up during the impact.

Since Mars lacks plate tectonics, rocks from its earliest history are preserved. On Earth, plate tectonics has recycled the initial crust that formed as our planet cooled. The geological trajectory followed by Mars and Earth before Earth developed plate tectonics may have been similar. These very ancient rocks from Mars gives us an analogue for understanding early earth geological processes.

But take a look and admire this info-graphic of the many rovers that have explored Earth, Moon, and Mars and given us so much exciting data to work with in the coming decades.

Source: NASA/JPL-Caltech.

3) Why Do These Fossil Shells Flip Their Spirals Every Few Millennia? Foraminifera are unicellular protists. Many species construct a coiled shell around the soft tissue. Most species may contain both right handed and left handed coiled shells. Scientists noticed that in a few species the direction of coiling is either predominantly right spiral or left spiral. And more interestingly, as they traced the species history through their fossil record, they found that the direction of coiling flipped between right handed and left handed every few thousands of years.

There are quite a few explanation put forth to explain this with changes in temperature of the sea water being a leading answer proposed in the early research of this phenomenon. But was it so? Or are we seeing speciation, the evolution of a new variant or species that then quickly spreads through the world’s oceans and becomes a common fossil shell to be discovered by paleontologists? A fascinating look at the evolution and ecology of foraminifera by Fanni Daniella Szakál.

Inspiration From The Galapagos Islands

From the famous Galapagos Islands comes news about a link between global warming and intensification of El Nino.

 

Imagery- Sentinel 2 Mosaic: Copernicus Browser

El Nino is a pattern of natural climatic variation which occurs when a weakening or reversal of easterly trade winds results in warm Pacific water piling up around the west coast of South America. This phenomenon depends upon subtle interactions between sea surface temperature, ocean currents, and winds, and climate models can’t reproduce these accurately. This means that often different models disagree on how El Nino might be affected by global warming.  Historical records don’t go too far back for us to assess whether there is a clear correlation between global warming and El Nino strength.

Now, a new method uses corals to tease out the temperature of sea water across centuries.

Coral skeletons grow by precipitating the mineral aragonite (CaCO3) in yearly growth bands. Scientists sampled these layers and measured the amount of element strontium and the ratio of heavy oxygen O18 to the lighter O16 in each layer. Aragonite incorporates less strontium and more O16 in warmer water. A sea surface temperature record stretching back a thousand years could be constructed.

The conclusion was that over the past 40 years, temperature swings have become more extreme in El Nino years. This finding from the Galapagos is complimented by another analysis of corals from Kiribati in the Central Pacific. Scientist have a taken a step towards a firmer understanding of the variability of this phenomenon which causes extreme weather events around the globe.

Which other important scientific studies have come out of the Galapagos?

Ahem… there is off course the visit in the year 1835 by a certain Mr. Charles Darwin. His observations there made an important but not an outsized contribution towards his theories of evolution. Darwin was just as comfortable driving home his point using the example of the domesticated pigeon as he was describing the finches and mockingbirds of the Pacific Islands.

Jeff Wallace of the University of Glamorgan in his brilliant introduction to the Wordsworth Classics of World Literature edition of The Origin of Species makes this observation: 

"Readers thus soon find themselves continually shuttled to and fro across the distance between the ‘familiar’ and the ‘other’, the domestic and the exotic, in a text which seems to disdain the very boundary lines between such distinctions. With bewildering and unpredictable variation, we encounter the women-eating barbarians of Terra del Fuego; the small Asiatic cockroach; the thickened stems of the common  and Swedish turnip; the climbing hooks  used by the trailing bamboo of the Malay Archipelago; the egg carrying folds of skin of pendunuculated centipedes; the northern and southern downs of the Weald; the sedimented beds of the mouth of the Mississippi during the glacial period. Darwin’s task at once scientifically central to his work and yet only to be achieved through a vivid and creative use of language, is to make strange or 'astonishing’ the detail of the close and familiar world, and to bring the most extraordinary facets of the alien and unknown within the realms of common understanding. His success, in a text whose fascination is undiminished if not growing as we enter the peculiar concerns of the millennium, lies ultimately in indicating that these contrasting worlds are in fact one world".  

The variation in the beak of the Galapagos ground finches and mockingbirds did inspire Darwin to think about common descent and natural selection, but the legendary finches don’t even feature in his book. The reason is he failed to label his collection systematically! On his return to England he did turn over his collection of wrens, blackbirds, and finches to the ornithologist John Gould.

To his surprise, Gould informed him that despite the great differences in their beak shape and size, the ‘wrens’ and ‘blackbirds’ were in fact all ground finches. His grab bag collection was actually a closely related group of 13 species of finches. Darwin did label his collection of four mockingbirds by island, and they too, as Gould found out, were distinct species. Gould added one more crucial piece of information. All these birds had close but not identical relatives on the American mainland.

Darwin eventually saw the significance of this observation and it became an important piece of evidence for his theory of common descent. He hypothesized that one founder population of these birds must have reached the Galapagos from the South American mainland and then diverged into distinct species on different islands. The standard depiction of an evolutionary tree with branches splitting away from a common node owes its origins to the rather disorganized collection of a wide thinking naturalist and the keen observation of an ornithologist.

Many decades later two scientists did label those finches correctly. And studied them in excruciating detail.

 I am referring to Rosemary and Peter Grant’s epic 40 year study of the morphological evolution of the ground finches of the Galapagos. These two scientists painstakingly measured the variation in beak size and shape over many generations. This they complimented by comparing their genetic differences, and using all this data pieced together a tale of their evolution through changing climate patterns and food availability. Their study has provided a powerful validation of Darwin’s idea that natural selection can shape morphologic change and specialization in populations.

Jonathan Weiner’s Pulitzer Prize winning book, The Beak of the Finch, captures this story vividly. A more updated description of this research has been provided by the two scientists themselves through their publication - 40 Years Of Evolution: Darwin’s Finches On Daphne Major Island.

The Galapagos Islands began forming about 4 million years ago. The Nazca tectonic plate is moving eastwards in the Pacific over a heat source situated deep in the earth’s mantle. The eastern edge of the Nazca Plate is sinking beneath the South American tectonic plate. Sporadic volcanism over the mantle hot spot has created a chain of volcanic islands which become younger towards the west. Volcanoes on the western islands are still active.Today, the islands and the surrounding waters are a marine reserve, offering some protection to its unique biodiversity.

Although geologically young, the specific geologic and bio-geographic circumstance of these islands have proven to be a fertile ground for science. From climate change, to evolution, to conservation biology, scientists working there continue to probe and tease out insights that are proving crucial in navigating the currents of a rapidly changing world.

Geology Word: Ooid

Ooids are sedimentary grains that form by precipitation of calcium carbonate around a nucleus. The nucleus may be an organic particle, or a mineral grain, or a shell fragment.

Repeated precipitation results in a concentric coating around the nucleus. Depending upon the saturation state of the sea and other variables like temperature, ooids may be made up of either calcite or aragonite. Both have the same chemical formula, CaCO3, but they differ in their atomic structure.

Ooids form in shallow seas where wave energy is vigorous. The turbulence results in the particles getting suspended and redeposited on the sea floor, resulting in an even coating around the nucleus. Beaches, tidal channels and regions near reefs are areas where ooids are accumulating today in tropical seas around the Bahamas, Caribbean, the Persian Gulf, and the Red Sea.

The picture shows an ancient ooid. This particle is around 350 million years old from the Mississippian Period, deposited in a warm shallow sea which occupied the State of Alabama in North America. I collected the rock sample on a field trip during my PhD days.

Interestingly, the nucleus is a foraminifera, a unicellular protist! Observe the coiled nature of the shell with its internal chambers visible. The shell of the foraminifera is also calcium carbonate and its precipitation is mediated by the organism. On the other hand, ooids results from an inorganic precipitation process, although there is evidence of bacterial involvement in some examples.

In the Precambrian (older than 539 million years ago), before animals evolved and began constructing shells, limestones were mostly made up of fine calcium carbonate mud. Precambrian calcium carbonate sand ( any sedimentary particle, irrespective of composition, between .063 mm and 2 mm is defined as sand) is rare, composed almost exclusively of ooids and a category of grains known as intraclasts (broken fragments of hardened sea floor).

The distribution of ooids and shell types through geologic history is a pointed reminder of how life has impacted the texture and composition of sedimentary rocks.

Deposited in high energy shoals, ooids help geologists reconstruct ancient environments and the paleogeography of continents which were flooded in the deep past.

Ooids are of interest to petroleum geologists too. The spherical grains result in a packing arrangement with high porosity (open spaces between grains). The localization of ooid layers in ancient sedimentary basins is keenly explored for their petroleum reservoir potential.

That was fun to write. I may turn this into a Geology Word series.

Across The Eparchean Unconformity

An unconformity is a gap in the recording of earth's history, similar to missing pages in a book. These breaks are more common than is realized. Stratigraphers, who organize geologic history, estimate that the time spans of non deposition exceed that of episodes of deposition. These gaps could be fleeting, as in a river meandering away and then reoccupying the old channel, or they could indicate conditions of non deposition and erosion lasting tens to even hundreds of millions of years.

Geologists recognize a hierarchy with the longer lasting breaks often pointing to major changes such as a mountain building phase accompanied by a prolonged sea level fall. As sediment deposition stops, erosion will often sculpt the top of the rock formation into an uneven surface. Eventually sediment deposition will resume. Geologists term such a break between the two phases of rock formation as an erosional unconformity.

Recently, the Geological Survey of India added the Eparchean Unconformity to its growing list of Geo-Heritage sites of national importance. This is one of those major long lasting erosional breaks seen in the Precambrian terrains of India. The chosen site is near Kalinjar Fort, in Banda District of Uttar Pradesh. Image Source: Geological Survey of India.

At this location, 1.2 billion year old sandstone of the Vindhyan Basin overlie 2.5 billion year old Bundelkhand granites marking more than a billion years of non deposition and erosion. While the amount of unrecorded time is significant, the real importance of this site is in the very different earth conditions represented by the older Bundelkhand granite as compared to the much younger Vindhyan sandstone. This unconformity marks the transition between the older Archean Eon and the younger Proterozoic Eon.

The Archean was a much hotter world. Melting of the earth’s mantle was producing large batches of silica rich magma which solidified to form buoyant continental crust, small rafts at first, growing into larger blocks as time went by. Vertical crustal movements created narrow depressions which got filled with lava and sediment eroded from nearby granitic highlands. These volcano-sedimentary successions were deformed and metamorphosed, and were preserved as enclaves within the granitic terrains.

By around 2.5 billion years ago, magmatic growth of continents petered out. Geologists estimate that around 70% of the present volume of continental crust was generated between 4 billion to 2.5 billion years ago.

As the mantle cooled, the more stable continental crust became the floor for a younger generation of sedimentary basins. The processes of chemical weathering and sediment transport became more prolonged on this wide gently subsiding continental terrain. As a result, waves and currents had more time to sort sediment by size, shape, and density before it was buried.

Thick deposits sorted by size into gravel, sand, and mud are the typical features of these younger basins. At places, ocean water saturated with calcium carbonate precipitated layers of calcite and aragonite sediment, preserved today as thick limestone. The biosphere was dominated by bacteria and unicellular eukaryotes, their morphology often imprinted on the rocks as wavy layers or as small mound shaped objects.

Magmatism and continental crustal growth continued less frequently in pulses coinciding with regions of plate convergence and orogeny. This phase, before large animal life evolved, lasting between 2.5 billion years and 539 million years ago is known as the Proterozoic Eon.

The newly listed National Geological Monument at Kalinjar Fort showcases these changing earth conditions.

For many decades though, exactly where to place the Eparchean Unconformity was something of a problem in Indian field geology. There was only sparse information on the absolute age of rocks and no consensus on the time span the Archean represented. Field mapping through Peninsular India indicated the presence of a distinct erosional break between deformed and metamorphosed rocks (crystalline basement) and younger flat lying or less deformed sediments. This appeared to be the natural horizon between the Archean and the Proterozoic.

But flat lying sedimentary sequences in different Indian basins, often referred to as Purana Basins, had been deposited at different times on older deformed rocks of varying ages. In some cases the foundation of the sub-horizontal sedimentary basins was indeed Archean. In other locations, the highly metamorphosed and deformed older rocks were actually Proterozoic. That meant that the Eparchean Unconformity, as earlier conceived, was not a useful marker of synchronous changes in earth processes. Many of these doubts are summarized early in this 1968 perspective on the Eparchean Unconformity by Dr. T V V G R K. Murthy, faculty at University of Sagar.

The dates for the Archean-Proterozoic transition have now been fixed at 2500 million years by the International Subcommisson of Precambrian Stratigraphy, without any condition regarding the rock type above and below the unconformity surface. As a result, the older criteria for identifying a particular erosional break as the Eparchean interval based on only rock types and extent of deformation was no longer tenable.

We finally have an answer for the question posed by Dr. Murthy so long ago:

“Does the eparchaean unconformity become evident by recognizing Archaean and Purana formations or are the Archaean and Puranas recognized by identifying the eparchaean unconformity”.

Today, better absolute dating of rocks makes differentiating Archean from Proterozoic rocks easier. The Eparchean Unconformity though is still an informal but useful term in Indian field geology signifying the cooling trajectory of the earth’s interior, with a hotter Archean transitioning to a cooler Proterozoic.

One of my earliest encounters with the Eparchean Unconformity occurred during my graduate studies. I was assigned to map an area of the Cuddapah Basin near Gani village in Andhra Pradesh, South India. I first traveled by the Dadar -Madras (now Chennai) Express to Guntakal. There, I transferred to a medium gauge train passing through small market towns connecting a vast hinterland an urban kid like me was unfamiliar with. The train chugged along eastwards towards Nandyal. For a couple of hours from Guntakal, we were firmly on Archean terrain, an agricultural landscape interrupted by small hillocks of granitic rocks. In a distance through the late morning haze we could see the low Cuddapah hills with their distinct tilted strata.   

The train crossed the Eparchean Unconformity and entered the Proterozoic through a breach in the Cuddapah hills at Dhone Junction. Ahead was the Cuddapah Basin stretching to the horizon.

We moved through quartzites, limestone, and shale, the three predominant sedimentary rock types of the shallow marine Proterozoic continental shelf. At Nandyal I had to hop onto a bus going to Kurnool. Gani village was midway between these two places. I remember arriving at Gani late evening to a warm welcome by my host Mr. Sivanand Rao. Next morning, with maps and aerial photos, hammer, and a geologic compass I started out. A half hour walk south of Gani were outcrops of the sedimentary rocks I had to map. It was a challenging and immensely satisfying introduction to field geology.

Picture shows a very youthful me and my field guide Yanganna standing in front of a quartzite hill. Captured by my accompanying batch mate Nitin Rane. June 1987.

Someday soon I hope to retrace my steps to little Gani village. The medium gauge tracks from Guntakal to Nandyal have long been replaced by standard gauge. Express trains pass through Nandyal, eventually swerving north towards Vishakapatam, or continuing south east towards the Coromandel Coast.

From Nandyal I may hire a car to Gani. I hope to meet Mr Sivanand Rao who so graciously hosted me over two field trips. I want to walk again towards the hills made of Paniam Quartzite, a sand deposit sorted and washed clean by powerful Proterozoic tides. Today, the Kurnool-Gani solar park stands near Gani, its tilted solar panels made up of ultra pure silica wafers making up an Anthropocene analogue of the more ancient quartz rich deposits.

Photomicrograph shows the Proterozoic Paniam Quartzite classified as a “super mature quartz arenite”. Notice the well rounded quartz grains outlined by iron oxide. Location - Gani anticline.

Beyond the quartzite hills are the Vempalle Formation dolomite, a vestige of the Proterozoic ocean I first waded in to begin my life long association with carbonate sedimentology. It will be a home coming of sorts for me.

Photomicrograph shows microbial layers in the Vempalle dolomite. Dolomite crystals of different size and shape have replaced the original organic mats. Location- Gani anticline.

I have one more incentive to relive this journey. The filter coffee at Guntakal railway station canteen. It is the very best.

Deccan Volcanism And Mass Extinction

What role did Deccan Volcanism play in the end Cretaceous mass extinction? 

There is widespread agreement that an asteroid struck what is now the Yucatan Peninsula region of Mexico 66.05 million years ago. The resulting environmental catastrophe precipitated an abrupt mass extinction, wiping out 70% of all species. In the rock record, a thin clay layer preserves evidence of the extraterrestrial origin of this impact event. It is referred to as the K-Pg boundary layer, K being Cretaceous and Pg referring to the Paleogene. The biota below the boundary layer is very different from fossil assemblages above it.

The earth at that time was also experiencing a major episode of basalt volcanism. This Large Igneous Province (LIP) is known as the Deccan Volcanic Province or the Deccan Traps. Volcanism occurred both before and after the impact event. Geologists have been divided on whether most the lava erupted after the asteroid hit or before it with different implications for the role of volcanism in the mass extinction. There was also an uncertain and incomplete assessment of the volumes of lava involved.

A new paper by Vivek S. Kale and coworkers titled “Spatio-temporal volume recalibration shows Deccan volcanism caused Terminal Cretaceous Mass Extinction” leaves no room for doubt about which side of the divide this group stands. The researchers examine over 80 lava sections across the Deccan Volcanic Province and recalculate the lava volume, assigning packages of lava a time bracket based of absolute dating, magnetic signatures, and fossils. They find that around 70% of the Deccan lava erupted in a time span of 300,000 years before the mass extinction.

Their results are summarized in this infographic. Source: Vivek S. Kale and coworkers, GSA Bulletin, November 2025.

The volume estimates of previous workers and their study is shown to the left. Also presented are marine paleo-environmental indicators represented by a paleo-temperature curve and carbon isotope curves spanning the mass extinction.

A short explanation of these curves will be useful.

For estimating temperature, geologists use the ratio of Oxygen 18/Oxygen 16 preserved in the shell (CaCO3) of planktonic organisms like foraminifera or calcareous nanoplankton. During warm phases, vigorous evaporation results in more of O18 escaping into the atmosphere, enriching the ocean slightly in the lighter O16. Calcium carbonate shell growth incorporate O18 and O16 without preference, maintaining the same O18/O16 ratio of sea water .

Warmer seas will result in shells having a lower O18/O16 ratio, than shells that grow in cooler water. The temperature is estimated from these measured oxygen isotope ratios.

The increase in sea surface temperature observed here, known as the Late Masstrichtian Warming Event (LMWE), has been linked to increased CO2 emissions during Deccan Volcanism.

Carbon isotope trends through time are presented by measuring the C13/C12 ratio of the target sample. ‘Bulk’ carbon means any carbon from hard parts as against carbon from organic material. It might mean just ground up limestone made up of a mixture of shells and lime mud. Jurassic onward, the proliferation of planktonic foraminifera has allowed geologists to select shells of just one or two species for their measurements. This makes it easier to screen for post depositional alteration which could reset the original oceanic C13/C12 ratio due to reaction with fluids of a different composition. It is more difficult to recognize these effects in a mixed sample, especially one with fine calcium carbonate mud.

Variation in C13/C12 ratio through time is generally regarded as an indicator of primary productivity. Photosynthesizing organisms have a strong preference for the lighter C12 to build organic tissue. The C13/C12 ratio of organic matter is strongly depleted compared to the sea water C13/C12 ratio. Similar to oxygen intake, growing shells preserve the C13/C12 ratio of the ocean. During environmental crises, reduced primary productivity enriches the ocean in the lighter isotope, also depressing the C13/C12 ratio of shells growing under these conditions.

The modest (relative to earlier Phanerozoic mass extinctions) carbon isotope excursion coincident with the K-Pg boundary suggests a decrease in primary productivity due to the extinction of some photosynthesizing groups. In contrast, the amplitude range of the isotope excursions seen during the Late Maastrichtian Warming Event can also be explained by a non-biological driver.

Carbon dioxide of volcanic origin is isotopically lighter than sea water. Pulses of volcanism may result in a large enough pool of lighter C12 dissolving in the ocean and shifting sea water C13/C12 ratio to the depleted values measured in Late Cretaceous shells.

Kale and coworkers argue that increased volume of lava eruption and emissions of CO2 and other gases in the Late Maastrichtian caused global warming and destabilized the biosphere. Ecosystems were already tottering when the asteroid delivered the knockout punch.

Just how stressed was the ecosystem? The most visible indicator of environmental changes impacting an ancient biosphere is the fossil record of that time period. Their paper however does not present any analysis of the Late Maastrichtian biota.

The several studies (Ref. 1, 2, 3) where from Kale and coworkers source the paleo-temperature and isotope record all unambiguously conclude that the LMWE had a limited impact on marine life. Species diversity did not change much. Fossil community structure remained stable, or where it did change, it reflected migration and range shifts. An example of this is L. Woelders and coworkers study of a Late Cretaceous section from the South Atlantic where they find fluctuating abundances of benthic foraminifera and dinoflagellates tracking climate and sea level cycles.

That is not to say that organisms were unaffected by this warming event. Other studies across the marine realm have documented biotic stress. Vincente Gilabert and coworkers find fragmentation of shells, abundance of low oxygen tolerant taxa, and dwarfing of some species, in their study of planktonic foraminifera of the Caravaca section in Spain. Similarly, Gerta Keller and coworkers study on planktonic foraminifera also reveals species dwarfism and abundance of high stress opportunistic species in the late Maastrichtian of the Indian Ocean and South Atlantic.

Nicholas Thibault and Dorothee Husson in a survey of the Late Cretaceous ocean point out that nanoplankton species numbers dropped during the LMWE, but diversity rebounded and increased with no significant extinction in the last 140,000 years of the Cretaceous. Michael Henehan and coworkers infer an increase in ocean acidity coinciding with the LTME based on reduced preservation of calcium carbonate shells on the sea floor. Shell preservation and sea water carbonate saturation return to pre event values in the last 200,000 years before the K-Pg impact event.

Do these observations on plankton species signal some sort of a beginning of the end of ecosystems across the entire marine biosphere? Since Deccan Volcanism spanned several hundred thousand years, could there have been a mass extinction if there had been no asteroid hit? It is conceivable that the prolonged volcanism could have resulted in a ratcheting of environment stress, eventually crossing tolerance thresholds.

This proposition has been hard to test because the asteroid crashed the Cretaceous party denying us an extended view of a volcanism only unfolding of history.

Still, we do have a good 300,000 year record before the impact of sedimentary deposition that coincided with the most voluminous phase of Deccan volcanism. Finer scale resolution of marine sections in the past few years allow us to examine global biotic trends from this time span.

As mentioned above, the impact of ocean warming seems to be quite limited to changes in shell size, temporary species abundance shifts, and adjustments by migration to favorable locales. Importantly, there is no signal of elevated extinction in diverse groups such as planktonic foraminifera, radiolarians, nanoplankton, ammonoids, bivalves, and gastropods in the final few hundred thousand years of the Cretaceous, when volcanism was at its peak (Ref 1,2,3,4).

A closer look at the environmental proxy data is revealing. After a warming phase, the paleo-temperature curve shows a 150 thousand year cooling trend in the terminal Maastrichtian until the K-Pg boundary. Mirroring this cooling trend is a distinct shift towards heavier carbon isotope values. This may be due to increasing primary productivity signaling a general amelioration of ocean conditions.

What could cause such a cooling despite ongoing voluminous volcanism and CO2 emissions? The answer may be enhanced weathering of all that fresh basalt which kept sequestering carbon dioxide during silicate weathering reactions.

There are two lines of observation that support increased weathering during end Cretaceous times.

The first observation is the recovery of calcium carbonate saturation level of the ocean. Weathering releases divalent cations like calcium and magnesium in to the ocean resulting in an excess positive charge (alkalinity) in sea water. The charge imbalance promotes dissociation of the poorly soluble CO2 gas (represented in equations as carbonic acid -H2CO3) and the formation of more stable negatively charged bicarbonate (HCO3) or carbonate (CO3) anions, raising carbonate saturation state. A decrease in dissolved gas leads to a draw down of atmospheric CO2 in to the ocean by air-sea gas exchange. Alkalinity can also be described as the buffering capacity of the ocean against excessive CO2 buildup.

Today, rapid atmospheric CO2 increase is a major concern for ecosystem and societal health. Research and small scale demonstrations to enhance ocean alkalinity are gaining importance as we try to replicate on human time scales what nature does over much longer time spans.

The other independent support for enhanced weathering comes from another isotope system, the osmium 187/osmium188 ratio. Continental crust is enriched in Rhenium187 which decays to Osmium187. Rocks like basalt derived directly from partial melting of the mantle have much lower Rhenium187 and consequently Osmium187. There is a progressive decrease in the Osmium187/Osmium188 ratio in Late Maastrichtian ocean sediments. This is consistent with weathering and increased supply of non-radiogenic Os188 from a fresh basalt source. Some of the decline can also be explained by a reduced supply of Os187 as large areas of radiogenic continental crust were blanketed by lava.

Late Cretaceous marine ecosystem health can also be assessed by looking at the difference in the C13/C12 ratio between surface dwelling planktonic and bottom dwelling benthic foraminifera shells. This isotope gradient is maintained by the biological pump, a transfer of organic carbon and nutrients from the sea surface to depths. Organic carbon is enriched in the lighter isotope compared to planktonic shells (as explained earlier). This organic matter sinks to the ocean depths and oxidizes, enriching the Dissolved Organic Carbon (DIC) pool in C12. Shells of benthic foraminifera (bottom dwelling) have a lower C13/12 ratio than planktonic calcifiers.

A noticeable C13/C12 gradient is present (Ref. 1, 2) in the latest Cretaceous ocean. It converges immediately above the K-Pg boundary, indicating a disruption of the marine carbon cycle only after the asteroid impact.

Environmental perturbations need not precipitate a terminal decline of ecosystems. My reading of the fossil record and the different proxies is that the effects of the LMWE were transient. Key biogeochemical cycles remained intact through the Late Maastrichtian implying that volcanism did not greatly diminish pelagic ecosystem function. The end Cretaceous marine biosphere shows recovery and resilience and not signs of a ‘critically damaged’ system.

David E. Fastovsky and Antoine Bercovici perceptively ask in their review of the terrestrial record of the K-Pg extinction- “how does one weaken an ecosystem in terms of its ability to withstand catastrophic, that is to say, very short-term events? In other words, this is not a ‘straw that broke the camel’s back’ situation, where preceding events play a big role in the eventual failure. Since large swaths were simply obliterated in the asteroid’s wake, and more distant regions experienced acute shock, it wouldn’t have mattered if ecosystems were weakened or hale and hearty.

I expressed a similar sentiment earlier in the post. To attribute a causal role to Deccan volcanism, there have to be signs that significant global extinction events occurred prior to the asteroid strike. The data just doesn’t point in that direction.

Earlier in earth history, the end Permian (251.9 million years ago) and the end Triassic (201.4 million years ago) mass extinctions have been more convincingly linked to massive environmental damage triggered by sustained igneous activity. The impact on the environment due to Deccan volcanism appears muted in comparison.

One reason could be that during the Permian and Triassic igneous episodes, magma intruded through thick sedimentary basins filled with limestone, organic rich shale, and sulfur bearing evaporites. Interestingly, the mass extinction coincided with the emplacement of thick sills, tabular bodies of magma injected parallel to the strata. As a result, a surge of volatiles emanating from baking sediment amplified volcanic emissions, overwhelming the earth’s natural buffering capacity. Both these mass extinctions are estimated to have unraveled in just tens of thousands of years.

In central Peninsular India, Deccan volcanism intersected older sedimentary basins only at the northern and southern fringes. Slow cooling shallow subsurface sills facilitating effective heat transfer to surrounding crust are absent. Instead, eruptions were fed by magma ascending rapidly along thin conduits (dikes) aligned in swarms. More relevant is that only sterile granitic rocks underlie the regions where the most voluminous eruptions took place.

Volatile bearing sedimentary rocks did play a role in the end Cretaceous mass extinction. But by a quirk of fate, it was the asteroid that found them. The Yucatan region of Mexico where the asteroid hit is underlain by hydrocarbon rich sediments, and sulfur rich salt. The impact released soot and sulfate aerosols in the atmosphere resulting in global cooling and reducing photosynthesis for weeks to months. Eventually, the dust settled upon a world utterly transformed. It was the beginning of a new world, one where flowering plants, song birds, and mammals diversified and flourished. From time to time, chance events have opened up entirely new avenues for evolution to explore.

It is pertinent to stress again the paramount importance of rates of processes in the context of our developing climate crises. Although in absolute terms humans may not emit more than past large volcanic events like the Cretaceous Deccan Volcanism or the Siberian Volcanism of late Permian times, we are emitting CO2 at a rate many times faster than these natural events. Carbon dioxide is building up in the atmosphere too quickly for weathering and ocean ecosystems to offset over the next few hundred years.

Lastly, Jurassic onward, the proliferation and evolution of plankton shell building organisms such as foraminifera and coccolithophores have played an important role in regulating ocean chemistry and as a buffer against atmosphere CO2 changes. This occurs by various processes. For one, the heavy shells act as ballast, exporting attached organic matter to depths before their degradation consumes oxygen in the uppermost oceanic layer. Much of this carbon gets buried, becoming a long term carbon sink.

The other important function is sustaining the ocean alkalinity balance. Precipitation of calcite and aragonite (CaCO3) removes positive charge from the ocean, in the form of divalent Ca cations, reducing alkalinity (see previous section on weathering). This alkalinity is recovered when calcareous skeletons, falling through the water column like an ocean snow, start dissolving when they sink below a threshold depth, releasing divalent Ca ions. This process known as carbonate compensation maintains the ocean’s capacity to absorb atmospheric CO2.

The cooling trend observed in the final 150,000 years of the Cretaceous suggests that the rates of emissions during Deccan volcanism likely remained relatively low. Silicate weathering on land and enhanced ocean alkalinity coupled to weathering runoff and carbonate compensation would have substantially offset Deccan CO2 emissions, moderating their environmental impact.

In the pre-Jurassic ocean, most carbonate skeleton producing organisms lived in the water column or on the sea bed of shallow continental shelves where the sea water was saturated with calcium carbonate. Skeletons did not dissolve in this environment and the biological contribution to ocean alkalinity was limited. This changed when the more geographically widespread plankton evolved biomineralization and their skeletons began settling down to depths where the ocean water is undersaturated with calcium carbonate. The calcareous plankton ecosystem has made post Jurassic oceans more resistant to external shocks, albeit over times scales of thousands of years.

By now readers may be wanting to ask- What about the dinosaurs? Did they persist right until the asteroid hit, or did they go extinct earlier? The avian branch of dinosaurs did survive. The following section is about the non-avian groups and the terrestrial fossil record.

There would have been an extirpation of local populations of dinosaurs and other fauna and flora living in areas directly affected by Deccan Volcanism. But what about the global record?

Terrestrial environments such as lakes and rivers which can preserve dinosaur fossils are patchily distributed. Frequent erosion also removes sediment, making the reconstruction of organic diversity and evolutionary trends much more difficult than for biota living in deep sea environments.

Despite these limitations, we do have some sedimentary sections which inform us about dinosaur life and the terrestrial biosphere of the Late Maastrichtian.

In their work on peat deposits from the Western Interior, U.S., Lauren O’Conner and coworkers estimate end Cretaceous paleo-temperatures based on configuration of preserved organic compounds. Besides the LMWE, they identify shorter warming and cooling phases in the final 100,000 years before the K-Pg boundary, likely driven by Deccan volcanism. However, the impact on the biota seems limited, with no decline in angiosperm diversity until the mass extinction.

Andrew J Flynn and coworkers study in New Mexico on the Naashoibito Member, a dinosaur rich sedimentary deposit, shows that dinosaurs were diverse and formed regionally distinct assemblages during the final few hundred thousand years before the asteroid strike.

The Hell’s Creek Formation spread over the States of North Dakota and Montana also offers insights in to the terrestrial ecosystems (Ref. 1, 2, 3) . Pollen and leaf assemblages attest to a healthy end Cretaceous flora. Both show a substantial and abrupt extinction at the K-Pg boundary. Vertebrate diversity also shows no decline throughout the Late Maastrichtian.

Mammals appear to the exception here. Gregory P Wilson and coworkers detailed survey of the Hell’s Creek Formation reveals two sequential changes in mammalian fauna 650,000 years and 200,000 years before the K-Pg boundary. Apart from changes in mammal assemblages, there is a decline in the abundance of metatherians. The authors link this decline to regional environmental changes, but don’t rule out the effect of Deccan volcanism.

Dinosaur fossils at the Hell Creek section have been found to within a few tens of cm of the K-Pg boundary, representing the last few hundred to a few thousand years of the Cretaceous. Their presence in numbers comparable to lower levels of the sedimentary section negates arguments that dinosaurs were either declining or had gone extinct well before the K-Pg impact event.

The icing on the cake is some recent work by Robert DePalma and coworkers on a debris layer in the Hell’s Creek section. These chaotic beds made up of boulders and fossils was deposited by a tsunami triggered by the asteroid hit. Among other organic remains, the researchers have found a fossil hind limb of an ornithiscian dinosaur. Their work was presented at the EGU 22 General Assembly 2022 in Vienna, Austria. The peer reviewed paper is yet to be published and some scientists have expressed skepticism and called for a more detailed examination of the fossils. But this initial announcement, along with other documented dinosaur fossil finds from North America, does strongly suggest that dinosaurs were well and alive until the very day the world changed 66.05 million years ago.

Geology Books

 These come highly recommended from the experts I follow- 

All released within the past year. 

The Whispers of Rocks - Anjana Khatwa: Blending indigenous stories and modern understanding of our world. 

Strata: Stories From Deep Time- Laura Poppick: Review by Edward Valauskas. 

A Little History of the Earth- Jamie Woodward- Kirkus Review.

I hope to read them soon. Happy reading :)

How Old Is Himalaya Topography?

 Is this true?

The answer is no. To be fair, aside from the click bait, the article itself does not make any such claim. It covers a new study on some igneous rocks from Arunachal Pradesh that formed during an earlier pre-Himalaya stage of the India Asia plate convergence. 

By mid late Cretaceous times, between 100 to 66 million years ago, the dense ocean lithosphere that was the front edge of the Indian plate was sinking underneath the Ladakh terrain, a splinter of Asian continental plate disconnected from the mainland. The subduction of the Indian plate triggered extensive melting in the deep subsurface, building over time a magmatic arc. The schematic below shows the plate tectonic scenario. 

Source: Sankar Chatterjee and coworkers: Gondwana Research.

This arc has been well studied in the Kohistan and Ladakh areas. The new work found that a body of granitic rock known as the Lohit pluton is also an eastern extension of the Kohistan Ladakh arc complex. 

Eventually, the oceanic crust of the Indian plate was consumed and the Indian continental crust collided with the Ladakh terrain, which by this time had sutured with the Asian mainland. Himalayan mountain building begins from this point on, roughly sometime after 50 million years ago. 

How do geologists know when topography began to form in the Himalayan region? Various dating methods tell us when a particular rock or mineral crystallized or cooled below a particular temperature. But how do we date the formation of topography? Sedimentary geology, my field of specialization, has played a big role in giving us insights into the tectonic and topographic evolution of the Himalaya. I'll summarize how the story of the formation of the different Himalaya ranges came to be written by these geologists. 

Due to inherited geologic history and subsequent conditions during continental collision, the entire region between Ladakh and the Himalaya front can be subdivided into six geologic terrains running along the length of the mountain arc. These are, from north to south,  the Gangdese magmatic arc, the Indus-Tsangpo suture zone, the Tethys Himalaya, the Greater Himalaya, the Lesser Himalaya, and the sub Himalaya (Siwalik). Each is made up of distinctive rock associations. 

The earliest topography began to from in the north, at the zone of contact between the two continental plates. Over time, there was a step wise progression of southwards topography formation. As each of the geologic terrains rose up, newly developed stream networks started eroding the rocks and delivered distinctive sediment mixtures to two types of sedimentary basins which had developed adjacent to the mountains. The Indus and Bengal basins at the two extremities of the Himalaya received sediments from the Indus and the Yarlung-Tsangpo/Brahmaputra. A second type of basin, known as a foreland basin formed to the south of the orogen. This moat like depression which runs parallel to the Himalaya, and is fed by streams running transverse to the ranges, also became an archive of material removed from the mountains. 

By carefully identifying the sedimentary grain types deposited in these basins and how their proportions change through the oldest to the youngest layers, geologists have been able to piece together the evolution of new topography and geologic provenance through time. 

I am presenting this reconstruction through a series of time slices which show topography formation in each of the geologic terrains and the resulting stream networks transporting the derived sediment from source to sink. The representation only shows the central foreland basin from Himachal Pradesh to Nepal. I have not shown the sedimentary history of the western and eastern basins. The trends at the Himalaya extremities are similar with some difference due to variations in the dominant geology of the contributing catchments. 

I have also not covered a short phase of basin formation in the Indus Tsangpo suture zone between 30 to 20 million years ago. This basin received sediments from both the Gangdese Arc and the Indian plate and was then uplifted to form the Indus Group ranges which include the famous Kailash mountain. For more details of this episode of Himalaya mountain building do refer to my post - Is Mount Kailash the Oldest Mountain in the Himalaya?

In the diagrams, the legend "foreland sandstone diagnostic grains" refers to the arrival of a suite of distinct grain types in the foreland basin. This signals the uplift and erosion of new rock types in the growing mountain range. Earlier formed ranges will in most cases continue to contribute sand, but it is the first appearance of a new grain type in successive strata that is the indicator of tectonic and topographic changes. 

The inspiration for these diagrams is from P.G. DeCelles and coworkers paper on the timing of the India Asia collision, published in the May 2014 issue of Tectonics.

A) Initial Continental Collision- 58-54 million years ago: What was the timing of the collision?

As long as Indian oceanic lithosphere was subducting under Asia, a deep trench and forearc basin separated the two continents. Sediments eroded from Asia were trapped in these depressions and could not travel on to the Indian continent. This situation changed on continental impact. By then the intervening basins were shallower, the Tethyan Ocean had started retreating and rivers originating on the Asian tectonic plate could now flow across the zone of collision and deposit sediments on the Indian continent. These Asia derived delta sediments rich in volcanic rock fragments eroded from the Gangdese Arc began to be deposited on the Indian continental shelf between 58-54  million years ago. TH, GH, and LH are the future Tethys Himalaya, Greater Himalaya, and Lesser Himalaya.

B) Proto Himalaya Stage- 45-30 million years ago. 

Preceding and during collision, slices of the Indian oceanic plate were thrust up and sandwiched between the two continental plates. Igneous rocks of this "suture zone" are made up of magnesium rich silicate minerals (olivine, pyroxene) and magnesium aluminum oxides (spinel). Only a remnant arm of the Tethys persists, but rivers can now flow across the Indian continent and deliver sediments to the foreland basin. This basin is located roughly over what is now the southern Lesser Himalaya and the Siwalik ranges. Gangdese Arc and suture zone derived sediments first appear in the foreland by 45 million years ago.

C) Early Himalaya Stage- 35-20 million years ago. 

A pulse of low grade metamorphic rock fragments start appearing in the early Oligocene to Miocene age foreland basin sediments. These are derived from the newly forming Tethyan fold and thrust belt. Volcanic rock fragments are now absent in the central foreland, suggesting that the growing Tethyan ranges are a barrier to rivers originating in the Gangdese Arc. The Indus and the Yarlung Tsangpo, initiated in the furrow of the suture zone and flowing parallel to the northern ranges before cutting across the rising mountain chain, continue to transport arc derived sediment to the basins at the western and eastern extremities. 

D) Main Himalaya Stage- From 20 million years ago.

Three pulses of mountain building are recognized during the Main Himalaya Stage. The early phase from 20 -11 million years ago records the uplift of the Greater Himalaya which are made up of high grade metamorphic rocks containing minerals like mica, feldspar, and garnet. Deformation continued southwards between 11-5 million years ago resulting in the formation of the Lesser Himalaya ranges. These shed low grade metamorphic and dolostone (magnesium calcium carbonate) detritus. 

Eventually the foreland basin got caught up in the progressing orogeny. New faults propagated southwards. Slices of the oldest foreland basin deposits were broken and accreted to the mountain front. These freshly exhumed rocks became the major source of sediments feeding the youngest depositional phase in the foreland. Pliocene-Pleistocene  (5 -0.5 million years ago) layers of the Siwalik hills are made up of rock fragments cannibalized from Eocene and Oligocene foreland strata. 

How is the age of the foreland basin sediments determined? The entire exercise of unraveling the topographic history of the Himalaya depends on that! 

These sediments have been dated using a variety of methods. Fossils provide age ranges for packages of sediment. Magnetic signals preserved in iron rich mineral grains are measured and pegged to an absolute date by comparing the magnetic pattern to a global magnetic chronology. Techniques such as fission track dating of zirconium silicate sand (zircon) tells geologists when that zircon in its source was being uplifted and cooled thus giving a fair idea of the time of its erosion, transport, and deposition as a sedimentary particle. 

Gathering all this age information on the sediments, the timing of the arrival of distinctive rock fragments and minerals in the foreland agrees well with the exhumation history of their provenance as deduced from bedrock geochronology. 

The growth of mountain chains is a long and complex process. The word "collision" may invoke ideas of near instant crustal response, but deformation and surface uplift moves rather slowly across strong and rigid plates. Surface and deep crustal process are linked. For example, the formation of the Tethyan fold and thrust belt resulted in crustal thickening and the deep burial and metamorphism of rocks that eventually became the Greater Himalaya. There was a long time lag between the initiation of collision and the main Himalaya pulse of uplift. The rise of  the Greater Himalaya took place a good 35 million years after the India Asia impact. 

Foreland basins give us valuable insights into the relationship between sedimentation and tectonics, but they too need careful evaluation. They are not static entities. Compare the proto Himalaya stage with the main Himalaya stage and you will notice that as mountain building moved southwards the location of the foreland shifted too in response to the migrating load of the thickened crust. This process continues to this day. If the region of the Siwalik hills was the foreland a few million years ago, today it is the Ganga alluvial plains. 

Orogeny and drainage impact the sedimentation patterns in the foreland. Different rivers breaking through along the mountain front may not be bringing uniform sediment mixtures at the same time. Hinterland differences in geology of the catchment results in variable sand composition along the length of the foreland. Take the example of river Ravi. It drains mostly the Lesser Himalaya and hence its brings with it low grade metamorphic sand grains. On the other hand, the Sutlej flows through a significant portion of the Greater Himalaya. High grade metamorphic grains make up a large proportion of its sand. 

Further, chemical reactions taking place in the subsurface may dissolve some types of minerals. This may create an apparent trend in sand composition through time, which may not accurately reflect the actual history of the provenance. 

Researchers need to be cognizant of these issues when they construct their answers. 

Understanding the timing of Himalaya uplift provides valuable insights into the geodynamic forces at play during continental collisions. But the interest in this question goes beyond geology. Climate scientists want to know more about the linkage between Himalaya evolution and the advent and shifts in the Asian monsoon. And the value of studying the Himalaya spills into many other areas. Orogeny exposed enormous volumes of fresh rock to chemical weathering, mobilizing nutrients and organic carbon which would then be sequestered in fluvial and marine environments. These elemental cycling and budgets are keenly studied by surface systems specialists. 

Visit the picturesque Kangra region in Himachal Pradesh. Climb the thick sandstone layers leading up to the historic Kangra fort. Go to nearby Jwalamukhi, where natural gas emanating from the deep keeps alight an eternal flame. Ascend the hills towards the famous town of Dharamshala. You will be traveling through the Miocene foreland basin. Sedimentary petrologists for decades have worked on these and other sites along the Himalaya frontal ranges amassing data on sandstone composition. Even as million dollar instrumentation keep revealing new facets of the earth, their main tool has remained the humble petrologic microscope mounted with a grain counting stage. From this labor of love has emerged the story of how and when the Himalaya came to be.

Fellow- Geological Society of India

I'm happy to announce that earlier this month I was elected a Fellow of the Geological Society of India in recognition of my efforts to introduce and popularize geology among the general public. 

 

As I came to know, some senior office bearers have been reading my blog and recommended my induction into the Society. I feel honored to be a part of this distinguished body. 

I want to thank you readers for your support and encouragement over the years. It has kept me seeking new topics to learn and write about. I started writing many years ago because I felt that there was a lack of popular style writings on the beauty of geology as a science and its relevance to society. Dramatic events such as earthquakes made news. But the field with its many sub specializations, and as a mode of inquiry into the history of the earth remained invisible to the lay audience. I aimed my writings to fill this lacunae. 

Interestingly, biologists were the first to start interacting with me. They were researchers interested in how landscapes impacted biodiversity and evolution. I have had many a fruitful exchange with them. Since then, my readership has expanded and I hear from people from diverse backgrounds. There is a sense of satisfaction that my collection of writings is being used as a resource by many science enthusiasts. A 17 year archive of my posts on varied geoscience topics is available on this blog for your perusal.

Let  me share an email I recently received from a student.

These are the moments when you think it has all been worth it.

Be sure to hum "He's A Jolly Good Fellow" when you are reading my posts. And get your friends to subscribe to this blog. Pronto!

Enigmatic Sedimentary Rock Darma Valley

A late Neoproterozoic to early Paleozoic section (~600-500 million years old) of the Tethyan Sedimentary Sequence is exposed around the villages of Dantu, Boun, and Philum, in the vicinity of the famous Panchachuli Glacier in Kumaon. The lower part, made up of low grade metamorphic rocks is accessible along the many local trails. The higher summits are made up of sandstone and conglomerate. These are harder to reach, but blocks eroded from the summit can be found in streams near Boun and Philum.

Two years ago I had posted a picture of a sandstone block showing convoluted folded layers and asked whether this folding was tectonic in origin or due to synsedimentary deformation of semi lithified sediment. In the latter case, shaking of the sea floor and mass movement of sediment due to an earthquake or a severe storm results in the sediment layers contorting and deforming in various ways. 

 Sandstone in stream bed near Boun village.

I could not decide between these alternatives although I favored a synsedimentary origin of these features. My reasoning was that such deformation appears local, since there were also examples of undeformed sandstone with delicately preserved primary bedding. The short wavelength folding observed in my example also is very different from the longer wavelength folds present in the lower part of the exposed Tethyan sequence. 

Last month on a trip to Boun village I came across another block from those high ridges which I think lends additional weight to the synsedimentary deformation scenario. 

You will notice that the block is made up of a conglomerate (pebbly layer) in the lower part, overlain by a beautifully cross bedded sandstone towards the top. The ten rupee coin gives a sense of scale. The lower half of the block is a classic flat pebble conglomerate. You will realize the meaning of the term as you read along. A more detailed examination of the conglomerate hints at an unusual mode of formation. The clues lie in the four boxes I have drawn. I will focus on them one by one to make my case. This is the cross section of the pebbly layer. The bedding plane view is not exposed in the boulder.

Before that, let me put up this picture of another conglomerate. This sample too has rolled down from the high ridges near Dantu village.

Sedimentologists will be confident in interpreting this as deposition in a gravelly stream or in the surf zone of a beach. The smooth and rounded shape of the cobbles is due to long transport from the source to the site of deposition, followed by the particles rubbing against each other in a high energy current and wave environment. 

Now take a look at the pebbles in Box 1. 

They have straight and jagged sides and pointed edges. This indicates very little transport and attrition before burial. The source rock of these pebbles must have been near by.

In fact, the source can be observed in Box 2.

The dark grey elongated pebbles were derived by the breakage of the bed in the lower part of the block. The dark grey layer has a fragmented fabric.  I have outlined in yellow some larger blocks of the remnant bed. They are surrounded by smaller broken pieces. It looks like a layer which hardened quickly on the sea floor broke due to a disturbance and yielded these flat pebbles. These pebbles are called intraclasts, since they are derived from a source from within the depositional environment. The slab like shape of the pebbles suggests breakage along parallel planes of weakness. The breakage is not due to tectonic overprinting since the overlying cross bedded sandstone is not affected.

Complete disarticulation of an early cemented layer will ultimately yield individual centimeter scale pebbles which make up the pebbly layer highlighted in Box 3. 

Notice the mostly horizontal disposition of the pebbles suggesting transport in a viscous laminar flow and quick burial. The sandy matrix has prevented pebbles from bumping into each other, thus preserving their sharp faces and edges. In contrast, constant exposure to waves and currents would have resulted in the sand being winnowed out and caused these platy pebbles to be rounded, imbricated and stacked at an angle.

Fine quartz and lime mud sediment was cemented by calcium carbonate on the sea floor within a few tens of centimeters of burial. This semi lithified layer then broke during an earthquake or when the sea floor was pounded during a severe storm. 

 Box 4 captures this transition from an in-place unbroken layer which show signs of breakage towards the top. 

Slope failure and mass movement of such a layer eventually resulted in complete breakage of the rigid bed and the formation of discrete flat chips which then were deposited as a flat pebble conglomerate. Box 1 to Box 4 represent different stages of the deformation and sedimentation process. The sharp contact of the layer in Box 3 with the underlying layer (see pic of the entire block) suggests that it may be material transported from an adjacent area where an equivalent bed was completely disarticulated. 

Occurrence of slope failure induced flat pebble conglomerates have been previously observed and reported from the Cambrian age Snowy Range Formation in  northern Wyoming and southern Montanan, U.S.A.

I have observed only one example of this during my recent visit and I have proposed only a tentative answer. Without observing and understanding the stratigraphic and sedimentologic context in an outcrop I cannot be certain that it is correct. 

The association of undeformed and deformed blocks does suggest that intermittent disturbances resulting in brittle and ductile deformation of semi hardened sediment masses alternated with quieter periods of sedimentation. The overlying cross bedded sandstone is an example of deposition during quieter phases.

Flat pebble conglomerates mostly form in sedimentary carbonate environments. This make sense since rapid cementation of the sea floor by calcium carbonate saturated sea water is common. This example though is from a predominantly siliciclastic setting where quartz rich silt lithified fairly rapidly.

These conglomerates also show a peculiar temporal range. They are common in Proterozoic and Cambrian age sequences, but become exceedingly rare in younger rocks. Paleoecologists suggest that this is due to the diversification of burrowing animals that took place during the Great Ordovician Biodiversification Event about 485 to 460 million years ago. 

The churning of sediment by bioturbation kept the sediment loose and granular and prevented frequent cementation of the sea floor and shallow buried layers. Carbonate intraclasts became rarer, forming only in more geographically restricted harsh hypersaline settings. Flat pebble conglomerates give us a glimpse in to the ecology and physical properties of the sea floor before the evolutionary radiation of burrowing macrofauna.

Geological processes and evolution interact and feed off each other.  Through earth history, the formation of diverse topography and chemical environments by geological circumstance have been triggers for evolutionary innovation.  In this example, the evolution of animals making deep vertical burrows resulted in the disappearance from the geologic record of a distinctive sedimentary rock type. Yet, the churning and resulting oxygenation of the sedimentary profile opened up new ecologic spaces for the colonization and diversification of a more complex web of marine communities. 

My quest for a more complete answer to the origin of these deformed sandstone continues. 

Stream near Baun village

On my next trip to Boun I will try to find more of these blocks to gather evidence in support of my theory. Or, who knows, try to find an easier route towards those high ridges! Stay tuned.

Further Reading:

1) Rapid Uplift - Field Photos: Folds- Tectonic Or Soft Sediment Deformation?

2) Thematic Posts - Rapid Uplift- Geological Processes and Evolution.

Shrinking Panchachuli Glacier

Over the past few years I have been regularly visiting the Panchachuli Glacier in the Kumaon Himalaya. Accompanying me are a group of geology enthusiasts from all walks of life. This is an outreach effort I have undertaken in collaboration with Deep Dive India. 

Every time I make it a point to walk to the glacier snout where the river Dhauliganga emerges from an ice cave. Every time my experience of approaching the glacial terminus is different. The Panchachuli Glacier is a shape shifter. I have to walk a little longer each time to reach the snout, negotiating the changing configuration of rubble mounds and streams.

I thought I would document the retreat of the snout over time using field photos from my visits beginning year 2017 and supplemented by Google Earth imagery going back to the year 2000. I won't keep you in suspense. The glacier snout has retreated by about 1 kilometer in the last 25 years. That is by 40 meters per year. But the rate has varied, with an acceleration in the past 8 years.

Let's begin with a synoptic view of the glacial valley.

 

The walk begins at village Dantu. It is about 6 kilometers to the present position of the snout. The red lines are edges of old lateral moraines. They are stable features and are easily recognizable in the satellite images. They are my fixed marker posts against which I will track the changing position of the snout. Distance to the snout in all images is estimated from marker post A. 

Year 2000- Google Earth Imagery.

Notice the curvilinear cracks (white arrow) near the snout. The glacier retreats by slices of ices cleaving off these cracks. Between the two marker posts is a smaller glacial valley. Semicircular depressions and ponds have formed on the glacier surface due to thawing of the ice. The distance to the snout from A is about 1.3 kilometers.

Year 2012- Google Earth Imagery.

Distance to the snout from marker post A is 840 meters indicating a retreat of 460 meters in 12 years, averaging about 38 meters per year. The smaller valley still has a fair amount of ice accumulation.   

Year 2017- Field Photo.

This was taken during my first visit from a high vantage point along the trail to "Zero Point", a popular trekking spot along an old lateral moraine. 

Year 2017- Google Earth Imagery.

Between 2012 and 2017, the snout has retreated a further 120 meters, an average retreat of about 26 meters per year. The frontal part of the smaller side glacier is now showing signs of collapse. Pronounced curvilinear cracks have appeared and glacier retreat has left behind rubble mounds producing an uneven topography at the front.

Year 2023- Field Photo.

Taken from close to the 2017 vantage point. 

Year 2023- Google Earth Imagery.

The snout is now just 280 meters from marker post A. This implies a retreat of about 430 meters between the years 2017 to 2023. The average rate of retreat is an astonishing 71 meters per year. There are signs of significant changes around the glacial terminus. The snout is now up-valley of marker post B. The smaller valley between the marker posts is almost completely ice free. A surface drainage has developed along this side valley and joins the main Dauliganga stream just downstream of the snout. The multiple small streams in the area is due to drainage finding its way around fresh mounds of rubble.

My most recent visit was earlier this month in May. Google Earth does not have imagery from 2025. But I will share a field photo taken from the high slopes looking down towards the snout.

 

The snout has retreated by tens of meters and is almost in line with marker post A, strongly suggestive of high rates of retreat persisting over the last couple of years. The area around the terminus is a degraded landscape with rubble heaps everywhere. We could not get very close to the ice cave this time due to time constraints but I assessed it would have been a more difficult passage negotiating the multiple streams and boulders. 

The rates of retreat that I have estimated are a little higher than those made by geologists from other parts of the Himalaya. A Ministry of Earth Sciences press release from 2023 has shared some data on Himalaya glaciers. The average retreat rate for Ganga Basin glaciers is about 15 meters per year, while that for Brahmaputra Basin is about 20 meters per year. 

A more detailed study of Gangotri, India's most famous glacier, shows a retreat rate of 20 meters per year. Significantly, the retreat accelerated in the past few years to about 33 meters per year, a pattern I too have observed for the Panchachuli Glacier. 

Himalayan rivers provide water security for hundreds of millions of people in the Indian subcontinent. Glacial runoff contributes a timely and significant amount of water to these rivers. The Indian government is building and planning scores of dams in the Indus, Ganga, and Brahmaputra basins with elaborate arrangements of water use and water sharing with different stakeholders. Given the massive ice loss and changing climatic patterns, it is imperative that detailed feasibility studies of these projects in terms of both safety, and near, mid, and long term projections of water availability are carried out. 

I'm putting up this final image taken in May 2025 from near the Dhauliganga stream looking towards the terminus. 

 

It captures nicely the long term changes that have taken place. The blue line marks the top of the glacier. Fresh collapse has exposed shiny ice walls. The brown line above is the crest of an old lateral moraine, several hundred feet higher than the present day glacier surface. 

Is there a better landscape to contemplate and appreciate climate change and the dynamic glaciers which have shaped our planet over past centuries?