Wednesday, April 19, 2017

Evolution Of The Konkan-Kanara Coastal Plain

The Konkan coastal plains is a beautiful getaway from west coast city life. Palm fringed beaches, quiet rivers and estuaries, betel nut plantations and forest tracts. Small villages and settlements dot the landscape. To the east, the coastal plains abut against the imposing Western Ghat escarpment.

How did this coastal plain of Maharashtra form? (Kanara refers to the stretch south of Maharashtra in the state of Karnataka).  I came across a paper by Mike Widdowson on the evolution of laterite in Goa. It also has a broader discussion on the conditions that led to the formation of geomorphology of the coastal lowlands extending all along the west coast of India.

Here it is summarized nicely in this figure below:

Source: Evolution of Laterite in Goa: Mike Widdowson  2009

After Deccan Volcanism ended, rifting of the Indian west coast and down faulting of the western side led to the formation of a west facing fault scarp. Erosion of this scarp over the early mid Cenozoic (from about 60 million years ago) has caused it to retreat eastwards. The Western Ghat escarpment is this retreated scarpThe coastal plain formed as an erosional surface that became broader and broader with the progressive eastward retreat of this cliff to the current location. The fault which caused the western side to subside thus lies in the Arabian Sea along the west coast.

In Mid-Late Miocene (~10 million years ago), a phase of humid climate resulted in intense chemical weathering of the basalts and pediment (rock debris) exposed along the coastal plains. This alteration of the basalts formed thick iron rich soils. The reddened and indurated crust of this soil is commonly termed laterite. In the Western coastal lowlands this laterite may be a few meters thick.

Subsequent uplift of the west coast and concomitant down cutting by west flowing rivers formed a dissected landscape composed of laterite capped mesas (table lands) and entrenched meandering streams. These mesas reach altitudes of 150-200 m in the eastern parts of the coastal plain. Nearer the coast they are about 50 -100 m above sea level. 

The western margin of India has seen multiple episodes of extensive laterite formation. The famous table lands of the hill stations of Panchgani and Mahabaleshwar are also made up of laterite. They occur at altitudes of around 1200 m to 1500 m.  However, this upland or high altitude laterite is much older, having formed about 60- 50 million years ago in the early Cenozoic, soon after Deccan volcanism ended. The Konkan and Goa lowland laterites point to another younger phase of laterization. Sheila Mishra and colleagues have identified two more surfaces in the Deccan Traps at 650 m ASL and 850 m ASL that preserve remnants of laterite cover. This suggests a complex polyphase history of denudation and chemical weathering and tectonic stability of the Sahaydri ranges of the Western Ghats.

The sea cliffs that one encounters as you travel along the Konkan and Goa coastline are a result of a late Cenozoic uplift. I remember with fondness a trek I did during my college days from the town of Ratnagiri south to the town of Malvan. There were absolutely majestic sections where we walked on the edge of laterite capped sea cliffs with the Arabian Sea heaving and thundering below us. Little coves and beaches of sparkling white sand lay between the cliffs. Here and there local fisherman had kept their fish catch to dry out in the sun. The pungent smell urged us on!

The satellite imagery below shows a section of the coastal plains from Ratnagiri in the north to Devgarh in the south. White arrows point to the laterite capped table lands dissected by stream networks. Orange arrows point to sea cliffs. Black arrows shows the Western Ghat escarpment.

This is a very interesting paper. Open Access.

Thursday, April 13, 2017

Oceanic Crustal Thickness Since The Breakup Of Pangea

Of interest:

Decrease in oceanic crustal thickness since the breakup of Pangaea - Harm J. A. Van Avendonk, Joshua K. Davis, Jennifer L. Harding and Lawrence A. Lawver

Earth’s mantle has cooled by 6–11 °C every 100 million years since the Archaean, 2.5 billion years ago. In more recent times, the surface heat loss that led to this temperature drop may have been enhanced by plate-tectonic processes, such as continental breakup, the continuous creation of oceanic lithosphere at mid-ocean ridges and subduction at deep-sea trenches. Here we use a compilation of marine seismic refraction data from ocean basins globally to analyse changes in the thickness of oceanic crust over time. We find that oceanic crust formed in the mid-Jurassic, about 170 million years ago, is 1.7 km thicker on average than crust produced along the present-day mid-ocean ridge system. If a higher mantle temperature is the cause of thicker Jurassic ocean crust, the upper mantle may have cooled by 15–20 °C per 100 million years over this time period. The difference between this and the long-term mantle cooling rate indeed suggests that modern plate tectonics coincide with greater mantle heat loss. We also find that the increase of ocean crustal thickness with plate age is stronger in the Indian and Atlantic oceans compared with the Pacific Ocean. This observation supports the idea that upper mantle temperature in the Jurassic was higher in the wake of the fragmented supercontinent Pangaea due to the effect of continental insulation.

Continental insulation refers to the idea that an unbroken continental crust such as that provided by a supercontinent may act as a blanket resulting in a slow build up of heat over tens to hundreds of millions of years in the underlying mantle. Eventual continental breakup will lead to enhanced magmatism and thicker ocean crust along these previously insulated regions.

The Pangaean paleogeography of the Triassic (252 million to 201 million years ago) is depicted in the map below. The distribution of continents is lopsided covering the sites of the future Atlantic and Indian Oceans.

 Source: Paleobiology Navigator

Wednesday, March 29, 2017

Exploring India's Paleogeography And Fossils Using The Paleobiology Database Navigator

I was directed to the Paleobiology Navigator by a tweet from @avinashtn .

Great fun! The Paleobiology Database is being maintained by an international non-governmental group of paleontologists. Contributing members add to it fossil occurrences from scientific publications.  The Paleobiology Database Navigator is a web mapping application managed by the University of Wisconsin-Madison that allows you to explore the geographic context of these fossil locations. You can filter the data based on age, taxonomy and geography. You can also generate diversity trends for the selected set.

I played around a bit with India specific fossil locations.

Paleozoic versus Mesozoic Basins

The figure below shows the distribution of fossil localities for the Paleozoic Era. India is shown as it is today and in its Paleozoic geography.

Source: Paleobiology Navigator

You can clearly see that fossils in Peninsular India are predominantly located in one narrow band in the center and east of the country. These are the Permian Gondwana basins. They are, starting from the westernmost and going eastwards, Satpura Basin, Son Valley Basin, Damodar Valley Basin and the Ranjganj Basin.  These are continental interior basins comprising river, lake and swamp environments. Most of India's coal deposits come from these basins. These basins are rich in plant fossils, and reptile and amphibians remains.

Now take a look at India's geographic position (arrow) during the Permian (298-252 million years ago). Peninsular India occupies an interior location within Gondwanaland, far away from any ocean. Tectonic stability through most of the Paleozoic meant lack of crustal movements. During this time, peninsular India was an erosional landscape until the Permian basin formation in the east.

The one Paleozoic fossil location in Rajasthan shown here represents early Permian marine sediments formed by the flooding of the western region by an arm of the Tethys sea.

And this database has still not added one important fossil location. This is the early Cambrian age locality near Jodhpur where sediments of the Nagaur Group are exposed. They contain trilobite trace fossils.  No basin development and sedimentation took place in Peninsular India from Mid-Cambrian to Permian times (530 million years to 298 million years). 

In contrast, look at the northern edge of India, where the Himalaya stand today. That margin was submerged under the Tethyan ocean. A thick pile of marine sediment accumulated right through the Paleozoic, forming the fossil rich Tethyan Sedimenary Sequence of the Himalaya.

Continental configurations changed in the Mesozoic (252 million to 66 million years ago). The figure below shows Mesozoic fossil locations and the Cretaceous paleogeography of India.

Source: Paleobiology Navigator

There is now a wide swath of fossil localities across Peninsular India. The dotted lines trace important linear depressions where sediments were deposited. The east west oriented Narmada rift zone (NRZ; Jurassic and Cretaceous) and the NW-SE oriented Pranhita Godavari zone (PGR; Triassic to Cretaceous) are important fossil repositories.  The eastern India basins continued accumulating sediment. To the west are the basins which formed in Gujarat and Rajasthan (Jurassic and Cretaceous). The Kutch rift (KR) is outline by dotted lines. And to the south east in Tamil Nadu, marine flooding of the eastern continental margin in the Cretaceous resulted in the deposition of richly fossiliferous sedimentary sequences.

All these basins ultimately owe their origin to the forces exerted on the crust as India pulled away (arrow) from Gondwanaland.  Seaways formed along these rifts and crustal depressions. The Mesozoic, especially the Jurassic and Cretaceous, was a time of global high sea levels. The western margin saw marine incursions from the nascent Indian Ocean, while the eastern margin was submerged by the waters of the newly formed Bay of Bengal.  River and lake systems also developed in more continental interior locations. The northern margin (Himalaya) was mostly a marine environment through the Mesozoic.

Marine versus Continental Interior Basins in Mesozoic Central India

The distribution of terrestrial organisms versus marine organisms can tell us about the extent of marine flooding into Peninsular Central India in the Mesozoic.

I created these maps by using localities of dinosaur fossils (above) to map the distribution of terrestrial sedimentary environments. I used localities of invertebrate marine organisms, namely,  brachiopods, echinoderms and ammonoids  to delimit the extent of marine environments along the Central Indian basins (below).

 Source: Paleobiology Navigator

You can see that terrestrial environments were present right across the Narmada rift zone, the Pranhita Godavari rift basin and in the western Indian basins also. In the western basins, some of the dinosaur fossils have been found in marginal marine settings comprising coastal and estuarine environments.

Deeper water marine environments as evidenced by brachiopod, echinoderm and ammonoid localities are however restricted to Gujarat, Rajasthan and western Madhya Pradesh. The Cretaceous Bagh Beds in Madhya Pradesh is the eastern most limit of Mesozoic marine flooding into Central India. Seaways did not extend into eastern parts of the Narmada rift basins.

Global and Indian Dinosaur Diversity Patterns

I used the Stats tool to create graphs of dinosaur diversity. The number of Genus per Stage is being used as a measure of diversity. Geologic time is subdivided in to bins. An Age is a bin spanning a few million years. Stage represents rock layers deposited in an Age. So, a diversity measure has been created by counting the number of dinosaur genus reported from successive bundles of rock layers, each representing a few million years of time.

Source: Paleobiology Navigator

The global diversity pattern shows episodes of diversification and decline in the Triassic, Jurassic and the Cretaceous. There appears to be a trend of increasing diversity through time with peak diversity in the Mid-Late Cretaceous. The Late Cretaceous extinction of dinosaurs forms the right side boundary.

The diversity measures in India show some differences with global trends. The number of Genus sampled are less. This is due to regional versus global sample. A smaller locale will generally have less of the total observed variation. The trends in diversity with time also is different from the global trajectories. There are a couple of reasons for this. First, this is a preservation artifact. Mesozoic terrestrial basins in India were receiving sediment only episodically. Depositional phases were interrupted by erosional hiatuses. Rock sections thus have been removed as well.   There was little to no sedimentation from Mid-Jurassic to Mid-Cretaceous in the Narmada rift basins. Hence, no fossils either. The lost diversity from this interval is irretrievable.

The second reason gives more hope. A couple of years ago, Dr. Dhananjay Mohabey of the Geological Survey of India gave a talk in Pune on Late Cretaceous dinosaurs of India. He mentioned that there are roomful of dinosaur fossils in government archives that are yet to be studied and catalogued. There is scope then to enhance our understanding of at least late Cretaceous dinosaur diversity of India.

I have barely scratched the surface. There are many more stories and patterns and trends in the Indian fossil record waiting to be teased out from this database. Dive in!

Saturday, March 18, 2017

Comments On The 1.6 Billion Year Old Red Algae From Central India

The Proterozoic Vindhyan sedimentary basin in Central India contains sediments ranging in age from 1.7 billion years to about 600 million years ago. Bengtson and colleagues report three dimensional preservation of cellular structures which they interpret as multicellular red algae. These fossils have been found in the Tirohan Dolomite dated to about 1. 6 billion years. Before this discovery, the earliest fossils of multicellular eukaryotes was the rhodophyte Bangiomorpha, dated to about 1.2 billion years.

The Tirohan Dolomite is exposed in the Chitrakoot region of Madhya Pradesh. The fossils occur in patches of carbonate sediment which was replaced by the calcium phosphate mineral apatite just after their deposition in a shallow marine setting. Phosphotization is often a very delicate process enablng the preservation of fragile cell structures.

Here is a picture of the cellular structures of red algae imaged by SEM (scanning electron microscope)

Source: Bengtson 2017

And another rendering of the three dimensional structure of the red algae imaged using Synchrotron-Radiation X-ray Tomographic Microscopy (SRXTM). The green objects inside the cell are interpreted to be organelles, components of eukaryotic cells which aid in different physiological functions. Prokaryotes (Bacteria) lack such organelles.

Source: Bengtson 2017

I don't want to dwell on this study too much. The paper is open access for those who want to explore further.

There are two side stories that I want to comment upon.

First. The Tirohan Dolomite and its fossil assemblage has a controversial past.

They were discovered about twenty years ago by Dr Rafat Azmi, a paleontologist working with the Wadia Institute of Himalayan Geology. He reported from the Rohtasgarh area in 1998 a rich trove of filamentous and spherical forms, and odd shaped mineral fragments. He interpreted the mineral fragments as "small shelly fossils" representing fragments of animal shells and the spherical forms as possible animal embryos. Later in 2006 he reported tubular forms which he interpreted as Cambrian animal taxa. The problem was that animals are thought to have evolved by the latest Neoproterozoic- early Cambrian (600 mya -540 mya), while the understanding then was that the Tirohan Dolomite is likely 1 billion to 1.5 billion years old. Azmi's interpretation carried two enormous implications; either a) the Tirohan Dolomite was much younger in age. This would have required a major revision of the ages of Vindhyan sediments or b) that the rocks were old (~1.5 billion years), but that animals evolved much earlier than the current fossil record indicated.

These very significant implications caught the attention of geologists and media alike. The Geological Society of India sent a team to investigate Dr. Azmi's claims. They reported that they were unable to find the fossils Dr. Azmi had claimed to have found.

 Memories of an earlier scandal in Indian palaeontology were still fresh. In the late 1980's Vishwajit Gupta of Punjab University was found guilty of fraud and plagiarism. He had been misreporting fossil discoveries from the Himalayas by using museum specimens from all over the world. He had  constructed an entirely fake narrative of Himalayan fossils and stratigraphy. Scientific journals were forced to retract his papers. The Paleontological Society of India produced a book authored by S. K Shah titled "The Himalayan Fossil Fraud".  Punjab University, disgracefully, allowed Dr. Gupta to remain in service till he retired in 2004.

Under this shadow, Azmi's fossils came under similar suspicion. Fortunately, Bengtson and colleagues in a study some years later confirmed that these fossils do exist in the Tirohan Dolomite. However, they sampled the Tirohan Dolomite at Chitrakoot and not its stratigraphic equivalent (Rohtas limestone) at Rohtasgarh where Dr. Azmi's initial claims came from. They established using absolute radiometric dating that the Tirohan Dolomite is 1.6 billion years old. And they showed that the forms, similar to those Dr. Azmi found, are not multicellular animals. The spherical forms were all likely gas bubbles. Some of the larger tubular forms were revealed in the present study as red algae. Animal evolution didn't take place that early after all. The claim of the "small shelly fossils" has not been resolved fully. Bengtson and colleagues work doesn't address them. Some other researchers though have interpreted them as non-biogenic mineral growths. The stratigraphy and broader fossil content of the Rohtas limestone from where Azmi collected his fossils firmly indicates that it is not Cambrian but Proterozoic in age. .  In this present paper, these scientists have named one of the red algal forms Rafatazmia chitrakootensis in honor of Dr. Razat Azmi.

The second comment I have is on multicellularity. These red algae are the oldest multicelluar eukaryotes found anywhere. Plants, Fungi, Protists (amoebas) are eukaryotes.  They share a common eukaryote ancestor which was unicellular. That means there was just one origin of the eukaryotic cell type. However, multicellularity has evolved many times independently in different branches of the eukaryote family.

Multicellularity comes in different flavors. In simple forms of multicellularity, organisms are made up of sheets and aggregates of cells sticking to one another. There is differentiation of somatic and reproductive cells. Communication between cells is limited. One important aspect is that all the cells are in direct contact with the environment, since in these organisms, nutrient transfer takes place by diffusion from the environment to the cell. More complex types of multicellularity require the evolution of not just cell to cell adhesion, but elaborate cell to cell communication systems and a division of labor i.e. cells specialized for different functions. Also, these organisms have a three dimensional arrangement of cells wherein only few cell types are in direct contact with the environment. Diffusion is not efficient enough to supply internal cells with all the necessary life support. Molecular conduits and tissues that facilitate bulk transport and circulation of nutrients need to evolve to build this type of multicellularity.

The figure below shows the many origins of the complex type of multicellularity (in red) in different eukaryotes branches.

Source: Andrew H Knoll 2011

Based on cell type, life is divided into two domains. The Prokaryotes (Bacteria and Archaea) have smaller simpler cells. Eukaryotes are generally larger and are made up of more complex cells. This cell type evolved by a symbiotic merger between two types of prokaryote cells. Prokaryote fossils have been found in rocks older than 3 billion years. The eukaryote fossil record begins in rocks younger than 2 billion years. The timing of the origin of eukaryotes is unclear. Estimates range from  2.5 billion to 1.5 billion years ago. These red algae fossils show that eukaryotes had already diverged into different branches by 1.6 billion years ago, which means that the unicellular ancestor of eukaryotes evolved before that. It also means that red algae took the road to multicellularity much earlier than animals.

Does complexity evolve necessarily whenever genetic potential is available or does it depend on ecologic opportunity? If the cellular machinery and the underlying genetic regulatory systems required for multicellularity evolved in the ancestors of red algae by 1.6 billion years ago, why did multicellular animals not evolve earlier as well? It could well be that there were ecologic conditions limiting the evolution of physiologically demanding creatures like animals. The end of Neoproterozoic ice-ages by about 650 million years ago and the break up of supercontinent Rodinia impacted sea water chemistry. Sea water oxygen increased to threshold levels permitting a more active life style. Increased weathering of continents brought into the oceans metals like zinc which are crucial for physiological functions. Creation of larger continental shelves and shallow water zones due to continental breakup provided varied ecologic spaces for diversification. Animal evolution was triggered in this ecological context.

Wednesday, March 8, 2017

Papers: Tectonics And Physical Volcanology Of Deccan Traps

There are plenty of research papers on the geochemistry of the Deccan Basalts. But nature lovers and trekkers like me come face to face not with chemistry but with the physical forms of lava and the structural elements of the volcanic pile.

I found this list of papers most useful. They have helped me sort out my confusions regarding lava morphology and taught me something about the structural fabric of the western margin of the Deccan Volcanic Province.

1) Near N–S paleo‑extension in the western Deccan region, India: Does it link strike‑slip tectonics with India–Seychelles rifting? - Achyuta Ayan Misra Gourab Bhattacharya, Soumyajit Mukherjee, Narayan Bose

This is a structural analysis of the fracture systems that cut across the western margin of the Deccan province.  The area of study is the coastal plains, about 100 km north and south of Mumbai. The Indian western margin is a rifted margin i.e. it formed by the breakup of India with Madagascar (88 million years ago) and then Seychelles (64 million years ago). This type of margin is formed by tensional forces splitting apart continents and so you would expect normal faults, wherein blocks of crust have moved down along inclined fault planes.  Except here, the researchers find evidence of strike slip movement along sub-vertical fault planes. This means crustal blocks slid past each other. This implies oblique rifting with components of both extension and transverse movement between India and Seychelles. There are some really revealing field photos of this transverse (strike slip) movements.

2) Geology of the Elephanta Island fault zone, western Indian rifted margin, and its significance for understanding the Panvel flexure- Hrishikesh Samant, Ashwin Pundalik, Joseph D’souza, Hetu Sheth, Keegan Carmo, LoboKyle D’souza, Vanit Patel

Wait a minute. There are normal faults with downthrown blocks in this region too. And from the famous Elephanta Island. The fault planes dip eastwards producing easterly downthrows. That means the easterly crustal block has moved down. Again, some good field photos of fault planes and slickensides ( fault surfaces which get a polished striated appearance due to the frictional movement of rocks). These faults with easterly downthrows are found all along the west coast.  There is one near the proposed site of the nuclear power plant at Jaitapur in southern Maharashtra, which shows signs of intermittent movement over the past fifty thousand years. So, there is a very practical reason for understanding these faults.

3) Deccan Plateau Uplift: insights from parts of Western Uplands, Maharashtra, India- Vivek. S Kale, Gauri Dole, Devdutta Upasani and Shilpa Patil Pillai

This is a study of part of the Deccan plateau. I visited this region a few weeks back.  Very useful information of the various fracture systems that cut across the stacks of lava and their significance in terms of recent (Quaternary) crustal movements and controls on the drainage systems. Well thought out block diagrams illustrate the authors ideas very clearly.

4) Pahoehoe–a'a transitions in the lava flow fields of the western Deccan Traps, India-implications for emplacement dynamics, flood basalt architecture and volcanic stratigraphy-  Raymond A. Duraiswami, Purva Gadpallu, Tahira N. Shaikh, Neha Cardin

Good explanations of the morphology of basalt lava flows.  I really liked the sketches showing the internal structure of lava flows and the emplacement of pahoehoe lava fields with its transformation into transitional and a'a type lavas. Very useful guide for my next outing into the Deccan basalts!

Saturday, February 25, 2017

Field Photos: Western Uplands And Giant Plagioclase Basalts

Last Sunday I visited Chavand fort near the town of Junnar, about 110 km north of Pune. This is a rugged terrain marked by several NW-SE oriented ridges separated by broad U shaped valleys. In the map below the black cross marks the location of Chavand. WGE refers to the Western Ghat Escarpment and KCB refers to the Konkan Coastal Belt. Trekkers familiar with this region will recognize the hill ranges, especially the Bhimashankar range and the Harishchandragad range. And near the town of Junnar is Shivneri fort, birthplace of the Maratha king Shivaji.

Source: Kale et. al, 2016

When Deccan volcanism ended, this region would have been a vast flat- to- gently undulating lava surface. At that time, some 60 million years ago, you could have walked from where Bhimashankar temple now stands to the present location of Harishchandragad along broadly the same elevation without the need to climb down several hundred feet or so into a valley and then climb up again. Over time however, south easterly flowing rivers and tributaries have gouged out grooves within this large plateau, dissecting it into a valley and ridge terrain.

The geomorphology of this region therefore reflects the creation of relief due to removal of material by erosion. This contrasts with other areas like the famous Basin and Range Province in western United States, where parallel faults have moved blocks of crust hundreds of feet to form a system of flat bottomed valleys (grabens) and flat topped ridges (horsts).

The Western Uplands end abruptly along the Western Ghat Escarpment, a sinuous west facing cliff overlooking the Konkan coastal plain. The escarpment is the edge of the Deccan Plateau.

There is another factor that has shaped this landscape. Take a look below at a satellite imagery of this area. The arrows mark fracture systems that have broken this plateau.

Erosion along these zones of weakened rock results in slabs of basalts peeling on rock faces. Over time, the result is a landscape that fragments into mesas, buttes and pinnacles. Chavand is one such mesa. Notice its straight edged polygonal shape suggestive of erosion along fracture planes.

Many of these fracture systems originated in the tensional forces that the western margin of India experienced during rifting and associated Deccan volcanism. After its separation from Madagascar around 88 million years ago, the Indian continent's rifted and the fractured western margin migrated over the Reunion hotspot, an unusually hot area of the mantle. The result, beginning around 68 million years ago was Deccan volcanism. Some of these linear structures, common in the area around Sangamner, are dikes. They are the pipes which brought up magma from deep subsurface chambers to the surface. Continued rifting of the western margin resulted ultimately in crustal blocks subsiding along a series of N-S oriented parallel faults. The Western Ghat Escarpment likely originated as a west facing fault scarp, but it would have been located as much as a 100 km to the west of its present location. Erosion over million of  years has resulted in an eastward retreat of this feature to where it stands now.

Picture below shows how hill ranges have been broken by a fracture system, resulting in isolated pinnacles.

And here is a picture of Chavand.

Recently my friend Vivek Kale and colleagues complied some very interesting geomorphologic, structural and sedimentology data to suggest that these western uplands have experience some tectonic movements during Quaternary times (past 2.58 million years). They emphasize that the Deccan plateau and Western Upland should not be regarded as a monolithic stable crust block. They point to three major fracture systems (F1, F5, F7 in the map below) which have segmented this part of the western upland. The central segment, i.e. the area north of Chavand, roughly between fracture systems F1 and F5 has moved up relative to the blocks to the north and south. The presence of sediments deposited in the Pravara river system and along F1 and some streams to the south  is evidence that the these blocks subsided somewhat, resulting in stretches of streams becoming sediment traps.

Source: Adapted in Kale et. al 2016 from Dole 2000, Dole et. al. 2002, Bondre 2006

These sediments represent deposition over the past 100,000 years or so. At some localities along the Mahalungi river, they have been deformed. Soft sediment deformation structures such as slumping, load structures and sand dykes have been recorded by Dole and colleagues. Such structures are evidence of ground shaking and sediment liquefaction and remobilization during earthquakes.  Also observed at one locality is reverse faulting. The faulting has been inferred to be of Holocene age, as recent as the past 10,000 years.

These structural movements have also disrupted and modified the antecedent drainage of this region. The map above shows several easterly flowing streams (R. Mahaludi, R. Adula, R. Mula, R. Madvi, R. Pushavati) in this region abruptly turning southeast as they intersect NW-SE trending fractures. The yellow overlay on the map indicates sedimentary deposits. The major fracture systems F1, F5, F7 likely reflect faults in the Precambrian continental crust underlying the Deccan volcanics. They have been rejuvenated in Quaternary times and have cut across and caused dislocations of the volcanic pile.

There are other interesting drainage features in this region. Many streams have stretches with potholes (Nighoj on R. Kukdi), cascades (stretches of R. Pushpavati, R. Mula, R. Ghod, R. Bhama) and entrenched meandering channels (R. Pravara, R. Mula, R. Ghod, R. Vel) all suggesting episodes of increased vigor of stream down cutting. Whether this is a climatic signal (e.g. increase in rainfall will increase water flow and stream erosive power) or is tectonically triggered (e.g. slight uplift and  tilting of land will increase stream gradient, resulting in more vigorous stream flow and down cutting), as Kale and colleagues have recently argued, is an active area of research.

Finally, the giant plagioclase basalts. Plagioclase, which belongs to the feldspar family of minerals, is a major component of basalts. The entire Deccan volcanic lava sequence is subdivided into three subgroups based on geochemical differences. The giant plagioclase basalt lava flows occur predominantly in the lower part of the volcanic sequence. The table on the left (Kale et. al. 2016) shows the geochemical stratigraphy of the Deccan volcanic sequence with the location of the giant plagioclase basalts (GPB). The GPB flows cap individual formations within the Kalsubai Subgroup. This has been  interpreted by many geologists to mean that they mark the final eruptions of a magmatic cycle. Because of their distinctive appearance these GPB flows have proved to be useful as marker horizons in stratrigraphic mapping.

The plagioclase crystals are greater than 1 cm and often have grown to several centimeters long. They are surrounded by a fine grained to glassy matrix. The picture below is a close up of a giant plagioclase basalt, showing tabular plagioclase phenocrysts (arrows).

Geologists agree that these giant crystals grew slowly in magma chambers tens of kilometers deep in the subsurface. These crystals were then brought to the surface by ascending magma, which then cooled rapidly on the surface forming a fine grained to glassy matrix. This two stage crystallization history is the reason for the two distinct grain sizes in this rock.

The picture below shows Chavand fort hill face. The giant plagioclase basalt lava flow makes up the shrubby gentler slope.  These basalts are vesicular (containing pits and holes due to trapped gas bubbles in the lava) and softer and have weathered to form the gentler slopes. The upper harder and more compact basalt forms cliffs.  

There are differing views however on how long these crystals were growing in the subsurface and what that implies for the mode and duration of Deccan eruptions. I"ll leave the details for another post. Meanwhile, here is another picture of the giant plagioclase basalt showing lath shaped plagioclase grains (1) and a rosette of plagioclase phenocrysts (2).

...And a few more pictures of the terrain as seen from the top of Chavand.

A large water tank dug out from the hard basalt

Mesa top grasslands give way to the rugged ranges of the Western Uplands. This is a north facing view with the Harishchandra range at the far end.

A south facing view with the Bhimashankar range with its forested plateaus.

..did I mention it was a pretty steep climb?

Until next time...

Monday, February 20, 2017

Aristotle And Darwin: Why?

A thought provoking passage from The Lagoon: How Aristotle Invented Science by Armand Marie Leroi-

The history of Western thought is littered with teleologists. From fourth-century Attica to twenty-first century Kansas, the Argument from Design has never lost its appeal. Aristotle and Darwin, however, share the most unusual conviction that though the organic world is filled with design there is no designer. But if the designer is dead for whose benefit is design? It's the prosecutor's question: cui bono?

Darwin answered that individuals benefit. Biologists have batted the question about ever since. The answers they've essayed are : memes, genes, individuals, groups, species, some combination or all of the above. Aristotle, however, generally appears to agree with Darwin: organs exist for the sake of the survival and reproduction of individual animals. This is why so much of his biology seems so familiar.

Yet there is a deep difference between Aristotle's teleology and Darwin's adapatationism, one which appears when we follow the chain of explanation that any theory of organic design invites. Why does the elephant have a trunk? To snorkel. Why must it snorkel? Because it's slow and lives in swamps. Why is it slow? Because it's big. Why is it big? To defend itself. Why must it defend itself? Because it wants to survive and reproduce. Why does it want to survive and reproduce? Because..

Because natural selection has designed the elephant to reproduce itself. Darwin gave teleology a mechanistic explanation. He halted the march of whys.

Aristotle was an eternalist. In his cosmos, organic beings were produced by a union of their parents. They in turn by a union of their parents.. continued into infinite regress. Organisms wanted to survive and reproduce so that their "kind" persist for ever. This static world had always existed. He saw divinity in immortality.

In Aristotle's world organic beings don't change and transform into anything else. He never did come around proposing a theory of transmutation or change or evolution. He had some of the raw material to advance such thoughts.

His extensive dissections and comparative anatomy had given him an understanding that life is arranged in a hierarchical manner. Dogs and foxes are more closely related to each other than either is to lions and leopards. Both, canids and felines though are part of a larger mammal family. Members within this family are more similar to each other than they are to members of reptiles. Aristotle recognized that there are groups within groups. He termed the basic taxonomic units as genos. These are part of the larger magiste gene. Ikthis (fish), entoma (insects),ornis (birds), zootoka tetrapoda (live bearing tetrapods- mammals), oiotoka tetrpoda (egg laying tetrapods -reptiles, amphibians) were some of his "greatest kinds" or  magiste gene.  Yet, he didn't ponder upon the relationship between life's groupings and never realized that this pattern is a tree like structure, resulting from common ancestry and subsequent lineage branching. The figure to the left is Darwin's sketch (Notebook B 1837) of the tree of life depicting common ancestry and the branching nature of life.

Aristotle was aware that there is variation within each "kind" or genos. Organisms within a genos varied in their eidos or form. He also had a theory of inheritance. Parents passed on their eidos to progeny, often not exactly. Progeny then, occasionally, could be somewhat different from parents. Moreover, his understanding of inheritance was surprisingly modern. A child might inherit her father's nose or her mother's nose but not something in between. So, he had a particulate view of inheritance. This also meant that traits remain stable and may pass on unaltered over many generations.  Father's and mother's contributions don't blend to form some average feature, but remain discrete. By this he did not mean that actual particles were being transmitted. Rather, traits were reproduced by the movement and heat of either the semen or 'menses' (menstrual fluid). Whichever was stronger determined whether the child resembled her father or mother.

Darwin never understood inheritance very well. He was sure that variation is the  fuel of evolution. But he was troubled that blending would wipe out variation in populations. He struggled with the problem of inheritance for decades.

Aristotle's limitation was that he didn't think anomalous features or differences offered any advantages to the individual. His view was that creatures are born within the limits of their physiology and there was no room for improvement. In that, he should have listened to Socrates! In a Greek society with very particular notions of beauty, Socrates, with his snub nose and flabby lips was an anomaly. He boasted though that his lips worked better than anyone else's. The mutant feature provided an advantage. Aristotle rejected such notions.

Perhaps this is why he couldn't take the next step.... That advantageous variation could be selected upon by nature. Possessors of that trait would on average leave behind more progeny. A certain new form would thus become more common over generations and while some other form disappeared. Some people argue that the lack of a fossil record may have been one reason why Aristotle never appreciated that organic beings have changed over time. However, as Leroi describes, Greek travelers and physiologoi (naturalists) from his time had written about fossil sea shells found high on mountains and fish imprints on stone. Theophrastus, his student and protege, describes dug up ivory. Dwarf elephants were among the many remains from the Pleistocene megafaunal fossil beds of Samos, Kos and Tilos islands.

To someone wedded to an unchanging cosmos, this may not have made any difference, Leroi argues.

It would be more than two millennia before Darwin and Wallace put all the pieces together.

Highly Recommended.

Thursday, February 9, 2017

Why Is There A "Lost Continent" Underneath Mauritius

Yes, the term "lost continent" brings up visions of a lost world full of fantastic creatures that once existed deep in the earth's past. Or, of a civilization that once was, but was swallowed up by rising seas and which now only remains on the margins of human memory.

The "lost continent" underneath Mauritius is making news. It is more accurate to say that there is continental crust underneath the oceanic lavas of Mauritius. And that continental crust is very old. Geologists found crystals of zircon in young lava that erupted on Mauritius about 5.7 million years ago. The age of the zircon is however Archean in age, between 2.5 billion - 3 billion years. That means the zircon crystal did not form in the young lava, but belongs to the older foundation of the island. They were extracted from this Archean crust by rising molten material and brought to the surface about 5. 7 million years ago.

The crust making up the earth continents is primarily made up of granitic and andesitic rocks and sedimentary cover. This crust is light and thick (30km-40 km) and it sticks above sea level.  On the other hand, the crust making  up the ocean basins is made up of basalt and is denser and thinner (~10 km). So, what is Archean continental crust doing in the middle of the Indian ocean, surrounded by Cretaceous-Cenozoic oceanic basaltic crust?

The answer lies in the way Gondwanaland broke up, or rather the way India broke away from Madagascar about 88 million years ago. This was a continuation of the progressive breakup of Gondwanaland that began in the late Jurassic about 150 million years ago. A large rigid continent need not break into two clean pieces. Very often, the edges splinter. Several smaller fragments of continental crust are left isolated near the edges of the two continents.

The map below shows these continental splinters scattered in the Indian ocean as Madagascar and India broke apart and drifted away from each other.

Source: Lewis D. Ashwal, Michael Wiedenbeck, and Trond H. Torsvik 2017

Mauritius is part of a series of splinters that collectively are called Mauritia. These splinters were part of the Archean continental nucleus that made up Madagascar and the western Dharwar craton in south India.

Here is the interesting part that many news reports haven't touched on. Look closely at the map above. Trace the Carlsberg Ridge southwards. The Indian plate, which is drifting northwards, lies to the east of this ridge and the African/Somali plate to the west. Today, Seychelles and Mauritius is on the African/Somali plate and Chagos and the Laccadives on the Indian plate. But, when the initial separation happened about 84 million years ago, Seychelles and most of Mauritia were on the northerly drifting Indian plate. This is because 84 million years ago, the plate boundary between the Indian and African plates was formed by sea floor spreading in the Mascarene basin.

This is depicted in the  paleo-geographic reconstruction below. At 65 million years, the CIR or Central Indian Ridge is where sea floor spreading is forming the Mascarene basin. Seychelles and Mauritia lie to the east of this ridge on the Indian plate.

Source: Shankar Chatterjee et. al. 2013

Later, beginning around 62 million years ago and continuing up to about 41 million years ago, the loci of sea floor spreading jumped eastwards. The result was the formation of new plate boundaries between the Seychelles and Laxmi Ridge (62 mya) and between Mauritius and Chagos/Laccadives (42 mya).  These "ridge jumps", as they are called, formed the Carlsberg Ridge and  transferred Seychelles and Mauritius on to the African/Somali plate. Continued northward drift of India coupled with sea floor spreading and the formation of new oceanic crust along the Carlsberg Ridge has formed the broad oceanic basin of the Arabian Sea/ Indian ocean.

The process of continental breakup involves extensional forces that stretch and thin the crust. Fault movements cause a subsidence of crustal blocks. Many of the splinters at the edge of major continental margins are such thinned downfaulted blocks. They thus often get submerged under the sea.

Open Access

Tuesday, January 31, 2017

Book- The Lagoon: How Aristotle Invented Science

Currently reading- The Lagoon: How Aristotle Invented Science by Armand Marie Leroi, Professor of Evolutionary Developmental Biology at Imperial College, London. 

"His ideas flow like a subterranean river through the history of our science, surfacing now and then as a spring with ideas that are apparently new but are, in fact, very old. 

This book is an exploration of the source :  the beautiful  scientific works that Aristotle wrote, and taught, at the Lyceum. Beautiful, but enigmatic too, for the very terms of his thoughts are so remote from us that they are hard to understand. He requires translation: not merely into English, but into the language of modern science. That, of course, is a perilous enterprise: the risk of misunderstanding him, of attributing to him ideas that he could not possible have had, is always there.

The perils are particularly great when the translator is a scientist. As a breed we make poor historians. We frankly lack the historical temper, the Rankean imperative to understand the past in its own right. Preoccupied with our own theories, we are inclined to see them in whatever we read" .

Promising start! Will keep you posted...

Saturday, January 21, 2017

Book: Indica- A Deep Natural History Of The Indian Subcontinent

I am not doing a general book review of Pranay Lal's book Indica: A Deep Natural History Of The Indian Subcontinent. For that, I recommend this fine literate piece by Pratik Kanjilal published in the Indian Express. And Prabha Chandran writes about it in the Huffington Post. Both are aimed at the general reader.

No one has, as far as I know, written critically about the science content of the book. I read through the book and have some comments on the geology.

Before I start, let me say that I enjoyed this book. Pranay Lal has read widely, traveled far, and has had immersive discussions with geologists and paleontologists.  The best sections of the book are when he is writing about the many fossil finds preserved in Indian sedimentary basins and their importance in interpreting paleo-geography, ecology and evolution. He certainly appears more comfortable writing about these themes than he is about geology.

There are many problems with the geology writing. Some are easy-to-fix errors, while others will, in my opinion, require some rethinking on the more effective presentation of ideas and processes.

Let's begin with the easy to fix errors-

1) Page 12: Ref: Structure of the earth-  "The innermost shell of the "core" was composed of iron  and nickel and was surrounded by a larger but less dense mass of molten iron called mantle". - The mantle which is the layer of the earth between the crust and the core is not molten. It is solid and is made up of silicates and not iron. The core itself has two layers, a solid inner core and an outer fluid layer made up of iron and nickel.

2) Page 13: Ref: Age of corals in Rajasthan and Kutch- " This coral colonized the seas about 380 million years ago". There are no 380 million year old sedimentary rocks in Rajasthan and Kutch (Devonian Period). This may be a typo. There are Jurassic age corals in Jaisalmer. They are about 170 million years old.

3) Page 45: Ref: Banded Iron Formations- "Both ferric iron and ferrous iron began to settle as successive bands at the bottom of the iron-rich seas and lakes as oxygen levels fluctuated. Once, deposited, the layers hardened one above the other and gave the appearance of a layered cake- thin strawberry-jam-coloured striations of highly oxidized iron (ferric oxide, Fe2O3) and dark coloured chocolate lines of less oxidized iron (ferrous oxide, FeO)" - In the vast majority of Banded Iron Formations the strawberry coloured striations are forms of silica, either chert or jasper. The dark coloured layers are hematite or magnetite ( ferric oxide Fe2O3). Ferrous iron (divalent) is usually in a dissolved state. Ferric oxides or hydroxide minerals and compounds form following oxidation of this dissolved ferrous iron. Some ferrous iron is trapped in carbonate and sulphide minerals.

4) Page 57: Ref: Coral mineralization- "When hard-bodied marine animals like corals evolved (around 2 to 1.7 billion years ago)" - Multicellular animals originated in the Neoproterozoic likely between 700 and 600 million years ago and acquired hard parts (mineral skeletons) by around 550 million years ago.

5) Page 57: Ref: Limestone formation- " ..the vast accumulation of shell and coral got pressed together into minerals like calcite and aragonite" - organisms combine Ca and CO3 ions to precipitate minerals like aragonite and calcite to build their shells. This accumulation of shells when pressed forms limestone rock.

6) Page 59: Ref: Picture of Cruziana- " This 565 million year old fossil is of Cruziana, one of the earliest multicellular animals and an ancestor of the trilobite which lived in shallow seas". Cruziana is an ichnofossil. It is a name for an impression of a particular shape made by trilobites disturbing the sediment on the sea floor (bioturbation). Cruziana is not an ancestor of the trilobite, it is evidence of the presence of trilobites. These ichnofossils from Rajasthan are in Cambrian age rocks and so have to be younger than 542 million years.

7) Page 60: Ref: Evolution of complex multicellular organisms and animals - " About 570 million years ago, a few enterprising organisms developed a new reproductive strategy - sex! Sex opened up a plethora of possibilities"  -  Sex evolved once in the unicellular eukaryote common ancestor of fungi, plants and animals more than a billion years ago. The oldest fossil evidence of a sexually reproducing multicellular organism is the protist Bangiomorpha pubescens. It is 1.2 billion years old. Preserved filaments show differential spore/gamete formation. So, sex evolved hundreds of millions of years before the evolution of animals.

8) Page 62: Ref: Animal family relationships- Comb jellies and jelly fish "evolved to become thin, pin-shaped worm like creatures with no arms or legs that wriggled on the bottom of the sea floor". The author is saying the creatures with bilateral symmetry arose from Cnetophores (comb jellies) and jelly fish (Cnidarians). Animal phylogeny reconstructed by genetic analysis shows that Cnetophores are a group which diverged from the animal family tree very early in its history. And Cnidarians and Bilaterans are sibling groups. They share a common ancestor. See this easy to understand essay by Jerry Coyne.

9) Page 154: Ref:  Bedaghat and  Makrana marble- The author says that the famous marble cliffs of Jabalpur (Bedaghat) and the Makrana marble used to build the Taj Mahal are Cretaceous in age.  He writes that Cretaceous sediment made up of calcium carbonate shells were deposited between 145 to 65 million years ago and were cooked by volcanic heat, which transformed these sediments into marble.  However, both these marble deposits are Proterozoic in age.  Calcium carbonate sediments accumulated in seas that covered Rajasthan and Central India in Proterozoic times. These deposits were then metamorphosed into marble during orogenic activity that took place during evolution of the Aravalli mountains (Makrana marble) and in Central India (Bedaghat marbles). Estimates are that deposition and metamorphism into marble took place between 2 billion to 1.5 billion years ago .

10) Page 131- Ref: Mid ocean ridges- "Deep sea trenches on the sea floor are the weakest points on the crust, made up as they are of a thin layer of rock and water above it.. " He goes on to explain that these are the spots where magma melts the crust and flows on to the surface creating new oceanic crust. Technically though, the term "deep sea trench" refers to places where tectonic plates are converging and oceanic crust is subducting underneath another tectonic plate. Lal on the other hand is describing regions where tectonic plates are spreading apart and new ocean crust is being generated. Such places are called "mid oceanic ridges".

11) Page 183: Ref:  Vivekanand rock as meeting place on Gondwana continents - "Geologists call the Vivekanand Rock memorial 'the Gondwana junction' because it marks a place where India, Madagascar, Sri Lanka, East Antarctica were once joined together". The Indian continental crust extends underneath the sea beyond the Vivekanand islet. The continental shelf edge, tens of kilometers away from the present day shoreline, is really the place where India would have been joined to Australia and Antarctica on the eastern margin and Madagascar on the western margin.

12) Page 210, 216, 222: Ref: Magma chambers in Deccan Traps- The authors points out examples of columnar jointing in basalts and calls them remnants of magma chambers. This is incorrect. Magma chambers are present several kilometers below the surface of the earth. If magma solidifies at this depth is won't be called a basalt (it will be called gabbro) and won't develop columnar jointing. These instances the author point out are either thick lava flows or volcanic plugs which have developed columnar jointing on account of cooling and shrinkage. Volcanic plugs are remnants of lava which solidifies in a volcanic vent at the surface.

13) Page 280: Ref: CO2 released by volcanism- " Most of the volume of CO2 in the atmosphere actually comes from volcanism and sea floor spreading. When sea floor spreading occurs, sediments on the ocean floor (including these shells) are dragged deep under the ocean floor where they heated and the trapped CO2 is released". At sea floor spreading centers volcanism releases CO2. Sediments and shells are dragged under the ocean floor at the other end of the plate at subduction zones. As they dive under, they get heated and release CO2.

Some longer discussions:

a) Page 18- Ore deposit formation- The author writes that collisions of meteorites during the early history of the earth up to 2.5 billion years ago kept puncturing the earth's crust releasing metals like iron from as deep as the core. These metals clumped together to form ore bodies. The early earth did go through a period of very heavy meteorite bombardment from 4.1 billion years ago up to 3. 8 known as the "Late Heavy Bombardment". Bombardment continued sporadically after that. No crust from this very early period is preserved as it kept getting smashed and recycled into the interiors. Any ore deposits that formed have also been destroyed.

The earth was much hotter then and after the easing of bombardment, intense magmatism from 3.8 billion to 2.5 billion years ago started forming the first continents. Geologists estimate that nearly 65% -70% of the present volume of the continental crust formed during this phase. The magmatism transferred metals from the mantle to the newly formed crust. 

A recent survey of  five years of research from 2011 to 2016 done by Indian geologists on ore deposits shows that not a single study invokes meteorite bombardment as the cause for ore concentration. Instead, internal forces like subduction zone magmatism, rift magmatism, hydrothermal circulation systems and near surface sedimentary processes are inferred. Now, there may be specific instances where meteorite impacts may have fractured the crust and initiated fluid circulation, but meteorite bombardment is a not a general explanation of metallogeny.

b) Chapter 7- Page 182 and subsequent pages- Pranay Lal discusses the breakup of Gondwanaland. How do continent breakup and what is the force that causes tectonic plates to move and drift for thousands of kilometer? He invokes volcanic eruptions as the cause of supercontinent breakup and the push exerted by magma upwelling through cracks as the force driving plate movement. He refers to a paper by Shanker Chatterjee and colleagues on the subject of India's epic northward journey after it broke up from Gondwana until it collided with Asia.

But an alternate view among geologists is that volcanic eruptions are the consequence of the break up of continents. And plate motion is driven not by the push of upwelling magma/lava but by the pull of cold dense lithosphere which sinks deep into the mantle at subduction zones. A perusal of the paper by Shanker Chatterjee shows that these scientists agree with this "slab pull" notion as the main force of plate motions. Mid oceanic ridge push is a secondary force. Unfortunately, the author does not even mention the slab pull force mechanism.

So, continents break up due to a variety of factors. Indeed, there could be an anomalous build up of heat underneath the continent, which thins and weakens the lithosphere (the rigid plate consisting of the crust and the upper part of the mantle). Hot buoyant mantle impinges the underside of the plate. At this point the mantle is still solid but can flow like silly putty. Continued stretching and thinning of the crust (caused by the slab pull force from a subduction zone at the other end of the plate) results in the underlying mantle decompressing. This results in the lowering of its melting point and magma generation. Magma rises through the fractures of the thinned crust and erupts on the surface.

In this view, volcanism did not prise apart fragments of Gondwanaland one by one. Rather, enormous episodes of volcanism like the Deccan Volcanic Event were triggered by rifting and Gondwana continents moving above anomalously heated portions of the mantle known as hot spots.

c) Page 261: Ref: Himalaya. Here is how Pranay Lal describes the rise of the Himalaya. "The Himalaya rose from below. The rubbing together of the immense plates and the monumental crushing and buckling of land produced a tremendous amount of heat and cause magma from below to ooze out of deep fissures which opened up on the surface. This melted and remelted granite, and pushed it upwards to the surface. As the granite slowly cooled, successive batches of molten granite thrust their way up,forcing the older granite slabs higher. Over time, this process created a pedestal for mountain building. Because the "cooking" process varied (different types of granite are cooked at various depths), the densities of rock slabs differed. This created large cracks or "faults" along places where the continental crust rasped, grinded and pushed slowly onward".

I didn't understand this at all.

Later he says that the Everest is made up of an initial four thousand odd meter foundation of granite overlain by another 3100 meter of sedimentary rock. The granite is 50-30 million years old while the sedimentary rocks are from the Paleozoic era (359 - 252 million years old).

During continental collision, there has been melting of the lower parts of the crust and this terrain has been intruded by pods and lenses of younger granite. Metamorphosed and partially melted Precambrian rock is the main component of the Greater Himalaya. In the Everest -Lhotse-Nuptsu region the granite intrusions are on a massive scale as described by Pranay Lal. These thick intrusive granite and high grade metamorphic rocks make up the base of Everest region. But, these younger granite intrusions are not this thick everywhere. They are present on a smaller scale along certain bands of the Greater Himalaya and are almost absent from the Lesser Himalaya.

Though the author may not mean it, phrases like "successive batches of molten granite thrust their way up, forcing the older granite slabs higher" may be misinterpreted by lay readers to mean that Himalayas formed as a result of magma pushing the crust up to form mountains. This is not how orogenic mountains like the Himalaya form.

As the Indian Plate collided with Asia it delaminated. You can think of this as the plate splitting into two tiers. The lower tier comprising the lower crust and upper mantle slid under Tibet. The upper tier impinged into Tibet and got squeezed, deformed and thickened. The Himalaya is this folded and faulted upper tier. The different Himalayan ranges are slices of the upper tier Indian crust stacked one on top of the other by a series of south moving thrust faults.

 The tectonic structure of the Himalaya with its geological divisions is summarized in the graphic below.

Source: Shankar Chatterjee et. al. 2013

What was the sequence of these thrust faulting events and how do they fit into the three pulses of mountain building that Pranay Lal mentions?  

Leaving the Tibet part aside, the Himalaya most people are familiar with are made up of four distinct geological terrains. I am listing them starting from the north and going south.  Tethyan sedimentary rocks (the ones making up the Everest and many other summits; These sediments  range in age from the Neoproterozoic to the Eocene- ~ 1000 million years to 40 million years, although the entire sequence is not exposed at one place), the Greater Himalaya Crystalline Complex (Proterozoic to Early Paleozoic, 1800 million years to 480 million years, with a younger imprint of metamorphism and granite intrusions), the Lesser Himalaya Sequence (Proterozoic to Cambrian; gneiss and low grade metamorphosed sediments, 1850 million years to 520 million years) and the Siwaliks which are Cenozoic sedimentary rocks deposited from around 15 million years to about 0.5 million year ago. The geological divisions roughly match up with the topographic divisions of the Greater Himalaya, the Lesser or Middle Himalaya and the Outer or Sub Himalaya.

The northern edge of the Indian plate was made up of  Proterozoic rocks, much as it is all across Peninsular India. This Proterozoic sequence was overlain by Paleozoic and Mesozoic sedimentary rocks. There is a more complete sequence of Paleozoic sediments in the Himalaya, since even though most of India was landlocked as part of Gondwanaland, the north edge of what was to become India was open to the Tethys sea all through the Paleozoic and Mesozoic.

As India collided with Asia:

1) Its continental crust impinged on the continental crust of Asia. The Neoproterozoic-Phanerozoic sedimentary cover was folded, faulted and scraped off and uplifted to form an early mountain range made up of the Tethyan sediments.

2) Horizontal shortening of the Indian crust during collision led to crustal thickening and rocks were subjected to high temperatures and pressures. They were metamorposed and partially melted into rocks known as migmatites and intruded by granites. Finally, compressive stresses broke the crust along a major fault known as the Main Central Thrust and uplifted this deeply buried terrain. The thrust moved crustal blocks upwards and southwards. Some geologists believe that the Great Himalaya Crystalline Complex is made up of hot soft rocks from the middle regions of the Indian crust which flowed towards the surface in response to the removal of  crustal cover by erosion. This ductile flow of rock from deep in the crust towards the surface is termed "channel flow" as hot soft rock is confined to a layer or channel between colder upper crust and a more rigid upper mantle.

Either way, with this thrusting and extrusion of high grade rock began the formation of the Greater Himalaya. The main activity of the Main Central Thrust is dated to between 16 million years to 25 million years or so. At about the same time the earlier uplifted Tethyan sediment detached themselves from the underlying crystalline basement and started sliding northwards along a major fault system known as the Southern Tibetan Detachment System.

3) As India continued to press into Asia, compressive stresses propagated southwards. Beginning around 16 million years to 11 million years, the terrain to the south of the Main Central Thrust began to get folded and faulted. Since it was further to the south from the collision zone, it did not experience the high levels of metamorphism and granite intrusions that the rocks of the Great Himalaya did. Eventually, these more distal rock formations were uplifted and moved southwards along the Main Boundary Thrust and associated faults to form the Lesser Himalaya.

4) The rise of the Greater Himalaya and the Lesser Himalaya loaded and depressed the crust in front of them in to a moat. In the alluvial plains, streams and lakes that formed were deposited sediments eroding from the rising Greater and Lesser Himalaya. These were the environments in which a lot of the mammalian evolution and diversification described in an earlier chapter took place. Beginning around a million years ago, maybe a little earlier, these sediments were folded, uplifted and thrust above the Gangetic alluvium along the Main Frontal Thrust to form the Siwalik ranges. The Main Frontal Thrust is still active, and Himalaya earthquakes which originate deep underground rupture along this fault plane. The Himalaya are growing southwards.

I am not writing a popular book for the lay public.  I realize I may have gone overboard with my Himalaya explanation and am not suggesting that Pranay Lal should include all this in his book. But any explanation should include at least the basic arrangement of the different lithologic terrains and their sequential uplift due to south progressing thrust faulting.

d)  Page 218 - Ref: Satpura mountains were uplifted due to the rise and push of magma leading up to the Deccan volcanism.- This is also a longer discussion but I'll stop on the geology aspects since this post is already too long. Let me refer to an article on the lack of pre- Deccan volcanic uplift in the Satpura region and elsewhere.  Many geologists have concluded that the uplift of the Satpura belt is not due to the push of magma. It occurred much later in the Cenozoic due to the various stresses on the Peninusular Indian crust.

e) I couldn't help elaborating on this:  Why do animals grow large? Pranay Lal mentions an intriguing evolutionary pattern seen in the fossil record. There is a trend towards an increase in body size in the early to mid Triassic following the Permian mass extinction. A second trend in increase in body size in seen in the Cretaceous when some lineages of dinosaurs evolved gigantism. The explanation given is that an increase in atmospheric oxygen levels favored an increase in body size.

Animals have a physiologically demanding lifestyle. That a certain threshold of oxygen will be required for them to prosper over the longer term is a given. It is too broad an explanation and it doesn't tell us why in a group one lineage evolved towards a larger size while a related lineage did not. In the two time periods that author points to, the reason why species evolved towards large size in the Triassic immediately after the Perman mass extinction may be different from why certain lineages evolved gigantism in the Cretaceous.

Let's take the case of body size trends during the early Triassic. Mass extinctions disproportionately cull larger bodied species . Since environmental condition are deteriorating rapidly, larger bodied species with  more energy expensive demands and slower reproduction rates cannot cope as well as species with a smaller body size. Survivor species in the aftermath of mass extinctions are small bodied, maybe as small as their biological limits. From this starting point, even if environmental conditions in the post mass extinction period are neutral in terms of favoring species with a particular body size, the only trend that will emerge is one towards a larger body size, since there is no room to get any smaller.

For Cretaceous too the oxygen hypothesis seems too pat. I could argue that Cretaceous was a time of high carbon dioxide levels. That would mean more food for plants. A lush healthy vegetated landscape means more food for herbivores, conditions favorable for evolution of larger size. Again this is a "just so story". Why did gigantism evolve in the Sauropoda? The answers may lie in phylogenetic heritage i.e. inheritance of ancestral characters which fortuitously proved advantageous, and evolutionary innovations that enabled them to acquire and utilize resources more efficiently than other groups. An avian style respiratory system enabled pnematization (air cavities in bones) of the axial skeleton. This evolved early in Sauropod history. A small head evolved because food was ingested without mastication. These two features enable a long neck to evolve (lighter head, lighter skeleton). A long neck enabled access to food not available to other herbivores.. a cascade of benefits due to inherited and newly acquired features.

Off course, these are ramblings about things that interest me. I have no expectation that Pranay Lal put all these details in his book.  

But, we need books like these to fire imaginations and inspire amateurs and students to go out and explore India's rich physical landscape. A good place to start will be the National Geological Monuments listed by the Geological Survey of India. A surge of visitor interest might put pressure on the government to expand protection to more sites of interest. The destruction of irreplaceable fossil sites and geological structures is a constant theme in Lal's book.

A second edition of this book will be welcome, but one that has gone scrutiny by a discerning geologist editor.

Wednesday, January 11, 2017

Trilobite Reproductive Biology: Insights From Pyritized Fossil Eggs

The delicacy of mineral replacement and the serendipity of finding something so small and fragile. This is spectacular.

Pyritized in situ trilobite eggs from the Ordovician of New York (Lorraine Group): Implications for trilobite reproductive biology - Thomas A. Hegna, Markus J. Martin and Simon A.F. Darroch

Despite a plethora of exceptionally preserved trilobites, trilobite reproduction has remained a mystery. No previously described trilobite has unambiguous eggs or genitalia preserved. This study reports the first occurrence of in situ preserved eggs belonging to Triarthrus eatoni (Hall, 1838) trilobites from the Lorraine Group in upstate New York, USA. Like other exceptionally preserved trilobites from the Lorraine Group, the complete exoskeletons are replaced with pyrite. The eggs are spherical to elliptical in shape, nearly 200 μm in size, and are clustered in the genal area of the cephalon. The fact that the eggs are smaller than the earliest-known trilobite ontogenetic (protaspis) stage suggests that trilobites may have had an unmineralized preliminary stage in their ontogeny, and that the protaspis shield formed only after hatching. The eggs are only visible ventrally with no dorsal brood pouch or recognized sexual dimorphism. The location of the eggs is consistent with where modern female horseshoe crabs release their unfertilized eggs from the ovarian network within their head. Trilobites likely released their gametes (eggs and sperm) through a genital pore of as-yet unknown location (likely near the posterior boundary of the head). If the T. eatoni reproductive biology is representative of other trilobites, they spawned with external fertilization, possibly the ancestral mode of reproduction for early arthropods. Because pyritization preferentially preserves the external rather than internal features of fossils, it is suggested that there is likely a bias in the fossil record toward the preservation of arthropods that brood eggs externally: arthropods that brood their eggs internally are unlikely to preserve any evidence of their mode of reproduction.

Thursday, January 5, 2017

Photomicrograph: Authigenic Feldspars From The Neoproterozoic - South India

Feldspars (plagioclase and alkali feldspars) are the most common minerals in the earth's crust. The vast bulk of them crystallize out of magma and lava. Feldspar also forms during metamorphic reactions. In sedimentary rocks they are commonly seen as detrital grains in sandstones. What is less appreciated is that they can also grow de novo in sediments during diagenesis i.e. during chemical reactions that take place as loose sediment reacts with fluids and gets transformed into rock.

  Authigenic twinned euhedral feldspar cross cutting mud clast

I noticed some lovely examples of such diagenetic or authigenic feldspars from the Neoproterozoic Banganapalli Formation from the Cuddapah Basin in South India during my M.Sc. dissertation project work. I recently got a chance to photograph my old thin sections again and I am posting some more photomicrographs of these authigenic feldspars below.
The Banganapalli Formation also termed the Banganapalli Quartzite is made up mostly of conglomerates and sandstones. They rest with an unconformable contact on the Paleoproterozoic Tadpatri Shale. There is spatial variability in the composition of the Banganapalli sediments. In my study area south of the village of Gani in Andhra Pradesh, the conglomerates and sandstone interfinger with limestones. These limestones appear a light purple in outcrop and are made up of carbonate mud with intermittent conglomerate layers and lenses of quartzite and jasper pebbles and cobbles, thin bands and layers of quartz sand along with limestone and siliciclastic mud intraclasts often showing a chaotic fabric (left). Fine clay layers are dispersed through the succession.

Near the contact between the Banganapalli limestones and the overlying Narji Limestone is an  intraclast conglomerate layer (right) made up of carbonate and siliciclastic intraclasts indicating rapid lithification of the sea floor and the subsequent disruption of hardened sea floor crusts during storms and seismic events.

The authigenic feldspars are present in this Banganapalli limestone succession. Feldspars are euhedral (well formed facets) and show contact twinning (separate crystals grow symmetrically forming mirror images across a common plane). They contain inclusions of calcite and clay. Mineral composition studies have shown that authigenic feldspars are either albite (sodium alumimium silicate) or orthoclase (potassium aluminium silicate). At that time (in the late 1980's)  I did not have access to an electron microprobe to accurately ascertain the composition of these feldspars. The twinning exhibited by these feldspars suggests to me that these are albite.

The feldspars cross cut intraclasts (arrow)

and calcite veins (arrow)

They grow in the carbonate mud matrix (arrow)

When did they form?

Feldspar crystals cross cut fractures and veins filled with calcite. This implies that they formed after lithification of the sediment. This could have occurred very early in the sediment history. Sea floor cementation and lithification of carbonate is commonly observed from the Proterozoic, which had calcium carbonate supersaturated oceans. The presence of intraclast layers with chaotically oriented clasts and cracks filled with detrital silica pebbles and sand indicate early lithification and disruption of sea floor. Calcite veins may simply suggest hardening and breakage of lithified sediment layers.  Sea water percolating through cracks in this early lithified sediment would have supplied sodium to the growing feldspars.

Alternatively,  the feldspars formed later under burial conditions. The Banganapalli Formation is made up of immature sandstone bodies containing plagioclase and alkali feldspar detrital grains. They show signs of dissolution and corrosion during diagenesis (in image below arrows point to partially dissolved feldspar grains).

Alkali released from the dissolution of detrital feldspars was transported by groundwater flow and used up in the growth of authigenic feldspars in adjacent limestones.

I'll leave the question open.

There are other interesting diagenetic features too in these sediments. The siliciclastic mud intraclasts show alteration to chlorite and glauconite and there is extensive neomorphic recrystallization of carbonate mud.

Authigenic feldspar are reported from sandstones too. They usually occur as tiny overgrowths on detrital feldspar grains. They are less well known from limestones. So, these unusually large euhedral authigenic feldspars stole the show for me.

Finally, the satellite image below shows the location of the limestone layers (black arrow) containing these authigenic feldspars. They occur on the south dipping limb of the Gani-Kalava anticline near the town of Nandayal.