Sunday, November 11, 2018

Stalactites And Other Calc Tufa Deposits Along Bageshwar Shama Road, Kumaon Himalaya

Traveling from Bageshwar to Shama, in Kumaon Uttarakhand, I came across a wondrous calc tufa deposit about a kilometer south of Kapkot village.

 The map below shows Bageshwar and Kapkot along Route 37. (Permanent Link).

Calc Tufa are calcium carbonate deposits which form on land in a subaerial environment. They are made up of the minerals calcite and, less commonly, aragonite. The most familiar of calcium carbonate deposits are sea floor and beach accumulations of shells and skeletons of marine organisms. Upon burial and hardening they turn into limestones. In the Proterozoic, before animals evolved the ability to biomineralize, vast thicknesses of limestones formed in the oceans by inorganic and bacterially mediated precipitation of calcium carbonate. Limestones that form in saline as well as fresh water lakes are also known.

Calc Tufa forms in the vicinity of springs, waterfalls, along river banks, caves and along hill slopes. They have a chalky texture, porous fabric and organic looking shapes. This is a result of calcium carbonate encrusting microbial, algal and moss colonies that inhabit these settings. Associated with these porous friable looking forms are more denser crystalline deposits. These are stalactites and various types of laminated and globular crusts. They are collectively called speleothems. They form generally in a cave setting by abiogenic precipitation from thin films of supersaturated water. This particular deposit containing both tufa and speleothems was along a steep hill slope with large cavities. The substrate rocks are the Mesoproterozoic age Deoban limestone and dolostones (made up of mineral dolomite). They are estimated to be around 1.5- 1.6 billion years old.

All along the exposure the rocks were shattered by prominent fracture zones. Rain water is weakly acidic. As it falls and moves through the cracks and fractures in these rocks it dissolves the minerals calcite and dolomite, becoming enriched in dissolved carbon dioxide (CO2) and calcium.  The partial pressure of CO2 (a measure of dissolved CO2 concentration) in this groundwater is more than the partial pressure of CO2 in the atmosphere. When groundwater enters a cave or emerges on a hill slope as a spring discharge, the lower partial pressure of CO2 in this open setting causes a degassing of CO2 from the groundwater. This results in the pH of the water to increase slightly, which in turn causes supersaturation of calcium carbonate in solution. Precipitation of calcium carbonate then begins on the cave walls and roof and on the hill slopes. It is possible that removal of CO2 by microbial photosynthesis may also be playing a role in triggering precipitation.

These tufa deposits occur at many places along the Bageshwar to Shama road. We finally stopped for a closer look at a largish looking deposit about a kilometer south of Kapkot. This was strictly road side geology on my part. We spent about half an hour at the deposit and so I am not presenting any detailed analysis or insights regarding this feature.

This is a complex deposit made up of varied types of tufa. We managed to photograph some beautiful calc tufa morphologies which I am posting below. My thanks to Pushkaraj Apte ( @pushkarajapte ) for contributing many of the photographs.

Lets get an idea of the size of the deposit. That's me, standing in front of the large cavern. You can see stalactites in the background.

 A peek inside the large cavity. It is about 3 meters in height and about 4 meters in width. I could have easily stood inside it. However, I did not enter it, fearing I would break some delicate mineral deposits which have formed on the floor of the cave.


Stalactites 1: The most striking of the formations are these stalactites. They range from thick columnar forms (1) which are more than a meter in length to smaller centimeter long thin delicate drips (2). The cave is damp. There is a thin film of water covering these columns suggesting ongoing mineral precipitation and growth of the stalactites. The floor of the cavern was also encrusted with deposits and partially covered with tufa debris.

 Stalactites 2: Along the hill slopes, exposed Deoban carbonate strata form ledges. Stalactites are growing on the undersides of these ledges. The bigger ones are about 1-2 feet in length.

 Botryoids:  At places botryoidal clusters (cave grapes) are seen. These hang from the roof (1) and accrete away from walls (2). They form by either radial or concentric growth of calcite (or aragonite) from a nucleation site. Each botryoid is about a centimeter or so in diameter.

Thin Platy Crusts: These thin (cm scale)delicate layers likely form in shallow films or pools of stagnant water on the floor of the cavity.

Flowstones?: These banded crusts  have formed on a slope from flowing water and likely represent abiogenic precipitation of calcite (flowstones). Alternatively they could be stromatolitic crusts formed by precipitation of calcite atop microbial sheaths and mats.

Calc Tufa:

Phytohermal Tufa: These are calcified moss deposits (a foot or so in height) which are formed on the floor of the cavity. They preserve the bushy morphology of the moss colonies. Calcite encrusted and eventually entirely replaced the moss colonies, turning them into fossilized organic structures.

Microhermal Tufa or Phytohermal Tufa: The thin tube like structures (few cm in length) of this calc tufa deposit suggests that it formed by mineral encrustation of filamentous algae or bacterial colonies. However, I cannot be sure. This too could be a moss colony.

Spongiform Tufa: Massive looking with dispersed holes. Such structures from by mineral encrusting organic matter (moss, microbial mats) draping the hillsides. The open spaces between the organic matter and decay of vegetation gives the deposit a sponge like texture. Some larger cavities (about 6 inches across) are lined with layered mineral deposits.

I found this broken piece along the road side next to the deposit. It is made up of small globular aggregates and columns which have accreted upon a substrate of spongiform tufa.

In this transverse section you can see clearly the calcium carbonate layers that have built up the column.

A cross section of the larger stalactites will also reveal its layered nature. Stalactites with such growth layers are of importance in reconstructing past climates. The oxygen in the calcite (CaCO3) provides the clue. Variations in the ratio of the two isotopes of oxygen (O18/O16) which are bound up in calcite are indicators of differences in the strength of rainfall. The lighter isotope (O16) is preferentially retained in the vapor phase. During phases of weak monsoons or drought, rain becomes enriched in the heavier isotope (O18). Calcite layers precipitated from this water will be enriched in the heavier isotope. In contrast, during strong monsoon phases, rain and groundwater becomes relatively enriched in the lighter isotope. As a result, calcite layers will inherit a 'lighter' oxygen isotope signal.

For the Indian subcontinent, reconstruction of the past variability of Asian monsoons going back hundreds to thousands of years, are based on precious few data points, spread rather sparsely across India. Recently, Gayatri Kathayat and colleagues published a study of Indian monsoon history over the past 5700 years based on the oxygen isotope record of cave stalactites from Sahiya in Uttarkhand, located about 200 km WNW of where we were. Judging by the size of some of the stalactites, I am guessing that deposition at this Kapkot site has been going on for a few hundred years at least. I wonder if this deposit can be a new paleo climate data source.

I did have another intriguing thought. Is the profusion of calc tufa deposits along road cuts in this region just a coincidence? Is it possible that blasting and cutting the hill side for building the road enhanced fractures and triggered collapse of blocks, resulting in the formation of caverns, and creating conditions favorable for calc tufa precipitation?   If so, then this deposit may be at most a hundred years old. Wild!

Sunday, November 4, 2018

Landscapes - Gogina To Munsiyari

I just finished a fabulous trek in the Kumaon Himalaya, Uttarakhand. We started from Gogina village. The route took us to Namik, then northwards along some high ridges towards Sudamkhan Pass. The plan was to turn eastwards at Sudamkhan Pass and walk along a high shepherd's trail towards Khaliya high ridges, and then descend towards Munsiyari. But some very nasty weather forced us to turn back from Sudamkhan. We then took a lower altitude route southeastwards, and after a few days walk ended up just near Birthi Falls roadhead. A taxi picked us there and took us on an hour long drive to the town of Munsiyari.

The map below shows the location of Gogina, Namik, Birthi and Munisyari.

We climbed from around 5000 ft starting at Gogina to a maximum of around 12, 500 feet near the Sudamkhan area. After Namik there was no village until  we reached Birthi, and so we passed through some glorious isolated wilderness areas, ascending from temperature broadleaf forests to alpine tundra like environs made up of grasslands and meadows and up to more bare rocky heights.

The terrain is made up of medium to high grade metamorphic rocks of the Greater Himalaya Sequence. I noticed amphibolites, mica, garnet and feldspar gnessises, mica schists, phyllites, calc silicates (metamorphosed clay bearing limestones) along the way. However, the pace of the trek required me to keep walking along with my friends... so I did not do much geology on this trek.

Below are some pictures I took during the trek.

1) A view of the Lesser Himalaya from Gyan Dhura village (before we reached Gogina).

2) A house in Namik Village

3) So many geological stories written in to the lovely building stones of Namik Village

4) We climbed above Namik Village and set up camp in this lovely meadow.

5) Walking through a lush forest above Namik

6) Above the tree line, a trail leads to a shepherd's lonely outpost.. route towards Sudamkhan Pass.

7) The desolate yet hauntingly beautiful landscape near Sudamkhan Pass.

8) That's me, taking in the stunning surroundings near Sudamkhan Pass.

9) Gneisses of the Greater Himalaya Sequence exposed along the high bare ridges.

10) At Bajemania meadows after we descended from the Sudamkhan area. Watching an afternoon storm build up in a distance.

11) Our pack horses enjoying a meal as fresh snow cover the higher slopes.

12) Autumn colors glow in the late evening sun.

13) The majestic Panchachuli Range at sunset. View from Munsiyari.

.. with promises to keep traveling in the Himalaya.

Wednesday, October 17, 2018

Dessication Cracks In Mars Lake Bed

This Comment and Reply published in the August 2018 issue of Geology is worth reading.

Desiccation cracks provide evidence of lake drying on Mars, Sutton Island member, Murray formation, Gale Crater: COMMENT Brian R. Pratt

Desiccation cracks provide evidence of lake drying on Mars, Sutton Island member, Murray formation, Gale crater: REPLY N. Stein; J.P. Grotzinger; J. Schieber; N. Mangold; B. Hallet; D.Y. Sumner; C. Fedo

The argument concerns the origin of polygonal shaped ridges found on the surface of mudstones deposited in a Martian lake. These millimeter to centimeter high ridges made of sand were interpreted as having formed by sand filling in dessication cracks that form on the surface of a drying lake bed. The alternate view argued is that the sand was injected into cracks formed during seismic events taking place on early Mars.

New about Mars is usually dominated by grand questions about evidence for extraterrestrial life. But scientists inch towards answering such bigger themes by working the nitty gritty. In this case, grit filling up cracks in a mudstone. Such a debate may seem arcane, but understanding these details matter. They form small but essential building blocks of a knowledge base, incrementally adding to the larger picture.

Mineralogy Of The Earliest Animal Shells

Carbonate sedimentology and evolution. Two of my favorite subjects converge in this interesting study published in the September 2018 issue of Geology.

Calcium isotope evidence that the earliest metazoan biomineralizers formed aragonite shells-
Sara B. Pruss; Clara L. Bl├Ąttler; Francis A. Macdonald; John A. Higgins

Ediacaran Cloudina and Namacalathus are among the earliest shell-forming organisms. The debated carbonate phase of their skeletons, high-magnesium calcite or aragonite, has been linked to seawater chemistry and pCO2, yet independent constraints on the original mineralogy are lacking. We present a new method to distinguish primary skeletal mineralogy using ╬┤44/40Ca values and trace element compositions of the skeletons and associated cements. Ca isotopes are useful because they are relatively insensitive to diagenetic alteration during burial, and they vary with the primary mineralogical phase of carbonate. We applied this method to microdrilled carbonate and cements associated with both Namacalathus and Cloudina skeletons from the Ediacaran Omkyk Member of the Nama Group in southern Namibia. These data demonstrate that both organisms originally produced aragonitic skeletons, which later underwent diagenetic conversion to calcite. We suggest that calcium isotopes can be used to further constrain unknown skeletal mineralogies through time and to reassess the relationship between seawater chemistry and the mineralogy of biocalcifiers.

Different animals groups acquired the ability to precipitate hard protective shells made up of calcium carbonate at different times between the latest Neoproterozoic (~ 580-541 mya) to early Paleozoic (541 - ~ 450 mya).  What is the larger picture of the evolution of the biomineralization in different animal groups and the mineralogy of the skeleton? Through geologic time the chemistry of sea water has oscillated from that favoring the precipitation of aragonite and high magnesium calcite to that favoring the precipitation of low magnesium calcite. From Late Neoproterozoic to Middle Cambrian sea water chemistry favored the precipitation of aragonite and high Mg Calcite. Animal groups which evolved skeletonization during this time largely adopted aragonite to build their skeletons. Animals which first evolved biomineralization in the Late Cambrian to Ordovician times, during the time of calcite seas, adopted calcite skeletons.

In most animal groups, skeletal mineralogy was conserved even when sea water chemistry changed later in history. There have been only rare instances of animals changing the mineral phase used to build its skeleton. I wrote about one such instance in the Mesozoic when some groups of molluscs switched from  aragonite to calcite. The most prominent example is from the Hippuritoidea (rudist) bivalves. The switch to calcite shells seems to have triggered an evolutionary diversification beginning in the Late Jurassic. By Late Cretaceous times rudist bivalves were so abundant that they displaced corals (which built their skeletons with aragonite) as the chief reef builders in the shallow shelf environments.

Fascinating topic!

Saturday, September 15, 2018

Geology Photo Contest

The Centre for Education and Research in Geosciences, a geology outreach group based in Pune, is organizing a photo contest. Entry is open to all. The selected entries will be exhibited during their annual Geology Week event to be held in Pune from October 8 to October 12. Last date for entry in September 21 2018.


.. and go to this webpage for details regarding the photo contest: Geology Photo Contest

Please share widely!

Tuesday, September 11, 2018

Before The Himalaya: Story Of A Late Cretaceous Subduction Zone

My article on the geology of the upper catchment of the Brahmaputra River in southern Tibet has been published in The Wire Sciences. This region is technically called a suture zone. It contains the remnants of the Tethys Ocean floor made up of basaltic oceanic crust and overlying deep sea sediments. These rocks were deformed and uplifted during the India Asia collision.

I focus on a paper on the Jiachala Formation by Hanpu Fu and colleagues published in a recent issue of Science China Earth Sciences. They use a technique known as detrital zircon geochronology to resolve a long standing problem about the age of that sedimentary deposit. In my article I explain how this technique works. I also elaborate on the broader story these deposits tell us about plate tectonics and the beginnings of the Himalaya.

An excerpt:

In the Cretaceous Period, the Indian plate, which had been moving northwards since the breakup of Gondwanaland, was approaching the Asian continent. The southern edge of the Asian continent was lighter continental crust, whereas the leading part of the Indian plate was denser oceanic crust. As a result, in the zone where the the two plates converged, the denser Indian plate slid below the Asian plate, forming a subduction zone.

As the Indian lithosphere sunk deeper into the mantle, it heated up and released water trapped in sediments and hydrated oceanic crust. This water penetrated the overlying Asian plate, lowering the melting point of its rocks and triggering magma generation. This buoyant magma rose through the Asian continental crust. Some of it reached the surface, resulting in extensive volcanism. The rest solidified in the subsurface, forming giant bodies of granite known as batholiths.

Such terrains have the grandiose name of magmatic arcs. The town of Leh and the surrounding settlements in the Indian region of Ladakh are situated partly on a magmatic arc.

Two sedimentary basins since developed south of this arc. Immediately adjacent to the magmatic arc was the forearc basin. A deeper depression, known as the trench, formed further away on the Indian oceanic lithosphere, at the junction where the Indian plate had slid under the Asian plate. Both were receiving sediments derived from the erosion of the Asian continent.

During subduction, slices of the Indian plate were scraped off and thrust to the surface. Such fault-bounded piles of sediment and oceanic crust are called accretionary wedges, and they, along with a chain of oceanic volcanoes  that formed to the west in the region between Ladakh and Kohistan, would have been the first island ranges formed in the Tethys Ocean between India and Asia.

 Read the complete article here.

Saturday, September 8, 2018

Geoscience Education Woes In India

Dilip Saha of the Indian Statistical Institute, Kolkata,  has written an editorial in Current Science on the many problems with geology education in India.

He identifies a lack of attention to field work and the quality of teachers as the two major weaknesses that need correction.

I agree with many of the points he has made. I had a very poor quality field training experience during my Master's education at University of Pune (now Savitribai Phule Pune University). That was somewhat compensated for by some very good classroom teaching. Across State Universities and local colleges, the quality of teaching suffers not just because subject experts are not up to the task, but because many departments are understaffed and don't have subject experts.  Often, just two or three faculty end up teaching all the subjects.

I will also add that besides the obvious improvements in field courses, teacher quality and pedagogy, a module on research ethics is desperately needed. This is not a geology specific issue. Plagiarism is a big problem in Indian academia. I occasionally mentor students from local colleges. I have found out, to my dismay, that copying and pasting material from a research paper in to one's thesis seems to be commonplace. Students don't even realize that they are crossing serious ethical lines.

Open Access.

Thursday, August 23, 2018

Proterozoic Upper Vindhyan Succession: Fossils And Age

I came across a remarkable paper by S. Kumar published in the December 2016 issue of The Palaeontological Society of India. It is an overview of all the published reports of megafossils from the Proterozoic age Vindhyan Basin of Central India. The findings read like a special issue of Retraction Watch. I am not implying any fraud or scientific misconduct (neither does S. Kumar), and the papers in question have not been retracted either. Rather, he concludes, based on a rereading of earlier reviews and new analysis, that there has been a widespread misinterpretation of data. Practically all megascopic features interpreted variously as trace fossils (burrows, trails, scratch marks), impressions and body fossils are not fossils. They are abiotic in origin. Only some carbonaceous megafossils represent the remains of large microbial communities.

The map below shows the outcrop pattern of the Vindhyan Basin.

Source: Candler C. Turner et. al. 2013

..and this figure summarizes our evolving understanding of the age range of the Vindhyan sedimentary succession.

Source: Geoffrey J. Gilleaudeau et. al. 2018

The most sensational and controversial of fossil reports from the Vindhyans are from the lower part of the succession. In the late 1990's and continuing in the early 2000's, Dr. Azmi, a researcher from the Wadia Institute of Himalaya Geology, reported microscopic embryo like globules,  rod like and filamentous microfabrics and oddly shaped mineral fragments. These fossils were found in the Rohtas Limestone and the Tirohan Dolomite of the Semri Group. He interpreted some of the globules to be animal embryos and the odd fragments as 'small shelly fossils', essentially bits and pieces of shells of marine animals. Based on this identification he claimed that the rocks were Late Neoproterozoic to Early Cambrian in age (~ 650 Ma to 520 Ma; Ma = mega annum = million years). This claim though has never gained wider acceptance.

First, since his discovery, improving geochronology of the Lower Vindhyans firmly brackets their age as between 1.7 Ga years to 1. 6 Ga (Ga= giga annum= billion years). And second, no Cambrian age animal fossils have been found from rocks younger than the Rohtas Limestone and Tirohan Dolomite. These shallow marine sediments should have yielded a prolific Cambrian fossil record. The consensus now is that those fossil remains are microbial and eukaryotic algal forms, and the odd shaped mineral fragments are just that; odd shaped inorganic mineral growths. Dr. Azmi's claim does not require a revision of Vindhyan stratigraphy.

An even more subversive fossil claim was made by A. Seilacher, P.K. Bose and F. Pfluger in 1998. They reported to have found burrows of triploblastic animals from the Chorhat Sandstone (dated to about 1.6 Ga) of the Lower Vindhyan Semri Group.  They accepted that the rocks are more than a billion years old and suggested that multicellular animals had evolved more than half a billion years before the appearance of the first animal body fossils in the Latest Neoproterozoic to Early Cambrian times. This claim too has been rejected by most workers. As S. Kumar writes in his review, the burrows have been shown  to be syneresis cracks, resulting from subaqueous shrinkage of sediment. They are not fossils. Molecular phylogeny indicates that multicellular animals originated between 800 Ma and 650 Ma and discernible megascopic fossils, in the form of traces, impressions, and body parts, appear first by around 560- 550 Ma. Any claim of megascopic metazoan 'fossils' in rocks older than 600 Ma needs to be treated with extreme skepticism.

In summary, most paleontologists now accept that there is no credible evidence of fossils of multicellular animals in the Vindhyan sediments. The youngest Vindhyan sediments are thought to be considerably older than the Cambrian (base of the Cambrian is 541 Ma).

When did Vindhyan sedimentation end? What is the age of the youngest Vindhyan rocks? This is one of the conundrums in Indian stratigraphy. Different types of data and methods of dating rocks have come up with conflicting ages.

Only three direct dates for the Upper Vindhyan Bhander Group are available. The Lakheri Limestone in the Rajasthan sub basin shows a date of 1073 Ma +- 210 Ma. The overlying Balwan Limestone has been dated to 866 Ma +- 180 Ma. About 60 m of sediment overlies the Balwan Limestone.  The Bhander Limestone in the Son Valley sub basin has yielded a date of 908 Ma +- 72 Ma. In the Son Valley, there is about 400 m of sediment overlying the Bhander Limestone. There is no direct method available yet to date these youngest rocks.

According to S. Kumar, the Maihar Sandstone, which is the uppermost Vindhyan strata exposed in the Son Valley, are around 650 Ma or slightly younger. The few non-carbonaceous remains that actually are fossils according to S. Kumar are found in these rocks. They are the microbial mat structure Arumberia and the body fossil Beltanelliformis minuta, which recently has been shown to be of cyanobacterial affinity. They are both Ediacaran age (637 Ma - 541 Ma) fossils. Kumar concludes that Vindhyan sedimentation in the Son Valley continued well into the Neoproterozoic, finally terminating in earliest Ediacaran times.

In contrast, an older age range is being hinted at by two very different types of data.  Bijaigarh black shales of the Kaimur Group have been dated to around 1200 Ma. The diamond bearing Majghawan kimberlite intrudes the Baghain Sandstone of the Kaimur Group near the town of Panna in Madhya Pradesh. It has been dated to about 1073 Ma. The Kaimur Group is dated between 1200 Ma and 1100 Ma. The kimberlite does not intrude the overlying Rewa and Bhander strata. They are younger than 1073 Ma, but apparently not much younger. The magnetic signatures frozen in the kimberlite match those preserved in the Rewa and Bhander rocks, suggesting that deposition of the Rewa and Bhander sediments did not persist for too long into the Neoproterozoic. The Bhander Limestone has recently yielded a date of 908 Ma (+- 72 Ma). Vindhyan sedimentation may have ended by 900 Ma.

Support for this scenario comes from the age of detrital zircon found in the Maihar Sandstone. The zircon has yielded a date of 1020 Ma. This date reflects the time when the zircon crystallized out of a magma. Zircons from this igneous rock were then eroded, transported and deposited in the Vindhyan basin. The Maihar Sandstone thus cannot be older than 1020 Ma. But, there is also an absence of zircons younger than 1020 Ma in the Maihar Sandstone. Geologists doing provenance work on the Vindhayans agree than the Aravalli orogenic belt to the west was an important source of sediments into the Vindhyan basin, as was the Central Indian Tectonic Zone (CITZ) to the south. Both of these belts could have yielded the 1020 Ma zircons, since they both contain igneous rocks of that age. The CITZ lacks igenous activity younger than this. But magmatism took place to the west of the Aravalli orogen between 873 Ma to 800 Ma (Erinpura Granite) and between 780 Ma and 750 Ma (Malani rhyolites and associated plutons). The erosion of these two igneous provinces could have provided a source of younger zircons to the Vindhyan basin. Their absence has been taken to imply that the Vindhyan Basin had closed by that time.

I am putting up these contrasting age signals here, but I don't really have a strong position on either of them. For one, geochronology of the Upper Vindhyans is still sparse and the few dates have come with large margins of error. I don't have the expertise to evaluate the magnetic data. The zircon data might seem to make a strong case for an earlier termination of Vindhyan sedimentation. But there is an alternate explanation for the lack of younger zircons in the uppermost Vindhyan sediments. The Erinpura Granite and the Malani igneous rocks lie to the west of the Aravalli belt. Orogenic activity  around 1 Ga, which deformed and uplifted the Delhi Group sediments, would have formed significant topography and a barrier to rivers flowing eastwards into the Vindhyan basin. Thus, a provenance cutoff rather than an end to sedimentation could explain the lack of younger age zircons.

Some workers have suggested that the Vindhyan sub basin to the west in Rajasthan closed earlier by around 900 Ma, but sedimentation continued until early Ediacaran times in the eastern parts of the basin exposed in the Son Valley in Madhya Pradesh.  Geoffrey J. Gilleaudeau and colleagues used variations in carbon isotopes of carbonate rocks to suggest age ranges for Vindhyan rocks in Rajasthan and the Son Valley. They found that carbon isotope values in carbonate formations in Rajasthan did not vary much and fitted the pattern observed for the late Mesoproterozoic (slightly older than 1 Ga) elsewhere around the globe. However, another study by Bivin George and colleagues of  the Balwan Limestone, which is the uppermost carbonate unit in the Rajasthan sub basin, showed substantial shifts in carbon isotope values through the section. The Balwan Limestone has been recently dated to about 866 Ma, but with a margin of error of +- 180 Ma! Bivin George and colleagues suggest that these variations match the 800 Ma globally synchronous Bitter Springs carbon isotope anomaly. Considering the presence of a 60 m shale layer above the Balwan Limestone, they put  the closure of the Vindhyan Basin in Rajasthan at around 770 Ma.

In Son Valley, carbon isotope values in the Bhander Limestone showed oscillations from enriched in the heavier isotope to sharply lighter values. This pattern is seen throughout the Neoproterozoic (younger than 1000 Ma) in sections elsewhere around the world. The sharp variations are thought to be due to large environmental perturbations that punctuated the Neoproterozoic from time to time.

During warmer climes, in healthy ecosystems, photosynthesizing marine organisms preferentially suck up the lighter carbon (C12) isotope. Sea water gets enriched in the heavier isotope (C13) which makes its way into carbonate minerals, resulting in 'heavier' or positive C isotope values of carbonate rock.  Environmental crises may cause a population crash of photosynthesizers. More of the lighter isotope is now available to enter carbonate minerals, resulting in  'lighter' or negative C isotope values of carbonate rock.

One problem in having a high confidence in this isotope curve matching is the inadequate geochronology of the Upper Vindhyans. Limestone ages in the Rajasthan sub basin have large margins of error (up to 200 Ma). Just one direct date of 908 Ma +- 72 Ma is available for the Bhander Limestone in the Son Valley sub basin. This means it is difficult to assign a particular pattern of variation to a specific Neoproterozoic interval. The technique has real potential but currently also underscores the need for sharper geochronology data.

However, the fossils Arumberia and Beltanelliformis found in the Maihar Sandstone support this scenario of a diachronous end to Vindhyan sedimentation. But there are niggling doubts too. The Sturtian (717 -643 Ma) and Marinoan glaciations (650 - 635 Ma) took place during the Neoproterozoic, covering much of the earth in ice. These glaciations ended with ice transported tillite deposits overlain by distinctive limestone or dolomite layers known as the 'cap-carbonates'. But there are no reports of unequivocal glacial deposits in the Rewa and Bhander sediments. Why their absence? Is that a hint that they are older than the Cryogenian Period i.e. older than 720 Ma, or is it because India was located at lower latitudes which were not glaciated? It is noteworthy that the latest Neoproterozoic (~ 590 Ma) Blaini Formation exposed in the Lesser Himalaya near Nainital and Mussoorie does contain glacially transported deposits.

But if the Upper Vindhyans are older than the Cryogenian, then what about the Ediacaran age fossils from the Maihar sediments?...

watch this space.

Saturday, August 18, 2018

Technology: Carbonate Sedimentology

Just a quick note to point out the range of instrumentation now available to sedimentologists for analysis of samples. This is from a study of Ordovician age sediments by Yihang Fang and Huifang Xu, published in the June 2018 issue of the Journal of Sedimentary Research.

An excerpt:

 A micro-laminated carbonate with alternating dolomite–calcite layers from the mid-Lower Ordovician St. Paul Group from the Central Appalachians in southern Pennsylvania was examined using optical microscopes, X-ray diffraction (XRD), scanning electron microscopy (SEM) with X-ray energy-dispersive spectroscopy, electron microprobe analysis (EPMA), scanning transmission electron microscopy (STEM), laser-induced fluorescence (LIF) imaging, short-wave infrared (SWIR) imaging, and X-ray fluorescence (XRF) imaging. The sample is composed mainly of two types of layers. Dolomite-dominated layers are darker in color, generally thinner, and contain detrital minerals such as quartz and feldspar. In contrast, calcite-dominated layers are lighter in color, thicker, and contain less detrital minerals supported by microcrystalline calcite matrix. In situ XRD, LIF, XRF, and SWIR results show that organic remnants are enriched in the dolomite layers. The coincided spatial distribution confirmed a positive correlation between dolomite and organic matter, and hence provide evidence for microbial-EPS-catalyzed formation of sedimentary dolomite.

That is a lot of heavy weight toys!!

The researchers were trying to come up with an answer to one of the long lasting problems in sedimentary geology; that of the origin of the mineral dolomite.

I am not going into this paper in detail. I have not read it.  Let me just say that it has been suspected that microbial communities living on the ocean floor and in the sediment column may catalyze the precipitation of dolomite and this study confirms that dolomite rich layers contain organic matter. The dolomite may form at the sediment sea water interface by precipitation directly out of sea water or more commonly by replacement of the aragonite or calcite sediments during shallow burial.

On a broader time scale, dolomite abundance through the Phanerozoic does seem to correlate well with episodes of lower atmospheric oxygen concentrations and consequently less oxygenated sea water. See this figure spanning the Phanerozoic Era ( McKenzie J.A. and Vasconcelos, C.  2009). This relationship between ocean anoxia and dolomite abundance may well hold for the Precambrian too.

Anaerobic bacteria can thrive under such anoxic conditions.  Sulfate ions in sea water are thought to be a hindrance for dolomite formation. Anaerobic bacteria  remove these ions from sea water and pore fluids by sulfate reducing respiration, thus creating conditions favorable for precipitation of the mineral. They may also present specific types of organic substrates which enable easy nucleation of dolomite crystals.

For more details on the origin of sedimentary dolomite, do see my post The Dolomite Problem- Peeking Under the Hood

Tuesday, July 31, 2018

Papers: Himalaya Foreland Basin Evolution

Foreland basins are moat like depressions that form in response to the loading of the crust by a rising fold and thrust mountain belt. They get filled up by sediment derived from the erosion of the mountain chain. The sediment composition of foreland basins and how it has changed through time is therefore an archive of the uplift and erosional history of the orogen. The Himalayan foreland basin contain a few kilometer thick pile of sediments. One broad scenario of foreland basin evolution goes like this. The oldest of these foreland sedimentary successions of Palaeocene-Mid Eocene age (~ 55- 45 my) are marine deposits formed in the ever narrowing Tethys Ocean. Uplift of the region due to the ongoing India-Asia collision resulted in the complete withdrawal of the sea. There was no sedimentation for 10-12 million years. After this depositional hiatus, sedimentation resumed in Early Oligocene times (~ 31 my) in a continental fluvial setting and continues till today.

This timing of events representing the transition from marine to continental conditions has been challenged by some workers. They have proposed that the depositional hiatus is of different magnitude at different places depending upon the effect of local tectonics. Estimates of the duration of the unconformity between marine strata and the overlying fluvial deposits range from 25 million years (early withdrawal of the sea) in the Kohat Basin of Pakistan to 3 million years (marine conditions persisting until around 31 million years ago) in the Subathu Basin of Himachal Pradesh, India. Continental depositional environments then formed between 28 million years ago to around 20 million years ago in different basins. River systems have deposited a thick succession of sediments since.

About 1 -0.5 million years ago these sediments were uplifted to form the well known Siwalik ranges. The locus of deposition shifted southwards. The alluvial plains of the Ganga river system is the present day foreland basin.

In this map of the Himalayan orogen, the southernmost belt, abbreviated as Ts, is the deformed foreland basin.

Source: An Yin 2006

..and in the satellite imagery, the foreland basin are the hill ranges to the south of the brown line. This line is a system of thrust faults that place the Lesser Himalaya on top of the foreland basin strata.

Provenance fingerprinting of the oldest foreland sediments indicate a Himalayan source. That tells us that incipient ranges in the collisional zone formed by around 50 million years ago. Since then, successive pulses of thrusting have uplifted terrains of differing composition. Their exhumation and erosion is recorded in the changing sediment mixture of the foreland basin strata. The schematic below reconstructs the Early Miocene to Pliocene uplift history of the Nepal Himalaya. The Dumri Formation and the Siwalik Group are the foreland basin sediment archive of this uplift.

Source: Peter G DeCelles et. al. 1998.

I've come across quite a few papers on these Himalaya foreland basin sediments. They focus on using a variety of techniques like sedimentary facies analysis, petrography, geochemistry and geochronology to unveil foreland basin geometry, foreland drainage patterns, paleo-climate and soil formation, and the timing of emplacement and erosion of different thrust sheets.

1) Evolution of the Paleogene succession of the western Himalayan foreland basin - B.P. Singh 2013.

2) Evolution of the Himalayan foreland basin, NW India : Yani Najman et. al. 2004.

3) Eocene-early Miocene foreland basin development and the history of Himalayan thrusting, western and central Nepal:  Peter G DeCelles et. al.  1998.

4) Neogene foreland basin deposits, erosional unroofing, and the kinematic history of the Himalayan fold-thrust belt, western Nepal: Peter G DeCelles et. al. 1998. (Behind Paywall).

5) Detrital geochronology and geochemistry of Cretaceous–Early Miocene strata of Nepal: Implications for timing and diachroneity of initial Himalayan orogenesis: Peter deCelles 2004. (Request full text).

6) The Os and Sr isotopic record of Himalayan paleorivers: Himalayan tectonics and influence on ocean chemistry: John Chesley 2000. (Request full text).

7) Early Oligocene paleosols of the Dagshai Formation, India: A record of the oldest tropical weathering in the Himalayan foreland: Pankaj Srivastava et. al. 2013.

Update: August 1 2018

Geologist Vimal Singh reminded me of some more studies on early foreland basin evolution as well as the excellent work of Rohtash Kumar on various aspects of the later Siwalik Group sediments. I am adding these papers to the list.

 8) Marine to continental transition in Himalaya foreland - Bera 2008

9) Reconstructing early Himalayan tectonic evolution and paleogeography from Tertiary foreland basin sedimentary rocks, northern India- Yani Najman and Eduardo Garzanti 2000 (Behind Paywall)

10)  Sedimentary  Architecture  of  Late Cenozoic  Himalayan  Foreland  Basin  Fill: An  Overview:  Rohtash Kumar et. al. 2011

A collection of Rohtash Kumar's papers on the Siwaliks can be viewed here.

I was somewhat unfamiliar with the Eocene-Early Miocene age deposits of the Himalayan foreland, and so was glad to have found studies that deal with the record of the earliest stages of Himalayan uplift.

Open access except where indicated.

Sunday, July 22, 2018

Mount Abu Geology

A reader asked me about the geology of Mount Abu and how it relates to the Aravalli fold mountains. Mount Abu is a popular tourist destination in the Sirohi district of Rajasthan. It forms a distinct elevated area ( ~ 5,500 feet ASL) and provides a much needed respite from the heat of the Rajasthan plains.

Here is my short note.

Mount Abu is a batholith. That means it is a big body of granite, made up of magma which cooled deep in the subsurface.

The satellite imagery below shows the Mount Abu hills in relation to the Aravalli fold mountains.

This magmatic episode is part of what is known as the Malani Igneous Suite. The term refers to an assortment of mostly felsic igneous rocks (magmas enriched in silica, aluminum and potassium) ranging from large intrusive bodies like the Mt Abu batholith, lava fields resulting from volcanism (Malani Rhyolites), and mafic (iron, magnesium, calcium rich magmas) and felsic dikes intruding the margins of the province. This was the result of a fairly prolonged phase of magmatism that affected the western margin of the Aravalli orogenic belt. The magmatic activity occurred between 780 -750 million years ago.  So, it is much younger than sedimentation and orogeny of the Delhi Supergroup (which ended about 1 billion years ago) and is not considered as part of the Aravalli/Delhi Supergroups. The thinking is that after Delhi orogeny, this part of the Indian craton underwent extension (was pulled apart), triggering granitic magmatism.

The detailed geological map shows Mount Abu batholith and other stratigraphic and tectonic elements of the Aravalli craton (continental block) and fold mountains. The Western Margin Fault roughly marks the zone of contact between the Aravalli craton to the east and the Marwar craton to the west. The Delhi orogeny is believed to have resulted from a convergence and suturing of these two cratons about 1 billion years ago.

Source: Joseph Meert and Manoj Pandit 2014

Here is a link  to an article on the Malani Igneous Suite with a useful map of the distribution of various igneous bodies in this province:

One additional point is that there were earlier magmatic episodes affecting the western margin of the Aravalli craton marking the culmination of the Delhi orogeny. This includes the 970 million year old granitoids of the Ambaji region, believed to represent crustal melting at the terminal stages of the Delhi orogeny. Younger than these are the post orogeny Erinpura Granite. This too was a protracted magmatic episode lasting the time span from 870 to 800 million years ago. Erinpura granites are collectively shown as post-Delhi granites in the map by Meert and Pandit. Note that both the Erinpura Granites and the Mt Abu batholith occur at the juncture between the Aravalli and Marwar cratons. Such a region would have lines of weaknesses inherited from earlier collision and deformation episodes making it susceptible to be reactivated as rifts and loci of magmatism.

Finally, why did the Mt. Abu granites that formed in the subsurface get elevated to heights of around 5,500 feet? The Aravalli mountains too have ridges which reach around 3000 feet. The Aravalli craton is bounded by two great NE-SW trending fault systems. The Great Boundary Fault to the east and the Western Margin Fault to the west. There are also innumerable NE-SW and some NW-SE trending fracture and fault systems (lineaments) which break up the crust into rigid blocks. Many of these originated during Archean and Proterozoic crustal deformation. Some new lineaments may have formed during the Jurassic breakup of Gondwanaland (A B Roy 2006).  Mount Abu may be an example of a block uplift, resulting from movements of the crust along these lineaments, perhaps in response to stresses originating from the India-Asia collision. The Aravalli fold mountains too have likely been rejuvenated and seen some recent uplift (Bhu 2014) These movements took place in the Neogene to Quaternary times, in the past 20 million years or so.

Saturday, July 14, 2018

Papers: Global Tectonics, Cryogenian Period, Himalaya Miocene Lakes

I've come across quite a few interesting papers on diverse topics in the past couple of weeks. Most of them are 'big question' themes, dealing with processes taking place on global scales. Here are the links.

Global Tectonics:

1) Subduction Initiation of the Wilson Cycle - In plate tectonics, the Wilson Cycle refers to cyclical ( frequency of 100's of millions of  years) breakup of continents and the opening and closing of ocean basins. But how is subduction initiated and new convergent plate boundaries formed? Some good examples from the Western Pacific margin and eastern Indonesia.

2) How Subduction Broke Up Pangaea - Was it top down forces.. i.e. the pull exerted by subducting slabs or was it the horizontal traction exerted by a convecting mantle (bottom up) that broke up the supercontinent?

3) Why is Africa Rifting? - Insights into the formation of the famous East Africa rift system.

4) Gondwana Large Igneous Provinces: distribution, diversity and significance - Synopsis of several papers that explore the link between prolonged magmatic episodes, tectonics, climate shifts and sedimentation patterns in Gondwana continents.


1) Snowball Earth climate dynamics and Cryogenian geology-geobiology - In the Cryogenian Period, between around 715 to 635 million years ago, the earth was blanketed in two prolonged glaciations. Before these glaciations, the earth was a microbial planet. The end of these glaciations is associated with the evolution of multicellular complex life. What were the conditions during the Cryogenian Period that influenced the evolution of life?


1) Oligocene‐Miocene Great Lakes in the India‐Asia Collision Zone - Mount Kailash is an important pilgrimage site for Hindus. The sediments that make up this mountain were deposited in narrow basins in the India-Asia collision zone. They preserve a record of surface environments and geodynamic mechanisms operating within the suture zone during the convergence of India with Asia.

All Open Access.

Friday, July 13, 2018

Sutlej Paleochannels- More Details

A while back I had written a post in response to a paper (Ajit Singh et. al. 2017) on the paleo-Sutlej river. The study used geochemical analysis to identify ancient channels of the river. Today, the Sutlej flows out of the Himalaya and joins the Indus River. But this study showed that until about 8000 years ago, the Sutlej (or at least a strand of the river) flowed along a different course. Its paleo-channels coincide with  the course of the river Ghaggar in Haryana and Rajasthan in northwest India. This topic is of relevance in studying how rivers may have impacted agricultural practices and settlement patterns of the Harappan Civilization. The geography of the river Ghaggar also matches that of the river Saraswati, described in the Rig Ved. There is therefore considerable interest in working out the detailed history of these rivers.

I had pointed out that an earlier paper by Liviu Giosan and colleagues has used topographic criteria to come to a similar conclusion as Ajit Singh and colleagues. Giosan's study stressed that today the Sutlej and Yamuna flow along deeply incised valleys that were cut in the early Holocene (~10,000 to 8700 years ago). The absence of such valleys in the region between the present day Sutlej and Yamuna indicates that the Ghaggar channel was not being fed by glacially sourced rivers since 10,000 -8700 years ago.

The relief rendition below is from Ajit Singh's paper. It shows clearly the incised valleys of the Sutlej and Yamuna. I had overlain the blue line and suggested that if the Sutlej had flowed into the Ghaggar in early Holocene, there should have been an incised valley along the blue line.

As it happens, Sanjeev Gupta, who was the lead scientist of the study by Ajit Singh and colleagues, is a reader of my blog. He emailed me and has provided more insights regarding these paleo-channels.

I am posting his comments below with his permission.

My comment from the earlier post- The modified relief rendition below also shows the course of the abandoned Sutlej incised valley. Note that this valley is much narrower than the Sutlej and Yamuna incised valleys. Also, trace these narrower incised valleys upstream and you can see that they originate in the Siwaliks. There are no deep extensive incised valleys along the route I have marked in blue. The Sutlej would have carved a prominent incised valley roughly along the blue route had it been flowing into the Ghaggar during most of the early and mid Holocene. Its absence suggests to me that the valley annotated as the abandoned Sutlej incised valley was really carved out in the earlier part of the Holocene by the smaller Ghaggar river originating in the Siwaliks.

Sanjeev Gupta's reply - Just to respond... we only see the incised channel in the SRTM where the valley is not completely infilled. Indeed where we see the valley in SRTM it is not the base of the incised valley. but a partially infilled valley. So along the blue line you have drawn there is likely to have been a valley but it is entirely infilled.

My comment (with regards to the 'abandoned Sutlej incised valley' in the above relief rendition) - (it)....was really carved out in the earlier part of the Holocene by the smaller Ghaggar river originating in the Siwaliks.

Sanjeev Gupta's reply- This is not possible because all the geochemistry signature is of the Sutlej - the base of the valley actually occurs in the stratigraphy.

We have some newer data that better constrain the timing of incision but I stress the topographic surface is not the base of the incised valley - that lies in the subsurface.


So, Sanjeev Gupta's view is that present day topography is not necessarily a more reliable guide to the course of these ancient rivers. Geochemical fingerprinting is the way to go.

Update July 22 2018:

Liviu Giosan posted a comment which led to an extended exchange with him regarding this topic. I have copied his comments below with his permission.

Me- Hi Liviu- was going to message you about my post when I saw your comment. Regarding Sanjeev Gupta's view, wondering if this is a matter of scale. The subsurface paleo valleys he says exist were much smaller valleys carved by a waning Sutlej and hence got infilled? Not quite the wider incision profile carved later by the river  along the present course.

Liviu Giosan - A matter of scale, time since abandonment, and location relative to a sediment source. Maybe I do not see Sanjeev's point. His work confirmed ours for holocene and in adition showed a pre-Holocene channel. A recent paper by Dave et al. showed the same thing for Yamuna.

Me- thanks. Can you send me the link to Dave et. al.?

Liviu Giosan - I can send you the paper. Please elaborate: what is Sanjeev's point?

Me- He hasn't elaborated. Those two short para's  i put up was all he emailed. I guess he is saying that there are incised valleys in the interfluve subsurface but are now infilled and so not picked  up by SRTM.

Liviu Giosan - That may be true but I doubt they will be all the way in filled unless they are old (preHolocene). After all his work shows a network of holocene streams in the region using SRTM. Further geochemistry we did  downstream so far is pretty (Clift et al. not cited by Sanjeev) shows no holocene contribution from Sutlej. Even more Sutlej incised since early Holocene making it almost impossible to feed shallow channels on the interfluve. This needs to be interpreted with all available evidence. Geochem is no silver bullet in isolation.


Sanjeev Gupta did mention to me that the point about topography is important but they did not have the space to address it in their paper. So, while there are differences in viewpoints about emphasis on techniques,  the overall data does point to a switch in the course of the Sutlej around 8-10 thousand years ago.

The paper on the paleo-Yamuna that Liviu Giosan mentioned is by Aditi Krishna Dave and colleagues. They examine detailed lithologs of the Chautang channel near the town of Hisar, Harayana, and with OSL (Optically Stimulated Luminesence which indicates the time of burial of sediment) age constraints propose that the Yamuna, which was flowing westwards and joining the Ghaggar-Hakra downstream of its confluence with the Sutlej, changed course and started flowing eastwards by 24 thousand years ago. I'll put up a short post on that shortly.

Friday, June 22, 2018

How Did Homo Sapiens Evolve?

Here is a very good summary of our evolving understanding of this question.

How did Homo Sapiens Evolve? - Julia Galway-Witham and Chris Stringer


Over the past 30 years, understanding of Homo sapiens evolution has advanced greatly. Most research has supported the theory that modern humans had originated in Africa by about 200,000 years ago, but the latest findings reveal more complexity than anticipated. They confirm interbreeding between H. sapiens and other hominin species, provide evidence for H. sapiens in Morocco as early as 300,000 years ago, and reveal a seemingly incremental evolution of H. sapiens cranial shape. Although the cumulative evidence still suggests that all modern humans are descended from African H. sapiens populations that replaced local populations of archaic humans, models of modern human origins must now include substantial interactions with those populations before they went extinct. These recent findings illustrate why researchers must remain open to challenging the prevailing theories of modern human origins.

..and on the status of Recent African Origin (RAO) versus Multiregionalism, the two competing theories of modern human origins

The apparently more deeply rooted origin of H. sapiens adds to the problem of how scientists can delineate H. sapiens from other species in an evolutionarily meaningful way and suggests that the species did not evolve as recently as previously envisaged. However, evidence of evolutionary novelty in the fossil record of putative H. sapiens still appears relatively recently in Africa, compared to a multiregional view of human evolution (in which H. sapiens began to evolve at least 1.8 million years ago). Within Africa, it is currently unclear whether the origin of H. sapiens was localized, as in some early RAO formulations, or involved a process similar to multiregional evolution, but operating purely within the continent (5, 13).

With only a few dissenters, the strictest versions of both RAO (which denies interbreeding with other lineages or species) and multiregionalism (which argues for an interbreeding network of one species over the past ~1.8 million years) are now generally regarded as falsified. Instead, two intermediate theories best accommodate the complex interactions between hominin taxa ~40,000 to 100,000 years ago (8, 14): RAO with hybridization (RAOWH) and the assimilation model (AM) (see the figure).

Source: How did Homo Sapiens Evolve? - Julia Galway-Witham and Chris Stringer 2018

Open Access.

Sunday, June 17, 2018

Mars Geology

Came across a couple of cracking papers on Mars geology

1) Ancient Martian aeolian processes and palaeomorphology reconstructed from the Stimson formation on the lower slope of Aeolis Mons, Gale crater, Mars - Steven G. Banham et. al. 2017

During its travels, the Mars Curiosity Rover has been taking some exquisite pictures of landscapes and rock outcrops. This study uses field photos of a sandstone body and analyses its ancient depositional setting.  The sandstone is a dune field, created and shaped by aeolian processes. The overall aim is to better understand the sedimentary environments on Mars, how they changed over time, and whether they could have been habitable environments.

Just take a look at the spectacular cross bedding and bed sets of the dune.

Source: Steven G. Banham et. al. 2017

And here is a depiction of the regional setting of the dune field

Source: Steven G. Banham et. al. 2017

2) A Field Guide To Finding Life On Mars- S. McMohan et. al. 2018

Excerpt: This paper reviews the rocks and minerals on Mars that could potentially host fossils or other signs of ancient life preserved since Mars was warmer and wetter billions of years ago. We apply recent results from the study of Earth’s fossil record and fossilization processes, and from the geological exploration of Mars by rovers and orbiters, in order to select the most favoured targets for astrobiological missions to Mars. We conclude that mudstones rich in silica and iron-bearing clays currently offer the best hope of finding fossils on Mars and should be prioritized, but that several other options warrant further research. We also recommend further experimental work on how fossilization processes operate under conditions analogous to early Mars.

I got to relearn a good bit about how depositional settings, mineralogy and geochemistry influence organic matter preservation.

Both Open Access.

Monday, June 11, 2018

550 Million Year Old Animal Tracks Preserved In Sediments

Late Ediacaran trackways produced by bilaterian animals with paired appendages - Zhe Chen, Xiang Chen, Chuanming Zhou2, Xunlai Yuan and Shuhai Xiao

Pretty exciting find this. One of the earliest evidence of the presence of bilateral animals on earth has been found in 550 - 540 million  year old sedimentary rocks from China. These are not body fossils. Rather a pair of pits impressed upon the sediment surface mark the movement of an animal with paired appendages.


Ediacaran trace fossils provide key paleontological evidence for the evolution of early animals and their behaviors. Thus far, however, this fossil record has been limited to simple surface trails and relatively shallow burrows. We report possible trackways, preserved in association with burrows, from the terminal Ediacaran Shibantan Member (ca. 551 to ca. 541 million years ago) in the Yangtze Gorges area of South China. These trace fossils represent the earliest known trackways. They consist of two rows of imprints arranged in poorly organized series or repeated groups. These trackways may have been produced by bilaterian animals with paired appendages, although the phylum-level phylogenetic affinity of the trace makers remains unknown. It is possible that the trackways and associated burrows were produced by the same trace maker, indicating a complex behavior involving both walking and burrowing. Together, these trackways and burrows mark the arrival of a new era characterized by an increasing geobiological footprint of bilaterian animals.

 Source: Zhe Chen 2018

To put this find in context to the broader trajectory of animal evolution. These sediments are 30-40 million years older than the Chengjiang Biota of China and the famous Burgess Shale of Canada. These two are known for their exceptional preservation of animal soft tissue, giving scientists a peek into the morphological diversity present at that time. However, animals did not suddenly originate during the time the Chengjiang Shale and the Burgess Shale were being deposited, as is sometimes misunderstood by some, or is deliberately and disingenuously argued by creationists.

Molecular clocks which can estimate the time of origin and divergence of organisms suggests that animals diverged from a common ancestor between 1000 million and 700 million years ago. Trace and body fossils start appearing by 600 million years ago.

I like to put up this figure which summarizes the fossil record of early animal evolution through the Late Neoproterozoic to Early-Mid Cambrian

Source: On The Origin of Phyla

You will notice an incremental increase in richness of the trace and body fossil record. The Cambrian "Explosion" was a geologically rapid diversification of the animal biosphere, fueled by a confluence of ecologic triggers. But in absolute years it took place over a 15-20 million year period.

Monday, May 21, 2018

W. Bengal Bangladesh- Geologic Controls On Arsenic Distribution In Ground Water

Science writer  Priyanka Pulla has written an excellent article exploring the geologic, socio-economic and technological issues related to the widespread arsenic contamination of groundwater in W. Bengal. Sadly, the government response to this crisis has been slow.

I thought I would elaborate on the geological question -  Why are Arsenic (As) levels much higher in shallower Holocene age aquifers and lower in the deeper Pleistocene age aquifer? The answer encompasses mineralogy, climate change, sea level changes and bacteria.

The ultimate source of As are high Himalayan rocks and Indo-Burman ranges with additional contributions from the Precambrian terrains of Peninsular India and the Siwalik hills.  Minerals like biotite, magnetite, illmenite, olivine, pyroxene, amphiboles contain As. These minerals release As when they undergo weathering in catchment areas and deposits of the alluvial plains. This As is absorbed on secondary minerals like Fe hydroxides like goethite. Such Fe hydroxides are authigenic, i.e. they grow in the shallow buried sediments of the alluvial plains. Under oxidizing conditions, As is immobile, sequestered in Fe hydroxides. However,  conditions may change, and these sediments may get overlain by or be redeposited in environments rich in organic material. Certain bacterial species living on this organic material break down these Fe hydroxides, using the oxygen for their metabolism, and releasing Fe and As into the groundwater. This is known as reductive dissolution of Fe hydroxides and is the principal mechanism for As entering the groundwater in the alluvial plains of Bangladesh and West Bengal.

During the Pleistocene.. 1) the high Himalaya was glaciated. Therefore, important sources of As like the Fe-Mg rich rocks of the Indus ophiolite belt (slices of oceanic crust that existed between India and Asia which have been thrust up during continental collision) and high grade metamorphic rocks such as schists and gneisses were covered in ice and not releasing sediment. Indian cratonic areas, the Siwalik foothills  and the Indo-Burman ranges were being eroded, but overall less As was making its way on to alluvial plains. 2) Since climate was cooler and drier, there was less organic material accumulating in sediment of alluvial plains. Conditions were oxidizing and As remained sequestered in Fe hydroxide minerals. 3) Sea level was much lower then. Almost the entire continental shelf was dry land. Ganga and Brahmaputra met the sea much to the south of present shoreline. Reducing environments like delta front marshes, ponds, estuaries, existed much to the south.

Sedimentary conditions changed by 12-15 thousand years ago. Glacial melt exposed As bearing rocks in high Himalaya. As a result, more As made its way on to alluvial plains. Importantly, sea level rose and flooded the continental shelf. The Pleistocene delta front reducing environments were drowned. Shorelines shifted northwards. The climate was warmer, encouraging vegetation growth. Reducing delta front environments like swamps, coastal marshes and lakes developed on previous alluvial plain sediments.

The map below shows the position of shorelines between 7 thousand and 4 thousand years ago along with the location of wells with high levels of As. This study focuses on Bangladesh but similar conditions existed in West Bengal as well. The sea has receded 2- 3 meters to its present location since 4 thousand years ago.  The delta front and shoreline belt that existed 4-7 thousand years ago is now a densely inhabited region .

 Source: Quaternary shoreline shifting and hydrogeologic influence on the distribution of groundwater arsenic in aquifers of the Bengal Basin- M. Shamsudduha, Ashraf Uddin 2007

Notice clustering of wells with high As along the past shorelines. Here, organic rich delta marshes and swamps developed. Bacterial reduction of Fe hydroxides released As in to groundwater.

As distribution also shows correlation with topography. This map shows high As levels in groundwater coinciding with topographic lows. Such low lying areas accumulate more fine sediment and organic material. Again, this will apply also to W. Bengal.

 Source: Quaternary shoreline shifting and hydrogeologic influence on the distribution of groundwater arsenic in aquifers of the Bengal Basin- M. Shamsudduha, Ashraf Uddin 2007

So, a change in climate and shifts in sedimentary environments in response to changing sea level from Pleistocene to Holocene exerted a strong control on As distribution in the alluvial plains of Bangladesh and W. Bengal.