20,000 Days In The Life Of A Clam

My Whatsapp profile description says, "what's a million years here and there".

It is a tongue in cheek acknowledgment of the vast spans of time geologists often have to contend with. If I am studying a rock that formed more than a billion years ago, a 5 to 10 million year uncertainty in nailing down its exact time of formation is acceptable. Uncertainty in estimating the time of formation may occur due to our as yet not so perfect understanding of the decay rate of various radioactive isotopes being used for dating, or due to limits of sensitivity of the instruments measuring the radioactive isotopes in the mineral. We are making great strides in measurement techniques with a 0.1 % accuracy now achievable. 

Sedimentary rocks are harder to date than igneous rocks since minerals with radioactive elements rarely form in them at the time of their deposition (some limestones and black shales are exceptions). Often, using some indirect methods we can bracket their maximum and minimum age. Take the example of the Alwar Group sedimentary rocks which occur in the northern Aravalli mountains in Rajasthan. They are estimated to have been deposited sometime between 2.1 billion years ago and 1.8 billion years ago, an uncertainty of 300 million years! We know from our understanding of sediment accumulation processes that deposition was not uniformly spread out over the 300 million years. Rather, the sequence of sediments would have been deposited in discrete pulses lasting 10 to 20 million years, separated by long phases of non-deposition. 

Amazingly, even though we don't know exactly when during the 300 million year interval these sediments came to be deposited, we can track fairly accurately what was happening then on a daily basis. Some strata of the Alwar Group are of shallow marine origin. As the daily tide flooded in and ebbed, a thin layer of sand was deposited during each of these high energy phases. During the slack phase in between, a thin layer of mud was deposited. Stacks of these tidal bundles made of a sand and mud couplet record the passage of daily tides. Observing the stacking pattern closely reveals even more details. Sets of bundles of thicker sand-mud couplets alternate with sets of thinner bundles. Each set formed during alternating spring (thick layers) and neep tide (thin layers) cycles. 

We can interpret the ancient record by comparing the patterns with those forming today in different settings. The principle "present is the key to the past" is used with some caution, but it works well in this case.

Such tidal rhythmites are fairly common in the geologic record. The pictures to the left is a section of a core from  Cretaceous age sediments laid down in an estuary. The sedimentary section is made up of sets of thicker silt layers capped by a darker mud layer, overlain by a set of thinner silt and mud couplets. Each silt layer represents deposition during the flood or ebb tide. During the slack period, stirred up organic rich mud settled down. Again, we don't know the exact age of the rock to a certainty of few hundred thousand years, but we can track daily events. Image source: G. Shanmugan; AAPG Bulletin v. 84, 2000.

Speaking of tides, the earth's rotation is slowing down due to tidal friction. Marine organisms like corals, brachiopods, and clams build a calcium carbonate skeleton to house their soft tissues. Their shell grows by a daily addition of a thin mineral layer. Geologists have studied the pattern of skeleton growth of Paleozoic corals. Besides daily growth bands, they can identify seasons too, as corals lay down thicker bands during the dry season and thinner bands during the wet phase. The number of days in the year are estimated by counting the number of daily bands in each season. It turns out that there were about 420 days in a year during the early and middle Silurian (between 443 million to 419 million years ago). By middle Devonian (roughly 370 million years ago), some 50 million years later, the number of days had reduced to about 410. Using bivalve shells, scientists estimate that there were 370 days in a year by Late Cretaceous times (about 80 to 70 million years ago). 

There are other examples closer to our own existence on earth of natural rhythms being preserved in rock. Geologists and climate experts routinely use mineral bands in cave stalagmites to understand variations in rainfall. A recent study by Gayatri Kathayat and colleagues from Uttarakhand, North India, reveals details of the course of Indian monsoons over thousands of years, mapping dry and wet phases lasting few centuries each. Despite an uncertainty of a few decades in the absolute age of each layer, it gives us a broad picture of climate change through the Holocene. 

We accept the fuzziness of our estimates of the age of an event while being able to sharply resolve the changes taking place in that cloud of uncertainty.

I could have named this post "Ode to Laminae", an appreciation of thin layers that form in tune with earth cycles and which preserve in their layering valuable information on ancient tides, earth moon dynamics, changing of seasons, and longer term climate change.  I just thought the title of the post and the paper it refers to is better click bait. 

Sometime in the Late Miocene (about 10 million years ago, what's a few tens of thousands of years here and there) a giant clam living on the western margin of the Makassar Strait (Indonesia) built a shell with daily growth increments. I will post the entire abstract of the paper below so you can get an idea of the details of ocean conditions scientists can tease out today with sophisticated instrumentation.  

 Iris Arndt et.al., 20,000 days in the life of a giant clam reveal late Miocene tropical climate variability.

Giant clams (Tridacna) are well-suited archives for studying past climates at (sub-)seasonal timescales, even in ‘deep-time’ due to their high preservation potential. They are fast growing (mm-cm/year), live several decades and build large aragonitic shells with seasonal to daily growth increments. Here we present a multi-proxy record of a late Miocene Tridacna that grew on the western margin of the Makassar Strait (Indonesia). By analysing daily elemental cycle lengths using our recently developed Python script Daydacna, we build an internal age model, which indicates that our record spans 20,916 ± 1220 days (2 SD), i.e. ∼57 ± 3 years. Our temporally resolved dataset of elemental ratios (El/Ca at sub-daily resolution) and stable oxygen and carbon isotopes (δ18O and δ13C at seasonal to weekly resolution) was complemented by dual clumped isotope measurements, which reveal that the shell grew in isotopic equilibrium with seawater. The corresponding Δ47 value yields a temperature of 27.9 ± 2.4 °C (2 SE) from which we calculate a mean oxygen isotopic composition of late Miocene tropical seawater of −0.43 ± 0.50 ‰. In our multi-decadal high temporal resolution records, we found multi-annual, seasonal and daily cycles as well as multi-day extreme weather events. We hypothesise that the multi-annual cycles (slightly above three years) might reflect global climate phenomena like ENSO, with the more clearly preserved yearly cycles indicating regional changes of water inflow into the reef, which together impact the local isotopic composition of water, temperature and nutrient availability. In addition, our chronology indicates that twice a year a rainy and cloudy season, presumably related to the passing of the ITCZ, affected light availability and primary productivity in the reef, reflected in decreased shell growth rates. Finally, we find irregularly occurring extreme weather events likely connected to heavy precipitation events that led to increased runoff, high turbidity, and possibly reduced temperatures in the reef.

Tell me geology isn't the coolest field of study.

LInks: India Aquifers, Early Bipedalism, Mars Geology

 Here are some interesting articles I read recently.

1) Mapping India's Aquifers.  Indian agriculture depends heavily on groundwater. To understand and manage this resource we need a good idea of the nature and extent of aquifers. Subodh Yadav, Joint Secretary, Department of Water Resources, River Development and Ganga Rejuvenation, Ministry of Jal Shakti, has written an informative article on the National Aquifer Mapping Program. Detailed reports are available to the public through the Central Ground Water Board, Aquifer Information and Management System page. Mapping and report availability is still work in progress.

2) Is Sahelanthropus the earliest biped? A good article by Brian Handwerk on the many questions spawned from a recent analysis of a 7 million year old femur fossil. Fossil remains named Sahelanthropus tchadensis were found nearly 20 years ago in Chad, and various studies have come to conflicting conclusions on whether Sahelanthropus could walk on two legs. Bipedalism is considered to be one of the key traits distinguishing members of the human branch from other apes and so there is a vital interest in understand the timing and circumstances of its evolution. 

3) Ground Penetrating Radar images from Mars Perseverance Rover. The indefatigable Mars Rover loaded with geological instruments is currently exploring the edge of the Jezero Crater on Mars. Here, rivers emptied into a large lake depositing sediment and building a delta. The first radar images show inclined sedimentary layers which could be the classic sign of a delta architecture or something else, scientists suspect. Read on! By Holly Ober, University of California, Los Angeles.

India Fossil Outcrops, Horse Domestication, Mars Landscapes

 From the past few days:

1) India is rapidly losing fossil rich outcrops to urbanization, expanding agriculture, mining, and unregulated fossil collection.

On International Fossil Day, October 23, 2021, the Paleontological Society of India, Pune Mumbai Student Chapter, organized a very informative online symposium on this topic. I have linked to part of the talks held that day. Paleontologist Dr. Rajani Panchang was the moderator. Several young researchers describe their field work in Kutch, Tamil Nadu, and Spiti Valley. Over the past several years, changes in land use and unchecked fossil removal has resulted in outcrop degradation and impoverishment.

.Video Permanent Link - India Fossil Outcrops .

Even though the Geological Survey of India and some local agencies have identified locations of geological importance, at present India does not have a law for the preservation of geoheritage sites. D.M. Banerjee writes about the struggle to get the Indian government to take up this issue seriously in his article Fate of Indian Geoheritage and Geopark Bill, published in the July 2021 issue of Current Science.

2) The origin of domestic horses has been a tough case to crack. It was long held using archeological evidence that horses were domesticated by the Botai Culture in Central Asia around 3500 B.C. But ancient DNA studies indicated that these early domesticated lines are not the ancestors of the modern domestic horse. Instead, the origin of the modern domestic horses have been tracked to the Volga-Don region in the Western Eurasian steppes between 2500 and 2000 B.C. The abstract of the paper is worth reading through- 

Domestication of horses fundamentally transformed long-range mobility and warfare. However, modern domesticated breeds do not descend from the earliest domestic horse lineage associated with archaeological evidence of bridling, milking and corralling at Botai, Central Asia around 3500 bc. Other longstanding candidate regions for horse domestication, such as Iberia and Anatolia, have also recently been challenged. Thus, the genetic, geographic and temporal origins of modern domestic horses have remained unknown. Here we pinpoint the Western Eurasian steppes, especially the lower Volga-Don region, as the homeland of modern domestic horses. Furthermore, we map the population changes accompanying domestication from 273 ancient horse genomes. This reveals that modern domestic horses ultimately replaced almost all other local populations as they expanded rapidly across Eurasia from about 2000 bc, synchronously with equestrian material culture, including Sintashta spoke-wheeled chariots. We find that equestrianism involved strong selection for critical locomotor and behavioural adaptations at the GSDMC and ZFPM1 genes. Our results reject the commonly held association between horseback riding and the massive expansion of Yamnaya steppe pastoralists into Europe around 3000 bc driving the spread of Indo-European languages. This contrasts with the scenario in Asia where Indo-Iranian languages, chariots and horses spread together, following the early second millennium bc Sintashta culture.

The paper is open access. And there is an easier to understand article in Nature as well. 

3) The remarkable range of technologies brought to bear on understanding the geology of Mars is giving some spectacular payoffs. Two studies caught my eye:

a) Mars' surface shaped by fast and furious floods from overflowing craters: Lake breach floods produced fast flowing streams that cut deep drainage valleys, reshaping the Mars landscape. Catastrophism has played a large role in the history of Martian surface evolution.

b) The Perseverance Rover rocks on!! The stunning images it has taken of rock outcrops on Mars is enabling geologists to reconstruct details of ancient sedimentary environments. N. Mangold and colleagues describe a delta lake system and flood deposits at Jezero Crater. 

Take a look at the details available to geologists for interpreting sedimentary processes and the rock history.

Source: N. Mangold et.al. 2021:  Perseverance rover reveals an ancient delta-lake system and flood deposits at Jezero crater, Mars.

On Mars, large crater lakes were sites of sediment deposition. Rivers meeting such craters dumped their sediment on the crater floor in lobes that expanded lakewards forming a delta. The architecture of the sedimentary layers within this delta environment has been vividly captured and described in this study. In the image, the bottomset strata are fine grained sediment deposited in waters ahead of the delta. The foreset strata represent deposition on the inclined growing delta front. And the foreset strata are deposits of rivers associated with the delta. The paper is quite detailed and a treat for sedimentologists. But the images can be enjoyed by all. Open Access too!  

Sedimentary Structures: Building Stones of Badami, Aihole And Pattadakal Temples

Is this sandstone slab in its original geological orientation (as when the sedimentary layers were deposited) or is it upside down? I'll answer this a little later, but first some background.

I recently visited the Chalukya style temples and rock cut monuments at Aihole, Pattadakal and Badami (6th -8th CE) in northern Karnataka and noticed some great sedimentary structures in the building stones. The term sedimentary structures refers to the shape and form sedimentary layers get sculpted into by the action of waves, currents, tides and wind during deposition of the sediment. The size of the deposited sedimentary particles and the orientation of layers are a reflection of both the vigor of the currents and waves and the direction of flow of water or wind.  

These monuments are made up of Neoproterozoic age (900-800 million year old) sandstones. Geologists have recognized using detailed sedimentological analysis that the sandstones formed mostly in a large braided river system that flowed in a northwesterly direction.

Between  roughly 1800 -800 million years ago, over the course of a billion years, the Indian continental crust sagged due to various tectonic forces to form several long lasting sedimentary basins. The Kaladgi Basin in which the Badami area sandstones were deposited is one such basin. The paleogeographic reconstruction below shows the position of the Indian continent at about one billion years ago and the location of the various sedimentary basins within it.

Source: Shilpa Patil Pillai, Kanchan Pande and Vivek S Kale: 2018: Implications of new 40Ar/39Ar age of Mallapur Intrusives on the chronology and evolution of the Kaladgi Basin, Dharwar Craton, India.

Much of this deposition took place in inland or epeiric seas that flooded the Indian continent. During intervals of sea level fall, rivers carved valleys and deposited coarse sediment. The Badami Cave sandstones are river deposits of the Kaladgi Basin. The stratigraphic column shows various sedimentary deposits of the Kaladgi Basin and their inferred environments of deposition.

Source: Shilpa Patil Pillai, Kanchan Pande and Vivek S Kale: 2018: Implications of new 40Ar/39Ar age of Mallapur Intrusives on the chronology and evolution of the Kaladgi Basin, Dharwar Craton, India.

The Badami braided river system was receiving sediment eroded from Archean age (>2.5 billion year old) rocks situated SE of the basin. These were granites, granodiorites, and low to medium grade metamorphic  rocks of the Dharwar craton (a large block of stable old continental crust). 

Land plants did not exist then. Weathered debris was moved quickly by surface flow into streams. Large sediment load, moving by traction i.e. by rolling and sliding on the stream bed, repeatedly choked the channels, forcing bifurcation of streams and formation of braids. Very broad braided rivers formed since there were no plants to stabilize banks.  The Badami sandstones (Cave Temple Formation) are technically known as arenites. This term indicates that the rock is made up of mostly coarse sand with very little finer sized mud. Accumulation of mostly coarser sand size and pebbly particles reflects a locale of repeated high discharges and vigorous currents which winnowed away the finer sized mud.  The braided river shown below as an example is from the Canterbury Plains of New Zealand.

 Source: Braided Rivers: What's the Story?

The Badami rocks preserve a record of  various subenvironments of this paleo-river. Picture shows channel and bar deposits in outcrop.

Source: Mukhopadhyay et. al. 2018; Stratigraphic Evolution and Architecture of the Terrestrial Succession at the Base of the Neoproterozoic Badami Group, Karnataka, India.

As river channels episodically migrated sideways and the basin floor subsided to accommodate more sediment, channel deposits and adjacent sand bars got stacked to form thick 'multi-story' sandstones. Each bed tells a story of a discrete depositional episode.

The arrangement of sand layers within each bed tells us about the subenvironments in which it formed and the energy and direction of water flow during deposition. I came across many types of these internal structures. I recognized tabular cross beds, trough cross beds, planar lamination and rippled beds. Water (or wind) can move & shape sand into piles or waves. Sand grains roll along the direction of flow, then avalanche down the steeper side (lee side) of the wave forming a layer inclined (cross) to the orientation of the main sand body. Successive avalanches form a set of cross beds. The graphic shows the formation of a set of cross beds.

 Source: Dr. Diane M Burns in Teaching Sedimentary Geology in the 21st Century.

Here is an example of cross beds from near the town of Badami.

And this one is from a building stone from Pattadakal temple.
 

Such cross beds were built by sediment avalanching on the lee side of migrating sand bars during high flow.

This picture show trough cross bedding from near the Badami cave complex. These represent the internal structure of migrating sinuous sand dunes on a channel floor. 

See this elegant explanation by Dawn Sumner, a sedimentologist at the University of California at Davis,  of how trough cross beds form.



Email subscribers who may not be able to see the embedded video, click on this link: Trough Cross Bedding Video.

And here is a beautiful example of trough cross bedding found in a Pattadakal temple building stone.

This is planar lamination on a slab at Pattadakal. The bed is constructed of parallel layers of coarse sand. It is interpreted to have been deposited in a high flow regime from sheets of water flowing over mid channel sand bars.
 

Ripples on a slab at Pattadakal. This is a rare preservation of a bedding surface showing rippled sand. Erosion usually cuts off the wavy upper part. These ripples indicate migration of small sand waves in a quieter flow regime on the channel floor.

Remember, cross beds are the inclined layers that form on the lee side of a ripple or wave or dune. Here are small cross sets on the floor of Aihole rock cut temple! These represent the cross beds formed by migration of small ripples. The ripples themselves have been eroded away. Arrows indicate the direction of water flow and cross bed accretion as ripples migrated.

Okay, let's go back to my first question. Is the slab I showed in the picture geologically upside down?

Yes it is. But how to tell?

As sand avalanches down the lee slope it forms a tail at the toe of the slope resulting in cross beds which become tangential to the floor. In picture the cross beds are tangential towards the top of slab i.e. that is actually the base.

Lets see at how the cross bed contact with the top and bottom bedding plane looks in an outcrop. Here is the original depositional orientation of cross beds manifest in this outcrop near Badami caves. They show a tail or tangential contact of the cross beds with the base. Since top of cross beds are not usually preserved they show a high angle contact truncated by upper bedding plane.

This slab is upside down too! Notice again the tangential contact of the cross beds (white arrow) is towards the top, which means that must have been the base. Yellow arrow points to high angle contact with the upper bedding surface. 

Towards the top of the exposed section of sandstone around Badami I came across some truly impressive examples of cross bedding. These particular exposures were on the crags opposite the four main Badami temples. There is a narrow passage past the archaeological museum and a short climb to the top. Take a look at these beauties!

These large cross beds reminded me of the inclined beds of wind blown sand dunes. Is it possible that abandoned sand bars were sculpted by wind in to big dunes? Or does this upper level sandstone represent, as a recent study suggests, the beginning of a marine incursion in to the basin? In this scenario, deposition of sand took place in high-energy shallow waters near the shore. These cross beds represent large migrating sand waves which were eventually shaped in to beach ridges and tidal bars.

The outcrops and building stones of these monuments mostly record the processes within the Badami braided paleo-river. 900 million yrs ago a complex of channels and bars, quieter pools and rippled sand beds existed where these temples stand today.

Do visit Aihole, Pattadakal and Badami and gaze at its splendid architecture and sculptures. But spare some time to appreciate the magnificent record of our natural history that these monuments preserve. 

Quiz- Is this slab upside down or in its true depositional orientation? 😉

Until next time....

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.

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.

The Bay Of Bengal Once Touched Sikkim

See this satellite imagery of the Himalaya.  The Indian State of Sikkim occupies the region just east of Darjeeling.

The Siwaliks (green arrows) appear as a forested linear band forming the southernmost hilly terrain of the Himalaya. The hills abut against broad alluvial plains. Rivers traversing the Himalaya carrying enormous sediment load encounter a gentler gradient upon exiting the hilly terrain. A loss of stream power results in sediment being dumped in the channel, so much so, that rivers get chocked on their own sediment. As a result, channels split and bifurcate forming a braided river system. These rivers  also suddenly change course, abandoning their channel and carving out new ones. Such course changes may occur during floods or by tilting of the land by structural movements.  Over time, the deposits of these ever changing rivers coalesce to form cone shape aprons of sediments known as alluvial fans. These rivers like the Kosi and the Tista, which flow transverse to the mountain range, meet an axial river like the Ganga and the Brahmaputra flowing parallel to the mountain front. The axial river flows into the Bay of Bengal.

The Siwalik hills were once these type of alluvial fans.  Just as today, during Miocene and Pliocene times, sediment was being deposited in front of the rising Himalayan mountains. Beginning about half a million years ago or so, these ancient alluvial fans were crumpled up and uplifted to form the Siwalik ranges. Active alluvial fan formation shifted southwards to its present locus. This process continues. In a few million years, the present day alluvial fans deposited by rivers like the Kosi and the Teesta will be deformed into a newer mountain range south of the Siwaliks. The Himalaya are growing southwards.

How do we know that the Siwaliks were once alluvial fans? Geologists rely on analogy, comparing the Siwalik sediments with what is accumulating in the present day alluvial fans. They find a striking similarity. Siwaliks are made up of alternations of coarse gravel layers and finer sand and silt layers with characteristic bed orientations and structures like cross beds and rippled sand. The gravel layers are inferred to be the river channel deposits while the finer sand and silt layers are the river bank, levee and floodplain deposits. An important finding made throughout the length of the Siwalik ranges has been the paleo-current directions preserved in the rocks.  Geologists have measured the orientation of bedding and ripple marks and found out that rivers were flowing south and south east i.e. perpendicular to the mountain chain. There is no evidence of an axial river like the Ganga in these Siwalik sediments. The thinking is that such an axial river must have flowed much to the south of the region of deposition of Siwalik sediments.

And what about evidence of a delta? Where did these Miocene and Pliocene rivers meet the sea? The logical geographic place to look for a coast would be towards the east. And in fact, that evidence has come from the Siwalik sediments of West Bengal and Sikkim. In a really interesting paper published recently in Current Science, Suchana Taral, Nandini Kar and Tapan Chakraborty describe sedimentary structures and marine trace fossils from Middle Siwalik sediments exposed along the Gish River and its tributaries in the Tista Valley. Siwalik rocks in the central and western part of the Himalaya show current structures that indicate south flowing rivers. In this easterly location however, the sediments show evidence of being deposited in a wave influenced environment. Sedimentary structures like wave ripple laminations and hummocky-swaley stratification indicate deposition in wave dominated marine bay.  Paleo-current indicators like ripple marks preserved on sandstone surfaces show a south as well as north directed current. This suggests an environment influenced by tides and north directed waves. Associated sediments show indicators of different delta environments like distributary channels, delta mouth bar and delta flood plain deposits.

Apart from current direction indicators, the sediments contain plant fossils indicative of mangrove vegetation and brackish water environments. They also contain trace fossils i.e. impressions and burrows made by creatures moving and disturbing the sediment surface. Cylindrichnus, Chondrites, Rosselia, Taenidium, Skolithos, Planolites are some of trace fossils reported in this study. The assemblage of trace fossils is similar to those reported from marine settings.

All this suggests that during the time of deposition of these Middle Siwalik sediments in Late Miocene-Pliocene times, about 5-10 million years ago, a branch of the Bay of Bengal had invaded as far north as present day Sikkim. Rivers carrying sediment from the Himalaya were debouching them in a delta and a shallow marine bay. The Sikkim Middle Siwalik strata are ancient deformed delta and marine deposits.  

A paleo-geographic reconstruction of this eastern part of these Siwalik depositional environments in shown below.

 Source: Suchana Taral, Nandini Kar and Tapan Chakraborty 2017

The  upper graphic shows the reconstructed delta and marine depositional environment. The lower graphic shows the regional paleo-geography. The pin shows the environmental location of the study area. The yellow rose diagram shows the paleocurrent directions measured in the Siwalik sediments.

Interestingly, some earlier work by geologists has shown that in Late Miocene times the Brahmaputra was flowing along a much more easterly route towards the Bay of Bengal. They used sand thickness and sand/shale ratios from wells drilled in the delta and found lobate sand bodies, which they inferred were brought in by a large river flowing from a ENE source. Their interpretation is shown in the graphic to the left (Uddin A. and Lundberg N. 1998). At the time the Shillong Plateau did not exist. The river flowed into the Bay of Bengal from the Upper Assam valley and through the Sylhet depression in to the Bengal Basin. The uplift of the Shillong Plateau in Pleistocene times forced the Brahmaputra to turn west and wrap itself around the newly emerging uplands.

Since Pliocene times, the tremendous amount of sediment being delivered by Himalayan rivers, coupled with Pleistocene sea level fall, has caused a retreat of this arm of the Bay of Bengal southwards.

In the satellite image below, based on the location of the Sikkim Siwalik deposits and other work on the Bengal Basin paleogeography, I have drawn in brown the coastline as it would have existed 5-10 million years ago. The ancient drainage systems are shown in blue. South directed arrows shows the extent of the growth of the Bengal/Bangladesh alluvial plains and delta and the retreat of the sea since then to its present location.

Pretty amazing finding.

Photomicrograph- Micro Fault Displacing Proterozoic Stromatolite Laminae

From the Paleoproterozoic Vempalle Dolomite near the village of Gani, Cuddapah Basin, South India,

This was my M.Sc dissertation area. Vempalle Dolomites got me fascinated with carbonate rock textures and diagenesis.

The image shows a micro fault displacing stromatolite laminae. Stromatolites are biosedimentary structures formed when sediment is either trapped within microbial sheets or when CaCO3 minerals like aragonite precipitate around the sheets that cover the sea floor. The microbial colonies grow in a variety of shapes and structures in response to the wave energy conditions. Flat sheet like structures like the one seen in outcrop from where I sampled this rock indicates a low energy regime.

Of interest here:

a) The presence of oolites associated with these lamellar stromatolites. Oolites form in high energy conditions where sediment grains are constantly rolled around and held in suspension for periods of time. This allows layers of calcium carbonate to precipitate around a nucleus resulting in a coated grain containing concentric rings of CaCO3. The presence of layers of oolites in a lamellar stromatolite rock suggests that oolites forming in high energy tidal channels and shoals were transported by storms onto adjacent lower energy settings such as these microbial covered tidal flats.

b) There is variation in the shape and size of dolomite crystals. This variation is not randomly distributed but is fabric selective. The fine grained stromatolite laminae has been replaced by fine grained dolomite. There is some patchy neomorphic (recrystallization) growth of this dolomitized mud into coarser irregular dolomite.  Pore spaces and sheet cracks and fractures are filled with coarser irregular shaped dolomite crystals.  Rhomb shaped dolomite crystals are associated with oolites. This suggests that the rock underwent multiple episodes of dolomitization. The fine grained stromatolite aragonite mud got replaced early by very fine grained dolomite crystals. Contemporaneously, sheet cracks and pores filled with a coarse irregular shaped dolomite crystals.  Both the saturation levels of the replacing fluid and the abundance of nucleation sites affect dolomite crystal shape and size. Finer grained substrates offer abundant nucleation sites resulting in finer grained dolomite. Crystals growing from supersaturated fluids form quickly and interfere with adjacent crystals resulting in irregular shaped interlocking textures.

Oolites made up of either aragonite or high Mg calcite crystals were replaced by rhomb shaped crystals. Rhombic shapes form when dolomite replaces coarser grained substrates or precipitates from fluids which are mildly saturated. In such instances there are fewer nucleation sites and individual crystals have a degree of freedom to grow crystal facets.


There is also chert (microcrystalline silica) in this rock. Its replaces oolites and is present in pores spaces and in fractures.

#ThinSectionThursday

Field Photos- Jaisalmer: Landscapes, History, Geology, Fossils

Jaisalmer was fun!

I was staying last weekend at the pretty amazing Suryagarh hotel. It is a relatively new hotel, but built in the style of an ancient fort. The interiors are stylishly crafted, with courtyards and sunlight corridors, and absolutely beautiful elegant use of lights in the evenings...the service was impeccable and the food was great too! I sampled quite a variety of cuisines, from the classic Halwai breakfast made in the Rajasthani style, to a lunch inspired by a hodge podge of different styles from Mediterranean to Thai, to another Rajasthani style meat and rice platter dinner. The highlight for me among the myriad dishes was a palate freshener made from basil, sweet melon (Mosumbi), lime and salt. It was liquid citrus basil chutney, served chilled. Simple, yet sublime!

Pune is green and black. Trees and that volcanic beast, the Deccan Basalts, give it those hues. Jaisalmer is buff, off-white, yellow, fawn, brown, rust, red, black. These are the colors of the clays and sandstones and limestones deposited in an immense sea which covered the area from the Jurassic to the early Cenozoic. I got to see only the Jurassic section, and that too only in a hurry.

Suryagarh had organized a long drive in the outback.. I guess that's the word that comes to my mind when facing an immense desert. The Thar stretches in all directions of Jaisalmer and a couple of km off-road you begin to sense the isolation.

An Oasis with the vast Thar behind.

We visited the smallish Khaba fort. The interesting history is at the base of the fort.

These are the ruins of the Paliwal community which is said to have moved to this area from Pali, Rajasthan few hundred years ago. They built a successful sustainable agricultural society, making clever use of the limited amount of available water. Then legend has it, they fell out of favor with some powerful locals and almost overnight abandoned their homes, migrating to several larger towns in Rajasthan.

All around, the Jurassic is inescapable. These are north tilted shales and sandstones. Only a few hundred feet of strata outcrop (on the surface), but there is more than five thousand meters of sediment in the Jaisalmer basin subsurface. Some layers are oil and natural gas reservoirs which ONGC has tapped into.

The shales, sandstones and limestones are fossil rich. I could spot ammonoids, belemnites and clams. In an isolated homestead belonging to a Bhil tribal family, we met this amazing old woman who scours the countryside for fossils and then sells them in the Jaisalmer market. This is her treasure trove!

.. and the Bhil family in their abode. The Bhils occupy a proud place in Rajasthan history, being most well known for the military support they gave to Maharana Pratap is his battles against the Mughal emperor Akbar. They, as most of the tribal societies in India are, quite marginalized, sustaining themselves on small farm plots and as day laborers.

Another type of remains of this ancient Jurassic life are ichnofossils. These are tracks and trails and burrows of worms and other creatures disturbing the sea floor and preserved as impressions and casts and molds. Here is a horizontal burrow system likely made by a polychaete worm like creature. The ichnofossils of the Jaisalmer area and their paleo-ecology have been well studied by paleontologists from Pune. Check out this paper co-authored by my friend P.K Sarkar from Fergusson College, Pune.

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More aspects of the geology and the physical processes prevailing in the Jurassic seas are seen in the building stones.

Here is a shell hash with pebbles, a concentration of coarse shells of molluscs and rock fragments, deposited on a Jurassic beach or in very shallow seas where strong wave action removes the finer clay and mud particles, leaving behind a lag of clean sand. Such coarse sands are of great importance to geologists. They are often quite porous and may end up hosting petroleum. So, the basic processes that control their distribution in ancient seas are studied intensely by sedimentologists.

And this great example of deposition in a Jurassic storm. The coarser pebbly layer contains "intraclasts" which are pieces of hardened sea floor that has been ripped up by storm waves, transported and then re-deposited. I could have stood and photographed the walls of the Suryagarh hotel all day. There is so much sedimentology to be learned, all literally written in stone.

The main source of water for these remote communities are these depressions where scanty rainfall accumulates. These make for a really soothing sight in the midst of the harsher surroundings.

Groundwater is usually found at depths of few hundred feet and is often saline. Aquifers are present in the top sand, as well as the Cenozoic, Cretaceous and Jurassic strata underneath. Back in the early Holocene some 8-10 thousand years ago, Rajasthan and Jaisalmer got a lot more  rain. Groundwater got stored in these deeper strata, but over the past few thousand years, the climate became drier, and these deep aquifers don't get replenished too often. Prolonged reaction of the water with the rock increases the salinity of the water.  But there are at places shallower lenses of fresh water which do get replenished from time to time. This community got lucky and has struck potable water at twenty odd feet. Here I am at one of the wells.

Another place of historical interest we visited was an ancient cemetery. Jaisalmer had trading links with Eurasia from medieval times and scattered through the countryside are tombs for the fallen traveler and important locals.

.. and off course a trip to the desert without sand dunes?.. This is part of the Sam Sand Dune National Park..

Overall, my visit was far too short, there is just a lot of history and geology to see around Jaisalmer. I did not even have the time to visit the world famous Jaisalmer fort, which is a UNESCO heritage site.  

Hey,  I am not really complaining! Who would, if you woke up to a view like this?

Some Interesting Papers On Himalayan Sedimentary Record By Indian Academics

A long time ago I mused that my perception was that in the context of Indian geology research, papers in top international sedimentary journals featured more Bengali geologists based in Universities in Bengal and eastern parts of India than from any other ethnic or linguistic region in India. That was fun lazy speculation on my part but perhaps not entirely wrong.

So, it is very refreshing to see a flurry of papers in recent issues of Sedimentary Geology from non Bengali research groups from other parts of the country as well!

1) Early Oligocene paleosols of the Dagshai Formation, India: A record of the oldest tropical weathering in the Himalayan foreland - Pankaj Srivastava, Subhra Patel, Nandita Singh, Toshienla Jamir, Nandan Kumar, Manini Aruche, Ramesh C. Patel

As the Indian collision with Asia progressed, the sea of Tethys shrank. Marine environments gave way to terrestrial basins. This work uses ancient soils that formed on these early terrestrial sediments during breaks in sedimentation to infer paleoclimate. The conclusion is that the climate was tropical with monsoonal conditions.

2)  Late Miocene–Early Pliocene reactivation of the Main Boundary Thrust: Evidence from the seismites in southeastern Kumaun Himalaya, India - Anurag Mishra, Deepak C. Srivastava, Jyoti Shah

The Himalayan terrain is broken up by several major thrust faults. Out of these, the Main Boundary Thrust brings into contact the Proterozoic Lesser Himalayas over the Cenozoic foreland basin sediments. The nature and reactivation history of the Main Boundary Thrust is not completely understood. A study of soft sediment deformation in foreland basin sediments in the vicinity of the Main Boundary Thrust documents sedimentation contemporaneous with seismic events.  Magnetostratigraphy indicate a Late Miocene-Early Pliocene reactivation of this major thrust fault.

3)  Exploring the temporal change in provenance encoded in the late Quaternary deposits of the Ganga Plain - Shailesh Agrawala, Prasanta Sanyal, Srinivasan Balakrishnan, Jitendra K. Dash

The rivers in the Ganges plains receive a lot of sediments from the Himalayas. But that is not their only source of sediment. Rivers like the Chambal, draining the Indian craton composed mainly of Precambrian granites and meta-sedimentary and meta-igneous rocks and the Late Cretaceous -Earliest Cenozoic Deccan Basalts contributes prodigious amounts of sediment especially to the Yamuna. A geochemical analysis of cored sediments from the Yamuna finds important contribution through the Pleistocene from both Himalayan and cratonic sources. Interestingly, there is a climatic control on the relative proportions of sediments received from the two sources. Himalayan sediments dominate during interglacial phases while cratonic sediments dominate during glacial phases. Expansion of mountain glaciers during cold intervals would have reduced supply of sediments from the Himalayas.

Field Photos: Fluvial Bedforms

The Deccan Volcanic Province in best known as a thick pile of basalt lava. It is igneous rock country. In Maharashtra, the plateau east of the western ghats is drained by rivers small and big flowing ultimately into the Bay of Bengal. Many of these rivers originate in the very humid mountainous region known as the western ghats. Eastwards in the more arid climatic zone prevailing over the plateau these rivers have deposited a lot of sediment throughout the Quaternary and possibly earlier.

Here is the regional context:

Source: Mishra et al 2003

I was visiting a mineral museum north of the town of Sangamner a couple of weeks ago and came across some superbly preserved fluvial bedforms  along a road cut just south of Sangamner. Fluvial bedforms are sedimentary structures formed as sediment is moved and is deposited by current action in the river bed.  If you zoom and pan southwards along National Highway 50 south of Sangamner in the embedded image below you can see a trace of a small tributary of the river Pravara.


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The sediments in the images below were deposited by that tributary. The river has incised or cut into its own deposits and the active channel today is about 10-15 meters below the section seen in the images. So these deposits form an ancient probably late Pleistocene fluvial terrace a few hundred meters wide.

1) View of fluvial bedforms with pebbly and sandy planar and cross stratification clearly seen. At right center is a pebbly cross bedded wedge, possibly a point bar deposit. So, the sediment you see was being rolled along the bed of the river. The morphology of the sediment surface was like a sheet of sand and pebbles (the planar layers are a cross section of these sheets). Here and there the sediment sheet was wrinkled into large waves (the large cross beds are likely the cross section of these large wave forms). Some sediment was being deposited along the banks along inclined surfaces forming the point bar deposits.

2) Another wide view of planar and cross beds.

3) Shallow cut and fill structures. These are common features in the river bed as pulses of high energy events such as floods may scour the sand in the river bed and then fill up the trough formed by sediment.

4) A small channel filled by gravel and sand overlain by planar and cross bedded sand and pebbly layers.

I couldn't stay long at the outcrop as traffic was zooming perilously close to me and we had to go some distance. These deposits though have been interpreted to preserve a record of Quaternary climate change. The entire section records a phase of mid-late Pleistocene - early Holocene aggradation in which sediment was deposited and the river channel built upwards. This was followed by a phase of incision later in the Holocene when the river cut into its own deposits leaving stranded terraces.

I hope this outcrop survives for long. Apart from aiding our understanding of fluvial geomorphology and climate change it is a superb teaching tool for sedimentary geology and geomorphology classes. Unfortunately intensive farming activity and excavations for construction are slowly degrading these fluvial deposits near Sangamner.