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.


Speleothems

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.

Brochure.



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

Please share widely!