Tuesday, October 25, 2016

Photomicrograph: Super Mature Quartz Arenites From Proterozoic Cuddapah Basin

One of the vivid memories of my Master thesis fieldwork in South India were a series of brightly reflecting hills. In the afternoons, the bare slopes of the hills were a blinding white and you had to wear dark sun glasses to minimize the glare.

These hills were made up of the Paniam Quartzite. This sedimentary sequence is part of the Neoproterozoic Kurnool Group which represents one megacycle of deposition in the long lasting Paleoproterozoic to Neoproterozoic Cuddapah Basin.  In sedimentary petrology terminology these white and bright sediments are quartz arenites, rocks made up mostly of the mineral quartz. In fact, they were super mature quartz arenites, i.e. they were made up of more than 90% quartz. I  point counted several samples and the percentage of quartz was around the 95%-96% mark.

Here is what they look like under the microscope. Notice how rounded the quartz grains are.

The white arrows in the photomicrograph below points to quartz cement which has precipitated between the grains. These cements are called overgrowths. They maintain the same optical orientation as the substrate quartz grain and hence in cross polarized light the detrital grain and the overgrowth appears as a single crystal unit. The detrital quartz grain is outlined by iron oxide dust which helps demarcate the contact between the grain and the later cement.

Here is another example of a super mature quartz arenite. The contact between the detrital grain and cement is again marked by a coat of dust. Notice the planar crystal facets of the quartz cement (white arrow) which contrasts nicely with the rounded detrital particles.

The example I have presented show only one generation of quartz overgrowth cement. There are instances where two generations of quartz overgrowth cements are present. Like the detrital grain, the first generation overgrowth has a coating of iron oxide or clay and is abraded. This indicates that the quartz grains have been derived from the erosion of older silica cemented sandstones. The original source of the quartz in these older sandstones were igneous or metamorphic rocks. After being eroded from these rocks and then transported and deposited, the quartz grains were overlain by silica cement (the first generation cement) and lithified into a sandstone.

Later (perhaps tens of millions of years later), this sandstone was uplifted and eroded. Disaggregation of grains during weathering broke of quartz sand particles along with attached fragments of cement. This cement overgrowth then got abraded and rounded during transport and acquired a dust coat. In its final site of deposition it was overlain by new silica overgrowth (the second generation cement). Abhijit Basu and colleagues present an interesting example of such "second cycle" or "recycled" quartz arenites from the sedimentary sequences of the Bastar Craton from Eastern India (Image to left: source Basu et al 2013).

A careful examination of quartz arenites and generations of silica cements can reveal a lot of useful information about uplift, erosion and recycling history of the earth's crust.

Quartz arenites are not restricted to the Proterozoic. They are common in younger age Paleozoic, Mesozoic and Cenozoic deposits too. They occur only sporadically in Archean age deposits. Thick sequences of quartz arenites become more common in the Proterozoic. This increase in the occurrence of quartz arenites in the Proterozoic has to do with the changing tectonics of the earth.

Among the common rock forming minerals, quartz is relatively chemically inert and is more resistant to physical breakdown during weathering and transport. In the Archean, sedimentary basins were generally linear troughs formed in front of island arcs. Due to these tectonically active conditions, the basin floor subsided quickly and detritus derived from weathering of igneous and metamorphic source rocks was deposited and buried before physical attrition and chemical dissolution could remove unstable minerals. The result was a mineralogically "immature" sandstone with the framework of the rock made  up of  quartz, feldpsars and volcanic and metamorphic rock fragments in different proportions . The sediments and associated volcanic material frequently got metamorphosed to a low grade "green mineral" assemblage of chlorite, actinolite and epidote. These deformed and metamorphosed successions embedded in Archean gneiss terrains are known as greenstone belts.

There are a few instances of quartz arenites in the Archean from terrains of the Canadian Province, the Baltic Shield in Russia and from the Bababudhan Group of the Dharwar Greenstone belt in South India. Many of these have been interpreted as a product of intense chemical weathering in Archean soils, wherein unstable pyroxenes, feldspars and meta-igneous and meta-sedimentary rock fragments were leached away, leaving behind a quartz rich residue. Sedimentary structures like cross bedding and ripple marks indicate shallow water environments of deposition where the sand was further subjected to physical attrition leaving behind a quartz rich sand deposit.

Such conditions of longer residence time and more intense chemical weathering in soil profiles and long periods of attrition and physical sorting by wave and tidal action became more common in basins of Proterozoic age. Phases of prolonged magmatism and heat loss from around 3 billion years ago to 2 billion years ago resulted in a cooler earth and one that now was made up of large rafts of granite/granodiorite crust which was buoyant and tectonically stable. Although the boundary between the Archean and the Proterozoic is pegged at around 2.5 billion  years ago, basin tectonic styles do not change abruptly. These were evolving conditions.

In Peninsular India, Proterozoic age sediments were deposited in two types of basins manifesting different tectonic styles. "Mobile Belts" are reminiscent of the older Archean greenstone belts in that they were tectonically active elements of the crust, perhaps forming in subducting settings at the boundary between two cratonic blocks. They are depressions which contain abundant volcano-sedimentary successions made up of volcanic flow and ash beds interlayered with  immature sand and mud and chemically precipitated silica and iron oxide layers.  These are interpreted as deeper water deposits. Some basins contain stromatolite limestone/dolomite. There are a few quartzite deposits too.  These may be the metamorphosed equivalents of quartz arenites which were deposited in shallow water environments.  These successions were subjected to metamorphism, deformation and intrusion by granitic plutons during orogenic episodes forming the "mobile belts" or fold belts.  The Aravalli and Delhi Group of sediments which make up the Aravalli mountain ranges in Rajasthan are a good example of these Early to Mid Proterozoic mobile belts.

Overlapping in time with the mobile belts but extending into the Neoproterozoic are the epicratonic basins, also known as the "Purana" basins. These basins experienced less volcanic activity and were subjected to less deformation and metamorphism than that seen in the mobile belts. They contain thick sequences of quartz arenites and limestones.  These basins were initiated by extension and rifting of the continental crust resulting in extensive shallow marine shelf areas.  Low relief Archean to Early Proterozoic source terrains made up of granitic and metamorphic rocks were subjected to intense chemical weathering. Since the basin floor subsided slowly in these passive margin basins, shallow water conditions prevailed for long periods of time. Quartz rich residues were transported and deposited as sand sheets in beach and tidal flat settings and as sand shoals in more open waters away from the shorelines. Wave action further sorted them into a texturally mature sand.

The Paniam Quartzite, whose afternoon glare I tried in vain to avoid, is a remnant of one of these vast sand sheets that occur across many epicratonic Proterozoic basins in Peninsular India. Other examples of this stable cratonic style of deposition include the Vindhyan Basin in Central India and the Kaladgi and Bhima Basins of South India.

The satellite image below shows the brightly reflecting slopes (white arrows) of this quartz arenite deposit around the village of Gani in Andhra Pradesh. The black dotted line is the contact between the Archean basement and the overlying Proterozoic Cuddapah Basin sediments. The linear structure is the left lateral Gani Kalava fault offsetting the Cuddapah Basin.

And here is one final photomicrograph of the Paniam quartz arenite showing well rounded detrital grains with faceted quartz overgrowths meeting in planar contact in the pore spaces.

Tuesday, October 18, 2016

Thursday, October 13, 2016

Sea Water Chemistry And Shell Mineralogy: Tales Of Mesozoic Bivalves

Years ago when I was in the second year of college, I along with friends, went for a fossil collection tour to the town of Ariyalur in South India. Rocks of Cretaceous age outcrop all around, and these strata have now become one of the most famous fossil localities in India.

We collected ammonites, echinoids, plant leaf impressions on clay and bivalves... lots and lots of bivalves.. In the picture below are the remains of my collection of molluscs. On the top left is an oyster with a clam clinging on to one of its valves. Bottom left is another oyster with its jagged valve margin. In the middle is a largish clam and to the right is an oyster whose layered shell structure is clearly visible.

I have some photomicrographs too taken from thin sections given to me by a friend.

In the above image the foliated shell microstructure of a piece of a bivalve can be clearly seen in cross polarized light.

And in the image below, a coarser prismatic crystal structure of a shell fragment is visible in the center of the image.

Most molluscs groups (including bivalves) in today's tropical seas built their skeletons using the CaCO3 polymorph aragonite. I say tropical seas, because molluscs with calcite skeletons are more common in temperate waters, such as for example in the marine communities living on the continental shelf of the southern coasts of Australia. In the Cretaceous seas though, even at tropical latitudes, calcite bivalves were common. In this apparent puzzle lies a very interesting story of climate change, sea floor spreading, changing sea water chemistry, the evolutionary decline and success of different bivalve groups during the Mesozoic, the emergence of bivalve reefs and the localization of hydrocarbon reservoirs.

Monday, October 3, 2016

Interview- Rosemary And Peter Grant On Watching Evolution In Action

Source: Quanta Magazine; Courtesy Peter and Rosemary Grant

Daphne Major in the Galapagos chain.

Yes, 40 years of field research on that half a square km size island, tracking, generation after generation, changes in body and beak size of different species of ground finches.  Lately, they have supplemented their morphologic and bird song data with genomic analysis to get an understanding of the genetic underpinnings of morphologic change.

Rosemary and Peter Grant interviewed about their epic evolution watch:

"The diminutive island wasn’t a particularly hospitable place for the Grants to spend their winters. At less than one-hundredth the size of Manhattan, Daphne resembles the tip of a volcano rising from the sea. Visitors must leap off the boat onto the edge of a steep ring of land that surrounds a central crater. The island’s vegetation is sparse. Herbs, cactus bushes and low trees provide food for finches — small, medium and large ground finches, as well as cactus finches — and other birds. The Grants brought with them all the food and water they would need and cooked meals in a shallow cave sheltered by a tarp from the baking sun. They camped on Daphne’s one tiny flat spot, barely larger than a picnic table.

...They visited Daphne for several months each year from 1973 to 2012, sometimes bringing their daughters. Over the course of their four-decade tenure, the couple tagged roughly 20,000 birds spanning at least eight generations. (The longest-lived bird on the Grants’ watch survived a whopping 17 years.) They tracked almost every mating and its offspring, creating large, multigenerational pedigrees for different finch species. They took blood samples and recorded the finches’ songs, which allowed them to track genetics and other factors long after the birds themselves died. They have confirmed some of Darwin’s most basic predictions and have earned a variety of prestigious science awards, including the Kyoto Prize in 2009".

indefatigable to the end..

"Do you plan to go back to Daphne?

RG: We stopped intensive work after 40 years, but we do plan to go back.

PG: The oldest person died at 122 years old. That means we have 40 more years".

Ground finches are off course the birds that Charles Darwin famously observed when on tour to the Galapagos, but infamously didn't mention in his book since he never labelled his samples according to their island location. He did borrow correctly labelled samples from other sailors and then had the ornithologist John Gould classify them. Gould's finding was that the islands finches were a group of sibling species despite their widely varying body and beak size and shape.

Habitats varied on different islands. Darwin realized that sometime in the distant past one ancestral population of finches must have immigrated from the South American mainland and then diverged into several morphologically distinct species, each a fit to its habitat. That was one of the threads of reasoning he weaved into a more comprehensive theory of common descent and evolution through natural selection.

The book, Beak of the Finch by Jonathan Weiner, is a bit dated but is still a riveting account of Rosemary and Peter Grant's research.

Update: @avinashtn alerted me to the Grant's book "40 Years of Evolution".