Geology Word: Ooid

Ooids are sedimentary grains that form by precipitation of calcium carbonate around a nucleus. The nucleus may be an organic particle, or a mineral grain, or a shell fragment.

Repeated precipitation results in a concentric coating around the nucleus. Depending upon the saturation state of the sea and other variables like temperature, ooids may be made up of either calcite or aragonite. Both have the same chemical formula, CaCO3, but they differ in their atomic structure.

Ooids form in shallow seas where wave energy is vigorous. The turbulence results in the particles getting suspended and redeposited on the sea floor, resulting in an even coating around the nucleus. Beaches, tidal channels and regions near reefs are areas where ooids are accumulating today in tropical seas around the Bahamas, Caribbean, the Persian Gulf, and the Red Sea.

The picture shows an ancient ooid. This particle is around 350 million years old from the Mississippian Period, deposited in a warm shallow sea which occupied the State of Alabama in North America. I collected the rock sample on a field trip during my PhD days.

Interestingly, the nucleus is a foraminifera, a unicellular protist! Observe the coiled nature of the shell with its internal chambers visible. The shell of the foraminifera is also calcium carbonate and its precipitation is mediated by the organism. On the other hand, ooids results from an inorganic precipitation process, although there is evidence of bacterial involvement in some examples.

In the Precambrian (older than 539 million years ago), before animals evolved and began constructing shells, limestones were mostly made up of fine calcium carbonate mud. Precambrian calcium carbonate sand ( any sedimentary particle, irrespective of composition, between .063 mm and 2 mm is defined as sand) is rare, composed almost exclusively of ooids and a category of grains known as intraclasts (broken fragments of hardened sea floor).

The distribution of ooids and shell types through geologic history is a pointed reminder of how life has impacted the texture and composition of sedimentary rocks.

Deposited in high energy shoals, ooids help geologists reconstruct ancient environments and the paleogeography of continents which were flooded in the deep past.

Ooids are of interest to petroleum geologists too. The spherical grains result in a packing arrangement with high porosity (open spaces between grains). The localization of ooid layers in ancient sedimentary basins is keenly explored for their petroleum reservoir potential.

That was fun to write. I may turn this into a Geology Word series.

Bengal Delta, Africa Rifting, India Sand Mining

A few readings for your perusal- 

1) The future of Bengal Delta. With this succinct title Dipen Bhattacharya has written an informative article on the origin and evolution of the Bengal Delta. The Bay of Bengal was created when India broke away from eastern Antarctica about 130 -120 million years ago in the early mid Cretaceous. The basin expanded as India drifted northwards. From Cretaceous to Oligocene times (120-25 million years ago) rivers from Peninsular India were providing most of the sediment being deposited in the Bay. Himalaya derived sediment started overwhelming Peninsular river input from about 25 million years ago. K.S. Krishna and coworkers have very elegantly demonstrated this in their study of sediment pathways in to the Bay of Bengal.

Dr. Bhattacharya has traced the evolution of the delta into more recent times, explaining the role of the Pleistocene ice ages in delta growth. The delta’s future too is at risk with dam building and ground water extraction amplifying the changes due to global warming induced sea level rise. Well worth reading.

2) Eastern Africa Is Splitting Apart, but Not Where We Expected.  Africa is tearing apart along a north south oriented corridor from the Red Sea to Mozambique. Plate motion has formed the famous rift valleys of Ethiopia, Kenya, and Tanzania, as the crust stretches and subsides along faults. Kimberley Cartier explains the geological set up of the region and the stages in which continents break apart with oceanic basins eventually forming along the initial zones of continental rifts.

Why this region of Africa is rifting is not all that easy to explain. If you look at the plate tectonic map of Eastern Africa and the adjacent Indian Ocean and Arabian Sea you will notice that the oceanic Somalia and Indian Ocean tectonic plates are pushing into East Africa. For continents to split and be pulled apart, there have to be extensional forces generated. These are usually provided at the locus of rifting by the mantle doming up, thereby breaking and pushing the lithosphere away, and by one end of the plate subducting underneath another overriding plate. The subducting oceanic crust becomes denser and heavier as it sinks deeper, pulling the rest of the plate with it.

After the breakup of Gondwanaland, the northerly movement of the India plate through the Cretaceous was sustained by the pull force of the northern edge of the India plate sinking under Asia. On the other hand, Eastern Africa is surrounded by plate spreading zones. There is no pull force available for eastern Africa, only the localized extensional stresses due to mantle upwelling.  Is that providing adequate horizontal traction at the base of the Africa Plate for the crust to break apart and stretch? For a deeper understanding into the mantle forces responsible for this, I will recommend J Micheal Kendall and Carolina Lithgow-Bertelloni ‘s article- Why is Africa Rifting?

3)  India’s rivers bear lasting scars from relentless sand mining. Some years ago I heard a podcast on Planet Money about a Jamaican beach that was stolen. An estimated 500 truckloads of sand was hauled away in the middle of the night. Sand is big business all over the world. Indian river beds too are being plundered for their sand to satisfy the demands of the booming construction industry. Sahana Ghosh explores how scientists are surveying Indian rivers using field observations and satellite data. They are trying to track down the amount of sand being extracted and the environmental impact of sand mining.

Photo: Insect Camouflage

 I nearly missed this insect as I was sipping my evening coffee.

The rust, black, and white pattern of the insect blends into the pink feldspar, biotite, and quartz of the granite table top. When the restaurant introduced these tables in the 1980’s, insects with a passing resemblance to the table colors survived the gaze of bird predators better than individuals of the same species not having that coloration. Granite colored insects reproduced more, and the match between the insect patterning and the table top became more fine tuned over time.

I made this up. It is what is known as a “Just So Story”, named after writer Rudyard Kipling’s Just So Stories for children. Kipling wrote imaginative fantastical explanations for how animals looked the way they do. The term made its way into biology and was especially used, rather derisively, by evolutionary biologist Stephen Jay Gould as a critique of evolutionary psychology. Gould complained that the field tends to come up with imaginative yet unsupported adaptationist explanations for every aspect of human behavior. They are Just So Stories. The criticism has extended to other areas of biology too. Such as one can make about my story of the insect.

How did this particular insect species get this coloration that matches the granite? Likely its historical origins lies in a very different environment. Perhaps adaptation through natural selection in an ecologic setting of leaves and colored pebbles did play a role in the evolution of this pattern. Or perhaps it is a side effect of some other developmental changes in the insect body plan. Whatever the explanation, it is only chance that its coloration matches the table top stone.

Across The Eparchean Unconformity

An unconformity is a gap in the recording of earth's history, similar to missing pages in a book. These breaks are more common than is realized. Stratigraphers, who organize geologic history, estimate that the time spans of non deposition exceed that of episodes of deposition. These gaps could be fleeting, as in a river meandering away and then reoccupying the old channel, or they could indicate conditions of non deposition and erosion lasting tens to even hundreds of millions of years.

Geologists recognize a hierarchy with the longer lasting breaks often pointing to major changes such as a mountain building phase accompanied by a prolonged sea level fall. As sediment deposition stops, erosion will often sculpt the top of the rock formation into an uneven surface. Eventually sediment deposition will resume. Geologists term such a break between the two phases of rock formation as an erosional unconformity.

Recently, the Geological Survey of India added the Eparchean Unconformity to its growing list of Geo-Heritage sites of national importance. This is one of those major long lasting erosional breaks seen in the Precambrian terrains of India. The chosen site is near Kalinjar Fort, in Banda District of Uttar Pradesh. Image Source: Geological Survey of India.

At this location, 1.2 billion year old sandstone of the Vindhyan Basin overlie 2.5 billion year old Bundelkhand granites marking more than a billion years of non deposition and erosion. While the amount of unrecorded time is significant, the real importance of this site is in the very different earth conditions represented by the older Bundelkhand granite as compared to the much younger Vindhyan sandstone. This unconformity marks the transition between the older Archean Eon and the younger Proterozoic Eon.

The Archean was a much hotter world. Melting of the earth’s mantle was producing large batches of silica rich magma which solidified to form buoyant continental crust, small rafts at first, growing into larger blocks as time went by. Vertical crustal movements created narrow depressions which got filled with lava and sediment eroded from nearby granitic highlands. These volcano-sedimentary successions were deformed and metamorphosed, and were preserved as enclaves within the granitic terrains.

By around 2.5 billion years ago, magmatic growth of continents petered out. Geologists estimate that around 70% of the present volume of continental crust was generated between 4 billion to 2.5 billion years ago.

As the mantle cooled, the more stable continental crust became the floor for a younger generation of sedimentary basins. The processes of chemical weathering and sediment transport became more prolonged on this wide gently subsiding continental terrain. As a result, waves and currents had more time to sort sediment by size, shape, and density before it was buried.

Thick deposits sorted by size into gravel, sand, and mud are the typical features of these younger basins. At places, ocean water saturated with calcium carbonate precipitated layers of calcite and aragonite sediment, preserved today as thick limestone. The biosphere was dominated by bacteria and unicellular eukaryotes, their morphology often imprinted on the rocks as wavy layers or as small mound shaped objects.

Magmatism and continental crustal growth continued less frequently in pulses coinciding with regions of plate convergence and orogeny. This phase, before large animal life evolved, lasting between 2.5 billion years and 539 million years ago is known as the Proterozoic Eon.

The newly listed National Geological Monument at Kalinjar Fort showcases these changing earth conditions.

For many decades though, exactly where to place the Eparchean Unconformity was something of a problem in Indian field geology. There was only sparse information on the absolute age of rocks and no consensus on the time span the Archean represented. Field mapping through Peninsular India indicated the presence of a distinct erosional break between deformed and metamorphosed rocks (crystalline basement) and younger flat lying or less deformed sediments. This appeared to be the natural horizon between the Archean and the Proterozoic.

But flat lying sedimentary sequences in different Indian basins, often referred to as Purana Basins, had been deposited at different times on older deformed rocks of varying ages. In some cases the foundation of the sub-horizontal sedimentary basins was indeed Archean. In other locations, the highly metamorphosed and deformed older rocks were actually Proterozoic. That meant that the Eparchean Unconformity, as earlier conceived, was not a useful marker of synchronous changes in earth processes. Many of these doubts are summarized early in this 1968 perspective on the Eparchean Unconformity by Dr. T V V G R K. Murthy, faculty at University of Sagar.

The dates for the Archean-Proterozoic transition have now been fixed at 2500 million years by the International Subcommisson of Precambrian Stratigraphy, without any condition regarding the rock type above and below the unconformity surface. As a result, the older criteria for identifying a particular erosional break as the Eparchean interval based on only rock types and extent of deformation was no longer tenable.

We finally have an answer for the question posed by Dr. Murthy so long ago:

“Does the eparchaean unconformity become evident by recognizing Archaean and Purana formations or are the Archaean and Puranas recognized by identifying the eparchaean unconformity”.

Today, better absolute dating of rocks makes differentiating Archean from Proterozoic rocks easier. The Eparchean Unconformity though is still an informal but useful term in Indian field geology signifying the cooling trajectory of the earth’s interior, with a hotter Archean transitioning to a cooler Proterozoic.

One of my earliest encounters with the Eparchean Unconformity occurred during my graduate studies. I was assigned to map an area of the Cuddapah Basin near Gani village in Andhra Pradesh, South India. I first traveled by the Dadar -Madras (now Chennai) Express to Guntakal. There, I transferred to a medium gauge train passing through small market towns connecting a vast hinterland an urban kid like me was unfamiliar with. The train chugged along eastwards towards Nandyal. For a couple of hours from Guntakal, we were firmly on Archean terrain, an agricultural landscape interrupted by small hillocks of granitic rocks. In a distance through the late morning haze we could see the low Cuddapah hills with their distinct tilted strata.   

The train crossed the Eparchean Unconformity and entered the Proterozoic through a breach in the Cuddapah hills at Dhone Junction. Ahead was the Cuddapah Basin stretching to the horizon.

We moved through quartzites, limestone, and shale, the three predominant sedimentary rock types of the shallow marine Proterozoic continental shelf. At Nandyal I had to hop onto a bus going to Kurnool. Gani village was midway between these two places. I remember arriving at Gani late evening to a warm welcome by my host Mr. Sivanand Rao. Next morning, with maps and aerial photos, hammer, and a geologic compass I started out. A half hour walk south of Gani were outcrops of the sedimentary rocks I had to map. It was a challenging and immensely satisfying introduction to field geology.

Picture shows a very youthful me and my field guide Yanganna standing in front of a quartzite hill. Captured by my accompanying batch mate Nitin Rane. June 1987.

Someday soon I hope to retrace my steps to little Gani village. The medium gauge tracks from Guntakal to Nandyal have long been replaced by standard gauge. Express trains pass through Nandyal, eventually swerving north towards Vishakapatam, or continuing south east towards the Coromandel Coast.

From Nandyal I may hire a car to Gani. I hope to meet Mr Sivanand Rao who so graciously hosted me over two field trips. I want to walk again towards the hills made of Paniam Quartzite, a sand deposit sorted and washed clean by powerful Proterozoic tides. Today, the Kurnool-Gani solar park stands near Gani, its tilted solar panels made up of ultra pure silica wafers making up an Anthropocene analogue of the more ancient quartz rich deposits.

Photomicrograph shows the Proterozoic Paniam Quartzite classified as a “super mature quartz arenite”. Notice the well rounded quartz grains outlined by iron oxide. Location - Gani anticline.

Beyond the quartzite hills are the Vempalle Formation dolomite, a vestige of the Proterozoic ocean I first waded in to begin my life long association with carbonate sedimentology. It will be a home coming of sorts for me.

Photomicrograph shows microbial layers in the Vempalle dolomite. Dolomite crystals of different size and shape have replaced the original organic mats. Location- Gani anticline.

I have one more incentive to relive this journey. The filter coffee at Guntakal railway station canteen. It is the very best.

Iran Oil, Flowering Plants, India Aquifers

Some readings and a podcast from the past few weeks- 

1) The Geological controls on Iran Oil- Geology lovers who like to explore satellite imagery would have surely noticed the landscape of southern Iran. The crust is wrinkled up into unending fold mountains. These have resulted due to the collision of the Arabian plate with the Eurasian plate. And that convergence earlier in history created a depression which filled  with organic rich mud, the source of all that Iranian oil reserves. Stephanie Pappas has written a nice primer on the quirk of geology that explains Iran’s oil bounty.

2) How Flowers Transformed Planet Earth-  “Both in the evolution of life on this planet, and in human culture, flowers have been a critical engine of connection and cooperation”. I had enjoyed David George Haskell’s earlier book, “The Forest Unseen”, in which he observed one square meter of forest floor through different seasons to track changes in fauna and flora and what that teaches us  about ecologic relationships. Viviane Callier talks to him about his new book on flowering plants and their significance in science and society.

3) Indian Hydrogeology- Groundwater is the lifeline of Indian agriculture. That makes understanding aquifers a critical aspect of exploring and utilization of this resource. Recently, Pune based groundwater researcher Dr. Himanshu Kulkarni was awarded the International Water Prize by the University of Oklahoma for his contributions to Indian hydrogeology. They span nearly 4 decades of work in the Deccan basalts, as well as other Indian geologic terrains. His work includes not just the science of aquifers, but also efforts on involving local communities in sharing and managing this resource.

Veena Srinivasan of Well Labs has a long conversation with Dr. Kulkarni about his life’s work. On a personal note, Himanshu was my senior in University. It was really good to hear about this recognition for his important contributions to Indian groundwater science.