Thursday, October 31, 2019

Geology Of India TV Series- Coral Islands Episode

There is a 13 part series on the geology of India being shown on DD National, a Government of India run channel. In 2016, Vigyan Prasar, an autonomous organization under Department of Science & Technology  had commissioned Pulse Media,  a New Delhi based television production company, to shoot and produce the series.

This is really a commendable decision to give earth sciences the attention it deserves and to try to bring this fascinating field to a broader audience.

But the one episode I've seen so far has been disappointing.

Last Saturday I saw the episode on Lakshadweep Islands and was shocked to see that its depiction of the geological evolution of the islands was factually wrong. The episode describes the formation of volcanic islands (Laccadive Ridge) in the Arabian Sea west of the State of Kerala. The map shows the location of the Laccadive Ridge and adjacent basins relative to the present west coast of India.

 Source: Kerala-Konkan Basin: Directorate General of Hydrocarbons, Govt. Of India.

The audience learns through narrative and a graphic that prolonged volcanism around 66 million years ago resulted in lava piling up on the sea floor and eventually sticking out above sea level forming islands. In the Quaternary Period ( beginning 2.6 million years ago), coral colonies then formed in the shallow water around these islands. The islands then subsided, leaving behind rings of coral reefs encircling deeper lagoons. This explanation (first proposed by Darwin) applies to younger oceanic volcanic regions like the S. Pacific where thermal subsidence over the past few hundred thousand to few million years has promoted the formation of the classic reef and atoll system.

Volcanism in the Laccadive region stopped by 60 million to 55 million years ago. Any thermal effects would have long vanished. In any case, there is no evidence that the Laccadive ridge ever was an island chain. It is considered a submarine volcanic ridge. This ridge actually originated when the Indian continent broke away from Africa. The western continental margin of India was faulted and a series of ridges and depressions were formed due to block movements along faults. The Laccadive Ridge is one such 'structural ridge'. These structures formed by late Cretaceous times (90 million to 66 million yrs ago), and may have been rejuvenated from time to time.

Volcanism then poured lava on top of this ridge and over adjoining regions too. As the sea floor rises here forming a topographic high, the seas above it are shallow. Conditions favorable for calcium carbonate shell secreting organisms have persisted for millions of years. As a result, on this undersea volcanic foundation, a thick pile of limestones has accumulated over the past 50 million years.  Sediments ranging in age from the Eocene to the Pleistocene underlie the present day coral reefs.

The cross section shows the stratigraphy (sedimentary sequence) of the Laccadive Ridge and adjacent regions.

Source: Kerala-Konkan Basin: Directorate General of Hydrocarbons, Govt. Of India.

The present system of living corals has nucleated on a foundation of Pleistocene limestone. They did not form surrounding 'volcanic islands'. Coral blocks and sand originating from dead corals and other shell producing organisms has been moved by currents and has piled up above sea level forming the Lakshadweep Islands. The initiation of coral growth is really tied to creation of appropriate water depths as a result of sea level changes occurring repeatedly over the Quaternary Period due to the waning and waxing of ice ages. It has nothing to do with subsiding volcanic islands.

I was really surprised to see that a factually wrong scenario passed the filters of the subject experts credited in the episode. Did they not peruse the final script? The episodes are of 25 minutes duration. I can understand a need for brevity and simplicity of explanation.  But scientific accuracy is more important and cannot be sacrificed in pursuit of brevity. I do hope accuracy is not a victim in the remaining episodes too. The series is being shown on Saturday evening at 530 pm IST on DD National.

Disclosure: Pulse Media had hired me as a consultant to do background research for this television series. Needless to say I am disappointed that the inputs I had sent regarding the geological evolution of Lakshadweep were not included in the episode.

Monday, October 14, 2019

Papers: Carbonate Sedimentology Tribute To Robert Ginsburg

Dr. Robert Ginsburg, who spent much of his career studying the geological evolution of the Florida Keys died recently. The Depositional Record has an open access special issue in his honor packed with papers on the modern and ancient carbonate rock record.

In Precambrian times (before 542 million years ago), the precipitation of calcium carbonate on the sea floor was influenced by the activity of microbes. Beginning around 530 million years ago, complex multicellular organisms evolved the ability to secrete calcium carbonate as a protective shell.  Since then, limestones have been forming by the aggregation of skeletons of marine organisms. They tell us about past biodiversity and the conditions in which these ancient organisms lived. Understanding the controls on the origin and accumulation of these sediments using present day examples provides useful analogues to interpret the past. Limestones (CaCO3) and dolostones (CaMg(CO3)2) are also important petroleum reservoirs.

The satellite image shows a portion of the Florida Keys carbonate platform.  It is made up of a low energy shoreline with plant stabilized mud flats, quiet sea grass covered lagoons, and towards the southern reaches, an arcuate coral reef system. The sunlight waters provide ideal conditions for a complex community of shell secreting organisms. Broken down shell fragments accumulate either in-situ or are distributed across the platform by waves and currents. Dr. Ginsburg wanted to know the details of these processes.

In the published issue, there are quite a few papers on carbonate depositional environments and the sediment production and distribution processes in action in the Bahamas shallow marine region. These are supplemented by examples from other parts of the world, including the Florida Keys.

... and don't forget to read the warm humorous tribute celebrating the life and work of Dr. Ginsburg written by Eugene Shinn.

"But, what would be his dissertation subject? Someone came up with a catalogue advertising Fellowships at the University of Miami in Florida. When he asked Jack Hough about Miami, Jack admitted he had never heard of the place. In fact, no one that far north had heard of the University of Miami. Bob drove south for the interview anyway. He was on a mission, an idea he wanted to test. He wanted to determine the process by which sediment became rock. It seemed straightforward enough. All you had to do was dig a hole or push a core tube a few feet into the sediment until it stopped. That would be the zone of transition from sediment to rock. He got the Fellowship (about $3,000) and began his research. Of course, the problem turned out to be more difficult than expected, so difficult in fact that many scientists are still working on it today".

One of his seminal contributions was a proposal to explain the cyclic nature of carbonate sedimentary deposits.  It is observed that many thick limestone sequences are made up of a repeated pattern of relatively deeper water subtidal sediments overlain by shallower water intertidal sediment. These couplets are stacked to form deposits that can be hundreds to even thousands of feet thick. Dr. Ginsburg suggested that this pattern arises due to the cyclical shifts in the reduction and expansion of source areas of carbonate sediment.

At times when the open sea area is large, healthy organic growth produces a large supply of skeletons. Breakdown of these skeletons produces carbonate mud. This mud is transported by currents and trapped along the shores causing accretion of tidal mud flats, which grow towards the open sea. This spread of tidal flats in turn eventually shrinks the size of the source region, reducing sediment supply and stopping tidal flat growth. Sediment production in this system then falters. Natural subsidence of the basin reestablishes water depths for optimum organic growth, again resulting in healthy sediment production, and the cycle restarts.

Here is the abstract of Dr. Ginsburg's model published in AAPG Bulletin in 1971.

Title: Landward Movement of Carbonate Mud: New Model for Regressive Cycles in Carbonates:

Repeated regressive cycles are characteristic of the Paleozoic shallow-water carbonates of North America; similar cycles are present, although less abundant, in Mesozoic and Cenozoic strata worldwide. Several of these cyclic carbonates contain major hydrocarbon reservoirs: Permian, Central Basin platform; Mississippian, Saskatchewan; Ordovician and Silurian, Montana. Studies of comparable recent deposits in Florida, the Bahamas, and the Persian Gulf suggest an alternative to the accepted tectonic explanation of these cycles.

The Florida Bay lagoon and the tidal flats of the Bahamas and Persian Gulf are traps for fine sediment produced on the large adjacent open platforms or shelves. The extensive source areas produce carbonate mud by precipitation and by the disintegration of organic skeletons. The carbonate mud moves shoreward by wind-driven, tidal or estuarine like circulation, and deposition is accelerated and stabilized by marine plants and animals.

Because the open marine source areas are many times larger than the nearshore traps, seaward progradation of the wedge of sediments is inevitable. This seaward progradation gives a regressive cycle from open marine shelf or platform to supratidal flat. As the shoreline progrades seaward the size of the open marine source area decreases; eventually reduced production of mud no longer exceeds slow continuous subsidence and a new transgression begins. When the source area expands so that production again exceeds subsidence a new regressive cycle starts.

The seaward progradation suggested by this model should be observable in ancient deposits.

This explanation of cyclicity is known as the autocyclic model, since all the feedbacks are internal to the system. The alternative explanation is called the allocyclic model. In this case, lithologic repeats are thought to result from changes in sea level caused by the growth and decay of polar ice caps due to cyclic changes in the solar radiation received by the earth (Milankovitch Cycles).

Dr. Ginsburg's work still generates a lot of debate.

Dive in!

Monday, October 7, 2019

Geology As A Socially Embedded Science

C.P. Rajendran writes on geology as a socially embedded science, and traces its historical development from a tool to exploit natural resources to present day concerns about sustainability.

"The bottom line of the arguments is that geology fortified by its unique narrative power and reasoning prowess which are the hallmarks of all historical sciences, cannot be seen from the perspectives of physics nor should it be treated as a derivative science. Geology is a ‘preeminent example of a synthetic science’, wherein the geologist employs a suite of logical techniques and tools to understand nature and its components. And, such reasoning powers that depend on the classical hermeneutical methods or interpretative logical procedures offer far superior methodology to find answers in a world of complexities and uncertainties that we now inhabit, be it safe disposal of nuclear waste, climate change or receding groundwater levels".

Fine essay. Do read.

Open Access.