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.
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.
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