Revising the Age of the Vindhyan Rocks

From Science Daily, a summary of research on the PreCambrian Vindhyan Group of rocks from central India. The Vindhyans are a sprawling east west oriented mountain chain. You encounter it when you travel on central railway from Pune to Delhi in the vicinity of Bhopal. All those flat lying, layered sandstones you see are the Vindhyans. The upper part of the Vindhyans, the Rewa and Bhander Groups have been thought to be about 700-500 million years old. This new research proposes that the Upper Vindhyans are about 1000 million years old, a revision of up to 500 million years. The bizarre part is that Science Daily never mentions that it is the upper part of the Vindhyans that have been studied and I had to dig into the abstract to settle that. Geologists are used to dates being revised by a few million years, but this revision if it stands is jaw dropping. Think about it for a minute. The entire Phanerozoic period from Cambrian to recent, defined by the first appearance of abundant skeletonized animal fossils is about 540 million years. This proposed correction represents about 11% of earth history. The images below depict the geological time scale and you get an idea about the ranges I am talking about. Notice how small a time period the Phanerozoic is compared with the PreCambrian.

How can such a massive revision come about and why were geologists so unsure and so off base regarding the age of the Vindhyans? The advantage when working in the Phanerozoic is the presence of fossils. We can use changes in fossil composition to subdivide the geological record into finer time periods which reflect changing ecology's, climates and sea-levels. The PreCambrian, a vast time period ranging from about 4.2 billion years to 542 million years has a very scanty fossil record. This combined with severe metamorphism -especially in the Archean, the earlier part of the PreCambrian- which destroys original sedimentary structures makes paleo-ecological reconstructions difficult. It has been hard to create a meaningful and fine scale subdivision of the rock record that reflects biological evolution and geological changes occurring worldwide. Geologists have been using continent scale thermal and tectonic events that may leave a characteristic and correlatable rock record to subdivide the PreCambrian but such events are far and few between. One such example though is the kimberlite magmatism that affected Peninsular India around 1100 million years ago. This magmatism includes the famous diamond bearing rocks of Panna district, Madhya Pradesh. This activity has left a marker horizon throughout the Peninsular Indian sedimentary basins that is used to identify the base of the NeoProterozoic. Overall, the situation is somewhat alleviated in the Proterozoic, the younger portion of the PreCambrian. Here there are examples of un-metamorphosed sediment from all over the world including India. More detailed studies are finding evidence of evolution preserved as trace fossils and early forms of skeletal organisms, especially in the NeoProterozoic. But the lack of a consistent fossil record has certainly limited the fine scale subdivision of PreCambrian earth history.

The report in Science Daily makes it seem as if the much older age proposed has come as a complete surprise. But that is not so. For long the Vindhyans, which are mainly composed of sandstone, shale and limestones were thought to have lacked minerals which could be used for absolute radiometric dating. But that has changed recently. Geo-chronological work on the Vindhyan group has been going on for several years now and many earlier studies have shown that the Vindhyans are much older. Many of this earlier studies had fixed the age of the lower part of the Vindhyans to be around 1600 million years old, leaving the age of the Upper Vindhyans somewhat unresolved. This current work though focuses on the Upper Vindhyans and uses two independent methods as cross-reference. I find the collective evidence quite persuasive. This may turn out to be one of those findings where geology textbooks may have to be altered.

Science Daily also points out some exciting implications of this finding. First, that it removes an objection to the Snowball earth theory, which says that from 790-630 million years ago, the earth was covered with glaciers that extended up to 10 deg of the equator. There is no physical evidence of this carved on the Vindhyans so pushing back the age of the Upper Vindhyans, making them much older than the snowball earth gets rid of this obstacle. Another implied paradigm shift is regarding the timing of the origin of multi cellular animals. Skeletal remains of metazoans begin appearing in the fossil record by 580 million years ago. The accepted date for the origin of metazoans based on fossils and molecular phylogeny is in the NeoProterozoic between 700-600 million years ago, and the body and skeletal fossil record of that and the slightly younger early Cambrian indicates several rapid evolutionary radiations whereby successive grades of complex animals evolved geologically rapidly and filled empty ecologic niches. On the other hand, some studies in the Vindhyans claim preservation of traces and burrows of triploblastic animals in rocks older than 1 billion years ago, dates which were not accepted by many geologists. No body fossils or skeletal remains of metazoans have been found in rocks this old anywhere. If these trace fossils do represent triploblasts then that would push the origin of complex animals much further back in time and implies a much more stately and slow evolution of complexity. In this view much of the evolution of complexity has been cryptic, not revealed by the fossil record perhaps because animals lacked hard skeletons. The relatively sudden appearance of body fossils by late NeoProterozoic -early Cambrian is then a preservation artifact. Animals already had evolved complex body parts, the explosion of fossils simply indicating a new ability to secrete skeletons. This latter view has gained in popularity among some geologists working on the Vindhyans. But the interpretations of the trace fossils from the Vindhyans has been criticized as not indicative of triploblasts, and basically most paleobiologists think that the metazoan body fossil patterns of the NeoProterozoic and early Cambrian are a faithful record of evolutionary history i.e. metazoans arose relatively late in the NeoProterozoic and evolved morphological complexity rapidly thereafter. Now a more reliable geo-chronological framework for the Vindhyans may open up these old findings to better scrutiny.