The Vindhyan basin, a vast crescent-shaped stretch of ancient geological land in central India, stretching from Bihar to Rajasthan, has a new date of birth. A team of Indian and American geologists has discovered that the basin — a depression in the earth’s crust spanning hundreds of kilometres — was formed more than one billion years ago, 500 million years earlier than previously believed.

The revised age of the Vindhyan basin buttresses evidence for the idea that virtually the entire surface of the earth was covered with snow and ice from about 700 million years to 635 million years ago — what scientists call the Snowball Earth theory. The new age, some geologists believe, will also add justification to the search for petroleum deposits deep under central India.

University of Florida geologist Joe Meert and his colleagues from India and the US analysed imprints of the earth’s magnetic field on dozens of samples of rocks — the size of wine corks — collected by drilling through 56 sites in Rajasthan and the Son Valley. They also studied a mineral called zircon embedded in sediments buried at various places in the Vindhyan basin.

Their results appear to converge on an age of about 1.02 billion years — much earlier than previous estimates which were based largely on fossils recovered from the sediments between 500 million years and 700 million years old.

“In modern geology, a revision of the age of basins by 500 million years is pretty unique,” said Joe Meert, an associate professor of geology at the University of Florida. The findings appear in the latest issue of the journal Precambrian Research.

“The revision is enormous — some 11 per cent of the total earth history,” said Abhijit Basu, a professor of geological studies at Indiana University. Basu was not part of this study but has long pursued research on the evolution of rocks on earth.

Scientists have debated the age of the Vindhyan basin — an area of about 104,000 square kilometres extending from Rajasthan through Uttar Pradesh and Bihar — for nearly a century. Some researchers had used fossils to suggest that the basin was formed about 500 million years ago, although a few studies over the past five years have pointed to rocks up to 1.6 billion years old.

“There was always some unease about the 500 million years age,” said Dheeraj Mohan Banerjee, a geologist at the University of Delhi and a member of the team that revised the age.

The new age of the Vindhyan basin justifies the search for petroleum deposits in central India

The lower layers of rock in the Vindhyan basin are known to be about one billion years old, Banerjee said. “The debate was centred mainly on the upper layers.”

The previously assumed young age of the basin was a source of irritation for geologists for another reason. It just didn’t fit in with the Snowball Earth theory. Over the past four decades, scientists have picked up telltale signatures of an intensely cold phase in the history of the earth when the oceans had frozen and temperatures in the equator were as cold as it is near the poles today.

Geologists have picked up these signatures in rocks from North America, Europe, Africa and northern Asia. “But they were missing in the Vindhyan basin. Now we know why — it formed long before the great freeze,” Banerjee said.

The revised age might also prompt scientists involved in the search for hydrocarbon deposits to view the Vindhyan basin in a new light. The age of underground layers of rock is an important factor in the search for petroleum deposits, said Banerjee.

Last year, an independent study by senior geochemist Pat Bickford of Syracuse University in the US had dated rocks from another ancient geological basin in India to about 1.02 billion years, pushing back its presumed age by 500 million years.

These revisions may now put a question mark on the hypothesis that the basins formed when a supercontinent called Rodinia broke up. Rodinia broke up about 700 million years ago, but the basins are much older than the split, Bickford said.