Forgotten Seascape: How an Eocene Shallow Sea Once Covered Assam’s Karbi‑Anglong

When the researchers from Nagaland University first trekked into the undulating hills of Karbi‑Anglong, they expected the usual granitic outcrops and laterite‑rich soils. What they found instead was a surprisingly continuous slab of limestone, shale and fine sandstone, studded with tiny, fossil‑filled shells that whispered of a very different past.

Led by Dr. T. K. Sharma of the Department of Geology, the team spent three weeks mapping outcrops around the Khasmahal and Koyakijong ranges. “At first glance you’d think you’re looking at typical hill‑top rocks,” Sharma recalls, “but the moment you start peering at the thin, wavy layers, the story changes.”

Laboratory analysis confirmed the presence of marine micro‑fossils – mainly foraminifera and scattered bivalve fragments – that are characteristic of shallow, warm‑water environments. In a few lucky spots, tiny ammonite whorls were even visible to the naked eye, a clear sign that seawater once lapped the very ground they now stand on.

Radiometric dating of volcanic ash layers interbedded with the sediments placed the formation firmly in the early Eocene, roughly 55 to 48 million years ago. In plain terms, that means the area was part of a broad, shallow sea that stretched across what is today Assam, Nagaland and parts of Meghalaya shortly after the Indian plate slammed into Eurasia.

The sedimentary package is surprisingly thick – estimates run close to 250 metres in some sections – suggesting that the marine conditions persisted for several million years. “It’s not just a fleeting tidal flat,” notes Sharma, “it’s a substantial marine basin that recorded a long, stable phase of deposition.”

Beyond the sheer novelty of a forgotten seascape, the discovery carries practical implications. Shallow marine carbonates are often good reservoirs for hydrocarbons, and the team hopes their findings will spark interest in further exploration for oil and gas in the region. Moreover, the fossil assemblage offers a valuable window into Eocene climate dynamics, a period marked by some of the warmest global temperatures of the Cenozoic era.

While the study is still undergoing peer review – the manuscript is slated for submission to the Journal of the Geological Society of India – the initial reaction from the broader scientific community has been enthusiastic. Dr. Priya Banerjee, a paleoclimatologist at the Indian Institute of Science, calls the work “a vital piece of the puzzle” for reconstructing the early Cenozoic seas of the Indian subcontinent.

So, the next time you hear locals speaking of the ‘green hills’ of Karbi‑Anglong, remember that beneath those verdant slopes lie the remnants of a marine world that vanished eons ago, leaving behind stone‑bound clues for us to decipher.