GUWAHATI

A significant swathe of present-day Assam was once under a warm, shallow tropical sea rich in microscopic marine life, a new study has revealed.

Two researchers from Nagaland University carried out a geological investigation of the Sylhet Limestone deposits in Karbi Anglong district’s Dillai Parbat to shed light on Assam’s sea link about 50 million years ago.

Sylhet Limestone, also known as Nummulitic Limestone, is a prominent middle-Eocene sedimentary rock formation (about 40–50 million years old) found in India’s northeastern region and Bangladesh. Composed of calcite, dolomite, and an abundance of marine microfossils, it is widely used as a raw material for cement, glass, and steel manufacturing.

Professor S.K. Srivastava of the university’s Department of Geology and research scholar Tsenchumo Lotha combined field investigations with laboratory analyses, including petrography and micropaleontology, for their study published in the peer-reviewed Journal of Geosciences Research.

The researchers found evidence that the limestone was deposited in a shallow, warm marine setting, “similar to a lagoon or near-shore shallow sea setting under a tropical to subtropical climate” with abundant oxygen and nutrients that supported diverse microscopic marine organisms.

They also examined limestone, shale and sandstone layers, sedimentary structures, and fossilised foraminifers preserved within the rocks to identify repeated fluctuations in sea level and changing energy conditions in the basin. Their findings indicated that ancient storms periodically disturbed the seafloor, leaving special patterns in the sediment that were later preserved as rock.

The researchers suggested that tectonic movements influenced the changing depositional conditions, providing valuable information for reconstructing the geological evolution of northeast India and improving understanding of carbonate sedimentation in the region.

“The evidence indicates that the limestone was deposited in a shallow marine, warm-water environment with favourable ecological conditions for micro-faunal growth,” Mr Srivastava said.