Guwahati: A geological investigation by Nagaland University (NU) researchers dated the Sylhet Limestone in Assam’s Karbi Anglong district to the Eocene epoch, about 56 to 33.9 million years ago, concluding the region was once a warm, shallow, nutrient-rich marine environment that supported diverse ancient marine life.The study focuses on the Dillai Parbat area and offers new insights into the limestone’s depositional environment, ancient sea conditions, rock formation and environmental history in northeast India.“We attempted to understand the depositional conditions by combining field observations and laboratory analysis. The evidence indicates that the limestone was deposited in a shallow marine, warm-water environment with favourable ecological conditions for microfaunal growth,” Srivastava said on Monday.Published in the peer-reviewed Journal of Geosciences Research, the paper was co-authored by ProfSK Srivastava of NU’s Department of Geology and research scholar Tsenchumo Lotha.The researchers said the findings improve scientific understanding of sedimentation processes, microfossil assemblages and environmental conditions, and indicate the observed changes were linked to tectonic movements.The team examined limestone, shale and sandstone layers, documenting sedimentary structures and field features, and conducted laboratory analysis of foraminiferal microfossils and mineral composition through microscopic examination. Based on the combined evidence, Srivastava said the area likely had sufficient oxygen and nutrients to sustain marine life.“The conditions were similar to a lagoon or a near-shore shallow sea setting under a tropical to subtropical climate. The findings also suggest that the sea level kept going up and down over time and the depositional environment experienced varying energy conditions. There were also storms, which made special patterns in the rocks,” Srivastava said.Srivastava said the work would also support correlation of carbonate sedimentation in similar basin settings in Northeast India.The researchers said earlier studies on Sylhet Limestone largely relied on limited datasets and field observations, and the new work addresses the gap through an integrated approach combining field observations, petrographic analysis and micropalaeontological investigations.Researcher Tsenchumo Lotha said, “Things are not always clear when we are in the midst of fieldwork. But when we connected the same with lab results, the overall picture started becoming clearer. That process was really important for understanding the environment of deposition.”NU vice-chancellor Prof Jagadish Kumar Patnaik congratulated the researchers, calling it a significant scientific contribution that enhances understanding of the geological history of the northeast.
