Petrography and stable isotope geochemistry of Middle Eocene Garampani limestone (Assam), India: implications for the depositional environment and diagenesis

Petrography and stable isotopic (carbon and oxygen) studies of the Middle Eocene Garampani limestone from Jamunagar limestone quarry of the Umrangso area, North Cachar Assam, were carried out to determine the environment of deposition and the level of diagenesis to improve our understanding of the climate at the time of its formation. The major petrographic types documented here include wackestone, packstone, and grainstone which fall into the type I and II categories of the triangular classification scheme indicating formation in a short-lived, high-energy environment that was dissipating at the time of deposition. The samples have a distinct signature that includes extensive diagenesis with varying intensities of compaction, recrystallization, pressure solution, and neomorphism. In occurrences of stratigraphically significant foraminiferal assemblages and palynofossils, the Garampani Limestone is assigned Middle Eocene age. Most of the samples produced positive δ13C ‰ ranging from 1.64 to 0.76 ‰ (PDB) and correlated with a transgressive phase. The negative δ18O values range from −6.25 to −4.67 ‰ (PDB) suggesting that the Middle Eocene was generally characterized by a warmer climate.