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A comprehensive data of whole‐rock and stable isotopic geochemistry along with microfossil assemblages of the carbonate rocks of the ophiolite mélange zone of the Indo–Myanmar orogenic Belt (IMOB), north‐east India are discussed, to determine the influence of terrigenous contamination during the formation of these carbonate rocks and also to understand their depositional environment and ages. These carbonate rocks contain a diverse fauna with the dominance of foraminiferal assemblages of planktonic foraminifera (Globotruncana sp. and Heterohelix sp., etc.) which indicates they were formed during the Santonian to Maastrichtian age. Based on chemical compositions, these carbonate rocks have been identified as limestone (CaO/MgO > 50.1) to slightly dolomitic limestone (9.1 < CaO/MgO < 50.1). Total rare earth element (REE) contents in these carbonates are variable (22.39–146.05 ppm). The Post‐Archean Australian Shale (PAAS)‐normalized REE + Y patterns of these carbonates exhibit seawater‐like REE patterns with LREE depletion and relative HREE enrichment with negative Ce anomalies (Ce/Ce* = 0.32–0.79) and positive Y (4.42–27.81 ppm) and Eu anomalies (Eu/Eu* = 1.11–1.86), suggesting that they were deposited under an oxygenated environment with contamination by hydrothermal activity. They are also depleted in δ13C 0/00 (PDB) (1.02–1.570/00) and δ18O 0/00 (PDB) (−6.37 to −9.00%) values which characterize marine precipitates. Eu anomalies and spread in negative δ18O 0/00 (PDB) values to a lesser extent of δ13C 0/00 (PDB) values of these carbonates suggest their formation was altered by diagenesis in the shallow marine environment. Our new whole‐rock and stable isotope geochemical characteristics, in conjunction with microfacies, suggest that the investigated carbonate rocks might have been formed in low‐energy environments, and deposited in neritic to bathyal palaeoenvironments during the Santonian to Maastrichtian interval.

In the present study, the rocks of the Disang Group of Manipur are evaluated for source rock potential for hydrocarbons. In these Paleogene rocks, amorphous organic matter is the most abundant component in association with other forms of organic matter. Other types of organic matters such as charcoal, partly biodegraded land plant fragments, black debris, spores and pollens are recorded as well. Rock-Eval and TOC analysis of the studied samples suggest that all the rocks have poor organic richness (TOC < 0.5 %) and poor hydrocarbon generation potential (S2 < 0.5 mgHC/g rock). The organic matter content is predominantly of Type III and Type IV. The T max and Productive Index values support the results of visual kerogen analysis. Most of the kerogen is of low level of maturity inferred from light colour of amorphous organic matter in some samples. This conclusion is also supported by Low T max values and low productive index obtained from these samples. TAI (Thermal Alteration Index) values above 3.5 suggest highly mature to overmature mixed palynofacies. The Disang Group of the Manipur appears poor to moderate gaseous hydrocarbons as far as hydrocarbon potential is concerned.