Abstract

• Occurrence of organic matters in the Cretaceous marine fossiliferous limestones, viz., Thaungpwet Taung limestones exposed at the Second Defiles of Ayeyarwady River and Paungchaung limestones exposed at the eastern foothill of Mt. Victoria are studied. The identified fossil assemblages indicate that the paleobiogeographic locations of the two units were in the northern Tethys Sea during Cretaceous. Both lithofacies and biofacies analysis reveal that the light bluish grey limestones (Thaungpwet Taung Formation) were deposited in an opened lagoonal basin in the shallow marine shelf underlain by the well-oxygenated subsoils and later, the organic matters accumulated in the limestones were eventually replaced by calcites during diagenesis. The dark brownish grey limestones (Paungchaung Limestone Formation) were deposited as distal carbonate ramps in deeper sea with euxinic floor and later enrich with organic matters due to the worldwide oceanic anoxic event (OAE) which is indicated by the high ratio of smaller planktic foraminifers and dinoflagellates to the smaller benthic foraminifers, and occurrence of iron sulphide (pyrite) in the chambers of planktons’ test. In addition, the planktic foraminiferal biostratigraphic zones recognized in the Paungchaung limestones indicate a coincidence with the mid-Cenomanian OAE which caused a large accumulation of organic carbon across the Tethyan realms. It is contrasted, that above two types of limestones have different amount of organic matters though a global event provided similar euxinic condition. Therefore, in can be regarded that the geometry of individual basin was major controlling factor to the physiochemical condition of each basin. Present study evidenced that Paungchaung limestones with high organic matters were accumulated during the mid-Cenomanian OAE occurred across the Tethys Sea. Indeed, these limestones underlie the Tertiary clastic sequences of Central Myanmar which are being drilled for the hydrocarbon industries.