Miocene deep water agglutinated foraminifera from offshore Krishna-Godavari Basin, India

摘要

The Krishna Godavari Basin is located in the East Coast Indian Peninsula containing 5 to 7 km of Cretaceous through Cenozoic terrigenous clastic sediments. The Miocene-Pliocene section in the offshore is known as the Ravva Formation, a major argillaceous facies with sandstone beds from which oil and gas are being produced. In this sequence, some cutting samples studied from four offshore wells were found to be rich in agglutinated benthic foraminifera, while planktic foraminifera were absent at these levels. These samples were studied in much detail for agglutinated foraminifera. A total of fifty-nine agglutinated foraminiferal species belonging to thirty-five genera have been identified. Two new species, Alveolophragmium indicum Govindan n. sp and Liebusella krishna Govindan n. sp., are described. All these species are reported for the first time from this region. The agglutinated benthic foraminiferal taxa bear close resemblance to those of Miocene agglutinated foraminifera assemblages reported from South Trinidad, the Falcon Basin of Venezuela, and offshore West Africa. The present report of alveolar genera, Alveovalvulina, Alveovalvulinella and Guppyella, and compressed, inner partitioned alveolar genera Pavonitina and Spiropsammia, together with other genera having complex reticulate inner structure as Cyclammina, Reticulophragmium and Alveolophragmium in this region are of great importance for the evaluation of biostratigraphy, paleoecology, paleobiogeography and burial history. Paleoecological analysis reveals three agglutinated foraminiferal biofacies/groupings, which characterize beyond outer shelf through bathyal slope (500 to 1500m) depth regimes in an oxygen minimum deep water setting for the deposition of these turbidite/ debris flow sediments A major marine transgression at the close of the Oligocene probably caused a moderately rich, more uniform distribution of deep water agglutinated species over this deep basin. Additional observations on the agglutinated benthic assemblage such as deformation of the tests, coloration of the tests and stratigraphic markers are also dealt with briefly.