The Genus Globotruncana and Biostratigraphy of the Lower Ariyalur Stage (upper Cretaceous) of Vridhachalam, South India

The Lower Ariyalur Stage (Upper Cretaceous) cropping out in the north of Vridhachalam, has been sampled for detailed foraminiferal study. Nine species and subspecies and one new species of the genus Globotrunrana and several benthonic species are identified. On the basis of the vertical distribution of these species, the Lower Ariyalur Stage, earlier described as a [single uniform formation, is subdivided into five biostratigraphic zones and subzones, 'commencing from the? Late Turonian and continuing up to the Lower Maestrichtian. This sequence of strata is correlated with the well known Upper Cretaceous formations of North America, Trinidad, Middle East, Europe and Australia. The systematic description and evolutionary trends of the species and subspecies of Globutruncana are given. On the basis of percentage distribution of the planktonic forms, the depth of the basin is calculated. There is indication of a shallowing of the Upper Cretaceous sea at the end of the Globutruncana linneiana trlcarinata zone (Lower Maestrichtian).