Study of Indian Miogypsinidae

Abstract

The succession of Indian Miogypsinidae from three closely sampled sections in Kutch, from core and cutting samples of some deep wells in Cauvery Basin, and from two samples from Saurastra, has been investigated in order to evaluate its value in biostratigraphic classification and correlation of the Oligo-Miocene sections. Fourteen specific units of Miogypsina, belonging to three subgenera - Miogypsinoides, Miogypsina s.s. and Lepidosemicyclina - are recognized on the basis of the X - V scale, in combination with a few other features. Very primitive species, M. cf. bermudezi and M. complanata, and highly advanced ones, such as M. excentrica and M. antillea, are present. As a whole the evolutionary trends in Indian Miogypsinidae are in accordance with the principle of nepionic acceleration, but there is a major contradiction in the case of early Miogypsinoides. The development trend in younger Miogypsinoides - from M. bantamensis via M. dehaartii to M. indica - is parallel to that of the succession of Miogypsina s.s., from M. gunteri via M. tani to M. globulina. Subconical cupulaeformis forms come into the picture at about the middle part of the range of M. dehaartii. The evolution in the Indian Lepidosemicyclina lineage, from M. thecideaeformis to M. exentrica via M. droogeri, took place within the range of M. globulina. M. globulina assemblages from India present a case of very slow or arrested evolution. Seven range zones (= lineage zones) could be based on the succession known from India. Study of associated planktonic foraminifera suggests that the rate of evolution in the Miogypsina s.s. - Lepidosemicyclina lineage, at least in the interval from M. tani to M. excentrica, is much higher than it is in planktonic forminifera. The advantages and limitations of using Miogypsinidae in time-stratigraphic correlations are discussed.