The limits of stratigraphic distribution of Glossopteris in India

Abstract

The literature reveals that controversy still persists over the lower and upper stratigraphic limits of the distribution of Glossopteris in Indian strata. A fresh approach is therefore made in this paper to evaluate these two limits from a critical reassessment of all published data, supplemented to some extent by field and laboratory investigations made by the author and some unpublished information obtained through personal communication. The lower limit is found to be considerably younger than the end of Talchir glaciation, which is not a sharp stratigraphic datum but seems to be a time-transgressive boundary represented by a succession of more than one boulder bed. The glacial phase was terminated in India, from the evidence of associated marine horizons, in the Early Permian (Sakmarian), although its onset might date back into the Carboniferous. This suggests the lower limit of Glossopteris in India somewhere high in the Sakmarian or at the base of the Artinskian. The upper limit is assessed from a reconsideration of the stratigraphic relations of the macroflora, microflora and vertebrate fauna accompanying the Permo-Triassic transition, particularly the Raniganj-Panchet sequence of Peninsular India. Results of recent research indicate the upper limit of the range of Glossopteris lies some way above the base of the Panchet Formation, i.e. at the summit of its lowermost section. This limit seems to coincide with the lower boundary of the Triassic. From the limits and pattern of the vertical distribution of Glossopteris, a simple scheme of floral zonation for the Indian lower Gondwana sequence is suggested. It is emphasized that the line of division between the lower and upper Gondwanas, which is conventionally placed at the upper boundary of the Panchet Formation from palaeontological and lithological criteria, should be defined on a biostratigraphic (particularly floral) basis, and accordingly lowered to the top of the Glossopteris-Dicroidium (transitional) zone. This zone is believed to be restricted to the lowermost beds of the Panchet Formation with very rare exceptions.