Stratigraphy, age and palaeoecology of Upper Gondwana equivalents of the Krishna-Godavari Basin, India

Abstract

Age equivalents of the Upper Gondwana sediments are exposed in the Krishna-Godavari Basin which comprises three depressions separated from one another by basement ridges. The sequence exposed in the West Godavari Depressions is classified into the Golapalli Sandstone (oldest), Raghavapuram Shale and Tirupati Sandstone (youngest) on the basis of lithologic characteristics. Likewise, in the Krishna Depression, towards the south, this sequence is classified into the Budavada Sandstone (oldest), Vemavaram Shale and Pavalur Sandstone. A number of shallow wells have been drilled on the outcrops. The dominantly arenaceous units, the Golapalli, Tirupati, Budavada and Pavalur sandstones, of the outcrop are represented by shales in the subsurface. The Raghavapuram and Vemavaram shales, however, continue to have the same lithology in the subsurface. It is concluded that the tripartite classification in the outcrops is not tenable in the subsurface and the representative units are probably lithological variants of a major argillaceous sequence. The exposed sequence is devoid of palynofossils. Plant megafossil, though scarce are considered referable to the Late Jurassic. The sequence met in the shallow wells contains abundant palynofossils of non-marine and transitional environments. The assemblages recorded from the dominantly argillaceous beds contain numerous taxa characteristic of an Early Cretaceous age. Hence, the sequences in two depressions are homotaxial and of Early Cretaceous age as is also inferred from microfaunal assemblages. An inter-relationship between the gymnosperm and phytoplankton populations has enabled to delineate some of the major environmental regimes which vary from non-marine in the west to shallow marine in the east through transitional swampy environments. The upland floral elements are predominant at the basin margin. The bulk of the terrigeneous matter was derived from areas located westwards. It is significant to note that the phytoplankton progressively increases due east and southeast. It could be as high as 20 percent near the western margin of the basin. If the sequences remain to be dominantly shaly in the subsurface of the present-day coastal area they are likely to possess good characters of source rocks of hydrocarbons, especially in view of the progressive increase in phytoplankton percentages and favorable depths of burial and palaeotemperatures which are evidenced by geophysical data and occurrence of extrusives in the outcrop are respectively.