290 INDIAN SUBCONTINENT Figure Geological map of the Singhbhum Craton (adapted with permission from Mahadevan TM, 1994; ß Geological Society of India, Bangalore) dominantly continental rift-basin deposits formed during the Early Permian and Middle Jurassic with minor marine inputs Adoption of such a definition helps in the finer tuning of the stratigraphic succession by removing the contemporaneous and other Jurassic-Creataceous formations, which are correlatable when the Jurassic Gondwana broke up and the Cretaceous-Eocene plume outburst occurred Recent studies favour adoption of smaller subdivisions of the Gondwana Supergroup instead of the two- and three-fold classifications based on floral assemblages Redefinition of the Gondwana Supergroup helps to identify three belts of the Gondwana basins These are: i E–W trending Damodar Valley basins Included in this belt is the narrow sub-parallel belt of subsidiary basins occurring north of the main belt ii NW–SE trending Son-Mahanadi Valley basins The belt widens and thickens in a northwesterly direction, meeting the extension of the Damodar Valley basins iii NW–SE trending Pranhita-Gondwana Valley basins Stratigraphic classification of the Gondwana formations developed in the three belts is given in Table The basal Talchir unit includes glacial tillites with shale and shale-siltstone rhythmites All the other units comprise sandstone with shale as the dominant lithology A number of coal seams (along with carbonaceous shale) occur in the Karharbari, Barakar, and Ranigani formations The Barren Measure, which occurs between the Barakar and the Raniganj is (as the name implies) devoid of any coal seam The Panchet and the Supra-Panchet are also coal-free sequences The latter is termed the Mahadeva Formation in the Satpura Range Sedimentation, Basin Morphology and Evolution Evidence from fossil flora and palynological studies indicate repeated changes in climate during the Gondwana sedimentation, which is also reflected in the nature of repetitive cycles of lithofacies After the initial cold glacial condition, the climate warmed up along with increased humidity, which varied between medium and very high This highly humid condition favoured enormous growth of plants The en masse deposition of plant remains ultimately led to the formation of coal seams The depositional condition changed to shallow freshwater with increased evidence of desiccation and salinity during later Panchet time Palaeoslope studies in the Damodar Valley provide inconsistent results, but a general north-northwesterly to northerly palaeoslope is suggested for these