ABSTRACT

The Mesozoic rocks of the Surendranagar district of Saurashtra (Gujarat), which overlie the Early Proterozoic basement and underlie the Deccan basalt, are interpreted to record the infilling of an Early Cretaceous failed riff. The rift-fill, comprising four formations (Than, Surajdeval, Ranipat, and Wadhwan, in ascending order), consists predominantly of an interbedded terrigenous assemblage of seven major and three minor lithofacies.

The lowermost Than Formation is mainly composed of coarsening upward successions of carbonaceous shale, including thin stringers of coal, interbedded siltstone and shale, and white and reddish crossbedded sandstone, interpreted to be the deposits of delta distributary channels in the proximity of shoreline. The succeeding Surajdeval Formation consists of lenticular bodies of crossbedded sandstone, local lenses of conglomerate, and laminated red shale and mudstone, exhibiting recurring fining-upward cycles; minor lithofacies include flaser-bedding and hummocky cross-stratification. These deposits are interpreted to have been laid down in the distal delta plain and nearshore tidal/coastal environment. The overlying Ranipat sandstone is mainly white, coarse, medium- to fine-grained, show ng profuse development of large and small scale planar- and trough-crossbedding, including locally herringbone crossbedding, probably deposited in subtidal/tidal and beach environment. The uppermost Wadhwan Formation contains lenses of fossiliferous limestone in the interbedded assemblage of pebbly, coarse to medium sandstone and mudstone and is interpreted to have been deposited in estuaries and embayments. Overall, the vertical lithic fill shows a progressive increase of coarse clastics from Than to Wadhwan, with a general increase of fine clastics laterally along the length of the basin in the direction of paleoflow and paleoslope from northeast to southwest.

It is suggested that the development and infilling of the Early Cretaceous Saurashtra failed rift, including subsequent eruption of Deccan basalt, coincided with the pericratonic rifting and opening of the Arabian sea to the west. Using the Saurashtra basin as a model, an idealized rift fill of the subsiding part should represent a succession of delta distributaries and tidal channels and interchannels in the lower part, grading up to nearshore coastal plain, embayments and estuaries, as shoreline advanced landward through time. The overall sequence tends to be transgressive through time and space.