| Abstract: | In two Proterozoic sandstones, of the Indian shield cross-stratification and cross-lamination are observed to grade continuously into parallel-laminations often bearing parting lineations. These are interpreted as having resulted from a gradual transition from ripple/dune to upper stage plane bed structures. During the transition the inclination of the cross-strata diminishes and their shape changes from concave-up to sigmoidal. The sigmoidal cross-strata are characterized by well defined topsets, foresets and toesets and the topsets bear parting lineations. In the course of the transition sigmoidal cross-strata may give way either to horizontal parallel-lamination or inclined parallel-lamination. In the former the toesets of the successive sigmoidal cross-strata thicken and the thickness of the cross-strata beyond the brink point (the junction between the topset and foreset) tends to become uniform until a plane-bed state is reached and horizontal parallel-laminations are formed. In the latter the topset of the successive sigmoidal cross-strata increases in length at the expense of the foreset and toeset until the brink point ceases to exist so that only the topset laminations prevail and appear as inclined parallel-laminations. These transitions presumably result from a gradual increase in flow intensity beyond the stability limits of ripples/dunes. The progressive morphological changes of the cross-stratification over the transition are attributed to changing fallout patterns on the lee face of the bedforms in response to increasing flow intensity. Preservation of the records of such transitions suggests an abundant supply of sand grade sediments from suspension during the transition, shaping the sediment concentration profile over the bedforms and facilitating turbulence suppression. The variation in the pattern of transition from sigmoidal cross-stratification to parallel-lamination may be the result of different rates of sediment feed from the prevailing suspended sediment load in the two instances. |
