MORPHOLOGY, INTERNAL STRUCTURE AND MECHANICS OF SMALL LONGITUDINAL (SEIF) DUNES IN AN AEOLIAN HORIZON OF THE PROTEROZOIC DHANDRAUL QUARTZITE, INDIA

The excellently preserved metre-scale, linear bedforms in an aeolian h orizon of the Proterozoic Dhandraul Quartzite, India, show oppositely dipping strata arranged in a zigzag pattern. The strata are dominantly of translatent type, deposited by along-crest migrating ripples prese rved on the flanks of dunes. The bedforms thus may be interpreted in a morphodynamic sense as longitudinal (seif) dunes. In order to determi ne the regional palaeoflow pattern, the migration directions of ripple s preserved at the top of sheet sandstones that are associated with th e dune cross-strata and internally show subhorizontal translatent stra ta were measured. A directionally varying flow with a mean direction n early parallel to the mean axial trend of the dunes is indicated. The kinematics of the dunes were thus largely the result of alternate oper ation of two oblique flow components, each of which was deflected at a dune crest into an along-crest flow on the downwind flank of the dune . The deflected flow formed along-crest migrating ripples, which in tu rn deposited climbing ripple strata. Alternate deposition on the two o pposite flanks resulted in near vertical accretion of the dunes, as is indicated by the zigzag pattern of stratal arrangement.