CSIR NET Dec 2015 Q116: Sedimentology

The Wasson and Hyde (1983) dune classification scheme plots dune type as a function of two variables: sediment supply (from limited to abundant, x-axis) and wind directional variability (from unidirectional to highly variable/multidirectional, y-axis). Barchan (crescentic) dunes form under limited sediment supply and low wind directional variability (strong unidirectional wind), corresponding to point A (lower-left). Longitudinal (linear/seif) dunes form under a somewhat greater sediment supply with increased wind directional variability (typically bidirectional wind regimes), corresponding to point B (intermediate position, higher on the variability axis). Transverse dunes form under abundant sediment supply with low wind directional variability (strong unidirectional wind), corresponding to point C (lower-right). Star dunes form under abundant sediment supply combined with high wind directional variability (complex, multidirectional wind regimes), corresponding to point D (upper-right). This gives A=Barchans, B=Longitudinal, C=Transverse, D=Star, matching option 4.

Option 1 (A=Longitudinal, B=Transverse, C=Star, D=Barchans) misassigns longitudinal dunes to the limited-supply/low-variability position that is actually characteristic of barchans, and misplaces star and barchan dune types relative to their true sediment-supply/wind-variability requirements. Option 2 (A=Transverse, B=Star, C=Barchans, D=Longitudinal) and option 3 (A=Star, B=Longitudinal, C=Transverse, D=Longitudinal) similarly scramble the diagnostic positions inconsistent with the Wasson-Hyde model. Option 4 is correct: it matches the well-established dune-type classification where barchans occupy the low-supply/low-variability corner, transverse dunes the high-supply/low-variability corner, star dunes the high-supply/high-variability corner, and longitudinal dunes an intermediate position with elevated wind variability.