CSIR NET Dec 2015 Q103: Sedimentology

Grain-size sorting and skewness statistics are classic tools (following Friedman, 1961, 1967) for discriminating depositional environments from sediment texture. River (fluvial) sediments are typically poorly sorted (mix of grain sizes carried by variable-competence flow) and show positive skewness (a tail of fine material/excess fines relative to a normal distribution), corresponding to R (poorly sorted) and X (positive). Glacial sediments (till, outwash) are very poorly sorted (extremely wide grain-size range from unsorted ice transport) and also tend toward positive skewness, corresponding to S (very poorly sorted) and X (positive). Aeolian (wind-blown, dune) sediments are very well sorted (wind is a highly selective transport agent) and typically show positive skewness (fine tail from suspension/saltation sorting), corresponding to Q (very well sorted) and X (positive). Coastal/beach sediments are well sorted by wave swash-backwash action and characteristically show negative skewness (loss/winnowing of fines, coarse tail), corresponding to P (well sorted) and Y (negative). This gives a-RX, b-SX, c-QX, d-PY, matching option 2.

Option 1 (a PX, b RY, c QY, d SY) misassigns well-sorted/negative-skew characteristics to river sediment and incorrectly gives glacial and aeolian sediments negative skewness, contrary to the well-established positive-skew signature of both environments. Option 2 is correct: it matches the classic Friedman sorting-skewness environmental fingerprints for river, glacial, aeolian, and coastal sediments. Option 3 (a SX, b RX, c PX, d QY) swaps the sorting categories of river and glacial sediments and incorrectly assigns aeolian sediment only 'well sorted' (P) rather than 'very well sorted' (Q). Option 4 (a SY, b RX, c PY, d QX) incorrectly gives river sediment negative skewness and swaps sorting/skewness pairings for coastal and aeolian environments relative to the established classification.