CSIR NET June 2016 Q57: Sedimentology
Deltaic sedimentary sequences (Gilbert-type / river-mouth deltas) show a characteristic coarsening-upward grain-size trend from prodelta to delta front to delta plain.
As a delta progrades seaward, fine prodelta muds are progressively overlain by coarser delta-front sands and then by delta-plain/distributary channel sands and gravels. So a vertical log through an advancing delta shows grain size increasing upward — the classic coarsening-upward (fining-downward) deltaic sequence. Option 3 is correct.
1) Incorrect — deltas do not show a uniform grain size. 2) Incorrect — this describes a fining-upward fluvial style sequence, opposite of deltaic progradation. 3) Correct — progradation stacks coarser facies over finer ones, producing a coarsening-upward profile. 4) Incorrect — grain size is not random; it follows a systematic progradational trend.