Page 207 - Sedimentology and Stratigraphy

194 Deltas

Fig. 12.18 A wave-dominated delta formed where wave activity reworks the sediment brought to the delta front to form
coastal sand bars and extensive mouth-bar deposits.

Fig. 12.19 Sand bars at the mouth of a wave-
dominated delta.

delta plain these will be peats or palaeosols, which After a number of changes in channel position the
represent a low clastic supply to this part of the plain active delta lobe may reoccupy an earlier position and
now that active lobe progradation has moved else- prograde over an older, compacted and submerged
where on the delta. The fringes of the delta lobe will delta-lobe succession. In cross-section the result is
be areas of slow, fine-grained deposition in shallow one coarsening-up delta-lobe succession built up on
water, while further offshore, carbonate facies may top of another. Repetition of this pattern has been
form over the toe of the delta. Abandonment facies recognised in the stratigraphic record and are referred
may show intense bioturbation because of the slow to as delta cycles, each ‘cycle’ representing the pro-
sedimentation rate. gradation of an individual delta lobe. The thickness of