Page 216 - Sedimentology and Stratigraphy

Barrier and Lagoon Systems 203

Fig. 13.6 A schematic graphic sedimentary log of
sandy beach deposits.

the sediment deposited on the foreshore and back- riers are composed of sand and/or gravel material and
shore region. The backshore area merges into the are largely built up by wave action. They may be
coastal plain and may show evidence of subaerial partially attached to the land, forming a beach spit,
conditions such as the formation of aeolian dunes or wholly attached as a welded barrier that comple-
and plant colonisation. tely encloses a lagoon, or can be isolated as a barrier
island in front of a lagoon. In practice, the distinction
between these three forms can be difficult to identify
13.3 BARRIER AND LAGOON in ancient successions and their sedimentological
SYSTEMS characteristics are very similar. Barriers (Fig. 13.9)
range in size from less than 100 m wide to several
13.3.1 Barriers kilometres and their length ranges from a few
hundred metres to many tens of kilometres (Davis &
Along some coastlines a barrier of sediment separates Fitzgerald 2004). The largest tend to form along the
the open sea from a lagoon that lies between the open coasts of large oceans where the wave energy is
barrier and the coastal plain (Fig. 13.8). Beach bar- high and the tidal range is small.