Page 214 - Sedimentology and Stratigraphy

Beaches 201

13.2 BEACHES of the foreshore, a back-beach gravel ridge that is a
distinctive feature of gravelly beaches. The clast com-
The beach is the area washed by waves breaking on position will vary according to local sediment supply,
the coast. The seaward part of the beach is the and may contain terrigenous clastic, volcaniclastic or
foreshore (11.1), which is a flat surface where bioclastic debris.
waves go back and forth and which is gently dipping At the top of the beach, a ridge, known as a berm,
towards the sea (Fig. 13.3). Where wave energy is marks the division between the foreshore and back-
sufficiently strong, sandy and gravelly material may shore area (Fig. 13.3). Water only washes over the
be continuously reworked on the foreshore, abrading top of the berm under storm-surge conditions. Sedi-
clasts of all sizes to a high degree of roundness, and ment carried by the waves over the berm crest is
effectively sorting sediment into different sizes (Hart & deposited on the landward side forming layers in the
Plint 1995). Sandy sediment is deposited in layers backshore that dip gently landward. These low-angle
parallel to the slope of the foreshore, dipping offshore strata are typically truncated by the foreshore strati-
at only a few degrees to the horizontal (much less fication, to form a pattern of sedimentary structures
than the angle of repose). This low-angle stratification that may be considered to be typical of the beach
of well-sorted, well-rounded sediment is particularly environment (Figs 13.3 & 13.4). The backshore area
characteristic of wave-dominated sandy beach may become colonised by plants and loose sand can
environments (Clifton 2003, 2006). Grains are typi- be reworked by aeolian processes.
cally compositionally mature as well as texturally Wave action in the lower part of the foreshore can
mature (2.5.3) because the continued abrasion in rework sand and fine gravel into wave ripples that
the beach swash zone tends to break down the can be seen on the sediment surface at low tide and
weaker clasts. can be preserved as wave-ripple cross-lamination.
On gravel beaches the water washed up the beach However, wave-formed sedimentary structures on
by each wave tends to percolate down into the porous the beach may be obliterated by organisms living in
gravel, and the backwash of each wave is therefore the intertidal environment and burrowing into the
weak. Clasts that are washed up the beach will there- sediment. This bioturbation may obscure any other
fore tend to build up to form a storm ridge at the top sedimentary structures.

Fig. 13.3 Morphological features of a beach comprising a beach foreshore and backshore separated by a berm; beach dune
ridges are aeolian deposits formed of sand reworked from the beach.