Page 219 - Sedimentology and Stratigraphy

206 Clastic Coasts and Estuaries

sediment and organic matter along the water’s edge.
In more temperate climates, saline-tolerant grasses,
shrubs and trees may play a similar role in trapping
sediment. Coarser sediment may enter the lagoon
when storms wash sediment over the barrier as
washover deposits, which are thin layers of sand
reworked by waves. Sand is also blown into the
water by onshore winds picking up material from
the dunes along the barrier.
An important characteristic of lagoons is their
water chemistry. Due to the limited connection to
open ocean, it is common for lagoon water to have
either higher or lower salinity than seawater. Low
salinity, brackish water (10.3) will be a feature of
lagoons in areas of high rainfall, local run-off of
fresh water from the coastal plain or small streams.
Mixing of the lagoon water with the seawater is
insufficient to maintain full salinity in these brackish
lagoons. In more arid settings the evaporation from
the surface of the lagoon may exceed the rate at
which seawater exchanges with the lagoon water
and the conditions become hypersaline (10.3), that
is, with salinities higher than that of seawater. If
salinities become very elevated, precipitation of
evaporite minerals will occur (3.2).
A lagoonal succession is typically mudstone, often
organic-rich, with thin, wave-rippled sand beds
(Boggs 2006) (Fig. 13.11). The deposits of lagoons Fig. 13.11 A schematic graphic sedimentary log of clastic
can be difficult to distinguish from those of lakes lagoon deposits.
with similar dimensions and in similar climatic set-
tings. The processes are almost identical in the two limited, occurring through widely spaced inlets and as
settings because they are both standing bodies of seepage through the barrier. Coarse sedimentation in
water. Two lines of evidence can be used to identify the lagoon will be largely restricted to washovers that
lagoonal facies. First, the fossil assemblage may indi- occur during storms. There is a strong likelihood of
cate a marine influence, and specifically a restricted the lagoon waters becoming either brackish or hyper-
fauna may provide evidence of brackish or hypersa- saline, depending upon the prevailing climate.
line water. Second, the association with other facies
is also important: lagoonal deposits occur above or
below beach/barrier island sediments and fully marine 13.4.2 Mesotidal coasts
shoreface deposits.
With the increased tidal range of mesotidal condi-
tions, more exchange of water between the lagoon
13.4 TIDES AND COASTAL SYSTEMS and the sea is required, resulting in more inlets form-
ing, breaking up the barrier into a series of islands
13.4.1 Microtidal coasts (Fig. 13.10). These inlets are the pathways for the
tidal flows and the currents within them can be
Under microtidal conditions wave action can main- strong enough to redistribute sediment. On the lagoon
tain a barrier system (Fig. 13.8) that can be more or side of the barrier sediment washed through the
less continuous for tens of kilometres. Exchange of channel is deposited in a flood-tidal delta
water between the lagoon and the sea may be very (Fig. 13.10). The water in the lagoon is shallow, so