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Coastal Carbonate and Evaporite Environments 229

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Fig. 15.4 Morphological features of a carbonate coastal environment with a barrier protecting a lagoon.

microtidal regimes, where they occur as laterally con- 15.2.2 Beach barrier lagoons
tinuous barriers parallel to the shoreline. In common
with barriers made up of terrigenous clastic material Lagoons form along carbonate coastlines where a
(13.3.1) they form in response to a slow rise in sea beach barrier wholly or partly encloses an area of
level. shallow water (Fig. 15.4). The character of the lagoon
An important difference between beaches made up deposits depends on the salinity of the water and this
of terrigenous clastic material and carbonate-rich in turn is determined by two factors: the degree of
beaches is the formation of beachrock in the latter. connection with the open ocean and the aridity of the
Carbonate in solution precipitates between sand and climate.
gravel material deposited on the beach and cement
the beach sediments into fully lithified rock. Beach- Carbonate lagoons
rock along the foreshore may act as a host for organ-
isms that bore into the hard substrate (11.7.2), a Carbonate lagoons are sites of fine-grained sedimen-
feature that may make it possible to recognise early tation forming layers of carbonate mudstone and
cementation of a beachrock in the stratigraphic wackestone with some grainstone and packstone
record. A prograding strandplain or barrier island beds deposited as washovers near the beach barrier.
generates a coarsening-upwards succession of well- Where a barrier island ridge is cut by tidal channels in
sorted, stratified grainstone and packstone. The a mesotidal regime, the tidal currents passing through
deposits are typically associated with lagoonal, supra- form flood- and ebb-tidal deltas in much the same way
tidal and inner shelf/ramp facies. as in clastic barrier island systems (13.3). The shape
At the top of the beach sands composed of bioclastic and internal sedimentary structures of these deposits
and other carbonate detritus may be reworked by are also similar on both clastic and carbonate coast-
wind to form aeolian dunes (8.4.2). When these lines, with lenses of cross-bedded oolitic and bioclastic
dune sands become wet calcium carbonate is locally packstone and grainstone formed by subaqueous
dissolved and reprecipitated to cement the material at dunes on flood-tidal deltas. The nature of the carbon-
the surface into a rock, which is often referred to as an ate material deposited on ebb- and flood-tidal deltas
aeolianite (Tucker & Wright 1990). Carbonate also depends on the type of material being generated in
precipitates around the roots of vegetation growing in the shallow marine waters: it may be bioclastic debris
the dune sands and may be preserved as nodular or oolitic sediment forming beds of grainstone and
rhizocretions (9.7.2) (McKee & Ward 1983). packstone (Fig. 15.4).