Page 245 - Sedimentology and Stratigraphy

232 Shallow Marine Carbonate and Evaporite Environments

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Fig. 15.7 Tide-influenced coastal carbonate environments.

cycle may be repeated many times if there is continued mud in warm climates tends to dry out and form a crust
subsidence along the coastal plain (Kendall & Harwood by syndepositional cementation. Repeated precipitation
1996). The displacive growth of the gypsum within of cements in this crust causes the surface layer to
the sediment is a distinctive feature of sabkhas, which expand and form a polygonal pattern of ridges, called
allows the deposits of these arid coasts to be distin- tepee structures or pseudoanticlines, a few tens of
guished from other marine evaporite deposits (15.5). centimetres across. As the pseudoanticlines grow they
Similar evaporite growth occurs within continental leave cavities beneath them, which are sites for the
sediments in arid regions (10.3). growth of sparry calcite cements. Smaller isolated
cavities and vertically elongate hollow tubes also
form in lime muds in intertidal areas due to air and
15.2.4 Intertidal carbonate deposits water being trapped in the sediment during the wet-
ting and drying process. Patches of calcite cement
Tidal currents along carbonate-dominated coastlines that grow in the cavities in the host of lime mud
transport and deposit coarse sediment in tidal chan- give rise to a fabric generally called fenestrae or
nels and finer carbonate mud on tidal flats. The tidal fenestral cavities. Vertical fenestrae may also result
channel sediments are similar in character to those from roots and burrows. Lime mudstones with small
found in tidal channels in clastic estuarine deposits cavities filled with calcite are sometimes called a
(13.6). The base of the channel succession is marked ‘birds-eye limestone’. The lithified crusts can be
by an erosive base overlain by a lag of coarse debris: reworked by storms and redeposited elsewhere.
this may consist of broken shells and intraclasts of A common feature of carbonate tidal flats is the
lithified carbonate sediment. Carbonate sands depos- formation of algal and bacterial mats, which trap
ited on migrating bars in the tidal channels form fine-grained sediment in thin layers to form the
cross-bedded grainstone and packstone beds (Pratt well-developed, fine lamination of a stromatolite
et al. 1992). As a channel migrates or is abandoned (3.1.3). Stromatolites may form horizontal layers or
the sands are overlain by finer sediment, forming beds irregular mounds on the tidal flats. Their distribution
of carbonate mudstone and wackestone. Bioturbation is partly controlled by the activity of organisms,
is normally common throughout. which either feed on the microbial mats or disrupt it
In the intertidal zones deposits of lime mud and shelly by bioturbation. Stromatolites tend to be better devel-
mud are subject to subaerial desiccation at low tide oped in the higher parts of the intertidal area that are
(Fig. 15.7). Terrigenous clastic mud remains relatively less favourable for other organisms that may graze on
wet when exposed between tidal cycles, but carbonate the mats.