Shallow-Marine Carbonates - Trace Fossils as Indicators of Sedimentary Environments - page 713

5.2.2 Upper Intertidal Microbial/Sand Flats
A broad coastal zone is occupied by microbial (cyanobacterial) mats with dif-
ferent morphologies ( Alsharhan and Kendall, 2003; Kendall et al., 2002 ). Such
microbial flats with a leather-like surface develop in a harsh and hypersaline
environment, which is uninhabitable for most burrowing organisms and there-
fore very few traces can be found here. Terrestrial arthropods are temporary
visitors of this transition zone and burrow underneath the mats while feeding
on decomposing mat material. Sand-filled shrinkage cracks along the margins
of mat polygons are preferred sites of moisture and weakness for entering the
substrate. Marine colonization by crustaceans and worms occurs in parallel with
diurnal inundation of seawater, resulting in a patchy distribution of sand or
sandy beaches. Volcano-like mounds, approximately 10 cm high and 4 cmwide
and with a 1-3 mm diameter axial canal, occur in the vicinity of tidal channels.
The columnar structures consist of alternating biolamination and pellet layers
and result from the work of polychaetes within the polygonal algal zone
( Shinn, 1972 ). Amphipods and ghost crabs ( Ocypode saratan ) are marine
animals with a semiterrestrial lifestyle and move up from the water and cross
the high-tide line to burrow into lower supratidal deposits. Amphipods are
known to produce Skolithos -, Arenicolites -, or Diplocraterion -like traces
( Gingras et al., 2008 ), whereas ghost crabs excavate large J-, U-, Y-shaped
or complexly branched burrows like Macanopsis and Psilonichnus (e.g., De,
2005 ). Besides scavenging, the ghost crabs may also rework organic-rich sand
(deposit-feeding) and produce pellets ( Fig. 12 B). Trackways and resting traces
may have a slight chance to become preserved, but this is unlikely for the up to
30 cm high sand towers build by some male ghost crabs to mark their territory
( Basson et al., 1977 ).
5.2.3 Lower Intertidal Sand/Mud Flats with Tidal Channels and
Beachrock
The substrate of this zone is extensively reworked by a diverse fauna, dominated
by elements of the meiofauna (e.g., foraminifers, nematodes, copepods, poly-
chaetes, and turbellarians; Fig. 12 C), several kinds of worms (e.g., polychaetes,
nemerteans, and sipunculids; Fig. 12 D), molluscs (gastropods, bivalves), crus-
taceans (e.g., amphipods, isopods, and decapods), insects (e.g., beetles), and
birds. Aragonite-cemented coastal sand, composed of cerithid gastropod shells,
may turn into well-developed beachrock ( Alsharhan and Kendall, 2003;
intensively burrowed mainly by crabs (probably Metopograpsus messor ) and worms. (F) Muddy
firmground surface on a lower intertidal mud flat with the paired opening of U- or Y-shaped crab
burrows. (G) Lower intertidal mud (salt) flat with circular and bow-shaped feeding traces (approx-
imately 1.2-1.6 m in diameter) of the Greater Flamingo ( Phoenicopterus roseus ). (H) Nesting site
and trackway (approximately 0.6 m wide) of the Green Turtle ( Chelonia mydas ). Photo taken at Ras
Al Hadd, Oman.