Shorefaces - Trace Fossils as Indicators of Sedimentary Environments - page 566

Skolithos Ichnofacies (see Figs. 2 and 3 ). However, the M. segregatis suite,
reflecting horizontal, intrastratal sediment-ingesting behaviors, seems to char-
acterize higher-energy upper shoreface and foreshore environments where
domiciles cannot be established, owing to shifting substrates and difficulty
of filter-feeding because of intense wave action (e.g., Aguirre et al., 2010;
Saunders et al., 1994; Seike, 2007, 2009 ).
2.3.3 Backshore
The backshore represents amixture of marine, marginal-marine, and non-marine
conditions. Typical environments include the beach backshore, coastal dunes,
washover fans, and supratidal flats. Frey and Pemberton (1987) indicated that
such environments are subject to extreme variations in energy levels, sediment
types, and physical and biogenic sedimentary structures. These settings are also
subject to temporal fluctuations between fresh and saline water. Marine
processes generally dominate during spring tides and storm surges, whereas
eolian processes and subaerial exposure of supratidal flats predominate during
neap tides and non-storm periods.
The topographically elevated position of this setting and its extended
periods of subaerial exposure preclude colonization by most benthic marine
animals. The only persistent, notable exceptions are amphibious crabs of the
family Ocypodidae and amphipods of the family Talitridae. Ocypodid crabs,
which include both scavengers and surficial deposit feeders, typically excavate
J-, Y-, or U-shaped dwelling burrows referable to the tracefossils Psilonichnus
( Fig. 10A ) and Polykladichnus ( Fig. 10B ). Talitrid amphipods construct vertical
burrows up to 25 cm deep in sand at the landward limit of the foreshore and feed
on seaweed washed up on the beach ( Dashtgard and Gingras, 2005 ). Other bio-
genic structures are generated by essentially terrestrial organisms and include
the vertical shafts of insects and spiders; the horizontal tunnels of other insects
and tetrapods; and the ephemeral tracks, trails, and fecal pellets of insects, rep-
tiles, birds, and mammals. Plant roots constitute another biogenic structure
common to the Psilonichnus Ichnofacies ( Fig. 10C ) and commonly penetrate
into the underlying foreshore deposits ( Fig. 9C ). The types of plants able to
exploit these substrates range from intertidal halophytes on the distal margins
of some washover fans to maritime and terrestrial grasses, weeds, vines, shrubs,
bushes, and trees on dunes.
3. SHOREFACE VARIABILITY
Except for textural variations, the upper shoreface-foreshore complex remains
relatively consistent in character over a wide range of shoreface morphody-
namic states. There are few facies clearly indicative of barred conditions,
as most depositional bodies have a low preservation potential ( Greenwood
and Mittler, 1985 ), and those that are preserved closely resemble the facies