Tidal Flats and Subtidal Sand Bodies - Trace Fossils as Indicators of Sedimentary Environments - page 522

The intensity of bioturbation is typically moderate ( Reineck and Singh,
1980 ). Alternation of sandstone and mudstone layers enhances preservation
of horizontal interface traces, such as those that typify the Cruziana Ichnofacies.
Common components are Cruziana , Rusophycus , Psammichnites , Protovirgu-
laria , Lockeia , Palaeophycus , Planolites , Helminthopsis , Helminthoidichnites ,
and Bergaueria ( Fig. 5 B and C). Clusters of Rusophycus are common in Early
Paleozoic tidal-flat deposits ( Fig. 5 B; M´ngano and Buatois, 2004a ). Vertebrate
trackways are typically preserved in sandy layers of the mixed intertidal flats.
Mesozoic examples include spectacular dinosaur tracks, commonly forming
megatrack sites ( Fig. 5 D; e.g., Avanzini et al., 2006; Lockley et al., 1992 ).
Microbial mats may be extensive, allowing a number of interactions, including
protection and undermat mining ( Baucon, 2008 ). Insect and polychaete burrows
are common, decapod burrows being particularly concentrated along mat mar-
gins ( Baucon, 2008 ). Substrate stabilization by microbial activity has also been
recognized in the stratigraphic record (e.g., Gerdes et al., 1985; Noffke, 2010;
Schieber, 2004 ), as also illustrated by the presence of wrinkle marks associated
with relict ripple troughs (e.g., M ´ ngano et al., 2002a ).
2.4 Sand Flats
Sand flats occupy the lower zones of tidal flats, and are dominated by bedload
transport of sand-sized sediment ( Fig. 2 ). They are the most variable intertidal
areas in terms of both sedimentary facies differentiation and trace-fossil content,
this variability being essentially controlled by the intensity of tidal currents in
combinationwithwave action ( M´ngano andBuatois, 2004b; Reineck andSingh,
1980 ). In macrotidal settings characterized by high-current velocities, migration
of large-scale bedforms (i.e., two-dimensional and three-dimensional dunes)
is the dominant process ( Boyd et al., 2006; Dalrymple, 2010; Dalrymple and
Rhodes, 1995 ). Deposits typically consist of medium- to thick-bedded, trough
and planar cross-bedded coarse- to fine-grained sandstone, and medium- to very
fine-grained sandstone with parallel lamination formed in the upper flow regime
( Fig. 6 A; Dalrymple, 2010; Dalrymple and Choi, 2007; Dalrymple et al., 1990 ).
Current-rippled fine-grained sandstone is less common. Under these conditions,
the lower intertidal zone is very difficult to distinguish from subtidal areas. In
lower-energy micro- to mesotidal settings, ripples are the dominant bedforms,
and deposits typically consist of current- andwave-rippled, cross-laminated, very
fine- and fine-grained sandstone, locally containing mud drapes and flaser bed-
ding. Flat-topped ripples, washout structures, erosional remnants and pockets,
and wrinkle marks are common ( Fig. 6 B).
On high-energy sand flats, rapidly migrating bedforms generally preclude
intense bioturbation (e.g., Baucon, 2008; M ´ ngano and Buatois, 2004a; Reineck
and Singh, 1980 ). Where present, ichnofaunas are dominated by vertical burrows
of suspension feeders or passive predators (e.g., Skolithos , Ophiomorpha , Areni-
colites , and Diplocraterion , illustrating the Skolithos Ichnofacies; Fig. 6 C-E).