Tidal Flats and Subtidal Sand Bodies - Trace Fossils as Indicators of Sedimentary Environments - page 537

However, in the shallow subtidal and shelf environments, different tidal
sand-body types have different ichnological signatures owing to variations in
substrate type, sediment mobility, water turbidity, type of organic matter (sus-
pended versus deposited), and salinity. In the shallow subtidal and on the inner
shelf, the Skolithos Ichnofacies tends to dominate the high-energy deposits
formed by subaqueous dunes and bars, the degree of bioturbation by these
suspension-feeders reflecting well-developed colonization windows. Com-
monly, muddy substrates at the toes of dunes are characterized by an impover-
ished detritus-feeding ichnofauna (impoverished Cruziana Ichnofacies). If the
sand bodies are emplaced on the inner shelf, a diverse deposit-feeder ichnofauna
develops along the margins of sand sheets, compound dune fields, and tidal
sand-ridges ( Cruziana Ichnofacies). However, if the sand bodies are developed
in restricted areas such as bays, delta fronts, or estuaries, they tend to contain
low-diversity ichnofaunas dominated by trophic generalists ( Gingras et al.,
2012; Tonkin, 2012 ).
5. SEQUENCE STRATIGRAPHY
Parasequences from wave- and tide-dominated settings contrast in their internal
bedset stacking pattern as a result of markedly different hydrodynamic energy
gradients. Parasequences in wave-dominated environments are characterized
by a coarsening-upward trend, whereas those representing shorelines adjacent
to tidal flats have a fining-upward character, recording the transitions from sand
flats to mixed flats and mud flats ( Fig. 16 B; Klein, 1977; Van Wagoner et al.,
FIGURE 16 Variability of tide-dominated parasequences. (A) Coarsening-upward sand-sheet
parasequence. (B) Fining-upward tidal-flat parasequence. See M´ngano and Buatois (2004b) for
details on the zonation of the tidal-flat parasequence.