Estuaries - Trace Fossils as Indicators of Sedimentary Environments - page 481

(cf. MacEachern and Gingras, 2007 ). The second major concordance is that firm
substrates are well colonized by low-diversity assemblages of infauna in
brackish-water zones, and so channel and valley margins should normally con-
tain omission suites of trace fossils that demarcate stratigraphic levels of trans-
gression across the SU. Another commonality exhibited by many of our
examples is the upward increase of bioturbation intensities and diversities,
which is a pattern associated with tidally influenced point bars and, to a lesser
degree, longitudinal bars.
In short, our modern and ancient analogs demonstrate that estuaries can be
ichnologically identified in the rock record. Importantly, it is apparent that the
type of estuary (wave dominated through to tide dominated) can be understood,
especially if taken in the context of the more general sedimentary facies
associations. There is some question as to whether or not tide-dominated
estuaries can be discerned from tide-dominated deltas. To answer this question,
a set of neoichnological studies in tide-dominated deltas need to be conducted;
for the time being, no detailed characterization exists. We predict that
limitations on larval recruitment in fluvially dominated settings will lead to
ichnological impoverishment basinward of the inner estuary and that, at least
on the map scale, the ichnology of the two systems will be discrete.
For finer-scale subdivisions, it is useful to contrast the ichnology of the
various estuaries. In particular, the sharp ichnological gradients and the presence
of more marine conditions in the outer estuary of wave-dominated estuaries set
them apart from the mixed-energy and tide-dominated estuaries. The most
important difference between wave- and tide-dominated estuaries is that
tide-dominated settings experience hydraulic energy conditions that far exceed
those imposed by tidal currents in either wave-dominated or mixed-energy
settings. A notable characteristic of tide-dominated estuaries is the enormous
volume of their tidal prisms. Large tidal prisms have the effect of effectively
mixing bay waters, such that water stratification is absent and lateral
salinity changes are gradual. Salinity stratification, eutrification, and rapid
salinity changes are, by contrast, more readily associated with the wave-
dominated end members. Larval recruitment is also profoundly influenced by
volumetrically large tide-water exchange, in that animal spat aremore evenlydis-
tributed over larger areas in macrotidal estuaries than in their microtidal
counterparts (e.g., Ayata et al., 2009; Bentley and Pacey, 1992 ). The gradient
of processes and salinities along the length of macrotidal estuaries leads to a
broad distribution of ichnological suites that may differ little from the
middle to the outer parts of the system. Finally, the resulting biogenic
structures in tide-dominated estuaries reflect organism responses that are more
profoundly influenced by the grain size of the substrate, sedimentation rates,
turbidity of the water column, and the overall hydraulic energy than are those
of organisms that inhabit wave-dominated estuaries.