Estuaries - Trace Fossils as Indicators of Sedimentary Environments - page 451

to observe in limited datasets (e.g., core). These features are consistently
observed in modern estuaries from the zone of abrupt sinuosity decrease and
channel-width expansion (see Fig. 1 ) to the estuary mouth. Within estuaries,
the following trends are noted. Tidal sedimentary structures decrease in
abundance landward and may be essentially absent in the inner estuary.
Brackish-water trace-fossil assemblages are increasingly marine in character
seaward, and bioturbated IHS are nominally limited to the inner estuary and,
more rarely, the fluvio-tidal transition (e.g., Clifton et al., 1976; Dalrymple
et al., 1992; Gingras et al., 1999; Hovikoski et al., 2008; Lanier and Tessier,
1998; Martinius and Van den Berg, 2011 ). Intertidal-flat deposits may be
expressed throughout the estuary, but they are most ichnologically distinctive
in the inner and middle estuary.
A strong influence on the distribution of estuary tracemakers is the range of
estuary morphologies, which are bracketed by the sedimentological end mem-
bers referred to as wave- and tide-dominated estuaries ( Fig. 1 A and B); between
the two end members reside “mixed” estuaries ( Fig. 1 C and D). Finally, the state
of estuarine fill (filled versus unfilled) has been used to classify estuaries more
specifically (e.g., Roy, 1994 ), although we consider these criteria to reflect the
later stage or stages of estuary filling, and assert that our ichnological criteria for
assisting in the identification of estuaries apply to the inferred sediment and the
terminal estuary fill.
2. THE ICHNOLOGICAL IDENTIFICATION OF ESTUARIES
The identification of estuaries in the geological record is consistent with at
least four ichnologically or sedimentologically discernible conditions. The
first is the presence of brackish-water sedimentation, which is strongly asso-
ciated with diminutive trace fossils in low-diversity associations. Secondly,
tidally influenced sedimentation, although not limited to estuaries, provides
evidence for sedimentation in marginal-marine settings. Thirdly, the pres-
ence of a transgressed valley floor may be ichnologically discernible by
occurrences of omission suites such as those of the Glossifungites or Trypa-
nites ichnofacies. Finally, transgressive backstepping of sedimentary envi-
ronments can be easily documented using ichnological datasets. It is
important to recognize that not all of the above characteristics may be rec-
ognized within a dataset. For example, the transgressed base of the estuary is
not always demarcated by omission suites, discerning a tidal signal from
core datasets may be challenging, and the outer part of the estuary—and
thereby stratigraphically higher units—may not display brackish-water
trace-fossil assemblages. However, the more of the criteria that can be estab-
lished for the system as a whole, the more probable is the interpretation of
estuary.