Estuaries - Trace Fossils as Indicators of Sedimentary Environments - page 482

6. CONCLUSIONS
Although there are a number of definitions for an estuary that are appropriate
depending on their context, we have chosen to emphasize a definition that suits
the identification of estuaries in the stratigraphic record as IVFs. The main cri-
teria for the identification of such IVF estuaries include the following:
1. The estuary fill is commonly encapsulatedwithin a valley, wherein the shared
contact represents a composite subaerial unconformity and a transgressive
surface (TS/SU), or a subaerial unconformity and a tidal-ravinement surface
(TRS/SU). This criterion may be established ichnologically through the
repeated identification of omission suites of trace fossils, commonly
representative of the Glossifungites or Trypanites ichnofacies. Note that
the SU does not host suites attributable to the Glossifungites Ichnofacies.
2. Brackish-water-associated strata dominantly represent the estuarine IVF.
By recognizing trends in trace-fossil sizes and diversities, and by comparing
the observed assemblages to their fully marine counterparts as a base line,
this criterion can be established ichnologically.
3. Estuaries are filled during transgression, which requires the recognition of
proximal and distal trends of trace-fossil size and diversity, as well as the
differentiation of trace-fossil suites that are consistent with fully marine,
brackish-water, and freshwater fluvial settings.
4. Estuaries receive sediment from both fluvial and tidal sources. Tidal cur-
rents influence the distribution of infauna largely through the transport of
larvae. The presence of a brackish-water fauna indicates larval transport into
the estuary and can be used as evidence for landward-directed suspended-
load tidal transport, which may or may not be accompanied by tidal bedload.
Additionally, certain ethologies can be used to infer the presence of tides.
Ichnological data are clearly important in the identification of estuaries. In spite
of the surprising range of sedimentological variability observed in estuaries, the
ichnological range is comparatively more constrained and therefore distinctive.
Trace-fossil assemblages are ideally suited to the recognition of brackish-water
conditions. The tidal bars within the estuaries tend to show trends of upward-
increasing bioturbation. Finally, trace-fossil omission suites are common along
the erosionally exhumed valley margins of estuaries.
ACKNOWLEDGMENTS
M. K. G.'s research is supported by an NSERC Discovery Grant (No. 238530) and ongoing
support from Nexen Inc., ConocoPhillips Canada, Devon Energy Canada, BP Canada, Statoil
and Shell. S. E. D.'s research is supported through an NSERC Discovery Grant (No. 341789)
and funding from Nexen Inc., Imperial Oil Ltd., Statoil and Suncor. J. A. M. is funded through
NSERC Discovery Grant No. 184293.