Estuaries - Trace Fossils as Indicators of Sedimentary Environments - page 458

increasing-upward bioturbate character ( Gingras and MacEachern, 2012;
MacEachern and Gingras, 2007 ).
Thomas et al. (1987) presented conceptual interpretations for a range of
inclined sand/mud alternations, referred to as IHS. Subsequent studies have
shown a strong association between IHS and tidally influenced bar deposits
( Fenies and Faugeres, 1998; Geier, 1995; Gingras et al., 1999; Hovikoski
et al., 2008; Leckie and Singh, 1991; Sisulak and Dashtgard, 2012; Van den
Berg, 1981 ). The presence of brackish-water trace-fossil assemblages coupled
with IHS ( Figs. 3 D and 4 E) has, since, become virtually synonymous with sedi-
mentation within estuaries (see MacEachern and Gingras, 2007 for a review).
This is, of course, an oversimplification, as similarly bioturbated IHS is present
in the distal parts of delta distributary channels (e.g., Sisulak and Dashtgard,
2012 ) and in other tidal channels as well (e.g., Choi et al., 2004; Dalrymple
and Choi, 2007; Pearson and Gingras, 2006 ).

2.3 Ichnological Evidence for Transgressive Incised Valley Fills
Trace fossils have proven to be useful in sequence-stratigraphic studies of
incised valleys. Discussion of ichnological applications in sequence-
stratigraphic analysis has centered around the use of substrate-controlled
ichnofacies as a means to identify and interpret the origin of strata-bounding
discontinuities. This includes occurrences of Glossifungites Ichnofacies-
demarcated discontinuities and, to a lesser degree, omission suites attributable
to the Teredolites and Trypanites ichnofacies ( Figs. 4 Band 5 A-C). The
conditions for the generation of omission suites marking stratigraphic discontinu-
ities and their association with transgressed sequence boundaries (transgressive
surfaces/composite subaerial unconformity, TS/SU) are outlined in MacEachern
et al. (2012) and are not recounted here. The established association of both the
Glossifungites and Teredolites ichnofacies with TS/SU is of importance because
these stratigraphic surfaces—if they are mappable and suggest a valley form—
may represent the floor and margins (i.e., the container) of the estuary. However,
where there are true freshwater fluvial deposits within the valley fill, the container
will not be marked by omission suites of the Glossifungites , Teredolites ,or
Trypanites ichnofacies.
Another application of estuary-associated trace-fossil assemblages is to
demonstrate the persistent transgressive nature of the estuary fill (summarized
in Fig. 5 ). Taken as a whole, trace fossils provide the required information to
determine whether stratigraphic successions have a transgressive or a regressive
character (e.g., Bann et al., 2004; MacEachern and Pemberton, 1994; MacEachern
et al., 2010; McIlroy, 2007; McIlroy et al., 2005; Pemberton et al., 1992 ). Rec-
ognizing the stacking pattern of estuary deposits hinges on the workers' ability
to establish proximal and distal trends based on trace-fossil size and diversity
and to differentiate ichnogenera that are consistent with marine, brackish-water,
and freshwater

settings (see discussion above). Although persistently