Estuaries - Trace Fossils as Indicators of Sedimentary Environments - page 473

dominated innermost 50 m from the mean high-tide level. Exceptions to this
generalization include relatively large (2-3 cm long, 1 cm wide), burrowing
amphipods with a propensity for making short, wide-diameter Skolithos -like
traces (up to 20 cm long). In high burrow densities, these can lead to the devel-
opment of cryptobioturbation ( Dashtgard and Gingras, 2005b ). In the upper
intertidal flats, Siphonichnus and Arenicolites are common (BI 2-3). In the

middle and lower intertidal zones of muddy tidal flats, the diversity of the infau-
nal community is generally low, although the intensity of burrowing is high
(BI

¼

3-4, Fig. 9 E-G; Pearson et al., 2007; Thurston, 1990 ). Typical burrows
include Arenicolites , Palaeophycus , Teichichnus , and Polykladichnus .The
outer part of muddy tidal flats and subtidal channels is reworked daily, and
infauna is likely to be rarely present.
In the outer estuary of Chignecto Bay, the muddy tidal flats are replaced by
sand-dominated deposits, and the tidal flats are replaced by tidally modulated
shorefaces (TMS; Dashtgard et al., 2006, 2009 ). Bioturbation is patchy in the
middle and lower intertidal zones of TMS (BI

¼

0-4; Fig. 9 H-J). Where mud
beds are present, Siphonichnus , Arenicolites , and Diplocraterion typically dom-
inate, and bioturbation values range from BI

¼

¼

1to4( Dashtgard et al., 2008 ). In
sand-dominated substrates, Skolithos- , Polykladichnus- , Palaeophycus- , and
rare Siphonichnus -like structures occur with bioturbation values of BI

0-2.
The animals residing in both the sandy and muddy substrates tend to live in
low-diversity communities and represent a mixture of small crustaceans and
bivalves, as well as large polychaetes. In the lower intertidal and subtidal zones,
Skolithos- , Cylindrichnus- , and Palaeophycus -like traces dominate.
In addition to TMS, the channels and subtidal portions of the outer estuary of
Chignecto Bay are dominantly erosional ( Amos and Asprey, 1979; Amos et al.,
1991 ). The deposits under the subtidal outer estuary are largely eroded by strong
tidal currents and have a low preservation potential. Large subtidal sand bars are
mapped in this part of the bay, yet little is known of the ichnological character of
these deposits. Rare Skolithos -generating Cerebratulus , fugichnia- and Siphon-
ichnus -producing razor clams have been observed. The resulting bioturbation
intensities are inferred to be low (BI

¼

¼ 0-1).

4. ANCIENT EXAMPLES—TRACE-FOSSIL DISTRIBUTION
4.1 Wave-Dominated Estuaries in the Viking Formation (Albian),
Alberta, Canada
Wave-dominated estuarine incised valleys have been described from a number
of Cretaceous stratigraphic units of Alberta, Canada, but the best-studied exam-
ples are from the Albian Viking Formation ( Fig. 10 ). The Viking Formation
contains at least five petroleum-producing deposits that are interpreted to reflect
estuarine IVF successions. The most extensively studied Viking IVF resides in
the Crystal Field ( Pattison, 1991; Pattison and Walker, 1994; Reinson et al.,