Shorefaces - Trace Fossils as Indicators of Sedimentary Environments - page 579

two subenvironments ichnologically. In such scenarios, physical characteristics
such as coarsening of sediment texture, decrease in thickness of fair-weather
deposits, and replacement of lower energy HCS beds by SCS beds may have
to be employed. In the case of QPL-bearing intervals, it is unclear whether such
a distinction is possible. In any case, vertical and lateral transitions between
shoreface “types” may occur, depending upon relative water depth, storm fre-
quency, storm intensity, and shoreline orientation in relation to dominant wave
attack. These conditions serve to vary the character of contemporaneous lower
and middle shoreface deposits along depositional strike, which may make differ-
entiation of these two subenvironments problematic.
Two types of tidal shorefaces also have been defined, depending upon the
tidal hydrodynamic conditions. TIS develop on coastlines affected by strong
tidal currents and are considered typical of strait margins and embayed coast-
lines. Sedimentological and ichnological criteria considered indicative of TIS
are confined to the lower shoreface and offshore as well as, to a lesser extent,
the middle shoreface. Across all three of these zones, grain sizes either remain
constant or increase slightly in the offshore direction and mud content of the
sediment is very low ( Dashtgard et al., in press; Frey and Dashtgard, 2011 ).
Sand accumulates as current-generated structures and plane beds in the lower
shoreface and offshore, although these structures are commonly obliterated
by pervasive bioturbation. Burrowing across the shoreface and into the offshore
is dominated by vertical structures ( Fig. 14C-E ) that are typical of the arche-
typal Skolithos Ichnofacies, marking a basinward shift in its paleodepositional
position. At the transition from the upper shoreface to the foreshore of the
TIS, it is common to find the Macaronichnus segregatis “toe-of-the-beach”
assemblage (cf. Saunders et al., 1994; Vakarelov et al., in press ).
TMS constitute the tidal end member of the wave-tidal spectrum for shore-
face settings ( Figs. 15 and 16 ). TMS are identified by (1) common interbedding
of sedimentary structures down the shoreface profile; (2) the notable absence of
an obvious middle shoreface zone; (3) an anomalously thick foreshore interval;
and (4) the presence of a low-diversity and low-density trace-fossil suite, con-
sisting of simple vertical and horizontal burrows ( Dashtgard et al., 2009 ). Trace
diversities remain consistent across the upper and lower shoreface, whereas
bioturbation intensities decrease landward.
ACKNOWLEDGMENTS
The authors would like to thank the Natural Science and Engineering Research Council of
Canada (NSERC) for research funding; Discovery Grant 184293 to J. A. M., Discovery Grant
341789 to S. E. D., Discovery Grant 0001380 to M. K. G., Discovery Grant 371662 to J.-P. Z.,
and Discovery Grant A0816 to S. G. P.. S. G. P. would like to thank the Canada Research
Chairs program for their support of his research. We are grateful to Ron Boyd for supplying
the copy of Figure 1 . The chapter benefited from a thoughtful review by Gary Hampson and
comments from Richard Bromley and Dirk Knaust. We thank them for their insights that
helped to improve the quality of this chapter.