Shorefaces - Trace Fossils as Indicators of Sedimentary Environments - page 575

The third criterion for recognizing TMS is the anomalously large thickness
of the foreshore and, to a lesser extent, the change in dip angle between the fore-
shore and the upper shoreface, particularly in gravel-prone settings. TMS fore-
shores are typically 4-5 m thick ( Dashtgard et al., 2009, in press; Vakarelov
et al., in press ), which is at least twice that of a microtidal, fair-weather
wave-dominated shoreface. Although foreshore thicknesses are significantly
less than the vertical tidal range under which TMS develop, the overthickened
foreshore does suggest an increased tidal range. For sandy foreshores, the con-
tact between the foreshore and the upper shoreface is gradational and the depo-
sitional angle of beds is reduced ( Ainsworth et al., 2008; Vakarelov et al., in
press ). A sharp change in dip angles between the foreshore and upper shoreface
is common in composite sand-and-gravel beaches, wherein the foreshore is
gravelly and the upper shoreface is sandy ( Clifton, 1981; Frey and Dashtgard,
2011; Jennings and Shulmeister, 2002; Kirk, 1980 ). It is worth mentioning that
distinctive sedimentary structures recording wave-tide interaction are docu-
mented in tidal flats (e.g., wave-generated tidal bundles, tidally modulated
HCS; e.g., Yang et al., 2008a,b ). Could it perhaps be possible that the same
or similar structures are generated on TMS? Even if not, TMS might form a
continuum with storm/wave-modulated tidal flats, each containing subtle sedi-
mentological and ichnological evidence for mixed process regime ( Fig. 16 ).
The remaining criteria for recognizing TMS are ichnological. For low and
moderately storm-influenced TMS in the rock record, the lower shoreface is
characterized by low BI values in sandstone beds and a dominance of deep-
penetrating traces such as Ophiomorpha and Thalassinoides ( Ainsworth
et al., 2008 ). In the lower shoreface of modern TMS, the trace suite is dominated
by vertical suspension- and surface-deposit-feeding traces, as well as horizontal
subsurface-deposit-feeding traces. Traces observed include Skolithos , Arenico-
lites , Planolites , Siphonichnus , Palaeophycus , and Polykladichnus ( Fig. 15 ).
BI values range between 0 and 3. The upper shoreface also exhibits burrowing
with the same range of traces as seen in the lower shoreface, yet the density of
burrowing is significantly reduced (BI

0-2). The foreshore is unburrowed.
Although our knowledge of the ichnological character of TMS is limited to only
a few examples, it appears that TMS can be characterized, ichnologically, by a
low-diversity and low-density suite consisting of simple vertical and horizontal
traces. Trace diversities remain consistent across the upper and lower shoreface,
whereas bioturbation intensities decrease landward.

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5. SUMMARY
The integration of ichnology and sedimentology provides a high-resolution depo-
sitional model for identifying wave-dominated foreshore through to offshore sub-
environments, whether they occur on strandplain, barrier islands, or even locations
in wave/storm-dominated deltas removed from the interference of distributary
channels ( Fig. 17 ). Wave-dominated (non-tidal) shorefaces can be grouped into
three main depositional complexes, each of which responds to a different set of