Shorefaces - Trace Fossils as Indicators of Sedimentary Environments - page 550

MacEachern, 2007; MacEachern and Bann, 2008; MacEachern et al., 2005,
2007; McIlroy, 2004, 2007; Moslow and Pemberton, 1988; Raychaudhuri and
Pemberton, 1992; Siggerud and Steel, 1999 ).
More recently, interest has focused on tidal effects on otherwise wave-
dominated shorefaces ( Ainsworth et al., 2008; Dashtgard et al., 2006, 2009, in
press; Desjardins et al., 2012; Frey and Dashtgard, 2011; Vakarelov et al., in
press ). Dashtgard et al. (in press) present models for two tidal shorefaces: tide-
influenced shorefaces (TIS) and tidally modulated shorefaces (TMS). TIS are
developed in settings with strong tidal currents, such as straits and bays. TMS
are developed in settings with large tidal ranges (

8 mmacrotidal and megatidal
settings), wherein wave regimes are displaced along the depositional profile as a
consequence of the rising and falling of tides. Although these models are new and
relatively untested, work by Ainsworth et al. (2008) and Vakarelov et al. (in press)
suggests that the criteria for recognizing tidal shorefaces are valid. Ongoing work
on these models promises to develop ichnological and sedimentological criteria
for distinguishing shoreface through to tidal-flat successions that occur along the
continuum of wave-tidal coastal environments.

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2. SHOREFACE SUBENVIRONMENTS
Coastal morphodynamics are subject to a wide range of variables depending upon
whether the shoreline is prograding due to net-sediment accumulation or due to
transgression. Estuaries and barrier-island systems, for example, are generally a
function of transgression, whereas shore-attached strandplains and deltas are
more apt to develop along prograding coastlines ( Fig. 1 ). Nevertheless, the shore-
face and adjacent environments, whether associated with progradation or ero-
sional shoreface retreat, can be grouped into three depositional complexes: the
offshore complex, the lower-middle shoreface complex, and the upper shore-
face-foreshore-backshore complex. Each complex corresponds to discrete com-
binations of physical processes and concomitant faunal responses, leading to
distinct combinations of primary physical and biogenic sedimentary structures.

2.1 Offshore Complex
Many sedimentologists adhere to the paradigm set by Reading and Collinson
(1996) who placed the offshore regime below effective (fair-weather) wave
base, but above maximum (storm-weather) wave base. The distribution of bio-
genic structures in the offshore is largely dictated by availability of food
resources and substrate consistency. In these regimes, food particles are deposi-
ted onto the bed and substrates are generally cohesive, leading to increased
interspecific competition and a corresponding increase in specialized feeding
strategies. The resulting ichnological suites are highly diverse and dominated
by deposit-feeding and grazing structures, attributable to the Cruziana Ichno-
facies ( Fig. 3 ).