Shorefaces - Trace Fossils as Indicators of Sedimentary Environments - page 578

physical parameters anddisplaysdiscrete ichnological characteristics.Withineach
depositional complex, the subenvironments can be reasonably differentiated
through a combination of physical and biogenic features.
The offshore complex is typically dominated by fair-weather conditions and
is characterized by ichnological suites attributable to the distal to archetypal
Cruziana Ichnofacies. Such suites display a predominance of deposit-feeding
traces and grazing/foraging traces, with rare dwelling traces of suspension
feeders. Storm effects are variable but ineffective in removing fair-weather
suites except under exceptionally high degrees of storm domination. The off-
shore typically displays an equilibrium suite of trace fossils, with variable over-
printing by opportunistic suites related to initial tempestite colonization.
The lower-middle shoreface complex is affected by both storm and fair-
weather conditions, and hence, the preserved record shows a high degree of
variability. Fair-weather (equilibrium) trace-fossil suites range from those charac-
teristic of proximal expressions of the Cruziana Ichnofacies in the lower shoreface
to the archetypal Skolithos Ichnofacies in the middle shoreface ( Figs. 2 and 3 ).
The upper shoreface-foreshore complex is also quite variable, but this is
mainly a consequence of sediment texture and morphodynamic configuration
of the shoreline. Preserved facies are remarkably similar over a wide range of
shoreface states and accumulate in response to fair-weather conditions. Storm
events are non-depositional and strongly erosive. In dissipative, lower energy sys-
tems, the trace-fossil suites are generally sparse and correspond to the archetypal
Skolithos Ichnofacies. In high-energy reflective systems, the upper shoreface-
foreshore transition may contain zones of Macaronichnus segregatis .
The main variations in the preserved record of a shoreface succession are
reflected in the lower-middle shoreface complex and can be attributed to vari-
ations in overall storm intensity, storm frequency, and relative water depth.
Three main “types” of shoreface successions can be recognized, although a
complete continuum exists. Erosionally amalgamated tempestites with minimal
preserved biogenic structures constitute the lower-middle shoreface complex of
strongly storm-dominated systems. Moderately storm-dominated systems lead
to lower-middle shoreface complexes that consist of stacked, erosionally based
storm beds with bioturbated tops consisting of opportunistic trace-fossil suites,
cross-cut by resident (fair-weather) suites. Weakly storm-affected shorefaces
constitute the lowest energy and most dissipative conditions, with the lower-
middle shoreface complex characterized by thoroughly bioturbated muddy
and silty sandstones recording nearly continuous fair-weather accumulation.
Discrimination between lower and middle shoreface deposits may be possible
on an ichnological basis, provided well-developed fair-weather suites are pre-
served. The boundary between the lower and middle shoreface corresponds to
the transition from suites attributable to the Cruziana Ichnofacies to those that
correspond to the Skolithos Ichnofacies ( MacEachern and Pemberton, 1992;
MacEachern et al., 1999b ). Under progressively higher degrees of storm domi-
nation, fair-weather suites are insufficiently preserved to accurately resolve the