Shorefaces - Trace Fossils as Indicators of Sedimentary Environments - page 557

Lower shoreface suites also contain subordinate numbers of permanent dwell-
ings such as horizontal Ophiomorpha borneensis ( Fig. 6A and B ), O. nodosa ,
O. irregulaire ( Fig. 6C ), as well as rare O . annulata , Skolithos , Conichnus ,
Diplocraterion habichi , D. parallelum ( Fig. 6D ), Bergaueria , and the passive
carnivore trace Palaeophycus tubularis . In addition, escape traces (fugichnia)
and those of various opportunistic organisms may also be common, generally
representing colonization of storm bed tops.
The dominance of deposit-feeding behaviors, with significant contribu-
tions of permanent dwellings associated with suspension-feeding organisms,
suggests that persistent wave shoaling (i.e., above fair-weather wave base)
is important in winnowing mud and moving it offshore, in suspending food
particles above the bed and in shifting sand at the sediment/water interface.
The resulting fair-weather trace-fossil suite corresponds to a proximal
expression of the Cruziana Ichnofacies (cf. MacEachern and Bann, 2008 ;
Fig. 3 ).
Storm events impart considerable control on the character of the lower
shoreface. Storms tend to uproot, destroy, and/or bury resident (fair-weather)
benthic communities ( Frey, 1990; Pemberton and Frey, 1984; Pemberton
et al., 1992b ). During poststorm recovery, initial colonization of the tempestite
generally records opportunistic dwelling/suspension-feeding structures, which
do not typically reflect the original resident community. Continued fair-
weather conditions may see a return to the original trace-fossil suite (see
Section 3 in the section on storms). Depending upon the degree of storm
dominance, the preserved record of the lower shoreface can be quite variable
( Dashtgard et al., in press; MacEachern and Pemberton, 1992; Plint, 2010;
Walker and Plint, 1992 ).
Some thin tempestites may escape thorough biogenic reworking under con-
ditions of high-frequency storms, moderate- to low-energy storm conditions,
and high sedimentation rates (cf. Wheatcroft, 1990 ). Saunders and Pemberton
(1986) and Saunders (1989) noted that thin storm beds common in the lower
shoreface deposits of the Appaloosa Sandstone of the Bearpaw-Horseshoe
Canyon Formation transition, Alberta, permitted Rhizocorallium jenense ,
Teichichnus rectus ,and Rosselia socialis to display reequilibration to each
successive sediment/water interface. R. socialis reflects this reequilibration by
vertical stacking of individual burrows. Similar observations were made in the
Pleistocene of Japan by Nara (1997) and the Permian of the Sydney Basin,
Australia by Bann et al. (2008) .
2.2.2 Middle Shoreface
The middle shoreface extends over the zone of shoaling and initial breaking of
waves ( Reinson, 1984 ) and is characterized by high oscillatory wave energy.
Longshore bars are commonly present near the upper portion of the middle
shoreface within intermediate (barred) states of shoreface morphodynamics