Shorefaces - Trace Fossils as Indicators of Sedimentary Environments - page 568

waves. The lower-middle shoreface complex straddles the transition from
proximal expressions of the Cruziana Ichnofacies to the Skolithos Ichnofacies,
further enhancing its variability. Although the offshore also receives sediment
during both storm and fair-weather conditions, the setting is generally fair-
weather dominated and resulting trace-fossil suites are representative of the
Cruziana Ichnofacies ( Figs. 2 and 3 ).
Storms and, to a lower degree, fair-weather waves are the dominant physical
processes operating on the lower and middle shoreface. As these deposits may
constitute the bulk of the ancient record of this portion of the shoreface ( Elliott,
1986; Kumar and Sanders, 1976; Reinson, 1984; Swift et al., 1985 ), it follows
that much of the observed trace-fossil variability may be attributable to variabil-
ity in the intensity and frequency of storm activity. Fluctuations in the sedimen-
tation rate are intimately associated with the episodic character of storm
deposition and this appears to control the ultimate preserved expression of
lower and middle shoreface.
Storm-induced variability in the preserved record of the lower and middle
shoreface is associated with the degree of storm influence (i.e., the interplay of
storm erosion versus tempestite deposition). By contrast, foreshore and upper
shoreface settings are characterized by preferential erosion rather than depo-
sition as a result of storms; most facies successions show erosionally bounded
fair-weather deposits. In general, it is possible to identify three principal
“types” of lower and middle shoreface successions: those that are strongly
storm-dominated (high energy), those that are moderately affected by storms
(intermediate energy), and those that are only weakly affected by storms or
dominated by fair-weather deposition (low energy) ( Dashtgard et al., in press;
MacEachern and Pemberton, 1992; MacEachern et al., 1999b ). A complete
intergradation exists, both along depositional strike (depending on the regional
paleogeography) and vertically through the facies succession (related to shal-
lowing and increasing effectiveness of storm action on the bed). Much of the
confusion that exists in defining the character of the lower and middle shoreface,
as well as in determining the boundaries between the two subenvironments,
appears to revolve around the variability imposed by the degree of storm
dominance and preservability of the fair-weather deposits ( MacEachern and
Pemberton, 1992 ).
The preserved record of strongly storm-dominated shorefaces is character-
ized by stacked, erosionally amalgamated tempestites (HCS, SCS, and QPL)
with few preserved biogenic structures ( Fig. 11 ). The preserved record of
moderately storm-dominated shorefaces is characterized by stacked tempestites
with preserved burrowed tops and/or fair-weather beds ( Fig. 12 ). Such
facies show a laminated-to-burrowed appearance (so-called lam-scram;
MacEachern and Pemberton, 1992 ). The preserved record of weakly storm-
affected shorefaces is characterized by little or no preserved tempestites and
a succession dominated by fair-weather deposits that impart a thoroughly
bioturbated appearance ( Fig. 13 ).