Shorefaces - Trace Fossils as Indicators of Sedimentary Environments - page 558

( Thom et al., 1986; Wright et al., 1979 ). Preserved deposits of longshore bars
are rarely intercalated within middle shoreface intervals, as under fair-weather
conditions bars migrate landward and weld to the foreshore ( Davidson-Arnott
and Greenwood, 1976; Davis, 1978 ). Middle shoreface sandstone beds tend to
be well sorted, winnowed of fines and fine- to medium-grained, with only minor
shale, siltstone, and shell layers ( Fig. 7 ). Preserved structures are predominantly
low-angle, wedge-shaped sets of parallel laminae, constituting SCS and lesser
HCS or QPL. Oscillation-ripple laminae, combined flow-ripple laminae, and
rarer trough cross-stratification are locally present. Storms have a strong influ-
ence on the middle shoreface, stronger than in the lower shoreface, and may
constitute much of its depositional record. The SCS or QPL tempestite beds
are typically erosionally amalgamated, becoming progressively more cannibal-
istic ( Aigner and Reineck, 1982 ) with increased shallowing and associated
enhancement of storm-induced scouring. As a result, the degree of bioturbation
is highly variable but generally quite low.
Ichnologically, the middle shoreface corresponds to a dominance of
suspension-feeding behaviors (constituting the Skolithos Ichnofacies; Fig. 3 ) over
deposit-feeding ethologies (the Cruziana Ichnofacies), although some traces of
the latter continue to persist as subordinate elements (e.g., Macaronichnus ,
Rosselia , Cylindrichnus )( Howard, 1972, 1975 ). The bulk of the suite consists
of ichnogenera such as Skolithos , Conichnus , Diplocraterion , Ophiomorpha
(commonly vertical components), Arenicolites , Bergaueria , Schaubcylindrich-
nus coronus , and Palaeophycus ( Fig. 7 ). Howard (1971a,b, 1975) and others have
noted that Ophiomorpha systems tend to shift from predominantly horizontal to
vertical orientations as energy levels increase. This shift in orientation may
accompany the transition from the lower to middle shoreface. Truncation of bur-
rows and fugichnia may be commonly associated with amalgamation of tempes-
tites ( Fig. 7A ).
Cruziana Ichnofacies elements include robust Rosselia ( R . socialis domi-
nates, Fig. 7D ), Cylindrichnus , and Siphonichnus . Frey (1990) noted that Cylin-
drichnus and Rosselia become more steeply inclined as energy levels increase.
This increase in inclination may also accompany the transition from the lower to
middle shoreface, mimicking the trend observed for Ophiomorpha .
As in the lower shoreface, variability in stormintensity and frequency strongly
affects the sedimentological and ichnological character of the middle shoreface.
As storm effects become more pronounced, the diversity and abundance of
deposit-feeding traces declines markedly. This appears to be attributable to the
abundance of well-winnowed sand and the general paucity of deposited food
for tracemakers of ichnogenera common to the Cruziana Ichnofacies. The pres-
ervation potential of biogenic structures in the middle shoreface is intrinsically
associated with the character of tempestite accumulation. In storm-dominated
conditions, as tempestites become progressively more cannibalistic, only deeply
penetrating traces (e.g., Ophiomorpha and fugichnia) may be preserved (see, for
instance, Fig. 7A-C ). It is not uncommon for storm-dominated middle shoreface