Shorefaces - Trace Fossils as Indicators of Sedimentary Environments - page 565

in the form of longshore bars, which migrate landward and weld to the beach-
face in response to the long-period waves generated during fair-weather condi-
tions ( Cook and Gorsline, 1972; Davidson-Arnott and Greenwood, 1976 ). Some
sediment, however, is permanently lost to the lower-middle shoreface, where it
is mobilized during storms into tempestites. It is not uncommon for upper shore-
face-foreshore complexes to acquire an intermediate, barred state during
extended storm seasons and return to largely unbarred reflective states during
fair-weather conditions ( Thom et al., 1986; Wright et al., 1979 ).
The presence of discrete zones of Macaronichnus segregatis is highly sig-
nificant to the interpretation of the nearshore-to-foreshore complex succession
as upper shoreface and foreshore deposits ( Saunders, 1989; Saunders and Pem-
berton, 1986 ). Clifton and Thompson (1978) originally described and defined
M. segregatis and attributed it to the deposit-feeding behavior of the opheliid
polychaete Ophelia limicina . In addition, the modern polychaete Euzonus
mucronata , also of the family Ophelidae , makes similar traces ( Pemberton
et al., 2001 ). Both of these modern tracemakers ingest sand as a means of
exploiting an epigranular food source of bacteria and other grain-attached
organic materials and subsequently excrete the “cleansed” sand to form the fill
of the structure. The polychaetes preferentially ingest quartz and feldspar sand
and avoid mica flakes and mafic grains; consequently, the fill of the structure
tends to be lighter in color than the surrounding host sediment.
In the intertidal and innermost surf zones of wave-exposed beaches, how-
ever, high oxygen windows occur as permanent, hydrodynamically sustained
features. There, oxygenated surface waters can circulate several meters into
the sand, well beneath the reach of surface wave disturbance. This is accompa-
nied by the filtering of tremendous volumes of dissolved and finely particulate
organic matter, virtually all of which is mineralized at depth, on passage
through the porous framework of the beach ( McLachlan et al., 1985 ). Under
these conditions, the more familiar, physically controlled habitat at the surface
is adjoined at depth by a second habitat. This second habitat is very stable, pre-
dictable, and possesses a potentially abundant and constantly replenishing food
supply. In modern oxygen-windowed beaches, only meiofauna and opheliid
polychaetes, such as Euzonus mucronata , have thus far been found to exploit
this unique, deep-sand habitat. In the Cadotte Member and other boreal beaches
of the Early Cretaceous, this is confirmed by the presence of Macaronichnus
( Fig. 9B and C ). In these ancient settings, the deep-window habitat was shared
by this specially adapted species of macrofaunal deposit feeder, most probably
opheliid or opheliid-like polychaetes similar to those in modern equivalents
( Clifton and Thompson, 1978; Saunders, 1989 ). The ability to exploit deep
habitats accounts for the preservation of their burrows in high-energy, nearshore
sand and sandy conglomerates ( Clifton, 1988; Clifton and Thompson, 1978;
Saunders, 1989 ).
In the normal Seilacherian ichnofacies scheme, the foreshore zone should be
dominated by the dwelling burrows of suspension feeders that characterize the