Shorefaces - Trace Fossils as Indicators of Sedimentary Environments - page 564

2.3.2 Foreshore
The foreshore is confined to the intertidal zone occupying the area of wave swash.
It also corresponds to the littoral zone in areas where tidal activity is minimal
(e.g., Baltic Sea and Mediterranean Sea). The swash and backwash mechanism
produces distinctive low-angle seaward-dipping planar-parallel laminations, typ-
ically in well-sorted, very well-winnowed, medium- to coarse-grained sandstone
( Fig. 9A ). Locally, however, textures may incorporate up to cobble-sized
particles. Stratification occurs as wedge-shaped sets with internal stratification
parallel to subparallel to the lower set contacts, commonly referred to as
swash-zone stratification. The set boundaries are not truncated, except by storm
erosion. Angular discordance of bed sets reflects the changing slope of the
prograding beachface during accretionary phases ( Reinson, 1984 ). Some convo-
lute laminae and flame structures may also be present ( Howard and Frey, 1984 ),
possibly reflecting wave-induced liquefaction during intense storm activity.
As sediment calibers increase to pebble and cobble sizes, internal stratifica-
tion becomes indistinct and is visible mainly as clast segregation ( Fig. 9B ).
Stratification, where visible, remains low-angle planar parallel. Conglomerates
are mostly clast-supported with a sand or granule matrix, although open
framework (no matrix) conglomerates are preserved locally. Unlike alluvial
conglomerates, foreshore conglomerates tend to display a good lateral continu-
ity of bedding ( Clifton, 1969 ) and clasts may vary considerably in size over the
entire foreshore interval, but are very well sorted within each bed ( MacEachern
and Hobbs, 2004 ). Elongate clasts may be imbricated, with long axes parallel to
depositional strike. Non-resistant clasts tend to be absent due to the intensity of
wave-induced erosion.
The diversity and abundance of trace fossils is low, due to the harsh envi-
ronmental conditions, continuously shifting substrates and overall low preser-
vation potential ( Howard and Frey, 1984 ). Most elements reflect the Skolithos
Ichnofacies, particularly the ichnotaxa Conichnus conicus (dwelling structure
of sea anemones), Ophiomorpha nodosa (vertical components), Skolithos line-
aris , and lesser Diplocraterion and Arenicolites , indicating the predominance of
suspension-feeding behaviors within deeply penetrating domiciles ( Fig. 3 ).
Some passively predaceous organisms inhabit the foreshore, which may pro-
duce Palaeophycus and possibly Schaubcylindrichnus , but these traces are shal-
lowly penetrating and, as such, are rarely preserved ( Howard and Frey, 1984 ).
In contrast to the lower-middle shoreface complex, the upper shoreface-
foreshore complex shows virtually no storm-induced deposition. Storm events
are characterized by pronounced erosion in this zone, where short-period waves
cut ridge and runnel systems and plane off the accretionary profile of the beach-
face ( Fox and Davis, 1978; Kumar and Sanders, 1976; Niedoroda et al., 1984;
Swift et al., 1985 ). The bulk of the eroded sediment is transported basinward to
the lower-middle shoreface complex. Much of it may be returned to the beach