Tidal Flats and Subtidal Sand Bodies - Trace Fossils as Indicators of Sedimentary Environments - page 525

degree of bioturbation in the point bars is lower than in tidal-flat deposits,
most likely reflecting higher rates of sedimentation along unstable channel
margins (e.g., Gingras et al., 1999; Reineck and Singh, 1980 ). Bivalve escape
traces may be present as a result of rapid sedimentation in channels and large
gullies ( Reineck and Singh, 1980 ). Overall, tidal-creek deposits tend to be
slightly more bioturbated than larger-scale tidal channels, being benthic
organisms remarkably different from those in the adjacent tidal flats
( Reineck and Singh, 1980 ). Associated exhumed compacted sediments may
contain dense populations of the crustacean Corophium volutator ( D¨ rjes,
1970; Reineck and Singh, 1980 ). Intense bioturbation by decapods and
polychaetes has been observed along the margins of tidal creeks dissecting
matgrounds ( Baucon, 2008 ).
3. SUBTIDAL ENVIRONMENTS
The subtidal zone either occurs immediately seaward of the intertidal area or is
separated from it by sandy barriers or barrier islands. It is a high-energy envi-
ronment characterized by high-current velocities ( Figs. 1 and 2 ; Dalrymple,
2010 ). In macrotidal settings, strong tidal currents tend to be the dominant sedi-
mentary process, thus large-scale bedforms develop in the subtidal region.
However, it should be noted that under microtidal or mesotidal conditions this
may not be the case, as for example in regions of the southern North Sea, which
are dominated by wave action weakening the tidal-current influence ( Antia
et al., 1994 ). In zones of actively migrating large-scale bedforms, conditions
are not favorable for benthos colonization and, as a result, few species are able
to survive in this setting ( Wilson, 1982, 1986 ). Modern studies and information
from the stratigraphic record indicate that faunal diversity increases toward
areas with smaller bedforms, and on the outer shelf where dunes tend to be
replaced by small ripples and interbeds with mud laminae. Parallel to this trend
in faunal diversity, variations in trophic types are apparent, with suspension
feeders being dominant in high-energy, shallow subtidal environments, and
deposit and detritus feeders becoming more abundant toward the outer shelf
( Buatois and M´ngano, 2011; Desjardins et al., 2012a; Wilson, 1982 ). Benthic
faunas in tide-dominated subtidal sand bodies are mostly controlled by the inter-
play of (1) salinity, (2) sediment mobility and hydrodynamic energy, (3) water
turbidity, (4) substrate type, and (5) food supply.
Shelf sand bodies are typically emplaced under normal-marine conditions and,
therefore, salinity fluctuation is not an important control factor. However,
in brackish-water subtidal settings, such as deltaic or estuarine tidal bars
( Fig. 1 ), salinity dilution may significantly affect benthic faunas (e.g., Carmona
et al., 2009 ).
Sediment mobility in the form of bedform migration essentially controls the
duration of the colonization window ( sensu Pollard et al., 1993 ). Under