Tidal Flats and Subtidal Sand Bodies - Trace Fossils as Indicators of Sedimentary Environments - page 517

marine organisms due to very high temperatures, long periods of exposure and
abnormal salinities. As a result, the highest density of biogenic structures in
tropical tidal flats is found in the lower intertidal zone ( Terwindt, 1988 ).
In colder areas, tidal flats may exhibit a high density of biogenic structures
in the upper intertidal zone (e.g., Yeo and Risk, 1981 ). Bathymetric displace-
ment of certain species along latitudinal gradients is common, as illustrated
by the bivalve Gemma gemma that lives in intertidal areas of northern North
America and in subtidal areas further south in order to avoid the hazards of high
temperatures ( Green and Hobson, 1970; Reise, 1985 ).
The substrate is a controlling factor operating at different scales, affecting
ichnology from overall trace-fossil distribution to trace-fossil morphology
( M ´ ngano et al., 2002a ). The substrate can be characterized in terms of compo-
sition and degree of consolidation, both features influencing the distribution and
preservation of biogenic structures. Vertical and horizontal differences in the
degree of substrate consolidation influence the diversity, abundance, and distri-
bution of intertidal organisms ( Newell, 1979; Reise, 1985 ). The pore-fluid con-
tent within the sediment can vary during a tidal cycle, the tidal-flat surface at
low tide being characterized by the presence of tidal pools within a commonly
emerged area, resulting in a range of substrate conditions along an isochronous
surface. As a result, organisms moving across the tidal flat may produce bio-
genic structures that display a considerable morphological variability, as illus-
trated by the bivalve trail Protovirgularia ( M´ngano et al., 1998 ). Erosional
exhumation of firm sediment layers or even hardgrounds due to tidal scouring
( Willis, 2005 ) is quite common in tide-dominated settings, resulting in the pres-
ence of certain elements of the Glossifungites Ichnofacies ( Buatois and
M´ngano, 2011 ).
Food supply displays a large-scale gradient along a transect from the supra-
tidal to lower intertidal region, with abundant organic particles concentrated on
the sediment surface in more proximal positions and abundant organic particles
in the water column in more agitated seaward settings. In particular, suspension-
feeder burrows are common in high-energy sand flats (e.g., M´ngano and
Buatois, 2004a ). Also, it has been suggested that some ichnotaxa (e.g., Syringo-
morpha ) may be associated with the exploitation of meiofauna and epigranular
bacteria that coat sand grains ( M ´ ngano and Buatois, 2004a ). In addition to gen-
eral trends, food supply is strongly variable at a smaller scale, resulting in high
spatial heterogeneity ( Buatois and M ´ ngano, 2011; M ´ ngano et al., 2002a ).
Water entering through ripple troughs and exiting through the ripple slopes
and crests, driven by wave pressure gradients (e.g., Webb, 1969 ), results in a
resorting of sediment with small grains and organic detritus being drawn into
the troughs. The distribution of organics at the scale of microtopography
accounts for the aggregation of meiofauna and invertebrate grazers in ripple
troughs ( Jansson, 1967; Newell, 1979 ). The grazing trail Psammichnites
grumula has been observed forming guided meanders along ripple troughs in
sand-flat deposits ( M´ngano et al., 2002b ).