Tidal Flats and Subtidal Sand Bodies - Trace Fossils as Indicators of Sedimentary Environments - page 526

conditions of continuous migration, the colonization window is closed or of
short duration, preventing bioturbation or allowing burrowing by opportunistic
organisms adapted to high-energy conditions, respectively. More extensive bio-
turbation typically results from longer-term colonization windows associated
with discontinuous migration or dormant forms ( Desjardins et al., 2010b ).
Water turbidity significantly influences the trophic type ( Buatois and
M´ngano, 2011 ). Clogging by abundant clay and silt particles in suspension
in the water column commonly precludes the establishment of suspension
feeders ( Perkins, 1974 ). Suspension-feeding is particularly inhibited in deltaic
settings ( Gingras et al., 1998; MacEachern et al., 2005 ) due to the presence of
buoyant mud plumes extending in a seaward direction, particularly under hypo-
pycnal conditions ( Bates, 1953; Kineke et al., 1996 ). In cases of extreme water
turbidity, primary production can be severely inhibited, resulting in an impov-
erishment not only of suspension feeders, but also of deposit and detritus feeders
( Leithold and Dean, 1998 ).
As in the case of intertidal settings, both the grain size and the degree of
consolidation of the substrate play an important role in controlling the subtidal
ichnofaunas. Some ichnotaxa tend to be restricted to certain types of substrate,
such as Skolithos and Ophiomorpha to sand-rich and Planolites to mud-rich
heterolithic intervals. Accumulation of fluid mud is quite common in tide-
dominated settings, imparting a substrate stress by reducing boundary shear
stress, which prevents benthic organisms from constructing permanent
structures or from actively backfilling tunnels ( Buatois and M´ngano, 2011;
MacEachern et al., 2005; Schieber, 2003 ). Fluid-mud deposits are typically
non-bioturbated or, more rarely, contain “mantle and swirl” biogenic structures
( Bhattacharya and MacEachern, 2009; Schieber, 2003 ).
The type of organic matter essentially controls feeding strategies.
Suspension-feeder burrows, such as Skolithos , tend to dominate under condi-
tions of relatively high energy conducive to a high content of suspended organic
particles in the water column (e.g., M´ngano and Buatois, 2004a; M´ngano
et al., 1996 ). Mud and silt layers rich in organic matter are typically bioturbated
by deposit and detritus feeders (e.g., Planolites , Cruziana ).
Five broad categories of subtidal sand bodies have recently been defined on
the basis of sedimentological and ichnological criteria: (1) compound dune
fields, (2) sand sheets, (3) sand ridges, (4) isolated dune patches, and (5) tidal
bars ( Fig. 1 ; Desjardins et al., 2012a ).
3.1 Compound Dune Fields
Compound dune fields comprise trains of large dunes with superimposed smal-
ler dunes, the crests of which are mainly oriented perpendicular to the predom-
inant tidal-current direction, and therefore accrete in a forward direction ( Fig. 8 ;
e.g., Ashley, 1990 ). This type of sand body was originally described from the
Eocene Baronia Formation in Spain by Mutti et al. (1985) as “tidal bars” but