Tidal Flats and Subtidal Sand Bodies - Trace Fossils as Indicators of Sedimentary Environments - page 528

packages display thickening- and coarsening-upward trends. Mud drapes are
common locally.
These deposits are typically non-bioturbated ( Fig. 9 A) or contain a few ver-
tical burrows (e.g., Skolithos , Ophiomorpha ), representing the Skolithos Ichno-
facies. Thin-bedded horizontal bottomsets may contain only scattered horizontal
burrows (e.g., Planolites ; Fig. 9 B). Compound dune deposits may gradationally
overlie heterolithic intervals containing a diverse ichnofauna, dominated by
horizontal trails and burrows mostly produced by deposit-feeding organisms
such as Rusophycus , Cruziana , Planolites , Phycodes , Asterosoma , and Hel-
minthoidichnites , illustrating the archetypal Cruziana Ichnofacies ( Desjardins
et al., 2012a ).
The lack of bioturbation in the sand body indicates moderate to high sedi-
mentation rates and erosion resulting from continuous and rapid bedform
migration. Flocculated clay and mud aggregates recorded in the mud drapes
may show restricted colonization by suspension feeders. Bioturbation in the
bottomset deposits suggests that the colonization window was relatively long.
The associated intensely bioturbated deposits containing the Cruziana Ichnofa-
cies reveal preferential preservation, low-energy conditions, and a more or less
continuous colonization window in a fully marine environment.
3.2 Sand Sheets
Sand sheets consist of compound dunes but differ from compound dune fields
because the former cover extensive areas of the shelf and show a decrease of
bedform sizes as a product of a streamwise change in the current velocity across
the sand sheet ( Desjardins et al., 2012a ). Local tidal flow-field velocities mainly
control the dune patterns in sand sheets, although other factors such as grain size
and water depth also play important additional roles in determining their final
configuration ( Flemming, 2000 ). The typical modern sand-sheet examples are
those of the North Sea and Irish Sea, which migrate under the influence of
strong tidal currents forming sediment transport paths that extend over hundreds
of square kilometers, developing suites of bedforms in the course that decrease
in size in conjunction with reductions in the current energy ( Belderson et al.,
1982; Van Landeghem et al., 2009 ). Along such transport paths, bedforms
are arranged in three zones: (1) a zone of large compound dunes forming the
sand-sheet core, (2) a zone of small compound dunes forming the sand-sheet front,
and (3) a zone of small dunes and sand ripples forming the sand-sheet margin
( Fig. 1 ; Desjardins et al., 2010a ). Where sand supply is abundant within areas of
large compound dunes, tidal sand ridges may develop, whereas areas of highest
energy are commonly characterized by sand ribbons, furrows, and gravel lags,
but sand ridges can also develop in these zones ( Stride et al., 1982 ).
Desjardins et al. (2010a) proposed a facies model for sand sheets based on
observations in the Early Cambrian Gog Group, western Canada, characterizing
each subzone in terms of its ichnology and sedimentology ( Figs. 10 and 11 A).