Tidal Flats and Subtidal Sand Bodies - Trace Fossils as Indicators of Sedimentary Environments - page 532

dunes of varied forms and sizes which migrate both on the stoss and lee sides of
the ridge. On modern shelves they typically occur in localized groups ( Snedden
and Dalrymple, 1999 ). Four conditions are required for sand-ridge formation:
(1) an initial irregularity, commonly a transgressed coastal sand body (e.g.,
Penland et al., 1988; Posamentier, 2002; Snedden and Dalrymple, 1999;
Snedden et al., 1999; Swift and Field, 1981 ), (2) sufficient sand supply, (3) a
current capable of moving sand, and (4) sufficient time for the sand to be
molded into a ridge. Tidal sand ridges in the North Sea were formed when
glacioeustatic lowstand deposits were reworked by tidal currents ( Belderson
et al., 1982 ). Sand ridges in the East China Sea (southern Yellow Sea) also
developed during the Pleistocene-Holocene transgression. These overlie del-
taic deposits ( Bern ´ et al., 2002; Wang et al., 2012 ) and become younger toward
the present location of the Yangtze River due to continued transgression ( Liu
et al., 1998 ).
The internal structures of tidal sand ridges comprise smaller-scale cross-
stratified beds deposited mainly on the lee sides ( Fig. 13 A). Paleocurrent direc-
tions determined from individual cross-stratified beds are bipolar but oriented at
an angle to the direction of sand-ridge accretion. Thickening- and coarsening-up
intervals are common and record progradation of dunes on the flanks of the
lower-energy lee-sides and in the swales between the ridges. The trace-fossil
content is mainly restricted to horizontal and subhorizontal structures in inter-
bedded sandstone and mudstone belonging to Cruziana , Dimorphichnus ,
Planolites (typically P. montanus ), Palaeophycus , Phycodes , Rusophycus ,
and Teichichnus (typically T. rectus ). Skolithos linearis is present in cross-
stratified beds.
Sand ridges display clear ichnological trends perpendicular to the ridge
axes. Bioturbation is virtually absent in the cross-stratified beds that make
up the cores of the tidal sand ridges, and only scarce Skolithos linearis are
present in medium- to thick-bedded cross-stratified sandstones, clearly indi-
cating closed to short-term colonization windows linked to rapidly migrating
bedforms ( Fig. 13 B). The lee-side deposits of sand ridges are characterized by
increased heterogeneity as recorded by pervasive interfingering of muddy
layers ( Fig. 13 C). These heterolithic deposits mark the transition from the
Skolithos Ichnofacies to a depauperate Cruziana Ichnofacies, being character-
ized by low-diversity suites dominated by Planolites and Teichichnus .Low
ichnodiversity and sparse bioturbation most probably reflect soupy substrates
in connection with fluid mud. The shelf deposits adjacent to the sand ridges
consist of mud-dominated heterolithic successions, containing abundant
arthropod (e.g., Cruziana , Dimorphichnus ,and Rusophycus ) and worm ( Pla-
nolites , Palaeophycus ,and Phycodes ) trace fossils ( Fig. 13 D and E). This
association is characteristic of low-energy conditions and abundant organic
particles settling from suspension onto the sea floor, thereby allowing
the development of the archetypal Cruziana Ichnofacies ( Desjardins et al.,
2012a ).