Tidal Flats and Subtidal Sand Bodies - Trace Fossils as Indicators of Sedimentary Environments - page 531

associated with flooding events. The sand-sheet front deposits are characterized
by smaller bedforms containing composite types of burrows (i.e. resulting from
multiple bioturbation events), with vertical burrows penetrating from various
colonization surfaces to develop pipe rocks associated with multiple pauses
in sedimentation. Multiple colonization events of sandy substrates by
suspension-feeding organisms are typical of the Skolithos Ichnofacies. Ichno-
fabrics of the mud-rich bottomsets contain a deposit-feeding fauna represented
by Planolites and Teichichnus , recording an impoverished Cruziana Ichnofa-
cies. Low ichnodiversity, even in the low-energy facies, is attributed to soupy
substrate conditions. Sand-sheet margin deposits are characterized by alternat-
ing episodic sandstone sedimentation and mud deposition under relatively
low-energy conditions. The presence of trace fossils of deposit feeders, which
fed on organic particles that fell out of suspension and accumulated on the sea
floor, is characteristic of lower-energy conditions and the development of the
Cruziana Ichnofacies. On the other hand, Rosselia within hummocky cross-
stratified sandstone reflects colonization after storms.

3.3 Tidal Sand Ridges
Sand ridges are elongated sand bodies that are larger and geographically more
stable in comparison to dunes, being oriented at an oblique angle to the strongest
current ( Figs. 1 and 12 ). A sand ridge is composed of dunes and compound

FIGURE 12 Integrated sedimentological and ichnological model for tidal sand ridges.