Eolian Environments - Trace Fossils as Indicators of Sedimentary Environments - page 418

Trace fossils play an important role in interpreting the paleoclimatic condi-
tions that are recorded in eolianites because they are direct reflections of the
episodic activity of organisms in the dunes throughout the year. Ekdale et al.
(2007) described the invertebrate ichnocoenoses in monsoonal deposits of the
Jurassic Navajo Sandstone in southern Utah. They found that the invertebrate
ichnocoenoses represented a community of shallow-burrowing, solitary (non-
social) insects that kept pace with the migrating dunes and flourished during
pluvial intervals that brought higher than usual precipitation to the Navajo erg.
4. ICHNOFABRICS
Because most trace fossils in eolianites are horizontal repichnia or very shallow
cubichnia, ichnofabrics in vertical cross section typically are poorly developed
or entirely absent. Coastal dunes, that are stabilized by dense vegetation, may
exhibit an ichnofabric dominated by pervasive rhizomorphs ( Fig. 5 )orby
highly localized social insect burrows, such as the cluster burrows and stellate
burrows in Quaternary eolianites in the Bahamas ( Curran, 2007 ). However, in
unstabilized, mobile sand dunes, one does not normally encounter complex,
composite ichnofabrics ( sensu Bromley and Ekdale, 1986 ), or high ichnofabric
indices ( sensu Droser and Bottjer, 1986 ). Permanent or semi-permanent dom-
ichnia or fodinichnia, which typically compose tiered ichnofabrics in other,
less-mobile substrates, are rare to absent in active dune environments.
There is one very interesting ichnofabric that is absolutely unique to the
loose sand substrate of mobile, subaerial dunes, but it is neither widely recog-
nized nor well understood. A number of vertebrate dune dwellers are highly
adapted to moving through loose, dry sand in the very same manner as many
aquatic organisms swim through water, namely by muscular undulations of
their elongate bodies. This process of infaunal locomotion through unconsoli-
dated sediment is known as sand swimming, and it is a behavior exhibited by
FIGURE 5 Plant trace fossils in ancient eolian deposits. Rhizoliths (plant-root traces with a thick
diagenetic halo) in Late Pleistocene eolianite of the South Alghero coast, Sardinia, Italy (photo-
graphs courtesy of Dirk Knaust). Rooted horizon in vertical outcrop section (A, image ¼ 1 m wide),
and bedding-plane view of three rhizoliths in cross section (B, image ¼ 20 cm wide).