Eolian Environments - Trace Fossils as Indicators of Sedimentary Environments - page 410

including spiders and scorpions that are preserved in well-sorted, medium-grained
sandstones” in eolian dune settings ( Hunt and Lucas, 2007 : 63). The ichnogenera
Octopodichnus and Paleohelcura (both arachnid trackways) are characteristic, but
their delicate nature (shallow surficial impressions in the sand) makes it possible
for them to be preserved only under special conditions. The authors report this
ichnofacies in Permian and Jurassic eolianites, and they note that these invertebrate
traces commonly co-occur in the same strata with the vertebrate traces that typify
the Chelichnus Ichnofacies.
The Entradichnus Ichnofacies was proposed “for the recurrent trace fossil
assemblage that typifies sparsely vegetated and unvegetated eolian dune fields
in arid climatic settings” ( Ekdale et al., 2007 : 570). The characteristic ichnogene-
ra (all of them invertebrate burrows) include Arenicolites , Digitichnus , Entrad-
ichnus , Palaeophycus , Planolites , Skolithos ,and Taenidium . The authors cite
numerous examples of this ichnofacies in eolianites of Permian, Triassic, Juras-
sic, Tertiary, and Quaternary age on several continents around the world, most of
which include the eponymous ichnogenus Entradichnus in the ichnoassemblage
( Ekdale and Picard, 1985 ). Buatois et al. (1998) report that other less common
ichnogenera, including the surficial trackways Octopodichnus and Paleohelcura ,
also are part of this recurrent trace-fossil association. Vertebrate tracks and track-
ways in eolian deposits are not excluded.
We regard the Entradichnus Ichnofacies of Ekdale et al. (2007) as the pri-
mary ichnofacies encompassing the spectrum of trace-fossil assemblages that
reflect eolian depositional environments throughout the geological record.
The much more narrowly defined trace-fossil assemblages that characterize
the Octopodichnus and Chelichnus ichnofacies of Hunt and Lucas (2007) are
subsumed within the Entradichnus Ichnofacies.
3. DEPOSITIONAL ENVIRONMENTS
3.1 Trace Fossils in Coastal Dunes
Coastal dune fields generally are localized in bands paralleling marine shore-
lines, and the component sand grains are reworked from beaches and tidal flats
into small, often well-vegetated dunes. They typically intergrade with the adja-
cent beach and tidal-flat facies, so the occurrence of occasional shell fragments
of marine organisms should not be unexpected in coastal-dune deposits; but the
trace-fossil associations are distinctly terrestrial.
Quaternary eolianites in the Bahamas are composed entirely of carbonate
grains, and they occur in coastal dunes that intergrade with beach and other
shallow-marine calcarentic deposits ( Curran and White, 2001; Knaust et al.,
2012 ). In this humid, tropical setting, rhizomorphs (i.e., plant-root penetration
structures) are ubiquitous and often are so pervasive that they obscure original
stratification and all other primary bedding features. The most prominent
rhizomorphs in Bahamian eolianites are identical to modern roots of railroad