Trace fossils in a Jurassic eolianite, Entrada Sandstone, Utah, USA

Trace fossils and bioturbate textures are present in eolian beds of the Entrada Sandstone (Jurassic) southeast of Moab, Utah. Three new ichnogenera and ichnospecies are described in this report. The most noticeable of the trace fossils are trails (Entradichnus meniscus n. ichnogen. and ichnosp.) which parallel bedding planes in cross-stratified sandstone with well-developed parting lineation. Trails are long, unbranched, and gently curved. Many specimens contain an internal structure of meniscate backfill. The trails are oriented parallel to the depositional dip of cross-strata, suggesting that their creators moved down the lee sides of dunes, pushing sediment back up behind them. Similar meniscate trails are produced in modern sand dunes by the larvae of tipulid insects ("crane flies'). A second trace fossil type consists of small, vertical burrows (Pustulichnus gregarious n. ichnogen. and ichnosp.) preserved as bumps in convex epirelief on cross-strata surfaces. These bumps may represent upward extensions of the meniscate trails described above, or they may represent shallow burrows made by sphecid insects ("sand wasps'). Larger, plug-shaped, vertical burrows containing laminated fill (Digitichnus laminatus n. ichnogen. and ichnosp.) are rare in the Entrada. Origin of these burrows is unknown. Moderate to thorough bioturbation of sandstone lenses also is present. These ancient dunes are believed to have been formed in a sand sea where deposition persisted for a long period of time. Deposition was probably within 30o of the paleoequator, and the climate was semiarid or arid and hot.