Skip to Main Content
Book Chapter

Trace Fossils in a Jurassic Eolianite, Entrada Sandstone, Utah, U.S.A.

By
A. A. Ekdale
A. A. Ekdale
Department of Geology and Geophysics, University of UtahSalt Lake City, Utah 84112
Search for other works by this author on:
M. Dane Picard
M. Dane Picard
Department of Geology and Geophysics, University of UtahSalt Lake City, Utah 84112
Search for other works by this author on:
Published:
January 01, 1985

Abstract

Most ancient eolianites possess a meager fossil record. Trace fossils and bioturbate textures are present, however, 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 (rails (Eniradichnus meniscus n. ichnogen. and ichnosp.) which parallel bedding planes in cross-stratified sandstone with we 11-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.

Sedimentary structures that together indicate an eolian origin for the sandstone are: (1) large-scale, high angle (mean of 22°), sweeping cross-stratification; (2) large-scale soft sediment deformation, including small-scale soft sediment faulting; (3) eolian ripple marks (large ripple index and high ripple symmetry index) parallel to the dip of foreset slopes of cross-strata; and (4) multiple parallel-truncation bedding planes. Paleocurrent measurements are unimodal and suggest that winds blew to the south and southeast. Eolian petrographic characteristics of the sandstone are: (1) bimodal textures; (2) frosting of grains; (3) rounded or well-rounded coarser grains; (4) minor matrix; (5) high quartz content; (6) dominantly calcite cement; and (7) moderately to well-sorted grains.

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 30° of the paleoequator, and the climate was semiarid or arid and hot.

You do not currently have access to this article.
Don't already have an account? Register

Figures & Tables

Contents

SEPM Special Publication

Biogenic Structures: Their Use in Interpreting Depositional Environments

H. Allen Curran
H. Allen Curran
Department of Geology Smith College, Northampton Massachusetts
Search for other works by this author on:
SEPM Society for Sedimentary Geology
Volume
35
ISBN electronic:
9781565761650
Publication date:
January 01, 1985

GeoRef

References

Related

A comprehensive resource of eBooks for researchers in the Earth Sciences

Close Modal
This Feature Is Available To Subscribers Only

Sign In or Create an Account

Close Modal
Close Modal