Trace Fossils in a Jurassic Eolianite, Entrada Sandstone, Utah, U.S.A.
A. A. Ekdale, M. Dane Picard, 1985. "Trace Fossils in a Jurassic Eolianite, Entrada Sandstone, Utah, U.S.A.", Biogenic Structures: Their Use in Interpreting Depositional Environments, H. Allen Curran
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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.
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Organisms of one sort or another today inhabit virtually every sediment environment on Earth, and the rock record tells us that this has been the case through the greater part of our planet’s history. Furthermore, organisms leave their mark in most sedimentary settings, either directly in the form of body fossils or indirectly as biogenic structures. In addition to their often profound modifying effects on substrates, ancient biogenic structures preserve a record of organism behavioral activity in response to substrate and other paleoenvironmental controls. Thus, biogenic structures can be highly useful as facies indicators and can provide valuable clues to the interpretation of paleodepositional environments. The purpose of this volume is to present a broad spectrum of case-book examples of the use of biogenic structures in the interpretation of depositional environments.