PALEOECOLOGICAL INFORMATION FROM PALEONTOLOGY AND SEDIMENTARY GEOLOGY OF INTERDUNAL SEDIMENTS OF THE EARLY JURASSIC GLEN CANYON GROUP IN AND AROUND DINOSAUR NATIONAL MONUMENT (DNM) IN NORTHEASTERN UTAH

The Early Jurassic Glen Canyon Group in and around DNM has revealed new sedimentological and paleontological information related to interdunal sediments that refine interpretations of the depositional environment of the Glen Canyon erg.

Carbonates of limited lateral extent (~100 m) occur within interdune sediments. Large, laminated, mound-like structures likely represent spring deposits with standing water. External molds of small, high-spired gastropods are relatively abundant within a carbonate ledge close to the spring mounds. These carbonate beds grade laterally into sandstone where the carbonates appear to be forming subsurface. Other carbonates, with greater lateral extent are immediately underlain by sandstone that contains abundant vertical and horizontal small diameter invertebrate burrows.

A new locality within the Glen Canyon preserves multiple arthropod (probably scorpion) trackways and small (~2cm across) Brasilichnium tracks. One bed contains hundreds of Brasilichnium pes tracks preserved in a small area. These ichnofossils are found on dune slipfaces of eolian cross strata. Although these trace fossils occur at different levels, all the bedding surfaces can be traced down dip into a lens of interdunal sediments.

Stacked bonebed horizons within another interdune preserve disarticulated, but often associated, remains of multiple individuals of coelophysoid theropod dinosaurs; along with poorly preserved leaf impressions in one extensively bioturbated sandstone horizon. The fossiliferous zones can be traced laterally into tridactyl trackway horizons and laminar mudstones of lacustrine and marginal facies. The bone bearing horizons occur at local diastems related to water table. Taphonomic analysis of the bonebeds will be helpful in interpreting the depositional processes and environmental characteristics of this locality.

The taxonomic diversity of the ichnofossils and the body fossils implies that a number of organisms made use of these environments, and the small size of some of the organisms (e.g. Brasilichnium and a sphenodont) suggests that at least some elements of the fauna were part of a resident population in perennial, albeit fluctuating, interdune wetlands. Some sedimentologic evidence, such as the carbonate spring mounds, support such an interpretation.