INTERDUNE FACIES CONTAINING A DINOSAUR BONE BED IN THE LOWER JURASSIC NUGGET SANDSTONE IN NORTHEASTERN UTAH

Just outside Dinosaur National Monument, in NE Utah, a recently discovered bone bed, the Saints & Sinners Quarry, has produced thousands of bones of small theropod dinosaurs. The bone bed occurs within an approximately 3 meter thick package of interdunal sediments located well within the thick, eolian sandstone member of the upper part of the Nugget SS. It is bounded above and below by large scale, high angle eolian crossbed sets. The interdunal sediments themselves are of very limited lateral extent, and in exposures only a few tens of meters away this interval is represented only by the truncation surface between dune sets. The interdunal sediments consist of a distinctive vertical succession of facies, a few meters in thickness.

The lower facies of the interdunal interval throughout the area of exposure consists of sandstones with small scale sedimentary structures, low angle cross laminae and shallow scour and fill troughs emphasized by thin, iron oxide crusts, that may represent adhesion ripples. The uppermost interdunal facies appears at one end of the outcrop area as an interval with sandstone layers of varying thickness separated by layers of fine sand and silt. Dinosaur tracks and oscillation ripples are present in this uppermost interval. The uppermost interval can be traced laterally along the outcrop directly into the bone bed. The layers of fine sediment thin and are lost in this lateral facies change to the bone bed, which consists of uniform sandstone with abundant bone. Bedding in the bone bed is obscure, but is revealed by weathering, as are fine, undulose laminae that suggest oscillation ripples. Massive sandstones immediately above the interdunal sequence are interpreted as eolian dune sands, but appear to lack large-scale cross laminae, probably the result of bioturbation.

We interpret the succession of facies within the interdunal sequence as evidence of a progressively wetter environment, with the wettest conditions at the time of deposition of the bone bed, when bodies of open water were present. The bioturbated dune sands immediately above the interdunes indicate a gradual return to drier conditions. Whether this represents an entirely local effect or is the result of more widespread climatic changes or some combination of both has not been determined.