CONTROLS ON THE SPATIAL DISTRIBUTION OF VERTEBRATE FOSSIL TRACKS IN THE NAVAJO SANDSTONE AT MOCCASIN MT, UTAH

Abundant vertebrate fossil tracks in the Navajo sandstone at Moccasin Mountain, Utah, are extraordinarily preserved on dune foresets deposited near angle of repose. Multiple levels of individual tracks, trackways, and trampled surfaces are visible in a variety of well exposed cross-section orientations between two first order bounding surfaces. The objective of this study is to identify controls on the distribution of tracks at this site. Development of a model is facilitated by the regular occurrence of tracks within an extended interval of foresets and their higher concentrations on several reactivation surfaces within that sequential record. This penecontemporaneous deformation provides a unique opportunity to constrain changes in the saturation state of the depositional surface of these Jurassic dunes.

An area of approximately 6000 square meters located between two first order bounding surfaces was mapped for dune architecture, tracks and trackways, and other deformation features. Details of the preservation of deformation structures were coded in the field by style of deformation, corresponding to different saturation states.

About 200 dune foresets were identified and measured in the study area. The dune prograded to the southwest. More than thirty of these foreset beds are trampled by track-making vertebrates. Previous researchers have identified six ichnotaxa in the study area with individual track sizes ranging from 2 to 30 cm. Trackways are oriented up, down, and across the slip face and are found in both plan view and in cross section. On average, about every fifth foreset is a trampled bed with a distance between trampled beds of 10 to 30 cm. Where individual tracks can be observed, a variety of deformation caused by the organism-sediment interaction is present. Spatial distribution of tracks and other deformation features are presented in a map and a series of cross sections through the study area. Preliminary assessment of the morphology of individual tracks suggests that surface moisture conditions were critical to the formation and preservation of the tracks. Damp to wet foresets coupled with repetitive episodes of trampling can be explained by reptile migration during monsoonal cycles.