Morphology and sediment deformation of downslope Brasilichnium trackways on a dune slipface in the Nugget sandstone of northeastern Utah, USA

Abstract

The presumed synapsid trackway known as Brasilichnium is known by numerous trackways from many localities throughout the eolian deposits of the Late Triassic to Early Jurassic erg that covered much of the western United States. These trackways occur primarily on the foreset beds of dunes. The overwhelming majority of the trackways were made by animals traveling up slope. A slab of Nugget Sandstone exhibited in the Department of Geology and Geophysics at the University of Utah preserves at least 17 Brasilichnium trackways, 13 going up slope and four going down slope. The downslope tracks have a distinctive morphology consisting of a raised anterior pressure pad ahead of, and a posterior collapse depression behind, a small, poorly preserved impression of the foot. The anterior pressure pad can be explained as a low angle thrust driven forward by the impact of the foot, whereas the posterior depression is a downslope slump as the foot was withdrawn. This interpretation is supported by the observation on an outcrop in Dinosaur National Monument of similar features in a Brasilichnium trackway that traverses the slope, and by studies of tracks made by other vertebrates in eolian sands. The enormous preponderance of upslope over downslope trackways in eolian sandstones remains an open question. The distinctiveness of the downslope track morphology and the fact that they are well preserved in the studied material, however, suggests that the answer is something more than coincidence or preservation bias.