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Dry-Season Tracks in Dinosaur-Triggered Grainflows

¹ 214 Bessey Hall, Department of Geosciences, University of Nebraska, Lincoln, NE 68588-0340; dloope1{at}unl.edu

Thousands of animal tracks are preserved in wind-blown cross-strata of the Lower Jurassic Navajo Sandstone at Coyote Buttes on the Arizona-Utah border, USA. Tracks deform thin grainflows that were deposited on the slip faces of large dunes. In cross-section, laminae (pin stripes) are smoothly folded, and are only very rarely broken. There is no sign of a central shaft left open when the trackmaker's foot was withdrawn from the substrate. At the top of each track, folded laminae are truncated by the pin stripe that marks the base of the next (younger) grainflow. Within some trackways, progressively younger tracks move up-section to younger grainflows. The folded, unbroken pin stripes and absence of a distinct shaft fill are inconsistent with a moist-sand (cohesive) substrate. Stratigraphic relationships between tracks and grainflows indicate that, as the animals moved across the slip face, they triggered dry avalanches, and then stepped on the newly deposited grainflows. Although scour by grainflows can remove shallow tracks, the loose packing (high porosity) of grainflows ensures that the feet of larger animals will penetrate well below the level of the next erosive surface. Although dry dune sand previously has been denigrated as a medium for track preservation, this example shows that dry eolian grainflows, due to their loose packing and their position in the zone of flow separation on the dune lee slope, can preserve abundant, clear tracks. A dry-sand origin of the tracks corroborates an earlier interpretation of the grainflows as December–February (dry season) deposits of cross-equatorial winds.

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