Footprints in Sand Dunes

["Jerry D. Harris" <jharris@dixie.edu>]

Another new paper:

Loope, D. 2006. Dry-season tracks in dinosaur-triggered grainflows. Palaios 
21(2):132-142. doi: 10.2110/palo.2005.p05-55.

ABSTRACT: 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central shaft left open when the trackmaker's foot was withdrawn from the 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.

   This paper is actually really, really cool...if you're in to 
sedimentology!

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Jerry D. Harris
Director of Paleontology
Dixie State College
Science Building
225 South 700 East
St. George, UT  84770   USA
Phone: (435) 652-7758
Fax: (435) 656-4022
E-mail: jharris@dixie.edu
and     dinogami@gmail.com
http://cactus.dixie.edu/jharris/

"Actually, it's a bacteria-run planet, but
mammals are better at public relations."
                                     -- Dave Unwin