VERTEBRATE-TRACK HYPOTHESIS FOR THE ORIGIN OF ENIGMATIC SEDIMENTARY STRUCTURES WITHIN EOLIAN CROSS-STRATA OF THE NAVAJO SANDSTONE AT ZION NATIONAL PARK, UTAH

Unusual sedimentary structures consisting of linear arrangements of vertically oriented, cylindrical deformations (about 5-cm in diameter) are preserved in large-scale cross-strata within the Jurassic Navajo Sandstone at two sites in Zion National Park, southwestern Utah. Individual cylinders within each series extend vertically as much as 2 m. Scores of these arrangements are present at each site. As many as 20 individual cylinders (evenly spaced about 5-10 cm apart) comprise each linear series. The cylinders always are present as a series; they do not occur as individuals. Laminae within cylinders are slightly concave-up, and are displaced downward less than 1 cm below corresponding laminae at the perimeters of the cylinders. All cylinders begin and end within the same stratigraphic interval: most have their upper termini a few 10’s of cm above a horizontal bounding surface. Some of the laminae near the upper termini of cylinders are folded and brecciated. All cylinders terminate downward within the uppermost 2 m of a thick (underlying) set of large-scale cross-strata.

We interpret the cylinders as fluid-escape structures. The even spacing of circles, however, makes each series strongly resemble a vertebrate trackway when it is viewed in plan on a single bedding plane. A few “normal” tracks (which penetrate and fold laminae only a few cm below the true tracking surface) are present in both the upper and lower set of cross-strata at both sites. We hypothesize that, during wet seasons, groundwater mounds formed below the large dunes. Zones of upward-seeping groundwater developed at the adjacent margins of interdune ponds. Tracking of animals across these zones produced inhomogeneities that became foci for upward-moving groundwater. Tracks may have ruptured surficial crusts that were held together by evaporite crystals. Rupture of such seals (recorded by the brecciated laminae?) may have abruptly steepened the groundwater pressure gradient, thereby triggering fluidization events. The presence of undisturbed lamine within cylinders shows that sand within the cylinders was not fluidized; instead, only a narrow ring of sand around each cylinder was disturbed by the upward flow. Laboratory-scale experiments to test our hypotheses are underway.