DYNAMIC LIQUEFACTION OF JURASSIC SAND DUNES AT WHITE POCKET, VERMILION CLIFFS NATIONAL MONUMENT, ARIZONA: PROCESSES, ORIGINS, AND IMPLICATIONS

An exceptional 3-D Jurassic Navajo Sandstone exposure of intense soft-sediment deformation is preserved at White Pocket, Vermilion Cliffs National Monument, Arizona. A dozen elongate northeast trending decameter-scale mounds and raised ridges are cored by deformed and upturned eolian dune sets, overlain by a massive blanket sandstone with breccia blocks. The mounds are spaced ~ 40-60 m and roughly perpendicular to the southerly paleoflow dune foreset directions. Oblique aerial photography using cameras mounted on a remote control airplane reveal features interpreted as liquefaction-induced ground failure. A shallow water table affected by differential dune loading facilitated lateral spreading and failure. The transition to steady-state flow liquefaction near the top of the shallow water table destroyed original sedimentary structure, creating a massive sand blanket that entrained brecciated blocks. In underlying dune sets, deformation progressed from initial cyclic mobility to strain-softening but ceased prior to steady state flow liquefaction. The dynamic deformation created mounds of water-pressurized, upwelled sediment spaced at tens of meters.

This textbook example of liquefaction supports theoretical and laboratory studies of deformation in saturated sand. The spatial extent, topographic relief, and intensity of deformation suggest that the seismic coda was of long duration, with shaking driven by surface/Love waves. Possible sources of the waves include: A) a large earthquake (> Mw 7-8) related to Jurassic subduction, or B) a bolide impact. The White Pocket example comprises a new class of geomorphic features with implications for studies on Earth and possibly on Mars, where both modern and ancient lithified dune fields have been subject to strong ground motion.