Dynamic liquefaction of Jurassic sand dunes: processes, Origins, and implications

摘要

Soft-sediment deformation occurs in ancient eolian dune deposits, but understanding its extent and the conditions and forces behind the deformation often remains elusive. Here we gain insight into the aerial extent and environmental conditions related to intense soft-sediment deformation, based on an exceptional three dimensional (3D) exposure of eolian dune deposits and preserved geomorphic landscape expressions in the Jurassic Navajo Sandstone at White Pocket, Vermilion Cliffs National Monument, Arizona. Deformation features include elongate northeast trending decameter-scale mounds and raised ridges cored by deformed and upturned eolian dune sets, overlain by a massive blanket sandstone with breccia blocks. The geomorphic mounds display ~40-60mspacing, roughly perpendicular to the southerly paleoflow dune foreset directions. The geometry of the deformation is imaged by oblique aerial photography using cameras mounted on a remote control airplane and high resolution panoramas with a robotic camera mount. We interpret the exquisitely preserved deformation features as liquefaction-induced ground failure, consistent with theoretical and laboratory studies of deformation in saturated sand. 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 ripped up during flow. The water-pressurized, upwelled sediment created the mounds (a relief inversion). In underlying deformed dune sets, deformation was intense but sedimentary structure was not obliterated as deformation progressed from initial cyclic mobility to strain-softening, but ceased prior to steady-state flow liquefaction. The spatial extent, topographic relief, and intensity of dynamic deformation suggest an origin of strong ground motion driven by long-duration surface waves of a large earthquake (> Mw 7-8), possibly related to back arc thrusting from Jurassic subduction of the Pacific plate under North America.