DYNAMIC SOFT-SEDIMENT DEFORMATION IN THE JURASSIC NAVAJO SANDSTONE EOLIANITE, MOAB, UTAH

The upper portion of the Jurassic Navajo Sandstone near Moab, Utah, contains extensive soft-sediment deformation (SSD) features. We developed an index to distinguish the multiple levels of SSD in this eolian unit, ranging from intact beds (SSD 0), up through intense and pervasive disturbances (SSD 5). In ascending order of deformation intensity, these levels are: SSD 1 – mostly intact host rock with small-scale bioturbation and/or cm-scale dish and fluid escape structures; SSD 2 – wavy, undulatory bedding and centi- to decimeter disturbances; SSD 3 - m-scale contorted and deformed eolian cross-bed sets (kinked, slumped, rolled, overturned, vertical, and detached); SSD 4 – remnant “pseudobedding” with relict blocks of eolian stratification within massive sandstone and/or large, internally massive pipes or masses; and SSD 5- laterally extensive, massive sandstone, meters to decameters thick, with a very smooth weathering expression. Many of these features, such as the deformed cross-bed sets and the massive sandstone beds are clearly genetically linked, but large distinct pipes occur sparsely.

Here, we highlight two localities, “Petrified Dunes” in Arches National Park and Seven Mile Canyon near Moab, that show the remarkable extent of the soft-sediment deformation. The “Petrified Dunes” preserve a dramatic, steep 60° contact of 20 m vertical height where sheared dune sets abut against deformed and massive sandstone. In Seven Mile Canyon, similarly steep contacts cut across multiple dune sets to juxtapose disrupted massive sandstone next to normal cross-bedded sandstone at irregular contacts.

Both individual and combinations of these stratabound features suggest dynamic deformation in the Jurassic eolian system nearly coincident with deposition, accompanied by high water table conditions and significant, strong, “violent” ground motion events that led to large-scale disruption near the eastern margin of the Navajo sand sea.