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Seismogenically induced fluidization of Jurassic erg sands, south‐central Utah

Large bodies of fluidized sandstone occur in the Jurassic Entrada, Carmel, Page and Navajo Formations at several locations in south‐central Utah. They are most abundant in the Entrada Sandstone, where they commonly occur in clusters, have a cylindrical form and have a sharp contact with their cross‐bedded host rock. These clastic pipes are as wide as 75 m and have exposed heights of as much as 100 m. Some of the Entrada pipes extend well into the underlying Carmel redbeds. Other clastic pipes in the Entrada Sandstone are less deformed and display various degrees of brittle‐to‐hydroplastic deformation and liquefaction. Clastic pipes in the Page and Navajo Sandstones are less common, but are similar in size and form to those in the Entrada and Carmel, and probably have a similar origin. Some massive sandstone bodies are irregular in form and have tongue‐like projections into the host rock, implying forcible injection of fluidized sand. Several pipe–host contacts in the Entrada Sandstone display small‐scale ring faults. Where relative displacement can be clearly demonstrated, pipe sandstones are invariably down‐faulted, locally as much as 5 m. At two sites, Carmel host rock is upwarped around the Entrada pipes. Stratified and cross‐bedded breccia blocks occur in many Entrada pipes, and preliminary petrographic analysis indicates that at least some of these breccia blocks are derived from the host rock. Homogeneous pipe sandstones are also petrographically similar to their Entrada host rock, suggesting that some pipes originate through fluidization of the fine‐grained Entrada. Fluidization of the Entrada must have occurred in a water‐saturated environment during early diagenesis but before complete lithification, most probably under considerable porewater pressure. Although there are no known modern analogues to these huge masses of structureless sandstone, they may have a small‐scale modern counterpart in earthquake‐induced sandblows. These features were most probably caused by large‐magnitude seismic events during the Middle Jurassic, although other possibilities cannot be ruled out at this point.