CLASTIC DIKE FORMATION IN SLACKWATER DEPOSITS TRIGGERED BY SURGING PLEISTOCENE MEGAFLOODS

Eastern Washington hosts numerous types of ductile and brittle soft-sediment deformation including multiple styles of clastic dikes. This project aims to classify styles of clastic dikes and physically model processes and triggers of clastic dike formation in eastern Washington. Several clastic dike locations in Washington (Touchet, Spokane, Lewiston, and Tucannon Valley) are “slackwater” deposits associated with margins of temporarily ponded lakes formed during Pleistocene megafloods. Though some sites are also in close proximity to active faults or large landslides the relationships between deformation features, orientation of brittle deformation, and sediments that host the deformation point towards the megafloods as triggers for fluidization and liquification of megaflood deposits. Over 200 meters deep, the Pleistocene megafloods flowed up to 6 m/s and surged up valleys providing an influx of saturated sediments and ample mass to form soft-sediment deformation and clastic dikes. For this study we also assume the turbulent megafloods would entrain air bubbles, so even though the sediments are saturated the dissolved and undissolved gas (air) would initially be present in the deposits. Modeling of saturated sand over partially-saturated sand and silt layers as well as water-lain sand and silt horizons produced clastic dikes and soft-sediment features. Key factors that produced injection clastic dikes are the infilling of sediment by debris flow (sediment rich megafloods), whereas extrusive dikes formed primarily by grain rotation during water and gas escape. Each model reproduced bubble-trains that provided a path for injection and extrusive clastic dikes. Laboratory experiments produced similar features and highlighted the importance of saturation of sediments as well as water/air escape-transport of sediments to cause clastic dikes, though some dikes may be infilling of existing fractures or bubble-train paths from sediments above.