Dr. Chad Pritchard
The Tucannon River valley in eastern Washington is home to a number of ductile and brittle soft-sediment deformation. This project aims to look at ductile deformation by classifying and comparing styles of clastic dikes in the field and to look at brittle deformation by analyzing small faults in localized outcrops. Clastic dikes are sedimentary intrusions into other sedimentary strata. There are three main types of clastic dikes: injection, extrusion, and sugtrusion. In addition, two subcategories of clastic dikes are described as sheeted dikes and fault injection dikes. Ductile and brittle deformation occurs in “slackwater” deposits associated with margins of glacial lakes and temporarily ponded lakes formed during Pleistocene megafloods. The Tucannon River valley provides a cross-sectional view of the interactions between slackwater deposits and surging flood waters. Pleistocene megafloods surged up valleys up to ~200 meters deep at meters per second providing an influx of sediment-laden flows and ample force to form soft-sediment deformation structures, including clastic dikes. The relationships between locations, soft-sediment deformation features, orientation of brittle deformation, and stratigraphy that host the deformation point towards the megafloods as triggers for fluidization and liquification of megaflood deposits. Fault planes were analyzed for strike, dip, and offset at two different locations, varying in grain size. At Smith Hallow Rd., faulting indicates principle stress coming form the sides, and at the Starbuck Quarry the principle stress was from above. Smith Hallow Rd. shows evidence of slumping, corresponding to ideas from Baker (1978). Laboratory experiments produced similar features to those observed in the field, highlighting 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.
Howard, Chelsi K., "Ductile and brittle deformation of slackwater deposits in the Tucannon River Valley" (2021). Geosciences Student Work. 3.
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