Pleistocene slack water deposits of the Missoula floods (the Touchet Beds) display dramatic evidence of soft-sediment deformation that may have resulted in part, from seismicity. Products of this deformation include flame and ball and pillow structures, syndepositional faulting on a variety of scales, and ubiquitous sheeted clastic dikes. The source of the stress or stresses that produced the deformation, especially as they relate to the formation of the dikes, has been the subject of debate for over 80 years.
Many of the features are similar to those observed in deep-water turbidites and may be readily attributed to the near instantaneous loading of the floodwaters and associated sediment. The clastic dikes, however, are not so easily explained. Theories on the origin of the dikes include upward injection of sediment due to liquefaction during seismicity, downward filling of ground cracks produced by melting of buried ice wedges, and downward filling of large-scale desiccation cracks. Recent research has revealed relationships between the dikes and country rock that require the rejection of these hypotheses in favor of downward injection of sediment into large-scale networks of ground cracks produced by landslides and lateral spreading. The presence of sheeted dikes of Touchet Bed sediment within the underlying Miocene and Pliocene bedrock implies a very forceful mode of injection. Earthquakes and hydraulic fracturing during flooding appear to provide the only reasonable mechanisms. The lack of a correlation of discreet fracturing events with individual floods argues in favor of seismicity as a mechanism. The unusual sheeted or compound structure of the large-scale clastic dikes of the Touchet beds implies multiple injections and represents a unique set of conditions that may have been triggered by earthquakes.