ANALYSIS OF PLAN-VIEW GEOMETRY OF CLASTIC DIKE NETWORKS IN MISSOULA FLOOD SLACKWATER SEDIMENTS (TOUCHET BEDS), SOUTHEASTERN WASHINGTON

Downward-injected networks of clastic dikes are present throughout the Missoula flood slackwater sediments (Touchet beds) of southeastern Washington. The dikes represent sediment-filled ground cracks that formed near the end of the cycle of Missoula floods. Regionally extensive networks of ground cracks are most commonly produced by desiccation, permafrost processes, or lateral spreading associated with seismicity. To compare the geometry of Touchet bed dikes with the patterns produced by these mechanisms, we analyzed satellite imagery of dike-related vegetation patterns, and cross-section and rare plan-view surface exposures of the dikes. Data were obtained for areas on the U.S. Dept. of Energy Hanford site, and in the valleys of the Snake, Walla Walla, and Touchet Rivers. The clastic dike networks form large diameter (~100 m), polygons with near-random orientations in the relatively flat topography between the present channel of the Columbia River and Rattlesnake Mountain. In the river valleys, the dikes are more closely spaced and generally form less random patterns with one or more preferred strikes. In areas where the location of the river during Touchet bed deposition is well constrained, a preferred strike is parallel or sub-parallel to the nearest channel. These observations are inconsistent with ground crack patterns produced by desiccation that are usually random but may be oriented perpendicular, rather than parallel, to the boundary of the cracked medium. Cracks produced by permafrost processes tend to form oriented orthogonal polygonal networks near stream channels and in lake beds. The geometry of these networks is a consequence of the change in thermal regime associated with proximity to the channel or shoreline. The crack network hosting the Touchet bed clastic dikes is most similar to fracture patterns produced by seismically-induced lateral spreading. Ground cracks associated with lateral spreading tend to parallel any nearby free-face and become progressively more random and polygonal with distance from the free-face and decrease in slope.