Pleistocene dikes in Tertiary rocks:
Downward emplacement of Touchet Bed clastic dikes into co-seismic fissures, south-central Washington
Andrew R. Neill, Erin H. Leckey, and Kevin R. Pogue
Clastic dikes are ubiquitous in the Touchet Beds which are the rhythmically-bedded slackwater deposits of the Late Pleistocene Missoula floods. Recently discovered outcrops that expose dikes penetrating through the Touchet Beds into Pliocene fluvial and lacustrine sediment of the Ringold Formation and Miocene Columbia River Basalt confirm that the dikes were emplaced downward into cracks. The presence of the dikes in older, more consolidated units invalidates many earlier hypotheses regarding the mechanism of emplacement. To further evaluate the mechanism and timing of dike emplacement, an extensive survey was conducted throughout the basin of Touchet Bed deposition. Data was gathered regarding dike distribution, geometry, and orientation. Suitable dikes were sampled for paleomagnetic analysis. Our survey discovered many new exposures where Touchet Bed clastic dikes are hosted by underlying units. Dikes are present in the Ringold Formation at several locations but only in the absence of a thick caliche horizon that generally caps the unit. Dikes are present in the Columbia River Basalt only in close proximity to the active Wallula fault zone (WFZ), a segment of the Olympic-Wallowa lineament. Unlike their counterparts in the overlying Touchet Beds, the dikes in the older units display a preferred orientation at most exposures. In some exposures, Touchet Bed dikes exploit older, breccia-filled fissures in the Columbia River Basalt. Reverse-polarity dikes from an earlier episode of emplacement intrude pre-Touchet Bed rhythmites along the lower Walla Walla and Yakima Rivers. These dikes terminate upward at an unconformity at the base of the overlying Touchet Beds. The reverse-polarity dikes and their host sediments may be evidence for an earlier (pre-780 Ka) episode of Missoula-type floods.
Data collected during our survey points to seismically-induced ground cracking or hydraulic fracturing as the only possible emplacement mechanism. We believe that the spatial relationship between the basalt-hosted dikes and the WFZ indicates that crack formation and dike emplacement were initiated by seismicity.