New Glacial Lake Missoula Chapter member Skye Cooley gave a talk on clastic dikes at our November meeting. Clastic dikes are sand-filled cracks that formed during the Pleistocene in flood-laid deposits across the Channeled Scabland of eastern Washington, northeastern Oregon, and western Idaho. The dikes were filled from above and contain conspicuous vertical sheets. Although none have been discovered in northwest Montana, they are numerous in flood-inundated valleys to the west. Their formation coincides with a dynamic period in the not too distant past when large earthquakes, catastrophic outburst floods, and rapid climatic shifts shaped the landscape. They are of interest because they may have formed in response to forces generated entirely by glacial outburst floods. Skye is working on a book titled “Clastic Dikes and Pleistocene Megafloods”, due out in 2020, and will be making a presentation at the Othello Sand Hill Crane Festival in March, so stay tuned. An abstract of his talk can be found below.
Typical Clastic Dike
Abstract
Thousands of clastic dikes occur in Ice Age flood deposits throughout the Channeled Scabland. The origin of these sand-filled cracks has been debated for nearly a century. The two competing explanations are earthquake shaking vs. rapid loading by outburst floods. The structures occur entirely within the margins of the Pleistocene floodway, nowhere above megaflood trimlines, or east of the Purcell Trench (former ice dam). This talk presents data on 3000 dikes penetrating 12 geologic units at 300 locations. The data indicate the dikes formed by hydraulic fracturing driven by floodwater loading. Vertical sheeting records punctuated growth during flood events. Growth of large dikes took centuries to millennia as new sheets, progressively sourced from younger and younger flood deposits, were added. In contrast to liquefaction-type dikes, these structures were filled from the top by sediment injected downward. The sheeted fills were sourced from turbulent, circulating currents within floods, not from fluidized sediment mobilized by shaking. The dikes appear unrelated to the Quaternary record of seismicity in the Columbia Basin.