Flood Count Controls Clastic Dike Width, Columbia Basin, WA
January 31, 2020
Pleistocene age clastic dikes in the Columbia Basin, WA grew incrementally by repeated injection of sediment into cracks. Injected sediments are called "fill bands" or "sheets". The model above is a simple representation of the more complex real-world system where cracks form by hydrofracture and sediment is injected into them under pressure. I believe cataclysmic Ice Age floods (Missoula floods) are the trigger for hydrofracturing. At least 40 major scabland floods flowed out of western Montana, across eastern Washington, and through Wallula Gap. About 10 floods were confined within the mainstem Columbia River Valley.
Each flood scoured and deformed the substrate. Each flood produced new fractures and a new fill bands (maybe just one, maybe a small set of bands, sometimes no bands). New fractures (new bands) follow the paths created by older ones; indeed reinjection along pre-existing planes of weakness is the rule. Some bands (especially wide ones) were split down the middle by later injections, thus one band becomes two. In many dikes it is possible to reconstruct the order of injection and division, at least approximately. New fractures only needed to descend a distance equal to the thickness of a flood bed (typically < 1m) to join up with older dikes. Most fill bands are only a few centimeters wide (average width is closer to 1cm for large numbers of dikes). Likewise, hydrofractures propagated into the substrate were thin and short, their dimensions preserved by stratified sandy fills that propped them immediately after they formed.
Flood rhythmite deposition and preservation is variably within the floodway. Rhythmite counts (flood counts) are highest in semi-protected valleys located just beyond high-velocity scabland channels. Sections containing the most complete local flood count records are found within the perimeters of slackwater lakes Lewis (Pasco-WW-Yakima Basins), Condon (Columbia Gorge below Wallula), and Allison (Willamette Valley). These same sections contain the widest dikes, that is, those with the most fill bands. Most floods, most disturbances, most fracture events, most fill bands.
Understanding fill bands is the key to understanding these dikes. Analyses appropriate for sheeted clastic dikes differ fundamentally with those for liquefaction features such as those described in the New Madrid Seismic Zone by USGS geologist Steve Obermeier. Sheeted dikes in the Columbia Basin of WA-OR-ID are simply not comparable with sand blows in Missouri-Indiana-Illinois.
There are lots more photos, figures, and maps of my original research on clastic dikes posted on my Instagram page (@skyecooleyartwork) and elsewhere on this website (skyecooley.com).