Clastic Dikes: Indicators of Climate during Late-Glacial Missoula Flooding?

Clastic dikes distributed throughout basins of south-central Washington (>10,000km2), formed during a single, narrow window of time near the end of the cycle of late-glacial Missoula flooding. Recent articles ascribe the dikes to earthquake-induced lateral spreading and top-down forceful injection. However, the dikes show characteristics common to both "seismites" and "ice wedge casts". They can be up to 2m in width, >10m in length, contain dozens of parallel fill-bands, are widely distributed, and form conspicuous polygonal networks and sheeted sills. They penetrate downward, far deeper than ice wedges. If seismites, then the shaking that accompanied each flood torrent was did not produce deep ground fractures; the dikes would terminate at contacts between flood beds. If cryogenic, then they formed during a brief period of unusually deep cold just prior to regional warming at 12,000calyrBP. It would seem likely that topographic basins in close proximity to an ice sheet margin (<200km) would tend to pool very cold air, yet ice wedges did not form in wet silt-sand rhythmites each time waters receded. The lack of ice wedge casts indicates that temperatures remained above periglacial (>0°C) during the ~3000yr-long flood episode. Data from pollen records, plant macrofossils, charcoal data suggest that the late-glacial Columbia Basin was a "tundra-like" grassland with sparse refugia of pine, fir, and spruce. Rodent burrows, mammal fossils, freshwater shells indicate that soils remained unfroze during decades-long interflood periods. While an exposed, stunted, grass-shrub community was present during flooding, perennial ground ice was not. True tundra requires both cold-adapted vegetation and frozen subsoils. A rich history of late-glacial food features is preserved, yet periglacial evidence is largely absent (solifluction/gelifluction, nivation hollows, sink holes). Conditions were subarctic during Missoula flooding, but not periglacial. If a periglacial zone existed further north, then it was narrowly confined to the CIS margin (<150km).