Truncations Control Clastic Dike Geometry


Truncations control dike width and apparent taper direction. What appears to be an upward-tapering, upward-injected clastic dike sourced ostensibly from a liquefied bed at depth is actually a sheeted, downward-intruded (per descendum) dike that has been partially truncated at several bedding contacts. Four flood beds are shown (A, B,C,D). They are unconsolidated, late Pleistocene rhythmites (Touchet Beds) composed of coarse sand to silt (coarse-grained bottoms, fine-grained tops). Several sheets of sediment comprise the clastic dike that cuts them. Vertical fill bands indicate the dike widened incrementally by new inputs of sediment (new sheets). The sheeting is truncated at several points along the dike's length, at bedding contacts. Sheet counts at 5 cross section lines show more sheets lower down and fewer sheets higher up. The dike appears to taper upward because it contains fewer bands in younger rhythmites.


This photograph - without my annotations - is the cover image of Fecht et al. (1999), a less than authoritative treatise on clastic dikes that, somehow, was published by Battelle Hanford. It is unclear if the authors of the 200+ page tome - Karl Fecht and Bruce Bjornstad - recognized the fundamental control bedding plane truncations have on dike geometry. I contend they did not.

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