Truncation Controls 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 bedding contacts. Four beds are shown (A, B,C,D). They are unconsolidated, late Pleistocene Touchet Bed rhythmites composed of coarse sand to silt (coarse-grained bottoms, fine-grained tops). The dike is comprised of several sheets of sediment (vertical fill bands), indicating it widened incrementally. Sheeting is truncated at several points along the dike's length, always at bedding contacts. Labeled cross sections show more sheets lower down and fewer sheets higher up.


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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