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Incremental Growth of Composite Clastic Dikes

Incremental growth of a sheeted clastic dike occurs over time, megaflood by megaflood. The figure above shows the growth of a composite clastic dike over the course of 9 separate Ice Age megafloods.

A few things to note about these crack-and-fill structures:

a.) The dikes are sediment-filled fractures that widened and deepened by the addition of new fill material in pulses ("fill bands" = "dikelets" = "sheets").

b.) The propagation and geometry of the dikes are governed by principles of fracture mechanics in natural materials (i.e., hydrofracture). Grainsize (actually porosity and permeability) of flood deposits (boulder-cobble channel gravels, laminated sands, sandy eddy bar gravels, silty-sandy slackwater rhythmites) governs the distribution, geometry, and sheeting in the dikes.

c.) Newly-deposited sediment is the source of new dike fills. The character of dike fills reflects sediment available at the ground surface during flooding (swept along by bottom currents or settled from slackwater).

d.) New fractures follow least-resistance pathways established by older injections.

e.) While new cracking and filling is shown here to occur with each successive flood, not all dikes grow at the same rate; fill band counts differ. And some floods produce no dikes. For example, few dikes are found in deposits of Waitt's small-volume, "late" floods that were confined to the mainstem Columbia River Valley.

f.) The early crack pattern tends to establish the geometry of the final dike.

g.) The final geometry created by new cracks simply following previous ones, as shown in the figure, results in an overall shape that closely resembles real dikes in the Channeled Scabland of Washington, Oregon, and Idaho.

h.) Each new crack is propagated and filled (propped) during megaflood events, not during interflood periods. A megaflood event can be broken into 4 phases: down-valley flooding + up-valley backflooding + slackwater periods + valley drainage. Models suggest a complete flood cycle spans a period of about a month. Rhythmite strata shows dikes initiate during more than one phase.

i.) If tempted to equate the pattern of repeated injections with earthquake recurrence, one must explain the overwhelming field evidence that indicates nearly all (if not all) soft sediment deformation in megaflood deposits is synsedimentary, that the dikes do not rise from a liquefied layer at depth (do not crosscut younger strata), and the absence of sheeted dikes in fine grained sediments of both Pliocene and Holocene age in Eastern Washington. These dikes formed during the Pleistocene (~1.8 Ma to ~12 ka), not before or since. They grew by processes inherent to that time - processes independent of >10 million years of Yakima Fold Belt growth and Quaternary activity on the OWL/Wallula, Hite, and Arlington-Shutler fault systems. They are not sand blows, nor seismites. They are flood injectites (Cooley, 2020).

j.) The happy-looking little fish is not to be trusted. All he knows is violence.

When megaflooding ceased, so did dike growth. The final shape resulting from 9 episodes of cracking and filling is shown at left.

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