DescriptionAt first glance it may not appear that the remote farming town of Starbuck, Washington, contains much geology. However, the unique slackwater deposits of the Tucannon Valley are perfectly exposed on the western side of town. The most notable feature of these deposits are the clastic dike structures and what we call the Starbuck Sand Flower which is over two meters tall. The creation of these beautiful clastic dike structures was likely the same that caused slackwater deposits.
Slackwater deposits are fine laminated silts and sands that accumulated in the valleys as the flood waters settled. These deposits were laid down in a fining up sequence, which means the clasts go from coarse to fine as you go up the strata. This is because the heavier gravel and sand-sized clasts settle out of suspension before the finer material. These laminated beds contained a lot of water due to their method of deposition, and this allows them to flow like molten plastic. Deposition was increased because of the Ice Age Floods which forced water up the valley, forming a temporary lake feature through the Tucannon Valley.
In order to identify the clastic dikes in front of you, look for vertical features that resemble tree branches. These tree-branch features near the top of the structure are what make up the structure of the sand flower.
The massive pressure from the overlying flood waters compressed the water-laden silts and sands, which in turn caused the finer, less dense material to flow upward. At first these structures seem to be associated with seismic activity, but if the geologic history of the area is taken into account, it’s clear that outburst floods were the cause of this plastic sediment flow.