Rocky Mountain Section - 72nd Annual Meeting - 2020

Paper No. 6-10
Presentation Time: 8:30 AM-4:30 PM


HOWARD, Chelsi K., Department of Geology, Eastern Washington University, Cheney, WA 99004 and PRITCHARD, Chad J., Department of Geology, Eastern Washington University, 130 Science Building, Cheney, WA 99004-2439

Clastic dikes are intrusions of sediments into layers of other sedimentary strata that are found in various places across eastern Washington. Three notable sites include Burlingame Canyon in Touchet, WA, Tacannon Valley near Starbuck, WA and Campion Park in Spokane, WA. At these locations, sand and fine-grained sediments interact in unique ways, which can potentially be reproduced in a laboratory setting. Clastic dikes are thought to be formed by either overburden or from seismic activity, and can be injection/sugtrusive (downward) or extrusive (upward). At eastern Washington occurrences, the clastic dikes were likely formed by the load force of glacial Lake Lewis in Tucannon Valley and Burlingame Canyon, and the load force of glacial Lake Columbia in Spokane. The estimated hydrostatic pressure of Lake Lewis (180-270 m at maximum depth) is 18 kg/cm2, without including suspended load and turbulence.

We were able to differentiate between linear clastic dikes, which are only one trunk with little or no splitting from the main trunk and branching dikes, which are multiple branches of dikes splitting from one main trunk. We recreated this environment by layering saturated sand below and on top of dry kaolinite clay, and slowly adding load force pressure up to 730 kg/cm2. While this showed evidence of air bubbles escaping to the surface from the top layer of sand, clastic dikes did not form. Additional tests involved striking the surface of the layered, saturated sand and kaolinite clay with a rock hammer at about 700 kg/cm2. This showed air bubbles escaping the softer sediments, and the clay filling the vacant trail left by air bubbles. The bottom layer of sand was then pulled up through the clay and out the top layer of sand, creating an extrusion-type clastic dike.

Boundary conditions of the box influenced the type of clastic dike formation. For example, linear dikes aggregated near the box edge due to higher confining stress and branching dikes were dominant in the middle of the box due to lower confining stress. Laboratory experiments produced features similar to those observed in the field and highlighted the importance of sediment saturation and bubble transport of sediments to cause extrusion clastic dikes.