Publication Date

Spring 2019

Degree Type


Degree Name

Master of Science (MS)




Robert B. Miller


Cascades, Chumstick formation, Deformation, Leavenworth fault zone, Structure, Swauk formation

Subject Areas



The North Cascades is an excellent area to study basins and strike-slip faults that formed during regional transtension. This structural, petrographic, and U-Pb geochronological study focuses on the Eocene Leavenworth fault (LVF), which separates Eocene non-marine clastic rocks of the Chumstick Formation and older Swauk Formation. The LVF has been interpreted as a dextral strike-slip fault active during Chumstick deposition and as a reverse fault. Poles to beds (n=207) in the Swauk Formation trend WNW, oblique to the ~320˚-striking LVF, and commonly dip SSW, defining a girdle and a fold axis plunging 4˚ toward 297˚. Beds (n=50) in the Chumstick Formation trend NW and generally dip NE, and poles to beds define a fold axis plunging 13˚ towards 306˚. Overall, 70% of folds in the formations trend NW to WNW, <30˚ counter-clockwise to the main strand of the LVF, compatible with dextral transpression. Most faults in the Swauk and Chumstick formations strike >45˚ or <30˚ to the main strand of the LVF. Their movement sense is unknown. Source terranes for the Swauk Formation likely include the Cascades crystalline core to the N and NE and a combination of other relatively local sources to the N, NE, and SW. Detrital zircon maximum depositional ages in the Swauk Formation range from 67.2 ± 1.9 to 50.46 ± 0.17 Ma, constraining the timing of deposition and early folding of the Swauk Formation. Distinctive ~91 Ma tonalitic clasts in Chumstick conglomerate were likely transported ~20 km from their source by dextral slip of the Leavenworth fault zone.