Tertiary stratigraphy and structure of the eastern flank of the Cascade Range, Washington

A ruling hypothesis for the central Cascade Range in Washington is that the Eocene arkosic formations, which are kilometers thick, were deposited in local grabens, such as the Chumstick Formation in the putative Chiwaukum graben. However, the formations are regional in extent and are preserved in less extensive northwest-trending synclines. The Chumstick Formation in the Peshastin syncline is a more proximal equivalent of the Roslyn Formation, which is preserved in the Kittitas Valley syncline 25 km to the southwest. The Chiwaukum structural low is partially bounded on the southwest by the Leavenworth fault zone, which consists of northwesterly striking, northeasterly verging reverse faults (with associated northwest-striking folds). The reverse faults and the hinges of the folds are cut by N-S, dextral strike-slip faults, which also partially bound the Chiwaukum structural low. Conglomeratic units in the Chumstick Formation are not proximal to either set of bounding faults. The Leavenworth fault occurs on the steeper northeastern limb of a northwesterly trending, basement-cored anticline. The Eagle Creek and Ainsley Canyon anticlines also have reverse faults on their steeper northeastern limbs. In the Puget Lowland, the Seattle reverse fault is in a similar anticline. The regional distribution of the Eocene formations and uplift of the Cascade Range are caused by folding of the Miocene Columbia River Basalt Group since 4 Ma. The remnant of a 4 Ma andesite on Natapoc Mountain shows that the present low topography of the Chiwaukum structural low is erosional and young.