AN ISOLATED CRUSH BRECCIA IN MAGNISON BUTTE, OR PART OF A LARGER FAULT SYSTEM IN EASTERN WASHINGTON?

Deciphering the Pre-Micoene geology of eastern Washington can be difficult due to the extensive Miocene basalt flows and Pleistocene megaflood deposits. Steptoes such as Magnison Butte located 11 km south of Reardan and 15 km west of Medical Lake, WA rise to an elevation of 915 m, and provide windows into the older bedrock. Field reconnaissance revealed that the dominant geologic units are argillite-shale, quartzite-argillite, and a fault-breccia separating the two. Using the three-point problem method, the orientation of a fault cutting across the butte and separating the two major geologic units was determined to strike 150ᵒ and dip 88ᵒ to the southwest. The eastern footwall side of the hill consists of argillite-shale and was mapped as the Neoproterozoic McHale Slate Formation. The western hanging wall side consists of quartzite-argillite and has been mapped as the Neoproterozoic Buffalo Hump Formation. This would require up to 500 meters of offset to remove the Stensgar Dolomite, if this unit also extends into Spokane County. An alternative hypothesis is that one of the main geologic units of Magnison Butte are the argillite and slate of the Togo Formation, which would require only minimal offset, up to meters. The fault-breccia contains fragments larger than 5 mm, it is classified as a crush breccia. The relatively large size correlates with offset of the Togo Formation, as greater offset would have reduced grain size in the fault plane. The fault plane has similar orientation to other north-south trending normal faults, which are approximately 48 to 50 Ma. Using steptoes as windows into the basement reveals a glimpse into the complex geologic history of eastern Washington.