PROJECTING VARIATIONS OF DEFORMATION IN THE SPOKANE, WA AREA BASED ON ISOLATED BUTTES

The isolated buttes seen in eastern Washington near Spokane are all interconnected, but buried by the Columbia River Basalt that was emplaced in the Miocene, approximately 16mya.The Buttes are made up of uplifted Eocene granitic rocks and Cretaceous fold and thrust deformation preserved in Cambrian to Mesoproterozoic rocks. Making multiple cross sections that show possible scenarios of pre-Neogene stratigraphy sheds light on the structural deformation required for the buttes to be where they are currently located. The starkest unconformity is that of the structurally offset contact between the Deer Trail Group and the Belt Super Group, which preserves the Proterozoic breakup of supercontinent Columbia. Locally, the possible continuation of the Jumpoff Joe Fault separates the Piegan Group (part of the Belt Super Group) and either the Buffalo Hump Formation (Windermere Group) or Cambrian quartzite, which shows a potential throw of up to 2km (3.2 miles). The Magnison Butte fault in places can displace up to 500m or as small as just tens of meters between the units of the Deer Trail Group. The Latah fault may displace up to 60m of basaltic stratigraphy and likely goes into basement rocks. Deciphering the stratigraphic ages and relationships between isolated buttes will provide the first larger scale view of the tectonic and structural history preserved in the basement rocks of the Spokane area.