STRUCTURAL RELATIONSHIP BETWEEN FOLDING AND FAULTING PRESERVED IN PROTEROZOIC BUTTES OF THE MEDICAL LAKE AREA, EASTERN WASHINGTON

Buttes of the Medical Lake area may help project fold and thrust style deformation into areas generally covered by younger geologic units. Specifically, Needham, Riddle, and Olsen Hills are made up Mesoproterozoic Belt Supergroup while the buttes to the West, such as Fancher Butte and Booth Hill, contain Neoproterozoic and possibly Cambrian rocks. These were likely thrust over the Belt Super group along what is regionally referred to as the Jump Off Joe fault, or similar structure. This poses an issue as reverse faults are not associated with younger rocks being thrust over older rocks. The fault is not well preserved due to the presence of Eocene granite exposed in Medical Lake as well as portions of Olsen, Riddle, and Needham Hills. Subsequent Miocene Columbia River Basalt flows also cover lower areas and obscure older rocks. Younger normal faults have dissected the area as well, in addition to the western Cheney Fracture Zone. Interpreted folding within the Medical Lake area identified gently northern plunging open-to-tight, upright-to-inclined folds that may be associated with Cordilleran thrusting.