Geology of a Part of the South-Central Cascade Mountains, Washington

Abstract

The center of the area investigated is 68 miles southeast of Seattle on the eastern slope of the south-central Cascade Mountains, Washington. The oldest rocks are pre-Tertiary igneous and metamorphic rocks which crop out in two north-westerly trending belts. The southern belt contains synkinematically metamorphosed greenschists, amphibolites and biotite schists which have been intruded by quartz dioritic rocks, and a tectonic complex of serpentinites and miscellaneous metamorphic rocks. The northern belt contains quartz dioritic gneisses, migmatites, and amphibolites which are in tectonic contact with low-grade metamorphic rocks. Unconformably overlying the pre-Tertiary rocks is the Eocene Naches Formation, a complex series of interbedded sedimentary and volcanic rocks at least 8000 feet thick. Feldspathic sandstones and basalts are most common in the lower and middle parts of the formation; rhyolitic rocks are found throughout the formation; andesitic rocks occur in the upper part of the formation. Unconformably overlying the Naches Formation are a variety of Oligocene-Miocene andesitic and basaltic rocks tentatively referred to as the Keechelus Andesitic Series. This in turn is Unconformably overlain by the Miocene-Pliocene Yakima Basalt, a series of basalt flows with local tuffaceous interbeds and palagonite breccias. The pre-Tertiary and Tertiary rocks have been intruded by a variety of magmatic types ranging in composition from rhyolite to gabbro. Structural trends are northwesterly across the northerly topographic alignment of the Cascade Mountains. The Tertiary structures parallel the pre-Tertiary structures and are largely controlled by them. Pre-Tertiary faulting, possibly thrusting, was intense, but exposures are poor, preventing measurement of exact displacement. At least one pre-Tertiary fault, marked by serpentinite lenses, had recurring movement during the Tertiary.