title: Mid-Tertiary Volcanic Rocks of the Timberwolf Mountain Area, South-Central Cascades, Washington
Creators: name: Shultz, Julianna M.
Publisher: name: Western Washington University
Year: 1988

description: <p>Over 3000 m of mid-Tertiary Cascade arc volcanics are exposed in the Timberwolf Mountain area of Washington's central Cascade Range. Arc rocks of the Ohanapecosh, Stevens Ridge, and Fifes Peak Formations overlie the Jura-Cretaceous Rimrock Lake inlier and sparse late Eocene(?) sedimentary rocks. Geochemical analyses of meta-basalts from the Russell Ranch and Indian Creek complexes that comprise the inlier indicate an island-arc or immature continental volcanic-arc tectonic environment for the inlier.</p>
<p>The early to mid Oligocene Ohanapecosh Formation is comprised of two facies within the study area. Distal, water-lain andesitic to dacitic lapilli-tuffs of the Wildcat Creek facies unconformably overlie Eocene sediments (and, in places, Jura-Cretaceous basement) and are gradational with proximal andesitic flows and lapilli-tuffs (debris flows?) of the Timberwolf Mountain facies. The change from distal to proximal facies indicates that an eastward migration of vents (from the type locality near Mt. Rainier) occurred during upper Ohanapecosh deposition.</p>
<p>Rhyolitic Stevens Ridge-equivalent rocks, informally named the tuff sequence of Rattlesnake Creek, disconformably overlie upper Ohanapecosh tuffs. This tuff sequence may have been derived in part from eruptions that led to the collapse of the Mt. Aix caldera, several kilometers west of the study area. Ash flows from the upper portion of the Rattlesnake Creek sequence are interbedded with andesitic flows from two Fifes Peak volcanoes.</p>
<p>Major and trace element analyses from a dissected Fifes Peak Formation shield volcano at Timberwolf Mountain and from Tieton Volcano (a Fifes Peak Formation stratovolcano centered south of the study area) suggest that the volcanoes had a similar magmatic history. Whole rock K/At dates from a flow from the basal portion of the shield volcano at Timberwolf Mountain and from a flow within the Tieton Volcano apron yielded ages of 24.0 ± 0.8 Ma and 26.0 ± 0.8 Ma, respectively. An anomalous, yet reproducible, age of approximately 8 Ma for a hypersthene augite andesite flow remnant at Timberwolf Mountain indicates late Miocene or Pliocene arc activity within the immediate vicinity of the study area.</p>
<p>The volcanic and magmatic evolution of the central Cascades is reflected by the differing eruptive styles and rock compositions of the Ohanapecosh, Stevens Ridge, and Fifes Peak Formations. Three stages of evolution are recognized that correspond to the above formations. Stage 1 consists of the deposition of thick sequences of andesitic tuffs originating from numerous vents (the Ohanapecosh Formation Stage). Stage 2 corresponds to the eruptive style of the rhyolitic Stevens Ridge Formation; this stage is characterized by voluminous rhyolitic eruptions, with some associated caldera collapses. Stage 3 eruptions of Fifes Peak Formation volcanics produced andesitic volcanoes that were intimately associated with Stage 2 rhyolitic, caldera-forming, eruptions. Abundant andesitic magmas of the Ohanapecosh Stage may have resulted from large volumes of basaltic magma underplating the upper crust and fractionating into less-dense andesitic melts. Continued underplating of the upper crust probably induced partial melting of the overlying crustal material,producing limited volumes of silicic melts (Stevens Ridge Stage). Andesitic eruptions of the Fifes Peak Stage punctuated Stage 2 eruptions when rhyolitic melts were depleted.</p>