U.S. DEPARTMENT OF THE INTERIOR TO ACCOMPANY MAP GQ-1681 U.S. GEOLOGICAL SURVEY GEOLOGIC MAP OF THE SPIRIT LAKE WEST QUADRANGLE, SKAMANIA AND COWLITZ COUNTIES, WASHINGTON By Russell C. Evarts and Roger P. Ashley INTRODUCTION Bishop. We are grateful to LedaBeth Gray, Brent D. Turrin, and James G. Smith for providing K-Ar and The Spirit Lake West quadrangle is located on the 40Ar/39Ar ages. Much of this work would have been western slope of the Cascade Range in southern impossible without the continuing logistical support Washington about 15 km north of Mount St. Helens. provided by Bobbie Myers and the staff of the David A. Bedrock consists of Oligocene volcanic and volcaniclastic Johnston Cascades Volcano Observatory. Discussions rocks and Oligocene to early Miocene shallow-level over the years with our U.S. Geological Survey colleagues plutonic rocks forming the core of the Tertiary Cascade Donald A. Swanson, James G. Smith, Virgil A. Frizzell, volcanic arc. Surficial deposits include drift representing Donal R. Mullineaux, and William E. Scott, with Paul E. at least two episodes of alpine glaciation as well as Hammond of Portland State University, and with William eruptive products of Mount St. Helens volcano M. Phillips and Michael A. Korosec of the Washington (Mullineaux and Crandell, 1981; Mullineaux, 1986; Division of Geology and Earth Resources have been Crandell, 1987). stimulating and informative. Technical reviews of the Repeated glacial advances during the Pleistocene manuscript by James G. Smith, and William J. Keith and sculpted the area into a terrain of considerable relief, i at comments on the Quaternary geology by Dwight R. bedrock exposures are generally restricted to outcrops in Crandell are especially appreciated. high-gradient streams due to the extensive and heavily vegetated surficial cover. Nearly all of the quadrangle is SUMMARY OF GEOLOGY within the area of devastation resulting from the May 18, The strata exposed in the Spirit Lake West 1980, eruption of Mount St. Helens (Lipman and quadrangle are part of a thick section of middle Tertiary Mullineaux, 1981). A laterally directed pyroclastic blast- subaerial volcanic rocks that underlies the Mount St. surge leveled old-growth forest in this area and buried the Helens area (Evarts and others, 1987). This section countryside under a stratigraphically complex blanket, as strikes approximately north-south and dips eastward at thick as 1 m, of ash, lapilli, and blocks (Hoblitt and others, low to moderate angles, forming the gentle western limb 1981; Moore and Sisson, 1981; Waitt, 1981; Fisher and of a major syncline whose axis lies about 15 km to the others, 1987). Subsequent erosion and salvage logging of east in the French Butte quadrangle (Walsh and others, the denuded slopes, however, has stripped much of this 1987; Swanson, 1989). The strata are lithologically cover as well as older surficial materials. Surficial deposits heterogeneous and stratigraphically complex. Distinctive were entirely removed from slopes above Spirit Lake and marker units are absent, and although unconformities along the North Fork Toutle River and South Coldwater have been recognized, their regional significance is Creek by passage of the rockslide-debris avalanche on the unknown; therefore, construction of a formal stratigraphic morning of May 18 (Voight and others, 1981; Fisher and framework was not attempted, and only lithologic and others, 1987). As a result, exposures of Tertiary bedrock local informal units are shown on this map. Isotopic age in the Spirit Lake West quadrangle and adjoining areas determinations are the most useful data for regional (Evarts and Ashley, 1993a) are unsurpassed in the correlations. The age data from this and adjacent areas western Cascade Range. This area thus provides an (table 1; Evarts and others, in press) indicate that the exceptional opportunity to examine stratigraphic and volcanic rocks in the quadrangle erupted between about structural details of the Tertiary volcanic arc. 36 and 27 Ma. Magmatic activity manifested as the ACKNOWLEDGMENTS epizonal Spirit Lake pluton continued until about 20 Ma. A 4°Ar/39Ar laser-fusion age for a dike cutting the pluton Weyerhaeuser Corporation freely allowed access to suggests very minor activity as young as about 9 Ma (table their lands in the western part of th3 quadrangle. We 1). were ably assisted in the field during the early stages of Many rocks of the quadrangle appear to represent mapping by Rick Bishop, Mike Covey, Jerry Infeld, Mike near-source depositional environments characterized by Lukk, and Carolyn Peterson. Laboratory support during abundant lava flows, pumiceous pyroclastic rocks, coarse­ the project was capably provided by Joe Ash, Mary grained epiclastic deposits, fine-grained subvolcanic Caress, Mike Covey, Jerry Infeld, Rick Pietropaoli, Scott intrusions, and zones of hydrothermal alteration (Cas and Petersen, Carolyn Peterson, and especially by Rick Wright, 1987; Smith, 1993). Stacks of lava flows such as those exposed near Coldwater Lake, for example, may though whether they actually vented to the surface is represent cross sections through the flanks of mafic shield unknown. Laterally continuous sections such as that volcanoes. The concentration of andesitic and basaltic extending from The Dome to Spirit Lake, however, were dikes, plugs, and hydrothermally altered areas at the more likely deposited on dispersal aprons low on the western end of Johnston Ridge possibly includes one or flanks of volcanoes or in intervolcanic lowlands. more exhumed subvolcanic vent zones that may have fed Chemical analyses (table 2) demonstrate that low- overlying lava flows. This area of preferentially east-west- potassium subalkaline basalt (less than 52 weight percent trending dikes appears to be the eastern margin of a large SiO2) and basaltic andesite (between 52 and 57 weight early Oligocene basaltic andesite volcano centered on a percent SiC^) dominate the section (fig. 1). Dacite and plutonic complex at Spud Mountain (Evarts and Ashley, rhyolite (greater than 63 weight percent SiC^), although 1990). Phaneritic intrusive rocks such as those that make abundant among stratigraphically higher rocks to the east up the sill complex on Windy Ridge (Twr) may well (Evarts and Ashley, 1993a), are rare in the Spirit Lake exemplify deeper zones of volcanic plumbing systems, West quadrangle. More than half of the samples are 45 50 55 60 65 70 75 SiO2, IN WEIGHT PERCENT Figure 1. versus SiC>2 (recalculated volatile-free) for volcanic and hypabyssal intrusive rocks from Spirit Lake West quadrangle showing classification according to International Union of Geological Sciences (LeBas and Streckeisen, 1991). Figure 2. AFM diagram for volcanic and hypabyssal intrusive rocks from Spirit Lake West quadrangle (A, Na2O+K2O; F, FeO+Fe2O3+MnO; M, MgO). Line separating tholeiitic and calc-alkaline rocks from Irvine and Baragar (1971). UJ O 01 6 lit U O o> 2 O 0) 45 50 55 60 65 70 75 80 Si02, IN WEIGHT PERCENT Figure 3. FeOYMgO versus SiC>2 (recalculated volatile-free) for volcanic hypabyssal intrusive rocks from Spirit Lake West quadrangle showing classification into tholeiitic and calc-alkaline rocks according to Miyashiro (1974). FeO*, total Fe and FeO. UJ O DC UJ CL h- UJ O 1 45 50 55 60 65 70 75 SiO,, IN WEIGHT PERCENT Figure 4. I^O versus SiC>2 (recalculated volatile-free) for volcanic and hypabyssal intrusive rocks from Spirit Lake West quadrangle [dots}. Low-, medium-, and high-potassium fields from Gill (1981, p. 6). Shaded area encompasses compositions of Quaternary volcanic rocks, exclusive of major stratovolcanoes, of southern Washington Cascade Range from Hammond and Korosec (1938). Trendlines shown for Quaternary stratovolcanoes Mount Ranier, Mount St. Helens, and Mount Adams based on data in Condie and Swenson (1973), Hildreth and Rerstein (1985), and Smith and Leeman (1987). classified as calc-alkaline on the AFM plot (fig. 2} of Irvine folding is otherwise poorly constrained, and deformation and Baragar (1971) whereas most are tholeiitic according may have taken place over an extended period (Evarts and to the FeOYMgO vs. SiO2 plot (fig. 3) of Miyashiro others, 1987). (1974). The Tertiary rocks tend to be lower in K2O than Faults and shear zones are common in the Johnston Quaternary volcanic rocks of equivalent SiO2 contents in Ridge-South Coldwater Creek area but scarce elsewhere. southern Washington (fig. 4). All are subvertical, and apparent offsets typically are A general westward coarsening of textures in the minor, only rarely exceeding 10 m. Generally, rocks on Spirit Lake pluton (informal name) suggests that it has both sides of faults are the same, and the sense of offset been tilted to the east along with its host rocks, so folding cannot be determined, but local slickensides record both of the Tertiary section must postdate 21 Ma, the strike-slip and dip-slip movements. Most faults are flanked crystallization age of the pluton (table 1). The age of by white to orange zones of hydrothermally altered rock, and some faults are occupied or crossed by Tertiary dikes. METAMORPHISM Therefore, many faults are probably relatively old and represent local small-scale adjustments to movements of Tertiary volcanic and intrusive rocks throughout the magma within Tertiary volcanic centers. Although fault southern Washington Cascade Range have been orientations vary considerably, north to north-northeast overprinted by zeolite-facies burial metamorphism (Fiske trends are most common, possibly reflecting