U.S. DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY Geologic map of the McCoy Peak quadrangle, southern Cascade Range, Washington by Donald A. Swanson1 Open-File Report 92-336 This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. 'U.S. Geological Survey, Department of Geological Sciences AJ-20, University of Washington, Seattle, Washington 98195 CONTENTS INTRODUCTION ........................ 1 5. Locations of samples collected in McCoy Peak ACKNOWLEDGMENTS ................... 1 {quadrangle ......................... 6 ROCK TERMINOLOGY AND CHEMICAL 6. Alkali-lime classification diagram for Tertiary CLASSIFICATION .................... 2 rocks in McCoy Peak quadrangle .......... 7 GENERAL GEOLOGY .................... 6 7. Plot of FeO*/MgO vs. SiO2 for rocks from TERTIARY ROCKS OLDER THAN INTRUSIVE McCoy Peak quadrangle ................ 7 SUITE OF KIDD CREEK ............... 7 8. Plot of total alkalis vs. SiO2 for rocks in McCoy Volcaniclastic rocks ................... 7 Peak quadrangle ..................... 8 Chaotic volcanic breccia ................ 8 9. Plot of K2O vs. SiO2 for rocks from McCoy Vitroclastic dacite and andesite breccia ...... 9 Peak quadrangle ..................... 8 Volcanic centers ...................... 10 10. Distrbution of pyroxene andesite and basaltic Regional dike swarm .................. 11 andesite dikes in French Butte, Greenhorn Age .............................. 13 Buttes, Tower Rock, and McCoy Peak INTRUSIVE SUITE OF KIDD CREEK ......... 13 quadrangles ........................ 12 Dikes ............................. 14 11. Rose diagrams of strikes of dikes and beds in Sills .............................. 16 McCoy Peak and other quadrangles ....... 13 Relation of dikes and sills ............... 16 12. Plcts of TiO2, FeO*, and MnO vs. SiO2 for Hornblende quartz diorite and granodiorite of rocks from McCoy Peak quadrangle ....... 13 McCoy Creek .................... 17 13. Generalized map showing distribution of Geometry of body ................. 17 intrusive suite of Kidd Creek ........... 15 Evidence relating the McCoy Creak 14. Comparison of analyses of Kidd Creek intrusive intrusion and the Kidd Creek suite done in two laboratories ........... 20 intrusive suite ............... 18 15. Comparison of adjusted analyses of Kidd Creek Relation of McCoy Creek intrusion to the intrusive suite with those of McCoy Creek radial dike and sill swarm ...... 18 intrusion .......................... 20 Age of intrusive suite of Kidd Creek ....... 21 16. Plot of glass vs. total olivine contents for high- STRUCTURE ........................... 21 K2O olivine basalt in McCoy Peak and ad­ Folds ............................. 21 jacent quadrangles ................... 24 Faults ............................. 22 17. Plot of Na6-0 and Ca&0 vs. crustal thickness Jumbo Creek zone ................. 23 (Plank and Langmuir, 1988) to compare QUATERNARY BASALT AND BASALTIC thickness of Tertiary crust with that of ANDESITE ......................... 23 other arcs ......................... 25 THICKNESS OF OLIGOCENE AND MIOCENE CRUST ............................ 25 TABLES DESCRIPTION OF MAP UNITS ............. 27 REFERENCES CITED ..................... 33 1. Chemical analyses of rocks from McCoy Peak CORRELATION OF MAP UNITS ............ 35 quadrangle ......................... 4 EXPLANATION OF MAP SYMBOLS ......... 36 2. Chemical analyses from Link (1985) ........ 19 3. Radiometric ages from the McCoy Peak FIGURES quadrangle ........................ 22 4. Modes of young olivine basalt in McCoy Peak 1. Map showing location of McCoy Peak quadrangle and adjacent quadrangles .............. 24 relative to other quadrangles and the Southern Washington Cascades Conductor .......... 2 2. Map of McCoy Peak quadrangle, showing PLATES localities mentioned in text .............. 3 3. Total alkali-silica classification diagram for rocks 1. Ge logic map of McCoy Peak in McCoy Peak quadrangle .............. 3 quadrangle ................ Separate sheet 4. Plot of phenocryst assemblage vs. SiO2 for rocks 2. Cress sections of McCoy Peak in McCoy Peak quadrangle .............. 6 quadrangle ................ Separate sheet GEOLOGIC MAP OF THE McCOY PEAK QUADRANGLE, SOUTHERN CASCADE RANGE, WASHINGTON By Donald A. Swanson INTRODUCTION scribed in this paper occurs in the northern part of the quadrangle. Very low-grade (zeolite-facies) metamorphism The McCoy Peak 7.5-minute quadrangle is centered pervades most of the Tertiary rocks and gives a greenish about 35 km northeast of Mount St. Helens and 25 km cast to many of the once-glassy volcaniclastic rocks. west of the crest of the Cascade Range in southern Wash* Propylitic alteration, local phyllic alteration, and sulfide ington (fig. 1). Geologic maps have recently been published mineralization affect two large areas and several smaller for the adjoining Tower Rock, French Butte, and Green* ones in the quadrangle. Quaternary flows and pyroclastic horn Buttes quadrangles (Swanson, 1989, 1991) and fin­ deposits, and Quaternary or Pliocene dikes, of olivine ished but not yet published for the Blue Lake quadrangle basalt and basaltic andesite occur in several places. Uncon- (D.A. Swanson, unpublished map, 1991). solidated deposits related to at least one episode of late The mapping and related geologic research are part of Pleistocene glaciation cover parts of the area. The quadran­ a project to tie the stratigraphy of the area near Mount St. gle is downwind from Mount St. Helens, and tephra from Helens (Evarts and Ashley, 1984, 1990a, 1990b, 1991, in eruptions of the past 50,000 years mantles most units press a-d; Evarts and others, 1987; Swanson, 1989, 1991) (Mullineaux, 1986), particularly in the southwestern part of into the now classic Tertiary stratigraphic section in the the area. Mount Rainier-White Pass area as defined by Fiske and Bedrock exposures are limited in many places, owing to others (1963) and modified by Vance and others (1987). dense forest cover and locally thick glacial drift, but overall This work will establish an improved regional geologic the number of exposures is much greater than farther west, framework for the southern Washington Cascades and because of less tephra cover. Many natural exposures occur complete a geologic transect across the west side of the along creeks and hillsides, particularly along the unfbrested Cascade Range (Swanson and Evarts, 1992). It will also upper parts of Langille and Juniper Ridges, and combined support a prospective geophysical transect linking coastal with road cuts permit adequate determination of most strati- Washington to the Columbia Plateau. In addition, the re­ graphic relations. search will test whether a major electrical conductivity Only small-scale (1:100,000 and smaller) reconnaissance layer in the upper crust, the Southern Washington Cascades geologic work had been done in the quadrangle before this Conductor (SWCC) of Stanley and others (1987) has a rec­ research, except for two thesis studies of the McCoy Creek ognizable influence on the geology of the area. The mining district (Simon, 1972; Link, 1985). Chief among the quadrangles being mapped lie within and astride the edge reconnaissance mapping was that by Hammond (1980), of the SWCC. Korosec (1987; included in Walsh and others, 1987), and The McCoy Peak quadrangle straddles the major east- J.G. Smith (unpublished map of the Yakima 2° sheet). The west drainage divide that separates the west-flowing Cispus new geologic map agrees in general with these earlier and Lewis Rivers (fig. 2). Most of the quadrangle is north studies but differs considerably in detail, particularly in the of the divide and is drained by the Cispus River, chiefly McCoy Creek area. via two major tributaries, Yellowjacket and McCoy Creeks. South of the divide drainage into the Lewis River is via Straight, French, and Quartz Creeks. Langille Ridge, ACKNOWLEDGMENTS Juniper Ridge, and most of the Cispus-Lewis divide are part of the Dark Divide Roadless Area in Gifford Pinchot I thank Chuyler Freeman (National Association of Geol­ National Forest and are accessible only by trail. ogy Teachers fellow), Gary Stoopes, and Barbara White The quadrangle is underlain chiefly by late Oligocene (my wife) for help in the field, and Jeff Marso for many and early Miocene volcanic deposits, both lava flows and discussions about the hornblende-bearing intrusive rocks. volcaniclastic rocks, that range from basaltic andesite to Barry Goldstein (University of Puget Sound) provided rhyolite but consist dominantly of basaltic andesite and advice on the glacial history of the area. Continued discus­ andesite. Numerous dikes and sills, chiefly of andesite and sions with Russ Evarts, Roger Ashley, and Jim Smith have low-SiO2 dacite, cut the older rocks, and a major sub- taught me a lot about Cascade geology and on occasion volcanic center with a radial dike and sill swarm first de­ have raised challenging alternative interpretations, and Russ 46°30' - 46 Figure 1. Index map showing location of McCoy Peak quadrangle relative to the three Holocene and late Pleistocene composite volca- noes in southern Washington; crest of Cascade Range; Pleistocene-Pliocene volcano at Goat Rocks; Southern Washington Cascades Conductor (SWCC; see text); and other 7-1/2' quadrangles where geologic mapping has been