Prepared in Cooperation with the Water Resources Program of the Yakama Nation Geologic Map of the Simcoe Mountains Volcanic Field, Main Central Segment, Yakama Nation, Washington By Wes Hildreth and Judy Fierstein Pamphlet to accompany Scientific Investigations Map 3315 Photograph showing Mount Adams andesitic stratovolcano and Signal Peak mafic shield volcano viewed westward from near Mill Creek Guard Station. Low-relief rocky meadows and modest forested ridges marked by scattered cinder cones and shields are common landforms in Simcoe Mountains volcanic field. Mount Adams (elevation: 12,276 ft; 3,742 m) is centered 50 km west and 2.8 km higher than foreground meadow (elevation: 2,950 ft.; 900 m); its eruptions began ~520 ka, its upper cone was built in late Pleistocene, and several eruptions have taken place in the Holocene. Signal Peak (elevation: 5,100 ft; 1,555 m), 20 km west of camera, is one of largest and highest eruptive centers in Simcoe Mountains volcanic field; short-lived shield, built around 3.7 Ma, is seven times older than Mount Adams. 2015 U.S. Department of the Interior U.S. Geological Survey Contents Introductory Overview for Non-Geologists ...............................................................................................1 Introduction.....................................................................................................................................................2 Physiography, Environment, Boundary Surveys, and Access ......................................................6 Previous Geologic Work ......................................................................................................................9 Methods................................................................................................................................................10 Geologic Setting ...........................................................................................................................................10 Tectonic Framework ...........................................................................................................................10 Pre-Pliocene Rocks ............................................................................................................................12 Mount Adams Volcanic Field ............................................................................................................12 Simcoe Mountains Volcanic Field .............................................................................................................13 Central Segment..................................................................................................................................13 Eastern Plateaus ........................................................................................................................14 Simcoe Mountains Highland ....................................................................................................14 Central Swath of High Vent Density ........................................................................................16 Swath of Lesser Vent Density ..................................................................................................16 Western Swath ...........................................................................................................................16 Northern Segment ..............................................................................................................................18 Southern Segment ..............................................................................................................................19 Composition of Eruptive Products ....................................................................................................19 Volcano Hazards .................................................................................................................................24 Age Relations Among Volcanic Fields in Southern Washington .........................................................24 Acknowledgments .......................................................................................................................................25 Introduction to Description of Map Units ................................................................................................25 Description of Map Units ............................................................................................................................27 Surficial Deposits And Basement Rocks ........................................................................................27 Volcanic Rocks ....................................................................................................................................28 References Cited..........................................................................................................................................68 Figures 1. Map showing regional location of intraplate Simcoe Mountains volcanic field in south-central Washington. ....................................................................................................3 2A. Map showing regional setting of map area and Yakama Nation in south-central Washington. .............................................................................................................................4 2B. Regional physiography as digital elevation model, illustrating transition from rugged Cascade Range to Yakima Fold Belt .....................................................................................5 3. Map showing drainage of Simcoe Mountains volcanic field and vicinity, including locations of geographic names mentioned in text and in Description of Map Units. .........................................................................................................................................7 4. Map showing simplified tectonic map of Pacific Northwest, highlighting Simcoe Mountains volcanic field draping southwest part of Yakima Foldbelt, east of Cascade Arc axis. ................................................................................................................11 5. Vent locations for central segment of Simcoe Mountains volcanic field. ..........................15 6. FeO*/MgO versus weight percent SiO2 for ~1,250 samples from central segment of Simcoe Mountains volcanic field. .....................................................................................20 7. K2O versus SiO2, both in weight percent, for ~1,300 samples from central segment of Simcoe Mountains volcanic field. .....................................................................................21 i 8. Total alkalies versus silica (TAS) diagram of LeBas and others (1986), showing conventional chemical pigeon-holing of volcanic rock names for ~1,300 samples from central segment of Simcoe Mountains volcanic field. .........................................22 9. Ba and Zr versus Nb, all in ppm, for ~1,250 samples from central segment of Simcoe Mountains volcanic field. ...................................................................................................23 Table 1. 40Ar/39Ar ages for basalt samples from Simcoe Mountains volcanic field....................................72 Appendixes Appendix A. Chemical data for Simcoe Mountains volcanic field, main central segment...................................................................................................................... web only Appendix B. Whole-rock potassium-argon ages for units within or overlapping Simcoe Mountains volcanic field, main central segment. .............................................76 ii Are they assigned, or can the countries pick their colors? When still molten, rhyolite lava is much stiffer than basalt. – What suits the character or the native waters best. Instead of flowing freely, it creeps along like warm road tar and Topography displays no favorites; North’s as near as West. seldom advances far from its vent. Additionally, as a result of its More delicate than the historians’ are the map-makers’ colors. stiffness, rhyolite can be much more explosive than basalt. Deep Elizabeth Bishop from The Map (1935) below ground, all lava contains dissolved gases (water, sulfur, and carbon dioxide), which try to bubble away as the lava rises buoyantly to the surface from its higher-pressure subsurface Introductory Overview for confinement. If the depressurizing ascent is rapid, the stiffness of rhyolite retards escape of the gas, which instead makes the Non-Geologists lava froth up into an explosive foam of ash and pumice when it emerges into the atmosphere. Rhyolite lavas of the Simcoe Mountainous parts of the Yakama Nation lands in south- Mountains were certainly accompanied by such explosive central Washington are mostly covered by basaltic lava flows eruptions of ash and pumice, but most deposits of such loose and cinder cones that make up the Simcoe Mountains volcanic material have since been eroded away. A few remnants of the field (figs. 1–5). The accompanying geologic map of the ash deposits survive near Tenie Creek and along Highway 97 central part of the volcanic field has been produced by the U.S. north of Satus Pass, and stream-reworked rhyolite pumice is Geological Survey (USGS) on behalf of the Water Resources present in the alluvium downstream along Satus Creek. Program of the Yakama Nation. The volcanic terrain stretches Its lack of fluidity also helps hot rhyolitic lava oozing continuously from Mount Adams eastward as