Scientific Investigations Map 2940 U.S. Department of the Interior Prepared in cooperation with Washington State Division of Geology and Earth Resources, U.S. National Park Service, and U.S. Forest Service Sheet 2 of 2 U.S. Geological Survey Pamphlets accompany map 123° 122° 121° 120° 121° 120° basalt lavas VOLCANIC 122° 121° 120° 49° 48°30’ erupt to form ARC 122° 121° 120° Mount Spickard oceanic plate BRITISH COLUMBIA MID-OCEAN 49° NORTHWEST OKANOGAN Ross Lake Fault Zone (System) Dome Ptarmigan River RIDGE Old Guard Glacier Mount Vedder Straight Peak Spine WESTERN Peak Peak Pk 49° Mountatn CANADA 5 BLOCK Point Baker Jack Mtn ocean floor basalt sedimentary Mt Baker ROBINSON MOUNTAIN UNITED METHOW Pasayten MOUNT BAKER [MB] rocks early Tertiary and Late Le Conte Glacier Twin 2000 [RM] STATES 10 Cretaceous metamorphism Sisters Jack METHOW BLOCK Mount 1600 San Juan BELLINGHAM Baker Mtn Robinson Mountain 4 CONCRETE 2000 Islands OCEANIC OCEANIC SU CHELAN BLOCK Methow CRUST rigid mantle B CRUST CASCADE 48°30’ Gardner Mtn LITHOSPHERE Creek DU Skagit C Fault Sedro 9 T rigid mantle DOMAIN Concrete CONTINENTAL TWISP plastic mantle plastic mantle IO Wolley Winthrop N early Late Cretaceous LITHOSPHERE 6 melting zone CONTINENTAL Sauk older STEHEKIN Washington (source of igneous rocks) Z melting zone metamorphism sedimentary O (more igneous rocks) Entiat DARRINGTON 1600 Pass Twisp MANTLE N OKANOGAN Mount rocks E 1200 P sediments SYSTEM U Misch LAKE LOBE G River Stehekin metamorphic Glacier pillow basalt E Lake rocks forming D T River a Peak Stillaguamish rr Darrington River dikes in METAMORPHIC g Vinegar Mt CHELAN S Glacier Chelan to Fault Fault O Bonanza n early Late Cretaceous 1200 U Peak gabbro Fault Pilchuck Peak -D 48° GLACIER N ultramafic rock e and Early Creataceous D SAUK RIVER [SR] TWISP [T] 2 3 v WENATCHEE BLOCK 48° i metamorphism Ophiolite layering ls SUBDUCTED OCEAN PLATE M Monroe 8 DOMAIN (OCEANIC LITHOSPHERE) t CORE MONROE Skykomish n OLYMPIC River F Chelan Figure 3. Sketch showing plate tectonic processes. The surface of the Earth is covered by many interlocking plates (lithosphere or crust). a MOUNTAINS 1 u l PUGET New crust forms at the mid-ocean ridge where oceanic plates move apart, allowing molten rock (magma) to reach the surface and erupt as t Z Figure 7. Looking south from east ridge of Mount Formidable [SRne]. Uplift and Skykomish Figure 8. View northwest from Bacon Peak [MBsw]. Rocks of Easton Terrane (Shuksan LOBE basalt lavas. An oceanic plate descends below a continental plate at a subduction zone where plates converge; rocks of both plates melt at o DOMAIN Early Cretaceous LEAVENWORTH n Leavenworth River 48° MELANGE BELTS erosion, especially glacial erosion, exposed rocks that formed deep in the Earth's crust. Greenschist) form Anderson Butte [MBsw]. A huge block of ultramafic rock (dunite) in e blueschist metamorphism depth to produce magma that rises toward the surface. Much of the magma collects in large masses (magma chambers) in the continental 800 ( Rocks of the Chelan Mountains terrane, including Cascade River Schist, form foreground SKYKOMISH RIVER [SK] CHELAN [C] D of Easton terrane Bell Pass Mélange underlies Twin Sisters [MBsw]. Mount Baker volcano dominates crust. Some magma reaches the surface to build a line of volcanoes (volcanic arc). Eventually the magma in the chambers cools and D cliffs. Underlying metamorphosed Marblemount pluton in Old Guard Peak [SRne] skyline. 47°30’ Snoqualmie Mount M SEATTLE Pass 7 crystallizes to become plutonic intrusive igneous rock. Sedimentary rocks on the ocean floor and some of the oceanic crust scrapes off at Leavenworth Stuart F extends along jagged ridge into middle foreground. The Cloudy Pass batholith, a Miocene Rattlesnake WENATCHEE Z Wenatchee depth to become metamorphic rocks. Blow-up shows idealized layering of oceanic lithosphere (ophiolite). ) Mtn White Chilwaukum Graben Cascade Arc pluton, supports Dome Peak [SRne] and Spire Point [SRne] in middle Yakima Enumclaw background. Glacier Peak volcano in the Cascade Magmatic Arc erupted on top of these River Figure 2. Map showing sources of geologic data for the Twisp 1:100,000 topographic Early Cretaceous and eroded older rocks. quadrangle and adjacent area. Line pattern indicates surficial geology modified from local Jurassic SNOQUALMIE PASS [SP] WENATCHEE [W] metamorphism 47° original sources using aerial photographs. 1. Barksdale, 1975; 2. Cater and Crowder, Fault Mount AREA OF MAP 1967; 3. Cater and Wright, 1967; 4. Dragovich and Norman, 1995; 5. Dragovich and (projected) River Ingalls Complex 47° Rainier others, 1997; 6. Haugerud, R.A., Mahoney, J.B., and Tabor, R.W., unpub. U.S. Geological Survey field maps (1990-2003); 7. Hopson and Mattinson, 1994 and C.A. Extensional Basins Contact—Dotted where Figure 9. Digital relief map showing maximum extent of Cordilleran 0 20 40 MILES uncertain Figure 1 Hopson, written commun., 2005; 8. Libby, 1964; 9. Miller, 1987; and 10. Tabor, 1961. Figure 4. Map showing major geologic structures and major Ice Sheet (white with blue contours; interval 200 m) in the North 0 40 80 KILOMETERS Columbia WASHINGTON metamorphic episodes in the North Cascades, Washington. NOTE: See Nontechnical Pamphlet for figures that are not on this map sheet Cascades during the Vashon stade; probable alpine glaciers on high peaks and beyond the margins of the main ice sheet are not shown Areas of multiple regional metamorphism are sketched from Fault—Dashed where field and laboratory data and descriptions in the literature. DDMFZ uncertain; dotted where (Waitt, 1972; Booth, 1990; Jon Riedel, written commun., 2005). Mount Figure 1. Map of northwestern Washington showing area of geologic map. Eight 1:100,000 topographic Boundaries are approximate. Domains shown are described Columbia Rver Basalt Group projected under cover Baker, Rattlesnake Mountain [SPnw], Jack Mountain [RMnw], and quadrangles that form geologic map are outlined and labeled. Quadrangle abbreviations and compass in Tabor and Haugerud (1999). or intruded by younger rocks many lesser peaks extended above the ice-sheet surface as nunataks. quadrants are used in the text as location codes to find places on the map, for example [MBse] is the 47° southeastern part of the Mount Baker quadrangle. Abbreviations for all quadrangles are shown here and along the edge of the geologic map. LIST OF MAP UNITS OROGENIC AND PRE-OROGENIC ROCKS EAST OF STRAIGHT CREEK FAULT [See technical or nontechnical pamphlets for unit descriptions. Note colors on the map vary as the underlying shaded- ROCKS UNIQUE TO WENATCHEE BLOCK relief base varies. Unit age in parentheses after the unit name is the age of assemblage or metamorphism for mélange and Ingalls terrane 122° 120° metamorphic units. Location codes, such as "[MBnw]," following a place name, unit name, or geologic feature name 121° Jis Ingalls terrane (Jurassic) 49° EXCELSIOR RIDGE FIGURE SYMBOLS FIGURE CORRELATION indicates a location in the northwest quadrant of the Mount Baker quadrangle. The eight 1:100,000-scale quadrangles are THRUST [Metamorphic and structural units shown with metamorphic or assemblage age; uncolored boxes outlined on the map, and quadrangle names and their abbreviations are labeled along the edges of the map, as well as on with dashed boundaries show protolith age. Asterisk indicates that box position represents protolith Jbi HC figures 1 and 5. The location code may consist of a quadrangle abbreviation or a combination of quadrangle and quadrant Resistant blocks of igneous and meta-igneous rocks Contact—Dotted where concealed and metamorphic age (see technical discussion)] abbreviations. A location code at the end of the rock descripton gives the unit location on the map] G Jbs SHUKSAN Resistant blocks of sedimentary rocks PAYSAYTEN CORRELATION OF MAP UNITS KC High-angle fault—Dotted where concealed UNCONSOLIDATED DEPOSITS Nason Terrane MOUNT HOZOMEEN X [For mélange and metamorphic units, position of boxes with solid boundaries indicates age of assemblage Pliocene, THRUST Unconsolidated deposits Pleistocene Knmg BAKER and Holocene Nason Ridge Migmatitic Gneiss (Late Cretaceous) or metamorphism, respectively; position of boxes with dashed boundaries indicates age of protolith.] MPS (Includes Quaternary NONGLACIAL DEPOSITS ROSS Thrust fault—Dotted where concealed volcanoes and Miocene and Qa Alluvium of valley bottoms (Holocene and Pleistocene) THRUST STRAIGHT (or) Pliocene gravel and breccia Kncs FAULT Chiwaukum Schist (Late Cretaceous) ROCKS OF Mount Baker (MB) at X and locally elsewhere) LAKE Robinson Mountain (RM) UNCONSOLIDATED DEPOSITS CASCADE MAGMATIC ARC Low-angle normal fault—Dotted where concealed Qu Alluvium (Holocene and Pleistocene) ROCKS IN WENATCHEE AND CHELAN BLOCKS Ma and Pliocene WELKER GHB Flood basalt Qt Talus deposits (Holocene and Pleistocene) Terrane overlap units and stitching plutons C Oligocene, Miocene, 1 Approximate age GLACIAL AND PK THRUST FAULT Antiformal fold axis and plunge—Dotted where TKsg Skagit Gneiss Complex (middle Eocene to Late Cretaceous) (millions of years) GLACIAL DEPOSITS NONGLACIAL DEPOSITS Geologic Age2 48°30’ concealed NONGLACIAL DEPOSITS ENTIAT Wi QTl Landslide deposits (Holocene, Pleistocene, and Pliocene?) CREEK Volcanic rocks Plutonic rocks QUATERNARY DARRINGTON AND TERTIARY TKso BP of the Cascade of theCascade Orthogneiss ZONE Magmatic Arc Magmatic Arc Qlh Lahars (Holocene and Pleistocene) Holocene Synformal fold axis and plunge—Dotted where 0.01 Qt Qlh T concealed TKsn Orthogneiss of The Needle Qa ROCKS OF LATE-