Prepared in cooperation with Washington State Division of Geology and Earth Resources, U.S. National Park Service, and U.S. Forest Service Geologic Map of the North Cascade Range, Washington By Ralph A. Haugerud and Rowland W. Tabor Technical pamphlet to accompany Scientific Investigations Map 2940 Looking south from the North Klawatti Glacier [Mbse]. In the right foreground, the glacier breaks into a heavily crevassed icefall where it descends steeply. Rock in the foreground knob is Eldorado Orthogneiss (unit TKgo), a 90 million-year-old stitching pluton, which here includes numerous dikes of light- colored pegmatite. Mount Buckner on the left skyline and Mount Forbidden hidden in clouds are also eroded from the Eldorado Orthogneiss (photographed in 1987). 2009 U.S. Department of the Interior U.S. Geological Survey CONTENTS Introduction.....................................................................................................................................................1 Using this report ....................................................................................................................................1 Map preparation ...................................................................................................................................1 Major sources of new data .................................................................................................................1 Acknowledgments ................................................................................................................................2 Geology of the North Cascade Range ........................................................................................................2 Major faults ............................................................................................................................................2 Darrington-Devils Mountain Fault Zone ...................................................................................3 Straight Creek Fault .....................................................................................................................4 Ross Lake Fault Zone ..................................................................................................................4 Pasayten Fault ..............................................................................................................................4 Geologic map units ...............................................................................................................................4 Orogenic and pre-orogenic rocks mostly west of the Straight Creek Fault ......................4 Orogenic and pre-orogenic rocks east of the Straight Creek Fault ....................................6 Rocks of late- and post-orogenic transtension ....................................................................10 Flood basalt and associated deposits ....................................................................................11 Rocks of the Cascade magmatic arc ......................................................................................11 Unconsolidated deposits ..........................................................................................................12 Description of map units .............................................................................................................................13 Unconsolidated deposits ...................................................................................................................13 Nonglacial deposits ...................................................................................................................13 Glacial deposits ..........................................................................................................................13 Glacial and nonglacial deposits ..............................................................................................14 Rocks of Cascade magmatic arc .....................................................................................................14 Flood basalt and associated deposits .............................................................................................15 Rocks of late- and post-orogenic transtension .............................................................................15 Extensional deposits .................................................................................................................15 Other rocks..................................................................................................................................16 Orogenic and pre-orogenic rocks mostly west of Straight Creek Fault ...................................16 Rocks southwest of Darrington-Devils Mountain Fault Zone ............................................16 Rocks in Darrington-Devils Mountain Fault Zone ................................................................16 Rocks northeast of Darrington-Devils Mountain Fault Zone ..............................................16 Orogenic and pre-orogenic rocks east of Straight Creek Fault ..................................................18 Rocks unique to Wenatchee block .........................................................................................18 Rocks in Wenatchee and Chelan blocks ...............................................................................18 Rocks unique to Ross Lake Fault Zone ...................................................................................20 Rocks in Methow block ............................................................................................................20 Rocks in Okanogan block .........................................................................................................21 References cited ..........................................................................................................................................22 Figures 1. Map of Northwestern Washington showing area of geologic map ............................sheet 2 2. Map showing sources of geologic data for Twisp 1:100,000 topographic quadrangle and adjacent area ............................................................................................................sheet 2 3. Sketch figure showing plate tectonic processes ...........................................................sheet 2 4. Map showing major geologic structures and major metamorphic episodes in the North Cascades, Washington ........................................................................................sheet 2 5. Generalized geologic map of North Cascades, Washinton ..........................................sheet 2 i 6. Geologic times scales relative to some geologic events in North Cascades, Washington ..................................................................................................................................3 7. Looking south from east ridge of Mount Formidable......................................................sheet 2 8. View northwest from Bacon Peak .....................................................................................sheet 2 9. Digital elevation model showing maximum extent of Cordilleran Ice Sheet in the North Cascades during the Vashon stade ...................................................................sheet 2 10. Sketch showing a north-south cross section of the Cascade Range in Washington ..................................................................................................................................2 11. Sketch of cross section showing how terranes (nappes) became stacked in the Northwest Cascades System along thrust faults during collision between tectonic plates .............................................................................................................................5 12. Granite of Cascades Pass family on the west wall of Nooksack cirque in foreground .........................................................................................................................sheet 2 13. Beds of sandstone (Eocene Swauk Formation) in older extensional deposits on south side of the east ridge of Summit Chief Mountain ............................................sheet 2 14. Disrupted sandstone, greenstone, and light-colored dikes in argillite matrix of western mélange belt unit TKwb, South Fork Stillaguamish River ..........................sheet 2 15. View east from Mount Index across Skykomish River valley to Gunn and Baring peaks, underlain by erosion-resistant tectonic slices of migmatitic gneiss in less-resistant sedimentary rocks of eastern mélange belt .....................sheet 2 16. Large lumps of erosion-resistant rock dotting serpentenite matrix of Helena- Haystack melange (units TKhm, TKhg) .........................................................................sheet 2 17. View of Eldorado Peak, looking northeast from divide between Sibley and Marble Creeks ...............................................................................................................................sheet 2 18. Sketch showing Cretaceous marine setting for accumulation of sediments that became rocks of Methow Ocean and younger deposits .....................................................9 19. View from Mount Winthrop south along Cascades Crest ...........................................sheet 2 20. Hozomeen greenstone in Methow block north of Little Beaver Creek .....................sheet 2 21. Ternary diagrams showing classification of igneous rocks by