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Geologic Map of the Mount Baker 30- by 60-Minute Quadrangle, Washington by R.W

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Geologic Map of the Mount Baker 30- by 60-Minute Quadrangle, Washington by R.W Geologic Map of the Mount Baker 30- by 60-Minute Quadrangle, Washington By R.W. Tabor, R.A. Haugerud, Wes Hildreth, and E.H. Brown Prepared in cooperation with the Washington State Department of Natural Resources, Division of Geology and Earth Resources Pamphlet to accompany Geologic Investigations Series I–2660 2003 U.S. Department of the Interior U.S. Geological Survey Geologic Map of the Mount Baker 30- by 60-Minute Quadrangle, Washington By R.W. Tabor1, R.A. Haugerud2, Wes Hildreth1, and E.H. Brown3 INTRODUCTION area: lithologic, time-stratigraphic, structural, and stratigraphic-structural (terranes). The plethora of terms is The Mount Baker 30- by 60-minute quadrangle encom- confusing to everyone, but the complexity of the geology passes rocks and structures that represent the essence of the allows little simplification. Much of the crust in northwest geology of the North Cascade Range (fig. 1, sheet 2; fig. 2, sheet Washington appears to be built of accreted terranes, hence 1). The quadrangle is mostly rugged and remote and includes many units have been given terrane names in the past (see much of the North Cascade National Park and several dedi- Tabor and others, 1987a,b, 1989; Brandon, 1989). On this cated wilderness areas managed by the U.S. Forest Service. map, even though most pre-Tertiary units are terranes or Geologic exploration has been slow and difficult. In 1858 probable terranes, where possible, we have used established George Gibbs (1874) ascended the Skagit River part way to lithologic or time-stratigraphic names. In discussion, we begin the geographic and geologic exploration of the North commonly move up the nomenclatural ladder to a more com- Cascades. In 1901, Reginald Daly (1912) surveyed the 49th prehensive terrane name. Within the main text describing parallel along the Canadian side of the border, and George each unit or group of units, we discuss the use of the appro- Smith and Frank Calkins (1904) surveyed the United States’ priate names. Names used in the General Geology overview side. Daly’s exhaustive report was the first attempt to synthe- derive from the more lengthy discussion. size what has become an extremely complicated geologic story. Modern geologic work began almost a half a century later when, in 1948, Peter Misch began his intensive study ACKNOWLEDGMENTS of the North Cascade Range (Misch, 1952, 1966, and see other references). His insights set the stage for all later work Many people have helped with field work in the diffi- in the North Cascades. Considerable progress in understand- cult terrane of the Mount Baker quadrangle: Michael Ort ing the North Cascades in light of modern plate tectonic theory (1984–1985), Patrick Goldstrand (1985), Carolyn has been made by E.H. Brown and his students. We have used Ortenburger (1985–1986), Janet Slate and Robert Fillmore much of their detailed geologic mapping (fig. 3, sheet 2). (1986), Kathleen Duggan (1987–1988), Scott Spees (1987), Although our tectonic reference frame has changed much Eric Roth (1988), Kris Alvarez (1990–1991), Tom Grundy with the recognition of plate tectonics and exotic terranes, (1990), Carmello Ferlito, David Maher, and Jim Misch’s observations prove to be remarkably accurate. Montgomery (1991), Chad Nelsen, Cathryn Dwyre, and Rob Our work in the Mount Baker quadrangle began in 1983 Osborn (1992). Hildreth began investigating the Quaternary as part of a project to map and compile the geology of the volcanic rocks in 1992; for field assistance he is grateful to Wenatchee and Concrete 1° by 2° quadrangles at 1:100,000 Kari Cooper, Mike Dean, Judy Fierstein, Ellen Lougee, Dave scale (fig. 1), work that we began in 1975. We have mapped in Tucker, and Patricia Weston. Many employees of North cooperation with the Division of Geology and Earth Resources, Cascades National Park and the Mount Baker Ranger District Washington State Department of Natural Resources. We have of the U.S. Forest Service have been helpful, in particular also benefited by the cooperation and helpfulness of the Craig Holmquist, Kevin Kennedy, Jerry Lee, and Bill Lester; National Park Service and the U.S. Forest Service. Jon Reidel has been particularly helpful. We are thankful that superb and ultimately cooperative helicopter pilots and their crews exist; thank you Tony and Sue Reese for our THE PROBLEM OF NAMES success and our safety. Previous workers, including the present authors, have We have discussed the geology of the map area with applied four kinds of names to geologic units in the map many. In particular we have enjoyed extended and some- times heated discussions with Derick Booth, Mark Brandon, Darrel Cowan, Joe Dragovich, Bob Miller, the late Peter 1 U.S.Geological Survey, Menlo Park, California 94025 Misch, Jim Monger, John Reidel, John Stacy, Jeff Tepper, 2U.S.Geological Survey, University of Washington, Seattle, Wash- ington 98195 and Joe Vance. Darrel Cowan, Derick Booth, and Bob Miller 3Department of Geology, Western Washington University, Bellingham, made many helpful suggestions for improvement of map Washington 98225 and text. 1 R.W. Tabor produced this digital map with GIS tech- suggested that the Ross Lake Fault Zone is a minor dis- nology using Alacarte (Wentworth and Fitzgibbon, 1991). location in an essentially uninterrupted cross section of a Many computer and (or) GIS experts helped, especially Mesozoic arc ranging from the deep roots in the North Tracey Felger, Todd Fitzgibbon, Patricia Helton, Eric Cascade core to the unmetamorphosed marine and terrestrial Lehmer, Bob Mark, Chad Nelson, Geoff Phelps, and Pahdy fore-arc deposits of the Methow terrane to the east. McCarthy. Many thanks to Carl Wentworth, who, no matter how busy, always answered questions about Alacarte. PRE-MID-CRETACEOUS ROCKS GENERAL GEOLOGY Rocks west of the Straight Creek Fault: the Northwest Cascades System Rocks in the Mount Baker quadrangle represent almost West of the Straight Creek Fault, the North Cascades all the geologic events recorded in the entire North Cascades: appear to be composed of two fundamental regional structural (1) pre-mid-Cretaceous assembly of Mesozoic and Paleozoic blocks separated by a complex tectonic belt and high-angle terranes that have different paleogeographic origins and faults (fig. 1, sheet 2; fig. 4, sheet 1; see also Tabor and others, structural and metamorphic histories (Tabor and others, 1989; Tabor, 1994; Tabor and Haugerud, 1999). The north- 1989; Tabor 1994), (2) mid-Cretaceous to Late Cretaceous eastern structural block, exposed primarily in the Mount Baker thickening by thrusting and pluton accumulation (Misch, quadrangle, is mostly composed of Paleozoic and Mesozoic 1966; McGroder, 1991; Brown and Walker, 1993; Haugerud volcanic arc and associated clastic wedge deposits along with and others, 1994), accompanied and followed by regional more thoroughly metamorphosed oceanic rocks, thrust in the metamorphism, (3) Eocene strike-slip faulting, extensional mid-Cretaceous into a series of nappes. The overall structure faulting, basin development, and continued metamorphism has been likened to a regional mélange by Brown (1987) and plutonism (Johnson, 1984, 1985; Brown, 1987; Miller who, modifying the earlier terminology of Misch (1966, p. and Bowring, 1990; Haugerud and others, 1991; Miller, 128), called rocks of this structural block the Northwest 1994), (4) growth of the Cascade magmatic arc in Oligocene Cascade System. As explained below, we now think that the to Holocene time (Vance and others, 1986, 1987; Smith, structure displays more order than regional mélange implies. 1993; Tabor and others, 1989), and (5) Quaternary glacial The southwestern block, exposed just south of the Mount erosion, drainage derangement, and deposition of glacial- Baker quadrangle, is mostly Mesozoic clastic rocks of derived sediments (Booth, 1987, 1990). submarine-fan origin and relatively unmetamorphosed We summarize this geology here. More detailed dis- oceanic rocks. Tabor and others (1989, 1993, 2002), Frizzell cussions of bedrock lithologies, young volcanic rocks of and others (1987), and Tabor (1994) described this block as Kulshan Caldera and Mount Baker volcanic center, uncon- the western and eastern mélange belts. solidated deposits and Quaternary history, complete with Four major nappes, stacked along folded thrusts, and more detailed references, follow. their probably autochthonous footwall make up the North- The Straight Creek Fault and the Ross Lake Fault Zone west Cascades System (fig. 4, sheet 1; fig. 5, sheet 2). The divide the rocks of the quadrangle into a core of deep-seated, structural stratigraphy of the Northwest Cascades System thoroughly metamorphosed rocks, flanked by less metamor- appears to be consistent over a wide area of northwest phosed rocks on either side (fig. 1, sheet 2; fig. 2, sheet 1). Washington. The rocks in the three lowermost nappes and These major faults are thought to be predominantly strike- the autochthon differ enough in lithology, structure, and slip (Misch, 1977a; Vance and Miller, 1981, 1992; Miller, metamorphic history to warrant consideration as separate 1994), though the rocks of the metamorphic core have been terranes (Tabor and others, 1989; Haugerud and others, uplifted 15 to 25 km relative to rocks on either side. The 1994), but the highest and youngest Gold Run Pass Nappe Straight Creek Fault, although now predominantly obliter- consists of slices of the lower nappes and autochthon. ated by Tertiary arc plutons, almost bisects the quadrangle. The stacked nappes have been
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