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by R.W. Tabor, V.A. Frizzell, Jr., D.B. Booth, and R.B. Waitt
The Cascade Range of Washington State, a western rampart of the North American Cordillera, comprises an older basement of accreted terranes and a cover of sedimentary and volcanic rocks. The Snoqualmie Pass quadrangle (fig. 1) lies at the north edge of the volcanic cover, where the regional structural uplift to the north elevated the older rocks to erosional levels. Most of the quadrangle is underlain by Tertiary rocks, but melanges of Paleozoic and Mesozoic rocks are exposed in structural highs in the northern part of the quadrangle.
North of the quadrangle, the north- trending Straight Creek fault cuts the mosaic of accreted terranes and mostly separates higher grade metamorphic rocks of the North Cascade crystalline core to the east from lower grade and unmetamorphosed rocks to the west. Rocks west of the fault are further partitioned by a Late Cretaceous and (or) Paleogene suture (the Helena-Haystack melange) and coincident Paleogene, high-angle Darrington-Devils Mountain Fault Zone. Rocks north and east of the suture are in the Northwest Cascade System of Misch (1966), as modified by Tabor and others (1989). Rocks in the suture zone itself comprise the Helena-Haystack melange of Tabor and others (1988, 1989; also Tabor, 1994), and rocks southwest of the suture form the western and eastern melange belts (Frizzell and others, 1987, Tabor and others, 1993). These latter named three fundamental western terranes are represented sparingly in the Snoqualmie Pass quadrangle.
The Straight Creek Fault, and its probable extensions in Canada and Alaska (Price and others, 1985, p. 3-48, 3-49), is a major strike-slip fault with post-mid-Cretaceous to pre-middle Eocene(?) right-lateral displacement variously estimated between 180 and 90 km (Misch, 1977; Frizzell, 1979; Vance and Miller, 1981). The zone of faulting, representing the probable southward extension of the Straight Creek Fault exposed in the Snoqualmie Pass quadrangle, has possibly also been the locus of strike-slip faulting in the Darrington-Devils Mountain Fault Zone which intersects the Straight Creek Fault at an acute angle (fig. 1). The Darrington-Devils Mountain Fault Zone, mostly coincident with the Helena-Haystack melange, is recognized in the Manastash Ridge area in the southwest corner of the quadrangle. Tabor (1987) and Tabor and others (1989, p. 13) discuss the possibility that the Darrington-Devils Mountain Fault Zone has been the most recently active structure, locally following the Straight Creek trend, but in fact truncating the earlier structure. We will continue to refer to the north-south zone of faulting as the Straight Creek Fault Zone (leaving out the and(or) Darrington-Devils Mountain Fault Zone).
Near the southeast corner of the quadrangle, the Straight Creek Fault Zone intersects the Olympic-Wallowa Lineament of Raisz (1945; see also Kienle and others, 1977; Tabor and Frizzell, 1979) in a complex of curving faults and folds. The Olympic-Wallowa lineament, as originally defined, is expressed physiographically from the Wallowa Mountains in Oregon to the Strait of Juan de Fuca between the Olympic Peninsula and Vancouver Island (see for example, Pike and Thelin, 1989). The lineament traverses the Snoqualmie Pass quadrangle almost diagonally from the southeast to northwest (fig. 1) and is expressed by a broad zone of faults and folds in rocks ranging in age from Jurassic to Miocene. The lineament is most strongly expressed in the faulted and folded rocks of the Manastash River Block (Generalized Geologic Map; Tabor and others, 1984, fig. 2) where the lineament meets the Straight Creek Fault Zone.
Faults in the Straight Creek Fault zone appear to swing southeastward into the lineament trend (Tabor and Frizzell, 1979). The lineament loses definition where it crosses the Cascade Crest just west of the the Straight Creek Fault, but it appears to be expressed in broad northwest-trending folds and faults in Miocene volcanic rocks between the South Fork of the Snoqualmie and White Rivers. The nature of the deep-seated structure or structures producing the lineament is uncertain. It may contain elements of structures of different ages, including the Darrington-Devils Mountain Fault Zone.
[Note added in press. Recent mapping and stratigraphic revisions by Eric Cheney (Cheney, 1999; see also Cheney, 2000) in the vicinity of Easton suggest that the Straight Creek Fault continues more or less straight south from the northern part of Lake Kachess. The interested reader may want to evaluate this new work.]
Pre-arc rocks of the North Cascades to the north of the Snoqualmie Pass quadrangle are offset right laterally across the Straight Creek Fault, and in the quadrangle terranes of the NORTHWEST CASCADE SYSTEM crop out east of the fault.
During the early to early middle Eocene, when the Straight Creek Fault and the Darrington-Devils Mountain Fault Zone were most active, regional extensional and transtensional faulting dominated the Pacific Northwest (Tabor and others, 1984; Johnson, 1985; Heller and others, 1987; Tabor, 1994). The faulting promoted the formation of local basins wherein fluvial feldspathic sediments and subordinate volcanic materials accumulated. By the middle Oligocene, movement on the major structures had waned and the north-trending Cascade volcanic arc was well established; it flooded the Cascade Range with calk-alkaline volcanic rocks, the-principle rocks exposed in the Snoqualmie Pass quadrangle.
HISTORY OF GEOLOGIC MAPPING AND ACKNOWLEDGMENTS
Geologic work in the Snoqualmie Pass 1:100,000 quadrangle began before the turn of the century with investigations of coal found along the Green River (Willis, 1886, p. 759-760). The basic geologic framework east of the Cascade Crest was established by Smith and Calkins (1906) in the Snoqualmie Pass 30-minute quadrangle, and subsequently many workers have drawn on their ideas. Fuller (1925) was one of the first to address the complexities of what we call the western melange belt and to develop the idea that some of the bodies of magma that ultimately formed the Tertiary batholiths vented to form the Tertiary volcanic cover rocks, a theme since expanded upon (Cater, 1960, 1969; Fiske and others, 1963; Tabor and Crowder, 1969). Hammond (1963) was the first to attack the difficult problems of regional correlation presented by post-Eocene volcanic rocks, and he has continued his regional studies in subsequent years (Hammond, 1977, 1980). Detailed mapping in the western foothills was begun on modern base maps by Vine (1962) and Gower and Wanek (1963).
Our work, begun in 1975, was part of a larger project that includes mapping the Wenatchee and Concrete 2 degree quadrangles at 1:100,000 scale. In the Snoqualmie Pass Quadrangle, Tabor, Frizzell, and Booth, with the ongoing assistance of Kathleen Ort, compiled and mapped most of the bedrock geology. Booth mapped the unconsolidated deposits in the northwestern third of the map area and Waitt mapped unconsolidated deposits elsewhere in the quadrangle. Our field work was considerably aided by the efforts of Sharon Allshouse and Eduardo Rodriquez in 1975, Jay Coburn and Ron Tal in 1976, Bill Gaum, Margaret Goddard, and Kim Marcus in 1977, Elizabeth L. Matthinson and Nora Shew in 1978, Steve Connelly, Stephen A. Sandburg, Fred Beall, Frederika C. Moser, and Susan Cook in 1981, and by Carol Eddy, Kathleen Ort, and Tad Schirmer in 1982.
M. Jean Hetherington, Steve Connelly, Bill Gaum, Mark Gordon, and Kathleen Ort also helped with lab and office work; Catherine R. McMasters provided drafting support. Dennis H. Sorg and Betty Hamachi made radiometric dating experiments feasible by supplying clean mineral separates. The late Jack Johnson, helicopter pilot extraordinary, flew us to the rocks in 1979 and 1981.
We benefited from conversations with many colleagues including Newell Campbell, Larry Chitwood, Eric Erickson, Sheri Goetsch, Howard Gower, Daryll Guzzi, Paul Hammond, Ralph Haugerud, Bob Miller, Bill Phillips, Don Turner, Joe Vance, Tim Walsh, Ray Wells, John Whetten, and Jim Yount. Donald A. Swanson provided unpublished data on the Columbia River Basalt Group. Jim Mattinson, Joe Vance, and Bob Zartman graciously discussed and shared isotopic data with us.
U.S. Department of the Interior
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