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Late Mesozoic Or Early Tertiary Melanges in the Western Cascades of Washington

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Late Mesozoic Or Early Tertiary Melanges in the Western Cascades of Washington Washington Division of Geology and Earth Resources Bulletin 77, 1987 129 LATE MESOZOIC OR EARLY TERTIARY MELANGES IN THE WESTERN CASCADES OF WASHINGTON Virgil A. Frizzell, Jr. U.S. Geological Survey 2255 North Gemini Flagstaff, Arizona 86001 Rowland W. Tabor Robert E. Zartman Charles D. Blome U.S. Geological Survey U.S. Geological Survey U.S. Geological Survey 345 Middlefield Road Denver Federal Center Denver Federal Center Menlo Park, California 94025 Denver, Colorado 80225 Denver, Colorado 80225 ABSTRACT Marine rocks west of the Straight Creek fault, from Snoqualmie Pass to the Pilchuck River, are highly deformed and varied in lithology. These rocks, referred to various formations or stratigraphic units by earlier workers, should be considered two lithologically distinct melange belts. Immediately west of the Straight Creek fault from the Weden Creek area to Snoqualmie Pass, the discontinuous melange of the eastern belt consists of strongly hornfelsed rocks including chert, metabasalt, graywacke, argiUite, marble, and ultramafic rocks. Marble near Skykomish yields Permian fusu1inids with Tethyan affinities. Zircons from tonalitic gneiss northwest of Skykomish are 190 million years old, and metagabbro near Snoqualmie Pass is 165 million years old. In the western foothills, the melange of the western belt consists of pervasively sheared argillite and graywacke studded with outcrop- to mountain-scale steep-sided phacoids of chert, limestone, pillow basalt, gabbro, and tonalite; ultramafic rocks are rare. Strikes of bedding and cleavage and trends of small fold axes are mostly north-northwest to north-northeast. Rare macrofossils in foliated sedimentary rocks and radiolarians in chert phacoids are Late Jurassic to Early Cretaceous in age. Tonalite associated with gabbro and diabase yields zircon U-Th-Pb ages of 150 to 170 million years in the Woods Creek area, at Mount Si, at Winter Lake, and on the North Fork of the Snoqualmie River. The general Late Jurassic age of both igneous and marine sedimentary components of this western belt suggests that most of these rocks were derived from a contemporaneous, if not common, oceanic sequence; the metaplutonic rocks therein may represent the oceanic basement of the sedimen­ tary rocks. Although we cannot rule out the possibility that the isolated tectonic blocks of metatonalite and metagabbro in the melanges are klippen of the proposed Haystack thrust to the northwest, we think that imbrication of the oceanic basement with its sedimentary and volcanic cover during formation of the melanges is a more probable origin for the blocks. The melange belts probably were formed by accretion, perhaps after the widespread Late Cretaceous metamorphic event in the Cascades to the east, but before the Eocene. Post-Late Cretaceous right-lateral strike­ slip faulting along the Straight Creek fault has moved the rocks to their present position. INTRODUCTION crop out west of the fault more or less between the Pre-Tertiary rocks in the central and northern Pilchuck River and the South Fork of the Snoqualmie Cascade Range of Washington consist of weakly meta­ River. morphosed to unmetamorphosed Paleozoic and Meso­ Previously, many of these rocks have been consid­ zoic sedimentary and igneous rocks west of the ered to be conventional stratified formations of sedi­ Straight Creek fault and medium- to high-grade meta­ mentary rocks and assigned a variety of formation morphic rocks east of the fault (Fig. 1). Some of the names and ages. (See table l of Dungan, 1974.) medium- to high-grade rocks east of the fault are Because of the intense penetrative deformation and described in a companion article (Tabor and others, scattering of exotic components in the rocks, we con­ this volume). We describe here two lithologically dis­ sider them to be melanges. They appear to constitute tinct belts of pervasively deformed marine rocks that two melange belts: an eastern belt that contains pre- 130 Selected papers on the geology of Washington 123° data suggest that both belts contain Paleozoic and Mesozoic components. Numerous workers, primarily students from the University of Washington, have investigated these and similar rocks; their endeavors, summarized briefly below, were of great value to us. Because much remains to be learned about the lithologies, ages, and relations of the rocks in the melanges, we refrain from using the formation names assigned by earlier workers, except when referring to work of others or correlations made by others with rocks outside the area of this report. Although Silberling and others (1984, p. C-22 - C-27) have correlated the melanges of the eastern and western belts with regional terranes called the San Juan and Olney Pass terranes, respectively, mapping and correlation of the melanges with regional terranes are still in progress. The origin of these disrupted oceanic Paleozoic and Mesozoic rocks and their subse­ quent accretion to the North American continent have EXPLANATION ~ been considered by many students of Northwest geol­ ECl) ogy (for instance, Danner, 1977; Davis and others, 0 Surficial deposits :, 1978; Coney and others, 1980; Vance and others, 0 u 5 1980; Whetten and others, 1980). N 0 We describe the melanges in two formats. The Intrusive rocks z w u general description of early work, naming and correla­ tions, overall structure, and age are discussed in the Sedimentary and volcanic rocks .,, :, text under "Description of Melange Belts." More 0 Cl)"' detailed lithologic descriptions of melange components l''~',':',, '''l" ~~ -oco ...J- 0 Plutonlc and metamorphic rocks I!? accompany Figure 2. New radiometric ages are given u ~~ uO in Appendix Tables Al and A2. -N nmmm 00 NW Melange Undifferentiated melange 0...J (/) <l "DEFINITION" OF MELANGE and marine sedimentary wn. and volcanic rocks ~ The term "melange", first applied on the West - -- Contact Coast by Bailey (1941, p. 30) to the Franciscan assem­ --- Fault blage "because of extreme contortion of the structures as well as the variability of the rock types," was popularized in the United States by Hsu (1968), whose work, also in the Franciscan, precipitated much discus­ Figure 1.-Simplified geologic map of northwestern sion. Although attempts have been made to formally Washington, showing location of melange belts in define the term "melange" (for example, Berkland and relation to other major rock groups and structures. others, 1972, p. 2296), participants at the 1978 Pen­ rose Conference on Melanges tried to arrive at a single dominantly chert and volcanic materials, and a west­ comprehensive definition without complete success. ern belt that contains predominantly elastic materials. However, the usage that grew from those discussions Two recent studies (Fox, 1983; Jones and others, (Silver and Buetner, 1980, p. 32 [after Hsu, 1968]) is 1978) describing the distribution and composition of applicable to rocks discussed in this paper. melanges on the West Coast omitted these rocks, even " 'Melange' is a general term though they underlie a significant area in western describing a mappable (at 1:25,000 or Washington. In a previous report (Frizzell and others, smaller scale), internally fragmented 1982), we indicated that the melange of the eastern and mixed rock body containing a belt contained both Paleozoic and Mesozoic compo­ variety of blocks, commonly in a per­ nents, whereas the melange of the western belt con­ vasively deformed matrix. The term tained mostly Late Jurassic and Early Cretaceous refers to rock mixtures formed by tec­ components. More recent field work and new fossil tonic movements, sedimentary sliding, Melanges in the Western Cascades I 3 I or any combination of such processes, nation is possible because the specimens have appar­ with no mixing process excluded. It ently been lost, Duror's descriptive report on the does not imply a particular mixing brachiopod and trilobite fauna (in Smith, 1916, p. 580- process. 'Melange' is a handy 582) includes line drawings. Duror concluded from her descriptive term, alerting the reader examination that the fossils were "Trenton age" and to the possibility that such a body had not been recorded in rocks younger than Ordovi­ may not conform to classic principles cian. Dedicated sleuthing by W. R. Danner (reported of superposition and stratigraphic in Galster, 1956, p. 20-21) failed to unearth either the succession [our emphasis]." specimens originally described by Duror or corrobora­ tive samples from Lowe Gulch, but the line drawings We would add to this general definition that exotic indicate that the fossils were correctly identified (J. T. blocks-tectonic inclusions detached from rocks for­ Dutro, Jr., personal commun., 1983). eign to the main body of the melange-should be Danner ( 1957) was successful, however, in produc­ present to distinguish a melange from a broken forma­ ing the first truly comprehensible regional strati­ tion (Hsu, 1968, p. 1065). graphic history of the pre-Tertiary rocks of western Washington. He (1957, p. 230-249) detailed the occur­ DESCRIPTION OF MELANGE BELTS rence of marble (discussed below) in the Skykomish area that he referred to his Stillaguamish Group Melange of the Eastern Belt (Danner, 1957); he has since called these rocks the The melange of the eastern belt consists predomi­ "Trafton sequence" (Danner, 1966, p. 363). Galster nantly of a mixture of mafic volcanic rocks and chert (I 956, p. 14-22) mapped rocks southwest of with minor argillite and graywacke. Marble is conspic­ Skykomish, which he also referred to the Stil­ uous locally, and large and small pods of ultramafic laguamish (now the Trafton). rocks occur throughout the belt. Mafic migmatitic Yeats ( 1964, p. 556) referred a prominent area of gneiss, metadiabase, and metatonalite are also present outcrop northwest of Skykomish to his (restricted) in the eastern belt. The rocks are considerably Gunn Peak Formation and Barclay Creek Formation. deformed, slightly metamorphosed (mostly to green­ The Gunn Peak Formation, composed predominantly schist facies), and locally overprinted to pyroxene of ribbon chert and quartzite (metachert) and contain­ hornfels by Tertiary plutons.
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