Isotopic Provenance of Paleogene Sandstones from the Accretionary Core of the Olympic Mountains, Washington

Isotopic provenance of Paleogene sandstones from the accretionary core of the Olympic Mountains, Washington

PAUL L. HELLER Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071 ROWLAND W. TABOR U.S. Geological Survey, 345 Middiefield Road, Menlo Park, California 94025 JAMES R. O'NEIL Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109-1063 DAVID R. PEVEAR Exxon Production Research Company, Houston, Texas 77252 MUHAMMAD SHAFIQULLAH Department of Geosciences, University of Arizona, Tucson, Arizona 85721 NANCY S. WINSLOW Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82070

ABSTRACT values (>0.710), moderately high S,80 values the trench as gravity flows, or far-traveled ter- (~+ 9) and Mesozoic age for detrital white ranes transported to their present site via Pacific Conventional modal sandstone data from mica of Olympic core rocks and sandstone of basin plates. Allochthoneity might be con- Paleogene units of the Pacific Northwest are the Yakutat terrane suggest a source in the strained by paleomagnetic studies; however, this not precise enough to pinpoint source areas high-rank metamorphic and plutonic rocks technique is limited by the suitability of the and constrain displacement histories of ac- from the eastern part of the Omineca Crystal- rocks to be studied and by difficulties in deter- creted sedimentary terranes. Isotopic prov- line Belt in southeastern British Columbia. mining displacement precisely. Compositional enance study, used in conjunction with Furthermore, rapid uplift of this source area studies can also be used to establish geologic ties traditional basin-analysis techniques, pro- during Eocene time is consistent with the dep- between sedimentary deposits and specific vides a powerful means of identifying source ositional age of the Olympic rocks. Sediment source areas to reconstruct transport or dis- areas. Analysis of Rb-Sr data in both whole- derived from this source area was trans- placement histories. Unfortunately, many clastic rock and detrital white micas of Paleogene ported westward by major river systems that deposits along the active margin of western sandstones from allochthonous units in the crossed the low-lying North Cascade Range North America have broadly similar composi- eastern core of the Olympic Mountains and and supplied the deposits of the autochtho- tions (Dickinson, 1982), and so determining coeval autochthonous sandstones from coast- nous units of the northern Washington coast specific source areas is equivocal. al Pacific Northwest shows that the Needles- and the offshore equivalents before the latter An alternate approach to pinpointing source Gray Wolf and Grand Valley lithic assem- were accreted to form the Olympic core. areas, and therefore displacement histories, is the blages of the core came from the same source Limited data from the Yakutat terrane sug- isotopic provenance technique (Heller and as sandstones of the Chuckanut Formation gest that it lay offshore of southern British Frost, 1988; Renne and others, 1990). This ap- and Puget Group in northern Washington. Columbia sometime during middle Eocene to proach, in conjunction with other basin-analysis The source of these units is isotopically early Oligocene time, consistent with paleo- techniques, can determine source areas to a de- distinct from the source for units farther magnetic and some paleontologic interpreta- gree not generally possible by most methods south, such as the Tyee Formation in Ore- tions, and subsequently migrated northward alone. We use this approach to determine the gon. Chemical compositions, conventional by plate motions. displacement history of fault-bounded sedimen- K-Ar age determinations, and oxygen- and tary terranes that comprise the core of the hydrogen-isotope compositions of white INTRODUCTION Olympic Mountains. By comparing mineralogic micas support this conclusion. and isotopic characteristics of sandstones in the Similar analyses of sandstones from the The core of the Olympic Mountains of core of the Olympic Mountains to other sedi- Western Olympic lithic assemblage and the northwest Washington (Figs. 1, 2) is a broad mentary units of the same age in the Pacific Yakutat terrane of southeastern Alaska sug- subduction complex that accreted to North Northwest and potential source areas through- gest that these two units have a similar America during Tertiary time (Stewart, 1970; out western North America, we can quite tightly source, but that they differ slightly from sand- Rau, 1973; Tabor and Cady, 1978b). As is typi- constrain the displacement and paleogeographic stones of the eastern Olympic core and au- cal for accretionary complexes, it is unknown to history of the region. In this paper, we use the tochthonous Washington units. what extent the Olympic core consists of locally time scale of Berggren and others (1985) as The overall composition of sandstones derived material reworked into the subduction correlated to the stratigraphy of the Pacific (lithic arkosic), and the very high initial-Sr zone, material transported long distances along Northwest by Rau (1981).

Additional material for this article (Appendix A) may be secured free of charge by requesting Supplementary Data 9204 from the GSA Documents Secretary.

Geological Society of America Bulletin, v. 104, p. 140-153, 10 figs., 3 tables, February 1992.

140

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Figure 1. Map of western North America showing location of Paleogene basins studied (stippled pattern), and possible metamorphic (ruled) and plutonic (+) source areas for sandstones of the Olympic Mountains.

GEOLOGIC SETTING

The Olympic Mountains (Fig. 2) consist of Cenozoic oceanic basalt and marine sedimentary rocks that have been deformed between the Juan de Fuca (née Farallon) plate and overrid- ing North American plate since middle Eocene time (Tabor and Cady, 1978a, 1978b). Within an oroclinally deformed horseshoe of oceanic basalts, there are clastic sedimentary packages of the Olympic core (Fig. 3), bounded by folded, imbricate thrust faults (Tabor and Cady, 1978a). Earliest deformation in the Olympic Moun- tains is related to docking of oceanic islands along the coastal zone during middle Eocene time at about 50 Ma (Snavely and Wagner, 1963; Snavely and others, 1968; Duncan, 1982). These volcanic rocks of the Crescent Formation (Fig. 3) are dominantly tholeiitic pil- low basalts interpreted to be of seamount, oceanic-ridge, or volcanic-arc origin (Cady, 1975; Duncan, 1982, Tabor, 1987). Continental affinities of clasts beneath and within the Cres- cent Formation suggest that the volcanic rocks erupted near the continent (Cady and others, 1972; Tabor and Cady, 1978a; Wells and others, 1984). Limited paleomagnetic data from the Crescent Formation on the east side of the Olympic Peninsula show no inclination anom- aly, suggesting that the unit has not been dis- north and northwest (Snavely in Muller and Elsewhere during this time in the Pacific placed far latitudinally (Beck and Engebretson, others, 1983) depositionally overlie the Crescent Northwest, clastic sedimentation was character- 1982). These observations, along with near Formation. This succession represents conform- ized by the rapid deposition of generally arkosic coincidence in age of the youngest volcanic able, more or less continuous, deposition from sandstones with conspicuous, relatively coarse- rocks (~51 Ma) and the oldest overlapping sed- latest Ulatisian (ca. 50 Ma) into Miocene time grained, white mica and moderate to high abun- imentary units around the Olympic periphery (ca. 12 Ma; Rau, 1981), which suggests docking dance of volcanic-lithic grains (Heller and (45-50 Ma; discussed below), indicate that the of the Crescent Formation prior to initial deposi- Ryberg, 1983; Heller and others, 1987). Along basaltic accumulations, no matter what their tion along the northern Olympic periphery at ca. the coast, these rocks of Eocene to Oligocene(?) origin, could not have traveled far at plate- 45-50 Ma. sedimentary basins (Fig. 2) include the Tyee tectonic rates before they accreted to the continent. Marine sedimentary rocks derived from the north and east overlap the Crescent Formation. Figure 2. Map of western Washington and Olympic South of the Olympic Mountains, in the Megler northwest Oregon showing major basins of Chuckanut core basin (Fig. 2), the oldest of these units are of Paleogene age (stippled pattern), Tertiary ac- Formation latest Ulatisian to Narizian age (—50-40 Ma; creted volcanic rocks (v), and other major Armentrout, 1981; Rau, 1981) and represent the tectonic elements. The locations of Belling- Puget progradation of the continental shelf across the ham (B), Seattle (S), Tacoma (T), and Port- Group accreted basaltic volcanic rocks (Heller and oth- land (P) are shown. ers, 1987). This overlap indicates that the Cres- Megler cent Formation had accreted by ca. 47 Ma. basin To the north, between the Olympic Moun- WASH. _ tains and the Strait of Juan de Fuca, Eocene Tyee ORE. and Oligocene marine units derived from the basin

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124° 123° ticular sandstone in the upper part (Figs. 4A, 4B). Sandstone beds are massive to graded, suggesting deposition by gravity flows, probably turbidity currents. Possibly these beds form parts Needles - Gray Wolf of submarine fans, trench-fill turbidites, and/or lithic assemblage slope deposits. In contrast, the Grand Valley lithic assemblage is sandier and contains centimeter- to-decimeter scale cross-bedding (Fig. 4C), laminations, rare flaser bedding, and other fea- tures (Fig. 4D). Many of these types of structures are common among shallow-marine shelf deposits. Depositional fades of the Elwha and Western Olympic lithic assemblages (Fig. 4E) have not been evaluated in detail. Stewart (1970) interpreted the westernmost parts of the Western Olympic lithic assemblage as submarine fans. The ages of core units are not well constrained, primarily because fossils are scarce. In the Needles-Gray Wolf lithic assemblage, Cady and others (1972) described a single late(?) Eo- cene macrofossil. Brandon and others (1988) reported a peak fission-track age of 39 ± 5 Ma from detrital zircons, consistent with a late Eo- cene age for the Needles-Gray Wolf unit. The Needles-Gray Wolf unit appears to be the high- Elwha lithic assemblage est structural slice in the Olympic core; therefore subduction of core rocks must have begun during or after the middle to late Eocene, after formation and docking of the continent-ward, Figure 3. Geologic map of Olympic Mountains showing Eocene through Oligocene sedimen- structurally higher, early(?) and middle Eocene tary rocks of the Olympic core (stippled pattern), distribution of volcanic rocks of the Crescent Crescent Formation (Tabor and Cady, 1978b). Formation of Eocene age (v), and major thrust faults (barbed lines). Sample locations are Elsewhere, the core rocks contain Tertiary shown with solid dots. Note that the major sedimentary units (the Needles-Gray Wolf, Grand fossils, some of which are Paleocene and Eocene Valley, Elwha, and Western Olympic lithic assemblages) are bounded by major thrust faults. (Tabor and Cady, 1978a, 1978b). The Grand Valley lithic assemblage contains foraminifers identifiable only as Tertiary (Tabor and Cady, 1978a). The Elwha lithic assemblage contains Formation and associated units in the Oregon ern Olympic lithic assemblages (Fig. 3; Tabor foraminifers of early(?) or middle Eocene age Coast Range, the sandstone of Megler and asso- and Cady, 1978a, 1978b). Core rocks range (Tabor and Cady, 1978a, 1978b). The Western ciated units in southwestern Washington, the from severely faulted, sheared with pervasive Olympic lithic assemblage contains both mol- Puget Group and Naches Formation east and slaty cleavage, to relatively undeformed, contin- lusks and foraminifera, ranging from at least late southeast of Seattle in the Cascade Range, and uous stratigraphic sequences that persist for Eocene to early Oligocene in age (Narizian to the upper part of the Chuckanut Formation many kilometers. Refugian; Gower, 1960; Rau, 1973). Large- north of Seattle. Degree of metamorphism in the eastern core scale folding is apparent in the accreted prism rocks varies, and it is greatest (lawsonite + pum- along with inverted age stacking (Tabor and Olympic Core pellyite + prehnite + quartz + white mica ± cal- Cady, 1978b; Tabor, 1987). cite ± chlorite) in an area central to the Eastern core rocks yield post-accretionary Sedimentary and metasedimentary rocks of core—mostly in the Elwha and Western Olym- metamorphic cooling ages (K-Ar whole rock) of the Olympic core range from Eocene age in the pic lithic assemblage (Tabor and Cady, 1978a; about 29 Ma (Tabor, 1972), indicating that they east to Miocene in the west (Rau, 1973, 1979; Brandon and Calderwood, 1990). Farther west, accreted before middle Oligocene time. The Tabor and Cady, 1978a). The age offshore of only laumontite has crystallized (Stewart, 1970; Western Olympic lithic assemblage may not accretionary material becomes even younger Tabor and Cady, 1978a). have been incorporated into the wedge until toward the modern trench (Snavely and Wag- In spite of tectonic disruption, inaccessibility, middle Miocene time, on the basis of plate re- ner, 1982). This study involves the Eocene to and locally heavy vegetation, sporadic preserva- constructions suggested by Fox (1983). Uplift of Oligocene sedimentary rocks of the central and tion of sedimentary structures and overall litho- the core rocks began about 12 Ma (middle Mio- eastern parts of the Olympic core (Fig. 2). The logic character of the deposits allows us to cene), based on the analysis of fission-tracks in eastern Olympic core consists of the Needles- generalize depositional environments. The apatite and an unroofing sequence recorded in Gray Wolf and Grand Valley lithic assemblages; Needles-Gray Wolf lithic assemblage is domi- upper Miocene deposits in the southern periph- farther to the west are the Elwha and the West- nantly mudstone with abundant intercalated len- eral rocks (Brandon and others, 1988).

Yakutat Terrane of Southeast Alaska of Paleocene(?) and early Eocene age (Plafker, weather oblique-transform fault system and is 1987). Overlying the basement, there are underthrusting older terranes to the north- Because some workers have suggested that unconformity-bounded clastic sequences of Eo- northwest (Bruns, 1983b; Plafker, 1987). The the Eocene strata of the Yakutat terrane in the cene through Quaternary age (Plafker, 1987). Tertiary displacement history of the Yakutat ter- northern Gulf of Alaska (Fig. 1) may have been The Paleogene sequence reflects an igneous and rane has been much debated (Bruns, 1983a, deposited off the coast of Washington (Plafker, high-rank metamorphic provenance, whereas 1984,1985; Chisholm, 1985; Keller and others, 1984; Van Alstine and others, 1985), we ana- the upper Cenozoic sequence was locally de- 1984; Plafker, 1984; Wolfe and McCoy, 1984). lyzed two sandstone samples from that terrane. rived (Plafker, 1987; Winkler and Plafker, Preliminary paleomagnetic data from the off- The Yakutat is a composite terrane of mostly 1981). Micaceous arkosic sandstones are present shore Yakutat well (Van Alstine and others, marine sedimentary and volcanic rocks that within the alluvial to shallow-marine Kulthieth 1985) suggest northward displacement of about have been accreting to southeast Alaska since Formation and partially equivalent marine units 13° latitude during post-early Eocene time rela- middle Tertiary time (Davis and Plafker, 1986; of middle Eocene to early Oligocene age. tive to cratonal North America. This would Plafker, 1987). Basement beneath the western At present, the Yakutat terrane is migrating place the terrane no farther south than Washing- two-thirds of the Yakutat terrane is oceanic crust northward along the Queen Charlotte-Fair- ton during Eocene time. Paleontologic data sug-

Figure 4. Photographs of sedimentary units within the core of the Olympic Mountains. (A) Needles-Gray Wolf lilhic assemblage showing stratigraphie character and fold within the unit. Taken at head of Milk Creek. (B) Needles- Gray Wolf lithic assemblage showing laminated medium-grained sandstone. Taken along trail on east side of Obstruction Peak. (C) Grand Valley lithic assemblage showing climbing ripples in fine-grained sandstone. Taken at Lake Lillian basin.

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E Figure 4. (Continued). (D) Grand Valley lithic assemblage showing large-scale load casts along base of sandstone bed. Taken along ridge east of Moose Lake. (E) Western Olympic lithic assemblage showing undeformed, laminated sandstones in foreground and tectonite blocks in slate in background. Taken along west side of Mount Olympus.

tive to North America during Eocene time (Engebretson and others, 1985). Snavely and others (1980) inferred, from offshore seismic data, the presence of a strike-slip fault off coastal Oregon along which the western edge of the Tyee basin (Figs. 1,2) may have been displaced D northward during Eocene time (compare with fig. 6 of Heller and others, 1987). This displaced gesting correlation with Tertiary rocks in west- least some of the Olympic core sequence may terrane riding on the Kula or Farallon plate ern Washington (Rau and others, 1983; Wolfe have been deposited elsewhere and subsequently could have reached the Olympic region in a few and McCoy, 1984) is consistent with this inter- incorporated into the accretionary wedge. Po- million years (Engebretson and others, 1985). pretation. In contrast, other microfaunal assem- tential source areas for the large volume of sed- Indeed, the northward component of plate mo- blages suggest that the early Eocene rocks of the imentary rocks in the Olympic core can be tion throughout the Tertiary was rapid enough Yakutat terrane have much more southerly af- found to the north, east, and south. Sediment that sedimentary rocks of the Olympic core finities and were deposited about 30° farther may have been derived from the Coast Plutonic could have been derived from as far south as south than present (Keller and others, 1984; von Complex of western British Columbia to the northern to central California. As we will show, Huene and others, 1985), at the paleolatitude of north (Fig. 1), an area that was rapidly uplifted sandstone in the Yakutat terrane in southeast southern California or Mexico. and eroded during Eocene time (Hollister, 1982; Alaska (Fig. 1) may be a related far-traveled The analyzed sandstones, collected by George Parrish and others, 1988). Detritus from the deposit as well. Plafker from dredge hauls, are from the upper Coast Plutonic Complex may have been trans- lower through upper Eocene part of the offshore ported to the south as turbidites flowing down SANDSTONE COMPOSITION OF THE sequence (see Appendix A),1 correlative to the the trench axis along the western margin of OLYMPIC CORE ROCKS Kulthieth Formation exposed onshore (Plafker, Vancouver Island. Alternatively, sediments may 1987; Plafker and others, 1980). have been transported from the east (Snavely Our initial approach to determining prove- and Wagner, 1963) by rivers that originated in, nance was to compare framework compositions POTENTIAL SOURCE AREAS or flowed through, the North Cascade Range of sandstones from the Olympic core to those (Fig. 1). A third possibility is that deposits in the from other Eocene basins in the Pacific North- In view of the overall translational nature of core of the Olympics were derived from the west. We collected fresh fine- to medium- much of the western edge of North America, at south (Fig. 1), transported by turbidites that grained sandstones from the Needles-Gray flowed north along the trench or transported en Wolf, Grand Valley, and Western Olympic lith- masse to the Olympic region on the Kula ic assemblages (Fig. 3) and made point counts to 'Appendix A may be secured free of charge by and/or Farallon plate, both of which had a sig- requesting Supplementary Data 9204 from the GSA compare with sandstones from the Chuckanut Documents Secretary. nificant northward component of motion rela- and Tyee Formations of mainland Washington

TABLE 1 . MEAN COMPOSITIONS OF EOCENE SANDSTONES FROM THE PACIFIC NORTHWEST unstable-lithic fragments (L = Ls + Lv), total lithic fragments (Lt = L + Qp), and total Unit Q-F-L Qm-F-Lt Qp-Lv-Ls name (std. dev. = lo) (std. dev. = la) (std. dev. = lo) quartz (Q). The data (Table 1) are plotted (Fig. 5) in Needles-Gray 51-33-16 42-33-25 40-2040 terms of mean values and one standard devia- Wolf lithic (5.9-5.6-4.1) (4.8-5.6-5.7) (10.1-9.1-8.7) assemblage tion for each of the units examined (Ingersoll (n = 14) and Suczek, 1979). Data for the Chuckanut Grand Valley 42-36-22 36-36-28 22-50-28 lithic assemblage (4.5-5.6-2.4) (2.4-5.6-4.6) (8.2-19.3-15.9) Formation, from Frizzell (1979), Johnson •5) 1983) are included. The data are striking in their Chuckanut 48-49-3 4549-6 49-13-38 overall similarity—most sandstones are of lithic Formation (9.9-9.6-1.0) (10.5-9.6-2.0) (15.5-15.4-10.1)

Chuckanut 4547-7 2746-27 68-16-16 units in the Olympic core overlap those from the Formation* (7.9-10.0-8.3) (5.9-9.9-13.3) (18.7-14.9-6.0) Chuckanut Formation and are similar to those

Sandstone of . appear to be slightly more enriched in volcanic- Megler and lower 37-45-18 3145-24 2648-26 lithic grains than the Needles-Gray Wolf rocks part of Mcintosh (1.8-7.1-6.7) (2.4-7.1-8.6) (4.5-7.0-5.0) Formation^ and Chuckanut Formation. Interpretation and

ISOTOPIC COMPOSITION OF OLYMPIC CORE ROCKS

The isotopic provenance technique (Heller and Frost, 1988) compares the stable and radiogenic isotope compositions of detrital grains with those of possible source areas. If potential source areas are distinguishable on the basis of age of formation, thermal history, and/or pedogenesis, then isotopic analyses may be used to Figure 5. Plots of sandstone composition from Paleogene units of the Pacific Northwest, determine sedimentary provenance. This tool is normalized to (A) total quartz (Q), feldspar (F), and unstable lithic fragments; and particularly useful where source areas have sim- (B) monocrystalline quartz (Qm), feldspar (F), and total lithic fragments (Lt). Mean and one ilar mineralogy, and produce sandstones of sim- standard deviation are shown for each of the units. Data are summarized in Table 1. ilar composition. Even minor differences in

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isotopic characteristics, however, may be appar- TABLE 2. ANALYSES OF WHITE MICAS ent. Both whole-rock and mineral separates may Sample MnOj k o CaO Ti02 NajO ai O MgO Si02 OH Total be studied by various isotopic systems to yield a FejOj 2 2 3

complete "fingerprint" of the clastic deposit. NG4-8-8-I 00.02 03.00 10.48 00.00 00.73 00.44 31.12 01.12 47.89 04.47 99.27 NG4-8-9-2 00.01 03.18 10.28 00.01 00.57 00.40 30.43 01.02 47.68 04.41 97.99 Ideally, coarse-grained, compositionally im- GV4-8-Ì [-Í 00.04 02.84 10.07 00.01 00.56 00.44 30.32 01.03 47.82 04.40 97.53 mature sandstones are used for isotopic analysis. CW4-8-17-5 00.03 02.86 10.08 00.00 00.65 00.61 31.17 00.91 47.22 04.42 97.96 YK-S1079-EG 00.03 03.73 10.18 00.00 00.84 00.50 30.32 00.97 47.10 04.40 98.07 They contain grains that are large enough to be CN4-8-12-3 00.00 01.99 09.81 00.01 00.94 00.56 31.71 01.04 48.55 04.50 99.12

separated easily from smaller grains that might Number of ions form authigenically. Immaturity implies rapid Mn Fe K Ca Ti Na Al Mg Si H20 Total erosion and burial which reduces exposure of NG4-8-8-1 0.002 0.302 1.792 0.000 0.074 0.114 4.918 0.223 6.420 2.000 15.848 grains to possible alteration during transport and NG4-8-9-2 0.001 0.324 1.780 0.001 0.058 0.105 4.868 0.206 6.470 2.000 15.816 deposition. The presence of unstable detrital GV4-8-U-1 0.005 0.291 1.747 0.002 0.057 0.115 4.861 0.208 6.505 2.000 15.790 CW4-8-17-5 0.004 0.295 1.745 0.001 0.067 0.159 4.980 0.184 6.405 2.000 15.841 grains in immature sandstones also indicates lit- YK-S10-79-EG 0.003 0.382 1.766 0.001 0.086 0.131 4.859 0.197 6.407 2.000 15.833 CN4-8-12-3 0.003 0.199 1.666 0.001 0.094 0.144 4.977 0.206 6.465 2.000 15.755 tle secondary alteration during weathering,

transport, and burial. Most changes that bring Note: samples are from the Needles-Gray Wolf (NG), Grand Valley (GV), and Western Olympic (CW) lithic ;; and the Yakutat lerrane (YK) and about geochemical partitioning also result in Cbuckanut Formation (CN). removal of unstable grains. Lastly, using sandstones with relatively little matrix (<20%) reduces problems caused by unrecognized second- 0.718 Figure 6. Rb-Sr system- /5\ ary alteration. atics for whole-rock sand- The abundance of unstable lithic grains in stone samples from Ter- some sandstones of the Olympic core suggests tiary units in Oregon and 0.714 _ // that the source rocks had not been severely western Washington, and A weathered and that the grains have not been modern Columbia River /o / " 1 multiply recycled. Although parts of the Olym- sands. Data from the CO CD pic core have been metamorphosed, samples Needles-Gray Wolf lithic oqcq ^ 00.71N 710 collected away from the most metamorphosed assemblage are grouped CO "ZTn V - /JK v part of the core show no evidence of significant together as are data from coo • Needles - Gray Wolf recrystallization. Mica and feldspar grains are the Tyee Formation and / O / lithic assemblage 0.706 clear and unaltered, and volcanic-lithic clasts the Herren unit of Eocene • Tyee Formation A Sandstone of Megler appear to be relatively fresh in thin section. In age in Oregon (Heller and + Herren unit addition, chemical analysis of white mica grains otters, 1985) and the mod- o Columbia River sands _l_ 1 1 1 1 1 i show them to be high in %K20 (>9%, Table 2), ern Columbia River sands 0.702 —1-, J 1 -1 1 suggesting little loss by secondary processes. (Peterman and Whetten, 0.0 0.4 0.8 1.2 1.6 2.0 2.4 1972). We studied white micas because previous 87Rb/86Sr studies of Eocene arkosic rocks in the northwest have shown that white mica is relatively common in many units, is generally unaltered, and others, 1985) in southwestern Washington, and autochthonous units on mainland Washington so is a stable reservoir for the isotopes of interest. what are undoubtedly allochthonous Eocene which includes the Chuckanut Formation and The white micas are coarse and have few inclu- sandstones from the Yakutat terrane in southern Puget Group; group 2, data from the eastern sions, allowing ease of separation. We avoided Alaska. Sample sites are given in Appendix A. Olympic core which includes the Needles-Gray heavy minerals because, although they make ex- Each sample of fresh, medium-grained sand- Wolf and Grand Valley lithic assemblages; cellent reservoirs for some elements, they are stone contained 5% or less white mica. We group 3, data from the Western Olympic lithic very low in abundance and are not hydraulically ground, washed, and sieved samples to separate assemblage and the Yakutat terrane in Alaska. equivalent to light minerals and so may repre- white micas from the 100 to 35 mesh fraction The results are presented in Table 3. sent local source areas that are not the same as (0.15 to 0.5 mm) and concentrated them further those supplying the bulk of the sediment (Dick- by magnetic separation, shaker table, and heavy RESULTS AND INTERPRETATION inson, 1970; Heller and Frost, 1988). liquids. Whole rocks and white-mica separates were analyzed for Rb-Sr and K-Ar at the Uni- Whole Rocks Methods versity of Arizona Laboratory of Isotope Geo- chemistry, oxygen- and hydrogen-isotope anal- Rb and Sr for whole-rock and mica samples The most comprehensive data come from the yses of white mica were made at the U.S. were analyzed with a VG-354 mass spectrome- Needles-Gray Wolf lithic assemblage, where we Geological Survey in Menlo Park, and electron ter. All 87Sr/86Sr ratios were normalized to sampled a transect across structure through most microprobe analyses of mica separates were 86Sr/88Sr = 0.1194 to correct for machine frac- of the unit (Fig. 3). We have also analyzed sam- done at Exxon Production Research Company. tionation. 85Rb/87Rb was taken to be 2.593, ples from the Grand Valley and Western Olym- Due to the limited number of samples, and and experimental errors (2a) are ±1% for pic lithic assemblages, autochthonous Eocene because of apparent geochemical similarity of 87Rb/86Sr and ±0.03% for 87Sr/86Sr. Replicate arkosic sequences in the Chuckanut Formation white mica from several of these units, we have analyses of NBS SRM 987 gave an average and Puget Group to the east in the Cascade organized the isotopic data into three groups for 87Sr/86Sr ratio of 0.71025 ± 0.00003. Range and the sandstone of Megler (Niem and the purposes of discussion. Group 1, data from Figure 6 compares Rb-Sr isotopic data of six

TABLE 3. ISOTOPIC ANALYSIS OF DETRITAL WHITE MICA FROM SANDSTONES AND WHOLE-ROCK SANDSTONES White Mica

Sample Rb ppm Sr ppm 87Rb/86Sr 87Sr/86Sr K-Ar (Ma) S180 SD Chemical Composition. Micas were ana-

NEEDLES-GRAYWOLF LITHIC ASSEMBLAGE lyzed with a CAMECA microprobe using U.S. Geological Survey standard muscovite B32. Six NG4-8-7-2A 349.5 60.2 16.86 0.75673 60.5 ± 1.4 NG4-8-7-3 368.7 48.3 22.22 0.77825 65.3 ± 1.5 representative samples were selected; from each, NG4-8-8-1 366 1 60.86 17.49 0.76228 73.0 ± 1.8 NG4-8-9-2 338.4 56.13 17.54 0.76439 73.4 ± 1.4 9.2 five grains were analyzed, using about 25 points NG4-8-I0-1 319.6 79.9 11.63 0.7525 68.8 ± 1.6 per grain. Only analyses which sum to >97% are NCM-IM 368.2 48 22.36 0.77977 67.5 ± 1.5 9.2 -42 NG4-8-15-1 394 42.8 26.82 0.78888 67.2 ± 1.6 9.0 -41 summarized in Table 2 (60% of analyses). The KAP-5-71* 329.4 37.7 25.44 0.76639 62.8 ± 1.4 9.3 -55 chemical analyses and structural formulae are GRAND VALLEY LITHIC ASSEMBLAGE averages of several grains and several points per GV4-8-I1-I 348.1 57.7 17.53 0.76005 61.7 ± 1.5 9.1 grain for each of the six sample locations (Table GV4-8-U-3 349.8 52.7 19.32 0.76672 65.6 ± 1.6 GV4-8-1M 61.9 ± 1.5 2). The standard deviations for almost all of the analyses are very small except A1 and Fe (as WESTERN OLYMPICS LITHIC ASSEMBLAGE Fe3+), which vary inversely, suggesting substitu- CW4-8-17-2 88.6 ± 2.1 CW4-8-17-5 297.7 109.1 7.92 0.74337 86.9 ± 2.0 tion in the octahedral sites. The Si/022 [Si 0.74337 CW4-8-17-5 295.4 109.1 7.86 ions/02o(OH)2] ranges from 6.4 to 6.5; these CW4-8-I7-7 284.1 112.6 7.33 0.74686 93.7 ± 2.2 CW4-8-17-9 283.3 122.2 6.73 0.7479 85.9 ± 2 are, therefore, phengitic muscovites (Guidotti, S86-24t 268.6 160.3 4.87 0.74519 1984). Other chemical properties, such as -0.5 YAKUTATTERRANE Mg+Fe/022,0.05 to 0.09 Ti/022, and Na in 8% YK-S10-79-EG 280.1 124.2 6.54 0.73755 83.1 ± Î.0 9.7 of the interlayer K sites are consistent with a -CHAN 16? YK-55-78-EG 281.7 112.5 7.27 0.74792 92.8 ± 2.2 relatively uniform population of grains consist- -CHAN22 S ing of phengitic muscovite. Individual grains ap- CHUCKANUT FORMATION pear to fall into one of two populations: one

CN4-8-12-1 364.4 39.4 26.99 0.79777 73.5 ± 1.7 9.5 -41 with high Fe (0.23/022), and one with low Fe CN4-8-12-3 357.6 39.9 26.07 0.77113 69.8 ± 1.6 9.3 -49 (0.10/022). Both populations are present in all CN4-8-14-2 342.7 42.3 23.56 0.76881 66.9 ± 1.6 9.0 -46 samples. PUGET GROUP Guidotti's (1984) plots illustrate the composi- PG4-S-4-2 66.3 I 1.6 10.4 PG4-8-4-3 314 52.1 17.57 0.78197 71.9 ± 1.7 10.0 tion of metamorphic micas; the analyses of our PG4-8-W 3523 48 21.38 0.78169 71.3 ± 1.7 9.3 study fall near the muscovite end of the phengite SANDSTONE OF MEGLER field. On Guidotti's figure 18, our analyses plot MS4-8-18-1 72.8 ± 1.7 9.3 -67 in the field of micas found in the muscovite + MS2-6-27-3" 383 43.28 25.8 0.7852 68.8 ± 1.7 MS2-6-27-7" 9.7 plagioclase zone, suggesting that they could be derived from a source that included high-rank •Sample collected by K. A. Pisciotto. ^Sample collected by P. D. Snavely, Jr., analyzed by Z. E, Petemtan. metamorphic rocks. This provenance is also ^Sample collected by G. Plafkei. suggested by the high Ti content of our samples "Data from Heller and others (1985). (Guidotti, 1984, p. 406). The analyses of this study do not clearly resemble those of coarse white micas reported from plutonic rocks, which are generally not phengitic (Speer, 1984; Sevigny whole-rock samples from the Needles-Gray from modern bottom sands collected along the and others, 1989). Wolf lithic assemblage with whole-rock data Columbia and Snake Rivers (Peterman and Rubidium and Strontium Contents. Com- from other Eocene sandstones of the Pacific Whetten, 1972). These modern lithic-arkosic parison of the abundance of rubidium and Northwest Although far from colinear, data sands (Whetten and others, 1969) are derived strontium in white micas from the mainland from the Tyee Formation, including one point from source areas to the east, including Idaho Washington basins (group 1), the eastern part of from the Herren unit of Fems and Brooks and British Columbia. The mixing field for the Olympic core (group 2), and the Western (1986) in the Blue Mountain region of Oregon, Needles-Gray Wolf sandstones does not line up Olympic-Yakutat units (group 3) suggests at form a steep trend. If the sandstones were de- with the trend from the Tyee Formation (Fig. least two different source areas (Table 3). White rived from a single source of uniform age, the 6), suggesting distinct source areas. Heller and micas from mainland Washington units and the Rb-Sr results should be colinear as long as iso- others (1985) concluded that the Tyee Forma- eastern Olympic core have overlapping values topic systematics were not changed during tion had a source in the Idaho batholith. The ranging from 39 to 80 ppm for Sr and 314 transport and deposition. Sandstones derived whole-rock Rb-Sr data, although limited, sug- to 394 ppm for Rb in contrast to the abundance from multiple and/or complex source areas, gest that the Needles-Gray Wolf lithic assem- of these elements in white micas from the however, yield a scatter of values defining a blage did not come from the same source area. Western Olympic-Yakutat units, which range broad array that results from a mixing of differ- One data point obtained for the sandstone of from 109 to 124 ppm Sr and 280 to 297 ppm ent rock units exposed in the source area Megler is included on this and other plots for Rb (Table 3). weighted by relative abundance, average Rb comparison, but we cannot determine if this unit These contents suggest that the eastern Olym- and Sr content, and initial strontium-isotope in southwest Washington was derived from sim- pic core and mainland Washington units derive ratios of minerals in each rock unit (Heller and ilar sources as the Olympic core rocks or the from a similar, if not the same, source area. The Frost, 1988). This data array overlaps results Tyee Formation and do not discuss it further. limited data from the Western Olympic lithic

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assemblage and Yakutat terrane suggest that Olympic core white micas is very similar to the these units may derive from a common source A. range of ages determined for autochthonous 0.800 and that it was different from the source of the • units in Washington (the Chuckanut Formation other units. 0.780 i and Puget Group). As with the Rb-Sr data, these Rb-Sr Systematica All of the Rb-Sr data for 1 • • data suggest that the sandstones of the eastern white micas from the Olympic core rocks and ^ 0.760 i CO Olympic core are, in part, derived from the same the Yakutat terrane appear to be approximately source as sandstones of the Chuckanut Forma- ¿g 0.740 • colinear on a Rb-Sr isochron diagram (Fig. 7 A). • Needles-Gray Wolf tion and Puget Group on mainland Washington. The data conform generally to a reference iso- ° 0.720 • Grand Valley In contrast, the few K-Ar ages obtained from chron of 135 Ma. The linearity represents a * Western Olympic the Western Olympic lithic assemblage are sig- 0.700 • Yakutat terrane provenance field that expresses the average age i * i ' i nificantly older than any from the eastern of white mica from the mixture of different 0 10 20 30 40 Olympic core units or the mainland Washington sources weighted by the volumetric contribution g 87Rb/®6Sr sandstones (Fig. 8). Ages of the Western Olym- from the various source rocks. It would be re- pic white micas range from 86 to 94 Ma with a markable for these white mica to come from 0.800 mean age of 89 Ma. Sparse data from the Yaku- very different source rocks of different initial-Sr - • »c° tat terrane in southeast Alaska overlap the range 0.780 - e values and age and yet still be relatively colinear o of the Western Olympic lithic assemblage, as O • % e today. The linear array of data, thus, is charac- 0.760 t 3 was found with the Rb-Sr data. This suggests, teristic of the source terrane, and although the 0 o but does not require, that the Western Olympic ¿5 0.740 © isochron approach to this data set may be in- lithic assemblage and the Yakutat terrane tapped valid for a number of reasons, it suggests a late ™ 0.720 • Needles-Gray Wolf similar, if not identical, source areas for white Mesozoic average age for the source area. These a Grand Valley mica. isotope results, however, do not indicate source 0.700 ° Tyee Fm. Stable-Isotope Analyses. Oxygen- and rocks of uniform isotopic age. 0 10 20 30 40 hydrogen-isotope analyses were made of 15 87 86 Rb/ Sr values for white micas from the q 87Rb/®6Sr separates of white mica. The data are reported in Western Olympic lithic assemblage and Yakutat Table 3 in the 8-notation in parts per mil relative terrane cluster and are lower than those of the 0.800 0 to the SMOW standard. A S180 value of 9.6 other units (Fig. 7A; Table 3), corresponding to « „Oj per mil was obtained for the NBS-28 quartz the distinction suggested by the bulk Rb and Sr 0.780 standard in the Menlo Park laboratory. Because values. Initial ratios derived from regression of J* 0.760 of the radiogenic isotope evidence for stability of • the data or from calculation based on K-Ar ages 00 coarse-grained minerals cited above and the (-68 Ma), are very high, well above 0.710. Such W 0-740 • Needles-Gray Wolf consistency of the stable isotope analyses regard- values strongly suggest ultimate derivation from • Grand Valley less of where in the section the samples were ™ 0.720 ° Chuckanut Fm. cratonal material. The source rocks probably lay 0 Puget Gp. collected, we consider the diagenetic influences 18 well east of the 0.706 line in Idaho, eastern 0.700 4 Sandstone of Megler on the S 0 values of these minerals to be min- Washington, and British Columbia (Armstrong, imal. Retention of original isotopic ratios by 1988; Armstrong and others, 1977). 0 10 20 30 40 micas in this environment would be expected 87Rb/86Sr The overlap in Rb-Sr ratios of the eastern because the rates of isotopic exchange between Olympic core units, along with their similar Figure 7. Rb-Sr isotopic systematics coarse-grained micas and water are very low at abundance of Rb and Sr, suggest that contained for detrital white micas collected from Ter- low temperatures (O'Neil, 1987). white mica was derived from similar source tiary sandstones of the Pacific Northwest. Replicate analyses of the oxygen-isotope ra- areas. Comparison with white mica from the (A) Units in the Olympic core and the Yaku- tios of the micas agreed to within ±0.2 per mil. Tyee Formation (Fig. 7B) shows consistently tat terrane of southwest Alaska. (B) Units of Such consistency attests to the homogeneity of higher 87Sr/86Sr ratios for the Olympics, indi- the Olympic core compared with the Tyee the samples, as it has been observed in many cating that these units were not derived from Formation of western Oregon. (C) Units stable-isotope laboratories that analyses of heter- precisely the same source area. In contrast, there of the Olympic core compared with autoch- ogeneous mineral separates are poorly reprodu- is an overlap of the data from the eastern Olym- thonous Tertiary sandstones of western cible, particularly when only a few milligrams pic core units with results from the mainland Washington. are analyzed. Furthermore, the total variation of Washington basins, the Chuckanut Formation, analyses of separates from different hand speci- and Puget Group (Fig. 7C), suggesting that mens of the same sandstone unit is only a few micas in these units may have been derived from tenths of a per mil at most. The uniformity of the the same source rocks, in agreement with bulk Grand Valley lithic assemblage nearly overlap oxygen-isotope analyses of samples means that Rb and Sr data (Table 3). those from the Needles-Gray Wolf rocks (Fig. the micas in each sandstone unit were fairly well K-Ar Ages. K-Ar ages for white mica from 8). The mean age for the Needles-Gray Wolf homogenized between the source area(s) and the the Needles-Gray Wolf lithic assemblage range lithic assemblage is virtually identical to that of site of deposition. Either the micas in the from 60 to 73 Ma with a mean of 68 Ma (Fig. the Tyee Formation (Fig. 8). The range of ages source(s) were uniform in composition or, more 8), significantly younger than the Rb-Sr refer- from the Tyee Formation, however, is remark- likely, the transport distance and/or cycling was ence isochron (Fig. 7A), suggesting that the ably limited (Heller and others, 1985), unlike sufficiently long to cause complete or extensive source area underwent multiple thermal-tectonic those from the eastern Olympic core. In con- mixing of two or more white mica populations. events. The ages of three samples from the trast, the spread of ages seen in the eastern The uniform oxygen-isotope composition of

leran miogeoclinal sediments in the subsurface. The high SlsO values of detrital white mica in Figure 8. K-Ar ages for this study probably reflect a similar origin. white micas from Paleo- gene sandstones of the DISCUSSION Pacific Northwest. Aver- age ages of the mainland This study demonstrates that isotopic analysis Washington units (Chuck- can discriminate between Eocene sandstone anut Formation and Puget units in the Pacific Northwest and can help to Group), and the Needles- define provenance. Within the resolution of Gray Wolf and West- available data, the isotopic composition of white ern Olympic lithic assem- mica, and to a lesser extent, the sandstone com- blages are shown. Tyee position of the Needles-Gray Wolf and Grand Formation data from Valley lithic assemblages in the core of the Heller and others, 1985. Olympic Mountains are similar to those of sand- North Cascade data from stones in the Chuckanut Formation and Puget pegmatites and schists. Group which crop out to the east in the Cascade Other data from Table 3. Range (Figs. 5,7,8). The isotopic compositions, although grossly similar, are different in detail Olympic from those of white micas in the sandstone of Mountains Megler in southwest Washington and the Tyee Formation in western Oregon (Figs. 7, 8). Iso- white micas from the Needles-Gray Wolf lithic southern Washington and Oregon, in general topic analyses of white mica from the Western assemblage (Fig. 9) suggests a consistent source. agreement with the results from the other iso- Olympic lithic assemblage and the Yakutat ter- As with the Rb-Sr and K-Ar data, the range topic systems. rane of southern Alaska may be different from of 3180 overlaps quite closely the range for the The observed 5lsO values of 9.0 to 9.3 per those in the eastern part of the Olympic core and Chuckanut Formation, and, to a lesser extent, mil for white mica in Needles-Gray Wolf sand- elsewhere in the Pacific Northwest (Table 3), most of the other Tertiary sandstone units in the stones is typical of white mica in metamorphic but their Rb-Sr ratios appear to fall into the Pacific Northwest (Fig. 9). Similarly, although rocks (for example, Garlick and Epstein, 1967; same linear array (Fig. 7A), suggesting that the the data are limited, there is a strong overlap of Frey and others, 1976) and at the high end for source area had white micas of similar, if not <5D values (Table 3) of micas from the Needles- granitoid micas. Thus, if the source was a grani- identical, age. Gray Wolf lithic assemblage (average = -46) toid rock, it most likely would be an S-type or and the Chuckanut Formation (average = -45). two-mica granite (Taylor, 1968; O'Neil and Provenance of the Eastern Part of the In contrast, one sample each from the sandstone Chappell, 1977). The observed range of white- Olympic Core of Megler (Table 3) in southwest Washington, mica values is within the observed range of the Tyee Formation in western Oregon, and the values for feldspar and biotite from the Meso- The similarity in composition of the Needles- Herren unit in eastern Oregon have 6D values of zoic batholith belt of western North America, Gray Wolf and Grand Valley lithic assemblages, -67, -64, and -65 per mil, respectively (Heller including the Idaho batholith (Criss and Taylor, Chuckanut Formation, and Puget Group sug- and others, 1985). These limited data suggest the 1983). In addition, Solomon and Taylor (1989) gests that these units derive from the same or a possibility that the source area for the units in indicated a belt of high <5lsO values in central very similar source. Units in the eastern Olym- northern Washington was different from that in Nevada that reflect the presence of thick Cordil- pic core likely are shallow-to-deep marine deposits and may be offshore equivalents of the fluvial Chuckanut Formation (Johnson, 1984a) and the deltaic Puget Group (Buckovic, 1979). +10.5 If so, then tectonic movements inferred by John- son (1984b) to explain the absence of offshore deposits related to the Chuckanut Formation are not required. A major finding of our study is that detritus derived from the Chuckanut and Puget Figure 9. S180 composition (in river systems may have been deposited on the per mil relative to SMOW) of white adjacent shelf, slope, and trench and were later micas from Paleogene sandstones incorporated into the subduction complex that of the Pacific Northwest. Data from makes up the core of the Olympic Mountains. Table 3. Although specific source area(s) for these sandstones are not identified, their general characteristics can be constrained well enough to eliminate some possible sources and to allow reconstruction of the transport system. On the Olympic basis of overall composition of the sandstone in Mountains the eastern Olympic core (lithic arkosic) and the

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coarse-grained nature and composition of the phosed granitic source rocks. Finally, interpreta- batholith as a significant source area. Transport white mica, we infer that the source included tion of fission-track ages and reset K-Ar ages in from either farther south, whether by trench axis plutonic or high-grade metamorphic rocks of the northern Okanagan area in southern British transport by turbidites or by relative plate mo- overall granitoid composition. Vance (1989) Columbia of between 45 and 60 Ma (Mathews, tions, is precluded by (1) the presence of several recognized distinctive kyanite-bearing sand- 1981; Sweetkind and Duncan, 1989; Sevigny depocenters along the trench axis that would stones of middle to late Eocene age, including and others, 1990) suggests that this potential likely dilute the more distal deposits, and, (2) the the Puget Group, that he linked to high-grade source area was rapidly uplifted during the time short time interval between deposition and ac- metamorphic sources in the southeastern part of of deposition of the Chuckanut Formation and cretion (<20 m.y.). the North Cascade Range in Washington. Pa- the units of the Olympic core. This apparent Sediments incorporated into the western leocurrent indicators in the Puget Group (Buck- match between composition of this source area Olympic core rocks could have been transported ovic, 1979) and the Chuckanut Formation and those of the eastern Olympic deposits, sug- south along a trench axis, but a source area to (Johnson, 1984a) indicate derivation from the gests to us that this region was an important the far north is not obvious. Rocks in western- east. Bucko vie (1979) proposed that Puget contributor of arkosic sediment now found in most Canada do not contain white mica in any Group sandstones were derived from the eastern the Eocene units of western Washington. A abundance and initial strontium-isotope ratios in Cascades or farther east. The rocks in the North similar conclusion was reached by Johnson western Canada are considerably lower than Cascades which supplied the kyanite probably (1984a), who proposed that the Okanagan those seen in the white micas of the Olympic did not supply the white mica because this min- region served as a major contributor of arkosic core (Armstrong, 1988). eral is uncommon in this source area, and, sediment to the Chuckanut Formation, primar- Our interpretation is that, like the eastern where present, its initial strontium-isotope ratios ily on the basis of sandstone composition and Olympic core, Western Olympic sandstones are far too low (Magloughlin, 1986). We have the reconstruction of the Chuckanut deposi- were derived generally from the Omineca Crys- not found kyanite in Olympic core rocks but tional system. talline Belt of southern British Columbia to the have found rare garnet and microcline, both east-northeast. The overall similarity of sand- probable indicators of metamorphic terranes. Provenance of the Western Part stone composition (Fig. 5) and mica chemistry Therefore, the Puget Group, Chuckanut Forma- of the Olympic Core (Table 2), and the colinearity of the Rb-Sr iso- tion, and Olympic core sandstones are of mixed topic values of white mica from the Western provenance, and the white mica came from Sandstones of the Western Olympic lithic as- Olympic lithic assemblage with those of sand- farther east or northeast. semblage are broadly similar in composition to stones in the eastern Olympic core (Fig. 7A) Mica chemistry, stable-isotope, K-Ar, and those in the eastern part of the Olympic core. suggest that similar, but not identical, source Rb-Sr data are compatible with source rocks in The most pronounced differences are the Rb areas were tapped. The K-Ar ages from micas in the Mesozoic and Cenozoic plutonic and meta- and Sr abundances and K-Ar ages for white the Western Olympic lithic assemblage are sig- morphic rocks that extend from the Idaho bath- micas, even though the Rb-Sr data are generally nificantly older than ages in the eastern Olym- olith up through western Canada (Armstrong, colinear with those of the other units. Isotopic pics. Although many exposed rocks in the 1988; Armstrong and others, 1977; Armstrong data from the Western Olympic lithic assem- Okanagan region have K-Ar systems reset to and Ward, 1991; Mathews, 1981; Medford and blage are significantly different from those of Late Cretaceous or early Tertiary age (Mathews, others, 1983; Fleck and Criss, 1985). Rb and Sr sandstones to the south, including the sandstone 1981; Sweetkind and Duncan, 1989), some values of white micas (Fig. 7) suggest that the of Megler and the Tyee Formation. Therefore, if K-Ar (muscovite) dates in metamorphic rocks source had very high initial 87Sr/86Sr ratios derived from the south, the sandstones of the of the Omineca Crystalline Belt are Early to (>0.710) and may have been Mesozoic in age. Western Olympic lithic assemblage likely have middle Cretaceous age (Parrish, 1979) as are In addition, K-Ar ages of the white mica indi- come from farther south than these penecon- U-Pb (zircon) ages in synkinematic granites of cate crystallization or thermal resetting during temporaneous units. the Monashee Mountains in southwestern Brit- latest Cretaceous to earliest Cenozoic time (Fig. A comparison of white-mica data with the ish Columbia (Sevigny and others, 1989). Other 8). The most likely source area is the southeast- Mesozoic plutonic belt of the western Cordillera suitable source rocks may have been exposed in ern margin of the Omineca Crystalline Belt in shows no reasonable match to the south. Iso- the Okanagan region during Eocene time. southern British Columbia. This region, just east topic compositions of white micas in the Idaho The subtle differences in K-Ar and Rb-Sr of of the Monashee Complex, is underlain by the batholith (Armstrong and others, 1977; Arm- white mica between the Western Olympic lithic Purcell Supergroup and older rocks of North strong and Ward, 1991) and its derived sand- assemblage and the eastern Olympic core rocks America (Armstrong, 1988; Sevigny and others, stones, such as the Tyee Formation and Herren may result from drainage reorganizations during 1989, 1990). The crystalline rocks include unit (Heller and others, 1985), are similar to the uplift of the Okanagan region and North Cas- white-mica-bearing plutons and high-grade Western Olympic lithic assemblage in sandstone cade Range. Regional extension and uplift are metamorphic rocks of Mesozoic age that have composition, but white mica from the latter has recorded in Eocene time throughout the Okana- been thermally reset to Late Cretaceous to early lower Rb and Sr contents, lower 87Sr/86Sr ra- gan region by the development of metamorphic Tertiary, as reflected by K-Ar ages, and most tios, and older K-Ar ages for contained white core complexes and associated structural fea- 87 86 importantly, contain very high initial Sr/ Sr mica (Table 3; Figs. 6, 7B). Farther south, the tures (Templeman-Kluit and Parkinson, 1986; ratios (>0.710; Medford and others, 1983; Arm- K-Ar thermal chronology (Evernden, 1970), Parrish and others, 1988; Sevigny and others, strong, 1988; Sevigny and others, 1989). Our S180 values of plutons (Masi and others, 1981) 1990). Uplift of the North Cascade Range took chemical and stable-isotope analyses of white and general lack of source areas for white mica place during middle to late Eocene time, as mica are consistent with regionally metamor- (Bateman, 1981) preclude the Sierra Nevada determined by K-Ar and fission-track ages

120° The Mesozoic metamorphic belt, including Figure 10. Paleogeograph- the Klamath Mountains, Blue Mountains, and ic reconstruction of the Pa- the North Cascade Range, was likely a pied- cific Northwest of the United mont area of relatively low relief at this time, States during early Eocene to because river systems flowing through this re- early Oligocene time. Major gion picked up only limited sediment Later up- Eocene sedimentary rocks lift of parts of the Mesozoic metamorphic belt (stippled pattern), accreted caused reorganization of the major drainage sys- volcanic rocks (v) of Eocene tems. This uplift probably began during latest age, and Mesozoic plutonic Eocene to early Oligocene time, which was rocks (+) and metamorphic characterized by major changes in composition rocks (ruled pattern) are and rate of sediment supply to coastal basins shown. Thick lines with ar- (Heller and others, 1987), development of re- rows represent major drain- gional erosion surfaces (Gresens, 1981), and up- age systems. Modern outline lift and erosion of the Cascade core. Tertiary of northwestern United -40° extension to the east also was responsible for States is shown with dotted migration and, in some cases, rotation of the pattern. Palinspastic distribu- volcanic fragments westward (Wells and Heller, tion of major tectonic ele- 1988), except for material outboard of the late ments from Heller and others Eocene trench which either continued to ac- (1987). crete, as did the Olympic core rocks, or migrated northward due to continued oblique conver- gence (Yakutat terrane).

CONCLUSIONS (Haugerud and others, 1988; Vance, 1989), the Paleogeography of the Continental Margin timing of deformation (Miller and Misch, 1963; The Olympic Mountains are a thick subduc- Johnson, 1985), the cessation of basin filling The Tertiary paleogeographic development of tion complex that formed, essentially at its pres- (Johnson, 1985), and the onset of regional ero- the Pacific Northwest, reconstructed in Figure ent location, along the leading edge of the sion (Gresens, 1981). Drainage reorganization 10, is based on the regional tectonic framework continent in early to middle Tertiary time. Prov- could lead to a slight change in isotopic charac- and available paleomagnetic studies (Heller and enance of clastic deposits in the accretionary ter of white mica by incorporating grains from others, 1987; Wells and Heller, 1988) and by wedge of the Olympic core was elucidated using an older and/or less radiogenic source. our provenance interpretation. The basis of the the isotopic provenance approach. The source reconstruction is that most of the rotation of the area for all of these deposits is characterized by Provenance of Micaceous Sandstones Oregon Coast Range is due to middle to late the composition of bulk sandstone and the iso- of the Yakutat Terrane Tertiary extension to the east, pivoting the range topic composition of the white mica. Abundant westward by clockwise rotation (Simpson and arkosic sandstone in most of the Eocene units of The provenance and evolution of the Yakutat Cox, 1977). This model is consistent with prov- the Pacific Northwest suggests a source of over- terrane is not well constrained by this study. The enance, paleomagnetic, and regional structural all granitic composition. Locally significant similarity in isotopic provenance of the Yakutat data (Heller and others, 1985; Wells and Heller, amounts of volcanic-lithic fragments suggest that terrane to the Western Olympic lithic assem- 1988). In our reconstruction, we presume that either volcanic-arc rocks or oceanic basalts were blage suggests a similar provenance. We suggest the volcanic basement beneath the Olympic also present in the source areas. Relatively that the Yakutat terrane lay somewhere close to, Mountains, the Yakutat terrane, Megler basin, abundant detrital white mica indicates that the possibly north of, the Olympic region during and the Oregon Coast Range are all part of a source included coarse-grained plutonic or high- middle Eocene time and tapped the eastern discontinuous seamount(?) chain that docked grade metamorphic rocks; phengitic muscovite source area(s) that later shed sediment into the against the early to middle Eocene trench in the compositions suggest the latter. Western Olympic lithic assemblage. The Eocene Pacific Northwest. By late Eocene time, the The source area for the white mica in all of basaltic basement below the sedimentary rocks trench stepped to a position outboard of most of the Olympic core rocks and the Chuckanut is probably of seamount origin (Davis and these volcanic fragments. Overlapping clastic Formation and Puget Group along the western Plafker, 1986), similar to that which accreted to deposits were derived from parts of the Meso- side of the Cascade Range is likely Mesozoic in Washington during Eocene time. The Yakutat zoic volcano-plutonic belt to the east. The Tyee age (Rb-Sr), and it underwent a tectono-thermal terrane likely represented the northern limit of basin deposits were derived mostly from the resetting event(s) in Late Cretaceous time the collided seamount province which includes Idaho batholith region, whereas the units to the (K-Ar). Initial strontium-isotope ratios are very the Crescent Formation basalts in the Olympic north were derived mostly from the Okanagan high (>0.710), and ôI80 values are moderately Mountains. By late Eocene time, or later, the region, which was undergoing rapid uplift and high (~+9). The most likely source area that fits Yakutat terrane began to migrate northward erosion during Eocene time due to regional these isotopic values and that contains moder- toward its present site. extension. ately abundant phengitic white mica is the

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Okanagan crustal shear in southern British Columbia: Canadian Jour- Snavely, P. D., Jr., and Wagner, H. C., 1963, Tertiary geologic history of nal of Earth Sciences, v. 14, p. 318-321. MANUSCRIPT RECEIVED BY THE SOCIETY JULY 23,1990 western Oregon and Washington: Washington State Division of Mines Van Alstine, D. R., Bazard, D. R., and Whitney, J. W., 1985, Paleomagnetism REVISED MANUSCRIPT RECEIVED JULY 12,1991 and Geology Report of Investigations, v. 22, p. 25. of cores from the Yakuiat well. Gulf of Alaska [abs.]: American Geo- MANUSCRIPT ACCEPTED JULY 18,1991

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