ABSTRACT. The Cowlitz and Hamlet Formations are deltaic to deep-marine units that fdled a forearc basin in western Washington and Oregon during the ...
MAGNETIC STRATIGRAPHY AND TECTONIC ROTATION OF THE UPPER MIDDLE EOCENE COWLITZ AND HAMLET FORMATIONS, WESTERN OREGON AND WASHINGTON DONALD R. PROTHERO DEPARTMENT OF GEOLOGY, OCCIDENTAL COLLEGE, LOS ANGELES, CA 90041
ELIZABETH NESBITT BURKE MEMORIAL MUSEUM, UNIVERSITY OF WASHINGTON, SEATTLE, WA 98195
ALAN R. NIEM AND DERIK KLEIBACKER DEPARTMENT OF GEOSCIENCES, OREGON STATE UNIVERSITY, CORVALLIS, OR 97331
ABSTRACT The Cowlitz and Hamlet Formations are deltaic to deep-marine units that fdled a forearc basin in western Washington and Oregon during the middle Eocene. The Cowlitz Formation is important for economic reasons, since the unit contains not only coal but also the only significant gas field in the Pacific Northwest. Dating of the Cowlitz Formation is crucial to the understanding of Pacific Coast Eocene faunas, since this unit is the basis of the "CowlitzCoaledo fauna" of the Weaver et al. (1944) time scale. Magnetic sampling of over 300 samples from 104 sites in seven different localities in northwest Oregon and southwest Washington yielded a stable remanence held mostly in magnetite, with slight overprints of hematite. Three sections are calibrated by ^Ar/^Ar dates, while the rest are intercorrelated by means of their molluscan and microfossil faunas and stratigraphic relationships. Some sections (e.g., the type section at Olequa Creek, or the section at Coal Creek, both in Washington) could be as young as Chron C16r, or 36.5 Ma (earliest late Eocene), while other sections (e.g., Germany Creek in Washington or Columbia River Mainline in Oregon) are as old as Chron C18r, or 41.0 Ma (middle middle Eocene). The Hamlet Formation is correlated with Chron C19r (42.0 Ma). All sections show clockwise tectonic rotation, some as low as 55 ± 9°, but others as great as 103 ± 7°; these results are consistent with the variable range of rotations reported in other Eocene units in western Oregon and southwestern Washington. Most of the localities show no significant northward translation. INTRODUCTION The Cowlitz and Hamlet Formations are marine and marginal marine units which filled the Oregonin Prothero, D R., ed., 2001, Magnetic Stratigraphy of the Pacific Coast Cenozoic: Pacific Section SEPM (Society for Sedimentary Geology), Book 91
Washington forearc basin during the middle to earliest late Eocene. After accretion of the basaltic seamount and oceanic crust province (i.e., Crescent and Siletz River volcanics) during the early and middle Eocene, the Cowlitz and Hamlet Formations represent large deltas and marine deposits prograded into the basin (Henriksen, 1956; Wells, 1981; Wells and Coe, 1985; Niem et al., 1992). In many places, the Cowlitz Formation is overlain by and locally interbedded with volcanic units, such as the Grays River volcanics in Washington (Phillips, 1987a, 1987b). In Oregon, the Cowlitz and Hamlet formations are interbedded with Grays River volcanics and are intruded by nearly contemporaneous Cole Mountain basalt (Kenitz, 1998; Niem et al., 1992, 1994; Niem and Niem, 1985, Fig. 3). The Cowlitz Formation was first named by Weaver (1912) for fossiliferous sandstones and siltstones exposed along the banks of the Cowlitz River near Vader, Washington (Fig. 1). In 1937, Weaver formally defined the type section of Cowlitz Formation as the 1200 m of exposures along Olequa Creek, and the 61 m of section at the Big Bend locality east of Vader. Henriksen (1956) expanded the definition of the Cowlitz Formation to include about 1670 m along both Olequa Creek and Stillwater Creek. Following Weaver's (1937) original description, Wells (1981) restricted the definition of the Cowlitz Formation to Henriksen's (1956) Olequa Creek Member, and referred the Stillwater Creek Member of Henriksen (1956) to the Mcintosh Formation. Recent mapping and section measurements by Payne (1998), Kleibacker (in prep.), and McCutcheon (in prep.) have informally subdivided the Olequa Creek member of the Cowlitz Formation into five units in Washington. In southwestern Washington, the Cowlitz
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organic-bearing siltstones, and claystones (unit 4 of Payne, 1998). These units are massive and intensely bioturbated. Shallow marine molluscan fossils are well preserved in this unit, but only locally abundant. A deep-marine laminated micaceous mudstone with some micaceous arkosic turbidite sandstone associated with channelized mudstone debris-flow deposits caps the sequence at the type section (unit 5 of Payne, 1998). Warren et al. (1945) and Warren and Norbisrath (1946) recognized the Cowlitz Formation in the Nehalem River Basin of northwest Oregon (Fig. 2), and Van Atta (1971) described it in greater detail, dividing it into an informal upper siltstone (mudstone) member about 70 m in thickness, and a lower sandstone member about 180 m in thickness. This friable arkosic micaceous sandstone has since been informally named the C&W sandstone member (after the Clark & Wilson exploration well—Bruer et al., 1984). The C&W sandstone is interpreted as a wave-dominated deltaic deposit formed on a highenergy shelf, with hummocky cross-stratification, soft-sediment deformation, minor coal beds, and carbonaceous bioturbated mudstones. Although it contains some shallow-water trace fossils, shelly fossils are absent (except as molds) due to leaching by groundwater (Berkman, 1990). In the subsurface, the C&W sandstone is overlain by the 350 m-thick deep-marine upper Cowlitz mudstone that contains bathyal late Narizian foraminifera (McKeel, 1983; McDougall, in Robertson, 1997; Niem et al., 1992). Underlying and laterally interfingering with the Cowlitz Formation in northwest Oregon is an informal unit described by Rarey (1986) as the Hamlet Formation (Fig. 3). As mapped and described by Niem and Niem (1985), Mumford (1988), Mumford and Niem (1992), and Niem et al. (1994), the Hamlet Formation consists of three informal members: 15 m of basal fossiliferous basaltic conglomerate and sandstone known as the Roy Creek member; a nearshore marine Sunset Highway member, consisting of about 40-300 m of micaceous lithic arkoses and subordinate basaltic sandstones, with shallow marine fossils such as Solen (razor clams) and other molluscs; and an upper Sweet Home Creek member, consisting of some thinly bedded micaceous arkosic turbidite sandstones within thick sequences of deepwater micaceous mudstones and siltstones which reach more than 1000 m in thickness in the subsurface (Niem and Niem, 1985; Niem et al., 1994). These slope mudstones are a lateral facies of the shelfal-deltaic C&W sandstone and the Sunset
Grays River volcanics
Mcintosh Formation
Figure 1. Index map showing general geology of the Eocene rocks in Washington (modified from Nesbitt, 1994). Formation consists of friable micaceous arkosic sandstone and subordinate siltstone, and volcaniclastic sandstone. The marginal marine facies includes massive bioturbated sandstones, with thin-bedded siltstones and thin coal seams. The Cowlitz Formation also yields brackish water, freshwater, and shallow marine molluscs (Nesbitt, 1995). In the lower parts of Olequa and Stillwater Creeks, the marine facies also crops out. This facies consists of fossiliferous micaceous glauconitic sandstones, with 76
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Highway member to the east (Fig. 3). In northwestern Oregon, the Hamlet Formation unconformably overlies the middle Eocene Tillamook Volcanics, and is unconformably overlain by the upper Eocenelower Oligocene Keasey Formation, and locally by the Cowlitz C&W sandstone member (Fig. 3). The Cowlitz Formation and its faunas have long been important, because they were the basis of the informal "Cowlitz-Coaledo" molluscan stage of Weaver et al. (1944). The age constraints on this fauna have been relatively imprecise, except that it was known to be middle Eocene in age, and also thought to be correlative with the late Narizian benthic foraminiferal stage (Nesbitt, 1995). However, the middle Eocene is 11.4 million years in duration (37.6-49.0 Ma, according to Berggren et al., 1995), and the Narizian stage spans most of the middle Eocene (Prothero, this volume), so these age assignments are not very exact. In addition, the Cowlitz faunas represent the last of the high-diversity tropical faunas of the middle Eocene "greenhouse" cli-
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Figure 3. Stratigraphic relationships between the Cowlitz and Hamlet Formations in northwest Oregon (after Niem et al., 1994)
77
C&W Sandstone was measured along a logging road of that name, which parallels Fall Creek. This section was described by Berkman (1990) and pictured by Niem et al. (1994, Fig. 29). The US 26 section of the Hamlet Formation, exposed in road cuts along that highway near Quartz Creek, was described by Mumford (1988) and Safley (1989), and illustrated by Niem et al. (1994, Fig. 27). The Elsie-Jewell Highway section that parallels the Nehalem River follows the sections measured by Nelson (1982). All these measured and sampled sections of Cowlitz and Hamlet formations are partial sections and represent a composite of these units. There may be more overlap and age refinement between sections if future sampling extends these sections to include the full thickness of both units. In the laboratory, the block samples were cored into cylinders on an air-cooled drill press. Samples that were too small or poorly indurated were cast into cylinders of Zircar aluminum ceramic. Measurements were made on a 2G cryogenic magnetometer equipped with an automatic sample changer. After measurement of NRM (natural remanent magnetization), samples were demagnetized at alternating fields (AF) of 25, 50, and 100 Gauss to determine the coercivity behavior of the magnetization, and to demagnetize any multi-domain grains. All samples were then thermally demagnetized at 300, 400, 500 and 600°C to remove overprints held in iron hydroxides, and determine how much remanence is held above the Curie point of magnetite. About 0.1 g of powdered rock from several samples was subjected to increasing isothermal remanent magnetization (IRM) to determine their IRM acquisition behavior. These were also AF demagnetized twice, once after having acquired an IRM produced in a 100 mT peak field and once after having acquired an anhysteretic remanent magnetization (ARM) in a 100 mT oscillating field. Such data are useful in conducting a modified Lowrie-Fuller test (Johnson et al., 1975; Pluhar et al., 1991).
mate before the late Eocene cooling (Nesbitt, 1995, in press). There are also economic reasons for studying the Cowlitz Formation. It is historically one of the more important coal-bearing units in Washington, as indicated by names such as Coal Creek (Weaver, 1937). In addition, the only major commercial gas field in the Pacific Noithwest (the Mist gas field, near Mist, Oregon) produces out of the Cowlitz Formation (Bruer, 1980; Armentrout and Suek, 1985; Alger, 1985; Niem et al., 1994). Cowlitz reservoir sandstones have average porosities of 25% and permeabilities of 200 md (millidarcys) (Armentrout and Suek, 1985). Ever since the discovery of the Mist gas field in 1979, over a dozen oil companies have drilled this potentially important energy resource. Precise correlation of the Cowlitz Formation to the global time scale, and to other similar-aged units in the Pacific Northwest, could be of great help in future petroleum exploration. Finally, magnetic data from these units are important for tectonic reasons as well. Most of the Eocene and Oligocene volcanic units of the Oregon and Washington Coast Ranges show a clockwise tectonic rotation of as much as 70-80° (Beck and Plumley, 1980; Magill and Cox, 1980; Magill et al., 1981; Wells et al., 1984; Wells and Coe, 1985; Wells and Heller, 1988; Wells, 1990). Paleomagnetic studies of the sedimentary rock units overlying and underlying these volcanics also show clockwise rotations, some by as much as 105° (Simpson and Cox, 1977; Bates et al., 1981; Prothero and Hankins, 2000; Prothero and Donohoo, 2001; numerous papers in this volume). However, Wells and Coe (1985) showed that the degree of rotation even within the same unit can vary widely from one small tectonic block to another, so it is important to define as many discrete paleomagnetic poles as possible to refine precisely when, where, and how much rotation has taken place within each tectonic block. METHODS Paleomagnetic samples were taken as oriented blocks of rock (3 samples per site) as densely as outcrops and exposures allowed. The Olequa Creek section follows the maps and sections of Weaver (1937) and Payne (1998). However, only units 3, 4, and 5 of Payne (1998) could be sampled. The Big Bend section follows Nesbitt (1995, Fig. 2). The Coal Creek section follows Weaver (1937). The two Germany Creek sections are described by Kleibacker (in prep.). The Columbia River Mainline section of the
RESULTS Rock magnetic analysis— Orthogonal demagnetization ("Zijderveld") plots of representative samples are shown in Figure 4. For both normal and reversed samples, a single-component remanence was held largely in magnetite, since the samples showed a low coercivity and lost all remanence above the Curie point of magnetite, 580°C (Fig. 4A-D). A few samples showed a higher coercivity, but since their remanence was gone by 78
cal component. AF = alternating field step (in Gauss); TT = thermal step (°C). Each division = 10 7 emu.
79
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Magnetic FIM (mT) Figure 5. IRM acquisition (ascending curve on right) and Lowrie-Fuller test (two descending curves on left) of representative powdered samples from the Cowlitz Formation. Open squares = IRM; solid squares = ARM. 600°C, this was probably due to overprinting by iron hydroxides (Fig. 4E). A few samples had an overprint which was removed by 300°C (Fig. 4F). IRM acquisition analysis (Fig. 5) showed that the samples nearly saturated at about 300 mT (millitesla), which is consistent with our results showing that magnetite is the primary carrier of remanence. But a slight increase in IRM intensity shows that some hematite was present as well. The modified Lowrie-Fuller test showed that the ARM was more resistant to AF demagnetization than the IRM, suggesting that the remanence is held in single-domain or pseudo-single-domain grains (Fig. 5). The mean directions for each site were estimated using the least squares method of Kirschvink (1980), and then calculated using Fisher (1953) statistics (see Table 1). As shown in Figure 6, the mean normal directions for each locality are antipodal to the reversed directions for the same localities. This positive reversal test shows that the overprinting has been removed, and the primary or characteristic remanence has been obtained.
Means of normal sites shown by capital letters; means of reversed site shown by lower case letters. Solid circles indicate ellipse of confidence of normal sites in the lower hemisphere; dashed circles indicate the ellipse of confidence of reversed sites in the upper hemisphere. Symbols as follows: C, c, normal and reversed means from Coal Creek; f, Columbia River Mainline; g, Germany Creek; N, Elsie-Jewell Highway; O, o, Olequa Creek; Q, q, Hamlet Formation at US 26. As can be seen from the plot, most means overlap in their circles of confidence, and the normal means are antipodal to the reversed means, and show a strong clockwise rotation.
Magnetic stratigraphy— Each site was ranked according to the scheme of Opdyke et al. (1977). Results for the type section at Olequa Creek are shown in Figure 7. The base of the section of unit 3 of Payne (1998) that contains a pumiceous lapilli tuff layer 40Ar/39Ar dated at 38.9 ± 0.01 Ma (Irving et al., 1996) is normal in polarity. Much of the overlying unit 3 is reversed in polarity. The remaining upper part of the section (upper unit 3 through unit 5) was normal in polarity. The short 80
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Figure 7. Magnetic stratigraphy of Cowlitz Formation on Olequa Creek and Big Bend (stratigraphy after Payne, 1998, and Nesbitt, 1995). Solid circles indicate Class I sites (Opdyke et al. 1977), which are statistically distinct from a random distribution at the 95% confidence level; dashed circles are Class II sites, in which one sample was missing or rejected, so statistics could not be calculated; open circles are Class III sites, where one site was divergent, but the other two gave a clear indication of polarity. Creek sections is shown in Figure 8. Each section is capped by a Grays River volcanic subaerial basalt flow 40Ar/39Ar dated at 39.53 ± 0.37 Ma and 40.09 ± 0.34 Ma in sedimentary strata of normal polarity. The remaining lower part of each is mostly reversed
60 m section at Big Bend (Fig. 7) is entirely normal in polarity. Based on Payne's (1998) mapping, it is correlated with the upper part of the normal magnetozone in unit 4. The magnetic stratigraphy of the two Germany 81
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