Field trip to the Wells Creek volcanics, North Fork Nooksack River

By Dave Tucker. February 4, 2010

The Wells Creek Volcanic Member of the Nooksack Formation (Tabor and others, 2003; see refs below) is found in the vicinity of Nooksack Falls, east of Glacier, Washington. These Middle Jurassic island arc rocks form steep yellowish cliffs along the Mount Baker Highway (Route 542) just east of the Excelsior Pass trailhead, where they crowd the highway against the deep gorge of the North Fork Nooksack River. If you have driven up the highway beyond Nooksack Falls, you have passed through the Wells Creek volcanic rocks. Tabor and others (2003) assigned these dacite and andesite lava flows, associated breccia, and tuffaceous rocks to the lowest member of the Nooksack Formation. This guide will showcase some exposures of these rocks, which erupted as part of an island chain in the Pacific Ocean about 180 million years ago (U-Pb dating; data in Tabor and others, 2003). The volcanics have been mined for metals and for beautiful massive greenstone (low-grade metamorphic lava) used as crushed rock and for retaining walls. The trip visits

• lava at Nooksack Falls and in a nearby quarry,
• a mine with acid-sulfide altered rocks along the Baker Highway,
• some paper- thin foliated slate up the road, part of the sedimentary rocks associated with the marine volcanics.

Significance: The Nooksack Formation is at the base of a stack of accreted terranes that make up the Northwest Cascades System (Brown, 1987). This is a group of fault-bounded terranes, accreted to the margin of North America during subduction of the seafloor, and stacked by thrust faults. Though the Nooksack is at the bottom of the stack, older rocks (Excelsior and Welker Peak) are thrust over top of them. Thus, the Wells Creek rocks are at the base of this stack in the northern Cascades. What lies beneath them is not known for certain.

The Wells Creek Volcanics Member is the unit labeled ‘Jwc’. Adapted from Tabor and others, 2003. Click to enlarge any figure.

Getting there: There are several stops in the field guide. Start out by going east on the Mount Baker Highway, continuing beyond the town of Glacier. The western-most outcrops of Wells Creek volcanics begin about 0.5 miles east of Mile Post 39, but they aren’t much to look at. They may have been prettier in Misch’s day; he describes them as weakly schistose (foliated) altered andesite breccia and massive greenstone (Misch, 1977).

To reach Nooksack Falls: turn right 0.6 miles east of mile post 40 (miles east of the junction with I-5) on the Wells Creek road (Forest Service Rd 33, gravel). Reach the falls in about 0.5 mile. Stop to see the mighty falls, and the rock upstream of the bridge if the river level is low, AND if the rocks aren’t too slick.

To the greenstone quarry: the road is gated just beyond the bridge at the falls, and usually opens July 1. Walk or bicycle beyond the gate 0.5 mi (0.8 km) to the obvious spur on the left. Walk up that a couple hundred feet into the small quarry.

To reach the exposures above the Nooksack Gorge and the mining prospect: continue east along the highway past the turn to Nooksack Falls. There are a couple of small pullouts on the south (river side) in 0.5 mile, beyond the Excelsior trailhead (outhouse at the trailhead). Use these pullouts, preferably the first. There is a larger pullout on the opposite side of the highway, too.

To the slate outcrop: continue another 0.5 mile beyond the highway pullouts at the old mine. The pullout is on the north side of the highway, so use extreme caution.

The only guide to the Wells Creek Volcanics I know of is part of a guide written by the venerable Peter Misch (Misch, 1977) for the 1977 GSA annual meeting. The description in Tabor and others (2003) has limited detail about the rocks themselves. There is an overview of the Nooksack Formation in Tabor and Haugerud (1999).

Nooksack Falls. Flow banding in the lava is visible at center, as dark horizontal bands on the pale rock. View from the end of the overlook trail.

Let’s start at Nooksack Falls, which drop 88 feet over a wall of Wells Creek lava. Before we look at the geology, I need to bring up grim facts. About a dozen people have been killed by going over the falls in the past half century. Most were probably trying to get a better view of the falls, although one woman went over when she slipped while brushing her teeth on the river bank. The interpretive sign at the falls parking lot has a list of victims, 1947 through 1999- there were earlier victims, and there have been more since. IT IS NOT WORTH YOUR LIFE to climb over the  fences in the hope of getting an even better view- there just aren’t any, unless you come cross country to the falls from the Deadhorse Road, not an easy task. Also, if you try to lay hands on the rock on the upstream side of the bridge, be very wary- they are smooth and usually slick from spray. A slip will be fatal. Vignette 74 in Geology of the North Cascades (Tabor and Haugerud, 1999) discusses very briefly the rocks here, but talks more about the erosional dynamics of the falls. Walk to the viewpoint on the north bank of the river to see the falls. With decent light, you will see thin dark horizontal bands in the gray rock between the splits of the falls. These are minerals or perhaps tiny vesicles aligned during flow of this lava 180 million years ago. Note that the rock tends to fracture along joints formed parallel to the flow banding. You can observe this as well, and a great variety of alluvial boulders, just upstream of the bridge…but if you venture close to the river channel, recall the warnings about being swept over the falls!

At the gate, just around the corner from the falls, a mining adit goes into the rock. Somebody thought of exploiting minerals along the hydrothermally altered rock of a joint (fracture) here. They probably met with little or no success, as the shaft is short, dark, low, and wet, and there’s little to see inside anyway.

The greenstone quarry on the Wells Creek Road. Bicycle for scale.

Go past the gate, afoot, abike, or acar (I know, that’s not a real word) if the gate is open. At the top of the second switchback, 0.25 mile (400 m) from the gate is a small exposure of really ratty-looking yellowish, hydrothermally altered rocks with nearly vertical foliation.  Nothing much to see here, other than to marvel at  the effect water, heat, and some pressure has on formerly perfectly respectful volcanic rocks. Sigh, ruined. One half mile beyond the gate (see instructions above) is a quarry in lovely greenstone. The 60 foot quarry face was active until about a decade ago. The rock here is a beautiful pure green. It is a low grade, nonfoliated metamorphic rock; the parent may have been the andesite or dacite lava ascribed to the Wells Creek member, but greenstone is generally believed to result from low- T metamorphism of more mafic basalt. I’ll buy andesitic lava as the parent.

Greenstone at the quarry. The white spots are lichens, not minerals. The dark spot near the pencil was probably pyroxene at one time. This rock was originally lava on the seafloor.

Greenstone is a greenschist facies rock. High pressure and warm fluids metamorphosed the original minerals into green minerals- epidote, chlorite, or actinolite. The rock here retains much of its original porphyritic texture- you can still see some glistening plagioclase grains, and the dark blobs were probably pyroxenes originally.

Go back down the road, out to the highway, and turn right (east). The next stop is just beyond the well-marked Excelsior trailhead. Pull over on the right at the first pullout. Enjoy the view straight down into the gorge of the Nooksack, cut into Wells Creek rocks all the way to the river.  The rocks here are mangled greenstones, faulted and jointed and extensively acid-sulfide altered. There is a lot of fine pyrite in most hand samples you will look at. This iron sulfide mineral is a pretty good indicator of sulfurous hot water circulating through hot rocks, perhaps a submarine hydrothermal vent, or during subduction and heating. Cross the road and head for the west edge of the little brushy indentation by the north side pullout.

The filled-in prospect along the highway, 0.1 mile east of the Excelsior trailhead. Lots of pyrite in these rocks.

Just beyond the brush screen is a filled-in mining prospect under the overhang; a single wooden prop is visible sticking out of the oozing mud. Just outside the entrance is a heap of slick gray clayey mud, perhaps the mine tailings. The gray rock is shot through with quartz veins, and glistens with tiny pyrite crystals. Silver is often associated with these types of metamorphic rocks, and perhaps that is what the miners hoped to find. The  Great Excelsior mine (Wolfe and others, 2004) in Wells Creek breccia across the Nooksack near the falls, produced gold until about 1917. Before you leave this place, cross the highway to the concrete wall, and look over the edge. Eeeeeeeeeeeeeeehaw! Took a lot of engineering there to hold up this highway!

The last stop is another 0.5 mile up the highway; you have to cross the highway and park on the north side. No volcanics there, but I couldn’t resist showing you the wafer-thin foliation in the slate at this rock wall.

Vertical foliation (flattened and aligned microscopic minerals) in this slate is paper thin.

This low grade metamorphic rock was at one time seafloor mud interbedded with the Wells Creek volcanic rocks, back when volcanism and sedimentation was occurring at a chain of marine volcanoes. The easternmost roadside exposure of Wells Creek rocks is only 0.25 miles beyond the slate. When you are done with the slate, look across the river at the wonderful columns of the Pinus Lake andesite, a Pleistocene lava flow that predates Mount Baker. We’ll go there another day, to lie on the brink and look DOWN that wall of lava columns!

REFERENCES

Brown, E., 1987, Structural geology and accretionary history of the Northwest Cascade System of Washington and British Columbia: Geological Society of America Bulletin, v. 99, p. 143-153.

Misch, P., 1977, Bedrock geology of the North Cascades. IN Brown, E. H.; Ellis, R. C., editors, Geological excursions in the Pacific Northwest: Western Washington University, p. 1-62.

Tabor, R., Haugerud, R., Hildreth, W., and Brown, E., 2003, Geologic map of the Mount Baker 30- by 60-minute quadrangle, Washington: Geologic Investigations Series I-2660. US Geological Survey. The map, cross sections and explanatory pamphlet are available online.

Wolfe, F., McKay, D., Jr., Norman, D.,and Brookshier, M., 2004,   Inactive and abandoned mine lands- Great Excelsior Mine, Mount Baker mining district, Whatcom County, Washington: Washington Division of Geology and Earth Resources Open File Report 2004-5.