A 10-meter-thick Mazama ash deposit in a Skagit River valley landslide-dammed lake

Bob Mooers at the outcrop beside the highway. Click any image to enlarge.

About 10 meters (What? 10 METERS?!) of Mount Mazama volcanic ash, erupted in the climactic Crater Lake caldera collapse, is preserved beside State Highway 20 in the Skagit River Valley. This field trip is based on a 2001 paper by Jon Riedel, the geologist at North Cascades National Park, and his colleagues Pat Pringle and Robert Schuster. This is a pretty quick stop, you can take a break from the drive (or ride) next time you are up the valley doing something else. I visited with my old friend Bob Mooers, author of a guidebook to western Washington waterfalls.

Getting there: The ash deposit is in a roadcut between Marblemount and Newhalem on WA Highway 20. From Marblemount, head up the valley. Beyond the entrance sign to North Cascades National Recreation area, watch for mile post 114 in another 2 miles. The roadcut is on  the north side of the highway 0.7 miles (1.1 km) further. There is a nearly hidden gravel road taking off to the north immediately beyond the roadcut, and a sort-of pullout on the river side of the road just beyond that. Park and walk back along the road a very short way to the outcrop. From Newhalem, drive west and watch for mile post 115 at Damnation Creek (the creek is often not signed – I’ll tell you a story about that at the end). Pull off on the little spur road 0.3 miles (0.5 km) west of the mile post.

The silt-sized homogeneous ash.

The 10-12 m high bank along the highway is all soft sediment. The roadcuts extend both north and south of the spur road for about 200 m, but the best place to examine the deposits is just west of the spur. I know, you can’t resist putting your nose on the outcrop, so go ahead and scramble up onto it- you can get pretty far up the bank by kicking steps. But first, A WORD OF CAUTION: the sediment here is almost pure rhyolitic volcanic ash; that means it consists of shards of abrasive siliceous glass. So, be careful you don’t get it into your camera or hand lens or eyes. Now, forewarned, grab a handful and notice how homogeneous and well sorted the deposit is, and light in color. What is the size of these grains? Use the classic field geologist’s test – put a few grains in your mouth. Sand-sized grains (between 1/16 and 2 mm) crunch in your teeth. Silt (1/256 – 1/16 mm) feels gritty on the tongue, but not so in your teeth. Clay-sized grains are too fine to feel in your mouth. Go ahead, test the sediment.The ash is about 10 meters thick along the road. A section on the other side of the river nearby suggests that there are another 8 meters or so of ash below the highway grade.

Scanning electron microscope image of pumiceous Layer O ash shards (collected on Mount Baker). Field of view is 200 microns across, or 2/100 mm.

Jon Riedel and his co-authors had this remarkable thickness of volcanic ash chemically analysed using the microprobe at Washington State University. It is virtually a dead ringer for the Mazama ash, also known as ‘Layer O’. This is the ash erupted from the Crater Lake caldera during its cataclysmic collapse, 600 km (360 miles) to the south of the Skagit River. The eruption showered a distinctive layer of ash a few centimeters thick throughout the northwest, easily recognized with only a little practice. In the immediate vicinity, Layer O is 2 cm thick when undisturbed. The ash layer provides a great time reference when it is found undisturbed in soils: the most commonly used published radiocarbon age for the eruption is 6845 radiocarbon years before present (¹⁴C years BP; Bacon, 1983), although another well-known determination is 6730 ¹⁴C years BP (Hallet and others, 1997). In calendar years, the eruption date is usually given as about 7640 years ago. The three calibrated ages derived from ¹⁴C dates of the Skagit River deposit yield 7880 (below the ash), and 7580 and 7220 calendar years ago from the ash layer. But, how did 2 centimeters grow into a deposit 10 meters thick?

Map showing Damnation Creek landslide. Mapping adapted from Riedel and others (2001); map base from Google Earth. Scale in lower left.

The study’s authors determined that a landslide fell from the north slope of the valley to dam the Skagit River.  The Damnation Creek landslide began at an elevation of 700 m (2300′), and fell about 580 m into the river valley. The deposit occurs just downriver from the thick accumulation of ash, and consists of blocks of angular schist, some of which are several meters across. The slide was big, piling up about 40 m (130 feet) above the river, and stretching around 1 km across the valley floor. The lake that resulted from the landslide was named ‘Lake Ksnea’ by the study authors, after the native name for a nearby location. The lake was estimated to be 30-40 meters deep at the downstream end, and stretched up the valley for 14 km (8.4 mi), to about the mouth of Gorge Creek at modern Gorge Dam. The lake began to fill with sediment. Three hundred years later, when the Crater Lake eruption dumped ash all over the countryside, the fine ash was promptly washed into the lake. Very little of the lake sediment is known to have survived, but fortunately a thick sequence is left where Damnation Creek flowed into the lake. Perhaps the ash fell on snow (other studies have suggested that the eruption occurred in fall or early winter); this makes sense as the deposit is so very homogenous, with little contamination by other alluvium. Riedel and others (2001) conclude that the ash was washed into the lake in only one or perhaps a few snow melt events in a single year. They also estimated the watershed drainage area of the lake to be 3300 km². In this area, assuming a primary tephra thickness of 2 cm, some 66×10⁶ m³ of ash was available for remobilizaton and sedimentation into the lake. The river has cut all the way down through the landslide deposit, but the study authors make no assumptions about how long the  lake may have persisted following deposition of the ash. Shovel Spur is a well-known rapid on the river at mile post 114,  downstream of the roadside deposit. The Skagit River in this area has some rowdy sections as it flows over and around boulders left behind by the landslide. (If you are a waterfall fan, buy my field partner Bob Mooers’ waterfall guide, which includes a description of this little rapid.)

The Skagit River forms a nice V as it races through Shovel Spur, a rapid at the site of the Damnation Creek landslide. Mile post 114.

Now, about that absent Damnation Creek highway sign. When Highway 20 was opened over the mountains, the new Department of Transportation sign at this creek crossing said ‘Darnation Creek’. Well, that didn’t wash with some persnickety people who insisted on accuracy (‘Damnation Creek’  predates the highway), so home made signs with the correct name began to appear. This was too much for the DOT, which removed the crude signs and put back ‘Darnation Creek’ signs. The cycle persisted for several years in the 1970s that I know of, but finally official signs with the correct name appeared. The debris flows of October 20, 2003 wiped out the road at Damnation Creek, and took the sign with it. The road was rebuilt, but the sign has not reappeared.

References:

Mini hoodoos in the ash, an erosional feature.

Bacon, C.R., 1983, Eruptive history of Mount Mazama and Crater Lake Caldera, Cascade Range, U.S.A.: Journal of Volcanology and Geothermal Research, v. 18, p. 57-115.

Hallet, D. J.; Hills, L. V.; Clague, J. J., 1997, New accelerator mass spectrometry radiocarbon ages for the Mazama tephra layer from Kootenay National Park, British Columbia, Canada: Canadian Journal of Earth Science, v. 34, p. 1202-1209.

Pringle, P, and Scott, K.M., 2001, see the last page of text at http://www.dnr.wa.gov/Publications/ger_psrc01_pringle_scott.pdf

Riedel, J. L.; Pringle, P.T.; Schuster, R. L., 2001, Deposition of Mount Mazama tephra in a landslide dammed lake on the upper Skagit River, State of Washington, U.S.A.: In White, J. and Riggs, N. eds., IAS Special Publication No. 30: Volcaniclastic sedimentation in lacustrine settings, p. 285-298. A portion of the paper is accessible via Google Books.