Volcanic ash interbedded in Chuckanut at Clayton Beach

VOLCANIC ASH AT CLAYTON BEACH, AND THE AGE OF THE CHUCKANUT FORMATION

By Dave Tucker. Posted Dec. 20, 2009

George Mustoe at the Clayton Beach tuff exposure

A thin bed of tuff (lithified volcanic ash) is preserved in the Chuckanut Formation, in a small cliff face at Clayton Beach. The beach is at the south end of Larrabee Park along Chuckanut Drive. Minerals in tuffs could be the ideal material  for dating the Chuckanut Formation; only one tuff dates has been published, and it is not from this location. An age of 49.9 give or take 1.2 million years was obtained from a tuff bed up on Lookout Mountain (east of Lake Samish) which is stratigraphically 2500 meters above the base of the Chuckanut. The 6-cm-thick Clayton Beach tuff is much closer to the base of the formation. Grains of the mineral zircon were collected from the Clayton Beach tuff by Sam Bowring, and were subjected to U-Pb radiometric dating. An age of 57 million years was obtained,  but according to Sam Johnson’s 1980s PhD research, all stratigraphic members of the Chuckanut Formation contain detrital zircon grains that give fission track ages as old as 56 Ma (million years old). ‘Detrital zircons’ are mineral grains that weather out of rocks and are deposited with a mix of sediments- they are not valid for accurate age determinations. George Mustoe reports “That age (57 Ma) is an important constraint, since it is the cooling date for igneous rock that was uplifted and weathered to provide a sediment source for the Chuckanut. To accept Bowring’s age, you have to throw out  Johnson’s numerous dates. I know Sam Bowring only was able to extract a tiny amount of material from the (Clayton) ash to get his U-PB date, and I wonder if he was possibly measuring some detrital grains, not phenocrysts from the tephra.”

The Clayton tuff is interbedded with steeply- bedded siltstone, and is overlain by a thin layer of black, coalified soil. Apparently, the ash fell onto flood deposits or perhaps a dry lake bed, and was then covered by a layer of leaves. If the U-Pb age of 56 Ma is due to contamination with detrital zircons (rather than zircons erupted from the volcano that produced the ash) then the ash layer is almost certainly not primary, as it was mixed with other sediment, and it was washed onto the sediments before burial and lithification. If the coal was an ancient soil, I’d think that the tuff would show more sign of disruption by burrowing animals or plants growing in the soil- no soil is preserved beneath the tuff. I’m open to comments or suggestions on this point. If you visit this place, PLEASE DON’T PICK AT THIS OUTCROP- it is a limited, rare one, there aren’t any fossils, and the tuff just crumbles to little bits in your hands anyway. The beach is a pretty fun place in its own right, with wonderful honeycomb structures eroded into the sandstone. There are a few fossil log casts in the sandstone, nice rock ledges to hang out on, and good swimming on a rising tide in summer.

The Clayton tuff is interbedded with steeply- bedded siltstone, and is overlain by a thin layer of

The 6-inch-wide yellow tuff at Clayton Beach is overlain by a dark coalified soil layer.

black, coalified soil. Apparently, the ash fell onto flood deposits or perhaps a dry lake bed, and was then covered by a layer of leaves. If the U-Pb age of 56 Ma is due to contamination with detrital zircons (rather than zircons erupted from the volcano that produced the ash) then the ash layer is almost certainly not primary, as it was mixed with other sediment, and it was washed onto the sediments before burial and lithification. If the coal was an ancient soil, I’d think that the tuff would show more sign of disruption by burrowing animals or plants growing in the soil- no soil is preserved beneath the tuff. I’m open to comments or suggestions on this point. If you visit this place, PLEASE DON’T PICK AT THIS OUTCROP- it is a limited, rare one, there aren’t any fossils, and the tuff just crumbles to little bits in your hands anyway. The beach is a pretty fun place in its own right, with wonderful honeycomb structures eroded into the sandstone. There are a few fossil log casts in the sandstone, nice rock ledges to hang out on, and good swimming on a rising tide in summer.

Getting there: The Clayton Beach trailhead is about 1/2 mile south of the entrance to LarrabeeState Park on Chuckanut Drive (State Route 11). Walk to the railroad tracks. Look for a trail heading straight through the woods toward the nearby beach. If you have found the right one, you’ll step down a short bank onto the beach at a small cove (at high tide). The exposure is on the rock wall a few feet to your right. if you miss the trail at the railroad tracks, stay on the main trail which angles down and left to the main sandy beach area. Walk north around the rocky points about 100 yards to the outcrop. It is pretty conspicuous, especially if you are looking for the yellow and black stripes in the rock.