Thursday, February 4, 2016

Possible Glacier Peak Tephra West of Puget Sound

I was recently traversing a bluff to assess geologic risk above the shores of Discovery Bay on the northeast side of the Olympic Peninsula. The bluff geology includes a glacial recessional drift unit capping the top of the bluff. Below that is a recessional outwash sand and gravel unit. Further down slope are much older non glacial sediments (Olympic) and a glacial drift unit from two ice ages ago (Possession). 

The story on the upper bluff is a bit complicated as the units were likely deposited when the Puget ice lobe was retreating. The site appears to be located where the glacier lake that had been located in what is now Puget Sound drained when the ice retreated enough to unblock the lake. Sea levels were tricky at this time period. The ice age was coming to an end but there was still enough global ice such that sea levels were much lower than today; however, on a local level the mass of glacial ice had isostatically lowered the local land surface and hence the local sea level relative to today was substantially higher. All in all a location with lots to contemplate while thinking about slope stability and route finding on steep slopes.

Silty clay at bluff top with sand and gravel below.

I noted some white material within the sand and gravel that obligated me to take a closer look. I am ever hopeful of finding a mammoth fossil. Instead it was cluster of white pebbles within the coasre sand.

I pulled some of the pebbles out and found that they were pebbles of tephra. Tephra is very fine volcanic ash. Some how an ash deposit was eroded and shaped into pebbles by flowing water and deposited on the northeast Olympic Peninsula (The site is on the Quimper Peninsula which is attached to the larger Olympic).

The pebbles of tephra pose an interesting puzzle. How did tephra get all the way across the Puget lowland from a volcano in the Cascade Range? Based on the location and estimated age of the glacial deposits as very late most recent ice age outwash, I suspect this tephra may be from a large eruption from a Glacier Peak from 13,100 years ago. Tephras have distinct chemical signatures and as such this tephra could lend an additional age control to the units where it is located.  


susan said...

That is very cool! Did you take a sample for analysis?

Dan McShane said...

Yes. I hope I can pass this problem off to some clever geologist. Eventually I will follow up on this find. The cool thing is its at a critical site for understanding the late stages of the glacial retreat.