MBVRC field trip to Baker Lake- an outwash dam, subaqueous lava and peperite.

By Dave Tucker March 12, 2011

Not gonna let a little rain stop us, right? Click to enlarge any image.

The weather gods reined in their downpour long enough for 22 people to enjoy the first MBVRC field trip of the year. Two van loads of us visited sites in the Baker River valley associated with the formation of Glacial Lake Baker and the basaltic Sulphur Creek lava that flowed into it. Yes, it was damp, but the downpour that had windshield wipers going at a fast clip as we headed up the Skagit valley, and that knitted a few concerned brows in the drivers’ rear view mirrors, had moderated to a mere slow-drenching drizzle as we arrived at Horseshoe Cove and left the vans for a few hours.

Outwash fan deposits at Burpee Hill. J. Saltonstall photo.

Field trip participants came from as far as Yakima. There were professional geologists, students, and teachers, but a majority were geo-amateurs, as one man called himself. We started by looking at the latest Pleistocene glacial deposits that form the Burpee Hill Fan, a sediment dam that plugged up the Baker River beginning around 12,200 14C years ago to form Glacial Lake Baker. We then moved along to see a subaerial deposit of the Sulphur Creek lava that flowed down Sulphur and Rocky Creeks into Baker River valley at 8800 14C years BP.

Subaerial Sulphur Creek lava on Baker Lake Road. J. Saltonstall photo

The flow and its source at Schreibers Meadow was the topic of last November’s first ever MBVRC fund-raiser field trip. The main focus of our trip on Saturday was the subaqueous facies of the lava flow. After reaching the Baker valley, the river of slow moving blocky basalt entered the 2-km-wide, 100-m-deep glacial lake, and crossed it completely to impinge on the east valley wall. We looked at lava that burrowed under soft lake floor clay, which was then rafted along on the back of the moving lava. We also saw the pervasively-fractured lava that cooled quickly under the lake, and which in places resembles lava that cooled against ice, or under a glacier. The lake sediments were themselves of considerable interest, as they changed in appearance and composition from 12,200 year old deep-water clay with no volcanic clasts to coarse sand and granules that is mostly derived from eroded Sulphur Creek lava and tephra, deposited as a delta in shallow water above the lava. We also saw several cm of Mazama ash, which fell into, or was washed into, the remnant of the glacial lake 6800 14C years BP.

Lava invaded and deformed lake sediment. A younger delta is at upper left. K. King photo.

Thanks to everyone who came along and supported MBVRC. All proceeds benefit MBVRC’s research funding program. Special thanks to Bernie Dougan from Whatcom Community College for volunteering to drive a van.

2 References:

Tucker, D. and Scott, K., 2009, Structures and facies associated with the flow of subaerial basaltic lava into a deep freshwater lake: The Sulphur Creek lava flow, North Cascades, Washington; Journal of Volcanology and Geothermal Research, v.185 p. 311–322. doi:10.1016/j.jvolgeores.2008.11.028

Baker Lake reservoir was drawn down 50 feet at the time of the trip. Umbrellas are essential in the winter! Kitty King photo.

Highly fractured subaqueous lava at Horseshoe Cove. J. Saltonstall photo

Peperite- lava mixes with, and bakes, lake sediment at Horseshoe Cove.

A pdf of this paper is available by email request: research@mbvrc.wwu.edu (cut and paste; this is not a link).

Also:

Tucker, D., Scott, K. and Lewis, D., 2007, Field guide to Mount Baker volcanic deposits in the Baker River valley: Nineteenth century lahars, tephras, debris avalanches, and early Holocene subaqueous lava, in Stelling, P., and Tucker, D.S., eds., Floods, Faults, and Fire: Geological Field Trips in Washington State and Southwest British Columbia: Geological Society of America Field Guide 9, p. 83-98, doi: 10.1130/2007.fl d009(04).

A layer of wood underlies lake clay; Mazama ash is the orange'blue layer. Delta sediment above. B. Rinck photo

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