A TRAVERSE GEOLOGIC MAP IN THE GLACIER PEAK WILDERNESS: FROM NORTH FORK SAUK RIVER TO GLACIER GAP

Glacier Peak, the least accessible and one of the most active of Washington’s North Cascades volcanoes, is composed of Cenozoic volcanic rocks that intruded Cretaceous metamorphic rocks which record mid- to lower- crustal deformation during Cretaceous accretion of terranes to North America. To investigate the relationship between Glacier Peak volcanism and pre-existing structural boundaries within the underlying crystalline rocks, we completed a geologic mapping traverse from the North Fork Sauk River trailhead to Glacier Gap, the route typically used by recreational hikers to access the Glacier Peak summit. The traverse covered areas in the Sloan Peak, Glacier Peak East, and Glacier Peak West 7.5-minutes quadrangles.

Thick (>5 meter) river alluvium and rock fall colluvium were observed along the North Fork Sauk River, with exposures of the Nason Ridge Migmatitic Gneiss dominating ridgelines. To the east, garnet-bearing Chiwaukum Schist and intercalated orthogneisses are present. Pressure-temperature estimates will provide insight into the metamorphic and structural interaction between the Nason Ridge Migmatitic Gneiss and Chiwaukum Schist. Pegmatites increase towards the east and a small, intermediate, biotite-rich granodiorite stock is present southwest of White Chuck Glacier. Ignimbrites sourced from Glacier Peak are contained within the White Chuck valley and likely represent valley fill during a previous eruption cycle. Ignimbrites contain Chiwaukum Schist clasts. These ignimbrites are overlain by minor pumice observed atop Glacier Gap, suggesting that while an earlier eruption may have generated pyroclastic flows to the south, a more recent (late Pleistocene?) eruption correlated with Glacier Peak pumice deposits was directed easterly with little southward transport of ejecta. The White Chuck Glacier is significantly smaller than mapped, covering approximately 0.6 km² as observed in 2017 compared to a map extent of 0.8 km² in 2002 and 2.4 km² in 1984 (based on the Glacier Peak West 7.5 minute quadrangle extent of the glacier). An unmapped glacial lake now occupies the foot of the glacier to the west; aerial imagery will be presented to further explore glacial trends in the area.