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U.S. Geological Survey
Geologic Investigation Series I-2592

Geologic Map of the Sauk River 30- by 60-Minute Quadrangle, Washington

By R.W. Tabor, D.B. Booth, J.A. Vance, and A.B. Ford



photo of Le Conte Glacier, Cascade Ranges, Washington
View south from Spider Mountain to Le Conte Glacier spilling down off Old Guard and Sentinel Peaks, carved from metamorphosed igneous rocks of the Triassic Marblemount pluton. The low line of jointed cliffs in the foreground are underlain by Cascade River Schist. Both units are in the Chelan Mountains terrane. On the left, glacier-covered Dome Peak rises behind Old Guard and the Le Conte Glacier. Its west ridge culminates in Spire Point (middle background, center right). Both Dome Peak and its ridge are eroded from the Miocene Cloudy Pass batholith. The skyline peak in the center of the photo is Glacier Peak volcano. All these peaks and ridges are in the Glacier Peak Wilderness Area. (Photograph by R.W. Tabor, 1958.)



The north-south-trending regionally significant Straight Creek Fault roughly bisects the Sauk River quadrangle and defines the fundamental geologic framework of it. Within the quadrangle, the Fault mostly separates low-grade metamorphic rocks on the west from medium- to high-grade metamorphic rocks of the Cascade metamorphic core. On the west, the Helena-Haystack melange and roughly coincident Darrington-Devils Mountain Fault Zone separate the western and eastern melange belts to the southwest from the Easton Metamorphic Suite, the Bell Pass melange, and rocks of the Chilliwack Group, to the northeast. The tectonic melanges have mostly Mesozoic marine components whereas the Chilliwack is mostly composed of Late Paleozoic arc rocks. Unconformably overlying the melanges and associated rocks are Eocene volcanic and sedimentary rocks, mostly infaulted along the Darrington-Devils Mountain Fault Zone. These younger rocks and a few small Eocene granitic plutons represent an extensional tectonic episode.

East of the Straight Creek Fault, medium to high-grade regional metamorphic rocks of the Nason, Chelan Mountains, and Swakane terranes have been intruded by deep seated, Late Cretaceous granodioritic to tonalitic plutons, mostly now orthogneisses.

Unmetamorphosed mostly tonalitic intrusions on both sides of the Straight Creek fault range from 35 to 4 million years old and represent the roots of volcanoes of the Cascade Magmatic Arc. Arc volcanic rocks are sparsely preserved east of the Straight Creek fault, but dormant Glacier Peak volcano on the eastern margin of the quadrangle is the youngest member of the Arc.

Deposits of the Canadian Ice Sheet are well represented on the west side of the quadrangle, whereas alpine glacial deposits are common to the east. Roughly 5000 years ago lahars from Glacier Peak flowed westward filling major valleys across the quadrangle.

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Download sheet 1 as a PDF file (12.1 MB) or as an Encapsulated PostScript (EPS) file (37.4 MB)

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Jump to USGS Data Series 188: Database for the Geologic Map of the Sauk River 30-Minute by 60-Minute Quadrangle, Washington (I-2592)

For questions about the content of this report, contact Rowland Tabor

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Maintained by: Michael Diggles
Created: June 11, 2003
Last modified: Tuesday, June 13, 2006 (mfd)