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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

### 2002

 

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| 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.) |

 

| **Summary**

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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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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| Department of the Interior | U.S.   Geological Survey |   URL of this page: https://pubs.usgs.gov/imap/i2592/  Maintained by: Michael Diggles  Created: June 11, 2003  Last modified: Tuesday, June 13, 2006 (mfd)