Geologic map of the Sauk River 30- by 60-minute quadrangle, Washington

IMAP 2592
See geospatial database at DS 188
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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.

Study Area

Publication type Report
Publication Subtype USGS Numbered Series
Title Geologic map of the Sauk River 30- by 60-minute quadrangle, Washington
Series title IMAP
Series number 2592
Subseries Geologic Investigation Series
ISBN 0607968710
DOI 10.3133/i2592
Edition -
Year Published 2002
Language ENGLISH
Contributing office(s) Earth Surface Processes Team
Description 1 map : col. ; 56 x 75 cm., on sheet 104 x 135 cm., folded in envelope 30 x 24 cm.; 1 data sheet (64 x 92 cm.); 1 pamphlet (67 p : map; 28 cm.).
Scale 100000
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