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Date of Award
Master of Science (MS)
Brown, Edwin H.
Babcock, R. Scott (Randall Scott)
Engebretson, David C.
Dominant bedrock units of the Deer Peaks area, northwestern Washington, include the Shuksan Metamorphic Suite, the Deer Peaks unit, the Chuckanut Formation, the Oso volcanic rocks and the Granite Lake Stock. Rocks of the Shuksan Metamorphic Suite (SMS) exhibit a stratigraphy of meta-basalt, iron/manganese schist, and carbonaceous phyllite. The shear sense of stretching lineations in the SMS indicates that during high pressure metamorphism the subduction zone dipped to the northeast relative to the present position of the rocks. Exposures of the SMS along Coal Mountain are consistent with interpretation of folding as an isoclinal anticline though discrimination between an F1 or an F2 structure is not possible. The Deer Peaks unit (DPu) (age unknown) consists of transitional calcalkaline tuffaceous andesite with minor interbedded dacite and less abundant slate and chert, all of which is intruded by diabase dikes. Trace element geochemistry is interpreted to indicate a transitional-to-arc back arc basin origin for the igneous protolith of the DPu. During rnetamorphism, a penetrative foliation (S ) was developed parallel to primary layering. S1 is folded into northeast-southwest trending F2 folds which are in turn folded about a south-southwest trending F3 axis. Metamorphic mineral assemblages recognized in the DPu are (in addition to quartz and albite):
1) Chlorite + Pumpellyite + Epidote + Actinolite
2) Pumpellyite + Calcium Carbonate + Actinolite
3) Epidote + Calcium Carbonate + Actinolite
Calcium carbonate is present as both aragonite and calcite. Metamorphism of the DPu is of high-P/low-T type, occurring at temperatures between 330oC and 350o C, and pressures between 7 and 11 Kbar. Rocks of the Deer Peaks unit are emplaced above those of the Shuksan Suite along the Deer Peaks thrust fault.
The Eocene Chuckanut Formation nonconformably overlies the metamorphic rocks in the study area. The Chuckanut Formation is intruded by quartz-diorite of the Higgins Mountain and Granite Lake stocks, and unconformably overlain by the 43.2 ± 1.9Ma Oso volcanic rocks. The Chuckanut Formation is folded into northwest trending open folds. Two northwest trending high angle faults juxtapose the Chuckanut Formation with metamorphic rocks in the area: the Higgins Mountain-Day Lake fault and the Segelson Lake shear zone. The Higgins Mountain-Day Lake fault is a sinistral strike slip fault.
Western North Cascades geology, Deer Peaks geology, Shuksan Metamorphic Suite
Western Washington University
Subject – LCSH
Geology, Structural; Metamorphism (Geology); Petrology--Washington (State)--Deer Peaks Region; Geology--Washington (State)--Deer Peaks Region
Deer Peaks Region (Wash.)
Copying of this document in whole or in part is allowable only for scholarly purposes. It is understood, however, that any copying or publication of this thesis for commercial purposes, or for financial gain, shall not be allowed without the author's written permission.
Reller, Gregory Joseph, "Structure and Petrology of the Deer Peaks Area Western North Cascades, Washington" (1986). WWU Graduate School Collection. 726.
Plate II Geologic Cross Sections For the Deer Peaks Area reduced.pdf (770 kB)
Reller Map Unnamed 1 reduced.pdf (4206 kB)
Reller Map Unnamed 2 reduced.pdf (3316 kB)