Synopses & Reviews
Published by the American Geophysical Union as part of the Field Trip Guidebooks Series, Volume 307.
The Cascade Range is an active north-trending volcanic arc at the western edge of North America (Figure 1). At the northern end of the range, between 47°N and 49°N, the average elevation increases, peaks become sharper, numerous small glaciers survive on the higher slopes, and volcanic rocks of the Cascade arc are scarce. This region is the North Cascades Range. The North Cascades are bounded on the west by the fore-arc basin of the Puget Lowland, on the south by the arc volcanic rocks of the Central Cascades, and on the southeast by the back-arc flood basalts of the Columbia Plateau. The geologic identity of the range is not so clearly defined to the north, but it is geographically bounded on the northeast by the Okanogan Ranges and on the northwest by the Fraser River, which separates the Cascades from the Coast Mountains.
Synopsis
Published by the American Geophysical Union as part of the
Field Trip Guidebooks Series.
The Cascade Range is an active north-trending volcanic arc at the western edge of North America (Figure 1). At the northern end of the range, between 47°N and 49°N, the average elevation increases, peaks become sharper, numerous small glaciers survive on the higher slopes, and volcanic rocks of the Cascade arc are scarce. This region is the North Cascades Range. The North Cascades are bounded on the west by the fore-arc basin of the Puget Lowland, on the south by the arc volcanic rocks of the Central Cascades, and on the southeast by the back-arc flood basalts of the Columbia Plateau. The geologic identity of the range is not so clearly defined to the north, but it is geographically bounded on the northeast by the Okanogan Ranges and on the northwest by the Fraser River, which separates the Cascades from the Coast Mountains.
Table of Contents
OVERVIEW OF THE GEOLOGY OF THE NORTH CASCADES 1
INTRODUCTION 1
Acknowledgments 1
THE GEOLOGY OF THE NORTH CASCADES RANGE 1
Present topography and glaciation 2
(I) Pre-Late Cretaceous terranes 3
North American craton 3
Quesnellia 3
Pre-Late Cretaceous terranes east of the Straight Creek fault 3
Methow terrane 3
Hozameen terrane (Hozameen Group) 3
Little Jack terrane 4
Jack Mountain Phyllite 4
Elija Ridge Schist 4
North Creek Volcanics 4
Chelan Mountains terrane 7
Napeequa unit 7
Twisp Valley Schist 7
Cascade River unit 7
Marblemount Meta Quartz Diorite 7
Skagit Gneiss 8
Nason terrane 8
Chiwaukum Schist 8
Ingalls terrane 8
Swakane terrane 8
Pre-Late Cretaceous terranes west of the Straight Creek fault 8
Northwest Cascades system (NWCS) 8
Grandy Ridge terrane 9
Chilliwack Group and Cultus Formation 9
Wells Creek Volcanics and Nooksack Group 9
Easton terrane (Easton Metamorphic Suite) 9
Shuksan Greenschist 9
Darrington Phyllite 9
Yellow Aster terrane (Yellow Aster Complex) 9
Western and eastern melange belts (WEMB) 9
Eastern melange belt 10
Trafton sequence 10
Western melange belt 10
Helena-Haystack melange 10
(II) Late Cretaceous to Eocene(?) orogeny 11
Deformation 12
in Methow terrane 12
in Chelan Mountains and Nason terranes 12
in NWCS 12
Metamorphism 12
in Nason terrane 12
in Chelan Mountains terrane 12
in NWCS 12
in WEMB 13
Syn- to late-orogenic plutons 13
(Ill) The Eocene event 13
Eocene faulting 13
Straight Creek fault 13
Darrington-Devils Mountain fault zone (DDMFZ) 13
Entiat fault 13
Ross Lake fault zone (RLFZ) 13
Ross Lake fault sensu stricto I3
Gabriel Peak tectonic belt 15
Hozameen-North Creek and Foggy Dew faults 15
Twisp Valley fault 15
Thunder Lake fault 16
Chewack-Pasayten fault 16
Eocene deposition 16
Chuckanut Formation 16
Swauk Formation 16
Barlow Pass Volcanics 16
Eocene high-level deformation 16
Eocene magmatism 16
Golden Horn batholith 16
Late lineated dikes 16
Eocene ductile deformation 17
Eocene K-Ar cooling ages 17
(IV) Cascade magmatic arc I7
Indexfamily 17
Snoqualmie family I7
Cascade Passfamily I7
TRIP LOG 34
Day 1. Spokane to Twisp and Winthrop 34
Day 2. Winthrop to upper Eagle Creek 36
Day 3. Upper Eagle Creek to Lake Juanita 37
Day 4. Lake Juanita to Stehekin and Stehekin Valley Ranch 38
Day 5. Stehekin Valley to Skagit Valley 40
Day 6. Diablo Lake to Da"ington 43
Day 7. Darrington to Rat Trap Pass, Suiattl~ Mountain and return 48
Day 8. Darrington to Gee Point area, Deer Creek Pass and return 51
Day 9. Darrington to Helena Ridge, Barlow Pass and Seattle 55
REFERENCES 57