This report provides new geologic mapping, compilation, and interpretation for
the more than 3 million acre Payette National Forest. The geologically
complex forest is divided by the Salmon River suture; before the Late
Cretaceous, the eastern and western parts had fundamentally different
geologic histories. The eastern part is underlain by Mesoproterozoic
to Cambrian(?) rocks of the Laurentian (Precambrian North American) continent.
Thick Mesoproterozoic units, which are at least in part equivalent in
age to the Belt Supergroup of northern Idaho and western Montana, underwent
Mesoproterozoic metamorphic and deformational events, including intrusion
of Mesoproterozoic plutons. During the Neoproterozoic to early Paleozoic,
the western edge of Laurentia was rifted. This event resulted in deposition
of rift-related Neoproterozoic to Lower Cambrian(?) volcanic and sedimentary
rocks above Mesoproterozoic rocks. The western part of the forest is
underlain by upper Paleozoic to lower Mesozoic island-arc volcanic and sedimentary rocks. These rocks comprise four recognized
island-arc terranes that were amalgamated and intruded by intermediate-composition
plutons, probably in the Late Jurassic and Early Cretaceous, and then
sutured to Laurentia along the right-lateral, transpressive Salmon River
suture in the Late Cretaceous. Metamorphic grade and structural complexity
of the rocks increase toward the suture from both sides. An older plutonic
series is intermediate in composition and preserved as elongated and
deformed bodies near the suture and as parts of roof pendants to younger
intrusions to the east. A younger magma series consists of undeformed,
marginally peraluminous plutons that formed east of the suture after
accretion. After suture-related compression, crustal extension resulted
in voluminous volcanic and plutonic rocks of the Eocene Challis magmatic
complex on the east side of the forest. Extension, from the Late Cretaceous
to post-Miocene, uplifted the area of the Idaho batholith relative to the western part of the forest and formed dominant
highlands along the Snake River. Having been reactivated by younger structures,
the Salmon River suture forms a north-trending topographic depression
along Long Valley, through McCall, to the Goose Creek and French Creek
drainages. During the last stages of metamorphism and deformation related
to the suture event, voluminous plutons of the Idaho batholith were intruded
east of the suture. Extensional basins also formed such that, in the
Miocene, the Columbia River Basalt Group and related basaltic lavas flowed
over most of the lower elevations on the western side of the forest and
redirected erosional debris into north-trending, fault-controlled drainages
and young sedimentary basins.
Version 1.0
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Posted June 2005 |
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