G145
Review Last Week
Terranes:
Formed
in a specific/unique environment
Geologist
can tell what environment rocks formed in so rocks give history of a region
Terrane accretion in NW
Period
of accretion mostly Mesozoic time
Occurred
due to the opening of the
Over
200 recognized terranes in NW Baja-Alaska
Varity
of terrane types
Average
distance traveled 2500km
Size
from tens to thousands of km long
Prior to accretion in the NW
Paleozoic:
BC,
WA, OR, ID, NV,
Wide
continental shelf accumulating thick marine sediments
Late Paleozoic through
Mesozoic
Terrane accretion of primarly
volcanic islands
Accretion
caused folding, faulting, and magma generation
Terranes that have added to the NW
Kootenay Arc ~180-120 mybp
North
Cascades Micro-continent
~50mybp
The Kootenary arc
Precambrian-Paleozoic
shelf material that is folded/sheared due to collision
subduction 180-120 mil years ago.
Represents
the westernmost edge of the ancient N. American Continent
Extends
from BC through Idaho Panhandle and eastern WA
Island
about size of CA
Rafted to
NW bay Kootenay subduction
“docked” about 100mybp
Closed Kootenay trench
Developed
new trench to the west
North Cascades Micro-continent
Fossils
indicate continent is from
Rafted to
N. America by
Second subduction zone on western edge of N. Cascades
Docked in
late Cretaceous to Eocene (50-60 mybp)
Closed
Today…
Oldest
rocks in OR
Part of
all this terrane accretion
Eastern Blue Mtns
Comprised of Paleozoic, Mesozoic, and Cenozoic rocks
Pz and Mz
rocks are accreted and are covered by Cz
volcanics
Accretion associated with
Deformation:
Synclines and Anticlines
Faults:
Reverse
Plutons: Stocks and Batholiths
Accretion of terranes in PZ and MZ was followed by the Late Cretaceous
Seaway
Seaway created environment of marine sediment deposition
Shale
Limestone
Conglomerates
Eastern Blues: 5 prominent Terranes
Recognized:
1) Grindstone Terrane
(smallest and oldest terrane)
Shallow backarc basin basin
Rocks include:
Limestones (coral
reefs)
Chert and shales (mud)
Devonian to Permian age fossils 380 mybp
to 250 mybp
2) Olds Ferry Terrane
(
Eastern most terrane and
trends to the NE
Volcanic Archipelago
Rocks include Ash, rhyolites,
basalt and andesite
Layered in with marine sandstones, shale and
limestone
Jurassic in age
3) Wallowa Terrane
Permian to Triassic in age
280-200 mybp
3 Basic groups of rocks
Seven Devils Volcanic Group
Hurwal Formation
Seven Devils Volcanic group
Volcanic
Docked W/
Large scale faulting sheared the block apart
7-Devils is the same as the Wrangellia
Terrane
Wrangella mtsà
Wallowa Mtns
Mid Triassic in age
~220 mybp
Represents coral reef environment
Lots of fossils
Brachiopods
Corals
Cephalopods
Hurwal Formation
Late Triassic
~210 mybp
Mudsà shale and siltstone
Deposited on top of Martin Bridge Limestone
4)
Granite Batholiths
Jurassic to Cretaceous
160-120 mybp
Wallowa batholith
5) Baker Terrane
Represents ocean floor and accretionary
wedge
Rocks are VERY folded and faulted due to subduction
zone and wedge
Three groups of rocks
Elkhorn Ridge Argillite
Burnt river Schist
Elkhorn Ridge Argillite
Permian to Triassic in age
280-200 mybp
Lightly metamorphosed mudstone with some chert
Represents ocean floor
Rich in Marine Fossils
Burnt River Schist
Metamorphosed mudstone
Represents Subduction zone environment
Mixed in with Elkhorn Ridge Argillite
Sequence represents subduction
of ocean floor
Creteceous
Seaway covers most of W. N.
America
Eastern Blues are still above water
Lots of clays silts and conglomerates are deposited
Beginning of Cenozoic
Cretaceous seaway begins to retreat from W. N. Amer.
No Cascades or Coast range
Shoreline east of present day Cascades
E. Blues above water and still uplifting
In Paleocene (~50 mybp) erosion begins to dominate
Okanogan Subcontinent docking and subduction
begins—
Many of the same terranes found in
the Western Blues as Eastern Blues
Grindstone (backarc)
Devonian through Permian age
Limestone, conglogmerate,
sandstone, mudstone
Lots of fossils
Baker Terrane (ocean floor)
New Terrane for the Western
Blues:
Izee Terrane: (forearc basin)
Mesozoic age
Greywacky
and arkose
Lots of fossils
Aldrich Mts
Hard to find
exposures
Main difference between
Eastern and Western is that most of Western Blues covered by Cenozoic
(tertiary) volcanics
Silicic volcanic arc creacted
by
Intrusive rocks (diorite plutons)
Extrusive: lava flows
but mostly ash
60 mil year of mammal and flora history covered and preserved
by ash deposits
Sedimentary deposits:
mudflows and lake deposits
Clarno Formation
~55-44 mybp (Eocene and Oligocene)
Andesite, ash tuff and volcanic
breccias
Most of Ochoco Mts
Vents include:
White and Black Buttes
Known as the “Nut Bed”
Some Mammals
Tapirs, Rhinos, Oredonts
36 mybp
Volcanic ash weathered to clay
Ignimbrite deposits
Very Colorful deposits but not well lithified
Over 6000 ft thick
Fossils represent warm wet temperate climate
Fossils include:
Rhinos, Tapirs, Camels, Oreodonts, Opossum, sabertooth
cats, turtles and lots of plants.
Miocene 17-12 mybp
Flood Basalts
Covers both Eastern and Western Blues
Deformation of basalts during Pliocene 5-1 mybp
In Western blues ~ 900-1000 ft thick
Two main formations
Picture Gorge basalts (17mybp)
Grand Ronde basalts
Mascall Formation
Same age as CRB’s (mixed into)
Ash deposits from Strawberry Volcanics
Air fall volcanic ash
Found around
Fossils: Camels,
Mercyhippus (3 toed horse), canines, and antelope
Uplift of Modern Wallowa Mtns
Mid-Miocene
Rattlesnake Ignimbrite
Late miocene
Sand, gravel and silts deposited by water
Topped by ash flow deposit from
Pleistocene
Glaciation of Higher mountains (Eastern Blues)
Ice formed and retreated several times
Left distinctive glacial topography
U-shaped valleys
Cirques
Moranes
Mining History
Gold
¾ of all
Lode and Placer
Primarily associated with granite batholiths
Copper & Silver
Located and mined with gold
Mercury
Clarno formation (cinnabar)
Thunder eggs