Blue Mountains - Geology of Oregon - page 20

ing, and Baker terrane rocks are no exception. The Part of the Wallowa terrane, the Seven Devils
terrane is a series of enormous coherent blocks sepa- Group of rocks has been compared to widespread
rated from each other by large-scale shear fault zones. terranes collectively called "Wrangellia", for extensive
The fine-grained rocks of this terrane were tightly exposures in the Wrangell Mountains of Alaska. This
compressed into folds during the late Triassic and early large terrane is a disjunct group of similar volcanic and
Jurassic. sedimentary rocks of Triassic age and older stretching
The northern-most terrane of the Blue Moun- southward from the Wrangell Mountains in southern
tains province is the Wallowa volcanic arc represented Alaska to include Chichagof Island, Queen Charlotte
by a five mile thick sequence of volcanic archipelago Islands, and Vancouver Island in Canada. The southern
rocks ranging from Permian to Jurassic in age which most piece of this string might be the Wallowa Moun-
were accreted behind the Baker terrane. Isolated tains. The scenario for the development of Wrangellia
exposures of the Wallowa terrane appear northeast of implies two separate and distinct tectonic processes.
Walla Walla, Washington, as well as south of Lewiston, During Triassic time, the various pieces of Wrangellia
Idaho, but continuous sections of this exotic block are

were a single island block situated somewhere in the

located in the Snake River Canyon where they can be tropical Pacific. The pieces were slowly being transport-

traced for over one hundred miles. ed toward North America on a conveyor belt-like

The older, volcanic rocks of the Wallowa tectonic plate where they collided with the West Coast.

terrane accumulated along the axis of an ancient island With the docking event, the block was sheared up by

archipelago. Fossiliferous sedimentary rocks of the large-scale faults similar to the San Andreas. Move-

Martin Bridge Limestone and shales and siltstones of ment along these faults "translated" or conveyed slices

the Hurwal Formation were later deposited on top of northward one by one. The Seven Devils volcanic rocks

volcanic rocks of the arc. The Martin Bridge and are typical of those from ocean margin island archipel-

Hurwal formations span the Triassic-Jurassic boundary agos, whereas the remainder of Wrangellia to the north

and reflect a shallow, warm shelf environment. Fossils has volcanic sequences frequently found in midocean,

of molluscs, corals, and swimming reptiles within the deep water environments. However, the overall similar-

Martin Bridge and Hurwal are similar to those found ity between the rocks and fossils of all the regions of

in rocks of the eastern Pacific as well as from central the terrane suggests there is a strong affinity between

Europe to the Himalayan Mountains. northeastern Oregon and Wrangellia.

Over the Hurwal Formation, fossil wood,
leaves, plant fragments, and invertebrates of the Coon Cretaceous

Hollow conglomerates and sandstones record the In the later Jurassic and early Cretaceous

nonmarine and nearshore environment that existed on between 160 and 120 million years ago, newly joined

the Blue Mountains island arc during the middle terranes of the Blue Mountains region were invaded by

Jurassic. Growth rings in petrified tree wood point to granitic batholiths of granodiorite and gabbro to

a temperate, seasonal climate with abundant rainfall cement them together with a granitic glue. These

and a short dry season before winter begins. The com- intrusions include the Bald Mountain and Wallowa

monly occurring ferns and quillworts grew along stream batholiths as well as several smaller bodies. A batholith

banks and in marshes. Molluscs and corals are found is a mass of coarse-grained crystalline rock which

mixed with plant fossils in shallow coastal sediments. melted upward from deep within the earth's mantle

As granites of the Wallowa batholith intruded these into the surrounding layers above. As the liquid magma

formations, heat and pressure locally altered them to slowly cooled, large crystals formed. Batholith roofs

marble and slate in late Jurassic and Cretaceous time. exposed by erosion have a surface area greater than 40

The Wallowa terrane is heavily sheared by square miles, and these two batholiths in northeastern

faults developed during the accretion against North Oregon certainly exceed that. The Bald Mountain

America during the late Mesozoic. The rocks are so batholith, northwest of Baker in the Elkhorn Moun-

heavily deformed that whole folds are locally over- tains, extends over 144 square miles, while the Wallowa

turned. Another major structural event affecting the batholith, forming the core of the Wallowa Mountains,

Wallowa terrane is shearing or faulting along three covers 324 square miles and is the largest in Oregon.

separate zones visible in the Snake River canyon. The It was during the early Cretaceous that the last

25 mile long north-south Oxbow shear zone shows exotic crustal blocks were attached or accreted to

displacement where the west side moved south and the
east side move north. During the early Jurassic the
total distance all three faults moved was several
hundred miles.