Geology Reference
In-Depth Information
the North American plate. After the oceanic slab was
carried down and melted, liquid rock on the surface
appeared as andesitic lavas of the Clarno Formation.
As the loose ash from these volcanoes mixed
with water, mud flows or "lahars" were formed. Lahars
flowed like viscous syrup to entomb both plant and
animal fossils in a broad band across eastern Oregon.
Individual, uninterrupted deposits of Clarno mudflows
up to 1,000 feet thick occur near Mitchell in Wheeler
County. Preserved in the sediments of the mudflows,
broad leaf plants record the wet, tropical climate of
eastern Oregon during the Eocene epoch. Avocado,
cinnamon, palm, and fig created a lush woodlands
interspersed with open, grassy plains and freshwater
lakes. A diversity of large land mammals, tapirs,
rhinoceros, titanothere, and oreodons shared the stage
with freshwater fish and snails. A peculiar aquatic
rhinoceros, not unlike a modern hippopotamus,
wallowed in shallow waterways to be preyed upon by
crocodiles and other large carnivores. The Clarno is
perhaps most famous for its "nut beds" where many
species of tropical and hardwood nuts, fruits, and seeds
are preserved in soft volcanic muds.
By early Oligocene time the Clarno volcanic
episode had subsided to be replaced by eruptions of a
new complexion in the John Day period about 36
million years ago. Only a brief interval of erosion
separates the two events. Most of the John Day volca-
noes were just to the west of the Blue Mountains
province near the present-day Cascade Range. Great
volumes of ash and dust erupted into the sky to settle
and be carried by streams and deposited in the broad
John Day basin. Volcanic vents at this time often
generated incandescent pyroclastic ash flows or "ignim-
Eocene palm leaf
Sabalites
from the Clarno
Formation is over 2 feet long (photo
courtesy Oregon Department of Geology
and Mineral Industries)
Tertiary volcanic events in eastern Oregon
opened in Eocene time around 44 million years ago
with andesitic and rhyolitic lavas erupting from a chain
of volcanic vents in the western Blue Mountains. Lavas,
accompanied by mud flows and successions of multi-
colored soils as well as tuffaceous sediments deposited
in lakes or by streams, make up the colorful blend of
rocks comprising the Clarno Formation. The source of
the Clarno andesitic volcanic activity is not well
understood but apparently relates to what is called the
Challis arc of Eocene volcanic vents extending diago-
nally northwest by southeast across Washington and
Idaho. Adjustments in the tectonic plates to the west
at this time produced a considerable bulk of material
being subducted from the west toward the east beneath
Many John Day volcanoes issued "ignimbrites" or
incandescent clouds of ash that flowed across the
landscape.
