Geology Reference
In-Depth Information
block was abandoned and a new one activated offshore
to the west where it is today.
The slow subsidence of the block created a
broad forearc trough along the western margin of
North America. From Eocene through Pliocene time
the basin was the recipient of deposits that blanketed
the earlier volcanic platform. Rivers draining the
Klamath Mountains and later the Idaho batholith
provided abundant sediments that accumulated in the
newly formed basin. During the early Eocene the
eastern edge of the subsiding coastal block that was to
become the Willamette Valley collected sandstones and
siltstones of the Flournoy Formation near Lorane,
Philomath, Falls City, the low hills around Camp
Adair, and in the southern valley. Where Eocene rocks
are exposed in the area north of Corvallis, rolling hills
contrast sharply with the flat valley floor elsewhere that
is covered with Pleistocene fill.
In the northern part of the valley these depos-
its were followed by middle Eocene Yamhill muds,
sands, and silts, mixed with ash and lavas from the
ancestral Cascades that were carried into the shallow
seaway. Within the Yamhill, shoals of limestones
around offshore banks formed the Rickreall and Buell
limestones containing broken mollusc shells, foraminif-
era, and calcareous algae intermixed with volcanic
debris. In the northern valley 2,000 feet of Nestucca
Formation deposited in a deep water setting extended
westward from McMinnville, while near-shore sands,
silts, and muds of the shallow marine Spencer Forma-
tion produced deltas along the margin. Found along
the western side of the valley from Eugene north to
Gales Creek in Washington County, Spencer sands are
covered by nonmarine tuffs and conglomerates of the
late Eocene Fisher Formation. Fossil plants from the
Fisher Formation southwest of Cottage Grove indicate
a warm, moist tropical climate where broad leaf plants
as the Aralia grew close to the shoreline. Beneath
Eugene almost a mile of upper Eocene silts and sands
of the Eugene Formation extend northward toward the
Salem hills. Marine molluscs, crabs, and sharks in this
formation suggest warm, semitropical seas. Sediments
of the Spencer, Fisher, and Eugene formations were
derived from the rapidly growing volcanics of the
Western Cascades.
Willamette Valley stratigraphy (after COSUNA, 1983)
Oligocene
The Oligocene ocean in the Willamette Valley
reached only as far south as Salem. The high-water
mark on the western shoreline is recorded by marine
sediments in the vicinity of Silverton and Scotts Mills
in Marion and Clackamas counties. In the Scotts Mills
Formation a transgressive, advancing seaway followed
by a regressive, retreating ocean chronicles storm
conditions, shallow water, and coastal swamps that gave
rise to thin layers of low-grade coal. Coal beds at
Wilhoit Springs and Butte Creek were deposited along
the margins of the sea as it retreated. Prior to the
arrival of the Columbia River lavas in the middle
Miocene, the Scotts Mills sediments were tilted east-
ward and severely eroded.
