Geology Reference
In-Depth Information
Willamette Valley under a shallow seaway. Volcanism
had increased considerably in the Western Cascades
showering tremendous quantities of ash directly into
the sea. Streams draining the mountains washed
additional volcanic material into the basin where it was
carried out onto shelf and slope by storm waves and
turbidity currents.
Miocene
Deposition contined uninterrupted from the
Oligocene into the early Miocene. The Miocene is
characterized by a rapidly retreating ocean as waters
receeded to the present limits with regional uplift and
continued rotation of the Coast Range block into the
Pliocene. With the shallowing of the forearc region, a
number of small basins on the shelf deepened toward
the west. Microfossils in the muds and silts of the Nye
Mudstone suggest they were deposited in deep cold
water of the basin at depths as much as 2,000 feet.
Fossiliferous sandstones and siltstones of the
Astoria Formation deposited here are among the best
known exposures on the central and northern coast. In
addition to a well-preserved fauna of fossil molluscs,
corals, barnacles, brachiopods, and crabs, Astoria
sediments are famous for remains of marine vertebrates
as sharks, turtles, whales, seals, and sea lions. Even
occasional ungulates or hoofed mammals were washed
Crinoids (sea-lilys) and echinoids (sea
urchins) inhabited the ocean during the
Keasey period.
Formation clastics, like those of the Tyee Formation of
the southern Coast Range, are "arkosic" or rich in
white mica, quartz, and potassium feldspar and proba-
bly had a similar source in the Idaho batholith. On the
shelf of the central basin, ash mixed with silts and
sands of the Alsea and Yaquina formations.
In the interval between 38 to 29 million years
ago, a number of intrusive bodies from batholith
sources below the forearc basin invaded the softer
marine sediments of the Coast Range. Later erosion,
aided by coastal uplift, stripped off sediments to expose
these Oligocene dikes and sills which form the major
peaks of the range today. The most prominent intrusive
bodies make up Marys Peak, Saddleback Mountain
southwest of Grande Ronde, Laurel Mountain west of
Dallas, Fanno Ridge, Sugarloaf and Stott mountains
north and northwest of Valsetz, Grass Mountain
northwest of Alsea, and Roman Nose Mountain south
of Mapleton.
Seal and skull from the Astoria Formation
Oligocene
Land areas along the margins of the ocean
emerged from the shrinking Oligocene ocean basin. By
the late Oligocene and early Miocene, uplift of the
Coast Range block was nearly complete with only the
central and north end of the coast and northern
