Geology Reference
In-Depth Information
Invertebrate fossils of the Astoria
Formation
into the middle Miocene seaway to become entombed
in the Astoria silts. Today the erosion of these sedi-
ments along the coast between Astoria and Lincoln
City continuously exposes fossils in the sea cliffs.
Middle Miocene time also saw the invasion of
Columbia River lavas along the northern coastal area.
Fifteen million year old basalts are known from Seal
Rocks, Yaquina Head, Depoe Bay, Cape Kiwanda,
Cape Lookout, Capes Meares, and Cape Falcon, and
Tillamook Head. The basalts found there today are
chemically and mineralogically identical to the Colum-
bia River lavas. The striking similarity between the two
suites of basalts has always been regarded as an enig-
ma, but it clearly suggests that they were derived from
the same magma beneath the Grande Ronde valley of
eastern Oregon.
Originally interpreted as having erupted from
local volcanoes on the coast, these puzzling basalts now
have been designated "invasive" rather than "intrusive".
The invasive mode of origin suggests that the coastal
Miocene basalts are the distal ends of flows that were
extruded elsewhere. Voluminous liquid masses of lavas,
flowing down the Columbia River valley and across the
coastal plain, temporarily ponded up in bays and
estuaries before advancing seaward as nearshore basins
filled to capacity. Tremendous hydrostatic pressure by
The Miocene baleen whale,
Cophocetus,
from the
Astoria Formation
