The continental terrace west of Oregon between 43° 50'N and
44° 40' N latitude is 50 to 55 miles wide. It consists of a continental
shelf, 16 to 35 miles wide, and a continental slope, 16 to 37 miles
wide. The eastern portion of the shelf is a smooth, sediment covered
area that slopes very gently west. The western portion of the shelf
contains four rocky bank areas. The banks are topographically irregular
and appear to be of structural origin. West of the banks the
shelf edge occurs at depths of 71 to 90 fathoms.
The continental slope extends from the edge of the shelf to the
abyssal plain at depths of 1530 to 1610 fathoms. A smooth upper slope
of less than three degrees extending to depths of 117 to 250 fathoms
occurs north and south of Heceta Bank. West of Heceta Bank the upper
slope is formed by a scarp that slopes 10° to 16° to 560 to 725
fathoms. West of the upper slope there is an area of irregular topography,
including benches, hills and scarps, which extends to depths
of 380 to 1100 fathoms. The lower part of the slope is formed by a
north-striking scarp which is 3000 to 6000 feet high and slopes 04° to
15°. The bathymetry indicates that the continental slope was formed
by step-type, block faulting.
Sediments form a thin surface layer over much of: the terrace.
Detrital sand, similar to the coastal sand, covers the shelf from the
shoreline to approximately 50 fathoms. The deeper areas on the shelf
and upper part of the slope are covered by glauconitic sands and silts
on the topographic highs and olive green, clayey silts in the topographic
lows. The intermediate and lower portions of the slope are
blanketed with olive-green, clayey silt. In these sediments the sand
fraction, which generally comprises less than five percent of the sample,
is composed chiefly of diatoms, Foraminifera, Radiolaria, and
sponge spicules. Sands are also present on the intermediate and deep
portions of the slope. Dredge hauls west of Newport obtained sand
composed mainly of detrital grains which may have been derived from
an underlying friable sandstone. Thin layers of sand occur in cores
from other portions of the slope. These sands may have been derived
by down slope movement of sediment from the upper slope and the
shelf.
Sedimentary rocks of Upper Miocene and Pliocene age crop out
on the shelf banks and on the continental slope. The banks consist of
a sequence of diatomaceous, clayey siltstones with interbeds and
concretions of calcareous siltstones. Glauconite sandstone, gray
wacke sandstone, and limestone breccia are exposed along with the
siltstone, on the northern end of Heceta Bank. Most of the rocks obtamed
from the slope are similar to those from the shelf. Friable,
wacke sandstone is exposed on the slope west of Newport.
Foraminifera, the sand fraction compositions, and textural analyses
all indicate that the sediment forming the siltstones from the
shelf were deposited at lower littoral to lower bathyal depths.
The sediments forming the rocks were deposited in one or more
sedimentary basins during the Miocene and Pliocene. The subsidence
continued until the. Late Pliocene when the area began to rise. The
area was uplifted as much as 1000 fathoms by the Late Pleistocene
when the shelf was eroded by transgressions and regressions resulting
from sea level changes. The last rise in sea level resulted in the erosion
of the shelf to its present form and the deposition of a thin layer
of sediment. Sand is presently being deposited on the shallow areas
adjacent to the continent, and silt and clay are being laid down on the
slope and the sheltered areas of the outer shelf. / Graduation date: 1965
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/28497 |
| Date | 23 April 1965 |
| Creators | Maloney, Neil Joseph |
| Contributors | Byrne, John V. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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