The continental margin off southern Oregon, which includes the
shelf and slope from Cape Blanco to the Oregon-California border,
exhibits a distinctive marginal-plateau structural pattern which divides
the margin into the continental shelf, the upper continental slope and
its associated benches, and the lower continental slope. Lutum transport
and deposition have dominated the sedimentary processes on the
margin since the start of Holocene time.
The structure of the southern Oregon margin is characterized by
north-south trending compressional folds, and near-vertical faults
which have been down-dropped to the west. Large-scale folds on the
upper slope have ponded sediments behind them resulting in the formation
of the Klamath Plateau, Cape Blanco Bench, and other bench-like
features. Development of the structural pattern is most likely a result
of the compressive underthrusting of the oceanic lithospheric plate
beneath the southernmost Oregon-northern California margin and the
crustal extension which exists throughout the nearby continent and
ocean basin.
Useful stratigraphic horizons within the late Pleistocene and
Holocene margin deposits include Mazama Ash (6600 years B.P.) and
several recognizable shifts in the abundance of Radiolaria and planktonic
Foraminifera, particularly one dating from 5000-4000 years
B.P. Holocene sedimentation rates vary from an average of 10 cm/
1000 years on the upper slope to an average of 50 cm/1000 years on
the lower slope, indicating that the lower slope is out-building and up-building
more rapidly than the upper slope. The paleo-depth range of
Pliocene fauna in sedimentary rocks from the margin suggests that
subsequent to their deposition both uplift and subsidence occurred on
the southern Oregon margin.
Sediments from the southern Oregon margin consist primarily
of olive gray lutite, gray lutite, and sand-silt layers. Olive gray
lutite is Holocene in age and is ubiquitous on the margin, with the
thickest accumulation (10 m average) found on the lower slope, while
the distribution of Holocene lutite on the upper slope is thin and patchy
(3-4 m or less). The gray lutite appears to be a late Pleistocene
deposit, and the sand-silt layers reflect both ages. The surface sediment
distribution pattern on the shelf consists of modern inner shelf
sand, modern central shelf mud, and mixed deposits of both types.
Relict deposits are present at the shelf edge. The lower slope consists
entirely of modern mud, but the surface sediment on the upper slope
and benches consists of both modern and relict deposits, and mixtures
of the two.
The mineralogy of the unconsolidated and consolidated sediments
from the margin indicates that the Klamath Mountains have been the
dominant source for these deposits since early Tertiary time. This
is reflected in the abundance of blue-green hornblende and other heavy
minerals indicative of the Mesozoic rocks of the Klamath Mountains;
the same source is suggested for the abundant chlorite found in the clay
fraction of margin sediments and rocks. There are indications in the
mineralogy of lower slope sediments which suggest that the Tertiary
strata of the southern Oregon Coast Ranges may be a secondary source
for the deposits in this environment. When compared to the upper
slope sediments, those from the lower slope have a higher feldspar
content, a higher pyroxene-to-amphibole ratio, and an apparently
higher illite content.
As a result of the Holocene rise in sea level, the deposition of
coarse clastics on the southern Oregon margin has been restricted to
the inner shelf. Consequently, only the fine-grained lutum discharged
from rivers is deposited on the outer margin environments. Submarine
topography, oceanographic conditions, and gravity are important
factors which effect transport and deposition of lutum on the margin.
A model of modern lutum transport by bottom turbid layers and
fine-particle suspensate is proposed for the southern Oregon margin.
Long-period swell is believed to be responsible for much of the
formation of bottom turbid layers on the shelf. Once formed, these
turbid layers move north and west over the shelf under the influence of
shelf currents, alternating tidal action, and gravity; upon reaching the
slope they are funneled into submarine valleys and deposited on the
lower slope and adjacent deep sea. Lutum deposited on the upper slope
is eventually re-suspended and transported by southerly bottom currents
into down-slope valleys; very little lutum remains behind on the
upper slope. Deposition of the fine-particle suspensate as well as
slumping and other gravitational processes contribute to the lower
slope sediments. The end result of modern lutum transport is the
continual up-building and out-building of the lower slope. / Graduation date: 1971
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/29282 |
| Date | 28 August 1970 |
| Creators | Spigai, Joseph John |
| Contributors | Kulm, L. D. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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