Cores taken from the ridge areas of the Gorda-Juan de Fuca
plate have a sedimentation rate that is appropriate for the study of
late Quaternary stratigraphy. An analysis of the clay and silt
mineralogy of the cores using X-ray diffraction methods and by
noting changes in the foraminiferan-radiolarian abundances in the
cores were utilized in developing a stratigraphic sequence.
The clay fractions of these sediments consists of chlorite,
illite and smectite. Cores taken from bathymetric highs contain,
on the average, less smectite than do the turbidites from the adjacent
lowlands. The low smectite content suggests eolian enrichment of
these sediments since dusts collected from the nearby continent also
have low smectite concentrations.
Changes in the relative abundances of radiolaria and foraminifera
are used to put biostratigraphic constraints on the correlation of
mineralogical datums. Two changes in the foraminiferan-radiolarian
ratios, marked by sharp increases in the abundance of radiolaria,
occurred at 12,500 years B.P. and 83,000 years B.P. as dated by
carbon-14 and sedimentation rate extrapolations, respectively. Such
faunal changes serve as an independent check of correlations of the
mineralogical datums.
In the 2 to 20 micron, silt fraction, quartz, chlorite, mica and
feldspar are the predominant minerals. Intervals in which the relative
abundance of quartz changes can be dated by carbon-l4 and
sedimentation rates, and related to late Quaternary climatic events.
The quartz-rich zones are synchronous with periods of high insolation, high stands of sea-level, and to a lesser degree with the
catastrophic floods of the Columbia River. The correlation with high
solar radiation reflects quartz enrichment of the sediment due to an
increased eolian contribution. The coincident high sea level stands
effectively decreased the sedimentation rate of quartz-poor continental
detritus that otherwise dilutes the eolian component. The
periodic floods of the Columbia River, caused by the failure of ice
dams, swept quartz-rich loess from eastern Washington down the
river and injected into the marine environment. Such sediment also
increased the quartz abundance in the quartz-rich zones on the
ridges. Thus, the late Quaternary stratigraphy of the cores can be
related to global late Quaternary climatic variations as well as to
events recorded on the adjacent continents.
The structural development of the Gorda-Juan de Fuca plate
over the last 10 million years can be explained by north-south
shortening coupled with the normal tectonism associated with a
spreading sea floor.
This hypothesis for the development of the plate is based on the
presently known magnetic anomaly pattern. A series of reconstructions
of this pattern back through the past 10 million years shows
that both the Gorda and Juan de Fuca portions of the plate have grown
steadily smaller. The incorporation of sequentially shorter Gorda
ridge anomalies into the Pacific plate appears to have led to the
northwest-southeast orientation of the Blanco Fracture Zone, with
consequent changes in the direction of spreading of the Juan de Fuca
Ridge. On the Juan de Fuca portion of the plate, the shortening was
accomplished by shear faulting in Cascadia Basin. Furthermore,
this faulting resulted in the rapid subduction of this portion of the
plate, which, in turn, produced a disconformity in the sediments of
Cascadia Basin. The reconstruction strengthens the notion that right
lateral strike slip motion between the Pacific and Gorda-Juan de Fuca
plate does, indeed, exist. / Graduation date: 1974
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/28569 |
| Date | 05 September 1973 |
| Creators | Phipps, James B. |
| Contributors | Kulm, LaVerne D., Heath, G. Ross |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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