A structure map was constructed of the continental shelf between
Cape Blanco and Coos Bay, Oregon, exclusively from an interpretation
of approximately 700 km of continuous seismic profiles.
At least ten discernible seismic units were mapped on the bases of
acoustic appearance, lateral continuity, angular unconformities, and
faults. The offshore units tentatively were correlated with onshore
geology and are believed to range in age from the latest Jurassic to
late Pleistocene. The sparker profiles reveal that the continental
shelf off southern Oregon has experienced significant late Tertiary
and Quaternary accretion due to deposition and tectonic uplift.
The oldest rock exposures, believed to be the latest Jurassic
in age, crop out on the inner continental shelf on the topographic
highs off Cape Blanco and Coquille Point. Erosional remnants of
probable Late Cretaceous turbidites and the middle Eocene sandstone
beds also are exposed on the bathymetric high on the inner
shelf southwest of Cape Arago. The initial emplacement of these
three uplifted structural blocks is probably a result of late Eocene
wrench faulting of the Port Orford shear zone and of the postulated
shear zone at Coquille Point.
No other early Tertiary sediments apparently are exposed on
this portion of the Oregon continental shelf, but they probably extend
seaward at depth on the continental margin. Middle Tertiary
strata are believed to be exposed on the outer shelf topographic high
southwest of Cape Arago.
Sediments of Miocene to Pliocene age were deposited throughout
much of the continental shelf that was surveyed. The greatest
amount of deposition occurred in a north-south trending basin between
Cape Blanco and Coquille Bank. Late to post-Pliocene tectonism
uplifted and exposed the older underlying rocks on the inner
shelf, which are probably of uppermost Jurassic to middle Tertiary
age. These same stresses also deformed the Mio-Pliocene sediments
into gently undulating structures on the inner shelf. The
greatest deformation occurred on the outer shelf and formed
Coquille Bank, a north-south trending, doubly plunging, asymmetrical
anticline. The terraces or benches on the upper continental
slope to the north and south of the Bank are structural features resulting from the doubly plunging anticline.
Eustatic sea level lowerings during the Pleistocene truncated
the shelf structures as deep as 130 m below present sea level. The
detritus was deposited as a wedge of sediments, which forms an unconformable
contact with the underlying strata on the outer shelf and
upper slope between Coos Bay and Coquille Bank. In areas of deposition
there is no distinct break between the shelf and the upper slope;
the former merely merges into the latter in a continuous convex
curve. In areas of nondeposition, the edge of the shelf is an erosional
and structural feature.
A possible buried channel was detected northwest of the mouth
of the Coquille River. This sediment filled channel is believed to be
an erosional remnant of a former course of the Coquille River during
a lower stand of sea level. / Graduation date: 1969
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/29429 |
| Date | 01 November 1968 |
| Creators | Mackay, Angus James |
| Contributors | Heinrichs, Donald F. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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