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FACIES (TIME TRANSGRESSIVE)
Elll SHELF
o SLOPE
IZ'J TRENCH
o ABYSSAL PLAIN
OUTER
HIGH
less deformed sediments, and essentially undeformed
shelf deposits cover the slope units with apparent angular
discordance. This model is illustrated in Figure 6.
The modern analog suggests that the Umpqua
Group and Tyee Formation were sequential parts of an
accreting continental margin, with depositional patterns
closely related to structural development.
V.E. ],I
Figure 6. The Umpqua Group may have been deposited
on an accreting continental slope and shelf in a
fashion similar to that proposed by Seely and others
(1974), with whose permission this illustration is
reproduced.
SYNTHESIS
One explanation of the structures in the Sutherlin
area could be that they were formed in a single deformational
event after deposition of the Roseburg sediments
and then uplifted and eroded prior to deposition of the
Lookingglass Formation. However, the change in rock
types going up in the stratigraphic column shows a logical
transition from the thin, rhythmic, turbidite slope
deposits of the Roseburg Formation to the deltaic or
shelf deposits of the nearby Tyee Formation. The large,
northeast-trending folds in the Sutherlin area were forming
while the Roseburg sediments were being deposited
on the continental slope. Decreasing deformation upward
in the section also indicates a transition to a more
stable shelf environment, and the relationship between
stratigraphy and structure suggests that deposition and
structural development occurred contemporaneously.
Currents carrying Roseburg sediments may well
have been controlled by the growing, northeasterlytrending
folds. By the time of deposition of the Tyee
Formation, these structures had become inactive, and
almost all were buried, although a few remaining topographic
highs on the Tyee sea floor locally affected flow
patterns, as near Drain. Turbidity flows originating
near the top of the slope to the south and east would
have been deflected by the developing ridges to flow
down trough axes; as each successive trough was filled,
or where the divide was low, flows would have overtopped
the adjacent downslope ridge and contributed
finer grained and progressively younger sediments to the
next lower trough. This model, like a series of baffles
across a slope, would explain both the Roseburg Formation
current directions and the northward change to
finer grained facies.
The modern continental margin of Oregon and
Washington (Silver, 1971, 1972; Carson and others,
1974; Kulm and Fowler, 1974) is comparable to this
model. Longitudinal ridges and troughs characterize the
present continental slope. Seismic profiles show these
ridges to be anticlinal and commonly bounded by steep
faults; the intervening troughs are filled with younger,
ACKNOWLEDGMENTS
Our thanks to Chris L. Nastrom, who assisted in
drafting, to Ewart M. Baldwin for many discussions of
Umpqua stratigraphy, and to our colleagues for critically
reading the manuscript and patiently listening to
our ideas.
REFERENCES
Baldwin, E.M., 1964, Thrust faulting in the Roseburg area,
Oregon: Oregon Department of Geology and Mineral Industries,
Ore Bin, v. 26, no. 10, p. 176-184.
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- - - 1975, Revision of the Eocene stratigraphy of southwestern
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(Continued, p. 146)
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OREGON GEOLOGY. VOL. 42. NO.8. AUGUST 1980