ABSTRACT
Seismic reflection
profiles,
industry well logs, and seafloor samples were used to construct
structure
contours of a late Miocene-early
Pliocene unconformity on the Oregon continental shelf and upper slope,
outlining a deformed elongate forearc basin. The unconformity is
probably 6-7.5 Ma, a worldwide hiatus found at northeast Pacific ODP
sites.
The unconformity is angular and probably subaerial on much of the
middle
to inner shelf, and could be traced farther seaward as a continuous
reflector
where it may be conformable or disconformable. Depths to the
unconformity
are up to 2.5 km, with thick Eocene to late Miocene strata below.
The current seaward edge of the basin forms a N-S-trending outer-arc
high,
but the basin may have formerly extended farther seaward. The
outer-arc
high prevented the bypassing of sediment onto the accretionary wedge
and
abyssal plain. The unconformity is deformed by active shelf
structures,
including late Pliocene-Pleistocene submarine banks which control the
position
of the shelf edge, and active faults including Daisy Bank and Nehalem
Bank
faults. The active Stonewall Bank anticline isolates a former remanent
of the basin, the Newport syncline. The landward boundary of the
basin is the uplifted Coast Range, but prior to late Miocene uplift,
the
basin extended farther east. Coast Range uplift caused the basin
axis to shift to the west, and may be related to a reduction of plate
convergence
rate and shallowing plate dip. The coastline has migrated little
from late Miocene to present. The Cascadia forearc basin may be
equivalent
to the Neogene Eel River basin off northern California, and analagous
to
the ancient Cretaceous Great Valley and the modern Java-Sumatra system,
with a deep forearc basin bound to the west by an active accretionary
wedge.
Filling of the Cascadia basin in the Pleistocene and erosion of the
continental
shelf during late Pleistocene lowstands resulted in the forearc
geometry
seen today. Comparison of offshore and onshore margin-parallel
uplift
rates reveal many similarities, including low uplift on the central
Oregon
margin which may represent low plate coupling.
OBJECTIVES
Figure 1. Tectonic map of the
Cascadia
subduction zone. Yellow box indicates study area and location of
Figure 2.
METHODS
A grid of multichannel migrated seismic reflection profiles was used to map out the current depth of the unconformable surface (Figure 2). The late Miocene-early Pliocene unconformity was initially identified from interpretations of benthic foramenifera (S. Drewry et al., unpublished data) taken from industry boreholes. Where the unconformity is angular on the inner to middle shelf, the surface could be easily
Figure 2. Map of the central and
northern Oregon continental shelf and slope. Positions of seismic
reflection datasets, seafloor samples, and exploratory wells used to
produce
the structure contour map of the late Miocene unconformity are shown.
traced along the network of profiles. Farther seaward, the
non-angular
reflectors may represent a disconformity or a correlative
conformity.
All available dated seafloor samples (Figure 2) were used to identify
Miocene
outcrops on the seafloor. The unconformity was found to be
continuously
traceable on much of the northern and central Oregon margin, with
correlation
to the north into Washington and south into southern Oregon more
complex.
The unconformity was
digitized
to produce an xyz file of unconformity position, with z initially in
seconds
two-way time, and later
converted to depth using appropriate velocities from seismic refraction
surveys and sonic logs. Depths shown here are below current sea
level.
The identification of a younger late Pliocene-Pleistocene surface
(angularly
unconformable in places) was used to sub-divide the post-Miocene
section
into Pliocene and Quaternary units. These were assigned
velocities
of 2.1 and 1.7 km/s respectively. The resulting xyz data were
used
to construct structure contours of the unconformity.
Figure 3. Multichannel migrated seismic reflection profile on the central Oregon inner shelf, due west of Cape Foulweather. This seismic profile clearly shows the angular late Miocene-early Pliocene age unconformity (PM) dipping to the west from the coastline. The unconformity truncates the middle Miocene Columbia River Basalt Group (CRBG) flows. Thick sequences of basinal strata of Eocene to late Miocene age are seen below the unconformity and increasing thicknesses above the unconformity to the west. The unconformity is angular throughout much of the inner and middle shelf and is presumed to have been eroded subaerially.
Figure 4. Multichannel migrated seismic reflection profile across the central Oregon shelf, west of Yaquina and Alsea Bays. This profile crosses Stonewall Bank anticline and Newport syncline to the east, a remanent of the broad early to mid Tertiary forearc basin. The angular late Miocene-early Pliocene unconformity (PM) truncates late and middle Miocene strata east of Stonewall Bank, including the Columbia River Basalt Group (CRBG). West of Stonewall Bank, strata are parallel with little sign of angular unconformity. The late Miocene-early Pliocene boundary is thought to be a disconformity or correlative conformity in this region. Miocene strata are uplifted and exposed on Stonewall Bank. The absence of significant thinning of Pliocene-Pleistocene strata across Stonewall Bank indicates that the anticline began to grow as recently as the late Pliocene or Pleistocene. Yeats et al. (1997) suggest that the bank is uplifted by a blind reverse fault capable of generating an Mw=6.8 earthquake.
Figure 5. Shaded relief image of the late Miocene-early Pliocene unconformity structure contour map on the central and northern Oregon continental shelf. The western edge of the image represents the current extent of basinal sediments, although the basin may have extended farther west and since been eroded. The unconformity reaches the seafloor at the eastern edge of the shaded image, where middle Miocene and older formations are exposed. The unconformity is deformed by Pliocene and Quaternary structures including the Stonewall Bank anticline, Newport syncline, uplifted submarine banks (Heceta, Nehalem, Siltcoos), the Nehalem Bank fault, and the left-lateral Daisy Bank fault. Cross sections of the unconformable surface, A-A', B-B', and C-C' are shown in Figure 6. D-D' represents the representative margin-parallel section used to compare relative offshore vertical motions with onshore uplfit rates (Figure 8). Uplift rate contours (mm/yr) determined from highway releveling surveys during the last 50 years are shown onshore in light blue. Notice the coincidence of negligible onshore uplift rates between 44.5° and 45.5° N with the deep basin deforming the unconformity (including the Newport syncline). Depths are below sea level.
Figure 6. Cross sections of the late Miocene-early Pliocene unconformity taken across the shaded relief image of Figure 5. The E-W profile, A-A', crosses the Stonewall Bank anticline and Newport syncline to the east. The unconformity is at a maximum depth of 2500 m in the axis of the syncline, but uplifted and eroded at Stonewall Bank. B-B' crosses Heceta Bank, one of several prominent and recently uplifted submarine banks on the Oregon margin. C-C' crosses the northward extension of the Newport syncline and the Nehalem Bank fault. This fault uplifts and truncates the unconformity and projects into Netarts Bay where it deforms late Pleistocene sediments.
Figure 7. Perspective view to the south of shaded relief image of the late Miocene-early Pliocene unconformity structure contour map, between 45.5° and 43.5° N. The present coastline would be located on the left of this image. This image clearly shows the young uplifted submarine banks of the Oregon margin, Stonewall, Siltcoos and Heceta, where Miocene strata are uplifted to the seafloor and eroded. The WNW-trending left lateral Daisy Bank fault also deforms the unconformable surface. This fault offsets Quaternary folds of the modern accretionary prism and abyssal plain sediments to the west. The unconformity reaches the seafloor at the eastern edge of the shaded image, and middle Miocene and older formations are exposed on the innermost shelf and onshore.
ASSUMPTIONS
The precise age of the unconformity is unknown, although benthic forams and seafloor samples suggest a late Miocene or early Pliocene age. We hypothesize that the unconformity represents a worldwide hiatus (NH6) in the late Miocene (6-7.5 Ma), recorded at ODP sites including those of the northeast Pacific (Keller and Barron, 1987). The unconformity (and correlative conformity) may be time transgressive. In some regions, the unconformity was difficult to trace (due to non-angularity or faulting), prodcuing some error in correlation. We assume that the angular unconformity was sub-aerial on the inner and middle shelf, and therefore initially at approximately the same depth parallel to the coastline. The current depth of this surface can therefore be used to determine relative uplift rates on the shelf.
BASIN GEOMETRY
The structure contour map
of the late Miocene-early Pliocene unconformity delineates a deformed
broad
elongate forearc basin underlying the current Oregon shelf and upper
slope.
Forearc basinal strata above the Pliocene-late Miocene unconformity
attain
thicknesses of up to 2500m off central Oregon, with even greater
thicknesses
of underlying Eocene to late Miocene sediments. The current
seaward
limit of basinal strata is marked by a N-S-trending outer-arc
high.
Strata underlying the late Miocene unconformity, and early Pliocene
strata
in some cases, do not thin or onlap against this structural high.
We therefore hypothesize that the forearc basin extended farther
seaward
prior to growth of this high. The basin continued to fill into
the
early Pleistocene, whereas sediments currently bypass the shelf and are
accumulating in slope basins and on the abyssal plain. Basin
strata
have been uplifted by late Pliocene and Quaternary structures and
subsequently
eroded. Paleo-water depths indicate shallowing up section,
suggesting
that sedimentation rates exceeded tectonic subsidence.
Prior to Coast Range uplift,
the forearc basin extended from the current continental slope to the
Willamette
Valley east of the Coast Range. The Coast Range marked the landward
boundary
of this forearc basin following uplift in the late Miocene.
Pliocene-Pleistocene
strata onlap against the late Miocene unconformity and dip westward
indicating
continued uplift of the Coast Range and growth of the basin from late
Miocene
to present. The position of the unconformity at the seafloor on
the
inner most shelf, close to the present coastline, suggests that the
coastline
has not migrated significantly from late Miocene to present. This
suggests that during Coast Range uplift and subsidence of the westward
forearc basin, the coastline has acted as a hingeline. With the
onset
of Coast Range uplift, the axis of the active forearc basin shifted to
the west. Uplift may have been associated with a shallowing subducting
plate due to changes in plate convergence rate.
MARGIN-PARALLEL UPLIFT RATES
Assuming the undeformed
late
Miocene unconformity was subaerially eroded on the inner shelf,
deformation
of this surface can be used to determine relative net uplift and
subsidence
rates along the margin. Comparisons were made with onshore uplift rates
from highway releveling surveys (Mitchell et al., 1994) and uplifted
Pleistocene
marine terraces (West, 1986). Low uplift rates from releveling
and
marine terraces between 44.5° and 45° N are coincident with the
deepest region of the offshore unconformity, the Newport
syncline.
To the south of Yaquina Bay, between 44.5° and 43°N, generally
higher onshore uplift rates coincide with the unconformity at shallower
depths. Between 44° and 45°N, the depth of the late
Miocene
unconformity shows many similarities to the pattern of uplift of the
80ka
marine terrace. North of 45°N, the depth to the offshore
unconformity
shallows, coincident with increasing uplift rates from highway
releveling.
Relative trends of uplift and subsidence along the margin have remained
similar from the latest Miocene to present. Low uplift rates in
central
Oregon may be a result of low coupling on the subduction zone in this
region,
and may represent a segment boundary, as suggested by Kelsey et al.
(1994).
Figure 8. Comparison of relative
uplift rates onshore and offshore. Onshore datasets are (a) uplift
rates
from highway releveling surveys for the last 50 years and (b) uplifted
late Pleistocene marine terraces (80-220 ka). The offshore
dataset
represents a cross section (D-D' of Figure 5) approximately parallel to
the margin and coastline of the late Miocene-early Pliocene
unconformity.
Note the low uplift rates or subsidence between 44.5° and 45°
N.
DEFORMATION OF THE LATE MIOCENE-PLIOCENE UNCONFORMITY
The unconformity is
deformed
by post-late Miocene structures on the continental shelf and upper
slope.
The current shelf break post-dates the basin geometry and deformation,
with synclinal basin axes and the outer-arc high (marking the seaward
extent
of forearc basin deposits) crossing this topographic boundary.
The
shelf break is in part controlled by the positions of major uplifted
submarine
banks, namely Heceta and Nehalem Bank, and intervening
embayments.
Structures underlying these banks are young - the absence of
significant
growth strata within much of the post-late Miocene unconformity
sequence
suggests a late Pliocene to Pleistocene age. Stonewall Bank
anticline
deforms the formerly broad basin and uplifts late Miocene strata to the
seafloor, with evidence of 1500-4000 m of eroded strata from the crest
of the anticline (Yeats et al., 1997). Absence of onlapping Pliocene
strata
against the late Miocene unconformity and thickening rather than
thinning
of much of the earlier Pliocene-Pleistocene sequence indicate that this
anticline bagan to form during the late Pliocene. Yeats et al.
(1997)
hypothesize that this structure is underlain by a blind reverse
fault.
Uplift of Stonewall Bank anticline isolates a remanent of the former
forearc
basin, the Newport syncline. The Nehalem Bank fault, due west of
Netarts and Tillamook Bays, uplifts the unconformity and Miocene strata
to the seafloor. This fault also deforms and offsets Quaternary
strata
and projects onshore as the Happy Camp Fault at Netarts Bay, which may
be structurally controlled. One of several WNW-trending
left-lateral
faults, which cross the Cascadia margin from the abyssal plain to the
shelf
edge, the Daisy Bank fault (Goldfinger et al., 1997), deforms the late
Miocene unconformity west of Stonewall Bank.
SUMMARY
The structure contour map
of the late Miocene-early Pliocene unconformity outlines a former broad
elongate forearc basin, possibly
analagous to the Eel River Basin off northern California, which has
subsequently been deformed by structures active during the Pliocene and
Quaternary. Prior to Coast Range uplift and erosion of the
western
edge of the basin, this forearc basin extended from the present
continental
slope to the Willamette Valley. Modern and ancient analogues of
deep
elongate forearc basins include the Java-Sumatra system and the
Cretaceous
Great Valley. Comparisons of the deformed unconformity with
onshore
datasets indicate similar variations in margin-parallel uplift
rates.
Coastal and offshore central Oregon is characterized by very low uplift
rates or subsidence, whereas uplift rates are higher to the north and
south.
Low uplift may be a result of low coupling on the subduction zone, and
this region may act as a segment boundary during subduction zone
earthquakes.
REFERENCES
Goldfinger, C., Kulm, L.D., Yeats, R.S., McNeill, L.C., and Hummon,
C.,
1997, Oblique strike-slip faulting of the central Cascadia submarine
forearc: Journal of Geophysical Research, v. 102, p. 8217-8243.
Keller, G., and Barron, J.A., 1987, Paleodepth distribution of
Neogene
deep-sea hiatuses: Paleoceanography, v. 2, p. 697-713.
Kelsey, H.M., Engebretson, D.C., Mitchell, C.E., and Ticknor, R.L.,
1994,
Topographic form of the Coast Ranges of the Cascadia margin in relation
to
coastal uplift rates and plate subduction: Journal of Geophysical
Research, v. 99, p. 12,245-12,255.
Mitchell, C.E., Vincent, P., Weldon II, R.J., and Richards, M.A.,
1994,
Present-day vertical deformation of the Cascadia margin, Pacific
northwest,
U.S.A.: Journal of Geophysical Research, v. 99, p. 12,257-12,277.
West, D.O., 1986, WNP-3 Geologic support services, Coastal terrace
study:
Report prepared for Washington Public Power Supply System.
Yeats, R.S., Kulm, L.D., Goldfinger, C., and McNeill, L.C., in
press,
Antecedent stream at Stonewall Bank: Slip rate on a blind fault
beneath
the Oregon continental shelf: Geological Society of America
Bulletin.
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