ML20151H093
| ML20151H093 | |
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| Site: | Trojan File:Portland General Electric icon.png |
| Issue date: | 04/12/1988 |
| From: | Alexis Nelson INTERIOR, DEPT. OF, GEOLOGICAL SURVEY |
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| NUDOCS 8808010139 | |
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s IMPLICATIONS OF LATE HOLOCENE SALT MARSH STRATIGRAPHY FOR EARTHQI
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RECURRENCE ALONG THE COAST OF SOUTH CENTRAL OREGON t
4 9950-04180
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(Earthquake Recurrence and Quaternary Deformation in the L
l Cascadia Subduction Zone, Coastal Oregon) i Alan R. Nelson Branch of Geologic Risk Assessment U.S. Osological Survey PO Box 25046, MS 966 Denver, Colorado 40225 I
J INTRODUCTION i
Repeated, great plate interface earthquakes have been postulated for the Cascadia subduction
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i one in western Wuhington and Oregon. The best evidence of the coseismic subsidence to be expected near the cout during great earthquakes is found in southwestern Wuhington where many 5
I exposures record repeated episodes of submergence of late Molocene marshes. Atwater and others have used consistent struigraphic relationships HC ages, and plant macrofosC1s from sequences of
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interbedded marsh pests and intertidal muds to show that the 6 marsh pests buried in the fut 4000
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j years throughout souhwestern Wuhington were submerged suddenly The late Holocene estuarine t
record in the central part of the subduction zone in Oregon is more difficult to interpret; there are I
sery few good exposures, and coring at some sites has produced evidence of a gradual rise of late
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Holocene sea level while sea tesel rise appears to be jerky at other sites.
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t MARS!! FORAMINIFERA AS SEA LEVEL INDICATORS
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One of our goils is to show if the peats we find interbedded with muds in cores from the central OreJton coast were submerged suddenly (coselimlealy),like those in Washington, Marsh j
foraminifera are more senaltise (o changes in sea lesel than many marsh plants and are easier to identify in cores. Thus, we have begun to use foraminifera faunu in cores from Oregon marshes i
j to test v riether our buried peats represent jerky (repeated coscismic) marsh subsidence. Because no i
studies of modern marsh foraminifera from the region hase been done, one of our first objectises is to show if modern marsh sutensironments at different elevations can be distinguished using i
foraminifers faunas.
Analyses of samples from surface transects c.f the first two of the Oregon marshes studied show the same strong correlat on between foraminiferal assemblage zones and sea tesel found in i
other marshes worldwide. Three (informal) assemblage zones can be recognized in the transects j
studied so far a high marsh sone, an upper low marsh none, and a low marsh mud flat tone, i
Tide-gauge data are not asaltable for either site, but the distribution of high and low marsh and t
j the position of mean high water can be estimated from macrofloras, The highest samples in each 4
transect, on the upland border, are barren of foraminifers. Samples from the upper part of the high-marsh tone are dominated by TrocAammina macrescens and TrocAsmmina inflora with letter i
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i numbers of bl.liammina fusca and Harlothrasmoldes wilberti. The pescentage of Af. fusca
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increnes towsids the base of the high marsh rene. Where sf. /usca becomes the dominant species oser T, ma rescens, T inflata, and other species we recognite an upper low-marsh none. In both transects this zone includes a 0.5 to I m high, nearly vertleal scarp resulting from modern erosion of the low marsh. In the low-mar:h mud flat none Af. fusca and Ammotium salsum co TrocAsmmina is absent, and ArorAax nana and calcareous species become incresslag abundant with
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I decrening elesation. On the basis of these preliminary analyses, we should be able to identify i
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former sudden changes in sea level of about 0.51.0 m in future analyses of cores from Oregon i
estuarles. Studies incorporating more accurate vertical control and more detailed sampling might 1
be able to resolve significantly smaller changes in sea level, i
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CHARACTER OF SEA LEVEL RISE INDICATED BY MARSH STRATIGRAPHY Preliminary coring and study of outcrops at 12 raarsh sites in seven tidal inlets yields i
conflicting evidence for the history of relative sea level along the south-central Oregon coast.
l Additional radiocarbon dates from most of these sequences are pending, l
i At iltes in the eastern arms of Cool Bay, one probable buried marsh surface is found in the upper i m of most cores overlying 4 6 m of uniform mud, in some cores both upper and lower L
1 contacts of peaty units are gradational, but in most cores the thickest peat bed has a fairly abrupt I
upper contact suggesting sudden submergence of a marsh. A spruce root from this buried surface i
i in Shinglehouse $1ough was dated at 340 "C yr8P. One interpretation of this type of marsh
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4 sequence is that sediment deposition rates in most tidal inlets have been low during all but the last I
few hundred years of the late Holocene and that for this reason no evidence (buried marsh I
j surfaces) of earlier sudden submergence events has been presersed. Another interpretation is that I
j no sudden changes in sea level have occurred.
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in contrast, at two sites in South Slough in western Coos Bay, cores show 6 8 abruptly buried
't marsh surfaces that are 0.4-1.2 m apart. Extensive coring in a small marsh along Winchester Creek J
revealed up to 8 buried marsh surfaces in sections 5 8 m thick. The 4 best deseloped surfaces can j
be correlated across the inlet. The uppermost buried surface has a modern "C age; it must have I
been buried by sedimentation following diking of the marsh Lower surfaces date at 460 (2.2 m) i j
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and 2380 (2.3 m) "C yrBP, indicating highly non uniform sedimentation rates. A core from Day Creek (described with C. Peterson and M. Darlenro, OSU),4 km to the north, had a similar 1
sequence of 6 buried surfaces. These sites are near the axis of the South Slough syncline, and j
tilted marine terraces on the west limb of the syncline document continued late Pleistocene folding i
j of this structure. Thus, the South Slough buried surfaces may record local Holocene coseismic j
faulting or folding rather than regional deformation of the central Oregon coast during great plate.
l interface earthquakes. Alternathely, sudden slip on flexure slip faults within the syncline might 1
t also occur primarily as a response to large subduction sone earthquakes.
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Coring in South inlet, an arm of the Siuslaw River estuary, shows that 4 m of fairly uniform j
j peat oserties 4 m of mud. Thl t>pe of marsh sequence suggests that late Holocene relatise sea.
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level rise was gradual with no abrupt changes in the type or rate of sedimentation. Subtle, gradua!
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lithologic changes within the peat section suggest only small, gradual changes in sea lesel. Abrupt i
i lithologic changes found in some cores farther up the valley of South Inlet probably record stream 1
flood egents.
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l Most cores in the Umpqua Riser estuary showed peaty beds in the upper 1.5 m of the cores.
but the upper and lower contacts of most beds were gradational. Abrupt contacts bounding some i
units could be due to sudden submergence or floodir't. At im Coot Rey, below 1.5 tii cal) inuds were found to 7 m depth in the cores. A "C age of 3.1 km from a depth of 6 m in one core l
l ladicates that the relathe rate of sea tesel rise here is twice the rate in Coos Bay, or that a great l
1 deal of differential compaction has taken place in these peat mud sequences.
A single buried marsh surface wu described in outcrop along the Coquille River estuary at a depth of 1.2 m. A small spruce stump rooted in the surface was dated at 290 "C )rBP. The l
estuarine muds that bury the surface are overlain by oserbank silts deposited by river flooding and
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by colian sands deri ed from dune fields to the west, s
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Thus, the most recent buried marsh surface, which may date from about 300 yrtP. appears to I
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be fairly widespread along this part cf the Oregon coast. Good evidence for earlier submergence I
events is found only in South Slough. To show whether or not the earlier submergence events l
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found in South Slough have regional extent emphasis in FY84 will be placed on coring less.
i protected sites in inlets with moderate-slae streams. The moderately high sedimentation rates in j
marshes at these sites shou ld hae allowed marshes to develop and be preserved following all major i
submergence events.
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i FLUVIAL TERRACES IN THE OREGON COAST RANGE: PRELIMINARY ASSESSMENT AS INDICATORS OF QUATERNARY DEFORMATION 9950 04180
[ Earthquake Recurrence and Quaternary Deformation in the Cascadia Subduction Zone, Coastal Oregon]
j Stephen F Personius U.S. Geological Survey a
1 Branch of Geologic Risk Assessment i
Box 25046, MS 966 Denver Federal Center i
Denver, CO 80225
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INTRODUCTION 1
The purpose of this study is to evaluate some of the effects of subduction along the 4
Cascadia subduction zone by examining the styles and rates of deformation of Quaternary i
deposits within the Oregon Coast Range (OCR). Extensive Quaternary deposits are relathely i
rare in the erosion dominated OCR; howeser, fluvial terraces along seseral Coast Range rivers i
appear to be well enough preserved for stratigraphic, chronologic, and tectonic anal > sis. The three thers examined in this study are the Umpqua River, the Smith Rher, a main tributary of the Umpqua, and the Siuslaw River. The Umpqua River has its headwaters in the Cascades; t
both the Smith and Siuslaw Rivers drain the western flank of the central OCR. This abstra:t l
will concentrate on the preliminary aspects of this study, including discussions of terrace i
geomorphology and stratigraphy, and some results of radiocarbon dating.
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GEOMORPHOLOGY I
J The poor preservation of fluvial terraces in most of the OCR refirets the processes that
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form these features. Most OCR terra:es are strath terraces, which are fluvial benches cut into bedrock, cosered by a thin seneer of flusial sediment. This t>pe of terrace is formed by i
fluvial downcutting in response to changes in base level. In the OCR, these changes are related j
i to custati: sea tesel changes and regional uplift. Strath terraces commonly do not form broad i
platforms along streams, so laterally extenshe, paired terraces of this t)pe are rarely p.esersed.
j OCR terraces are commonly presersed as scattered unpaired remnants, usua!!y restricted to the j
i insides of meander bends and along wider parts of ther valleys, and less commonly in j
abandoned meander loops. All the thers examined in this study are flowing in deeply incised j
valleys, which indicates that uplift of the OCR has been an ongoing,long. term process, I
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STRATIGRAPHY I
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Exposures of fluvial terrace sediments along the Umpqua, Smith, and Siuslaw Rhers show a l
remarkably conshtent stratigraphic sequence. They typically consist of a 12.m. thick sandy l
pebble gruel that oserlies a cut bedrock bench; this grastiis in turn oserlain by a 2.$.m. thick i
sitt or sandy sitt. An exception to this sequence is seen in terraces %ery near the Coast, where i
i the sediments generally consist of much thi:Ler deposits of sand and sitt. These near.constal deposits are probably oserthickened by trapping of sediment in estuaries during periods cf 4
i higher sea level. Howeser, seseral high (>90 m). well esposed fluvial terra:es near the coast i
show a thickened, but stratigraphically similar sequence of sitt oser graseloser bedrock, I
suggesting that processes of flusial terrace formation are similar along the length cf Coast l
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I have interpreted the gravel facies as bedload sediment deposited in channels, and the silt
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facies as overbank sediment deposited during periodic flooding. The modern river channels are flowing directly on bedrock except in estuarine settings near the coast. Terraces surfaces appear
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to be reoccupied only rarely by channel deposits, but are frequently reoccupied during seasonal flooding. This is evident because the sitt units are remarkably uniform stratigraphically; they are generally massive or weakly stratified, with only minor thin, discontinuous sand and sandy grael interbeds. Of over 40 exposures examined so far, only one outcrop showed a gravel deposit at the surface of a terrace deposit. Because the modern rivers are flowing directly on bedrock, contemporary uplift of the Coast Range is assumed. This uplift eventually results in raising the surface of the terrace beyoad the reach of flood waters, scnd the terrace surface is abandoned. The massive nature of the silt facies and general hek of buried soils within these deposits suggests that overbank sedimentation occurs at regular intervals at fairly high rates until the terrace surface is abandoned.
PRELIMINARY RESULTS Umpqua River Fluvial terraces are intermittently present along the length of the Umpqua River. Terraces are presently being examined from near the mouth of the river near Reedsport to Coles Valley, 160 river kilometers upstream. Terrace remnants vary in height, from the modern floodplain to over 100 m above modern river level. Correlation of these scattered remnants is difficult, but a dating program of radiocarbon and theimoluminescence (TL) analyses is being undertaken in an attempt to identify possible terrace deformation and to calculate rates of downcutting. Several radiocarbon dates have been obtained on charcoal in the lower terrace: Terracss about 13-15 m above river level, 120-130 krn ups: ream are 7-10 ka. A terrace of similar height on Scholfield Slough,15 km upstream from its confluence with the Umpqua near Reedsport, has a
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radiocarbon age of >26 ka. This relationship suggests that the coast may be subsiding relative to the inland Coast Range. Alternatively, this relationship may be explained by a decreasing stream gradient and subsequent convergence of terraces as the river approaches base level.
Additional radiocarbon and TL dates and terrace profiles will be used to further analyze these problems.
Smith River The Smith River is a major tributary of the Umpqua River; the confluence of these two rivers is just upstream from the town of Reedsport, about 18 km from the mouth of the Umpqua River. The dralnage basin of the Smith River is much smaller than that of the Umpqua River, and is subsequently shorter and has a much steeper gradient than the Umpqua River. Several radiocarbon dates obtained on charco?! indicate that rates of downcutting are substantially faster on the Smith River. A 3 km terrace surface 47 km upstream from the confluence is about 30 m above river level, whereas a correlative terrace surface 10 km upstream from the confluence is only about 10 m above river level. This relationship again suggests decreasing stream gradients and(or) subsidence near the coast. Additional dates and terrace elevations are pending.
Siuslaw River Terraces along the Siuslaw River were the subject of studies by Schlicker and Deacon (1974) and Adams (1984). They both concluded that a high terrace surface on the north side c.,f the river showed apparent westward tilt that may I ave been related to active folding. My studies along the Siuslaw River show that this "surface" is actually several terrace levels that
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may have been incorrectly mapped as a single surface. Terrace profiles are currently being
i constructed in order to assess possible deformation. Unfortunately, the only well preserved terrace surfaces along the Siuslaw river are those preserved at great height (80-110 m) above the l
k' modern river level. The degree of soil development on these surfaces (several-meter-thick Bt horizons with 2.5 YR colors, complete weathering of in situ gravel clasts) suggests that they may 4
be several hundred thousand years old. This would suggest that these deposits are probably substantially older than the estimite of 100 ka of Adams (1984), and that they are beyond the range of TL dating.
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FLUVIAL MORPHOLOGY OF THE OREGON COAST
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9950 04180
[ Earthquake Recurrence and Quaternary Deformation in the Cascadia Subduction Zone, Coastal Oregon)
Susan Rhea U.S. Geological Survey Branch of Geologic Risk Assessment Box 25046, MS 960, Denver Federal Center Denver, CO 80225 i
INTRODUCTIUN The purpose of this study is first, to determine the feasibility of usms river profile and sur-rounding topographical relationships to identify where geologic controls exist, and second, to expose regions of current uplift on the Oregon Coast, River course and elevation change for 22 rivers and from the U.S. Geological Survey Lengths varied from un) der 25..n from 100 m oa some of the sherter tributarie=, to over 1600 m on the longer rivers. The ratio of relief i
to length ranged from.2 to 3.0E The ratio of the area above the river profile within a rectangle i
deSned by relief and length to that below the profile ranged from 1.33 to 13, with a mean of 3.8.
Changes in river slope and valley character were compared to geologic information as presented on Peck's 1961 "Geologic Map of Oregon West of the 12188 Meridian", U.S. Geologic Survey Map l
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I 325. Peck's map was the most complete reference for geologie information over the entire Oregon coast, and included sufficient detail along river drainages to explain most of the anomalies on the drainages.
BACKGROUND A river's natural development is from steep slopes in the headlands, where there is little water volume and erosive capability, to flat slopes at the mouth where water volume, and hence erosive capability, have increased. When the river system is in equilibrium, there will be a smooth transition from the steep head to the flat mouth as the river system balances energy (discharge and elevation change) and work (sediment load and degradation). Where the change in slope is irregular, there may be a change in water discharge (an increase in discharge results in a decrease in slope), lithologic change, or tectonic motion. A change in lithology could be expressed in river course change, a change in meander behavior, a change in valley shape, and change in river slope. Downstream of uplift there is downcutting and increased sinuousity if the uplift rate is slow enough, or entrenched meanders if the rate is too fast. The morphology upst'.eam of an uplifted region resembles an area of subsidence, having flooded channels, bank erosion, and generally flattened slope. Since the river system is always eroding toward equilibrium, tectonic effects are not observed for long periods of time, unien thry are an ongoing procen.
DISCUSSION i
River elevation and slope versus length profiles were constructed for 22 rivers. The theoretical
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profile for each river was also generated, theory anticipating a smooth exponential decrease in i
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slope with length. Abrupt decreases in slope, or inflection points, and broad slope convexities were compared to discharge and geologic changes along the rivers' courses. Overall, increased discharge resulted in decreased slope downstream, but there were places where a major tributary joined the main channel and no increase in slope occurred. On several rivers there were increases 4
in slope at tributary junctions, opposite to expectations. Nearly all of the slope irregularities were coincident with geologic contacts and intrusions, such as massive basalts adjacent to estuarine and marine sediments. Generally, inflections occurred as the tiver bed encountered a resistant formation within a less resistant formation, such as a mafic intrusion into marine sediments. However, not all of the anomalous slope patterns could be explained. For example, a steep section on Siletz River was through a very resistant mafic intrusion, and the river flattened on the more erodable marine sedimentary rocks. Other factors also influence river behavior, one of those being topographical changes, which is in turn generated from tectonic movement. Therefore, although river changes correlate with geologic changes, causality can be aml.iguous.
A more significant observation from the river and valley data was that headwaters were very steep and associated valleys wide, there were slope convexities in the middle sections with ar.sociated narrow, deep valleys, and many rivers ended with slope increases. (See examples in following figure.)
Interpretation of the slope increases at the mouths included tectonic movement either in the form of base level lowering at the coast, or uplift further inland. If base level lowering at the coast is accepted, the flat section upriver could only have been eroded during a long period of tectonic stability, an unlikely possibility given the tectonic history of the Oregon coast. If,instead, the slope i
convexities midriver were caused by uplift 50 to 100 km landward of the ccast, both the steepening downstream and flattening upstream were explained. The uplift must be an ongoing process since i,
these river features are a present landform, i
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Siletz River Rogue River j
Figure 1. River pronles for four rivers studied on the Oregon Coast. Dotted line represents ' ideal',or
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theoretical proSle. Valley proSles at several places along the river are also included, demonstrating entrenching on downstream flat sections of rivers, Convax proSles toward mouths of rivers suggest ongoing uplift within 50 to 100 km of the coast.
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