ML20151P543

From kanterella
Jump to navigation Jump to search
Fluvial Terraces in Oregon Coast Range:Preliminary Assessment as Indicators of Quaternary Deformation
ML20151P543
Person / Time
Site: Trojan File:Portland General Electric icon.png
Issue date: 06/15/1988
From: Personius S
INTERIOR, DEPT. OF, GEOLOGICAL SURVEY
To:
Shared Package
ML20151H012 List:
References
NUDOCS 8808100085
Download: ML20151P543 (3)
v • d • e


Text

.

FLUVIAL TERRACES IN THE OREGON COAST RANGE: PRELIMINARY ASSESShfENT

! AS INDICATORS OF QUATERNARY DEFORMATION

( 9950-04180 (Earthquake Recurrence and Quaternary Deformation in the Cascadia Subduction Zone, Coastal Oregon]

Stephen F. Personius U.S. Geological Survey Branch of Geologic Risk Assessment Box 25046, MS 966, Denver Federal Center Denver, CO 80225 INTRODUCTION The purpose of this study is to evaluate some of the effects of subduction along the Cascadia subduction zone by examining the styles and rates of deformation of Quaternary deposits within the Oregon Coast Range (OCR). Extensive Quaternary deposits are relatively rare in the erosion-dominated OCR; however, fluvial terraces along several Coast Range rivers appear to be well enough preserved for stratigraphic, chionologic, and tectonic analysis. The three rivers examined in this study are the Umpqua River, the Smith River, a main tributary of the Umpqua, and the Siuslaw River. The Umpqua River has its headwaters in the Cascades; both the Smith and Siuslaw Rivers drain the western flank of the central OCR. This abstract will concentrate on the preliminary aspects of this study, including discussions of terrace geomorphology and stratigraphy, and some results of radiocarbon dating.

GEOMORPHOLOGY The poor preservation of fluvial terraces in most of the OCR reflects the processes that form these features. Most OCR terraces are strath terraces, which are fluvial benches cut into bedrock, covered by a thin veneer of fluvial sediment. This type of terrace is formed by fluvial downcutting in response to changes in base level. In the OCR, these changes are related to eustatic sea level changes and regional uplift. Strath terraces commonly do not form broad platforms along streams, so laterally extensive, paired terraces of this type are rarely preserved.

OCR terraces are commonly preserved as scattered unpaired remnants, usually restricted to the insides of meander bends and along wider parts of river valleys, and less commonly in abandoned meander loops. All the rivers examined in this study are flowing in deeply incised valleys, which indicates that uplift of the OCR has been an ongoing, long-term process.

STRATIGRAPHY Exposures of fluvial terrace sediments along the Umpqua, Smith, and Siuslaw Rivers show a remarkably consistent stratigraphic sequence. They typically consist of a 1-2-m-thick sandy pebble gravel that overlies a cut bedrock bench; this gravel is in turn overlain by a 2-5-m-thick silt or sandy silt.

An exception to this sequence is seen in terraces very near the coast, where the sediments generally consist of much thicker deposits of sand and sitt. The.se near-coastal deposits are probably overthickened by trapping of sediment in estuaries during periods of higher sea level. However, several high (>90 m), well exposed fluvial terraces near the coast show a thickened, but stratigraphically similar sequence of silt over gravel over bedrock, suggesting that processes of fluvial terrace formation are similar along the length of Coast Range rivers.

8808100085 880615 PDR ADOCK 05000344 P PDR

I have interpreted the gravel facies as bedload sediment deposited in channels, and the silt

/ 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 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 gravel 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 beyond the reach of flood waters, and the terrace surface is abandoned. The massive nature of the sitt facies and general lack 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 thermoluminescence (TL) analyses is being undertaken in an attempt to identify possible terrace deformation and to calculate rates of downcutting. Several radiocarbon m above river level, dates have been obtained on charcoal in the lower terraces.

120-130 Terraces abou km upstream 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 radiocarbon age of >26 ka. This relationship suggests that the coast may be subsiding re:ative 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 drainage 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 charcoal indicate that rates of downcutting are substantially faster on the Smith River. A 3 ka 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.

terrace elevations are pending. Additional dates and 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 of the river showed appare st westward tilt that may have been related to active folding. My studies along the Siuslaw River show that this "surface" is actually several terrace levels that may have been incorrectly mapped as a single surface. Terrace profiles are currently being l

l I

l

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 I 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 ma be several hundred thousand years old. This would suggest that these deposits are probably substantially older than the estimate of 100 ka of Adams (1984), and that they are beyond the range of TL dating.

- _ _ ., ,, m,, , , . - . _ , . . ,--.--, , , , . - ,,.- , _ _ _ , - _ _ . ,_wy,,,--.m-,,,_q,,,v,y- - , -v- .~+, , . , , --

Retrieved from "https://kanterella.com/w/index.php?title=ML20151P543&oldid=2965656"