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Holocene strath terraces, climate change, and active tectonics; the Clearwater River basin, Olympic Peninsula, Washington State

Karl W. Wegmann and Frank J. Pazzaglia
Holocene strath terraces, climate change, and active tectonics; the Clearwater River basin, Olympic Peninsula, Washington State
Geological Society of America Bulletin (June 2002) 114 (6): 731-744


The approximately 400 km (super 2) Clearwater River basin, located on the Pacific flank of the actively uplifting Olympic Mountains of western Washington State, contains a well-preserved flight of Holocene fluvial terraces. We have collected a large data set of numeric ages from these terraces that is used to elucidate the geomorphic, fluvial, active tectonic, and climatic processes that operate at Holocene spatial and temporal scales. Detailed field mapping reveals three prominent Holocene straths and their overlying terrace deposits. Terrace ages fall into three broad ranges: ca. 9000-11 000 yr B.P. (Qt4), 4000-8000 yr B.P. (Qt5), and 0-3000 yr B.P. (Qt6). Terrace deposit stratigraphy, sedimentology, and age distributions allow us to consider two alternative models for their genesis. The favored model states that the terrace ages are coincident with lateral incision of the Clearwater channel, emplacement of the terrace alluvium, and the carving of the straths. Vertical incision of the Clearwater channel was primarily relegated to the brief ( approximately 1000 yr) intervals when we have no record of terraces. Alternatively, the straths were carved as the channel incised vertically during the brief time periods between dated terrace deposits, and the terrace ages record a subsequent long time of alluviation atop the straths and concomitant termination of vertical incision. In both models, we envision a Clearwater River channel at or near capacity with a temporally variable rate of both lateral and vertical incision. Small deviations from this at-capacity condition are driven by variations in the liberation and delivery of hillslope sediment to the channel. We consider several causes for variable hillslope sediment flux in this tectonically active setting including Holocene climate change and ground accelerations related to earthquakes. Holocene rates of vertical incision are reconstructed along nearly the entire Clearwater Valley from the wide distribution of dated terraces. Incision rates clearly increase upstream, mimicking a pattern documented for Pleistocene terraces in the same basin; however, the rates are 2-3 times those determined for the Pleistocene terraces. The faster Holocene incision rates may be interpreted in terms of an increase in the rates of rock uplift. However, we favor an alternative explanation in which the Holocene rates represent a channel rapidly reacquiring its stable, graded concavity following protracted periods of time in the Pleistocene when it could not accomplish any vertical incision into tectonically uplifted bedrock because the channel was raised above the bedrock valley bottom by climatically induced alluviation. These results illustrate how, even in tectonically active settings, representative rates of rock uplift inferred from studies of river incision should be integrated over at least one glacial-interglacial cycle.

ISSN: 0016-7606
EISSN: 1943-2674
Serial Title: Geological Society of America Bulletin
Serial Volume: 114
Serial Issue: 6
Title: Holocene strath terraces, climate change, and active tectonics; the Clearwater River basin, Olympic Peninsula, Washington State
Affiliation: Washington State Department of Natural Resources, Division of Geology and Earth Resources, Olympia, WA, United States
Pages: 731-744
Published: 200206
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 90
Accession Number: 2002-043000
Categories: Quaternary geology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table, sketch maps
N47°37'00" - N47°37'30", W124°13'00" - W124°12'00"
Secondary Affiliation: Lehigh University, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 200214
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