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Abstract for pub number 707

Abstract

Various types of mass movement features are found in the drainage basin of the East Fork Coquille River in the southern Oregon Coast Range. The distribution and forms of mass movement features in the area are related to geologic factors and the resultant topography.

The Jurassic Otter Point Formation, a melange of low-grade metamorphic and marine sedimentary rocks, is present in scattered outcrops in the southwest portion of the study area but is not extensive. The Tertiary Roseburg Formation consists primarily of bedded siltstone and is compressed into a series of west to northwest-striking folds. The overlying Looking-glass, Flournoy, and Tyee Formations consist of rhythmically bedded sandstone and siltstone units with an east to northeasterly dip of 5-15º decreasing upward in the stratigraphic section. The units form cuesta ridges with up to 2000 feet of relief.

The distribution of mass movements is demonstrably related to the bedrock geology and the study area topography. Debris avalanches are more common on the steep slopes underlain by Flournoy Formation and Tyee Formation sandstones, on the obsequent slope of cuesta ridges, and on north-facing slopes.

Soil creep occurs throughout the study area and may be the primary mass movement form in siltstone terrane, through soil creep was not studied in detail. slump-earthflow, rockfalls, and rock slumps also occur in the study area though less extensively than debris avalanches.

Stratigraphy and bedrock attitude contributed to the pre-historic occurrence of a major landslide involving Flournoy and Tyee Formation bedrock. The Sitkum landslide dammed the East Fork Coquille River, forming a substantial lake which is now filled with sediments. The form and size of the Sitkum landslide is similar to other landslides which have dammed drainages in the Coast Range, including Loon Lake, Triangle Lake, and Drift Creek.

Comparisons with the Loon Lake landslide, which has a known radiocarbon date, provide estimated dates of 3125 years B.P. for the Sitkum landslide and 10,300 years for the Triangle Lake landslide.