EPISODIC MOVEMENT OF SEDIMENT OFF MANCOS SHALE SLOPES NEAR HANKSVILLE, WAYNE COUNTY, UTAH

Between Caineville and Hanksville, Utah, the Fremont River flows through country underlain by the Tununk and Blue Gate Members of the Cretaceous Mancos Shale, and receives drainage from about 1,000 km² of this formation. Badland topography on both sides of the river consists of narrow ridges separated by broad pediments. The ridges are from 10 to 30 m high with side slopes ranging from 35 to 40 degrees. Two distinct and episodic processes are responsible for moving sediment down and away from these slopes: mass movement and water-flow. A thirty-year erosion pin study has shown that the weathered crust moves by a combination of down-slope creep, averaging 5.9 cm/yr, and mudflows. At least 65 % of this mass movement occurs in the winter months (November – May).

Recent sediment trap studies show that about 95 % of sediment movement away from the toe of the slope and across pediments is accomplished by isolated and localized high-intensity summer convective storms. Inspection of weather records and personal observation suggests that accelerated mass movement on the slopes has a 20 year recurrence, while transport away from the slopes, across pediments, and toward the river recurs more frequently.