MANCOS SHALE LANDSCAPES: SCIENCE AND MANAGEMENT OF BLACK SHALE TERRAINS - - A MULTI-AGENCY PROJECT

Land managers and scientists of the USGS, BLM, and BOR are collaborating to begin addressing the issue of how best to manage terrains underlain by the Cretaceous Mancos Shale. While we are focused on trying to understand and quantify the natural processes of weathering and erosion that result in the movement of salts and selenium from their sources to the Colorado River, some data comparing natural and anthropogenic processes are being generated. A 550-foot core of the lower Mancos was obtained from the Gunnison Gorge National Conservation Area (GGNCA) for stratigraphic control and a chemical baseline. Selenium content varies between 6.1 and <0.1 ppm. The highest concentrations are in the transition zone between the Juana Lopez and Montezuma Valley Members. Sulfur content, a proxy for salinity, varies between 7.9 and 0.18% (Se and S are not strongly correlated). Mancos soils from the GGNCA and the Factory Butte area of central UT as well as pond sediments in the Badger Wash area, CO have Se and S content ranges between 12.1 and 0.2 ppm and 13.5 and <0.03 %, respectively. Secondary salts that form crusts on the soils are predominantly sodium sulfates and have Se contents up to 150 ppm. Bulk soil density which influences both runoff and plant yield appears to be independent of position on either disturbed or undisturbed hill slopes, but does increase beneath single track motorcycle trails. Rainfall simulations on both undisturbed and slightly disturbed (several passes with a motorcycle) hillslopes of approximately equal slope show a possible correlation between slope aspect and either runoff or sediment yield. Rates of hillslope sheet wash and creep on undisturbed hillslopes also appear to be independent of aspect but do vary by season (rainfall intensity and amount). Biological soil crusts (BSC) which both stabilize and fertilize the soils appear to be almost ubiquitous as either visible crusts or those indicated by the presence of chlorophyll a. The highest values of chlorophyll a (mean = 0.025 mg/g) were found where litter cover was > 6%, visible BSC cover was > 1%, and rock cover was > 3%. Dust generated in a designated off-highway vehicle use area of the GGNCA was significantly higher during a period of moderate use as opposed to a period of low use (93 vs. 5.2 µg/m3/hr). These data, when fully integrated, will help to guide land-use decisions.