NATURALLY OCCURRING ASBESTOS IN U.S. FOREST SERVICE SYSTEM LANDS

Keywords: naturally occurring asbestos, ophiolites, ultramafic intrusions, banded iron deposits Naturally occurring asbestos (NOA) is a geologic hazard that is found in all nine regions of the U.S. Forest Service. Recent work by several state agencies and the U.S. Environmental Protection Agency has resulted in the recognition of a NOA hazard threshold of 0.25% for NOA field samples. This is problematic in that all serpentinites usually have, at the very least, trace amounts of chrysotile. Therefore the agency geologists are tackling this problem by evaluating the NOA hazard and risk within a geologic setting context. For example, in the Western United States the national forests have large bodies of serpentinites that are within ophiolites in the North Cascades, Blue Mountains of Oregon, Klamath Mountains in Oregon and California, Coast Ranges of California and the Sierra Nevada. A similar geologic setting is also found in southeastern Alaska. In evaluating the NOA in this geologic setting (accretion of exotic terranes to North America) the geologists can identify areas where field assessments are needed. Similarly, the East Coast national forests have ophiolites that were docked against Laurentia in late Paleozoic time leaving a geologic setting of an accreted continental micro-plate demarcated by the Blue Ridge Ophiolite in the Appalachian Mountains. The second geologic setting is the ultramafic intrusions found in Montana, the Rainy Creek Complex near Libby and the Stillwater Complex in southwestern Montana. Both complexes have significant amounts of NOA created through the intrusion process of the ultramafic body and/or subsequent faulting. The third geologic setting is the banded iron deposits that have undergone intrusions and regional scale faulting resulting in NOA. Banded iron formations are found in southwestern Montana (i.e., Stillwater Complex) and in the Great Lake states of Minnesota, Wisconsin and Michigan. By tackling this problem with solid field methods and a thorough understanding of the regional geologic history, the NOA geologist can be expeditious in characterizing and evaluating NOA hazards.