The Belt Association promotes and coordinates geologic study of the Proterozoic Belt Supergroup of the northwestern United States, and the equivalent Purcell Group in southwestern Canada. An objective of the Association is to provide funds for geologic research by senior undergraduate students and graduate students working in Belt terrane.
The Association is comprised of a 9-member board of directors, elected for three-year terms. There is no general membership. The Association sponsors a Belt Symposium every decade. Belt Symposium IV was held in 2003 in Salmon, Idaho. The Symposium proceedings were published as SEPM Special Publication 86, 2007. The field guides to the 2003 meeting were published in Northwest Geology, v. 32, 2003. Informal sessions are held each winter in Spokane, Washington, to discuss results of on-going research in the Belt.
What is the Belt
The Belt-Purcell Supergroup is a package of sedimentary rocks deposited in a Mesoproterozoic basin over what are now portions of Alberta, British Columbia, Montana, Idaho, and Washington. The low metamorphic grade of Belt rocks in much of their outcrop area allows a rare opportunity to study Proterozoic sedimentation processes.
The Belt sediments accumulated to perhaps 50,000 feet in thickness near the Washington-Idaho line, and thin eastward to the vanishing point in central Montana. The sediments are not seen to thin to the west; instead they are abruptly truncated by an 700 Ma. rift which split the basin. The western portion of the basin is now thought to be part of either Australia or Siberia.
Convergent tectonism along the west margin of the North American plate started in the Mesozoic and continues to the present day. This convergence resulted in the accretion of the 'exotic' terranes that make up most of Oregon and Washington, and the attendant compression and plutonism caused widespread disruption of the Belt rocks.
The study of Belt rocks has focused on three primary goals. First is an understanding of Proterozoic sedimentation processes. Second is the correlation of faulted and folded blocks to allow piecing together the history of tectonic adjustments. Third is the locating of economic ore deposits. These goals are interdependent.
Proterozoic sedimentation processes have only poor modern analogues. The land was bare of vegetation and the air contained less oxygen than it does today. No burrowing organisms were present to disrupt the sediments once they were deposited. Thus, sedimentary structures such as mudcracks, ripple marks, and salt casts are extremely well preserved. The weather and even plate tectonics appear to have operated under different rules. The best modern analogue we have for Proterozoic sediment deposition is alluvial outwash fans in sparsely vegetated desert terrains. These do not approach the vast scale of deposition in the Belt basin. In addition, no modern analogue exists for a basin with the size and geometry of the Belt basin, suggesting that the mechanisms of Proterozoic plate tectonics differed from those of the Phanerozoic.
Belt rocks host world-class ore deposits, most notably the Sullivan volcanogenic massive-sulfide deposit, and the Coeur d'Alene Ag-Pb-Zn district.
"The Belt Association, incorporated for the purpose of encouraging research on the economically significant and intellectually challenging Mesoproterozoic Belt-Purcell Supergroup, holds its annual review of new work. This is an informal session where anyone with an interest in the Belt is invited to listen or share new data, observations, and conjectures."
Current Research in the Belt
Both the Idaho Geological Survey and the Montana Bureau of Mines and Geology have active mapping projects in the Belt Supergroup. Edmap projects by geoscience graduate students have been incorporated with these mapping initiatives. Project areas include the Bonners Ferry 30 x 60 minute quadrangle in northern Idaho and the Salmon 30 x 60 minute quadrangle in eastern Idaho and western Montana. For more information contact Reed Lewis at the Idaho Geological Survey.
Detrital zircon age analyses
Much of the recent research on the Belt Supergroup has focused on isotopic study of provenance, including U-Pb dating of detrital zircons, Hf isotopes of these dated grains, and Nd isotopic analysis of fine-grained Belt strata.
The Association offers $500 to $1000 research grants each year to one or more graduate students whose theses deal with Belt-Purcell rocks. Grant applications are due March 1.
Preliminary Geologic Map of the Sandpoint 30 x 60 minute quadrangle, Idaho and Montana, and the Idaho part of the Chewelah 30 x 60 Minute quadrangle Reed S. Lewis, Russell F. Burmester, Roy M. Breckenridge, Mark D. McFaddan, and William M. Phillips, 2008, Idaho Geological Survey, Digital Web Map 94, scale 1:100,000.
Geologic Map of the Homer Youngs Peak Quadrangle, Lemhi County, Idaho and Beaverhead County, Montana, by Jeffery D. Lonn, Reed S. Lewis, Russell F. Burmester, and Loudon R. Stanford, 2008: Idaho Geological Survey, Digital Web Map 95, scale 1:24,000.
Metallogeny of Mesoproterozoic sedimentary rocks in Idaho and Montana—Studies by the Mineral Resrouces Program, U.S. Geological Survey, 2004-2007: J.M. O'Neill, editor, U.S. Geological Survey Open-File Report 2007-1280, 27 p.
Proterozoic Geology of Western North America and Siberia, 2007, edited by P.K. Link and R.S. Lewis: SEPM Special Publication 86. 257 p.
Tobacco Root Geological Society, Field Conference at the Belt Symposium IV, Missoula, Montana, and Salmon, Idaho, August 7-12, 2003: Northwest Geology, vol. 32, 223 p. (available from the Idaho Geological Survey, MISC-15; also available from the Tobacco Root Geological Society).
Tobacco Root Geological Society, 26th Annual Field Conference, Wallace, Idaho, 2001, Out of the Flood and into the Panhandle—Northern Idaho Geology: Northwest Geology, vol. 30, 81 p. (available from the Idaho Geological Survey, MISC-13; also available from the Tobacco Root Geological Society).
Geologic Guidebook to the Belt-Purcell Supergroup, Glacier National Park and Vicinity, Montana and Adjacent Canada, edited by Paul Karl Link, 1997, (2nd edition), 173 p. (available from the Idaho Geological Survey, MISC-6).
Geology and ore deposits of the trans-Challis fault system/Great Falls tectonic zone: Tobacco Root Geological Society 15th Annual Field Conference, 121 p. (available from the Idaho Geological Survey, MISC-7; also available from the Tobacco Root Geological Society).