The Belt Association

The Belt Association promotes geologic study of the Proterozoic Belt Supergroup of the northwestern United States, and the equivalent Purcell Group in southwestern Canada. 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. Belt Symposium V was also held in Salmon in conjunction with the Tobacco Root Geological Society in 2013. Papers and field guides were published in Northwest Geology, v. 42, in 2013 and in Geological Society of America Special Paper 522 in 2016. The 2023 meeting will also take place in Salmon.

There is no general membership. Informal field trips are held to discuss results of ongoing research in the Belt. To be placed on the email list for these trips and Belt Association news, contact Reed Lewis.

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 a 700 Ma. rift which split the basin. The western portion of the basin is now thought to be part of either Australia, Antarctica, China, 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.

Selected Publications

Belt Basin: Window to Mesoproterozoic Earth, 2016: Geological Society of America Special Paper 522, J.S. MacLean and J.W. Sears, eds.

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 and Montana Bureau of Mines and Geology).

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 and Montana Bureau of Mines and Geology).

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).