Introduction to the Belt Supergroup
Rocks of the Middle Proterozoic Belt Supergroup (1470 to 1400 Ma) hosts the Silver Valley ore deposits and underlies much of the country north and west of Salmon, Idaho. In Idaho these rocks belong to the Yellowjacket and Hoodoo Formations, the Lemhi Group, and the Swauger Formation. The Belt Supergroup is world famous and enigmatic for several reasons. The following is a quote from Don Winston, University of Montana.
"….for the past twenty five years I have concentrated on stratigraphy and sedimentology of the Middle Proterozoic Belt Supergroup ….. The Belt was deposited in a huge intracratonic basin that stretched across western Montana , northern Idaho and into eastern Washington and Canada . It was filled by sand, silt, clay, and carbonate sediments that locally reach more than 18 km thick. The sedimentary structures of these rocks are beautifully preserved, since they were deposited 1.5 to 1.4 billion years ago. They reveal a world of pure sedimentary processes unfettered by plants and animals. " They....."reveal a world of gigantic sheetflood alluvial aprons and shallow seas or lakes, for which there are no modern counterparts."
-- Dr. Don Winston (http://www.umt.edu/geosciences/faculty/winston/winston.htm)
Geology, Age and Extent of Belt Supergroup
The Belt Supergroup is an immense package (15-20 km thick) of sedimentary rocks that were deposited on Archean and Paleoproterozoic crystalline basement from 1470 to 1400 million years ago. Figure 1 shows the geographic extent of the Belt Supergroup. The Belt Supergroup extends southward from Canada , where it is called the Purcell Supergroup, through western Montana and into northern and central Idaho. The Belt Supergroup extends westward into eastern Washington. Most of the spectacularly glacially sculpted rocks in Glacier National Monument are Belt Supergroup.
Figure 1. The location of Belt Supergroup exposures, as well as the correlative Purcell Supergroup and the younger Windermere Supergroup, exposed only in Canada .
The Belt Supergroup has been subdivided into four major units, from older to younger, 1) the Lower Belt, 2) the Ravalli Group (Yellowjacket Formation and Hoodoo Quartzite), 3) the Piegan Group (Apple Creek Formation, also formerly the middle Belt carbonate), and 4) the Missoula Group (Gunsight Formation and Swauger Quartzite) (See Figure 2).
The Lower Belt is comprised predominantly of the Prichard Formation which is a deep-water facies of a southwest-derived fine-grained clastic wedge. Some zircons in the Prichard Formation are non-North American and were likely derived from the eastern Australian craton.
The Ravalli Group (Yellowjacket Formation and Hoodoo Quartzite) is a largely subaerial clastic wedge, with the same southwestern provenance.
The Piegan Group (Apple Creek Formation, formerly the Middle Belt Carbonate, Winston, 2007), consists of cyclic carbonate-siliciclastic deposits.
Finally the Missoula Group (Gunsight Formation and Swauger Quartzite) contains another fluvial clastic wedge that was derived from the south from a different river system than supplied the Ravalli Group. Figure 3 shows a possible paleogeographic setting of the Belt Supergroup, with Siberia to the west, and Australia to the southwest (Sears, 2007).
Figure 2: Stratigraphic column of the Belt Supergroup.
Because of the extent of the Belt Supergroup, the described rock types and formation names vary depending upon location. For example, Mesoproterozoic rocks in east-central Idaho are separated from the Belt Supergroup by the Bitterroot Batholith and major thrust faults (Winston & Link, 1993). Correlations between the east-central Idaho rocks and the Belt Supergroup are contentious (Link et al.,2007).
What we do know is that the same populations of detrital zircons are present in both the Idaho rocks and the Belt Supergroup, and that the upper Missoula Group Bonner Formation strongly resembles the Swauger Formation in Idaho.
Geologic History of Belt Supergroup
Belt Supergroup Tectonic Setting
The Belt Supergroup rocks were deposited in the largest Middle Proterozoic basin on the North America continent. The Belt-Purcell basin was intracratonic and periodically connected to the world ocean. At other times, it was enclosed and formed lacustrine deposits (Ross and Villeneuve, 2003). The current accepted model, based on geochemistry and sedimentology, is that the basin formed as a result of hinterland rifting inboard of a compressional orogenic belt, within a 1.45 Ga Middle Proterozoic supercontinent, the precursor of Rodinia (Winston and Link, 1993). Plate reconstruction for this time period is not easy and is an active area of research.
Figure 3: Plate reconstruction map at the time of Belt Supergroup deposition.
As rifting initiated and the basin formed, rapid subsidence of the basin was caused by loading the crust with basaltic sills that intruded the Pritchard Formation. Subsequent faulting and folding within the Idaho-Montana thrust belt, plus intrusion of Neoproterozoic, Ordovician, Cretaceous and Eocene magma (covered in following modules) have affected the Belt Supergroup.
Sedimentology of the Belt Supergroup
As mentioned in the introductory statements by Don Winston, the sedimentary structures preserved in the Belt Supergroup are striking. Figure 4 shows the principal sediment types and structures found in the Belt Supergroup and the interpretation of the depositional environments.
Figure 4: Principle sediment types and structures occurring in the Belt Supergroup. (After Winston and Link, 1993 and Winston, et. al., 1999)
Where did all the fine-grained sediment come from?
One extraordinary thing about the Belt Supergroup is that it mainly accumulated in a rapidly subsiding underfilled sedimentary basin where there was abundant accommodation space for the sediment to aggrade vertically. This basin did not produce the kinds of sedimentary structures that are found in marine shoreface deposits during the Paleozoic. Such successions contain erosional surfaces or incised valleys (sequence boundaries) that are related to local and global sea-level changes. Sandstones and mudstones of the Ravalli and Missoula Groups display cyclic aggrading contacts between sedimentation events driven ultimately by floods in large fluvial systems. They do not contain tidal cross bedding, or marine truncation surfaces or incised valleys. Thus, deposition was largely in a huge lake basin, where accommodation space was not limiting and sediment could build up to create the extraordinarily thick Belt Supergroup.
An annotated PDF of the Belt Supergroup. - The first few slides of this show are from the article by Link et al (2007) and show updated correlation schemes between Idaho and the main Belt Supergroup basin.
Evans, K.V., Aleinikoff, J.N., Obradovich, J.D., and Fanning, C.M., 2000, SHRIMP U-Pb geochronology of volcanic rocks, Belt Supergroup, western Montana; evidence for rapid deposition of sedimentary strata: Canadian Journal of Earth Sciences, v. 37, no. 9, p. 1287-1300.
Link, P.K., Fanning, C.M., Lund, K.I., and Aleinikoff, J.N., 2007 in press, Detrital zircons, correlation and provenance of Mesoproterozoic Belt Supergroup and correlative strata of east-central Idaho and southwest Montana: in Link, P.K., and Lewis, R.S., eds., SEPM Special Publication 86, Proterozoic geology of western North America and Siberia, p.101-128.
Orr, W. N. and Orr, S. L., 2002, Geology of the Pacific Northwest, 2nd Edition, Chapter 2: Omineca-Intermontane Province: McGraw-Hill Higher Education, New York, NY, p. 19-47.
Ross, G.M., and Villeneuve, M., 2003, Provenance of the Mesoproterozoic (1.45 Ga) Belt basin (western North America): another piece in the pre-Rodinia paleogeographic puzzle: Geological Societyof America Bulletin, v. 115, p. 1191-1217.
Sears, J. W., and Price, R.A., 1978, The Siberian connection; a case for Precambrian separation of the North American and Siberian cratons: Geology, v. 6, p. 267-270.
Sears, J.W., 2007, Rift destabilization of a Proterozoic epicontinental pediment: A model for the Belt Purcell basin, North America: in Link, P.K., and Lewis, R.S., eds., SEPM Special Publication 86, Proterozoic geology of western North America and Siberia, p. 55-64.
Winston, Don, 2007, Revised stratigraphy and depositional history of the Helena and Wallace Formations, Mid-Proterozoic Piegan Group, Belt Supergroup, Montana and Idaho, in Link, P.K., and Lewis, R.S., eds., SEPM Special Publication 86, Proterozoic geology of western North America and Siberia, p. 65-100.
Winston, Don, and Link, P.K., 1993, Middle Proterozoic rocks of Montana, Idaho, and Washington: The Belt Supergroup: in Reed., J., Simms, P., Houston, R., Rankin, D., Link, P., Van Schmus, R., and Bickford, P., eds., Precambrian of the conterminous United States: Boulder, Colorado, Geological Society of America, The Geology of North America , v. C-3, p. 487--521.
Winston, Don, P.K. Link , and Nate Hathaway, 1999, The Yellowjacket is not the Prichard and other heresies: Belt Supergroup Correlations, Structure and Paleogeography, east-central Idaho: in Hughes, S.S. and G.D. Thackray, eds., Guidebook to the Geology of Eastern Idaho: Pocatello, Idaho Museum of Natural History, p. 3-20.
Continue to Module 3 - Passive Margin