Mesoproterozoic Belt Supergroup

Belt Supergroup stratigraphy

The Belt Supergroup has been subdivided into four major units, from older to younger, 1) the Lower Belt, 2) the Ravalli Group, 3) the Piegan Group, and 4) the Missoula Group.

The Lower Belt is composed predominantly of the Prichard Formation, which is a deep-water facies of a southwest-derived fine-grained clastic wedge. Some detrital zircons in the Prichard Formation are non-North American and were likely derived from the eastern Australian craton.

The Ravalli Group is a largely subaerial clastic wedge, with the same southwestern provenance.

The Piegan Group consists of cyclic carbonate-siliciclastic deposits.

Finally, the Missoula Group (including Lemhi Group, Swauger, Lawson Creek, and Apple Creek formation rocks in east-central Idaho) contains another fluvial clastic wedge that was derived from the south from a different river system than supplied the Ravalli Group.

Belt strata in the Idaho panhandle are correlative with the Lower Belt, Ravalli Group, and Piegan Group, which is the oldest part of the Belt Supergroup (modified from Lonn et al., 2020).

In central and east-central Idaho, Belt Supergroup rocks belong to the Lemhi Group and overlying Swauger, Lawson Creek, and Apple Creek formations, which are thought to be equivalent in age to the Missoula Group, which is the youngest part of the Belt stratigraphy elsewhere. These rocks are coarser grained than their Montana age-equivalents of the Missoula Group. Figure modified from Lonn et al. (2020).

Belt Supergroup tectonic setting

Belt Supergroup rocks were deposited in the largest Middle Proterozoic basin on the North American continent. Most workers interpret that the Belt basin was formed as an intracratonic basin and periodically connected to the world ocean. At other times, it was enclosed and formed lacustrine deposits (Ross and Villeneuve, 2003). Though most workers agree that the basin formed during rifting of the 1.45 Ga Middle Proterozoic supercontinent Nuna, there is extensive debate about whether extension resulted in formation of an ocean basin and complete continental breakup or if rifting failed prior to forming a major ocean basin. Plate reconstruction for this time period is not easy and is an active area of research.

Hypothesized plate tectonic configuration of supercontinent Nuna at ca. 1.6-1.3 Ga, showing the reconstructed position of the Belt basin (modified from Brennan et al., 2021). In this model, Australia and Antarctica are placed outboard (west in modern coordinates relative to Belt basin) of Laurentia.

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 Paleozoic time, such as 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 is interpreted to have been largely in a huge lake basin, where accommodation space was not limiting and sediment could build up to create the extraordinarily thick Belt Supergroup.

• Detrital zircon data and measured paleocurrent indicators suggest that the Lower Belt basin received sediment from the east and west (in modern coordinates), including likely from part of the Gawler craton in Australia (Stewart, 2009).
• During the late stages of deposition, detrital zircons indicate a shift to a predominantly southerly source from southwestern Laurentia (Link et al., 2016).