The Belt Supergroup was deposited in an intracratonic basin, occupied during much of its history by alluvial aprons that sloped down to a landlocked sea. Belt rocks are here analyzed in terms of 13 "sediment types," descriptive taxa in which the metamorphic overprint is filtered out and the original sedimentary aspects are emphasized. The lower Belt records maximum transgression of the Belt sea, during which turbidite sand and pelagic mud were deposited in the central part of the basin, carbonate mud accumulated on its eastern margin, and coarse conglomerate accumulated along its fault-bounded southern side. The Ravalli Group records extensive alluvial apron/mudflat progradation from the south and west. The middle Belt carbonate, representing a second large transgressive eriod, is characterized by terrigenous-carbonate cycles in the eastern part of the basin and by turbidite sand and mud, derived from the west, in the deeper, locally slumped, western part of the basin. The Missoula Group represents a series of northwest-facing prograded alluvial aprons alternating with transgressive mudflat and shallow-water deposits. The Garnet Range Formation, near the top of the Missoula Group, represents incursion of open marine waters into the Belt basin.

The Belt basin subsided as a group of at least four large crustal blocks, separated by three nearly east-west fault lines and a northwest-trending fault line. Differential subsidence of the blocks is recorded in abrupt thickness changes and soft sediment deformation along the fault lines. Cretaceous thrusting formed western and eastern thrust belts which are continuous across the blocks, but which are segmented and broken along the east-west lines. Tertiary extensional dislocations along the lines.

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