The Belt Supergroup was deposited in a Middle Proterozoic intracratonic basin, occupied during much of its history by alluvial aprons that sloped down to a landlocked sea. Rocks from the Ravalli Group, middle Belt carbonate and Missoula Group have been classified into thirteen sediment types that are arranged in long-lived facies tract patterns. These sediment types and their sedimentologic interpretations are as follows: (1) gravel--braided stream gravel bars high on alluvial aprons; (2) cross-bedded sand--braided stream channels on upper and middle alluvial aprons; (3) flat-laminated sand--sheetfloods on sandflats on middle and lower alluvial aprons; (4) tabular silt--storm deposits on submerged mudflats; (5) even couple--distal sheetfloods on sandflat surfaces; (6) pin h-and-swell couple--subwavebase turbidite deposition in the Belt sea; (7) pinch-and-swell couplet--subwavebase underflow and interflow settle-out; (8) even couplet--traction accumulation followed by suspension settle-out from episodic floods that crossed exposed and submerged mudflats; (9) lenticular couplet--submerged mudflat surfaces reworked by waves; (10) microlamina--subaqueous surfaces of minimal accumulation, locally coated with organic material; (11) coarse sand and intraclast--small beaches and shoals; (12) discontinuous layer--combined traction and suspension settle-out on the sandflats or on alluvial apron surfaces during flood wane; and (13) carbonate mud--precipitated carbonate mud and terrigenous sand and mud accumulation mostly on shallow, submerged bottoms.

The lower Belt records maximum transgression of the Belt sea. Turbidite sand and pelagic mud were deposited across the central part of the basin. Carbonate mud was precipitated at times on its eastern side, and coarse conglomerate accumulated along its fault-bounded southern margin. The Ravalli Group records progradation of mudflats and alluvial aprons from the south and west across much of the basin. The middle Belt carbonate was deposited during a second large transgressive period, when terrigenous-to-carbonate cycles formed across the eastern part of the basin and turbidite sand and mud derived from the west was deposited in the deeper, locally slumped, western part of the basin. The Missoula Group represents a series of alluvial aprons that prograded northward into the basin, sepa ated by 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.

A tectonic hypothesis proposes that the Belt basin was cut by at least four major growth fault zones which are based on linear trends of abrupt stratigraphic thickness changes that coincide with local patches of soft sediment deformation. Three nearly east-west fault lines are, in northward sequence, the Perry, Garnet, and Jocko lines. The fourth line, the Townsend line, trends northwestward from the Perry to the Jocko line. The inferred fault lines partly enclose at least four major crustal blocks that subsided during Middle Proterozoic time, forming the Belt basin. The crystalline Dillon block lies south of the Perry line, the southern basin boundary. Between the Perry line and the Garnet line are the Helena embayment block on the east and the Deer Lodge block on the west, separated by the Townsend line. The Ovando block lies north of the Deer Lodge block. North of the Ovando block is the Charlo block (or blocks). Cretaceous to Eocene folds and thrusts formed western and eastern thrust belts, separated by the central part of the Belt basin, which was not intensely deformed. Thrusts and folds are long and continuous within the Precambrian crustal blocks, but are broken locally along the east-west lines. Later Cenozoic extensional faulting produced distinctive domains over the Precambrian blocks and dislocations along the Precambrian lines. Because the central part of the basin is only mildly deformed, its relatively intact stratigraphic framework provides the basis for much of the sedimentologic interpretation.

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