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Ancient Fluvial Systems
Fluvial Systems of the Precambrian Belt Supergroup, Montana and Idaho, U.S.A.
Fluvial quartzite and argillite wedges hundreds of meters thick extend from the fault-bounded south side of the Belt Basin out across the basin for hundreds of kilometers, forming parts of the late Precambrian Belt Supergroup of northwestern Montana, northern Idaho, northeastern Washington, and the Purcell of southeastern British Columbia. From investigations of the Bonner and Mount Shields Formations (upper Belt Missoula Group), five rock types can be identified that correspond to five major depositional tracts on alluvial fan, distal flat, and sea margin surfaces. The conglomeratic rock type contains coarse pebble lenses in matrix supported sand, and accumulated on the proximal parts of the fan surface close to the south bounding faults. Here braided streams deposited gravel in low, longitudinal channel bars and deposited poorly sorted muddy sand between the channels. The conglomeratic rock type passes down the fan to the coarse, crossbedded rock type, containing well sorted, coarse grained, feldspathic quartzose sand in 10 to 40 cm tabular unimodal crossbeds. This rock type represents mostly linguoid and transverse bars that migrated down broad, braided stream channels, working from side to side across the fan surface. Farther down fan, accumulated the fine, horizontally laminated rock type consisting of even beds 10 to 30 cm thick, within which are vertically graded sedimentation units of fine horizontally laminated sand that commonly passes up to rippled crossbedded sand capped by red argillite. This rock type accumulated from traction transported fine sand carried by shallow sheet wash floods across the distal parts of the fan. Suspended wash load continued out onto flats where flow slowed and ponded. Graded couplets of fine sand and silt to clay accumulated from the nearly standing water which repeatedly drained, forming the fine sand to clay couplets of the red argillite rock type. Seaward from the red argillite rock type, where the surfaces were not repeatedly drained, the sedimentary couplets are finer grained, thinner, and the clay was diagenetically altered to chlorite, forming the green argillite rock type. The model derived from the Mount Shields and Bonner appears to apply with minor modifications to other fluvial units of the Belt.
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