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Abstract

In the absence of land plants, broad pediments may have formed stable landforms that beveled Proterozoic continents. Braided streams would have transported a thin layer of clastic sediment across such Proterozoic epicontinental pediments. The Proterozoic pediment–braidplain system may be represented by extremely flat regional unconformities beneath locally preserved, supermature, braidplain sandstones. Continental rifting would have destabilized Proterozoic epicontinental pediments by funneling runoff along rift axes to create large rivers, which otherwise were not favored in the Proterozoic landscape. The sedimentological history and detrital–zircon provenance of the intracratonic Mesoproterozoic Belt–Purcell basin of western North America may be described in terms of destabilization of a late Paleoproterozoic to early Mesoproterozoic epicontinental pediment by a three–armed rift system with the Belt–Purcell basin at its center. A model using a Siberia–Laurentia–Australia paleocontinental reconstruction implies that the sedimentary veneer of the pediment washed down the western branch of the rift system to enter the Belt–Purcell basin at a point source on its western side. Capture of clastic sediment in delta fans on the western side of the basin permitted clean carbonate to precipitate on the northeast side. Reconfiguration of the basin by renewed rifting appears to have changed composition, grain size, and sedimentary provenance during deposition of the Missoula Group (upper Belt–Purcell Supergroup).

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