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The Bonner Formation of the Missoula Group probably represents a braided stream sequence that prograded over sea-margin mud flats from southwestern Montana, northward across western Montana. In the Pioneer Mountains close to its source, the Bonner is both crossbedded, coarse grained, feldspathic sandstone (quartzite), reflecting subaqueous dunes in low flow regime, and horizontally laminated sand-supported conglomerate, representing deposition by transitional flow. Northwestward near Missoula, the Bonner is chiefly directionally crossbedded, coarse feldspathic sandstone (quartzite), representing subaqueous dunes. Farther northwestward, near Superior, the Bonner is mostly horizontally laminated, fine to medium feldspathic sandstone with common directionally oriented, rippl -drift cross laminae and sparse crossbeds. Here the Bonner was deposited by transition flow so shallow that upon slowing to lower regime, the bed forms altered to ripples, not dunes. Here, the Bonner also contains oscillation-rippled, mud-cracked argillite and a few stromatolite beds, indicating mudflat deposition.
By integrating these areas, one can interpret the Bonner to have been a giant pediplain or fan traversed by braided streams that were large enough near their sources to carry small cobbles in transition flow. Near Missoula, the streams were carrying mostly coarse sand in a lower flow regime and probably were dispersing across the pediplain. By the time they reached the vicinity of Superior, the braided streams were very shallow and were carrying medium to mostly fine sand, and were interdigitating with extensive mud flats bordering a sea. The wedge of Bonner may reflect renewed movement in the late Precambrian along the Willow Creek fault zone.
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