Abstract

The valley and alluvial cone of the antecedent Madison River in the Missouri Flats area of SW Montana lie in a half-graben bounded by a prominent prehistoric fault scarp. Footwall-sourced alluvial fans and fans derived from a major transfer zone have prograded onto the oldest of a prominent suite of three paired terraces of Holocene age which bound the modern braided floodplain of the river. The terraces have been cut into the Pleistocene alluvial cone and are cut by the prehistoric fault scarp. Levelling studies on the terrace surfaces in the hangingwall reveal that movement on the fault has caused surface tilting, erosion and deposition. A suite of terraces present in the footwall of the fault shows backtihing consistent with footwall uplift having occurred.

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