ABSTRACT

Lateral migration fluvial meander belts preserves wider than normal coarse-sediment bodies, asymmetric meander belts, that have a complex architecture recording the history of channel movement. Detailed investigations of the morphology and deposits of asymmetric meander belts of the Madison River and its South Fork, southwestern Montana (using topographic mapping, ground-penetrating radar profiling, coring, sonar, and radiocarbon dating) have been undertaken in an attempt to show how they are influenced by tectonic tilting and faulting, base-level change, catchment evolution, and climate change. The asymmetrical form of the meander belts resulted from a regional tectonically induced lateral bias to channel position over at least the last 10 ky. Radiocarbon ages from abandoned channel ills increase progressively up-tilt away from the active meander belts. The South Fork meander loops decreased in wavelength and channel width through the Holocene, but no systematic changes in channel size have been recognized in the Madison meander belt. These results indicate that the changes in discharge patterns of the two rivers differed, and this difference may be related to local climate change and drainage evolution in their catchments. Episodes of incision during meander-belt evolution have isolated abandoned meander loops with restricted age ranges on terraces above the modern flood plains. The terraces are extensive on the up-tilt side of both meander belts and on the footwalls of intrabasinal faults, with only small terrace remnants on the down-tilt side of the meander belts The distribution of terraces and large abandoned meander loops in the South Fork shows that channel-belt asymmetry did not develop by progressive lateral migration of the meander-belt axis, as previously suggested, but rather that the flood plain episodically decreased in width, with the down-tilt margin fixed in space. Some of the Madison River terraces, however, do show evidence of down-tilt migration of the river. Preservation of diatomite in downstream parts of both meander belts suggests periodic transgression by an ancestral Lake Hebgen. Changing lake shore position may explain some of the episodes of incision, and is probably associated with large seismic events and surface deformation.