HOLOCENE MEANDER-BELT EVOLUTION IN AN ACTIVE EXTENSIONAL BASIN, SOUTHWESTERN MONTANA

Lateral migration of 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 inve stigations of the morphology and deposits of asymmetric meander belts of the Madison River and its South Fork, southwestern Montana (using t opographic mapping, ground-penetrating radar profiling, coring, sonar, and radiocarbon dating) have been undertaken in an attempt to show ho w they are influenced by tectonic tilting and faulting, base-level cha nge, catchment evolution, and climate change. The asymmetrical form of the meander belts resulted from a regional tectonically induced later al bias to channel position over at least the last 10 ky. Radiocarbon ages from abandoned channel fills 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 systemati c 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 meande r 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 te rrace remnants on the down-tilt side of the meander belts. The distrib ution of terraces and large abandoned meander loops in the South Fork shows that channel-belt asymmetry did not develop by progressive later al migration of the meander-belt axis, as previously suggested, but ra ther that the flood plain episodically decreased in width, with the do wn-tilt margin fixed in space. Some of the Madison River terraces, how ever, do show evidence of down-tilt migration of the river. Preservati on of diatomite in downstream parts of both meander belts suggests per iodic transgression by an ancestral Lake Hebgen. Changing lake shore p osition may explain some of the episodes of incision, and is probably associated with large seismic events and surface deformation.