GEOLOGIC MAP OF THE NEZ PERCE DRAINAGE BASIN, SOUTHWESTERN MONTANA

A geologic map of the Nez Perce drainage basin in southwestern Montana offers an understanding of the developing paleotopography of the area. Four gravel units were mapped in the basin, and Eocene volcanics underlie and intrude the western units, providing a means of bracketing their age with Ar39/Ar40. The western portion (headwaters) of the Nez Perce basin is underlain by three gravel units. The central portion of the basin is underlain by Archean metamorphic bedrock with little or no overlying gravels. A fourth group of gravels forms terraces and a Holocene floodplain in the downstream, eastern portion of the basin. The oldest gravel is exposed along the northwestern divide, adjacent to the Highland mountains, and is the least extensive of the gravels. It contains dispersed cobbles and boulders up to 6 m in diameter composed of local metamorphic gneisses, amphibolites and Quadrant sandstone, which are absent in the two younger units. Few cobbles between the boulders suggest that this was a debris flow. Lithologic and topographic evidence suggest that flow was derived from the north. A second gravel unit, which overlies this deposit and forms most of the western basin adjacent to the Divide Basin, is composed of relatively well sorted, bedded, and rounded clasts that include red, purple and brown quartzites and sandstones, similar to lithologies from the Pioneer Mountains to the west. Faulting between this gravel and Archean gneisses to the east, with the absence of gneiss clasts in the fluvial gravel suggests a western source. The third gravel is reworked from the second unit: It extends downslope from the second gravel and has a similar lithology, but smaller and more broken clasts than those in the second unit. It is unclear if these units are facies of the same deposit or are two separate deposits due to faulting between them. A fourth unit in the downstream part of the basin contains locally derived gravel clasts that are the smaller and more angular than those of the older units in the basin. This gravel forms terraces and floodplain of Nez Perce Creek. The three older gravel units have extrabasinal sources, are unrelated to the modern drainage, and formed as extensional basins evolved during the Tertiary. The youngest gravel was deposited after the present drainage basin formed during the Quaternary and Holocene.