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Abstract

A geologic map of the Nez Perce drainage basin in southwestern Montana offers an understanding of the developing paleotopography of the area following the Laramide orogeny, ca. 65 ma. In this project, a small drainage basin was studied in detail, focusing on the geomorphology, structure, lithology, and ages of the various rock units. Based on the results of these studies and the relationships found among four major gravel units mapped within the basin, the basin development was approximated. The youngest unit is a fine-grained (mean diameter = 8.23 cm) channel deposit within Nez Perce Creek, sourced from within the basin, based upon gneiss and quartzite derived from the present drainage. The next-youngest unit is a fine-grained (mean diameter = 10.92 cm) alluvial sheet deposit forming broad flat surfaces adjacent to Nez Perce Creek in the central basin, which was also sourced from within the present drainage based upon gneiss and quartzite derived from the present drainage. The next unit, a coarse-grained (mean diameter = 22.90 cm) Miocene gravel creating the western divide of the basin, was sourced from the Highland Mountains to the north. The oldest unit, a coarse-grained (mean diameter = 39.88 cm) debris flow, is interlayered with Eocene basalt and rhyolite tuff, allowing its age to be bracketed to ca. 48 ma using Ar39/Ar40. Based on unique lithology, this unit was sourced from the Pioneer Mountains to the east. One large fault separates Archean gneiss bedrock on the east side of the basin from younger gravel deposits on the west. Several younger normal faults within the divide gravel indicate tectonic activity more recent than the Miocene time. This evidence of recent faulting, along with a lack of evidence of geomorphology shaped by fluvial erosion, indicates that basin development was more recent than previously assumed (Reynolds, 1979; Rupple, 1993) and was controlled more by structural than by fluvial forces.

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