DEVELOPMENT OF QUATERNARY ALLUVIAL FANS AND PIEDMONT GLACIER MORAINE FANS ALONG THE MADISON RANGE FRONT, SOUTHWEST MONTANA

Tectonically active extensional mountain fronts in the northern Rocky Mountains are often characterized by development of Quaternary alluvial fans and piedmont glacier moraines. Eight fan-shaped depositional landforms with multiple morphologies present at major drainage basin outlets along the western Madison Range front, southwest Montana were studied through Google Earth analysis and field reconnaissance. Fans were parameterized by slope, area, surface morphology, and deposit types; drainage basins by slope, area, bedrock lithology, and presence of glacial deposits/features. Fan-shaped landforms include: 1) large sheetflood alluvial fans, 2) large debris-flow alluvial fans, 3) small debris-flow alluvial fans, and 4) piedmont terminal/ground moraine fans.

Drainage basin area corresponds directly to alluvial fan area and slope. Large (45.3-67.9 km²), low-gradient (1.3-2.6°) sheetflood fans possess the largest (43.3-156.7 km²) drainage basins (Cedar and Indian Creek); small (1.0-1.6 km²), steep (5.6-5.9°) debris-flow fans have the smallest (3.6-6.0 km²) drainage basins (Shell and South of Deer Creek). Larger (3.9-12.4 km²), moderate slope (3.7-4.1°) debris-flow fans characterize drainages of intermediate size (9.8-11.7 km²) (Mill and Tolman Creek), with some evidence of tilted sandstone and shale lithology controlling debris flow generation. Fan-shaped piedmont glacial moraines show no relationship to drainage basin and lithologic parameters and were deposited upon older Pleistocene alluvial fans (Wolf and South Fork of Indian Creek).

We suggest along-strike (spatial) and vertical (temporal) variation in deposition of coarse-grained fan deposits reflects both tectonic and climatic controls on sediment production and delivery and is more complex along active extensional mountain fronts developed in high latitudes under glacial conditions.