ALLUVIAL FAN MORPHOLOGY, CHRONOLOGY, AND FAULTING IN THE SOUTHERN BEAVERHEAD MOUNTAINS, IDAHO

The Beaverhead fault is a normal fault located north of the eastern Snake River Plain in southeastern Idaho. The two most southern fault segments are the Blue Dome and Nicholia which are 25 and 42 km long respectively. The Blue Dome segment is south of the Nicholia segment where the segment boundary between the two is defined by a right step in the range front fault. The segments display distinct fan geomorphologies rooted in seismic activity contrasts and Middle-Late Pleistocene climatic variability. We use lidar-based geomorphic mapping and pedogenic carbonate data, coupled with optically stimulated luminesce (OSL) and cosmogenic radionuclide (CRN) geochronology, to reveal the fan histories.

Geomorphic mapping reveals contrasts between the fan sequences and morphologies of the two segments. The Blue Dome segment has five alluvial fan units (Q1-Q5, youngest to oldest). Q1-Q5 fans mark the mouths of several larger canyons, while small, generally younger fans (Q1-Q2) mark the mouths of minor drainages on the range front. The Nicholia segment has four alluvial fans (Q1-Q4), with a clear morphologic sequences at major canyon mouths.

The relative alluvial fan chronology is rooted in mapping and carbonate rind data collected from 23 trenches. Carbonate rinds were measured on 50 pebble clasts from each trench. Mean clast thickness and thickness ranges are coupled with morphologic relationships to correlate fan surfaces within and between the two study segments. Alluvial fan ages and fault chronologies are to be further constrained with 23 OSL ages and 8 CRN ages on fan boulders. The numerical chronology will provide robust calibration of carbonate rind accumulation rates.

We infer that the differences in fan morphologies are rooted in contrasting fault activity between the two segments. Fault scarps cut the Q2-Q4 fans in the Nicholia segment, but have not been observed cutting any of the fans in the Blue Dome segment. The simpler Nicholia fan sequence appears to reflect burial of older fan surfaces and more frequent enhancement of accommodation space through fault hanging wall subsidence, while the Blue Dome segment fan sequence appears to reflect widespread aggradation followed by episodic climatically driven incision.