QUATERNARY STREAM AND FAN TERRACES IN THE BOISE FOOTHILLS OF THE IDAHO BATHOLITH MOUNTAINS

Drainages of the Idaho batholith mountains along the NE margin of the western Snake River Plain, which are tributary to the Boise River, have responded to faulting and base level drop during the Quaternary. In Hulls Gulch, response to said conditions is recorded in the form of numerous remnant gravel/cobble dominated deposits which mantle unlithified Miocene-Pliocene fluvial and lacustrine sands. Remnants in Hulls Gulch were grouped into four terrace levels (A, B, C, and D) using slope profiles and deposit similarities. The upper level “A” is a fan surface (capping the ridges in the area), which graded to a broad floodplain, while the lower levels are stream gravels deposited as the fan was incised. About 1.6 km upstream from the foothills front, remnants of all four terraces are present and have heights above the present stream of 50 m (A), 35 m (B), 23 m (C), and 12 m (D). Terraces A, B, and C are considered to be time equivalent in a sequential fashion to the Gowen, Sunrise, and Whitney terraces of the Boise River because they occur at the same relative positions above their respective stream levels. Using the age constraints Othberg provided for the Boise River Terraces (based on ages of overlying basalt flows, combined with glacial stages), age estimates for the studied terraces are; A: 415-560 ka, B: 215-310 ka, and C: 132-198 ka. Faults displace terrace A, and no other, restricting the time of the 12 m displacement to 560-215 ka.