VARIATIONS IN RATES OF QUATERNARY DENUDATION IN THE RAINBOW BASIN AREA, MOJAVE DESERT, CONSTRAINED BY IRSL GEOCHRONOLOGY ON FLUVIAL TERRACE AND FAN DEPOSITS

The landscape of the Rainbow Basin area in the Mojave Desert developed below a denuded bajada that was composed of sediment shed southward from the Mud Hills, which is an erosional upland formed in a Miocene basin-fill sequence (“Mbf”). Late Quaternary tectonism has led to denudation of the Mud Hills bajada and renewed exhumation of the Mbf. To determine the timing and rate of denudation and topographic development in the Mud Hills, we used post-infrared, infrared luminescence dating (pIR-IRSL) to measure the age of the incised bajada, as well as a lower fluvial terrace level developed within the exhumed Mbf. The Mud Hills bajada is composed of an approximately 20 meter-thick gravelly alluvial sequence overlying an unconformity cut in Mbf. Three pIR-IRSL dates from this deposit ranged from 61 ± 8.5 ka to 71.3 ± 8.8 ka. The lower terrace level within Rainbow Basin is composed of terrace alluvium that varies in thickness from less than 1 meter to 6 meters overlying straths and meter scale cut-and-fill structures cut into the Mbf. Two pIR-IRSL ages from this lower terrace were 29 ± 3.9 ka and 33.7 ± 3.9 ka. The straths and cut-and-fill structures are only 1 to 5 meters above active bedrock-eroding stream channels, whereas the unconformity under the bajada is approximately 54 meters above the bedrock-eroding channels. The sample sites align along orogenic strike and therefore indicate substantially different denudation magnitudes at the same distance from the source area of the bajada alluvium and of the terrace alluvium. Relatively little stream incision into Mbf has occurred since approximately 30 ka at rates of 0.02 to 0.2 mm/yr; contrastingly greater stream incision into Mbf occurred between about 60 ka and 30 ka at a rate of 1.2 to 3.6 mm/yr. Detailed mapping and GPS surveying of the base of the bajada deposit show that it has been warped down across the southern front of the Mud Hills, which may be the result of deformation above a blind fault that has contributed to uplift of the Mud Hills. Ongoing work is focusing on documenting the amount and style of deformation of the 60 to 70 ka bajada deposit and better understanding the variation in incision rate.