RATES OF LATE CENOZOIC LANDSCAPE EVOLUTION CONSTRAINED BY (U-TH)/HE DATING OF CLINKER, POWDER RIVER BASIN, WYOMING AND MONTANA

The modern Rocky Mountain landscape has developed through extensive stream incision since the Pliocene, but constraints on the timing and rates of erosion are limited. Rare volcanic ash deposits in river terraces provide snapshots of former landscapes, but do not provide sufficiently high temporal resolution to deduce long-term changes in incision rate or the relationship between erosion and climate. The geology of the Powder River basin provides a unique opportunity to address spatial and temporal patterns of stream incision. Numerous coal seams in the Paleocene Fort Union Formation within the basin have caught fire during late Cenozoic incision, with lowering of the groundwater table and exposure of the coal to the atmosphere. We use (U-Th)/He ages of clinker to constrain erosion, with the assumption that coal must be near Earth's surface to burn. Ages of 51 in-situ samples range from 7 ka to 1 Ma, and a Pliocene sedimentary deposit contains clasts of clinker as old as 4 Ma. Clinker preferentially formed during interglacial periods, indicating that incision rates are higher and/or coal fires are more frequent during warm, dry times. Average rates of incision are >300 m/Ma, among the highest documented for the central Rocky Mountains region. Highest rates of erosion are found in the headwaters of streams, suggesting that knickpoint propagation upstream may be an important erosional process. Overall, the clinker ages demonstrate that rates of landscape evolution in the central Rocky Mountains have been temporally and spatially variable, controlled by climate and drainage network morphology.