PLEISTOCENE EXHUMATION RATES OF WYOMING INTERMONTANE BASINS FROM (U-TH)/HE DATING OF CLINKER
Natural burning of Tertiary coal beds has produced thermally metamorphosed rocks (clinker) over large regions of basins and plains in Wyoming, Montana and North Dakota. Being more resistant to erosion than surrounding Tertiary shales and sandstones, clinker forms distinctive geomorphic features and frequently caps escarpments with tens to hundreds of meters of relief. Coal is ignited by several processes, including range fires and spontaneous combustion, but it only burns when above the water table. Regional erosional exhumation lowers the water table relative to bedrock, exposing structurally deeper levels of coal to burning. Burning resets low-temperature thermochronometers in detrital minerals in and above the coal beds, providing an age of burning, and in some cases constraining the rate of fluvial incision and erosional exhumation in the region.
We measured (U-Th)/He ages of detrital zircons in clinker from the Powder River Basin of northeastern Wyoming. Two samples from the DeSmet clinker on the western edge of basin are 65 ± 10 and 12 ± 3 ka (former 36 m above latter). On the eastern side of the basin in the Wyodak clinker of the Rochelle Hills, ages are 513 ± 41 ka, 486 ± 38 ka and 478 ± 39 ka (replicates), 207 ± 17 ka, 191 ± 15 ka and 206 ± 16 ka (replicates), and 175 ± 88 ka. The Wyodak coal and clinker in the Rochelle Hills dips gently to the west, exposing a structural depth of 125 m over 8 km. Zircon (U-Th)/He ages in the Little Thunder Creek escarpment region decrease systematically from 513 to ~175 ka in an east-to-west transect through about 60-70 m of structural depth. These ages are concordant with, but generally more precise than, zircon fission-track ages from other samples from the region (Coates and Naeser, 1984). To the extent that these time-transgressive ages represent lowering of the water table relative to bedrock due to regional erosion, they can be used to estimate a regional erosion rate of about 0.1-0.2 mm/yr for this part of the basin. These rates are consistent with apatite fission-track data from boreholes in the southern Powder River basin (Naeser, 1992), which, if interpreted in terms of exhumation alone, require post-Late-Miocene erosion rates of at least 0.1 mm/yr. We are currently dating samples from other dipping or stacked sequences of clinker units elsewhere in the region for erosion-rate constraints.