Clinker geochronology, the first glacial maximum, and landscape evolution in the northern Rockies

Late Cenozoic erosion in the Powder River Basin of northern Wyoming and southern Montana has exhumed numerous coal beds to shallow depths where they burn naturally, forming erosion-resistant metamorphic rocks called clinker. Because most clinker forms tens of meters from the surface, its formation age records the timing and rate of exhumation through this depth, which can be used to constrain incision and lateral backwasting rates and the evolution of topographic relief. Zircon (U-Th)/He ages from ∼100 distinct clinker units provide several insights into the geomorphic evolution of the region. Ages of in-situ clinker range from as old as 1.1 Ma to as young as 10 ka, but most formed in one of the last three interglacial periods, reflecting either changes in fluvial incision caused by glacial-interglacial cycles or other climatic effects on rates of natural coal burning. Most clinker older than ca. 200 ka is either detrital or >∼200 m above local base level. Detrital clinker atop a broad strath terrace in the northern part of the basin provides a maximum age of 2.6 ± 0.2 Ma for terrace formation. This corresponds to the onset of major Northern Hemisphere glaciation interpreted from marine records, suggesting that the terrace formed by lateral erosion of the landscape as rivers were overwhelmed with sediment during the earliest Plio-Pleistocene glacial episode. The overall correlation of in-situ clinker ages with elevation above local base level can be interpreted with a simple model for shallow exhumation ages that requires increasing incision and topographic relief over at least the past ∼1 Myr at rates of ∼0.1-0.3 km/Myr, assuming typical clinker formation depths of 20-40 m.