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

[摘要] Late Cenozoic erosion in the Powder River Basin of northern Wyomingand southern Montana has exhumed numerous coal beds to shallow depths where theyburn naturally, forming erosion-resistant metamorphic rocks called clinker. Becausemost clinker forms tens of meters from the surface, its formation age records thetiming and rate of exhumation through this depth, which can be used to constrainincision and lateral backwasting rates and theevolution of topographic relief. Zircon (U-Th)/He ages from ~100 distinctclinker 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 mostformed in one of the last three interglacial periods, reflecting either changesin fluvial incision caused by glacial-interglacial cycles or other climatic effectson rates of natural coal burning. Most clinkerolder 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 providesa maximum age of 2.6 ± 0.2 Ma for terrace formation. This corresponds to the onsetof major Northern Hemisphere glaciation interpreted from marine records, suggestingthat the terrace formed by lateral erosion of the landscape as rivers were overwhelmedwith sediment during the earliest Plio-Pleistocene glacial episode. The overallcorrelation of in-situ clinker ages with elevation above local base level can beinterpreted with a simple model for shallow exhumation ages that requires increasingincision and topographic relief over at least the past ~1 Myr at rates of ~0.1–0.3km/Myr, assuming typical clinker formation depths of 20–40 m.

[学科分类] 地质学

[关键词] Fire safety;fatal fires;smoke alarms