July 2011 in GSA Today: Clinker geochronology

July GSA Today science article authors Peter W. Reiners of the University of Arizona and colleagues have developed and successfully carried out a novel, extraordinary technique for learning how efficiently river channels cut and increase local topographic relief: They have used the exposure of "clinker" deposits in combination with highly refined dating techniques. Clinkers are baked coals; baking naturally occurs in shallow depths (tens of meters) and when the clinkers are exhumed during erosion and the development of topographic relief, they are resistant and the rate of exposure can be timed using the U-Th/He isotopic system found in zircons within the clinkers.

In this application, the Reiners and colleagues determined aspects of the pace of recent evolution of Powder River Basin in northern Wyoming and southern Montana. Ages of in-situ clinkers range from as old as 1.1 million years to as young as 10,000 years, but most formed in one of the last three interglacial periods, reflecting either changes in fluvial downcutting caused by glacial-interglacial cycles or other climatic effects on rates of natural coal burning. Clinker deposits atop a broad terrace in the northern part of the Powder River Basin provide a maximum age of 2.6 plus or minus 0.2 million years 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 clinker ages with elevation above local base level suggests generally increasing incision and topographic relief in the Basin over at least the last one million years, at rates of ~0.1-0.3 km/million years.

Provided by Geological Society of America