HOT DATES WITH CLINKER: A TOOL TO ASSESS LANDSCAPE EVOLUTION IN THE POWDER RIVER BASIN

In the Powder River Basin (PRB) of southeast Montana and northeast Wyoming, gently-dipping coal beds exposed by regional downwasting have burned naturally from at least the Pliocene to the present. More than four thousand square kilometers of reddish clinker deposits - formed by the baking, fusing, and melting of layers of sediment originally lying above burned coal beds - cap ridges and escarpments throughout the dissected landscape of the PRB. It is literally a landscape formed by fire - albeit many separate fires in many different places over the past few million years.

(U-Th)/He and fission-track ages of zircon grains from baked sandstones in clinker provide new insights about rates of regional erosion as well as the episodic advance of coal fires into hillslopes. Older, resistant clinker layers up to 60 meters thick, formed by the burning of thick coal beds, control downwasting, and limited new data indicate that narrow rims of thinner clinker on the sides of valleys between these thick layers are much younger. This pattern suggests that downcutting of streams and backwasting of slopes dominate over downwasting of the clinker-supported flat divides between drainages.

We are concentrating on dating clinker from the Wyodak-Anderson and Knobloch coal zones of the Fort Union Formation in two areas: the Rochelle Hills east of Wright, Wyoming, and the Tongue River Valley near Ashland and Birney, Montana. Additional ages have been obtained on clinker samples from the Felix and Lake de Smet coal zones in the Wasatch Formation. Recently determined (U/Th)/He ages of clinker from these areas range from as old as ~ 1.1 Ma to as young as 10 ka. These ages generally agree with fission-track ages of clinker analyzed in the early 1980's, but have established a higher precision record of the spatial-temporal pattern of ancient coal fires in the region. Dates so far indicate about 200 to 400 meters of downcutting in the past million years by major river systems in the basin. Our ongoing (U-Th)/He dating analyses will enable us to better map the pattern of fluvial incision and slope retreat in the PRB over space and time, and to weigh the relative importance of uplift, variations in climate, and base level change in exhuming the PRB during Late Cenozoic time.