DEDUCING CLIMATE SIGNALS IN QUATERNARY LANDSCAPE EVOLUTION IN THE CENTRAL ROCKY MOUNTAINS

Climate may be the dominant factor affecting landscape evolution during the late Cenozoic, but progress on deducing climate signals in stream incision records requires new suites of landform ages that are precise, accurate, and sufficiently numerous for statistical comparison with detailed climate records. The geology of the Powder River basin in Wyoming and Montana provides an unusually good opportunity for dating landforms that reflect stream incision: numerous coal seams in the Paleocene Fort Union Formation within the basin have caught fire during late Cenozoic incision, with lowering of the groundwater table and exposure of the coal to dry, oxygen-rich conditions. We use zircon (U-Th)/He ages of the clinker to constrain erosion, with the assumption that coal must be near Earth’s surface to burn. The precision of ages (generally <10% error) and density of ages (>80 sites with many replicates) allow statistical analyses of the relationship between landscape evolution and climate in the basin since 1 Ma. The probability density function of the ages for the entire basin shows two temporal patterns: 1) a bias toward younger ages because of erosion of older clinker, and 2) episodic occurrence of coal fires corresponding with times of high eccentricity in Earth’s orbit and low global ice volume. A spectral analysis of the probability density function shows peaks in spectral density at ~100 kyr and ~250 kyr periodicities. Ages farther than 50-km from the Bighorn Mountain front show stronger statistical correlations with climate records than ages close to the mountain front, suggesting that glaciers in the stream headwaters do not provide the primary climatic control on downstream incision rates. This is contrary to previous studies that have tied the formation of strath terraces in Rocky Mountain basins to cycles of upstream glacial erosion.