APATITE (U/TH)/HE GEOCHRONOLOGY AND EROSIONAL HISTORY OF GRAND AND BATTLEMENT MESAS, WESTERN COLORADO: IMPLICATIONS FOR LATE CENOZOIC EROSIONAL HISTORY OF THE UPPER COLORADO RIVER BASIN

New apatite (U/Th)/He data, along with radiometric dating of late Miocene basaltic lava flows, constrain the late Cenozoic erosional history of the upper Colorado River Basin in western Colorado. Cooling ages were determined for apatite samples from upper Cretaceous and lower Tertiary sandstone outcrops along the Colorado River in the vicinity of Grand and Battlement Mesas. The mesas are capped by 11-9 Ma basalt flows, and represent a late Miocene paleo-surface. Based on the elevations of the lava flows and of the underlying apatite-bearing sandstones, the samples were buried 1.2-1.7 km ca. 10 Ma.

For the Grand Mesa samples, cooling ages (n=18) range from 123.75 +/- 9.45 to 8.68 +/- 0.61 Ma. The highest elevation samples (1650-1830 m) produced mostly mid-Tertiary cooling ages that indicate the samples were buried insufficiently to completely reset the sample cooling age. For the lowest elevation (1480 m) and most deeply buried sample (~1.6 km deep), cooling ages (n=5) average 18.6 +/- 1.4 Ma with one outlier age of 53.2 +/- 3.7 Ma. Inverse HeFTy time-temperature modeling is ongoing, and will be used in conjunction with geologic constraints such as the ca. 10 Ma Grand Mesa paleo-surface to evaluate regional late Cenozoic erosion. San Juan Volcanic Field ignimbrite events represent the final phase of sediment burial in the region, and the final ignimbrite event occurred ca. 23 Ma. Modeling of apatite (U/Th)/He cooling ages will provide insight on the timing of subsequent Neogene erosional episodes.

New data on regional erosion events is crucial for testing ideas of late Cenozoic mantle-driven differential uplift and erosion in the Colorado Rockies, and the effects of river integration events. Early Miocene cooling ages could reflect fluvial erosion and formation of paleovalleys that were filled by late Miocene Grand and Battlement Mesa lava flows. Alternatively, late Miocene cooling ages could reflect a pulse of fluvial incision triggered by Colorado River integration through Grand Canyon ca. 6-5 Ma.