CONSTRAINTS ON LATE CRETACEOUS UPLIFT AND DENUDATION OF THE SOUTHWESTERN COLORADO PLATEAU FROM U-PB DATING OF LACUSTRINE LIMESTONE

The uplift and denudation of the Colorado Plateau is important in reconstructing the geomorphic evolution of western North America. A Late Cretaceous (64 ± 2 Ma, n=3) U-Pb age for the Long Point limestone, which overlies the regionally significant erosional surface developed on Permo-Triassic formations, suggests an early uplift of the southwestern Colorado Plateau. U-Pb analyses of three separate outcrops of Long Point limestone gave ages of 64.0 ± 0.7, 60.5 ± 4.6, and 66.3 ± 3.9 Ma, which are considerably older than a fossil-based, late Paleocene-middle Eocene age for the limestone. Samples of the Long Point Limestone were dated using the isotope dilution isochron method on well-preserved carbonates having high uranium and low lead concentrations. Secondary cement that normally obstructs the absolute age measurement was found to contain little uranium, and based on the three isochron results yielding the same age but having markedly different U and Pb concentrations, we interpret the U-Pb ages to be accurate. This approach demonstrates the potential of using U-Pb dating of lacustrine limestones in paleontological and landscape evolution studies. A 64 Ma age for the Long Point limestone, and the deep denudation of the Coconino Plateau almost down to the Kaibab Limestone level by this time, has important implications for the geomorphic evolution of the Grand Canyon region. It implies that the southwestern Colorado Plateau and Kaibab arch had already uplifted by 64 Ma and that any Late Cretaceous-aged paleocanyon in the Long Point area (eastern Grand Canyon region) west of the Kaibab arch had not yet penetrated down past the top of the Kaibab Limestone by 64 Ma.