TIMING AND GEOMETRY OF THE ‘GREAT DENUDATION’ OF THE GRAND STAIRCASE OF THE SOUTHERN COLORADO PLATEAU FROM COMBINED GEOLOGIC AND APATITE THERMOCHRONOLOGIC DATA

The evolution of the landscape of the southern Colorado Plateau has been a topic of interest for well over a century. Clarence Dutton first proposed that a “Great Denudation” had occurred sometime during the Tertiary, stripping Mesozoic strata away from the region surrounding the Grand Canyon to form the iconic Grand Staircase of southern Utah and the high relief landscape we see today. Numerous studies have focused on the denudational history of the Plateau, often using low-temperature apatite thermochronology, in an effort to constrain the time and therefore the mechanism by which it gained in elevation while maintaining minimal deformation in comparison to the surrounding regions. This study uses a combination of geologic surficial constraints and inverse modeling of 10 new and 45 published samples with apatite He data (15 of which also have apatite fission track data and 2 of which have ⁴He/³He profile data) to produce a comprehensive model of the Cenozoic landscape evolution of the southern region of the Colorado Plateau. We find three main times of denudational cooling: 1) during the time of the Laramide orogeny (70-50 Ma), 2) during the ignimbrite flare-up and initiation of Basin and Range extension (30-15 Ma), and 3) Pliocene-present day erosion. Denudational cooling via cliff retreat during the Laramide orogeny is focused on the southernmost and westernmost margins of the plateau and near the Kaibab Uplift. Denudational cooling during the ignimbrite flare-up is focused farther east, near the Little Colorado River corridor and along a proposed East Kaibab paleovalley in the Eastern Grand Canyon that was likely cut by an ancestor of the Little Colorado River. Denudational cooling since the Pliocene is focused along the modern Colorado River, particularly in Marble Canyon, and near the base of the present day Grand Staircase. Based on these results we propose a multi-stage uplift model, with some uplift occurring during each of these periods and driving drainage reorganization events that further enhanced denudation and drove pulses of cliff retreat across the southern Colorado Plateau.