Timing of erosional episodes in the Marble Canyon and Vermilion Cliffs region from apatite (U-Th)/He thermochronology

The history of incision of the Marble Canyon segment of Grand Canyon, and the rate of retreat of the Vermilion Cliffs portion of the Grand Staircase, are examined using a synthesis of new and published low-temperature thermochronology datasets. I test models for a 50-70 Ma Marble Canyon and interpretations that postulate SW to NE progressive denudational stripping of the Colorado Plateau via cliff retreat. The main goal is to evaluate the timing of carving of Marble Canyon. Thermal history models were generated using HeFTy for 14 samples that range in elevation from 940 m (river level) to 2052 m (top of the Vermilion Cliffs) to 2420 m (top of the Kaibab uplift). I find that samples from these different elevations record different components of a multi-stage cooling history for this region. Samples along the Colorado River in Marble Canyon show that rocks resided above 110 °C until 30 to 40 Ma and cooled to near-surface conditions between 25 and 6 Ma, with progressively younger cooling ages upstream. Hence this portion of the Colorado Plateau was covered with approximately 2 kilometers of Mesozoic rock until 30 Ma, precluding the presence of a 50-70 Ma paleocanyon. Intermediate elevation samples on the Kaibab rim of Marble Canyon show rapid 30-15 Ma cooling due to regional denudation as the Cretaceous section was eroded, but the pattern of cooling reflects carving of a 25-15 Ma paleocanyon across the Kaibab uplift, rather than a simple SW to NE cliff retreat. Post 10 Ma cooling along the river in Marble Canyon, further denudation of the top of the Kaibab uplift, and in the headwaters of Kanab Creek, indicate that the modern Grand Staircase cliff retreat accelerated in the last ca. 10 Ma, but with geometry initiated in the last 25 Ma due to base level caused by the 25-15 Ma paleocanyon. Major findings are that: 1) Marble Canyon was carved quickly in the last ~10 Ma. 2) An early stage of cooling seen in many samples is attributed to cliff retreat initiated by carving of East Kaibab paleocanyon 25-15 Ma; 3) Formation of the Grand Staircase accelerated in the last ca. 10 Ma once the Colorado River was integrated through Marble Canyon area in the last 6 Ma. 4) Current interpretations for episodic incision 70-50, 25-15, and post- 10 Ma seem most consistent with a combination and interplay between vertical incision of paleocanyons that then triggers cliff retreat of rims.Earth and Planetary ScienceMastersUniversity of New Mexico. Dept. of Earth and Planetary SciencesKarlstrom, KarlCrossey, LauraKelley, Shar