EARLY PLEISTOCENE (< CA. 2 MA) DENUDATION OF NORTHEAST ARIZONA INFERRED FROM RECONSTRUCTION OF THE WHITE MESA PALEODRAINAGE

Geologic mapping and geochronology show that alluvium post-dating cutting of Grand Canyon and integration of the Colorado River system records rapid denudation of a large region south of Glen Canyon. The White Mesa alluvium crops out at two localities along a 57-km SW-descending slope from White Mesa across Crooked Ridge to The Gap, 60-km NE of Grand Canyon. The 20–50 m thick alluvium accumulated in a low-energy suspended sediment fluvial system within the bedrock-bounded White Mesa paleovalley system. The paleovalley formed in Jurassic−Cretaceous strata and is presently up to 1 km above the Colorado River base level at Glen Canyon. Gravel composition, detrital zircon data, and paleochannel orientation demonstrate that the sediment is reworked from local Cretaceous bedrock—now mostly removed—and an unknown source that contained exotic clasts likely sourced from the San Juan Mountains. ⁴⁰Ar/³⁹Ar dating of an interbedded tuff at 1.993 ± 0.002 Ma and minimum detrital sanidine ages of 2.02 ± 0.02 Ma (n=2) from near the base show that White Mesa alluvium was deposited at ca. 2 Ma. Local geomorphic relations establish that the alluvium is older than the 1.2−0.8 Ma Bishop-Glass Mountain tuff.

Reconstruction of the White Mesa alluvium paleodrainage basin suggests that it was bounded on the N and W by the San Juan and Colorado rivers, which were near their present position and depth by ca. 1.36 Ma (Wolkowinsky and Granger, 2004). The White Mesa paleovalley must have extended south of The Gap along the strike valley of the Echo Cliffs monocline based on modern topographic constraints; it joined the ancestral Little Colorado River or Moenkopi Wash, a tributary of the Little Colorado River, near Cameron, AZ. The volume of Cretaceous strata removed after 2 Ma south of the San Juan River is substantial. The area exceeded 10,000 km² and volume was substantially larger than 3,000 km³, assuming a thickness of 300 m. This rapid denudation resulted in up to a 100 km southward shift of the drainage divide between the San Juan-Little Colorado River, cutting of Chinle and Monument valleys, and removal of the northern portion of the ancestral Little Colorado River drainage basin.