SEARCH FOR THE ANCESTRAL COLORADO RIVER

The purpose of this study is to systematically investigate the gravel deposits of southern Utah and northern Arizona in order to determine if any of these gravels were deposited by an Ancestral Colorado River (ACR) that flowed north of the Grand Canyon before it became integrated to the Gulf of California at ~5.5-6 Ma. Included among the almost 50 sites visited are (from west to east): Kanab Creek Goosenecks, Little Cedar Knoll, Johnson Canyon, Kitchen Corral Wash, Wire Pass, White House trailhead, Dive Butte, Cedar Mountain, Kaibab Plateau, Paria Plateau, Navajo Bridge, Gap, Crooked Ridge, White Mesa, and Red Lake. All of the gravel sites eastward to, and including, the Paria Plateau - except for the Kaibab Plateau - contain colorful, pebble- to cobble-size, quartzite gravels typical of those shed southward from the Claron or other Late Cretaceous to Eocene formations. No exotic gravels of any kind were found on the Kaibab Plateau - only Timpoweap residual material was observed. Rounded, disc-shaped, porphyritic, volcanic pebbles and cobbles of Colorado River type were found on the Paria Plateau at Joe's Ranch water-tank site, and on Cedar Mountain along the power-line towers road, but both of these gravel occurrences are not in situ. The Claron-type quartzite gravels are concentrated on highs along present-day stream systems draining from the north toward the Colorado River (e.g., Paria River, Kaibab Gulch), and away from these drainages the gravels pinch out or get sparse. While Claron-type gravels exist along the eastern edge of the Paria Plateau, they have not been identified across the Colorado River on the Kaibab or Kaibito Plateaus. This suggests transportation by stream systems after the Colorado River was in its present position. Investigation of gravel types from White Mesa to the Gap discredit the notion that an ACR once flowed from that direction. Miocene incision by the San Juan River suggests an ACR near its present position, and barbed tributaries in Marble Canyon suggest that the Little Colorado River may have flowed northward to an ACR sometime before 6 Ma. Both imply a possible mid- to late-Miocene basin in the Glen Canyon region, but the evidence for such an ACR and basin remain elusive. A map, description, photographs of, and gravel samples from, the visited sites will be displayed at this workshop-type poster paper.